Electronics > Projects, Designs, and Technical Stuff
Cruise control, PID and integral wind-up
Circlotron:
Just thinking, if a normal analog PID setup was used in a car cruise control then if a hill came along and the car slowed somewhat then recovered, it would overshoot the original speed and then settle down to the set speed again. This overshoot "problem" as I see it is not a deficiency in the circuit but simply the car catching up to the position it would have been at if it had not slowed down.
The question is - how can you make an analog system that doesn't overshoot greatly, that once it reaches the set speed again it dumps the integral wind-up? In other words, is happy once it regains the set speed rather than the lost position.
jeremy:
A leaky integral term is a simple way. For every unit time, subtract some small value from the absolute value of the integral error. Essentially the integral term will always decay to zero.
You could also not use the integral term at all. In some systems the I term can actually increase error (although I have no idea about cruise control)
IanB:
--- Quote from: Circlotron on April 30, 2019, 01:16:09 am ---Just thinking, if a normal analog PID setup was used in a car cruise control then if a hill came along and the car slowed somewhat then recovered, it would overshoot the original speed and then settle down to the set speed again.
--- End quote ---
This is not necessarily the case.
--- Quote ---This overshoot "problem" as I see it is not a deficiency in the circuit but simply the car catching up to the position it would have been at if it had not slowed down.
--- End quote ---
I don't think this is how control loops work. The control loop doesn't know the vehicle's position--it is not using GPS, for example. It only knows the instantaneous speed of the car.
--- Quote ---The question is - how can you make an analog system that doesn't overshoot greatly, that once it reaches the set speed again it dumps the integral wind-up? In other words, is happy once it regains the set speed rather than the lost position.
--- End quote ---
This is easy, it is all a matter of tuning. If you tune for an over-damped control response then there will be no overshoot.
Phoenix:
Cruise control these days is digital. Anti windup is as simple as limiting a variable or stopping the integral summation if the output is >100%.
Similar can be done with an opamp pid using a zener diode across the integrator capacitor.
Sent from my G8441 using Tapatalk
David Hess:
Cruise controls or at least the analog ones do suffer from windup and will overshoot when recovering but the effect is usually well controlled.
Integrator windup is ameliorated either by including anti-windup circuits or using a topology which does not suffer as much from it like a transconductance error amplifier. Some operational amplifiers include "clamping" inputs which can be used to limit windup. The old LM301A operational amplifier is a favorite of mine because one of its compensation pins provides access to the transconductance node and can be used for clamping.
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