Capacitive sensor and method for operating a capacitive sensor
Patent Information
- Application Number
- EP2023702583
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-03-08
- Filing Date
- 2023-01-30
- Publication Date
- 2025-01-15
AI Technical Summary
Capacitive pressure sensors in devices like smartphones and wearables face challenges in detecting device malfunctions due to environmental exposure, which can cause offset errors from liquids, and existing self-test methods are not effective in identifying short circuits that can lead to measurement errors.
A capacitive sensor with a MEMS element and an ASIC element that uses a capacitive Wheatstone bridge circuit to determine reference capacitance values, allowing for self-testing without additional detection components, by applying control signals to supply lines and evaluating capacitance values to detect short-circuit resistances.
Enables simple self-testing of capacitive sensors, identifying potential measurement errors and allowing for timely replacement to prevent consequential damage, with no need for dedicated detection components and minimal circuitry expenditure.
Smart Images
Figure 1.1
Abstract
Description
[0001] Description
[0002] title
[0003] Capacitive sensor and method for operating a capacitive sensor
[0004] The invention relates to a capacitive sensor. The invention further relates to a method for operating a capacitive sensor. The invention further relates to a computer program product.
[0005] State of the art
[0006] Pressure sensors in devices such as smartphones, smartwatches, fitness trackers, and wearables, as well as in the IoT market, are becoming increasingly popular. Use cases are not limited to weather detection and altitude estimation, but are also expanding with new ones emerging, such as high-precision ground level detection for 911 emergency call locations.
[0007] For reliable functionality, it is becoming increasingly important to ensure functionality throughout its entire service life. A key feature for this is the ability to detect device malfunctions through a self-test. It is known that pressure sensors are exposed to the environment and can therefore come into contact with fluids. This can cause adverse offsets in pressure measurements due to the presence of mass in the form of the fluid, which could be, for example, water, oil, sweat, etc.
[0008] Self-tests are known for sensors in which the excitation of a MEMS element is used to subsequently check measured values. US 2010 / 180687 A1 discloses a pressure sensor with switchable sensor elements in which reference elements are read indirectly by switching them between branches of a Wheatstone bridge for self-testing.
[0009] Disclosure of the invention
[0010] It is an object of the present invention to provide an improved capacitive sensor.
[0011] The object is achieved according to a first aspect with a capacitive sensor, comprising: a MEMS element with a capacitive Wheatstone bridge circuit; and an ASIC element, wherein the ASIC element is designed to determine a measured value of at least one reference capacitance of the capacitive Wheatstone bridge circuit, wherein control signals for determining the reference capacitance can be applied to a respective supply line of the capacitive Wheatstone bridge circuit, wherein the ASIC element is designed to read out and evaluate the measured values of the reference capacitances, wherein a resistance value of at least one short-circuit resistance of the capacitive sensor can be determined from the values of the reference capacitances.
[0012] The proposed method exploits the fact that a measured capacitance value is distorted by a short-circuit resistor. The value of the measured reference capacitances is not actually of interest; rather, the values of the reference capacitors are used as indicators to detect short circuits that may occur due to production-related factors. Evaluation software can be hard-wired into an ASIC, for example, to perform a self-test of the capacitive sensor.
[0013] This advantageously provides a capacitive sensor that can be used to easily perform a self-test. This advantageously eliminates the need for an additional dedicated detection component, as the capacitive pressure sensor can be used with existing hardware components. This makes it possible to determine whether a measurement process using the capacitive sensor may be faulty.
[0014] The object is achieved according to a second aspect with a method for operating a capacitive sensor, comprising the steps:
[0015] - Applying a control signal to a supply line of a capacitive Wheatstone bridge circuit;
[0016] Determining capacitance values of reference capacitances of capacitive Wheatstone bridge circuits;
[0017] - Evaluation of the determined capacitance values of reference capacitances of capacitive Wheatstone bridge circuit; and
[0018] Determining at least one short-circuit resistance from the evaluated capacitance values of the reference capacitances based on a comparison with a defined reference value.
[0019] The proposed method can be used in production or in the field, whereby in case of a failure the sensor (e.g. an impact sensor in the automotive sector) is replaced in order to avoid subsequent damage.
[0020] The object is achieved according to a third aspect with a computer program product with program code means, configured to carry out the proposed method when it runs on a proposed capacitive sensor or is stored on a computer-readable data carrier.
[0021] Advantageous developments of the proposed capacitive sensor and the proposed method are the subject of dependent claims.
[0022] Advantageous developments of the capacitive sensor provide that a first short-circuit resistance between a first supply line of the capacitive Wheatstone bridge circuit and ground potential, a second short-circuit resistance between a second supply line of the capacitive Wheatstone bridge circuit and ground potential and a third short-circuit resistance between the first supply line and the second supply line of the capacitive Wheatstone bridge circuit can be determined.
[0023] A further advantageous development of the capacitive sensor provides that a serial resistance in the first supply line and / or the second supply line can also be determined.
[0024] A further advantageous development of the capacitive pressure sensor provides that a control signal for controlling measuring capacitances can be generated using the ASIC element. This advantageously minimizes circuitry complexity.
[0025] A further advantageous development of the capacitive sensor provides that a defined reference value of a relation between the determined reference capacitances is used to determine the at least one short-circuit resistance (RKS).
[0026] Further advantageous developments of the capacitive sensor provide that the at least one short-circuit resistance can be determined in a range between approximately 100 kQ and approximately 1 MQ.
[0027] The invention, along with further features and advantages, is described in detail below with reference to several figures. The figures are primarily intended to illustrate the principles essential to the invention.
[0028] Disclosed method features result analogously from corresponding disclosed device features, and vice versa. This means, in particular, that features, technical advantages, and embodiments relating to the capacitive sensor result analogously from corresponding embodiments, features, and advantages relating to the method for operating a capacitive sensor, and vice versa.
[0029] In the figures: Fig. 1 shows a perspective view of an embodiment of a proposed capacitive sensor;
[0030] Fig. 2 is a cross-sectional view through the embodiment of the proposed capacitive sensor of Fig. 1;
[0031] Fig. 3 a basic block diagram of the proposed capacitive
[0032] Sensors in normal operation;
[0033] Fig. 4, 5 basic circuit diagrams of the proposed capacitive sensor in a test operation;
[0034] Fig. 6 is a circuit diagram showing various resistances that can be determined using the proposed method;
[0035] Fig. 7 is a time sequence diagram showing a basic sequence of the proposed method for operating a capacitive sensor; and
[0036] Fig. 8 is a time sequence diagram showing a basic sequence of the proposed method for operating a capacitive sensor.
[0037] Description of embodiments
[0038] A capacitive sensor is proposed, which can be designed, for example, as a barometric capacitive pressure sensor. In such a capacitive sensor, pressure is measured using a MEMS element in which both variable capacitances (pressure measuring elements) and fixed reference capacitances are incorporated, arranged in a capacitive Wheatstone bridge circuit. While some faults and failures of the capacitive sensor can be detected during the measurement of the entire Wheatstone bridge, the proposed capacitive sensor proposes individual capacitance measurements to find more potential problems. Advantageously, the proposed capacitive sensor does not require dedicated elements or signal processing chains; a proposed test process can be implemented, for example, with a specific configuration of the ASIC element.
[0039] In the embodiment of the proposed capacitive sensor 100 shown in Fig. 1, it can be seen that an MEMS element 10 is arranged on an ASIC element 20. A damping element 15, e.g., in the form of Semicosil, is preferably arranged between the MEMS element 10 and the ASIC element 20. The capacitive sensor 100 and the ASIC element 20 are covered and thus protected by a cap element 30 (e.g., a metal cover). Other embodiments of the capacitive sensor 100 with different layers of the MEMS element 10 and ASIC element 20 are also conceivable and not shown in the figures.
[0040] Fig. 2 shows a cross-sectional view of the capacitive sensor 100 of Fig. 1.
[0041] Barometric pressure is measured using the MEMS element 10, which implements a full capacitive Wheatstone bridge circuit 21, as shown in Fig. 3. Two measuring elements of the capacitive bridge circuit 21 are variable measuring capacitances CMI, CM2 and are intended for pressure measurement. The other two elements represent capacitances with a fixed capacitance value, which are used as reference capacitances CRefi, CRef2 in the proposed capacitive sensor 100.
[0042] Bonding wires (not shown) are used for the purpose of electrical contacting, in particular for electrical contacting between the ASIC element 20 and the substrate 1 (circuit board) arranged underneath, as well as between the ASIC element 20 and the MEMS element 10. Furthermore, the bonding wires can also be used to electrically connect the ASIC element 20 to the capacitive bridge circuit 21.
[0043] The capacitive bridge circuit 21 is controlled by electrical control signals A1, A2, which can, for example, be square-wave signals with frequencies from approximately 30 kHz to approximately 70 kHz. A first electrical control signal A1 is applied to a first supply line D1, and a second electrical measurement signal M2 is applied to a second supply line D2 of the capacitive bridge circuit 21.
[0044] Fig. 3 shows a conventional capacitive sensor 100 with the capacitive bridge circuit 21 in "normal operation," in which all bridge capacitances are determined for the purpose of a barometric pressure measurement. A first measuring capacitance CMI can be seen, which is controlled by a control element (e.g., ASIC element 20) with the first control signal A1. A capacitance value of the first measuring element CMI is determined, e.g., by a calculation using a downstream evaluation element 40 (e.g., a microcomputer) of the ASIC element 20.
[0045] Fig. 4 shows an embodiment of the proposed capacitive sensor 100 in a test mode in which only a single reference capacitance ÜRefi, CRef2 is determined successively, whereby in this case only the second reference capacitance CRef2 is determined. For this purpose, a second control signal A2 is applied to the second supply line D2, and a measurement signal of the second reference capacitance CRef2 is read out at a terminal S1, whereby a terminal S2 is separated from the evaluation element 40 by an open switch element 11. The first supply line D1 is connected to ground potential GND during the measurement process.
[0046] The evaluation element 40 comprises an amplification element 41 (e.g. low noise amplifier), a downstream A / D converter 42 and a downstream digital signal processor 43. The functioning of such an evaluation element 40 is known per se and is therefore not explained in more detail here.
[0047] Fig. 5 shows the proposed capacitive sensor 100, also in test mode, although in this case only the first reference capacitance ÜRefi is determined. For this purpose, a first control signal A1 is applied to the first supply line D1, and a measurement signal of the first reference capacitance ÜRefi is read out at a terminal S2, wherein a terminal S1 is separated from the evaluation element 40 by an open switch element 12. The second supply line D2 is connected to ground potential GND. The measurement signal is fed to the evaluation element 40 for evaluation. The second supply line D2 is connected to ground potential GND during the measurement process. Fig. 6 shows the capacitive Wheatstone bridge circuit 21 with potential low-ohm short-circuit resistors RKSI ... RKSII, which can be determined using the proposed method.These can be located either between the first supply line D1 and ground potential GND, and / or between the second supply line D2 and ground potential GND, and / or between the first supply line D1 and the second supply line D2. Resistance values of the short-circuit resistors can, for example, be between approximately 100 kΩ and approximately 1 MΩ, but other ohmic resistance values for the low-ohm short-circuit resistors RKS are also conceivable.
[0048] Some system failures can be detected by direct measurement of the capacitive Wheatstone bridge circuit 21. Others cannot, such as short circuits in the drive section of the capacitive Wheatstone bridge circuit 21. These short circuits can significantly reduce the bandwidth of the drive signal, which can lead to excitation of the resonant modes of the capacitive sensor 100 and prolong the system's settling time. This can adversely lead to, for example, offset errors and / or an increase in noise. Since the occurrence of such effects can also depend on temperature and pressure, they may be difficult to detect.
[0049] With the proposed method, such short circuits can be easily detected by measuring the reference capacitances CRen, CRef2 individually.
[0050] The short circuits according to Figure 4 provide different measured values for the two reference capacitances Cpen , CRef2.
[0051] For a short circuit between the first supply line D1 (or the second supply line D2) to ground potential GND, the measured value of the corresponding reference capacitance Cpen (or CRef2) is reduced by a reduction factor R according to the following formula:
[0052] R = 1 / 2 * Rs / (Rs + RKS) (1) with:
[0053] RKS Ohmic short-circuit resistance
[0054] Rs ... . Ohmic series resistance between the drive circuit and the first or second supply line D1 , D2
[0055] The capacitances and series resistances Rs are typically matched during manufacturing. Therefore, if a difference between the measured values exceeds a certain reference value, it can be assumed that one or more electrical short circuits are present.
[0056] In a typical application of the proposed capacitive sensor 100, it is therefore sufficient to determine and verify a difference between the reference capacitances ÜRefi, CRef2. This can be done, for example, by reading the two reference capacitances ÜRefi, ÜRef2, calculating the absolute or relative difference between the two reference capacitances ÜRefi, ÜRef2 and comparing it with a predefined reference value Rw.
[0057] If the predefined reference value Rw is exceeded, a fault is detected (at least one short circuit is present). If the reference value Rw is exceeded, the capacitive sensor 100 is identified as a good part (no short circuit is present).
[0058] Fig. 7 shows a flowchart of the fault test for short circuits between the first supply line D1 or the second supply line D2 and ground potential GND:
[0059] In a step 210, a measurement of the first reference capacitance ÜRefi is carried out.
[0060] In a step 220, a measurement of the second reference capacitance ÜRef2 is carried out.
[0061] In a step 230, the reference capacities ÜREfi and ÜRef2 are compared. In a step 240, a check is performed to determine whether a result of the following formula:
[0062] CRefl — CRef2 / CRefl (1) is greater than a predefined reference value R w .
[0063] If this is not the case, it is detected in a step 250 that the capacitive sensor 100 is error-free.
[0064] If this is the case, it is recognized in a step 260 that the capacitive sensor 100 has at least one short-circuit resistor RKS.
[0065] Fig. 8 shows a sequence of a method for detecting a short circuit between the first and second supply lines D1, D2.
[0066] To detect a short circuit between the first supply line D1 and the second supply line D2, the measured value of both measuring capacitances CMI, CM2 is reduced by a reduction factor R as follows:
[0067] R = Rs / (Rs + RKS / 2) (2)
[0068] In this case, the short-circuit resistance can be determined by directly comparing one and / or both values with a reference value Rw. This reference value Rw can be a fixed value for the capacitive sensor 100, with the reference value Rw preferably being determined for individual production batches of the capacitive sensor 100.
[0069] The given formulas (1), (2) are only examples and can be replaced and / or supplemented by other formulas.
[0070] In a step 310, a measurement of the first reference capacitance CRefi is performed. In a step 320, a measurement of the second reference capacitance CRef2 is performed.
[0071] In a step 330, the reference capacities ÜRefi, ÜRef2 are compared with each other.
[0072] In a step 240, a check is carried out to determine whether all conditions of the following formula are met:
[0073] ÜRefi < upper threshold AND ÜRefi > lower threshold AND CRef2 < upper threshold AND CRef2 > upper threshold
[0074] If all four conditions are met, it is determined in a step 360 that the capacitive sensor 100 is error-free.
[0075] If the four conditions are not met, it is detected in a step 350 that the capacitive sensor 100 has at least one short-circuit resistor RKS.
[0076] Advantageously, the proposed method can be used in a final test of a manufacturing process of capacitive pressure sensors as well as in the normal operation of such sensors, but can also be used for other capacitive sensors.
[0077] An example of generating the fixed value for a production batch would be to determine a distribution of the reference capacities ÜRefi, ÜRef2 and set a threshold value such that a specific distribution of the reference capacities is identified as a good part. For example, this could be a 6-sigma normal distribution.
[0078] The proposed method can be implemented, for example, as a self-test during production testing. It could also be executed later in the field. In this case, the comparison value / threshold can be stored in non-volatile memory in the production unit.
[0079] The proposed method can be used, for example, as a test procedure during production testing to eliminate faulty sensors. Additionally, it can optionally also be used later in the product as a test to detect faults during the lifetime of the capacitive sensor 100.
[0080] The explained detection principle can also be applied to embodiments of the capacitive sensor 100 that differ from the one explained above, e.g., for capacitive sensors 100 characterized by a different number of pads and with a different position of the wire bonds between the bridge circuit 21 of the MEMS element 10 and the ASIC element 20.
[0081] In all of the variants described, the electrical control signal A1, A1 can be embodied, for example, as an electrical current, voltage, or charge signal, whereby a capacitance value of the measuring capacitance CM1, CM2 can be determined. The signal shape of the control signal A can be, for example, rectangular, sinusoidal, or otherwise.
[0082] The proposed method can preferably be implemented as software executed at least partially on the ASIC element 20 or at least partially externally thereto, thereby supporting easy adaptability of the method. Alternatively, the proposed method can be implemented at least partially or entirely in hardware.
[0083] Advantageously, the proposed method can be implemented as a computer program that runs on the ASIC element 20 of the capacitive sensor 100 or is stored on a computer-readable data carrier.
[0084] In summary, the present invention proposes a capacitive sensor and a method for operating a capacitive pressure sensor, with which a self-test is possible in a simple manner, whereby a status of the capacitive sensor and the admissibility of measuring processes can be advantageously assessed.
[0085] The person skilled in the art will modify and / or combine the features of the invention in a suitable manner without deviating from the essence of the invention.
Claims
Claims 1. Capacitive sensor (100) comprising: a MEMS element (10) with a capacitive Wheatstone 'seeing bridge circuit (21); and an ASIC element (20), wherein the ASIC element (20) is configured to determine a measured value of at least one reference capacitance (ÜRefi , ÜRef2) of the capacitive Wheatstone bridge circuit (21), wherein control signals (A1, A2) for determining the reference capacitance (ÜRefi , CRef2) can be applied to each of the supply lines (D1 , D2) of the capacitive Wheatstone bridge circuit (21), wherein the ASIC element (20) is configured to read and evaluate the measured values of the reference capacitances (ÜRefi , CRef2), wherein a resistance value of at least one short-circuit resistance (RKS) of the capacitive sensor (100) can be determined from the values of the reference capacitances (ÜRefi , ÜRef2).
2. Capacitive sensor (100) according to claim 1, wherein a first short-circuit resistance (RKSI) between a first supply line (D1) of the capacitive Wheatstone 'see bridge circuit and ground potential (GND), a second short-circuit resistance (RKS2) between a second supply line (D2) of the capacitive Wheatstone 'see bridge circuit and ground potential (GND) and a third short-circuit resistance (RKSS) between the first supply line (D1) and the second supply line (D2) of the capacitive Wheatstone 'see bridge circuit (21) can be determined.
3. Capacitive sensor (100) according to claim 1 or 2, characterized in that a series resistance (Rs) can be determined in the first supply line (D1) and / or the second supply line (D2). A capacitive sensor (100) according to any one of the preceding claims, characterized in that a control signal (A1, A2) for controlling measuring capacitances (CMI, CM2) can be generated by means of the ASIC element (20). A capacitive sensor (100) according to any one of the preceding claims, characterized in that a defined reference value (Rw) of a relation between the determined reference capacitances (ÜRefi, ÜReß) is used to determine the at least one short-circuit resistance (RKS). A capacitive sensor (100) wherein the at least one short-circuit resistance (RKS) can be determined in a range between approximately 100 kΩ and approximately 1 MΩ.A capacitive sensor (100) according to one of the preceding claims, wherein, for determining a second reference capacitance (ÜRef2), a second control signal (A2) can be applied to the second supply line (D2) and a measurement signal of the second reference capacitance (CRef2) can be read out at a first terminal (S1), wherein a second terminal (S2) is separated from an evaluation element (40) by an open switch element (11) and the first supply line (D1) can be connected to ground potential (GND) during the measurement process; and wherein, for determining the first reference capacitance (ÜRefi), the second control signal (A2) can be applied to the first supply line (D1) and a measurement signal of the first reference capacitance (ÜRefi) can be read out at the second terminal (S2), wherein a first terminal (S1) is separated from the evaluation element (40) by an open switch element (12) and the second supply line (D2) can be connected to ground potential (GND) during the measurement process.Method for operating a capacitive sensor (100), comprising the steps:. - Applying a control signal (A1 , A1 ) to a supply line of a capacitive Wheatstone bridge circuit (21); Determining capacity values of reference capacities (ÜRefi , ÜReß) capacitive Wheatstone bridge circuit (21); - Evaluating the determined capacity values of reference capacities (ÜRefi , ÜReß) capacitive Wheatstone bridge circuits (21); and Determining at least one short-circuit resistance (RCR) from the evaluated capacitance values of the reference capacitances by comparison with a defined reference value (Rw). A method according to claim 8, wherein a value of a first short-circuit resistance (RCR1) between a first supply line (D1) and ground potential (GND), and / or a value of a second short-circuit resistance (RCR2) between a second supply line (D2) and ground potential (GND), and / or a value of a third short-circuit resistance (RCRSS) between the first and the second supply lines (D1, D2) is determined. A method according to claim 9, wherein a value of a series resistance (Rs) in the first supply line (D1) and / or the second supply line is determined. A method according to any one of claims 8 to 10, wherein a 6-sigma distribution is used to determine whether the capacitive sensor (100) is good or bad.A method according to any one of claims 8 to 11, wherein the method is carried out at defined times or cyclically. A computer program configured with program code means for carrying out the method according to any one of claims 8 to 12 when it runs on an ASIC element (20) of a capacitive sensor (100) or is stored on a computer-readable data carrier.