System and method for impedance measurement

Abstract

A system (1) for impedance measurement comprises: - a first voltage source (2), adapted to generate an alternating first supply voltage with reference to ground; - a shield node (4) connected to the first voltage source (2) via a first source impedance (6); - a sense node (5) coupled to the shield node (4) via a sense-shield impedance (3); - a reference portion (10) comprising at least one reference impedance (12, 16) and being at least connectable between the sense node (5) and ground; and - a control unit (25), wherein a detection impedance (30) is couplable between the sense node (5) and ground, and the control unit (25) is adapted to: - perform a primary measurement and a secondary measurement while the first voltage source (2) is active, wherein at least one reference impedance (12, 16) is connected between the sense node (5) and ground in at least one measurement; and - determine the detection impedance (30) at least based on at least one reference impedance (12, 16) and on results of both measurements.

Description

System for Impedance MeasurementTechnical field

[0001] The invention relates to a system for impedance measurement, and to amethod for impedance measurement. Background of the Invention

[0002] In various situations, a detection impedance, which may have resistive,capacitive and / or inductive properties, needs to be measured. One example are so- called capacitive sensors which are used to detect the presence and optionally the position of an object or body part. The object represents a detection impedance between a sensor electrode and ground. The impedance may in general be referred to as a complex impedance with a real part or resistance representing the resistiveproperties and an imaginary part or reactance representing the capacitive and / orinductive properties. Alternatively, the impedance can be represented by an absolute value and a phase angle. Although measuring an impedance is theoretically straightforward, difficulties arise from additional impedances associated with the measurement equipment itself. On the one hand, any voltage source that is coupled to an unknown impedance-to-be-measured has an internal impedance, which is effectively connected in series with the impedance-to-be- measured. Furthermore, there will oftentimes be a non-negligible parasitic impedance of the measurement equipment, which is effectively connected parallel to the impedance-to-be-measured. Since these impedances are unknown like the impedance-to-be-measured, they have a falsifying influence on the measurement result that is difficult to account for. Object of the invention

[0003] It is thus an object of the present invention to provide means for preciseimpedance measurement.

[0004] This problem is solved by a system according to claim 1.General Description of the Invention

[0005] The invention relates to a system for impedance measurement. The term“system” is not to be construed in any limiting way. Various components of thesystem, which are mentioned hereinafter, can be connected in a permanent way,and may be disposed in a common housing. The system could also be referred to as a device for impedance measurement.

[0006] The system comprises a first voltage source, which is adapted to generatean alternating first supply voltage with reference to ground. Although this is referred to as a “first” voltage source, there are embodiments in which this is the only voltage source. The first voltage source (and the second voltage source mentioned below) is adapted to generate a voltage in a controllable way. However, the invention does not require this to be an “ideal” voltage source in that the voltage is practically independent of the current. Specifically, the first voltage source is adapted to generate a first supply voltage, which is an alternating voltage. In some embodiments, this may be a periodic waveform, while in other embodiments, it may be non-periodic, for example a pseudo-random waveform. Here and in the following, “ground” refers to ground potential or any common reference potential. It also includes any object that is permanently or temporarily at ground potential.

[0007] The system further comprises a shield node connected to the first voltagesource via a first source impedance. Here and in the following, “connected” preferably refers to a conducting connection. However, in some cases, this could also refer to a capacitive and / or inductive coupling. Again, the term “first” source impedance does not imply that there has to be an additional source impedance. Although the first source impedance is referred to as a dedicated element, it may physically be integrated into the first voltage source, either entirely or partially. When the first voltage source is activated and generates the first supply voltage, an alternating voltage is transferred through the first source impedance to the shield node. The shield node may belong to, or be identical to, a shield electrode of a capacitive sensor.

[0008] Also, the system comprises a sense node coupled to the shield node via asense-shield impedance. “Coupled” here and in the following may refer to electrically conductive connection. Preferably though, the coupling may additionally or alternatively be a capacitive and / or inductive coupling, i.e., a coupling through electromagnetic fields. One could say the sense-shield impedance characterises the coupling between the sense node and the shield node. The sense node may belong to, or be identical to, a sense electrode of the abovementioned capacitivesensor. Due to the abovementioned arrangement, the first voltage source, the first source impedance, the shield node, the sense-shield-impedance, and the sense node are connected in series. When the first voltage source generates the alternating first supply voltage, an alternating signal is transferred from the shield node to the sense node through the sense-shield impedance.

[0009] It will be noted, that there may further be a parasitic impedance e.g. betweenthe shield node and ground. Such a parasitic impedance may possibly not beassociated with a specific element. It is an impedance that is unwanted, but generally cannot be avoided. The parasitic impedance may depend not only on thephysical components of the system as such, but also on the position of the systemas a whole and on its surroundings.

[0010] The system also comprises a reference portion comprising at least onereference impedance and being at least connectable between the sense node and ground. The reference portion can be linear or branched. It is either permanently connected between the sense node and ground or can at least temporarily be connected therebetween. Apart from the reference impedance, which is preferablyknown a priori to the measurements, the reference portion may comprise one ormore additional elements, either active or passive.

[0011] Furthermore, the system comprises a control unit. The control unit may beconnected to one or several of the abovementioned elements. However, at least one of the abovementioned elements may be physically integrated into the control unit. The control unit may receive signals from an element and / or control an element. Also, it may internally perform logical operations and / or calculations. The controlunit may comprise (or be implemented as) a microcontroller. At least some functionsof the control unit may be software-implemented.

[0012] A detection impedance is couplable between the sense node and ground.The detection impedance can be caused by an object or body part that is to be detected. Based on the detection impedance, the presence, position, and / or composition of an object may be detected. The detection impedance can be coupled between the sense node and ground. One could also say that the sense node canbe coupled to ground via the detection impedance. As a rule, the detectionimpedance is not part of the system as such. The detection impedance (or an object causing the detection impedance) may be in physical contact, possibly electricallyconductive contact, with the sense node. However, in many cases, there may be no physical contact. The detection impedance can also be referred to as an impedance to be measured. When it is coupled between the sense node and ground, it is inparallel to the reference portion. Also, it is in series to the first voltage source, thefirst source impedance, the shield node, the sense-shield-impedance, and the sense node.

[0013] The control unit is adapted to at least perform the following operations.These may be performed sequentially, either in the order in which they are mentioned or in a different order. However, at least two steps could be performedat least partially at the same time, i.e., simultaneously.

[0014] The control unit performs a primary measurement and a secondarymeasurement while the first voltage source is active, wherein at least one reference impedance is connected between the sense node and ground in at least one measurement. The respective reference impedance may be connected directly or indirectly to the sense node, and to ground. As will be explained later, at least one voltage, preferably a plurality of voltages is measured in each measurement. The terms “primary” and “secondary” do not indicate an order or sequence, although it is possible that the primary measurement is performed before the secondary measurement. However, the sequence could be inversed, or the measurements could be performed simultaneously. During both measurements, the first voltage source is active and supplies the first supply voltage. However, the first supply voltage could have different properties during the primary measurement and thesecondary measurement, respectively. Accordingly, an alternating signal istransmitted though the shield node, the sense node, and the detection impedance to ground. Also, since at least one reference impedance is connected between the sense node and ground, it is connected to in parallel to the detection impedance. The respective reference impedance may be connected between the sense node and ground only during the primary or the secondary measurement, or during both measurements. Through the sense node, the reference impedance is also connected to the first voltage source. The two measurements may be performed for different states of the system, but they may also be performed in different ways, i.e., the measurements differ although the state of the system is unchanged.

[0015] In another step, the control unit determines the detection impedance at leastbased on at least one reference impedance and on results of both measurements. In this context, “determine” is to be understood in that the control unit deduces, finds and / or calculates the detection impedance. Preferably, the control unit is adapted to determine the detection impedance as a complex parameter having a real part or resistance and an imaginary part or reactance. As will be explained below, the detection impedance can be determined based on additional values. Either way, both the primary measurement and the secondary measurement are used for determining the detection impedance. The determination process also uses knowledge about at least one reference impedance. In other words, the referenceimpedance is either explicitly or implicitly used for determining the detectionimpedance. The detection impedance can be determined with a high accuracy despite the abovementioned first source impedance and a parasitic impedance between the shield node and ground, which are not known a priori. This is possible due to the reference portion and the at least one reference impedance therein. There are various possibilities how the first and secondary measurement can be conducted and how the detection impedance can be determined, as will be explained below.

[0016] In a preferred embodiment, the control unit is adapted to:- measure a primary shield-node voltage of the shield node and a primarysense-node voltage of the sense node in the first measurement;- measure a secondary sense-node voltage of the sense node in the secondmeasurement; and- determine the detection impedance at least based on the primary shield-nodevoltage, the primary sense-node voltage and secondary sense-node voltage. In other words, in the primary measurement, the control unit measures a primary shield-node voltage of the shield node and a primary sense-node voltage of the sense node. In the secondary measurement, the control unit measures a secondary sense-node voltage of the sense node. Preferably, these and other voltages are measured with respect to a common potential, specifically with respect to ground potential. It is understood that the control unit either is connected to the sense node and shield node, or these nodes are integrated into the control unit. As will be explained below, additional voltages can be measured. Also, it is possible that morethan one primary or more than one secondary voltage is measured. The control unit determines the detection impedance at least based on the primary shield-node voltage, as well as the primary and secondary sense-node voltage. As will be explained below, the detection impedance can be determined based on additional values.

[0017] Preferably, the control unit is adapted to:- also measure a secondary shield-node voltage in the secondarymeasurement; and- determine the detection impedance also based on the secondary shield-nodevoltage. In this embodiment, the control unit measures a (primary or secondary) shield-node voltages and a (primary or secondary) sense-node voltage in each of the primary and secondary measurement.

[0018] In some cases, especially if the imaginary part or the real part of theimpedances can be neglected, it may be sufficient to measure only the amplitude ofa voltage, e.g. the shield-node voltage or the sense-node voltage. In other cases,the phase angle of the voltage or the real part and imaginary part of the voltageneed to be measured. In a preferred embodiment, the control unit is adapted tomeasure at least one voltage by a complex voltage measurement. In other words, the measured voltage is regarded as a complex value with a real part and an imaginary part. This may be done for every measured voltage.

[0019] In addition to determining the detection impedance, the control unit may beadapted to determine the sense-shield impedance. This may be possible incombination with any of the abovementioned embodiments. Determining the sense- shield impedance may not be necessary for determining the detection impedance but can be useful for diagnostic purposes.

[0020] According to one embodiment, the reference portion comprises a switcharrangement with a first reference impedance which is connectable to ground via a switch. Specifically, the first reference impedance may be connected to the sense node and to the switch. The switch, in turn, is connected to ground. When the switch is closed, the first reference impedance is connected between the sense node andground. In one embodiment, the reference portion only comprises the switch arrangement.

[0021] There may be various possibilities how to determine the detectionimpedance using the switch arrangement. According to one preferred option, thecontrol unit is adapted to:- activate the first voltage source;- perform the primary measurement while the switch is open;- perform the secondary measurement while the switch is closed; and- determine the detection impedance also based on the first referenceimpedance. During the primary measurement, the switch is open, wherefore the first reference impedance does not influence the measurement. During the secondary measurement, the switch is closed, wherefore the first reference impedance is connected in parallel to the detection impedance. Based on the different voltage values measured in the first and secondary measurement, and the primary measurement impedance, the detection impedance can be deduced.

[0022] According to another embodiment, the reference portion comprises avoltage arrangement with:- a second voltage source adapted to provide an alternating second supplyvoltage with reference to ground;- a second reference impedance connected between the sense node and areference node; and- a second source impedance between the second voltage source and thereference node. The term “second reference impedance” is not to be construed in that this embodiment can only be used together with the abovementioned first reference impedance. Rather, the reference portion may only comprise the voltage arrangement. The second voltage source may be constructed identical or similar to the first voltage source. Due to the abovementioned arrangement, the second voltage source, the second source impedance, the reference node, the second reference impedance, and the sense node are connected in series. Although thesecond source impedance is referred to as a dedicated element, it may physicallybe integrated into the second voltage source, either entirely or partially. Like the shield node and the sense node, the reference node is either connected to thecontrol unit or is integrated into the control unit.

[0023] The first supply voltage and the second supply voltage may be non-periodicvoltages, e.g., pseudo-random voltages. In another embodiment, at least one supplyvoltage is a periodically alternating voltage. This could be a sine wave, whichcomprises only one frequency, but it could also be an oscillation with upperharmonics, e.g., an approximated sine wave, a square wave, or a filtered square wave.

[0024] There may be various possibilities how to determine the detectionimpedance using the voltage arrangement. According to one preferred option, the control unit is adapted to:- also measure a primary reference-node voltage of the reference node in theprimary measurement and / or also measure a secondary reference-nodevoltage of the reference node in the secondary measurement; and- determine the detection impedance also based on the primary reference-nodevoltage and secondary reference-node voltage, and the second reference impedance. Herein, “also” indicates that the primary and / or secondary reference-node voltageis measured in addition to the primary and secondary sense-node voltage and theprimary (and possibly secondary) shield-node voltage. Likewise, the detection impedance is determined based on the first and secondary reference-node voltage and the second reference impedance in addition to the abovementioned sense-node voltages and shield-node voltages. In some embodiments, both the primary and secondary reference voltage are determined. In other embodiments, only one reference voltage is determined.

[0025] One embodiment provides that the control unit is adapted to:- apply the first supply voltage and the second supply voltage having a first setof complex amplitudes;- perform the primary measurement with the first set of complex amplitudes;- apply the first supply voltage and the second supply voltage having a secondset of complex amplitudes different from the first set; and- perform the secondary measurement with the second set of complexamplitudes. Each complex amplitude corresponds to a “real” amplitude and a phase angle, or a real part and an imaginary part, respectively. The first set and the second set of amplitudes are different in that the amplitude of at least one voltage is different. In each set of amplitudes, one amplitude may be zero. Also, the complex amplitudes in one of the sets may be identical. In any case, the frequency of both voltages is the same. Both supply voltages may comprise a plurality of frequencies, e.g., corresponding to an approximated sine wave or the like. In this case, the frequency spectrum of both voltages is preferably identical. In this embodiment, both voltages are applied simultaneously. One could say that during each measurement, the detection impedance, and parts of the system itself are subjected to a voltage that results from a superposition of the first and second supply voltage. However, since at least one complex amplitude is different during the secondary measurement, the same is true for the superposition. Therefore, the shield-node voltage, the sense- node voltage and the reference-node voltage will generally be different in the primary measurement as compared to the secondary measurement. This, in combination with different nodes experiencing different superpositions of the supply voltages, allows to deduce the detection impedance. In this embodiment, it is preferred that the primary and secondary shield-node voltage, the primary and secondary sense-node voltage, and the primary and secondary reference-node voltage are measured and used for determining the detection impedance.

[0026] Another embodiment provides that the control unit is adapted to:- apply the first supply voltage having a first frequency and the second supplyvoltage having a second frequency different from the first frequency;- perform the primary measurement at the first frequency; and- perform the secondary measurement at the second frequency.While the in the previous embodiment, both supply voltages have the same frequency, they have different frequencies in this embodiment. Both supply voltages may be sine waves, each having a single frequency, but they may also compriseupper harmonics, e.g. corresponding to an approximated sine wave, a square wave,or the like. The first and secondary measurement can be performed without changing any of the supply voltages and even simultaneously. The shield-nodevoltage, the sense-node voltage and the reference-node voltage can be measured in a frequency-selective manner, e.g., using a narrow band pass. Thus, a frequency component caused by the first supply voltage can be selectively measured, as well as a frequency component caused by the second supply voltage. This embodiment is highly advantageous in that it may save time, because there is no need to change the setting of a switch or to alter the supply voltages in between the first and secondary measurement.

[0027] In some embodiments, the control unit is adapted to determine the detectionimpedance based on a model that takes the at least one source impedance intoaccount. In other embodiments, the source impedance is small in comparison to the other impedances. In such an embodiment, the control unit may be adapted to determine the detection impedance based on neglecting at least one source impedance. This simplifies the underlying mathematical calculations and can reduce the processing time. Also, it may reduce the number of voltages that need to be measured. For instance, with regard to the abovementioned embodiment that uses different frequencies, it is normally necessary to measure the primary and secondary shield-node voltage, and the primary and secondary reference-node voltage. However, if the source impedances are neglected, it is sufficient to measure the primary shield-node voltage and the secondary reference-node voltage (in addition to the sense-node voltages).

[0028] While some embodiments provide that the reference portion comprises onlythe switch arrangement or only the voltage arrangement, it is also possible to combine both. According to one embodiment, the reference portion comprises the switch arrangement and the voltage arrangement connected in parallel between the sense node and ground.

[0029] When using a layout with both the switch arrangement and the voltagearrangement, the system could be operated in various ways. For instance, the switch could be left open, thereby effectively eliminating the switch arrangement, and the system could be operated like the abovementioned embodiment that only has the voltage arrangement. Another embodiment provides that the control unit is adapted to:- apply the first supply voltage having a first frequency and the second supplyvoltage having a second frequency different from the first frequency;- perform the primary measurement, with the switch open, by measuring a firstprimary sense-node voltage at the first frequency, a second primary sense- node voltage at the second frequency, the primary shield-node voltage at the first frequency, and the primary reference-node voltage at the second frequency;- perform the secondary measurement, with the switch closed, by measuring afirst secondary sense-node voltage at the first frequency, a second secondary sense-node voltage at the second frequency, the secondary shield-nodevoltage at the first frequency, and the secondary reference-node voltage at the second frequency; and- determine the second reference impedance at least based on the first primary,second primary, first secondary and second secondary shield-node voltage,the primary and secondary sense-node voltage, and the primary and secondary reference-node voltage.

[0030] Although this embodiment requires different frequencies and differentswitching states of the switch, it enables to determine the second reference impedance, which is otherwise assumed to be known a priori. If the actual value of the second reference impedance differs from the supposed value, this cannot be detected in the aforementioned embodiments and may cause a systematic error in the calculation of the detection impedance, and the sense-shield impedance, if applicable. This embodiment, on the other hand, determines the second reference impedance. Then, if the actual value differs significantly from the design value, this can be taken as an indication that any results based on the design value are unreliable or unusable. It is also conceivable that the detection impedance (and possibly the sense-shield impedance) can be determined (partially) based on the actual value second reference impedance, which would increase the reliability of the result.

[0031] As already mentioned above, any of the supply voltages may be a sine wavewith a single frequency. However, at least one supply voltage may comprise asuperposition of periodic oscillations, e.g. of harmonic oscillations, corresponding toa plurality of frequencies. Examples for this would be an approximated sine wave, a square wave or a filtered square wave. With the supply voltage comprising a plurality of frequencies, the same is true for the shield-node voltage, the sense-node voltageand, where applicable, the reference-node voltage. Under these circumstances, it is preferred that the control unit is adapted to separately evaluate a plurality of frequencies of at least one measured voltage. In other words, each the voltages, e.g., the first and secondary sense-node voltage, the first and secondary shield- node voltage etc., can be evaluated for each frequency individually. Theoretically, each evaluation should yield the same value for the detection impedance. In practice, though, there may be different results which can be used to find a mean value, identify outliers, or the like.

[0032] The invention also provides a method for impedance detection, using:- a first voltage source, adapted to generate an alternating first supply voltagewith reference to ground;- a shield node connected to the first voltage source via a first sourceimpedance;- a sense node coupled to the shield node via a sense-shield impedance;- a reference portion comprising at least one reference impedance and being atleast connectable between the sense node and ground; and- a control unit,wherein the method comprises:- coupling a detection impedance between the sense node and ground;- the control unit performing a primary measurement and a secondarymeasurement while the first voltage source is active, wherein at least one reference impedance is connected between the sense node and ground in at least one measurement; and- the control unit determining the detection impedance at least based on at leastone reference impedance and on results of both measurements.

[0033] All these terms have been explained above with reference to the inventivesystem and therefore will not be explained again. The steps of the inventive methodmay be executed in the sequence in which they are mentioned, but also in a differentsequence or simultaneously. Preferred embodiments of the inventive methodcorrespond to those of the inventive system.Brief Description of the Drawings

[0034] Further details and advantages of the present invention will be apparentfrom the following detailed description of not limiting embodiments with reference tothe attached drawing, wherein:Fig. 1 is a schematic view of a first embodiment of an inventive system with adetection impedance coupled thereto; Fig.2 is a schematic view of a second embodiment of an inventive system with a detection impedance coupled thereto; and Fig.3 is a schematic view of a third embodiment of an inventive system with a detection impedance coupled thereto. Description of Preferred Embodiments

[0035] Fig.1 is a schematic view of a first embodiment of an inventive measurementsystem 1. The system comprises a first voltage source 2, which is connected to ground. On the other side, it is connected to a shield node 4 via a first source impedance 6, which represents the combination of an impedance associated with the first voltage source 2 as such, and a wiring impedance. The shield node 4 is coupled to a sense node 5 via a sense-shield impedance 3. The shield node 4 and the sense node 5 can be considered as parts of a so-called capacitive sensor. There is a parasitic impedance 7 between shield node 4 and ground. The system 1 also comprises a reference portion 10 that is connected to the sense node 5. In this embodiment, the reference portion 10 consists of a switch arrangement 11, which comprises a first reference impedance 12 that is connected to ground via a switch 13. Fig.1 shows the switch 13 in open state, i.e., the first reference impedance 12 is isolated from ground. The sense node 5 is coupled to ground via a detection impedance 30 that is to be measured. Various functions of the system 1 are controlled by a control unit 25, which is connected to the shield node 4, the sense node 5, the first voltage source 2 and the switch 13 by connections not shown here for sake of simplicity. Some functions of the control unit 25 may be software- implemented.

[0036] In order to measure the detection impedance 30, the control unit 25activates the first voltage source 2 so that it generates a periodic first supply voltage,for example a sine wave, an approximated sine wave, a square wave or a filtered square wave. The sense-shield impedance 6 transfers the periodic waveform to the shield node 4. The sense-shield impedance 3 forms together with the detection impedance 30 a voltage divider.

[0037] For a primary measurement, control unit 25 opens the switch 13, as shownin Fig.1. A complex primary sense-node voltage V^^^^^,^on sense node 5 is measured. Also, a complex shield-node voltage V^^^^^^,^on shield node 4 is measured. For a secondary measurement, control unit 25 closes the switch 13, which connects the first reference impedance 12 in parallel to the detection impedance 30. A complex secondary sense-node voltage V^^^^^,^on sense node 5 and a complex shield-node voltage V^^^^^^,^on shield node 4 are measured. Then,the complex impedance Z^ of the detection impedance 30 can be found out by thefollowing calculation: V^ − VX =^^^^^,^ ^^^^^,^^V^^^^^,^V^^^^^^ − VX =,^ ^^^^^,^^V^^^^^,^X^ − XZ = Z ^^ ^^^X^where Z^^^is the complex impedance of the first reference impedance 12. The complex impedance Z^of the sense-shield impedance 3 is found by: VZ ^^^^^^,^ − V ^^^^^,^^ = Z^ V ^^^^^,^This value can for example be used for diagnostics purposes.

[0038] The advantage of including the measured complex voltage on the shieldnode 4 in the calculation is that the error introduced into the measurement by thefirst source impedance 6 and the parasitic impedance 7 is substantially eliminated.

[0039] Fig.2 shows a second embodiment of an inventive system 1. In thisembodiment, the reference portion 10 comprises no switch arrangement 11, but avoltage arrangement 15. The voltage arrangement 15 comprises a secondreference impedance 16 that is connected to the sense node 5. On the other side,it is connected to a second voltage source 19 via a reference node 18 and a secondsource impedance 17. As the first embodiment, the first voltage source 2 is activatedto generate a periodic waveform, for example a sine wave, approximated sine wave or square wave or filtered square wave. The shield impedance 6 transfers the periodic waveform to the shield node 4.

[0040] There are two options how the detection impedance 30 and the sense-shield impedance 3 can be determined. In a first option, the second voltage source 19 generates a periodic second supply voltage, e.g. a sine wave, an approximatedsine wave, a square wave, or a filtered square wave, which has the same frequencyas the first supply voltage of the first voltage source 2.

[0041] For a primary measurement, the control unit 25 sets the complex voltagesof the first voltage source 2 and the second voltage source 19 to a first set of complex amplitudes. One of the two amplitudes can be zero. The primary sense- node voltage V^^^^^,^and the primary shield-node voltage V^^^^^^,^are measured. Also, a primary reference-node voltage V^^^,^on the reference node 18 is measuredor known a priori. The voltage V^^^,^ is e.g. known a priori if the source impedance17 can be neglected, and the output voltage of second voltage source 19 is knownby for example using a voltage source with known output amplitude. For asecondary measurement, at least one supply voltage is set to a different complex amplitude, optionally to zero. The secondary sense-node voltage V^^^^^,^and the secondary shield-node voltage V^^^^^^,^are measured. Furthermore, a secondary reference-node voltage V^^^,^on reference node 18 is measured or known a priori.Then the complex impedance Z^ of detection impedance 30 is found out by thefollowing calculation: V^^^ = V^^^^^^,^ − V^^^^^,^V^^^ = V^^^^^^,^ − V^^^^^,^V^^^ = V^^^,^ − V^^^^^,^V^^^ = V^^^,^ − V^^^^^,^V^^^^^^,^ ^V V ^^ ^^^−^^^^V^^^V^^^V^^^where Z^^^is the complex impedance of the second reference impedance 16. The complex impedance Z^of the sense-shield impedance 3 is found by:

[0042] According to another option, both voltage sources 2, 19 generate a periodicwaveform. However, the first voltage source 2 generates a first supply voltage at afirst frequencyand the second voltage source 19 generates a second supplyvoltage at a second frequency f^ that is different from the first frequency f^. For thisoption, all measurements can be performed in parallel, which is advantageous ifmeasurement timing is critical.

[0043] The detection impedance 30, the sense-shield impedance 3, and thereference impedance 16 are each considered to be a combination of a capacitanceand a resistance connected in parallel. According to one possibility, the sourceimpedances 6, 17 are so small that they can be neglected, and the following methodcan be applied to calculate the detection impedance 30 and the sense-shield impedance 3.

[0044] In a primary measurement, the control unit 25 measures a primary sense-node voltage V^^^^^,^ on the sense node 5 at the first frequency f^ and a primaryshield-node voltage V^^^^^^,^ on shield node 4 at the first frequency f^. In a secondarymeasurement, which can be performed parallel to the primary measurement, asecondary sense-node voltage V^^^^^,^ is measured on sense node 5 at the secondfrequency f^. A (secondary) reference-node voltage V^^^,^ is measured on referencenode 18 at the second frequency f^ or known a priori. The following quantities K^and K^are calculated: V^^^^^^ − VK =,^ ^^^^^,^^V^^^^^,^V^ − VK ^^,^ ^^^^^,^^ = Y^^^,^V^^^^^,^where Y^^^,^ is the complex admittance of the second reference complex impedance16 at the second frequency f^.Then, with the following definitions:K^^ = Re(K^) ; K^^ = Im(K^);K^^ = Re(K^) ; K^^ = Im(K^);ω^ = 2πf^; andω^ = 2πf^and G^^^being the parallel conductance and C^^^the parallel capacitance of thesecond reference impedance 16, it is possible to calculate C^^, the parallelcapacitance, and G^^, the parallel conductance of the sense-shield impedance 3 asfollows:with Y^^ = K^ – (Gcp + j ∙ ω^ ∙ Ccp) ; j^ = −1the parallel capacitance C^and the parallel conductance G^of the detection impedance 30 are calculated as follows: G^ = Re(Y^^)Im(Y^) C =^^ω^

[0045] According to another option, at least one source impedance 6, 17 is notneglected. Again, all measurements can be done in parallel which is advantageous if measurement timing is critical. Again, the detection impedance 30, the sense-shield impedance 3, and the reference impedance 16 are each considered to be acombination of a capacitance and a resistance connected in parallel. The detectionimpedance 30 and the sense-shield impedance 3 can be calculated as follows.

[0046] In a primary measurement, a primary sense-node voltage V^^^^^,^ on thesense node 5, a primary shield-node voltage V^^^^^^,^ on the shield node 4, and aprimary reference-node voltage V^^^,^ on the reference node 18 are measured at thefirst frequency f^. In a secondary measurement, a secondary sense-node voltageV^^^^^,^ on the sense node 5, a secondary shield-node voltage V^^^^^^,^ on the shieldnode 4, and a secondary reference-node voltage V^^^,^ on the reference node 18 aremeasured at the second frequency f^. The following quantities are calculated:A^ = V^^^^^,^B^ = V^^^^^,^ − V^^^^^^,^V^^^^^,^^ ∙ Y^^^,^A^ = V^^^^^,^B^ = V^^^^^,^ − V^^^^^^,^L^ = ^V^^^,^ − V^^^^^,^^ ∙ Y^^^,^where Y^^^,^ is the complex admittance of the second reference impedance 16 at thefirst frequencyand Y^^^,^ is the complex admittance of the second referenceimpedance 16 at the second frequency f^. With:A^^ = Re(A^) ; A^^ = Im(A^);A^^ = Re(A^) ; A^^ = Im(A^);B^^ = Re(B^) ; B^^ = Im(B^);B^^ = Re(B^) ; B^^ = Im(B^);L^^ = Re(L^) ; L^^ = Im(L^);L^^ = Re(L^) ; L^^ = Im(L^);ω^ = 2πf^; andω^ = 2πf^the parallel capacitance C^and the parallel conductance G^of the detectionimpedance 30, as well as the parallel capacitance C^^ and the parallel conductanceG^^ of the shield-node impedance 3 can be calculated with the following matrixoperation: G^ A^^ −A^^ ∙ ω^ B^^L^^C ^^^ A A ∙G^^^ ^ = ^ ^^ ^^ ω^ B^^L^^A^^ −A^^ ∙ ω^ B^^ ^L^^^ C^^ A^^ A^^ ∙ ω^ B^^L^^where ||-1means the inverted matrix ||.

[0047] Fig.3 shows a third embodiment of an inventive system 1, which can beregarded as a combination of the first and second embodiment in that the referenceportion 10 comprises a switch portion 11 and a voltage portion 15. The detectionimpedance 30 and / or the sense-shield impedance 3 can be measured similarly asin the first embodiment. The second reference impedance 16 is used for diagnosingthe validity of the measurement result of detection impedance 30 and / or the sense-shield impedance 3 as follows: The second reference impedance 16 is measuredand checked against its designed value. If the measured value deviates from thedesigned value, then it can be concluded that the measurement of the otherimpedances 3, 30 is likely to be wrong and must be discarded.

[0048] The measurement of impedances detection impedance 30 and / or thesense-shield impedance 3, and the measurement of the second referenceimpedance 16 can be done in parallel, thereby saving time. All three aforementionedimpedances 3, 16, 30 are considered to be a combination of a capacitance and aresistance. Both voltage sources 2, 19 generate a periodic waveform, for examplea sine wave, approximated sine wave or square wave or filtered square wave. The first voltage source 2 generates a first frequencyand the second voltage source19 generates a frequency f^, wherein the first frequency f^ is different from thesecond frequency f^. For a primary measurement, the control unit 25 opens theswitch 13. The control unit 25 measures a first primary sense-node voltageV^^^^^,^(f^) at the first frequency f^, a second primary sense-node voltage V^^^^^,^(f^)at the second frequency f^, a primary shield-node voltage V^^^^^^,^on shield node4 at the first frequencyand a primary reference-node voltage V^^^,^(f^). onreference node 18 at the second frequency f^.

[0049] For a secondary measurement, the control unit 25 closes the switch 13,which connects the first reference impedance 12 in parallel to the detection impedance 30. The control unit 25 measures a first secondary sense-node voltageV^^^^^,^(f^) at the first frequencya second secondary sense-node voltageV^^^^^,^(f^) at the second frequency f^ on sense node 5, a secondary shield-nodevoltage V^^^^^^,^(f^) on shield node 4 at the first frequencyand a secondaryreference-node voltage V^^^,^(f^) on reference node 18 at the second frequency f^.

[0050] Y^^^,^ is the complex admittance of the second reference compleximpedance 16 at the first frequency f^ and Y^^^,^ is the complex admittance of thesecond reference complex impedance 16 at the second frequency f^. Then, theparallel capacitance C^and the parallel conductance G^of the detection impedance30, the parallel capacitance C^ and the parallel conductance G^ of the shield-nodeimpedance 3, as well as the parallel capacitance C^and the parallel conductance G^of the sense-shield impedance 16 are calculated with the following operations:Y=Y^^^^ Y^^^,^ ∙Y^ − Y^Y = Y ∙V^^^^^,^(f^) ^^ ^^^ V^^^,^(f^) − V^^^^^,^(f^)

[0051] The parallel capacitance C^ and the parallel conductance G^ of impedance30 and the parallel capacitance C^and the parallel conductance G^of the sense-shield impedance 3 and the parallel capacitance C^ and the parallel conductanceG^of the reference impedance 16 are calculated with the operations below:G^^ = Re(Y^^)Im(Y^) C =^^^ω^G^ = Re(Y^^)G^ = Re(Y^^^) − G^

[0052] An alternative to calculate C^ and G^ is to replace the calculated C^, G^above by the a-priori known values. The impedance 17 is neglected in the calculations above.

[0053] In the examples above, the detection impedance 30 and the sense-shieldimpedance 3 are considered to be a capacitance and a resistance connected inparallel. If any of these impedances 3, 30 is instead considered to be a seriesconnection of a resistance and a capacitance, the parallel resistance / capacitance circuit can be transformed into the series form at the frequency of interest. The sameapplies if the second reference impedance 16 is considered to be a seriesconnection of a resistance and a capacitance. Likewise, any of the abovementioned impedances 3, 16, 30 can be replaced by a combination of an inductance and a resistance.

[0054] The voltage sources 2, 19, which generate a periodic waveform in theexamples above, may also generate a superposition of periodic waveforms with different frequencies, and the control unit 25 may perform a complex voltagemeasurement for each of the different frequencies. This can be done for redundancypurposes, or for example if the measurement disturbed by an external disturbancesource on at least one of the different frequencies above.

[0055] For the case that only the capacitive part of the complex impedances aboveare relevant, and resistive parts in the circuits can be neglected, a complex voltage measurement is not necessary and can be replaced by an AC voltage measurement. Also, in this case the voltage sources 2, 19 do not need to generateperiodic waveforms, but could, e.g., generate pseudo-random waveforms.

[0056] The voltage source 2, 19 may at least partially be integrated into the controlunit 25, which may comprise a microcontroller. Then, the periodic waveforms canpreferably be generated by the microcontroller using its PWM outputs. The complexvoltages can be measured with the microcontroller ADC and a software IQdemodulator. The calculations are preferably done in software, including the matrix inversion above.

[0057] Similar to the third embodiment, various options described with reference tothe first and second embodiment can be combined for diagnostics or redundancy purposes.List of Reference Symbols 1system2 voltage source3 sense-shield impedance4 shield node5 sense node6 first source impedance7 parasitic impedance10 reference portion11 switch arrangement12 first reference impedance13 switch15 voltage arrangement16 second reference impedance17 second source impedance18 reference node19 second voltage source25 control unit30 detection impedance

Claims

Claims1. A system (1) for impedance measurement, comprising:- a first voltage source (2), adapted to generate an alternating first supplyvoltage with reference to ground; -a shield node (4) connected to the first voltage source (2) via a first sourceimpedance (6); -a sense node (5) coupled to the shield node (4) via a sense-shield impedance(3); -a reference portion (10) comprising at least one reference impedance (12,16) and being at least connectable between the sense node (5) and ground; and -a control unit (25),wherein a detection impedance (30) is couplable between the sense node (5) and ground, and the control unit (25) is adapted to: -perform a primary measurement and a secondary measurement while thefirst voltage source (2) is active, wherein at least one reference impedance (12, 16) is connected between the sense node (5) and ground in at least one measurement; and -determine the detection impedance (30) at least based on at least onereference impedance (12, 16) and on results of both measurements.

2. A system according to claim 1, the control unit (25) being adapted to:- measure a primary shield-node voltage of the shield node (4) and a primarysense-node voltage of the sense node (5) in the first measurement; -measure a secondary sense-node voltage of the sense node (5) in thesecond measurement; and -determine the detection impedance (30) at least based on the primary shield-node voltage, the primary sense-node voltage and secondary sense-node voltage.

3. A system according to any of the preceding claims, wherein the control unit (25)is adapted to: -also measure a secondary shield-node voltage in the secondarymeasurement; and- determine the detection impedance (30) also based on the secondary shield-node voltage.

4. A system according to any of the preceding claims, wherein the control unit (25)is adapted to determine the sense-shield impedance (3).

5. A system according to any of the preceding claims, wherein the referenceportion (10) comprises a switch arrangement (11) with a first reference impedance (12) which is connectable to ground via a switch (13).

6. A system according to any of the preceding claims, the control unit (25) beingadapted to: -perform the primary measurement while the switch (13) is open;- perform the secondary measurement while the switch (13) is closed; and- determine the detection impedance (30) also based on the first referenceimpedance (12).

7. A system according to any of the preceding claims, wherein the referenceportion (10) comprises a voltage arrangement (15) with:- a second voltage source (19) adapted to provide an alternating secondsupply voltage with reference to ground;- a second reference impedance (16) connected between the sense node (6)and a reference node (18); and -a second source impedance (17) between the second voltage source (19)and the reference node (18).

8. A system according to any of the preceding claims, wherein the control unit isadapted to: -also measure a primary reference-node voltage of the reference node (18) inthe primary measurement, and / or also measure a secondary reference-nodevoltage of the reference node (18) in the secondary measurement, and- determine the detection impedance (30) also based on the primary reference-node voltage and / or the secondary reference-node voltage, and the second reference impedance (16).

9. A system according to any of the preceding claims, wherein the control unit (25)is adapted to: -apply the first supply voltage and the second supply voltage having a first setof complex amplitudes; -perform the primary measurement with the first set of complex amplitudes;- apply the first supply voltage and the second supply voltage having a secondset of complex amplitudes different from the first set; and- perform the secondary measurement with the second set of complexamplitudes.

10. A system according to any of the preceding claims, wherein the control unit (25)is adapted to: -apply the first supply voltage having a first frequency and the second supplyvoltage having a second frequency different from the first frequency;- perform the primary measurement at the first frequency; and- perform the secondary measurement at the second frequency.

11. A system according to any of the preceding claims, wherein the control unit (25)is adapted to determine the detection impedance (30) based on neglecting at least one source impedance (6, 17).

12. A system according to any of the preceding claims, wherein the referenceportion (10) comprises the switch arrangement (11) and the voltage arrangement (15) connected in parallel between the sense node (5) and ground.

13. A system according to any of the preceding claims, wherein the control unit (25)is adapted to: -apply the first supply voltage having a first frequency and the second supplyvoltage having a second frequency different from the first frequency;- perform the primary measurement, with the switch (13) open, by measuringa first primary sense-node voltage at the first frequency, a second primarysense-node voltage at the second frequency, the primary shield-node voltage at the first frequency, and the primary reference-node voltage at the second frequency; -perform the secondary measurement, with the switch (13) closed, bymeasuring a first secondary sense-node voltage at the first frequency, asecond secondary sense-node voltage at the second frequency, the secondary shield-node voltage at the first frequency, and the secondary reference-node voltage at the second frequency; and- determine the second reference impedance (16) at least based on the firstprimary, second primary, first secondary and second secondary shield-nodevoltage, the primary and secondary sense-node voltage, and the primary andsecondary reference-node voltage.

14. A system according to any of the preceding claims, wherein at least one supplyvoltage comprises a superposition of periodic oscillations corresponding to a plurality of frequencies, and the control unit (25) is adapted to separately evaluate a plurality of frequencies of at least one measured voltage.

15. A method for impedance detection, using:- a first voltage source (2), adapted to generate an alternating first supplyvoltage with reference to ground; -a shield node (4) connected to the first voltage source (2) via a first sourceimpedance (6); -a sense node (5) coupled to the shield node (4) via a sense-shield impedance(3); -a reference portion (10) comprising at least one reference impedance (12,16) and being at least connectable between the sense node (5) and ground; and -a control unit (25),wherein the method comprises: -coupling a detection impedance (30) between the sense node (5) andground; -the control unit (25) performing a primary measurement and a secondarymeasurement while the first voltage source (2) is active, wherein at least one reference impedance (12, 16) is connected between the sense node (5) and ground in at least one measurement; and -the control unit (25) determining the detection impedance (30) at least basedon at least one reference impedance (12, 16) and on results of both measurements.

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