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

Frontiers in Robotics and AI, 2022 · DOI: 10.3389/frobt.2021.702860 · Published: January 20, 2022

Simple Explanation

This research focuses on improving walking ability for people with movement impairments using a combination of robotic exoskeletons and functional electrical stimulation (FES). This combination, called a hybrid exoskeleton, uses both robotic assistance and muscle activation to help with movement. The study integrates a cable-driven lower-limb exoskeleton with FES to enable treadmill walking at a constant speed. The electrical stimulation activates specific muscle groups around the knee joint, while electric motors adjust the knee joint's stiffness. For the hip joint, electric motors are used to control movement. The system's performance was tested on able-bodied individuals walking on a treadmill. The results showed that this hybrid approach, using both robotic assistance and muscle stimulation, is feasible for treadmill walking.

Study Duration

Not specified

Participants

Three able-bodied individuals

Evidence Level

Level 5, Experimental study

Key Findings

1
The controllers developed in this study achieved repeatable and consistent kinematic joint trajectories as a function of the gait cycle.
2
Consistent joint kinematic patterns were obtained across all participants for both walking speeds.
3
The integration of FES and cable-driven exoskeletons holds the potential to customize the human interaction to restore or improve function in individuals with movement disorders by achieving repetitive and coordinated walking.

Research Summary

This paper presents a hybrid lower-limb exoskeleton that combines functional electrical stimulation (FES) and electric motors to assist with treadmill walking. The system uses a nonlinear robust controller for FES of the quadriceps and hamstrings, an integral torque feedback controller for knee joint stiffness, and a robust sliding-mode controller for hip joint kinematic tracking. Experiments on able-bodied individuals demonstrated the feasibility of the approach, showing repeatable kinematic joint trajectories and consistent gait patterns at different walking speeds.

Practical Implications

Rehabilitation Potential

The hybrid exoskeleton has the potential to improve walking ability and build muscle capacity in individuals with movement deficits.

Customizable Assistance

Coupling kinematic and stiffness controllers for FES and electric motors can influence each individual’s gait kinematics and foot trajectories across different walking speeds.

Community Ambulation

Advances in control methods and wearable devices are needed to increase the participant’s gait speed and endurance toward achieving community ambulation after injury.

Study Limitations

1
Muscle activation input delay negatively influences walking performance.
2
The stability analysis provides conservative, sufficient control gain conditions.
3
Measurement of torque about the knee joint using load cells can be prone to noise.

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