Redundant Haptic Interfaces for Enhanced Force Feedback Capability Despite Joint Torque Limits

In this paper, an actuator saturation compensation method (ASCM) is proposed to enhances the force feedback capability of a redundant haptic interface (RHI) by leveraging its kinematic redundancy. This method acts in the null-space of the Jacobian matrix of the RHI and distributes the overloaded actuator's torque among the available unsaturated actuators at the joints. This method empowers design engineers to utilize smaller actuators that have lower rotor inertia and friction in the design of new haptic interfaces. This is advantageous because having low apparent inertia and friction is a requisite for truthfully recreating the feeling of moving in free space. By employing ASCM, the required torque for rendering a stiff environment will be optimally distributed among smallcapacity actuators that otherwise become saturated. Moreover, manipulability enhancement of the RHI along the direction of the task is proposed as a tertiary objective - the primary objective is force reflection and the secondary objective is actuator saturation compensation. The tertiary objective acts if the primary and secondary objectives are feasible, and the haptic interface still has remaining redundancy. Simulation examples are provided throughout the paper to demonstrate the concept. Also, experimental results with a four degree-offreedom (Dof) planar haptic interface are reported that verify the practicality of the proposed method.