DE10231946A1 - Method for measuring the level of a fluid in a container and corresponding level sensor - Google Patents

Abstract

The present invention relates to a method for measuring the fill level of a fluid, in particular a liquid, in a container by measuring the average charging current of one or more capacitors which are periodically charged and discharged. The invention also relates to a corresponding fill level sensor which is suitable for carrying out the method and which has a DC voltage source (11), at least one capacitive sensor element (14) which has a measuring electrode (13) and a counter electrode (15), an electronic switch (12), which can be switched periodically between a first position for charging the measuring electrode (13) and a second position for discharging the measuring electrode (13), and means (18) for measuring the average charging current flowing between the DC voltage source (11) and the measuring electrode (13) includes.

Description

The invention relates to a method for measuring the level a fluid, in particular a liquid, in a container and an appropriate one to carry out level sensor suitable for the method.

In different areas It is important in technology to check the level of fluids, for example of liquids or pasty Materials, to be measured and further processed, for example in electronic form or display. The measurement of the liquid level is an example in fuel, cooling water, oil or Brake fluid containers from Motor vehicles, in underfloor tanks of petrol stations or in a wide variety Storage containers and Reactors in the chemical or food industry called. Level sensors can on various physical measurement principles are based. So are hydrostatic or pneumatic sensors known which measure the height of a liquid level measure the hydrostatic pressure of a liquid column above the sensor. Radar sensors or optical sensors are also known, which the height a liquid level over the Running time of short radar or Determine light pulses that reflect on the surface of the liquid become. Other optical sensors for level measurement rework the principle of a light barrier. A particularly wide range of applications have mechanical level sensors found because they are simple to build and are therefore inexpensive to manufacture. With mechanical level sensors becomes the position of a swimmer who is in the liquid of interest immersed, measured and over converted a resistance piston into an electrical signal. For the transfer the level lever sensors usually become from the float to the resistance piston or dip tubes used.

Hydrostatic sensors, radar sensors or optical sensors are complex and expensive and are therefore not suitable for areas of application which are exposed to high cost pressures, as is the case, for example, in the motor vehicle industry. The inexpensive mechanical sensors in turn have the disadvantage that the movable mechanical components are susceptible to wear and can be damaged if subjected to excessive mechanical stress. Another disadvantage of mechanical level sensors is that they cannot be used if, for example, packing elements are to be installed in a container for flammable liquids to prevent fires or explosions. Such fillers can be obtained, for example, from a product developed by the applicant under the product name "eXess" and in the European patent application EP-A-0 699 474 described area-like molded part exist.

Level sensors are also known which work with capacitive sensor elements. With such capacitive measuring methods, the capacitance of a capacitor formed from measuring electrodes is used as a measure of the fill level. This takes advantage of the fact that the capacitance of the capacitor also changes when the dielectric between the measuring electrodes changes. This results in the capacitance C _{x of} a capacitor which is immersed in a liquid with the relative dielectric constant ε _r over a section of its length I _m and has air as a dielectric (ε _r of air ≈ 1) on the remaining section I _g
C x = kε 0 (I G + I m ε r ).
where k is an electrode constant into which - for example in the case of a plate capacitor - essentially the width and the spacing of the capacitor plates are included, and eo are the absolute dielectric constant.

Usually have capacitive level sensors at least one capacitive sensor element, which consists of an excitation electrode and a sensor electrode. At the excitation electrode a sinusoidal or rectangular Voltage signal applied. As receiver acts a current-voltage amplifier, for example a transimpedance amplifier. capacity changes are often also in the capacitive sensor element of a level sensor in an oscillator circuit by measuring the capacitance of the sensor element dependent Vibration frequency can be determined.

Problematic for the measurement accuracy of the known capacitive level sensors are particularly parasitic Capacities, how they occur on the lines to the electrodes or stray capacities, such as by using several capacitive sensor elements, for example by Adjacent electronic components are created. Therefore, are conventional level sensors mostly with additional ones Provide shield electrodes and have elaborate compensation technology implemented in analog technology and evaluation circuits. For these reasons, capacitive level sensors are in the automotive sector, for example, not yet with the price widely used mechanical sensors competitive.

The present invention is therefore based on the technical problem, a simple and inexpensive method for level measurement and to provide a corresponding, cost-effective fill level sensor, the design of which can easily be adapted to the special requirements of a wide variety of application areas.

This problem is solved by the procedure for measuring the level of a fluid in a container according to the present Claim 1 and the level sensor according to the present Claim 7. Advantageous developments of the invention are the subject the dependent Expectations.

The invention accordingly relates to a method for measuring the fill level of a fluid in a container, wherein at least one capacitive sensor element, which comprises a measuring electrode and a counter electrode, is periodically charged and discharged again with a constant voltage, and the mean charging current is measured in the process. The level of the fluid in the container can be determined from the measured average charging current. This makes use of the fact that when a DC voltage U _{g is applied} in a pulsed manner to a discharged capacitor of capacitance C _x , a high charging current initially flows, which then decreases exponentially towards zero. The relationship applies to the average charging current I _M , which can be tapped, for example, via a low-pass circuit in the supply line to the measuring electrode by measuring the voltage drop across a suitable measuring resistor.
I M = C x U G N,
where N is the number of charge cycles per unit of time.

The subject of the invention is furthermore a level sensor with a DC voltage source, at least one capacitive sensor element, which has a measuring electrode and a counter electrode, a switch, which periodically between a first position for charging the measuring electrode and a second position for discharging the measuring electrode is and means for measuring the between the DC power source and the measuring electrode flowing medium charging current.

The method according to the invention and the device according to the invention have numerous advantages. So is the capacitive sensor element of the level sensor according to the invention particularly easy to set up. Instead of an excitation electrode and the capacitive sensor element merely has a sensor electrode a measuring electrode with a simple, advantageous on Ground counter electrode cooperates. From and to the capacitive Sensor element are only digital signals for switching the Transfer sensor element. The actual measurand in the The lead to the measuring electrode is the low-frequency charging current, whose mean is evaluated. There are therefore no complex shield electrodes or complex evaluation circuits to compensate for parasitic capacitances. The fill level sensor according to the invention can be predominantly with digital components, so that the previously used capacitive Measurement methods usual and relatively expensive analog electronics can be avoided. The fill level sensor according to the invention is about it easy to manufacture. Namely, it can be based on inexpensive processes from the electronics industry, for example the production of PCB, the assembly and testing the circuits.

The capacitive sensor is advantageously charged and discharged via one electronic switch.

The electronic switch is particularly advantageously switched at a defined frequency, so that, according to the above equation, the average charging current I _{M is} directly proportional to the sought capacitance C _{x of} the sensor element.

The electronic switch can be discreet for example, can be realized with FET transistors or as an analog switch be trained. The electronic switch is particularly advantageous however designed as a clockable logic gate, wherein the logic gate executes particularly preferably in CMOS technology, since the quiescent current is one such CMOS gate low and compared to that to be measured average charging current is negligible.

According to a particularly preferred embodiment several capacitive sensor elements are used in the invention, in particular several sequentially controllable capacitive sensor elements are provided. The level sensor according to the invention is accordingly segmentable, the measuring section consisting of one segment or can be composed of numerous segments. It is possible Assemble measuring sections of several meters from individual segments, without the known level sensors necessary elaborate Screening measures required would. By using several capacitive sensor elements, the measurement accuracy of the level sensor according to the invention is also simple scale. A given measuring section can be used to increase the Accuracy can be divided into short individual segments. The precision elevated accordingly proportional to the number of segments. It is also possible, for example the Measuring electrode as an elongated Execute electrode element. Depending on the angle of the electrode element to the surface of the fluid to be measured let yourself then also influence the measurement accuracy. Segmented level sensors are particularly suitable when monitoring the levels of multiphase liquids Need to become. On segmented level sensor allows also a particularly simple calibration at the transition of the liquid surface or -interface of one segment to the next segment.

If several sensor elements are used, a switch is preferably assigned to each sensor element, wherein the switches are particularly advantageous via a common clock line are sequentially switchable.

When using multiple sensor elements is also preferred that the sensor elements have a common, grounded Have counterelectrode, so that the manufacturing costs continue reduced.

The level sensor according to the invention can be not just on a rigid support, for example a circuit board, but particularly advantageous is the sensor, for example as a printed circuit, on a flexible or semi-flexible carrier educated. For example, a film can be used as a carrier material. So that the liquid sensor also simply curved surfaces can be assembled as often for example with tank containers occurrence. In the manufacture can the conductor structures, for example, printed on continuous film and the electronic components, for example by means of adhesive technology be applied. The slides can subsequently to the required Cut length and made up.

The invention is described below Reference to in the attached Exemplary embodiments shown in the drawings explained in more detail.

The drawing shows:

1 a schematic representation of the measuring principle on which the method according to the invention and the level sensor according to the invention are based;

2 a schematic representation of the dependence of a capacitor capacity on the level of a fluid;

3 a circuit diagram, which the interpretation of the switch of the 1 shows as electronic logic gate;

4 a schematic overall circuit diagram of a level sensor consisting of four capacitive sensor elements; and

5 a top view of a strip-shaped level sensor with four capacitive sensor elements.

In 1 is a schematic diagram of the circuit of a preferred embodiment of the level sensor according to the invention 10 shown. The level sensor 10 has a DC voltage source 11 on, the output voltage U _g , which is typically between 3 and 15 volts, via a switch 12 to a measuring electrode 13 a capacitive sensor element 14 can be created. The sensor element 14 also has a simple counter electrode which is at ground potential 15 on. The desk 12 has two positions, in which in 1 position shown the capacitive sensor element 14 is charged. The measuring electrode is in the second (not shown) position 13 also on ground 16 and is therefore discharged. Is symbolic in the insert 17 hinted that the switch 12 is periodically switched back and forth between the two positions, so that the DC voltage U _g in the form of rectangular pulses on the measuring electrode 13 is applied. Via a current measuring device 18 is in the supply line 19 by the voltage source 11 to the measuring electrode 13 leads, the average charging current I _m measured. The average charging current I _m is, as already explained in the introduction, at a constant voltage U _g and a predetermined number N of charging cycles per unit of time directly to the capacitance C _{x of} the capacitive sensor element 14 proportional. The dependence of the capacity on the fill level is explained below.

In 2 schematically shows a situation in which the capacitive sensor element 14 partially in a liquid 20 is immersed with a relative dielectric constant ε _r > 1. Between the measuring electrode 13 and the counter electrode 15 there is therefore a liquid dielectric on a section I _{m of} the capacitor length 20 with ε _r > 1, while there is a gaseous dielectric on the remaining section I _g 21 (Air) with a relative dielectric constant ε _r ≈ 1 between the electrode plates 13 . 15 located. The total capacitance C _{x of} the capacitive sensor element 14 This results in a parallel connection of the two resulting partial capacities as follows: C x = kε 0 (I G + I m ε r ).

Do you know the total measuring distance 1 the level can be determined from the determination of I _m by determining the capacitor capacitance C _x and using the relation I _g = I - I _m . The capacity in turn can be determined from the average charging current. Depending on the capacitance C _x , more or less charge flows to the electrodes in a time predetermined by the frequency (number N of charging cycles per unit time). This charge transport corresponds to a charge current, the mean value of which is proportional to the capacity. The charging current itself has an exponentially falling characteristic for each voltage pulse. Therefore one becomes advantageous in the ammeter 18 ( 1 ), for example, smooth the current profile using a first-order low-pass filter, as described below with reference to the in 3 circuit shown is explained in more detail.

3 shows schematically a preferred embodiment of the switch 12 , which in the example shown as a CMOS logic gate 22 is trained. 3 shows a NAND gate, it is irrelevant for the charging principle of the capacitor, which logic type (AND, NOR, NAND, NOR, etc.) is used. The CMOS gate 22 is made up of p-channel MOSFETs 23 and n-channel MOSFETs 24 builds. The two entrances 25 . 26 of the gate are controlled by a (not shown) microprocessor with a frequency CLK ₁ of 300 kHz or a frequency CLK ₂ of 100 Hz. According to the truth table of the NAND gate one finds that when the input 26 is equal to "0" (ie LOW), that at the connection 27 DC voltage signal present with that at the connection 25 clock frequency CLK applied to the capacitive sensor 14 leading exit 28 appears. In the intervals in which the exit 28 with that through the entrance 25 predetermined clock is at "0" (LOW), the n-channel MOSFETs pull the output 28 against those at the connection 29 lying mass 16 and the capacitive sensor element 14 will be unloaded. The one when charging the sensor element 14 flowing charging current can than over a measuring resistor 30 generated voltage drop are tapped and digitized by means of an (not shown) analog / digital converter for further data processing. For smoothing the charging current while on the sensor element 14 applied rectangular voltage pulse, has the ammeter 18 advantageously also a suitably dimensioned capacitor 31 on so that a low pass 1 , Order is formed and on resistance 30 essentially the low-frequency current component is detected.

4 now shows a circuit of a further embodiment of the level sensor according to the invention. Components that in connection with the embodiment of the 1 to 3 described components correspond or perform a comparable function, are designated by the same reference numerals. The level sensor 10 the 4 has a measuring section which is made up of four capacitive sensor elements connected in parallel 14a - 14d is formed. Every sensor element 14a - 14d is a switch designed as a CMOS logic gate 12a - 12d assigned, which in turn is clocked via a (not shown) microprocessor. At the in 4 shown embodiment, a circuit was realized in which only a single control line 32 for sequential control of the capacitive sensor elements 14a - 14d is needed. This is done before each input of the logic gate 12a - 12d a D flip-flop 33a - 33d switched whose Q output 34a - 34d the input signal for both the logic gate designed as a NAND gate 12a - 12d as well as for the D input of the next D flip-flop. In the example shown, the "clock inputs" of the flip-flops 33a - 33d from the control line 32 controlled with a frequency CLK ₂ of 100 Hz. Is at the input of the first flip-flop 33a If a signal is applied, the first significant flank of the 100 Hz clock is used Q -Output "0" (LOW) and the first NAND gate 12a for the over the line 35 supplied measuring clock CLK ₁ , which again has a frequency of 300 kHz, transparent. At the same time is due to the second flip-flop 33b a HIGH signal. The second flip-flop takes over on the next significant edge 33b the LOW signal of the first flip-flop 33a while at the same time at the input of the first flip-flop 33a there is again a HIGH signal and this flip-flop and the NAND gate 12a blocks again with a HIGH signal at the output. In this way, the LOW signal can be passed on by any number of flip-flops, so that any number of capacitive sensor elements with a control line 14 can be controlled.

The in 4 level sensor shown 10 is also via a third, advantageously also by a microprocessor via a line 36 provided clock CLK ₃ controlled. The third clock signal represents the start signal for the first electrode and is dependent on the total number of capacitive measuring elements. If the measuring pulse at the last flip-flop ( 34d in 4 ) has arrived, for the first significant edge on the first flip-flop ( 34a ) there is a new signal. The third clock frequency CLK ₃ thus essentially results from the second clock frequency divided by the number of capacitive sensor elements, which in the example shown leads to a third clock frequency of 25 Hz.

In addition to the three clock lines 32 . 35 and 36 become independent of the number of capacitive sensor elements 14 only that to the voltage source 11 leading management 19 and a return line 37 for grounding the counter electrodes of the capacitive sensor elements 14a - 14d needed.

The choice of the three clock frequencies CLK ₁ , CLK ₂ and CLK ₃ depends in particular on the capacitance ranges to be measured, the number of capacitive sensor elements and other boundary conditions such as the desired measurement accuracy. Depending on the electrode geometry and the relative dielectric constant of the fluid to be examined, the measuring frequencies CLK ₁ can be chosen in a wide range, for example between a few kHz and a few MHz. The optimal value of the second clock frequency CLK ₂ for switching the pulse from one flip-flop to the next depends on the transient response of the charging current at the measuring resistor 30 and can be, for example, between a few Hz and a few kHz.

In 5 is finally a top view of a strip-shaped fill level sensor according to the invention 10 shown, which corresponds to the circuit diagram of the 4 four capacitive sensor elements 14a - 14d having. Each sensor element has an elongated measuring electrode 13a - 13d on, each by non-conductive areas 38a - 38d from a common counter electrode 15 are separated. The electrodes are on a carrier, for example a flexible film 39 printed and by means of vias 40a - 40d connected to lines (not shown) on the back of the carrier. On the back of the carrier are also the corresponding (in the representation of 5 also not recognizable) components that each capacitive sensor element 14a - 14d are assigned, in particular the CMOS logic gates and the corresponding flip-flops. There are connection pins in the upper area of the level sensor 41 arranged for connection to a microprocessor.

Claims (12)

Method for measuring the level of a fluid in a Container, being one at least one capacitive sensor element, which is a measuring electrode and comprises a counter electrode, periodically with a constant one Tension charges and unloaded again the measures the average charging current and from the measured mean Charging current the level of the fluid in the container certainly. Method according to claim 1, characterized in that the capacitive sensor via a electronic switch charges and unloads. Method according to claim 2, characterized in that one with the electronic switch a defined frequency switches. Procedure according to a of claims 1 to 3, characterized in that one has several capacitive sensor elements used. Method according to claim 4, characterized in that the capacitive sensor elements sequentially controlled. Method according to claim 5, characterized in that the capacitive sensor elements over a drives common clock line. Level sensor with a DC voltage source ( 11 ), at least one capacitive sensor element ( 14 ), which is a measuring electrode ( 13 ) and a counter electrode ( 15 ), an electronic switch ( 12 ), which periodically between a first position for charging the measuring electrode ( 13 ) and a second position for discharging the measuring electrode ( 13 ) is switchable, and means ( 18 ) for measuring the between the DC voltage source ( 11 ) and the measuring electrode ( 13 ) flowing average charging current. Level sensor according to claim 7, characterized in that the electronic switch ( 12 ) as a tactable logic gate ( 22 ) is trained. Level sensor according to one of claims 7 or 8, characterized in that a plurality of sequentially controllable capacitive sensor elements ( 14a - 14d ) are provided. Level sensor according to claim 9, characterized in that each sensor element ( 14a - 14d ) a switch ( 12a - 12d ) is assigned, the switches ( 12a - 12d ) via a common clock line ( 32 ) can be switched sequentially. Level sensor according to one of claims 9 to 10, characterized in that the sensor elements ( 14a - 14d ) a common counter electrode ( 15 ) exhibit. Level sensor according to one of claims 7 to 11, characterized in that the sensor as a printed circuit on a flexible carrier ( 39 ) is trained.