Fault tolerance promises higher system reliability and safety for mechanical systems as it has for computer hardware and software, nuclear reactors and aircraft industry. This work addresses the fault tolerance capability gained by robotic manipulators as their design is enhanced by serial and parallel mechanisms, kinematic redundancy and dual actuators. As these design options improve a robotic system's tolerance to faults, it becomes necessary to evaluate the degree of fault tolerance gained by the system. This paper develops a measure for fault tolerance capability of a robotic system and provides several example robots to illustrate its use.

As a robot loses some of its capabilities due to component failure, fault-tolerant controllers isolate the faulty unit and complete the task with minimum performance loss. However, as the robot undergoes a failure and accumulates large errors, controller torque requirements usually increase and may even exceed actuator capacities. This problem can be avoided in redundant serial manipulators if the redundancy of the system is exploited to redistribute the actuator torques. This is accomplished by implementing the controller in the end-effector space. The computed torque method and PID feedback control are used in the controller. When torque demand on an actuator exceeds the allowable limit, that actuator is isolated and the required load is distributed among the remaining actuators. This controller not only avoids the saturation problem, but also allows the robot to eliminate errors and track the original trajectory. Application of the method is demonstrated on a four degree-of-freedom serial manipulator.>

When a fault-tolerant robot fails, its fault responsive system detects and identifies the failure. During the recovery process, reconfiguration of the system isolates the fault, and a new system model and a suitable controller attempt to completely compensate for the faulty condition without interrupting the robot's operation. In this paper, the authors address the recovery process for fault-tolerant serial robots when they experience actuator failure. For this purpose, the authors consider three controllers based on PID feedback, sliding control, and parameter adaptation methods. It is shown that the sliding control implemented with a boundary layer reduces the system errors efficiently when the errors are large, and the controller behaves like an ordinary PID feedback as the errors get smaller. Additionally, when failures cause uncertainty in system parameters, inclusion of parameter identification capability in the controller design is suggested. Although the work is valid for a general robot, simulation results are presented on a four-axis robot.>