Capacity Planning, Aggregate Scheduling, Master Schedule, and Short-Term Scheduling - PowerPoint PPT Presentation

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Capacity Planning, Aggregate Scheduling, Master Schedule, and Short-Term Scheduling

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  1. Capacity Planning, Aggregate Scheduling, Master Schedule, and Short-Term Scheduling Capacity Planning 1. Facility size 2. Equipment procurement Long-term Aggregate Scheduling 1. Facility utilization 2. Personnel needs 3. Subcontracting Intermediate-term Intermediate-term Master Schedule 1. MRP 2. Dis-aggregation of master plan Short-term Short-term Scheduling 1. Work center loading 2. Job sequencing

  2. Scheduling Low-Volume Systems Loading - assignment of jobs to process centers Sequencing - determining the order in which jobs will be processed Job-shop scheduling Scheduling for low-volume systems with many variations in requirements

  3. Assigning jobs to work centers Considerations Job priority (e.g., due date) Capacity Work center hours available Hours needed for job Approaches Gantt charts (load & scheduling) - capacity Assignment method - job to specific work center Loading Jobs in Work Centers

  4. Loading Two Approaches to loading: - Finite Loading – limit the amount of work that is released to a given work center on the basis of capacity consideration. Input/output control is a method for keeping track of assigned resource (input) and the work completed by a resources (output). This helps identify a backlog (input rate in greater that output rate). The remove a backlog to slow down the input.

  5. Loading - Infinite loading – assign job to center regardless of available capacity. This requires that proper planning has been done to make sure enough capacity is available to take care of all work to be done.

  6. Scheduling Definition: Determination of when something is to be done and the tasks and activities required to do it. Establishing the timing of the use of equipment, facilities and human activities in an organization Scheduling aids in on-time completion Direct link to value perceived by customers Scheduling improves the utilization of the firm’s resources Direct link to productivity

  7. Strategic Implications of Short-Term Scheduling By scheduling effectively, companies use assets more effectively and create greater capacity per dollar invested, which, in turn, lowers cost This added capacity and related flexibility provides faster delivery and therefore better customer service Good scheduling is a competitive advantage which contributes to dependable delivery

  8. The Goals of Short-Term Scheduling Minimize completion time Maximize utilization (make effective use of personnel and equipment) Minimize WIP inventory (keep inventory levels low) Minimize customer wait time

  9. Scheduling Services Considerations Scheduling services different from manufacturing Inability to store or inventory services Random nature of customer requests for service Point of customer contact Back-office operations Front-office operations

  10. Qualitative factors Number and variety of jobs Complexity of jobs Nature of operations Quantitative criteria Average completion time Utilization (% of time facility is used) WIP inventory (average # jobs in system) Customer waiting time (average lateness) Choosing a Scheduling Method

  11. Forward and Backward Scheduling Forward scheduling: begins the schedule as soon as the requirements are known jobs performed to customer order schedule can be accomplished even if due date is missed often causes buildup of WIP Backward scheduling: begins with the due date of the final operation; schedules jobs in reverse order used in many manufacturing environments, catering, scheduling surgery

  12. Scheduling Difficulties Variability in Setup times Processing times Interruptions Changes in the set of jobs No method for identifying optimal schedule Scheduling is not an exact science Ongoing task for a manager

  13. Minimizing Scheduling Difficulties Set realistic due dates Focus on bottleneck operations Consider lot splitting of large jobs

  14. Sequencing Sequencing: Determine the order in which jobs at a work center will be processed. Priority rules: Simple heuristics used to select the order in which jobs will be processed. Job time: Time needed for setup and processing of a job.

  15. Priority Rules for Dispatching Jobs First come, first served The first job to arrive at a work center is processed first Earliest due date The job with the earliest due date is processed first Shortest processing time The job with the shortest processing time is processed first Longest processing time The job with the longest processing time is processed first Critical ratio The ratio of time remaining to required work time remaining is calculated, and jobs are scheduled in order of increasing ratio. FCFS EDD SPT LPT CR

  16. Assumptions of Priority Rules The setup of jobs is known Setup time is independent of processing sequence Setup time is deterministic There will be no interruptions in processing such as: Machine breakdowns Accidents Worker illness

  17. Process first job to arrive at a work center first Average performance on most scheduling criteria Appears ‘fair’ & reasonable to customers Important for service organizations Example: Restaurants First Come, First Served Rule

  18. SPT and LPT Rule SPT – Schedule a job with shortest processing time first LPT – Schedule a job with longest processing time first

  19. Process job with earliest due date first Widely used by many companies If due dates important If MRP used Due dates updated by each MRP run Performs poorly on many scheduling criteria Earliest Due Date Rule

  20. Advantages of the Critical RatioScheduling Rule Use of the critical ratio can help to: determine the status of a specific job establish a relative priority among jobs on a common basis relate both stock and make-to-order jobs on a common basis adjust priorities and revise schedules automatically for changes in both demand and job progress dynamically track job progress and location

  21. Two Work Center Sequencing Johnson’s Rule: technique for minimizing completion time for a group of jobs to be processed on two machines or at two work centers. Minimizes total idle time Several conditions must be satisfied

  22. Johnson’s Rule Conditions Job time must be known and constant Job times must be independent of sequence Jobs must follow same two-step sequence Job priorities cannot be used All units must be completed at the first work center before moving to second

  23. Johnson's Rule - Scheduling N Jobs on Two Machines 1. List all jobs and their processing times each machine. 2. Select first the job with the shortest processing time. If the shortest processing time lies with the first machine, the job is scheduled first; if with the second machine, the job is scheduled last. 3. Once a job is scheduled, eliminate it. Apply steps 2-3 to the remaining jobs until all jobs are assigned. The process will move toward the center of the sequence.

  24. Limitations of Rule-Based Dispatching Systems Scheduling is dynamic; therefore, rules need to be revised to adjust to changes in process, equipment, product mix, etc. Rules do not look upstream or downstream; idle resources and bottleneck resources in other departments may not be recognized Rules do not look beyond due dates

  25. Number of Servers Single Server Multiple Servers Examples: Multiple Single Servers Examples:

  26. Queue Discipline First come -- first served (FCFS): Multiple Priorities: Examples:

  27. Project Management A project is a set of activities aimed at meeting a goal, with a defined beginning and end.

  28. Project Management with Certain Time Estimates Summary of steps: Determine activities that need to be accomplished Determine precedence relationships and completion times Construct network diagram Determine the critical path Determine early start and late start schedules

  29. Project Scheduling When the Times of Activities are Uncertain Summary of steps: Determine the activities that need to be accomplished Determine the precedence relationships and completion times Construct the network diagram Determine the critical path Determine the early start and late start schedules Calculate the variances for the activity times Calculate the probability of completing by the desired due date

  30. Project Scheduling with Time Uncertainty The “Heuristic” approach to dealing with timing uncertainty Based on understanding of individual activities as conforming to a “beta” distribution Take three time estimates Optimistic - What is the (realistic) fastest we can get an activity done? Pessimistic - What is the (realistic) worst case scenario for delay? Most likely - What is our “most likely” estimate?

  31. Process job with earliest due date first Widely used by many companies If due dates important If MRP used Due dates updated by each MRP run Performs poorly on many scheduling criteria Earliest Due Date Rule

  32. Best Methods among Priority Rule • SPT and EDD are superior in all measure. SPT is generally the best for minimizing flow and average number of jobs.

  33. Process job with smallest CR first Performs well on average lateness Critical Ratio (CR) • Ratio of time remaining to work time remaining Time remaining = CR Work days remaining Due date - Today' s date = Work (lead )time remaining

  34. Advantages of the Critical RatioScheduling Rule Use of the critical ratio can help to: determine the status of a specific job establish a relative priority among jobs on a common basis relate both stock and make-to-order jobs on a common basis adjust priorities and revise schedules automatically for changes in both demand and job progress dynamically track job progress and location

  35. Two Work Center Sequencing Johnson’s Rule: technique for minimizing completion time for a group of jobs to be processed on two machines or at two work centers. Minimizes total idle time Several conditions must be satisfied

  36. Johnson’s Rule Conditions Job time must be known and constant Job times must be independent of sequence Jobs must follow same two-step sequence Job priorities cannot be used All units must be completed at the first work center before moving to second

  37. Limitations of Rule-Based Dispatching Systems Scheduling is dynamic; therefore, rules need to be revised to adjust to changes in process, equipment, product mix, etc. Rules do not look upstream or downstream; idle resources and bottleneck resources in other departments may not be recognized Rules do not look beyond due dates

  38. Queuing Theory • Where is waiting? • Service facility • Fast-food restaurants • post office • grocery store • bank Manufacturing Equipment awaiting repair Phone or computer network Product orders

  39. Measures of System Performance • Average number of customers waiting • Average time customers wait • System utilization

  40. Application of Queuing Theory • We can use the results from queuing theory to make the following types of decisions: • How many servers to employ • Whether to use a single fast server or a number of slower servers • Whether to have general purpose or faster specific servers Goal: Minimize total cost = cost of servers + cost of waiting

  41. System Characteristics • Number of servers • Arrival and service pattern • Infinite Population – The arrival is not affected by waiting line (public places, e.g. gas station) • Finite Population – The arrival is affect by waiting line. • Queue discipline