Abstract:
A data input device is provided with a film-based pressure sensor build from a first carrier film, a second carrier film and a spacer arranged between the carrier films for keeping them at a distance from one another. The spacer has an opening delimiting an active zone, in which first and second electrodes are arranged in such a way that, in response to a compressive force acting on the active zone, an electrical contact is established between the first and second electrodes. A control circuit able to operate in at least a first and a second mode of operation is configured so as to measure, in the first mode of operation, a quantity indicative of electrical resistance between the first and second electrodes for detecting an amount or a position of a compressive force acting on the active zone and, in the second mode of operation, a quantity indicative of a capacitance for detecting a person or an object approaching thereto.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to data input devices and more particularly to data input devices including a film-based pressure sensor for human-device interaction. 
       BACKGROUND OF THE INVENTION 
       [0002]    Data input devices are commonly used in conjunction with electronic appliances to feed them with various kinds of data, including e.g. control data influencing directly or indirectly the behavior of the appliance, data that are processed by the appliance and/or simply stored. 
         [0003]    It is known to construct data input devices based upon film-type pressure sensors. A film-type pressure sensor comprises two carrier films, which are arranged at a certain distance from one another by means of a spacer. The spacer is provided with at least one opening that defines an active zone of the sensor, in which the two carrier films face one another. Inside this active zone, at least two electrodes are arranged in such a way on the carrier films, that electrical contact is established between the electrodes when the two carrier films are pressed together under the action of a compressive force acting on the sensor in the active zone. 
         [0004]    Depending on the application of such a pressure sensor, a layer of semiconducting material may be disposed between the electrodes, so that the sensor shows a gradual pressure sensitive behavior, that is to say its resistance varies gradually or even continuously as a function of the force applied. The layer of semiconducting material may comprise a material whose internal electrical resistance varies as a function of compression or of deformation of the layer or a material whose surface structure confers to the layer a surface resistance that is reduced following an increase in the number of points of contact with a conducting surface of an electrode, against which the layer of semiconducting material is pressed under the action of the compressive force. 
         [0005]    WO 2004/049364 relates to a data input device comprising several keys arranged in at least two rows. A unidirectional position detector of film-type construction is associated with each row of keys Each unidirectional position sensor enables the detection of the actuated key along the direction of the unidirectional position detector. The unidirectional position sensors are interconnected in such a way that a control circuit can detect in which row a key has been actuated. 
         [0006]    A different kind of sensors is based upon capacitive sensing. U.S. Pat. No. 3,896,425 discloses an electrical proximity detector that senses the changes in the contents of a defined sensitive volume. The detector comprises an antenna that is driven by an oscillator and that emits an electric field into the sensitive volume. A person or an object intruding into the sensitive volume causes a change of the electric field of the antenna, which is detected by the detector. To shape the electric field of the antenna the detector comprises a first shield, driven by the oscillator with a signal of same amplitude and phase as the signal of the antenna, and a second, grounded shield. 
         [0007]    Other sensors based on electric field or “capacitive” sensing have been proposed by J. Smith et al. in “Electric Field Sensing for Graphical Interfaces”, IEEE Computer Graphics and Applications, Issue May/June 1998, 54-60, as a human-computer interface. The interfaces are based upon an array of electrodes to detect the gestures of a user. 
         [0008]    The above-mentioned documents are herewith included herein by reference. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention provides an improved data input device. 
         [0010]    The invention further provides an improved method for operating a data input device. 
         [0011]    A data input device is provided with a film-based pressure sensor comprising a first carrier film, a second carrier film and a spacer arranged between the carrier films for keeping them at a distance from one another. The spacer has an opening delimiting an active zone, in which first and second electrodes are arranged in such a way that, in response to a compressive force acting on the active zone, an electrical contact is established between the first and second electrodes. A control circuit able to operate in at least a first and a second mode of operation is configured so as to measure, in the first mode of operation, a quantity indicative of electrical resistance between the first and second electrodes for detecting an amount or a position of a compressive force acting on the active zone and, in the second mode of operation, a quantity indicative of a capacitance for detecting a person or an object approaching thereto. It should be noted that the terms “first” and “second” are primarily used for denoting the modes of operation and that the terms should not be understood as indicating an order of the modes of operation in time. The control circuit may operate in the first mode of operation before or after operating in the second mode of operation. For instance, the control circuit could first operate in the second mode of operation and switch to the first mode of operation only if a person or object approaching the active zone has been detected. 
         [0012]    According to a first aspect of the invention the first carrier film has a first electrode applied thereon and the second carrier film has a second electrode applied thereon. The first and second electrodes face each other in the active zone in such a way that, in response to compressive force acting on the pressure sensor at the active zone, the first and second carrier films are pressed together and an electrical contact is established between the first and second electrodes. The control circuit is configured so as to measure, in the first mode of operation, a quantity indicative of electrical resistance between the first and second electrodes for detecting an amount of compressive force acting on the active zone and, in the second mode of operation, a quantity indicative of the capacitance of a capacitor formed by the first electrode and its surroundings, for detecting a person or an object approaching thereto. For sake of conciseness, we use the phrase “capacitance of the first electrode” instead of “capacitance of a capacitor formed by the first electrode and its surroundings”. The surroundings of the first electrode may for instance comprise a part of a user&#39;s body, such as their hand or finger. The quantity indicative of capacitance can be any physical quantity that is linked to the capacitance by the laws of physics, such as, for instance, amplitude of a current, amplitude of a voltage or impedance. For instance, amount of current that flows into the first electrode for a given voltage applied to the latter depends on and is therefore indicative of the capacitance the first electrode. 
         [0013]    The data input device according to the first aspect of the invention preferably comprises a first module for measuring the quantity indicative of electrical resistance, dedicated to the first mode of operation, and a second module for measuring the quantity indicative of a capacitance of the first electrode, dedicated to the second mode of operation. 
         [0014]    The first module can, for instance, comprise a current source operationally connected, in the first mode of operation, to the first and second electrodes so as to create a current through the first and second electrodes if an electrical contact is established between the first and second electrodes, and a voltage measurement circuit operationally connected, in the first mode of operation, to the first and second electrodes for measuring a voltage between them, the voltage being indicative of electrical resistance between the first and second electrodes. Alternatively or additionally, the first module can comprise a voltage source operationally connected, in the first mode of operation, to the first and second electrodes for creating a voltage between them and a current measurement circuit operationally connected, in the first mode of operation, for measuring a current flowing through the first and second electrodes, the current being in this case indicative of electrical resistance between the first and second electrodes. 
         [0015]    The second module can, for instance, include an AC voltage source operationally connected, in the second mode of operation, at least to the first electrode for applying an oscillating voltage to the first electrode and a current measurement circuit operationally connected, in the second mode of operation, for measuring an electrical current flowing into the first electrode, the electrical current flowing into the first electrode being indicative of a capacitance of the first electrode. Preferably, the AC voltage source is operationally connected, in the second mode of operation, to the first and second electrodes so as to apply voltages of substantially same phase and amplitude to the first and second electrodes. In this case, the potential difference between the first and the second electrodes remains close to zero, so that the second electrode shields the first electrode. 
         [0016]    The data input device according to the first aspect of the invention advantageously comprises a switching unit switching the control circuit between at least the first and second modes of operation. 
         [0017]    It will be appreciated that the first and/or the second electrode may comprise a pressure sensitive layer, e.g. a semiconducting layer arranged in facing relationship with the respective other one of the first and second electrodes in the active zone. The pressure sensitive layer may comprise a material whose internal electrical resistance varies as a function of compression or of deformation or a material whose surface structure confers to the layer a surface resistance that is reduced following an increase in the number of points of contact with a conducting surface of an electrode, against which the layer of semiconducting material is pressed under the action of a compressive force. 
         [0018]    Preferably, the control circuit is configured so as to output a first output signal responsive to the quantity indicative of electrical resistance and a second output signal responsive to the quantity indicative of a capacitance of the first electrode. The control circuit may comprise, for instance, a first and a second output and be configured so as to output the first output signal at the first output and the second output signal at the second output. 
         [0019]    The first and second electrodes are usually formed integrally but according to a variant of the invention, the first electrode is subdivided into at least two electrode portions. In this case, the at least two electrode portions may be electrically separated at least in the second mode of operation so that the quantity indicative of a capacitance of the first electrode can be determined separately for each one of the at least two electrode portions. 
         [0020]    According to a second aspect of the invention a method is proposed for operating a data input device as described above. Such a method comprises the measuring of a quantity indicative of electrical resistance between the first and second electrodes for detecting an amount of compressive force acting on the active zone and the measuring of a quantity indicative of a capacitance of the first electrode for detecting a person or an object approaching thereto. Preferably, the measuring of a quantity indicative of a capacitance of the first electrode includes applying an oscillating voltage to the first electrodes and measuring an electrical current flowing into the first electrode in response to the applied oscillating voltage. Most preferably, the measuring a quantity indicative of a capacitance of the first electrodes includes applying an oscillating voltage to the second electrode, the oscillating voltage applied to the second electrode having substantially same phase and amplitude as the oscillating voltage applied to the first electrode. 
         [0021]    A third aspect of the invention concerns a data input device with a film-based pressure sensor of slightly different configuration. According to the third aspect of the invention, the first carrier film has a first electrode and a second electrode applied thereon; the second carrier film has a third electrode applied thereon. The first and second electrodes face the third electrode in the active zone in such a way that, in response to a compressive force acting on the active zone, the first and second carrier films are pressed together and an electrical contact is established between the first and second electrodes via the third electrode. This data input device further comprises a control circuit able to operate in at least a first and a second mode of operation, the control circuit being configured so as to measure, in the first mode of operation, a quantity indicative of electrical resistance between the first and second electrodes for detecting an amount of compressive force acting on the active zone and, in the second mode of operation, a quantity indicative of a capacitance of at least one of the first, second and third electrodes for detecting a person or an object approaching thereto. The at least one of the first, second and third electrodes preferably designates, for instance, the first electrode, the first and second electrodes, or the third electrode. 
         [0022]    A data input device according to the third aspect of the invention may also comprise a first module for measuring the quantity indicative of electrical resistance, dedicated to the first mode of operation, and a second module for measuring the quantity indicative of a capacitance dedicated to the second mode of operation. 
         [0023]    The first module may comprise, for instance a current source operationally connected, in the first mode of operation, to the first and second electrodes so as to create a current through the first and second electrodes if an electrical contact is established between the first and second electrodes via the third electrode, and a voltage measurement circuit operationally connected, in the first mode of operation, to the first and second electrodes for measuring a voltage between them, the voltage being indicative of electrical resistance between the first and second electrodes. Alternatively or additionally, the first module may comprise a voltage source operationally connected, in the first mode of operation, at the first and second electrodes for creating a voltage between them and a current measurement circuit operationally connected, in the first mode of operation, for measuring a current flowing through the first and second electrodes, the current being indicative of electrical resistance between the first and second electrodes. 
         [0024]    The second module preferably includes an AC voltage source operationally connected, in the second mode of operation, at least to the at least one of the first, second and third electrodes for applying an oscillating voltage to the at least one of the first, second and third electrodes and a current measurement circuit operationally connected, in the second mode of operation, for measuring an electrical current flowing into the at least one of the first, second and third electrodes, the electrical current flowing into the at least one of the first, second and third electrodes being indicative of a capacitance of the at least one of the first, second and third electrodes. Most preferably, this AC voltage source is operationally connected to the first, second and third electrodes so as to apply voltages of substantially same phase and amplitude to the first, second and third electrodes, in the second mode of operation. 
         [0025]    The data input device according to the third aspect of the invention may further comprise a switching unit switching the control circuit between at least the first and second modes of operation. 
         [0026]    Advantageously, at least one of the first, second and third electrodes comprises a pressure sensitive layer arranged in facing relationship with a respective other one or respective other ones of the first, second and third electrodes in the active zone. 
         [0027]    The control circuit is preferably configured so as to output a first output signal responsive to the quantity indicative of electrical resistance and a second output signal responsive to the quantity indicative of a capacitance of the at least one of the first, second and third electrodes. Most preferably, the control circuit comprises at least a first and a second output, the control circuit being configured so as to output the first output signal at the first output and the second output signal at the second output. 
         [0028]    As in the data input device according to the first aspect, the electrodes in the data input device according to the third aspect of the invention are normally formed integrally. Nevertheless, the at least one of the first, second and third electrodes can be subdivided into at least two electrode portions that are electrically separated at least in the second mode of operation. In this case, the quantity indicative of the capacitance of the at least one of the first, second and third electrodes can be determined separately for each one of the at least two electrode portions. 
         [0029]    According to a fourth aspect of the invention it is proposed a method operating a data input device according to the third aspect of the invention. Such a method comprises the measuring of the quantity indicative of electrical resistance between the first and second electrodes for detecting an amount of compressive force acting on the active zone and the measuring the quantity indicative of a capacitance of the at least one of the first, second and third electrodes for detecting a person or an object approaching thereto. Preferably, the measuring of a quantity indicative of the capacitance of the at least one of the first, second and third electrodes includes applying an oscillating voltage to the at least one of the first, second and third electrodes and measuring an electrical current flowing into the at least one of the first, second and third electrodes in response to the applied oscillating voltage. Most preferably, the measuring of the quantity indicative of a capacitance of the at least one of the first, second and third electrodes includes applying an oscillating voltage to the other one or the other ones of the at least one of the first, second and third electrodes, the oscillating voltage applied to the other one or other ones of the at least one of the first, second and third electrodes having same phase and amplitude as the oscillating voltage applied to the at least one of the first, second and third electrodes. 
         [0030]    According to a fifth aspect of the invention a data input device is proposed that comprises a film-based position sensor. The position sensor comprises a first carrier film, a second carrier film and a spacer arranged between the first and second carrier films for keeping the first and second carrier films at a distance from one another, and comprising an opening delimiting an active zone of the position sensor. The first carrier film has a first electrode and a second electrode applied thereon; the second carrier film has a third electrode applied thereon. The first electrode includes a series of resistively connected first conductors. The third electrode faces the first electrode and the second electrode in the active zone in such a way that, in response to a compressive force acting on the active zone, the first and second carrier films are pressed together and an electrical contact is established between one or more of the first conductors and the second electrode via the third electrode. The data input device according to the fifth aspect of the invention further comprises a control circuit able to operate in at least a first and a second mode of operation, the control circuit being configured so as to measure, in the first mode of operation, a quantity indicative of a position of resistive coupling between the first electrode and the second electrode for detecting a position of a compressive force acting on the active zone and, in the second mode of operation, a quantity indicative of a position of capacitive coupling between the first electrode and the second electrode for detecting a position of a person or an object approaching to the data input device. 
         [0031]    According to a preferred embodiment of the data input device according to the fifth aspect of the invention, the second electrode comprises a series of conductively connected second conductors, the first and second conductors being arranged so as to interdigitate. 
         [0032]    To obtain the position of a compressive force or a capacitive coupling in two dimensions, the film-based position sensor may comprise a plurality of active zones, delimited by an opening or openings of the spacer. The first carrier film has a first electrode and a second electrode applied thereon in each one of the active zones and the second carrier film has a third electrode applied thereon in each one of the active zones. Each first electrode includes a series of resistively connected first conductors. Each third electrode faces its respective first and second electrodes in the respective active zone in such a way that, in response to a compressive force acting on an active zone, the first and second carrier films are pressed together and the third electrodes provides an electrical contact between one or more of the first conductors of the respective first electrode and the respective second electrode. The control circuit is in this case able to operate in at least a first and a second mode of operation and configured so as to measure, in the first mode of operation, a quantity indicative of a position in two dimensions of resistive coupling between said first electrode and said second electrode for detecting a position in two dimensions of a compressive force and, in the second mode of operation, a quantity indicative of a position of capacitive coupling between said first electrode and said second electrode for detecting a position of a person or an object approaching to said data input device. 
         [0033]    Data input devices referred to hereinabove include but are not limited to keyboards, keypads, touch screens, touch panels, computer mice, etc. In case of devices with keys, one or several keys may be associated to each active zone of the device. Detecting of the position (in one or in two dimensions) of the compressive force can in this case be considered equivalent to determining which key has been actuated. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    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 accompanying drawings, wherein: 
           [0035]      FIG. 1  is a schematic representation of a data input device according to the first aspect of the invention; 
           [0036]      FIG. 2  is a schematic representation of a data input device according to the third aspect of the invention; 
           [0037]      FIG. 3  is an equivalent circuit diagram of a module for measuring a quantity indicative of capacitance; 
           [0038]      FIG. 4  is an illustration of the capacitances intervening during the measurement of a quantity indicative of capacitance; 
           [0039]      FIG. 5  is an illustration of an example of a possible layout of the first and second electrodes in the device of  FIG. 2 ; 
           [0040]      FIG. 6  an illustration of another example of a possible layout of the first and second electrodes in the device of  FIG. 2 ; 
           [0041]      FIG. 7  is a schematic cross sectional view of a pressure sensor that can be used in a device according to the fifth aspect of the invention; 
           [0042]      FIG. 8  is a schematic illustration of a data input device that uses the pressure sensor of  FIG. 7 ; 
           [0043]      FIG. 9  is an equivalent circuit diagram of the device of  FIG. 8  during the determination of the position of capacitive coupling. 
           [0044]      FIGS. 10 and 11  are schematics of a circuit of a data input device with a 2D position sensor; 
           [0045]      FIGS. 12 and 13  are schematics of a circuit of another data input device with a 2D position sensor; 
           [0046]      FIG. 14  is a schematic illustration of the impedance of the first electrode varying as a function of the distance of an actuating member. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0047]      FIG. 1  shows a data input device according to the first aspect of the invention. The device  10  comprises pressure sensor  12  of film-type construction with an active zone  14 . The pressure sensor  12  comprises first and second carrier films  16 ,  18 , made of substantially flexible, electrically insulating material, such as e.g. PET, PEN, PI or the like. A spacer  20  is sandwiched between the first and second carrier films  16 ,  18 , thereby arranging the carrier films the distance corresponding to the spacer thickness from one another. The spacer can also be made of any substantially flexible, electrically insulating material. The spacer is provided with an opening that delimits the active zone  14  of the pressure sensor  10 . In the active zone  14 , the first carrier foil  16  carries a first electrode  22  on its inward-facing side, while the second carrier foil  18  carries a second electrode  24  on its inward-facing side. The first electrode  22  comprises a conductive layer  26  applied directly on the first carrier foil  16  and a pressure sensitive layer  28  facing towards the second electrode. The second electrode is provided by a conductive layer. The electrodes are preferably printed ones. 
         [0048]    The first and second electrodes  22 ,  24  are connected to a control circuit  30  by leads  32  and  34 . The control circuit  30  comprises a first module  36  for measuring a quantity indicative of electrical resistance between the first and second electrodes  22 ,  24 , a second module  38  for measuring a quantity indicative of the capacitance of the first electrode and a switching unit  40  for connecting alternatively the first or the second module to the electrodes  22 ,  24 . 
         [0049]    The first module  38  comprises a current source  42  whose terminals can be connected to the first and second electrodes, respectively, through the switching unit  40 . A reference resistor  44  and a voltage measurement circuit  46  are connected in parallel to the current source  42 . In operation, i.e. in the first operating mode, the current source tries to create a defined current between its terminals. As long as the electrodes  22  and  24  are separated from each other, i.e. when the compressive force acting on the pressure sensor is not sufficient for pressing the carrier films together in the active zone  14 , the current can flow only through the reference resistor  44  because of the very high input impedance of the voltage measurement circuit  46 . If however, the electrodes  22  and  24  are in contact, the total resistance connected in parallel to the voltage measurement circuit drops so that the voltage necessary to keep the defined current upright drops. In other words, the potential difference measured by the voltage measurement circuit is indicative of the resistance between the first and second electrodes  22 ,  24 . In response to the measurement, the first module outputs a first signal on the first output  48 . 
         [0050]    The second module  38 , dedicated to the “capacitive” measurement, comprises a AC voltage source  50 , providing an oscillating signal to both the first and second electrodes  22 ,  24  through the switching unit  40 . A current measurement circuit  52  measures the current flowing into the first electrode. The amount of current that may flow into the first electrode depends on and is therefore indicative of the capacitance of a capacitor formed by the first electrode and its surroundings. The second module outputs a second signal on the second output  54  of the control circuit, depending on the measured current. 
         [0051]      FIG. 2  shows a data input device according to the third aspect of the invention The device  210  comprises pressure sensor  212  of film-type construction with an active zone  214 . The pressure sensor  212  comprises first and second carrier films  216 ,  218 , made of substantially flexible, electrically insulating material, such as e.g. PET, PEN, PI or the like. A spacer  220  is sandwiched between the first and second carrier films  216 ,  218 , thereby arranging the carrier films the distance corresponding to the spacer thickness from one another. The spacer can also be made of any substantially flexible, electrically insulating material. The spacer is provided with an opening that delimits the active zone  214  of the pressure sensor  210 . In the active zone  14 , the first carrier foil  216  carries a first electrode  222  and a second electrode  224  on its inward-facing side, while the second carrier foil  218  carries a third electrode  225  on its inward-facing side. The first and second electrodes  222 ,  224  are provided by a conductive layer applied directly on the first carrier foil  216 . The third electrode  225  comprises a pressure sensitive layer facing towards the first and second electrode  222 ,  224 . The electrodes are preferably printed ones. 
         [0052]    The electrodes  222 ,  224 ,  225  are connected to a control circuit  230  by leads  232 ,  234  and  235 , respectively. The control circuit  230  comprises a first module  236  for measuring a quantity indicative of electrical resistance between the first and second electrodes  222 ,  224 , a second module  238  for measuring a quantity indicative of the capacitance of the first and second electrodes  222 ,  224  and a switching unit  240  connecting alternatively the first or the second module to the electrodes  222 ,  224  and  225 . 
         [0053]    The first module  238  comprises a current source  242  whose terminals can be connected to the first and second electrodes, respectively, through the switching unit  240 . A reference resistor  244  and a voltage measurement circuit  246  are connected in parallel to the current source  242 . In operation, i.e. in the first operating mode, the current source  242  tries to create a defined current between its terminals. As long as the compressive force acting on the pressure sensor is not sufficient for pressing the carrier films together in the active zone  214 , the current can flow only through the reference resistor  244  because of the very high input impedance of the voltage measurement circuit  246 . If however, the carrier films are pressed together, both the first electrode  222  and the second electrode  224  gets into contact with the third electrode  225 . As a consequence, the total resistance connected in parallel to the voltage measurement circuit  246  drops, so that the voltage necessary to keep the defined current upright drops as well. In other words, the potential difference measured by the voltage measurement circuit  246  is indicative of the resistance between the first and second electrodes  222 ,  224 . In response to the measurement, the first module  236  outputs a first signal on the first output  248 . The second module  238 , dedicated to the “capacitive” measurement, comprises a AC voltage source  250 , providing an oscillating signal to electrodes  222 ,  224  and  225 , through the switching unit  240 . A current measurement circuit  252  measures the current flowing into the first and second electrodes  222 ,  224 . The amount of current that may flow into these electrodes depends on and is therefore indicative of the capacitance of a capacitor formed by the first and second electrodes and its surroundings. The second module  238  outputs a second signal on the second output  254  of the control circuit, depending on the measured current. 
         [0054]    The data input device may be operated alternately in the mode of operation associated to the resistance measurement and in the mode of operation associated to the “capacitance” measurement. Those skilled will understand that the duration of these measurement modes may be equal or different. Furthermore, the frequencies of the different modes of operation may be equal or different. For instance it is possible that the data input device is operated in the mode of operation associated to the resistance measurement only half often as in the mode of operation associated to the “capacitance” measurement, or vice versa. There may be other modes of operation of the data input device, such as, for instance an error diagnose mode, in which the data input device checks for possible short circuits or circuit interruptions. 
         [0055]      FIG. 3  shows an alternative embodiment of a module capable of measuring a quantity indicative of resistance between the first and second electrodes. The module  336  comprises a voltage source  343  connected in series with a current measurement circuit  345 . In the first measurement mode, the voltage source applies a potential difference between the first and second electrodes. If an electrical contact is established between these, either directly or via a third electrode, the resistance between the electrodes decreases and the current measured by the current measurement circuit increases according to Ohm&#39;s law. The module  336  could be substituted to module  36  in  FIG. 1  and to module  236  in  FIG. 2 . Those skilled will be aware of other electric circuits that can measure a quantity indicative of resistance. 
         [0056]      FIG. 4  schematically shows the capacitances intervening during the “capacitive” measurement in the case of a data input device  210  as in  FIG. 2 . Of the data input device  210 , only the electrodes  222 ,  224  and  225  are shown. During the “capacitive” measurement, one measures a quantity indicative of the capacitance formed by the first and second electrodes  222 ,  224  and their surroundings. As illustrated, a variety of (virtual) capacitors have to be considered. For instance, the first and second electrodes form a capacitor with the third electrode. If a user approaches to the data input device with any portion of their body, e.g. their finger  56 , the capacitance of the capacitor formed by the first and second electrodes  222 ,  224  with the surroundings changes, and this change is detected by the data input device. It should be noted that if the electrodes  222 ,  224  and  225  are all driven with a signal of same amplitude and phase, they are remain at substantially the same electric potential during the measurement. Consequently, the capacitances of the capacitors formed by the first and second electrodes, the first and third electrodes and the second and third electrodes remain substantially constant. In particular, the third electrode shields the first and second electrodes from changes in the electric field that occur behind the third electrode, as seen from the first and second electrodes. 
         [0057]      FIGS. 5 and 6  show two possible layouts of the first and second electrodes  222  and  224 . In  FIG. 5 , both electrodes comprise a number of conductors with interconnected first ends and free second ends so as to be of substantially comb-like appearance. The conductors are arranged substantially parallel to one another, the conductors of the first electrode interdigitating with the conductors of the second electrode. In the embodiment of  FIG. 6 , the electrodes comprise a number of concentrically arranged conductive ring portions. Starting at the central point of the arrangement, the conductive ring portions belong alternately to the second and the first electrodes. It shall be noted that several other electrode layouts could be used for putting the present invention into practice. 
         [0058]      FIG. 7  shows a schematic of a film-based position sensor  712  of a data input device according to the fifth aspect of the invention. The position sensor  712  comprises first and second carrier films  716 ,  718  spaced apart by a spacer  720 . The spacer  720  is provided with an opening delimiting an active zone  714  of the position sensor  712 . The first carrier film  716  carries a first electrode  722  and a second electrode  724  on its inward-facing surface; the second carrier film  718  carries a third electrode  725  on its inward-facing surface. The third electrode  725  faces the first electrode  722  and the second electrode  724  in the active zone  714 . The second and third electrodes are conductive electrodes. A top schematic view of the first and second electrodes is given in  FIG. 8 . Both the first and second electrodes  722 ,  724  are essentially comb-shaped. The first electrode  722  includes a series of resistively connected first conductors  758  arranged substantially parallel one to another. The resistive connection of the first conductors  758  is provided by a resistive strip  760 . The second electrode  724  includes a series of conductively connected conductors  762 , that interdigitate with the conductors of the first electrode  722 . The third electrode  725  is not connected. 
         [0059]    If a compressive force acts locally onto the position sensor, the first and second carrier films  716 ,  716  are pressed together and an electrical contact is established between one or more of the conductors  758  and the second electrode  724  via the third electrode  725 . By measuring the resistance between one of the terminals  764  and  766  of the first electrode  722  and the second electrode  724  one can find the position where the first and second electrodes are in contact and thus where the force is acting on the sensor. More generally, one can measure any quantity indicative of the resistance between one of the terminals  764  and  766  and the second electrode  724  to obtain that information. In the data input device represented schematically in  FIG. 8 , this is achieved in that voltage source  743  of the control circuit  730  applies a DC potential difference between the terminals  764  and  766  of the first electrode  722 . Voltage measurement circuit  746  then measures the voltage on the second electrode, which depends on the location of the contact point between the electrodes  722 ,  724  and  725 . 
         [0060]    The data input device  710  can also operate in “capacitive” detection mode. The alternating voltage source  750  of the control circuit  730  then applies an oscillating potential difference between the terminals  764  and  766  of the first electrode  722 . The resulting oscillating voltage is measured at voltage measurement circuit  746  and indicates the location of capacitive coupling between the first and second electrodes  722  and  725 . An equivalent circuit diagram representing this situation is given in  FIG. 9 . Capacitive coupling between the first and second electrodes  722  and  724  is illustrated as impedance Z Z . The connection point  757  of impedance Z Z  to the first electrode corresponds to the point, where capacitive coupling is highest, e.g. due to the proximity of a user&#39;s hand or finger. Because of the very high impedance of the voltage measurement circuit  746 , the voltage U 0  detected by the voltage measurement circuit be approximated as U 0 ˜U applied ·Z X′ /(Z X +Z X′ ), where U applied  is the voltage applied by the alternating voltage source  750 . As the total impedance Z X +Z X′  of the first electrode is known, this expression immediately yields Z X′  (and thus the position of capacitive coupling) as a function of the measured voltage U 0 . 
         [0061]    The control circuit  730  comprises a switching unit  740  for switching between capacitive detection mode and resistive detection mode. In  FIG. 8 , the voltage measurement circuit  746  is shown being common to the two modes of operation. The control circuit  730  might, however, comprise separate voltage measurement circuit dedicated to a respective mode of operation of the circuit. 
         [0062]      FIGS. 10 and 11  schematically illustrate a data input device  1010  comprising a plurality of position sensors  712  arranged in parallel. Data input device  1010  detects the position of a compressive force or capacitive coupling in two dimensions. The first electrodes  722  of the position sensors  712  are connected between two common terminals  1064  and  1066 . The second electrodes  724  are resistively interconnected in series by means of a resistive strip  1061  between terminals  1065  and  1067 . 
         [0063]    Measuring the y-position of a compressive force or capacitive coupling is illustrated in  FIG. 10 . A voltage is applied between the terminals  1065  and  1067  and the resulting voltage U 0y  is measured at terminal  1066  (or at terminal  1064 ). The applied voltage is preferable a DC voltage in the case of the resistive measurement (for finding the y-position of the compressive force) and an AC voltage in case of the capacitive measurement (for finding the y-position of capacitive coupling). Measuring the x-position of a compressive force or capacitive coupling is illustrated in  FIG. 11 . A voltage is applied between the terminals  1064  and  1066  and the resulting voltage U 0x  is measured at terminal  1067  (or  1065 ). The applied voltage is preferable a DC voltage in the case of the resistive measurement (for finding the x-position of the compressive force) and an AC voltage in case of the capacitive measurement (for finding the x-position of capacitive coupling). 
         [0064]      FIGS. 12 and 13  illustrate a variant of the data input device represented in  FIGS. 10 and 11 . In data input device  1210 , the layout of the position sensors  712  differs from the layout previously discussed. The first electrodes  1222  comprise respectively a series of first conductors  1258  that are resistively interconnected. Unlike in the previous embodiments, the resistive interconnection is not provided through a continuous resistive strip but through discrete resistors  1260 . The second electrodes comprise respectively a series of second conductors  1262  that are conductively interconnected. The first and second conductors protrude into the active zones of the position sensor and form therein interdigitating configurations. The first electrodes  1222  are connected between two common terminals  1264  and  1266 . The second electrodes  1224  are resistively interconnected in series between terminals  1265  and  1267  by means of discrete resistors  1261 . 
         [0065]    Measuring the y-position of a compressive force or capacitive coupling is illustrated in  FIG. 12 . A voltage is applied between the terminals  1265  and  1267  and the resulting voltage U 0y  is measured at terminal  1266  (or at terminal  1264 ). The applied voltage is preferable a DC voltage in the case of the resistive measurement (for finding the y-position of the compressive force) and an AC voltage in case of the capacitive measurement (for finding the y-position of capacitive coupling). Measuring the x-position of a compressive force or capacitive coupling is illustrated in  FIG. 13 . A voltage is applied between the terminals  1264  and  1266  and the resulting voltage U 0x  is measured at terminal  1267  (or  1265 ). The applied voltage is preferable a DC voltage in the case of the resistive measurement (for finding the x-position of the compressive force) and an AC voltage in case of the capacitive measurement (for finding the x-position of capacitive coupling). 
         [0066]      FIG. 14  schematically illustrates the impedance between the first electrode and ground as a function of the distance of an actuating member (e.g. the user&#39;s finger, hand or any other body part) to the data input device. It is assumed, for the purpose of this explanation, that the first electrode is driven with a voltage at a fixed frequency. In  FIG. 14 , the distance decreases from left to right along the horizontal axis. If the actuating member is far away from the device, its impedance is maximum. As the actuating member approaches (without getting in contact with the device at this moment), the capacitance between the electrode and ground increases, so that the impedance decreases. As the actuating member gets even closer, it eventually presses onto the device, whereby the first and second electrodes are brought into contact. The capacitance remains now substantially constant but the resistance between the first and second electrodes now decreases with increasing pressure on the sensor. Consequently, the impedance decreases towards a minimum value. It should be noted that the device is advantageously switched from capacitance detection mode (second mode of operation) to resistance detection mode (first mode of operation) about when the actuating member comes into contact with the carrier film.