WO1999027338A1

WO1999027338A1 - Method for producing a thick film sensor and such thick film sensor

Info

Publication number: WO1999027338A1
Application number: PCT/BE1998/000182
Authority: WO
Inventors: Leo A. J. Gypen
Original assignee: Alliance Europe, Naamloze Vennootschap
Priority date: 1997-11-25
Filing date: 1998-11-24
Publication date: 1999-06-03
Also published as: AU1327599A; BE1011568A3

Abstract

Method for producing a thick film sensor, whereby at least one resistive element (2 and 3) in the shape of a thick film resistor and at least two electrical conductors (4 to 10) which are connected to this element (2 and 3) are provided on a substrate (1) of steelplate, provided with an electric insulating layer, characterized in that the steelplate of the substrate (1) is first enamelled or coated with ceramic material, and in that it is only put in the desired shape later on.

Description

Method for producing a thick film sensor and such thick film sensor.

The present invention concerns a method for producing a thick film sensor, whereby at least one resistive element in the shape of a thick film resistor and at least two electrical conductors which are connected to this element are provided on a substrate of steelplate, provided with an electric insulating layer.

Such thick film sensors are used among others for measuring forces. In order to produce these thick film sensors, piezo-resistive elements are provided on the substrate which is equipped with an insulating layer. The substrate must be elastically bendable, such that it is bent by the exerted forces and such that the piezo- resistive elements are deformed, as a result of which their resistance changes. From these changes in resistance can be derived the corresponding force components .

It is already known to use steel coated with an insulating glass or enamel layer as a substrate instead of the conventional ceramic aluminate material. For steelplate is more elastic than ceramic material and when it is not coated it can be easily worked with standard machine tools, so that it can be put in the required shape without any problems with for example narrow zones to allow for the necessary elastic bending.

Moreover, substrates of coated steelplate allow for savings on the assembly, since the substrate is used as a frame, as a support for the electric network and as a support for the resistive elements. The latter can be provided directly on the substrate in the shape of a paste, for example on the basis of bismuth ruthenium oxides, by means of a silkscreen machine or such. The temperature sensitivity of these sensors is relatively restricted.

According to known methods, the substrate is first put in the desired shape and enamelled only afterwards which, depending on the shape, can be relatively difficult.

The invention aims to remedy said disadvantage and to provide a method for producing a thick film sensor which is relatively simple, irrespective of the shape.

This aim is reached according to the invention in that the steelplate of the substrate is first enamelled or coated with ceramic material, and in that it is only put in the desired shape later on.

The desired shape can be obtained by means of punching or by means of water jet cutting, but it is preferably obtained by means of laser cutting.

The desired shape can be obtained both before and after one or several resistive elements and two or more conductors have been provided, but these elements and conductors are preferably provided on the enamelled substrate before the substrate is put in the desired shape .

One or several resistive elements and two or more conductors can be provided by means of printing with silkscreen pastes which have resistive and conductive characteristics respectively, and they can be burnt in after the screen-printing.

The present invention also concerns a thick film sensor produced according to the method of any of the preceding embodiments .

In particular, the invention also concerns a thick film sensor containing a substrate of steelplate, at least one electric insulating layer made of enamel or ceramic material provided upon it, and a number of resistive elements in the shape of thick film resistors and electrical conductors which connect these elements with one another.

The known power sensors only allow to measure forces which are perpendicularly directed onto them and they cannot carry out any three-dimensional measurements.

Such a three-dimensional measurement is required for certain applications, however, such as in what is called the "smartpen", a pen to record the biometrical characteristics of a signature or in other words the dynamics of the movement when putting a signature.

It is known to use a power sensor made of stainless steel in such a pen, upon which strain gauges are glued.

However, this power sensor is relatively large, which is disadvantageous in order to build it in in a writing pen.

The present invention aims a thick film sensor which excludes the above-mentioned and other disadvantages and which can thus measure for example forces and moments in three dimensions, and which is relatively compact and hence very suitable to be used in the above-mentioned smartpen.

This aim is reached according to the invention in that the substrate contains a central part which is connected to an outer part which surrounds the central part at least partially by means of at least three elastically deformable bridges and whereby an inner resistive element is provided on each bridge which is connected to an outer resistive element on the above-mentioned outer part by means of a conductor.

On the central part can then be provided a middle conductor with which the three inner resistive elements are connected with one end, whereas they are connected to an outer resistive element with their other end by means of another conductor.

The outer part may consist of three arms which are each connected to the central part by means of a bridge, whereby an outer resistive element is provided on each arm.

In another embodiment, the outer part may consist of a ring which is connected to the central part by means of the three bridges, whereby the outer resistive elements are provided on the ring which are connected to an inner resistive element with one end by means of a conductor, and which are preferably connected to an additional conductor on the ring with their other end.

Preferably, the resistive elements have the same resistance when they are unloaded.

The resistive elements can be piezo-resistive elements, so that the thick film sensor can be used for example to measure three-dimensional forces.

In order to better explain the characteristics of the invention, the following two embodiments of a method for making a thick film sensor and of a thick film sensor according to the invention are described as an example only without being limitative in any way, with reference to the accompanying drawings, in which:

figure 1 shows a top view of a thick film sensor according to the invention; figure 2 shows a top view analogous to that of figure 1, but for another embodiment of a thick film sensor according to the invention.

The thick film sensors represented in the figures mainly consist of a substrate 1 and three piezo-resistive elements 2 and three piezo-resistive elements 3 in the shape of thick film resistors, and conductors 4 to 10 connecting all the piezo-resistive elements 2 and 3 with one another.

The substrate 1 consists of a central part 11 which is connected with three elastically deformable bridges 12 with an outer part 13 situated around the central part 11. As is represented in figures 1 and 2, this central part 11 can be provided with an opening 14 in its centre.

In the sensor represented in figure 1, the outer part 13 of the substrate 1 consists of three separate arms which are each connected to a bridge 12 respectively and which are connected to the central part 11 by means of this bridge 12. These arms extend concentrically in the same sense of rotation as of the bridge 12 with which they are connected.

A conductor 4 is situated on the central part 11 and extends over two thirds of a circle around the centre thereof, in particular around the opening 14. The three piezo-resistive elements 2 which are connected onto it are situated on the three bridges 12 respectively and are directed with their longitudinal direction outwards and at angles of 120°. The other conductors 5 to 10 and the piezo-resistive elements 3 are like three arms bent in the same direction and are provided on the three arms of the outer part 13.

In the sensor represented in figure 2 however, the outer part 13 is a ring which is connected to a bridge 12 in three evenly distributed places on this ring.

The first conductor 4 which is provided on the central part 11 forms a ring around the centre thereof, with three outward directed projections situated on the bridges 12 and onto which the piezo-resistive elements 2 are connected whose longitudinal direction is concentrically directed around the above-mentioned conductor 4 on these bridges 12.

The other conductors 5 to 10 and the piezo-resistive elements 3 are provided like three double- folded arms with a part on a bridge 12 and a part on the ring forming the outer part 13. The piezo-resistive elements 3 are situated in the outer part of these arms and thus on the part 13, whereby their longitudinal direction is concentric to the middle conductor 4.

The temperature sensitivity of the piezo-resistive elements 2 and 3 is relatively low in both sensors.

The piezo-resistive elements 2 and 3 are made such that they have the same resistance when unloaded.

The above-described sensors are made as follows:

First, the substrate 1 is made by enamelling a steelplate in an oven or by coating it with ceramic material. Naturally, enamel or ceramic material are used to this end with which can be formed a sufficiently insulating dielectric layer on the steelplate.

The steelplate can be pre-treated, for example it can be degreased, pickled and nickel-plated in the case of ordinary enamellable steel types or it can be roughened by rubbing it or blasting it, either or not accompanied by pre-oxidation in the case of stainless steel types. The enamel or ceramic material can be provided in one or several layers.

The steelplate can be enamelled in the shape of a piece which is bigger than the sensor to be formed and which is for example square or continuous in the shape of a coil which is cut to pieces during or after the enamelling

(what is called coilcoat enamelling) .

The enamel or ceramic material can be provided on the steelplate in various manners, for example it can be sprayed on it in liquid shape, it can be coilcoated, electrostatically sprayed (wet or dry) , powder coated, or it can be provided by means of electrophoresis or by means of screen printing. In the latter case, it is even possible to enamel not the entire surface of the steelplate, but predominantly those places which will form the substrate 1 of the sensor after it has been shaped.

Suitable enamels or ceramic materials are among others alkali-free or alkali-poor enamels or ceramic materials, as well as enamels in which the alkali elements have been immobilized.

The conductors 4 to 10 are preferably provided before the piezo-resistive elements 2 and 3 are provided.

These conductors 4 to 10 are formed by providing a paste on the basis of silver palladium in strips and by subsequently burning it in the enamel layer of the substrate 1, for example at a temperature of about 640 °C for some ten minutes. Also pastes on the basis of silver or on the basis of gold can be used to this end.

The paste can be provided by means of screen printing with for example a 200 -mesh screen of stainless steel and having an emulsion thickness of 18 micrometres.

Not only between the piezo-resistive elements 2 and 3, but also near ends of the piezo-resistive elements 3 which cannot be connected is provided a short conductor 8, 9 or 10 forming contact surfaces for the connection of the sensor in an electrical network. On these contact surfaces can be soldered electrical wires with for example a solder of 62% pewter, 36% lead and 2% silver. Also bonding by means of conductive glues are among the possibilities.

The piezo-resistive elements 2 and 3 are formed in an analogous manner. For each of the elements 2-3 is provided a strip of what is called resistance paste on the substrate 1, overlapping with both ends on the ends of conductors 4 to 10, which is subsequently burnt in.

The paste can be provided by means of screen printing, for example with a 200-mesh screen and with an emulsion thickness of 18 micrometres.

A suitable paste is a paste of bismuth ruthenium which is burnt in at a temperature of for example 640°C for ten minutes .

The emulsion paste is provided for example with a dry layer thickness of about 22 micrometres, which will amount to 12 micrometres after it has been burnt in.

The Gauge factor of the piezo-resistive elements 2 and 3, i.e. the relation between the resistance variation delta R and the product of the resistance and the relative deformation, preferably amounts to 10.

The piezo-resistive elements 2 and 3 as well as the conductors 4 to 10 which are connected to these piezo- resistive elements 2 and 3, are provided on the substrate 1 according to the above-described pattern which takes into account the above-described final shape which the substrate 1 should have in the finished sensor. The final step in the production consists in providing the required shape to the substrate 1 with a central part 11, whereby the piezo-resistive elements 2 are situated in easily deformable parts of the substrate 1, namely the bridges 12.

This is obtained by means of laser cutting.

The appliance, capacity, cutting speed and gas pressure are selected such that fissures in the enamel coating occurring during the cutting are avoided.

As a result of forces exerted on the central part 11 of the substrate 1, while its outer part 13 is being held, the relatively narrow and easily deformable bridges 12 and thus the three inner piezo-resistive elements 2 will deform, which will cause a change in the resistance thereof. However, there will be practically no deformation of the outer piezo-resistive elements 3, so that electronic effects such as temperature variations, etc. can be compensated.

When the piezo-resistors are connected in a Wheatstone bridge via the conductors 4 to 10 included, forces can be measured threedimensionally, i.e. in the direction X, Y and Z, as a function of time. Naturally, also two- dimensional and unidimensional forces can be measured.

The substrate 1 can also be formed by means of punching or cutting with a water jet under high pressure instead of by means of laser cutting.

According to a variant, the substrate 1 is put in the required shape immediately after the enamelling, before the conductors 4 and the piezo-resistive elements 2-3 are provided.

Naturally, the shapes represented in the figures are merely examples and the substrate 1 as well as the conductors 4 of the resistive elements 2 and 3 can be formed in numerous other shapes.

The sensor cannot only be used in what is called the smartpen. It can also be used in other applications whereby unidimensional or multidimensional forces, moments, pressures or accelerations have to be measured. For example it can be used as a shock or acceleration sensor for controlling an airbag in vehicles.

Also, several forces and moments can be measured simultaneously by providing more than three active piezo- resistive elements (radially directed in figure 1) .

The sensor can also be used as a mass sensor to measure a weight .

If one or several screen-printed resistive elements are provided instead of the piezo-resistive elements, whose resistance changes depending on the concentration of a specific gas such as NO_x or CO, the thick film sensor can be used for measuring concentrations of these gases. It can be made according to the above-described method, but with another shape than the one represented in the figures.

Claims

Claims .

1. Method for producing a thick film sensor, whereby at least one resistive element (2 and 3) in the shape of a thick film resistor and at least two electrical conductors (4 to 10) which are connected to this element (2 and 3) are provided on a substrate (1) of steelplate, provided with an electric insulating layer, characterized in that the steelplate of the substrate (1) is first enamelled or coated with ceramic material, and in that it is only put in the desired shape later on.

2. Method according to claim 1, characterized in that the substrate (1) is put in the desired shape by means of laser cutting.

3. Method according to claim 1, characterized in that the substrate (1) is put in the desired shape by means of cutting with a water jet under high pressure and/or by means of punching .

4. Method according to any of claims 1 to 3 , characterized in that the substrate (1) is put in the desired shape after the resistive element (2 and 3) and the conductors (4 to 10) have been provided.

5. Method according to any of claims 1 to 4 , characterized in that the resistive element (2 and 3) and the conductors (4 to 10) are provided on the substrate (1) in the shape of a screen-printing paste which is burnt in after the screen-printing.

6. Method according to any of claims 1 to 5 , characterized in that a piezo-resistive element is provided as a resistive element (2 and 3) .

7. Method according to any of claims 1 to 6, characterized in that a number of piezo-resistive elements (2 and 3) in the shape of thick film resistors as well as conductors (4 to 10) which connect these elements (2 and 3), are provided on the substrate (1) .

8. Method according to claim 7, characterized in that both the resistive elements (2 and 3) as well as the conductors (4 to 10) which are connected to these resistive elements (2 and 3), are provided on the substrate (1) according to a pattern which takes into account the final shape which the substrate (1) should have in the finished sensor.

9. Thick film sensor made according to the method of any of the preceding claims.

10. Thick film sensor containing a substrate of steelplate, at least one electric insulating layer made of enamel or ceramic material provided upon it, a number of resistive elements in the shape of thick film resistors and electrical conductors which connect these elements with one another, characterized in that the substrate (1) contains a central part (11) which is connected to an outer part (13) which surrounds the central part (11) at least partially by means of at least three elastically deformable bridges (12) and whereby an inner resistive element (2) is provided on each bridge (12) which is connected to an outer resistive element (3) on the above-mentioned outer part (13) by means of a conductor (5, 6, 7) .

11. Sensor according to claim 10, characterized in that a middle conductor (4) is provided on the central part (11) with which the inner resistive elements (2) are connected with one end, whereas they are connected to an outer resistive element (3) with their other end by means of another conductor (5, 6, 7) .

12. Sensor according to claim 10 or 11, characterized in that the outer part (13) of the substrate (1) consists of three arms which are each connected to the central part (11) by means of a bridge (12) , whereby an outer piezo- resistive element (3) is provided on each arm.

13. Sensor according claim 10 or 11, characterized in that the outer part (13) of the substrate (1) consists of a ring which is connected to the central part (11) by means of the bridges (12) , whereby the outer resistive elements (3) are provided on the ring, which are connected to an inner resistive element (2) with one end by means of a conductor (5, 6, 7) .

14. Sensor according to any of claims 10 to 13, characterized in that the outer resistive elements (3) are connected to the ring with their ends directed away from the inner resistive elements (2) on a conductor (8, 9, 10) forming an additional contact surface.

15. Sensor according to any of claims 10 to 14, characterized in that the resistive elements (2 and 3) have the same resistance when unloaded.

16. Sensor according to any of claims 10 to 15, characterized in that the resistive elements (2 and 3) are piezo-resistive elements.