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66 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, JANUARY 1975 A A Feedback Controlled Stimulator That Delivers Controlled Displacements or Forces to Cutaneous Mechanoreceptors JOHN BYRNE, MICROMANIPULATOR MOANG COILVIBRATOR MEMBER, IEEE Abstract-Based on a design by the root locus method and utilizing both displacement and force feedback an instrument is described which can be used to deliver either controlled displacement or controlled force stimuli to the skin. Examples of the stimulator operated in each of its two modes are given. DISPLACEMENT TRANSOUCER (LVDT) t- -FORCE TRAlNSD)UCER STIMULATING PROBE B INTRODUCTION To study the neural activity elicited by mechanical stimulation of the skin, a tactile stimuli must not only simulate natural stimuli encountered in everyday life but also must be precisely controlled. Of the several stimulators which have been used in electrophysioMONI, (OSCILLSOOPE TAPE) logical investigations of mechanoreceptors the moving coil vibrator has proven to be particularly useful because it can be precisely Fig. 1. A. Side view of the instrument used to stimulate the skin B. Block diagram of the control system. The instrument can be controlled. To deliver controlled displacement stimuli to cutaneous operated in one of two modes: controlled displacement or controlled force. mechanoreceptors several investigators have combined moving coil vibrators with displacement feedback [1], [2]. These systems have, however, suffered from several drawbacks. One difficulty is that one Differential Transformer (LVDT), whose core is connected to the constantly needs to adjust the stimulating probe height to compenvia the horizontal bracket shown. Force stimulating probe assembly sate for cupping of the skin with repeated stimulation. In addition a relevant physiological parameter, the force delivered to the mechano- is monitored by a miniature pressure transducer mounted on the receptor is not controlled but varies depending upon whether the end of the stimulating probe assembly. The actual probe used here 13 mm long, 500 ,um in diameter epoxy coated stainless steel rod mechanoreceptor lies above soft tissue or above bony structure. The iswhich is attached to the diaphragm of the force transducer with a significance of this effect was- illustrated by Werner and Mountcastle small bead of epoxy. The instrument measures 16.5 cm from the top [1]. When displacement was used as an independent variable the the vibrator to the tip of the stimulating probe. The of moving typical power function describing the mechanoreceptor stimulus- vibrator is 6.4 coil cm in diameter. response relationship was not observed for mechanoreceptors over Fig. 1B shows a block diagram of the control system and illustrates bone. The same study showed that in general when force instead of how it can be operated in one of two configurations. In the controlled displacement was used as the stimulus parameter the stimulus- displacement mode an input voltage signal results in a displacement response relationship tended to be linear. The use of force as the to the signal and independent, within limits, of is which proportional stimulus parameter would therefore not only minimize the effects which the force creates as it is displaced. In this mode the the probe over various skin of stimulating areas with different mechanical a function generator is applied to the input of the con-trol of output properties but may also lead to a single uniform mathematical output voltage of the displacement transducer is function describing the response properties of a particular class of system andto the the compared input voltage signal so as to produce an error signal. mechanoreceptors. is The and fed to the moving coil vibrator. For error signal amplified This paper describes a controlled displacement stimnulator to which and data analysis oscilloscope display during experiments analog several modifications have been added so as to be able to control to the and resultant force are voltages corresponding displacement force as well. The stimulator can be operated in the standard controlled displacement mode with force simu:ltaneously monitored or provided, In the controlled force mode an input voltage waveform by using an additional force feedback loop the stimulator can also results in a force which is proportional to that waveform and indedeliver controlled force stimuli. The stimulator is in principle similar pendent, within limits, of the distance the probe must travel to to one previously described by Chubbuck [3] but differs in that it generate that force. The feedback scheme here is similar to that preutilizes modern force transducers and readily available components. viously described for the controlled displacement mode. The design also permits those investigators who are currently using moving coil vibrators to easily add feedback control. A preliminary Dispiacement Transducer description of this instrument has previously been presented [4]. An LVDT is an electromechanical transducer which produces an electrical output proportional to the displcement of a separate ELEMENTS AND DESIGN OF THE CONTROL SYSTEM moveable core. I have used a LVDT (Schaevitz Engineering 050 an output voltage amplitude which is linearly Fig. 1A shows a side view of the instrument. The stimulator is DCB) that provides to proportional displacements up to 1200 ,m with a frequency remounted on a micromanipulator so it can be easily positioned over response of the LVDT is any desired area of the skin. The electromechanical stimulator is a sponse from dc to 500 Hz. The frequency a determined of fixed excitation the by frequency voltage and the moving coil vibrator with a stimulating probe assembly attached to time constant of the demodulator RC filter. Since the reciprocal of the moving coil. Displacement is monitored with a Linear Variable the filter time constant is much smaller than the frequency of the excitation, the filter time constant predominates and the LVDT can Manuscript received October 8, 1973; revised May 1, 1974. This be approximated by a first order lag network with a 3 dB break frework was supported in part by N.I.H. Bioengineering Training Grant quency at 500 Hz. This pole (P2) is illustrated in Fig. 2. NIGMSGM 01066 and by P.H.S. Grantt NS 09361. The author was with the Department of Bioengineering, the Polytechnic Institute of New York, Brooklyni, N. Y., and the Department of Neurobiology and Behavior, the Public Healtlh Research Institute of Force Transducer the City of New York, New York, N. Y. He is now with the Departmenits of Physiology and Psychiatry, the College of Physicians and Surgeons of Columbia University, New York, N. Y., and the Department of The force transducer must be small and light, exhibit low diift, Mental Hygiene, the New York State Psychiatric Institute, New York, have a good frequency response, and have a diaphragm displacement N. Y. FORCE I 67 COMMUNICATIONS jw ZP Z2 350 300 250 200 150 100 P2 -ci 3100 ~~~~~Zc _2 , .- 3000 Boo 700 500 400 u 300 200 too -0 iSO 200 250 300 pi 350 jwLA Fig. 2. Root locus plot for the controlled displacement system. Pc and c are the complex poles of the moving coil vibrator and P2 iS the pole introduced by the displacement transducer. Zc and Z2 are the zeros introduced by the displacement compe-nsating network. As the forward loop gain is increased the complex poles approach the negative real axis. much less than the smallest displacement to be measured by the LVDT in the experimental situation. I used two different force transducers. Initially, a pressure sensitive transistor (Stow Laboratories PT22) was employed. This device has the physical dimensions of a TO-46 can, weighs 1/3 g, has a frequency response from dc to 100 kHz and a full scale diaphragm displacement of 0.05 micra. The particular pressure sensitive transistor (Pitran) which was used provides an output voltage which is linearly proportional to point forces up to approximately 15 g with forces as small as 0.1 g being easily resolved. The one severe limitation of the Pitran, especially when the system is used in the controlled force mode is its high temperature coefficient which is typically 10% full scale output per °C. Although the Pitran was operated in a temperature controlled environment, it was nevertheless required to continuously adjust the bias level. To avoid this difficulty a miniature pressure transducer manufactured by Sensotec (Sensotec MI6BWv-100 mm Hg) was utilized in later experiments. The Sensotec has a temperature coefficient of .02% full scale output per °C, similar physical dimensions as the Pitran, a frequieney response fronm dc to 30 kHz and a full scale diaphragm displacement of 38 pm. The one disadvantage of this device is the large full scale diaphragm displacement which limits the accuracy 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. Since the vibrator can be approximated by a second order spring, mass, damper system, published charts [5] of these systems can be used to determine the vibrator pole location. With the LVDT and stimnulating probe connected, step inputs were applied to the vibrator. By observing the maximum percent overshoot and peak time of the underdamped response a complex pole (Pl,P1) at 32 i j 318 was readily calculated and plotted in Fig. 2. Incorporating the vibrator into a conventional positional control system would only further degrade the undesirable transient response, since any increase in the forward loop gain would cause a further outward migration of the complex pole. An alteration or adjustment of the control system in order to provide a suitable performance was therefore necessary. Displacement Compensating Network When the locus of roots does not result in a suitable root configuration one must add a co'mpensating network in order to alter the locus of the roots as the gain is varied. Therefore one may utilize the root locus method [5], [6] and determine a suitable compensator network transfer function so that the root locus results in the desired closedloop configuration. -300 - w Fig. 3. Root locus plot of the instrument whlen operated in the controlled force mode. Pa and Pb are the significant poles of the controlled displacemeiit system with a fixed displacement forward loop gain. Zc and Z2 are the zeros of the displacement compensating network while P3 is the pole of the force compenisating network. Since the primary concern here is to eliminate the vibrator's underdamped response while not degrading the rise time, it is desirable to move the roots toward the negative real axis. As a minimum condition, a settling time. (ts) of 12 ms and a damping ratio of .7 was desired. Assuming that the complex poles dominate the response, the closed loop damping constant (¢wn) would therefore equal 4/t8 or 333 rad/s. The addition of a lead compensating network will bring the roots to the desired damping ratio and wn. With this type of compensation, -the zero of the compensating network is usually placed directly below the desired root location and was therefore set to 333 rad/s (Zc Fig. 2). An additional zero (Z2) was added to cancel the effect of the pole (P2) introduced by the LVDT. By placing this zero near Z1 it also provides the root locus of the dominant poles (P1,Pl) with some protection from the effect of any additional unknown poles which miight lie on the negative real axis to the left of Z2. It is clear fronm the root locus in Fig. 2 that the displacement transient response can easily be adjusted by an appropriate setting of the displacement forward-loop gain. Force Compensating NVetwork With the roots of the displacement control system now fixed and with the addition of force feedback, one can add an additional compensating network to improve systenm perforniance. I have used a standard lag type compensator. This network introduces a dominan't pole near the jw axis and provides damping which helps reduce high frequency oscillations arising from mechanical resonances in the probe assembly. A simplified root locus plot of the controlled force system is illustrated in Fig. 3. The new pole (P3) introduced is shown with the compensator zeros (Zl,Z2) and the significant poles (Pa,Pb) of the controlled displacement system with fixed displacement forward-loop gain. It was assuimed the pole P2 (Fig. 2) moved sufficiently far on the negative real axis to neglect its effect. The root locus plot illustrates that a suitable da'mped transient response can be simply obtained by adjusting the force forward-loop gain. The use of the root locus method assuiies a linear time-invariant system. While this assutmption is sounid for the conitrolled displacement loop it is not strictly valid for the control force system where negative inputs cause the probe to uncoupled from the skin. The method is nevertheless 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 operational amplifier (Torque Systems Type PA-ill). When the Sensotec IEEE TRANSACTIONS ON BIOMEDICAL 68 .15M K\XIOOK l 47K K1-^ | INPUT ; ENGINEERING, JANUARY 1975 i X~~~~~~~~~rOLLE FORE DISPLXEMENT FEEDOACK~~~~~~~~~~~~~~~~~~I 22K FORRC FEEEAK L~~~~~~~~~~~~~~~~~~FR ZERO 20K IOK zi7 IOOK FIE POSITONADJUST 115 1-158 /) 68Kl 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, resistances are in U. B 1125 p L---J-.. I lg 1 30 mv t A DGsploceFnent CONTROLLED DISPLACEMENT _ 2 CONTROLLED FORCE -< _ ,f~~- -- I 125 y 5p Force Response _XIt |30 -V Fig. 5. Intracellular recordings from two mechanoreceptor cells (A and B) illustrating examples.of the stimulator operated in the controlled displacement and controlled force modes. A, and Bo are two examples from different experiments of controlled displacement steps. In A2 and B2 controlled force stimuli are delivered to the skin at identical locations as the stimuli in A1 and B1. force transducer is used, its output is fed to a differential Amplifier with a gain of 10 (Analog Devices Type 146K). The output of this amplifier is then connected to amplifier Al with the 470 kQ summing resistor reduced to 4.7 kQ. The instrument is powered by a +15V 3.7 A power supply (ACDC Electronics type OA1SD3.7). The poles and zeros of the compensators are generated with the RC networks R1Cj (Z,), R2C2 (Z2), and R3C3 (P3). The capacitor in the feedback network of amplifiers Al, A3, A5 and A6 was added to reduce high frequency noise. The circuit diagram also shows a fine position potentiometer which enables the stimulating probe to be precisely lowered to the structure which is to be stimulated. To protect the force transducer against excessive overloads a circuit may be added which causes the probe to be automatically retraeted when a preset force is reached. In fabricating the stimulating probe assembly, care should be exercised to minimize weight and to avoid mechanical resonances. APPLICATIONS I have used this stimulator to study the properties of mechanoreceptor neurons of the marine mollusc Aplysia. Fig. 5 gives examples from two experiments and illustrates the operation of the stimulator in each of its two modes with the resultant neural discharge recorded from the cell body of a mechanoreceptor neuron (4,7,8). In the controlled displacement mode (Fig. 5Aj, B1) 4 s duration 5 ms rise time step indentations are applied to the skin. In both A1 and B1 the initial force overshoots and then gradually decays to a steady state level. In the controlled force mode (Fig. 5A2, B2) step force stimuli 4 s in duration with 20 ms rise times are applied to the skin. Here the system compensates for the mechanical properties of the skin by generating an early fast and then slower rate of rise in the initial portion of the displacement waveforms. Although the late phases of the neural response in both modes are similar in the controlled displacement mode, Fig. 5A, and B1 illustrate a higher initial discharge frequency which is most probably a result of the mechanoreceptor responding to the initial force overshoot. Preliminary studies suggest that the difference in the rise times (5 ms and 20 ms) cannot be discriminated by the mechanoreceptors. The basic displacement control system presented here has also been used in our laboratory to drive other vibrators. When stronger stimuli or heavier probe arrays are required the Ling Model 201 vibrator has been used [9]. In these cases the circuit shown in Fig. 4 has been modified by changing R2 to 15 kQ and C1 to .002 uf. 69 COMMUNICATIONS ACKNOWLEDGMENT I would like to thank Dr. V. Castellucci and Dr. E. Kandel for much helpful discussion on all aspects of this work and Drs. J. Bongiorno, Jr., S. Deutsch and G. Weiss for reviewing an earlier draft of this paper. REFERENCES [11 G. Werner and V. B. Mountcastle, "Neural activity in mechanoreceptive cutaneous afferents: stimulus-response relations, Weber functions, and information transmission," J. Neurophysiology, vol. 28, pp. 359-397, 1965. [21 A. Iggo and A. R. Muir,in"The structureJ. and function of a slowly adapting touch corpuscle hairy skin," Physiology, vol. 200, pp. 763-796, 1969. [31 J. G. Chubbuck, "Small-motion biological stimulator," APL Technical Digest, vol. 5, pp. 18-23, May-June 1966. [4] J. Byrne, "Receptive fields and response properties of Aplysia mechanoreceptors," Ph.D. Thesis, Polytechnic Institute of New York, 1973. S. C. Gupta and L. Hasdorff, Fundamentals oJ Automatic Control. 15J New York: John Wiley, 1970. W. R. Evans, "Control system synthesis by root locus method," [61 AIEE Trans., vol. 69, pp. 66-69, 1950. Kandel, "Receptive fields response 17] J. Byrne, V.ofCastellucci and E. R. properties mechanoreceptor neurons innervating siphon skin and mantle shelf in Aplysia," J. Neurophysiol., vol. 37, pp. 1041-1064, 1964. [8] V. Castellucci, H. Pinsker, T. Kupfermann and E. R. Kandel, "Neuronal mechanisms of habituation and dishabituation of the gillwithdrawal reflex in Aplysia," Science. vol. 167, pp. 1745-1748, 1970. 19] E. P. Gardner and W. A. Spencer, "Sensory funneling I. Psychophysical observations of human subjects and responses of cutaneous mechanoreceptive afferents in the cat to patterned skin stimuli," J. Neurophysiology, vol. 35, pp. 925-953, 1972. An Inexpensive Device for Determining Sinus Node Function 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 measuring the refractory period of the cardiac conducting system has been described. All components used in the device may be purchased for less than $150 and commercial equipment capable of performing similar tasks costs over two-thousand dollars. The undesired features of previous methods have been eliminated and results of using the device in clinical settings have been given. I. INTRODUCTION The introduction of a catheter ;method [1], 2] for recording activity of the specialized conduction system in man has stimulated new interest in the electrophysiology of cardiac conduction. Elec- trical activity from the bundle of His has been used to separate the PR interval of the scalar electrocardiogram into a segment representing conduction between the sinus and A-V nodes, and an interval representing conduction in the ventricular specialized conducting system [31, [4]. According to standard terminology these measurements are referred to as the P-H interval representing conduction time from the P-wave of the scalar electrocardiogram to the His bundle potential and the H to V interval representing the conduction time from the His bundle to the onset of the QRS complex in the His bundle electrogram or scalar electrocardiogram [1]. The human atrioventricular (A-V) conducting system has been further defined through the studies of Wit et al. by the introduction of premature atrial beats [5]. The response of the human conducting system to the 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 University of Arkansas, Fayetteville, Ark. 72701. Engi-neering, J. K. Bissett is with the Veterans Administration, Little Rock, Ark. 72201. of conduction delay both above and below the His bundle as the impulse becomes progressively more premature. The initial prolongation of P to H time produced by premature atrial beats following a basic drive stimulus is referred to as the relative rcfractory period of the A-V node. The effective refractory period of the A-V node refers to the latest atrial premature beat which fails to conduct to the His bundle. The functional refractory period of the A-V node is defined as the minimal interval between two successive His bundle responses both propagated from the atrium. Measurements made utilizing His bundle recordings have permitted separation of the refractory periods of the A-V node from those of the ventricular specialized conducting systems [5]. The response of the atrioventricular conducting system to the introduction of premature atrial beats has been utilized in the study of clinical arrhythmias. Bigger and colleagues have demonstrated the importance of a delay in atrioventricular conduction tirne produced by premature atrial beats in the initiation of paroxysmal superventriculac tachycardia [6] [7]. Other investigators have demonstrated the importance of this mechanism in the initiation of paroxysmal tachycardia in the Wolff-Parkinson-White syndrome [8]. Additional studies have defined defects in the A-V conducting system in patients with clinical electrocaridographic abnormalities [9]. The results of these investigations have demonstrated the importance of information gained through electrical physiological study of the conducting system. The limitations of this method, however, have included the need for costly and complex equipment to introduce premature atrial stimuli. In most instances determination of the refractory periods in the human A-V conducting system has required the utilization of 2 coupled pulse generators with timed relay circuits to inhibit the drive stimuli after the premature stimuli to facilitate measurement [8]. This has the added disadvantage of capturing the heart rate and pacing at a higher than normal rate. This communication describes an inexpensive device which determines the function of the sinus node and refractory periods of the A-V conducting system by the introduction of single timed premature atrial contractions [10], [11]. II. DESI GN CRITERIA In studying the response of the atrioventricular conducting system to the introduction of premature atrial beats, the desired measurement is that period of time between the R-wave peak and the next closest point where an artificially applied stimulus (through an electrode implanted in the right atrium) no longer causes a premature ventricular contraction. This is the effective refractcry period of the A-V node. Further, it is desirable to wait at least 8-10 beats before applying a new stimulus. An accuracy of ±2 msec is desired to permit scanning of the entire cardiac cycle to sequentially measure the refractory periods of the atrium, A-V node, and ventricular conducting systems. Also, alignment of the stimulating pulse in reference to the patient's ECG before actual stimulation occurs is necessary. Output pulse amplitude should vary between 3 and 8 volts and pulsewidth between 0.5 and 4.0 milliseconds. Leakage current must be less than 10 imicroamps. A normal heart beating at 70 BPM has a TP interval of 150-200 ms but because the heart rate exhibits a wide variation among individuals the location of the stimulating pulse must be adjustable over a 500 millisecond range. The input to the device is provided through ECG surface electrodes (usually a modified LEAD II or V connection) positioned such that the R-wave is large with respect to the P and T waves. III. DEVICE DESCRIPTION The device was designed to stimulate the myocardium with a single pulse at a preset time delay following the R wave. The myocardium was then repeatedly stimulated with decreasing shorter time delays until the stimulus no longer produced a premature