James WEBB Space Telescope Deployment Tower Assembly Deploying Anomaly and Lessons Learned
Abstract
During ground testing the Deployment Tower Assembly (DTA) on the James Webb Space Telescope occasionally failed to maintain the specified preload at the end of either stowed or deployed positions after the motor current was turned off. The design of this preload device relies on a magnetic detent brake attached to the motor rotor of a planetary gear motor. To study this characteristic a stepper motor model with dynamic loading conditions is developed using VISSIM program and the key parameters (i.e., Coulomb torque, magnetic detent torque, viscous torque, etc.) are derived by comparing the measured and the simulated data. The analytical tool illustrates that rotor backspin is caused by normal stepper motor response to an instantaneous transition from an energized holding state to a magnetic detent. Irregularities in both the magnetic detents and the various frictional elements within the system will cause variable holding capabilities that explain why backspin does not always occur. The key lesson learned is that magnetic detent holding capability is very sensitive to rotor dynamics and should not be based on torque capability from a test where loading is applied slowly (quasi-static torque).
- Publication:
-
ESA Special Publication
- Pub Date:
- September 2015
- Bibcode:
- 2015ESASP.737E..23T