Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-06-30
2002-10-22
Yockey, David F. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C256S013000
Reexamination Certificate
active
06469729
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to laser marking devices. More particularly, it relates to laser marking devices for marking substrates such as labels with date codes, lot numbers and related information to permit a manufacturer to track products. Typically such marking systems are used to mark alpha-numeric characters on the desired substrate whether the product itself, a label applied to the product or packaging in which the product is shipped. In the typical laser marking device the surface to be printed is moved past the laser beam which is scanned across it in a manner to apply the alpha-numeric information. The surface to be marked should move such that it is always within the depth of focus (focal plane) of the marking system. For the typical surface, which is planar, the optics can be preset or automatically adjusted to maintain a given focal distance such that high quality characters can be marked thereon. When, however, the motion of the object or the curvature of the object is such that the surface moves in and out of the focal plane, the quality of the characters printed is reduced. This can result in a change in the height, spacing and/or blurring of the characters making the code difficult or impossible to read. Obviously, this is not acceptable when the information being marked relates to important information such as plant location, date codes and lot numbers.
An example of a scanning type laser marking device is disclosed in U.S. Pat. No. 5,734,412 to Hasebe et al. As illustrated therein, a laser marking device including a laser, a scanner and a lens are employed for marking characters on a work piece “W” which is conveyed past the scanning laser beam. As illustrated in
FIG. 1
of that patent the work piece is planar and all that is required is to maintain a constant focal distance to the work piece as the beam is scanned thereacross to create the desired markings.
Such devices would have quality problems with respect to work pieces or substrates which are not planar and/or which have a variable focal distance from the optics. In such a case, the markings on the substrate would be blurred, vary in height or spacing and could otherwise be unreadable thereby rendering the characters unsatisfactory for use.
The subject of focal distance and the correction thereof in a laser scanning system is discussed in some detail in U.S. Pat. No. 5,754,328 to Cobb et al. In this patent, laser scanning systems are classified into three types: objective scanners, pre-objective scanners and post-objective scanners.
FIGS. 1A and 1B
of the Cobb patent illustrate objective scanners utilizing a simple lens to focus a beam of light onto a work piece or part
12
. Scanning is accomplished by either moving the lens or moving the part. Such a system does not provide any correction for focal distance.
FIG. 2
of the Cobb patent illustrates a pre-objective scanner employing a moving mirrored surface
22
, typically a galvanometer or rotating mirrored polygon, to reflect a laser beam onto a lens
20
. The lens then focuses the beam onto the work surface
12
. As indicated in Cobb, a major advantage of pre-objective scanning is its high speed and its ability to have a flat field image. Disadvantages include that the lens is somewhat complex. Again, pre-objective scanners are typically used with flat work pieces.
Finally, in
FIG. 3
, Cobb discusses a post-objective scanner in which the scanning occurs after the beam passes through a lens
30
. Note in
FIG. 3A
that this post-objective arrangement causes the laser beam
125
to be perfectly focused along an arc
31
but out of focus at various points on the planar surface of the work piece
32
; as for example, at points
34
and
38
. The Cobb patent basically describes an improvement of the post-objective scanner type shown in
FIG. 3
in that it discloses a method and apparatus for astigmatically correcting the scanning so that the beam is focused along the planar surface
32
of the work piece.
FIG. 7
of Cobb illustrates the improvement which, in large part, consist of tilting an objective lens
70
in a “mid-objective scanner system” by an angle alpha prior to the scanning mechanism
50
. Before reaching the focal point the laser beam is again folded 90° by use of a concave mirror
77
having a defined radius of curvature. The radius of curvature
78
corrects the field of curvature in the image plane
75
of the work piece to be marked resulting in a flat field of depth for marking on the planar work piece.
Although Cobb discloses changes to the field of depth of a scanned laser beam, it provides a solution which differs significantly from that required to provide a variable field of focus for arcuate work pieces such as bottles, cans and labels which are applied to bottles which are basically cylindrical in cross section. Cobb also does not address the further issues that arise if the work pieces remain in motion along a conveyor belt or carrier wheel while it is being marked.
It is accordingly, an object of the present invention to provide a pre-objective scanner system for laser marking on moving arcuate substrates. It is a further object of the invention to provide such a system with a variable field of focus which matches the variable distance of the arcuate surface from the lens as the surface passes by the lens thereby to insure that the alpha-numeric characters marked on the surface are of uniform height, spacing and quality.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification and drawings.
SUMMARY OF THE INVENTION
According to a preferred embodiment of the present invention, a pre-objective laser marker is disclosed in which a laser beam is scanned by an optical scanner, such as a mirror, a mirrored polygonal surface or an acousto-optical deflector across the surface of an optical element such as a spherical lens. From the lens, the beam is scanned onto a work surface to be marked, which surface is moving relative to the lens and which has a variable image distance from the lens due, for example, to the fact that it is mounted on an arcuate surface moving transversely to the lens while rotating as, for example, a label applying device. To match the variable image distance of the surface to the optics of the marking system, the lens is tilted as a function of the desired variation in the image distance. The result is a variable field of focus for the laser marker which, if it closely matches the variable image distance, results in the printing of high quality alpha-numeric characters on the work surface or substrate.
The invention also encompasses the ability to accurately synchronize a scanning device to the position and velocity of the product to be marked as it moves across the focusing lens. This synchronization permits the optimum placement of the laser beam on a tilted or aspheric focusing lens such that the laser focal point accurately matches the optimal surface position on the product for marking. This is accomplished by using an encoder that tracks the transverse motion of the arcuate surface as it moves along a carrier wheel, which is added to precalculated correction data that measures the rotation of the arcuate surface on an individual carrier mounted on the carrier wheel. In other words, an error signal, based on undetected motion, has been added to the scanner driver signal based on the motion detected by the encoder resulting in a scanner that tracks the product. Concurrently, as the scanner tracks the product, the laser beam is being scanned across the tilted optics in synchronization with the product. The result is a laser code optimally printed on moving and curved objects.
In more detail, a device for marking a relatively moving substrate by scanning a laser beam along an optical path generally defines a length that terminates at the substrate. The length of the optical path varies during marking. The marking device also includes a laser source for producing the laser beam and an objective lens with a surface disposed in th
Bootzin Joel H.
Piper Rudnick
Videojet Technologies Inc.
Yockey David F.
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