DISCRETE EVENT SIMULATION

Chapter 6 - Sim u la t io n Mo d e llin g of To o l Pla n n in g Sy ste m

on different workcenters and they flow according to the process plans. When the part arrives, the required tool as specified in the process plan, is issued by the tool store. Each process consumes a certain amount of tool life. Such a manufacturing scenario is regarded as a discrete type of simulation. A typical simulation model is represented by the ENTTnES, ATTRIBUTES, EVENTS, QUEUES, ACTIVITIES and STATES [Carrie, 1988]. These are explained below.

ENTITIES - The ’parts’ and ’ordered tools’ form the entities in this model. There is also a dummy entity used for initialising and advancing the simulation clock.

ATTRIBUTES - Each of the above mentioned entities have their own attributes. The values of these attributes change as the entities flow through the system during the simulation. For e.g. the entity ’part’ has the following ten attributes associated with it. 1. Event time 2. Event code 3. Part number 4. Batch size 5. Operation number 6. Workcenter number 7. Operation time 8. Tool code 100

Ch apter 6 - Sim u la t io n Mo d e llin g of To o l Pla n n in g System

9. Tool required quantity 10. Order number

EVENTS - Event occurs when a certain activity begins or ends. Each entity has ’event time’(attribute 1) and ’event code* (attribute 2). The ’event code’ represents an activity that needs to be carried out when the system clock reaches the ’event time’ (attribute

1). The entities are chronologically queued in the ’event queue’ and are executed in increasing order of the ’event time’. Some of the important events handled by the system are described below. The event occurs when,

(a) the part is loaded on the workcenter.

(b) the part is unloaded from the workcenter at the completion of the operation. (c) the purchase requisitions are sent for tools (or when the tools are ordered). (d) the tools are received by the tool stores.

(e) the part begins to wait in the ’waiting queue’ due to tool shortages. (f) the part ends its waiting process and is loaded on the workcenter.

There are other events such as checking the tool stock levels. If the tool stock levels are below the minimum required, then those tools are purchased and restocked. These events take place according to the tool restocking rules. Each model has a unique tool replenishment strategy, leading to different nature of events being built for each model.

Chapter 6 - Sim u la t io n Mo d e llin g of To o l Pla n n in g System

The ’event code’ assists in recording many time dependent variables. For e.g. the production delays (’total_wait’) can be calculated by taking the difference between the ’start’ and ’wait’ times.

QUEUES - The simulation engine consists of a queue called as the ’event’ queue and the other two types are ’arrive’ and ’wait’ queue. ’Event’ queue has all the entities of the system arranged chronologically, irrespective of the nature of the entity. This process of arranging the entities in a sequence is carried out by the simulation engine. Each workcenter in the model has one ’arrive’ and one ’wait’ queue. When the part arrives at the workcenter , it is put in the ’arrive’ queue. The tools required for that operation are then issued from the tool stores. If there are any tool shortages, then the part is transferred to the ’wait’ queue and is held until the tools become available. The ’event’ queue may consist of any entities (parts or tools on order), whereas, the queues at the workcenter contain only parts as entities.

ACTIVITIES - Activities are the processes such as the part being machined at the workcenter or the tools being procured. Every activity has a certain length of time which is taken into account while calculating the activity finish time. This is handled by the engine routines. For e.g. the tool procurement time is added to the time at which the order is placed and the tools are received when the system clock attains the resultant time.

Chapter 6 - Sim u la t io n Mo d e llin g of To o l Pla n n in g Sy ste m

STATES - Each entity has a ’state’, for e.g. when the parts are loaded, the machine is set to ’busy’ and at the completion of the operation, it is set to ’idle’. Similarly, when the order is placed for procurement of certain type of tool, then the entity ’tool on order’ has the flag ’ordered ?’ which is set to ’yes*. As soon as the tools are received, the flag is set to ’no’. With this facility, the activities can be logically controlled.