IRTF TCS Servo concepts I

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

2. 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….

3. 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

4. What PIDs do • PIDs close a position loop

5. 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.

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

7. 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.

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

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

10. 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)

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

12. “stick/slip” and nonlinear disturbances

13. Other nonlinear disturbances

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

15. 20Hz PID position updates What a PID might do • We could easily get this with a PID alone. Peaks excite resonances velocity Desired velocity High inertia case position time

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

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

18. Lead Lag circuit

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

20. Why we shouldn’t use the bull gear encoder feedback for HP velocity control 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.” Bull gear Torsion difference preload 3.7Hz LP filter + sum Pwr amp Rect/ lead/lag/sum HP velocity loop2 3Hz HP filter Tach inc encoder