Teng-Hu Cheng

Stationary cycling induced by switched functional electrical stimulation control

Functional electrical stimulation (FES) is used to activate the dysfunctional lower limb muscles of individuals with neuromuscular disorders to produce cycling as a means of exercise and rehabilitation. In this paper, a stimulation pattern for quadriceps femoris-only FES-cycling is derived based on the effectiveness of knee joint torque in producing forward pedaling. In addition, a switched sliding-mode controller is designed for the uncertain, nonlinear cycle-rider system with autonomous state-dependent switching. The switched controller yields ultimately bounded tracking of a desired trajectory in the presence of an unknown, time-varying, bounded disturbance, provided a reverse dwell-time condition is satisfied by appropriate choice of the control gains and a sufficient desired cadence. Stability is derived through Lyapunov methods for switched systems, and experimental results demonstrate the performance of the switched control system under typical cycling conditions.

Closed-Loop Asynchronous Neuromuscular Electrical Stimulation Prolongs Functional Movements in the Lower Body

Neuromuscular electrical stimulation (NMES) is commonly used in rehabilitative settings and is also used for assistive purposes to create functional movements, where it is termed functional electrical stimulation (FES). One limitation of NMES/FES is early onset of muscle fatigue. NMES-induced fatigue can be reduced by switching between multiple stimulation channels that target different motor units or synergistic muscles (i.e., asynchronous stimulation). However, switching stimulation channels introduces additional complexity due to the need to consider the switching dynamics and differing muscle response to stimulation. The objective of this study was to develop and test a closed-loop controller for asynchronous stimulation. The developed closed-loop controller yields asymptotic tracking of a desired trajectory for a persons knee-shank complex despite switching between stimulation channels. The developed controller was implemented on four able-bodied individuals with four-channel asynchronous stimulation as well as single-channel conventional stimulation. The results indicate that asynchronous stimulation extends the duration that functional movements can be performed during feedback control. This result is promising for the implementation of asynchronous stimulation in closed-loop rehabilitative procedures and in assistive devices as a method to reduce muscle fatigue while maintaining a persons ability to track a desired limb trajectory.

Ensuring network connectivity for nonholonomic robots during decentralized rendezvous

In a multi-robot system, robots are typically required to collaborate over a communication network to achieve objectives cooperatively. Due to the limited communication and sensing capabilities on each robot, the cooperative objective must be accomplished while ensuring that specified robots stay within each others sensing and communication ranges and that the overall network remains connected. In this paper, a dipolar navigation function and corresponding time-varying continuous controller is developed for repositioning and reorienting a group of wheeled robots with nonholonomic constraints. Only local sensing feedback information from neighbors is used to navigate the robots and maintain network connectivity, which indicates that communication is available when required for various tasks, but communication is not required for navigation. Simulation results demonstrate the performance of the developed approach.

Decentralized formation control with connectivity maintenance and collision avoidance under limited and intermittent sensing

A decentralized switched controller is developed for dynamic agents to perform global formation configuration convergence while maintaining network connectivity and avoiding collision within agents and between stationary obstacles, using only local feedback under limited and intermittent sensing. Due to the intermittent sensing, constant position feedback may not be available for agents all the time. Intermittent sensing can also lead to a disconnected network or collisions between agents. Using a navigation function framework, a decentralized switched controller is developed to navigate the agents to the desired positions while ensuring network maintenance and collision avoidance. Simulation results are provided to illustrate the performance of the developed controller.

Tracking control of a human limb during asynchronous neuromuscular electrical stimulation

Neuromuscular electrical stimulation (NMES) is defined as the use of an electrical stimulus to elicit muscle contractions and is commonly used in rehabilitative settings. NMES is also used for assistive purposes to create functional movements where it is termed functional electrical stimulation (FES). One limitation of NMES/FES is early onset of fatigue due to the nonselective, spatially fixed, synchronous activation of motor units. Asynchronous stimulation can reduce NMES-induced fatigue; however, one limitation of asynchronous stimulation is that switching between stimulation channels may introduce discontinuities due to a differing response to stimulation by each group of recruited motor units. Thus, there is a need to design a controller which considers the switching dynamics and muscle response to stimulation during asynchronous stimulation. A closed-loop feedback controller is developed in this paper to yield semi-global asymptotic tracking of a desired trajectory for a persons knee-shank complex during asynchronous stimulation. The result is promising for the implementation of asynchronous stimulation in assistive devices as a method to reduce fatigue while tracking a desired trajectory.

Decentralized event-triggered control for leader-follower consensus

A decentralized event-triggered control scheme for leader-follower consensus is developed. The approach aims to reduce inter-agent communication while ensuring asymptotic leader-follower consensus. The objective is achieved by designing a control algorithm that uses the estimates of neighbors states for feedback and only updates the estimates through communication when events are triggered. The update events are determined by a decentralized trigger condition, designed from the stability analysis, such that the control ensures asymptotic leader-follower consensus with only intermittent communication. The effectiveness of reducing inter-agent communication is shown by developing a positive constant lower bound of the minimum inter-event interval. Since switched dynamics are considered, Zeno execution is proven to be avoided, and Lyapunov-based convergence analysis is provided to ensure the asymptotic leader-follower consensus. Simulation results are provided to demonstrate the effectiveness of the developed control strategy.

Switched Control of Cadence During Stationary Cycling Induced by Functional Electrical Stimulation

Functional electrical stimulation (FES) can be used to activate the dysfunctional lower limb muscles of individuals with neurological disorders to produce cycling as a means of rehabilitation. However, previous literature suggests that poor muscle control and nonphysiological muscle fiber recruitment during FES-cycling causes lower efficiency and power output at the cycle crank than able-bodied cycling, thus motivating the investigation of improved control methods for FES-cycling. In this paper, a stimulation pattern is designed based on the kinematic effectiveness of the riders hip and knee joints to produce a forward torque about the cycle crank. A robust controller is designed for the uncertain, nonlinear cycle-rider system with autonomous, state-dependent switching. Provided sufficient conditions are satisfied, the switched controller yields ultimately bounded tracking of a desired cadence. Experimental results on four able-bodied subjects demonstrate cadence tracking errors of 0.05 ±1.59 and 5.27 ±2.14 revolutions per minute during volitional and FES-induced cycling, respectively. To establish feasibility of FES-assisted cycling in subjects with Parkinsons disease, experimental results with one subject demonstrate tracking errors of 0.43 ±4.06 and 0.17 ±3.11 revolutions per minute during volitional and FES-induced cycling, respectively.

Cadence control of stationary cycling induced by switched functional electrical stimulation control

Cycling induced by functional electrical stimulation (FES) is an effective means for exercise and rehabilitation of individuals suffering from neurological disorders such as stroke, spinal cord injury, and cerebral palsy. To achieve FES-cycling, potential fields are alternately applied across muscle groups in the lower extremities. Alternating stimulation of various muscle groups according to the crank position makes the FES-cycling system a switched system with autonomous state-dependent switching. This paper examines FES-cycling from a switched systems analysis perspective. Specifically, a switched sliding mode controller is developed to yield approximate tracking of a desired cadence despite an uncertain, nonlinear cycle-rider model. Cadence tracking is proven via a common Lyapunov-like function and experimental results are provided to demonstrate the performance of the switched controller.

Decentralized event-triggered control of networked systems-part 1: Leader-follower consensus under switching topologies

A decentralized controller that uses event-triggered scheduling is developed for the leader-follower consensus problem under switching communication topologies. To reduce inter-agent communication, a feedback controller is designed based on state estimates of neighboring agents that are updated by scheduled communication. The state estimates are updated when the network topology switches or a decentralized trigger condition is met. The trigger condition is designed such that the control requires reduced inter-agent communication for feedback while still achieving leader-follower consensus under switching topologies. Since the control strategy produces switched dynamics, analysis is provided to show that Zeno behavior is avoided by developing a positive constant lower bound on the minimum inter-event interval. A Lyapunov-based convergence analysis is also provided to indicate asymptotic convergence of the developed control methodology.

Switched Tracking Control of the Lower Limb During Asynchronous Neuromuscular Electrical Stimulation: Theory and Experiments

Neuromuscular electrical stimulation (NMES) induces muscle contractions via electrical stimuli. NMES can be used for rehabilitation and to enable functional movements; however, a fundamental limitation is the early onset of fatigue. Asynchronous stimulation is a method that can reduce fatigue by utilizing multiple stimulation channels to segregate and switch between different sets of recruited motor units. However, switching between stimulation channels is challenging due to each channel’s differing response to stimulation. To address this challenge, a switched systems analysis is used in the present work to design a controller that allows for instantaneous switching between stimulation channels. The developed controller yields semi-global exponential tracking of a desired angular trajectory for a person’s knee-joint. Experiments were conducted in six able-bodied individuals. Compared to conventional stimulation, the results indicate that asynchronous stimulation with the developed controller yields longer durations of successful tracking despite different responses between the stimulation channels.