project specification

Clutch spring knee exoskeleton

An exoskeleton that reduces joint loading, muscle fatigue, or metabolic demand of walking. This is an exoskeletal architecture for augmenting running leveraging a spring–clutch exoskeleton consisting of a custom interference clutch with an integrated planetary gearbox. Specifications for the exoskeletal joint were derived from existing biomechanical data. The design also includes a control system to enable clutch locking and unlocking throughout a running gait cycle.

Specifications

Mass 710 g
Diameter 85 mm
Thickness 49 mm
Holding torque 190 (T) (Nm)
Radial load 4780 (Fr) (N)
Axial load 4050 (Fa) (N)
Engagement time 28.6 (Dteng) (ms)
Resolution 1.8 (deg)
Range of motion 30 (deg)
Battery 2000 mAh single cell lithium polymer battery
Processor 1 AtMega168PA AVR microcontroller
Sensors 1 AtMega168PA AVR microcontroller
1 MEMS* accelerometer
1 Reflective encoder
Data storage MicroSD

Overview

This is an exoskeletal architecture for augmenting running leveraging a spring–clutch exoskeleton consisting of a custom interference clutch with an integrated planetary gearbox.

Specifications for the exoskeletal joint were derived from existing biomechanical data. The design also includes a control system to enable clutch locking and unlocking throughout a running gait cycle. 

The elastic element of the exoskeleton consists of thin strips of semirigid unidirectional “E” fiberglass laminate located proximal and distal to the lockable exoskeletal joint. The net system constitutes a compression spring, characterized by the geometry of these elastic elements. 

The exoskeletal joint consists of an interference clutch (chosen for its high ratio of holding torque to mass) in series with a planetary gearbox which decreases the load on the clutch plates and increases effective resolution, compensating for the discretized engagement of the interference clutch. 

Electronics are housed in a lateral subassembly with only a single pair of wires necessary to connect to the solenoid. A quadrature phase encoder for measuring the exoskeletal joint angle and a break beam for measuring solenoid position are both interfaced optically. The sensor suite is completed by a three-degrees of freedom sagittal plane inertial measurement unit. 

References

Introduces the project and its motivation. Describes the elastic element design, clutched exoskeletal joint, and electrical systems. Discusses the results of the research and mechanical validation.

G. Elliott, A. Marecki, H. Herr. - Journal of Medical Devices, September 2014.

Paper investigates the hypothesis that the addition of an external elastic element at the knee during the stance phase of running results in a reduction in knee extensor activation so that total joint quasistiffness is maintained.

G. Elliott, A. Marecki, H. Herr, et al. - IEEE International Conference on Rehabilitation Robotics, June 2013.

Describes the design of two elastic leg exoskeletons that act in parallel with the wearer’s legs. Tests hypotheses by measuring GRFs, exoskeletal compressions, and metabolic rates.

A. Grabowski, H. Herr. - Journal of Applied Physiology, May 2009.

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