Capacitive sensor device

A capacitive sensor device for measuring properties of a medium to be measured when this medium surrounds and flows past the sensor. The sensor has a housing of metal, an opening in the housing transverse to the flow of said medium to be measured which is adapted to house at least one capacitive electrode, and where the electrode(s) of the sensor is/are electrically insulated from said medium to be measured by means of a glass ceramic material that is arranged to be flush with the outer surface of the sensor.

The present invention relates to a capacitive sensor device for measuring properties of a medium to be measured when this medium surrounds and flows past the sensor, where the sensor has a housing of metal, and where the electrode(s) of the sensor is/are electrically insulated from said medium to be measured by means of a glass ceramic material that is arranged to be flush with the outer surface of the sensor.

The prior art includes a number of devices for capacitive measurement of properties of a medium to be measured which flows, for example, through a pipeline. However, there has long been a desire to provide a capacitive sensor that is robust, resistant to any corrosive medium to be measured, and in addition provides increased sensitivity in comparison with the known solutions.

Accordingly, the present invention is characterised in that an opening in the housing transverse to the flow of the medium to be measured is designed to house at least one capacitive electrode, and that the electrode(s) of the sensor is/are electrically insulated from said medium to be measured by means of said glass ceramic material that is arranged to be flush with the opposite faces of the sensor housing.

In an alternative embodiment, the sensor is characterised in that an opening in the housing transverse to the flow of said medium to be measured is designed to house at least two electrodes which are electrically insulated from each other at a fixed distance and from said medium to be measured by means of said glass ceramic material that is arranged to be flush with the opposite faces of the sensor housing.

According to another embodiment in which there are at least two electrodes, the electrodes of the sensor, when viewed from one face of the sensor housing to the opposite face, consist of a first capacitive sensor electrode, a common reference electrode that is in galvanic contact with the sensor housing, and a second capacitive sensor electrode.

In an alternative embodiment in which the sensor has at least two electrodes, the electrodes of the sensor, when viewed from one face of the sensor housing to the opposite face, consist of a first capacitive sensor electrode, a first counter-phase or shield electrode, a common reference electrode that is in galvanic contact with the sensor housing, a second counter-phase or shield electrode and a second capacitive sensor electrode.

In another alternative embodiment, the electrodes of the sensor, when viewed from one face of the sensor housing to the opposite face, consist of a) a capacitive sensor electrode that is surrounded in the same plane by a ring-shaped counter-phase or shield electrode, and b) a common reference electrode.

According to yet another embodiment, the electrodes of the sensor, when viewed from one face of the sensor housing to the opposite face, consist of a) a first capacitive sensor electrode that is surrounded in the same plane by a first ring-shaped counter-phase or shield electrode, b) a common reference electrode, and c) a second capacitive sensor electrode that is surrounded in the same plane by a second ring-shaped counter-phase or shield electrode, the reference electrode being common to the electrodes in both point a) and point b).

It is an advantage if the counter-phase or shield electrodes are made in the form of rings having an internal diameter smaller than the diameter of the capacitive electrode and an outer diameter greater than the diameter of the capacitive electrode.

It would also be advantageous to allow the electrodes of the sensor to be disc-shaped with a conical or polygonal contour.

When the electrodes of the sensor comprise counter-phase or shield electrodes, these electrodes will preferably have a larger surface area than the capacitive sensor electrodes.

Furthermore, it would be advantageous if the sensor housing, seen in cross-section, were given an aerodynamic form.

Said medium to be measured may, for example, consist of one fluid or several fluids in a mixture. The term “fluid” in this context is to be understood in its widest sense, and includes, e.g., liquid, gas, a mixture of liquid and gas (also including air), liquid and/or gas containing particles (e.g., sand), or consist of, e.g., powder or a powder composition, optionally in connection with a fluid, as for instance air.

In its simplest form, the device may consist of a single electrode1which is embedded in a glass ceramic material2in the sensor housing3. A wire connection4leads to the outside of the sensor housing3via a duct5. The sensor housing has a wire connection6. Thus, the sensor housing3will in this case act as a reference electrode.

In the embodiment shown inFIG. 2there are two capacitive electrodes7,8which are embedded in a glass ceramic material2, and wires9,10run from the electrodes7,8via a wire duct11to the outside of the housing3. In this case too, the housing3will form a reference or counter-electrode to the electrodes7,8.

In the embodiment shown inFIG. 3two capacitive electrodes12,13are provided, and between them a reference electrode14which preferably, but not necessarily, is on the same potential as the housing3. Here too, the electrodes12-14are embedded in a glass ceramic material2in an opening in the housing3.

The electrodes12-14are connected to the outside of the housing3via respective wires15,16,17, which run through a duct18in the housing3.

In the fourth embodiment shown inFIG. 4, the sensor has a total of five electrodes placed in the opening19in the housing3. Also in this case, all the electrodes are embedded in a glass ceramic material, and here too the glass ceramic material2will be flush with opposite faces3′,3″ of the housing3. When viewed from left to right inFIG. 4, i.e., from the face3′ to the face3″, there is a first capacitive sensor electrode20, a first counter-phase or shield electrode (so-called “Guard” electrode)21, a common reference electrode22, a second counter-phase or shield electrode23and a second capacitive sensor electrode24. The counter-phase or shield electrodes21,24will in effect create a form of screen or reference platform for the capacitive electrodes, and the housing3will form a counter-electrode. The respective electrodes20-24are connected to the outside of the sensor housing3via respective wires25-29which run through a duct30in the housing3.

InFIG. 6the sensor is shown in perspective, as it will in fact look for all the embodiments shown inFIGS. 1-4. InFIG. 5the sensor has been placed purely schematically in a pipeline, and where the opposite faces3′,3″ are essentially parallel to the flow direction of the medium flowing though the pipeline31.

FIG. 7shows a typical, but not necessarily limiting exemplary embodiment of the circuits which could control the electrodes20-24when there is a total of five electrodes. The reference numerals32and33indicate respectively a first and a second oscillator which are connected to a first and second capacitive electrode20,24. The reference numerals34and35indicate counter-phase/shield driver electronics to drive the electrodes21,23in opposite phase to the electrodes20,24so as to obtain an efficient screening and create a platform or base for the electrodes20and24. This also allows the detection field to be pushed further out into the medium. Like the housing3, the electrode22is connected to earth. The reference numeral36indicates a signal circuit connected to the oscillators32and33to detect signal variations and thus provide an expression of properties of the medium flowing past the sensor.

The fact that the sensor is actually two-sided means that increased sensitivity is obtained in relation to what is previously known in connection with capacitive sensors which traditionally are placed in, for example, the pipeline wall.

The electrodes of the sensor are preferably disc-shaped, and may optionally be given a conical or polygonal contour, preferably matching the shape of the opening19in the housing3. It would be advantageous, with the object of causing minimum turbulence, to give the sensor housing, insofar as possible, an aerodynamic form. This is indicated to some extent in FIG.5. In an alternative embodiment, the electrodes21,23may be ring-shaped and have an inner diameter smaller than the diameter of the respective, adjacent capacitive electrode, and an outer diameter greater than the diameter of the capacitive electrode.

For the sake of simplicity, the electrodes have not been included in FIG.8andFIG. 9, but it will be seen that, in particular in the embodiment shown inFIG. 4, a capacitive field CF is obtained which extends from one side3′ of the sensor housing to the opposite side3″, as indicated in FIG.9and also indicated to a certain extent in FIG.8.

It is important to note that when the glass ceramic material2is used, the distance between the electrodes, if there are two or more electrodes, will be fixed and will therefore be independent of pressure and temperature.

The electrodes21and23, as shown in connection with FIG.4andFIG. 7, prevent the sensor electrodes20and24from being affected by each other and will also provide a fixed capacity that is small is relation to the measurement capacity provided by the sensor electrodes20and24. The electrodes21and23can expediently be connected to an extra screen in a measuring cable (not shown) and also to associated electronic equipment34,35, as shown in FIG.7.

As indicated inFIG. 9, the measuring field will extend from one sensor electrode through the glass ceramic material, then through the material to be measured, and then through the glass ceramic material to the other sensor electrode.

As shown inFIGS. 4 and 7, the counter-phase electrodes can preferably have a greater diameter than the sensor electrodes20,24and will thus cause the measuring field to reach further into the medium to be measured than would be the case if such electrodes21,23were not used.

If only one electrode is used, as shown inFIG. 1, without the use of the electrodes21,23and22, the measuring field will be between the sensor housing that is exposed to the medium to be measured and the sensor electrode1that lies insulated in the glass ceramic material2. By using a two-sided electrode of this kind, where both sides are exposed to the medium to be measured, the capacity provided by the sensor will in fact be great, with a measuring field that is contiguous with the sensor housing3.

In the solution that can be seen from eitherFIG. 2orFIG. 3, where there are two sensor electrodes7,8;12,13, these electrodes will, as shown, also both lie insulated in the glass ceramic material2, and the field will extend between both sides of the sensor, as shown and described in connection withFIGS. 8 and 9.

FIGS. 10aand10bshow an embodiment which differs slightly from what has been shown in the preceding figures. The sensor housing is indicated by the reference numeral37in these figures, and has a through opening37′ in which the sensor electrodes38,39and40are placed, and where the opening is filled with glass ceramic material41which surrounds and holds the electrodes apart at a fixed distance. The electrodes of the sensor, when viewed from one face37″ of the sensor housing to the opposite face37′″, consist of a capacitive sensor electrode38that is surrounded in the same plane by a ring-shaped counter-phase or shield electrode39. These two electrodes have a common reference electrode40. The actual housing may have an earth potential or a potential in common with the reference electrode. The counter-phase or shield electrode, a so-called “Guard” electrode, helps to ensure that the main field from the capacitive electrode is pushed further out into the medium to be measured, whereby the sensitivity of the sensor is also increased.

An alternative embodiment of that shown inFIG. 10can be seen in FIG.11. In this figure, the sensor housing is indicated by the reference numeral42and has a through opening42′, in which the sensor electrodes43-47are placed, and where the opening is filled with a glass ceramic material48which surrounds and holds the electrodes apart at a fixed distance. The electrodes of the sensor, when viewed from one face42″ of the sensor housing to the opposite face42′″, consist of a capacitive sensor electrode43which is surrounded in the same plane by a ring-shaped counter-phase or shield electrode44. These two electrodes have a common reference electrode45. A second capacitive sensor electrode46that is surrounded in the same plane by a second ring-shaped counter-phase or shield electrode47is placed at a distance from the reference electrode45, but the reference electrode45will be common to all the electrodes43,44and46,47. The actual housing42may have an earth potential or a potential in common with the reference electrode. Here too, the counter-phase or shield electrodes44,47, so-called “Guard” electrodes, will help to ensure that the main field from the respective, adjacent capacitive electrode43, respectively46, is pushed further out into the medium to be measured, thereby also increasing the sensitivity of the sensor. The advantage of the embodiment shown inFIG. 11is that the sensor in this case, compared with the embodiment shown inFIG. 10, has considerably greater sensitivity because of the possibility of two-sided detection.