Synchronous Position and Compliance Regulation on a Bi-Joint Gait Exoskeleton Driven by Pneumatic Muscles

A previously developed pneumatic muscles' (PMs) actuated gait exoskeleton (with only knee joint) has been demonstrated in achieving appropriate actuation torque, range of motion (ROM), and control bandwidth for task-specific gait training. While the adopted multi-input-multi-output (MIMO) sliding mode (SM) strategy has preliminarily implemented simultaneous control of the exoskeleton's angular trajectory and compliance, its efficacy with human users during gait cycles has not been investigated. This article presents an improved bi-joint gait rehabilitation exoskeleton (BiGREX) with integrated human hip and knee joints. The results with 12 healthy subjects demonstrated that the system's compliance can be effectively adjusted while guiding the subjects walking in predefined trajectories. Note to Practitioners -This article was motivated by achieving compliant interaction between PM-actuated exoskeletons and human when conducting task-specific gait training. Due to the intrinsic nonlinearity of PM, it is challenging to establish a mathematical model to precisely predict real-time compliance of the powered joints. This article suggests a new strategy that adopts the average pressure of flexor and extensor PMs as the feedback to synchronously realize the joint position control and compliance regulation. A novel experimental approach was adopted to validate the system capability on adjusting the compliance from human users' perception. This article provides a new insight between the controlled PM pressure and the desired joint compliance, which would be essential for the future design of PM-actuated exoskeletons.