DE10133692A1 - Method of measuring level of liquid in container involves forming indication signal from variable capacitance of capacitor or characteristic impedance of immersion sensor - Google Patents

Abstract

The method involves arranging at least one sensor to be at least partly immersed in the liquid. The sensor passes an electrical signal depending on the depth of immersion or the liquid level to an evaluation device and has a capacitor or characteristic impedance with variable capacitance determined by the immersion depth. The evaluation device forms a level indication signal from the electrical signal. Independent claims are also included for the following: (1) A sensor, especially for implementing the method; (2) A level measurement device.

Description

State of the art

The invention is based on a method or a sensor and a device with at least two sensors Level measurement of a liquid in a container, according to the genus of secondary claims. Level meter for various liquids are already known and are manufactured in numerous technologies. The well-known Level meters work for example according to mechanical, electrical, thermal, capacitive, inductive or frequency-modeled procedures. For example, Fuel measurement in a vehicle tank resistance sensors used, by means of a float Resistance ratio of a divider depending on Level in the liquid container is changed. This Resistance ratio is used for the evaluation of the Level or the amount of liquid. Mechanical sensors have the disadvantage, however, that their mechanics are relative One is particularly sensitive in rough everyday operation Motor vehicle. On the other hand, the encoder signal fluctuates the vibrations during the trip very strong, so that further measures need to be taken to address the to compensate for constantly changing measured values. Purely electric  Measuring sensors, on the other hand, have the disadvantage that they are built complex. Especially with containers with irregular geometry, the measurement result is not without further linearizable so that the scale for one Level indicator is more difficult to manufacture.

Advantages of the invention

The inventive method or the sensor and the Device according to the invention with at least two sensors with the characteristic features of the siblings In contrast, claims has the advantage that the water level the liquid with the help of changing an electrical Size is detected. This is done after a pure electrical measuring method, so that mechanically moving parts are not required.

By those listed in the dependent claims Measures are advantageous training and Improvements to the method specified in the main claim possible. It is particularly advantageous that the sensor analog, frequency or time signal for the level delivers, the signal due to the mechanical structure the sensor is essentially independent of strong ones Shocks and wave movements in the Liquid container.

It is also advantageous that the sensor is in the form of a coaxial capacitor or waveguide is constructed, an air cell being provided between two electrodes is in the liquid according to the level can penetrate. Because of the relatively narrow air cell strong fluctuations in level advantageously largely suppressed.  

In particular, by shaping the waveguide a commercially available air cell cable is advantageous achieved that this sensor attached to almost every container shape can be adapted, for example, a simple linear or logarithmic scaling on one Display device is possible. So when training the sensor as a waveguide also by means of a high-frequency signal or by means of a short electrical pulse the position of the Impedance jump at the transition between air and the Liquid medium or the change of Standing wave ratio on the waveguide or Signal transit time advantageous as a measure of the level be determined.

A differential sensor is also used to calibrate the measured value usable, for example, full of the liquid is surrounded and thus a corresponding reference signal delivers. This sensor can also be found in the tank pipe, for example or a drain pipe can be arranged, for example a desired liquid drain completely monitor.

Since the liquid between the two electrodes of the Sensor acts as a dielectric, due to the specific property of the liquid found whether undesirable admixtures of another, other liquid are present. For example, on this way it can be determined whether there is water in the There is fuel and, if applicable, how high the proportion is.

In a particularly advantageous embodiment of the invention the sensor has the form of a delay Waveguide. This can reduce the running time of the electrical Signals are increased, so when evaluating the  Pulse echoes or other, especially high-frequency electrical properties simpler, slower and therefore less expensive electronic components in one Evaluation circuit can be used.

drawing

Embodiment of the invention are in the drawing shown and in the description below explained.

Fig. 1 shows a container with a plurality of sensors, and Fig. 2 shows the structure of a sensor in a spatial representation, and in Figs. 3 and 4 embodiments are shown of a sensor or the associated waveguide. In FIG. 5, an evaluation circuit is shown for a level sensor, which operates by means of electric 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 3 a, 3 b are shown, but there may also be more, for example 4 sensors 3 , which monitor different tank areas. The container 1 is e.g. B. 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, which is arranged, for example, in an outlet pipe 11 of the container 1 . 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 bushings 12 are provided in particular for the installation and removal of the sensors 3 and for the cable bushing for the electrical lines 9 a, 9 b.

The electrical lines 9 are connected to an evaluation device 7 which essentially has a measuring device which, for. 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, insulation 25 is preferably applied around the second electrode 24 . The ends immersed in the liquid 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 measure 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 properties that change when the air cells 22 are filled with the liquid 2 by feeding in a high-frequency voltage . 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. The deformation of the waveguide can therefore also be adapted to complicated container shapes. This enables the volume or the mass determination to be determined 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 several locations on the container 1 , for example to compensate for inclination and acceleration dependencies of the measurement signal in the case of containers 1 that can be moved. 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 realized, for example, by designing the reference sensor 4 as a piece of the suction, flow or return line or at the lowest 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 at 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 the 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 _{0 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 ₀

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 takes into account the dependence of the change in capacitance dCx on the change in fill length due to the medium, the geometry and the material of the sensor 3 . 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 determining the amount of liquid or the level in the container 1 uses the reflection of high-frequency waves at jumps in impedance in the waveguide. The part of the sensor 3 immersed in the liquid 2 suddenly changes its length-specific capacitance Cx and thus its impedance Z 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 is 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 , this must be provided with an appropriate 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 selected current and voltage values are chosen so that no hazard can arise.

A third alternative measurement method for the fill level of the liquid 2 in the container 1 consists in that the correction factor KM is determined with the aid of 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.

The determination of the container content is thereby - as well in the second alternative evaluation method - to the pure Volume determination depending on the filling medium and the Temperature expansion of the container contents. However, it is first evaluation method rather a mass determination and thus 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 'and the length-specific capacitance C' according to the formula

T = √L'C '  

The impedance Z of the conductor is according to the formula

Z = √L '/ C'

certainly. In a delay waveguide, the Runtime of electrical signals is therefore preferably increased are increased by increasing L 'or C'. In addition, at Adjustment using a delay waveguide the scale (e.g. linearization or logarithmization of a irregularly shaped container) also by the variation of L 'and C' over the sensor length. It makes sense to do this by taking suitable measures Ratio of L 'and C' over the sensor length kept constant in order to evaluate the high frequency (HF) - or pulse signals no additional, possibly disturbing To get reflections of the signal.

The delay waveguide can preferably be used in the following designs outlined in FIG. 1, with suitable adaptations which are described in more detail in the following exemplary embodiments.

In the embodiment of Fig. 3 is the inner conductor of the waveguide a long 1-layer coil 31 on an insulating rod core 32. The length-specific inductance L '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 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) is not to falsify the measurement due to an excessive level within the sensor, and strong sloshing movements of the medium do not interfere with portable containers should.

The surface of the coil and the inner surface of the jacket tube form the capacitance of the waveguide. To adjust the scale, the length-specific inductance L 'can be varied by the winding density of the coil. The possibly desired proportional change in the length-specific capacitance C 'is already caused by a thinning 36 of the coil winding due to the surface area of the coil conductor becoming smaller and can still 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 cylinder 42 and one outside 43 of the coil 41 arranged together form the sheathed 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 necessary for damping containers which can be moved in order to prevent an extreme sloshing movement within the sensor. Extreme slanting of the liquid level within the sensor would otherwise lead to the undetectability of the pulse echo.

To prevent eddy currents, the sheathed conductors can be slit open in the longitudinal direction and, if necessary, sealed by insulation 45 (or 45 a, 45 b), or 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 to considerably longer delay times compared to the first design and thus to simpler evaluation circuits.

In an advantageous embodiment, support rods 47 can be installed as coil carriers in the filling space 46 (or 46 a, 46 b). These can also 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. B. as a coating of the sheathed conductor, such a high-frequency suitable ferromagnetic material. 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. When pulse echo method generates a short electrical pulse I1 in a pulse shaper 51, which communicates with the measuring clock oscillator 51 in a connection. The electrical pulse I1 or successive pulses is or are supplied to the sensor via an impedance matching 52 . A reflection takes place at the impedance step SP1 and SP2 at the transitions of the filling media 53 , 54 and at the short-circuited end SP3 of the sensor, for example. After the double running time 2 T, 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 the pulse I1, the signal SP1a reflected at the jump in impedance SP1 occurs first, then the signal SP2a reflected in the other jump in impedance and finally the signal SP3a reflected at the end of the sensor. The reflected signals SP1a, 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 , 67 or 68 , the transit time 2 T can be separated for each medium be converted into a logical pulse length. The signal curves that occur are indicated above the individual components 57 to 68 . The counting pulses are provided by a clock oscillator 69 , which is also connected to the counters 70 , 71 and 72 via a frequency divider. The pulse length can thus be converted into a number of pulses by a simple gate circuit for a clock oscillator 69 .

The sensor voltage can be used in a further evaluation method or the echo signal measurable at the input of the sensor directly digitized or because it is quickly on each other emitted pulses I1 even with the reflected signal SP1a, SP2a, SP3a by a repeatable with high frequency Signal acts, also through an expansion or Sampling procedures are stretched and digitized. For example, a time extension is also suitable scanning method known in DE-P 198 24 047. The digitized echo signal is characteristic of the Runtime of the pulse echo, or the fill level in the sensor. The Detection of the echoes and the conversion into level and / or Water content can then take place in a process computer. At The process computer can use a level sensor for a motor vehicle Be part of the engine control unit.

For precise determination of the signal transit time or the fill level can also use other parameters such as evaluation of the Slope or the pulse height can be used or  at least when generating correction signals be taken into account. Furthermore, a direct Form comparison of the measurement signal or parts of the Measurement signals or from parameters obtained from the measurement signal with measurement signals or parts obtained from calibration measurements of this or perform parameters thereof. It can still between the two closest measurement signals or from Measurement signals or parameters can be interpolated. The for Funding required are for example in Processor or an associated control unit included.

A combination of those described above Signal evaluation with the use of a sensor Delay waveguide is also possible.

Claims (32)

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 ) delivers an electrical signal dependent on the depth of immersion in the liquid ( 2 ) or its fill level to an evaluation device ( 7 ) via a line ( 9 ), characterized in that the sensor ( 3 ) has a capacitor with a variable capacitance or has wave resistance, the capacity, the wave resistance or their changes being determined by the installation depth in the liquid ( 2 ), and the evaluation device ( 7 ) forming an indication signal for the amount of liquid or the liquid level from the received electrical signals. 2. The method according to claim 1, characterized in a Range of liquid level resulting impedance jump leads to reflections with standing waves in the waveguide, which are evaluated. 3. The method according to claim 1 or 2, characterized in that the position of the liquid level in the waveguide  by varying the high-frequency signal fed in is determined, between the liquid level and a known measuring point the voltage or current nodes or bellies taking into account the frequency and the Propagation speed can be counted or set. 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 duration of the signal fed in with its echo on open and shorted end of the waveguide is measured. 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 a reference sensor ( 4 ) with defined dimensions can be used for calibration, which is completely flowed through by the liquid ( 2 ). 8. The method according to claim 7, characterized in that the sensor signal can be used to detect admixtures in the liquid ( 2 ). 9. The method according to claim 7 or 8, characterized in that a correction factor (KM) is determined from the output signal of the reference sensor ( 4 ), 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 Level of a tank of a motor vehicle with Fuel is fillable, is used. 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 ). 13. The method according to any one of the preceding claims, characterized in that a calibration signal is formed from the measured values of the individual sensors ( 3 ), which is compared with the measured signal. 14. Sensor, in particular for performing the method according to one of the preceding claims, characterized in that the sensor ( 3 ) has a transducer in the form of a delay waveguide. 15. Sensor according to claim 14, characterized in that the delay waveguide is length-specific Has inductance and a length-specific capacitance. 16. Sensor according to claim 14 or 15, characterized in that the sensor ( 3 ) is a differential sensor. 17. Sensor according to claim 14, 15 or 16, 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 supporting 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 14, 15, 16 or 17, characterized in that the sensor ( 3 ) provides an analog signal for the level. 19. Sensor according to any one of claims 14 to 18, characterized in that the sensor ( 3 ) is constructed in the form of a coaxial capacitor or a waveguide, a lead wire having a first electrode ( 21 ) and a metallic sheath a second electrode ( 24 ) form, between which there is essentially the liquid to be metered ( 2 ). 20. Sensor according to one of claims 14 to 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 14 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 characterized in that the waveguide is in the form of a Delay waveguide is constructed over which the Runtime of electrical signals can be extended. 23. Sensor according to one of the preceding claims, characterized characterized that the length-specific inductance and the length specific capacity of the Delay waveguide through constructive measures in the way you want. 24. Sensor according to any one of the preceding claims, characterized characterized that the inner conductor of the Delay waveguide as a single-layer coil on one insulating rod core is built. 25. Sensor according to claim 24, characterized in that there is a jacket tube with small holes that the Surrounding 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 jacket tube are slotted in the longitudinal direction and are sealed by insulation or by several Longitudinal segments are built. 27. Sensor according to one of the preceding claims, characterized characterized that predeterminable components of the Jacket tube or additional support rods or additional Partitions 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 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 supplied 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 one of the preceding claims, characterized characterized that evaluation means are available that comprise at least one processor and means for Digitization of the sensor voltage and other means for extended signal sampling. 31. Sensor according to claim 30, characterized in that the means for digitizing the sensor voltage and extended signal sampling a sampling Include control device. 32. Sensor according to claim 31, characterized in that additional means for comparing the shape of the measurement signal or off derived values with the measurement signal accordingly formed calibration signals are present and the Comparison results in signal evaluation be taken into account.