Transcript F5 mirror support servo

f/5 Mirror Support Servo
Operation
MMT Observatory
D. Clark
4/6/2007
Necessary Conditions for Proper
Mirror Support Operation
• Hardpoints properly locate and define the mirror location
w.r.t. the mirror cell
• Hardpoint force measurements are linear w.r.t. the actual
mirror position
• The axial support plane and the plane of the mirror back
surface are parallel; in addition, all axial support
actuators are at the same height w.r.t. the back surface
in their nominal operating position
• The lateral actuator support locations and the CG plane
are coplanar
• The lateral hardpoints ideally are mounted in the CG
plane
F5 Axial Servo Block Diagram
Σ
SERVO
CIRCUIT
FORCE BIAS
AIR
VALVE
x200
HARD
POINT
LOAD
CELL
1 OF 3 IDENTICAL SERVO LOOPS
F5 Lateral Servo Block Diagram
Σ
SERVO
CIRCUIT
FORCE BIAS
Tangent Rod
x200
LOAD
CELL
Zero Adj
x200
LOAD
CELL
Zero Adj
Tangent Rod
AIR
VALVE
Circuit Operation
• An adjustable setpoint equal to the “assembled”
hardpoint force is used as the force reference
voltage.
• The servo loop PI gain forces the mirror support
air transducers to cancel the gravity load of the
mirror.
• A “soft-start” is provided to keep the loop
integrator from windup at turn-on.
• If insufficient air pressure is present, the air
valve outputs are switched off.
Overall Block Diagram
Current Design Features
• Input DC is locally regulated on-board
• Precision voltage references used for load cell
excitation and generation of bias-offset and
force-reference circuits
• Both bias-offset and force-reference voltages
are fully adjustable for all servo circuits
• Anti-windup and graceful shutdown for force
loop integrators
• Extensive use of SMT; parts we “expect” to
replace are socketed
• uP-based DAQ for live support system telemetry
Design Improvements
• Reduce the number of adjustments; only
force-reference is really necessary
• Add a single connector for convenient
attachment of test equipment, instead of
hunting all over the board for test points
• Improve the board layout for better signal
integrity in the DAQ system
• Upgrade to an Ethernet-capable uP for
faster, simpler DAQ communications
Conclusion
• The f/5 mirror support servo is in some
ways superior to its counterpart in the f/9
mirror system
• The DAQ available in the f/5 system is a
valuable add-on at little cost overall
• Improvements can and should be pursued
for follow-on wide-field secondaries, but
the basic design is sound, and proven to
work