Submicrometer precision for lasers in space | Astronomy
The Fraunhofer ILT has designed a laser for measuring methane concentrations, which will be used in 2021/2022 in the German-French satellite Merlin. For this purpose, we developed an assembly station with position stability in the nanometer range.
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Whether extremely precise, dynamic or robust - Steinmeyer Mechatronik offers a wide range of positioning solutions for laser technology. The Fraunhofer Institute for Laser Technology ILT in Aachen had entrusted the Dresden-based positioning specialists with a particularly challenging project: the development of an assembly station with positional stability in the nanometer range.
When Albert Einstein described stimulated emission in 1916 and thus provided the theoretical basis for the laser (Light Amplification by Stimulated Emission of Radiation), Steinmeyer Mechatronik was already looking back on a 44-year company history. Decades were to pass before the first functioning laser was built: It was not until 1960 that the efforts were crowned with success. Today - more than 100 years after Einstein's description of the phenomenon - laser technology is one of the key technologies and it is impossible to imagine industry, medicine, science and everyday life without it. Regardless of the industry or application: the basic prerequisite for an optimal result is exact alignment or positioning. "The combination of high dynamics and simultaneously high repeatability often determines success," knows Elger Matthes, Development and Product Management at Steinmeyer Mechatronik, and adds, "To avoid contamination on the optics, positioning systems for laser applications must also meet high cleanliness requirements."
High-performance positioning solutions for laser technology
Steinmeyer Mechatronik is one of the world's leading manufacturers of high-precision positioning solutions for optical and precision mechanical systems. For laser technology, the Dresden-based company offers systems that fully meet the demanding tasks. In addition to the production of standard products, the company's core competence includes the realization of individual special projects. Together with the customer, the positioning experts develop innovative concepts that are precisely tailored to the respective application. "Years of experience in this field have shown us that only a perfectly coordinated system brings the desired success," emphasizes Matthes. In addition to systems that bring optics, laser optical assemblies, focus and mirror deflection units, and laser safety elements into the correct position, the portfolio therefore also includes other crucial components. These include a laser drilling head developed specifically for the industry. This trepanning optics enables the drilling of micro holes with precisely defined geometries.
Assembly station with nanometer stability
One example of the Dresden-based company's high innovative strength and solution competence is the development of an assembly station with nanometer stability for the Fraunhofer Institute for Laser Technology ILT. For active measurement of the methane concentration in the Earth's atmosphere, the Aachen-based laser specialists have designed a laser that is to be used in the Franco-German MERLIN satellite (Methane Remote Sensing LIDAR mission) starting in 2021/2022. The laser system consists of a large number of individual components that are joined together using an innovative soldering technique known as optical soldering. The novel joining process offers decisive advantages for the journey into space: The strong connection effortlessly withstands even high shocks and vibrations up to 25 times the acceleration due to gravity, thus providing the sensitive laser technology with the necessary robustness to survive the jerky rocket launch undamaged. In addition, no adhesive is used. This means that no gases can escape that could contaminate the sensitive mirror surfaces.
Stability out of this world
To connect the laser's various components with pinpoint accuracy, they must be aligned in the beam path and fixed with submicrometer precision. "Our task was to develop an outgassing-free system for aligning and holding the individual optical elements," Matthes said. "The greatest challenge here was the required position stability. Only deviations of a maximum of 50 nm were permitted. On the other hand, no special requirements were placed on accuracy and dynamics." The dual 4-axis system (XYZ-Phi) designed by Steinmeyer Mechatronik roughly brings the parts to be joined into position and then fixes them so that absolutely nothing can move. Finally, a hexapod takes over the fine positioning in the submicrometer range before the soldering process can begin.
The air must get out
The stability of a positioning system is influenced by many factors. These include control oscillations, settlement effects, displacement of lubricant, natural frequency as well as cross-coupling of other axes. For absolute standstill, all these influences must be eliminated. But how? Steinmeyer Mechatronik's solution is as simple as it is effective: clamping - using air bearings. "Air bearings are not normally associated with sluggishness and stability, but instead stand for smooth, gliding movements and extremely good run-off values," explains Elger Matthes. The trick: The air has to come out. "If you bleed the bearings in a controlled manner, you get an incredibly smooth system. With relatively simple methods, we thus achieve stability in the nanometer range." So the X and Y axes were equipped with air-bearing slides. Once the system is positioned, it is lowered by releasing the air from the vertical bearings and fixed in place by venting the horizontal bearings.
Drives for standstill
To decouple the drive from the guides, the horizontal axes are driven by toothed belts. These have a certain stiffness only in the direction of movement and thus have no effect on the stability of the overall system. A ball screw drive with stepper motor was selected for the vertical. "Thanks to its self-retaining in full step, the stepper motor makes an optimum contribution to stability," says Matthes. "Also advantageous are the high rigidity of the ball screw and the high efficiency for lifting the loads." The rotary motion is handled by a worm rotary table. This is self-locking and very rigid. In combination with a stepper motor, absolute standstill is thus achieved even on the fourth axis as soon as the air bearings have been vented. The result: a position holding accuracy of 0.05 µm - proven with a capacitive sensor in the fully assembled system.
Maximum flexibility and optimum protection
A second Y-traverse with Z-axis and rotary axis is provided for a second hexapod. Both Y-beams can be moved out of the working area to ensure maximum accessibility. In addition, an opening in the support wall facilitates access to the component from all sides. Since soldering is carried out with a UV laser, all aluminum parts were provided with a bilathal coating. This absorbs ultraviolet radiation and minimizes scattered radiation. To protect the optical surfaces from contamination, all particle sources were also enclosed with metal sheets and only outgas-free lubricants were used. talen bearings fixed.
Everything from a single source
Like all Steinmeyer Mechatronik products, the positioning system for the Fraunhofer ILT was developed and produced in-house. All departments - from development to manufacturing and assembly to the test department - work closely together under one roof at the Dresden site. This allows synergies to be optimally utilized and specific customer requirements to be realized quickly and easily - regardless of whether it is a single functional sample, projects with quantities of a few dozen or series with several thousand units per year.Assembly was carried out in the company's own clean room by specially trained personnel. "With a cleanroom area of 150 m², we are able to manufacture everything from individual assemblies to series components. The large area allows us to operate several workstations in parallel, thus ensuring exceptional flexibility," Matthes makes clear, adding, "When it comes to packaging, we rely on well-known standards. Thus, we ensure that the components reach their destination in the same condition in which they left our cleanroom."
Positioning technology with radiance
Redefining the limits of what is possible: This is the challenge that the positioning experts at Steinmeyer Mechatronik set themselves - with success, as the example of the assembly unit for the Fraunhofer ILT shows. Extensive knowledge of the industry and many years of expertise benefit the Dresden-based company just as much as flexible production processes and state-of-the-art manufacturing technology. In this way, the company optimally meets the diversity of applications as well as the demanding positioning tasks in the field of laser technology.
Author: Christoph John, Development Manager, Steinmeyer Mechatronik