Methods for impedance measurement of electrical components

Abstract

The invention relates to a method for repeatedly determining, at several intervals, an electrical signal characteristic of the magnitude and / or change of an impedance that changes under the influence of a magnetic, electric, and / or electromagnetic field, which in turn is altered by an object approaching the impedance. For each interval, the impedance (Z) is measured. X ) is subjected to a sinusoidal excitation voltage, resulting in an induced measuring current (I x ). A reference impedance is subjected to a compensation voltage that is 180° out of phase with the excitation voltage, resulting in a compensation current (I). REF ) comes. The difference from the measuring current (I X ) and the compensation current (I REFThe input of an analog or digital integrator (42) is fed to the input of an analog or digital integrator, the output of which provides the magnitude of the compensation voltage. The output of the integrator (42) is the signal characteristic of the magnitude and / or change of the impedance.

Description

[0001] The invention relates to a method for measuring the impedance of electrical components and in particular to a method for determining, at several intervals and thus repeatedly, an electrical signal characteristic of the size and / or change of an impedance, wherein the impedance can change, for example, under the influence of a magnetic, electrical and / or electromagnetic field, which can change on the one hand due to an object approaching the impedance.

[0002] Depending on the application, the repeated determination of the impedance of electrical components installed in a vehicle is of great importance. Examples include detecting the driver's hand gripping the steering wheel or seat occupancy detection.

[0003] Due to various influencing factors or compensation measures, the measurement signals used for impedance measurements are not free of offsets. For example, externally connected EMC filters can cause such offsets.

[0004] The offset signal component can sometimes be significantly larger than the useful signal component. Therefore, digital processing of the measurement signal typically requires the use of analog-to-digital converters (ADCs) with a comparatively high resolution and bit count, as well as a high permissible input voltage. This, however, is excessively complex and therefore disadvantageous. If the maximum resolution of the ADCs used is limited, the resolution of the measurement signal is also severely limited, which is particularly disadvantageous for comparatively large signals.

[0005] A method for measuring the impedance of electrical components is known from US 2021 / 0 081 073 A1. According to this method, the measurement signal is amplified using a transimpedance amplifier, which means that comparatively large measurement signals can be processed. However, this poses the risk that the analog-to-digital converters required for digital processing of the measurement signal should have a comparatively high resolution and bit count, as well as a high permissible input voltage, which entails the correspondingly high complexity described above.

[0006] US 9 575 105 B1 describes a method and a device for measuring complex impedances.

[0007] Finally, DE 10 2013 227 225 A1 discloses current-based charge compensation in a capacitive touch sensor. The object of the invention is to provide a method of the type mentioned above that allows the signals representing the impedance to be kept low without compromising the accuracy of the measurement.

[0008] To achieve this object, the invention proposes a method for determining, at several intervals and thus repeatedly, an electrical signal characteristic of the size and / or change of an impedance, which changes under the influence of a magnetic, electrical and / or electromagnetic field, which in turn changes due to an object approaching the impedance, wherein in the method - per interval - the impedance is subjected to a sinusoidal excitation voltage, - a measuring current is induced in the impedance as a result of the excitation voltage, - a reference impedance of known size is subjected to a compensation voltage that is 180° out of phase with the excitation voltage and - a compensation current is induced in the reference impedance as a result of the compensation voltage, - the difference between the measuring current and the compensation current is fed to the input of an analogue or digital integrator, the output of which, after a digital-to-analogue conversion if necessary, provides the magnitude of the compensation voltage, - the compensation voltage is generated and - the output of the integrator (42) is the signal characteristic of the magnitude and / or change of the impedance.

[0009] According to the invention, a reference impedance is used to determine the magnitude of the impedance. The reference impedance is excited with a compensation voltage and then induces a compensation current, which is subtracted from the measuring current induced by the excitation voltage for the impedance to be measured. The difference signal is fed to a digital delta integrator, which integrates the difference and determines the compensation voltage at its output, which in turn is used to excite the reference impedance. The magnitude of the compensation voltage is characteristic of the magnitude of the impedance and / or its change. Instead of a single reference impedance, several different reference impedances can be selected, depending on the magnitude of the impedance to be measured.In this way, the difference signal is minimal, which in turn leads to the advantages mentioned above regarding the high resolution in the digital part despite the limited number of bits and input voltage of the ADC.

[0010] The different reference impedances can optionally be controlled via a multiplexer. Alternatively, the reference impedance can also be adjusted in terms of its magnitude. In this case, the use of a superimposed control loop is also recommended, with the goal of normalizing the measurement result to 1.

[0011] In a further advantageous embodiment of the invention, it can be provided that the difference signal from the difference between the measuring current and the compensation current, if necessary after amplification, is subjected to an analog-to-digital conversion and then fed to a digital IQ demodulator, the I and Q signal components of which are fed to the input of a digital integrator, and that to form the compensation voltage, the I and Q signal components integrated over time are modulated in a digital IQ modulator, the output of which is connected to a digital-to-analog converter which outputs the compensation voltage.

[0012] The impedance to be measured is, in particular, a capacitive seat occupancy sensor whose electrodes approach one another under the influence of weight, thus causing a change in capacitance (see, for example, US Pat. No. 8,896,326 B2, which also describes the evaluation principle for determining the magnitude of the impedance using IQ demodulation, as is also advantageously used according to the invention). A further area of ​​application of the method according to the invention is the detection of whether the driver is gripping the steering wheel. For this purpose, individual electrode surfaces are arranged in the steering wheel, which, together with the vehicle chassis, form a capacitor whose dielectric is influenced by the hand, resulting in a change in capacitance.

[0013] The invention is explained in more detail below using various embodiments and with reference to the drawings. In detail: Fig. 1 is a block diagram of an impedance measurement circuit which is not the subject of the invention, Fig. 2 is a block diagram of a circuit for impedance measurement, which is also not the subject of the invention, Fig. 3 is a block diagram of a first embodiment of a circuit for impedance measurement, Fig. 4 a block diagram of a second embodiment of a circuit for impedance measurement and Fig. 5 a block diagram of a third embodiment of a circuit for impedance measurement.

[0014] Before referring to the three embodiments of the invention ( Fig. 3 to 5) is discussed, should be based on the Fig. 1 and Fig. 2 the concept of impedance measurement according to the invention will be discussed in principle.

[0015] Fig. Figure 1 shows the block diagram of an example circuit. An impedance Z to be measured Xis excited by a sinusoidal voltage signal generated in a digital part 12 of the circuit 10. By means of a DAC 14, the digital sinusoidal signal is converted into an analog sinusoidal signal and, after low-pass filtering in a low-pass filter 16, is applied by a driver 18 to the impedance Z to be measured. X In response to this, the impedance Z X a current I X which is mirrored in a current mirror 20. The mirrored current I X After a current-to-voltage conversion in an IV converter 22 (for example, a shunt resistor) and a bandpass filtering in a bandpass filter 24, the signal is fed to an amplifier 26, whose output signal is converted by an ADC 28 into a digital signal for processing in the digital part 12. In the digital part 12, an IQ demodulation takes place in an IQ demodulator 30 to determine the real part and the imaginary part of the complex induced current I XAfter further filtering in a digital filter 32, two signals are available which are representative of the magnitude of the impedance Z X and can be further processed externally of the circuit 10.

[0016] Due to interference suppression measures, for example for EMC protection, the input 34 of the typically integrated circuit 10, to which the impedance Z to be measured is X connected, with in relation to the impedance to be measured Z X This results in offsets in the measurement signal, which should be compensated for in order to be able to measure with high resolution using relatively simple means. Therefore, the mirrored current I XA compensation current is subtracted, thus compensating for the offset. As a result of this compensation, very small signals remain, which, after amplification with an ADC 28 with a limited number of bits and a limited input voltage, can still be converted into a digital signal with sufficiently high resolution and further processed in the digital section 12. The compensation current is generated in the digital section 12 and subjected to IQ modulation in an IQ modulator 36. It is then converted into the analog compensation current by a DAC 38.

[0017] Circuit 10 operates intermittently and at intervals. The measured current value in digital section 12 determined during one of these intervals can be used to operate IQ modulator 36 with the parameters of IQ demodulator 30 to generate the digital compensation current.

[0018] In Fig. 2 is another circuit 40 for measuring an impedance Z X As far as the individual components of the circuit 40 correspond to those of the circuit 10 of the Fig. 1 correspond or are equal, they are in Fig. 2 with the same reference numerals as in Fig. 1 marked.

[0019] The difference between circuit 40 and Fig. 2 compared to that according to Fig. 1 consists in the integration of the I and Q signal components at the output of the IQ demodulator 30 by a (delta) integrator 42, which integrates both the I and Q signal components. The output signals of the integrator 42 are fed to the IQ modulator 36 to generate the digital compensation current, which is converted via the DAC 38 into the analog compensation current, which in turn is derived from the measurement current I X is subtracted. The function for the I output of the IQ modulator is as follows: I=−sin(ωt)IΣ

[0020] The following equation results for the Q output of the IQ modulator: Q=−cos(ωt)QΣ

[0021] Here I Σ , Q Σ the outputs of the Delta Integrator 42.

[0022] If the impedance Z X no longer changes from interval to interval, the demodulator 36 no longer outputs any signals (signal zero), which means that the impedance value last calculated by the integrator 42 continues to apply. If the impedance value changes, the output signals of the demodulator 36 deviate from zero. This changes the integrator output 42 and thus also the compensation current, which in the next measuring interval is determined by the measuring current I X is subtracted. If the impedance Z X has not changed, zero signals are again generated at the output of the demodulator 36.

[0023] A first embodiment of a circuit 50 according to the invention for measuring an impedance Z that potentially changes during operation X is in Fig. 3. Here too, those components of the circuit 50 which correspond to those of the circuit 10 according to Fig. 2 are equal or correspond to these, in Fig. 3 with the same reference numerals as in Fig. 2 are marked.

[0024] In the first embodiment according to Fig. 3, a reference impedance Z is used to generate the compensation current. REF excited and their induced current I REF used. For this purpose, a digital compensation voltage is generated in the digital part 12 by means of the IQ modulator 36, which, after a DAC conversion in the DAC 38 and, if necessary, filtering in a low-pass filter 52, is connected to the reference impedance Z by means of a driver 54. REF is created.

[0025] The induced current I REF mirrored and from the mirrored measuring current I X subtracted in a known manner.

[0026] In the digital part 12, the I and Q signal components of the IQ demodulator 30 are again integrated, with the output signals of the integrator 42 being subjected to an IQ transformation in a circuit 50. The reason for this is that the outputs of the delta integrator for the I and Q signal components of the IQ demodulator 30 may have to be swapped, depending on whether the reference impedance Z REF consists only of a capacitor or only of a resistor. If Z REFwere realized only with the help of a capacitor, then the I output of the delta integrator 42 would have to be connected to the inverted Q input of the IQ modulator 36 and the Q output of the delta integrator would have to be connected to the I input of the IQ modulator 36. If Z REF exclusively from a resistor, then similar to Fig. 2 the I output of the delta integrator 42 is connected to the I input of the IQ modulator 36 and the Q output of the delta integrator 42 is connected to the Q input of the IQ modulator 36. For “mixed cases” where Z REF has both an imaginary, e.g. capacitive component and a real, e.g. resistive component, a transformation is required so that changes in the capacitance and resistance of the impedance Z to be measured XIf only the imaginary part (Q component) or only the real part (I component) of the IQ demodulator 30 changes in the measurement result, the compensation must be phase-neutral. For this purpose, the IQ transformation circuit 57 is used to rotate the phase shift due to Z. REF , resulting in a total of -180°. The transformation equation for the I-output of the IQ modulator is as follows: I=−A sin(ωt)[IΣcos(φ)+QΣsin(φ)]

[0027] The following equation results for the Q output of the IQ modulator: Q=−A cos(ωt)[−IΣsin(φ)+QΣcos(φ)].

[0028] Here I Σ , Q Σ the outputs of the delta integrator. The phase φ generates the phase shift due to Z REF to achieve phase neutrality is necessary: φ=atan(2πƒSigCRef1 / RRef)

[0029] The amplitude A is: A=0.5 / RNorm1 / RRef2+(ωCRef)2

[0030] Using the parameter R NORM IQ normalization is performed. This means that when the capacity C X the impedance to be measured Z X equal to the normalization value R NORM = 1 / ωCx (or the resistance R X the impedance to be measured Z X equal to the parameter R NORM = R X ), the measurement result of the imaginary or real part becomes one. This amplitude normalization can be performed using the IQ transformation or it can be done outside the control loop at the output of the delta integrator 42, as in Fig. 3 shown at 58.

[0031] A second embodiment of a circuit 60 for measuring an impedance Z X is in Fig. 4. Here too, the individual components of the circuit 60, provided they correspond to or are similar to those of the circuit 50, are Fig. 4 with the same reference numerals as in Fig. 3 are designated.

[0032] The difference between circuit 60 and that according to Fig. 3 is in the switchability of the reference impedance to different sized reference impedances Z REF1 and Z REF2 It is also possible to select between several discrete reference impedances. The switching is carried out by means of a multiplexer 62, which receives a control signal from the digital part 12 and adjusts the compensation voltage to the selected one of the several reference impedances Z REF1 , Z REF2 , ... is switched. The induced compensation current I REF a current mirror. After that, the sequence behaves exactly as in Fig. 3 shown.

[0033] A third embodiment of a circuit 70 for measuring an impedance Z X is in Fig. 5. Here too, the individual components of the circuit 70, provided they correspond to or are similar to those of the circuit 60, are Fig. 5 with the same reference numerals as in Fig. 4 are designated.

[0034] In contrast to the circuit 60 after Fig. 4 is switched on at circuit 70 according to Fig. 5 is not switched to different discrete impedance references, but rather a reference impedance Z REF used, whose capacity C REF and their resistance R REF are virtually infinitely adjustable. The control signals come from the digital part 12. Then the digital controller 72 outputs the R X corresponding value and as Q-component the Z X corresponding value. This means that R X = R REF and C X = C REF However, Cx may be larger than the value for C REF maximum adjustable value and R Xlarger than that for R REF largest possible adjustable value, the specifications for the digital controller 72 would have to be less than 1, e.g. 0.1, whereby the output of the digital controller 72 would then output an I component and a Q component, each of which is 1 / 10 of the set value for C REF or R REF be, R X and Cx of the impedance to be measured Z X are therefore larger by a factor of 10 than that output by the digital controller 72. This reference impedance Z, which is implemented, for example, as a capacitance and resistance decade, REF is adjusted using a higher-level control loop so that the I,Q output of the delta integrator 42 assumes the value one in the steady state. The measured value of the unknown impedance Z X is then adjusted by the Z REF -decade. The advantage of the sensor architectures mappings of the Fig. 2 to 4 is that nonlinearities in the signal chain (IV converter 22, bandpass filter 24, amplifier 26, ADC 28) do not affect the measurement accuracy. Fig. The sensor shown in Figure 5 is also insensitive to nonlinearities in the signal and reference paths (DAC 14, DAC 38, current mirror 20, current mirror 56). The measurement accuracy is only affected by the Z REF -decade. LIST OF REFERENCE SYMBOLS 10 Circuit 12 Digital part 14 DAC 16 low-pass filters 18 drivers 20 current mirrors 22 IV converters 24 bandpass filters 26 amplifiers 28 ADC 30 IQ demodulator 32 digital filters 34 Entrance 36 IQ modulator 38 DAC 40 circuit 42 Delta Integrator 50 circuit 52 low-pass filters 54 drivers 56 current mirrors 57 IQ transformation circuit 58 IQ normalization circuit 60 circuit 62 multiplexers 70 circuit 72 digital controller Z REF1 Reference impedance Z REF2 Reference impedance Z X impedance to be measured I X Measuring current I REF Reference current

Claims

[1] Method for the determination, at several intervals and thus repeatedly, of an electrical signal characteristic of the size and / or change of an impedance which changes under the influence of a magnetic, electric and / or electromagnetic field which in turn changes due to an object approaching the impedance, wherein in the method - per interval - the impedance (Z X ) is subjected to a sinusoidal excitation voltage, - in the impedance (Z X ) as a result of the excitation voltage a measuring current (I X ) is induced, - a reference impedance (Z REF ) of known size is subjected to a compensation voltage that is 180° out of phase with the excitation voltage and - in the reference impedance (Z REF ) as a result of the compensation voltage a compensation current (I REF ) is induced, - the difference between the measuring current (I X ) and the compensation current (I REF ), optionally after amplification, is fed to the input of an analogue or digital integrator (42), the output of which, optionally after digital-to-analogue conversion, provides the magnitude of the compensation voltage, - the compensation voltage is generated and - the output of the integrator (42) is the signal characteristic of the magnitude and / or change of the impedance. [2] Method according to claim 1, characterized by several reference impedances (Z REF ) each with a known size, whereby depending on the size of the compensation voltage one of these reference impedances (Z REF ) is supplied with the compensation voltage and as a result a compensation current (I REF ) delivers. [3] Method according to claim 1, characterized by that the size of the reference impedance (Z REF) is variably adjustable and is changed by means of a controller (72) which has an input receiving the signal at the output of the integrator (42) and an output, the output of the controller (72) providing the signal characteristic of the size and / or change in the impedance. [4] Method according to one of claims 1 to 3, characterized by that the difference signal is the difference between the measuring current (I X ) and compensation current (I REF), optionally after amplification, is subjected to an analog-to-digital conversion and then fed to a digital IQ demodulator (30), the I and Q signal components of which are fed to the input of a digital integrator (42), and in that, to form the compensation voltage, the I and Q signal components integrated over time are modulated in a digital IQ modulator (36), the output of which is connected to a digital-to-analog converter (38) which outputs the analog compensation current. [5] Method according to one of claims 1 to 4, characterized by that the difference signal is the difference between the measuring current (I X ) and compensation current (I REF), optionally after amplification, is subjected to an analog-to-digital conversion and then fed to a digital IQ demodulator (30), the I and Q signal components of which are fed to the input of a digital integrator (42), and in that, to form the compensation voltage, the I and Q signal components integrated over time are modulated in a digital IQ modulator (36), the output of which is connected to a digital-to-analog converter (38) which outputs the analog compensation current and, in addition, in a superimposed control loop with the digital controller (72), the I and Q components of the reference impedance are set as a function of the IQ output of the digital integrator (42) according to the control specifications for the I and Q components.

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