Measurement system and sensor circuit for capacitive sensors

Abstract

A sensor circuit for capacitive sensors comprises a DC reference driver generating a DC reference signal, a sensor driver generating an AC signal having an output port outputting the AC signal to feed a capacitive sensor and a current output port outputting a measurement current corresponding to the AC signal, and a plurality of sensor ports connected to respective capacitive sensors. The sensor circuit has a switch matrix between the sensor ports and the drivers, which has for each sensor port a switch path comprising an RT switch, a sensor switch and a load resistance. The switch matrix and the capacitive sensors are DC decoupled by DC decoupling means between them. The sensor circuit can perform a self-test: the sensor driver outputs a DC signal, such that a voltage difference is formed between the DC reference driver and the sensor driver, which is applied to the switch matrix. The current flowing through the switch matrix during the self-test is measured. The invention also relates to a measurement system having the sensor circuit and the capacitive sensors.

Description

TECHNICAL FIELD

[0001] The invention relates to a sensor circuit for a capacitive sensor, comprising a DC reference driver for generating a DC reference signal, a sensor driver for generating a sinusoidal signal for feeding a capacitive sensor, a plurality of sensor ports for connecting to one capacitive sensor each, and a switch matrix for establishing a connection of the sensor ports to the drivers. The invention further relates to a capacitive measuring system for evaluating a capacitive sensor, comprising a sensor, a sensor circuit for a capacitive sensor, and a DC decoupling between the sensor circuit and the sensor. The invention further relates to a corresponding method for performing an integrated self-test of safety-relevant components of a sensor circuit for a capacitive sensor. BACKGROUND

[0002] Capacitive sensors and corresponding measuring systems or sensor circuits are applied in many areas of daily life, for example as contactless sensors or touch sensors. They are particularly common in the automotive sector and in vehicles. Capacitive sensors are used, for example, when it is necessary to check whether a vehicle driver is holding the steering wheel and is able to or is actively driving the vehicle. In the case of autonomous or semi-autonomous systems, it is checked at regular intervals whether the vehicle driver has his hands on the steering wheel in order to determine that the vehicle driver is awake and can manually drive the vehicle if necessary.

[0003] Another field of application in the automotive sector is the recognition of seat occupancy. When a passenger is sitting in a seat in the vehicle, he must fasten his seat belt. For this purpose, on the one hand the seat occupancy is detected and on the other hand it is checked whether the seat belt lock is fastened by the seat belt. When the seat is occupied, this must always be the case. There are various capacitive sensors that perform this check.

[0004] For example, DE 112010004513 B4 proposes a system for recognizing occupancy in a vehicle, in which not only the overall occupancy of a seat is recognized, but it is also possible to distinguish whether a person or another object with significantly less weight than a person is on the seat. In addition to a pure capacitive detection of the presence of a person or an object, the possibility of recognizing a relative force is proposed. For example, a plurality of sensors and one shield electrode are used here, which can be interconnected in different ways using a switch matrix.

[0005] Since the use of such capacitive sensors in the automotive sector relates to safety-relevant components, it is necessary to check all components of these components, in particular whether the switches for switching the individual internal switch paths are functioning properly. SUMMARY

[0006] This task is accomplished by a sensor circuit having the features of claim 1, a capacitive measurement system having the features of claim 9, and a method having the features of claim 15.

[0007] Within the scope of this invention, it has been recognized that checks on individual components within a sensor circuit can be performed through integrated self-tests. Importantly, this ensures the proper functioning of switches and resistors used in the circuit paths of the switch matrix. This invention allows for reliable and simple checks of these components without the need for additional parts.

[0008] In one aspect, the present invention relates to a sensor circuit for capacitive sensors, comprising a DC reference driver for generating a DC reference signal, a sensor driver for generating a sinusoidal signal, a plurality of sensor ports for connecting to each capacitive sensor, and a switch matrix for establishing connections between the sensor ports and the driver. The sensor driver has an output port for outputting a sinusoidal signal for feeding the capacitive sensor. It also includes a current output port for outputting a measured current corresponding to the sinusoidal signal output at the output port. This measured current can be used to measure and inspect safety-related components. The switch matrix has a line path for each sensor port, the line path comprising an RT switch (a switch close to the DC reference driver), a sensor switch, and a load resistor. The RT switch and the load resistor are preferably connected in series between the DC reference driver and the corresponding sensor port. The sensor switch is connected between the sensor port and the sensor driver.

[0009] This switch matrix enables the connection of one of the sensor ports of the sensor circuit to a sensor driver, such that during operation of the sensor circuit and for measuring the connected sensor, a sinusoidal signal output by the sensor driver reaches the capacitive sensor connected at the corresponding sensor port. Other sensor ports are decoupled from the sensor driver by disconnecting the sensor switches in their respective line paths. A DC reference signal from a DC reference driver is used to supply the other sensor ports to raise them to a defined potential. For this purpose, the RT switch is closed in series with the load resistor.

[0010] A change in the impedance of a capacitive sensor causes a change in the current discharged by the sensor driver. This current is additionally measured at the current output port, such that the change in impedance is reflected in the change in current. The measured current at the current output port is fed to an evaluation unit, which may preferably include an analog signal processing unit and a filter to provide a signal to an analog-to-digital converter, such that the current signal can be further processed into a digital signal, for example.

[0011] A DC coupling device arranged between the switching matrix and the capacitive sensor ensures DC decoupling. However, it is permissible for the AC signal or sinusoidal signal output by the sensor driver used to detect the capacitive sensor. DC coupling devices only suppress DC signals. This can be done, for example, by using a capacitor (Kondensator) or a capacitor.

[0012] According to the present invention, the sensor circuit is designed to perform a self-test, wherein the sensor driver outputs a DC signal. Therefore, the sensor driver is designed to output not only an AC signal or a sinusoidal signal but also a DC signal at its output terminal. A voltage difference is formed between the DC reference driver and the sensor driver by generating a DC voltage, which is applied to the switching matrix. Here, during the self-test, the switching path between the DC reference driver and the sensor driver is closed, wherein the switching path includes a series-connected RT switch, a sensor switch, and a load resistor. The generated voltage difference causes a measurable current to flow through the switching matrix. Preferably, the current is tapped and measured at the current output port of the sensor driver. This is done, for example, by an evaluation unit.

[0013] This allows for the inspection of components in the switch path of the switch matrix for each sensor port, where the load resistor, RT switch, and sensor switch are assigned to the sensor port. Each sensor port is inspected sequentially.

[0014] In order to measure only the safety-related components in the switching path for a single sensor port, the other sensor ports of the sensor circuit are isolated from the two drivers, namely the DC reference driver and the sensor driver, by disconnecting the corresponding RT switch and sensor switch belonging to that sensor port.

[0015] On the other hand, the present invention relates to a capacitive measurement system for evaluating capacitive sensors, comprising a sensor, preferably having sensor electrodes, a sensor circuit for the capacitive sensor, and a DC decoupling device between the sensor circuit and the sensor. The sensor circuit herein includes a DC reference driver for generating a DC reference signal, a sensor driver for generating and outputting an AC signal, such as a sine wave, at an output port to feed the sensor, a sensor port for connecting to the sensor, and a switching matrix for establishing connections between the sensor port and the two drivers, namely the DC reference driver and the sensor driver. The sensor circuit preferably includes multiple sensor ports to enable the manipulation and evaluation of multiple capacitive sensors.

[0016] According to the present invention, the sensor driver has a current output port for outputting a measured current corresponding to an AC signal or sinusoidal signal output at the output port. The switch matrix includes an RT switch, a sensor switch, and a load resistor connected in series with the RT switch. These components allow the sensor port to be connected to a DC reference driver and / or the sensor driver.

[0017] According to the present invention, the sensor circuit is designed to perform a self-test, wherein the sensor driver outputs a DC signal, causing a voltage difference to be formed between the DC reference driver and the sensor driver, which is applied to the switching matrix. During the self-test, the current flowing through the switching matrix can be measured, for example, by evaluating the measured current output at the current output port.

[0018] To establish a voltage difference between the DC reference driver and the sensor driver during self-test, this is achieved by closing the RT switch and the sensor switch at the relevant sensor port. The sensor ports form a switching path. Preferably, the associated RT switch and sensor switch are disconnected for all other sensor ports, making these ports unconnected and decoupled from the driver. Therefore, it is preferable to always measure only one switching path of the sensor port.

[0019] Another aspect of the invention relates to a corresponding method and a computer program product having program code for performing the steps of the method when the program code is executed on a computer, and to a storage medium having a computer program stored thereon, which, when executed on a computer, causes the computer program to perform the method described herein. The method can also be implemented entirely or partially in hardware.

[0020] The switch matrix of the sensor circuit connects the sensor driver to various capacitive sensors whose impedances are to be measured sequentially. The functionality and safety-related components of the switch matrix must be checked periodically for the sensors in the vehicle. The measurement system according to the invention is based on the principle of controlling sensors with AC signals to measure their impedance. There is no DC path between the sensor port and the ground (GND) or voltage supply device. This is ensured by a DC decoupling device. To check the switch matrix and resistors in the circuit, a self-test (BIST) is preferably performed cyclically between two measurements. For this purpose, only existing circuitry from the sensor circuit where impedance measurement is applied is used. No additional measurement circuitry, comparators, or switches are required.

[0021] The self-test is performed using a DC voltage, which is output by the sensor driver and, if necessary, by a shielded driver (Schirm-Treiber) for the shielded electrodes, instead of an AC signal. Current is measured at the current output using the same signal path also used for impedance measurement. Current is measured for different switching positions of the RT switch and the sensor switch. When one switch is open, it must be approximately zero. If both switches are closed, the (digital) current must be within defined limits, which are stored in the memory unit and can be determined during production testing.

[0022] Preferred embodiments of the invention are described in the dependent claims. It should be understood that, without departing from the scope of the invention, the above-described features and the features explained below can be used not only in the combinations specified, but also in other combinations or individually. In particular, the method and computer program product can be performed according to the designs described in the dependent claims for sensor circuits and measurement systems.

[0023] According to the present invention, safety-related components of the sensor circuit are checked during self-testing, thereby enabling the diagnosis of the integrity of the measurement path of the measurement system. Here, for example, a portion of the sensor circuit used in an IC and in practical applications for AC capacitance measurement is used for DC resistance measurement. No additional components, such as comparators, measuring devices, or switches, are required.

[0024] In a preferred embodiment, the sensor circuit includes at least four sensor ports, preferably at least eight sensor ports. Such circuitry reflects most application requirements of capacitive sensors. Thus, sensor elements with four or eight capacitive sensors can be used. In various cases, it has proven advantageous for the sensor circuit to have 16 sensor ports, allowing a total of 16 capacitive sensors to be connected.

[0025] A preferred embodiment of the sensor circuit specifies that the switch matrix is ​​switched during self-test so that the individual sensor ports are operated alternately. This allows for the examination of all sensor ports, as well as the switching and load resistances between the sensor ports and the sensor drivers, and between the sensor ports and the DC reference driver.

[0026] In another preferred embodiment, other sensor ports that are not manipulated and whose associated components are not inspected are set to a defined potential. They are particularly preferably decoupled from the switch matrix.

[0027] In another preferred embodiment of the sensor circuit, the sensor switch and / or RT switch for the switching path of the relevant sensor port are switched to different switching positions during self-test. In one embodiment, both the RT switch and the sensor switch are closed. In this case, the determined current value or current measurement value or its digitized value must be within defined limits. These limits can be fixed or determined during production testing after production. If these limit values ​​are determined during production testing, narrower limits can be achieved because manufacturing-related tolerances can be calibrated. These limit values ​​are preferably stored in non-volatile memory.

[0028] If at least one of the two switches in the switching path (RT switch and sensor switch) is open, the current value must be zero.

[0029] The sensor circuit according to the invention has the following advantages: existing circuitry for impedance measurement from capacitive sensors can be used for self-testing. This integrated self-test is called "BIST." This name was also used by experts during this period. The term stands for "Built-In Self-Test." The advantage of this BIST measurement is that no additional voltage comparator, measuring device, or switch is required. Therefore, the BIST test should be performed very simply and at low cost. The BIST test is also very robust because no additional components are likely to be error-prone. The BIST measurement is performed using a DC voltage, wherein the DC voltage is output by the sensor driver in place of an AC signal, such as a sinusoidal signal. The current is measured using the same signal path also used for impedance measurement.

[0030] Using BIST measurements, not only can the switches in the switch matrix be checked, but also their resistances. Furthermore, various error modes (Fehlermodi) in the external wiring can also be detected. For example, a short circuit in a capacitor connected to the sensor port can be identified. A short circuit in the capacitor causes an increase in DC current, and is therefore detected. Line breaks, such as those in the lead-in line (Zuleitung) between the sensor circuit and the capacitive sensor, can also typically be identified.

[0031] In a preferred embodiment, the sensor circuit is designed to have a DC driver input and a sensor driver input. A DC signal can be fed into a DC reference driver at the DC driver input and then output by the driver. An external signal can be supplied to the sensor driver at the sensor driver input, where the sensor driver input outputs this external signal. A sinusoidal signal or another AC signal can be fed into the sensor driver input for applied impedance measurement. For BIST testing, a DC signal, instead of an AC signal, can be fed into the sensor driver input, where the DC signal is used and output during self-test. It is also readily possible in this way to enable the sensor driver to output not only AC or sinusoidal signals but also DC signals.

[0032] In a preferred embodiment, the sensor circuit is designed to also supply and operate the shielding electrode of the sensor element, or a shielding electrode belonging to a capacitive sensor. The sensor circuit preferably includes a shielding driver for generating a shielded sinusoidal signal or a shielded AC signal, wherein the shielded sinusoidal signal or shielded AC signal is output at a shielded port to which such a shielding electrode can be connected. The shielding electrode is preferably connected in parallel with the capacitive sensor, enabling measurements to be performed to improve sensor accuracy. Furthermore, shielding electrodes are typically used to suppress parasitic capacitance and other parasitic effects in impedance measurements of the sensor.

[0033] The sensor circuit preferably includes a schrm-switchload port, which is also configured to connect to the shielding electrode, and the sensor circuit has an assigned schrm-switchload port for each sensor port for connection to the sensor. A switching load path is formed between the schrm-switchload port and each sensor switching load port, which includes a switching load switch and a switching load resistor. The switching load path can be integrated into the switch matrix of the sensor circuit. Each sensor switching load port is connected to the associated schrm-switchload port through this switching load path. The switching load resistor can be formed by one or more resistors arranged in series. In practice, a value of 2.5 kOhm has proven advantageous.

[0034] In a preferred embodiment of the sensor circuit, it is specified that each switched load path can be checked during self-test. The shielded driver is preferably designed to output a shielded DC signal used during self-test. The shielded DC signal is preferably not equal to the DC signal of the sensor driver, and particularly preferably it is smaller than the DC signal of the sensor driver. In this way, a voltage difference is formed between the two drivers, thereby driving current through the switched load path during self-test, and the switched load resistance and the switched load switch can be checked. For this purpose, a shielded check path is formed by closing the sensor switch and establishing a connection between the shielded port and the shielded switched load port, as well as a connection between the sensor port and the sensor switched load port. For checking, the switched load switch is closed to check the switched load resistance. The measured current fed into the sensor driver at the current output port is measured. The comparison of the current with the limit value allows the conclusion that the switched load resistance is fault-free.

[0035] When the load changer is open, the measured current value must be close to or equal to zero. In this way, the load changer can also be reliably checked for faultiness by performing these two measurements using the load changer while it is closed and open.

[0036] In a preferred embodiment of the measurement system, the sensor circuit is designed to perform self-tests on all sensor ports, preferably sequentially. In a particularly preferred embodiment, a self-test is performed on the corresponding sensor port after each measurement of the impedance of a capacitive sensor.

[0037] Preferably, the sensor circuit of the measurement system has multiple sensor ports, preferably at least four, and particularly preferably at least eight. In this way, multiple capacitive sensors can be connected to the sensor circuit and checked through a self-test.

[0038] In another preferred embodiment, a filter network is arranged between the sensor circuit and the sensors, preferably providing a filter for each capacitive sensor. In a preferred embodiment, a filter is arranged between each sensor and each sensor port of the sensor circuit, the filter including a coupling capacitor, which preferably forms a DC coupling device. The filter is particularly preferably designed as an EMV filter (electromagnetic compatibility filter), comprising a series inductor and an additional parallel capacitor.

[0039] In another preferred embodiment, the measurement system includes a shielding electrode, which is preferably connected in parallel to the sensor. Particularly preferred is that the shielding electrode and the capacitive sensor form a sensor element. Also preferred is a sensor element comprising a shielding electrode and multiple capacitive sensors.

[0040] Preferably, the sensor circuit of the measurement system has a shielded driver for generating a shielded sinusoidal signal or a shielded AC signal, wherein the shielded sinusoidal signal or shielded AC signal is output at a shielded port of the sensor circuit, where a shielded electrode is connected. In this way, sensor accuracy can be improved, for example, by preventing parasitic capacitance and its effects.

[0041] In a preferred embodiment, the sensor circuit of the measurement system includes two switched load ports connected to each other via a switched load path having a switched load switch and a switched load resistor. One of the switched load ports is a shielded switched load port for connection to a shielded electrode. The second switched load port is a sensor switched load port for connection to the sensor. If multiple sensor ports exist in the sensor circuit, the sensor circuit preferably includes a corresponding sensor switched load port for each sensor port, which is assigned to that sensor port. The sensor port is particularly preferably connected to the assigned sensor switched load port. This connection can also be formed, for example, in a filter arranged between the sensor circuit and the sensor.

[0042] A preferred embodiment of the measurement system includes an evaluation unit, which is preferably part of a sensor circuit. The evaluation unit particularly preferably has an analog signal processing unit and outputs the measurement signal for AD conversion to, for example, an AD converter. The evaluation unit preferably also includes amplifiers and / or filters to preprocess the measurement signal for AD conversion accordingly. Attached Figure Description

[0043] The invention will now be described and explained in more detail using selected embodiments with reference to the accompanying drawings. Wherein:

[0044] Figure 1 The sensor circuit and sensor elements and multiple sensors according to the present invention are shown;

[0045] Figure 2 The invention is shown to have Figure 2 The measurement system of the sensor circuit in the middle;

[0046] Figure 3 It shows Figure 1 Alternative implementation methods for sensor circuits in [the system / system];

[0047] Figure 4 The schematic circuit of another embodiment of a sensor circuit with a sensor is shown;

[0048] Figure 5 A schematic diagram of the implementation scheme of the sensor circuit on the IC is shown.

[0049] Figure 6 It shows the method for using according to Figure 1 The principle and flow of the integrated self-test method for sensor circuits; and

[0050] Figure 7 It shows according to Figure 3 A method for integrating and self-testing safety-related components of sensor circuits. Detailed Implementation

[0051] Figure 1 Sensor circuit 20 is shown, which includes a DC reference driver 22 and a sensor driver 24. Sensor circuit 20 includes a switch matrix 30 arranged between the two drivers and respective sensor ports 40, where each sensor 50 can be connected. The output 26 of the DC driver can be connected to each sensor port 40 via an internal wiring path of the switch matrix 30. The same applies to the output port 28 of the sensor driver 24, which can be switched to each sensor port 40.

[0052] The sensor driver 24 has a current output port 29 at which a measured current corresponding to the signal at the output port 28 is measured, for example, a copy of that signal. An optional evaluation unit 21 may be connected to the current output port 29. This evaluation unit may be an analog signal processing unit or an analog signal processor (ASP) such that the signal is available at the measurement output 23, which can be processed by an AD converter.

[0053] The switch matrix 30 includes multiple switches and a load resistor 32 to interconnect the driver outputs with the respective sensor ports 40. The DC driver output 26 can be interconnected with the sensor ports 40 via the load resistor 32 and a series-connected RT switch 34. When the RT switch 34a is closed, a DC reference signal output from the DC reference driver 22 is applied to the sensor port 40. Therefore, the sensor port 40a is raised to a DC voltage potential. Simultaneously, the sensor switch 36a is open, thus eliminating the connection to the sensor driver 24.

[0054] To measure the connected sensor at sensor port 40b, the corresponding sensor switch 36b is closed, while the RT switch 34b is opened. Therefore, the AC signal output by sensor driver 24 at output port 28 is forwarded to sensor port 40b, allowing measurement using capacitive sensor 50.

[0055] Sensor circuit 20 may include a DC decoupling device 62, which can be switched to the appropriate path, for example, via RT switch 34, if a DC signal should not be supplied to the connected sensor port 40. The DC decoupling device 62 is preferably arranged in a filter 60, which is positioned between the sensor element 52 having multiple capacitive sensors 50 and the sensor circuit 20. The connection between the sensor circuit 20 and the sensors 50 is made via the filter 60. The DC decoupling device 62 can be formed, for example, by a series capacitor arranged in the lead-in line to the sensor 50. The filter 60 shown herein also includes a series inductor 64 at the filter input and an EMV capacitor connected to ground (GND). 66. In this way, unwanted interference signals are decoupled.

[0056] After measuring the sensor 50 connected to one of the sensor ports 40, such as sensor port 40b, as Figure 1 As shown, sensor port 40b is decoupled from sensor driver 24 by sensor switch 36b being turned off. Then, RT switch 34b is closed, causing sensor port 40b to return to the DC voltage potential of DC reference driver 22. DC decoupling device 62 prevents DC voltage signals from being transmitted to sensor 50. All other sensor ports 40 are then preferably operated sequentially so that measurements can be performed using the corresponding sensor 50. RT switch 34 and sensor switch 36 are switched accordingly.

[0057] After each measurement cycle, a self-test is preferably performed on each individual sensor port 40. Figure 1 The inspection of sensor port 40b is shown. For this purpose, switch path 38 (dashed line) is formed in switch matrix 30. Both RT switch 34b and sensor switch 36b are closed. A DC signal (instead of an AC signal) is generated by means of sensor driver 24, which is preferably greater than the DC reference signal of DC reference driver 22. Due to the voltage difference between the two drivers, the current flowing through load resistor 32b is also output at current output port 29, and can be filtered by means of analog signal processing in evaluation unit 21, amplified, and digitized using an analog-to-digital converter arranged at measurement output terminal 23. Except for sensor port 40b, all other sensor ports 40 are unconnected because the corresponding RT switch 34 and sensor switch 36 are open.

[0058] During self-test (BIST), the associated RT switch 34 and sensor switch 36, as well as the load resistor 32, preferably 500 Ohms, are checked for each sensor port 40. Initially, both switches 34 and 36 are closed simultaneously. The output voltage of the sensor driver 24 is preferably set to its maximum voltage, for example, 1.9V. Therefore, the output voltage is 0.5V higher than the output voltage of the DC reference driver 22, which is preferably 1.4V. The current supplied by the sensor driver 24 is output as a measurement current at the current output port 29 and amplified in the evaluation unit 21, where a high-pass filter, which may be present, is bridged during BIST execution to enable the measurement of the DC signal. The digitized current value is compared with a limit value determined during production testing and which can be stored in memory. Preferably, the measured value must lie between these two limits from the production test.

[0059] The self-test measurements for each sensor port 40 are repeated twice, with one of the two switches, RT switch 34 or sensor switch 36, disconnected. For both measurements, the current must be close to zero. Therefore, it can be checked whether the digital measurement tapped at current output port 29 is less than the lower limit.

[0060] Therefore, this circuit can be used to check the functionality of all RT switches 34 and sensor switches 36. It can also be used to check the value of the load resistor 32.

[0061] Figure 2 A measurement system 10 is shown, comprising: a sensor circuit 20, a sensor element 52 having a plurality of sensors 50, and a filter 60 connected between the sensor element 52 and the sensor circuit 20. In the embodiment shown here, the sensor circuit 20 has eight sensor ports, allowing all eight sensors 50 of the sensor element 52 to be measured. The sensor circuit 20 includes a switch matrix 30 to interconnect the outputs of a DC reference driver 22 and a sensor driver 24 with the respective sensor ports 40. An evaluation unit 21 is connected to a current output port 29, the output signal of which is output to an AD converter 12. The AD converter 12 may include a comparator unit to check, for example, a limit value stored in a memory unit 14, against a digitized current measurement. A processing unit 16 can compare the digitized current value with the limit value and generate a corresponding evaluation signal or output an error signal. Additionally, the processing unit 16 can control the sensor circuit 20 and open or close corresponding switches.

[0062] In the embodiment of the measurement system 10 shown here, a DC source 17 is arranged at the input 25 of the DC driver, which generates a DC signal and thus feeds it to the DC reference driver 22. A signal source 18 is arranged at the sensor driver input 27 of the sensor driver 24, which can output an AC signal and alternatively output a DC signal. Therefore, an AC signal, such as a sine wave, or a DC signal can be fed to the sensor driver 24. The latter is used for self-testing.

[0063] Figure 3 An alternative embodiment of the sensor circuit 20 is shown, which additionally includes a shield driver 70 connected to the shield electrode 54 of the sensor element 52. The shield electrode 54 and the sensor 50 are respectively represented as RC networks. The relationship between the shield electrode 54 and the sensor 50 can also be represented using an RC network, such as... Figure 3 As shown in the image.

[0064] The shielded driver 70 generates a shielded AC signal during measurement, which is output to the shielded electrode 54 via the shielded port 72 and the filter 60. In the filter 60, a node 68 is arranged after the inductor 64, from which a line with another series inductor 64 leads to the shielded switching load port 74 of the sensor circuit 20.

[0065] Each sensor port 40 is assigned a sensor switching load port 76, which is connected to the sensor port 40 at another node 69 in the filter 60. A switchable connection to a switching load resistor 77 exists between the shielded switching load port 74 and the corresponding sensor switching load port 76. In the embodiment shown here, the switching load resistor 77 consists of two single resistors and a switching load switch 78 connected in series. Different measurements can be performed through the corresponding interconnections and different switching states of the switching load switch 78, RT switch 34, and sensor switch 36. In this example of a capacitive seat occupancy sensor, appropriate interconnections allow not only the identification of basic seat occupancy but also the performance of qualitative force measurements.

[0066] Therefore, the switching load switch 78 and switching load resistor 77 are also safety-related components, and their functionality must be checked using BIST. Similarly, when checking sensor switch 36 and RT switch 34, a DC signal is fed to shielded driver 70, causing it to output a DC signal. A voltage difference is created between shielded driver 70 and sensor driver 24, both operating in DC, causing current to flow through switching load path 80, as shown by the dashed line. By opening and closing switching load switch 78 and sensor switch 36, the functionality of switching load path 80d and its components can be checked for each sensor port 40. Thus, line breaks in switching load path 80, such as a defective inductor 64 in the path, can also be identified.

[0067] Figure 4 A specific embodiment of sensor circuit 20, designed as a sensor IC, is shown. Sensor circuit 20 has a total of eight sensor ports 40, four of which are connected to a reference element 90 with four input terminals. In addition to the other four sensor ports 40, there are corresponding sensor switching load ports 76, which are connected via a filter 60 to a sensor element 52 having four sensors 50. The sensor element also includes a shielding electrode 54, which is connected to the shielding port 72 via the filter 60. The shielding switching load port 74 is internally connected to the sensor switching load port 76, as shown... Figure 3 As shown. The host microcontroller 92, connected to the sensor circuit 20, is responsible for controlling the sensor circuit 20, providing necessary signals, and receiving measurement results or pre-processed measurement current from the sensor driver 24.

[0068] Figure 5 An implementation of an integrated module, an IC module 94, with sensor circuitry 20, is shown, on which eight capacitive sensors 50 and shielding electrodes 54 can be connected. Corresponding sensor ports 40, 76, 72, and 74 are connected to the left side. The IC module 94 also includes a signal source module 96, which provides AC signals, such as sine waves and DC signals, to the respective drivers of the sensor circuitry 20. An analog signal processor 98 further processes the measurement signal output at the current output port 29 of the sensor driver 24 and provides it to the control unit 100, which is fed by a clock generator 102. The individual modules are fed via a voltage supply module 104. Other implementations of the IC module are possible. However, the IC shown here represents a very compact configuration.

[0069] Figure 6The principle flow of a method for integrating safety-related components of a sensor circuit for a capacitive sensor is shown. A corresponding implementation of the sensor circuit has been described above. The method includes the following steps:

[0070] In the first step S10: the RT switch 34 and sensor switch 36 of the switch path 38 of the switch matrix 30 for the selected sensor port 40 are closed. In another step S12 of the method: the sensor switch 36 and RT switch 34 of the other switch paths 38 of the switch matrix 30 are disconnected. Following step S14, which outputs a DC reference signal to the DC reference driver 22, is step S18, which outputs a DC signal to the sensor driver 24, wherein this DC signal is different from the DC reference signal and is preferably larger. Step S20 involves measuring the measurement current provided at the current output port 29 of the sensor driver 24. In step S22, the measured current is compared with a limit value.

[0071] In optional step S24, the RT switch 34 and / or sensor switch 36 of the selected switch path 38 are disconnected. Steps S14 to S22 are repeated for each switch position, wherein, preferably, only one of the two switches 34 and 36 is disconnected. These optional steps are as follows: Figure 6 As shown by the dashed line in the image.

[0072] Another preferred embodiment of the method includes disconnecting the two switches of the selected switch path 38 of the switch matrix 30 in step S26. In the next step, another switch path is selected and the method restarts at step S10. Here, the method is performed for all sensor ports 40 of the sensor circuit 20.

[0073] Figure 7 It shows that it can be followed according to Figure 6 A special implementation of the self-testing method. When using a shielded electrode in a sensor element with multiple sensors, the self-testing is performed according to... Figure 7The method is as follows: In step S30, all RT switches 34 and sensor switches 38 for all switching paths 36 of sensor port 40 are disconnected. In step S32, sensor port 40 is selected to check the associated sensor switching load port 76. Next is step S34, which includes disconnecting the switching load switches 78 of all other switching load paths 80. For the selected sensor port 40, in step S36, the corresponding sensor switch 36 is closed. Step S38 specifies that a DC shielding signal is output at the shielding driver 70. In step S40, a DC signal, different from the DC shielding signal and preferably larger, is output at the sensor driver 24. Step S42 includes measuring the measurement current provided at the current output port 29 of the sensor driver 24. Next is step S44, which compares the measured current with a limit value.

[0074] In an alternative implementation, further optional steps of the method are performed. Step S46 involves disconnecting the switching load switch 78 of the selected switching load path 80. Step S48 involves measuring the measurement current supplied at the current output port of the sensor driver 22. This step is followed by step S50, which specifies that the measured current is compared with a limit value.

[0075] Optionally, the process may include further steps. In step S52, the switching load switch 78 and sensor switch 36 for the selected sensor port 40 are disconnected. Step S54 includes selecting another switching load path 80 for the sensor circuit 20. Optionally, this is followed by... Figure 7 Other steps of the method.

[0076] Another equally preferred embodiment of the method specifies in step S60 that the measured current is digitized before comparison with the limit value. In step S62, it is checked whether the digital value of the measured current conforms to one or more of the corresponding limit values. Step S64 involves outputting a verification signal to the output unit.

[0077] The invention has been fully described and explained using the accompanying drawings and specification. This description and explanation should be understood as exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other embodiments or variations will be apparent to those skilled in the art upon use of the invention and upon precise analysis of the drawings, disclosure, and subsequent claims.

[0078] In the claims, the words "comprising" and "having" do not exclude the presence of other elements or steps. The indefinite articles "a" or "an" do not exclude the presence of a plural. A single element or unit can perform the function of multiple units mentioned in the patent claims. Elements, units, devices, and systems can be implemented partially or entirely in hardware and / or software. The mere mention of some measures in several different dependent claims should not be construed as an advantageous combination of these measures. Computer programs can be stored / sold on non-volatile data carriers. Computer programs can be sold together with and / or as part of hardware, for example, via the Internet or through wired or wireless communication systems. Reference numerals in the patent claims should not be construed as restrictive.

[0079] Figure Labels

[0080] 10 Measurement System

[0081] 12 AD converter

[0082] 14 storage units

[0083] 16 processing units

[0084] 17 DC Power Supply

[0085] 18 signal sources

[0086] 20 Sensor Circuit

[0087] 21 Evaluation Units

[0088] 22 DC Reference Driver

[0089] 23 Measurement Output Terminal

[0090] 24 Sensor Drivers

[0091] 25 DC driver input terminals

[0092] 26 DC driver output

[0093] 27 Sensor driver input terminal

[0094] 28 Output Ports

[0095] 29 Current output port

[0096] 30 Switch Matrix

[0097] 32 Load resistor

[0098] 34 RT switch

[0099] 36 Sensor Switch

[0100] 38 Switch Path

[0101] 40 sensor ports

[0102] 50 sensors

[0103] 52 Sensor Components

[0104] 54 Shielding Electrodes

[0105] 60 Filter

[0106] 62 DC decoupling device

[0107] 64 Inductors

[0108] 66 EMV capacitor

[0109] 68 nodes

[0110] 69 nodes

[0111] 70 Shielded Driver

[0112] 72 shielded ports

[0113] 74 Shielded load switching port

[0114] 76 Sensor switching load port

[0115] 77 Switching load resistors

[0116] 78. Switching the load switch

[0117] 80 Switch load path

[0118] 90 Reference Element

[0119] 92 host microcontroller

[0120] 94 IC Module

[0121] 96 Signal Source Module

[0122] 98 Analog Signal Processor

[0123] 100 Control Unit

[0124] 102 Clock Generator

[0125] 104 Voltage Supply Module

Claims

1. A sensor circuit for a capacitive sensor (50), the sensor circuit comprising: DC reference driver (22) used to generate DC reference signal, A sensor driver (24) for generating an AC signal, the sensor driver having an output port (28) for outputting an AC signal for feeding the capacitive sensor (50) and a current output port (29) for outputting a measurement current corresponding to the AC signal output at the output port. Multiple sensor ports (40) for connecting to each capacitive sensor (50), A switch matrix (30) for establishing the connection between the sensor port (40) and the driver, wherein the switch matrix (30) has a switch path for each sensor port (40), the switch path having an RT switch (34), a sensor switch (36), and a load resistor (32), and A DC decoupling device between the switch matrix (30) and the capacitive sensor (50) is used to DC decouple the switch matrix (30) from the capacitive sensor (50). The sensor circuit (20) is designed to perform a self-test in which: The sensor driver (24) outputs a DC signal, causing a voltage difference to form between the DC reference driver (22) and the sensor driver (24), which is applied to the switching matrix (30). -Measure the current flowing through the switch matrix (30) during the self-test.

2. The sensor circuit according to claim 1, characterized in that, During the self-test, the switch matrix (30) is switched such that the individual sensor ports (40) are operated in an alternating manner.

3. The sensor circuit according to claim 1 or 2, characterized in that, The sensor switch (36) and / or the RT switch (34) of the switching path are switched to different switching positions during self-test.

4. The sensor circuit according to claim 1 or 2, characterized in that, The sensor circuit (20) has a DC driver input (25) and a sensor driver input (27) to feed a DC signal to the DC reference driver (22) and an AC signal and / or a DC signal to the sensor driver (24).

5. The sensor circuit according to claim 1 or 2, characterized in that, The sensor circuit (20) includes a shielded driver (70) for generating a shielded AC signal, wherein the shielded AC signal is output to a shielded port (72) where a shielded electrode (54) can be connected.

6. The sensor circuit according to the preceding claim, characterized in that, The sensor circuit (20) includes a shielded switching load port (74) for connection to the shielded electrode (54), and the sensor circuit has an assigned sensor switching load port (76) for each sensor port (40) for connection to the sensor (50), wherein a switching load path (80) including a switching load switch (78) and a switching load resistor (77) is formed between the shielded switching load port and each sensor switching load port, such that each sensor switching load port and the shielded switching load port are interconnected.

7. The sensor circuit according to the preceding claim, characterized in that, Each switching load path (80) can be checked in a self-test, wherein the shielded driver (70) outputs a shielded DC signal in the self-test.

8. The sensor circuit according to claim 2, characterized in that, Other sensor ports (40) are at a defined potential.

9. The sensor circuit according to claim 2, characterized in that, Other sensor ports (40) are decoupled from the switch matrix (30).

10. The sensor circuit according to claim 4, characterized in that, During self-testing, the sensor driver (24) is fed a DC signal and outputs the DC signal.

11. The sensor circuit according to claim 5, characterized in that, The shielding electrode (54) is connected in parallel with the capacitive sensor (50), enabling measurements to be performed to improve sensor accuracy.

12. The sensor circuit according to claim 7, characterized in that, The shielded DC signal is not equal to the DC signal of the sensor driver (24).

13. The sensor circuit according to claim 7, characterized in that, The shielded DC signal is less than the DC signal of the sensor driver (24).

14. A capacitive measurement system for evaluating capacitive sensors, comprising: A sensor (50) with sensor electrodes, a sensor circuit (20) for the capacitive sensor (50), and a DC decoupling device between the sensor circuit (20) and the sensor. The sensor circuit (20) includes: DC reference driver (22) for generating DC reference signal; A sensor driver (24) is used to generate and output an AC signal at the output port to feed the sensor (50); Sensor port (40) for connecting to the sensor; A switch matrix (30) is used to establish the connection between the sensor port (40) and the two drivers; in, - The sensor driver (24) has a current output port (29) for outputting a measured current corresponding to the AC signal output at the output port (28); - The switch matrix (30) includes an RT switch (34), a sensor switch (36), and a load resistor (32) for connection to the sensor port (40); - The sensor circuit (20) is designed to perform a self-test in which the sensor driver (24) outputs a DC signal, causing a voltage difference to be formed between the DC reference driver (22) and the sensor driver (24), the voltage difference being applied to the switching matrix (30); and -Measure the current flowing through the switch matrix (30) during the self-test.

15. The measurement system according to the preceding claim, characterized in that, A filter network is arranged between the sensor circuit (20) and the sensor (50).

16. The measurement system according to claim 14 or 15, characterized in that, The current flowing through the switch matrix (30) during the self-test is measured at the current output port (29) of the sensor driver (24).

17. The measurement system according to claim 14 or 15, characterized in that, The measurement system has shielded electrodes, and the sensor circuit (20) includes a shielded driver (70) for generating a shielded AC signal output to a shielded port (72), wherein the shielded electrodes (54) are connected to the shielded port (72).

18. The measurement system according to the preceding claim, characterized in that, The sensor circuit (20) includes two switching load ports connected to each other via a switching load path (80) including a switching load switch (78) and a switching load resistor (77). One of the switching load ports is a shielded switching load port (74) for connection to a shielding electrode (54), and the second switching load port is a sensor switching load port (76) for connection to the sensor (50).

19. The measurement system according to claim 14 or 15, characterized in that, The sensor circuit (20) includes an evaluation unit (21).

20. The measurement system according to claim 14 or 15, characterized in that, A filter (60) is arranged between the sensor circuit (20) and the sensor (50).

21. The measurement system according to claim 15, characterized in that, The filter network includes coupling capacitors.

22. The measurement system according to claim 21, characterized in that, The coupling capacitor is a DC decoupling device.

23. The measurement system according to claim 17, characterized in that, The shielding electrode (54) is connected in parallel with the sensor (50), enabling measurements to be performed to improve sensor accuracy.

24. The measurement system according to claim 18, characterized in that, Each sensor port (40) is assigned a corresponding sensor switching load port.

25. The measurement system according to claim 18, characterized in that, The sensor port (40) is connected to the assigned sensor switching load port.

26. The measurement system according to claim 19, characterized in that, The evaluation unit is an analog signal processing unit that outputs a measurement signal for AD conversion.

27. The measurement system according to claim 19, characterized in that, The evaluation unit includes an amplifier and / or a filter.

28. The measurement system according to claim 20, characterized in that, The filter (60) includes a DC coupling device (62).

29. The measurement system according to claim 20, characterized in that, The filter (60) is an EMV filter, which includes a series inductor (64) and a parallel capacitor.

30. A method for integrated self-testing of safety-related components of a sensor circuit (20) for a capacitive sensor (50), wherein the sensor circuit (20) includes a DC reference driver (22) for generating a DC reference signal, a sensor driver (24) for generating and outputting an AC signal at an output port (28) to feed the sensor, a plurality of sensor ports (40) for connecting to each sensor, a switch matrix (30) for establishing connections between the sensor ports (40) and the driver, and a DC decoupling device (62) between the switch matrix (30) and the capacitive sensor for DC decoupling from the capacitive sensor, the method comprising a self-test having the following steps: a. Close the RT switch (34) and the sensor switch (36) for the switching path of the sensor port (40); b. Disconnect the RT switch (34) and sensor switch (36) of the other switch paths of the switch matrix; c. Output a DC reference signal at the DC reference driver (22); d. Output a DC signal that is different from the DC reference signal at the sensor driver (24); e. Measure the measurement current supplied at the current output port (29) of the sensor driver (24); f. Compare the measured current with the limit value.

31. The method according to the preceding claim, characterized in that... It also includes the following steps: h. Disconnect the RT switch (34) and / or the sensor switch (36) of the switch path (38); i. Repeat steps c to f of the method according to claim 30.

32. The method according to claim 30 or 31, characterized in that... It also includes the following steps: - Disconnect the RT switch (34) and sensor switch (36) of one of the switch paths (38) of the switch matrix (30); - Select another switch path.

33. The method according to claim 30 or 31, wherein the method includes additional self-testing steps: - Disconnect the RT switch (34) and sensor switch (36) for all switch paths (38) of the sensor port (40); - Select sensor port (40) to check the relevant switching load path (80); -Disconnect all other load switching switches for the load switching paths (78); - Close the sensor switch (36) for the selected sensor port (40); - Close the load switching switch for the selected load switching path; - Output a DC shielding signal at the shielded driver (70); - Output a DC signal that is different from the DC shielding signal at the sensor driver (24); - Measure the measurement current supplied at the current output port (29) of the sensor driver (24); - Compare the measured current with the limit value.

34. The method according to the preceding claim, characterized in that... It also includes the following steps: - Disconnect the load switching switch (78) of the selected load switching path (80); - Measure the measurement current supplied at the current output port (29) of the sensor driver (24); - Compare the measured current with the limit value.

35. The method according to the preceding claim, characterized in that... It also includes the following steps: - Disconnect the switching load switch (78) and sensor switch (36) for the selected sensor port (40); - Select another load switching path (80); - Perform the steps of the method according to claim 33 or 34 for the other load switching path.

36. The method according to claim 30 or 31, characterized in that... It also includes the following steps: - The measured current is digitized before being compared with the stated limit value; - Check whether the measured current value meets the corresponding limit value; - Output a verification signal to the control unit (100).

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