Servo tuning

After Al and Brian fixed the El drive alignment they handed Polarbear off to Sean for servo verification and tuning. Polarbear has 3 distinct servo loops. GCP sends position commands to a position loop, which sends velocity commands to a velocity loop, which sends current commands to a current loop. The current controller is factory autotuned once it's hooked up to the motors, but the velocity and position loops must be hand tuned.

To tune and verify each servo Sean repeated two basic tests: network analysis and impulse response, looking for the same things we do in bolometers - bandwidth, resonances, time constants, etc. He runs the tests largely by hand - turning the knob on a function generator over 30 seconds or so.

In their servo system, we have some margin to play. As long as we remain stable, we can trade between overshoot/settling time and bandwidth. Since we want to do fast scans with out needing much pointing precision, Sean cranked up the position loop bandwidth from .22Hz to .5Hz - the spec is .25Hz.


The velocity response of our telescope has a resonance at 3Hz due to elasticity in the drive train:

This sweep goes from .1Hz to about 15Hz. The measured velocity not coming up to command velocity on one side is due to a problem with the DAQ that acquired the data. At 3Hz the measured velocity phase slips about 90 degrees behind the command.