Method for detecting presence of human hand in grip region of steering wheel

Abstract

The invention relates to a method for detecting the presence of a human hand (1) in a gripping region of a steering wheel (2) by means of a capacitive sensor system, the presence of the hand (1) in a detection region of a sensor arrangement comprising at least one sensor electrode (4) connected to the input / output (3.1) of the sensor electronics (3) causes a change in a capacitive coupling (CHand) of the sensor electrode (4) to a reference potential relative to a reference state in which the hand (1) is not present, and wherein the metallic steering wheel core (5) is connected to ground (GND), according to the invention, the method is characterized in that a measure ([delta] UHumid) for an environmental impact (CHumid, RHumid) on a capacitive and ohmic coupling between the sensor electrode (4) and the steering wheel core (5) caused by water storage in the steering wheel (2) is determined by means of at least one diagnostic measurement, and is used to correct the determination of the capacitive coupling (CHand) of the sensor electrode (4) to the reference potential.

Description

Technical Field

[0001] The invention relates to a method for detecting the presence of a human hand in the gripping area of ​​a steering wheel by means of a capacitive sensor system, wherein the presence of a human hand in a detection area of ​​a sensor structure comprising at least one sensor electrode connected to an input / output of sensor electronics causes a change in the capacitive coupling of the sensor electrode to a reference potential relative to a reference state in the absence of the hand, and wherein the metallic steering wheel core is connected to ground potential. Background Art

[0002] The sensor electrodes used here are single-layer or multi-layer structures of electrically conductive sensor mats in the steering wheel rim. These sensor systems are also referred to as "HOD sensors" (Hands On Wheel Detection). The presence of a hand can be detected here via a capacitive measurement. The fact that there is a capacitive coupling between the hand and the sensor electrode, whose capacitance value C Hand The task of the associated sensor electronics or the measuring method using the sensor electronics is to detect the capacitance C Hand The particular difficulty here is that the capacitor C Hand In particular, the change to be detected is very small compared to other internal capacitances of the steering wheel. For this reason, the so-called integration method is used to measure it, in which a small charge quantity is transferred from the sensor element, whose capacitance value is relatively small and variable, to an integration capacitor with a known fixed and significantly larger capacitance value in a plurality of consecutive cycles. After a predetermined number of such integration cycles, the voltage at the integration capacitor is then measured with the aid of an A / D converter and processed into a sensor amplitude, which in turn is a value for the capacitance C Hand Other factors that may also influence the measured capacitance must therefore be kept as constant as possible. For this purpose, in particular the steering wheel core is placed at a fixed potential.

[0003] Sensor systems for steering wheels of motor vehicles for detecting hands with a capacitive sensor structure are known in principle from the prior art. Thus, for example, DE 203 09 603 U1 discloses a steering wheel with sensor electrodes arranged in a sector shape, which make it possible to detect the approach and / or departure of a human hand and associate it with a specific area of ​​the steering wheel.

[0004] WO 2016 / 087279 A1 discloses a steering wheel for a motor vehicle, which has a capacitive sensor system, and the sensor structure of the sensor system can at least sometimes also be operated as a heating structure. In addition, the steering wheel has a metallic steering wheel core, which is operated as an active shielding element and can be charged with a defined potential in order to reduce interfering capacitive coupling of the sensor structure to the steering wheel core.

[0005] Due to the capacitance C between the hand and the corresponding sensor electrode Hand Significantly greater capacitance C between the steering wheel core and the sensor electrode Core , potential changes in the steering wheel core have a disturbing effect on the HOD measurement. The connection of the steering wheel core to a fixed potential is implemented in the sensor electronics to avoid this problem. This potential connection, which is usually implemented at ground potential GND, typically has C GND >100nF capacitor.

[0006] The sensor electrodes and the steering wheel core are usually connected via a plug connector to a cable harness which leads to the HOD electronics and is likewise connected to the HOD electronics via a plug connector.

[0007] This actual situation and the relevant physical quantities are schematically illustrated in FIG. 2 according to the prior art.

[0008] The substrate and filler materials used for constructing the sensor electrodes and for constructing the haptics of the steering wheel are usually materials that absorb water under high humidity ambient conditions. The water stored in these materials has a significant influence not only on their electrical conductivity but also on their dielectric constant. As a result, the capacitive and ohmic coupling between the individual layers of the sensor electrodes and their coupling to the steering wheel core and / or the steering wheel heater is affected by the water storage. This coupling Figure 1 , where for reasons of clarity only the coupling C between the sensor electrodes and the steering wheel core is shown. Humid and R Humid .

[0009] The fact that water storage has a significantly greater influence on the HOD measurement than different gripping states can prove problematic here. In addition, the influence of the gripping state itself is also changed, so that when performing a simple value range analysis, it is possible to erroneously identify a gripping state that is believed to be different from the actual one.

[0010] Although usually environmental sensors (such as NTC) are installed in the steering wheel and evaluated. However, these sensors are usually only used for protection, not for compensation. Since they are separate sensors, the influence is also not sufficiently linked to the HOD measurement to make compensation of the HOD measurement value possible in real time. Summary of the invention

[0011] The method according to the invention has the following advantages over the previously described prior art: a diagnostic method is specified which correlates the influences on the HOD measurement and thus enables a correction of the HOD measurement for compensation of environmental influences in real time in order to avoid the described errors.

[0012] This is achieved according to the invention by determining, by means of at least one diagnostic measurement, the environmental influences C on the capacitive and ohmic coupling between the sensor electrode and the steering wheel core caused by water storage in the steering wheel. Humid and R Humid The measure of ΔU Humid and is used to correct the determination of the capacitive coupling of the sensor electrode to the reference potential.

[0013] In a preferred embodiment of the method according to the invention, the diagnostic measurement comprises a capacitor with a capacitance C via a resistor R Humid Determination of the discharge during a certain waiting time Δt, wherein the measure ΔU Humid is the voltage U after the initial voltage U0 and the waiting time Δt Humid The difference between .

[0014] The diagnostic measurement is advantageously carried out in a cyclically repeated manner with a predefined sampling rate.

[0015] A further advantageous embodiment of the method according to the invention consists in

[0016] The following description of possible implementations follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention is described below with reference to the accompanying drawings, wherein:

[0018] Figure 1 : shows a schematic illustration of a HOD sensor system for implementing the method according to the invention;

[0019] FIG. 2 : shows a schematic illustration of a HOD sensor system according to the prior art and the relevant physical quantities. DETAILED DESCRIPTION

[0020] As can be seen in FIG. 2 , which shows the prior art, in a conventional HOD sensor system at least one sensor electrode 4 of a sensor structure of a steering wheel 2 is connected to an input / output 3 . 1 of sensor electronics 3 .

[0021] The capacitive coupling C of the sensor electrode 4 to the hand 1 due to the presence of the human hand 1 in the detection area of ​​the sensor electrode 4 is detected via this input / output 3 . 1 of the sensor electronics 3 . Hand In contrast, the capacitance C between the sensor electrode 4 and the steering wheel core 5 core The steering wheel core 5 itself is connected via one or more wires 6 to a fixed potential, which is usually ground GND. This connection usually has a capacitance of C GND >100 nF. Single and multiple contact-connections of the steering wheel core 5 are known, wherein multiple contact-connections increase the robustness of the connection.

[0022] exist Figure 1 2 shows a design of a HOD sensor system for carrying out a method variant according to the invention. The basic structure of the sensor system and the method for detecting the presence of a hand in the gripping area of ​​a steering wheel are identical to those described above with reference to FIG.

[0023] Additionally shown is the coupling C between the sensor electrodes and the steering wheel core. Humid and R Humid Therefore, the total capacitance C coupled between the sensor electrode 4 and the steering wheel core 5 is C=C core +C Humid .

[0024] To carry out the method according to the invention, the sensor electrode 4 is firstly connected via the input / output 3 . 1 of the sensor electronics 3 with a defined voltage U HOD = U0 is charged. This connection is then disconnected, for example, by switching the input / output 3.1 of the sensor electronics 3 to an input. The charge stored on the sensor electrode 4 can then be charged via the resistor R Humid After a constant waiting time Δt, it then drops to the value U Humid The voltage of is measured via the input / output 3 . 1 of the sensor electronics 3 .

[0025] A capacitor with capacitance C is connected to a resistor R Humid The discharge is carried out according to the following formula:

[0026]

[0027] In a dry environment, the resistance R Humid is actually infinite, so that the voltage U determined in this way Humid ≈U0.

[0028] Therefore, the changes measured under real conditions, ie with water stored in the material used,

[0029] ΔU Humid =U0-U Humid

[0030] Is for environmental impact C Humid and R Humid The measure of C Hand .

[0031] Since the same electronics and sensor components are used according to the invention as for the HOD measurement itself, ΔU Humid Measuring U with HOD HOD Related and due to C Hand The independence of can be used as a correction factor.

[0032] If the diagnosis is carried out cyclically with a sufficient sampling rate, compensation of environmental influences is possible in real time with respect to the runtime.

Claims

1. A method for detecting the presence of a human hand (1) in the gripping area of ​​a steering wheel (2) by means of a capacitive sensor system, wherein: The presence of a human hand (1) in the detection area of ​​a sensor structure comprising at least one sensor electrode (4) connected to an input / output terminal (3.1) of the sensor electronics (3) causes a capacitive coupling C of the sensor electrode (4) to a reference potential. Hand The invention relates to a change in a reference state in which a hand (1) is not present, and in which a metallic steering wheel core (5) is connected to ground potential (GND), characterized in that environmental influences C on the capacitive coupling and ohmic coupling between the sensor electrode (4) and the steering wheel core (5) caused by water storage in the steering wheel (2) are determined by means of at least one diagnostic measurement. Humid and R Humid The measure of ΔU Humid and is used to calibrate the capacitive coupling C between the sensor electrode (4) and the reference potential Hand of determination.

2. The method according to claim 1, characterized in that The diagnostic measurement comprises a capacitor with capacitance C connected via a resistor R Humid The discharge is determined within a certain waiting time Δt, and the measurement ΔU Humid is the difference between the initial voltage U0 and the voltage U after the waiting time Δt Humid The difference between .

3. The method according to claim 1 or 2, characterized in that: The diagnostic measurement is performed in a cyclically repeated manner at a predefined sampling rate.

Images