Method for detecting a presence of a human hand in a gripping area of a steering wheel

Abstract

A method for detecting a presence of a human hand (1) in a gripping area of a steering wheel (2) by means of a capacitive sensor system, wherein the presence of the human hand (1) in a detection range of a sensor structure comprising at least one sensor electrode (4) connected to an input / output (3.1) of sensor electronics (3) brings about a change in a capacitive coupling (CHand) of the sensor electrode (4) at a reference potential as compared with a reference state without the presence of the hand (1), and wherein a metal steering-wheel core (5) is connected to a ground potential (GND), is characterized according to the invention in that, by means of at least one diagnostic measurement, a measure (ΔUHumid) of the environmental influences (CHumid, RHumid) on the capacitive and the ohmic coupling between the sensor electrode (4) and the steering-wheel core (5) attributable to water retention in the steering wheel (2) is determined and is used for correcting the determination of the capacitive coupling (CHand) of the sensor electrode (4) at the reference potential.

Description

[0001] Method for detecting the presence of a human hand in a gripping area of ​​a steering wheel

[0002] The invention relates to a method for detecting the presence of a human hand in a gripping area of ​​a steering wheel by means of a capacitive sensor system, wherein the presence of the human hand in a detection area of ​​a sensor structure comprising at least one sensor electrode connected to an input / output of a sensor electronics causes a change in a capacitive coupling of the sensor electrode to a reference potential compared to a reference state without the presence of the hand, and wherein a metallic steering wheel core is connected to a ground potential.

[0003] The sensor electrodes used here are a single- or multi-layer structure of conductive sensor mats in the steering wheel rim. These sensor systems are also known as HOD sensors (Hands On Wheel Detection). The presence of a hand can be detected using a capacitive measurement. This exploits the fact that there is a capacitive coupling between the hand and the sensor electrode, whose capacitance Cnand depends on the grip state. The task of the associated sensor electronics or a measurement method used by them is to detect a change in the capacitance Cnand caused by a change in the grip state. The particular difficulty here is that the capacitance Cnand and, in particular, the changes to be detected are very small compared to other internal capacitances of the steering wheel.For this reason, so-called integration methods are often used to measure them. In these methods, small amounts of charge are transferred from the sensor element, whose capacitance is relatively small and variable, to an integration capacitor with a known, fixed and significantly larger capacitance in several consecutive cycles. The voltage across the integration capacitor after a predetermined number of such integration cycles is then measured using an A / D converter and processed into a sensor amplitude, which in turn is a direct measure of the capacitance Cnand. Other influences that can also affect the measured capacitance must therefore be kept constant as much as possible. For this purpose, the steering wheel core, in particular, is placed at a fixed potential.

[0004] Sensor systems for motor vehicle steering wheels for hand detection using capacitive sensor structures are generally known from the prior art. For example, DE 203 09 603 LI1 shows a steering wheel with sector-arranged sensor electrodes that allow the approach and / or distance of a human hand to be detected and assigned to a specific area of ​​the steering wheel.

[0005] WO 2016 / 087279 A1 discloses a steering wheel for a motor vehicle with a capacitive sensor system whose sensor structure can also be operated, at least temporarily, as a heating structure. The steering wheel further comprises a metallic steering wheel core that can be operated as an active shielding element and subjected to a defined electrical potential to reduce disruptive capacitive coupling between the sensor structure and the steering wheel core.

[0006] Due to the significantly larger capacitance Ccore between the steering wheel core and the respective sensor electrode compared to the capacitance Cnand between a hand and a respective sensor electrode, a potential change in the steering wheel core would have a disruptive effect on the HOD measurement. To avoid this problem, the steering wheel core is connected to a fixed potential in the sensor electronics. This potential connection, which is generally connected to a ground potential GND, typically has a capacitance of CGND > 100 nF. The sensor electrodes and the steering wheel core are usually connected via connectors to a wiring harness that leads to the HOD electronics and is also connected to it via connectors.

[0007] This fact as well as the relevant physical quantities are shown schematically in Fig. 2 of the state of the art.

[0008] The substrates and filler materials used for the installation of the sensor electrodes and for the design of the steering wheel's haptics are generally materials that absorb water under ambient conditions with high humidity. The water stored in these materials has a significant influence on both their electrical conductivity and their dielectric constant. Therefore, both the capacitive and ohmic coupling between the individual layers of the sensor electrodes, as well as their coupling to the steering wheel core and / or a steering wheel heater, are influenced by the water storage. The coupling is illustrated in Fig. 1, although for reasons of clarity, only the couplings C Humid and R HU mid between a sensor electrode and the steering wheel core.

[0009] A problem here is the fact that the influence of water retention on the HOD measurement can be significantly greater than the influence of different grip conditions. Furthermore, the influence of the grip condition itself is also altered to such an extent that a simple range analysis may incorrectly identify a supposedly different grip condition than the actual one.

[0010] Although environmental sensors, such as NTC thermistors, are typically installed and evaluated in the steering wheel, these typically serve only as a safeguard, not as a compensation. Furthermore, since these sensors are separate, their influence is not sufficiently correlated with the HOD measurement to allow for real-time compensation of the HOD measurements.

[0011] The method according to the present invention has the advantage over the previously described prior art of demonstrating a diagnostic method that correlates with the influence on the HOD measurement and thus enables a correction of the HOD measurement to compensate for environmental influences in real time in order to avoid the described errors.

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

[0013] In a preferred embodiment of the method according to the invention, the diagnostic measurement comprises determining the discharge of a capacitor with the capacitance C via a resistor R Humid in a specified waiting time At, where the dimension U Humid the difference between an output voltage U o and the voltage U Humid after the waiting time At.

[0014] The diagnostic measurement is advantageously carried out cyclically with a specified sampling rate.

[0015] Further advantageous embodiments of the method according to the invention will become apparent from the following description of a possible embodiment of the invention.

[0016] The invention is explained below with reference to the accompanying drawings, in which:

[0017] Fig. 1 : a schematic representation of a HOD sensor system for carrying out a method according to the invention

[0018] Fig. 2: a schematic representation of a state-of-the-art HOD sensor system and the relevant physical quantities

[0019] As can be seen in Fig. 2, which shows the prior art, in a conventional HOD sensor system the at least one sensor electrode 4 of the sensor structure of the steering wheel 2 is connected to an input / output 3.1 of the sensor electronics 3.

[0020] Via this input / output 3.1 of the sensor electronics 3, a change in the capacitive coupling Cnand between the sensor electrode 4 and the hand 1 caused by the presence of a human hand 1 in a detection area of ​​the sensor electrode 4 can be detected. The capacitance Ccore between the sensor electrode 4 and the steering wheel core 5, in contrast, is significantly larger. The steering wheel core 5 itself is connected to a fixed potential, which is generally a ground potential GND, via one or more lines 6. This connection has a capacitance of typically CGND > 100 nF. Both single and multiple contacting of the steering wheel core 5 is known, with multiple contacting increasing the robustness of the connection.

[0021] Fig. 1 shows an embodiment of a HOD sensor system for implementing a variant of the method according to the invention. The basic structure of the sensor system and the method for detecting the presence of a hand in a gripping area of ​​the steering wheel are identical to those previously described with reference to Fig. 2. In addition, the couplings C Humid and R HU mid between a sensor electrode and the steering wheel core. The total capacitance C of the coupling between the sensor electrode 4 and the steering wheel core 5 is therefore C = Ccore + C Humid .

[0022] To carry out the method according to the invention, the sensor electrode 4 is first supplied with a defined voltage U via the input / output 3.1 of the sensor electronics 3 H0D - U ocharged. This connection is then separated, e.g. by switching the input / output 3.1 of the sensor electronics 3 to input. The charge now stored on the sensor electrode 4 can then be discharged via the resistor R Humid After a constant waiting time At, the pressure is then set to the value U Humid dropped voltage across input / output 3.1 of sensor electronics 3 is measured.

[0023] The discharge of a capacitor with the capacity C via a resistor Rnumid occurs according to the equation

[0024] In dry environments the resistance R Humid practically infinite, so that the voltage U determined in this way Humid « u0ist.

[0025] The change measured under real conditions, ie with water embedded in the materials used is therefore a measure of the environmental influences C Humid and R Humid , but largely independent of C Hand. Since according to this invention the same electronics and sensors are used as for the HOD measurement itself, U Humid with the HOD measurement U H0D and can be due to the independence of C Hand be used as a correction factor.

[0026] If this diagnosis is carried out cyclically with a sufficient sampling rate, real-time compensation of environmental influences is possible during runtime.

Claims

Patent claims 1 . Method for detecting the presence of a human hand (1) in a gripping area of ​​a steering wheel (2) by means of a capacitive sensor system, wherein the presence of the human hand (1) in a detection area of ​​a sensor structure comprising at least one sensor electrode (4) connected to an input / output (3.1) of a sensor electronics (3) causes a change in a capacitive coupling CHand of the sensor electrode (4) with a reference potential compared to a reference state without the presence of the hand (1), and wherein a metallic steering wheel core (5) is connected to a ground potential (GND), characterized in that by means of at least one diagnostic measurement a measure MJ Humid for the environmental influences C caused by water retention in the steering wheel (2) Humid and R HUmid on the capacitive and ohmic coupling between the sensor electrode (4) and the steering wheel core (5) and is used to correct the determination of the capacitive coupling CHand of the sensor electrode (4) with the reference potential.

2. Method according to claim 1, characterized in that the diagnostic measurement comprises determining the discharge of a capacitor with the capacitance C via a resistor R Humid in a specified waiting time At, and that the measure U Humid the difference between an output voltage U o and the voltage U Humid after the waiting time At.

3. Method according to claim 1 or 2, characterized in that the diagnostic measurement is carried out cyclically and repeatedly at a predetermined sampling rate.

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