Detecting Capacitance and Sensitivity Faults in a Capacitive Sensor

Abstract

Embodiments of the present disclosure relate to detecting capacitance faults and sensitivity faults in a capacitance sensor. A capacitance sensor includes a first conductive structure; a second conductive structure opposite to the first conductive structure, wherein the second conductive structure is movable relative to the first conductive structure in response to an external force acting thereon, and wherein the second conductive structure is capacitively coupled to the first conductive structure to form a first capacitor, the first capacitor having a first capacitance that varies with a change in the distance between the first conductive structure and the second conductive structure; a signal generator configured to apply a first electrical signal step at an input or an output of the first capacitor to cause a first voltage transient response at an output of the first capacitor; and a diagnostic circuit configured to detect a fault in the capacitance sensor by measuring a first time constant of the first voltage transient response and detecting a fault based on the first time constant.

Description

Background Art

[0001] Capacitive sensors have many applications and can be used as pressure sensors, acoustic sensors, microphone sensors, etc. The capacitive sensor can be a microelectromechanical system (MEMS) capacitive sensor in any of the above applications.

[0002] Capacitive faults are known fault modes in capacitive sensors. For example, when the membrane of the capacitive sensor ruptures, when conductive particles enter between the conductive backplates of the capacitive sensor, or when there is water between the conductive backplates. Sensitivity faults can also occur due to the presence of water between the conductive backplates, weakening of the membrane (i.e., softening), or when the space between the backplates is accidentally filled with water. In both cases, the sensor function can be impaired due to a change in the desired sensor sensitivity and / or an increase in the noise level. The above capacitive faults can cause a change in the capacitance of the capacitive sensor and / or a change in the voltage dependence of the capacitance (i.e., the sensor sensitivity).

[0003] Therefore, an improved device capable of detecting and diagnosing capacitance and sensitivity faults in a capacitive sensor may be needed. Summary of the Invention

[0004] An embodiment provides a capacitive sensor, comprising: a first conductive structure; a second conductive structure opposite to the first conductive structure, wherein the second conductive structure is movable relative to the first conductive structure in response to an external force acting thereon, wherein the second conductive structure is capacitively coupled to the first conductive structure to form a first capacitor having a first capacitance that varies depending on a change in the distance between the first conductive structure and the second conductive structure, wherein the first capacitance represents the external force; a signal generator configured to apply a first electrical signal step at an input or output of the first capacitor to cause a first voltage transient response at an output of the first capacitor; and a diagnostic circuit configured to detect a fault in the capacitive sensor by measuring a first time constant of the first voltage transient response and detecting a fault based on the first time constant.

[0005] An embodiment provides a capacitive sensor, the capacitive sensor comprising: a first conductive structure; a second conductive structure opposite to the first conductive structure, wherein the second conductive structure is movable relative to the first conductive structure, wherein the second conductive structure is capacitively coupled to the first conductive structure to form a first capacitor having a first capacitance that varies depending on a change in the distance between the first conductive structure and the second conductive structure, wherein the first capacitor comprises a first terminal coupled to a bias supply voltage and a second terminal coupled to a first DC voltage set by a bias resistance circuit; a third conductive structure opposite to the first conductive structure, wherein the third conductive structure is movable relative to the first conductive structure, wherein the third conductive structure is capacitively coupled to the first conductive structure to form a second capacitor having a second capacitance that varies depending on a change in the distance between the first conductive structure and the third conductive structure, wherein the second capacitor comprises a first terminal coupled to a bias supply voltage and a second terminal coupled to a second DC voltage set by a bias resistance circuit; a common-mode buffer configured to receive the first DC voltage, the second DC voltage, and a reference voltage and generate a differential signal based on a difference between a common-mode voltage of the first DC voltage and the second DC voltage and the reference voltage; a signal generator configured to apply a first electrical signal step at an input or an output of the first capacitor and at an input or an output of the second capacitor to respectively cause a first voltage transient response in the first DC voltage and a second voltage transient response in the second DC voltage; and a diagnostic circuit configured to detect a fault in the capacitive sensor by measuring a first time constant of the differential signal corresponding to the first voltage transient response and the second voltage transient response and detecting a fault based on the first time constant. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Embodiments are described herein with reference to the accompanying drawings.

[0007] Figure 1 A cross-sectional view of a MEMS element of a capacitive sensor according to one or more embodiments is shown;

[0008] Figure 2 is a block diagram of a capacitive sensor according to one or more embodiments;

[0009] Figure 3A and Figure 3B is a schematic block diagram of a capacitive sensor readout circuit according to one or more embodiments;

[0010] Figure 4A and Figure 4B are respectively from Figure 3A and Figure 3B signal diagrams obtained from the capacitive sensor readout circuit of;

[0011] Figure 5A and Figure 5Bis a schematic block diagram of another capacitive sensor readout circuit according to one or more embodiments; and

[0012] Figure 6 is a schematic block diagram of another capacitive sensor readout circuit according to one or more embodiments. DETAILED DESCRIPTION

[0013] In the following, details are set forth to provide a more thorough explanation of the exemplary embodiments. However, it will be apparent to those skilled in the art that the embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form or schematic diagrams rather than in detail to avoid obscuring the embodiments. Furthermore, unless otherwise specifically stated, the features of the different embodiments described below may be combined with each other.

[0014] In addition, in the following description, equivalent or similar elements or elements having equivalent or similar functions are denoted by equivalent or similar reference numerals. Since identical or functionally equivalent elements are given the same reference numerals in the drawings, repeated descriptions of elements having the same reference numerals may be omitted. Thus, the descriptions provided for elements having the same or similar reference numerals are interchangeable.

[0015] It should be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, no intervening elements are present. Other words used to describe the relationship between elements should be interpreted in a similar manner (e.g., "between" versus "directly between", "adjacent" versus "directly adjacent", etc.).

[0016] In the embodiments described herein or shown in the drawings, any direct electrical connection or coupling (i.e., any connection or coupling without additional intervening elements) can also be achieved by an indirect connection or coupling (i.e., a connection or coupling having one or more additional intervening elements), or vice versa, as long as the general purpose of the connection or coupling (e.g., transmitting a certain signal or transmitting a certain information) is substantially maintained. Features from different embodiments can be combined to form additional embodiments. For example, variations or modifications described with respect to one of the embodiments can also apply to other embodiments, unless stated to the contrary.

[0017] The term "substantially" can be used herein to account for minor manufacturing tolerances (e.g., within 5%) that are considered acceptable in the industry without departing from the aspects of the embodiments described herein.

[0018] In the present disclosure, expressions including ordinal numbers such as "first", "second", etc. may modify various elements. However, such elements are not limited to the above expressions. For example, the above expressions do not limit the order and / or importance of the elements. The above expressions are only used to distinguish one element from other elements. For example, a first box and a second box represent different boxes, although they are both boxes. For another example, a first element may be referred to as a second element, and similarly, a second element may also be referred to as a first element without departing from the scope of the present disclosure.

[0019] One or more aspects of the present disclosure may be implemented as a non-transitory computer-readable recording medium having recorded thereon a program embodying a method / algorithm for instructing a processor to execute the method / algorithm. Accordingly, the non-transitory computer-readable recording medium may have electronically-readable control signals stored thereon that cooperate with (or are capable of cooperating with) a programmable computer system to cause the corresponding method / algorithm to be executed. The non-transitory computer-readable recording medium may be, for example, a CD-ROM, a DVD, a Blu-ray disc, a RAM, a ROM, a PROM, an EPROM, an EEPROM, a flash memory, or an electronic memory device.

[0020] Each element of the present disclosure may be configured by implementing dedicated hardware or a software program on a memory, and the software program controls the processor to execute the functions of any component or a combination thereof. Any component may be implemented as a central processing unit (CPU) or other processor that reads and executes a software program from a recording medium such as a hard disk or a semiconductor memory device. For example, instructions may be executed by one or more processors, such as one or more CPUs, digital signal processors (DSPs), general microprocessors, application-specific integrated circuits (ASICs), field-programmable logic arrays (FPGAs), programmable logic controllers (PLCs), or other equivalent integrated or discrete logic circuit systems.

[0021] Accordingly, the term "processor" as used herein refers to any of the foregoing structures or any other structure suitable for implementing the techniques described herein. Accordingly, the techniques described in the present disclosure may be implemented at least in part in hardware, software, firmware, or any combination thereof. For example, various aspects of the described techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, or any other equivalent integrated or discrete logic circuit systems, as well as any combination of such components.

[0022] Controllers that include hardware can also perform one or more of the techniques of the present disclosure. A controller that includes one or more processors can use electrical signals and digital algorithms to perform its receiving, analyzing, and controlling functions, which can also include calibration functions. Such hardware, software, and firmware can be implemented within the same device or in separate devices to support the various techniques described in the present disclosure. The software can be stored on a non-transitory computer-readable medium such that the non-transitory computer-readable medium includes program code or program algorithms stored thereon that, when executed, cause the controller to perform the steps of a method via a computer program.

[0023] A signal processing circuit and / or a signal conditioning circuit can receive one or more signals from one or more components and perform signal conditioning or processing on them. As used herein, signal conditioning refers to manipulating a signal such that the signal meets the requirements of the next stage for further processing. Signal conditioning can include conversion from analog to digital (e.g., via an analog-to-digital converter), amplification, filtering, transformation, biasing, range matching, isolation, and any other process required to make the signal suitable for processing after conditioning.

[0024] Accordingly, the signal processing circuit can include an analog-to-digital converter (ADC) that converts analog signals from one or more sensor elements into digital signals. The signal processing circuit can also include a digital signal processor (DSP) that performs some processing on the digital signals.

[0025] Embodiments relate to the diagnosis of electrical faults in capacitive sensors, and more precisely to the diagnosis of capacitance and sensitivity faults in microelectromechanical systems (MEMS) capacitive sensors, which are some of the critical fault modes of the device. The diagnosis can be applied to any capacitive sensor, including a single capacitive sensor or a sensor that uses two capacitors (e.g., a top capacitor C Top and a bottom capacitor C Bot ) to sense and generate a measurement signal representing a physical quantity (e.g., pressure, acoustic wave, vibration, or any other alternating current (AC) external force) measured by the capacitive sensor.

[0026] The diagnostic circuit performs its diagnosis by measuring some selected electrical parameters of the MEMS capacitive sensor, such as one or more of its time constants, and then provides this information as an output diagnosis. A capacitance or sensitivity fault, if present, will cause an offset of one or more time constants beyond a tolerance range or threshold. The diagnostic circuit is configured to measure the time constants and, once they exceed the tolerance range, provide this information as an output diagnosis (e.g., as a fault indicator).

[0027] Figure 1 A cross-sectional view of the MEMS element of a capacitive sensor according to one or more embodiments is shown. In particular,Figure 1 Shown is a MEMS element 11 of a dual-backplate capacitive sensor that can be implemented as a MEMS microphone, but the embodiment is not limited thereto. For example, the MEMS capacitive sensor can also be used as a pressure sensor.

[0028] The MEMS element 11 of the dual-backplate capacitive sensor includes three electrodes, including a conductive membrane 12 and two conductive backplates 13 and 14. The membrane 12 is movable, and the two conductive backplates 13 and 14 can be movably fixed in a stationary position. In other embodiments, one or more of the backplates 13 and 14 can also be movable.

[0029] A top capacitor CTop is formed between the top backplate 13 and the membrane 12, and a bottom capacitor CBot is formed between the bottom backplate 14 and the membrane 12. Depending on the distance between the corresponding electrodes (i.e., the thickness of the gap g1 or g2) changing in response to an external force (e.g., pressure or acoustic wave (sound)) applied to the movable conductive structure (e.g., the membrane 12), the capacitance of each capacitor CTop and CBot changes. For example, the capacitance is calculated according to the formula Q = CV, where Q is the charge in coulombs, C is the capacitance in farads, and V is the potential difference between the electrodes of the capacitor in volts.

[0030] Different types of readout circuits can be used to measure the change in capacitance. For example, the readout circuit can utilize constant change readout, during which the voltage V is measured while the charge Q remains constant. The change in capacitance caused by the change in the thickness of the gaps g1 and g2 results in a change in the voltages Vtop and Vbot stored across the capacitors, and the measurement circuit can measure these voltages as sensor signals. Specifically, the voltages across each capacitor CTop and CBot represent the sensor signals, which can be measured by the readout circuit and associated with a physical quantity such as pressure or acoustic wave (sound). Therefore, the voltage is the variable to be measured. Each capacitor can be read out independently, or the average value of the capacitors can be read out using a common-mode buffer.

[0031] In this example, the membrane 12 can move closer to one backplate as it moves away from the other backplate, thereby changing the capacitance of each capacitor CTop and CBot. The movement of the membrane 12 is caused by an external force (e.g., pressure or acoustic wave (sound) or vibration) applied to the membrane 12. These external forces are oscillating or AC external forces whose polarity or direction changes with time.

[0032] It should also be understood that the embodiments are not limited to double-backplate capacitive sensors and can also be applied to single-capacitor sensors consisting of two electrodes or capacitive sensors having two or more capacitors, including double-capacitor sensors in which two outer electrodes are movable and an intermediate electrode is movably fixed. Regardless of the type of capacitive sensor, at least two electrodes are used to form at least one capacitor. Each capacitor is thus formed by two electrodes, at least one of which is movable relative to the other electrode in response to an external force applied thereto. One electrode of the capacitor can be referred to as a reference electrode, while the other electrode of the capacitor can be referred to as a counter electrode.

[0033] An isolation material 15 is also provided to provide electrical isolation between the conductive elements 12, 13, and 14.

[0034] Conductive materials such as metal particles or water droplets can penetrate the MEMS element 11 and form a conductive path (e.g., a short circuit) between otherwise electrically isolated portions of the MEMS element 11. Unwanted conductive paths can have a negative impact on the performance of the MEMS element 11. Capacitance and / or sensitivity faults can also be caused by defects within the integrated circuit itself, which can occur throughout the life cycle of the device. The following embodiments provide additional readout circuitry for detecting capacitance and / or sensitivity faults and possibly compensating for the detected faults to restore or otherwise improve the functionality of the capacitive sensor.

[0035] Figure 2 is a block diagram of a capacitive sensor 200 according to one or more embodiments. The capacitive sensor 200 includes a MEMS element 11 and a signal processing circuit 20, which is configured to receive a sensor signal Ssense (e.g., voltages Vtop and Vbot) from the MEMS element 11, perform signal processing on the sensor signal Ssense, and output the processed sensor signal as a measurement signal Smeas at a signal output (SignalOut). A readout circuit is provided between the MEMS element 11 and the signal processing circuit 20. The readout circuit receives a signal from the MEMS element 11 and can include a biasing circuit (e.g., a resistive biasing circuit) for providing a signal to the signal processing circuit 20. The readout circuit is at Figure 3A , Figure 3B , Figure 5A , Figure 5B and Figure 6is shown. The capacitance sensor 200 further includes a fault diagnosis circuit 30 configured to receive the sensor signal Ssense during a diagnostic operation and detect one or more types of faults based on the sensor signal Ssense. The faults may include a capacitance fault or a sensitivity fault of the MEMS element 11. In response to detecting a fault, the fault diagnosis circuit 30 is configured to generate an error signal Serr and output the error signal Serr at an Error Out.

[0036] To detect an electrical fault, the fault diagnosis circuit 30 is configured to measure one of the electrical parameters affected by a capacitance fault or a sensitivity fault, compare the electrical parameter with a predetermined error threshold or with a predetermined tolerance range having a minimum threshold and a maximum threshold, and generate the error signal Serr in response to the electrical parameter crossing (e.g., exceeding) the predetermined error threshold or deviating from the predetermined tolerance range. The electrical parameter affected by a capacitance fault may be the RC time constants of the capacitors CTop and CBot measured in response to a capacitance diagnostic injection signal (e.g., with respect to Rbias1 and Rbias2, respectively). The RC time constants are proportional and thus represent the capacitances of CTop and CBot. The electrical parameter affected by a sensitivity fault may be the difference between the RC time constants of the capacitors (i.e., CTop or CBot) measured in response to a sensitivity diagnostic injection signal. The difference between the RC time constants measured in response to a sensitivity diagnostic injection signal corresponds to the voltage dependence of the capacitor, which in turn corresponds to the sensor sensitivity itself. A capacitance fault will cause a shift in the corresponding electrical parameter measured at the output of the MEMS element 11 to outside the predetermined tolerance range. Similarly, a sensitivity fault will cause a shift in the corresponding electrical parameter measured at the output of the MEMS element 11 to outside the predetermined tolerance range. The fault diagnosis circuit 30 is configured to monitor and detect these types of faults during a diagnostic operation.

[0037] The error signal Serr may be provided to an additional diagnostic circuit (not shown) configured to perform further analysis on the capacitance sensor 200 to determine the cause or source of the fault.

[0038] The capacitive sensor 200 further includes a controller 40 configured to perform diagnostic operations on the MEMS element 11. The controller 40 may be configured to generate a control signal Sreduce that is configured to control one or more circuit components at the input or at the output of the MEMS element 11 to reduce the sensor signal Ssense that may be caused by a sensed external force (such as pressure or acceleration). Reducing the sensor signal Ssense caused by the external force enables the electrical parameters of the target fault to be measured and evaluated. Methods of reducing the sensor signal Ssense include setting the input bias voltage Vbias applied to the input of the MEMS element 11 to zero, reducing the resistance value of the bias resistor Rbias arranged at the output of the MEMS element 11 to increase the 3dB frequency of the high-pass filter formed by Rbias and the capacitors CTop and CBot, thereby effectively filtering the sensor signal Ssense, or performing the readout of the sensor signal Ssense using a common-mode buffer.

[0039] The controller 40 further includes a signal generator 50 configured to inject one or more diagnostic signals Sinject into the MEMS circuitry (i.e., at its input or output) after the sensor signal Ssense has been reduced by one or more of the above methods. The injected signal Sinject causes a response at the outputs of the capacitors CTop and CBot, and the fault diagnosis circuit 30 is configured to measure electrical parameters indicative of a capacitance fault or a sensitivity fault based on the response to the injected signal Sinject. It should be understood that although the signal generator 50 is shown integrated with the controller 40, the two may be separate elements.

[0040] The injected signal Sinject includes at least one electrical signal step (e.g., a voltage step or a current step). Thus, the injected signal Sinject is a step signal, and the electrical signal step is a signal transition from one signal level to a second level that causes a response at the outputs of the capacitors CTop and CBot. The signal transition may be a step-up transition (i.e., a rising edge) or a step-down transition (i.e., a falling edge).

[0041] To test for capacitance and sensitivity faults, the controller 40 may transmit the injected signal Sinject or timing information indicating the injection time or the trigger time of the injected signal or its components to the fault diagnosis circuit 30. Specifically, the fault diagnosis circuit 30 detects the trigger time of the electrical step (i.e., the rising edge or the falling edge between two predefined signal levels) of the injected signal Sinject, or receives the trigger time directly from the controller 40. Alternatively, the fault diagnosis circuit 30 may be integrated with the controller 50 and thus automatically know the trigger time.

[0042] In either case, the fault diagnosis circuit 30 uses the trigger time of the electrical signal step of the injected signal to calculate the relevant RC time constant. For example, the RC time constant can be calculated from the time when the signal generator 50 triggers the corresponding electrical step to the time when the measured signal reaches a predefined threshold. Thus, the fault diagnosis circuit 30 receives information indicating the trigger time of the electrical step.

[0043] The controller 40 can also control the predefined threshold used by the comparator circuitry to measure the RC time constant. The time constant is the amount of time it takes for the transient response of the capacitor to change from a first power value to a certain percentage of its second power value. In other words, it is the amount of time it takes for the voltage across the capacitor to change from one extreme value to a certain percentage of its second extreme value. The predefined threshold is for the latter and represents a certain percentage of its second extreme value. For example, the predefined threshold can be set to approximately 63% of the maximum power value and last for one time constant during a rising response transient, and can be set to approximately 37% of the maximum power value and last for one time constant during a falling response transient. Thus, the controller 40 can control the predefined threshold used by the comparator circuitry to determine when the measured signal reaches a certain level after the trigger time used to calculate the RC time constant.

[0044] Figure 3A and Figure 3B are schematic block diagrams of a capacitive sensor readout circuit according to one or more embodiments. In particular, Figure 3A illustrates a capacitive sensor readout circuit 300 implementing one technique for detecting capacitive faults, Figure 3B illustrates a capacitive sensor readout circuit 300 implementing another technique for detecting capacitive faults. The differences between these three detection techniques include: the position of the injection node at which the injected signal (i.e., the electrical signal step) causes a response at Vtop and Vbot; and / or the type of the injected signal (e.g., a voltage signal or a current signal). The readout circuit 300 is a constant charge (CC) readout circuit for reading the signal of a capacitive MEMS element (e.g., MEMS element 11). Thus, the charge of each capacitor CTop and CBot remains constant, while the voltage across the capacitor changes based on the change in capacitance.

[0045] The DC bias voltage source 10 provides a DC bias voltage Vbias to the conductive structures of the MEMS element (i.e., the (multiple) backplates and the (multiple) membranes). The AC external forces F AC1 and F AC2 represent external forces (e.g., pressure or acceleration) that are applied and act on the conductive structures, thereby causing a change in capacitance at the capacitors CTop and / or CBot.

[0046] An amplifier 21, such as a programmable gain amplifier or a buffer amplifier, is configured to receive voltages Vtop and Vbot from the MEMS element as sensor signals. The amplifier 21 is part of the signal processing circuit 20 and can be the start of its signal processing chain. For example, the amplifier 21 can be configured to receive the sensor signals (i.e., voltages Vtop and Vbot) and provide the amplified sensor signals to a discrete-time ADC (not shown) arranged downstream along the signal processing chain of the signal processing circuit 20.

[0047] The voltages Vtop and Vbot at the output of the MEMS element are set via very high-ohm bias resistors Rbias1 and Rbias2, respectively, and a reference voltage Vref1. During the sensing operation, the bias resistors Rbias1 and Rbias2 are set to very high ohmic values to reduce the noise at the input of the amplifier 21. For example, the bias resistors Rbias1 and Rbias2 can initially be set anywhere between 1 Gohm and 500 Gohms, depending on the noise suppression requirements and bandwidth requirements of the readout circuit. The higher the resistance, the higher the noise suppression providing a higher SNR. However, it is conceivable that other resistors can be used. In this example, the bias resistors Rbias1 and Rbias2 are maintained equal or substantially equal and can be adjustable to reduce the sensor signals so that capacitance and / or sensitivity faults can be tested and detected.

[0048] The reference voltage Vref1 is input at a common node between the two bias resistors Rbias1 and Rbias2. After a charging time given by the resistance values of Rbias1 and Rbias2 and the time constants of Ctop and Cbot, the Vtop and Vbot DC voltages at the MEMS output are set by the reference voltage Vref1. However, if a capacitance or sensitivity fault occurs at one of the capacitors Ctop or Cbot, the time constants of Ctop and Cbot can be affected. For example, larger capacitors typically have a longer transient response and thus a higher time constant. Conversely, smaller capacitors typically have a shorter transient response and thus a lower time constant. Therefore, the time constants of Ctop and Cbot can be measured and evaluated by the fault diagnosis circuit 30 to detect capacitance or sensitivity faults. Specifically, the time constants of Ctop and Cbot can be measured by causing a signal response at Vtop and Vbot, respectively, by an electrical signal step and measuring the rise time (i.e., charging time) or fall time (i.e., discharging time) of Vtop and Vbot against a corresponding time constant threshold Vref2.

[0049] The capacitive sensor readout circuit 300 includes a controller 40 and a signal generator 50. The controller 40 and the signal generator 50 generate various control signals and injection signals for performing fault diagnosis. The controller 40 generates control signals, including: a clock signal SCLK, a time constant threshold signal Vref2 for setting the time constant threshold Vref2 to be used by the threshold comparators 31 and 32, an electrical step timing signal Sstep indicating the time when an electrical step is triggered, a configuration signal Vbias for controlling the supply of the DC bias voltage source 10 to the MEMS element 11, and configuration signals Rbias1 and Rbias2 for adjusting the resistance values of the bias resistors Rbias1 and Rbias2.

[0050] The configuration signals Vbias, Rbias1, and Rbias2 are used as a control signal Sreduce to reduce the sensor signal Ssense that may be caused by a sensed external force (such as pressure or acceleration). The methods for reducing the sensor signal Ssense include setting the input bias voltage Vbias applied to the input of the MEMS element 11 to zero, reducing the resistance values of the bias resistors Rbias1 and Rbias2 to increase the 3dB frequency of the high-pass filter formed by Rbias and the capacitors CTop and CBot, thereby effectively filtering the sensor signal Ssense, or performing the readout of the sensor signal Ssense using a common-mode buffer (see Figure 6 ).

[0051] The controller 40 can adjust the time constant threshold Vref2 according to the type of the injected electrical signal step and the injection point within the circuit. It should also be noted that although the controller 40 is shown as providing the clock signal SCLK, the system clock can also generate and provide the clock signal SCLK.

[0052] The signal generator 50 generates an injection signal Sinject, and the injection signal Sinject has an electrical signal step with a step rise or a step fall. The injection signal Sinject can be applied as a voltage step at the node Vref1 (see Figure 3A ), applied as a voltage step at the DC bias voltage source 10 as Vbias (see Figure 3B ), or applied as a current step at the nodes Vtop and Vbot (see Figure 5A) Applying the injection signal Sinject at the node Vref1 can generate a low-pass filter time evolution in the transient responses of the capacitors Ctop and Cbot. Applying the injection signal Sinject as Vbias at the DC bias voltage source 10 can generate a high-pass filter time evolution in the transient responses of the capacitors Ctop and Cbot. If a voltage step is applied as Vbias at the DC bias voltage source 10, the sensor signal Ssense should be reduced by decreasing the resistance values of the bias resistors Rbias1 and Rbias2 or by performing the readout of the sensor signal Ssense using a common-mode buffer, because while the injection signal (i.e., the electrical signal step) is injected and the signal response is evaluated by the fault diagnosis circuit 30, the sensor signal is maintained in a reduced state.

[0053] Once the sensor signal Ssense is placed in a reduced state, the signal generator 50 generates an injection signal Sinject having an electrical signal step. In Figure 3A the example, the resistance values of Rbias1 and Rbias2 have been reduced to 100 kohms, and the electrical signal step (i.e., the step rise) is injected at the node of Vref1. In other words, Vref1 as the injection signal is increased by the signal generator 50 from the first voltage level to a higher second voltage level, and causes the capacitors Ctop and Cbot to start charging. As a result, the electrical signal step as the transient response causes an exponential signal increase at the outputs Vtop and Vbot of the capacitors Ctop and Cbot.

[0054] The fault diagnosis circuit 30 includes a threshold comparator 31 that compares the output voltage Vtop with a time constant threshold Vref2 set by the system controller 40. The time constant threshold Vref2 can be set to a voltage value corresponding to the time constants of the capacitors Ctop and Cbot, and in the absence of any faults, the capacitance values of the capacitors Ctop and Cbot should be substantially equal. The threshold comparator 31 is configured to receive Vtop and Vref2 and change its output to Vstop1 when Vtop is equal to or greater than Vref2. For example, the threshold comparator 31 can change its output from 0V to 5V. The change in the output indicates that the capacitor Ctop has been charged to a certain point in response to the injection signal, i.e., Vtop has crossed the time constant threshold Vref2, which represents the time constant of the capacitor Ctop. The output of Vstop1 stops being allocated to the capacitor Ctop and the counter 34a that is used to measure its time constant.

[0055] Similarly, the fault diagnosis circuit 30 includes a threshold comparator 32 that compares the output voltage Vbot with a time constant threshold Vref2 set by the system controller 40. The threshold comparator 31 is configured to receive Vbot and Vref2 and change its output when Vbot is equal to or greater than Vref2. The threshold comparator 32 is configured to receive Vbot and Vref2 and change its output to Vstop2 when Vbot is equal to or greater than Vref2. For example, the threshold comparator 32 can change its output from 0V to 5V. The change in the output indicates that the capacitor Cbot has been charged to a certain point in response to the injection signal, i.e., Vbot has crossed the time constant threshold Vref2, which represents the time constant of the capacitor Cbot. The output of Vstop2 stops the counter 34b that is assigned to the capacitor Cbot and used to measure its time constant.

[0056] The fault diagnosis circuit 30 further includes a processing circuit 33 that includes counters 34a, 34b that operate (i.e., count up) based on a clock signal SCLK. The counters 34a, 34b start when an electrical signal step is triggered. Thus, the electrical step timing signal Sstep is the counter start signal received by the counters 34a, 34b to trigger the counters 34a, 34b to start counting.

[0057] The counter 34a is configured to stop counting when the threshold comparator 31 outputs the voltage Vstop1 to indicate that Vtop has crossed the time constant threshold Vref2. The timing value of the counter 34a at the time of receiving the voltage Vstop1 represents the time constant TCtop of the capacitor Ctop. The counter 34b is configured to stop counting when the threshold comparator 32 outputs the voltage Vstop2 to indicate that Vbot has crossed the time constant threshold Vref2. The timing value of the counter 34b at the time of receiving the voltage Vstop2 represents the time constant TCbot of the capacitor Cbot.

[0058] The processing circuit 33 of the fault diagnosis circuit 30 includes a fault detector 35 configured to compare the measured time constants TCtop and TCbot (e.g., the final count values of a counter) with a predetermined threshold range including a minimum threshold TCmin and a maximum threshold TCmax according to Equations 1 and 2. The minimum threshold TCmin can be set to be greater than zero. The minimum threshold and the maximum threshold can be set to be within a certain percentage (e.g., within 10%) of the expected time constant. If the fault detector 35 determines that any of the time constants TCtop and TCbot is outside the predetermined threshold range (i.e., less than the minimum threshold TCmin or greater than the maximum threshold TCmax), the fault detector 35 generates an error signal indicating a capacitor fault. Otherwise, the fault detector 35 determines that there is no capacitor fault at the capacitor Ctop or Cbot and can output a "no fault" signal. The time constant of the error signal may exceed the limit of the predetermined threshold range, thereby indicating a fault in the capacitor.

[0059] TCmin ≤ TCtop ≤ TCmax Equation 1

[0060] TCmin ≤ TCbot ≤ TCmax Equation 2

[0061] In Figure 3B the example, the resistance values of Rbias1 and Rbias2 are again reduced to 100 kohms to place the sensor signal in a reduced state. However, an electrical signal step (i.e., a step down) is injected at the DC power supply via the input voltage Vbias. In other words, Vbias as the injection signal is reduced from a first voltage level to a lower second voltage level by the signal generator 50 and causes a signal response at the outputs of the capacitors Ctop and Cbot. As a result, the electrical signal step as a transient response causes an exponential signal increase at the outputs Vtop and Vbot of the capacitors Ctop and Cbot.

[0062] The fault diagnosis circuit 30 monitors the values of Vtop and Vbot during their transient responses in a similar manner as described above for Figure 3A described.

[0063] By applying the injection signal as a two-step electrical signal instead of as a one-step electrical signal, Figure 3A and Figure 3BThe capacitive sensor readout circuit 300 can also be used to test for sensitivity faults. As used herein, a two-step electrical signal includes an electrical step-up from a first signal level to a second signal level and an electrical step-down from the second signal level to the first signal level. The triggering of the two electrical signal steps is separated by a predetermined amount of time controlled by the controller 40. As previously described, one of the above methods for placing the sensor signal in a reduced state is implemented prior to applying the two-step electrical signal.

[0064] Figure 4A is for testing sensitivity faults according to one or more embodiments Figure 3A Signal diagram of various signals present in the capacitive sensor readout circuit 300. The signals include a two-step electrical signal Sinject injected by the signal generator 50 at the node Vref1, the transient response of the output voltage Vtop in response to the two-step electrical signal Sinject, and the transient response of the output voltage Vbot in response to the two-step electrical signal Sinject.

[0065] As previously described, the fault diagnosis circuit 30 uses the threshold comparators 31 and 32 and the counters 34a and 34b to measure the time constants of the capacitors Ctop and Cbot. However, since the injected signal Sinject is a two-step electrical signal, the fault diagnosis circuit 30 is configured to measure the time constants of the capacitors Ctop and Cbot for each electrical signal step. As a result, the fault diagnosis circuit 30 will determine a first time constant TCtop1 for the electrical step-up and a second time constant TCtop2 for the electrical step-down for the capacitor Ctop. Similarly, the fault diagnosis circuit 30 will determine a first time constant TCbot1 for the electrical step-up and a second time constant TCbot2 for the electrical step-down for the capacitor Cbot.

[0066] Because the time constant threshold Vref2 for measuring the time constant of the voltage rise is different from the time constant threshold Vref2 for measuring the time constant of the voltage drop, it is necessary to adjust the time constant threshold Vref2 for each electrical signal step. The controller 40 can be configured to adjust the time constant threshold Vref2 according to different signal steps. For example, the controller 40 can set the time constant threshold Vref2 to the value Vref2_up to increase the transient response, and can set the time constant threshold Vref2 to the value Vref2_down to reduce the transient response. During the charging or discharging step, the values Vref2_up and Vref2_down can be set to approximately 63% of the final voltage value of the capacitor. For example, the controller 40 can set the time constant threshold Vref2 to an appropriate value when the corresponding electrical signal step is triggered.

[0067] The threshold comparator 31 generates a counter stop value when Vtop becomes equal to or greater than Vref2_up in response to an electrical step rise, and generates a counter stop value when Vtop becomes equal to or less than Vref2_down in response to an electrical step fall. Similarly, the threshold comparator 32 generates a counter stop value when Vbot becomes equal to or greater than Vref2_up in response to an electrical step rise, and generates a counter stop value when Vbot becomes equal to or less than Vref2_down in response to an electrical step fall. The counter values at the counters 34a and 34b represent the corresponding time constants TCtop1, TCtop2, TCbot1, and TCbot2.

[0068] The difference between the two measured time constants of the capacitor Ctop or Cbot can be used to detect a sensitivity fault of the capacitor. For example, the fault detector 35 can calculate the difference between the time constants and compare the difference with a predetermined difference threshold range including a minimum difference threshold ΔTCmin and a maximum difference threshold ΔTCmax according to Equation 3-6.

[0069] ΔTCtop = |TCtop1 - TCtop2| Equation 3

[0070] ΔTCbot = |TCbot1 - TCbot2| Equation 4

[0071] ΔTCmin ≤ ΔTCtop ≤ ΔTCmax Equation 5

[0072] ΔTCmin ≤ ΔTCbot ≤ ΔTCmax Equation 6

[0073] The minimum threshold and the maximum threshold can be set to be within a certain percentage (e.g., within 10%) of the expected difference threshold. If the fault detector 35 determines that any one of the differential time constants ΔTCtop and ΔTCbot is outside the predetermined difference threshold range (i.e., less than the minimum threshold ΔTCmin or greater than the maximum threshold ΔTCmax), the fault detector 35 generates an error signal indicating a sensitivity fault. Otherwise, the fault detector 35 determines that there is no sensitivity fault at the capacitor Ctop or Cbot and can output a "no fault" signal. The error signal can identify which differential time constant exceeds the limit of the predetermined difference threshold range, thereby indicating the faulty capacitor.

[0074] Figure 4B for testing sensitivity faults according to one or more embodiments Figure 3A - 3BSignal diagrams of various signals present in the capacitive sensor readout circuit 300. The signals include a two-step electrical signal Sinject injected by the signal generator 50 via Vbias at the DC bias voltage source 10, the transient response of the output voltage Vtop in response to the two-step electrical signal Sinject, and the transient response of the output voltage Vbot in response to the two-step electrical signal Sinject.

[0075] Because the time constant threshold Vref2 for measuring the time constant of the voltage rise is different from the time constant threshold Vref2 for measuring the time constant of the voltage drop, it is necessary to adjust the time constant threshold Vref2 for each electrical signal step. The controller 40 can set the time constant threshold Vref2 to the value Vref2_up to increase the transient response and can set the time constant threshold Vref2 to the value Vref2_down to decrease the transient response.

[0076] The fault diagnosis circuit 30 performs similar measurements as described above with reference to Figure 4A to determine the corresponding time constants TCtop1, TCtop2, TCbot1, and TCbot2, and then performs a fault assessment according to Equation 3-6.

[0077] Figure 5A The capacitive sensor readout circuit 500A shows another technique for detecting capacitive faults. The capacitive sensor readout circuit 500A is similar to the capacitive sensor readout circuit 300, except that the signal generator 50 is configured to inject a current signal instead of a voltage signal as the electrical signal step in order to cause a transient voltage response at the capacitors Ctop and Cbot. The function of the fault diagnosis circuit 30 is the same as that described above with reference to the capacitive sensor readout circuit 300.

[0078] In Figure 5A the example, the sensor signal is placed in a reduced state by one of the above methods. In this example, the signal generator 50 is implemented as two current sources 51 and 52 and two switches SW1 and SW2, which are configured to inject current steps at the nodes Vtop and Vbot. The switches SW1 and SW2 are controlled by the controller 40, which synchronously opens or closes the switches, for example, via the corresponding injection control signals Sinject (SW1 and SW2). When the switches SW1 and SW2 are closed, the current steps at the nodes Vtop and Vbot cause a linear increase (i.e., linear charging) of the voltages Vtop and Vbot across the capacitors Ctop and Cbot. The rise times of the voltages Vtop and Vbot can be monitored similarly as described above to determine the time constants TCtop and TCbot of the capacitors Ctop and Cbot, respectively. The fault diagnosis circuit 30 operates in the same manner as described above forFigure 3A In a similar manner as described, the time constants TCtop and TCbot are compared with a predetermined threshold range to determine whether there is a capacitance fault. When switches SW1 and SW2 are re-opened, capacitors Ctop and Cbot exponentially discharge due to bias resistors Rbias1 and Rbias2.

[0079] Figure 5B A capacitive sensor readout circuit 500B implementing another technique for detecting sensitivity faults is shown. In this example, signal generator 50 is implemented as four current sources / sinks 51 - 54 and four switches SW1 - SW4. Current sources 51 and 52 are configured to provide current steps at nodes Vtop and Vbot when switches SW1 and SW2 are closed. Current sinks 53 and 54 are configured to absorb current steps from nodes Vtop and Vbot when switches SW3 and SW4 are closed. A two-step electrical signal can be generated by first generating a first current step using current sources 51 and 52, where switches SW1 and SW2 are synchronously closed and switches SW3 and SW4 are synchronously open, and then generating a second current step using current sinks 53 and 54, where switches SW1 and SW2 are synchronously open and switches SW3 and SW4 are synchronously closed. Controller 40 is configured to control the states of switches SW1 - SW4 via corresponding injection control signals Sinject(SW1 - SW4).

[0080] When switches SW1 and SW2 are closed, the current steps at nodes Vtop and Vbot cause a linear increase (i.e., linear charging) in the voltages Vtop and Vbot across capacitors Ctop and Cbot. The rise times of voltages Vtop and Vbot can be monitored in a similar manner as described above to determine the time constants TCtop1 and TCbot1 of capacitors Ctop and Cbot, respectively. When switches SW3 and SW4 are closed, the current steps at nodes Vtop and Vbot cause a linear decrease (i.e., linear discharging) in the voltages Vtop and Vbot across capacitors Ctop and Cbot. The fall times of voltages Vtop and Vbot can be monitored in a similar manner as described above to determine the time constants TCtop2 and TCbot2 of capacitors Ctop and Cbot, respectively. Controller 40 also adjusts the value at Vref2 between Vref2_up and Vref2_down based on whether the transient response caused by the signal step is an increasing (charging) or decreasing (discharging) transient response, as described in conjunction with Figure 4A and Figure 4B similarly described.

[0081] The function of fault diagnosis circuit 30 is the same as that previously referenced for capacitive sensor readout circuit 300 and in conjunction with Figure 4A and Figure 4BThe detection of the described sensitivity faults is the same as described.

[0082] The capacitive sensor readout circuit 500B can also be used to test for capacitive faults by using only current sources 51 and 52 (i.e., by keeping switches SW3 and SW4 open and keeping current sinks 53 and 54 disconnected). In this way, a single electrical step injection signal as shown Figure 5A can be generated.

[0083] Figure 6 is a schematic block diagram of a capacitive sensor readout circuit 600 according to one or more embodiments. In particular, the capacitive sensor readout circuit 600 includes a common-mode buffer 36 that performs sensor signal readout for fault diagnosis. By using the common-mode buffer 36 in this way, the sensor signal Ssense is always in a reduced state in the fault diagnosis circuit 30. Therefore, the signal Sreduce is not required.

[0084] The common-mode buffer 36 includes two input terminals electrically connected to DC voltages Vtop and Vbot, respectively. The common-mode buffer 36 is configured to internally generate an average signal representing the average of these two inputs (i.e., the average of the voltages Vtop and Vbot). The common-mode amplifier 36 also includes an input terminal electrically connected to a common-mode voltage Vref1 (i.e., the node coupled between bias resistors Rbias1 and Rbias2). The common-mode buffer 36 is configured to generate a differential output signal Vdiff representing the difference between the average signal (i.e., the common mode between Vtop and Vbot) and the common-mode reference signal (i.e., the common-mode voltage Vref1).

[0085] The signal generator 50 is configured to apply a single-step or two-step electrical injection signal in any of the above-described ways, e.g., by applying a signal at Vbias or Vref1. The electrical signal step causes a transient response at the differential output signal Vdiff of the common-mode buffer 36, and this transient response can be evaluated to determine the average time constants of capacitors Ctop and Cbot. The fault diagnosis circuit 30 includes a threshold comparator 37 that receives the differential output signal Vdiff and a time constant threshold Vref2, and generates a counter stop signal Vstop when Vdiff becomes equal to or greater than Vref2. The counter stop signal Vstop stops the counter 34 of the processing circuit 33, which is started by an electrical step timing signal Sstep when the electrical signal step is triggered. The count value at which the counter 34 stops represents the average time constants of capacitors Ctop and Cbot. According to the above-described embodiments, the fault detector 35 compares the measured average time constants with a predetermined threshold range to determine whether a fault exists.

[0086] Although various embodiments have been described, it will be apparent to those of ordinary skill in the art that there are many more embodiments and implementations within the scope of the present disclosure. Accordingly, the invention is not limited except as by the appended claims and their equivalents.

[0087] Regarding the various functions performed by the above-described components or structures (components, devices, circuits, systems, etc.), unless otherwise specified, the terms used to describe such components (including references to "means") are intended to correspond to any component or structure that performs the specified function of the described component (i.e., is functionally equivalent), even if not structurally equivalent to the disclosed structure that performs the function in the exemplary implementations of the invention shown herein.

[0088] In addition, the following claims are hereby incorporated into the detailed description, where each claim can stand on its own as a separate exemplary embodiment. While each claim can stand on its own as a separate exemplary embodiment, it should be noted that although a dependent claim can refer to a particular combination with one or more other claims in the claims, other exemplary embodiments can also include combinations of the dependent claim with the subject matter of each other dependent or independent claim. Such combinations are presented herein unless it is stated that a particular combination is not intended. In addition, it is intended that the features of a claim be included in any other independent claim, even if that claim does not directly depend on that independent claim.

[0089] It should also be noted that the methods disclosed in the specification or claims can be implemented by an apparatus having components for performing each of the corresponding acts of these methods.

[0090] In addition, it should be understood that the disclosure of multiple acts or functions in the specification or claims may not be construed as being in a particular order. Accordingly, the disclosure of multiple acts or functions does not limit these to a particular order unless such acts or functions are not interchangeable for technical reasons. In addition, in some embodiments, a single act can include or can be divided into multiple sub-acts. Such sub-acts can be included and are part of the disclosure of that single act unless explicitly excluded.

[0091] In summary, although various exemplary embodiments have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made, which will achieve some of the advantages of the concepts disclosed herein without departing from the spirit and scope of the present invention. It will be apparent to those skilled in the art that other components performing the same functions can be appropriately substituted. It should be understood that other embodiments can be used and structural or logical changes can be made without departing from the scope of the present invention. It should be mentioned that features explained with reference to specific figures can be combined with features of other figures, even in those figures not explicitly mentioned. Such modifications to the general inventive concept are intended to be covered by the appended claims and their legal equivalents.

Claims

1. A capacitive sensor, comprising: A first conductive structure; A second conductive structure, opposite to the first conductive structure, wherein the second conductive structure is movable relative to the first conductive structure in response to an external force acting on the second conductive structure, and wherein the second conductive structure is capacitively coupled to the first conductive structure to form a first capacitor, the first capacitor having a first capacitance that varies with a change in the distance between the first conductive structure and the second conductive structure, and wherein the first capacitance represents the external force; A signal generator configured to apply a first electrical signal step at an input or an output of the first capacitor to cause a first voltage transient response at an output of the first capacitor; And A diagnostic circuit configured to detect a fault in the capacitive sensor by calculating a first time constant of the first voltage transient response and detecting a fault based on the first time constant; Wherein the first time constant is calculated based on a time from when the first electrical signal step triggered by the signal generator reaches a reference threshold by an output voltage representing the first voltage transient response.

2. The capacitive sensor according to claim 1, wherein the first electrical signal step causes the first capacitor to charge to a maximum voltage or discharge to a minimum voltage.

3. The capacitive sensor according to claim 1, wherein the first electrical signal step is a step-up signal step or a step-down signal step.

4. The capacitive sensor according to claim 1, wherein: The diagnostic circuit includes a comparator, a counter, and a fault detector, The comparator is configured to receive the output voltage representing the first voltage transient response and compare the output voltage with the reference threshold to generate a comparison result having one of two comparator values, The counter is configured to stop counting when the comparison result changes from a first comparator value to a second comparator value and output a stop counter value representing the first time constant, and The fault detector is configured to compare the stop counter value with a predetermined threshold range and generate an error signal in response to the stop counter value being outside the predetermined threshold range.

5. The capacitive sensor according to claim 1, wherein: The diagnostic circuit includes a comparator and a fault detector, The comparator is configured to receive the output voltage representing the first voltage transient response and compare the output voltage with the reference threshold to detect the first time constant, and The fault detector is configured to compare the first time constant with a predetermined threshold range and generate an error signal in response to the first time constant being outside the predetermined threshold range.

6. The capacitance sensor according to claim 5, wherein the diagnostic circuit includes a counter configured to output a first counter value representing the detected first time constant, and the fault detector is configured to compare the first counter value with the predetermined threshold range and generate the error signal in response to the first counter value being outside the predetermined threshold range.

7. The capacitance sensor according to claim 1, wherein: the diagnostic circuit is configured to compare the first time constant with a predetermined threshold range and generate an error signal in response to the first time constant being outside the predetermined threshold range.

8. The capacitance sensor according to claim 7, wherein the predetermined threshold range includes a minimum threshold and a maximum threshold, and the first time constant is outside the predetermined threshold range if the first time constant is less than the minimum threshold or greater than the maximum threshold.

9. The capacitance sensor according to claim 1, wherein: the signal generator is configured to apply a second electrical signal step at the input or output of the first capacitor to cause a second voltage transient response at the output of the first capacitor, wherein the direction of the second voltage transient response is opposite to the direction of the first voltage transient response, and the diagnostic circuit is configured to detect the fault in the capacitance sensor by measuring a second time constant of the second voltage transient response and detecting the fault based on the first time constant and the second time constant.

10. The capacitance sensor according to claim 9, wherein the diagnostic circuit is configured to calculate a difference between the first time constant and the second time constant, compare a magnitude of the difference with a predetermined threshold range, and generate an error signal in response to the magnitude of the difference being outside the predetermined threshold range.

11. The capacitance sensor according to claim 9, wherein: the diagnostic circuit includes a comparator, at least one counter, and a fault detector, the comparator is configured to receive output voltages representing the first voltage transient response and the second voltage transient response, compare the output voltages with a first reference threshold to detect the first time constant, and compare the output voltages with a second reference threshold to detect the second time constant, the at least one counter is configured to output a first counter value representing the detected first time constant and output a second counter value representing the detected second time constant, and the fault detector is configured to calculate a difference between the first counter value and the second counter value, compare a magnitude of the difference with a predetermined threshold range, and generate an error signal in response to the magnitude of the difference being outside the predetermined threshold range.

12. The capacitance sensor according to claim 1, further comprising: A third conductive structure, opposite to the first conductive structure, wherein the third conductive structure is movable relative to the first conductive structure, and wherein the third conductive structure is capacitively coupled to the first conductive structure to form a second capacitor, the second capacitor having a second capacitance that varies with a change in the distance between the first conductive structure and the third conductive structure, wherein the signal generator is configured to apply the first electrical signal step at an input or an output of the second capacitor to cause a second voltage transient response at an output of the second capacitor, the diagnostic circuit is configured to detect the fault in the capacitive sensor by measuring a second time constant of the second voltage transient response and detecting the fault based on the first time constant and the second time constant.

13. The capacitive sensor according to claim 12, wherein the signal generator is configured to simultaneously apply the first electrical signal step to the first capacitor and the second capacitor such that the first voltage transient response and the second voltage transient response are caused by the same electrical signal step.

14. The capacitive sensor according to claim 12, wherein: the diagnostic circuit is configured to compare the first time constant and the second time constant with a predetermined threshold range and generate an error signal in response to the first time constant being outside the predetermined threshold range or the second time constant being outside the predetermined threshold range.

15. The capacitive sensor according to claim 12, wherein: the signal generator is configured to apply a second electrical signal step at an input or an output of the first capacitor to cause a third voltage transient response at an output of the first capacitor, wherein a direction of the third voltage transient response is opposite to a direction of the first voltage transient response, the signal generator is configured to apply a second electrical signal step at an input or an output of the second capacitor to cause a fourth voltage transient response at an output of the second capacitor, wherein a direction of the fourth voltage transient response is opposite to a direction of the second voltage transient response, the diagnostic circuit is configured to detect the fault in the capacitive sensor by measuring a third time constant of the third voltage transient response, measuring a fourth time constant of the fourth voltage transient response, and detecting the fault based on the first time constant, the second time constant, the third time constant, and the fourth time constant.

16. The capacitive sensor according to claim 15, wherein the diagnostic circuit is configured to calculate a first difference between the first time constant and the third time constant, compare the magnitude of the first difference with a predetermined threshold range, calculate a second difference between the second time constant and the fourth time constant, compare the magnitude of the first difference with the predetermined threshold range, compare the magnitude of the second difference with the predetermined threshold range, and generate an error signal in response to the magnitude of the first difference being outside the predetermined threshold range or in response to the magnitude of the second difference being outside the predetermined threshold range.

17. The capacitive sensor according to claim 1, further comprising a controller, wherein the first capacitor includes a first terminal coupled to a bias supply voltage and a second terminal coupled to a DC voltage set by a bias resistor circuit, and wherein the controller is configured to drive the bias supply voltage to zero and control the signal generator to apply the first electrical signal step at the output of the first capacitor to cause the first voltage transient response when the bias supply voltage is zero.

18. The capacitive sensor according to claim 1, further comprising a controller, wherein the first capacitor includes a first terminal coupled to a bias supply voltage and a second terminal coupled to a DC voltage set by a bias resistor circuit, and wherein the controller is configured to reduce the resistance of the bias resistor circuit and control the signal generator to apply the first electrical signal step at the input or output of the first capacitor to cause the first voltage transient response when the resistance of the bias resistor circuit is reduced.

19. A capacitive sensor, comprising: a first conductive structure; a second conductive structure, opposite to the first conductive structure, wherein the second conductive structure is movable relative to the first conductive structure, wherein the second conductive structure is capacitively coupled to the first conductive structure to form a first capacitor, the first capacitor having a first capacitance that varies with a change in the distance between the first conductive structure and the second conductive structure, wherein the first capacitor includes a first terminal coupled to a bias supply voltage and a second terminal coupled to a first DC voltage set by a bias resistor circuit; a third conductive structure, opposite to the first conductive structure, wherein the third conductive structure is movable relative to the first conductive structure, wherein the third conductive structure is capacitively coupled to the first conductive structure to form a second capacitor, the second capacitor having a second capacitance that varies with a change in the distance between the first conductive structure and the third conductive structure, wherein the second capacitor includes a first terminal coupled to the bias supply voltage and a second terminal coupled to a second DC voltage set by the bias resistor circuit; a common-mode buffer configured to receive the first DC voltage, the second DC voltage, and a reference voltage, and generate a differential signal based on a difference between a common-mode voltage of the first DC voltage and the second DC voltage and the reference voltage; A signal generator configured to apply a first electrical signal step at an input or output of the first capacitor and at an input or output of the second capacitor to cause a first voltage transient response in the first DC voltage and a second voltage transient response in the second DC voltage, respectively; and A diagnostic circuit configured to detect a fault in the capacitive sensor by calculating a first time constant of the differential signal corresponding to the first voltage transient response and the second voltage transient response and detecting the fault based on the first time constant; wherein the first time constant is calculated based on the time from the first electrical signal step triggered by the signal generator to when the differential signal reaches a reference threshold.

20. The capacitive sensor according to claim 19, wherein: The diagnostic circuit is configured to compare the first time constant with a predetermined threshold range and generate an error signal in response to the first time constant being outside the predetermined threshold range.

21. The capacitive sensor according to claim 19, wherein: The signal generator is configured to apply a second electrical signal step at an input or output of the first capacitor to cause a third voltage transient response in the first DC voltage, wherein the direction of the third voltage transient response is opposite to the direction of the first voltage transient response, The signal generator is configured to apply a second electrical signal step at an input or output of the second capacitor to cause a fourth voltage transient response in the second DC voltage, wherein the direction of the fourth voltage transient response is opposite to the direction of the second voltage transient response, The diagnostic circuit is configured to detect the fault in the capacitive sensor by measuring a second time constant of the differential signal corresponding to the third voltage transient response and the fourth voltage transient response and detecting the fault based on the first time constant and the second time constant.

22. The capacitive sensor according to claim 21, wherein the diagnostic circuit is configured to calculate a difference between the first time constant and the second time constant, compare a magnitude of the difference with a predetermined threshold range, and generate an error signal in response to the magnitude of the difference being outside the predetermined threshold range.

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