Abstract
The use of robotic manipulators in remote and sensitive areas calls for more robust solutions when handling joint failure, and the industry demands mathematically robust approaches to handle even the worst case scenarios. For both serial and parallel manipulators torque failure is indeed a worst case scenario. Thus, a systematic analysis of the effects of external forces on manipulators with passive joints is presented. For serial manipulators we find under what conditions the robot is conditionally equilibrated, that is, equilibrated with respect to a specific external force. These conditions are, as expected, very restrictive. The serial, or subchain, case serves as a good platform for analyzing parallel manipulators. In parallel manipulators passive joints can appear as a design choice or as a result of torque failure. In both cases a good understanding of the effects that passive joints have on the mobility and motion of the parallel manipulator is crucial. We first look at the effects that passive joints have on the mobility of the mechanism. Then, if the mobility considering passive joints only is not zero we find a condition similar to the serial case for which the parallel manipulator is conditionally equilibrated with respect to a specific external force.
Highlights
Serial and parallel robots are widely used in remote and harsh environments where humans cannot or do notHow to cite this paper: Pham, C.D., From, P.J. and Gravdahl, J.T. (2014) A Geometric Approach to the Design of Serial and Parallel Manipulators with Passive Joints
For serial manipulators we find that for certain configurations the manipulator remains conditionally equilibrated with respect to a specific external force, such as gravity, even after joint failure occurs
This must be handled in the control algorithms as there is no way to guarantee fault tolerance through a fault tolerant design of the mechanism
Summary
How to cite this paper: Pham, C.D., From, P.J. and Gravdahl, J.T. (2014) A Geometric Approach to the Design of Serial and Parallel Manipulators with Passive Joints. In this case it can resist a wrench in an arbitrary direction either through kinematic constraints or through actuator torques We obtain this if the manipulator, considering the passive joints only, has mobility equal to zero, i.e. we do not want the passive joints to allow any motion when the active joints are locked. If the fault tolerance problem is not addressed in the design process it must be handled in the control of the manipulator in the case of such an occurrence In this case the serial and parallel manipulators are treated in a similar manner and we search the configuration space of the manipulator for a set of joint positions for which the manipulator remains equilibrated for a given external force. This allows us to verify resistance with respect to a specific external force, as opposed to a type or class of forces
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