Capacitive sensing device for hands-off detection

Abstract

A sensing and heating arrangement for capacitive detection of an object in an automotive vehicle, comprises 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, said heater circuit configured to supply a heating current to the first and second terminals of said heater element, an AC decoupling element connected between the first and second terminals of said heater element and said heater circuit; and a capacitive sensing unit configured for using said heater element as first antenna electrode, said capacitive sensing unit comprising a sensing circuit including a signal generating unit configured for applying an AC measurement signal to one of said first and second terminals of said heater element and a signal sensing circuit including a current measuring means configured to sense a current flowing into the heater element in response to the applied AC measurement signal. The AC decoupling element comprises a vehicle clock spring which is mounted between the steering wheel and the steering column of the vehicle.

Description

Capacitive Sensing Device for Hands-Off DetectionTechnical field

[0001] The present invention generally relates to a sensing and heating arrangement for capacitive detection of an object, more particularly to a Hands-Off detection sensor for an Advanced Driver Assistance System.

[0002] More specifically, the present invention relates to a capacitive sensor device using a heating member as functional electrode for detecting the presence or absence of a person’s hand on the steering wheel of a car (hands-off or hands-on detection) and a steering wheel comprising such a sensing and heating arrangement.Background of the Invention

[0003] Hands-Off Detection (HOD) sensors are generally used in automotive vehicles to provide information to an Advanced Driver Assistance System (ADAS) regarding the position of the driver hands on the steering wheel. Such HOD can be used to ensure that the driver is in control of the vehicle at all times, which is a fundamental principle of the Vienna Convention on Road Traffic.

[0004] HOD sensors typically use a capacitance measurement device for detecting detect the presence of the driver’s hand on or in proximity of the steering wheel. As used herein, the term “capacitance measurement device” designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person’s body, etc.) upon an electric field.

[0005] A capacitance measurement device generally comprises at least one antenna electrode, to which an oscillating electric voltage signal is applied and which thereupon emits an electric field into a region of space proximate to the antenna electrode. In the case of a HOD sensor, the antenna electrode is mounted on the steering wheel and layered around the steering wheel rim or skeleton so as to cover the steering wheel rim. A hand of the vehicle driver in the proximity of the antenna electrode modifies the capacitance between the antenna electrode and ground and this the influence on the electric field is detected by the sensing circuit. The capacitive coupling strength which is dependent on the proximity of the hand to the antenna electrode may e.g. be determined by applying an alternating voltage signalto an antenna electrode and by measuring the current flowing from that antenna electrode either towards ground. This current may be measured by a transimpedance amplifier, which is connected to the sensing electrode and which converts the current flowing into the sensing electrode into a voltage proportional to the current.

[0006] Some capacitance measurement devices are designed as sense-only capacitive sensors having a single sense electrode. Also, quite often capacitive sensors are used that comprise a sense electrode and a so-called “guard electrode” that are proximally arranged and mutually galvanically insulated from each other. This technique of “guarding” is well known in the art and is frequently used for intentionally masking, and thus shaping, a sensitivity regime of a capacitive sensor. To this end, the guard electrode is kept at the same electric AC potential as the sense electrode. As a result, a space between the sense electrode and the guard electrode is free of an electric field, and the guard-sense capacitive sensor is insensitive in a direction between the sense electrode and the guard electrode.

[0007] Specifically in HOD systems, a guard electrode is often employed for shielding one or more sense electrodes from inner parts of a vehicle steering wheel such as the steering wheel rim or skeleton. Such a sense-guard HOD system comprises two conductive textile electrodes to be mounted on the grounded steering wheel rim. One of the conductive textile electrodes is layered around the steering wheel rim so as to cover it, thereby forming a guard layer. The other conductive textile electrode is then layered around the guard layer without entering in contact with the latter, thereby forming a sense layer. These sense layer and guard layer are typically kept at a predetermined distance from each other by means of a spacer.

[0008] An AC signal is applied to the guard electrode and to a transimpedance amplifier connected to the sense electrode. As an object approaches the sensor, it becomes capacitively coupled with the sense layer, resulting in a measurable shift in the output of the transimpedance amplifier. Conversely, as the guard and sense electrodes are kept at similar electric potentials, parasitic capacitance between the sense electrode and the grounded rim is prevented. In other words, the guard electrode capacitively shields the sense electrode from the grounded rim, thereby improving the reliability of the capacitance measurement device.

[0009] Further to the HOD sensor electrodes, a steering wheel is often equipped with a steering wheel heater to improve the comfort of the driver. Such a steering wheel heater comprises a heating member, usually a textile material with conductive heater wires embedded therein, which is to be mounted on the steering wheel rim and a heater control unit configured to apply an appropriate heater current to the heating member. It is clear that in such a case, the integration of the heater member and one or more capacitive electrodes in the steering wheel is complicated and quite costly.

[0010] It has therefore been proposed to reduce the number of different layers by using the heater member of the steering wheel heater as one of the electrodes of the capacitive sensor. In such a system, the wire-based heater, which is already fitted in the steering wheel to improve the comfort of the driver, may be used as the guard electrode or even as the sense electrode.

[0011] During heater operation, the heater member is coupled by the heater control unit between vehicle battery voltage Vbat and ground in order to apply a heating current to the heater member. On the other hand, the capacitive sensing operation requires an alternating voltage signal to be applied by the HOD electronic control unit (HOD ECU) to the heater member to be used as guard or sending electrode. In order to properly operate, the heater member therefore needs to be AC decoupled from the vehicle ground, the vehicle battery and the heater electronic control unit (HCU).

[0012] In some Heater-as-Guard or Heater-as-Sense systems, the decoupling is done by switches, e.g. by switching of transistors (AC&DC-decoupling), which are connected between the vehicle battery, the HCU and ground. When the switches / transistors open the circuit, the heater circuit components are decoupled from the heater element and the capacitive measurement may be started. The disadvantage of this kind of decoupling is of course that the heating and detecting operations have to be done in sequence, which reduces the availability of the heating and HOD function since only one of the two may be operated at any time.

[0013] In order to alleviate this problem, combined heater and capacitive sensing systems have been proposed, in which the heater element is AC decoupled from the heater circuit comprising the vehicle battery, the HCU and ground by means of an inductive decoupling element such as a common mode choke. Common modechokes include at least two wire windings, usually having the same number of turns, wound on a common ferrite core. The at least two wire windings work as simple wires against differential mode current flowing through the common mode choke windings in opposite current directions. Against common mode current flowing through the common mode choke windings in the same current direction, the at least two wire windings work as an inductor having a large impedance. For this reason, the common mode choke (CMC) is often employed to provide for AC-decoupling of the heating member from a heating current supply.

[0014] This means that if a common mode choke is connected between the terminals of the heater circuit and the terminals of the heater element, an AC signal applied to the heater element will be blocked by the common mode choke and can not flow to the vehicle battery or to ground. On the other hand, a DC heating current from the heater circuit can pass the common mode choke freely, so that the combined system may simultaneously be operated in heating and HOD sensing mode. While this AC decoupling by means of a common mode choke increases the availability of both the heating and sensing function of the combined heater and sensor element, it is noted that the common mode choke is a costly component, which increases the overall cost of the heating ad sensing system. Furthermore, the size of a common mode choke renders its integration quite difficult in the limited space available in the steering wheel.Object of the invention

[0015] It is therefore desirable to provide a combined heater and HOD sensing system without the above described shortcomings.General Description of the Invention

[0016] In one aspect of the invention, a sensing and heating arrangement for capacitive detection of an object in an automotive vehicle, comprises 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, said heater circuit configured to supply a heating current to the first and second terminals of said heater element, an AC decoupling element connected between the first and second terminalsof said heater element and said heater circuit; and a capacitive sensing unit configured for using said heater element as first antenna electrode, said capacitive sensing unit comprising a sensing circuit including a signal generating unit configured for applying an AC measurement signal to one of said first and second terminals of said heater element and a signal sensing circuit including a current measuring means configured to sense a current flowing into the heater element in response to the applied AC measurement signal.

[0017] According to the invention, the AC decoupling element comprises a vehicle clock spring which is mounted between the steering wheel and the steering column of the vehicle. A clock spring is a vehicle’s steering system component responsible for maintaining an electrical connection between the steering wheel and the airbag, horn, and other electrical components. It is a coiled wire wound around and housed in a protective casing. As the steering wheel is turned, the clock spring uncoils and recoils, allowing the wires to stretch and contract without breaking.

[0018] Clock springs generally consist of a flat multicore cable wound in a spiral shape. Due to this spiral shape, the clock spring exhibits a self-inductance which can be advantageously used as inductive decoupling element. The present invention therefore proposes to use this vehicle spring clock, i.e. a component which is already present in the vehicle, to AC decouple the heater element from the heater circuit. The clock spring, which is a standard component in every vehicle, is used as decoupling element (due to its self-inductance) which means that the decoupling element generates no additional costs in usage.

[0019] It will of course be noted that due to the clock spring being arranged between the heater circuit is mounted in the vehicle remote from the steering wheel and connected via the clock spring to the steering wheel mounted heater element. The HCU is thus not mounted in the steering wheel and accordingly the HCU does not take additional space in the limited steering wheel mounting space.

[0020] It is noted that the capacitive sensing unit may be configured as sense only sensor, in which the heater element is to only active antenna electrode of the sensing system. In preferred embodiments, the capacitive sensing unit is configured as a sense-guard system, in which a guard electrode is used to shape the electricalfield generated around the antenna electrodes and to shield the sensing antenna electrode from underlying grounded structures such as the steering wheel rim.

[0021] In such sense-guard embodiments, said capacitive sensing unit comprises a second antenna electrode, said second antenna electrode arranged on said heater element at a predetermined distance from said heater element by means of a spacer, and wherein the signal generating unit is configured for applying the AC measurement signal to both one of said first and second terminals of said heater element and the second antenna electrode. Both the said heater element and the second antenna electrode are thus kept at a same AC potential so that the heater element and the further antenna electrode form together a sense-guard detection unit.

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

[0023] In an alternative configuration, in which the heater element is used as sensing electrode, the antenna electrodes are mounted on the steering wheel, such that the second antenna electrode is, in operation, directed away from an object to be sensed. In this case the heater element is directed towards the object to be sensed and operated as sense electrode.

[0024] In another aspect of the invention, a sensing and heating arrangement for capacitive detection of an object in an automotive vehicle, comprises 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, said heater circuit configured to supply a heating current to the first and second terminals of said heater element, an AC decoupling element connected between the first and second terminals of said heater element and said heater circuit; and a capacitive sensing unit comprising a first antenna electrode and second antenna electrode, said first and second antenna electrodes being arranged at a predetermined distance one above the other by means of a spacer, a signalgenerating unit configured for applying an AC measurement signal to said first antenna electrode and / or said second antenna electrode and a signal sensing circuit including a current measuring means 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 said first or second antenna electrodes is formed by said heater element and the AC decoupling element comprises a vehicle clock spring.

[0026] In yet a further aspect, the invention relates to an automotive vehicle comprising a steering wheel with a steering wheel rim, said steering wheel being mounted on a steering wheel column, and a sensing and heating arrangement as described above. The heater element respectively the first and second antenna electrodes are mounted in the steering wheel, preferably layered around the steering wheel rim, wherein the heater circuit is mounted in the vehicle remote from the steering wheel. The heater circuit is connected to the first and second terminals of the heater element via the vehicle clock spring arranged between the steering wheel and the steering wheel column. It will be appreciated that the sensing circuit of the capacitive sensing unit is preferably arranged in a hub of the steering wheel.

[0027] One of the first and second antenna electrodes is layered around the steering wheel rim so as to cover it, thereby forming a guard layer. The other antenna electrode is then layered around the guard layer without entering in contact with the latter, thereby forming a sense layer. These sense layer and guard layer are typically kept at a predetermined distance from each other by means of a spacer.

[0028] The skilled person will appreciate, that the heater element may e.g. comprise a printed heater with a conductive heating trace printed on a dielectric substrate or a wire based heater comprising a textile material with conductive heater wires embedded therein. Of course, any heater construction that is suitable for integration into the steering wheel is also possible. The said first antenna electrode and / or said second antenna electrode are preferably conductive textile electrodes but printed electrodes may also be used as well.

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

[0030] The skilled person will finally appreciate, that the heater element and / or the first or second antenna electrode may be divided in a plurality of zones, such that a capacitive detection may be performed for each zone individually. Such multi-zone sensors are well known in the art and allow for instance to detect the location of the object, e.g. a hand, on the steering wheel rim or depending on the arrangement of the different sensing zones to distinguish between a simple one touch inn one location and a multi-location touch corresponding to a grasp of the steering wheel.Brief Description of the Drawings

[0031] Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawing, wherein:Fig.1 shows a schematic electrical circuit representing a sensing and heating arrangement in a sense-guard configuration;Fig.2 shows a schematic electrical circuit representing a sensing and heating arrangement in a sense-only configuration.Description of Preferred Embodiments

[0032] Fig.1 is a schematic electrical circuit representing a sensing and heating arrangement in a sense-only configuration with the heater element used as guard electrode.

[0033] The electrical circuit shown in Fig. 1 is divided in three different sections, one part being integrated in the steering wheel of a vehicle, one section being integrated in the vehicle outside of the steering wheel, and the clock spring arrangement arranged between the steering wheel and the steering wheel column.

[0034] Inside of the steering wheel, the system comprises a heater element 3 arranged on the steering wheel rim or steering wheel skeleton 1 . The heater element 3 is used as a guard electrode and shields a sense electrode 2 arranged above the heater element against the underlying grounded skeleton 1. The heater element 3is connected to the HOD ECU for applying the AC sensing signal 4. This AC sensing signal is also applied to the sense-electrode 2 by means of e.g. a transimpedance amplifier 4, which acts as current measurement device for detecting the current flowing into the sense 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 Circuit Unit) which is placed inside the vehicle remote from the steering wheel. The HCU is equipped with at least one switch 6, typically realized by power transistors. The HCU controls the temperature by controlling the power (PWM duty cycle or voltage or current) on the heater element 3 during the heating operation of the heater element.

[0036] The heater element 3 is also connected to the HOD ECU which is configured for applying an AC guard signal 5 to the heater element 3. The HOD ECU and the steering wheel skeleton 1 are typically connected to vehicle ground or to the same potential for sensing accuracy and robustness reasons.

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

[0038] Due to the spiral configuration of the electrical traces, the electrical traces on the clock-spring have self-inductances. The guard signal 5 generates a current flow through the heater to the environment, here mainly to the skeleton 1 via the parasitic capacitance network. The guard signal current will also flow through the clock-spring 7 and the harness 8 to Ground and VBat.

[0039] In case the switch 6 is open, the guard signal can only have a functional voltage and phase if the impedance of the HCU in the described path plus peripheric EMC components is high enough. In case the switch 6 is closed the impedance is too low (short to VBat or GND). The self-inductance 7 of the clock spring however generates a sufficient impedance to reduce the current flow in the direction of the HCU independent of the switch-states 6. The clock spring therefore sufficiently AC decouples the heater element so that a concurrent sensing and heating operastion is possible.

[0040] Fig.2 shows a schematic electrical circuit representing a sensing and heating arrangement in a sense-only configuration, in which the heater element 3 is used as sense electrode.

[0041] As with the previous embodiment, the electrical circuit shown in Fig. 2 is divided in three different sections, one part being integrated in the steering wheel of a vehicle, one section being integrated in the vehicle outside of the steering wheel, and the clock spring arrangement arranged between the steering wheel and the steering wheel column.

[0042] In this embodiment, the arrangement does not contain a dedicated sense electrode but the heater is used as sense-heater 2. The sense-heater 2 is integrated inside the steering wheel and connected to the HOD ECU which is configured for applying the AC sensing signal 4 to the sense-heater 2. The sense-heater 2 is also connected to a current measurement device 4, e.g. a transimpedance amplifier 4, for detecting the current flowing into the sense heater 2 in response to the applied AC measurement signal.

Claims

Claims1 . Sensing and heating arrangement for capacitive detection of an object in an automotive vehicle, comprising- 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, said heater circuit configured to supply a heating current to the first and second terminals of said heater element,- an AC decoupling element connected between the first and second terminals of said heater element and said heater circuit; and- a capacitive sensing unit configured for using said heater element as first antenna electrode, said capacitive sensing unit comprising a sensing circuit including a signal generating unit configured for applying an AC measurement signal to one of said first and second terminals of said heater element and a signal sensing circuit including a current measuring means configured to sense a current flowing into the heater element in response to the applied AC measurement signal; characterized in that said AC decoupling element comprises a vehicle clock spring.

2. Sensing and heating arrangement according to claim 1 , wherein said capacitive sensing unit comprises a second antenna electrode, said second antenna electrode arranged on said heater element at a predetermined distance from said heater element by means of a spacer, and wherein the signal generating unit is configured for applying the AC measurement signal to both one of said first and second terminals of said heater element and the second antenna electrode.

3. Sensing and heating arrangement according to claim 2, wherein in operation, said second antenna electrode is directed towards an object to be sensed, and wherein said sensing circuit is configured to sense a current flowing into the second antenna electrode in response to the applied AC measurement signal.

4. Sensing and heating arrangement according to claim 2, wherein in operation, said second antenna electrode is directed away from an object to be sensed.

5. Sensing and heating arrangement for capacitive detection of an object in an automotive vehicle, comprising- 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, said heater circuit configured to supply a heating current to the first and second terminals of said heater element,- an AC decoupling element connected between the first and second terminals of said heater element and said heater circuit; and- a capacitive sensing unit comprising a first antenna electrode and second antenna electrode, said first and second antenna electrodes being arranged at a predetermined distance one above the other by means of a spacer, a signal generating unit configured for applying an AC measurement signal to said first antenna electrode and / or said second antenna electrode and a signal sensing circuit including a current measuring means configured to sense a current flowing into the second antenna electrode in response to the applied AC measurement signal;- wherein one of said first or second antenna electrodes is formed by said heater element characterized in that said AC decoupling element comprises a vehicle clock spring.

6. Sensing and heating arrangement according to any one of the preceding claims, wherein said heater element comprises a printed heater with a conductive heating trace printed on a dielectric substrate or a wire based heater comprising a textile material with conductive heater wires embedded therein and / or wherein said first antenna electrode and / or said second antenna electrode is a conductive textile electrode.

7. Sensing and heating arrangement according to any one of the preceding claims 2-6, wherein said current measuring means is connected to the first antenna electrode and the second antenna electrode, and wherein the current measuring means preferably includes a transimpedance amplifier.

8. Automotive vehicle comprising a steering wheel with a steering wheel rim, said steering wheel being mounted on a steering wheel column, and a sensing and heating arrangement according to any one of the preceding claims, wherein theheater element respectively the first and second antenna electrodes are mounted in the steering wheel, preferably layered around the steering wheel rim, wherein the heater circuit is mounted in the vehicle remote from the steering wheel, and wherein the heater circuit is connected to the first and second terminals of the heater element via the vehicle clock spring arranged between the steering wheel and the steering wheel column.

9. Automotive vehicle according to claim 8, wherein the sensing circuit of the capacitive sensing unit is arranged in a hub of the steering wheel.

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