Design of Hybrid Phase Sliding Mode Control Scheme for Lower Extremity Exoskeleton

Lingling Chen,Xiaowei Song,Chao Wang,Jie Wang

doi:10.3390/app9183754

Abstract

Aiming at a pneumatic artificial muscle (PAM) lower extremity exoskeleton, a control mechanism based on hybrid phase sliding mode control (SMC) is proposed. First of all, the human gait cycle is mainly divided into the swing phase and stance phase, and the lower extremity exoskeleton phase models are established by the Euler–Lagrange method, respectively. Secondly, the lower limb exoskeleton is inevitably affected in the diverse working environment, and the exoskeleton model has nonlinear and strong coupling characteristics, which both increase the control difficulty. In this situations, a robust sliding mode control method is designed based on an Extended State Observer (ESO). Thirdly, the pneumatic muscle takes time to contract and relax, and then the joint input torque cannot jump when the gait phase changes, hence, the smoothing switching of the assistive control scheme is introduced to solve it. The smoothing switching time is detected by a phase detector, and the phase detector is designed by the plantar pressure information. Finally the comparative simulation shows that this control strategy has the advantages of fast time, high control precision and no jump during control torque switching. Pneumatic artificial muscle contraction rate curve shows that the pneumatic muscles’ motion range meets the control requirement of the exoskeleton.

Highlights

In BLEEX, a hybrid control strategy [14] is put forward to control the lower limb exoskeleton in the swing phase or stance phase respectively, where the sensitivity amplification controller (SAC) is employed for the swing leg, the position control strategy is applied for the stance leg
For a Load-Carrying lower extremity exoskeleton, the position control approach is applied in the stance phase, and the following control method is used in the swing phase [16]
The oscillating leg rotates around the hip joint P0 ( x0, y0 ); During the stance phase, the thigh and upper torso are approximated as the fixed axis motion rotating around the foot, the axis is between the foot and the ground P0 ( x0, y0 ), and P16 represents the center of mass of the torso

Summary

Introduction

The lower extremity exoskeleton is a wearable device with an active actuator that provides active torque to the operator and helps the operator to accomplish subjective willing movements.Exoskeletons can be used in many fields, such as military [1,2], construction [3,4], service [5,6], and medical [7,8,9] , which have attracted scholars’ extensive attention from all over the world [10].In recent decades, different research institutions have adopted different control mechanisms for the exoskeleton [11,12], the Hybrid Control Strategy [13] among them is applied in exoskeleton systems for Human-Performance and obtains improved performance successfully. In BLEEX, a hybrid control strategy [14] is put forward to control the lower limb exoskeleton in the swing phase or stance phase respectively, where the sensitivity amplification controller (SAC) is employed for the swing leg, the position control strategy is applied for the stance leg. Another Hybrid Control Strategy is presented to control a hydraulically actuated lower limb exoskeleton, where the H∞ control method is designed to improve the performance under all kinds of uncertainties. In [17], for different dynamic models of a gait

Methods

Results

Conclusion