WO2025154016A1 - Method and circuit for capacitive sensing - Google Patents

Abstract

A method and circuit for capacitive sensing in hands-off detection applications. The method comprises positioning a first switch and a second switch in a first configuration to cause a reference capacitor to charge to a reference voltage; positioning the first switch and the second switch in a second configuration to transfer the charge of the reference capacitor with a sensor electrode; measuring a voltage of the reference capacitor in the first configuration and a voltage of the reference capacitor in the second configuration; and comparing the voltage of the reference capacitor in the first configuration to the voltage of the reference capacitor in the second configuration.

Description

METHOD AND CIRCUIT FOR CAPACITIVE SENSING

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63/622,119, filed January 18, 2024, which is incorporated herein by reference in its entirety.

FIELD

[0002] The present teachings relate to a method and circuit for capacitive sensing. The capacitive sensing may be used for hands-off detection, such as in a steering wheel of a vehicle.

BACKGROUND

[0003] Some vehicles include systems for detecting occupant presence in a seat or behind the steering wheel. These features are becoming more ubiquitous with the development of autonomous driving features, typically requiring confirmation of occupant presence and alertness behind the steering wheel.

[0004] Some systems use two supply voltages in capacitive measurements and do not utilize shield signal to suppress physical and environmental influence on the same measurement, such as described in U.S. Patent No. 9,823,798 B2.

[0005] It would be desirable to provide a method and circuit for capacitive sensing that includes electrode shielding.

[0006] It would be desirable to provide a method and circuit for capacitive sensing that mitigates potential errors introduced by parasitic sensor capacitance and electromagnetic noise from environmental interference sources.

SUMMARY

[0007] The present teachings relate to a method which may address at least some of the needs identified above. The method may be used for hands-off detection via capacitive sensing.

[0008] The method may comprise positioning a first switch and a second switch in a first configuration to cause a reference capacitor to charge to a reference voltage. [0009] The method may comprise positioning the first switch and the second switch in a second configuration to transfer the charge of the reference capacitor with a sensor electrode.

[0010] The method may comprise measuring a voltage of the reference capacitor in the first configuration and a voltage of the reference capacitor in the second configuration.

[0011] The method may comprise comparing the voltage of the reference capacitor in the first configuration to the voltage of the reference capacitor in the second configuration. A voltage differential may indicate proximity of a human body.

[0012] Said measuring may be performed by an analog-to-digital converter.

[0013] The sensor electrode may be grounded in the first configuration.

[0014] Said comparing may be performed by a controller.

[0015] The charge of the reference capacitor may be transferred to the sensor electrode through a separation impedance and a charging impedance.

[0016] The separation impedance may be in a range of about 100 ohms to about 3 kilo-ohms. The charging impedance may be in a range of about 3 kilo-ohms to about 200 kilo-ohms.

[0017] The separation impedance may be a resistor or a combination of one or more resistors, capacitors, and inductors.

[0018] The charging impedance may be a resistor or a combination of one or more resistors, capacitors, and inductors.

[0019] The voltage may be measured a pre-determined period of time after the first and second switches are positioned to the second configuration.

[0020] The present teachings relate to a circuit which may address at least some of the needs identified above. The circuit may be used for hands-off detection via capacitive sensing.

[0021] The circuit may comprise: a reference capacitor chargeable to a reference voltage, a sensor electrode in selective communication with the reference capacitor, a first switch that selectively connects and dis-connects the reference capacitor relative to a source of the reference voltage, and a second switch that selectively connects and dis-connects the sensor electrode to ground.

[0022] In a first configuration of the first and second switches, the reference capacitor may be charged to the reference voltage. In a second configuration of the first and second switches, proximity of a human body to the sensor electrode may cause a change in capacitance of the reference capacitor.

[0023] The circuit may further comprise an analog-to-digital converter that measures the capacitance of the reference capacitor.

[0024] The circuit may further comprise a shield electrode for shielding the sensor electrode from environmental interference sources.

[0025] The circuit may further comprise a shield electrode buffer located between the shield electrode and the sensor electrode for modulating a signal magnitude.

[0026] The shield electrode buffer may be a unity amplifier.

[0027] The charge of the reference capacitor may be transferred to the sensor electrode through a separation impedance and a charging impedance.

[0028] The separation impedance may be in a range of about 100 ohms to about 3 kilo-ohms.

The charging impedance may be in a range of about 3 kilo-ohms to about 200 kilo-ohms.

[0029] The separation impedance may be a resistor or a combination of one or more resistors, capacitors, and inductors.

[0030] The charging impedance may be a resistor or a combination of one or more resistors, capacitors, and inductors.

[0031] The analog-to-digital converter may be connected to the source of the reference voltage.

[0032] The circuit may further comprise a controller.

[0033] The circuit described in one or any combination of the foregoing paragraphs may be used in the method described in one or any combination of the foregoing paragraphs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a schematic view of a capacitive detection circuit according to the present teachings.

DETAILED DESCRIPTION

[0035] The present teachings provide for an improved method and circuit for capacitive sensing. The capacitive sensing may be used for hands-off detection. The circuit may be included in a steering wheel, although the present teachings also contemplate location of the circuit, particularly at least the sensor electrode of the circuit in a seat, gear shifter, door panel, or other components within the cabin interior. The rest of the circuit may be disposed near the sensor electrode or at some distance therefrom. Preferably, the entire circuit is disposed within the steering wheel.

[0036] The circuit may rely upon a single reference voltage. In this regard, the reference voltage used to charge the reference capacitor and the reference voltage used for comparison purposes, as described herein, may be the same. Thus, the circuit of the present teachings may avoid inaccuracies that may arise between multiple different voltage sources. Even different voltage sources adapted and configured to provide the same voltage may deviate from the target voltage.

[0037] The circuit may utilize a shield electrode. In this regard, the effects of parasitic sensor capacitance and electromagnetic noise from environmental interference sources may be eliminated or at least substantially mitigated. Noise may originate from various electronic components within the vehicle, such as the instrument cluster.

[0038] The circuit may comprise one or more components, including: a sensor electrode, a shield electrode, a reference capacitor, a first switch, a second switch, an analog-to-digital convertor, a shield electrode buffer, a separation impedance, a charging impedance, or any combination thereof. One or any combination of the foregoing may be electrically connected, via a wired connection. Although, it is contemplated that in some aspects, such electrical connections may be selectively disconnected, via, e.g., one or more switches. The present teachings contemplate that the circuit may comprise additional elements not explicitly described herein. It is contemplated that some or all of the components are disposed on a circuit board or may be integrated into one or more material elements of the steering wheel or other vehicle component described herein.

[0039] The circuit may comprise a reference capacitor. The reference capacitor may be in the form of a fixed value capacitor. Exemplary fixed value capacitors are described in A low-cost, stable reference capacitor for capacitive sensor systems, IEEE Transactions on Instrumentation and Measurement (Volume: 45, Issue: 2, April 1996, Pages 526-530), incorporated herein by reference in its entirety. The reference capacitor may have a capacitance in the range of about 10 pico-farads to about 3 nano-farads.

[0040] The circuit may comprise a sensor electrode. The sensor electrode may function to detect electromagnetic energy from an occupant. The sensor electrode may be located, preferably, under a trim surface of the steering wheel (directly underneath the trim surface). The sensor electrode may have a capacitance of about 10 pico-farads to about 3 nano-farads.

[0041] The circuit may comprise a shield electrode. The shield electrode may function to limit the sensing area of the sensor electrode. That is, generally limiting the sensing area to where the occupant contacts the steering wheel, and exclude sensing in directions from which electromagnetic interference may originate from. The sensor electrode may be closer to steering wheel surface relative to the shield electrode or on the same plane as the shield electrode. The shield electrode may have a capacitance of about 10 pico-farads to about 3 nano-farads.

[0042] The circuit may comprise one or more switches. Preferably, the circuit may comprise two switches. The switches may be in the form of single pole / double throw switches. The switches may be mechanical switches or electronic switches. The switches may be momentary switches or latched switches. Preferably, the switches are electronic switches.

[0043] The circuit may comprise one or more impedance components. The impedance components may function to impose an impedance on the circuit. The impedance components may be in the form of resistors or any combination of one or more of resistors, capacitors, and inductors (e.g., in the form of a sub-circuit).

[0044] The impedance components may include a separation impedance device. The separation impedance device may be located between the electrodes and an analog-to-digital converter. The separation impedance device may be located between the electrodes and the reference capacitor. The separation impedance device may have an impedance of about 100 ohms to about 3 kilo-ohms.

[0045] The impedance components may include a charging impedance device. The charging impedance device may be located between the electrodes and the separation impedance device. The charging impedance device may be located between the electrodes and ground. The charging impedance device may have an impedance of about 3 kilo-ohms to about 200 kilo-ohms. [0046] The circuit may comprise an analog-to-digital converter. The analog-to-digital converter may function to convert an analog signal (e.g., voltage) to a digital signal. The digital signal may be read and processable by a controller, processor, or the like.

[0047] The circuit may comprise a shield electrode buffer. The shield electrode buffer may function to synchronize the electrical potential between the shield electrode and the sensor electrode. The shield electrode buffer may be located between the shield electrode and the sensor electrode. It is believed that this assures no charge transfer between these two electrodes and therefore sensor electrode is shielding the circuit from influence of material in between these two electrodes and behind shield electrode.

[0048] The present teachings contemplate a method of capacitive sensing with the circuit described herein.

[0049] The capacitance measurement may be performed via measurement of a voltage change (e.g., drop) due to charge distribution between the reference capacitor and sensor capacitance using one reference voltage.

[0050] Most preferably, the sensor signal may be buffered and supplied to shield electrode to eliminate harmful effects of parasitic sensor capacitance and electromagnetic noise.

[0051] In a first action, the electrodes may be disposed at or near the surface of a vehicle component (e.g., steering wheel) and the system may be ready, when energized by the vehicle, to detect an occupant touching the steering wheel. It is contemplated that when the vehicle electronics are active (e.g., vehicle motor is turned on or vehicle auxiliary power is on) and an occupant is present and touching the steering wheel, the circuit may detect such presence and the present method may be constantly cycling during operation of the vehicle.

[0052] In the first active step, the first switch and second switch may be in a first position and the reference capacitor may be charged to the reference voltage. It is contemplated that the same reference voltage may be used as reference voltage for the analog-to-digital converter. This can assure measurement accuracy is not sensitive to any significant reference voltage variations. Through a charging impedance device and a second switch, the electrodes are discharged (grounded).

[0053] In the second active step, the first switch and second switch are in a second position. The reference capacitor transfers charge via separation impedance and charging impedance to the sensor electrode. After a predefined time, typically about few microseconds up to few hundred milliseconds, the analog-to-digital converter measures voltage of the reference capacitor. It is contemplated that any change in the sensor electrode caused by human body proximity (e.g., touching the steering wheel) may cause change its capacitance change and may affect this measured voltage.

[0054] During this second active step, the first switch connects the reference capacitor to the analog-to-digital converter, and the second switch is in a high impedance position. After the second active step concluded, the whole cycle repeats and starts from the first active step. It is contemplated that the total time to complete on cycle can range from about 3 microseconds to about 300 milliseconds. Preferably, it may be about 5 milliseconds or less.

[0055] It is also contemplated that via a shield electrode buffer (also known as a unity amplifier), the shield electrode is brought synchronously to same potential as sensor electrode. This ensures that no charge transfers between these two electrodes and therefore sensor electrode shielding from influence of material in between these two electrodes and behind shield electrode. [0056] FIG. 1 is a schematic view of a capacitive detection circuit 10 according to the present teachings.

[0057] The capacitive detection circuit 10 includes electrodes 12 configured and adapted to sense proximity of a human body. In some examples described herein, the electrodes 12 may be located in a steering wheel for hands-off detection, but the present teachings are not intended to be limited to this example. Other locations of the electrodes 12 are envisioned, including in a seat, a gear shifter, or other component within the cabin. The electrodes 12 include a sensor electrode 14 and a shield electrode 16.

[0058] The capacitive detection circuit 10 includes a first switch 18 and a second switch 20, each having a first position 1 and a second position 2. In the first position 1, the electrodes 12 are connected to ground 22 and a reference capacitor 24 is connected to a reference voltage source 26. In this regard, the reference capacitor 24 becomes charged with a reference voltage. In the second position 2, the electrodes 12 and the reference capacitor 24 are connected, and the foregoing are connected to an analog-to-digital converter 28. In this regard, the electrodes 12 will cause a change in capacitance and thus, the measured voltage, when a human body comes into proximity of the electrodes 12. The reference voltage source 26 is also connected to the analog- to-digital converter 28. The reference voltage can be compared to a voltage measured by the analog-to-digital converter 28 while the first and second switches 18, 20 are in the second position 2. Where a human is in proximity to the electrodes 12, the voltage drops below the reference voltage and comparison with the reference voltage will indicate such proximity. The comparison may be performed by a controller (e.g., a microcontroller) (not shown).

[0059] Between the sensor electrode 14 and the shield electrode 16 is a shield electrode buffer 30 (e.g., a unity amplifier) to bring the shield electrode 16 to the same potential as the sensor electrode 14 and thus, assure no transfer of charge between the sensor and shield electrodes 14, 16 and assuring shielding of the sensor electrode 14 from material between the electrodes 12 and behind the shield electrode 16.

[0060] The capacitive detection circuit 10 includes a separation impedance device 32 between the first switch 18 and the electrodes 12, and a charging impedance device 34 between the second switch 20 and the electrodes 12. The separation and charging impedance devices 32, 34 may be resistors or circuits including any combination of resistors, capacitors, and inductors. The separation impedance device (“DC impedance” or “resistance”) 32 may be in the range of about 100 ohms to about 3 kilo-ohms. The charging impedance device 34 may be in the range of about 3 kilo-ohms to about 200 kilo-ohms.

[0061] The explanations and illustrations presented herein are intended to acquaint others skilled in the art with the invention, its principles, and its practical application. The above description is intended to be illustrative and not restrictive. Those skilled in the art may adapt and apply the invention in its numerous forms, as may be best suited to the requirements of a particular use.

[0062] Accordingly, the specific embodiments of the present invention as set forth are not intended as being exhaustive or limiting of the teachings. The scope of the teachings should, therefore, be determined not with reference to this description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The omission in the following claims of any aspect of subject matter that is disclosed herein is not a disclaimer of such subject matter, nor should it be regarded that the inventors did not consider such subject matter to be part of the disclosed inventive subject matter. [0063] Plural elements or steps can be provided by a single integrated element or step. Alternatively, a single element or step might be divided into separate plural elements or steps.

[0064] The disclosure of “a” or “one” to describe an element or step is not intended to foreclose additional elements or steps. For example, disclosure of “a switch” does not limit the teachings to a single motor. Instead, for example, disclosure of “a switch” may include “one or more switches.”

[0065] While the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be used to distinguish one element, component, region, layer or section from another region, layer, or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings.

[0066] Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0067] Any of the elements, components, regions, layers and/or sections disclosed herein are not necessarily limited to a single embodiment. Instead, any of the elements, components, regions, layers and/or sections disclosed herein may be substituted, combined, and/or modified with any of the elements, components, regions, layers and/or sections disclosed herein to form one or more embodiments that may be or may not be specifically illustrated or described herein. [0068] The disclosures of all articles and references, including patent applications and publications, testing specifications, are incorporated by reference for all purposes. Other combinations are also possible as will be gleaned from the following claims, which are also hereby incorporated by reference into this written description.

REFERENCE NUMERALS

[0069] 10 Capacitive detection circuit

[0070] 12 Electrodes

[0071] 14 Sensor electrode

[0072] 16 Shield electrode

[0073] 18 First switch

[0074] 20 Second switch

[0075] 22 Ground

[0076] 24 Reference capacitor

[0077] 26 Reference voltage source

[0078] 28 Analog-to-digital converter

[0079] 30 Shield electrode buffer

[0080] 32 Separation impedance device

[0081] 34 Charging impedance device

Claims

What is claimed is:

Claim 1 : A method for hands-off detection via capacitive sensing, the method comprising: positioning a first switch and a second switch in a first configuration to cause a reference capacitor to charge to a reference voltage; positioning the first switch and the second switch in a second configuration to transfer the charge of the reference capacitor with a sensor electrode; measuring a voltage of the reference capacitor in the first configuration and a voltage of the reference capacitor in the second configuration; and comparing the voltage of the reference capacitor in the first configuration to the voltage of the reference capacitor in the second configuration, whereby a voltage differential indicates proximity of a human body.

Claim 2: The method according to Claim 1, wherein said measuring is performed by an analog-to-digital converter.

Claim 3: The method according to Claim 1 or Claim 2, wherein the sensor electrode is grounded in the first configuration.

Claim 4: The method according to any one of the preceding claims, wherein said comparing is performed by a controller.

Claim 5: The method according to any one of the preceding claims, wherein the charge of the reference capacitor is transferred to the sensor electrode through a separation impedance and a charging impedance; wherein the separation impedance, in a range of about 100 ohms to about 3 kilo-ohms, is a resistor or a combination of one or more resistors, capacitors, and inductors; and wherein the charging impedance, in a range of about 3 kilo-ohms to about 200 kilo-ohms, is a resistor or a combination of one or more resistors, capacitors, and inductors. Claim 6: The method according to any one of the preceding claims, wherein the voltage is measured a pre- determined period of time after the first and second switches are positioned to the second configuration.

Claim 7: A capacitive detection circuit for hands-off detection, the capacitive detection circuit comprising: a reference capacitor chargeable to a reference voltage, a sensor electrode in selective communication with the reference capacitor, a first switch that selectively connects and dis-connects the reference capacitor relative to a source of the reference voltage, and a second switch that selectively connects and dis-connects the sensor electrode to ground; and wherein in a first configuration of the first and second switches, the reference capacitor is charged to the reference voltage and in a second configuration of the first and second switches proximity of a human body to the sensor electrode causes a change in capacitance of the reference capacitor.

Claim 8: The capacitive detection circuit according to Claim 7, further comprising an analog-to-digital converter that measures the capacitance of the reference capacitor.

Claim 9: The capacitive detection circuit according to Claim 7 or Claim 8, further comprising a shield electrode for shielding the sensor electrode from environmental interference sources.

Claim 10: The capacitive detection circuit according to Claim 9, further comprising a shield electrode buffer located between the shield electrode and the sensor electrode for modulating a signal magnitude. Claim 11: The capacitive detection circuit according to Claim 10, wherein the shield electrode buffer is a unity amplifier.

Claim 12: The capacitive detection circuit according to any one of Claims 7 through 11, wherein the charge of the reference capacitor is transferred to the sensor electrode through a separation impedance and a charging impedance; wherein the separation impedance is in a range of about 100 ohms to about 3 kilo-ohms; and wherein the charging impedance is in a range of about 3 kilo-ohms to about 200 kilo-ohms.

Claim 13: The capacitive detection circuit according to Claim 12, wherein the separation impedance is a resistor or a combination of one or more resistors, capacitors, and inductors; and wherein the charging impedance is a resistor or a combination of one or more resistors, capacitors, and inductors.

Claim 14: The capacitive detection circuit according to any one of Claims 7 through 13, wherein the analog-to-digital converter is connected to the source of the reference voltage.

Claim 15: The capacitive detection circuit according to any one of Claims 7 through 14, further comprising a controller.

Claim 16: Use of the capacitive detection circuit according to any one of Claims 7 through 15 in the method according to any one of Claims 1 through 6.