Abstract:
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-integralderivative 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.
