Development of Precision Controller for Thirty Meter Telescope Actuator

Abstract

The Thirty Meter Telescope (TMT) mirror segment actuator is a soft actuator designed by Jet Propulsion Laboratory (JPL), Pasadena USA. As a part of India‟s in-kind contribution, all required 1600 actuators will be manufactured in India and will be supplied to TMT consortium. Parallel to the effort to prototype actuator in India, a project has also been initiated to study TMT actuator in details. This thesis is a result of an M.Tech project carried out at ITCC laboratory of IIA with the aim to develop a precision controller for the TMT actuator and study its performance.

The actuator controller has been implemented around a single board computer called SBC6845. Actuator drive board comprises the power electronics to handle the voice coil motor (VCM), off-loader as well as snubber stepper motors. The driver board also contains required electronics to get the position and other feedbacks from different sensors. An integrated PCB, suitable for our application was designed and fabricated by applying recommendations and standard practices of PCB design. The board consists of VCM power amplifier, Stepper motor driver, Decoder, and related circuitry. The closed loop PID control is implemented for position loop using the feedback from linear optical encoder. Another closed loop is introduced around off-loader and the feedback comes from current sensor. Additional sensors like force sensor, vibration sensor, limit switches etc, are implemented to monitor and control the performance of the actuator.

Performance of actuators such as steady state response, tracking error, disturbance rejection, slewing rate, tracking rate, performance under dynamically induced load has been carried out with the prototype actuator in the ITCC laboratory of Indian Institute of Astrophysics, Bangalore. The tuning of closed loop is done by Relay Autotuning method. And the best tuned closed loop system gives a rise time less then 30ms against the 120ms rise time in open loop mode. The control bandwidth achieved is 50Hz. The displacement error or the steady state position error of the control system is found to be about 8.74nm RMS. The tracking rate of 260nm/s is achieved with the tracking error around 7 nm RMS against the requirement of 4.4 nm RMS. Also the offloading algorithm is implemented to reduce the power consumption by VCM due to static loads. The whole control system has been successfully realized.

Parallelly we also worked on the actuator modelling and control simulation. Our this endeavour will help in predicting the behaviour of actuator and controller in different dynamic conditions. So far we have understood different aspects of the TMT actuator and derived different control parameters. We also derived an effective transfer function model of the actuator which relates the input voltages to the output position of the shaft. A customized control system simulator in Matlab/Simulink has also been built. This simulator tool can be used to study the performance of the actuator model in time and frequency domain, as well as with the variable loads. In order to include other parameters like sensor noise, filters, auto-tuning capability, frequency domain analysis etc, this simulator will be improved in near future.