Design of an FPGA-Based Fuzzy Feedback Controller for Closed-Loop FES in Knee Joint Model

Area of Science:

• Biomedical Engineering
• Rehabilitation Technology
• Control Systems

Background:

• Functional electrical stimulation (FES) aids spinal cord injury (SCI) patients in restoring motor function.
• Open-loop FES strategies lead to early muscle fatigue.
• Closed-loop FES systems offer a solution to mitigate fatigue by adapting stimulation.

Purpose of the Study:

• To develop and implement a digital fuzzy feedback controller (FFC) for a closed-loop FES system.
• To reduce early muscle fatigue in FES-assisted rehabilitation.
• To accurately control limb movement, specifically knee extension, during FES therapy.

Main Methods:

• A fuzzy logic controller (FLC) was modeled in MATLAB Simulink.
• The FLC was translated into digital logic using Verilog HDL for FPGA implementation.
• A digital FFC was embedded onto an Intel FPGA (DE2-115) board for real-time verification.
• The system controlled knee extension by regulating stimulus pulse width based on a reference angle.

Main Results:

• The digital FFC achieved precise control of knee extension, reaching target angles with minimal overshoot (1.4°) and steady-state error (0.4°).
• The FFC utilized only 4% of FPGA logic elements and processed data rapidly (238 µs per update).
• The controller's high processing speed allowed for timely updates to stimulation parameters each cycle.

Conclusions:

• The implemented digital FFC is effective for closed-loop FES applications.
• This approach significantly reduces muscle fatigue and enhances motor function recovery in SCI patients.
• The system demonstrates high accuracy and efficiency, paving the way for real-world FES rehabilitation devices.