The feasibility of space-based fault-tolerant robot joint design with a dual-motor, single-output differential-based mechanical drive system is investigated. The mathematical model of the differential system is developed, and the inherent nonlinear dynamic characteristics for the differential are reduced to linear state equations through variable substitutions. Nonlinear phenomena such as gearbox forward/backdrive efficiency, motor friction/stiction, and torque limiting are included. Simulations have been performed for various joint failure conditions. A scaled-down differential testbed has been designed and built to validate the analytical results. Simulation and test results demonstrate that the design is capable of sustaining a single failure and absorbing the failure disturbance, and continuing to be operational with the remaining single drive mode.>

An analysis of an improved joint drive system for the shuttle remote manipulator system (SRMS), capable of sustaining a single actuator failure, is presented. This system employs a differential gear train with dual input actuators driving a single load. The mathematical model for the drive system includes: gearbox flexibility and damping, motor damping, high gear ratio, and high load impedance. The nonlinear dynamic equations for the system reduce to linear form without approximation. Effects of gearbox placement and load sharing are examined. A feedback system that improves overall response is discussed. Simulation results show that the design is able to sustain a single electromechanical actuator failure without impeding the joint's performance. Plans for a hardware implementation to verify the system are given.>