LU100737B1

LU100737B1 - Diagnostics for Capacitive Sensor

Info

Publication number: LU100737B1
Application number: LU100737A
Authority: LU
Inventors: Laurent Lamesch
Original assignee: Iee Sa
Priority date: 2018-03-20
Filing date: 2018-03-20
Publication date: 2019-10-01

Abstract

A capacitance measurement circuit (100) for determining a complex electric current of at least one capacitive sense-guard sensor comprises a periodic signal voltage source (12), wherein each guard electrode is electrically connectable to the signal voltage source (12), a current measurement circuit (10) comprising at least one measurement channel (26) to determine a measurement current through a sense electrode, and at least one diagnostics channel (34) to determine, with reference to an AC ground potential (16), a diagnostic current flowing through a guard electrode and a sense electrode of a capacitive sensor, and a remotely controllable switching unit (42), which in a measurement switching state is configured to electrically connect the sense electrode of the capacitive sensor to the measurement channel (26), and in a diagnostic switching state is configured to electrically connect the sense electrode to the diagnostics channel (34).

Description

P-IEE-492/LU 1 LU100737

Diagnostics for Capacitive Sensor

Technical field [0001] The invention relates to a capacitance measurement circuit fordetermining a complex electric current of at least one capacitive sensor, to acapacitive sensing system comprising such capacitance measurement circuit, to amethod of operating such capacitive sensing systems with regard to capacitivesensor diagnosis, and to a software module for controlling automatic execution ofsuch method.

Background of the Invention [0002] Capacitive measurement and/or detection systems have a wide range ofapplications, and are among others widely used for the detection of the presenceand/or the position of a conductive body in the vicinity of an electrode of thesystem. A capacitive sensor, called by some electric field sensor or proximitysensor, designates a sensor, which generates a signal responsive to the influenceof what is being sensed (a person, a part of a person’s body, a pet, an object, etc.)upon an electric field. A capacitive sensor generally comprises at least oneantenna electrode, to which is applied an oscillating electric signal and whichthereupon establishes an electric field into a region of space proximate to theantenna electrode, while the sensor is operating. The sensor comprises at leastone sensing electrode - which could comprise the one or more antenna electrodesthemselves - at which the influence of an object or living being on the electric fieldis detected.

[0003] The different capacitive sensing mechanisms are, for instance, explainedin the technical paper entitled “Electric Field Sensing for Graphical Interfaces” byJ. R. Smith et al., published in IEEE Computer Graphics and Applications, 18(3):54-60, 1998. The paper describes the concept of electric field sensing as used formaking non-contact three-dimensional position measurements, and moreparticularly for sensing the position of a human hand for purposes of providingthree-dimensional positional inputs to a computer. Within the general concept ofcapacitive sensing, the author distinguishes between distinct mechanisms herefers to as "loading mode", "shunt mode", and "transmit mode", which correspond

P-IEE-492/LU 2 LU100737 to various possible electric current pathways. In the "loading mode", an oscillatingvoltage signal is applied to a transmit electrode, which builds up an oscillatingelectric field to ground. The object to be sensed modifies the capacitance betweenthe transmit electrode and ground. In the “shunt mode”, which is alternativelyreferred to as “coupling mode”, an oscillating voltage signal is applied to thetransmit electrode, building up an electric field to a receive electrode, and thedisplacement current induced at the receive electrode is measured, whereby thedisplacement current may be modified by the body being sensed. In the “transmitmode”, the transmit electrode is put in contact with the user's body, which thenbecomes a transmitter relative to a receiver, either by direct electrical connectionor via capacitive coupling.

[0004] The capacitive coupling is generally determined by applying an alternatingvoltage signal to a capacitive antenna electrode and by measuring the currentflowing from the antenna electrode either towards ground (in the loading mode) orinto the second electrode (receiving electrode) in case of the coupling mode. Thiscurrent is usually measured by means of a transimpedance amplifier, which isconnected to the sensing electrode and which converts a current flowing into saidsensing electrode into a voltage, which is proportional to the current flowing intothe antenna electrode.

[0005] Some capacitive sensors are designed as sense-only capacitive sensorshaving a single sense electrode. Also, quite often capacitive sensors are used thatcomprise a sense electrode and a guard electrode that are arranged close to eachother and are mutually insulated from each other. This technique of “guarding” iswell known in the art and is frequently used for intentionally masking, and thusshaping, a sensitivity regime of a capacitive sensor. To this end, the guardelectrode is kept at the same electric AC potential as the sense electrode. As aresult, a space between the sense electrode and the guard electrode is free of anelectric field, and the guard-sense capacitive sensor is insensitive in a directionbetween the sense electrode and the guard electrode.

[0006] By way of example, patent document US 8,354,936 B2 describes acapacitive passenger detector for a vehicle. The capacitive passenger detectorincludes a main electrode, a sub-electrode and a guard electrode. The mainelectrode and the sub-electrode are separated apart from each other, and

P-1EE-492/LU 3 LU100737 disposed in a seat of a vehicle. The guard electrode is disposed between the mainelectrode and a body of the vehicle, and separated apart from the main electrode.A sensitive characteristic measurement unit is configured for applying analternating voltage signal to the main electrode, the sub-electrode and the guardelectrode selectively or totally and for converting a current generated in the mainelectrode, the sub-electrode and the guard electrode to a voltage, respectively.The capacitive passenger detector further comprises a controller that defines acurrent flowing through the guard electrode to be a reference current when avoltage of the main electrode and a voltage of the guard electrode have the samepotential. The controller defines a current flowing direction of the current flowingthrough the guard electrode to be a negative direction when the voltage of themain electrode is higher than the voltage of the guard electrode. The controllerdefines the current flowing direction of the current flowing through the guardelectrode to be a positive direction when the voltage of the main electrode is lowerthan the voltage of the guard electrode. The controller corrects the voltage of themain electrode based on the current flowing through the guard electrode so that acorrected voltage of the main electrode is set to be a passenger determinationdata. Even when a potential difference is generated between the main electrodeand the guard electrode, the controller detects the capacitance of the passengercorrectly.

[0007] Capacitive sensing systems which are used in the control of airbagsystems or other safety-related applications may be considered as safety-relevantsystem components. It may thus be necessary to monitor the good functioning ofthe different components of the sensor (sensing electrode and/or guard electrode)in order to rule out a false reading by the capacitive occupancy or proximitydetection system.

[0008] It has been proposed in the prior art to furnish capacitive measurementcircuits with diagnostic means for detecting a capacitive sensor interruption, inparticular a guard electrode interruption.

[0009] For instance, international application WO 2017/129552 A1 describes acapacitance measurement circuit for determining a sense current of a capacitivesensor with a sense electrode and a guard electrode. The capacitancemeasurement circuit comprises a periodic signal voltage source, a sense current

P-IEE-492/LU 4 LU100737 measurement circuit configured for determining the sense current with reference toa reference voltage and at least one remotely controllable switch member. Theconfiguration is such that in the first switching state, the at least one switchmember electrically connects the sense current measurement circuit to theperiodic measurement voltage for providing a first reference voltage, and in thesecond switching state, the at least one switch member electrically connects thesense current measurement circuit to a second reference voltage that is differentfrom the first reference voltage. By intentionally changing the reference voltageused for determining the sense current by connecting the sense currentmeasurement circuit to the second reference voltage that is different from the firstreference voltage, a signal can be generated by the sense current measurementcircuit that can be indicative of an electrical interruption, wherein the interruptionmay include any interruption of electrical connections between the respectivesense and guard cabling and connecting members.

Object of the invention [0010] It is therefore an object of the present invention to provide an improvedand as comprehensive as possible diagnostic concept for a capacitive sensingsystem, in particular for use in automotive applications, that is particularly suitablefor multichannel capacitive sensing systems and that can be realized with lowhardware effort and at moderate costs.

General Description of the Invention [0011] In one aspect of the present invention, the object is achieved by acapacitance measurement circuit for determining a complex electric current of atleast one capacitive sensor including at least one electrically conductive senseelectrode and at least one electrically conductive guard electrode that arearranged close to each other. The at least one electrically conductive senseelectrode and at least one electrically conductive guard electrode may e.g. bemutually galvanically separated from each other. However this is not arequirement. For example, a resistive connection between sense and guardelectrodes can be used for diagnostics purpose.

P-IEE-492/LU 5 LU100737 [0012] The capacitance measurement circuit comprises at least one periodicsignal voltage source, a current measurement circuit and a remotely controllableswitching unit [0013] The at least one periodic signal voltage source is configured for providingan alternating measurement voltage at an output port, wherein each guardelectrode is electrically connectable to the output port for receiving the periodicmeasurement voltage.

[0014] The current measurement circuit comprises at least one measurementchannel and at least one diagnostics channel. The at least one measurementchannel includes a measurement current-to-voltage converter that is configured todetermine, with reference to the measurement voltage, a measurement currentthrough a sense electrode that is indicative of a position of an object relative to thecapacitive sensor. The at least one diagnostics channel includes a diagnosticscurrent-to-voltage converter that is configured to determine, with reference to anAC ground potential, a diagnostic current flowing through a guard electrode and asense electrode of a capacitive sensor that is connected to the diagnosticscurrent-to-voltage converter.

[0015] The remotely controllable switching unit comprises, for each capacitivesensor, a plurality of operatively coupled switching members. In a measurementswitching state with regard to the capacitive sensor, the switching unit isconfigured to electrically connect the sense electrode of the capacitive sensor to asignal input port of a measurement current-to-voltage converter. In a diagnosticswitching state with regard to the capacitive sensor, the switching unit isconfigured to electrically connect the sense electrode to a signal input port of adiagnostics current-to-voltage converter. In the diagnostic switching state, thesense electrode is unconnected to the measurement current-to-voltage converter.In the measurement switching state, the sense electrode is unconnected to thediagnostics current-to-voltage converter.

[0016] One advantage of the proposed capacitance measurement circuit is thatmeans are provided to diagnose the capacitive sensor and sensor wiring bymeasuring the complex sense-to-guard impedance of the capacitive sensor.

P-IEE-492/LU 6 LU100737 [0017] Another advantage is that an AC ground connection of the capacitivesensor can be diagnosed by the proposed capacitance measurement circuit whenthe sense electrode is connected to the signal input port of a diagnostics current-to-voltage converter.

[0018] When a capacitive sensor is not used for determining a measurementcurrent through its sense electrode that is indicative of a position of an objectrelative to the capacitive sensor, an AC potential can be kept at or close to ACground potential, which can be advantageous under certain circumstances.

[0019] The phrase “being configured to”, as used in this application, shall inparticular be understood as being specifically programmed, laid out, furnished orarranged. The phrase “operatively coupled switching members”, as used in thisapplication, shall in particular be understood as switching members that areconfigured to essentially change their switching states simultaneously.

[0020] Preferably, the at least one capacitive sensor is configured for beingoperated in loading mode.

[0021] The proposed capacitance measurement circuit is advantageouslyapplicable particularly in vehicles. The term “vehicle”, as used in this application,shall particularly be understood to encompass passenger cars, trucks, tractor unitsand buses.

[0022] Preferably, the measurement current-to-voltage converter and/or thediagnostics current-to-voltage converter comprise a transimpedanceamplifier (TIA) whose function is to convert a current provided at a signal input portinto an output voltage that is proportional to the determined current. A TIA isconfigured to convert the input current with reference to a reference voltage that isprovided to a reference input port. Preferably, the transimpedance amplifier isbased on one or more operational amplifiers.

[0023] In preferred embodiments, the capacitance measurement circuit isconfigured to generate an output signal that is indicative of a sensor interruption ifthe magnitude of a determined diagnostic current flowing through a senseelectrode and a guard electrode of a capacitive sensor that is connected to thediagnostics current-to-voltage converter is less than a predefined threshold value.

P-IEE-492/LU 7 LU100737

The output signal can beneficially be transferred, for instance to a higher-rankingcontrol unit of a vehicle, for initiating further action.

[0024] Preferably, for each capacitive sensor, a demultiplexer member isprovided in the switching unit that is configured, in a measurement switching statewith regard to a respective capacitive sensor, to keep a terminal of a switchingmember for electrically connecting the sense electrode to a signal input port of adiagnostics current-to-voltage converter at the converter end to guard potential ofthe respective capacitive sensor. By that, terminals of the switching member,which is in an open switching position when the capacitive sensor is in ameasurement switching state, can be kept at the electric potential of the guardelectrode. Thus, a parasitic capacitance of the switching member can virtually bereduced to zero, and a measurement error due to parasitic capacitance of theswitching member can be minimized.

[0025] In preferred embodiments of the capacitance measurement circuit, foreach capacitive sensor, a demultiplexer member is provided in the switching unitthat is configured to, in a measurement switching state with regard to a respectivecapacitive sensor, keep a terminal of a switching member for electricallyconnecting the sense electrode to a signal input port of the diagnostics current-to-voltage converter at the converter end at guard potential of the respectivecapacitive sensor.

[0026] Further, the demultiplexer member provided in the switching unit isconfigured, for each capacitive sensor whose sense electrode is unconnected to ameasurement current-to-voltage converter, to either connect a terminal of aswitching member for electrically connecting the sense electrode to an input portof a diagnostics current-to-voltage converter at the converter end to guardpotential of the capacitive sensor, or to connect the terminal of the switchingmember for electrically connecting the sense electrode to the signal input port ofthe diagnostics current-to-voltage converter at the converter end to AC groundpotential.

[0027] This embodiment is particularly beneficial in multichannel applications,which include more than two capacitive sensors and may comprise only onediagnostics current-to-voltage converter and one measurement current-to-voltageconverter for cost reasons. In this case, time of operational availability can be

P-IEE-492/LU 8 LU100737 increased, as the capacitance measurement circuit can enable to set an ACpotential of the sense electrodes of the capacitive sensors to AC ground potentialeven if a respective capacitive sensor is not connected to either the measurementcurrent-to-voltage converter or to the diagnostics current-to-voltage converter.

[0028] Preferably, the current measurement circuit comprises, for each capacitivesensor, a diagnostics output port that is electrically connected to a sense electrodeof the capacitive sensor. With a capacitive sensor in the diagnostic switching state,the diagnostics output port can provide a complex electrical signal for furtherdiagnosis.

[0029] In such embodiments, the current measurement circuit may furthercomprise means, for each capacitive sensor, to keep the diagnostics output port atguard potential of the respective capacitive sensor, which can beneficiallyeliminate a systematic error of measurement due to the diagnostics output portbeing connected to the sense electrode of the capacitive sensor.

[0030] In preferred embodiments of the capacitance measurement circuit, theremotely controllable switching unit forms part of a microcontroller, by which areliable and simple remote control of the remotely controllable switching unit and,in this way, a reliable diagnosis of a capacitive sensor can be enabled.Microcontrollers that are suitably equipped and include, for instance, a processorunit, a digital memory unit, a microcontroller system clock, a demultiplexer unit andanalog-to-digital converters are nowadays readily available in many variations.

[0031] Preferably, the remotely controllable switching unit is configured toperiodically switch, for each capacitive sensor and in a manner that is coordinatedamong the capacitive sensors, between a measurement switching state and adiagnostic switching state. In this way, reliable operation of the capacitancemeasurement circuit can be accomplished.

[0032] In another aspect of the invention, a capacitive sensing system isprovided. The capacitive sensing system includes an embodiment of thecapacitance measurement circuit disclosed herein, a switch remote control unitand a plurality of capacitive sensors.

[0033] The switch remote control unit is configured for remotely controlling theremotely controllable switching unit. Each capacitive sensor of the plurality of

P-IEE-492/LU 9 LU100737 capacitive sensors includes at least one electrically conductive sense electrodeand at least one electrically conductive guard electrode that are arranged close toeach other. As stated above, the at least one electrically conductive senseelectrode and at least one electrically conductive guard electrode may be mutuallygalvanically separated from each other, but this is not a requirement. For example,a resistive connection between sense and guard electrodes can be used fordiagnostics purpose.

[0034] The capacitance measurement circuit comprises one periodic signalvoltage source that is configured for providing an alternating measurement voltageat an output port, wherein each guard electrode is electrically connected to theoutput port for receiving the periodic measurement voltage.

[0035] The current measurement circuit includes one measurement channel withone measurement current-to-voltage converter that is configured to determine, oneat a time and with reference to the measurement voltage, a measurement currentthrough a sense electrode of one of the plurality of capacitive sensors that isindicative of a position of an object relative to the respective capacitive sensor.

[0036] The current measurement circuit further comprises a plurality ofdiagnostics channels, each one being configured to determine, with reference toan AC ground potential, a diagnostic current flowing through a guard electrode anda sense electrode of a different one of the plurality of capacitive sensors.

[0037] In this way, a capacitive sensing system with reliable operation and a highpercentage of operational availability can be accomplished, including the benefitsthat have been described beforehand in context with the disclosed capacitancemeasurement circuit.

[0038] Preferably, the switch remote control unit forms part of a microcontroller,by which a reliable and simple remote control of the remotely controllableswitching unit and, in this way, a reliable diagnosis of a capacitive sensor can beenabled. Most preferred, both the switch remote control unit and the remotelycontroUaWe switching unit form part of the same microcontroller. In that way, shortcontrol paths between the switch remote control unit and the remotely controllableswitching unit can be achieved, which are less susceptible to electromagneticinterference.

P-IEE-492/LU 10 LU100737 [0039] In the preferred embodiments, the capacitive sensing system furtherincludes a signal processing unit that is configured for processing at least one outof an output signal of the measurement current-to-voltage converter with referenceto the periodic measurement voltage and an output signal of the diagnosticscurrent-to-voltage converter with reference to the periodic measurement voltage.In this way, an improved signal-to-noise ratio (SNR) can be achieved for the outputsignal of the measurement current-to-voltage converter and/or the output signal ofthe diagnostics current-to-voltage converter.

[0040] Further improvement of the SNR can be achieved if the capacitive sensingsystem includes a demodulation circuit that is configured for demodulating at leastone out of an input signal of the measurement current-to-voltage converter withreference to the periodic measurement voltage and an input signal of thediagnostics current-to-voltage converter with reference to the periodicmeasurement voltage.

[0041] In case that the demodulation circuit is configured for demodulating theinput signal of the measurement current-to-voltage converter, the demodulationcircuit is preferably connected to the signal input port of the measurement current-to-voltage converter and the remotely controllable switching unit is accordinglyconfigured to electrically connect the sense electrode of the capacitive sensor tothe demodulation circuit.

[0042] In yet another aspect of the invention, a method of operating thecapacitive sensing system disclosed herein with regard to capacitive sensordiagnosis is provided.

[0043] The method at least comprises steps of controlling the remotely controllable switching unit to electrically disconnectthe sense electrode of a capacitive sensor to be diagnosed from the signalinput port of the measurement current-to-voltage converter, controlling the remotely controllable switching unit to electrically connect thesense electrode of the capacitive sensor to be diagnosed to the signal inputport of the diagnostics current-to-voltage converter, controlling the remotely controllable switching unit to electrically connect thesense electrode of another capacitive sensor to the signal input port of the

P-IEE-492/LU 11 LU100737 measurement current-to-voltage converter and to disconnect the senseelectrode of this capacitive sensor from guard potential or from AC groundpotential, controlling the remotely controllable switching unit to electrically connect thesense electrodes of the remainder of the plurality of capacitive sensors to ACground potential, determining a sense current value through the sense electrode of thecapacitive sensor to be diagnosed by the diagnostics current-to-voltageconverter, comparing the determined sense current value with at least onepredetermined threshold value, generating an output signal that is indicative of a sensor interruption if thedetermined sense current value is smaller than the predetermined thresholdvalue.

[0044] In preferred embodiments of the method, the steps are executed andrepeated in a periodic manner after swapping to a next capacitive sensor of theplurality of capacitive sensors to be diagnosed for each cycle of steps until allcapacitive sensors of the plurality of capacitive sensors are diagnosed.

[0045] In yet another aspect of the invention, a software module for controllingautomatic execution of steps of an embodiment of the method disclosed herein isprovided.

[0046] The method steps to be conducted are converted into a program code ofthe software module, wherein the program code is implementable in a digitalmemory unit of the capacitive sensing system or of a separate control unit and isexecutable by a processor unit of the capacitive sensing system or of a separatecontrol unit.

[0047] The software module can enable a robust and reliable execution of themethod and can allow for a fast modification of method steps.

[0048] These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

[0049] It shall be pointed out that the features and measures detailed individuallyin the preceding description can be combined with one another in any technically

P-IEE-492/LU 12 LU100737 meaningful manner and show further embodiments of the invention. Thedescription characterizes and specifies the invention in particular in connectionwith the figures.

Brief Description of the Drawings [0050] 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 illustrates an electric equivalent circuit diagram of a possibleembodiment of a single-channel capacitance measurement circuit inaccordance with the invention,

Fig. 2 illustrates an electric equivalent circuit diagram of an alternativeembodiment of a single-channel capacitance measurement circuit inaccordance with the invention,

Fig. 3 illustrates an electric equivalent circuit diagram of a possibleembodiment of a multichannel capacitive sensing system in accordancewith the invention,

Figs. 4-6 illustrate electric equivalent circuit diagrams of alternative possibleembodiments of a multichannel capacitive sensing system inaccordance with the invention, and

Fig. 7 is a flow chart of a method of operating the capacitive sensing systempursuant to Fig. 6.

Description of Preferred Embodiments [0051] In the different figures, the same parts are always provided with the samereference numerals. Therefore, these parts are usually described only once.

[0052] Fig. 1 schematically illustrates an electric equivalent circuit diagram of apossible embodiment of a single-channel capacitance measurement circuit 100 inaccordance with the invention. The capacitance measurement circuit 100 servesfor determining a complex electric current of a single capacitive sensor configuredin loading mode that includes an electrically conductive sense electrode and anelectrically conductive guard electrode that are arranged close to each other, and

P-IEE-492/LU 13 LU100737 that are mutually galvanically separated from each other. The sense electrode anda respective sense wiring resistance are represented in Fig. 1 by a sense node 18,and the guard electrode and a respective guard wiring resistance are representedby a guard node 20. A sense-to-guard impedance 22, which is substantially ofcapacitive nature, is electrically connected between the sense node 18 and theguard node 20. In this specific embodiment, the sense-to-guard impedance 22may have a capacitance of about 1 nF, but capacitive sensors with a higher orlower sense-to-guard capacitance are also contemplated.

[0053] The single-channel capacitance measurement circuit 100 includes aperiodic, namely sinusoidal, signal voltage source 12 that is configured forproviding an alternating measurement voltage at an output port 14 with respect toan AC ground potential 16. The guard electrode is electrically connected to theperiodic signal voltage source 12 for receiving the periodic measurement voltage,from where it is transferred to the guard node 20.

[0054] The single-channel capacitance measurement circuit 100 includes acurrent measurement circuit 10 that comprises a measurement channel 26including a measurement current-to-voltage converter 28 and a diagnosticschannel 34 including a diagnostics current-to-voltage converter 36. Both themeasurement current-to-voltage converter 28 and the diagnostics current-to-voltage converter 36 comprise a transimpedance amplifier (TIA), whose function isto convert a complex electric current provided at a signal input port 30, 38 into anoutput voltage that is proportional to the determined sense current. Both the TIAsare configured to convert the sense current with reference to a reference voltagethat is provided to a reference input port 32, 40. The reference input port 32 of themeasurement current-to-voltage converter 28 is electrically connected to theoutput port 14 of the signal voltage source 12. The reference input port 40 of thediagnostics current-to-voltage converter 36 is electrically connected to AC groundpotential 16.

[0055] Thus, the measurement current-to-voltage converter 28 is configured todetermine, with reference to the measurement voltage, a measurement currentthrough the sense electrode that is indicative of a position of an object relative tothe capacitive sensor. An object approaching the sense electrode is represented inthe electric equivalent circuit diagram of Fig. 1 by an unknown impedance 24 that

P-IEE-492/LU 14 LU100737 is connected to AC ground potential 16, which for instance may be a vehicleground potential. If the grounded object approaches the sense electrode, theunknown impedance 24 changes in that at least its capacitive portion increases,and the measurement current flowing between the sense electrode and groundpotential and, by that, an amplitude of the TIA output voltage signal is increased,indicating a closer proximity of the object to the capacitive sensor.

[0056] The diagnostics current-to-voltage converter 36 is configured to determine,with reference to AC ground potential 16, a diagnostic current flowing through theguard electrode and the sense electrode of a capacitive sensor when connected tothe diagnostics current-to-voltage converter 36.

[0057] Furthermore, the capacitance measurement circuit 100 includes aswitching unit 42 that is remotely controllable by a switch remote control unit (notshown). The switching unit 42 comprises a plurality of two operatively coupledswitching members 44, 46. The switching members 44, 46 are operatively coupledin that the one switching member 44 changes a switching position from open toclosed while the other switching member 46 changes the switching position fromclosed to open and vice versa.

[0058] A first one 44 of the switching members 44, 46 is positioned between thesense node 18 and the signal input port 30 of the measurement current-to-voltageconverter 28. A second one 46 of the switching members 44, 46 is positionedbetween the sense node 18 and the signal input port 38 of the diagnostics current-to-voltage converter 36.

[0059] In a measurement switching state with regard to the capacitive sensor, theswitching unit 42 is configured to electrically connect the sense electrode of thecapacitive sensor to the signal input port 30 of the measurement current-to-voltageconverter 28 by controlling the first switching member 44 into a closed switchingstate. At the same time, the second switching member 46 is controlled into anopen switching state. The measurement current-to-voltage converter 28 measuresthe unknown current flowing through the unknown impedance 24. The voltage atan output port 88 of the measurement current-to-voltage converter 28 is indicativeof the unknown impedance 24 and can be measured, recorded and evaluated. Ifnecessary, a signal processing circuit, for example a synchronous rectifier and

P-1EE-492/LU 15 LU100737 lowpass filter, may be inserted between the output port 88 of the measurementcurrent-to-voltage converter 28 and an evaluation unit (not shown).

[0060] It is noted that AC coupling capacitors or impedances for limiting electriccurrents during a potential sensor short circuit to any external nodes or in case ofan ESD (electrostatic discharge) event can be inserted in series with most of themembers of the capacitance measurement circuit 100.

[0061] In a diagnostic switching state with regard to the capacitive sensor, theswitching unit 42 is configured to electrically connect the sense electrode to asignal input port 38 of the diagnostics current-to-voltage converter 36 bycontrolling the second switching member 46 into a closed switching state. At thesame time, the first switching member 44 is controlled into an open switchingstate. In the diagnostic switching state, the sense electrode is kept at or close toAC ground potential, and, simultaneously, the sense node AC ground level andthe sense-to-guard impedance 22 of the capacitive sensor can be diagnosed asfollows.

[0062] The diagnostics current-to-voltage converter 36 is measuring a currentflowing from the signal voltage source 12 through the guard node 20, the sense-to-guard impedance 22, the sense node 18 and the second switching member 46,and into the signal input port 38 of the diagnostics current-to-voltage converter 36.The output voltage at an output port 90 of the diagnostics current-to-voltageconverte 36 can be measured, recorded and evaluated. If necessary, a signalprocessing circuit, for example a synchronous rectifier and lowpass filter, may beinserted between the output port 90 of the diagnostics current-to-voltageconverter 36 and an evaluation unit (not shown). Furnished with an evaluation unit,the capacitance measurement circuit 100 is configured to generate an outputsignal that is indicative of a sensor interruption if the magnitude of a determineddiagnostic current flowing through the sense electrode and the guard electrode ofthe capacitive sensor is less than a predefined threshold value.

[0063] As the voltage of the signal voltage source 12 is known a priori, the sense-to-guard impedance 22 can be calculated. As the reference input port 40 of thediagnostics current-to-voltage converter 36 is grounded, the sense node 18 is alsogrounded. As the diagnostics current-to-voltage converter 36 measures the currentinto its signal input port 38, a quantitative statement can also be made with regard

P-IEE-492/LU 16 LU100737 to sense electrode AC ground level diagnostics: If no electric current is detected,there is an issue within the measurement path, which may lead to the senseelectrode not being properly connected to AC ground potential 16. The currentmeasurement circuit 100 comprises a diagnostics output port 92 that is electricallyconnected to the sense electrode of the capacitive sensor. If desired, a moreprecise diagnostic can be applied by measuring the voltage at the diagnosticsoutput port 92, which is equal to the voltage at the sense node 18.

[0064] In the following, other embodiments of capacitance measurement circuitsin accordance with the invention and of capacitive sensing systems comprisingsuch capacitance measurement circuit will be described with reference to Figs. 2to 6. In order to avoid unnecessary repetition, only differences to a respectivepreceding embodiment will be described.

[0065] Fig. 2 illustrates an electric equivalent circuit diagram of an alternativeembodiment of a single-channel capacitance measurement circuit 200 inaccordance with the invention. The single-channel capacitance measurementcircuit 200 comprises two modifications compared to the embodiment as shown inFig. 1.

[0066] For the connected capacitive sensor, a demultiplexer member 48 isprovided in the remotely controllable switching unit 42. The remotely controllableswitching unit 42 is configured, by controlling the demultiplexer member 48 and inthe measurement switching state with regard to the capacitive sensor, to keep aterminal of the switching member 46 for electrically connecting the sense electrodeto the signal input port 38 of the diagnostics current-to-voltage converter 36 at theconverter end to guard potential of the capacitive sensor. This preventsintroducing a measurement error by a parasitic capacitance of the switchingmember 46, as a voltage difference across terminals of the switching member 46is virtually reduced to 0 V.

[0067] In a diagnostic switching state with regard to the capacitive sensor, theremotely controllable switching unit 42 is configured, by controlling thedemultiplexer member 48, to connect the terminal of the switching member 46 atthe converter end to the signal input port 38 of the diagnostics current-to-voltageconverter 36.

P-IEE-492/LU 17 LU100737 [0068] Moreover, the current measurement circuit 200 comprises means, for eachcapacitive sensor, to keep the diagnostics output port 92 at guard potential of therespective capacitive sensor, which can beneficially eliminate a systematic error ofmeasurement due to the diagnostics output port 92 being connected to the senseelectrode of the capacitive sensor. In this particular embodiment, the meanscomprise two resistors 54, 56 and a capacitor 58 arranged in a T configuration.

[0069] Fig. 3 illustrates an electric equivalent circuit diagram of a possibleembodiment of a multichannel capacitive sensing system 300 in accordance withthe invention, comprising a plurality of two capacitive sensors. The multichannelcapacitive sensing system 300 includes a capacitance measurement circuit 200pursuant to Fig. 2, and a duplicated version 200' of this capacitance measurementcircuit 200, wherein the periodic signal voltage source 12 is common to bothcapacitance measurement circuits 200, 200'. In other embodiments, each of thecapacitance measurement circuits may include a periodic signal voltage source ofits own. Each capacitance measurement circuit 200,200' includes onemeasurement channel 26, 26' and one diagnostics channel 34, 34' for each one ofthe two capacitive sensors.

[0070] The multichannel capacitive sensing system 300 includes amicrocontroller 60, and the remotely controllable switching unit 42 forms part of themicrocontroller. In turn, the microcontroller 60 comprises a switch remote controlunit 62 that is configured for remotely controlling the remotely controllableswitching unit 42.

[0071] The microcontroller 60 comprises a processor unit 64, a digital datamemory unit 66 to which the processor unit 64 has data access, and a plurality ofanalog-to-digital converters (ADCs, not shown), and is also employed to serve asan evaluation unit 70.

[0072] The remotely controllable switching unit 42 is configured, by control of theswitch remote control unit 62, to control the switching members 44, 44', 46, 46'such that each capacitive sensor is connected either in the measurementswitching state or in the diagnostic switching state.

[0073] The remotely controllable switching unit 42 is further configured, by controlof the switch remote control unit 62, to periodically switch, for each capacitive

P-IEE-492/LU 18 LU100737 sensor and in a manner that is coordinated among the capacitive sensors,between the measurement switching state and the diagnostic switching state.

[0074] Although in this particular embodiment the multichannel capacitive sensingsystem 300 includes a plurality of two capacitive sensors, it is also contemplatedthat the multichannel capacitive sensing system 300 may include a plurality ofmore than two capacitive sensors.

[0075] Fig. 4 illustrates an electric equivalent circuit diagram of an alternativepossible embodiments of a multichannel capacitive sensing system 400 inaccordance with the invention. The difference of the multichannel capacitivesensing system 400 compared to the multichannel capacitive sensing system 300pursuant to Fig. 3 is that there is only one measurement current-to-voltageconverter 28.

[0076] In multichannel multiplexed measurement applications, time is neither lostby the sense-to-guard diagnostics nor by the sense electrode AC ground leveldiagnostics. Usually, the measurement current-to-voltage converter 28 andassociated processing circuit is more expensive than the diagnostics current-to-voltage converter 36 due to the required measurement accuracy, which is lowerfor the diagnostics current-to-voltage converter 36. Therefore, only onemeasurement current-to-voltage converter 28 is used in the multichannelcapacitive sensing system 400 pursuant to Fig. 4, while there is one diagnosticscurrent-to-voltage converter 36,36' per capacitive sensor. The switchingconfiguration is the same as for the multichannel capacitive sensing system 300pursuant to Fig. 3, except that the remotely controllable switching unit 42 isconfigured, controlled by the switch remote control unit 62, to only put onecapacitive sensor into a measurement switching state at the same time. As thediagnostics can be executed for each capacitive sensor which is currently notmeasuring, in parallel to the measurement, the diagnostics does not add time to arequired total measurement time.

[0077] Fig. 5 illustrates an electric equivalent circuit diagram of anotheralternative possible embodiments of a multichannel capacitive sensing system 500in accordance with the invention. The difference of the multichannel capacitivesensing system 500 compared to the multichannel capacitive sensing system 400

P-IEE-492/LU 19 LU100737 pursuant to Fig. 4 is that there is only one measurement current-to-voltageconverter 28 and only one diagnostics current-to-voltage converter 36.

[0078] This embodiment is cost-efficient, however, for more than two capacitivesensors, a case that is contemplated as well, the remotely controllable switchingunit 42 has to be configured, controlled by the switch remote control unit 62, toonly put one capacitive sensor into a diagnostic switching state at the same time,leaving the sense nodes 18, 18' of the other capacitive sensors at guard potential.Consequently, time is lost by not being able to perform measurements anddiagnostics at the same time.

[0079] Fig. 6 illustrates an electric equivalent circuit diagram of anotheralternative possible embodiments of a multichannel capacitive sensing system 600in accordance with the invention. The difference of the multichannel capacitivesensing system 600 compared to the multichannel capacitive sensing system 500pursuant to Fig. 5 is that a demultiplexer member 50, 50' that is provided in theremotely controllable switching unit 42 for each capacitive sensor comprises anadditional terminal.

[0080] In a measurement switching state with regard to a respective capacitivesensor, each one of the demultiplexer members 50, 50' having an additionalterminal is configured to keep the terminal of the switching member 46, 46' forelectrically connecting the sense electrode to a signal input port 38 of thediagnostics current-to-voltage converter 36 at the converter end at guard potentialof the respective capacitive sensor.

[0081] Further, for each capacitive sensor whose sense electrode is unconnectedto the signal input port 30 of the measurement current-to-voltage converter 28, thedemultiplexer members 50, 50' are configured to either connect the terminal of theswitching member 46, 46' for electrically connecting the sense electrode to asignal input port 38 of the diagnostics current-to-voltage converter 36 at theconverter end to guard potential of the capacitive sensor, or to connect theterminal of the switching member 46, 46' for electrically connecting the senseelectrode to the signal input port 38 of the diagnostics current-to-voltageconverter 36 at the converter end to AC ground potential 16.

P-IEE-492/LU 20 LU100737 [0082] By that, a remedy is provided for a loss in operational availability existingfor the capacitive sensing system 500 pursuant to Fig. 5 in a case of more thantwo capacitive sensors, and high operational availability can be achieved despitethe hardware saving.

[0083] In the following, an embodiment of a method of operating the capacitivesensing system 600 pursuant to Fig. 6 with regard to capacitive sensor diagnosiswill be described. A flowchart of the method is given in Fig. 7. In preparation ofusing the capacitive sensing system 600, it shall be understood that all involvedunits and devices are in an operational state and configured as illustrated in Fig. 6.

[0084] The microcontroller 60 is furnished with a software module 68 forcontrolling automatic execution of steps of the method. Steps to be conducted areconverted into a program code of the software module, wherein the program codeis implementable in the digital data memory unit 66 of the microcontroller 60 and isexecutable by the processor unit 64 of the microcontroller 60.

[0085] In a first step 72 of the method, the remotely controllable switching unit 42is controlled by the switch remote control unit 62 to electrically disconnect thesense electrode of a capacitive sensor to be diagnosed from the signal inputport 30 of the measurement current-to-voltage converter 28. At substantially thesame time and in an immediately following step 74, the remotely controllableswitching unit 42 is controlled by the switch remote control unit 62 to electricallyconnect the sense electrode of the capacitive sensor to be diagnosed to the signalinput port 38 of the diagnostics current-to-voltage converter 36.

[0086] Then, in another step 76, the remotely controllable switching unit 42 iscontrolled by the switch remote control unit 62 to electrically connect the senseelectrode of another capacitive sensor to the signal input port 30 of themeasurement current-to-voltage converter 28 and, in an immediately followingstep 78, to disconnect the sense electrode of this capacitive sensor from guardpotential or from AC ground potential 16.

[0087] The remotely controllable switching unit 42 is controlled by the switchremote control unit 62 to electrically connect the sense electrodes of the remainderof the plurality of capacitive sensors to AC ground potential 16 in a further step 80.Then, in a further step 82, a complex sense current value through the sense

P-IEE-492/LU 21 LU100737 electrode of the capacitive sensor to be diagnosed is determined by thediagnostics current-to-voltage converter 36. After that, the determined sensecurrent value is compared with a predetermined threshold value by themicrocontroller 60 serving as the evaluation unit 70 in a next step 84. An outputsignal that is indicative of a sensor interruption is generated in another step 86 ifthe magnitude of the determined sense current value is smaller than thepredetermined threshold value.

[0088] After a step of swapping, for each cycle of steps, to a next capacitivesensor of the plurality of capacitive sensors that is to be diagnosed, the above-described steps are executed and repeated in a periodic manner until allcapacitive sensors are diagnosed.

[0089] While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and description are to beconsidered illustrative or exemplary and not restrictive; the invention is not limitedto the disclosed embodiments.

[0090] Other variations to be disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimed invention, from a studyof the drawings, the disclosure, and the appended claims. In the claims, the word“comprising” does not exclude other elements or steps, and the indefinite article“a” or “an” does not exclude a plurality, which is meant to express a quantity of atleast two. The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measures cannotbe used to advantage. Any reference signs in the claims should not be construedas limiting scope.

P-IEE-492/LU 22 LU100737

List of Reference Symbols 10 current measurement circuit 54 resistor 12 signal voltage source 56 resistor 14 output port 58 capacitor 16 AC ground potential 60 microcontroller 18 sense node 62 switch remote control unit 20 guard node 64 processor unit 22 sense-to-guard impedance 66 digital data memory unit 24 unknown impedance 68 software module 26 measurement channel 70 evaluation unit 28 measurement current-to-voltage 88 output port MCVC 28 converter (MCVC) 90 output port DCVC 36 30 signal input port 92 diagnostic output port 32 reference input port 100 single-channel capacitance 34 diagnostics channel measurement circuit 36 diagnostics current-to-voltage 200 single-channel capacitance converter (DCVC) measurement circuit 38 signal input port 300 multichannel capacitive sensing 40 reference input port system 42 remotely controllable switchingunit 400 multichannel capacitive sensingsystem 44 switching member 500 multichannel capacitive sensingsystem 46 switching member 600 multichannel capacitive sensing 48 demultiplexer member system 50 demultiplexer member

Method steps 72 disconnect sense electrode of capacitive sensor to be diagnosed from MCVCsignal input port 74 connect sense electrode of capacitive sensor to be diagnosed to DCVCsignal input port 76 connect sense electrode of another capacitive sensor to MCVC signal inputport 78 disconnect sense electrode of this capacitive sensor from guard or ACground potential 80 connect sense electrodes of remainder of capacitive sensors to AC groundpotential

P-IEE-492/LU 23 LU100737 82 determine sense current value of sensor to be diagnosed 84 compare determined sense current value with threshold 86 generate output signal

Claims

A capacitance measuring circuit (100) for determining a complex electrical current from at least one capacitive sensor having at least one electrically conductive sensing electrode and at least one electrically conductive guard electrode disposed proximate to each other, the capacitance sensing circuit (100) comprising: - at least one periodic capacitor Signal voltage source (12) adapted to provide an AC measurement voltage at an output port (14), each guard electrode being electrically connectable to the output port (14) to receive the periodic measurement voltage, - a current sense circuit (10) comprising at least one measuring channel (26) with a measuring current / voltage converter (28) adapted to determine, with respect to the measuring voltage, a measuring current through a detecting electrode indicating a position of an object relative to the capacitive sensor , and at least one diagnostic channel (34) having a diagnostic stream / S voltage converter (36) adapted to determine, with respect to an AC ground potential (16), a diagnostic current flowing through a guard electrode and a sense electrode of a capacitive sensor which is applied to the diagonal current / voltage converter (16). 36), and - a remotely controllable switching unit (42) having a plurality of functionally coupled switching elements (44, 46) for each capacitive sensor, wherein in a measurement circuit state with respect to the capacitive sensor, the switching unit (42) is adapted to connect the detection electrode of the capacitive sensor Sensor to electrically connect to a signal input port (30) of a sense current / voltage converter (28), and wherein in a diagnostic circuit state with respect to the capacitive sensor, the switching unit (42) is adapted to connect the sense electrode to a signal input port (38 ) of a diagnostic current / voltage converter (36).

2. A capacitance measuring circuit (100) according to claim 1, wherein the capacitance measuring circuit (100) is adapted to generate an output signal indicative of a sensor fault when the magnitude of a detected diagnostic current provided by a sense electrode and a guard electrode of a a capacitive sensor connected to the diagnostic current / voltage converter (36) is less than a predetermined threshold value.

3. capacitance measuring circuit (100) according to claim 1 or 2, wherein for each capacitive sensor, a demultiplexer element is provided in the switching unit is hen-hen, which is designed in a measuring circuit state with respect to a respective capacitive sensor, a connection of a Schaltele-element electrical connection of the detection electrode with a sig naleingangsport a diagnostic current / voltage converter at the converter end to keep the protection potential of the respective capacitive sensor.

4. capacitance measuring circuit (200) according to claim 1 or 2, wherein for each capacitive sensor, a demultiplexer element (48) in the switching unit (42) is provided, which is designed to - in a measurement circuit state with respect to a respective capacitive sensor holding a terminal of a switching element (46) for electrically connecting the detection electrode to a signal input port (38) of the diagnosis current / voltage converter (36) at the transducer end at the protection potential of the respective capacitive sensor, and - for each capacitive sensor whose detection electrode is not connected to a measuring current / voltage converter (28) has a connection of a switching element (46) for electrically connecting the detection electrode to a signal input port (38) of a diagnostic current / voltage converter (36) at the converter end to the protective pole connect the potential of the capacitive sensor or the connection of the switching element (46) for electrical connection of the detection electr o-de with the signal input port (38) of the diagnostic current / voltage converter (36) at the converter end aufzuschalten the AC ground potential (16).

A capacitance measurement circuit (200) according to any one of the preceding claims, wherein the current sense circuit (10) comprises for each capacitive sensor a diagnostic output port (92) electrically connected to a sense electrode of the capacitive sensor.

The capacitance measurement circuit (200) of claim 5, wherein the current measuring circuit (10) includes means (54, 56, 58) for each capacitive sensor to maintain the diagnostic output port (92) at the protection potential of the respective capacitive sensor.

7. capacitance measuring circuit (200) according to one of the preceding Ansprü-che, wherein the remotely controllable switching unit (42) forms part of a Mikrocon- troller (60).

A capacitance measuring circuit (100) according to any preceding claim, wherein the remotely controllable switching unit (42) is adapted to periodically switch between a measurement circuit state and a diagnostic circuit state for each capacitive sensor in a manner coordinated between the capacitive sensors ,

A capacitive detection system (300), comprising: - a capacitance measurement circuit (200) according to any one of the preceding claims, - a remote control unit (62) adapted to remotely control said remote controllable switching unit (42), and - A plurality of capacitive sensors, each capacitive sensor having at least one electrically conductive detection electrode and at least one electrically conductive guard electrode, which are arranged close to each other and mutually galvanically separated from each other, and the capacitance measuring circuit (200) comprises: - a periodic signal voltage source (12) adapted to provide an alternating measuring voltage at an output port (14), each guard electrode being electrically connected to the output port (14) for receiving the periodic measuring voltage, and wherein - the current measuring circuit (10) comprises: - a Measuring channel (26) with a measuring current / voltage converter (28), which is designed to einze ln and with respect to the sense voltage, determining a sense current through a sense electrode of one of the plurality of capacitive sensors indicative of a position of an object relative to the respective capacitive sensor, and a diagnostic channel (34) having a diagnostic current / voltage converter (36), which is configured to determine, with respect to an AC ground potential (16), a diagnostic current flowing through a guard electrode and a sense electrode of each capacitive sensor of the plurality of capacitive sensors.

The capacitive sensing system (300) of claim 9, wherein the current sensing circuit (10) includes a plurality of diagnostic channels (34, 34 '), each diagnostic channel (34, 34') including a diagnostic current-to-voltage converter (36, 36 ') which is adapted to determine, with respect to an AC ground potential (16), a diagnostic current flowing through a guard electrode and a sense electrode of another sensor of the plurality of capacitive sensors.

The capacitive sensing system (300) of claim 9 or 10, wherein the remote controller (62) forms part of a microcontroller (60).

12. A capacitive detection system (300) according to any one of claims 9 to 11, further comprising a signal processing unit which is set out, at least one output signal of the measuring current / voltage converter (28) with respect to the periodic measuring voltage and Minim least to process an output signal of the diagnostic current / voltage converter (36) with reference to the periodic measurement voltage.

The capacitive detection system (300) of any one of claims 9 to 12, further comprising a demodulation circuit adapted to receive at least one of an input signal of the measurement current-to-voltage converter (28) with respect to the periodic measurement span and to demodulate an input signal of the diagnostic current / voltage converter (36) with respect to the periodic measurement voltage.

A method of operating the capacitive sensing system (600) of any of claims 9 to 13 with regard to the diagnosis of a capacitive sensor, the method comprising at least the steps of: - controlling the remotely controllable switching unit (42) in such a manner that the sensing electrode a capacitive sensor to be diagnosed, is electrically isolated (72) from the signal input port (30) of the measuring current / voltage converter (28), controlling the remotely controllable switching unit (42) in such a way that the detection electrode the capacitive sensor to be diagnosed is electrically connected (74) to the signal input port (38) of the diagnostic current / voltage converter (36), controlling the remotely controllable switching unit (42) in such a way that the sensing electrode of another capacitive sensor to the signal input port (30) of the measuring current / voltage converter (28) electrically connected (76) and the detection electrode di (78) is isolated from the protection potential or AC ground potential (16), controlling the remotely controllable switching unit (42) such that the sense electrodes of all remaining ones of the plurality of capacitive sensors are electrically connected to the AC ground potential (16). the (80), - determining (82), by the diagnostic current / voltage converter (36), a sense current value through the sense electrode of the capacitive sensor to be diagnosed, - comparing (84) the detected A sense current value having at least one predetermined threshold, and - generating (86) an output signal indicative of a sensor failure if the detected sense current value is less than the predetermined threshold.

The method of claim 14, wherein steps (72) to (86) are repetitively and periodically performed after each cycle of steps (72) to (86) of the next capacitive sensor of the plurality of capacitive sensors, at which a diagnosis is to be made has been made until the diagnosis is made on all capacitive sensors.

16. Software modules (68) for controlling the automatic execution of the steps of the method according to claim 14 or 15, wherein the executed method steps are converted into a program code of the software module (68), wherein the program code in a digital data storage unit (66) of the Capacitive detection system (600) or a separate control unit and can be implemented by a processor unit (64) of the capacitive detection system (600) or a separate control unit.