CA1106639A - Vibrating diaphragm fluid pressure sensor device - Google Patents

Abstract

Abstract A fluid pressure responsive transducer is disclosed of the kind having a thin flat vibrating diaphragm serving as a wall of a continuous pres-sure chamber for converting a variable fluid pressure magnitude directly into a variable frequency electrical signal. The thin flat diaphragm, secured only at its periphery to the chamber, becomes progressively stiffer as it is pro-gressively deformed by fluid pressure loading more and more from its flat or unstressed position. A dual cavity configuration of the pressure chamber is formed by a rigid divider wall with a restricted passage therethrough. The effects of disturbing acoustic waves which might otherwise be generated in-terior of the fluid chamber or within pneumatic lines coupled thereto are thus suppressed.

Description

This application is a diyision of our Canadian patent application Serial No. 299,986 filed March 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 func~ion of fluid pressure loading. Most important, the device has an output frequency variation substantially greater than prior ar-t devices over pressure ranges of interest particularly in air da~a and altitude sensing systems.
In more particularity, the device of the prior Frische patent utili-~es a pressure chamber ha~ing a wall defined by a flat diaphragm uniformly restrained at its periphery and subjected to fluid pressure dif~erences 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 ac~ing 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. The diaphragm may properly be considered as a spring-mass mechanical system, and it can there~ore be driven at a characteristic resonant Erequency which is a function of its ef-fective mass and spring stiffness. As the diaphragm is deformed to a lesser or greater degree by changes in gas pressure, its stiffness changes and its mechanically resonant frequency changes as a truefunction oE applied pressure. Thus, the flat diaphragm system pro-vides the desired pressure-to-frequency conversion characteristic needed for digital pressure measurement applications.
The vibrating 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. However, 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 cer-tain circum-stances, interfere with the degree of precision of pressure measurement ob-tainable by the device. 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 prior 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 improved vibrating diaphragm fluid pres-sure sensor in which the effects of disturbing acoustic waves which might otherwise be present within the interio:r of the fluid chamber or within pneu-matic lines coupled thereto are eliminated, Like the deYices of the priorFrische patent, the inventIon includes a thin flat vibrating diaphragm divid-ing 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 alternatively be a steady reference pressure or a second variable pressure.
The vibrating diaphragm has the appropriate thinness, surface area, and resi-liency that its resonant frequency changes in accordance with the relative magnitudes of 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 providing a corresponding output signal. According to the invention, a rigid wall is sup-plied 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 ~luid 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 sub-stantially 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, in fluid pressure measuring apparatus of the kind including a vibratable common wall disposed between first and second chambers, at least said first chamber .

' being adapted for coupling to a source of fluid pressure for providing a dif-ferential pressure acting across said vibratable common wall, said vibratable common wall having a resonant frequency changing in accordance with changes in the differential pressure acting thereupon, means responsive to vibration of said v:ibratable common wall for driving same over a predetermined range of operating frequencies and for providing an output signal corresponding there-to, the improvement comprising: rigid wall means affixed within said first chamber for dividing said first chamber into first and second cavity means, said first cavity means being bounded in part by said vibratable common wall, said second cavity means being adapted for coupling to said source, and res-tricted orifice means for mutually coupling said first and second cavity means through said rigid wall means whereby pressure variation frequencies below a predetermined frequency are coupled into said first cavity means to act upon said vibratable common wall, whereas pressure variation noise frequencies above said predetermined frequency are excluded by said first and second cav-ity means and by said restricted orifice means from acting upon said vibra table common wall.
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 o~ 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 as-sociated ~easurement circuit showing electrical in~erconnections with the driver mechanism of Figure 1.
Figure 3 is a graph useful in explaining the operation of the in-vention.
Referring to Figure 1, the invention includes a flat, circular, , ' resilient metal diaphragm 4 which is formed integrally at one end of a gene-rally cylindrIcal l~all member 25, Though diaphragm 4 is prefcrably forrned as an integral part of wall mem~er 25, the diaphragm may alternatively be a sep-arate member if uniformly welded at its periphery, as by electron beam welding, to ~all member 25. Wall member 25 :is provided witll upper and lower annular flange members 2 and 50 encompassing an annular recessed region 14 between 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 on the axis of hollow cylindrical 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 example, of one particular metallic substance, the choice being dictated largely by the stable resiliency requirements of diaphragm 4. Since diaphra~n 4 must have minimum internal hysteresis characteristics, the diaph-ragm and its associated elements are constructed of Be-Cu or alternatively of a commercially available alloy of nickel, iron and chromium sold under the trade mark Ni-Span C by the International Nickel Co. 9 Inc., Huntington Alloy Products Division. Use of such a material having substantially a zcro tempe-rature coefficient of Young's modulus over the operating range of temperatures is preferred.
The cavity-defining elements described in the fore-going are ulti-mately formed into an integral unit by generating an annular bond 51 between the lower annular flange member 50 and a second annular flange member 52 for-med as part of base member 54, as by electron beam welding. Before the weld 51 is actually formed, two or more relatively large openings 16, 7~ are bored ~ 3 ~

through the cylindrical wall me~ber 15, Also, an annular gro~Je is formed in the inner wall 3 to accommodate ~-ring 13; the latter forms a hermetic seal between wall 3 and the adjacent outer wall 15. By virtue of the openings 16, 74 and orifice 21 in the rigid divider wall 18, there are no significant long term pressure differences on the opposite sides of divider wall 18. It will be appreciated that the configuration employing 0-ring 13 and the elements providing walls 3, 15 permits ready assembly of the parts ultimately unified by seal 51.
Reentrant portion 66 is equipped with an axial bore within which is sealed, as by epoxy cement, an extension 65 of an insulating support element 24 composed of phenolic or a conventional compressed, molded plastic, for example. Element 24 provides support, because o-f 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 divider wall 18 is fastened by an epoxy seal 19 to portion 12 of insulator 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 sides.
A~ 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 ~ith 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 variable pressure whose magnitude is to be measured. Provision is also made through base member 54 for the supply of driving electrical current to driver pick~off coil 8, as will be further described. For this purpose, the rigid conductor 60 extends through a conventional glass-to-metal seal 61.

Epoxy cylinder 62 within a bore through base member 54 seryes to stabili~e the lead 63 to prevent shorting, ConductoT 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 flow through coil 8. In this 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 between base 54 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 ~Ihen Be-Cu is used in the cavity-defining elements. It will be seen that several major cavities are formed within casing 1. A first cavity pro-vides an isolated chamber, 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 interiGr of a cup-shaped 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 11 so as to cut the conductors of coil g when the armature position oscillates vertically.
For this purpose, the magnet assembly is mounted in a hub 6, being sealed therein at 5 ky solder~ for example hub 6 bein~ affixed to the center of the yibra~ory diaph~agm 4, ~ y applying a sinusoid~l electrical signal of proper frequency to terminals 63, 73 of coil 8 (see ~igure 2), diap~ragm 4 is caused to vibrate at its natural mechanIcal resonance frequency. The transducer response reaches a resonant peak when the driving frequency is equal to the mechanical resonance frequency of diaphragm ~1, the latter being d~termined 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 ~igure 2, such that the back electromotive force generated as the magnet assem~ly moves ~ith respect to coil 8 is connec-ted back to the input of driver amplifier 82, ~n 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 member 54 to terminals 85, ~8 of a bridge circuit 88, 95, 96. The output of the bridge circuit a* terminals- 98, 9~ is connected via leads 83 *o the input of amplifier 82 in order to amplify the unbalance 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 circuit operates : as a self-resonant electromechanical oscillator ~hich oscillates at the natu-ral resonance frequency of diaphragm 4. The output of amplifier 82 may be further supplied by leads 84, 100 *o be amplified by an output amplifier 87 to provide an amplified signal whose frequency is the desired function of pressure to a utili~ation circuit such as a counter and display 101. In order to main-tain the bridge circuit accurately balanced, coil 88, which forms the fourth leg of the bridge circuit, may also be disposed iD *he cavi*y C, thereby to be subjected to the same thermal environment as coil 8.

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'' '- ,' '" , , ' . '' ' The pre~ent invention has been described with Tespec~ to a diaph-ragm driving and velocity detecting arrangement including a permanent magnet assembly moving with the diaphragm and cooperating with a fixed coil. It will be appreciated by those skilled in the art that other possible techniques for driving the resonant diaphragm and for detecting its motion may include a moving coil and a fi~ed permanent magnet assembly, an electrostatic driver with variable 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 and 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 beha~ior of diaphragm 4 of the present invention.
Some further theoretical considerations will be useful in understanding the problems overcome by the present in~ention. To measure an absolute pressure with the prior Fiische device, a first side of the vibrating diaphragm 4 is exposed to a reference vacuum. Hence, depending on the quality of the vacuum, there is essentially no acoustic response from that first side of the vib-rating diaphragm within the ~acuum. The acoustic response of the gas on thepressuri~ed side of the vibrating diaphragm depends on the moiecular weight and temperature of the gas and the shape of its associated chamber that is inherently an acoustic resonator 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 of 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 produces a change in the acoustic response seen by _ g _ .

, the vibrating diaphragm and, hence, the Yibxating diaphragm oscillates at dif-ferent frequencies when tne deyice is operated at the same pressure and temp-erature~ but wit~ different gas media. The aforementioned phenomena is com-monly referred to as density sensItlvity. To reduce the effect of operating the vibrating sensor diapl~ragm witll various gases, the resonance frequency of the gas filled chamber can be made much higher than the range of operation of the vibrating diaphragm. The arther away 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 acoustical reflecting surface 18a accordingto the present invent~on is placed close (much less than one quarter wave length) to the vibrating diaphragm surface 4a. Since the speed of sound in a gas is also dependent upon gas temperature, the chamber resonance frequency is inherently also a function of temperature. However9 the close reflecting sur-faces 4a, 18a also 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 acoustical phenomena occurring within the gas fil-led pressure cavity, because the gas itself acts like a pneumatic spring. The stiffness of this pnewnatic 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 ~ibrating 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 volwne displaced by the vibrating diaphragm oscillation is small compared to the chamber volume, and so that the ~ 10 -spring action of the gas does not become a significant portion of the total stirfness of the vibrating diaphTagm. Hence, the optimum chamber geometry depends upon a balance of these two restrictions tailored cmpirically to a particular combination 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 exampleJ 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 aircraf~ fuselage, or the like. ~y 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 pneumatics of a vibrating diaphragm pressure sensor. These are acoustical reflections from surfaces within the sensor chamber and pneumatic stiffness of the gas wi~hin the chamber. These two phenomena are controlled according to the present invention by means of pro-per chamber geometry and size to produce optimum performance of the vibratingdiaphragm pressure sensor.
Both of these effects can be described mathematically by means of the classical wave equation:

C2V 2 0 _ ~ t where ~ is *he velocity potential, V is the conventional operator, t is time and C is the sonic velocity as defined by:

, :: :

C = ~/ KRT

V m where 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 equation and momentum equations for a compressible, zero viscosity gas.
For an axially-symmetric~ cylindrical hollow resonator, the ~wave equation is:

2 2 2 ~ 0 + 1 ~0 + ~ 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 dis~urbing pheno-mena, acoustical reflec~ion 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 ~ and strike ~he bounding surface 18a, being reflected therefrom. Hence, after the acoustic waves are reflected from surface 18a, they travel back toward diaphragm 4. ~hen the waves return to thc surface 4a of diaphragm 4, they may or may not be in phase with the motion of the dia~
phragm and new waves may be generated; hence, the in-phase or resonant waves ~end to add energy to diaphragm 4, tending to reinforce its oscillation; or ,~

they may be ou-t-of-phase or anti-resonant and tend to take energy from the diaphragm and to oppose its oscillation. The acoustic wa~es travel at the speed of sound through ~he gas medium, which speed depends upon the tempera-ture and molecular ~eight of the gas. Thus, for a given gas at a constant temperature, the time necessary ~or a wave to travcl from the diaphragm sur-face ~la to the reflecting surface 18a and back to the diaphragm surface 4a depends on the distance traveled. ThereFore, the distance from the diaphragm surface ~a to the reflecting surface 18a determines, at least for constant temperature conditions, whe~her the reflected wave diminishes or amplifies the oscillation of diaphragm 4.
The gas medium also acts as a pneumatic spring attached 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 ~ to the total volume of the chamber.
As this ratio becomes larger, the pneumatic stiffness becomes great; as ~he sensor chamber decreases in volume, the stiffness of the gas becomes a signif-icant portion of the total stifness 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.
Therefore, it is sèen that the optimum charnber configuration con-sists of a compromise with respect to the foregoing acoustic phenomena. T~e 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 of pneumatic inputs to the sensor so as to eliminate -- , ; ' ,~ :
.

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associated acoustical dis~urbances.
The first two of these items involve determining the proper chamber geometry based upon a ~alance of the acoustic reflections and pneumatic stiff-ness. The last item involves the use of the ser:ies and parallel cavi-ties separated by orifices to control the frequency of acoustic waves that enter the sensor cavlty adjacent diaphragm 4.
To reduce the molecular weight or density sensitivity, the cavity B
must in general be effectively nlade 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 R 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 of a closed resonator such as cavity B is obviously related to its length; however, this resonance depends upon the temperature 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 4 is afected differently by the acoustic waves. When different gases are used as the media, the molecular weight becomes a variable, and hence, the sensor - operates at a slightly different frequency for a given pressure and tempera-ture for differen~ gases. This is molecular weight sensiti~ity or density sensitivity.
To decrease molecular weight sensitivity, the distance to be travel-1'1 -' - ~ , ed by the acoustic wave between sur~aces 4a and 18a is made very short. This allows the occurrence of a small change in the wave velocity with a minimum effect on the diaphragm response. The shortening of the distance traveled by the acoustic wave can also be thought of as increasing the resonant frequency of the cavity sucK that it is always much higher than the operating frequency of the diaphragm. Since the two parameters that cause the wave velocity to change are gas molecular weight and temperature, the reduction of the effect of a change in wave velocity reduces the sensitivity of the sensor to changes in molecular weight and temperature. It should be noted that the metai 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 effects. 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 how close the reflecting surfaces 4a and 18a may be made. If the volume of the chamber becomes too small in proportion to the volume displaced by the diaphragm vibration, then 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 cha~ber 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 attached to the diaphragm, and as the ratio of cavity volume to displacement volume becomes smaller, the stiffness of the spring increases. As this spring becomes stiffer, more energy is required to move diaphragm 4. Hence, to reduce the molecular weight sensitivity of the absolute pressure sensor, the sensor chamber must have the reflecting surface 18a as close to the diaphragm surface 4a as possible, and at the same time _ 15 -retain a reas-onable volume in the chamb,er B~ The precise balance of these effects, and hence, the necessary cavity geometry depends arbitrarily upon the pressure range to be measured and the characteristics of the particular dia-phragm to be used.
The final area o~ concern is acoustic filtering. Acous-tic interfer-ence waves 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 other parts of the overall system and waves generated within the sensor and reflected from a discontinuity such as a restriction in the associated pneu-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 wa~es so as to isolate the sensor from external interference.
Accordingly, it is seen that the invention employs a novel configu-ration having a self-oscillating diaphragm 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 chamber and similar noise signals arising within or reflected into the sensor through the pneumatic signal supply line 69, and 2. pneumatic stiffness of the gas within the chambers A, B, 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 diaphragm 4 by 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 sensor to acoustic resonance effects. Divider wall 18 locates ~ 16 -~A~j orifice 21 so as to provide restricted communication ketween cayities B and C.
The combination of cavities B and C with orifice 21 and orifice 68 provides an acoustic low pass filter that passes the desired low frequency signals that are truepressureinformation signals "~hile filtering out all high frequency pressure noise components.
In this manner, it is seen that the invention uses common internal parts 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 transducer yielding an output suitable for use in digital con-10 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 1 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 l, the diaphragm serving in common a lower configuration like that of Figure 4 and an upper mirror image configuration affixed to annular flange 2 and above diaphra~m 4.
Practical devices such as that of Figure l are quite 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 20 effecti~e gas-filled 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 beir,g about 0.050 inches thick at the location ~f the orifice. The internal diameter of orifice 68 was about 0.125 inches. In view of the small size of such a device, the advantage of :Eorming 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 divider wall supporting interior parts of the sensor may be affixed to reentran~ part 66, forming a first sub-assembly. The magnet 10, ll may be affixed to diaphragm , ~, -

4, forming a second sub-assembl.y. ~sing 0-ring 13, the two s~b-assemblies may then readily be mated prior to forming seal 51. The arrangement whereby the annular electron beam seal 51 is effected as far as possible from the thin diaphragm 4 permits assembly of the cavity system without damaging thermal distortion, including possible asymmetric distortion of diaphragm 4. It will be understood by those skilled in the art that the dimensions and ratios used in drawing Figures 1 and 2 are selected with the view of providing drawings that are most fully benefi.cial in clearly illustrating the invention, and that the invention is not at all li.mited to dimensions or dimensional ratios ex-pressed or implied in this speci-fication.
Figure 3 provides curves at 1, 30, and 90 inches of mercury of the gain 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 freqllencies somewhat above dot-ted line 104 are heavily attenuated by the filtering action of the orifice-cavity system.
While 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 invention in it.s broader aspects.

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In fluid pressure measuring apparatus of the kind including a vibra-table common wall disposed between first and second chambers, at least said first chamber being adapted for coupling to a source of fluid pressure for providing a differential pressure acting across said vibratable common wall, said vibratable common wall having a resonant frequency changing in accord-ance with changes in the differential pressure acting thereupon, means res-ponsive to vibration of said vibratable common wall for driving same over a predetermined range of operating frequencies and for providing an output sig-nal corresponding thereto, the improvement comprising: rigid wall means af-fixed within said first chamber for dividing said first chamber into first and second cavity means, said first cavity means being bounded in part by said vibratable common wall, said second cavity means being adapted for coupling to said source, and restricted orifice means for mutually coupling said first and second cavity means through said rigid wall means whereby pressure vari-ation frequencies below a predetermined frequency are coupled into said first cavity means to act upon said vibratable common wall, whereas pressure vari-ation noise frequencies above said predetermined frequency are excluded by said first and second cavity means and by said restricted orifice means from acting upon said vibratable common wall.

2. Apparatus as described in Claim 1 wherein: said vibratable common wall takes the form of diaphragm means substantially flat when at rest, and said rigid wall means is disposed substantially parallel to said diaphragm means when at rest and spaced therefrom by a finite distance substantially less than one quarter of a wave length of the highest normal operating fre-quency of vibration of said diaphragm means.

3. Apparatus as described in Claim 2 wherein the ratio of the volume between said rigid wall means and said diaphragm means relative to the volume of gas displaced between the maximum positive and negative deflections of said diaphragm means is so selected as to prevent fluid resistance to compressibil-ity from affecting the amplitude of vibration of said diaphragm means.