WO2002006776A2

WO2002006776A2 - Method and sensor for measuring the level of a liquid in a receptacle

Info

Publication number: WO2002006776A2
Application number: PCT/DE2001/002609
Authority: WO
Inventors: Walter Bauer
Original assignee: Robert Bosch Gmbh
Priority date: 2000-07-14
Filing date: 2001-07-12
Publication date: 2002-01-24
Also published as: WO2002006776A3; DE10133692A1

Abstract

The invention relates to a method for measuring the level of a liquid in a receptacle, whereby at least one sensor (3) is arranged inside the receptacle (1). The sensor (3) is provided in the form of an optical waveguide with an air cell (22) located between both electrodes (21, 24) in which a certain quantity of the liquid (2) flows according to the level of liquid. The volumetric quantity or the level in the receptacle (1) can be determined by determining the capacitance or the change thereof as a function of the immersed sensor (3). As an evaluation method, either a change in capacity is measured or the sensor (3) is subjected to the action of a high-frequency signal and wave properties are used for determining the fluid. For example, the resonance frequency, the standing wave ratio, the frequency of a standing wave, which is generated in a defined manner, between the liquid and the feeding point, the impedance or another electrical property can all be utilized for measuring. According to the shape of the tank, a number of sensors (3) can also be arranged inside the receptacle (1) and they can be evaluated individually or by connecting them in parallel.

Description

Method and sensor for level measurement of a liquid in a container

State of the art

The invention is based on a method or a sensor and a device with at least two sensors for measuring the level of a liquid in a container, according to the type of the independent claims. Level meters for various liquids are already known and are manufactured in numerous technologies. The known level meters work, for example, according to mechanical, electrical, thermal, capacitive, inductive or frequency-modeled methods. For example, resistance sensors are used for fuel measurement in a vehicle tank, with the aid of a float

Resistance ratio of a divider is changed depending on the level in the liquid container. This resistance ratio is used to evaluate the level or the amount of liquid. However, mechanical sensors have the disadvantage that their mechanics are relatively sensitive, especially in the rough everyday operation of a motor vehicle. On the other hand, the encoder signal fluctuates very strongly due to the vibrations during the journey, so that further measures must be taken to compensate for the constantly changing measured values. Purely electric Measuring sensors, on the other hand, have the disadvantage that they are complex. In the case of containers with an irregular geometry in particular, the measurement result cannot readily be linearized, so that the scale for a level indicator is more difficult to produce.

Advantages of the invention

The method according to the invention or the sensor as well as the device according to the invention with at least two sensors with the characterizing features of the independent claims has the advantage that the level of the liquid is detected with the aid of a change in an electrical variable. This is done according to a purely electrical measuring method, so that mechanically moving parts are not required.

The measures listed in the dependent claims allow advantageous developments and improvements of the method specified in the main claim. It is particularly advantageous that the sensor supplies an analog, frequency or time signal for the level, the signal being essentially independent of strong vibrations and wave movements in the liquid container due to the mechanical structure of the sensor.

It is also advantageous that the sensor is constructed in the form of a coaxial capacitor or waveguide, an air cell being provided between two electrodes, into which the liquid can penetrate according to the level. Due to the relatively narrow air cell, strong level fluctuations are advantageously largely suppressed. In particular, by designing the waveguide in the form of a commercially available air cell cable, it is advantageously achieved that this sensor can be adapted to almost any container shape in such a way that, for example, simple linear or logarithmic scaling on one

Display device is possible. When the sensor is designed as a waveguide, the position of the impedance jump at the transition between air and the liquid medium or the change in the

Standing wave ratio on the waveguide or the signal transit time can advantageously be determined as a measure of the level.

A difference sensor can also be used to calibrate the measured value, which, for example, is completely surrounded by the liquid and thus supplies a corresponding reference signal. This sensor can, for example, also be arranged in the tank pipe or a drain pipe in order, for example, to continuously monitor a desired liquid drain.

Since the liquid between the two electrodes of the sensor acts as a dielectric, it can be determined on the basis of the specific property of the liquid whether undesirable admixtures of another, different liquid are present. For example, it can be used in this way to determine whether there is water in the fuel and, if appropriate, how high the proportion is.

In a particularly advantageous embodiment of the invention, the sensor has the form of a delay waveguide. As a result, the transit time of the electrical signals can be increased, which in the evaluation of the Pulse echoes or other, in particular high-frequency electrical properties, simpler, slower and therefore more cost-effective electronic components can be used in an evaluation circuit.

drawing

Embodiments of the invention are shown in the drawing and explained in more detail in the following description.

Figure 1 shows a container with several sensors and Figure 2 shows the structure of a sensor in a spatial representation and in Figures 3 and 4 embodiments of a sensor or the associated waveguide are shown. FIG. 5 shows an evaluation circuit for a fill level sensor that works by means of an electrical pulse-echo method.

Description of the embodiments

According to FIG. 1, at least one sensor 3 is attached inside or outside of an arbitrarily shaped container 1, for example a fuel tank of a motor vehicle, which can monitor the entire container or different areas of the container 1. Sensors 3a, 3b are shown, but there may also be more, for example 4 sensors 3, which monitor different tank areas. The container 1 is, for example, partially filled with a liquid 2, which also rises to approximately the same level within the sensors 3. At the bottom of the container 1, a reference sensor 4 is arranged, through which the liquid 2 flows completely. In an alternative embodiment, a further sensor 5 is provided, for example in a Outlet pipe 11 of the container 1 is arranged. A pump 6, which is arranged in front of the outlet pipe 11, can be provided in the case of a fuel tank for the fuel delivery. Likewise, the container 1 has a tank cap 13, which is used to fill the container 1. Further

Feedthroughs 12 are provided in particular for the installation and removal of the sensors 3 and for the cable feedthrough for the electrical lines 9a, 9b.

The electrical lines 9 are with a

Evaluation device 7 connected, which essentially has a measuring device, the z. B. measures the capacity of the at least partially immersed sensors 3 with the aid of known measuring methods and delivers a corresponding display signal via a line 10 to a display 8. Alternative possible measured variables are, for example, the wave properties of the coaxial waveguide.

FIG. 2 shows a partial view of a sensor 3, which is essentially constructed as a coaxial capacitor or waveguide. Such waveguides are commercially available, for example, as coaxial air cell cables and are used for the transmission of high-frequency electrical signals. The waveguide has a first

Electrode 21, which is designed as a central conductor. A second electrode 24 is arranged around the first electrode 21 and is held coaxially to the first electrode 21 by a supporting skeleton 23. The support skeleton 23 has one or more air cells 22, so that when the waveguide is immersed in the liquid, it can flow between the two electrodes 21, 24, since the end of the waveguide is not closed. To protect against electrical short circuits, an insulation 25 is preferably applied around the second electrode 24. The in the Liquid-immersed ends are also isolated. The structure and structure of the waveguide is designed such that it is semi-rigid and can therefore also be laid in an angular, curved or loop shape inside the container 1 or, for example, also in the outlet pipe 11, along the outer wall or also away from it , In this way it is possible to transform the measured value in such a way that a linear, logarithmic or other suitable scaling on the display 8 is possible as desired. Known measuring methods are used to determine the level in the container 1, the capacitance between the electrodes being measured in the simplest case. The sensor capacitance can also be used to generate a dependent frequency or pulse duration in a corresponding circuit (oscillator, RC or LC timing element). An alternative evaluation circuit can, by feeding in a high-frequency voltage, the wave properties such as the standing wave ratio, the frequency of a defined standing wave between the liquid and the feed point, the impedance or other electrical

Measure properties that change when the air cells 22 are filled with the liquid 2. Furthermore, the waveguide is designed in such a way that it does not significantly change its electrical properties when it is deformed in the desired directions. Due to the deformation of the waveguide, an adaptation to complicated container shapes can also be achieved. This makes it possible to determine the volume or the mass determination linearly with the filling length of the sensor 3.

In a further embodiment of the invention, it is also provided that, when measuring the capacitance, a plurality of sensors 3 are connected in parallel and arranged at a plurality of points in the container 1, in order, for example, to also dependence on the inclination and acceleration of the measurement signal portable containers 1 to compensate. Furthermore, to compensate for the substance dependencies, provision is optionally made to install a reference sensor 4, which is preferably of identical design and has a known length. This reference sensor 4 is arranged such that it is immersed as completely as possible in the liquid 2. This can be achieved, for example, by designing the reference sensor 4 as a piece of the suction, flow or return line or at the deepest point of the container 1, preferably in the area of a suction point for the pump 6. It is also provided that the Absolute value of the reference sensor 4 is used to compensate for a quantity falsification by undesired admixtures, for example water in the fuel. The proportion of the amount added (water) can also be determined, as will be explained in more detail below. It is also possible to recognize a sediment if an additional sensor has been arranged accordingly. If there is a water layer on the bottom of the container 1, the sensor can be flushed with the fuel at intervals to prevent incorrect measurement and thus freed from the water.

The mode of operation of sensor 3 is explained in more detail below using different evaluation methods.

After a first evaluation method, the capacitance C of the sensor 3 is measured. It is composed of a constant basic capacitance C _{Q of} the sensor 3 filled with air and an additional capacitance dC which increases linearly with the filling quantity (mass or volume at constant temperature) and is determined by the dielectric constant of the liquid 2 and by the geometric structure of the sensor. dC C - C

If there are several sensors 3 connected in parallel, the capacitance values of the individual sensors must be added. The filling quantity V thus results

V = Ks (s) * dC / KM,

where for the change in volume dV depending on the immersion length ds the conversion factor Ks (s) according to the formula

Ks (s) = dV (s) / ds

is determined. The shape can continuously influence Ks (s) over the entire length due to the inclined position. For linearization of the signal, Ks (s) should preferably be kept constant by adjusting the inclined position over the entire fill level. The factor KM is a proportionality factor that determines the dependency of the

Changes in capacitance dCx from the change in fill length due to the medium, the geometry and the material of the sensor 3 are taken into account. KM is calculated using the formula:

KM = dCx / dl

Thus, all parameters for determining the volume V for the liquid 2 in the container 1 are known.

A second alternative evaluation method for the

Determination of the amount of liquid or the level in the container 1 uses the reflection of high-frequency waves on impedance jumps in the waveguide. The part of the sensor 3 immersed in the liquid 2 changes its length-specific capacitance Cx and thus its impedance Z abruptly between the immersed and non-immersed part of the sensor. The impedance Z is calculated for the waveguide using the formula

Z = (Lx / Cx) ⁰ ' ⁵

where Lx is the length-specific inductance and Cx is the length-specific capacitance of the waveguide. The measuring principle consists in that a high-frequency signal is fed in at the non-immersed end of the sensor 3, which is partially reflected by the jump in impedance, that is to say on the surface of the liquid of the partially immersed sensor 3, so that a partially standing wave is produced. In order to avoid a disturbing reflection at the end of the sensor 3, it must be equipped with an appropriate one

Terminating resistor. By measuring or by varying the feed frequency and fixing the nodes and bellies of this standing wave, the distance of the water level in relation to the measurement point (feed point) can be deduced. The ratio of the maximum and minimum current along the waveguide (standing wave ratio) can also be used to draw conclusions about the changed length-specific capacitance Cx and thus the dielectric constant of the liquid 2. Of course, the chosen electricity and

Voltage values selected so that no hazard can arise.

A third alternative measuring method for the level of the liquid 2 in the container 1 is that the

Correction factor KM is determined using the reference sensor 4. The correction factor KM is determined depending on the current filling medium using the following formula:

KM = dCref / lref determined, the values dCref and lref being the reference values for the capacitance and length of the reference element 4. If this correction factor KM is used in the formula for the first evaluation method, the result is for the volume

V = Ks * lref * dC / dCref.

As a result, as in the second alternative evaluation method, the determination of the container content becomes purely volume-dependent depending on the filling medium and the temperature expansion of the container contents. On the other hand, the first evaluation method is more a mass determination and therefore independent of temperature.

Finally, a fourth evaluation method for volume determination can be provided. The specific constants KM are assumed to be known for two different filling media or any mixing ratios of these two. The reference sensor 4 determines the current value of KM. This allows the mixing ratio of the two components, e.g. B. can be determined by interpolation. The method thus also offers the possibility, for example, of determining the water content in the fuel of a vehicle tank or possibly identifying incorrect or different types of fuel.

In a further embodiment, the sensor has the form of a delay waveguide. The transit time T for electrical signals in a conductor is calculated from the length-specific inductance L ^Λ and the length-specific capacitance C ^Λ according to the formula

The impedance Z of the conductor is according to the formula

Z = V / C

certainly. In a delay waveguide, the transit time of electrical signals can therefore preferably be increased by increasing L ^Λ or C ^Λ . In addition, when using a delay waveguide, the scale can be adjusted (eg linearization or logarithmization of an irregularly shaped container) by varying L ^Λ and C over the sensor length. Appropriately, the ratio of L and C ^{Λ is} kept constant over the sensor length by means of suitable measures, so that no additional, possibly disturbing reflections of the signal are obtained when evaluating the radio frequency (HF) or pulse signals.

Preferably, the delay waveguide in the in

Figure 1 outlined following designs are used, with suitable adaptations, which are described in more detail in the following embodiments.

In the exemplary embodiment according to FIG. 3, the inner conductor of the waveguide is a long 1-layer coil 31 on an insulating rod core 32. The length-specific inductance L ^{λ is} significantly increased by the construction of the inner conductor of the coaxial waveguide as a coil. In the space between the surface of the coil and the jacket tube is the filling medium 33 which can penetrate into the interior through small holes 34 along and / or below and above the jacket tube 35. Several holes along the casing tube, at least in the lower area, are necessary if a sediment of an undesired medium (e.g. water) the measurement should not be falsified by an excessive level within the sensor, and strong sloshing movements of the medium should not interfere with portable containers.

The surface of the coil and the inner surface of the jacket tube form the capacitance of the waveguide. For adaptation of the scale length-specific inductance L ^Λ can be varied by the winding density of the coil. The possibly desired proportional change in the length-specific capacitance C ^Λ is already caused by the thinning surface of the coil conductor when the coil winding is thinned 36 and can be completed by changing the coil or jacket tube diameter 37.

In the exemplary embodiment according to FIG. 4, the inner conductor of the waveguide is a long 1-layer coil 41 with a hollow core. One inside 42 and one outside 43 of the coil 41 arranged conducting cylinder together form the

External conductor. In contrast to the design according to FIG. 3, the small bores 44 in the jacket or in the inner conductor of the sensor are required for damping containers which can be moved to prevent an extreme sloshing movement within the sensor. Extreme slant of the

Liquid levels inside the sensor would otherwise make the pulse echo undetectable.

To prevent eddy currents, the sheathed conductors can be slit open in the longitudinal direction and, if necessary, sealed by insulation 45 (or 45a, 45b), or can consist entirely of several longitudinal segments. As in the exemplary embodiment according to FIG. 2, only the outer jacket 43 can be constructed from a plurality of longitudinal segments. The larger diameter of this structure leads Compared to the first design, the delay times are considerably longer and the evaluation circuits are simpler.

In an advantageous embodiment, support rods 47 can be installed in the filling space 46 (or 46a, 46b) as coil supports. These can simultaneously be used as partitions 48 for filling rooms arranged in a sector. To further increase the length-specific inductance, these supporting and dividing walls or dividing bars can be produced from a ferromagnetic material suitable for high frequencies. It can also be in another suitable location, e.g. such a high-frequency ferromagnetic material can be installed as a coating on the sheathed conductor. The same applies to the adjustment of the scale as for the design one according to FIG. 3.

An evaluation circuit for a pulse-echo method is preferably suitable as the evaluation device. Such an evaluation circuit is shown in FIG. 5. In the pulse-echo method, a short electrical pulse II is generated in a pulse shaper stage 51, which is connected to the measuring clock oscillator 51a. The electrical pulse II or successive pulses is or are fed to the sensor via an impedance matching 52. Each on

Impedance jump SP1 and SP2 at the transitions of the filling media 53, 54 and at the short-circuited end SP3 of the sensor, for example, a reflection takes place. After the double running time 2T, a signal is registered as an echo at the input of the sensor, which is measurable. This signal is shown in block 55. It can be seen that after pulse II the signal SPla reflected at the jump in impedance SP1 occurs first, then the signal SP2a reflected at the other jump in impedance and finally the signal SP3a reflected at the end of the sensor. The reflected signals SPla, SP2a, SP3a are processed in a voltage follower 56 and compared in comparators 57, 58, 59 with threshold values S1, S2 and S3. Using flip-flops 60, 61 or 62 and a subsequent gate logic 63, 64 or 65, with which the signal components caused by the reflections can be separated from one another and a pulse length measurement 66, 61 or 68, the transit time 2T can be separated for each medium in a logical pulse length can be converted. The signal curves that occur are indicated above the individual components 57 to 68. The counting pulses are provided by a timing clock oscillator 69, which is also connected to the counters 70, 71 and 72 via a frequency divider. The pulse length can therefore be converted into a number of pulses by a simple gate circuit for a timing clock oscillator 69.

In a further evaluation method, the sensor voltage or the echo signal that can be measured at the input of the sensor can be directly digitized or, since pulses II that are emitted quickly on one another, the reflected signal SPla, SP2a, SP3a is also a signal that can be repeated at high frequency, also by means of a Stretching or sampling processes are temporally stretched and digitized. A scanning method known for example in DE-P 198 24 047 is also suitable for the temporal expansion. The digitized echo signal is characteristic of the duration of the pulse echo or the level in the sensor. The detection of the echoes and the conversion into fill level and / or water content can then be carried out in a process computer. In the case of a fill level sensor for a motor vehicle, the process computer can be part of the engine control unit.

Other parameters such as evaluating the slope or the pulse height can also be used to determine the signal propagation time or the fill level at least be taken into account when generating correction signals. Furthermore, a direct form comparison of the measurement signal or of parts of the measurement signal or of parameters obtained from the measurement signal with measurement signals or parts thereof or parameters thereof obtained from calibration measurements can be carried out. It is also possible to interpolate between the two closest measurement signals or from measurement signals or parameters. The means required for implementation are contained, for example, in the processor or an associated control unit.

A combination of the signal evaluation described above with the use of a sensor with delay waveguide is also possible

Claims

Expectations

1. A method for level measurement of a liquid (2) in a container (1), wherein at least one sensor (3) is arranged inside or as a communicating tube outside the container (1) so that it is at least partially immersed in the liquid, the Sensor (3) via a line (9) delivers an electrical signal dependent on the immersion depth in the liquid (2) or its fill level to an evaluation device (7), characterized in that the sensor (3) has one in its

Capacitance variable capacitor or characteristic impedance, the capacitance, the characteristic impedance or its changes being determined by the installation depth in the liquid (2), and the evaluation device (7) forms an indication signal for the quantity of liquid or the liquid level from the received electrical signals.

2. The method according to claim 1, characterized in that an impedance jump occurring in the region of the liquid level in the waveguide leads to reflections with standing waves which are evaluated.

3. The method according to claim 1 or 2, characterized in that the position of the liquid level in the waveguide is determined by varying the high-frequency signal fed in, the voltage or current nodes or bellies being counted or set between the liquid level and a known measuring point, taking into account the frequency and the speed of propagation.

4. The method according to any one of claims 1 to 3, characterized in that at least a short electrical pulse is given to the waveguide (3) for a known measuring point and that the transit time of the echo signal up to the reflected liquid level is measured and evaluated.

5. The method according to claim 4, characterized in that the transit time of the fed-in signal is measured with its echo at the open and short-circuited end of the waveguide.

6. The method according to any one of the preceding claims, characterized in that the length and / or position of the sensor (3) in the container (1) can be used to adapt the measured value to a scale.

7. The method according to any one of the preceding claims, characterized in that for calibration

Reference sensor (4) with defined dimensions can be used, which is completely flowed through by the liquid (2).

8. The method according to claim 7, characterized in that the sensor signal for detecting admixtures in the liquid (2) can be used.

9. The method according to claim 7 or 8, characterized in that from the output signal of the reference sensor (4) Correction factor (KM) is determined, which is independent of the current filling medium.

10. The method according to claim 9, characterized in that in the case of two different liquids (2) whose specific correction factors (KM) are known, the mixing ratio is preferably determined by water and fuel.

11. The method according to any one of the preceding claims, characterized in that it is used to determine the fill level of a tank of a motor vehicle which can be filled with fuel.

12. The method according to any one of the preceding claims, characterized in that the acceleration and / or the inclined position of the container (1) is determined from the measured values of the individual sensors (3).

17. Sensor, in particular for carrying out the method according to one of the preceding claims, characterized in that the sensor (3) is a coaxially constructed waveguide, that the waveguide has a central first electrode (21) around which by means of a support skeleton (23) a second electrode (24) is arranged coaxially, and that between the two electrodes (21, 24) at least one air cell (22) is formed, into which the liquid (2) can penetrate according to the fill level in the container (1).

18. Sensor according to claim 17, characterized in that the sensor (3) provides an analog signal for the level.

19. Sensor according to claim 17 or 18, characterized in that the sensor (3) is constructed in the form of a coaxial capacitor or a waveguide, a lead wire forming a first electrode (21) and a metallic sheathing forming a second electrode (24), between which there is essentially the liquid to be measured (2).

20. Sensor according to claim 17, 18 or 19, characterized in that the waveguide is an air cell cable, at least one air cell (22) being formed between the first electrode (21) and the second electrode (24).

21. Sensor according to one of claims 17 to 20, characterized in that the sensor (3) can be operated as a waveguide for a high-frequency signal and that the change in impedance (Z) is a measure of the liquid volume.

22. Sensor according to one of the preceding claims, characterized in that the waveguide is constructed in the form of a delay waveguide, via which the transit time of electrical signals can be extended.

23. Sensor according to one of the preceding claims, characterized in that the length-specific inductance and the length-specific capacitance of the

Delay waveguide can be determined by design measures in the desired manner.

24. Sensor according to one of the preceding claims, characterized in that the inner conductor of the delay waveguide is constructed as a single-layer coil on an insulating rod core.

25. Sensor according to claim 24, characterized in that a jacket tube with small holes is present, which surrounds the surface of the single-layer coil, whereby the surface of the coil and the inner surface of the jacket tube form the length-specific capacitance of the waveguide.

26. Sensor according to claim 24, characterized in that the conductors of the tubular casing are slotted in the longitudinal direction and are sealed by insulation or are constructed from a plurality of longitudinal segments.

27. Sensor according to one of the preceding claims, characterized in that predeterminable components of the casing tube or additional support rods or additional partitions are made of high-frequency ferromagnetic material.

28. Sensor according to one of the preceding claims, characterized in that an evaluation circuit is provided which comprises means for carrying out a pulse-echo method, with at least three comparators (57, 58, 59) which the sensor signal comprising the reflected signal components compare with predefinable thresholds (S1, S2, S3) and with downstream flip-flops (60-65), which form a gate logic for signal separation and means for pulse length measurement (66-72) for determining the transit time of the reflected signals.

29. Sensor according to claim 28, characterized in that the means for measuring the pulse length comprises at least three logic elements (66, 67, 68) to which the signal components caused by the reflections can be fed and downstream counters (70, 71, 72), the three logic elements (66, 67, 68) and the counters (70, 71, 72) are connected to a clock oscillator (69).

30. Sensor according to any one of the preceding claims, characterized in that evaluation means are present which comprise at least one processor and means for digitizing the sensor voltage and further means for temporally stretched signal sampling.

31. Sensor according to claim 30, characterized in that the means for digitizing the sensor voltage and for temporally stretched signal sampling comprise a scanning control device.

32. Sensor according to claim 31, characterized in that additional means for comparing the shape of the measurement signal or quantities derived from the measurement signal with correspondingly formed calibration signals are provided and the comparison results are also taken into account in the signal evaluation.

33. Device for level measurement according to one of the preceding claims, characterized in that irregularly shaped and portable containers (1) are equipped with several sensors (3) for different partial volumes, which are connected in series or in parallel so that measurement errors due to movement or inclined position of the Container (1) are corrected approximately.