Abstract:
An apparatus and a method for controlling a touch panel are disclosed herein, the apparatus includes an object detection module and an adjusting device. The object detection module can detect a position of at least one object contacting the touch panel. A position analyzer recognizes position of the object and the adjusting device can set the touch panel to a predetermined position according to the result recognized by the position analyzer.

Description:
BACKGROUND 
     1. Technical Field 
     The disclosure generally relates to a touch panel, and more particularly, to a multi-touch detection method for a touch panel. 
     2. Description of Related Art 
     Types of conventional touch panels mainly comprise resistive touch panels, capacitive touch panels, infrared rays touch panels, and surface acoustic wave touch panels. Generally, resistive touch panels, such as four lines type or five lines type touch panels, can only detect a single touch action at the same time in use because voltage variations of conductive films are detected by using an analogical method. When users touch resistive touch panels in a multi-touch action, an erroneous operation may be generated. 
     US patent applications No. US2006/0097991 and US2008/0158181 disclose structures of capacitive touch panels capable of performing multi-touch detection, which generally include two transparent conductive layers respectively disposed on opposite side surfaces of transparent glass substrates. According to the product resolution, two transparent conductive layers need to be respectively processed through a photolithography process. Conducting wires formed on the same transparent conductive layer are disposed apart and in parallel to each other. The conducting wires formed on one of the transparent conductive layers are perpendicular to that formed on another transparent conductive layer. During operation, by repeating to scan each of the conducting wires, variations of capacitances thereof are analyzed to determine the coordinates of contact points of users&#39; fingers. 
     However, the difficult photolithography processes are necessary for forming the foregoing capacitive touch panels. The product yield may be low due to the difficult photolithography processes. The driving method is complex for recognizing the contact point on touch panel. Accordingly, although capacitive touch panels can be applied for detecting multi-touch action, the high cost limits the scope of the application. 
     SUMMARY 
     An embodiment of the disclosure provides a multi-touch detection method for determining the coordinates of contact points while the contact points are very close. 
     An embodiment of the disclosure provides a multi-touch detection method for a touch panel. The touch panel includes a first conductive layer and a second conductive layer which are overlapped. The first conductive layer has a plurality of first electrodes disposed along a first-axis direction, and the second conductive layer has a plurality of second electrodes disposed along a second-axis direction. The multi-touch detection method includes following steps. The second electrodes are sensed to obtain a first voltage function when a first voltage is provided to the first electrodes. The second electrodes are sensed to obtain a second voltage function when the first voltage is provided to a first portion of the first electrodes and is not provided to a second portion of the first electrodes. Positions of a first contact point and a second contact point in the second-axis direction is calculated by using the first and the second voltage functions. 
     Another embodiment of the disclosure provides a multi-touch detection method for a touch panel, wherein the touch panel includes a first conductive layer and a second conductive layer which are overlapped. The first conductive layer has a plurality of first electrodes disposed along a first-axis direction. The second conductive layer has a plurality of second electrodes and third electrodes, and the second electrodes and the third electrodes are respectively disposed at different sides of the second conductive layer along the first-axis direction. The multi-touch detection method includes following steps. The second electrodes are sensed to obtain a first voltage function when a first voltage is provided to a first portion of the first electrodes and is not provided to a second portion of the first electrodes. The third electrodes are sensed to obtain a second voltage function when the first voltage is provided to the second portion of the first electrodes and is not provided to the first portion of the first electrodes. A position corresponding to an extreme value of the first voltage function is deemed as a position of the first contact point in the second-axis direction. A position corresponding to an extreme value of the second voltage function is deemed as a position of the second contact point in the second-axis direction. 
     In an embodiment of the disclosure, the first conductive layer and the second conductive layer are anisotropic in electric conductivity. For example, a low impedance direction of the second conductive layer is the first-axis direction, and a low impedance direction of the first conductive layer is the second-axis direction. In an embodiment of the disclosure, the first conductive layer and the second conductive layer are conductive films formed with carbon nanotubes arranged substantially in parallel. 
     Based on the above, in an embodiment of the disclosure, the multi-touch detection method reads the first voltage function including the first and the second contact points while the two contact points are very close, and then reads the second voltage function including the first contact point, simultaneously obtaining the position of the first contact point, by driving a portion of the electrodes of the conductive layer. Finally, the position of the second contact point is calculated by the first and the second voltage functions. In another embodiment of the disclosure, a set of electrodes is disposed at each of the right and left sides of the conductive layer. The multi-touch detection method can still obtain the positions of the first and the second contact points through the electrode sets respectively disposed at the right and left sides of the conductive layer by driving a portion of the electrodes of the conductive layer while the two contact points are very close. 
     In order to make the aforementioned and other features and advantages of the disclosure more comprehensible, embodiments accompanying figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
         FIG. 1  is a schematic diagram showing an assembly structure of a resistive touch panel according to an embodiment of the disclosure. 
         FIG. 2  illustrates voltage functions sensed by the touch panel of  FIG. 1  according to an embodiment of the disclosure. 
         FIG. 3  illustrates voltage functions sensed by the touch panel of  FIG. 1  according to an embodiment of the disclosure. 
         FIG. 4A  and  FIG. 4B  illustrate a multi-touch detection method according to an embodiment of the disclosure. 
         FIG. 5A ,  FIG. 5B , and  FIG. 5C  illustrate a multi-touch detection method according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram showing an assembly structure of a resistive touch panel  100  according to an embodiment of the disclosure. Cartesian coordinate system is introduced in  FIG. 1 , which includes X-axis direction, Y-axis direction, and Z-axis direction perpendicular to one another. First electrodes  114  and second electrodes  124  are respectively and simply shown by five electrodes in  FIG. 1 . However, in practice, the numbers of first electrodes  114  and second electrodes  124  can be determined based on the area of touch panel or the application field. 
     As shown in  FIG. 1 , the touch panel  100  is formed by a first conductive film  110  and an partially overlapping second conductive film  120 . The first conductive film  110  and the second conductive film  120  are adhered by a ringed adhesive layer  130 . There are a plurality of insulating spacers  132  uniformly distributed between the first conductive film  110  and the second conductive film  120 , such that the two conductive films  110  and  120  are separated by a constant distance. 
     The first conductive film  110  includes a substrate  111  and a first conductive layer  113 , wherein the first conductive layer  113  is adhered to the surface of the substrate  111  by an adhesive layer  112 . A plurality of first electrodes  114  are disposed at one side of the first conductive layer  113  along a first-axis direction such as the X-axis direction. Herein, distances between adjacent two of the first electrodes  114  are identical, and the first electrodes  114  are respectively electrically connected to the first conductive layer  113 . The ends of the first electrodes  114  extend to the center of the lower edge of the first conductive film  110  for transmitting signals to the outside. 
     The second conductive film  120  includes a substrate  121  and a second conductive layer  123 , wherein the second conductive layer  123  is adhered to the surface of the substrate  121  by an adhesive layer  122 . A plurality of second electrodes  124  are disposed at one side of the second conductive layer  123  along a second-axis direction such as the Y-axis direction. Distances between adjacent two of the second electrodes  124  are identical, and the second electrodes  124  are respectively electrically connected to the second conductive layer  123 . The second electrodes  124  are connected to conducting wires  125  which are arranged in parallel at the right side of the second conductive film  120 . The conducting wires  125  extend along the edge at the right side of the second conductive layer  123 , and the ends of the conducting wires  125  extend to the center of the lower edge of the second conductive film  120  for transmitting signals to the outside. 
     The touch panel  100  further includes a flexible printed circuit board  140  which has a plurality of metal connecting points  141 , and there is a notch  131  in the center of the lower edge of the ringed adhesive layer  130 . During the assembly, the notch  131  corresponds to the flexible printed circuit board  140 , and the upper and the lower metal connecting points  141  of the flexible printed circuit board  140  can be electrically connected to the ends of the conducting wires of the first conductive film  110  and the second conductive film  120 , such that external electronic signals can be transmitted to the first electrodes  114  of the first conductive film  110  and the second electrodes  124  of the second conductive film  120 . 
     In an embodiment of the disclosure, the substrates  111  and  121  used in the touch panel  100  include transparent materials, such as polyethylene (PE), polycarbonate (PC), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), or thin glass substrates. The materials of the ringed adhesive layer  130  and the adhesive layers  112  and  122  may be thermal-cured glue or UV-cured glue. 
     Taiwan patent publication (No. TW 200920689), “Apparatus And Method For Synthesizing Films of Carbon Nanotubes”, discloses a method for synthesizing films of carbon nanotubes. By the method, films of carbon nanotubes which are conductive can be generated, and the method can also be applied to fabricate transparent and conductive films because the films are fabricated by drawing through super vertical-aligned carbon nanotube array in the method. 
     In order to enhance the reliability of the touch panel  100  and reduce the frame width of the touch panel  100 , the first conductive layer  113  and the second conductive layer  123  of the embodiment of the disclosure are formed by conductive films of carbon nanotubes through the above method. However, during the drawing process, the long chain-shaped carbon nanotubes are substantially arranged in parallel along the drawing direction, and the conductive films of carbon nanotubes have lower impedance in the drawing direction. The first conductive layer  113  and the second conductive layer  123  are anisotropic in electric conductivity. The impedance of the conductive films in the direction perpendicular to the drawing direction is about 50˜350 times of that of the conductive films in the drawing direction. The surface resistance of the conductive films is between 1 KΩ and 800 KΩ based on the positions and the direction of the measurement. 
     As shown in  FIG. 1 , in the embodiment of the disclosure, the first conductive layer  113  has a main conductive direction D 1  such as the drawing direction of the original conductive film, and the second conductive layer  123  has another main conductive direction D 2 . In the present embodiment, the main conductive direction D 1 , i.e. the low impedance direction, of the first conductive layer  113  and the main conductive direction D 2  of the second conductive layer  123  are substantially perpendicular to each other. For example, the low impedance direction D 2  of the second conductive layer  123  is the X-axis direction, and the low impedance direction D 1  of the first conductive layer  113  is the Y-axis direction. Herein, the impedances of the first conductive layer  113  and the second conductive layer  123  in the direction perpendicular to the main conductive direction are about 100 to 200 times of those of the first conductive layer  113  and the second conductive layer  123  in the main conductive directions D 1  and D 2 . 
     Following embodiments simply show two contact points as examples when the touch panel  100  operates. However, during practical operation, the multi-touch detection method in the embodiments of the disclosure can be suitable for a plurality of contact points. 
       FIG. 2  illustrates voltage functions sensed by the touch panel  100  of  FIG. 1  according to an embodiment of the disclosure. The second electrodes  124  of the second conductive layer  123  are supplied with a second voltage such as a ground voltage Vss. When the ground voltage Vss is provided to each of the second electrodes  124 , the sensing circuit (not shown) can sequentially sense each of the first electrodes  114  of the first conductive layer  113  one by one. When one of the first electrodes  114  is sensed, the other first electrodes  114  which are not sensed yet are provided with a first voltage such as a system voltage Vdd. Accordingly, the voltage function of the X-axis can be obtained according to the positions of each of the first electrodes  114  (corresponding to the X-axis position) and the sensed voltages.  FIG. 2  illustrates an exemplary case that the touch panel  100  has two contact points. In the positions of the contact points, the first conductive layer  113  and the second conductive layer  123  are electrically connected. Because the electric conductivity of the first conductive layer  113  is anisotropic, the voltages of the X-axis positions x 1  and x 2  of the two contact points are pulled down, and the voltages of the other positions are substantially maintained at the level of the system voltage Vdd. Accordingly, the positions corresponding to two extreme values of the X-axis voltage function are respectively deemed as the positions of the first contact point and the second contact point in the X-axis direction. Herein, the extreme value is a relative minimum. 
     Similarly, when one of the second electrodes  124  of the second conductive layer  123  is sensed, the first electrodes  114  of the first conductive layer  113  are supplied with the system voltage Vdd. At this time, the sensing circuit (not shown in the figures) can sequentially sense each of the second electrodes  124  one by one. When one of the second electrodes  124  is sensed, the other second electrodes  124  which are not sensed yet are provided with the ground voltage Vss. Accordingly, the voltage function of the Y-axis can be obtained according to the positions of each of the second electrodes  124  corresponding to the Y-axis position and the sensed voltages. Because of the anisotropic electric conductivity of the second conductive layer  123 , the voltages of the Y-axis positions y 1  and y 2  of the two contact points are pulled up, and the voltages of the other positions are substantially maintained at the level of the ground voltage Vss. Accordingly, the positions corresponding to two extreme values of the Y-axis voltage function are respectively deemed as the positions of the first contact point and the second contact point in the Y-axis direction. Herein, the extreme value is a relative maximum. 
     The curve of the continuous function illustrated in  FIG. 2  is a schematic diagram. In practice, the voltage read by the first electrodes  114  and the second electrodes  124  are discrete values. Using discrete values to obtain the relative maximum and/or the relative minimum of the voltage function is well-known for those ordinarily skilled in the art, and it will not be described again herein. 
       FIG. 3  illustrates voltage functions sensed by the touch panel  100  of  FIG. 1  according to an embodiment of the disclosure.  FIG. 3  is similar to  FIG. 2 , the difference therebetween lies in that, the Y-axis positions y 1  and y 2  of the two contact points are very close, such that two waveforms of the Y-axis positions y 1  and y 2  are overlapped to form a larger waveform in the Y-axis voltage function. Accordingly, after sensing the second electrodes  124 , the sensing circuit (not shown) only obtains an extreme value in the Y-axis voltage function. The system may erroneously determine the position corresponding to the extreme value as the positions of the two contact points, i.e. dotted line circles in  FIG. 3 . However, the exact positions of the two contact points are the Y-axis positions y 1  and y 2 . This type of sensing error may be overcome by sensing methods performed in following embodiments. 
     First Embodiment 
       FIG. 4A  and  FIG. 4B  illustrate a multi-touch detection method according to a first embodiment of the disclosure. In the present embodiment, the second conductive layer  123  of the touch panel  100  has a plurality of electrodes  124  and  124 ′ disposed along the Y-axis direction. The electrodes  124  and  124 ′ are respectively disposed at different sides of the second conductive layer  123  along the X-axis direction, such as the left side and the right side of the second conductive layer  123  shown in  FIG. 4A . The detail not described in the present embodiment can refer to the embodiments of  FIG. 1  to  FIG. 3 . When the electrodes  124  and/or  124 ′ are/is provided with the second voltage such as the ground voltage Vss, the first electrodes  114  are sensed to obtain the voltage function along the X-axis. The positions corresponding to two extreme values of the X-axis voltage function are respectively deemed as the position x 2  of the first contact point p 1  and the position x 1  of the second contact point p 2  in the X-axis direction. In the foregoing, when the first electrodes  114  are sequentially sensed, the other first electrodes  114  which are not sensed yet are provided with the first voltage such as the system voltage Vdd. 
     When the sensing error as described in  FIG. 3  occurs, following steps are proceeded to obtain the Y-axis positions y 1  and y 2  of the two contact points or the approximate positions thereof. First, the system voltage Vdd is provided to drive a first portion of the first electrodes  114  but not provided to a second portion of the first electrodes  114 . In  FIG. 4A  and  FIG. 4B , the first electrodes  114  are divided into two portions. However, in other embodiments, the first electrodes  114  may be divided into three or more portions. During the process of driving the first electrodes  114 , the system voltage Vdd can be sequentially provided to each portion of the first electrodes  114 . In addition, the portions of the first electrodes  114  which are not provided with the system voltage Vdd yet can be coupled to other reference voltages or be floated. In the present embodiment, the portions of the first electrodes  114  which are not provided with the system voltage Vdd yet are coupled to the ground voltage Vss. 
     Referring to  FIG. 4A , when the system voltage Vdd is provided to the right half portion of the first electrodes  114 , and the ground voltage Vss is provided to the left half portion of the first electrodes  114 , the second electrodes  124  are sensed to obtain a first voltage function of the Y-axis. When the second electrodes  124  are sequentially sensed, the ground voltage Vss is provided to the second electrodes  124  which are not sensed yet. Regarding the contact point on the left side of the touch panel  100 , the first conductive layer  113  is not provided with voltages for pulling up, such that the contact point rarely provides contribution to the first voltage function of the Y-axis. Accordingly, the position y 1′  corresponding to the extreme values of the first voltage function can be deemed as the position y 1  of the contact point on the right side of the touch panel  100  in the Y-axis direction. 
     Referring to  FIG. 4B , the system voltage Vdd is provided to the left half portion of the first electrodes  114  but not provided to the right half portion of the first electrodes  114 . When the system voltage Vdd is provided to the left half portion of the first electrodes  114 , and the ground voltage Vss is provided to the right half portion of the first electrodes  114 , the electrodes  124 ′ are sensed to obtain a second voltage function of the Y-axis. When the electrodes  124 ′ are sequentially sensed, the ground voltage Vss is provided to the electrodes  124 ′ which are not sensed yet. Regarding the contact point on the right side of the touch panel  100 , the first conductive layer  113  is not provided with voltages for pulling up, such that the contact point rarely provides contribution to the second voltage function of the Y-axis. Accordingly, the position y 2 ′ corresponding to the extreme values of the second voltage function can be deemed as the position y 2  of the contact point on the left side of the touch panel  100  in the Y-axis direction. 
     Therefore, even if the Y-axis positions y 1  and y 2  of the two contact points are very close, the Y-axis positions of the two contact points can still be respectively sensed in the present embodiment. It should be noted that, in the present embodiment, the case, “the Y-axis positions y 1  and y 2  of the two contact points are very close,” is exemplary, and thereby those ordinarily skilled in the art can be taught based on the present embodiment and analogize to other condition. For example, two sets of electrodes can be disposed at two sides of the first conductive layer  113  of the touch panel  100  in the Y-axis direction. By the way,  FIG. 4B  does not show the electrodes at the upper side. By sequentially providing the ground voltage Vss to the upper and lower half portions of the electrodes  124 , even if the X-axis positions x 1  and x 2  are very close, the X-axis positions x 1  and x 2  of the two contact points can still be read respectively by the electrodes disposed at the two sides of the first conductive layer  113 . 
     Second Embodiment 
     In consideration of the volume of products, the electrodes can simply be disposed at a single side of the first conductive layer  113  and a single side of the second conductive layer  123 .  FIG. 5A ,  FIG. 5B  and  FIG. 5C  illustrate a multi-touch detection method according to a second embodiment of the disclosure, wherein symbols PY 1 -PY 13  represent the second electrodes  124  of the second conductive layer  123 . The detail not described in the present embodiment can refer to the embodiments of  FIG. 1  to  FIG. 3  and  FIG. 4A  to  FIG. 4B . When one of the second electrodes  124  of the second conductive layer  123  is sensed, all of the first electrodes  114  of the first conductive layer  113  are supplied with the first voltage such as the system voltage Vdd. When the second electrodes  124  are sequentially sensed, the other second electrodes  124  which are not sensed yet are provided with the second voltage such as the ground voltage Vss. According to the position of each of the second electrodes  124  corresponding to the Y-axis position and the sensed voltages, the voltage function P( 1 +2) which is formed by the two waveforms of the contact points p 1  and p 2  overlapped between the positions y 1  and y 2  can be obtained. 
     Following steps showing how to obtain the Y-axis positions y 1  and y 2  of the two contact points p 1  and p 2  or the approximate positions thereof. First, the system voltage Vdd is provided to a first portion of the first electrodes  114  but not provided to a second portion of the first electrodes  114 . In  FIG. 5B , the first electrodes  114  are divided into two portions. However, in other embodiments, the first electrodes  114  may be divided into three or more portions. In addition, the portions of the first electrodes  114  which are not provided with the system voltage Vdd yet can be coupled to other reference voltages or be floated. In the present embodiment, the portions of the first electrodes  114  which are not provided with the system voltage Vdd yet are coupled to the ground voltage Vss. 
     Similar to  FIG. 4A ,  FIG. 5B  illustrates that when the system voltage Vdd is provided to a portion of the first electrodes  114  (the right half portion in  FIG. 5B ) and is not provided to another portion of the first electrodes  114  (the left half portion in  FIG. 5B ), the second electrodes  124  are sensed to obtained a voltage function P 1 . Next, the positions of the contact points p 1  and p 2  in the Y-axis direction are calculated by the voltage functions P( 1 +2) and P 1 . It will be described in detail as follows. 
     Referring to  FIG. 5C , regarding the contact point p 2  on the left side of the touch panel  100 , the first conductive layer  113  is not provided with voltages for pulling up, such that the contact point p 2  rarely provides contribution to the voltage function P 1 . Accordingly, the position corresponding to the extreme values of the voltage function P 1  can be deemed as the position y 1  of the contact point p 1  on the right side of the touch panel  100  in the Y-axis direction. 
     In the present embodiment, a correction parameter r is provided, and then the voltage function P 1  is multiplied by the correction parameter r to obtain a voltage function P 1 ′, i.e. P 1 ′=r×P 1 . The voltage function P 1 ′ can represent the Y-axis voltage function corresponding to the only one contact point p 1  on the touch panel  100 . The correction parameter r may be generated through a lookup table. By providing the lookup table in the present embodiment, the lookup table is searched to obtain the correction parameter r according to the position x 2  of the contact point p 1  in the X-axis direction. 
     An equation P 2 =P( 1 +2)−r×P 1  is calculated to obtain a voltage function P 2 , and then the position corresponding to an extreme value of the voltage function P 2  is deemed as the position y 2  of the contact point p 2  in the Y-axis direction. Herein, the extreme value is a relative maximum. Therefore, even if the Y-axis positions y 1  and y 2  of the two contact points p 1  and p 2  are very close, the Y-axis positions of the two contact points can still be respectively sensed in the present embodiment. It should be noted that, in the present embodiment, the case, “the Y-axis positions y 1  and y 2  of the two contact points are very closed,” is just one of examples to explain the disclosure, and thereby those of ordinary skilled in the art can be taught based on the present embodiment and analogize to other condition. For example, when the X-axis positions x 1  and x 2  are very close, the ground voltage Vss is provided by the methods “complete driving” and “partial driving”. Next, the voltage function which is formed by the two waveforms of the contact points p 1  and p 2  overlapped on the X-axis and the voltage function corresponding to the only one contact point p 1  are obtained. Finally, the positions (or the approximate positions) of the contact points p 1  and p 2  on the touch panel  100  in the X-axis direction are calculated by the two foregoing voltage functions. 
     In other embodiments, the correction parameter r may be unnecessary, such that the preparation for the lookup table is omitted, and the complexity of calculating is reduced. That is, the foregoing step of “calculating the equation P 2 =P( 1 +2)−r×P 1 ” is modified as the step of “calculating equation P 2 =P( 1 +2)−P 1 ” to obtain the voltage function P 2 , and further obtain the position y 2  of the contact point p 2  in the Y-axis direction. 
     Third Embodiment 
     In the present embodiment, a step similar to that of the second embodiment is adopted to obtain the voltage functions P( 1 +2) and P 1 . The difference between the present embodiment and the second embodiment lies in the formula for calculating the positions of the contact points p 1  and p 2  in the Y-axis direction by using the voltage functions P( 1 +2) and P 1 . 
     In the present embodiment, the position corresponding to an extreme value, which is a relative maximum herein, of the voltage function P( 1 +2) is deemed as a central position pm, and the position corresponding to an extreme value, which is a relative maximum herein, of the voltage function P 1  is deemed as the position of the contact point p 1 . In this case, the central position pm is located between the positions of the contact points p 1  and p 2 , such that when the central position pm and the position of the contact point p 1  are given, the position of the contact point p 2  can simply be obtained by the midpoint equation. For example, the equation p 2 =2×pm−p 1  is calculated to obtain the position of the contact point p 2 . Compared with that in the second embodiment, the error in the present embodiment is larger but the operation can be substantially simplified. 
     To sum up the foregoing embodiments, the voltage function P( 1 +2) including the contact points p 1  and p 2  is read while the two contact points are very close. Next, the voltage function P 1  including the contact point p 1  is read, simultaneously obtaining the position of the first contact point p 1 , by driving a portion of the electrodes of the conductive layer. Finally, the position of the contact point p 2  is calculated by the voltage functions P( 1 +2) and P 1 . In the first embodiment, a set of electrodes is disposed at each of the right and left sides of the second conductive layer. The multi-touch detection method can still obtain the positions of the contact points p 1  and p 2  through the electrode sets respectively disposed at the right and left sides of the second conductive layer by driving a portion of the electrodes of the first conductive layer while the two contact points are very close. 
     Although the disclosure has been described with reference to the above embodiments, it will be apparent to one of the ordinary skill in the art that modifications to the described embodiment may be made without departing from the spirit of the disclosure. Accordingly, the scope of the disclosure will be defined by the attached claims not by the above detailed descriptions.