FR2512201A1 - CIRCUIT FOR THE CONTINUOUS MEASUREMENT OF THE FILLING LEVEL OF A TANK FILLED AT LEAST PARTIALLY WITH LIQUID - Google Patents

Abstract

THIS DEVICE FOR THE CONTINUOUS MEASUREMENT OF THE FILLING LEVEL OF A TANK FILLED WITH LIQUID, IN PARTICULAR OF A FUEL TANK, USES IN A SERIES CIRCUIT A HEATED MEASURING RESISTOR, WHICH IS IMMERSED IN THE LIQUID AND EMERGING AND A COMPENSATION RESISTANCE R WHICH IS IMMERSED IN THE LIQUID EVEN AT THE MINIMUM LEVEL OF THE LIQUID, AND AN ANALYSIS CIRCUIT V WHICH PROCESSES THE MEASUREMENT SIGNAL AND THE COMPENSATION SIGNAL WHICH ARE BROUGHT TO IT, FORMING A DIFFERENCE. THE CURRENT OF THE CONSTANT CURRENT SOURCE ST WHICH SUPPLIES THE MEASURING RESISTANCE R AND THE COMPENSATION RESISTANCE R IS CONTROLLED AT A LIMITED CURRENT THROUGH A REACTION CIRCUIT D.

Description

The invention relates to a circuit for measuring

  continuous monitoring of the filling level of a filled tank

  at least partially of liquid, in particular of a reservoir

  see fuel, which comprises a measurement resistor having a relatively high temperature coefficient, connected to a constant current source and

  can be heated by this source, and which is immersed

  gée in the liquid, a compensation resistance with relatively high temperature coefficient, also connected to a constant current source, which is also immersed in the liquid, an operating circuit, which processes the measurement signal which is supplied to it and the compensation signal by forming a difference and which determines an indication corresponding to the filling level at each instant, the current passing through the measurement resistor being controlled as a function of the temperature which

  reigns above the free surface of the liquid.

  In motor vehicles, it is increasingly desired to obtain, apart from the indication of the filling level of the fuel tank, an indication

  analog filling level for cooling water

  cooling and for oil Compared to limit switches with switching or contact, which serve, for example to warn of a minimum filling level going down, analogical indications give the possibility of filling the level as a precaution of

find faults.

  Analog level measuring devices which use the resistance variation of a heated resistance sensor, having a high temperature coefficient and placed between the minimum level and the level

  maximum are generally known.

  We already know by patent applications; the Federal Republic of Germany DE-OS 14 73 132 ,; 24 55 198, 27 18 295 and DE-AS 21 40 963 and 28 41 889 devices which are composed of two resistors, of which

  only one is heated depending on the filling level

  wise at a temperature higher than that of the medium which surrounds it while the second resistance is used to capture the temperature of the liquid and to adjust the level signal of the liquid Insofar as it is a question of capturing the filling level of a variable temperature liquid, such compensation cannot be dispensed with because the temperature of the measuring resistor results from the temperature of the liquid and the above-mentioned upper temperature.

  example, a device with a single resistance, without resistance

  compensation, can have the same ohmic value 1 o resulting in the case of a low temperature of the liquid and a low filling level as in the case of a higher temperature and a high level -To eliminate this ambiguity, it is absolutely necessary to have

  a sensor fitted with a compensation resistor.

  DE-OS 24 55 198, 14 73 132 patent applications

  and DE AS 21 40 963, 28 41 889 describe circuits intended for

  born to form a filling signal as well compensated

  as possible, in which voltage signals, conve-

  amplified are treated in relation to each other or subtracted from each other In the

  two known methods of interpretation, however,

  owing to a considerable dependence of the level signal on the temperature, in particular of the signal reduced to a low level, that is to say when the measurement sensor has emerged. The reason for this fact resides in the fact that, not only the measurement and compensation part of the sensor has a resistance as a function of temperature but also in the fact that the thermal resistance of the measurement part which emerges from the sensor is a function of the

temperature.

  In patent application DE-AS 28 41 889, a modification against the current of the heating and measuring currents for the temperature above the liquid is described for a sensor which includes resistors connected in parallel. The control circuit existing controls the current from the current source that feeds

  sensors depending on the prevailing temperature

  above the free surface of the liquid The influence described on the current is exactly reversed for an oil / water sensor in which, for reasons of precision and to obtain a large measurement effect, a nickel-iron film of 0 is worn. , 1 mm in diameter at a temperature rise of approximately 1000 C. Furthermore, it is desirable for cost reasons not to have

  to use an additional adjustment circuit.

  The invention aims to improve a circuit according to DE-AS 28 41 889 so that it can be constructed in a simpler manner and therefore less storytelling and that it however works with better precision. The invention solves this problem by the fact that a circuit of the kind defined at the beginning of this specification is characterized in that the compensation resistor is arranged so as to be immersed in the liquid even at the minimum level of this liquid, in this that the compensation resistance and the measurement resistance are connected in series to a common constant current source and that an adjustable feedback circuit is provided between

  the input of the compensation signal interpretation circuit

  sation and the constant current source.

  According to the invention, the sensor is advantageously taken as a base in two parts operating with a constant current, with a subtraction analysis.

  such a sensor arrangement, there is a resis-

  dual, function of temperature so that the corresponding filling level signal of the liquid

  at a low liquid level (partial measurement resistance-

  The reason for this is that the thermal resistance of the part of the measuring resistance which has emerged and which is thus influenced by the surrounding air

  (gaseous medium) decreases with increasing temperature

  ture In the presence of a higher environment temperature, one must therefore use a higher heating current than in the presence of a lower environment temperature to achieve a rise in temperature which brings the same effect U to a given filling level This can be achieved inexpensively and

  precise by an adjustable reaction of the UK signal on the neck

  rant of heating of the sensor (constant current at the origin) in a direction such that a signal UK corresponding to a higher temperature leads to the chosen increase of the heating current so that one obtains values

  Ua = US VKUK constants throughout the time range

  peratures for the same levels of liquids.

  The invention will be described with more

  precision in the continuation concerning an example of realization

  tion shown in the drawing in which:

  Fig l is a block diagram of the arrangement

measurement sitive; and

Fig 2 is a diagram of the circuit.

  The device for continuous measurement of the filling level of a tank filled with liquid comprises a sensor S placed in the tank, which is formed by two resistors RM and R, connected in series. The measurement resistance RM immerses in the liquid Here, the liquid level can vary between "max" and "min" At liquid level "max", the measurement resistance T RM is fully immersed At level "min", the measurement resistance is not

more surrounded by liquid.

  The compensation resistor RK connected in series is constantly fully immersed in the liquid. A sensor-S indicated and composed of a resistance wire in nickel or ferronickel of 0.1 0.2 mm in diameter Depending on the impedance and the desired level of liquid level the sensor element can be in the form of a stick or be wrapped around a load-bearing element. The sensor S is supplied by a constant current source ST This constant current source delivers an intensity i of constant value as long as the temperature of the liquid contained in the tank has a

  constant value The value of the intensity i is independent

  the level of the liquid in the tank.

  The intensity ia a value such that the resistances RM and RK which both have a relatively high coefficient of temperature are heated For a sensor built for oil and water, in this case a heating takes place causing an elevation of temperature of approximately 1000 C compared to that of the gaseous medium present in the tank when the level

liquid is lowered.

  On the sensor S whose resistance is Rs = RK + Rm, a measurement signal Us is formed which is

  transmitted by a conductor a to the positive input of an am-

differential amplifier V 1.

  On the concentration resistance RK, appears the compensation signal UK which is transmitted by a conductor b at the input of an amplifier V 2 La

  output of this amplifier V 2 is connected by a conduc-

  c at the negative input of the differential amplifier

tiel V 1.

  The output of the differential amplifier V 1 delivers the indication signal U = U V x U or V R _ RM Ka S K K

K RK RK

  To compensate for variations in temperature

  ture of the liquid, the output of the amplifier V 2 is con-

  connected by a conductor d to the constant current source

  ST Through this reaction, which is before-

  carefully carried out in the form of a positive reaction, the constant current source STK is controlled in such a way that a UK signal corresponding to a temperature

  higher liquid leads to a corresponding rise

  weighting of the heating current i so that constant values Ua = Us -Vx x UK are obtained throughout the

  temperature range for the same liquid levels.

  The adjustable reaction compensates for the temperature dependence of the RM resistance emerged with respect to

  in the gaseous medium (air) above the liquid.

  The gaseous medium is more or less saturated with vaporized liquid.

  risé This in turn generates a dependence of the thermal resistance with respect to the liquid. As a result, the output signals from the sensor, for example in the presence of water and oil, are different. liquid level, we

  must therefore be able to have different degrees of reaction

  tion and this reaction must be adjustable.

  In addition, there may still be an indi-

  temperature cation for which line b on which the UK compensation signal occurs is connected via a line e to an amplifier V 3 whose output delivers a temperature indication signal

  UT for an indicating device not shown.

  The circuit shown in Fig 2 consists of universally known and easily achievable partial circuits such as voltage stabilization, a current source, an amplifier and therefore does not need to be described in excessive detail The constant current source ST K according to FIG. 1 comprises a transistor T and an operational amplifier B 1 With their peripheral elements, a precise adjusted current source is obtained. The operational amplifier B is

  rule in R 1 on a precise supply voltage On ob-

  therefore holds with a resistance R 1 of precision, an intensity i of precise value which is sent to the sensor S. The device for producing the reference voltage composed of the resistors R 4, R 5 and the Zener diodes Dl and D 2 mounted in cascade, gives a stable reference value over wide ranges of voltages

  the accumulator and, of temperature The voltage of polari-

  basic position of transistor T is produced by resistors

  tances R 2 and R 3 of a voltage divider The positive input of the operational amplifier B is connected to the voltage divider formed by the resistors R 7 and R 6, which is connected to the reference voltage The input negative of operational amplifier B is supplied through

  resistance R 8; by the supply voltage

  health on resistance Ri In addition, resistance

  R 12 is inserted in the peripheral circuit.

  The output of the operational amplifier B is connected through a resistor R 9 to the base of the transistor T 1. The reaction of the compensation signal UK as a function of temperature is ensured by the resistors R 10 and Rl 'The operational amplifier B 2 forms with its

  peripheral elements an amplifier for the UK signal.

  It works with the necessary amplification V_ RM + K The signal UK is transmitted to the input K R

  positive through resistance R 21 and resistance R 22.

  The negative input is connected to resistors R 23 and R 24 * Its output is connected to the base of transistor T 2 whose emitter is connected to resistor R 25 The collector

  transmits the amplified UK signal to resistors R 1 o, R 1 ,.

  With this circuit arrangement, it is possible to obtain a coupling relatively devoid of other reactions.

  stages and the reaction of the VK x UK signal can be intro

  picks up in the current source against the plus line. The differential amplifier is formed by

  the operational amplifier B 3 and its components

  spherical It is used to ensure the combination U VK x U

S KK

  It amplifies the differential signal in the ratio R 29 / R 27 = R 28 / R 26 so that the resulting liquid level signal Uh uses a large part of the range

of battery voltage.

  The signal Us of the sensor is transmitted through the resistor R 26 to the positive input of the operational amplifier B while its negative input is connected

  through resistor R 27 at the emitter of transistor T 2.

  Resistor R 29 is connected between the negative input and

the exit.

  The operational amplifier B 4 forms with its

  peripheral elements an amplifier which allows the indi-

  cation of the temperature of a determined part of the UK compensation signal in a spread form For this purpose, its positive input is connected to the transistor T 2 through

  resistor R 30 Resistor R 31 is connected to the-

  negative battery supply line, resistors R 33 and R 34 are connected to the negative input

33 34

of the operational amplifier.

  At the output, you can take the voltage UT which

gives the temperature indication.

  The transistors T 3 and T 4 form with their peripheral elements, the resistors R 13, R 14, R 15, R 16, R 17, R 18, R 19 and R 20 as well as the diode D 4 a voltage regulator which produces precise reference voltages

  required for setting the function point-

amplifiers.

  The capacitor C and the diode D form in

1 3

  quality of peripheral elements, with resistance R 12, overvoltage protection for voltage

  supply of operational amplifiers B 1 B 4.

  An example is given below of a list of values and types of components included in the

  construction of the circuit of Fig 2.

  List of components R o 1 10 _ + 5% TK 100 ppm

R 1.6 K 6 _ "

2

R 3 1.3 K _ / x "R 4

R 4 270 _ / 1 "

R 620 I '-

R 1.8 K _J 1 _

6

R 7 ca 3.2 K and 1- "

R 10 K el t "Rg 6.8 K _n"

R i O ca 20 K _ 1 t-

Rl ca 700 / t "il

-15 _ / AL

se 1 Watt f il R 12

2 5 1 22 O 1

R 13 300 5%

R 1 K

R 15 setting approx Ca 0.1 K

R 16 4, 3 K - / 'L + 1%

R 5.6 K JL

R 6.2 K

R 19 2.4 K

R 1.5 K

R 20 K

R 22 100 K JL

R 20 K

23

R 100 K

R 1 K

R 26 10 K

R 27 10 K

R 62 K -e-t-

R 29 62 K

R 30 10 K

R 31 51 K

R 33 51 K

R 34 10 K

C 1,100 t LF

D 1 ZPD 8.2

D 2 ZPD 5.1

D 3 Zy 20

D ZPD 5.1

BD 876

BC 237

BC 635

BC 237

1 LM 2902

  il 1 il 1 il 'T 1 T 2 T 3 T 4 Bl B 2 B 3 B 4

Claims (2)

  1 Circuit for continuous level measurement   filling a tank filled at least partially-   Lement of liquid, in particular of a fuel tank, and which comprises a measurement resistor (RM) having a relatively high temperature coefficient, connected to a constant current source (STK) and capable of being heated by this source, and which is immersed in the liquid, a compensating resistor (RK) with relatively high temperature coefficient, also connected to a constant current source (STK), which is immersed in the liquid, an operating circuit, which processes the signal of measurement which is brought to it and the compensation signal by forming a difference and which determines a corresponding indication   at the filling level of this moment, the current crossing   the measurement resistance (RM) being ordered inde-   depending on the temperature above the   free surface of the liquid characterized in that the resistance   compensation element (RK) is arranged so as to be immersed in the liquid even at the minimum level of this liquid, in that the compensation resistance (k) and the measurement resistance (R,) are connected in series to a source common constant current S S) and   that an adjustable feedback circuit is provided between the   input of the interpretation circuit (V 1) of the com-   thinking (UK) and the constant current source.   2 Device according to claim 1, charac-   terized in that the compensation resistance (RK) is   further connected to a temperature indicator (UT).