Journal of Laser Applications 11 199-205 (1999) (ISSN 1042-346X/99/11(5)/1/7)
J. M. Lee, K. G. Watkins, W. M. Steen
Laser Group, Department of Engineering, University of Liverpool Brownlow Hill, Liverpool L69 3GH, United Kingdom
P. C. Russell, G. R. Jones
Centre for Intelligent Monitoring Systems, Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool L69 3GJ, United Kingdom
Abstract: The analysis of acoustic waves produced during laser materials processing is widely carried out for characterising, monitoring and controlling the process. Traditional methods used in analysing the acoustic data such as RMS signal strength and variation analysis, frequency spectrum analysis are based on signal intensity modulation which offers the advantage of inherent simplicity. However, these basic intensity monitoring systems tend to be sensitive to spurious changes in intensity resulting from variations in the various electrical components within the system and the noise of the industrial environment. In addition, it is known that the acoustic intensity is dependent on the distance and observation angle between its source and acoustic sensor. In this paper, a new type of acoustic analysing method based on chromatic modulation is presented. Chromatic modulation offers not only much more acoustic information, precise distinction of the ambiguous signals and robust monitoring in the laser processing system but it could also make control of the laser process easier by simple chromatic output factors. It is also shown in this paper how a chromatic modulation technique may be utilised as an acoustic analysis method to monitor the laser cleaning process. Consequently, it was found that the chromatic acoustic analysis method could be successfully applied for surface monitoring in the laser cleaning process and could provide correct monitoring information in spite of sensor stand-off variation.