Lockable Lower-Limb Exoskeleton Based on a Novel Variable-Stiffness Joint: Reducing Physical Fatigue at Squatting

Abstract

Some special tasks require human operations and cannot be performed by robots or other autonomous equipment, such as special industrial assembly and surgical procedures in small and crowded spaces. Workers/surgeons in these cases tend to have physical fatigue. In this study, a novel variable-stiffness joint based on positive pressure was proposed, and a torque model was established. The locking torque variation, step response, and energy consumption were evaluated in comparison with a torque motor. A lockable lower-limb exoskeleton based on the variable-stiffness joint was developed, and wearable tests were conducted to evaluate a voice recognition interface and supporting performance. The locking torque of the variable-stiffness joint could be continuously varied from 0 Nm to 26 Nm with the air pressure ranging from 1.6 bar to 5.5 bar. The settling time was 0.328 s in the step response experiment. With a load of 6 Nm, the variable-stiffness joint can realize an energy consumption reduction of 75.01% compared with using a torque motor. Moreover, the lockable lower-limb exoskeleton can realize a 35–60% reduction in the average muscle activation in each subject (aged 22–57) to maintain squatting postures at three different knee angles (paired t-test, P < 0.01). The proposed exoskeleton system has good mobility, low energy consumption, and easy-to-control features, showing great potential in supporting the weight of workers/surgeons during long-term operations.