CLICK ON The X-Upper Right of Each Window

Otherwise, to end the session

CLICK ON The X-Upper Right of Main Window.

7. MANUFACT.GPS

Simulation of an electronics manufacturing system.

Problem Statement

A manufacturing department of an electronics company makes digital watches. In the dispatch department, the watches are packed by an automatic packing machine, in display packets, in the quantities ordered by retailers. The order size is given by the following function.

Order Size

Frequency .10 .25 .30 .15 .12 .05 .03

The mean time between order arrivals is 15 minutes, exponentially distributed. The packing time per order is 120 seconds plus 10 seconds per watch packed in the order. The manufacturing department produces the digital watches in lot sizes of 60 units, in 455 minutes.

Simulate 5 days of the company operation to provide the following information:

1.The average number of orders waiting in the packing department.

2.The quantity of watches dispatched each day.

3.The distribution of transit times of orders.

Listing

; GPSS World Sample File - MANUFACT.GPS, by Gerard F. Cummings

***********************************************************************

*Manufacturing Company *

***********************************************************************

*Time Unit is one hour *

Sizeorder FUNCTION RN1,D7 ;Order size

.10,6/.35,12/.65,18/.80,24/.92,30/.97,36/1.0,48 Transit TABLE M1,.015,.015,20 ;Transit time Number TABLE X1,100,100,20 ;No. packed each day Ptime VARIABLE .0028#P1+0.0334 ;Packing time Amount EQU 1000 ;Initial stock amount

Stock STORAGE 4000 ;Warehouse holds

;4000 units

***********************************************************************

GENERATE (Exponential(1,0,0.25)) ;Order arrives ASSIGN 1,1,Sizeorder ;P1=order size

TEST GE S$Stock,P1,Stockout ;Is stock sufficient? LEAVE Stock,P1 ;Remove P1 from stock

QUEUE Packing

SEIZE Machine ;Get a machine DEPART Packing

ADVANCE V$Ptime ;Packing time RELEASE Machine ;Free the machine

SAVEVALUE 1+,P1 ;Accumulate no. packed TABULATE Transit ;Record transit time TERMINATE

Stockout TERMINATE

***********************************************************************

GENERATE 0.75,0.08334,1 ;Xact every 40+/-5 mins ENTER Stock,60 ;Make 60, Stock

;increased by 60

Stockad TERMINATE

***********************************************************************

GENERATE 8 ;Xact every day TABULATE Number

SAVEVALUE 1,0

TERMINATE 1

***********************************************************************

GENERATE ,,,1,10 ;Initial stock xact ENTER Stock,Amount ;Set initial stock TERMINATE

***********************************************************************

The model is organized into several segments. After the function, variable, and Storage Entities are defined, there are four more model segments. Transactions in the top segment represent orders, transactions in the next segment add lots of 60 watches to the inventory, transactions in the next segment tabulate daily sales and time the overall simulation in days, and the transaction in the bottom segment initializes the stock level to 1000.

Time units are in hours. The stock level is represented by S$Stock, the current Storage Content in the Storage Entity Stock. Therefore, we use LEAVE to take from inventory, ENTER to add to it. Unfilled orders are stockouts. The warehouse space available is 4000. The stock level, represented by S$Stock, is initialized to 1000 when the simulation starts by a high priority transaction in the final segment of the model. This transaction will be the very first one to move in the simulation..

Running the Simulation

To run the simulation and create a Standard Report,

CHOOSE File / Open

and in the dialog box

SELECT MANUFACT

and then

SELECT Open

Next, the simulation must be created.

CHOOSE Command / Create Simulation

then

CHOOSE Command / START and in the dialog box, replace the 1.

TYPE 5

and

SELECT OK

The simulation will end when 5 transactions have entered the TERMINATE 1 Block. This represents 5 days of operation.

When the simulation ends, GPSS World writes a report to the default report file, Manufact.1.1. As discussed in Chapter 1, the Report extension will vary depending on saved simulations and previously existing reports. For our purposes, we will assume that this is the first time the simulation has been created and run giving an extension of 1.1.

This report will be automatically displayed in a window. If you close the window, you can reopen it by using the GPSS World File / Open in the Main Menu. Then

you should choose Report in the "Files of type" drop down box at the bottom of the window. GPSS World reports are written in a special format. If you wish to edit the report, you will have to copy its contents to the clipboard and from there into a word processor. You will not be able to open the file directly in a word processor.

Discussion of Results

We can answer the original questions from data in the Standard Report:

∙The average number of orders waiting in the packing department was only 0.12. This is taken from the report for the Ave. Cont. value under the Queue Entity Packing.

∙The distribution of transit times of orders is given in the Table Transit.

∙The distribution of watches dispatched each day is given in the Table Number.

Inside the Simulation

Let us now explore the ending condition of the simulation which generated the Standard Report above. If you are not at the end of the simulation, please Retranslate the model and run it again.

Let's use the SHOW Command to look at some System Numeric Attributes. First, the number of orders received.

CHOOSE Command / SHOW

and in the dialog box

TYPE N1

SELECT OK

You’ll see the results (169) in the Status Line of the Main Window and in the Journal Window as well. Next, the number of times a stockout occurred.

CHOOSE Command / SHOW

and in the dialog box

TYPE N$Stockout

SELECT OK

Now, let’s open some graphics windows.

CHOOSE Window / Simulation Window / Storages Window

This is the Storages Window. The stock level is represented by the current content of the Storage Entity which you will see is 898. You will find this value under Storage In Use in the Storages Window.

The current stock level is quite high. If we cannot increase sales, we should reduce the production cutoff level.

Let’s explore the other graphics windows, but first close the Storages Window.

CLICK ON The X-Upper Right of Storages Window

then

CHOOSE Window / Simulation Window / Facilities Window

This shows that the utilization of the packing machine was low. The color of the icon tells us that the machine is currently in use. Take a look at other values that are available to you in the detailed side of this window. Perhaps a less expensive machine would do. Let’s look at the Transit Table

CHOOSE Window / Simulation Window / Table Window and in the drop-down box use the down arrow to find Transit

CLICK ON The down arrow in the Table selection box

CLICK ON Transit

SELECT OK

This shows the distribution of order transit times. Next, look at the Number Table.

CHOOSE Window / Simulation Window / Table Window and in the drop-down box the Table name Number is selected.

SELECT OK

The Number Table shows the histogram of watches packed each day. Before we go further, let’s close up the windows we have opened so far.

CLICK ON The X-Upper Right of Graphics Windows

Now, let’s do some magic. Let’s create an order when the inventory is near its maximum level. First, open the Blocks Window.

CHOOSE Window / Simulation Window / Blocks Window

Now, use your mouse to select the first TERMINATE Block in the model (the one immediately following the TABULATE Block or Block 12). Place the mouse pointer over the Block and

CLICK ON The Block Icon of the TERMINATE Block

Did you see the Block light up? Good. Now, let’s put a Stop Condition on the Block.

CLICK ON The Place Icon in the Debugging Toolbar

CHOOSE Command / START

and in the dialog box, replace the 1.

TYPE 5,NP

and

SELECT OK

When the simulation stops, move the active transaction up to the order entry segment.

CHOOSE Command / Custom

and in the dialog box

TYPE TRANSFER ,2

SELECT OK

Now, watch the new order by Stepping through the simulation.

PRESS p

several times. This is only the tip of the iceberg. We can apply ANY Block statement to the active transaction in manual simulation mode. We can manually SPLIT new transactions, make Facility and Storage Entities unavailable to represent an outage, we can even use EXECUTE to initiate any Block that exists in the model.

Feel free now to experiment with the parameters of the manufacturing system. You may feel that a different scheme may be best. Many judgments of the behavior of a system are subjective, and your notion of optimization may depend on many other factors. For this reason, it is not always safe to rely blindly on someone else’s opinion.

You may stop here or choose to go on to the next model.

If you wish to go on to the next lesson, close all windows related to this model.

CLICK ON The X-Upper Right of Each Window