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Proportional-Integral-Derivative (PID) controllers

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61–65 of 65 posts

Re: Proportional-Integral-Derivative (PID) controllers

#61
post #19

PIDs are great but notoriously hard to tune. They require deep insight into the underlying physical phenomena to get right. They are also rather rigid and cannot adjust well to a changing environment (temperature and humidity can fluctuate dramatically between summer and winter in some climates). Of course, no one tunes them by hand anymore for these reasons, relying instead on optimization techniques like particle s…

> we will replace most PIDs with a small neural network for almost all industrial applications

A similar argument was made in the early 90's/late 80's for using Fuzzy Logic (https://en.wikipedia.org/wiki/Fuzzy_logic) instead of classical control algorithms, including PID.

I'm sure many here will remember the late Bob Pease of National Semiconductor writing articles[1] against this, mainly due to the inability of the designer to predict the behavior of the system. Believe it or not, being able to logically reason about how an algorithm that's controlling tens of thousands of $$$ of product in process is actually important.

[1] https://www.electronicdesign.com/technologies/embedded/digit...

Re: Proportional-Integral-Derivative (PID) controllers

#62
post #39

Jesus christ. The wretched control system paper we had for B-Tech computer science! The text book jumped right on to the integrals and derivations without even a whisper on what the thing is supposed to be useful for! I want to understand this so much now, but the memories of that paper is such a turn off!

It's very dated by now, but The Control System Design Guide by George Ellis is a pretty good hands-on book about designing and analyzing control systems. It goes well beyond the basic PID stuff, but I found it very helpful when I was getting a Mechatronics certificate 20+ years go. TBH, it's the most accessible book on the subject that I've ever read.

Re: Proportional-Integral-Derivative (PID) controllers

#63
I think the more valuable topic to study is feedback control loop in general as a part of control theory: gain, stability/oscillation damping, overshoot, hysteresis/minimum commit, and noise filtering strategies (low pass, Kalman, etc). Those are just very useful general concepts applicable to any kind of long running systems.

The actual PID controllers ,however, are actually really finicky to tune, and hard to reason about directly. Good for simple linear systems, but it falls apart as real life systems tend to be complex and nonlinear.

Re: Proportional-Integral-Derivative (PID) controllers

#65
post #47

Earlier quoted context omitted.

> I think that PID hits a certain sweet spot between cleverness, ease of implementation and practical utility that makes it catnip for the typical programmer's mind. It's easy to implement, but hard to tune. PID controllers can be built from analog pneumatic components, and often are.[1] This predates computer control. The I term is called "Reset" and the D term is called "Rate" in classical control. [1] https://cont…

Pretty easy to tune really - almost never use derivative, increase the proportional until oscillations occur then halve it, add a little integral, not so much it oscillates.

PI controllers (D = 0) are not hard to tune, but if the system has enough inertia to need a D term, then it gets complicated.
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