I sort of understand using PID to set a POT value via a DC motor. The feedback is stable and static.

My robocar needs to stay in a path while moving. The feed back - change in the echo response from the wall - is dynamic and is a function of the car's speed and direction. The PIC is plenty fast. The sonar response is not "dynamic" in this sense. My response delay problem is in the physical reality of the steering mechanism and the physics of car motion.

Another issue may be that the HPWM signal is not "really" to radio control spec. It is not a 50Hz pulse of width varying between 1msec and 2msec with 256 gradations. The frequency is more than 50 per second. The arguments I use for HPWM I arrived at empirically. I do not have an appropriate oscilloscope to accurately measure what I am producing.

I need a better understanding of what you are suggesting.

Ken