GB1562141A - Devices for monitoring the liquid level in plastics containers - Google Patents

Description

(54) IMPROVEMENTS IN DEVICES FOR MONITORING THE LIQUID LEVEL IN PLASTICS CONTAINERS (71) We, VDO ADOLF SCHINDL- ING AG, of 103 Grafstrasse, Frankfurt/ Main, Germany, a body corporate organized according to the laws of Germany, 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 a device for monitoring the liquid level in plastics containers or vessels, in particular in motor vehicles, having a capacitive sensor which is arranged on the outside of the container wall and which forms an oscillation determining element of an oscillator the output of which is connected with a display stage.

Known devices for monitoring the liquid level in plastics containers, such as for example in brake fluid containers or in containers for the washing liquid of a windscreen wiping installation, generally comprise a float and an electrical switch actuated by the said float and connected in a warning circuit. These devices have the disadvantage that they occupy a relatively large volume and are in particular not suitable as additional equipment for attaching to containers which are already installed.

Furthermore, monitoring units having a capacitive sensor or feeler are known.

These sensors have two electrodes which are insulated from each other and are coaxial. The electrodes are fitted in a retaining arrangement, they project into the inner space of the container and are electrically connected with an oscillator, forming an oscillating element of the oscillator. The volume of these devices is, it is true, substantially smaller than that of the above described devices, but the ability to use these devices as additional equipment for attaching to containers already installed still leaves much to be desired. The latter is due in particular to the working of the containers by boring, filling, cutting or the like operation and to sealing problems occurring especially in the case of thinwalled plastics containers.If access to the interior of the container is possible, then the sensor can be fitted in the wall of the container together with its retaining system for example by means of a nut adapted to be screwed on to retaining means projecting into the inner space of the container, the retaining means being formed with a screw thread. If, on the other hand, access to the interior of the container is limited or is not possible at all, such as is frequently so in the case of washing liquid and brake fluid containers, then fluid-tight securing of the sensor is no longer possible.

It is an object of the present invention to obviate partially or wholly the above disadvantage by providing a device which can be mounted or assembled simply and rapidly and in fluid-tight manner at any desired container wall, and in particular is suitable as additional equipment for attaching to containers already installed.

According to the present invention there is provided a device for monitoring the liquid level in plastics containers, including a device for monitoring the liquid level in plastics containers including: a capacitive sensor having a pair of flat electrodes arranged in juxtaposition in one plane on a thin insulating sheet, said sheet being adapted to be secured to the wall of the container; an oscillator, an oscillation determining element of which is formed by said capacitive sensor, said oscillator also including one or more integrated circuits having a high input impedance; and display means for indicating the output from the oscillator.

Due to the employment of an oscillator having one or more integrated circuits with high input impedance, it becomes possible to employ a capacitor of extremely small capacity, i.e. a disc or plate capacitor, the average distance between the plates of which is relatively large. In this way however, it is no longer necessary to arrange the electrodes within the container or in such manner that they project into the container, and it becomes possible to obtain a monitoring device which, without aperturing the container wall, can be secured on the outside of the latter and is therefore suitable as additional equipment to be fitted to a container.

It has been found that by arranging the two electrodes on the thin insulating sheet, considerable advantages are achieved both from the aspect of manufacturing techniques and also from that of assembly techniques. It has been found to be especially advantageous to provide the thin insulating sheet with a self-adhering surface. This greatly simplifies assembly of the sensor to the outside of the container. Of course, it would also be possible to secure the sensor with a retaining tape or band made from metal or plastics and extending about the container, but in that case (if further, costly components are not employed), it will generally be necessary to keep in stock a variety of retaining bands of varying length for employment with containers of varying size.Further improvement and simplification of assembly of the device to the container wall can be achieved by combining the electrodes and the oscillator in a structural unit.

The oscillator circuit or circuits may be produced by the CMOS Technique. It has been found to be advantageous to construct the oscillator (by the CMOS Technique) from two logic elements connected in cascade. A feedback circuit comprising the capacitive circuit may be connected between the output of the second logic element and the inputs to the two logic elements. In particular in the case of an oscillator assembled in this matter, it would become possible to dispense with a screened or shielded line between the oscillator and the two electrodes. This has an advantageous effect on production costs. Each logic element may be a NAND-gate. Such NAND-gates are components which are extremely inexpensively commercially obtainable, thereby permitting manufacture of an inexpensive oscillator.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawing, wherein the sole figure is a circuit diagram of one preferred form of device for monitoring the liquid level in a plastics container. Referring to the drawing the device for monitoring the liquid level in a plastics container 1 comprises a capacitor 2 having two electrodes 3 and 4 and constituting one of the oscillation determining elements of an oscillator 5.

The two electrodes 3 and 4 are in the form of two rectangular foil pieces arranged in juxtaposition in one plane and constituting, together with a thin plastics sheet 6, a structural unit the side of which facing the container 1 is provided with a self-adhering layer.

The capacitor 2 is electrically connected to the oscillator 5 by means of conductors 7 and 8. The oscillator 5 comprises two NAND gates 9 and 10 connected in cascade. A feedback circuit is connected between the output of the NAND-gate 10 and the inputs to the NAND gates 9 and 10 via the capacitor 2 and resistors 11 and 12 respectively. The output of the oscillator 5 is connected to a monitoring lamp 15; through a rectifier 13 and an amplifier 14.

If the liquid level is above the two electrodes 3 and 4, the oscillator 5 is damped and does not oscillate, with the result that the lamp 15 is not lit. However, as soon as the liquid has fallen below the two electrodes 3 and 4, the oscillator 5 begins to oscillate and the lamp 15 is lit to indicate that the minimum level in the container 1 has been reached.

WHAT WE CLAIM IS: 1. A device for monitoring the liquid level in plastics containers including: a capacitive sensor having a pair of flat electrodes arranged in juxtaposition in one plane or a thin insulating sheet, said sheet being adapted to be secured to the wall of the container; an oscillator, an oscillation determining element of which is formed by said capacitive sensor, said oscillator also including one or more integrated circuits having a high input impedance; and display means for indicating the output from the oscillator.

2. A device according to claim 1 wherein the thin insulating sheet is provided with a self-adhering surface.

3. A device according to claim 1 or 2, wherein the oscillator is manufactured by the CMOS technique.

4. A device according to claim 3, wherein the oscillator comprises two logic elements connected in cascade.

5. A device according to claim 4, wherein each logic element is a NANDgate.

6. A device according to claim 4 or 5, wherein a feedback circuit comprising the capacitive sensor is connected between the output of the second logic element and the inputs to the two logic elements.

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. having one or more integrated circuits with high input impedance, it becomes possible to employ a capacitor of extremely small capacity, i.e. a disc or plate capacitor, the average distance between the plates of which is relatively large. In this way however, it is no longer necessary to arrange the electrodes within the container or in such manner that they project into the container, and it becomes possible to obtain a monitoring device which, without aperturing the container wall, can be secured on the outside of the latter and is therefore suitable as additional equipment to be fitted to a container. It has been found that by arranging the two electrodes on the thin insulating sheet, considerable advantages are achieved both from the aspect of manufacturing techniques and also from that of assembly techniques. It has been found to be especially advantageous to provide the thin insulating sheet with a self-adhering surface. This greatly simplifies assembly of the sensor to the outside of the container. Of course, it would also be possible to secure the sensor with a retaining tape or band made from metal or plastics and extending about the container, but in that case (if further, costly components are not employed), it will generally be necessary to keep in stock a variety of retaining bands of varying length for employment with containers of varying size.Further improvement and simplification of assembly of the device to the container wall can be achieved by combining the electrodes and the oscillator in a structural unit. The oscillator circuit or circuits may be produced by the CMOS Technique. It has been found to be advantageous to construct the oscillator (by the CMOS Technique) from two logic elements connected in cascade. A feedback circuit comprising the capacitive circuit may be connected between the output of the second logic element and the inputs to the two logic elements. In particular in the case of an oscillator assembled in this matter, it would become possible to dispense with a screened or shielded line between the oscillator and the two electrodes. This has an advantageous effect on production costs. Each logic element may be a NAND-gate. Such NAND-gates are components which are extremely inexpensively commercially obtainable, thereby permitting manufacture of an inexpensive oscillator. The present invention will now be described in greater detail by way of example with reference to the accompanying drawing, wherein the sole figure is a circuit diagram of one preferred form of device for monitoring the liquid level in a plastics container. Referring to the drawing the device for monitoring the liquid level in a plastics container 1 comprises a capacitor 2 having two electrodes 3 and 4 and constituting one of the oscillation determining elements of an oscillator 5. The two electrodes 3 and 4 are in the form of two rectangular foil pieces arranged in juxtaposition in one plane and constituting, together with a thin plastics sheet 6, a structural unit the side of which facing the container 1 is provided with a self-adhering layer. The capacitor 2 is electrically connected to the oscillator 5 by means of conductors 7 and 8. The oscillator 5 comprises two NAND gates 9 and 10 connected in cascade. A feedback circuit is connected between the output of the NAND-gate 10 and the inputs to the NAND gates 9 and 10 via the capacitor 2 and resistors 11 and 12 respectively. The output of the oscillator 5 is connected to a monitoring lamp 15; through a rectifier 13 and an amplifier 14. If the liquid level is above the two electrodes 3 and 4, the oscillator 5 is damped and does not oscillate, with the result that the lamp 15 is not lit. However, as soon as the liquid has fallen below the two electrodes 3 and 4, the oscillator 5 begins to oscillate and the lamp 15 is lit to indicate that the minimum level in the container 1 has been reached. WHAT WE CLAIM IS:

1. A device for monitoring the liquid level in plastics containers including: a capacitive sensor having a pair of flat electrodes arranged in juxtaposition in one plane or a thin insulating sheet, said sheet being adapted to be secured to the wall of the container; an oscillator, an oscillation determining element of which is formed by said capacitive sensor, said oscillator also including one or more integrated circuits having a high input impedance; and display means for indicating the output from the oscillator.

2. A device according to claim 1 wherein the thin insulating sheet is provided with a self-adhering surface.

3. A device according to claim 1 or 2, wherein the oscillator is manufactured by the CMOS technique.

4. A device according to claim 3, wherein the oscillator comprises two logic elements connected in cascade.

5. A device according to claim 4, wherein each logic element is a NANDgate.

6. A device according to claim 4 or 5, wherein a feedback circuit comprising the capacitive sensor is connected between the output of the second logic element and the inputs to the two logic elements.

7. A device for monitoring the liquid

level in plastics containers constructed and arranged to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.