GB2129565A

GB2129565A - Pressure and temperature transducer

Info

Publication number: GB2129565A
Application number: GB08328674A
Authority: GB
Inventors: Michel Valdois
Original assignee: Flopetrol Services Inc
Current assignee: Flopetrol Services Inc
Priority date: 1982-10-29
Filing date: 1983-10-27
Publication date: 1984-05-16
Also published as: FR2535458B1; GB8328674D0; FR2535458A1; JPS59132328A

Abstract

A pressure and temperature transducer, of the type comprising a diaphragm (12) having a sensitive part which is deformable under the effect of the pressure to be measured and at least one electrical resistor (16) fixed on one of the sides of the diaphragm, undergoing variations in electrical resistance under the action of pressure variations, further comprises diaphragm temperature measurement means made up of at least one electrical resistor fixed on the sensitive part of the diaphragm, the resistance of this electrical resistor being dependent on the diaphragm temperature variations and practically insensitive to said pressure variations. The resistor may comprise two resistive elements (24, 26) attached to the diaphragm (12) in respective regions of equal tensile and compressive strains. Alternatively (Figure 6) at least one resistor may be attached to a region lying astride the inflection locus of the diaphragm. Pressure and temperature may be measured by the same resistive elements, by means of selective circuit arrangements. <IMAGE>

Description

SPECIFICATION Pressure and temperature transducers The present invention relates to a pressure transducer comprising temperature measurement means insensitive to pressure variations. The pressure transducer is of the type having a diaphragm which is elastically deformable by the pressure and on which electrical resistors have been deposited.

For pressure measurements, use is currently made of transducers made up of a generally circular flexible diaphragm which deforms under the effect of the pressure. Electrical resistors are fixed on the diaphragm to detect and measure the deformations.

They can be deposited directly on the diaphragm by vacuum evaporation or by cathode sputtering. The resistors can also be deposited on a flexible substrate such as Mylar. This substrate is then glued on the diaphragm which is elastically deformable under the action of pressure variations. The resistors can take on different forms, the most current being in the form of combs or circles. The face of the diaphragm supporting the resistors is kept under vacuum within a case. Under the action of pressure, the resistors undergo modifications in their dimensions in the same manner as the diaphragm. This results in corresponding variations in their electrical resistance. These resistances are measured to determine, after calibration, the corresponding pressure variations. Generally, four electrical resistors connected in a Wheatstone bridge are used.In this case, the voltages at the terminals of the bridge are measured.

In practice, the resistors are not only sensitive to pressure variations but also to temperature variations. The result is that, at constant pressure, a voltage variation is picked up at the Wheatstone bridge terminals when the temperature varies. To compensate the thermal effects, additional electrical resistors are currently placed in the measurement circuit. The resistances and temperature coefficients of these additional resistors are chosen so as to obtain an overall temperature compensation. To accomplish this, the unbalance voltage of the Wheatstone bridge as a function of temperature is measured with the pressure remaining constant. The voltage variation law at the terminals of the Wheatstone bridge as a function of temperature is thus determined.Compensation resistors are then added so as to cancel or keep constant the voltage at the bridge measurement terminals when the tempera- ture varies.

The compensation resistors are generally located outside of the case containing the diaphragm and Wheatstone bridge resistors under vacuum. It is to be noted that the compensation of the thermal effects thus obtained is only a static compensation.

In fact, the compensation resistors are not placed in the same location as the measurement resistors forming the Wheatstone bridge. In addition, the nature of the compensation resistors and measurement resistors is different: the first are much more massive than the measurement resistors. The result is that the temperature variations are not felt as rapidly on the compensation resistors as on the measurement resistors.

It is an object of the invention to provide a pressure transducer corresponding better than those of the prior art to the requirements of engineering practice, notably in that it does not have the above drawbacks. It is a particular object of the invention to measure the temperature of the diaphragm on which are deposited the pressure measurement resistors so as to provide, by means of the same transducer, a temperature measurement associated with a pressure variation measurement. The temperature compensation which can be carried out on the pressure measurements, after the calibration of the transducer, is a dynamic compensation.More precisely, the invention provides a pressure transducer of the type comprising a diaphragm having a sensitive part which is deformable under the effect of the pressure to be measured and at least one electrical resistor fixed on one of the sides of said diaphragm, undergoing variations in electrical resistance under the effect of variations in said pressure, and including diaphragm temperature measurement means made up of at least one electrical resistor fixed on said sensitive part of the diaphragm,the resistance of said means being dependent on the diaphragm temperature variations and practically insensitive to said pressure variations.

According to one embodiment, two electrical resistors are used whose dimensional variations due to pressure variations are equal and opposite. The result is that the variations in electrical resistance are due only to the temperature, if the total resistance of the two resistors is measured.

According to a second embodiment, the two electrical resistors used are the resistors of one of the two arms of Wheatstone bridge used for measuring the pressure variations. It can be noted that the temperature measurement can be carried out alternately by means of the two bridge arms.

According to a third embodiment, a single electrical resistor is used which is located on a part of the geometrical locus of the diaphragm inflection points or overlapping this locus.

Pressure transducers in accordance with this invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 represents schematically the sensitive part of a pressure transducer comprising temperature measurement elements; Figure 2 represents the circuit diagram used in connection with the transducers shown schematically in Figures 1 and 6; Figure 3 represents, in section and for only half of the diaphragm, the deformation of the transducer diaphragm under the action of pressure; Figure 4 shows the arrangement of the resistors on the diaphragm for another particular embodiment; Figure 5 is the circuit diagram of the transducer of Figure 4; and Figure 6 shows two other alternative arrangements for the electrical resistors used for temperature measurement.

In Figure 1 is shown schematically, in a cutaway view, the sensitive part of a pressure transducer.

This part comprises a cylinder 10 closed at its upper end by a thin disk 12. The pressure to be measured is exerted along the arrows P. The outside of the disk 12 is contained in an enclosure 14 represented very schematically by a dotted line in Figure 1. The inside of the enclosure 14 is placed under air vacuum. The thickness of the disk 12 (or diaphragm) depends on the values of the pressure to be measured. The thicker the disk, the higherthe pressures which can be measured and vice versa. On the outside of the disk 12, i.e. on the side opposite the face of the disk subjected to pressure, resistors 16, 18,20 and 22 have been deposited. These resistors may be deposited directly on this face by vacuum evaporation or by cathode sputtering.Another method consists in depositing them previously on a flexible substrate, such as Mylar for example, and then gluing them on the outside of the disk. The electrical resistors can have any shape. The shape shown in Figure 1 is currently used, as are configuratuions in the form of combs or straight shapes. These electrical resistors constitute the pressure measurement means. In fact, the disk 12 undergoes elastic deformation when pressure is applied on its internal face. This results in a modification of the geometrical dimensions of the resistors 16 to 22 and, consequently, a variation in their electrical resistance. The electrical resistors 16 to 22 are connected in a Wheatstone bridge as shown in Figure 2. The two bridge arms are made up, on the one hand, ofthe resistors 16 and 22 and, on the other, of the resistors 18 and 20.The bridge is supplied with electric current between terminals 34 and 36 by means of an electric current source 28 such as a battery or a stabilized voltage source. The measurement of the bridge unbalance voltage is carried out between terminals 30 and 32. This unbalance voltage provides a measurement of the pressure variations. This type of pressure transducer having electrical resistors connected in a Wheatstone bridge is used currently in industry.

According to another embodiment of the invention, two resistors 24 and 26 are deposited on the outer surface of the disk 12 at locations chosen so that the dimensional variations of the two resistors 24 and 26 are equal and opposite when the pressure varies and when the temperature is constant. Hence, by connecting these two resistors in series and measuring their total electrical resistance, the changes in resistance are due only to temperature variations and no longer to pressure variations. The electrical connection of the two resistors 24 and 26 is shown in the right-hand part of Figure 2. These two resistors 24 and 26 are connected in series, on the one hand to a current generator 38 and, on the other, to a voltmeter 40 making it possible to measure the voltage variations at the terminals of both the resistors 24 and 26.

To position these two resistors on the disk 12, one of the resistors is deposited at a location undergoing a compression while the other is deposited at a location undergoing an extension. Referring to Figure 3, 12a and 12b represent schematically half of the disk respectively at rest and deformed under the action of pressure. The center of the disk is represented by 0 and its radius by OR = r. The deformation of a circular plate is characterized by two zones, one under compression which is in fact a ring on the periphery of the plate, and the other under extension made up of a disk having a radius equal to 0.628r.

The circle of radius equal to 0.628r is the inflection circle which is the locus of the points on the surface of the deformed disk, characterized by a change in the sign of the slope. This is shown by the arrows 42.

If a resistor is deposited in the part of the disk under compression, i.e. on the ring between the circles of radius rand 0.628r, the resistance of this resistor will drop when the disk 12 changes from position 1 2a to 12b under the action of the pressure. On the other hand, a resistor deposited in the part of the disk under extension will see its resistance increase. The reduction in the geometrical dimensions of one can compensate for the increase in the geometrical dimensions of the other. Consequently, by connecting these two resistors in series, the total resistance of the two resistors will not be modified under the effect of the pressure. On the other hand, their total resistance remains sensitive to temperature variations which modify substantially in the same manner the dimension of these resistors.In other words, these two resistors constitute temperature measurement means independent of the deformations of the disk 12 and hence independent of pressure variations. To position the two temperature measurement resistors so that they are insensitive to pressure two methods are possible. Firstly, it is possible to proceed experimentally by depositing strain gages on the zones under compression and extension, keeping the temperature constant. In this case, it is sufficient that, for a deformation of the disk 12 such as 12b, a strain gage placed in the zone under compression should furnish a measurement equal but opposite to that of a strain gage placed in the extension zone. It is also possible to proceed by calculation. To accomplish this, it is necessary to calculate the deformations undergone by the disk 12 under the action of pressure.The mathematical study of the deformations of a circular plate with a fixed edge is set forth, for example, in the book entitled "Th6orie des Plaques et Coques" ("Theory of Plates and Shells") by S. Timoshenko and S.

Woinowski-Krieger, published by the Librairie polytechnique Ch. Be ranger in 1961. This mathematical study is found in Paragraph 16, Pages 54 to 56 of that work.

It can be noted that the diaphragm 12 supporting the resistors is not necessarily circular. It can, for example, have the form of an ellipse. In this case, reference will no longer be made to an inflection circle but to an inflection ellipse. In general, reference will be made to the locus of the inflection points.

Figure 4 shows schematically another arrangement of the resistors making it possible to carry out a temperature measurement independent of pressure.

For this purpose, use is made of two of the four resistors 44,46, 48 and 50. The shape of these resistors can, as previously, be of any type. In Figure 4 has been represented a resistor shape of a type customarily used. These four resistors 44 to 50 are connected in a Wheatstone bridge for pressure measurement as described earlier. The inflection circle with a radius equal to 0.628r is represented by 52. It is noted that the two resistors 44 and 46 are represented in the extension zone of the disk whereas the two resistors 48 and 50 are placed in the compression zone. The two resistors 44 and 48 are positioned so that, at constant temperature, the reduction in the geometrical dimensions of the resistor 48 are compensated by the increase in the dimensions of resistor 44. The same is done for the two resistors 46 and 50.Thus, by measuring the variations in the electrical resistance of the assembly of the two resistors 44 and 48 connected in series, one obtains a temperature measurement independent of pressure variations. The same applies to the assembly made up of the two resistors 46 and 50 connected in series.

Referring to Figure 5, which represents the circuit diagram of the transducer of Figure 4, the resistors 44 to 50 are connected in a Wheatstone bridge. The unbalance voltage of the bridge is measured between terminals 54 and 56. This voltage provides a pressure measurement. The two resistors 44 and 48 are connected in series between terminals 58 and 60.

The same is done for the resistors 46 and 50 between the terminals 62 and 64. The two terminals 58 and 62 can be shorted simultaneously with the terminals 60 and 64 thanks to contacts 66 and 68. A magnetic core 72 is connected to the contact 66 and surrounded by an excitation coil 74. The latter is connected, on the one hand, to a terminal with a constant voltage or ground 76 and, on the other, to the terminal 78 of a multiplexing control 80. A magnetic core 69 is connected to the contact 68 and surrounded by an excitation coil 70. The latter is connected, on the one hand, to the ground 76 and, on the other, to the terminal 78 of the multiplexing control 80. The second terminal 82 of th is multiplexing control is connected to an excitation coil 84 whose other end is connected to the ground 76.A magnetic core 88 is placed inside the coil 84 and is connected to a rigid rod 90 having two contacts 92 and 94. The rod 90 has an insulating part 86 so as to isolate the two contacts 92 and 94 electrically. The two contacts 92 and 94 make it possible to measure alternately the total resistances, on the one hand, of the resistors 44 and 48 connected in series and, on the other hand, of the resistors 46 and 50 connected in series. For this purpose, between the contact 92 and 94 are connected conventional resistance measurement means (not shown) such as a voltmeter, on the terminals of the contacts 92 and 94. By exciting the coil 74 using the multiplexing control 80, the terminals 58 and 62, on the one hand, and the terminals 60 and 64, on the other, are short-circuited. The Wheatstone bridge made up of the resistors 44 to 50 is thus obtained.

The unbalance voltage of the bridge between the terminals 54 and 56 is then measured so as to obtain a pressure measurement. Then, the electric current in the coils 70 and 74 is cut off and the coil 84 is excited in one direction so as to place the contacts 92 and 94 in connection with the terminals 96 and 100 respectively. It is then possible to measure the variations in the electrical resistance of the bridge arm constituted by the resistors 44 and 48. This measurement provides a temperature measurement. By exciting the coil 84 in the other direction, by reversing the electric current, a link is established between the contact 92 and the terminal 98, on the one hand, and between the contact 94 and the terminal 102 on the other. It is then possible to measure the resistance of the second bridge arm made up of the resistors 46 and 50. A second temperature measurement is thus obtained.Instead of reversing the current in the coil 84, it would be possible to use a second coil (not shown) connected electrically to the multiplexing control 80 and acting on the bar 90 in the direction opposite the coil 84. By cutting off the current in the coil 84, it is again possible to excite the coils 70 and 74 so as to carry out a pressure measurement, and so on.

The electric power supply of the bridge is provided in a conventional manner between the terminals 58 and 60 or 62 and 64. However, if the temperature measurements are carried out alternately with the two bridge arms, the arm whose resistance is being measured must be supplied with electric current. To accomplish this, the bridge power supply is switched between the terminals 58 and 62, on the one hand, and 60 and 64 on the other. It should be noted that this switching is not necessary if only one bridge arm is used for temperature measurement. In this case, the electric power supply is provided at all times on the terminals of this arm (for example between 58 and 60 or between 62 and 64).

Figure 6 represents schematically another possible configuration of the resistors for a pressure transducer allowing simultaneous temperature measurement. In this figure are represented four resistors 104, 106, 108 and 110 connected in a Wheatstone bridge for the measurement of pressure variations. The resistors are connected as shown in the left-hand part of Figure 2. The temperature measurement means, independent of pressure variations, are represented according to one embodiment by an electrical resistor 112 deposited on an arc of the inflection circle 116 of the disk which is deformed under the action of pressure. By definition, the inflection circle does not undergo any deformation. Hence, whatever the deformation of the disk 12, the resistor 112 does not change geometrical dimensions.Thus, by measuring its resistance, one obtains a temperature measurement. The width of this resistor, which is small in practice (about 1 millimeter), has been exaggerated in Figure 6 to better illustrate the embodiment. The resistor used for measuring the temperature can also be deposited along a radius of the disk, overlapping the inflection circle, as represented by the resistor 114. The part 118 of the resistor located in the compression zone will undergo a reduction in length whereas the part 120 located in the extension zone will undergo an increase in length. Consequently, the resistor 114 will conserve, on the whole, the same geometrical dimensions and, hence, the same resistance. The measurement of this resistance will therefore provide an indication of the temperature independent of pressure variations undergone by the disk 12. This measurement can be carried out with the circuit diagram shown in the right-hand part of Figure 2. In place of the resistors 24 and 26 there will of course be either the resistor 112 or the resistor 118.

The transducer just described consequently provides a pressure measurement associated with a temperature measurement. It is noted that the temperature is measured at the same location as the pressure measurement. In addition, the resistors used for temperature measurement can be of the same type as those used for pressure measurement.

Hence, the inertia of these resistors as a function of temperature is the same for all the resistors. It is also noted that the resistors are all on the diaphragm and that there are no longer any compensation resistors located outside of the case represented schematically by 14 in Figure 1. Based upon the determination of variations in the resistance of the resistors used for pressure measurement as a function of temperature, it is possible to correct the pressure value measured as function of temperature. It is thus possible to obtain a temperature-compensated value of the pressure as well as a pressure-independent value of the temperature. Temperature compensation of pressure values is a dynamic compensation if suitable calculation means are associated with the transducer allowing rapid correction of the pressure values measured as a function of temperature.

The present invention is of course not limited to the illustrative embodiments represented and described here as nonlimitative examples. In the embodiments just described a particular form of resistor has been represented. This form of resistor can of course be different. Also, in every case four resistors connected in a Wheatstone bridge for pressure measurement have been represented. It is evident that, in principle, a single resistor is sufficlient for pressure measurement even though, in practice, most transducers have resistors connected in a Wheatstone bridge. It is also evident that one arm of the Wheatstone bridge can have more than two resistors. It is also to be noted that the form of the resistors for temperature measurement can be different from the form used for pressure measurement.

Claims

1. Pressure transducer of the type comprising a diaphragm having a sensitive part which is deformable under the effect of the pressure to be measured and at least one electrical resistor fixed on one of the sides of said diaphragm, undergoing variations in electrical resistance under the effect of variations in said pressure, and including diaphragm temperature measurement means made up of at least one electrical resistor fixed on said sensitive part of the diaphragm, the value of the resistance of said means being dependent on the diaphragm temperature variations and practically insensitive to said pressure variations.

2. Pressure transducer according to claim 1, wherein said temperature measurement means comprise at least two electrical resistors connected in series and fixed on said sensitive part of diaphragm on each side of the locus of the inflection points so that the dimensional variations of said resistors are compensated during the deformation of the diaphragm under the effect of pressure variations.

3. Pressure transducer according to claim 2 further comprising electrical resistors forming the two arms of a Wheatstone bridge for pressure measurement, wherein said resistors of said temperature measurement means are the resistors of at least one of the two arms of the Wheatstone bridge.

4. Pressure transducer according to claim 3, including means for measuring alternately a parameter characteristic of the unbalance of the bridge to provide a pressure measurement and a parameter characteristic of the electrical resistance of at least one of the two bridge arms to provide a temperature measurement, said means comprising a multiplexing control.

5. Pressure transducer according to claim 1, wherein said electrical resistor of said temperature measurement means matches the shape of the locus of the diaphragm inflection points and is deposited on at least one part of said locus of the inflection points.

6. Pressure transducer according to claim 1, wherein the electrical resistor of said temperature measurement means has a symmetrical shape in relation to the locus of the diaphragm inflection points.

7. Pressure transducer according to claim 6, wherein said resistor has a straight shape with a longitudinal axis perpendicular to the locus of the diaphragm inflection points.

8. Pressure transducer according to any of the preceding claims, wherein said temperature measurement means are made up of at least one electrical resistor formed by the depositing of a material directly on the diaphragm or on a substrate glued on the diaphragm.

9. Pressure transducer according to any of the preceding claims, wherein said diaphragm is circular, the locus of the diaphragm inflection points being a circle.

10. Pressure transducer substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

11. Pressure transducer substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

12. Pressure transducer substantially as hereinbefore described with reference to Figure 6 of the accompanying drawings.