Compliant Control and Compensation for A Compact Cable-Driven Robotic Manipulator

Cable-driven robotic manipulators are desirable for medical applications, for their form factor flexibility after separating actuation from the distal end. However, intended to work under high spatial constraints such as dental or other surgical applications, severe cable elongations will raise control challenges from inaccuracy to excessive compliance. It is critical to proactively regulate the system compliance, in order to achieve both compliant behavior to avoid tissue damage, and rigid behavior necessary for dental drilling. Both ends of this challenges have been extensively studied in literature, with rigidity achieved by cable elongation compensation, and virtual compliance regulation by impedance control. However, each approach worked within its own turf, with very little being studied in how to blend the two sources of compliance strategically. In this work, blending virtual compliance modulated by impedance control with transmission compliance induced by cable elasticity was investigated and demonstrated in a modified design of our proprietary dental manipulator. It was shown that direct application of impedance control in a cable-driven system would not bluntly increase compliance, and may cause instability. Instead, we proposed a compliance-blending framework with Cartesian-space super-positioning of cable motion compensation and impedance control, and validated the efficacy on the 6-DOF dental manipulator platform. Desirable results were achieved using highly common approaches in both impedance control and cable compensation, making the proposed approach applicable to a wide range of cable-driven robotic systems for impedance control.