Optimal Control of a Variable Stiffness Mechanism for the Hammering Task

Variable stiffness actuators can be used in a variety of robotics applications for dynamic manipulation tasks such as hammering a nail or flipping a burger. Here, a semi-active variable stiffness mechanism is used for enhancing the dynamic manipulation of a stiff robot. The mechanism is attached to the end-effector of a robot and its performance is investigated over a hammering task. The hammering task is posed as an optimal control problem to maximize the velocity of the hammer-head while hitting the nail. The problem is solved to obtain an optimal end-effector trajectory of the robot and the desired mechanism stiffness as a function of time. The performance of the mechanism is then tested for three different stiffness settings on the obtained trajectory. Through experiments, we show that the mechanism can achieve higher velocities in the variable stiffness setting when compared to the stiff actuation.