Capacitive sensor for measuring the level of a substance in a tank

Abstract

A capacitive sensor for measuring the level of a substance in a tank is disclosed, comprising a substrate with a plurality of layers (1a, 1b, 1c, 1d, 1e), wherein each layer (1a, 1b, 1c, 1d, 1e) comprises one or more planes of components, such as measurement electrodes (2), a microcontroller (3) and traces (4) for connecting the measurement electrodes (2) to the microcontroller (3). According to the invention, the capacitive sensor comprises a first layer (1a) comprising a plane comprising a plurality of linearly distributed measurement electrodes (2), all traces (4) for connecting the measurement electrodes (2) to the microcontroller (3) have the same shape and length, and are surrounded by at least one guard plate (8) and optionally at least one shield plate (7), both made of an electrically conductive material.

Description

Technical Field

[0001] The present invention relates to a capacitive sensor for measuring the level of a substance (typically a liquid) in a tank or reservoir.

[0002] The capacitive sensor of this invention aims to provide the best level of reliability for measurements, as well as excellent protection against electromagnetic noise, which can distort the accurate measurement of liquid levels in a tank.

[0003] The capacitive liquid level sensor of the present invention for substances in tanks is intended for use in the measurement systems industry of industrial installations, and more specifically, in systems for measuring the liquid level in tanks typical of the automotive industry, where higher accuracy is required in the measurement standards. Background Technology

[0004] Capacitive systems are known to measure the liquid level in a tank based on the arrangement of electrodes immersed in a liquid present in the tank and the measurement of the capacitance change between the electrodes.

[0005] Some of these systems, such as the one described in document WO 2008080865 A2, are based on the introduction of two linear electrodes parallel to the direction of liquid level change in the tank. These systems have the advantage of high resolution in measuring liquid level changes, but they also have some drawbacks. First, without a reference electrode, where one reference electrode is fully immersed in the liquid medium and the other is in the air, the liquid level cannot be determined because, with a constant liquid level, the capacitance between the electrodes varies with the dielectric properties of both the medium and the insulating substrate on which the electrodes are placed. These properties are affected by factors such as instantaneous humidity or temperature, or more slowly due to the gradual aging of the insulating material. Second, the linear electrodes act like antennas, capable of capturing electromagnetic interference that can be conducted to the sensor's measurement controller, leading to erroneous readings or even complete inability to measure. Third, as the liquid level rises, the trace leading to the "wet" reference electrode also couples with the liquid, causing a change in the capacitance of the immersed reference electrode, thus introducing a difficult-to-correct nonlinearity in the response.

[0006] Another type of capacitive level sensor exists, such as the one described in document EP 1573280 B1, which is based on a discrete array of electrodes arranged along the direction of liquid level change in the tank. This type of capacitive sensor solves the aforementioned problems with linear electrodes because the discrete electrode array is unlikely to form an antenna due to its geometry, and further allows for redundant reference electrodes (in both the immersion and dry media) because they allow for a strategy based on sequentially measuring the “distributed” capacitance values ​​between each electrode in the queue (acting as the measuring electrode or the first armature of the capacitor) and the remaining electrodes in the queue, which are “collectively” connected to a single and identical known potential, thus acting as the second armature of the capacitor. Electrodes with high capacitance face the liquid medium, while those electrodes that show capacitance comparable to or close to their initial value in air (which can be stored in the controller’s memory via an independent reference for each electrode) do not face the medium.

[0007] However, this type of capacitive sensor based on a discrete electrode array also has some drawbacks, stemming from the fact that conductive traces from the corresponding input of the controller to each measuring electrode must be present on the surface opposite the motherboard substrate (the printed circuit board). These traces are coupled to the remaining electrodes, the liquid medium, and any other adjacent conductors along the length of the traces at different potentials. This presents several problems. If the electrode not performing the measurement is connected to the system ground as is normally, a considerable base capacitance (the capacitance when not facing the liquid medium) is added to the electrode in measurement mode, and this capacitance increases if there are other nearby conductors grounded as is normally the case. These base capacitances relative to ground, as well as any other parasitic capacitances introduced by the traces, are different for each electrode and, moreover, vary significantly with temperature or humidity, as these cause changes in the dielectric properties of the substrate. This results in variations in response comparable to those caused by changes in the liquid medium level, which are different for each electrode and follow different patterns of variation, making it very difficult to establish an effective compensation law for the entire sensor. Furthermore, the connection traces are coupled to the dielectric, and once the liquid level exceeds the electrode level, the response of each electrode will continuously change with the liquid height, resulting in different final values ​​for each electrode, similar to what happens with base capacitance. The connection traces are also susceptible to external electromagnetic interference and capacitive coupling with external grounding components.

[0008] Purpose of the invention

[0009] To address the aforementioned drawbacks, the present invention relates to a capacitive sensor for measuring the liquid level of a substance in a tank.

[0010] The capacitive sensor for measuring the liquid level of a substance in a tank, the objective of this invention, comprises a motherboard (PCB) having multiple layers. Each layer includes one or more regions having components such as measuring electrodes, electronic signal conditioning circuitry, and traces for connecting the measuring electrodes to the electronic signal conditioning circuitry.

[0011] Therefore, the capacitive sensor includes a first layer, which in turn includes a region containing a plurality of linearly distributed measuring electrodes.

[0012] In a novel manner, in the capacitive sensor target of the present invention, all traces for connecting the measuring electrodes to the electronic signal conditioning circuit are:

[0013] - Including the same shape and length, and;

[0014] - Surrounded by at least one protective plate and optionally at least one shielding plate, both made of conductive material, the protective plate is subjected to a protective voltage to prevent coupling between the trace and the dielectric and with any adjacent conductors at different potentials, which could potentially affect the capacitance measured by the measuring electrodes; the shielding plate is grounded for electromagnetic shielding of the trace.

[0015] Protective plates and shielding plates are different names for plates made of conductive materials used to shield the traces on the mainboard of a capacitive sensor and other optional components.

[0016] At least one protective plate and / or at least one shielding plate may be made of copper and / or tin.

[0017] The protective plate can be connected to a protective voltage, so that if the electrodes and corresponding traces are set at the measuring voltage, the remaining traces can be set to the same voltage as the protective plate by an electronic signal conditioning circuit.

[0018] At least one shielding plate can be grounded.

[0019] Since all traces have the same shape and length, they all have the same underlying capacitance associated with adjacent conductors, and thus they also exhibit the same evolution of the dielectric constant relative to environmental variations (e.g., variations caused by temperature or humidity) within a relatively narrow margin. This is advantageous for possible calibration or compensation strategies.

[0020] Furthermore, the traces surrounded by the shielding plate allow for the prevention or at least minimization of coupling between the traces used to connect the electrodes and the medium (e.g., the liquid medium) being measured, enhancing the “equalization” effect of the electrode response. In addition, the inclusion of the shielding plate provides protection against electric and electromagnetic fields that may affect the measurement, providing a high-precision sensor specifically designed for the automotive industry, where higher precision sensor measurements are required.

[0021] This allows for the prevention or at least minimization of undesirable coupling between the trace and other adjacent conductors in the printed circuit board (PCB), thus minimizing the impact on the measurement of variations in the characteristics of the printed circuit board substrate. Without a metal plate (protective plate and / or shielding plate), equalization of the base capacitance when the sensor is not facing the medium will be performed at the cost of increasing the base capacitance, which is extremely detrimental to the signal-to-noise ratio.

[0022] Furthermore, it allows for the prevention or minimization of the coupling of external electromagnetic interference with the trace (“antenna effect”).

[0023] In short, a combination of traces of the same shape and length with a shielding (protective) plate can achieve the following:

[0024] - Smaller, more balanced base capacitor;

[0025] - The response of each measuring electrode is more uniform because they face the medium, are not affected by the other "wet" electrodes, and do not change further as the liquid level gradually rises;

[0026] - Greater immunity to any type of external interference, robustly achieving a signal-to-noise ratio sufficient to enable sensor devices to function correctly.

[0027] The highly uniform response of the different measuring electrodes in a capacitive sensor, regardless of their position within the sensor, and the stability provided by reducing the drift of the substrate's dielectric properties with temperature, humidity, or aging (the main cause of drift), mean that there is no need for a reference electrode at the far or near end of the structure, as is the case in those structures that consist of linear electrodes rather than multiple sensor elements, or in other structures that, even if they consist of discrete sensor elements, suffer significant drift (which can even reach the magnitude of the useful signal caused by the presence of liquid). Therefore, comparison with an immersed reference electrode and with another non-immersed reference electrode are necessary to determine whether the change in electrode readings is due to the presence of liquid.

[0028] Thus, for this invention, the capacitance value of each measuring electrode can be simply compared with a typical reference value for each measuring electrode stored in the memory of the electronic signal conditioning circuit to determine whether a given electrode is facing the medium. This is equivalent to performing an "automatic zeroing" of the system when the measuring electrode is in contact with air, subtracting the value measured under that condition from the output value of each measuring electrode, thereby zeroing the output "in air," and then using the subtraction calculated thereby as the output signal.

[0029] Another advantage of this solution is that, for the same reason mentioned above, it is independent of the dielectric constant of the fluid or medium being measured; the sensitivity limit is marked by the residual change in the readings of each electrode in a “dry” state, which is very small. In fact, different liquids will provide different absolute capacitance values ​​for each electrode, but the comparison method will work as long as the difference from the initial value (“dry” electrode) is above the system's noise threshold.

[0030] According to a first embodiment of the present invention, the capacitive sensor includes a second layer, a third layer, and a fourth layer. The second layer includes a shielding plate; the third layer includes traces for connecting measuring electrodes to an electronic signal conditioning circuit; and the fourth layer includes a protective plate. The first and second layers are the outer layers of a motherboard (PCB), and the third layer is located between the second and fourth layers.

[0031] According to a possible embodiment of the invention, the capacitive sensor includes a second layer including a shielding plate and a third layer including a protective plate, as well as traces for connecting measuring electrodes to an electronic signal conditioning circuit. Each trace is inserted into a corresponding elongated window formed in the protective plate. The first and second layers are the outer layers of a motherboard (PCB), and the third layer is located between the first and second layers.

[0032] The above-described embodiment has the advantage of simplifying the motherboard (and thereby reducing its cost) by unifying the layer including the traces with the board including the protective plate.

[0033] Considering that each time the electrode (and corresponding trace) is set to the measurement voltage, the remaining trace can be set to the same voltage (protective voltage) as the shielding plate by a switching device, so that the remaining trace is as if it were part of the shielding plate and not interrupted, thus inserting the trace into the shielding plate has almost no negative impact on the shielding function of the plate. This solution is particularly suitable for sensors with low measurement levels and correspondingly short sensor elements with few electrodes and short traces, because in these types of sensors, on the one hand, the geometric and electrical interference of the shielding plate due to the insertion of the trace is minimized, and on the other hand, due to the shortness of the trace, their capacitance relative to any adjacent conductor is lower than that of the wet electrode.

[0034] According to one aspect of the invention, a grounding electrode is arranged between every two measuring electrodes. All grounding electrodes are physically and electrically connected to a single and identical potential.

[0035] The features described in the previous paragraph avoid the need, as is the case with other capacitive sensors in the prior art, for the medium to be conductive and act as a “virtual electrode” that needs to be grounded or somehow connected to a circuit for the circuit diagram to work. This is a clear drawback of the strategy, given that not all media of interest are conductive (including, for example, oil or diesel or gasoline-type fuels, which are insulators).

[0036] Based on the feature of inserting a grounding electrode between the measuring electrodes, the measuring electrodes, the traces not being measured, and the protective plate are preferably set at a protection voltage to prevent the traces from coupling with the dielectric; and the grounding electrodes are preferably grounded so that they act as a second armature.

[0037] According to another aspect of the invention, in addition to the measuring electrodes of the first layer, the capacitive sensor also includes an additional layer comprising three or more linearly distributed measuring electrodes, wherein the first layer and the aforementioned additional measuring electrode layer are outer layers of the motherboard (PCB). This feature improves the sensitivity of the capacitive sensor. Preferably, the measuring electrodes of the first layer and the additional layer are interconnected in pairs, thereby increasing the surface area of ​​the armature.

[0038] According to possible embodiments, the signal conditioning electronics are arranged in a region of the first layer adjacent to the region including the measuring electrodes. In the case of the first embodiment, the fourth layer preferably includes a shielding plate for the signal conditioning electronics, wherein the shielding plate for the electronic signal conditioning circuit is at a voltage different from that of the protective plate, preferably at the system ground voltage.

[0039] As an alternative to the scheme mentioned in the previous paragraph, the electronic signal conditioning circuit can be arranged in a region located in the second layer and attached to a shielding plate arranged in the layer.

[0040] As described in the previous paragraph, the first layer may include a shielding plate for electronic signal conditioning circuitry, wherein the shielding plate for electronic signal conditioning circuitry is at a different potential than the measuring electrodes.

[0041] As an alternative to the scheme mentioned in the previous paragraph, the third layer (including the traces) may include a shield for electronic signal conditioning circuitry, wherein the shield for electronic signal conditioning circuitry is at a different potential than the protective plate and / or the traces used to connect the measuring electrodes to the electronic signal conditioning circuitry, typically at ground voltage. Attached Figure Description

[0042] The following figures have been included as part of the description of at least one embodiment of the present invention.

[0043] Figure 1An exploded perspective view of the different layers of a motherboard constituting a capacitive sensor according to a first embodiment of the present invention is shown.

[0044] Figure 2 A schematic exploded plan view of the different layers of the motherboard of the capacitive sensor according to the first embodiment is shown.

[0045] Figure 3 A schematic exploded plan view of the different layers of the motherboard of the capacitive sensor according to the second embodiment is shown.

[0046] Figure 4 An exploded perspective view of the different layers of a motherboard constituting a capacitive sensor according to a third embodiment of the present invention is shown.

[0047] Figure 5 A schematic exploded plan view of different layers of the motherboard of the capacitive sensor according to the second and third embodiments is shown.

[0048] Figure 6 A schematic exploded plan view of the different layers of the motherboard of the capacitive sensor according to the fourth embodiment is shown. Detailed Implementation

[0049] As described above, the present invention relates to a capacitive sensor for measuring the liquid level of a substance in a tank.

[0050] The capacitive sensor of the present invention is intended to be built on a motherboard (PCB) consisting of multiple stacked layers (1a, 1b, 1c, 1d, 1e).

[0051] Each layer of the motherboard (1a, 1b, 1c, 1d, 1e) may include one or more boards for performing different functions.

[0052] according to Figure 1 and Figure 2 The first embodiment of the capacitive sensor shown has a motherboard comprising four stacked layers (1a, 1b, 1c, 1d).

[0053] The first sensing layer (1a) comprises two regions. Multiple linearly distributed measurement electrodes (2) are located within the first region. The electronic signal conditioning circuit (3) of the motherboard is disposed in the second region of the first layer (1a).

[0054] The second shielding layer (1b), opposite to the first sensing layer (1a), comprises two plates made of conductive material. One of these plates is a shielding plate (7) used to shield the traces (4) of the electronic signal conditioning circuit (3) that connects each measuring electrode (2) to the main board from external electric and electromagnetic fields. The other of these plates, also a shielding plate (7) in the region adjacent to the aforementioned plates, is used to shield the electronic signal conditioning circuit (3) located in the first layer (1a).

[0055] The third layer (1c), which is adjacent to the second layer (1b) above it, includes the area where the aforementioned traces (4) connect each measuring electrode (2) to the electronic signal conditioning circuit (3).

[0056] The fourth layer (1d) of the capacitive sensor is located between the first layer (1a) and the third layer (1c). This fourth layer (1d) comprises two regions. One of these regions includes a plate made of conductive material, which is a protective plate (8) for shielding the third layer (1c) including the trace (4), thereby preventing or minimizing coupling of the trace (4) to the dielectric, to the measuring electrode (2), or to any adjacent conductor at a different potential. Preferably, the protective plate (8) can be connected to a protective voltage substantially equal to the measuring voltage, such that all traces are at the same voltage, thereby determining whether an electrode is facing the dielectric by comparing the capacitance value of each measuring electrode with a typical reference value of each measuring electrode stored in the memory of the electronic signal conditioning circuit. The other region includes a shielding plate (7) for shielding the electronic signal conditioning circuit (3) located in the first layer (1a).

[0057] Each preferably rectangular measuring electrode (2) arranged one next to the other along its long side is connected to the node (5) of the trace (4) via a conductive element (not depicted) through a corresponding hole (6) in the fourth layer (1d) where a protective plate (8) is located. The hole (6) is metallized to establish the conduction, so that the measuring electrode (2) is connected to the electronic signal conditioning circuit (3), thereby enabling the connection between the layers by means of the metallized hole (6).

[0058] In all embodiments of the capacitive sensor target of the present invention, all traces (4) have the same length, regardless of whether the nodes (5) are located at different heights for connection to the corresponding measuring electrodes (2).

[0059] Similarly, in all embodiments of the capacitive sensor target of the present invention, all traces (4) have the same shape.

[0060] Figure 3 A second embodiment of the capacitive sensor target of the present invention is illustrated schematically.

[0061] The second embodiment differs from the first embodiment in that a ground electrode (2') is provided between every two measuring electrodes (2) in the first layer (1a). All ground electrodes (2') are preferably wider than the measuring electrodes (2) and interconnected around the ground electrodes (2'), as follows: Figure 3 As can be seen in the image. Thus, the grounding electrode (2') is connected with one and the same potential (normally grounded).

[0062] Thus, according to this second embodiment, the ground electrode (2') group behaves like the second armature of a capacitor, with the first armature being the corresponding measuring electrode (2).

[0063] Figure 4 and Figure 5 A third embodiment of the capacitive sensor target of the present invention is illustrated schematically.

[0064] The third embodiment differs from the first embodiment in that, in the third embodiment of the capacitive sensor, the second layer (1c) and the fourth layer (1d) of the first embodiment are integrated into a single third layer (1c).

[0065] The third layer (1c) of the third embodiment includes a plurality of traces (4) inserted into a corresponding elongated window (9) or elongated channel made in the protective plate (8).

[0066] Furthermore, in this third embodiment, the electronic signal conditioning circuit (3) can be located in the first layer (1a) (as occurred in the first embodiment) or in the second layer (1b) including the shielding plate (7). Additionally, as... Figure 5 As can be seen, in this embodiment, the ground electrode (2') in the first layer (1a) is used, and the configuration is also optional.

[0067] Figure 6 A fourth embodiment of the capacitive sensor target of the present invention is illustrated schematically.

[0068] The fourth embodiment differs from the first embodiment in that it has an additional layer (1e) on the motherboard, which is located on the outside adjacent to the second layer (1b), on the side opposite to the side of the third layer (1c) that contacts the second layer (1b).

[0069] Therefore, like the first layer (1a), this additional layer (1e) is the outer layer of the motherboard.

[0070] Similar to the first layer (1a), the additional layer (1e) is a sensing layer that allows for improved system sensitivity by replicating the useful surfaces of the measurement electrodes (2). Therefore, the additional layer (1e) comprises a plurality of linearly distributed measurement electrodes (2) that are connected to corresponding traces (4) of the third layer (1c) via nodes (5) approached through perforations (6) in the shielding plate (7) of the second layer (1b), in which case the shielding plate (7) of the second layer (1b) becomes a shielding plate similar to a protective plate (8). For this last embodiment, as... Figure 6 As can be seen, it also adopted Figure 3The second embodiment, which includes a ground electrode (2') between the sensing electrodes (2) in both the first layer (1a) and the additional layer (1e), is more suitable for use with insulating or low-conductivity fluids.

[0071] This section has described four embodiments of the invention with the aid of the accompanying drawings, each with its own features. However, the capacitive sensor target of the present invention can be achieved by combining features of different embodiments, thereby creating various alternative embodiments having typical features of two or more of the embodiments described above.

Claims

1. A capacitive sensor for measuring the liquid level of a substance in a tank, comprising a main board having multiple layers (1a, 1b, 1c, 1d, 1e), wherein each layer (1a, 1b, 1c, 1d, 1e) includes one or more regions comprising a measuring electrode (2), an electronic signal conditioning circuit (3), and traces (4) for connecting the measuring electrode (2) to the electronic signal conditioning circuit (3), wherein the capacitive sensor includes a first layer (1a) comprising a region comprising a plurality of linearly distributed measuring electrodes (2), and all traces (4) for connecting the measuring electrode (2) to the electronic signal conditioning circuit (3): - Each includes the same shape and length, and; - Surrounded by at least one protective plate (8) and at least one shielding plate (7), both made of conductive material, the protective plate (8) is configured to be protected against voltage to prevent coupling between traces (4) that affects the capacitance measured by the measuring electrode (2), and the at least one shielding plate (7) is grounded for electromagnetic shielding of the traces (4), and in, The plurality of layers further includes: a second layer (1b) comprising a shielding plate (7); a third layer (1c) comprising the trace (4) for connecting the measuring electrode (2) to the electronic signal conditioning circuit (3); and a fourth layer (1d) comprising a protective plate (8), wherein the first layer (1a) and the second layer (1b) are outer layers of the motherboard, and wherein the third layer (1c) is disposed between the second layer (1b) and the fourth layer (1d).

2. A capacitive sensor for measuring the liquid level of a substance in a tank, comprising a main board having multiple layers (1a, 1b, 1c, 1d, 1e), wherein each layer (1a, 1b, 1c, 1d, 1e) includes one or more regions comprising a measuring electrode (2), an electronic signal conditioning circuit (3), and traces (4) for connecting the measuring electrode (2) to the electronic signal conditioning circuit (3), wherein the capacitive sensor includes a first layer (1a) comprising a region comprising a plurality of linearly distributed measuring electrodes (2), and all traces (4) for connecting the measuring electrode (2) to the electronic signal conditioning circuit (3): - Each includes the same shape and length, and; - Surrounded by at least one protective plate (8) and at least one shielding plate (7), both made of conductive material, the protective plate (8) is configured to be protected against voltage to prevent coupling between traces (4) that affects the capacitance measured by the measuring electrode (2), and the at least one shielding plate (7) is grounded for electromagnetic shielding of the traces (4), and in, The plurality of layers further includes a second layer (1b) and a third layer (1c), the second layer (1b) including a shielding plate (7), and the third layer (1c) including a protective plate (8) and traces (4) for connecting the measuring electrode (2) to the electronic signal conditioning circuit (3), wherein each trace (4) is inserted into a corresponding elongated window (9) made on the protective plate (8), wherein the first layer (1a) and the second layer (1b) are the outer layers of the motherboard, and wherein the third layer (1c) is disposed between the first layer (1a) and the second layer (1b).

3. The capacitive sensor according to claim 1 or 2, characterized in that, A ground electrode (2') is provided between every two measuring electrodes (2), wherein all ground electrodes (2') are connected to one and the same potential.

4. The capacitive sensor according to claim 3, characterized in that, The grounding electrode (2') is grounded.

5. The capacitive sensor according to claim 1 or 2, characterized in that, The electronic signal conditioning circuit (3) is configured to measure the capacitance of the measuring electrode (2) at a measuring voltage by connecting the remaining measuring electrodes (2) to a single and identical voltage.

6. The capacitive sensor according to claim 1 or 2, characterized in that, The electronic signal conditioning circuit (3) is configured to measure the capacitance between each measuring electrode (2) and the ground electrode (2').

7. The capacitive sensor according to claim 1 or 2, characterized in that, In addition to the first layer (1a), the capacitive sensor also includes an additional layer (1e), which in turn includes a plurality of linearly distributed measuring electrodes (2), wherein the additional layer (1e) of the first layer (1a) and the measuring electrodes (2) is the outer layer of the motherboard.

8. The capacitive sensor according to claim 1 or 2, characterized in that, The electronic signal conditioning circuit (3) is arranged in the region adjacent to the region including the measuring electrode (2) in the first layer (1a).

9. The capacitive sensor according to claim 8, characterized in that, The fourth layer (1d) includes a shielding plate (7) for the electronic signal conditioning circuit (3), wherein the shielding plate (7) for the electronic signal conditioning circuit (3) is at a different potential than the protective plate (8).

10. The capacitive sensor according to claim 1 or 2, characterized in that, The electronic signal conditioning circuit (3) is arranged in a region located in the second layer (1b) and attached to a shielding plate (7) arranged in the region.

11. The capacitive sensor according to claim 10, characterized in that, The first layer (1a) includes a shielding plate (7) for the electronic signal conditioning circuit (3), wherein the shielding plate (7) for the electronic signal conditioning circuit (3) is arranged adjacent to the region including the measuring electrode (2).

12. The capacitive sensor according to claim 10, characterized in that, The third layer (1d) includes a shielding plate (7) for the electronic signal conditioning circuit (3), wherein the shielding plate (7) for the electronic signal conditioning circuit (3) is grounded.

13. The capacitive sensor according to claim 1 or 2, characterized in that, The protection voltage is a measured voltage.

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