Nonlinear RISE-Based Control for a Gravity Balanced Exoskeleton: Evaluation of the Design and Control Principle

Abstract

In this paper we present a new control approach called RISE (Robust Integral of the Sign of Error) to control a three DOF exoskeleton designed at the Lebanese University for the assistance and rehabilitation of the human lower limb. The main purposes of this work are: (1) testing this novel control approach called RISE, (2) testing the gravity balancing design and show its advantages, in simplifying the control system and enhancing its performance. The extension and flexion of the knee joint in a seated position is considered as a case study in three different contexts: a passive rehabilitation, an assistance as needed and a resistive rehabilitation. The human torque acting on the robot is taken into consideration through a sequence of realistic torque generated values. We have shown that the RISE controller ensures a good convergence of the trajectory. By comparing a gravity balanced and unbalanced design, we have proven that the control torque is significantly reduced by the gravity balancing mechanism.