Abstract:
The friction drive system for printing, plotting or cutting graphic images on strip material includes a feedback for a drive motor driving a plurality of friction wheels for advancing strip material in a longitudinal direction. The feedback signal includes a short-term response component and a long-term response component to accurately pinpoint the exact longitudinal location of the strip material. The short-term response component is generated by comparing a motor encoder signal from a motor encoder secured to the drive motor with a commanded longitudinal position of the strip material and passing the resultant differential error signal through an all pass filter. The long-term response component is generated by comparing a detecting encoder signal from a detecting encoder secured to a device detecting the actual longitudinal position of the strip material with the commanded longitudinal position of the strip material and passing the differential error signal through a low pass filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to friction drive systems such as printers, plotters and cutters that feed strip material therethrough for generating graphic images and, more particularly, to friction drive systems which accurately track the longitudinal position of the strip material. 
     2. Background Art 
     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. Spring pressure urges the pinch rollers and material against the friction wheels. During plotting, printing, or cutting, the strip material is driven by the friction wheels back and forth in the longitudinal or X-coordinate direction in accordance with a commanded position for the strip material. As the strip material is advanced back and forth in the longitudinal direction, a pen, printing head, or cutting blade is driven over the strip material in the lateral or Y-direction. 
     These systems have gained substantial favor due to their ability to accept plain (unperforated) strips of material in differing widths. However, the existing friction feed systems experience several problems. One problem is that the existing systems do not compare the commanded position of the strip material and the actual position of the strip material. Thus, if a longitudinal slippage or creep error in the X-coordinate direction occurs with the strip material moving either too slowly or too fast, respectively, the system is not aware of the discrepancy between the commanded position and the actual position of the strip material. This potential discrepancy is not detected until the plot is completed and results in inaccurate final work product. This problem is most pronounced in long plots, i.e. those two or more feet in length, and those in which the strip material moves back and forth in the X-coordinate direction with respect to a tool head such as a plotting pen, print head, or cutting blade. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to ensure that the actual longitudinal position of the strip material is substantially identical to the commanded longitudinal position of the strip material in a friction drive system. 
     According to the present invention, a friction drive apparatus for feeding strip material in a longitudinal direction along a feed path includes a motor encoder secured to a drive motor that rotates friction wheels for advancing the strip material longitudinally and a detecting means for detecting the longitudinal position of the strip material. The motor encoder generates a motor encoder signal, indicative of the rotational movement of the drive motor and friction wheels. The detecting means generates a detecting encoder signal indicative of the actual longitudinal position of the strip material. The motor encoder signal is compared with the commanded position signal and the difference is filtered and defined as a filtered motor encoder position error signal or a short-term error signal component. The detecting encoder signal is also compared to the commanded position of the strip material with the difference filtered to remove high frequencies to result in a filtered detecting encoder position error signal or a long-term error signal component. The short-term error signal component and the long-term error signal component are then combined to result in a position error signal that is used as a feed back for the closed loop control system. 
     In the preferred embodiment of the present invention, the strip material includes an encoder pattern printed on the strip material and the detecting means includes an illuminator and a sensor to track the encoder pattern of the strip material to provide the microprocessor with the detecting encoder signal. 
     One advantage of the present invention is that the position error signal has improved accuracy over both the low frequency and the high frequency ranges because the short term accuracy of the friction wheels and the long term accuracy of the longitudinal feed provide highly reliable signals under all feed conditions. 
     Another advantage of the present invention is that the actual longitudinal position of the strip material is compared with the commanded position of the strip material. 
     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 
     FIG. 1 is an exploded, side elevational view schematically showing a friction drive apparatus; 
     FIG. 2 is a top plan view of a base assembly of the friction drive apparatus of FIG. 1 with the strip material shown in phantom and schematically illustrating the closed loop control system with a position error signal being fed back to a drive motor; 
     FIG. 3 is an enlarged, schematic side view of the strip material of FIG. 2 with a detecting means tracking an encoder pattern printed on the strip material; 
     FIG. 4 is a graph showing the response curves of a low pass and an all pass filters for the friction drive apparatus of FIG. 2; 
     FIG. 5 is a graph showing the response curves of a low pass and a high pass filters for the friction drive apparatus of FIG. 2; 
     FIG. 6 is an enlarged, schematic side view of the strip material of FIG. 2 with the detecting means tracking an encoder track printed on the strip material, according to another embodiment of the present invention; 
     FIG. 7 is an enlarged, schematic plan view of the strip material of FIG. 2 with the encoder pattern printed thereon, according to another embodiment of the present invention; and 
     FIG. 8 is a top plan view of a base assembly of the friction drive apparatus of FIG. 1 with the strip material shown in phantom and of the control system, according to a further embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an apparatus  10  for plotting, printing, or cutting strip material  12  includes a cover assembly  14  and a base assembly  16 . The strip material  12  includes an encoder pattern  18  and a pair of longitudinal edges  20 ,  22 , as best seen in FIG.  2 . The strip material is moving in a longitudinal or X-coordinate 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-coordinate direction, substantially perpendicular to the longitudinal or X-coordinate direction and the feed path  24 . The cover assembly  14  also includes a plurality of pinch rollers  30  that are disposed along the longitudinal edges  20 ,  22  of the strip material  12 . The base assembly  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 corresponding plurality of pinch rollers  30 . 
     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 . The motor drive  40  may be a servo-motor with a drive shaft being connected to a motor encoder  44  for detecting rotational movement thereof. A motor encoder signal x m  from the motor encoder  44  is communicated to a microprocessor  50 . 
     The apparatus  10  also includes a detecting means  54  for tracking an actual longitudinal position of the strip material  12 . The detecting means  54 , in the preferred embodiment of the present invention, includes a first illuminator  56  which can be a laser diode  60  with a lens  62  for emitting and focusing a light beam onto the encoder pattern  18  and a first optical sensor  64 , such as a photo diode  66 , for sensing the encoder pattern  18 , as shown in FIG.  3 . The detecting means  54  in the preferred embodiment also includes a second illuminator  70  and a second optical sensor  72  spaced approximately ninety degrees (90°) out of phase with the first illuminator  56  and first optical sensor  64 . A detecting encoder signal x d  from the optical sensors  64 ,  72  of the detecting means  54  is communicated to the microprocessor  50 , as shown in FIG.  2 . 
     In operation, the drive motor  40  rotates the friction wheels  34 ,  36  which together with the pinch rollers  30  engage the strip material  12  to advance it back and forth along the feed path  24  in the longitudinal or X-coordinate direction, as shown in FIG.  1 . As the strip material  12  moves in the longitudinal or X-coordinate 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 tool employed. As the motor drive  40  rotates the friction wheels  34 ,  36 , the motor encoder  44  tracks the rotational movement of the drive motor  40  and sends the motor encoder signal x m  to the microprocessor  50 , as best seen in FIG.  2 . 
     As the strip material is fed along the feed path  24 , the detecting means  54  reads the encoder pattern  18  on the strip material  12  to track the actual longitudinal position of the strip material  12  in the X-coordinate direction. The optical sensors  64 ,  72  read the encoder pattern  18  to result in a logic-readable encoder information, such as, for example, a quad b encoder signals. These signals are then communicated to the microprocessor  50 . The microprocessor  50  receives the two position signals x m , x d , one from the motor encoder  44  and one from the detecting means  54 , conveying data regarding the motor position and the actual longitudinal position of the strip material  12 , respectively. The microprocessor  50  then compares each position signal x m , x d  with the commanded longitudinal position input x c  from input  74 . The comparison between the motor encoder signal x m  and the commanded position x c  yields a potential discrepancy between the two signals expressed as a first error signal ε m . Comparison between the detecting encoder signal x d  and the commanded position x c  yields a second error signal ε d . The error signals ε d  and ε m  are then filtered through low and all pass filters  76 ,  78 , respectively, which can be internal to the microprocessor  50 . The low pass filter  76  removes high frequencies from the detecting encoder error signal ε d  and allows low frequencies to pass through. The filtered signals ε fm  and ε fd  are combined, as best seen in FIG. 4, and further processed, if necessary, by means of an amplifier  82  to define a single actual longitudinal position error signal ε p  that is fed back to drive motor  40  to complete a closed loop feedback system. The position error signal ε p  is added to correct the longitudinal position gradually without ruining the final product. 
     Alternatively, the all pass filter  78  can be eliminated, thereby combining the filtered detecting encoder position error signal ε fd  with the motor encoder position error signal ε m  to result in the longitudinal position error signal ε p . Additionally, the all pass filter can be replaced with a high pass filter to remove low frequencies from the motor encoder error signal ε m  and allow high frequencies to pass through as the filtered motor encoder position error signal ε fm , as shown in FIG.  5 . 
     The longitudinal position error signal ε p  fed to the motor is accurate over both the low and high frequencies, and therefore provides motor feedback response accurate over the long-term and short-term strip material positions. The present invention maximizes the accuracy of each error signal ε fm  and ε fd  to achieve greater accuracy in determining the actual longitudinal position of the strip material. The motor encoder signal x m  is much more accurate for instantaneous displacements of the strip material  12  driven by the drive motor  40 . However, over the long-term, the accuracy of the motor encoder signal x m  decreases because in the long-term, the strip material may slip relative to the friction wheels  34 ,  36  driven by the drive motor  40 , thereby resulting in a discrepancy between the motor encoder reading and the actual position of the strip material. Therefore, the error ε m  resulting from the difference between the motor encoder position signal x m  and commanded position signal x c  is used to provide short-term displacement of the strip material. 
     Additionally, the detecting encoder signal x d  provides greater accuracy over the long-term as the detection means  54  tracks the movement of the strip material  12 . Once the two filtered signals are combined, as shown in FIGS. 2,  4  and  5 , the resulting position error ε p  accurately tracks both the short-term transient movement of the strip material and the long-term large scale movements thereof and has greater accuracy over both, high and low frequencies. 
     Referring to FIG. 6, in one alternate embodiment of the present invention, only one illuminator  56  is used with a plurality of reflectors  86  to produce a second beam image on the encoder track  18 . Referring to FIG. 7, in another embodiment of the present invention, a second encoder pattern  88  is printed on the strip material  12  with a ninety degree (90°) spacing or one quarter (¼) line spatial spacing with respect to the first encoder pattern  18 . 
     Referring to FIG. 8, in a further embodiment of the present invention, the detecting means  54  is a free running sprocket wheel  92  to accommodate perforated strip material. The sprocket wheel  92 , including a plurality of pins  94  to engage punched holes  96  formed in the strip material  12 , is placed under the strip material so that the strip material  12  rotates the wheel as the strip material moves through the apparatus. There is no drive connected to the sprocket wheel  92 , and the wheel inertia is kept very low so that the material  12  is able to rotate the wheel  92  without impeding motion due to acceleration or friction. A detecting encoder  98  tracks the rotational position of the sprocket wheel  92  and sends the detecting encoder signal x d  to the microprocessor  50 . 
     Additionally, the present invention can be implemented in a printing, plotting or cutting apparatus  110  having multiple friction wheels  34 ,  36 ,  134  being driven by multiple drive motors  40 ,  140 , as shown in FIG.  8 . In this alternate embodiment, each motor  40 ,  140  has a servo-loop including motor encoders ( 44 ,  144 ) and filters ( 76 ,  78 ,  176 ,  178 ) configured and operating analogously to the feedback system described above and shown in FIG. 2 except that differential command signals can be added to the longitudinal position signal x c  for steering the strip material. 
     Use of other detecting means, such as optically readable encoders or, magnetic encoders cooperating with printed or magnetic tracks on the material, or free running pin or star wheels, is also possible. 
     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, the all pass, high pass and low pass filters are shown incorporated into the microprocessor. However, the all pass, high pass and low pass filters can be separate from the microprocessor. Also, the encoder pattern  18  can be printed on either side of the strip material or in the central portion thereof.