Diagnostics for capacitive sensors

Abstract

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

Description

Technical Field

[0001] The present invention relates to a capacitance measurement circuit for determining a complex current of at least one capacitive sensor, to a capacitive sensing system comprising such a capacitance measurement circuit, to a method of operating such a capacitive sensing system with respect to capacitive sensor diagnostics, and to a software module for controlling the automatic execution of such a method. Background Art

[0002] Capacitive measurement and / or detection systems have a wide range of applications and are particularly widely used to detect the presence and / or position of an electrical conductor in the vicinity of an electrode of the system. A capacitive sensor, also known as an electric field sensor or proximity sensor, is a sensor that generates a signal in response to the effect of something being sensed (a person, a part of a person, a pet, an object, etc.) on an electric field. A capacitive sensor typically includes at least one antenna electrode to which an oscillating electrical signal is applied when the sensor is in operation and which then establishes an electric field in a region of space proximate to the antenna electrode. The sensor includes at least one sensing electrode—which may include one or more antenna electrodes themselves—at which the effect of an object or a living being on the electric field can be detected.

[0003] For example, different capacitive sensing mechanisms are explained in a technical paper entitled "Electric Field Sensing for Graphical Interfaces" published by JR Smith et al. in 1998 in IEEE Computer Graphics and Applications (18(3):54-60). The paper describes the concept of electric field sensing for making contactless three-dimensional position measurements, and more specifically, for sensing the position of a human hand for the purpose of providing three-dimensional position input to a computer. Within the general concept of capacitive sensing, the authors distinguish between different mechanisms, referring to "load mode", "shunt mode" and "transmit mode" corresponding to various possible current paths. In "load mode", an oscillating voltage signal is applied to the transmitting electrode, which builds an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmitting electrode and ground. In "shunt mode" (alternatively also called "coupling mode"), an oscillating voltage signal is applied to the transmitting electrode, thereby building an electric field to the receiving electrode, and the displacement current induced at the receiving electrode is measured, so that the displacement current can be modified by the sensed body. In "transmit mode", the transmit electrode is in contact with the user's body, which then becomes a transmitter relative to the receiver, either by direct electrical connection or via capacitive coupling.

[0004] Typically, the capacitive coupling is determined by applying an alternating voltage signal to the capacitive antenna electrode and by measuring the current flowing from the antenna electrode (in load mode) to ground or into the second electrode (receiving electrode) in the case of coupling mode. This current is typically measured with the aid of a transimpedance amplifier that is connected to the sensing electrode and converts the current flowing into the sensing electrode into a voltage that is proportional to the current flowing into the antenna electrode.

[0005] Some capacitive sensors are designed as sense-only capacitive sensors with a single sense electrode. Also, capacitive sensors are often used that include a sense electrode and a guard electrode that are arranged close to each other and insulated from each other. This "guarding" technique is well known in the art and is frequently used to intentionally mask the sensitivity range of a capacitive sensor and thus shape it. To this end, the guard electrode is maintained at the same AC potential as the sense electrode. As a result, the space between the sense electrode and the guard electrode is free of an electric field, and the guard-sense capacitive sensor is insensitive in the direction between the sense electrode and the guard electrode.

[0006] For example, patent document US 8,354,936 B2 describes a capacitive passenger detector for a vehicle. The capacitive passenger detector includes a main electrode, a sub-electrode and a guard electrode. The main electrode and the sub-electrode are separated from each other and are arranged in a seat of the vehicle. The guard electrode is arranged between the main electrode and the body of the vehicle and is separated from the main electrode. The sensitive characteristic measurement unit is configured to selectively or completely apply an alternating voltage signal to the main electrode, the sub-electrode and the guard electrode, respectively, and is used to convert the current generated in the main electrode, the sub-electrode and the guard electrode into a voltage. The capacitive passenger detector also includes a controller, which defines the current flowing through the guard electrode as a reference current when the voltage of the main electrode and the voltage of the guard electrode have the same potential. When the voltage of the main electrode is higher than the voltage of the guard electrode, the controller defines the current flow direction of the current flowing through the guard electrode as a negative direction. When the voltage of the main electrode is lower than the voltage of the guard electrode, the controller defines the current flow direction of the current flowing through the guard electrode as a positive direction. The controller corrects the voltage of the main electrode based on the current flowing through the guard electrode, thereby setting the corrected voltage of the main electrode as the passenger determination data. Even when a potential difference is generated between the main electrode and the guard electrode, the controller can correctly detect the capacitance of the passenger.

[0007] Capacitive sensing systems used in the control of airbag systems or other safety-related applications may be considered safety-related system components. Therefore, it may be necessary to monitor the good functionality of the different components of the sensor (sensing electrodes and / or guard electrodes) in order to exclude false readings of the capacitive occupancy or proximity detection system.

[0008] It has already been proposed in the prior art to provide a capacitive measuring circuit with a diagnostic device for detecting an interruption of a capacitive sensor, in particular an interruption of a guard electrode.

[0009] For example, international application WO 2017 / 129552 A1 describes a capacitance measurement circuit for determining a sensing current of a capacitive sensor having a sensing electrode and a guard electrode. The capacitance measurement circuit includes a periodic signal voltage source, a sensing current measurement circuit configured to determine the sensing current with reference to a reference voltage, and at least one remotely controllable switch member. The construction is such that in a first switching state, at least one switch member electrically connects the sensing current measurement circuit to a periodic measurement voltage for providing a first reference voltage, and in a second switching state, at least one switch member electrically connects the sensing current measurement circuit to a second reference voltage different from the first reference voltage. By intentionally changing the reference voltage for determining the sensing current by connecting the sensing current measurement circuit to a second reference voltage different from the first reference voltage, a signal that can indicate an electrical interruption can be generated by the sensing current measurement circuit, wherein the interruption can include any interruption of the electrical connection between the corresponding sensing cable and the guard cable and the connecting member. Summary of the invention

[0010] Purpose of the Invention

[0011] The object of the present invention is therefore to provide an improved capacitive sensing system and a diagnostic concept which is as comprehensive as possible for a capacitive sensing system, in particular for automotive applications, which is suitable in particular for multi-channel capacitive sensing systems and can be implemented with low hardware effort and moderate costs.

[0012] General Description of the Invention

[0013] In one aspect of the invention, the object is achieved by a capacitance measurement circuit for determining a complex current of at least one capacitive sensor, the at least one capacitive sensor comprising at least one conductive sensing electrode and at least one conductive guard electrode arranged close to each other. The at least one conductive sensing electrode and the at least one conductive guard electrode may, for example, be electrically separated from each other. However, this is not required. For example, a resistive connection between the sensing electrode and the guard electrode may be used for diagnostic purposes.

[0014] The capacitance measurement circuit includes at least one alternating signal voltage source, a current measurement circuit and a remote controllable switch unit.

[0015] At least one alternating signal voltage source is configured for providing an alternating measurement voltage at the output port, wherein each guard electrode is electrically connectable to the output port for receiving the alternating measurement voltage.

[0016] The current measurement circuit includes at least one measurement channel and at least one diagnostic channel. At least one measurement channel includes a measurement current-to-voltage converter configured to determine a measurement current indicating a position of an object relative to the capacitive sensor through the sensing electrode with reference to the alternating measurement voltage. At least one diagnostic channel includes a diagnostic current-to-voltage converter configured to determine a diagnostic current flowing through a guard electrode and a sensing electrode of the capacitive sensor connected to the diagnostic current-to-voltage converter with reference to an AC ground potential (alternating current ground potential).

[0017] The remotely controllable switch unit comprises a plurality of operatively coupled switch members for each capacitive sensor. In a measuring switch state with respect to the capacitive sensor, the switch unit is configured to electrically connect a sensing electrode of the capacitive sensor to a signal input port of a measuring current-to-voltage converter of one of the at least one measuring channel. In a diagnostic switch state with respect to the capacitive sensor, the switch unit is configured to electrically connect the sensing electrode to a signal input port of a diagnostic current-to-voltage converter of one of the at least one diagnostic channel. In the diagnostic switch state, the sensing electrode is not connected to the measuring current-to-voltage converter. In the measuring switch state, the sensing electrode is not connected to the diagnostic current-to-voltage converter.

[0018] One advantage of the proposed capacitance measurement circuit is that a means is provided for diagnosing the capacitive sensor and the sensor wiring by measuring the complex sense-protection impedance of the capacitive sensor.

[0019] Another advantage is that the AC ground connection of the capacitive sensor can be diagnosed by the proposed capacitance measurement circuit when the sensing electrode is connected to the signal input port of the diagnostic current-voltage converter.

[0020] When the capacitive sensor is not used to determine a measurement current through its sensing electrodes indicating the position of an object relative to the capacitive sensor, the AC potential may be maintained at or close to AC ground potential, which may be advantageous in certain circumstances.

[0021] As used in this application, the phrase "configured to" should be specifically understood as being specifically programmed, laid out, supplied or arranged. As used in this application, the phrase "operably coupled switch members" should be specifically understood as switch members configured to change their switch states substantially simultaneously.

[0022] Preferably, the at least one capacitive sensor is configured to operate in a load mode.

[0023] The proposed capacitance measurement circuit is advantageously particularly suitable for use in vehicles.As used in this application, the term "vehicle" should be understood in particular to include passenger cars, trucks, tractor units and buses.

[0024] Preferably, the measuring current-to-voltage converter and / or the diagnostic current-to-voltage converter comprises a transimpedance amplifier (TIA) whose function is to convert the current provided at the signal input port into an output voltage proportional to the determined current. The TIA is configured to convert the input current with reference to a reference voltage provided to a reference input port. Preferably, the transimpedance amplifier is based on one or more operational amplifiers.

[0025] In a preferred embodiment, the capacitance measurement circuit is configured to generate an output signal indicating a sensor interruption if the magnitude of the determined diagnostic current flowing through the sensing electrode and the guard electrode of the capacitive sensor connected to the diagnostic current-voltage converter is less than a predefined threshold value. The output signal can be advantageously transmitted to a higher-level control unit of the vehicle, for example, for initiating further actions.

[0026] Preferably, for each capacitive sensor,

[0027] - a first switch member of the plurality of operatively coupled switch members is configured to, in a measuring switch state with respect to the capacitive sensor, electrically connect a sensing electrode of the capacitive sensor to a signal input port of a measuring current-voltage converter;

[0028] - a second switch member of the plurality of operatively coupled switch members is configured to electrically connect the sensing electrode to the signal input port of the diagnostic current-voltage converter in a diagnostic switch state with respect to the capacitive sensor, the second switch member comprising a first terminal and a second terminal, wherein in the diagnostic switch state with respect to the capacitive sensor, the first terminal of the second switch member is operatively coupled to the sensing electrode and the second terminal of the second switch member is operatively coupled to the signal input port of the diagnostic current-voltage converter; and

[0029] - the switch unit comprises a demultiplexer member which is configured to connect the second terminal of the second switch member to the guard potential of the respective capacitive sensor, i.e. to the potential of the guard electrode, in the measuring switch state with respect to the respective capacitive sensor. In this way, the second terminal of the second switch member, which is in the open switch position when the capacitive sensor is in the measuring switch state, can be kept at the potential of the guard electrode. As a result, the parasitic capacitance of the switch member can be reduced to zero in practice and the measurement error due to the parasitic capacitance of the switch member can be minimized.

[0030] In a preferred embodiment of the capacitance measurement circuit, for each capacitive sensor,

[0031] - a first switch member of the plurality of operatively coupled switch members is configured to, in a measuring switch state with respect to the capacitive sensor, electrically connect a sensing electrode of the capacitive sensor to a signal input port of a measuring current-voltage converter;

[0032] - a second switch member of the plurality of operatively coupled switch members is configured to electrically connect the sensing electrode to the signal input port of the diagnostic current-voltage converter in a diagnostic switch state with respect to the capacitive sensor, the second switch member comprising a first terminal and a second terminal, wherein in the diagnostic switch state with respect to the capacitive sensor, the first terminal of the second switch member is operatively coupled to the sensing electrode and the second terminal of the second switch member is operatively coupled to the signal input port of the diagnostic current-voltage converter; and

[0033] - the switching unit comprises a demultiplexer member configured to connect the second terminal of the second switching member to a guard potential of the respective capacitive sensor in a measuring switching state with respect to the respective capacitive sensor.

[0034] Furthermore, the demultiplexer means provided in the switching unit is configured to connect the second terminal of the second switching means to a guard potential of the capacitive sensor or to AC ground potential for each capacitive sensor whose sensing electrode is not connected to a measuring current-voltage converter.

[0035] This embodiment is particularly beneficial in multi-channel applications that include more than two capacitive sensors and that may include only one diagnostic current-voltage converter and one measuring current-voltage converter for cost reasons. In this case, since the capacitance measurement circuit enables the AC potential of the sensing electrode of the capacitive sensor to be set to the AC ground potential, the time of operational availability can be increased even if the corresponding capacitive sensor is not connected to the measuring current-voltage converter or the diagnostic current-voltage converter.

[0036] Preferably, the current measurement circuit includes a diagnostic output port for each capacitive sensor, the diagnostic output port being electrically connected to a sensing electrode of the capacitive sensor. When the capacitive sensor is in a diagnostic switch state, the diagnostic output port can provide a complex electrical signal for further diagnosis.

[0037] In such an embodiment, the current measurement circuit may also include a device for each capacitive sensor that maintains the diagnostic output port at the guard potential of the corresponding capacitive sensor, which can beneficially eliminate systematic errors in the measurement caused by the diagnostic output port being connected to the sensing electrode of the capacitive sensor.

[0038] In a preferred embodiment of the capacitance measuring circuit, the remotely controllable switch unit forms part of a microcontroller, by means of which a reliable and simple remote control of the remotely controllable switch unit can be achieved and in this way a reliable diagnosis of the capacitive sensor can be achieved. Microcontrollers suitably equipped and comprising, for example, a processor unit, a digital storage unit, a microcontroller system clock, a demultiplexer unit and an analog-digital converter are readily available today in many variants.

[0039] Preferably, the remotely controllable switch unit is configured to periodically switch each capacitive sensor and in a coordinated manner between the capacitive sensors between the measuring switch state and the diagnostic switch state. In this way, a reliable operation of the capacitance measuring circuit can be achieved.

[0040] In another aspect of the present invention, a capacitive sensing system is provided. The capacitive sensing system includes the embodiments disclosed herein of a capacitance measurement circuit, a switch remote control unit, and a plurality of capacitive sensors.

[0041] The switch remote control unit is configured to remotely control the remotely controllable switch unit. Each capacitive sensor of the plurality of capacitive sensors comprises at least one conductive sensing electrode and at least one conductive guard electrode arranged close to each other. As described above, the at least one conductive sensing electrode and the at least one conductive guard electrode may be electrically separated from each other, but this is not required. For example, a resistive connection between the sensing electrode and the guard electrode may be used for diagnostic purposes.

[0042] The capacitance measurement circuit comprises an alternating signal voltage source configured to provide an alternating measurement voltage at an output port, wherein each guard electrode is electrically connected to the output port for receiving the alternating measurement voltage.

[0043] The current measurement circuit includes a measurement channel having a measurement current-to-voltage converter configured to determine, one at a time and with reference to a measurement voltage, a measurement current through a sensing electrode of one of the plurality of capacitive sensors indicating a position of an object relative to the corresponding capacitive sensor.

[0044] The current measurement circuit also includes a plurality of diagnostic channels, each of the diagnostic channels being configured to determine a diagnostic current flowing through the guard electrode and the sense electrode of a different one of the plurality of capacitive sensors with reference to an AC ground potential.

[0045] In this way, a capacitive sensing system having reliable operation and a high percentage of operational availability may be achieved, which includes the benefits that have been previously described in conjunction with the disclosed capacitance measurement circuit.

[0046] Preferably, the switch remote control unit forms part of a microcontroller, by means of which a reliable and simple remote control of the remotely controllable switch unit can be achieved and in this way a reliable diagnosis of the capacitive sensor can be achieved. Most preferably, both the switch remote control unit and the remotely controllable switch unit form part of the same microcontroller. In this way, a short control path between the switch remote control unit and the remotely controllable switch unit can be achieved which is not susceptible to electromagnetic interference.

[0047] In a preferred embodiment, the capacitive sensing system further comprises a signal processing unit configured to process at least one of the output signal of the measuring current-voltage converter with reference to the alternating measuring voltage and the output signal of the diagnostic current-voltage converter with reference to the alternating measuring voltage. In this way, an improved signal-to-noise ratio (SNR) can be achieved for the output signal of the measuring current-voltage converter and / or the output signal of the diagnostic current-voltage converter.

[0048] A further improvement of the SNR may be achieved if the capacitive sensing system comprises a demodulation circuit configured for at least one of demodulating an input signal of a measuring current-to-voltage converter with reference to the alternating measurement voltage and demodulating an input signal of a diagnostic current-to-voltage converter with reference to the alternating measurement voltage.

[0049] In case the demodulation circuit is configured for demodulating the input signal of the measuring current-voltage converter, the demodulation circuit is preferably connected to the signal input port of the measuring current-voltage converter, and the remotely controllable switch unit is therefore configured to electrically connect the sensing electrode of the capacitive sensor to the demodulation circuit.

[0050] In yet another aspect of the present invention, a method of operating the capacitive sensing system disclosed herein is provided with respect to capacitive sensor diagnostics.

[0051] The method comprises at least the following steps:

[0052] - controlling the remotely controllable switch unit to electrically disconnect the sensing electrode of the capacitive sensor to be diagnosed from the signal input port of the measuring current-voltage converter,

[0053] - controlling the remotely controllable switch unit to electrically connect the sensing electrode of the capacitive sensor to be diagnosed to the signal input port of the diagnostic current-voltage converter,

[0054] - controlling the remotely controllable switch unit to electrically connect the sensing electrode of another capacitive sensor to the signal input port of the measuring current-voltage converter and to disconnect the sensing electrode of the capacitive sensor from the protection potential or from the AC ground potential,

[0055] - controlling the remotely controllable switch unit to electrically connect the sensing electrodes of the remaining capacitive sensors of the plurality of capacitive sensors to the AC ground potential,

[0056] - determining a sensed current value of a sensing electrode of a capacitive sensor diagnosed by a diagnostic current-to-voltage converter,

[0057] - comparing the determined sensed current value with at least one predetermined threshold value,

[0058] - If the determined sensed current value is less than a predetermined threshold value, generating an output signal indicating a sensor interruption.

[0059] In a preferred embodiment of the method, for each cycle of the steps, after switching to the next capacitive sensor to be diagnosed among the plurality of capacitive sensors, the steps are performed and repeated in a periodic manner until all capacitive sensors among the plurality of capacitive sensors are diagnosed.

[0060] In yet another aspect of the present invention, a software module for controlling the automatic execution of the steps of the embodiments of the methods disclosed herein is provided.

[0061] The method steps to be performed are converted into program code of a software module, wherein the program code can be stored in a digital storage unit of the capacitive sensing system or a separate control unit and can be executed by a processor unit of the capacitive sensing system or a separate control unit.

[0062] The software module may enable a robust and reliable execution of the method and may allow rapid modification of method steps.

[0063] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0064] It should be pointed out that the features and measures described in detail individually in the preceding description can be combined with one another in any technically meaningful manner and represent further embodiments of the invention. The description particularly characterizes and explains the invention in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Other details and advantages of the invention will become apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings, in which:

[0066] Figure 1 shows an electrical equivalent circuit diagram of a possible embodiment of a single-channel capacitance measurement circuit according to the present invention,

[0067] Figure 2 shows an electrical equivalent circuit diagram of an alternative embodiment of a single-channel capacitance measurement circuit according to the present invention,

[0068] Figure 3 shows an electrical equivalent circuit diagram of a possible embodiment of a multi-channel capacitive sensing system according to the present invention,

[0069] Figure 4-Figure 6 An electrical equivalent circuit diagram showing an alternative possible embodiment of a multi-channel capacitive sensing system according to the present invention, and

[0070] Figure 7 is based on Figure 6 A flow chart of a method of operating a capacitive sensing system. DETAILED DESCRIPTION

[0071] In the different figures, identical elements are always provided with the same reference symbols. Therefore, these elements are usually described only once.

[0072] Figure 1 The electrical equivalent circuit diagram of a possible embodiment of a single-channel capacitance measurement circuit 100 according to the present invention is schematically shown. The capacitance measurement circuit 100 is used to determine the complex current of a single capacitive sensor configured in load mode, the single capacitive sensor comprising a conductive sensing electrode and a conductive guard electrode arranged close to each other and electrically separated from each other. The sensing electrode and the corresponding sensing wiring resistance are Figure 1 1 is represented by sense node 18, and the guard electrode and corresponding guard wiring resistance are represented by guard node 20. A sense-guard impedance 22, which is substantially capacitive in nature, is electrically connected between the sense node 18 and the guard node 20. In this particular embodiment, the sense-guard impedance 22 may have a capacitance of approximately 1 nF, although capacitive sensors with higher or lower sense-guard capacitances are also contemplated.

[0073] The single-channel capacitance measurement circuit 100 includes an alternating (i.e., sinusoidal) signal voltage source 12 configured to provide an alternating measurement voltage relative to an AC ground potential 16 at an output port 14. A guard electrode is electrically connected to the alternating signal voltage source 12 for receiving the alternating measurement voltage, which is transmitted from the alternating signal voltage source 12 to a guard node 20.

[0074] The single-channel capacitance measurement circuit 100 includes a current measurement circuit 10, which includes a measurement channel 26 having a measurement current-to-voltage converter 28 and a diagnostic channel 34 having a diagnostic current-to-voltage converter 36. Both the measurement current-to-voltage converter 28 and the diagnostic current-to-voltage converter 36 include a transimpedance amplifier (TIA) whose function is to convert a complex current provided at a signal input port 30, 38 into an output voltage proportional to a determined sense current. The two TIAs are configured to convert the sense current with reference to a reference voltage provided to a reference input port 32, 40. The reference input port 32 of the measurement current-to-voltage converter 28 is electrically connected to the output port 14 of the signal voltage source 12. The reference input port 40 of the diagnostic current-to-voltage converter 36 is electrically connected to the AC ground potential 16.

[0075] Therefore, the measuring current-to-voltage converter 28 is configured to determine the measuring current through the sensing electrodes indicative of the position of the object relative to the capacitive sensor with reference to the measuring voltage. Figure 1 The electrical equivalent circuit diagram of the sensor is represented by an unknown impedance 24, which is connected to the AC ground potential 16, which may be, for example, the vehicle ground potential. If a grounded object approaches the sensing electrode, the unknown impedance 24 changes in that at least its capacitive part increases, and the measurement current flowing between the sensing electrode and the ground potential increases, so that the amplitude of the TIA output voltage signal increases, which indicates a closer proximity of the object to the capacitive sensor.

[0076] The diagnostic current-to-voltage converter 36 is configured to determine, with reference to the AC ground potential 16 , a diagnostic current that flows through the guard electrode and the sense electrode of the capacitive sensor when the capacitive sensor is connected to the diagnostic current-to-voltage converter 36 .

[0077] In addition, the capacitance measuring circuit 100 includes a switch unit 42 that can be remotely controlled by a switch remote control unit (not shown). The switch unit 42 includes a plurality of two operatively coupled switch members 44, 46. The switch members 44, 46 are operatively coupled because one switch member 44 changes the switch position from open to closed, while the other switch member 46 changes the switch position from closed to open, and vice versa.

[0078] A first one of the switch members 44, 46 is located between the sensing node 18 and the signal input port 30 of the measuring current-to-voltage converter 28. A second one of the switch members 44, 46 is located between the sensing node 18 and the signal input port 38 of the diagnostic current-to-voltage converter 36.

[0079] In the measurement switch state about the capacitive sensor, the switch unit 42 is configured to electrically connect the sensing electrode of the capacitive sensor to the signal input port 30 of the measuring current-voltage converter 28 by controlling the first switch member 44 to enter the closed switch state. At the same time, the second switch member 46 is controlled to enter the open switch state. The measuring current-voltage converter 28 measures the unknown current flowing through the unknown impedance 24. The voltage at the output port 88 of the measuring current-voltage converter 28 indicates the unknown impedance 24 and can be measured, recorded and evaluated. If necessary, a signal processing circuit, such as a synchronous rectifier and a low-pass filter, can be inserted between the output port 88 of the measuring current-voltage converter 28 and the evaluation unit (not shown).

[0080] Note that an AC coupling capacitor or impedance for limiting current during a potential sensor short circuit to any external node or in the event of an ESD (electrostatic discharge) event may be inserted in series with most components in the capacitance measurement circuit 100 .

[0081] In the diagnostic switch state for the capacitive sensor, the switch unit 42 is configured to electrically connect the sensing electrode to the signal input port 38 of the diagnostic current-voltage converter 36 by controlling the second switch member 46 to enter the closed switch state. At the same time, the first switch member 44 is controlled to enter the open switch state. In the diagnostic switch state, the sensing electrode is maintained at or close to the AC ground potential, and at the same time, the sensing node AC ground level of the capacitive sensor and the sensing-protection impedance 22 can be diagnosed as follows.

[0082] The diagnostic current-to-voltage converter 36 measures the current flowing from the signal voltage source 12 through the guard node 20, the sense-to-guard impedance 22, the sense node 18, and the second switch member 46, and into the signal input port 38 of the diagnostic current-to-voltage converter 36. The output voltage at the output port 90 of the diagnostic current-to-voltage converter 36 can be measured, recorded, and evaluated. If necessary, a signal processing circuit, such as a synchronous rectifier and a low-pass filter, can be inserted between the output port 90 of the diagnostic current-to-voltage converter 36 and an evaluation unit (not shown). The capacitance measurement circuit 100 supplied with the evaluation unit is configured to generate an output signal indicating a sensor interruption if the magnitude of the determined diagnostic current flowing through the sensing electrode and the guard electrode of the capacitive sensor is less than a predefined threshold.

[0083] When the voltage of the signal voltage source 12 is known a priori, the sensing-guard impedance 22 can be calculated. When the reference input port 40 of the diagnostic current-to-voltage converter 36 is grounded, the sensing node 18 is also grounded. When the diagnostic current-to-voltage converter 36 measures the current entering its signal input port 38, quantitative statements about the diagnostics of the sensing electrode AC ground level can also be made: if no current is detected, there is a problem within the measurement path, which can result in the sensing electrode not being properly connected to the AC ground potential 16. The current measurement circuit 100 includes a diagnostic output port 92, which is electrically connected to the sensing electrode of the capacitive sensor. If desired, a more precise diagnosis can be applied by measuring the voltage at the diagnostic output port 92 equal to the voltage at the sensing node 18.

[0084] In the following, reference will be made to Figures 2 to 6 Further embodiments of a capacitance measuring circuit according to the invention and a capacitive sensing system comprising such a capacitance measuring circuit are described. In order to avoid unnecessary repetitions, only the differences from the corresponding previous embodiments will be described.

[0085] Figure 2 FIG. 2 shows an electrical equivalent circuit diagram of an alternative embodiment of a single-channel capacitance measurement circuit 200 according to the present invention. Figure 1 Compared to the embodiment shown in , the single-channel capacitance measurement circuit 200 includes two modifications.

[0086] For the connected capacitive sensor, a demultiplexer member 48 is provided in the remote controllable switch unit 42. The remote controllable switch unit 42 is configured to keep the terminals of the switch member 46 at the converter end for electrically connecting the sensing electrode to the signal input port 38 of the diagnostic current-voltage converter 36 at the protection potential of the capacitive sensor by controlling the demultiplexer member 48 and in the measurement switch state with respect to the capacitive sensor. Since the voltage difference between the terminals across the switch member 46 is reduced to 0V in practice, this prevents the introduction of measurement errors caused by the parasitic capacitance of the switch member 46.

[0087] In the diagnostic switch state with respect to the capacitive sensor, the remotely controllable switch unit 42 is configured to connect the terminal of the switch member 46 at the converter end to the signal input port 38 of the diagnostic current-voltage converter 36 by controlling the demultiplexer member 48 .

[0088] In addition, the current measurement circuit 200 includes a means for each capacitive sensor to maintain the diagnostic output port 92 at the guard potential of the corresponding capacitive sensor, which can advantageously eliminate systematic measurement errors caused by the connection of the diagnostic output port 92 to the sensing electrode of the capacitive sensor. In this particular embodiment, the means includes two resistors 54, 56 and a capacitor 58 arranged in a T configuration.

[0089] Figure 3 FIG. 2 shows an electrical equivalent circuit diagram of a possible embodiment of a multi-channel capacitive sensing system 300 including a plurality of two capacitive sensors according to the present invention. Figure 2 A capacitance measurement circuit 200 and a replica 200' of the capacitance measurement circuit 200, wherein the alternating signal voltage source 12 is common to both capacitance measurement circuits 200, 200'. In other embodiments, each of the capacitance measurement circuits may include its own alternating signal voltage source. Each capacitance measurement circuit 200, 200' includes one measurement channel 26, 26' and one diagnostic channel 34, 34' for each of the two capacitive sensors.

[0090] The multi-channel capacitive sensing system 300 comprises a microcontroller 60 , and the remotely controllable switch unit 42 forms a part of the microcontroller. In turn, the microcontroller 60 comprises a switch remote control unit 62 configured for remotely controlling the remotely controllable switch unit 42 .

[0091] The microcontroller 60 comprises a processor unit 64 , a digital data storage unit 66 to which the processor unit 64 has data access, and a plurality of analog-digital converters (ADCs, not shown), and is also used to function as an evaluation unit 70 .

[0092] The remotely controllable switch unit 42 is configured to control the switch members 44 , 44 ′, 46 , 46 ′ by controlling the switch remote control unit 62 so that each capacitive sensor is connected in a measuring switch state or in a diagnostic switch state.

[0093] The remotely controllable switch unit 42 is further configured to periodically switch each capacitive sensor and in a coordinated manner between the capacitive sensors between the measuring switch state and the diagnostic switch state by controlling the switch remote control unit 62 .

[0094] Although in this particular embodiment, the multi-channel capacitive sensing system 300 includes a plurality (two) of capacitive sensors, it is also contemplated that the multi-channel capacitive sensing system 300 may include a plurality of more than (two) capacitive sensors.

[0095] Figure 4 FIG. 4 is an electrical equivalent circuit diagram of an alternative possible embodiment of a multi-channel capacitive sensing system 400 according to the present invention. Figure 3 Compared to the multi-channel capacitive sensing system 300 , the multi-channel capacitive sensing system 400 is different in that there is only one measuring current-to-voltage converter 28 .

[0096] In multi-channel multiplexed measurement applications, time is not wasted on either sense-guard diagnostics or sense electrode AC ground level diagnostics. Typically, the measurement current-to-voltage converter 28 and associated processing circuitry are more expensive than the diagnostic current-to-voltage converter 36 due to the lower measurement accuracy required for the diagnostic current-to-voltage converter 36. Figure 4 Only one measuring current-to-voltage converter 28 is used in the multi-channel capacitive sensing system 400, while each capacitive sensor has a diagnostic current-to-voltage converter 36, 36'. Figure 3 The multi-channel capacitive sensing system 300 is the same as the multi-channel capacitive sensing system 300, except that the remotely controllable switch unit 42 is configured to be controlled by the switch remote control unit 62 to place only one capacitive sensor in the measuring switch state at a time. Since the diagnosis and measurement can be performed in parallel for each capacitive sensor that is not currently being measured, the diagnosis does not add time to the total measurement time required.

[0097] Figure 5 FIG. 5 shows an electrical equivalent circuit diagram of another alternative possible embodiment of a multi-channel capacitive sensing system 500 according to the present invention. Figure 4 Compared to the multi-channel capacitive sensing system 400 , the multi-channel capacitive sensing system 500 differs in that there is only one measurement current-to-voltage converter 28 and only one diagnostic current-to-voltage converter 36 .

[0098] This embodiment is cost effective, but, for more than two capacitive sensors, in the same envisaged case, the remotely controllable switch unit 42 must be configured to be controlled by the switch remote control unit 62 to simultaneously place only one capacitive sensor in the diagnostic switch state, while the sensing nodes 18, 18' of the other capacitive sensors remain at the protection potential. Therefore, time is wasted because measurements and diagnostics cannot be performed simultaneously.

[0099] Figure 6 FIG. 6 is an electrical equivalent circuit diagram of another alternative possible embodiment of a multi-channel capacitive sensing system 600 according to the present invention. Figure 5Compared to the multi-channel capacitive sensing system 500 , the multi-channel capacitive sensing system 600 differs in that the demultiplexer member 50 , 50 ′ provided for each capacitive sensor in the remotely controllable switch unit 42 includes an additional terminal.

[0100] In the measuring switching state with respect to the corresponding capacitive sensor, each of the demultiplexer members 50, 50' having additional terminals is configured to maintain the terminals of the switch members 46, 46' at the converter end at the protection potential of the corresponding capacitive sensor, the switch members 46, 46' being used to electrically connect the sensing electrodes to the signal input port 38 of the diagnostic current-voltage converter 36.

[0101] Furthermore, for each capacitive sensor whose sensing electrode is not connected to the signal input port 30 of the measuring current-voltage converter 28, the demultiplexer member 50, 50' is configured to connect the terminals at the converter ends of the switch members 46, 46' for electrically connecting the sensing electrode to the signal input port 38 of the diagnostic current-voltage converter 36 to the guard potential of the capacitive sensor, or to connect the terminals at the converter ends of the switch members 46, 46' for electrically connecting the sensing electrode to the signal input port 38 of the diagnostic current-voltage converter 36 to the AC ground potential 16.

[0102] Thus, in the case of more than two capacitive sensors, Figure 5 The capacitive sensing system 500 provides a remedy for the loss of operational availability that exists, and high operational availability can be achieved despite the hardware savings.

[0103] In the following, the operation of the capacitive sensor diagnosis will be described. Figure 6 An embodiment of a method of a capacitive sensing system 600 is shown in FIG. Figure 7 In preparing to use the capacitive sensing system 600, it should be understood that all units and devices involved are in an operational state and as Figure 6 is configured as shown in .

[0104] The microcontroller 60 is supplied with a software module 68 for controlling the automatic execution of the steps of the method. The steps to be performed are converted into a program code of the software module, wherein the program code can be implemented in a digital data storage unit 66 of the microcontroller 60 and can be executed by a processor unit 64 of the microcontroller 60.

[0105] In a first step 72 of the method, the remotely controllable switch unit 42 is controlled by the switch remote control unit 62 to electrically disconnect the sensing electrode of the capacitive sensor to be diagnosed from the signal input port 30 of the measuring current-to-voltage converter 28. Almost simultaneously and immediately thereafter, at step 74, the remotely controllable switch unit 42 is controlled by the switch remote control unit 62 to electrically connect the sensing electrode of the capacitive sensor to be diagnosed to the signal input port 38 of the diagnostic current-to-voltage converter 36.

[0106] Then, in a further step 76, the remotely controllable switch unit 42 is controlled by the switch remote control unit 62 to electrically connect the sensing electrode of another capacitive sensor to the signal input port 30 of the measuring current-voltage converter 28, and subsequently in a step 78, the sensing electrode of the capacitive sensor is disconnected from the protection potential or from the AC ground potential 16.

[0107] In a further step 80, the remotely controllable switch unit 42 is controlled by the switch remote control unit 62 to electrically connect the sensing electrodes of the remaining capacitive sensors of the plurality of capacitive sensors to the AC ground potential 16. Then, in a further step 82, the complex sensed current value through the sensing electrode of the capacitive sensor to be diagnosed is determined by the diagnostic current-voltage converter 36. Thereafter, in a next step 84, the determined sensed current value is compared with a predefined threshold value by the microcontroller 60 acting as the evaluation unit 70. If the magnitude of the determined sensed current value is less than the predetermined threshold value, an output signal indicating a sensor interruption is generated in a further step 86.

[0108] For each cycle of the steps, after the step of switching to the next capacitive sensor to be diagnosed among the plurality of capacitive sensors, the above steps are performed and repeated in a periodic manner until all the capacitive sensors are diagnosed.

[0109] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

[0110] Other variations of the embodiments to be disclosed may be understood and effected by those skilled in the art in practicing the claimed invention, based on a study of 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" does not exclude a plurality of at least two. The fact that certain measures are recited in mutually different dependent claims does not mean that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

[0111] Reference numerals list

[0112] 10 Current measurement circuit 54 resistor

[0113] 12 Signal voltage source 56 Resistors

[0114] 14 Output port 58 Capacitor

[0115] 16 AC ground potential 60 Microcontroller

[0116] 18 Sensing Node 62 Switch Remote Control Unit

[0117] 20 protection nodes 64 processor units

[0118] 22 Sense-protect impedance 66 Digital data storage unit

[0119] 24 Unknown impedance 68 Software module

[0120] 26 measuring channels 70 evaluation units

[0121] 28 Measurement current-voltage converter (MCVC) 88 Output port MCVC28

[0122] 30 Signal input port 90 Output port DCVC36

[0123] 32 Reference input port 92 Diagnostic output port

[0124] 34 Diagnostic channels 100 Single channel capacitance measurement circuit

[0125] 36 Diagnostic Current-Voltage Converter (DCVC) 200 Single Channel Capacitance Measurement Circuit

[0126] 38 signal input ports 300 multi-channel capacitive sensing system

[0127] 40 Reference Input Port 400 Multi-Channel Capacitive Sensing System

[0128] 42 Remotely controllable switch unit 500 Multi-channel capacitive sensing system

[0129] 44 Switching component 600 Multi-channel capacitive sensing system

[0130] 46 Switch component

[0131] 48 Demultiplexer component

[0132] 50 Demultiplexer component

[0133] Method steps

[0134] 72 Disconnect the sensing electrode of the capacitive sensor to be diagnosed from the MCVC signal input port

[0135] 74 Connect the sensing electrode of the capacitive sensor to be diagnosed to the DCVC signal input port

[0136] 76 Connect the sensing electrode of another capacitive sensor to the MCVC signal input port

[0137] 78 Disconnect the sensing electrode of the capacitive sensor from the protection potential or AC ground potential

[0138] 80 Connect the sensing electrodes of the remaining capacitive sensors in the capacitive sensor to the AC ground potential

[0139] 82 Determine the sensing current value of the sensor to be diagnosed

[0140] 84 Compare the determined sensing current value with the threshold value

[0141] 86 Generate output signal

Claims

1. A capacitance measurement circuit for determining a complex current of at least one capacitive sensor, the capacitive sensor comprising at least one conductive sensing electrode and at least one conductive guard electrode arranged close to each other, the capacitance measurement circuit comprising: at least one alternating signal voltage source (12) configured to provide an alternating measurement voltage at an output port (14), wherein each guard electrode is electrically connectable to the output port (14) for receiving the alternating measurement voltage; - a current measurement circuit (10), the current measurement circuit (10) comprising: at least one measurement channel (26), the at least one measurement channel (26) comprising a measurement current-voltage converter (28), the measurement current-voltage converter (28) being configured to determine, with reference to the alternating measurement voltage, a measurement current through the sensing electrode indicating the position of an object relative to the capacitive sensor; and at least one diagnostic channel (34), the at least one diagnostic channel (34) comprising a diagnostic current-voltage converter (36), the diagnostic current-voltage converter (36) being configured to determine, with reference to an AC ground potential (16) (alternating current ground potential), a diagnostic current flowing through a guard electrode and a sensing electrode of the capacitive sensor connected to the diagnostic current-voltage converter (36), and - a remotely controllable switch unit (42), the remotely controllable switch unit (42) comprising a plurality of operatively coupled switch members (44, 46) for each capacitive sensor, wherein, in a measuring switch state with respect to the capacitive sensor, the switch unit (42) is configured to electrically connect the sensing electrode of the capacitive sensor to a signal input port (30) of the measuring current-voltage converter (28) of the at least one measuring channel (26), and wherein, in a diagnostic switch state with respect to the capacitive sensor, the switch unit (42) is configured to electrically connect the sensing electrode to a signal input port (38) of the diagnostic current-voltage converter (36) of the at least one diagnostic channel (34).

2. The capacitance measurement circuit according to claim 1, wherein: The capacitance measurement circuit is configured to generate an output signal indicating a sensor interrupt if the magnitude of the determined diagnostic current flowing through the sense electrode and the guard electrode of the capacitive sensor connected to the diagnostic current-to-voltage converter (36) is less than a predefined threshold.

3. The capacitance measurement circuit according to claim 1 or 2, wherein: For each capacitive sensor, a first switch member (44) of the plurality of operatively coupled switch members (44, 46) being configured to electrically connect the sensing electrode of the capacitive sensor to the signal input port (30) of the measuring current-voltage converter (28) in the measuring switch state with respect to the capacitive sensor; - a second switch member (46) of the plurality of operatively coupled switch members (44, 46) is configured to electrically connect the sensing electrode to the signal input port (38) of the diagnostic current-to-voltage converter (36) in the diagnostic switch state with respect to the capacitive sensor, the second switch member (46) comprising a first terminal and a second terminal, wherein, in the diagnostic switch state with respect to the capacitive sensor, the first terminal of the second switch member (46) is operatively coupled to the sensing electrode and the second terminal of the second switch member (46) is operatively coupled to the signal input port (38) of the diagnostic current-to-voltage converter (36), and The switching unit comprises a demultiplexer member (48, 50) configured to connect the second terminal of the second switching member (46) to a guard potential of a guard electrode of the respective capacitive sensor in the measuring switching state with respect to the respective capacitive sensor.

4. The capacitance measurement circuit according to claim 1 or 2, wherein: For each capacitive sensor, a first switch member (44) of the plurality of operatively coupled switch members (44, 46) being configured to electrically connect the sensing electrode of the capacitive sensor to the signal input port (30) of the measuring current-voltage converter (28) in the measuring switch state with respect to the capacitive sensor; - a second switch member (46) of the plurality of operatively coupled switch members (44, 46) is configured to electrically connect the sensing electrode to the signal input port (38) of the diagnostic current-to-voltage converter (36) in the diagnostic switch state with respect to the capacitive sensor, the second switch member (46) comprising a first terminal and a second terminal, wherein, in the diagnostic switch state with respect to the capacitive sensor, the first terminal of the second switch member (46) is operatively coupled to the sensing electrode and the second terminal of the second switch member (46) is operatively coupled to the signal input port (38) of the diagnostic current-to-voltage converter (36), and - the switch unit comprises a demultiplexer member (50) configured to, - in the measuring switching state with respect to the corresponding capacitive sensor, connecting the second terminal of the second switching member (46) to the guard potential of the guard electrode of the corresponding capacitive sensor as well as - for each capacitive sensor whose sensing electrode is not connected to a measuring current-to-voltage converter (28), connecting the second terminal of the second switching member (46) to a guard potential of the capacitive sensor or connecting the second terminal of the second switching member (46) to an AC ground potential (16).

5. The capacitance measurement circuit according to claim 1 or 2, wherein: The current measurement circuit (10) includes a diagnostic output port (92) for each capacitive sensor, the diagnostic output port (92) being electrically connected to a sensing electrode of the capacitive sensor.

6. The capacitance measurement circuit according to claim 5, wherein: The current measurement circuit (10) includes means (54, 56, 58) for each capacitive sensor, the means (54, 56, 58) maintaining the diagnostic output port (92) at the guard potential of the respective capacitive sensor.

7. The capacitance measurement circuit according to claim 1 or 2, wherein: The remotely controllable switch unit (42) forms part of a microcontroller (60).

8. The capacitance measurement circuit according to claim 1 or 2, wherein: The remotely controllable switch unit (42) is configured to periodically switch each capacitive sensor and in a coordinated manner between the capacitive sensors between a measuring switch state and a diagnostic switch state.

9. A capacitive sensing system comprising: - a capacitance measuring circuit as claimed in any one of the preceding claims, - a switch remote control unit (62), the switch remote control unit (62) being configured to remotely control the remotely controllable switch unit (42); as well as - a plurality of capacitive sensors, each capacitive sensor comprising at least one conductive sensing electrode and at least one conductive guard electrode arranged close to each other, wherein each guard electrode is electrically connected to the output port (14) of the alternating signal voltage source (12) for receiving the alternating measurement voltage, And among them, - The current measurement circuit (10) comprises: a measuring channel (26) having a measuring current-to-voltage converter (28) configured to determine, one at a time and with reference to the alternating measurement voltage, a measuring current through a sensing electrode of one of the plurality of capacitive sensors, the measuring current being indicative of a position of the object relative to the respective capacitive sensor, and - a diagnostic channel (34) having a diagnostic current-to-voltage converter (36) configured to determine a diagnostic current flowing through the guard electrode and the sense electrode of each of the plurality of capacitive sensors with reference to an AC ground potential (16).

10. The capacitive sensing system of claim 9, wherein: The current measurement circuit (10) includes a plurality of diagnostic channels (34, 34'), each diagnostic channel (34, 34') including a diagnostic current-to-voltage converter (36, 36') configured to determine a diagnostic current flowing through a guard electrode and a sense electrode of a different one of the plurality of capacitive sensors with reference to an AC ground potential (16).

11. The capacitive sensing system according to claim 9 or 10, wherein: The switch remote control unit (62) forms part of the microcontroller (60).

12. The capacitive sensing system according to claim 9 or 10, further comprising a signal processing unit configured to process at least one of an output signal of the measuring current-to-voltage converter (28) with reference to the alternating measurement voltage and an output signal of the diagnostic current-to-voltage converter (36) with reference to the alternating measurement voltage.

13. The capacitive sensing system according to claim 9 or 10, further comprising: A demodulation circuit is configured to at least one of demodulate an input signal of the measurement current-to-voltage converter (28) with reference to the alternating measurement voltage and demodulate an input signal of the diagnostic current-to-voltage converter (36) with reference to the alternating measurement voltage.

14. A method for operating a capacitive sensing system according to any one of claims 9 to 13 regarding capacitive sensor diagnostics, the method comprising at least the following steps: - controlling the remotely controllable switch unit (42) to electrically disconnect (72) the sensing electrode of the capacitive sensor to be diagnosed from the signal input port (30) of the measuring current-voltage converter (28), - controlling the remotely controllable switch unit (42) to electrically connect (74) the sensing electrode of the capacitive sensor to be diagnosed to the signal input port (38) of the diagnostic current-voltage converter (36), - controlling the remotely controllable switch unit (42) to electrically connect (76) the sensing electrode of another capacitive sensor to the signal input port (30) of the measuring current-voltage converter (28) and to disconnect (78) the sensing electrode of the capacitive sensor from a protection potential or from an AC ground potential (16), - controlling the remotely controllable switch unit (42) to electrically connect (80) the sensing electrodes of the remaining capacitive sensors of the plurality of capacitive sensors to an AC ground potential (16), - determining (82) by means of the diagnostic current-to-voltage converter (36) a value of a sensed current through the sense electrode of the capacitive sensor to be diagnosed, - comparing the determined sensed current value with at least one predetermined threshold value (84), and - If the determined sensed current value is less than said predetermined threshold value, generating (86) an output signal indicating a sensor interruption.

15. The method according to claim 14, wherein: For each cycle of steps (72) to (86), after switching to the next capacitive sensor to be diagnosed among the plurality of capacitive sensors, steps (72) to (86) are performed and repeated in a periodic manner until all capacitive sensors are diagnosed.

16. A program product (68) for controlling the automatic execution of the steps of the method according to claim 14 or 15, wherein: The method steps to be performed are converted into program code of the program product (68), wherein the program code can be implemented in a digital data storage unit (66) of the capacitive sensing system or a separate control unit and the program code can be executed by a processor unit (64) of the capacitive sensing system or a separate control unit.

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