Acoustic Process Control for Laser Material Processing

Transcription

1 Acoustic Process Control for Lser Mteril Processing Opticl microphone s novel er for industril mnufcturing Blthsr Fischer, Wolfgng Rohringer, Nils Pnzer nd Sestin Hecker Listening to n industril process cn revel relevnt informtion. As complementry pproch to opticl process surveillnce, monitoring the coustic emissions is promising lterntive. Yet, coustic detectors hve not een extensively estlished in process control, one reson eing their limiterequency ndwidth. Overcoming the disdvntges of existing microphones, novel lser-sed coustic sensor opens new possiilities in industril coustic process monitoring. In order to chieve cost-effective high-qulity production process, utomtion with miniml down time is required. Mnufcturing close to process tolernces cn drsticlly increse yield, ut usully requires close wtch on the production qulity. This is most elegntly done vi in-line, in-process monitoring ecuse potentil errors re immeditely recognized nd corresponding mesures cn e tken through either humn opertor or, in the cse of closed loop, y the mchine itself. Process monitoring is of prticulr importnce when smll chnge in process prmeters cn signify considerle loss of qulity. This holds for mny production processes ut is especilly true for lser mteril processing, such s welding, dicing, or dditive mnufcturing. Lser process monitoring cn e chieved using photodiodes, cmers, spectrometers, pyrometers or chrge sensors [1, 2]. Rdition-emitting res re the plsm plume, the melting zone nd the reflected lser em. All of them deliver diverse process informtion, distinguishle y their different wvelength. Sem trcking, for exmple, is sed on the projection of Fig. 1 Sensor hed connected to n opticl fier: centerpiece of the opticl microphone is the miniture Fry-Pérot etlon shown on the right side of the picture. Sound enters the etlon through the ir gp etween the mirror surfces. Smll size nd roust dimensioning of the glss elements prevent the influence of mechnicl virtions to e elow the sensor self-noise throughout the full mesurement rnge. structured light nd the detection of the reflecterction. Another exmple for the use of n uxiliry light source is opticl coherence tomogrphy (OCT), n interferometric technique tht is using proe em to scn the surfce, which cn generte in-line informtion out the sem qulity or the keyhole depth [3]. Recently, opticl coherence tomogrphy hs een successfully used to determine the penetrtion depth correlting it with the mesured keyhole depth [4]. Comining different monitoring methods will generte much more preferle nd comprehensive picture of the process. An lterntive method to chrcterize nd monitor n industril process is to mesure its sound nd ultrsound emissions. Acoustic process control systems employ contcting nd non-contcting cousticl sensors. Contcting systems mesure structure-orne process emissions. They work well for mny pplictions ut lwys require physicl contct with the workpiece. In n utomted production environment, this is often not possile. In contrst, stte-of-the-rt noncontcting coustic systems mke use of irorne sound nd ultrsound process emissions. When employing conventionl cpcitive microphones, these systems struggle with interfer- Compny XARION Lser Acoustics Vienn, Austri Meet us t Int. Lser Symposium & Int. Symposium Tilored Joining XARION Lser Acoustics GmH is young high-tech compny, which develops nd mrkets novel lser-coustic sensor. The dvntges of the trnsducer refrining from ny mechniclly moving prts include liner frequency response nd rod ultrsounrequency detection ndwidth in oth ir (1 MHz) nd liquids (25 MHz). The compny is sed in Vienn nd employs 20 people. The key mrkets comprise coustic metrology, industril process control, non-destructive mteril testing nd medicl imging WILEY-VCH Verlg GmH & Co. KGA, Weinheim Lser Technik Journl 5/

2 ence from surrounding industril noise due to the nrrow mesuring rnge of microphones, typiclly limited to 100 khz ndwidth. Experiments hve shown tht high-frequency irorne ultrsound my e used to monitor keyhole welding [2]. In this pproch, piezoelectric trnsducer mesures the ultrsound emissions during the process. Results show tht the emitted ultrsound chnges t the trnsition point from therml conduction welding to keyhole welding [2]. However, in order to chieve the necessry sensitivity with ir-coupled piezoelectric receivers, they need to e designed in highly resonnt wy, leding to so-clled cod (prolongtion of signl due to ringing, mechnicl resonnce) nd significnt reduction of the frequency ndwidth. A sustntil prt of the process informtion is lost due to the limited ndwidth. Furthermore, cpcitive nd piezoelectric trnsducers re susceptile to electromgnetic interference. These limittions my serve s possile explntion s to why coustic process monitoring is not yet in wide use in industril production. The solution would e n ir-coupled (nd non-contct) coustic trnsducer, which is not susceptile to loud ckground noise. The ttriutes of this coustic process monitoring technology should e: In-line process monitoring without disturing or influencing the process Esy implementtion on existing plnts lser mirror Fig. 2 Principle of opertion of the opticl microphone. The ultrsonic signl is detected opticlly y the chnge of the refrctive index within Fry-Pérot etlon. Sttisticl roustness of the mesurements. No influence y noise from ckground or neighoring mchines, or from electromgnetic interference Resistnce to dust, dirt nluids Low cross-sensitivity; prmeterto-e-monitored should e uniquely identifile. A len mount of dt in order to reduce the complexity of the control softwre. Opticl microphone senses ultrsound process emission The principle of the opticl microphone is to directly ssess chnges in the density of the opticl medium. The opertionl principle is illustrted in Fig 2. The core of the microphone cpsule consists of Fry-Pérot cvity, sound mirror n opticl interferometer mde of two semi-trnsmissive mirrors, rrnged t distnce mtching multiple of the lser s hlf wvelength. This leds to constructive interference of the trnsmitted lser em. Smll chnges in the density of the opticl medium cuse chnges in the opticl index of refrction nd, therefore, in the lser s propgtion speed nd in its wvelength. Thus, the (fixed) distnce etween the two mirrors will not stisfy the condition for constructive interference nymore, resulting in chnge in the trnsmitted nd the reflected lser intensity. This is mesured with photodiode. The use of this multipss interferometer enles oth the high sensitivity nd the smll size of the opticl microphone. If used in coustic process monitoring, the system consists of two min components: Amplitude (.u.) Amplitude (.u.) Time (s) Frequency (khz) Time (s) Frequency (khz) Fig. 3 Acoustic emissions from lser welding. Spectrogrm of coustic emissions during lser welding process. Crossjet ckground noise covers the wnted signl for frequencies up to 150 khz (). Zooming into the frequency rnge from 150 khz to 550 khz revels the signture of the lser welding process nd shows swtooth pttern, chrcteristic for the monitored lser welding depth (). 22 Lser Technik Journl 5/ WILEY-VCH Verlg GmH & Co. KGA, Weinheim

3 the coustic detection system, including the ll-opticl sensor hed nd the driver unit comprising lser nd detector the nlog-to-digitl converter supporting high-speed cquisition, the softwre nd dt mngement system, enling in-line process control with the mesured dt When listening to industril process emissions, the detection of high ultrsound eyond 100 khz ecomes crucil. Given the fct tht the humn er only detects frequencies up to pproximtely 17 khz, nd mny technicl ultrsound devices, such s utomotive prking ids, operte t few tens of khz, irorne coustic frequencies ove 100 khz re somewht exotic fielor tody s stte of the rt. Nevertheless, there re four importnt resons why the ultr-high frequency rnge is of relevnce to coustic process monitoring. First, ecuse significnt process informtion is hidden in the very high ultrsounrequencies. Second, disturing ckground noise usully is of prticulrly high intensity in the udile coustic frequency rnge or the ner ultrsound rnge. In contrst, the spce eyond 100 khz is usully comprtively quiet. Third, the sorption for high ultrsound frequency in ir is sustntil nd mounts to pprox. 15 db per 10 cm in ir (t 1 MHz). This mens tht simple sptil isoltion of mchinery cn e used rther thn coustic insultion. This llows, for exmple, mchine inside hll contining mny mchines without the neeor coustic insultion from the other mchines. Finlly, reverertion time is function of frequency. Up to 100 khz, it is virtully impossile to relize reverertion times shorter thn 10 ms except in perfect free-field conditions, which is never the cse in industril environments with mny cousticlly reflective surfces. As mtter of fct, typicl reverertion times for udio frequencies my mount to severl hundreds of milliseconds. For the ccurte temporl resolution of n industril process usully consisting of rpid sequence of coustic events, it is impertive to monitor ultr-high coustic frequencies. The physicl principle of the opticl microphone leds to outstnding properties compred to cpcitive microphone or piezoelectric trnsducer. Without ny moving or deformle mechnicl prts (such s memrnes or deformle crystls) there is no limittion induced y the resonnce properties of mss-spring system. This results in wide coustic detection ndwidth of the opticl microphone spnning from 5 Hz to 1 MHz in ir, nd up to 25 MHz in liquids. In ddition, ecuse the sensor hed is ll-opticl, strong electromgnetic rdition cnnot influence the detection signl. Emitted ultrsound revels informtion out lser weld penetrtion depth The opticl microphone hs een successfully tested in numer of process control settings, such s dynmic ftigue testing, drilling nd mchining, product inspection, ut specificlly for vrious lser mnufcturing processes such s dicing, dditive mnufcturing, nd welding. All of these processes were shown to feture rodnd ultrsound emissions. The conducted tests show promising results with regrds to the extrction of process informtion from these high-frequency emissions. Fig. 3 shows typicl dtset from metl sheet lser welding process. The opticl microphone records the coustic signl, nd rel-time dt cquisition system processes the dt strem nd performs short-time Fourier trnsform (STFT) of recorded signl segments to construct spectrogrm. It revels common sitution during lser welding: in the udio-coustic rnge nor ultrsounrequencies elow 200 khz, the most prominent feture is the coustic emission from the crossjet, strong ir lst tht protects the weld- weld with crcks weld without crcks lser fulty weld good weld opticl microphone Acoustic signl Time Time Fig. 4 Crck detection during glss welding. ) Mesurement setup. An ultrshort-pulse lser scns over the workpiece to estlish weld. Vrying process prmeters (pulse energy, focus position, etc.) my led to crcks in the vicinity of the weld. ) Upper pnel: imge of workpiece contining good nulty welds. Lower left pnel: in-process coustic emission from crcks during welding. Lower right pnel: coustic emissions from good weld, scled to the sme mplitude rnge WILEY-VCH Verlg GmH & Co. KGA, Weinheim Lser Technik Journl 5/

4 = lser focus dimeter glss opticl microphone ing optics. While the lser welding process does contriute signl in this frequency rnge, its spectrum extends up to 800 khz. The opticl microphone cn trget these high-frequency emissions for process monitoring (Fig. 3). This is in contrst to coustic monitoring of the lower-frequency regime, where the superposition of the signl with crossjet noise mkes it hrd to extrct the detils of the process informtion. Fig. 3 lso demonstrtes the correltion etween process prmeters nd the coustic emission. During the depicted process, the welding penetrtion depth hs een vried. The signl mplitude recorded with the opticl microphone is proportionl to the penetrtion depth, feturing sw-toothshped time dependence, which reproduces the chosen modultion of the welding lser power. In n in-line setting, this dt cn e used to implement feedck nd regulte process prmeters in rel time. High-frequency ultrsounor precision monitoring of glss welding Acoustic signl Lser z position (mm) Fig. 5 Monitoring of lser focus position during glss welding. ) Principle. Vrying xil displcement of the processing lser during lterl scnning leds to chnge of the spot size nd therefore the intensity t the welding level. ) Mesured coustic signl. The mplitude of the emitted coustic signl is in excellent pproximtion qudrtic function of the intensity t the welding level, where the lser light is sored. Fitting the signl integrted in the high-frequency region, where coustic ckground is mostly sent, llows the determintion of the focus position with su-micron ccurcy. One of the more recent pplictions of lser welding is its use for glss joining. Glss is chllenging mteril due to its low therml conductivity. Uneven heting during lser processing my esily cuse stress-induced crcks. Tight control of process prmeters is therefore crucil. In n experimentl setup, the opticl microphone hs een successfully pplied to detection of such stress-induced crcks cused y fulty process prmeters. A schemtic of the mesurement setup is shown in Fig. 4. During the welding process, the lser ws scnned over the work piece to generte severl stright, prllel welds evenly distriuted over the smple surfce. Vrying process prmeters long the pth of the detection lser cused res with fulty welds exhiiting crcks. Listening with the opticl microphone revels tht these crcks re ccompnied y short ursts of ultrsound emission (Fig. 4). Correlting mesurement time with the position of the lser spot enles n in-line detection nd locliztion of these crcks. It ws found tht some crcks were formed severl milliseconds fter the welding process, which might e due to the cooling of the specimen fter the lser em hs pssed over it. In ddition, the emitterequency components nd the signl durtion were oserved to e correlted with the crck size. It ws found tht these differences minly mnifest for ultrsounrequenices eyond 100 khz. Su-micron monitoring of processing lser focl position Even in well-ligned em shping systems for processing lsers, therml effects nd mechnicl hysteresis my cuse drifts in the focl plne of the lser spot. During processing, such drifts cuse vrition in the size of the lser spot t the position of the work piece nd therefore chnges in the deposited intensity. This effect is especilly severe for high-precision processes utilizing smll focl spot dimeters, such s lser dicing or glss welding processes, where slight chnges of energy deposition my cuse defects s discussed ove. Specificlly, devitions of 10 µm cn led to crcks. Hence, checking nd redjusting focus t regulr intervls is required. Often, this is still ccomplished y vrying the focus position on test piece nd visul inspection. Listening to coustic emissions provides n lterntive: using the circumstnce tht the mplitude of the coustic signl emitted y the process is proportionl to the intensity deposited t the smple surfce llows to keep trck of the lser s focl plne. With conventionl microphones, however, this is extremely chllenging: for high-precision lser processing, usully ultrshort-pulse lsers with repetition rtes of up to 500 khz re employed. These lsers cuse short, rodnd coustic signls, with most of the energy distriuted over frequencies eyond conventionl microphone s detection rnge. The opticl microphone, on the other hnd, cn use these emissions to determine focus with surprisingly high ccurcy. Fig. 5 shows mesurement in glss welding setup, where the focl spot hs een shifted during processing, nd coustic emissions hve een recorded. Mximum signl corresponds to correct focusing. Using the known velocity with which the focus is displced, one cn correlte the reltive focl shift with mesurement time, resulting in determintion of the correct focl position. A su-µm precision hs een experimentlly demonstrted. Since this process cn e utomted, the opticl microphone cn help in reducing mchine downtimes, nd expensive rejects cn e minimized. A novel er for process monitoring These exmples provide smll suset of the opticl microphone s pplictions for industril process control. They should demonstrte tht coustic process monitoring using high-frequency irorne ultrsound is potent technique, complementry to estlished 24 Lser Technik Journl 5/ WILEY-VCH Verlg GmH & Co. KGA, Weinheim

5 ville methods nd well suiteor evluting nd controlling lser mteril processing. DOI: /ltj [1] D. Y. You, X. D. Go, S. Ktym: Review of lser welding monitoring, Science nd Technology of Welding nd Joining 19 (2013), 3, [2] M. Bstuck et l.: Üerwchung und Qulitätssicherung des Lserschweiß prozesses ultrhochfester Krosseriestähle durch integrierte ZfP-Verfhren; DGZfP-Jhrestgung [3] N. D. Dupriez, C. Truckenrodt: OCT for Efficient High Qulity Lser Welding. Lser Tech. J. 13 (2016) 3, [4] J. J. Blecher, C. M. Glrith, C. vn Vlck, T. A. Plmer, J. M. Frser, P. J. L.Wester et l.: Rel time monitoring of lser em welding keyhole depth y lser interferometry, Science nd Technology of Welding nd Joining 19 (2014) 7, Authors Blthsr Fischer ws orn in Switzerlnd, where he studied physics. He moved to Vienn in 2001 to complete Tonmeister degree t the University of Music. He received his PhD in photonics from Vienn University of Technology, efore joining Philips Sound Solutions s R&D memer. In 2012, Blthsr founded Xrion Lser Acoustics, sed on his invention, the memrne-free opticl microphone. In the course of his creer he hs won severl wrds, including the Fehrer Prize nd the Leiinger Innovtion Awrd. Wolfgng Rohringer studied physics t TU Wien. After receiving his PhD in 2014 from the Institute of Atomic nd Sutomic Physics, conducting reserch on proing ultrcold quntum gses with integrteier optics, he joined coopertion project etween Xrion Lser Acoustics nd the Medicl University of Vienn s PostDoc. The project successfully demonstrted photocoustic microscopy with the opticl microphone. Since 2016, he works s reserch nd development engineer t Xrion. Nils Pnzer studied mechnicl engineering nd mngement t TU München, finishing the Diplom in the field of production technology. Since 2015, he is studying medicine t Medicl University of Vienn. He supports Xrion on projectcontrcted sis. Sestin Hecker studied medicl engineering t University of Stuttgrt with n emphsis on opticl systems nd micro system technology. Since 2017 he is doctorl cndidte t Grdute School of Excellence dvnced Mnufcturing Engineering (GSME), doing reserch on methods of process monitoring in lser micro mchining. Dr. Blthsr Fischer, XARION Lser Acoustics GmH, Ghegstrße 3, A-1030 Wien, Phone / , Fx / , WILEY-VCH Verlg GmH & Co. KGA, Weinheim Lser Technik Journl 5/