Introduction to Arena. A Simple Simulation. Model1. We examine a simple model: parts arrive at a server, are served, and depart the system. There will be an "Arrival Event", a "Departure Event” and an "End Simulation Event".
A Simple Simulation
We examine a simple model: parts arrive at a server, are served, and depart the system. There will be an "Arrival Event", a "Departure Event” and an "End Simulation Event".
The Arrival Event simply sets up a part, which might be enqueued - if the server is busy - or might be given to the server - if the server is idle. In either case the Arrival Event triggers a subsequent Arrival Event. In case the server is idle, it will also trigger a "Departure Event", for the part just given to the server. The execution of the "Departure Event" triggers either another "Departure Event" - if the queue is non-empty - or the "Falling Asleep” of the server.
The "End Simulation Event" simply terminates the simulation, and may or may not have any relation to any other event.
Start Arena. Choose File/New.
Move the cursor to the left “tongue” and click on “Attach…” A dialog will appear, choose Common.tpo from the list - the “Common Panel”. You may choose others.
The first module we need is the “Arrive Module”, corresponding to the “Arrival Event”: click on the “Arrive” icon on the Common Panel, move the cursor to the model window - which you may have maximized after File/New - and click near the left end.
The next step involves setting up the properties of the Arrive Module. Double-clicking on “Arrive” brings up the “Arrive” dialog. Change the name from “Arrive1” to “InDoor”.
Note the “Batch Size” of 1: one item will arrive each time there is an arrival. We need to choose the “interarrival distribution”: in “Time Between”, open the list, choose “EXPO(Mean)”. Replace “Mean” by 5.0 - 5.0 time units will be the mean between arrivals.
We will chose a time attribute: write “Time of Arrival” in the “Mark Time Attribute” small window.
We now need to determine how we are going to leave: in the “Leave Data Area”, click on “Connect”. The box will change… How???
Save in “My Documents” under “YourNameModel1”, just to be safe…
At this point, we can pick and place a “Server Module”:
A little care with placement will result in a configuration similar to the one above. Double-click on “Server” to bring up the “Server” dialog box.
We will replace “Server1” by “Machine”, and we choose “Machine_R” from the “Resource” list in the “Server Data” area. We must now choose a “Process Time”.
From the list, choose TRIA(Min, Mode, Max) replacing Min by 1, Mode by 4, and Max by 8 - a triangular distribution. Choose “Connect” in the “Leave Data” area.
The “Server” dialog should look like this.
We now need to attach and specify the “Depart Module”.
Change “Depart1” to “OutDoor”; in the “Count Area” choose “Individual Counter”, and in the “Tally Area” choose “Individual Tally”.
Type “Production” in the “Counter” window; type “Flowtime” in the “Tally” window, and choose “Time of Arrival” from the “Attribute” list.
Our system is now set up, except for making it do something. We now choose a “Simulate” model, followed by two separate instances of the “Animate” model.
Choose a title and an analyst name.
Give a “Length of Replication” of 20. The “Simulate” model should now look like:
The two instances of the “Animate” model will look like the picture in the following slide: the “Animate” dialog box appears on double-clicking the “Animate” box, while the “Plot” one appears on double-clicking the little plot window right above the “Animate” box. Fill in the spaces as indicated.
Choose “Resource” and “Number Busy” for the second one. Adjust the display as you like.
You can now run the model.
It will terminate almost immediately, giving you the choice to see the results. If you choose to examine the results, you will get a text window with a fair amount of data… that you have to make sense of.
Exercises: Simulation with Arena, Ch. 3; the previous slides took you through much of Ch. 4, and should be useful.
Make multiple replications, by just asking for them in the Simulate Model main dialog. Compare the Summary Reports from various replications. Would a single run be adequate to predict the behavior of the system?
2) Reduce the interarrival times to one half of the original: just replace EXPO(5.0) by EXPO(2.5) in the “Time Between” box of the “Arrive” module. What does the simulation look like? What does the report look like?
3) Lengthen the run from 20 minutes to 600 minutes - the “Simulate” module, “Length of Replication” box. You will have to change the “Time Range” in the plots, the “Maximum”, the “Number in Queue” and the “# History Points” in the “Number in Queue”.
4) In the “Process Time” box of the Server module append a +2 to the expression appearing there. Run the model for 1440 minutes (24 hrs) of simulated time, watching the queue-length plots - for which you will have to change the “Time Range”, and you will have to edit the “Expressions” changing the Maximum to 60.
For visual effects, you can click on the line for the queue and drag it to the left; you can reduce the size of the entity picture by double-clicking on it above the “Simulate” module and reducing its “Size Factor”.
What is happening?
5) a) Add a second machine right after the first machine: all parts exiting the first machine go to the second for further processing. Processing times at the second machine are the same as at the first (i.e. same distribution). Gather all statistics as before PLUS time in queue, queue length and utilization at the second machine.
b) Immediately after the second machine, add an “Inspector”: the inspection will take a constant 5 minutes, and will have a 20% chance of failure. All parts exit the system regardless of pass/fail. Count the numbers that fail, that pass, and gather statistics on the time in queue, queue length and utilization at the inspection station.
c) Add plots to track the queue length and number busy at all three stations; run the simulation for 600 minutes.
What can you conclude?