Scheduling is about assigning an appropriate number of workers to the jobs during each day of work. It is used for determining when an activity should start or end, depending on its (1) duration, (2) predecessor activity (or activities), (3) predecessor relationships, (4) resource availability, and (5) target completion date of the project.
Production environments can be characterized in a number of ways such as if the production is continuous (process industry) or job oriented (discrete manufacturing), if the products are made to stock or made to order.
In discrete manufacturing, shops are generally characterized as either flow shops or job shops. In a flow shop, orders are performed in a fixed sequence through the machines and other resources of the work centers in the shop whereas in a job shop, orders go through the work centers in arbitrary patterns. Scheduling in a job shop is therefore more complex than scheduling in a flow shop and the job shop is the more general case.
The type of demand that drives the production also affects scheduling. If the demands are known for a long time into the future, scheduling is easier than if demand is uncertain and changes must be handled on short notice. Production that is made to stock is often easier to schedule as the stock can be used as a buffer that gives freedom when scheduling. If the produced goods are delivered directly to customers, no such buffer exits since the customer expects that agreed upon delivery times are kept.
At a more detailed level, three components must be analyzed to determine viable scheduling approaches for the production environment. These are the production resources, the orders and operations, and materials and subparts.
Production resources
Production resources are everything that is required to perform the production. It can be for example personnel, machines or tools and other equipment. Resources with similar skills and capabilities are often grouped into work centers. Four characteristics can be used to describe resources: functionality, capacity, availability and cost.
- Functionality – The functionality of a resource describes what operations it can perform. This is determined by for example the skills and competence of the personnel or the capability of the machine considered.
- Capacity – The capacity of a resource can be described by how many jobs the resource can perform at the same time and by how effectively it performs an operation. It is possible that resources have the same functionality but perform the same operation with different efficiency. For instance, a specialized resource can often perform an operation suitable for it faster than a general purpose resource.
- Availability – This characteristic describes when the resource is available to perform operations. The availability of a resource to perform an operation is determined by when the resources is open (which shifts that are applied to the resource) and what other operations that are requiring the resource’s capacity.
- Cost – To perform production in a resource always incurs a cost. This cost must often be considered when scheduling. It is in most cases desirable to perform work where it incurs the least cost.
Orders and operations
Orders and operations describe what should be produced by a production environment and those activities that must be performed to accomplish this. The structure of orders and operations is hierarchical; an order contains operations and possibly also depends on other orders. Both orders and operations have costs associated with them. As the operations of an order are executed they accumulate cost that is derived from the costs of the production resources. The costs of the operations are then accumulated in the orders. Orders that are under execution are called work in process or in-process inventory. It is often desirable to have as few orders in process as possible and to keep the cost associated with those orders as low as possible.
- Orders – Orders can be of different types such as customer orders which are associated with a specific delivery to a customer and/or a production order that can either satisfy the need of a customer order or be stored in stock for later use. Orders can have different states such as planned, in process or finished. Information associated with an order can be for instance release date, due date, quantity and/or priority.
- Operations – An operation describes a basic activity or task that should be performed. The operations contained in an order have precedence relations between them that describe the sequence they should be performed in. The information associated with an operation varies with the production environment, but some sort of description; which production resource or work center that should perform it and the expected processing time and quantity. These last two attributes are always required. This information can then be extended with information such as set-up time, post-production time and required tools. During order execution it is also necessary to keep track of the state of the operation such as not ready, started or finished. In more advanced production environments other possibilities might exist such as splitting an operation and performing it in parallel on more than one resource or having alternate resources that can perform an operation. It can also be possible to interrupt an operation to start another more important operation, this is called preemption. When the next operation should start can also vary, for instance after some specified quantity is produced or some specified time has elapsed on the current operation.
- Materials and subparts – Materials and subparts that are required to perform production are specified per operation. Materials can be raw materials such as steel or wood or goods like nuts and bolts. Subparts are more refined components that can either be bought from a supplier or manufactured by the company and then stored for later use. To keep track of materials and subparts and to determine when operations can be performed information such as available quantity, expected deliveries and allocations to orders and operations must be maintained.