World Academy of Science, Engineering and Technology International Journal of Medical, Health, Pharmaceutical and Biomedical Engineering Vol:7 No:12, 2013
Close Loop Controlled Current Nerve Locator H. A. Alzomor, B. K. Ouda, A. M. Eldeib
Abstract—Successful regional anesthesia depends upon precise
International Science Index Vol:7, No:12, 2013 waset.org/Publication/9996703
location of the peripheral nerve or nerve plexus. Locating peripheral nerves is preferred to be done using nerve stimulation. In order to generate a nerve impulse by electrical means, a minimum threshold stimulus of current “rheobase” must be applied to the nerve. The technique depends on stimulating muscular twitching at a close distance to the nerve without actually touching it. Success rate of this operation depends on the accuracy of current intensity pulses used for stimulation .In this paper, we will discuss a circuit and algorithm for closed loop control for the current, theoretical analysis and test results is discussed and results is compared to previous techniques.
Keywords—Close Loop Control, Constant Current, Nerve Locator.
I. INTRODUCTION
T
HE regional anesthesia is preferred to global anesthesia when applicable due to its advantages including safety, low cost and patient preference. Historically, nerve blocks were performed using anatomical landmarks as a guide as to where to insert the needle and then eliciting paraesthesia. When the locating needle touched the nerve the patient experienced a sensation like ‘pins and needles’ or an ‘electric shock like sensation. The disadvantages of using a paraesthesia technique are the theoretical increase in the risk of nerve damage by the needle touching the nerve, its reliance on a subjective sensation that patient experiences and the lack of an objective response that the anesthetist can use [1]. Nerve Locator is the most commonly used technique in most developed countries for locating peripheral nerves [2]. The technique depends on stimulating muscular twitching at a close distance to the nerve without actually touching it [3]. Successful regional anesthesia depends upon precise location of the peripheral nerve or nerve plexus [3]. In order to generate a nerve impulse by electrical means, a threshold stimulus of current must be applied to the nerve “rheobase” [2]. Further the needle tip is from the nerve, a proportionately greater current is needed to stimulate the nerve, and vice versa. Based on this fact, the amount of current and pulse duration required to stimulate the nerve indicates how far the needle tip from the nerve is. The load resistance varies with the tissue impedance and the position of the electrode from the tip of the needle. Current delivered to the nerve has to be maintained constant regardless of the tissue
impedance and the distance between the needle tip and the electrode. II. PREVIOUS WORK Several implementations for generating a fixed current for nerve locator and stimulators were proposed before like using a current mirror with variable control voltage which achieves a relative error up 5% and lacks a closed loop control as in [4] or using operational amplifier with step up transformer for setting the current and operational amplifier for current feedback as in [5] or using a µ-controller to drive 16 Bit Digital to Analog Converter "DAC" which its output is used to drive voltage to current converter circuit implemented with two operational amplifier as in [6]. Previous techniques suffer from bad current precision or lack of information about the current intensity accuracy. III. OUR PREVIOUS WORK A. Design Idea In order to control the stimulating current, a controlled current sink circuit was developed [7]. The current sink circuit consists of a Pulse Width Modulation PWM signal with a duty cycle proportional to the required current and a low pass filter for converting the PWM to the DC voltage. An Operational Amplifier Op-Amp and Darlington transistor are used for buffering the DC voltage which is applied to high precision resistor to control the amount of sink current, a transistor is used to interrupt the current path in order to control the pulse width and frequency of the pulse. The PWM and interrupt resistor timings are generated using timer interrupts on the µcontroller. Circuit is shown in Fig. 1 and simulation result for the current against control Voltage is shown in Fig. 2.
H. A. Alzomor is with the Almanar for Electronic Systems, Cairo, Egypt (phone: +202-358-35215; e-mail:
[email protected]). B.K. Ouda is with the Department of Faculty of Biomedical Engineering, Cairo University, Egypt (e-mail:
[email protected]). A. M. Eldeib is with the Department of Faculty of Biomedical Engineering, Cairo University, Egypt (e-mail:
[email protected]).
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World Academy of Science, Engineering and Technology International Journal of Medical, Health, Pharmaceutical and Biomedical Engineering Vol:7 No:12, 2013
is 0.6 Volts. V is 820ohm m. is 5.1Kohm m.
0.00086
0.00141 at 1st step 0
iis 0 Volts. 0.0 00141
0.00086 0.00086 0.00141
whhen =5mA is 0.2Voolts. 0.005 0.00 0141 0.000086
International Science Index Vol:7, No:12, 2013 waset.org/Publication/9996703
0.6
0.00141 0.2 0 4.356
Control
.
volltage dynam mic range = =3.756 Volts. Voltage step= = 5/256=0.01995 Volts. Dynamic rangge steps = 3.756/0.0195 = 192 1 Steps. Minimum currrent step = 5m mA/192 = 26µ µA.
Fig.. 1 Previous woork current conttrol circuit
(5)
-
C. Previous Work W Impairm ments Current was controlled uusing open loop l control circuit whhich is not capable of compensation n for variatio ons in component valuues with time aand due to tem mperature channge. 1) A 8 bit µ--controller waas used for generating g thee pulse width modu ulation PWM signal for vo oltage control which limits the reesolution of thhe minimum current c step. mum current sttep. 2) Low resolution of minim IV. PROPPOSED DESIGN N o the impairments of the old deesign a In order to overcome new design was proposed witth the followinng improvemeents Fig. 2 The currrent following through t load aggainst control vooltage
B. Previous Work Mathem matical Analyssis R is given by the The current following inn the load R1 foollowing equattion IR
IR
IR
(1) (2) (3)
froom (1), (2), an nd (3) (4)
A. Closed Looop Control A closed lo oop control system wass implemented by c following in the con ntrol resistor and a use meeasuring the current it to modify th he PWM signnal used to generate g the control c vooltage. A highh side currentt measuremennt is used insttead of meeasuring the current follow wing into thee control resistor to takke the base current of thhe pulse conntrol transistoor into consideration. The correctioon action is done on two step ps; the 1st tim me after settting the new current a corrrection valuee proportionall to the errror is added to t the PWM aand in the connsecutive corrrective steeps one PWM M step is addeed / subtracted d from the prrevious PW WM value forr correction. T This way the system locks to the desired value quuickly and avvoids oscillatiions for conseecutive mented to redduce oscillatioons by steeps. A hystereesis is implem defining a thresshold of errorr to start the correction process. Figg. 4 shows thee flow chart foor the feedbacck algorithm.
takes values v from 0 to 0.2 Vooltages in satturation reegion. is 5 Volts.
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World Academy of Science, Engineering and Technology International Journal of Medical, Health, Pharmaceutical and Biomedical Engineering Vol:7 No:12, 2013
5m mAmp is conttrolled using 3 control circu uits each of them t is tun ned with minimum current precision suittable to its ran nge. c Fig. 3 showss the proposeed design witth feedback control cirrcuit. The sim mulation resuults for the 3 current rang ges are sho own in Fig. 5.. V. PROPO OSED DESIGN MATHEMATIC CAL ANALYSIS S Mathematicall Analysis is done for Q4 in Fig. 3 wh hile the sam me analysis iss applicable fo or Q2 and Q3. (6)
International Science Index Vol:7, No:12, 2013 waset.org/Publication/9996703
(7)
B. Hi Precision PWM Conntrol A 32 bit µ-controller µ w was used forr implementiing the mbedded prog gram and 16 bit timer useed for generatting the em vo oltage control PWM sign nal which alllows high prrecision co ontrol voltage.
is about 0.06Volts 0 and is abou ut 0.6Volts. We calculateed the start aand stop conttrol voltage, control c Vooltage range, current range, number of steps s per each h range an nd minimum current c step foor each range of the three ranges, r ressults is shown n in Table I. The range of control voltagge from 0 to 3.3 3 Volt is con ntrolled usiing 19200 step p Pulse widthh modulation. The current range r from 0 to 5mAmp is i divided into o three ran nges and the control voltagge for the beg ginning of thee range an nd the end of each e range is ccalculated, fro om dynamic raange of thee control volttage for each current rang ge we calculatted the nuumber of con ntrol steps aand min currrent step an nd the miinimum preciision ratio for each range which is the ratio between step current in thatt range to th he minimum current c nge. vaalue in that ran
Fig. 4 Currennt close loop conntrol
Fig. 5 Simulation S resuults for the 3 cuurrent ranges
Fig. 3 Propossed Hi precisionn close loop currrent control sysstem
C. High Reso olution Current Step Low values of Current needs n more precision p than n High vaalues of currrent, for exam mple an errorr of 26µ-Am mp –the m minimum curreent step- at 5m mAmp is 0.52 2% while an error e of 26 6µ-Amp at 50µ-Amp 5 is 52%, in ord der to increaase the prrecision of cu urrent at low values v the wh hole range fro om 0 to
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World Academy of Science, Engineering and Technology International Journal of Medical, Health, Pharmaceutical and Biomedical Engineering Vol:7 No:12, 2013
Control Volltage (Volt) 0.6 656808949 2.9 908918193 0.7 788008825 3.1 171796724 0.8 853007373 3.0 064105549
IR1 ((Amp) 0.00005 0.00025 0 0.0003 0 0.0013 0 0.0013 0.005
Vce (Volt) 0.06 0.06 0.06 0.06 0.06 0.06
TABLE I CURRENT RAN NGES, CONTROL VOLTAGE RANGE ES AND MAX ERR ROR IN EACH RAN NGE Vbe Rcc Current ran Control Voltag ge nge number of Rb (Ω) (Volt) (Ω Ω) Range (volt)) (Amp) voltage stteps 0.6 120 012 180000 2.2521092433 0.0002 2 131033 0.6 120 012 180000 0.6 244 42 100000 2.3837878999 0.001 138699 0.6 244 42 100000 0.6 616 20000 2.2110981766 0.0037 7 128644 0.6 616 20000
Min. currrent step (A Amp)
Min. Precision P ratio in n range %
1.52637E-08
0.0 031%
7.21033E-08
0.0 024%
2.87624 4E-07
0.0 022%
VI. PROPOSED R DESIIGN TESTING AND A RESULTS
International Science Index Vol:7, No:12, 2013 waset.org/Publication/9996703
The current is i measured th hrough a calib brated high prrecision 10 0KΩ resistor using a 100M MHz with 8 biit vertical reso olution, reesults is shown n in Table II.
Set Current mAm mp 0.1 0.3 1 2.5 5
TABLE II ERROR R IN CURRENT Measurred current mAm mp 0.1 0.298 0.992 2.52 5
Errorr% 0% % 0.8% % 0.8% % 0.8% % 0% %
The actual errror is higher than the theo oretical error showed s in n Table I becaause of the acccuracy of the passive comp ponents ussed is 1%. Taable III showss the measureements of the current in n the proposed design com mpared with measured m don ne on a w range of th wide he nerve locattors available in the markett [2]. TA ABLE III CURRENT INTENSSITY IN THE PROPOSED DESIGN COMPARED O TO DEV VICES IN E MARKET THE Target Current in ntensity, mAmp 5 1 0.3 0.1 Proposed d Design 5 0.992 0.298 0.1 Stimplex HNS 12 5 0.99 0.3 0.1 MultiSim 0.29 0.09 m Vario 4.9 9 0.98 9 Plexiival 4.9 9 0.98 0.3 0.11 1 Plexy ygon 4.9 9 0.98 0.29 0.09 9 Stimplex HNS 11 4.9 9 1.1 0.4 0.2 Multisem 0.98 0.3 0.1 m Sensor 5
The design is fabricated and a tested at Faculty F of Vetterinary Cairo Universitty on the supeerficial head neerves and deep limbs neerves of colt,, and the pro ocess of nerv ves stimulatio on was su uccessfully do one. Device iss currently beeing test at faaculty if M Medicine Cairro University y. Picture fo or the device after asssembly is sho own in Fig. 6.
Fiig. 6 Proposed ddesign after asssembly
VII. CO ONCLUSION A design wass proposed to iimplement clo osed loop conttrol for controlling the pulsating currrent of nervee locator, the whole cu urrent range is divided on thhree bands with different ab bsolute cu urrent precisio on for each raange to avoid d high error ratio r in cu urrent at low current c valuess. The proposed design sho owed a sig gnificant imprrovement in ccurrent intensiity accuracy against a ouur previous design [7] and is comparable with the bestt Nerve loccators in the market m [2]. ACKNOW WLEDGMENT The authors would w like too thank the Egyptian inform mation tecchnology dev velopment auuthority ITID DA for fundin ng the pro oject and Pro of. Mohamed Barakat for guiding the tests t at Faaculty of Veterrinary. REFEERENCES [1] [2] [3] [4]
[5]
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Dr Anand M. M Sardesai and Dr Uma Iyer, “Nerve Stimulaation for Peripheral Nerrve Blockade." ATOTW, A 2009, #1 149 August ,PP 1-8. D. Jochum , G. G Iohom, D. P. D Diarra, F. Loughnane, L. J. Dupree and H. Bouaziz, “An n Objective asseessment of nerv ve stimulators used u for peripheral nerv ve blockade” Anaaesthesia, 2006, 61, 6 PP 557–564. Phil Dalrymp ple and Subbiah h Chelliah “Elecctrical Nerve Lo ocators”, British Journal of Anaesthesia, 2006, Volume 6 Number 1, PP 32 2-36. L Jader A. De Lima and Adriano S. Cordeiro, “A Low-Cost urrent Control”, IEEE Neurostimulattor with Accuurate Pulsed-Cu Transactions on o Biomedical Enngineering, vol. 49, 4 no. 5, May 2002, 2 pp. 497-500. C Yan Lu, Kai-Yu Tong, A. A B. Rad, Danieel H. K. K. W. Eric Cheng, Chow, and Danny Sutanto, “D Development of a Circuit for Fu unctional
World Academy of Science, Engineering and Technology International Journal of Medical, Health, Pharmaceutical and Biomedical Engineering Vol:7 No:12, 2013
[6]
International Science Index Vol:7, No:12, 2013 waset.org/Publication/9996703
[7]
Electrical Stimulation” IEEE Transactions on Neural Systems And Rehabilitation Engineering, vol. 12, No. 1, March 2004 pp 43-47. Subaryani D H Soedirdjo, Mervin T Hutabarat, “Microcontroller-based Transcutaneous Electrical Nerve Stimulator with 8 Bit Cascade DAC” International Conference on Instrumentation, Communications, Information Technology, and Biomedical Engineering (ICICI-BME), 2009 pp1-5. H. A. Alzomor, B. K. Ouda “Low Cost,Compact and Pulsated Constant Current Microcontroller Based Nerve Locator” Cairo 6th International Biomedical Engineering Conference, Dec. 2012, pp. 87–90.
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