Design and evaluation of a torque-controllable knee joint actuator with adjustable series compliance and parallel elasticity

Compliant actuators are increasingly being designed for wearable robots (WRs) to more adequately address their issues with safety, wearability, and overall system efficiency. The advantages of mechanical compliance are utilized in a new actuator designed to exploit inherent gait dynamics. Unlike any other orthotic power unit, it combines Variable Stiffness Actuator (VSA) and Parallel Elasticity Actuation (PEA) unit into a single modular system. This way, the actuator has the potential to provide the benefits of VSA when net-positive work is necessary and efficiently store energy during energetically conservative tasks. A novel real-time torque controller allows the two units to work together throughout the gait cycle. The design aspects and experimental evaluation of the actuator and its low-level torque controller are presented in this paper. The actuator characterization, carried out in two benchmarking environments, highlights the actuator's high torque density and favorable energetic performance, providing evidence for its applicability in a standalone or multiple-joint lower limb orthoses.