VIBRATION FATIGUE DESIGN METHODOLOGY OF A LARGE SCALE HEAVY DUTY ROBOT

Transcription

1 ICSV14 Carns Australa 9-12 July, 2007 VIBRATION FATIGUE DESIGN METHODOLOGY OF A LARGE SCALE HEAVY DUTY ROBOT Jong Hw Seo 1, Jae Chul Hwang 1, Yong Won Cho 1, Dong Il Km 1, Hong Jae Ym 2 1 Robotcs Technology Lab, Mechatroncs & Manufacturng Technology Center SAMSUNG Electroncs Co., LTD. Suwon, South Korea 2 School of Mechancal and Manufacturng Engneerng, Kookmn Unversty SeongBuk-Gu, Seoul, South Korea jonghw.seo@samsung.com Abstract The objectve of the present study s to develop a desgn methodology for the large scale heavy duty robot to meet the desgn requrements of vbraton and stress levels n structural components resultng from exposure of system modules to LCD (Lqud Crystal Dsplay) processng envronments. Vbratons of the component structures sgnfcantly nfluence on the moton accuracy and fatgue damage. To analyze and desgn a heavy duty robot for LCD transfer, FE and mult-body dynamc smulaton technques have been used. The lnks of a robot are modeled as flexble bodes usng modal coordnates. Nonlnear mechancal propertes such as frcton, complance of motors, gears and bearngs were consdered n the flexble mult-body dynamcs model. Varous desgn proposals are nvestgated to mprove structural desgn performances usng the dynamc smulaton model. Desgn senstvty analyses wth respect to vbraton and stresses are carred out to search an optmal desgn. An example of an 8th-Generaton LTR (LCD Transfer Robot) s llustrated to demonstrate the proposed methodology. Fnally, the results are verfed by real experments ncludng stran measure test and vbraton test. 1. INTRODUCTION LCDs are wdely used to TV s, computers, moble phones, etc. because they offer some real advantages over other dsplay technologes. They are thnner and lghter and draw much less power. Recently, the sze of raw glass s greatly ncreased n a new generaton LCD (Lqud Crystal Dsplay) technology. In order to handle bgger and heaver glasses, t s necessary to develop a large scale LTR (LCD Transfer Robot) to support varous complcated LCD fabrcaton processes. It wll cause many dffcult desgn problems such as vbraton and hgh stresses due to heaver dynamc loads, resultng n naccurate transfer moton and fatgue cracks. Therefore t s necessary to establsh a methodology for predctng deflectons, vbratons, and dynamc stress tme hstores usng vrtual computer smulaton model. An ntegrated desgn smulaton method would be useful to valdate a baselne desgn and to propose new mproved desgns. In ths paper an ntegrated computer smulaton methodology s presented to predct deflectons, dynamc stresses due to vbratons desgn, based on the exstng FEM and flexble body dynamcs technology. The proposed methodology s appled to the LTR that handles 8th-Generaton LCD glasses. Vbraton analyss s performed and valdated wth the vbraton modal test to dentfy and to recapture the nherent phenomenon n the system. Some flexble components n the LTR may experence severe 1

2 vbraton to cause fatgue damage due to large dynamc loads. Modal characterstcs are used to consder structural flexblty n flexble mult-body dynamc smulatons. Tp deflecton of the end-effector can be calculated to see f desgn requrements are met. Dynamc loads and dynamc stress hstores can be obtaned from the dynamc smulaton. Stress levels are nvestgated at the crtcal areas to predct f fatgue cracks mght occur. If the stress level s not n a safe regon, desgn change should be made based on the computer smulaton results and desgn senstvty study. Then a prototype LTR s bult and tested for desgn valdaton. The present paper descrbes the CAE-based durablty analyss that s beng mplemented and developed at SAMSUNG, to predct fatgue damages correspondng to durablty tests. The proposed methodology can be used to develop a new large scale LTR n the early desgn stage. 2. INTRODUCTION OF LCD-TRANSFER ROBOT Fgure 1 shows varous types of LTRs. Telescope type LTR conssts of a base frame, a R-frame, two Z-frames, two artculated arms wth slender hands as shown n Fg.1(a). The frame structures are fabrcated wth cast ron and alumnum. Hands wth slender fngers are made of lghtweght composte materals. It also has two arms (upper and lower arms) to handle 2 glasses smultaneously. The LTR has a cylndrcal workspace to transfer glasses for varous processes. For precson control of handlng the glasses, statc deformaton at the tp of the fnger must be less than 10mm. Snce the jonts whch connect the arms and lnks nclude bearngs and gears, jont complance must be consdered to predct the statc deformaton at the tp. Flexbltes of the arm tself are also mportant to both statc and dynamc deformaton, because the arm s a knd of cantlever type structure wth a large lumped mass at the tp. Fgure 1 shows varous types of LTRs. LTR s supposed to repeat mllons of cycles to perform LCD fabrcaton processes n lfe. Therefore, t has to pass the physcal tests to ensure the survvablty of the robot system when subjected to statc and cyclc loadngs. The durablty test s a cyclc loadng apparatus that evaluates the durablty characterstcs of the component structure. Among the many dfferent tests, one of the most crtcal s the hand moton of stretchng out and pullng n wth z-frame s vertcal moton. The crtcal moton smulates the jerkng and twstng mpact that an arm support bracket mght experence when runnng wth large glasses loaded. The arms and hands are synchronzed and moved at a speed of about 4m/s. (a) Telescopc type (b) Gate type (c) Lnk type Fgure 1. LCD Transfer robots (LTR) Snce LTR repeats mllons of cycles of partcular loadng and unloadng wth varous confguratons, t may result n fatgue falures at the crtcal stress area. In ths paper, to predct statc and dynamc deformaton at the tp of the fnger and crtcal stress levels ncludng vbraton of the LTR, flexble mult-body dynamc smulatons are presented. Frames, lnks, arms are modelled as flexble bodes. Statc and dynamc deformaton s assumed to be very small, therefore, wthn the elastc range. To represent the flexblty, vbraton normal modes and statc correcton modes are obtaned from the fnte elements vbraton and statc analyss for each flexble component. To represent the jont complance, sprng and damper force elements are used nstead of knematc jont elements. 2

3 3. FLEXIBLE MULTI-BODY DYNAMICS The man advantage of usng modal coordnates n flexble mult-body dynamcs s the reducton n the number of generalzed coordnates that must be ncluded n the analyss. Two types of modes are used n component mode synthess for flexble mult-body dynamcs [1-2]. One s a normal mode. The other s a statc mode. All used normal modes and statc modes must be normalzed to have the same magntude and be orthogonalzed to be ndependent to each other. 3.1 Knematcs of Flexble Components A typcal flexble component s shown n Fg.2. Fgure 2. Global dsplacement of a pont p n a flexble component The flexble component s dscretzed nto a large number of fnte elements. The global poston of a pont p n a flexble part can be represented as r = R + A u = R + A ( u + u ) (1) p Where R s the global poston vector of the X -Y -Z body reference frame, A s the coordnate transformaton matrx from the body reference frame to the global nertal frame, u o s the ntal poston vector of the pont p from the body reference frame, and u f s the dsplacement vector due to deformaton. The dsplacement vector u f can be approxmated by a lnear combnaton of deformaton modes lke Equaton (2). M j = 1 o f u = Ψ η = ψ η (2) f j j Where Ψ = Ψ ( x, y, z ) = [ Ψt, Ψ r ] s a modal matrx and ψ j s the correspondng deformaton mode of a flexble part. η = η (t) s a 6N 1 modal vector and η j s modal coordnates, M s the number of modal coordnates. The deformaton modes can be normal modes, statc modes, or combnaton of normal and statc modes. Used M modes should be lnearly ndependent to each other. 3.2 Flexble Mult-body Dynamc Equatons As shown n Fg. 2, the nodal poston vector of a typcal pont p n the global reference frame can thus be wrtten as Equaton (3) usng Equaton (2) r p = R + A u = R + A ( uo + Ψt η ) p = π + Ψr η π (3) Where u + = uo Ψt η and the rotatonal dsplacement π p of nodal pont p s defned by Ψ r η. The 3

4 combned set of knematc and drvng constrants of the mult-body dynamc system may be wrtten n the form [1-2] Φ (q, t ) = 0 (4) T T T T T T T Where the generalzed coordnates q = [ q, q ] = [ r, π, η, t s the tme, Φ s the r f ] constrant equaton. Usng the Lagrange Multpler Theorem [1], varatonal equatons of moton of the mult-body system may be obtaned by summng all bodes and constrants n the system as n the matrx form of Equaton (5). M Φ * q Φ 0 T q q&& λ Ths s a mxed system of dfferental-algebrac equatons of moton for consderng the elastc effect of the mechancal system. To solve mxed dfferental algebrac equatons, many numercal algorthms have been developed [1]. Usng the Equaton (5), dynamc stress hstory of a flexble component can be calculated [3]. = Q γ * (5) 4. DYNAMIC MODELING OF AN LCD-TRANSFER ROBOT 4.1 Modelng of a Flexble Parts The LTR system shown n Fg. 1(a) s modeled wth 86 rgd bodes, 30 flexble bodes, knematc jonts, and force elements [1]. For parallel rectlnear moton of the fnger and hand bracket, a tmng belt at each jont s modeled to drve at constant speed rato. As shown n the Fg.3, to represent the elastcty and dampng of the belt, sprng and dampng forces are approxmated to be proportonal to dsplacement and velocty of the belt length change. Even the jont complance s modeled n a smlar way wth sprng and damper elements. Major components such as arms and lnks are made of cast ron or cast alumnum. Those structural components can be assumed to be lnear elastc durng normal operaton. However, such a small elastc deformaton may cause vbraton and repeated dynamc stresses resultng n naccurate transfer moton and fatgue cracks. Therefore t s necessary to establsh a methodology for predctng the deformaton, vbraton, and dynamc stress tme hstores usng vrtual computer smulaton model. Fgure 3. Dynamc modelng of LTR arm system Component mode synthess technque [1-3] explaned n the prevous secton can be used for effcent computer smulaton n large rgd body gross moton wth small elastc deformaton. Fgure 4 shows the 1 st vbraton mode shapes of flexble components n the telescopc LTR n Fg.1(a). Also 4

5 Fg.4 shows the component mode numbers used n the flexble mult-body dynamc analyss, as explaned n prevous secton. (a) Fnger (36) (b) Arm1 (18) (c) Arm-Frame (42) (d) Z1-Frame (24) (e) Z2-Frame (24) (f) R-Frame (36) Fgure 4. 1 st Vbraton modes of flexble components (Component modes used for dynamc smulaton) 5. ANALYSIS AND EXPERIMENT FOR 8G-LTR DESIGN 5.1 System Vbraton Modes Snce major structural components such as arms and lnks are modelled as flexble bodes wth a few vbraton normal modes wth proper knematc jonts and force elements, fundamental vbraton modes of the total LTR system can be nvestgated. (a) Experment set-up for modal analyss (b) Vbraton mode shape from test results Fgure 5. Modal deformaton of the telescopc LTR by modal test Analytcal vbraton modes are compared wth the expermental test results for valdaton. Fgure 5 shows the test set up for modal test of the LTR system. Comparson wth the modal test results showed that smulaton results correlate well wth the test results. 5

6 5.2 Desgn Analyss and Improvement for Vbraton Desgn problems such as tp deflecton and fatgue crack can be nvestgated wth the vald smulaton model. Among the varous process events for LCD glass transfer moton, stretchng out and pullng n motons of the hands wth glass loaded are the most crtcal moton to cause severe vbraton and hgh stresses at the supportng bracket structure. Usng the proposed flexble mult-body smulaton technology, the crtcal moton s regenerated to nvestgate how large deflecton and stresses occur durng the operaton. Snce we have a vald smulaton model, we can nvestgate varous desgn proposals. Through desgn senstvty studes the R-frame at the base of the LTR s known to be a crtcal component. To ncrease bendng and twstng stffness, heght and wdth of the beam cross secton s enlarged, and rbs are added as explaned n Fg.6. Even alumnum materal s replaced wth hgh strength steel to ncrease the Young s modulus. Fgure 6. Desgn study to reduce the vbraton by dynamcs smulaton Fgure 6 shows the comparson of vbraton dsplacements durng the smulated motons between the orgnal baselne desgn and the new mproved desgn. 50% reducton of the vbraton level s observed even at the prototype test. 5.3 Stress Analyss and Desgn Improvement by Dynamc Smulaton As the sze of raw glass tends to become larger for productvty and manufacturng cost compettveness, LTRs need to be faster and bgger to handle the larger and heaver glasses wth hgher speed. It may result n ncreased dynamc loads causng fatgue cracks due to dynamc stresses. Fgure 7 shows an example of the fatgue cracks due to dynamc loads at the supportng frame structure n the LTR fatgue test. Usng the flexble mult-body dynamc smulaton, cause and effect for the fatgue crack can be analyzed pror to adopt n an actual spot. To reduce the level of dynamc stress at the crtcal area, the shape and thckness of the structure must be redesgned based on the valdated smulaton model. Expermental tests are executed to valdate the accuracy of dynamc stresses predcted n vrtual computer smulatons, as shown n Fg.8 Fgure 7. An example of the crack fatgue 6

7 Fgure 8. Stress experment and dynamc analyss for fatgue desgn Fgure 9 shows the comparson of dynamc stresses between the orgnal desgn and varous desgn proposals wth dfferent desgn shape and metal thckness. The stress measure pont of the part s dot crcle area n the Fg.8. Fgure 9. Stress analyss and desgn mprovement for fatgue 5.4 Handlng Accuracy and Desgn Optmzaton If dynamc loads are ncreased, t mght deterorate the accuracy of the precson transfer moton due to deflecton and deformaton of major structural components [4]. The proposed smulaton methodology can be used to evaluate the deflecton at the tp of the fnger. By nvestgatng the smulaton results for the baselne desgn, sgnfcant desgn parameters are dentfed and an optmal desgn can be found to mprove the accuracy of the precson transfer moton. Fgure 10 shows the vertcal deflectons at the tp of the hand fnger for varous desgns durng the specfed LTR moton. Tp deflecton for the baselne desgn of the LTR s 40 mm that exceed the desgn specfcaton requrement 10 mm. Expermental tests usng the laser tracker are carred out to valdate the smulatons. The dynamc smulaton and DOE (Desgn of experments) method were used to fnd an optmal desgn satsfyng the desgn target of maxmum deflecton wthn 10 mm. 7

8 Fgure 10. Optmzed desgn for handlng accuracy by smulaton 6. CONCLUSIONS A computer smulaton methodology was presented for vbraton and fatgue analyss of the LTR system. Varable ampltude mult-axal loadng condtons can be generated to nvestgate any structural deflecton, vbraton, and fatgue. Flexble components are modelled usng component mode synthess technque. To represent the jont complance and belt flexblty, sprng and damper force elements are ntroduced wth proper approxmaton. To have a vald smulaton model, vbraton modes of the total LTR system s compared wth the modal test results. Comparson of the analyss and test results shows that they correlate well wth each other. Deflecton of the tp of the end-effector s nvestgated wth the proposed methodology. To reduce the tp defecton, a better desgn can be developed wth the smulaton model. Fatgue crack falure can be predcted wth the baselne desgn. To prevent the fatgue falure at the crtcal area, stresses are reduced by changng the structural desgn. The results of the vrtual durablty assessment are qute good, and show good correlaton wth the areas of falure on the test. The value of beng able to predct servce lves based on results obtaned exclusvely n the vrtual doman s obvous. The proposed methodology can be used to develop another type of LTR system. ACKNOWLEDGMENT Ths work was partly supported by the Seoul Research & Busness Development Program (Grant No ). REFERENCES [1] Shabana, A. A., Dynamcs of Multbody Systems 2 nd Edton, 1998, Cambrdge Unversty Press. [2] Wu S.C, Haug, E.J. and Km S.S., A Varatonal Approach to Dynamcs of Flexble Multbody Systems, CCAD Techncal Report 86-15, 1986, Unversty of Iowa. [3] Ym, H.J., "Computatonal methods for dynamc stress analyss of mechancal system", Techncal Report R-84, Center for Smulaton and Desgn Optmzaton of Mechancal systems, 1990, Unversty of Iowa. [4] Hwang, J.C., Byun, K.S., Cho, Y.W., Improvement of the Path Accuracy of the 7 th Generaton LCD Glass Transfer Robot, Internatonal Conference on Mechatroncs and Informaton Technology, 2005, Chongqng, Chna. 8