A wide range of architectures is now available to laser system designers for beam positioning. These architectures allow the designers to solve many different applications. This article investigates the design, advantages, consequences, and comparative application of the 3-axis scan head technology, as opposed to the 2-axis scan head architecture.
In a regular set-up of a two-axis laser scanning system, a collimated beam is reflected by the X- and Y-axis scanning mirrors prior to crossing the threshold of the focusing lens. The collimated beam is directed on the work surface through the focusing lens. Rotations of the X- and Y-axis scanning mirrors indicate the movement of the focused spot over a flat field.
Among other factors, the focusing lens also influences the size of the field and the size of the spot. F-theta lenses have been particularly configured for this purpose. Such an arrangement is referred to as a pre-objective scanning system because the laser makes contact with the X- and Y-axis scanning mirrors before the focusing (objective) lens, as indicated in Figure 1.
Figure 1. Image Credit: FARO Technologies Inc.
This design is highly effective, provided the diameter of the field size and the diameter of the laser beam are proportionally small. For instance, pre-objective scanning is useful for applications where less than 20 mm beam diameters with less than 300 mm field size are used.
As demand increases for the field size, so does the diameters of scan mirrors and laser beams to maintain a numerical aperture (NA) that is consistent with a tiny focused spot. It would be costly, bulky, and impractical to use F-theta scan lenses for high-value laser beams. Therefore, one solution is to use a 3-axis scanning system technology.
Within a 3-axis scanning system, the XY mirrors are placed after the final focusing lens, and therefore, they are referred to as a post-objective scanning system. It is not necessary to increase the size of the lens because the laser beam does not move on the objective lens. But this kind of configuration does not create the required flat field. Therefore, a third axis (Z-axis) of motion is introduced in the form of a linear lens translator, and this helps in securing a flat field.
A telescope is used by standard laser systems to boost the scope of a laser beam to a diameter that is in accordance with the required NA. The distance between the objective lens(s) and the telescope input lens(s) determines the focus distance of the system. To achieve dynamic control across the focus distance, the input lens(s) is installed onto a linear lens translator (that is, the third axis), as shown in Figure 2.
Figure 2. Image Credit: FARO Technologies Inc.
A focused laser spot throughout a flat field is realized by coordinating the motion of the linear lens translator with the rotations of the X and Y scanning mirrors.
The XY scanning system should be connected to a controller to perform laser application tasks. This controller also operates the laser and provides real-time synchronicity of the scanners as well as the laser. The controller is connected to a software or firmware that offers a continual data stream to the hardware.
User data, in the form of text, graphics, barcodes, etc, is fragmented to create particular commands for each axis and the laser. The controller should offset geometric distortions that are inherent with the lens and the arrangement of XY mirrors. This can be achieved by using a lookup table. An image is produced by modifying the user coordinates in association with the data in the lookup table, and this image does not distort in the working field.
Usually, the user data is presented in a two-dimensional (2D) format. This concept works very well with the pre-objective systems because graphics and data processing can be siphoned into X and Y channels of data for placing each mirror. Complete data is required to push the Z-axis mechanism in the 3-axis post-objective system. Therefore, the lookup table helps in producing a third coordinate for the Z-axis.
For each XY coordinate, Z ordinate is calculated, which will be utilized to direct the laser beam at the required Z-axis position. A previous understanding of the optical configuration is integral to calculate these ordinates. During the entire laser scanning process, these values are used to create a signal for the Z-axis mechanism.
Changing Field Sizes
With the help of this architecture, many lookup tables can be cataloged to use the optical system at various focus distances. The interval between the working field and the scanning mechanism is controlled through a static adjustment of the lens spacing. The effectual action of the Z-axis provides real-time and continuous focusing to sustain a flat field.
To manage the system remotely, the operator has to simply nominate the relevant table and then set the static adjustment of the expander lens position to the matching distance. When the lookup table data correlates with the hardware’s physical setup, the system will focus properly. Any variation will indicate the occurrence of focus errors at the work plane.
This kind of optical system lacks telecentric compensation. The angle of the beam to the flat field is governed by the mirror’s rotation angle. One outcome is the increased field size with the distance from the scan mirrors. This feature implies that 3-axis scanning systems are more flexible when compared to the 2-axis systems, because working fields for objects with different sizes can be achieved without making any changes to the lenses or hardware.
The optics’ design usually implements the required attributes to create a constant beam diameter at the X scan mirror. Therefore, the system’s NA sets the distance to the flat field by the beam diameter on the X mirror. Both spot size and field size scale proportionally, since the focused spot diameter is already set by the NA. For instance, if the field size is increased by two-fold, the spot size will also increase by two-fold.
To date, the discussion has been restricted to 2D scanning since the 3-axis technique is used to focus the laser spot in a 2D field. Most significantly, the optics and controls facilitate in focusing the beam in a 3D volume. Moreover, 3D data can be embedded in the lookup table, so that the Z-axis ordinates represent a predefined contoured surface.
Using the rendering software, 3D data can also be transferred directly to each axis, allowing arbitrary access to any point inside the addressable space. This also includes the modifications of the Z-axis to help focus on irregular or contoured non-flat surfaces. In such cases, intricate calculations need sophisticated software for non-distorted delivery of beams onto a 3D shape.
FARO Technologies offers complete laser system solutions for system integrators and OEMs. Smart scan heads with ultra-precision are paired with advanced marking software and Ethernet-based controllers to achieve a complete laser system that has streamlined communication.
The 2D and 3D scan heads from FARO Technologies offer large field sizes and avoid the need to integrate gantries, large scanning lenses, or XY tables. Excellent precision, bandwidth, and flexibility in 2-axis and 3-axis components can be achieved through sophisticated optical position detector galvanometer technology.
This information has been sourced, reviewed and adapted from materials provided by FARO Technologies Inc.
For more information on this source, please visit FARO Technologies Inc.