  This paper investigates the mechanism of various faults of momentum exchange
devices. These devices are modeled as a cascade electric motor EM - variable
speed drive VSD system. Considering the mechanical part of the EM and the VSD
system, the potential faults are reviewed and summarized. Thus with a clear
understanding of these potential faults, a general fault model in a cascade
multiplicative structure is established for momentum exchange devices. Based on
this general model, various fault scenarios can be simulated, and the possible
output can be appropriately visualized. In this paper, six types of working
condition are identified and the corresponding fault models are constructed.
Using this fault model, the control responses using reaction wheels and single
gimbal control moment gyros under various fault conditions are demonstrated.
The simulation results show the severities of the faults and demonstrate that
the additive fault is more serious than the multiplicative fault from the
viewpoint of control accuracy. Finally, existing fault-tolerant control
strategies are brief summarized and potential approaches including both passive
and active ones to accommodate gimbal fault of single gimbal control moment
gyro is demonstrated.
