PID autotune does not work

[Morbo]

Hello,

 

why does aurotune not work in the attached vi? Plus if I change the setpoint, shouldnt the PID output change as well? I.e., if say the pump needs to pump more, has to spin faster? or at least, have to ramp up once to fill to a new level and then oscillate again, since the amplitude is the same.

 

Its a squarewave on the input, the help says, when the process is oscillating, outotune should come up with something. The squarewave kind of oscillates just around 2 levels?

 

Thanks,

 

MorboPID 01 autotune.vi

Edited May 9, 2015 by Morbo

[viSci]

Go to the PID Tank Simulation example as a good starting point.

[Morbo]

added feedback. But autotune is still not working. It just flickers and the results are NaN (not a number)PID 01 autotune feedback.vi

[PiDi]

There is absolutely no way to create the controller for something that does not respond to controller output. In your design the controller does not output to the plant - the plant is only generating square wave, it is all the controller "see" and there is nothing it can do about it, because there is no feedback from controller to plant.

[Morbo]

but it could be some alternating demand and the controller adds or subtracts? Then it sees squarewave plus its response?

 

Or would I have to do a shift register of sorts?

 

Seems it does not do much better with a sinewave.

PID 02 autotune feedback.vi

Edited May 10, 2015 by Morbo