Single Channel Hybrid FES Gait Assist System

Abstract

Loss of mobility due to lower limb paralysis is common consequence of thoracic level spinal cord injury (SCI). In the US there are approximately 253,000 persons with SCI. The wheelchair is the most common form of mobility for individuals with paraplegia but there remains a need for assistive technology that can enable paraplegics to walk and reach in the periphery of wheelchair. A new concept is presented that combines functional electrical stimulation (FES) with an energy storing orthosis (ESO) that contains a fluid power system to store and transfer energy during the gait cycle. Elastic energy storage elements on the orthosis hip and knee joints hold the leg in a flexed equilibrium position. Stimulation of the quadriceps extends the knee, placing excess energy in both the equilibrium spring and an energy transfer element. The stored energy is transferred to the hip where it is discharged and used to extend the hip against its equilibrium spring which also aids in forward progression. A new step is initiated by releasing the hip and knee joints from the straight leg position to the flexed position. The concept is realized using gas springs and pneumatic cylinders. Gas springs act as flexed energy storage elements. Lower air cylinder and the tubing acts as an accumulator and the upper cylinder acts as hip joint actuator. The system uses 2 way proportional solenoid actuated pneumatic valves for control during extension. The conceptual design of the ESO was completed and implemented in a dynamic simulation model (MSC ADAMS) and in a benchtop prototype for engineering measurements. Of the 14 joules of energy available from quadriceps, 8.9 joules of energy is utilized for doing work against springs and inertial forces; 5.4 joules is stored in pneumatic system; of which 1.4 joules is required for hip extensions and the remaining will be used for forward progression. No studies were conducted with human subjects. A hydraulic fluid power system was investigated for better control and braking possibilities but was not adopted because of difficulities in accumulator design and high fluid friction losses. A Matlab code was used to calculate the torques required at joints to support standing. Commerical braces are being used for improved user comfort. A wrap spring brake is being designed to maintain standing posture without FES or any active energy input. Technical feasibility of the ESO prototype will be evaluated using two subjects with paraplegia.