Capacitive sensing device for hand-off detection

Abstract

A sensing and heating device for capacitive detection of an object in a motor vehicle includes a heater element having a first terminal and a second terminal, a heater circuit including a heating power source and a heater electronic control unit (HCU), the heater circuit configured to supply a heating current to the first and second terminals of the heater element, an AC decoupling element connected between the first and second terminals of the heater element and the heater circuit; and a capacitive sensing unit configured to use the heater element as a first antenna electrode, the capacitive sensing unit comprising a sensing circuit and a signal sensing circuit, the sensing circuit comprising a signal generating unit configured to apply an AC measurement signal to one of the first and second terminals of the heater element, the signal sensing circuit includes a current measurement device configured to sense a current flowing into the heater element in response to an applied AC measurement signal. The AC decoupling element includes a vehicle clock spring mounted between a steering wheel and a steering column of the vehicle.

Description

Technical Field

[0001] The present invention generally relates to sensing and heating devices for capacitance detection of objects, and more specifically to hands-off detection sensors for advanced driver assistance systems.

[0002] More specifically, the present invention relates to a capacitive sensor device that uses a heating element as a functional electrode for detecting the presence or absence of a human hand on a car steering wheel (hand off or hand on detection), and a steering wheel including such a sensing and heating device. Background Technology

[0003] Hands-off detection (HOD) sensors are commonly used in motor vehicles to provide information to advanced driver assistance systems (ADAS) about the position of the driver's hands on the steering wheel. This HOD helps ensure that the driver always has control of the vehicle, a fundamental principle of the Vienna Convention on Road Traffic.

[0004] HOD sensors typically use capacitance measurement devices to detect the presence of the driver's hands on or near the steering wheel. As used herein, the term "capacitance measurement device" refers to a sensor that generates a signal in response to the effect of the sensed object (a person, a part of a person's body, etc.) on an electric field.

[0005] Capacitance measuring devices typically include at least one antenna electrode to which an oscillating voltage signal is applied, and which then emits an electric field into a spatial region adjacent to the antenna electrode. In the case of a HOD sensor, the antenna electrode is mounted on the steering wheel and stacked around the steering wheel rim or frame to cover the rim. The driver's hand approaching the antenna electrode modifies the capacitance between the antenna electrode and ground, and this effect on the electric field is detected by sensing circuitry. The capacitive coupling strength, depending on the proximity of the hand to the antenna electrode, can be determined, for example, by applying an alternating current voltage signal to the antenna electrode and measuring the current flowing from the antenna electrode to ground. This current can be measured by a transimpedance amplifier connected to the sensing electrode and converting the current flowing into the sensing electrode into a voltage proportional to the current.

[0006] Some capacitance measurement devices are designed as capacitance-only sensors with a single sensing electrode. Moreover, capacitance sensors typically employ a configuration comprising sensing electrodes arranged close together and electrically insulated from each other, along with a so-called "shield electrode." This "shielding" technique is well-known in the art and is frequently used to intentionally mask and thus shape the sensitivity region of the capacitance sensor. For this purpose, the shield electrode is kept at the same AC potential as the sensing electrode. Therefore, there is no electric field in the space between the sensing electrode and the shield electrode, and the shielded capacitance sensor is insensitive in the direction between the sensing electrode and the shield electrode.

[0007] Specifically, in a HOD (Hard Distress Device) system, protective electrodes are typically used to shield one or more sensing electrodes from internal components of the vehicle steering wheel, such as the steering wheel rim or frame. This sensing-protected HOD system includes two conductive fabric electrodes to be mounted on the grounded steering wheel rim. One of the conductive fabric electrodes is stacked around the steering wheel rim to cover it, forming a protective layer. The other conductive fabric electrode is then stacked around the protective layer without contacting it, forming a sensing layer. These sensing and protective layers are typically maintained at a predetermined distance from each other by spacers.

[0008] An AC signal is applied to the guard electrode and a transimpedance amplifier connected to the sensing electrode. When an object approaches the sensor, it becomes capacitively coupled to the sensing layer, causing a measurable shift in the output of the transimpedance amplifier. Conversely, because the guard electrode and the sensing electrode maintain similar potentials, parasitic capacitance between the sensing electrode and the ground rim is prevented. In other words, the guard electrode shields the sensing electrode from the ground rim capacitance, thereby improving the reliability of the capacitance measurement device.

[0009] In addition to HOD sensor electrodes, steering wheels are typically equipped with steering wheel heaters to improve driver comfort. These heaters include a heating element, usually a fabric material with conductive heater wires embedded within it, mounted on the steering wheel rim. The heater control unit is configured to apply an appropriate heater current to the heating element. Clearly, in this case, integrating the heater element and one or more capacitor electrodes into the steering wheel is complex and quite expensive.

[0010] Therefore, it has been proposed to reduce the number of different layers by using the heater component of the steering wheel heater as one of the electrodes of the capacitive sensor. In such a system, the wire heater, which is already installed in the steering wheel to improve driver comfort, can be used as a protective electrode or even as a sensing electrode.

[0011] During heater operation, the heater assembly is connected between the vehicle battery voltage Vbat and ground via the heater control unit to apply heating current to the heater assembly. On the other hand, capacitive sensing operation requires the HOD electronic control unit (HOD ECU) to apply an AC voltage signal to the heater assembly as either a shielding electrode or a transmitting electrode. Therefore, for proper operation, the heater assembly must be decoupled from vehicle ground, vehicle battery, and the heater electronic control unit (HCU) AC.

[0012] In some heater-as-Guard or heater-as-Sense systems, decoupling is achieved through switching, such as by switching a transistor connected between the vehicle battery, HCU, and ground (AC & DC decoupling). When the switch / transistor disconnects the circuit, the heater circuit components are decoupled from the heater element, and capacitance measurement can begin. The disadvantage of this decoupling is that heating and sensing operations must be performed sequentially, which reduces the availability of both heating and HOD functions, as only one can operate at any given time.

[0013] To mitigate this problem, a combined heater and capacitive sensing system has been proposed, in which the heater element is AC decoupled from the heater circuit, which includes the vehicle battery, HCU, and ground, via an inductive decoupling element, such as a common-mode choke. The common-mode choke comprises at least two wire windings, typically having the same number of turns, wound around a common ferrite core. The at least two wire windings act as simple conductors, resisting differential-mode currents flowing through the common-mode choke windings in opposite current directions. For common-mode currents flowing through the common-mode choke windings in the same current direction, the at least two wire windings act as inductors with high impedance. For this purpose, common-mode chokes (CMCs) are typically used to provide AC decoupling between the heating element and the heating current source.

[0014] This means that if the common-mode choke is connected between the terminals of the heater circuit and the terminals of the heater element, the AC signal applied to the heater element will be blocked by the common-mode choke and cannot flow to the vehicle battery or ground. On the other hand, the DC heating current from the heater circuit can freely pass through the common-mode choke, allowing the combined system to operate simultaneously in heating and HOD sensing modes. While this AC decoupling via the common-mode choke increases the availability of both heating and sensing functions in the combined heater and sensor elements, it should be noted that the common-mode choke is an expensive component, increasing the overall cost of the heating and sensing system. Furthermore, the size of the common-mode choke makes its integration within the limited space available in the steering wheel quite difficult. Summary of the Invention

[0015] Therefore, it is desirable to provide a combined heater and HOD sensing system that does not have the aforementioned drawbacks.

[0016] General description of the invention In one aspect of the invention, a sensing and heating device for capacitance detection of an object in a motor vehicle includes: - A heater element having a first terminal and a second terminal, - A heater circuit, comprising a heating power supply and a heater electronic control unit (HCU), the heater circuit being configured to supply heating current to the first and second terminals of the heater element. - An AC decoupling element, connected between the first and second terminals of the heater element and the heater circuit; and - A capacitance sensing unit configured to use the heater element as a first antenna electrode, the capacitance sensing unit including a sensing circuit and a signal sensing circuit, the sensing circuit including a signal generating unit configured to apply an AC measurement signal to one of the first terminal and the second terminal of the heater element, the signal sensing circuit including a current measuring device configured to sense the current flowing into the heater element in response to the applied AC measurement signal.

[0017] According to the present invention, the AC decoupling element includes a vehicle clock spring mounted between the steering wheel and the steering column of a vehicle. The clock spring is a component of the vehicle's steering system responsible for maintaining the electrical connection between the steering wheel and the airbag, horn, and other electrical components. It is a coiled wire wound and housed within a protective housing. As the steering wheel is turned, the clock spring unfolds and rewinds, allowing the wire to extend and retract without breaking.

[0018] Clock springs are typically composed of a flat, multi-core cable wound in a helical shape. Due to this helical shape, clock springs exhibit self-inductance, which can be advantageously used as an inductive decoupling element. Therefore, this invention proposes using a vehicle clock spring (i.e., a component already present in vehicles) to decouple heater elements from the heater circuit AC. The use of a clock spring, a standard component in every vehicle, as a decoupling element (due to its self-inductance) means that the decoupling element does not incur additional costs during use.

[0019] Of course, it should be noted that, because the clock spring is positioned between the steering wheel and the heater circuit is mounted away from the steering wheel in the vehicle, it is connected to the heater element mounted on the steering wheel via the clock spring. Therefore, the HCU is not mounted in the steering wheel and thus does not occupy additional space within the limited steering wheel mounting space.

[0020] Note that the capacitive sensing unit can be configured as a sensing-only sensor, where the heating element is used only for the active antenna electrode of the sensing system. In a preferred embodiment, the capacitive sensing unit is configured as a sensing protection system, where the protection electrode is used to shape the electric field generated around the antenna electrode and shield the sensing antenna electrode from the underlying ground structure (e.g., a steering wheel rim).

[0021] In this type of sensing protection embodiment, the capacitive sensing unit includes a second antenna electrode, which is disposed on the heater element at a predetermined distance from the heater element by means of a spacer. The signal generating unit is configured to apply the AC measurement signal to one of the first and second terminals of the heater element and both the second antenna electrode. Therefore, both the heater element and the second antenna electrode maintain the same AC potential, such that the heater element and the other antenna electrode together form a sensing protection detection unit.

[0022] In one embodiment where the heater element is used as a protective electrode, the antenna electrode is mounted on the steering wheel such that the second antenna electrode is guided toward the object to be sensed during operation. The sensing circuit is then configured to sense the current flowing into the second antenna electrode in response to an applied AC measurement signal.

[0023] In an alternative configuration where the heater element is used as a sensing electrode, the antenna electrode is mounted on the steering wheel, such that the second antenna electrode is guided away from the object to be sensed during operation. In this case, the heater element is guided toward the object to be sensed and operates as a sensing electrode.

[0024] In another aspect of the invention, a sensing and heating device for capacitance detection of an object in a motor vehicle includes: - A heater element having a first terminal and a second terminal, - A heater circuit, including a heating power supply and a heater electronic control unit (HCU), the heater circuit being configured to supply heating current to the first and second terminals of the heater element. - An AC decoupling element, connected between the first and second terminals of the heater element and the heater circuit; and - A capacitance sensing unit, comprising: a first antenna electrode and a second antenna electrode, the first antenna electrode and the second antenna electrode being arranged on each other at a predetermined distance by means of a spacer; a signal generation unit configured to apply an AC measurement signal to the first antenna electrode and / or the second antenna electrode; and a signal sensing circuit including a current measuring device configured to sense a current flowing into the second antenna electrode in response to the applied AC measurement signal.

[0025] In this arrangement, one of the first or second antenna electrodes is formed by the heater element, and the AC decoupling element includes a vehicle clock spring.

[0026] In another aspect, the present invention relates to a motor vehicle comprising a steering wheel having a steering wheel rim and the sensing and heating device described above, the steering wheel being mounted on a steering column. A heater element, a first antenna electrode, and a second antenna electrode are respectively mounted in the steering wheel, preferably stacked around the steering wheel rim, wherein a heater circuit is mounted in the vehicle away from the steering wheel. The heater circuit is connected to a first terminal and a second terminal of the heater element via a vehicle clock spring arranged between the steering wheel and the steering column. It should be understood that the sensing circuit of the capacitive sensing unit is preferably arranged in the hub of the steering wheel.

[0027] One of the first and second antenna electrodes is stacked around the steering wheel rim to cover it, thus forming a protective layer. Then, the other antenna electrode is stacked around the protective layer without contacting it, thus forming a sensing layer. These sensing and protective layers are typically maintained at a predetermined distance from each other by spacers.

[0028] Those skilled in the art will understand that the heater element may, for example, include a printed heater having conductive heating traces printed on a dielectric substrate, or a wire heater comprising a fabric material in which conductive heater wires are embedded. Of course, any heater configuration suitable for integration into the steering wheel is also possible. The first antenna electrode and / or the second antenna electrode are preferably conductive fabric electrodes, but printed electrodes may also be used.

[0029] It should also be noted that the current measuring device is preferably connected to the first antenna electrode and the second antenna electrode, and preferably includes a transimpedance amplifier.

[0030] Those skilled in the art will eventually understand that the heater element and / or the first or second antenna electrode can be divided into multiple regions, allowing capacitance detection to be performed individually for each region. Such multi-region sensors are well known in the art and allow, for example, the detection of the position of an object (e.g., a hand) on the rim of a steering wheel, or the differentiation between a simple single touch in one location and multiple touches corresponding to a grip on the steering wheel, depending on the arrangement of the different sensing regions. Attached Figure Description

[0031] Further details and advantages of the invention will become apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings, in which: Figure 1 A schematic circuit of a sensing and heating device, shown in a sensing protection configuration, is illustrated. Figure 2A schematic circuit of a sensing and heating device, shown as a sensing-only configuration, is illustrated. Detailed Implementation

[0032] Figure 1 This is a schematic circuit representing a sensing and heating device with only sensing configuration, wherein the heater element serves as a protective electrode.

[0033] Figure 1 The circuit shown is divided into three different parts: one part is integrated into the vehicle's steering wheel, another part is integrated into the exterior of the vehicle's steering wheel, and the clock spring device is arranged between the steering wheel and the steering column.

[0034] Inside the steering wheel, the system includes a heater element 3 disposed on the steering wheel rim or steering wheel frame 1. The heater element 3 serves as a protective electrode and provides shielding for the sensing electrode 2 disposed above the heater element against the grounded frame 1 below. The heater element 3 is connected to the HOD ECU for applying an AC sensing signal 4. This AC sensing signal is also applied to the sensing electrode 2 via, for example, a transimpedance amplifier 4, which functions as a current measuring device for detecting the current flowing into the sensing electrode 2 or the heater element 3 in response to the applied AC measurement signal.

[0035] The heater element 3 is also connected via its terminals to the HCU (Heating Electronics Unit), which is located away from the steering wheel inside the vehicle. The HCU is equipped with at least one switch 6, typically implemented by a power transistor. During the heating operation of the heater element, the HCU controls the temperature by controlling the power (PWM duty cycle, voltage, or current) on the heater element 3.

[0036] The heater element 3 is also connected to the HOD ECU, which is configured to apply the AC protection signal 5 to the heater element 3. For reasons of sensing accuracy and robustness, the HOD ECU and the steering wheel frame 1 are typically connected to vehicle ground or to the same potential.

[0037] Typically, each steering wheel is electrically connected to the vehicle via a clock spring. The heating current flowing from the HCU through heater 3 is also guided through the clock spring. The DC current passes through these traces without substantial loss.

[0038] Due to the spiral configuration of the electrical traces, the electrical traces on the clock spring have self-inductance. The protection signal 5 generates a current that flows to the environment through the heater, primarily via a parasitic capacitance network to the frame 1. The protection signal current also flows to ground and VBat through the clock spring 7 and the wiring harness 8.

[0039] With switch 6 open, if the impedance of the HCU plus the peripheral EMC components in the described path is high enough, then the protection signal can only have the functional voltage and phase. With switch 6 closed, the impedance is too low (short-circuited to VBat or GND). However, the self-inductance 7 of the clock spring generates sufficient impedance to reduce current flow in the direction of the HCU, regardless of switch state 6. Therefore, the clock spring sufficiently AC decouples the heater element, allowing simultaneous sensing and heating operations.

[0040] Figure 2 A schematic circuit of a sensing and heating device in a sensing-only configuration is shown, wherein heater element 3 is used as a sensing electrode.

[0041] As in the previous embodiments, Figure 2 The circuit shown is divided into three different parts: one part is integrated into the vehicle's steering wheel, another part is integrated into the exterior of the vehicle's steering wheel, and the clock spring device is arranged between the steering wheel and the steering column.

[0042] In this embodiment, the device does not include dedicated sensing electrodes, but a heater serves as the sensing heater 2. The sensing heater 2 is integrated inside the steering wheel and connected to the HOD ECU, which is configured to apply an AC sensing signal 4 to the sensing heater 2. The sensing heater 2 is also connected to a current measuring device 4, such as a transimpedance amplifier 4, for detecting the current flowing into the sensing heater 2 in response to the applied AC measurement signal.

Claims

1. A sensing and heating device for capacitance detection of an object in a motor vehicle, comprising: - A heater element having a first terminal and a second terminal, - A heater circuit, including a heating power supply and a heater electronic control unit (HCU), the heater circuit being configured to supply heating current to the first and second terminals of the heater element. - An AC decoupling element, which is connected between the first and second terminals of the heater element and the heater circuit; as well as - A capacitance sensing unit configured to use the heater element as a first antenna electrode, the capacitance sensing unit including a sensing circuit and a signal sensing circuit, the sensing circuit including a signal generating unit configured to apply an AC measurement signal to one of the first terminal and the second terminal of the heater element, the signal sensing circuit including a current measuring device configured to sense the current flowing into the heater element in response to the applied AC measurement signal; The AC decoupling element is characterized in that it includes a vehicle clock spring.

2. The sensing and heating apparatus of claim 1, wherein the capacitance sensing unit includes a second antenna electrode disposed on the heater element and spaced a predetermined distance from the heater element by means of a spacer, and wherein the signal generation unit is configured to apply the AC measurement signal to one of the first terminal and the second terminal of the heater element and both the second antenna electrode.

3. The sensing and heating device according to claim 2, wherein, In operation, the second antenna electrode is guided toward the object to be sensed, and the sensing circuit is configured to sense the current flowing into the second antenna electrode in response to an applied AC measurement signal.

4. The sensing and heating device according to claim 2, wherein, During operation, the second antenna electrode is guided away from the object to be sensed.

5. A sensing and heating device for capacitance detection of an object in a motor vehicle, comprising: - A heater element having a first terminal and a second terminal, - A heater circuit, including a heating power supply and a heater electronic control unit (HCU), the heater circuit being configured to supply heating current to the first and second terminals of the heater element. - AC decoupling element, which is connected between the first and second terminals of the heater element and the heater circuit; as well as - A capacitive sensing unit, comprising a first antenna electrode and a second antenna electrode, a signal generation unit and a signal sensing circuit, wherein the first antenna electrode and the second antenna electrode are arranged on each other at a predetermined distance by means of a spacer; The signal generation unit is configured to apply an AC measurement signal to the first antenna electrode and / or the second antenna electrode; The signal sensing circuit includes a current measuring device configured to sense the current flowing into the second antenna electrode in response to an applied AC measurement signal. - wherein one of the first antenna electrode or the second antenna electrode is formed by the heater element. The AC decoupling element is characterized in that it includes a vehicle clock spring.

6. The sensing and heating device according to any one of the preceding claims, wherein, The heater element includes a printed heater or a wire heater, wherein the printed heater has conductive heating traces printed on a dielectric substrate, the wire heater includes a fabric material with conductive heater wires embedded therein, and / or wherein the first antenna electrode and / or the second antenna electrode are conductive fabric electrodes.

7. The sensing and heating device according to any one of claims 2-6, wherein, The current measuring device is connected to the first antenna electrode and the second antenna electrode, and wherein the current measuring device preferably includes a transimpedance amplifier.

8. A motor vehicle comprising a steering wheel having a steering wheel rim and a sensing and heating device according to any one of the preceding claims, the steering wheel being mounted on a steering column, wherein a heater element, a first antenna electrode, and a second antenna electrode are respectively mounted in the steering wheel, preferably stacked around the steering wheel rim, wherein a heater circuit is mounted in the vehicle away from the steering wheel, and wherein the heater circuit is connected to the first and second terminals of the heater element via a vehicle clock spring disposed between the steering wheel and the steering column.

9. The motor vehicle according to claim 8, wherein, The sensing circuit of the capacitance sensing unit is arranged in the hub of the steering wheel.

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