Patent Publication Number: US-2011062656-A1

Title: Media stocker, paper feeding device, and method of controlling posture of recording media

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority from Japanese Patent Application No. 2009-213832 filed on Sep. 15, 2009, the entire content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a media stocker, a paper feeding device, and a method of controlling the posture of recording media. 
     BACKGROUND 
     A media stocker configured to stock a large number of recording media vertically lifts and lowers a paper feeding table to thereby bring recording media on the paper feeding table into contact with a pickup roller. Therefore, unlike a system for rotating the paper feeding table to press the recording media against the pickup roller described in, for example, JP-A-2005-330027 (document 1), the media stocker can stock an enormous number of recording media exceeding several hundred recording media, for example, four hundred recording media. 
     On the other hand, in recent years, recording mediums, for example, label sheets embedded with RFID media capable of performing communication by RFID are developed and put to practical use. Among such recording media, there are recording media in which the RFID media are embedded while being shifted from the center portion between an upstream side end and a downstream end in a conveying direction of the recording media. 
     The thickness of the RFID media is not so large and is, for example, about 0.1 mm. However, when the recording media in which the RFID media are arranged to be shifted are set in the media stocker, the RFID media having such thickness are stacked and the recording media incline to be higher on a side where the RFID media are provided. 
     On the other hand, in the technical field of the media stocker, it is a technical common sense that the posture of a recording medium at the top to be picked up is most desirably horizontal. This is because, if the recording medium at the top inclines, a conveying direction of recording media conveyed by the pickup roller also inclines following the inclination of the recording medium at the top. Such inclination causes a deficiency in a separating mechanism located on the downstream side in the conveying direction of the pickup roller and configured to separate only the recording medium at the top. This deficiency causes a conveyance failure. As the deficiency in the separating mechanism, for example, the recording medium at the top cannot be correctly separated or, depending on the structure of the separating mechanism, a skew occurs in the recording medium at the top. 
     The present invention is devised in view of such points and it is an object of the present invention to make it possible to maintain horizontal, even when media having different thicknesses on the upstream side and the downstream side in the conveying direction are stacked and stocked, for example, when the recording media having the RFID media in a position shifted from the center are used, the posture of a recording medium at the top of the stacked recording media. 
     SUMMARY 
     According to an aspect of the present invention, there is provided a media stocker including: a paper feeding table configured to support plural recording media in a stacked state; a lifting and lowering mechanism configured to lift and lower the paper feeding table; a bottom plate attached to the paper feeding table and configured to receive the recording media placed thereon and support the recording media such that a downstream side end in a conveying direction of the recording media on the paper feeding table moves up and down relatively to an upstream side end in the conveying direction; and a control section configured to make an angle of the bottom plate adjustable such that a surface of the recording medium at the top extends along a direction substantially orthogonal to a lifting and lowering direction of the paper feeding table. 
     According to another aspect of the present invention, there is provided a paper feeding device including: a paper feeding table configured to support plural recording media in a stacked state; a lifting and lowering mechanism configured to lift and lower the paper feeding table; a bottom plate attached to the paper feeding table and configured to receive the recording media placed thereon and support the recording media such that a downstream side end in a conveying direction of the recording media on the paper feeding table moves up and down relatively to an upstream side end in the conveying direction; a control section configured to make an angle of the bottom plate adjustable such that a surface of the recording medium at the top extends along a direction substantially orthogonal to a lifting and lowering direction of the paper feeding table; and a pickup roller configured to pick up the recording medium at the top placed on the paper feeding table and convey the recording medium at the top in a predetermined conveying direction. 
     According to still another aspect of the present invention, there is provided a method of controlling the posture of recording media in a media stocker including: a paper feeding table configured to support plural recording media in a stacked state; a lifting and lowering mechanism configured to lift and lower the paper feeding table; and a bottom plate attached to the paper feeding table and configured to receive the recording media placed thereon and support the recording media such that a downstream side end in a conveying direction of the recording media on the paper feeding table moves up and down relatively to an upstream side end in the conveying direction, the method including adjusting an angle of the bottom plate such that a surface of the recording medium at the top extends along a direction substantially orthogonal to a lifting and lowering direction of the paper feeding table. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall front view of a paper feeding device according to an embodiment of the present invention; 
         FIG. 2  is a plan view of an example of a recording medium attached with an RFID medium; 
         FIG. 3  is a side view of a paper feeding table and a lifting and lowering mechanism therefor; 
         FIG. 4  is a front view of the paper feeding table and the lifting and lowering mechanism therefor; 
         FIG. 5  is a schematic diagram for explaining the position of a shaft as a swing center of a bottom plate of the paper feeding table; 
         FIG. 6  is a schematic diagram for explaining a role and optimum restoring force of coil springs included in a control section; 
         FIG. 7  is a front view of a paper feeding table according to another embodiment of the present invention; 
         FIG. 8  is a front view of a paper feeding table according to still another embodiment of the present invention; and 
         FIG. 9  is a perspective view of the paper feeding table. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an overall front view of a paper feeding device according to an embodiment of the present invention. In a paper feeding device  31  according to this embodiment, a pickup roller  41  and a separating mechanism  51  are attached to a media stocker  101 . The paper feeding device  31  stocks, for example, a recording medium  11  shown in  FIG. 2  in the media stocker  101 . 
       FIG. 2  is a plan view of an example of the recording medium  11 . The recording medium  11  has an RFID medium  12  capable of performing communication by RFID. The RFID medium  12  includes, although not shown in  FIG. 2 , an IC chip configured to execute the communication by RFID and an antenna connected to the IC chip. In the recording medium  11 , the RFID medium  12  is arranged to be shifted to a downstream side end  11 D in a conveying direction indicated by an arrow in  FIG. 2  from a center portion C between an upstream side end  11 U and the downstream side end  11 D in the conveying direction. The thickness of the RFID medium  12  in the recording medium  11  attached with the RFID medium is, for example, about 0.1 mm. 
     As shown in  FIG. 1 , the paper feeding device  31  picks up and conveys, with the pickup roller  41 , the recording medium  11  at the top among recording media  11  stocked in the media stocker  101 , separates, with the separating mechanism  51 , the recording medium  11  at the top from the other recording media  11 , and feeds the recording medium  11  at the top to a paper feeding port (not shown) of an image forming apparatus. 
     The pickup roller  41  feeds, with pickup conveyance by friction, not only the recording medium  11  at the top but also the recording media  11  located under the recording medium  11  at the top to the separating mechanism  51 . 
     The pickup roller  41  is located near RFID media  12  in the recording media  11  stocked in the media stocker  101  (near a side having larger thickness of the recording media  11  stocked in the media stocker  101 ). In other words, the pickup roller  41  is located on one end side in the conveying direction of the recording media  11 . 
     The separating mechanism  51  includes a conveying roller  52  and a separating roller  53  opposed to each other via a feeding path SP for the recording media  11 . The conveying roller  52  rotates when the pickup roller  41  starts rotation in order to pick up and convey the recording media  11 . The conveying roller  52  applies conveying force to the recording media  11  picked up and conveyed by the pickup roller  41 . The separating roller  53  rotates following the conveying roller  52  when the conveying roller  52  starts rotation. The separating roller  53  stops the rotation when the recording media  11  picked up and conveyed by the pickup roller  41  reach a position immediately before the separating mechanism  51 . 
     Specifically, the paper feeding device  31  stops the rotation of the separating roller  53  in the position immediately before the recording medium  11  at the top and the recording media  11  located under the recording medium  11  at the top reach the separating mechanism  51 . Therefore, the separating mechanism  51  can stop the recording media  11  located under the recording medium  11  at the top by bumping the recording media  11  located under the recording medium  11  at the top against the separating roller  53 . In other words, the separating mechanism  51  separates only the recording medium  11  at the top from the other recording media  11 . The recording medium  11  at the top separated from the other recording media  11  is conveyed by the conveying roller  52  to the paper feeding port (not shown) of the image forming apparatus. 
     Only the recording medium  11  at the top separated from the other recording media  11  is conveyed in this way because conveying force applied to the recording medium  11  at the top by the rotation of the conveying roller  52  is stronger than frictional force between the recording medium  11  at the top and the stopped recording medium  11  located right under the recording medium  11  at the top. In this way, the paper feeding device  31  loosens, with the separating mechanism  51 , the plural recording media  11  picked up and conveyed by the pickup roller  41  and separates and conveys only the recording medium  11  at the top. 
     The rotation of the separating roller  53  is stopped by a one-way clutch (not shown). It is determined by using, for example, a sensing value of a sensor (not shown) arranged between the pickup roller  41  and the separating mechanism  51  that the recording media  11  reaches the position immediately before the separating mechanism  51 . 
     The separating mechanism  51  does not always have to be the system explained above and may be, for example, a reverse roller system. 
     The media stocker  101  is explained below.  FIG. 3  is a side view of a paper feeding table  103  and a lifting and lowering mechanism  104  therefor.  FIG. 4  is a front view of the paper feeding table  103  and the lifting and lowering mechanism  104  therefor. 
     In the media stocker  101 , the paper feeding table  103  is attached to one base plate  102  of sheet metal to be freely lifted and lowered. The lifting and lowering mechanism  104  allows the paper feeding table  103  to be freely lifted and lowered. Therefore, the lifting and lowering mechanism  104  is explained and then the paper feeding table  103  is explained in detail below. 
     The lifting and lowering mechanism  104  includes a pair of lifting and lowering rollers  105  attached under the paper feeding table  103  to be rotatable around a horizontal rotation axis and a pair of rear lifting and lowering rollers  106  located on the opposite side of the paper feeding table  103  across the base plate  102 . In the lifting and lowering mechanism  104 , the rear lifting and lowering rollers  106  are attached to a single roller holding member  107  of sheet metal to be rotatable around a horizontal rotation axis. The lifting and lowering mechanism  104  couples and fixes the paper feeding table  103  and the roller holding member  107  via elongated communication holes  108  of a long hole shape formed in three streaks in the base plate  102  and extending in the vertical direction. Therefore, the lifting and lowering mechanism  104  supports the paper feeding table  103  with the pair of lifting and lowering rollers  105  located on the front side of the base plate  102  and the pair of rear lifting and lowering rollers  106  located on the rear side of the base plate  102  to allow the paper feeding table  103  to be freely lifted and lowered along the base plate  102 . From the viewpoint of supporting fulcrums for supporting the paper feeding table  103 , sections where the pair of lifting and lowering rollers  105  are set in contact with the base plate  102  are supporting fulcrums located at the bottom and sections where the pair of rear lifting and lowering rollers  106  are set in contact with the base plate  102  are supporting fulcrums located at the top. 
     The lifting and lowering mechanism  104  includes a pair of belt winding mechanisms  109  located on both sides of the base plate  102 . The belt winding mechanisms  109  are mechanisms obtained by laying endless belts  111  over a pair of upper and lower pulleys  110 . The pulley  110  located in the lower part rotates with power from a driving source (not shown) to rotate the endless belts  111 . Therefore, the lifting and lowering mechanisms  104  couple and fix coupling sections  112  provided in the roller holding member  107  to the respective pair of endless belts  111  to transmit the rotation of the endless belt  111  to the roller holding member  107  and lift and lower the paper feeding table  103 . 
     The paper feeding table  103  is explained in detail below. 
     The paper feeding table  103  has a bottom plate  113 . The bottom plate  113  receives the plural recording media  11  directly placed thereon and supports the recording media  11 . The paper feeding table  103  swingably holds the bottom plate  113 . Specifically, in the paper feeding table  103 , a shaft  114  orthogonal to a pickup and conveying direction of the recording media  11  by the pickup roller  41  is located in a center portion of the bottom plate  113 . The bottom plate  113  is attached rotatably around the shaft  114 . This makes the bottom plate  113  swingable. 
       FIG. 5  is a schematic diagram for explaining the position of the shaft  114  as a swing center of the bottom plate  113  of the paper feeding table  103 . As shown in  FIG. 5 , the bottom plate  113  of the paper feeding table  103  includes the shaft  114  attached to the paper feeding table  103  between a pair of coil springs  116  functioning as urging members. The arrangement position of the shaft  114  of the bottom plate  113  is further examined. In this embodiment, in the paper feeding table  103 , the shaft  114  is arranged in the center portion of the bottom plate  113 . As width in upstream and downstream directions in the conveying direction of the recording media  11 , the width of the bottom plate  113  and the width of the recording medium  11  coincide with each other. Therefore, the position of the shaft  114  as the swing center of the bottom plate  113  is located in the center portion C of the recording media  11  (see  FIG. 5 ). What is important in this embodiment is that the shaft  114  of the bottom plate  113  is located in the center portion C in the upstream and downstream directions in the conveying direction of the recording medium  11 . 
     However, the shaft  114  of the bottom plate  113  does not always have to be located in the precise center portion C in the upstream and downstream directions in the conveying direction of the recording media  11 . The shaft  114  of the bottom plate  113  only has to be arranged on a vertical surface between the RFID media  12  of the recording media  11  placed on the bottom plate  113  and the upstream side end  11 U or the downstream side end  11 D in the conveying direction of the recording media  11  in a direction far from the RFID media  12 . In  FIG. 5 , a range in which the shaft  114  of the bottom plate  113  should be arranged is indicated by DR. 
     Therefore, in the paper feeding table  103 , in a state in which the recording media  11  are set such that the RFID media  12  are located further on the downstream side in the conveying direction than the center portion of the recording media  11  (a state in the upper part of  FIG. 5 ), the shaft  114  of the bottom plate  113  only has to be arranged on the vertical surface between the RFID media  12  and the upstream side end  11 U in the conveying direction of the recording media  11  in a direction far from the RFID media  12 . Conversely, when the recording media  11  are set such that the RFID media  12  is located further on the upstream side in the conveying direction than the center portion of the recording media  11  (a state in the lower part of  FIG. 5 ), the shaft  114  of the bottom plate  113  only has to be arranged on the vertical surface between the RFID media  12  and the downstream side end  11 D in the conveying direction of the recording media  11  in a direction far from the RFID media  12 . 
     The paper feeding table  103  includes a control section  115  configured to control a rotation angle of the bottom plate  113  such that the surface of the recording medium  11  at the top extends along a direction orthogonal to the lifting and lowering direction of the paper feeding table  103 , more specifically, to bring an inclination angle of the recording medium  11  at the top, which inclines according to the thickness of the RFID media  12 , with respect to the horizontal surface closer to zero degree. The control section  115  has the pair of coil springs  116  as urging members. The coil springs  116  are compression coil springs configured to accumulate urging force in a compressed state, arranged on both the upstream and downstream sides of the recording media  11  placed on the bottom plate  113  across the shaft  114 , and urge the bottom plate  113  upward. The coil springs  116  have the same number of turns and the same urging force. The coil springs  116  are separated from the shaft  114  by the same distance. Therefore, the bottom plate  113  is maintained horizontal in a vacant state in which no recording medium  11  is placed on the bottom plate  113 . 
     In such a configuration, the media stocker  101  stocks and stores a large volume of recording media  11  and prepares for pickup and conveyance by the pickup roller  41 . To stock the recording media  11 , the media stocker  101  places the recording media  11  on the bottom plate  113  of the paper feeding table  103 . In this way, a process for stacking the plural recording media  11 , in which the RFID media  12  are provided to be shifted from the center portion between the upstream side end and the downstream side end in the conveying direction, and holding the recording media  11  with the paper feeding table  103 , which is freely lifted and lowered, is performed. 
       FIG. 6  is a schematic diagram for explaining a role and optimum urging force of the coil springs  116  included in the control section  115 . It is assumed that, in the paper feeding table  103 , the recording media  11  are set such that the RFID media  12  are located further on the downstream side in the conveying direction than the center portion C of the recording media  11 . 
     The thickness of the recording media  11  is different across the center portion thereof on a side on which the RFID media  12  are provided and a side on which the RFID media  12  are not provided. Therefore, as the number of stacked recording media  11  increases, the recording media  11  inclines to be higher on the side on which the RFID media  12  are provided (see  FIG. 6A ). 
     On the other hand, the weight of the recording media  11  is different by the weight of the RFID media  12  across the center portion thereof on the side on which the RFID media  12  are provided and the side on which the RFID media  12  are not provided. As the recording media  11  as a whole, this weight difference increases in proportion to an increase in the number of stacked recording media  11 . Therefore, the media stocker  101  according to this embodiment maintains the posture of the recording medium  11  at the top horizontal making use of such a weight difference that occurs between the upstream side and the downstream side in the conveying direction across the center portion C of the recording media  11 . Specifically, in an example shown in  FIG. 6 , since the RFID media  12  are located further on the downstream side in the conveying direction than the center portion C of the recording media  11 , the weight on the downstream side in the conveying direction is larger than the weight on the upstream side in the conveying direction. Therefore, the coil spring  116  arranged on the downstream side in the conveying direction is compressed and the coil spring  116  arranged on the upstream side in the conveying direction is stretched. The bottom plate  113  inclines to fall to the downstream side in the conveying direction. As a result, the posture of the recording medium  11  at the top is maintained horizontal. 
     However, whether the posture of the recording medium  11  at the top is maintained horizontal depends on a value of a sum of the restoring force of the compressed coil spring  116  and the restoring force of the stretched coil spring  116 . If such restoring forces of the coil springs  116  are set to optimum restoring forces, it is possible to maintain the posture of the recording medium  11  at the top horizontal. On the other hand, if the restoring forces of the coil springs  116  are too strong, as shown in  FIG. 6C , the inclined state of the recording media  11  is not completely eliminated and the recording medium  11  at the top still keeps the inclined state. If the restoring forces of the coil springs  116  are too weak, as shown in  FIG. 6D , the recording media  11  incline in the opposite direction and the recording medium  11  at the top also inclines in the opposite direction. 
     Therefore, in selecting the coil springs  116 , first, a difference between the thickness on the side on which the RFID media  12  are provided and the thickness on the side on which the RFID media  12  are not provided and a difference between the weight on the side on which the RFID media  12  are provided and the weight on the side on which the RFID media  12  are not provided are measured in the recording media  11 . Then, the coil springs  116  having a spring rate at which a compression amount of the pair of coils springs  116  obtained when load equivalent to the measured weight is applied is equal to the measured thickness dimension only has to be selected. Further, to maintain the posture of the recording medium  11  at the top horizontal irrespectively of the number of stacked recording media  11  on the bottom plate  113  of the paper feeding table  103 , it is desirable to use the coil springs  116  having a progressive rate at which the load and the compression amount linearly change. 
     In addition, as in this embodiment, the shaft  114  of the bottom plate  113  is located in the center portion C in the upstream and downstream directions in the conveying direction of the recording media  11 . Then, even if the recording media  11  are set such that the RFID media  12  are located further on the downstream side in the conveying direction than the center portion C of the recording media  11  in the paper feeding table  103 , it is possible to maintain the posture of the recording medium  11  at the top horizontal under the same condition. Conversely, even if the recording media  11  are set such that the RFID media  12  are located further on the upstream side in the conveying direction than the center portion C of the recording media  11  in the paper feeding table  103 , it is possible to maintain the posture of the recording medium  11  at the top horizontal under the same condition. 
     In this way, a process for controlling an angle of the bottom plate  113  configured to receive the recording media  11  placed thereon and support the recording media  11  in the paper feeding table  103  to bring an inclination angle of the recording medium  11  at the top, which inclines because of the thickness of the RFID media  12 , with respect to the horizontal surface closer to zero degree is performed. 
     As explained above, according to this embodiment, as a result of maintaining the posture of the recording medium  11  at the top horizontal, the direction of pickup and conveyance of the recording media  11  by the pickup roller  41  is also maintained horizontal. Thereafter, it is possible to correctly perform separation and feeding in the separating mechanism  51 . This makes it possible to prevent a conveyance failure of the recording media  11 . 
     Another embodiment of the present invention is explained with reference to  FIG. 7 . Components same as those explained with reference to  FIGS. 1 to 6  are denoted by the same reference numerals and signs and explanation of the components is omitted. 
       FIG. 7  is a front view of the paper feeding table  103  according to this embodiment. The paper feeding table  103  according to this embodiment is applied to the media stocker  101  of a system for setting the recording media  11  such that the RFID media  12  are located further on the downstream side in the conveying direction than the center portion C of the recording media  11  in the paper feeding table  103 . 
     In the media stocker  101  shown in  FIG. 1  and the like, the bottom plate  113  is swingably provided in the paper feeding table  103 . On the other hand, in the media stocker  101  according to this embodiment, the shaft  114  of the bottom plate  113  is located on the distal end side of the paper feeding table  103  and the bottom plate  113  is provided rotatably around the shaft  114 . In the media stocker  101  according to this embodiment, the control section  115  is located in a position where the control section  115  urges the bottom plate  113  upward further on the downstream side in the conveying direction of the recording media  11  than the vertical surface passing the shaft  114  of the bottom plate  113 . 
     In such a configuration, when the recording media  11  are stacked on the bottom plate  113  of the paper feeding table  103 , the coil springs  116  are compressed by the weight of the RFID media  12  of the recording media  11 . Therefore, the recording media  11  are prevented from inclining to be higher on the downstream side in the conveying direction because of the thickness of the RFID media  12 . As a result, it is possible to maintain the posture of the recording medium  11  at the top horizontal. 
     Concerning the media stocker  101  of a system for setting the recording media  11  such that the RFID media  12  are located further on the upstream side in the conveying direction than the center portion C of the recording media  11 , the paper feeding table  103  can be applied by interchanging the positions of the shaft  114  of the bottom plate  113  and the coil springs  116  in the upstream and downstream directions in the conveying direction of the recording media  11 . 
     The shaft  114  of the bottom plate  113  only has to be arranged on a vertical surface between the RFID media  12  of the recording media  11  placed on the bottom plate  113  and the upstream side end or the downstream side end in the conveying direction of the recording media  11  in a direction far from the RFID media  12 . 
     A still another embodiment of the present invention is explained with reference to  FIGS. 8 and 9 . Components same as those explained with reference to  FIGS. 1 to 6  are denoted by the same reference numerals and signs and explanation of the components is omitted. 
       FIG. 8  is a front view of the paper feeding table  103  according to this embodiment.  FIG. 9  is a perspective view of the paper feeding table  103 . As in the embodiment explained with reference to  FIG. 7 , the paper feeding table  103  according to this embodiment is applied to the media stocker  101  of the system for setting the recording media  11  such that the RFID media  12  are located further on the downstream side in the conveying direction than the center portion C of the recording media  11  in the paper feeding table  103 . 
     In the media stocker  101  according to this embodiment, a deflecting member  121  is provided in the paper feeding table  103  instead of the bottom plate  113  and the coil springs  116 . The deflecting member  121  is an elastic member formed to be bent in a U shape, a piece on the lower side of which is fixed to the paper feeding table  103  and a piece on the upper side of which is the bottom plate  113  configured to receive the recording media  11  placed thereon and support the recording media  11 . In the deflecting member  121 , a deflecting free end FE side of the bottom plate  113  is arranged on a side on which stacking thickness is increased by the RFID media  12  in the recording media  11  placed on the bottom plate  113 . Therefore, in the case of this embodiment, the free end FE of the bottom plate  113  is arranged on the downstream side in the conveying direction of the recording media  11 . 
     In such a configuration, when the recording media  11  are stacked on the bottom plate  113  of the paper feeding table  103 , the deflecting member  121  is compressed by the weight of the RFID media  12 . Therefore, the recording media  11  are prevented from inclining to be higher on the downstream side in the conveying direction because of the thickness of the RFID media  12 . As a result, it is possible to maintain the posture of the recording medium  11  at the top horizontal. 
     Concerning the media stocker  101  of the system for setting the recording media  11  such that the RFID media  12  are located further on the upstream side in the conveying direction than the center portion C of the recording media  11 , the paper feeding table  103  can be applied by reversing the direction of the deflecting member  121  in the upstream and downstream directions in the conveying direction of the recording media  11 . 
     The control section  115  may control a swinging angle of the bottom plate  113  according to driving of an actuator (not shown). Specifically, the actuator is controlled to be driven to maintain the posture of the recording medium  11  at the top horizontal, whereby the swinging angle of the bottom plate  113  is controlled. 
     In this embodiment, as the recording media  11  stocked by the media stocker  101 , the recording media  11  in which the RFID media  12  are provided to be shifted from the center portion between the upstream side end and the downstream side end in the conveying direction are explained. On the other hand, the recording media  11  do not always have to have the RFID media  12 . The operational effects of the media stocker  101  according to this embodiment are realized as long as the recording media  11  have different thicknesses on the upstream side and the downstream side across the center portion in the conveying direction and have larger weight on the side on which the thickness is larger. 
     In addition, in this embodiment, the structure in which the bottom plate  113  rotates around the fixed shaft  114  is explained. On the other hand, the bottom plate  113  does not always need the physical shaft  114  in the rotation center thereof. Specifically, the bottom plate  113  may swing around an imaginary shaft. Moreover, the shaft in this case does not always have to be fixed. As an example, the shaft may be an imaginary shaft, the position of which shifts according to the swing of the bottom plate  113 . 
     Further effects and modifications can be easily derived by those skilled in the art. Therefore, a wider aspect of the present invention is not limited by the specific details and the representative embodiment represented and described above. Therefore, various modifications are possible without departing from the spirit or the scope of the general concept of the invention defined by the appended claims and their equivalents.