CAPACITIVE SENSOR, METHOD FOR MANUFACTURING SUCH A SENSOR AND MOULD PROVIDED WITH SUCH A SENSOR

The invention relates to a capacitive sensor for measuring electrical properties of a material placed close to the capacitive sensor, comprising a first electrode, a second electrode and a third electrode which is placed between the first electrode and the second electrode and which is earthed in the situation of use.

Capacitive sensors of this type are known. They are used for instance to determine the degree of curing of an article manufactured from epoxy in a mould, so that the article can be removed from the mould at an optimum degree of curing. The known sensors are generally embodied as a dielectric film onto which the electrodes are printed. The drawback here is that they can only be used once and that they are left behind in the article, which can have an adverse effect not only on the strength but also the appearance of the article.

The capacitive sensor according to the invention does not have this drawback and has the feature that the first electrode, the second electrode and the third electrode comprise three metal plates running at least almost parallel to each other, wherein at least one edge of each metal plate lies in a predetermined reference plane. The plates can then be arranged in the mould, wherein the reference plane lies against the article.

A favourable embodiment of the inventive capacitive sensor has the feature that the third electrode is larger than the first and the second electrode so that a scatter field is almost only formed close to the reference plane, which can considerably increase the sensitivity of the measurement. Larger is here understood to mean that the third electrode is significantly larger, thus effectively preventing a scatter field.

A further favourable embodiment with which a robust and readily reproducible capacitive sensor can be realized has the feature that a

dielectric is placed between the first and the third electrode and between the second and the third electrode. A significant additional advantage is that no material which could adversely affect measurement can now penetrate between the first and the third electrode and between the second and the third electrode.

A further favourable embodiment has the feature that the first electrode, the second electrode and the third electrode are manufactured from a flexible material, such as for instance a metal film. A dielectric which is flexible and has a constant thickness is preferably arranged between the first and the third electrode and between the second and the third electrode, so that the capacitive sensor can be made into almost any conceivable form and can thus be applied at practically any location.

According to a further aspect of the invention, a particularly favourable embodiment has the feature that the capacitive sensor comprises a package consisting of a first electrode together with a dielectric, a third electrode together with a dielectric, a second electrode together with a dielectric and a third electrode together with a dielectric in rolled-up state. The package in rolled-up state is preferably accommodated in a tubular holder such that an outer end of the tubular holder at least almost coincides with the predetermined reference plane. The tubular holder is preferably manufactured from metal, wherein a good shielding against electrical interference fields is obtained in very simple manner, while the tubular holder made from metal can moreover be readily fixed, for instance screwed, into for instance a mould.

The invention also relates to a method for manufacturing a capacitive sensor. The inventive method has the feature that a package is assembled from four layers manufactured from a flexible conductive material and four layers of dielectric, that the package is subsequently provided with connections, rolled up and pushed into a tubular holder such that the connections protrude on a first side from the tubular holder.

A particularly favourable realization with which a robust capacitive sensor is obtained, which moreover generates very readily reproducible measured values, has the feature that the tubular holder is then impregnated with a plastic, for instance a curable epoxy resin. A vacuum is here preferably created beforehand in the tubular holder, thereby achieving an almost complete filling.

A further favourable realization has the feature that a second side lying opposite the first side is then processed such that one side of the three electrodes always at least almost coincides with the processed second side, wherein the processed second side forms the reference plane. This reference plane does not necessarily have to be flat. The sensor can for instance be arranged in a mould, wherein the part of the capacitive sensor protruding into the mould can be ground or cut, optionally together with the mould, into a form such that it forms an integral part of the inner wall of the mould. While the capacity of the sensor does change due to the grinding or cutting, the relevant stray capacitance remains practically unchanged, so that re-calibration of the sensor can generally be dispensed with.

The invention also relates to a mould for manufacturing objects from a curable plastic such as epoxy resin, provided with a capacitive sensor as specified in the foregoing paragraphs.

The invention will now be further elucidated on the basis of the following figures, wherein:

Fig. 1 A shows a possible embodiment of a capacitive sensor according to the invention;

Fig. 1B shows an improved embodiment of a capacitive sensor according to the invention;

Fig. 2A shows a schematic cross-section of a package of electrodes suitable for rolling up;

Fig. 2B shows a schematic top view of a rolled-up package of electrodes accommodated in a tubular holder;

Fig. 3A is a schematic side view of a rolled-up package of electrodes accommodated in a tubular holder;

Fig. 3B is a schematic side view of an alternative embodiment of a rolled-up package of electrodes accommodated in a tubular holder; Fig. 4 is a schematic side view of a mould provided with sensors according to the invention.

Fig. 1 A shows a possible embodiment of a capacitive sensor according to the invention. The capacitive sensor consists of a first plate 1, a second plate 2 and an earthed plate 3 place therebetween. An alternating voltage is fed to plate 1 using a signal generator 4. Owing to the presence of earthed plate 3 the field generated by this alternating voltage can only reach plate 2 via a scatter field indicated schematically with arrows. An AC voltmeter 5 connected to plate 2 will therefore make a certain movement when signal generator 4 is switched on. If a dielectric 6 is introduced into the scatter field, AC voltmeter 5 will then make a greater movement, depending on the dielectric constant of dielectric 6. In this way it is possible to measure the dielectric constant of dielectric 6. It is also possible to embody AC voltmeter 5 as a phase-sensitive AC voltmeter. In this case the dielectric losses of dielectric 6 can also be determined in a manner self-evident to a skilled person. It is further noted that the scatter field is shown in Fig. 1 A only on the top side of the capacitive sensor, but that it does in fact occur all around it, which has an adverse effect on the sensitivity of the measurement. Fig. 1B shows an improved embodiment of a capacitive sensor according to the invention, wherein earthed plate 3 is given a larger form except for the top side of the capacitive sensor where the edges of first plate 1 , second plate 2 and earthed plate 3 together define a reference plane. Because earthed plate 3 is larger, a scatter field will form almost exclusively on the top side whereby the dielectric properties of a dielectric can there be determined with maximum sensitivity. On the other sides earthed plate 3 must protrude well outside the other plates, for instance over a distance corresponding to ten times the mutual distance between the plates.

A layer of dielectric, for instance kapton film, is preferably arranged between first plate 1 and earthed plate 3 and between second plate 2 and earthed plate 3, whereby a cohesive package is obtained, the reference plane of which is moreover closed.

It is also possible to feed a direct voltage to plate 1 and to connect a DC voltmeter to plate 2. The specific resistance of a material lying against the reference plane can then be measured. This may for instance be important in situations wherein curing of an epoxy mixture is being monitored. When the epoxy mixture is almost cured the specific resistance is high and the temperature is generally also high. Without the presence of earthed plate 3 a leakage current through the dielectric could affect the measured values in this situation. With the sensor according to the invention this is not the case since measurement takes place only via the scatter field located outside the dielectric.

Fig. 2A shows a schematic cross-section of a package 7 of electrodes suitable for being rolled up. The package consists of a first electrode 1 together with a dielectric, a third electrode 3 together with a dielectric, a second electrode 2 together with a dielectric and a third electrode 3 together with a dielectric, wherein third electrodes 3 are significantly larger than the other electrodes, with the exception of one side which will form the reference plane on which measurement takes place. It will be apparent that the package can be assembled differently in the detail. The first electrode 1 together with the dielectric in the figure can thus be placed below the lower third electrode 3 together with the dielectric without the package hereby changing essentially in the rolted-up situation. The dielectric consists for instance of kapton film which is flexible and can advantageously be produced in a predetermined thickness.

Fig. 2B shows a schematic top view of a rolled-up package 7 of electrodes accommodated in a tubular holder 8. The free space shown in the figure is preferably filled with an epoxy resin in a vacuum moulding process in a manner self-evident to a skilled person.

If measurement takes place using a high-frequency alternating voltage, it may be advantageous to first fold double and then roll up package 7, whereby the self-induction of the rolled-up package is almost no longer a factor.

Fig. 3A shows a schematic side view of a rolled-up package of electrodes 7 accommodated in a tubular holder 8 and provided with connections 9 for connection of a signal generator, an AC voltmeter and an earth. The top side 10 forms the reference plane, in which plane the edges of first electrode 1, second electrode 2 and third electrode 3 lie in one plane for the purpose of measuring a dielectric place thereon. Fig. 3B shows a schematic side view of an alternative embodiment of a rolled-up package of electrodes 7 accommodated in a tubular holder 8, wherein top side 10 is ground in concave shape so that the dielectric properties of an object curved in this way can be determined with maximum precision and maximum sensitivity.

Fig. 4 shows a schematic side view of a mould 11 provided with sensors 12a,12b,12c,12d according to the invention. The sensors are screwed into mould 11 , whereafter the interior of mould 11 undergoes a further processing such that the top sides 10a, 10b, 10c, 10d of sensors 12a,12b,12c,12d in fact form part of the interior of mould 11. The filling of mould 11 can now be monitored precisely with sensors 12a,12b,12c,12d and the curing of for instance an epoxy mixture can then be followed in a per se known manner so that mould 11 can be opened at a suitable moment and an object manufactured in mould 11 can be removed. Mould 11 , including sensors 12a,12b,12c,12d can then be used again.