Patent Document

[0001]    The present invention relates to friction drive apparatus such as printers, plotters and cutters that feed strip material for producing graphic images and, more particularly, to a method for calibration of friction drive apparatus and a method for automatic alignment of strip material therein.  
         BACKGROUND OF THE INVENTION  
         [0002]    Friction, grit, or grid drive systems for moving strips or webs of sheet material longitudinally back and forth along a feed path through a plotting, printing, or cutting device are well known in the art. In such drive systems, friction (or grit or grid) wheels are placed on one side of the strip of sheet material (generally vinyl or paper) and pinch rollers, of rubber or other flexible material, are placed on the other side of the strip, with spring pressure urging the pinch rollers and material against the friction wheels. During plotting, printing, or cutting, the strip material is driven back and forth, in the longitudinal or X-direction, by the friction wheels while, at the same time, a pen, printing head, or cutting blade is driven over the strip material in the lateral or Y-direction.  
           [0003]    These systems have gained substantial favor due to their ability to accept plain (unperforated) strips of material in differing widths. However, the existing friction drive apparatus experience several problems. One problem that occurs in friction drive apparatus is a skew error. The skew error will arise as a result of strip material being driven unevenly between its two longitudinal edges, causing the strip material to assume a cocked position. The error is integrated in the lateral or Y-direction and produces an increasing lateral position error as the strip material moves along the X-direction. The error is often visible when the start of one object must align with the end of a previously plotted object. In the worst case, such lateral errors result in the strip drifting completely off the friction wheel. The skew error is highly undesirable because the resultant graphic image is usually destroyed.  
           [0004]    Most material strips are inserted manually into the friction drive systems. During the manual insertion, it is essentially impossible to place the material strip perfectly straight in the friction drive apparatus. Therefore, the existing systems typically use at least three feet of strip material until the strip material is straightened with respect to the friction drive apparatus. This manual alignment procedure has numerous drawbacks. First, it results in excessive material consumption and waste thereof. Second, the procedure is time consuming. Additionally, manual alignment is not always effective. Therefore, there is a need to reduce wasteful consumption of strip material during loading thereof into the friction drive apparatus and to ensure proper alignment of the strip material within the friction drive apparatus during operation.  
         SUMMARY OF THE INVENTION  
         [0005]    It is an object of the present invention to provide an apparatus and a method for automatically aligning strip material in a friction drive apparatus at the onset of an operation without excessive strip material waste.  
           [0006]    It is another object of the present invention to provide an apparatus and a method for properly calibrating two sensors that detect an edge of the strip material in the friction drive apparatus with respect to each other.  
           [0007]    According to the present invention, a friction drive apparatus incudes an edge detection system having a first sensor and a second sensor for determining a lateral position of a longitudinal edge of a strip material. The friction drive apparatus also includes first and second friction wheels advancing the strip material in a longitudinal direction that are rotated by independently driven motors which are driven independently in response to position of the longitudinal edge of the strip material detected by the sensor disposed behind the friction wheels with respect to the direction of motion of the strip material.  
           [0008]    The friction drive apparatus also includes instructions for automatically aligning the strip material in the friction drive apparatus upon loading of the strip material and instructions for calibrating the second sensor with respect to the first sensor of the edge detection system. The automatic alignment procedure includes steps of advancing the strip material in the longitudinal direction a predetermined aligning amount while the strip material is steered with respect to the controlling sensor to eliminate any lateral deviations of the strip material from the feed path. The calibration procedure calibrates the second sensor with respect to the first sensor to eliminate any potential offset that may have been introduced during assembly and installation of the sensors.  
           [0009]    One advantage of the present invention is that it eliminates the need for an operator to manually align the strip material. The automatic alignment reduces the amount of wasted strip material as compared to a manual alignment operation and results in time savings and improved quality of the final graphic product. Another advantage of the present invention is that the calibration procedure provides additional accuracy to the proper alignment of the strip material and also improves quality of the final graphic product.  
           [0010]    The foregoing and other advantages of the present invention become more apparent in light of the following detailed description of the exemplary embodiments thereof, as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is an exploded side elevational view schematically showing a friction drive apparatus, according to the present invention;  
         [0012]    [0012]FIG. 2 is a schematic plan view of a bottom portion of the friction drive apparatus of FIG. 1 with the strip material shown in phantom;  
         [0013]    [0013]FIG. 3 is a schematic, perspective view of an edge detection system of the friction drive apparatus of FIG. 2 with the strip material shown in phantom;  
         [0014]    [0014]FIG. 4 is a schematic representation of a strip material moving properly along a feed path for the strip material in the friction drive apparatus of FIG. 2;  
         [0015]    [0015]FIG. 5 is a schematic representation of the strip material deviating from the feed path of FIG. 4 and a correction initiated by adjusting the relative speeds of drive motors;  
         [0016]    [0016]FIG. 6 is a schematic representation of the strip material deviating from the feed path of FIG. 4 and a further correction initiated by adjusting the relative speeds of the drive motors;  
         [0017]    [0017]FIG. 7 is a schematic representation of the strip material being loaded into the friction drive apparatus of FIG. 1;  
         [0018]    [0018]FIG. 8 is a high level logic diagram of an automatic alignment procedure of the strip material subsequent to being loaded into the friction drive apparatus as shown in FIG. 7;  
         [0019]    [0019]FIG. 9 is a schematic representation of the strip material being steered into a proper alignment position in accordance with the automatic alignment procedure of FIG. 8;  
         [0020]    [0020]FIG. 10 is a schematic representation of the strip material being further steered into a proper alignment position in accordance with the automatic alignment procedure of FIG. 8;  
         [0021]    [0021]FIG. 11 is a high level logic diagram of a calibration procedure for the edge detection system of the friction drive apparatus of FIG. 1;  
         [0022]    [0022]FIG. 12 is a schematic representation of an alternate embodiment of the edge detection system with the strip material moving along the feed path in the drive apparatus of FIG. 1;  
         [0023]    [0023]FIG. 13 is a schematic representation of another alternate embodiment of the edge detection system with the strip material moving along the feed path in the drive apparatus of FIG. 1; and  
         [0024]    [0024]FIG. 14 is a schematic representation of a wide strip material moving along the feed path in the drive apparatus of FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Referring to FIG. 1, an apparatus  10  for plotting, printing, or cutting strip material  12  includes a top portion  14  and a bottom portion  16 . The strip material  12 , having longitudinal edges  20 ,  22 , as best seen in FIG. 2, is moving in a longitudinal or X-direction along a feed path  24 . The top portion  14  of the apparatus  10  includes a tool head  26  movable in a lateral or Y-direction perpendicular to the X-direction and the feed path  24 . The top portion  14  also includes a plurality of pinch rollers  30  that are disposed along the longitudinal edges  20 ,  22  of the strip material  12 . The bottom portion  16  of the apparatus  10  includes a stationary or roller platen  32 , disposed in register with the tool head  26 , and a plurality of friction wheels  34 ,  36 , disposed in register with the pinch rollers  30 .  
         [0026]    Referring to FIG. 2, each friction wheel  34 ,  36  has a surface for engaging the strip material  12 , and is driven by a motor drive  40 ,  42 , respectively. Each motor drive  40 ,  42  may be a servo-motor with a drive shaft connected to a shaft encoder  44 ,  46  for detecting rotation of the drive shaft. Each encoder  44 ,  46  is connected to a decoder  50 ,  52 , respectively. Each decoder  50 ,  52  is in communication with a processor  54 . The apparatus  10  also includes an edge detection system  55  that operates in conjunction with the motors  40 ,  42  to automatically align the strip material  12  and to minimize skew error during operation. The edge detection system  55  includes a first sensor  56  and a second sensor  58  for tracking the longitudinal edge  20  of the strip material  12 , with sensors  56 ,  58  being disposed on opposite sides of the friction wheels  34 ,  36 . Each sensor  56 ,  58  is in communication with the processor  54  via associated circuitry  62 ,  64 , respectively. The processor  54  also communicates with each motor drive  40 ,  42  to complete a closed loop system.  
         [0027]    Referring to FIG. 3, the edge detection system  55  further includes a first light source  66  and a second light source  68  positioned substantially above the first and second sensors  56 ,  58 , respectively. Each sensor  56 ,  58  includes a first and second outer edges  72 ,  74  and first and second inner edges  76 ,  78 , respectively, with first and second stops  82 ,  84  disposed substantially adjacent to each respective outer edge  72 ,  74 . In the preferred embodiment of the present, invention each sensor  56 ,  58  includes a plurality of pixels  92  arranged in a linear array with a central pixel  94  being disposed in the center of the plurality of pixels  92  and defined to be a center reference position. Also, in the preferred embodiment of the present invention, the associated circuitry  62 ,  64  includes a pulse shaper and a serial to parallel converter (not shown).  
         [0028]    During normal operation, as the strip material  12  is fed along the feed path  24  in the longitudinal or X-direction, the friction wheels  34 ,  36  and the pinch rollers  30  are urged together and engage the strip material  12 , as best seen in FIGS. 1 and 2. The motor drives  40 ,  42  rotate the friction wheels  34 ,  36 , respectively, at substantially the same speed to ensure that both longitudinal edges  20 ,  22  of the strip material  12  progress along the feed path  24  in the X-direction simultaneously. As the strip material  12  moves in the longitudinal or X-direction, the tool head  26  moves in a lateral or Y-direction, either plotting, printing, or cutting the strip material depending on the specific type of the tool employed.  
         [0029]    The sensor  58 , disposed behind the friction wheels  34 ,  36  with respect to the strip material motion indicated by the arrow, detects and ensures that the strip material  12  does not move laterally in the Y-direction. Referring to FIG. 3, each pixel  92  that is exposed to light emitted from the light source  68  generates photo current, which is then integrated. A logic “one” from each pixel  92  indicates presence of light. Pixels that are shielded from light by the strip material  12 , do not generate photo current and result in a logic reading of “zero”. A bit shift register (not shown) outputs serial data, one bit for each pixel starting with the first pixel, adjacent to the outer edge  74  of the sensor  58 . The output is then shaped and input into a counter (not shown). The counter counts until the serial data reaches at least two logic “zeros” in succession. Two logic “zeros” in succession indicate that the edge  20  of the strip material  12  has been reached and the counter is stopped. The position of the edge  20  of the strip material  12  is then established and used to reposition the strip material  12 . This procedure is repeated every predetermined time interval. In the preferred embodiment of the present invention, the predetermined time interval is approximately every 250 micro-seconds. Thus, with proper longitudinal positioning of the strip material, that is, with no Y-position error, the sensor  58  is half covered, and the motor drives  40 ,  42  rotate friction wheels  34 ,  36  simultaneously at the same speed, as shown in FIG. 4.  
         [0030]    Referring to FIG. 5, a Y-position error occurs when the strip material  12 , for example, moves to the right exposing more than one half of the sensor  58 . When more than one half of the sensor  58  is exposed, the sensor  58  and its associated circuitry generate a positional output to the processor  54  via the associated circuitry  64 , as best seen in FIG. 2, indicating that the strip material  12  is shifted to the right. Once the processor  54  receives such a positional output from the sensor  58 , the processor  54  imposes a differential signal on the signals to the motor drives  40 ,  42  to increase the speed of the motor drive  40 , driving friction wheel  34 , and to decrease the speed of the motor drive  42 , driving friction wheel  36 . The differential signal and resulting differential velocities of the friction wheels vary in proportion to the Y-direction error detected by the sensor  58 . As the motor drives  40 ,  42  rotate friction wheels  34 ,  36  at different speeds, the front portion of strip material  12  is skewed to the right, as indicated by the arrow, and the rear portion of the strip material is skewed to the left to cover a greater portion of the sensor  58 . As the skewed strip material  12  continues to move in a longitudinal or X-direction, more of the sensor  58  becomes covered.  
         [0031]    When half of the sensor  58  is covered, as shown in FIG. 6, the sensor  58  indicates that it is half-covered and the motor processor  54  reduces the differential signal to zero. At this instant, the strip material  12  is skewed as shown, but moves directly forward in the X-direction because the motor drives  40 ,  42  are driving the friction wheels at the same speed. In effect, the skewed position of the strip material causes the Y-position error at the sensor  58  to be integrated as the strip material moves forward in the X-direction. Once an area greater than one half of the sensor  58  is covered, the sensor  58  sends a signal to the processor  54  indicating that more than half of the sensor  58  is covered and the processor  54  imposes a differential signal on the signals to the motor drives  40 ,  42  to decrease the speed of the motor drive  40  and friction wheel  34  and increase the speed of the motor drive  42  and friction wheel  36 . The difference in rotational speeds of the friction wheels  34 ,  36  now turns and skews the strip material to the left, in the direction of the slower rotating friction wheel  34 , as indicated by the arrow, which begins to uncover sensor  58 . The differential rotational speed of the friction wheels  34 ,  36  continues until the strip material  12  covers only one half of the sensor  58  and the differential signal from the processor fades out. The processor  54  then applies equal drive signals to the motor drives  40 ,  42  and the friction wheels  34 ,  36  are driven at the same rotational speed.  
         [0032]    The strip material  12  again moves in the X-direction. If at this time the strip material is still skewed in the Y-direction, because the processor is under-damped or over-damped, the forward motion in the X-direction will again integrate the Y-position error and the sensor  58  will signal the processor to shift the strip, material back to a central position over the sensor  58  with corrective skewing motions as described above. The skewing motions will have the same or opposite direction depending upon the direction of the Y-position error.  
         [0033]    When the feed of the strip material  12  in the X-direction is reversed, control of the Y-position error is switched by the processor  54  from the sensor  58  to the sensor  56 , which now disposed behind the friction wheels  34 ,  36  with respect to the strip material  12  motion. The Y-position error is then detected at the sensor  56 , but is otherwise controlled in the same manner as described above.  
         [0034]    To avoid sudden jumps in either plotting, printing, or cutting operations, the increasing or decreasing speed commands are incremental. Small increments are preferred so that the error is corrected gradually.  
         [0035]    Referring to FIG. 7, the strip material  12  is loaded into the friction drive apparatus  10  and automatically aligned prior to starting an operation. The strip material  12  is placed into the friction drive apparatus  10  such that the first longitudinal edge  20  of the strip material  12  is in contact with the first and second stops  82 ,  84 . In that position, the strip material  12  is covering more than half of both the first and second sensors  56 ,  58 . The friction drive apparatus  10  is then turned on to perform an automatic alignment procedure  96  resident in memory, as shown in FIG. 8. First, the friction drive apparatus  10  saves the initial X-axis alignment position of the strip material  12 , as indicated by B 2 . Then, the friction drive apparatus  10  advances the strip material  12  a predetermined aligning distance, steering the strip material in accordance with the above steering procedure, as indicated by B 4  and shown in FIGS. 9 and 10.  
         [0036]    In the preferred embodiment of the present invention, the strip material  12  is displaced approximately twelve inches (12″). As the strip material  12  is advanced forward the predetermined aligning distance, the exact position of the first longitudinal edge  20  of the strip material  12  with respect to the second sensor  58  is continuously monitored. In the preferred embodiment of the present invention, the exact position of the first longitudinal edge  20  is checked approximately every two hundred fifty (250) micro-seconds with the processor  54  retrieving the information from the sensors approximately every millisecond. At the end of the movement of the strip material  12  the predetermined aligning distance, if the first longitudinal edge  20  of the strip material  12  has been centered with respect to the second sensor  58 , at least a minimum number of times during the periodic checks, the friction drive apparatus  10  is to assume that the strip material  12  is aligned with respect to the second sensor  58 , as indicated by B 6 , B 8 .  
         [0037]    If the first longitudinal edge  20  of the strip material  12  is not aligned when the strip material  12  is advanced the predetermined aligning distance, the strip material feed direction is reversed and the strip material  12  is returned to its original position, as indicated by B 10 . If the edge  20  is aligned, the friction drive apparatus  10  displaces the strip material  12  the predetermined aligning distance in a reverse direction to the initial X-axis position that was previously saved, as indicated by B 12 . During the reverse movement, the strip material  12  is shifted in accordance with the above steering scheme by the first sensor  56 . Thus, the friction drive apparatus  10  monitors and saves the exact position of the first longitudinal edge  20  of the strip material  12  with respect to the first sensor  56 , as indicated by B 14 . In the preferred embodiment of the present invention, processor  54  of the friction drive apparatus checks the exact position of the first longitudinal edge  20  of the strip material  12  every millisecond during the reverse advance of the strip material  12 . If the first longitudinal edge  20  of the strip material  12  has been centered with respect to the first sensor  56  for at least a minimum number of times, the friction drive apparatus  10  is to assume that the strip material  12  is aligned with respect to the first sensor  56 , as indicated by B 16 . If it was determined that the strip material is aligned with respect to the first sensor  56 , the procedure is completed, as indicated by B 18 .  
         [0038]    If the first longitudinal edge of the strip material  12  is not aligned with respect to the first sensor  56 , the result is that the strip material  12  is not aligned. If it was determined that the strip material  12  is not aligned, as indicated by B 20 , the automatic alignment procedure  96  is repeated. In the preferred embodiment of the present invention, the automatic alignment procedure  96  is repeated three (3) times before an error signal is displayed, as indicated by B 22 . Every time the automatic alignment procedure is performed, the internal counter is incremented by one (not shown). Typically, the friction drive apparatus  10  according to the present invention, does align the strip material  12  within the three (3) attempts.  
         [0039]    Although the automatic alignment procedure  96  ensures that the strip material  12  is substantially parallel to the feed path  24  and is centered with respect to the controlling sensor, the first time the automatic alignment procedure  96  is activated in the friction drive apparatus  10 , it does not ensure that the first and second sensors  56 ,  58  are calibrated with respect to each other and therefore does not ensure that when the direction of strip material feed is reversed the graphic lines coincide.  
         [0040]    Referring to FIG. 11, a sensor calibration procedure  98 , resident in memory, ensures that the first and second sensors  56 ,  58  are calibrated with respect to each other at the onset of the friction drive apparatus operation. Subsequent to the initial automatic alignment procedure  96 , the initial X-axis calibration position of the strip material  12  is saved, as indicated by C 2 . The strip material  12  is then advanced forward a predetermined calibration distance in the X-axis direction, as indicated by C 4 . In the preferred embodiment, the predetermined calibration distance is approximately sixteen inches (16″). As the strip material  12  is advanced forward, the friction drive apparatus  10  steers the strip material  12  to maintain proper alignment with respect to the second sensor  58  in accordance with the above lateral error correcting scheme. Once the strip material  12  has been advanced the predetermined calibration distance, the first and second sensors  56 ,  58  are read to establish a first sensor forward position and a second sensor forward position, as indicated by C 6 . Subsequently, a first difference is taken between the first sensor forward position and the second sensor forward position, as indicated by C 8 . Then, the strip material  12  is advanced the predetermined calibration distance in a reverse X-axis direction to the saved X-axis calibration position, as indicated by C 10 , with the lateral error correction scheme maintaining the strip material  12  aligned with respect to the first sensor  56 . Once the strip material  12  is returned to its original position, the first and second sensor positions are read again to establish a first sensor reverse position and a-second sensor reverse position, as indicated by C 12 . Then, a second difference is calculated between the first sensor reverse position and the second sensor reverse position, as indicated by C 14 . Subsequently, the second sensor  58  is adjusted by a sensor adjustment such that the center reference position of the second sensor  58  is decremented if the first difference and the second difference are both positive and incremented if the first difference and the second difference are both negative, as indicated by C 16 , C 18  and C 20 , C 22 , respectively.  
         [0041]    The new adjusted second sensor  58  position reflects an offset, if any, between the center pixel  94  of the first sensor  56  and the center pixel  94  of the second sensor  58  that was potentially introduced during assembly and installation of the sensors  56 ,  58 .  
         [0042]    In the preferred embodiment of the present invention, the sensor adjustment is an average of the first and second differences. Thus, the center reference position  94  of the second sensor  58  is moved from the central pixel either toward the outer edge  74  or the inner edge  78  by a certain number of pixels, as established by the sensor adjustment. However, although the preferred embodiment of the present invention defines the sensor adjustment to be an average of the first and second differences, the sensor adjustment can be defined to equal to the first difference.  
         [0043]    Subsequent to incrementing or decrementing the center position  94  of the second sensor  58  by the sensor adjustment, the sensor adjustment is compared to a maximum threshold adjustment, as indicated by C 24 . If the sensor adjustment exceeds the maximum threshold adjustment, then there is an error, as indicated by C 25 . If the sensor adjustment is smaller than the minimum threshold adjustment, then the counter is reset as indicated by C 26 , and the calibration procedure is repeated. The maximum threshold adjustment is provided to ensure that the sensor adjustment does not shift the center reference position of the sensor  58  too far from the center of the sensor  58 , thereby inhibiting steering ability of the sensor  58 .  
         [0044]    However, if the first difference and the second difference are substantially zero, then the counter is incremented, as indicated by C 28 , and checked if it exceeds five, as indicated by C 30 . If the counter exceeds five, then the calibration is completed, as indicated by C 32 . However, if the counter is less than five, the calibration procedure  98  is repeated until there is no substantial difference between the readings of sensors  56 ,  58  at least five times in a row.  
         [0045]    Once the second sensor adjustment is determined, the microprocessor applies the adjustment to the second sensor  58  in all subsequent operations.  
         [0046]    Referring to FIG. 12, in an alternate embodiment, sensors  56 ,  58  can be positioned along an edge  99  of a stripe  100  marked on the underside of the strip material  12 . The stripe  100  is spaced away in a lateral direction from either of the longitudinal edges  20 ,  22  of the strip material  12  and extends in the longitudinal direction. The Y-position error is detected by the sensors  56 ,  58  and corrected in the manner described above with the edge  99  of the stripe  100  functioning analogously to the longitudinal edge  20  of the strip material  12 . The automatic alignment procedure  96  and the calibration procedure  98  are performed analogously with the stops  182 ,  184  being spaced away from the outer edges  72 ,  74  of the sensors  56 ,  58 , respectively.  
         [0047]    Referring to FIG. 13, another alternate embodiment uses a pair of sensors  156 ,  158  disposed at predetermined positions in front of the friction wheels  34 ,  36 , as viewed in the direction of motion of the strip material  12 . A steering reference point  102  is defined at a predetermined distance behind the friction wheels, as viewed in the direction of motion of the strip material  12 . Based on the inputs from sensors  156 ,  158 , the processor  54  determines a lateral error at the steering reference point  102 . If it is determined that there is no error at the steering reference point  102 , the friction wheels are driven simultaneously. However, if it is determined that there is a skewing or lateral error at the steering reference point  102 , the processor  54  steers the motor drives and subsequently the friction wheels to straighten the strip material  12  in the manner described above.  
         [0048]    The present invention provides a method and apparatus for automatically aligning the strip material  12  in the friction drive apparatus  10 . This eliminates the need for an operator to manually align the strip material  12 . Typically, manual alignment results in excessive amounts of wasted strip material and does not always provide error free final graphic products. Therefore, the automatic alignment procedure of the present invention translates into savings of operator time, strip material savings and improved quality of the final graphic product. The calibration procedure of the present invention provides additional accuracy to the proper alignment of the strip material and improves quality of the final graphic product.  
         [0049]    The sensors  56 ,  58 ,  156 ,  158  used in the preferred embodiment of the present invention are digital sensors. One type of digital sensor that can be used is a linear sensor array model number TSL401, manufactured by Texas Instruments, Inc., having a place of business at Dallas, Tex. In another embodiment of the present invention, large area diffuse sensors can be used with A/D converters replacing the pulse shaper and serial to parallel connector. These sensors preferably have an output proportional to the illuminated area. This can be accomplished with the photoresistive sensors, such as Clairex type CL700 Series and simple No. 47 lamps. Alternatively, a silicon photo diode can be used with a diffuser-window about one half of an inch (½″) in diameter and a plastic lens to focus the window on the sensitive area of the diode, which is usually quite small compared to the window. Still other types of optical, magnetic, capacitive or mechanical sensors can be used. The light source  66 ,  68  is either a Light Emitting Device (LED) or a laser.  
         [0050]    While a variety of general purpose micro processors can be used to implement the present invention, the preferred embodiment of the present invention uses a microprocessor and a Digital Signal Processor (DSP). One type of the microprocessor that can be used is a microprocessor model number MC68360 and a digital signal processor model number DSP56303, both manufactured by Motorola, Inc., having a place of business in Austin, Tex.  
         [0051]    Although the preferred embodiment of the present invention depicts the apparatus  10  having the friction wheels  34 ,  36  disposed within the bottom portion  14  and the pinch rollers  30  disposed within the top portion  16 , the location of the friction wheels  34 ,  36  and pinch rollers  30  can be reversed. Similarly, the sensors  56 ,  58  can be disposed within the top portion  16  of the apparatus. Moreover, although the wheels  34 ,  36  are referred to as friction wheels throughout the specification, it will be understood by those skilled in the pertinent art that the wheels  34 ,  36  can be either friction, embossed, grit, grid or any other type of wheel that engages the strip material. Furthermore, although FIG. 7 depicts the strip material  12  being loaded up against stops  82 ,  84 , the strip material can be placed at any location over the sensors  56 ,  58  and the strip material will be aligned.  
         [0052]    Although FIGS.  3 - 6  show one friction wheel associated with each longitudinal edge of the strip material, a lesser or greater number of friction wheels driving the strip material can be used. Referring to FIG. 14, for wide strip material  212  used with larger printers, plotters and/or cutters, in the preferred mode of the present invention, a third friction wheel  104  is used to drive the middle portion of the strip material  212 . The third friction wheel  104  is coupled to the first friction wheel  34 . The force of the pinch roller  30 , shown in FIG. 1, corresponding to the third friction wheel  104 , is lower to avoid interference with the lateral steering of the strip material  212 . However, the third friction wheel  104  is activated to reduce longitudinal positional error of the strip material  212 .  
         [0053]    While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art, that various modifications to this invention may be made without departing from the spirit and scope of the present invention. For example, predetermined calibration and aligning distances can vary. Also, although the preferred embodiment of the present invention provides stops  82 ,  84  for ensuring that the strip material is positioned over the sensors  56 ,  58  when the strip material  12  is placed into the friction drive apparatus  10 , the stops  82 ,  84  are not necessary as long as the longitudinal edge  20  of the strip material  12  or the edge  99  of the stripe  100  of the strip material  12  is positioned over the controlling sensor. Additionally, the aligning function can be performed when the Y-axis position of the longitudinal edge of the strip material is taken either continuously or intermittently and the steering of the strip material does not need to be performed simultaneously with the Y-axis position measurement. Similarly, the aligning method can be performed regardless whether the strip material is moved continuously or intermittently in the course of a work operation.

Technology Category: b