Open-loop stiffness control of overconstrained mechanisms/robotic linkage systems

A novel approach for the control of task-space stiffness characteristics in systems consisting of a superabundance of kinematically dependent inputs is proposed. When there are more input actuations than operational degrees of freedom, internal preloads can be generated that produce effective restoring forces in the face of displacement or disturbances imposed on the system. Examples of this excessive actuation can be found in certain modes of structurally overconstrained parallel manipulators and in antagonistically structured serial manipulators. The input loads are synthesized offline (prior to operation) and entered as a feedforward component so that the desired effective loads on objects are obtained, and (simultaneously) significant disturbances at the task level can be largely rejected in an open-loop fashion. This reduces the burden and shortcomings of standard feedback schemes. Moreover, a layered feedback scheme is used to compensate for small perturbations and unmodeled dynamics. The open-loop task-based stiffness control scheme as applied to structurally parallel mechanism/robotic linkage systems is investigated. The scheme´s applicability as a programmable active compliance device is also discussed