Assembly Line Control Models in Automotive Industry

ASSEMBLY LINE CONTROL MODELS IN AUTOMOTIVE INDUSTRY 1. Hampl and P. Skvor INORGA - Institute Jor Industrial Manag em ent AutomatIzation, L etenska 17, 11 806 Prague 1, Czechoslovakia

following main functional areas: 1. INTRODUO!TION

(1) Production pla nning, ( 2) Production scheduling, (3) Production monitoring and reporting, (4 ) Production control, (5) Quali ty control.

The paper deals with assembly line model within the framework a production control and information computer system consisting of five basic functional areas. These areas and their mutual interactions are being briefly reviewed including the proposed computer hardware and communications equipment.

2.1 Production planning

The plan is formed generally for three planning periods ahead, the first period plan being compulsory, the other two periods data being of perspective nature. As a planning period one calendar month was chosen. For this plannnig period necessary parts, subassemblies and assemblies to be manufactured are calculated for different work centers and shops.

A more detailed attention is paid to the description of the production control area where the problem of finding an optimum sequence of different vehicle types and options that are to be completed assembly line was solved. The relevant mathematical model is based on the daily vehicle schedule and considers assembly timing data of different vehicle types, loading of each work center and technological restrictions. The control of movements of selected parts and conveyor control are integral parts of the algorithm. 2.

The planning of manufacuture of parts and subassemblies and/or assemblies differs for batch (rota cycle) production and flow line production, respectively. Considering current stock levels of finished manufactured parts and bought out items, lead times and delivery times and other planning standards are determined the process of calculations of requirements is based on the vehicle build requirement given by volume of basic type of vehicle ordered, volume of spare parts and parts for supplies for reject and direct takings from

BASIC CONCEPTS AND FUNCTIONS OF THE PRODUCTION CONTROL AND INFORMATION SYSTEM

The total system problem area was divided according the nature of various production, planning and control activities into the 367

368

J. Hampl and P . Skvo r

production process. 2.2 Production scheduling Similar approach to that of planning process is applied for production scheduling in metallurgical shops, press shops, mechanical and assembly shops but contrary to the planning process a time period shorter than one month is used. The production scheduling process for the area body shop - paint shop final assembly line follows up the calculated one month vehicle build plan and sales subsystem requirements (one month order file). Due to the nature of production process in this area no optimatization techniques were applied the calculation of schedules. The vehicle build schedule represents disjunction of the required volume of vehicles to be built and manufactured and/or bought out items delivery restrictions prevailing in the production process. The production schedule is formed by succesive sorts of considered monthly order book. Using production planning standards and bill of material explosion tables the daily schedules for different work centers and succesive shops are formed out of the three days vehicle built schedule file. The three days file at the same time permits to decide allocation of substite orders in case of paint shop failure. The requirements for material and transfers to the lines are derived from the three days order file. 2.3 Production monitoring and reporting creates a necessary feedback for the production scheduling and control. In the body shop - paint shop - assembly line area the

production monitoring a reporting system provides the collection and processing of data on real number of parts and subassemblies manufactured, number of bodies produced and number of painted bodies (in different body type breakdown). The order follow up is carried out along the assembly line area (including body number tracking) from the order allocation point to the finished car shipping area. The collection of production data on trims, wheels, eneines, transmissions and axes of different types is provided as well as processing of these data into the form necessary for production control and (i.e. necessary accumulations, explosions, deriation calculations and reporting and failure signalling are carried out). The production monitoring and reporting data records will be also utilized for the payroll calculations. 2.4 Production control Assignement of orders to painted bodies in the body shop - paint shop - assembly line is done by the use of the created three days order file. This order assignment is based on the line capacity possibilities, such that all different work stations be maximum utilized. The algorithm applied is described in more detail in Part 4 of this paper. The order assignment procedure takes into consideration the material status on conveyor and inventory carrying sections. This function is linked with the successive shop control, the preparation of vehicle documentation and preparation of documents for inventory carrying section automatic take down. Out of other interesting control functions the assignments of colours before actual

As sembl y li ne co nt r o l mod e l s

spraying can be mentioned. As a part of production control process in this area also production reporting and distribution of control information to selected work stations in shops is performed.

2.5 Quality control function provides monitoring of quality in manufacturing shops, work and adjustment centers, material and parts stock yards and warehouses and guarantee analysis and follow up. This functional area objective is to collect information and data for production and preproduction management on the defectivity of parts so as to permit design and technological modification. 3. INFORMATION SYSTEM HARDWARE EQUIPMENT Some of the above mentioned functions have been at the present time partially implemented in A.Z.N.P. works using the IBM 360/30 computer. To provide computing and communication facilities for further functions in other areas two control computer systems IBM 5/7 are installed with the data installed with the data collection terminals 2790 scattered directly in work shops. As a result, part of the system functions described ab ove will be processed by the control computers IBM S/7, the latter part will be processed by the central IBM 360/30 computer. The mutual interacti~n in the processing of outputs and data retransmittion between IBM 36 0 /3 0 and IBM S/7 computers will be ensured by the ma gn e tic t a pe da t a files ex1<.1' . \ 1. r

11 "

369

change. 4.

GENERAL FORJl'lU LATI ON OF i'l:UlTIPRODUCT ASSEMBLY LI NE CONTRO L .'i ROBLll1

In this part of t h e pa p er a detailed description of a ma thema tic a l model of multi product a s s embly line is presented. This pa rt belongs to an importa nt area of production control in automotive industry, featuring problems t hat have not been solved in Cz echoslov aki a by use of a process c ontrol computer so far. An assembly line i s defined a s an organization of technol ogical process during which course assembly of components ( parts, subas s Emblies) into a fin al, finis h ed product is performed. Due to organiza tiona l a nd technological reasons t h e a s s embly line is usually divided into a number of work stations.These will be assumed as elementery units of assembly process. A work station (are a ) i s usually defined by a particul a r loc a tion a nd positi on in t echn olo gical sequence. Accordin g ( 0 its work objective t h e work st a tion i s equi pped by necess a r y tools, materi a ls and work force of required professional qua lific a ti ons. Let us as s ume a n a ssembly line divided into m work are a s, ea ch of t h em be i ng 0: leng th Ij ( j = 1,2, •.••••• , m). Generally, t h e work s t a t ions may be of different size , bu t f or the sake of simplicity l e t us a ssume lj' the work st a tion leng th, being expressed as an integer numb er; its practical interpret a tion ma y be for example a certain number of po sitions of ma in transfer conveyor. In our case this number den otes number of products as sembled simult a neou sl y

3 70

J. Hampl a nd P . Skvo r

in the j-th work station. The other pa rameter of each work station is Dj' the number of workers assigned to the j-th work station. The transfer of products between different work stations along the line is usually performed by a transport conveyor, which also sets rhythmical rate of output. The time interval between the arrival of two succesive products to the work station is usually called line work (denoted by symbol t). This is the time necessary for the assembled product to proceed by a step, i.e. by one conveyor position. The work capacity of j-th work station may be obtained using the work station area length Ij and ~he number of workers D as j

This is the model of the situ~ ation on the assembly line derived from the length and work force level of particular work area. Further on, an opposite variable Kj (PLAN) will be introduced, representing work capacity derived from the production plan, i.e. work capacity required to assemble the planned volume of products. The succession of assembly activities on the line are given technological sequencing requirements, i.e. by a compulsory sequence of prescribed m~nufactu~­ ing operations (i.e. elementary work unit tasks). These operations have been assigned to particular work areas and their volume is characterized by the length of their performance times. The usual way how to express the volume of work

is to specify the number of standard performance minutes required. As mentioned above, technological operations at one work station area may be performed by one or more (~.) workers. J If the assembly line is designed only for one type of Droduct the technological time for j-th work station area is denoted by T j ( j = 1, 2 •••• , m). The sum of performance times of all work tasks performed on the whole assembly line is usually called the product performance time. Methaphorically, the term performance time could also be used for the variable T j of j-th work station area. An assembly line need not to be used for assembly of a single product exclusively but may be used for simultaneous pr oduction o~ several products, or rahter several types (modifications) of one product. However, approximately same sequence of technological opera tions and line design (including the division into work stations) must remain unchanged this must hold but the performance times of different types may vary, of course. Hence, in our further exp la nations a multiproduct assembly line, technologically designed for simultaneous assembly of ~ different types (modifications) of a certain product will be considered. Let us denote different modifications by index i ( i = 1,2, •••• n) a nd assume that technologically required performance time of different assembly operations expressed by a number of performance minutes required will be specified in a form of a matrix ( Ti j ) for all i j

= 1,2, = 1,2,

... ...

, ,

n m

Assembly line cont r ol mod e ls

rhe next

t~rm

to intr odu ce is v a ri-

a ble if . ,,,hich re'Oresents the level J

37 1

(I)

Vi. :: V . + V. , - V . , J J J,~ J, ~j.

-

of lo a ding ( utiliz a tion) of j-th

From (3 ) it follows th a t if t h e

work station. V. will be obt a ined J b y summine u"9 t!Echnological per-

d if f erence

form a nce tioes of all

differ~nt

V.J, 1

-

9roducts tha t are to be formed

t'len

during a cert a in time interval in the

y'.>V. ,1 J

j-th work a rea; 1.

V. =

J

"J L.

( 2)

V.J, 1

V.

J, l . J

- V.J,

Ij

>

0

.(.0

k=l then

where in j ex k denotes the relative position of the product in j-th area and the auxiliary variable V rejk presents a part of work load (utilization) o! j-th work area capacity b y k-th product.

1

.

(5 )

This means practic a lly tha t by a rrival of a ] roduct of higher p erformance tirr.e then t ha t of the depa rting one the work station loading

The relative "9os ition index k of work area

VI. " V. J J

may obtain values k =

in the ap ; ro p ri a te s te p becoces increased a nd vice vers a . qence, if

1,2, ...... , Ij. If k :: 1 then the

the p roducts coming to t h e assembly

product is just incoming to the 1st

line have

position of the

ap ~ ropriate

work

area it follows tha t k :: lj. Similarily it me ans that the product is located on the l a st po sition of j-th

diff~rent

re rforman ce

timings (as given by te chno logic a l time matrix (T ;) the loading of i " different work stations fluctu a tes. The objective of production process

work area, i.e. it will move into

control is to

the immedi a tely fol l owing (j+l) work

flu c tuation s , i.e. rr.a ximum l 08.ding

stqtion during the next step (move)

even l y bal a nced with t h e work a re a

of the a ssembly line conveyor.

c apa cit y . ~he

The decisive f a ctors of per-

achiev~

diff~~ence

minimum

between the re a l

form a nce timings for calcula tion of

work capacit y o f t h e a rea a nd area

c hanges in work a rea loading a re the

loadi ng (utiliz a tion) acco rding to

timinrs in both extreme positions,

(3) will be c a lled s l a ck

i.e. the quantities Vj,l' V j , Ij' r e specti ve l y. rhe loading of the

~his

j-th work a rea is increased b y the quantity V. 1 at t h e arrival time J, of the c roduct into the 1st position.

ca~acity.

quantity is consequently

~sed

for decision making as to which o f Dossible product -ntions) will be put

ty p es (mod ific a to the

asse~bly

line. This decision is o ad e under tota~

At the same time the j-th work

the constraint tha t t h e

station area is being left b y a

lo a ding V. must not exceed the j-th J work area real c a pa city, i.e.

r roduct with relative po sition index k equa l

to Ij. As a result

the lo a ding of a rea is decreased for all

VI. • (I) J j=1,2, ..•.•. , m.

b y quantity V"ll .. Hence the J .J resulting lo ad ing after this step

As the ~ roduct's arrival ti~e to the

will be

multi~roduct assembly line (i.e.

i-th

372

Jo Hampl and Po Skvor

~ o difica~icn

n r~ive~

st ~ tion

of 1st work

to 1st position a re a ) the

v ~ lue

To 1 is "B ut f e r

equality: K

j

~

K

j

( r IA ~l)

•

1,

= mJ.i,l

Vl ,.1.'

5 . ?RACTI:AL TESTS OF The

and more g enerally for a vector

,

algorit;~

for k = 1,2, •••.... , Ij' T~is

c 8.lcul a tion cp.n be , 'er-

The

= 1,2,

se~uence

.•• , m in the cycle.

model does not consider

t h e product arriv a l

time to the

work area sta tion. It was rroved that it is sufficient to sequence of

~ ro ~ ucts

of

k~ow

the

; erfor~ance

timing length max l} 8.nd this verifies the !) erformance timing of ~ o dification

each

d~ring

experimental control

of 8. ssembly line of Skoda cars in A.Z.N.P. works in l':ladb Bolesl a v.

formed consecutively f or all work 2.reas j

de s cribed model of

assembly line control

was applied in a decision making

= V;tJ ,_1...-.. -1

V '"'j,k

~bove

muJti~roduct

AIGCRIT~:

for a ll wc rk

st2. tions.

I'heroducti0n control experiment was carried out in A.Z.N.P. works enabled

p r ~ ctic a l

o p er a tional

verification of the ma thematical model of assem >ly line control includine the

re~ated

functions of in-

for m3 tion s y stem (r8cording of bodies coming to the assembl y line, o s er a tor's ~ctivities by system initiation and/or fai:ure, control of conveyor's stocka ge branches

Using the v a lues of techno-

a ccording to the required

o~tions

logical perform a nce timings in j-th

of sub-assemblies, etc.). The fact

row of (To 0) matrix

that the s y stems people and shop

lJ

~he

cap ~ city

m~nagement

requirement for j-th work area for

floor

scheduling interv a l x ma y be calcu-

verify beforeha nd the use of nrocess

lated

control com p uter for t h e actual

ex pression

fro~

could test 8.nd

assembl y line control contributed significantly to the possibility of p l a nned early informa tion system

n

i: i = 1

To

lJ 0

•

PlO (x)

implementation.

•

By simple comparison of real work capacity K

j

from the relation (1),

with c a pacity requirements derived fro~ Ko

(:PIAN) a ccording to relaJ tion (9) the possibility of ful-

fillment of the appropriate time period x I roduction schedule may be evaluated. Also critical work stations, that are not

in

position

to fulfil the plan, can be found. The [ roduction schedule can be fulfilled under existing work capacities if holds t h e following un-