BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a scanner, and in particular to a scanner with cover opened to a predetermined angle. 
         [0003]    2. Description of the Related Art 
         [0004]    Referring to  FIG. 1   a , a conventional scanner  10  comprises a main body  5  and a cover  7 . The cover  7  can be opened to an angle α, exceeding 90° for easy operation, however, when erected or hung on a wall, as shown in  FIGS. 1   b  and  1   c , to avoid damage, the open angle of the cover  7  must be limited. In general, the open angle of the cover  7  is between 30° to 60° when the scanner  10  is erected. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    An embodiment of a scanner of the invention comprises a main body, a cover and a limiting mechanism. The cover is rotatably disposed on the main body to cover a scanned object. The limiting mechanism is disposed between the main body and the cover. When the main body is horizontal, the limiting mechanism does not function and the cover can assure a workable angle. When the main body is erected, the limiting mechanism limits the cover to a predetermined angle. 
         [0006]    The limiting mechanism comprises a first hook rotatably disposed on the main body and a second hook corresponding to the first hook and fixed to the cover. When the main body is horizontal, the first hook does not engage the second hook and the cover can assure a workable angle. When the main body is erected, the first hook engages the second hook and the cover is limited to the predetermined angle. 
         [0007]    The main body has a inclined surface. When the main body is horizontal, the first hook leans on the inclined surface without engaging the second hook. When the main body is erected, the first hook rotates from the inclined surface by gravity to engage the second hook. 
         [0008]    The first hook comprises a pivot portion rotatably disposed on the main body and a first hook portion connected to the pivot portion and engaging the second hook. 
         [0009]    The pivot portion and the first hook portion can be formed integrally. In another aspect, the first hook portion can be rotatably connected to the pivot portion. The first hook further comprises an extension spring or a torsion spring connecting the first hook portion and the pivot portion, whereby the first hook portion is positioned to the pivot portion. 
         [0010]    The first hook further comprises a rib disposed on the pivot portion and abutting the first hook portion, whereby the first hook portion is positioned to the pivot portion. 
         [0011]    The main body has a through hole, and the pivot portion has a shaft joined to the through hole, whereby the pivot portion is rotatably disposed on the main body. 
         [0012]    The shaft has a curved groove with a first end and a second end, and a protrusion disposed in the through hole such that when the shaft is joined to the through hole, the protrusion is in the curved groove. When the main body is horizontal, the protrusion abuts the first end, whereby the first hook does not engage the second hook. When the main body is erected, the first hook is rotated by gravity and the protrusion abuts the second end, whereby the first hook engages the second hook and the cover is opened to the predetermined angle. 
         [0013]    The second hook comprises a support portion fixed to the cover and a second hook portion connected to the support portion and engaging the first hook. 
         [0014]    The second hook portion and the support portion can be formed integrally. In another aspect, the second hook portion can be rotatably connected to the support portion. The second hook further comprises an extension spring or a torsion spring connecting the second hook portion and the support portion, whereby the second hook portion is positioned to the support portion. 
         [0015]    The second hook further comprises a rib disposed on the support portion and abutting the second hook portion, whereby the second hook portion is positioned to the support portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0017]      FIGS. 1   a ,  1   b  and  1   c  depict a conventional scanner; 
           [0018]      FIG. 2  is a schematic view of an embodiment of a scanner of the invention; 
           [0019]      FIG. 3  is an enlarged view of the limiting mechanism of  FIG. 2 ; 
           [0020]      FIGS. 4   a ˜ 4   c  depict motion of the limiting mechanism when the scanner is horizontal; 
           [0021]      FIGS. 5   a ˜ 5   d  depict motion of the limiting mechanism when the scanner is erected; 
           [0022]      FIGS. 6   a  and  6   b  depict another embodiment of a scanner of the invention; 
           [0023]      FIGS. 7   a  and  7   b  depict another embodiment of a scanner of the invention; 
           [0024]      FIGS. 8   a ˜ 8   c  are schematic views of the first hook and the second hook of the embodiment of  FIGS. 7   a  and  7   b;    
           [0025]      FIG. 9  depicts another embodiment of a scanner of the invention; 
           [0026]      FIG. 10  depicts another embodiment of a scanner of the invention; and 
           [0027]      FIG. 11  depicts another embodiment of a scanner of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Referring to  FIG. 2 , a scanner  100  of the invention comprises a main body  200 , a cover  300  and a limiting mechanism  400 .  FIG. 3  is an enlarged view of the limiting mechanism  400 . The limiting mechanism  400  comprises a first hook  410  and a second hook  420 . The first hook  410  is rotatably disposed on the main body  100 . The second hook  420  is fixed to the cover  300 . The first hook  410  and the second hook  420  are substantially L-shaped and capable of engaging each other. The first hook  410  comprises a first hook portion  412  and a pivot portion  414  rotatbly connected to the main body  200  with respect to a shaft B. The second hook  420  comprises a second hook portion  422  and a support portion  424  fixed to the cover  300 . The second hook portion  422  is connected to the support portion  424 . The first hook portion  412  and the second hook portion  422  engage each other. 
         [0029]      FIGS. 4   a ˜ 4   c  depict motions of the limiting mechanism  400  when the scanner  100  is horizontal. When the main body  200  is horizontal, the first hook  410  leans on an inclined surface  220  of the main body  200 . The inclined surface  220  has a proper angle with respect to the scanning plane  230 , whereby the first hook  410  cannot engage the second hook  420 . When the cover  300  is rotated about a shaft A and lifted upward the first hook  410  cannot constrain the cover  300 , whereby the cover  300  can be opened to an appropriate workable angle, as shown in  FIGS. 4   b  and  4   c.    
         [0030]      FIGS. 5   a ˜ 5   d  depict motion of the limiting mechanism  400  when the scanner  100  is erected. In  FIG. 5   a , when the main body  200  is erected, the first hook  410  is inclined by a proper angle with respect to a vertical plane due to the inclined surface  220 . The first hook  410  is rotated by gravity to contact with the second hook portion  422 . When the cover  300  is rotated about the shaft A and opened, the second hook portion  422  engages the first hook portion  412  to limit the open angle of the cover  300  as shown in  FIGS. 5   b ˜ 5   d.    
         [0031]    In the embodiment, when the main body  200  is erected, the angle of the inclined surface  220  with respect to the vertical plane (the scanning plane  230 ) ensures that the first hook  410  rotates to engage the second hook  420 .  FIGS. 6   a  and  6   b  depict another embodiment of a scanner of the invention. In this embodiment, a curved groove  413  is formed on the shaft B. The curved groove  413  has a first end  4131  and a second end  4132 . The main body  200  has a through hole  240  in which a protrusion  242  is disposed. When the shaft B engages the through hole  240 , the protrusion  242  is in the curved groove  413  and capable of abutting the first end  4131  or the second end  4132 . When the main body  200  is horizontal, the first hook  410  is rotated clockwise by gravity. At this time, the first end  4131  abuts the protrusion  242 , whereby the first hook  410  is inclined with respect to the scanning plane  230  as shown in  FIG. 6   a . When the main body  200  is erected, the first hook  410  is rotated counterclockwise by gravity and the second end  4132  abuts the protrusion  242 , whereby the first hook  410  engages the second hook  420  and the cover  300  is opened to a predetermined angle. 
         [0032]      FIGS. 7   a  and  7   b  depict another embodiment of the invention. The embodiment is similar to the embodiment shown in  FIG. 2 . Only the first hook  410  is different from the previous embodiment. The first hook portion  412  is pivoted on the pivot portion  414 , and an extension spring  416  connects the first hook portion  412  and shaft B. A pair of ribs  417  is disposed on the pivot portion  414  and abuts the first hook portion  412 . When the extension spring  416  pulls the first hook portion  412 , the first hook portion  412  abuts the rib  417  and is positioned to the pivot portion  414 .  FIGS. 8   a ˜ 8   c  depict the first hook  410  engaging the second hook  420  of the embodiment.  FIG. 8   a  depicts the cover  300  opened to a maximum angle when the main body  200  is erected. If the cover  300  is further opened, the extension spring  416  is extended and the second hook portion  422  forces the first hook portion  412  to rotate with respect to the pivot portion  414 , whereby the cover  300  is opened to a maximum angle as shown in  FIGS. 8   b  and  8   c . In this way, when force is exerted on the cover  300 , damage to the limiting mechanism  400  is avoided. 
         [0033]      FIG. 9  depicts another embodiment of a scanner of the invention. The embodiment is similar to the embodiment shown in  FIG. 7   a . Only the extension spring  416  is replaced by a torsion spring  418 . Operation of the embodiment is the same as the embodiment of  FIG. 7   a.    
         [0034]    In the two embodiments, the first hook  410  comprises two individual elements, the first hook portion  412  and the pivot portion  414 . Similarly, the second hook  420  can also comprise two individual elements.  FIG. 10  depicts another embodiment of the invention. The second hook portion  422  is pivoted on the support portion  424 . Two ribs  425  are disposed on opposite sides of support portion  424 . A torsion spring  428  connects the second hook portion and the support portion  424 . The second hook portion  422  abuts the rib  425  and is positioned to the support portion  424 . As the motion of the embodiment is the same as the previous two embodiments, description is omitted. 
         [0035]      FIG. 11  depicts another embodiment of the invention. The embodiment is similar to the embodiment shown in  FIG. 10 . Only the torsion spring  428  is replaced by an extension spring  426  connecting the second hook portion  422  and the support portion  424 . 
         [0036]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
A scanner comprises a main body, a cover and a limiting mechanism. The cover is rotatably disposed on the main body to cover a scanned object. The limiting mechanism is disposed between the main body and the cover. When the main body is horizontal, the limiting mechanism does not function and the cover assures a workable angle. When the main body is erected, the limiting mechanism limits the cover to a predetermined angle.