Robust device for measuring moisture and fertilizer content in soils

Abstract

Device for measuring the water content and fertilizer content of soils, wherein the device has the following features: - two planar capacitors (1a, b) formed on the top and bottom surfaces of a first plastic printed circuit board (2), each planar capacitor (1a, b) consisting of a copper layer with interlocking structures, and the printed circuit board (2) being covered on all sides with a water-impermeable plastic layer (3), - a second printed circuit board (4) comprising an electronic circuit (5) configured to evaluate capacitance changes of the planar capacitors (1a, b), wherein the first printed circuit board (2) is signal-contacted and mechanically connected to the second printed circuit board (4), - a porous protective body (6) that encloses the first circuit board (2), - an inner sleeve (7) in which the assembly of the first printed circuit board (2) and the second printed circuit board (4) is arranged, wherein the second printed circuit board (4) is encapsulated 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 2 tapered and opposing ribs (11) arranged on the inner wall of the inner sleeve (7).

Description

[0001] 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. Hereinafter, the term "soil" refers to agriculturally used land.

[0002] Devices of this type are known, for example, from the following publications: US 2019 / 0 271 656 A1, DE 25 33 507 A1 and DE 10 2020 003 382 B4.

[0003] The design and operation of measurement and automation systems is the responsibility of measurement and automation technicians in fields other than agriculture. Such specialists are generally not available in agriculture, or only for farms with large areas where the aforementioned problems with soil structure do not exist anyway.

[0004] Therefore, one requirement for an irrigation and fertilization system is that the sensors used are very robustly constructed so that even untrained personnel do not damage them when inserting them into the ground. Damaged sensors deliver no or incorrect readings, inevitably leading to incorrect application of water and fertilizer.

[0005] However, there is another cause for incorrect measurements and the resulting incorrect dosages: Soils can have different structures. In fields and greenhouses, the soils are usually optimally filled in with respect to their depth and have a consistent structure across their surface. In other words, the soils are essentially structured the same everywhere in the field. If the moisture and / or fertilizer content is measured in such soil using a sensor, this measurement will also be representative of other areas, provided water and / or fertilizer have been applied uniformly.

[0006] However, there are also soils that exhibit a very uneven structure with respect to their depth. Such soils are found, for example, in settlements with houses interspersed with gardens. These gardens are increasingly used for growing fruit and vegetables. Often, these soils are covered with only a thin layer of topsoil suitable for planting. The underlying layers are often formed by the accumulation of materials such as construction debris, formwork boards, or unknown materials. This highly inhomogeneous subsoil causes water and / or water containing fertilizer to penetrate the soil very differently. Thus, there can be almost impermeable zones, but also zones with larger cavities where water seeps away very quickly.

[0007] The structure of the deeper soil layer in the aforementioned settlement soils is no longer modifiable. Even if the overlying layer consists of optimal potting soil, the described, significantly different water penetration behavior remains unchanged. Therefore, even precise sensors are useless for dosing water and fertilizer, as there are no suitable locations in such soils where representative measurements can be taken.

[0008] Experts are aware that these circumstances lead to serious measurement errors. This also explains why highly precise irrigation technology, including accurate fertilizer dosing, does not produce the theoretically expected effect on plant growth and yield under such conditions, as the dosage is based on incorrect measurements.

[0009] In summary, it can be stated that soils with such a different structure cannot yet be irrigated or fertilized according to their needs.

[0010] The only way to avoid over- or under-irrigation, or over- or under-fertilization, in soils with highly variable structures is to measure parameters at as many locations as possible and then implement selective irrigation and / or fertilization based on those measurements. However, the associated effort and costs are currently so high that this method has not yet become widespread. If more sensors are required, the resulting additional costs may outweigh the benefits of improved, i.e., more precise, irrigation and fertilization.

[0011] Therefore, there is a need to reduce the effort required for site-specific irrigation and fertilization. The first step towards this is the provision of very cost-effective and robust devices that can simultaneously determine the water and ion content of the soil at a large number of locations.

[0012] Another requirement is a small measuring volume. This means, for example, that there are devices that measure soil moisture using microwaves. These microwave devices measure a volume of approximately 500 to 5000 cubic centimeters of soil. Given the specific soil conditions mentioned earlier, significant material variations could occur within this measuring volume. Consequently, different volumes of material within the measuring volume will have varying water and fertilizer contents. The percolation rate of water within the measuring volume can also differ. Experts understand that such a measurement method is bound to produce inaccurate readings, as it is crucial to measure the water and fertilizer content as close as possible to the roots. This is only possible with a device that has a small measuring tip, i.e., a small measuring volume.

[0013] Microwave measurements present another problem: if the device is positioned near roots, the microwaves penetrate them. 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 when calculating the necessary amount of water. The same errors occur when calculating the required fertilizer concentration. Furthermore, it should be noted that roots also grow within the microwave field, which can further distort the measurements.

[0014] In summary, it can be stated that a device for measuring moisture and fertilizer content in soils must have a wide variety of design and measurement properties for the inhomogeneous soil structures described above, some of which are mutually exclusive.

[0015] The device, hereinafter referred to as the sensor, should have the following features and properties: a. Rod-shaped construction with a diameter of less than 3 cm, to be easily inserted into a hole in the ground created with a conventional drill and a spiral drill bit. b. Maximum volume of the sensor head approx. 2 x 2 x 2 cm to be able to measure close to roots. c. High mechanical robustness so that the rod-shaped sensor is not damaged when inserted into the hole in the ground and when removed at certain time intervals, as the hole in the ground is not drilled anew each time. d. Low manufacturing costs to be able to use a large number of sensors if needed.

[0016] The aforementioned requirements and properties are achieved or solved with a device according to claim 1.

[0017] The device has the following features: Two planar capacitors 1a, b, formed on the top and bottom surfaces of a first plastic printed circuit board 2, each planar capacitor 1a, b consisting of a copper layer with interlocking structures. Preferably, these structures are two comb-like electrodes whose teeth interlock but do not touch, thus forming a planar capacitor. However, other interlocking structures can also be used.

[0018] The first circuit board 2 is covered on all sides with a waterproof plastic layer 3, i.e. the side edges of the circuit board are also covered so that the copper layer is safely protected from corrosion.

[0019] An electronic circuit 5 is arranged on a second circuit board 4, which is configured to evaluate changes in the capacitance of the planar capacitors 1a, b. The first circuit board 2 is connected to the second circuit board 4 via signal transmission and is mechanically connected.

[0020] 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 water-soluble fertilizer to the planar capacitors via capillary action and distribute them evenly across the entire surface of the planar capacitors. Even distribution means that no unwetted areas form on the planar capacitors. This is achieved by ensuring that at least the contact surfaces between the protective body 6 and the plastic layer 3 are very finely porous. In addition, the protective body 6 must be pressed against the plastic layer 3 with a predetermined contact pressure by a mechanical means.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 aforementioned requirements are met; textile fibers or felts are preferably suitable for this purpose.

[0021] The assembly of the first printed circuit board 2 and the second printed circuit board 4 is inserted into an inner sleeve 7 and fixed 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 led to the outside. The potting compound 8 is filled until the second printed circuit board 4 and the electronic circuit 5 are completely covered. The potting compound makes the inner sleeve mechanically very stable and protects the circuit from corrosion.

[0022] Once the potting compound 8 has hardened, the protective body 6 is attached to the first circuit board. The protective body 6 is slotted for this purpose.

[0023] The inner sleeve 7 is held in a mechanically stable outer tube 9. The inner sleeve 7 can either be 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 that are large enough to ensure that the protective body 6 has extensive contact with the ground, allowing water to penetrate or be drawn in by capillary action through the protective body material.

[0024] 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.

[0025] However, it is necessary to replace the protective cover at predetermined intervals because microorganisms may clog the capillaries and thus distort the measurement result. This presents the problem that the old, possibly even somewhat encrusted, protective cover 6 must be removed from the circuit board 2 and a new protective cover 6 fitted. This replacement is best carried out in the field so that the hole from which the sensor was extracted remains intact. Therefore, it is essential to prevent damage to the plastic layer 3 when removing the protective cover 6.

[0026] Damage is avoided according to the invention by providing at least two opposing, tapered ridges 11 on the inner wall of the inner sleeve 7. The ridges act as an assembly aid when attaching the protective body and as an anti-rotation device when removing it, as explained below: When the protective body is attached, the points press into the soft material and expand it to the cross-section of the ridges. When the protective body is to be removed, e.g., after a few months, the ridges act as an anti-rotation device, 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 ridges 11 also prevent the protective body from rotating when the plastic cap 10 is screwed on or off.

[0027] According to an advantageous embodiment of 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. As a result, the protective body can be used for a longer period of time.

[0028] According to an advantageous embodiment of claim 3, the electronic circuit 5 is configured to selectively connect the planar capacitors 1a, b. This makes it possible to double the surface area of ​​the planar capacitor. In other words, during normal operation, the water content and the ion content can be measured simultaneously. When both planar capacitors are connected, either the water content or the ion content can be measured with higher accuracy.

[0029] According to an advantageous embodiment of claim 4, the outer tube has a thread at its upper end for attaching an extension tube.

[0030] According to an advantageous embodiment of claim 5, length markings are provided on the outer tube. These length markings are useful when the sensors are to be inserted at the same or different depths, for example, to record a profile of the water content in the soil.

[0031] The device is explained in more detail below with reference to drawings. Fig. Figure 1 shows a combination of two planar sensors with evaluation electronics, Fig. Figure 1a shows an enlarged view of a planar sensor, Fig. Figure 2 shows the planar sensors with a partially attached protective cover, Fig: 3 shows a sensor housing with a protective cap made of plastic, Fig. Figures 4a and b show the open and closed protective body in one design option. Fig. Figure 5 shows a cross-section of the inner sleeve with the second circuit board without electronic circuitry. Fig. Figure 6 shows an agriculturally used soil with a uniform soil structure and Fig. Figure 7 shows agriculturally used soil with an uneven soil structure. Reference symbol list 1a, b Planar capacitors 2 first circuit board 3 plastic layers 4 second circuit board 5 electronic circuit 6 porous protective bodies 7 Inner sleeve 8 Potting compound 9 Outer pipe 10 plastic caps 11 Anti-rotation device

[0032] The Fig. Figure 1 shows a combination of two planar sensors 1a, b mounted on a first printed circuit board 2. This is a double-sided copper-coated plastic board, commonly used in the prior art and therefore very cost-effective to manufacture.

[0033] 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 encased in a thin plastic layer 3. Plastics that are particularly waterproof, such as polyvinylidene fluoride, are selected for this purpose. A wide range of plastics and coating technologies are available to the expert. In principle, the plastic layer must be highly resistant to water, fertilizers, and microorganisms. The thinner the layer, the more accurate the measurement results.

[0034] The Fig. Figure 2 shows the first circuit board 2 with the planar sensors 1a, b and a protective cover 6, which is only partially attached to the first circuit board 2 for illustrative purposes. This protective cover has a dual function: It protects the mechanically very sensitive plastic layer 3 from damage and ensures good, uniform wetting of the planar sensors 1a, b. This property is particularly important because only a uniform film of moisture on the planar sensors 1a, b enables a meaningful capacitance measurement.

[0035] 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 moisture-tight with a potting compound 8. The inner sleeve 7 has an opening at its upper end for the connecting cable for the electronic circuit 5.

[0036] The inner sleeve 7 is fitted into a mechanically stable outer tube 9, in this example glued in place. The porous protective body 6 is shown with the dashed line. It can be seen that the protective body 6 projects a few millimeters into the inner sleeve 7. Preferably, the potting compound 8 forms the mounting stop for the protective body 6.

[0037] The outer tube 9 has a thread for attaching the plastic cap 10. The plastic cap 10 also has openings (not shown here) so that the porous protective body 6 comes into contact with the surrounding soil. This allows capillary action to draw water and fertilizer ions to the planar capacitors.

[0038] 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 sensitive thin plastic layer. The importance of this feature was already mentioned in the introduction. The following positive effects are achieved by means of the anti-rotation device 11, which simultaneously exerts a certain pressure on the two halves of the slotted protective body 6: When the protective body is inserted, the pointed ridges 11 press into the soft protective body 6. This compresses the protective body 6 slightly, ensuring that the protective body has the required good mechanical contact with the planar capacitors and that a uniform liquid film is formed.

[0039] If, after a few weeks or months, the protective body needs to be replaced because the capillaries are no longer optimally permeable, the protective body must be removed by hand in the field. The ribs 11 then act as an anti-rotation device and only allow the protective body to be removed along the longitudinal axis of the device. In this embodiment, the inner sleeve 7 is manufactured by injection molding. The ribs 11 are manufactured simultaneously in this process. This helps to keep overall manufacturing costs low.

[0040] The Fig. 4a and Fig. Figure 4b illustrates how the protective body 6 encloses the first circuit board 2.

[0041] The Fig. 5 shows the cross-section of the object of the Fig. 3. It can be seen that the second circuit board 4 is held centrally in two grooves. These serve only as an assembly aid and can also be omitted. Since these grooves are also produced in the same operation during the injection molding of the inner sleeve, there are no additional costs. Reference numeral 11 indicates the pointed ridges, whose special function as guides and anti-rotation devices has been described above.

[0042] The Fig. 6 and Fig. Figure 7 shows soils with the different structures described above. It is clear to the expert that, with a soil structure according to Fig. 7. Measurements must be taken at several points, and then watering and fertilizing must be differentiated accordingly.

Claims

[1] Device for measuring the water content and the 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) having an electronic circuit (5) designed to evaluate capacitance changes of the planar capacitors (1a, b), wherein the first circuit board (2) is signal-contacted and mechanically connected to the second circuit board (4), - a porous protective body (6) enclosing the first circuit board (2), - 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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