PID P-Only Controller Introduction

An advantage of P-only control is that it is simple to tune and is a good option for integrating systems. The drawback with P-only controllers is that there is often persistent offset of the process variable away from the desired set point.

[@aireenchoo8104]

Hello. Thanks for making this video. Why won't the P-only control have an offset for an integrating system?

[@apm]

+Aluha Carpenter, great question! The system acts as the integrator so the PID controller doesn't need one. Check out this additional material, especially the offset worksheet (PDF) in this web page: apmonitor.com/che436/index.php/Main/LectureNotes28

[@seablue1085]

Thanks

[@saeed6296]

why do we have to subtract derivative term ?

[@apm]

See Derivative Kick section: apmonitor.com/pdc/index.php/Main/ProportionalIntegralDerivative

[@taharehmansiddiqui813]

Can you Explain why the offset cannot be eliminated ?

[@apm]

+Taha Siddiqui I covered this in additional detail later in the course at: apmonitor.com/che436/index.php/Main/LectureNotes28 P-only control won't have offset for an integrating system, however. For a non-integrating system, one way to think about it is that as the PV approaches the new setpoint (SP), the error approaches zero. The PID equation for just a P-only controller is u = ubias + Kc * (SP-PV). Therefore the error will go to zero and the value of u (controller output) will go back to the prior steady state ubias value and the prior operating point. An integral term is used to remove the offset.

[@taharehmansiddiqui813]

+APMonitor.com Thank you very much :)