GB2386688A

GB2386688A - A solid state sensor for liquid level measurement

Info

Publication number: GB2386688A
Application number: GB0206726A
Authority: GB
Inventors: John Evan Griffiths; Nicholas J Evans
Original assignee: FOZMULA Ltd
Current assignee: FOZMULA Ltd
Priority date: 2002-03-21
Filing date: 2002-03-21
Publication date: 2003-09-24
Also published as: GB0206726D0

Abstract

Apparatus and method for sensing the level of a liquid in a vessel comprises using a capacitive sensor to sense the liquid level, inputting a signal from the sensor to a comparator (D) in a digital circuit so it is compared with a ramped signal to determine the appropriate numbers of resistors (A) to be switched on by transistors (B) in order to drive a gauge (H) to indicate liquid level. The arrangement is typically used in a fuel gauge system. The sensor system uses a capacitive sensor but the circuit mimics the output of a resistive sensor with defined ohmic values, thus temperature compensation may be achieved. The sensor may also comprise radar, ultrasonic or reed switches to sense the liquid level.

Description

1 2386688

A Solid State Sensor for Liquid Level Measurement The present invention relates to a liquid level sensor and in particular, although not exclusively, to a capacitive solid state liquid level sensor for a fuel tank.

5 It is known to provide a liquid level sensing device for a fuel tank, the device having a resistive output. Such a device may have a float attached to one end of a swinging arm, the other end of the swinging arm connected to a pivot. As the fuel level changes in the tank the float rises or falls and the swinging arm operates a variable resistor at the pivot which provides a resistive electric output. The resistive output has defined ohmic values 10 and is connected to a dial gauge for displaying the fuel level in the tank. Known dial gauges require a resistive signal to be used.

It is further known to provide a capacitive liquid level sensor such as the device disclosed in GB-A-2295254 which comprises two electrodes arranged to form a 15 capacitor. The device is placed in the fuel tank so that the fuel acts as a dielectric for the capacitor. As the fuel level changes the amount of dielectric between the electrodes changes which correspondingly changes the capacitance of the capacitor. The capacitive output may be converted to a quasi-resistive output by Field Effect Transistors (PET)

utilising a pulse width modulated signal.

capacitive liquid level sensor has the advantage that there are no moving parts which may wear out or may become dislodged and cause damage. This may be particularly important for an off road vehicle such as a mechanical digger which may sustain significant bumping and jolting. A capacitive sensor also has fewer parts which 25 simplifies manufacturing of the device. One of the main problems associated with the prior capacitive liquid level sensor is the variance in the output signal, for example due to temperature variations. This variance may give a false reading of the fuel level indicated on the dial gauge.

30 Further whilst a quasi-resistive output can be created using a pulse width modulated signal, such signals have been found to be incompatible with certain dial gauges such

that the sensor must be specifically calibrated when it is connected to a specific dial gauge. This inevitably increases production costs, and makes replacement and repair somewhat problematic.

5 What is required is a liquid level sensing device which can create the output of a true resistive sensor with deemed ohmic values and which is independent of temperature.

The device should provide an approximate linear output over the temperature range of at least -40 C to 85 C.

10 According to the invention there is provided apparatus for sensing the level of a liquid in a vessel for holding liquids comprising a sensor to sense the liquid level and a digital electronic circuit wherein the output from the sensor is converted to a pure resistive output of the electronic circuit.

15 Such an apparatus can provide a pure resistive output required by a dial gauge. The sensor can be used with any dial gauge having the appropriate ohmic range without calibration being necessary. In this way, the sensor can be a drop-in replacement for a..

swing-arm type sensor.

20 In a preferred embodiment the sensor is a capacitance sensor. À À Preferably one electrode of the capacitor is a tube open at both ends and the other electrode is axially arranged therewithin such that the liquid within the vessel is an electrolyte between the electrodes. In use the sensor is arranged to detect the level of 25 liquid from a minimum to a maximum.

Preferably the electronic circuit has a plurality of resistors arranged in series. This chain of resistors simulates a defined ohmic sensor. In a preferred embodiment the resistors are arranged such that they may be switched on or off so as to combine their resistive 30 values in a binary manner. Advantageously each resistor in the chain has a different ohmic value.

Preferably the electronic circuit has a plurality of electronic latches. The latches provide the means to select the binary combination of resistors.

Preferably the electronic circuit has a comparator. The comparator further providing a 5 means of selecting the combination of resistors.

in a preferred embodiment the electronic circuit has a plurality of field effect

transistors. The transistors may be arranged in parallel with the resistors to selectively include each resistor in the resistive chain.

In an alternative embodiment of the electronic circuit an analogue to digital convertor is arranged in parallel with the chain of resistors.

an. Preferably the electronic circuit drives a dial gauge, the gauge having a resistive input to À..

15 receive a signal from the output of the electronic circuit. When the sensor is placed in À...:.

the tank the gauge provides a visual indication of the amount of fuel in the tank.

Advantageously the electronic circuit may have a light emitting diode. The light À. '.

emitting diode may provide a visual indication that the tank is empty or near empty..

20. Other features of the invention will be apparent from the following description of a

preferred embodiment shown by way of example only in the accompanying drawings, in which; 25 - Figure 1 is a diagrammatic side representation of a capacitive sensor according to the present invention.

- Figure 2 is a diagrammatic end representation of a capacitive sensor according to the present invention.

- Figure 3 is a diagrammatic representation of a circuit for controlling the output of the 30 sensor according to the present invention.

Referring firstly to Figure there is shown a capacitive sensor in accordance with the present invention, generally designated 10. 'l'he sensor 10 has a tubular outer electrode 12 and a inner electrode 14 arranged axially therewithin. 'l'he lower end of the outer electrode 12 has a plug 16 to accurately locate the inner electrode 14. The outer 5 electrode 12 has an upper hole l 8 to permit air to enter and exit a gap 20 between the outer electrode 12 and the inner electrode 14. The outer electrode 12 also has a lower hole 22 to permit fuel to enter and exit the gap 20. The sensor 10 has a circular flange 24 for attachment to a fuel tank (not shown). The electronics for the sensor 10 are located in a housing 26 mounted on the flange 24. In use the sensor 10 is mounted in a 10 substantially upright orientation.

Figure 2 illustrates an end view of the sensor according to the present invention. The axial arrangement of the outer electrode 12, inner electrode 14, plug 16, flange 24 and housing 26 is illustrated. À À 15.... .

Figure 3 is a diagrammatic representation of a circuit for controlling the output of the fuel level sensor according to the present invention. The circuit comprises a bank of resistors generally indicated A, a bank of Field Effect Transistors (FETs) generally

indicated B. two latches generally indicated C, a comparator D, a capacitor E, one diode À ' 20 F. one light emitting diode G. a dial gauge El and two further resistors I. The chain of À..

resistors are used to simulate a variable resistor with defined ohmic values such as that found in a swing-arm fuel level sensor. The resistors are switched in a binary arithmetic manner so that a small number of resistors can produce small ohmic charges at each switch action. For example, a chain of eight resistors represents an eight bit binary 25 counter which is capable of producing 255 steps. The sensor detects a change of fuel level by sensing a change of capacitance. The change in capacitance is translated into a digital signal proportionate to the fuel level. This signal is thus converted to a DC signal retaining the fuel level proportionality. The DC signal is compared with a ramped signal by the comparator to determine the appropriate numbers of resistors to be 30 switched in order that the driven gauge indicates the fuel level. The electronic circuit of Figure 3 is a preferred embodiment for the output circuit of the sensor and provides a pure resistive output.

s The bank of transistors B selectively include resistors A into the circuit in the following manner. For reference, the resistance value of each resistor is indicated by the number adjacent each resistor. The resistance value of 1 unit is included into the circuit by the 5 corresponding FET whilst the other seven resistors are bypassed by their corresponding FETs. The resistance value of 2 units is included into the circuit by its corresponding PET whilst the other seven resistors are bypassed by their corresponding FETs. The resistance value of 3 units is included into the circuit by the corresponding FETs to the 1 unit and 2 units resistance value whilst the other six resistors are bypassed by their 10 corresponding FETs. This method of binary counting is continued until the total resistance value of 255 units is achieved.

The electronic circuit of Figure 3 may be designed to operate with any one of the industry standards 240Q - 33Q, 10Q - 180Q, OQ - 90Q or 90Q - OQ for dial gauges A..

15 or any other resistance span. Since the electronic device is digital its operation is À...:.

independent of temperature.

Whilst a preferred embodiment for switching of the output resistor chain has been À. À disclosed using a capacitive sensor it will be appreciated that a number of alternative..

20 methods for sensing the fuel level may be used. For example radar, ultrasonics or reed...

switches. Whilst a preferred embodiment for the device has been described it will be appreciated that many other designs of the sensor and electronic circuit exist that would have the 25 desired effect of the invention with the proviso that the circuit has a resistive output...DTD:

Claims

1. Apparatus for sensing the level of a liquid in a vessel for holding liquid comprising a sensor to sense the liquid level and a digital electronic circuit wherein the 5 output from the sensor is converted to a pure resistive output of the electronic circuit.

2. Apparatus according to claim I wherein the sensor is a capacitance sensor.

3. Apparatus according to claim 2 wherein one electrode of the capacitor is a tube 10 and the other electrode is axially arranged therewithin, the tube having open ends such that in use the liquid within the vessel is an electrolyte between the electrodes whose level varies.

4. Apparatus according to any preceding claim wherein the electronic circuit has Àt..

15 a plurality of resistors arranged in series between the positive and negative sides of the..

circuit. . ,

5. Apparatus according to claim 4 in which the resistors are adapted to be selectively À.

combined in a binary manner to increase the range of resistive output..

.

6. Apparatus according to claim 4 or claim 5 wherein the electronic circuit has a plurality of electronic latches arranged in parallel with the series of resistors.

7. Apparatus according to claim 6 wherein the electronic latches comprise a plurality 25 of field effect transistors arranged in parallel with the series of resistors.

8. Apparatus according to claim 6 or claim 7 wherein the electronic circuit has a comparator arranged in parallel with the latches.

30

9. Apparatus according to claim 4 or claim S wherein the electronic circuit has an analogue to digital convertor.

10. Apparatus according to any preceding claim and further including a dial gauge to receive a pure resistive signal from the output of the electronic circuit.

1 1. Apparatus according to claim 10 wherein the electronic circuit has a light emitting 5 diode arranged in parallel with the dial gauge.

12. In combination a sensor, a fuel tank and a dial gauge according to any preceding claim.

10

13. A method of sensing the level of liquid in a vessel, comprising the steps of i) using a sensor having an output signal to detect the level of liquid in the vessel; ii) using a digital electronic circuit to convert the output signal of the sensor into a pure resistive output.

A. ' 15

14. A method according to claim 14, further comprising the step of iii) using the..

resistive output to drive a gauge.

15. A method according to claim 14 or 15 in which the digital circuit comprises a À series of resistors..

20.

16. A method according to claim 16 in which the resistors are adapted to be combined in a binary manner to increase the range of resistive output.

17. Apparatus for sensing liquid level constructed and arranged substantially as 25 described herein with reference to the accompanying drawings.