David,

PWM is a form of DAC. Provided that the PWM signal is low pass filtered, a reasonable (but somewhat noisy) and cheaply produced DC signal will result. In the case of motor control, the motor provides the low pass filtering. Possibly a PWM cannot provide the large dynamic range required by this project. I haven't closed-loop controlled motor speed before, but I have closed-loop controlled pressure and temperature using a PID(Proportional Integral Derivative) algorithm with a PLC. Could this be used? I think it could be somewhat easily implemented in a uC. Assembly would be tough, but C would be feasible.

Cory Spackman