IRTF TCS Servo concepts I Peter Onaka Note: this is a compilation of information blatantly copied from multiple sources. 11/4/03

TCS Servo primary functions Tracking = primarily velocity control –Most important = Only time when we are actually acquiring science data. Offset = small position move –Finishes with track mode, observer waiting… Slew = large position move –Observer waiting….

Why is tracking most important? Poor tracking means –Smeared image/spectra –May render AO useless –Image quality invalidated –Bad or poor data –* because all or our instruments integrate on object Poor Offset means –Wasted time –Possible failure to acquire data (sequences) Poor Slew means –Wasted time

What PIDs do PIDs close a position loop

Ground based telescopes A PID alone will probably not be adequate. We also have a more complicated drive arrangement. We need to close a velocity AND position loop.

What does the velocity loop do? Resonances Position loop Slew and Feedforward

What does each loop do? We shouldn’t expect a PID alone to work well for velocity control due to “disturbances and nonlinearities” affecting the dynamics of the servo.

What disturbances? It’s important to understand that the disturbances have a power spectrum (they have frequency terms).

Dynamic wind effects Mauna Kea Power at and above telescope resonances of 3.5Hz (RA) and 8.5Hz (DEC)

Wind effect PSD (VLT model) Torque magnitude could still be significant Power at and above telescope resonances of 3.5Hz (RA) and 8.5Hz (DEC)

Frequency/tuning challenge Decrease to avoid resonances Increase for disturbance correction But remember “Increase bandwidth”

“stick/slip” and nonlinear disturbances

Other nonlinear disturbances

How to fix nonlinear disturbances Position loop is low bandwidth High bandwidth velocity loop is “ essential”

What a PID might do We could easily get this with a PID alone. velocity position 20Hz PID position updates Desired velocity time High inertia case Peaks excite resonances

The old servo Position command pulse rate Tach Pwr amp Rect/ lead/lag/ sum 3Hz HP filter preload Bull gear 3.7Hz LP filter + sum sum Pwr amp Rect/ lead/lag/ sum 3.7Hz HP filter preload Torque split inc encoder Position error counter 14 bit DAC Integrator + feed- forward Limiters + ramp gen Feedforward & offset Position loop LP velocity loop HP velocity loop1 HP velocity loop2

LP velocity loops Rect = unipolar for anti-backlash Lead/lag = PI controller plus R/C lead circuit Sum = velocity command, tach HP and preload Tach Bull gear Pwr amp Rect/ lead/lag/ sum 3Hz HP filter preload HP velocity loop1

Lead Lag circuit

Lead Lag = stiffness This stiffness increase compensates for torque disturbances (wind, cable wrap loading, stick-slip etc.)

Why we shouldn’t use the bull gear encoder feedback for HP velocity control Tach Pwr amp Rect/ lead/lag/ sum 3Hz HP filter preload Bull gear 3.7Hz LP filter + sum inc encoder HP velocity loop2 Ev: ” It was discovered early on in the development of the TCS, that there existed a significant amount of torsion between a given motor and the bull gear twisting of motor shaft). The simplest method of reducing the affect of this torsion was to create two separate servo loops, one for each motor/tachometer combination in the frequency range where this torsion affect dominates.” Torsion difference