A novel stiffness node controller which enables simultaneous regulation of torque and stiffness in multi-muscle driven joints

Biological systems are generally able to change joint stiffness by co-activation of antagonistic muscles. A close study of the stiffness generated in an antagonistically actuated, muscle-driven hinge joint has shown that in dependence of some muscle and joint parameters, positions in the angular joint space can be found for which different levels of co-activation do not generate any change in stiffness (stiffness nodes). Consequently, for joint positions in the vicinity of such a node, any stiffness controller is expected to fail. This paper proposes a novel stiffness node controller whose objective is to shift the stiffness node away when the joint approaches the angular position of the node. In order to test the ability of the node controller, a simulation of simultaneous torque and stiffness control is performed. In the simulation, two different cases will be examined. In the first case, the joint position is in the vicinity of a node and the node controller is switched off. In this situation the torque and stiffness controllers alone are not able to reach their desired values. In the second case the node controller is switched on which leads to a node displacement and - as a consequence - the controllers reach their requested values.