An Efficient Closed-Form Method for Optimal Hybrid Force-Velocity Control

This paper derives a closed-form method for computing hybrid force-velocity control. The key idea is to maximize the kinematic conditioning of the mechanical system, which includes a robot, free objects, a rigid environment and contact constraints. The method is complete, in that it always produces an optimal/near optimal solution when a solution exists. It is efficient, since it is in closed form, avoiding the iterative search of our previous work. We test the method on 78,000 randomly generated test cases. The method outperforms our previous search-based technique by being from 7 to 40 times faster, while consistently producing better solutions in the sense of robustness to kinematic singularity. We also test the method in several representative manipulation experiments.

I. INTRODUCTION

Contact constraints help human manipulation with improved precision and dexterity. For example, when cutting a piece of wood with a band saw, we often slide the wood along a guide rail to position it accurately. Hybrid Force-Velocity Control (HFVC) naturally suits such tasks. The velocity control (high-stiffness) can drive the system precisely, avoiding the need to finely balance the forces. The force control (low-stiffness) can avoid excessive internal force and maintain desired contacts. A properly designed HFVC could keep both advantages.