Precision controller for segmented mirror telescope actuator: Control and tuning

The segmented mirror telescopes are built using small hexagonal mirrors positioned and aligned by the three actuators and six edge sensors per segment to maintain the shape of the primary mirror. The global loop controller does this by sending commands to the actuators to move within few nanometers. These commands are executed at individual actuator locally by a precision controller. The paper describes our effort to develop a precision actuator controller at India TMT Coordination Centre (ITCC) laboratory, Indian Institute of Astrophysics, Bangalore. The actuator controller is implemented around a single board computer called SBC6845. We have also designed and developed a customized actuator drive board which comprises power electronics to handle the voice coil motor (VCM), off-loader as well as snubber stepper motors. In addition to this, drive board also contains a decoder, current sensor and related circuitry to get the position and other feedbacks. The closed loop proportional-integral-derivative controller (PID controller) is implemented for position loop using the feedback from linear optical encoder. Another closed loop is introduced around off-loader with the feedback from current sensor. The tuning of position loop is done by two different methods based on Relay Auto tuning and System Identification. By making use of above controller and the optimum gain, several experiments have been conducted to test the performance of prototype soft actuator. The Actuator along with its best tuned controller gives the steady state position error around 2.44nm RMS. At the tracking rate of 300nm/s, we could achieve RMS position error of 4.04nm, which is better than what is required. These results and other details of controller and its tuning are presented.