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It is a black art because it depends on the load and the symmetry of (in the case of temperature controllers) the heat/cool cycles (try an oven with nitrogen injection cooling!). It is always a trade-off since some systems cannot tolerate any overshoot and therefore require different parameters to those that can even though they behave the same in relation to transients. You can formally calculate PID parameters but they are, at best, a "first best guess" in real world applications and unless you are dealing with an ideal load, will need to be tweaked with things like prop-band and dead-band. This is all you need to know about calculating PID. http://www.dcnz.com/resources/tutorials/pid.pdf The rest is knowledge of the system and experience with similar systems which give you the Control Engineers equivalent of "Code Smell". It is similar in many ways to debugging which is why it is difficult to explain by the experienced engineer.

Even after thre semesters of controls study in my BSEE program and multiple projects with manually tuned PID systems, I still feel like it's something of a black art. I've met oldschool engineers at my clients' offices with a preternatural talent for tuning PIDs, but when I asked them what their approach was, they couldn't really describe it. Just "intuition". What's your favorite book, tutorial, or other resource on tuning PIDs by hand? I've read all the Wikipedia articles and free online tutorials presented by college undrgrads or companies with a product to sell. I'm looking for the real deal here, the authentic guide to tuning mastery!

It doesn't help trying to tune when the implementation does not match the learned theory. Take, for example, tuning drives. The few times that I've been involved with the tuning, the proportional and derivative have been unit-less values and the integral in milliseconds (specifically, the rate at which the PID was calculated). As an aside, I just got out of a sales pitch during which the hardware expert described the controls drawing of their PID controller as "something the Austrians came up with while drinking one night".

I don't think it is as different as you might think. With temp controllers you reduce power approaching setpoint since you only need to put enough energy in to compensate for heat loss (synonymous with your holding current and, generally, it is a current unless they are gas heaters). You need far less energy to maintain a temperature than you need to get there (overcome thermal inertia). Similarly with motors. You need more energy to drive to position (overcome mechanical inertia) than hold it for exactly the same reasons. That's the thing about PID tuning. It doesn't matter what the system is (3 fat ladies on a see-saw). It's an empirical method to map a systems final output solely in terms of its input to arrive at an approximate model of the system - regardless of what the units may be (deg/C, deg/rad, or lemons).

Here's one person's opinion that I think is to be taken into account : http://lavag.org/topic/17009-labview-2013-favorite-features-and-improvements/?p=105003

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