Limit cycles and stiffness control with variable stiffness actuators

Variable stiffness actuators realize highly dynamic systems, whose inherent mechanical compliance can be properly exploited to obtain a robust and energy-efficient behavior. The paper presents a control strategy for variable stiffness actuators with the primarily goal of tracking a limit cycle trajectory and a desired stiffness, which is a function of the output position of the actuator. The proposed controller guarantees that the load, attached at the output of the actuation system, asymptotically tracks the desired cyclic motion and the desired stiffness even in presence of external disturbances. Experimental tests on the rotational variable stiffness actuator vsaUT-II validate the control strategy.