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Topic 28

Topic 28. Flexible Assembly Systems. Each job has an unique identity Make to order, low volume environment Possibly complicated route through system Very difficult. Limited number of product types Given quantity of each type Mass production High degree of automation Even more difficult!.

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Topic 28

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  1. Topic 28 Flexible Assembly Systems IE 514

  2. Each job has an unique identity Make to order, low volume environment Possibly complicated route through system Very difficult Limited number of product types Given quantity of each type Mass production High degree of automation Even more difficult! Job Shops Flexible Assembly IE 514

  3. Flexible Assembly Systems • Sequencing Unpaced Assembly Systems • Simple flow line with finite buffers • Application: assembly of copiers • Sequencing Paced Assembly Systems • Conveyor belt moves at a fixed speed • Application: automobile assembly • Scheduling Flexible Flow Systems • Flow lines with finite buffers and bypass • Application: producing printed circuit board IE 514

  4. Sequencing Unpaced Assembly Systems • Number of machines in series • No buffers • Material handling system • When a job finishes moves to next station • No bypassing • Blocking • Can model any finite buffer situation IE 514

  5. Cyclic Schedules • Schedules often cyclic or periodic: • Given set of jobs scheduled in certain order • Contains all product types • May contain multiple jobs of same type • Second identical set scheduled, etc. • Practical if insignificant setup time • Low inventory cost • Easy to implement IE 514

  6. Minimum Part Set • Suppose l product types • Let Nk be target number of jobs of type k • Let z be the greatest common divisor • Then is the smallest set with ‘correct’ proportions • Called the minimum part set (MPS) IE 514

  7. Defining a Cyclic Schedule • Consider the jobs in the MPS as n jobs • Let pij be as before • A cyclic schedule is determined by sequencing the job in the MPS • Maximizing TP = Minimizing cycle time IE 514

  8. MPS Cycle Time Example Buffer! IE 514

  9. Sequence: 1,2,3 (ii) (iii) (i) (i) (ii) (ii) (ii) (iii) (iii) (i) (i) (i) (ii) (ii) (ii) (iii) (i) (i) (ii) (ii) 1 2 3 4 5 6 7 8 Cycle Time = 3 IE 514

  10. Sequence: 1,3,2 (i) (ii) (ii) (iii) (iii) (i) (i) (ii) (ii) (iii) (iii) (i) (i) (ii) (ii) (iii) (iii) (i) (i) (ii) (ii) (iii) 1 2 3 4 5 6 7 Cycle Time IE 514

  11. Minimizing Cycle Time • Profile Fitting (PF) heuristic: • Select first job j1 • Arbitrarily • Largest amount of processing • Generates profile • Determine which job goes next IE 514

  12. PF: Next Job • Compute for each candidate job • Time machines are idle • Time job is blocked • Start with departure times: IE 514

  13. Nonproductive Time • Calculate sum of idle and blocked time • Repeat for all remaining jobs in the MPS • Select job with smallest number • Calculate new profile and repeat IE 514

  14. Discussion: PF Heuristic • PF heuristic performs well in practice • Refinement: • Nonproductive time is not equally bad on all machines • Bottleneck machine • Use weight in the sum IE 514

  15. Discussion: PF Heuristic • Basic assumptions • Setup is not important • Low WIP is important  Cyclic schedules good • Want to maximize throughput  Minimize cycle time  PF heuristic performs well IE 514

  16. Discussion: FMS • Flexible Manufacturing Systems (FMS) • Numerically Controlled machines • Automated Material Handling system • Produces a variety of product/part types • Scheduling • Routing of jobs • Sequencing on machines • Setup of tools • Similar features but more complicated IE 514

  17. Discussion: Solution Methods • Formulated as ‘simple’ sequencing • Can apply branch-and-bound • In general constraints make mathematical programming formulation difficult • PF heuristic easy to generalize IE 514

  18. Additional Complications • The material handling system does not wait for a job to complete Paced assembly systems • There may be multiple machines at each station and/or there may be bypass  Flexible flow systems with bypass IE 514

  19. Topic 29 Paced Assembly Systems IE 514

  20. Paced Assembly Systems • Conveyor moves jobs at fixed speeds • Fixed distance between jobs • Spacing proportional to processing time • No bypass • Unit cycle time • time between two successive jobs IE 514

  21. Grouping and Spacing • Attributes and characteristics of each job • color, options, destination of cars • Changeover cost • Group operations with high changeover • Certain long operations • Space evenly over the sequence • Capacity constrained operations (criticality index) IE 514

  22. Objectives • Minimize total setup cost • Meet due dates for make-to-order jobs • Total weighted tardiness • Spacing of capacity constrained operations • Pi(l) = penalty for working on two jobs l positions apart in ith workstation • Regular rate of material consumption IE 514

  23. Grouping and Spacing Heuristic • Determine the total number of jobs to be scheduled • Group jobs with high setup cost operations • Order each subgroup accounting for shipping dates • Space jobs within subgroups accounting for capacity constrained operations IE 514

  24. Example • Single machine with 10 jobs • Each job has a unit processing time • Setup cost • If there is a penalty cost IE 514

  25. Example Data IE 514

  26. Grouping • Group A: Jobs 1,2, and 3 • Group B: Jobs 4,5, and 6 • Group C: Jobs 7,8,9, and 19 • Best order: A  B  C IE 514

  27. Grouped Jobs A B C Due date  Order A  C  B IE 514

  28. Capacity Constrained Operations A C B IE 514

  29. Topic 30 Flexible Flow Systems IE 514

  30. Flexible Flow System with Bypass IE 514

  31. Flexible Flow Line Algorithm • Objectives • Maximize throughput • Minimize work-in-process (WIP) • Minimizes the makespan of a day’s mix • Actually minimization of cycle time for MPS • Reduces blocking probabilities IE 514

  32. Flexible Flow Line Algorithm • Three phases: • Machine allocation phase • assigns each job to a specific machine at station • Sequencing phase • orders in which jobs are released • dynamic balancing heuristic • Time release phase • minimize MPS cycle time on bottlenecks IE 514

  33. Machine Allocation • Bank of machines • Which machine for which job? • Basic idea: workload balancing • Use LPT dispatching rule IE 514

  34. Sequencing • Basic idea: spread out jobs sent to the same machine • Dynamic balancing heuristic • For a given station, let pij be processing time of job j on ith machine • Let IE 514

  35. Dynamic Balancing Heuristic • Let Sj be the jobs released before and including job j • Define • Target IE 514

  36. Minimizing Overload • Define the overload of the ith machine • The cumulative overload is • Minimize IE 514

  37. Release Timing • MPS workload of each machine known • Highest workload = bottleneck • MPS cycle time  Bottleneck cycle time • Algorithm • Step 1: Release all jobs as soon as possible • Step 2: Delay all jobs upstream from bottleneck as much as possible • Step 3: Move up all jobs downstream from the bottleneck as much as possible IE 514

  38. Example IE 514

  39. Data IE 514

  40. Machine Allocation IE 514

  41. Workload IE 514

  42. Overload IE 514

  43. Overload Matrix IE 514

  44. Dynamic Balancing First Job IE 514

  45. Selecting the Second Job • Calculate the cumulative overload where IE 514

  46. Cumulative Overload Selected next IE 514

  47. Final Cycle • Schedule jobs 4,5,1,3,2 • Release timing phase • Machine 4 is the bottleneck • Delay jobs on Machine 1, 2, and 3 • Expedite jobs on Machine 5 IE 514

  48. Topic 31 Lot Sizing IE 514

  49. Lot Sizing • Domain: • large number of identical jobs • setup time/cost significant • setup may be sequence dependent • Terminology • jobs = items • sequence of identical jobs = run IE 514

  50. Applications • Continuous manufacturing • chemical, paper, pharmaceutical, etc. • Service industry • retail procurement IE 514

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