PRESSURE TRANSDUCERS AND METHODS OF CONVERTING A VARIABLE PRESSURE INPUT

Abstract: A quartz pressure transducer (10) is disclosed which includes diaphragms (14,16,18) having deposited metal layers (20,22,24,26,32,34) on both sides thereof. Since the layers are all on the diaphragms, the sensor disclosed herein is highly resistant to errors caused by changes in temperature, while retaining excellent resistance to errors caused by acceleration or vibration forces. The transducer may be constructed as either an absolute pressure sensor or as a differential pressure sensor, and in the primary embodiment contains a reference capacitance which may be used by appropriate electronic circuitry toprovide compensation for acceleration or vibration forces, making the output of the pressure transducer a highly accurate, highly sensitive indication of pressure sensed.

Description: The present invention relates to pressure transducers and to methods of converting a variable pressure input. The invention is applicable to such transducers and methods for providing a variable output capacitance indicative of variable pneumatic pressure acting on the transducers. The term pressure transducer as used herein includes both an absolute pressure sensor, which is supplied with a single pressure input, and a differential pressure sensor, which is supplied with two pressurs and determines the difference between the two pressures. Quartz pressure transducers have been used for some time in the avionics field to convert a number of different sampled pressures to electronic signals which may be analyzed to provide information about aircraft altitude, Mach number, indicated air speed, angle of attack and slip or yaw angle. Since information on both military and commercial aircraft must be highly accurate, pressure transducers must be highly reliable and relatively immune from noise which may be caused by acceleration, turbulence, and temperature. A system for measuring such pressures has three basic components, the first of which is a Pitot tube which measures an air pressure at a location on the exterior of the aircraft, and has a pneumatic line leading from the Pitot tube to the interior of the aircraft. The second component in the pressure measurement apparatus is the transducer which converts pressure into an electrical signal, with a typical transducer being of a fused quartz type such as that shown in US Patent No.3,962,921, to Lips. Finally, the third component of the system is electronic circuitry which has an input the variable capacitance from the quartz pressure transducer. Such circuitry is shown in US Patent No. 3,790,910, to McCormack. Although several configurations of quartz pressure transducers are available, virtually all the designs are similar to the Lips patent design, and utilize two diaphragms with substantially equal natural frequencies, with the diaphragms being superposed for movement in phase and in unison under accelerating forces to minimize the effect of acceleration and vibration on the sensor. Ceramic and metallic transducers have also been made using the same principles as fuxed quartz transducers, and it should be understood that the principles of the invention disclosed herein may also be applicable to such transducers. More recently, a three element transducer has been manufactured by the Garrett Corporation of Los Angeles, California. By utilizing three elements two sets of capacitor electrodes may be arranged, with one of the sets of capacitor electrodes being responsive to changes in pressure, and the other set of capacitor electrodes being immune to changes in pressure. Since both sets of capacitor electrodes are equally responsive to acceleration or vibration, the circuity for the sensor may be so arranged and configured as to reduce the sensitive of the assembly to acceleration and vibration error. This approach, while providing some degree of improvement in reduced sensitivity to acceleration and vibration, has not resulted in a reduction of error caused by temperature variation. Therefore, it is apparent that a need exists for a pressure transducer which will have substantially greater immunity to temperature variations. It is desirable that the improved transducer be so free of temperature error as to greatly reduce the requirement for temperature compensation in the electronic circuitry accompanying the transducer. While the improved transducer should have substantially increased immunity to temperature variation, it must also retain a high degree of immunity to vibration and acceleration forces. All of these desirable effects must be accomplished without degraduation or desensitization of the output of the pressure transducer. Finally, it is desirable that the improved transducer be adaptable for use both as an absolute sensor and also as a differential sensor, whereby the sensor will detect the difference between two pressures applied to it. An object of the invention is to provide an improved pressure transducer having great resistance to temperature error while capable of providing a highly accurate indication of pressure supplied to the transducer. It has now been realised that there are two basic causes of temperature error in quartz pressure transducers - thermal curvature of quartz diaphragms due to the presence of an electrode on only one side of diaphragm, and differing physical characteristics of the capacitor elements used in the pressure transducer. It has been realised that if only one side of a quartz diaphragm is plated with a deposited metal electrode, that diaphragm will bend slightly when subjected to temperature variation, in a manner similar to a bimetallic strip. Even more importantly, if one of the elements of the transducer is thicker than another, it has been realised that physical change with temperature will not be consistent with that of the thinner element. According to one aspect of the invention, there is provided a transducer comprising first, second and third diaphragms characterised in that each said diaphragm has an electrically conductive layer on each major surface thereof. Preferably, said second diaphragm is between said first and third diaphragms. Preferably, the electrically conductive layers on said second diaphragm, and those layers on the first and third diaphragms which face said second diaphragm, are connected as electrodes. Said electrodes preferably provide a first capacitance between the first and second diaphragm and a second capacitance between the second and third diaphragm. Preferably,one of said diaphragms is mounted to respond to a variable pressure input to cause a change in at least one of said first and second capacitances in response to change in said pressure. Preferably, each diaphragm is of fused quartz. According to a further aspect of the invention, there is provided a method of converting a variable pressure input into a capacitive electrical output indicative of said variable pressure input, comprising: providing a housing portion; mounting a first diaphragm adjacent to and spaced away from said housing portion; depositing a first electrode on the side of said first diaphragm facing said housing portion; depositing a second electrode on the side of said first diaphragm away from said housing portion; mounting a second diaphragm adjacent to and spaced away from the side of said first diaphragm having said second electrode deposited thereon; depositing a third electrode on the side of said second diaphragm facing said first diaphragm; depositing a fourth electrode on the side of said second diaphragm away from said first diaphragm; mounting a third diaphragm adjacent to and spaced away from the side of said second diaphragm having said fourth electrode deposited thereon; depositing a fifth electrode on the side of said third diaphragm facing said second diaphragm; depositing a sixth electrode on the side of said third diaphragm away from said second diaphragm; sealing the areas between the edges of said first, second, and third diaphragms and said housing portion; applying said variable pressure input across one of said first, second, and third diaphragms; measuring a first capacitance across said fourth and fifth electrodes; and measuring a second capacitance across said second and third elements, said first and second capacitances being usable to determine the value of said variable pressure input. The present invention in its preferred embodiment includes four elements, three of which are diaphragms carrying electrodes for use in pairs to form capacitors. One of the three diaphragms is sensitive to pressure supplied to the pressure transducer. The remaining two diaphragms are reference diaphragms, which may be used in the pressure transducer to eliminate errors due to acceleration or vibration encountered by the pressure transducer during operation. Since all three diaphragms will be subject to the same acceleration and vibration forces, and since all three diaphragms are superposed for movement in phase and in unison under such forces, electrical measurements can be taken from the diaphragms which are responsive to pressure variations independent of movement of the diaphragms caused by acceleration and vibration forces. Since the pressure sensor of the preferred embodiment of the present invention has two capacitances as an output, one of the capacitances is responsive to pressure and the other is a reference capacitance. By analyzing the change in reference capacitance and subtracting an appropriate electrical signal from the signal generated by the pressure sensitive capacitance, an accurate reading of capacitance change due to pressure alone may be determined. In addition, the pressure transducer of the preferred embodiment of the present invention is much less susceptible to error caused by temperature changes because of a design feature of the transducer not previously known in the art. The deposited metal electrodes are deposited only on the diaphragms and are not deposited on the housing, which is thicker than the diaphragms. The reason for this is that the greatest amount of temperature error in transducers is caused by unequal response to temperature change of an electrode deposited on a diaphragm and an electrode deposited on the housing. The illustrated embodiment is the first transducer designed to overcome this problem. In an alternate embodiment, five elements are used, with the additional element being an additional housing portion which may be desired in certain applications of the transducer. Additionally, two diaphragm configurations are disclosed, one of which has the pressure sensitive diaphragm located between two reference diaphragms, the other of which has the pressure sensitive diaphragm located next to two adjacent referenced diaphragms. Accordingly, it may be perceived that a pressure transducers constructed according to the preferred embodiment of the present invention will not only have substantially better immunity to acceleration and vibration forces, but will also be immune to error induced by temperature change during operation of the transducer. The pressure transducer is adaptable to use both as absolute sensor and as a differential sensor. It may also be used in a two housing configuration, the first of which has a housing portion located only on one side of the diaphragms, and the second of which has a housing portion located on both sides of the diaphragms. A highly accurate, highly sensitive pressure transducer thus provides the above significant advantages without substantial disadvantage. The present invention will be better understood with reference to the accompanying drawings which illustrate preferred embodiments of the invention and in which: Figure 1 is a cross-sectional view of a transducer constructed according to a first embodiment of the present invention and having one housing portion, with the pressure sensitive diaphragm located adjacent to the two reference diaphragm; Figure 1A is the equivalent electrical schematic of the transducer shown in Figure 1; Figure 2 is a cross-sectional view of an alternative embodiment of the present invention illustrating a pressure transducer having two housing portions and having the pressure sensitive diaphragm mounted between the two reference diaphragms; and Figure 2A is the equivalent electrical schematic of the transducer shown in Figure 2. The present transducers illustrated in Figures 1 and 2 are preferably constructed of quartz or silica glass, and consist of three thin diaphragms with either one or two housing sections. In the past, diaphragms and housing sections were secured together by a bonding cement to provide a substantially leak-proof seal against loss of fluid pressure applied to the transducer, but in the preferred embodiments they are fused together along the dotted lines in order to provide a more uniform and permanent bond between the various diaphragms and housing sections. In Figure 1, a transducer 10 is illustrated which is comprised of a housing base portion 12, a pressure sensitive diaphragm 14, and two reference diaphragms 16 and 18. In the embodiment shown in Figure 1, the pressure sensitive diaphragm 14 is adjacent the two reference diaphragms 16, 18 rather than being disposed between the two reference diaphragms 16, 18. The reference diaphragms 16, 18 are disposed between the pressure sensitive diaphragm 14 and the housing portion 12. For convenient fabrication it is preferable that the pressure sensitive diaphragm 14, the reference diaphragms 16, 18 and the housing portion 12 be discs of circular configuration, although it will be apparent to those skilled in the art that such circular configuration is not an absolute requirement for practicing the present invention. Also, the pressure sensitive diaphragm 14 and the reference diaphragms 16, 18, are relatively thin in comparison with their diameters, thereby making the diaphragms more subject to vibration in a first mode as compared with any other exciting frequency that the transducer 10 will be subjected to. Therefore, the pressure diaphragm 14 and the reference diaphragms 16, 18 will move in phase and in unison whenever the transducer 10 is subjected to accelerating or vibratory forces. The pressure sensitive diaphragm 14 and the reference diaphragms 16, 18, each have metal electrodes deposited on both sides thereof in a configuration which is preferably circular, and does not extend to the edges of the diaphragms. The deposited metal electrodes are formed prior to construction of the transducer 10 by the bonding or fusing operations described above. The metal electrodes are made from a noble metal such as gold which may be deposited on the diaphragms easily. The pressure sensitive diaphragm 14 has an electrode 20 on the side of the pressure sensitive diaphragm 14 facing away from the reference diaphragm 16, and a second deposited metal electrode 22 on side of the pressure sensitive diaphragm 14 facing the reference diaphragm 16. The deposited metal electrode 20 on the pressure sensitive diaphragm 14 will not be used to form a capacitor, but is deposited on the surface of the pressure sensitive diaphragm 14 to ensure that the pressure sensitive diaphragm 14 will not bend when subjected to heating or cooling due to unequal expansion, as is the case where a diaphragm has an electrode deposited only on one side thereof. The reference diaphragm 16 has a deposited metal electrode 24 on the side of the reference diaphragm 16 facing the pressure sensitive diaphragm 14, and an electrode 26 on the side of the reference diaphragm 16 facing away from the pressure sensitive diaphragm 14. Unlike the electrodes 20, 22 on the pressure sensitive diaphragm 14, the electrodes 24, 26 on the reference diaphragm 16 are connected together by conductor means 28 extending through a passage way 30 in the reference diaphragm 16. The passageway 30 in the diaphragm 16 is relatively small, but allows for a thin wire to extend through the aperture 30 without touching the conductor means 28. The thin wire will be discussed in detail below. The reference diaphragm 18 has a deposited metal electrode 32 on the side of the reference diaphragm 18 facing the reference diaphragm 16, and a second deposited metal electrode 34 on the side of the reference diaphragm 18 away from the reference diaphragm 16. The reference diaphragm 18 has two passageways 36, 38, extending therethrough. The passageway 36 is aligned with the passageway 30 extending through reference diaphragm 16. The deposited metal electrodes 32, 34 on the reference diaphragm 18 are connected by a conductor means 40 extending through the passageway 36 and a conductor means 42 extending through the passageway 38. The housing portion 12 is located adjacent the side of the reference diaphragm 18 facing away from the reference diaphragm 16. The housing portion 12 has a passageway 44 therethrough which is aligned with the passageway 36 in the reference diaphragm 18 and the passageway 30 in the reference diaphragm 16. The housing portion 12 has a second passageway 46 which is aligned with the passageway 38 extending through the reference diaphragm 18. The housing portion 12 has a third passageway 48 opposite a portion of the deposited metal electrode 34 having no aperture or passageway nereby in the reference diaphragm 18. A thin conducting wire 50 extends through the passageways 44, 36 and 30 contained in the housing portion 12, the reference diaphragm 18, and the reference diaphragm 16, respectively, and leads to the deposited metal electrode 22 on the side of the pressure sensitive diaphragm 14 facing the reference diaphragm 16. The other end of the conducting wire 50 is connected to a terminal A mounted on the side of the housing portion 12 away from the reference diaphragm 18. A second conducting wire 52 extends through the passageway 46 and the passageway 38 contained in the housing portion 12 and the reference diaphragm 18, respectively, and leads to the deposited metal electrode 26 on the reference diaphragm 16. The other end of the conducting wire 52 is connected to a terminal B mounted on the side of the housing portion 12 away from the reference diaphragm 18. A third conducting wire 54 extends through the passageway 48 in the housing portion 12 and is connected at one end to the deposited metal electrode 34 on the reference diaphragm 18 and at the other end to a terminal C mounted on the side of the housing portion 12 away from the reference diaphragm 18. It should be noted that the conducting wires 50, 52, and 54 are extremely fine wire, and in no way interfere with movement of the pressure sensitive diaphragm 14 or the reference diaphragms 16, 18. The terminals A, B, and C are sealed to the housing portion 12. The housing portion 12 includes a seal-off tube 56 having an aperture which extends through the housing portion 12 into the chamber between the pressure sensitive diaphragm 14, the reference diaphragms 16, 18, and the housing portion 12. For an absolute pressure sensor, pressure will be applied to the side of the pressure sensitive diaphragm 14 containing the deposited metal electrode 20, and the chamber hereinabove described will be evacuated and the seal-off tube 56 will be sealed. For construction of a differential pressure sensor, pressure will be applied as above to the side of the pressure sensitive diaphragm 14 containing the deposited metal electrode 20, and a second pressure will be applied to the transducer 10 through the seal-off tube 56. In Figure 1A the electrical equivalent of the transducer 10 in Figure 1 is illustrated. Two capacitances are formed, the first being capacitance C₁ which is between terminals A and B, and the second capacitance C₂ which is located between the terminals B and C. The capacitors C₁ and C₂ of the transducer 10 shown in Figure 1 are so connected as to share a common terminal, namely terminal B. Due to the design of the transducer 10 shown in Figure 1, the capacitance C₁ will change as pressure is applied to the transducer 10, and of course will also change as the transducer 10 is subjected to acceleration or vibration forces. For example, as pressure applied to the transducer 10 increases, the pressure sensitive diaphragm 14 will move towards the reference diaphragm 16, causing capacitance C₁ to increase. The capacitance C₂ will not change as pressure is applied to transducer 10 but will change as the transducer 10 is subjected to acceleration or vibration forces. Therefore, it is apparent that the capacitor C₂ may be used by circuity accompanying the sensor in order to compensate for error caused by acceleration or vibration in the change of capacitance of the capacitor C₁, and the output of the system may be thus compensated to eliminate any sensitivity to errors caused by acceleration or vibration forces. Additionally, since each of the diaphragms in the transducer 10 have electrodes deposited on both sides thereof, and since the electrodes used to form capacitors are deposited only on diaphragms and not on the housing portion 12, the transducer 10 will be substantially less subject to error due to change in temperature of the transducer 10. This represents a substantial step in the art, since the circuity accompanying the transducer 10 needs far less compensation for temperature error. In Figure 2, a sensor 60 is illustrated which is constructed in a manner similar to the transducer 10 shown in Figure 1. The transducer 16 Figure 2 has two structural differences. The first is that the transducer 60 Figure 2 has a first housing portion 62 and second housing portion 64. The second difference of the transducer 60 shown in Figure 2 from the transducer 10 shown in Figure 1 is that a pressure sensitive diaphragm 66 is mounted between a reference diaphragm 68 and a second reference diaphragm 70. The transducer 60 shown in Figure 2 is also preferably constructed by fusing the assembly including the housing portions of 62, 64 and the pressure sensitive diaphragm 66 and the reference diaphragms 68, 70 together to form a single unitary assembly, with the fused portion being shown in dotted lines. The pressure sensitive diaphragm 66 has a deposited metal electrode 72 on the side of the pressure sensitive diaphragm 66 facing the reference diaphragm 68, and a second deposited metal electrode 74 on the side of the pressure sensitive diaphragm 66 facing the other reference diaphragm 70. The reference diaphragm 68 has a deposited metal electrode 76 on the side of the reference diaphragm 68 facing the pressure sensitive diaphragm 66, and a second deposited metal electrode 78 on the side of the reference diaphragm 68 facing the second housing portion 64. The reference diaphragm 70 has a deposited metal electrode 80 on the side of the reference diaphragm 70 facing the pressure sensitive diaphragm 66, and a second deposited metal electrode 82 on the side of the reference diaphragm 70 facing the housing portion 62. The reference diaphragm 68 has a passageway 84 therethrough, which passageway 84 has a conductor means 86 extending therethrough to connect the deposited metal electrodes 76, 78 on the two opposing sides of the reference diaphragm 68. The passageway 84 through the reference diaphragm 68 is of sufficient diameter to allow a wire to extend therethrough without touching the conducting means 86. Likewise, the reference diaphragm 70 has a passageway 88 therethrough, with passageway 88 having a conductor means 90 therethrough connecting the deposited metal electrodes 80, 82 on the opposite sides the reference diaphragm 70. The first housing portion 62 has a passageway 92 therethrough which passageway is located for alignment with the passageway 88 through the reference diaphragm 70. The housing portion 62 has a second passageway 94 extending therethrough above a portion of the deposited metal electrode 82 on the reference diaphragm 70. The second housing portion 64 has a passageway 96 therethrough which is aligned with the passageway 84 contained in the reference diaphragm 68. The second housing portion 64 has a second passageway 98 therethrough which extends toward the deposited metal electrode 78 on the reference diaphragm 68. A thin conductive wire extends through passageway 98 in the second housing portion 64 from the deposited metal electrode 78 on the reference diaphragm 68 to a terminal A mounted on the outside of the second housing portion 64. A wire 102 extends through the passageway 96 in the second housing portion 64 and through the passageway 84 in the reference diaphragm 68, with the wire 102 leading from the deposited metal electrode 72 on the pressure sensitive diaphragm 66 to a terminal B mounted on the exterior of the second housing portion 64. Likewise, a third wire 104 extends through the passageway 92 in the first housing portion 62 and the passageway 88 in the reference diaphragm 70, and the wire 104 leads from the deposited metal electrode 74 on the pressure sensitive diaphragm 66 to a terminal C mounted on the exterior of the first housing portion 62. Finally, a fourth wire 106 extends through the passageway 94 in the first housing portion 62 and leads from the deposited metal electrode 82 on the reference diaphragm 70 to a terminal D mounted on the exterior of the first housing portion 62. As is the case with the transducer 10 shown in Figure 1, the terminals on the transducer 60 in Figure 2 are sealed to the housing portions. Pressure is applied to the transdcuer 60 through an aperture 108 contained in the second housing portion 64, which pressure will be in the chamber comprising the areas between the second housing portion 64, the reference diaphragm 68, and the pressure sensitive diaphragm 66. For an absolute pressure sensor, a sealed-off tube 110 having an aperture extending through the first housing portion 62 would be sealed after a vacuum is drawn in the chamber comprised of the areas between the pressure sensitive diaphragm 66, the reference diaphragm 70, and the first housing portion 62. For a differential pressure sensor, a second pressure would be supplied to the seal-of tube 110, and would therefore be applied to the area above described. Figure 2A illustrates the electrical schematic of the capacitors formed by the transducer 60 shown in Figure 2 with a first capacitance C₁ being between terminals A and B, and a second capacitance C₂, being between terminals C and D. In the transducer shown in Figure 2, the capacitors C₁ and C₂ are not connected together. Since the pressure sensitive diaphragm 66 is located between the reference diaphragms 68, 70, it is apparent that both C₁ and C₂ will change as pressure supplied to the transducer 60 changes. As pressure increases, it is apparent that C₂ will increase and C₁ will decrease due to movement of the pressure sensitive diaphragm 66 towards the reference diaphragm 70 and away from the reference diaphragm 68. In both the embodiments shown in Figures 1 and 2, the output circuitry would then use either the ratio on the difference of C₁ and C₂ to determine pressure sensed, with compensation for acceleration and vibration errors being inherently included in the sensor design of Figure 2 also since the pressure sensing diaphragm 66 and both reference diaphragms 68, 70 are subjected to the same forces. It is therefore apparent that the embodiment shown in Figure 2 has two structural differences from the embodiment shown in Figure 1, namely first that the pressure sensitive diaphragm in Figure 2 is between the two reference diaphragms rather than adjacent the diaphragms as in Figure 1. The second difference is that the transducer 60 in Figure 2 has a fifth element which is the second housing portion 64, which element may be mandated by certain transducer design requirements. In addition, both capacitances C₁ and C₂ vary as pressure changes. It is important to note that the sensor of Figure 1 could also include a second housing portion 64, if it were desired to effect the capacitor arrangement of Figure 1 with the second housing portion. The transducer 60 of Figure 2 shares the same advantages of the transducer of Figure 1, namely an immunity to errors caused by temperature in addition to immunity to errors caused by acceleration or vibration forces. It may therefore be appreciated that the transducers disclosed herein possesses significant advantages over the art and that they are not only highly accurate and highly sensitive, but immune to error caused by temperature variation, unlike transducers previously known in the art. Since the preferred embodiments of the present invention may be constructed easily using existing technology at relatively little extra cost, it may be appreciated that they are highly desirable avionic components.

Claims: A pressure transducer comprising first, second and third diaphragms (14,16,18,66,68,70) characterised in that each said diaphragm has an electrically conductive layer (20,22,24,26,32,34,72,74,76,78,80,82) on each major surface thereof. A transducer according to claim 1, wherein said second diaphragm (16,66) is between said first (14,68) and third (18,70) diaphragms. A transducer according to claim 2, wherein the electrically conductive layers (24,26,72,74) on said second diaphragm, and those layers (22,32,76,80) on the first and third diaphragms which face said second diaphragm, are connected as electrodes. A transducer according to claim 3, wherein said electrodes provide a first capacitance (C₂) between the first and second diaphragms and a second capacitance (C₁) between the second and third diaphragm. A transducer according to claim 4, wherein one of said diaphragms is mounted to respond to a variable pressure input to cause a change in at least one of said first and second capacitances in response to change in said pressure. A transducer according to any one of the preceding claims wherein each diaphragm is of fused quartz. A method of converting a variable pressure input into a capacitive electrical output indicative of said variable pressure input, comprising: providing a housing portion; mounting a first diaphragm adjacent to and spaced away from said housing portion; depositing a first electrode on the side of said first diaphragm facing said housing portion; depositing a second electrode on the side of said first diaphragm away from said housing portion; mounting a second diaphragm adjacent to and spaced away from the side of said first diaphragm having said second electrode deposited thereon; depositing a third electrode on the side of said second diaphragm facing said first diaphragm; depositing a fourth electrode on the side of said second diaphragm away from said first diaphragm; mounting a third diaphragm adjacent to and spaced away from the side of said second diaphragm having said fourth electrode deposited thereon; depositing a fifth electrode on the side of said third diaphragm facing said second diaphragm; depositing a sixth electrode on the side of said third diaphragm away from said second diaphragm; sealing the areas between the edges of said first, second, and third diaphragms and said housing portion; applying said variable pressure input across one of said first, second, and third diaphragms; measuring a first capacitance across said fourth and fifth electrodes; and measuring a second capacitances across said second and third elements, said first and second capacitances being usable to determine the value of said variable pressure input.

Inventors: SHKEDI, ZVI
Applicants: ALLIED-SIGNAL INC. (A DELAWARE CORPORATION)
Application year: 1986
Main classification: G01L9
IPC codes: G01L9, G01L19