Future Events List Example

From Sigma

Jump to: navigation, search

We will follow the changes in a typical future events list by examining a simulation of a machine center with three identical machines (numbered 0, 1, and 2) and two workers (worker 0 and worker 1).

The types of events that might occur in this example are the ARRIVAL of the next part at the machining center, a machine "STARTing" or "FINISHing" work on a part, a BREAKDOWN of a machine, and a broken machine being REPAIRED. An actual simulation model would, of course, have other types of events.

For each event that pertains to a specific machine and/or operator, the machine number followed by the operator number (if appropriate) are listed as event attributes. At a particular time during a simulation run, the future events list might look like the one pictured in Table 2.1. The future events are logically sorted according to times that events are scheduled to occur. Here time will be measured in minutes.

The current time in this example is 3.00, and the ARRIVAL of a part has just occurred at the center. This ARRIVAL event has "scheduled" the next ARRIVAL event to occur at time 3.37. We can determine the status of each machine by scanning down the future events list and checking what lies in the future for each machine. (Recall that the machine number is designated by the first event attribute.) Machine 0 is due to FINISH processing the part it is currently working on at time 3.20, so machine 0 must be busy. Likewise, machine 1 is busy and due to FINISH working on a part at time 3.40. Finally, machine 2 will be REPAIRED at time 3.43, so it is currently being fixed by the repair crew. We can see from the future events list in Table 2.1 that when the part arrived at time 3.00 none of the machines were available to start working on it. Thus, the part will join other parts in a queue waiting to be processed.

Future Events List for a System with Three Machines 
(Time = 3.00)
Time	Event Type	Event Attributes
3.00	ARRIVAL	
3.20	FINISH	        0,1
3.35	BREAKDOWN	1
3.37	ARRIVAL	
3.40	FINISH	        1,0
3.43	REPAIRED	2
9.01	BREAKDOWN	0

Note that machine 0 is due to experience its next BREAKDOWN at time 9.01 and machine 1 is due for a BREAKDOWN at time 3.35 - before it can FINISH its current operation. Therefore, when machine 1 breaks down at time 3.35, the FINISH event for this machine at time 3.40 will have to be cancelled.

To see how this machining center simulation might proceed, we will now advance the current time to 3.20 and execute the FINISH event on machine 0. Looking at the second attribute of this FINISH event, we see that operator 1 becomes idle. Since we know that there is at least one part waiting, we can immediately START processing the next part. A new START event for machine 0 has been scheduled to occur at the current time of 3.20 with operator 1. The future events list is now like Table 2.2.

Future Events List for a System with Three Machines
(Time = 3.20).
Time	Event Type	Event Attributes
3.20	FINISH	        0,1
3.20	START	        0,1
3.35	BREAKDOWN	1
3.37	ARRIVAL	
3.40	FINISH    	1,0
3.43	REPAIRED	2
9.01	BREAKDOWN	0

We next execute the START event for machine 0 at time 3.20 with operator 1. Suppose that a stored (or randomly generated) machine processing time for machine 0 is 1.20 minutes, then the FINISH event for this machine will be scheduled to occur 1.20 minutes from the current time of 3.20 or at time 4.40. The future events list after executing the START event at time 3.20 is shown in Table 2.3.

Future Events List for a System with Three Machines
(Time = 3.20)
Time	Event Type	Event Attributes
3.20	START	        0,1
3.35	BREAKDOWN	1
3.37	ARRIVAL	
3.40	FINISH	        1,0
3.43	REPAIRED	2
4.40	FINISH	        0,1
9.01	BREAKDOWN	0

Next, we advance the time to 3.35 and execute the BREAKDOWN event for machine 1. This BREAKDOWN event will cause the FINISH event for machine 1 scheduled at time 3.40 to be cancelled. We will assume here that the part is destroyed when the machine breaks down and that the worker becomes available for other work. If it takes five minutes for a repair crew to repair machine 1, the future events list after the BREAKDOWN occurs is like that in Table 2.4. (Note the REPAIRED event for machine 1 has been scheduled at 8.35, five minutes beyond the current time of 3.35.) The simulation will now advance to time 3.37 when the next ARRIVAL event will occur.

Future Events List for a System with Three Machines
(Time = 3.35)
Time	Event Type	Event Attributes
3.35	BREAKDOWN	1
3.37	ARRIVAL	
3.43	REPAIRED	2
4.40	FINISH	        0,1
8.35	REPAIRED	1
9.01	BREAKDOWN	0


Back to About

Personal tools