Integration, Sensing, and Control of a Modular Soft-Rigid Pneumatic Lower Limb Exoskeleton

This article presents the system integration, sensing, and control of a novel modular soft-rigid pneumatic exoskeleton for lower limb. The proposed exoskeleton consists of three soft hinges (to drive the hip, knee, and ankle joints) and four rigid links (aligned with the waist, thigh, crus, and foot). Each soft hinge is made of and actuated by a customized bidirectional curl pneumatic artificial muscle (CPAM), whereas the links are three-dimensional printed. Each of the rigid links combined with its lower soft hinge (if any) is made into an independent soft-rigid module, that is, the waist-hip, thigh-knee, crus-ankle, and foot modules. With each of the modules are multiple sensors integrated, including two pressure sensors for detecting the inflating pressures, and two flex sensors and an inertia measurement unit for estimating the bending angles of the soft hinges via data fusion. Through a data-fitted angle-torque-pressure relationship of the CPAM, the actuation torque is estimated. An external electropneumatic control system is also developed. The double closed-loop control system consisting of pressure servos and position/torque controllers is designed to control the bending angles and actuation torques of the exoskeleton hinges. Experiment shows good motion controllability of the proposed exoskeleton in the range of motion of a gait cycle.