Rotator Servo Tuning on the 2.5m Telescope

Sloan Digital Sky Survey Telescope Technical Note 20060111

R. French Leger
January 11, 2006

On January 10, 2006, Wendell Jordan, Bob Pfaffenberger and I investigated the servo ringing in the rotator motor. The PID parameters for all three axis were recorded when we started this exercise this morning.

Az
  P=160
  I=4
  D=700

Alt
  P=120
  I=4
  D=600

Rot
  P=120
  I=12
  D=600

We altered the PID values for the rotator until we got a good output signal on the Glentek amp. The final values for the rotator PID that gave us the best signal and got rid of the motor ringing were:

Rot
  P=120
  I=4
  D=350

We will leave it at these values and monitor the rotator's behavior. We did not change the values for the az or alt. We also ran these tests at different alt angles with a cartridge mounted. Once we felt that we had a good setting we tested with the imager mounted and the system worked well.

We changed the PID values on the rotator axis motor due to a change in its torque values from 350 ft-lb before we re-greased the bearing last summer to 450 ft-lb after:

Old PID values
Azimuth P=160
I=4
D=700

Altitude P=120
I=4
D=600

Rotator P=120
I=12
D=600

New rotator values, az and alt were unchanged

Rotator P=120
I=4
D=350

This was done to eliminate the motor "ringing" during acceleration due to servo oscillations.