Robust device for measuring moisture and fertilizer content in soils

Abstract

The invention relates to a robust device for measuring the water available to plants in agriculturally used soils. The device is also intended to measure the fertilizer content. The device comprises the following features: two planar capacitors (1a, b) and an electronic circuit (5) configured to evaluate changes in the capacitance of the planar capacitors (1a, b). The circuit (5) is housed in a robust protective casing. The planar capacitors (1a, b) are covered with a water-permeable protective body. An anti-rotation device (11) prevents damage to the planar capacitors (1a, b) when the protective body (6) is replaced.

Description

[0001] The invention relates to a robust device for measuring the water available to plants in agricultural soils. The device is also intended to measure the fertilizer content. The term soil is understood below to refer to agriculturally used soil.

[0002] In fields other than agriculture, the setup and operation of measurement and automation systems is the responsibility of measurement and automation technicians. Such specialists are generally unavailable in agriculture, or only on farms with large areas, where the aforementioned problems with soil structure don't exist anyway.

[0003] Accordingly, one requirement for an irrigation and fertilization system is that the sensors used are very robustly constructed so that even untrained personnel cannot damage them when inserting them into the soil. Damaged sensors will provide no or incorrect measurement results, which inevitably leads to incorrect dosing of water and fertilizer.

[0004] However, there is another cause for incorrect measurements and the resulting incorrect dosage: Soils can have different structures. In fields and greenhouses, soils are usually optimally filled in terms of depth and have a consistent structure across the entire area. In other words, the soils have essentially the same structure at every point in the field. If the moisture and / or fertilizer content is measured in such a soil using a sensor, this measured value is also representative of other areas, provided water and / or fertilizer have been applied evenly.

[0005] However, there are also soils that have a very uneven structure in terms of their depth. Such soils can be found, for example, in housing developments with houses that have gardens between them. These gardens are increasingly being used for growing fruit and vegetables. Often these soils are only covered with a thin layer of soil that is suitable for growing plants. The underlying layers are often created by the addition of material such as building rubble or formwork boards, or by unknown material. This highly inhomogeneous subsoil means that water and / or water containing fertilizer penetrates the soil at very different rates. This means that there can be zones that are almost impermeable to water, but also zones with larger cavities into which the water seeps away very quickly.

[0006] The structure of the deeper soil layer in the residential soils described above is no longer changeable. Even if the overlying layer consists of optimal potting soil, the described highly variable water penetration behavior remains unchanged. Therefore, even precise sensors are of no use for dosing water and fertilizer, as there are no suitable locations in such soils where representative measurements can be taken.

[0007] Experts know that these conditions can lead to serious measurement errors. This also explains why high-precision irrigation technology, including precise fertilizer dosing, fails to deliver the theoretically expected effect on plant growth and harvest under such circumstances, because the dosing is based on incorrect measurements.

[0008] In summary, it can be stated that soils with such a diverse structure cannot yet be irrigated or fertilized as needed.

[0009] The only way to prevent over- or under-irrigation, or over- or under-fertilization, in soils with highly variable structures is to measure at as many locations as possible and then apply selective irrigation and / or fertilization. However, the associated effort and cost are so high that this method has not yet become widespread. If more sensors are required, the resulting additional financial outlay may outweigh the benefits of improved, i.e., more precise, irrigation and fertilization.

[0010] There is therefore a need to reduce the effort required for site-specific irrigation and fertilization. The first step towards this goal is the provision of highly cost-effective and robust devices that can simultaneously determine the water and ion content of the soil at a wide range of locations.

[0011] A further requirement is a small measuring volume. This means the following: There are, for example, devices that measure soil moisture using microwaves. These microwave devices record a measuring volume of approximately 500 to 5,000 cubic centimeters of soil. Given the special soil conditions mentioned above, large differences in material could occur within the measuring volume. As a result, the different material volumes within the measuring volume have different water and fertilizer contents. The infiltration rate of the water within the measuring volume can also vary. Experts are aware that this type of measuring method is bound to result in inaccurate measurements, because it is important to measure the water and fertilizer content as close to a root as possible. This is only possible with a device that has a small measuring tip, i.e. a small measuring volume.

[0012] There's another problem with measuring using the microwave method: If the device is positioned near roots, the microwaves also penetrate these roots. This means the water content of the roots is also measured. However, this amount of water is already being used by the plant to which these roots belong. This leads to significant errors in calculating the necessary water supply. The same errors arise when calculating the required fertilizer concentration. It should also be noted that roots also grow within the microwave field, which also distorts the measured values.

[0013] In summary, it can be stated that a device for measuring moisture and fertilizer content in soils for the inhomogeneous soil structures described above must have a wide variety of structural and metrological properties, some of which are mutually exclusive.

[0014] The device, hereinafter referred to as the sensor, should have the following features and characteristics: a. Rod-shaped construction with a diameter of less than 3 cm so that it can be easily inserted into a hole in the ground created with a conventional drill and a twist drill. b. The volume of the sensor head should be a maximum of approximately 2 x 2 x 2 cm to allow measurements close to roots. c. High mechanical robustness to ensure that the rod-shaped sensor is not damaged when inserted into the hole and when removed at certain intervals, since the hole is not drilled anew each time. d. Low manufacturing costs to allow the use of a variety of sensors, if needed.

[0015] The above-mentioned requirements and properties are achieved or solved with a device according to patent claim 1.

[0016] The device has the following features: Two planar capacitors 1a, b are formed on the top and bottom sides of a first plastic circuit board 2, each planar capacitor 1a, b consisting of a copper layer with interlocking structures. Preferably, these are two comb-like electrodes whose prongs interlock but do not touch, thus forming a planar capacitor. However, other interlocking structures can also be used.

[0017] The first circuit board 2 is covered on all sides with a water-impermeable plastic layer 3, ie the side edges of the circuit board are also covered so that the copper layer is safely protected from corrosion.

[0018] An electronic circuit 5 is arranged on a second circuit board 4, which is designed to evaluate capacitance changes of the planar capacitors 1a, b. The first circuit board 2 is signal-connected and mechanically connected to the second circuit board 4.

[0019] The first circuit board 2 is encased in a porous protective body 6, which serves as contact protection for the mechanically sensitive structures of the planar capacitors. Furthermore, the protective body is designed to conduct water and fertilizer dissolved in the water to the planar capacitors by capillary action, where it distributes it evenly over the entire surface of the planar capacitors. Uniform distribution means that no unwetted areas form on the planar capacitors. This is achieved by ensuring that at least the contact surfaces of the protective body 6 with the plastic layer 3 are very fine-pored. Furthermore, the protective body 6 must be pressed onto the plastic layer 3 with a predetermined contact pressure by a mechanical measure.The material for the porous protective body and the contact pressure is selected by the person skilled in the art in such a way that the above-mentioned requirements are met; textile fibers or felt are preferably suitable for this purpose.

[0020] The composite of the first circuit board 2 and the second circuit board 4 is inserted into an inner sleeve 7 and secured with a potting compound 8. The inner sleeve has an opening at one end through which the connecting cable of the electronic circuit 5 is routed to the outside. The potting compound 8 is filled to such an extent that the second circuit board 4 with the electronic circuit 5 is completely covered. The potting compound makes the inner sleeve mechanically very stable and protects the circuit from corrosion.

[0021] Once the potting compound 8 has cured, the protective body 6 is placed onto the first circuit board. For this purpose, the protective body 6 is slotted.

[0022] The inner sleeve 7 is held in a mechanically stable outer tube 9. The inner sleeve 7 can be either tightly fitted or glued in place. The outer tube 9 has a thread on the side of the protective body 6, onto which a mechanically stable plastic cap 10 is screwed. The plastic cap has several recesses large enough to allow the protective body 6 to have extensive contact with the ground, allowing water to penetrate or be drawn in due to the capillary action of the protective body material.

[0023] As described above, the first circuit board 2 with the plastic layer 3 is mechanically very sensitive. This sensitivity is due to the fact that the copper structures should be as fine as possible and the plastic layer very thin. The finer the copper structures and the thinner the plastic layer, the greater the measurement effect.

[0024] However, the protective body must be replaced at predetermined intervals because microorganisms may clog the capillaries and thereby distort the measurement result. This creates the problem that the old, possibly even slightly encrusted, protective body 6 must be removed from the circuit board 2 and a new one must be replaced. This replacement is best done in the field to preserve the hole from which the sensor was extracted. Therefore, it is necessary to prevent damage to the plastic layer 3 when removing the protective body 6.

[0025] According to the invention, damage is prevented by providing at least two tapered webs 11 opposite one another on the inner wall of the inner sleeve 7. The webs act as assembly aids during insertion and as anti-twist devices when removing the protective body, as explained below: When the protective body is inserted, the tips press into the soft material and expand it to the cross-section of the webs. If the protective body is to be removed, for example, after a few months, the webs act as anti-twist devices, so that the protective body can only be pulled off the first circuit board in a straight line. This prevents damage to the plastic layer 3. The webs 11 also prevent the protective body from twisting when screwing on or off the plastic cap 10.

[0026] According to an advantageous embodiment according to claim 2, the plastic cap 10 is made of antibacterial plastic. This has the advantage of slowing down the penetration of microorganisms into the protective body. This allows the protective body to be used for a longer period of time.

[0027] According to an advantageous development according to claim 3, the electronic circuit 5 is designed to selectively interconnect the planar capacitors 1a, b. This opens up the possibility of doubling the area of ​​the planar capacitor. In other words, during normal operation, the water content and the ion content can be measured simultaneously. If both planar capacitors are interconnected, the water content or the ion content can be measured with greater accuracy.

[0028] According to an advantageous development according to claim 4, the outer tube has a thread at its upper end in order to attach an extension tube.

[0029] According to an advantageous development according to claim 5, length markings are provided on the outer tube. The length markings are useful when the sensors are to be installed at the same or different depths, for example, to record a profile of the water content in the soil.

[0030] The device is explained in more detail below using drawings. Fig. 1 shows a combination of two planar sensors with an evaluation electronics, Fig. 1a shows an enlarged view of a planar sensor, Fig. 2 shows the planar sensors with a half-attached protective body, Fig: 3 shows a sensor housing with a protective cap made of plastic, Fig. 4a, b shows the opened and closed protective body in one design option, Fig. 5 shows a cross section of the inner sleeve with the second circuit board without electronic circuit, Fig. 6 shows an agricultural soil with a uniform soil structure and Fig. Figure 7 shows an agricultural soil with an uneven soil structure. List of reference symbols 1a, b Planar capacitors 2 first circuit board 3 plastic layer 4 second circuit board 5 electronic circuit 6 porous protective body 7 inner sleeve 8 Potting compound 9 Outer tube 10 plastic caps 11 Anti-twist device

[0031] The Fig. Figure 1 shows a composite of two planar sensors 1a, b formed on a first printed circuit board 2. This is a double-sided copper-coated plastic plate, which is commonly used in the prior art and can therefore be manufactured very cost-effectively.

[0032] The Fig. Figure 1a shows an enlarged view of the planar sensor 1a with a comb structure. The same comb structure is located on the back of the plastic plate. The entire first circuit board 2 is completely covered with a thin plastic layer 3. Particularly waterproof plastics, such as polyvinylidene fluoride, are selected for this purpose. Experts have access to a wide range of plastics and coating technologies for this purpose. Fundamentally, the plastic layer must be highly resistant to water, fertilizers, and microorganisms. The thinner the layer, the more accurate the measurement results.

[0033] The Fig. Figure 2 shows the first circuit board 2 with the planar sensors 1a, b, with a protective body 6, which is only half-attached to the first circuit board 2 for illustrative purposes. This protective body has a dual function: It protects the mechanically very sensitive plastic layer 3 from damage and ensures good, even wetting of the planar sensors 1a, b. This property is particularly important because only a uniform moisture film on the planar sensors 1a, b enables meaningful capacitance measurements.

[0034] The Fig. Figure 3 shows a sensor housing with an inner sleeve 7, in which the second circuit board 4 with the electronic circuit 5 is arranged and sealed with a moisture-tight potting compound 8. The inner sleeve 7 has an opening at its upper end for the connection cable for the electronic circuit 5.

[0035] The inner sleeve 7 is fitted into a mechanically stable outer tube 9, glued in this example. The porous protective body 6 is shown by the dashed line. It can be seen that the protective body 6 protrudes a few millimeters into the inner sleeve 7. Preferably, the potting compound 8 forms the mounting stop for the protective body 6.

[0036] The outer tube 9 has a thread for attaching the plastic cap 10. Furthermore, the plastic cap 10 has openings (not shown here) that allow the porous protective body 6 to come into contact with the surrounding soil. This conducts the water and fertilizer ions to the planar capacitors through capillary action.

[0037] As already described in the explanation of the device according to claim 1, a special device is required that allows the protective body to be changed without damaging the delicate, thin plastic layer. The importance of this feature was already mentioned in the introduction. The anti-twist device 11, which simultaneously exerts a certain amount of pressure on the two halves of the slotted protective body 6, achieves the following positive effects: When inserting the protective body, the pointed webs 11 press into the soft protective body 6. This slightly compresses the protective body 6 so that the protective body has the necessary good mechanical contact with the planar capacitors and a uniform liquid film is formed.

[0038] If experience shows that the protective body needs to be replaced after a few weeks or months because the capillaries are no longer optimally permeable, the protective body must be removed manually by a person in the field. The webs 11 then act as an anti-twist device and only allow removal of the protective body along the length of the device. In this embodiment, the inner sleeve 7 is manufactured by injection molding. The webs 11 are also manufactured during this process. This allows overall manufacturing costs to be kept low.

[0039] The Fig. 4a and Fig. 4b illustrate how the protective body 6 encloses the first circuit board 2.

[0040] The Fig. 5 shows the cross section of the object of Fig. 3. It can be seen that the second circuit board 4 is held centrally in two grooves. These serve only as assembly aids, but can also be omitted. Since these grooves are also created during the injection molding of the inner sleeve in the same process, no additional costs arise. Reference numeral 11 indicates the tapered webs, whose special function as guides and anti-twist protection is described above.

[0041] The Fig. 6 and Fig. 7 show soils with the different structures described above. It is clear to the expert that with a soil structure according to Fig. 7 must be measured at several locations and then watered and fertilized accordingly.

Claims

[1] Device for measuring the water content and fertilizer content of soils, the device having the following features: - two planar capacitors (1a,b) formed on the top and bottom of a first printed circuit board (2) made of plastic, each planar capacitor (1a,b) consisting of a copper layer with interlocking structures and the printed circuit board (2) being coated on all sides with a water-impermeable plastic layer (3), - a second circuit board 4 has an electronic circuit (5) which is designed to evaluate capacitance changes of the planar capacitors (1a, b), wherein the first circuit board (2) is in signal contact with the second circuit board (4) and is mechanically connected, - a porous protective body (6) enclosing the first circuit board, - an inner sleeve (7) in which the composite of the first circuit board (2) and the second circuit board (4) is arranged, wherein the second circuit board (4) is potted on all sides with a potting compound (8), - an outer tube (9) in which the inner sleeve (7) is arranged, wherein the outer tube (9) has a thread onto which - a plastic cap (10) with openings is screwed on, which fixes the protective body (6) and - at least two tapered and opposing webs (11) arranged on the inner wall of the inner sleeve (7). [2] Device according to claim 1, wherein the plastic cap (10) is made of antibacterial plastic. [3] Device according to claim 1 or 2, wherein the electronic circuit (5) is designed to selectively interconnect the planar capacitors (1a, b). [4] Device according to at least one of the preceding claims, wherein the outer tube (9) has a thread at its upper end for attaching an extension tube. [5] Device according to at least one of the preceding claims, wherein the outer tube (9) has length markings.

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