PID speed control

Hi, There are three variables in PID loop – Kp,Ki, Kd. Once we tune to get the value for one desired speed, do we have to tune again if we change the desired speed?

no, the same PID constants are used

while what infinity dragons said is generally true, you want to tune the PID loop for the general value you want to be running at in competition, the if you use X speed to tune the constants, those constants will work best for X speed, and may behave slightly differently at Y speed. We have never experienced a problem with this in VEX (because the motor speeds are between 0 and 2500 or so rpm) but in FRC we tuned a fly wheel PID for 100 or so RPM and found it was not very stable at 5000 rpm

For the most part you shouldn’t have any issues tuning it at x speed and then running it at y, especially in vex. Just make sure it is stable at x, because their is the possibility for an increase in speed to amplify the effect of an unstable PID loop.