GB2323671A

GB2323671A - Vessel contents sensor

Info

Publication number: GB2323671A
Application number: GB9706411A
Authority: GB
Inventors: David Albert Skye
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1997-03-27
Filing date: 1997-03-27
Publication date: 1998-09-30
Anticipated expiration: 2017-03-27
Also published as: GB2323671B; GB9706411D0

Abstract

A vessel contents sensor 1 is provided for non-intrusive sensing of the level of the contents, such as a liquid 6, of a vessel 3. The sensor is a capacitive sensor with an input plate 10 and a pick-up plate 14. A third common plate 12 which acts as a common reference can also be provided. The plates are disposed on a sloping support 8 which is suitable for supporting the base 4 of the vessel. The support is angled to the horizontal such that the input plate is higher than the pick-up plate. The signal from the pick-up plate provides an output indicating the gradual depletion of the level of the liquid in the vessel as the liquid gradually uncovers the input plate.

Description

VESSEL CONTENTS SENSOR This invention relates to a vessel contents sensor for nonintrusive sensing of the level of the contents of a vessel.

This invention particularly relates to the situation where the contents of the vessel is a liquid although the contents could equally be a solid such as a powder.

In a situation where liquids are used in part of a process, such as replenisher solutions in a photoprocessing machine, it is desirable to be able to detect when the liquid level is low and also when the supplying vessel is empty.

Capacitive level sensors are known which have two sensor electrodes which are separated by a material which acts as a dielectric, for example, the liquid in a container. US patent no. US 4,970,468 describes equipment for detecting a liquid surface using a vertically moveable probe and a rotatable metal plate on which the container is placed. US patent no. US 5,477,727 describes a capacitive level sensor for a container in which two electrodes are disposed in patterns over the surface of the vertical wall of a container of a liquid.

UK patent application no. GB 2033961A describes an electronic capacitive liquid level gauge which uses the method of discriminating between the electrical capacitance to earth, or to a common sensor, of discrete sensors placed vertically in the liquid. The electrical capacitance varies when the sensors are above or below the surface of the liquid.

UK patent application no. GB 1489293A describes an arrangement of condenser plates on the side of a liquid container which enables easy connection to an alarm circuit.

Other known methods of detecting liquid levels in a container without using a probe inside the container include the use of ultrasonic vibration.

The present invention has the advantage of enabling an analogue output to be provided with greater variation in signal per change in volume than the known liquid level sensors. The present invention also provides a sensor on which vessels can be placed without connection to the sensor. Also, the vessel is angled which enables more of the contents of the vessel to be removed.

Accordingly to the present invention there is provided a vessel contents sensor for non-intrusive sensing of the level of a contents of a vessel comprising a capacitive sensor having two plates, a first input plate and a second pick-up plate, wherein the plates are disposed on a support suitable for supporting the base of a vessel, the support being at an angle to the horizontal such that one plate is higher than the other plate.

Preferably, the first input plate is disposed on the support higher than the second pick-up plate such that the first input plate is uncovered as the contents of the vessel decreases before the pick-up plate is uncovered.

Preferably, the input plate is supplied with a medium frequency square wave. Most preferably, the frequency of the square wave is about 500kHz.

Preferably, a third plate is provided as a common reference between the input plate and the pick-up plate to reduce background noise.

Preferably, the output received from the pick-up plate is amplified and filtered. Most preferably, the output from the pick-up plate is rectified to produce a DC output.

Preferably, the output drives an analogue meter which shows the decrease in level of the contents of the vessel.

An embodiment of the vessel contents sensor for nonintrusive sensing of the level of a contents of a vessel is now described with reference to the accompanying drawings in which: Figure 1 is a side elevation of the vessel contents sensor together with a vessel; Figure 2 is a block diagram of the electrical apparatus of the vessel contents sensor; and Figure 3 is a graph showing the variation of output voltage of the vessel contents sensor against the volume remaining in a vessel.

Referring to the drawings, a vessel contents sensor (1) is provided for non-intrusive sensing of the level of the contents of a vessel (3). The contents of the vessel (3) is generally liquid (6); however, the contents could be a solid in the form of a powder. The sensor (1) is particularly suitable for situations where liquids are used in part of a process, such as the replenisher solutions in a photoprocessing machine.

The sensor (1) has a sloping support (8) on which the base (4) of the vessel (3) can be placed. The slope of the support can be determined for stability of a particular vessel shape. The support (8) has three plates (10, 12, 14) mounted on it on which the base (4) of the vessel (3) rests.

The first plate (10) is an input plate which is supplied with a medium frequency square wave of about 500kHz from an oscillator (16). A second plate (14) is a pick-up plate which picks up the wave emitted from the input plate (10) as the wave is coupled through the liquid (6) in the vessel (3). A third plate (12) is a common plate which acts as a common reference between the input plate (10) and the pickup plate (14).

The frequency of the square wave can be higher or lower than 500kHz. Frequencies of 50Hz and its harmonics should be avoided and the frequency should be kept low enough to avoid signal attenuation through the cable although low capacitive cable can be used.

The three plates (10, 12, 14) are disposed on the sloping support (8) such that the input plate (10) is highest on the sloping support (8), the pick-up plate (14) is lowest on the sloping support (8) and the common plate (12) is between the input plate (10) and the pick-up plate (14). The common plate (12) could equally be below the pick-up plate (14).

The high impedance output from the pick-up plate (14) is fed through an amplifier (18) and a tuned filter (20). The amplifier (18) may include a band pass filter removing the need for a separate filter (20). The output is then rectified by a rectifier (22) to produce a DC level output (24).

In use, when the vessel (3) is full of a liquid (6), the medium frequency square wave from the input plate (10) is coupled through the liquid (6) and the pick-up plate (14) has a high pick-up level resulting in a high DC level output (24). This output level will remain unchanged throughout the bulk of the supply of the liquid (6). As the level of the liquid (6) approaches the base (4) of the vessel (3), the highest plate, which is the input plate (10), starts to become uncovered by the liquid (6). At this point, the coupling between the input plate (10) and the pick-up plate (14) is reduced and the resulting DC output level (24) will start to fall. As progressively less area of the input plate (10) is covered by the liquid (6), the DC output level (24) continues to fall until it reaches almost 0 as the vessel (3) becomes empty and the input plate (10) is fully uncovered.

The provision of a sloping support (8) on which a vessel (3) rests improves the emptying of the vessel (3) as the liquid collects in the bottom corner of the vessel (3).

Figure 3 shows the results of the emptying of a liquid (6) from a 5 litre vessel (3). Figure 3 shows the drop in DC output voltage (24) as the volume remaining in the vessel (3) falls to 0.

The DC output signal (24) can be used in different ways to suit the application of the sensor (1).

(a) the output signal can drive a conventional analogue meter which acts as a "fuel gauge" for the level of the liquid (6) in the vessel (3); (b) the output signal can drive a digital display in a similar way; (c) the output signal can drive a series of LEDs or other discrete indicators to provide "low", "very low" and "out" warnings.

(d) the output signal can be used to generate an audible warning of a low level or empty vessel (3); (e) the output signal can be fed to an analogue-to digital convertor for use with computer or embedded micro-controller applications; (f) the output signal can be used to provide "inhibit" signals if an empty vessel (3) can not be tolerated by the system in which it is used.

The sensor (1) has the advantage that it is fail safe in that a break in the signal will result in the DC output level (24) being 0 and the sensor (1) will assume that the vessel (3) is empty and the warning signals will be activated.

The important features of the sensor (1) are that the sensor (1) is non-intrusive and therefore there is no risk of contamination of the liquid (6) in the vessel (3) and no electrical connections are made to the vessel (3). The plates (10, 12, 14) are mounted on the support (8) and the vessel (3) can easily be removed from the support (8) without disconnecting electrical connections. The analogue output is such that low level warnings can be given before the vessel (3) is completely empty. The angle of slope of the support (8) can be varied to increase or decrease the sensitivity of the sensor (1). The sensor (1) also has the advantage of having very low power consumption and is relatively low-cost due to a low number of components.

Modifications and improvements can be made to the foregoing without departing from the scope of the present invention.

Claims

1. A vessel contents sensor for non-intrusive sensing of the level of a contents of a vessel comprising a capacitive sensor having two plates, a first input plate and a second pick-up plate, wherein the plates are disposed on a support suitable for supporting the base of a vessel, the support being at an angle to the horizontal such that one plate is higher than the other plate.

2. A vessel contents sensor as claimed in claim 1, wherein the first input plate is disposed on the support higher than the second pick-up plate such that the first input plate is uncovered as the contents of the vessel decreases before the pick-up plate is uncovered.

3. A vessel contents sensor as claimed in claim 1 or claim 2, wherein the input plate is supplied with a medium frequency source.

4. A vessel contents sensor as claimed in claim 3, wherein the medium frequency source is in the form of a square wave.

5. A vessel contents sensor as claimed in claim 3, wherein the frequency of the source is about 500kHz.

6. A vessel contents sensor as claimed in any of the preceding claims, wherein a third plate is provided as a common reference between the input plate and the pick-up plate to reduce background noise.

7. A vessel contents sensor as claimed in any of the preceding claims, wherein the output received from the pick-up plate is amplified and filtered.

8. A vessel contents sensor as claimed in any of the preceding claims, wherein the output from the pick up plate is rectified to produce a DC output.

9. A vessel contents sensor as claimed in any of the preceding claims, wherein the output from the pick up plate drives an analogue meter which shows the decrease in level of the contents of the vessel.

10. A vessel contents sensor substantially as hereinbefore described with reference to the accompanying drawings.