🚀 Hey Jugglebot Enthusiasts! 🤖 I'm thrilled to finally share this update of the actuator journey with all of you! Your insights, curiosity, and passion have been pivotal in this progression. If you were designing this actuator, what would YOU do differently? Or better yet, if you could see one feature or capability added to these actuators, what would it be? 🧐 Don't forget, if you want to learn more about the project, or to lend a helping hand, check out our Zulip site, here: pdj.zulipchat.com/
@@MichaelMantion Hmm interesting. I'm in two minds about this. Do you think just making the music a bit quieter would be better? Or would you prefer just none at all?
Love the series and the focus on the process of basing the design in proper design requirements. Inspiring thank you, clearly you are on a great path with the project 👊👊👍
Super cool project! Immediately got a couple ideas as well. Sorry if these are already named, but here we go: - Route the extend and retract spring via the same wire, so kind of get a capstan drive. - Make the sleeves for the inner bearings slightly concave. to make them self align to the rod? Maybe also make a groove in the middle of them to make a channel for the wire, to turn that issue into a feature. It might remove an extra bearing for routing the wire. - Make the moving bearings three sets at a 90deg angle, to get a kind of rollercoaster setup. - Add a fan to the output shaft of the motor, that spins a cooling fan over the brake resistor. Well now that that's out in the world so I don't think about it again, good luck!
You should look at 1-1.5mm Dyneema rope for your cable, very light, very flexible, very strong, low friction, low elongation/elasticity and easy to splice to create loop or termination
Kevlar would be better as dyneema does exhibit some degree of creep when maintained under tension for long periods. Kevlar is absolutely stable. Also is it possible to arrange it as a continuous loop running back to the capstan (motor) where it would be anchored by a tensioning screw widget. Would you ease the termination clamp problem.
For the bearings, I would suggest a delrin or Teflon roller. You can get a pack of these easily, marketed as 3d printer rollers. They would stay on the bearings better, resist abrasion, and have a self-lubricating effect with the tubes. For the synchromesh cable, you may be able to design a clamping plate similar to those used for bike cables.
Very good suggestions, cheers! I think the bike clamping plate will probably be the final solution for this problem; super simple, small and light. Great thinking!
The wheels used by 3D printers to roll on 2020 could also be used. The wheels are really cheap. There are also different shapes. Maybe a concave shape is better for rolling on the carbon fiber. You can buy them with a groove. Or if you can control the spinning modify them with a static spinning drill bit and slowly turning the wheel.
Some of these have a Delrin "v" groove profile that would give you 2 points of contact per bearing on your carbon tube. This is the first thing that jumped to mind when I saw your printed sleeves....I would imagine that this would keep things located a little more positively (at least, short of a concave profile to match the tube radius) . You might be able to use 3 of the same on the captured end of the moving tube in place of the 2 guide rollers you currently have riding one of the outside tubes...basically the same thing you have holding that moving tube inside your printed end, but flipped inside out. That would both keep it centered AND prevent axial rotation given the 2 contact points inside the V. Anyway super interesting project, and very promising.
For the cables pulleys You can absolutely do the compression with 2 pulleys : Use the first pulley to first route the cable throught any point of the desired destination line, and then put a pulley at that point to get the destination line First step is always possible due to a point and a line always being in the same plane, and same for 2 intersecting lines in the second step Good luck with your project
For your linear bearings, a lot of newer industrial bearings use metalized ceramics. Systems that used to be rows of ball bearings on hardened steel rails have become ceramic sleeves which fit snug to the rail. I don't know if that applies to carbon fiber tubing, or much for your project - but a good lool at linear bearings might give you some ideas for how to stabilize your tube. I would even recommend going with some kind of graphite coating on the rods that can act as a dry lubricant. An oil or grease probably won't fit the application well, but can make a world of difference in longevity. Gears that run for decades with oil in them won't last a day without it. A little can go a very long way. You could look at just trying a teflon sleeve or set of pads that form to the wall of the tube. Play around with contact area and pressure to get what you need. The resin binder for the carbon fiber is probably going to be easy to wear. You may want to consider adding or trying to identify tubing made with a harder material. A silicon dioxide coating may improve wear characteristics (or perhaps an alumina spinel - quartz/saphire coat). If you want to go aerospace, you could try a sputtered titanium nitride coating. Not sure how well (or horrible) it would fuse - but if you're looking to go off the wall with it - that would be a direction. For the string, you might actually look into amarid/kevlar. The nomenclature to use is "tow" when looking for strings of fabric material (same with carbon fiber) - I spent days losing my mind looking for a carbon fiber string because I didn't know that was called a "tow". Well - a tow is unbraided, so I suppose not technically a string/line. You might look at fishing lines for off the shelf solutions and even talk to those communities about line routing.
The TPU sleeve over the bearing will work way better if you design them to fully engage with the surface area of the carbon fiber tubes. Make them like a circular indent like doing a sphere revolve cut in your cad program where the sphere is the same size as your carbon fiber rods. It will also help hold them centered if you add a bit either side of the bearing on the flat side that helps lock them in, like a U shape that comes down about a millimeter from the outside diameter of the bearing towards the center of the diameter. Hope this helps. 👍👍
Super glue probably doesn't stick well to TPU. Scotch Weld PR40 adheres very well to TPU. You could also print the TPU sleeves to be concave on the outside so they are self centering
you should be able to hold the synchromesh cable by having two small metal plates that clamp together with two bolts, when you tighten the bolts it will crush the cable in between the plates and hold onto it real tight, I am taking inspiration from how electric cables are held into some fixtures, its often molded into the plastic in consumer electronics so that when the chassis of whatever is screwed together the two mating surfaces pinch the cable to act as strain relief on where the cable is attached inside.
When adding heat inserts, as soon as you pull the iron away, flip the part over and press the face with the insert (and still hot plastic) directly against your workbench for 3-5 seconds. The flat surface of the workbench will keep the small raised ring from forming around the insert as it cools, so you won't need to shave it off manually. It will also help make sure the insert is aligned perpendicular to the face. Alternatively, you can press a small block of copper of aluminum (like a spare heatsink) against the fresh insert to accomplish the same task.
Just lying here, thinking that there are a couple of things that i might be able to usefully comment on... a bit off the wall, but i wonder if using two motors, although more complex, might give you control over things like stretch in a string, and providing some kind of dynamic control over the string action and loading... with encoders on the motors, and optical "zeroing" sensors, you get calibration on every cycle... i wonder if some kind of silicone 'oil' impregnating the string might improve lifetime ... indeed, steel cable with a 'dry' lubricant might work well, even on the smallish radius pulleys? I guess that a lubricant could be 'wet', if the actuator 'tube' was enclosed....anyway, sick in bed, just the ramblings of an amateur....
Interesting idea with two motors! I'm not sure it'd be worth the added complexity in my case though 🤔 Silicone lube on the string is a neat idea! Since this video I've done a heap of testing with different strings (video to come eventually...) and have found dyneema to be really good; high strength, low wear over repeated cycles around small pulleys, low stretch etc. Cheers for the input, and I hope you get better soon!
*Old Design Issues* - Bearings wore out quickly. - String clamping led to wear. - Assembly required hammering, causing parts to crack. - Not modular; any issue with one actuator affects the entire system. - Bowden tubes kinked and had slop. *New Design Improvements* - Strings internally routed, making it sleek and modular. - Motor contained within the actuator for easy replacement. - Clamping mechanisms simplify assembly. - TPU sleeves minimize wear on bearings. *Performance Metrics* - Endurance: Over 5 hours of continuous runtime. - Speed: Max 3.4 m/s. - Precision: Less than 0.06 mm. - Lifting Force: Almost 7 kg. *Future Improvements* - Use motor body as the spool for the string. - Consider using synchromesh cable. - Replace TPU sleeves with heat shrink. - Modify bottom cap to reduce torsion. *Community Engagement* - Set up a Zulip site for better communication. *Positive Points (Sorted by Importance)* 1. *Modularity:* The new design allows for easy replacement of individual actuators, reducing downtime. 2. *Endurance:* Over 5 hours of continuous runtime, passing 50,000 cycles with minimal wear. 3. *Speed:* Achieved a maximum speed of 3.4 m/s, meeting the design requirements. 4. *Precision:* Less than 0.06 mm, close to the theoretical limit of 8.4 micrometers. 5. *Lifting Force:* Capable of lifting almost 7 kg, exceeding expectations. 6. *Ease of Assembly:* New clamping mechanisms eliminate the need for hammering parts together. 7. *Reduced Wear:* TPU sleeves and internal routing of strings minimize wear and tear. 8. *Community Engagement:* Set up a Zulip site for better communication and sharing of ideas. *Negative Points (Sorted by Importance)* 1. *String Wear:* The string broke during precision testing, indicating potential durability issues. 2. *Torsion Issue:* When fully compressed, the actuator has little resistance against torsion. 3. *TPU Sleeve Problems:* TPU sleeves can slip off and are not perfectly smooth or symmetric. 4. *Complex String Routing:* The design requires complex string routing, which could be simplified. 5. *Braking Limitations:* Difficulty in slowing down the actuators, indicating a need for better braking mechanisms. 6. *Assembly Cracks:* The old design required hammering, leading to cracked parts. 7. *Old Design Wear:* Bearings and strings in the old design wore out quickly, affecting longevity.
@dine9093 tbh even if it were ChatGPT-made, I reckon it's still pretty good! I wouldn't think it'd be so effective just from (presumably) the transcript?
Awesome design, I am working on a project that needs a linear actuator and nothing I have found online will suit my needs. I cannot believe it never occurred to me that I could make my own, thank you for inspiring me.
Definately use the dual wheel design on all 3 outter shafts to stop torsional forces. And for braking you can use another mini actuator to press a cylindrical bearing down against the wheels in the bearing block and the harder it presses the more it brakes the bearing and an electrical actuator is very percise and fast so with the right algorithms it can precisely brake the movement.
Hello. Harrison. I'm writing because I think I can help you a little, with an idea. You talked about being able to tighten and change your strings on your Linear Actuators. when you tune the strings on a guitar, you turn a small handle. very simple mechanical structure. What might not be so good about it is the weight. But it can certainly be made very light in weight. Thanks for a great channel. Finn
To terminate the synchromesh cable the first thing that came to my mind is a two-piece metal clamp style endpiece with interlocking ridges...like a bike brake/gear cable clamp.
Regenerative breaking can be done with a 3 phase rectifier and some way to either store the energy or dissipate the energy. A good place to start is looking up "delta alternator diode layout". Delta alternators have nearly identical wiring to most brush-less motors and you can use the same type of rectifier layout because of this. The trick however is using PNP transistors instead of diodes. By using PNP transistors, you can monitor the motor driver's voltages at the transistor's base and limit the current of the rectifier when the motor driver is engaged. Keep going on the project, very interested to see where this goes!
Thankfully the ODrive is capable of handling the electrical stuff for regen braking, but the problem is that my electrical knowledge is subpar right now so I need to learn/figure out what batteries or brake resistors to use. I've tried the naive "increase brake resistor power, keeping nominal resistance the same" but that didn't work for some reason 🤷♂
Can it be done with less than 3 pulleys? Referring to the green string, my thought is to have the first pulley be slightly tilted and the correct diameter to stretch the distance between the motor pulley edge and the upper central pulley edge, so that the intermediate pulley essentially does the job of both pulleys you have right now, but it keeps the string in line coming off of the motor pulley and the upper pulley.
Hi @harrisonlow - great work! I wanted to point your attention to a type of string that has much higher strength called "spectra" or "dyneema". used most commonly in sailing, paragliding and kitesurfing, such strings can support 100s of kilograms in rather thin gauges. The fibers are also cut-resistant but you might have a heat problem with them and might consider lubrication to control that.
Thanks for the suggestion! I've actually since swapped to dyneema (0.8 mm) and I love it! 😍 Super strong, really thin and flexible and barely any creep. I actually have a video in the works on my testing of a bunch of strings 😁
@110:30. For the bearing surfaces, can you use an o-ring of something like teflon or viton? The viton would even give some resistance to tube rotation.
You should get a force gauge so you can get an accurate force reading for both extension and contraction; At least more accurately than having to binary search the max load.
You know what: I actually totally forgot that I have a force gauge lying around. I've been wanting to make a little box to house an LCD display and have a data-out port for datalogging but have been putting it off for the longest time. I completely forgot I have it! Maybe this is the push I need to finally get that sorted out. I'll add it back up my list. Cheers for the reminder 😊
It looks like a space elevator! For faster stopping you can use electromagnetic brakes - one that goes on the motor shaft and stops the shaft when you put power into it. You can use the motor to brake, if it is an AC or stepper motor, just applying a DC voltage to the windings should work as a brake, but with a DC motor you either have to reverse the input voltage or switch the circuit to a large load (resistor, light bulb or something) to do regenerative braking.
You could consider using a very large super capacitor as a "breaking load" for the ESC. If you dump into the capacitor and then pull power back out, you should end up with a net loss in power usage, thus the batteries will be at net discharge without worrying about chemistry issues.
You could reduce the number of pulleys by putting the motor axis at a right angle to the axis of the actuator. If you make the drive spool the right diameter and offset it a little (so one side is in the center and the other side is in line with the side tube) you would only need the one pulley at the far end of the 'blue string.'
Loving seeing this develop,thanks for sharing. Couple of quick thoughts... 1. Could you use concentric tubes, so one large and one small instead of four small? Also, can you get carbon fibre tubes in square section, which will help with the twist?
Cheers! I briefly toyed with the idea of using one large CF tube for the support structure (rather than three small ones) but I wasn't a huge fan of it for two main reasons: - Large CF tubes are super expensive - I don't like how opaque they are; I like being able to see the "inner workings" of the actuator. I've come to find that having an "open" design is also super helpful for fixing issues as they arise The square-section tubing is a good idea! I wonder how that would affect the geometry of the upper "bearing block"? I'll have a think about this. Thanks for the thoughts!
Consider nylon for instead of TPU for the bearing sleeves, it will make it more rigid / stiff / precise. You might also consider engaging more of the small linear rods with bearings.
Great project! What about dynema as the string? Very low stretch, very high strength. Standard braid so making sliced loops and terminations is easy. Also, compression cones for attaching a possibility.
I've actually recently been doing a TON of testing of different string materials and dyneema is looking quite good! What do you mean by compression cones? I'm not familiar with that term
the cheapest way to terminate the sychromesh cable would probably be a simple ring-tongue terminal that you crimp on the end. this way you already got a nice point to screw them into place. only problem i see with this is maybe some backlash you get, if you don't get the length exactly right on both cables, creating some dead space until the slack is picked up. Some kind of idler/tension pulley that you can adjust somewhere near the motor can solve this. I'm thinking some kind of set screw that changes the position of one of the pulleys, so you snug it up once and the whole system is ready to go until something else gives, stretches etc
I’d be curious what the accuracy of this (or the next iteration) would be with a bunch of consecutive movements that are all in the same direction, like 5 random points chosen to be consecutively descending or ascending, and measuring primarily the relative deltas that are then achieved _between_ the points, and then comparing that to the requested point deltas. I would not be surprised if your consecutive deltas end up showing far higher precision when moving repeatedly in the same direction without the driving string having to change. If this is the case it could be a good indication that some undesirable inaccuracy is coming from the slack transferring between the two strings, and you might be able to design movement paths around that (ie. preferring multiple consecutive movements in the same direction if at all possible)
10:55 Weld the end of the cable to a connector that connects to your bolt or, alternatively, use a larger bolt as the cable end connector, drill a hole through it horizontally, feed the cable through, sandwich it between two nuts, and tighten down.
For the termination of the cable you can put it through a tube, with a hole for a bolt, fixed to the piece at the end. Then you just have to press it with the bolt. I think it's the easiest way you can do it.
You can redirect the string with only two bearings for the green string, and (at least in the section you showed when talking about it) one bearing on the blue line. You should only need one pulley to change a strings direction. The cable clming off the first redirect pulley is in the exact same situation geometrically as it is coming off the spool (unwrapping from a cylinder with a vertical axle), only offset slightly.
Basically you should only need one pulley with it's groove/side face coming off tangent to the spool (and the axle horizontal) and redirecting the string straight up the tube in one case, and in the second instance to the central redirect. For your bearing on cf issue, bearings with delrin sleeves are an off the shelf part (look up delrin v wheels).
Realy nice project would like to see more of this. One safety approved could be add some end stopper for that middle pipe so it dosent fly out of when string cut off.
The subscribe button is just one click away 😜 I do like that idea, but it's also kinda fun not knowing if the central tube will shoot out or not 😁 (I probably will add safety features like this to the completed platform though. Depends how often/severely it happens in testing) Cheers for the feedback 😊
I suggest you to put the bearings in different orientations, 3 to 3 with some angle (I sopose 60 degrees in between) to get more points of contact with the bars and to prevent them of moving around its axes. Great design, thank you!
Very good improvements! I am looking to make some actuators for a very different application, but endurance is important to me, keeping cost in mind. I'd very very keen on seeing you implement your planned improvements.
Yeah that PID will need some updating once the Stewart Platform is working. My current settings are "just-good-enough" values for preliminary testing 😊 Cheers!
For the cables, perhaps a wire rope end stop with set screws for easy adjustment, or a threaded cable stud that you could adjust the tension of with a nut.
String routing without the two pulleys parallel to the motor shaft should be doable, as long as the pulley taking the string parallel to the actuator axis has the string enter/exit tangent to the shaft it should still spool fine. This does assume there isn't a large number of wraps when the actuator is at full travel, but as a rule of thumb anything up to 5deg off the pulley centreline should be fine without guides. I'm not saying this will be elegant, but it should be possible. If the torsion restraint engages all 3 carbon tubes, you can probably get rid of one of the sets of rollers at the top end. this will make the actuators stiffer at low extension, but if you have a desired limit then the minimum separation can be set to achieve that and everywhere else will be better. Threading strings through tubes can be done with compressed air, add a small flag or rag to the end if it has trouble to give it more to blow against.
You should be able to get the pulley count down to 2 per string: * for the extension string, just rotate the existing bottom-of-tube pulley around the tube's axis until the input string is tangent to the drive pulley, and remove the other motor-side pulley. * for the retraction string, chose a random point "A" in the plane of the drive pulley ~3cm away, draw a line from "A" tangent to the drive pulley in "B"; then set a point "C" on the central axis ~1cm above the pulley; the string should follow the path "BA" "AC" then up the central axis from "C". The 2 pulleys are semicircles at "A" and "C", tangent to "BA" and "AC", and "AC" "central axis" (in CAD, create a plane from each pair of lines and just add the circle) [for clearance, you might need to swap the extension and retraction drums]
Maybe you could try using carbon fibre yarn (tow), which won't stretch over time, causing different problems. As someone below already said, Teflon rollers are self-lubricating and will last forever. And if it would be acceptable to have the motor axis perpendicular to the axis of the actuator, thus eliminating all the pulleys, would probably eliminate all the slack which may be generated due to the wear of the pulleys. Alternatively, you may want to make the pulleys from teflon, too...
Interesting! I imagine there would be a host of other problems with carbon fibre tow, especially with the weird routing that I'm subjecting these strings to. I'm glad you suggested it though - I didn't even know this was a thing before now 😊 Cheers for the suggestion of teflon rollers - putting in an order for these is on my to-do list for today
instead of string have you tried a high-strength fishing line. It may last longer due to it being designed for reeling up and casting and the tensile strength of a large bluefin on the end of it?
What of you had the motor oriented 90° from the direction of travel, such that it forms the "leg" of an "L" shape? It's not *quite* as sleek, design-wise, but it would eliminate the need for many of the pulleys.
Cheers! Are you thinking these could be used to stop the torsion problem? I can see how this would also (probably) make the central tube quite a bit more stiff as well. I wonder if it'd be worth the extra weight? I'll put in an order for some and test them out. Cheers for the idea!
@@harrisonlow in my experience it should help with the torsion issue. I do believe, as you have planned, indexing on all 3 rods for the torsion issue would be best. I made the suggestion to keep the movement smooth and add support. Thanks and best of luck!
Haha now I'm not so convinced that bracing against all three support rods is best - I like the solidity that a linear rail would provide. I'll have to wait and see how the testing goes once they arrive. Cheers!
Wow, great project! For the braking, rather than resistors, dumping current into a partially-discharged battery should absorb the energy without so much temperature rise. The circuitry would be more tricky, but you should be able to find a lot of info out there under DC motor regenerative braking. You could use the battery as the power source, connected to a charging circuit to keep it from running down. Dumping current into a battery would cut the heat a lot, because it would be storing energy in the battery chemistry vs just dissipating it.
Yeah I think batteries are the way to go long-term. I just need to sit down and digest all the info online about it. Last time I found myself looking into Power Management Systems and charge/discharge rates I quickly felt like I was in over my head 😅 Cheers for the input!
To use the Synchomesh cable a set screw that holds and a tension screw that pulls the cable could be nice. Not sure if you tested bearing sleve on the rod? Love the idea with using the motor housing
EDIT: I see on Zulip that the same suggestion has already been made! You can save your eyeballs on this post! It's possible to use fewer pulleys. The trick is to imagine a plane for each pulley; right through the centre of the pulley and parallel to the pulley face. Each plane defines where the string coming into and going out of the pulley can lie. The trick is to intersect the pulley planes with the entry and exit points (pulley tangents) for each successive pulley. Dodgy blender model that visualizes the idea: ua-cam.com/video/vnq_sSShIdY/v-deo.html
Kevlar String? Might be a bit more wear resistant than nylon. According to other posts on Reddit, Synchromesh cable can be terminated with standard cable crimp connections.
Good idea! I wonder how small these mechanisms could be made, though? I'd need one tensioner on each end of the central CF tube, and I want them to be as small and light as possible
for terminating the synchromesh, look up wedge sockets. if a bolt into a 3d printed part is strong enough to terminate the current string I'd bet a 3d printed wedge socket would work for the synchromesh here
Pardon my ignorance, but is there any reason you're not using a linear beearing for the centeral shaft? I guess the CF might not be so compatable with direct contact from the bearings. Cool project regardless! Also for the synchromech cable, as a stab in the dark solution, could you try crimping it in an electrical ring terminal?
String is very stretchy. Under loads the braid tightens, eventually that is permanent. You had no way to compensate for that. For terminating the cable, which may also require stretch compensation, look at how it is done on steel cables. Crimp on a terminal to prevent fraying, but not for a load, or for loads, slip on a bigger terminal than loop the cable and put it back into the terminal before crimping. in electrical applications sometimes soldering is used, instead of crimping. I don't know what that cable is, but perhaps epoxy would work, maybe with crimping.
MBS on soft goods (like string/rope, webbing, slings, etc.) is for the most optimal loading scenario. Tight bend radii (like found in knots) will significantly reduce the actual breaking strength
9 months late, but could you use a pinch bolt for the syncromesh cable? The sort of pinch bolt that you might use for the braided cable on your bicycle brakes.
To terminate the synchro wire you could use a electronics standoff thread that into the plastic. Drill a small cross hole in the standoff. Thread wire through the cross hole and use a set screw to hold the wire. This would make it easy to change and Tighen.
Fascinating idea! I actually think we've figured it out over on Zulip 😁 A simple clamp built-in to the printed parts is easy to adjust/fasten with a single M3 bolt and is super sleek and effective! Cheers for the suggestion! I like the creativity 😊
Speed: overheating brake resistors. Use a cosinus curve (speed) at the end instead of feading all that power to a break resistor. Have had the same problem with a 220 Watt motor on a frequention speed control, it tripped above a specific speed while with that cos curve it did that at about a double speed. Hope this helps.
That's a clever idea! I can see how that would go a long way to getting higher max speeds. For better or worse, when Jugglebot is in operation, it's fairly likely it will (semi) often be sent commands that require it to stop and start really suddenly, and I'm not sure I'd always be able to smooth them out like this, but I will try to keep this in mind while I work on the control side of things. Definitely food for thought! Cheers 😊
One thing about your string strength test, how did you attach it to the weights? Something I didn't know until very recently is that how you knot a rope will affect *the rope's* strength, a lot. A tight bend can cut the strength down with over half. This is true for ropes, and should apply to strings as well, though probably to a lesser degree. The theory is basically this: When you have a straight rope at tension it pulls equally on the entire cross section of the rope. But if you have the rope in a tight curve the part of the rope on the inner part of that bend, obviously can't be in as much tension as the outer part of the rope. Therefore, all the weight lands on fewer fibres. Strings have fewer fibres and a much smaller radius, so they would be much less effected by this, but in your testing the string failed right at the knot precisely because it's the weakest part of the string. The redirection of the string shouldn't be a problem, even a small bearing has a large enough diameter to not be a problem with such a thin string, but the attachment at the very end could *possibly* be improved to get more strength out of your existing string. Either by clamping it so it's straight at the end, or using a better knot. My source, and more details, would be Kyle Hill's recent youtube video "The Figure 8 Knot Has Never Failed. Why?" It's about climbing ropes in particular, but it goes into the details of how knots and bends affect the strength of a rope at any diameter, and briefly mentions what diameter bend would be considered "no bend at all" from the strength standpoint. It's relative to the strings diameter so only the knotting/attachment will really be a problem.
Just got to the bit with the tensioning screw, that might just be weakening the string, hard to be certain. The rule of thumb is that the pin should be four times the diameter of the string to not weaken it. I know it doesn't matter for your application in specific, but in general wrapping the string around a pin like that can remove a fair bit of the strings strength. I can't imagine the threads biting into the string helps any there either. Why not simply weld the synchromesh cable into a loop, then use a tensioning pulley like you would on a timing belt for a 3d printer? Alternatively, something like a crocodile clip could possibly work. Though I'd be a bit concerned about the cable twisting over time and loosening. You could also simply wrap it around a pulley that you then lock instead, giving you an easy thumb screw to tighten or loosen it. It'd be bulky then though. If it wasn't clear, I didn't see your previous video on this system so my suggestions don't take any considerations you mentioned in that one in mind.
@@Sylfa Interestingly the string in the raw strength test actually failed somewhere in the middle of the string, quite far away from either knot! I do agree about the thread of the bolt not helping here: the string gets rather munged up after I tighten and loosen it a few times in the same spot 😬 I do like the idea of having the string being a single loop, though I'm not sure how to ensure the motor has a solid "grip" on the string? My understanding of belt-driven CNC machines-which use a similar concept-is that the belt is very tightly routed around the motor's output shaft, which has a toothed gear to match the profile of the belt. If I'm using plain string I have no idea how to ensure that the string doesn't slip on the spool? (though maybe this could be seen as a safety feature, where the string slips instead of snapping?). I can kind of imagine how it could work with the synchromesh cable thanks to its strange profile, though even then it might be tricky if printing the parts and using the smaller synchromesh cable (tolerances would be tricky here). Cheers for the input!
@@harrisonlow Yeah, I was thinking of the syncromesh cable for that suggestion. Using a string in a full loop, hrm. I guess, you *could* try 3d-printing a soft screwlike holder that winds the thread and keeps it from overlapping itself, but honestly it'd be slipping the thread through which would ruin the friction, and thread. The only real way I can think of doing it with a loop of string would be to have the loop just barely longer than what you need, then tightening it enough so it's got enough friction. Not sure that's possible. A timing belt would work better than that I'm sure, but I can't see that working unless everything's in-line so you'd have to restructure it entirely. Don't think it'd be worth it for your application. That's what CNC machines use though, at least the ones I'm aware of. In case you weren't aware, the timing belts are basically a flexible cogwheel, allowing you to put it over cogs to have no slipping, you only need enough tension so that it doesn't slip out of the teeth. As to it snapping in the middle, that's a good sign that what you're doing is not a problem, I guess strings really don't have the same issue as thicker ropes. Or if they do, it's minimal. Strangely enough I could have sworn I saw the string snap at the weight in the video, guess my mind was just playing tricks on me. 😂 Yeah, I think the only real consideration is to avoid jamming the thread into the screw, to avoid chewing up the string over time. At least if you don't need the full strength the motor can output.
I'm not sure that just having enough tension would work, though a simple solution like wrapping rubber (or heat-shrink?) over the spool might be enough to stop it from slipping. Just re-checked the "strength" footage and it definitely snaps in the middle (when doing the direct string strength test). No idea why it doesn't snap at the knots 🤷♂️
without a better metrology toolbox, you can measure repeatability by using a fixed object and shims, move away and come back to brush the object and confirm with shims more precisely, it could be challenging depending on how much position authority/rigidity the controller has, if it's too elastic the shims could lack feeling
That's a good idea! I considered setting the actuator up to touch an object and return back to it (ideally without knocking it over) but couldn't think of a way of using that to create actual measurements (rather than a simple "under-shot" / "bang-on" / "knocked over"). Using shims is a great idea! I do wonder about the elasticity of the system masking the true result, though
The Actuator would resist better bending forces if you moved 3 of the roller to the bottom of the moving carbon rod. You can remove one of the 3 pulleys if you replace 2 pulleys in x and y plane by one pulley that is in a plane that is tilted 45 degrees from X and Y. You can double the strings to increase the breaking strenght.
For braking or regenerative braking I have three ideas, peltiers are fun because they produce less heat as resistors, you could also make your own resistor out of plastic embedded metal wire sold in hardware stores for next to nothing, or aluminium foil for example. For regenerstive braking you buy a 3 dollar 3 phase to 2 phase rectifier or build your own from diodes then the power goes into a dc motor controller to adjust braking power then into a step up converter. I never managed to test this idea even though I wired all the electronics but I still think it might work. Good luck.
Interesting ideas! For better or worse, I'd (currently) prefer to stick with using the ODrives and figuring out how to fix this issue "properly". I know that there's some way to do this with my current setup, but I'm just goosing something and can't figure out what it is! Cheers for the suggestions 😊
I am new to python coding and that endurance test counter would be interesting to see.. But this whole project is amazing. Thank you for documenting and sharing it!
The way that counter worked was to simply add 1 each time the actuator reached the end of its movement 😊 FWIW, I have a bunch of the code up on Zulip so check out the link in the description if you're interested to see it. And if you want any specific code, just let me know and I'll happily share it with you 😁
The precision could be improved by changing the winding mechanism because, currently, the line is winding up on itself and unpredictably changing the diameter of the winding pulley.
Amazing design and great video, thank you for sharing all of this knowledge with us! Did you think about using square CF tubing instead of round for the middle shaft? It could use 2x4 bearing, giving much better stability sideways and it would be resistant to rotating.
I honestly didn't even know that square CF tubing was a thing! (despite my supplier selling it 😅). I'm hesitant to go for square tubing though because I don't want to add even more bearings to the bearing block, though I do like the resistance to rotation. Others have suggested hexagonal CF tubing and I quite like that idea, but I can't find a supplier in Aus...
@@harrisonlow I bought some Hexagonal tubes from China for an old project, I think that should work with 2x3 bearings,, but honestly I love redundancy so I would add more bearings (but also this increases complexity, resistance and weight). I think I've seen some triangular tubing too, using that might be the ultimate solution: you can keep the current 2x3 bearing setup, all faces would be covered and it should be resistant to rotation.
To terminate the cable you could make a clamping block to clamp around the cables helix with a 3d print In two halfs, and the to tension just a through bolt fully threaded pushing up against the fixed part of the assembly but running through the 3d printed cable clamp, turning the bolt would then increase tension, almost like a chain tensioner on industrial equipment
Interesting idea! Do you think this could be made super small? I'd need one of these on each end of the central CF tube so they need to be quite small and light
you could do a combined testing aiming at measuring the endurance compared to the amount of mechanical energy deployed, the weight lifted times the distance * how long it took, in g per meter per second, same as it was a conveyor belt
Yeah for a while I was considering doing an "output power" test (Force x Velocity) but ultimately decided it would be too tedious to run and I didn't want to have to deal with more broken strings as a result of overheating/failing brake resistors
Measure with mirror (front surface if possible) and laser. Adjust resolution based on distance. Used to Align aerial camera systems this way to a few microns.
If the motor can is used as a pulley, the string wraps decrease cooling surface area for the motor and the heat could soften polymer string. High loads could flex a thin motor can and pop the magnets off. Not likely to cause failure, just something to watch for if weird symptoms show up. Great iteration.
Interesting take on that solution! I hadn't considered either of those failure modes before. I don't think the decreased cooling surface would be very noticeable because the circumference of the motor is quite large compared to the lengths of string I'm dealing with here, though the oblique loads on the body of the motor could well cause problems (I'm not familiar enough with BLDC motors to know if this is likely or not). Cheers for the input!
Maybe you can get rid of the roller bearings completely by changing the design to slide bearings/shaft guides? I found some relatively low cost composite parts at IGUS, they rate their shaft guides up to 5m/s. I don’t know if the carbon fiber shaft will be an issue, they should be able to tell you ^^
Wow those are pretty decent speeds! I think I'd have the same problem I had with linear bearings though - the tolerance of my carbon fiber tubes aren't good enough. Maybe it's worth seeing how expensive it'd be to get super fine tolerance tubes 🤔
In terms of stiffness I don't know if that would change a lot but instead of having 6 rollers on the fixed part, why not put 3 at the top end of the fixed part, and 3 on the bottom end of the moving bar. This way the moving part is guided by the whole overlaping length with the fixed part, thus stiffer
I toyed with this idea a lot in the development of these actuators and ultimately decided against it for a few reasons: - The "guide" CF tubes, while quite stiff, can flex a bit if they're loaded at their midpoint - I want to reduce the weight of the "end effector" of the leg as much as possible, since this is the part that's moving the most (and fastest) - After experiencing how much wobble there can be in the fully-extended tip of the leg (if things aren't tuned quite right) I'm worried about the stiffness I'd be able to get out of the system you're describing when the leg is fully extended. Maybe the solution is a simple one: reduce the "operative" length of the leg. Others have been suggesting using linear rails that run along the support CF tubes and I wonder if that would be the best of both worlds: increased bending stiffness for midpoint loads, (relatively) low weight, high stiffness for when the leg is fully extended, etc...
You can slow down the motors passively rather fast by short circuiting the windings, in which case 100% of the energy generated is immediately used for braking. Depending on the circuit, simply turning off all power transistors will have this effect, if the generated current can be dumped back into the motor through a diode for example. Or you can drive them to 100% power with opposite polarity, it slows the motor down at the highest possible rate.
Do you happen to know if either of these options is possible with ODrives? I don't have much experience with BLDC motors so much of this is new to me 😅
@@harrisonlow My main experience is with stepper motors and my understanding that 3 phase AC is fundamentally the same. Most drivers seem handle them like constant speed motors with speed control loop hot-glued on top, however. ODrive doesn't seem to be an exception, given it that it has great difficulty working with open loop setups. It's purely a firmware issue though. I'm in the process of designing an actuator of my own, except its specced for 80 kgf of thrust and uses stepper motors with belt drive. Unloaded acceleration is virtually infinite and the speed is voltage limited to 3 m/s or thereabouts. I'm sticking to 80V to keep the budget from ballooning.
@@harrisonlow It's for a consumer 6DOF platform. The idea is simple enough: a closed loop Nema 32 stepper driving a belt which directly attaches to an output shaft. It's all stock parts. The question of cost is in finding minimum acceptable quality suppliers. You can always build a platform using Siemens and Kuka parts for quarter million bucks.
2 ideas (i am at 7th minute of the video ) springs at both ends for helping the breaking and reversing directions , and i think you can use just one string for both directions , just tie it as a complete loop
I do like the idea of having the string being a single loop, though I'm not sure how to ensure the motor has a solid "grip" on the string? This is similar to bow belt-driven CNC's work, where a belt is very tightly routed around the motor's output shaft, which has a toothed gear to match the profile of the belt. If I'm using plain string I have no idea how to ensure that the string doesn't slip on the spool? (though maybe this could be seen as a safety feature, where the string slips instead of snapping?). I can kind of imagine how it could work with the synchromesh cable thanks to its strange profile, though even then it might be tricky if printing the parts and using the smaller synchromesh cable (tolerances would be tricky here). Springs would definitely be helpful for (hopefully) stopping complete failures if the motor can't slow down fast enough, though I don't know if I like that they'd necessarily reduce the "operating" length of the actuator 🤔 Cheers for the ideas!
@@harrisonlow for the loop string , and how to hold it on the motor , i was thinking that if you make 3-5 turns around a screw type of gear on the motor. then the same length of string would be released on one side of the gear that was pulled from the other . the number of turns around that gear i think that can hold the string in a steady position without slipping . i dont know how fast that can work without giving you a knot :)
Yeah I'd have to give that a test. So-called "friction knots" can be super solid, and I imagine that adding grooves for the outer spiral of the synchromesh cable would make it even stronger again. This is becoming quite a promising idea! 🤔
Rope expert and research engineer here. Quoted strength of ropes and lines are almost always a theoretical strength for when the line is kept perfectly straight. If there is a knot, intentional or otherwise, strength is dramatically reduced, by 50 to 80%. How much exactly depends on the type of fiber in the line, the specific knot used, and other factors. So yeah. The moment you tie a knot in a line, you've lost at least half it's strength! The reason is simply the sharp bend radii inside the knot. The fibers of the line that are on the outside of the bend take a longer path, and thus take more or all of the load, and thus break before the fibers on the inside of the radius are helping. This problem is worse with modern high-performance fibers that have very low stretch like UHMWPE (Dyneema, Spectra), LCP, PBO and others; and less bad with stretchier fibers like Nylon, Polypropylene or Polyester. Dyneema often looses 70 to 80% of quoted strength if it has knots in it! There are two ways to engineer around this. One in to use splices instead of knots. Splicing is a bit of a specialized skill, but not that hard to learn, especially for 12-braid Dyneema which is what I suspect you're using (and what I'd recommend). The other way is to use large diameter diverters. A diverter in this context could be a short piece of steel axle with a smooth surface that the line wraps around a few turns before terminating. "Large diameter" is maybe >20 times the diameter of the line you're using, more is better but diminishing returns. That way the force is dissipated over a longer part of the line, and thus it looses less strength than in the very sharp diameter of a knot. After the diverter you can use a knot. A good splice will reduce the strength of a line by only 5 or 10%, and a diverter will asymptotically approach 0% loss as the diameter approaches infinity. Dyneema is an amazing fiber, but it has one big weakness: It is very heat sensitive, above about 70°C it quickly starts loosing strength. This makes a large difference in knots that aren't tightened properly. Once load is applied to a loose knot the knot will tighten very quickly, causing lots of friction heating and often melting the line, even at relatively low loads. Dyneema is very abrasion resistant in general however, as long as the abrasion isn't rapid enough to accumulate heat. I'd be more than happy to answer questions or otherwise help out :)
Very interesting! Cheers for all the info there 😊 I'd love to know your thoughts on a tensile-testing rig that I plan to build to compare my different string/cable options. I've written up a brief summary of what I plan to do on Zulip (I'll post the link in a reply in case YT eats it), and of course I'm happy to give details for anything you'd like to know
@@harrisonlow No worries! Have been watching your progress on an off over the last year, very impressive :) Glad if I can help out! How do you currently keep the strings taught? You obviously don't want slack, but also your mechanism will bind if they are too tight. Do you just tighten them at the screw-terminal shown at 10:45? Or do you have a tightener somewhere? If you've shown that in some other video I've missed it. If not, then a mechanism like that might improve your repeatability and consistency, though at the cost of complexity. ------------------------------------- Kevlar (Aramid) is not a bad choice. Aramid typically has very high coefficient of friction, the opposite of Dyneema. It's typically not quite as bad as Dyneema for loosing strength in knots, but almost. Aramid is very sensitive to UV light, and degrades very quickly if exposed to sunlight (several percent loss of strength per day). Aramid is very heat resistant however, will never melt, but instead only slowly start to char around 500°C. Dyneema (UHMWPE) is stronger, less stretchy, and much slipperier (coefficient of friction around 0.05 to 0.08, similar to teflon) than Aramid. This last property can be both good and bad, but often makes it difficult to tie knots in. You can usually get hold of fine Dyneema braid in sport-fishing stores, sold as high-quality fishing line. Parachuters and paragliders also use fine Dyneema braids, see for instance: www.liros.com/catalog/en/dc-35-p3399/ Beware that Dyneema spun for different purposes will have different heat treatments and coatings, giving them very different properties. The paragliding line linked above for example is heat-treated to be very compact and very stiff, and is pre-streched and then heat-set, in order to minimize creep. Dyneema is almost impervious to sunlight (after 10+years of tropical sunlight it will only have a slight surface fuzziness, with very little loss in strength), chemicals (it is essentially polyethylene) or abrasion. It's only flaw is it's low melting point. Steel will be stiffer than any polymer fiber, and susceptible to material fatigue. Steel is about the same strength as Dyneema per diameter, but since Dyneema has a specific gravity of ~0.97, steel is only about 1/8th as strong per weight. It is also prone to wearing though blocks and pullies. To prevent fatigue you should make sure the diameter of all blocks and pullies is large relative to the diameter of the cable. To mitigate these factors, try to find a cable made up of as many thin strands as possible: 133 strand (7*19) cable is usually available. Try a sport-fishing store again, they use them for the last meter or so of their line to prevent some types of fish to chew through the line. Aside from which type of fiber is used, the line construction also has a large impact on the properties of a line. In the closeup of your screw-terminals around 10:45, it looks like your Kevlar is a laid 2-strand construction. This is one of the cheapest ways to manufacture thin lines, and has just about the highest lumpiness (least smooth surface) and highest stretch of any method. A hollow-braid like a 8-braid or 12-braid gives a much smoother, less stretchy line. Another issue with laid lines are that they tend to twist or un-twist over time, changing their properties as the fibers moves around. They will also behave differently if they are threaded through a helical path that is clock-wise vs counter-clockwise, since the twist in the lay has an inherent chirality. But this might be too deep down the rabbit hole, if you haven't encountered any such problems yet :) If some of your lines get over-twisted and some under-twisted however, this might be the issue. Now, I haven't spent all that much time analyzing your design, so I only have a low confidence in my suggestions. Feel free to ignore or explain why I'm wrong :) But it seems to me that you would like a [1] smooth (in order to minimize losses in pullies, stiction and noise), [2] low stretch (to minimize slack and play), [3] long lasting line, [4] chirally symmetrical (ie robust against twisting, ie braided rather than laid), that is [5] easy to tie off at the ends. Laid 2-strand Kevlar gives you the second and last of those (being high-friction, it will hold knots or your screw-terminals, though it will loose much strength there, and it will degrade over time from UV): A braided Dyneema line will give you the first three, but might require a more involved solution for the last point. You might be able to find a thin kevlar braid too, though I don't have a good suggestion for where to look off the top of my head. Maybe a sewing thread for firefighting/motorcycle gear? Most sewing threads are laid 2 or 3 strand, though some are braided. An industrial sewing supplier might have some. Hope that helps :) I don't understand what the syncromesh cables are helpful for in your case. I'm not familiar with them at all, but what I gather is that they are designed to stop slippage, rather like the teeth of a timing belt. In you case the cable is fixed at each end (right?), and doesn't drive any wheels, (right?) So why is the synching feature helpful? ------------------------------- As for building a tensile-testing rig: You built a beautiful machine already by the looks of things! For testing cables, lines, and knots, a longer working throw is useful. Beware the recoil when it breaks! Check out the channel HowNot2 here on UA-cam: ua-cam.com/video/8P3GK21Mdtk/v-deo.html They do lots of pull testing in an amature rig, in the linked video I even make a cameo 🙃 If using Dyneema you are likely to run into the problem of the line slipping out of knots or your end-terminals, since it is so slippery. Build your test rig such that you can test this as well as the lines themselves. For testing the lines themselves; pull two smooth large diameter (
Wow! Awesome info here. Cheers! Re. synchromesh: my understanding is that it's more flexible (in bending), stiffer (longitudinally) and stronger than similarly-sized plain steel cable, though I could be completely mistaken here. I guess we'll find out when I put them head-to-head! (You're right that [in this design] the "no-slip" feature wouldn't really be used) Haha yeah the tensile testing rig I made before was quite pleasing to look at, though it was very weak (entirely a result of the pissy little motors that drove it). I also don't have it anymore (built it for someone else) so I'm keen to build another better one! I used to watch a lot of HowNot2's videos! I've got a brother who's quite into highlining/rope swings and his interest bled over a bit into me for a while there (though highlining scares the shit out of me 😅) Re. the testing rig: Do you think large diameter wooden dowel would work as a mandrel on which to wrap the cables? I don't have many power tools and wood would be the cheapest/easiest option for me here. I could have a look around Bunnings for some metal tubing/rods, but wood would be much simpler. Something else that is fairly important to me is to be able to test the cables with as short a length as possible. Particularly for the synchromesh cable because it's so expensive, I want to minimise waste as much as possible during these tests. Do you have any thoughts/suggestions for how I could do that? Again, big thanks for all the info 😊
@@harrisonlow Cheers :) Re: Syncromesh: Steel cable comes in many different alloys, tempers, pre-stretch etc, and I'm not surprised if Syncromesh uses some fancy high-tech version. Could you unwrap the coil from it, and just use the cable? Steel cable is more flexible (in bending) the more strands it's made up of, much like electrical cable. It's longitudinal stretch is mostly a function of the alloy/temper/pre-stretch etc. I don't think the coil on the Syncromesh contributes much to it's strength, but I could be wrong. Note that steel cable will fail for your application when it starts to deform plastically, rather than when it breaks outright. How close that is to the ultimate yeild strength depends on the alloy/temper/pre-stretch etc. Do you want the cable to be stiff like the syncromesh? I don't understand why. Maybe I misunderstood your comment at 10:26. Steel cable is difficult to terminate neatly, especially if you want it to be easily adjustable. Swaged ends and turn-buckles are the go-to methods in industry, but difficult to apply at that small scale. Re: Testing rig: Sure, a wooden dowel might work, with somewhat reduced repeatability. The lines will score the surface of the wood, and the wood grain will deform, causing high-spots etc, but maybe not too bad, depending on the type of wood. Could you use the shank of an M24 bolt or something like that? Should be easy to source from a local hardware store. To reduce waste, use smaller diameter diverters, and place them close together. If you have a spool of the stuff you're testing you don't have to cut it; just affix the end, wrap it around the first and then the second diverter, affix, and just let the spool dangle. That way you won't waste the length that is wrapped around the second diverter. Just mark the last bit as compromised when you're done testing. Difference between Stretch and Creep: Everything stretches, but stretch can be split into several categories. You're obviously familiar with Elastic and Plastic deformation. High quality polymer lines do have some Elastic deformation, but don't typically deform Plastically before break. They do get temporarily elongated after seeing a high load, but will shrink back close to original length with some time and use. This slow rebound can sometimes cause people to over-tighten things, but is rarely a problem. Creep however, is a slow, permanent elongation that occurs after seeing a constant load over a long period of time, similar to why bicycle chains need to be tightened from time to time. Most polymers see some Creep, though the extent varies. The least amount of Creep (and incidentally just about every other type of stretch too) easily available today is probably something like the paragliding line I linked to above ( www.liros.com/catalog/en/dc-35-p3399/ ). During production it is stretched close to it's breaking point, and then heat-treated into it's glass-transition region, thus locking in the elongation, and effectively preventing most Creep.
I would absolutely let you know. They look so cool, it is tempting to create a project just to make use of them. I am thinking light weight quadruped robot. Or bouncing bipedal balancing bot. If they can handling gravity fast enough for juggling, they should be able to handle gravity fast enough for bouncing?@@harrisonlow :)
@@whitneydesignlabs8738 Both ideas sound awesome! Some things to keep in mind when drafting ideas: 1. These actuators have quite a bit of rotational inertia. If you pull the push-rod out (quickly) and suddenly stop it, the whole actuator twists around fairly aggressively (thanks to the rotational inertia of the motor). Thankfully I've found a very cool solution to this issue for Jugglebot (will be in a future video) but it's worth keeping in mind 2. I don't need to worry about the weight of the actuators, since I've got the motor (by far the heaviest part) right at the base, so it moves around as little as possible. I feel very confident that the actuators would be able to "jump" themselves off the ground, but you might not have a huge amount of leeway for the mass of whatever other bits are needed for the robot. I love the idea of a quadruped, and it'd be handy to be able to scale this design down to fit a smaller robot. If you do go down this path, keep me posted on Zulip (link in description) and I'll happily lend a hand where possible 😊
A solution for cables could be nitinol, while it's a lot stronger than steel wire, it's also very flexible; the only problem with it, is that it's superelastic, which is what gives it its formidable properties of bending and withstand extreme forces without snapping, the downside that I wanted to mention by this, is that it can stretch and contract like a rubber-band, which would reduce precession if too much weight or force is applied to the cable.
Hmm yeah I don't know if I like the sound of "superelastic" for this purpose; I want these to be as stiff as possible so that they can perform the fast accelerations that will (probably) be needed when Jugglebot is up and running
@@harrisonlow Another solution would be Cu-Al-Ni alloy cables, while it's better when it comes to precision under extreme forces, the alloy can be exposed to corrosion, making it less viable in humid environments.
In the end, the best solution for stronger cables, would be superelastic alloys, as they permanently remove metal fatigue, while also being incredibly flexible, the cost of this of cause, is that they tend to expand and contract under exposure and release of extreme forces, reducing the overall precision. The precision of the accurator, with superelastic alloys, can be improved by increasing the diameter of the cable, this though comes with another cost, which would be acceleration.
@@harrisonlow but if you want stiffness over material fatigue, while also staying flexible, rope in general is best for this; I don't know any special kind of rope that's stronger than other kinds of rope, other than silk rope, but this makes the project really expensive very quickly.
My first thought for terminating a wire with variable tension would be electric guitar saddles? not sure if you could just use them out of the box or if you'd have to make something custom tho
Good idea! I wonder how small these mechanisms could be made, though? I'd need one tensioner on each end of the central CF tube, and I want them to be as small and light as possible
When there is no wire left on the pulley and When there is a lot of wire wound The outer diameter will be different. How did you solve this? Also, due to the difference, the tension in the string will not be linear. That's a big problem.
Good question! There are a few things going on here: 1. In the actual actuators I'm using, I've tried to only have as much string as is necessary. This means that even when fully wound, there are only ~6 winds around the pulley, which isn't really enough to cause more than a single overlap. The testing actuator had longer strings to make it easier/faster to recover from breakages. 2. The string is so thin that even if overlap occurs, it doesn't really matter 3. Related to 2, the string has a slight amount of stretch, so it can tolerate slight increases in tension
Have you considered dyneema string? Available through bow shops (used for all modern archery), it has virtually zero stretch and not too much creep and possibly too much strength.
Interesting! I recall looking into Dyneema thread after the last video and don't quite remember why I lost interest in it. It might've been hard to find in super thin threads? I didn't know it's used in archery! Maybe that'd be my "in" to finding some. If you have any experience in the matter, how do you think dyneema would compare to synchromesh cable (shown at 10:30 in the vid)? Cheers for the input!
For the syncomesh you could use a crimp style ring terminal with heatshrink. Instead of heatsrink could you use ceramic bearings? Or make a uhmw wheel for the bearings
The trouble with these is that they don't (inherently) give any way to easily adjust the tension in the line, and I'm not sure if these would be the best way to terminate the cable at the spool end, due to the fairly small sizes in that area. Thankfully I actually think we've figured it out over on Zulip 😁 A simple clamp built-in to the printed parts is easy to adjust/fasten with a single M3 bolt and is super sleek and effective! Do you think ceramic bearings would abrade the central CF tube? My (limited) experience with ceramic is that it's super abrasive 🤔
🚀 Hey Jugglebot Enthusiasts! 🤖
I'm thrilled to finally share this update of the actuator journey with all of you! Your insights, curiosity, and passion have been pivotal in this progression.
If you were designing this actuator, what would YOU do differently? Or better yet, if you could see one feature or capability added to these actuators, what would it be? 🧐
Don't forget, if you want to learn more about the project, or to lend a helping hand, check out our Zulip site, here: pdj.zulipchat.com/
Please don't add music. I really wanted to see how this was made but just couldn't with the music.
@@MichaelMantion Hmm interesting. I'm in two minds about this. Do you think just making the music a bit quieter would be better? Or would you prefer just none at all?
Would love to make a design for you but i just cant do things for free anymore
Love the series and the focus on the process of basing the design in proper design requirements. Inspiring thank you, clearly you are on a great path with the project 👊👊👍
Cheers Martin!
Super cool project! Immediately got a couple ideas as well.
Sorry if these are already named, but here we go:
- Route the extend and retract spring via the same wire, so kind of get a capstan drive.
- Make the sleeves for the inner bearings slightly concave. to make them self align to the rod? Maybe also make a groove in the middle of them to make a channel for the wire, to turn that issue into a feature. It might remove an extra bearing for routing the wire.
- Make the moving bearings three sets at a 90deg angle, to get a kind of rollercoaster setup.
- Add a fan to the output shaft of the motor, that spins a cooling fan over the brake resistor.
Well now that that's out in the world so I don't think about it again, good luck!
You should look at 1-1.5mm Dyneema rope for your cable, very light, very flexible, very strong, low friction, low elongation/elasticity and easy to splice to create loop or termination
Kevlar would be better as dyneema does exhibit some degree of creep when maintained under tension for long periods. Kevlar is absolutely stable. Also is it possible to arrange it as a continuous loop running back to the capstan (motor) where it would be anchored by a tensioning screw widget. Would you ease the termination clamp problem.
For the bearings, I would suggest a delrin or Teflon roller. You can get a pack of these easily, marketed as 3d printer rollers. They would stay on the bearings better, resist abrasion, and have a self-lubricating effect with the tubes. For the synchromesh cable, you may be able to design a clamping plate similar to those used for bike cables.
Very good suggestions, cheers!
I think the bike clamping plate will probably be the final solution for this problem; super simple, small and light. Great thinking!
The wheels used by 3D printers to roll on 2020 could also be used. The wheels are really cheap. There are also different shapes. Maybe a concave shape is better for rolling on the carbon fiber. You can buy them with a groove. Or if you can control the spinning modify them with a static spinning drill bit and slowly turning the wheel.
Why not linear bearings? Recirculating ball type used by many machine tool makers for linear movement. Robust, accurate, hard wearing and cheapish.
@@seeker1015 Weight to stiffness ratio is worse (if you include the rails) than carbon fiber. More weight means: requires bigger motors.
Some of these have a Delrin "v" groove profile that would give you 2 points of contact per bearing on your carbon tube. This is the first thing that jumped to mind when I saw your printed sleeves....I would imagine that this would keep things located a little more positively (at least, short of a concave profile to match the tube radius) . You might be able to use 3 of the same on the captured end of the moving tube in place of the 2 guide rollers you currently have riding one of the outside tubes...basically the same thing you have holding that moving tube inside your printed end, but flipped inside out. That would both keep it centered AND prevent axial rotation given the 2 contact points inside the V. Anyway super interesting project, and very promising.
For the cables pulleys
You can absolutely do the compression with 2 pulleys :
Use the first pulley to first route the cable throught any point of the desired destination line, and then put a pulley at that point to get the destination line
First step is always possible due to a point and a line always being in the same plane, and same for 2 intersecting lines in the second step
Good luck with your project
For your linear bearings, a lot of newer industrial bearings use metalized ceramics. Systems that used to be rows of ball bearings on hardened steel rails have become ceramic sleeves which fit snug to the rail. I don't know if that applies to carbon fiber tubing, or much for your project - but a good lool at linear bearings might give you some ideas for how to stabilize your tube.
I would even recommend going with some kind of graphite coating on the rods that can act as a dry lubricant. An oil or grease probably won't fit the application well, but can make a world of difference in longevity. Gears that run for decades with oil in them won't last a day without it. A little can go a very long way.
You could look at just trying a teflon sleeve or set of pads that form to the wall of the tube. Play around with contact area and pressure to get what you need. The resin binder for the carbon fiber is probably going to be easy to wear. You may want to consider adding or trying to identify tubing made with a harder material. A silicon dioxide coating may improve wear characteristics (or perhaps an alumina spinel - quartz/saphire coat).
If you want to go aerospace, you could try a sputtered titanium nitride coating. Not sure how well (or horrible) it would fuse - but if you're looking to go off the wall with it - that would be a direction.
For the string, you might actually look into amarid/kevlar. The nomenclature to use is "tow" when looking for strings of fabric material (same with carbon fiber) - I spent days losing my mind looking for a carbon fiber string because I didn't know that was called a "tow". Well - a tow is unbraided, so I suppose not technically a string/line.
You might look at fishing lines for off the shelf solutions and even talk to those communities about line routing.
The TPU sleeve over the bearing will work way better if you design them to fully engage with the surface area of the carbon fiber tubes. Make them like a circular indent like doing a sphere revolve cut in your cad program where the sphere is the same size as your carbon fiber rods. It will also help hold them centered if you add a bit either side of the bearing on the flat side that helps lock them in, like a U shape that comes down about a millimeter from the outside diameter of the bearing towards the center of the diameter. Hope this helps. 👍👍
Super glue probably doesn't stick well to TPU. Scotch Weld PR40 adheres very well to TPU. You could also print the TPU sleeves to be concave on the outside so they are self centering
you should be able to hold the synchromesh cable by having two small metal plates that clamp together with two bolts, when you tighten the bolts it will crush the cable in between the plates and hold onto it real tight, I am taking inspiration from how electric cables are held into some fixtures, its often molded into the plastic in consumer electronics so that when the chassis of whatever is screwed together the two mating surfaces pinch the cable to act as strain relief on where the cable is attached inside.
When adding heat inserts, as soon as you pull the iron away, flip the part over and press the face with the insert (and still hot plastic) directly against your workbench for 3-5 seconds. The flat surface of the workbench will keep the small raised ring from forming around the insert as it cools, so you won't need to shave it off manually. It will also help make sure the insert is aligned perpendicular to the face. Alternatively, you can press a small block of copper of aluminum (like a spare heatsink) against the fresh insert to accomplish the same task.
That's a *really* good idea! Thanks heaps for sharing it!
Just lying here, thinking that there are a couple of things that i might be able to usefully comment on... a bit off the wall, but i wonder if using two motors, although more complex, might give you control over things like stretch in a string, and providing some kind of dynamic control over the string action and loading... with encoders on the motors, and optical "zeroing" sensors, you get calibration on every cycle... i wonder if some kind of silicone 'oil' impregnating the string might improve lifetime ... indeed, steel cable with a 'dry' lubricant might work well, even on the smallish radius pulleys? I guess that a lubricant could be 'wet', if the actuator 'tube' was enclosed....anyway, sick in bed, just the ramblings of an amateur....
Interesting idea with two motors! I'm not sure it'd be worth the added complexity in my case though 🤔 Silicone lube on the string is a neat idea! Since this video I've done a heap of testing with different strings (video to come eventually...) and have found dyneema to be really good; high strength, low wear over repeated cycles around small pulleys, low stretch etc.
Cheers for the input, and I hope you get better soon!
*Old Design Issues*
- Bearings wore out quickly.
- String clamping led to wear.
- Assembly required hammering, causing parts to crack.
- Not modular; any issue with one actuator affects the entire system.
- Bowden tubes kinked and had slop.
*New Design Improvements*
- Strings internally routed, making it sleek and modular.
- Motor contained within the actuator for easy replacement.
- Clamping mechanisms simplify assembly.
- TPU sleeves minimize wear on bearings.
*Performance Metrics*
- Endurance: Over 5 hours of continuous runtime.
- Speed: Max 3.4 m/s.
- Precision: Less than 0.06 mm.
- Lifting Force: Almost 7 kg.
*Future Improvements*
- Use motor body as the spool for the string.
- Consider using synchromesh cable.
- Replace TPU sleeves with heat shrink.
- Modify bottom cap to reduce torsion.
*Community Engagement*
- Set up a Zulip site for better communication.
*Positive Points (Sorted by Importance)*
1. *Modularity:* The new design allows for easy replacement of individual actuators, reducing downtime.
2. *Endurance:* Over 5 hours of continuous runtime, passing 50,000 cycles with minimal wear.
3. *Speed:* Achieved a maximum speed of 3.4 m/s, meeting the design requirements.
4. *Precision:* Less than 0.06 mm, close to the theoretical limit of 8.4 micrometers.
5. *Lifting Force:* Capable of lifting almost 7 kg, exceeding expectations.
6. *Ease of Assembly:* New clamping mechanisms eliminate the need for hammering parts together.
7. *Reduced Wear:* TPU sleeves and internal routing of strings minimize wear and tear.
8. *Community Engagement:* Set up a Zulip site for better communication and sharing of ideas.
*Negative Points (Sorted by Importance)*
1. *String Wear:* The string broke during precision testing, indicating potential durability issues.
2. *Torsion Issue:* When fully compressed, the actuator has little resistance against torsion.
3. *TPU Sleeve Problems:* TPU sleeves can slip off and are not perfectly smooth or symmetric.
4. *Complex String Routing:* The design requires complex string routing, which could be simplified.
5. *Braking Limitations:* Difficulty in slowing down the actuators, indicating a need for better braking mechanisms.
6. *Assembly Cracks:* The old design required hammering, leading to cracked parts.
7. *Old Design Wear:* Bearings and strings in the old design wore out quickly, affecting longevity.
Wow! What a thorough summary!
definitely not openAI output.
Good boy. You paid attention. Have a chocolate fish.
@dine9093 tbh even if it were ChatGPT-made, I reckon it's still pretty good! I wouldn't think it'd be so effective just from (presumably) the transcript?
@@harrisonlow It would if it also used your previous videos transcripts. keep up the cool work though!
It’s a shame I’ve only just today discovered your channel, your communication of your development process is very inspirational to me!
Rest assured, there's still a lot of development to go! 😁
Awesome design, I am working on a project that needs a linear actuator and nothing I have found online will suit my needs. I cannot believe it never occurred to me that I could make my own, thank you for inspiring me.
Definately use the dual wheel design on all 3 outter shafts to stop torsional forces. And for braking you can use another mini actuator to press a cylindrical bearing down against the wheels in the bearing block and the harder it presses the more it brakes the bearing and an electrical actuator is very percise and fast so with the right algorithms it can precisely brake the movement.
Use a collet block to terminate the synchromesh cable. The bonus is that a collet will draw it tighter as your cinch down on it.
This is awesome!! I've been needing some scalable linear actuators for a project. These look Great!
Awesome! Be sure to check out the Printables link in the description for all the files. Let me know if you make one - I'd love to see it!
Hello. Harrison. I'm writing because I think I can help you a little, with an idea.
You talked about being able to tighten and change your strings on your Linear Actuators. when you tune the strings on a guitar, you turn a small handle. very simple mechanical structure. What might not be so good about it is the weight. But it can certainly be made very light in weight. Thanks for a great channel. Finn
To terminate the synchromesh cable the first thing that came to my mind is a two-piece metal clamp style endpiece with interlocking ridges...like a bike brake/gear cable clamp.
Regenerative breaking can be done with a 3 phase rectifier and some way to either store the energy or dissipate the energy. A good place to start is looking up "delta alternator diode layout". Delta alternators have nearly identical wiring to most brush-less motors and you can use the same type of rectifier layout because of this. The trick however is using PNP transistors instead of diodes. By using PNP transistors, you can monitor the motor driver's voltages at the transistor's base and limit the current of the rectifier when the motor driver is engaged. Keep going on the project, very interested to see where this goes!
Thankfully the ODrive is capable of handling the electrical stuff for regen braking, but the problem is that my electrical knowledge is subpar right now so I need to learn/figure out what batteries or brake resistors to use. I've tried the naive "increase brake resistor power, keeping nominal resistance the same" but that didn't work for some reason 🤷♂
Can it be done with less than 3 pulleys? Referring to the green string, my thought is to have the first pulley be slightly tilted and the correct diameter to stretch the distance between the motor pulley edge and the upper central pulley edge, so that the intermediate pulley essentially does the job of both pulleys you have right now, but it keeps the string in line coming off of the motor pulley and the upper pulley.
Yep, I now realise that this is very doable with only two pulleys per string. Good thinking!
@@harrisonlow Awesome. Thanks for replying to my comments. Can't wait for the next update!
Hi @harrisonlow - great work! I wanted to point your attention to a type of string that has much higher strength called "spectra" or "dyneema". used most commonly in sailing, paragliding and kitesurfing, such strings can support 100s of kilograms in rather thin gauges. The fibers are also cut-resistant but you might have a heat problem with them and might consider lubrication to control that.
Thanks for the suggestion! I've actually since swapped to dyneema (0.8 mm) and I love it! 😍 Super strong, really thin and flexible and barely any creep. I actually have a video in the works on my testing of a bunch of strings 😁
@110:30. For the bearing surfaces, can you use an o-ring of something like teflon or viton? The viton would even give some resistance to tube rotation.
You should get a force gauge so you can get an accurate force reading for both extension and contraction; At least more accurately than having to binary search the max load.
You know what: I actually totally forgot that I have a force gauge lying around. I've been wanting to make a little box to house an LCD display and have a data-out port for datalogging but have been putting it off for the longest time. I completely forgot I have it!
Maybe this is the push I need to finally get that sorted out. I'll add it back up my list. Cheers for the reminder 😊
For the brakes, use magnetic damper less work on the brake, plus it will give it a shove back the other way.
It looks like a space elevator! For faster stopping you can use electromagnetic brakes - one that goes on the motor shaft and stops the shaft when you put power into it. You can use the motor to brake, if it is an AC or stepper motor, just applying a DC voltage to the windings should work as a brake, but with a DC motor you either have to reverse the input voltage or switch the circuit to a large load (resistor, light bulb or something) to do regenerative braking.
You can measure thousands of cycles with pretty much arbitrary precision if you use a Pi HD camera and the right lens and focus on the target area(s).
I hate to tell you you are not getting mm precession with a single pi camera. You would be incredibly lucky to get cm precision
What an excellent project update. I love the collaboration happening, thanks for sharing.
You could consider using a very large super capacitor as a "breaking load" for the ESC. If you dump into the capacitor and then pull power back out, you should end up with a net loss in power usage, thus the batteries will be at net discharge without worrying about chemistry issues.
You could reduce the number of pulleys by putting the motor axis at a right angle to the axis of the actuator. If you make the drive spool the right diameter and offset it a little (so one side is in the center and the other side is in line with the side tube) you would only need the one pulley at the far end of the 'blue string.'
This is a lot better than the geometrical fiddling that I suggested.
Loving seeing this develop,thanks for sharing. Couple of quick thoughts... 1. Could you use concentric tubes, so one large and one small instead of four small? Also, can you get carbon fibre tubes in square section, which will help with the twist?
Cheers!
I briefly toyed with the idea of using one large CF tube for the support structure (rather than three small ones) but I wasn't a huge fan of it for two main reasons:
- Large CF tubes are super expensive
- I don't like how opaque they are; I like being able to see the "inner workings" of the actuator. I've come to find that having an "open" design is also super helpful for fixing issues as they arise
The square-section tubing is a good idea! I wonder how that would affect the geometry of the upper "bearing block"? I'll have a think about this. Thanks for the thoughts!
Consider nylon for instead of TPU for the bearing sleeves, it will make it more rigid / stiff / precise. You might also consider engaging more of the small linear rods with bearings.
Great project!
What about dynema as the string? Very low stretch, very high strength. Standard braid so making sliced loops and terminations is easy. Also, compression cones for attaching a possibility.
I've actually recently been doing a TON of testing of different string materials and dyneema is looking quite good!
What do you mean by compression cones? I'm not familiar with that term
the cheapest way to terminate the sychromesh cable would probably be a simple ring-tongue terminal that you crimp on the end. this way you already got a nice point to screw them into place. only problem i see with this is maybe some backlash you get, if you don't get the length exactly right on both cables, creating some dead space until the slack is picked up. Some kind of idler/tension pulley that you can adjust somewhere near the motor can solve this. I'm thinking some kind of set screw that changes the position of one of the pulleys, so you snug it up once and the whole system is ready to go until something else gives, stretches etc
I’d be curious what the accuracy of this (or the next iteration) would be with a bunch of consecutive movements that are all in the same direction, like 5 random points chosen to be consecutively descending or ascending, and measuring primarily the relative deltas that are then achieved _between_ the points, and then comparing that to the requested point deltas.
I would not be surprised if your consecutive deltas end up showing far higher precision when moving repeatedly in the same direction without the driving string having to change.
If this is the case it could be a good indication that some undesirable inaccuracy is coming from the slack transferring between the two strings, and you might be able to design movement paths around that (ie. preferring multiple consecutive movements in the same direction if at all possible)
That's a very good point! Either way, I'm super happy with sub-millimetre precision (let alone < 0.06 mm!)
10:55 Weld the end of the cable to a connector that connects to your bolt or, alternatively, use a larger bolt as the cable end connector, drill a hole through it horizontally, feed the cable through, sandwich it between two nuts, and tighten down.
For the termination of the cable you can put it through a tube, with a hole for a bolt, fixed to the piece at the end. Then you just have to press it with the bolt.
I think it's the easiest way you can do it.
You can redirect the string with only two bearings for the green string, and (at least in the section you showed when talking about it) one bearing on the blue line.
You should only need one pulley to change a strings direction. The cable clming off the first redirect pulley is in the exact same situation geometrically as it is coming off the spool (unwrapping from a cylinder with a vertical axle), only offset slightly.
Basically you should only need one pulley with it's groove/side face coming off tangent to the spool (and the axle horizontal) and redirecting the string straight up the tube in one case, and in the second instance to the central redirect. For your bearing on cf issue, bearings with delrin sleeves are an off the shelf part (look up delrin v wheels).
Realy nice project would like to see more of this.
One safety approved could be add some end stopper for that middle pipe so it dosent fly out of when string cut off.
The subscribe button is just one click away 😜
I do like that idea, but it's also kinda fun not knowing if the central tube will shoot out or not 😁 (I probably will add safety features like this to the completed platform though. Depends how often/severely it happens in testing)
Cheers for the feedback 😊
I suggest you to put the bearings in different orientations, 3 to 3 with some angle (I sopose 60 degrees in between) to get more points of contact with the bars and to prevent them of moving around its axes.
Great design, thank you!
Very good improvements! I am looking to make some actuators for a very different application, but endurance is important to me, keeping cost in mind. I'd very very keen on seeing you implement your planned improvements.
Look into the PID loop of your actuators, all the other math seemed spot on to me. Cool video btw!
Yeah that PID will need some updating once the Stewart Platform is working. My current settings are "just-good-enough" values for preliminary testing 😊
Cheers!
For the cables, perhaps a wire rope end stop with set screws for easy adjustment, or a threaded cable stud that you could adjust the tension of with a nut.
I love the first 15 seconds!
Brainstorming is our human superpower 100%.
🇦🇺🤜🏼🤛🏼😎🍀
String routing without the two pulleys parallel to the motor shaft should be doable, as long as the pulley taking the string parallel to the actuator axis has the string enter/exit tangent to the shaft it should still spool fine. This does assume there isn't a large number of wraps when the actuator is at full travel, but as a rule of thumb anything up to 5deg off the pulley centreline should be fine without guides.
I'm not saying this will be elegant, but it should be possible.
If the torsion restraint engages all 3 carbon tubes, you can probably get rid of one of the sets of rollers at the top end. this will make the actuators stiffer at low extension, but if you have a desired limit then the minimum separation can be set to achieve that and everywhere else will be better.
Threading strings through tubes can be done with compressed air, add a small flag or rag to the end if it has trouble to give it more to blow against.
You should be able to get the pulley count down to 2 per string:
* for the extension string, just rotate the existing bottom-of-tube pulley around the tube's axis until the input string is tangent to the drive pulley, and remove the other motor-side pulley.
* for the retraction string, chose a random point "A" in the plane of the drive pulley ~3cm away, draw a line from "A" tangent to the drive pulley in "B"; then set a point "C" on the central axis ~1cm above the pulley; the string should follow the path "BA" "AC" then up the central axis from "C". The 2 pulleys are semicircles at "A" and "C", tangent to "BA" and "AC", and "AC" "central axis" (in CAD, create a plane from each pair of lines and just add the circle) [for clearance, you might need to swap the extension and retraction drums]
Maybe you could try using carbon fibre yarn (tow), which won't stretch over time, causing different problems. As someone below already said, Teflon rollers are self-lubricating and will last forever. And if it would be acceptable to have the motor axis perpendicular to the axis of the actuator, thus eliminating all the pulleys, would probably eliminate all the slack which may be generated due to the wear of the pulleys. Alternatively, you may want to make the pulleys from teflon, too...
Interesting! I imagine there would be a host of other problems with carbon fibre tow, especially with the weird routing that I'm subjecting these strings to. I'm glad you suggested it though - I didn't even know this was a thing before now 😊
Cheers for the suggestion of teflon rollers - putting in an order for these is on my to-do list for today
instead of string have you tried a high-strength fishing line. It may last longer due to it being designed for reeling up and casting and the tensile strength of a large bluefin on the end of it?
What of you had the motor oriented 90° from the direction of travel, such that it forms the "leg" of an "L" shape? It's not *quite* as sleek, design-wise, but it would eliminate the need for many of the pulleys.
Congratulations! The actuators look great. How about "rod track guide" bearings to run against the carbon fiber rod?
Cheers!
Are you thinking these could be used to stop the torsion problem? I can see how this would also (probably) make the central tube quite a bit more stiff as well. I wonder if it'd be worth the extra weight? I'll put in an order for some and test them out. Cheers for the idea!
@@harrisonlow in my experience it should help with the torsion issue. I do believe, as you have planned, indexing on all 3 rods for the torsion issue would be best. I made the suggestion to keep the movement smooth and add support. Thanks and best of luck!
Haha now I'm not so convinced that bracing against all three support rods is best - I like the solidity that a linear rail would provide. I'll have to wait and see how the testing goes once they arrive.
Cheers!
This is a cool project and the discussion gets me thinking. Great job. :)
Have you tried terminating the line with pinch points similar to the way you clamp the carbon fiber rods?
Wow, great project! For the braking, rather than resistors, dumping current into a partially-discharged battery should absorb the energy without so much temperature rise. The circuitry would be more tricky, but you should be able to find a lot of info out there under DC motor regenerative braking. You could use the battery as the power source, connected to a charging circuit to keep it from running down.
Dumping current into a battery would cut the heat a lot, because it would be storing energy in the battery chemistry vs just dissipating it.
Yeah I think batteries are the way to go long-term. I just need to sit down and digest all the info online about it. Last time I found myself looking into Power Management Systems and charge/discharge rates I quickly felt like I was in over my head 😅
Cheers for the input!
To use the Synchomesh cable a set screw that holds and a tension screw that pulls the cable could be nice.
Not sure if you tested bearing sleve on the rod?
Love the idea with using the motor housing
EDIT: I see on Zulip that the same suggestion has already been made! You can save your eyeballs on this post!
It's possible to use fewer pulleys. The trick is to imagine a plane for each pulley; right through the centre of the pulley and parallel to the pulley face. Each plane defines where the string coming into and going out of the pulley can lie. The trick is to intersect the pulley planes with the entry and exit points (pulley tangents) for each successive pulley.
Dodgy blender model that visualizes the idea: ua-cam.com/video/vnq_sSShIdY/v-deo.html
Kevlar String? Might be a bit more wear resistant than nylon. According to other posts on Reddit, Synchromesh cable can be terminated with standard cable crimp connections.
Just commenting for the algorithm, great video brother!
Haha cheers 😁
for cable tensioning with the syncromesh maybe look towards guitar cable tensioning. Its easy to swap strings and readjust tension
Good idea! I wonder how small these mechanisms could be made, though? I'd need one tensioner on each end of the central CF tube, and I want them to be as small and light as possible
Neat concept and build. Look forward to seeing your updates. Cheers!
for terminating the synchromesh, look up wedge sockets. if a bolt into a 3d printed part is strong enough to terminate the current string I'd bet a 3d printed wedge socket would work for the synchromesh here
Pardon my ignorance, but is there any reason you're not using a linear beearing for the centeral shaft? I guess the CF might not be so compatable with direct contact from the bearings. Cool project regardless!
Also for the synchromech cable, as a stab in the dark solution, could you try crimping it in an electrical ring terminal?
String is very stretchy. Under loads the braid tightens, eventually that is permanent. You had no way to compensate for that.
For terminating the cable, which may also require stretch compensation, look at how it is done on steel cables. Crimp on a terminal to prevent fraying, but not for a load, or for loads, slip on a bigger terminal than loop the cable and put it back into the terminal before crimping.
in electrical applications sometimes soldering is used, instead of crimping. I don't know what that cable is, but perhaps epoxy would work, maybe with crimping.
rotate the motor orientation so that the string starts out pointing in the right direction. try looking at braided fishing line.
Instead of bleeding the power from braking into resistors, use a capacitor bank to rapidly charge and discharge during the reversals.
For cable termination a barrel nut that's cross drilled could work similar to what is used of frogslegs bicycle brakes
MBS on soft goods (like string/rope, webbing, slings, etc.) is for the most optimal loading scenario. Tight bend radii (like found in knots) will significantly reduce the actual breaking strength
Interestingly the string in the raw strength test actually failed somewhere in the middle of the string, quite far away from either knot!
9 months late, but could you use a pinch bolt for the syncromesh cable? The sort of pinch bolt that you might use for the braided cable on your bicycle brakes.
To terminate the synchro wire you could use a electronics standoff thread that into the plastic. Drill a small cross hole in the standoff. Thread wire through the cross hole and use a set screw to hold the wire. This would make it easy to change and Tighen.
Fascinating idea! I actually think we've figured it out over on Zulip 😁 A simple clamp built-in to the printed parts is easy to adjust/fasten with a single M3 bolt and is super sleek and effective!
Cheers for the suggestion! I like the creativity 😊
5:38 so obviously we lean into that. I don't know if you realize it, but you're making mech suit bones. bones have twists in them
Speed: overheating brake resistors. Use a cosinus curve (speed) at the end instead of feading all that power to a break resistor.
Have had the same problem with a 220 Watt motor on a frequention speed control, it tripped above a specific speed while with that cos curve it did that at about a double speed. Hope this helps.
That's a clever idea! I can see how that would go a long way to getting higher max speeds. For better or worse, when Jugglebot is in operation, it's fairly likely it will (semi) often be sent commands that require it to stop and start really suddenly, and I'm not sure I'd always be able to smooth them out like this, but I will try to keep this in mind while I work on the control side of things. Definitely food for thought! Cheers 😊
One thing about your string strength test, how did you attach it to the weights? Something I didn't know until very recently is that how you knot a rope will affect *the rope's* strength, a lot. A tight bend can cut the strength down with over half. This is true for ropes, and should apply to strings as well, though probably to a lesser degree.
The theory is basically this: When you have a straight rope at tension it pulls equally on the entire cross section of the rope. But if you have the rope in a tight curve the part of the rope on the inner part of that bend, obviously can't be in as much tension as the outer part of the rope. Therefore, all the weight lands on fewer fibres.
Strings have fewer fibres and a much smaller radius, so they would be much less effected by this, but in your testing the string failed right at the knot precisely because it's the weakest part of the string.
The redirection of the string shouldn't be a problem, even a small bearing has a large enough diameter to not be a problem with such a thin string, but the attachment at the very end could *possibly* be improved to get more strength out of your existing string. Either by clamping it so it's straight at the end, or using a better knot.
My source, and more details, would be Kyle Hill's recent youtube video "The Figure 8 Knot Has Never Failed. Why?" It's about climbing ropes in particular, but it goes into the details of how knots and bends affect the strength of a rope at any diameter, and briefly mentions what diameter bend would be considered "no bend at all" from the strength standpoint. It's relative to the strings diameter so only the knotting/attachment will really be a problem.
Just got to the bit with the tensioning screw, that might just be weakening the string, hard to be certain. The rule of thumb is that the pin should be four times the diameter of the string to not weaken it. I know it doesn't matter for your application in specific, but in general wrapping the string around a pin like that can remove a fair bit of the strings strength. I can't imagine the threads biting into the string helps any there either.
Why not simply weld the synchromesh cable into a loop, then use a tensioning pulley like you would on a timing belt for a 3d printer? Alternatively, something like a crocodile clip could possibly work. Though I'd be a bit concerned about the cable twisting over time and loosening. You could also simply wrap it around a pulley that you then lock instead, giving you an easy thumb screw to tighten or loosen it. It'd be bulky then though.
If it wasn't clear, I didn't see your previous video on this system so my suggestions don't take any considerations you mentioned in that one in mind.
@@Sylfa Interestingly the string in the raw strength test actually failed somewhere in the middle of the string, quite far away from either knot! I do agree about the thread of the bolt not helping here: the string gets rather munged up after I tighten and loosen it a few times in the same spot 😬
I do like the idea of having the string being a single loop, though I'm not sure how to ensure the motor has a solid "grip" on the string? My understanding of belt-driven CNC machines-which use a similar concept-is that the belt is very tightly routed around the motor's output shaft, which has a toothed gear to match the profile of the belt. If I'm using plain string I have no idea how to ensure that the string doesn't slip on the spool? (though maybe this could be seen as a safety feature, where the string slips instead of snapping?). I can kind of imagine how it could work with the synchromesh cable thanks to its strange profile, though even then it might be tricky if printing the parts and using the smaller synchromesh cable (tolerances would be tricky here).
Cheers for the input!
@@harrisonlow Yeah, I was thinking of the syncromesh cable for that suggestion. Using a string in a full loop, hrm. I guess, you *could* try 3d-printing a soft screwlike holder that winds the thread and keeps it from overlapping itself, but honestly it'd be slipping the thread through which would ruin the friction, and thread.
The only real way I can think of doing it with a loop of string would be to have the loop just barely longer than what you need, then tightening it enough so it's got enough friction. Not sure that's possible. A timing belt would work better than that I'm sure, but I can't see that working unless everything's in-line so you'd have to restructure it entirely. Don't think it'd be worth it for your application.
That's what CNC machines use though, at least the ones I'm aware of. In case you weren't aware, the timing belts are basically a flexible cogwheel, allowing you to put it over cogs to have no slipping, you only need enough tension so that it doesn't slip out of the teeth.
As to it snapping in the middle, that's a good sign that what you're doing is not a problem, I guess strings really don't have the same issue as thicker ropes. Or if they do, it's minimal. Strangely enough I could have sworn I saw the string snap at the weight in the video, guess my mind was just playing tricks on me. 😂
Yeah, I think the only real consideration is to avoid jamming the thread into the screw, to avoid chewing up the string over time. At least if you don't need the full strength the motor can output.
I'm not sure that just having enough tension would work, though a simple solution like wrapping rubber (or heat-shrink?) over the spool might be enough to stop it from slipping.
Just re-checked the "strength" footage and it definitely snaps in the middle (when doing the direct string strength test). No idea why it doesn't snap at the knots 🤷♂️
without a better metrology toolbox, you can measure repeatability by using a fixed object and shims, move away and come back to brush the object and confirm with shims more precisely, it could be challenging depending on how much position authority/rigidity the controller has, if it's too elastic the shims could lack feeling
That's a good idea! I considered setting the actuator up to touch an object and return back to it (ideally without knocking it over) but couldn't think of a way of using that to create actual measurements (rather than a simple "under-shot" / "bang-on" / "knocked over"). Using shims is a great idea! I do wonder about the elasticity of the system masking the true result, though
The Actuator would resist better bending forces if you moved 3 of the roller to the bottom of the moving carbon rod.
You can remove one of the 3 pulleys if you replace 2 pulleys in x and y plane by one pulley that is in a plane that is tilted 45 degrees from X and Y.
You can double the strings to increase the breaking strenght.
For braking or regenerative braking I have three ideas, peltiers are fun because they produce less heat as resistors, you could also make your own resistor out of plastic embedded metal wire sold in hardware stores for next to nothing, or aluminium foil for example. For regenerstive braking you buy a 3 dollar 3 phase to 2 phase rectifier or build your own from diodes then the power goes into a dc motor controller to adjust braking power then into a step up converter. I never managed to test this idea even though I wired all the electronics but I still think it might work. Good luck.
Interesting ideas! For better or worse, I'd (currently) prefer to stick with using the ODrives and figuring out how to fix this issue "properly". I know that there's some way to do this with my current setup, but I'm just goosing something and can't figure out what it is!
Cheers for the suggestions 😊
I am new to python coding and that endurance test counter would be interesting to see.. But this whole project is amazing. Thank you for documenting and sharing it!
The way that counter worked was to simply add 1 each time the actuator reached the end of its movement 😊
FWIW, I have a bunch of the code up on Zulip so check out the link in the description if you're interested to see it. And if you want any specific code, just let me know and I'll happily share it with you 😁
The precision could be improved by changing the winding mechanism because, currently, the line is winding up on itself and unpredictably changing the diameter of the winding pulley.
use some really strong fishing line! unless maybe the stretching of the string could be a problem
Amazing design and great video, thank you for sharing all of this knowledge with us!
Did you think about using square CF tubing instead of round for the middle shaft? It could use 2x4 bearing, giving much better stability sideways and it would be resistant to rotating.
I honestly didn't even know that square CF tubing was a thing! (despite my supplier selling it 😅). I'm hesitant to go for square tubing though because I don't want to add even more bearings to the bearing block, though I do like the resistance to rotation. Others have suggested hexagonal CF tubing and I quite like that idea, but I can't find a supplier in Aus...
@@harrisonlow I bought some Hexagonal tubes from China for an old project, I think that should work with 2x3 bearings,, but honestly I love redundancy so I would add more bearings (but also this increases complexity, resistance and weight). I think I've seen some triangular tubing too, using that might be the ultimate solution: you can keep the current 2x3 bearing setup, all faces would be covered and it should be resistant to rotation.
To terminate the cable you could make a clamping block to clamp around the cables helix with a 3d print In two halfs, and the to tension just a through bolt fully threaded pushing up against the fixed part of the assembly but running through the 3d printed cable clamp, turning the bolt would then increase tension, almost like a chain tensioner on industrial equipment
Interesting idea! Do you think this could be made super small? I'd need one of these on each end of the central CF tube so they need to be quite small and light
you could do a combined testing aiming at measuring the endurance compared to the amount of mechanical energy deployed, the weight lifted times the distance * how long it took, in g per meter per second, same as it was a conveyor belt
Yeah for a while I was considering doing an "output power" test (Force x Velocity) but ultimately decided it would be too tedious to run and I didn't want to have to deal with more broken strings as a result of overheating/failing brake resistors
Measure with mirror (front surface if possible) and laser. Adjust resolution based on distance. Used to Align aerial camera systems this way to a few microns.
If the motor can is used as a pulley, the string wraps decrease cooling surface area for the motor and the heat could soften polymer string. High loads could flex a thin motor can and pop the magnets off. Not likely to cause failure, just something to watch for if weird symptoms show up. Great iteration.
Interesting take on that solution! I hadn't considered either of those failure modes before. I don't think the decreased cooling surface would be very noticeable because the circumference of the motor is quite large compared to the lengths of string I'm dealing with here, though the oblique loads on the body of the motor could well cause problems (I'm not familiar enough with BLDC motors to know if this is likely or not).
Cheers for the input!
As I thought about it more, the string might aid cooling. Capturing motor heat via conduction when wound and releasing it by convection when unwound.
very smart, thank you for sharing.
What's the current string made out of? Dyneema?
Maybe you can get rid of the roller bearings completely by changing the design to slide bearings/shaft guides? I found some relatively low cost composite parts at IGUS, they rate their shaft guides up to 5m/s. I don’t know if the carbon fiber shaft will be an issue, they should be able to tell you ^^
Wow those are pretty decent speeds! I think I'd have the same problem I had with linear bearings though - the tolerance of my carbon fiber tubes aren't good enough. Maybe it's worth seeing how expensive it'd be to get super fine tolerance tubes 🤔
In terms of stiffness I don't know if that would change a lot but instead of having 6 rollers on the fixed part, why not put 3 at the top end of the fixed part, and 3 on the bottom end of the moving bar. This way the moving part is guided by the whole overlaping length with the fixed part, thus stiffer
I toyed with this idea a lot in the development of these actuators and ultimately decided against it for a few reasons:
- The "guide" CF tubes, while quite stiff, can flex a bit if they're loaded at their midpoint
- I want to reduce the weight of the "end effector" of the leg as much as possible, since this is the part that's moving the most (and fastest)
- After experiencing how much wobble there can be in the fully-extended tip of the leg (if things aren't tuned quite right) I'm worried about the stiffness I'd be able to get out of the system you're describing when the leg is fully extended. Maybe the solution is a simple one: reduce the "operative" length of the leg.
Others have been suggesting using linear rails that run along the support CF tubes and I wonder if that would be the best of both worlds: increased bending stiffness for midpoint loads, (relatively) low weight, high stiffness for when the leg is fully extended, etc...
You can slow down the motors passively rather fast by short circuiting the windings, in which case 100% of the energy generated is immediately used for braking. Depending on the circuit, simply turning off all power transistors will have this effect, if the generated current can be dumped back into the motor through a diode for example. Or you can drive them to 100% power with opposite polarity, it slows the motor down at the highest possible rate.
Do you happen to know if either of these options is possible with ODrives? I don't have much experience with BLDC motors so much of this is new to me 😅
@@harrisonlow My main experience is with stepper motors and my understanding that 3 phase AC is fundamentally the same. Most drivers seem handle them like constant speed motors with speed control loop hot-glued on top, however. ODrive doesn't seem to be an exception, given it that it has great difficulty working with open loop setups. It's purely a firmware issue though. I'm in the process of designing an actuator of my own, except its specced for 80 kgf of thrust and uses stepper motors with belt drive. Unloaded acceleration is virtually infinite and the speed is voltage limited to 3 m/s or thereabouts. I'm sticking to 80V to keep the budget from ballooning.
@@michaelbuckers Very cool! I'd love to see that actuator once you've finished it! Do you plan to publish any info on it?
@@harrisonlow It's for a consumer 6DOF platform. The idea is simple enough: a closed loop Nema 32 stepper driving a belt which directly attaches to an output shaft. It's all stock parts. The question of cost is in finding minimum acceptable quality suppliers. You can always build a platform using Siemens and Kuka parts for quarter million bucks.
2 ideas (i am at 7th minute of the video )
springs at both ends for helping the breaking and reversing directions , and i think you can use just one string for both directions , just tie it as a complete loop
I do like the idea of having the string being a single loop, though I'm not sure how to ensure the motor has a solid "grip" on the string? This is similar to bow belt-driven CNC's work, where a belt is very tightly routed around the motor's output shaft, which has a toothed gear to match the profile of the belt. If I'm using plain string I have no idea how to ensure that the string doesn't slip on the spool? (though maybe this could be seen as a safety feature, where the string slips instead of snapping?). I can kind of imagine how it could work with the synchromesh cable thanks to its strange profile, though even then it might be tricky if printing the parts and using the smaller synchromesh cable (tolerances would be tricky here).
Springs would definitely be helpful for (hopefully) stopping complete failures if the motor can't slow down fast enough, though I don't know if I like that they'd necessarily reduce the "operating" length of the actuator 🤔
Cheers for the ideas!
@@harrisonlow for the loop string , and how to hold it on the motor , i was thinking that if you make 3-5 turns around a screw type of gear on the motor. then the same length of string would be released on one side of the gear that was pulled from the other .
the number of turns around that gear i think that can hold the string in a steady position without slipping .
i dont know how fast that can work without giving you a knot :)
Yeah I'd have to give that a test. So-called "friction knots" can be super solid, and I imagine that adding grooves for the outer spiral of the synchromesh cable would make it even stronger again. This is becoming quite a promising idea! 🤔
Rope expert and research engineer here.
Quoted strength of ropes and lines are almost always a theoretical strength for when the line is kept perfectly straight. If there is a knot, intentional or otherwise, strength is dramatically reduced, by 50 to 80%. How much exactly depends on the type of fiber in the line, the specific knot used, and other factors.
So yeah. The moment you tie a knot in a line, you've lost at least half it's strength!
The reason is simply the sharp bend radii inside the knot. The fibers of the line that are on the outside of the bend take a longer path, and thus take more or all of the load, and thus break before the fibers on the inside of the radius are helping. This problem is worse with modern high-performance fibers that have very low stretch like UHMWPE (Dyneema, Spectra), LCP, PBO and others; and less bad with stretchier fibers like Nylon, Polypropylene or Polyester. Dyneema often looses 70 to 80% of quoted strength if it has knots in it!
There are two ways to engineer around this. One in to use splices instead of knots. Splicing is a bit of a specialized skill, but not that hard to learn, especially for 12-braid Dyneema which is what I suspect you're using (and what I'd recommend). The other way is to use large diameter diverters. A diverter in this context could be a short piece of steel axle with a smooth surface that the line wraps around a few turns before terminating. "Large diameter" is maybe >20 times the diameter of the line you're using, more is better but diminishing returns. That way the force is dissipated over a longer part of the line, and thus it looses less strength than in the very sharp diameter of a knot. After the diverter you can use a knot.
A good splice will reduce the strength of a line by only 5 or 10%, and a diverter will asymptotically approach 0% loss as the diameter approaches infinity.
Dyneema is an amazing fiber, but it has one big weakness: It is very heat sensitive, above about 70°C it quickly starts loosing strength. This makes a large difference in knots that aren't tightened properly. Once load is applied to a loose knot the knot will tighten very quickly, causing lots of friction heating and often melting the line, even at relatively low loads. Dyneema is very abrasion resistant in general however, as long as the abrasion isn't rapid enough to accumulate heat.
I'd be more than happy to answer questions or otherwise help out :)
Very interesting! Cheers for all the info there 😊
I'd love to know your thoughts on a tensile-testing rig that I plan to build to compare my different string/cable options. I've written up a brief summary of what I plan to do on Zulip (I'll post the link in a reply in case YT eats it), and of course I'm happy to give details for anything you'd like to know
Link to Zulip thread:
pdj.zulipchat.com/#narrow/stream/399279-General/topic/String.2FCable.20Testing/near/390033444
@@harrisonlow No worries! Have been watching your progress on an off over the last year, very impressive :) Glad if I can help out!
How do you currently keep the strings taught? You obviously don't want slack, but also your mechanism will bind if they are too tight. Do you just tighten them at the screw-terminal shown at 10:45? Or do you have a tightener somewhere? If you've shown that in some other video I've missed it. If not, then a mechanism like that might improve your repeatability and consistency, though at the cost of complexity.
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Kevlar (Aramid) is not a bad choice. Aramid typically has very high coefficient of friction, the opposite of Dyneema. It's typically not quite as bad as Dyneema for loosing strength in knots, but almost. Aramid is very sensitive to UV light, and degrades very quickly if exposed to sunlight (several percent loss of strength per day). Aramid is very heat resistant however, will never melt, but instead only slowly start to char around 500°C.
Dyneema (UHMWPE) is stronger, less stretchy, and much slipperier (coefficient of friction around 0.05 to 0.08, similar to teflon) than Aramid. This last property can be both good and bad, but often makes it difficult to tie knots in. You can usually get hold of fine Dyneema braid in sport-fishing stores, sold as high-quality fishing line. Parachuters and paragliders also use fine Dyneema braids, see for instance: www.liros.com/catalog/en/dc-35-p3399/
Beware that Dyneema spun for different purposes will have different heat treatments and coatings, giving them very different properties. The paragliding line linked above for example is heat-treated to be very compact and very stiff, and is pre-streched and then heat-set, in order to minimize creep.
Dyneema is almost impervious to sunlight (after 10+years of tropical sunlight it will only have a slight surface fuzziness, with very little loss in strength), chemicals (it is essentially polyethylene) or abrasion. It's only flaw is it's low melting point.
Steel will be stiffer than any polymer fiber, and susceptible to material fatigue. Steel is about the same strength as Dyneema per diameter, but since Dyneema has a specific gravity of ~0.97, steel is only about 1/8th as strong per weight. It is also prone to wearing though blocks and pullies. To prevent fatigue you should make sure the diameter of all blocks and pullies is large relative to the diameter of the cable. To mitigate these factors, try to find a cable made up of as many thin strands as possible: 133 strand (7*19) cable is usually available. Try a sport-fishing store again, they use them for the last meter or so of their line to prevent some types of fish to chew through the line.
Aside from which type of fiber is used, the line construction also has a large impact on the properties of a line. In the closeup of your screw-terminals around 10:45, it looks like your Kevlar is a laid 2-strand construction. This is one of the cheapest ways to manufacture thin lines, and has just about the highest lumpiness (least smooth surface) and highest stretch of any method. A hollow-braid like a 8-braid or 12-braid gives a much smoother, less stretchy line. Another issue with laid lines are that they tend to twist or un-twist over time, changing their properties as the fibers moves around. They will also behave differently if they are threaded through a helical path that is clock-wise vs counter-clockwise, since the twist in the lay has an inherent chirality. But this might be too deep down the rabbit hole, if you haven't encountered any such problems yet :) If some of your lines get over-twisted and some under-twisted however, this might be the issue.
Now, I haven't spent all that much time analyzing your design, so I only have a low confidence in my suggestions. Feel free to ignore or explain why I'm wrong :) But it seems to me that you would like a [1] smooth (in order to minimize losses in pullies, stiction and noise), [2] low stretch (to minimize slack and play), [3] long lasting line, [4] chirally symmetrical (ie robust against twisting, ie braided rather than laid), that is [5] easy to tie off at the ends. Laid 2-strand Kevlar gives you the second and last of those (being high-friction, it will hold knots or your screw-terminals, though it will loose much strength there, and it will degrade over time from UV): A braided Dyneema line will give you the first three, but might require a more involved solution for the last point. You might be able to find a thin kevlar braid too, though I don't have a good suggestion for where to look off the top of my head. Maybe a sewing thread for firefighting/motorcycle gear? Most sewing threads are laid 2 or 3 strand, though some are braided. An industrial sewing supplier might have some.
Hope that helps :)
I don't understand what the syncromesh cables are helpful for in your case. I'm not familiar with them at all, but what I gather is that they are designed to stop slippage, rather like the teeth of a timing belt. In you case the cable is fixed at each end (right?), and doesn't drive any wheels, (right?) So why is the synching feature helpful?
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As for building a tensile-testing rig: You built a beautiful machine already by the looks of things! For testing cables, lines, and knots, a longer working throw is useful. Beware the recoil when it breaks! Check out the channel HowNot2 here on UA-cam: ua-cam.com/video/8P3GK21Mdtk/v-deo.html They do lots of pull testing in an amature rig, in the linked video I even make a cameo 🙃
If using Dyneema you are likely to run into the problem of the line slipping out of knots or your end-terminals, since it is so slippery. Build your test rig such that you can test this as well as the lines themselves. For testing the lines themselves; pull two smooth large diameter (
Wow! Awesome info here. Cheers!
Re. synchromesh: my understanding is that it's more flexible (in bending), stiffer (longitudinally) and stronger than similarly-sized plain steel cable, though I could be completely mistaken here. I guess we'll find out when I put them head-to-head! (You're right that [in this design] the "no-slip" feature wouldn't really be used)
Haha yeah the tensile testing rig I made before was quite pleasing to look at, though it was very weak (entirely a result of the pissy little motors that drove it). I also don't have it anymore (built it for someone else) so I'm keen to build another better one!
I used to watch a lot of HowNot2's videos! I've got a brother who's quite into highlining/rope swings and his interest bled over a bit into me for a while there (though highlining scares the shit out of me 😅)
Re. the testing rig: Do you think large diameter wooden dowel would work as a mandrel on which to wrap the cables? I don't have many power tools and wood would be the cheapest/easiest option for me here. I could have a look around Bunnings for some metal tubing/rods, but wood would be much simpler.
Something else that is fairly important to me is to be able to test the cables with as short a length as possible. Particularly for the synchromesh cable because it's so expensive, I want to minimise waste as much as possible during these tests. Do you have any thoughts/suggestions for how I could do that?
Again, big thanks for all the info 😊
@@harrisonlow Cheers :)
Re: Syncromesh: Steel cable comes in many different alloys, tempers, pre-stretch etc, and I'm not surprised if Syncromesh uses some fancy high-tech version. Could you unwrap the coil from it, and just use the cable? Steel cable is more flexible (in bending) the more strands it's made up of, much like electrical cable. It's longitudinal stretch is mostly a function of the alloy/temper/pre-stretch etc. I don't think the coil on the Syncromesh contributes much to it's strength, but I could be wrong.
Note that steel cable will fail for your application when it starts to deform plastically, rather than when it breaks outright. How close that is to the ultimate yeild strength depends on the alloy/temper/pre-stretch etc.
Do you want the cable to be stiff like the syncromesh? I don't understand why. Maybe I misunderstood your comment at 10:26.
Steel cable is difficult to terminate neatly, especially if you want it to be easily adjustable. Swaged ends and turn-buckles are the go-to methods in industry, but difficult to apply at that small scale.
Re: Testing rig: Sure, a wooden dowel might work, with somewhat reduced repeatability. The lines will score the surface of the wood, and the wood grain will deform, causing high-spots etc, but maybe not too bad, depending on the type of wood. Could you use the shank of an M24 bolt or something like that? Should be easy to source from a local hardware store.
To reduce waste, use smaller diameter diverters, and place them close together. If you have a spool of the stuff you're testing you don't have to cut it; just affix the end, wrap it around the first and then the second diverter, affix, and just let the spool dangle. That way you won't waste the length that is wrapped around the second diverter. Just mark the last bit as compromised when you're done testing.
Difference between Stretch and Creep: Everything stretches, but stretch can be split into several categories. You're obviously familiar with Elastic and Plastic deformation. High quality polymer lines do have some Elastic deformation, but don't typically deform Plastically before break. They do get temporarily elongated after seeing a high load, but will shrink back close to original length with some time and use. This slow rebound can sometimes cause people to over-tighten things, but is rarely a problem.
Creep however, is a slow, permanent elongation that occurs after seeing a constant load over a long period of time, similar to why bicycle chains need to be tightened from time to time. Most polymers see some Creep, though the extent varies. The least amount of Creep (and incidentally just about every other type of stretch too) easily available today is probably something like the paragliding line I linked to above ( www.liros.com/catalog/en/dc-35-p3399/ ). During production it is stretched close to it's breaking point, and then heat-treated into it's glass-transition region, thus locking in the elongation, and effectively preventing most Creep.
would v grove bearings be simpler than trying to add a print over a normal bearing?
Great work! Thanks for keep us posted. I can think of many uses for this kind of actuator in robotics etc.
Cheers! If you ever use them in anything, I'd love to know 😊
I would absolutely let you know. They look so cool, it is tempting to create a project just to make use of them. I am thinking light weight quadruped robot. Or bouncing bipedal balancing bot. If they can handling gravity fast enough for juggling, they should be able to handle gravity fast enough for bouncing?@@harrisonlow :)
@@whitneydesignlabs8738 Both ideas sound awesome! Some things to keep in mind when drafting ideas:
1. These actuators have quite a bit of rotational inertia. If you pull the push-rod out (quickly) and suddenly stop it, the whole actuator twists around fairly aggressively (thanks to the rotational inertia of the motor). Thankfully I've found a very cool solution to this issue for Jugglebot (will be in a future video) but it's worth keeping in mind
2. I don't need to worry about the weight of the actuators, since I've got the motor (by far the heaviest part) right at the base, so it moves around as little as possible. I feel very confident that the actuators would be able to "jump" themselves off the ground, but you might not have a huge amount of leeway for the mass of whatever other bits are needed for the robot.
I love the idea of a quadruped, and it'd be handy to be able to scale this design down to fit a smaller robot. If you do go down this path, keep me posted on Zulip (link in description) and I'll happily lend a hand where possible 😊
A solution for cables could be nitinol, while it's a lot stronger than steel wire, it's also very flexible; the only problem with it, is that it's superelastic, which is what gives it its formidable properties of bending and withstand extreme forces without snapping, the downside that I wanted to mention by this, is that it can stretch and contract like a rubber-band, which would reduce precession if too much weight or force is applied to the cable.
Hmm yeah I don't know if I like the sound of "superelastic" for this purpose; I want these to be as stiff as possible so that they can perform the fast accelerations that will (probably) be needed when Jugglebot is up and running
@@harrisonlow Another solution would be Cu-Al-Ni alloy cables, while it's better when it comes to precision under extreme forces, the alloy can be exposed to corrosion, making it less viable in humid environments.
In the end, the best solution for stronger cables, would be superelastic alloys, as they permanently remove metal fatigue, while also being incredibly flexible, the cost of this of cause, is that they tend to expand and contract under exposure and release of extreme forces, reducing the overall precision. The precision of the accurator, with superelastic alloys, can be improved by increasing the diameter of the cable, this though comes with another cost, which would be acceleration.
@@harrisonlow but if you want stiffness over material fatigue, while also staying flexible, rope in general is best for this; I don't know any special kind of rope that's stronger than other kinds of rope, other than silk rope, but this makes the project really expensive very quickly.
My first thought for terminating a wire with variable tension would be electric guitar saddles? not sure if you could just use them out of the box or if you'd have to make something custom tho
Good idea! I wonder how small these mechanisms could be made, though? I'd need one tensioner on each end of the central CF tube, and I want them to be as small and light as possible
Whats the name of the jazz song playing in the background from 13:35?
When there is no wire left on the pulley and
When there is a lot of wire wound
The outer diameter will be different. How did you solve this?
Also, due to the difference, the tension in the string will not be linear.
That's a big problem.
Good question! There are a few things going on here:
1. In the actual actuators I'm using, I've tried to only have as much string as is necessary. This means that even when fully wound, there are only ~6 winds around the pulley, which isn't really enough to cause more than a single overlap. The testing actuator had longer strings to make it easier/faster to recover from breakages.
2. The string is so thin that even if overlap occurs, it doesn't really matter
3. Related to 2, the string has a slight amount of stretch, so it can tolerate slight increases in tension
Have you considered dyneema string? Available through bow shops (used for all modern archery), it has virtually zero stretch and not too much creep and possibly too much strength.
Interesting! I recall looking into Dyneema thread after the last video and don't quite remember why I lost interest in it. It might've been hard to find in super thin threads? I didn't know it's used in archery! Maybe that'd be my "in" to finding some.
If you have any experience in the matter, how do you think dyneema would compare to synchromesh cable (shown at 10:30 in the vid)?
Cheers for the input!
tienes que usar cuerda estática de escalada, pero no se si la hay de ese diametro.
For the syncomesh you could use a crimp style ring terminal with heatshrink. Instead of heatsrink could you use ceramic bearings? Or make a uhmw wheel for the bearings
The trouble with these is that they don't (inherently) give any way to easily adjust the tension in the line, and I'm not sure if these would be the best way to terminate the cable at the spool end, due to the fairly small sizes in that area. Thankfully I actually think we've figured it out over on Zulip 😁 A simple clamp built-in to the printed parts is easy to adjust/fasten with a single M3 bolt and is super sleek and effective!
Do you think ceramic bearings would abrade the central CF tube? My (limited) experience with ceramic is that it's super abrasive 🤔
if the back tip will be attached to 3 other rods, then you can use 3 large bearing only on the top
Great project, thanks for sharing. why wont you just use 'Delrin V groove Dual Bearing Wheels' instead of inventing the wheel? :)