We describe the methods used to control 0.1 meter diameter crystal retarders on the Daniel K. Inouye Solar Telescope (DKIST). The DKIST retarders are required to arrive at, or pass through, requested positions within 7.407 microseconds of the expected time of arrival, with requested rotational velocities of up to 1,800 degrees per second. The motor control system has a Logistic distribution of the time error centered on the expected time of arrival with an RMS error of 1.232 microseconds at that angular velocity. We discuss the use of servo-cycle time compensation techniques, the advantages and disadvantages of the Parabolic Velocity Transfer used to transition between requested positions and velocities, and the methods used to tune both the motor control system and the time coordination system. We evaluate the motor controller’s performance with statistical methods and discuss the unexpected Logistic distribution of the arrival errors.
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