Capacitive level sensor and water tank for a motor vehicle with a level sensor

Abstract

Capacitive fill level sensor with a circuit board (1) which has sensor areas (2) and an evaluation unit (3), as well as an electrical connection (4) and a housing (5), wherein two sensor areas (2) are provided on the two sides of the circuit board (1) which are opposite one another and have two pairs of measuring electrodes (61, 61b, 62a, 62b), characterized in that first measuring electrodes (61a, 61b) and a first shield electrode (71) surrounding them are arranged on one side of the circuit board (1), which are driven in phase synchronism, and on the other side second measuring electrodes (62a, 62b) and a second shield electrode (72) surrounding them are arranged, which are driven in phase opposition to the first.

Description

[0001] The invention relates to a capacitive fill level sensor according to claim 1 and a water tank for a motor vehicle with a capacitive fill level sensor according to claim 5.

[0002] Capacitive level sensors are not only used in process measurement technology, but also in automotive engineering, where they are used for fuel level indicators, as well as in cooling water and detergent containers.

[0003] DE 10 2013 010 708 A1 describes a capacitive level switch that can be mounted in any position. The sensor electrodes and the evaluation unit are arranged together on a circuit board (single-board sensor). The active (sensor) surfaces and a counter electrode are surrounded by ground planes. The active surface(s) and the counter electrode are asymmetrically designed. In particular, the active surface is interrupted, and the counter electrode is dumbbell-shaped.

[0004] The asymmetrical structure, because interference signals do not cancel each other out, as well as the ground connection, which is unfavorable both in terms of signal technology and interference immunity, are considered to be disadvantages.

[0005] DE 690 01 151 T2 shows a capacitive level sensor in which the measuring electrodes act as frequency-determining capacitors, which is considered to be poorly immune to interference and therefore disadvantageous. Furthermore, the complexity is relatively high. The counter electrodes are grounded, which is also considered disadvantageous.

[0006] DE 10 2013 005 963 A1 describes a capacitive level sensor with a measuring electrode, a counter electrode, and two shielding electrodes, all of which are strip-shaped. The impedance of a capacitance located between the shielding electrodes and the counter electrode is evaluated as the ratio of the measuring capacitance to the sum of the shielding capacitances, which requires external shielding. The sensor is designed for mounting on the outer housing wall.

[0007] WO 2008 062 146 A1 describes a capacitive level sensor suitable as an immersion probe with two sets of electrodes driven in antiphase, each with a lower and an upper measuring electrode. Once the lower measuring electrodes are completely covered, the level can be calculated ratiometrically from the ratio of the measured values ​​of the two measuring electrode pairs, known as the phase angle (see page 9, equation 1). An active shield electrode is omitted here.

[0008] The object of the invention is to improve the interference immunity, in particular to provide an arrangement with symmetrical measuring electrodes without electrodes at ground potential.

[0009] The object of the invention is achieved by the features of claim 1. The dependent claims relate to the advantageous embodiment of the invention. The essential idea of ​​the invention is to drive symmetrical measuring electrodes in antiphase, i.e., to apply a push-pull signal, and to actively shield them to avoid ground connection, whereby all measuring electrodes (pairs) are to be shielded by a single shield electrode (pair).

[0010] For evaluation, the high-frequency currents from two measuring electrodes driven in antiphase, with one pair of measuring electrodes serving as a reference, are added (not subtracted) to create a metrologically closed circuit that eliminates the need for a ground electrode (reference potential) on both the sensor and supply sides. An arrangement according to the invention comprises a pair of shielding electrodes and at least two pairs of measuring electrodes (six electrodes), with four electrodes each active simultaneously.

[0011] The measuring electrodes are enclosed by the in-phase driven shield electrodes, which are at least arranged in the immediate vicinity of the measuring electrodes.

[0012] A high-frequency generator according to the invention has (at least) six outputs, the first two outputs driving the shield electrodes and the other four outputs driving the two pairs of measuring electrodes (see Fig. 4).

[0013] The advantages of the invention include improved noise immunity and the avoidance of galvanic coupling with the fluid. Furthermore, the arrangement is cost-effective to manufacture and easy to install.

[0014] The invention is explained in more detail with reference to the drawings. Fig. 1 shows a level sensor according to the invention in an isometric view, Fig. 2 shows the front of a preferred electrode arrangement with 8 electrode pairs, Fig. 3 shows a cross-section through the electrode system of the sensor. Fig. 4 shows a circuit with two pairs of measuring electrodes and an evaluation unit.

[0015] The Fig. Figure 1 shows the most important elements of a level sensor according to the invention in an isometric view. Visible are a carrier 1 with an electrode system 2, which may have a non-conductive sheath to prevent galvanic coupling with the medium, a screw thread for mounting the level sensor in a designated liquid container, and a housing part 5, which houses an evaluation unit 3 (only indicated here) and an electrical connection 4.

[0016] The Fig. Figure 2 shows a preferred electrode arrangement with eight measuring electrode pairs, with only the electrodes 61a to 61h arranged on the front being visible. The partner electrodes, designated 62a to 62h, are located on the rear and are enclosed by a second shield electrode 72 designed like the first shield electrode 71. All electrodes can be designed as copper surfaces on or in a printed circuit board 1 using multilayer technology. The measuring electrodes 61a-61h arranged on the front are enclosed by the shield electrode 71, so that they are shielded from the opposite second electrode system 62a-62h. Only the connecting lines of the shield electrode 72 are shown. Of course, all 16 measuring electrodes and the two shield electrodes 71 and 72 must have connecting lines to the evaluation unit 3 (not shown here). These lines have been omitted for clarity.The multilayer technology design is advantageous because it allows for a well-shielded cable routing against the other side of the circuit board, with the two opposing shield electrodes driving the unwanted parasitic capacitances of the sensor without these contributing noticeably to the measurement.

[0017] This also effectively suppresses conductive deposits on the sensor area 2. This is achieved because all electrodes located on one side of the circuit board 1 are driven with the same alternating voltage and in the same phase position.

[0018] The shield electrode 71 and its counter electrode 72 can be completely closed as shown, but can also be interrupted at several points, preferably at the top and bottom.

[0019] The Fig. Figure 3 shows a section through the sensor's electrode system, showing the opposing measuring electrodes 61a and 62a, as well as the shielding electrodes 71 and 72. Due to the sectional view, only a single pair of measuring electrodes 61a, 62a is visible here.

[0020] The shielding electrodes 71 and 72 are additionally connected by an internal connection designated iV, arranged on the inside of a multilayer printed circuit board, in order to ensure better shielding of the sensitive measuring electrodes 61a, 61b from each other.

[0021] Additionally, parasitic capacitances caused by buildup, the housing, and the medium are shown, and their suppression is explained. The capacitances between the measuring electrode and the associated shield electrode are effectively suppressed because they are driven in phase by their active shielding.

[0022] The capacitances between the shield electrodes have no effect because they are driven with low resistance and are not actively measured.

[0023] The effective medium capacity C between the measuring electrodes Medium is measured without the need for a container connected to ground (which is quite advantageous) because the opposite pair of measuring electrodes 61a, 62a are controlled in antiphase, whereby the virtual zero potential 12 shown in dashed lines is formed in the medium.

[0024] In addition to effective suppression of buildup, reliable limit level detection or, in the case of several electrode pairs, level measurement is also achieved.

[0025] The Fig. 4 shows an exemplary circuit arrangement with two pairs of measuring electrodes 61a, 62a and 61b, 62b driven in antiphase with the active shield electrodes 71 and 72, which are driven by the generator outputs 8a and 8b with a phase shift of 180°.

[0026] The representation of the electrode system is schematic and should be understood as a longitudinal section.

[0027] The two signal sources 8a and 8b can be implemented via I / O ports of a microcontroller. All electrodes in the upper half of the image are supplied with a phase shift of 0°, and their counter electrodes in the lower half of the image are supplied with a phase shift of 180°. The contacts of the two changeover switches S1 and S2, which serve both to switch the measuring electrode pairs (61, 62) and to synchronously rectify the measuring signals, are labeled abcd.

[0028] For a measurement of the first electrode pair 61a, 62a, both switches S1, S2 are switched back and forth between contacts a and b in synchronism with the driving signal source 8a and 8b, whereby the phase-selective rectification mentioned above takes place.

[0029] The same happens when evaluating the second pair of electrodes 61b - 62b, whereby their signals are also rectified.

[0030] Of course, all of these processes can be controlled by a microcontroller.

[0031] The voltage curve at the tapped voltage nodes at electrodes 61a, 62a, 61b, 62b, and 71, 72 depends on the internal and external capacitances not shown here. The only capacitance shown here is the capacitance C formed by the medium to be detected. M . The antiphase control results in the already mentioned virtual zero potential 12 (symmetry line).

[0032] Switches S1 and S2, acting as synchronous rectifiers, generate a differential current at the input of amplifier 9, which is additively composed of the measured currents of the measuring electrode pairs 61a, 62a and 61b, 62b, respectively. If, at a specific time, a positive signal is tapped at the measuring electrode 61a with switch S1 in position a), a negative signal is simultaneously tapped by the antiphase control at the measuring electrode 62a, acting as the counter electrode, with switch S2 in position a).

[0033] In switch position b, the opposing electrodes 61a, 62a are contacted in the inverted phase position. This results in phase-synchronous rectification of the two electrode currents. In conjunction with the subsequent difference formation at amplifier 9, this results in the overall system summing the measurement signals derived from the respective measurement capacitances, regardless of whether switching occurs at 0° or 180°.

[0034] MS1 is the measurement signal tapped by the upper electrodes 61a, 61b, and MS2 is the measurement signal tapped by the lower electrodes 62a, 62b. Then, in one phase position, MS1 - (-MS2) = MS1 + MS2, and in the inverted phase position, MS2 - (-MS1) = MS2 + MS1.

[0035] With regard to interference sensitivity, this arrangement results in considerable advantages, since the interference is essentially present as common-mode interference, so that both signal paths are also influenced in common mode and are suppressed again by the common-mode rejection of the subsequent amplifier 9, while the useful signal is present as a differential-mode signal.

[0036] It is easy to understand that for the two in Fig. 4 shown electrode pairs 61a, 62a and 61b, 62b 2 x 4 synchronously controllable switching contacts, namely for S1 the contacts a, b, c, d, and for S2 the contacts a, b, c, d are required.

[0037] For eight electrode pairs, 2 x 16 switching contacts S1a, S1b...S1p and S2a, S2b... S2p are required.

[0038] The differential amplifier 9 shown represents a particularly simple design. It features calibration inputs 11 weighted by different resistors, which can be supplied with a high or low signal from the IO ports of a microcontroller. The output signal of amplifier 9 can, for example, be connected to the input of a conventional ADC of a microcontroller and fed there for further intelligent signal processing. Reference symbol 1 circuit board 2 Sensor area with an electrode system of measuring and shielding electrodes 3 Evaluation unit 4 Electrical connection 5 housings 61 measuring electrodes 61a...61h, which are applied with phase position 0° 62 measuring electrodes 62a...62h, which are applied with a phase position of 180° 71 Shield electrode, which is applied with phase position 0° 72 Shield electrode, which is applied with phase position 180° 8a Generator outputs (signal source) that generate the phase position 0° 8b Generator outputs (signal source) that generate the phase position 180° 9 amplifiers, amplifier circuits, differential amplifiers 10 synchronous rectifiers 11 calibration inputs 12 virtual zero potential C M , measuring capacity (C Medium ) S1, S2 Phase-synchronous driven changeover switches for the electrodes with the corresponding switch contacts S1a, S1b, ... S1p and S2a, S2b, ... S2p

Claims

[1] Capacitive level sensor with a circuit board (1) which has sensor areas (2), and an evaluation unit (3), as well as an electrical connection (4) and a housing (5), wherein two sensor areas (2) are provided on the two sides of the circuit board (1) opposite each other, which have two pairs of measuring electrodes (61, 61b, 62a, 62b), characterized by that first measuring electrodes (61a, 61b) and a first shielding electrode (71) surrounding them are arranged on one side of the printed circuit board (1), which are driven in phase synchronism, and on the other side second measuring electrodes (62a, 62b) and a second shielding electrode (72) surrounding them are arranged, which are driven in antiphase to the first. [2] Capacitive level sensor according to claim 1, characterized bythat the high-frequency currents of the antiphase driven measuring electrode pairs (61a, 61b, 62a, 62b) are added to generate a metrologically closed circuit in which the connection to a reference potential by a ground electrode is omitted on both the sensor side and the supply side. [3] Capacitive level sensor according to claim 1, characterized by that the evaluation unit (3) has phase-synchronous switches (S1, S2) connected to the measuring electrodes (61 and 62), which serve to rectify the signals coming from the measuring electrode pairs. [4] Capacitive level sensor according to one of the preceding claims, characterized by that an amplifier circuit (9) is provided which is designed for current measurement and has at least one calibration input (11). [5] Washer water tank for a motor vehicle with a level sensor according to one of the preceding claims.

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