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

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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.

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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

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What PIDs do

• PIDs close a position loop

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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.

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

• We shouldnt expect a PID alone to work well for
  velocity control due to disturbances and
  nonlinearities affecting the dynamics of the
  servo.

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What disturbances?

• Its important to understand that the
  disturbances have a power spectrum (they have
  frequency terms).

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Dynamic wind effects
Mauna Kea
Power at and above telescope resonances of 3.5Hz
(RA) and 8.5Hz (DEC)
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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)
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Frequency/tuning challenge
Increase bandwidth

• But remember

Decrease to avoid resonances
Increase for disturbance correction
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stick/slip and nonlinear disturbances
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Other nonlinear disturbances
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How to fix nonlinear disturbances
Position loop is low bandwidth
High bandwidth velocity loop is essential
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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
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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
Rect/ lead/lag/sum
LP velocity loop
HP velocity loop2
3Hz HP filter
Tach
Position loop
inc encoder
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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
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Lead Lag circuit
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Lead Lag stiffness

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

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Why we shouldnt 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