Abstract:
A capacitive pressure includes a laminated arrangement with a first flexible, electrically insulating carrier film carrying a first capacitor electrode, a second flexible, electrically insulating carrier film carrying a second capacitor electrode and a flexible, electrically insulating spacer film sandwiched between the first and second carrier films, where the spacer film has a through-hole or recess therein, with respect to which the first and second capacitor electrodes are arranged opposite one another, in such a way that the first and second electrodes are brought closer together by resilient bending of the first and/or second carrier film into the through-hole or recess under the action of a compressive force acting on the pressure sensor.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a capacitive pressure sensor, e.g. for use as an input device for human-appliance interaction (touchpad, keypad, slider, pressure sensing mat, etc.). 
       BRIEF DESCRIPTION OF RELATED ART 
       [0002]    Capacitive pressure sensors as such are well known in the art. Such a sensor generally comprises a capacitor, whose capacitance varies as a function of pressure. It is, for instance, known to built a capacitive switch, comprising a first capacitor electrode made of bulk metal and a second capacitor electrode also made of bulk metal, arranged at a certain distance from the first capacitor electrode by an insulating foam spacer. As the first and second electrodes are brought closer together under the action of a compressive force acting on the pressure switch, the capacitance of the capacitor increases. An evaluation circuit detects this increase of capacitance. If the capacitance exceeds a certain predefined threshold, the evaluation circuit triggers some action associated with the capacitive switch. Such capacitive switches are, for instance, used in computer mouse buttons. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    The present invention provides a capacitive pressure sensor, which is robust and can be manufactured at low costs. 
         [0004]    The capacitive pressure sensor comprises a laminated arrangement with a first flexible, electrically insulating carrier film carrying a first capacitor electrode, a second flexible, electrically insulating carrier film carrying a second capacitor electrode and a flexible, electrically insulating spacer film sandwiched between the first and second carrier films. The spacer film has a through-hole or recess therein, with respect to which the first and second capacitor electrodes are arranged opposite one another, in such a way that the first and second electrodes are brought closer together by resilient bending of the first and/or second carrier film into the through-hole or recess under the action of a compressive force acting on the pressure sensor. The capacitive pressure sensor is advantageously configured and arranged so that a short-circuit between the first and second capacitor electrodes is prevented even for relatively high pressure. This is the case, for instance, if at least one of the first and second capacitor electrodes is arranged on the surface of the respective carrier film that faces away from the spacer film. In this configuration, the carrier layer itself prevents contact between the electrodes. In another suitable configuration, the spacer film does not have a through-hole therein but a recess, whose depth is inferior to the thickness of the spacer film. If the spacer film has a through-hole therein, if the first capacitor electrode is arranged on the surface of the first carrier film that faces the spacer film and if the second capacitor electrode is arranged on the surface of the second carrier film that faces the spacer film, a short-circuit may be avoided by a dedicated electrically insulating layer arranged on at least one of the first and second capacitor electrodes. 
         [0005]    An advantage of a laminated capacitive pressure sensor as recited above is that it can be produced with low thickness, e.g. in the range from 0.1 to 1 mm, more preferably in the range from 0.2 to 5 mm. Typically, the carrier films and the spacer film have a thickness ranging from 25 μm to some hundreds of μm. The reduced thickness of such laminated capacitive pressure sensor makes it interesting for a broad range of applications, e.g. in pressure-sensing mats for detecting and/or classifying a passenger on a vehicle seat, in keypads or touchpads for electronic appliances (mobile phone, personal digital assistant, handheld game console, computer, and so forth). 
         [0006]    According to a preferred embodiment of the invention, the first and or the second carrier film and/or the spacer film comprises one or more layers made of thermoplastic polymer material, such as e.g. PET, PEN, PI, PEEK, PES, PPS, PSU and mixtures thereof. Combining different materials allows one to tailor the flexibility, shear and tear resistance, and to improve sensor reliability. The electrodes are preferably conductive polymer thick film electrodes, formed by printing of conductive ink onto the first and/or the second carrier film. Preferably, the flexible spacer film is configured as a double-sided adhesive. 
         [0007]    Most preferably, the gap between the first and second capacitor electrodes (i.e. the opening or recess) does not comprise a foam material arranged therein but is only filled with gas. Conveniently, this gas is air; nevertheless, other gases (e.g. N 2 , Ar, CO 2  or mixtures thereof) are also suitable. 
         [0008]    Advantageously, the capacitive pressure sensor comprises an evaluation circuit operatively connected to the first and second capacitor electrodes and configured for determining a quantity indicative of capacitance (and thus of the pressure) between the first and second capacitor electrodes. Preferably, the evaluation circuit is configured for operating in two modes of operation: in the first mode of operation, the evaluation circuit determines a quantity indicative of capacitance between the first capacitor electrode and ground and, in the second mode of operation, the evaluation circuit determines a quantity indicative of capacitance between the first and second capacitor electrodes. Those skilled will appreciate that such a capacitive pressure sensor combines proximity sensing (in the first mode of operation) with pressure sensing (in the second mode of operation) 
         [0009]    As will be appreciated, the invention is not limited to a capacitive pressure sensor comprising a single pair of capacitor electrodes, which is of course the simplest embodiment. The first carrier film could carry, for instance, a plurality of first capacitor electrodes, each one of the first capacitor electrodes being arranged opposite a common second capacitor electrode. Alternatively, both the first and the second carrier films could carry a plurality of capacitor electrodes, each one of the capacitor electrodes on the first carrier film being arranged opposite a respective one of the capacitor electrodes on the second carrier film. Other variants for arranging first and second capacitor electrodes (e.g. first and second capacitor electrodes offset with respect to one another; first electrodes arranged in groups, wherein the members of a group are arranged opposite a common second electrode; etc.) are deemed within the reach of those normally skilled in the art. 
         [0010]    As will be apparent to those skilled in the art, a capacitive pressure sensor as generally described hereinbefore can be manufactured by applying the first capacitor electrode onto the first flexible carrier film and the second capacitor electrode onto the second flexible carrier film, providing a flexible spacer film with an opening or recess; and laminating together the first first flexible carrier film carrying the first capacitor electrode, the spacer film and the second flexible carrier film carrying the second capacitor electrode in such a way that the first and second capacitor electrodes are arranged opposite one another with respect to the opening or recess. 
         [0011]    As shall be appreciated, the carrier films, the spacer the electrodes, as well as any other layers or components of the capacitive pressure sensor according to the present invention may be made of transparent, semi-transparent or translucent material, in such a way that the input device may be back-illuminated and/or positioned on top of a display screen. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings, wherein: 
           [0013]      FIG. 1  is a schematic cross-sectional view of a laminated capacitive proximity and pressure sensor, connected to an evaluation circuit; 
           [0014]      FIG. 2  is a cross-sectional view of a variant of the capacitive proximity and pressure sensor shown in  FIG. 1 ; 
           [0015]      FIG. 3  is an illustration of different examples of electrically insulating patterns; 
           [0016]      FIG. 4  is a schematic cross-sectional view of a laminated pressure sensor carried out as a capacitive touchpad; 
           [0017]      FIG. 5  is a schematic cross-sectional view of a variant of the capacitive touchpad of  FIG. 4 ; 
           [0018]      FIG. 6  is a schematic cross-sectional view of a laminated capacitive touchpad according to another embodiment; 
           [0019]      FIG. 7  is a schematic cross-sectional view of a variant of the touchpad represented in  FIG. 6 ; 
           [0020]      FIGS. 8   a - 8   c  are illustrations of examples of linear layouts for the first capacitor electrodes; 
           [0021]      FIGS. 9   a - 9   d  are illustrations of examples of circular layouts for the first capacitor electrodes; 
           [0022]      FIGS. 10   a - 10   c  are illustrations of examples of layouts for the first and second capacitor electrodes for detecting position or movement in 2 dimensions. 
       
    
    
       [0023]    It should be noted that the drawings are not to scale. In particular, no scale should be derived from the human finger depicted in certain of the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIG. 1  shows a first example of a laminated capacitive proximity and pressure sensor  10 . The device comprises first and second carrier films  12 ,  14 , made of substantially flexible, electrically insulating material, such as e.g. PET, PEN, PI or the like. A double-sided adhesive layer  16  is sandwiched as a spacer film between the first and second carrier films  12 ,  14  so as to keep these apart from one another. The double-sided adhesive layer  16  is provided with an opening  18  therein, which delimits an active zone of the proximity and pressure sensor  10 . In the active zone, the first carrier foil  12  carries a first capacitor electrode  20  on the side directed towards the second carrier film  14 , while the second carrier film  14  carries a second capacitor electrode  22  on the side directed towards the first carrier film  12 . The first and second capacitor electrodes  20 ,  22  are formed from conductive material (e.g. silver ink) applied directly on the first and second carrier films  12 ,  14 , respectively. The second capacitor electrode has a layer  24  of electrically insulating material (dielectric, e.g. PET, PEN, PI, etc.) formed thereon. 
         [0025]    The right-hand side of  FIG. 1  shows an evaluation circuit  26  connected to the first and second capacitor electrodes  20 ,  22  by leads  28 ,  30 . The evaluation circuit  26  comprises a microprocessor, an application-specific integrated circuit (ASIC) or a programmable chip, configured so as to operate in at least a first and a second mode of operation. 
         [0026]    The evaluation circuit  26  determines, while in the first mode of operation, a quantity indicative of a capacitance between the first capacitor electrode  20  and ground and, while in the second mode of operation, a quantity indicative of a capacitance between the first capacitor electrode  20  and the second capacitor electrode  22 . The evaluation circuit  26  may operate in the first mode of operation before and/or after operating in the second mode of operation. The evaluation circuit  26  may cyclically switch between the modes of operation, e.g. several times per second. Preferably, however, the evaluation circuit  26  remains in the proximity-sensing mode (first mode) until the proximity of a body having an electric-field-changing property is detected. Alternatively, the evaluation circuit  26  could remain in the pressure-sensing mode (second mode) until a force or pressure exceeding a predefined threshold has been detected. It shall be noted that the recited “quantity indicative of a capacitance” can be any physical quantity that is linked to the capacitance by the laws of physics, such as, for instance, amplitude and/or phase of a current, amplitude and/or phase of a voltage, charge, impedance, and so forth. 
         [0027]    The first mode of operation is associated to sensing an object having an electric-field-influencing property in the vicinity of the first capacitor electrode  20 , e.g. a user&#39;s finger  32 , a conductive stylus, or the like. In the first mode of operation, the evaluation circuit  26  keeps the first and second capacitor electrodes  20 ,  22  essentially at the same electric potential, so that the electric field substantially cancels between the first and second electrodes  20 ,  22 . The second electrode  22  thus acts as a driven shield for the first electrode  20  and the sensitivity of the latter is directed away from the second electrode  22 . If an oscillating voltage is applied to the first capacitor electrode  20 , an oscillating electric field to ground is built up. The object to be sensed modifies the capacitance between the first capacitor electrode  20  and ground, which is sensed by the evaluation circuit  26 . It should be noted that in the first mode of operation detecting the proximity of the object to be sensed does not require the object touching or being in contact with the proximity and pressure sensor  10 . 
         [0028]    The second mode of operation is associated with sensing pressure exerted on the sensor  10  by some kind of actuator, such as e.g. the user&#39;s finger  32  or stylus (in order to detect the amount of pressure exerted upon the active zone of the sensor  10 ). In the second mode of operation, the evaluation circuit  26  essentially determines the capacitance of the capacitor formed by the first and the second capacitor electrodes  20 ,  22 . It is well known that the capacitance of a capacitor depends upon the distance between its electrodes. In the illustrated case, the distance between the first and second capacitor electrodes  20 ,  22  decreases with increasing pressure exerted upon the pressure sensor  10 . As a consequence, the capacitance between the capacitor electrodes increases, which is detected by the evaluation circuit  26 . 
         [0029]      FIG. 2  shows a variant of the proximity and pressure sensor of  FIG. 1 . The construction is the same, except that the first capacitor electrode  20 , like the second capacitor electrode  22 , has formed thereon a layer  24  of electrically insulating material. Those skilled will appreciate that patterning one of the electrically insulating layers  24  allows tailoring the response of the proximity and pressure sensor  10  in the second mode of operation. As long as the electrically insulating layers  24  are spaced from one another (i.e. for low pressures exerted by the user) the pattern has no significant influence on sensor response. However, as the pressure increases the electrically insulating layers  24  come into contact and a contact surface forms. Patterning the insulating layer  24  thus results in that the minimum distance between the first and second electrodes  20 ,  24  is not constant on the contact surface. Accordingly, the capacitance increase is different from the case where the insulating layers  24  are both of uniform thickness. Examples of patterned insulating layers  24  are shown in  FIG. 3 . 
         [0030]      FIGS. 4 to 6  show various examples of a capacitive pressure sensor  10  carried out as a touchpad. The touchpad  10  of  FIG. 4  comprises a laminated structure of a first carrier film  12 , a second carrier film  14 , a spacer  16 , sandwiched between the first and second carrier films  12 ,  14  so as to keep them spaced apart, and a protective thermoplastic film  34 . The spacer  16  has a matrix-like arrangement of openings  18  therein, which define keys of the touchpad  10 . To each key is associated a pair of a first capacitor electrode  20  and a second capacitor electrode  22  arranged on the first and second carrier films  12 ,  14 , respectively. Each first capacitor electrode  20  is arranged opposite its second-capacitor-electrode counterpart  22 , with respect to the associated opening  18  of the spacer  16 . The first capacitor electrodes  20  are arranged on the side of the first carrier film that faces the spacer film  16  and the second carrier film  14 . The second capacitor electrodes  22 , however, are arranged on the side of the second carrier film that faces away from the spacer film  16  and the first carrier film  12 . The protective thermoplastic film  34  is laminated onto that same side of the second carrier film, so to prevent contamination of the second capacitor electrodes. In the embodiment of  FIG. 4 , a short-circuit between any one of the first capacitor electrodes and the corresponding second capacitor electrode is effectively prevented due to the presence of the insulating second carrier film  14  between the first and second capacitor electrodes. 
         [0031]    In the touchpad  10  of  FIG. 5 , the first and second capacitor electrodes  20 ,  22  are arranged on the interior sides of the first and second carrier films  12 ,  14 , respectively. Instead of openings carried out as through-holes as in  FIGS. 1 ,  2  and  4 , the spacer  16  of  FIG. 5  has a plurality of recesses  19  therein, whose depth is inferior to the thickness of the spacer. As a result, the second capacitor electrodes  22  are separated from the first capacitor electrodes not only by gas-filled gaps but also by those portions of the spacer film  16  that define the bottom of recesses  19 . 
         [0032]      FIG. 6  shows a touchpad  10 , in which the comprises a laminated arrangement of a first carrier film  12 , a second carrier film  14  and a spacer film  16 , sandwiched between the first and second carrier films  12 ,  14  so as to keep these spaced apart. The spacer  16  has openings  18  therein, which define the active zones (“keys”) of the touchpad  10 . To each key is associated a first capacitor electrode  20  arranged on the first carrier film  12 . A common second capacitor electrode  22  extends over all the keys of the touchpad  10 . The touchpad  10 . To prevent short-circuits each one of the first capacitor electrodes is covered with a thin electrically insulting layer  24 . 
         [0033]      FIG. 7  shows a variant of the touchpad of  FIG. 6 . In this variant, it is the common second capacitor electrode  22 , which is covered with a thin electrically insulating layer. Moreover, the touchpad  10  of  FIG. 7  has an opening  18  that defines a common active zone, in which at least some of the first capacitor electrodes  20  are arranged. The present variant is especially suitable for applications in which a user presses on the first and/or the second carrier film and performs a continuous sliding movement while maintaining the pressure. It should be noted that the first capacitor electrodes could be arranged along a line, a curve or in a grid-like configuration.  FIGS. 8   a - 8   c  and  9   a - 9   d  show several possible layouts of the first capacitor electrodes in top view. 
         [0034]    The touch pads of  FIGS. 4-7  are advantageously connected to an evaluation circuit (not shown), which determines, in a first mode of operation, a quantity indicative of capacitance between individual ones of the first capacitor electrodes  20  and ground and, in a second mode of operation, a quantity indicative of a capacitance between individual ones of the first capacitor electrodes  20  and the corresponding second capacitor electrode(s). 
         [0035]    In the first mode of operation, the position of a user&#39;s finger could, for instance be detected by determining, for each one of the first capacitor electrodes, the quantity indicative of capacitive coupling between this electrode and ground. The position may e.g. be computed as the centroid of the positions of the first capacitor electrodes, weighed with the corresponding quantity indicative of capacitance. The first mode of operation is suitable, for instance, when the user controls a cursor (e.g. on the display of an appliance). The second mode of operation is associated to actuation of a key of the touchpad, e.g. by a user&#39;s finger or a stylus. 
         [0036]    In  FIGS. 8   a - 8   c  the first capacitor electrodes are arranged along a straight line, whereas in  FIG. 9   a - 9   d,  they are arranged in a circle. In the arrangements of  FIGS. 8   a,    8   b,    9   a  and  9   b,  the first capacitor electrodes  20  are separately connectable to an evaluation circuit. Accordingly, it is possible to detect the position of the user&#39;s finger in both the first and second modes of operation. In the arrangements of  FIG. 8   c,    9   c  and  9   d,  the first capacitor electrodes are not separately connected to the control circuit. Instead, there are three groups of first capacitor electrodes  20 . The first capacitor electrodes  20  of each group are conductively interconnected. Along the active zone, a first capacitor electrode of the first group is followed by one of the second group, which is, in turn, followed by one of the third group, after which the succession starts again with a first capacitor electrode of the first group. In these configurations, detection of the (absolute) position of a user&#39;s finger or stylus is not possible. Nevertheless, such slider can detect a movement of the user&#39;s finger or stylus (in both modes of operation). When the user&#39;s finger or stylus moves from the left to the right in  FIG. 8   c  or in the clockwise sense in  FIGS. 9   c  and  9   d,  the succession of the groups of first capacitor electrodes that have increased capacitive coupling to ground or to the second capacitor electrode is 2-3-1 (and cyclically continued). When the user&#39;s finger moves from the right to the left in  FIG. 8   c  or in the clockwise sense in  FIGS. 9   c  and  9   d,  the succession of the groups of first capacitor electrodes that have increased capacitive coupling to ground or to the second capacitor electrode is 3-2-1 (and cyclically continued). Given the reduced number of external connectors, the configurations of  FIGS. 8   c,    9   c  and  9   d  is particularly interesting if the absolute position does not need to be known, e.g. for navigating through list-based menus (scrolling through a list of items displayed and selecting an item to enter a sub-menu or start a certain function). The action of selecting an item from the list can e.g. take place when the user presses on the slider with a force that causes the quantity indicative of capacitance between the first and second capacitor electrodes to exceed the predetermined threshold. 
         [0037]      FIGS. 10   a - 10   c  schematically show possible layouts for the first and second capacitor electrodes for detecting position or movement in 2 dimensions. 
         [0038]    In  FIGS. 10   b  and  10   c,  the electrodes  20 ,  22  are configured as elongated conductive strips arranged in parallel. The first capacitor electrodes  20  extend crosswise to the second capacitor electrodes  22  so as to form a grid-like configuration. 
         [0039]    In  FIG. 10   a,  the electrodes are configured as individual discs disposed in rows and columns; to each first capacitor electrode  20  is associated, in facing relationship with respect to the spacer. The first capacitor electrodes are conductively interconnected along the columns and the second capacitor electrodes are conductively interconnected along the rows. 
         [0040]    In  FIGS. 10   a  and  10   b,  each line or column is separately connectable to a control circuit. Accordingly, it is possible to detect the position of the user&#39;s finger or stylus compressing locally pressure sensor  10  by determining the amount of capacitive coupling between the rows and the columns. 
         [0041]    In  FIG. 10   c,  the rows and columns are not separately connectable to a control circuit. Instead, there are three groups of rows and three groups of columns. The electrodes of each group are conductively interconnected. In direction along the columns, a row of the first group is followed by one of the second group, which is, in turn, followed by one of the third group, after which the succession starts again with a row of the first group. Similarly, in direction along the rows, a column of the first group is followed by one of the second group, which is, in turn, followed by one of the third group, after which the succession starts again with a column of the first group. A touchpad as shown in  FIG. 10   c  is not capable of detecting (absolute) position of the point of application of a force. Nevertheless, such touchpad can detect movement of the point of application of a force. The direction of the movement perpendicular to the rows can be determined from the succession of the groups of columns, which have increased capacitive coupling to the rows on the other carrier film. Likewise, the direction of the movement perpendicular to the columns can be determined from the succession of the groups of rows, which have increased capacitive coupling to the columns on the other carrier film.