11. ROBOTFMS.GPS

Simulation of a robot operated FMS.

Problem Statement

An experimental, robot operated, flexible manufacturing system, has two computer numerical control machine tools, an arrival area, and a finished parts area. Components arrive every 150 seconds, exponentially distributed, and are machined on both machines in sequence. The robot takes 8±1 seconds to grip or release components, and 6 seconds to move components from the arrival area to the first machine. Processing time, on the first machine is normally distributed, with a mean of 60 seconds and a standard deviation of 10 seconds. The robot takes 7 seconds, to move from the first machine to the second machine. Machining time on the second machine is 100 seconds, exponentially distributed. Finally, the robot takes 5 seconds, to move components from the second machine, to the finished parts storage area.

Simulate the manufacturing cell operation, for 75 completed parts.

1.Find the distribution of transit times of jobs.

2.Find the utilization of the robot, and the machine tools.

3.Find the maximum storage areas required, in the cell.

Listing

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

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

*Experimental Manufacturing Cell *

*Two CNC machines and one Robot *

*One arrival area and one finished parts area *

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

RMULT 78863

Transit TABLE M1,100,100,20 ;Record lead time

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

GENERATE (Exponential(1,0,150)) ;A job arrives QUEUE One ;Arrival queue

SEIZE Robot ;Get the robot DEPART One ;Depart the queue

ADVANCE 8,1 ;Robot grips the job ADVANCE 6 ;Robot moves to machine 1 ADVANCE 8,1 ;Robot place the job RELEASE Robot ;Free the robot QUEUE Two ;Wait in next queue SEIZE Machine1 ;Get first machine DEPART Two ;Depart the queue

ADVANCE (Normal(1,60,10)) ;Process time RELEASE Machine1 ;Free machine 1

QUEUE Three ;Join queue for machine 2 SEIZE Robot ;Get the robot

DEPART Three ;Depart the queue ADVANCE 8,1 ;Robot grips part ADVANCE 7 ;Robot moves to machine 2 ADVANCE 8,1 ;Robot places the part RELEASE Robot ;Free the robot QUEUE Four ;Join queue machine 2 SEIZE Machine2 ;Get machine 2 DEPART Four ;Depart the queue

ADVANCE (Exponential(1,0,100)) ;Process 2 RELEASE Machine2 ;Free machine 2

QUEUE Five ;Queue for exit station SEIZE Robot ;Get the robot

DEPART Five ;Depart the queue ADVANCE 8,1 ;Robot grips the part ADVANCE 5 ;Robot moves to exit ADVANCE 8,1 ;Robot places the part RELEASE Robot ;Free the robot TABULATE Transit ;Transit time TERMINATE 1 ;Job is completed

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Transactions represent jobs, which pass serially through each step in the operation.

The Table Transit collects the distribution of job completion times.

Running the Simulation

To run a simulation and create a Standard Report,

CHOOSE File / Open and in the dialog box

SELECT ROBOTFMS

and then

SELECT OK

Next, create the simulation.

CHOOSE Command / Create Simulation

then

CHOOSE Command / START

and in the dialog box, replace the 1.

TYPE 75

SELECT OK

The simulation will end when 75 Transactions have entered the

TERMINATE 1 Block. This represents the completion of 75 jobs.

When the simulation ends, GPSS World writes a report to the default report file, RobotFMS.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

From the Standard Report, we see that the Robot, Machine1, and Machine2 Facilities had utilizations of 36%, 33% and 64%, respectively. The Queue Entity Four registered a notably long average waiting time of 179.7 seconds. This queue represents jobs waiting for Machine2.

The order completion times are represented in the Table

Transit. The mean was 451.8 seconds, with a standard deviation of around 250.8.

The maximum storage requirements required for each Queue

Entity totals 13 for this simulation. This is the total of all Queue

Entity Maximum values. Maximum values can vary widely from run to run. Therefore, we would want to complete several more simulations before reporting a space requirement, in the form of a confidence interval. Confidence intervals are provided by the ANOVA Command, which is discussed in Lesson 13 of this manual and Chapter 6 of the GPSS World Reference Manual.

Inside the Simulation

Let's 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.

Now let’s open some graphics windows.

CHOOSE Window / Simulation Window / Facilities Window

This is the Facilities Window. It gives information on machine usage. Machine2 has the highest utilization.

To view the histogram of job completion times,

CHOOSE Window / Simulation Window / Table Window and since the Transit Table is the only one in this model

SELECT OK

Close the Table and Facilities Windows once you have examined them.

CLICK ON The X-Upper Right of Each Window

Occasionally, you may find it desirable to rerun simulations after adding Tables, Qtables, or other automatic data collection Entities. It is always possible to create and report your own statistics using Variable, Fvariable, Bvariable, Matrix, and Savevalue Entities. They, and other simulation related values can be viewed via the Expressions Window as well. Also, the I/O Stream Blocks and String Commands can be used to create reports that are formatted as you wish with the selected information from your simulation.

Let’s take a look at where Transactions are.

CHOOSE Window / Simulation Window / Blocks Window

and look at the history of Transactions in the simulation. Look at the total Block entry counts. This model is rather simple with no TRANSFER Blocks or alternate destinations. Let’s watch

the dynamics. Often, a new Active Transaction is chosen when the current Transaction enters an ADVANCE Block. Open an

Expressions Window on the Active Transaction number.

CHOOSE Window / Simulation Window / Expressions Window

The Edit Expressions Window opens. When you enter the value in the second box in this dialog box, you should place the

mouse pointer in the small box and click once. Do not press e

to go from the first to the second box as GPSS World then assumes that all values have been entered. Now, in the dialog box for the Label field

TYPE Active Transaction Number

and in the Expression field

TYPE XN1

CLICK ON View

and

CLICK ON Memorize

If we memorize all the expressions, we'll be able to close the window and open it again later with easy retrieval of all the values. Also if you save the simulation the values in the Expressions Window will be saved with the simulation if they have been memorized.

SELECT OK

and

CHOOSE Command / START

and in the dialog box, replace the 1.

TYPE 2000

SELECT OK

If you want to slow things down, interrupt the simulation and use the Step Command. Feel free to interact with the simulation environment. When you are done, halt the simulation.

PRESS o

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

Otherwise, to end the session