Abstract:
Disclosed is an apparatus for sensing the feeding of individual sheets of paper in which the feeding of two or more overlapping sheets can be detected in a sheet feeding apparatus for continuously transferring sheets of paper one by one along a predetermined paper traveling path. The apparatus employs a support plate which allows the sheets of paper to slide, and a contact arm adapted to contact the support plate and to be moved upward by a height corresponding to the thickness of the sheet of paper when the sheet of paper passes between the support plate and the contact arm, instead of using an expensive rollers, bearings or the like as a means for sensing the thickness of a sheet of paper being fed. Accordingly, the rotational angle of the contact arm enables the thickness of a sheet of paper being fed to be sensed, so that the feeding of two or more overlapping sheets of paper between a driving roller and a driven roller is always detected correctly and reliably even when foreign matter is stuck to the rollers. Further, the paper feed sensing apparatus has a simple structure and malfunctions less.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for sensing the feeding of individual sheets of paper, and more particularly, to an apparatus for sensing the feeding of individual sheets of paper in which the feeding of two or more overlapping sheets can be detected in a sheet feeding apparatus for continuously transferring sheets of paper one by one along a predetermined paper traveling path 
     2. Description of the Related Art 
     In general, such a sheet feeding apparatus can be adapted for use in combination with, for example, a printing machine in a printing house, an automatic teller machine (ATM) in a bank, and the like. The sheet feeding apparatus is equipped with a driving roller and a driven roller which contact each other, and has a sheet feeding scheme in which a sheet of paper is fed between the driving roller and the driven roller such that they are compressed against the sheet of paper 
     For example, in the case of a printing machine in a printing house, individual sheets of paper to be printed are transferred from section to section so that different processes are performed in sequences and if the individual sheets of paper are not correctly supplied, there may be a paper jam or printing may be adversely affected. In addition, in the case of an automatic teller machine (ATM), the number of bills of cash and bank notes dispensed by the ATM must be accurately checked. 
     When a sheet feeding apparatus for transferring individual sheets of paper does not supply sheets of paper one by one but rather feeds two or more sheets of paper at a time, an error occurs in the sheet feeding apparatus during its feeding operation and this can lead to further problems as well. Thus, it is required to always check the thickness of a sheet of paper fed between two corresponding rollers pressed against the sheet of paper. 
     FIG. 1 is a schematic longitudinal sectional view illustrating the construction of a typical sheet feeding apparatus given as an example to explain a conventional individual paper feed sensing apparatus. 
     Referring to FIG. 1, the sheet feeding apparatus is provided with a plurality of pairs of driving roller  12  and a driven roller  14  which are supported on a frame  10  The outer surfaces of the driving roller  12  and the driven roller  14  in each pair are in contact with each other to discharge to the outside a sheet of paper P fed therebetween along a predetermined paper traveling path. The driving rollers  12  are mechanically coupled to each other through a power transfer means such as gears, belts or the like, and rotate by means of a separate drive motor (not shown) to feed a sheet of paper along the predetermined paper traveling path such that the driving rollers  12  and the driven rollers  14  are compressed against the sheet of paper. 
     In the meantime, the sheet feeding apparatus as constructed above includes a paper feed sensing apparatus for detecting the thickness of a sheet of paper being fed. The paper feed sensing apparatus functions to check a space between the driving rollers  12  and the driven rollers  14  which are in contact with each other. There are also diverse kinds of paper feed sensing apparatuses, but almost all paper feed sensing apparatuses employ a paper feed sensing mechanism in which they detect a physical variation in the space between the driving rollers  12  and the driven rollers  14  and then output the detected result to the outside through an electronic sensor  18 . 
     For this purpose, a driving roller  12  and a driven roller  14  coupled to the sensor  18  are replaced by a metal driving roller  20  and a support bearing  22 , respectively, which have been machined very precisely. The outer surface of the metal driving roller  20  is treated very precisely and its concentricity is very precise. The support bearing  22  serves as a driven roller whose outer surface is in contact with that of metal driving roller  20 . Of course, the support bearing  22  is manufactured such that its outer surface is also treated very precisely to have more precise concentricity. 
     The metal driving roller  20  and the support bearing  22 , machined precisely as mentioned above, are coupled to the sensor  18 . Accordingly, when there is a variation in the thickness of a sheet of paper passing between the metal driving roller  20  and the support bearing  22 , the distance between the center axes of rotational motion of the metal driving roller  20  and the support bearing  22  are also changed, which is detected by the sensor  18 . 
     However, in the case of such a conventional paper feed sensing apparatus, when the metal driving roller  20  and the support bearing  22  are not manufactured with sufficient precision, the apparatus does not operate reliably. Generally, a sheet of paper has a thickness of 0.1 mm or less. Thus, if either the metal driving roller  20  or the support bearing  22  is slightly eccentric or its outer surface is slightly non-uniform, the thickness of a sheet of paper being fed cannot be detected correctly. 
     For example, when a sheet of paper having a thickness of 0.1 mm passes between the metal driving roller  20  and the support bearing  22  in a state in which external foreign matter is adhered to the outer surface of either the metal driving roller  20  or the support bearing  22 , the sensor  18  detects the total thickness obtained by adding the thickness of the sheet and that of the foreign matter. At this time, if the thickness of the foreign matter is 0.1 mm, the sensor  18  will recognize one sheet of paper as two sheets of paper. 
     Moreover, the sensor  18  measures the distance between the rotational center axes of the metal driving roller  20  and the support bearing  22  to detect a variation in the distance. Accordingly, if foreign matter adhered to the outer surface of either the metal driving roller  20  or the support bearing  22  has an irregular thickness or pattern, there occurs a variation in the distance between the rotational center axes of the metal driving roller  20  and the support bearing  22 , which may make it impossible to measure the distance between the rotational center axes thereof. Further, when the support bearing is constructed as a ball having a number of balls fit into a space between the rotational center axis and the outer surface of a wheel portion, the space between the rotational center axis and the outer surface of the wheel portion is not constant on a microscopic level. 
     As a result, for the above-mentioned conventional paper feed sensing apparatus, there is the problem in that it must employ a high-priced metal driving roller  20  and support bearing  22  whose outer surface is machined very precisely and whose concentricity is very precise, thereby increasing the manufacturing cost. In addition, there is a limitation of measurement in that when foreign matter is adhered to the outer surface of either the metal driving roller  20  or the support bearing  22 , measurement is inaccurate. There arises a further problem in that a separate device for preventing the adhesion of foreign matter is required, thereby increasing both the complexity of the paper feed sensing apparatus and the frequency of malfunction. 
     SUMMARY OF THE INVENTION 
     To solve the above-described problems, it is a primary object of the present invention to provide an apparatus for sensing the feeding of individual sheets of paper adapted for use with a sheet feeding apparatus, which can simplify the structure of the sheet feeding apparatus, reduce the frequency of malfunction, and enable the feeding of two or more overlapping sheets of paper between driving rollers and driven rollers to always be detected correctly and reliably irrespective of presence of foreign matter on the rollers and without using expensive rollers, bearings or the like as a means for sensing the thickness of a sheet of paper being fed. 
     To accomplish the primary object of the present invention, there is provided an apparatus for sensing the feeding of individual sheets of paper which is adapted for use with a sheet feeding apparatus including a plurality of pairs of a driving roller and a driven roller which continuously feed a plurality sheets of paper one by one along a predetermined paper traveling path, the apparatus for sensing the feeding of individual sheets of paper comprising a support plate mounted between and adapted to support a sheet of paper passing between the driving roller and driven roller of each of the plurality of pairs of a driving roller and a driven roller along the predetermined paper traveling path, the support plate having a plurality of driving roller through-holes formed thereon which allows the plurality of pairs of a driving roller and a driven roller to contact each other therethough, a rotating shaft rotatably mounted over the top surface of the support plate, parallel to the support plate, an extending contact arm fixedly mounted on the rotating shaft with one end portion thereof extending toward the support plate to contact the top surface of the support plate, the extending contact arm being moved upward from the top surface of the support plate by a height corresponding to the thickness of the sheet of paper when the sheet of paper passes along the support plate thus causing the rotating shaft to rotate, an amplifying section operatively coupled to the rotating shaft to amplify an amount of rotation of the rotating shaft, and a sensor operatively coupled to the amplifying section for sensing the amount of rotation amplified by the amplifying section, wherein the feeding of two or more overlapping sheets of paper between the pairs of a driving roller and a driven roller is detected. 
     Also, the apparatus may further comprise a cover plate disposed between the support plate and the rotating shaft in parallel with and spaced apart from the support plate, thus defining the predetermined paper traveling path between the support plate and the cover plate and guiding the sheet of paper passing on the support plate along the predetermined paper traveling path, the cover plate having a plurality of driven roller through-holes which allow parts of lower portions of the driven rollers to pass therethrough so that the outer surfaces of both the driving rollers and the driven rollers contact each other, and a plurality of extending contact arm through-holes which are each disposed between two driven roller through-holes and allow bottom portions of the extending contact arms to pass therethrough so that the bottom portions of the extending projections contact the top surface of the support plate. 
     The amplifying section preferably includes a pivoting rod fixedly mounted at one end thereof on one end of the rotating shaft and extending perpendicular to the rotating shaft, a connecting shaft fixedly connected to the other end of the pivoting rod and disposed parallel to the rotating shaft, and a link member rotatably connected at one end to the connecting shaft and disposed parallel to the pivoting rod. 
     Also, the sensor is preferably a magnetic sensor with a rotational center axis is rotatably connected to the other end of the link member. 
     The length of the end portion of the extending contact arm extending toward the support plate is preferably smaller than that of the pivoting rod. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
     FIG. 1 is a schematic longitudinal sectional view illustrating the construction of a typical sheet feeding apparatus; 
     FIG. 2 is a partially cut-away perspective view illustrating the construction of a paper feed sensing apparatus according to an embodiment of the present invention which is in an assembled state; 
     FIG. 3 is an exploded perspective view illustrating the construction of the paper feed sensing apparatus of FIG. 2; 
     FIG. 4 is a cross-sectional view taken along line IV—IV in FIG. 2, which shows a sheet feeding construction in which a sheet of paper is transferred between a driving roller  12  and a driven roller  14 , 
     FIG. 5 is a cross-sectional view taken along line V—V in FIG. 2; and 
     FIGS. 6,  7  and  8  are views illustrating the operation of a paper feed sensing apparatus according to an embodiment of the present invention 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 2 is a partially cut-away perspective view illustrating the construction of a paper feed sensing apparatus according to an embodiment of the present invention which is in an assembled state. 
     Referring to FIG. 2, a plurality of driving rollers  12  and driven rollers  14  are fixedly mounted on a driving shaft  29  and a driven shaft  28 , respectively, each of which is rotatably mounted at one end to a frame  10 . The driving rollers  12  and the driven rollers  14  function to transfer a sheet of paper. FIG. 2 shows only rollers where the sheet feed sensing apparatus of the present invention is installed. 
     As shown in FIG. 2, a sheet feed sensing apparatus according to the present invention includes a cover plate  31  and a support plate  33 , a rotating shaft  35 , an extending contact arm  41 , a pivoting rod  26 , and a link member  37 . 
     The cover plate  31  and support plate  33  are fixedly mounted to the frame  10  to allow a sheet of paper to be fed through a passage formed therebetween. The rotating shaft  35  is disposed in parallel with the driven shaft  28  over the top surface of the cover plate  31  and is rotatably mounted at one end to the frame  10 . The extending contact arm  41  is fixedly mounted on the rotating shaft  35  and one end portion thereof passes through the cover plate  31  to contact the top surface of the support plate  33 . The pivoting rod  26  is fixedly mounted at one end portion on the one end of the rotating shaft  35 . And, the link member  37  is relatively rotatably coupled at one end to the other free end portion of the pivoting rod  26  for connecting a magnetic sensor  39  to the free end portion of the pivoting rod  26 . 
     Further, the driving rollers  12  and the driven rollers  14  contact each other through the cover plate  31  and the support plate  33 . 
     The support plate  33  has a curved surface and a uniform thickness to correspond to a paper traveling path. Also, the support plate  33  is located between the driving rollers  12  and the driven rollers  14  and has a number of driving roller through-holes  47  (see FIG. 3) formed thereon so that the driving rollers  12  and the driven rollers  14  can contact each other through the roller through-holes  47 . Of course, the shape of the support plate  33  may be diversely modified according to various embodiments. 
     The driving roller through-holes  47  (see FIG. 3) are provided such that they receive the upper portions of the driving rollers  12  to allow the outer surfaces of both the driving rollers  12  and the driven rollers  14  to contact each other, and, as shown in FIG. 4, the driving rollers  12  are inserted into the driving roller through-holes  47  by the thickness of the support plate  33 . Therefore, the upper portions of the driving rollers  12  inserted into the driving roller through-holes  47  lie substantially flush with the top surface of the support plate  33 , and a sheet of paper passing between the driving rollers  12  and the driven rollers  14  beyond the driving roller through-holes  47  is not bent upward nor downward. 
     The cover plate  31  is spaced apart from and parallel to the support plate  33  to define a predetermined paper traveling path therebetween, and is fixedly mounted to the frame  10 . As shown in FIG. 3, the cover plate  31  guides a sheet of paper passing on the support plate  33  along the predetermined paper traveling path and has a number of driven roller through-holes  45  and extending contact arm through-holes  49  formed thereon. 
     The driven roller through-holes  45  (see FIG. 3) are provided such that they receive the lower portions of the driven rollers  14  to allow the outer surfaces of both the driving rollers  12  and the driven rollers  14  to contact each other. Also, the extending contact arm through-holes  49  are provided such that they receive the extending projections  54  of the extending contact arms  41  to allow a contact surface  43  (see FIGS. 3 and 5) of the extending projections  54  to contact the top surface of the support plate  33 . 
     The rotating shaft  35 , which is disposed in parallel with the driven shaft  28  over the top surface of the cover plate  31 , is rotatably mounted at one end to the frame  10 , and fixedly mounts the plurality of extending contact arms  41  thereon. 
     The extending contact arms  41  are fixedly mounted on the rotating shaft  35  to each correspond to one of the extending contact arm through-holes  49 . Each of the extending contact arms  41  includes the extending projection  54  which extends toward the top surface of the support plate  33  from one side of the outer surface of the extending contact arm like a cam. The extending projection  54  passes though an extending contact arm through-hole  49  to allow a contact surface thereof to contact the top surface of the support plate  33 . Also, as shown in FIG. 5, the bottom surface of the extending projection  54  of the extending contact arm  41  is curved so that the contact surface  43  thereof, which contacts the top surface of the support plate  33  is rounded. 
     Accordingly, the support plate  33  and the cover plate  31  are fixedly mounted to the frame  10  such that they define a predetermined paper traveling path therebetween. The driving rollers  12  and the driven rollers  14  contact each other through the driving roller through-holes  47  and the driven roller through-holes  45 . In addition, the extending projections  54  of the extending contact arms  41  pass through the extending contact arm through-holes  49  to allow the contact surfaces  43  thereof to contact the top surface of the support plate  33 . After that, the extending projections  54  wait for a sheet of paper to come along the predetermined paper traveling path defined between the support plate  33  and the cover plate  31 . 
     In the meantime, the pivoting rod  26  is fixedly mounted at one end portion to one end of the rotating shaft  35 , perpendicular to the rotating shaft  35 , to thus rotate together with the rotating shaft  35 . Also, the other end portion of the pivoting rod  26  is coupled to the magnetic sensor  39  through the link member  37 . 
     The magnetic sensor  39  is a well-known sensor which generates electrical current when a center shaft  51  (see FIG. 6) thereof rotates to detect the passing of a sheet of paper between the driving rollers  12  and the driven rollers  14  and transmits the detected result to an external controller (not shown). A body of the magnetic sensor  39  is fixedly mounted to the frame  10 , and the sensor shaft  51  thereof is fixedly connected to an end of the link member  37 . 
     The link member  37  acts to convert the pivotal movement of the pivoting rod  26  into the rotational movement of the sensor shaft  51  of the magnetic sensor  39 , which is coupled at one end to a free end portion of the pivoting rod  26  through a connecting shaft  53  thereof and is rotatably coupled at the other end to the center axis of the magnetic sensor  39 . At this time, the connecting shaft  53  of the link member  37  is rotatably coupled to the link member  37  and is fixedly mounted to the free end portion of the pivoting rod  26 . Here, the pivoting rod  26 , the connecting shaft  53  and the link member  37  constitute an amplifier for amplifying an amount of rotation of the rotating shaft  35  to axially rotate the sensor shaft  51  of the magnetic sensor  39 . 
     Therefore, when the rotating shaft  35  rotates by a certain angle in the direction of an arrow “a”, the pivoting rod  26  pivots in the direction of an arrow “b” around the rotating shaft  35 , which causes the sensor shaft  51  of the magnetic sensor  39  to rotate in the direction of an arrow “c”. The pivotal movement of the pivoting rod  26  in the direction of the arrow “b” occurs when a sheet of paper passes between the top surface of the support plate  33  and the contact surfaces  43  of the extending projection  54  which are in contact with each other. 
     Consequently, the paper feed sensing apparatus of the present invention detects an ascending degree of the contact surfaces  43  (see FIG. 5) which are moved upward by a height corresponding to the thickness of the sheet of paper being fed by means of the magnetic sensor  39  when a sheet of paper passes between the support plate  33  and the extending contact arms  41 . Of course, the greater the ascending degree of the contact surfaces  43 , the larger an angle of rotation of the sensor shaft  51  of the magnetic sensor  39 . 
     FIG. 3 is an exploded perspective view illustrating the construction of the paper feed sensing apparatus of FIG.  2 . 
     As shown in FIG. 3, the support plate  33  with a certain thickness has a plurality of driving roller through-holes  47  formed uniformly spaced apart thereon. Each of the driving roller through-holes  47  is of a quadrangular shape and a part of an upper portion of a corresponding driving roller  12  is moved upward and inserted into each driving roller through-hole  47  by the thickness of the support plate  33 . 
     The cover plate  31  positioned over the support plate  33  also has a plurality of driven roller through-holes  45  of a quadrangular shape formed thereon to correspond to one of the plurality of driving roller through-holes  47 . Each of the driven roller through-holes  45  functions as a passage for making it possible for a part of the lower portion of the driven rollers  14  to downwardly pass therethrough so that the outer surfaces of both the driving rollers  12  and the driven rollers  14  contact each other. 
     Also, each of the extending contact arm through-holes  49  of a rectangular shape, which is disposed between a pair of driven roller through-holes  45 , functions as a passage for making it possible for the extending projections  54  of the extending contact arms  41  to pass therethrough so that the contact surface  43  of the extending projections  54  comes into contact with the top surface of the support plate  33 . According to other embodiments, the extending contact arm through-hole  49  and the driven roller through-hole  45  may be formed as an integrated single hole. 
     The extending projection  54  of the extending contact arm  41  fixedly mounted on the rotating shaft  35  is bent upward so that the bottom surface thereof, i.e., the contact surface  43 , is curved. The contact surface  43  comes into contact with a sheet of paper being fed along the predetermined paper traveling path defined between the support plate  33  and the cover plate  31  so that it is upward pushed to be spaced apart from the support plate  33 . In this way, since the contact surface  43  is curved, a sheet of paper can easily enter a contact boundary portion between the extending contact arm  41  and the support plate  33 . 
     In the meantime, the rotating shaft  35  fixedly supporting the extending contact arm  41  and the contact surface  43  are spaced apart from each other by a distance of “d” as shown in FIG.  6 . The aim of this is to enable the rotating shaft  35  to rotate in response to the upwardly rotational movement of the extending projection  54  as well as to make it possible for the contact surface  43  to press against the top surface of the support plate  33  by means of a load of the extending projection  54 . 
     FIG. 4 is a cross-sectional view taken along line IV—IV in FIG. 2, which shows a sheet feeding construction in which a sheet of paper is transferred between a driving roller  12  and a driven roller  14 . 
     Referring to FIG. 4, it can be seen that a driving roller  12  and a driven roller  14  are guided into the driving roller through-hole  47  and the driven roller through-hole  45 , respectively, to contact each other. At this time, the upper portion of the driving roller  12  lies substantially flush with the top surface of the support plate  33 . Accordingly, in the case where a sheet of paper (p) is fed along the predetermined paper traveling path defined between the support plate  33  and the cover plate  31 , although the sheet of paper passes the upper portion of driving roller through-hole  47 , it can be continuously transferred without being depressed downwardly. 
     FIG. 5 is a cross-sectional view taken along line V—V in FIG.  2 . 
     Referring to FIG. 5, the extending projection  54  of the extending contact arm  41  fixedly mounted on the rotating shaft  35  is guided into the extending contact arm through-hole  49  so that its contact surface  43  comes into contact with the top surface of the support plate  33 . The contact surface  43  of the bottom portion of the extending projection  54  is always biased downward in the direction of an arrow “f” relative to the top surface of the support plate  33  by the load of the extending projection. That is, in the extending contact arm  41 , the extending projection side is heavier by virtue of its asymmetrical structure due to the extending projection  54 , so that the contact surface  43  presses down against the top surface of the support plate  33  with a force corresponding to the weight of the extending projection  54 . 
     However, since the force applied downward in the direction of an arrow “f” through the contact surface  43  is smaller than the feeding force of a sheet of paper (p), it does not interfere with the movement of the sheet of paper. Accordingly, when the sheet of paper (p) is slid between the contact surface  43  of the extending contact arm  41  and the top surface of the support plate  33 , the extending projection  54  is pushed upward by the thickness of the guided sheet of paper (p) so that the contact surface  43  ascends by a height corresponding to the thickness of the sheet of paper (p) while being pressed against the top surface of the sheet of paper (p) downward in the direction of an arrow “f”. 
     The operation of a paper feed sensing apparatus according to an embodiment of the present invention will be described in detail hereinafter with reference to FIGS. 6,  7  and  8 . 
     FIGS. 6,  7  and  8  are cross-sectional views illustrating the operation of the sheet feeding apparatus according to the present invention. 
     FIG. 6 shows the operation just before a sheet of paper (p) is inserted between the support plate  33  and the cover plate  31 . 
     Referring to FIG. 6, the sheet of paper (p) is fed along a predetermined paper traveling path between the support plate  33  and the cover plate  31  in the slant direction of an arrow “i” so as to be slid beneath the bottom end portion of the extending projection  54 . At this time, the contact surface  43  of the extending contact arm  41  is in contact with the top surface of the support plate  33 , and the rotatably moving rod  26  and the magnetic sensor  39  are maintained in an initially set state. 
     Referring to FIG. 7, as the sheet of paper (p) is fed along the predetermined paper traveling path between the support plate  33  and the cover plate  31  in the slant direction of an arrow “i”, it first reaches the contact surface between the driving roller  12  and the driven roller  14  and then is slid beneath the bottom end portion of the extending projection  54  by means of the feeding force applied thereto from the driving roller  12  and the driven roller  14 . At this time, the extending projection  54  is pushed upward by the thickness of the guided sheet of paper (p) while being rotated upward, which causes the rotating shaft  35  to angularly rotate in the counterclockwise direction of an arrow “k”. 
     As the rotating shaft  35  rotates by a certain angle, the pivoting rod  26  pivots around the rotating shaft  35 , and the link member  37  rotatably mounted to a free end of the pivoting rod  26  rotates in the counterclockwise direction relative to the pivoting rod  26 , which causes the sensor shaft  51  of the magnetic sensor  39  to rotate in the direction of an arrow “m”. 
     At this time, one sheet of paper passes beneath the bottom end portion of the extending projection  54 , which means a normal operational state. Thus, a degree of angular rotation of the sensor shaft  51  is within a normal range, so that an erroneous signal is not generated. 
     On the other hand, as shown in FIG. 8, in the case where two overlapping sheets of paper (p) are slid beneath the bottom end portion of the extending projection  54 , the ascending height of the extending projection  54  is twice that in the case of FIG.  7 . Accordingly, the rotational angles of both the rotating shaft  35  and the sensor shaft  51  of the magnetic sensor  39  are also twice those in the case of FIG. 7, so that an error signal is generated. 
     It should, of course, be noted that in the case of the magnetic sensor  39 , the rotational angle of the sensor shaft  51  required for generating an error signal could be determined when adjusting the settings of the paper feed sensing apparatus. 
     As described above, the paper feed sensing apparatus according to the present invention has the advantage of detecting a variation in the thickness of a sheet of paper being fed between the driving roller  12  and the driven roller  14  along the predetermined paper traveling path between the support plate  33  and the cover plate  31  by detecting a variation in the distance between the top surface  43  of the support plate  33  and the contact surface of the extending projection  54  of the extending contact arm  41  which is biased toward the top surface of the support plate  33 , so that the machined very precisely rollers of the conventional art are not required. 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various modifications may be made without departing from the spirit of the invention. The scope of the invention, therefore, is to be determined solely by the appended claims.