GB2059071A

GB2059071A - Capacitive pressure transducer

Info

Publication number: GB2059071A
Application number: GB8030293A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-09-21
Filing date: 1980-09-19
Publication date: 1981-04-15
Also published as: FR2466012B1; IT1132979B; SE8006587L; GB2059071B; FR2466012A1; BR8005996A; DE2938205A1; JPS5654331A; IT8024775A0

Abstract

The transducer comprises a profile- etched metallic diaphragm (7) the peripheral zone (6) of which is left unetched for mounting the diaphragm on an Al2O3 base plate (1) in an air-tight and pressure-tight manner; and a thin metallic film (2) applied to the adjacent surface of the base plate, the diaphragm (7) and the metallic film (2) constituting a capacitor which varies with the separation between the diaphragm and the metallic film and hence with ambient pressure. The capacitor determines the frequency of an oscillator 13. <IMAGE>

Description

SPECIFICATION Capacitive pressure transducer and evaluation circuit The invention relates to a capacitive pressure transducer, in particular for the detection of the intake-air pressure of an internal combustion engine, having a metallic diaphragm which is deformed in dependence on pressure and opposite which there is arranged a metallic surface, which is electrically insulated with respect to the diaphragm, the metallic surface and the diaphragm forming the two electrodes of an electrical capacitor.

In electronic controlling and regulating systems for internal combustion engines, absolute-pressure transducers are required for measuring the pressure in the air-intake pipe. In addition to conventional requirements, such as a high operating-temperature range, such pressure transducers should, above all, fulfil the requirements of smallness of size and insensitivity to acceleration, in order that the pressure transducer may be mounted directly in the air-intake pipe of the internal combustion engine. It is an object of the present invention to provide such a transducer.

There is provided by the present invention a capacitive pressure transducer, in particular for detecting the intake-air pressure of an internal combustion engine, comprising a metallic diaphragm which is deformed in dependence on pressure, and opposite which there is arranged a metallic surface which is electrically insulated with respect to the diaphragm, this metallic surface and the diaphragm forming the two electrodes of an electrical capacitor, wherein the metallic surface, in the form of a thin conductive film, is applied to one side of a plate of insulating material, and the diaphragm comprises a profile-etched metallic plate, whose peripheral zone, which is not removed by the etching process, is attached to the plate of insulating material, with respect to which it forms an air-tight and pressure-tight seal.

The use of a profile-etched diaphragm instead of a conventional grooved diaphragm has the advantage that profile-etched diaphragms can be manufactured with great precision from almost any resilient material and that the material used can be matched to the coefficient of thermal expansion of the support.

Moreover, the characteristic of the pressure transducer, such as the dependence of the capacitance C on the pressure p can be easily matched to actual requirements by corresponding selection of the etched profile.

It is particularly advantageous if, as in a further embodiment of the invention, the other side of the plate of insulating material remote from the diaphragm carries an evaluation circuit, in particular in the form of a thin-film circuit. This may advantageously be in the form of a resistance-capacitance oscillator, in particular a multivibrator with an operational amplifier.

Embodiments of the invention are described hereinafter and are shown in the drawing, in which; Figure 1 is a cross-sectional view of a first embodiment of a capacitive pressure transducer according to the present invention, Figure 2 is a cross-sectional view of a second embodiment of a pressure transducer according to the present invention when subjected to an external pressure p = 0, and Figure 3 is a cross-sectional view of the pressure transducer of Figure 2, but subjected to an increased external pressure p relative to its internal pressure, Figure 4 is an evaluation circuit which provides an output frequency

Figure 5 is another evaluation circuit with an operational amplifier.

The pressure transducer shown in Figure 1 has a thin supporting plate 1 of insulating material, which is manufactured from Al2O of ceramic material and carries in the middle of one of its two upper sides a thin-film metallic coating 2. The thin film is connected by means of a contact 3, which extends through the plate 1, to the conductor path 4, which is arranged on the opposite side of the plate 1 and is also in the form of a thin film.

On the side of plate 1 to which the thin film 2 is applied there is provided a conductor path 5 also in the form of a thin film which, as a closed ring, surrounds the central thin film 2, a clearance being provided between the conductor path 5 and the thin film 2. The peripheral zone 6 of a diaphragm 7, which is profile-etched from a metallic plate, is attached by soldering to the annular conductor path 5 to form a pressure-tight seal. The diaphragm defines a cavity 8, which is exhausted. The base 9 of the diaphragm 7 process, together with the opposing thin film 2, forms an electrical capacitor whose capacitance C is increased the more the external pressure p acting upon the base 9 of the diaphragm 7 increases and bends the base 9 against the thin film 2.

The diaphragm 7, which operates as the second electrode of the capacitor, is connected by means of a through connection 11 to a contact rail 12, which also is applied by a thin-film technique and which is part of an evaluation circuit comprises a plurality of components, of which two, designated as 14 and 15, are shown in the drawing.

The embodiment of the pressure transducer shown has the advantage that the pressure transducer occupies only a small amount of space, is substantially insensitive to temperature, and can easly be mounted in the air-intake pipe of an internal combustion engine, where it provides an electrical quantity which is variable in dependence on the intake-air pressure, preferably a pressure-dependent frequency f.

In the embodiment shown in Figures 2 and 3, as in the embodiment of Figure 1, a small ceramic plate 1, serving as a support, and a thin-film electrode 2 applied to the ceramic plate 1, are provided. In contrast to the first embodiment, the diaphragm 20, produced from a profile-etched metallic plate, is in this case provided with an annular groove 23 about its peripheral zone 21 by which the diaphram 20 is soldered pressure-tight to a conductor path 22 which is annular in plan,the annular groove 23 being so deeply etched that, at the base of the groove 23 there is formed a thin wall portion 24 which is easily deformable, as is evident from Figure 3, and whose restoring forces are insubstantial.In the central 25, surrounded by the groove 23, the diaphragm 20 is situated opposite, and only a short distance e from, the thin-film electrode 2 when the external pressure p = 0 and the inner chamber 26 defined by the diaphragm 20 is exhausted. In the case shown in Figure 2, where = 0, a relatively high value of capacitance of the capacitor formed by the central region 25 and the opposite thin-film electrode 2 is achieved owing to the small distance e.

If a pressure p acts in the manner shown in Figure 3 upon the diaphragm 20, the central region 25 is brought into very close proximity with the thin-fil m electrode 2, and the resultant capacitance between these two electrodes is consequently substantially increased.

The capacitance C between the two electrodes 2 and 25 and its behaviour in dependence on the external pressure p can be influenced in a simple manner by the depth of etching of the groove 23 and of the free frontal area in the central region 25.

For evaluation of the pressure-dependent capacitance Cit is possible to use the resistance capacitance oscillator shown diagrammatically in Figure 4, in which a voltage whose frequency f is produced at the outputteminal A. The oR5tput A is connected to the input of the oscillator by means of a feedback resistor R, which can be adjusted, and which, together with the variable capacitance C of the pressure transducer according to Figure 1 or Figure 2 determines the oscillating frequency f.

Figure 5 shows another evaluation circuit, which may be designated as a multivibratorwith an operation amplifier. In this case an alternating voltage Ua, whose repetition rate f = 1/2 RC 1 n (1 + 2 R1/R2), is provided at the output A of an operational amplifier 0.

Claims

1. A capacitive pressure transducer, in particular for detecting the intake-air pressure of an internal combustion engine, comprising a metallic diaphragm, which is deformed in dependence on pressure, and opposite which there is arranged a metallic surface which is electrically insulated with respect to the diaphragm, this metallic surface and the diaphragm forming the two electrodes of an electrical capacitor, wherein the metallic surface in the form of a thin conductive film, is applied to one side of a plate of insulating material, and the diaphragm comprises a profile-etched metallic plate, whose peripheral zone which is not removed by the etching process, is attached to the plate of insulating material, with respect to which it forms an air-tight and pressure-tight seal.

2. A pressure transducer according to claim 1, wherein the metallic plate has a central zone of reduced thickness, which is produced by etching.

3. A pressure transducer according to claim 1 or 2, wherein the diaphragm has a central zone which is defined with respect to the peripheral zone by an etched groove which extends about this peripheral zone.

4. A pressure transducer according to any of claims 1 to 3, wherein the plate of insulating material is composed of ceramic material.

5. A pressure transducer according to claim 4, wherein the ceramic material is Awl203.

6. A pressure transducer according to any of claims 1 to 5, wherein the plate of insulating material carries on its other side remote from the diaphragm an evaluation circuit.

7. A pressure transducer according to claim 6, wherein the evaluation circuit is in the form of a thin-film circuit.

8. A pressure transducer according to claim 6 or 7, wherein the evaluation circuit is in the form of a resistance-capacitance oscillator.

9. A pressure transducer according to claim 8, wherein the oscillator is an operational amplifier based multivibratorthe repetition rate of the output of which is dependent on the resistance and capacitance values of the resistance capacitance network of the oscillator.

10. A pressure transducer according to claim 8, wherein the oscillator comprises a variable frequency generator the frequency output of which is dependent on the resistance and capacitance values of the resistance capacitance network of the oscillator.

11. A capacitive pressure transducer, substantially as hereinbefore described with reference to Figure 1 orto Figures 2 and 3 ofthe accompanying drawings.

12. A capacitive pressure transducer substantially as hereinbefore described with reference to Figure 1 or to Figu res 2 and 3 and to Figure 4 or Figure 5 of the accompanying drawings.