We develop an assembly robot system for assembling the flexible belt-shaped subject. An image processing method is developed to recognize the belt-shaped subject. This method is able to determine the grasping point and grasping angle for piking up a subject by a multiple hands unit. CAD information is used to determine the grasping point. The multiple hands unit is developed, which is able to grasp all grasping points of a subject at a time. In addition, the image processing method is used to judge whether a subject is fastened accurately at right position or not during the assembly

Kajihara, Yasuhiro

Matsuki, Katsunori

Miyazaki, Satoshi

Munesawa, Yoshiomi

Osaki, Hirokazu

Uchiyama, Hiromitsu

Okayama University Digital Information Repository

Development of Assembly Robot System for Flexible Belt-Shaped Subject

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering, Okayama University, Vo1.35. Nos.1&2. pp.163-168. March 2001Development ofAssembly Robot Systemfor Flexible Belt-Shaped SubjectHiromitsu UeHIYAMA*, Satoshi MIYAZAKI", Hirokazu OSAKI"Yasuhiro KAJIHARA", Yoshiomi MUNESAWA* and Katsunori MATSUKI"""(Received December 14, 2000)We develop an assembly robot system for assembling the flexible belt-shaped subject. Animage processing method is developed to recognize the belt-shaped subject. This method isable to determine the grasping point and grasping angle for piking up a subject by amultiple hands unit. CAD information is used to determine the grasping point. The multiplehands unit is developed, which is able to grasp all grasping points of a subject at a time. Inaddition, the image processing method is used to judge whether a subject is fastenedaccurately at right position or not during the assembly.1. INTRODUCTIONVarious kinds of flexible belt-shaped subjects are used in the assembly processes. Although the introduction ofrobots has promoted automation of many assembly processes, assembly of soft subject has not been automated yet. [I]It is the one of most difficult problems to be solved for advancement of the automation. Therefore, we tried todevelop an assembly robot system, for the purpose of assembling the flexible belt-shaped subject. Firstly, on thebasis of the CAD figure, grasping positions on a subject are decided so that the shape of the subject could be kelpremain unchanged during assembly work. Next, a unit of multiple hands is designed, which is able to grasp pluralgrasping positions at a time. An image processing method is developed to recognize the shape of subject beforegrasping, and determine the grasping positions on a subject comparing with the data obtained by CAD figure. [2] [3)An inspection method is also developed for judging whether each subject has been assembled accurately at a rightposition or not. Finally, an assembly robot system is developed by using the above mentioned methods and themultiple hands unit.2. PROPOSED METHOD2.1 AssumptionsFollowing conditions are assumed.<i) Assembly is possible even if a subjects is curved or distorted in a vertical plane.(H) The stiffness coefficient and the weight per length of a subject are uniform.(iii) Flexible subject is supplied one by one to a flat working table without being twisted or overlapped. r.] Thesubject is picked up and assembled to a rigid object fixed on another working table or JIG.******Department of Systems EngineeringGraduate School of Natural Science and TechnologyUchiyama Manufacturing Corporation163164 Hiromitsu UCHIYAMA, Satoshi MIYAZAKI, Hirokazu OSAKI Yasuhiro KAJIHARA, Yoshiomi MUNESAWA and Katsunori MATSUKI2.2 SymbolsSome key symbols are shown as below.Dt (DII, DJI) : Coordinate of R-th pixel on the contour line of belt-shaped subject. (e=1,Z, ... ,N)N : Number of pixel (e) on contour line of an subject.ANI : Opening angle (Fig.3) of pixel (e) .2.3 Determination of Grasping PointsThe grasping position is determined form the CAD figure of a subject so that the deformation of subject shouldbe minimalized. Additionally, the number of grasping position is also paid attention to be minimized as possible asthey could.2.3.1 Modeling of distortion of the subjectFig.1 shows a model of distortion. There is two inflection points where the flexure is beginning and ended. Asymbol DE represents the distance from j-th point on the subject to a virtual line connecting start point and end oneof the flexure. >Then, the evaluation value MD of the flexure is defined by eq. ( 1 ) . [5](1)The amount of flexure increases by growing of the evaluation value MD.Fig.1 An evaluation model of the flexure2.3.2 Determination of grasping pointsGrasping point is determined by following process considering the flexure.(i) Grasp from the upper side at the convex part. (Prevention of flexure)(ii) Grasp the inflection point in horizontal plane. (Prevention of twist and spacing)(iii) The above mentioned position is determined from the evaluation value MD of all flexures under theassumption that a subject would be held at a time.(1) MD <LMDThe amount of twist and spacing between the consecutive grasping points are examined to evaluate whether oneor the other grasping point could be eliminated. If MD is smaller than a pre-examined constant value LMD, thensuch point is eliminated from candidates to minimize the number of the multiple hands unit. The actual graspingposition is chosen from candidates considering following conditions.(Z) LMD:;;; MD :;;; HMDIf above condition is satisfied, this section could be assembled. So two grasping points are determined.(3) HMD <- MDIf MD exceeds a constant value HMD, the point which becomes the maximum of DSj is chosen additionally as agrasping point. Here, LMD and HMD are given by measuring real belt-shaped subject.2.4 Image Processing Method for the Recognition of the Belt-Shaped Subject2.4.1 Composition of the assembly robot system(a) This system in Fig.2 is composed of the image processor, robot with 6 degrees of freedom, computer, threeDevelopment of Assembly Robot System for Flexible Belt-Shaped SubjectCCD cameras and switching unit.(b) The multiple hands unit hold up aligned subject, and move it to another working table-2 for assembling thesubject.(c) Camera Cl takes an image at upper side from the working table-l to recognize overall picture of a subject.Camera C2 is set at upper side from the other working table-2 and used for the inspection. Camera C3 is set athorizontal side. They are used to determine the grasping position precisely by the multiple hands unit to grasp asubject.165CCD Camera 3 9CCDCamera 2 --WorkingTable>-2JIGMultiple_~~~~""-I Hands unitSubject~-:,,=-~'--J CCDCamera 1•'mo,_ i'_b_ar_d -'ComputerFig.2 Assembly robot system2.4.2 Determination of a reference pointCorners of flexible belt-shaped subject usually could keep the original shape, because the stiffness coefficient isbigger than that of other flat segment. So corners of subject are chosen as reference point which will be matchedwith the CAD figure. Therefore, a method is required for recognizing corners on the contour line. The corner couldbe recognized by measuring the opening angle.(1) Estimation of opening angle at a cornerThe opening angle is calculated by CAD figure and contour line in the input image. The image processing iscarried out to make out the contour line in the input image and CAD figure. Next, the opening angle AN, at pixel f.is defined by eq. (2 ) .1(DJ -DJ) (DJ -DJ )1 180ANt = arctan t-NA t _ arctan t.NA t x-Dlt _NA -Dl t Dl t• NA -Dl t 1fContour pointFig.3 Calculation of opening angleWhere, two points (position D'.SA, D"SA) are set at intervals away NA pixels from the point D, on the contourline: NA is given experimentally.166 Hiromitsu UCHIYAMA, Satoshi MIYAZAKI, Hirokazu OSAKI Yasuhiro KAjIHARA, Yoshiomi MUNESAWA and Katsullori MATSUKI(2) The identification of cornerThe corner is identified based on the opening angle AN,. However, each subject has several corners. So, theopening angle of every points on the contour line are shown in a figure in Fig.4. The vertical axis represents theopening angle ANI, and horizontal axis represents l-th pixel on· the contour line. The minimum value of openingangle AN, is searched in this graph. The pixel with the minimum value AN, is chosen and set as corner Q. CornerC. satisfies the following equations. Where, the constant values AS and NC are given by pre-examination.ANt ~AS, (MS. sf.sME., OsASs1r)IME. -Ms.1 ~NCc. = MS~E.{4Nt}~ 1t"~Abll,~8-oo MS. MEtNumber of pixel on the contour lineFig.4 Opening angle of pixel on the contour line2.4.3 Determination of grasping points and grasping angleGrasping point on a subject is not always parallel to the horizontal Hne <i-axis). It is often inclined. Therefore,the direction of every robot hands should be adjusted so that they can grasp a subject accurately. Without thisadjustment any robot hand could not hold the grasping point accurately. The angle of inclination HAt at graspingpoint Hk is given by eq. ( 6 ) . The angle HAt is given as the mean value around the grasping point. Where, thesymbol a and S are constant values given by experiments.t.. t.. DJ-DJ~ 4J ~ arctan m-S m.SHA _ m~. m _ :::.m•...::.~--=-.-'-__D_iJ""m_""s_-_D_iJ""m"".s-L• 2a+l 2a +1Contour PointFig.5 The grasping angle HAt of the grasping point lIt2.5 Grasping Procedure by Multiple Hands UnitAs the accurate positioning is required to assembly belt-shaped subject assured by following procedure.2.5.1 Picking up by robot handsOnly two consecutive grasping points are held at first. Then the subject is lifted up so that other segments of thesubject are sagged downward by their own weight. After that, the bottom face of the multiple hands unit is turneduntil it becomes parallel to the vertical plane.Development of Assembly Robot System for Flexible Belt-Shaped Subject 1672.5.2 Adjustment of posture of subjectThe angles at both side at the grasping position (a) are calculated and represented by symbols DLat and DRat.Where, angle DRat is made by horizontal axis and the right side segment of the subject sagged downward. The idealangles at the same point are calculated by using the CAD figure, and they are represented by symbols BLat andBRat. Considering the difference between DLat and DRat, the unit of multiple hands unit is rotated angle at aroundits tool axis. The rotational angle is given by eq. ( 7 ) . The posture of subject could approach to vertical by thisprocedure. After the execution of above mentioned procedure, all hands could be closed.Ok _ (BLOk -DLOk)+(BROk -DROk)22.5.3 ,Recognition of other grasping positionsAfter subject is grasped with the multiple hands unit completely, it is positioned to the object by the followingprocedure.<D An image is taken from the bottom face of multiple hands unit. The contour line of subject is calculated byimage processing by usin~ outlining.® The corners of subject are recognized.@ The reference points are searched from the corners.@ Grasping positions hold by multiple hands unit are calculated.@ The number of pixel on contour line is calculated from the reference point to every grasping points.@ The contour line of the rigid subject on a JIG is obtained by using outlining.(J) On the basis of @ and ®, the positions for putting on the subject are precisely determined.@ The subject is positioned actually. Where, the segments changing in vertical plane are positioned in advance.2.5.4 Assembly by multiple hands unitIn order to assemble the subject. multiple hands unit should do the motion of grasp and insert at the same time. Each gripper of multiplehands unit has the following function. While holding the segment by gripping fingers, pushing lever goes down in order to insert thesubject.(Fig.6) It is positioned to assemble the subject by these function, after multiple hands unit move on the JIG.I~E Pusing levers<E- Gripping fingers~I Flxible subject<o-JIGFig.6 Function of grippers2.6 InspectionAn image of subject is taken from its upwards by the camera, after the assembly of a belt-shaped subject. Then,the contour line of the subject is detected by the contour-tracing method. On the other hand, a virtual border line isdrew around the periphery of the rigid object fixed on JIG. If the belt-shaped subject has been assembled accurately,its contour line never protrude the virtual border. If any portion of subject protrudes the virtual contour line, then thatportion is faulty segment. Therefore, eq. ( 8) is defined in order to evaluate the accuracy of assembly.MJ - -xlOON ( 8 )Where, N is the length of virtual contour line and M is the length of segments protruding from the virtual contourline.168 IIlromllsu L·CHIY:\~I:\. SalOshl ~1IY:\Z:\Kl. Hlrokalu OS.\I\I Yasuhlro I\.-\JIH.\R.-\. Yoshiomi ~IL":\ES.-\ \H and I\alsunori ~I.HSlI\l3. APPLICATION EXAMPLEA assembly robot system is shown in Fig.7. This system is composed of one industrial robot with 6.D.O.F. (,jTheassembly subject is also shown in Fig.7. As a result, the averaged operation time for one subject was about 120 sec,and success rate was about 60 %. Main factor causing failure is that each gripper grasped wrong position. For thesake of flexibility of the subject, the grasping positions are moved slightly comparing planned positions when thesubject suspended. Therefor, some grippers grasped the slightly wrong positions. In order to minimize the error ofgrasping position, preliminary numerical analysis of the object is required from the viewing point of deflection.ceo camare C 2CCD camare C 1RobotMultiple hands unitV Subject~~ Work Table-l--Work Table-2 and JIGFig.7 Assembly robot system4. CONCLUSIONIn this research we developed a robot system which could assemble the flexible belt-shaped subject.Firstly, grasping positions on subject are determined to keep the shape of subject. Secondly, a multiple hands unitwhich can hold all grasping positions are developed to assemble subject accurately and to judge that it has beenassembled accurately. Finally, an assembly robot system is constructed by using the proposed method and themultiple hands unit.REFERENCE[1 J K.Hattori, et al.: "A Method for recognition and array of Twisted Belt-like Object Using Image Processing",The Japan Society of Mechanical Engineers, Lecture papers of General meeting' Lecture meeting of 35-thChuugoku & Shikoku Area Branch, (1997), pp.333-334.[2J R.Nevatla: Machine perception, (1986)[3] J.Hasegawa, et al.: Image processing on personal computer, (1993)[4 J S.Akaki, et al.: "Arraying and Inspecting Method for Twisted Belt-like Object Using Image Processing", TheJapan Society of Mechanical Engineers, Lecture papers of General meeting' Lecture meeting of 36-th Chuugoku& Shikoku Area Branch, (1998), pp.329-330.[5] K.Matsuki: "Assembly Method for Flexible Objects with Three Dimensional Belt-shape", 2000 graduationthesis of Mechanical Engineering, Faculty of Engineering, Okayama University.[6J Mitsubishi Electric Co., MELFA RV-N10 operating manual, (1993)

Development of Assembly Robot System for Flexible Belt-Shaped Subject

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