Capacitive sensor, method for reading out a capacitive sensor field and method for producing a capacitive sensor field

A capacitive sensor (20) includes a capacitive sensor field (2), the capacitive sensor field (2) having a plurality of discrete electrodes (4) which are coupled to discrete leads (8). The leads (8) of a first electrode (41) are guided such that they are capacitively coupled with at least one second electrode (42). A first signal (Cm1) is detected at a first lead (8) which is coupled with the first electrode (41), and a second signal (Cm2) is detected at a second lead (8) which is coupled with a second electrode (42). The capacity (Cf1, Cf2) of the first electrode (41) or of the second electrode (42) is determined using a predetermined calculation formula which takes the first signal (Cm1), the second signal (Cm2) and the capacitive coupling between the second electrode (4) and the first lead (8) coupled with the first electrode (41) into account.

RELATED APPLICATIONS

This application corresponds to PCT/EP2013/056877, filed Apr. 2, 2013, which claims the benefit of German Application No. 10 2012 006 546.9, filed Apr. 2, 2012, the subject matter, of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to a capacitive sensor having an evaluation unit and a sensor field comprising a plurality of discrete electrodes, and to a method of reading out such a capacitive sensor field.

TECHNICAL BACKGROUND

Two-dimensional capacitive sensor fields are in many cases used as control panels (touch pads) for vehicle functions, for example to operate a radio or a navigation system. The capacitive sensor field is preferably arranged in the instrument panel of a motor vehicle and is combined with a display or a mask displaying the corresponding vehicle functions. According to the prior art, such touch pads are produced using relatively thick circuit boards to minimize crosstalk between the leads of the electrodes and the electrodes themselves and to permit an accurate capacity measuring at the respective electrode. The capacity to be measured is generated by the interaction between the respective electrode of the capacitive sensor field and an object lying in an isolated manner on a contact plate and assumed to be grounded, for example a pin specifically provided therefor or the finger of a user. To convert the capacities measured at the electrodes of a capacitive sensor field into appropriate position and/or proximity signals which can be transmitted to the vehicle electronics for further evaluation, a capacitive sensor usually comprises in addition to the sensor field an evaluation unit or evaluation electronics which can of course also be integrated into other electronic components of the vehicle.

The thick circuit boards used to manufacture known sensor fields represent on the one hand a cost factor. On the other hand, a thick and accordingly mechanically rigid circuit board leads to restrictions with respect to the application possibilities of the capacitive sensor field. For example, the adaptation of the sensor field to a desired spatial shape of the touch pad is possible to a limited extent only, without the rigid circuit board having to be adapted already during manufacture to this spatial shape, e.g. to a determined convex or concave surface of the touch pad.

SUMMARY

The object of the present invention is to specify an improved capacitive sensor, an improved method of reading out a capacitive sensor field, and an improved method of manufacturing a capacitive sensor field.

According to one aspect of the invention, a method of reading out a capacitive sensor field is specified, this sensor field having a plurality of discrete electrodes. Each electrode of the capacitive sensor field is coupled to a discrete lead extending from the respective electrode up into a connection area of the sensor field. A capacity of the corresponding electrode can be read out through the respective discrete lead. The plurality of electrodes of the sensor field comprises at least one first electrode the lead of which is guided such that this lead is capacitively coupled with at least one second electrode. A first signal is detected at a first lead coupled with the first electrode, and a second signal is detected at a second lead coupled with the second electrode. The capacity of the first electrode or the capacity of the second electrode is determined by evaluation of a predetermined calculation formula, this calculation formula taking the first signal, the second signal and the capacitive coupling between the second electrode and the first lead coupled with the first electrode into account.

In contrast to the known technical approaches, according to aspects of the invention, the method does not pursue the approach to minimize a capacitive coupling between the leads and the electrodes; it rather consciously accepts this coupling. The inaccuracies thus generated during reading out of the corresponding capacities are arithmetically compensated for by the use of an appropriate calculation formula. It is advantageously possible to dispense with the use of accordingly thick circuit boards to keep crosstalk between the electrode leads and the electrode as low as possible. The calculation formula incorporates, among other things, a capacitive coupling, preferably a capacity between the second electrode and the first lead which is coupled with the first electrode. The capacity between the second electrode and the first lead can be determined empirically, for example. It is however also possible to calculate this capacity on the basis of the geometry used and the materials used (in particular on the basis of the dielectric constant thereof). By consciously accepting a capacitive coupling between the leads of the electrodes and those electrodes in the proximity of which these leads are guided, it is possible to fall back on a plurality of manufacturing techniques for manufacturing an appropriate capacitive sensor field. It is thus for example possible to apply the electrodes and all leads in a layered printing technique by means of conductive or non-conductive paint using appropriate insulating layers. A flexible plastic film, as it is sufficiently thin, can be used as a substrate. This flexible support (substrate) permits an easy adaptation of the shape of the capacitive sensor field to the spatial shape of a control element of a motor vehicle, for example. A control element (touch pad) including the appropriate capacitive sensor field can thus flexibly be adapted to the outer shape of an instrument panel of a motor vehicle and be integrated therein. According to aspects of the invention, such a manufacturing method furthermore offers advantages with respect to the known conventional methods which use relatively thick circuit boards as a substrate.

According to a further aspect of the invention, an advantageous method of reading out a capacitive sensor field is specified, the plurality of electrodes of the sensor field comprising at least one third electrode the lead of which is guided such that this lead is capacitively coupled with the first electrode, the second electrode and, if necessary, with further electrodes. The detection step furthermore comprises in such a method the detection of a third signal at a third lead coupled with the third electrode. If necessary, further signals are detected at further leads which are coupled with the further electrodes. These further leads are capacitively coupled with the third electrode, for example. The capacity of the first, the second or the third electrode is determined by evaluation of an appropriate calculation formula, this calculation formula which is to be respectively determined individually for the first, the second or the third electrode taking the first, the second and the third signal and, if necessary, the further signals at the further leads into account. The calculation formula(s) furthermore incorporate(s) a capacity between the second electrode and the lead of the first electrode, a capacity between the third lead and the first electrode, a capacity between the third lead and the second electrode, and, if necessary, a capacity between the third lead and the further electrodes coupled with this lead. Complex sensor fields can also be reliably read out using such a method.

To determine the calculation formulas mentioned, a linear system of equations can advantageously be formulated which can be solved analytically or numerically and the result of which shows the corresponding calculation formulas for calculating the first, the second and, if necessary, the third capacity. A first, a second and, if necessary, a third equation are present in this linear system of equations for the first, the second and, if necessary, for the third signal. The first up to the third equation is formulated by analyzing the corresponding capacity network of the capacitive sensor. The capacities along a path defined by the corresponding first, second or, if necessary, third lead from the first, second and, if necessary, third electrode up to the output thereof in a corresponding connection area of the sensor field are taken into account here. The capacities appearing in this path are initially the first, the second and, if necessary, the third capacity of the first, the second or, if necessary, of the third electrode. Furthermore, the capacities between the first, the second and, if necessary, the third lead and the electrodes coupled with these leads are taken into account.

According to a further aspect of the invention, a capacitive sensor is specified, which has an evaluation unit and a sensor field comprising a plurality of discrete electrodes. Each of these discrete electrodes is respectively coupled to a discrete lead which extends from the respective electrode up into a connection area. The respective lead is preferably directly electrically connected to the corresponding electrode. The lead is for example directly printed onto the electrode, and the electrode is in this way brought into contact with the lead. The capacity of the respective electrode can be read out through the corresponding lead. The capacitive sensor field comprises at least one first electrode the lead of which is guided such that this lead is capacitively coupled with at least one second electrode. The evaluation unit is designed to detect a first signal at a first lead coupled with the first electrode, and a second signal at a second lead coupled with the second electrode. The capacity of the first electrode or of the second electrode is determined using a calculation formula which is predetermined and preferably stored in the evaluation unit. This calculation formula takes the first signal, the second signal and the capacitive coupling between the second electrode and the lead of the first electrode into account. This calculation formula preferably takes a capacity between the second electrode and the lead of the first electrode into account. As already mentioned, this capacity can be determined both empirically and theoretically on the basis of the geometry and materials used.

Advantageously, the sensor field of the capacitive sensor can furthermore have a plurality of outer electrodes adjoining a connection area of the capacitive sensor field. These outer electrodes are preferably arranged in at least one portion of the periphery of the sensor field. These outer electrodes separate the connection area from a plurality of inner electrodes. The leads of the inner electrodes are guided such that they are capacitively coupled with at least one of the outer electrodes and reach to the connection area preferably arranged at the edge of the sensor field. According to such an embodiment of the capacitive sensor, an inner electrode is a first electrode, and an outer electrode is a second electrode.

The leads of the inner electrodes can furthermore be guided at least across a partial area of a surface of the outer electrodes, the leads of the inner electrode being advantageously separated from the outer electrode by an electric insulating layer. With regard to the flexibility and the costs of the capacitive sensor, it is advantageous if the electrodes, the leads and/or the insulating layer are layers manufactured in a printing process.

According to a further aspect of the invention, a method of manufacturing a capacitive sensor as explained according to the aspects above of the invention is specified. In such a manufacturing method, the discrete electrodes, the electric insulating layer and the leads are applied one after another onto a flexible substrate using a printing process.

Further advantages of the capacitive sensor and of the method of manufacturing such a capacitive sensor according to aspects of the invention have already been mentioned in view of the method according to the invention and are thus not repeated.

DETAILED SPECIFICATION

FIG. 1shows a simplified perspective view of the front side of a capacitive sensor field2which comprises a plurality of discrete electrodes4(for clarity reasons, only some of the electrodes4are provided with reference numerals). The electrodes4are arranged on a preferably transparent substrate6which can be a sufficiently thin and thus flexile plastic film or a thin flexible circuit board, for example.

The electrodes4are discrete, i.e. in a plane in which the capacitive sensor field2extends, the electrodes4are spaced apart and electrically separated from each other. Each of the electrodes4is coupled to a lead8which is also discrete. This lead8is preferably partially printed onto the corresponding electrode4and is thus electrically connected or coupled therewith. The lead8reaches from the electrode4up into a connection area10of the capacitive sensor field2. The leads8to those electrodes4which do not directly adjoin the connection area10are guided across the surface of the electrodes4adjoining the connection area10and are furthermore electrically separated with respect to the corresponding electrodes4by an insulating layer12. The electrodes4, the leads8and the insulating layers12are preferably manufactured in a layered printing process using appropriate conductive or non-conductive paint.

The capacitive sensor field2which is preferably integrated into a control panel of a motor vehicle and is for example used to operate a radio or a navigation system is operated by contact with the finger14or a pin provided therefor. It is possible to realize a touch pad by combining such a capacitive sensor field2with a mask defining for example different control elements, or with a display lying on top. By contact or movement of the finger14or of the pin, respectively, predetermined functions can be carried out, for example for adjusting the volume of a radio.

FIG. 2shows the capacitive sensor field2already known fromFIG. 1from its rear side in a simplified and perspective cutout view. The capacitive sensor field2comprises outer electrodes42which directly adjoin the connection area10. Due to these outer electrodes42, the inner electrodes41arranged in an interior of the capacitive sensor field are separated from the connection area10located at the outer edge of the sensor field2. The leads8of the outer electrodes42are guided directly into the connection area10starting therefrom. In contrast thereto, the leads8of the inner electrodes41are guided across the outer electrodes42into the connection area10and are separated from the corresponding outer electrode42by an insulating layer12. Due to this layout of the routing of the leads8of the inner electrodes41, a capacitive coupling is generated between this lead8and the corresponding outer electrode42. The resulting capacity mainly depends on the dimensions of the overlap area between the lead8and the outer electrode42, the thickness of the insulating layer12, and the material of this insulating layer12(in particular the dielectric constant). This inductive coupling is taken into account when evaluating a first and a second signal Cm1and Cm2. The first signal Cm1is tapped at a first lead8which is connected to an inner electrode41also referred to as a first electrode. The second signal Cm2is tapped at a second lead8which is connected to an outer electrode42also referred to as a second electrode. A calculation formula taking this capacitive coupling into account and correcting the latter is determined by analyzing the capacitive network present between the leads8and the electrodes4.

FIG. 3shows a simplified equivalent circuit diagram of the capacitive network between the inner and outer electrodes41,42(referred to as first and second electrodes) and the leads8thereof. The first signal Cm1is applied to the first lead which is connected to the first and inner electrode41. The capacity of the first electrode41is referred to as Cf1and is generated when a finger14or a pin assumed to be grounded enters into interaction with this first electrode41. As the lead8to this first and inner electrode41is guided across the outer and second electrode42the capacity of which is referred to as Cf2(seeFIG. 2), a capacitive coupling is generated between this lead8and the outer electrode42. The capacity between the lead8and the second electrode42is referred to as Ck12in the equivalent circuit diagram ofFIG. 3.

The second signal Cm2is detected at a second lead connected to the second and inner electrode42. The capacity applied to the second and inner electrode42changes when the finger14or the pin assumed as grounded enters into interaction with this second and inner electrode42.

It can be taken from the equivalent circuit diagram shown inFIG. 3that the first signal Cm1detected at the first lead is determined as follows by the capacities shown in the equivalent circuit diagram:

Accordingly, the following dependence on the capacities shown in the equivalent circuit diagram ofFIG. 3is obtained for the second signal Cm2applied to the second lead:

The equations shown for the first and the second signal Cm1, Cm2form a linear system of equations which can be solved for this quantity to determine the capacity Cf1, Cf2of the first and the second electrode41,42, respectively. For the capacity Cf1of the first electrode41, the following calculation formula is obtained:

The capacity Cf1of the first electrode41can thus be determined on the basis of the first and the second signal Cm1, Cm2and on the basis of the capacity Ck12between the second lead8which is coupled with the second electrode42, and the first electrode41. Accordingly, the following calculation formula applies to the capacity Cf2of the second electrode42:

For a capacitive sensor field2in which the leads8of the inner electrodes41are merely guided across a number of electrodes (more specifically across the outer electrodes42), the analytical solution represented can be found for determining the corresponding capacities Cf1, Cf2of the electrodes41,42. In case a sensor field2however comprises electrodes the leads8of which are guided across a multitude of electrodes4, an analytical solution can usually no longer be found for determining the corresponding capacities of the electrodes4so that it is necessary to fall back on numerical methods.

In the following, an exemplary system of equations for a sensor field2is to be specified, in which a third electrode4is present the lead8of which is guided across a first and outer electrode41and across an adjoining second and inner electrode42. At first, an equivalent circuit diagram is obtained in this case which is shown inFIG. 4.

The signals Cm1, Cm2and Cm3are detected at a first up to a third lead. As the lead to the third electrode is guided across the first electrode (with the capacity Cf1) and across the second electrode (with the capacity Cf2), the further capacities Ck13and Ck23are obtained in addition to the already known capacity Ck12, which result from the capacitive coupling of the lead extending to the third electrode (with the capacity Cf3) with the first electrode (Ck13) and the second electrode (Ck23).

In the capacity network shown inFIG. 4, the following equations which form a linear system of equations can be formulated for the signals Cm1, Cm2and Cm3detected at the first up to the third lead:

By a numeric solution of this system of equations, the terms for the first up to the third capacity Cf1, Cf2and Cf3can in turn be found, which represent corresponding calculation formulas.

FIG. 5shows a simplified view of the signal propagation on the basis of the signals Cm1to Cm4detected at the leads, which are evaluated in accordance with a method according to aspects of the invention referred to as electrode crosstalk compensation inFIG. 5, so that the capacities Cf1to Cf4applied to the corresponding electrodes4can be calculated. These capacities Cf1to Cf4are evaluated in a further step referred to as electrode capacity evaluation to determine, e.g., the position of the finger14on the capacitive sensor field2and to carry out a corresponding function on the basis of the position signal or proximity signal output in this step.

FIG. 6shows a simplified view of a cutout of a center console16of a motor vehicle, which comprises a capacitive sensor20in addition to different control elements18. The capacitive sensor20comprises a capacitive sensor field2and an evaluation unit22connected thereto, which is preferably arranged in the interior of the center console16(or also at another place of the motor vehicle).