IRTF TCS Servo concepts I
Peter OnakaNote: 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
resonancesIncrease 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 servoPosition command pulse rate
Tach
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
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