LU100745B1

LU100745B1 - System for Grounding and Diagnostics

Info

Publication number: LU100745B1
Application number: LU100745A
Authority: LU
Inventors: Laurent Lamesch; Michael Pütz
Original assignee: Iee Sa
Priority date: 2018-03-27
Filing date: 2018-03-27
Publication date: 2019-10-01

Abstract

The invention relates to a system (1) for grounding and diagnostics, comprising a conductive frame (2) for mounting a capacitive sensor. In order to provide means for ensuring and monitoring a grounded condition of a conductive frame, the invention provides that the system further comprises a diagnostics circuit (10), by which the frame (2) is AC grounded and which comprises an electric source (11) connected to the frame (2) via a first line (5) and adapted to apply a diagnose signal to the frame (2), the diagnostics circuit (10) being adapted to provide at least one quantity that depends on the diagnose signal and on a ground connection of the frame (2).

Description

System for Grounding and Diagnostics

Technical field [0001] The invention relates to a system for grounding and diagnostics and to adiagnostics circuit, e.g. to be used in conjunction with a capacitive sensor.

Background of the Invention [0002] Capacitive sensors today are used for a vast variety of applications, likeinput devices (e.g. touchpads, capacitive sliders, touch wheels, etc ), proximitysensors or occupant detection systems.

[0003] There are many different types of capacitive sensors known in the art, butmost of them rely on the following principle. A sensing electrode is disposed sothat an approaching object (person, hand, finger or the like) changes thecapacitance of the sensing electrode with respect to ground. The object of mayalso account for dielectric losses, wherefore the sensing electrode in general hasan impedance with a resistance and a reactance, both of which may be influencedby the object. The changing impedance is measured by a measurement circuit.For instance, the sensing electrode may be connected to an alternating voltage,e.g. a square wave voltage, and the current through the electrode, which dependson its impedance, can be converted by the measurement circuit into a voltage.This voltage is indicative of the impedance and thus may be used to determinewhether an object is near the sensing electrode.

[0004] In many applications, the sensing electrode is mounted on a conductingstructure, which may be referred to as a frame. For instance, in occupant detectionsystems, the sensing electrode is normally disposed within the vehicle seat. Ingeneral, an electric field is formed between the sensing electrode and the metalcomponents of the vehicle seat, which may be referred to as the seat frame.Therefore, the impedance on the one hand depends on the proximity of an object,but also on the grounded condition of the seat frame, i.e. whether the seat frameproperly connected to ground (i.e. the vehicle body) or not (i.e. has a floatingpotential). While it is possible to successfully detect an object even if the seatframe is not grounded, the capacitive sensor has to be calibrated for a certaincondition of the seat frame (grounded or not grounded), which afterwards has to be maintained during the operation of the capacitive sensor. If the conditionchanges, this can lead to false detections or non-detections, respectively. Sinceoccupant detection systems are mostly connected to safety-relevant systems likeseatbelt reminders, airbags and the like, any malfunction of the capacitive sensorhas to be avoided.

Object of the invention [0005] It is thus an object of the present invention to provide means for ensuringand monitoring a grounded condition of a conductive frame.

[0006] This problem is solved by a system according to claim 1 and by adiagnostics circuit according to claim 15.

General Description of the Invention [0007] The invention provides a system for grounding and diagnostics. In thiscontext, "grounding" refers to applying, establishing and/or maintaining aconnection to ground.

[0008] The system comprises a conductive frame for mounting a capacitivesensor. The frame is for mounting a capacitive sensor, i.e. in operational state, acapacitive sensor is directly or (usually) indirectly mounted on the conductiveframe. The frame, which normally is made of metal, may in particular be a vehicleseat frame on which one or several capacitive sensors of an occupant detectionsystem can be mounted or it may be a steering wheel rim of a steering wheel onwhich at least one sensor for hand detection is mounted. Either way, since thecapacitive sensor is to be mounted on the conductive frame, the capacitance andthe impedance of the capacitive sensor are influenced by the presence of theframe. In particular, the impedance depends on whether the frame is groundednot.

[0009] The system further comprises a diagnostics circuit, by which the frame isAC grounded and which comprises an electric source connected to the frame via afirst line and adapted to apply diagnose signal to the frame, the diagnostics circuitbeing adapted to provide at least one quantity that depends on the diagnose signaland that enables diagnostics of a grounded condition of the frame. Here and in thefollowing, "AC grounded" means connected to ground so that an alternating current can flow between ground and the AC grounded element. The frame is ACgrounded by the diagnostics circuit, which implies that the diagnostics circuit itselfhas to be AC grounded. As the frame is AC grounded, it has a well-defined electricpotential, wherefore any detrimental effect on measurements by the capacitivesensor can be avoided. Here and in the following, "connected" refers to either adirect connection or an indirect connection via at least one intermediate element.

[0010] However, correct operation of the capacitive sensor depends on whetherthe grounded condition of the frame can be maintained. Therefore, the diagnosticscircuit is also adapted for diagnostics of the grounded condition. It comprises aelectric source, which is connected to the frame via a first line. The first line can beany kind of conductor, like a conductor path on a printed circuit board, a wire orthe like. The electric source may be connected directly or indirectly, i.e. via at leastone intermediate element, to the first line. In general, it has a wiring resistance,which possibly cannot be neglected. The electric source, which may be any kind ofvoltage or current source, optionally in combination with other elements, isadapted to generate and apply a diagnose signal to the frame via the first line. Thediagnose signal may be an AC signal or a DC signal. In general, the "response" tothe diagnose signal depends on whether the frame is grounded and if so, how it isgrounded. For instance, the response may depend on an external groundconnection of the frame and on the integrity of the AC connection to ground via thediagnostics circuit. The diagnostics circuit is adapted to provide at least onequantity that depends on the diagnose signal and on a ground connection of theframe. Herein, "providing" the quantity may in particular refer to measuring thequantity or outputting the quantity for measurement by another device. The at leastone quantity is normally a voltage or a current. It may be considered as aresponse to the diagnose signal, wherefore it depends on the diagnose signal, e.g.on an amplitude and/or a frequency of the diagnose signal. Also, the at least onequantity is influenced by a ground connection of the frame, e.g. an external groundconnection and/or the (AC) ground connection via the diagnostics circuit.Therefore, by measuring the respective quantity, it is possible to performdiagnostics of the ground connection.

[0011] While references made to "a" diagnose signal, the parameters of thediagnose signal may be changed over time and/or it may be temporarily interrupted. In such cases, the diagnose signal could also be considered as asequence of signals.

[0012] According to one embodiment, the diagnostics circuit is adapted to provideat least one quantity that enables determination of at least one of a groundimpedance or a ground shift voltage that the frame has with respect to ground. Ingeneral, the relation of the frame to ground may be described by a groundimpedance and/or a ground shift voltage. The ground impedance is the impedancebetween the frame and ground, which may comprise a resistance and areactance, due to resistive, capacitive and/or inductive effects. If the frame isconnected to ground, the ground impedance and the ground shift voltage arecharacteristic of an external ground connection. The ground shift voltagerepresents a voltage shift between the frame (or a structure to which the frame isconnected) and ground. Referring to a vehicle seat or a steering wheel, a non-zeroground shift voltage may be due to currents flowing through the groundingconnections, which have a non-negligible impedance. It should be noted thatdepending on the respective situation, the ground impedance and/or the groundshift voltage may be negligible or zero. In a case where there the frame has noexternal ground connection, the ground shift voltage is undefined and the groundimpedance may be purely capacitive (typically below 200 pF). By providing theabovementioned quantity, which can be measured by the diagnostics circuit itselfor by another device, at least a partial diagnostics of the relation of the frame toground is possible. For instance, if the ground shift voltage is determined, this maybe used to determine an external influence on the diagnostics or to identify a shortcircuit between the frame and a voltage source like the car battery. In a situationwhere the frame is supposed to be electrically isolated from ground and thereforeshould have a floating potential, an unwanted connection to ground can bedetected. Also, if the ground impedance is determined, this may be used to identifya defect in the external ground connection, e.g. an open circuit or the like. Thereare several different ways how the ground impedance or the ground shift voltagecan be determined. One example is to apply a voltage signal or a current signal asthe diagnose signal and to detect a voltage at a specific point within thediagnostics circuit. This voltage depends on the diagnose signal itself, impedanceswithin the diagnostics circuit, the ground impedance and the ground shift voltage.

By sequentially applying two DC signals and measuring the voltage, the groundimpedance as well as the ground shift voltage can be calculated (strictly speaking,this is an approximation, which is valid, though, if the wiring resistance of the firstline - plus, where applicable, that of the second line - can be neglected). Likewise,a single AC signal may be applied, which allows to determine the groundimpedance, assuming that the ground shift voltage has no frequency componenthaving the same frequency as the AC signal.

[0013] The electric source may be a first voltage source. The first voltage sourcemay be an AC voltage source adapted to apply an AC voltage as the diagnosesignal or a DC voltage source adapted to apply a DC voltage. Alternatively, theelectric source may be a current source. It may either be an AC current sourceadapted to apply an AC current as the diagnose signal or it may be a DC currentsource adapted to apply a DC current as the diagnose signal. If the electric sourceis an AC voltage source or AC current source, it is preferred that it is adapted toprovide a frequency that is different from a measurement frequency used tooperate the capacitive sensor. If present, the upper harmonics of the two signalsshould also be different. However, in general it may be sufficient if there is nosignificant correlation between the diagnose signal and the signal used formeasurement, e.g. one may be sinusoidal while the other is a pseudo-randomphase shift keyed signal. The same criterion applies to any AC current sources orAC voltage sources mentioned below. As long as the frequencies are different asdescribed, measurement operation of the capacitive sensor and operation of thediagnostics circuit may be performed simultaneously. Otherwise, operation of thecapacitive sensor needs to be interrupted while the diagnostics circuit is inoperation.

[0014] In particular, but not exclusively, when the electric source is a first voltagesource, it may be connected to the first line via a first impedance element. The firstimpedance element can be any element or circuit that has a non-zero or non-negligible impedance. In general, it may be a resistive, capacitive or inductiveelement or a combination of these, e.g. a parallel connection and/or seriesconnection. If the first voltage source is a DC voltage source, the first impedanceelement is preferably a resistive element (e.g. a resistor). If the first voltage sourceis an AC voltage source, the first impedance element is preferably a capacitive element (e g o capacitor) or a combination of a cap a ns rive element ano a resistiveelement It is understood that the first impedance element gwes rise to a veltagedrop if a ου??ent flows through it The voltage drop is characteristic of the currentand, tot a given voltage of the first voltage source the current so turn ischaracteristic of the total impedance to which the voltage is applied {and possiblyother voltages e g The above-mentioned ground shift voltage). Therefore if ispossible to deduce characteristics like the ground shift veltage and tne groundimpedance from tne voltage drop at the first impedance element if the electeesource is a current source, the first impedance element may be omitted Thecunen? applied by the current source in general leads to a voltage drop at eachelement st flows through. For instance the gre-imd impedance leads to a voltagedrop that in turn, has an influence on the voltage at a specific point within thediagnostics Circuit. Ttss voltage may also be influenced by a non -zero ground shiftvoltage Therefore, a voltage measurement tor several measurements) within thediagnostics circuit may be used to identify the ground impedance sno.for theground shift voltage.

[GOto] The electee source may be connected to the fust fins via a first node,which ss AC grui.înded In general lhe first node ss only a reference point along anelectee connection between the electric source and the first fine However, apartfrom being connected to the first fins and the electee source, the first nude has aconnection to ground via a capacitive element (e g a capacitor^ i e st ss ACgrounded This may be realised by a conductor branching off a connectionbetween tne electric source and the first fine In this embodiment, at least one (andpossibly the only) AC- ground connection of ths diagnostics circuit is realised viathe first nude It is understood that if the diagnose signal is an AC signal anelectric current flaws through the first node ano the above-mentioned capacitiveelement to ground fl the diagnose signal te a DC signal, no current should flowthrough the first node and the capacitive element to ground If, however, asigrafinant direct current flow ss detected, (his can be attributed to a short Circuit orsome defect of the capacitive element. If ths diagnostics circuit comprises trie firstimpedance element, the first impedance element ;s connected between theelectric source and the first node [0016] Preferably, the diagnostics circuit is adapted to provide a first voltage of afirst output, which is connected to the first node. The first output may, for example,be connected to an input of a microcontroller or similar measurement device that isadapted to measure the first voltage, evaluate the result, e.g. by comparison withpredefined threshold values, and optionally output an error signal or warningsignal. The measurement device may be part of the diagnostics circuit or may beexternal to the diagnostics circuit.

[0017] According to one preferred embodiment, a second impedance element isconnected between the first node and the first line. In other words, the first node isconnected to the first line (and thus to the frame) via the second impedanceelement. Like the first impedance element, the second impedance element can beany element or circuit that has a non-zero or non-negligible impedance. It may bea resistive, capacitive or inductive element or a combination of these, e g. aparallel connection and/or series connection. If the diagnose signal is a DC signal,the second impedance element is preferably a resistive element (e g. a resistor) ora parallel connection of a resistive element and a capacitive element. If thediagnose signal is an AC signal, the second impedance element may be acapacitive element (e g. a capacitor). In case of a DC signal, the presence of sucha second impedance element may be employed to detect a short circuit betweenthe first node and ground. Under normal conditions, the total resistanceencountered by the first signal should be greater than the resistance of the secondimpedance element. However, when there is an unwanted DC connectionbetween the first node and ground, the total resistance may drop to a value belowthe resistance of the second impedance element. This, in turn, can be detectede.g. by measuring the first voltage at the first output.

[0018] In particular in those cases where the external ground connection isknown and well-defined, it may be sufficient if the diagnostics circuit is connectedto the frame by the first line only. In other situations, it may be preferable that thediagnostics circuit is connected to the frame by a second line. Although referenceis made to a single diagnostics circuit, those parts of the diagnostics circuit thatare connected to the first fine and those parts that are connected to the secondline are usually only connected with each other via the frame, i.e. there are normally no electric connections between these two parts within the diagnosticscircuit itself.

[0019] Preferably, the second line is connected to a second node, which is atleast AC grounded. In other words, the diagnostics circuit can be AC grounded atthe second node, either exclusively or in addition to the first node being ACgrounded, so that the frame can be AC grounded via the second node and,optionally, via the first node. It may be advantageous to have an AC groundconnection at both the first node and the second node to provide someredundancy. As the diagnose signal is applied by the electric source, at least apart of the corresponding signal flows through the first line, the frame and thesecond line to the second node and from there to ground. The second node is atleast AC grounded, which includes the possibility that it is also DC grounded. Thelatter configuration is in particular preferred if the diagnose signal is a DC signal.Such a DC ground connection may be realised by a resistive element alone or bya parallel connection of a capacitive element and a resistive element.

[0020] According to one embodiment, the diagnostics circuit is adapted tomeasure a difference between the first voltage and a second voltage of a secondoutput connected to the second node. Since the first node and the second nodeare connected via the first line, the frame and the second line, the first and secondline and the frame can be assumed to be intact if the difference between the firstand second voltage is below a certain threshold. Therefore, the integrity of the firstand second line and the frame can be verified by a single measurement. In thiscontext, the electric source may be a voltage source or a current source asdescribed above and the diagnose signal may be an AC signal or a DC signal.

[0021] In some embodiments, the first output is directly connected to the firstnode. In other embodiments, the first output is connected to the first node via athird impedance element. This may e g. be the case when no DC current isallowed to flow through the frame. This means that no DC path may be openbetween the frame and ground. Therefore the first, second and/or third impedanceelement have to be chosen so that no such DC path is open. Since one possibilityfor such a DC path would be through the first line, the first node and the firstoutput, either the second impedance element (if present) or the third impedanceelement has to comprise a capacitance that is only permeable for AC signals. (0022] ÄGcerding to one embodiment, the electric source comprises a firsttrarisimpedance amplifier, having a reference input connected io a second vcitscesource sad a signal input connected to the first node. By the characteristics of thetransimpedance amplifier, the voltage al signai input follows the voltage at triereference input, which is defined by the second voltage source Thetransimpedance amplifier also has an output where a voltage is provided that isproportional to the current flowing through the signal input if for instance, aconnection between the frame and the first node via the first line i$ defective, thecurrent through the signal input becomes negligible and thus also the voltage atthe output The signai input may be connected to the first node via the firstimpedance element. Further, first node is preferably connected to the first line viatrie second impedance element and is connected to the first output via the thirdimpedance element The fksi. second and third impedance element are preferablychosen so that no DC path exists between the frame and ground.

[0023j in combinaison with the above-mentioned transimpedance amplifier, thediagnostics circuit preferably comprises a second hne as described aboveAccording to one preferred embodiment, the second line is .AC grounded, i e.connected to ground via a capacitive element, it may be connected to theabovementioned second node, which is AC grounded. According to anotherembodiment, the second line is connected to a third voltage source via acapacitance element One might also say that the second line is capacitivelycoupled or AC coupled to the third voltage source, it is understood that the thirdvoltage source is an alternating voltage source The third voltage source could bea guard voltage source of a measurement circuit. of the capacitive sensor and thecapacitance element could be the capacitance between the corresponding guardelectrode and trie frame. Tire diagnostics circuit may be adapted to, in a first step,activate the third voltage source and deactivate the second voltage source, and, ma second step, activate the second voltage source In the first step, a current issupposed to flow into the signal mpul of the transimpedance amplifier, if. howeverthe connection between the third voltage source and the signal input is damaged,the current may become negligible. However, such a negligible current could alsobe due to the ground impedance being very low, wherefore the current, originatingfrom the third voltage source almost entirely flows through the ground impedance to ground However, in the second step, the third voltage source is deactivatedand by activating the second valloge source, the voltage at the signal input followsthe voilage at the reference input Under these circumstances, a non-negligiblecurrent flows through the signal input unless tne connection between the signalinput and the frame is interrupted. 10024] According tu one embodiment, the diagnostics circuit may comprise) twotransimpedance ampliners More specifically, the diagnostics circuit comprises (inaddition to the first transimpedance amplifier) a second iransimpedance amplifierhaving a reference input connected io a fourth voltage source and a signal inputconnected to a third node that ts AC grounded and connected to the second line,while the diagnostics circuit is adapted to provide (and optionally measure) a thirdvoltage ot a third output connected to the third node, in particular, the diagnosticscircuit may compose two pans that are similar or even identical in setup Thus, thereference input of the first transimpedance amplifier may be connected to thesecond voltage source while its Signal input is connected to the first node via thefirst impedance element, white the first node in turn is connected to the first fine viathe second impedance element and to the first output via the third impedanceelement Similarly. the reference input of ths second transimpedance amplifiermay be connected to thia fourth voltage source while its Signal input is connectedto the third node v-a a fourth impedance element, while the third node m turn isconnected to the second via a fifth impedance element and fo the thud output viathe sixth impedance element Ths first to sixth impedance element are preferablyadapted to provide an AC- only path between the frame and ground. Thisembodiment allows for an exact measurement of the respective resistances of thefirst line and the second line as well as the ground impedance. e g by applyingmethods described m WO 1999/069003 Al and WO 2000/048019 Al Reternng toFig 1 in WO 2000043010 A1; for example, the voltage sources 100 and 101 areequivalent to the first and second veltage source m this embodiment In thepresent case, currents are measured with the first and second transimpedanceamplifier While in WO 2000/048010 At. three capacitances are calculated, thesame principle can be applied to calculating the ground impedance, the resistanceof the first line (plus the impedance of the first impedance element and of thesecond impedance element, where applicable) and the resistance of the second line (plus the impedance of the fourth impedance element and of the fifthimpedance element, where applicable). Thus, an equation system similar to theone shown in WO 2000/048010 A1, which is based on network analysis andKirchhoffs rules, can be applied. Since the impedances of the first, second, fourthand fifth impedance element are known, these can be eliminated to calculate thewiring resistances of the first and second line. Applying the principles of WO1999/059003 A1 to the present case, the first and second voltage sources applytwo signals of same frequency but opposite polarity. The amplitude is adjusted sothat the currents are the same (with opposite polarity) and no current flows fromthe frame to ground.

[0025] The invention also provides a diagnostics circuit for a conductive frame formounting a capacitive sensor, wherein the diagnostic circuit is adapted for ACgrounding the frame and comprises an electric source adapted for beingconnected to the frame via a first line and to apply a diagnose signal to the frame,the diagnostics circuit being adapted to provide at least one quantity that dependson the diagnose signal and on a ground connection of the frame. All these termshave been explained above with respect to the inventive system and therefore willnot be explained again.

[0026] Preferred embodiments of the inventive diagnose circuit correspond tothose of the inventive system.

Brief Description of the Drawings [0027] Further details and advantages of the present invention will be apparentfrom the following detailed description of not limiting embodiments with referenceto the attached drawing, wherein:

Fig. 1 is a schematic view of a first embodiment of an inventive system;

Fig. 2 is a schematic view of a second embodiment of an inventive system;

Fig. 3 is a schematic view of a third embodiment of an inventive system;

Fig. 4 is a schematic view of a fourth embodiment of an inventive system;

Fig. 5 is a schematic view of a fifth embodiment of an inventive system;

Fig. 6 is a schematic view of a sixth embodiment of an inventive system; and

Fig. 7 is a schematic view of a seventh embodiment of an inventive system.

Description of Preferred Embodiments [0028] Fig.1 shows a first embodiment of an inventive system 1 for grounding anddiagnostics, which may be used in connection with a occupant detection system ora hands-on detection system in a vehicle. It comprises a frame 2 (represented bya frame node), which can be a seat frame or a steering wheel rim, respectively,and a diagnostics circuit 10. The frame 2, on which a capacitive sensor (notshown) is to be mounted, has an external ground connection characterised by aground impedance 3 and a ground shift voltage 4. The ground impedance 3 is theimpedance between the frame 2 and ground, and can be resistive, capacitive,and/or inductive. The ground shift voltage 4 is present inside the wiring of a vehicledue to currents flowing through the grounding connections which have a non-negligible impedance.

[0029] The frame 2 is connected to the diagnostics circuit 10 via a first line 5having a first wiring resistance 6 and being connected to a first circuit port 10.1and via a second line 7 having a second wiring resistance 8 and being connectedto a second circuit port 10.2. The diagnostics circuit 10 is adapted for a situationwhere a DC current may be injected into the frame 2, which is not always allowedor possible, and where the external ground connection is undefined (i.e. present,absent or not known).

[0030] A first electric source 11, which in this case is a first voltage source 12, isconnected via a first impedance element 13, in this case a resistor, to a first node14, which is AC grounded via a first capacitor 16. The first node 14 is alsoconnected to a first output 15 and is connected to the first line 5 via a secondimpedance element 17, which in this case is a parallel connection of a first resistor18 and a second capacitor 19. The first output 15 may be connected to an ADCinput of a micro controller (which is not shown here). The second line 7 isconnected to a second node 20, which is AC grounded via a third capacitor 21 andDC grounded via a second resistor 22. In this embodiment, the frame 2 is ACgrounded via first and second capacitors 16, 19 as well as via third capacitor 21whereby a redundant AC grounding is provided. It would be possible to omit e.g.first and second capacitors 16, 19. The first voltage source 12 may be an AC voltage source or a DC voltage source. The resistors 18, 22 may be omitted if it isan AC voltage source.

[0031] In order to perform diagnostics, the first voltage source 12 applies andiagnose signal with at least two different DC voltage levels. For each of thesevoltage levels, the voltage at the first output 15 is measured (either by thediagnostics circuit 10 or by some external measurement device). This voltage isdefined by the known voltage level of the first voltage source 12, the first andsecond impedance element 13, 17, the unknown first and second wiring resistance6, 8, the ground impedance 3 and the unknown ground shift voltage 4. Byevaluating the results of the two measurements, the equivalent source voltage andequivalent source resistance of a series connected voltage source and resistance(Thévenin equivalent circuit), connected between ground and the first circuit port10.1 can be calculated. The calculated source resistance comprises a contributionfrom the first wiring resistance 6, whereby the remaining resistance between theframe 2 and ground must be smaller than the calculated source resistance. Thecalculated source voltage can in addition be used to diagnose a short circuitbetween the first circuit board 10.1 or frame 2 to an external voltage source whichhas a larger voltage level than the ground shift voltage 4, for example batteryvoltage.

[0032] Measurement of the voltage at the first output 15 also allows to determinea total impedance and a total resistance. To this respect, the first resistor 18allows to detect a short circuit between the first node 14 and ground, as the totalmeasured resistance must always be higher than the resistance of the first resistor18.

[0033] Alternatively, the first voltage source 12 can also supply an AC voltagesignal. Preferably, the frequency of the AC voltage signal is different from themeasurement frequency of the capacitive measurement system (and preferablytheir respective harmonics are different). In general, it may be sufficient if there isno significant correlation between the diagnose signal and the signal used formeasurement. This allows the parallel operation of the capacitive measurementsystem and the diagnostics circuit 10 without creating a disturbance between thetwo. If the two frequencies are the same, both measurements have to be performed alternatlngiy Similar requirements apply io ottier AC voilage sourcesand AC current sources referred to below [0034] By generating an AC signal of known characteristics. the influence of theground shift voltage 4 on the diagnostics measurement result is eliminated,provided that mere is no correlation between the signals generated by first voltagesource 12 and the ground shift voltage 4 In order to measure the equivalentsource resistance above, only one AC voltage level generated by the first voltagesource 12 and one measurement un output 15 is sursoient.

[0035] Fig. 2 shows a secund embodiment of an inventive system 1.. which islargely identical to ths embodiment shown in fig 1 and therefore will not beexplained in detail again However, the first impedance element 13 has beenomitted and the first electric source 11 is a DC or AG current source 23 of knownoutput current level. All measurements described for the system 1 in Fig. 1 can beapplied m the same manner to the system 1 in Fig 2 by setting the appropriatecurrent level and AC or DC operation mode of current source 23 [0036] Fig 3 shows a third embodiment of an inventive system 1, which is largelyidentical to the embodiment shown in fig 1 However the secund node 20 isconnected to a secund output 24 which may be e g. connected to the samemicrocontroller as fire first output IS According to this embodiment no sequentialDC measurements are required. The first voltage source 12 applies a DC voltage,and the voltage difference between tne first and second output 15. 24 is measuredand evaluated if the voltage difference is below a predetermined threshold, theconnections between the circuit ports 10.1, 10 2 and frame .2 are deemed to beintact. Similar to the second embodrererft in Fig.2. the first voltage source 12 couldbe replaced by a current source 23 and first impedance element 13 could beomitted, [0037] Flg<4 shows a fourth embodiment of an inventive system 1, which qgaln islargely similar to the embodiment shown in fig 1. However, this embodiment isdesigned for a situation where a no DC current may be injected into the frame 2.and where the external connection between frame and ground is undefined[present, absent or nut known} in order to prevent any DC currant flowingbetween the frame 2 and ground, the first and second impedance element 13. 17are either purely capacitive or a series connection oi a capacitance and a resistance. The same applies to a third impedance element by which the firstoutput 15 is connected to the first node 14. Also, the DC ground connection via thesecond resistor 22 has been eliminated. In this embodiment, the first voltagesource 12 of course has to be an AC voltage source. In this embodiment, the firstvoltage source 12 could be replaced by a current source and the first impedanceelement 13 could be omitted. The AC grounding and diagnostics is similar to theembodiment of Fig. 1, except that an AC only path exists between the frame 2 andground. Here, too, it would be possible to connect the second node 20 to thesecond output 24 in order to perform a difference measurement of two voltages.

[0038] Each of the embodiments shown in figs. 1 to 4 can be simplified for thecase that the frame 2 is already connected in a defined way to ground externally,via the impedance 3. In this case, the third capacitor 21 and the second resistor 22can be omitted.

[0039] Fig.5 shows another embodiment of an inventive system 1 for a situationwhere a DC current is not allowed to be injected into the frame 2, and where theexternal connection between frame 2 and ground is undefined (present, absent ornot known). Here again, the first, second and third impedance element 13, 17, 25are chosen so that an AC only path exists between the first circuit port 10.1 andground. The diagnostics circuit 10 comprises a first transimpedance amplifier 26having a signal input 26.1, a reference input 26.2 and an output 26.3. The signalinput 26.1 is connected via the first impedance element 13 to the first node 14while the reference input 26.2 is connected to a second voltage source 27. Thesecond line 7 is connected to a third voltage source 29 via a fourth capacitor 28.Optionally, the third voltage source 29 can be the guard voltage source of acapacitive loading mode measurement circuit, and the fourth capacitor 28 can bethe capacitance between a guard electrode and the frame 2. In this case, secondwiring resistance 8 has 0 Ohms.

[0040] In a first step, the third voltage source 29 generates an AC voltage, andthe second voltage source 27 is deactivated. A current flows through the fourthcapacitor 28, the second wiring resistance 8, the first wiring resistance 6, thesecond impedance element 17, and the first impedance element 13 into the signalinput 26.1 of the first transimpedance amplifier 26 and generates a correspondingoutput voltage on the output 26.3. If there is a break in the connection between the first circuit port 10.1 and frame 2, no current flows into the first transimpedanceamplifier 26 and there is no output voltage on the output 26.3, which can be usedas a criteria to output an error signal or the like. Another cause for a negligiblecurrent into the signal input 26.1 may however also be that the ground impedance3 is so low that substantially all the current provided by third voltage source 29flows into ground. Therefore, a second measurement step is performed. Thesecond source 27 is switched on and generates an AC signal with a frequencywhich is preferably different from the capacitive measurement frequency (andthereby from the signal of the third voltage source 29, if this is the guard voltagesource). Thereby, the first transimpedance amplifier 26 helps keeping the frame 2at AC ground for the capacitive measurement frequency. Alternatively, if thefrequency is the same, the third voltage source 29 must be temporarily switchedoff, which also interrupts the capacitive measurement. Since the voltage at thesignal input 26.1 follows the voltage of the reference input 26.2, it is substantiallydefined by the second voltage source 27. Therefore, the current flowing throughthe reference input 26.2, and thereby the voltage on the output 26.3, is indicativeof the current flowing through the first wiring resistance 6. Therefore, a potentialinterruption can be inferred from the voltage level of output 26.3. The first output15 is optional in this embodiment and allows the detection of a direct short of thefirst capacitor 16, as in the embodiments of fig. 1 to 4.

[0041] The embodiment of Fig.5 can be simplified for the case where the frame 2is already connected in a defined way to ground externally via the groundimpedance 3. In this case, the fourth capacitor 28 and third voltage source 29 canbe omitted.

[0042] Fig.6 shows an embodiment which allows the exact measurement of thewiring resistances 6 and 8, and the ground impedance 3. Regarding thecomponents connected to the first line 5, this embodiment is identical to theembodiment shown in fig. 5. However, a signal input 36.1 of a secondtransimpedance amplifier 36 is connected via a fourth impedance element 30 to athird node 32 which in turn is connected via a fifth impedance element 35 to thesecond line 7. Like the first node 14, the second node 32 is AC grounded via a fifthcapacitor 34 and is connected to a third output 33 via a sixth impedance element38. The reference input 36.2 of the second transimpedance amplifier 36 is connected to a fourth voltage source 37. The second transimpedance amplifier 36and the fourth voltage source may be considered as parts of a second electricsource 31. The first and the fourth voltage source 27, 37 each generate ACvoltages with a frequency which is different from the capacitive measurementfrequency. This allows the transimpedance amplifiers 26, 36 to help to keep thecircuit ports 10.1, 10.2 at AC ground at the capacitive measurement frequency. Ifany of the frequencies of voltage sources 27, 37 is the same as the capacitivemeasurement frequency, the diagnostics and capacitive measurement cannot beperformed at the same time.

[0043] By setting AC voltage sources 27, 37 to different levels, the wiringresistances 6, 8 and the ground impedance 3 can be determined by applying themethods described in WO 1999/059003 A1 or WO 2000/048010 A1, respectively.

[0044] Fig.7 shows a seventh embodiment of an inventive system 1, which maybe regarded as a combination of the embodiments of Fig. 1 and Fig.5. The secondvoltage source 27 generates an AC voltage on the signal input 26.1 oftransimpedance amplifier 26. Similar to the embodiment in Fig.1, the outputvoltage of output 26.3 is indicative of the equivalent series impedance of aThévenin equivalent circuit as described for the embodiment in Fig.1.

List of Reference Symbols 1 system 2 frame 3 ground impedance 4 ground shift voltage 5,7 line 6, 8 wiring resistance 10 diagnostics circuit 10.1,10.2 circuit port 11,31 electric source 12, 27, 29, 37 voltage source 13, 17, 25, 30, 35, 38 impedance element 14, 20, 32 node 15,24,33 output 16,19,21,28,34 capacitor 18, 22 resistor 23 current source 26, 36 transimpedance amplifier 26.1,36.1 signal input 26.2, 36.2 reference input 26.3, 36.3 output

Claims

A grounding and diagnostic system (1) comprising: - a conductive frame (2) for mounting a capacitive sensor, and - a diagnostic circuit (10) through which the frame (2) is AC grounded and which has an electrical circuit Source (11) connected to the frame (2) via a first line (5) and configured to apply a diagnostic signal to the frame (2), the diagnostic circuit (10) being configured to provide at least one size depends on the diagnostic signal and a ground connection of the frame (2).

A system according to any one of the preceding claims, characterized in that the diagnostic circuit (10) is arranged to provide at least one quantity representing the determination of a grounding impedance (3) and / or a ground displacement voltage (4), the frame (2) with respect to ground has enabled.

3. System according to one of the preceding claims, characterized in that the electrical source (11) is a first voltage source (12) or a current source (23).

4. System according to one of the preceding claims, characterized in that the electrical source (11) with the first line (5) via a first impedance element (13) is connected.

A system according to any one of the preceding claims, characterized in that the electrical source (11) is connected to the first line (5) via a first node (14) which is AC grounded.

A system according to any one of the preceding claims, characterized in that the diagnostic circuit (10) is arranged to provide a first voltage of a first output (15) connected to the first node (14).

A system according to any one of the preceding claims, characterized in that a second impedance element (17) is connected between the first node (14) and the first line (5).

8. System according to any one of the preceding claims, characterized in that the diagnostic circuit (10) to the frame (2) via a second line (8) is connected.

A system according to any one of the preceding claims, characterized in that the second line (7) is connected to a second node (20) which is at least AC grounded.

A system according to any one of the preceding claims, characterized in that the diagnostic circuit (10) is arranged to measure a difference between the first voltage and a second voltage of a second output (24) connected to the second node (20).

A system according to any one of the preceding claims, characterized in that the first output (15) is connected to the first node (14) via a third impedance element (25).

A system according to any one of the preceding claims, characterized in that the electrical source (11) comprises a first transimpedance amplifier (26) having a reference input (26.2) connected to a second voltage source (27) and a signal input (26.1), which is connected to the first node (14).

A system according to claim 12, characterized in that the second line (7) is connected to a third voltage source (29) via a capacitive element (21).

A system according to claim 12, characterized in that the diagnostic circuit (10) comprises a second impedance amplifier (36) having a reference input (36.2) connected to a fourth voltage source (37) and a signal input (36.1) connected to a third node (32) which is AC grounded and connected to the second line (8) and that the diagnostic circuit (10) is configured to provide a third voltage of a third output (33) connected to the third node (32) ,

A diagnostic circuit (10) for a conductive frame (2) for mounting a capacitive sensor, wherein the AC grounding diagnostic circuit (10) of the frame (2) is configured to include an electrical source (11) for connection to the frame (2) is configured via a first line (5) and to apply a diagnostic signal to the frame (2), the diagnostic circuit (10) being configured to provide at least one size dependent on the diagnostic signal and a ground connection of the frame (2) ,