Analytical study on improving intrinsic stiffness modulation of biarticularly actuated manipulators through infinity norm resolution

Modulation of intrinsic stiffness is a key factor to achieve high performances while guaranteeing safety in human-robot interaction. Variable Stiffness Actuators (VSA) allow for simultaneous position and stiffness control of a joint through the use of two actuators, and are therefore used in the design of intrinsically compliant manipulators. It is known that biarticular actuators, actuators spanning two consecutive joints, employed in a redundant configuration improve stiffness modulation in manipulators. In respect to manipulators designed with monoarticular VSAs, the ones employing added biarticular VSAs are characterized by a higher stiffness regulation capability. However, this redundancy resulting from the presence of biarticular actuator leads to an optimization problem. In our previous work, we proposed a closed form infinity norm solution to address this problem of actuator redundancy resolution in biarticularly actuated manipulators. This infinity norm approach, in comparison with the traditionally used Moore-Penrose pseudoinverse, achieves a higher end effector maximum force. In this work, the advantage of the infinity norm resolution in end effector intrinsic stiffness modulation of manipulators employing redundant biarticular actuators is analyzed.