Electrical – How to feedforward be used with PID for motion control

Can you instead make a tacho using a slotted opto switch and a disc with perforations. Alternatively you could get some kind of "once per rev" signal from a cheap inductive probe and a small magnet on the shaft.

Once you have this pulse waveform coming back you need to compare its frequency with a frequency that is equivalent to your demand. You mention you have a potentiometer as the "demand" - this could be used in a circuit such as a Voltage-to-frequency converter.

In effect, your demand has been translated to a frequency and that frequency is what you expect to get from your tacho (described above) when it has settled at the correct speed. Using a simple Exclusive or gate or the type II phase comparator as used in a 4046 phased lock loop would work - are you allowed to use logic chips?

You shouldn't be using the PID output as a velocity command in your system, as it will simply become unstable and eventually oscillate between one end limit and the other, or have no response.

Using a linear actuator as you are, you should use the output of the PID as a position command, and have the actuator follow the output that way. By setting the PID min and max limits, the end of travel switches are only safeties if something goes horribly wrong.

Also, if you detect that your PID is at either limit (which it might have to be if the response is very slow), you want to inhibit the integral term for as long as it is at a limit so that you don't suffer from integrator wind-up.

PID's work fine on motor controls, true, but I use them industrially to control temperatures easily on systems with several minutes of lag. Tuning is everything... Remember the basics, Proportional gain will only get you (approximately) half to three quarters way to the setpoint, Integral gain will take you the rest of the way. Derivative tempers the change.

For temperature control loops with a long lag, I find it much easier to ramp the setpoint up slowly to the final setpoint rather than jump it immediately, it allows a tighter tuning of the PID without having it go into oscillation.