ES2924949A1 - SYSTEM TO MEASURE HUMIDITY IN A MEDIUM (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

System for measuring humidity in a medium that includes a generator (1) that injects power into the medium, in the water absorption radio frequency band between 2 GHz and 130 GHz, through an input port (4), transferring said power to an output port (5), connected to a power meter (2). The degree of moisture in the ground will influence the power transferred, the measurement of which is delivered to the output (8) of the power meter (2). On the other hand, between the signal injected into the input port (4) and the one received at the output port (5) there will be a phase difference related to the behavior of the terrain and the degree of humidity, which will be recorded as a measurement at the output (9) of the phase comparator (3). The system does not require direct contact between its conductive elements and the ground, thus increasing the robustness of the sensor against external agents. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

SYSTEM TO MEASURE HUMIDITY IN A MEDIUM

TECHNICAL SECTOR

The present invention falls within the precision agriculture sector, in the field of humidity sensors. The invention employs power transfer in the water absorption band.

One application of the present invention is the measurement of soil moisture in order to optimize the cultivation of plants, although it can be used for other applications that require obtaining soil moisture measurements.

BACKGROUND OF THE INVENTION

The main moisture measurement techniques in agricultural applications are based on the use of capacitive sensors, reflectometry techniques, measurement of electrical conductivity or optical techniques in the infrared range.

Patent documents CN207516300U, JP2012112801A or GB2398637A are examples of capacitive sensors. Its foundation is to use an electronic design that contains a capacitor to which an alternating voltage is applied. Said capacitor, which has at least two electrodes separated by a dielectric, is the one that acts as a sensor element. The medium or terrain from which you want to obtain moisture will be part of that dielectric.

The applied alternating voltage is usually in the MHz frequency range, as in WO2007002994A1, where the design operates at 10 MHz. This reduces the complications inherent in higher frequency electronic designs, which require increased design precision. of the geometry of the circuit and its electromagnetic characteristics at high frequencies, as well as properly selecting the electronic components for high frequencies.

Since the ground is going to form part of the dielectric of the capacitive sensor, depending on the general characteristics of the ground, such as humidity, affected the characteristics of said dielectric and therefore, its capacity. These capacity changes in the sensor can be measured as voltage changes in an impedance divider in which the capacitive sensor participates, as in documents WO2007002994A1, US2015330932A1 or CN101281152A. Other options are to record the changes in the measurement of a balanced impedance bridge that includes the capacitive sensor, or to measure the charging and discharging times of the capacitor.

As a summary of the above, in the measurement techniques that use capacitive sensors, the fundamental component of the sensor is a capacitor with at least two terminals and a dielectric, the objective of the sensor being to relate the changes in its capacity with the humidity of the land.

Another of the humidity measurement techniques in agriculture is based on reflectometry, which is based on launching an excitation signal into the medium and recording the characteristics of the reflected signal due to the fact that an impedance change occurs in the sensor-interface. medium. This excitation can be a voltage pulse or it can be high-frequency pulses, like in radar.

One way of injecting the voltage pulse into the medium is by using two electrodes in contact with the medium and then measuring the characteristics of the reflected signal on them. The pulse can be injected periodically, forming a square signal. The characteristics to be measured can be the amplitude of the reflected voltage, the delays in the arrival of the reflections with respect to the excitation instant or the changes in the received frequency spectrum compared to the spectrum of the excitation signal. Examples of this technique are shown in patent documents US6104200A, EP0297604A2 or US6441622B1.

Designs based on time domain reflectometry (TDR) techniques focus on recording the propagation delay between the injected or incident pulse and the reflected pulse, in addition to measuring characteristics of the reflected signal with respect to the incident signal, such as changes in amplitude or change in slope of the reflected pulses due to the effect of the terrain surrounding the sensor, as in documents US6657443B2, US6831468B2 or US8285503B1. In this type of design, the precision of the timing or synchronization control electronics is especially important and represents an added difficulty.

On the other hand, there are high-frequency designs based on reflectometry that include electromagnetic guidance geometries through which high-frequency pulses are injected, as in a radar. These high-frequency pulses will be reflected when they interact with the medium, such as the soil whose humidity is to be measured, so that by analyzing the reflection received it is possible to characterize the medium.

The high frequency TRIME system from the manufacturer IMKO is based on this combination of reflectometry and a guided system. TRIME uses time domain reflectometry (TDR), launching 1 GHz pulses and then characterizing the reflections that occur. The first TRIME publications date back to 1998 [1,2] and patent document WO2019214924A1 contains an example of the geometry used. Other designs have been derived from this system, such as those published in patent documents such as CN101210899A or CN201007712Y.

In addition to the techniques described for humidity measurement, there are also designs based on the measurement of electrical conductivity, such as those shown in documents US10247687B2, CN207036751U, or designs that employ optical measurement techniques, using wavelengths from the infrared , as in document US6079433A, where designs based on the emission of infrared radiation are shown, to then measure the characteristics of the received signal after being influenced by the terrain.

These techniques record the joint behavior of all the components of the terrain, so that, if the main changes in that behavior are mainly due to changes in humidity, it is possible to relate the measurements to the value of the humidity of the terrain.

References

[1] Wegehenkel M., Using TRIME-TDR for the determination of soil water dynamics on sandy soils. Journal of Plant Nutrition and Soil Science (1998). DOI:

10.1002/jpln.1998.3581610512

[2] Wegehenkel M., The validation of soil water balance models based on measurements of soil water contents by gravimetry and by TRIME-TDR and of soil water suctions determined by tensiometer. Journal of Plant Nutrition and Soil Science (1998). DOI: 10.1002/jpln.1998.3581610513

EXPLANATION OF THE INVENTION

The present invention proposes a system for measuring humidity in a medium according to the independent claim more generally. Particular embodiments are defined in the dependent claims. The system uses the absorption and transfer of electromagnetic power as a technique to measure the humidity of a medium (soil, terrain). The system measures the transfer of radio frequency power from an input port coupled to the medium to another output port, also coupled to the medium. The power transferred between the ports will be reduced by the absorption of power by the medium, so that the power measured at the output port will be reduced compared to that at the input port depending on the humidity of the medium. The term port is understood as the connection zone of controlled geometry in a high frequency application. Specifically, the term port is the cross section that defines the border or junction area between two structures that transport high-frequency electromagnetic power. Therefore, this technique employs the opposite phenomenon to that used in reflectometry techniques: transfer instead of reflection. In this document high frequency is to be understood as 1 GHz or more.

To maximize power absorption when measuring humidity, the humidity measurement system will apply an excitation to the input port aimed at enhancing this effect of maximum absorption by water. Water, as a lossy dielectric material, has a notable electromagnetic power absorption or dissipation effect at microwave frequencies, that is, above 1 GHz, being especially noticeable in the frequency range between 2 GHz and 130 GHz for a temperature of 17 °C, reaching a maximum absorption around the frequency of 20 GHz. Therefore, this will be the ideal operating frequency range for the system to measure humidity, below the electromagnetic spectrum of infrared radiation.

TRANSFERRED POWER MEASUREMENT

The humidity measurement system will have, on the one hand, a power generator module connected to an input port, which can be a radiofrequency oscillator that will generate a continuous sinusoidal voltage with a zero average value. By output, which can be a circuit that transforms the received power into a useful signal, such as a voltage that is a function of the received power.

It should be noted that the system is based on the transfer of power from one port to another. The electrical power simultaneously involves the magnitudes voltage and current or impedance, which cannot be analyzed in isolation, so that the electrical power at an instant can be defined as the product of the instantaneous voltage times the instantaneous current. Based on this foundation, it must be taken into account that, in general, to obtain the maximum transfer of electrical power from a generator to a load, the impedance matching principle must be met. In the case where resistive impedances are used, the generator and load impedances must be the same.

These impedance matching considerations must be taken into account in system design. From the point of view of the power generator module, to obtain the maximum power transfer, the impedance of the generator must be adapted to the impedance offered by the input port. Equivalently, to transfer maximum power to the power meter module, the output port impedance must be matched to the impedance offered by the power meter. The influence of the medium to be measured will cause a phenomenon of power absorption and deviation from the impedance matching principle at each point, influencing the power transfer recorded by the power meter module.

The system object of this invention has the advantage that it allows power to be injected continuously over time, since it measures the transfer of power from one input port to another output port. Compared to reflectometry techniques that require switching between transmission of incident pulses and detection of reflected pulses, the invention reduces design complexity by avoiding additional amplification, switching, and time synchronization elements since pulses do not have to be distinguished. reflected incidents.

The system object of this invention offers the advantage of not needing a conductive material in contact with said medium, minimizing corrosion problems in metallic elements, since it couples electromagnetic power to the medium through an alternating signal whose average voltage is zero.

The system object of this invention does not alter the characteristics of the medium and avoids problems of polarization of the medium, since the alternating signal coupled to the medium is of high frequency and has zero average voltage. As continuous voltages are not applied to the medium in a sustained manner, there are no problems of polarization of the medium due to ion displacement, avoiding having to subsequently compensate said polarization, for example, by applying an opposite voltage. In addition, the applied excitation frequencies are very high, in the order of GHz, which, together with the very low ionic mobility of the soil components at those frequencies, means that the moisture measurement system does not alter the characteristics of the soil during the measurement. measurement.

The response of the system to measure humidity is very fast: once the excitation power is activated at the input port, the power is received practically instantaneously at the output port since, since it is only necessary to measure the transfer of power from one port to another, the delay is primarily related to the propagation time of the electromagnetic field from the input port to the output port.

The moisture metering system allows for rapid on/off excitation at the inlet port to timely sample moisture at the outlet port and turn off the excitation to save energy. This is due to the fast propagation of the power to the output port without perceptible transient effects, due to the combination of the high excitation frequency used, in the GHz range, with the low mobility of the ground ions at that frequency. excitation frequency.

From the point of view of the medium to be measured, the humidity measurement system allows the excitation to be continuously applied and, therefore, allows rapid variations in humidity and activity to be detected continuously over time and with temporal precision. of the medium while the excitation is active. This is a consequence of the rapid response of the system to measure humidity and its null influence on the medium during the measurement.

MEASUREMENT OF THE INTRODUCED PHASE

Following the described mechanism of signal transfer from an input port to an output port, embodiments of the system to measure humidity can incorporate a input and the signal obtained at the output port. This provides additional information about the effect on the signal transfer in terms of the phase shift introduced. Therefore, the previously explained transferred power measurement alone, as well as in combination with this introduced phase shift measurement, can be used as joint information or used as two different humidity measurement techniques.

As an example of the operation of the phase comparator circuit, if the voltage of the oscillating signal that is transmitted through the input port is expressed as an alternating voltage of the form A rc os (2 n ft + 9 T ) and the voltage of the signal which is received at the output port as AR-cos( 2 -nf-t+ 9 R), where f is the frequency, t is the time, and 9 ^t and 9 ^r are the additional phases of the transmitted and received signal, respectively, the device The phase comparator will generate at its output a voltage proportional to the phase shift between both signals, which can be expressed as Vd = k (^R - 9 ^t ), where k is a constant.

BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, a set of drawings is attached as an integral part of said description where, for illustrative and non-limiting purposes, the following has been represented :

Fig. 1.- Shows an explanatory diagram of the impedance matching principle for maximum power transfer from the generator to the power meter, whose internal impedance acts as the circuit load.

Fig. 2.- Shows an explanatory diagram of the power transfer process from the generator to the power meter, whose internal impedance acts as a load, including the medium from which humidity is to be obtained.

Fig. 3.- Shows a schematic plan view with a possible embodiment of the system for measuring humidity in a medium according to the present invention.

Fig. 4.- Shows a schematic plan view with another possible embodiment of the NUMERICAL REFERENCES

1 voltage generator.

2 Power meter whose internal impedance is R 0 .

3 Phase detector.

4 Input terminal in the middle.

5 Middle output terminal.

6 Circuit Substrate.

7 Common conductor plane.

8 Power meter output terminal.

9 Phase detector output terminal.

10 Processor.

11 Medium.

12 input port.

13 Outlet port.

DETAILED DESCRIPTION OF THE INVENTION

In view of the aforementioned figures and according to the adopted numbering, Figs. 1 and 2 allow detailing the physical foundations on which the invention is based and then show in Figs. 3 and 4 two possible preferred embodiments of the invention, based on the aforementioned physical foundations.

The present invention uses the principle of end-to-end electrical or electromagnetic power transfer. The optimal situation of electrical power transfer in the case of resistive impedances is described in Fig. 1. In said figure a voltage generator 1 is shown, whose effective value of electromotive force is Vg and which is directly connected to a load R 0 which represents the internal impedance of the power meter 2. The generator and load impedances are resistive and matched, that is, they have an identical value R 0 . Under these impedance matching conditions, the power transfer to the load P is maximum and is P = Vg2/(4Ro).

In the embodiment of the present invention, a medium 11 will be included, from which humidity is to be obtained, between the generator 1 and the power meter 2, whose internal impedance is Ro, as indicated in Fig. 2. Said embodiment includes a generator to medium 11 through an input port consisting of input terminal 4 and common conducting plane 7. Power propagates through medium 11 to an output port consisting of output terminal 5 and common conducting plane 7 , to be delivered to the power meter 2. The effect of the medium 11 is to reduce the power P transferred to the power meter 2 due to the effect of the physical characteristics of the medium 11, including humidity, so that the power delivered to power meter 2 will be less than the optimal case described in Fig. 1, ie P < Vg2/(4Ro).

Based on these principles, Fig. 3 shows a possible preferred embodiment of the moisture meter system to be developed with printed circuit manufacturing technology. The striped areas correspond to the input terminal 4, the output terminal 5 and the common conductive plane 7, and are conductive surfaces and the unscratched area corresponds to the dielectric substrate 6 on which the conductive elements rest, of which the input 4 and output 5 terminals and the common plane 7 form part. Said substrate 6 ends in a point to facilitate its insertion into the ground, that is, the medium 11 whose humidity is to be measured. The set does not require direct contact between the conductive surfaces and the ground to be measured, so it can include an insulating protective cover, thus increasing the robustness of the system to measure humidity against external agents. Said assembly will be buried in the medium 11, including the input 4 and output 5 terminals, although the end opposite the tip, where the horseshoe-shaped element that interacts with the medium 11 to measure humidity, can be found. protrude slightly from the ground. The conductive element comprised between elements 4 and 5, in the shape of a horseshoe, is not a high-frequency equipotential surface and offers a distribution of voltage and current amplitudes that is variable throughout its area, a distribution responsible for generating a field electromagnetic field in the environment of the horseshoe. The common conductor plane 7 is approximately equipotential in the extension of its surface, minimizing the emission of the electromagnetic field in the area comprised by said conductor and reducing spurious effects of high-frequency noise that may affect the electronic components of blocks 1, 2 and 3 located in that area.

The indicative numbers of Fig. 3 correspond to those used in Figs.

1 and 2, so that a signal generator circuit, represented by voltage generator 1, whose generator impedance is Ro, injects power between input terminal 4 and common terminal 7. The alternating signal generated by generator 1 will have The power injected into the input port, composed of input terminal 4 and common conducting plane 7, will propagate to the output port, composed of output terminal 5 and common conducting plane 7, so the medium to be measured, the soil, will influence this propagation. The power meter 2, with an internal impedance R 0 , will be connected to the output port, that is, between the output terminal 5 and the common terminal 7. Said power meter will offer at its output terminal 8 a useful magnitude related to with the measurement, such as a voltage proportional to the registered power.

Fig. 3 includes a phase detector 3, which measures the phase shift between the signal coming from the generator 1 and the one entering the power meter 2, offering at its output terminal 9 a useful magnitude related to said phase shift, such as a voltage proportional to the detected phase shift. These useful magnitudes of power and phase shift are transferred to a processor 10, which processes them to determine a humidity value in the medium 11. Said processor receives, at a certain moment, information on the power transferred from the generator 1 to the power meter 2 at output terminal 8 and, optionally, offset information at terminal 9. Processor 10 will convert this information to a moisture value. To perform this conversion, there are different options. One of them is that the processor 10 stores a correspondence table whose input is the power received at the terminal 8 or the offset received at 9 and its output is the corresponding humidity. This table must be previously generated for each type of medium from which the humidity is to be obtained. Another option is for the processor to store physical parameters of the terrain calibration and implement a mathematical formula by which, based on the power received in 8 and/or the offset received in 9, together with the physical parameters of the medium, it calculates the value moisture. This mathematical formula can be a polynomial model that obtains the humidity in percent as: h = ko ki ( v-vo) k2 ( v-vo)2 ... + kn ( v-vo) n, where v is the voltage (proportional to power), offered by the power meter 2 at terminal 8 and v 0 a constant whose value corresponds to the voltage recorded at terminal 8 for 0% humidity. The ki coefficients are related to the physical characteristics of the environment, so that, based on various ground calibration measurements, as many as ki coefficients are to be calculated, said ki coefficients are obtained and these are stored in the processor configuration. Taking into account that the voltage at terminal 8 of power meter 2 decreases with humidity, the simplest linear approximation calculates humidity using ko = 100 and ki = -100, so that h = 100 - 100 ( v-vo ), expression that in practice works is unique and it is possible to define alternative formulas based on electromagnetic parameters of the ground such as the dielectric constant and the conductivity of the medium, parameters that are stored in the processor configuration. Processor 10 may obtain moisture values using only the power measurement, only the offset measurement, or combining both power and offset measurements, depending on how the operation is implemented in the processor.

Fig. 4 shows another possible preferred embodiment of the humidity meter following the same conventions that have been explained for Fig. 3. The only differences lie in the geometry of the input 4 and output 5 terminals. In this case the conductor rectangular associated with input terminal 4 can be assimilated to a transmitting antenna and the one connected to output terminal 5, to a receiving antenna. In the same way as explained for Fig. 3, the power to be transmitted is injected into the input port, formed by the input terminal 4 and the common conductor plane 7, while the power to be received will be measured at the output port, formed by output terminal 5 and common conductor plane 7.

For both Fig. 3 and Fig. 4, the geometry of the elements that make up the design must be carefully elaborated, since it is a high-frequency design, with which the electrical characteristics, as well as the dimensions of the Conductive and dielectric elements used will influence the transfer of power from one point to another in the circuit. Mention that, in Fig. 4, although the input 4 and output 5 terminals are not linked by a conductive material, they are linked electromagnetically at high frequencies.

To carry out the design optimally, we will start from the specification of the generator impedance R 0 , which we will assume to be purely resistive, so that generator 1 must be connected to a transmission line or waveguide with characteristic impedance R 0 . In this way, the generator voltage Vg, when applied through its generator impedance R 0 on the characteristic impedance line Ro, becomes a power wave of value P = Vg2/(4Ro) that is transported to the generator. input port formed by the input terminal 4 and the conductive plane 7. The transmission line can be made with microstrip technology ( micmstrip ), adjusting the width and separation of the conductive elements depending on the conductivity and dielectric constant of the substrate with in order to obtain a characteristic line impedance value Ro. Structure Dimensions be adjusted so that the impedance seen at the output port, formed by the output terminal 5 and the conductive plane 7, in conditions of absence of humidity, is as close as possible to the reference impedance R 0 . The power received at the output port will be guided with another section of transmission line or waveguide, with characteristic impedance R 0 , to the power meter 2, whose input impedance will coincide with the reference impedance R 0 . In the event that any of the interconnection points described work with an impedance other than the reference R 0 , the power transferred from the generator to the power meter will be less, so these effects must be considered or included. impedance matching stages to maximize power transfer.

Both embodiments admit variants in their geometry, but the key concepts of the invention are: using an input port 12 to the medium 11 (terminal 4 and common plane 7) to which power is going to be injected in the absorption radiofrequency range of the water and an outlet port 13 of the medium 11 (terminal 5 and conductive plane 7), from which the reading of the power transferred after being affected by the medium 11 will be obtained, in addition to obtaining (optionally) phase information through of the phase detector 3.

Fig. 5 shows an example of voltages obtained at terminal 8 in an actual test carried out with this moisture meter system. Said voltages are proportional to the power received in the power meter 2 and enter the processor 10 to be transformed into humidity values with the operations described above.

The graph shown in Fig. 6 shows the values calculated by the processor and since it is a real test, carried out on cultivated land, it can be seen how moisture is lost over several days, forming a descending stepped graph if viewed from left to right. This is due to the fact that the loss of humidity is more pronounced during part of the day (falling flanks) than during the night (flat areas of the step). This behavior is reversed at the moment the terrain irrigation system is activated, performing irrigations at regular intervals that are shown as ascending steps and abrupt increases in humidity in the form of peaks each time the irrigation is activated. Activating irrigation over several days causes soil moisture to rise again, as shown on the right side of Fig. 6.

Having sufficiently described with the previous examples, different aspects to put the invention into practice, protection is claimed for the following system for measuring humidity in a medium.

Claims (14)

1. System for measuring humidity in a medium (10) characterized in that it comprises: a voltage generator (1) configured to generate a high frequency voltage signal; a power meter (2) configured to measure a power signal; an input port (12) comprising an input terminal (4) which, in operation, is within the means (11), where the input terminal (4) is electrically connected with the voltage generator (1) to deliver a high frequency power signal to the medium (10); an output port (13) comprising an output terminal (5) which, in operation, is within the means (11), where the output terminal (5) is electrically connected with the power meter (2) to receive a high frequency power signal from the medium (10); a processor (10) configured to process the power signal delivered to the medium (11) and the measured power signal of the medium (11) and, based on said power signals, determine a humidity value in the medium (11) . 2. System for measuring humidity in a medium (11) according to claim 1, further comprising a phase detector (3) electrically connected to the input terminal (4) of the input port (12) and to the input terminal output (13) of the output port (13), where the phase detector (3) is configured to measure the phase difference between signals from both terminals, where the processor (10) is configured to process the phase difference measured by the phase detector (3) and perform an adjustment of the humidity value in the medium (11) additionally as a function of the measured phase difference. 3. System for measuring humidity in a medium (11) according to claim 1 or 2, wherein the voltage generator (1) generates an alternating signal whose average voltage is null. 4. System for measuring humidity in a medium (11) according to any one of claims 1 to 3, wherein the input terminal (4) and the output terminal (5) are connected by a conductive material. 5. System for measuring humidity in a medium (11) according to any of the connected by a non-conductive material. 6. System for measuring humidity in a medium (11) according to any of claims 1 to 5, comprising a common conducting plane (7) electrically connected to the voltage generator (1), to the power meter (2) , with the phase detector (3), acting as a voltage reference. 7. System for measuring humidity in a medium (11) according to any of claims 1 to 6, where the processor comprises a memory where a table of correspondences between a power value and an associated humidity value is stored. 8. System for measuring humidity in a medium (11) according to any of claims 2 to 7, comprising a correspondence memory where information is stored between an offset value and a humidity value; where the processor (10) is configured to access the correspondence memory. 9. System for measuring humidity in a medium (11) according to any of claims 1 to 7, where the processor (10) is configured to calculate the humidity value based on a set of physical parameters of the medium (11) and the measured power. 10. System for measuring humidity in a medium (11) according to claim 9, wherein the processor (10) is configured to access a plurality of physical parameters for calibration of the medium (11) and to calculate the humidity value, it is performs the application of a mathematical formula based on a polynomial model. 11. System for measuring humidity in a medium (11) according to any of claims 1 to 11, comprising a substrate (6) with pointed geometry. 12. System for measuring humidity in a medium (11) according to claim 11, wherein the substrate (6) comprises an insulating protective cover. 13. System for measuring humidity in a medium (11) according to any of claims 1 to 12, where the voltage signal is within the frequency range between 2 GHz and 130 GHz. 14. System for measuring humidity in a medium (11) according to claim 13, wherein the voltage signal is 20 GHz.