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 AIR VALVE x200 FORCE BIAS HARD POINT LOAD CELL 1 OF 3 IDENTICAL SERVO LOOPS

F5 Lateral Servo Block Diagram Σ SERVO CIRCUIT x200 FORCE BIAS LOAD CELL Tangent Rod 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