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

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. Power at and above telescope resonances of 3.5Hz (RA) and 8.5Hz (DEC)
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. Peaks excite resonances
What a PID might do
We could easily get this with a PID alone.
Peaks excite resonances
velocity
Desired velocity
20Hz PID position updates
High inertia case
position
time

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