Designers of laser systems have a wide range of architectures for beam positioning at their disposal which allows them to solve a variety of applications. Throughout this paper, there will be an investigation of the design, benefits, consequences, and comparative use of the 3-axis scan head technology as compared to 2-axis scan head architecture.
In a standard set-up of a two-axis laser scanning system, before crossing the threshold of the focusing lens, a collimated beam is reflected by the X and Y-axis scanning mirrors. The beam is focused on the work surface via the lens. Revolutions of the X and Y mirrors means that there is motion of the focused spot across a flat field.
Amongst other factors, the lens also impacts the size of the spot as well as the size of the field. F-theta lenses are specifically configured for this purpose. This arrangement is known as a pre-objective scanning system due to the fact the laser comes into contact with the scanning mirrors before the focusing (objective) lens. (See figure 1).
This architecture is most effective if the diameter of the laser beam and the field size are proportionally small. For example, pre-objective scanning is beneficial for applications using beam diameters of less than 20 mm with a field size of less than 300 mm.
As requirements for the field size increase, so must the scan mirrors and laser beam diameters to preserve a numerical aperture (NA) consistent with a small focused spot. Using F-theta scan lenses for laser beams of increased value would be costly, large, and impractical. Therefore, consideration should be given to a solution using a 3-axis scanning system technology.
The XY mirrors are positioned after the final focusing lens in a 3-axis scanning system, thus, they are known as a post-objective scanning system. There is no need to increase the size of the lens since the laser beam does not move on the objective lens; however, this configuration does not generate a flat field. So, introducing a third axis (Z-axis) of motion in the form of a linear lens translator enables and secures a flat field.
Common laser systems use a telescope to increase the scope of a laser beam to a diameter compatible with the necessary NA. The focus distance of the system is determined by the distance between the telescope input lens(s) and objective lens(s). Dynamic control across the focus distance is achieved by installing the input lens(s) onto a linear lens translator (the third axis). (See Figure 2).
By coordinating the movement of the linear lens translator with the revolutions of the X and Y scanning mirrors, a focused laser spot throughout a flat field is achieved.
To carry out laser application tasks, the XY scanning system must be coupled with a controller, which is also operating the laser. The controller gives synchronicity of the scanners and laser in real-time which is linked to firmware or software that provides the hardware with a continual stream of data. User data (in the form of barcodes, graphics, text, etc.,) is fragmented to generate specific commands for the laser and each axis. It is also necessary that the controller compensates for geometric distortions characteristic of the lens and the XY mirror arrangement. This can be accomplished using a lookup table. By adjusting the user coordinates in correlation with the information in this table achieves an image that remains undistorted in the working field.
Typically, the user data is put forward in a 2-dimensional format. This works efficiently with the pre-objective systems as processing of the data or graphics can be siphoned into X and Y channels of data for positioning each mirror. Complete information is necessary to drive the Z-axis mechanism in the 3-axis post-objective system. Therefore, using the lookup table enables the creation of a third coordinate for the Z-axis. For every XY coordinate, calculation of a Z ordinate is carried out which will be used to focus the laser beam at the requisite Z-axis position. Prior knowledge of the optical configuration is fundamental to the calculation of these ordinates. Throughout the laser scanning process, these values are used to produce a signal for the Z axis mechanism.
Changing Field Sizes
Utilizing this architecture, various lookup tables can be cataloged to operate the optical system at numerous focus distances. A static adjustment of the lens spacing regulates the interval between the scan mechanism and the working field. The effectual action of the Z axis gives constant real-time focusing to maintain a flat field.
To control the system at a distance, the operator need only nominate the table that is relevant and subsequently set the static expander lens position adjustment to the distance that matches. When the data in the lookup table agrees with the physical set up of the hardware, the system will focus correctly. Any disparity will mean that focus errors occur at the work plane.
There is no telecentric compensation present in this type of optical system. The beam’s angle to the flat field is determined by the rotation angle of the mirror. One outcome is an increase in the field size with the distance from the scan mirrors. This trait means that 3-axis scanning systems are more flexible than 2-axis systems, as working fields for objects that differ in size can be realized without changing hardware or lenses.
Traditionally, the design of optics incorporates the necessary attributes to generate a constant beam diameter at the X scan mirror. Thus, the numerical aperture (NA) of the system is settled the distance to the flat field by the diameter of the beam on the X mirror. Since the focused spot diameter is set by the NA, field size and spot size scale proportionally. For example, if the field size is doubled, the spot size will also double.
So far, the discussion has been limited to 2-dimensional scanning as the 3-axis technique is used to focus the laser spot in a 2-dimensional field. Notably, the controls and optics allow for the focusing of the beam in a 3-dimensional volume. It is possible to embed 3-dimensional information in the lookup table so that the Z-axis ordinates stand for a predefined contoured surface.
Moreover, 3-dimensional data can be sent directly to each axis using the rendering software; this enables random access to any point within the addressable space. Additionally, this is including Z-axis modifications to permit focus on contoured or irregular non-flat surfaces. In these instances, complex calculations require advanced software for non-distorted beam delivery onto a 3D shape.
Complete laser system solutions for OEMS and system integrators are available via FARO. Smart scan heads with ultra-precision are coupled with Ethernet-based controllers and state-of-the-art marking software for a complete laser system with streamlined communication. FARO’s 3D and 2D scan heads provide large field sizes without the need for integrating XY tables, large scanning lenses, or gantries. Superior bandwidth, flexibility, and accuracy in 2 and 3-axis components are made possible by advanced optical position detector galvanometer technology.
This information has been sourced, reviewed and adapted from materials provided by FARO Technologies Inc.
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