Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1998-09-28
2001-10-16
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S037000
Reexamination Certificate
active
06302506
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to such a machine and method that constructs text or images from individual marks created-on the printing medium, in a two-dimensional pixel array, by a pen or other marking element or head that scans across the medium. The invention is particularly related to printers that operate by the thermal-inkjet process—which discharges individual ink droplets as a printhead slews across a print medium and a method for compensating for non uniform Printhead motion. As will be seen, however, certain features of the invention are applicable to other scanning-head printing processes as well.
BACKGROUND OF THE INVENTION
The operation of any scanning-head device in slewing the printhead across the medium to discharge ink droplets does present some obstacles to precise positioning of the printed marks, and also to best image quality. In order to describe these obstacles it will be helpful first to set forth some of the context in which these systems operate.
In many printing devices, position information is derived by automatic reading of graduations along a scale or so-called “encoder strip” (or sometimes “codestrip”) that is extended across the medium. The graduations typically are in the form of opaque lines marked on a transparent plastic or glass strip, or in the form of solid opaque bars separated by apertures formed through a metal strip.
Such graduations typically are sensed electrooptically to generate an electrical waveform that may be characterized as a square wave, or more rigorously a trapezoidal wave. Electronic circuitry responds to each pulse in the wavetrain, signaling the pen-drive (or other marking-head-drive) mechanism at each pixel location—that is, each point where ink can be discharged to form a properly located picture element as part of the desired image.
These data are compared, or combined, with information about the desired image—triggering the pen or other marking head to produce a mark on the printing medium at each pixel location where a mark is desired. As will be understood, these operations are readily carried out for each of several different ink colors, for printing machines that are capable of printing in different colors.
In addition to this use of the encoder-derived signal as an absolute physical reference for firing the pens, the frequency of the wavetrain is ordinarily used to control the velocity of the pen carriage. Some systems also make other uses of the encoder signal—such as, for example, controlling carriage reversal, acceleration, mark quality, etc. in the end zones of the carriage travel, beyond the extent of the markable image region.
Now, standardized circuitry for responding to each pulse in the encoder-derived signal is most straightforwardly designed to recognize a common feature of each pulse. Thus some circuits may operate from a leading (rising) edge of a pulse, others from a trailing (falling) edge—but generally each circuit will respond only to one or the other, not both.
Such circuits have been developed to a highly refined stage. Accordingly it is cost-effective and otherwise desirable to employ one of these well-refined, already existing circuits relative to such compensation; however, in adapting such a preexisting design, several problems arise.
(a) Encoder dimensional tolerances—As noted earlier, the position of the pen carriage unit is monitored by a detector that reads a position encoder. In this regard, encoder-reading circuitry processes the encoder reader data to produce pulses each time the carriage unit moves a fixed distance, usually on the order of 1/75th or 1/1 50th of an inch. However, the drop placement on the print medium must be placed on spatial boundaries that are much smaller, such as down to 1/50,000th of an inch or even smaller. Unfortunately, the position encoder is not nearly this precise. Moreover, other problems are associated with the encoder dimensional tolerances. For example, the encoder-reading circuitry is triggered from the falling edges of the initial encoder-derived wavetrain. The alternating opaque markings and transparent segments (or solid bars and orifices) of the encoder strip are arranged in time alignment with the signals that result from reading of those features by a transmissive optical emitter/detector pair.
It will be understood that, in selecting the point at which a mark should be made, it is possible to make allowance for the nominal width of the transparent segment. For example, the firing of a pen could be delayed by a period of time automatically calculated from the nominal width of the transparent segment divided by the carriage velocity. Although both these pieces of information are available during operation of the system, the results of this method would be unsatisfactory because of preferred manufacturing procedures for creation of the encoder strip . These procedures arise from economics related to dimensional requirements, as follows.
Thus, in making a encoder strip, the dimension which is most important to hold to highest precision is the overall periodicity of the alternating opaque bars and transparent segments—i.e., the periodicity dimension that gives rise to a full wavelength of the wavetrain. The two internal dimensions of each mark-and-transparent-segment pair—namely, the length of the bar and the length of the transparent segment—are much less important.
In a unidirectional printing machine, only the distance between falling edges (or alternately rising edges) has any importance, provided only that (1) the distance from each falling edge to its next associated rising edge is great enough to permit the sensing apparatus to recognize the falling edge; and (2) the distance from each rising edge to its next associated falling edge is great enough to permit the sensing apparatus to reset itself in preparation for sensing the failing edge.
More specifically, the dimensional accuracy of the encoder-strip features are plus-or-minus only one percent for the full periodic pattern width, but plus-or-minus ten to twenty percent for the opaque bar width alone. While it would be entirely possible to manufacture an encoder strip with much finer precision in the internal dimensions just mentioned, an encoder strip so made would be substantially more expensive.
(b) Time-of-flight and analogous misalignment effects—A certain amount of time elapses between the issuance of a mark-command pulse to a print head and the mark actually being created on the printing medium. For instance, in an inkjet printer, some time elapses between: the issuance of a fire-command pulse—approximately at an encoder-wavetrain falling edge—to a pen nozzle and the instant when a resulting ink drop actually reaches the medium.
During this time, however, the carriage and pen continue to move across the printing medium—and, in the case of an inkjet device, so does the ink drop, even after leaving the pen. The initial velocity component of the drop along the scanning axis or dimension, when scanning forward, is very closely equal to the carriage velocity; this velocity likely decreases while the drop travels in the orthogonal axis or dimension toward the printing medium—but nevertheless, some forward movement or displacement of the ink drop along the scanning axis does occur before the drop reaches the medium to form an ink spot.
In a unidirectional printing machine, this delay is substantially inconsequential, once the carriage velocity of the pen is constant, for all the ink drops are offset in this same manner by very nearly the same distance, and in the same direction. In other words, the entire image is offset together along the scanning axis; but this does not matter to the resulting printed image because there are no relative offsets within the image—and therefore no discontinuities, no distortions of image features, etc.
Thus time-of-flight and analogous misalignment effects impede the creating hi
Barlow John
Hewlett--Packard Company
Huffman Julian D.
Potts Jerry R.
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