Abstract:
A double speed intermittent film pull down for a movie projector ( 100 ) comprises a sprocket ( 122 ), which advances the film ( 114 ) a frame at a time. A star wheel ( 140 ) is mounted on a common shaft ( 132 ) with the sprocket ( 122 ). A cam ( 134 ) turns the sprocket ( 122 ) every other rotation of the cam ( 134 ). A shutter ( 106 ) is mounted on a common shaft ( 132 ) with the cam (134) and interrupts light to the frame of the film ( 114 ). The common shaft ( 132 ) for the cam ( 134 ) and shutter ( 106 ) turns at approximately twice the speed of the common shaft ( 132 ) for the star wheel ( 140 ) and sprocket ( 122 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to commonly-assigned copending U.S. Patent application Ser. No. 09/672,272, filed Sep. 28, 2000, entitled AN IMPROVED MOTION PICTURE FILM PROJECTOR, by Ehme et al., the disclosure of which are incorporated herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates in general to a drive mechanism for a motion picture projector and in particular, to a modified Geneva Mechanism which advances a star wheel in one-eight rotation of the driver.  
         BACKGROUND OF THE INVENTION  
         [0003]    In a typical theatrical movie projector, film is advanced through a projection gate at a frame rate of 24 frames/second. In other words, every {fraction (1/24)} second, a new film frame is “pulled down” into the projection aperture and the image contained on the film frame is projected onto the screen. During the time the film frame is being “pulled down,” a shutter closes and the screen is dark. After the frame has come to rest in the projection aperture, the shutter opens.  
           [0004]    The reason that the shutter must close during pull down is that the projected movie image would be degraded if the moving film were projected onto the screen. Therefore, the projected movie image necessarily “flickers” as the shutter opens and closes. It has been found that a flicker rate of 24 Hz produces a noticeable flicker and is objectionable to the audience. This problem is much less noticeable at a flicker rate of 48 Hz. For this reason, it is common to use a shutter which closes again while the film frame is motionless in the projection gate. From the standpoint of flicker, this results in a good quality movie projection.  
           [0005]    Another important aspect of movie projection quality is screen brightness. While closing the shutter twice per frame is good from the standpoint of flicker, it is bad from the standpoint of screen brightness. To achieve high screen brightness while still having a shutter rate of 48 Hz, the duration of the time the shutter is closed in comparison to the time that it is open should be as short as possible. But the length of time the shutter is closed is determined by the time required for film pull down. So screen brightness can be improved by reducing the film pull down time.  
           [0006]    For example, in a typical projector, the film is advanced by a sprocket mounted on the shaft of a four point Geneva Mechanism star, which is, in turn, driven intermittently by a Geneva Mechanism pinwheel. Such a drive is described in U.S. Pat. No. 1,774,789, issued Sep. 2, 1930. The Geneva Mechanism driver shaft rotates at a speed of 24 revs/sec, which advances the film a rate of 24 frames/sec. This type of drive is still standard in the motion picture projection industry. In such an intermittent drive, the time required for pull down is one-fourth of the time of one complete rotation of the driver shaft. The driver shaft, completes one rotation in {fraction (1/24)} second (approximately 40 ms). Thus, pull down occurs in approximately 10 ms.  
           [0007]    Since the shutter is designed to be closed twice per frame and the duration of each shutter closure is equal to the pull down time, the shutter is closed one-half the time, approximately 20 ms out of every 40 ms. Clearly, if pull down could be accomplished in half the time required by a standard Geneva Mechanism (one-eight rotation of the driver shaft instead of one-fourth rotation), the shutter would then need to be closed twice for only 5 ms each time, a total of 10 ms out of every 40 ms. This would obviously be a big improvement for screen brightness.  
         SUMMARY OF THE INVENTION  
         [0008]    According to one aspect of the present invention, a double speed intermittent film pull down for a movie projector comprises a sprocket, which advances the film a frame at a time. A star wheel is mounted on a common shaft with the sprocket. A cam turns the sprocket every other rotation of the cam. A shutter is mounted on a common shaft with the cam and interrupts light to the frame of the film. The common shaft for the cam and shutter turns at approximately twice the speed of the common shaft for the star wheel and sprocket.  
           [0009]    According to one embodiment of the invention, the standard Geneva intermittent drive mechanism, which achieves star wheel advance in one-fourth rotation of the driver, is modified to advance star wheel in one-eight rotation of the driver. This is accomplished by causing the driver to rotate at double speed while also reciprocating axially, causing the driver pin to engage the star wheel slot only every second time around.  
           [0010]    It is necessary for these two shutter intervals to be nearly equal in duration in order to limit perceived flicker. Therefore, since one blanking period must be approximately one-fourth of the frame period in order to blank the projected image as the film moves, the other blanking period must be of essentially the same duration.  
           [0011]    The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a perspective view of a prior art movie projector;  
         [0013]    [0013]FIG. 2 is a top view of a prior art Geneva Mechanism;  
         [0014]    [0014]FIG. 3 is a perspective view of a driver and cam assembly according to the present invention;  
         [0015]    [0015]FIG. 4 is a perspective view of the driver shown in FIG. 3 with a counter weight;  
         [0016]    [0016]FIG. 5 is a perspective view in partial section which shows rotating hubs; and  
         [0017]    [0017]FIG. 6 is a perspective view of the entire mechanism. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The traditional prior art motion picture film projector, also known as a movie projector  100 , is illustrated in FIG. 1, where a beam of light  116  generated by arc lamp  102 . Projector  100  uses elliptical reflector  104  as beam shaping optics to focus light beam  116  past shutter  106  and through aperture plate  108  to illuminate a frame of the film  114 . The film frame is then imaged by projection lens  110  onto screen  112 . Each frame is sequentially moved through the film gate (not shown) and past the aperture plate  108  by sprocket  122 , which is driven by Geneva Mechanism  120 , whose main components are star wheel  140  and driver  130 .  
         [0019]    The conventional Geneva Mechanism  120 , shown in greater detail in FIG. 2, comprises a driver  130  and a star wheel  140 , which together drive a load, such as film, in a controlled intermittent fashion, while driver  130  itself is driven continuously with essentially constant rotary motion. The rotational motion of star wheel  140  includes an index followed by a dwell for each revolution of driver  130 .  
         [0020]    Driver  130  typically includes a drive shaft  132 , a restraining cam  134 , and a drive arm bearing a drive pin  136 . Driver  130  is typically attached to a flywheel (not shown) and a drive motor or gear train (not shown), which provide a constant angular velocity input to driver  130 .  
         [0021]    Star wheel  140  comprises a shaft  146 , which is attached at one end to star wheel  140 , and at the other end to a load to be driven. In the case of a motion picture projector, a sprocket  122  is attached to the shaft  146 , and the sprocket in turn engages the perforations of the film, thereby transferring the intermittent drive motion to the film. In the typical Geneva Mechanism, the star wheel  140  comprises a number of straight slots  142 , the center lines of which extend radially outward from the center of rotation, and where straight slots  142  are positioned at equal angles about the center of rotation. In between straight slots  142  are a number of concave surfaces  144 . Concave surfaces  144  and straight slots  142  alternate around the periphery of star wheel  140  and are equal in number.  
         [0022]    During an indexing motion, drive pin  136  enters one of straight slots  142 , and then angularly accelerates star wheel  140  about its center of rotation. This acceleration continues until the midpoint of the indexing motion, where drive pin  136  crosses the line joining the centers of rotation of star wheel  140  and driver  130 . At this point star wheel  140  begins an angular deceleration which continues until drive pin  136  exits straight slot  142 . The star wheel  140  attains its maximum angular velocity at the mid-index position, while both at the beginning and end of index in its angular velocity is zero. As the star wheel  140  depicted in FIG. 2 has four straight slots, its complete motion for one index from rest to peak velocity and back to rest corresponds to 90° of rotation.  
         [0023]    Following the indexing motion there occurs a dwell period, during which drive pin  136  is not in engagement with star wheel  140 . Driver  130  then rotates to complete one revolution such that it subsequently returns to a position of initial engagement with the next straight slot  142  of star wheel  140 . During the dwell period, the star wheel  140  is restrained from any rotation by the engagement of one of its concave surfaces  144  with convex surface of restraining cam  134  of driver  130 .  
         [0024]    The method of operation of a standard Geneva Mechanism is well known. The method of operation of the present invention is similar, except that the driver is caused to undergo a reciprocating axial motion in addition to its rotating motion. These two motions must occur simultaneously and in an exact phase relation to each other in order for the driver pin to engage the star wheel only every second rotation. The mechanism that causes the combined axial and rotary motion of the driver are intended to be only exemplary. Other designs which accomplish the same purpose are intended to fall within the scope of this invention.  
         [0025]    In FIG. 3 a driver is shown, which is comprised of head  210  and shaft  215 . Rigidly clamped to the shaft is cam/gear assembly  220 . Cam/gear assembly  220  is comprised of gear  225 , which is pressed onto hub  230 . Slot  235  is cut into hub  230  and is cylindroidal in shape. Another gear, not shown in FIG. 3, will engage gear  225  to cause the driver to rotate. The purpose of the cylindriodally shaped slot is to cause the driver to reciprocate axially. This will become clear from the discussion below.  
         [0026]    [0026]FIG. 4 shows counterweight  245  mounted on shaft  215 . Key  240  insures that counterweight  245  will rotate along with shaft  215 , but allows it to move freely in an axial direction. Counterweight  245  has a hub section  246  with a cylindroidally shaped slot  250 . Slot  250  and slot  235  are in opposed phase relation with each other which will cause the axial motion of the counterweight to be exactly equal but opposite in direction to the axial motion of the driver.  
         [0027]    [0027]FIG. 5 shows sleeve  255 , cutaway in section view, which fits rotatably over hubs  230  and  246 . Two round pins  260  fit in diametrically opposed holes in sleeve  255  and extend inwardly into slot  235 . Clip  265  fits in grooves  266  and retains pins  260  in place. Two round pins  270  fit in diametrically opposed holes in sleeve  255  and extend inwardly into slot  250 . Clip  275  fits in grooves  276  and retains pins  270  in place. The diameter of the pins is approximately 0.001 inches less than the width of the slots so that as the sleeve is rotated, the pins will be free to slide in the slots. The sleeve has a rectangular groove  280  which is just 0.001 inches wider than the thickness of fixed plate  290 . This allows sleeve  255  to rotate but prevents it from moving axially. Gear  285  is pressed onto sleeve  255 .  
         [0028]    Rotation of the sleeve by one-quarter turn causes the driver to retract axially and the counterweight to move forward axially by an equal amount. Rotation of the sleeve by an additional 290 degrees causes the driver and counterweight to return to their original axial positions. Alternatively, if the sleeve were prevented from moving rotationally, then rotation of the driver would produce exactly the same results. One rotation of the driver would be accompanied by two axial cycles of motion. However, according to this embodiment the driver moves through only one-half of an axial cycle in one complete rotation. For this to happen, sleeve rotates at three-quarters of the speed of the driver.  
         [0029]    [0029]FIG. 6 shows the mechanism for causing the sleeve to rotate at three-quarters of the speed of the driver. Change gear  292  is comprised of two gears  295  and  300 , rigidly and concentrically attached to shaft  310 , which is driven rotationally by means of a motor. Change gear  295  engages gear  225 . Gear  300  engages gear  285 . The ratio of the diameters of these gears is chosen to produce rotation of the sleeve at three-quarters of the rotation of the driver. The gear ratio of gear  225  to gear  295  is 1:2. The gear ratio of gear  300  to gear  285  is 3:2. The combined ratio is the product of these two ratios, 3:4. So the sleeve is caused to rotate at three-quarters of the speed of the driver, thereby causing the driver to reciprocate axially once for every two rotations.  
         [0030]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.  
       PARTS LIST  
       [0031]    [0031] 100 . Projector  
         [0032]    [0032] 102 . Arc lamp  
         [0033]    [0033] 104 . Reflector  
         [0034]    [0034] 106 . Shutter  
         [0035]    [0035] 108 . Aperture plate  
         [0036]    [0036] 110 . Projection lens  
         [0037]    [0037] 112 . Screen  
         [0038]    [0038] 114 . Film  
         [0039]    [0039] 116 . Light beam  
         [0040]    [0040] 120 . Geneva Mechanism  
         [0041]    [0041] 122 . Sprocket  
         [0042]    [0042] 130 . Driver  
         [0043]    [0043] 132 . Drive shaft  
         [0044]    [0044] 134 . Restraining cam  
         [0045]    [0045] 136 . Drive pin  
         [0046]    [0046] 140 . Star wheel  
         [0047]    [0047] 142 . Straight slots  
         [0048]    [0048] 144 . Concave surfaces  
         [0049]    [0049] 146 . Shaft  
         [0050]    [0050] 210 . Head  
         [0051]    [0051] 215 . Shaft  
         [0052]    [0052] 220 . Cam/gear assembly  
         [0053]    [0053] 225 . Gear  
         [0054]    [0054] 230 . Hub  
         [0055]    [0055] 235 . Slot  
         [0056]    [0056] 245 . Counterweight  
         [0057]    [0057] 246 . Hub section  
         [0058]    [0058] 250 . Slot  
         [0059]    [0059] 255 . Sleeve  
         [0060]    [0060] 260 . Pins  
         [0061]    [0061] 265 . Clip  
         [0062]    [0062] 266 . Grooves  
         [0063]    [0063] 270 . Pins  
         [0064]    [0064] 275 . Clip  
         [0065]    [0065] 276 . Grooves  
         [0066]    [0066] 280 . Grooves  
         [0067]    [0067] 285 . Gear  
         [0068]    [0068] 290 . Plate  
         [0069]    [0069] 292 . Gear  
         [0070]    [0070] 295 . Gear  
         [0071]    [0071] 300 . Gear  
         [0072]    [0072] 310 . Shaft