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f/5 Mirror Support Servo Operation

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

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f/5 Mirror Support Servo Operation

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  1. f/5 Mirror Support Servo Operation MMT Observatory D. Clark 4/6/2007

  2. 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

  3. F5 Axial Servo Block Diagram Σ SERVO CIRCUIT FORCE BIAS AIR VALVE HARD POINT LOAD CELL x200 1 OF 3 IDENTICAL SERVO LOOPS

  4. F5 Lateral Servo Block Diagram Σ SERVO CIRCUIT FORCE BIAS AIR VALVE Tangent Rod LOAD CELL x200 Zero Adj LOAD CELL x200 Zero Adj Tangent Rod

  5. 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.

  6. Overall Block Diagram

  7. 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

  8. 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

  9. 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

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