GB2127555A - Liquid-level detecting device - Google Patents

Abstract

A device for measuring the level of a liquid in a tank comprises a plurality of resistive probes 10 disposed at successive levels on an insulating substrate 40, each probe having a temperature coefficient other than zero such that its resistance depends whether it is immersed to a greater or lesser extent in the liquid, and each probe being fed separately with electric current so that the voltage across it gives an indication of the extent to which it is immersed in the liquid. The voltage across the probes may be scanned and stored on energizing the probes, and then again after an interval. The ratio of voltages for any probe is indicative of the degree of immersion, and the corresponding value of liquid may be read from a read only memory and added to the readings for the other probes. <IMAGE>

Description

SPECIFICATION Liquid-level detecting device This invention relates to a device for detecting a liquid level, for example inside a fuel tank which may be mounted in a motor vehicle.

We have previously proposed liquid-level detecting devices without moving parts, using thermistors which, according to whether they are immersed or not, supply a signal corresponding to a discrete level inside the tank, an interpolation between discrete levels being effected by an existing flowmeter on the vehicle.

In this manner a counter, preset initially to the amount of fuel which has entered the tank, is decremented by the pulses originating from the flowmeter placed in the admission conduit to the carburettor and reset systematically at each of the levels defined by the successive thermistors. It is thus possible to obtain an indication of the level at any moment. The disadvantage of such a system lies in the fact that it requires the vehicle to have a flowmeter.

We have previously proposed arrangements comprising a single wire probe fed with current or voltage, the potential at the terminals of this depending on the level of the liquid. It was possible to obtain this by using a wire with a high temperature coefficient and by exploiting the fact that the immersed wire is cooled, thus causing a modification in its resistance and hence a different voltage at its terminals depending on the length of wire immersed.

There is a difficulty in this process, however, due to the length of the wire which, since it does not have a constant linear temperature coefficient, necessitates the introduction of complex correction factors into the memories of the data-handling microprocessor. On the other hand, since the feed voltage is applied to the whole of the wire, the voltage gradient is slight and errors may result.

In accordance with this invention, there is provided a device for measuring the level of a liquid in a tank, comprising a plurality of resistive probes disposed on an insulating substrate one above the other, each probe having a temperature coefficient of resistance other than zero and a resistance of which depends whether it is immersed to a greater or lesser extent in the liquid, and each probe being fed separately with electric current and the whole being immersed in the tank in a general direction which is other than horizontal. This device eliminates the above difficulties whilst improving precision and avoiding the need for a flowmeter when this is not required for other purposes.

An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic elevation of a measuring probe assembly according to the invention; Figure 2 is a diagram of an electronic processing circuit associated with the probe assembly and likewise; Figure 3 is an operating flow chart of the processing circuit; and Figure 4 is a representation of the voltage at the terminals of an individual probe, depending on time.

As can be seen in Figure 1, a plurality of probes 10 are disposed one below the other on an insulating substrate 20 and staggered in the horizontal direction in such a manner as to permit a slight overlapping between them. The lower ends 30 are united and connected to the same potential point, namely ground. The other ends are each connected separately to a contact blade situated, for example, in the upper portion of the substrate: seven probes are thus represented in this example and these ends are numbered from 1 to 7. The whole of the pick-off with its connections may advantageously be constructed in the form of a printed circuit. This is placed in a receptacle 40 having only a single calibrated aperture 41, situated in the lower portion, communicating with the outside. Thus the internal liquid level is little effected by rapid variations in the level of the main tank.

This level detector may comprise as many wire probes as necessary. The length of these is immaterial but preferably and in order to simplify the calculation, an effor is made to determine it in such a manner that the variation in level from one end to the other is expressed by a proportional variation in the capacity of the liquid tank. If not, a conversion table may be used. Thus each probe defines a precise quantity and the more probes there are, the greater the precision in the measurement. In practice, the limit is determined by price.

A block diagram of the control electronics is illustrated in Figure 2. The probes 10 are fed separately with voltage from a single source of voltage V, by way of resistors R" R2, ........

These may be identical or not according to whether it is desired to match the pick-offs to one another by regulating the voltage gradients to the same value. It is likewise possible to use a separate current source for each of the probes as illustrated in the Figure, or a current source starting from a voltage V which can easily be obtained by selecting very large resistors R1,R2, R3... Rn in comparison with the resistance of the probes. The voltages V1, V2, V3... Vn are applied to the inputs of an analogue multiplexor M, the output of which is connected to an analogue/digital converter A. The result of the conversion effected on 7 bits is applied to the input bus of a microprocessor ,uP which directs all the operations.The generating and the switching of the voltage or of the current I is effected inside a control box C connected to the microprocessor.

The resistors Tri ... Rn may advantageously be situated on the substrate 20 thus forming an integral part of the pick-off.

The operating flow chart is given in Figure 3. In a first period, at the stage 100, the probes are fed.

The voltage at the terminals of the probe 1 is then taken off, at the stage 101, then converted into a binary number by the A/D converter and finally stored inside the microprocessor. The same operations are carried out through the stages 102 and 103 for all the probes of the pick-off. Then, at the stages 104 and 105, the microprocessor effects a return to the main program for a period t, (of the order of a few seconds). At the end of this time, the previous operations are repeated from stage 106 to stage 108. Then at stage 109, the probes are de-energized.

Thus the stores of the microprocessor have stored 2 voltage values for each probe, corresponding the first to the voltage at the terminals of a cold probe and the second to the voltage at the terminals of a heated probe. When a resistive wire probe is traversed by a current, its internal temperature increases. Since the wire has a high temperature coefficient, the resistance is modified and this is the more so, the more the probe is heated. This is what Figure 4 shows. At the time to, the voltage at the terminals is V0 and at the end of a time t1, it reaches Va if it is outside the liquid and Vb if it is inside. Va > Vb if the temperature coefficient of the probe is positive.

The voltage values Vj corresponding to the different degrees of immersion of the probe are spread out between these two extreme positions.

Since the probe is, by definition, very sensitive to temperature, the voltage V0 measured at the time to depends essentially on the climatic environment and therefore cannot be constant.

For this reason, the relationship of the two voltages picked up at the time to and at the time t, is used, which only depends on the rise in the temperature of the probe when it is traversed by a current. It should likewise be noted that the nominal resistance of the probe may be any value, thus eliminating any original adjustment necessitated by variations in resistivity of the wire.

Let us return to the flow chart of Figure 3 where it is seen that the microprocessor effects successively the ratio of the voltages picked up at the terminals of the probes. At stage 110, S'1 K,= S1 is computed and this ratio is stored at stage 111.

At stage 112, S'2 K22 S2 is computed and this ratio is stored at stage 11 3.

At stage 1 14, S'n Kn= Sn is computed and this ratio is stored at stage 1 15.

Then, taking into account the fact that the ratio K is the greater the more the probe S dissipates its heat outside the liquid, K, is first tested at stage 1 6 in order to find out whether this ratio is greater than a certain value N corresponding to a probe totally emerged. If this is not the case, then the probe is partially in the liquid; the microprocessor will then seek in the read-only store the correspondence between the ratio K, and the liquid volume in litres, and then will add to this quantity found, the number of litres corresponding to the n-1 probes (stages 11 7 and 1 18). The remaining quantity of liquid is thus available in a read-write store.

If K, > N, the probe is totally emerged. The microprocessor then tests the following probe S2 at stage 1 19 and the process previously described is repeated at stages 120 and 121 if K2 < N (the number of litres corresponding to the n-2 probes is then added). If K2 > N, there is a movenent on to the following probe and so on up to the last probe, called Sn in our example. If Kn < N (stage 122) the correspondence between this ratio and the liquid volume in litres is sought at stage 123; no other operation has to be carried out because, since this probe is the last, the number found in the read-only store corresponds directly to the number of litres remaining in the tank. If Kn > N, the tank is empty.

After a sufficient time, necessary for the cooling of the probes, the program is carried out again by the microprocessor.

It is possible to insert precise warnings at one or more selected levels, as indicated at stage 1 24.

To compensate for any irregularities in the capacity of the tank, the probes need not necessarily be rectilinear and may have such a curvature that their variation in resistance depending on their degree of immersion corresponds to a particular relationship. This special configuration may be used in the case where the capacity of the read-only store of the microprocessor is inadequate or simply if the data handling system does not comprise one.

Claims (8)

Claims

1. A device for measuring the level of a liquid in a tank, comprising a plurality of resistive probes disposed on an insulating substrate, one above the other, each probe having a temperature coefficient of resistance other than zero and a resistance of which depends whether it is immersed to a greater or lesser extent in the liquid, and each probe being fed separately with electric current and the whole being immersed in the tank in a general direction which is other than horizontal.

2. A device as claimed in Claim 1, in which the probes are staggered in such a manner that when one probe is totally immersed in the liquid, the lower end of the next probe above dips into the liquid.

3. A device as claimed in Claim 2, in which the probes are of an unequal length thus enabling a better adaptation to a particular shape of tank to be obtained.

4. A device as claimed in Claim 3, in which the probes are not rectilinear, to enable a lack of linearity between the depth of the liquid in the tank and its volume to be compensated.

5. A device as claimed in Claim 4, comprising means arranged to sample the voltage at the terminals of each probe successively, and to transmit this voltage to a system permitting its storage then its utilization.

6 A device as claimed in Claim 5, in which said means is arranged, for independence from different ambient conditions, to measure the voltage at the terminals of each probe firstly at one instance and then at a predetermined time afterwards, with subsequent computing of the ratio of these voltage measurements.

7. A device as claimed in Claim 6, comprising a read-only memory providing a correspondence between said ratio and the liquid volume.

8. A liquid level measuring device substantially as herein described with reference to the accompanying drawings.