Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-01-29
2001-10-30
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S041000, C400S352000, C400S354000
Reexamination Certificate
active
06310638
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to carriage drive systems for printing and scanning devices, and more particularly, to an apparatus and method for reducing vibrations of a carriage during movement along a carriage path.
In inkjet printing systems and document scanning systems a carriage is moved relative to a media to either print or scan the media. In an inkjet printing system, the carriage carries an inkjet pen which ejects ink drops onto the media as the media is moved along a media path. In a document scanning system the carriage carries an optical sensor which detects ink markings or characters on the media as the carriage moves relative to the media. To achieve accurate printing or scanning, it is important to know or maintain an accurate positional relationship between the carriage and the media.
In inkjet printing it is important that the carriage scan the inkjet pen smoothly across the media with minimum vibration so that ink dots can be accurately placed. Conventional inkjet printers print 300 dots per inch or 600 dots per inch. In addition, printers which print at 1200 dots per inch are being sought. As the number of dots per inch increase, the dot size has decreased. Precise dot positioning of the smaller dots at increasing dot density leads to higher quality images. In particular, such positioning of colored dots is leading to near photographic image quality. One challenge in striving to achieve such improved image quality is the adverse impact of carriage vibrations. Dot placement errors as small as 5 microns cause noticeable defects in print quality.
FIG. 1
shows two overlapping circles
12
having a common first size. Each circle
12
represents an inkjet printing dot of a first size. Such size is largely exaggerated here for purposes of illustration.
FIG. 2
shows two overlapping circles
14
having a common second size which is smaller than the first size. Again, each circle
14
represents an inkjet printing dot of a second size, and such size is largely exaggerated for purposes of illustration. In each example, the dots
12
and dots
14
overlap by a common percentage of their respective diameters (e.g., 20%). The absolute distance of overlap is larger for the larger dots
12
than for the dots
14
. The overlap of dots
12
is a distance x. The overlap of dots
14
is a distance y. For purposes of illustration, assume that dots
14
are half the size of dots
12
and that y=0.5x.
Consider now a situation where the carriage vibrates during printing along an axis
16
. If the vibration amplitude along axis
16
is much smaller than the distance x, then the impacts of the vibration will not adversely impact the dot placement accuracy, and thus not adversely image the image quality. As the vibration amplitude along axis
16
approaches the distance x, however, more white space occurs on the media in the vicinity of the dots
12
intersection. Taken over an entire image, the effect appears as a banding of lighter and darker areas of the image.
FIG. 3
shows an exemplary image
18
exhibiting such banding.
Given the same amount of vibration amplitude, the impact to an image formed of the smaller dots
14
is more adverse than to an image formed with the dots
12
. For example, a vibration amplitude of 0.25x may be acceptable for printing using dots
12
. The same vibration amplitude equals 0.5y and may cause unacceptable banding when printing with the dots
14
. Such bands occur within an image at the frequency of vibration of the carriage along the axis
16
. In general, the smaller dot size and higher resolution of advancing ink jet printers require more accurate placement of dots to achieve expected image quality improvements.
Any vibrations displacing the carriage relative to the media can potentially reduce printing/scanning accuracy. Typical sources of vibration are external vibrations which move the whole printer or scanner, and internal sources which are coupled to the carriage or media. This invention is directed toward internal vibrations which are coupled to the carriage. Efforts to reduce the impact of the vibrations have included reducing the magnitude of the vibrations generated by the drive system. This is achieved, for example, by using a smoother running carriage motor or by achieving more accurate meshing of teeth between drive belt and motor. Another approach is to stiffen the carriage system (i.e., increase the resonant frequency of the carriage and carriage rod so that the vibrations have less impact on the carriage). This is achieved, for example, by increasing precision of the carriage bearing, increasing the size of the carriage, or increasing diameter of the carriage rod. All of these solutions add significant expense to the system. Accordingly, there is need for a relatively low cost, yet effective solution for eliminating or reducing the carriage vibrations or the impact of such vibrations.
SUMMARY OF THE INVENTION
A carriage drive system includes a carriage driven along a carriage rod under a force generated by a drive motor through a drive belt. The carriage includes a roller or a sled which runs along a track. Thus, the carriage includes three regions of external contact: the carriage to drive belt connection, the carriage to carriage rod connection, and the carriage to track connection. It is desired that the carriage move along the carriage rod without rotational vibration about the carriage rod, without vibrational offset perpendicular to the carriage rod, and without back and forth vibration along the carriage rod. This invention is directed toward isolating the carriage from rotational vibrations introduced to the carriage through a carriage rod or carriage track.
With regard to the carriage rod, the carriage rest on the carriage rod at carriage V-bearings. The V-bearing connection is open at the bottom. One advantage of such carriage placement is that the carriage does not encounter the entire surface of the carriage rod. This allows the carriage rod to be mounted to a housing at intermittent points along the underside portion of the carriage rod away from the carriage. (Rather than having the rod mounted to the housing at only the end points of the rod). Such mounting of the carriage rod increases stiffness of the carriage rod. In addition the distance between the carriage rod and print media is more uniform over the length of the rod.
According to an aspect of this invention, a magnetic preloader is used to apply a magnetic force biasing the carriage toward the carriage rod. Such magnetic preloader is located in the vicinity of the carriage V-bearings. Preloading the V-bearing connection between the carriage and the carriage rod reduces vibrations from (i) stiff members attached to the carriage such as ink supply tubes; (ii) high acceleration rates of the carriage relative to the carriage rod; (iii) vibrational chatter of the V-bearings along the carriage rod; and (iv) dynamics between the carriage drive motor attachment point, the carriage center of gravity and the carriage center of friction.
According to another aspect of the invention, at the carriage to track connection a restoring force is applied to bias the carriage toward the carriage track. At high slew velocities of the carriage, a local discontinuity, such as a bump, encountered by a carriage roller causes an acceleration of the roller away from the track surface. The magnitude of the acceleration is proportional to the effective slope of the discontinuity multiplied by the carriage slew velocity. The effective slope is the slope of the discontinuity on the roller's surface. The upward rotation causes a torque rotating the carriage around the carriage rod. Such torque is proportional to the moment of inertia times the acceleration at the roller. Gravity causes a restoring torque opposing this rotational torque. The gravitational restoring force is proportional to gravity times the distance from the carriage rod center of gravity to the carriage rod.
According to an aspect of this invention, an additional restoring to
Cameron James M.
Heiles Tod S.
Barlow John
Hewlett--Packard Company
Stewart Jr. Charles W.
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