Entity Operations

An entity operation defines what happens to an entity when it enters a location.

For modeling purposes, the exact nature of the operation (machining, patient check-in, or whatever) is unimportant. What is essential is to know what happens

in terms of the time required, the resources used, and any other logic that impacts system performance. For operations requiring more than a time and resource des- ignation, detailed logic may need to be defined using if–then statements, variable assignment statements, or some other type of statement (see Section 7.4.8, “Use of Programming Logic”).

An entity operation is one of several different types of activities that take place in a system. As with any other activity in the system, the decision to include an entity operation in a model should be based on whether the operation impacts entity flow in some way. For example, if a labeling activity is performed on enti- ties in motion on a conveyor, the activity need not be modeled unless there are sit- uations where the labeler experiences frequent interruptions.

Consolidation of Entities

Entities often undergo operations where they are consolidated or become either physically or logically connected with other entities. Examples of entity consoli- dation include batching and stacking. In such situations, entities are allowed to simply accumulate until a specified quantity has been gathered, and then they are grouped together into a single unit. Entity consolidation may be temporary, al- lowing them to later be separated, or permanent, in which case the consolidated entities no longer retain their individual identities. Figure 7.6 illustrates these two types of consolidation.

Examples of consolidating multiple entities to a single entity include

• Accumulating multiple items to fill a container.

• Gathering people together into groups of five for a ride at an amusement park.

• Grouping items to load them into an oven for heating.

In ProModel, entities are consolidated permanently using the COMBINE com- mand. Entities may be consolidated temporarily using the GROUP command.

Attachment of Entities

In addition to consolidating accumulated entities at a location, entities can also be attached to a specific entity at a location. Examples of attaching entities might be

FIGURE 7.6 Consolidation of entities into a single entity. In (a) permanent consolidation, batched entities get

destroyed. In (b) temporary consolidation, batched entities are preserved for later unbatching.

before after

(a)

before (b) after

FIGURE 7.7 Attachment of one or more entities to another entity. In (a) permanent

attachment, the attached entities get destroyed. In (b) temporary attachment, the attached entities are preserved for later detachment.

befor e

before (a)

(b)

after

after

attaching a shipping document to an order ready for delivery, assembling wheels to a chassis, or placing items into a container. The difference between attaching entities and consolidating entities is that entities become attached to an existing base or main entity that must be present at the location. It is the presence of the main entity at the location that triggers the routing of the entities to be attached. In consolidation, entities are combined in whichever order they happen to enter the location. Like consolidation, attachment may be temporary, where the attached entities are later detached, or permanent, where the attached entities are destroyed and only the initial entity to which they were attached continues. Figure 7.7 illus- trates these two types of attachment.

Examples of attaching entities to another entity include

• Attaching component parts to a base assembly.

• Delivering a completed order to a waiting customer.

• Loading material into a container.

In ProModel, entities are attached to another entity using the LOAD

command for temporary attachments or the JOIN command for permanent attachments. Cor- responding LOAD and JOIN routings must also be defined for the entities to be loaded or joined.

Dividing Entities

In some entity processes, a single entity is converted into two or more new enti-ties. An example of entity splitting might be an item that is cut into smaller pieces or a purchase order that has carbon copies removed for filing or sending to accounting. Entities are divided in one of two ways: either the entity is split up into two or more new entities and the original entity no longer exists; or additional entities are merely created (cloned) from the original entity, which continues to exist. These two methods are shown in Figure 7.8.

FIGURE 7.8 Multiple entities created from a single entity. Either (a) the entity splits into multiple entities (the original entity is destroyed) or (b) the entity creates one or more entities (the original entity continues).

before after

(a)

before

after (b)

Examples of entities being split or creating new entities from a single entity include

• A container or pallet load being broken down into the individual items comprising the load.

• Driving in and leaving a car at an automotive service center.

• Separating a form from a multiform document.

• A customer placing an order that is processed while the customer waits.

• A length of bar stock being cut into smaller pieces.

In ProModel, entities are split using a SPLIT statement. New entities are created from an existing entity using a CREATE statement. Alternatively, entities can be con- veniently split or created using the routing options provided in ProModel.