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# pneumatic servo control

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I would like to implement servo control using a low pressure pneumatic actuator. There are various companies offering valves to 'complete' systems. I am inclined to use a Textron R-DDV valve at this time with a standard pneumatic cylinder.

Obviously similar to a hydraulic system but using much lower pressure (100 psi) & a compressible fluid.

I am looking for advice on how best to model for a compressible fluid (high compliance coupling with varying spring rate depending on position) & any other associated pitfalls.

Anyone out there have experience with this?

Thank You

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I think the first thing is not to have too high expectations of what you can accomplish. For about 20 years now, I have been hearing claims that the problem of tight position control of pneumatic systems has been "solved", but I've never really seen it pan out.

If I were tackling this, I would consider it like a motor/load system with a very compliant motor coupling. With very soft control implemented, I would characterize the resonant frequency at various points of travel, including both extremes.

For my first cut, I would implement a notch filter to compensate for the lowest frequency resonance. It may work well enough over the entire range of frequencies. If it works well at the design frequency but not for significantly higher frequencies, then you will need to consider a gain scheduling regime that changes the filter as a function of position. Remember that when you are changing gains, you need to make sure that you change all 5 parameters -- the 4 notch coefficients plus the Ix30 proportional gain -- in the same servo cycle, so this will need to be done in a foreground task.

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For those of you who don't know what a foreground task is, you can use PLC0, PLCC0, or Open Servo as your foreground program. Make sure I5=1 or =3 to use foreground PLCs or PLCCs. You can control the rate of execution of PLC0 and PLCC0 with I8, I8=0 being the fastest possible rate of execution (RTI=servo rate when I8=0).