Throwing, hitting, jumping or kicking are often referred to as explosive movements since they require the sudden release of large amounts of energy to be successful. Instead of using large and powerful motors to achieve such movements, researchers are turning to compliant actuators with elastic components capable of passively storing and releasing energy. Varying the stiffness of the actuator can be interesting to go from highly compliant actuators that are safe for human-robot interactions to stiffer actuators that are optimized for the task at hand. Exploring how stiffness impacts task performance is highly complex and is usually done through trial and error.

Instead, Braun et al. propose a framework that optimizes the control of actuator stiffness and torque automatically. Demonstrations are performed using a robot arm in simulation and reality on a ball throwing task (see video below). Interestingly, controlling the torque and stiffness independently leads to better performance than systems where stiffness can not be independently controlled.

Currently, the authors are implementing the proposed framework on anthropomorphic variable stiffness devices with many degrees of freedom, such as the DLR Hand-Arm System. This work provides a blueprint for achieving optimal control in the next generation of robotic devices where variable stiffness actuation is likely to play a dominant role.