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 xe2x80x9cWxe2x80x9d 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 xe2x80x9cmid-objective scanner systemxe2x80x9d by an angle alpha prior to the scanning mechanism 50. Before reaching the focal point the laser beam is again folded 90xc2x0 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.
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 the optical path to direct the laser beam onto the substrate. A scanner in the optical path scans the beam across the surface of the objective lens, so that the objective lens produces a variable image distance across the surface of the objective lens. A controller for synchronizing the scanning system to the motion of the substrate is also provided and includes an encoder for determining a first component of the movement of the substrate and a memory for storing data representing a second component of movement of the substrate. The controller combines the first and second components for driving the scanner. Due to the resulting combined signal, the variable image distance closely conforms to the variation in the length of the optical path, and the laser beam is maintained at a desired marking distance as it is scanned over the substrate.