Background

One significant challenge of developing a controller for functional electrical stimulation systems is the time-variant nonlinear dynamics of the neuromusculoskeletal system. In the conventional methods, the stimulation intensity is adjusted by a controller; however, the stimulation frequency is always constant. The previous studies have shown that the stimulation frequency is effective in fatigue formation.

Objectives

A simultaneous modulation of the stimulation intensity and frequency is proposed to improve the joint controllability and muscle endurance. The presented control method determines pulse width (PW), amplitude, and frequency of the electrical stimulation signal, synchronously. Three different modulations are applied for control of the knee joint to show the superiority of the proposed modulation.

Methods

The stimulation intensity is controlled by the PW and pulse amplitude of the electrical signal using an adaptive fuzzy terminal sliding mode controller and a fuzzy logic controller, respectively. Also, a fuzzy logic controller is applied to adjust the stimulation frequency. The proposed method is utilized to control the knee joint movement using quadriceps femoris muscles for ten paraplegic subjects.

Results

Two different test protocols are defined to evaluate the presented method: A protocol for testing the controllability and another protocol for evaluating the produced muscle endurance. The average value of the root mean square of the tracking error was 6.4° for the proposed method which is 5.1° and 9.6° less than PW modulation and synchronous PW and amplitude modulation, respectively. The average time duration of the knee full extension was 96 sec for the proposed method which is 17 and 26 sec more than PW modulation and synchronous PW and amplitude modulation, respectively.

Conclusions

The experimental results show that control performance and tracking ability of the joint reference trajectory are improved by using the simultaneous modulation of PW, amplitude, and frequency.