Calculating force distributions for redundantly actuated tendon-based Stewart platforms
Abstract Completely and redundantly restraint tendon-based Stewart platforms demand for a distribution of tendon forces to control the platform on a given trajectory. Thus, position control has to be extended by a tendon force controller which generates continuous and feasible force values. The computation of such force distributions can be formulated as a constrained optimization problem. Solving the problem is numerically expensive and requires an algorithm which is capable to be integrated into a realtime environment. In this paper, different algorithms for tendon force distribution are proposed and investigated with respect to their usability on a realtime system. In a future modified version of Segesta it is planned to add an eighth tendon. The platform can be basically guided using position control in the domain of tendon lengths. Following a trajectory, intermediate poses for every time step are calculated. For these points, the inverse kinematics delivers the corresponding tendon lengths. Since the actual tendon lengths are available from sensors, feedback control is used to guide the platform. This basic control concept provides satisfying results at low velocities. For higher accelerations and velocities it was observed that the platform begins to 'wobble' due to slack tendons. To prevent slackness and also to limit forces, tension has to be controlled within lower and upper bounds. The calculation of a force distribution is theoretically.
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