Silent Steps: Mitigation and Analysis of Stepper Motor Induced Jitter on the RST

On the Roman Space Telescope (RST), expected to launch in early 2027, the High Gain Antenna (HGA) is a critical element of the mission that transmits data from L2 to Earth at 500 Megabits/second. Hybrid stepper motors were chosen to control the HGA because of their small step angle. But stepper motors also induce a dynamic response that results in image jitter. The motor's torque signature includes pulses as the rotor poles pass through the magnetic detents that are used to provide stable unpowered holding torque. Several low frequency modes, mostly associated with deployable subsystems, are excited, resulting in increases in optical jitter. This paper covers the necessary mitigations included in the design and nonlinear analyses performed that were required to demonstrate that mission jitter requirements with respect to the HGA disturbances are met. The magnitude of jitter is mitigated in a multi-tiered approach. Requirements on the motors and control electronics balance the need for unpowered stability with that for smooth stepping. The dynamics of the observatory are improved with dampers to reduce the effect of jitter. Finally, control algorithms are implemented that balance image jitter against the need to maintain ground station pointing. The expected optical performance is analyzed regularly as the design is updated and test results are collected. Custom tools developed for RST are used to develop test correlated nonlinear models. These models are coupled with finite element model derived dynamic models to perform end-to-end nonlinear transient analyses. Uncertainty analyses are used to better understand worst-case outcomes. Ultimately the analysis is used to verify that mitigations are successful in helping the observatory meet system level requirements.