JOURNAL OF AUTOMATIC CONTROL, UNIVERSITY OF BELGRADE, VOL 15(SUPPLEMENT), 2005 ©
Novel Electronic Stimulator for Functional Electrical Therapy Nikola Jorgovanović, Strahinja Došen and Ratko Petrović
Abstract - The novel electronic stimulator for Functional Electrical Therapy (FET) is an apparatus for promotion of the recovery of functioning in humans with sensory-motor lesion in the central nervous system having impact to movement. Recent clinical studies demonstrated that the recovery of functioning was contributed by the following: 1) electrical stimulation of efferent nerves that augment and/or generate missing functions of the upper limb, 2) electrically induced strong input to the central nervous system via afferent nerves, 3) intensive exercise by the same paretic upper limb, and 4) increased awareness of being able to accomplish the task. The apparatus, therefore, comprises a multi-channel pulse generator designed for surface electrodes, and the novel controller that mimics spatial and timing synergies of muscle activation found in able-bodied subjects when performing functional tasks. Index Terms - neural prosthesis, electrical stimulator, functional electrical therapy
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I. INTRODUCTION
EURAL PROSTHESIS (NP) can be described as a bypass between paralyzed sensory-motor systems and central nervous system. The NP should restore functions that are lost or diminished as a result of injury or illness. Nowadays, NP is used, with various successes, for standing, walking, reaching, grasping and holding in subjects after stroke and spinal cord injuries [1,2]. The NP has a role to generate controlled bursts of electrical charge and deliver those via electrodes to tissues. We describe here a new electronic stimulator that comprises eight channels that are designed to deliver programmable bursts of compensated monophasic current controlled pulses. The stimulator integrates an easy to program interface. The stimulator is for use with surface electrodes. The stimulator supports state control and it supports sensory inputs and complex patterns of stimulation. The technical requirements for the stimulators follow the clinical findings with the use of the Belgrade grasping system [3]. As a result of fast growing technology and achievements in integrated electronics, there is significant increase in the speed and complexity of modern microcontrollers. Using new microcontrollers with integrated memory modules and peripherals, it is possible to design single-chip controlled stimulators. The integrated microcontroller modules can Manuscript originally received April 26. Received in the final form on June 5, 2005. R. Petrović is with the Faculty of Engineering, University of Novi Sad, Novi Sad, Serbia
[email protected] N. Jorgovanović is with the Faculty of Engineering, University of Novi Sad, Novi Sad, Serbia
[email protected] S. Došen is with the Faculty of Engineering, University of Novi Sad, Novi Sad, Serbia
[email protected]
replace specialized hardware circuits that were typically applied in earlier designs. The stimulator which we designed and realized integrates: 1) 8 stimulation channels, 2) monopolar stimulation, 3) generations of current controlled monophasic compensated pulses, 4) programmability (stimulation frequency, pulse width, stimulation amplitude, amplitude rise and fall time, independent set up of delay and duration of stimulation pulse train for each channel; archiving of stimulations protocols and parameters; calibration mode useful for finding motor points; easy-to-use user interface), 5) battery supply and compact design (small dimension and weight), 6) wireless link to PC, 7) fulfillment of international standards for medical devices. Original concepts implemented in this stimulator can be seen in its structure, applied technical and technological solutions and provision of complete software control [4]. One similar prototype was designed by UNA Company for Neurodan A/S, Aalborg, Denmark and brought to the market under the name Actigrip®. The Actigrip device supports functional electrical therapy in post-stroke subjects, and mostly relies on the single switch command that triggers a sequence that mimics timing of muscle activations that were preprogrammed. Our new stimulator is primarily made for treatment of walking and standing; yet, it can be programmed for other applications. The stimulator can also be modified for applications in clinical electrical therapy. II. HARDWARE STRUCTURE The stimulator hardware structure was realized with one circuit board housing almost all hardware, including LCD display and connectors. Keyboard, LED diodes, and battery holder are integrated in the device front panel. The stimulator consists of four functional units: 1) microcontroller, 2) power supply unit, 3) user interface and 4) output pulse generator. The stimulator block scheme is presented in figure 1. Hardware is designed as a general platform that can be used for realization of various types of neural prosthesis with wide range of possible applications. For that purpose analog and digital input channels are implemented. External myoelectric command interface is also realized. It connects to one analog and one digital stimulator’s channel [1]. A. Microcontroller The microcontroller is the main part of the stimulator (figure 1). The microcontroller controls all of the
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PETROVIĆ R., JORGOVANOVIĆ N., DOŠEN S., NOVEL ELECTRONIC STIMULATOR FOR FET
Fig 1. The block diagram of the structure of the stimulator.
stimulator’s functional units (e.g. pulse generator, power supply unit and user interface). After analyzing the needs for our stimulator, we have chosen 18F452 Microchip© module. It is 16-bit RISC processor with maximum rate of 10MIPS. Small consumption (less then 10 mA with clock rate 20MHz and Vcc=3.4V), wide range for power supply voltages (from 2V to 5.5V) and wide processor clock (0 to 40MHz) make this microprocessor very suitable for battery -supplied devices. This microcontroller unit fulfills all requirements for our design. B. Power Supply Unit Battery supplied devices are intrinsically safe (the patient is isolated from power supply voltage). They also give mobility. Eventual problems with these devices are possible variations of battery voltage. For the most often used batteries, (e.g. alkaline, NiMH) the voltage varies from 0.9V to 1.6V (full battery cell). Thus, four battery cells can provide supply voltage between 3.6V and 6.4V. To guaranty proper continuous work of the device, it is necessary that all electronic components work with supply voltage beneath 3.6V (3.45V was adopted). . The power supply unit provides power supply for all electronic circuits in the stimulator. It provides negative voltage supply for LCD polarization and constant current supply for LCD back light. Step-up DC/DC converter provides supply for the pulse generator module. It is realized as two-stage boost converter that can increase voltage up to 100V with one and 300V with two stages. It is sufficient for various clinical applications. C. User Interface User interface consists of LCD display and a keyboard. The display has 80 characters arranged in four rows. It allows readable printout without any shortcut. LCD backlight helps in reading the text, but also spends a lot of battery energy. Because of that, the backlight is under
software control, which allows both better consumption of battery energy and comfortable usage of LCD. The keyboard consists of six buttons. The buttons are: left arrow, right arrow, up arrow, down arrow, Enter and Escape. With the keyboard and LCD, the stimulator is fully menu driven device and thus easy to use for an end user (figure 3). D. Pulse Generator Pulse generator produces monophasic compensated current pulses (figure 2). The pulse generator consists of five functional parts: 1) tact generator; 2) digital to analog (D/A) converter; 3) decoder and demultiplexer; 4) constant current source; and 5) circuit for generating relaxation pulses. The pulses are generated sequentially, one for each channel. For subsequent pulse, the D/A converter is fed with stimulation amplitude value. The microcontroller sends an address of channel at which will be generated pulse. Then, it generates tact signal for stimulation pulse using its CCP modules (in negative logic). CPP modules (Capture Compare PWM) provide very fast outputs with resolution of 1 µs. During low level of CCP1 the output of D/A converter is passed to current source, which generates constant current pulse. When CCP1 goes high, the positive pulse ends and the other CCP module - CCP2 goes low for very short time. That drives circuit for generating relaxation pulses, which provides compensation pulse. III. COMMUNICATION INTERFACE Communication interface provides communication between stimulator and PC. Wireless infrared communication is used in order to isolate the patient from the main power supply. Infrared link is implemented in accordance with IRDA standard. Through this communication link, it is possible to download/upload stimulator parameters. Communication is supported with specially designed PC software that provides support for all
JOURNAL OF AUTOMATIC CONTROL, UNIVERSITY OF BELGRADE phases of functional electrical therapy [5].
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memory. On that way the user can easily change the stimulator functionality and target application.
V. CONCLUSION
Fig 2. Stimulus pulse shape (pulse width 250 µs, amplitude 10 mA)
IV. FIRMWARE Stimulator hardware is designed to enable various applications. Main characteristics that support wide range of possible applications are: maximal number of stimulation channels is 8, output voltage can be increased up to 300V, maximal amplitude of current pulses is 150mA, pulse width can be varied from 10us to 1ms and frequency of stimulation can be set in range 20Hz - 100Hz. Different applications use different stimulation protocols with specific sets of stimulation parameters and their ranges. This requires different menu options and user messages for each application.
The FES stimulator presented in this paper is not bound to particular clinical application, but provides universal platform for therapeutic application in the process of neurorehabilitation. It is microprocessor system for electrophysiology and rehabilitation, based on microcontroller (figure 4). Original concepts in this device can be seen in its structure, applied technical and technological solutions and provision of complete software control. The stimulator has small energy consumption. With those technical solutions the stimulator enables continuous work during three days, in four-channel stimulation mode, with one charging of four NiMH batteries. In comparison, Compex Motion [6] stimulator provides 8 hours of functioning. The special ability that the stimulator provides is the easy way of set up of stimulation parameters by using stimulator keyboard and menu system, or by PC application via IC link. Simply simply changing its firmware can change the target stimulator application. It is provided by specialized procedure called boot loader, which is integrated in the stimulator. With that approach we designed the stimulator, which is very easy to use and has a universal propose.
Stimulator firmware is realized to support one specific application. Thus, if one wants to use the stimulator for new application, the firmware has to be changed. The microcontroller used in this device, has the ability to reprogram its FLASH memory. We used that feature to program special low level routine that provides the user with the option to download new firmware into device (via infrared link). Firmware is divided in two parts: 1) stimulation firmware; and 2) programming firmware – boot loader. The stimulation firmware is main program that drives the stimulator device. When this part is active, the stimulator runs in normal (user) mode. The stimulator functionality can be changed if user replaces this firmware part. The boot loader is part of the firmware, which stays inactive during normal mode. Its task is to reprogram the device. The boot loader is placed in the lowest locations of free FLASH memory and takes only 3% of it. The user cannot change this part of FLASH. The stimulator firmware is located in the rest of the FLASH. To activate the boot loader, user has to choose boot loader option in the main stimulator menu. When the boot loading procedure is started, the device connects to PC, receives new firmware (machine code) and writes it into FLASH
Fig. 3. Novel electronic stimulator for electrotherapy
The described FET stimulator was distributed to more than 15 clinical facilities. The FET stimulator is today in pilot use in several rehabilitation institutions (Institute for rehabilitation “Dr Miroslav Zotovic” in Belgrade, SCG; Neuron, Kuopio, Finland; Rehabilitation Institute of Slovenia, Ljubljana; and Brønderslev and Hammel Rehabilitation centers in Denmark). The product was CE marked. The firmware was realized for different applications: grasping, standing, walking, therapy, and the EMGN system (sensory and motor evoked potentials).
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PETROVIĆ R., JORGOVANOVIĆ N., DOŠEN S., NOVEL ELECTRONIC STIMULATOR FOR FET The work on the project was partly supported by the Ministry of Science and Environment Protection of Republic of Serbia, Belgrade.
REFERENCES [1] [2] [3] Fig 4. “Inside the FET stimulator” view.
[4]
ACKNOWLEDGMENT We acknowledge the support of Dejan B. Popović, from SMI, Department of Health Science and Technology, Aalborg University, Denmark, and Faculty of Electrical Engineering, Belgrade.
[5] [6]
M. Popović, D. B. Popović, L. Schwirtlich, T. Sinkjær, “Clinical Evaluation of Functional Electrical Therapy (FET) in chronic hemiplegic subjects”, Neuromod, 7(2)2004. M. Popović, D. B. Popović, T. Sinkjær, A. Stefanović, L. Schwirtlich, “Clinical Evaluation of Functional Electrical Therapy in Acute Hemiplegic Subjects”, J Rehab Res Develop, 40(5):443-454, 2003. D.B. Popović, M. Popović (1998) "Belgrade grasping system", J Electronics BanjaLuka 2: 21-28. N. Jorgovanović “Control of functional electrical stimulation for neurorehabilitation in subjects with CNS impairment”, Ph.D. thesis, Faculty of Engineering, University of Novi Sad, Novi Sad, 2003. (in Serbian) S. Došen “FETStudio – Software system for support of functional electrical therapy ”, J Automatic Control, University of Belgrade 15(Sup):31-34 2005 (this issue) T. Keller, M. Popović, I. Pappas, P. Müller “Transcutaneous functional electrical stimulator - Compex Motion". ISAO Artificial Organs 26(3): 219 – 223, 2002.
