Systems technology for highly dynamic beam scanning

Task

Multi wavelength optics to process composites
© Fraunhofer IWS Dresden
Multi wavelength optics to process composites

Only a decade ago, scanning systems were exclusively used for marking tasks with pulsed lasers. Today it is possible to deflect 6 kW cw power laser beams with galvanometer driven tilting mirrors. Scanner technology today is applied for laser beam welding, sublimation cutting of metals, textiles and composite materials and to dynamically shape the beam for laser surface refinement.

Suitable system technology has to meet different technical requirements. There are numerous possibilities to design a system and also many system builders. The selection of the best solution is often difficult for the user. Fraunhofer engineers therefore endeavored to classify the processing systems based on scientific considerations.

Solution

Principle of multi wavelength optics to process composites
© Fraunhofer IWS Dresden
Principle of multi wavelength optics to process composites
X-Y-plot during generation of a 90° corner with target speed of v = 15 m s-1
© Fraunhofer IWS Dresden
X-Y-plot during generation of a 90° corner with target speed of v = 15 m s-1
Contour deviation when generating a 90° corner with different scanner systems
© Fraunhofer IWS Dresden
Contour deviation when generating a 90° corner with different scanner systems

Highly dynamic axes systems for kW laser radiation have a different optics design. There are differentiations between preobjective and post-objective scanning. The focus diameter is relevant for the process and is determined by the beam quality of the laser, the beam aperture and the focal length of the optical system.

The size of the processing field and the working distance to the focal plane are also characteristic parameters. The later defines besides the mirror mass also the dynamic properties of the beam spot on the part. Commercially available systems achieve processing speeds of up to 20 m s-1. However, scanner systems are limited with respect to maximum dynamics.

To compare processing systems technical specifications can be used. It is also important however, to consider type and flexibility of the axes control systems, which are critical to whether or not a system can be used for a particular application. Fraunhofer engineers developed measurement routines to compare the dynamics of different scanner systems. Dynamic behavior, characterized by positioning deviations are quantified based on reference contours.

In the most simple case target and actual position data are compared. If the system does not provide external positioning information reference engravings are measured using fast pulsed lasers. The dynamic behavior during harmonic oscillations is characterized with amplitude-frequency dependencies (Bode diagram). To compare beam scanner systems from different manufacturers, it was also necessary to develop a family of control modules for using each of the systems.

Results

1D laser scanner optics with scanner-position-depending laser power modulation
© Fraunhofer IWS Dresden
1D laser scanner optics with scanner-position-depending laser power modulation

Galvanometer scanners were used for laser macro materials processing and the gained experience provided input for implementing a database. The database holds technical specifications of beam scanners that are commercially available and also those that are used at IWS. The database user can call upon additional modules to calculated achievable spot diameters and intensities when using lasers of different beam qualities.

If data is available such as processing field dimensions, spot diameter and dynamics, the user has the tools to design the ideal scanner system based on this information. Criteria can be weighted to derive recommended solutions from the pool of commercially available systems. In some cases the recommendation will be to use a very special optics design. For such applications our high-speed laser-processing group has an excellent track record of successful industry projects to develop application adapted scanner systems.