PID control loops - closed loop feedback

[kevinw]

I believe the integral control portion should offset the initial setting of pid, but I would set it to 0, as this will most likely reduce settling time, overshoot, etc.

As for the digital response, my team saw something similar until we realized we were reading the wrong potentiometer value on the robot. We were reading a centered potentiometer, so when the input was neutral, the motor would not move. When the input was forward, the arm went forward, but the feedback wasn't in place to slow the arm down as it approached it's desired position because we were monitoring the wrong joint. I would recommend adding some debugging code to display both your arm pot value and your joystick value - this will also help you check to make sure the scaling is the same.

It may also be a function of the gain settings. One suggestion is to comment out the integral and derivative control inputs, leaving only the proportional control effort and attempt to tune it so that it's satisfactory. This is the simplest loop, and often is enough to get the job done well. If you find you have error that can not be overcome (due to gravity, etc.), then you may have to add in integral control back in, or go back to a full PID. Either way, I try to start as simple as possible and add in complexity only as needed.

Hope this helps.