Characterisation of Laser Cutting for an Adaptive Control Environment

Huang, Ming-Yaw (1994) Characterisation of Laser Cutting for an Adaptive Control Environment. PhD thesis, University of Glasgow.

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Abstract

The reactive gas assisted laser cutting process is influenced strongly by characteristics such as: power mode (i.e. CW or pulsed), power density, transverse mode, beam polarisation; set-up of operating system such as: beam delivery optics, nozzle type, assist gas type and gas pressure, beam-focusing, laser beam operating position (offset), and feedrate; and characteristics of target material such as: conductivity, diffusivity, melting temperature, boiling temperature, latent heat of melting and vaporisation, plasma formation, exothermic chemical reaction with reactive gas O2, and relevant viscosities. So many parameters are involved that inevitably laser cutting is a highly nonlinear process and consequently difficult to analyse or predict. Relationships between the operational parameters and cut quality were investigated empirically. As long as the cut can be initiated, change of focal point offset distance does not greatly influence the size of the heat affected zone (HAZ). The kerf surface roughness is slightly increased when the offset distance is increased. The best focal point position, for cutting mild steel up to 6 mm, is on the surface. Increasing the assist gas pressure reduces the HAZ size slightly, reduces dross attachment and has little effect on the kerf surface roughness. The effects are more pronounced on the mild steel than stainless steel because of the higher viscosity of liquid phase stainless steel. A slower feedrate increases both the HAZ size and the roughness of the kerf surface; therefore, feedrate is chosen as the major control parameter in the cutting system. spark cone images from the workpiece lower surface are analysed. An intense uniform cone was observed when a clean cut was performed. Sparser spark cone images were characteristic of poorer quality cuts where dross attachment disturbs the gas-metal mixture flow. A hierarchically structured environment that integrates a knowledge-based expert system, adaptive process control and pattern recognition techniques for controlling the laser cutting process are developed. Knowledge of the laser cutting process for different materials is organised and encoded into a rule-based system. An adaptive control algorithm based on on-line recursive parameter estimation and on-line control law synthesis was adopted for the highly non-linear cutting process control. Cutting speed was selected as the major control variable. Irradiance emitted from the cut front was used for the feedback signal to this adaptive controller. The irradiance signal feeds the recursive parameter estimator for system identification. Techniques of pattern recognition, which have been well developed in coherent optics, were applied to the cutting quality analysis by characterising the exit spark cone images of the gas assisted laser cutting process. Images from the cutting processes were grabbed, edge enhanced and correlated with a synthetic discriminant function (SDF) filter which was synthesised from reference images known to give good cut quality. The SDF discrimination performance was enhanced by incorporating the Wiener filter into its construction, such that the in-class image and out-of-class image were integrated into a single filter. Results from digital simulations based on these pattern recognition algorithms are also presented.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: C R Chatwin
Keywords: Mechanical engineering, Optics
Date of Award: 1994
Depositing User: Enlighten Team
Unique ID: glathesis:1994-75982
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 17:09
Last Modified: 19 Nov 2019 17:09
URI: http://theses.gla.ac.uk/id/eprint/75982

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