Implementations of a Four-Level Mechanical Architecture for Fault-Tolerant Robots
with Del Tesar, Chetan Kapor and Dev Sreevijayan

Introduction - This work develops a four-level mechanical architecture for fault-tolerant robots devised in concert with personnel of the Telerobotics Program at NASA, Johnson Space Center. The description includes a specific example for each level of the architecture. In most cases where the examples include actual hardware implementations, NASA headquarters funded the development. The goal for this work is developing and testing technology applicable to future NASA missions, including: Mars exploration, planetary surveillance, and space station operations. The level of performance and versatility expected from space robots makes it imperative that particular emphasis be placed on the question of architecture. Traditionally, mechanical systems have had monolithic architectures that do not permit easy repair and replacement, nor the fluent incorporation of advances in component technologies. Ease of repair and replacement are directly linked to the availability of the system and is, therefore, a matter of immediate concern to space systems. The deficiencies of such a philosophy, or lack of one, are best offset by aiming for an architecture that is highly structured and modular. A true modular architecture helps reduce life cycle costs and frees the designer to quickly prototype and develop actual operational systems for future space missions. Fault tolerance in a robotics context may be defmed as the capability of the system to sustain a failure and still continue operation without significant impact on the manipulator payload or its immediate environment. Graceful degradation is often inadequate as a requirement and uncontrolled motion must at all costs be avoided. Fault tolerance is assured by the incorporation of protective redundancies and their organization into an effective and responsive architecture. While the causes of unreliability do not disappear, their effects are counteracted by the capability of the system to mask the failure or reconfigure itself in the event of component failure. Redundancy is demanded by twin operational considerations for space operations: enhanced performance provided by redundant systems and the ability to tolerate faults. Note these redundancies are active redundancies; operational at all times and enhancing the performance of the robot through the optimization of secondary criteria. The concept of masking redundancy, where the effects of intramodular failure are not felt downstream from the module, is utilized in deriving mechanical architectures that provide protective redundancies at multiple levels.

The Four-Level Architecture - Tesar et al. present the conceptual outline of a total architecture based on modularity principles. While the issue of architecture depends ultimately on the context and specific tasks envisaged of the robot, it is possible to devise a broad architectural scheme that is based on the requirement that the system be two-fault tolerant . The resulting architecture, in its most general form, is capable of providing a masking redundancy at the first level and a dynamic or reconfiguring redundancy to tolerate the second fault. The organization of these redundancies may be conceived in four levels, constituting a subsumptive architecture. The four levels are: 1) extra actuators per joint (e.g., prime mover duality) 2) extra joints per degree of freedom (redundantly actuated parallel structures, e.g., 4-legged spherical shoulder) 3) extra DOF per arm (redundant manipulators) 4) extra arms per manipulator system (e.g., dual arm robots, four-fingered hands, etc.) (1) Dual Redundant Actuators: Existing robotic actuator systems could suffer failure in a variety of modes. Without complete functional duality, it becomes impossible to mask all the probable failure modes. It is, therefore, imperative that a dual actuator set driving a single joint be designed. These actuator sets can be in either of two distinct configurations: torque summing or velocity summing. In the torque summing configuration, the actuators share a common angular velocity and sum their torques. In the velocity summing configuration, the actuators share a common torque and sum their angular velocities. (2) Parallel-Structured Modules: The second level of the fault-tolerant architecture comprises parallel-structured modules under force redundancy. These are closed-loop (kinematic) structures with more joints than Degrees Of Freedom (DOF). In most such structures, the driving actuators are placed at the base joints such that the system has minimal inertial forces. It is also possible, in accordance with the subsumption principle embodied in the architecture, to provide Level I fault tolerance at each of the actuated joints, though this will initially come at the expense of increased complexity and the need for more careful design. (3) Redundant Serial Manipulators: The third level of the architecture is characterized by a superabundance of kinematically independent inputs. In general, these structures possess more than the six degrees of freedom necessary for arbitrary spatial motion. Most existing manipulators do not possess redundancy, much less, hyper-redundancy, but it is nevertheless interesting to study the effects of kinematic structure on the fault tolerance capabilities of the manipulator. Such a discussion should naturally lead to the problem of designing serial manipulators that possess kinematic fault tolerance, which may be defined as the ability of a mechanism to retain a useful workspace following a change in its kinematic structure due to failure. Paredis et al. investigated the design of serial manipulator structures that provide fault tolerance for specified paths. (4) Multiple Ann Systems: Dual or multiple arm manipulator systems constitute the fourth and highest level of the subsumptive architecture. The dual arm system at this level of generality has individual arms reflecting the subsumption principle up to and including the third level. This duality at the fourth level is a direct reflection of the anthropomorphic paradigm and provides for fault tolerance. The analysis of such structures is siriiilar to those of serial and parallel manipulators, depending on the degree of operational coupling between the arms. The following sections describe development work at each of the four levels. Next Page ->