Machine Vision based Micro-crack Inspection in Thin-film Solar Cell Panel

Transcription

1 Sensors & Transducers Vol. 179 ssue 9 Sepember 2014 pp Sensors & Transducers 2014 by FSA Publshng S. L. hp:// Machne Vson based Mcro-crack nspecon n Thn-flm Solar Cell Panel Zhang Ynong Pu Janao Fang Janjun College of Auomaon Bejng Unon Unversy Bejng Chna Tel.: fax: E-mal: zdhynong@buu.edu.cn zdhjanao@buu.edu.cn Receved: 26 Aprl 2014 /Acceped: 29 Augus 2014 /Publshed: 30 Sepember 2014 Absrac: Thn flm solar cell consss of varous layers so he surface of solar cell shows heerogeneous exures. Because of hs propery he vsual nspecon of mcro-crack s very dffcul. n hs paper we propose he machne vson-based mcro-crack deecon scheme for hn flm solar cell panel. n he proposed mehod he crack edge deecon s based on he applcaon of dagonal-kernel and cross-kernel n parallel. Expermenal resuls show ha he proposed mehod has beer performance of mcro-crack deecon han convenonal ansoropc model based mehods on a cross-kernel. Copyrgh 2014 FSA Publshng S. L. Keywords: Edge deecon Dagonal-kernel Cross-kernel Heerogeneously exured Solar cell Mcro-crack nspecon. 1. nroducon Snce varous assembly lnes became auomac he machne vson sysems have rapdly spread o he semconducor dsplay meal and seel ndusres and have been used successfully n varous felds. The man objecve of he machne vson sysem used n solar cell manufacurng s o deec he defecs on surface of solar cell panels. n hs paper we propose he mcro-crack deecon scheme for hn flm solar cell panels. The proposed mehod s o apply he edge dffuson model based on a dagonalkernel and convenonal cross-kernel n parallel. The edge dffuson model was frs nroduced by Peron-Malk n mage processng for edge deecon and scale-space descrpon [1]. has been wdely used as an adapve edge preservng smoohng echnque for edge deecon mage resoraon mage smoohng mage segmenaon and exure segmenaon. The radonal machne vson defec deecon algorhms based on spaal doman have been appled o mber nspecon [3] carpe nspecon [4] and meal nspecon [5] and based on frequency doman have been appled o fabrc [6] wafer [7] and seel qualy nspecon [8]. The dsadvanage of hese mehods s ha hey can be appled only f he surface for nspecon s unform or repevely paerned. However he hn flm solar cell consss of varous crysals so he surface of hn flm solar cell has heerogeneous exures. Therefore he convenonal algorhms canno be appled o hs problem. Tsa e al. nroduced he edge dffuson model based defec deecon scheme n whch edge dffuson model was based on cross-kernel [2]. n hs paper we appled a dagonal kernel n parallel wh Tsa e al. mehod [2]. shows beer performance for defec deecon. Ths paper s organzed as follows. Secon 2 overvews he mcro-crack properes n hn flm solar cell panel. Secon 3 overvews he convenonal and proposed edge dffuson model. Secon 4 shows hp:// 157

2 Sensors & Transducers Vol. 179 ssue 9 Sepember 2014 pp he expermenal resuls from hn flm solar cell surface mages. The performance comparson wh he Tsa e al. mehod s also dscussed. Secon 5 gves he concluson of our research. 2. Mcro-Crack Properes n Thn Flm Solar Cell Panel 2.1. Mcro-crack Properes n Thn Flm Solar Cell Panel mages The mcro-crack n hn flm solar cell panels has wo man feaures: low gray-level and hgh graden magnude [2]. Fg. 1 dsplays a paral solar cell panel mage ha conans a dagonal mcro-crack on he cener of he mage. f he wdh of he crack s wde hen he gray-value of he pxels s low. Generally n he semconducor and dsplay ndusres here are wo defec deecon schemes of he auomaed vsual nspecon sysem. The frs mehod s o use he mage subracon and he second mehod s o use he sascal properes n RO (regon of neres). n he former he reference and es mages are subraced and he dfference area s deeced whle n he laer he dfference beween he reference and es mages s descrbed by sascal model. Fg. 2. Defec deecon scheme by sascal Properes n RO. Fg. 3. Defec deecon scheme by mage subracon. Fg. 1. Mcro-crack n hn flm solar cell The Lmaons of Convenonal Algorhms RO for nspecon s se up usng reference mage and he defec s deeced by sascal properes of he RO n es mage (e.g. mean sandard devaon ec.). These wo mehods requre ha brghness of RO n he es and reference mage should be unform. f he varaon of gray-value of RO on es mages for each producon s large hen he abovemenoned approaches are no avalable. Fg. 2 shows he defec deecon scheme by sascal properes n RO and Fg. 3 shows ha he defec deecon scheme by mage subracon. As can be seen from Fg. 1 he surface of hn flm solar cell panel s very rregular and vares wh each produc. Therefore deecon of he mcro-crack on hese wafers s a very dffcul ssue. To solve hs problem Tsa e al. nroduced he edge dffuson model based defec deecon scheme ha was ansoropc model based on cross-kernel [2]. 3. Edge Dffuson Model 3.1. Mcro-crack Properes n Thn Flm Solar Cell Panel mages Edge dffuson model was frs nroduced by Perona-Malk n mage processng for scale-space descrpon and edge deecon. has been used as an adapve edge-preservng smoohng process for 158

3 Sensors & Transducers Vol. 179 ssue 9 Sepember 2014 pp exure segmenaon and mage segmenaon. s dffuson fler usng he pxel nformaon from cross-kernel [1]. n he edge dffuson model (1) denoes he gray-level a coordnaes of a dgal mage a eraon. The cross-kernel represen he gradens of four neghbors n norh souh eas and wes drecons respecvely.e. 4 ( ) ( ) ( ) / 4 1 x y + c x y x y + (1) 1 Edge dffuson model uses he cross-kernel n Eq. (2) and smoohes he neghbors n he norh souh eas and wes drecon pxels. However f we use he graden magnude drecly he edge wll be smoohed and herefore we canno preserve he edge. Hence we need he dffuson coeffcens y + 1) ( x + 1 (2) c g( ) (3) 3.2. Tsa Chan and Chao Dffuson Model The mcro-crack n hn flm solar cell panels has boh low gray-level and hgh graden magnude n mage. The Tsa e al. nroduced he edge dffuson based defec deecon scheme ha was ansoropc model based on cross-kernel [2]. The proposed mehod smoohs he mcro-crack and preserves he pxel-value of all pxels n he faulless regon. Afer hs process he subracon beween dffused mage and orgnal mage wll sgnfcanly nensfy he mcro-crack n dfference mage. Afer ha he mcro-crack n dfference mage can be deeced. The dffuson coeffcens funcon of he Tsa e al. dffuson model for mcro-crack deecon s gven by 1 2 ( ) 1 K g f + f ( ) (4) x y and f s gven as f 0 / 255 (5) where f s he normalzed gray-level of an orgnal mage wh 8-b dsplay.e. and K s he regularzaon parameer. The dfference mage beween orgnal mage and dffused mage s hen defned as 0 T ( (6) n order o segmen mcro-crack defecs n he dfference mage we use he smple sascal conrol lm o se up he hreshold. s gven by 0 H 255 f ( > μ oherwse + C σ (7) where μ and σ are he mean and sandard devaon of he dfference mage and C s a conrol consan. n order o remove he nosy pxels n he bnary mage he nose-removal process proceeds as follows. f H + H + H ( x or H + H + H y + 1) 0 or H y + 1) + H + H ( x or H + H + H ( x + 1 y + 1) 0 hen le H 0( reanhe defec pon) else le H 255( removehe nosy pon) end (8) Afer pos-processng some pxels of he mcrocrack are also erased. n order o refll he dsconneced mcro-crack regon a fllng process s carred ou rgh afer he nose-remove process. f H 0 and H ( x + y + j) 0 hen le H ( x + y + ) 0 for any ( x + y + ) N( end 3.3. Proposed Edge Dffuson Model (9) The convenonal dffuson model uses four neghbors n he norh souh eas and wes. Therefore reflecs he four neghbors pxel nformaon well bu canno reflec he dagonal pxel nformaon [2]. The mcro-crack n hn flm solar cell can occur n all drecons. Therefore he convenonal mehod 159

4 Sensors & Transducers Vol. 179 ssue 9 Sepember 2014 pp has a drawback ha canno reflec he dagonal pxel nformaon. n hs paper we propose o apply he edge dffuson based on a dagonal-kernel n parallel wh convenonal cross-kernel. The dfference mage beween orgnal and dffused mages of he norh souh eas and wes drecons s defned n Eq. (10) as dagonal kernel o Tsa-Chang-Chao mehod. shows beer performance for defec deecon. Expermenal resuls have shown ha he proposed edge dffuson model can be well appled o he mcro-crack deecon of hn flm solar cell panels. NSWE 0( ( (10) NSWE n Eq. (11) he dfference mage beween orgnal and dffused mages of he dagonal drecons s hen defned as dag ( dag 0 T (11) T And he dagonal-kernel represen he gradens of four neghbors n norheas souheas norhwes and souhwes drecons respecvely.e. (a) ( x + 1 y + 1) (12) ( x + 1 y + 1) The reconsrucon mage beween cross-kernel dffused mage and dagonal-kernel dffused mage s hen defned as + ( NSWE dag (13) 4. Expermenal Resuls n hs secon we presen he expermenal resuls. We use he sensed mage under fron LED llumnaon and evaluae he proposed algorhm performance. Fg. 4 shows he resul of expermen. We can check he edge pxels ha are a comparson beween he resul of convenonal mehod and he resul of proposed mehod. 5. Conclusons We dscuss edge dffuson model for mcro-crack defec deecon n hn flm solar cell panel. The ansoropc model s wdely used echnque n he feld of mage processng. The hn flm solar cell panel consss of varous crysals so he surface of solar cell panel has heerogeneous exures. Therefore he convenonal algorhms canno be appled o hs problem. Tsa e al. announced he edge dffuson model based defec deecon scheme ha ulzed ansoropc model based on cross-kernel. n hs paper we appled a (b) (c) Fg. 4. Performance evaluaon (a) mcro-crack mage (b) Tsa e al. s mehod (c) proposed mehod. Acknowledgemens Ths work was suppored by Research of hgh precson laser scrbng machne (11102JA1201) Scence and Technology Program of Bejng Muncpal Commsson of Educaon (KM ) and Talen srong school projec of Bejng Unon Unversy under Gran BPHR2012C06 Bejng Chna. References [1]. P. Perona and J. Malk Scale-space and edge deecon usng ansoropc dffuson EEE Transacons on Paern Analyss and Machne nellgence Vol. 12 ssue pp [2]. D. Tsa C. Chang and S. Chao Mcro-crack nspecon n heerogeneously exured solar wafers usng ansoropc dffuson mage and Vson Compung Vol. 28 No. 3 March 2010 pp [3]. R. W. Conners C. W. McMlln K. Ln and R. E. Vasquez-Espnosa denfyng and locang surface defecs n wood EEE Transacons on Paern Analyss and Machne nellgence PAM pp

5 Sensors & Transducers Vol. 179 ssue 9 Sepember 2014 pp [4]. L. H. Sew and R. M. Hogdson Texure measures for carpe wear assessmen EEE Transacons on Paern Analyss and Machne nellgence Vol. 10 ssue pp [5]. K. V. Ramana and B. Ramamoorhy Sascal mehods o compare he exure feaures of machned surfaces Paern Recognon Vol. 29 No pp [6]. J. Escofe M.S. Mllan H. Abrl and E. Torreclla nspecon of fabrc ressance o abrason by Fourer analyss Proceedngs of SPE Vol pp [7]. T. Ohshge H. Tanaka Y. Myazak T. Kanda H. chmura N. Kosaka and T. Tomoda Deec nspecon sysem for paerned wafers based on he spaal-frequency flerng EEE/CHMT European nernaonal Elecronc Manufacurng Technology Symposum 1991 pp [8]. K. Wlsch A. Pnz and T. Lndeberg Auomac assessmen scheme for seel qualy nspecon Machne Vson and Applcaons Vol. 12 No pp Copyrgh nernaonal Frequency Sensor Assocaon (FSA) Publshng S. L. All rghs reserved. (hp:// 161