Download Controlled Stimulator That Delivers Con- A trolled

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