Magazine Articles
The articles below are from the Spring 2010 Issue of the AILU Magazine
Laser-based machining of fine feature scales for high precision optical position encoders
High-resolution Optical Position Encoders (OPE) are used for measuring and recording displacements of moving parts, for automatic positioning in machine tools and for the correction of errors in the relative motion of machine tool parts. Providing high-accuracy position feedback makes them an essential device for manufacturing assembly systems such as high quality printing and state of the art remotely controlled medical robots.
An OPE has two main components: a scale and a read-head. The encoder read-head comprises a light source, a detector array and two diffraction gratings that must be aligned to the same precision as the marks on the scale grating used in reflection. For optimum 1st order reflectivity the diffraction gratings must have a sinusoidal profile. The article describes how this was achieved by laser, by exploiting thermally generated surface distortions to shape the metal surface in a single step.
The laser-generated sinusoidal grating grooves were made in a 150 µm thick spring steel substrate coated by layers of copper and nickel, with a final top layer of gold of less than 400 nm. The AFM image (see illustration) shows the overall groove profile to be sinusoidal as expected, with a depth of ~ 615 nm. The depth implies that the laser beam had reached through to the nickel layer.
The conclusions reached indicated that a direct write process for encoder scale fabrication had been developed at commercially-viable rates. The results show that it is possible to reliably generate 4 μm-pitch gratings with features of height 415 nm or 615 nm. This process has the potential to manufacture sinusoidal reflective scales with the characteristics (4 μm width and ~200 ± 10 nm depth) and high production rates required.
Stéphanie Giet, Charalampos Michakis, Jonathan Parry and Duncan Hand - Heriot Watt University, Department of Physics, Edinburgh, Matthew Kidd, Alexander Ellin, Jonathan Shephard, Nick Weston - Renishaw plc, Edinburgh
IMAGE: 4 μm-pitch grating machined on a multilayered metallic substrate with gold top layer. The groove width was also 4 μm. Courtesy - Renishaw plc
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Laser joining for photovoltaic module production
The enormous growth of the photovoltaic solar cell industry in recent years has led to an increasing demand for flexible and efficient production technologies. Laser edge isolation is currently the only standard laser manufacturing process for mono- and multicrystalline solar cells but there at many more laser processes under development. These processes have the potential to meet market and production-driven demands for new solar cell designs that offer higher efficiency and manufacturing productivity.
One of these processes is related to the interconnection of solar cells to solar modules. Currently single solar cells with dimensions of up to 156x156 mm² are electrically and mechanically interconnected by tin-plated copper ribbons to large modules. Due to the decreasing thickness of silicon solar cells below 200 μm contactless joining processes for module production are required to avoid increased scrap by breakage.
Welding is a completely new technique in solar module manufacturing and although there is currently no experience of its use in this application it is an interesting technology with respect to future developments. Attractions include a x10 reduction in processing time and the elimination of flux (though corrosive, there is to date no evidence of it having a negative impact).
Laser processes provide many advantages for the manufacturing of solar cells in certain processing steps. This holds for the processing of the silicon cells as well as for the interconnection of the single cells to complete modules. Laser joining, soldering in particular, provides low energy solutions for high speed packaging with high yield and high quality. With new laser sources and integrated process control systems an optimized thermal management of the interconnection process could be provided, which will be necessary for future thinner solar cells.
Felix Schmitt, Malte Schulz-Ruthenberg, Alexander Olowinsky and Arnold Gillner - Fraunhofer Institute for Laser Technology ILT, Aachen, Germany
IMAGE: Clamping for laser soldering by: (a) mechanical downholders; and (b) pressurized air.
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