Abstract:
A touch panel includes a light-transmissible, piezoelectric substrate having a first surface and a second surface opposite to the first surface of the piezoelectric substrate, a first light transmissible, resistor layer provided on the first surface of the substrate, and a second light transmissible, resistor layer provided on the second surface of the substrate. A portion of the piezoelectric substrate, upon being pressed, has an electric resistance locally decreasing. The touch panel, even having a simple construction, can avoid reflections of external light and suppress a Newton&#39;s ring.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a touch panel used for inputting data to an electronic apparatus, and to an input device including the touch panel. 
     BACKGROUND OF THE INVENTION 
     As electronic apparatuses have had multi-functions, the apparatuses employ light-transmissible touch panels mounted on screens of displays, such as liquid crystal displays, of the apparatuses. A touch panel is simply manipulated by a user to allow him/her to view and select a character, an icon, and a symbol displayed on the screen with a finger or a dedicated pen for activating desired functions. 
     A conventional touch panel disclosed in Japanese Patent Laid-Open Publication No. 7-169367 will be explained.  FIG. 5  is a cross sectional view of the conventional touch panel. A light transmissible upper resistor layer  3  made of, for example, indium tin oxide is provided on the lower surface of an upper substrate  1  made of light transmissible film. Similarly, a light transmissible lower resistor layer  4  made of, for example, indium tin oxide is provided on the upper surface of a lower substrate  2  made of light transmissible film. Plural dot spacers  5  made of insulating resin are provided at equal intervals on the upper surface of the lower resistor layer  4 . A pair of upper electrodes (not shown) are provided on both ends of the upper resistor layer  3  while a pair of lower electrodes (not shown) are provided on both ends of the lower resistor layer  4  and extend in a direction orthogonal to the upper electrodes. The upper substrate  1  is bonded at its outer edge to the upper end of a frame-shaped spacer  6  by an adhesive layer (not shown) provided on the uppermost surface of the spacer  6 . The lower substrate  2  has an outer rim bonded to the lower end of the spacer  6  by an adhesive layer (not shown) provided on the lower surface of the spacer  6 . This arrangement allows the upper resistor layer  3  to face the lower resistor layer  4  at a predetermined space  7  between the layers. 
     The touch panel is mounted to a screen of a display, such as a liquid crystal display, and the upper and lower electrodes are connected to a control circuit (not shown) in an electronic apparatus. 
     A user presses the upper substrate  1  with a finger or a touch pen while viewing the screen of the display through the touch panel. The upper substrate  1  accordingly deflects and causes a pressed portion of the upper resistor layer  3  to contact the lower resistor layer  4 . The control circuit supplies voltages to the upper and lower electrodes and measures a voltage ratio between the electrodes so as to determine the position of the pressed portion. The control circuit activates various functions of the electronic apparatus circuit according to the detected position. 
     The conventional touch panel is however constructed with a number of components, such as the upper substrate  1 , the upper resistor layer  3 , the space  7 , the lower resistor layer  4 , and the lower substrate  2 . These components reflect external light at upper and lower surfaces of the components, thus reducing transparency. This disturbs the user&#39;s view of the display through the touch panel. 
     The space  7  may generate an optical interference pattern, such as a Newton&#39;s ring. The Newton&#39;s ring may be reduced by filling the space  7  with silicone oil or the like, however, this process increases the overall number of processes for manufacturing the touch panel, and thus makes the touch panel expensive. 
     SUMMARY OF THE INVENTION 
     A touch panel includes a light-transmissible, piezoelectric substrate having a first surface and a second surface opposite to the first surface of the piezoelectric substrate, a first light transmissible, resistor layer provided on the first surface of the substrate, and a second light transmissible, resistor layer provided on the second surface of the substrate. A portion of the piezoelectric substrate, upon being pressed, has an electric resistance locally decreasing. 
     The touch panel, even having a simple construction, can avoid reflections of external light and suppress a Newton&#39;s ring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of a touch panel according to an exemplary embodiment of the present invention. 
         FIG. 2  is an exploded perspective view of the touch panel according to the embodiment. 
         FIG. 3  is a schematic view of an input device according to the embodiment. 
         FIG. 4  is an exploded perspective view of the input device of the embodiment. 
         FIG. 5  is a cross sectional view of a conventional touch panel. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a cross sectional view of a touch panel  101  according to an exemplary embodiment of the present invention.  FIG. 2  is an exploded perspective view of the touch panel  101 . An upper resistor layer  12  made of light transmissible material, such as indium tin oxide or tin oxide, is formed by, for example, vacuum sputtering on the upper surface  11 A of a light-transmissible, piezoelectric substrate  11  made of piezoelectric material, such as poly-vinylidene fluoride, and has a thickness ranging from 150 μm to 200 μm. A lower resistor layer  13  made of light-transmissible material, such as indium tin oxide or tin oxide, is deposited on the lower surface  11 B of the substrate  11  by, for example, vacuum sputtering. The upper resistor layer  12  has an upper surface  12 C coated with a hard coat layer  14  made of transparent insulating resin, such as acrylic resin, polyester resin, or epoxy resin. The lower resistor layer  13  has a lower surface  13 C coated with an insulating layer  15  made of transparent insulating resin, such as acrylic resin, polyester resin, or epoxy resin. A pair of upper electrodes  12 A and  12 B made of electrically conductive material, such as silver or carbon, are provided on left and right ends of the upper resistor layer  12  by printing. A pair of lower electrodes  13 A and  13 B are provided on front and rear ends of the lower resistor layer  13 . This arrangement provides the touch panel  13 . The upper electrodes  12 A and  12 B are arranged in a direction D 1  orthogonal to a direction D 2  in which the lower electrodes  13 A and  13 B are arranged. 
     When a voltage is applied to the piezoelectric substrate  11  made of poly-vinylidene fluoride and having a film sheet shape in its thickness direction, a molecular structure of the substrate changes from random orientation into regular lattice orientation. When a voltage is applied between the resistor layers  12  and  13 , the substrate  11  is polarized, and, for example, the upper surface  11 A has a positive charge, while the lower surface  11 B has a negative charge. While being polarized, the piezoelectric substrate  11  is pressed, and becomes conductive in a thickness direction  11 C at the pressed portion. More specifically, the pressed portion locally has an electric resistance smaller than before the pressing. An electric resistance of an area surrounding the pressed portion changes less than that of the pressed portion, or may not change. That is, the pressed portion of the substrate  11  has the electric resistance smaller than other areas including the surrounding area. 
       FIG. 3  is a schematic view of an input device according to the embodiment.  FIG. 4  is an exploded constructive view of the input device. The touch panel  101  is mounted on a screen  102 A of a display device  102 , such as a liquid crystal display, in an electronic apparatus. The upper electrodes  12 A and  12 B and the lower electrodes  13 A and  13 B are coupled to a control circuit  20  implemented by a microcomputer or the like, thus providing the input device. 
     An operation of the input device including the touch panel of the embodiment will be described. A user presses a position P 1  on the upper surface of the hard coat layer  14  with a finger or a touch pen while viewing the screen  102 A of the display device  102  through the touch panel  101 . Upon being pressed thorough the hard coat layer  14  and the upper resistor layer  12 , the substrate  11  is compressed beneath the position P 1 . The pressing operation decreases distances between molecules in lattice arrangement at the pressed portion of the substrate  11 , and accordingly makes an electric resistance of the pressed portion smaller than other portions which are not pressed. 
     Then, the control circuit  20  supplies a voltage between the upper electrodes  12 A and  12 B of the upper resistor layer  12 . The control circuit  20  then measures a potential at the pressed position of the upper resistor layer  12  through the lower electrodes  13 A and  13 B of the lower resistor layer  13  and through the pressed portion of the substrate  11 . The control circuit  20  determines the position of the pressed portion along the direction D 1  according to a ratio of the measured potential to the supplied voltage. 
     Then, the control circuit  20  supplies a voltage between the lower electrodes  13 A and  13 B of the lower resistor layer  13 . The control circuit  20  then measures a potential at the pressed portion of the lower resistor layer  13  through the upper electrodes  12 A and  12 B of the upper resistor layer  12  and through the pressed portion of the substrate  11 . The control circuit  20  determines the position of the pressed portion along the direction D 2  according to a ratio of the measured potential to the supplied voltage. 
     As described above, the control circuit  20  determines the pressed position P 1  based on the positions along the directions D 1  and D 2 . The control circuit activates various functions of the electronic apparatus. 
     As set forth above, the touch panel  101  of the embodiment includes the substrate  11 , the upper resistor layer  12 , the lower resistor layer  13 , the hard coat layers  14 , and the insulating layer  15 , and thus, does not have the inner space  7  between the resistor layers shown in  FIG. 6 . This arrangement prevents the touch panel  101  from reflecting external light, such as sun light or artificial light, and generating a Newton&#39;s ring. Hence, the user clearly views the screen  102 A of the display device  102  through the touch panel  101 . 
     When a voltage is applied between the resistor layers  12  and  13 , the piezoelectric substrate  11  of poly-vinylidene fluoride is polarized so that molecules are regularly oriented in lattice form. Hence, when the control circuit  20  applies a voltage between the upper resistor layer  12  and the lower resistor layer  13 , the pressed portion of the piezoelectric substrate  11  vibrates by piezoelectric effect. The vibration is then transmitted via the finger or the touch pen to the user, hence creating a click feel upon the pressing operation. 
     The voltage supplied by the control circuit  20  for determining the pressed position is about 5V. The control circuit may apply a voltage of several tens of volts higher than the voltage for the determining between resistor layers  12  and  13  when detecting that the touch panel  101  is pressed. The higher the voltage applied between resistor layers  12  and  13 , the more the substrate  11  vibrates. Hence, the high voltage provides a clear click feel. 
     The hard coat layer  14  and the insulating layer  15  on the resistor layers  12  and  13  prevent the upper resistor layer  12  from being worn away due to the pressing operation, and improve electrically-insulating properties. 
     In the touch panel  101  according to the embodiment, the upper electrodes  12 A and  12 B are arranged in the direction D 1  orthogonal to the direction D 2  in which the lower electrodes  13 A and  13 B are arranged. The directions D 1  and D 1  may not be orthogonal to each other. As long as the directions are non-parallel to each other, the control circuit  20  determines the pressed position P 1 . 
     In the touch panel  101  of the embodiment, the surfaces  11 A and  11 B of the substrate  11  are entirely covered with the upper resistor layer  12  and the lower resistor layer  13 , respectively. The upper resistor layer  12  may have a comb-like shape, i.e. may have stripes extending in parallel to each other and having respective one ends joined together, while the lower resistor layer  13  may have a comb-like shape, i.e., may have stripes extending orthogonally to the stripes of the upper resistor layer  12  and having respective one ends joined together.