A CAPACITIVE SIGNAL
PICK(F DEVICE
JUN 11 19S58
by
e
R
AeR
WARREN W. BENJAMIN
Submitted in Partial Fulfillment
of the Requirements f or the
Degree of Bachelor of Science
at the
MASSACHUSETTS INSTITUTE2?
&TECHNOLOGY
June 1958
Signature of Author
uepartment of Economics. A
1958
Certified by
Thesis Supervisor
Accepted by
Chairman, Departmental Committee on Theses
ACKf OWIEDGENS1T
The author wishes to express his gratitude to Dr. E. B. Dane
and Mr. Frederick Hopewell for suggesting the subject of this thesis
and providing laboratory space and materials.
Appreciation is
directed to Professor Robert K. Mueller for his guidance as thesis
advisor and to Mr. Francis Merenda for his technical assistance.
Also appreciation is expressed to the personnel of the Instrmentation Laboratory of the Massachusetts Institute of Technology for
countless favors over a long period of time.
..
...
.
............
A oli
......
....
A CAPACITIVE SIGNAL
PiC 10FF DEVICE
by
WAEEN W. BfJAMIN
Submit ted to the Department of 3Economics on May 26th, 1958,
in partial fulfillment of the requirements for the degree of
Bachelor of Science.
ABSTRACT
An investigation has been made of a gaseous discharge transducer for possible use as a Capacitance Signal Pickoff device that
will convert the angular displacement of a two-degree-of-freedom
gyro wheel, about its combined input-output axes, to- a de voltage
that is proportional to these angular changes.
The possible appli-
cation and limitations of the device are examined, and experimental
evidence is presented to show the feasibility of the device.
The
sensitivity and accuracy are found to be a function of the ampli-V
tude and frequency of the R.F. supply voltage,
is the frequency
increases, amplitude variations in the excitation supply voltage
become less important while the sensitivity decreases,
The device
is designed to measure angular displacement, and it is found to
have merit for this purpose,
TABLE CF CONTENTS
Pate Nmuber
Chapter I
Introduction
1
Chapter II Pickoff Considerations -
5
Chap-ter III Preliminary Investigation
9
Chapter IV
Final Investigation
16
Chapter V
Results and Conclusions
20
Appendix A
Data from which figures 5and 6
were drawn
22
Appendix B'Derivation of the Input Impedance for
the Difference Amplifier shown in
figure 8
25
Appendix C Data from which figure 8 was drawn
27
Bibliography
28
TABLE OF TIGURES
Figure 1
Basic Transducer
2
Figure 2
Characteristics of Basic Transducer
2
6
Figure 3 Pickoff Configaration
Figure 4
Figure
5
Figure 6
Basic Experimental Circuit
10
Sensitivity vs Excitation at 250 E
11
Sensitivity vs Excitation at 600
C
12
Figure 7 Differential Amplifier
15
Figure 8
17
Transducer Characteristics
CHAPTER I
INTRODUCTION
If a tube containing two internal electrodes is filled with
an inert gas at a low pressure and is then excited by a radio frequency electrical field of sufficient magnitude to cause ionization of the gas within the tube, a de voltage arises between the
two electrodes.
This phenomenon was discovered independently by
L. Rhode1 in 1932 and by K. S. Lion
in 1938.
Three masters
theses2,3,4 have been presented at M.I.T. on this subject under
the supervision of the late Professor K. S. Lion.
The first of these three theses, presented by J. W. Sheets
in 1949 examines the effect of varying the position of a movable
excitation electrode around a gaseous discharge tube (see fig. 1).
This mechanical movement gives rise to a do voltage between the
two internal electrodes that varies linearly with the position of
the movable excitation electrode (see fig. 2).
Sheets attributed
this phenomenon to variation of electron energies in the plasma
thereby causing a difference of charge to appear between the two
electrodes which in turn gives rise to different potentials on the
electrodes.
The second of these theses, presented by G. H. Wayne,3 describes an application of the gaseous discharge transducer to an
ultra-micrometer.
Wayne presents data which show that the R.F.
excitation frequency affects sensitivity and that as the frequency
increases the effect of voltage variations becomes less important.
dP2-
4
The third of these theses, presented by W. W. Woods, investigates the phenomenon still further.
His results contradict pre-
viously advanced theories and show that the electron energies in
one part of the gaseous discharge do not vary appreciably with
respect to those in another part as the position of the discharge
tube is varied in the exciting field.
No complete explanation of the gaseous discharge phenomenon
confirmed by quantitative analysis has been put forth, but recent
22
investigations by Chenot and K. S. Lion2 suggest that the phenomenon may be caused by the following mechanism:
consider that the
two internal electrodes are probes inserted in the ionized gas.
If
there is no external conducting path between the electrodes, each
electrode will assume the potential of an isolated probe.
Now,
since the drift velocity of the relectrons is much higher than that
of the ions in the ionized gas, each isolated probe will take on
a charge that is slightly negative with respect to the potential
of the surrounding ionized gas.
If the ionized. gas is caused by
a do field, a stationary potential difference could arise between
the two probes as a function of differing electron and ion temperatures surrounding each probe.
If the ionized gas is caused by an ac field, a comlication
arises because the internal electrodes are capacitively coupled to
the external exciting electrodes
--
as shown by Wood.0
The result
is that the internal electrodes are not isolated probes and that
their potentials oscillate in time with the frequency of the exciting ac field.
With a given external electrode configur~tion, the
magnitude and place of the ac potential on each electrode will be
a function of the electric Parameters of the equivalent circuit as
seen by each electrode,
Thus, a variation of capacitances (along
4
with the inherent nonlinear character of the probes ) C or C2will
produce asymmetrical eurrents from the ionized gas to the electrodes
and result in net cha;rges on each electrode which lead to a difference of potential between them.
These conditions suggest the essen-
tial reasons that make possible the use of a gaseous discharge tube
as capacitance pickoff device.
To detect gyro wheel angular displacement, a pickoff device
is used which consists of a gaseous discharge tube and capacitors
0 and 02.
The asymmetry caused by varying these capacitors pro.
duces an output dc voltage that appears between the two internal
electrodes.
CH4PTR II
PIOKOFF
ONSIDERATIONS
The aim of this chapter is to discuss the use of a set of
capacitors, used in conjunction with a gaseous discharge tube, as
a signal pickoff transducer.
--
An important requirement of a pickoff
the term "pickoff" being used f or that part of a gyroscopic de-
vice whose function it is to transform the output of the device into
a suitable signal -- is that it gauge the desired quantity without
appreciably affecting the quantity or disturbing the gyroseople de-
5
vice.
For the purpose of this thesis, the desired quantity refers
to the angular displacement of a two-degree-of-freedom gyro wheel
about its combined input-putput axes.
The method of angular measure-
ment to be used consists of mounting four fixed metal plates of
equal area in close proximity to the gyro wheel as shown in fig. 3.
The gyro wheel can be at ground potential via a sliding brush
assembly in contact with the outside diameter of the gyro wheel.
These four plates, separated by a distance 6 from the gyro wheel,
form four capacitors all mounted in the plane of one of the input~o
output axes,
A similar set of four capacitor plates is mounted
in the 1ane of the other input-dutput axis.
This is neither dis-
cussed nor shown since it is a similar system and therefore all
that is pertinent to one system applie
to the other.
It is to be
noted that each fixed capacitor plate is connected in parallel to
the capacitor plate mounted diagonally opposite to it so as to form
a total of two capacitors instead of four.
This diagonal parallel
arrangement is used so that motion of the gyro wheel perpendicular
o-5-
GYRO WHEEL
8
C,
C2
FG.3
PICK-OFF
CONFIGURATION
-6-
A
ak
to the capacitor plates will not produce a change in total capacitances Cl and C2 . That is, when b
is decreased on one side of the
is increased by the same amount on the other side of
1 x.Area
C will not change value. The
the wheel; and since C=
gyro wheel, 6
same conditions hold true for capatitor 02*
However, when the gyro wheel is displaced 0 degrees, both
and 8{ decrease an equal amount with a resulting increase in
6
magnitude of capacitance Cl .As &l
ad
{- decrease, 6 2 and
' in-
2
crease with. a resulting decrease in magnitude of capacitance C2'
Both capacitances Cl and 02 change by the same magnitvAe but in
opposite senses.
A similar analysis holds true for angular displace-
ments of negative 6 degrees.
For the given gyro wheel configuration, as shown in fig, 3,
R = 1.65 inches; therefore, for an angular displacement of one
second arc
--
which is equal to 4.848 microinches
--
6 will change
eight microinches.
Abt::'6RAO = (1.65")C.000004848);Z:
8 microinches
Now, since this movement of eight microinches appears as
Ab's in all four capacitance plates, the resulting change in the
magnitude in total capacitance is four times what it would be had
only one plate been used.
If the area of each capacitance plate
is 3.5 cm2 and 8 is equal to .0762 cm, both Cl and C2 will be 8.10
micro-microfarads.
The problem then is to detect changes in capacitance of the
order of .005 micro-.microfarads and convert these changes to a do
voltage that will be proportional to either a positive or negative
angular displacement of a two-degree-of-freeom gyro wheel about
-7-
its combined input-ontput axese 'This proportional voltage can then
be used to correct a gyrO system to a null position via a servo
system.
To determine the feasibility of the capacitance pickoff device for possible application in the above specified system, it is
necessary to determine the following important properties:
1. Sensitivity
For given capacitance changes,
-
what is
the corresponding change in the da voltage output of the capacitance pickoff device2
2.
Null drift
--
If the two capacitances in question (Cl
and 02) remain of equal value, does the de voltage output of the
transducer remain at sero3
3.
TUncertainty -- Are there any fluctuations in the de
voltage output with a fixed capacitance configarationt
-8-w
=M!
....
-77
13
CAPTR III
PRELIMINARY INTESTIGAT ION
In order to illustre the basip priciples of operation
of the capacitance pickoff device, a simplified circuit, shown in
fig. 4, was constructed.
This circuit utilizes a readily available
NE-2 neon bulb ,as the basic gaseous discharge transducer.
The
radio frequency seurce used is a Hewlett-Paelard Model 4000 signal
generator, the 6AQ5 tube is employed as a conventional power amplitier, and T1 is an auto transformer utilizing a core from a television flyback transformer.
C
make up a series resonant BI.O circuit.
and the two 0B2 gas tubes
The purpose of the two QB2
gas tubas is to insure that the gaseous discharge tube excitation
voltage will have a relatively stable amplitude.
When the two 032
tubes ionize, together withfl 1 they form the resistance part of the
RWL circuit,
These tubes, when ionized, act similarly to a resist-
ance except that the voltage drop across the two tubes remains relatively constant as the current passing through them increases or
decreases.
6
This is true, however, only if the voltage appearing
across the tubes is high etough to maintain ionization.
By varying the value of R, the amplitude of the excitation
voltage appearing across the NFr2 neon bulb can be varied.
graph shown in fig.
5
The
presents data taken for three different
age excitation magnitudes at a frequency of 250 XC.
lwt-
The do voltage
output of the transducer is plotted against changes in Cy while C2
is held constant.
The graph shown in fig. 6 presents the same kind
of data except that the frequency is 6oo XE.
-9-M
Higher frequencies
BA SIC
EX PER IMENTAL
CIFR C'Ui T
Fig. 4
D.C. OUT
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IM
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6AQ5
NE -2
6o80pf
CI
C2
2- 30ppf
270 A
.022
250
220K
50
150.A
+ 2 25V
4-
Fig. 5
SENSITIVITY vs EXCITATION
-4
VOLTS at 250 K.C.
18
17
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Fig. 6
SENSITIVITY vs EXCITATION
78
VOLTS at 6-00 K.C(
17
16~
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0
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r
r
rrw'
+
0
were npt tried because a variable high frequency source was not
available.
During these tests, the exeitation electrode was set
at an arbitrary position; and therefore, when the two capacitors
In
were of equal magnitude, a dc voltage output was produced.
any final system, the exeitation electrode would be set at a
position correspondin& to X = 0, as shown in fig. 1, so that when
isM equal to C2 the de voltage output would be zero.
As shown in the graphs of fig. 5 and fig. 6, the device is
quite linear from -'2 volts to +.5 volts; and the sensitivity is a
function of both excitation amplitude and frequency.
When the excitation voltage, at 250 30, is equal to
500
volts peek-to-peak (as measured on a Tektronix Oscilloscope),
the
sensitivity is 5 millivolts for a change in capacitance of .005
micro-mierofarads.
Inereasing the frequency reduces the sensi-
tivity to 4 millivolts for a change in capacitance of .005 micromicrofarads,
but the effect of amplitude variations in the excit-
ing electrode voltage becomes less pronounced.
the findings of Wayne.
This corroborates
3
Increasing the magnitude of the exciting electrode voltage
beyond
500
volts peak-to-peak results in greater sensitivity; how-
ever, tube life is considerably shortened.
The output impedance of the gaseous discharge transducer
is on the order of one megohm.
Therefore, the device used to measure
the dc voltage output must have an input impedance of ten megohms
or higher in order to avoid loading effects,
Since it is the difference between the two internal electrode voltages that is pertinent, a difference amplifier (as shown
-13-
in fig. 7) was constructed whose input impedance is greater than
ten megohms (see Appendix 3).
The cathode-follower action of the
difference amplifier provides an impedance match so that a Sanborn
recorder can be used without affecting the trensduber output4
order to eliminate the possibility of any temperature variation
affecting the difference amplifier, boron carbon resistors were
used because of their almost zero temperature coefficient,
T.
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CHAPEfl IT
INVESTIGATIC&
VAL
In this chapter a commercially available gaseous discharge
transducer is examined and its characteristies are shown graphisally.
Then the problem of keeping the excitation supply voltage at a
constant amplitude is examined, and attempts to solve it are presented.
In Chapter III an fE-2 neon bulb was used as the basic
gaseous discharge tube.
A specially made transducer, manufactured
by the Decker Aviation Corporation, was on hand and so a plot of
its characteristics was also made and is presented in fig. 8. The
linearity displayed was f ound to be qui.te good and the sensitivity
obtained was 10 millivolts for a change in capacitance of .005
micro-microfarads.
A check on the uncertainty error and the null drift revealed
results that were not within allowable limits.
Using a Sanborn
Recorder (Type 150) driven by the difference amplifier shown in
fig. 7, the uncertainty was k3 millivolts and the null drift was
8 millivolts in eight hours.
A check of excitation voltage ampli-
tude showed variations and dritt which were correlated with actual
uncertainty and null drift,
Now, from the graphs shown in figs.
5
and 6 of Ohapter III,
it becomes apparent that variations in the amplitude of the radio
freqency excitation voltage cause wide variations in the de output voltage of the transducer.
In order that the uncertainty of
the de voltage output of the gaseous discharge transducer be kept
wwwwwwwamm"Wwwom .mmmmmm-om W-MMWAMMOO", "7,
Fig.8
CHARACTERISTICS
TRANSDUCER
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as low as possible, a constant amplitude radio frequency source
is necessary.
Not only must the amplitude of the radio frequency
source be constant over a long period of time, but also it must
be unaffeeted by transient changes.
Very little has been written
about constant amplitude sine waves that is applicable to this
particular problem, Since in this case the amplitude of the excitation supply voltage is 380 volts pp. and the frequency is
250 EC.
(The Decker Aviation Corporation recommends these values
for long tube life and high sensitivity.)
The high voltage re-
quirement demands the use of a step-up transformer and thus inherent thermal sensitivity is introduced.
The 250 EC requirement
megns that the transformers and coils must be wound since no com-
mercially manufactured transformers and/or coils are available at
this frequency.
A self-contained oscillator amplifier was designed utilizing
a feedback system to correct for amplitude variations.
Using con-
ventional techniques, the output voltage is sampled and fed back
to control the bias of the oscillation.
As the amplitude of the
output increases, the feedback loop causes the bias to increase
and thus reduces the amplitude of oscillation.
Both the oscillator coil and the output transformer were
wound using Litz wire and polystyrene coil forms.
Since the
oscillator anmplifier must have long term stability, one percent
boron carbon resistors were used in order to reduce the effects
of temperature,
The oscillator amplifier was not completed at the time of
this writing.
However, it was felt that the problem of building
-18-
frEI~a.hftMIEI~ImI*m.*m~*.
I
I
AN
Mill
"T"
Ovin
ANN.'!
IRA.
a constant amplitude oscillatoi amplifier was beyond the scope
of this thesis
--
this constituting an independent problem within
the realms of engineering probability.
Preliminary results using the oscillator amplifier show
that the uncertainty can be reduced to Al millivolt, but that the
long run null drift remained on the order of 8 millivolts for an
eight-hour test.
-19-
... ....
BRSULTS AD CONCLUSIONS
The capacitance signal piekoff device was designed to measure
the angular displacement of a gyro wheel by converting this displacement to a change in capacitance.
measwed
using
This capacitance change is then
a gaseous discharge tube.
The results obtained in
this thesis show that the gaseous discharge tube will convert ehanges
in capacitance to a do voltage proportional to either a positive
or negative angular displacement.
It has been shown both by previous work9 and by this thesis
that, in order for the gaseous discharge transducer to have a low
uncertainty error, the radio frequency supply voltage must have a
constant amplitude which at 250 EC requires an accuracy of one part
in -one thousand using the Decker Aviation transducer.
The sensitivity at 250 ED was found to be 10 millivolts for
a change of .005 micro-nicrofarads which corresponds t-o one second
of arc of angular displacement.
The problems of signal output uncertainty and null drift
have not been resolved; but, as- pointed out in Chapter IT, the
design of a constant amplitude amplifier is within engineering
possibilities.
The null drift poses a more serious problem in
that the drift is probably due to temperature caused component
changes.
More experimentation would be necessary in order to de-
termine the exact source of the drift.
Current signal generators used at the Instrumentation Laboratory produce 100 millivolts for an angular displacement of 1 milli-
,~*
'9sr!~5~
a
macu
radian.
I
Using the capacitanee pickoff transducer, the sensitivity
is 2000 millivolts for 1 milliradian of angular displacement.
While
this is a significant improvement, it must be remembered that the
data collected for this thesis was obtained under static conditions,
the capacitors being fixed.
The effects of a rotating wheel might
well introduce periodic errors not observable under static conditions.
These considerations probably require more attention; however, any
frequency components derived from wheel asymmetry could be filtered
out.
In any further research, an investigation should be made
of the characteristics of the transducer using frequencies in the
range of five megacycles.
While the sensitivity will be decreased,
it might prove worth while to sacrifice sensitivity for less null
drift and uncertainty.
Since the most serious drawback of the capacitance pickoff
is the null drift, it
is suggested that Zener diodes (when they
become available) be used to clip the output of the radio frequency
excitation voltage.
This clipped sine wave could then be reshaped
using a conventional filter.
The effects of temperature drift might
well be lessened with this approach.
In sumation, the capacitance pickoff was designed to measure
angular displacement of a gyro wheel and convert this angular dis-
placement to a dc voltage that would be proportional to either positive or negative displacement.
The pickoff device was found to
accomplish this purpose with the limitation of excessive null drift
which in the copinion of the author is a solvable engineering problem,
-21-
singe
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Data from which figa
5
a4 6 were drawn.
Fig, 5
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400 volts pp.
13.8
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400 volts pp.
11.9
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10.7
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-139
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-1.0
6oo x
500 voltsr PP.
7-7
27.5
-1.5
500 volts PP.
600 xE
6oo im
7.0
.27.5
-2.0
500 volts PP-
27.5
2.5
-.
500
volts pp.
500 volts PP.
6.5
.5
600
Ec
600 E0
600 Ec
6oo mc
600 E
600 EL
600 xc
600 E
600 XC
6oo
6o,o 130
APPThDI
B
Derivation of the Input Impedance,for the
Difference Amplifier shown in figare 8.
Using an incremental model shown below of one-half of the
difference amplif ler is analyzed.
where:
=5.3K
r
p
= 165
+p
,RSR.=-470
Re
1
R
=4.?meg
g
9
se
+ RE+RL)+Ri
=i(r+R)+R(1iP=i(rP
and since eg
iRg
p(R-
=p(rp
pil
Rx)
i
iR-
-
-
aR)-
i9R
-
r 9 o-
i (r
~r
+
1
+
B) +
+RK+
+
IR
-iR,
RL+RJ
T1 rp
'' ) 1p
=4.5K
-
p+1)
-
R i
R)
+
+RR (+1)
= 0
= 0
=0
Now,
substitution of equation 2 into equation 1 gives:
L1(pR
in =gl't+
ra
in
+r
e
in
i
p
-
Bg)
+
+L(pR
g+
-
B1 )
rp + IL + RKp+
Using the values given in the above figure:
Z g
............
24 megohms.
1)
AIPFKDUC
Data from which figur
8 was drawn.
Yrequency
C2
Vout
ppfd
ppfd
volts
380 volts pp4
11.00
10
2.0
250 EC
380 volts pp.
10.72
10
1.5
250 E
380 volts pp.
10.52
10
1.0
250 EC
380 volts pp.
10.26
10
.5
250 XC
380 volts pp.,
10.00
10
0
380 volts pp.
9.74
10
.5
380 volts pp,
9.52
10
-1.0
250 EM
380 volts pp.
9424
10
-1.5
250 EM
380 volts pp.
9.00
10
-2.0
250 E
Excitation Voltage
di7m
250 E0
250 E
B LBLIOGRAPFY
1.
Lion, Kurt
,: "Mechanital-]1leetric Transducer"; The Review
of Scientific Instrumente, Volume 27, No. 4, pp. 222-225,
April 1956,
2o
Sheets, John
.,
8.M. Physics, M.I.T. 1947:
'An Experimental
Study of a Rectifier Effect in High Frequency Discharges".
3,
Wayne, George H,, S.M. Electrical Engineering, M.I.T. 1949;
"Investigation of a Glow-Lamp Ultra-Micrometer".
4. Woods, W. William, 5.M. Electrical Engineering, M.I.T. 1951:
"A Gaseous Discharge Mechano-Electric Transducer".
5.
"An Introduction to the Study of Gyro-
Weems, William R.:
scopic Instruments", pp. 7-.
6.
Francis, T. J:
"Fundamentals of Discharge Tube Circuits";
Methuen's Monographs on Physical Subjects.
UN~ Nt
-