Inlämningsuppgift SIM
THE LEEDSSIM FACTORY
The LeedsSim factory is a traditional assembly facility working as a subcontractor to the
telecommunications industry. Their main product is a switchboard cabinet used in the 3 rd
generation (3G) network base stations. The product is available in two versions: Standard and
Delux. The company has been very successful in increasing sales and with the 3G expansion
taking off in Europe, orders are piling up. On average 40% of the orders are for Delux and 60%
are for Standard. Unfortunately, the operations department has had some problems with
reaching the desired (and necessary) productivity. They have therefore decided to seek your
help to create a simulation model of the involved processes as a tool for analyzing and
improving the production process.
To find the right level of detail in the model description, they urge you to start with a simple
model and then successively add more details until a suitable model with the right level of
complexity is obtained. The simulation should be run over a two-month period of 8-hour
workdays (eight weeks 5 working days per week).
A schematic flowchart of the manufacturing process is depicted below.
Storage of
inbound material
Parallel operations
(Workstation 1-2)
Assembly
(Workstation 3)
Painting
(Workstation 4)
Inspection
(Workstation 5)
1
3
4
5
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Raw material arrives by truck once every week (on Monday morning) to the storage area for
inbound goods. Every shipment contains 21 units each of three different raw materials, A, B and
C. A cabinet consist of one unit each of these three raw materials. Workstation 1 requires one
unit each of raw material A and B to produce subcomponent 1. Workstation 2 requires one unit
of raw material C to produce subcomponent 2. The subcomponents made in workstations 1 and
2 are the same for both Standard and Delux. The difference between the two products
materialize in the Assembly and Painting operations. In Workstation 3 (the Assembly operation)
subcomponents 1 and 2 are assembled into a cabinet. The assembly operation is different for
Standard and Delux as shown by the difference in processing times in Table 1.
The painting operation in Workstation 4 also follow different time distributions for standard
and Delux cabinets, see Table 1. Only one cabinet at a time can be painted, and there is no place
to store painted cabinets in the paint shop. If the cabinet is not removed, the paint shop is
blocked and no new cabinets can be painted (hint: to prevent items from leaving the painting
operation use the “Activity, service” block in the discrete event library).
The inspection of painted cabinets is done in exactly the same way for Standard and Delux. The
inspection is carried out by a worker with special training. At the moment, only one employee in
the company has the necessary training to carry out inspections. The production manager at
LeedsSim has collected data regarding the inspection times. After having performed a chisquare-test, he concluded that the inspection time can be modeled as an exponentially
distributed variable with mean 1.2 hours.
After completed and approved inspection, the finished cabinets leave the factory. The cabinets
are immediately transported by truck either to a central warehouse or directly to the customer. It
is noteworthy that currently each workstation can only process one item at a time. Moreover, the
physical layout of the plant allows for buffers of components before and after Workstation 1,2,3
and 5, but only before Workstation 4 (the paint shop).
The internal logistics within the plant is such that the transportation times for incomplete
cabinets can be neglected. However, to transport fully assembled cabinets to and from the paint
shop, a special type of forklift truck is needed. The transportation time from the assembly line to
the paint shop is exponentially distributed with a mean of 45 minutes. The transportation time
between the paint shop and the inspection station on the loading dock is normally distributed
with mean 60 minutes and standard deviation of 15 minutes. Cabinets that pass inspection are
immediately picked up by the contracted transportation company. After a completed delivery,
the forklift truck always returns to the strategically located parking area to get new instructions.
The travel times between the parking area and its various destinations are negligible. There is
currently one forklift truck available in the plant.
In order to adequately describe the process, it is also important to recognize that not all painted
cabinets pass inspection. On average, 10% of the Standard cabinets and 30% of the Delux
cabinets fail inspection. A cabinet that fails the inspection is put aside in a buffer and then
transported with the forklift truck to be reworked at another facility (you do not need to model
the rework facility). The transportation time to the rework facility is approximately 30 minutes,
and can for our purposes be considered constant (times to and from the parking area are still
considered to be zero). The orders for the forklift truck are to prioritize; firstly, transportation
from the paint shop to the inspection, secondly, transportation from the inspection to the rework
facility, and lastly, transportation from the assembly to the painting operation (hint: when using a
labor pool or resource block the order in which you place the connectors determines the priorities of the
workers).
The performance measures that LeedsSim wants to track are the number of cabinets produced of
Standard and Delux, respectively, and the average capital tied up in work in process inventories
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(number of units of raw material in the production system). However, they also want to keep
track of:
 The inventories at different parts of the workshop, including inbound storage (the
maximum, the mean and the standard deviation).
 The cycle time for a job before and after assembly. More precisely, (1) the cycle time for a
unit of raw material measured from the instance it arrives to the inbound storage area
until one of each subcomponent reaches the buffer before workstation 3, (2) the cycle
time for Standard and Delux cabinets, respectively, measured from the moment one of
each subcomponent reaches the buffer before workstation 3 until the inspection is
completed (both the mean and standard deviation).
 Utilization of workstations and equipment like the forklift truck.
Table 1 Estimated processing time distributions
Workstation
Product
Processing
time distribution
1
Standard, Delux
Triangular
2
Standard, Delux
Lognormal
3
Standard
Triangular
Delux
Triangular
Parameter values (hours)
Min=1; Max=2; Most likely=1.5
Mean=2; Std.Dev = 0.5
Min=1; Max=3; Most likely=2
Min=1.5; Max=4; Most likely=2.5
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Standard
Delux
Triangular
Triangular
Min=2; Max=4; Most likely=3
Min=2; Max=4; Most likely=3.5
5
Standard, Delux
Exponential
Mean=1.2
Issues that LeedsSim want you to analyze and provide answers to under your contract
1) Describe the production process graphically in a flow chart and verify it with one of
LeedsSim’s representatives (=övningsassistenterna). Be careful to include as much of the
logic as possible. The purpose is to make sure that you fully understand the process
before you start building the Extend model. LeedsSim have experienced problems with
consultants that have not validated the models they have built, which have been costly
for the company.
2) Build a simulation model of the production process in Extend.
3) Use the simulation model to analyze the existing production process.
Run the simulation 30 times with different random seeds and estimate the mean of the
relevant performance measures analyzed above with 95% confidence intervals, that is:
a) The number of Standard and Delux cabinets being produced.
b) The utilization of all resources.
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c) The cycle time for a unit of raw material. The cycle times for Standard and Delux,
respectively.
d) The inventory levels in all buffers.
(Hints: LeedsSim is only interested in the cycle times, etc expressed in work hours, you do not have to
take into consideration that production is shut down for 16 hours a day. This applies to the entire
assignment. To efficiently collect the data and analyze it: Use the Resource Stats, Activity Stats,
Queue Stats and Mean & Variance Stats blocks, check “Append new updates” and click on confidence
interval. For the number of cabinets produced use a “Mean and variance” block, connect it to the exit
block, check the dialog options “calculate for multiple simulations”, “calculate confidence interval”
and “use number of inputs-1”. Do not use the “M&V Stats block” to estimate the throughput.
4) How can the production process be improved? LeedsSim needs to increase productivity
(and thereby reduce costs). Based on your understanding of the production process,
suggest at least two design changes and try them out! What are the benefits in expected
total cost for capital tied up in inventory, cycle times and throughput. What are your
conclusions? What is your recommendation to LeedsSim as to how to improve their
operations?
Clearly, your improvement suggestions are associated with implementation costs.
However, your analysis does not need to include a specification of these costs. LeedsSim
is at this stage only interested in the improvement potential of different solutions. The
costs associated with their implementation is weighed in at a later stage, when the
decision about which changes to implement is made.
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Redovisning av ert uppdrag och formalia
Uppgiften utförs i grupper om 3-4 teknologer och redovisas i form av en välskriven och
välstrukturerad teknisk rapport. Rapporten ska beskriva problemet (ert uppdrag), er ansats för
att angripa/lösa det, er analys samt era resultat och slutsatser. Bilagor kan användas för att
redovisa tekniska beräkningar eller data som ni använder för er analys. De konkreta uppgifterna
1) – 4) måste givetvis besvaras ordentligt. Relevanta Extendmodeller ska skickas till Fredrik
Olsson och refereras till i rapporten. VIKTIGT! Innan ni skickar modellerna måste de läggas in
en zipfil. Denna zipfil ska namnges med gruppmedlemmarnas namn. Ange också i ert e-mail
alla gruppmedlemmarnas namn. Rapporten med alla gruppmedlemmarnas namn på framsidan
lämnas i pappersformat till Fredrik Olsson. Utnyttja handledningstillfällena genom att komma
väl förberedda!
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