CA1141560A - Vibrating beam pressure sensor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A sensor providing an output indicating pressure including means to excite a vibrating beam at its natural frequency, and means to load the beam in response to pressure to alter the natural frequency of the beam. The frequency output is sensed by a capacitor pick off mounted on the same side of the beam as the means to excite the beam.
Changes in the output frequency provides an indication of the pressure being measured.

Description

S~

VIB!~ATING BE,l\M PRESSllRE Sl N~SOR
A(.i~GROUND Ol: '1`11~ INVEN'I'ION
1. Field of the In~ention The present invention relates to digital output pressure sensors using a vibrating ~eam as a sensing elcment.

2. I'r;or Art In the prior art, various sensors whictl use vibrating beams have becn advanced. Usually some type of dig;tal pulse is provided to indicate the force or pressure derived force on the In beam. Some of the sensors utilize piezoelectric sensing, such ias United States Patent No 's 3,470,~nO and 3,479,536. ~ type of pivoting beam that is constrained in its path of movement is also shown in United States Patent No. 3,649,857.
An additional patent relating to vibrating beams is Patent No. 3,664,237 which is of general interest. Capacitance sensing of vibrating beams is illustrated ;n lJ.S. Patent No.'s

3,187,579 and 3,762,223. In both of these patents the sensing capaci-tor plate is on the opposite side of a member from the drive coil.
A piezoelectric beam which flexes transversely becausc 2() of accelerations in the mass at the end oF the heam, or bccausc of impact of micro meteroids is shown in Unite~ States Patcnt No.
3,304,773. ~n acceleromcter using a vihrilting i)e~m witll ca;),lcit.lllcc sensing is sJlown ;n ll.S, I'atcllt No. 3,505,8~.fi.
lJnitcd States l'atcnt No. 4,149,~22 issuc(l Al)ril 17, 1'.)7'9 shows a vibratory wire pressure sensor which discloses a thin wire th~t is held undcr a spring load to create a ~retens iOII, and is lo-~clcd by ~ressurc so that the natural frcqucncy of the wirc ci~anges. Ii~c lever which is utilized to load the vibrating wire is mountcd with a cross flexure connection that pcrmits -rclativcly Frce l);votal movc-ment. For sensing, a current from an oscillator is ~)in~ tllrougil tl-le wire and thi s current reacts with a magnctic fickl From .l ~)crTnarlcllt magnct therc1)y callsirlg thc wire to movc. Iilis prodllccs a b.lck lMI:
and positive Fccdi)ack at the currcnt generating oscillat(>r cir(lJit sustains the vibration of the wirc. Ihus tllc s(llliOr (liicloscd in 35 I'atcllt No. ~,l49,~22 docs not utilizc call:lcitivc~ tyl~c iull~

SUMMARY OF THE INVENTION
The present invention relates to a pressure sensor and more particularly to a pressure sensor which loads and affects the natural frequency of a vibrating beam. rhe frequency is sensed and provided to the output in the form of digital pulses that can provide a direct digital measurement or readout of the pressure being measured.
The beam is loaded through a pivotally mounted lever, as shown, which aids in isolating unwanted stresses during loading of the beam.
The excitation for the beam is from a coil driven by an oscillator, and the sensing is capacitance. Tnere is, thus little interaction between the excitation and the sensing signals, which enhances accuracy.
In the particular embodiment shown, the excitation coil and the pick-off capacitor plate are mounted on the same side of the vibrating beam, to simplify mounting, and also to simplify adjustments as to the spacing between the drive coil, the sensing capacitor, and the vibrating beam. Other advantages include fewer parts, better control on parasitic resonances, and components of sensing circuit may be mounted on the pick-off assembly.
With the vibrating beam mounted as disclosed there is substantially no current in the beam, so that the beam has little undesirable effect on the drive coil or the pick-off capacitor perforrnance.
BRIEF ~ESCRIPTION OF THE ~RAWINGS
Figure l is a top plan view of the pressure sensor including sensing means made according to the present invention;
Figure 2 is an elevational view taken as on line 2--2 in Figure l with parts broken away;
Figure 3, which is on the same sheet as Figure 2, is a fragmentary sectional view taken as along line 3--3 in Figure 2;
Figure 4, which is on the same sheet as Figure l, is a view taken on line 4- 4 in Figure 3;
Figure 5 is a sectional view taken as on line 5--5 in 3~ Figure l;
FIgure 6 is a fragrnentary top plan view of a rnodified form of the invention;

~.~ 4~

Figure 7, which is on the same sheet as Figure 2, is a side view of the coil and pick-up assembly used with the device of Figure 6 taken as on line 7--7 in Figure 6;
FIgure 8 is a view taken as on line 8--8 in Figure 7; and Figure 9 is a schematic representation of a simplified drive and sensing circuitry used with the sensor of the present invention.
D~SCRIP~ION OF THE PREFERRED EMBODIMENT
- In Figures 1 and 2, a general layout of a sensor made according to the present invention is shown. Such a sensor assembly is indicated generally at 10, and includes an outer housing 11 that provides a base for mounting the various components. The housing base indicat~d at 12 is a flat plate, and the housing includes upright walls 13 and 14. The exterior surface of housing 11 preferably has a silicone rubber coating approximately .062 inches in thickness deposited thereon to dampen exterior vibrations. Walls 13 are provided with apertures and mounting means for mounting a pair of bellows indicated at 15 and 16, respectively.
This bellows 16 has an end collar 17 that forms a small ring which fits partially within and is retained in a recess in a clamp 23A. The clamp 23A slidably fits over and may be slid along a tang 23 which forms a portion of a pivoting, loading lever assembly indicated generally at 24. The tang 23 extends between the bellows 15 and 16. The bellows 15 has an end ring 17A which partially fits with a recess in tang 23 and is thus connected to the tang 23. The bellows 15 and 16 transfer to the tang 23 loads from pressure which expand the bellows in an axial direction. The tang 23 has dove tail edge portions over which the clamp 23A mates and slides. The bellows are of ordinary design used with sensors and normally are at a rest or balanced position as shown. A first pressure inlet 20 is connected to the bellows 15, and a second pressure inlet 21 is open to the bellows 16. The bellows, of course, when subjected to pressure will tend to expand axially and extend their inner ends adjacent the tang 23 and clamp 23A. Thus, as shown, differentials in pressure will cause a shift in the position of the tang 23.
The clamp 23A can be slid and adjusted transversely to the -1~4~5~

direction of movement of the bellows along tang 23 so that the moment of bellows 16 about the hinge like pivot shown at 27 can be shifted with respect to the moment of the bellows 15 about hinge 27. This movement of the clamp 23A along the tang 23 permits adjustment of the force from one bellows or the other so that whcn equal pressurc is applied to bellows 15 and 16, the resultant force (moments) is zero.
The lever assembly 24, as shown in plan vicw, includes a mounting block 25 that in turn is attached to the base wall 12 of the housing 11. ~he lever assembly 24 is made from a unitary piece of material. ~ moving lever portion 30 is joined to block 25 through a hinge-like pivot shown at 27. The pivot is formed as a substantially reduced cross section thickness joining the block 25 and portion 30 along the axis of pivot. Thus the lever assembly has a mounting portion 25 and a pivoting lever portion 30. The pivoting lever portion 30 clears the base 12 of the housing so that it may pivot under pressure differentials. Suitable spacers may be placed under block 25 to raise the lever portion 30 away from base 12.
'rhe lever portion 30 pivots about the hinge or pivot por-tion 27, which has little stiffness to pivoting, but has great stiffness resisting twisting of the lever portion 30 about an axis parallel to the plane of pivotal movemellt. 'I'his stiffness i~ ul-io effective against acceleration in a directioll along tlle l)ivotal axis.
'I'he lover portion is connected to tang 23 by a connector section 31.
A loading beam indicated generally at 35 is made of a single block of magnetic material, preferably Ni-Span C, alloy ~n2, ~h. Cl, t~ C/G ~r~ C7 ~ V 7Q
~*~e-by the International ~ickel Company. I,oading beam 35 inclucles a center vibrating beam section 42 that is positioned between and integral with first and second isolator sections 43 ancl ~4 which support the vibrating beam section 42. Isolator section 43 is mounted at its outer encl opposite its junction to the vibrating beam section as at 36 to a support lug 37 which can be part o~ the hlock 25.
l'he outer end of isolator section 43 may also be separately mountecl to the base 12 of the housing. The connection to support 37 is a fixecl connection, such as so~dering or brazing. 'I'he outer encl of thc isolator section 43 can also be pinned or fastenecl with su-itable cap screws.

~4~s~n l`he outer end of the isolator section 44, which is the opposite encl of the loading beam 35, is mountcd as at 40 to a support 41 protruding from the pivoting lever portion 30. The mounting can be made in any desired manner.
The sensor assembly is acceleration balanced on at least two axis, that is, an acceleration in the "Y" axis or the "X" axis shown in Figure l will have little or no effect as the sum of the torque moments on the sensor side of the pivot axis is the same as the sum of the torque moments on the bellows side. The sensor is not so balanced in the "Z" axis (shown in Figure 7) as the stiffness of the pivot is effective in substantially reducing acceleration effects from a force applied along this axis. It is also possible to balance the torque moments about this axis if desired. Due to tolerances, every sensor assembly has a somewhat different torque moment. The effect of these tolerances with respect to acceleration preferably is minim~ed by depositing small amounts of a material, preferably solder in the areas near pivot 27 to equalize the torgue moments about that axis as shown at 27A in Figure l.
l'he vibrating beam section 42 is thus supported by isolator sections ~3 and 44 between supports 37 and 41. 7'he beam section 42 is thus subjected to forces that are exerted by movement of the pivoting lever portion 30. The isolator sections 43 and 44 are each rclieved in its center ~ortion to form a pair of thin strll~s.
It can be seen therefore that .my movcment of the tang 2-, between the bellows 15 and 16 because of deflections or movements of the bellows will cause thc tang 23 to pivot the pivoting lcver portion 30 ahout pivot 27. l`he movement causes a change in thc tension or compression stress or loading of the vibrating beam section 42. The change in loading of the vibrating beam section 42 will change its resonant frequency of vibration The change ;n resonant frequency will be proportional to the pressurc differential present in pressure inlets 20 and 21, and thus the bellows 15 and lG.
One pressure inlet can be open to atmosphere or be evacuated, so thc value of an individual pressure can be meiasured as well. I'urtllcr, one bellows may be removed.
l'hc beam is driven or excited into resonance through thc use of a coil which in turn is excited by an AC signal controllcd at ~4~5~C) resonant frequency. ~urther, at rest the beam is nominally balanced, not in compression nor tension; thus is not subject to creep and relax-ation of the supporting structure or the beam which,contribute to sta-bility problems. Preferably, the beam is operated at lower stresses within the elastic limits of the beam material.
The isolators 43 and 44 each include a mass 43A and 44A
extending perpendicular to the beam section 42 and each has spaced isolation springs 43B and 43C and 44B and 44C, respectively. These isolators decouple the vibrating beam section 42 from the en~ mountings to reduce energy losses which lower the beam "Q". It should be noted that small stress reducing radii are used at the junction between vibrating beam section 42 and the masses 43A and 44A.
In the design of the beam section 42 and isolators 43 and 44 it is important that no single element has a natural frequency that can be easily driven by the beam section 42 itself which must sweep a wide frequency range to be useful. If the other elements have a natural frequency which can be driven by the beam section 42, the beam section 42 will tend to excite such element in the system, particularly if its natural frequency is equal to or in some multiple of (i.e. 1, 2, 3, and 4 times) beam freq-lency. When this happens the offectiveness of the isolators is reduced thus reducing the beams "Q"
and changing its frequency. The result of this is a sharp ~is-continuity in the beam's linearity or smoothness with applied stress.
In addition, it can be seen that the center beam can he easily clriven at its resonant frequency and two times its resonant frequeTlcy, since with each half cycle of the beam there is one pull on the ends of the vibrating beam section 42. The beam deflects to opposite sides of its at rest plane during operation.
In practice, it is not possible to make the isolators so low in frequency t1lat they cannot be driven by the vibrating bearn since they would have to be very long and th-in. 'Ihu.s the ovcr.lll scnsor size would be large and the system would be clifficult to manufactllre.
lt is possible as discussed above, for the vibratiTlg beanl-;ection to drive the isolator springs at tlleir seconcl, or -third or higher o~der harmonics.
Also, in practice it is not possible to makc tllese isolator springs so high in frequency that they cannot he drivcn and still providc 1~4~5~à0 the necessary low frequency isolation. Thus it can be seen that the isolator spring frequency must be chosen to provide a "window" so as not to interfere with the vibrating beam section 42 through its stressed frequency range. That is, when stress is appliecl to the vibrating beam section 42 its natural frequency and twice this fre-quency should not coincide with prominent resonant frequencies of the isolator springs 43B and 43C and 44B and 44C. The widest "window"
is provided when the fundamental isolator spring frequency for each of these four springs is greater than the highest vibrating beam 10 section frequency, which occurs at full scale stress, but lower than twice the rest frequency of the vibrating beam section 42.
A specific example of the beam section 42 includes a beam section .0047 inches thick, with approximate limits of not less than .001 inches and not greater than .010 inches. The beam section is c .250 inches long, with approximate limits of not less than .10 inches and not greater than .50 inches and it is .042 inches wide, with approximate limits of not less than .020 and not greater than .100.
The unstressed natural frequency is directly proportional to the beam thickness and inversely proportional to the square of the beam length and is about 14.5 Kl-lz using a suitable material such as Ni-Span C. Ni-Span C is used for its uniquo com~ination of excellent spring properties and substantially zcro temperature coefficient over a wide range of temperatures. Other natural froquencies could have been chosen.
I`lle change in frequency is determiTIecl by the stress in the beam which is generated by the force provided by the lever assembly 24. In one example, this is about 2.5 pouncls provided by one-half inch diameter bellows at one atmosphcre. The stress in the vibrating beam section 42 can be adjusted without changing the heam section's unstressed frequency by changing the width. In thc cxample, the stress was acljusted to about lO,OOO psi using Ni-Span C. 'I'his stress is ideal since it is very low compared to the yielcl strength of Ni-S~all C
and thus an extremely low system hysteresis errors res~llt. In adclition, long term changes in frequency arc not cncountcrecl. Thc rcs-llting changc ~4~S~C~

in frequency of vibrating beam section 42 is about 2300 llz or about 16% of full scale frequency which can be provided either by putting the beam in compression or in tension. Thus, for this example, a zero to onc atmosphere pressure change yields a resonant frequency change from 14.5 ~Iz up to 16.8 Kllz.
For the isolator springs, as discussed above, the frequencies are adjusted in the same manner as for the vibrating beam section 42, to be greater than 16.8 ~z but less than two times 14.5 KHz or 29 KHz. In practice it has been found that an isolator spring frequency of between 22 ~llz to 24 ~Iz works well. The springs 43B
and 43C and 44B and 44C are made .012 inches thick by .315 inches long for simpler construction. The isolation mass (43A and 44A) is thus adjusted to yield an isolator system frequency of 2600 Hz, which provides an excellent isolator to vibrating beam section frequency ratio and thus a very low transmissibility.
The mechanical construction of the coil and pick-off is shown in Figures 1 through 4, in particular. As shown, a ceramic mounting block 50 is mounted onto a disc member 51. The disc member is metal and can be bra~ed to the block S0 in a usual manner. If desirecl, the block may be pinned to the disc. The disc member 51 fits within a recess 52 in the base 12 (see Pigure 2) of the housing, so that the block 50 protrudes into the housing. The disc can be rotated in the recess 52 to a desired position. Thc disc is then soldere~ into place to hold it securely in rotational position during use. 'I'he block 50 is madc of a nonmagnetic, nonelectrical conclucting solid matcrial such as machinable sintered alumina or a similar ccrlmic material. As shown, the block 50 inclucles a raised portion 53 that has a surface 53A which is closely adjacent and parallcl to the plane of the vibrating beam section 42. The block 53 fits between the ends of isolator sections 43 and 44. Block 53 has a cylinclrical recess 54 defined therein extending inwardly from 1 surface 57 opposite from the surface 53A (see ~igurc 3). A manclrcl mcmber 55 is mounted in this recess, and can be boncled to the hlock 53 by elcctrically conductive epoxy or solder. The mandrel 55 is preferahly made from mag-netic stainless steel, and preferably a high "mu" or high magnetic pcr-s~n - meability material.
The mandrel has a disc end 56 and between the surface 57 of the bloek 53 and the dise 56 there is a eoil indicated at 58 that is wound in plaee. Thisis generally a single wound coil having a suitable number of turns to generate the neeessary n~agnetic foree for driving the heam as will be explained.
On faee 53A of the bloek 53 that is adjaeent and facing the vibrating beam section 42 an eleetrieally conductive strip 61 forming a eapaeitor plate is sereened onto the eeramic material and then fired in plaee. The eiapaeitor plate 61 is shown in ~igure 1.
Additional e]eetrieal eomponents ean be mounted direetly to the surflee of the ceramic bloek 50, and the various components eonnected to the eapaeitor plate by sereened on strips of con~uetive materia]. Sueh eomponents are identified subsequently in this speeifieat:ion.
The surfaee 57 of the bloek 53 whieh is adjacent to the eoil 58, and on an opposite side of the bloek 53 from the capaeitor plate 51 may be metalized to provide for a shield to prevent inter-ferenee from the eurrent in the eoil 58 from aEfeeting the eapaeitance sensed between the v;brating heam seetion 42 and the eap.lcitor plate 61.
it can be seen th~t by rotating the dise 51 the spacing hetween the ear)aeitor pIate .ancI the vibratillg l-enm sectioIl 42 c~n be ndjusted at tlle time of assembly. 'I'he l)lock 50 r~ .lte or dis~
51 are instnlled after the heaIn 35 is in pl;Ice~ 'I'lle s~.acillg betwee plate 61 nnd the vihrating be~Im seetion 42 is about .0()5 inches.
The v-ibrating beam section 42 o-f tlIe be.Im forms the otller plate of a variable eapaeitor in conj~mction with pl~te Gl. 'I'he vibrating beatn section 42 is grounded. Note tha-t hlock 50 may be supplied with circuit feed through pin 50A and connector openiIlgs 5()I3 if desired.
In Figures 6 and 7 a modif;ed form oi' the corIstructiorl is shown. 'I'he beatn 35 iarld the lever assembIy 24 (in particlIlar thc pivot-ing portion 30) and the bellows mounting are substantirIlly tlle simIe.
The mounting bloek 25 is modified for mountiIlg a modified drive .alld ~ick-o~f sensor assembly 7~. A sllart 75 is molJnted in a provide(I
operIing in the block 25 as can be .secn in l:igure G and suitable set -S~C~

screws 76 permit the longitudinal adjustment of thc shaft 75 rclative to the vibrating beam section 42. The shaft 75 in turn is attached to a disc 79 wh:ich has a rod 78 extending coil support rnandrcl outwardly therefrom. The disc 79 can be bonded to the end of the shaft 75 in the usual manner with epoxy or other bonding material. A coil 82 is l~ound onto the mandrel 78 between the inner surfacc of the disc 79 and a surface 83 of a cerarnic disc pick-up member 84. It should be noted that the coil mandrel 78 extends into a reccss in thc ceramic disc 84, a desired distance, and this mandrel 78 is suitably fastened to the ceramic pick-up to hold the two parts together and in place after the coil has been wound into place on the coil mandrcl.
The ceramic disc 84 as shown also has electrically conductive surfaces shown by shading in ligure 8 thereon thereby permitting the circuitry or portions thereof to be mounted proximatc to the beam. The components mounted on the disc 84 prefcrably include a capacitor plate 85 which corresponds to the plate 61, with sui.table connecting strips 86 which lead to a thick film hybrid resistor 87, and thick film capacitor 88 which can be cemented to and clcctrically connected to the str:ips 86. Suitable contact poi.nts ZO 87~ and 88~ cnn bc p:rovided for connecti.ng to thc othcr cnds o-f the capacitor 88 and resistor 87. As can be scen, the hyb-r;d capacitor 88 and thc roxistor 87 are mounted closc:ly adj.lcent to tllc cal)a(:itor platc 85, and protrude fron) tl-e ccramic disc mcmbcr so that tlle vibriating beam section 42 fits between these two components. The vibrating beam section 42 .is spaced closely :froTn, and generally parlllcl to thc capac:itor plate 85.
In operation, the beam in c:ithcr Form of the invcntion is vibrated hy energi~ing the associated coi]. ~ milgnetic flux is generated and causcs the vibrat:illg bcam sect iOIl 42 to deflcct in oppositc d:irections gcncrally porpcndicul.:lr to :its iongitudirlal plane. 'l'hc mandrc:l 55 or 78 extends through tho ccraMic hlock 53 or 84 a substanti.al distancc so that tho magnetic ficld is couplcd to the beam section 42. T}le changes in cap.lcit~lncc oi~ thc capacitor arc scnscd and in effcct control thc dr;vc circuit to rcs()llate thc bc.lrn scction .Is well as providc a frc~ucncy outl~ut. ~ny charlgc in diFferential pressure causcs a shift of the tang which results :in S~

pivoting lever portion 3n about the pivot axis 27 to change the compression or tension load on vi~rating beam section ~2, causing a change in the resonant frequency of the be~n. The change will be sensed by the changing of the frequency of the signal from the capacitor formed between vibrating beam section 42 and either plate 61 or plate 85, depending on the sensor that is positioned a~jaccnt the beam. This change in signal is representative of the clifferential in pressure in relation to the reference pressure frequency. Preferably the beam is driven at its fundamental frequency, however, the capacitor pick-off permits relocation of the pick-off capacitor as to be sensitive to any desired harmonic of the fundamental frequency or other desired frequcncy .
Figure 9 shows a simplified electrical schematic diagram of a circuit for deriving a frequency output signal from the vibrating beam of the present invention. The circuit can be in several different embodiments. In Figure 9~ vibrating beam section 42 is shown supported between isolators 43 and 44 as described previously. A capacitive pick-up electrode shown at 61 (it coul,~ also be electrode 85) is spaced from vibrating bearn section 42 to form a variable capacitor.
The circuit shown in Figure 9 includes a supply voltage terlninal 9(), a ground terminal 91, and an output terminal 92. A
source (not shown) of a supply voltage V+ is connected to supply voltage terminal 90. Connected betweon terrninal 90 and ground term,inal 9l are a resistor 93 ancl coil 58 (or 82). A l)C current f'lows thlougl resistor 93 and coil 58 to produce a steady magnetic field out of mandrel or core 54 (or 75). 'I'he steady magnetic field produced by the DC current through coil 58 could be replaced by a permaneTIt magnet, if desired.
Also connected-~between terminal 90 and pick-off electrode 61 is a bias resistor 88 (shown mounted on the ceramic disc iTI Figure 7).
Bias resistor 88 provides a nc bias current at junction 9$ whele resistor 88 ancl electrode 61 are connected. ~s a result, the volt.Lge across the variable capacitor Eorrned by vibrating beam sectioll ~2 and pick-off' electrode 61 is a funct-ion of -the I~C current allCI thc fre(luerlcy of vibr.ltiorl of vibr.lting bearn section 42.
The signal derived ~rom junction 95 is provided tllro~lgh .4~S~

-l2-a filter capacitor S7 (shown mounted in Figure 7) to an amplifier 97.
The amplifier output of amplifier 97 is supplied to output terminal 92 as the output signal of the circuit. The output of amplifier 97 is .llso connected through feedback ca~acitor 98 to junction ~9 which is the junction of resistor 93 and drive coil 58.
When the circuit is turned on the magnetic field from coil 58 will shiFt the beam section 42. ~he capacitance between the beam section 42 and plate 61 affects the i.nput to amplifier 97 through bias resistor 88 and capacitor 87. The output of amplifier 97 changes the feedback signal on capacitor 98 which in turn affects the current through coil 5~. Any change in current through coil 58 changes the magnetic field through vibrating beam section 42 and the vi.brating beam section 42 will again shift. The vibrating ~eam section 42 thus is excited to its natural -frcqucncy and the out~ut of amplifier 97 wi.ll vary at lS this frequency. The components are selected to provide appropriate phasing to provide the oscillations at the desired frequency range.
The circuit shown in Figure 9 therefore derives a time varying output signal whose fre4uency is a function of the frequency of bcam section 42. This output signal can then be converted to a digital or an analog signal and provides an indication of tlc tension or compression in beam section 42 which cllanges the frccluency and wllich .is a function of the prcssure to be scnsod.
Tlle actual circuit components for thc ampl:if:ier 97 alld othcr componellts for convcrting thc signal at output tcrmin.l]
92 to a digital OI' analog signal csln be mounted as shown schcmatically in ligure 1 OT if dcsired all of the components may le mounte~ on block 50 as shown in Figurc 4.
If low frequency vibrations from external sources ~such as aircraft vibrations if the sensor is used in an aircraft) causc low frequcncy osc-illation in thc OUtpllt signal an(l xuitablc filtcrs can bc used to cl:iminate any SIIC}l unwanted oscil.l.ltions.

Claims (19)

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

1. A pressure sensor providing an output signal that is proportional to the natural frequency of a vibrating beam having a longitudinal axis, comprising;
a base;
means to mount one end of said beam with respect to said base;
an actuator;
a first end of said actuator being operably coupled to an opposite end of said beam from the attachment of said beam to said base;
means to apply a pressure signal to said actuator to cause said actuator to change the force exerted on said beam by said actuator in direction along the longitudinal axis of said beam;
drive means having a longitudinal axis substantially normal to the longitudinal axis of the beam to provide a driving signal to oscillate said beam at its natural frequency;
and pick-off means to sense the frequency of oscillation of said beam, said drive means and pick-off means being fixed relative to each other and coupled as a unit to said base on the same side of the beam.

2. The pressure as claimed in Claim 1 wherein said drive means comprises electromagnetic drive means.

3. The combination as specified in Claim 2 wherein said pick-off means in a plane substantially parallel with the beam comprises a capacitor plate.

4. The pressure sensor as claimed in any of Claims 1, 2 or 3 wherein aid actuator comprises a lever pivotally mounted on said base.

5. The pressure sensor as claimed in Claim 1 wherein the actuator comprises a lever pivotally mounted on said base, and wherein said lever extends along its pivot axis substantially more than the width of said beam in the same direction, a support block for said lever mounted on said base, said support block and said lever being formed from a single piece of material, said pivotal mounting of said lever comprising a section of material substantially reduced in thickness in direction perpendicular to the pivot axis and to the longitudinal axis of the beam to form a hinge action pivot elongated along the pivot axis between said lever and said support block.

6. The combination as specified in Claim 5 wherein the surface of said block of material adjacent said vibrating beam is larger in transverse dimension than the portion of the vibrating beam with which it is used and electrical components coupled electrically to said capacitor plate and mounted directly on said block of material in position not aligned with the vibrating beam.

7. The pressure sensor as claimed in Claim 1 wherein said drive means comprises an electromagnetic driver including a current carrying coil, a mandrel, said coil being wound on said mandrel, a block of electrically nonconductive material being mounted on said mandrel between said coil and said vibrating member, said block having one surface adjacent said beam.

8. The pressure sensor as claimed in Claim 7 wherein said mandrel includes a portion extending into said block of material a substantial distance to provide for a path of conduction of magnetic flux to a position adjacent to said vibrating beam.

9. The pressure sensor as claimed in Claim 7 wherein the surface of said block of material adjacent said coil has a metalized layer on the side thereof facing said coil.

10. The pressure sensor as claimed in Claim 1 wherein said means to apply load is a force.

11. The pressure sensor as claimed in Claim 1 or Claim 2 wherein the pick-off comprises a block of electrically non-conductive material having a capacitor plate deposited thereon and facing the vibrating beam, said vibrating beam having a flat surface facing said capacitor plate.

12. The pressure sensor as claimed in Claims 1, 2 or 3 wherein substantially no current is passed through the vibrating beam.

13. The pressure sensor as specified in Claim 1 wherein said vibrating beam comprises a center beam section having a natural frequency which changes with stress in the center beam section and having first and second ends; beam section to the means responsive to pressure whereby the tension in said center beam section changes from a first stress condition when the pressure to be measured is at a minimum to a fully stressed condition when the pressure to be sensed is at a maximum; center beam section being mounted to said base and coupled to the actuator at its first and second ends through first and second spring assemblies forming first and second isolators, respectively; one end of the first isolator being connected to the first end of said center beam section and the other end of the first isolator being connected to the base; one end of said second isolator being connected to the second end of said center beam section and the other end of the second isolator being coupled to the actuator; said spring assemblies comprising the isolators each having a fundamental natural frequency greater than the natural frequency of the center beam section when the center beam section is fully stressed, and lower than twice the natural frequency of the center beam section when the center beam section is at a substantially unstressed condition.

14. The pressure sensor as claimed in Claim 13 wherein said isolators each comprise a pair of generally parallel, spaced apart spring blade members joined together at the opposite ends thereof, and wherein each of said blade members individually has a fundamental natural frequency greater than the natural frequency of the center beam section when the center beam section is fully stressed and lower than twice the natural frequency of the center beam section when the center beam section is at a substantially unstressed condition.

15. The combination as specified in Claim 14 wherein the means to join each pair of blade members adjacent the center beam section comprises a mass selected to provide an isolator frequency that is substantially lower than the lowest natural frequency of the center beam section during use.

16. The pressure sensor of Claims 1, or 2 wherein said actuator comprises a pivoting lever made of metal which pivots about a lever axis perpendicular to the longitudinal axis of the beam to load the beam in tension, said lever being subject to acceleration forces in direction tending to pivot the lever about the lever pivot axis, and thereby change the loading on the beam, and a selected mass of solder physically adhering to said lever in position relative to the lever pivot axis to balance the moments created about the lever pivot axis when the lever is subjected to acceleration forces.

17. A sensor providing an output signal that is pro-portional to the natural frequency of an elongated vibrating beam comprising a base; means to mount one end of said beam with respect to said base; an actuator mounted on said base and having a first end connected to an opposite end of said beam from the attachment of said beam to said base; means to apply load to said actuator to cause changes in the stress in said beam in direction along the longitudinal axis of said beam; a coil and pick-off sensor assembly mounted on said base, said pick-off sensor comprising a capacitor plate adjacent one side of said beam and said coil being on an opposite side of said capacitor plate from said beam; means connecting said coil in an energization circuit to provide oscillating flux to vibrate said beam and means connecting said capacitor in said circuit to control oscillation of said flux in response to capacitance changes between said beam and capacitor plate.

18. A pressure transducer of the vibrating member type wherein said vibrating member is oscillated at a frequency dependent upon the stress in the vibrating member and the frequency of oscilla-tion is sensed as a measurement of pressure having: a base; an elongated vibrating member having first and second ends; means to mount said vibrating member relative to said base at one end thereof;
first means sensitive to a pressure signal coupled to the vibrating member to change the stress in said vibrating member as a function of the pressure signal; electromagnetic driver means for controlling oscillation of said vibrating member responsive to the pressure signal thereon; and second means to sense the oscillation of the vibrating member comprising a block of electrically nonconductive material disposed on the base and having a capacitor plate deposited thereon and positioned adjacent the vibrating member, said vibrating member having a flat surface facing said capacitor plate which, responsive to the pressure signal, moves toward and away from the capacitor plate during oscillation.

19. In a pressure transducer of the vibrating beam type having a base, means to drive a vibrating beam to oscillate at its natural frequency, which varies as a function of the stress in the vibrating beam, means to sense the frequency of oscillation of the beam as a measurement of pressure and having means responsive to pressure to be sensed to exert a tension on the beam as a function of pressure, the improvement comprising: a vibrating beam comprising a center beam section having a natural frequency which changes with stress in the center beam section; means to mount a first end of said center beam section relative to the base; means to mount a second end of said center beam section to the means responsive to pressure whereby the tension in said center beam section changes from a first stress condition when the pressure to be measured is at a minimum to a fully stressed condition when the pressure to be sensed is at a maximum; said means to mount the first and second ends of said center beam section comprising first and second spring assemblies forming isolators, respectively; one end of the first isolator being connected to the first end of said center beam section and the other end of the first isolator being connected to the base;
one end of said second isolator being connected to the second end of said center beam section and the other end of the second isolator being connected to the means responsive to pressure; said spring assemblies comprising the isolators each having a fundamental natural frequency greater than the natural frequency of the center section when the center beam section is fully stressed, and lower than twice the natural frequency of the center beam section when the center beam section is at its first stress condition.