Optimal control for maximizing link velocity of robotic variable stiffness joints

In this paper we evaluate the potential of Variable Stiffness Actuation to utilize its inherent joint elasticity and capability to adjust the intrinsic joint stiffness. These abilities make it possible to realize fundamentally different motion control schemes in comparison to intrinsically stiff robots. In this paper we treat the problem of how to generate optimally fast link side velocity at a certain time instant by fully exploiting the elastic energy transfer effects between motor, joint elasticity, stiffness adjustment mechanism, and link. Based on optimal control theory we show that it is possible to significantly and optimally exceed the motor maximum velocity by appropriate motor commands. We solve the problem for models of increasing complexity in order to consecutively elaborate the core insights into the chosen problem. Finally, we present experimental results with a VIA joint prototype, confirming the correctness of the developed formalism.