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Publication Date: 2005-Jan-13

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Martin A.K. Schwan: AUTHOR


A method and apparatus for reducing light center printing by applying static and or dynamic spatial compensation that produces a strobe period extension based upon the number of actuated print elements and their distribution along the print head which ensures that a minimum energy level is transferred to each actuated print element. A plurality of data masks may be applied to divide the actuated print elements into subgroups for smaller strobe periods which together provide the minimum energy level to each print element but avoids overheating individual print elements which do not require the longest calculated strobe period. The method may be combined with existing history compensation logic and or the calculations performed within the printer processor or external to the printer.

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Method and Apparatus for Non-Uniform Print Density

Compensation of Commonly Energized Print Elements


1.  Field of the Invention

The invention relates to print density compensation for printing systems.  More particularly, the invention relates to a method and apparatus for compensation of lightened center printing resulting from power losses related to simultaneous actuation of a plurality of interconnected print elements.

2.  Description of Related Art

Printers, for example thermal or thermal transfer printers, use a print head comprised of a line of print elements coupled to a common power supply and or ground bus.  Selected individual print elements along the line are actuated by a control processor to print the desired indicia, one line at a time.

Each of the selected print elements comprising the current line may be activated for a common strobe period to activate thermal properties of the media or of a transfer print ribbon located between the print head and the media.  The strobe period is selected to be long enough to create a desired indicia but not so long that the individual print elements overheat, causing further transfer to the media as it is indexed, resulting in streaking.  History logic schemes used to minimize streaking utilize accumulators within the printer control processor to track recent activation of a print element, thereby limiting the number of strobe periods a print element receives if it has been recently activated.  Another way to limit streaking effects from over-heated print elements is to limit the media throughput rate, allowing the print head elements more time to cool between sequential activations but reducing overall printing speed.

A phenomenon known as light center printing occurs when the desired indicia calls for a large percentage of the print elements of a print head to be actuated simultaneously.  Light center printing produces a lightened gradient that is maximized at print elements located the farthest in-line from the bus connections at either side of the print head, i.e. the center of the print head.

As shown in Figure 1, each successive actuated print element P1-Pn away from the print head energizing interconnections with the printer, here shown as main bus connection(s) 4, shares a common feed and return bus that may be modeled as a parallel electrical network increasing in number of components with each additional actuated print element P.  Transistor switches or the like, for enabling or disabling each print element P according to the desired print line, also add a power loss to each print element P.  For circuit demonstration purposes, the switches are omitted.  The electrical network forms an impedance ladder with a variable impedance operating as a shunt with increasing power loss for each additional actuated print element P resulting from rising current levels and the cumulative feed and return trace resistances R inline along the common feed and r...