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A Feedback Controlled Stimulator That Delivers Con-trolled Displacements or Forces to Cutaneous

Mechanoreceptors

JOHN BYRNE, MEMBER, IEEE

Abstract-Based on a design by the root locus method and utilizingboth displacement and force feedback an instrument is describedwhich can be used to deliver either controlled displacement orcontrolled force stimuli to the skin. Examples of the stimulatoroperated in each of its two modes are given.

INTRODUCTION

To study the neural activity elicited by mechanical stimulation ofthe skin, a tactile stimuli must not only simulate natural stimuliencountered in everyday life but also must be precisely controlled.Of the several stimulators which have been used in electrophysio-logical investigations of mechanoreceptors the moving coil vibratorhas proven to be particularly useful because it can be preciselycontrolled. To deliver controlled displacement stimuli to cutaneousmechanoreceptors several investigators have combined moving coilvibrators with displacement feedback [1], [2]. These systems have,however, suffered from several drawbacks. One difficulty is that oneconstantly needs to adjust the stimulating probe height to compen-sate for cupping of the skin with repeated stimulation. In addition arelevant physiological parameter, the force delivered to the mechano-receptor is not controlled but varies depending upon whether themechanoreceptor lies above soft tissue or above bony structure. Thesignificance of this effect was- illustrated by Werner and Mountcastle[1]. When displacement was used as an independent variable thetypical power function describing the mechanoreceptor stimulus-response relationship was not observed for mechanoreceptors overbone. The same study showed that in general when force instead ofdisplacement was used as the stimulus parameter the stimulus-response relationship tended to be linear. The use of force as thestimulus parameter would therefore not only minimize the effectsof stimulating over various skin areas with different mechanicalproperties but may also lead to a single uniform mathematicalfunction describing the response properties of a particular class ofmechanoreceptors.

This paper describes a controlled displacement stimnulator to whichseveral modifications have been added so as to be able to controlforce as well. The stimulator can be operated in the standard con-trolled displacement mode with force simu:ltaneously monitored orby using an additional force feedback loop the stimulator can alsodeliver controlled force stimuli. The stimulator is in principle similarto one previously described by Chubbuck [3] but differs in that itutilizes modern force transducers and readily available components.The design also permits those investigators who are currently usingmoving coil vibrators to easily add feedback control. A preliminarydescription of this instrument has previously been presented [4].

ELEMENTS AND DESIGN OF THE CONTROL SYSTEM

Fig. 1A shows a side view of the instrument. The stimulator ismounted on a micromanipulator so it can be easily positioned overany desired area of the skin. The electromechanical stimulator is amoving coil vibrator with a stimulating probe assembly attached tothe moving coil. Displacement is monitored with a Linear Variable

Manuscript received October 8, 1973; revised May 1, 1974. Thiswork was supported in part by N.I.H. Bioengineering Training GrantNIGMSGM 01066 and by P.H.S. Grantt NS 09361.The author was with the Department of Bioengineering, the Poly-

technic Institute of New York, Brooklyni, N. Y., and the Departmentof Neurobiology and Behavior, the Public Healtlh Research Institute ofthe City of New York, New York, N. Y. He is now with the Departmenitsof Physiology and Psychiatry, the College of Physicians and Surgeonsof Columbia University, New York, N. Y., and the Department ofMental Hygiene, the New York State Psychiatric Institute, New York,N. Y.

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, JANUARY 1975

A MICROMANIPULATOR

MOANG COILVIBRATOR

DISPLACEMENT TRANSOUCER (LVDT)

t- -FORCE TRAlNSD)UCERSTIMULATING PROBE

B

Fig. 1. A. Side view of the instrument used to stimulate the skinB. Block diagram of the control system. The instrument can beoperated in one of two modes: controlled displacement or controlledforce.

Differential Transformer (LVDT), whose core is connected to thestimulating probe assembly via the horizontal bracket shown. Forceis monitored by a miniature pressure transducer mounted on theend of the stimulating probe assembly. The actual probe used hereis 13 mm long, 500 ,um in diameter epoxy coated stainless steel rodwhich is attached to the diaphragm of the force transducer with asmall bead of epoxy. The instrument measures 16.5 cm from the topof the moving coil vibrator to the tip of the stimulating probe. Thevibrator is 6.4 cm in diameter.

Fig. 1B shows a block diagram of the control system and illustrateshow it can be operated in one of two configurations. In the controlleddisplacement mode an input voltage signal results in a displacementwhich is proportional to the signal and independent, within limits, ofthe force which the probe creates as it is displaced. In this mode theoutput of a function generator is applied to the input of the con-trolsystem and the output voltage of the displacement transducer iscompared to the input voltage signal so as to produce an error signal.The error signal is amplified and fed to the moving coil vibrator. Fordata analysis and oscilloscope display during experiments analogvoltages corresponding to the displacement and resultant force areprovided, In the controlled force mode an input voltage waveformresults in a force which is proportional to that waveform and inde-pendent, within limits, of the distance the probe must travel togenerate that force. The feedback scheme here is similar to that pre-viously described for the controlled displacement mode.

Dispiacement Transducer

An LVDT is an electromechanical transducer which produces anelectrical output proportional to the displcement of a separatemoveable core. I have used a LVDT (Schaevitz Engineering 050DCB) that provides an output voltage amplitude which is linearlyproportional to displacements up to 1200 ,m with a frequency re-sponse from dc to 500 Hz. The frequency response of the LVDT isdetermined by the frequency of a fixed excitation voltage and thetime constant of the demodulator RC filter. Since the reciprocal ofthe filter time constant is much smaller than the frequency of theexcitation, the filter time constant predominates and the LVDT canbe approximated by a first order lag network with a 3 dB break fre-quency at 500 Hz. This pole (P2) is illustrated in Fig. 2.

Force Transducer

The force transducer must be small and light, exhibit low diift,have a good frequency response, and have a diaphragm displacement

FORCEMONI,

(OSCILLSOOPE I TAPE)

COMMUNICATIONS

Z2 ZP

_2 , ~~~~~ZcP2-ci

Fig. 2. Root locus plot for the controlled displacement system. Pc andc are the complex poles of the moving coil vibrator and P2 iS the poleintroduced by the displacement transducer. Zc and Z2 are the zerosintroduced by the displacement compe-nsating network. As the forwardloop gain is increased the complex poles approach the negative real axis.

much less than the smallest displacement to be measured by theLVDT in the experimental situation. I used two different forcetransducers. Initially, a pressure sensitive transistor (Stow Labora-tories PT22) was employed. This device has the physical dimensionsof a TO-46 can, weighs 1/3 g, has a frequency response from dc to100 kHz and a full scale diaphragm displacement of 0.05 micra. Theparticular pressure sensitive transistor (Pitran) which was usedprovides an output voltage which is linearly proportional to pointforces up to approximately 15 g with forces as small as 0.1 g beingeasily resolved. The one severe limitation of the Pitran, especiallywhen the system is used in the controlled force mode is its high tem-perature coefficient which is typically 10% full scale output per °C.

Although the Pitran was operated in a temperature controlledenvironment, it was nevertheless required to continuously adjust thebias level. To avoid this difficulty a miniature pressure transducermanufactured by Sensotec (Sensotec MI6BWv-100 mm Hg) wasutilized in later experiments. The Sensotec has a temperature coeffi-cient of .02% full scale output per °C, similar physical dimensionsas the Pitran, a frequieney response fronm dc to 30 kHz and a fullscale diaphragm displacement of 38 pm. The one disadvantage of thisdevice is the large full scale diaphragm displacement which limits theaccuracy of displacement measurements made with the LVI)T.

Moving Coil Vibrator

The moving coil vibrator is a Ling Model 120 (Ling Electronics).This device has a 3Q coil and a total stroke length of 2.5 mm. Sincethe vibrator can be approximated by a second order spring, mass,damper system, published charts [5] of these systems can be used todetermine the vibrator pole location. With the LVDT and stimnulat-ing probe connected, step inputs were applied to the vibrator. Byobserving the maximum percent overshoot and peak time of theunderdamped response a complex pole (Pl,P1) at 32 i j 318 wasreadily calculated and plotted in Fig. 2. Incorporating the vibratorinto a conventional positional control system would only furtherdegrade the undesirable transient response, since any increase in theforward loop gain would cause a further outward migration of thecomplex pole. An alteration or adjustment of the control system inorder to provide a suitable performance was therefore necessary.

Displacement Compensating Network

When the locus of roots does not result in a suitable root configura-tion one must add a co'mpensating network in order to alter the locusof the roots as the gain is varied. Therefore one may utilize the rootlocus method [5], [6] and determine a suitable compensator networktransfer function so that the root locus results in the desired closed-loop configuration.

jw350

300

250

200

150

100

-0

iSO

200

250

300

350

jwLA-300- w

Fig. 3. Root locus plot of the instrument whlen operated in the con-trolled force mode. Pa and Pb are the significant poles of the con-trolled displacemeiit system with a fixed displacement forward loopgain. Zc and Z2 are the zeros of the displacement compensating net-work while P3 is the pole of the force compenisating network.

Since the primary concern here is to eliminate the vibrator'sunderdamped response while not degrading the rise time, it isdesirable to move the roots toward the negative real axis. As a mini-mum condition, a settling time. (ts) of 12 ms and a damping ratio of.7 was desired. Assuming that the complex poles dominate theresponse, the closed loop damping constant (¢wn) would thereforeequal 4/t8 or 333 rad/s. The addition of a lead compensating networkwill bring the roots to the desired damping ratio and wn. With thistype of compensation, -the zero of the compensating network is usu-ally placed directly below the desired root location and was thereforeset to 333 rad/s (Zc Fig. 2). An additional zero (Z2) was added tocancel the effect of the pole (P2) introduced by the LVDT. By plac-ing this zero near Z1 it also provides the root locus of the dominantpoles (P1,Pl) with some protection from the effect of any additionalunknown poles which miight lie on the negative real axis to the leftof Z2. It is clear fronm the root locus in Fig. 2 that the displacementtransient response can easily be adjusted by an appropriate settingof the displacement forward-loop gain.

Force Compensating NVetwork

With the roots of the displacement control system now fixed andwith the addition of force feedback, one can add an additional com-

pensating network to improve systenm perforniance. I have used a

standard lag type compensator. This network introduces a dominan'tpole near the jw axis and provides damping which helps reduce highfrequency oscillations arising from mechanical resonances in theprobe assembly. A simplified root locus plot of the controlled forcesystem is illustrated in Fig. 3. The new pole (P3) introduced is shownwith the compensator zeros (Zl,Z2) and the significant poles (Pa,Pb)of the controlled displacement system with fixed displacement for-ward-loop gain. It was assuimed the pole P2 (Fig. 2) moved suffi-ciently far on the negative real axis to neglect its effect. The rootlocus plot illustrates that a suitable da'mped transient response canbe simply obtained by adjusting the force forward-loop gain. Theuse of the root locus method assuiies a linear time-invariant system.While this assutmption is sounid for the conitrolled displacement loopit is not strictly valid for the control force system where negativeinputs cause the probe to uncoupled from the skin. The method isnevertheless usefuil in this case sinice it simplifies the analysis and

yields reasonable results for positive command inputs.

FABRICATION

A complete circuit diagram of the control system is shown in Fig. 4.Amplifiers Al to A5 are integrated circuit operational amplifiers(Burr Brown Type 3500 A) with amplifier A6 being a power opera-tional amplifier (Torque Systems Type PA-ill). When the Sensotec

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.- u3100 3000 Boo 700 500 400 300 200 too

pi

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, JANUARY 1975

.15M l K\XIOOK | ^47KK1-INPUT

; X~~~~~~~~~rOLLE FORE i

22K DISPLXEMENT FEEDOACK~~~~~~~~~~~~~~~~~~I

FORRC FEEEAKL~~~~~~~~~~~~~~~~~~FR ZERO 115 /)1-158

IOOKzi7 IOK 20K 68KlFIE POSITONADJUST

DISPLACEMENT, ..MONITOR

470K

FOR§CoE 3 3=* 15 - 15

Fig. 4. Schematic diagram of the stimulator electronics. Decimal values of capacitance are in uF; others are in pF, resist-ances are in U.

B1125 p

A CONTROLLED DISPLACEMENT

DGsploceFnent _

Force

2 CONTROLLED FORCE

-< _ ,f~~- -- I 125y

5p

Response _XIt |30 -V

Fig. 5. Intracellular recordings from two mechanoreceptor cells (Aand B) illustrating examples.of the stimulator operated in the con-trolled displacement and controlled force modes. A, and Bo are twoexamples from different experiments of controlled displacement steps.In A2 and B2 controlled force stimuli are delivered to the skin atidentical locations as the stimuli in A1 and B1.

force transducer is used, its output is fed to a differential Amplifierwith a gain of 10 (Analog Devices Type 146K). The output of thisamplifier is then connected to amplifier Al with the 470 kQ summingresistor reduced to 4.7 kQ. The instrument is powered by a +15V3.7 A power supply (ACDC Electronics type OA1SD3.7). The polesand zeros of the compensators are generated with the RC networksR1Cj (Z,), R2C2 (Z2), and R3C3 (P3). The capacitor in the feedbacknetwork of amplifiers Al, A3, A5 and A6 was added to reduce highfrequency noise. The circuit diagram also shows a fine position poten-tiometer which enables the stimulating probe to be precisely loweredto the structure which is to be stimulated. To protect the forcetransducer against excessive overloads a circuit may be added whichcauses the probe to be automatically retraeted when a preset forceis reached. In fabricating the stimulating probe assembly, care shouldbe exercised to minimize weight and to avoid mechanical resonances.

APPLICATIONS

I have used this stimulator to study the properties of mechano-receptor neurons of the marine mollusc Aplysia. Fig. 5 gives examplesfrom two experiments and illustrates the operation of the stimulator

in each of its two modes with the resultant neural discharge recordedfrom the cell body of a mechanoreceptor neuron (4,7,8). In thecontrolled displacement mode (Fig. 5Aj, B1) 4 s duration 5 ms risetime step indentations are applied to the skin. In both A1 and B1 theinitial force overshoots and then gradually decays to a steady statelevel. In the controlled force mode (Fig. 5A2, B2) step force stimuli4 s in duration with 20 ms rise times are applied to the skin. Here thesystem compensates for the mechanical properties of the skin bygenerating an early fast and then slower rate of rise in the initialportion of the displacement waveforms. Although the late phases ofthe neural response in both modes are similar in the controlled displace-ment mode, Fig. 5A, and B1 illustrate a higher initial discharge fre-quency which is most probably a result of the mechanoreceptorresponding to the initial force overshoot. Preliminary studies suggestthat the difference in the rise times (5 ms and 20 ms) cannot bediscriminated by the mechanoreceptors.The basic displacement control system presented here has also

been used in our laboratory to drive other vibrators. When strongerstimuli or heavier probe arrays are required the Ling Model 201vibrator has been used [9]. In these cases the circuit shown in Fig. 4has been modified by changing R2 to 15 kQ and C1 to .002 uf.

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L---J-.. I lg

t 1 30 mv

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ACKNOWLEDGMENT

I would like to thank Dr. V. Castellucci and Dr. E. Kandel formuch helpful discussion on all aspects of this work and Drs. J.Bongiorno, Jr., S. Deutsch and G. Weiss for reviewing an earlierdraft of this paper.

REFERENCES

[11 G. Werner and V. B. Mountcastle, "Neural activity in mechano-receptive cutaneous afferents: stimulus-response relations, Weberfunctions, and information transmission," J. Neurophysiology, vol.28, pp. 359-397, 1965.

[21 A. Iggo and A. R. Muir, "The structure and function of a slowlyadapting touch corpuscle in hairy skin," J. Physiology, vol. 200, pp.763-796, 1969.

[31 J. G. Chubbuck, "Small-motion biological stimulator," APL Tech-nical Digest, vol. 5, pp. 18-23, May-June 1966.

[4] J. Byrne, "Receptive fields and response properties of Aplysiamechanoreceptors," Ph.D. Thesis, Polytechnic Institute of NewYork, 1973.

15J S. C. Gupta and L. Hasdorff, Fundamentals oJ Automatic Control.New York: John Wiley, 1970.

[61 W. R. Evans, "Control system synthesis by root locus method,"AIEE Trans., vol. 69, pp. 66-69, 1950.

17] J. Byrne, V. Castellucci and E. R. Kandel, "Receptive fields responseproperties of mechanoreceptor neurons innervating siphon skin andmantle shelf in Aplysia," J. Neurophysiol., vol. 37, pp. 1041-1064,1964.

[8] V. Castellucci, H. Pinsker, T. Kupfermann and E. R. Kandel, "Neu-ronal mechanisms of habituation and dishabituation of the gill-withdrawal reflex in Aplysia," Science. vol. 167, pp. 1745-1748, 1970.

19] E. P. Gardner and W. A. Spencer, "Sensory funneling I. Psycho-physical observations of human subjects and responses of cutaneousmechanoreceptive afferents in the cat to patterned skin stimuli,"J. Neurophysiology, vol. 35, pp. 925-953, 1972.

An Inexpensive Device for Determining Sinus NodeFunction and the Refractory Periods of Cardiac Conduction

Systems

N. M. SCHMITT, MEMBER, IEEE, It. V. BOYD,AND JOE K. BISSETT

Abstract-A simple, inexpensive device capable of measuringthe refractory period of the cardiac conducting system has beendescribed. All components used in the device may be purchasedfor less than $150 and commercial equipment capable of performingsimilar tasks costs over two-thousand dollars. The undesiredfeatures of previous methods have been eliminated and resultsof using the device in clinical settings have been given.

I. INTRODUCTION

The introduction of a catheter ;method [1], 2] for recordingactivity of the specialized conduction system in man has stimulatednew interest in the electrophysiology of cardiac conduction. Elec-trical activity from the bundle of His has been used to separate thePR interval of the scalar electrocardiogram into a segment repre-senting conduction between the sinus and A-V nodes, and an intervalrepresenting conduction in the ventricular specialized conductingsystem [31, [4]. According to standard terminology these measure-ments are referred to as the P-H interval representing conductiontime from the P-wave of the scalar electrocardiogram to the Hisbundle potential and the H to V interval representing the conductiontime from the His bundle to the onset of the QRS complex in theHis bundle electrogram or scalar electrocardiogram [1]. The humanatrioventricular (A-V) conducting system has been further definedthrough the studies of Wit et al. by the introduction of prematureatrial beats [5]. The response of the human conducting system tothe introduction of premature atrial beats results in the development

Manuscript received March 12, 1974; revised August 6, 1974.N. M. Schmitt and R. V. Boyd are with the Department of Electrical

Engi-neering, University of Arkansas, Fayetteville, Ark. 72701.J. K. Bissett is with the Veterans Administration, Little Rock, Ark.

72201.

of conduction delay both above and below the His bundle as theimpulse becomes progressively more premature. The initial pro-longation of P to H time produced by premature atrial beats follow-ing a basic drive stimulus is referred to as the relative rcfractoryperiod of the A-V node. The effective refractory period of the A-Vnode refers to the latest atrial premature beat which fails to conductto the His bundle. The functional refractory period of the A-Vnode is defined as the minimal interval between two successive Hisbundle responses both propagated from the atrium. Measurementsmade utilizing His bundle recordings have permitted separationof the refractory periods of the A-V node from those of the ventricu-lar specialized conducting systems [5].The response of the atrioventricular conducting system to the

introduction of premature atrial beats has been utilized in the studyof clinical arrhythmias. Bigger and colleagues have demonstratedthe importance of a delay in atrioventricular conduction tirne pro-duced by premature atrial beats in the initiation of paroxysmalsuperventriculac tachycardia [6] [7]. Other investigators havedemonstrated the importance of this mechanism in the initiationof paroxysmal tachycardia in the Wolff-Parkinson-White syndrome[8]. Additional studies have defined defects in the A-V conductingsystem in patients with clinical electrocaridographic abnormalities[9].The results of these investigations have demonstrated the impor-

tance of information gained through electrical physiological studyof the conducting system. The limitations of this method, however,have included the need for costly and complex equipment to intro-duce premature atrial stimuli. In most instances determinationof the refractory periods in the human A-V conducting system hasrequired the utilization of 2 coupled pulse generators with timedrelay circuits to inhibit the drive stimuli after the premature stimulito facilitate measurement [8]. This has the added disadvantage ofcapturing the heart rate and pacing at a higher than normal rate.

This communication describes an inexpensive device which deter-mines the function of the sinus node and refractory periods of theA-V conducting system by the introduction of single timed prematureatrial contractions [10], [11].

II. DESIGN CRITERIA

In studying the response of the atrioventricular conducting systemto the introduction of premature atrial beats, the desired measure-ment is that period of time between the R-wave peak and the nextclosest point where an artificially applied stimulus (through anelectrode implanted in the right atrium) no longer causes a pre-mature ventricular contraction. This is the effective refractcryperiod of the A-V node. Further, it is desirable to wait at least 8-10beats before applying a new stimulus. An accuracy of ±2 msecis desired to permit scanning of the entire cardiac cycle to sequen-tially measure the refractory periods of the atrium, A-V node, andventricular conducting systems. Also, alignment of the stimulatingpulse in reference to the patient's ECG before actual stimulationoccurs is necessary. Output pulse amplitude should vary between3 and 8 volts and pulsewidth between 0.5 and 4.0 milliseconds.Leakage current must be less than 10 imicroamps. A normal heartbeating at 70 BPM has a TP interval of 150-200 ms but becausethe heart rate exhibits a wide variation among individuals thelocation of the stimulating pulse must be adjustable over a 500millisecond range. The input to the device is provided through ECGsurface electrodes (usually a modified LEAD II or V connection)positioned such that the R-wave is large with respect to the P andT waves.

III. DEVICE DESCRIPTION

The device was designed to stimulate the myocardium with asingle pulse at a preset time delay following the R wave. The myo-cardium was then repeatedly stimulated with decreasing shortertime delays until the stimulus no longer produced a premature

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