ANALYSIS: E-MOBILITY
ends. Burrs on the cut surfaces, remnants of insulation and imprecisely bent hairpins can influence the welding process and affect quality. To achieve an optimal weld seam, the control of the automated manufacturing process must be adjusted, for example, if the ends of the hairpins have a vertical offset or there is a gap between the two ends being welded. Image processing is generally
used to account for these parameters in the process. Berlin-based Scansonic, which develops processing optics for autobody construction, has adopted an innovative approach to the laser welding of copper hairpins with its line of RLW-S optics. High-performance scanner drives are used to deflect the laser beam to maximise shape accuracy of the oscillation function, even at high frequencies. The result is a reproducible and highly reliable process. Two functional modules integrated into the optics work together to determine the position of the hairpins. The optics use conventional
RLW-S laser optics have been successfully implemented in the production of electric motors
achieve a safe and stable laser- welding process.
Absorption depends upon temperature Economical laser sources with scalable power ranges emit beams in the infrared wavelength range at 1,030 or 1,070nm. At these wavelengths, copper absorbs only around 5 per cent of laser light at room temperature. The degree of absorption then increases to around 15 per cent shortly before the melting temperature is reached, and ultimately reaches nearly 100 per cent when a vapour capillary, or keyhole, has formed. However, this process is extremely dynamic – for example, the keyhole can briefly close and the vapour pressure can cause the ejection of molten material. This spattering must be prevented
because the ejected material can lead to short-circuits within the stator, or to other defects. Spattering can be effectively
avoided if the welding speed is higher than 20m/min. Achieving this for the start of the process requires a specific adjustment of the relevant process parameters, such as laser power, speed and focus size. This stabilises the keyhole and practically eliminates spattering.
Image processing for optimal results The geometry and positioning of the hairpin ends can have an enormous effect on the result of the welding process. The hairpins have a rectangular cross-section of only a few square millimetres. Before being inserted in the stator laminations, they are cut to length, bent and insulation is stripped from the
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camera-based image processing. Because the surfaces of the individual hairpins reflect varying degrees of light, they appear in the greyscale image with widely varying levels of brightness and may not be reliably recognised. Illuminating the surfaces from different angles significantly improves recognition. Because the camera system provides images only from the front side, gaps between the ends can be detected, but the system cannot identify a vertical offset. Laser line triangulation is useful for detecting an offset between the two ends being welded. In this process, a sensor projects a straight laser line onto the surface of the two hairpins. The light is reflected and measured by the receiving element in the sensor. Using an angular offset between the projection unit and the receiving element, the sensor can measure the object’s height profile. The height offset is then identified and taken into account in the subsequent welding process.
“Depending upon the motor design, several hundred hairpins must be welded with one another”
Successful demonstration on the production line The reliable and high-quality welding of hairpins offered by such optics is only half of the equation for large-scale production, however. In addition to process reliability, the method must also be compatible with the 60-second cycle time customary in the automotive industry. To increase speed, a large scanning field could be used, which would allow all hairpin pairs to be welded without moving the stator. In practice, however, this approach leads to quality problems caused by the angular offset in the various positions of the stator.
With this in mind, Scansonic
focused on process quality and stability when developing its RLW-S system. The company’s developers used a smaller scanning field and adapted the camera’s field of view for optimal effect. This ensures optimal positioning of the laser beam processing point on the component, as well as the position measurement of this point via the camera. To weld a complete stator, the unit must be rotated. The desired cycle time can be achieved by using a powerful rotary axis. Standardised processing optics can then take full advantage of their technological capabilities. Using this approach, Scansonic RLW-S laser processing optics has already demonstrated flawless operation on the production lines of a major automotive manufacturer. l
Pravin Sievi is a product owner at Scansonic MI in Berlin and is responsible for remote welding solutions.
SUMMER 2022 LASER SYSTEMS EUROPE 17
Scansonic
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