GB1562139A

GB1562139A - Strain gauge transducer provided with a temperatrue sensor

Info

Publication number: GB1562139A
Application number: GB4171577A
Authority: GB
Original assignee: Schlumberger SA
Current assignee: Schlumberger SA
Priority date: 1976-10-08
Filing date: 1977-10-06
Publication date: 1980-03-05
Also published as: FR2367280B1; NL7711021A; DE2745263A1; BE859016A; IT1085284B; FR2367280A1

Description

(54) STRAIN GAUGE TRANSDUCER PROVIDED WITH A TEMPERATURE SENSOR (71) We, SCHLUMBERGER INDUSTRIE, a French body corporate of 12 Place des Etats-Unis, 92120 Montrouge, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to transducers of physical parameters of the deposited strain gauge type, and more particularly concerns such a transducer equipped with a temperature sensor, as well as its method of fabrication.

It is known that deposited strain gauge transducers are usable for the measurement of physical parameters such as force, pressure and displacement. These transducers generally comprise a sensing element constituted by a membrane or a beam, of insulating material or material rendered insulating by a deposit of an insulating substrate such as silica or alumina, on which membrane are deposited under vacuum a group of four gauges and their connections so as to form an electrical resistance bridge. When the membrane is being subjected to the deformation resulting from the application of the physical parameter to be measured, there results an imbalance of the bridge gauges by variation in their resistance and an output signal representative of the parameter under consideration is produced.

For various reasons, this sensing element is also influenced by the temperature, and this influence must be corrected to avoid it being taken as a signal indicative of variation of the physical parameter itself. This correction is most often achieved by the insertion of thermosensitive elements, serv ing as a temperature sensor, disposed in the neighbourhood of the sensing element and connected in the electric measuring circuit associated with the transducer. The disadvantage of this solution is that the temperature of the sensor is not strictly identical to that of the sensing element, and that, particularly, a rapid variation in ambient conditions produces a significant transitory error while the sensor and the sensing element together have not reached a thermal equilibrium. To avoid this error, it is therefore desirable that the temperature sensor is integrated with the sensing element of the transducer, so as to be subjected to the same ambient thermal conditions.

The object of the invention is therefore to provide a deposited strain gauge transducer equipped with a temperature sensor integrated with the sensing element, as well as a method of fabrication for the simultaneous production of this sensing element and its temperature sensor.

According to the invention, a method of making a strain gauge transducer having a temperature sensor comprises the steps of: a) depositing successively on a deformable insulating substrate a layer of metal alloy, whose electrical resistance varies with strain and a first, thin, metallic layer; b) depositing a second, thick, metallic layer on said first, thin layer; and c) selectively removing parts of the layers thus deposited so as to leave on the insulating substrate at least one region of the alloy, which region forms a strain gauge, and portions of said thick layer, which portions form electrical connections for said strain gauge; temperature sensor of the transducer being formed by a further region of the alloy covered by the thin metallic layer but not by the thick metallic layer.

This process has the advantage of allowing at the same time the operations of depositing the gauges, and the simultane ous production of the temperature sensor.

Furthermore, since this sensor is in practice preferably realised with the help of the same materials as those used for the manufacture of the bridge of gauges, it is of relatively low cost and improves the performance of the transducer, while being readily reproducible in mass production.

The invention will be better understood by referring to the description which will follow, in relation to the attached drawing, which represents, by way of non-limitative example, a transducer produced in accordance with the invention. On this drawing: Fig. 1 is a plan view of the sensing element of such a transducer Fig. 2 is a section following a median plane II-II.

First of all, the usual process for the manufacture of a transducer not equipped with a temperature sensor will be described, and afterwards it will be shown how this process is modified, according to the invention, to obtain the sensor. Referring to Figs 1 and 2, on a membrane 10 of material insulated by a deposit of silica or alumina, one first deposits under vacuum a thin layer 11, of the order of 0.02,u in thickness, of the material required for the gauges, for example, an alloy of nickelchrome or of constantan, of which the gauge coefficient is between 2 and 3; this coefficient being the relationship between the relative variation in resistance and the relative variation in elongation. On top of this alloy layer 11, a first, thin, metal layer 12, where electrical resistance varies with temperature, is then deposited, of the order of 0.04,u in thickness, preferably of gold. Then this first, metallic layer is reinforced by an electrolytic deposit of a second, thick, metallic layer 13, of the order of 1 its in thickness, preferably of gold, but which can be another good conducting metal, such as platinum, silver or nickel.

By masking by means of photosensitive resins protecting the places to be preseared, one proceeds to a local attack on the stratified collection of the layers 11, 12 and 13 in such a way as to leave this stratification only following the configuration shown at 15. Then, by a selective chemical attack, one uncovers on the one hand the metallic alloy 11 in four parts J1 to J4 which make up the four gauges of the measuring bridge, and on the other hand the thick, metallic layer 13 following four paths 14 which form the connections between the four gauges. This collection 15 makes up a measuring bridge which can be connected to its supply circuit and its output circuit by gold wires applied by thermo-compression on the conductors at the areas enlarged for that purpose 16 to 19, chosen to make up the four terminals of the bridge The choice of gold to realise the two metallic layers 12, 13 of the stratification is motivated by its qualities of resistance to deterioration and its suitability of welding by thermo-compression.

The gauges J1 to J4 are sensitive to the deformation of the membrane under the effect of the physical parameter to be measured. In general J1 and J2 work under tension and J3 and J4 under compression.

One could think of realising the temperature sensor in the same way, but the alloy of the gauges is chosen such that its resistance varies strongly with deformation but little with temperature. It is necessary, on the contrary, to choose for this sensor a material whose resistivity varies strongly with the temperature so that correction is easy, and so that its temperature coefficient is great in relation to the effects of deformation.

According to the invention, the temperature sensor is realised during the manufacture of the measuring bridge by using the first, thin, metallic layer 12 in the following way: by the technique of photosensitive masks, or any other equivalent means, the desired location of the sensor 20 on the membrane 10 is protected locally, in order to prevent the depositing of the second, thick, metallic layer in this zone. Afterwards, this same zone is protected by resin or any other body capable of resisting throughout the chemical attack, which produces the gauges with their connections; however, it is ensured that the metallic layers are not removed around the whole of the periphery of the zone 20, but are left in two regions 21,22 which will serve as connections for the sensor 20 with the output circuit.

The membrane 10 is thus provided with its bridge of gauges J1 to J4 and with its temperature sensor 20, this last having been realised at the same time as the fabrication of the bridge without the method having to be modified, with the exception of the selective additional protection of the place 20 during the operation of depositing the second, metallic layer 13 and the places 20, 21, 22 during the operation of chemical attack.

Another solution could consist of using for the sensor 20 the layers 12 and 13 at the same time, but the resistance obtained in this case would be too low to Provide the hoped for correction signal. It would also be possible to realise the sensor 20 by chemical attack on the single thick layer 13, but this operating mode would require a supplementary operation and would not permit the obtaining of a metallic layer of a uniform thickness, of the order of sev eral hundreths of a micron, nor is it easily reproducible and it is very much preferable to operate by masking the first thin layer.

WHAT WE CLAIM IS: 1. A method of making a strain gauge transducer having a temperature sensor, comprising the steps of: a) depositing successively on a deformable insulating substrate a layer of metal alloy whose electrical resistance varies with strain, and a first, thin, metallic layer; b) depositing a second, thick, metallic layer on said first, thin, layer; and c) selectively removing parts of the layers thus deposited so as to leave on the insulating substrate at least one region of the alloy, which - region forms a strain gauge, and portions of said thick layer, which portions form electrical connections for said strain gauge; the temperature sensor of the transducer being formed by a further region of the alloy covered by the thin metallic layer but not by the thick metallic layer.

2. A transducer according to claim 1, wherein said step of selectively removing parts of the layers is arranged to leave on the substrate a bridge of predetermined configuration of said strain gauges.

3. A method according to claim 1 or claim 2, wherein said step of selectively removing parts of the layers is arranged to leave regions which constitute the connections for said temperature sensor.

4. A strain gauge transducer produced by the method of any one of claims 1 to 3.

5. A transducer according to claim 4, wherein the first, thin, layer and the second, thick layer are of the same metal.

6. A transducer according to claim 5, wherein said metal is gold, platinum, silver or nickel.

7. A transducer according to any one of claims 4 to 6 connected to a measuring circuit, wherein said temperature sensor is connected in this circuit so as to correct the indications of the transducer as a function of the variations of temperature to which it is subjected.

8. A method of making a strain gauge transducer having a temperature sensor, the method being substantially as herein described.

9. A strain gauge transducer substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. eral hundreths of a micron, nor is it easily reproducible and it is very much preferable to operate by masking the first thin layer. WHAT WE CLAIM IS:

1. A method of making a strain gauge transducer having a temperature sensor, comprising the steps of: a) depositing successively on a deformable insulating substrate a layer of metal alloy whose electrical resistance varies with strain, and a first, thin, metallic layer; b) depositing a second, thick, metallic layer on said first, thin, layer; and c) selectively removing parts of the layers thus deposited so as to leave on the insulating substrate at least one region of the alloy, which - region forms a strain gauge, and portions of said thick layer, which portions form electrical connections for said strain gauge; the temperature sensor of the transducer being formed by a further region of the alloy covered by the thin metallic layer but not by the thick metallic layer.