Typewriting machines – Means auxiliary to typewriting function – Indicator means
Reexamination Certificate
1999-02-19
2001-07-03
Hilten, John S. (Department: 2854)
Typewriting machines
Means auxiliary to typewriting function
Indicator means
Reexamination Certificate
active
06254292
ABSTRACT:
RELATED PATENT DOCUMENTS
Related documents are coowned U.S. Pat. No. 5,276,970 of Wilcox, and U.S. Pat. No. 4,789,874 of Majette—and also U.S. patent application Ser. No. 08/657,722 in the names of Armiñana et al., issued as U.S. Pat. No. 5,992,969. Each of these documents in its entirety is incorporated by reference into this present document.
FIELD OF THE 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 a scanning thermal-inkjet machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. It is most particularly applicable to large-format printer/plotters.
BACKGROUND OF THE INVENTION
(a) Encoders in incremental printing—Most large-format incremental printers use a linear encoder in determining and controlling printhead-carriage position and called “codestrip”, tensioned along the scan-axis structure, and an encoder sensor that is assembled on the carriage—with a groove for the strip.
The sensor electrooptically reads markings on the taut strip. Associated electronics generates electronic pulses for interpretation by circuitry in the printer.
Some early tensioned encoder strips were all plastic, adequate for small, desktop printers but not for larger printer/plotter machines. Other early strips were glued to the carriage-supporting “beam” structure, but such a solution gave up the advantages of a separate tensioned strip—including much easier assembly and disassembly, on the assembly line as well as in the field.
Representative work of recent years in codestrip refinement appears in the Wilcox and Armiñana documents mentioned above. Such work in electronic interfacing appears in the Majette patent.
(b) Alignment—Accurate readings, and also minimization of noise in operation, require good alignment between the strip and sensor. Maintaining such performance reliably over the life of a product requires avoiding friction and wear—which in turn makes alignment even more important.
In the evolution of large-format printer/plotters, recent developments have tended toward use of these devices to print wider and wider mechanical drawings and posters. Of course these applications require wider-bed printing machines with correspondingly longer codestrips.
Alignment, however, is progressively more difficult for longer codestrips, partly because of the tendencies to sag under the influence of gravity and twist slightly due to very small variations in mounting angle at each end of the strip. A particularly problematic cause of misalignment is vibration in the working environment.
Vibration sources include impacts from nearby industrial construction, heavy motor traffic, elevators within the building and the like. Nevertheless, for codestrips of the type introduced in the Armiñana document, alignment has been under good control heretofore in systems having modest overall carriage travel—below about one meter (roughly three feet).
(c) The one-meter barrier—More recently it has been noted that performance for strips spanning about 107 cm (3½ feet) is acceptable, but only marginally so. A current generation of these machines requires encoder strips with spans of 152 cm and 183 cm (five and six feet respectively). In a machine of this size the associated long dimensions of the strip cause failures in functional-vibration tests, particularly in large-amplitude harmonic movement near the middle of the strip.
This vibration can produce bad readings from the sensor. For instance the counter may miss counting one or more scale graduations on the encoder strip. The result can be significant errors in a printed image.
In cases that are even more serious, vibration causes complete disassembly of the sensor system—as the strip jumps entirely out of the sensor groove. In such cases trained service personnel may be required to restore normal operation.
Damage to the strip can occur, and the sensor too may require repair. To prevent such problems the system is programmed to shut down the carriage servocontrol motor if the sensor system is able to detect that it has lost count of the encoder graduations—as for example if it loses the pulse train completely.
If such a loss of count occurs while the carriage is near either end of the mechanism, and moving rapidly toward that end, this safety override may not have enough time to stop the carriage before it reaches the end bulkhead. Considerable damage to the carriage and other parts of the mechanism can result.
For machines of modest size it is sufficient to provide a mechanical limiter that simply retains the strip within the sensor groove. The limiter and its installation represent undesirable added cost.
This simple solution, moreover, has proven inadequate for a strip over 1½ m long. Even though retained within the sensor, the strip undergoes oscillations large enough to make sensor measurements erratic and unreliable.
People familiar with this field will understand that the “barrier” suggested in the title of this subsection is not an abrupt step at precisely one meter. Rather the difficulty in achieving satisfactory codestrip arrangements increases progressively over a considerable range from, perhaps, less than one meter to two or possibly three meters. Nevertheless there is a clear qualitative difference, between lengths under one meter and lengths of, say, several meters.
(d) An overconstrained problem—The encoder strip is a rather simple mechanical article, but those skilled in the art will recognize that this seeming simplicity may be very deceptive. The strip interacts in subtle ways with several different complex components of the system.
As a result, it is not at all obvious how to overcome the difficulties outlined above. Some of the more-evident candidate solutions are impractical, due to certain persistent constraints.
The progressively larger machine formats, even below the one-meter barrier suggested earlier, have called for greater tension in the strip. Beyond that barrier, simple increase of tension in the strip is unacceptable.
One reason is that higher tension could potentially introduce safety concerns. Another reason is that higher tension in the strip can cause small twists and other irregular deformations in the associated mechanism. Even if microscopic, such interference with the straightness and structural integrity of the guide-and-support rods and beam can throw off the positional calibration of the whole carriage drive system.
Such potential damage can be difficult to detect, and the design cost of reevaluating the entire mechanical system for such potential damage is in itself severe. If found, such a problem can be compensated only by strengthening the entire structure. Beefing up the mechanism in that way, in turn, would entail additional weight and cost.
Another complication is that addition of stiffening elements or any other attachment to the strip itself would be extremely awkward, since the sensor groove is very narrow. Of course it is important not to add anything to the strip, or next to it, that might pose even greater risk of damage than the strip itself poses—that is to say, catastrophic failure modes must be evaluated as carefully as routine operation.
Thus a supporting ledge below the codestrip (reasonably remote from the moving sensor) might be useful, although costly, but it would not resolve the problem of the strip moving upward. A “ceiling” strip immediately above the codestrip, to correct that deficiency, does not appear practical since the encoder sensor—moving at high speed—could strike such a component.
It has been suggested to return to the approach of using adhesive to secure a strip to a solid beam structure. As mentioned earlier, however, that approach has associated inefficiencies and high costs. Such a beam-mounted encoder strip is also difficult to install and remove.
Using small screws or bolts to fix a thin metal strip along the base would be even more und
Ashen & Lippman
Hewlett -Packard Company
Hilten John S.
Nolan, Jr. Charles H.
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