Design Of An Automatic Laser Focus Following System For Metal Curved Surface Processing

A defocusing amount means the ray-axis distance from a laser focus to a workpiece surface.Only by precisely controlling the defocusing amount can achieve optimal results when laser cutting,engraving,welding and milling a metal workpiece with a random surface.The existing methods to control focus following are based on the distance measurement methods of the capacitance,the laser triangulation or the machine vision.However,the capacitance method is disturbed by the high temperature metal vapor and its error increases significantly with the workpiece curvature;the laser triangulation method can not balance the accuracy and the cost of the measurement system;and the machine vision depth-of-field method has poor real-time and low accuracy.In addition,the focus-following accuracy of the above methods are limited since the focal length of the lens through high-power laser always varieties based on the thermal lens effect.Therefore,in this paper a control method of focus-following is proposed,which adopts the signal of metal plasma ultraviolet(UV)radiation intensity caused from the laser ablating the metal workpiece and hence has higher accuracy due to the self-adaption of the thermal lens effect.Based on the novel method,an automatic laser focus-following system for metal curved surface processing is designed and realized,as follows:Firstly,the spectral of laser-induced plasma is theoretically analyzed,and then a AlGaN wide-band-gap UV photoelectric sensor and a metal test material are optimally selected.Furthermore,the focus-following system is designed and realized,in which the UV signal acquisition module simultaneously provides the first-grade and the second-grade amplified outputs to balance the sensitivity and the laser-power response range.Secondly,the UV acquisition module output signals varying with the defocusing amount is experimentally obtained with the slant pull method using a ultra-planar Fe target,and is compared with the Fe+ characteristic-spectral-line intensity signals of the laser induced breakdown spectroscopy(LIBS)synchronously obtained by a optical fiber spectrometer.The results show the two sets of signals have a great mapping relation,which proves that the error caused by the plasma radiation interference of N,O,C,H from the air and the organic pollutants can be neglected.Furthermore,the interference of the laser power and the side-blown protective gas flow rate is measured and analyzed.Finally,the semi-empirical formula of the defocusing amount and the UV-module output voltage is deduced using the above experiment data.Based on the formula,an improved hill climbing algorithm is proposed to improve the focus-following real time.To test the accuracy of the algorithm,the engraving is performed on an iron workpieces with a curvature radius of less than 4 cm,using an 1064-nm-wavelength 100-ns-pulsewidth 20-kHz-frequency 15-W-power Gaussian laser with a amplitude fluctuation of 5%.A engraving depth of 913.3±30.9 μm is observed using a confocal microscope.