I. Introduction
The mass deployment of robotic solutions in manufacturing causes huge energy demand. For example, of the total electrical energy usage in production processes of automotive industries is consumed by industrial robots [1]. For ecological and economic reasons, this motivates research on reducing the energy cost of industrial robots. Furthermore, with the extensive deployment of compliant robots expected in the near-future for human–robot collaboration, medical, and civil services, etc., this imperative to save energy is likely to become even more critical. Variable impedance actuators (VIAs) are believed to be the key for the next generation of robots to interact safely with uncertain environments and provide better performance in cyclic tasks and dynamical movements [2]. For example, the physical compliance incorporated in variable stiffness actuators (VSAs) (e.g., using elastic components such as springs) enables energy storage, which can be used to 1) absorb external energy introduced into the system (e.g., from collisions) to enhance safety, and 2) amplify output power by releasing stored energy as and when required by the task [3].