Feasibility of a Hydraulic Power Assist System for Use in Hybrid Neuroprostheses

Abstract

Feasibility of using pressurized hydraulic fluid as a source of on-demand assistive power for hybrid neuroprosthesis combining exoskeleton with functional neuromuscular stimulation was explored. Hydraulic systems were selected as an alternative to electric motors for their high torque/mass ratio and ability to be located proximally on the exoskeleton and distribute power distally to assist in moving the joints. The power assist system (PAS) was designed and constructed using off-the-shelf components to test the feasibility of using high pressure fluid from an accumulator to provide assistive torque to an exoskeletal hip joint. The PAS was able to provide 21 Nm of assistive torque at an input pressure of 3171 kPa with a response time of 93 ms resulting in 32° of hip flexion in an able-bodied test. The torque output was independent of initial position of the joint and was linearly related to pressure. Thus, accumulator pressure can be specified to provide assistive torque as needed in exoskeletal devices for walking or stair climbing beyond those possible either volitionally or with electrical stimulation alone.

Figure 1

Diagram of the PAS system. Note that P1 and P2 can connect to any hydraulic circuit. In this study, the PAS is connected to a single hydraulic actuator via valve P1.

Figure 2

PAS bench testing setup. The PAS hydraulic circuitry is connected to the rotary actuator consisting of right hip cylinder (RHC) connected between the exoskeleton upright and rack and pinion transmission to transmit torque. The upright is attached to a Biodex dynamometer to measure flexion torque. Pressure, torque, angle, and angular velocity data are read into an xPC Simulink real time environment.

Figure 3

Simulated PAS operation. Hip angle of 25° was achieved before activation and assistive power of PAS was simulated to drive hip to 60° of flexion.

Figure 4

Output isometric torque as a function of supplied pressure at 0 and 60° of hip flexion. R ² of 0.99 is representative of both curves.

Figure 5

Cylinder pressure differential and torque change (a) during isokinetic (120°/s) movement of the hip (b). Power assist was activated at time = 2 seconds.

Figure 6

Able-bodied PAS test. Subject simulated FNS by flexing hip to approximately 45° (a). PAS was then activated to continue flexing the hip to approximately 77° with minimal user assistance or resistance (b).