An Adaptive Sliding Mode Variable Admittance Control Method for Lower Limb Rehabilitation Exoskeleton Robot

Abstract

As passive rehabilitation training with fixed trajectory ignores the active participation of patients, in order to increase the active participation of patients and improve the effect of rehabilitation training, this paper proposes an innovative adaptive sliding mode variable admittance (ASMVA) controller for the Lower Limb Rehabilitation Exoskeleton Robot. The ASMVA controller consists of an outer loop with variable admittance controller and an inner loop with an adaptive sliding mode controller. It estimates the wearer's active muscle strength and movement intention by judging the deviation between the actual and standard interaction force of the wearer's leg and the exoskeleton, thereby adaptively changing admittance controller parameters to alter training intensity. Three healthy volunteers engaged in further experimental studies, including trajectory tracking experiments with no admittance, fixed admittance, and variable admittance adjustment. The experimental results show that the proposed ASMVA control scheme has high control accuracy. Besides, the ASMVA can not only increase training intensity according to the active muscle strength of the patient during positive movement intention (so as to increase active participation of the patient), but also increase the amount of trajectory adjustment during negative movement intention to ensure the safety of the patient.

Review on Control Strategies for Lower Limb Rehabilitation Exoskeletons

“…muscle voluntary force as controller input), wearers do not have to push on the exoskeleton after counteracting their own limb's inertia and gravity. Instead, the dynamic response of the device is directly according to the muscle force [50], [51] or even muscle force intention only [52], [53]. It can also be deemed as reshaping the admittance of the human-machine system [54].…”

“…The kinematic approach can be in the form of plain trajectory tracking [168], admittance control [54], some of the biological-signal-based control [121], and so on. The controller for executing the kinematic command can be simple PID [41], or with some sophisticated method such as model-based control, which directly regulate torque output by input-output linearization controller in underactuated model [12], or by feedback linearization controller in fully-actuated model [50].…”

“…Involvement of dynamics model or not is also a key factor in motion control algorithm design. There are three types of frequently used dynamics model for LLE: 1) simple one DoF mass-spring-damper model [169], 2) fully-actuated dynamics model [50], [51], and 3) floating-base hybrid dynamics model [12], [74]. The first one can be used to characterize the SEA or describe the machine side of the exoskeleton.…”

A Study on the Response of the Rehabilitation Lower Device using Sliding Mode Controller

“…Thanks to the development of computers over the years, simulation has become an indispensable tool for synthesizing control laws in mechanical systems, allowing the verification of the behavior of control laws on a virtual model of an automatic system called simulation model. Currently, the PID controller is widely applied [7], especially in robot control, to support rehabilitation for stroke patients with the advantages of simple and easy to control structure [8]. The disadvantage is that it is difficult to find the K p , K i , K d coefficients when the load changes.…”

Modeling and Finite-Time Sliding Model Control of a Lower Limb Exoskeleton for Use in Rehabilitation

“…Various applications include on-line control of robot motions and forces, trajectory design and optimization, design of robot mechanisms [12]. Various controllers are designed for the control of lower limb exoskeleton system like Sliding Mode Controller (SMC) [13], adaptive control [15], Lyapunov based control [14], etc. It is knows that SMC is better in uncertainty rejection, remarkable properties of accuracy, robustness, and easy tuning and implementation.…”