Witness Tuto

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2012 Lanner Group Limited

WITNESS Getting Started Material


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Page I 2012 Lanner Group Limited

Table of Contents

Part I Preface 1

Part II What is new? 2

Part III Getting installed and running 3

Part IV Licensing your copy 9

Part V Getting Started 10

................................................................................................................................... 101 Simulation

................................................................................................................................... 112 The WITNESS Product Family

.......................................................................................................................................................... 11WITNESS

.......................................................................................................................................................... 11WITNESS Scenario Manager

.......................................................................................................................................................... 12WITNESS Documentor

.......................................................................................................................................................... 12WITNESS Optimizer

................................................................................................................................... 123 Lanner Group

Part VI Building your first model 14

................................................................................................................................... 151 Stage 1

................................................................................................................................... 192 Stage 2

................................................................................................................................... 233 Stage 3

................................................................................................................................... 274 Stage 4

................................................................................................................................... 335 Stage 5

................................................................................................................................... 346 Summary

Part VII Case study: JETTY.MOD 35

Part VIII Conducting a Simulation Project 44

................................................................................................................................... 441 Overview

................................................................................................................................... 442 Establish objectives

................................................................................................................................... 453 Scope and level of model detail

................................................................................................................................... 454 Data Collection

................................................................................................................................... 465 Structuring the model

................................................................................................................................... 466 Building the model

................................................................................................................................... 477 Running the model

................................................................................................................................... 478 Generating reports

................................................................................................................................... 489 Testing the model

................................................................................................................................... 4810 Experimentation

................................................................................................................................... 4911 Documentation

................................................................................................................................... 4912 Presentation of results and implementation
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Part IX Modeling With WITNESS 51

................................................................................................................................... 511 Elements: the WITNESS building blocks

................................................................................................................................... 512 Discrete Elements

.......................................................................................................................................................... 51Parts

.......................................................................................................................................................... 52Buffers

.......................................................................................................................................................... 52Machines

.......................................................................................................................................................... 52Labor

.......................................................................................................................................................... 53Conveyors

.......................................................................................................................................................... 53Paths

.......................................................................................................................................................... 53Tracks

.......................................................................................................................................................... 53Vehicles

................................................................................................................................... 533 Power & free elements

.......................................................................................................................................................... 53Network

.......................................................................................................................................................... 53Section

.......................................................................................................................................................... 54Station

.......................................................................................................................................................... 54Carrier

................................................................................................................................... 544 Continuous Processing Elements.......................................................................................................................................................... 54Fluids

.......................................................................................................................................................... 54Processors

.......................................................................................................................................................... 54Tanks

.......................................................................................................................................................... 55Pipes

................................................................................................................................... 555 Logical Elements and Modules

.......................................................................................................................................................... 55Attributes

.......................................................................................................................................................... 55Variables

.......................................................................................................................................................... 55Files

.......................................................................................................................................................... 55Distributions

.......................................................................................................................................................... 56Functions

.......................................................................................................................................................... 56Part Files

.......................................................................................................................................................... 56Shifts

.......................................................................................................................................................... 56Modules

................................................................................................................................... 566 Graphical & Reporting elements

.......................................................................................................................................................... 56Pie Charts

.......................................................................................................................................................... 56Timeseries

.......................................................................................................................................................... 57Histograms

.......................................................................................................................................................... 57Reports

................................................................................................................................... 577 Manipulating elements: Rules, Expressions and Actions

.......................................................................................................................................................... 57Rules

.......................................................................................................................................................... 57Expressions

.......................................................................................................................................................... 57Actions

Part X Using and enhancing a WITNESS model 59................................................................................................................................... 591 WITNESS Model Storage Formats

................................................................................................................................... 592 Help

................................................................................................................................... 593 Enhancing the screen display

................................................................................................................................... 604 Useful buttons

Part XI Example models 62

................................................................................................................................... 621 Dem1.mod - Simple Assembly Model
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Page III 2012 Lanner Group Limited

................................................................................................................................... 662 Dem2.mod - Simple Logistics Model

Part XII Model Gallery 69

................................................................................................................................... 691 Typical Manufacturing Model

................................................................................................................................... 702 Typical Warehouse Model

................................................................................................................................... 713 Office Model

................................................................................................................................... 724 Call Centre Business Process

................................................................................................................................... 735 Airfield Logistics

................................................................................................................................... 746 Tracks and Vehicles

................................................................................................................................... 757 Garage Forecourt

................................................................................................................................... 768 Chemical Processing Plant

Part XIII Further learning resources 77

................................................................................................................................... 771 Learning More

.......................................................................................................................................................... 77Support

Part XIV Appendix 79

................................................................................................................................... 791 System requirements

................................................................................................................................... 792 Hardware Dongle (On a local computer)

................................................................................................................................... 833 Installation of License Server

.......................................................................................................................................................... 87Running WITNESS on Client Machines

Part XV Copyright & trademarks 88

Index 0
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Page 1


1 Preface

Dear User,

Thank you for choosing WITNESS from Lanner Group. With Lanner simulation, business managers

can model, analyze and optimize processes to make superior decisions in a risk-free environment.

Lanner simulation is the key to improving productivity, efficiency and reducing costs. The aim of

this document is to make your first experience with our latest build of WITNESS highly productive.

Feedback on ways we can improve your experience and use is always appreciated.

Yours sincerely

David J ones

CEO


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2 What is new?

WITNESS 12

Simplified WITNESS Installation

Windows 7 compliance

64-bit platform support

WITNESS 'Visual Studio 2010' style U/I delivering the maximum modeling area on one and two

screen wide-angle displays

Support of two screen displays

Dock tool windows to the edge of a screen

Float windows over or outside a screen.Minimize tool windows along the edge of a screen.

Display windows on different monitors.

Tile windows in the modeling window.

Reset window placement to the default layout.

Undo/Redo

Full screen display option

'Mouse over' enhance tool tips

'Mouse wheel / CTRL' zoom in/out

Go online to see more on earlier releases http://www.lanner.com/en/media/witness/whats_ new.cfm
http://www.lanner.com/en/media/witness/whats_new.cfm


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3 Getting installed and running

To install WITNESS you first need to know whether you are installing a stand-alone copy on a single PCor whether it is to be linked into a network security system. Halfway through the installation process you

will be prompted as to whether you are using a Hardware Dongle ( on the local computer) or License

Server ( on a local network) . P lease note that if you make the wrong response at this stage, it is not a

problem, as it is possible to alter the choice in a simple dialog when WITNESS is run.

The current platforms for WITNESS are Windows XP, Windows Vista and Windows 7 please be

aware that some of these require certain service packs to be applied see system requirementsfor full

details.

The current licensing systems for WITNESS are Hardware Dongle ( USB) and Licence Server

( Reprise ) . Older security systems also exist but are being actively retired. If you have a differentsystem or unsure, please contact your Lanner representative.

For stand-alone systems secured by a Hardware Dongle, there is no server program necessary, as the

licence will be linked to the dongle device itself.

For users of a Licence Server system, the license server program will need to be installed on a server

before the software can be licensed and run. You may still install WITNESS on an individual PC before

this is done, but the security setup can only be completed after the license server installationand is

running, as the name of the server and the appropriate licence code will need to be entered.

Please Note: A License Server is a local network license, not a Global license; it does not give the

user the right to move or use the software to other countries.A License Server is not licensed tooperate on a virtual machine environment. P lease contact the Lanner support deskif this is required.

After installation all WITNESS systems need a licence code to work for details on how to obtain this

please read the section on licensing your copy

WITNESS is supplied by downloading from the internet( or can be supplied on DVD by request

please contact your Lanner representative) . In either case, run the setup.exe file in the root directory of

the image or the DVD, then follow on-screen instructions. When using a DVD the install program may

start automatically. We strongly recommend that you install WITNESS on each system that is required

to run it. It is possible to install WITNESS on a central server, but this will greatly reduce the

performance and responsiveness of the software.

WITNESS can be installed in the following configurations: Full WITNESS ( Commercial) , Full

WITNESS ( Educational) or Run Only. Run Only WITNESS licence's do not allow changes to be made

to model logic using the WITNESS dialogs all OK buttons are disabled. The Full Commercial and Full

Educational options offer exactly the same level of functionality in WITNESS, but the correct option

must be chosen to work with issued dongle / license. If you are unsure then please contact your Lanner

representative. The distinction between systems allows Lanner to recognise the version in which

models have been created to ensure academic licence's are not used for commercial work.
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A Full WITNESS software license enables WITNESS, Scenario Managerand Documentor. Your licence

may be extended to allow WITNESS Optimizer.

Installation1. Run the setup.exe file and follow the on screen instructions. The installation should take lessthan 5 minutes.

The on-screen instructions will offer you the following choices:

Manufacturing or Service edition and Options( optional modules and examples)

Manufacturing or Service Version of WITNESS: The English version of WITNESS is available in two

editions. The difference between the 2 editions is only certain names used in the software. The

manufacturing edition calls the elements machines, parts, buffers and talks about parts being shipped

or scrapped. The service edition calls the elements activities, entities and queues and talks aboutentities being served or lost. Choose the appropriate edition for the industry you are to model.

In order to make use of Optimizer you will need to have purchased the necessary additional license.

Licensing options- Hardware Dongle ( on the local computer) or License Server ( local network

license)

The current main licensing systems for WITNESS are the ( USB) Hardware Dongle and the

( Reprise Network) Licence Server. Older security systems also exist but are being actively retired.

If you have a different system please see the appropriate section in this document or contact Lanner

if you are unsure.

Full WITNESS for commercial or educational use or a reduce 'Runtime' only version

Full WITNESS ( Commercial) , Full WITNESS ( Educational) or Run Only. Run Only WITNESS

licences do not allow changes to be made to model logic using the WITNESS dialogs all OK

buttons are disabled. The Full Commercial and Full Educational options offer exactly the same level

of functionality in WITNESS but the correct option must be chosen to work with issued dongle /

license. If you are unsure then please contact your Lanner representative. The distinction between

systems allows Lanner to recognise the version in which models have been created to ensure

academic licences are not used for commercial work.


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2. The second step to get WITNESS running is to get a security code for the supplied Hardware


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Dongle( to operate on local computer) or link via your local network to a WITNESS License

Server.
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4 Licensing your copy

WITNESS requires you to register your copy with Lanner and receive a license security code. Lanner

offers two licensing security technologies, hardware dongle ( on local computer) or License Server( Local Network) .

Hardware Dongle is a 'standalone' installation enables WITNESS to run on a selectedlocal PC.

License Server is a floating license that permits an agreed number of users to runWITNESS across a company network. The license server is a machine remote from the WITNESS

users machines that serves multiple licenses to users across the network. When the agreed number

of WITNESS copies is running on the network, no more can be started until a license is freed by

another user closing down WITNESS.

Please Note:

A License Server is a local network license, not a Global license; it does not give the user the right to

move or use the software to other countries.

A License Server is not licensed to operate on a virtual machine environment. P lease contact the

Lanner support deskif this is required.

Installation instructionsfor:

Hardware Dongle ( on local machine) Click here

License Server ( local network) Click here


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R. D. Hurrion, Engineering Journal of Operations Research

WITNESS is Lanner Group s simulation software package. It is the culmination of more than a

decade s development experience with computer-based simulation. This experience has led us to

evolve a visual, interactive and interpretive approach to simulation without the need for compilation.

There are currently more than 6,500 WITNESS systems in use worldwide, in organizations ranging

from automotive to pharmaceutical, aerospace to electronics, hospitals to banks, airports to defense

and more. The WITNESS Manufacturing Performance Edition is the version of WITNESS specially

designed for manufacturing applications. It is ideally suited to a variety of production and storage

layout and logistical modeling scenarios.

5.2 The WITNESS Product Family

5.2.1 WITNESS

WITNESS Can Provide The Right Answer For Your Business

- Reduces Risk in Decision Making - Accurately predicting how proposed changes to a facility or

process will work.

- Supports Investment Decisions - P rovides evidence to identify profitable ideas and avoid costly

mistakes.

- Communicates the Optimal Solution to Decision Makers - Enables scenarios to be compared and the

best option for the business to be graphically reported.

- Generates an Operational Asset - Simulations deployed as robust applications become an asset in

operational decision making.

WITNESS is an established dynamic process simulation tool used by thousands of organizations to

validate a proposed facility or business process, to achieve a desired process performance metric or to

support continuous process improvement activities.

How WITNESS Works

WITNESS works by enabling you to represent a real world process in a dynamic animated computer

model. You see exactly how the process is operating as time advances. A simulation model can

incorporate all the variability of real life experience ( variable reliability, process times, resource

efficiency etc.) . This ability to include real life variability is a key advantage over spreadsheet models

where averages must be used. WITNESS includes a quick3D capability that is present in Full

WITNESS. Quick3D allows the creation of a 3D view of a WITNESS model using Lanner standard

library objects. For more information on WITNESS go to http://www.lanner.com/en/witness.cfm

5.2.2 WITNESS Scenario Manager

A module included as standard with both the manufacturing performance edition and the service and

process performance edition of WITNESS.

It offers a highly flexible framework for experimentation and analysis including:

Choice of model, run length, number of replications
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Specification of model parameters for experimentation

Parameter groups for linked input values

Full factorial simulation runs with progress indicators

Comprehensive statistics generated automatically for each model run

Flexible charting

Wizards to help

Professional SQL Server database repository

Option to add non simulation parameters such as investment level

Compare different experiments on different models with aliasing to align statistics for

comparison and charting.

For a full walkthrough of the Scenario Manager functionality please look at the example analysis in the

documentation directory ( found under the WITNESS directory) after installation.

5.2.3 WITNESS Documentor

The Documentor is a module included as standard with all WITNESS editions

It allows you to create a simple formatted version of the information included in a WITNESS model.

Accessed off the Model menu it offers an interface to select model elements and the details required in

the report. In this way it is easy to view the elements used, notes created, logic rules and actions for

part or all of a model in a single report.

5.2.4 WITNESS Optimizer

WITNESS Optimizer finds the best solutions for your simulation model. You choose a measure of

performance which is fully customizable, set the parameters that are allowed to change and the

optimizer will perform experiments intelligently to find the best solutions. It is an intelligent helper and

will work tirelessly to improve your results.

Using the latest in sophisticated mathematical techniques, it offers an easy-to-use interface and the

presentation of optimal results in a selection of useful and innovative tables and graphs. 5.3 Lanner Group

Lanner simulation software enables business process improvement for world leading organizations.

With Lanner simulation, business managers can model, analyse and optimize processes to makesuperior decisions in a risk-free environment. Lanner simulation is the key to improving productivity,

efficiency and reducing costs.

Lanner's advanced simulation technology is supplied to simulation professionals through its

WITNESS brand. Lanner's L-SIM brand has quickly established itself as the leading embedded

simulation engine used in enterprise suites from the top software solutions suppliers. Lanner's

technology is also embedded within its expanding range of individually badged, task specific

simulation and planning applications. Based in the UK with subsidiaries and partners in Europe, The


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Americas and the Far East, Lanner applications are used by more than 3,500 companies globally.

Please visit the Lanner website for the latest product and support information :

Contact our web site for support and information: www.lanner.com
http://www.lanner.com/


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6 Building your first model

Introduction

This is a description of how to build and use a simple WITNESS model. The model demonstrates

concepts, and is not meant to represent a real industrial system. Much larger and more detailed

models can be developed in WITNESS. The model is built incrementally. In this way you can ensure

that each stage of the model is correct before you go on to the next stage, and it is possible to see

clearly the effects of any change that you make.

In stages 1 and 2, you build a simple model containing machines and buffers, and understand how to

run a model and obtain results. In stage 3 you add a simple conveyor and a further machining

operation and in stage 4 you add further characteristics - breakdowns and labor. By stage 5 the model

is complete, and you will learn how to view your model running in three dimensions. With even a

small increase in the complexity of the model it becomes difficult to calculate the statistics manually

and it is seen that simulation is essential to determine throughput.

If you don t want to build the model, but would like to see the incremental approach to model

building, run models STAGE1.MOD to STAGE4.MOD in the demo\tutorial subdirectory of your

WITNESS installation. These model files correspond to the model at the end of stages 1 to 4.

Model summary

In the model, Widgetsare produced, washed and weighed. After each operation parts are passed to a

buffer in front of the next operation. After production they travel on a conveyor to a packing machine,

are packed in twos and then leave the model. A technician is required to operate the packing machine

and is also used to repair the wash machine when it breaks down.


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( called Machine001) appears selected in the simulation window.

Hold down the left mouse button whilst the cursor is inside the net and you can drag the new

element around the screen to position it. Click outside the net to remove it.

The machine is now part of the model you are building.

Add a buffer by clicking on the buffer icon in the designer elements window. Once it is selected

move the cursor into the layout window and click again to place the display which represents the

buffer. Drag the buffer to the position that you want.

Add a part in the same way.

The three elements for this first stage are now all added to the model and the next step is to add the

detail needed to run the model.

Process Flow Logic

The next step in this model is to define logic rules linking these elements together. The simplest way

toinput these rules is by the toolbars and the mouse. There are several types of connection rules in

WITNESS ( please see the Quick Reference Book for full details) . Common ones include push and

pull, percent and sequence rules. Less common ones also exist to offer complex rules which will

match attributes or conditions, if rules to enter complex decision logic, etc.

We will begin by using simple PUSH and PULL rules to describe the flow of parts through the system.

Machine rule information

Select the machine Machine001by clicking on its icon. Now click on the visual pullbutton on

thestandard toolbar:

The default rule should be pull leave this in this case but note that others are selectable from

the pull down combo box.

Click on the Part001display and then the WORLDbutton and you should see the following:

Hin t: When you click on the Part001 display , it may appear that nothing has happened. There is

a small message at the bottom left of your screen that shows the current part type selected. It

initially say s 'Current part: ALL', and will change as the part is selec ted.

WORLD is a term used by WITNESS to indicate parts that are outside the model, and hence will be

created and brought into the model by this command.


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see the names of all of the elements as you add them to your model. If it is not already open, you

should open it now by using the element selector button from the toolbar.

In the element selector window, select Part001. Now click on Part001 again ( don't double-click

or it will open up a far more complex data form that we will look at later on) , and you will be ableto type in a new name. Call it Widget.

Repeat the process to change the names of :-

Machine001 to Produce

Buffer001 to QWash.

The model has not changed in terms of how it will perform, simply in terms of how it looks and what

things are called. Check this out by re-running your model and ensuring that the results are correct.

Note the use of the Begin button to rewind the simulation back to time zero in order to re-run a

simulation.


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This corresponds to the model STAGE1.MOD.

6.2 Stage 2

Now you have created one machine and one buffer, and checked that the model works so far, you can

add more model elements.

The value of building a model in incremental stages like this cannot be emphasized too strongly. It is

much easier to isolate mistakes and potential problems and it enables you to understand what is

happening much more clearly. Additions and changes can be made at any time and these are

incorporated immediately without the need to reset your model.

You can continue building your model using the part and machine that you have already used in the

Basic set of designer elements. However, as a quick way of selecting machines with different

appearances, there are some alternatives available in the 'More Buffers' and 'More Machines' designer

groups. First of all select the 'More Machines' designer group.

Add another machine to the model by clicking on Mc_ Washin the designer elementswindow

and positioning the new machine to the right of the existing buffer.

In terms of its logical behavior, this machine is exactly the same as the machine that you added

previously; however, you will see that it's display has been configured differently.

Change the name of this machine from the element selector as before.

Its new name is : Wash

When prompted, enter the Cycle Time : 4

Now add another buffer following the WASH machine. This time use the 'Bf_ Count' designer

element from the 'More Buffers' designer group; you will find that this displays a count of how

many parts are in the buffer rather than showing the parts. Change the name of this buffer in the

usual way:

Name : QWeigh

Add another machine based on the Mc_ Weigh machine in the 'More Machines' group, and as

before set:

Name : Weigh

When prompted, enter the Cycle Time : 3

Rules

In order to make the model run, the last step is to enter the input and output rules which control the

flow of parts through the model in the same way as in stage 1.

Click on the Wash machine to select it.


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( note Wash is displayed in the selectbox on the standard toolbar) .

Click on the visual pullbutton.

Then click on the QWashbuffer display.

Click on OKto accept the rule Pull QWash

Then for the WASH machine:

Use the visual pushbutton to create the rule Push QWeigh

Next, for the WEIGH machine:

Use the visual pullbutton to create the rule Pull QWeigh

Use the visual pushbutton to create the rule Push SHIP

( use the SHIP button on the VISUAL RULE Bar)

SHIP is an expression used by WITNESS to represent things that are outside the model. It is similar

to the WORLD expression that we saw earlier. When a part is pushed to SHIP , it leaves the model.

Rearrange the elements on the screen if necessary. It should now look something like this :

Anticipated results

You know that the time taken to process a widget now is :

3 minutes to produce

4 minutes to wash

5 minutes to weigh

Therefore the first widget will be produced ( shipped) after 12 minutes and one every 5 minutes

thereafter ( due to the limiting rate of the Produce operation) .

Therefore it can be calculated that after 100 minutes 18 will have been shipped. Use the statistics


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report as in stage one to confirm this for a run of 100 minutes.

Modifying the model display

You may wish to modify the way elements are displayed to enhance the look of the model.

Element displays can be selected and dragged around the screen with the mouse. You can also move

the display of several elements at a time by drawing a net around the elements with the mouse and

then dragging the net.

WITNESS allows you to lock displays on windows or display layers, and also to link the various

display items for an element together. To lock displays use the view/layersoption or the window/

controloption.

Many display items can be changed directly through selecting on the screen and using the toolbar

buttons e.g. fonts, colors and layers. All other items have their displays altered using the display

toolbar for an element. Element displays can also be stretched and deleted, this is done using the

mouse.

To delete a graphical display of an element, select the graphic to remove using the right mouse button

to access the context menu. Now select the Delete Graphicsoption to remove the graphic from the

window.

To stretch a graphic, select it with the left mouse button, then use the handles that appear around the

graphic as well as holding the Control ( Ctrl) button to change its size. Handles will only appear in

Graphical Editing mode and if the graphic is resizable. To enable the graphical editing mode use the

View\Graphical Editingmenu.

As an example of changing the graphical display, we will modify the icon for the Produce machine.

Right Click on the Produce machine icon and select 'Update Graphic...' from the menu with the left

hand mouse button.

You should see the following:


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Double click on the icon on the dialog to show the picture gallery. Then choose icon number 3 and

click OK to select from the gallery and Update to update the icon display.

Your model display should now look like this :



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6.3 Stage 3

Now we will add a conveyor and the packing machine to finish the production line.

To create a conveyor select the Transport tab in the designer elements window. Click onConveyorin

the designer window to select it and then click below Produce in the layout window.

Click again in order to position the conveyor. In order to create the correct layout for the conveyorselect it again and then use the Ctrlkey on the keyboard in conjunction with the mouse left hand

button to drag the nodes of the conveyor to the correct places. The mouse is positioned correctly to

move a node when the 4 way arrow display changes to a 2 way arrow display. Selecting and dragging

in this way the directional arrows in the center of each section of conveyor ( with the Ctrlkey)

creates additional sections and allows the creation of corners.

Lay out your conveyor roughly as shown with the co-ordinate displays turned on ( Window/Co-

ordinatesmenu item) note that the dimensions are shown for accurate depiction. Note also that the

display grid/display/snap to grid are all options please see on-line help for full details.


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Add another machine to the model by returning to the basic tab in the designer elements

window and clicking on machineand positioning the new machine to the right of the end of the

conveyor.

Change the name of the machine in the usual way. Name : Pack

Detailing an Element

You are about to enter the Detail form for the new machine. This is your first introduction to a very

powerful facility that allows you to change the characteristics and behavior of WITNESS elements.

Elements can be detailed in several ways:

The easiest way to access and change the details of an element is to double-click on the element

displayed on the screen ( that is, the machine icons or the buffer numerical display) .

Other options include:

Clicking once on the element s icon, then clicking on the detailbutton on the standard toolbar

Using the right mouse button on the element in the element selector and selectingdetail.

Change the detail of the new Packmachine by double clicking on its icon. Enter:


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Type of Machine : Batch ( select from pull down list)

Batch Min. : 2

Cycle Time : 7( Note that we have always input a cycle time previously by waiting to be

asked. You can now see how you would change the previous answers to adjust the model.)

Press the OKbutton to accept changes to the Pack Detail form.

The packing machine packs two widgets together. Often this would be classified as an assembly

machine but in this instance where it is the end of the line we will leave it as a batch machine.

Change the detail of the conveyor by double clicking it. Enter:

Length in parts : 40

Rules

Click on theWeighmachine to select it.

Click on the visual push button.

Delete SHIP in the rule

Then click on the Conveyorpath display.

Click on OKto accept the rule PushConveyor001

Click on the Packmachine to select it.

Click on the visualpullbutton.

Then click on the Conveyorpath display.

Click on OKto accept the rule Pull Conveyor001

Click on the visual push button.

Click on SHIPin the rule bar

Click on OKto accept the rule Push SHIP

Running the Model

Run the model again for 100 time units entering the index time of the conveyor when prompted to

be 1.0

After 100 time units we would expect just 8 to have been shipped when we take into account the time

spent on the conveyor and all the machines.

Adding a Variable CounterTo create a variable to record and display the output fromPackon the screen choose the designer

element VInteger( integer variable on the variables tab) and position on the screen as you have

done with other elements.

Change its name to NumberShipped( no spaces) in the usual way. You could also change its

name by double-clicking on it to go to the Detail page, and then changing its name there.


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We will now use this to count widgets that are packed.

Double click on the Packmachine to select it.

Click on the actions on outputbutton.

Enter: NumberShipped=NumberShipped+1 This is an action that occurs for every part the leaves the Pack machine;

it increases the value of our variable by one for each part counted.

( it can help to select the element name from the element names assistant on the assistant

toolbar

see quick reference guide/helptext for details)

Click on the OKbutton to accept this statement.

Click on the OKbutton to accept the Packdetail.

Experiment with the way that the variable is displayed, for example:-

Draw a net around the variable ( name and value) to select it all

Change the font ( button on the display edit toolbar) to be larger and bold

Change the font color ( another button on the display edit bar) .

The screen should now look like this:

Run the model and see the counter display change as parts are packed.


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6.4 Stage 4

Now we will add a breakdown pattern ( with variability) for the Washmachine and labor

requirements into the model.

Add a labor element to the model from the basic tab in the designer elementswindow. P osition in

the layout window.

Change the name of this element in the usual way. Enter:

Name : Technician

Random sampling

In real life, operations are subject to a certain degree of variability. You can introduce this variability

into your model by using distributions. WITNESS provides a wide range of distributions for you to

choose from ( or lets you define your own, if you wish to use your own data) .

WITNESS uses random number streams to sample from distributions. You may either specify these

or leave WITNESS to assign different random number streams in different places necessary for

repeatable and proper experimentation.

Double click on the Washmachine and select the Breakdownstab.

Click on the add breakdownbutton ( circled in red below) to add the default New Breakdown.

Change the breakdown modeto busy timefrom the pull-down list as shown below.

We will then need to tell the model how frequently the machine breaks down, and how long it takes

to repair.

Locate the cursor in the time between failuresfield of the breakdown modesection of the


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screen. Now you are going to use the distribution assistantto define this.

In the element selector window, select the 'Assistants' tab.

Expand the 'Distributions' heading to show all distributions available.

Right-click the UNIFORMdistribution ( scroll down if necessary) and select Insert withDistribution Wizard.

Enter the limits as 100and 300, and press the Previewbutton. This can be very useful to

visualize the shape of the distribution, especially when more complex distributions are used.

Click OKto accept the information entered.

Click on the Repair Timefield.

Use the same procedure as before to add a Triangledistribution

Enter the parameters asMinimum=30, Most Likely=60 and Maximum=120.

Click OKto accept the data entered & OKagain to close the machine dialog.

This means that the Washmachine now calculates:

the time between breakdownsusing the uniform distribution with equal likelihood of any time

between 100 and 300 minutes

the repair timeusing the triangle distribution to sample a time between 30 minutes and 2 hours


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This corresponds to the model in STAGE4.MOD.

Running the Model

Run the model for 5000 time units ( use the batch key to run fast if you wish) .

As you might predict from the figures for breakdown, the wash machine is not keeping up with the

work rate.

Note the queue of widgets in front of the wash machine the number of parts in this buffer is growing

steadily.

The statistics for the wash machine ( use the right hand mouse button menu to access statistics)

show us that the time spent waiting for the technician to come to repair the machine is small

compared to breakdown times and therefore the main problem is the reliability of the machine itself.

There are a number of alternative options. We'll look at just two.

First let us try adding a second wash machine.

Double click on the Washmachine and change the quantity to 2.

Reposition the display by dragging the second main icon below the first note that as the display

items are locked the other queues and icon representing the second wash machine move with the

main icon.


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Your display should look as follows:

Running the model to 5000 now indicates that the wash is no longer the bottleneck of the system

however the bottleneck has just moved downstream.

A powerful way to see this is to draw a net around all of your elements, and right-click to selectstatistics. This will initially show statistics for the Widget, and by clicking the double right arrows you

can move on to buffers, machines etc. With the machines shown, press the Chart States button.

This gives the graphical representation of how each machine was occupied, as shown below.


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The model statistics show that the machine Weigh is blocked for a large percentage of the time and

that the Pack machine is waiting for labor for a large percentage of the time. Therefore it is unlikely

that any extra Weigh or Pack machines are needed. We could consider more labor at this point, and

may have to however there may be other alternatives. Although the Produce machine is fully used,

we can see from the screen that the buffer in front of Weigh is getting full; so we may get more

throughput by clearing this.

And here is the power of simulation we can try out what-if scenarios to evaluate many alternatives.

What if we could improve the reliability of the Wash machine. It may be worth a Six Sigma project in

this area. But is it? Let us try what if the interval between breakdowns could be increased to

between 150 and 300 minutes or even between 300 and 500 minutes.

Try these experiments by altering the parameters in the uniform distribution on the breakdown tab

for the Wash machine.

The first has no great beneficial value. However if the second were possible then it would cure all

throughput problems and even remove the need for a second wash machine ( try this too!) .

This type of sensitivity analysis is a great result from simulation. What are the key production levels

that require more machinery or labor can improved control help e.g. prioritizing work at different

areas? All ideas can be tried.

Of course in reality this model would be more complex. It may include shift patterns, raw material

deliveries or availability and complex production rules. All manner of complexity can be included and

tried in models.


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6.5 Stage 5

Looking at the model in three dimensions

We have built our WITNESS model as a two dimensional representation of the facility drawn in plan

view. This can be extremely powerful for understanding the layout and the flow. In particular the use

of status colors to show what each machine is doing can add a great deal of extra information to the

display.

However, there are times when it is useful to see the model in three dimensions. Each of the two

dimensional shapes in the WITNESS picture gallery is associated with a 3D shape. By using the

Model/Quick 3D menu option, a three dimensional view of your model will be created. As you learn

more about using WITNESS, you will discover how you can make adjustments to this 3D display.

Beyond that, the WITNESS VR module is available to extend this to produce fully realistic 3D

simulations of your own facility.

You will notice that in the default 3D view that you created has all equipment at floor level, which may

of course be correct. It is quite simple to adjust the height of items in the 3D display, and indeed to

create additional floors if necessary.

Let us suppose that the Produce machine, and the buffer before the Washers are at an elevated

position two metres above ground. We will place this on a different 'layer' in the simulation model.

Draw a net around the machine and the buffer, and change them from being on the 'Simulation Layer'

to the layer called 'Floor plus 2 metres'. You will see that a number of additional layers are defined in

the Startup model.

If you now run the Model/Quick 3D option again, you will find that machine and buffer appear at anelevated position as shown below. The 3D window contains a number of toolbar buttons to allow you

to navigate around the 3D world and change its appearance. You will find details about how to use

these options in the Help system.


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6.6 Summary

We hope you have seen how easy it is to build models in WITNESS, and how quick changes can lead

to quick results.

Remember that you can use models in a group environment; to get new ideas, develop them, and

obtain consensus by being able to try them out. This manual contains relatively simple models whichintroduce WITNESS, although WITNESS is capable of handling large, real life problems as well.

The complete, visual and interactive environment of WITNESS is unique and really does allow

managers and engineers to obtain the benefits that simulation can offer.

As well as interactive experimentation WITNESS offers a number of methods to conduct experiments

automatically please see the help text for details of experimentation and analysis.

The WITNESS Optimizer offers extended experimentation with algorithms that provide intelligent

searches for good solutions. If you have the WITNESS Optimizer you can try it with this model.


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7 Case study: JETTY.MOD

This model is a representation of ajettywhere ships arrive to deliver a range of products. The crane

labor element is used by thejettymachine to represent the constraint that only one ship may unload

at any one time due to only one crane being available. Ships can only pass through the lock undercertain tideconditions. The tide is represented by the tide processor element which has rising and

warning levels to set the state of the tide.

To open this model, start WITNESS and then use the file/opencommand or the open filebutton

on the standard toolbar to open J ETTY.MOD in the DEMO directory beneath your WITNESS

installation.

Use the Windowcommand to view different windows which contain additional information.

A histogram of mooring delays can be seen by using the associated viewfrom the histogram icon

at the top left of the window entitled Facility Overview. The window entitledJ etty Utilization

displays a series of pie charts showing jetty utilization s.


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Lock timing for one ship 20 minutes

Inter-arrival rate for ships NEGEXP( 60,1)

Crane Unloading time for each ship 150 minutes

Capacity of Outer Harbor Holding area for inbound ships 2

Capacity of Inner Harbor Holding area for inbound ships 3

Capacity of Inner Harbor Holding area for outbound ships 2

Number of Cranes 1

Tidal SystemHigh tide to low tide in 5 hours. Low tide

to high tide in 5 hours. 1 hour at full tide.

Lock operation depth above 3 meter depth

The inter-arrivals rate NEGEXP ( 60,1) means that, on average, one ship arrives every 60 minutes

( but spread randomly) . This random arrival pattern is represented by the Negative Exponential

distribution which is one of WITNESS s many standard distributions. The 1 represents pseudo

random number stream 1. You could think of this as an electronic die which is rolled whenever a value

needs to be used. In this way, different arrival intervals are set as the simulation runs. WITNESS

experiments are repeatable in that every run will generate the same samples of random numbers ( in

this case, the same arrival pattern) .

Again, the distributions and accuracy of timings and so on can be vital to the simulation results. If a

timing can vary between ten and twenty minutes, then you may need to model this.

Dealing with uncertain data

When data is uncertain, the best course of action is to use the best guess data available, but

make sure that sensitivity analysis on this data is carried out in the experimentation.

For example, if a timing is given as 10 minutes but it could be anywhere between 5 and 15

minutes, then ( if you cannot time it easily) use the value 10 in the model but do some model

runs with values of 5 and 15, and perhaps several values in between. If the different timings make

a significant difference to your results, then it is important that you establish the data more

accurately. The simulation model has proved that you need to know this data if you are to predict

what-if information.

However, if the different timings make little or no difference, then why waste time and energy in

establishing the data more accurately?

Let us now look at the base experiment with the model. A suitable run time for this model is one

hundred days. Please note that many simulation models should be run for a long time. This example

model contains a particularly unpredictable distribution in the negative exponential. It is this that

means that the model must be run for a fairly long time to cater for all the inherent variability in the

process.


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If your machine is not powerful enough and is tak ing too long to perform the ex periments then t ry

shorter run lengths; although the results will not matc h the ones shown here exact ly, the comparisons

between runs will probably s till largely be indicative of the true choices . However, please be aware of

the problems of run lengths which are too short.

Several of the standard texts on simulation give full explanations of how to establish run length ( or in

the case of non-equilibrium situations, the number of replications with different random number

settings to be run) . In essence, you need to run the model until the model has experienced all the

variability a simple test of this is to look at the variability of the results over successive time

segments. That is, if the result from each 1000 minutes over a 10,000 minute run is the same, then

1000 is possibly the right length of run for further experiments. However, this can be misleading ( for

example, if the data contains one event that will only happen in a million time units then a million or

more may be necessary) .

To run the model for 100 days you can enter 100*24*60 in the box in the executetoolbar at the

bottom of the screen and then click on either the runor the batch( fast forward) button. Thesimulation model will stop at the entered time ( as long as the clockbutton by the side of the box

is depressed) .

To view results, display other windows by using the windowscommand. You can also look at the

standard result tables by using the reports, select all the elements in the simulationsection of

the element selector and then click on the statisticsbutton. The forward and backward arrow

keys in the reports dialogs enable you to move through the different pages of the reports.

As explained above, the results shown below represent a run of the base model data for one hundred

days, which is assumed to be sufficient in this case. This raises the question of which results should

be examined. In this case the key result is the level of business that is, the number of ships

unloaded versus the number rejected ( and consequently lost to the competition) .

Other interesting results include the average time that a ship takes to be unloaded ( from entering the

harbor) . This represents the level of service, which could affect the decision to return.

Other statistics will guide us as to the key options for change by indicating high or low utilization s of

different facilities, showing where bottlenecks occur in a process and where investment may help.


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comparative runs may also be wrong.

Now we can examine the options open to the Harbor Operations Manager. In most simulation projects

there are usually a number of real-life options favored by the experience of the management of the

facility. This experience may uncover other data which may be relevant and need to be included in amodel. Often this experience will help you to decide the most useful options for experimentation.

However, it is also your duty as a simulation practitioner to explore the model that you have created;

to understand the relationships between the model parameters and to get to know the full dynamics of

the process. By playing with the model you can learn a lot which, in conjunction with other factors

such as experience, should help you to determine the best way ahead.

Some options may be ruled out as impractical or too costly. Cost is often to be taken into account.

In this example we will first simply look at the options which may increase the number of ships

served.

You may care to think about this yourself before proceeding. What options do you think may lie open

to the Harbor Manager ?

Some options include :

i. Increasing the number of unloading cranes

ii. Increasing the speed of the unloading cranes

iii. Increasing the capacities of the different harbor areas to accept more ships ( 3 separate

options here)

iv.Increasing the capability of the lock to handle ships for longer periods of time


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v. Increase the capacity of the lock

vi.Dredge the channels to the lock so that the lock can operate at lower levels.

There may well be other options which are dependent on the type of goods to be unloaded alternative unloading mechanisms, different traffic organization in the inner harbor, and so on.

The results show that with the current situation the jetties are waiting for a crane for a long time.

Therefore our first experiment here may be to try adding another crane.

Double-click on the word craneon the jetty layout the jetty dialog should appear. Change the

quantity of cranes to 2 and click on the OK button.

Now run the model ( first beginand then batchthe model) to the 100 day mark again.

The results this time show the following:

Scenario Title Base Case Run Two Cranes

Ships Served 956 1355

Ships Rejecting Harbor because full 1450 1055

Average Turnaround Time 1102 mins 647 mins

Crane Utilization 99.86% 70.68%

Lock Utilization 26.64% 37.67%

Berth Waiting for Crane 64.62% averaged over

3 jetties

14.69%

Berth Blocked for ship leaving 1.58% averaged over

3 jetties

22.69%

The comparisons show us that investing in a second crane would increase the number of ships that

can be shipped by 41.7%.

However the results also show that if this is done, the bottleneck is no longer the crane operation.

With berths being blocked for over 20% of the time there is now a bottleneck beyond this operation.

We can now try increasing the number of ships allowed in the inner harbor in the outgoing queue or

perhaps try increasing the availability of the lock.

To reset the capacity of the outward queue in the inner harbor, double click on the appropriate

queue and enter a capacity of 3 ( for example) instead of 2.

To change the capability of the lock, double click on the tideprocessor element, select the

warninglevelspage and alter both the risingand the fallinglevel to 2. ( You can do this by

clicking on the level, altering the value in the box above and then updating the level) .


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Try each of these options singly and then both together, keeping 2 cranes in each case. This table shows the

evaluations of all five scenarios so far:

Scenario BaseCase 2 3 4 5

DATA

Number of Cranes 1 2 2 2 2

Outgoing Harbor Queue

Capacity

2 2 3 2 3

Lock availability in hours out of

11

5 5 5 7 7

RESULTS

Ships Shipped 956 1355 1379 1696 1705

Ships Rejecting Harbor

because full

1450 1055 1031 714 705

Average Turnaround Time 1102 647 626 488 473

Crane Utilization 99.86% 70.68% 71.93% 88.44% 88.91%

Lock Utilization 26.64% 37.67% 38.33% 47.14% 47.39%

Berth Waiting for Crane ( jettyaverage) 64.62% 14.69% 14.17% 20.74% 21.02%

Berth Blocked for ship leaving

( jetty average)

1.58% 22.69% 12.82% 7.11% 1.94%

This shows that if the channels to the lock can be dredged to enable it to operate for 7 hours out of 11

instead of 5, this has a much bigger effect than enabling more queuing space in the inner harbor for

outgoing ships.

However all these results must be balanced with the practicality of the solution and the costs.

For this example we shall establish some costs:

Cost of an additional crane is $10Million ( depreciated over 500 days)

Cost of additional labor for new crane $1000 per day

Cost of dredging channels to lock $2,000 per day

Cost of increasing outgoing harbor queue to 3 ( through new organization/flow buoy installation/

extra drainage) $1,000 per day


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Profit cleared on normal operations for unloading one boat $4,000

Cost of third crane would be the same as the second

Cost of increasing other queuing capacity areas by one in the model is the same as for theoutgoing harbor queue.

Cost of building a second lock $10Million ( depreciated over 500 days)

Lanner s challenge to you is to establish the best option for this harbor, based on

the information given above. You can submit the result to [email protected] for

verification.


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8 Conducting a Simulation Project

8.1 Overview

Projects which involve simulation have several unique aspects which must be managed particularlycarefully to ensure their success. The topics in this chapter outline a typical sequence of events in a

project, using a practical methodology:

establishing objectives

deciding the scopeand level of detail in the model

collecting data

structuringthe model

buildingthe model

runningthe model

generating reportstestingthe model

experimentingwith the model

documentingthe model

presenting the resultsand implementingthem

8.2 Establish objectives

This is the first and most important phase of any simulation project. The aim of any simulation project

should be to make a better business decision. You, as simulation modeler, must understand this

business decision as it is likely to have important implications for the content of your simulation

model.

For example,consider two models of exactly the same production cell built for two entirely different

reasons.

The first model is built in order to discover the theoretical maximum throughput of the cell as it is

subjected to different product mixes. The model therefore contains detailedinformation on part

availability ( contained within a part file) and the cell's production schedule ( contained within a data

file) . Labor required to operate the cell at the maximum rate is not a consideration.

The second model is built to investigate the number of operators required to operate the cellefficiently. The model therefore contains detailed information about labor priorities, job interruptions,

and shiftpatterns. In this case, the production schedule might assume less importance.

Thus the content of the models is driven by the business decision which needs to be made. There is

no requirement to represent every single feature of the real world production cell in either model.


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8.3 Scope and level of model detail

The scope of a simulation model refers to where it begins and where it ends. For example, consider a

model of a typing pool. The objective of the model is to decide how many typists are required in order

to provide a particular document delivery time. The typing pool could be considered in isolation.

Parts arrive in an in-tray ( buffer) . Each typist takes documents from the in-tray, processes them

and pushes them out of the model ( to SHIP) .

Alternatively, the typing pool could be considered part of the paper-flow of the entire organization. This

involves shifting the start and end points of the simulation model to encompass more of the real world.

It is important to limit the scope of the model as far as possible. With regard to the level of detail

contained within a model, the golden rule is to model the minimum necessary in order to

achieve the model's objective.

At the beginning of the model-building process, small additions to the model lead to large increases in

its accuracy. As the model becomes more detailed, however, each subsequent addition adds less tothe model's accuracy. In fact, it could be argued that the addition of unnecessary detail could lead to

an eventual fall in the accuracy of the model. This is particularly true where you are trying to model

human behavior, which is often inconsistent. The graph below shows a typical relationship between

the level of detail of a model and the model's accuracy.

It is possible to use WITNESS elementsto represent combinations of real world processes and

therefore to model a process at a 'higher level'. For instance, a manufacturing cell or even an entire

manufacturing plant could be represented solely as a WITNESS machine.

8.4 Data Collection

Information for a model is likely to fall into one of three categories:

1. Available - data is readily available and it is in an appropriate format that the model can use

immediately.

2. Not available but collectable - data is either in an incorrect format or it has not been

collated before. You might need to perform a small work study in order to collect this type of

data ( for example, timing certain processes manually) .


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8.7 Running the model

After defining, displaying and detailing the elements of your model, you can run it immediately, then

modify it by adding, changing or deleting elements. You can then run the model again in order to

assess the impact of these changes.

This ability to build a model incrementally, testing each section as you go, is a powerful aid to

productivity, and generates confidence in the validity of your model.

You can run the model in different modes, from step-by-step ( with full screen display) to a

batched time in the future ( with no screen display) .

There are many WITNESS features which aid analysis, including standard report tables and graphs

( which list the basic mathematical behaviors of all elements in the model automatically) , meteor

trails, elements flows and process views. You can also create timeseries, pie charts, histograms and

customized report tables and expressions in WITNESS.

8.8 Generating reports

When you have built and run your model, you can use WITNESS reports to help you choose between

alternative modeling scenarios.

For example:

In order to increase the utilization of machinery, you may be most interested in the proportion of

the time that machines spent in an idle state compared to the time they spent in a busy state

during a simulated shift.

In order to increase the throughput of your operation, you may be most interested in the number of

parts processed during a simulated day s operation.

In order to rationalize your work force, you ll be interested in the proportion of time that operators

of each grade spend busy.

In order to reduce wastage, you may be interested in the quantity of parts that were scrapped

during the simulated period.

In order to avoid losing customers because your queues are too long, you ll be interested in the

number of customers who failed to enter for this reason ( number of parts rejected) .

In order to estimate the life of your vehicles, you ll be interested in the total distance they travel

in a simulated week.

You can choose different types of report to run and you can output the report to a printer, display it on

screen, or save it to various types of file ( including CSV and HTML formats) . You can copy all

standard reports and graphs to the Windows clipboard and paste them into other applications.


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The length of the run should be determined by a number of factors. The most important factor is that a

reasonable sample of random numbersis taken from each of the random number streams used in

the model. Each run should aim to use at least 10-15 numbers from each stream. If one stream is

being used to calculate a breakdown interval of between 1 and 2 weeks then a run length of between

20 and 30 weeks would be necessary.

Another factor is the reporting periodof the real-life situation being modeled. It makes little sense to

calculate an optimum run length of 3 weeks and 1 day if you need to compare your model results with

a real-life situation which reports every 30 day period.

It is important to run any model with random activity by using several different sets of random number

streams before you can place any confidence in the model's results. Otherwise it is possible that the

results obtained are solely the consequence of one set of the random number streams chosen rather

than any model changes that you have made. You should compare each set of results; if you find any

uncharacteristic values, you should review and assess them and, if necessary, discard them.

You can use the model/random numberscommand to reset random streams from antithetic toregular, or regular to antithetic. Alternatively, you may use the model/experimentoption to automate

the run of a model which uses number streams.

8.11 Documentation

It is a good idea to document the way in which you built the model, as it makes it easier to understand

if you ( or someone else) examines it at a later date. Such documentation should include the model

structure diagram. WITNESS also provides other facilities for model documentation, either within the

model itself, or externally to a file or a printer.

You can attach notesto most element detail dialogs and display the notes in the simulation window.

You can use these to enter descriptions of how each part of the model should work. You can also usean exclamation mark in rules and actions in order to insert commentsabout the purpose of the rule

or action.

The sources of data used, the assumptions made and the results obtained should also form part of

the model documentation. If the project is documented as it proceeds then the documentation will

prove to be a less onerous task. It is recommended that any project documentation is completed

before the presentation of results as there is often less inclination to document a project which has

been laid to rest.

The WITNESS Documentor module allows the creation of a report from the names, notes and other

property fields within a model.

8.12 Presentation of results and implementation

The method of presentation for results depends on the size of the simulation project and the culture of

your organization.

An animated model provides an effective communication tool to support business decisions,

particularly if you have enhanced its graphical display.


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9 Modeling With WITNESS

This section provides an introduction to the basic building blocks of WITNESS and how you can use

them to build a model. The description is designed to give you an overall appreciation of the product

before introducing you to model examples where WITNESS s processes and features are explored inmore detail.

9.1 Elements: the WITNESS building blocks

A business or commercial operation might produce a number of different parts using a variety of

machines, conveyors, vehicles, other equipment and people.

A WITNESS model uses the same combination of parts, people, machines and other simulation

devices, called modeling elements, in order to simulate the operation being studied.

There are two main versions of WITNESS which offer different terminologies for manufacturing and for

service and process applications. This book is designed for people principally modeling manufacturing

applications. However the elements give scope for modeling other applications too. For example it is

possible to model logistical operations.

It is also possible to treat the manufacturing terminology in the product abstractly modeling people

or telephone calls as parts to be processed by different activities represented by machines, and so

on. For customers principally modeling non-manufacturing activities we recommend the Service and

Process Performance edition of WITNESS. Please contact Lanner for details.

As you become more familiar with the capabilities of each element, you will find that virtually any

aspect of your operation can be meaningfully represented.

9.2 Discrete Elements

These are displayed as dynamic icons and represent tangible elements in the real-life situation being

studied.

9.2.1 Parts

Parts flow through the model. They can represent, for example:

products ( cars, engines, etc) .

product batches.

a project progressing through a large corporation.

calls in a telephone exchange.

tiny electronic components or whole computers.

Parts can be:

displayed in different ways as an icon or a text description.

characterized by a particular set of attributes ( for example, weight, length, color) , which can

be fixed ( for every part of the same type) or variable ( for each individual part of the same

type) .


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handled in different ways ( batched, created singly or in lots, changed into other parts, many

parts can be combined into one part, or one part can be split into several parts) .

filled with fluid and emptied of fluid.

9.2.2 Buffers

These are places where parts can be held. For example:

parts awaiting an operation on a factory floor.

people in a queue.

the space containing aircraft waiting to land.

a hopper containing components at an assembly station.

Buffers can:

have parts arranged within them according to different ordering methods ( for example, first-in

first-out or by priority) .

be tied directly to machines ( dedicated buffers) .

hold parts for a specified minimum time. In this way buffers can be used to simulate equipment

such as ovens and operations such as settling or cooling.

hold parts for a specified maximum time after which they attempt to leave the buffer. An

example of a use of buffers In this way is to be used to simulate shelf life of components after

which they must be scrapped.

9.2.3 Machines

These are powerful elements which are used to represent anything that takes parts from somewhere,

processes them and sends them on to their next destination. For example:

a machine tool, lathe or a press.

a complete shop or a single supermarket checkout.

an organization which handles a project then passes it on.an entire plant or an individual work cell.

Machines can:

be one of seven types, handling parts in a variety of ways.

model such factors as the time a machine takes to process a part, time between breakdowns,

multiple setups, multiple stations, multiple cycles, time to repair breakdowns and to set a

machine up, as well as labor for these events.

fill and empty parts with fluid.

9.2.4 Labor

This element can be used to model both human and physical resources ( for example, tools, peopleor equipment) which may be required by other elements for processing, setting up, repair, cleaning

and so on. Controlling labor use is usually very important in modeling. There are many different

options available in WITNESS e.g. complex rules for allocation and the ability to take labor resources

away from an element so that a more important task can be completed for another element ( resource

pre-emption) .


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9.2.5 Conveyors

These are used to move parts from one fixed point in the model to another over time, both belt and

roller conveyors can be represented. There are two types of conveyors.

Fixed conveyors maintain a constant distance between parts. If the conveyor stops, the

distance between the parts on the conveyor remains the same.Queuing conveyors allow parts to accumulate. If the conveyor becomes blocked, the parts will

slide together until the conveyor is full.

9.2.6 Paths

A path is an element that parts or labor units can travel along in order to get from one element to

another element. You can use it to represent the length and the physical route of a real life journey in

your model.

9.2.7 Tracks

These are the paths that vehicles follow when transporting parts. They also define points at which

vehicles may load, unload or park. You can model unidirectional or bi-directional tracks.

9.2.8 Vehicles

These represent vehicles, ( for example, Automatic Guided Vehicles, cranes, forklift trucks) which

transport parts. You can specify:

a variety of destinations and priorities for vehicles.

the time a vehicle should stop at the end of a track before going on to the next track.

a length of time the track remains busy after a vehicle has left it ( for example, to avoid

collision with the long train of the front vehicle) .

a maximum speed for a vehicle on a track.

the speed of a vehicle when loaded and unloaded.

vehicle acceleration and deceleration.the time it takes for parts to be loaded and unloaded from a vehicle.

9.3 Power & free elements

Power and free elements allow you to model the specialized form of material transportation systems.

There are four types of power and free elements.

9.3.1 Network

The network groups a set of sections, stations and carriers together. There are two types of network:

self-powered and section-powered, and the type affects the behavior of sections and carriers within

the network.

9.3.2 Section

This is the path that carriers move along, the section must be part of a network. The network

determines the behavior of a section.


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9.4.4 Pipes

These are the elements used to connect processors and tanks. Fluids flow through pipes at a given

rate. Pipes can:

be cleaned according to certain criteria.

break down according to certain criteria.change the name and color of the fluid on input or output.

have negative flow rates.

flow with or without an output.

9.5 Logical Elements and Modules

These represent the data and reporting aspects of the model ( that is, control and information) . They

enable you to handle data easily, customize reports and build more complex logic into WITNESS

models.

9.5.1 Attributes

These are characteristics of a specific part or labor unit. For example, the number of cylinders in an

engine could be held in an attribute, and you could then use this attribute to determine the amount of

time required for tuning and adjustment. Each attribute may hold an integer, or a real number, or a

string, or a reference to another WITNESS modeling element.

9.5.2 Variables

Variables are values which can be accessed from anywhere in the model. For example, a variable

could be used to record the value of items in an inventory. In addition to variables that you de

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