Closed-Loop Torque and Kinematic Control of a Hybrid Lower-Limb Exoskeleton for Treadmill Walking

Abstract

Restoring and improving the ability to walk is a top priority for individuals with movement impairments due to neurological injuries. Powered exoskeletons coupled with functional electrical stimulation (FES), called hybrid exoskeletons, exploit the benefits of activating muscles and robotic assistance for locomotion. In this paper, a cable-driven lower-limb exoskeleton is integrated with FES for treadmill walking at a constant speed. A nonlinear robust controller is used to activate the quadriceps and hamstrings muscle groups via FES to achieve kinematic tracking about the knee joint. Moreover, electric motors adjust the knee joint stiffness throughout the gait cycle using an integral torque feedback controller. For the hip joint, a robust sliding-mode controller is developed to achieve kinematic tracking using electric motors. The human-exoskeleton dynamic model is derived using Lagrangian dynamics and incorporates phase-dependent switching to capture the effects of transitioning from the stance to the swing phase, and vice versa. Moreover, low-level control input switching is used to activate individual muscles and motors to achieve flexion and extension about the hip and knee joints. A Lyapunov-based stability analysis is developed to ensure exponential tracking of the kinematic and torque closed-loop error systems, while guaranteeing that the control input signals remain bounded. The developed controllers were tested in real-time walking experiments on a treadmill in three able-bodied individuals at two gait speeds. The experimental results demonstrate the feasibility of coupling a cable-driven exoskeleton with FES for treadmill walking using a switching-based control strategy and exploiting both kinematic and force feedback.

Toward a wearable monitor of local muscle fatigue during electrical muscle stimulation using tissue Doppler imaging

“…By integrating muscle fatigue as feedback, EMS protocols can be better refined to optimize the desired stimulation outcomes. This might be particularly useful for stimulating muscles effectively over a prolonged period, such as when using a powered exoskeleton coupled with functional EMS (Chang et al, 2022 ). By gauging the level of fatigue recovery between bouts of EMS, stimulation could be limited to periods when the muscle has recovered enough to generate sufficient torque.…”

Soft pneumatic muscles for post-stroke lower limb ankle rehabilitation: leveraging the potential of soft robotics to optimize functional outcomes

“…These innovative approaches have demonstrated promising results, particularly in enhancing attention levels and user engagement, especially when heavier grasping weights are involved. Including haptic stimulation has proven to be instrumental in improving rehabilitation outcomes and promoting active user participation ( Li et al, 2021 ; Chang et al, 2022 ). The collective progress in exoskeleton-assisted rehabilitation systems is poised to significantly impact hand rehabilitation outcomes, fostering greater patient engagement and participation.…”

Motorized FES-cycling and closed-loop nonlinear control for power tracking using a finite-time stable torque algorithm

“…Further, functional electrical stimulation (FES) is a therapeutic approach that evokes artificial muscle contractions by applying electrical stimuli across skeletal muscles, which can provide improvements in muscle strength, blood flow, bone mineral density, and range of motion Doucet et al (2012); Reed (1997); Peckham and Knutson (2005). FES has been combined with electrical motors to develop hybrid rehabilitation machines that can mitigate muscle fatigue and prolong the benefits of muscle stimulation by exploiting the electric motor's torque reliability for locomotion Ho et al (2014); Chang et al (2017); Nataraj et al (2017); Alibeji et al (2017); Chang et al (2022), upper-limb rehabilitation McCabe et al (2015); Rouse et al (2018), and FES-cycling Bellman et al (2017); Duenas et al (2019); Ghanbari et al (2019); Chang and Duenas (2019); Chang et al (2020). Motorized FES-cycling has been recommended for lower-limb rehabilitation in individuals with limited function who otherwise would not be able to engage in physical activity, since stationary cycling reduces the risks of falling (e.g., compared to assisted walking).…”

“…Adaptive-based control methods involving switched dynamics have been recently introduced to compensate for model uncertainties and improve tracking performance using iterative learning Ghanbari et al (2019), repetitive learning Duenas et al (2019), and concurrent learning Casas et al (2020) approaches. Power tracking controllers were designed to track predetermined desired torque trajectories using impedance and admittance techniques Chang et al (2022); Cousin et al (2019), Cousin et al (2020) and a model-based feedback approach Hunt et al (2004). However, existing power tracking controllers usually implement predetermined desired torque trajectories and thus are prone to experience degraded performance due to the timevarying muscle dynamics and fatigue.…”

“…However, validation of the approach in Sheng et al (2020) for cycling is still needed to ensure robust measurements. Impedance and admittance controllers for FES-cycling have implemented an indirect torque approach in which changes in the rider's torque influence the cadence trajectory or vice versa (i.e., changes in cadence impact the torque trajectory) Chang et al (2022); Cousin et al (2019); Cousin et al (2020). However, it remains unclear how to exploit estimates of the active muscle torque to adjust the desired torque trajectory in real-time independently and without affecting the cycle's cadence.…”