GB2190500A - Liquid level sensor - Google Patents

Abstract

A liquid level sensor comprises a silicon strain gauge (4) acted on by a float (2) to vary the strain gauge output in accordance with the liquid level (3). Two vertically spaced strain gauges may be provided to give maximum and minimum level readings or several strain gauges may be provided each fixed to a respective float. The float may be elongated and incorporate a weight and may be arranged between vertically spaced strain gauges to give a progressive output or suspended from the sensor. In an alternative arrangement where there is no contact between the float and sensor the float carries spaced magnets which coact with respective magnets outside the container to bend cantilever beams carrying the strain gauges. <IMAGE>

Description

SPECIFICATION Liquid level sensor This invention relates to sensing liquid levels, particularly but not exclusively for sensing the levels of liquid e.g. oil andwaterin a motorvehicle and the like.

Various liquid level detectors have been proposed and are currently used for remotely sensing the level of a liquid in a motorvehicle. For example in the petrol tank the petrol level is determined by a float which is pivotally mounted and which moves a wiper across a wipertrack in a variable resistor, the resistance varying in accordance with the level of petrol inthetank.

For measuring the level of oil in the crankcase of an internal combustion engine, it is known to have a resistance element heated by passing an electrical currentthrough it and measuring its resistance. Its resistance in the oil will be different to its resistance outofthe oil because the oil has a significantly greater cooling effect on the resistance element than does the surrounding air when the oil level has dropped.

Other liquid level sensors includefloatswitches for detecting the liquid level and having an operating mechanism for actuating a switch regardless ofthe position ofthe float.

Another liquid level detector for a motorvehicle has a float with flexible conductor sheets in sliding contact with two resistance faces of a vertical rule.

Yet another liquid level detector this time for a storage battery, has a small float emitting light and a photocell for detecting the light and an optical fibre terminal.

Another mechanism for measuring liquid level inside a container includes transforming a float movement into a rotary motion which controls the movement of a magnet in the vicinity of a Hall effect generator.

Yet anothertype of liquid level sensorfor electrically conducting liquid comprises copperfoil capacitor electrodes on non-conducting container wall.

All the above mentioned ideas for sensing liquid levels have the general disadvantagethattheyare not equally applicable to a variety of different applications. Thus in general it is not possible to apply a liquid level sensor which is suitable for measuring the oil in the crankcase of an internal combustion engine to measuring the water level in a radiator or even the water level in a water reservoir for a windscreen washer. This is because eitherthe temperature ranges are different or the physical nature ofthe liquids is differentandwill notworkor not work so well with one type of sensor compared with another.

It is an object of the present invention to provide a liquid level sensor which can be applied equally well to a number of different applications yet which is simple and cheap to manufacture and rugged and reliable in operation.

According to the present invention a liquid level sensor comprising a float operatively connected with a silicon strain gauge devicewherebyto apply a strain to the device which varies in dependence upon the level of the liquid.

In one embodiment the float is a free floating mass located in a tube and the strain gauge device is strained via the float at a predetermined liquid level in the tube.

There can be more than one strain gauge device; forexampleto provide maximum and minimum level indications. In the maximum position the float will be lifted by the liquid to strain an up strain gauge device and in the minimum position will strain a second strain gauge device underthe influence of the weight of the float in free air. Thus the float although buoyant in the liquid will have a weight out of the liquid sufficient to provide an indication by straining the second strain gauge device.

In a further embodiment the float can be secured to the strain gauge device so that both its buoyancy will give an indication and its weight out of the liquid will provide a different indication through the same strain gauge device. There could be several such floats mounted on respective strain gauge devices along the length of tube orothersupportto give a progressive indication of level for example in a petrol tank.

In orderthatthe invention can be clearly understood reference will now be madetothe accompanying drawings in which: Figures ito 6show schematically various embodiments of liquid level sensors according to embodiments ofthe present invention, and Figure 7shows in greater detail a silicon strain gauge device suitable for use in any ofthe embodiments of Figures 1 to 6; Referring to the drawings Figures 1 to 5 show various forms of liquid level sensors according to embodiments of the present invention. In its simplestform as shown in Figure 1 the liquid level sensor comprises a tube 1 housing a float 2 shown floating on a liquid whose level is indicated generally at3.Secured inthewall ofthetube 1 at positionS is shown asilicon strain gaugesensor4which is connected to sensing electronics circuitry 6 by means of lead 7.

As shown the float 2 is being pressed againstthe underside of the silicon strain gauge sensor4 thereby changing the output signal to the electronics 6which provides an indication 8 of the fact. When the liquid level 3 falls so that the float no longer bears against the underside of the strain gauge sensor4 then the output 8 changes to indicate that fact. As can be clearly understood the level of liquid 3 can rise up above the sensor4 so thatthe sensor4 becomes immersed in the liquid and the electronics 6 and output 8 can be arranged to provide one outputwhen the float 2 is pressing against the sensor 4 and a different output when it is not, thereby providing a "minimum" liquid level sensor.

Turning now to Figure 2there is shown the tube 1 this time carrying two silicon strain gauge devices 10 and 11 both connected to sensor electronics 12 by leads 13 and 14. Thefloat 15 is shown floating on liquid indicated generally by the level 16. Thistime the float has a weight 17. It need not be a discrete weightfor indeed the float could have a buoyancy of say 0.7 timesthat of the fluid by being made of material of that density.

When the liquid 16 rises so thatthe float bears againstthe underside of sensor 10 the output 18 of the electronics 12 gives a first indication corresponding for example to a "maximum" liquid level and when the liquid level falls so thatthefloat rests on top of the strain gauge sensor 11 and is no longer supported bythefluid then the weight ofthe float 15will cause the sensor 11 to provide a changed output so thatthe electronics 12 at its output 18will provide a different output representing a "minimum" liquid level.

At any point in between the silicon strain gauge sensors, as illustrated in the drawing,thenathird signalwill be outputat 18 representing an "acceptable" or "normal" liquid level.Thiscouldfor example be used as a dip-stick in an engine oil level indication system.

Referring nowto Figure 3 there is shown an arrangementwhereseveral silicon strain gauge sensors such as 20 are each secured in the wall ofthe tube 1 and each have fixed to them a float 22 at a location indicated by 23. Each silicon strain gauge sensor such as 20 is connected by a lead such as 24to indicating electronics 25 having an output 26.

As can be readily understood the changing level of liquid will progressively provide the electronics with changing outputs from the respective silicon strain gauges so that a progressive liquid level can be determined by the electronics 25. This could for example be used as a fuel gauge sensor in a motor vehicle.

Referring to Figure 4there is shown anotherform of liquid level sensing device comprising a tube 1 housing an elongate float 30 incorporating a weight 31. The float is arranged between upper silicon strain gauge device 32 and lower silicon strain gauge device 33 both of which are connected by leads 34 and 35to electronics 36 which has an output 37. As can be readily understood as the liquid level rises in the tube 1, the upward force on the float 31 increases until it equals the downwards force due to theweight ofthe float atwhich pointthere is no net force on either strain gauge 32 or 33.Up to that point strain gauge 33 has been bearing the weight of thefloat and produces an output accordingly, this output slowly decreasing until the float is just buoyant in the liquid. Asthe liquid level increases furtherthe float becomes pushed upwards against the silicon strain gauge sensor 32 which then begins to produce a progressively increasing output to the electronics 36.

This liquid level sensor can be used in a manner similar to that of Figure 3 to indicate a progressive change in liquid level, once again for example for monitoring the level of fuel in a fuel tank of a motor vehicle.

Figure 5 shows another embodiment comprising a tube 1 housing a float 40 which is attached at position 41 to a silicon strain gauge sensor 42 secured in the side wall ofthe tube. The output ofthe sensor42 is connected to electronics 43 buy a lead 44.

The liquid level 45 shown inthedrawing givesthe float40 some buoyancy and has produced some strain in the sensor 42 so that output 46 indicates "halffull". As the level 45 falls there will be less and less upward force on the float 40 and therefore less and less force on the sensor 42so that the output 46 changes accordingly. Similarly as the level risesthe upward force produced bythefloat40 increases so the output 46 increases accordinglyfrom the sensor.

This arrangement can be used to give a progressive indication of level eg. for a fuel tank.

In all the arrangements so far described the float comes into contact with the silicon strain gauge sensor either by being directly connected to it or by bearing against it. In either of these designs the silicon strain gauge sensor needs to be rugged and well protected against shock and abuse.

As an alternative it is proposed to provide an arrangement in which no contact between the sensor and the float takes place. Refrring to Figure 6 the tube 1 contains a float 60 which has embedded in its upper end a magnet 61 and at its lower end a second magnet 62. On the wall ofthe tube 1 is mounted a first silicon strain gauge sensor 63 and second silicon strain gauge sensor64. These sensors are in the form of cantilever beams and attheirfree ends they are provided respectively with magnets 65 and 66.

The ends of the tube 1 are provided with respective bars 67 and 68to prevent the float floating out or falling out of the tube 1. The silicon strain gauge sensors 63 and 64 are connected by respective leads 69 and 70 to electronics 71 which provides an output 72.

Inthe position shown in Figure6the liquid has fallen below the bottom ofthetube and is therefore not visible in the drawings in these circumstances the float 60 has fallen to the bottom of the tube, in which position the magnet 62 is directly in alignment with the magnet 66 and so the beam of the silicon sensor 64 is deflected thereby producing an output 72.

When liquid rises up the tube to a certain level the float will rise until the magnet 62 is not in alignment with the magnet 66 but the magnet 61 is brought into alignmentwith the magnet 65. Under these conditions the beam of the sensor 63 is deflected magnetically and thus another output is provided at 72.

This device could be used to provide minimum and maximum level indications in for example the oil reservoir of an internal combusion engine.

In a modification it would be possible to achieve a similar result with only one strain gauge and one magnet in the float. Thus for examplethe strain gauge sensor 64 could provide a first output at 72 in a position shown in the drawing and a second output when thefloatfloats away in to its upper position because the strain gauge sensor will no longer be deflected and thus a different output 72 will be produced.

In the arrangement shown in Figure 6the silicon strain gauge sensors are prevented from any form of contact with the float 60 and can in fact be housed with a plastics housing on the outside ofthetubel, not only to prevent external damage butto completely seal the strain gauge sensors from the environment in which the tube will be used. Thus they can be hermetically sealed from that environment. This could for example be useful in a corrosive environment such as the electrolyte of a lead acid accumulator batteryfora motorvehicle.

The silicon strain gauge sensors shown in the embodiments just described can take several forms.

One suitable form is shown in Figure 7. Referring to Figure 7 the sensor comprises a cantilever beam 50 formed by a thin strip-like silicon element. The thin cantilever has a piezo-resistive strain gauge bridge 52 implanted in its surface 51. The strain gauge is connected to four contacts 53 on a portion 54 of enlarged area on the silicon element.

Two of the contacts 53 receive "power in" in the form of a dc voltage and the remaining two contacts provide an outputvoltagewhich varies in accordance with the bending of the cantilever beam 50. Thefour leadsfrom the contacts representthe various leads7,13,14,24,34,35,44,69 and70 inthe embodiments described with reference to Figures 1 to6.

In Figure 7 is shown a pad 55 on the end ofthe beam which will bearagainstthefloatinthe embodiments of Figures 1,2 and 4where the device of Figure 7 is used as the sensors 4,10,11,32, and 33.

Inthe embodiments of Figures3 and 5thenthe float will be attached to the free end of the cantilever beam. In any of the arrangements were the float comes into contact with the beam then some form of protection against excessive movement of the end of the beam will be required in orderto preventthe beam being broken. This can be intheform of partof the silicon structure formed as "tethers" and indicated by the reference numerals 56 and 57 in Figure 7. These are formed during the manufacture ofthe sensor. Alternatively protection can be applied afterwards as part of the packaging of the sensor. For example the package could have walls which prevent excessive movement of the tip of the beam.

Claims (1)

1. A liquid level sensor comprising a float operatively connected with a silicon strain gauge devicewherebyto apply a straintothedevicewhich varies in dependence upon the level ofthe liquid.