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jitach
July 20th, 2009, 09:54 AM
Hello Everyone,

I'm currently trying to implement a PID control on a fatigue loading machine. What I'm trying is creating a cyclic (sinusodial) load input as a pressure value, which is my SP, and try to track this by controlling a proportional hydraulic valve with PID.

I'm now pretty on the way of controlling the system for f=0.01Hz (load frequency). I have tuned the system with Ziegler-Nichols method (although I do not have much experience, I tried to implement the basics) However, I need a higher frequency to complete my loadings in a reasonable time (i.e 1 Hz), but when I start to increase the frequency, all I obtain is a chattering system with big deviations in the pressure and sometimes impact loading.

The question is; how can I modify a PID controller to track faster inputs? Does the maximum trackable frequency depend on the natural frequency of the overall system? If so, what can I do the increase the natural frequency ? (i.e will stiffening the frame help? etc.)

Thanks for your patience in reading this.

Peter Nachtwey
July 20th, 2009, 11:17 AM
Use feed forwards, Make sure the hydraulic design is controllable. You ability to track the the target depends on power, natural frequency and the PLC sample time. The natural frequency is determined by the area of the piston, the load and the volume of oil that is being compressed. As a PLC programmer you can't do much about any of this but you can add feed forwards.

This is not an easy project for someone that isn't experienced. Part of the experience is knowing if the design is good.
My company gets involved with these kinds of projects all the time. Most of the time it is an educational experience for the system integrator.

You can't violate Newton's laws of motion. The commonly violated law is the 3rd on about equal and opposite reactions.
Chances are the problems you have are not going to be solvable in a PLC.