Note: Descriptions are shown in the official language in which they were submitted.
This application is a division of our Canadian patent application
Serial ~o. 299,98G filed ~iarch 29, 1978.
The present invention pertains to fluid pressure responsive appara-
tus and more particularly concerns a vibrating diaphragm sensor apparatus for
converting fluid pressure magnitude directly into an electrical signal whose
frequency varies as a function of that applied fluid pressure.
The immediate prior art vibrating diaphragm fluid pressure sensor is
that of the R. H. Frische United States Patent 3,~56,508, issued July 22, 1969,
and assigned to Sperry Rand Corporation. In this prior Frische patent, ante-
cedent concepts for pressure sensors generally unsuited for application in
aircraft digital air data and altitude sensing systems are also discussed.
The device of the former Frische patent overcomes limitations of such prior
art transducers by use of a simple, flat diaphragm not requiring association
with a vibrating wire. Further, it directly measures gas pressure rather than
gas density with the change in the diaphragm vibrating frequency resulting
from changes in the mechanical spring rate of the diaphragm as a function of
fluid pressure loading. Most important, the device has an output frequency
variation substantially greater than prior art devices over pressure ranges of
interest particularly in air data and altitude sensing systems.
In more particularity, the device of the prior Frische patent utili-
zes a pressure chamber having a wall defined by a flat diaphragm uniformly
restrained at its periphery and subjected to fluid pressure differences betwe-
en one side and the other. The-diaphragm becomes stiffer in a non-linear
fashion the farther it is deformed from its flat or unstressed position by
the varying pressure of fluid acting on one of its sides. Thus, the diaphragm
deforms easily for the first several increments of applied fluid pressure but,
as the pressure is progressively increased, additional deformation progessi-
vely diminishes. ~he diaphragm may properly be considered as a spring-mass
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: "
mechanical system, and it can theTefore be driven at a characteristic resonant
frequency which is a function of its effective mass and spring stiffness. As
the diaphragm is deformcd to a lesser or greater degree by changes in gas
prcssure, its stiffness changes and its mechanically resonant frcquency changes
as a truefunction of applied pressure. Thus, the flat diaphragm system pro-
vides the desired pressure-to-frequency conversion characteristic needed for
digital pressure measurement applications.
The vibr~ting diaphragm sensor of the prior Frische patent has been
widely accepted as a reliable and accurate means for measuring gas pressure,
many problems associated with the structural design of the vibrating diaphragm
itself and with thermal and vibration isolation from the environment having
been generally resolved. The pressure chamber geometry is determined largely
by factors inherent in the design and successful manufacture of the vibrating
diaphragm. Hol~ever, it is found that the vibrating nature of the device may
give rise to acoustic waves within the interior of its gas chamber or within
the pneumatic lines coupled to the sensor which waves, under certain circum-
stances, interfere with the degree of precision of pressure measurement ob-
tainable by the deYice. Inherently, the vibrating diaphragm pressure sensor
operates over a frequency range dependent upon the range of gas pressures to
be measured, and~therefore the acoustic waves generated are of varying fre-
quencies and amplitudes. These acoustic waves and their reflections can cause
the prlor art vibrating diaphragm sensor to be unstable or inaccurate depend-
ing upon the selected chamber geometry, and the present invention derives
from an appreciation of these undesired acoustical effects upon the total
performance of the vibrating diaphragm gas pressure sensor.
The present invention is an improvcd vibrating diapllragm fluid pres-
sure sensor in ~thich the effects of disturbing acoustic waves which might
otherwise be pres~nt ~.~tithi~ the in~erlor of the fluid chamber or within pneu-
, - 2 -
matic lines coupled thereto are eli~inated. Like the devices of the prior
Frische patent, the invention includes a thin flat vibrating diaphragm
dividing the enclosure into two chambers, one being subjected to a first
fluid pressure and the other being subjected to a second Fluid pressure which
may alterna-tively be a steady reference pressure or a second variable pres-
sure. The vlbrating diaphragm has the appropriate thinness, surface area,
and resiliency that its resonant frequency changes in accordance with the
relative magnitudes oE the aforementioned first and second fluid pressures. A
circuit acting with the vibratory diaphragm as a self-tuned oscillator includes
a means for driving the diaphragm substantially at the resonant frequency of
the latter over a predetermined range of operating frequencies and for pro-
viding a corresponding output signal. According to the invention, a rigid
wall is supplied in one of the chambers, dividing it into two cavities, one
cavity being disposed adjacent the vibratory diaphragm itself and the other
being connected to the variable pressure fluid input line. A restricted
passage or orifice in the rigid dividing wall provides communication between
the two cavities. The relative volumes of the cavities and the orifice
dimensions are selected such as to provide an acoustic filter for suppressing
acoustic wave resonances, preventing them from adversely affecting the
normal resonance vibrations of the diaphragm. The diaphragm and the rigid
divider wall are disposed in substantially parallel relation and are
separated by a distance significantly less than a quarter wave length at the
highest normal operating frequency of vibration of the diaphragm, thereby
widely separating the cavity acoustic resonances from the highest diaphragm
operating frequency consistent with the compressibility effects of the gas
on the vibrating diaphragm.
According to a broad aspect of the invention there is provided a
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fluid pressure sensor comprising: a pair of chambers having a ~hin common
vibratable diaphragm therebetween, one of said chambers being coupled to a
source of fluid pressure thereby to provide a differential pressure acting
across said diaphragm having a resonant frequency changing in accordance with
changes in said differential pressure ovcr a predetermined range of pressures,
means for sensing and driving said diaphragm at said resonant frequency and
providing an output signal corresponding thereto, rigid wall means dividing
said one chamber into a plurality of cavities, one of said cavities cooperat-
ing with said diaphragm and another thereof with said fluid pressure source,
said wall means being substantially coextensive with and substantially
parallel to said diaphragm and spaced therefrom by a distance less than a
quarter wave length of the highest resonant frequency of said diaphragm, and
orifice means in said wall means coupling said cavities to said fluid pressure
source.
The invention will now be further described in conjunction with
the accompanying drawings, in which:
Figure 1 is an elevation view in cross section of a preferred form
of the invention.
Figure 2 illustrates, on an enlarged scale, a portion in cross
section of the Figure 1 apparatus and includes the wiring diagram of an
associated measurement circuit showing electrical interconnections with the
driver mechanism of Figure 1.
Figure 3 is a graph use~ul in explaining the operation of the
invention.
Referring to Figure 1, the invention includes a flat, circular
y~
resilient metal diaphragm 4 which is formcd integrally at one end of a gene-
rally cylindrical ~all member 25, Though diaphragm 4 is preferably formed as
an integral part of ~all member 25, the diaphragm may alteTnatively be a sep-
arate member if uniformly welded at its periphery~ as by electron beam welding,
to ~all member ~5. ~Vall member 25 is provided with upper and lower annular
flange members 2 and 50 encompassing an annular recessed region 14 bet~een the
flange members. Interior of the cylindrical wall 3 of wall member 25 is dis-
posed a second hollow cylindrical member 15 which is normally formed integ-
rally with a generally circular base member 54. A round reentrant portion 66
located Oll ~he axis of hollow cyllndrical member 15 and formed integrally on
the interior surface of base 54 thereof extends toward diaphragm 4 and serves
as a support element for other essential parts of the invention yet to be des-
cribed.
The elements of the invention thus far discussed are preferably
formed, for e~ample, of one particular metallic substance, the choice being
dictated largely by the stable resiliency requirements of diaphragm 4. Since
diaphragm 4 must have minimum internal hysteresis characteristics, the diaph-
ragm and i~s associated elements are constructed of Be-Cu or alternatively of
a commercially available alloy of nickel, iron and chromiurn sold under the
trade mark ~i-Span C by the International Nickel Co., Inc., Huntington Alloy
Products Division. Use of such a material having substantially a zero tempe-
rature coefXicient of Young's modulus over the operating range of temperatures
is preferred.
The cavity-defining elements described in the fore-going are ulti-
mately forrned into an integral unit by generating an annular bond 51 between
the 101~er annular flange member 50 and a second annular flange member 52 for-
med as part of base member 54, as by electron beam welding. ~efore the weld
51 is actually formed, two o~ ~ore reIatively large Gpenings 16, 74 are bored
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,. . ' ' , : -
~ '
~$~
through the cylindrical wall mel~er 15~ Also, an annular groove is formed in
the inner wall 3 to accommodate 0-ring 13; the latter forms a hermetic seal
bet~een wall 3 and the adjacent outer wall 15. By virtue of the openings 16,
74 and orifice 21 in the rigid divider ~all 18, there are no significant long
term pressure di~ferences on the opposite sides of divider wall 18. It ~
be appreciated that the configuration employing 0-ring 13 and the elements
providing ~alls 3, 15 permits ready assembly of the parts ultinlatcly unified
- by seal Sl.
Reen~rant portion 66 is equipped with an axial bore within which is
sealed, as by epoxy cement, an extension 65 of an insulating support elemen~
24 composed of phenolic or a conventional compressed, molded plastic, for
example. Element 24 provides support, because of its inverted truncated coni-
cal portion 20 on cylindrical portion 64, for rigid divider wall 18 and for a
bobbin portion 7 supporting, in turn, the driver pick-off coil 8 above divider
wall 18. Divider wall 18 may also be formed of a compressed molded plastic
and is fastened at its periphery 17 by an epoxy bond to hollow cylinder 15.
A central aperture of rigid divid^r wall 18 is fastened by an epoxy seal 19 to
portion 12 of insula~or 24, bobbin 7 and coil 8 being supported above portion
12. Divider wall 18 is supplied with a calibrated orifice 21 connecting the
cavities on each of its s;des.
At the axis of the cavity system, an axial bore 68, forming a res-
tricted orifice is formed through base member 54, which bore 68 communicates
with the interior of the device through radial bore 67 in reentrant portion
66. Bore 68 is coupled through the extended steel pipe or tube 69 to the
source of ~ariable pressure whose magnitude IS to be measured. Provision is
also made through base member 54 for the supply of driving electrical current
to driver pic~-off coil 8, as will be further described. For this purpose,
the rigid conductor 6~ extends through a conventional glass-to-metal seal 61.
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,
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Epoxy cylinder 62 l~ithin a bo-re through base member 54 se-rves to stahilize the
le~d 63 to prev~nt shorting. Conductor 70 is similarly arranged with respect
to seal 71 and cylinder 63. As will be further described in connection with
Figure 2, conductors 22, 23 are respectively bonded to conductors 60, 70, and
permit current flo~ through coil 8. In ~his manner, external connection to
coil 8 is provided at outer terminals 63, 73.
At the periphery of base member 54, an annular electron beam weld
53 is made be-tween base 5~ and a cup-shaped outer casing 1. Casing 1 may be
composed of Ni-Span C when the cavity-defining elements are of that material
or of Cu wh~n Be-Cu is used in the cavity-definin~ elements. It will be seen
that several major cavities are formed within casing 1. A first cavity pro-
vides an isolated chaJ.,ber, not being connected to the other cavities; this is
the reference cavity A formed between casing 1, base member 54, cylindrical
member 25, and diaphragm 4. In a static pressure application, the reference
cavity or chamber A is evacuated. On the other hand, should it be desirable
to employ the invention as a differential pressure measuring device, a fixed
or variable pressure input similar to input pipe 69 may be readily provided
near the periphery of base member 54 for communication with cavity A just
below flange member 52, for example. The other three cavities B, C~ and D
cooperatively form a second major chamber, as will be further discussed.
The stationary driver pick-off coil 8 cooperates with a magnet
assembly 9, coil 8 being supported within an annular hollow portion interior
of a cup-shapcd magnetic pole piece integral with and surrounding a reentrant
annular pole piece, as is seen also in the enlarged view of Figure 2. The
magnet 9 assembly thus provides an intense radial magnetic field running from
annular pole 10 outward to the opposite polarity annular pole ll so as to cut
the conductors of coil 8 when ~he armature position oscillates vertically.
Por this purpose, the magnet assembly is mounted in a hub 6, being sealed
.
. ' . ' , ' '
;3~
therein at 5 by soldex, for examplc hub 6 bein~ affixed to the center of the
vibratory diaphra~m 4,
By applying a sinusoidal electrical signal ol proper frequency to
terminals 63, 73 of coil 8 ~see Figure 2~, diaphragm 4 is caused to vibrate at
i-ts natur.ll mcchanical resonance fre~uelZcy. The transducer response reaches a
resonant peak ~hcn the driving frequency is equal to the mechanical resonance
frequency oF diaphragm 4, the latter being dctermined by the pressure applied
via tube 69. It is therefore possible to connect the driver, pick-off coil 8
in a feed back circuit as shown in Figure 2, such that the back electromotive
force generated as the magnet assembly moves with respect to coil 8 is connec-
ted back to the input of driver amplifier 82. In this configuration, the
closed loop sensor and amplifier circuit oscillates substantially at the elec-
tromechanical resonance frequency of the system, and the frequency of oscilla-
tion changes as a function of the pressure across diaphragm 4. The terminals
63, 73 of coil 8 are connected via leads 60, 70 through the glass seals 61, 71
in the base memb~r 54 to terminals 85, 98 of a bridge circuit 88, 95, 96. The
output of the bridge circuit at terminals 98, 99 is connected via leads 83 to
the input of amplifier 82 in order to amplify the un~,alance or feed back elec-
tromotive force signal and to apply it to coil 8 as a driving signal via the
bridge circuit and leads 80, 81. In this way, the closed loop circui~ operates
as a self-resonant electromechanical oscillator which oscillates at the natu-
ral resonance frequency of diaphragm 4. The output of amplifier 82 may be
further supplied by leads 84, 100 to be amplified by an output ampliEier 87 to
provide an amplified signal whose fre~uency is the desired function of pressure
to a utilization circuit such as a counter and display 101. In order to main-
tain tl-e bridge circuit accurately balanced, coil 88, which forms the fourth
leg of the bridge c;rcuit, may also be disposed in the cavity C, thereby to be
subjected to the same thermal environment as coil 8.
The ~resent inYention has been descrihed with respect to a diaph-
ragm driving and velocity detecting arrangement i:r,cluding a permanent magnet
assembly moving ~ith the diaphragm and cooperating with a fi~ed coil. It will
he appreciated ~y those sk;lled in the art that other possible techniques for
driving the resonant diaphragm alld for detecting its motion may include a
moving coil and a fi~ed pe-rmanent magnet assembly, an electrostatic ~river
witll variablc capacity detection, moving armature elements, piezoelectric
transducers, or a fixed coil cooperating directly with the diaphragm wherein
the diaphragm is fabricated, for instance, from the alloy Ni Span C, thereby
eliminating the mass of the magnet from the diaphragm.
The general theory ar.d the principles of operation of a peripherally
clamped vibrating diaphragm system as used in the present invention had been
adequately presented in the aforementioned Frische patent and applied equally
well with respect to the behavior of diaphragm ~ of the present invention.
Some fu~ther theoretical considerations will be useful in understanding the
problems overcome by -the present invention. To measure an absolute pressure
with the prior Flis~he device, a first side o-f the vibrating diaphragm 4 is
exposed to a reference vacuum. ~lence, depending on the quality of the vacuum,
there is essentially no acoustic response from that first side of the vib-
rating diaphragm within the vacuum. The acoustic response of the gas on thepressuri~ed side of the vibrating diaphragm depends on the molecular weight
and temperature of the gas and the shape of its associated chamber that is
inherently an acoustic reson~tor at some predetermined frequency.
That resonance frequency depends upon the speed of sound in the gas
medium of the chamber or, assuming a constant temperature, upon -the molecular
weight o~ the gas medium. Thus, a change in the molecular weight of the gas
produces a change in the resonance frequency in the gas chamber. A change in
this resonance frequency pro~uces a change in the acoustic response seen by
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the v;brating di.lphra~m and, hence, the vibrating diaphragm oscillates at dif-
ferent frequencies when the device is operated at the same pressure and temp-
erature, but wi.h differen~ gas media. The aforementioned phenomena is com-
monly referred to as density sensitivity. To reduce the effcct of operating
the vibrating sensor cliaphrl~gm with various gases~ the resonance frequency of
the gas filled chcm~ber can be made much higher than the range of opcration of
the vibrating diaph-ragm. The farther a-~ay from the vibrating diaphragm fre-
quency range the gas filled chamber resonance is moved, the smaller the effect
of a slight change in the chamber resonance frequency. To increase the reson-
ance frequency of the chamber, the acoustlcal reflecting surface 18a accordingto the present invention is placed close (much less than one quarter wave
length) to the vibrating diaphra~n surface 4a. Since the speed of sound in a
gas is also dependent upon gas temperature, the char,ber resonance frequency is
~inherently also a function of temperature. However, ~he close reflecting sur-
faces 4a, 18a a]so reduce that portion of the temperature sensitivity of the
sensor device related to the acoustic phenomena. There is additionally a
temperature sensitivity related to the vibrating diaphragm 4 itself; however,
the temperature sensitivity of a sensor device with close reflecting walls 4a,
18a is much less complex than the temperature sensitivity of the prior Frische
device.
There is a second accustical phenomena occurring within the gas fil-
led pressure cavity, because the gas itself acts like a pneumatic spring. The
stiffness of this pneumatic spring depends on the gas pressure within the
chamber and upon the ratio of the volume of the chamber to the volume dis-
placed per cycle of oscillation of the vibrating diaphragm. Therefore, there
is a limit to how close the reflecting walls 4a, 18a of the chamber can be.
; The chamber must be large enough that the volume displaced by the vibrating
diaphragm oscillation is small compared to the chamber volume, and so that the
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spring ~ction of the gas does not become a significant portion of the total
stirfness of the vibrating diaphragm. Hence, the optimum chamber geometry
depends upon a balancc of these two restr;ctions tailored emplrically to a
particular comhination of pressure range and vibrating component characteris-
tics.
~ nother problem with the vibrating diaphragm pressure sensor is re-
lated to noise within associated pneumatic lines. This noise may include
acoustic waves generated by other system components or acoustic waves genera-
ted by the vibrating diaphragm itself and reflected by discontinuities in the
pneumatic line back to the sensor, for example, by a coupling of reduced dia-
meter in the pneumatic line, Other sources of acoustic wave disturbances may
be related to the aircraft pressure system, the location of pressure ports on
the aircraft fuselage, or the like. By use of multiple cavity configurations
and orifices as provided in the present invention, these acoustic waves can
be prevented from entering the primary sensor chamber. Thus, there are two
phenomena associated with the pne-~latics of a vibrating diaphragm pressure
sensor. These are acoustical reflections from surfaces within the sensor
chamber and pneumatic stiffness of the gas within the chamber. These two
phenomena are controlled according to the present invention by means of pro-
per chamber geometry and si~e to produce optimum performance of the vibratingdiaphragm pressure sensor.
Both of these effects can be described mathematically by means of
the classical wave equation:
2 2
~ t
where 0 is the velocity potential, V is the conventional operator, t is time
and C is the sonic velocity as defined by:
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:-, ., , . . : ,: : :, .. .
- - , . . . . .
.: . - - ::: :
:, .: '. - ~ . . - ' -
. . -. :
... : ,
C = K~T
\,
here K is the ratio of specific heats~ R is the universal gas constant, m is
the molecular weight of the gas, and T is the absolute temperature, all in
consistent units. The wave equation may be developed by combining the contin-
uity cquation and momentllm equations for a compressible, ~ero viscosity gas.
For an a~ially-symmetric, cylindrical hollow resonator! the wave equation is:
2 2 2
~ 0 + 1 ~0 ~ ~ 0 = 1 ~ 0
~r2 r ~r ~z2 C2 ~t2
where r is the hollow resonator radial dimension, and z is the axial dimension
of the hollow resonator. A conventional technique for the separation of vari-
ables may be used to solve -the above equation. For use in a mathematical
model analysis of the vibrating diaphragm, the variationals of kinetic energy
and potential energy are calculated from the results of the wave equation sol-
ution. These energy changes are then used in conjunction with the calculated
energy of the diaphragm to determine its frequency of oscillation. The model
solves for the minimum energy condition of the system.
This mathematical model accounts for the two basic disturbing pheno-
mena, acoustical reflection and pneumatic stiffness. Acoustic waves are al-
ternating high and low pressure regions moving through the gas medium. Acous-
tic waves are generated by the vibration of the diaphragm 4. These waves move
through the chamber B and strike the bounding surface 18a, being reflected
therefrom. Hence, after the acoustic waves are reflected from surface 18a,
they travel back toward diaphragm 4. When the waves return to thc surface ~a
of diaphragm 4, they 1nay or may not be iTI phase with the motion of the dia-
phragm and new waves may be generated; hence> the in-phase or resonant waves
tend to add energy to diaphragm 4, tending to reinforce its oscillation; or
..
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' ' ' :
-:
they may be out-of-phase or anti-resonant and tend to take energy from the
diap~ragm and to oppose its oscillation. The acoustic ~aves travel at the
speed of sound through the gas medium, which speed depends upon the tem.pera-
ture and molecular ~eight of the gas. Thusl for a given gas at a constant
temperature, the time neccssary for a wave to travel from the diaphragm sur-
face ~la to tlle reflecting surface l~a and back to the diaphragm surface ~a
clepends on the distance traveled. Thcrefore, the distance from the diaphragm
surface 4a to the reflecting surface l~a determines, at least for constant
temperature conditions, whether the reflected wave diminishes or amplifies
the oscillation of diaphragm 4.
The gas medium also acts as a pneumatic spring a~tached to diaphragm
4. As the diaphragm 4 oscillates, it acts on the gas in the sensor chamber.
The stiffness of the gas medium is determined by the ratio of the delta vol-
: ume caused by oscillation of diaphragm 4 to the total volume of the chamber.
As this ratio becomes larger, the pneumatic stiffness becomes great; as the
sensor chamber decreases in volume, the stiffness of the gas becomes a signif-
icant portion of the total stiffness of diaphragm 4. This phenomena may be
thought of as a reflection of an acoustic wave from a reflecting surface very
close to the diaphragm as compared to the wave length of the acoustic wave.
That is, a region one wave long fills the entire chamber.
~ `herefore, it is seen that the optimum chamber configuration con-
sists of a compromise with respect to the foregoing acoustic phenomena. The
optimum compromise depends upon the operating pressure range and the charac-
teristics of the vibrating diaphragm. The areas to be improved by control-
ling the acoustic phenomena are:
~ 1. molecular weight sensitivity of the absolute sensors,
; 2. temperature sensitivity of the absolute sensors, and
3. filtering ~f prleumatic inputs to the sensor so as to eliminate
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: . . ' ~ .
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associatcd acoustical disturbances.
The first two of these items invol~e determining the proper chamber
geometry based upon a balance of the acoustic reflections and pneu~atic stiff-
ness. The last item involves the use of the series and parallel cavities
scparatecl by orifices to control the frequency of acoustic waves that cnter the
sensor cavity adjacerlt d;aphragm 4.
Io reducc the molecular weigh-t or density sensitivity, the cavity B
must in gcncral be effectively made smaller than in past practice. Any closed
circular cylindrical resonator has a reinforcing standing wave when its length
(distance between reflecting surfaces) is equal to the length of one half wave
and an interference wave when its length is one quarter wave length. The sen-
sor cavity B with the small orifice 21 for an entry port appears as a closed
tube, with small secondary effects related to the orifice opening. The reson-
ant frequency o a closed resonator such as cavity B is obviously related to
its length; however, this resonance depends upon the temperat-re of the gas
and the molecular weight of the gas. The frequency of the chamber resonance
is independent of the gas pressure. The temperature and molecular weight of
the gas determine the acoustic wave velocity for the gas. As the acoustic
wave velocity changes, the resonance frequency of the cavity changes. Also,
waves of any frequency take a different length of time to travel from the dia-
phragm 4, to rebound from a reflecting surface 18a, and to return to the dia-
phragm; thus, as the sonic velocity of the gas changes, the diaphragm ~ is
affected differently by the acoustic waves. ~hen different gases are used as
the media, the molecular weight becomes a variable, and hence7 the sensor
operates at a slightly different frequency for a given pressure and tempera-
ture for different gases. This is molecular weight sensitivity or densi-ty
sensitivity.
To decrease molecular weight sensitivity, the distance to be travel-
- 1'1 -
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'
,
ed by the acouStic wave bet~een sur~aces 4a and l8a is made yery short. ~his
allows the occurrence of a small change in the l~ave velocity with a minimum
effect on the diaphragm response. The shorteni~g of the distance traveled by
the acoustic l~ave can also he thought of as increasing the resonant frequency
of the cavity such that it is always much h:igher than the operating frequency
of the diaphr.lgm. Since the t~o parameters that cause the ~ave velocity to
change are gas molecular weight and temperature, the reduction of the effect
of a change in ~ave velocity reduces the sensitivity of the sensor to changes
in molecular ~eight and temperature. It should be noted that the metal of
diaphragm 4 has an inherent small temperature sensitivity and, hence, the
foregoing change in chamber geometry reduces only the temperature sensitivity
related to the acoustical ef-fects. In fact, the total temperature sensitivity
is actually slightly increased in magnitude, but it is simplified from being
a dual function of pressure and temperature to substantially a function of
pressure only.
The pneumatic spring effect limits ho~i close the reflecting surfaces
4a and l&a may be made. If the volume of the chamber becomes too small in
proportion to the volume displaced by the diaphragm vibration, *hen the stiff-
ness of the gas becomes a significant portion of the total diaphragm stiffness.
As the diaphragm 4 oscillates, it pumps gas in and out of chamber B through
orifice 21. As the chamber volume becomes smaller, more and more energy is
required for the diaphragm to compress and to pump the gas. This gas stiff-
ness can be thought of as a spring a-ttached to the diaphragm, and as the ratio
of cavity volurne to displacement volume becomes smaller, the stiffness of the
spring increases. As this spring becomes stiffer, more energy is required to
move diaphragm 4. Ilence, to reduce the molecular ~eight sensitivity of the
absolute pressure sensor, the sensor charmber must have the reflecting surface
18a as close to the diaphragm surface ~a as possible, and at the same time
- 15 -
retain a reasonable volume in the chamber B. Thc precise balance of these
effects, and hence, the necessary cavity geometry depends arbitrarily upon the
pressure range to be measured and the charac-teristics of the particular dia-
phragm to ~)c used.
I`he final area of concern is acousti.c f;.ltering. ~coustic interfer-
ence wa~cs in the pneumatic lines may cause -the sensor to be unstable and in-
accurate. These acoustic waves come from two sources, outside of the sensor
in othcr parts of the overall system and waves generated within the sensor and
reflected from a discontinuity such as a restriction in the associated pneu-
lQ matic system. To help control this problem, an acoustic filter is incorporated
in the sensor chamber configuration. Both orifices and combinations of cavity
volumes can be used to filter these waves so as to isolate the sensor from
external interference.
Accordingly, lt is seen that the invention employs a novel configu-
ration having a self-oscillating diaphragrn separating the cavities A and B of
Figure 1, cavities B, C, and D forming effectively a single resonant chamber.
The shape and volume of the latter chamber are selected to minimize two im-
portant adverse effects:
1. acoustic noise reflections arising from surfaces within the
latter charnber and similar noise signals arising ~ithin or reflected
into the sensor through the pneumatic signal supply line 69, and
2. pneumatic stif-fness of the gas within the cha-mbers A, ~, and C.
The selected configuration provides a wide separation of the undesired acous-
tic resonances and the range of operating frequencies of diaphragm 4. The
configuration provides a divider wall 18 with a surface 18a separated from
di.apilragm 4 b~ a distance much less than one quarter wave length at the high-
est normal operating frequency of diaphragm 4, thereby minimizing the sensiti-
vity of the s~nso~ t3 ~coustic resonance effects. Divider wall 18 locates
.
,. . .. . .
~ 156~
orifice 21 so as to provide restricted communication between cavities B an~ C.
The co~bination of ca~ities B and C with or-ifice 2l and or;fice 68 provides an
acotlstic lo~ pass filter that passcs the desircd low frequency signals that
are truepressureinformation signals, ~hile filtering out all high frequency
pressure noise eomponents.
In this manner, it is seen thclt the invention uses common internal
p~rts in compensating for undesired density and acoustic noise effects present
in prior art pressure sensors. The invention is not only useful as an abso-
lute pressure ~ransducer yielding an output suitable for use in digital con-
trol or instrumentation systems, but may readily be adapted to measure dif-
ferential pressure values with respect to two varying input pressures. In the
latter application, the outer casing l may be discarded and the new configu-
ration would substantially take the form of a mirror image arrangement about
the plane of diaphragm 4 in Figure 1, the diaphragm serving in common a lower
configuration like that of Figure 'l and an upper mirror image configuration
affixed to annular flange 2 and above diaphragm 4.
Practical devices such as that of Figure 1 are ~uite small, involv-
ing cavities of very small volume. For example, in one typical form of the
invention, the volume of cavity A was about 0.084 cubic inches, while the
effective gas-rilled volume below diaphragm 4 was about 0.271 cubic inches.
Orifice 21 was formed by a bore about 0.029 inches in diameter, divider wall
18 being about 0.050 inches thick at the location of the orifice. The internal
diameter of orifice 68 was about 0.125 inches. In view of the small si~e of
such a device, the advantage of forming the cavity system by first machining
diaphragm ~4~ and cylindrical wall ~2~ 14, 50) portions and separate base ~54)
and cylindrical wall (15) portions is apparent. The coil and dividcr wall
supporting interior parts of the sensor may be affixed to reentrant par-t 66,
forming a first sub-~sse~Iy. The ~agnet 10, 11 may be affixed to diaphragm
~f~
4, forming a second s~lb-assembly. ~sing 0-ring 13, the two s~h-assemblies
may then readily ~)e mated prior to forming seal 51. The arrangement ~hereby
the annular clectron beam seal 51 is eFfected as far as possible from the thin
diaphra~n 4 pcrmits assembly of the cavity system without damaging thermal
clistortion, including possible asylllmetric distortion of diaphragm 4. It will
be understood by those skillecl in the art that the dimensions and ratios used
in drawing ~igures l and 2 are selected with the view oF providing drawings
tha-t are most fully beneficial in clearly illustrating the invention, and that
the inven-tion is not at all limited to dimensions or dimensional ratios ex-
pressed or implied in this specification.
Figure 3 provides curves at 1, 30~ and 90 inches of mercury of thegain and phase angle characteristics of the novel transducer system, showing
gain and phase characteristics of interest, especially at the vertical dotted
lines 104 and 105. Low frequency -true pressure data signals are substantially
unaffected by the invention, while signals of frequencies somewhat above dot-
ted line 104 are heavily attenuated by the Filtering action of the orifice-
cavity system.
I~hile the invention has been described in its preferred embodiments,
it is to be understood that the words which have been used are words of des-
cription rather than of limitation and that changes within the purview of theappended claims may be made without departing from the true scope and spirit
of the invcntion in its broader aspects.
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