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Good day,

I was confronted with the task to develop a new controller for pneumatic cylinders. The control channels are displacement and force. A load cell is used to measure the force (analog signal). The displacement measurement takes place via a scale on the cylinder. In old applications a proportional flow control valve was used as actuator. However, these are probably poorly available and/or no longer available in sufficient quality.

My question would therefore be whether I can also realize the control with fast-switching directional control valves? I have also found methods where both sides of the cylinder are pressure controlled.

Maybe someone with more experience in pneumatic control can give me a hint which method is the most effective and modern for my problem. For starters, the recommendation of suitable valves would also be enough for me ;)

Update: I started installing the force regulator this week, which is actually working quite well. I'm using an MPYE valve from Festo, which is non-linear (link). Next month I will get the hardware to use a different valve that should work better. Right now it's just a simple p controller with an offset to compensate for the non-linearity of the valve. Unfortunately, I won't be able to test the position controller because I won't get the hardware approved (please don't ask why). However, I am confident that the position controller will work similarly well with an adjusted p-value.

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  • $\begingroup$ It will be difficult if not impossible to control to both load AND displacement. But you could use the more important one as your control feedback, and just set limits on the other that will cause a halt or despressurization if trespassed $\endgroup$
    – RC_23
    May 11 at 12:25

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I have tried to do positional control using Clippard miniature valves, which have a fast switching time (a few milliseconds), but I couldn't achieve any great accuracy, due to the sheer number of variables - the final position of the piston depends on the balance of various forces, some of which are outside your control.

The simplistic approach of just shutting off the air feed when the piston is near the desired position doesn't work; you have to apply pressure to both sides of the piston to try and get a positive stop, and the amount & timing of the back pressure will have to change depending of the position of the piston within the cylinder, and the length & size of piping between the valve and cylinder, since this affects the volume of air that is being compressed, and hence the force on the load.

So achieving a good positional accuracy is a complex problem.

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