The active control of segmented mirrors requires actuators to move the segments in response to perturbations. Each segment of the University of California Ten Meter Telescope has three of its six rigid-body degrees of freedom actively controlled; piston and tilt about two axes. The system design requires the actuator to carry a load that varies as the telescope moves from zenith to horizon. The maximum load is one third of the segment mass, about 150kg. The system design also needs actuator adjustment resolution less than 20nm over a range of 3mm with a 2µm/sec response rate. Actuators which satisfy these requirements have been designed, built, and tested. A torque motor turns a screw shaft whose axial load is taken by a roller thrust bearing. Simultaneously the screw drives a roller nut to position the mirror segment. The roller screw converts rotary to linear motion with nanometer smoothness over a large dynamic range. A stick-slip behavior in the thrust bearing makes the mechanical system non-linear for small motions. Each actuator has a microprocessor-controlled servo loop and the servo loop algorithm compensates for this non-linear behavior. The actuator design and servo loop algorithm are described and the results of servo loop performance tests are given.
"Actuators For A Segmented Mirror Control System", Proc. SPIE 0444, Advanced Technology Optical Telescopes II, (3 November 1983); doi: 10.1117/12.937988; https://doi.org/10.1117/12.937988