PID Control: Varying saturation range?

Hey folks,

I need your help and/or opinion on this problem:

I am trying to control the shaft speed of a hydro motor by regulating its displacement in Simscape. However, depending on the actual shaft speed, the range in which the displacement is allowed to be set, changes. For a normalized displacement control signal 0...1: At speeds that are higher than the nominal shaft speed of the motor (as described in the motors data sheet), the allowable displacement decreases from 1 to zero (at an actual_speed/nominal_speed ratio of ~ 1.7). How can I implement that logic in e.g. a PID control?

A first attempt consisted of implementing a switch block, which would check if the control signal of the PID block was higher than allowed (by comparing with the speed ratio described above, extracted from a lookup table). However, that´s a bad solution since it does not feed the information about the limitation back to the controller, so the controller might still be trying to set displacement = 1, but only some "allowable" displacement = (e.g.) 0.5 reaches the motor.

Is there a way, for example, to set varying saturation limits for the control signal output?

Looking forward to your replies and solutions!

Kind regards

Felix

Answer 1

[Fangjun Jiang]

Saturation Dynamic

[Felix Schönig]

Thanks, I did not know this block. However, the way I see it, the controller itself does not get an information about the varying saturation limits. But does he even need that info? I guess you might be right. Controller tries displacement = 1, displacement = .5 reaches the motor block after running through the dynamic saturation block and any knowledge about dynamic saturation limits would not change the controller's behaviour. Is that right?

[Fangjun Jiang]

PID itself is linear. Any nonlinear behavior you have to do it outside of it.