Circuit for measuring complex AC resistance

Abstract

The invention relates to a circuit for measuring complex AC resistance, comprising a signal source (14) designed to generate an AC excitation signal, and a first signal path (20), a second signal path (22) and a third signal path (24), into each of which the AC excitation signal from the signal source (14) is fed in parallel with each other. The alternating current excitation signal is fed into the first signal path (20) or into the second signal path (22) and into the third signal path (24) at least temporarily with a phase offset in relation to the alternating current excitation signal generated by the signal source (14). The first signal path (20) has a high-pass filter (30) and an AC resistor to be detected in series with the high-pass filter (30). A measurement signal tapped at a measurement point (32) between the high-pass filter (30) and the AC resistance to be detected is mixed with the AC excitation signal in the second signal path (22) and further demodulated to obtain a first demodulated measurement signal. The measurement signal tapped at the measurement point (32) is also mixed with the alternating current excitation signal in the third signal path (24) and further demodulated to obtain a second demodulated measurement signal. A signal evaluation unit (56) receives the demodulated measurement signal and determines an imaginary part and a real part of the AC resistance to be detected based on the demodulated measurement signal.

Description

Technical Field

[0001] The invention relates to a circuit for measuring complex AC resistance.

[0002] This circuit can be used as an impedance sensor in various applications. Background Art

[0003] Document EP 3 512 099 B1 discloses a capacitive sensor having a sensor electrode, wherein the sensor electrode is also connected to a signal generating circuit for generating an output signal to the sensor electrode, and wherein the sensor electrode is also connected to a signal evaluation circuit configured to evaluate an input signal from the sensor electrode. The signal evaluation circuit has a synchronous rectifier, wherein the synchronous rectifier comprises a first switch and a second switch, which are connected to a signal generator and are configured to switch alternately in phase with the input signal. With this known sensor, only the imaginary part of the complex AC resistance can be measured.

[0004] Other capacitive sensors are known from US 2015 / 0323372 A1 and DE 10 2012 201 226 B4. The latter document discloses a probe for a capacitive level sensor, which has a measuring impedance, a reference impedance, a first rectifier and a second rectifier, two measuring resistors, a ground connection and a connection configured for connecting a multi-core connecting line, wherein the connecting line has a first core for transmitting a transmission signal and four other cores for transmitting a DC voltage signal. This circuit is relatively complex.

[0005] DE 10 2018 209 904 A1 discloses a fill level sensor or limit level sensor with temperature compensation. The sensor has a processing unit for processing a measurement signal generated by the sensor and a reference unit for generating a reference signal, wherein the processing unit and the reference unit each have a signal conversion unit with temperature-dependent signal conversion. Summary of the invention

[0006] The object of the present invention is to provide a circuit for measuring an unknown complex AC resistance, which has a simple structure and can be used to measure the imaginary part and the real part of the AC resistance.

[0007] The object is achieved by providing a circuit for measuring a complex AC resistance, the circuit comprising a signal source designed to generate an AC voltage excitation signal, a first signal path, a second signal path and a third signal path, the AC voltage excitation signal of the signal source being fed into each of the first signal path, the second signal path and the third signal path in parallel with each other, wherein the AC voltage excitation signal is fed into the first signal path or the second signal path and the third signal path in a manner that is at least temporarily phase-shifted compared to the AC voltage excitation signal generated by the signal source, wherein the first signal path comprises a high-pass filter and an AC resistance to be measured connected in series therewith, wherein a measurement signal tapped at a measurement point between the high-pass filter and the AC resistance to be measured is mixed with the AC voltage excitation signal in the second signal path and further demodulated to obtain a first demodulated measurement signal, and is mixed with the AC voltage excitation signal in the third signal path and further demodulated to obtain a second demodulated measurement signal, and further comprises a signal evaluation unit that receives the demodulated measurement signal and determines the imaginary part and the real part of the AC resistance to be measured from the demodulated measurement signal.

[0008] The circuit according to the present invention can be used to measure an unknown AC resistance, which is connected to a measuring electrode for example for a reference potential, wherein the reference potential can be a reference potential of a signal source. The circuit has a signal source that generates an AC voltage excitation signal, wherein the spectrum of the signal source is preferably selected so as to utilize a wide spectrum or a signal band. The AC voltage excitation signal is fed into a first signal path, which has a high-pass filter and an AC resistance to be measured connected in series therewith. The high-pass filter and the AC resistance to be measured form an AC voltage divider. The AC voltage excitation signal is fed into a second signal path and a third signal path while being fed into the first signal path. The measurement signal for measuring the complex AC resistance is tapped between the high-pass filter and the AC resistance to be measured. The circuit according to the present invention also provides at least temporarily setting a phase offset. When the phase offset is turned on, an AC voltage excitation signal that is phase-rotated compared to the AC voltage excitation signal generated by the signal source is fed into the first signal path or the second signal path and the third signal path. The AC resistance can be measured alternately when the phase offset is turned on and off. For example, the phase shift can be 0° (off), 90°, 180° or 270°, including -90°, -180° or -270°. The measurement signal tapped at the measuring point between the high-pass filter and the AC resistance to be measured is mixed with the AC voltage excitation signal in the second signal path and the third signal path. When the phase shift is switched on, the phase-shifted measurement signal is mixed and demodulated with the non-phase-shifted AC voltage excitation signal, or the non-phase-shifted measurement signal is mixed with the phase-shifted AC voltage excitation signal, depending on whether the phase shift is present in the first signal path or in the second and third signal paths. The phase shift can be achieved by a phase shifter. Mixing and demodulation are comparable to amplitude demodulation. The signal evaluation unit receives the demodulated measurement signals, which may have been low-pass filtered, and determines the imaginary and real parts of the AC resistance to be measured based on these signals, which is achieved by the temporarily switched-on phase shift, which allows not only the amplitude of the AC resistance to be determined, but also the phase of the AC resistance to be determined.

[0009] The circuit structure according to the present invention is simple, so the reactive component and active component of the complex AC resistance can be simply measured.

[0010] Preferred embodiments are specified in the dependent claims or described below.

[0011] In a preferred embodiment, the second signal path may include a first diode and a first measuring capacitor, wherein the measuring capacitor is connected to a measuring point between the high-pass filter and the AC resistor to be measured, and the third signal path may include a second diode and a second measuring capacitor, wherein the second measuring capacitor is connected to a measuring point between the high-pass filter and the AC resistor to be measured, wherein the first diode and the second diode are arranged to alternately pass half waves of the AC voltage excitation signal.

[0012] Two measuring capacitors together with two diodes form a simple device for mixing the measurement signal tapped at the measurement point with the corresponding AC voltage excitation signal guided in the second signal path and the third signal path, and for demodulating the signal thus mixed. The diode is connected to the measurement point between the high-pass filter and the complex AC resistor via the measuring capacitor. The circuit is configured so that during the positive half-wave of the AC voltage excitation signal of the signal source, one diode is conductive and a current flows through the associated measuring capacitor, whereby the latter is charged to a measurement voltage proportional to the AC voltage divider by the current of the measuring capacitor. During the negative half-wave of the AC voltage excitation signal of the signal source, the other diode is conductive, so that the charging current flows through the associated measuring capacitor. The measurement signal at the measurement point is separated from the DC component of the AC voltage excitation signal by the high-pass filter and the two measuring capacitors.

[0013] Preferably, the first measuring capacitor and / or the second measuring capacitor has a capacitance in the picofarad range.

[0014] Furthermore, an inverter may be arranged in the second signal path or the third signal path, the inverter being connected to the signal source and connected upstream of the first diode or the second diode to invert the AC voltage excitation signal from the signal source, ie, rotate it by 180°.

[0015] In this embodiment, the two diodes can be arranged with the same polarity in the second signal path and the third signal path. However, it is also possible to connect two diodes with opposite polarities to the signal source and optionally couple a bias voltage, so that no inverter is required.

[0016] Furthermore, the high-pass filter may comprise a capacitor, in particular only one capacitor.

[0017] This further simplifies the circuit in terms of complexity. The capacitance of the capacitor of the high-pass filter can be selected according to the expected value range of the AC resistance to be measured. It is preferred if the maximum possible voltage swing is achieved between the measurement signal level at low measurement impedance and high measurement impedance.

[0018] It is understood that additional filters and protection components can be arranged between the high-pass filter and the measuring electrodes connected to the complex AC resistor in order to optimize the EMC characteristics of the circuit.

[0019] The spectrum of the AC voltage excitation signal that can be generated by the signal source preferably includes frequencies in a wide frequency band. Preferably, depending on the signal source used, the frequencies that can be generated are in the range of 100kHz to 200MHz. For example, the circuit can operate at a frequency between 5-50MHz. A voltage controlled oscillator (VCO) controlled by a microcontroller can be used as a signal source, wherein the control voltage depends on the input range of the voltage controlled oscillator and is used to generate a broadband output spectrum at the output of the voltage controlled oscillator. Alternatively, an oscillating signal source contained in the microcontroller can be modulated accordingly to output a broadband spectrum instead of modulating the voltage controlled oscillator.

[0020] Preferably, the signal source is configured to generate an AC voltage excitation signal with a time-varying frequency. For this purpose, the signal source can be configured, for example, in the form of a swept frequency generator. This makes it easy to improve the EMC performance of the circuit.

[0021] Further preferably, the circuit can include a first low-pass filter and a second low-pass filter, into which the demodulated measurement signal is fed. The low-pass filter can be used to separate the AC voltage signal component from the demodulated measurement signal. An analog-to-digital converter (ADC) can be arranged downstream of the low-pass filter, which digitizes the measurement signal. It is also possible to use only one ADC, wherein the above-mentioned optional subtraction stage is performed by an analog stage before digitization.

[0022] The signal evaluation unit can preferably be designed to subtract the demodulated measurement signals from one another. The advantage here is that on the one hand the measurement signal is increased and on the other hand external interference signals are eliminated. The complex AC resistance can then be determined based on the difference between the measurement signals.

[0023] Furthermore, according to the present invention, there is provided an impedance sensor comprising a measuring electrode to which a complex AC resistance to be measured is connected relative to a reference potential, and comprising the circuit according to one or more of the above-described embodiments.

[0024] The impedance sensor according to the invention can be used for various applications. In particular, the impedance sensor is not only able to detect the capacitance between the measuring electrode and a reference potential (in particular ground potential), but also the conductive coupling between the measuring electrode and the reference potential determined by the real part of the complex AC resistance. This makes it possible, for example, to distinguish objects or detect contamination.

[0025] For example, impedance sensors can be used as fill level sensors to continuously monitor liquid media or bulk materials, for example in tanks. The measuring electrode can be designed as a rod. A conductive sheath or a conductive wall of the tank can serve as a counter electrode. The measuring electrode and the counter electrode form a capacitor. The value of the capacitor changes continuously with the fill level in the tank and can be measured by the circuit according to the invention. Conductive contaminants, such as biofilms or other deposits in the tank, can be detected via the conductivity measurement values ​​(conductance) of the circuit according to the invention. However, changes in the medium properties can also be detected via the conductivity measurement values, which are reflected in the conductivity of the process medium.

[0026] Impedance sensors can also be used as limit level switches for process media. For example, the measuring electrode can be designed as a cover. If the process medium comes into contact with the cover of the limit level switch, the measured capacitance changes significantly, which triggers a switching event. This makes it possible, for example, to implement dry-running protection or overflow protection for pumps when filling open containers. Here, the conductivity measurement value can also be used to detect contamination or changes in the process medium.

[0027] Furthermore, impedance sensors can be used as proximity sensors, for example in the automation industry. Similar to limit level switches, a switching event can be triggered. For example, as soon as an object comes closer to the measuring electrode, the capacitance between the measuring electrode and a reference potential (e.g. ground potential) changes. If the signal change due to this capacitance is large enough, a switching event can be triggered. Conductive measurement can be used to distinguish objects. However, it can also be used to detect conductive deposits on the measuring electrode of the proximity sensor. This makes it possible to continue to detect objects while, for example, a water film or puddle is forming on the proximity sensor.

[0028] Other possible uses of the impedance sensor according to the invention are in the field of human-machine interaction in the form of buttons, sliders or rotary encoders. Furthermore, the impedance sensor according to the invention can be used as a flow monitor, where properties of the process medium are monitored that are related to the capacitive or conductive coupling of the sensor.

[0029] Further advantages and features will become apparent from the following description and accompanying drawings.

[0030] It is understood that the features mentioned above and those yet to be explained below can be used not only in the combination specified in each case but also in other combinations or alone, without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Exemplary embodiments of the present invention are shown in the accompanying drawings and will be described in more detail below with reference to the accompanying drawings. In the accompanying drawings: Figure 1 The principle of the circuit diagram of an AC voltage divider with an AC resistor is shown; Figure 2 A circuit diagram showing a first exemplary embodiment of a circuit for measuring a complex AC resistance; Figure 3 Another circuit diagram showing another exemplary embodiment of a circuit for measuring a complex AC resistance; Figure 4 Another circuit diagram showing another exemplary embodiment of a circuit for measuring a complex AC resistance; Figure 5 Another circuit diagram showing another exemplary embodiment of a circuit for measuring a complex AC resistance; Figure 6 Another circuit diagram showing another exemplary embodiment of a circuit for measuring a complex AC resistance; Figure 7 Another circuit diagram showing another exemplary embodiment of a circuit for measuring a complex AC resistance; Figure 8 another circuit diagram showing another exemplary embodiment of a circuit for measuring a complex AC resistance; and Fig. 9 Schematic diagram showing possible uses of an impedance sensor. DETAILED DESCRIPTION

[0032] Figure 1 The principle of a circuit diagram showing an AC voltage divider with an unknown (especially variable) AC resistance Z, Figure 1 1. The unknown AC resistance Z is connected to the measuring electrode 2 relative to the reference potential 3. In order to measure the unknown AC resistance Z, the measuring electrode 2 is connected to the signal source 4 via the internal coupling capacitor CK, such as Figure 1 5 in the figure. The reference potential 3 can be the reference potential of the signal source 4. The coupling capacitor CK together with the unknown AC resistance Z forms an AC voltage divider for the voltage of the signal source 4. Ideally, the frequency spectrum of the signal source 4 should be selected to utilize a wider signal frequency band. On the one hand, this helps to distribute the output power over many frequencies, and on the other hand, it reduces the sensitivity of the measurement to external signal sources. The measurement voltage V of the AC voltage divider formed by the coupling capacitor CK and the AC resistance Z to be determined is tapped at the measurement point between the coupling capacitor CK and the AC resistance Z. Figure 1 In the embodiment, the measuring voltage V is tapped directly at the measuring electrode 2 in parallel with the AC resistor Z.

[0033] refer to Figures 2 to 4 , exemplary embodiments of circuits are described below, by means of which the complex AC resistance Z can be measured in a simple manner.

[0034] Figure 2A circuit 10 is shown for measuring the complex AC resistance of a measuring electrode 12 connected to a reference potential, such as a reference Figure 1 as described. Figure 2 The AC resistance Z is not shown.

[0035] The circuit 10 includes a signal source 14, which is designed to generate an AC voltage excitation signal. Figure 2 In the exemplary embodiment of the invention, the signal source 14 has a voltage controlled oscillator (VCO) 16. A microcontroller (µC) 18 generates a control voltage for the voltage controlled oscillator 16. The control voltage depends on the input range of the voltage controlled oscillator and is used to generate a broadband output spectrum at the output of the voltage controlled oscillator 16. The control voltage can be output via a digital-to-analog converter integrated in the microcontroller 18. Alternatively, a pulse width modulated (PWM) signal can also be generated by the microcontroller 18 and converted (for example, by means of a low-pass filter) into a low-frequency control voltage for the voltage controlled oscillator. The control signal of the voltage controlled oscillator can be varied within a sweep time. The time course of the control voltage can be selected so that the spectrum generated by the voltage controlled oscillator has optimal EMC characteristics. For example, the sweep time can vary from a few milliseconds to a few seconds.

[0036] The spectrum of the AC voltage excitation signal output by the voltage controlled oscillator 16 can be in the range of several kHz to several MHz. For example, the spectrum can start from 70 MHz and extend to a frequency of 150 MHz. The broadband AC voltage signal spectrum is used to improve the EMC characteristics of the entire circuit 10. On the one hand, the radiated energy is distributed over many frequencies, and on the other hand, the sensitivity to interference from external signal frequencies is reduced.

[0037] Starting from the signal source 14 , the circuit 10 includes a first signal path 20 , a second signal path 22 , and a third signal path 24 , into each of which the AC voltage excitation signal output by the signal source 14 is fed in parallel with each other.

[0038] The AC voltage excitation signal is fed into the first signal path at least temporarily in a phase-shifted manner. For this purpose, a phase shifter 26 (Δφ) is arranged in the first signal path 20. The phase shifter 26 ensures that its output signal has a phase shift compared to the AC voltage excitation signal fed from the signal source 14. The phase shifter 26 can be controlled by the microcontroller 18, as shown by the discontinuous line 28. The microcontroller 18 can specify how large the phase shift should be between the input signal fed into the signal path 20 and the output signal at the output of the phase shifter 26. Typically, the phase shift is set to 0°, 90°, 180° or 270°. However, other phase shifts can also be applied.

[0039] The output signal of the phase shifter 26 is coupled to the high pass filter 30. As shown in the exemplary embodiment to be described later, the high pass filter 30 can be designed as a capacitor corresponding to Figure 1 The high-pass filter 30 forms an AC voltage divider together with the complex AC resistance to be measured externally and connected to the measuring electrode 12, as described above with reference to Figure 1 At the measuring point 32, as shown in reference Figure 1 As mentioned above, the measuring point may be located directly on the measuring electrode 12 , and a measuring voltage may be tapped relative to a reference potential (eg, a reference potential of the signal source 14 ), and the complex AC resistance may be determined based on the measuring voltage as described below.

[0040] If the high pass filter 30 is designed as a capacitor, the capacitance of the capacitor of the high pass filter 30 is selected according to the expected value range of the complex AC resistance to be measured. The aim is to achieve the largest possible voltage swing between the AC voltage levels of the measurement signal at low and high measurement impedances.

[0041] It should be understood that additional filters and protection elements may be arranged between the high-pass filter 30 and the measuring electrode 12 , for example in order to optimize the EMC properties of the circuit 10 .

[0042] The AC voltage excitation signal output by signal source 14 is fed into second signal path 22 and third signal path 24 in parallel with first signal path 20. In the exemplary embodiment shown, the AC voltage excitation signal has no phase shift in signal paths 22 and 24 relative to the output signal of signal source 14.

[0043] The AC voltage measurement signal tapped at the measuring point 32 is mixed with the AC voltage excitation signal fed into the second signal path 22 in the first mixer-demodulator element 34 arranged in the second signal path 22, and the mixed signal is further demodulated. Similarly, the AC voltage measurement signal tapped at the measuring point 32 is mixed with the AC voltage excitation signal fed into the third signal path 24 in the second mixer-demodulator element 36, and the mixed signal is demodulated. The first mixer-demodulator element 34 includes a first mixer 38 and a first demodulator 40. The second mixer-demodulator element 36 includes a second mixer 42 and a second demodulator 44. The first mixer-demodulator element 34 and the second mixer-demodulator element 36 are preferably designed to be similar to an amplitude demodulator or an envelope detector. They may include a diode-capacitor device, as shown in the exemplary embodiment described later. If the mixer-demodulator elements 34, 36 are designed as diode-capacitor arrangements, the inverter 46 can be arranged in one of the signal paths 22 or 24, where in the signal path 24 the inverter rotates the phase of the AC voltage excitation signal fed into the signal path 24 by 180°. Thus, the diodes in the first mixer-demodulator element 34 and the second mixer-demodulator element 36 can be arranged with the same polarity as one another. If the mixer-demodulator elements 34, 36 are designed as diode-capacitor arrangements, the respective diodes have a junction capacitance that is as low as possible, and the respective capacitors typically also have a capacitance in the picofarad range.

[0044] The respective demodulated measurement signals output by the demodulators 40 and 44 are fed to respective low-pass filters 48, 50, which separate the respective measurement signals from the HF components. The dimensions of the low-pass filters 48 and 50 are determined according to the desired step response of the circuit 10 to the change in the complex AC resistance to be measured. Furthermore, the low-pass filtered AC voltage measurement signals are fed to respective analog-to-digital converters 52, 54, which digitize the measurement signals. When determining the dimensions of the low-pass filters 48, 50, the anti-aliasing criteria of the analog-to-digital converters 52, 54 may also be taken into account.

[0045] The digitized measurement signals are evaluated in a signal evaluation unit 56, which can be integrated in the microcontroller 18 here, in order to calculate the imaginary and real parts of the AC resistance to be determined. Preferably, the digitized measurement signals supplied to the signal evaluation unit 56 are subtracted from one another. The measurements are carried out temporarily, for example alternately, with and without phase shift and according to the phase shift set on the phase shifter 26, from which the imaginary signal component or the real signal component of the complex AC resistance to be measured can be specifically determined.

[0046] Figure 3 An exemplary embodiment of a circuit 10a is shown, which is Figure 2The components of circuit 10a that are identical or equivalent to the components of circuit 10 are provided with Figure 2 The same reference numerals are used in the drawings.

[0047] In the following, only the differences between the circuit 10 a and the circuit 10 are described.

[0048] In the circuit 10a, the phase shifter 26 is not arranged in the first signal path 20, but in the signal paths 22 and 24. Therefore, in the circuit 10a, when the phase shift is set by the phase shifter 26, the measurement signal tapped at the measurement point 32 (without phase shift compared to the AC voltage excitation signal output by the signal source 14) is mixed with the phase-shifted AC voltage excitation signal in the signal paths 22 and 24.

[0049] Thus, for circuit 10a, as for circuit 10, the real and imaginary parts of the unknown alternating resistance Z connected to the measuring electrode 12 with respect to the reference potential can be determined.

[0050] Figure 4 Another exemplary embodiment of a circuit 10b for measuring complex AC resistance is shown. Figure 2 The components of the circuit 10b which are the same as or equivalent to the components of the circuit 10 are provided with Figure 2 In the following, only the differences between the circuit 10b and the circuit 10 are described.

[0051] One difference relates to the design of the signal source 14. Instead Figure 2 The circuit 10b has an oscillating signal generator 16 contained in a microcontroller 18 as a signal source 14, which is modulated accordingly to output a broadband spectrum. According to the microcontroller 18, the output AC voltage excitation signal frequency is in the kHz to lower MHz range. For example, the spectrum output by the oscillating signal generator 16 can start from 150kHz and extend to a frequency of 2MHz. It is preferred that the signal source 14 is integrated into the microcontroller 18, especially for a compact circuit design in the area of ​​a human-machine interface, for example in the form of a button, etc.

[0052] Furthermore, in the circuit 10 b , the analog-to-digital converters 52 , 54 , the phase shifter 26 , and the inverter 46 are also integrated into the microcontroller 18 .

[0053] refer to Figures 5 to 8 , describes a further exemplary embodiment of a circuit that operates according to the same basic principles as circuits 10, 10a and 10b. The circuits described below are provided with components that are the same or equivalent to the components of circuits 10, 10a, 10b. Figures 2 to 4 The same reference numerals are used in the drawings.

[0054] Figure 5 A circuit 100 for measuring a complex AC resistance Z is shown, which is connected to a measuring electrode 12 relative to a reference potential of a signal source 14. The measurement signal is tapped at a measuring point 32. The signal source 14 may be a reference Figure 2 or reference Figure 4 Design as described.

[0055] Figures 2 to 4 The high-pass filter 30 in FIG. 1 is designed as the capacitor 60 in the circuit 100 . Figures 2 to 4 The mixer-demodulator elements 34, 36 in the embodiment are designed here exclusively as diode-capacitor arrangements with a diode (D1) 62 and a measuring capacitor 64, which are connected to the measuring point 32 via the measuring capacitor 64, or with a diode (D2) 66, which is connected to the measuring point 32 via a measuring capacitor 68. The AC voltage excitation signal of the signal source 14 is fed into the diodes 62 and 66 in the second signal path 22 and the third signal path 24, in parallel with the coupling to the first signal path 20, which contains an AC voltage divider consisting of the capacitor 60 and the AC resistance Z to be measured. The anti-phase AC voltage excitation signal of the signal source 14 is connected to the diode D1 via an inverter 46, which is arranged here in the second signal path 22. The diodes 62 and 66 are connected to the signal source 14 with the same polarity.

[0056] The diode 66 (D2) conducts during the positive half-wave of the AC voltage excitation signal of the signal source 14, and current flows through the measuring capacitor 68. The current through the capacitor 68 charges the capacitor 68 to a measuring voltage proportional to the AC voltage divider composed of the capacitor 60 and the AC resistor Z. During the negative half-wave of the AC voltage excitation signal of the signal source 14, the diode 62 conducts due to the inverter 46. When the diode 62 conducts, the charging current flows through the measuring capacitor 64.

[0057] The voltage at the measuring point 32 of the measuring electrode 12 is separated from the DC voltage component of the signal source 14 by the capacitors 60, 64 and 68. Therefore, the corresponding measured voltage at the input of the low-pass filter 48, 50 is equal to the charge of the capacitors 64 and 68 plus the AC voltage signal component. The low-pass filter 48, 50 separates the AC voltage signal component from the measurement signal of the capacitor charge of the capacitors 64, 68. The low-pass filtered measurement signal can then be digitized in the analog-to-digital converters ADC1 and ADC2. For example, Figure 5 The phase shifters indicated by "+-90°" in FIG. 2 can be connected in the second signal path 22 and the third signal path 24. The measurement signals digitized by the analog-to-digital converters ADC1 and ADC2 can then be evaluated in an evaluation unit as described above to determine the real and imaginary parts of the complex AC resistance Z.

[0058] In the evaluation circuit, as described above, the difference between the digitized measurement signals is preferably formed. This not only increases the resulting measurement signal, but also eliminates external interfering signals.

[0059] Diodes 62 and 66 are preferably housed in a common housing to reduce temperature differences between diodes 62 and 64 .

[0060] Figure 5 An advantage of the circuit 100 in is that the resulting measured voltage has a positive sign etc. and can therefore easily be further processed, for example with the aid of an analog-to-digital converter ADC1.

[0061] Figure 5 The resistors R1 and R2 in FIG. 6 are used to transfer the bias current through the diodes 62 and 66 to a reference potential. The reference potential can be a reference voltage source or a reference potential of the signal source 14, such as Figure 5 Resistors R1 and R2 also reduce or prevent signal drift due to bias current.

[0062] Figure 6 shows a circuit 100a which is Figure 5 A variation of circuit 100 in FIG. Figure 5 The components of the circuit 100a which are the same as or equivalent to the components of the circuit 100 in FIG. Figure 5 In the following, only the differences from the circuit 100 will be described.

[0063] In circuits 100a and 100, two diodes 62 and 66 are connected to Figure 5 The diodes 62 and 66 are connected to the signal source 14 with opposite polarity, but have the same polarity as each other. A bias voltage (BIAS) may also be required. The bias voltage ensures that the diodes 62 and 66 enter the conductive state when the voltage level at the cathode of each diode is sufficiently lower than the bias voltage. The current through the diodes 62 and 66 is additionally limited by the corresponding resistors R1 and R2.

[0064] The capacitors 64, 68 of the circuits 100 and 100a may be dimensioned to be substantially the same in terms of their capacitance. However, they may also have different dimensions.

[0065] Over the frequency range of the signal source 14 , the resistors R1 , R2 or the input resistance of the low pass filters 48 , 50 may be many times greater than the AC resistance of the capacitor 66 of the high pass filter 30 .

[0066] The junction capacitance of diodes 62 and 66 should be as small as possible.

[0067] exist Figure 6 In the embodiment of the present invention, a phase shifter may be provided at position 70.

[0068] In order to determine the complex AC resistance Z, in addition to measuring the measurement voltage at the measurement point 32 which is in phase with the AC voltage excitation signal of the signal source 14, a phase-shifted measurement signal is also required. For this purpose, Figure 5 As shown, the AC voltage excitation signal is coupled into the signal paths 22, 24, for example offset by 90°. By turning the phase offset on or off, measurements with and without phase offset can be performed alternately. The measured voltage at the measuring point 32 can be determined within a certain period of time, for example, within the time period (swing period) required to pass a certain spectrum. The measurement is then repeated with a phase offset of, for example, +90° or -90°. The two measurement signals obtained in this way can be used as measured variables of the complex AC resistance for the amplitude and phase calculation of the AC resistance Z.

[0069] Figure 7 and Figure 8 Further exemplary embodiments of circuits 100b and 100c are shown, wherein for Figure 5 The same or comparable components as those of the circuit 100 in Figure 5 The same reference numerals are used. Only the differences from circuit 100 are described.

[0070] exist Figure 7 In the embodiment, the diodes 62 and 66 are arranged with polarities opposite to each other in the second signal path 22 and the third signal path 24, respectively. Therefore, the inverter 46 can be omitted.

[0071] The same applies to Figure 8 In the circuit 100c, two diodes 62 and 66 are connected to Figure 7 Opposite polarities are arranged in the respective signal paths 22 and 24 .

[0072] Fig. 9 An impedance sensor 200 is shown having a measuring electrode 12 and a circuit 210 connected to the measuring electrode 12. The circuit 210 may be the circuit 10, 10a, 10b, 100, 100a, 100b, 100c.

[0073] exist Fig. 9In an exemplary embodiment of the present invention, the impedance sensor 200 is used as a filling level sensor to continuously monitor the filling level of a liquid medium 220 or a bulk material 220 in a tank 230. For this purpose, the measuring electrode 12 can be designed as a rod. The conductive sheath 13 or the wall of the tank 230 can be used as a counter electrode, as long as the wall of the tank 230 is at least partially conductive. The measuring electrode 12 and the counter electrode form a capacitor. The value of the capacitor changes continuously with the filling level in the tank 230 and can be measured by the circuit 210. Conductive contaminants, such as biofilms or other deposits in the tank 230, can be detected by the sensor electronics of the circuit 210 through the real part, i.e. the conductive component of the measured AC resistance. However, changes in the medium properties reflected in the conductivity of the medium 220 can also be detected by the conductive measurement value.

[0074] Another application of an impedance sensor designed with or connected to the circuit 10, 10a, 10b, 100, 100a, 100b, 100c can be designed as a limit level switch. For example, the measuring electrode 12 can be designed as a cover. If the process medium contacts the cover of the limit level switch, the measured capacitance changes significantly, which triggers a switching activity. This makes it possible to implement dry running protection or overflow protection for pumps when filling open containers. Here, the conductivity measurement value can also be used to detect contamination or changes in the process medium.

[0075] Another possible use of the impedance sensor with circuit according to this exemplary embodiment is as a proximity sensor in the automation industry. Similar to the limit level switch, the switch application is triggered according to the value of the measured AC resistance. As soon as an object gets closer to the measuring electrode 12, the capacitance between the measuring electrode 12 and the ground potential changes. If the signal change due to this capacitance is large enough, a switch activity can be triggered. The conductivity measurement can be used to distinguish objects. However, it can also be used to detect conductive deposits on the measuring electrode 12 of the proximity sensor. This makes it possible to continue to detect objects during the formation of a water film or puddle on the proximity sensor.

[0076] A further use of an impedance sensor having a circuit according to any of the above exemplary embodiments is in the field of human-machine interaction in the form of buttons, sliders or rotary encoders. Furthermore, an application as a flow monitor is possible, in which properties of the process medium are monitored that are related to the capacitive or conductive coupling of the impedance sensor.

Claims

1. A circuit for measuring complex AC resistance, comprising a signal source (14) configured to generate an AC voltage excitation signal, a first signal path (20), a second signal path (22) and a third signal path (24), wherein the AC voltage excitation signal of the signal source (14) is fed to each of the first signal path (20), the second signal path (22) and the third signal path (24) in parallel with each other, wherein: The AC voltage excitation signal is fed into the first signal path (20) or the second signal path (22) and the third signal path (24) in a manner that is at least temporarily phase-shifted compared to the AC voltage excitation signal generated by the signal source (14), wherein the first signal path (20) includes a high-pass filter (30) and an AC resistor to be measured connected in series therewith, wherein a measurement signal tapped at a measurement point (32) between the high-pass filter (20) and the AC resistor to be measured is mixed with the AC voltage excitation signal in the second signal path (22) and further demodulated to obtain a first demodulated measurement signal, and is mixed with the AC voltage excitation signal in the third signal path (24) and further demodulated to obtain a second demodulated measurement signal, and further includes a signal evaluation unit (56) that receives the demodulated measurement signal and determines the imaginary part and the real part of the AC resistor to be measured from the demodulated measurement signal.

2. The circuit according to claim 1, wherein The second signal path (22) comprises a first diode (62) and a first measuring capacitor (64), wherein the measuring capacitor (64) is connected to the measuring point (32) between the high-pass filter (30) and the AC resistor to be measured, and the third signal path (24) comprises a second diode (66) and a second measuring capacitor (68), wherein the second measuring capacitor (68) is connected to the measuring point (32) between the high-pass filter (30) and the AC resistor to be measured, wherein the first diode (62) and the second diode (66) are arranged to alternately pass half waves of the AC voltage excitation signal.

3. The circuit according to claim 2, wherein: The first measuring capacitor (64) and / or the second measuring capacitor (68) have a capacitance in the picofarad range.

4. The circuit according to any one of claims 1 to 3, wherein: An inverter is arranged in the second signal path (22) or the third signal path (24), the inverter is connected to the signal source (14) and connected upstream of the first diode (62) or the second diode (66), and the inverter inverts the AC voltage excitation signal from the signal source (14).

5. The circuit according to any one of claims 1 to 4, wherein the high pass filter (30) comprises a capacitor (60).

6. The circuit according to any one of claims 1 to 5, wherein: The frequency spectrum of the AC voltage excitation signal generated by the signal source (14) includes frequencies in the range of 100 kHz to 200 MHz.

7. The circuit according to any one of claims 1 to 6, wherein the signal source (14) is configured to generate an AC voltage excitation signal having a variable frequency.

8. The circuit of any one of claims 1 to 7, wherein the phase shift is an odd multiple of 90°.

9. The circuit according to any one of claims 1 to 8, further comprising a first low-pass filter (48) and a second low-pass filter (50), into which the demodulated measurement signal is fed.

10. The circuit according to any one of claims 1 to 9, further comprising a first analog-to-digital converter (52) and / or a second analog-to-digital converter (54) for digitizing the demodulated and optionally low-pass filtered measurement signal.

11. The circuit according to any one of claims 1 to 10, wherein: The signal evaluation unit (56) subtracts the demodulated measurement signals from one another.

12. An impedance sensor comprising a measuring electrode (12) and a circuit (10; 10a; 10b; 100; 100a; 100b; 100c) according to any one of claims 1 to 11, the complex AC resistance to be measured being connected to the measuring electrode (12) relative to a reference potential.

13. Use of the impedance sensor (200) according to claim 12 for detecting the filling level of a medium (220) in a container (230), and / or as a limit level switch, and / or for detecting changes in medium properties, and / or as a proximity sensor, and / or for distinguishably detecting different media and / or objects, and / or as a button or as a slider or a rotary encoder.

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