GB2411474A

GB2411474A - A liquid level sensing device

Info

Publication number: GB2411474A
Application number: GB0404139A
Authority: GB
Inventors: William George Jones
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2004-02-25
Filing date: 2004-02-25
Publication date: 2005-08-31
Also published as: GB0404139D0

Abstract

A device for sensing the level of fuel in a fuel tank 10 is disclosed. The device is operable to vary the displacement of a float support member 24, by a distance that is different to the actual change in distance between an upper and lower surfaces 7 and 8 of the fuel tank. In a preferred embodiment two spaced apart gearwheels are used, each of which is rotatably supported by the float support member and is engaged with teeth formed on a fixed support arm 30 and teeth formed on a moving compensation arm 50. In such an arrangement the actual movement of the compensation arm is twice that of the movement of the float support member.

Description

241 1 474

A LIQUID LEVEL SENSING DEVICE

This invention relates to the sensing of the level of a liquid in a reservoir and in particular to the sensing of the level of fuel in a motor vehicle fuel tank.

It is well known to provide a motor vehicle with a mechanism for sensing the level of the fuel in a fuel tank fitted to the motor vehicle.

The majority of passenger vehicle fuel level indication systems are just that, they only indicate the remaining level of fuel in the tank, regardless of the technology used to sense it. However, factors may combine to give the operator the impression that the indicating gauge is showing a near absolute value for the contents of the tank. For example, some gauges display volumetric values (litres, gallons) associated with the fractional indicators on analogue gauges or some use displays with digital readings.

The introduction of blow moulded plastic fuel tanks has further complicated matters. The traditional pressed steel fabricated fuel tank was reasonably stable in shape and suffered only slightly from deformation due to fuel vapour ?5 pressurization of the tank. The introduction of plastic tanks has led to problems with the stability of the tank shape in both manufacture and use. For example, moulding process variability affects the wall thickness and tank depth and these affect the resulting volume of the as-new 0 tank.

Plastic tank stiffness is also less than that of a steel tank, this leads to greater deflection under pressurization and, as the tank ages, especially with monolayer, rather than multilayer plastic tanks, the absorption into the tank polymer of fuel (gasoline) hydrocarbons, leads to a swelling of the tank which may have a measurable effect on the volume of the tank.

One of the ways in which designers have tried to accommodate the variability of the plastic tank is to provide a means to reference the fuel level sensor to the bottom face of the fuel tank. This bottom referencing is able to ensure that the operator always sees a representation of the fuel left in the tank and will help to avoid unexpected run-out of fuel.

An example of a bottom referenced fuel level sensor is shown in Fig.1 and is described in detail hereinafter, suffice it to say that such an arrangement suffers from the disadvantage that the device used to sense the level of the fuel in the tank has to be set low in the fuel tank in order to prevent a large volume of fuel remaining in the fuel tank when the fuel sensor indicates empty and this results in the float associated with the fuel sensor often being submerged by a large amount when the fuel tank is full. The mechanism used will therefore produce an indication of no fuel usage initially because as the housing moves up it merely reduces the amount by which the float is submerged until the float reaches the surface of the fuel or contacts the upper surface of the tank and the will then be an indication of rapid fuel usage as the fuel level falls and the housing rises.

It is an object of the invention to provide a liquid sensing means which provides improved performance in a cost effective manner.

According to a first aspect of the invention there is provided a liquid level sensing device for measuring the 3tj level of a liquid in a reservoir having an upper surface and a lower surface, a support arm fixedly attached to one of the upper and lower surfaces, a float support member moveably connected to the support arm for relative motion therebetween, a float arm pivotally supported by the float support member for rotation about a pivot axis, the float arm having a float fastened to a free end thereof for movement in relation to the level of fluid in the reservoir so as to move the float arm accordingly, a sensor means connected to the float arm and arranged to provide an output based upon the relative position of the float with respect to the pivot axis of the float arm and a compensation arm lo operatively connected to the other of the upper and lower surfaces of the reservoir and connected to the float support member so as to move the float support member relative to the support arm when the one of the upper and lower surfaces moves relative to the other of the upper and lower surfaces is wherein the compensation arm is connected to the float support member such that it moves the float support member relative to the support arm a different distance to any actual change in distance between the upper and lower surfaces.

The sensing device may be a fuel level sensor mechanism.

The compensation arm may be operatively connected to the other of the upper and lower surfaces of the reservoir by being attached to the respective surface of the reservoir.

Alternatively, the compensation arm may be operatively connected to the other of the upper and lower surfaces of the reservoir by being biased into engagement with the other of the upper and lower surfaces of the reservoir Preferably, the support arm may be fixedly attached to the upper surface of the reservoir and the compensation arm is operatively connected to the lower surface of the reservoir.

The compensation arm may be connected to the float support member by means of at least one wheel rotatably supported by the float support member and driveably connected to the compensation arm.

The support arm may be connected to the float support member by means of at least one wheel rotatably supported by the float support member and driveably connected to the lo support arm.

The compensation arm and the support arm may both be driveably connected to at least one common wheel.

There may be at least two common wheels each having an axis of rotation that is offset with respect to the axis of rotation of the or each of the other of the common wheels.

Alternatively, the compensation arm may be driveably connected to a first wheel and the support arm is driveably connected to a second wheel, the first and second wheels having a common axis of rotation.

The first and second wheels may form a gear pair.

The diameter of the first wheel may be different to the diameter of the second wheel.

There may be more than one gear pair interposed between the compensation arm and the support arm, each having an axis of rotation that is offset with respect to the axis of rotation of the or each of the other of the gear pairs.

The driveable connection between the compensation arm and the or each wheel may be by means of a geared drive, the compensation arm having a number of teeth formed thereon for engagement with complementary teeth formed on the or each wheel.

The driveable connection between the support arm and the or each wheel may be by means of a geared drive, the support arm having a number of teeth formed thereon for engagement with complementary teeth formed on the or each wheel.

lo The sensor means may be one of a variable resistor, a potentiometer and a variable capacitance device.

According to a second aspect of the invention there is provided a fuel tank for a motor vehicle having a liquid level sensing device in accordance with said first aspect of the invention.

The invention will now be described by way of example with reference to the accompanying drawing of which:

Fig.1 is a cross-section through a prior art fuel

tank showing a prior art fuel level sensor mechanism; Fig.2 is a crosssection similar to that of Fig.1 but showing a fuel level sensor mechanism according to a first embodiment of the invention; Fig.3 is an enlarged cross section through part of the fuel level sensor mechanism shown in Fig.2; Fig.4 is a diagrammatic representation of a second embodiment of a fuel level sensor mechanism according to the invention; Fig.5 is a composite diagrammatic representation of the fuel level sensor mechanism shown in Fig.4 indicating how the mechanism compensates for deflection of the bottom of the fuel tanks flg.b is a diagrammatic representation of a third embodiment of a fuel level sensor mechanism according to the invention; Fig.7A is an end view of a gear pair according to the invention; and Fig.7B is a side view of the gear pair shown in Fig.7A.

With reference to Fig.1 there is shown a prior art

plastic fuel tank 10 containing fuel up to a level 12 and with a fuel level sensor mechanism 14 positioned in the tank 10. The sensor mechanism 14 is introduced into the tank through an aperture 16 in the upper surface 7 of the tank 10 and this aperture 16 is subsequently closed by a closure plate 18.

The sensor mechanism 14 has a float 20 mounted at the end of a float arm 22. The sensor mechanism 14 also has a float support member in the form of a housing 24 which contains a resistor card 26 forming a variable resistance 2, sensor means. The float arm 22 is pivotally connected to the housing 24 for rotation about a substantially horizontal axis.

As the fuel level 12 in the tank varies, the float 20 will follow the changing fuel level and the arm 22 will rotate, causing electrical contacts which move with the arm 22 to move relative to the resistor card 26 which has a resistive track 32 formed thereon. As a result, a varying electrical output will be sent along electrical conductors (not shown) to a remote fuel level indicating device (not shown). /1 11

The housing 24 is slidingly supported on a support arm or track 30 which is attached to the upper surface 7 of the fuel tank 10 by means of a support bracket 29.

The housing 24 has a compensation arm extending downwardly therefrom for engagement with an attachment device 25 fixed to the lower surface of the tank 10. The compensation arm is an interference fit in the attachment device 25 and so once engaged the housing 24 will move with lo the lower surface 8 of the tank 10.

This type of device is referred to as bottom referenced because the housing 24 is linked to the lower surface 8 of the tank 10. Instead of being fixed the compensation arm could be merely biased against the lower surface.

In order to ensure that a reliable output is obtained from the sensor mechanism 14 the housing 24 must be positioned relatively low in the tank 10 to allow for the fact that it will be moved upwards when the tank 10 empties.

This is because the lower surface 8 of the tank 10 will bow downwards by perhaps 20 mm when subject to a full load of fuel but will return to a higher position as the fuel load lightens.

Unfortunately, and as previously discussed, this results in the float 20 being submerged when the tank 10 is full and so is unable to provide any measurement of change in fuel level until the float reaches the surface of the fuel or contacts the upper surface 7 of the fuel tank 10.

However, because the housing 24 is connected directly to the lower surface 8 of the tank 10, it will move rapidly upward as fuel is used from the full tank condition and so, if the float 20 is in contact with the upper surface 7 or is floating on the fuel surface, the sensor means formed by the resistor card 26 will indicate a rapid use of fuel.

With reference to Figs. 2 and 3 there is shown a first embodiment of a fuel level sensing mechanism 14 according to the invention. The mechanism 14 is similar to that previously described and is fitted in the plastic fuel tank 10 in the same manner through the aperture 16 in the upper surface 7 of the tank 10.

The sensor mechanism 14 has, as before, the float 20 mounted at the end of the float arm 22 and has the float lo support member in the form of a housing 24 which contains the resistor card 26 forming the sensor means slidingly supported on the support arm 30 and, as before, the support arm 30 is attached to the upper surface 7 of the fuel tank by means of the support bracket 29. As before, the float arm 22 is pivotally connected to the housing 24 and movement of the float 20 relative to the pivot axis of the float arm 22 is used to provide an indication of a change in the level 12 of the fuel in the fuel tank 10.

However, in this case, the housing 24 does not have a compensation arm extending downwardly therefrom for engagement with an attachment device 25 fixed to the lower surface of the tank 10. Instead the housing 24 is slidingly engaged with a compensation arm 50 which is an interference fit in the attachment device 25 and moves with the lower surface of the tank 10.

The compensation arm 50 has a number of teeth 51 formed thereon which form a gear rack. The teeth 51 of the gear rack are engaged with complementary teeth 42 formed on a gearwheel 40. The gear wheel 40 is rotatably supported by a shaft 41 which is engaged with apertures (not shown) in the housing 24 and is also engaged with teeth 31 formed on the support arm 30. It will be appreciated that a friction drive could also be used but that a geared drive provides greater certainty of motion as there is no risk of slippage.

As shown in Fig.3, the housing 24 has a first guideway 24a which is slidingly engaged with the support arm 30 and a second guideway 24b in which is slidingly engaged the compensation arm 50. In this way the housing 24 is prevented from rocking relative to the support arm 30 and the compensation arm 50 is stably supporting for sliding motion relative to the housing 24.

It will be appreciated that with such an arrangement lO the vertical movement of the housing 24 relative to the support arm 30 is half of the displacement of the lower surface 8.

This is because XT = Xl + X2 or X2 = XT - Xl where: XT = vertical displacement of the compensating arm 50; Xl = vertical displacement of compensation arm relative to axis of rotation of the gearwheel 40; and X2 = vertical displacement of housing 24 relative to the support arm 30.

Because the support arm 30 and the compensation arm 50 are engaged with the same gearwheel 40 then the angle of rotation of the gearwheel 40 is the same and so Xl = X2.

Therefore if XT = 20mm, Xl will be lOmm and X2 will be lOmm.

This allows the housing 24 to be mounted higher in the fuel tank lO because the housing 24 will only travel upwards half of the distance of the lower surface 8 and so there is a reduced risk of the housing 24 moving so far from the lower surface 8 that a large amount of fuel will remain in the fuel tank lO when the float arm 22 reaches the lower limit of the track 32 on the resistor card 25. This means reran that the float 20 need not be submerged when the tank is full but, in an ideal case, can rest upon the upper surface 7 of the tank 10. Therefore, as soon as fuel is used, a use of fuel signal can be immediately provided because the upward movement of the compensation arm 50 due to the use of fuel will result in an upward movement of the housing 24.

This upward movement of the housing 24 will cause the float arm 22 to rotate relative to the housing 24 because the float 20 is abutting the upper surface 7 of the tank 10 and JO so cannot move upwards. Consequently, the float arm 22 moves along the track 32 and a change in output is provided to the remote fuel level indicating device (not shown).

Because of the ratio between the compensation arm 50 l5 and the housing 24, the rate of fuel usage is more representative of the actual fuel usage than is the case if the housing 24 is attached directly to the lower surface 8.

It will be appreciated that as the support arm 30 is fastened to the upper surface 7 of the tank 10 and the compensation arm 50 is operatively connected to the lower surface 8 of the tank 10, the actual change in distance between the upper and lower surfaces 7 and 8 of the tank is the same as the relative displacement between the compensation arm 50 and the support arm 30. Therefore, as the housing 24 moves a different distance relative to the support arm 30 than the compensation arm 50, then the housing 24 can be said to move relative to the support arm a different distance to any actual change in distance between the upper and lower surfaces 7 and 8.

Although the invention has been described with respect to a sensor means using a resistor card 26 having a resistance track 32 thereon it will be appreciated that it is not so limited and that other types of sensor such as a potentiometer or capacitance device whether of a linear or rotary form could be used. It will also be appreciated that the compensation arm could be biased against the lower surface rather than being attached to it.

With particular reference to Figs. 4 and 5 there is shown a second embodiment of the invention which in many respects is similar and for which the same principle of operation is used.

However, in this case, instead of there being a single gearwheel interposed between the support arm and the compensation arm there are two separate gearwheels G1 and G2 which have axes of rotation which are offset with respect to one another.

The two gearwheels G1 and G2 are rotatably supported by a housing 124 to which a float arm 22 is pivotally connected for rotation about a pivot point 'P'. The float arm 22 has, as before, a range of travel 'R' between a notional full or upper position and a notional empty or lower position.

The two gearwheels G1 and G2 are both driveably engaged with a fixed support arm 130 and with a moving compensation arm 150. Therefore, as before, the displacement XT of the compensation arm 150 relative to the support arm 130 is double that of the corresponding relative displacement between the housing 124 and the support arm 130.

Operation of the fuel level sensor mechanism will not be described again because it is identical to that previously described with respect to the first embodiment.

Fig.5 shows a fuel tank having upper and lower surfaces 7 and 8 and indicates in a graphical manner the operation of the fuel level sensor mechanism. On the left hand side there is shown the situation when the fuel tank is full and on the right hand side there is shown the situation when the fuel tank is nearly empty.

"A" is the empty float height of the tank assembly) "B" is the change in bottom surface 8 position) "C" is the new float height as compensated for by the mechanism according to the invention) and "D" is the change in height of the housing 124 and 0 hence the change in height of the lower limit of movement of the float due to the action of the mechanism according to the invention.

It can clearly be seen that the distance "D" on Fig.5 is less that the distance "B" and in fact "D" is half that of "B" using the mechanism described above.

Once again the mechanism provides improved full tank performance while reducing any potential loss of empty tank performance.

In the two embodiments described above the support arm and the compensation arm are both engaged with a common gearwheel and so the ration X2:XT is 0.5. However, this need not be the case and the support arm 30, 130 and the compensation arm 50, 150 can be engaged with gearwheels of differing diameter fastened together for common rotation.

Figs. 7A and 7B show a gear pair 300 which could be used in such an arrangement. The gear pair 300 has a first gear wheel 301 for engagement with the compensation arm 50, and a second gearwheel 302 for engagement with the support arm 30, 130. Both of the gear wheels 301, 302 are driveably connected to a common shaft 303 so that they both rotate the same amount and have a common axis of rotation.

It will be appreciated that the interconnection between the gearwheels 301, 302 and the two arms 30, 130, 50, 150 could be reversed so that the gear wheel 301 is engaged with the support arm 30, 130 and the second gearwheel 302 is engaged with the compensation arm 50, 150 but for the purpose of explanation the first arrangement will be used as

an example.

The first gearwheel 301 has an effective diameter D1 lo and the second gearwheel 302 has an effective diameter D2.

As before the relative motion X2 of the housing 24, 124 relative to the support arm 30, 124 will be equal to X2 = XT - X1.

Assuming that the movement X1 of the compensation arm 50, 150 relative to the axis of rotation of the gearwheels 301, 302 produces an angular rotation of the shaft 303 equal to then it can be shown that: X1 (3.142 * D1) * and X2 (3.142 *D2) * as is the same for both of the gearwheels 301, 302 then X1/D1 = X2/D2 and consequently X1 = X2 *(D1/D2).

Therefore XT = X2 + X2*(D1/D2) So if D1 = 25mm and D2 = 15mm XT = X2 + X2 *(25/15) = 2.66X2 and the ration X2/XT = 0.375.

That is to say the housing 24, 124 will only move 0.375 mm relative to the support arm 30, 130 (and hence relative to the upper surface 7) for every mm of movement of the compensation arm relative to the support arm 30, 130.

Therefore if the lower surface 8 of the tank moves up 20 mm the housing 24, 124 will only move up 7.5mm.

It will be appreciated that various alternative ratios could be used depending upon the particular fuel tank and the position of the compensation arm within the fuel tank.

lo It will be appreciated that the lower surface of the fuel tank will not move a uniform amount and that therefore the actual position at which movement of the lower surface is sensed will effect the operation of the mechanism. By carrying out experimental work in which the actual deflection of the lower surface at the position where the deflection is to be sensed by the compensation arm it is possible to determine a ratio which produces the best full and empty tank performance.

It will be further appreciated that various alternative gear arrangements could be used to provide a similar ratio effect between the movement of the housing and the movement of the lower surface of the fuel tank.

With reference to Fig.6 there is shown a third embodiment of the invention which uses a lever arrangement rather than a gear arrangement to provide the required ratio.

A fuel tank 200 has an upper surface 207 and a lower surface 208. A support arm 230 is attached to the upper surface 207. The support arm 230 slidingly supports a moveable float support member in the form of a housing 224 to which is pivotally attached a float arm 222 having a float 220 at a free end thereof. The housing 224 supports a rotary potentiometer (not shown) which is connected to the inner end of the float arm 222 so that rotation of the float arm 222 will produce a change in the output from the potentiometer.

The housing 224 is moveable relative to the support member 230 by a compensation arm 250 which has one end operatively connected to the lower surface of the fuel tank by being biased there against by a spring (not shown), although the compensation arm could alternatively be attached to the lower surface.

The compensation arm 250 is connected to the housing 224 by a lever system such that vertical movement of the compensation arm 250 moves the housing 224 up and down the support arm 230.

Because of the lever arrangement employed the housing will only move approximately 50% of the distance relative to the support arm 230 that the compensation arm 250 moves.

This is because the connection of the housing 224 by means of a strut 252 to a cross beam 251 is at a radius 'rl' from the pivot axis of the cross beam 251 and the position of connection of the compensation arm 250 to the cross beam 251 is at a distance 'rl' +'r2' where 'rl' = 'r'2.

If the lengths 'rl' and 'r2' are changed differing ratios can be produced.

In general the ratio of housing movement relative to lower surface movement is proportional to the distance of the positions of connection of the strut 252 and the compensation arm 250 to the cross beam 251.

As before this ratio between the movement of the housing 224 relative to the support arm 230 and the actual movement of the lower surface is used to provide the fuel level sensor mechanism with an improved performance compared to a fixed linkage between the lower surface of the tank and the housing.

Although the invention has been described with reference to embodiments in which the compensation arm is operatively connected to the lower surface of the tank it will appreciated that it could also be used to sense the motion of the upper surface of the tank if required by fixing the support arm to the lower surface of the tank and Jo operatively connecting the compensation arm to the top surface of the tank.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to a number of specific embodiments it is not limited to these embodiments and that various alternative embodiments or modifications to the disclosed embodiments could be made without departing from the scope of the invention.

Claims

Claims 1. A liquid level sensing device for measuring the level of a

liquid in a reservoir having an upper surface and a lower surface, a support arm fixedly attached to one of the upper and lower surfaces, a float support member moveably connected to the support arm for relative motion therebetween, a float arm pivotally supported by the float support member for rotation about a pivot axis, the float lo arm having a float fastened to a free end thereof for movement in relation to the level of fluid in the reservoir so as to move the float arm accordingly, a sensor means connected to the float arm and arranged to provide an output based upon the relative position of the float with respect to the pivot axis of the float arm and a compensation arm operatively connected to the other of the upper and lower surfaces of the reservoir and connected to the float support member so as to move the float support member relative to the support arm when the one of the upper and lower surfaces moves relative to the other of the upper and lower surfaces wherein the compensation arm is connected to the float support member such that it moves the float support member relative to the support arm a different distance to any actual change in distance between the upper and lower surfaces.
2. A sensing device as claimed in claim 1 wherein the sensing device is a fuel level sensor mechanism.
3. A sensing device as claimed in claim 1 or in claim 2 wherein the compensation arm is operatively connected to the other of the upper and lower surfaces of the reservoir by being attached to the respective surface of the reservoir.
4. A sensing device as claimed in claim 1 or in claim 2 wherein the compensation arm is operatively connected to the other of the upper and lower surfaces of the reservoir by being biased into engagement with the other of the upper and lower surfaces of the reservoir.
5. A sensing device as claimed in any of claims 1 to 4 wherein the support arm is fixedly attached to the upper surface of the reservoir and the compensation arm is operatively connected to the lower surface of the reservoir.

lo
6. A sensing device as claimed in any of claims 1 to wherein the compensation arm is connected to the float support member by means of at least one wheel rotatably supported by the float support member and driveably connected to the compensation arm.
7. A sensing device as claimed in any of claims 1 to 6 wherein the support arm is connected to the float support member by means of at least one wheel rotatably supported by the float support member and driveably connected to the support arm.
8. A sensing device as claimed in claim 7 when dependent upon claim 6 wherein the compensation arm and the support arm are both driveably connected to at least one common wheel.
9. A sensing device as claimed in claim 8 wherein there are at least two common wheels each having an axis of rotation that is offset with respect to the axis of rotation of the or each of the other of the common wheels.
10. A sensing device as claimed in claim 7 when dependent upon claim 6 wherein the compensation arm is driveably connected to a first wheel and the support arm is driveably connected to a second wheel, the first and second wheels having a common axis of rotation. r) rN
11. A sensing device as claimed in claim 10 wherein the diameter of the first wheel is different to the diameter of the second wheel.
12. A sensing device as claimed in claim 10 or in claim 11 wherein the first and second gearwheels form a gear palr.
lo13. A sensing device as claimed in claim 12 wherein there is more than one gear pair interposed between the compensation arm and the support arm, each having an axis of rotation that is offset with respect to the axis of rotation of the or each of the other of the gear pairs.
14. A sensing device as claimed in any of claims 6 to 13 wherein the driveable connection between the compensation arm and the or each wheel is by means of a geared drive, the compensation arm having a number of teeth formed thereon for TO engagement with complementary teeth formed on the or each wheel.
15. A sensing device as claimed in any of claims 6 to 13 wherein the driveable connection between the support arm and the or each wheel is by means of a geared drive, the support arm having a number of teeth formed thereon for engagement with complementary teeth formed on the or each wheel.
16. A fuel tank for a motor vehicle having a liquid level sensing device as claimed in any of claims 1 to 15.
17. A liquid level sensing device substantially as described herein with reference to the accompanying drawing.
18. A fuel tank for a motor vehicle substantially as described herein with reference to the accompanying drawing.