Compared with laser cutting, laser welding and other laser processing industries, the development of laser cleaning is obviously sluggish. The main reason is that the efficiency of laser cleaning is not high and the cost performance is low. Laser cleaning efficiency is mainly determined by laser power and scanning speed. With the rapid development of domestic lasers, pulsed lasers have reached 1KW pulses, and continuous lasers have reached 50KW. The laser power is high, the laser spot size is large, and the diameter of the high-power 1000W pulsed laser beam reaches 30mm to 40mm.
Laser cleaning and scanning components are mainly vibrating mirror motors and a very small number of octagonal rotating mirrors. The vibrating mirror motors are loaded with reflectors to deflect the laser spot back and forth to realize laser line scanning. The larger the laser spot, the larger the mirror size, and the stronger the yaw inertia, resulting in slower yaw speed. Taking the HurryScan series products of Scanlab manufacturer as an example, the scanning speed of the 10mm spot galvanometer is about 12m/s, the scanning speed of the 20mm spot is about 6m/s, and the scanning speed of the 30mm spot is about 1.2m/s. The higher the laser power, the larger the laser spot and the slower the scanning speed. Efficient cleaning requires high laser power and fast scanning speed, and the galvanometer cannot meet the requirements of high-power lasers. At the same time, due to the reciprocal deflection scanning of the galvanometer motor, the laser scans to the edge area and decelerates to zero and then accelerates in the opposite direction, resulting in a long stay of the laser spot in the edge area and a long time of laser thermal action, resulting in serious edge overburning. Therefore, the vibrating mirror motor has two obvious defects in the application of high-power laser cleaning, namely, slow scanning speed and serious edge overheating. Galvo motors are used more for low power laser marking rather than high power cleaning applications.
Figure 1 Schematic diagram of octahedral mirror scanning
The technology of eight-sided rotating mirror (or other number of surfaces such as six-sided rotating mirror, four-sided rotating mirror, etc.) is based on the expired foreign patents of the last century, that is, a reflector is installed on each of the eight sides of the octagonal prism, and the laser beam is transmitted at a certain incident angle. When it is obliquely incident on the side reflector of the prism, the direction of the incident light remains unchanged, and the prism rotates along the center. When the laser spot passes across the mirror surface of the side mirror, the incident angle of the incident laser relative to the side mirror changes, and the reflected laser forms a scan. For the octagonal mirror, when the laser scans to the ridge line between adjacent side mirrors, the laser needs to be turned off, and the laser can only be turned on after the laser spot completely enters the side mirror, otherwise the edge of the side mirror will be damaged, and it will take a long time Afterwards the entire reflector will be broken. The laser spot itself has a certain size. When the laser spot slides from one end edge of the side reflector to the other end, the central angle ɑ of the edge track relative to the center of the octagon mirror is the actual light output angle of the side reflector, and the light output angle of one rotation is 8*ɑ. Introduce the laser utilization rate β=N*a/360, where N is the number of polygonal mirrors, where N=8.As shown in Figure 1, when the size of the reflector on the side of the prism is 20*18mm, when the laser runs from P1 to P2, the utilization of the prism is β=8*28.72/360=63.8%.The cleaning efficiency of the equivalent 100W pulsed eight-sided rotating mirror is the same as that of the 63.8W pulsed galvanometer, and the cleaning efficiency of high-power pulsed laser will be lower. At the same time, since the laser is turned off in the edge area, and the laser is turned on when the edge of the new side mirror is scanned, the first pulse effect will appear at the moment the laser is turned off and turned on, resulting in the problem of overburning at the edge of the laser. Two obvious defects in the application of octagonal mirrors to high-power laser cleaning are low laser utilization and the problem of edge overburning still exists. The octagonal mirror is mostly used for radar scanning, the radar laser power is low, and the laser does not need to be turned off in the area where the ridge lines intersect.
Figure 2 Left: scanning image of the galvanometer system
Figure 2 Right: MASM system scan
Aiming at the problems of slow scanning speed and edge overheating and cleaning, the Institute of Surface and Interface Science and Technology of Jiangsu University took 5 years to successfully develop a high-speed multi-array scanning system MASM (Multi-Array Scan Mirror), which uses a unidirectional high-speed DC motor as The power part compresses and shapes the incident laser spot in space, and finally scans and emits it in the form of line laser, and the laser scanning line speed can reach 100m/s. As shown in Figure 2, the scanning speed of the galvanometer is set to 15m/s, the speed of the MASM system is set to 50m/s, the moving speed of the platform in the vertical line scanning direction is 90mm/s, and the laser is always on, that is, the laser utilization rate is 100%. The distance between adjacent lines of the galvanometer is 0.81mm, and the distance between adjacent lines of the MASM system is 0.18mm.
Left: EDS comparison diagram of indicator light
Right: Laser EDS comparison chart
Figure 3 EDS effect diagram
As shown in Figure 3, the edge extinction EDS subsystem (Edge Deal System) in the MASM system can completely solve the problem of edge overburning and control the smooth transition of the laser from the edge to the non-edge area. The left side of Figure 3 shows the non-dull to extinction state, indicating that both ends of the red light are weakened; the right side of Figure 3 shows the contrast between the left and right ends after cleaning, and the matte area transitions smoothly to the substrate. Tang Faquan, the chief technical engineer responsible for the innovation and development of cleaning technology, introduced that "the robot-applied MASM system can meet the requirements of 30-spot to 40-spot laser, and the scanning speed can still reach 50m/s, which is very suitable for high-power pulsed laser cleaning applications." The development of MASM array scanning mirror system provides an effective solution for high-power high-speed laser cleaning applications and low-power hand-held weld seam cleaning.
