FR2858401A1 - Liquid level measuring procedure e.g. for steam oven water tank uses two electrical windings to induce magnetic flux and measure voltage - Google Patents

Abstract

Passing an electrical signal through a primary winding (6) positioned close to the tank containing a liquid so that the induced magnetic flux penetrates the tank, measuring the voltage at the terminals (14, 16) of a secondary winding (8) lying close to the primary one and the tank, and interpreting the measurement to determine the degree to which the tank is filled. Passing an electrical signal through a primary winding (6) positioned close to the tank containing a liquid so that the induced magnetic flux penetrates the tank, measuring the voltage at the terminals (14, 16) of a secondary winding (8) lying close to the primary one and the tank, and interpreting the measurement to determine the degree to which the tank is filled. The two windings are of insulated copper wire on a support (2) of a synthetic material with an aperture (4) that can receive the tank; they lie parallel to one another and can be placed round the tank or along one side. The signal from the secondary winding is rectified, filtered and interpreted by a resonant electronic circuit.

Description

The present invention relates to a method for measuring a level of

  liquid in a tank and a corresponding device.

  A field of application of the present invention is the measurement of a water level in a tank of synthetic material. The water may be replaced by a similar liquid, for example an antifreeze liquid, a beverage, etc. As an example, mention may be made of measuring the level in a water tank of a steam oven . This is the application at the origin of the present invention.

  In a water tank of a steam oven the measurement of liquid level can be done according to different techniques: measurement by float, measurement 10 ultrasound, capacitive measurement .... Depending on the case, these means require a sensor attached to the reservoir (So an electrical connection on the tank if it is removable), an opening on the top for viewing the level or have a significant cost. In the (most common) case of a float measurement, this float is placed in a vertical guide inside the tank. No actual measurement is made but a detection of the water level in the reservoir when the float reaches a given altitude. Various devices can be implemented to detect the passage of the float in a given position. Generally a magnet is encapsulated in the float. An electronic card with most often four contacts ILS (Soft Blade Switch) is fixed on a side 20 of a support for receiving the tank. When the latter is in place on its support, the float faces the electronic card and more precisely to these contacts ILS.

  The cost price of this device (float incorporating a magnet, guide in the tank, ILS contacts, ...) is relatively high. In addition, as previously indicated, a water level is not measured here, but the passage of the liquid surface is found at predetermined levels.

  The present invention therefore aims to reduce the cost of a device for giving an indication of a liquid level in a tank. Advantageously, the invention will provide continuous indication of this level of liquid.

  For this purpose, it proposes a method for measuring a liquid level, in particular water or the like, in a tank, characterized in that it comprises the following steps: sending an electrical signal in a first winding arranged near the tank so that the magnetic flux induced by the circulation of an electric current in this winding penetrates inside the tank, - measuring the voltage at the terminals of a second winding arranged near the first winding and tank, and - interpretation of the voltage measurement to determine the fill rate of the tank when it is in place.

  In this way, a transformer is produced, the magnetic core of which consists essentially of the reservoir and the liquid it contains.

  The voltage at the terminals of the secondary winding is then linked to the magnetic coupling, depending on the magnetic properties of the core, between the windings. Its value therefore depends on the kernel. The variation of the liquid level in the tank modifying the magnetic properties thereof, these variations are then noticeable at the level of the secondary winding. The material in which the reservoir is made is preferably such that magnetic coupling variations between an empty reservoir and a full reservoir are sensitive. The magnetic conductance of the reservoir will be chosen to be compatible with the magnetic coupling variations that one wishes to observe.

  For a better efficiency of the measuring method according to the invention, the windings are preferably substantially parallel to one another. In the measurement method, for the interpretation of the signal received at the terminals of the second winding, provision is made for example that this signal is rectified and filtered.

  The present invention also relates to a device for measuring a liquid level, in particular water or the like, in a tank, characterized in that it comprises: a first winding arranged near the tank so that the magnetic flux induced by the circulation of an electric current in this winding penetrates inside the tank, - a second winding, placed near the first winding, - means for sending an electric signal in the first winding, and an electronic circuit for measuring and interpreting the voltage across the second winding.

  Such a device allows the implementation of the method described above.

  Both coils preferably surround the tank. In this way we optimize the coupling between the two coils.

  An alternative embodiment may however provide that the two coils are disposed near the tank in a plane substantially parallel to a side of the tank.

  The reservoir is for example intended to take place on a support and then advantageously makes the two windings on this support.

  It is also possible to provide an embodiment in which the windings are made on a piece of synthetic material inside which the reservoir fits.

  A preferred embodiment provides that both coils are both made of insulated copper wire and have substantially the same number of turns arranged on a single layer.

  The signal sent at the primary circuit must be variable. It is then proposed that the means for sending an electrical signal into the first coil comprises a modulated signal generator, for example square signals.

  The measuring and interpreting means comprise for example a resonant circuit tuned according to the frequency of the signal sent in the first winding. In this embodiment, the resonant circuit is preferably tuned to an odd multiple frequency of the frequency of the signal sent in the first coil, for example at a frequency triple the frequency of this signal.

  The means for measuring and interpreting the voltage at the terminals of the second winding may also comprise means for performing a rectification of this voltage as well as filtering means.

  The present invention also relates to an assembly consisting of a removable reservoir and a support for receiving the reservoir, characterized in that it further comprises a measuring device as described above.

  Finally, the present invention relates to a steam oven, characterized in that it comprises a measuring device as described above.

  Details and advantages of the present invention will emerge more clearly from the description which follows, given with reference to the appended schematic drawing in which: FIG. 1 represents a reservoir support for the implementation of a method according to the invention, FIG. 2 schematically represents an electric circuit used for the implementation in the present invention. FIGS. 3 and 4 show the field lines crossing the support of FIG. 1 when this support receives an empty reservoir, respectively a solid reservoir, FIG. schematically shows the gain of the circuit of Figure 2 as a function of the frequency of the signal applied thereto, and Figure 6 shows an alternative embodiment of the support of Figure 1.

  Figure 1 shows a tank support for a steam oven.

  The present invention is hereinafter described with reference to a steam oven tank but it can of course be adapted to other applications. This support is intended to receive a tank of substantially parallelepiped shape. It also has a parallelepiped shape of dimensions slightly greater than those of the reservoir and one of its faces has an opening 4 on the front of the support intended to allow the introduction and extraction of the reservoir in its support.

  Two windings are also noted around the support 2. A first winding, called the primary winding 6, is, in the example shown in the drawing, on the bottom side, opposite the opening 4, of the reservoir support 2 while that the other winding, called the secondary winding 8, is arranged, parallel to the first, a few centimeters thereof towards the front of the tank support 2. Each of these coils comprises for example twenty turns and is made using an insulated copper wire with a diameter of 0.2 mm. These coils are made on a single layer. The wires making these coils are connected to an electronic card. Between the support 2 of the reservoir and this electronic card, these son are twisted two by two (a pair of son by winding).

  The primary windings 6 and secondary 8 form a transformer whose magnetic core is the reservoir when it is in place in its support 2. Thus, if a voltage is imposed across the primary winding 6, there appears a voltage at terminals of the secondary winding 8 which is linked to the magnetic coupling, and therefore to the core, between the two windings. The voltage measured at the terminals of the secondary winding 8 will therefore depend on the properties of the core which vary according to the level of liquid in the tank.

  For a steam oven the liquid used is of course water. However, any liquid level having a magnetic conduction property can be measured. The tank will preferably be made of synthetic material. A tank with too high magnetic conductivity should be avoided. It must be avoided that the magnetic conductivity of the liquid is negligible compared with that of the reservoir so that a variation of the level of liquid in the reservoir produces a substantial change in the conductivity of the assembly formed by the reservoir and the liquid. that it contains.

  Figure 2 schematically shows the primary windings 6 and secondary 8 integrated in an electrical circuit used for the implementation of the invention. The terminals of the windings have been identified by references 10, 12, 14 and 16. The direction of winding is related to the direction of travel of the electric current. It is therefore possible to reverse the winding direction as long as it is done for both solenoids. The terminals 10, 12 of the primary winding are fed directly by a microcontroller arranged on the aforementioned electronic card. The current in this primary winding 6 is limited to a few microamperes by a resistor R1 connected in series with the primary winding 6. For example, a resistance of the order of kΩ, for example R1 = 1.5 kΩ, will be chosen.

  The numerical value of the resistor R1 is given for purely illustrative and non-limiting purposes. The same applies to all the digital data of the present description. These values are preferred values for the precise application to a particular steam oven and can therefore vary substantially for example for a steam oven of different sizes or for another application area.

  Since the current is very low in the primary winding, the magnetic flux induced by this primary winding 6 will also be low. To then obtain at the terminals 14, 16 of the secondary winding 8 measurable voltage values, a capacitor Cr is associated with the secondary winding to form a resonant circuit and thus amplify the signal.

  Let Ve be the voltage at the input of the electrical circuit of FIG. 2, that is to say the voltage at the terminals of the assembly formed by the resistor R1 and the primary winding 6 connected in series. The voltage Ve corresponds for example to a square wave signal of 5 V with a frequency of 500 kHz. This signal is for example generated directly by an output of the microcontroller. When a 4 MHz microcontroller quartz is used, the frequency of 500 kHz can be obtained by a predivision by 4 with a timer initialized for a division of frequency by 2. The output of this signal is made on a logical port.

  If it is desired to reduce the current flowing in the primary winding 6, for example by increasing the value of the resistor R1, it is then necessary to increase for example the number of turns to maintain the sensitivity of the whole circuit.

  At the secondary level, the value of the capacitance Cr is chosen such that the assembly formed by the secondary winding 8 and the capacitance Cr form a resonant circuit tuned to an odd multiple frequency of the frequency of the primary signal. For example, a triple resonant frequency of the primary frequency, that is to say 1.5 MHz, is chosen. We then have, for example, Cr = 47 pF. Of course, this value varies with the number of turns of the coils and corresponds to a given winding section.

  At the terminals of the resonant circuit, a voltage Vr is obtained. Note also on the secondary circuit the presence of a diode 18, a capacitor Cf and a resistor Rf. These components perform, in a known manner, a single alternation rectification and then a filter enabling a continuous output voltage (signal) Vs to be obtained. Cf and Rf are arranged in parallel and are separated from the resonant circuit by the diode 18. Any other rectification / filtering scheme can also be applied.

  As a numerical example, the following values can be given: Cf = 1 ptF and Rf = 100 kQ. These values make it possible to obtain efficient filtering of the 1.5 MHz signal at the output of the resonant circuit with a response time of less than one second. The diode 18 is for example a diode 1N4148 which can be chosen here for its speed, its low capacitance and its cost.

  The operating principle of this device is as follows. The inductance of the secondary circuit comprising the resonant circuit formed by the secondary winding 8 and the capacitance Cr is composed on the one hand of the inductance of the secondary winding 8 but also of a part of the inductance of the primary winding 6, this part of the inductance of the primary winding 6 being a function of the magnetic coupling between the two coils. A coupling variation between the primary and secondary windings 6 thus generates a variation of inductance seen from the secondary circuit which in turn causes a variation of the resonance frequency of the resonant circuit influencing the amplitude of the signal Vr and therefore consequently the output voltage Vs.

  The modification of the coupling between the primary 6 and secondary 8 coils is illustrated by FIGS. 3 and 4. FIG. 3 corresponds to the presence in the support 2 of a reservoir containing no liquid while FIG. 4 corresponds to a support 2 containing a tank full of water. Between these two figures there is a variation in the number of field lines 20 passing through the two coils.

  In this way, for a tank of 50x150x200 mm dimension and corresponding coils of 50x150 mm of surface having 20 turns each and spaced 4 cm apart, with the values of components and signals previously given, a variation of voltage at Vs is obtained. in the order of 500 mV between a tank full of water and an empty tank. It is therefore easy to deduce the level of liquid in the reservoir as a function of this voltage Vs. It can also be noted the presence or absence of the reservoir which results in a variation of the voltage Vs of the order of 60 mV.

  The physical reasons for these voltage variations are briefly explained below.

  When the primary winding 6 is traversed by a variable current, it creates a magnetic flux. Part of this flux p will cross the two coils.

  Another part will close without crossing the secondary winding 8: it is the leakage flux pfj. If the core (tank filled with more or less water) is of high magnetic permeability, the leakage flux (Pfl is low and the flow leads p is important.

  The magnetic flux induced by the primary winding 6 creates in the secondary winding 8 an induced current which gives rise to an induced secondary flux which tends to oppose the flux p which gave rise to it. Thus, at the secondary winding 8, a secondary leakage flux qf2 created by the secondary current is obtained which is in opposition to the orientation of the induction flux.

  In a magnetic circuit of the transformer type, the greater the magnetic coupling between the windings, the lower the magnetic losses, and the greater the amplitude of the voltage across the secondary circuit. Lenz's law allows to calculate the voltage at the terminals of a winding crossed by a magnetic flux. Using this law of Lenz and the magnetic flux conservation law, we obtain a system of equations which makes it possible to show that the equivalent inductance of the secondary circuit consists of the inductance of the secondary winding 8 minus one value. which is called leakage inductance. Let L2 be the inductance of the secondary winding 8 and Lf2 be its leakage inductance. Then, at the secondary circuit, there is a resonant circuit whose resonance frequency is deduced from the formula 10 (L2-Lf2) Cr o2 = 1. Resonant frequency is therefore a function of magnetic leakage.

  In our numerical example, the resonant circuit is realized to obtain a resonance frequency of 1.5 MHz. The circuit thus formed must also be sensitive to variations in inductance. It is therefore necessary that the inductive value is preponderant on the capacitive value. Since the inductive value is small and can not increase the capacitance, a high resonance frequency is therefore chosen.

  In the above explanations, it is unnecessary to take into account the Joule losses which are related to the resistance of the coils 6 and 8 and are constant regardless of the magnetic core.

  Figure 5 illustrates the variation of gain G as a function of frequency.

  A first curve 22 shows the gain variation for a tank filled with water. The curve 24 corresponds to a tank filled only half while the curve 26 corresponds to a tank containing only air.

  Arrow 28 illustrates the decrease in gain G when moving from a full tank of water to an empty tank. In order for this gain drop to correspond to a drop in level, the resonance frequency of the resonant circuit is slightly greater than the frequency corresponding to the first harmonic of the input signal Ve. So in this case, the frequency of the input signal is 500 kHz, so the first odd harmonic is 1.5 MHz. The secondary circuit then has for example a resonance frequency, depending on the level of filling of the reservoir, between 1.501 and 1.515 MHz.

  In summary when the water level drops in the tank, the primary winding 6 is observed to increase the leak flow tfl and decrease the p flow through the two windings. As a result, the induced electromotive force decreases leading to a decrease in the voltage at the terminals of the secondary winding 8. At the level of the secondary winding 8, it is observed in parallel that the leakage flux pf2 increases, thus causing an increase in the inductance of the leak. The equivalent inductance of the secondary circuit decreases causing the increase of the resonance frequency of the resonant circuit. The gain of the resonant circuit therefore decreases causing a drop in the voltage at the terminals of the secondary winding 8.

  Experiments have shown that the presence of minerals in the reservoir water has little influence on the measurement and hardly disturbs it. Similarly, the presence of a metal shroud around the measuring system can create variations in the values of the voltages measured but the voltage variations are not affected. Variations in temperature (for use in a steam oven) also do not affect the measurement.

  Figure 6 shows an alternative embodiment. There is shown in this figure a reservoir 30. On one side thereof, a support 32 carries two coils 36 and 38. These coils extend over almost the entire height of the tank. They do not surround it but are arranged parallel to a side of it. There is thus also a coupling of the two coils.

  However, the voltage variations between an empty tank and a full tank of water are lower than in the first embodiment described above.

  The invention as described above allows, with the use of a few simple electronic components and two coils, to achieve a sufficiently accurate measurement of a liquid level in a tank. The method described, and the corresponding device, allow to have a continuous measurement (Vs varies continuously). The accuracy of the measurement here is solely related to the accuracy of the acquisition system. It is further noted that the measurement is made without any electrical connection to the tank within which the liquid level is measured without contact with the liquid. The invention is suitable both for a tank with a lid and a tank without lid.

  In addition to measuring the level of liquid in the tank, the method and device described above can detect the presence or absence of the tank in its support.

  The measuring device described above also has the advantage, compared to the known device of the prior art to have a lower cost.

  The present invention is not limited to the embodiments described above by way of non-limiting examples. It also relates to all the variants within the reach of those skilled in the art within the scope of

claims below.

Claims (15)

  1. A method for measuring a liquid level, in particular water or the like, in a tank, characterized in that it comprises the following steps: 5 - sending an electrical signal in a first winding (6) arranged near the reservoir so that the magnetic flux induced by the circulation of an electric current in this coil (6) penetrates inside the tank, - measuring the voltage across a second winding (8) disposed near the first winding (6) and the reservoir, and - interpretation of the voltage measurement to determine the fill rate of the tank when it is in place.   2. Measuring method according to claim 1, characterized in that the coils (6, 8) are substantially parallel to each other.   3. Measuring method according to one of claims 1 or 2, characterized in that the measured signal representative of the voltage across the second winding is rectified and filtered.   4. A device for measuring a liquid level, in particular water or the like, in a tank, characterized in that it comprises: a first winding (6) arranged near the tank so that the magnetic flux induced by the circulation of an electric current in this winding (6) penetrates inside the tank, - a second winding (8), placed near the first winding (6), - means for sending an electrical signal in the first winding, and an electronic circuit for measuring and interpreting the voltage across the second winding (8).   5. Measuring device according to claim 4, characterized in that the two windings (6, 8) surround the tank.   6. Measuring device according to claim 4, characterized in that the two windings (6, 8) are arranged near the reservoir (30) in a plane substantially parallel to a side of the reservoir (30).   7. Measuring device according to one of claims 4 to 6, characterized in that the reservoir is intended to take place on a support (2) and in that the two coils (6, 8) are formed on the support ( 2).   8. Measuring device according to one of claims 4 to 6, characterized in that the coils (6, 8) are formed on a piece of synthetic material within which the reservoir is interlocked.   9. Measuring device according to one of claims 4 to 8, characterized in that the two coils (6, 8) are both made of insulated copper wire and have substantially the same number of turns arranged on a single layer .   10. Measuring device according to one of claims 4 to 9, characterized in that the means for sending an electrical signal in the first coil (6) comprises a modulated signal generator, for example square signals.   11. Measuring device according to one of claims 4 to 10, characterized in that the measuring and interpreting means comprise a resonant circuit (8, Cr) tuned according to the frequency of the signal sent to the first winding. (6).   Measuring device according to Claim 11, characterized in that the resonant circuit (8, Cr) is tuned to an odd multiple frequency of the signal frequency sent in the first winding (6), for example at a triple frequency of the frequency of this signal.   13. Measuring device according to one of claims 4 to 12, characterized in that the means for measuring and interpreting the voltage across the second winding (8) comprises means (18) for performing a recovery of this voltage as well as filtering means (Cf, Rf).   14. A set consisting of a removable reservoir and a support for receiving the reservoir, characterized in that it further comprises a device for   measurement according to one of claims 4 to 13.   15. Steam oven, characterized in that it comprises a device for   measurement according to one of claims 4 to 13.