Abstract
Stationary motorized cycling assisted by functional electrical stimulation (FES) is a popular therapy for people with movement impairments. Maximizing volitional contributions from the rider of the cycle can lead to long-term benefits like increased muscular strength and cardiovascular endurance. This paper develops a combined motor and FES control system that tasks the rider with maintaining their cadence near a target point using their own volition, while assistance or resistance is applied gradually as their cadence approaches the lower or upper boundary, respectively, of a user-defined safe range. Safety-ensuring barrier functions are used to guarantee that the rider’s cadence is constrained to the safe range, while minimal assistance is provided within the range to maximize effort by the rider. FES stimulation is applied before electric motor assistance to further increase power output from the rider. To account for uncertain dynamics, barrier function methods are combined with robust control tools from Lyapunov theory to develop controllers that guarantee safety in the worst-case. Because of the intermittent nature of FES stimulation, the closed-loop system is modeled as a hybrid system to certify that the set of states for which the cadence is in the safe range is asymptotically stable. The performance of the developed control method is demonstrated experimentally on five participants. The barrier function controller constrained the riders’ cadence in a range of 50 ± 5 RPM with an average cadence standard deviation of 1.4 RPM for a protocol where cadence with minimal variance was prioritized and used minimal assistance from the motor (4.1% of trial duration) in a separate protocol where power output from the rider was prioritized.
Highlights
IntroductionStationary cycling assisted by functional electrical stimulation (FES) can lead to long-term benefits for people with movement impairments due to neurological conditions such as stroke, spinal cord injury, traumatic brain injury, cerebral palsy, multiple sclerosis, and others Johnston et al (2008); Ferrante et al (2008); Hooker et al (1992); Janssen et al (2008); Trevisi et al (2012)
In contrast to the barrier function controller, the electric motor controller for the 3M controller is coupled with functional electrical stimulation (FES) stimulation via the angular position state z1, so that the motor is inactive whenever FES is active, and vice versa
Protocol B was designed to show how assistance from the motor can be reduced by selecting a wider safe range, thereby encouraging more volitional contributions from the rider
Summary
Stationary cycling assisted by functional electrical stimulation (FES) can lead to long-term benefits for people with movement impairments due to neurological conditions such as stroke, spinal cord injury, traumatic brain injury, cerebral palsy, multiple sclerosis, and others Johnston et al (2008); Ferrante et al (2008); Hooker et al (1992); Janssen et al (2008); Trevisi et al (2012). Electric motor support should be minimized in lieu of torque produced by the rider’s muscles via either FES or their own volition, which leads to higher intensity training by increasing the rider’s heart rate and oxygen uptake Hooker et al (1992). The objective of this work is to design controllers for both the electric motor and FES stimulation that facilitate volitional cycling by minimizing machine assistance while ensuring that the rider’s cadence is constrained to a userdefined range
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