Planning Lecture 6

Welcome to Planning & Scheduling! This interactive session includes information on how to build a winning partnership with production, a step-by-step implementation plan, and detailed information on the daily role of the planner. As always, we present real-world illustrations on the fundamentals of Planning and Scheduling to help you increase productivity and lower overall costs — which means faster repairs, less downtime, and greater availability.

The objective of Module 6 is to review the following:

• • Long term, medium term, short term scheduling
  • Types and categories of schedules
  • Master production schedules
  • Manufacturing schedules

Types of Schedules

In general, scheduling is performed in three stages: short-,medium-, and long-term. The activity frequency of these stages are implied by their names.

Long-term Scheduling Long term scheduling is performed when a new process is created. The long-term scheduler limits the number of processes to allow for processing by taking the decision to add one or more new jobs, based on FCFS (First-Come, first-serve) basis or priority or execution time or Input/Output requirements. Long-term scheduler executes relatively infrequently.

Medium-term Scheduling Medium-term scheduling is a part of the swapping function. This scheduler works in close conjunction with the long-term scheduler. It will perform the swapping-in function among the swapped-out processes. Medium-term scheduler executes some what more frequently.

Short-term Scheduling Short-term scheduler is also called as dispatcher. Short-term scheduler is involved whenever an event occurs, that may lead to the interruption of the current running process. Short-term scheduler executes most frequently.

Acting as the primary resource allocator, the longterm scheduler admits more jobs when the resource utilization is low, and blocks the incoming jobs from entering the ready queue when utilization is too high.

When the main memory becomes over-committed, the medium-term scheduler releases the memory of a suspended (blocked or stopped) process by swapping (rolling) it out.

In summary, both schedulers control the level of multiprogramming and avoid (as much as possible) overloading the system by many processes and cause  ‘‘thrashing’’ (more on this later).

Types and Categories of Schedules

There are different types of scheduling which are usually used as approaches in the manufacturing units. Some of them are-

• Manual Scheduling- This kind is done manually by a scheduler, where the scheduler analyses all the resources and then assign necessary activities to satisfy constraints.
• OR/Mathematical/Exact- In this approach, the scheduler uses all mathematical programming like linear, non linear, dynamic, integrated/ mixed and network analysis. The scheduler also uses probabilistic methods like queuing theory and inventory theory.
• Heuristic Search- In this approach the scheduler generates schedules that are good enough, constructive and repair. The scheduler searches for limited breath and depth, look ahead methods hill climbing and neighbourhood methods. This kind of approach is bottleneck and constraint driven with minimum conflicts, priority dispatch and work allocation rules.
• Advanced algorithms- In these advanced algorithms, the scheduler uses genetic algorithms, simulated annealing (intensification vs diversification, exploitation vs exploration), expert systems, neural networks, fuzzy systems and mixes of all or any of the methods for scheduling.
• Expert systems & AI- The scheduler operates the systems with the help of tools for managing uncertainty and complexity, uses GUI, visualization techniques, messaging and communications. He should also have domain specific support like database support, information systems and advisory/decision support.

Master Production Scheduling

Master Production Scheduling (MPS), otherwise known as an over-all schedule, focuses on a planning horizong, divided into equal time periods (time buckets, such a by month). It includes a plan for the production of individual commodities such as staffing, inventory, etc for the allotted time period. A MPS usually dictates when and how much of each product is going to be produced, based on criteria such as demand, capacity, and inventory availability.

MPS aids in decision making by generating a set of output data based on forecast demand, production costs, inventory money, customer needs, production lead time, and capacity. The resulting output information includes the amounts to produce, staffing requirements, quantity of product available to promise, and projected available funds for production. It also sets the expectations of the revenue that the business is likely to generate.

Manufacturing and Operation Scheduling or Detailed scheduling

Manufacturing Scheduling (sometimes called detailed scheduling or production scheduling) focuses on a shorter horizon than MPS. It also fixes a time and date to each operation in a continuous timeline rather than in time buckets, defining the start and completion time-frame for each process. Both subsequent stages of production planning and control depend on this timeline which makes it a very valuable asset in the production process. Scheduling looks to optimize the use of time; from each piece of work involved to project planning to customer delivery. The goal is to maximize thruput (output) and on-time delivery, within the constraints of equipment, labor, storage, and inventory capacity. This usually includes focusing on maximizing utilization of critical bottleneck resources by minimizing changeovers and cleanout and also avoiding material starvation.  In order to schedule more efficiently, there are a variety of methodologies and tools that planners can apply.