rubber film. Using a subject and with the probe
open-circuited (not on scale) 60 volts and 350
ma. was read, rubber covered; and 52 volts and
350 ma. with the probe uncovered. Under normal
test conditions with the probe not touching (0.4
meter reading) 5.2 volts and 36 ma. was read,
rubber, covered; and 5.2 volts and 36 ma., un-
covered. With the probe making firm contact
with the chest wall 6.4 volts and 40 ma, was read,
rubber covered; and 1 volt and 66 ma. uncovered
and not motorboating. Under this condition of
touching the subject with the probe rubber
covered and uncovered, the carrier frequency
shifts from the 50 KHz normal operating fre-
quency to 178 KHz. In addition, with the probe
uncovered, and touching the subject, an unstable
condition occurs in which the instrument starts
motorboating at 6.6 Hz, 6 volts and 90 ma. is
read and a shock is reported.

It must be pointed out that this instrument
was designed for industrial applications. Its use
in this study was primarily for research purposes
and in the hands of inexperienced or untutored
personnel, this device may be hazardous, particu-

larly if the subject or patient is also connected to
other medical instrumentation.

VII. Summary

We have described a capacitive displacement
transducer with a linear response and constant
sensitivity for a frequency range of 0-1,000 Hz.
Its application to measurement of chest wall
motions was verified using static displacements
from flat and curved surfaces and both metal
and human tissue.

The transducer has been used to obtain record-
ings of apex motion, heart sounds, brachial and
radial pulses.

Its advantages are that it is noncontacting and
linear; that it has a wide frequency response and
can easily be calibrated to obtain quantitation
of actual motion.

Problems encountered in use were the mechani-
cal positioning of the probe including respiratory
motions in the measurement and the need to
insulate the probe with a film of thin plastic to
insure against any electrical hazards to the
subject.



Phono

ACG

Carotid

ECG

-J4

Ficure 5. An apexcardiogram (ACG) taken with a conventional piezoelectric transducer. ECG, Carotid and Phono
taken with conventional tranducers.

BRACHIAL

Figure 6. Brachial artery pulse and its derivative 4,.

Phono taken by piezo-electric transducer.

8


Appendix B

Range of Probes:

Six probes are available for use with the in-
strument. The standard probes have a flat,
circular inner electrode surrounded by a guard
ring. The electrode and guard ring are sepa-
rated by an insulating sleeve. The probes are
screwed into a probe holder mounted on the end
of a coaxial cable.

The range of the probes for the instrument is:

Normal FSD
Probe | Bffective prove radius | (Distance and Vibration)
‘Inches Inches ‘Millimeters

A 0.0223 6.001, 0.025,

B 0.050 0.005, 0,125

Cc 0.0707 0.010 0.25,

D 0.158 0.050 1.25

E 0.223, 0.100 2.50

F 0.500 0.500 12.5

It should be noted that full scale deflection
(FSD) of the meter corrésponds to 1 volt at the
output of the filter. Therefore, the sensitivity
for the F Probe is 1 volt per 0.500 inches or 2
volts per inch deflection over the range of the
probe, i.e., 0 to 0.500 inches. The sensitivity for
the D Probe, one particularly useful for chest
wall motions (apexcardiograms), jis 1 volt per

0.050 inches or 20 volts per inch over the range
0 to 0.050 inches.

BIBLIOGRAPHY

4. Compara, Damian, Bulletin De V’Acad. Pol. d Se.
et al., Litt., CLd. Mid. 1949.

2. Cauesus, H. Early stage of arteriosclerosis in cardio-
grams, 1959, Warsaw State Enterprise of Medical
Publications, U.S. Joint Publications Research Service
R-1995-D/B Dee. 5, 1961, p. 1-81.

8, Berson and PIPBERGER. Measurements of chest wall
vibrations due to the activity of the heart. Journal
of Applied Physiology 21(2) March 1966.

4, Date Groom, Leonardo H. MEDINA, Y. T. SIRVONEN.
The proximity transducer, The American Journal of
Medical Electronics, October-December 1964, p. 261-
265.

5, A. Bencurmon and E, G. DiaMonp. ‘The normal and
abnormal apexcardiogram. Its physiologic variation
and its relation to intracardiac events. American
Journal of Cardiology September 1963, p. 368-382.

6. J. N. Berry, A new method for recording cardiac,
hepatic and other pulsations, movements and thrills.
American Heart Journal, January 1966, p. 16-25.

7. The recording of heart sounds and vibrations, I.
Historical Review and description of a new electronic
direct-contact vibration pickup. Exp. Med. Surg. 14,
239-254, 1955.

8. Lyre Pererson, Methods in medical research VII,
Robert Rushmer, editor, Year Book Medical Pub-
lishers, p. 3-4, 1966.



Appendix A

See Fig. 1.
WHERE:
V:=Source potential.
V.=Amplifier output.
R =Grid resistor of amplifier input stage.
C,=Standard capacitance (of reactance X,).
C.=Probe/structure capacitance (unknown, of
reactance X,).
i, =Current passed by Standard.
is=Current fed back by probe capacitance
structure.
e =Potential across R.
A =Amplifier gain,
THEN:
Vo=A-eand e=(i,—ig) -R.
A>>I and C, is comparable with C,.
THEREFORE:
e>0 and hence, for finite R, (ii—i:)->0, that
is, =i.
NOW:
Voir: Xu (if e=0).

AND:
Vi=ii- Xs (if e=0).
Vo ik Xi
Vi i! OU
BUT:
i;=io-*» Vo=V, - Su
ip=ig Vo=HVi oa

BUT:
V, and X, are constant.

PUTTING:
K for V./Xs, Vo=K - Xu.
NOW:
X, =sb and C, « 1/d, where d is the plate
separation.
V.=K’d where K’ is a new constant of pro-
portionality.
The amplifier output (V.) is thus directly
proportional to the probe/structure separation
(d) and is independent of the amplifier gain.



mv/mm DEFLECTION

200
175
150
125
100

a}

50

25

EFFECT OF CURVED SURFACE ON LINERITY

0.100 .°)
a é
090} 6
a)
o80F n
. é
6
an | ‘ @
a
Pe
2 .060F
gS
ge 050
ws
2
z 0 @ FLAT
a DIAMETER
Sasol a 1.2cm CURVED
‘ = 10.5cm CURVED
a © 16.5cm CURVED
020 , 6
6
ooo
0 8 1 r L n n n L L n
0 0010 020 030.040 050-060 070 080-090 0.100
DIAL GAUGE
Ficure 8. Effect of curved surface on linearity.
PROBE DI-1140
VIBRATION METER B731A
2 OCT 64 CAL at DTMB
e +1
- 2» _g qo
° 7
e
rears pl eer
2 3 4 5678910 2 3 4 5 6789100 2 3 4 5 67891000

Ficure 4, Frequency response characteristics.

7

Odb

DECIBELS re.185 mv



AM 68-3

BIOMEDICAL APPLICATIONS OF A COMMERCIAL
CAPACITANCE TRANSDUCER

Edward Podolak
James B. Kinn
Edwin E. Westura, M.D.

Approved by Released by

°o
wd
P. V. Srecet, M.D.
Feverau Air Surcron

She! A “N\e-hhe

Sranuey‘R. Mouter, M.D.
Cuzer, Auromeprican APPLICATIONS
Division

March 1968

DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
Office of Aviation Medicine



‘os Ul UMOYS eqold ‘6 BEND

10


HEETPEEEEEY Ish
Phono . . .

dy he AJ

i
Displacement 4
Apexcardiogram
| |
d, la My IK
MT
Carotid ‘

eco i a “—~

Fiaure 7. A displacement apexcardiogram (capacitance transducer) with its first d, and second d, derivatives.
The carotid pulse and phono taken with conventional transducers.

ene ed oe ee > Se
SI .

Phono AN J Nw .

3.62mm/sec. i :

Qe ¥ anne BRGREEEEE

Displacement \ | |

Apexcardiogram | : iP '
|
|
i

d
2

ae

Carotid
Acceleration

ECG —~ ——

Freure 8. Comparison of the second derivative d, of the displacement apexcardiogram
with the accelerometer output.

9



v,

IL
Lal
Vip
C;, is
Ie r AMPLIFIER
R rf GAIN A Vo
PO

STANDARD C; HAS REACTANCE Xs
UNKNOWN C, HAS REACTANCE X,

DISTANCE METER READING
(INCHES)

SCHEMATIC DIAGRAM

Ficure 1, Simplified schematic diagram.

EFFECT OF TISSUE, CALIBRATION USING FINGER

020030 040 050060070080 100
DIAL GAUGE (INCH)

Ficure 2. Effect of tissue, calibration using finger.

6


These tests verify the use of this device for
this application. The obvious lack of any load-
ing feedback to the motion under study is as-
sured because of the non-contacting feature. High
magnetic fields had no measurable effect on the
device’s performance.

IV. Dynamic Characteristics

The frequency response characteristics were
determined at the U.S. Navy Ship Research and
Development Center in Washington, D.C., using
their shake tables. The results of this test are
shown in Figure 4. The motion of the top of a
calibrated accelerometer mounted on the shake
tables was monitored and based on its calibration,
the peak-to-peak displacement was calculated. In
order to determine that the top of the accele-
rometer followed the motion of the table, a meas-
urement to the table was made at 700 Hz as
shown in the figure. The fall off of the response
at the high end is due to a 1,000 Hz low-pass
filter used in the output circuit of the vibration
meter, and the low-end calibration was limited
by the frequency characteristics of the shake table
itself.

V. Results

The major applications of this device investi-
gated thus far have been measurements of chest
wall motion at the apex, point of maximum
precordial impulse (apexcardiogram). <A typical
apexcardiogram (ACG) taken with a piezoelec-
tric transducer is shown in Fig. 5, where the
ACG, carotid pulse and phono were taken with
conventional transducers. A displacement apex-
cardiogram of the same subject taken with the
capacitance device is shown in Fig. 7 along with
its first and second derivatives. Brachial arterial
pulses (capacitance transducer) are shown in Fig.
6. No difficulty was experienced in measuring
vessel pulses in both the brachial and radial
arteries. Good results are dependent largely on
patient cooperation and experience in placement
of the probe.

Because of wide dynamic range and frequency
response, it is possible to place the probe at the
apex and with the subject breathing obtain a
respiration tracing with a small variation super-
imposed on it (the apexcardiogram). Then by
having the subject hold his breath and then in-
creasing the gain of our recorder we could mag-
nify this apexcardiogram and eliminate the curve
of respiration. Also, with and without breathing

depending on the range of the probe, low fre-
quencies could be filtered out and heart sounds
obtained.

The extended range and frequency response
allowed electronic differentiation of the apex and
brachial displacements for velocity and accelera-
tion of the motion. An example of this is pre-
sented in Figure 7 and a comparison between
acceleration made in this way and directly, using
a sensitive miniature accelerometer* coupled to
a special calibrated amplifier} is shown in Figure
8.

VI. Discussion

Because this distance measurement requires
the use of a disk capacitor plate rather than a
point, the measurement inherently has some
“averaging” of information. That the apex
motion does not move so as to maintain a parallel
relationship with this capacitor disk is also
reason to expect some distance averaging. Ex-
actly what this effect does to the frequency re-
sponse and accuracy of the measurement must.
be investigated further or determined.

Hazards Involved

Subjects have occasionally reported burning
sensations at the leg electrode (used for ground
return) when other instruments such as on ECG
were attached. The use of telemetry circumvented
this problem.

When the instrument is used alone and with the
probe making firm contact on the dry chest wall
a burning sensation has been reported at the
probe end. A thin plastic or rubber film has
been wrapped around the probe and use success-
fully, ie, with the probe making firm contact
with the chest wall, no subject has ever reported
any sensation or shock. However, the transducer
must be recalibrated because of the additional
dielectric material added.

Peak-to-peak voltage (guard ring to ground)
and current measurement* (10 ohms in series
with the guard ring circuit at the probe end)
have been made for two conditions of the probe
and three conditions of the test procedure for
determining the hazard involved. The probe
was used “uncovered” and “covered” with a thin

*Wilcoxon Accelerometer, Model 137.

tIthaco Amplifier, Model 252.

*Tektronix 555 Scope, Type D Input, 1 Meg. Shunted
47 put.


BIOMEDICAL APPLICATIONS OF A COMMERCIAL
CAPACITANCE TRANSDUCER

I. Introduction

The detection and measurement of chest wall
motions have been investigated in the past, using
many different transducers (1, 2, 3, 4, 6, 7).
These transducers can be divided into two
groups—contacting and noncontacting. Experi-
ence is greatest using contacting transducers of
various types (piezo-electric crystals, condenser
microphones, carbon microphones, etc.). The
problems inherent in these methods include the
Jack of DC response, lack of a linear response
proportional to distance and distortion of the
measurement due to contact.

In order to overcome these problems, non-
contacting transducers have been developed using
body capacitance (4) and photo-electric detection
of light reflected from the vibrating surface (6).
Again, linear response proportional to distance
has been a problem with these devices. Berson
and Pipberger reported a photo-electric method
that detects the motion of a light-weight target
riding on the chest (3).

Excellent reviews of the past methods in this
area have been made by Groom (7), and Berson
and Pipberger (3) have reviewed the instrumen-
tation used and the problems of transducer
design.

It is the purpose of this paper to report the
use of a new commercially available noncontact-
ing capacitive transducer* which produces a
linear output proportional to distance and has
a frequency response from DC to 10 KHz.

IL. Principle of Operation

The design is centered on a high-gain
feedback amplifier. An input reference signal
consists of the current passed by a standard
capacitor connected to a 50 KHz oscillator. A
feedback loop is completed by the capacitance
formed between a probe and the structure under
test, and feeds a current to the amplifier input

#B731A Vibration Meter, Southfield Electronic Sales, Ine.
21250 10% Mile Road Southfield, Michigan 48075.

in opposition to the reference signal. The am-
plifier output voltage is therefore inversely pro-
portional to the probe/structure capacitance and
this, in turn, is inversely proportional to the
probe/structure separation. Hence, the 50
KHz output from the amplifier has a mean
amplitude determined by the mean distance be-
tween the probe and test structure, and is ampli-
tude-modulated to an extent depending on the
peak-to-peak vibration of the structure. A feed-
back voltmeter rectifies the 50 KHz signal and
indicates mean distance. A simplified schematic
js shown in Figure 1. A more detailed circuit
analysis is given in Appendix A.

Il. Verification for Use in Physiological
Measurements

Although this device produces a linear output
using flat metal surfaces, it had to be shown that
its use can be extended to chest wall and pulse
measurements. Under these conditions one side
of the capacitor is skin which may have a curva-
ture. Two sets of measurements were made to
determine linearity when (1) skin is used as one
side of the capacitor and when (2) curved sur-
faces are used,

To verify the use of this transducer on skin
surfaces the probe surface was placed parallel
to the skin on a finger which had been taped to
a table top. The movement of the probe was
monitored by means of a dial gauge. The probe
separation as read on the distance meter and the
dial gauge were both adjusted to read 0.100
inches and increments of about 0.010 inches were
made as shown in Figure 2.

‘A second series of measurements were made
to determine the effect of curved surfaces on the
linearity. To do this the measurements were
again referenced to the dial gauge and metal
curved and flat surfaces were used as one plate
of the capacitor. The results in Figure 3 show
the response for the various surfaces with maxi-
mum deviation again at the small separation.



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