GB2125167A - Electrical fluid pressure transducer diaphragms - Google Patents

Abstract

The displaceable diaphragm of an inductive or capacitive pressure transducer is constituted by a plastics or rubber-like material impregnated, enclosing or coated with a magnetic or conductive material. The magnetic material can be in dust form enclosed or impregnated in the diaphragm. The conductive material can be a coating of aluminium or gold. In either case the diaphragm can be of synthetic or natural rubber or any synthetic material having similar properties.

Description

SPECIFICATION A pressure transducer Technical Field The present invention relates to a pressure transducer and is particularly concerned with a pressure transducer having an electrical element and a component that is adjustable to cause a change of an electrical parameter across the terminals of the electrical element. If the electrical element is an electrical coil magnet then the adjustable component acts to vary the reluctance of the magnetic circuit thus changing the inductance of the coil. If the electrical element is a capacitor then movement of the adjustable component effects a change in the capacitance of the capacitor. The component may for example constitute one plate of the capacitor.

Where the component is a reluctance varying component then it should have magnetic properties, and where the component is intended to vary the capacitance of a capacitor then it should have capacitive characteristics.

Background Art The component is displaceable in response to variations in fluid pressure which are intended to be detected by the pressure transducer, and it has been previously proposed that the displaceable component should be metallic. An example of a material currently used in the production of a displaceable component is solid bar stainless steel, such as 431 stainless steel, which is a magnetic stainless steel. It has been found that displaceable components for use in pressure transducers cannot be machined to a thickness of less than 0.007 otherwise serious manufacturing problems are encountered and production losses occur.

In a metallic displaceable component the first regime of deformation occurs as the centre of the component bends due to overall pressure; the component being clamped at its circumference.

With a component of 1.375 inches diameter, as the components centre deforms to a distance of 0.005 inches radial stresses take over and thereafter the deflection at the component centre becomes considerably reduced. This effect produces non-linearity of the deflection/pressure characteristic of the pressure transducer.

Statement of Invention and Advantages It is an aim of the invention to improve the sensitivity of the deflection/pressure characteristic of a pressure transducer and according to one aspect of the invention there is provided a pressure transducer comprising a casing of magnetic material having two facing pole pieces separated by a gap, two electrical coil magnets housed in the casing to surround their respective pole pieces, a displaceable component mounted in the casing to extend across the gap and pole pieces, displacement of the component towards one or other of the other pole pieces being in response to variations in fluid pressure at outer surfaces of the casing to change at least one electrical parameter of the coils, and passages extending from said outer surfaces of the casing to opposite surfaces of the displaceable component, in which the displaceable component comprises magnetic material mounted between the pole pieces on a plastics diaphragm.

The diaphragm may be made of any material of a plastics form such as for example silastomer rubber, natural rubber, synthetic rubber, condensate of teraphlalic acid, Kapton, Kevlar and polytetrafluorethylene (PTFE). Examples of suitable magnetic materials for the displaceable component are magnetic dust, such as is used in magnetic tape, soft iron filings suspended in cold setting resin, radio metal and mumetal.

In order to reduce corrosion of the magnetic material of the displaceable component, the magnetic material may be encased between two plastic diaphragms. In order to strengthen the displaceable component the plastics diaphragm having the magnetic material mounted thereon, or alternatively the two plastics diaphragms having the magnetic material mounted therebetween, may be secured between two annular members which are themselves mounted in the casing. An advantage of this construction is that the displaceable component can be manufactured and sold as a readily replaceable unit. An advantage of encasing the magnetic material between two plastics diaphragms is that it provides environmental protection to the magnetic material.

According to another aspect of the invention there is provided a pressure transducer comprising a casing having a gap therein, two opposed spaced capacitor plates located in the gap and electrically insulated from the casing, a displaceable component having capacitive characteristics mounted in the casing to extend across the gap between the spaced capacitor plates to provide with said capacitor plates two capacitors, displacement of a component towards one or other of the capacitor plates being in response to variations in fluid pressure at outer surfaces of the casing, to change at least one electrical parameter of the said two capacitors, and passages extending from said outer surfaces of the casing to opposite surfaces of the displaceable component, in which the displaceable component comprises material having capacitive characteristics mounted between the said capacitor plates on a plastics diaphragm.

The plastics diaphragm may conveniently be manufactured from any of the materials previously mentioned in connection with the pressure transducer including the two electrical coil magnets.

In order to provide the plastics diaphragm with capacitive characteristics it may be vacuum coated with any suitable material such as for example aluminium or gold. This coated capacitive plastics diaphragm may be strengthened by being mounted between two annular members as previously mentioned in connection with the electrical coil magnet pressure transducer.

Figures in the Drawings Two embodiments of the invention will now be described by way of example with reference to the accompanying illustrative drawings in which: FIGURE 1 is a sectional elevation of one pressure transducer of the invention; FIGURE 2 is a sectional elevation of a displaceable component for the transducer of Figure 1; FIGURE 3 is a diagram of a circuit including the transducer of Figure 1; FIGURE 4 is a sectional elevation of another pressure transducer of the invention; FIGURE 5 is a sectional elevation of a displaceable component for the transducer of Figure 4; and FIGURE 6 is a diagram of a circuit including the transducer of Figure 4.

Detailed Description of Drawings Referring particularly to Figures 1 to 3, a pressure transducer includes a casing of magnetic material formed in two separable parts 2 and 4.

These parts 2 and 4 have respective electrical coil magnets 6 and 8 located in suitable recesses to surround pole pieces 10 and 1 2. The casing parts 2 and 4 are machined to have central recesses 14 and 1 6 which define a central gap when the parts 2 and 4 are connected together at their annular surfaces 18 8 and 20 surrounding the recesses 14 and 1 6. Passages 22 and 24 extend through the casing parts 2 and 4 from opposite outer surfaces to the inner recessed surfaces. Projections 26 on the surfaces 1 8 are dimensioned and located to mate with recesses 28 on the surfaces 20 when the two parts 2 and 4 are brought together to form the casing.

A disc 30 of magnetic dust is secured to a flexible diaphragm 32 of silastmer rubber by a suitable adhesive. The disc 30 is located centrally between the two pole pieces 10 and 1 2 with the diaphragm 32 extending outwardly between the annular surfaces 18 and 20. The casing parts 2 and 4 are then brought together to locate the projections 26 in the recesses 28 thereby trapping and slightly tensioning the flexible diaphragm 32 and locating the disc 30 in the casing gap spaced from and between the pole pieces 10 and 12. A variation in the pressure difference in the gap on opposite sides of the diaphragm 32 will cause the diaphragm 32 to flex so as to move the disc 30 towards one or other of the pole pieces 10 and 12 thereby causing a corresponding variation in the inductance of the electrical coil magnets 6 and 8.

Referring to Figure 3, the coils 6 and 8 are connected to form two arms of a Wheatstone bridge circuit the other two arms of the circuit being formed by the two halves of a centre-tapped secondary coil 34 of a transformer 36. The primary coil of the transformer 36 is connected to an A.C. source alternating at 1000 Hz. The centre tap of the secondary coil 34 is connected to the connecting point 38 of the coils 6 and 8 by a suitable volt meter arranged to indicate signals of up to 500 millivolts. Any variation in the pressure difference between the casing outer surfaces 38 and 40 will produce a corresponding variation in pressure on the opposite sides of the diaphragm 32 which will cause the diaphragm 32 to move towards one or other of the pole pieces 1 0 and 12.

This will cause a corresponding variation in the inductance of the two coil magnets 6 and 8 which will cause corresponding variation in the metre reading on the Wheatstone bridge circuit.

Referring to Figure 2, in a preferred diaphragm construction, the diaphragm 32 is secured between two annular members 42 and 44 which are screwed together. The casing parts 2 and 4 are suitably modified to receive these annular members 42 and 44. An important advantage of this modified diaphragm construction is that it provides a self-supporting diaphragm construction which can be produced and sold as an independent, separate replacement component for a pressure transducer. It will be appreciated that the diaphragm 32 is slightly tensioned when located in the annular members 42 and 44, and that this tensioning does not have to be carried out when the diaphragm is located in the transducer casing. if desired, the disc 30 may be encased between two flexible diaphragms 32 which are then secured between either the casing parts 2 and 4 or the annular members 42 and 44.

An advantage of this arrangement is that it reduces corrosion of the disc 30. As shown in Figure 2, the annular members 42 and 44 are provided with complementary projections 26 and recesses 28 similar to the transducer parts 2 and 4.

Figures 4 to 6 illustrate a second embodiment of the invention, and for clarity corresponding parts of the first and second embodiments will be given the same reference numerals.

Referring to Figure 4, a casing has two parts 46 and 48 machined to have central recesses 14 and 1 6 which define a central gap when the parts are connected together at annular surfaces 1 8 and 20. Capacitor plates 50 and 52 are secured to respective parts 46 and 48 in the recesses 14 and 16 by an epoxy type cement 54. Connector wires 56 for the plates 50 and 52 extend through passages in the parts 46 and 48 and are secured in position by the cement 54.

A flexible diaphragm 32 is vacuum coated with a suitable material to provide the diaphragm with capacitive characteristics. Aluminium and gold are examples of such suitable materials. This vacuum coating 58 is dimensioned to extend completely across the recesses 14 and 1 6 and the annular surfaces 1 8 and 20 of the casing parts 46 and 48.

The diaphragm is secured between these casing parts 46 and 48 in the same manner as described hereinbefore in connection with the first embodiment.

Referring to Figure 5, this capacitive diaphragm 32 can be mounted between two annular members 42 and 44 in the same manner as described in connection with the first embodiment. The vacuum coating 58 will extend completely across the gap between the annular members 42 and 44.

Referring particularly to Figures 4 and 6, the capacitor plates 50 and 52 and the vapour coated diaphragm 32 constitute two capacitors 60 and 62 which are connected to form part of a Wheatstone bridge circuit. The other two arms of the Wheatstone bridge circuit are the centretapped secondary coils of a transformer 64. The primary coil of the transformer 64 is energised by an A.C. source having a frequency of 60 KHz. The opposite terminals of the transformer secondary coil are connected to the respective capacitor plates 50 and 52 by rectifiers 66 and the centre tap 68 of the transformer secondary coil is coupled to the vapour coated diaphragm 32 by a voltmeter (not shown) having a suitable voltage scale. Passages 22 and 24 extend between the outer surfaces 38 and 40 of the casing to the recesses 14 and 1 6 on opposite sides of the flexible diaphragm 32.

This second embodiment operates in a similar manner to the previously described first embodiment. A variation in the pressure difference between the outer surfaces 38 and 40 produces a corresponding variation on opposite sides of the diaphragm 32 causing this diaphragm to move towards one of the capacitor plates 50 or 52. This produces a corresponding variation in the capacitance of the capacitors 60 and 62 thereby producing a change in the reading of the voltmeter of the Wheatstone bridge circuit.

The flexible diaphragm 32 can be pre-tensioned by known manufacturing techniques before it is mounted between either the casing parts 46 and 48 or the annular members 42 and 44. This technique of manufacturing and pre-tensioning the diaphragm 32 in a suitable jig before mounting it in a transducer ensures that radial pre-tensions are basically equal before the diaphragm 32 is finally positioned between the casing parts 46 and 48 or the annular members 42 and 44.

Claims (14)

1. A pressure transducer comprising a casing of magnetic material having two facing pole pieces separated by a gap, two electrical coil magnets housed in the casing to surround their respective pole pieces, a displaceable component mounted in the casing to extend across the gap and pole pieces, displacement of the component towards one or other of the other pole pieces being in response to variations in fluid pressure at outer surfaces of the casing to change at least one electrical parameter of the coils, and passages extending from said outer surfaces of the casing to opposite surfaces of the displaceable component, in which the displaceable component comprises magnetic material mounted between the pole pieces on a plastics diaphragm.

2. A pressure transducer as claimed in Claim 1 in which the magnetic material is encased between two plastics diaphragms.

3. A pressure transducer as claimed in Claim 1 or Claim 2, in which the plastics diaphragm having the magnetic material mounted thereon, or the two plastics diaphragms having the magnetic material mounted therebetween, is or are secured between two annular members which are themselves mounted in the casing.

4. A pressure transducer as claimed in any preceding Claim in which the magnetic material for the displaceable component comprises one or more of the following materials:-- magnetic dust, soft iron filings suspended in cold setting resin, radio metal and metal.

5. A pressure transducer comprising a casing having a gap therein, two opposed spaced capacitor plates located in the gap and electrically insulated from the casing, a displaceable component having capacitive characteristics mounted in the casing to extend across the gap between the spaced capacitor plates to provide with said capacitor plates two capacitors, displacement of a component towards one or other of the capacitor plates being in response to variations in fluid pressure at outer surfaces of the casing to change at least one electrical parameter of the said two capacitors, and passages extending from said outer surfaces of the casing to opposite surfaces of the displaceable component, in which the displaceable component comprises material having capacitive characteristics mounted between the said capacitor plates on a plastics diaphragm.

6. A pressure transducer as claimed in Claim 5, in which the plastics diaphragm is vacuum coated with the said material having capacitive characteristics.

7. A pressure transducer as claimed in Claim 6 in which the plastics diaphragm is vacuum coated with aluminium or gold.

8. A pressure transducer as claimed in Claim 6 or Claim 7 in which the plastics diaphragm is mounted between two annular members which are themselves mounted in the casing.

9. A pressure transducer as claimed in any preceding Claim in which the diaphragm is made of one or more of the following materials: silastomer rubber, natural rubber, synthetic rubber, condensate or teraphlalic acid, Kapton, Kevlar and polytetrafluorethylene (PTFE).

10. A Wheatstone Bridge signal indicating circuit in which two of the bridge members consist of said two electrical coil magnets of the transducer of Claim 1.

11. A Wheatstone Bridge signal indicating circuit in which two of the bridge members consist of said two capacitors of the pressure transducer of Claim 5.

12. A pressure transducer substantially as herein described and shown in Figures 1 and 2 of the accompanying drawings.

1 3. A pressure transducer substantially as herein described and shown in Figures 4 and 5 of the accompanying drawings.

14. An electrical circuit substantially as herein described and shown in Figure 3 of the accompanying drawings.

1 5. An electrical circuit substantially as herein described and shown in Figure 6 of the accompanying drawings.