Functional Electrical Stimulation (FES) is the application of electrical impulses on muscle fiber to induce an involuntary muscle response or contraction for rehabilitation. FES combined with an electric motor can facilitate muscle recovery and rehabilitation through the implementation of a closed-loop control system. The purpose of this research is to see the effects of decoupled leg control on muscle recovery for people with neurological conditions. To do so, a stationary cycle with a decoupled crank is used for people to pedal on, and each leg is stimulated separately. This research was performed at the Nonlinear Controls and Robotics Laboratory (NCR) at the University of Florida.
The purpose of this research is to see the effects of decoupled leg control on muscle recovery for people with neurological conditions
Note: the full research can be found here
My research at the NCR Lab focused on the development of a nonlinear sliding mode controller for a cycle with decoupled crank shaft (i.e. there is no mechanical engagement between the two pedals). The control objective was to track position and angular velocity to simulate standard pedaling conditions with a constant phase difference of 180 degrees.
The model was divided into Dominant and Non-dominantsubsystems that were controlled independently of each other.
The mathematical model for the switched cycle-rider dynamics was developed using the torques produced by each component about the crank axis. All states depend exclusively on the crank angles, qdom and qnon, for the dominant and the non-dominant subsystems, respectively. This model takes into consideration:
- Torques about the crank axis by the electric motor
- Torques about the crank axis by the rider, separated into:
- Passive torques, including inertial, centripetal, gravitational and viscoelastic tissue forces
- FES induced muscle contributions, including the total contribution for each muscle group (right and left hamstrings, glutes and quadriceps)
- Disturbances in the load
3. Torques about the crank axis by the cycle.
A Lyapunov-like function is used to prove stability and tracking to the desired cadence and position for the combined FES-Motor system. Two controllers, electric motor signal and muscle activation signals are developed for each leg given that the rider system is divided into right and left.
Electric motors are used to assist the rider in the kinematic dead zones, such as with the knee fully extended and knee fully contracted, where none of the muscle groups stimulated using FES has significant impact to create motion. Besides these dead zones, the electric motors are also active during the first ten seconds of the process in order to crank up the cycle speed to the desired cadence for muscle stimulation to be the most effective.
Each muscle group is only stimulated on its peak performance range, and is not stimulated where it would provide countering forces to the desired motion. This is done through the implementation of switching signals that depend on the crank angle, determining when each muscle group will be active. The final application can be seen below:
Experimental results on two able-bodied individuals show the feasibility and stability of the closed-loop controller. The results can be seen below. Please refer to the linked research paper for more details about the development, implementation and results of this project.
 F. Esquivel Estay, C. Rouse, M. Cohen, C. Cousin, and W. E. Dixon, “Cadence and Position Tracking for Decoupled Legs during Switched Split-Crank Motorized FES-Cycling,” American Control Conference, Philadelphia, PA, July 2019.