Hi Henrik,

I have had a high degree of success with this project today.
I disconnected the motor and replaced the feedback voltage with another pot as you suggested. I was able to get some very nice PWM signals showing on the scope. I think a lot of my problems last night were caused by a dodgy MOSFET. It failed under load this morning and the whole system fired up nicely once it was replaced.

To tune the system I added two more pots; one for Kp and one for Kd. I also connected the LCD which helped me greatly in seeing what was happening. The motor I am using doesn't have alot of inertia so Kp and Kd terms are relatively small for a tightly tuned system response. So far, so good.

I am a little confused with your velocity feedforward section. pid_velocity_cmd is initially set to zero and your program comment states that this variable is "set by the main program." From what I can find, this variable is only used in the calculation for pid_Vel_ff and pid_Acc_FF. If this is the case, it will always remain zero (as will pid_Vel_FF, pid_Acc_FF and pid_LastVelocity_Cmd). Am I missing something?

For my trials I ended up setting pid_Vel_FF manually just so the output could overcome what little insertia the system does have.

There is still a fair bit left to do on this project but before I do much more I need to transfer this circuit from my breadboard to a PCB. I am sure a lot of the instability I am seeing on my scope can be attributed to the breadboard (switching currents, ground loops etc.)

Your return comments on the feedforward situation would be greatly appreciated.

Cheers
Barry