Patent Document

FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to the field of surface mount assembly machines and more particularly to a tape feeder providing highly repeatable and accurate advancement of a component-carrying tape.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the surface mount assembly field, component-carrying tapes are used to store and deliver electronic components for use in populating circuit boards or other substrates using surface mount processes. These component-carrying tapes have pockets sequentially arranged along the length of the tape for carrying various electronic components and perforations along an edge of the tape for use in advancing the tape. The distance between the pockets is referred to as the pitch of the tape. A tape feeder is typically used to provide automated delivery of the components to surface mount equipment, such as a pick-and-place machine. The tape feeder typically comprises a feed sprocket that engages the perforations in the tape, a motor to provide a driving force, a drive train to transfer force from the motor to the sprocket, and a control system to control the rotation of the motor and consequently, the advancement and positioning of the tape.  
         [0003]     Surface mount components continue to get smaller, and, in order to increase efficiency, it is desirable to decrease the pitch (i.e., the space between pockets in the tape). Smaller components and reduced pitch require more precise positioning of the tape by the tape feeder so that the pick-and-place machine, which has a small head, can pick up the components. Existing tape feeders, however, often lack the precision and repeatability to accurately present these smaller components typically having dimensions of 0.04 inches or less. Also, tape feeders designed to handle small components and small pitch sizes are typically complex and costly to produce.  
       SUMMARY OF THE INVENTION  
       [0004]     In an exemplary embodiment of the invention, a tape feeder precisely advances a component-carrying tape to present sequential electronic components disposed at a pitch in the tape to a pick-and-place machine. In the exemplary tape feeder, a feed sprocket, and an encoder disc are operatively associated with each other and rotatably disposed on a common axis. A motor is operatively connected to the feed sprocket to repetitively rotate the feed sprocket over an angle corresponding to the pitch of the component-carrying tape. An encoder is disposed to read the encoder disk and provide a feedback signal indicating the angular position of the feed sprocket. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The invention is described below with reference to the accompanying drawings, of which:  
         [0006]      FIG. 1  is a side view, partially in section, showing a tape feeder according to an exemplary embodiment of the invention;  
         [0007]      FIG. 2  is a sectional view of the tape feeder of  FIG. 1  taken generally along axis A-A shown in  FIG. 1 ;  
         [0008]      FIG. 3  shows an encoder disc according to an exemplary embodiment of the invention; and  
         [0009]      FIG. 4  shows a detailed view of the encoder disc of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]     The present invention is a tape feeder  1  with a low complexity architecture that drives a component-carrying tape  30  by engaging perforations (not shown) along an edge of the component-carrying tape  30 , providing component positioning that is highly accurate and repeatable. Referring to  FIGS. 1 and 2 , in an exemplary embodiment of the invention, a feed sprocket  10  is attached to a worm gear  20  that rotates around a fixed axis  25  (shown in  FIG. 2 ) on a pair of ball bearings  26 . The ball bearings  26  are spring loaded and biased in the axial direction to remove radial and axial play. The feed sprocket  10  comprises a plurality of teeth  12  disposed around its periphery, such that the arc length between the teeth  12  is essentially equal to the spacing between the perforations in the edge of the component-carrying tape  30 . The feed sprocket  10  may, for example, be mounted on a hub of the worm gear  20  or attached to a side face of the worm gear  20 . Feed sprocket  10  and worm gear  20  are operatively associated with each other, such that they rotate together about the axis  25  defined by the ball bearings  26 .  
         [0011]     The feed sprocket  10  and worm gear  20  are mounted in a housing  50 . The feed sprocket  10  and the worm gear  20  are positioned with respect to the upper tape feed track  3  such that the teeth  12  engage the feed holes in the component-carrying tape  30  riding in the upper tape feed track  3 . The upper tape feed track  3  is formed in the housing  50  to guide the component-carrying tape  30 . Upper tape feed track  3  directs the tape  30  over the feed sprocket  10  at a window  55  where components are removed from the tape  30 . After the components are removed, the empty tape  30  is guided through a lower tape feed track  3 A where the emptied tape  30  exits the tape feeder  1 .  
         [0012]     The worm gear  20  is driven by a worm shaft  21  mounted by a pair of ball bearings (not shown) in a worm shaft mounting block  23  and coupled to a DC gear motor  22 . The mounting of the worm shaft  21  and motor  22  assembly is adjustable to limit backlash between the worm shaft  21  and worm gear  20 . This adjustment is made by sliding the worm shaft mounting block  23  toward the worm gear  20  and keeping its right surface against the mating surface on the housing to maintain the square relationship of the worm shaft  21  and worm gear  20 . When the location of zero backlash is found, two screws are inserted through the worm mounting block  23  to lock the block and thus the worm shaft  21  in place. DC power is selectively provided to the motor  22  to rotate the worm gear  20  and feed sprocket  10 , and thereby advance the component-carrying tape  30 . DC power is discontinued to maintain the position of the worm gear  20  and the feed sprocket  10 , and thereby stop the component-carrying tape  30  so that a pick-and-place machine can remove a component from the component-carrying tape  30 . Thus, the angular position of the worm gear  20  and the feed sprocket  10  are controlled by applying and interrupting power to the motor  22 .  
         [0013]     An encoder disc  40  is mounted to the worm gear  20  via a hub to rotate together with the sprocket  10  and the worm gear  20  on the same ball bearing axis. The encoder disc  40  is operatively associated with the worm gear  20  and feed sprocket  10 , such that its angular position is consistent with the angular positions of the worm gear  20  and feed sprocket  10 . An encoder  46  is mounted in the housing  50  and positioned to read the encoder disc  40 .  
         [0014]     As shown in  FIGS. 3 and 4 , the encoder disc  40  has a primary ring of finely spaced lines  41  on a face of the encoder disc  40 , extending radially at essentially equal angular intervals. The lines  41  are read by the encoder  46 , which generates an electronic pulse that is used to interpret the angular position of the encoder disc  40 . Quadrature output can be used to multiply the number of encoder pulses into a higher number of “counts” to improve position resolution. The angular position of the worm gear  20  and feed sprocket  10  are equivalent to the angular position of the encoder disc  40 , and are therefore also determined by the encoder  46 . The encoder disc  40  has a very large number of lines  41 , substantially more lines than there are teeth on the feed sprocket (e.g., more than ten times as many lines as teeth, and preferably about 2500 distinct, essentially equally spaced lines). The substantially greater number of lines  41  enable very precise measurement of the angular position of the encoder disc  40  and therefore, the angular position of the operatively associated feed sprocket  10 . From a plurality of angular position measurements, the angular velocity of the feed sprocket  10  can be determined, and therefore, the speed and position of the component-carrying tape  30  can be precisely determined.  
         [0015]     Optionally, a secondary ring with a relatively smaller number of equally spaced lines  42 , as compared to the number of lines  41 , may be provided on the encoder disc  40 . The number of lines  42  matches the typical number of feed strokes accomplished by one complete revolution of the feed sprocket  10 . These lines  42  may be used as a reference point on the feed sprocket  10  after each feed stroke.  
         [0016]     A processor (not shown), such as a microcomputer, can count the electronic pulses or “counts” that are generated by the encoder  46  as a result of the lines  41  passing the encoder  46 . By counting the lines  41  from a known start-point (e.g., lines  42 ), the processor can monitor the feed sprocket position and use software to control the motor  22  to effect an exact and repeatable sprocket feed. An improvement in precision is gained by having the encoder  46  on the axis of the feed sprocket  10 , rather than on the motor  22 , as is typical. Also, because the encoder disc  40  can use lines  42  as a known start point for each feed stroke, cumulative errors from successive feed strokes can be prevented. Additionally, because the closely spaced lines  41  can be used to accurately determine the position and angular velocity of the feed sprocket  10 , the DC power to the motor  22  can be discontinued at the appropriate time to compensate for hysteresis in the motor  22  and worm gear  20 .  
         [0017]     Referring again to  FIGS. 1 and 2 , a tape cover plate  51  forms a portion of the housing  50  positioned over the upper tape feed track  3  to retain the tape  30  in operative engagement with the feed sprocket  10 . As described above, the components on the component-carrying tape  30  can be accessed through the window  55  by a pick-and-place head (not shown) of an assembly machine. To access the components, a thin cover tape  31  must be removed from the component-carrying tape  30 . When the tape  30  is first loaded, the cover tape  31  is peeled back from the tape  30  in the window  55  and threaded around a pulley  54  to a pull-off wheel  56  which is turning opposite from the direction of travel of the component-carrying tape  30 . On the outer diameter of the pull-off wheel  56 , a tire  57  is in frictional contact with the cover tape  31 . The tire  57  is composed of a resilient material, such as urethane. The cover tape  31  is pulled off of the component-carrying tape  30  and back by rotating the pull-off wheel  56 . The pull-off wheel  56  may be rotated, for example, by a belt  59 , which transmits power from the worm gear  20 . The belt  59  rides in a groove  52  on a hub of the worm gear  20  and is coupled to pull-off wheel  56 . A spring wheel  58  is biased toward the tire  57 , pressing the cover tape  31  into the tire  57  to ensure that the tire  57  adequately grips the cover tape  31  being pulled and expelled.  
         [0018]     The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.

Technology Category: 4