Method for sensing the presence of a human hand in a gripping region of a steering wheel

Abstract

The invention relates to a method for sensing 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 human hand (1) in a detection region of a sensor structure that comprises at least one sensor electrode (4) connected to a first input / output (3.1) of a sensor electronics unit (3) resulting in a change to 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), with a metal core (5) of the steering wheel being connected to a ground potential (GND). According to the invention, the method is characterised in that a diagnosis of the connection of the steering wheel core (5) to the ground potential (GND) is carried out during runtime by connecting at least a first contact point (5.1) of the steering wheel core (5) to a second input (3.2) of the sensor electronics unit (3), which continuously monitors the applied 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 a human hand in a detection area of ​​a sensor structure comprising at least one sensor electrode connected to a first 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 a hand, and wherein a metallic core of the steering wheel 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 this 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. The impedance ZGND of this potential connection, which is generally connected to a ground potential GND, typically has a capacitive component of CGND > 100 nF. The sensor electrodes and the steering wheel core are usually connected via connectors to a cable 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. 3 of the state of the art.

[0008] Faulty contacts or cable breaks in the potential connection of the steering wheel core can lead to significant changes in the impedance ZGND and in particular in the capacitive component CGND of the same and thus to incorrect measurement results.

[0009] The method according to the present invention has the advantage over the previously described prior art of avoiding such errors.

[0010] This is achieved according to the invention in that a diagnosis of the connection of the steering wheel core to the ground potential is carried out at runtime by connecting at least one contact point of the steering wheel core to a second input of the sensor electronics, which carries out a continuous monitoring of the applied potential.

[0011] Further advantageous embodiments of the method according to the invention emerge from the further subclaims and from the following description of a possible embodiment of the invention.

[0012] The invention is explained below with reference to the accompanying drawings, in which:

[0013] Fig. 1 : a schematic representation of a HOD sensor system for carrying out a method according to the invention in a first variant

[0014] Fig. 2: a schematic representation of a HOD sensor system for carrying out a method according to the invention in a first variant

[0015] Fig.3: a schematic representation of a state-of-the-art HOD sensor system and the relevant physical quantities

[0016] As can be seen in Fig. 3, 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 a first input / output 3.1 of the sensor electronics 3.

[0017] 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 via one or more lines 6 to a fixed potential, which is generally a ground potential GND. This connection has an impedance ZGND with a capacitive component of typically CGND > 100 nF. Both single and multiple contacts of the steering wheel core 5 are known, with multiple contacts increasing the robustness of the connection. However, a targeted diagnosis of this connection, in particular to detect an interruption, is not yet known. In Fig.Figure 1 shows a first embodiment of a HOD sensor system for implementing a method according to the invention in a first variant. The basic structure of the sensor system and the method for detecting the presence of a hand within a gripping area of ​​the steering wheel are identical to those previously described with reference to Figure 3.

[0018] In addition, there is also a connection here between one of the lines 6 connecting the steering wheel core 5 to the ground potential GND and a further, second input 3.2 of the sensor electronics 3. Using this second input 3.2 of the sensor electronics 3, a capacitive voltage divider measurement is carried out, in which the capacitive coupling between a contact point 5.1 of the steering wheel core 5 and the sensor electrode 4 via the capacitance Ccore is used. For this purpose, the sensor electrode 4 is supplied with a control signal, which, due to the capacitive coupling Ccore, is also followed by the voltage Ucore applied to the steering wheel core 5. If the connection to the steering wheel core at one of the nodes 6.1 or 6.2 of line 6 is interrupted, the ratio of the voltage divider and thus the measured voltage Ucore changes. A loss of the steering wheel connection to the ground potential GND can be detected via a value range diagnosis.

[0019] However, the voltage divider is dominated by ZGND or CGND (CGND > 100 nF, Ccore ~ ​​1 nF). To reliably detect an open circuit at node 6.2, the connection to the fixed potential GND must be interrupted during the diagnostic measurement, which is achieved via an additional switch 8. Otherwise, the measured voltage change would be too small and thus not robust against other influences on the measurement.

[0020] Fig. 2 shows an alternative embodiment of an HOD sensor system for carrying out a method according to the invention. In contrast to the embodiment shown in Fig. 1, in addition to the connection of a first contact point 5.1 of the steering wheel core 5 to the second input 3.2 of the sensor electronics 3, there is also a further connection of a second contact point 5.2 of the steering wheel core 5 to a further, third input 3.3 of the sensor electronics 3. However, the line 6' of this further connection, in which the node 6.1 is located here, is not directly connected to the ground potential GND, unlike the line 6 connected to the second input 3.2, in which the node 6.2 is located. An interruption in the connection to the node 6.1 or 6.2 can be detected through a short-circuit diagnosis.

Claims

Patent claims 1. A 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 a first input / output (3.1) of a sensor electronics system (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 core (5) of the steering wheel is connected to a ground potential (GND), characterized in that a diagnosis of the connection (6) of the steering wheel core (5) with the ground potential (GND) is carried out at runtime by connecting at least one first contact point (5.1) of the steering wheel core (5) to a second input (3.2) of the sensor electronics system (3), which continuously monitors the applied potential.

2. Method according to claim 1, characterized in that a capacitive voltage divider measurement is carried out by means of the second input (3.2) of the sensor electronics (3), in which the capacitive coupling between the steering wheel core (5) and the sensor electrode (4) via the capacitance Ccore is used.

3. Method according to claim 2, characterized in that the sensor electrode (4) is supplied with a control signal which, due to the capacitive coupling Ccore, is also followed by the voltage Ucore applied to the steering wheel core (5).

4. Method according to claim 2 or 3, characterized in that the connection (6) of the steering wheel core (5) to the ground potential (GND) during the diagnostic measurement via an additional switch (8) is interrupted. Method according to claim 1, characterized in that a second contact point (5.2) of the steering wheel core (5) is connected to a third input (3.3) of the sensor electronics (3), and in that an interruption in the connection (6) of the steering wheel core (5) to the ground potential (GND) or in the connection (6') of the steering wheel core (5) to the third input (3.3) of the sensor electronics (3) is detected by a short-circuit diagnosis.

Images