Illumination methods for optical wear detection

Transcription

1 Illumination methods fo optical wea detection 1 J. Zhang, 2 P.P.L.Regtien 1 VIMEC Applied Vision Technology, Coy 43, 5653 LC Eindhoven, The Nethelands jianbo.zhang@gmail.com 2 Faculty Electical Engineeing, Mathematics and Compute Science, Univesity of Twente, PO Box 217, 7500 GA Enschede, The Nethelands p.p.l.egtien@utwente.nl Abstact. The pape pesents some esults of a study on optical wea detection. The focus of the pape is on the illumination, to optimize the contast of the images. Vaious illumination methods ae compaed: bight field vesus dak field illumination, and vaious kind of light souces: lase light, diffuse light and ing-light. Keywods: wea detection, illumination. 1. Intoduction Wea plays an impotant ole in detemining the life span of poducts o machine elements. The lifetime of a component depends on wea. Detection and monitoing of wea ae athe impotant in tibological eseach as well as in industial applications. Some typical examples ae: measuement of dynamics of wea pocesses, engineeing suface inspection, coating failue detection, tool wea monitoing and so on. With wide and extensive use of wea esistant mateials, wea itself is becoming smalle and smalle, in some pecision application, fo instance, down to nanomete dimensions. Due to dynamics and complex natue of a wea pocess, measuement of wea is usually conducted offline, i.e. duing measuement the wea pocess needs to be inteupted and the specimen to be emoved fom the teste peiodically to measue the evolution of wea as a function of time, numbe of cycles, o sliding distance. Reliable online detection and monitoing of wea, in which the wea pocess is not inteupted and the wea envionment (tempeatue, humidity, lubication etc.) is not changed, emains a challenge to tibological eseach as well as to the industy. The main objective of the cuent eseach poject is to do online wea detection and monitoing by an imaging system, the fist element of which is image acquisition. One aspect that is often ovelooked in compute vision is the acquisition of adequate images: a pope illumination should have sufficient contast and be effective in evealing the featues of inteest of the specimen (hee wea phenomena), in ode to identify small suface defects on an othewise textued suface. 2. Measuement setup Figue 1 shows the measuement set-up, consisting of a otay table with a pin-shaped wea intende, and an imaging system which is a combination of a video zoom micoscope and a monochome CCD senso. The pin is mounted on a stiff leve and held stationay wheeas the disk otates. The load applied on the test sample is vaied by emoving o adding weights on the leve. A sevo moto contol with a ea-mounted optical encode ensues accuate indexing. The weaing suface is monitoed continuously by the imaging system. 37

2 Fig. 1. Pictue of the wea testing and monitoing appaatus As an illustation, figue 2 shows a sequence of images taken fom a specimen duing a wea test. They clealy demonstate that suface defects can be obseved by changes in contast and textue. The goal of this pat of the eseach is to find the optimal illumination stategy allowing the detection of small defects. Fig. 2. Sequence of pictues duing a wea pocess, conducted by a steel pin on a otating specimen of aluminium. 3. Results The econstuction of wea paametes fom a two-dimensional adiometic image equies undestanding of how light eflects fom a ough suface. Obviously, a adiometic image of a suface is not identical to a geometic image (height image), as is clealy demonstated in figue 3. The uppe pat of this figue shows the intensity image of a textued suface with a scatch. The ight side pesents the intensity distibution along the line indicated in the left side. The lowe pat of figue 3 shows the topogaphical image of the same suface detail, measued by a white light intefeomete. The left pat is a gey-coded epesentation of this depth image, and the ight pat shows the depth pofile along the line indicated at the left side. Although the pictues have slightly diffeent scales and the lines ae not exactly along the same position, it is clea that both pofiles diffe substantially, showing that an intensity image is not the same as a geometic image. 38

3 Fig. 3. Compaison of adiometic and topogaphic images of the same suface, and the coesponding intensity pofile and height pofile. Vaious models have been investigated to descibe the elation between suface popeties on the one hand and intensity distibution of the images on the othe, fo diffeent light souces and diffeent illumination stategies [2]. In the expeiments a ing light illuminato is applied. Fom the geomety of the setup an illumination model can be deived. Figue 4 shows the geomety and the illumination fo vaious distances h between the ing light illuminato and the suface unde test. Note that fo the lage values of h the egion of inteest is unifomly illuminated. Fig. 4. Geomety of the ing illumination (left) and the coesponding illumination model (ight). 39

4 Assuming a nomally distibuted suface height, a eflection model has been developed, descibing the suface eflection and hence the intensity as seen by the camea. As an example, figue 5 shows adiance cuves along the x-axis fo diffeent suface oughness σ α fo h = 20 mm and = 16 mm. Obviously, oughness σ α has a significant effect on the adiance: lage oughness gives lage adiance in the diection ( θ = 0, φ = 0 ). This is due to the fact that specula eflection is assumed to occu on each suface facet. Fo oughe sufaces, light is scatteed in moe diections and moe light eflects into the diection ( θ = 0, φ = 0 ), wheeas fo smoothe sufaces light ays ae moe eflected to a small ange of specula diections. In addition, ove a specific ange of suface oughness (0.4 < σ α < π/3), unifom eflection is achieved. Fig. 5. Ring-light modelling. Left: suface built up fom andomly oiented patches; ight: calculated adiance (nomalized) along the x-axis viewed into the diection θ = 0, fo diffeent suface oughness. Apat fom bight-field illumination, as in the pevious example, also dak-field illumination has been investigated. In this appoach, the specimen is illuminated fom the side, unde a naow angle. In that case, only a small pat of the light is eflected upwads into the diection of the camea. Howeve, when the suface shows iegulaities, fo instance due to wea, moe light will aive at the camea, so it can be expected that scatches could be detected much bette by using dak-field illumination. Fo this pupose a special dak-field ing-light adapte was mounted on the micoscope (figue 6). Fig. 6. Special ing-light adapte to ceate dak-field illumination. Left: photogaph of the adapte; ight: diection of light ays [3]. The dak-field ing-light souce tansmits light in adial diection ove the full cicumfeence. To enhance contast in a paticula diection, optical masks of vaious angles have been mounted in font of the dak-field illuminato. The idea is to disciminate between scatches unning in diffeent diections and also to disciminate between scatches and the oiginal (linea) textue. 40

5 Figue 7 shows a typical esult, in which fou types of illumination ae compaed. The oiginal suface has a gooved machined stuctue. Two scatches ae applied: one oiented in almost the same diection as the machined gooves, anothe unde an angle of about 45 degees. Fig. 7. Fou types of illumination fo the same egion of inteest. Top left: bight field illumination; top ight: dak-field illumination all aound; down left: dak-field, with 90 degees mask; down ight: same, with 30 degee mask. 4. Discussion Bight field illumination gives good esults fo athe lage suface defects. By pope filteing, the suface aea of the won egion can be detemined athe easily [2, 4]. Smalle defects give lowe contast, hence detection becomes moe difficult o impossible. Dak-field illumination eveals bette contast in this case, as shown in figue 6. In ode to bette disciminate between linea textue and linea defects, an optical mask is applied, that let pass the incident light ove a esticted segment only. Howeve, the naowe this filte, the lowe the aveage intensity. Futhe investigations aim to eplace the optical masks by digital filtes, and to automatically optimize the width and the oientation of the filte. Acknowledgements The authos like to thank A.P de Vies and C.W Cheng fo the design and implementation of the optical system and the special accessoies. Refeences [1] J. Zhang, M. Kosten and P. P. L. Regtien, A vision system fo online wea detection, Poceedings XVII IMEKO Wold Congess, Dubovnik, Coatia, pp , [2] J.Zhang, Detection and monitoing of wea using imaging methods, PhD thesis Univesity of Twente, The Nethelands, Septembe 8, 2006; ISBN [3] Schott optical components, Gemany. [4] J. Zhang, P.P.L. Regtien, M.J. Kosten, Monitoing of dy sliding wea using factal analysis, 10th TC-10 IMEKO Confeence on Technical Diagnostics, 9-10 June, 2005, Budapest, Hungay 41