I think you may have described displacement a bit incorrectly. Displacement is a change in position (with direction included). You mentioned that displacement was tangent to position over time, but being tangent means that displacement would have a time component, which is doesn’t. It seems like you got displacement by taking the derivative of position... but that wouldn’t make sense. It’s an old video so I don’t expect you to update, but I don’t want anyone else getting confused! Great video otherwise, thanks Tom!
Hi Anoop, Thank you for the comment. Since we are only expressing the magnitude component of acceleration we are expressing acceleration in scalar form. Later on in the video I graphically display the vector form of acceleration as the change in velocity. Or if we are representing change in speed, then we would be discussing scalar quantity that is the magnitude component of velocity, since speed only has a magnitude component. www.av8n.com/physics/acceleration.htm
When a system vibrates, its motion can be expressed as A*Sinωt. - A is the amplitude of vibration (length unit) and ω the frequency in rad/s. On differentiating twice, we get (-) Aω2*Sinωt. The value Aω2 is m/s2 (unit of acceleration) and when divided by 9.81 (acceleration due to gravity) gives the 'g' value of this vibration. This is how I explain the term 'g' . Expressing acceleration in terms of 'g' has the advantage of not worrying about its units (in SI units or Brit or American!!). Tom. Do you agree with this explanation??
@@adambodom Hi Adam, 1 g = 9806.6, so 5 mm/s^2 would be virtually negligible and detecting this level with most typical accelerometers and data acquisition systems would be difficult as you would be well below the noise threshold of most measurement devices/systems.
why in last random vibration "psd vs frequency" graph for +3dB/octave the Grms became twice at 40 Hz ?? as per you, G should get double at 80 Hz as +6dB/octave means doubling the amplitude (as per your dB chart)
The statement resonant frequency for a device under test is not fully correct. Most systems are complex and have many integrated components. The result is a stack of resonant frequencies. To measure the resonant frequencies of a device under test, one must place response accelerometers on each of the integrated components on the device under test. You would then perform a flat low amplitude sine sweep. Once you perform the sine sweep, you will then perform a flat low amplitude random test
Hi Jack, Some things to keep in mind: Are you only going to run vibration tests or also shock? What is your worst case payload or test specimen's weight and dimensions? What is your worst case G level, GRMS, Displacement, Velocity? What is the test frequency ranges for your vibration profiles? Will you run 3 axes, single axis vertical or horizontal, simultaneous axes (2 axis, 3 axis, 6DOF etc), do you want to run vibe with another environmental stress such as temperature, humidity, etc (Combined environment)? What types of vibration tests will you perform: Will you run Sine, Random, Sine-on-Random, etc? If you are running shock, are you performing classical shock pulses (Half-Sine, TPS, Trapezoid, etc)? Will you perform SRS? Will you perform time replication shock and vibration? What is your pulse duration, shape, frequency, etc? How about your building? Can your test lab location structurally handle a vibration test system? Will you require air isolation or will you build a seismic mass? I hope this helps. If the above info/terms are new to you or you need more help, feel free to reach out to me in one of the links in the description section of the video.
This video content was helpful but i had an extremely difficult time following the video because he was not fluid in his speaking, and it drove me nuts it took me and hour and a half to watch.
I think you may have described displacement a bit incorrectly. Displacement is a change in position (with direction included). You mentioned that displacement was tangent to position over time, but being tangent means that displacement would have a time component, which is doesn’t. It seems like you got displacement by taking the derivative of position... but that wouldn’t make sense. It’s an old video so I don’t expect you to update, but I don’t want anyone else getting confused! Great video otherwise, thanks Tom!
Lower video production quality than your current videos, but the content of this video is indeed high quality
Hi Anoop,
Thank you for the comment. Since we are only expressing the magnitude component of acceleration we are expressing acceleration in scalar form. Later on in the video I graphically display the vector form of acceleration as the change in velocity. Or if we are representing change in speed, then we would be discussing scalar quantity that is the magnitude component of velocity, since speed only has a magnitude component. www.av8n.com/physics/acceleration.htm
Very Good Information
Nice video. A lot of detail for one video. Will need to watch again to absorb.
Thanks Tom
Great lesson on vibration testing!
thank you so much.
When a system vibrates, its motion can be expressed as A*Sinωt. - A is the amplitude of vibration (length unit) and ω the frequency in rad/s. On differentiating twice, we get (-) Aω2*Sinωt. The value Aω2 is m/s2 (unit of acceleration) and when divided by 9.81 (acceleration due to gravity) gives the 'g' value of this vibration. This is how I explain the term 'g' . Expressing acceleration in terms of 'g' has the advantage of not worrying about its units (in SI units or Brit or American!!). Tom. Do you agree with this explanation??
Hi Ananth,
I agree. That is a great way to express vibration acceleration in mathematical terms!
Tom Resh . What's the difference of expressing it in 'g' or mm/s2?
If the value in mm/s2 is 5, what will be the expression in 'g'?
@@adambodom Hi Adam, 1 g = 9806.6, so 5 mm/s^2 would be virtually negligible and detecting this level with most typical accelerometers and data acquisition systems would be difficult as you would be well below the noise threshold of most measurement devices/systems.
to validate your sine sweep data responses.
thanks
why in last random vibration "psd vs frequency" graph for +3dB/octave the Grms became twice at 40 Hz ??
as per you, G should get double at 80 Hz as +6dB/octave means doubling the amplitude (as per your dB chart)
The statement resonant frequency for a device under test is not fully correct. Most systems are complex and have many integrated components. The result is a stack of resonant frequencies. To measure the resonant frequencies of a device under test, one must place response accelerometers on each of the integrated components on the device under test. You would then perform a flat low amplitude sine sweep. Once you perform the sine sweep, you will then perform a flat low amplitude random test
How do you find a resonant frequency for a device under test?
Pretty good content, but the audio peaks completely destroyed my ears. Guess that has to do with resonant frequencies too!
🤣
GOOD
Acceleration is a vector quantity .
Also see: www.labworks-inc.com/engineering_info/sine_vib_test.htm
I am trying to figure out a vibe table to purchase, but don't know what to get. Any advice you can give me?
Hi Jack, Some things to keep in mind: Are you only going to run vibration tests or also shock? What is your worst case payload or test specimen's weight and dimensions? What is your worst case G level, GRMS, Displacement, Velocity? What is the test frequency ranges for your vibration profiles? Will you run 3 axes, single axis vertical or horizontal, simultaneous axes (2 axis, 3 axis, 6DOF etc), do you want to run vibe with another environmental stress such as temperature, humidity, etc (Combined environment)? What types of vibration tests will you perform: Will you run Sine, Random, Sine-on-Random, etc? If you are running shock, are you performing classical shock pulses (Half-Sine, TPS, Trapezoid, etc)? Will you perform SRS? Will you perform time replication shock and vibration? What is your pulse duration, shape, frequency, etc? How about your building? Can your test lab location structurally handle a vibration test system? Will you require air isolation or will you build a seismic mass? I hope this helps. If the above info/terms are new to you or you need more help, feel free to reach out to me in one of the links in the description section of the video.
@@tomreshtesting Thank you
This video content was helpful but i had an extremely difficult time following the video because he was not fluid in his speaking, and it drove me nuts it took me and hour and a half to watch.
Great lesson on vibration testing!
Thanks Tom