GB2120482A - Liquid level monitoring device - Google Patents

Abstract

A device for monitoring the level of a liquid in a tank comprises a plurality of thermistors 51,....mounted at different levels and each having different resistance values according to whether it is covered or not covered by the liquid. Changes in resistance values are detected to reset a fuel quantity register R to the corresponding reading. A flowmeter is arranged to decrement the reading between successive such resettings, and a differentiation system D1...D4 takes into account the direction of displacement of the level of the liquid, to index the register R to the correct values. <IMAGE>

Description

SPECIFICATION Liquid level monitoring device This invention relates to a device for detecting or monitoring the level of a liquid in a tank, for example containing fuel for an engine particularly a vehicle engine.

Numerous devices are known for measuring the level of a liquid in a tank. The majority use a float which, held by a system of rods or slides, carries a slide which rubs on a resistive element thus modifying its resistance depending on the level. These devices have many disadvantages and mainly that of comprising a frictional contact which is therefore subject to wear. This mechanism, without being very complicated, may nevertheless develop play or jam. Also, most of the time the precision of these devices is inadequate which is particularly annoying above all when they are associated with onboard computers which give an hourly or kilometric consumption as well as a cruising range for the vehicle.

More precise devices are also known but their price is well above that which is acceptable for large-scale mass-production (e.g. capacitive or pneumatic devices). Devices likewise existing enabling a particular level to be detected by means of a thermaliy-dependent resistor carrying an electrical current, the resistance value of this resistor depending upon whether or not it is immersed in the liquid.

In accordance with this invention, there is provided a device for monitoring the level of a liquid within a tank, comprising a plurality of temperature dependent resistor, means for mounting said resistors at discrete levels within said tank, said resistors being arranged to provide electrical outputs indiating when the liquid is at the respective said levels, and a register arranged to count and deduct flow pulses supplied by a flowmeter associated with an outlet of the tank and further arranged to be reset by said electrical outputs.

The device thus makes use of a flowmeter, which is in any event required if an on-board computer is to determine the instantaneous consumption, cruising range, etc., and thus does not involve additional cost. The device is entirely static, in not requiring any moving parts.

Embodiments of this invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a probe of a level monitoring device in accordance with this invention; Figure 2 shows a probe of an alternative device in accordance with this invention; Figure 3 is a sectional view of a fuel tank; Figure 4 is a graph showing, depending on time, some practicai cases of resetting and restarting of a fuel quantity register of the devices; Figure 5 shows flow diagrams of the devices; Figure 6 is a block diagram of the electronic circuit of the devices; Figure 7 shows a flow diagram explaining the generation of an alarm in the event of an incomplete filling of the fuel tank.

Referring to Figure 1, a probe of a level monitoring device (in a vehicle fuel tank) comprises 4 sensors each consisting of a parallel connection of a fixed resistor 4 and a thermosensitive resistor or thermistor 5 having a positive temperature coefficient. The fixed resistors may be soldered or deposited in a thin layer or thick layer on a substrate 2 of insulating material (for example Ceramic or glass). The thermistors 5 are soldered to the terminals of the fixed resistors 4 and the whole assembly may be covered with a varnish resistant to hydrocarbons. The assemblies consisting of the resistors 4 and 5 are connected in series and therefore connected to one another by a conductor 6 which is fixed by one of its ends to the bottom of the tank and thus electrically connected to earth.The other end emerges from the tank to be connected to a fixed resistor 7, itself connected to a positive potential V. The terminal strip thus constructed is situated inside a tube 1 of any material having, in its lower portion, a calibrated hole 3 bringing the interior of the tube into communication with the tank, to enable a level to be obtained within the tube which is less disturbed than outside because of a delay effect thus achieved.

Figure 3 illustrates the introduction of such a probe into a tank 8 comprising a suction outlet 12 for the liquid in the path of which a flowmeter 11 is situated. This tank is closed by a stopper 1 0 actuating a contact 9, the purpose of which will be demonstrated below.

The operation of the probe shown in Figure 1 is as foilows. When the tank is empty, the thermistors 5 and the resistors 4 are fed with current through the resistor 7. The thermistors heat up, their resistance is very great, and the voltage E at the junction of the probe with the resistor 7 is given by: V(R4,+ R42 + R43 + ... R4) E R7 + R4,+ R42 + R43 + ... R4 if the resistors R4 are selected so that R4 < R5 in the heated state. If the probe is fed with a constant current more simply: E -- I (R41 + R42 + R43 + R4,).

When an assembly R4, R5 is covered by the liquid, R5 cools and its value becomes very small in comparison with that of R4, thus reducing the voltage E correspondingly.

The examination of this by a simple electronic circuit enables the precise moment when the liquid covers or uncovers a thermistor to be known and hence, according to their distribution inside the tank, the level to be determined and as a result, the amount of liquid present.

By way of example, Figure 6 illustrates an electronic circuit comprising a microprocessor 4 which receives, on the one hand, pulses originating from the flowmeter 11 and, on the other hand, logic signals ABCD supplied by 4 voltage comparators (T1 to T4) each of which switches over when the voltage E at the terminals of the probe is higher than a respective reference voltage determined by a resistor bridge included in that comparator.

In this example it is assumed that, the tank being full, the voltage E is at a minimum and that the liquid has reached the level 400 covering the whole of the probe, the three other levels being situated at 300, 200 and 100. If the level drops slightly, the probe sensor S1 is no longer covered by the liquid and the thus increased voltage E causes the comparator T1 to switch over and so pass to "1". If the level increases slightly, this same comparator passes to "0", the level remaining substantially the same. Thus, for the same level of liquid, the comparator in question (here T1) may be at "1" or "0" according to the direction of displacement of the level.The tables below indicate the states to which the various comparators change at the different levels, the left hand table corresponding to a progressive raising of level and the right hand table corresponding to a progressive lowering of level.

ABCD HF ABCD HF 400 0 0 0 0 0 1 400 1 0 0 0 1 0 300 1 0 0 0 0 1 300 1 1 0 0 1 0 200 1 1 0 0 0 1 200 1 1 1 0 1 0 100 1 1 1 0 0 1 100 1 1 1 1 1 0 Thus it will be seen that one and the same combination exists to indicate 2 different levels. For example: "1000" appears twice in the table, for 300 and 400 according to the direction of displacement of the level of the liquid. In order to distinguish between them, 4 differentiators D1 to D4 are used associated with 4 inverters 11 to 14 enabling a positive "pulse" to be obtained at the output H or F of one of the 2 OR gates 1 and 2, according to whether the signal at the output of a comparator T is passing from "0" to "1 " or from "1 " to "O". This "pulse", applied to an input of the microprocessor, will trigger a request for interruption of the program in progress.

In the above table, the states of the two outputs H and F have been indicated according to the direction of displacement of the level. Each transient signal "1" determines the progress of a particular program, once the request for interruption has been taken into account by the microprocessor. The two corresponding flow diagrams are shown in Figures 5-1 and 5-2.

The states 100 and 200 respectively characterise the start of the interruption programs 1 and 2.

The stages 101, 102, 104, 106, 108 and 201, 202, 204, 206, 208 correspond to the test of the inputs A, B, C and D and according to the reply determine either the passage to the following test or the indexing of the fuel quantity register to the required number. (This register, R Figure 6, may possibly be outside the microprocessor, but in any case its contents are non volatile.) If the result of the test is still negative, it means that the right combination has not been found and that one of the comparators is no longer working. In this case, an alarm is set off (states 110 and 210) and the interruption return effected (1 1 1 and 21 1).

A third flow diagram is illustrated in Figure 5-3. The state 300 corresponds to the start of an interruption triggered by a flowmeter pulse which leads, in the stage 301, to the deduction of a unit in the quantity register, then to the interruption return at 302.

Thus it will be seen that outside the periods when a particular number is imposed into the register, which permits the resetting of this, the pulses originating from the flowmeter are regularly deducted from the quantity register. This implies that at any moment, the number in the register is the precise representation of the amount of liquid contained within the tank. This is true even when the engine is stopped because the contents of the register are then transferred into a permanent store (either an EEPROM, or a RAM with autonomous feed).

Difficulties may occur during the filling of the tank because this is generally effected with the engine stopped and therefore without any feed to the microprocessor. Solutions are provided for cases (presented in Figure 4) liable to occur in the course of use of the vehicle. Q1, Q2, Q3 and 04 represent the 4 levels to which the register is systematically reset. These are represented on the graph by encircled dots. The crosses indicate stops and starts with or without filling.

At the time to, the register stores a quantity Q34, then since the fuel diminishes, the register is reset to the level 03. Then, stop. Then start with an amount Q23, resetting to 02, then stop with an amount Q12, then refilling at the time t,.

Referring to Figure 3 and to Figure 6, it will be seen that there is a contact 9 which is normally open and which closes when the tank stopper is removed. This has the effect of feeding the microprocessor and of supplying it with a signal Y indicating that the tank stopper has been removed.

This logical level "0" is likewise applied, by means of a resistor R2, to a relay L, the other end of which is connected to the positive terminal of the battery. This relay is thus energised, latching itself via a contact C2 and a resistor R1, and a contact C1 then ensuring feed to the microprocessor. Contact C3 represents the ignition key. The resistors R1 and R2 are of the same order of magnitude as the resistance of the relay which picks up for a voltage of 6V. The object of the resistor R2 is to prevent a short circuit if, by chance, the tank stopper is removed and the ignition key is in the closed position. The filling is therefore effected at the time t1, until the tank is full. The quantity register then contains the amount Q4, after two successive resettings at Q2 and Q3.

At the time t2, after resetting to 100, the tank is not completely refilled and the last resetting takes place at Q3. Referring to the flow diagram of Figure 7, the use of the signal Y is shown. This subprogram is situated in one of the main loops of the microprocessor program and comprises a starting stage 400 then a test of the signal Y at 401. If Y is not present, the return to the main program is effected. If the tank is open, the signal Y exists and the following test at 402 relates to the state of the comparators. If the tank is not full, ABCD f 0000, a flashing alarm or a "buzzer" (Al Figure 6) is triggered at the stage 404 then, return of sub-program. On the other hand, if the tank is full ABCD = 0000, the alarm is stopped at the stage 403.

The object of this is to warn the driver that if full is not reached, he himself should intervene in the computer to enter manually the amount indicated at the pump which will thus be added to the amount previously included in the quantity register.

It will be noted in the flow diagram of Figure 7 that once the alarm has been set off it will not be stopped when the tank stopper is replaced, which forces the driver to intervene.

In fact, as seen in Figure 6, since the contact 9 is open (tank stopper in position) the signal Y changes back to "1" by means of the resistor R2, the relay L and the microprocessor remaining fed, the one by its selfholding contact C2 and the other by the contact Cl.

When the ignition contact C3 closes, the relay L drops out again because it is no longer fed. C1 and C2 open, the microprocessor is fed by C3. This maintains the alarm system energized until the user has entered in the register the amount displayed at the pump.

Another solution, in the case of a partial filling, does not warn the user that he has to enter manually the amount displayed at the pump, but temporarily suspends the calculations which cause the amount of fuel to appear, until an automatic resetting occurs. This has the advantage of simplicity because no intervention by the user is necessary. This is effected simply by eliminating the alarm devices Al of Figure 6 and using the control signal to inhibit the calculations. Stage 404 of Figure 7 becomes: "Inhibition of calculations" and stage 403: "Stop inhibition". Stage 111 of Figure 5-1 is converted into: "Stop inhibition", the "Return of Interruption" being transferred to the following stage.

Figure 2 shows another probe in accordance with this invention, comprising a plurality of thermistors 5. One terminal of each thermistor is connected to earth and the other is connected to a respective resistor 71, 72 7,... 7n themselves connected to the positive terminal of the battery.

The voltage existing at the junction between the thermistor and its fixed resistor 7n relative to earth is marked E, E2, E3 . . En. This voltage is low when the thermistor is immersed in the liquid and thus cooled and high when it is not immersed in the liquid. The circuit diagram is the same as Figure 6.

The voltages E,, E2,E3... En are applied to the inputs of the comparators Tr, T2, T3 . . . Tnl the reference voltage between Emin and Emax being the same for all.

Claims (6)

1. A device for monitoring the level of a liquid within a tank, comprising a plurality of temperature dependent resistor, means for mounting said resistors at discrete levels within said tank, said resistors being arranged to provide electrical outputs indicating when the liquid is at the respective said levels, and a register arranged to count and deduct flow pulses supplied by a flowmeter associated with an outlet of the tank and further arranged to be reset by said electrical outputs.

2. A device as claimed in Claim 1 , further comprising means for detecting the direction of variation in the liquid level when the liquid covers or uncovers each of said resistors.

3. A device as claimed in Claim 1 or 2, further comprising a switching device activated by any absence of a stopper of the tank to provide an alarm signal, if the tank is then not completely refilled, obliging the user to enter manually the qantity of liquid which has been supplied.

4. A device as claimed in Claim 1 or 2, in which removal of a stopper of the tank is arranged to inhibit flowmeter pulse calculations unless the tank is completely refilled or until an automatic resetting said register occurs upon lowering of the liquid to a said discrete level.

5. A device as claimed in any preceding claim, in which said resistors are mounted on an insulating plate fixed inside a receptacle formed with a restricted orifice at its lower end.

6. A liquid level monitoring device substantially as herein described with reference to Figures 1 to 2 together with Figures 3 to 7 of the accompanying drawings.