Tube-based Model Predictive Control of An Input Delayed Functional Electrical Stimulation

Abstract

Functional electrical stimulation (FES) is an external application of electrical currents to elicit muscle contractions that can potentially restore limb function in persons with spinal cord injury. However, FES often leads to the rapid onset of muscle fatigue, which limits performance of FES-based devices due to reduction in force generation capability. Fatigue is caused by unnatural muscle recruitment and synchronous and repetitive recruitment of muscle fibers. In this situation, over-stimulation of the muscle fibers further aggravates the muscle fatigue. Therefore, a motivation exists to use optimal controls that minimize muscle stimulation while providing a desired performance. Model predictive controller (MPC) is one such optimal control method. However, the traditional MPC is dependent on exact model knowledge of the musculoskeletal dynamics and cannot handle modeling uncertainties. Motivated to address modeling uncertainties, robust MPC approach is used to control FES. A new robust MPC technique is studied to address electromechanical delay (EMD) during FES, which often causes performance issues and stability problems. This paper developed a novel tube-based MPC for controlling knee extension elicited through FES. In the tube-based MPC, the EMD compensation controller was chosen to be the tube that reduced the error between the nominal MPC and the output of the real system. Regulation experiments were performed on an able-bodied participant, and the controller showed robust performance despite modeling uncertainties.