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A System for Automatic Assembly of Different Products
A System for Automatic Assembly of Different Products A. Arnstrom. P. Grondahl, The Swedish Institute of Production Engineering Research/ The Royal Institute of Technology - Submitted by G. Sohlenius ( I ) , The Royal Institute of Technology
Summary 0. systetl, f o r f l e x i b l e automatic assembly has been b u i l t a t PS-Lab IVF/KTH. The system i s capable c f assembling a number o f d i f f e r e n t products and v a r i a n t s i n small batches ( < l h ) . Automatic set-up between batches, m a t e r i a l s handling/feeding and e r r o r d e t e c t i o n / r e c o v e r y are c a r r i e d o u t a u t o m a t i c a l l y . The system c o n s i s t s o f f o u r i n d u s t r i a l robots, a p a l l e t conveyor, moulded f i x t u r e s , interchangeable g r i p p e r s and f i n y e r s , standardized p a l l e t s f o r feeding o f p a r t s , p a r t l y standardized magazines, conventional and f l e x i b l e feeders. The paper deals w i t h the philosophy behind t h e system. The system concept and t e c h n i c a l s o l u t i o n s a r e described. Examples o f products t h a t are assembled i n the system are given. Conclusions based on the experience from the system are drawn.
INTRODUCTION The Production Systems Laboratory IVFiKTH represents a coo p e r a t i o n between the Stockholm branch o f The Swedish I n s t i t u t e o f Production Engineering Cesearch (IVF) and the Dept of Manufacturing Systems a t the Royal I n s t i t u t e o f Technology (KTH). This paper deals w i t h the l a b o r a t i v e p a r t o f the research program on F l e x i b l e kutoioatic Assembly, FAA, c a r r i e d o u t a t PS-Lab IVF/KTH. PROPEfTIES OF THE
PILOT
SYSTEM FOR FAA AT PS-LAB IVF/KTH
The f o l l o w i n g was decided regarding t h e system:
I t should r u n a u t o m a t i c a l l y d u r i n g 2 - 3 s h i f t s . Manual a c t i v i t i e s should be concentrated t o day-time. Reason: The c a p i t a l t i e d up i n equipment should be u t i l i z e d as much as possible. This goal should be reached w i t h o u t having people working a t n i g h t s . This means t h a t manual operations must be separated from machine operations. I t should be p o s s i b l e t o assemble d i f f e r e n t v a r i a n t s and/or products.
Reason: This i s a must i n most companies i n order t o g e t the above mentioned u t i l i z a t i o n . I t should accept small batches ( < l h )
Reason: In order t o avoid i n v e n t o r y and increase the p o s s i b i l i t y o f producing d i r e c t l y t o customer demand the system should be able t o assemble small batches according t o orders i n s t e a d o f l a r g e batches according t o prognosis.
A l l r o b o t s have changeable g r i p p e r s o r f i n g e r s . D i f f e r e n t kinds o f feeding equipment system docked t o t h e conveyor frame. p e r i p h e r a l equipment c o n s t i t u t e s one c o n t r o l l e r does not c o m u n i c a t e w i t h i t i s p o s s i b l e t o communicate w i t h a Series 1 ) .
can be placed around the Each r o b o t and i t s nodule. The r o b o t o t h e r modules. tlowever, host computer (IBM
Advantages w i t h the system c o n f i g u r a t i o n Non- indexed t r a n s p o r t s between s t a t ions.
Using a c o n t i n o u s l y d r i v e n conveyor means non-indexed t r a n s p o r t s between the s t a t i o n s . Therefore we w i l l a u t o m a t i c a l l y g e t a queue a t the s t a t i o n w i t h the longest c y c l e time f o r a c e r t a i n batch. This means two t h i n g s : 1 . The t r a n s p o r t time i n t o t h i s s t a t i o n from t h e preceding s t a t i o n w i l l be minimized. 2. Thanks t o the b u f f e r (queue) the time l o s s due t o v a r y i n g c y c l e ( s e r v i c e - ) time w i l l be reduced compared t o an indexing conveyor w i t h constant c y c l e time. (The v a r i a t i o n s come from d i f f e r e n c e s i n search time, i n s e r t i o n time e t c ) . "Redundant" assembly s t a t i o n s . More than one docking- o r assembly-station a t each r o b o t g i v e s two advantages: 1 . While assembly i s going on i n one s t a t i o n t h e o t h e r ( s ) a t t h a t r o b o t can be set-up f o r the next batch. [ h i s means low time l o s s f o r set-up. 2. Each r o b o t can e a s i l y be combined w i t h a d d i t i o n a l equipment such as presses, automatic screw-drivers, glue p i s t o l s e t c . Changeable assembly sequence
It should be used f o r products w i t h s h o r t l i f e - t i m e . Reason: As the market demands more and more d i v e r s i f i e d products, the p o s s i b i l i t y o f e s t i m a t i n g product volumes i - 3 years i n advance decreases.
The assembly sequence can e a s i l y be changed by l e t t i n g t h e m i n i p a l l e t s go m r e than once round the system. This r e q u i r e s coding o f the p a l l e t s o r o f the f i x t u r e s . Capacity changes
The above mentioned p o i n t s lead t o the f o l l o w i n g demands on the system:
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The system should be capable o f automatic set-up between batches.
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M a t e r i a l s handling and feeding should be automatic.
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6 o n i t o r i n g and e r r o r recovery should be automatic.
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The system should be very f l e x i b l e regarding capacity, geometry and assembly sequence.
The system i s b u i l t f o r research purposes and t o study problems regarding both s i n g l e s t a t i o n s and c o o r d i n a t i o n o f s t a t i o n s . We t h e r e f o r e decided t o b u i l d the system w i t h f o u r r o b o t s connected by a conveyor.
The system can be g i v e n h i g h f l e x i b i l i t y regarding c a p a c i t y s i n c e i t i s p o s s i b l e t o choose between having a l l f o u r r o b o t s assemble the same product o r , f o r example, having h a l f t h e system assemble one product and the o t h e r h a l f o f t h e system assemble another. This makes i t p o s s i b l e t o handle peaks i n demand f o r each product. To d i v i d e the system i n t o two separate flows r e q u i r e s t h a t t h e I B M robots can l i f t f i x t u r e s from p a l l e t s on one s i d e o f the system and place them on empty p a l l e t s on the o t h e r side. Independent modules No d i r e c t communication between modules i s possible, however, information can be t r a n s f e r r e d by t h e m i n i - p a l l e t s . The c o n t r o l system on nodule l e v e l can o n l y conrnunicate w i t h the h o s t computer and v i c e versa. This means t h a t changes can be made i n one module w i t h o u t a f f e c t i n g o t h e r modules.
D e s c r i p t i o n o f the system Experiment o b j e c t s The l a t t e r p a r t o f the paper i l l u s t r a t e s the system. The system c o n t a i n s f o u r i n d u s t r i a l robots, two ASEA 6/2 and two IBM 7545. The r o b o t s a r e surrounded by a c o n t i n o u s l y d r i v e n chain conveyor from EUAE. The conveyor c a r r i e s a number o f standard m i n i p a l l e t s which i n t u r n c a r r y p a r t - o r variant-dependent f i x t u r e s . The f i x t u r e s are f i t t e d t o the m i n i - p a l l e t s by a standardized mounting and are changeable by one o f the robots. The f i x t u r e s a r e moulded i n polyurethane. The m i n i - p a l l e t s r e s t on t h e conveyor chain. Along the conveyor t h e r e a r e t e n docking s t a t i o n s which a r e a c t i v a t e d by the a r r i v a l o f a m i n i - p a l l e t . There a r e two s t a t i o n s a t each I B M r o b o t and t h r e e a t each ASEA robot.
Annals of the CIRP Vol. 34/1/1985
The system has been b u i l t f o r research purposes, problem "generating and s o l v i n g " and t e s t i n g p r i n c i p l e s . I t has n o t been b u i l t f o r any special product. A t present we use the f o l l o w i n g products f o r experimentation.
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A f a m i l y o f a i r motors c o n s i s t i n g o f d i f f e r e n t v a r i a n t s . ( A t l a s Copco). An o i l pump. (Micromatic).
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AUTOl4ATIC OPERATIONS
drawn:
Automatic assembly operations
Degrees of freedom
The systems i s r a t h e r conventional when i t comes t o assembly operations. Worth mentioning i s the changing o f the grippers. One o f t h e ASEA robots has a r e v o l v i n g g r i p p e r head. The o t h e r ASEA r o b o t has a g r i p p e r w i t h changeable f i n g e r s . The l a t t e r system i s a l s o used i n one o f t h e I B M robots w h i l e t h e o t h e r 1BM r o b o t has changeable g r i p p e r s using an electro-magnetic holder
I n order t o be a b l e t o use d i f f e r e n t feeding techniques and magazines, i t i s p r e f e r a b l e t h a t robots w i t h 6 degrees o f freedom should be used.
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Special purpose equipment can be docked a t the docking s t a t i o n s . For the a i r motors we have f o r example a press f o r the pressing o f r o t o r s i n the b a l l - b e a r i n g s . For the oil-pump we have a screwd r i v e r - s t a t i o n a t one ASEA robot. Automatic feeding and m a t e r i a l s hand1 i n 9 Our main idea f o r f l e x i b l e feeding and t r a n s p o r t i s t o use standardized p a l l e t s which are docked t o t h e system by an automatic guided v e h i c l e (AGV). These p a l l e t s can be used f o r t r a n s p o r t o f p a r t s , p a r t s i n magazines, f i x t u r e s , g r i p p e r s , t o o l s , minor feeders w i t h p a r t s etc. The p a l l e t s have automatic connection o f compressed a i r , s i g n a l comnunication and 24 v o l t s e l e c t r i c i t y supply when docking i s made t o the conveyor frame. One manual a c t i v i t y i n t h e system w i l l be r i g g i n g o f these f e e d i n g - p a l l e t s f o r the unmanned n i g h t s h i f t and p l a c i n g them i n t h e s t o r e from where they can be fetched by t h e AGV.
Simul a t i on The mental a b i l i t y o f humans t o r e a l i z e how f o r example a 5degree-of-freedom-robot i s able t o move, i s r e s t r i c t e d . CAD s i m u l a t i o n i s one p o s s i b l e s o l u t i o n t o a v o i d mistakes due t o this. Lack o f h a n d l i n g area I n most FAA-systems developed f o r small batch production o f d i f f e r e n t products, t h e number o f robots i s normally n o t s e t by p r o d u c t i o n volume demands. The handling area of m o T 1 R b : s normally r e s t r i c t s the number o f d i f f e r e n t components t h a t can be presented t o the robot. I f , as i n our case, one wants s t a t i o n s where new components can be docked w h i l e t h e system i s working, the p o s s i b l e number o f presented components i s reduced. In other words: I t i s the number o f components i n a c e r t a i n product t h a t o f t e n defines t h e necessary number o f robots. One s o l u t i o n (though n o t favourable, see below) i s t o have d i f f e r e n t components on the same p a l l e t / s t a t i o n and/or stick-magazines. Another way i s t o increase t h e handling area f o r the robots. Feeding
The docking arrangement f o r the f e e d i n g - p a l l e t s can a l s o be used f o r o t h e r feeding equipment. Ile have f o r example t r i e d a r e programmable b e l t conveyor feeder. Wipers were used t o b r i n g the p a r t s i n a n a t u r a l r e s t i n g p o s i t i o n . A 256 diode a r r a y camera and an 8 b i t personal computer compared an a c t u a l scan w i t h p r e t a u g h t scans. I n t h i s way n a t u r a l r e s t i n g p o s i t i o n s were i d e n t i f i e d . Normally we use t h e standard f e e d i n g - p a l l e t s w i t h magazines. Exchange o f p a l l e t s can be done w i t h o u t i n t e r r u p t i n g the assembly operations as each r o b o t has a t l e a s t two assembly stations.
There i s no good s o l u t i o n f o r the feeding problem. Product s p e c i f i c magazines are normally expensive and c r e a t e a m a t e r i a l s handling /storage/ c o n t r o l problem. Flexible/programmable feeders a r e normally b u l k y and decrease t h e p o s s i b l e number o f presented components. They are a l s o d i f f i c u l t t o empty and fill up w i t h old/new components. We t h i n k t h a t a p o s s i b l e s o l u t i o n i s t o have the components s t o r e d i n one l a y e r on a f l a t surface. The l a y e r s a r e stacked on each o t h e r y i s i o n i s needed t o determine t h e o r i e n t a t i o n .
Automatic set-up between batches
Unbalance
Exchange o f f i x t u r e s
Uhen r o b o t s are used i n s e r i e s i t i s hard o r impossible t o balance t h e r o b o t s together. This i s due t o a/ d i f f i c u l t i e s i n a s s i g n i n g assembly operations f r e e l y between robots, and b/ varying c y c l e time due t o adaptive f u n c t i o n s and e r r o r recovery proceedures. As many products are assembled i n t h e system and as engineering time i s expensive, we t h i n k unbalance should be accepted.
The m i n i - p a l l e t s on t h e conveyor a r e t h e standard ones from t h e conveyor manufacturer. Each m i n i - p a l l e t c a r r i e s a p a r t - o r batch-dependent f i x t u r e which consequently has t o be changed when a new batch i s t o be assembled. This i s done by one o f the IBM r o b o t s which has one o f i t s g r i p p e r s designed f o r t h i s task. A t t h e present stage t h e f i x t u r e s are stacked i n magazines w i t h i n t h e system. I f necessary they could be t r a n s p o r t e d t o and from the assembly system on the p r e v i o u s l y described feedingpall e t s . Exchange o f g r i p p e r s Grippers and f i n g e r s a r e both permanently r i g g e d i n magazines o r holders w i t h i n the system and transported t o and from t h e system on the f e e d i n g - p a l l e t s . Program exchange There i s a c e n t r a l h o s t o r comnunications computer i n the system. However, each r o b o t works independently d u r i n g the assembly o f each batch. The r o b o t s count asse;bly cycles. When " f i n i s h e d products" equals " s t a r t e d " minus r e j e c t e d " assemblies, set-up of t h e system i s i n i t i a t e d by t h e c e n t r a l computer. This orders l o a d i n g of set-up programs ( g r i p p e r and f i x t u r e exchange, p a l l e t exchange e t c ) and l o a d i n g of assembly programs f o r the next batch. The t o t a l set-up time between two batches i s l e s s than 2 min. Automatic e r r o r recovery Automatic m o n i t o r i n g and e r r o r recovery i s rudimentary a t the present stage. Some examples:
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I n d u c t i v e sensors a r e used t o v e r i f y the r e s u l t o f a g r i p p e r change
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I n d u c t i v e sensors check whether the g r i p p e r has picked up a p a r t o r not. F i b e r o p t i c s i n the g r i p p e r v e r i f y the r e s u l t o f an i n s e r t i o n operation.
The l a t t e r s o l u t i o n i s used when i n s e r t i n g t h e blades i n t h e a i r motor r o t o r . A f t e r t h r e e t r i a l s w i t h o u t success, the r o b o t l i f t s o u t t h e motor and p u t s i t aside. The m i n i - p a l l e t i s sent away empty t o s t a r t a new round w i t h a new motor. Should t h r e e t r i a l s on t h r e e products f a i l , the system i s a u t o m a t i c a l l y s e t up f o r a new product/batch. CONCLUSIONS Uhen working w i t h t h e system we have o f course experienced prob'lems. From some o f these problems general conclusions can be
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Gripper changing
We change g r i p p e r s w i t h i n t h e assembly cycles. This o f course increases t h e time r e g a r d l e s s o f which g r i p p e r changing system i s being used. (A f a s t r e v o l v e r head g r i p p e r i s an exception). We a r e i n favour o f more frequent use o f s e r v o - c o n t r o l l e d p a r a l l e l jaw g r i p p e r s as these w i l l decrease the t o t a l changing time. Conipl iance The number o f papers and discussions d e a l i n g w i t h a c t i v e and passive insertion/compliance problems made us suspect t h a t i n s e r t i o n could be a problem. Ile have had however no problems w i t h t h i s and, a c t u a l l y , n o t seen any products t h a t needed RCC o r s i m u l a r equipment. The i n s e r t i o n problem c e r t a i n l y e x i s t s , b u t our experience i n d i c a t e s t h a t the frequence o f t h i s i n p r a c t i s e i s small. The reason could be t h a t t h e compliance i n the robots i s enough t o avoid the problem. E r r o r recovery As small batches w i t h d i f f e r e n t products a r e assembled e r r o r s w i l l occur r a t h e r often. The common e r r o r s , and s p e c i a l l y those which a r e caused by the components, should be detected autom a t i c a l l y and a l s o recovered a u t o m a t i c a l l y . This i n d i c a t e s t h a t p r o g r a m a b l e sensors should be used ( v i s i o n e t c ) . I t a l s o p o i n t s o u t the need f o r a s u f f i c i e n t number o f I/O:s. The primary memory o f the r o b o t c o n t r o l system must be l a r g e enough so t h a t a number of r o u t i n e s f o r e r r o r recovery can be stored. As e r r o r recovery r o u t i n e s g e t m r e s o p h i s t i c a t e d some knowledge based d i a g n o s t i c system c o u l d be necessary. Comnunication We have a l o t of d i f f e r e n t c o n t r o l l e r s i n o u r system. These have a very poor communication a b i l i t y due t o d i f f e r e n t standards. One s o l u t i o n (which we have p r a c t i s e d ) i s t o use t h e I/O:s as d i g i t a l b i t s . This o f course i s a very crude way and creates a need f o r a l a r g e number o f I/O:s ( 1 3 0 + 30). I t i s t h e r e f o r e e s s e n t i a l t o s e t t l e some k i n d o f standard f o r communication, and r e s u l t s of the ongoing work w i t h t h i s w i l l be r e a l l y welcome. ACKNOWLEDGEMENTS The research program on f l e x i b l e automatic assembly c o n s i s t s o f a t h e o r e t i c a l p a r t and a l a b o r a t i v e p a r t . The r e s u l t o f t h e l a t t e r i s described i n t h i s paper. Seven persons a t PS-Lab
IVF/KTH work w i t h the program. They are, beside the authors: R o l f Eriksson, E r k k i Krank, Lars Landqvist, Marcus Papinski and Johan Panten. Prograniinable feeding equipment has been developed i n cooperation w i t h Ake Wernersson and Dan Anorke, Dept o f Optimization and Systems Theory, The Royal I n s t i t u t e of Technology. Work w i t h v i s i o n i n the system i s c a r r i e d o u t by Anders Pettersson, Dept o f Instrumentation Laboratory, The Royal I n s t it u t e o f Technology
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B u i l d i n g o f the system has been financed by The t r a d e A s s o c i a t i o n o f the Swedish mechanical and e l e c t r i c a l engineering I n d u s t r y (MEKAN), The National Swedish Board f o r Technical Development (STU) and The National Board of U n i v e r s i t i e s and Colleges (UHA). Fundamental equipment has been borrowed from, g i v e n by o r rented a t reduced p r i c e from the f o l l o w i n g companies: ANT AUTOMATION, ASEA AB, ATLAS COPCO, BEGHER, EWAB, IBM and ROBOTSVETS AUTOMATION.
Fig 2 Mini-pallet with f i x t u r e i n docking s t a t i o n . Robot l i f t i n g o u t f i n i s h e d a i r motor.
Fig 3 Robot w i t h electromagnetic g r i p p e r holder. Magazine f o r d i f f e r e n t grippers. Magazine f o r d i f f e r e n t fixtures.
Fig 4
S i x g r i p p e r s on a r e v o l v i n g head.
Fig 5
A gripper with changeable f i n g e r s i s also used t o h o l d a screw-driver
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Fig 6
Docking s t a t i o n f o r f e e d i n g - p a l l e t s d e l i v e r e d by a d r i v e r l e s s v e h i c l e . Automatic connection o f a i r power, e l e c t r i c s i g n a l s and e l e c t r i c power t o t h e pallet when i t i s docked.
F i g 10
rig 7
D i f f e r e n t k i n d s o f magazines: A box f o r b u l k s t o r e d components ( t o f e e d e r ) , s p r i n g powered magazines and g r a v i t y magazines.
Fig 8
D i f f e r e n t products t r i e d : Large and small a i r motor and an o i l pump.
Fig 9
Each o f t h e experiment products c o n t a i n s 11-12 p a r t s .
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I n s e r t i o n o f a blade i n t h e r o t o r o f an a i r motor by one o f the g r i p p e r s i n t h e m u l t i p l e g r i p p e r head. The " w i r e s " a r e o p t i c a l f i b e r s making i t p o s s i b l e t o check ( w i t h opened g r i p p e r ) i f t h e i n s e r t i o n was successful o r has t o be repeated. A f t e r t h r e e t r i e s w i t h o u t success the r o b o t l i f t s o u t the o b j e c t and a new one i s s t a r t e d up t o g e t t h e d e s i r e d number of camp1p t e products.
Summary 0. systetl, f o r f l e x i b l e automatic assembly has been b u i l t a t PS-Lab IVF/KTH. The system i s capable c f assembling a number o f d i f f e r e n t products and v a r i a n t s i n small batches ( < l h ) . Automatic set-up between batches, m a t e r i a l s handling/feeding and e r r o r d e t e c t i o n / r e c o v e r y are c a r r i e d o u t a u t o m a t i c a l l y . The system c o n s i s t s o f f o u r i n d u s t r i a l robots, a p a l l e t conveyor, moulded f i x t u r e s , interchangeable g r i p p e r s and f i n y e r s , standardized p a l l e t s f o r feeding o f p a r t s , p a r t l y standardized magazines, conventional and f l e x i b l e feeders. The paper deals w i t h the philosophy behind t h e system. The system concept and t e c h n i c a l s o l u t i o n s a r e described. Examples o f products t h a t are assembled i n the system are given. Conclusions based on the experience from the system are drawn.
INTRODUCTION The Production Systems Laboratory IVFiKTH represents a coo p e r a t i o n between the Stockholm branch o f The Swedish I n s t i t u t e o f Production Engineering Cesearch (IVF) and the Dept of Manufacturing Systems a t the Royal I n s t i t u t e o f Technology (KTH). This paper deals w i t h the l a b o r a t i v e p a r t o f the research program on F l e x i b l e kutoioatic Assembly, FAA, c a r r i e d o u t a t PS-Lab IVF/KTH. PROPEfTIES OF THE
PILOT
SYSTEM FOR FAA AT PS-LAB IVF/KTH
The f o l l o w i n g was decided regarding t h e system:
I t should r u n a u t o m a t i c a l l y d u r i n g 2 - 3 s h i f t s . Manual a c t i v i t i e s should be concentrated t o day-time. Reason: The c a p i t a l t i e d up i n equipment should be u t i l i z e d as much as possible. This goal should be reached w i t h o u t having people working a t n i g h t s . This means t h a t manual operations must be separated from machine operations. I t should be p o s s i b l e t o assemble d i f f e r e n t v a r i a n t s and/or products.
Reason: This i s a must i n most companies i n order t o g e t the above mentioned u t i l i z a t i o n . I t should accept small batches ( < l h )
Reason: In order t o avoid i n v e n t o r y and increase the p o s s i b i l i t y o f producing d i r e c t l y t o customer demand the system should be able t o assemble small batches according t o orders i n s t e a d o f l a r g e batches according t o prognosis.
A l l r o b o t s have changeable g r i p p e r s o r f i n g e r s . D i f f e r e n t kinds o f feeding equipment system docked t o t h e conveyor frame. p e r i p h e r a l equipment c o n s t i t u t e s one c o n t r o l l e r does not c o m u n i c a t e w i t h i t i s p o s s i b l e t o communicate w i t h a Series 1 ) .
can be placed around the Each r o b o t and i t s nodule. The r o b o t o t h e r modules. tlowever, host computer (IBM
Advantages w i t h the system c o n f i g u r a t i o n Non- indexed t r a n s p o r t s between s t a t ions.
Using a c o n t i n o u s l y d r i v e n conveyor means non-indexed t r a n s p o r t s between the s t a t i o n s . Therefore we w i l l a u t o m a t i c a l l y g e t a queue a t the s t a t i o n w i t h the longest c y c l e time f o r a c e r t a i n batch. This means two t h i n g s : 1 . The t r a n s p o r t time i n t o t h i s s t a t i o n from t h e preceding s t a t i o n w i l l be minimized. 2. Thanks t o the b u f f e r (queue) the time l o s s due t o v a r y i n g c y c l e ( s e r v i c e - ) time w i l l be reduced compared t o an indexing conveyor w i t h constant c y c l e time. (The v a r i a t i o n s come from d i f f e r e n c e s i n search time, i n s e r t i o n time e t c ) . "Redundant" assembly s t a t i o n s . More than one docking- o r assembly-station a t each r o b o t g i v e s two advantages: 1 . While assembly i s going on i n one s t a t i o n t h e o t h e r ( s ) a t t h a t r o b o t can be set-up f o r the next batch. [ h i s means low time l o s s f o r set-up. 2. Each r o b o t can e a s i l y be combined w i t h a d d i t i o n a l equipment such as presses, automatic screw-drivers, glue p i s t o l s e t c . Changeable assembly sequence
It should be used f o r products w i t h s h o r t l i f e - t i m e . Reason: As the market demands more and more d i v e r s i f i e d products, the p o s s i b i l i t y o f e s t i m a t i n g product volumes i - 3 years i n advance decreases.
The assembly sequence can e a s i l y be changed by l e t t i n g t h e m i n i p a l l e t s go m r e than once round the system. This r e q u i r e s coding o f the p a l l e t s o r o f the f i x t u r e s . Capacity changes
The above mentioned p o i n t s lead t o the f o l l o w i n g demands on the system:
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The system should be capable o f automatic set-up between batches.
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M a t e r i a l s handling and feeding should be automatic.
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6 o n i t o r i n g and e r r o r recovery should be automatic.
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The system should be very f l e x i b l e regarding capacity, geometry and assembly sequence.
The system i s b u i l t f o r research purposes and t o study problems regarding both s i n g l e s t a t i o n s and c o o r d i n a t i o n o f s t a t i o n s . We t h e r e f o r e decided t o b u i l d the system w i t h f o u r r o b o t s connected by a conveyor.
The system can be g i v e n h i g h f l e x i b i l i t y regarding c a p a c i t y s i n c e i t i s p o s s i b l e t o choose between having a l l f o u r r o b o t s assemble the same product o r , f o r example, having h a l f t h e system assemble one product and the o t h e r h a l f o f t h e system assemble another. This makes i t p o s s i b l e t o handle peaks i n demand f o r each product. To d i v i d e the system i n t o two separate flows r e q u i r e s t h a t t h e I B M robots can l i f t f i x t u r e s from p a l l e t s on one s i d e o f the system and place them on empty p a l l e t s on the o t h e r side. Independent modules No d i r e c t communication between modules i s possible, however, information can be t r a n s f e r r e d by t h e m i n i - p a l l e t s . The c o n t r o l system on nodule l e v e l can o n l y conrnunicate w i t h the h o s t computer and v i c e versa. This means t h a t changes can be made i n one module w i t h o u t a f f e c t i n g o t h e r modules.
D e s c r i p t i o n o f the system Experiment o b j e c t s The l a t t e r p a r t o f the paper i l l u s t r a t e s the system. The system c o n t a i n s f o u r i n d u s t r i a l robots, two ASEA 6/2 and two IBM 7545. The r o b o t s a r e surrounded by a c o n t i n o u s l y d r i v e n chain conveyor from EUAE. The conveyor c a r r i e s a number o f standard m i n i p a l l e t s which i n t u r n c a r r y p a r t - o r variant-dependent f i x t u r e s . The f i x t u r e s are f i t t e d t o the m i n i - p a l l e t s by a standardized mounting and are changeable by one o f the robots. The f i x t u r e s a r e moulded i n polyurethane. The m i n i - p a l l e t s r e s t on t h e conveyor chain. Along the conveyor t h e r e a r e t e n docking s t a t i o n s which a r e a c t i v a t e d by the a r r i v a l o f a m i n i - p a l l e t . There a r e two s t a t i o n s a t each I B M r o b o t and t h r e e a t each ASEA robot.
Annals of the CIRP Vol. 34/1/1985
The system has been b u i l t f o r research purposes, problem "generating and s o l v i n g " and t e s t i n g p r i n c i p l e s . I t has n o t been b u i l t f o r any special product. A t present we use the f o l l o w i n g products f o r experimentation.
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A f a m i l y o f a i r motors c o n s i s t i n g o f d i f f e r e n t v a r i a n t s . ( A t l a s Copco). An o i l pump. (Micromatic).
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AUTOl4ATIC OPERATIONS
drawn:
Automatic assembly operations
Degrees of freedom
The systems i s r a t h e r conventional when i t comes t o assembly operations. Worth mentioning i s the changing o f the grippers. One o f t h e ASEA robots has a r e v o l v i n g g r i p p e r head. The o t h e r ASEA r o b o t has a g r i p p e r w i t h changeable f i n g e r s . The l a t t e r system i s a l s o used i n one o f t h e I B M robots w h i l e t h e o t h e r 1BM r o b o t has changeable g r i p p e r s using an electro-magnetic holder
I n order t o be a b l e t o use d i f f e r e n t feeding techniques and magazines, i t i s p r e f e r a b l e t h a t robots w i t h 6 degrees o f freedom should be used.
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Special purpose equipment can be docked a t the docking s t a t i o n s . For the a i r motors we have f o r example a press f o r the pressing o f r o t o r s i n the b a l l - b e a r i n g s . For the oil-pump we have a screwd r i v e r - s t a t i o n a t one ASEA robot. Automatic feeding and m a t e r i a l s hand1 i n 9 Our main idea f o r f l e x i b l e feeding and t r a n s p o r t i s t o use standardized p a l l e t s which are docked t o t h e system by an automatic guided v e h i c l e (AGV). These p a l l e t s can be used f o r t r a n s p o r t o f p a r t s , p a r t s i n magazines, f i x t u r e s , g r i p p e r s , t o o l s , minor feeders w i t h p a r t s etc. The p a l l e t s have automatic connection o f compressed a i r , s i g n a l comnunication and 24 v o l t s e l e c t r i c i t y supply when docking i s made t o the conveyor frame. One manual a c t i v i t y i n t h e system w i l l be r i g g i n g o f these f e e d i n g - p a l l e t s f o r the unmanned n i g h t s h i f t and p l a c i n g them i n t h e s t o r e from where they can be fetched by t h e AGV.
Simul a t i on The mental a b i l i t y o f humans t o r e a l i z e how f o r example a 5degree-of-freedom-robot i s able t o move, i s r e s t r i c t e d . CAD s i m u l a t i o n i s one p o s s i b l e s o l u t i o n t o a v o i d mistakes due t o this. Lack o f h a n d l i n g area I n most FAA-systems developed f o r small batch production o f d i f f e r e n t products, t h e number o f robots i s normally n o t s e t by p r o d u c t i o n volume demands. The handling area of m o T 1 R b : s normally r e s t r i c t s the number o f d i f f e r e n t components t h a t can be presented t o the robot. I f , as i n our case, one wants s t a t i o n s where new components can be docked w h i l e t h e system i s working, the p o s s i b l e number o f presented components i s reduced. In other words: I t i s the number o f components i n a c e r t a i n product t h a t o f t e n defines t h e necessary number o f robots. One s o l u t i o n (though n o t favourable, see below) i s t o have d i f f e r e n t components on the same p a l l e t / s t a t i o n and/or stick-magazines. Another way i s t o increase t h e handling area f o r the robots. Feeding
The docking arrangement f o r the f e e d i n g - p a l l e t s can a l s o be used f o r o t h e r feeding equipment. Ile have f o r example t r i e d a r e programmable b e l t conveyor feeder. Wipers were used t o b r i n g the p a r t s i n a n a t u r a l r e s t i n g p o s i t i o n . A 256 diode a r r a y camera and an 8 b i t personal computer compared an a c t u a l scan w i t h p r e t a u g h t scans. I n t h i s way n a t u r a l r e s t i n g p o s i t i o n s were i d e n t i f i e d . Normally we use t h e standard f e e d i n g - p a l l e t s w i t h magazines. Exchange o f p a l l e t s can be done w i t h o u t i n t e r r u p t i n g the assembly operations as each r o b o t has a t l e a s t two assembly stations.
There i s no good s o l u t i o n f o r the feeding problem. Product s p e c i f i c magazines are normally expensive and c r e a t e a m a t e r i a l s handling /storage/ c o n t r o l problem. Flexible/programmable feeders a r e normally b u l k y and decrease t h e p o s s i b l e number o f presented components. They are a l s o d i f f i c u l t t o empty and fill up w i t h old/new components. We t h i n k t h a t a p o s s i b l e s o l u t i o n i s t o have the components s t o r e d i n one l a y e r on a f l a t surface. The l a y e r s a r e stacked on each o t h e r y i s i o n i s needed t o determine t h e o r i e n t a t i o n .
Automatic set-up between batches
Unbalance
Exchange o f f i x t u r e s
Uhen r o b o t s are used i n s e r i e s i t i s hard o r impossible t o balance t h e r o b o t s together. This i s due t o a/ d i f f i c u l t i e s i n a s s i g n i n g assembly operations f r e e l y between robots, and b/ varying c y c l e time due t o adaptive f u n c t i o n s and e r r o r recovery proceedures. As many products are assembled i n t h e system and as engineering time i s expensive, we t h i n k unbalance should be accepted.
The m i n i - p a l l e t s on t h e conveyor a r e t h e standard ones from t h e conveyor manufacturer. Each m i n i - p a l l e t c a r r i e s a p a r t - o r batch-dependent f i x t u r e which consequently has t o be changed when a new batch i s t o be assembled. This i s done by one o f the IBM r o b o t s which has one o f i t s g r i p p e r s designed f o r t h i s task. A t t h e present stage t h e f i x t u r e s are stacked i n magazines w i t h i n t h e system. I f necessary they could be t r a n s p o r t e d t o and from the assembly system on the p r e v i o u s l y described feedingpall e t s . Exchange o f g r i p p e r s Grippers and f i n g e r s a r e both permanently r i g g e d i n magazines o r holders w i t h i n the system and transported t o and from t h e system on the f e e d i n g - p a l l e t s . Program exchange There i s a c e n t r a l h o s t o r comnunications computer i n the system. However, each r o b o t works independently d u r i n g the assembly o f each batch. The r o b o t s count asse;bly cycles. When " f i n i s h e d products" equals " s t a r t e d " minus r e j e c t e d " assemblies, set-up of t h e system i s i n i t i a t e d by t h e c e n t r a l computer. This orders l o a d i n g of set-up programs ( g r i p p e r and f i x t u r e exchange, p a l l e t exchange e t c ) and l o a d i n g of assembly programs f o r the next batch. The t o t a l set-up time between two batches i s l e s s than 2 min. Automatic e r r o r recovery Automatic m o n i t o r i n g and e r r o r recovery i s rudimentary a t the present stage. Some examples:
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I n d u c t i v e sensors a r e used t o v e r i f y the r e s u l t o f a g r i p p e r change
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I n d u c t i v e sensors check whether the g r i p p e r has picked up a p a r t o r not. F i b e r o p t i c s i n the g r i p p e r v e r i f y the r e s u l t o f an i n s e r t i o n operation.
The l a t t e r s o l u t i o n i s used when i n s e r t i n g t h e blades i n t h e a i r motor r o t o r . A f t e r t h r e e t r i a l s w i t h o u t success, the r o b o t l i f t s o u t t h e motor and p u t s i t aside. The m i n i - p a l l e t i s sent away empty t o s t a r t a new round w i t h a new motor. Should t h r e e t r i a l s on t h r e e products f a i l , the system i s a u t o m a t i c a l l y s e t up f o r a new product/batch. CONCLUSIONS Uhen working w i t h t h e system we have o f course experienced prob'lems. From some o f these problems general conclusions can be
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Gripper changing
We change g r i p p e r s w i t h i n t h e assembly cycles. This o f course increases t h e time r e g a r d l e s s o f which g r i p p e r changing system i s being used. (A f a s t r e v o l v e r head g r i p p e r i s an exception). We a r e i n favour o f more frequent use o f s e r v o - c o n t r o l l e d p a r a l l e l jaw g r i p p e r s as these w i l l decrease the t o t a l changing time. Conipl iance The number o f papers and discussions d e a l i n g w i t h a c t i v e and passive insertion/compliance problems made us suspect t h a t i n s e r t i o n could be a problem. Ile have had however no problems w i t h t h i s and, a c t u a l l y , n o t seen any products t h a t needed RCC o r s i m u l a r equipment. The i n s e r t i o n problem c e r t a i n l y e x i s t s , b u t our experience i n d i c a t e s t h a t the frequence o f t h i s i n p r a c t i s e i s small. The reason could be t h a t t h e compliance i n the robots i s enough t o avoid the problem. E r r o r recovery As small batches w i t h d i f f e r e n t products a r e assembled e r r o r s w i l l occur r a t h e r often. The common e r r o r s , and s p e c i a l l y those which a r e caused by the components, should be detected autom a t i c a l l y and a l s o recovered a u t o m a t i c a l l y . This i n d i c a t e s t h a t p r o g r a m a b l e sensors should be used ( v i s i o n e t c ) . I t a l s o p o i n t s o u t the need f o r a s u f f i c i e n t number o f I/O:s. The primary memory o f the r o b o t c o n t r o l system must be l a r g e enough so t h a t a number of r o u t i n e s f o r e r r o r recovery can be stored. As e r r o r recovery r o u t i n e s g e t m r e s o p h i s t i c a t e d some knowledge based d i a g n o s t i c system c o u l d be necessary. Comnunication We have a l o t of d i f f e r e n t c o n t r o l l e r s i n o u r system. These have a very poor communication a b i l i t y due t o d i f f e r e n t standards. One s o l u t i o n (which we have p r a c t i s e d ) i s t o use t h e I/O:s as d i g i t a l b i t s . This o f course i s a very crude way and creates a need f o r a l a r g e number o f I/O:s ( 1 3 0 + 30). I t i s t h e r e f o r e e s s e n t i a l t o s e t t l e some k i n d o f standard f o r communication, and r e s u l t s of the ongoing work w i t h t h i s w i l l be r e a l l y welcome. ACKNOWLEDGEMENTS The research program on f l e x i b l e automatic assembly c o n s i s t s o f a t h e o r e t i c a l p a r t and a l a b o r a t i v e p a r t . The r e s u l t o f t h e l a t t e r i s described i n t h i s paper. Seven persons a t PS-Lab
IVF/KTH work w i t h the program. They are, beside the authors: R o l f Eriksson, E r k k i Krank, Lars Landqvist, Marcus Papinski and Johan Panten. Prograniinable feeding equipment has been developed i n cooperation w i t h Ake Wernersson and Dan Anorke, Dept o f Optimization and Systems Theory, The Royal I n s t i t u t e of Technology. Work w i t h v i s i o n i n the system i s c a r r i e d o u t by Anders Pettersson, Dept o f Instrumentation Laboratory, The Royal I n s t it u t e o f Technology
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B u i l d i n g o f the system has been financed by The t r a d e A s s o c i a t i o n o f the Swedish mechanical and e l e c t r i c a l engineering I n d u s t r y (MEKAN), The National Swedish Board f o r Technical Development (STU) and The National Board of U n i v e r s i t i e s and Colleges (UHA). Fundamental equipment has been borrowed from, g i v e n by o r rented a t reduced p r i c e from the f o l l o w i n g companies: ANT AUTOMATION, ASEA AB, ATLAS COPCO, BEGHER, EWAB, IBM and ROBOTSVETS AUTOMATION.
Fig 2 Mini-pallet with f i x t u r e i n docking s t a t i o n . Robot l i f t i n g o u t f i n i s h e d a i r motor.
Fig 3 Robot w i t h electromagnetic g r i p p e r holder. Magazine f o r d i f f e r e n t grippers. Magazine f o r d i f f e r e n t fixtures.
Fig 4
S i x g r i p p e r s on a r e v o l v i n g head.
Fig 5
A gripper with changeable f i n g e r s i s also used t o h o l d a screw-driver
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Fig 6
Docking s t a t i o n f o r f e e d i n g - p a l l e t s d e l i v e r e d by a d r i v e r l e s s v e h i c l e . Automatic connection o f a i r power, e l e c t r i c s i g n a l s and e l e c t r i c power t o t h e pallet when i t i s docked.
F i g 10
rig 7
D i f f e r e n t k i n d s o f magazines: A box f o r b u l k s t o r e d components ( t o f e e d e r ) , s p r i n g powered magazines and g r a v i t y magazines.
Fig 8
D i f f e r e n t products t r i e d : Large and small a i r motor and an o i l pump.
Fig 9
Each o f t h e experiment products c o n t a i n s 11-12 p a r t s .
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I n s e r t i o n o f a blade i n t h e r o t o r o f an a i r motor by one o f the g r i p p e r s i n t h e m u l t i p l e g r i p p e r head. The " w i r e s " a r e o p t i c a l f i b e r s making i t p o s s i b l e t o check ( w i t h opened g r i p p e r ) i f t h e i n s e r t i o n was successful o r has t o be repeated. A f t e r t h r e e t r i e s w i t h o u t success the r o b o t l i f t s o u t the o b j e c t and a new one i s s t a r t e d up t o g e t t h e d e s i r e d number of camp1p t e products.