Dynamic frictional constraints for robot assisted surgery

Collaborative, as opposed to autonomous, control strategies are used within the majority of commercially available, surgical robotic systems. Amongst these, active constraints and virtual fixtures, where assistance is in the form of regulation applied to the motion of surgical tools, offer an effective means to maximise both user and robot capabilities. Conventional active constraint approaches, however, are likely to result in active forcing of the tools when used within a dynamically changing surgical environment. It is posited that such behaviour inherently reduces a surgeon´s control over the procedure, and therefore compromises patient safety and clinical acceptance. Utilising a friction model to enforce constraints ensures that energy is never introduced into the system; however frictional constraints suffer from problems once penetration of a constrained region has occurred. A frictional constraint formulation is proposed which eliminates this by redirecting a user´s motion, guiding him towards the surface. Experimental validation shows that the proposed constraint significantly improves a user´s path-following performance over unassisted cases, while approaching the performance benchmark of a viscoelastic active constraint.