Patent Publication Number: US-6662139-B2

Title: Drawing apparatus and method of attaching balance weights

Description:
This is a Division of application Ser. No. 09/249,088 filed Feb. 12, 1999, now U.S. Pat. No. 6,505,142. The disclosure of the prior application is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a drawing apparatus for performing drawing on a recording material such as a plate mounted on a drum and a method of attaching balance weights for adjusting a balanced state at the time of rotating the drum. 
     2. Description of the Background Art 
     A drawing apparatus for performing drawing upon irradiation of various types of photosensitive materials with light has been used. In a drum-type drawing apparatus, a photosensitive material is attached to a recording drum which is rotatable in a primary scanning direction, and a recording head comprising a laser diode and the like is moved in a secondary scanning direction parallel to a rotary shaft of the recording drum, to perform drawing on the photosensitive material. 
     FIG. 37 is a schematic perspective view of a recording drum of a conventional drawing apparatus. A recording drum  1  shown in FIG. 37 is so constructed that a plate  2  made of aluminum can be mounted thereon as a photosensitive material. A fixing clamp  4  for fixing one end of the plate  2  and a movable clamp  5  for fixing the other end of the plate  2  are provided on an outer peripheral surface of the recording drum  1 . The movable clamp  5  is formed so as to be movable on the outer peripheral surface of the recording drum  1  depending on the size of the plate  2 . The recording drum  1  to which the plate  2  is attached is rotated at relatively low speed around the rotary shaft  3 , so that drawing is performed on the surface of the plate  2 . 
     In recent years, it has been needed to improve the efficiency of drawing processing on the plate  2 . Therefore, it has been examined whether the recording drum  1  to which the plate  2  is attached is rotated at high speed to perform a drawing operation. 
     However, the plate  2  made of aluminum is heavier than a film material or the like, and is attached in an offset state to a part of the outer peripheral surface of the recording drum  1 . When the recording drum  1  is rotated at high speed, therefore, a centrifugal force developed by the plate  2  causes an unbalance force to be exerted on the rotary shaft of the recording drum  1 . The fixing clamp  4  and the movable clamp  5  are provided on the outer peripheral surface of the recording drum  1 . The fixing clamp  4  and the movable clamp  5  respectively have corresponding masses. When the recording drum  1  is rotated at high speed, similarly to the plate  2 , therefore, centrifugal forces developed by the fixing clamp  4  and the movable clamp  5  cause an unbalance force. 
     When the unbalance force  5  caused by the plate  2 , the fixing clamp  4  and the movable clamp  5  is exerted on the recording drum  1 , forced vibration may occur in bearings for supporting the rotary shaft  3  of the recording drum  1  when the recording drum  1  is rotated at high speed, so that the recording drum may develop a fault. The production of such an unbalance force presents no problem when the recording drum  1  is rotated at low speed, as in the conventional example, while presenting a large problem as the recording drum  1  is rotated at high speed in order to make the drawing operation efficient. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a drawing apparatus in which no forced vibration occurs by an unbalance between centrifugal forces at the time of rotation and a method of attaching balance weights. 
     A drawing apparatus according to the present invention is a drawing apparatus for drawing an image on a recording material, comprising a cylindrical drum having a rotary shaft and an outer peripheral surface, driving means for rotating the drum around the rotary shaft, a fixing member provided on the outer peripheral surface of the drum for fixing the recording material to the outer peripheral surface of the drum, a plurality of balance weights provided so as to be movable and fixable along a circumference whose center coincides with the rotary shaft of the drum, and position calculation means for calculating, on the basis of information relating to the recording material and the fixing member, the positions of the plurality of balance weights for canceling an unbalance force caused by centrifugal forces developed by the fixing member and the recording material at the time of rotating the drum. 
     In the drawing apparatus, the recording material is mounted on the outer peripheral surface of the cylindrical drum by the fixing member, and is rotated by the driving means. An image is drawn onto the recording material which is rotated as the drum is rotated. The fixing member and the recording material which are provided on the outer peripheral surface of the cylindrical drum respectively develop centrifugal forces in different directions at the time of rotating the drum. Therefore, the unbalance force caused by the centrifugal forces developed by the recording material and the fixing member is exerted on the rotary shaft of the drum. 
     On the other hand, the plurality of balance weights are provided along the circumference whose center coincides with the rotary shaft of the drum. Each of the balance weights develops the centrifugal force by the rotation of the drum. The plurality of balance weights are arranged such that a composite force of the centrifugal forces developed by the balance weights is balanced with the unbalanced force caused by the recording material and the fixing member. Even when the drum is rotated, therefore, the unbalance force is prevented from being exerted on the rotary shaft. Therefore, it is possible to prevent the drum from developing a fault by forced vibration occurring in the rotary shaft of the drum. The plurality of balance weights are provided so as to be movable and fixable along the circumference whose center coincides with the rotary shaft of the drum. Even when the weight and the size of the recording material are changed, therefore, it is possible to cancel the unbalance force caused by the recording material and the fixing member by changing the positions of the plurality of balance weights. 
     The position calculation means calculates the positions of the balance weights on the basis of the information relating to the recording material and the fixing member. Even when a different type of recording material is used, therefore, it is possible to calculate the most suitable position of the balance weight depending on the type. 
     The cylindrical drum may further have two end surfaces disposed on opposite sides thereof, and the plurality of balance weights may comprise at least two balance weights provided on each of both the end surfaces of the drum so as to be movable and fixable along the circumference. 
     In this case, at least two balance weights are provided on each of both the end surfaces of the drum. Even when the weight and the size of the recording material are changed, therefore, it is possible to cancel the unbalance force caused by the recording material and the fixing member by changing the positions of the balance weights. 
     An annular groove whose center coincides with the rotary shaft may be formed on each of both the end surfaces of the drum, and the plurality of balance weights may be provided so as to be movable and fixable along the annular groove. 
     In this case, it is easy to move and position the plurality of balance weights by providing the annular grooves. 
     Each of the plurality of balance weights may include a magnet which is attractable to each of both the end surfaces of the drum. 
     The balance weight includes a magnet, thereby making it possible to easily move and fix the balance weights on both the end surfaces of the drum without using a special structure. 
     The drawing apparatus may further comprise movement means for moving the plurality of balance weights to the positions calculated by the position calculation means. 
     In this case, the movement means can move the plurality of balance weights. Therefore, it is possible to rotate the drum at high speed without causing forced vibration in the rotary shaft of the drum irrespective of the type of the recording material. 
     The movement means may comprise engagement means for engaging one of the balance weights provided on the drum, and control means for rotating the drum by activating the driving means in a state where the balance weight is engaged by the engagement means, to move the balance weight to the position calculated by the position calculation means. 
     The engagement means engages one of the balance weights, to stop the balance weight in the position. In this state, the driving means rotates the drum, thereby making it possible to move the position of the balance weight on the end surface of the drum to a predetermined position. 
     The drawing apparatus may further comprise storage means for storing the position information of each of the balance weights on each of the end surfaces of the drum, and the control means may rotate the drum by activating the driving means to the position where the balance weight is to be engaged by the engagement means on the basis of the position information stored in the storage means. 
     In this case, the storage means stores the position information of the balance weights on the end surface of the drum. Therefore, the control means takes out the position information from the storage means, and refers to the position information to rotate the drum, so that the engagement means can easily engage the balance weights. 
     The drawing apparatus may further comprise detection means for obtaining the position information of each of the balance weights on each of the end surfaces of the drum, and the control means may rotate the drum by activating the driving means to the position where the balance weight is to be engaged by the engagement means on the basis of the position information obtained by the detection means. 
     In this case, the position information of each of the balance weights on each of the end surfaces of the drum is obtained by the detection means. The control means rotates the drum on the basis of the obtained position information of the balance weight, so that the engagement means can easily engage the balance weight. 
     The drawing apparatus may further comprise input means for inputting the information relating to the recording material. 
     The information relating to various types of recording materials are inputted using the input means. The position calculation means calculates the positions of the plurality of balance weights on the basis of the information inputted from the input mean. When a different type of recording material is used, therefore, it is possible to arrange the plurality of balance weights in the most suitable positions irrespective of the type of the recording material by inputting the information. 
     The position calculation means may calculate, on the basis of the information relating to the recording material and the fixing member, an unbalance amount caused by the recording material and the fixing member, and calculate the positions of the plurality of balance weights such that a weighting amount caused by the plurality of balance weights is balanced with the unbalance amount. 
     Consequently, it is possible to easily calculate the positions of the plurality of balance weights such that the weighting amount caused by the plurality of balance weights are balanced with the unbalance amount caused by the rotation of the fixing member and the recording material. 
     The drawing apparatus may further comprise information detection means for detecting the information relating to the recording material mounted on the drum. 
     In this case, the information relating to the recording material mounted on the drum is detected, and the positions of the plurality of balance weights are calculated such that the unbalance force caused by the centrifugal forces exerted on the drum is canceled on the basis of the detected information. Even when the setting of the information relating to the recording material is erroneous, forced vibration is prevented from occurring in the drum by the unbalance force at the time of rotating the drum. 
     The information detection means may detect the size of the recording material mounted on the drum. 
     In this case, the size of the recording material mounted on the drum is detected. Even when the setting of the size of the recording material is erroneous, therefore, forced vibration is prevented from occurring by the unbalance force at the time of rotating the drum. 
     The drawing apparatus may further comprise unbalance detection means for detecting the presence or absence of an unbalance force caused by the centrifugal forces exerted on the drum. 
     In this case, the presence or absence of the unbalance force caused by the centrifugal forces exerted on the drum is detected, thereby making it possible to prevent the forced vibration from occurring by the unbalance force at the time of rotating the drum. 
     The unbalance detection means may comprise driving release means for releasing a driving force of the driving means, to bring the drum into a free state from the driving means, rotation detection means for detecting whether or not the drum is rotated after being brought into a free state by the driving release means, and judgment means for judging the presence or absence of the unbalance force on the basis of the result of the detection by the rotation detection means. 
     In this case, the driving force of the driving means is released, so that the drum is brought into a free state from the driving means. When the unbalance force exists, the drum is rotated. Therefore, it is possible to judge the presence or absence of the unbalance force by detecting whether or not the drum is rotated. 
     The judgment means may judge that the unbalance force exists when the rotation of the drum is detected, while rotating the drum through a predetermined angle by activating the driving means and then releasing the driving force of the driving means by the driving release means when no rotation of the drum is detected, and judging that the unbalance force exists when the rotation of the drum is detected, while judging that the unbalance force does not exist when no rotation of the drum is detected. 
     When the drum is rotated after being brought into a free state, it is possible to judge that the unbalance force exists. Even in a case where the unbalance force exists, when the direction of the unbalance force coincides with the direction of gravity when the drum is brought into a free state, the drum is not rotated. When it is not detected that the drum is rotated, therefore, the drum is rotated through a predetermined angle by the driving means, and is then brought into a free state. When the drum is rotated in this state, it can be judged that the unbalance force exits. When the drum is not rotated in this state, it can be judged that the unbalance force does not exist. 
     In a drawing apparatus for drawing an image on a recording material while rotating a cylindrical drum on which the recording material is mounted by a fixing member, a method of attaching unbalance weights according to another aspect of the present invention is a method of attaching on the drum a plurality of balance weights for canceling an unbalance force caused by centrifugal forces developed by the recording material and the fixing member at the time of rotating the drum, which comprises the steps of calculating, on the basis of information relating to the recording material and the fixing member, the positions of the plurality of balance weights for canceling the unbalance force, the positions of the plurality of balance weights being determined along a circumference whose center coincides with a rotary shaft of the drum, and attaching the plurality of balance weights to the calculated positions. 
     In the method of attaching the balance weights, it is possible to attach the balance weights along the circumference whose center coincides with the rotary shaft of the drum such that the unbalance force caused by the fixing member and the recording material is canceled. Consequently, it is possible to perform drawing while rotating the drum at high speed without causing forced vibration in the rotary shaft of the drum. 
     The plurality of balance weights may be at least two balance weights provided on each of both the end surfaces of the drum along the circumference. 
     In this case, it is possible to mount at least two balance weights on each of both the end surfaces of the drum so as to cancel the unbalance force caused by the fixing member and the recording material. Consequently, it is possible to perform drawing while rotating the drum at high speed without causing forced vibration in the rotary shaft of the drum. 
     The step of calculating the positions may comprise the steps of calculating, on the basis of the information relating to the recording material and the fixing member, an unbalance amount caused by the recording material and the fixing member, and calculating the positions of the plurality of balance weights such that a weighting amount caused by the plurality of balance weights is balanced with the unbalance amount. 
     Consequently, the unbalance amount caused by the recording material and the fixing member is calculated for each drawing processing of various types of recording materials, and the positions of the plurality of balance weights are calculated such that the weighting amount caused by the balance weights is balanced with the unbalance amount. The balance weights are attached to both the end surfaces of the drum on the basis of the calculated positions of the balance weights, thereby making it possible to prevent forced vibration from occurring in the rotary shaft of the drum. 
     A drawing apparatus according to still another aspect of the present invention is a drawing apparatus for drawing an image on a recording material, which comprises a cylindrical drum having a rotary shaft and an outer peripheral surface, driving means for rotating the drum around the rotary shaft, a fixing member provided on the outer peripheral surface of the drum for fixing the recording material to the outer peripheral surface of the drum, and a pair of balance weights, each of which is provided at each of ends of the drum so as to be movable and fixable along a circumference whose center coincides with the rotary shaft of the drum and a radius of the circumference. 
     In the drawing apparatus, the recording material is attached to the outer peripheral surface of the cylindrical drum by the fixing member, and is rotated by the driving means. Drawing is performed onto the recording material which is rotated as the drum is rotated. The fixing member and the recording material which are provided on the outer peripheral surface of the cylindrical drum respectively develop centrifugal forces in different directions at the time of rotating the drum. Therefore, the unbalance force caused by the centrifugal forces developed by the recording material and the fixing member is exerted on the rotary shaft of the drum. 
     On the other hand, the pair of balance weights is provided at each of both the ends of the drum. Each of the balance weights develops a centrifugal force by the rotation of the drum. The pair of unbalance weights is arranged such that the composite force of the centrifugal forces developed by the pair of balance weights is balanced with the unbalance force caused by the recording material and the fixing member. Even when the drum is rotated, therefore, the unbalance force is prevented from being exerted on the rotary shaft, so that the drum can be prevented from developing a fault by forced vibration occurring in the rotary shaft of the drum. The pair of balance weights provided at each of both the ends of the drum is provided so as to be movable and fixable along the circumference and the radius of the drum. Even when the weight and the size of the recording material are changed, therefore, it is possible to cancel the unbalance force caused by the recording material and the fixing member by changing the position of each of the balance weights in the radial direction and the circumferential direction. 
     The cylindrical drum may further have two end surfaces disposed on opposite sides thereof, and each of the pair of balance weights may be provided on each of the end surfaces of the drum. 
     In this case, each of the balance weights is provided on each of the end surfaces of the drum so as to be movable and fixable along the circumference and the radius of the drum. Even when the weight and the size of the recording material are changed, therefore, it is possible to cancel the unbalance force caused by the recording material and the fixing member by changing the position of each of the balance weights in the radial direction and the circumferential direction. 
     Each of the pair of balance weights may include a magnet which is attractable to each of both the end surfaces of the drum. 
     By composing the balance weights of the magnet, the balance weights can be easily moved on both the end surfaces of the drum and fixed thereto without using a special structure. 
     The drawing apparatus may further comprise position calculation means for calculating, on the basis of information relating to the recording material and the fixing member, the positions of the pair of balance weights for canceling an unbalance force caused by centrifugal forces developed by the fixing member and the recording material at the time of rotating the drum, and movement means for moving the pair of balance weights to the positions calculated by the position calculation means. 
     In this case, the position calculation means calculates the positions of the balance weights on the basis of the information relating to the recording material and the fixing member. Even when a different type of recording material is used, therefore, it is possible to calculate the most suitable position of the balance weight depending on the type. Consequently, the movement means can move the pair of balance weights. Therefore, it is possible to rotate the drum at high speed without causing forced vibration in the rotary shaft of the drum irrespective of the type of the recording material. 
     The position calculation means may calculate the respective positions, along the radius and the circumference, of the balance weights on each of the end surfaces of the drum, and the movement means may comprise engagement means to be engaged with one of the balance weights provided on each of the end surfaces of the drum, the engagement means being movable along the radius, and control means for moving the engagement means to transfer the balance weight to the calculated position along the radius on the end surface of the drum, and for rotating the drum by activating the driving means in a state where the balance weight is engaged with the engagement means, to transfer the balance weight to the calculated position along the circumference on the end surface of the drum. 
     In this case, the engagement means is engaged with each of the balance weights, and is moved in the radial direction, to define the position, along the radius on the end surface of the drum, of the balance weight. Further, the driving means rotates the drum in a state where the engagement means is engaged with the balance weight, so that the position, along the circumference on the end surface of the drum, of the balance weight is defined. Consequently, the balance weight is moved to a predetermined position on the end surface of the drum, thereby making it possible to cancel the unbalance force caused at the time of rotating the drum. 
     The drawing apparatus may further comprise storage means for storing the position information of each of the balance weights on each of the end surfaces of the drum, and the control means may move the engagement means and rotates the drum on the basis of the position information stored in the strage means, whereby the engagement means being engaged with the balance weight. 
     In this case, the storage means stores the position information of the pair of balance weights on the end surface of the drum. Therefore, the control means takes out position information of the balance weights from the storage means, refers to the position information, to move the engagement means in the radial direction, and rotates the drum, thereby making it possible to easily engage the engagement means with the balance weights. 
     The drawing apparatus may further comprise input means for inputting the information relating to the recording material. 
     The information relating to various types of recording materials may be inputted using the input means. The position calculation means calculates the positions of the pair of balance weights on the basis of the information inputted from the input means. When a different type of recording material is used, therefore, the information is inputted, thereby making it possible to arrange the pair of balance weights in the most suitable positions irrespective of the type of the recording material. 
     The position calculation means may calculate, on the basis of the information relating to the recording material and the fixing member, an unbalance amount caused by the recording material and the fixing member, and calculate the positions of the pair of balance weights such that a weighting amount caused by the pair of balance weights is balanced with the unbalance amount. 
     Consequently, it is possible to easily calculate the positions of the pair of balance weights such that the balance weights are balanced with the unbalance amount caused by the rotation of the fixing member and the recording material. 
     In a drawing apparatus for drawing an image on a recording material while rotating a cylindrical drum on which the recording material is mounted by a fixing member, a method of attaching balance weights according to a further aspect of the present invention is a method of attaching to the drum a pair of balance weights for canceling an unbalance force caused by centrifugal forces developed by the recording material and the fixing member at the time of rotating the drum, which comprises the steps of calculating the positions where the pair of balance weights, each of which is provided at each of ends of the drum so as to develop centrifugal forces for canceling the unbalance force exerted on the drum, and attaching the pair of balance weights to the calculated positions. 
     In the method of attaching the balance weights, it is possible to mount the pair of balance weights at both the ends of the drum such that the unbalance force caused by the fixing member and the recording material is canceled. Consequently, it is possible to rotate the drum at high speed without causing forced vibration in the rotary shaft of the drum, to perform drawing. 
     Each of the pair of balance weights may be provided on each of both end surfaces of the drum. 
     In this case, it is possible to attach each of the pair of balance weights on each of both the end surfaces of the drum such that the unbalance force caused by the fixing member and the recording material is canceled. Consequently, it is possible to rotate the drum at high speed without causing forced vibration in the rotary shaft of the drum, to perform drawing. 
     The step of calculating the positions may comprise the steps of calculating, on the basis of information relating to the recording material and the fixing member, an unbalance amount caused by the recording material and the fixing member, and calculating the positions of the pair of balance weights such that a weighting amount caused by the pair of balance weights is balanced with the unbalance amount. 
     In this case, the unbalance amount caused by the recording material and the fixing member is calculated for each drawing processing of various types of recording materials. Further, the positions of the pair of balance weights are calculated such that the weighting amount caused by the balance weights is balanced with the unbalance amount. The balance weights are attached to both the end surfaces of the drum on the basis of the calculated positions of the balance weights, thereby making it possible to prevent forced vibration from occurring in the rotary shaft of the drum. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing the construction of a drawing apparatus according to a first embodiment of the present invention; 
     FIG. 2 is a schematic view of a principal part of the drawing apparatus shown in FIG. 1; 
     FIG. 3 is a perspective view of a recording drum; 
     FIG. 4 is a side view of the recording drum shown in FIG. 2; 
     FIGS.  5 ( a ) and  5 ( b ) are schematic views showing an unbalance force caused by centrifugal forces developed by a movable clamp and a plate in the recording drum, and diagrams showing an unbalance force vector; 
     FIGS.  6 ( a ) and  6 ( b ) are schematic views showing a balance force caused by centrifugal forces developed by balance weights in the recording drum, and diagrams showing a balance force vector; 
     FIG. 7 is a schematic view of a drawing apparatus according to a second embodiment of the present invention; 
     FIG. 8 is a schematic side view of a recording drum in a drawing apparatus according to a third embodiment of the present invention; 
     FIG. 9 is a schematic front view of a drawing apparatus in the fourth embodiment of the present invention; 
     FIG. 10 is a schematic side view of the drawing apparatus shown in FIG. 9; 
     FIG. 11 is a front view showing one end of a recording drum shown in FIGS. 9 and 10; 
     FIG. 12 is a perspective view of a balance weight used for the drawing apparatus shown in FIGS. 9 and 10; 
     FIGS.  13 ( a ) and  13 ( b ) are front views showing the structure and the operation of a weight position adjuster shown in FIGS. 9 and 10; 
     FIG. 14 is a side view showing the structure and the operation of a conveying unit shown in FIG. 10; 
     FIG. 15 is a side view showing the structure and the operation of the conveying unit shown in FIG.  10 . 
     FIG. 16 is a flow chart showing an example of operations of the drawing apparatus shown in FIGS. 9 and 10; 
     FIG. 17 is a flow chart showing an example of operations of the drawing apparatus shown in FIGS. 9 and 10; 
     FIG. 18 is a flow chart showing unbalance detection processing; 
     FIG. 19 is a flow chart showing another example of operations of the drawing apparatus shown in FIGS. 9 and 10; 
     FIGS.  20 ( a ) and  20 ( b ) are front views and side views showing another example of a balance weight attaching method; 
     FIGS.  21 ( a ) and  21 ( b ) are front views and side views showing another example of a balance weight attaching method; 
     FIGS.  22 ( a ) and  22 ( b ) are front views and side views showing another example of a balance weight attaching method; 
     FIGS.  23 ( a ) and  23 ( b ) are front views and side views showing another example of a balance weight attaching method; 
     FIGS.  24 ( a ) and  24 ( b ) are front views and side views showing another example of a balance weight attaching method; 
     FIG. 25 is a cross-sectional view showing a first example of the position where a balance weight is attached in the balance weight attaching method shown in FIG. 24; 
     FIG. 26 is a cross-sectional view showing a second example of the position where a balance weight is attached in the balance weight attaching method shown in FIG. 24; 
     FIG. 27 is a cross-sectional view showing a third example of the position where a balance weight is attached in the balance weight attaching method shown in FIG. 24; 
     FIG. 28 is a schematic view showing the construction of a drawing apparatus according to a fifth embodiment of the present invention; 
     FIG. 29 is a schematic view of a principal part of the drawing apparatus shown in FIG. 28; 
     FIG. 30 is a perspective view of a recording drum; 
     FIG. 31 is a side view of the recording drum shown in FIG. 29; 
     FIGS.  32 ( a ) and  32 ( b ) are side views and front views of a weight position adjuster in the drawing apparatus; 
     FIGS.  33 ( a ) and  33 ( b ) are schematic views of an unbalance force caused by centrifugal forces developed by a movable clamp and a plate in the recording drum, and diagrams showing an unbalance force vector; 
     FIG. 34 is a schematic view showing a balance force caused by centrifugal forces developed by balance weights in the recording drum; 
     FIGS.  35 ( a ) and  35 ( b ) are front views and side views showing another example of a balance weight attaching method; 
     FIGS.  36 ( a ) and  36 ( b ) are front views and side views showing another example of a balance weight attaching method; and 
     FIG. 37 is a perspective view of a recording drum in a conventional drawing apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic view of a drawing apparatus in a first embodiment of the present invention, and FIG. 2 is a schematic view of a principal part of the drawing apparatus shown in FIG.  1 . 
     In FIG. 1, the drawing apparatus comprises a cylindrical recording drum  1 . The recording drum  1  is supported so as to be rotatable around a rotary shaft  3  having both its ends supported on bearings. A driving mechanism comprising a pair of pulleys  9  and  11 , a belt  10  and a motor  12  is connected to one end of the rotary shaft  3 . The driving mechanism transmits the rotation of the motor  12  to the rotary shaft  3  through the pair of pulleys  9  and  11  and the belt  10 , to rotate the recording drum  1 . 
     In FIG. 2, a recording head  120  is disposed in a position opposite to an outer peripheral surface of the recording drum  1 . The recording head  120  comprises a plurality of laser diodes  100 . The recording head  120  is attached to a guide  130  so as to be movable, and is moved in a secondary scanning direction B in synchronization with the rotation of the recording drum  1 . 
     The current position in a primary scanning direction A of the plate  2  is detected on the basis of a detection signal outputted from a rotation angle detector  13  attached to the rotary shaft  3  of the recording drum  1 . The current position in the secondary scanning direction B of the recording head  120  is detected on the basis of a detection signal outputted from a linear encoder  150 . 
     The plurality of laser diodes  100  in the recording head  120  are driven by a laser diode driving circuit portion  160 . The laser diode driving circuit portion  160  comprises a plurality of laser diode driving circuits  110  corresponding to the plurality of laser diodes  100  in the recording head  120 . 
     An image signal generator circuit  180  generates a serial image signal VDS. A serial/parallel converter  170  converts the serial image signal VDS generated by the image signal generator circuit  180  into parallel image signals VD 1  to VDn, and respectively feeds the image signals VD 1  to VDn to the plurality of laser diode driving circuits  110  in the laser diode driving circuit portion  160 . Consequently, each of the laser diodes  100  in the recording head  120  is driven by the corresponding laser diode driving circuit  110 , to irradiate the plate  2  with laser light. 
     FIG. 3 is a perspective view of the recording drum, and FIG. 4 is a side view of the recording drum. A fixing clamp  4  and a movable clamp  5  for attaching the plate  2  are disposed on the outer peripheral surface of the recording drum  1 . The fixing clamp  4  is fixed to the outer peripheral surface of the recording drum  1 , and the movable clamp  5  is formed so as to be movable on the outer peripheral surface of the recording drum  1  depending on the size of the plate  2 . A fixing clamp balance member  8  is attached to the outer peripheral surface of the recording drum  1  on the opposite side of the fixing clamp  4 . The fixing clamp balance member  8  is formed so as to have an approximately equal weight to that of the fixing clamp  4 . Consequently, the fixing clamp balance member  8  develops a centrifugal force which is balanced with a centrifugal force developed by the fixing clamp  4  at the time of rotating the recording drum  1 , thereby preventing an unbalance force from being produced. 
     Annular grooves  6  whose center is the rotary shaft  3  are respectively formed on both end surfaces of the recording drum  1 . Two balance weights  7  are disposed inside the annular groove  6  on each of the end surfaces. The balance weight  7  is composed of a permanent magnet, and is attracted to the end surface of the recording drum  1  made of a metal by its magnetic force. The balance weight  7  is movable to an arbitrary position of the annular groove  6 . A hole  7   a  into which an adjusting pin  16  of a weight position adjuster  15  for adjusting the position of the balance weight  7  is to be inserted is formed on the surface of the balance weight  7 . The balance weights  7  are arranged such that the recording drum  1  is rotatable in a state where the centrifugal force developed by the fixing clamp  4  is balanced with centrifugal forces developed by the balance weights  7 . The arrangement and the operation of the balance weights  7  will be described in detail later. 
     In FIG. 1, a pair of weight position adjusters  15  is arranged opposite to both the end surfaces of the recording drum  1 . The weight position adjuster  15  has an adjusting pin  16  which can advance and retreat to and from the end surface of the recording drum  1 . The adjusting pin  16  extends at the time of adjusting the position of the balance weight  7 , and enters the hole  7   a  of the balance weight  7 , to inhibit the movement of the balance weight  7 . When the recording drum  1  is rotated in a state where the adjusting pin  16  is inserted into the hole  7   a  of the balance weight  7 , the end surface of the recording drum  1  is rotated and moved while sliding relative to the balance weight  7 . Consequently, the position in the circumferential direction of the balance weight  7  on the end surface of the recording drum  1  is moved. 
     A controller  14  receives a detection signal from the rotation angle detector  13 , and controls each of operations of the motor  12  for rotating the recording drum  1  and the pair of weight position adjusters  15 . 
     An input unit  18  inputs information relating to the size, the weight, and so forth of the plate  2 . A position calculator  19  calculates the position where the balance weight  7  is disposed on the basis of data representing the physical properties of the plate  2  which is inputted from the input unit  18 . Further, a storage  20  stores the position where the balance weight  7  is disposed. 
     In the present embodiment, the recording drum  1  corresponds to a drum in the present invention, the driving mechanism corresponds to driving means, the movable clamp  5  corresponds to a fixing member, the balance weight  7  corresponds to a balance weight, and the annular groove  6  corresponds to an annular groove. Further, the position calculator  19  corresponds to position calculation means, the controller  14  corresponds to control means, the weight position adjuster  15  corresponds to engagement means, the storage  20  corresponds to storage means, and the input unit  18  corresponds to input means. 
     Description is now made of an operation for adjusting the positions of the balance weights  7  in the drawing apparatus. The operation for adjusting the positions of the balance weights  7  is performed by the step of calculating an unbalance force, the step of calculating the position where the balance weight is disposed, and the step of adjusting the position of the balance weight on the basis of control carried out by the controller  14 . 
     When drawing processing of a new plate which differs in size and weight from the plate in preprocessing, information relating to the size, the weight, and so forth of the plate is previously inputted from the input unit  18 . Thereafter, the following steps are carried out. 
     (1) Step of Calculating Unbalance Force 
     When the recording drum  1  is rotated at high speed, the fixing clamp  4 , the movable clamp  5 , the plate  2 , and the fixing clamp balance member  8  which are disposed on the outer peripheral surface of the recording drum  1  respectively develop centrifugal forces directed outward along the radius of the recording drum  1  from the positions of their respective centers of gravity. Generally, the members are not arranged in consideration of a balance among the respective centrifugal forces. Therefore, an unbalance force is caused by variations in the directions in which the centrifugal forces are exerted and the sizes thereof. The direction in which the unbalance force is exerted periodically varies as the recording drum  1  is rotated. Therefore, forced vibration occurs in the rotary shaft  3  of the recording drum  1 , causing the recording drum to develop a fault. 
     Therefore, the position calculator  19  calculates as follows the unbalance force exerted on the recording drum  1  on the basis of the inputted information relating to the size, the mass, and so forth of the plate  2  in addition to information relating to the position, the mass, and so forth of each of the members. 
     FIG. 5 is a schematic view (a) showing an unbalance force caused by the centrifugal forces developed by the movable clamp and the plate in the recording drum, and a diagram (b) of an unbalance force vector. In the following description, equations expressing the centrifugal force developed by the recording drum  1 , the unbalance forces produced in the recording drum  1 , and a balance force for canceling the unbalance force include a common term of rotation angular velocity. 
     In the following description, the common term of angular velocity is omitted. Consequently, the unbalance force and the balance force from which the term of angular velocity is omitted are respectively referred to as an unbalance amount D and a balance amount B. 
     The fixing clamp  4  and the fixing clamp balance member  8  are arranged such that their respective centrifugal forces are balanced with each other. Therefore, the fixing clamp  4  and the fixing clamp balance member  8  are excluded from the following calculation of the unbalance amount D. 
     Furthermore, in the following description, the rotary shaft  3  is taken as a center of rotation, and an X-Y coordinate system using as the Y-axis a direction passing through the center of rotation and the fixing clamp  4  and using as the X-axis a direction perpendicular to the Y-axis is fixed on the end surface of the recording drum  1 . Further, the circumferential direction in a counterclockwise direction from the Y-axis is taken as a θ direction. 
     As shown in FIG.  5 ( a ), when the recording drum  1  is rotated at high speed, the plate  2  develops a centrifugal force Fg in its center of gravity G. An angle in the circumferential direction of the centrifugal force Fg developed by the plate  2  is taken as θg. 
     The movable clamp  5  develops a centrifugal force Fc in its center of gravity. Let θc be an angle in the circumferential direction of the centrifugal force Fc developed by the movable clamp  5 . 
     A composite amount of the centrifugal force Fg developed by the plate  2  and the centrifugal force Fc developed by the movable clamp  5  is exerted as the unbalance amount D on the rotary shaft  3  of the recording drum  1 . 
     In FIG.  5 ( b ), letting Dx and Dy be a component in the X-axis direction and a component in the Y-axis direction of the unbalance amount D, respectively, Dx and Dy are found by the following equations from the centrifugal forces Fg and Fc:                    Dx   =     Fgx   +   Fcx                 =         Fg   ·   sin                   θ                 g     +     Fc   ·     sin        (     π   -     θ                 c       )                       =         Mg   ·   Rg   ·   sin                   θ                 g     +       Mc   ·   Rc   ·   sin                   θ                 c                     (   1   )                     Dy   =     Fgy   +   Fcy                 =         Fg   ·   cos                   θ                 g     +     Fc   ·     cos        (     π   -     θ                 c       )                       =         Mg   ·   Rg   ·   cos                   θ     +       Mc   ·   Rc   ·   cos                   θ                 c                     (   2   )                         
     Mg: mass of plate 
     Mc: mass of movable clamp 
     Rg: distance from rotary shaft to center of gravity G of plate 
     Rc: distance from rotary shaft to center of gravity of movable clamp 
     In the equations (1) and (2), Fgx and Fgy respectively indicate a component in the X-axis direction and a component in the Y-axis direction of the centrifugal force Fg, and Fcx and Fcy respectively indicate a component in the X-axis direction and a component in the Y-axis direction of the centrifugal force Fc. 
     Furthermore, letting θ D  be an angle in the circumferential direction in which the unbalance amount D is exerted, the unbalance amount D and the angle θ D  in the circumferential direction are respectively expressed by the following equations: 
     
       
           D ={square root over (Dx 2 +Dy 2 )}  (3)  
       
     
     in case of Dx&lt;0               θ   D     =       π   2     -       tan     -   1            Dy   Dx                 (   4   )                         
     in case of Dx≧0               θ   D     =         3   2        π     +       tan     -   1            Dy   Dx                 (   5   )                         
     Consequently, the unbalance amount D and the direction in which the unbalance amount is exerted are found. 
     (2) Step of Calculating Position Where Balance Weight is Disposed 
     When the unbalance amount D and the direction in which the unbalance amount is exerted are found, the position calculator  19  calculates as follows such a balance amount B as to cancel the unbalance amount D. 
     FIG. 6 is a schematic view (a) showing an unbalance force caused by centrifugal forces developed by the balance weights in the recording drum, and is a diagram (b) of a balance force vector. In FIG.  6 ( a ), the balance amount B is so set as to be equal to the unbalance amount D, and to be exerted in the opposite direction to the unbalance amount D. The balance amount B is a composite amount of centrifugal forces Fw developed by two balance weights  7  on each of the end surfaces of the recording drum  1 , that is, a composite amount of centrifugal forces Fw developed by four balance weights on both the end surfaces thereof. The mass of each of the balance weights  7  is previously determined. The distance Rw from the center of the rotary shaft  3  to the center of gravity of each of the balance weights  7  is defined by the annular groove  6 . 
     The balance weights  7  are arranged symmetrically with each other on both the end surfaces of the recording drum  1 . An angle θw between the two balance weights  7  is adjusted, to find a desired balance amount B. The two balance weights  7  shall be arranged upon being equally distributed on both sides in the direction in which the balance amount B is exerted. 
     Letting Mw be the mass of each of the balance weights  7 , the centrifugal force Fw developed by the balance weight  7  is expressed by the following equation: 
     
       
           Fw=Mw×Rw   (6)  
       
     
     A composite amount of the centrifugal forces Fw developed by the four balance weights  7  arranged on both the end surfaces of the recording drum  1 , that is, the balance amount B is expressed by the following equation:              B   =       4   ·   Mw   ·   Rw   ·   cos                       θ   w     2               (   7   )                         
     The balance amount B may be equal to the unbalance amount D previously found. The angle θw between the two balance weights  7  is found by the following equation on the basis of this relationship:                θ   w     =     2   ·       cos     -   1            (     D     4   ·   Mw   ·   Rw       )                 (   8   )                         
     By the foregoing steps, respective angles (attaching angles) θ 1  and θ 2  in the circumferential direction from the Y-axis of one of the balance weights and the other balance weight on each of the end surfaces of the recording drum  1  are found by the following equation:                θ   1     =       θ   D     +   π   -       θ   w     2               (   9   )                         
     
       
         θ 2 =θ 1   +θ   w   (10)  
       
     
     (3) Step of Adjusting Position of Balance Weight 
     When the attaching angles θ 1  and θ 2  of the two balance weights  7  are found by the foregoing steps (1) and (2), the balance weights  7  are moved to positions at the found attaching angles. 
     In FIG. 1, the attaching angles θ 1  and θ 2  of the balance weights  7  on the end surface of the recording drum  1  at the time of terminating previous drawing processing are stored on the storage  20 . The controller  14  drives the motor  12  on the basis of information relating to the current position where the recording drum  1  is rotated and information relating to the attaching angles of the balance weights  7  from the rotation angle detector  13 , to rotate the recording drum  1  so that the adjusting pin  16  of the weight position adjuster  15  and the hole  7   a  of one of the balance weights  7  coincide with each other. The adjusting pin  16  is extended, and is inserted into the hole  7   a  of the balance weight  7 . In this state, the recording drum  1  is rotated, to relatively move one of the balance weights  7  to the position at the attaching angle θ 1  found in the above-mentioned step (2). Thereafter, the adjusting pin  16  is caused to retreat. Consequently, the adjustment of the position of one of the balance weights  7  is terminated. 
     The controller  14  then rotates the recording drum  1  so that the hole  7   a  of the other balance weight  7  coincides with the adjusting pin  16  of the weight position adjuster  15 . The adjusting pin  16  is extended, and is inserted into the hole  7   a  of the balance weight  7 . In this state, the recording drum  1  is rotated, to relatively move the other balance weight  7  to the position at the attaching angle θ 2  found in the above-mentioned step (2). Thereafter, the adjusting pin  16  is caused to retreat. Consequently, the adjustment of the position of the other balance weight  7  is terminated. 
     By the foregoing steps, the four balance weights  7  are so disposed as to be balanced with the centrifugal forces developed by the plate  2  and the movable clamp  5 . Consequently, the recording drum  1  can be rotated at high speed without causing forced vibration in the bearings of the rotary shaft  3 . 
     FIG. 7 is a schematic view of a drawing apparatus according to a second embodiment of the present invention. In the drawing apparatus according to the second embodiment, a position detector  17  for detecting the position of a balance weight  7  is provided. Examples of the position detector  17  include an electrooptical sensor or a magnetic sensor. 
     The position detector  17  detects, at the step of adjusting the position where the balance weight  7  is disposed, the position of the balance weight  7  before position adjustment. A recording drum  1  is rotated such that an adjusting pin  16  of a weight position adjuster  15  and a hole  7   a  of the balance weight  7  coincide with each other on the basis of the detected position of the balance weight  7 . Thereafter, the position of the balance weight  7  is adjusted by the same operations as those in the first embodiment. 
     The position detector  17  is thus provided, to eliminate the necessity of providing a storage for storing the position of the balance weight  7  for each drawing processing on a plate  2 , so that the construction of the drawing apparatus is simplified. 
     In the present embodiment, the position detector  17  corresponds to detection means in the present invention. 
     FIG. 8 is a schematic side view of a recording drum in a drawing apparatus according to a third embodiment of the present invention. The drawing apparatus according to the third embodiment differs from the drawing apparatuses according to the first and second embodiments in a balance weight position adjusting operation. In the drawing apparatus according to the third embodiment, a balance weight can be moved to a predetermined position in the following manner without storing the position where the balance weight is attached and provision means for detecting the position of the balance weight. 
     In FIG. 8, balance weights  21  are arranged inside an annular groove  6 , and a tapered notch  21   a  is formed on an end surface in the circumferential direction of each of the balance weights  21 . The notch  21   a  guides an adjusting pin  16  of a weight position adjuster  15  such that the adjusting pin  16  always abuts against a predetermined position of the balance weight  21 . 
     When an attaching angle of the balance weight is calculated by a position calculator  19  as in the above-mentioned first and second embodiments, the program proceeds to a position adjusting step. At the step, the adjusting pin  16  of the weight position adjuster  15  is first extended into the annular groove  6 . The recording drum  1  is rotated at least once, and is then stopped such that a fixing clamp  4  of the recording drum  1  is at the position of the origin in the direction of rotation. Consequently, the two balance weights  21  are aligned in the position of the origin in the direction of rotation by the adjusting pin  16  on each of the end surfaces of the recording drum  1 , as shown in FIG.  8 . 
     The recording drum  1  is then moved, to insert the adjusting pin  16  into the notch  21   a  between the two balance weights  21 . The recording drum  1  is rotated such that the adjusting pin  16  abuts against one of the balance weights  21 , to move one of the balance weights  21  to a position at an attaching angle θ 1  inside the annular groove  6  of the recording drum  1 . Consequently, the adjustment of the position of one of the balance weights  21  is terminated. 
     In the same manner as described above, the recording drum  1  is rotated such that the adjusting pin  16  abuts against the other balance weight  21 , to move the other balance weight  21  to a position at an attaching angle θ 2 . Consequently, the adjustment of the position of the other balance weight  21  is terminated. In the present embodiment, the balance weight  21  can be thus moved to a predetermined position where the balance weight  21  is attached without previously storing and detecting the position of the balance weight  21 . 
     Although the drawing apparatuses according to the first to third embodiments are so constructed as to adjust the positions of the balance weights  7  or  21  using the weight position adjuster  15 , a worker may find the positions where the balance weights  7  or  21  are disposed and then directly attach the balance weights  7  or  21  to the positions on both the end surfaces of the recording drum  1 . In this case, the necessity of the weight position adjuster  15  is eliminated, so that the construction of the drawing apparatus is simplified. 
     Although in the above-mentioned first to third embodiments, description was made of an example in which the two balance weights  7  or the two balance weights  21  are arranged on each of the end surfaces of the recording drum  1 , the present invention is not limited to the same. For example, at least two balance weights  7  or  21  may be arranged per end surface. By increasing the number of the balance weights  7  or  21 , the size of each of the balance weights can be decreased. Therefore, arrangement becomes easy, and fine adjustment becomes easy. 
     Although in the above-mentioned first to third embodiments, the balance weights  7  or  21  are disposed inside the annular grooves  6 , they may be directly attached to the end surfaces of the recording drum  1  without providing the annular grooves  6 . 
     Furthermore, the balance weight  7  or  21  is not limited to a magnet. For example, the balance weight may have a structure in which it is mechanically attached to the end surface of the recording drum  1 . 
     FIG. 9 is a schematic side view of a drawing apparatus according to a fourth embodiment of the present invention, and FIG. 10 is a schematic side view of the drawing apparatus shown in FIG.  9 . 
     In FIGS. 9 and 10, the drawing apparatus comprises a cylindrical recording drum  1 . The recording drum  1  is rotated in a direction indicated by an arrow A (a primary scanning direction) around a rotary shaft la by a rotation driving device  200 . The rotation driving device  200  comprises a servo motor for applying a driving force to the rotary shaft  10  of the recording drum  1  and an encoder for detecting the amount of rotation of the recording drum  1 . 
     A plate  2  made of aluminum is mounted as a photosensitive material on an outer peripheral surface of the recording drum  1 . One end of the plate  2  is fixed to the outer peripheral surface of the recording drum  1  by a plurality of front end clamps  40 , and the other end of the plate  2  is fixed to the outer peripheral surface of the recording drum  1  by a plurality of rear end clamps  50 . The plurality of front end clamps  40  correspond to the fixing clamp  4  in the first embodiment, and the plurality of rear end clamps  50  correspond to the movable clamp  5  in the first embodiment. 
     As shown in FIG. 10, annular grooves  6  are respectively formed on both end surfaces of the recording drum  1 , and two balance weights  7  are attached so as to be movable in the circumferential direction inside each of the annular grooves  6 . A weight position adjuster  700  for moving the balance weights  7  is arranged on a rotation driving device  200 . 
     As shown in FIG. 9, a recording head  120  comprising a plurality of laser diodes  100  is disposed ahead of the recording drum  1 . The structures and the operations of the recording head  120 , a laser diode driving circuit portions  160 , a serial/parallel converter  170 , and an image signal generator circuit  180  are the same as the structures and the operations of the corresponding units in the drawing apparatus shown in FIG.  2 . 
     As shown in FIG. 10, a clamp driving device  300  is provided behind the recording drum  1 . The clamp driving device  300  is used for attaching the rear end clamps  50  on the recording drum  1 , detaching the rear end clamps  50  from the recording drum  1 , and releasing the front end clamps  40  on the recording drum  1 . 
     The clamp driving device  300  comprises a pair of clamp arms  310  which are swingable in a direction indicated by an arrow C. As shown in FIG. 9, a driving bar  311  is attached between the pair of clamp arms  310 , and a plurality of first driving devices  320  are attached to the driving bar  311 . The first driving device  320  is provided with a driving pin  321  for fixing and releasing the rear end clamps  50 , two holding pins  322  for holding the rear end clamps  50 , and a release pin  323  for releasing the front end clamps  40  at the time of attaching the plate  2 . The clamp driving apparatus  300  comprises a second driving device (not shown) for releasing the front end clamps  40  at the time of detaching the plate  2 . 
     As shown in FIG. 10, a conveying unit  900  is disposed so as to be swingable in a direction indicted by an arrow R above the recording drum  1 . The conveying unit  900  comprises a first plate carry-in conveying path  91  and a second plate carry-out conveying path  92 . At the time of carrying the plate  2  into the recording drum  1 , the plate  2  is supplied onto the recording drum  1  through the first conveying path  91  in the conveying unit  900 . At the time of carrying the plate  2  out of the recording drum  1 , the plate  2  detached from the recording drum  1  is carried outward through the second conveying path  92  in the conveying unit  900 . 
     A punching device  600  for punching a positioning hole in the plate  2  is disposed at a front end of the conveying unit  900 . The plate  2  is supplied to the punching device  600  through the first conveying path  91  in the conveying unit  900  before being supplied onto the recording drum  1 , so that the positioning hole is formed at a front end of the plate  2 . The positioning hole in the plate  2  is engaged with a positioning pin (not shown) provided on the outer peripheral surface of the recording drum  1 . 
     A controller  400  shown in FIG. 10 comprises a CPU (Central Processing Unit), a storage device, an input-output interface, and so forth, and controls the respective units of the drawing apparatus. An operation panel  500  for a worker entering various type of information relating to the size of the plate  2 , for example, and various types of instructions is connected to the controller  400 . 
     FIG. 11 is a side view showing one of end surfaces of the recording drum  1 . As shown in FIG. 11, an annular grove  6  is formed on the end surface of the recording drum  1 . Two balance weights  7  each having a hole  7   a  are attached in the annular groove  6  so as to be movable in the circumferential direction. 
     FIG. 12 is a perspective view of the balance weight  7 . As shown in FIG. 12, the balance weight  7  comprises a weight main body  70  and a pair of resin plates  71 . The resin plates  71  are respectively attached to both side surfaces of the weight main body  70 . The balance weight  7  slides on an end surface of the resin plate  71  inside the annular groove  6  of the recording drum  1 . The weight main body  70  contains a magnet  72 . Further, a position detecting dog  73  projecting perpendicularly to a surface, on which the hole  7   a  is formed, of the weight main body  70  is attached. 
     FIG. 13 is a front view showing the structure and the operation of a weight position adjuster  700 . 
     In FIG. 13, aluminum is used as the material of the recording drum  1  in order to make the recording drum  1  lightweight. A thin metal plate  6   a  is screwed to a bottom surface of the annular groove  6  in the recording drum  1 . The magnet  72  contained in the balance weight  7  attracts the metal plate  6   a , so that the balance weight  7  is fixed in the annular groove  6 . 
     The weight position adjuster  700  comprises a driving device main body  701 . The driving device main body  70  is provided with a cam driving motor  702 . A cam  703  is attached to the cam driving motor  702 . A driving plate  704  is attached to the driving device main body  701  so as to be rotatable around a rotary shaft  705 . One end of the driving plate  704  is urged in a direction indicated by an arrow S 1  by a spring  706 . A U-shaped notch  707  is formed at the other end of the driving plate  704 . 
     A driving pin  708  is attached to the driving device main body  701  so as to be slidable in the horizontal direction by an attaching member  709 . An engaging pin  710  which is engaged with the U-shaped notch  707  of the driving plate  704  is provided at a rear end of the driving pin  708 . 
     A reflection type balance weight detecting sensor  711  for detecting the position detecting dog  73  of the balance weight  7  is attached to a front surface of the driving device main body  701 . A reflection type driving plate detecting sensor  712  for detecting the driving plate  704  is attached to an upper surface of the driving device main body  701 . 
     As shown in FIG.  13 ( a ), the driving plate  704  is urged in a direction indicated by the arrow S 1  by the spring  760  in its initial state. Consequently, the driving pin  708  is spaced apart from an end surface of the recording drum  1 . At this time, one end of the driving plate  704  is detected by the driving plate detecting sensor  712 . 
     The balance weight  7  in the annular groove  6  of the recording drum  1  is detected by the balance weight detecting sensor  711 . As shown in FIG.  13 ( b ), the cam  703  is rotated through a predetermined angle by the cam driving motor  702  at the time of driving the balance weight  7 . Consequently, the driving plate  704  is rotated in a direction indicated by an arrow S 2  against an urging force of the spring  706 . As a result, the driving pin  708  is pressed out toward the end surface of the recording drum  1 , and is inserted into the hole  7   a  of the balance weight  7 . 
     By rotating the recording drum  1  in this state, the balance weight  7  can be moved in the circumferential direction in the annular groove  6  of the recording drum  1 . 
     Thereafter, when the cam  703  is returned to the initial state by the cam driving motor  702 , the driving plate  704  is rotated in the direction indicated by the arrow S 1  by the urging force of the spring  706 . Consequently, the driving pin  708  retreats, and is detached from the hole  7   a  of the balance weight  7 . The balance weight  7  is fixed in the annular groove  6  by a magnetic force of the magnet  72 . 
     FIG. 14 is a side view showing the structure of the conveying unit  900 . 
     The conveying unit  900  comprises the first plate carry-in conveying path  91 , the second plate carry-out conveying path  92 , a conveying path switching mechanism  93 , a first conveying mechanism  94 , and a second conveying mechanism  95  inside a unit main body  900   a.    
     The conveying path switching mechanism  93  is constituted by a conveying path switching motor  930 , a gear  931 , a cam gear  932 , a cam follower  933 , and a cam follower guide  934 . The gear  931  is attached to the conveying path switching motor  930 , the cam gear  932  is meshed with the gear  931 , and the cam follower  933  is fixed to the cam gear  932 . The cam follower  933  is engaged with the cam follower guide  934 . The cam follower guide  934  is fixed to the unit main body  900   a , and the unit main body  900   a  is supported so as to be swingable by a predetermined supporting member (not shown), centered around its rear part. 
     When the conveying path switching motor  930  is rotated, the cam gear  932  is rotated through the gear  931 , and the cam follower guide  934  is moved up and down by the cam follower  933  fixed to the cam gear  934 . Consequently, the conveying unit  900  swings in a direction indicated by an arrow R. 
     The first conveying mechanism  94  is constituted by a conveying roller driving motor  940 , pulleys  941   a ,  941   b ,  941   c  and  941   d , a belt  942 , and three conveying rollers  943 ,  944 , and  945 . The pulley  941   a  is attached to the conveying roller driving motor  940 , and the pulleys  941   b ,  941   c , and  941   d  are respectively attached to the conveying rollers  943 ,  944 , and  945 . Torque developed by the conveying roller driving motor  940  is transmitted to the pulleys  941   b ,  941   c , and  941   d  through the belt  942  from the pulley  941   a . Consequently, the conveying rollers  943 ,  944 , and  945  are rotated. 
     The second conveying mechanism  95  is constituted by a conveying roller driving motor  950  and conveying rollers  951  and  952 . The conveying rollers  951  and  952  are rotated by the conveying roller driving motor  950 . 
     A nip roller  963  is disposed in close proximity to the conveying roller  943  in the first conveying path  91 . The nip roller  963  is supported so as to be swingable by a swinging member  962 . A gear  961   a  is attached to a nip roller driving motor  960 . The gear  961   a  is meshed with a gear  961   b  attached to the swinging member  962 . When the nip roller driving motor  960  is rotated, the swinging member  962  swings through the gears  961   a  and  961   b , and the nip roller  963  is brought into contact with the conveying roller  943 . Consequently, the plate  2  is held by the conveying roller  943  and the nip roller  963 . 
     A plate front end detecting sensor  970  for detecting a front end of the plate  2  is disposed at a front end of the first conveying path  91 , and a plate rear end detecting sensor  971  for detecting a rear end of the plate  2  is disposed at the center of the first conveying path  91 . 
     The operations of the conveying unit  900  will be described while referring to FIGS. 14 and 15. 
     When the plate  2  is carried into the recording drum  1 , the plate  2  is set on the first conveying path  91  in the conveying unit  900 , and a positioning hole is formed at the front end of the plate  2  by the punching device  600  shown in FIG. 10, after which the conveying unit  900  is swung toward its lower position by the conveying path switching motor  930 , as shown in FIG.  14 . The nip roller  963  is spaced apart from the conveying roller  943  by the nip roller driving motor  960 . 
     A clamp arm  310  is swung, to bring the first driving device  320  near the outer peripheral surface of the recording drum  1 , and press a rear end of the front end clamp  40  by the release pin  323  of the first driving device  320 . Consequently, a clearance is formed between the front end of the front end clamp  40  and the outer peripheral surface of the recording drum  1 . 
     In this state, the conveying roller driving motor  940  is rotated, the plate  2  is conveyed toward the recording drum  1 , the front end of the plate  2  is inserted between the front end of the front end clamp  40  and the outer peripheral surface of the recording drum  1 , and the positioning hole of the plate  2  is fitted in the positioning pin on the recording drum  1 . 
     After the conveying roller driving motor  940  is stopped, the clamp arm  310  is returned in the opposite direction, to separate the release pin  323  of the first driving device  320  from the front end clamp  40 . Consequently, the front end of the plate  2  is fixed to the outer peripheral surface of the recording drum  1  by the front end clamp  40 . 
     Thereafter, the conveying roller driving motor  940  is rotated, the recording drum  1  is rotated, and the plate  2  is wound around the outer peripheral surface of the recording drum  1 , after which the rear end of the plate  2  is fixed to the outer peripheral surface of the recording drum  1  by the rear end clamp  50 , as shown in FIG.  10 . 
     When the plate  2  is carried out of the recording drum  1 , the rear end clamp  50  is released by the first driving device  320  in the clamp driving device  300 , and the recording drum  1  is then rotated in the opposite direction. As shown in FIG. 15, the front end of the plate  2  is inserted between the conveying rollers  951  and  952  in the second conveying mechanism  95  in the conveying unit  900 , and the conveying rollers  951  and  952  are rotated by the conveying roller driving motor  950 , to pull the plate  2  into the second conveying path  92  in the conveying unit  900 . Thereafter, the front end clamp  40  on the recording drum  1  is released by a second driving device  324  in the clamp driving device  300 , to carry the plate  2  outward through the second conveying path  92  in the conveying unit  900 . 
     FIGS. 16 and 17 are flow charts showing an example of operations of the drawing apparatus shown in FIGS. 9 and 10. 
     The controller  400  shown in FIG. 10 stores the current position of each of the balance weights  7  on the end surface of the recording drum  1  and the size of the precious plate. 
     A worker sets the plate  2  on the first conveying path  91  in the conveying unit  9  shown in FIG. 10 (step S 1 ), and enters the size of the plate  2  from the operation panel  500  (step S 2 ). 
     As shown in FIG. 14, the conveying unit  900  is then swung downward, and the plate  2  is conveyed onto the recording drum  1  through the conveying path  91 , to fix the front end of the plate  2  to the outer peripheral surface of the recording drum  1  by the front end clamp  40  (step S 3 ). 
     Thereafter, the plate  2  is mounted on the outer peripheral surface of the recording drum  1  while rotating the recording drum  1  (step S 4 ). At this time, the controller  400  detects the size of the plate  2  on the basis of the timing at which the front end of the plate  2  passes through the plate front end detecting sensor  970  shown in FIG. 14, the timing at which the rear end of the plate  2  passes through the plate rear end detecting sensor  971 , and the transport speed of the plate  2  (step S 5 ). 
     The controller  400  then judges whether or not the size of the plate  2  inputted from the operation panel  500  differs from the size of the previous plate (step S 6 ). 
     When the size of the plate  2  inputted from the operation panel  500  differs from the size of the previous plate, the controller  400  calculates the position where each of the balance weights  7  is disposed on the basis of the size of the plate  2  inputted from the operation panel  500  (step S 7 ). A method of calculating the position where each of the balance weights  7  is disposed is the same as that in the first embodiment. 
     Each of the balance weights  7  attached to both end surfaces of the recording drum  1  is moved to the calculated position where it is disposed by the weight position adjuster  700  (step S 8 ). In this case, the controller  400  stores the current position of each of the balance weights  7 . Therefore, the recording drum  1  is first rotated, to move one of the balance weights  7  to the position of the driving pin  708  in the weight position adjuster  700 . As shown in FIG. 13, the driving pin  708  in the weight position adjuster  700  is inserted into the hole  7   a  of the balance weight  7 . In this state, the recording drum  1  is rotated, to move the balance weight  7  to the calculated position where it is disposed. Similarly, the other balance weight  7  is moved to the calculated position where it is disposed by the weight position adjuster  700 . 
     Unbalance detection processing, described later, is then performed (step S 9 ). When the size of the plate  2  inputted from the operation panel  500  is the same as the size of the previous plate at the step S 6 , the unbalance detection processing is performed without calculating the position where each of the balance weights  7  is disposed (step S 7 ) and moving the balance weight  7  (step S 8 ). 
     When the unbalance of the recording drum  1  is detected (step S 10 ), the controller  400  performs processing after unbalance detection, as described later (step S 11 ). Thereafter, drawing processing is performed by the recording head  120  shown in FIGS. 9 and 10 (step S 12 ). 
     According to the above-mentioned operations of the drawing apparatus in the present embodiment, the unbalance of the recording drum  1  is automatically detected, so that an unbalance is prevented from occurring in the recording drum  1  by an input error of the size of the plate  2  by the worker. Consequently, the drawing apparatus is prevented from being damaged by forced vibration in the recording drum  1 . 
     FIG. 18 is a flow chart showing the unbalance detection processing. 
     The excitation of a servo motor included in the rotation driving device  200  is first released, to bring the recording drum  1  into a free state (step S 11 ). At this time, if an unbalance occurs in the recording drum  1 , the recording drum  1  is subtly rotated. An output pulse of the encoder included in the rotation driving device  200  is detected, to detect whether or not the recording drum  1  is rotated (step S 12 ). 
     When the recording drum  1  is rotated, it is judged that an unbalance occurs in the recording drum  1  (step S 17 ). Even in a case where an unbalance occurs in the recording drum  1 , the recording drum  1  is not rotated when an unbalance amount vector of the recording drum  1  coincides with the direction of the gravity of the recording drum  1  when the excitation of the servo motor is released. 
     When the recording drum  1  is not rotated, the recording drum  1  is rotated through an angle of 90° by the rotation driving device  200  (step S 13 ). The excitation of the servo motor in the rotation driving device  200  is released, to bring the recording drum  1  into a free state (step S 14 ). 
     In this state, it is detected whether or not the recording drum  1  is rotated (step S 15 ). When the recording drum  1  is rotated, it is judged that an unbalance occurs in the recording drum  1  (step S 17 ). On the other hand, when the recording drum  1  is not rotated, it is judged that the recording drum  1  is in a balanced state (step S 16 ). 
     The unbalance of the recording drum  1  can be thus reliably detected. In this example, when the recording drum  1  is rotated by the first release of the excitation of the servo motor, the recording drum  1  is rotated through an angle of 90°. An angle of rotation is not limited to 90°. For example, it may be another angle. 
     Examples of processing after the unbalance detection at the step S 11  include the following two types of processing. The first processing is for returning the plate  2  to the second conveying path  92  in the conveying unit  900 , as shown in FIG.  15 . In this case, the worker replaces the plate  2  returned to the second conveying path  92  in the conveying unit  900  with another plate having an inputted size, or sets the plate  2  on the first conveying path  92  in the conveying unit  900  again, to enter the correct size of the plate  2  from the operation panel  500  again. 
     The second processing is for calculating the position where each of the balance weights  7  on both end surfaces of the recording drum  1  is disposed on the basis of the size of the plate  2  detected at the step S 5 , and moving the balance weight  7  to the calculated position. Thereafter, the unbalance detection processing shown in FIG. 18 may be performed again. 
     FIG. 19 is a flow chart showing another example of operations of the drawing apparatus shown in FIGS. 9 and 10. In FIG. 19, processing previous to the step S 31  is the same as the processing at the steps S 1  to S 5  shown in FIG.  16 . 
     After the step S 5  shown in FIG. 16, the controller  400  judges whether or not the detected size of the plate  2  differs from the size of the previous plate (step S 31 ). When the detected size of the plate  2  differs from the size of the previous plate, the position where each of the balance weights  7  is disposed on both the end surfaces of the recording drum  1  is calculated on the basis of the detected size of the plate  2  (step S 32 ). Each of the balance weights  7  on both the end surfaces of the recording drum  1  is moved to the calculated position (step S 33 ). 
     Thereafter, drawing processing is performed by the recording head  120  shown in FIGS. 9 and 10 (step S 34 ). When the size of the plate  2  detected at the step S 31  is the same as the size of the previous plate, drawing processing is performed by the recording head  120  shown in FIGS. 9 and 10 (step S 34 ) without calculating the position where each of the balance weights  7  is disposed (step S 32 ) and moving the balance weight  7  (step S 33 ). 
     The unbalance detection processing (step S 9 ) and the processing after unbalance detection (step S 11 ) may be performed similarly to the operations shown in FIG. 17 before the drawing processing at the step S 34 . 
     According to the above-mentioned operations of the drawing apparatus in the present embodiment, the size of the plate  2  is automatically detected, and the balance weight  7  is disposed in the position where no unbalance occurs in the recording drum  1  on the basis of the detected size of the plate  2 . Therefore, an unbalance is prevented from occurring on the recording drum  1  by an input error of the size of the plate  2  by the worker. Consequently, the drawing apparatus is prevented from being damaged by forced vibration in the recording drum  1 . 
     Although in the above-mentioned embodiment, the length of the plate  2  is detected by the plate front end detecting sensor  970  and the plate rear end detecting sensor  971  in the conveying unit  900 , a sensor for detecting the width, the thickness, and so forth of the plate  2  may be provided in the conveying unit  900  or the like. 
     Although in the above-mentioned embodiment, the worker enters the size of the plate  2  as information relating to the plate  2  and detects the size of the plate  2 , and calculates the position where each of the balance weights  7  is disposed on the basis of the inputted size of the plate  2  or the detected size of the plate  2 , the worker may enter the size of the plate  2  and the other information relating to the weight thereof, for example, as the information relating to the plate  2 , detect the size of the plate  2  and the other information relating to the weight thereof, and calculate the position where each of the balance weights  7  is disposed on the basis of the inputted information relating to the plate  2  or the detected information relating to the plate  2 . 
     FIGS. 20 to  24  are diagrams showing other examples of a balance weight attaching method, where (a) is a front view, and (b) is a side view. 
     In the example shown in FIG. 20, cylinders  101  made of a metal are respectively extended at both ends of a recording drum  1 . Two balance weights  7  are disposed so as to be movable and fixable in the circumferential direction on an inner peripheral surface of each of the cylinders  101 . Each of the balance weights  7  is composed of a permanent magnet, and is attracted to the inner peripheral surface of the cylinder  101  by its magnetic force. 
     A hole  7   a  into which the adjusting pin  16  of the weight position adjuster  15  shown in FIG. 1 can be inserted is formed in each of the balance weights  7 . When the recording drum  1  is rotated in a state where the adjusting pin  16  is inserted into the hole  7   a  of the balance weight  7 , the balance weight  7  is moved in the circumferential direction along the inner peripheral surface of the cylinder  101 . After position adjustment, the balance weight  7  is fixed to the inner peripheral surface of the cylinder  101  by the magnetic force. 
     In the example shown in FIG. 21, annular grooves  61  along the circumference of a recording drum  1  are respectively formed near both ends of an outer peripheral surface of the recording drum  1 . An engagement part  75  in an inverted T shape of two balance weights  7  is mounted on each of the annular grooves  61  so as to be movable and fixable in the circumferential direction. Each of the balance weights  7  is composed of a permanent magnet, and is attracted to an inner surface of the annular groove  61  of the recording drum  1  made of a metal by its magnetic force. 
     A hole  7   a  into which the adjusting pin  16  of the weight position adjuster  15  shown in FIG. 1 can be inserted is formed in each of the balance weights  7 . In this example, the weight position adjuster  15  shown in FIG. 1 is arranged ahead of or behind the recording drum  1 . When the recording drum  1  is rotated in a state where the adjusting pin  16  is inserted into the hole  7   a  of the balance weight  7 , the balance weight  7  is moved in the circumferential direction along the annular groove  61  on the outer peripheral surface of the recording drum  1 . After position adjustment, the balance weight  7  is fixed to the annular groove  61  by the magnetic force. 
     In the example shown in FIG. 22, auxiliary members  62  in a disk shape are respectively fixed to both ends of a rotary shaft  3  of a recording drum  1 . An annular groove  63  along the circumference whose center is the rotary shaft  3  of the recording drum  1  is formed on one surface of each of the auxiliary members  62 . Two balance weights  7  are attached so as to be movable and fixable in the circumferential direction inside each of the annular grooves  63 . An inner surface of the annular groove  63  in the auxiliary member  62  is made of a metal. Each of the balance weights  7  is composed of a permanent magnet, and is attracted to the inner surface of the annular groove  63  in the auxiliary member  62  by its magnetic force. 
     A hole  7   a  into which the adjusting pin  16  of the weight position adjuster  15  shown in FIG. 1 can be inserted is formed in each of the balance weights  7 . When the auxiliary member  62 , together with the recording drum  1 , is rotated in a state where the adjusting pin  16  is inserted into the hole  7   a  of the balance weight  7 , the balance weight  7  is moved in the circumferential direction along the annular groove  63  in the auxiliary member  62 . After position adjustment, the balance weight  7  is fixed to the annular groove  63  in the auxiliary member  62  by the magnetic force. 
     In the example shown in FIG. 23, two arm members  65  are attached to each of both ends of a rotary shaft  3  of a recording drum  1  so as to be rotatable and fixable around and to the rotary shaft  3 . The arm member  65  is constituted by a cylindrical attachment part  65   a  fitted in the rotary shaft  3  and a long arm  65   b  and a short arm  65   c . A balance weight  7  is fixed to an end of the long arm  65   b , and a balancer  66  is provided at an end of the short arm  65   c . The balance  66  keeps the balance of the arm member  65  when there is no balance weight  7 . The attachment unit  65   a  in the arm member  65  is composed of a permanent magnet, and is attracted to the rotary shaft  3  made of a metal by its magnetic force. 
     A hole  7   a  into which the adjusting pin  16  of the weight position adjuster  15  shown in FIG. 1 can be inserted is formed in each of the balance weights  7 . When the rotary shaft  3 , together with the recording drum  1 , is rotated in a state where the adjusting pin  16  is inserted into the hole  7   a  of the balance weight  7 , the arm member  65  is rotated around the rotary shaft  3 , and the balance weight  7  fixed to the arm member  65  is moved along the circumference of the recording drum  1 . After position adjustment, the arm member  65  is fixed to the rotary shaft  3  by the magnetic force. 
     The arm member  65  may be attached to the rotary shaft  3  so as to be rotatable and fixable by respectively providing the rotary shaft  3  and the attachment part  65   a  of the arm member  65  with ratchet structures instead of forming the attachment part  65   a  of the arm member  65  of a permanent magnet. 
     In the example shown in FIG. 24, an annular groove  68  is formed at the center of an outer peripheral surface of a recording drum  1 . Two balance weights  7  are disposed so as to be movable and fixable in the circumferential direction inside the annular groove  68 . Each of the balance weights  7  is composed of a permanent magnet, and an inner surface of the annular groove  68  of the recording drum  1  is made of a metal. It is preferable that the balance weights  7  are provided such that the surfaces of the balance weights  7  and the outer peripheral surface of the recording drum  1  are flush with each other. 
     As shown in FIG. 25, each of the balance weights  7  may be fixed to the bottom surface of the annular groove  68  of the recording drum  1  by its magnetic force. As shown in FIG. 26, each of the balance weights  7  may be fixed to a side surface of the annular groove  68  of the recording drum  1  by its magnetic force. Further, as shown in FIG. 27, flanges  69  may extend inward from upper ends of both side surfaces of the annular groove  68 , and each of the balance weights  7  may be fixed to the inner surface of the flange  69  by its magnetic force. As shown in FIGS. 25 to  27 , a hole  7   a  into which the adjusting pin  16  of the weight position adjuster  15  shown in FIG. 1 can be inserted is formed in each of the balance weights  7 . 
     In the example shown in FIG. 24, the weight position adjuster  15  is arranged ahead of or behind the recording drum  1 . When the recording drum  1  is rotated in a state where the adjusting pin  16  is inserted into the hole  7   a  of the balance weight  7 , the balance weight  7  is moved in the circumferential direction along the annular groove  68  of the recording drum  1 . After position adjustment, the balance weight  7  is fixed in the annular groove  68  of the recording drum  1  by its magnetic force. In the example shown in FIG. 24, an unbalance force exerted on the recording drum  1  can be canceled by the two balance weights  7 . 
     FIG. 28 is a schematic view of a drawing apparatus in a fifth embodiment of the present invention, and FIG. 29 is a schematic view of a principal part of the drawing apparatus shown in FIG.  28 . 
     In FIG. 28, the drawing apparatus comprises a cylindrical recording drum  1 . The recording drum  1  is rotatably supported so as to be movable around a rotary shaft  3  having both its ends supported on bearings. A driving mechanism comprising a pair of pulleys  9  and  11 , a belt  10  and a motor  12  is connected to one end of the rotary shaft  3 . The driving mechanism transmits the rotation of the motor  12  to the rotary shaft  3  through the pair of pulleys  9  and  11  and the belt  10 , to rotate the recording drum  1 . 
     In FIG. 29, a recording head  120  is disposed in a position opposite to an outer peripheral surface of the recording drum  1 . The recording head  120  comprises a plurality of laser diodes  100 . The recording head  120  is attached to a guide  130  so as to be movable, and is moved in a secondary scanning direction B in synchronization with the rotation of the recording drum  1 . 
     The current position in a primary scanning direction A of a plate  2  is detected on the basis of a detection signal outputted from a rotation angle detector  13  attached to the rotary shaft  3  of the recording drum  1 . The current position in the secondary scanning direction B of the recording head  120  is detected on the basis of a detection signal outputted from a linear encoder  150 . 
     The plurality of laser diodes  100  in the recording head  120  are driven by a laser diode driving circuit portion  160 . The laser diode driving circuit portion  160  comprises a plurality of laser diode driving circuits  110  corresponding to the plurality of laser diodes  100  in the recording head  120 . 
     An image signal generator circuit  180  generates a serial image signal VDS. A serial/parallel converter  170  converts the serial image signal VDS generated by the image signal generator circuit  180  into parallel image signals VD 1  to VDn, and respectively feeds the image signals VD 1  to VDn to the plurality of laser diode driving circuits  110  in the laser diode driving circuit portion  160 . Consequently, each of the laser diodes  100  in the recording head  120  is driven by the corresponding laser diode driving circuit  110 , to irradiate the plate  2  with laser light. 
     FIG. 30 is a perspective view of the recording drum, and FIG. 31 is a side view of the recording drum. A fixing clamp  4  and a movable clamp  5  for attaching the plate  2  are disposed on the outer peripheral surface of the recording drum  1 . The fixing clamp  4  is fixed to the outer peripheral surface of the recording drum  1 , and the movable clamp  5  is formed so as to be movable on the outer peripheral surface of the recording drum  1  depending on the size of the plate  2 . A fixing clamp balance member  8  is attached to the outer peripheral surface of the recording drum  1  on the opposite side of the fixing clamp  4 . The fixing clamp balance member  8  is formed so as to have an approximately equal weight to that of the fixing clamp  4 . Consequently, the fixing clamp balance member  8  develops a centrifugal force which is balanced with a centrifugal force developed by the fixing clamp  4  at the time of rotating the recording drum  1 , thereby preventing an unbalance force from being produced. 
     One balance weight  7  is disposed on each of both end surfaces  1   a  of the recording drum  1 . The balance weight  7  is composed of a permanent magnet, and is attracted to the end surface  1   a  of the recording drum  1  made of a metal by its magnetic force. A hole  7   a  into which an engaging pin  17  of a weight position adjuster  15  is to be inserted is formed on the surface of the balance weight  7 . The balance weight  7  is arranged in a position on the end surface  1   a  which is rotatable in a state where centrifugal forces developed by the respective members which are exerted on the rotary shaft  3  of the recording drum  1  are balanced with each other. The arrangement and the operation of the balance weights  7  will be described in detail later. 
     A pair of weight position adjusters  15  is arranged opposite to both the end surfaces  1   a  of the recording drum  1 . FIG. 32 is a side view (a) and a front view (b) of the weight position adjuster  15 . In FIGS. 28 and 32, the weight position adjuster  15  has an engaging pin  17  which can advance and retreat to and from the end surface  1   a  of the recording drum  1 . The engaging pin  17  is so moved as to advance and retreat to and from the balance weight  7  by a cylinder  36 . The cylinder  36  is supported on a supporting member  38 . The supporting member  38  is engaged with a feed screw  39  extending in a direction parallel to the end surface  1   a  of the recording drum  1 , and is moved back and forth in a direction C parallel to the end surface  1   a  of the recording drum  1  as the feed screw  39  is rotated. The feed screw  39  is composed of a ball screw, and both ends thereof are respectively supported by bearings  40 . A motor  41  is connected to one end of the feed screw  39 . The motor  41  is rotatable forward and backward. The feed screw  39  is rotated by the rotation of the motor  41 . Consequently, the cylinder  36  and the engaging pin  17  which are supported on the supporting member  38  are moved back and forth in the direction C parallel to the end surface  1   a  of the recording drum  1 . 
     A controller  23  receives a detection signal from the rotation angle detector  13 , and controls each of the operations of the motor  12  for rotating the recording drum  1  and the pair of weight position adjusters  15 . 
     An input unit  24  inputs information relating to the size, the weight, and so forth of the plate  2 . A position calculator  25  calculates the position where the balance weight  7  is disposed on the basis of data representing the physical properties of the plate  2  which is inputted from the input unit  24 . Further, a storage  26  stores the position where the balance weight  7  is disposed. 
     In the present embodiment, the recording drum  1  corresponds to a drum in the present invention, the driving mechanism corresponds to driving means, the movable clamp  5  corresponds to a fixing member, and the balance weight  7  corresponds to a balance weight. Further, the position calculator  25  corresponds to position calculation means, the controller  23  corresponds to control means, the weight position adjuster  15  corresponds to engagement means, the storage  26  corresponds to storage means, and the input unit  24  corresponds to input means. 
     Description is now made of an operation for adjusting the positions of the balance weights  7  in the drawing apparatus. The operation for adjusting the positions of the balance weights  7  is performed by the step of calculating an unbalance force, the step of calculating the position where the balance weight is disposed, and the step of adjusting the position of the balance weight on the basis of control carried out by the controller  23 . 
     When drawing processing of a new plate which differs in the size and the weight from the plate in preprocessing, information relating to the size, the weight, and so forth of the plate is previously inputted from the input unit  24 . Thereafter, the following steps are carried out. 
     (1) Step of Calculating Unbalance Force 
     When the recording drum  1  is rotated at high speed, the fixing clamp  4 , the movable clamp  5 , the plate  2 , and the fixing clamp balance member  8  which are disposed on the outer peripheral surface of the recording drum  1  respectively develop centrifugal forces directed outward along the radius of the recording drum  1  from the positions of their respective centers of gravity. Generally, the members are not disposed in consideration of a balance among the respective centrifugal forces. Therefore, an unbalance force is caused by variations in the directions in which the centrifugal forces are exerted and the sizes thereof. The unbalance force is periodically changed in the direction in which it is exerted as the recording drum  1  is rotated. Therefore, forced vibration occurs in the rotary shaft  3  of the recording drum  1 , causing the recording drum  1  to develop a fault. 
     Therefore, the position calculator  25  calculates as follows the unbalance force exerted on the recording drum  1  on the basis of the inputted information relating to the size, the mass, and so forth of the plate  2  in addition to the information relating to the position, the mass, and so forth of each of the members. 
     FIG. 33 is a schematic view (a) showing an unbalance force caused by the centrifugal forces developed by the movable clamp and the plate in the recording drum, and a diagram (b) of an unbalance force vector. In the following description, equations expressing the centrifugal force produced in the recording drum  1 , the unbalance force exerted on the recording drum  1 , and a balance force for canceling the unbalance force include a common term of rotation angular velocity. In the following description, the common term of rotation angular velocity is omitted. Consequently, the unbalance force and the balance force from which the common term of rotation angular velocity is omitted are respectively referred to as an unbalance amount D and a balance amount B. 
     The fixing clamp  4  and the fixing clamp balance member  8  are arranged such that their respective centrifugal forces are balanced with each other. Therefore, the fixing clamp  4  and the fixing clamp balance member  8  are excluded from the following calculation of the unbalance amount D. 
     Furthermore, in the following description, the rotary shaft  3  is taken as a center of rotation, and an X-Y coordinate system using as the Y-axis a direction passing through the center of rotation and the fixing clamp  4  and using as the X-axis a direction perpendicular to the Y-axis is fixed on the end surface of the recording drum  1 . Further, the circumferential direction in a counterclockwise direction from the Y-axis is taken as a θ direction. 
     As shown in FIG.  33 ( a ), when the recording drum  1  is rotated at high speed, the plate  2  develops a centrifugal force Fg in its center of gravity G. An angle in the circumferential direction of the centrifugal force Fg developed by the plate  2  is taken as θg. 
     The movable clamp  5  develops a centrifugal force Fc in its center of gravity. Let θc be an angle in the circumferential direction of the centrifugal force Fc developed by the movable clamp  5 . 
     A composite amount of the centrifugal force Fg developed by the plate  2  and the centrifugal force Fc developed by the movable clamp  5  is exerted as the unbalance amount D on the rotary shaft  3  of the recording drum  1 . 
     In FIG.  33 ( b ), letting Dx and Dy be a component in the X-axis direction and a component in the Y-axis direction of the unbalance amount D, respectively, Dx and Dy are found by the following equations from the centrifugal forces Fg and Fc:                    Dx   =     Fgx   +   Fcx                 =         Fg   ·   sin                   θ                 g     +     Fc   ·     sin        (     π   -     θ                 c       )                       =         Mg   ·   Rg   ·   sin                   θ                 g     -       Mc   ·   Rc   ·   sin                   θ                 c                     (   11   )                     Dy   =     Fgy   +   Fcy                 =         Fg   ·   cos                   θ                 g     +     Fc   ·     cos        (     π   -     θ                 c       )                       =         Mg   ·   Rg   ·   cos                   θ                 g     +       Mc   ·   Rc   ·   cos                   θ                 c                     (   12   )                         
     Mg: mass of plate 
     Mc: mass of movable clamp 
     Rg: distance from rotary shaft to center of gravity G of plate 
     Rc: distance from rotary shaft to center of gravity of movable clamp 
     In the equations (11) and (12), Fgx and Fgy respectively indicate a component in the X-axis direction and a component in the Y-axis direction of the centrifugal force Fg, and Fcx and Fcy respectively indicate a component in the X-axis direction and a component in the Y-axis direction of the centrifugal force Fc. 
     Furthermore, letting θ D  be an angle in the circumferential direction in which the unbalance amount D is exerted, the unbalance amount D and the angle θ D  in the circumferential direction are respectively expressed by the following equations: 
     
       
           D ={square root over (Dx 2   +Dy   2 )}  (13)  
       
     
     in case of Dx&lt;0               θ   D     =       π   2     -       tan     -   1            Dy   Dx                 (   14   )                         
     in case of Dx≧0               θ   D     =         3   2        π     +       tan     -   1            Dy   Dx                 (   15   )                         
     Consequently, the unbalance amount D and the direction in which it is exerted are found. 
     (2) Step of Calculating Position Where Balance Weight is Disposed 
     When the unbalance amount D and the direction in which the unbalance amount is exerted are found, the position calculator  25  calculates as follows such a balance amount B as to cancel the unbalance amount D. 
     FIG. 34 is a schematic view showing an unbalance force caused by centrifugal forces developed by the balance weights in the recording drum. In FIG. 34, the balance amount B is so set as to be equal to the unbalance amount D, and to be exerted in the opposite direction to the unbalance amount D. The balance amount B is a composite amount of centrifugal forces Fw developed by two balance weights  7  on both the end surfaces of the recording drum  1 . 
     The mass of each of the balance weights  7  is previously determined. Letting Mw be the mass of each of the balance weights  7 , and letting Rw be the distance from the center of the rotary shaft  3  to the center of gravity of each of the balance weights  7 , a centrifugal force Fw developed by the balance weight  7  is expressed by the following equation: 
     
       
           Fw=Mw×Rw   (16)  
       
     
     A composite amount of the centrifugal forces Fw developed by the two balance weights  7  arranged on both the end surfaces of the recording drum  1 , that is, the balance amount B is expressed by the following equation: 
     
       
           B =2· Fw =2 ×Mw×Rw   (17)  
       
     
     The balance amount B may be equal to the unbalance weight D previously found. When the mass of the balance weight  7  is constant, the distance Rw in the radial direction of the balance weights  7  from the rotary shaft  3  which satisfies the relationship expressed by the equation (17) may be found by the following equation: 
     
       
           Rw=B /(2 ·Mw )  (18)  
       
     
     Angles in the circumferential direction of the balance weights  7  (attaching angles) θw are found by the following equation: 
     
       
         θ w =θ D +π  (19)  
       
     
     (3) Step of Adjusting Position of Balance Weight 
     When the distance Rw in the radial direction of the two balance weights from the rotary shaft  3  and the attaching angles θw in the circumferential direction of the balance weights  7  are found by the foregoing steps (1) and (2), the balance weights  7  are moved to the found positions in the radial direction and the circumferential direction. 
     In FIG. 28, the distance Rw in the radial direction of the balance weights  7  and the attaching angles θw in the circumferential direction of the balance weights  7  on the end surfaces of the recording drum  1  at the time of transmitting the previous drawing processing are stored on the storage  26 . The controller  23  drives the motor  12  on the basis of information relating to the current position where the recording drum  1  is rotated and information relating to the position where the balance weight  7  is disposed from the rotation angle detector  13 , and rotates the recording drum  1  so that the position in the vertical direction of the hole  7   a  of the balance weight  7  coincides with the position in the vertical direction of the engaging pin  17  of the position adjuster  15 . Further, the motor  41  is driven, and the feed screw  39  is rotated, to move the supporting member  38  along the radius on the end surface  2   a  of the recording drum  1  so that the positions of the hole  7   a  of the balance weight  7  and the engaging pin  17  coincide with each other in the horizontal direction. 
     In this state, the engaging pin  17  is extended, and is inserted into the hole  7   a  of the balance weight  7 . The feed screw  39  is driven, and the engaging pin  17  is moved in the radial direction, to move the balance weight  7  to a position at the distance Rw in the radial direction that is found in the above-mentioned step (2). 
     Furthermore, the recording drum  1  is rotated in a state where the engaging pin  17  is inserted into the hole  7   a  of the balance weight  7 , to relatively move the balance weight  7  to the position at the attaching angle θw in the circumferential direction which is found in the above-mentioned step (2). Thereafter, the engaging pin  17  is caused to retreat. Consequently, the adjustment of the position of the balance weight  7  is terminated. The positions of the balance weights  7  are simultaneously adjusted on both the end surfaces  1   a  of the recording drum  1 . 
     By the foregoing steps, four balance weights  7  are disposed such that they are balanced with the centrifugal forces developed by the plate  2  and the movable clamp  5 . Consequently, the recording drum  1  can be rotated at high speed without causing forced vibration in the bearings of the rotation shaft  3 . 
     The end  1   a  of the recording drum  1  and the surface to which the balance weight  7  is attracted are not limited to flat surfaces. For example, they may be respectively provided with grooves extending along the circumference whose center is the rotary shaft  3  of the recording drum  1 . In this case, the grooves on the end surface  1   a  and the surface to which the balance weight  7  is attracted are meshed with each other, thereby making it possible to prevent the balance weight  7  from being moved in the radial direction and separated by the rotation at high speed of the recording drum  1 . 
     Although in the drawing apparatus according to the above-mentioned embodiment is so constructed as to adjust the positions of the balance weights  7  using the weight position adjuster  15 , a worker may find the positions where the balance weights  7  are disposed, and then directly attach the balance weights  7  to the positions on both the end surfaces of the recording drum  1 . In this case, the necessity of the weight position adjuster  15  is eliminated, so that the construction of the drawing apparatus is simplified. 
     Furthermore, the balance weight  7  is not limited to a magnet. For example, the balance weight may have a structure in which it is mechanically attached to the end surface of the recording drum  1 . 
     FIGS. 35 and 36 are diagrams showing another example of a method of attaching balance weights, where (a) is a front view, and (b) is a side view. 
     In the example shown in FIG. 35, auxiliary members  62  in a disk shape are respectively fixed to both ends of a rotary shaft  3  of a recording drum  1 . One balance weight  7  is disposed so as to be movable and fixable in the circumferential direction and in the radial direction on one surface of each of the auxiliary members  62 . The auxiliary member  62  is made of a metal. Each of the balance weights  7  is composed of a permanent magnet, and is attracted to the auxiliary member  62  by its magnetic force. 
     A hole  7   a  into which the engaging pin  17  of the weight position adjuster  15  shown in FIG. 28 can be inserted is formed in each of the balance weights  7 . When the engaging pin  17  is moved along the radius whose center is the recording shaft  3  of the recording drum  1  in a state where the engaging pin  17  is inserted into the hole  7   a  of the balance weight  7 , the balance weight  7  is moved in the radial direction on the auxiliary member  62 . When the auxiliary member  62 , together with the recording drum  1 , is rotated in this state, the balance weight  7  is moved along the circumference whose center is the rotary shaft  3  of the recording drum  1  on the auxiliary member  62 . After position adjustment, the balance weight  7  is fixed to the auxiliary member  62  by the magnetic force. 
     In the example shown in FIG. 36, one arm member  65  is attached to each of both ends of a rotary shaft  3  of a recording drum  1  so as to be rotatable and fixable around and to the rotary shaft  3 . The arm member  65  is constituted by a cylindrical attachment part  65   a  fitted in the rotary shaft  3  and a long arm  65   b  and a short arm  65   c . A balance weight  7  is fixed to the long arm  65   b  so as to be movable and fixable, and a balancer  66  is provided at an end of the short arm  65   c . The balance  66  keeps a balance of the arm member  65  when there is no balance weight  7 . The attachment part  65   a  of the arm member  65  is composed of a permanent magnet, and is attracted to the rotary shaft  3  made of a metal by its magnetic force. The long arm  65   b  of the arm member  65  is made of a metal. Each of the balance weights  7  is composed of a permanent magnet, and is attracted to the long arm  65   b  of the arm member  65  by its magnetic force. 
     A hole  7   a  into which the engaging pin  17  of the weight position adjuster  15  shown in FIG. 28 can be inserted is formed in each of the balance weights  7 . When the engaging pin  17  is moved along the radius whose center is the rotary shaft  3  of the recording drum  1  in a state where it is inserted into the hole  7   a  of the balance weight  7 , the balance weight  7  is moved in the radial direction on the long arm  65   b  of the arm member  65 . When the rotary shaft  3 , together with the recording drum  1 , is rotated in this state, the arm member  65  is rotated around the rotary shaft  3 , and the balance weight  7  attached to the arm member  65  is moved along the circumference of the recording drum  1 . After position adjustment, the arm member  65  is fixed to the rotary shaft  3  by the magnetic force. 
     The arm member  65  may be attached to the rotary shaft  3  so as to be rotatable and fixable by providing the attachment part  65   a  of the arm member  65  with a ratchet structure instead of forming the attachment part  65   a  of the arm member  65  of a permanent magnet. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.