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[DPRG] When to PID?
Subject: [DPRG] When to PID?
From: David M Wilson
davidmw at tx.rr.com
Date: Fri Mar 16 00:04:04 CDT 2007
Are there any 'rules of thumb' that can be applied to a design to help
determine if it is a good / poor candidate for software based PID motor
control?
These seem like a significant challenge for PID control of wheel RPM
(measured using BEMF) or for my tuning ability:
- Frequent wheel speed changes are needed for steering control, including
'immediate' wheel stop and reversal.
- If the requested speed can not be achieved at one wheel, slow the
remaining wheels appropriately.
- Motors that accelerate slowly and that require a significant reduction /
reversal of power to halt that acceleration once acceleration is
established. There is a significant 'dead zone' in which the power can be
reduced with every control loop but the wheel will continue to accelerate
(but at a decreasing rate of acceleration).
- Motors that respond best (fastest) to intentional power overshoots early
in each acceleration run. If a power setting of 150 will maintain an
unloaded 200 RPM, spiking the motor at a power setting of 200 - 250 and
decreasing the power to 150 as the target speed is reached provides the best
performance.
- The PID controller can hit peak power while accelerating but it usually is
achieved by accumulating lots of error and happens too late to avoid a huge
overshoot. Or the P gain is so high that oscillations are a sure thing.
- It may be necessary to dump accumulated error before each maneuver in an
effort to speed up transitions but this could result in choppy control.
- Dropping the 'I' and creating a PD controller creates a system that favors
a set point of zero. By design the PD controller cuts power each time the
error reaches zero and this does bad things to top speed and torque when I
often need to run the motors full out.
- Adding a constant or computed value as the 'base power setting' to the PD
formula can shift the natural set point but performance isn't very
consistent at different power settings or loads.
For the PID experts - does this sound like a tuning challenge or just a bad
fit for PID?
David Wilson
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