KEYWORDS: Tunable filters, Feedback control, Control systems, Digital signal processing, Electronic filtering, Repetition frequency, Signal processing, Mirrors
In ground-based mid-infrared observations the background radiation must be removed. Chopping is a background removal method requiring fast switching of the observation field. For MIMIZUKU, the mid-infrared instrument for the TAO telescope, we have developed a cold chopper which switches the observing field by tilting a movable mirror inside MIMIZUKU, instead of tilting the large secondary mirror.
We require a short transition time, sufficient amplitude, high frequency and steadyness for observation in the chopper movement.
With Repetitive Control we significantly increase performance by iteratively improving a feedforward trajectory and continously adapting to changes in the nonlinear dynamics.
This allows for much shorter transition time (<30 ms) and more freedom in the design of a feedback controller. Furthermore, repetitive disturbances originating from the cryo-cooler can be countered thus improving stability on sky.
Controller design, stabilisation, choice of reference trajectory, real-time computability and performance trade-offs are subjects in this research.
MIMIZUKU, the mid-infrared instrument for the 6.5-m telescope at the University of Tokyo Atacama Observatory (TAO), employs a cold chopper to perform chopping, which tilts a mirror placed on the internal cold optics at about 30 K. The mirror rotates around two orthogonal axes, and its tilt angle is controlled by the balance between the restoring force of the flexural pivots and the magnetic force driven by the coils in the system. In this study, we developed a final-product model of the chopper and tested its onboard performance in MIMIZUKU. This experiment showed that the mirror could be operated with a stability of 3.83×10−4 and 3.29×10−4 degrees, and a transition time of 31.2 and 32.2 milliseconds for each rotation, when both rotations were driven at 5 Hz with an amplitude of 0.59 degrees, satisfying the performance requirements.
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