Circuitry for alternating heating and capacitance measurement runs and related methods

Abstract

The invention relates to a circuit and a method for performing an alternating heating operation and a capacitance measuring operation by means of a common heating line, comprising the steps of: performing a heating operation, by means of a switching of a control circuit, a first pair of switching elements and a second pair of switching elements are in a conductive state, the first and second switching elements are connected in series with the heating line; the heating line is conductively connected to one of two different heating potentials via the first and second switching elements, respectively, such that a heating current is applied to the heating line; the control circuit triggers a switching of the first and second switching elements to a measuring operation, such that both conductive connections are interrupted multiple times in the measuring operation; performing a measuring operation, in which a detection circuit applies an alternating voltage of an alternating voltage source to the heating line to determine a measuring capacitance of the heating line with respect to a reference potential; performing a test phase, in which the control circuit switches a test circuit such that a test impedance is connected to the measuring capacitance, and the detection circuit detects at least a related change of the measuring capacitance and / or determines a total impedance.

Description

Background Technology

[0001] Steering wheels and driver's seats in motor vehicles are typically equipped with electric heaters as a comfort feature. For this purpose, heating wires are routed through the grip areas, particularly the rim of the steering wheel or the seat cushion and backrest. The heating wires pass through the seat cushion and backrest. For safety reasons, but also to achieve the additional comfort function, the ability to perform detection on contact or at least proximity is required, such as so-called "hands-on detection," which concerns monitoring grip on the steering wheel rim or performing driver-passenger identification, which, for example, involves activating or deactivating specific comfort functions specific to the seat position. Therefore, it has come to be that the heating wire, in a non-heating phase, is used as an electrode in a so-called measurement run for capacitive proximity detection. Since heating runs are typically performed with pulse-width modulated heating current, there are phases where no heating current is present for the measurement run. To avoid interference from "all sides" during measurement operation—that is, to prevent interference caused by all electrodes providing heating current and heating voltage from entering the heating wire, which acts as a capacitor electrode, and the reference electrode (or ground potential)—it is known, for example, from DE 11 2014 002 044 T5, to isolate the heating wire on all electrodes from the electrodes providing different heating potentials during measurement operation using a field-effect transistor. Switching devices, especially field-effect transistors, have parasitic capacitances that interfere with the determination of the actual measured capacitance. US 2010 / 0033851 A1 proposes to support the insulation effect of disconnecting the switching device during measurement operation by adding an impedance (especially a diode), wherein additionally, a shielding signal can be applied to the connection line between the diode and the switching device. This solution has the disadvantage that the added impedance, especially the diode, affects the heating current, particularly by having ohmic losses, and therefore the heating voltage cannot be optimally converted into the electrical heating power (Joule heating) emitted by the heating wire. Another problem when using heated wires as electrodes for capacitive proximity detection is that the electrodes are subjected to high thermal loads and thermal expansion fluctuations due to the change in measurement operation between heated and non-heated operation. This can ultimately lead not only to failure of the heated wires but also to fluctuations in the sensitivity of the detection circuit connected to the heated wires. Summary of the Invention

[0002] Against this backdrop, the object of the present invention is to provide a circuit system capable of alternating between a heating measurement mode and a capacitance measurement mode, wherein, in addition to effectively utilizing the heating current during heating operation, the reliability of capacitive proximity detection during measurement operation is at least ensured or even improved.

[0003] This objective is achieved by the circuit system of claim 1. Other features, embodiments, characteristics, and advantages will be apparent from the dependent claims, the specification, and the drawings. The method of alternating operation of heating measurement mode and capacitance measurement mode via a common heating line according to the invention, and the application of the circuit device according to the invention, are the subject of the additional independent claims.

[0004] This invention relates to a circuit system for alternating operation of a heating mode and a capacitance measurement mode when using a common heating wire, wherein during heating operation, a heating current fed from two electrodes located at different heating potentials is applied to the heating wire, which is, for example, a resistance wire (e.g., nichrome wire), wherein the heating voltage on the heating wire decreases. The circuit arrangement has a pair of first switching elements and a pair of second switching elements. Preferably, the first switching elements are constructed of transistors, preferably each of which is a field-effect transistor, and generally preferably of a metal-oxide-semiconductor field-effect transistor (MOS-FET). More preferably, the first and second switching elements are implemented by transistors, preferably each of which is a field-effect transistor, and generally preferably of a MOS-FET. Here, the heating wire is connected to the first and second switching elements such that during heating operation, the first and second switching elements are simultaneously in a conductive state, and the first and second switching elements are connected in series with the heating wire. Here, the heating wire is conductively connected to one of two different heating potentials, for example, one side is the vehicle ground potential, and the other side is the battery positive potential, respectively, via the first switching element and the second switching element connected to the first switching element via a conductor segment. During heating operation, a heating current is applied to the heating wire by connecting the first and second switching elements. If at least one of the first and second switching elements is in a non-conductive or open state, no heating current is applied. Therefore, the heating power of the heating wire can be adjusted by periodically switching and changing the duration of the corresponding heating operation, for example by controlling at least one or all of the switching elements with a pulse width modulation signal.

[0005] According to the invention, a detection circuit is also provided for determining the measured capacitance of the heating wire relative to a reference potential (e.g., a reference electrode or vehicle ground potential) by applying an AC voltage from an AC voltage source to the heating wire during measurement operations other than heating operation. Changes in the measured capacitance can, for example, detect the approach of a vehicle passenger or at least the approach of a vehicle passenger's hand. Different methods are known for determining this type of measured capacitance. According to the invention, the method used herein includes: reliably detecting the measured capacitance by applying an AC voltage to the heating wire, which serves as a emitting electrode. An amplitude modulation detection circuit provides a high-frequency AC current (e.g., 20 kHz) to the capacitor to be measured formed by the heating wire and detects the generated reactive current.

[0006] In a frequency modulation (FM) detection circuit, the capacitor to be measured is interconnected with an inductor to form an oscillating circuit, as part of an LC oscillator, and its frequency is measured by comparing it with a reference. In another variant of the FM detection circuit, the measuring capacitor is part of an astable multivibrator. Preferably, the detection circuit is designed to measure the change in current between a heated conductor and an AC voltage source caused by an applied AC voltage during measurement operation, thereby determining the corresponding measuring capacitor based on the phase shift between the AC voltage and the current change. For example, in the case of signal amplification by a measurement amplifier, the current change is measured based on the voltage drop across a resistor (shunt) in a parallel circuit.

[0007] The invention also includes a control circuit to switch the first and second switching elements from heating operation to measurement operation, during which the first and second switching elements are in an open state, such that the two connections of the heating wire, which conduct electricity relative to different heating potentials during heating operation, are interrupted multiple times during measurement operation. This multiple interruption of the two heating potentials has the following advantages: it is particularly effective for capacitive decoupling of the heating wire relative to the heating potential, and the multiple interruptions reduce parasitic capacitance on the connections to the heating potentials, where the switching elements should now be considered as series capacitive impedances. It also allows for better utilization of the AC voltage detection circuit because the first switching elements are symmetrically separated, for example, unlike the asymmetrically connected diodes in the prior art, and this separation affects both current directions of the AC current generated during measurement operation. This facilitates and improves the determination of the measurement capacitance by means of AC voltage, particularly by a preferred method of detecting phase shift.

[0008] According to the invention, the control circuit is further configured to, during a test phase in a measurement run, switch a test circuit, including at least one third switching element and a test impedance, such that the test impedance is connected to the measurement capacitor, and the detection circuit detects at least the relevant changes in the measurement capacitor and / or the total impedance composed of the measurement capacitor and the test impedance. For example, only the qualitative detection of changes in the measurement capacitor caused by connecting the test impedance. In another embodiment, a quantitative measurement of the total impedance generated by the measurement capacitor and the test impedance is provided to enable calibration of the detection circuit by means of a predetermined test impedance. In a simple embodiment, only the temporal consistency between changes in the measurement capacitor and changes in the switching state is studied. By setting the test phase and the associated construction features, the functional integrity of the test heating line and the detection circuit can be extended with a relatively simple technical construction of the heating circuit and the measurement circuit, and calibration of the detection circuit or downstream evaluation device can be performed in a quantitative study, for example, by determining changes in the measurement capacitor or the total impedance generated during the test phase. Preferably, the duration of the test phase is shorter than the duration of the measurement run between two consecutive heating runs. Outside the test phase, at least the third switching element is in the off state. For example, the duration of the test phase is less than 1 / 10 of the total duration of the measurement run including the test phase.

[0009] In one embodiment, the test phase occurs only during periods excluding manual approach to the steering wheel, such as when the driver's cab is locked and unoccupied. In another embodiment, the test phase is performed only if no approach is detected in a previous measurement run occurring outside of the test phase. In one embodiment, the test phase is performed at the start of a measurement run. In alternating heating and measurement runs, it can be configured such that at least one measurement run without a test phase is performed among several measurement runs, preferably multiple measurement runs without a test phase.

[0010] The connection includes, for example, a measuring capacitor connected in series with a test impedance. However, it is preferably configured such that the test impedance belonging to the test circuit is connected in parallel to the measuring capacitor during the test phase.

[0011] Preferably, during the testing phase, the third switching element electrically connects the heating wire to a reference potential via a test impedance. Preferably, the test impedance is a capacitor with a predetermined test capacitance. To avoid parasitic capacitance, the third switching element is preferably a bipolar transistor.

[0012] Preferably, the circuit system is designed such that heating operation and measurement operation are performed alternately. For example, the control circuit is configured to generate pulse width modulation control signals for the first and / or second switching elements. Furthermore, a microcontroller is provided, for example, to change the duty cycle of the pulse width modulation control signals according to the desired and / or predetermined heating power.

[0013] A preferred embodiment of the circuit system according to the invention also includes a shielding circuit configured to apply an AC voltage from an AC voltage source to at least the conductor segment between the respective first and second switching elements during measurement operation. In this case, the term AC voltage is reused based on the fact that the AC voltage applied to the heating wire and the AC voltage applied to the conductor segment during measurement operation are substantially consistent in amplitude, frequency, and phase to achieve optimal shielding.

[0014] According to a preferred embodiment, at least the first switching element is a transistor, particularly a field-effect transistor, and a shielding circuit system is provided to ensure that an AC voltage is applied to the control terminals, such as the base or gate, of the relevant transistor during measurement operation, thereby achieving particularly effective shielding. Here, the AC voltage and / or the first switching element are designed to exclude switching operations of the first switching element during measurement operation.

[0015] According to a preferred embodiment, the detection circuit is supplemented with a compensation circuit to compensate for the temperature-dependent blocking behavior of the first switching element, particularly when the first switching element is configured as a field-effect transistor and cannot completely prevent temperature-dependent reactive current. To compensate for this, the compensation circuit is specifically configured to, for example, change the operating point of the measuring amplifier measuring the alternating current in a temperature-dependent manner, thereby counteracting the change in blocking behavior. For this purpose, the compensation circuit has, for example, a microcontroller-controlled reference circuit forming an R-2R network.

[0016] The present invention also relates to the use of a circuit system of one of the foregoing embodiments in a motor vehicle, wherein the heating wire is integrated into the steering wheel of the motor vehicle, for example, integrated into the steering wheel rim.

[0017] The present invention also relates to a method for alternately performing heating operation and capacitance measurement operation by means of a common heating line, the method comprising the following steps.

[0018] During heating operation, the control circuit switches a pair of first switching elements and a pair of second switching elements to a conductive state. In this heating operation, the first and second switching elements, along with the heating wire, are connected in series. Furthermore, during heating operation, the heating wire is electrically connected to one of two different heating potentials via the first switching element and the second switching element connected to the first switching element via a conductor segment, such that a heating current is applied to the heating wire due to the different heating potentials.

[0019] In the next step, the control circuit triggers the first and second switching elements to switch from heating operation to measurement operation, during which the first and second switching elements are in an open state. Thus, the two conductive connections of the heating wire relative to different heating potentials during heating operation are interrupted multiple times during measurement operation. During measurement operation, the measurement capacitance of the heating wire relative to a reference potential is determined by applying an AC voltage to the heating wire via a detection circuit. Subsequently, preferably, a switch from measurement operation to heating operation occurs; more preferably, the heating mode and measurement mode are operated alternately.

[0020] According to the present invention, during the test phase of measurement operation, a test circuit is switched by a control circuit, the test circuit including at least one third switching element and a test impedance, such that the test impedance is connected to a measuring capacitor, and a detection circuit detects at least the relevant changes in the measuring capacitor and / or the total impedance constituted by the measuring capacitor and the test impedance. For example, only the change in the measuring capacitor caused by connecting the test impedance is qualitatively detected. In another embodiment, a quantitative measurement of the total impedance generated by the measuring capacitor and the test impedance is provided to enable calibration of the detection circuit by means of a predetermined test impedance. In a simple embodiment, only the temporal consistency between changes in the measuring capacitor and changes in the switching state is studied.

[0021] By setting up a test phase and related construction features, the functional integrity of the test heating wire and detection circuit can be extended through a relatively simple technical construction of the heating and measurement circuits. Furthermore, calibration of the detection circuit or downstream evaluation device can be performed in quantitative studies, such as by determining the change in measured capacitance or the total impedance generated during the test phase. Preferably, the duration of the test phase is shorter than the duration of the measurement run between two consecutive heating runs. Outside of the test phase, at least a third switching element is in the off state. For example, the ratio of the duration of the test phase to the total duration of the measurement run including the test phase is less than 1 / 10.

[0022] In one embodiment, the test phase occurs only during periods excluding manual approach to the steering wheel, such as when the driver's cab is locked and unoccupied. In another embodiment, the test phase is performed only if no approach is detected in a previous measurement run occurring outside of the test phase.

[0023] In one embodiment, a test phase is performed at the start of a measurement run. In alternating heating and measurement runs, it can be configured such that at least one measurement run without a test phase is performed among several measurement runs; preferably, multiple measurement runs without a test phase are performed.

[0024] The connection includes, for example, a measuring capacitor connected in series with a test impedance. However, it is preferably configured such that the test impedance belonging to the test circuit is connected in parallel to the measuring capacitor during the test phase.

[0025] Preferably, during the testing phase, the third switching element electrically connects the heating wire to a reference potential via a test impedance. Preferably, the test impedance is a capacitor with a predetermined test capacitance. To avoid parasitic capacitance, the third switching element is preferably a bipolar transistor.

[0026] In a preferred embodiment of the method according to the invention, an AC voltage from an AC voltage source is applied to the conductor segment via a shielding circuit during measurement operation. In this case, the term AC voltage is used based on the fact that the AC voltage applied to the heating wire and the AC voltage applied to the conductor segment during measurement operation are substantially consistent in amplitude, frequency, and phase to achieve optimal shielding.

[0027] According to a preferred embodiment, at least the first switching element is implemented by a transistor, particularly a field-effect transistor, wherein a shielding circuit is designed to apply an AC voltage to the control terminals, such as the base or gate, of the relevant transistor during measurement operation to achieve particularly effective shielding. Here, the AC voltage and / or the first switching element are designed to exclude switching operations of the first switching element during measurement operation.

[0028] According to a preferred embodiment of the method, during measurement operation, the detection circuit generates a current change between the heated conductor and the AC voltage source due to the application of an AC voltage, thereby determining the corresponding measurement capacitance based on the phase shift between the AC voltage and the current change.

[0029] According to a preferred embodiment of this method, the temperature-dependent blocking behavior of the first switching element is compensated during detection, particularly when the first switching element is configured as a field-effect transistor and cannot completely prevent temperature-dependent reactive current. To compensate for this, the detection circuit is supplemented with a compensation circuit that alters the operating point of the measuring amplifier measuring changes in AC current in a temperature-dependent manner, thereby counteracting the change in blocking behavior. For this purpose, the compensation circuit has, for example, a microcontroller-controlled reference circuit forming an R-2R network. Attached Figure Description

[0030] The invention will now be explained in detail with reference to the accompanying drawings. The drawings are to be understood as illustrative only and represent preferred embodiments and variations. The drawings show:

[0031] Figure 1 It is a schematic plan view of a steering wheel with a heating wire integrated into the steering wheel and belonging to the circuit system according to the invention;

[0032] Figure 2A schematic diagram of the circuit system according to the present invention. Detailed Implementation

[0033] Figure 1 The use of the circuit system 1 according to the invention in a steering wheel 10 of a motor vehicle (not shown) is illustrated. A heating wire 2 (e.g., a resistance wire, such as a nichrome wire) is integrated into the grip area 20 (here, the steering wheel rim) of the steering wheel to, on the one hand, heat the grip area 20 for the vehicle passenger B gripping the steering wheel 10 during the heating operation of the circuit system 1, and on the other hand, perform capacitive detection of contact or proximity of the vehicle passenger B's hand to or near the grip area 20. For safety reasons, but also to achieve additional comfort functions, this capacitive detection is provided for contact and at least one proximity to perform, for example, so-called hand-on detection, which relates to monitoring whether the steering wheel rim is being gripped, or to perform driver-passenger identification, here involving, for example, activating or deactivating specific comfort functions specific to the seat position. Figure 1 As shown, under heating operation, different heating potentials V are applied to heating wire 2. H+ V H- The heating current. For example, V H- This is the vehicle's grounding potential. During measurement operation, an AC voltage V is applied to the heating wire 2 via the circuit mechanism 1 according to the present invention. AC .

[0034] Figure 2 A circuit system 1 is schematically shown that uses a common heating line 2 for alternating heating operation and capacitance measurement operation. Here, during heating operation, the circuit is controlled by two different heating potentials V. H+ V H-The heating current supplied is applied to the heating wire 2, where the heating voltage drops. For this purpose, the circuit device 1 includes a pair of first switching elements 3a, 3b and a pair of second switching elements 4a, 4b. Here, the first switching elements 3a, 3b are each composed of field-effect transistors, particularly latching field-effect transistors, preferably metal-oxide-semiconductor field-effect transistors (MOS-FETs). The second switching elements 4a, 4b are also implemented by transistors, preferably by field-effect transistors, and typically preferably by MOS-FETs. Here, the heating wire 2 is connected to the first switching elements 3a, 3b and the second switching elements 4a, 4b such that during heating operation, both the first switching elements 3b and the second switching elements 4a, 4b are simultaneously conductive, and the first switching elements 3a, 3b and the second switching elements 4a, 4b are connected in series with the heating wire 2. In this configuration, heating wire 2 is electrically connected to two different heating potentials V via first switching elements 3a and 3b and second switching elements 4a and 4b connected to the first switching elements 3a and 3b via conductor segments 5a and 5b. H+ V H- One of the switching elements 3a, 3b, and 4a, 4b is selected, causing a heating current to be applied to the heating wire 2. If at least one of the first switching elements 3a, 3b and the second switching elements 4a, 4b is in a non-conductive or open state, no heating current is applied. This is achieved, for example, by means of a pulse width modulation (PWM) control signal from the microcontroller 13. a or PWM b The microcontroller controls at least one or all switching elements 3a, 3b; 4a, 4b to periodically switch and vary the duration of the corresponding heating operation, thereby adjusting the heating power of the heating line 2. The microcontroller is associated with control circuits 6a, 6b, 13 and controls the control circuit components 6a, 6b assigned to the switching elements 3a, 3b; 4a, 4b. During heating operation, switching element 14, belonging to the test circuits 14, 15 described in detail below, is switched to the off state.

[0035] According to the present invention, a detection circuit 9 is also provided to detect the AC voltage V of the AC voltage source 12 applied to the heating wire 2 during measurement operations other than heating operations. AC The measurement capacitance of heating wire 2 relative to a reference potential (e.g., vehicle ground potential 16) is determined, in which case the AC voltage source 12 is a sine wave generator controlled by microcontroller 12. The change in this capacitance can be used to detect, for example, the approach of vehicle passenger B, or at least the approach of vehicle passenger B's hand. Here, the detection circuit 9 is designed to measure the capacitance due to the applied AC voltage V during measurement operation. AC The change in current generated between the heating conductor 2 and the AC voltage source 12, so as to thereby utilize the AC voltage VAC The capacitance is determined by the phase shift between the current and the current change. Specifically, the current change is measured based on the voltage drop at the parallel circuit resistor 8 (shunt) after the signal is amplified by the measurement amplifier of the detection circuit, and the measurement result of the current change is transmitted to the microcontroller 12.

[0036] The switching from heating operation to measurement operation is achieved by the microcontroller 13 in cooperation with control circuit components 6a and 6b. This ensures that during measurement operation, the first switching elements 3a and 3b and the second switching elements 4a and 4b are in the off state, thus allowing the heating wire 2 to switch relative to different heating potentials V during heating operation. H+ V H- The two conductive connections were interrupted several times during the measurement operation.

[0037] Regarding the two heating potentials V H+ V H- Multiple interruptions have the following advantages: except for heating wire 2 relative to the heating potential V H+ V H- Capacitive decoupling is particularly effective, and multiple interruptions reduce the interaction with the heating potential V. H- V H- The parasitic capacitances on the connection, where switching elements 3a, 3b; 4a, 4b should now be considered as series capacitive impedances, and the AC voltage V can be better utilized. AC The detection circuit 9 is designed for this purpose because the first switching elements 3a and 3b are symmetrically separated, unlike the asymmetrically connected diodes in the prior art. This separation affects both current directions of the alternating current generated during measurement operation, which helps and improves the measurement using the alternating voltage V. AC The capacitance is determined, particularly by a preferred method of detecting phase shift. This is because the microcontroller 13 uses a pulse-width modulation (PWM) control signal. a or PWM b The control circuit components 6a and 6b, and the circuit system 1, are designed to operate alternately in heating mode and measurement mode. The microcontroller 13 adjusts the pulse width modulation control signal PWM according to the desired and / or predetermined heating power. a or PWM b Duty cycle.

[0038] In the circuit system shown according to the present invention 1, a shielding circuit 7 is also provided, which is configured to, during measurement operation, supply power not only to the conductor segments 5a, 5b between the corresponding first switching elements 3a, 3b and the second switching elements 4a, 4b, but also to the control terminals G of the first switching elements 3a, 3b. a G b Apply AC voltage V from AC voltage source 12 ACIn this context, the term AC voltage is considered to refer to the fact that the AC voltage applied to heating wire 2 and the AC voltage applied to conductor segments 5a and 5b during measurement operation are substantially the same in amplitude, frequency, and phase to achieve optimal shielding.

[0039] To compensate for the temperature-dependent blocking behavior of the first switching elements 3a and 3b, the detection circuit 9 is supplemented with a compensation circuit 11 to compensate for the temperature-dependent reactive current or temperature-dependent blocking behavior of these first switching elements 3a and 3b. Here, the compensation circuit 11 is configured to change the operating point of the measuring amplifier of the detection circuit 9 in a temperature-dependent manner, thereby counteracting the change in blocking behavior. For this purpose, the compensation circuit has, for example, a reference circuit constituting an R-2R network, which is connected to the microcontroller 13 used to control the compensation.

[0040] According to the invention, control circuits 6a, 6b, 13 are further configured to switch test circuits 14, 15 (including a third switching element 14 and a test impedance 15) during a test phase of a measurement run, such that the test impedance 15 connects to the measurement capacitor, and the detection circuit 9 detects at least the relevant changes in the measurement capacitor and / or the total impedance constituted by the measurement capacitor and the test impedance. For example, only the change in the measurement capacitor caused by connecting the test impedance 15 is qualitatively detected. In another embodiment, a quantitative measurement of the total impedance generated by the measurement capacitor and the test impedance 15 is provided to enable calibration of the detection circuit by means of a predetermined test impedance. In a simple embodiment, only the temporal consistency between changes in the measurement capacitor and changes in the switching state is studied. By setting up a test phase and related construction features, the functional integrity of the test heating line 2 and the detection circuit 9 can be extended with a relatively simple technical construction of the heating circuit and the measurement circuit, and calibration of the detection circuit 9 or downstream evaluation device can be performed in a quantitative study, for example, by determining changes in the measurement capacitor or the total impedance generated during the test phase. Preferably, the duration of the test phase is shorter than the duration of the measurement run between two consecutive heating runs. During the measurement run outside the test phase, at least the third switching element 14 is in the off state. To inspect the heating wire 2 along its entire length, an AC voltage VAC is fed to one end of the heating wire 2, and a test impedance 15 is connected to the other end. Thus, the test impedance 15, associated with test circuits 14 and 15, is connected in parallel to the measurement capacitor during the test phase. Here, during the test phase, the third switching element 14 electrically connects the reference potential 16 to the heating wire 2 via the test impedance 15. Here, the test impedance 15 is a capacitor with a predetermined test capacitance. To avoid parasitic capacitance, the third switching element 14 is a bipolar transistor. For example, the ratio of the duration of the test phase to the total duration of the measurement run including the test phase is less than 1 / 10.

[0041] The timing sequence of the heating and measurement runs is not shown in detail. In one embodiment, the test phase occurs only during periods when there is no hand proximity to the steering wheel, such as when the driver's cab is locked and unoccupied. In another embodiment, the test phase is performed only if no proximity is detected in a previous measurement run outside of the test phase. In one embodiment, the test phase is performed at the start of the measurement run. In alternating heating and measurement runs, it can be configured such that at least one measurement run without a test phase is performed among several measurement runs, preferably multiple measurement runs without a test phase.

Claims

1. A circuit system (1) for alternating heating operation and capacitance measurement operation, comprising: A pair of first switching elements and a pair of second switching elements; A heating wire (2) is connected to the first switching element and the second switching element such that during heating operation, the first switching element and the second switching element are in a conductive state, the first switching element and the second switching element are connected in series with the heating wire (2), and the heating wire (2) is conductively connected to one of two different heating potentials via one of the first switching elements and one of the second switching elements connected to the first switching element via a conductor segment, such that a heating current is applied to the heating wire (2); The detection circuit (9) is used to determine the measured capacitance of the heating wire (2) relative to the reference potential (16) by applying an AC voltage (VAC) from an AC voltage source (12) to the heating wire during measurement operations other than heating operation. The test circuit has at least one third switching element (14) and a test impedance (15). The control circuit is configured to switch the first and second switching elements from heating operation to measurement operation, wherein during the measurement operation, the first and second switching elements are in an open state, such that the two conductive connections of the heating wire (2) with respect to different heating potentials during the heating operation are interrupted multiple times during the measurement operation, and the control circuit is also configured to switch the test circuit during the test phase within the measurement operation, such that the test impedance (15) is connected to the measurement capacitor, and the detection circuit (9) detects at least the relevant changes in the measurement capacitor, and / or the total impedance consisting of the measurement capacitor and the test impedance (15); The test impedance (15) is connected in parallel to the measurement capacitor during the test phase; During the testing phase, at least one of the third switching elements (14) conductively connects the heating wire (2) to the reference potential (16) via the test impedance (15); The test impedance (15) is a capacitor with a predetermined test capacitance.

2. The circuit system (1) according to claim 1, wherein the third switching element (14) is a bipolar transistor.

3. The circuit system (1) according to claim 1 further comprises a shielding circuit (7) configured to apply at least the AC voltage (VAC) of the AC voltage source (12) to the conductor segment during measurement operation.

4. The circuit system (1) according to claim 3, wherein at least the first switching element is a transistor, and the shielding circuit (7) is configured such that an AC voltage (VAC) during measurement operation is applied to the control terminals of the respective transistors.

5. The circuit system (1) according to claim 4, wherein the first switching element is a field-effect transistor.

6. The circuit system (1) according to claim 4, wherein the control terminal is a base or a gate.

7. The circuit system (1) according to claim 1, wherein the detection circuit (9) is designed to measure the current change generated between the heating wire (2) and the AC voltage source (12) due to the application of an AC voltage (VAC) during measurement operation, so as to determine the measurement capacitance based on the phase shift between the AC voltage (VAC) and the current change, and to detect the change in the measurement capacitance and / or determine the total impedance during the test phase.

8. The circuit system (1) according to claim 1, wherein the detection circuit (9) is further provided with a compensation circuit (11) to compensate for the temperature-related blocking behavior of the first switching element.

9. Use of the circuit system (1) according to any one of claims 1-8 in a motor vehicle, wherein the heating wire (2) is integrated into the steering wheel (10) of the motor vehicle.

10. A method for alternating heating and capacitance measurement using a common heating line (2), comprising the following steps: During the heating operation, a pair of first switching elements and a pair of second switching elements are in a conductive state by switching by the control circuit. The first switching elements and the second switching elements are connected in series with the heating wire (2). The heating wire (2) is conductively connected to one of two different heating potentials via one of the first switching elements and one of the second switching elements connected to the first switching element via a conductor segment, so that a heating current is applied to the heating wire (2). The control circuit triggers the first and second switching elements to switch from heating operation to measurement operation, wherein during the measurement operation, the first and second switching elements are in an open state, so that the two conductive connections of the heating wire (2) with respect to two different heating potentials are interrupted multiple times during the measurement operation. A measurement operation is performed in which the measured capacitance of the heating wire (2) relative to a reference potential (16) is determined by applying an AC voltage (VAC) from an AC voltage source (12) to the heating wire (2) via a detection circuit (9); The test phase is performed during the measurement operation, and the test circuit includes at least one third switching element (14) and a test impedance (15). The control circuit switches the test circuit such that the test impedance (15) is connected to the measurement capacitor, and the detection circuit (9) detects at least the relevant changes in the measurement capacitor and / or the total impedance consisting of the measurement capacitor and the test impedance (15). The test impedance (15) is connected in parallel to the measurement capacitor during the test phase; During the testing phase, at least one of the third switching elements (14) conductively connects the heating wire (2) to the reference potential (16) via the test impedance (15). The test impedance (15) is a capacitor with a predetermined test capacitance.

11. The method according to claim 10, wherein, The third switching element (14) is a bipolar transistor.

12. The method of claim 10, wherein during measurement operation, the AC voltage (VAC) of the AC voltage source (12) is applied to the conductor segment via the shielding circuit (7).

13. The method of claim 10, wherein at least the first switching element is a transistor, and an alternating current voltage (VAC) is applied to the control terminals of the transistor during measurement operation.

14. The method of claim 13, wherein the first switching element is a field-effect transistor.

15. The method of claim 13, wherein the control terminal is a base or a gate.

16. The method according to claim 10, wherein during measurement operation, the detection circuit (9) measures the current change generated between the heating wire (2) and the AC voltage source (12) due to the application of an AC voltage (VAC) in order to determine the measurement capacitance based on the phase shift between the AC voltage (VAC) and the current change, and detects the change in the measurement capacitance during the test phase, and / or determines the total impedance.

17. The method of claim 10, wherein the temperature-dependent blocking behavior of the first switching element is compensated during measurement operation.

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