Interleave pulse modulation for thermal printers

Details Interleave pulse modulation for thermal printers Interleave pulse modulation for thermal printers

2000-01-04

2002-04-23

Barlow, John (Department: 2853)

Incremental printing of symbolic information

Ink jet

Controller

C347S010000, C347S011000, C347S012000, C347S014000, C347S015000

Reexamination Certificate

active

06375300

ABSTRACT:

TECHNICAL FIELD
The present invention relates in general to a thermal dot printer, and in particular to a thermal dot printer and a method for controlling a thermal dot printer using interleave pulse modulation for heating up the printing elements of the thermal printhead of the thermal dot printer.
BACKGROUND INFORMATION
Various kinds of dot printers are known in the art. So-called thermal dot printers employ thermal energy to form images or characters on a media. Generally speaking, such thermal dot printers operate by either applying thermal energy to the media or to a heat sensitive coating on the surface of the media to alter the characteristics of the media or the characteristics of the heat sensitive coating, or by thermally energizing a thermally sensitive hot melt wax ink ribbon to transfer ink from the ribbon to the media.
Typically, a thermal dot printer contains a thermal printhead for printing characters or images on a media, a drive system for moving the paper across the printhead, a print logic for outputting character print signals to the printhead, and optionally, a drive system for moving the printhead across the paper.
The thermal printhead usually includes a plurality of print positions arranged in either vertical or horizontal lines. Each print position includes a printing element connected to wires. When electrical power is applied to the wires, the printing element increases in temperature. At a certain temperature, the printing element causes a visible dot to appear on the media being printed. A group of closely spaced dots represents a character or symbol.
When using a conventional thermal matrix printer in which the printing elements of the thermal matrix printhead are arranged in the form of a matrix, the printing elements are selectively heated to form a character as the thermal matrix printhead travels in the printing direction at a predetermined pitch across the paper or medium path. As such, one character is formed by the group of dots each time the thermal matrix printhead is traveled by a predetermined number of dots. Once an entire row of characters is printed, the paper advances so that another row of information can be printed as the process is repeated.
While conventional thermal matrix printers are still used in some applications, modern printers are more likely to employ a thermal in-line printhead. A thermal in-line printhead is a stationary printhead that uses a series of dot printing elements configured in a horizontal line across the width of the paper. As such, the printhead remains stationary with respect to the paper. The number of printing elements is a function of the print quality and the width of the paper. As opposed to a thermal matrix printer, which prints a single character then moves a predetermined amount before printing another character, an in-line printhead selectively prints a horizontal row of dots across the paper at once. The drive system, comprising a stepper motor and a system of gears and rollers, continuously moves the print media at a predetermined rate along a paper path allowing the sequential printing of multiple rows of dots. Thus, all the characters in the row are formed as multiple rows of dots are printed across the media.
A typical in-line printhead may have several hundred print elements. Typically these elements are divided into groups of elements. For example, a printhead configuration of 640 elements might be divided into five groups of 128 elements. Each of the five groups could be activated separately with separate control lines. Separate control lines are desirable because the power required to activate all 640 elements exceeds the capacity of power supplies typically used in in-line printers. With typical power supplies, three groups of elements can be activated at a time. Thus, in a printhead configuration of 640 elements, it is possible to combine the control lines into two “banks” of groups. The first bank contains three groups of elements (or 384 elements comprising the left column of the paper) and the second bank contains two groups of elements (or 296 elements comprising the right column of the paper). Both banks of elements are used to print one row of 640 dots.
For printing a first row of dots, therefore, a first set of printing elements is activated in the first bank of elements. After printing the first set of dots, the first set of printing elements is deactivated, and a second set of printing elements from the second bank of elements has to be activated for printing the rest of the row of dots. After printing the second set of dots, the second set of printing elements is deactivated. This process is repeated to print another row of dots until the entire row of characters is printed on the media. During the printing process, the print media is continually moving across a media or paper path. The rate that the dots are printed out corresponds to the distance that it is moved while printing. Once the row of characters is printed, the paper is advanced so that another row of characters or other information can be printed as the process is repeated.
The size and shape of the dots is a function of the shape of the printing element, the temperature of the printing element and the length of time the printing element is applied to the media or to the ribbon.
For printing a dot on the media, the respective printing element has to be supplied with electrical power for a sufficient length of time to heat up the printing element up to a predetermined printing temperature and to keep this printing temperature for a predetermined length of time. More precisely, for printing a dot, the printing element must rise to the predetermined printing temperature which is sufficient to alter the characteristics of the medium and stay at this temperature for a predetermined period of time to complete the printing of the dot. Consequently, the temperature of the printing element and the amount of heat applied by this printing element is dependent upon the level of drive current supplied to the printing element and the length of time the drive current is being supplied to the printing element.
A well known method for heating up the printing elements is “normal pulsing”, i.e. a printing element is supplied with a single long drive current pulse of predetermined pulse width and pulse amplitude. The graph of such a pulse is illustrated in FIG.
3
. The y or vertical axis represents the pulse amplitude and the x or horizontal axis represents units of time. In a thermal in-line printer, normal pulsing is performed by supplying a long drive current pulse to a first set of printing elements in the first bank of dots, and then supplying a long drive current pulse to a second set of printing elements in a second bank of dots. Consequently, not every element will be activated in a bank of elements. A “set” of elements refers to those elements within a particular bank of elements that will be activated or used in the process to print a row of dots. The set of elements will vary within the banks depending on the characters in the row to be printed.
FIG. 3
shows an example for “normal pulsing” of the current pulses to two successive sets of printing elements in a printhead. The pulse curve for the first set in the first bank of elements is
304
. The pulse curve for the second set in the second bank of elements is
306
. As an example of normal pulsing, a printing element of the first set of printing elements is turned on for 10 units of time (first current pulse
301
). After completion of pulse
301
, i.e. when the first set of dots is printed, a second current pulse
302
is supplied to a second set of printing elements. Since the row of dots is comprised of both sets of dots, the entire row is printed once both sets of dots have been printed.
The process is repeated for the next row of dots until the entire row of characters is printed on the media. During this time, the paper or media is constantly moving along the paper path. The sets of dots are printed separately while the media is moving. Thus, there is a slight

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