Implementations of a Four-Level Mechanical Architecture for Fault-Tolerant Robots, Page 4

Level IV - Multiple arm robots represent the fourth and highest level in the fault-tolerant architecture. In the event of one arm failing, the extra arms could assume the task responsibilities. Fault tolerance at this level clearly demonstrates the need for active redundancies that provide increased capabilities as well as fault tolerance. The extra arms can provide additional task performance capabilities during normal operating conditions. A number of researchers have shown both experimental and theoretical work in cooperative manipulation using dual-arm robots. Notable examples include: manipulating single rigid objects single flexible objects , and manipulating two objects. a dual-arm robot with 17 DOF handling a thin plate demands a balance of motion and force control representative of dual-arm operations. Multiple performance criteria form the basis for this balance. These criteria emphasize task-based performance indicators derived from the physical description of the manipulator. The origins of these criteria are from foundation activity in high speed mechanisms for production machinery . There, the issues of precision and modeling of complex non-linear structures forced the development of a geometric understanding for mechanical structures and how to represent them with efficient analytical tools. Thomas and Tesar showed that the concept of kinematic influence coefficients (used in systems with 1 DOF) were effective in spatial manipulator structures with N DOF. The criteria formulations emphasize efficiency and portability. With currently available computational hardware, decisions based on several of these criteria are possible in real-time. Given the rapid pace of advancements in computational speed, it will soon be possible to employ the entire suite of performance criteria in a real-time decision making process. The constraint criteria involve rapidly calculated elementary formulations. The robot's physical limitations form the basis for these criteria. These limitations restrict joint travels, joint speeds, joint accelerations, and joint torques. The Jacobian matrix forms the basis for the geometric performance criteria. These criteria are task independent and based only on the geometry of the robot, thus these criteria are formulated once for each robot with no need for reformulation if the task changes". The force transmission criterion represents the magnification that the joint torques will undergo in their transformation to the end-effector space. As such, it may be used to indicate configurations which generally minimize the joint torques required to fulfill the output forces. In the context of dual-arm cooperating operation, accurate force and torque control at the end-effectors is essential.

Conclusions - This paper described a four-level fault tolerant mechanical architecture devised and implemented in concert with NASA. Redundant systems provide the fault tolerance, but the main tenet of this architecture is that all redundancies actively increase the performance of the system. The four levels are: 1. redundant actuators, 2. parallel-structured modules, 3. serial kinematic redundancy, and 4. multiple arm systems. Developing and testing technology applicable to future NASA missions, including: Mars exploration, planetary surveillance, and space station operations is the goal for this work. The first and most fundamental level of the architecture is at the actuator. At this level, this paper recommends achieving fault tolerance with functional duality and describes a prototype actuator symmetrical about its centerline with complete duality between its right and left halves. Each half contains an armature, rare-earth magnets, a resolver, a brake, and a Ferguson's paradox epicyclic gear train. The actuator is low-weight and mechanically stiff. If one side fails, the other side can operate above it's continuous power rating for a finite length of time to mask the effects of the failure. The next layer of the architecture incorporates parallel modules. These parallel modules include platform structures such as the well-known Stewart platform. This paper describes a fault tolerant knuckle that could form the base module for one leg of a Stewart platform. Currently, this knuckle is implemented as a fault-tolerance testbed. It uses two independent servo systems per DOF for Level I mechanical redundancy at each axis. Each servo system consists of a clutch, a brushless resolver, a brake, a Hall-effect sensor, and a 3-phase brushless DC motor. The system will simulate a wide variety of different fault conditions for testing fault detection, identification, and recovery algorithms. The third layer of the architecture is serial kinematic redundancy. Extra degrees of freedom in the kinematic chain provide the fault tolerance at this level. A number of researchers study redundancy resolution of serial kinematic chains. This paper presented a novel method based on joint-level perturbations, the notion of a six-axis wrist, and sequential filters. This method will resolve kinematic redundancy based on multiple performance criteria in an extremely efficient manner. The paper presents an example of a simulated robot with 21 DOF suffering simultaneous failures locking 4 of its joints. The redundancy resolution algorithm automatically reconfigures in response to the fault and maintains the desired EEF path. Even for a robot with this very-high degree of redundancy the space applications, including: astronaut assist, tool site preparation, module change-out, tile and solar panel inspection, and tactile manipulation. Multiple arm robots represent the fourth and highest level in the fault-tolerant architecture. Fault tolerance at this level clearly demonstrates the need for active redundancies that provide increased capabilities as well as fault tolerance. A multiple arm robot should use its extra arms. Dual-arm operations demand a balance of force and motion control. This paper represents the position that multiple performance criteria should form the basis for this balance. These criteria emphasize task-based performance indicators derived from the physical description of the robot. The paper presents a number of geometric criteria based on the Jacobian matrix and derives a force transmission criterion.