DE102014011037A1 - Sensor electrode for a capacitive proximity sensor - Google Patents

Abstract

A capacitive proximity sensor (15) and a sensor electrode (17, 18) for such are indicated. The sensor electrode (17, 18) comprises an elongated, planar electrode conductor (23) made of electrically conductive plastic material. For complete or partial compensation of the length-dependent loss of sensitivity of the sensor electrode (17, 18) due to the specific ohmic resistance of the electrically conductive plastic material, the surface area of the conductor cross-section of the electrode conductor (23) in the longitudinal direction of the electrode conductor (23) is varied and / or the electrode conductor (12) at a plurality of spaced-apart contact points (25, 28, 31, 32) with a metallic lead (26, 29) contacted.

Description

The invention relates to a sensor electrode for a capacitive proximity sensor, in particular for use in a motor vehicle. It further relates to a capacitive proximity sensor with such a sensor electrode.

Capacitive proximity sensors are often used in automotive technology for the detection of obstacles in the travel of movable motor vehicle parts, such as anti-trap in a motorized adjustment device for a side window, a tailgate or a convertible top. Capacitive sensors are further used in automotive technology for the detection of positioning commands, which gives a vehicle user contactless by means of a hand or foot movement.

For example, this is off DE 10 2010 049 400 A1 a capacitive proximity sensor, by means of which a contact opening command for the automatic opening of a tailgate is detected without contact. The proximity sensor in this case comprises two elongate sensor electrodes which are mounted one above the other in the vehicle transverse direction on the rear bumper of the motor vehicle. The vehicle user issues the opening command by kicking under the rear bumper with one foot.

Conventionally, either flat conductors or round conductors are used as sensor electrodes for capacitive proximity sensors of the type described above. For example, in the case of DE 10 2010 049 400 A1 known proximity sensor, the upper sensor electrode as a flat conductor and the lower sensor electrode formed as a circular conductor. In both cases, the actual electrode conductor of the respective sensor electrode is usually formed of metal, in particular copper.

Out EP 2 159 917 A1 On the other hand, a capacitive proximity sensor is known whose sensor electrodes are formed from electrically conductive plastic. Such plastic electrodes are characterized by low weight and rational manufacturability and are therefore of considerable advantage over conventional sensor electrodes with a metallic electrode conductor. In addition, plastic electrodes manufacturing technology can be easily made in existing plastic parts such. B. integrate bumper, whereby the assembly costs for the associated sensor can be reduced and a high reliability is achieved. In particular, a faulty mounting of the sensor electrodes and a faulty detachment of the sensor electrodes from the vehicle part can be virtually ruled out.

However, plastic electrodes, unlike corresponding metal electrodes, typically have a sensitivity that decreases markedly over the length of the sensor electrode. Comparable approaches of an object (in particular a body part) to the sensor electrode are thus detected to different degrees by the capacitive sensor, depending on whether the object near the connection side or far from the connection side is approximated to the sensor electrode. In some applications of a capacitive sensor, this length-dependent loss of sensitivity of the sensor electrode may be desirable and - as in EP 2 689 976 A1 and DE 10 2011 112 274 A1 is described - be exploited specifically to determine the longitudinal position of the approach.

In most applications, however, the length-dependent loss of sensitivity of the sensor electrode is undesirable because it increases the risk of detection errors of the proximity sensor. In these applications, plastic electrodes often can not be used.

The invention has for its object to provide an easy and rational producible, but at the same time effective sensor electrode and an associated capacitive sensor.

With respect to a sensor electrode for a capacitive proximity sensor, this object is achieved by the features of claim 1. With respect to a capacitive proximity sensor comprising at least one such sensor electrode, the object is achieved by the features of claim 7. Advantageous embodiments and further developments of the invention are in the dependent claims and the description below.

The sensor electrode has an elongated, planar electrode conductor of electrically conductive plastic material. According to a first aspect of the invention, the surface area of the conductor cross-section of the electrode conductor is varied in the longitudinal direction of the electrode conductor, so that the length-dependent loss of sensitivity of the sensor electrode caused by the specific (ohmic) resistance of the electrically conductive plastic is completely or partially compensated. According to a second aspect of the invention, for the same purpose and with the same effect, the electrode conductor is contacted at a plurality of spaced apart contact points with a metallic lead. The two aspects of the invention can be used individually within the scope of the invention, but also in combination with one another.

The length-dependent sensitivity is in this case, for example, by means of the sensor electrode measured signal strength of the sensor signal, which is caused by a certain object at a certain longitudinal position of the sensor electrode at a certain distance approximated. Conveniently, this sensitivity is normalized by giving it in relation to the maximum signal strength that causes a comparable approach event (ie, the same object approaching the same distance) at any longitudinal position of the sensor electrode. Again, the above-mentioned object may also be a body part.

Both aspects of the invention are based on the common idea that the advantageous use of electrically conductive plastic for the electrode conductor of the sensor electrode becomes possible without the previous limitations, if it is possible to compensate for the disadvantages associated with this material. As is known, the disadvantages of plastic electrodes are due to the comparatively high specific resistance of the plastic material. However, the measurable ohmic resistance of the sensor electrode is correlated with the specific resistance of the plastic material on the one hand over the length of the formed within the electrode conductor current path between a considered longitudinal position of the sensor electrode and the terminal side of the sensor electrode, and on the other hand via the conductor cross section of the electrode conductor. The measurable ohmic resistance of the sensor electrode is greater, the longer the current path and the smaller the conductor cross-section. This in turn is based on the knowledge that the negative influence of the high specific resistance can be compensated on the one hand by deliberate shortening of the current paths within the electrode conductor, and on the other hand by influencing the conductor cross section. Furthermore, the sensitivity of the sensor electrode is also influenced by the size of its surface, so that the negative influence of the high specific resistance can also be varied by varying this surface.

Preferably, the electrode conductor is designed such that the surface area of its conductor cross-section increases with increasing distance to the nearest contact point. The contact point of the electrode conductor is the or each point at which the electrode conductor is contacted with a metallic lead. If the sensor electrode has only one contact point, this contact point is also always the nearest point of contact.

The surface area of the conductor cross-section is preferably increased by increasing the width of the electrode conductor with increasing distance to the nearest contact point.

In an expedient embodiment, the electrode conductor forms a continuous coherent surface. In an alternative embodiment of the invention, however, the electrode conductor can also be designed such that it has holes or cuts. In particular, the electrode conductor can be formed in the context of the invention by a net-like conductor structure. In these cases, the surface area of the conductor cross-section is optionally increased by the fact that the holes or incisions are formed or arranged with increasing distance to the nearest contact point with decreasing size and / or density.

In order to shorten the current paths within the electrode conductor, the latter has, in an expedient embodiment, a contact point at both longitudinal ends. The electrode conductor is therefore contacted at both longitudinal ends. The two contact points are expediently contacted by means of two separate metallic leads. The separate supply lines can thereby be used not only for signal transmission to the sensor electrode, but also - in the context of a diagnostic function - for testing the electrical continuity of the current paths formed over the leads and the entire length of the sensor electrode. As a result, wire breaks or other contact errors can be detected.

Additionally or alternatively, the electrode conductor is guided in parallel in an advantageous embodiment of the invention, a metallic lead over at least part of its length. This supply line is in this case contacted at at least two spaced-apart contact points with the electrode conductor.

The proximity sensor according to the invention comprises an electronic control and evaluation unit and at least one associated sensor electrode of the type described above. The control and evaluation unit is formed in particular by a microcontroller with a control and evaluation program (firmware) implemented therein. Alternatively, however, it can also be formed by a non-programmable hardware circuit, in particular an ASIC.

Embodiments of the invention will be explained in more detail with reference to a drawing. Show:

1 in a rough schematic side view of a rear part of a motor vehicle, which is provided with a two elongated sensor electrodes having capacitive proximity sensor for contactless opening of a tailgate, and

2 in a schematic representation of the proximity sensor according to 1 with one of the two sensor electrodes, an object for the determination of the length-dependent loss of sensitivity at a certain distance along the sensor electrode along, and in a schematic diagram the course of the length-dependent sensitivity to a longitudinal position along the sensor electrode,

3 - 5 in illustration according to 2 the proximity sensor with various other embodiments of the sensor electrode and the respectively associated course of the length-dependent sensitivity.

Corresponding parts and sizes are always provided with the same reference numerals in all figures.

1 shows a tail 1 of a motor vehicle 2 equipped with a movable vehicle door in the form of a tailgate 3 is provided. The tailgate 3 is pivotable around a top edge 4 of the stern 1 hinged. By pivoting the tailgate 3 can the same along a (in 1 indicated by an arrow) Verstellwegs P reversible between an open position 5 and a closed position 6 to be moved. In the open position 5 and the closed position 6 is the tailgate 3 each shown with dashed lines. With solid lines is the tailgate 3 in an (arbitrarily chosen) intermediate position 7 between the open position 5 and the closed position 6 shown.

For automatic adjustment of the tailgate 3 is the motor vehicle 2 with an actuator 10 fitted. The adjusting device 10 includes an electric servomotor 11 and a (in 1 only indicated) actuating mechanism 12 over which the servomotor 11 mechanically with the tailgate 3 coupled to their adjustment. For controlling the servomotor 11 includes the adjusting device 10 also an electronic engine control unit 13 ,

For non-contact detection of opening commands of a motor vehicle user for the tailgate 3 is the motor vehicle 2 further with a capacitive proximity sensor 15 fitted. The proximity sensor 15 includes an electronic control and evaluation unit 16 and two sensor electrodes 17 and 18 ,

The two sensor electrodes 17 and 18 are parallel and substantially superimposed on the (not visible from the outside) rear of a rear bumper 19 of the motor vehicle 2 arranged here and each extend horizontally in the vehicle transverse direction over a large part of the vehicle width.

During operation of the proximity sensor 15 gets to the sensor electrodes 17 and 18 through the control and evaluation unit 16 an alternating electrical voltage applied, under the effect of each of the two sensor electrodes 17 . 18 in one of the sensor electrodes 17 . 18 each upstream volume of space (hereinafter referred to as detection space 20 respectively. 21 designated) generates an electric field. How out 1 is recognizable, are the sensor electrodes 17 and 18 designed and aligned so that the detection space 20 the upper sensor electrode 17 starting from the bumper 19 extends substantially in a horizontal direction, while the detection space 21 the lower sensor electrode 18 the bumper 19 flanked essentially on the underside.

When a body part, especially a foot of a vehicle user to the bumper 19 approximated and here in the detection rooms 20 and 21 is introduced, this body part influenced by the sensor electrodes due to the electrical conductivity of the human body tissue and the capacitive coupling of the body tissue with the substrate 17 . 18 outgoing electric field and thus the at the sensor electrodes 17 . 18 measurable capacity. The control and evaluation unit measures the measure (hereinafter "sensor signal") for the capacity 16 For example, the current strength of the current flowing at a predetermined voltage to the sensor electrode displacement current.

In order to signal a door opening request, the vehicle user conventionally performs a kick-like foot movement under the bumper 19 , The temporal capacity change caused thereby is calculated by the control and evaluation unit 16 detected. If the course of the detected sensor signal and thus the measurable change in capacity correspond to stored triggering criteria, the control and evaluation unit interprets 16 the capacity change as the opening command and outputs a corresponding opening signal O to the engine control unit 13 from this, by controlling the servomotor 11 the opening of the tailgate 3 causes.

2 shows roughly schematically the proximity sensor 15 with the control and evaluation unit 16 as well as by way of example with one of the two sensor electrodes 17 . 18 ,

As can be seen from this illustration, the sensor electrode comprises 17 . 18 a flat, along a longitudinal direction elongated electrode conductor 23 , The electrode conductor 23 is made of electrically conductive plastic, such as polyaniline (PAni) and has in the embodiment according to 2 the shape of an elongated rectangular strip of constant width.

At one longitudinal end 24 has the sensor electrode 17 . 18 a contact point 25 made of metal (in particular copper), via which a supply line 26 electrically with the electrode conductor 23 connected is. Furthermore, the sensor electrode 17 . 18 in the execution according to 2 at the opposite longitudinal end 27 another contact point 28 on, over the one more, from the supply line 26 separate supply line 29 with the electrode conductor 23 electrically contacted.

Preferably, the electrode conductor 23 isolated from the environment by means of a layer of electrically insulating plastic, in order to short-circuit the electrode conductor 23 avoid with surrounding vehicle parts and the electrode conductor 23 Protect against damaging environmental influences (water, dirt, etc.). In an expedient embodiment, the sensor electrodes 17 . 18 , and in particular their respective electrode conductor 23 in the also made of plastic bumper 19 integrated.

About the supply lines 26 and 29 , which are formed as ordinary insulated wire or stranded conductors made of copper, is the sensor electrode 17 . 18 (Specifically, their electrode leads 23 ) electrically with the control and evaluation unit 16 connected.

Due to the comparatively high specific resistance of the electrode conductor 23 used plastic material has the sensor electrode 17 . 18 a sensitivity varying over its length. Characteristic of the sensitivity is the signal strength of the control signal, the approach of a certain object 30 to a certain distance A to a certain longitudinal position x of the sensor electrode 17 . 18 causes. Other than in the scheme according to 2 shown, the distance A is in this case perpendicular to the surface of the electrode conductor 23 measured.

In the following, "(length-dependent, normalized) sensitivity" S denotes a measured variable, for the determination of which the signal strength of the sensor signal is determined by the given longitudinal position x of the sensor electrode 17 . 18 in the particular distance A approximated object 30 is normalized to the maximum signal strength (ie, divided by the maximum signal strength), which is the approximation of the same object 30 at the same distance A at any longitudinal position x of the sensor electrode 17 . 18 causes. As in 2 implied, is the object 30 in particular galvanically or at least capacitively coupled to ground. With this item 30 it may in particular be a body part of the human body, in particular a foot or a hand act.

To determine the sensitivity curve, for example - as in 2 schematically indicated - the object 30 while maintaining the distance A parallel to the longitudinal direction of the sensor electrode 17 . 18 be moved, wherein continuously the sensor signal of the proximity sensor 15 is detected. The one in the diagram of 2 shown course of the sensitivity S results here in that the maximum of the detected curve of the sensor signal is set by the normalization described above to the value one. The standardization has the advantage that the values of the sensitivity S are largely independent of the object used for their determination 30 and the distance A, in which this object 30 to the sensor electrode 17 . 18 is approximated. The normalized sensitivity S can theoretically assume values between one and zero or be given as a percentage between 100% and 0%.

A conventional plastic electrode with a strip-shaped planar electrode conductor of constant width, which is contacted only one longitudinal end with a metallic lead, has a length-dependent, normalized sensitivity S ', which has its maximum (and thus the value of one) at the contact point assigned to this longitudinal end and decreases with increasing distance to this contact point. The typical course of this sensitivity S 'of conventional sensor electrodes is shown in the diagram according to FIG 2 entered for comparison with dashed line.

As 2 can be seen, is by the second contact point 28 at the second longitudinal end 27 of the electrode conductor 23 the effect is achieved that there the sensitivity S is also at least approximately raised to the value of 1. Between the two contact points 25 and 28 Adopts the sensitivity of the sensor electrode 17 . 18 according to 2 from. However, the loss of sensitivity is considerably less pronounced here than in the case of a conventional sensor electrode contacted only on one side. The sensitivity loss caused by the comparatively high specific resistance of the electrode material is thus partially compensated.

In the execution according to 3 includes the sensor electrode 17 . 18 in addition to the contact points 25 and 28 further contact points 31 and 32 , The contact points 25 . 28 . 31 and 32 are here at regular intervals over the length of the electrode conductor 23 distributed. In the execution according to 3 is the second supply line 29 unavailable. Instead, the supply line 26 over the entire length of the sensor electrode 17 . 18 parallel to the electrode conductor 23 guided and with all contact points 25 . 28 . 31 and 32 contacted. Unlike in the illustration according to 3 indicated, is the supply line 26 preferably over the entire length of the sensor electrode 17 . 18 with the electrode conductor 23 mechanically connected.

As from the diagram of 3 shows, by (compared to 2 ) increased number of contact points 25 . 28 . 31 and 32 the course of the sensitivity S more evenly aligned with a constant curve with the value S = 1 as in the embodiment according to 2 , Due to the high resistivity of the conductor material of the electrode conductor 23 caused loss of sensitivity is thus compensated even more.

In the embodiment according to 4 is the electrode conductor 23 similar to a conventional sensor electrode, only at the longitudinal end 24 in the (here only) contact point 25 with the (here only) supply line 26 contacted. In the case of this configuration of the sensor electrode 17 . 18 to compensate for expected loss of sensitivity is according to 4 the electrode conductor 23 designed such that its width with increasing distance to the longitudinal end 24 steadily increasing. Through this - trapezoidal - shape of the electrode conductor 23 is achieved that also the conductor cross-section and relevant for the field generation surface of the electrode conductor 23 with increasing distance to the longitudinal end 24 increase. As from the diagram of 4 shows, by this measure, an at least approximately constant course of the sensitivity S over the entire length of the sensor electrode 17 . 18 reached.

In the embodiments according to 2 to 4 forms the electrode conductor 23 each a completely filled, seamless contiguous area. Deviating from this is the electrode conductor 23 in the embodiment according to 5 with holes 33 Mistake. The remaining electrode conductor 23 , which again has a constant width over its entire length in the illustrated example, thus forms a grid-like conductor structure.

In order at the thus designed electrode conductor 23 to achieve the same effect as in the embodiment according to 4 namely one with increasing distance to the (again single) contact point 25 increasing surface area of the conductor cross section and an increasing effective surface, are the holes 33 designed so that their size - with the same density of holes 33 - With increasing distance to the contact point 25 decreases. Alternatively or in addition to this, the density of the holes can also be used 33 with increasing distance to the contact point 25 be lowered. Like the diagram too 5 can be seen, is also a substantially constant course of the sensitivity S over the entire length of the sensor electrode by this measure 17 . 18 achieved.

The sensor electrodes 17 and 18 of the proximity sensor 15 can be the same, especially after each of the in the 2 to 5 illustrated embodiments, be constructed. Alternatively, the sensor electrodes 17 and 18 also be constructed differently. In particular, one of the two sensor electrodes 17 or 18 be designed in a conventional manner.

The invention will be particularly apparent in the embodiments described above, but is not limited thereto. Rather, numerous other embodiments of the invention may be inferred from the claims and the foregoing description. In particular, the basis of the 2 to 5 shown measures to compensate for the caused by the resistivity of the electrode material sensitivity loss are combined with each other as desired.

LIST OF REFERENCE NUMBERS

1

Rear

2

motor vehicle

3

tailgate

4

top edge

5

open position

6

closed position

7

intermediate position

10

locking device

11

servomotor

12

actuating mechanism

13

Engine control unit

15

Proximity sensor

16

Control and evaluation unit

17

sensor electrode

18

sensor electrode

19

bumper

20

detection space

21

detection space

23

electrode conductor

24

longitudinal edge

25

contact point

26

supply

27

longitudinal end

28

contact point

29

supply

30

object

31

contact point

32

contact point

33

hole

P

adjustment

O

opening signal

A

distance

x

longitudinal position

S

sensitivity

S '

sensitivity

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102010049400 A1 [0003, 0004]
• EP 2159917 A1 [0005]
• EP 2689976 A1 [0006]
• DE 102011112274 A1 [0006]

Claims (7)

Sensor electrode ( 17 . 18 ) for a capacitive proximity sensor ( 15 ), with an elongated, flat electrode conductor ( 23 ) made of electrically conductive plastic material, wherein for complete or partial compensation of the caused by the specific ohmic resistance of the electrically conductive plastic material, length-dependent loss of sensitivity of the sensor electrode ( 17 . 18 ) - the area of the conductor cross-section of the electrode conductor ( 23 ) in the longitudinal direction of the electrode conductor ( 23 ) is varied and / or - the electrode conductor ( 12 ) at a plurality of spaced apart contact points ( 25 . 28 . 31 . 32 ) with a metallic feed line ( 26 . 29 ) is contacted. Sensor electrode ( 17 . 18 ) according to claim 1, wherein the area of the conductor cross-section of the electrode conductor ( 23 ) with increasing distance to the nearest contact point ( 25 . 28 . 31 . 32 ) increases. Sensor electrode ( 17 . 18 ) according to claim 2, wherein the width of the electrode conductor ( 23 ) with increasing distance to the nearest contact point ( 25 . 28 . 31 . 32 ) increases. Sensor electrode ( 17 . 18 ) according to claim 2 or 3, wherein the electrode conductor ( 23 ) Holes ( 33 ) or incisions, and wherein the size and / or density of the holes ( 33 ) or incisions with increasing distance to the nearest contact point ( 25 . 28 . 31 . 32 ) decreases. Sensor electrode ( 17 . 18 ) according to one of claims 1 to 4, wherein the electrode conductor ( 23 ) at both longitudinal ends ( 24 . 27 ) each have a contact point ( 25 . 28 ), and in these contact points ( 25 . 28 ) by means of two separate metallic leads ( 26 . 29 ) is contacted. Sensor electrode ( 17 . 18 ) according to one of claims 1 to 5, wherein the electrode conductor ( 23 ) a metallic feed line ( 26 ) is parallel led over at least a part of its length, and wherein this supply line ( 26 ) at at least two spaced contact points ( 25 . 28 . 31 . 32 ) with the electrode conductor ( 23 ) is contacted. Capacitive proximity sensor ( 15 ) with an electronic control and evaluation unit ( 16 ) and at least one sensor electrode ( 17 . 18 ) according to one of claims 1 to 6.

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