Author name used in this publication: Kai-Yu Tong2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Cheng, KWE

Chow, DHK

Lu, Y

Rad, AB

Sutanto, D

Tong, KYR

IEEE Transactions on Neural Systems and Rehabilitation Engineering

Author name used in this publication: Kai-Yu Tong2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING , VOL. 12, NO. 1, MARCH 2004 43Development of a Circuit for FunctionalElectrical StimulationK. W. Eric Cheng, Yan Lu, Kai-Yu Tong, A. B. Rad, Daniel H. K. Chow, and Danny Sutanto, Senior Member, IEEEAbstract—This paper examines the various design of a multiple-purpose portable functional electrical stimulator which is used insurface stimulation of paralyzed muscle of patients with stroke andresults in limb activation. The functionality, circuit performanceand reliability of the circuits will be examined. Analysis, design,and experimental results are presented.Index Terms—Functional electrical stimulation (FES), resonantcircuit, transformerless.I. INTRODUCTIONFUNCTIONAL electrical stimulation (FES) is a form oforthotic/therapeutic treatment that applies transcutaneouselectrical current to initiate contractions in muscles, and is com-monly used for individuals with spinal-cord injuries (SCIs) orstroke. FES has been used to facilitate upper and lower ex-tremity mobility, improve respiratory function, restore boweland bladder function, restore male sexual function, and to treatand help prevent secondary complications such as muscle at-rophy, spasticity, pressure ulcers, deep venous thrombosis, con-tractures, and bone demineralization. [1]–[3]For FES, a controlled electrical stimulus is applied to motorunits/nerves to elicit a muscle contraction in an attempt torestore functional movements of a paralyzed musculoskeletalsystem. Several FES stimulators with microprocessor ormicrocontroller have been developed to improve lower andupper limb functions in subjects after SCI or stroke [4]. Mostof the proposed systems have a more or less fixed designand lack of an open architecture. They generally operatewith preprogrammed stimulation patterns that are stored in alookup table. Often, a single sensor combined with a controlalgorithm either triggers preprogrammed stimulation sequencesor scales and reads the stimulation parameters out of a lookuptable [5]. A transformer is also needed to step-up the voltage.The drawback is that this increases the device size and cost,and electromagnetic interference due to the transformer. Thedesign of the transformer is also needed to handle the smallmark-space ratio of the pulse. The wide range of amplitude isalso restricted because of the fixed transformer turns-ratio.Manuscript received August 15, 2001; revised August 27, 2001. This workwas supported by the Research Committee of the Hong Kong Polytechnic Uni-versity under Project G-YC55.K. W. E. Cheng, Y. Lu, A. B. Rad, and D. Sutanto are with the Departmentof Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom,Hong Kong (e-mail: eeecheng@polyu.edu.hk).K.-Y. Tong and D. H. K. Chow are with the Jockey Club Rehabilitation En-gineering Center, The Hong Kong Polytechnic University, Hung Hom, HongKong.Digital Object Identifier 10.1109/TNSRE.2003.819936Various control strategies and circuit design have been devel-oped to provide enhanced functionality, repeatability, and a widerange of stimulation parameters for FES stimulator [3] in orderto provide predictability of muscle responses. In general, com-plicated circuitry and control method are needed for this appli-cation which, in turn, imposes designs that are bulky, expensive,and high-power consumption. This paper presents two viablesolutions to the problem. The required components are smalland the required input voltage is low. The circuits have threedegrees of controllability which are amplitude, pulsewidth, andfrequency and are ideal candidates for improvement of the FEScircuit.II. FES CIRCUIT USING A STEP-UP TRANSFORMERConventionally, FES circuit is designed by using an oscil-lator which generates necessary pulse by using analogue elec-tronics. The output waveforms including amplitude, frequency,and pulsewidth can be regulated. The output is then stepped upto the required voltage by a step-up transformer.A. Circuit DescriptionFig. 1 shows the schematic diagram of the circuit. It can bedivided into two parts. The first part consists of two integratedcircuit (IC) timers 555 and some attached components such asresistors, capacitors, and diodes. The first 555 (IC1) is a mono-stable oscillator and the second 555 (IC2) is an astable multi-vibrator. The sensor input is reserved for external trigger signalsuch as a microswitch. The output of the astable multivibratoris a series of pulses. The width duty cycle and the amount ofthe pulses (pulse frequency) can be controlled by adjusting thevalue of the resistors ( , , ) and capacitors ( and ).The second part of the circuit consists of four operational ampli-fiers (OP1–4), a transistor, a transformer, and a set of discretecomponents. OP1 is used as an error amplifier. OP2 is to am-plify the signal to drive the transformer T1. OP3 and OP4 arethe current-feedback network. The pulse amplitude can be regu-lated by . The transformer is used to further step up the outputvoltage. The function of this part is to transfer the output pulsesof the astable multivibrator into a series of current pulses whoseamplitude can be up to 100 mA. A current feedback loop is in-cluded to ensure the current amplitude.B. Experimental WaveformsThese waveforms, as shown in Fig. 2 are taking at loadk , switching frequency Hz. The inductanceof the transformer is 12.2 mH and 4.8 H for the primaryside and secondary side. The transformer used is: 50 : 20001534-4320/04$20.00 © 2004 IEEEAuthorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 14, 2009 at 04:17 from IEEE Xplore.  Restrictions apply.44 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING , VOL. 12, NO. 1, MARCH 2004Fig. 1. Typical circuit of a transformer-based FES.Fig. 2. Typical waveforms of a transformer-based FES. (Channel 1: output ofOP2; Channel 2: output of the transformer.)using an Rm8 core. It can be seen that the output waveformshas a negative voltage for the recovery of the transformer’smagnetizing current.III. FES CIRCUIT USING ZERO-VOLTAGE SWITCHINGRESONANT TECHNIQUEA. Circuit DescriptionFig. 3 shows the schematic diagram of the circuit. The cir-cuit is based on resonant converter [6] which has been used inpower conversion but rarely used in medical electronics. Fig. 4shows the idealized waveforms of the circuit. It consists of twotransistors and a set of resonant components and . , ,Fig. 3. Proposed resonant circuit for FES.and are amplitude regulating components. The operation ofthe circuit can be derived in the following stages of operation:State 1 : Both transistors are turned off. ischarged with current in . The equation is, therefore(1)State 2 : When rises to , and start toresonate. The circuit equations can be described as(2)(3)where and . This stateterminates after a quarter of resonant period.State 3 : Since is already being off, the currentcannot pass through to because the body-diodeof will no longer conduct. is, therefore, electricallydisconnected from the circuit. The voltage of is, there-fore, maintained at(4)Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 14, 2009 at 04:17 from IEEE Xplore.  Restrictions apply.CHENG et al.: DEVELOPMENT OF A CIRCUIT FOR FES 45Fig. 4. Idealized waveforms of the double-mode circuit.Therefore, the amplitude of is controlled by and.State 4 : This state can be terminated by turningon . The resonance of and as described in State2 then resumes. The state equations can be written as(5)(6)when reaches zero and cannot resonate to negativebecause the body-diode of conducts.State 5 : The magnetic energy stored in isneeded to be reset to zero before the whole operation cyclefinishes. is now discharged linearly by . The equa-tion is shown to be(7)State 6 : No operation occurs for and .and is then energized by as a low-frequencyresonance which is not the concern of this paper becausethey do not contribute the necessary output for the FES.B. Description of the ApplicationSurface electrodes will be affixed to different musclegroups of the upper extremity for stimulation and monitoringthe muscle activities an electromyographic (EMG) system.The muscle activities through the control system is used totrigger/control proposed device and generating the musclecontractions through electrical stimulation by deriving thevoltage across . The accuracy and the reproducibility of thestimulator will also be evaluated. It can be seen from Fig. 3,that the width of the waveform mainly depends on the durationof State 3. The durations of the States 1, 2, and 4 are usuallydesigned to be much smaller compared to the duration ofState 3. The amplitude of the signal depends on which iscontrolled by through the following:(8)Fig. 5. Experimental waveforms of the proposed circuit.where is derived by equating the input and output energiesof the system, which as(9)where , ,, andThe three degrees of freedom of control of the stimulationsignals are summarized as follows:1) pulse amplitude is adjusted by ;2) pulsewidth is controlled by the off-state of orapproximately equal to ;3) pulse frequency is varied by controlling the fre-quency of the cycle—the frequency of gate signals ofand .C. Experimental VerificationThe stimulator is designed according to the following specifi-cations: input voltage V, minimum value of pulsewidths, amplitude V V, and frequencyHz kHz. The parameters used are: mH,nF, mH, and F. The circuithas been tested for these specifications. Fig. 5 shows the experi-mental waveforms. The generated signal can give up to 100-mApulse current for stimulation. The three control stimulation pa-rameters are working well.Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 14, 2009 at 04:17 from IEEE Xplore.  Restrictions apply.46 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING , VOL. 12, NO. 1, MARCH 2004IV. DISCUSSIONThe two proposed circuits have been analyzed, built andtested. The performance of two circuits is very similar andboth can provide the required pulse pattern for use as anFES. The first circuit is an analogue electronic circuit, whichrequires a transformer to step up the voltage. The transformeris required to step up the voltage from 9 to 200 V. The designis critical because it needs to handle a pulse current of at least100 mA. The transformer is also the most bulky and expensivecomponent in the circuit. The component count is also high,therefore, in practice, surface-mount devices are needed.The second circuit is based on a zero-voltage switching reso-nant techniques. The main feature of the circuit is that no trans-former is needed. Therefore, it obviously has the advantage ofno large magnetic components. The resonant componentand and the dc choke are very small as it can be seenin Section III-C. The component count is also very small. Thepulse currents are mainly derived from and, therefore, the se-lection of is very critical. must be large enough to supportthe energy required to the load (patient). The present selectionof 68 nF is sufficient for the operation as an FES.V. CONCLUSIONTwo flexible electronic circuits have been developed for FES.The generated signal has a wide variation of the pulsewidth,controllable amplitude, and frequency. Both of the advantageand disadvantage have been discussed. The resonant circuit pro-posed has a preferred feature and require less component and noneed to install a transformer. It also has a low component count.The circuit is simple and is operated from a 9-V voltage supplyor battery.ACKNOWLEDGMENTThe authors would like to thank Dr. K. F. Fok and Dr. D. Fongof the Tuen Mun Hospital, Hong Kong, for their advice.REFERENCES[1] D. Graupe and K. H. Kohn, “Transcutaneous functional neuromuscularstimulation of certain traumatic complete thoracic paraplegics forindependent short-distance ambulation,” Neurolog. Res., vol. 19, pp.323–333, June 1997.[2] G. M. Yarkony, E. J. Roth, G. Cybulski, and R. J. Jaeger, “Neuromus-cular stimulation in spinal cord injury II: Prevention of secondary com-plications,” Arch. Phys Med. Rehab., vol. 73, pp. 195–200.[3] J. O. Teeter, C. Kantor, and D. Brown, Functional Electrical Stimulation(FES) Resource Guide for Persons With Spinal Cord Injury or MultipleSclerosis. Cleveland, OH: Cleveland FES Center, 1995.[4] M. H. Grant, J. F. Keating, A. C. B. Smith, M. Delargy, and B. J. An-drews, “The use of functional electrical stimulation to assist gait in pa-tients with incomplete spinal cord injury,” Disability Rehab., vol. 14, pp.93–97, 1992.[5] J. J. Abbas, “Feedback control of coronal plane hip angle in paraplegicsubjects using functional neuromuscular stimulation,” IEEE Trans.Biomed. Eng., vol. 38, pp. 687–698, 1991.[6] K. W. E. Cheng and P. D. Evans, “Parallel-mode extended-periodquasiresonant convertor,” Proc. Inst. Elect. Eng. B, vol. 138, no. 5, pp.243–251, Sept. 1991.K. W. Eric Cheng graduated from the University ofBath, Bath, U.K., in 1987 and received the Ph.D. de-gree from the same university in 1990.Before he joined The Hong Kong PolytechnicUniversity, Hung Hom, Hong Kong, in 1997, hewas a Principal Engineer with Lucas Aerospace,Birmingham, U.K. He is currently a Professor andthe Director of the Power Electronics ResearchCenter, The Hong Kong Polytechnic University. Hehas authored more than 100 published papers andseven books. His research interests include powerelectronics, medical electronics, drives, and magnetics.Professor Cheng is a Chartered Engineer and a Corporate Member of theInstitution of Electrical Engineers.Yan Lu was born in Nanchang, China. He receivedthe B.Sc. degree in electric engineering from theJiangxi University of Technology, Jiangxi, China,in 1985, and the M.S. degree in control engineeringfrom the Shanghai University of Technology,Shanghai, China, in 1988. He is currently workingtoward the Ph.D. degree at The Hong Kong Poly-technic University, Hung Hom.He was with the Tongji University of Shanghaiuntil 2000. His research interests include the fieldsof power electronics and nonlinear control.Kai-Yu Tong received the Ph.D. degree in bioengi-neering from the University of Strathclyde, Glasgow,U.K., in 1998.He spent four months as a Research Fellow at theUniversity of Strathclyde and participated in a jointproject with the Spinal Cord Injury Unit, SouthernGeneral Hospital, Glasgow, U.K. He joined the Reha-bilitation Engineering Center, The Hong Kong Poly-technic University, Hung Hom, as a Postdoctoral Re-search Fellow in 1999 and as an Assistant Professorin 2001. His research interests include the control ofFES for upper and lower extremity functions, sensor development for FES con-trollers, artificial intelligence, and stroke rehabilitation.A. B. Rad received the B.Sc. degree in engineeringfrom the Abadan Institute of Technology, Abadan,Iran, in 1977, the M.Sc. degree in control engineeringfrom the University of Bradford, Bradford, U.K., in1986, and the Ph.D. degree in control engineeringfrom the University of Sussex, Brighton, U.K., in1988.He is currently a Professor with the Department ofElectrical Engineering, The Hong Kong PolytechnicUniversity, Hung Hom. His research interestsinclude autonomous systems, adaptive control, andintelligent process control.Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 14, 2009 at 04:17 from IEEE Xplore.  Restrictions apply.CHENG et al.: DEVELOPMENT OF A CIRCUIT FOR FES 47Daniel H. K. Chow received the B.Sc. degree in me-chanical engineering and the M.Phil. degree in bio-engineering from the University of Hong Kong, in1985 and 1988, respectively, and the Ph.D. degreein bioengineering from the University of Strathclyde,Glasgow, U.K., in 1992.He is currently an Associate Professor with theJockey Club Rehabilitation Engineering Center, TheHong Kong Polytechnic University, Hung Hom.His research interests include ergonomics, spinebiomechanics, movement analysis, and rehabilitationtechnology.Danny Sutanto (M’88–SM’88) received the B.Eng.and Ph.D. degrees from the University of WesternAustralia, Crawley, Western Australia, in 1978 and1981, respectively.In 1981, he joined GEC Projects, Australia, asa Power System Analyst. In 1982, he joined theSchool of Electrical Engineering, University of NewSouth Wales, New South Wales, Australia. Since1996, he has been with The Hong Kong PolytechnicUniversity, Hung Hom, where he is now a Professorin electrical engineering. His research interestsinclude power system analysis, power system economics, voltage stability,harmonics, power electronics and computer-aided education.Dr. Sutanto is the IEEE PES Region 10, Regional Representative.Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on July 14, 2009 at 04:17 from IEEE Xplore.  Restrictions apply.

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