Wireless Distributed Architecture for Therapeutic FES: Metrology for muscle control M. Toussaint, Jr. * D. Andreu ** P. Fraisse *,** * LIRMM-CNRS, University of Montpellier 2, France (e-mail: philippe.fraisse@lirmm.fr, mickael.toussaint@lirmm.fr). ** INRIA, Sophia Antipolis, France (e-mail: david.andreu@inria.fr) Abstract: This paper presents a Functional Electro-Stimulation distributed architecture based on a wireless network, for therapeutic training of disabled patients. On this distributed architecture, the movement (of disabled members) is artificially controlled by means of a global controller which pilots a set of stimulation units. The closed loop control system we developed for controlling muscle is based on a high order sliding mode method. In such wireless network- based control, the variable delay introduced by the network must be taken into account to ensure the stability of the closed loop. Thus, in order to characterize the medium on which the control is performed, we carried out accurate measurements of the architecture performances (stack-crossing, round-trip time, etc.). We then propose the use of a Kalman filter to predict the communication delay evolution, with the aim to exploit it within the closed loop control. 1. INTRODUCTION Electrical stimulation can generate an artificial contrac- tion of skeletal muscles by applying sequences of electrical pulses to sensory-motor system via electrodes which can be placed on the skin, [Kralj et al., 1980], or implanted [Guiraud et al., 2006]. Transcutaneous (surface) electri- cal stimulation is a technique widely applied for physical therapy, sports training and clinical purposes. It can be used for muscle atrophy treatment, muscle force training, endurance training, pain treatment and functional move- ment therapy [Keller et al., 2006]. Functional Electrical Stimulation (FES) concerns the restoration of a functional movement in disabled patients. FES applications applied to lower limbs include foot drop correction, single joint control, cycling, standing up, walking... FES applications applied to upper limbs allow hand grasping enhancement. A wide range of disabilities are concerned with FES, they include spinal cord injuries, stroke, multiple sclerosis; cere- bral palsy and Parkinson’s disease [Prodanov et al., 2003] and both children and adults are concerned. Two distinct objectives may be targeted when using those techniques, depending on the type of disorder: chronic assistance or acute training. FES can be applied for example for walking assistance and training in post-stroke hemiplegic patients, as well as for standing and gait restoration in paraplegic patients [Heliot et al., 2007]. Physiological effects of FES- assisted verticalization in paraplegic patients include for example: prevention of muscle atrophy, promotion of re- nal functions, improvement of joint range of motion, well being, improved digestion, bowel and bladder functions, retardation of bone-density loss, decreased spasticity, re- duced risks of pressure sores, improved cardiovascular health, improved skin and muscle tone [Cybulski and Jaegger, 1986]. The FES used in the framework of exercise was termed Functional Electrical Therapy (FET). In this context of FET, stimulators which are used are wire-based and centralized ones; each electrode is con- nected to a central controller by means of wires (the num- ber of wires for each electrode depending on its number of poles). Those electrodes being used for stimulation or recording (external sensors based on EMG for example), of the sensory-motor system activity. When dealing with a set of (multipolar) electrodes, those wires constitute an important constraint for the patient mobility, and thus for the training. This constraint must be removed to propose realistic solutions for rehabilitation applications to be used by physiotherapist and/or the patient himself. As a conse- quence we designed a distributed FES architecture based on distributed stimulation units (DSU) [Andreu et al., 2005]. Each DSU is a device composed of digital and analogue parts, the latter being connected to the (multipolar) elec- trode used to stimulate the muscle. A DSU can be con- figured, programmed and remotely operated [Souquet et al., 2007]. It embeds in particular a 3-layer protocol stack according to the reference given by the structure of the reduced OSI model. These layers are the Application layer, the Medium Access Control layer (MAC) and the Physical layer. The physical layer ensures the telecommu- nication over the 2.4 GHz Radio Frequency (RF) based wireless medium (used in considered experimental setup). The MAC layer ensures a deterministic medium shar- ing [Godary et al., 2007]. The application layer supports configuration, programming and remote operating (from start/stop requests to online stimulation control) of the stimulation unit. On this distributed architecture, the movement (of dis- abled members) is artificially controlled by means of a global controller which pilots a set of DSUs. For instance, it sets dynamically the parameters of the stimulation, like the stimulation frequency, the pulse amplitude and Proceedings of the 17th World Congress The International Federation of Automatic Control Seoul, Korea, July 6-11, 2008 978-1-1234-7890-2/08/$20.00 © 2008 IFAC 11588 10.3182/20080706-5-KR-1001.3796