A Muscle-First, Electromechanical Hybrid Gait Restoration System in People With Spinal Cord Injury

Abstract

The development of a hybrid system for people with spinal cord injuries is described. The system includes implanted neural stimulation to activate the user's otherwise paralyzed muscles, an exoskeleton with electromechanical actuators at the hips and knees, and a sensory and control system that integrates both components. We are using a muscle-first approach: The person's muscles are the primary motivator for his/her joints and the motors provide power assistance. This design philosophy led to the development of high efficiency, low friction joint actuators, and feed-forward, burst-torque control. The system was tested with two participants with spinal cord injury (SCI) and unique implanted stimulation systems. Torque burst addition was found to increase gait speed. The system was found to satisfy the main design requirements as laid out at the outset.

Transmission Comparison for Cooperative Robotic Applications

“…The use of multiple planetary stages represented a unique setup amongst similar work. In comparison to our previous work [17], the motor used in this work had half the terminal resistance (0.226 Ohms vs. 0.573 Ohms)-thereby decreasing the resistive losses in the motor. At 15 A (the maximum current for both motors), this change in resistance decreases the motor resistive losses (and thereby decreasing heat production) from 128 W to 51 W. In context, these actuators are capable of 200-250 W maximum output mechanical power, while consuming 300-320 W of electrical power under similar load.…”

“…As a point of comparison, an exoskeleton with bilaterial hip and knee actuators capable of 40 Nm peak torque was able to restore gait to 0.22 m/s to a subject with a T10 complete SCI [32]. Other projects have reported peak torques in a similar range-36 Nm [17], 30 Nm [33], and 60 Nm [23]. For this work, a torque requirement was not defined-the objective was to achieve the necessary joint velocities (which defined the necessary gearing) as described above.…”

“…Each actuator was the same size (231.8 mm long, 108 mm wide, and 76.7 mm deep) and mass (2.1 kg), and was small/light enough to be considered for biomedical exoskeleton applications. 50:1 overall gearing was targeted for each design-at 28.8 V (system voltage used in prior work [17]), the motor would spin at 3860 RPM without any load, and 463 deg/sec. output after the 50:1 reduction.…”

“…A similar idea has been explored in hybrid systems for people with complete SCI where the biological torque component is supplied by Neuromuscluar Electrical Stimulation (NMES). Previous work in this area of hybrid human/orthotic gait restoration came in a variety of architectures, with the bracing ranging from passive exoskeletons [13], to semi-active orthoses with variable joint constraints [14][15][16], to fully active exoskeletons [17][18][19][20].…”

“…The style of transmissions used varies widely. Examples include a single planetary gearbox plus one belt reduction stage [23], a single stepped planetary gearbox [24], a one stage belt/chain reduction [25], and a harmonic drive [17,19].…”