翻译~~激光束用热成像过程控制切割质量的监控和保证.pdf

Quality monitoring and assurance for laser beam cutting using a thermographic process control H Haferkamp M Goede A von Busse Laser Zentrum Hannover e V Hollerithallee 8 30419 Hannover Germany ABSTRACT This paper presents an optical method of process control based on the thermographic detection of signals from the process zone At the Laser Zentrum Hamiover e V a monitoring system based on a high temperature camera has been developed to enable on line monitoring of the cut quality During the cutting process the temperature field was observed on line using a thermocamera The camera images were analysed afterwards using specific digital image analysis The crucial task of the work is the adequate application of analysing and visualisation techniques to extract significant information from the measured signals and to fmd correlations to the cutting quality Investigations have been carried out on cutting metal sheet materials such as steels and titanium For the investigations different process parameters such as laser power or process gas pressure were varied Influences of the optical set up on the signal were studied Results show that the temperature distribution in the process zone is strongly connected to the cut quality e g dross attachment surface roughness of the cut kerf width of the kerf Observation of the temperature and temperature gradients at the cutting front allow a determination of the resulting cut quality Keywords laser cutting quality monitoring process control thermography image processing 1 INTRODUCTION Laser beam cutting has been well established in many fields of industrial applications and has reached a high grade of productivity and flexibility Currently almost 40 of all laser material processing applications are cutting applications 2 Laser beam cutting of metals is restricted to a narrow range of parameters in order to provide good cut surface quality The interaction between light and material is very sensitive to inner and outer influences Process integrated quality techniques have to be applied to ensure high quality products and a cost effective use of laser beam cutting machines In the past a number of scientific investigations have been carried out to develop an efficient process monitoring and control system for industrial use but at present suitable systems are still not available3 The systems and sensors currently available are only able to solve machine and process specific subproblems or they are not robust enough with respect to industrial requirements A high variety of materials and geometries require a flexible detection unit for process monitoring Quality monitoring systems which enable the operator to immediately identify a decrease in quality during the process rather than in a subsequent quality inspection are still in the stage of development For the development of an on line detection system for machining quality assurance during laser beam cutting there are several basic technical criteria to be considered The process diagnostic system should be highly adaptable to different materials with different thicknesses and geometries as well because the laser beam cutting process shows a high flexibility in geometry and material Besides it is important to be able to perform quality monitoring independent of the processing d irection Considering the high cutting speeds achievable a real time detection of the cutting quality has to be possible Moreover the beam quality and propagation should not be influenced by the diagnostic system For an increase of productivity and process reliability there must be a reliability of the information on the machining quality There are some physically observable parameters which can be used for the observation of the cutting quality4 5 6 light emission from the cutting front sound emission from the process zone temperatures and temperature distribution laser light penetrating through the cut kerf and direction and intensity of the shower of spark below the workpiece In this paper investigations are described which have been carried out in order to examine the possibility to extract important information of the cutting quality using a high temperature camera and the heat radiation emitted from the process zone Part of theEuropto Conference on Optical Measurement Systems for Industrial Inspection Munich Germany June 1999 SPIE Vol 3824 0277 786X 99 1 0 00 383 2 EXPERIMENTAL INVESTIGATIONS The system is based on measuring the temperature disthbution in the process zone during laser beam treatment by detecting the emitted heat radiation co axially to the laser beam using a high temperature camera From the camera signals spatially highly resolved and detailed information of the cutting process can be obtained The set up of the thermographic system is shown in Figure 1 camera 1data cquisitioj I mirror fiber connection cuthng nozzle workpiece Figure1 Set up of the thermographic system The laser used for the investigations performed was a lamp pumped solid state laser Rofin Sinar CWO2O with a maximum output power of 2000 W The laser beam is guided to the working unit by an optical fiber with a diameter of 600 j tm Itis collimated by a collimating lens and focused on the workpiece by a focusing lens focal lengths 80 mm after being guided by a 90 tilted mirror The mirror is selectively coated for reflection of radiation in a narrow band around the Nd YAG laser working wavelength 1064 mu incident at an angle of 45 to the mirror s surface Heat radiation is emitted from the cutting front and the process zone during the cutting process which passes through the cutting nozzle and is then collimated by the focusing lens This radiation is transmitted through the 90 tilted mirror and further reaches the high temperature camera The camera operates at a standard video frequency of 50 Hz for half images and its CCD chip is sensitive to radiation within the range of 0 9 1 3tm The image size is 600 x 800 pixels The images are stored digitally by a personal computer PC and then analysed off lirie7 Numerically controlled x y translation stages are used for the workpiece handling Consecutive measurements on 0 5 mm and 1 0 mm CrNi steel with nitrogen and oxygen as assist gases as well as on 1 0 mm Ti with argon as assist gas have been performed The cutting speed and the pressure of the assist gas were changed in order to vary the achievable machining quality Starting from the evaluation of the emitted heat radiation the following points should be considered changes of the radiation source and the beam positioning control of the power density and the resulting temperature distribution on the surface of the workpiece control of quality determining parameters roughness dross attachment control of the width of the cutting ken control of pitching and complete cutting and the extension of the heat affected zone The ranges for these variations in the process parameters are shown inTable1 The focus was positioned on the surface of the material 384 collimating lens heat Ilber 6 08768 42 Vb Table 1 Processparametersfor the experimental investigations CrNimaterial thickness0 5 mm 1 0 mm gas IgaspressureN2 I0 8 1 6 MPa 02 0 4 1 1 MPa cutting speed 1 500 6000mm s Timaterial thickness1 0 mm gas IgaspressureAr I0 7 1 7 MPa cutting speed 3000 7000nim s With commonly used methods the cutting quality was examined and evaluated according to DIN 2320 part 1 to 5 For the analysis of the thermographic images it is necessary to translate the temperature information into a corresponding grey scale value or colour information A calibration of the system is necessary For the calibration of the detection unit a measuring device was realised based on a tungsten lamp as radiator at a defined temperature By varying the current in the tungsten band it is heated up to a particular temperature with characteristic radiation properties In the course of the analysis of the temperature images two methods were used on one hand characteristics were extracted independent of the temporal course during processing with a particular set of parameters Especially characteristics based on spatially highly resolved and detailed information were considered On the other hand characteristics of the temporal course of the signal were studied Figure 2 shows a typical temperature field and illustrates the investigated characteristics Table 2 and Figure 3 demonstrate the characteristics of the temporal course of the signal S C original image characteristic II size of colour fields 6 09445 24 Vb Figure2 Characteristics ofthe temperature fields ci ci c C ci E colour value at cutting front characteristic lV relation colour value center lateral characteristic V relation of colour field sizes characteristic VI relation of colour mean values characteristic Ill characteristic I mean value of colours 100 90 80 70 60 Ill dGV d bc i I 40 L I dGV dt Table 2 Characteristics of the temporal course of the temverature fields Isignal rise at the beginning of the process IIstandard deviation ofthe signal III slope of the signal Ivfmal level of the signal 0 0 00 20 40 60 81 01 2 time s 4 Figure 3 Characteristic temporal course of the mean grey value ofthe temperaturefield images 385 3 RESULTS 3 1 Correlation of image characteristics to process parameters The investigations performed show that when different process parameters were varied particular characteristics showed significant correlation to the cut quality obtained The characteristic features prove to be a sufficiently precisemethod for controlling cut quality depending on cutting speed and gas pressure when laser fusion cutting chrome nickel steel with thickness of 0 5 ni m and 1 0 mm It is obvious that splitting the temperature image into the three colour channels red green and blue as well as the extraction of local image information due to the spatially highly resolved measurement techniques are advantageous since significant changes possibly are not detectable with global image parameters orintegral signals When varying cutting speed for example judgement of cutting quality is possible by determmation of the sizeof the blue colour field Control of the cutting process can be based on quotation of a threshold value as lower limit for theblue value In Figure 4 the size of the different colour fields and the colour value at the cutting front is shown in dependence on cutting speed seperating 0 5 mm CrNi steel A large blue colour field as well as a highvalue for the colour blue at the cutting front correspond with a good cut quality Figure 4 Correlation ofimage characteristics to cut quality When oxygen cutting of CrNi steel using the laser cut quality can be correlated to different image parameters when varying cutting speed Process control is possible by declaration of an intervalfor optimum colour values When varying process gas pressure correlations between image characteristicsand machining quality have only conditionally been found so that the investigated characteristics so far are not sufficient for a process control When varying the cutting speed separating CrNi steel and Ti the temporal developmentof the mean grey value of the temperature field images was studied Results show that the signal has to be followed over a certain time interval so that the standard deviation of the signal in consideration of the slope of the signal canbe determined This standard deviation is directly correlated to the roughness of the cut This is shown in Figure5 The other characteristics under investigation have not shown a significant correlation to machining quality 14 12 6 45005000 55006000 cutting speed mm mini cuttingspeed mm mini Figure5 Standard deviation of signal and corresponding roughness of the cut 386 0 5 mm CrNi steel size of colour fields dependent on cutting speed 0 5 mm CrNi steel colour values at cutting front dependent on cutting speed 300 150 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 cutting speed mm mm x 20003000 400050006000 7000 cutting speed mm mm 0 cc 0 0 cc C cc I 4000 5000 6000 E In conclusion several parameters of the thermographic images can be found that are directly correlated to the machining quality The temporal development of the signal contains important information 3 2 Correlation of faulty machining parameters to temperature images Iii the fundamental studies on process parameters and characteristics constant machining parameters and consequently constant cut results have been correlated to the respective temperature images In further studies faulty machining parameters as maladjustment of the laser beam axis to the gas jet inhomogeneous beam profile different surface conditions 0 1 the workpiece faulty distance nozzle workpiece and faulty focal position have been chosen in order to determine their influence on the temperature images If the laser beam axis is not centred with respect to the gas jet one sided rough cut edges with roughness values Rz up to 1 00 m and increased dross attachment occur In order to correlate this phenomena to the temperature images the laser beam axis was maladjusted to the gas jet with a distance of about 1 0 of the nozzle diameter Results show that a comparing inspection of the two edges is suitable for detection of asymmetries within the machining process Figure 6 illustrates this During the complete sequence the mean grey value of the left edge upper part of the image is higher than the one of the right edge lower part of the image In accordance the Rz value of the left cut edge is always higher than the one of the right cut edge Maladjustmentof laser beam axis to gas jet In another series of investigations the beam profile was changed by maladjustment of the tilted mirror Figure 7 shows that an inhomogeneous beam profile results in an asymmetric temperature image Changes of the beam source the beam positioning system and the optical components affect the power density distribution in the beam and lead to errors in the imaging of the beam on the surface of the workpiece which can be detected by the thermographic images during the process 387 Profile curve of cuttii Figure 6 Detection offaulty machiningparameters maladjustmentofbeam axis mean grey value of image halfs within a image sequence Inhomogeneous power density distribution homogeneous beam profile 6 09447 24 Vb thermographic images during the process 388 Figure 7 Detection offaulty machining inhomogeneousbeam profile When materials are processed using laser radiation the result of the machining process depends strongly on the laser beam power which is absorbed by the workpiece The absorption coefficient is affected among other things by the surface condition soiling coating Different absorption conditions call for different optimum process parameters which have to be adjusted to the existing conditions The surface of the workpiece was dyed black at different positions and the change in the thermographic images was observed when passing these positions The left part of Figure 8 shows how the different absorption conditions affect the thermographic detection The characteristic kidney like shape is maintained but the difference image shows a resulting higher heat influenced zone and different temperature values at the cutting front For the realisation of a process control this means that for the difference image a certain interval has to be defmed in which the corresponding values have to be Figure 8 Detection offaulz y machining differentsurface condition and different focal position inhomogeneous beam profile changed surface condition of workpiece higher absorption due to black dying changed focal position of laser beam focus on workpiece surface optimum parameters without black dying RflQ44L74Vb largerHAZ focus position above workpiece surface F no separation LZH The efficiency of the expulsion of the molten material strongly depends on the distance of the nozzle to the surface of the workpiece and the focal position of the laser beam In order to investigate the effect of changes in the distance nozzle workpiece in a series of studies the workpiece was insufficiently mounted so that the high gas pressure lead to bending of the sheet and in consequence to a higher distance ofthe nozzle to the surface In Figure 8 on the right side the influence of this phenomena on the corresponding temperature image is shown A characteristic is the change in shape of the temperature field from the kidney like shape to a more circular contour In addition higher colour values indicate unsuccessful cutting Control of the focal position when realising a complete process control system is possible by control ofthe shape ofthe temperature field The cutting width should be as small as possible and knowledge of its size is important when cutting pieces with narrow tolerance Figure 9 shows that by detennining the distance of the peaks of the half moon or kidney like shaped temperature field determination of the cutting width is possible There is a linear correlation between the distance in the image in pixels and the real cutting width in ni The accuracy of this correlation depends on the resolution of the imaging and the sharpness ofthe contour When cutting a closed contour it is indispensable to drill an initial hole A spatially controlled measurement of the pitching process has a great importance in addition a reduction of time for this process is directly connected to the knowledge of the point when the workpiece has been separated successfully and the cutting process could start In a series of investigations the pitching process was observed with the thermographic system The size of the hole is given directly by the image and the detection system can determine the successful pitching In the first image the centre of the detected circular temperature distribution is determined and followed over the sequence A decrease of the grey value at this point indicates separation of the workpiece and creation of the keyhole left diagram in Fi