METHOD FOR PRODUCING A FOIL-BASED PRESSURE SENSOR

Abstract

A method for producing a foil-based pressure sensor includes the steps of: providing a bottom element with a bottom foil and at least one bottom electrode disposed on the bottom foil; providing a plurality of top elements, each top element having a top foil with at least one top electrode disposed under the top foil, a combined area of the top elements being smaller than an area of the bottom element; and individually positioning and at least indirectly connecting the top elements to the bottom element so that at least one top electrode of each top element is disposed over at least one bottom electrode of the bottom element to form a sensor cell that is adapted to be activated when a pressure on the sensor cell exceeds a turn-on point. The turn-on point is adjusted by selecting a position of a top element from a plurality of positions.

Claims

1. A method for producing a foil-based pressure sensor, comprising: providing a bottom element with a bottom foil and at least one bottom electrode disposed on the bottom foil, providing a plurality of top elements, each top element comprising a top foil with at least one top electrode disposed under the top foil, a combined area of the top elements being smaller than an area of the bottom element, individually positioning and at least indirectly connecting the top elements to the bottom element so that at least one top electrode of each top element is disposed over at least one bottom electrode of the bottom element to form a sensor cell that is adapted to be activated when a pressure on the sensor cell exceeds a turn-on point, wherein, for at least one sensor cell, the turn-on point is adjusted by selecting a position of a top element from a plurality of positions in which the sensor cell works, but which differ by their turn-on point.

2. A method according to claim 1, wherein the turn-on point of at least one sensor cell is adjusted by selecting one of a plurality of horizontal positions of a top element with respect to the bottom element, in which horizontal positions the sensor cell works, but which differ by their turn-on point.

3. A method according to claim 1, further comprising connecting at least one top element to the bottom element via at least one spacer element so that the spacer element is interposed between the top foil and the bottom foil.

4. A method according to claim 1, wherein at least one top element is provided with the spacer element being connected to the top foil before the top element is connected to the bottom element.

5. A method according to claim 1, wherein at least one spacer element comprises an adhesive material, which adhesive material is bonded to the bottom element to connect the top element to the bottom element.

6. A method according to claim 1, wherein the spacer element comprises an opening in which at least a portion of the at least one top electrode is disposed.

7. A method according to claim 1, wherein the bottom element comprises first alignment marks and the first alignment marks are used to determine the position of a top element with respect to the bottom element.

8. A method according to claim 7, wherein at least one top element comprises second alignment marks and the first and second alignment marks are used to determine the position of the top element with respect to the bottom element.

9. A method according to claim 1, wherein at least one top element comprises a vertically extending connector element in electrical contact with at least one top electrode, which connector element is brought into contact with at least one bottom electrode by connecting the top element to the bottom element, whereby a permanent electrical connection between the top electrode and the bottom electrode is established.

10. A method according to claim 1, further comprising, prior to positioning and connecting a top element to the bottom element, selecting one of a plurality of possible orientations about a vertical axis of the top element with respect to the bottom element.

11. A method according to claim 1, wherein the turn-on point is adjusted by selecting one of a first position of a top element, in which at least one top support structure extending downwards from the top foil in the vicinity of a top electrode is disposed vertically opposite at least one bottom support structure extending upwards from the bottom foil in the vicinity of a bottom electrode and a second position, in which the at least one top support structure and the at least one bottom support structure are horizontally offset to each other.

12. A method according to claim 11, wherein the first and second position correspond to different orientations about the vertical axis.

13. A method according to claim 1, wherein the turn-on point is adjusted by selecting for a given sensor cell one of a plurality of top elements having different properties.

14. A method according to claim 13, wherein at least two top elements have top foils with different flexibility.

15. A foil-based pressure sensor, comprising: a bottom element with a bottom foil and at least one bottom electrode disposed on the bottom foil, and a plurality of top elements, each top element comprising a top foil with at least one top electrode disposed under the top foil, a combined area of the top elements being smaller than an area of the bottom element wherein the top elements are disposed above the bottom element and at least indirectly connected to the bottom element so that at least one top electrode of each top element is disposed over at least one bottom electrode of the bottom element to form a sensor cell that is adapted to be activated when a pressure on the sensor cell exceeds a turn-on point, and, for at least one sensor cell, a position of a top element is selected from a plurality of positions in which the sensor cell works, but which differ by their turn-on point.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawing, wherein:

[0031] FIG. 1 is a perspective view of a first embodiment of an inventive pressure sensor prior to assembly;

[0032] FIG. 2 is a top view of a portion of the pressure sensor from FIG. 1;

[0033] FIG. 3 is a sectional view along the line III-III in FIG. 2;

[0034] FIG. 4 is respective view of a second embodiment of an inventive pressure sensor;

[0035] FIG. 5 is a sectional view of along the line of V-V in FIG. 4;

[0036] FIG. 6 is top view of a third embodiment of an inventive pressure sensor; and

[0037] FIG. 7 is a sectional view according to the line VII-VII in FIG. 6.

DETAILED DESCRIPTION

[0038] FIGS. 1-3 schematically show a first embodiment of a foil-based pressure sensor 1, which may be used for occupancy detection in a vehicle seat. The sensor 1 comprises a bottom element 2 with a bottom foil 3 that extends in a horizontal plane defined by a first horizontal axis X and a second horizontal axis Y. A vertical axis Z may correspond to the direction of gravity when the sensor 1 is installed in the vehicle seat, but the sensor 1 could also be aligned differently. The bottom foil 3 may e.g. be made of flexible plastic material, silicone or rubber. The bottom element 2 comprises two terminals 10 that are connected to a first conductor path 5 and a second conductor path 7, respectively. The first conductor path 5 connects two first bottom electrodes 4, while the second conductor path 7 connects two second bottom electrodes 6. A third conductor path 9 connects four third bottom electrodes 8. All bottom electrodes 4, 6, 8 are disposed on an upper side of the bottom foil 3 and may e.g. be made of conductive ink printed onto the bottom foil 3 or of metal foil laminated onto the bottom foil 3. Each third bottom electrode 8 is disposed in the vicinity of a first bottom electrode 4 or a second bottom electrode 6, respectively. In this embodiment, the overall shape of the bottom element 2 corresponds to a fork or a letter “Y”, but this is just by way of example.

[0039] The sensor 1 further comprises four top elements 20, each of which comprises a top foil 21 that may be made of the same material as the bottom foil 3. A top electrode 22 is disposed on an underside of the top foil 21. Like the bottom electrodes 4, 6, 8, the top electrode 22 may e.g. be made of conductive ink or metal foil. The top foil 21 of each top element 20 has a rectangular shape. On an underside of the top foil 21, each top element 20 comprises a spacer element 23. The spacer element 23 may also be made of flexible plastic, silicone or rubber foil, but normally has a greater thickness along the vertical direction Z than the bottom foil 3 and the top foil 20. The outer dimensions of the spacer element 23 correspond to those of the top foil 21. Each spacer element 23 has a rectangular cutout or opening 24, inside which the respective top electrode 22 is disposed. The top elements 20 are pre-manufactured before they are assembled with the bottom element 2. Each spacer element 23 may comprise a double-sided adhesive lining so that during the manufacturing process of the top elements 20, the spacer element 23 is laminated or bonded to the top foil 20.

[0040] To complete the production of the pressure sensor 1, each pre-manufactured top element 20 is positioned on the bottom element 2 and connected thereto by a bonding process using the adhesive lining of the respective spacer element 23. Since the top elements 20 are separate from each other, each of them can be positioned individually, which enables a high degree of precision. To facilitate this precise positioning, the bottom element 2 comprises a plurality of first alignment marks 11 and each top element 20 has corresponding second alignment marks 26. The respective first and second alignment marks 11, 26 are optical marks that are printed on the respective foil 3, 21. The top foil 21 and the spacer element 23 can be transparent or translucent so that the first alignment marks 11 are visible through the top elements 20. By aligning the first and second alignment marks 11, 26 the horizontal position of the respective top element 20 with respect to the bottom element 2 can be adjusted accurately.

[0041] When assembled, each top element 20 together with the bottom element 2 form a sensor cell 30, one of which is shown in FIGS. 2 and 3. The top electrode 22 is disposed over both the first bottom electrode 4 and the third bottom electrode 8. Due to the presence of the spacer element 23, the top electrode 22 is vertically spaced apart from either bottom electrode 4, 8 when no pressure is acting on the sensor cell 30, i.e. when the sensor 1 is in unloaded state. This changes, however, when an external pressure p.sub.ext exceeds a turn-on point as shown in FIG. 3. The upper part of FIG. 3 shows the top element 20 in a first horizontal position Al with respect to the bottom element 2, in which the top electrode 22 is disposed symmetrically with respect to the bottom electrodes 4, 8. By elastic deformation of the top foil 21, the top electrode 22 is brought into contact with the bottom electrodes 4, 8, thereby establishing an electrical contact so that a current I can flow between the first and third bottom electrode 4, 8. By exceeding the turn-on point, the sensor cell 30 is activated. The lower part of FIG. 3 shows the top element 20 in a second horizontal position A2 with respect to the bottom element 2, in which the top electrode 22 is disposed non-symmetrically with respect to the bottom electrodes 4, 8. The first and second positions A1, A2 differ by a horizontal offset s along the first horizontal axis X. Although the pressure p.sub.ext and the elastic deformation of the top foil 21 are the same as in the upper part of FIG. 3, the top electrode 20 only makes contact with the third bottom electrodes 8. Since there is no electrical contact between the top electrode 20 and the first bottom electrode 4, the sensor cell 30 is not activated. This is only possible by exceeding a significantly higher turn-on point.

[0042] Since the turn-on point can depend on the horizontal position of the top element 20 with respect to the bottom element 2, individual positioning of the top elements 20 allows to accurately determine the turn-on points of their respective sensor cells 30. For example, if the first and second alignment marks 11, 2611, 26 are brought into congruence, this corresponds to a symmetric position of the top electrode 22 with a turn-on point that can be determined in advance by experiments. However, if the first and second alignment marks are horizontally offset from each other, as shown in FIG. 2, this corresponds to a non-symmetric position of the top electrode 22 with a different turn-on point that can also be determined experimentally in advance.

[0043] Apart from allowing for an individual positioning of the top elements 20 and an accurate determination of the turn-on point, it is understood that the inventive concept with small, separate top elements 20 leads to a significantly reduced material usage since the top foil 21 and the spacer element 23 are only needed for the area of the top elements 20, which is significantly smaller than the area of the bottom element 2.

[0044] FIGS. 4 and 5 show a second embodiment of a sensor 1 (or rather a portion thereof). In this embodiment, the top electrode 22 extends horizontally beyond the opening 24 in the spacer element 23 and is electrically connected to a connector element 27 that is also a part of the top element 20. The connector element 27 extends vertically downwards from the top electrode 22 and its vertical thickness is chosen so that in assembled state, a permanent electrical connection is established between the top electrode 22 and a first bottom electrode 4, as can be seen in FIG. 5. In the unloaded state, which is shown in FIGS. 4 and 5, the top electrode 22 is disposed vertically spaced apart from a second bottom electrode 6. When a pressure p.sub.ext acts on the sensor cell 30, the top foil 21 is elastically deformed and when the pressure p.sub.ext exceeds a turn-on point, an electrical contact is established between the top electrode 22 and the second bottom electrode 6.

[0045] FIGS. 6 and 7 show a third embodiment of a sensor 1 with a sensor cell 30 that is similar to the one shown in FIGS. 2 and 3. However, the top element 20 comprises six top support structures 28 extending downwards from the top foil 21 and the bottom element 2 comprises six corresponding bottom support structures 12. On the one hand, the presence of the top support structures 28 influences the deformed and of the top foil 21, but this effect is normally small if the total area of the top support structures 28 is much smaller than the area of the opening 24. The right side of FIGS. 6 and 7 shows a first orientation B1, each top support structure 28 is disposed vertically opposite a corresponding bottom support structure 12. Therefore, when the top foil 22 is elastically deformed, the top support structures 28 abut the bottom support structures 12 which leads to a significant increase of the stiffness of the sensor cell 30. At this point, the top electrode 22 is still out of contact with the bottom electrodes 4, 6, i.e. the sensor cell 30 is not activated. This is only possible by a significant increase of the pressure p.sub.ext.

[0046] The left side of the FIG. 6 shows a second orientation B2 of the top element 20 about the vertical direction Z, which differs from the first orientation B1 by a rotation of 180° about the vertical axis Z. In this orientation, all top support structures 28 are horizontally offset with respect to the bottom support structures 12. However, when at least one top support structure 28 abuts the bottom element 2 and/or at least one bottom support structure 12 abuts the top element 20, further deformation of the top foil 21 is only possible with significantly increased pressure p.sub.ext. However, this occurs at a significantly greater deformation than in the first orientation B1. By properly adjusting the thickness of the top electrode 22, the bottom electrodes 4, 8 and the top support structure 28, it is possible that the sensor cell 30 is activated before the top support structure 28 gets into contact with the bottom element 2. In other words, the first orientation B1 corresponds to a significantly higher turn-on point than the second orientation B2.

[0047] It should be noted that in all shown embodiments, the turn-on point can also be influenced by other parameters. For example, different top elements 20 with different properties could be available for each sensor cell 30. In the production process, one of these top elements 20 can be chosen, thereby influencing of the turn-on point of the sensor cell 30. For example, the top elements 20 could have openings 24 with different shapes and/or sizes. Also, they could have top foils 21 made of different materials or having different thicknesses.