PRESSURE TRANSDUCER

Abstract: A pressure transducer (12) having a housing (14, 16), a pressure port (24) through said housing (14), a cylindrical recess (22) in said housing (14, 16) and communicating with said pressure port (24). A pair of nested bellows (50, 64) is acted upon by the pressure at said pressure port (24) such that the displacement of one of said bellows (64) is amplified by the displacement of the other of said bellows (50). Means is included for temperature effects and non-linearity. The present invention provides a simple durable pressure transducer (12) having an output of increased accuracy. The present invention is especially well suited for pressure measurements related to internal combustion engines.

Description: Description Pressure Transducer 5 Technical Field This invention relates generally to pressure and displacement sensing devices and more particularly to bellows-type transducers for sensing pressure. 10 Background Art In many applications it is desired that pressure sensors be of high accuracy. This has been somewhat difficult to achieve for applications such as on internal combustion engines in which the sensor is 15 exposed to a wide temperature range, ε cck, vibration and significant pressure transients. Most existing pressure sensors are of three types: compressible chamber or fluid types, compressible crystal types, and strain gauge types. 20 The typical strain gauge type pressure sensor generally includes resistive elements placed upon an expansible diaphram such that deflection of the ciaphram induces a change in the resistance of the resistive elements. This alteration in resistivity can be measured and 25 related to pressure. Such pressure sεr.scrε tend to be somewhat fragile and of limited accuracy owing to the , small displacement of the diaphram. The compressible crystal type pressure sensor typically employs pi«ezo-resistive elements exposed to 30 the pressure desired to be measured. Such sensors are quite accurate, however, achieving uniformity at a low .manufacturing cost has proven somewhat difficult. The most common and generall least expensive type of pressure sensor is the compressible chamber or 35 fluid type. Typically, these incorporate a bellows or sealed spring system exposec. to the pressure source of interest. The contraction or expansion of the bellows can serve either to move an indicator or establish an electrical signal as a measure of the pressure. Most 5 of these sensors are relatively inaccurate owing to the fact that small changes in pressure cause a very small deflection in the bellows. One approach to the solution of this problem was presented by A.B. Newton in U.S. Patent 2,274,254 10 issued February 24, 1942. Newton proposed that deflection of a first bellows, exposed to the pressure source of interest, act upon a pivoted link to secure mechanical amplification of the deflection of the first bellows. It would be advantageous were such a system 15 more compact and free of dependency upon the use of a link. The present invention is directed to overcoming one or more of the problems as set forth above. 20 Disclosure of the Invention In one aspect of the present invention a sensor has a first compressible member with first and second end portions and a second compressible member 25 with first and second end portions in fluid communication one of the first and second end portions of the first member. One of the end portions of the second member is displaced in response to movement of one of the first and second end portions of the first 30 member. Means is provided for converting movement of one of the first and second end portions of the second bellows into an indication of pressure or displacement. Existing axially compressible member type pressure transducers do not optimize accuracy and 35 simplicity. In the present invention mechanical - _. - a plification of the movement of the axially compressible member is attained establishing a pressure measurement of increased accuracy. The means for accomplishing this is quite simple and compact being a pair of nested axially compressible members in a metallic housing. The present invention includes digital output, dampening of the axially compressible members, simplicity of calibration and temperature compensation. Brief Description of the Drawings Fig. 1 shows a cross sectional view of the present invention taken along the longitudinal axis, both bellows are fully extended in this view; Fig. 2 corresponds to Fig. 1 and shows a detail thereof with both .sets of bellows in partial compression; Fig. 3 shows a cross sectional view of a second embodiment of the present invention; and Fig. 4 shows a third embodiment of the present invention suited for measuring displacement of an article of interest. Best Mode for Carrying Out the Invention Referring to the drawings, a measuring sensor embodying principles of the present invention is generally indicated by the reference number 10. Preferably, as will be detailed subsequently, the measuring device 10 is utilized as a pressure transducer 12, however, the displacement measuring device 10 has other uses as will become apparent to those skilled in the art following an examination of the description below. In the preferred embodiment, the pressure transducer 12 has a housing 13 defined by a first housing member 14 and a second housing member 16. The first housing member 14 has a probe portion 18 which is insertable into a region 19 the pressure of which it is desired to measure. The probe portion 18 has threads and corresponding to a threaded aperture in a housing 20 surrounding the region of interest 19. The pressure transducer 12 can be screwed into this aperture, positioning the pressure transducer 12 to measure the pressure of the region of interest 19. The probe portion 18 includes seals such as O-rings 21, and forms a pressure-tight connection between the pressure transducer 12 and the region of interest 19. Opposite the probe portion 18, the first housing 14 defines a cylindrical recess 22. Preferably, this cylindrical recess 22 is of somewhat larger cross-sectional diameter than the probe portion 18. A pressure port 24 extends coaxially with the cylindrical recess 22 from an outer face 26 of the probe portion 18 to the cylindrical recess 22. A push plate 28 is positioned within the cylindrical recess 22. This push plate 28 is a disc which is coaxial with and of slightly smaller diameter than the cylindrical recess 22 and is axially movable along the recess 22. The push plate 28 has a pressure port face 30 adjacent the pressure-port 24 and a bellows face 32 opposite the pressure pσr face 30. An annular support member 34 of relatively thin wall thickness and substantially equal diameter to the push plate 28 is connected to the bellows face 32 of the. push plate 28. A plunger assembly 35 is positioned within the pressure port 24. The plunger assembly 35 is in contact with, but not connected to the push plate 28. Hence, an increase in the pressure to which the probe portion 18 is exposed causes displacement of the plunger 36 toward the cylindrical recess 22 which in turn causes displacement of the push plate 28. As shown in Fig. 4, the plunger assembly 35 may consist of a single cylindrical element. The preferred embodiment for the plunger assembly 35, shown in Fig. 1, is slightly more complicated. A series of cylindrical force transmitting elements 36 are disposed in abutting relationship in an outer portion of the pressure port 24. A cylindrical plunger 37 is positioned in a larger, inner portion of the pressure port 24, the plunger 37 being intermediate and in abutment with the force transmitting elements 35 and the push plate 28. The plunger 37 has a narrowed neck portion 39 adjacent a drain port 41 in said first housing member 14. This drain port 41 communicates with a sump for the fluid in the region of interest 19- Hence, as fluid from the region of interest 19 passes the force transmitting elements 36 it reaches the narrowed neck portion 39 and is drained into the sump. The communication of the sump with the pressure port 24 serves as a vent allowing unpressurized air - from the sump to communicate with the pressure port face 30 of the push plate 28. The second housing member 16 is threaded into the cylindrical recess 22 of the first housing member 14. The second housing member 16 is generally annular in shape and has opposite ends, a bellows end 38 and a connector end 40. The bellows end 38 includes a neck portion 42 of smaller diameter than an adjacent threaded portion 44. This difference in diameters defines an annular recess 46 intermediate the second member bellows end 38 and the first housing member 16. The bellows end 38 of the second housing member 16 also has a planar ring-shaped face 48 opposing the bellows face 32 of the push plate 28. BAD ORIGINAL A first axially compressible member, such as a first bellows 50, has first and second ends 52,54 and is connected at its first end 52 to the second member bellows-attachment end 38. This connection serves as a seal against fluid flow and consequently is effected by brazing, epoxy bonding or in some other suitable manner. The first bellows 50 is coaxial with the first and second housing members 14,16 and defines a first chamber 51 within said housing 14,16. The second end 54 rests substantially flush against the push plate bellows face 32. The second end 54 has a slight projection 56 mating with a corresponding detent 58 in the push plate 28. Preferably the second end 54 is not attached to the push plate 28, but is biased thereagainst by the outward spring force of the first bellows 50. Inwardly projecting from the second end 54 of the first bellows 50, and coaxial therewith, is a hollow, open-ended cylindrical guide 60, best seen in Fig. 2. This guide 60 has a venthole 62 at a position adjacent the first bellows second end 54. A second axially compressible member, such as a second bellows 64, has first and second ends 66,68 and is coaxial with and extends into said first bellows 50. The first end 66 of the second bellows 64 is sealingly affixed to the second member connector end 40. A second chamber 69 within said housing 13 is defined by said second bellows 64. The second end of the second bellows 68 further defines a cylindrical piloting member 70 which is sized for positioning the cylindrical guide 60 totally therewithin. The second bellows 64 is piloted over this guide 60 and frees the second bellows 64 to expand and contact relative to the first bellows 50 while remaining axially aligned with both the first bellows 50 and the cylindrical recess 22. The first and second bellows 50,64 are in sealed fluid communication. That is, the first bellows 50 when compressed or extended tends to alter the pressure on a fluid in communication with the second bellows 64 such that the second bellows 64 extends or retracts in response to the movement of the first bellows 64 so as to maintain the system in equilibrium. Generally, then, the use of the term "sealed fluid communication" indicates that altering the extension of the first bellows 50 acts on a fluid to produce a compensating alteration of the extension of the second bellows 64. Both of the bellows 50,64 have folding portions 72 which are preferably fabricated, of a beryllium-copper alloy, a metal which has a long flex-life. The end portions 52,54,66,68 of the two bellows 50,64 may also be fashioned of the same beryllium-copper alloy or of some other substance which may be sealingly attached to the folding portions 72 by brazing or the like. As detailed above, in the preferred embodiment of the present invention, two bellows 50,64 are utilized. It should be understood that other types of expansible-contractable members could be utilized without departing from the principle of the present invention. In the embodiment detailed in Fig. 1 the bellows 50,64 provide the spring force necessary to overcome, with due compression, the force exerted on the push plate 28. Alternatively, as shown in Fig. 2, a spring 110 may be positioned intermediate the push plate bellows face 32 and the second housing member 16, this spring 110 being used to supply the countering force to the push plate 28. The bellows 50,64 may then be manufactured with a relatively small spring constant. Means may be included for adjusting the force applied by the spring 110 to the push plate 28. IGINAL A wiper block 76 is rigidly attached to the cylindrical piloting member 70. A wiper block guide 77 extends from and is rigidly connected to the connector end 40 of the second housing member 16. This wiper block guide 77 is, in cross section, C-shaped and includes rails 78 engageable in rail accepting portions 79 of said wiper block 76. The wiper block 76 is then guidingly translatable along said wiper block guide 77. The wiper block guide 77 then serves to support both the first and the second bellows 50,64. A wiper 80 is connected to the wiper block 76 and contacts a resistive strip 81 running longitudinally along the wiper block guide 77. The resistive strip 78 and the wiper block 76 are so arranged that the wiper 80 rides on the resistive strip 78 over the entire travel of the second bellows 64. An analog-to-digital converter 82 is attached to the connector end 40 of the second housing member 16 at a location interior to the second bellows 64. The pressure transducer 12 includes power input conductors 84 and a signal output circuit 86 for the A/D converter 82. A resistance measuring circuit 88 extends from the A/D converter 82, to the wiper 80, to the resistive strip 78, and back to the converter 82. An annulus or sealed region 90 is defined by the first bellows 50, the second bellows 64 and the second housing member 16. That is, a sealed region exists within the first bellows 50 and outside the second bellows 64. For the purpose of this description, that portion of the inside of the second housing member 16 with which the first bellows 50 is sealingly connected is deemed a portion of the first bellows 50. This region 90 is occupied by an incompressible fluid which is effectively inert and which is not so viscous as to significantly impair relatively free movement of the bellows 50,64. Preferably this fluid is Silicon 200 Fluid manufactured by DOW Corning of Midland, Michigan. Other suitable fluids will be recognized by those skilled in the art. It is important this fluid be conditioned to remove all entrained air during the assembly of the pressure transducer 12. The second housing member 16 includes an increased I.D. portion 92 at its connector end 40. The juncture of this increased I.D. portion 92 with the remainder of the second housing member 16 defines a seat 94 to which the first end 66 of the second bellows 64 is sealingly affixed. A disc-shaped substratum 96 is positioned within this increased I.D. portion 92 and against the second bellows 64. An annular plug 98 is press fit into the increased I.D. portion 92 and serves to retain the substratum 96 in position. A rubber grommet 100 is positioned v/ithin the annular plug 98 and seats against the substratum 96. Preferably, the rubber grommet 100 and the annular plug 98 are integral. The power input and signal output conductors 84,86 pass through the rubber grommet 100, the substratum 96, and into the analog-to-digital converter 82. The volume external to the first bellows 50 and the volume internal to the second bellows 64 are vented one to the other to prevent pressure imbalances from affecting the performance of the bellows system 50,64. Preferably, this is achieved by use of a vent 108 joining these two volumes as shown in Figs. 1 and 2. Alternatively, each of these volumes could be individually vented to the region surrounding the transducer 12. The resistance measuring circuit 88 includes means to compensate for the effects of varying temperature on the pressure transducer 12. Preferably this means includes a thermistor having a positive temperature coefficient. A thermistor can be utilized to counter movement of the wiper 80 across the resistive strip 78 resulting from temperature induced changes in the volume of the fluid in the sealed region 90. Means other than the resistance measuring circuit 88 may be utilized for the measurement of the displacement of the second bellows 64. For example, the wiper block 76 could be replaced with a magnet and a Hall-effect linear output transducer could be positioned adjacent the analog-to-digital converter 82 for monitoring the position of the magnet. Other suitable systems will be recognized by those skilled in the art. The annular recess 46 has a radial boundary 104 defined by the threaded portion 44 of the second housing member 16. This radial boundary 104 serves as a stop and prevents movement of the support member 34, and hence the push plate 28, more than a set amount in the direction of the connector end 40 of the second housing member 16. This serves to prevent damage to the pressure transducer 12 resulting from overcompression of the bellows 50,64. It is preferable that the second bellows 64 have a cross-sectional area significantly smaller than that of the first bellows 50. A ratio of these cross sectional areas of about 1:4 is preferred. As will be recognized by those skilled in the art, it is important that the bellows system be stable through the frequency range of vibration and high frequency pressure transients to which it will be exposed in use. It is preferable that the bellows system be insensitive to vibration in the frequency of between 18 and 1000 Hz. The cylindrical guide 60 with its venthole 62 will serve as a dashpot to dampen response to vibration. In an alternative embodiment, shown in Fig. 3, a bi- etallic disc 106 is positioned intermediate the plunger 37 and the push plate 28. The bi-metallic disc 106 has a temperature sensitive radius of curvature, the thermal expansion curve of which is selected to compensate for the temperature sensitivity of the bellows arrangement. Such a selection is well within the ability of one skilled in the art. The bimetallic disc 106 should be sufficiently rigid that there is no significant deflection of the disc 106 in response to a compressive force being applied to the disc 106. Industrial Applicability In the operation of the present invention, the probe portion 18 is threaded through a housing containing the region of interest 19. Pressure within this region forces the plunger assembly 35 within the pressure port 24 toward the push plate 28. It is the plunger assembly 35, rather than the pressurized fluid in the region of interest 19, that acts directly on the push plate 28. The use of this intermediary plunger assembly 35 permits the pressure transducer 12 to measure high pressure without the need for the bellows 50,64 being strong enough to counter the large force that would exist were the fluid of interest imposed across the full cross-sectional area of the push plate 28. As a result, the first bellows 50 has a relatively low spring constant and is thusly of increased sensitivity. Displacement of the push plate 28 compresses the first bellows 50. This in turn acts upon the incompressible fluid within the sealed region 90 and induces axial compression of the second bellows 64. As the first and second bellows 50,64 are of different cross-sectional areas, the relative displacements of the two bellows 50,64 are not identical. In the preferred embodiment, the ratio of the cross-sectional areas is 4 to 1, hence axial compression of the bellows 50,64 occurs in the ratio of 1 to 4. That is, for each 1 mm displacement of the first bellows 50, the second bellows 64 is displaced 4 mm. This mechanical amplification provides increased resolution of the pressure measurement. As is the case with virtually all pressure transducers, the response of the present invention is not perfectly linear over all temperatures and pressures. The temperature-induced inaccuracies are primarily the result of expansion and contraction of the fluid intermediate the first and second bellows 50,64. This is accommodated through use of a thermistor located within the pressure transducer 12, preferably within analogue-to-digital converter 82. This thermistor is selected to have the correct positive thermal coefficient of resistivity to counter the decrease in resistivity of the resistance measuring circuit 88, owing to the positive thermal coefficient of expansion of the fluid in the sealed region 90. Alternatively, and as described previously, a bi-metallic disc 106 may be used to counter this temperature dependency. Compensation for non-linearity of response of the bellows system 50,64, owing to such factors non-constant spring rate of the bellows 50,64 can be achieved through the use of suitable electronics. Such " BUREA 0i P a compensating circuit can be positioned intermediate the resistance measuring circuit 88 and the A/D converter 82. An additional means by which compensation for the non-linear response of the bellows system 50,64 may be achieved involves trimming of the resistive strip 81. The resistive strip 81 may be produced so as to have a resistance that is a non-linear function of its length. The non-linearity of the resistance can be selected so as to compensate for the non-linearity of the force-displacement relationship of the bellows system 50,64. In manufacture of the pressure transducer 12 of the present invention, adjustment of the output to obtain the correct value is easily achieved. With the transducer 12 complete and the second housing member 16 screwed into the first housing member 14 to approximately the correct position, the transducer 12 is exposed to a known pressure source. The position of the second housing 16 is then altered until the correct output is attained, at which point the threads are staked. The present invention may also be utilized as a displacement measuring device 10. In this embodiment, as shown in Fig. 4, the probe portion 18 is secured adjacent the object whose displacement it is desired to measure. The plunger 36 in this embodiment is extended to contact the object of interest at a position when that object is furthest from the displacement measuring device 10. Subsequent motion of the object toward the displacement measuring device 10 will displace the push plate 28 and compress the bellows 50,64 as detailed previously. The displacement measuring device as shown in Figs. 1-4 is able to measure displacements on the order of a few -BUREA millimeters. Those skilled in the art could, in light of the teaching of the present disclosure, increase the allowed compressibility of the bellows 50,64 to permit measurements of greater displacement ranges. Other aspects, objects, advantages of the present invention may be obtained from a study of the drawings, the disclosure and the appended claims.

Claims: Claims 1. A pressure sensor (12) comprising: a housing (13) ; 5 a first axially compressible member (50) having first and second ends (52,54), said first end (52) being fixed relative to said housing (14,16); and a second axially compressible member (64) having first and second ends (66,68), said second 0 member being in sealed fluid communication with said first member (50) , said first end (66) of said second member (64) being fixed relative to said housing (13) . 2. The pressure sensor (12), as set forth in 5 claim 1, wherein said first and second members (50,64) are bellows. 3. The pressure senεor (12), as set forth in claim 1, wherein said first and second members (50,64) 0 are of different cross-sectional areas. 4. The pressure sensor (12), as set forth in claim 1, wherein a substantially incompressible fluid provides the sealed fluid communication between the 5 first and second members (50,64). 5. The pressure sensor (12), as set forth in claim 1, wherein said first and second members (50,64) are of different cross-sectiόnal areas and displacement O of said second end (54) of said first member (50) induces a displacement of different magnitude of said second end (68) of said second member (64) . 5 OMPI 6. The pressure sensor (12), as set forth in claim 3, wherein said second member (64) is disposed at least partially within said first member (50) . 7. The pressure sensor (12) , as set forth in claim 6, wherein said first member (50) and said second member (64) define a sealed region (90) intermediate said members (50,64). 8. The pressure sensor (12) , as set forth in claim 7, wherein said sealed region (90) contains a substantially incompressible fluid. 9- The pressure sensor (12) , as set forth in claim 8, wherein said housing (13) has a probe portion (18) and a pressure port (24) , said pressure port (24) extending through said housing (13) from said probe portion (18) to a position adjacent the second end (54) of said first member (50) . 10. The pressure sensor (12), as set forth in claim 9, further including a plunger assembly (35) positioned in said pressure port (24) , in contact with the second end of said first member and being movable in response to the pressure extent at said probe portion (18) . 11. The pressure sensor (12) , as set forth in claim 9, further including a push plate (28) intermediate the second end of said first member (54) and said pressure port (24) . -BU REA 12. The pressure senεor (12) , as set forth in claim 11, further including a plunger (36) positioned in said pressure port (24) , adapted to substantially prevent fluid flow therepast, and being in contact with said push plate (28) . 13. The pressure sensor (12) , as set forth in claim 9, wherein said housing (13) further includes: a first housing portion (14) having said probe portion (18) and a cylindrical recess (22) , said pressure port (24) extending between said cylindrical recess (22) and said pressure port (24); and a second housing member (16) having the respective first ends (52,66) of said first and second members (50,64) fixed thereto. 14. The pressure sensor (12), as set forth in claim 13, wherein the respective first " and second ends (52,66) of said first and second members (50,64) are sealingly attached to said second housing portion (16) and said housing member 16 and said first and second members (50,64) defining said sealed region (90) positioned interior said first member (50) and exterior said second member (64) . 15. The pressure sensor (12), as set forth in claim 14, further including means for generating a signal (88) which is a function of displacement of said second end (68) of said second axially compressible member (64) . 16. The pressure sensor (12), as set forth in claim 14, including means (82,86,88) for relating displacement of said second end (68) of said second member (64) to the pressure at said probe portion (18) . 17. The pressure sensor (12) , as set forth in claim 16, wherein said means (82,86,88) provides an electrical signal directly proportional to the pressure at said probe portion (18) . 18. The pressure sensor (12) , as set forth in claim 14, wherein said second housing member (16) is adjustably attached to said first housing member (14) . 19. The pressure sensor (12) , as set forth in claim 13, further including: an elongate guide (60) extending axially from said second end (54) of said first member (50) toward said first member first end (52) ; and a piloting member (70) positioned in the second end (68) of said second member (64) ,. extending toward the first end (66) of said second member (64) and in contact with said elongate guide (60) . 20. The pressure sensor (12), as set forth in claim 19, further including: a wiper (80) connected to and movable with said piloting member (70); and a resistive element (78) attached to said pressure sensor (12) and in slidable contact with said wiper (80) . 21. The pressure sensor (12) , as set forth in claim 16, wherein said first and second axially extensible members (50,64) are bellows. 22. A sensor (10) , comprising: first and second axially compressible members (50,64) each having first and second ends (52,54/66,68), said first and second members (50,64) defining first and second chambers (51,69) with said second member (64) being positioned within said first chamber (51) ; means (28,35) for axially displacing one of the first member ends (52,54); and means for axially displacing one of the second member ends (66,68) in response and relative to displacement of said one end (52,54) of said first member (50) . 23. The sensor (10), as set forth in claim 22, further including a housing (13) attached to said first and second members (50,64). 24. The sensor (10) , as set forth in claim 23, wherein said first and second members (50,64) and said housing (14,16) define a sealed region (90)-, intermediate said first and second axially compressible members (50,64); and further including a fluid positioned in said region (90) . 25. The sensor (10), as set forth in claim 24, wherein said fluid is incompressible and the cross-sectional area of second member (64) is less than the cross-sectional area of said first member (50) . 26. The sensor (10) , as set forth in claim 25, wherein said sensor (10) includes means (28,36) for axially moving said one end (54) of said first member (50) in response to a change in pressure to which said sensor (10) is exposed. 27. The sensor (10) , as set forth in claim 26, further including means (82,86,88) for indicating the pressure to which said sensor (10) is exposed relative to the magnitude of compression of said second member (64) .

Inventors: FUZZELL JOE E
Applicants: CATERPILLAR TRACTOR CO, FUZZELL JOE E
Application year: 1981
Main classification: G01L9
IPC codes: G01L7, G01L9