PID for velocity control

[SuperBK]

We want to implement a PID for velocity control of the left and right motors. We understand how a position PID works, but for a velocity control, when the motor reaches the desired velociy, the error will be zero, therefore, it will only have the I and D terms to keep it driving. Is that enough? In this case would the integral sum never be cleared?

About resolution: Right now we get around 1200 counts per second at full speed from the gear tooth sensors. If we wait for 100ms that will give us a speed range of 0-120. Should the PID calculation only run at the same rate? If the PID loop runs ever 26 ms, it will aleady have run three times before we have a speed update. Whats a good balance of PID loop time to data update time?

Is the 26.2ms main loop stable enough to run the PID in or should it have its own timer?

Thanks,
Brian