To PID or not to PID

Actually open loop control always _retains_ the stability characteristics of the plant -- you can't stabilize an unstable plant with open-loop control, and you can't destabilize a stable plant. As soon as you add feedback then you move the system poles around (in a linear system), and that affects stability.

What closed loop control does for you is allow you to take a plant that is inaccurate by itself, measure it with something cheap and accurate, and drive the plant accurately with the measurement.

You can do this in reverse, too. The most interesting closed-loop system I work on is a rate-integrating gyro that's used as a rate sensor. The way this is done is that the gyro is built with very quick, accurate torquers to force the gyro to precess. The rate measurement is done by servoing the gyro wheel to the center of it's travel, then reporting the command to the torquer.

On Sat, 09 Jul 2005 09:08:35 -0700, Tim Wescott proclaimed to the world:

True. Did I infer otherwise? You can destabilize a plant in an open loop system if the final control elements are sticking. The same is true in a closed system too, more so in fact.

An interesting way of looking at it. I would put it thus: A close loop system's focus is on the value of the controlled process directly, while an open loop infers the process. (excuse the excessive use of the word "infer")

I've seen this before and am trying to remember where. Perhaps it was being used to auto syncro generators in a grid. A similar system I built for a looped chain conveyor system where the load was shared between motors positioned throughout the loop. The chain slack was the problem. For some reason I do a lot of correcting designs after they have been installed in plants and they do not work properly. This system had VSDs for each motor with a common speed signal. The original designer had supposed that the motors would run at the same speed, sharing the load. Actually the motor with the heavier load had more slippage and ended up taking all of the load and all the slack ended up in one take up bin. The conveyor still worked but was dragging all of the chain around a turn which wore the guides. If the designers had specified synchronous motors it might have worked. I changed it so that the feedback was motor load, not speed and put sensors in the chain slack bins to trim the loads as necessary to maintain slack in each bin. Each motor load ended up being trimmed to take just the load of moving the chain for the section it handled. Crude, but sometimes crude works the best.

Be well,

HoP

The preceding message represents personal opinions and/or advice that may prove incorrect or harmful. But then maybe not. Feel free to disregard.

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In materials lab in college, we had a screw press that measured applied force with a platform scale. The motor-driven screw applied strain at a constant rate, and we were supposed to record the force by balancing and reading the scale every 20 seconds. The lab instructor insisted that accurate readings were possible (though he never offered to demonstrate). I rigged a sensor to measure the scale arm's position, a solenoid to hold it centered, and a strip-chart recorder with timing marks to record the current needed to do that. The servo amplifier I built wasn't in the books, but I needed it to deal with the prominent resonances in the mechanical system. We had great lab reports!

There's a name for that design philosophy. "null measurement", I think.

Jerry