Embodying Desired Behavior in Variable Stiffness Actuators

Variable stiffness actuators are a class of actuators with the capability of changing their apparent output stiffness independently from the actuator output position. This is achieved by introducing internally a number of compliant elements, and internal actuated degrees of freedom that determine how these compliant elements are perceived at the actuator output. The introduction of a mechanical compliance introduces intrinsic, passive oscillatory behavior to the system, but rather than trying to minimize this effect, the question arises if it can be exploited for the actuation of periodic motions. In this work, we propose a strategy to control the variable stiffness actuator optimally, with respect to a cost criterion, to a desired periodic motion of the output. In particular, the cost criterion provides a measure of embodiment of the desired behavior in the passive behavior of the variable stiffness actuator, i.e., the variable stiffness actuator is controlled such that its passive behavior is as close as possible to the desired behavior and thus that the control effort is minimized.