Incremental printing of symbolic information – Ink jet – Medium and processing means
Reexamination Certificate
1998-09-29
2001-01-30
Hilten, John S. (Department: 2854)
Incremental printing of symbolic information
Ink jet
Medium and processing means
C400S582000, C400S583400
Reexamination Certificate
active
06179419
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to media handling systems for inkjet printing devices, and more particularly to a media handling system which carries a media sheet along an endless loop belt.
Inkjet printing devices eject ink drops onto a media sheet from a plurality of nozzles. The ink drops form symbols, characters or graphics as desired. Placement of the drops is important for achieving desired print quality and resolution. To achieve accurate dot placement, the timing of each nozzle ejection is precisely controlled relative to the position of the media sheet. The media sheet is carried along a media path and passed through a print zone where ink drops are ejected onto the media sheet. To achieve accurate dot placement the motion of the media sheet is to be accurately controlled.
In conventional media handling systems, the media sheet is either stepped or continuously advanced. In a stepping embodiment, one or more rows of dots are printed at a time onto the media sheet while the media sheet is stationary. This occurs, for example, as an inkjet printhead is scanned across the media sheet. Alternatively, this occurs while a page wide array of inkjet nozzles ejects ink drops. Upon completion of printing to the one or more rows, the media sheet is advanced a known media advance distance. Another one or more rows of dots then are printed. For embodiments in which the media sheet is continuously advanced, the media sheet is moving while the inkjet printhead is scanning across the media sheet. For either approach, dot placement accuracy is affected by media advance accuracy.
This invention is directed toward a method and apparatus for improving media advance accuracy in a belt driven media handling subsystem for an inkjet printing apparatus.
SUMMARY OF THE INVENTION
According to the invention, a media handling system having an endless loop belt, which carries a media sheet through a print zone, achieves improved media advance accuracy by including closed loop feedback control. According to various embodiments, the position of either a drive shaft which rotates the endless belt or the position of the endless belt itself is monitored to provide feedback to a drive motor. The drive motor is linked to the drive shaft through a gear train. The endless belt is rotated by the drive shaft, either directly, or through rollers mounted to the drive shaft.
According to one aspect of the invention, feedback of the drive shaft is achieved by encoding the drive shaft or including a code wheel which turns with the drive shaft. The encoding is sensed and fed back to a controller as an indication of the angular position of the drive shaft. The controller uses the angular position to factor out motor errors or gear train errors. In a servo control loop the angular position serves as a servo error correction of the motor and gear train errors. By accurately controlling the drive shaft which rotates the belt, the belt position is accurately controlled.
To derive accurate media advance distance from the control of the drive shaft, the belt characteristics are measured, (e.g., average belt thickness). Profiles of expected wear of the belt over time also can be programmed into a calculation of media advance distance. Further, a profile of changes in belt thickness with changes in temperature also can be programmed into the calculation. Additional factors such as drive shaft runout or other shape characteristics also may be measured and used to achieve accurate media advance distance.
According to a preferred embodiment of the invention, the endless belt length is an integer multiple of the drive shaft diameter. Characteristics of the belt then may also be factored into the calculation to derive an accurate media advance distance. Exemplary belt factors include average belt thickness, thickness as a function of belt length, and how such factors change with age, wear or temperature.
According to an alternative embodiment where the belt length is not an integer multiple of the drive shaft circumference, the belt includes an indexed position which is related to an indexed or home position of the drive shaft. The index position of the belt is marked with a notch, an opening, a magnetic strip, a conductive strip, a bump or another feature which serves to distinguish the index position on the belt from other positions along the length of the belt.
According to an alternative aspect of the invention, rather than sense the drive shaft position, the belt position itself is sensed. The belt position serves as a feedback parameter for correcting motor error, gear train error, drive shaft error, and any other error occurring in the drive structures between the motor and the belt.
According to an aspect of the invention, the belt position is sensed by including a reflective encoder strip along the backside of the belt. An optical sensor then reads the encoder strip to monitor belt position. Alternatively, a sensor including CCD elements may be used, or a pick-up wheel and rotary encoder may be used. The belt position is detected from either the same side of the belt which carries the media sheet or the opposite side of the belt.
One advantage of the feedback control is that media advance accuracy is improved. A beneficial effect is that dot placement accuracy is improved and print quality is increased. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.
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Elgee Steven B.
Kelley Richard A
Rasmussen Steve O
Smith Brooke E
Chau Minh H.
Hewlett-Packard
Hilten John S.
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