WO2007049005A1

WO2007049005A1 - Optical fluid level detector

Info

Publication number: WO2007049005A1
Application number: PCT/GB2006/003874
Authority: WO
Inventors: Roger John Hazelden; Robert Andrew Pinnock
Original assignee: Trw Limited
Priority date: 2005-10-26
Filing date: 2006-10-19
Publication date: 2007-05-03
Also published as: GB0521800D0; EP1941248A1

Abstract

An optical liquid level sensor comprises a source of optical radiation, an optical detector sensitive to the optical radiation, and at least one optical element forming a surface which is positioned within a container and in which the light source directs radiation towards the surface to form a pattern which is dependent upon the level of the fluid the detector comprises an array of M detecting elements, where M is greater than 1, which elements capture an image of the pattern of radiation formed by the source upon the surface whereby the level of the fluid can be determined by analysing the image captured by the detector array. The device may analyse from the pattern any meniscus formed between surface and floor and from this determine the condition of the fluid.

Description

OPTICAL FLUID LEVEL DETECTOR

This invention relates to an optical fluid level detector. It in particular but not exclusively relates to a level detector capable of providing information about the level of fluid in a container. It is envisaged that the invention may be especially suited to detecting the level of fuel in a vehicle fuel tank, or perhaps the level of oil or brake fluid in a reservoir or sump.

Many types of level sensor are known. One particular type employs a light source from which light is transmitted, a detector for detecting the transmitted light, and an optical element which defines an optical path along which the light from the source can be carried to the detector. The element includes at least one surface from which the light is reflected when in contact with air towards the detector. Similarly, when in contact with the liquid total internal reflection will not occur and the light passes through away from the detector. This face is located above the lowest level of the tank and below a level that the fluid will reach as the container is filled. The change from reflection to transmission will occur as the level in the container is varied, which in turn changes the intensity of light reaching the detector, and hence the voltage output from the detector. This intensity can therefore be used to provide information about the level of the fluid.

In an alternative, two elements are provided which are spaced apart to define a gap which can be filled by the fluid as the level rises. The elements are arranged to guide light from one to the other across the gap and on to the detector when the fluid is present. When the level is lower than the gap, total internal reflection again occurs sending light away from the detector. As with the fjLrst mentioned design this changes the intensity of light reaching the detector to provide information on fluid level.

According to a first aspect of the invention there is provided an optical liquid level sensor comprising: a source of optical radiation; an optical detector sensitive to the optical radiation; and at least one optical element forming a surface which is positioned within a container and in which the light source directs radiation towards the surface to form a pattern which is dependent upon the level of the fluid; characterised in that the detector comprises an array of M detecting elements, where M is greater than 1, which elements capture an image of the pattern of radiation formed by the source upon the surface whereby the level of the fluid can be determined by analysing the image captured by the detector array.

The surface may be a transmissive surface, by which we mean an interface through which at least some of the light from the source passes on its way to the detecting elements, or a reflective surface by which we mean a surface from which at least some of the light is reflected on its way to the detecting elements. It may be the external surface of the optical element, or an internal surface. The level of fluid will alter the pattern of light that passes through the surface or is reflected from the surface.

The surface may be positioned within the container so that different portions become submerged as the fluid level rises.

By providing a detector array which captures an image of pattern of light on the surface, either of the light reflected from it or transmitted through it, the level can be calculated without the need to calibrate for the intensity of the light source provided that the relative illumination of the overall surface is not varied. This provides a considerable and previously unforeseen benefit when compared with prior art systems that use a single photodetector and measure a single intensity value.

The reflecting surface may comprise an external face of the optical element or it may comprise an internal face.

The surface may be viewed directly by the detector such that it directly images the pattern. Alternatively, the pattern may itself be reflected in another surface before it is incident upon the detector.

The detector may comprise a linear array of detector elements, by which we mean a row of detector elements. There may be any number of elements greater than one, but a most preferred arrangement could use at least 16, at least 32, preferably at least 64 and most preferably 128 elements .

Alternatively, the detector array may be a two dimensional array. For example, it may comprise a rectilinear grid of N * M elements, where N and M are greater than 1 (the linear array can be considered to be such a grid where N = I) .

A lens may be provided between the surface on which the pattern is to be observed and the detector array. The lens may be positioned such as to focus an image of the pattern reflected from (or passing through) the reflective surface onto the detector array. This allows the array to "see" the pattern of light from the reflector. It may be a fresnel lens, which can conveniently be of plastic or glass material. It may be moulded to shape, which is convenient as it is low cost compared with a conventional ground lens. This is possible in this application since the quality of the image need not be that high.

It may be a cylindrical or rod lens which focuses the light into a one- dimensional strip that is imaged by the detector array.

The lens may be part of the optical element, e.g. it may be molded into a face of the optical element.

An electronic processing means may be provided which is adapted to process the output from the detector elements of the detector array. It may be adapted to identify the position along the array at which the light incident. on the array changes from high intensity to a lower intensity. This will, depending on the design, correspond to the level at which the fluid reaches up the optical element.

For example, each element of the array may produce an output voltage or current having a magnitude that is a function of the intensity of light incident upon it. The processing means may determine the point along the array at which the voltage or current magnitude changes corresponding to the level of light.

An abrupt transmission from high to low magnitude output from the detector elements may occur, and this can be recorded as the level of the fluid. A knowledge of the geometry of the apparatus enables this measurement to be made.

Several refinements are envisaged. In one refinement, a number of measurements may be taken at different times and the average measurement of level taken. This may, for example, allow changes in level that occur when a container of fluid is moving to be compensated. An example of this is application to a vehicle fuel tank in which the fluid will slop around as the vehicle accelerates, decelerates or travels around corners .

The number of samples used to form the average and/or the time between each sample may be varied. When used to measure the level of fluid in a fuel tank of a vehicle they may be varied according to the operating state of the vehicle. If the vehicle is stationary less samples and/or shorter time intervals between them may be used so that a quick reading is obtained. Alternatively, if the vehicle is in motion or accelerating or cornering more samples or a longer interval between samples could be used.

The processing unit may select the time intervals/number of samples automatically in response to information about the vehicles state. It could take information from various sensors to do this, such as an ignition sensing wire, vehicle speedometer, ABS sensors, yaw sensors etc.

To allow for the effect of travelling up a hill, a very large number of samples or interval could be used to give an average over, say 5 or more minutes or perhaps a given distance travelled. This will prevent false readings being taken on a gradient.

Whilst a simple measure of level based on identifying the transition from light to dark in the pattern of light on the surface has been described a more detailed analysis could be performed that may provide additional information.

In one example, a detailed analysis of the sharpness of transition from light to dark could be made. This transition may be blurred due to the meniscus that will form at the interface between the fluid and the surface on which the pattern is formed. Changes in the nature of this transition determined by analysing the output of the detector elements in the region of the transitioned may be used to determine the shape of the meniscus and/or its size and hence determine one or more properties of the fluid. This could therefore be used to determine the condition of the fluid, such as an engine oil by determining its viscosity. Protection is sought through this application for such condition monitoring.

Several different types of optical element may be used as follows. It is to be noted that in each case, a surface is provided on which a pattern of light can be imaged by the detector array and which gives information about the fluid level.

In a first arrangement, the optical element may. comprise a diffuse reflecting surface imaged by the detector array. In a simple example, this could be a planar surface that is roughened and extends down into the fluid container with the detector imaging a strip along its length.

Where the reflecting surface is an internal surface, it may comprise an internal surface of an optical element arranged to provide total internal reflection of light from the source, onto the detector when in air yet to transmit light away from the detector out of the element when the surface is submerged in the fluid. This works best if the refractive index of the material is matched to that of the fluid, and if the angle of incidence of the light on the surface is chosen to allow total internal reflection when in air.

Where the surface is an internal surface, an additional reflective surface may be provided between the light source and that surface. This additional surface may be oriented to reflect light evenly from the source onto the surface and may therefore be diffuse. It may be coated with a diffuse reflective coating or may, for example, be roughened.

The optical element may therefore comprise a prism and the two reflecting surfaces may be provided by opposite faces of the prism. This may have a wedge shape having an upper face where the light source and detector array are positioned and comprising opposite planar reflecting faces that taper towards one another away from the upper face. The angle of the two faces should be chosen to reflect light emitted by the source from one face to the other and then back to the source. When the wedge is submerged the total internal reflection will cease and light will transmit out of the wedge.

In a similar arrangement the optical element may comprise a conical element which narrows in diameter towards the end furthest from the source/detector array may be provided. The reflecting surface will be made up of pairs of locations on the surface which are located diametrically opposite the axis of revolution of the conical element.

There may be two optical elements, each having an optically transmissive face spaced from a corresponding optically transmissive face on the other to define a gap which may contain the fluid. The light may be adapted to pass through the gap when the faces are index matched by fluid in the gap, but otherwise be prevented from passing through by total internal reflection.

Where two elements are provided which are spaced to provide the gap, they may each comprise a wedge shaped element. The reflecting surface, or another surface which either reflects or transmits light in the path from light source to detector array may have a stepped profile.

There will now be described, by way of example only, various embodiments of the present invention with reference to the accompanying drawings of which:

Figure l(a) is a plan view of a first embodiment of a liquid level detector in accordance with a first aspect of the invention

Figure l(b) is a graph showing the output of the detector elements of the embodiment of Figure l(a) at a first, low, level of fluid;

Figure l(c) is a graph showing the output of the detector elements of the embodiment of Figure 1 at a second, higher, level of fluid;

Figure 2 is a plan view of a second embodiment of a liquid level detector in accordance with a first aspect of the invention

Figure 3 is a plan view of a third embodiment of a liquid level detector in accordance with a first aspect of the invention;

Figure 4 is of a fourth embodiment of a liquid level detector in accordance with a first aspect of the invention; and

Figures 5 (a) and 5(b) illustrate the pattern of light imaged on the detector array for (a) sharp focusing optics and a very viscous fluid and (b) either lower quality focusing optics or a more viscous fluid.

Example 1 As shown in Figure l(a) , an optical level detector comprises of two optical elements 110, 120, each of which comprises a transparent prism of plastic or glass, that are positioned adjacent one another and extend downwards from an upper surface of a fluid container 130 such as a vehicle fuel tank. A horizontal bottom face 111, 121 of each of the prisms

110, 120 is located towards the bottom of the container 130, and two planar, vertically extending, faces 112, 122 of the prism are positioned adjacent one another to define an elongate vertically extending channel 140.

A sloping side 113,123 (by which we mean non-vertical) of each prism opposing the vertically aligned faces 112,122 extends downwardly at a non-vertical orientation such that the top of each prism is wider than the base. The sloping side 113,123 of one of the prisms is coated with a layer of diffuse material which, as will become apparent, makes the surface reflect diffusely.

A light source 150, such as a light emitting diode, is positioned above a top face of the one of the prisms 119. The source emits light which may be visible or infra-red, and which is guided into the prism 110 through a diffuser (not shown) in such a way that the light is substantially uniform in its illumination of the sloping face of the prism 110. This light is then reflected from the sloping face onto the vertical face on the other side of the prism 110.

The other prism 120 is provided with a reflective coating on its sloping face 122 which provides specular reflection. It may, for example, be silvered to provide a mirrored surface. A detector array 160 is located above this prism 120 which receives light reflected from the mirrored surface. This will correspond to light that has passed across the gap from the first prism 110. A fresnel lens (not shown) of moulded plastic is provided between the detector array 160 and the second prism 120 to guide any light that exists the prism onto the detector array such that the detector sees an image of the light reflected from the mirrored surface. By suitable geometry and component sizing, the array will capture an image along its length of a corresponding linear strip of the entire mirrored surface. The lens may focus the surface of the reflector onto the array, but it could work without it being in focus. In an alternative, a cylindrical or rod lens could be used to focus the whole of the surface down into a thin strip that is imaged by the detector array. This has the advantage that light from the whole surface is focused onto the array.

In the example shown, the detector array 160 is a one dimensional array of detecting elements or pixels. That is that the array is a group of pixels arranged on next to the other in a line. In a preferred arrangement the array consists of 128 detection pixels and associated read-out circuitry. Other arrays could of course be used. They may have more or less pixels, and they may be two dimensional rather than all arranged in a line. A grid of pixels could be provided.

An electronic unit, not shown, drives the light source and receives the output from each of the detector elements of the detector array. An image is captured by reading out all of the values of the pixels at a given time and storing the values in an electronic memory (not shown) . This electronic unit then processes the images seen by the array to determine the level of the fluid.

In use, fluid in the tank such as petrol or diesel will enter the space 140 between the two prisms 110,120. It will fill the space up to a level dependent upon the level of the fluid in the container, with air occupying any remaining unfilled space. Above the level of the fuel, light will be transmitted poorly across the space 140 between the two prisms 110,120 because the refractive index of plastic or glass differs greatly from that of air. The output from those detector elements of the array that are imaging the corresponding region of the mirrored surface will therefore be low. On the other hand, below the level of the fluid the amount of light transmitted will be greater as the fuel acts to index match the two facing surfaces of the prisms 110,120 . The output of the detector elements imaging the part of the mirrored surface corresponding to regions below the surface will therefore be higher.

The image seen by the one-dimensional array for different fuel levels can be seen in Figures l(a) and l(b) . In effect the image corresponds to the pattern of light that is seen on the sloping surface of the prism 120, which in turn depends on the pattern of light seen to pass across the space 140. To determine the fuel level, the one dimensional image is analysed to determine the centre of the region where the image shows a transition from dark to light.

Figure l(b) shows the pattern of light imaged by the detector at a low fluid level, and Figure l(c) shows the corresponding pattern of light at a higher level.

This apparatus has an advantage over a prior art technique of simply looking at the overall intensity of light that is incident upon a single detector. There is, for example, no need to calibrate the intensity of the output of the LED. Also, any errors due to dirt on the reflectors or damage to the reflectors can be identified and taken into consideration.

The image may or may not include a sharp transition from light to dark which indicates the level. Instead it may see a gradual transition. This will be the case if the reflector of the other prism is not completely specular or if the focusing of the lens is imperfect.

Example 2

An alternative arrangement is shown in Figure 2 . This device again has two optical elements 210, 220 which each comprise a prism. As with the embodiment of Figure 1, they are again positioned one next to the other such that a gap 240 is formed between two facing surfaces of the prisms. In this case, rather than the faces defining the gap being parallel and vertical, both prisms define a surface which extends downwardly and inwardly so that the gap 240 narrows from top to bottom. The facing surface of the one prism 210 is coated with a diffuse reflecting surface. A light source 240 illuminates this surface from above, and the diffuse surface reflects light uniformly across the gap 230 towards the other facing surface of the other prism. This surface is mirrored with either a diffuse or specular reflective surface. A receiver 250 is located above the second surface to capture an image of the pattern of light reflected from the second surface.

When the prisms are partially immersed in fluid, only the portion above the fluid level will reflect light from the source 240 on to the receiver 250. Below the surface, the light will be scattered from the fluid surface and a very small amount reach the receiver.

Both the first and second prisms of this example (and the first example) may be separate elements as shown in the accompanying figures or may comprise parts of a single optical element. For example, the gap and the two facing surfaces may be formed by an open ended hole which extends through the centre of a block of material. Example 3

In this embodiment of an optical fluid level detector, an optical element is provided which comprises a substantially conical rod 310 which tapers in diameter from an upper end 311 towards it base 312. The base 312 of the rod is immersed in the fluid, with the fluid level rising up the outside of the rod as the tank is filled. The rod 310 may, as shown in the enlarged view of the rod, have faceted sides, by which we mean that rather than smoothly tapering from a first diameter at the upper end to a second, smaller diameter, at the lower end, its diameter changes in a series of steps. Each step is defined by an angled region of outer wall interconnecting a region which is angled by a lesser amount away from the vertical. A light source 320, such as an LED, transmits light down into the rod 310 and a detector 330 array is positioned so that it collects light from the rod to form an image of the light reflected back from the rod. A screen 340 prevents direct transmission of light from the source 320 to the detector 330.

The steps provide regions at which light is reflected from one side of the rod to the other and back up to the receiver. In the absence of fluid, each step reflects and appears as a bright band on the receiver array separated by a dark band. When fluid is surrounding the rod 310, it index matches to the rod material and so the light will escape from the submerged step rather than reflecting internally.

Example 4

This embodiment is similar in operation to that of Figure 2, but rather than two prisms defining two prism surfaces a single prism 410 that has a reflecting surface 411 is provided which is inclined away from the vertical. This is provided with a diffuse coating and illuminated by a light source 420. A detector array 430 captures an image of the pattern of light reflected from the surface 411. A baffle 440 prevents light from the source directly illuminating the receiver.

On the portion of the surface 411 that is above the level of the fluid, the light will be reflected to the receiver array 430 and appear as a bright region. On the portion below the surface, the light will be diffused and scattered by the fluid and so only a small amount will reach the detector. This will be apparent as a darker region. The transition from lighter to darker region in the image captured by the detector array will provide a measure of the level of the fluid on the surface.

An advantage of this embodiment over Examples 1 and 3 is that the prism element does not need to be transparent. The reflecting surface could be provided by a surface of the container itself for example, perhaps coated with a specular coating or simply roughened.

In each example, the level of fluid is determined by analysing the pattern of light on the surface. This is performed by monitoring the output of the elements in a multi-element detector array. This provides more information than can be obtained by taking a single measurement of the overall amount of light reflected.

For a very viscous fluid and with suitable focusing optics, a sharp transition in the pattern corresponding to the level of the fluid can be seen. An example of this is shown in Figure 5 (a) of the accompanying drawings. The transition from light to dark occurs across only 1 detector element 512. With less precise focusing a smoother transition will occur as shown in Figure 5(b) which gives a gradual change across multiple detector elements 510, 511, 512. In each case, a threshold of transition from high to low output can be used as the measure of the level. In the case of a more viscous fluid and sharp focusing optics, the transition will not be sharp but will correspond to the meniscus formed on the surface by the fluid where it meets the surface. This will appear similar to the transition shown in Figure 5(b) for imperfect optics. By monitoring the relative levels of the outputs of each of the detector elements in the region of the transition the size and shape of this meniscus can be determined and this can be used to monitor the condition of the fluid.

Claims

1. An optical liquid level sensor comprising: a source of optical radiation; an optical detector sensitive to the optical radiation; and at least one optical element forming a surface which is positioned within a container and in which the light source directs radiation towards the surface to form a pattern which is dependent upon the level of the fluid; characterised in that the detector comprises an array of M detecting elements, where M is greater than 1, which elements capture an image of the pattern of radiation formed by the source upon the surface whereby the level of the fluid can be determined by analysing the image captured by the detector array.

2. An optical liquid level sensor according to claim 1 in which the surface is a transmissive surface, through which at least some of the light from the source passes on its way to the detecting elements.

3. An optical liquid level sensor according to claim 1 in which the surface is a reflective surface from which at least some of the light is reflected on its way to the detecting elements.

4. An optical liquid level sensor according to any preceding claim in which the surface is an external surface.

5. An optical liquid level sensor according to any preceding claim in which the surface is positioned within the container so that different portions become submerged as the fluid level rises .

6. An optical liquid level sensor according to any preceding claim in which the detector comprises a linear array of detector elements,

7. An optical liquid level sensor according to any one of claims 1 to 5 in which the detector array comprises a two dimensional array.

8. An optical liquid level sensor according to any preceding claim in which a lens is provided between the surface on which the pattern is to be observed and the detector array.

9. An optical liquid level sensor according to claim 8 in which the lens is a moulded fresnel lens.

10. An optical liquid level sensor according to claim 8 or 9 in which the lens is part of the optical element.

11. An optical liquid level sensor according to any preceding claim when dependent on claim 6 or 7 which further includes an electronic processing means which is adapted to process the output from the detector elements of the detector array to identify the position along the array at which the light incident on the array changes from a first intensity to a second, lower intensity.

12. An optical liquid level sensor according to claim 11 in which each element of the detector array produces an output voltage or current having a magnitude that is a function of the intensity of light incident upon it and the processing means determines the point along the array at which the voltage or current magnitude changes corresponding to the level of light.

13. An optical liquid level sensor according to claim 12 in which the electronic processing means is adapted to analyse the sharpness of the transition from light to dark across the elements of the detector array so as to determine the shape and/or size of the meniscus at the interface between fluid and surface on which the pattern is formed and hence determine one or more properties of the fluid.

14. An optical liquid level sensor according to any preceding claim in which the optical element comprises a diffuse reflecting surface imaged by the detector array.

15. An optical liquid level sensor according to any one of claims 1 to 13 in which the reflecting surface is an internal surface of the optical element arranged to provide total internal reflection of light from the source onto the detector when in air yet to transmit light away from the detector out of the element when the surface is submerged in the fluid.

16. An optical liquid level sensor according to any preceding claim in which the element comprises a prism and in which two reflecting surfaces are provided by opposite faces of the prism, the prism having a wedge shape having an upper face where the light source and detector array are positioned and comprising opposite planar reflecting faces that taper towards one another away from the upper face, the angle of the two faces being chosen to reflect light emitted by the source from one face to the other and then back to the source.

17. An optical liquid level sensor according to any one of claims 1 to 15 in which the optical element comprises a conical element which narrows in diameter towards the end furthest from the source/detector array, the reflecting surface being made up of pairs of locations on the surface which are located diametrically opposite the axis of revolution of the conical element.

18. An optical liquid level sensor according to any one of claims 1 to 15 which includes two optical elements, each having an optically transmissive face spaced from a corresponding optically transmissive face on the other to define a gap which may contain the fluid, the light being adapted to pass through the gap when the faces are index matched by fluid in the gap, but otherwise being prevented from passing through by total internal reflection.

19. An optical liquid level sensor according to claim 18 in which each of the two elements comprises a wedge shaped element.

20. An optical liquid level sensor according to the reflecting surface, or another surface which either reflects or transmits light in the path from light source to detector array may have a stepped profile.