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Issue No:05/03/2

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"There cannot be a crisis next week. My schedule is already full"
-Henry Kissinger

In recent times, a number of organizations are focusing on increasing business performance by improving scheduling like scheduling of delivery vehicles, scheduling of salesman visits to different towns, scheduling of operations on shop floor, scheduling of aircrafts, scheduling of activities for launching a spaceship etc. The concern is assigning a given set of machines and possibly other resources to a set of tasks in order to execute the tasks optimally subject to certain constraints, such as deadlines, availability of resources, priorities for some tasks over others or machine capabilities.

Scheduling is simply the allocation of resources over time to perform a set of tasks. Production being an integral part of a company catches a lot of attention and faces major scheduling/rescheduling issues.

There are two basic steps in production scheduling

Step I: Defining the problem: It includes understanding of the shop floor layout, machine capacities, routings, alternate resources, processing constraints, various shop floor events and the existing and desired ability to deal with these events (like change in resource, change in material availability, change in demand).

Above all the scheduling objective needs to be spelt out clearly. E.g., Scheduling objectives can be minimization of WIP, due date performance, maximization of throughput, level loading etc.

Step II: Model building and deployment:
It includes a number of smaller steps to deploy a user-friendly solution. It is important to remember that the solution should help scheduler to

  • Schedule the tasks optimally at the shop floor
  • Provide activity level visibility - start time, expected due date, resource utilization
  • Build and evaluate various scheduling scenarios
  • Reschedule quickly in the event of late demand, resource overload or unplanned machine breakdown or late material

To build a robust scheduling solution, there are number of approaches/ algorithms like "disjunctive graph model", "polynomial algorithms", "dynamic programming algorithms", "polynomial approximation schemes", "branch and bound methods", "decomposition approaches", "simulated annealing", "heuristics models", "theory of constraints" etc.

The right model framework varies from company to company based upon shop floor complexities and scheduling objectives.

Largely, scheduling issues can be tackled best if the users are involved in the schedule construction process and they interact with the schedule to input their subjective preferences like sequencing constraints, due date preferences etc., and explore interactively the possible trade-offs between different schedule criteria.

Major benefits of a good schedule are

  • Better plant item utilization - means that plant equipment is less idle and capital invested is performing better.
  • Low unplanned overtime levels - controlled due to better due-date performance and less of expediting cost for late running orders
  • Low stock levels - balanced material flow helps in reducing WIP and lowering of storage requirement and associated costs
  • Better due date performance - improves stock availability, enhances ability to retain customers and reduces costs for servicing demand with decrease in rush dispatches
  • Ability to identify the impact of changes quickly to the production plan such as additional, high priority orders, and mechanical failure
  • Low cost of labor by reducing instances of idle-labor

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