Tuning PID loops can be tricky when theory and reality don't see eye to eye.

One application years ago was for controlling hydraulics on sand trucks. For those of you that don't have the pleasure of driving on ice covered roads for 6 months of every year, sand and salt are applied to the roads to break up the ice and give cars a little traction.

The control loop monitored the truck speed and read the driver's setting for lbs/mile to adjust the speed of an auger motor feeding sand to a "spinner" on the back of the truck.

The spec also called for a "burst" mode, where they wanted a momentary increase to something like twice the set rate. When the truck nears an intersection, they want more sand. Sounds good, just double the rate. Except that the truck is idling in the intersection, not enough RPMs to keep the hydraulics working at a steady pressure.

Lots of trial and error to get that one working...