Patent Publication Number: US-6986615-B2

Title: Backlash reduction

Description:
BACKGROUND OF INVENTION 
     Paper advance error (“stitch” error) in a TIJ printer can result from a combination of drive train backlash together with coasting of the driven transport as the drives decelerate. This IP proposes a two-part paper advance profile to remedy this problem. The paper is advanced short of the final intended position. The final paper advance is made as a series of discrete steps which take up any backlash that may have occurred during the first advance. 
     Most low-cost TIJ printers advance paper incrementally through the print zone, such as by using a stepper motor or a position-controlled servo motor driving a shaft via a geartrain. Because the geartrain is designed for low cost manufacture, there is inevitably accumulated backlash in the drive train. This backlash can be a source of error in precision paper advances between carriage scans. Typical precision requirements are single standard of deviation errors of 20 μm for a paper advance of about 10 mm. Any backlash in the system can contribute an error if the motor deceleration occurs more rapidly than the driven roll deceleration. This is possible because the driven rolls are large diameter with significant inertia and low frictional drag. In this event, the load (driven roll) will “coast” through the geartrain backlash and stop at some indeterminate position. 
     SUMMARY OF INVENTION 
     Embodiments substantially reduce this error source by advancing the substrate short of the next printing position by a predetermined amount, such as N motor steps or encoder units. Embodiments then more slowly advances the substrate in increments to the next printing position, such as by advancing a stepper motor by N additional steps or by advancing a position-controlled servo motor by N encoder units. The value for N can be predetermined to be greater than the total possible backlash error in the drive train. As a result, the driven roll can be in some indeterminate position within the range of backlash error. If the driven roll has not coasted ahead at all, then it will be advanced by N increments and will be parked in the correct position for the next carriage pass. If the driven roll has coasted ahead, by, for example, M increments, where M&lt;N, due to system backlash and load inertia, then the motor will advance M increments until all the backlash has been cleared from the drive train and then both motor and driven roll will advance N−M increments in unison to arrive at the desired park position. In either case, the driven roll ends up at the desired final position without any backlash error contribution. 
     Embodiments assume that the load will not overshoot the drive train when the drive train makes a series of very short, low velocity increments. In other words, once the backlash has been cleared during the N discrete increments, the load stays in synchronism with the drive train; that is, overshoot errors are negligible because the load is not given sufficient kinetic energy. In embodiments using stepper motors, motor steps can be either accomplished, for example, as full steps, half-steps, and microsteps, depending on the sophistication of the stepper driver circuit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustrating a printer with a substrate transport system in which embodiments of the invention can be employed; 
         FIG. 2  is a schematic illustrating the components of embodiments of the invention; 
         FIG. 3  is a schematic illustrating the components of embodiments of the invention in a more abstract fashion; and 
         FIG. 4  is a schematic illustrating the method implemented by embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     While embodiments are described in terms of printers and ink jet printers, it should be readily apparent that embodiments can be applied to other types of machines in which backlash take-up can introduce error into positioning. Thus, the description of the embodiments that follows is exemplary in nature and is not intended to narrow the scope of the claims. 
     With reference to the accompanying FIGS., a printer  1  arranged to print on a substrate  2 , such as paper, includes a substrate transport system  10  including a drive motor  11  and a driven roll  12 . Interposed between drive motor  11  and driven roll  12  in embodiments is a gear train  13  or the like that transfers drive from the motor  11  to the roll. As a result of gaps between teeth in the gear train  13 , among other things, backlash arises, which can cause errors in substrate placement. The drive motor  11 , in embodiments, is a stepper motor driven by a controller  20  that includes a stepper motor drive circuit  21 . It should be recognized that a position-controlled servo motor that can be advanced by encoder units could be substituted for the stepper motor; for ease of description, however, a stepper motor will be discussed. 
     To take up backlash in the transport system  10 , embodiments advance the substrate to a point  31  short of an intended final destination  32 . For example, embodiments can advance the substrate  2  N motor steps short  31  of a next printing position  32 . The distance between the stopping point  31  and the intended final destination  32  can be greater than a total possible backlash error in the drive train  13  between the drive motor  11  and the driven roll  12 . Thus, in the example above, the value for N would be greater than the total possible steps the motor  11  would have to make to take up the backlash error in the drive train  13 . 
     At the stopping point  31 , the driven roll  12  can be in some indeterminate position within the range of backlash error. The motor  11  then slowly advances the substrate  2  to the intended final destination  32 , taking up the backlash in the process. In the example above, the stepper motor  11  makes N additional steps forward to the next printing position  32 . If the driven roll  12  has not coasted ahead at all, then it will be advanced by N steps and will be parked in the correct position for the next carriage pass. If the driven roll  12  has coasted ahead, for example, M steps, (where M&lt;N) due to system backlash and load inertia, then the motor  11  will advance M steps until all the backlash has been cleared from the drive train  13 . Both motor  11  and driven roll  12  will then advance N−M steps in unison to arrive at the intended final destination  32 , the desired park position for the next print. In either case, the substrate  2  and the driven roll  12  ends up at the desired final position  32  without any backlash error contribution. 
     Embodiments rely on the proposition that the load, i.e., the substrate  2  and driven roll  12 , will not overshoot the drive train  13  when the drive train  13  makes a series of very short, low velocity steps. Once the backlash has been cleared during the N discrete steps, the load stays in synchronism with the drive train  13 ; thus, overshoot errors are negligible because the load never is given sufficient kinetic energy. Where stepper motors are used, motor steps can be either accomplished as full steps, half-steps, or microsteps, depending on the sophistication of the stepper driver circuit  21 . 
     In a more abstract explanation, as represented, for example, by schematic  FIG. 3 , embodiments include a backlash reduction apparatus comprising a drive motor  11 , a drive train  13  driven by the motor  11 , and at least one substrate transport mechanism  12  connected to the drive train  13  and driven by the motor  11  through the drive train  13 . In embodiments, the drive motor  11  is a stepper motor, the drive train  13  is a gear train, and the substrate transport mechanism  12  is at least a driven roller. The apparatus is controlled by a controller  20  comprising a substrate advancer  22  in communication with the stepper motor  11 , the substrate advancer  22  emitting control signals to the stepper motor  11  that cause the substrate  2  to move to a point  31  short of an intended destination  32 . Embodiments can also include a substrate position sensor  24  to which the substrate advancer  22  can respond, though such position sensors are not necessarily needed. 
     The controller  20  also includes a substrate final advancer  23  in communication with the stepper motor  11 . Embodiments can include one or more substrate position sensors  24  connected to the controller, but such position sensors are not necessarily required. The substrate final advancer  23  sends control signals to the stepper motor  11  that cause the substrate  2  to continue to the intended destination  32 . In embodiments, the signals from the substrate advancer  23  cause the stepper motor  11  to stop the substrate  2  a predetermined number of steps, such as N steps, where N is a whole number, from the intended destination  32 , the predetermined number of steps being greater than a total possible backlash error in the drive train  13 . The substrate final advancer signals then cause the stepper motor  11  to advance by the predetermined number of steps, taking up remaining backlash and moving the substrate  2  to the intended final destination  32  of the substrate (the printing position). The substrate advancer  22  and the final advancer  23  can be responsive to substrate position sensors  24  connected to the controller  20 , though such position sensors  24  are not necessarily needed, and can be part of or in communication with a drive motor control circuit  21 . 
     In the abstract or in the concrete, the backlash reduction apparatus will execute a method comprising advancing a substrate to a point short of a final intended position (block  102 ) and finally advancing the substrate to the final intended position (block  103 ), thereby taking up backlash in a substrate transport system, as seen, for example, in  FIG. 3 . The method can also include monitoring substrate position (block  101 ) and sending substrate position information to a controller that initiates the advancing and final advancing of the substrate, and finally advancing can include advancing the substrate at a lower speed than the speed at which the substrate was advanced to the point short of the final intended destination. Finally advancing can be done by advancing the substrate incrementally from the point short of the final intended destination to the final intended destination, as by providing a stepper motor  11 , providing a substrate transport driven by the stepper motor  11 . 
     While the invention has been described with reference to the structures and embodiments disclosed herein, it is not confined to the details set forth, and encompasses such modifications or changes as may come within the purpose of the invention.