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Estimation of Production Rate in Flexible Assembly Systems
Estimation of Production Rate in Flexible Assembly Systems H. Makino ( I ) , Yamanashi University; M. Tominaga, Sony Corp./Japan Received on January 4,1995
A bstrac? Production planning of a new product starts from the estimation of the production volume per month or per year The construction of the standardized flexible assembly systems should be designed to satisfy the required cycle time Because a flexible assembly system consists of several sub-systems such as feeding, transport, assembly and transfer ones, the total cycle time of the whole system depends upon the cycle times and repeating numbers of each operation In this report the cycle time and the consequent production rate are estimated for typical flexible assembly systems Each factor affecting the results is discussed
Keywords : Assembly machine, Flexibility, Productivity
1.
introduction
Flexible assembly system was born in order to automate small-lot-large-variety products When dedicated systems were used, it was said that the economical solution for assembly automation was only Justified for mass production of e.g. 100000 pieces per month or more. However, not so many single products reach such a high volume production. Usually the products of car industries and electric industries have less figure than this Nowadays, flexible assembly line using assembly robots can deal with a small volume production of e.g. 20000 pieces per month. The question is that how this figure be made smaller without losing economical justification? The solution is t o develop "long cycle time" assembly systems. If the cycle time of a system is 30 seconds, then it 20000 pieces per month corresponds to production in one shift Thus, if 5000 pieces per month production is required, long cycle time of 2
I
minutes' system should be developed Tnis does not mean to develop a slow speed machine The machines or robots should move in their maximum speed Cycle time for each assembly operation should be minimized, and many operations should be combined The system will be complicated The general rules for planning and scheduling of assembly systems are discussed in papers [4 51 In this report the factors that aftect to the total cycle time are discussed Cycle times ot each individual sub-system are analyzed Some of them are critical, and others are not The degree of influence of each factor also depends upon the degree of complexity, or, simply saying, upon the number of assembled parts in a station Calculation of each elemental cycle time is made in case of a commercial robotic assembly system In this [l] Figure 1 shows a typical layout of it report, the system is called "Sl" system
1100401
Fia 1
Annals of the CIRP Vol. 44/1/1995
I
TvPical Lavout of a Flexible Assemblv Svstem
7
Element of Flexible Assemblv Svstem
2.
Flexible assembly system consists following four major sub-systems [3].
of
the
Feeding Transport Assembly (process) Transfer The word "tray" is used here for the parts tray which transports oriented parts from the feeding apparatus to the assembly station, while the word "pallet" is used for the transfer pallet which transfers assembly fixtures from station to station The following symbols are defined.
I I
b
M
A
1
I
2 Parts Orienting and F e e d
p : numbers of parts (of a kind) set on a tray n : numbers of kinds of parts assemb1ed.m a station rn : numbers of fixtures on a transfer pallet
If a part is used in an assembly in plural, then the number of the part is counted in rn For instance, if four identical screws are used in an assembly and the number of assemblies on a pallet IS three, then rn = 12. 3.
Cvcle Time Analvsis
To make it clear, the total cycle time of the whole system is called "tact time" In the following estimation, elemental operation which affects to the tact time of the system is marked and which does not affect is shown with with no mark I ,
3.1. Feeding A vibratory orienting and feeding apparatus is used in the system. One of the six kinds of parts is selected and dropped onto the tray which is vibrated three-dimensionally to accommodate parts in holes or grooves of the tray (Fig 2) The cycle of the sub-system is as follows: Load empty tray Change over and reset Vibratory accommodation of parts onto tray Unload filled tray Total feeding cycle time t f = 180 s The cycle time of the system is 180 seconds in average. However, for a particular parts tray the throughput time for it may be doubled, because a .different parts tray is possibly waiting at the entrance buffer of the apparatus. The feeding cycle time does not affect to the tact time under the condition that feeding and transport are made in time and at least one full
8
Fia. 3
Parts Transport
tray !or each kind of parts are waiting at the tray buffer of the station 32 Transport The transport of trays consists of two functions I e conveying and setting The former IS made by conveyor motion, and the latter IS made by the robot (Fig 3) The elemental operations are Tray transport (by conveyor) Transport tray from feeder to station Load tray to station buffer area Transport tray from station to feeder Transport time 1 t p 7 = 360 s Tray change (by robot) * Unload empty tray to conveyor Pick up filled tray from buffer area and load it to parts presenting position Transport time 2 t p 2 = 16 s The motion time of the robot affects to the tact time. The total transport time in tact time is,
fp2 x n x m / p where p is the average number of parts on a tray.
3 3 Assembly process The typical assembly sequence !s as !olIov~s tor n kind of parts ro?ate tool turret tor m fixtures pick up part if emp?y skip to tne next point place process continue cont!nue Assembly operation IS not only a mere pick-andp!ace motion but also it !ncludes s o v e assembly processing e g screw driving and spring setting Although the cycle time of pure p&p motion for 300 mm s?roke is 0 6 to 0 7 seconds, it needs longer time for assen?bly processing The empirical estimation for the processing time with p&p motion is as !ollo\~s P&p and processing ' S i mple pick -and-place ' Screw driving * Spring set Gear meshing Precise positioning ' Point skip Average assembly time 1
2 3-3 0 s 40 s 35 s 40 s 35 s 10 s
Fig 4
Assembly Processing
t a l = 3.0 s
"Point skip" occurs when there are some vacant points on the tray In the case according to the signal of the absence check, the robot moves to ths next point The accommodation efficiency is usually greater than 90 4b and the time for this skip motion is counted as 0 1 second per part
Tool change time for each part: ' Rotate tool turret tap = 1 0 s 3.4. Transfer Transter cycle is as follows (Fig. 5): * A
Fiu 5. Pallet Transfer
Relief assembled pallet and flow out Accept next pallet into position and locate t t 7 = 3.0s Transfer time 1
If n = 5, m = 2 and p = 50, then,
Transfer pallet to the next station t t 2 = 100 s Transfer time 2
Because this is for two sets of assembly built, net tact time for one product is,
Transfer time which affects the tact time is t t 7 only.
T = 3.2
T /
+ 30 +
5
+ 3 = 41 2 ti
m = 20.6 s
It occurs once in a tact time.
3 5 Tact time The total tact time of the system is given by the following formula
T = t p 2 x n x m l p + fa1 x n x m + t a p x n +tt1
Usually 10 yo margin is in account and the gross tact time is,
T / m x 110 36 = 22.66 s It corresponds to the production of 26500 pieces per month per one shift.
9
From the above analysis the effects of each subsystem are summarized as follows ( 1 ) The most busy apparatus is the assembly robot The robot motion gives the critical path If the robot moves all over the tact time without stoppage. it is most effective for production It the robot is waiting something e g , the reach of tray, etc , then it causes loss time
( 2 ) The assembly tact time consists of four terms tray change assembly, tool change, and pallet change The first three are done by the robot, and the last term is done by the transfer sub-system In the case of dedicated system, the last term is almost comparable as the assembly time However, in the case of flexible system, the weight of the transfer system is lessened by n x rn times (3! Tray chapge time is reduced by increasing p. and tool change time is reduced by increasing rn, relatively (4) This calculation was made under the assumption that transport and transfer were made in sufficiently short trme If the number of parts n becomes larger, transfer will become less significant but transport will be a problem The design of transport system dominates the design of small volume production system
5. Smaller Volume Production
Svst-
The manufacturer of the system S1 has developed a new system [2] recently In this report the system is called "52" system S2 is designed to assemble up to thirty parts in a station Table 1 points out the difference between S1 and S2, and Figure 6 shows the plan view of S2 .Table 1 Comparison between sy-stem S1 and $2 Svstem
1
s1 30000
....................................... Tact Time (s) ..................................... 10-30 ................................. Parts per 3-5 Station .................................. 6 x 1 .................................... 350+250
I
s2
I
5000
I
<= 30
I
180
1.. ...........................
I
6 x 3 .....................................
I
400+250
The major design changes from S1 to S2 are: (1) Turret changer (tool changer changer) is applied. (2) Two loops of vertically circulating tray conveyors are installed
10
F g _ 6 l e _ x i b i eAssemblv Svstem for Small Volume
7. C o n c l u s i o n Cycle time evaluation for a typical flexible assembly system has been made and the factors that affect to the tact time are discussed In the small volume assembly system, the production rate is dominated by the assembly processing time Other functions like feeding. transport and transfer are less counted in the tact time However, it the numbers of assembled parts in a station become larger, transport function must be checked Every part should be presented in time at the assembly area The key system in flexible assembly is the transport system, which did not appear or was neglected in the dedicated assembly system
Refer e n c e s [ l ] Fujimori, T , 1990, Developmsnt of flexible assembly system SMART, 21 st International Symposium on Industrial Robot, 75-82 [2] Fujimori, T ,1994, Effectiveness of Factory Automation Which Leads to Value Added Manufacturing, 25th International Symposium on Industrial Robot, OD-S1-13-20 [3] Makino, H and Arai T , 1994, New Developments in Assembly Systems, ClRP Annals VOl 431211 994, 501-512 [4] Varl Brussel H 1990, Planning and Scheduling of Assembly Systems ClSP Annals VOI 39/2/f 990, 637-644 [5] Wiendahl, H P I 1989, Shortening the Manufacturing Cycle of Products with Many Variants by Simultaneous Assembly Engineering, 10th International Conference on Assembly Automation, 11-29
A bstrac? Production planning of a new product starts from the estimation of the production volume per month or per year The construction of the standardized flexible assembly systems should be designed to satisfy the required cycle time Because a flexible assembly system consists of several sub-systems such as feeding, transport, assembly and transfer ones, the total cycle time of the whole system depends upon the cycle times and repeating numbers of each operation In this report the cycle time and the consequent production rate are estimated for typical flexible assembly systems Each factor affecting the results is discussed
Keywords : Assembly machine, Flexibility, Productivity
1.
introduction
Flexible assembly system was born in order to automate small-lot-large-variety products When dedicated systems were used, it was said that the economical solution for assembly automation was only Justified for mass production of e.g. 100000 pieces per month or more. However, not so many single products reach such a high volume production. Usually the products of car industries and electric industries have less figure than this Nowadays, flexible assembly line using assembly robots can deal with a small volume production of e.g. 20000 pieces per month. The question is that how this figure be made smaller without losing economical justification? The solution is t o develop "long cycle time" assembly systems. If the cycle time of a system is 30 seconds, then it 20000 pieces per month corresponds to production in one shift Thus, if 5000 pieces per month production is required, long cycle time of 2
I
minutes' system should be developed Tnis does not mean to develop a slow speed machine The machines or robots should move in their maximum speed Cycle time for each assembly operation should be minimized, and many operations should be combined The system will be complicated The general rules for planning and scheduling of assembly systems are discussed in papers [4 51 In this report the factors that aftect to the total cycle time are discussed Cycle times ot each individual sub-system are analyzed Some of them are critical, and others are not The degree of influence of each factor also depends upon the degree of complexity, or, simply saying, upon the number of assembled parts in a station Calculation of each elemental cycle time is made in case of a commercial robotic assembly system In this [l] Figure 1 shows a typical layout of it report, the system is called "Sl" system
1100401
Fia 1
Annals of the CIRP Vol. 44/1/1995
I
TvPical Lavout of a Flexible Assemblv Svstem
7
Element of Flexible Assemblv Svstem
2.
Flexible assembly system consists following four major sub-systems [3].
of
the
Feeding Transport Assembly (process) Transfer The word "tray" is used here for the parts tray which transports oriented parts from the feeding apparatus to the assembly station, while the word "pallet" is used for the transfer pallet which transfers assembly fixtures from station to station The following symbols are defined.
I I
b
M
A
1
I
2 Parts Orienting and F e e d
p : numbers of parts (of a kind) set on a tray n : numbers of kinds of parts assemb1ed.m a station rn : numbers of fixtures on a transfer pallet
If a part is used in an assembly in plural, then the number of the part is counted in rn For instance, if four identical screws are used in an assembly and the number of assemblies on a pallet IS three, then rn = 12. 3.
Cvcle Time Analvsis
To make it clear, the total cycle time of the whole system is called "tact time" In the following estimation, elemental operation which affects to the tact time of the system is marked and which does not affect is shown with with no mark I ,
3.1. Feeding A vibratory orienting and feeding apparatus is used in the system. One of the six kinds of parts is selected and dropped onto the tray which is vibrated three-dimensionally to accommodate parts in holes or grooves of the tray (Fig 2) The cycle of the sub-system is as follows: Load empty tray Change over and reset Vibratory accommodation of parts onto tray Unload filled tray Total feeding cycle time t f = 180 s The cycle time of the system is 180 seconds in average. However, for a particular parts tray the throughput time for it may be doubled, because a .different parts tray is possibly waiting at the entrance buffer of the apparatus. The feeding cycle time does not affect to the tact time under the condition that feeding and transport are made in time and at least one full
8
Fia. 3
Parts Transport
tray !or each kind of parts are waiting at the tray buffer of the station 32 Transport The transport of trays consists of two functions I e conveying and setting The former IS made by conveyor motion, and the latter IS made by the robot (Fig 3) The elemental operations are Tray transport (by conveyor) Transport tray from feeder to station Load tray to station buffer area Transport tray from station to feeder Transport time 1 t p 7 = 360 s Tray change (by robot) * Unload empty tray to conveyor Pick up filled tray from buffer area and load it to parts presenting position Transport time 2 t p 2 = 16 s The motion time of the robot affects to the tact time. The total transport time in tact time is,
fp2 x n x m / p where p is the average number of parts on a tray.
3 3 Assembly process The typical assembly sequence !s as !olIov~s tor n kind of parts ro?ate tool turret tor m fixtures pick up part if emp?y skip to tne next point place process continue cont!nue Assembly operation IS not only a mere pick-andp!ace motion but also it !ncludes s o v e assembly processing e g screw driving and spring setting Although the cycle time of pure p&p motion for 300 mm s?roke is 0 6 to 0 7 seconds, it needs longer time for assen?bly processing The empirical estimation for the processing time with p&p motion is as !ollo\~s P&p and processing ' S i mple pick -and-place ' Screw driving * Spring set Gear meshing Precise positioning ' Point skip Average assembly time 1
2 3-3 0 s 40 s 35 s 40 s 35 s 10 s
Fig 4
Assembly Processing
t a l = 3.0 s
"Point skip" occurs when there are some vacant points on the tray In the case according to the signal of the absence check, the robot moves to ths next point The accommodation efficiency is usually greater than 90 4b and the time for this skip motion is counted as 0 1 second per part
Tool change time for each part: ' Rotate tool turret tap = 1 0 s 3.4. Transfer Transter cycle is as follows (Fig. 5): * A
Fiu 5. Pallet Transfer
Relief assembled pallet and flow out Accept next pallet into position and locate t t 7 = 3.0s Transfer time 1
If n = 5, m = 2 and p = 50, then,
Transfer pallet to the next station t t 2 = 100 s Transfer time 2
Because this is for two sets of assembly built, net tact time for one product is,
Transfer time which affects the tact time is t t 7 only.
T = 3.2
T /
+ 30 +
5
+ 3 = 41 2 ti
m = 20.6 s
It occurs once in a tact time.
3 5 Tact time The total tact time of the system is given by the following formula
T = t p 2 x n x m l p + fa1 x n x m + t a p x n +tt1
Usually 10 yo margin is in account and the gross tact time is,
T / m x 110 36 = 22.66 s It corresponds to the production of 26500 pieces per month per one shift.
9
From the above analysis the effects of each subsystem are summarized as follows ( 1 ) The most busy apparatus is the assembly robot The robot motion gives the critical path If the robot moves all over the tact time without stoppage. it is most effective for production It the robot is waiting something e g , the reach of tray, etc , then it causes loss time
( 2 ) The assembly tact time consists of four terms tray change assembly, tool change, and pallet change The first three are done by the robot, and the last term is done by the transfer sub-system In the case of dedicated system, the last term is almost comparable as the assembly time However, in the case of flexible system, the weight of the transfer system is lessened by n x rn times (3! Tray chapge time is reduced by increasing p. and tool change time is reduced by increasing rn, relatively (4) This calculation was made under the assumption that transport and transfer were made in sufficiently short trme If the number of parts n becomes larger, transfer will become less significant but transport will be a problem The design of transport system dominates the design of small volume production system
5. Smaller Volume Production
Svst-
The manufacturer of the system S1 has developed a new system [2] recently In this report the system is called "52" system S2 is designed to assemble up to thirty parts in a station Table 1 points out the difference between S1 and S2, and Figure 6 shows the plan view of S2 .Table 1 Comparison between sy-stem S1 and $2 Svstem
1
s1 30000
....................................... Tact Time (s) ..................................... 10-30 ................................. Parts per 3-5 Station .................................. 6 x 1 .................................... 350+250
I
s2
I
5000
I
<= 30
I
180
1.. ...........................
I
6 x 3 .....................................
I
400+250
The major design changes from S1 to S2 are: (1) Turret changer (tool changer changer) is applied. (2) Two loops of vertically circulating tray conveyors are installed
10
F g _ 6 l e _ x i b i eAssemblv Svstem for Small Volume
7. C o n c l u s i o n Cycle time evaluation for a typical flexible assembly system has been made and the factors that affect to the tact time are discussed In the small volume assembly system, the production rate is dominated by the assembly processing time Other functions like feeding. transport and transfer are less counted in the tact time However, it the numbers of assembled parts in a station become larger, transport function must be checked Every part should be presented in time at the assembly area The key system in flexible assembly is the transport system, which did not appear or was neglected in the dedicated assembly system
Refer e n c e s [ l ] Fujimori, T , 1990, Developmsnt of flexible assembly system SMART, 21 st International Symposium on Industrial Robot, 75-82 [2] Fujimori, T ,1994, Effectiveness of Factory Automation Which Leads to Value Added Manufacturing, 25th International Symposium on Industrial Robot, OD-S1-13-20 [3] Makino, H and Arai T , 1994, New Developments in Assembly Systems, ClRP Annals VOl 431211 994, 501-512 [4] Varl Brussel H 1990, Planning and Scheduling of Assembly Systems ClSP Annals VOI 39/2/f 990, 637-644 [5] Wiendahl, H P I 1989, Shortening the Manufacturing Cycle of Products with Many Variants by Simultaneous Assembly Engineering, 10th International Conference on Assembly Automation, 11-29