DE3110295A1 - Pressure transducer - Google Patents

Description

Pressure transducer

The invention relates to a pressure transducer for supplying an electrical signal corresponding to a (acting) Pressure by means of variable capacitance elements, their impedances with a shift or deflection accordingly the pressure vary.

A previous capacitive differential pressure transducer is z. B. described in JA-AS 23916/74. In the case of a pressure sensor according to this JA-AS, a cell is in two metal parts divided; a metal foil or layer is formed on each metal part via an insulator, and a measuring membrane, which forms a capacitor plate, is through the two metal parts together with the metal layers held. Through pressures that act on pressure reduction diaphragms on both sides of the pressure sensor body, the measuring membrane is deflected, so that changes in the capacities of the two variable capacitance elements between the measuring membrane and the metal layers. On the basis of these changes in capacitance, an electrical Signal derived, which corresponds to the (acting) pressure.

A previous converter circuit for such a pressure sensor is z. B. in JA-Gbm 21643/72 described. This previous one Converter circuit uses an oscillator to generate an AC voltage signal, the amplitude of which is set by a control unit and the two variable capacitance elements of a pressure sensor of the named type is created. An output voltage which is proportional to a difference between the currents flowing through the variable capacitance elements by two rectifier elements connected to the two capacitance elements with opposite polarities and supplied a smoothing circle. At the same time, a voltage proportional to the sum of the currents is obtained

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. and compared with a reference voltage. The resulting differential voltage is fed back to the control unit. If the sizes or values of the circuit components are expediently selected, a signal is obtained which corresponds to the ratio between the difference and the sum of the first and second variable capacity element.

The previous capacity pressure sensor described uses a flat membrane as a measuring membrane, which is between the both metal parts is braced. However, this pressure sensor is sensitive to thermal deformation due to the different Expansion coefficients of the sensor body and the measuring membrane susceptible. In addition, it may be subject to mechanical deformation, if the whole Pressure sensor is clamped by means of clamping flanges. Furthermore, the sensor body consists of two separate metal parts, which have to be welded together with the measuring diaphragm in between, which leads to an increase in manufacturing costs leads.

The disadvantage of the previous converter circuit is that its output signal changes with changes in the ambient temperature fluctuates. The rectifier elements provided in the converter circuit can namely have different temperature characteristics which affect the output signal. In addition, must in this converter circuit in the variable capacitance elements flowing currents can be set to a constant size, so that the Circuit construction becomes complicated as a whole.

The object of the invention is in particular to create a pressure transducer with a pressure sensor or sensor, which, while avoiding the deficiencies of the state of the art, has a simple structure and is suitable for thermal and mechanical influences Deformation is insensitive.

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This pressure transducer should also have a transducer circuit, which is an electrical supplied by the pressure sensor Stably amplify the measurement signal and transmit the amplified signal to an acceptance instrument.

Furthermore, a pressure sensor should be used in this pressure transducer be provided, the variable capacitance elements are held in a simple manner, while their electrodes are led out from the sensor body under electrical insulation.

This object is achieved by the features characterized in the attached patent claims.

A special structural feature of the invention is the use of a capacitance pressure sensor in which a first cantilever element forming a capacitor plate in a measuring space of the pressure sensor housing is arranged, a through hole formed in the housing extends from the measuring space on either side or extending to one side of the housing, a pressure relief membrane mounted on either side or on one side of the housing is coupled to the cantilevered element by means of a rod and on both sides of the first cantilevered element Elements a second and a third fixed condensate filter plate · self-supporting or cantilevered construction are arranged.

A second structural feature of the invention consists in the combined use of said capacitance pressure sensor and a converter circuit in which two variable capacitance elements formed by said capacitor plates via a changeover or changeover switch alternating with an oscillator circuit for generating Clock change signals can be connected according to the impedances of the first or second variable capacitance element

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and of which a signal corresponding to the ratio between the difference and the sum of the frequency signals is available.

A third structural feature of the invention is that the three cantilevered elements mentioned by means of a "Packing glass" in a sealing cap for tightly enclosing a liquid in the measuring space of the sensor body are set so that they are electrically isolated from each other.

According to the invention, various errors that inevitably occur during the measurements can also be detected by means of minor Compensate for variations well.

In the following, preferred embodiments of the invention are explained in more detail with reference to the accompanying drawings. It demonstrate:

Fig. 1 is a perspective view of a capacitance pressure sensor with features according to the invention,

Fig. 2 is a section along the line A-A in Fig. 1,

Figures 3a and 3b are front views of a movable and a fixed electrode, from the side of the pressure decrease diaphragms seen here

Fig. 4 is a fragmentary view, on an enlarged scale, showing the relationship between the movable electrode and any fixed electrode,

FIGS. 5 and 6 are schematic representations of a specific example of a possibility for compensation of output (signal) changes due to different more effective surfaces of the pressure reduction membranes,

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Fig. 7 is a block diagram of one for the inventive Capacitance pressure sensor suitable converter circuit,

8 is a detailed circuit diagram of a particular embodiment the converter circuit,

9 shows a clock or timing diagram for explaining the mode of operation of the converter circuit according to FIG. 8,

Figs. 10a and 10b are sectional views of main parts of modified ones Embodiments of the invention,

Figs. 11 and 12 are sectional views of the main parts of the next modified embodiments of the invention and

Figures 13a and 13b are a side view and a front view, respectively a modified solid electrode according to the invention.

The capacitance pressure sensor shown in Fig. 1 for the pressure transducer according to the invention has a metal, cylindrical housing 1, which has a through hole 2, which extends from the outer surface of the housing · ses 1 extends to its center and forms a measuring space, and is provided with a bore 3, which the bore 2 cuts at a right angle and which is used for the installation of components in the housing 1. After assembly, the bore 3 is closed with a plug made of the same metal as that of the housing 1. On both side surfaces of the case 1, corrugations of the same configuration as those mounted on the side surfaces are provided Pressure reduction diaphragms 5 and 6 are provided. From the centers of the corrugated areas, holes 7 and 8 go to From the measuring room.

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The arrangement has a cylindrical metal housing 9 in which measuring elements, such as capacitor plates or the like. are mounted and which has a through hole 10 which communicates with the measuring space 2 in the housing 1.

i A tube or a sleeve 11 with a substantially Cup-shaped configuration is as a sealing or insert cap (flank cap) in the upper opening of the bore 10 used. The sleeve 11 contains a capacitor plate as a movable electrode and two opposing ones Capacitor plates 13 and 14 as fixed electrodes, the same on both sides of the capacitor piktte 12 Are arranged spaced. These capacitor plates are mutually insulated by a glass filling 15 attached. The electrodes can be made of metal or by vapor deposition of a metal on the surface of a Elements made of glass, ceramic or the like. Be made.

In the following, the relationship between the movable electrode 12 and the fixed electrodes 13 and 14 will be detailed explained. 3a and 3b show the electrodes 12, 13, 14, seen from the pressure decrease membrane 5 and 6 respectively. For example, as shown on a somewhat exaggerated scale in the enlarged illustration of FIG. 4, the fixed electrode 13 (or 14) is relative to the movable electrode 12 arranged so that the space between them widens towards the lower end of the latter electrode and the ratio of the initial distance between the electrodes to the displacement of the movable electrode 1 2 over the entire length of the fixed electrode 13 (or 14) is always constant. This ensures that the displacement of the movable electrode 12 under a pressure differential and the resulting output have a linear relationship. At the base or pedestal parts of electrodes 12, 13 and 14

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lines 12a, 13a and 14a are connected, which are isolated from the housing 1 and led out of this.

The movable electrode 12 is in a manner to be described in more detail via rods 16 and 17 with the pressure reduction membranes 5 or 6 connected. The rods 16 and 17 are each in advance in the center of the membrane concerned 5 and 6 welded and introduced through the holes 7 and 8 in the measuring chamber 2, if the membranes 5 and 6 can be mounted on the housing 1. At the bottom of the movable electrode 12 is an insulator Metal attachment 18 for attaching the rods 16 and 17 attached. The free ends of the rods 16 and 17 are under With the aid of the bore 3 in the housing 1 introduced into the extension 18 and at this by means of a in the Bore 3 welded introduced welder. Then the plug 4 is pressed in to close the bore 3.

The measuring space 2, the bore 10 and other interior spaces are filled with an incompressible liquid, unä Both sides of the housing 1 are attached or attached in the manner shown in dash-dotted lines in FIG. Clamping flanges 1Θ and 20 attached.

A lead low melting point glass is used as glass 15 to fill the sleeve 11, which has a softening or melting temperature, for example 500 ⁰ C and an expansion coefficient of about 7x10 / ⁰ C. The use of such a glass prevents the flexibility or pliability of the electrodes 12, 13 and 14 from being impaired by the heat generated during casting. The tubular sleeve consists, for example, of an iron-nickel alloy, the expansion coefficient of which is slightly greater than that of the glass

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This ensures that at temperatures below the melting or sealing temperature of the glass 15 through the sleeve 11 a compressive force is always exerted on the glass 15, whereby the fixing of the electrodes stabilized, and the airtight seal of the liquid in the measuring space is improved.

As the two fixed electrodes and the movable electrode arranged between them by means of the glass package are set in the sealing cap, the electrodes can be easily fixed. Especially when using a With low-melting glass, the electrodes can be fixed without impairing their elasticity. if If the temperature coefficient of the sealing cap is chosen to be somewhat greater than that of the glass, the holding strength can be reduced of the electrodes and the airtight sealing of the liquid enclosed in the measuring space is improved will.

The operation of the capacitance pressure sensor according to the invention with the structure described above is explained below. When the pressure decrease diaphragms 5 and 6 are applied with pressures P ₁ and P ₂ , so that the movable electrode 12 is displaced by the pressure difference, the distances between the movable electrode 12 and the fixed electrodes 13 and 14 change so that the capacitances C ₁ and C_ change differentially between them.

In general, a converter circuit connected downstream of the pressure sensor generates an output signal corresponding to the Differential pressure or pressure difference an electrical signal which

C. - C ₀

is proportional. To achieve an output signal with

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a linear relationship to the pressure difference, however, it is necessary to arrange the electrodes so that the capacitances C and C ₂ between movable electrode 12 and fixed electrodes 13 and 14 have the relationships to each other to be described.

According to FIG. 4, D _{Q is} the distance between the movable electrode 12 and the fixed electrode 13 at the tip or underside of the latter without the action of pressure, with E the deflection of the movable electrode at the position of the tip of the electrode 13, with d » the distance between the electrodes 12 and 13 in a given position without the action of pressure and e denotes the deflection of the movable electrode in the given position. If the fixed electrode 13 is arranged relative to the movable electrode 12 so that the ratio of its clearance to the deflection of the movable electrode 12 in a non-pressurized state is always constant in any position over the entire length of the fixed electrode 13, the following relationship can be established :

e (S) E

Dl (D

The capacitance C ₁ between the movable electrode 12 and the fixed electrode 13 can be expressed by the following relationship:

, 'fs

(2)

In the above equation: S = electrode area, £ = dielectric constant in vacuum and £ = specific inductive capacity of the enclosed liquid. Using equation (1), equation (2) can be rewritten as follows:

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- "16 -

- = Γ ^ε ° ' ^ε d ^ε ο · ^ε ' ^ρ ο Γ ι

. _O. _E.

S dT ^{as =} ST-TP (3)

with K = constant.

In a similar way, the capacitance C ₂ between the movable electrode 12 and the fixed electrode 14 can be determined according to the following equation:

_c _ ^ε ο- ^{£ ρ} ο Γ j, _{ds =} _l

2 D _{n +} EJS d _n D _{0 +}

E W

If the relationships or equations (3) and (4) in the expression

^C 1

₊ c ₂

are used, results in:

^C l - ^C 2 ^D o " E. ^D o ⁺ H E. ^C l ^{+ C} 2 K κ ^D ( ^D o - lc. ^D o ⁺ B.

(5)

Since D- is a constant and E is a pressure difference are proportional, the output signal is proportional to the differential pressure or pressure difference. In this Case it is therefore not necessary to pass the output signal through linearize an electrical circuit.

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In the miniaturization of such a differential pressure pressure sensor result in slight differences in the effective surface area and spring constant between the membranes 5 and 6 show deviations in the output signal that are no longer negligible. This means that the Internal pressure changes with a change in volume of the enclosed liquid with a change in temperature, wherein which via the rods 16 and 17 with the membranes 5 and 6 connected movable electrode 12 with creation a change in the output signal is deflected in one direction.

A way to compensate for changes in output signal due to differences in effective surface area and other factors between the two membranes is to reduce the thickness of the membranes. Through this however, the field of application of the device is limited. A known measure that is inconsistent with this Disadvantage is, is illustrated for example in Fig. 5; there is a difference between the forces acting on both diaphragms 21 and 22 are compensated for by relocating the force application point to a rod23. Here, a clamping piece 26 for attaching the rods 24 and 25 of the membranes 21 and 22 to the rod is arranged in such a way that that it can be rotated in the direction of the arrows. The force application point is adjusted by slightly turning the clamping piece 26.

Since the pressure sensor according to the invention, the membranes 5 and 6 and the movable electrode 12 are connected to one another by the rods 16 and 17, the described Achieve compensation also by arranging a rotatable clamping piece 26 of the type shown in FIG. The same result can, however, be achieved even more easily achieve that the rods 16 and 17 are bent in the manner shown schematically in FIG.

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A converter circuit suitable for the pressure sensor according to the invention is described below with reference to FIG. The dot-dashed block 27 corresponds to the capacitance pressure sensor according to FIGS movable electrode 12 shown. The block 28 drawn in dash-dotted lines represents a changeover or changeover switch made up of two switch units 28a and 28b, which are opened and closed alternately and in a complementary manner. An oscillator 29 with a variable or controllable frequency can be alternately connected to the variable resistance elements 27a and 27b via the changeover switch 28 to generate first and second frequency signals f ₁ and f ~ corresponding to the changes in impedance of the capacitance elements 27a and 27b depending on the changeover of the changeover switch 28 to create. A computing unit 30 counts a certain number of pulses or periods of the two frequency signals f .. and f ₂ from the oscillator 29, depending on which it emits a signal which represents the ratio between the difference and the sum of these frequency signals. An output unit 31 is used to output the signal from the arithmetic unit 30 after this signal has been amplified.

The two variable capacitance elements 27a and 27b can alternately be connected to the oscillator 29 via the switch units 28a and 28b, depending on their actuation periods. The closing periods of the two switch units 28a and 28b are also referred to below as the first and second periods. In the first period, the variable frequency oscillator 29 supplies the first frequency signal f ₁ corresponding to the capacitance C _{1 of} the first variable capacitance element 27a, while the oscillator 29 _{supplies the second frequency signal f 2} corresponding to the capacitance C _{1 of} the second capacitance element 27b in the second period . If the oscillator 29 with

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variable frequency an oscillator is used, which generates signals with frequencies that correspond to the capacitance Cj and C "are inversely proportional to the first and second variable capacitance element 27a and 27b, respectively the frequencies f ... and f "of the two frequency signals by

fj ~ η and
^{1 C}

The arithmetic unit 30 supplies the output unit 31 to the frequencies f... And f. In the first and second periods, respectively related duty cycle signals. Depending on the signals from the arithmetic unit 3 0, the output unit delivers 31 an analog signal D with a

proportional value.

FIG. 8 is a detailed circuit diagram of the converter circuit according to the invention shown schematically in FIG. 7. In FIG. 8, the parts corresponding to the parts of FIG. 7 are denoted by the same reference numerals as before and are therefore not explained in more detail. The circuit contains complementary MOSFET switches Q ₁ and Q ₂ - The fixed electrodes 13 and 14 are connected to _{one input side 29a of the variable frequency oscillator 29 via switches Q 1} and Q ″, respectively, while the movable electrode 12 is connected to the other Input side 29b of the oscillator 29 is connected. The oscillator 29 consists, for example, of an RC oscillator with a complementary MOSFET inverter amplifier made up of field effect transistors Q ₃ and Q ₄ and a complementary MOSFET inverter amplifier.

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stronger from field effect transistors. _{Q 5} and Q _g. In the circuit according to FIG. 8, the input side 29a is at the source and drain electrodes of the field effect transistors Q ₁ . and Q, while the input side 29b is connected to the first gate electrodes of the field effect transistors Q ₃ and Q ₄ and, via a resistor, to the source and drain electrodes of the field effect transistors Q ₃ and Q and to the first gate electrodes of the field effect transistors Q ₅ and Qg is connected. The source electrodes and the second gate electrodes of the field effect transistors Q- and Q ₁ - are connected to a positive current source + E, while the drain and second gate electrodes of the field effect transistors Q. and Q are connected to a negative current source -E The connection point between the field effect transistors Q ₅ and Q is connected to an output terminal 29c, from which the first and second frequency signals are transmitted to the arithmetic unit 30 in synchronism with the opening and closing of the switches Q and Q ". The oscillation frequency of the oscillator 29 depends on the sizes of the electrostatic capacitances C ₁ and C _{2 of} the two variable capacitance elements 27a and 27b connected between the input sides 29a and 29b. The oscillator 29 generates the first and second frequency signals in accordance with the reciprocal values of time constants which are determined by the electrostatic capacitances C ₁ and C_ and the resistance value of the resistor R.

The computing unit 30 contains an n-bit counter Q_ (n = positive integer) and complementary MOSFET switching gates Q ₈ and Q _g . The two frequency signals are input to the counter Q_. The most significant bit output signal of the counter Q_ is applied to the gate electrodes of the switches Q ₁ and Qy of the changeover switch 28 and is supplied to a first gate electrode of the complementary MOSFET switches Q ₈ and Qg. The source electrode and the second gate electrode of the switch Q _g are connected to the positive current source + E, while the drain- and second gate-

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Electrode of the switch Q are connected to the negative power source -E. The drain and source electrodes of the switch and Qg Q _g are respectively connected together, wherein the connecting point to a smoothing circuit composed of resistors R .. and R ₂ and capacitors C, and C is connected. ₄

If, with this arrangement, the output signal of the counter Q _{7 of} the arithmetic unit 30 has the level "O", the switch Q ₀ and the switch Q. of the changeover switch 28 remain in the closed or through-connected state. In this state, the first frequency signal with the frequency f .., which is related to the reciprocal _{value of the time constant determined by the capacitance C 1 of} the pressure sensor 27 and the resistance R of the oscillator 29, is sent to a clock gate input CL of the counter Q ₇ created. If in the course of the counting process of the counter Q ₇ the output signal changes to the level "1", the switch Q "opens or blocks, while the switch Qg * closes or switches through and the switches Q ₁ and Q _{2 of} the switch 28 into the Go to the locked state or to the switched-through state. In this state, the second frequency signal with the frequency f ₂ , which is determined according to the reciprocal of the time constant of the capacitance C _{2 of} the pressure sensor 27 and the resistance R of the oscillator 29, is applied to the clock input CL of the counter Q ₇ . When the output signal of the counter Q _{7 changes} to level "O" in the course of its counting process, counting of the first frequency signal is resumed. In this way, the first and second frequencies are counted alternately _{by the counter Q 7.}

FIG. 9 is a timing diagram for explaining the operation of the converter circuit according to FIG. 8. In FIG Q _{g of} the arithmetic unit 30 denotes output waveforms

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net. More precisely: the counter Q ₇ counts the first and second frequency signals f .. or f- in steps of N (corresponding to the full count of the counter Q ₇ ) in order to then change the state of the most significant bit. Accordingly, a time period T ₁ , during which the first frequency signal is supplied, is determined by T ₁ =

-ψ-, and a time period T-, during which the second frequency signal is generated, is determined by

N
T- = -ψ-. The two frequency signals are thus on the

in Fig. 9 at A shown manner from the oscillator 29 alternately supplied.

In the arithmetic unit 30, the switches Qg and Q- are each _{closed or switched on during a time period T 1} or T-, so that a pulse duty factor signal is supplied which alternates the voltages + E and -E during the time periods T ₁ and T - Assumes, as shown at B in FIG. This output signal is smoothed by the smoothing circuit from the resistors R .. and R- and the capacitors C ₃ and C ₄ , so that the arithmetic unit 30 the

proportional signal supplies. Since a smoothed potential V_ of the smoothing circle is an average value of the output waveform B according to Fig. 9 results in:

T. χ (+ E) + Tx (-E) T ₁ - T-

V _n = - ± 1 = _i £ E

D T + TT + T

¹ I ¹ I. ^T l ^Γ 2

N N

from the relations T ₁ = - = - and T- = · = - can be deduced

1 2

Derive the voltage:

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N N

⁷ I. ' ⁷ Z ^f 2 - ^f l

N ₊ N f + f

1 ^{1 line}

This voltage is fed to a two-wire transformer, indicated by the dot-dash block 31a in the output unit 31, which emits a current signal. This arrangement in which the output sides 31b and 31c are used in common as power source terminals and signal output terminals is known per se. A field effect transistor Q _1n forms a constant current device, and a diode ZD ₁ supplies the constant voltage + E and -E for the output unit 31, the changeover switch 28, the oscillator 29 with variable frequency and the arithmetic unit 30. To a non-inverting terminal of an operational amplifier Q. ₁₁ , said smoothed voltage V _{n is} applied while its inverting terminal is applied with a voltage obtained by dividing the constant voltages + E and -E by resistors R ₁ , R and R 1 -. The output signal of the operational amplifier Q ₁₁ is supplied as a drive signal to an output transistor Q ₁₂ to control the output current through the output terminals 31b and 31c. A resistor R, is as a feedback resistor between

the output transistor Q ₁₂ and the output terminal 31c are switched. The feedback voltage supplied by the feedback resistor R 1 is fed back negatively or in the reverse direction with subtraction from the smoothed voltage V, whereby the output current is controlled or adjusted. In this way, a current signal of a _{magnitude proportional to the smoothed voltage V D} is supplied to the output terminals 31b and 31c.

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The described pressure transducer according to the invention works satisfactorily in itself. However, if a static pressure to be measured of more than, for example, 100 kg / cm ² acts on the incompressible liquid such as silicone oil located in the measuring chamber 2, errors due to the slight compression of the measuring chamber 2 should preferably be compensated for. When the silicone oil is compressed while reducing its volume, the pressure-reducing diaphragms 5 and 6 deform, thereby adversely affecting the measurement accuracy and the measurement range. To provide such compensation, the modified embodiments shown in Figures 10 (a) and 10 (b) showing major parts of modified pressure transducers are provided.

If, in the case of the capacities C. and C ~, which represent quantities measured in the unpressurized state, a static pressure of about 100 kg / cm ² acts on the liquid enclosed in the measuring space 2, the capacities C ₁ and C "UmAc ₁ or Ac ₂ , ie the distances between the electrodes increase. These differences or deviations Ac. and Ac “are preferably compensated. In the embodiment of the invention shown in FIG. 10 (a), an inner end section 11a of the tubular sleeve 11 is made thinner than its outer end section 11b for this purpose, so that the static pressure can act on the end section 11a in the manner indicated by the arrows A. The electrodes 13 and 14, which have moved apart, are therefore brought _{closer to each other by the correct amount in accordance with the differences Ac 1} and Ac ₂ , as shown by the broken lines in FIG. 10 (a).

Conversely, when the spacing between the electrodes due to the described, high static pressure decrease, that is, when the capacitances C ₁ and C ₂ to Ac ₁ ¹ and Ac "zoom, of the sleeve 11 should preferably be of the middle part 11c to be thinner than its inner

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and outer end portions 11a and 11b, respectively (see FIG. 10b). The high static pressure acts on the through the Arrows B in Fig. 10 (b) on the central portion 11c. The thickness of the sleeve 11 can be through Estimation of the capacity differences due to the high static pressure to which the pressure transducer is exposed is to determine.

In FIGS. 11 and 12, two further modifications are shown in accordance with of the invention shown. In this case, in the pressure transducer according to the invention, the displacement or Deflection measuring devices illustrated. With this pressure transducer, it is recommended to detect errors due to a change in temperature 'To compensate, which primarily leads to changes in the spring constant of the movable electrode 12 as well of membranes 5 and 6. These factors cause an immediate change in the measuring range, d. H. they lead to measurement errors.

According to Fig. 11, the sleeve 11 is also filled with glass 15, i. H. poured out, but there is essentially the tubular or annular sleeve 11 from a Metal, such as Kovar, which has the same coefficient of expansion as glass. At the outer end of the sleeve 11 is an annular collar 11a is formed which encloses part of the electrodes 12 to 14. In this embodiment the electrodes 12 to 14 are inserted close to the end section of the sleeve 11.

The sleeve 11 is inserted into the housing 9 with a seal or welded into it. The material of the housing 9 is preferably chosen so that its thermal Coefficient of expansion is smaller than that of the metal sleeve 11 and "of the glass 15. For this purpose, the Housing 9 can be made from Invar, for example.

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In the deflection measuring device with the structure described, the annular collar 11a expands with increasing temperature increasingly outward. Due to the choice of the thermal expansion coefficients of the relevant components the compressive force acting on the upper part of the glass filling 15 increases as the temperature rises. There the compressive force acts only on the upper part of the glass panel 15, the fixed electrodes 13 and thus moved apart at their inner ends. The distance the fixed electrode 13 or 14 to the movable electrode 12 increases thereby / while the measuring range increases (span) reduced. The construction described is therefore suitable for a deflection measuring or sensing device, at which an increase in temperature allows an expansion of the measuring range. With this device, the measuring range change well corrected.

If, on the other hand, the coefficient of thermal expansion of the housing 9 is larger than that of the sleeve 11 and the glass filling 15, namely when the housing 9z. B. off is made of stainless steel, the compressive force acting on the collar 11a gradually decreases as the temperature increases away. This means that the on the collar 11a acting tensile stress increased. As a result, the distance between the free end portions of the electrodes enlarge.

This construction can therefore be effective as a deflection measuring device can be used in which an increase in temperature leads to a reduction in the distance or measuring range leads. In this case, the change in the measuring range can also be corrected well.

In the embodiment described, by choosing the thermal expansion coefficient of the materials Displacement or deflection direction of the free ends of the fixed electrodes 13 and 14 are determined. Measurement errors due to temperature changes can be

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ßige choice of the position and the configuration of the collar 11a be compensated. A correspondingly modified construction is shown in FIG. The direction of compensation is opposed to that in the embodiment according to FIG.

According to Fig.12 is at the lower portion of the tubular Sleeve 11 is formed with an annular collar 11b. When the thermal The expansion coefficient of the housing 9 is smaller than that of the glass filling 15 and the sleeve 11, increases (with increasing temperature) the on the collar 11b corresponding part of the glass filling 15 acting compressive force, so that the distance between the electrodes 12 and 13 (or 14) scaled down to compensate.

On the other hand, when the coefficient of thermal expansion of the housing 9 is greater than that of the glass filling 15 and the sleeve 11, is at the level of the collar 11b lying part of the glass filling 15 is reduced, so that the distance between the electrodes 12 and 13 (or 14) enlarged for compensation.

While in the described embodiments of the ring. If collar 11a or 11b is formed on the sleeve, it can also be formed on the inside of the housing.

According to a further feature of the invention, measurement errors due to the influence of gravity, i. H. as a result can be compensated for by positional errors of the pressure transducer. This is because the pressure transducer is not always perpendicular Position used. With the previous pressure transducers, it is not possible to detect such measurement errors that are due to the weight based on the liquid enclosed in the measuring space to compensate. If, in extreme cases, the pressure transducer is below is rotated at a right angle with respect to the vertical position, the entire weight of the trapped acts Liquid, such as silicone oil, on one of the membranes 5 or 6. Here it is obvious that gravity

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directly affects the measurement in an undesirable manner.

According to the invention, such measurement errors by means of a slight Modification can be compensated or compensated for. Such a modification is shown below with reference to Fig. 13 (a) and 13 (b) explains which a fixed electrode 13 (14) show in side view or in front view. The electrodes 13 and 14 shown in Figs. 13 (a) and 13 (b) are respectively provided with a cutout 13a or 14a. This means that the electrodes 13 and 14 have narrower sections 13a or 14a, around which they move under the action of gravity can bend in the same direction.

Since the specific weights of the individual components and the incompressible liquid, such as silicone oil, as well as the Distance between the pressure decrease diaphragms 5 and 6 are known in advance, the narrower portions 13a and 14a expediently designed so that the displacement or deflection of the movable electrode 12 of those of the fixed electrodes 13 and 14 is the same. In this way, measurement errors due to the influence of gravity can be avoided be well compensated.

Since the above-described capacitance pressure sensor according to the invention, the two fixed electrodes and the movable electrodes arranged in between are designed as self-supporting or cantilevered elements the pressure sensor is hardly influenced by thermal deformation of the electrodes, so that it has excellent temperature characteristics or characteristic curve. Even if the pressure sensor housing is due to the tension on the mounting flanges undergoes a deformation, are the changes in the distance between the movable electrode and the fixed electrodes small due to the deformation, because the electrodes are only supported on one side and theirs are supported Sections are close together. In addition, the pressure sensor housing is designed as a one-piece construction, so that its manufacturing costs compared to the previous one

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Pressure sensors of the same general type, the housing of which is formed in two parts, are low.

The converter circuit according to the invention is for changes in ambient temperature insensitive, so that it is stable or unaffected by a mechanical deflection able to amplify and deliver an electrical signal. Since this converter circuit also, in contrast to the previous circuit of this type, no impedance elements in a bridge circuit using Has rectifier elements, an adjustment of the characteristics of the circuit components is not necessary. About that In addition, the converter circuit has a simplified circuit structure, thereby improving its reliability will.

Although the capacitance pressure sensor according to the invention is described above as a differential pressure measuring device, in which pressure reduction diaphragms are arranged on both sides of the housing, the invention is the same Applicable to a pressure transmitter or transmitter, in which such a membrane is only on one side of the Pressure sensor housing is provided. In the illustrated embodiments of the invention, there are also the electrodes isolated from the pressure sensor housing. However, the invention can be implemented with an electrical circuit in which the movable electrode and the pressure sensor housing are not isolated from each other.

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Claims (16)

Claims Pressure transducer, characterized by a capacitive or capacity pressure sensor with a in the center of its Housing provided, filled with an enclosed liquid measuring space, cantilevered by a or cantilevered in the measuring space arranged movable electrode, through one of the measuring space to at least o.incr side of the C! ohaur; oH © rut. jerky, continuous. Bore through a pressure reduction element arranged on at least one side of the housing, through a SLab coupling the pressure reduction element via the bore to the movable electrode and through two cantilevered, fixed electrodes placed on opposite sides of the movable electrode are and together with this two variable capacity 130065/0713 form elements, with an electrical signal corresponding to an (acting) pressure as a function changes in the capacities of the first and second variable capacitance element can be generated. 2. Pressure transducer according to claim 1, characterized by a converter circuit with a changeover switch for selecting the first or second variable capacitance element, by a variable frequency oscillator for generation two frequency signals, each of the impedance of the first and second variable capacitance element correspond, which in turn are connected to the oscillator via the switch, and by a computing unit to provide a signal corresponding to the difference and the sum of the first and second Frequency signal. 3. Pressure transducer according to claim 1 and 2, characterized in that that the converter circuit comprises the changeover switch, the oscillator and the arithmetic unit with a counter, when counting a predetermined number of periods of the first or second frequency signal from the oscillator inverted its output signal for switching the changeover switch, and that a circuit is provided to to let the counter deliver a duty cycle signal which is the ratio between the difference and the Corresponds to the sum of the first and second frequency signal. 4. Pressure transducer according to claim 2, characterized in that the changeover switch of the converter circuit has two complementary ones MOSFET switches (or MOS field effect transistors) has, which are connected with the first or second input to the (relevant) fixed electrodes are. 13006 5/0713 5. Pressure transducer according to claim 3, characterized in that the variable frequency oscillator has a first complementary MOSFET inverter amplifier with third and fourth complementary MOSFET (MOS field effect transistor) and a second complementary MOSFET inverter amplifier with fifth and sixth complementary ones MOSFET has that a first output terminal of the switch to source and drain electrodes of fifth and sixth MOSFET and a second output terminal of the same to first gate electrodes of the third and fourth MOSFET connected and via a resistor with source and drain electrodes of third and fourth MOSFET and the first gate electrodes of fifth and sixth MOSFET are connected that source and second gate electrodes of the third and fifth MOSFET are connected to a positive current source that Drain and second gate electrodes of fourth and sixth MOSFET connected to a negative power source and that a connection point between the fifth and sixth transistor to an output terminal of the oscillator connected. 6. Pressure transducer according to claim 5, characterized in that the arithmetic unit has a counter unit whose clock input is connected to the output terminal of the oscillator and its output for the most significant bit is connected to the gate electrodes of the first and second MOSFET or MOS field effect transistor, a eighth and a ninth complementary MOS field effect transistor, whose first gate electrodes with the counter output for the most significant bit are connected while the source and second gate electrodes of the eighth Transistor are connected to a positive current source and drain and second gate electrodes of the ninth Transistor are connected to a negative power source while continuing to drain and source electrodes of the eighth and ninth transistor are connected to a connection point, and a smoothing circuit on- 130065/0713 whose input is connected to the connection point and the two resistors connected in series, with a first terminal of the first resistor connected to the Connection point is connected, as well as two series-connected and via the second resistor (parallel) switched capacitors, wherein a first terminal of the first capacitor is a branch between first and second resistor is connected, while first and second terminal of the second capacitor are connected to the output terminals of the computing unit. 7. Pressure transducer according to claim 6, characterized in that an output unit is connected to the output terminals of the arithmetic unit is connected, which has a two-wire signal transmitter for outputting a current signal as a function of signals at the output terminals of the computing unit and an operational amplifier comprises whose non-inverting input terminal is connected to a first output terminal of the arithmetic unit is that a Zener diode to determine the size of the positive and negative Stromquellensspannung.vorvors with the anode of the Zener diode connected to the negative power source and its cathode to the positive Current source are connected that a second formed at the junction of the first and second capacitor Output of the arithmetic unit is connected to the negative power source that between the anode and cathode the Zener diode has a voltage divider for applying a voltage of a predetermined magnitude to an inverting one Input of the operational amplifier is connected that an output transistor with an output of the operational amplifier is connected and that a transistor in constant current configuration for supplying a current is switched to the Zener diode. 130065/0713 8. Pressure transducer according to claim 1, characterized in that a sealing cap for sealing the enclosed Liquid is provided in the measuring space that the sealing cap at least partially with a glass filling for holding the movable electrode and the fixed one Electrodes under electrical insulation of the same is provided and that an electrical signal accordingly an acting pressure of changes in the capacities of the first and second variable capacitance elements can be derived is. 9. Pressure transducer according to claim 8, characterized in that the glass filling consists of a low-melting glass consists. 10. Pressure transducer according to claim 8, characterized in that the sealing cap is made of a material whose (thermal) expansion coefficient corresponds to that of the glass filling or slightly exceeds. 11. Pressure transducer according to claim 8, characterized in that the sealing cap is substantially cup-shaped with a Side wall is formed and that the side wall has at least one thinner portion, which with a Part of the pressure sensor housing defines at least one free space which is connected to the measuring space stands. 12. Pressure transducer according to claim 8, characterized in that that the sealing cap is substantially cup-shaped in a side wall and that the side wall is a Has annular collar, so that the thinner part of the sealing cap together 'with a part of the pressure sensor housing defines at least one free space. 130065/0713 13. Pressure transducer according to claim 12, characterized in that that the free space is in connection with the measuring space. 14. Pressure transducer according to claim 12, characterized in that that the free space is in connection with the ambient atmosphere. 15. Pressure transducer according to one of claims 12, 13 and 14, characterized in that the coefficient of thermal expansion the sealing cap, the glass filling and the mentioned part of the pressure sensor housing for compensating . ren of measurement errors due to temperature changes are appropriately chosen and that the coefficient of thermal expansion of the pressure sensor housing is different from that of the glass filling and the sealing cap. 16. Pressure transducer according to claim 1, characterized in that the two fixed electrodes each have one Have thinner section and at their free ends essentially depending on the influence of gravity the enclosed liquid are movable due to an inclination angle of the central axis of the measuring space. 130065/0713