FR2508641A1 - IMPROVED QUARTZ DIFFERENTIAL PRESSURE TRANSDUCER - Google Patents

Abstract

THIS TRANSDUCER INCLUDES A CENTRAL BODY 22 BORED WITH ONE HOLE AND TWO ELASTIC INSULATING MEMBRANES 30, 32 ARRANGED ON EACH OTHER OF THE CENTRAL BODY AND GLUED ON THEIR PERIPHERY TO THE SIDES OF THE CENTRAL BODY BY GLASS JOINTS 34 , 36 IN A SPACED ARRANGEMENT TO FORM A SEALED CAVITY 40. A COLUMN 50 SLIDING FITTED IN THE HOLE IS FIXED TO THE MEMBRANES SO THAT THE MOVEMENT OF EACH MEMBRANE IS DEPENDENT ON THAT OF THE OTHER. ELECTRODES 60A, 60B ARE APPLIED ON THE INNER FACES OF THE MEMBRANES AND COOPERATING ELECTRODES 80 ARE ARRANGED ON BOTH FACES OF THE CENTRAL ORGAN TO FORM TWO CAPACITORS WHICH VARY, ACCORDING TO THE DIFFERENTIAL PRESSURE APPLIED TO THE MEMBRANES AND EGG, OPPOSED.

Description

DIFFERENTIAL PRESSURE TRANSDUCER IN

PERFECTED QUARTZ

  The present invention relates to differential pressure transducers and, in particular, to transducers

capacitive pressure.

  Pressure sensors often include at least one flexible membrane that carries a metallized or electric part

  trode which cooperates with a base plate of another member

  brane having a similar metallized part to form a pressure-sensitive capacitor These pressure transducers have an inherent drawback, namely, the

  that the variation in capacity depending on the pressure

  is not linear Prior art capacitive transducers and / or pressure measurement devices have used various techniques to render this variation

  of capacity as a function of more linear pressure.

  Among these techniques, one can mention that which consists

  remove parts of the electrodes to create a configuration

  complicated electrode ration and that of using

  read electronic circuits to match non-linear

  electronic rites to the inherent non-linear function of the pressure sensor so that the output signal

  global is more linear When incorporating configurations

  elaborate electrode and complicated electronic circuits, the complexity of the sensor is increased, the sensor

  is more expensive and its reliability is reduced.

  The present invention relates to a sensor or trans-

  capacitive pressure driver comprising a central member

  crossed by a hole and a pair of elastic elastic membranes

  three-fold insulation which is liable to deform in response to the difference in pressure applied on both sides and

  other of these membranes and are arranged in em alignment

  pounded relative to each other on either side of the gold

  central gane around the hole These membranes are glued, in

  a spaced relative arrangement, alongside the central organ

  tral by a glass type seal, thus forming

  between them a sealed cavity The transducer also includes

  a small column mounted sliding in the hole and

  stained on the lower or inner faces of the two membranes

  to make sure that the displacement of each of the mem-

  branes is dependent on that of the other; a first pair of conductive layers or electrodes are applied to each side of the central organ and a second pair of conductive layers are applied to the interior surface

  of each membrane to form a first capacitor

  pressure sensitive in cooperation with the conductive layers

  corresponding trices arranged on the central organ of

  so that the change in capacity of the first conden-

  sator is approximately equal and opposite to the change in capacitance of the second capacitor The transducer include

  in addition, signal generating means, operating in re-

  response to the values of the first and second capacitors to generate an output signal so as to minimize the non-linear component of the capacitance change as a function of pressure so that the output signal

  generated in response to the difference between the pressures applied

  on the outside of the elastic membranes is con-

  considerably more linear than in the transducers of the

prior art.

  Therefore, one of the general purposes of this

  invention is to provide a different pressure transducer

  which produces a more linear output signal than

  known transducers Another object of the present invention

  tion is to provide a capacitive pressure transducer which considerably reduces the above-mentioned non-linearities and which does not require the use of complex electronic compensation circuits Yet another object of

  the invention is to produce a pressure transducer capa-

  citive with increased sensitivity.

  An advantage of the present invention arises from the symmetry

  sorts membranes and their corresponding electrodes.

  This relationship allows the device of the present invention to exhibit insensitivity to temperature changes

  as well as a high degree of rejection in common mode.

  Other characteristics of the invention will appear

  reading the following description and examining

  appended drawings in which: FIG. 1 is a sectional view of a capacitive differential pressure transducer applying the present invention; Fig 2 is a plan view of one of the elastic membranes, this view showing the configuration of electrodes which it carries;

  Fig 3 is a plan view showing a configuration

  ration of electrodes arranged on the central organ;

  Fig 4 shows another configuration of elect

  trodes arranged on the central organ;

  Fig 5 is a sectional view showing the trans-

  pressure conductor in a bent position; Fig 6 shows a capacitive measurement bridge; Fig 7 shows a diode measurement bridge; and Fig. 8 is a schematic sectional view on a smaller scale of the transducer showing another operating situation.

  We will now refer to Figs 1 to 4 which represent

  feel the main elements of a pressure transducer The pressure transducer 20 comprises a central support member 22 traversed by an opening 24 In this embodiment, the central support member is constituted by

  a circular disc designed to be supported on its edge.

  The support for the edge of the transducer has not been represented in FIG. 1 but it has been schematically represented in the

  Fig 5 The central support member 22 is manufactured, preferably

  rence, in a material which has a practically zero hysteresis characteristic and a low coefficient of thermal expansion Materials of this type are, in particular, alumina,

  "Pyrex" and molten quartz Although, in the re-

  preferred embodiment of the invention, a support member is used

  central 22 circular, other geometric shapes are also

  Lely usable The transducer further comprises two smaller disks which constitute the flexible membranes and 32 which are preferably arranged coaxially with respect to the center of the central member 22 The flexible membranes 30 and 32 are preferably manufactured in the same material as the central support member 22 The membranes

  are maintained in a parallel relative arrangement, es-

  paced from each other and from the central support member 22 by a pair of annular seals of a known type, such as annular seals made of glass frit 34 and 36 The seals annular sealing 34 and 36 can be deposited on the central support member in a known manner, for example by screen printing, vacuum deposition

and / or sputtering.

  To prevent the central support member 22 from bending when pressure is applied to the exterior surfaces

  membranes 30 and 32, it is preferable to manufacture this or-

  gane central support 22 so that it is more rigid than each of the membranes 30 and 32 In addition, to reduce the

  thermal effects, it is desirable to manufacture the mem-

  branes and the central support member in the same material

  in addition, also, to reduce the mechanical stresses

  in the sensor, it is desirable that the characteristics

  mechanical characteristics of the material used for seals 34 and 36 are similar to the characteristics of the membranes

  and the central support member A ty-

  pique is a molten quartz glass, such as the trans-

  glass fiber G 1015 manufactured by Vitta Corpora-

  tion Damberry, Connecticut, EUA The membranes 30 and 32, the seals 34 and 36 and the central support member 22

  cooperate to delimit between them a sealed chamber 40-

mentally sealed.

  The manometric capsule 20 further comprises a central spacer, coupling member or baluster which freely crosses the opening 24 and is connected to the undersides of the membranes 30 and 32 The balancer 50 is joined to the membranes by seals of glass type 52 and 54 The length of the baluster 50 and the thickness of the joints 52 and 54 are established in a known manner so that

  only at rest, i.e. when no pressure is applied

  folded to the membranes, the membranes are kept in a relative parallel arrangement and are essentially free of stresses. The transducer 20 further comprises several electrodes which are glued or fixed in a

  otherwise to the surfaces of the central member 22 and to the mem-

  branes 30 and 32 These electrodes form at least two con-

  pressure sensitive densifiers The specific configuration

  electrodes that form these detector capacitors

  pressure may vary For the purpose of illustration,

  presented two such electrode configurations.

  dra, however, that the configurations of the electrodes located

  between the membrane 30 and the support member 22 and between the mem-

  brane 32 and the central support member 22 are substantially

  identical for any specific configuration.

  We will now refer to Fig 2 which represents a

  plan view of a typical electrode 60 a disposed on the surface

  inner face 56 of the membrane 30 An identical electrode b, which has been shown in FIG. 1, is disposed on the inner surface 58 of the membrane 32 More precisely,

  Fig 2 shows a bottom plan view of the mem-

  brane 30 The membrane 30 comprises an annular electrode 60 a disposed concentrically inside the seal 34 The electrode 60 a comprises conductors 62 a and 64 a

  conductors 62 a and 64 a extend from the electrode an-

  ring 60 has up to 9 edges of the membrane 30 Fig 2 also shows the relative arrangement of the seal 52 and

the electrode 60 a.

  In a first embodiment of the invention, the configuration of the electrodes arranged on the central support member and on the membranes forms a first pressure sensing capacitor C and a second sensing capacitor

  pressure Cs 2 In another embodiment, the confi-

  guration of electrodes forms first and second condensa-

  reference sensors Crl 'Cr 2 in combination with first and second pressure sensing capacitors Cs 1 and Cs 2 * We will now refer to Fig 4 which represents a

  electrode configuration that can be used on the

  central gane to form two pressure sensing capacitors c 51 and C in combination with the electrodes 60 a sl S 2 and 60 b This configuration can be used with either of the bridged circuits shown in Figs 6 and 7 In this configuration, the central support member 22

  -5 carries on each of its faces an annular electrode 80 dis-

  laid centrally connected to a conductor 82 extending radially The electrode 80 is located inside the annular seal 34 or, respectively, 36 and surrounds

hole 24.

  Fig 3 shows another configuration of electrodes

  which allows to form two pressure sensing capacitors

  sion Cs 1 and Cs 2 and two reference capacitors Cri and Cr 2.

  More specifically, each face of the central support member carries an annular electrode 90 disposed centrally and which surrounds the opening 24 A conductor 92 extends radially from the electrode 90 to the edge of the support member

  22 The support member 22 further comprises a second elect

  trode 94 in C shape almost completely closed which surrounds

  part of the annular electrode 90 The electrode 94 comprises

  carries another conductor 96 extending radially It can be demonstrated that the deformation between the electrodes 60 a, 60 b and is greater than the deformation between the electrodes a, 60 b and 94 Consequently, the capacitors formed between the electrodes 60 a , 94 and, respectively, 60 b, 94 are

  called reference capacitors Cr since their ca-

  capacity does not change appreciably while the

  capacitors formed between the electrodes 60 a, 90 and, respec-

  60 b, 90 which are arranged centrally in the re-

  pressure sensitive region of membranes 30 and, respectively-

  ment, 32 are called pressure sensitive capacitors Cs During the manufacture of the pressure transducer 20,

  it is preferable to orient the conductors 82 and, respectively

  vement, 92, 94 of both configurations of electrodes so that they are also spaced from the conductors 62 a,

  64 a, 62 b, 64 b, which are respectively located on the underside

  membrane 30 and on the inside of the

membrane 32.

  We will now refer to Figs 6 and 7 which represent

  two measuring bridges which can be connected in one way

  known method to the electrodes forming the capacitors of the transducer 20 The measurement bridge 100 shown in FIG. 6 includes four capacitors C 1-C 4 The circuit of the measurement bridge shown in Fig 6 can be used with one

  either of the two capacitive electrode configurations.

  As shown in Fig 6, the capacitors C 1 and C 2 correspond to the pressure sensing capacitors C 1

  and C 52 The capacitors C 3 and C 4 correspond to the conden-

  reference sators which can be capacitors of

  reference located inside the transducer 20 or the con-

  densifiers external to the transducer We can demonstrate that

  the output voltage of bridge 100 is proportional to the values

  their of its capabilities -This relationship is given by the-

  equation (1) below: EL F 1 / c C 1 (1 / C 1 + 1 / C 2) 1 / c 3 / <1 c C 3 + 1 / C 4> (1) Fig 7 represents a other measuring bridge 110 which

  includes, among others, diodes D 1 to D 4 which are connected

  pressure sensor capacitors C 1 and C 2 The output voltage of this bridge is likewise proportional to the

  values of its capacities and is given by equation (2) above

below: E - i (2) o C 1 C 2

  The measuring bridges of Fig 6 and 7 can be used

  to measure variations in capacitance of a transducer

  pressure sensor with a single pressure sensing capacitor

  as well as the multiple pressure sensor capacitors of the present invention In the case of the pressure sensor of the prior art which has a single

  pressure capacitor, the output voltage of the bridge

  pacities of Fig 6 is given by equation (1) Furthermore,

  since the right term of equation (1) does not

  not laugh, a constant H is combined with the output voltage to provide a modified output voltage E'0 The equation

  (3) shows the resulting relationship of the output voltage.

  E 't' 1 / C 1 / (1 / c 1 + 1 / C 2) = C + C (3 o il 2 C 1 + 2

  It will be recalled that the variable capacitance C 1 can be ex-

  awarded by: c 1 A (4) C 1 d 1 + Z Sd 1 in which A is a constant of proportionality and \ d 1 is the change of the nominal spacing d 1 between the

  plates forming the capacitor C 1.

  By substituting equation (4) in equation (3) and by introducing the term: X = A (d + A dl) (5) C 2 we can rewrite equation (3) in the form: E ii (6) 1 + A / C 2 (d 1 + d 1) 1 + X

  By developing equation (6), using the develop-

  binominal, we obtain equation (7) which shows clearly-

  ment that the output voltage E 'generated by the pressure detector of the prior art varies so

  linear as a function of displacement.

  E On + x 2 + (7) It can be shown that the separation between the plates of the condenser is proportional to the pressure applied; therefore, equation (7) also shows that the output voltage varies in a non-linear fashion as a function of pressure. In contrast, the output voltage generated by the

  transducer 20 in combination with either of the circuits

  cooked in the bridge of Figs 6 and 7 varies much more

  linear as a function of the change in separation distance

tion of the capacitor plates.

  By way of example, if the transducer 20 is connected in the bridge circuit of FIG. 6 and if it is assumed that the capacitors C 1 and C 2 are respectively constituted by the

  capacitor C 1 and through capacitor Cs 2, the output voltage

  tie is E 'given by equation (8) below: Cs 1 A 1 / (d 1 + L d 1 ET + c 2 A 1 / (d 1 d) A (d + d) (8) Cs Cs 2 (1 22 2 It will be recalled that the initial values of the capacitors C

  and Cs 2 are equal This relation implies that the cons-

  aunts of proportionality A 1 and A 2 are also equal as are the initial spacings d 1 and d 2 In addition, we can also demonstrate that the displacements L \ d 1 and âd 2 are

equal and opposite.

  The output voltage E'0, as expressed in the e-

  equation (8) can thus be expressed in the form: d + a d

E ', 2 2 (9)

  o 2 d 2 Equation (9) shows that the output voltage El 'is linearly proportional to the change in the spacing between the plates of each capacitor Cs 1, Cs 2 Similarly, we can see that the output voltage is proportional to the pressure difference applied A similar relationship

  showing the improvement in the linearity of the output voltage

  tie can be calculated for the in-bridge diode circuit of

Fig 7.

  In addition, a very advantageous feature of the

  present invention resides in that it produces a high degree

  rejection mode in common mode, that is to say that the manometric capsule and its electronic circuits only respond

  pressure differences and not at the level of the absolute pressure

  read applied This characteristic results from the symmetry of the membranes 30 and 32 and their electrodes which produces a first order elimination of the deformation of the membranes

  compared to the support 22 In many employment situations

  fold, the capsule is subjected to a strong basic pressure and

  must measure deviations from this base pressure.

  This operating situation has been shown in Fig 8 which shows the case where the basic pressures applied

  P and P are practically identical In response to these pres-

  sions, the membranes deform as shown Being

  born that the pressure sensing capacitors Csi and Cs 2 change in the same way, the bridge circuits 100 and 110

  remain balanced and no output signal is produced

  tie until the applied pressures have changed slightly

  rement to produce a pressure difference.

  This situation is in clear opposition to the function

  operation of a device of the prior art of the type

  consisting of a transducer having a single capacitor

  pressure sensor disposed between a membrane, such as membrane 30 or 32, and a support such as support 22 Dans

  the common mode situation, i.e. when the pres-

  applied sions Pl and P 2 are equal, the transducer generates

  an output signal due to the change in the value of the

  pressure sensor capacitor.

  By using in combination with the bridge a capsule

  nominal 20 in which an electrode configuration as shown in FIGS. 2 and 3 is arranged on the membranes 30 and 32 and on the support 22 and in the case where the nominal values of the reference capacitors Cs 1 and C 2 and those pressure sensitive capacitors C and C 52 are equal, all capacitors change by the same value and in the same direction in response to variations in

  temperature, thus keeping bridge 100 in a condi-

  balanced balance Bridge 110 provides the same compensating effect

tion of the temperature.

  In operation, the pressure capsule 20 is placed in a housing which clamps the peripheral edge

  of the support member 22 and thus forms two receiving chambers

  pressure trices separated by the support member 22 such

  so that a first pressure is applied to the surface

  membrane pressure sensor 30 and that a second press

  ion is applied to the pressure receiving surface of the membrane 32 When no pressure is applied to them or when equal pressures are applied to them, the membranes are kept at equal distances d 1 and d 2 from the organ

  support 22 If there is a pressure difference, the mem-

  branes deform, as shown in Fig 5 As an example, if the pressure Pl is greater than the pressure P 2, the spacing between the membrane 30 and the central support member 22 decreases while the spacing between the membrane 32 and the central support member 22 increases An output voltage is generated in accordance with the above equations which produces a much more linear relationship than in the past between the change in capacity and the different pressure

applied.

  Many changes and modifications can be

  brought to the embodiments described above of the

  vention without going beyond its framework Therefore,

  the scope of the invention should not be considered limited

  that to the extent defined by the claims

attached.

Claims (9)

  1 A capacitive pressure transducer (20) characterized in that it comprises: a central member (22) through which a hole (24) passes; two electrically insulating elastic membranes (30, 32) capable of bending in response to the pressure applied to them, arranged in vertical alignment with one another and arranged on either side of the hole and glued to the sides of the central body by a seal (34, 36)   of the glass type in a spaced arrangement so as to form   sea between them a sealed cavity (40); a baluster (50) received sliding in the hole and fixed to the membranes of   so that the displacement of each of the membranes is dependent   before the displacement of the other; first two layers   ductrices (80, 90, 94) applied to the opposite faces of the central member; and two second conductive layers (60 a,   b) applied to the interior surface of each of the mem-   branes to form, in cooperation with the conductive layers   trices arranged on the central organ, first and second capacitors (C 51, Cs 2) varying according to the pressure   so that the capacity changes of the first condensa-   are approximately equal and opposed to changes   capacity of the second capacitor.   2 transducer according to claim 1, characterized in that   that the capacities of the first and second capacitors will   pressure-dependent when no pressure is applied to the membranes or only at equal pressures   applied to them are approximately equal.   3 transducer according to claim 2, characterized in that it further comprises reference electrodes (60 a,   60 b, 94) arranged on the membranes (30, 32) and on the or-   central gane (22) so as to form two capacitors   of reference (Cr, C) practically insensitive to pressure applied.   4 transducer according to claim 3, characterized in that it further comprises a housing for carrying the central member (22) and for defining in cooperation with it means for transmitting a first pressure applied to one of the membranes and to transmit a second pressure applied to the other membrane.   A capacitive pressure transducer (20) characterized in that it comprises: a central member (22) through which a hole (24) passes; two electrically insulating elastic membranes (30, 32) capable of bending in response to the pressure applied to them, arranged in vertical alignment with one another and arranged on either side of the hole and glued to the sides of the central body by a seal (34, 36)   of the glass type in a spaced arrangement so as to form   sea between them a sealed cavity (40); a baluster (50) received sliding in the hole and fixed to the membranes of   so that the displacement of each of the membranes is dependent   before the displacement of the other; first two layers   ductrices (80, 90, 94) applied to the opposite faces of the central member; two second conductive layers (60 a,   b) applied to the interior surface of each of the mem-   branes to form, in cooperation with the conductive layers   trices arranged on the central organ, first and second capacitors (C 51, Cs 2) varying according to the pressure   so that the capacity changes of the first condensa-   are approximately equal and opposite to the changes in capacitance of the second capacitor; and signal generating means (100; 110) operating in response to the capacitances of the first and second capacitors (C 51, Cs 2 J for generating   an output signal so as to minimize the com   nonlinear posing of the capacity variation as a function   pressure so that the resulting output signal   dré is significantly more linear in response to the difference of   pressure applied to elastic membranes.   6 transducer according to claim 5, characterized in that it further comprises reference capacitors (C r, Cr 2) associated with each of the variable capacitors in pressure function.   7 transducer according to claim 6, characterized in that the reference capacitors are located inside from the sealed cavity (40).   8 transducer according to claim 7, characterized in that it further comprises a housing for carrying the central member (22) and for defining in cooperation with it means for transmitting a first pressure applied to one of the membranes and to transmit a second pressure applied to the other membrane.   9 A capacitive pressure transducer (20) characterized in that it comprises: a central member (22) through which a hole (24) passes; two electrically insulating elastic membranes (30, 32) capable of flexing in response to the pressure applied to them, arranged in vertical alignment one   with each other and arranged on either side of the hole and col-   linked to the sides of the central member by a seal (34, 36) of the glass type in a spaced arrangement so as to form between them a sealed cavity (40); a baluster (50) slidably received in the hole and fixed to the membranes so that the displacement of each of the membranes is dependent on the displacement of the other; and electrodes (60 a, 60 b, 80; 90, 94) arranged on the inner faces of the members and on the two faces of the central member to form at least two pressure sensing capacitors (Cs 1, Cs 2); and measuring means (100) forming a capacitance bridge connected to the   minus two pressure sensing capacitors for generating   drer a pressure signal in response to the change of space-   at least two between the two pressure sensing capacitors so that the bridge capacitance circuit is maintained in a balanced configuration and thus its output signal is zero when the spacing   between the two pressure sensing capacitors is ap- approximately equal.