Scheduling

1. Scheduling “Oh no! Not another course with Hofmann!”

2. Types Of Scheduling • Demand scheduling is for customers: It assigns customers to a definite time for an order or service. • Workforce scheduling is for employees: It determines when employees work. • Operations schedulingcombines workforce scheduling with job scheduling. • Assigning jobs to workstations. • Assigning people to workstations. • Assigning people to jobs. (Typical with most office work.) Often, several jobs must be scheduled at one or more workstations. Typically, a variety of tasks can be performed at each workstation. (Especially using One-Worker, Many Machines)

3. Flow Shops and Job Shops Product-Focused/Line flow (Flow Shops) • High volume and low variety, medium- to high-volume production utilizing line or continuous processes • Easier to schedule because the work flow is common. • Process-focused/Flexible flow(Job Shops) • Low-to medium-volume production utilizing job shop or batch processes • High variety of products with lowervolume output • Harder to schedule because work flows are more varied.

4. Problems in Scheduling Job Shops • Scheduling a wide variety of low-volume jobs. • Time to complete each job varies since each job is often unique. • Different jobs have different due dates. • Demand is less predictable. • A wide variety of equipment and tools are needed.

5. Scheduling in Job Shops • 1. Jobs are usually assigned to equipment or to work stations. • Workers are assigned to jobs or to work stations • Jobs must be sequenced. (Determining the order in which jobs are accomplished.) • Work-in-progress needs to be tracked.

6. Gantt Charts • Charts to monitor the progress of work and/or view the utilization of workstations. • The Gantt Progress Chartgraphically displays the current status of each job or activity relative to its scheduled completion date. • The Gantt Workstation Chart/Machine Chart shows when and where work or people are assigned. • You can see which workstations, equipment, or other productive assets, are being utilized or idle.

7. GANTTPROGRESS CHARTSHOWS JOBSTATUS (PROGRESS) Current Date • • Colored bars show job status (ahead, on time or behind). • Brackets show estimated job duration, start and end times.

8. GANTT WORKSTATION CHART(Machine Chart)Shows when vehicles are being utilization or are idle. X indicates equipment is not being utilized & not available. Blanks indicate equipment is available to be scheduled.

9. Gantt Workstation Chart for Hospital Operating Rooms Work assignments/shifts for Doctors

12. Emergency Room Work Assignments http://associate.com/gantt/

13. SCHEDULING TERMS Job Loading means assigning jobs to people or machines. Wait Time is that portion of Job Flow Time that a job is available but not being worked on. (In the Queue) Job Flow Time: Time that a job is in the system. It starts when a job becomes available to be worked on. (Wait time + work time) Past Due is a measure of job lateness. (Days late, or % of jobs late. Often expressed as the average number of days late.) Makespan time is the job flow time required to complete a group (batch) of jobs. For example, completing multiple jobs for one customer. Utilization= (Time spent working on a job) / (Job Flow Time) (How long it took you to do it ÷ how long you had it)

14. Example I gave you an assignmenton Tuesday (Job Loading), and it was due today. • Job Flow Time was four days. • If you did it the same day I assigned it… • Wait time was zero • If you did it just before class … • Wait time was 4 days • If it took you 30 minutes to do it, regardless of when … • Utilization was (0.5 hrs / 32) = 1.5% • (Four 8-hour work days = 32hours.) • Utilization is what % of the time you had the job (flow time) did you spend working on it.

15. SCHEDULING STRATEGIES 1) Forward Scheduling: Starts at the current timeand works forward to schedule in the future • Typical in a job shop environment • EG: Dentist office or Car repair facility • They look forward in time to see when you (or job) canbe scheduled. • Tends to cause higher inventory costs in manufacturing. 2) Backward Scheduling: The focus is on the job due date. You back up from the due date to determine the start date. • Typical in a repetitive type production environment (line flow) • Assembly-line situation where jobs are customer specific • Some high variety, low volume situations such as construction.

16. Job Shop Dispatching Single-Workstation Scheduling Single-Dimension Rules • First Come, First Served (FCFS) • Earliest Due Date (EDD) • Shortest Processing Time (SPT) • Longest Processing Time (LPT) Multiple-Dimension Rules (we are not covering these) • Critical Ratio (CR) • Slack Remaining per operation (S/RO) Multiple-Workstation Scheduling • Johnson’s Rule

17. JOB SEQUENCING

18. FIRST-COME, FIRST SERVED Jobs are scheduled as they come in. 4 0 14 0 16 0 30 0 42 28 48 26 154 54 FLOW TIME = The time a job spends in the system. Total flow time = 154 days LATENESS: The time (hours, days or weeks) that jobs are late getting back to customers. AVERAGE DAYS LATE = (Total days late / # Jobs) = (54 / 6) = 9 days AVERAGE COMPLETION TIME = (Total Flow Time / # Jobs) = (154 / 6) = 25.7 days AVERAGE # JOBS IN SYSTEM = (Total Flow Time / Total Work Time) = 154 /48 = 3.2 jobs

19. Jobs are sequenced according to when they are due. Earliest Due Date 12 0 14 0 20 0 30 5 44 14 48 14 168 33 AVERAGE DAYS LATE = (Total days late / # Jobs) = (33 / 6) = 5.5 days AVERAGE COMPLETION TIME = (Total Flow Time / # Jobs) = (168 / 6) = 28 days AVERAGE # JOBS IN SYSTEM = (Total Flow Time / Total Work Time) = 168 /48 = 3.5 jobs

20. Shortest job is done first, and the longest job is done last. Shortest Processing Time 2 0 6 0 12 0 22 0 34 20 48 18 124 38 AVERAGE DAYS LATE = (Total days late / # Jobs) = (38 / 6) = 6.3 days AVERAGE COMPLETION TIME = (Total Flow Time / # Jobs) = (124 / 6) = 20.6 days AVERAGE # JOBS IN SYSTEM = (Total Flow Time / Total Work Time) = 124 / 48 = 2.58 jobs

21. Longest Processing Time Longest job is done first. 14 0 26 12 36 11 42 20 46 12 48 30 212 85 AVERAGE DAYS LATE = (Total days late / # Jobs) = (85 / 6) = 14.1 days AVERAGE COMPLETION TIME = (Total Flow Time / # Jobs) = (212 / 6) = 35.3 days AVERAGE # JOBS IN SYSTEM = (Total Flow Time / Total Work Time) = 212/ 48 = 4.42 jobs

22. A Comparison of four Methods Earliest Due Date has the least lateness and thus has the best customer service. It also results in the lowest total inventorybecause jobs go back to the customer the quickest. Shortest Processing Time has the lowest average job completion time and the least # of jobs in the system. Thus it also has the lowest work-in-processinventory. First Come, First Serve is considered the most fair by customers, but it has no other advantage.

23. Which Method To Use? • Shortest Processing Time may have the best measures, but it discriminates against the longest jobs. • Longest jobs may be the most important or bring the biggest revenues. • If longest jobs are the most important, then consider using the LPT method. • First-Come, First Served is perceived by most customers to be the fairest method. • The method used should be based on customer situation, risk, revenues, job importance, etc.

24. Multiple-Phase Situations Multiple Phase (same as in queuing theory) • There are multiple work stations (processing stops) Johnson’s Rule • Easily used for two-phase work flows. • With three or more phases the scheduling techniques become complex. Job Shop Dispatching (Assigning jobs to stations) • Used by most job shop companies with three or more job phases. • The decision about which job to process next at a particular workstation is made using priority rules at each workstation when it becomes available.

25. Johnson’s Rule for 2-phase Scheduling 1. Select the job with the shortest phase time. 2. If the shortest phase time is in phase one, schedule that job first. If it is in phase two, schedule that job last. In case of a tie (equal phase times) the decision is arbitrary. 3. Eliminate the job just scheduled and repeat steps 1 & 2 until all jobs are scheduled. E D C A B F RESULTS

26. SequencingwithJohnson’s Rule At the Morris Machine Co. Eliminate M2 from consideration. The next shortest time is M5 at workstation #2, so schedule M5 next to last. Eliminate M3 from consideration. The next shortest time is M2 at Workstation 1, so schedule M2 first. Shortest time is 3 hours at workstation 2, so schedule job M3 last. Eliminate M1 and the only job remaining to be scheduled is M4. Eliminate M5 from consideration. The next shortest time is M1 at workstation #1, so schedule M5 next. Time (hr) Motor Workstation 1 Workstation 2 Sequence = M2 M1 M4 M5 M3

27. Workstations M2 M1 M4 M5 M3 Idle—available 1 Gantt Chart for the Morris Machine Co. Repair Schedule (4) (12) (15) (10) (5) for further work M2 M1 M4 M5 M3 2 Idle Idle (5) (22) (16) (8) (3) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Hours Sequencing • Johnson’s Rule minimizes the idle time at station 2 and gives the fastest repair time for all five jobs. • No other sequence will produce a lower make-span time.

28. Manufacturing ProcessFlexible Flow / Job Shop Shipping Department Raw Materials Legend: Batch of parts Workstation Job-shop scheduling can be complex. This is only six work stations.

29. SCHEDULING JOBS FOR MULTIPLE WORKSTATIONS • Each workstation is treated independently using priority sequencing rules. • When a workstation becomes idle, an appropriate priority rule is applied to the jobs waiting for that operation, and the job with the highest priority is selected. • When that operation is finished, the job is moved to the next work station in its routing, where it waits until it again has the highest priority at that station. • Identifying the best priority rule to use at a particular work station (phase) in a process is a complex problem. • Computer simulation models are effective tools to determine which priority rules work best in a given situation. • Computer simulation is rarely used in smaller-volume operations

30. Scheduling in Multiple Workstation (flexible flow) situations Different jobs travel different routes. Each workstation is treated independently. When a job arrives at a given workstation, it joins that queue and is given a priority according to the rule being used at that station.

31. Scheduling and the Supply Chain • Your Production scheduling should be integrated with your supply chains. • Demand Forecasts for your products affect your entire supply chain and thus should be shared with your suppliers. • Location of inventory (inventory placement) is strategic and long-term, affecting your supply chains. • Available-To-Promise decisions should be checked with suppliers for availability of materials and components. • Scheduling should be a part of, rather than reactive to, the supply chain.

32. Scheduling Customers for Service • Backlogs • Customers are in a queue for services and are given a due date for the fulfillment of an order. • Or allow a backlog to develop as customers arrive. Customers may never know exactly when their orders will be fulfilled. • Appointments • Specific times for service • Reservations • Specific times for customers to occupy or use service facilities

33. Scheduling Employees • Fixed Schedule • Workers on shift work or fixed hours. • Rotating Schedule • Individual schedules vary from week to week. • Problems: • Holidays / consecutive days off / personal days / vacation days / sick days / labor union agreements

34. Unique Situation Scheduling • EG: Scheduling faculty, courses & rooms. • Faculty have personal preferences & needs • Morning vs. Evening classes • Four-day, Three-day and Two-day schedules • Classroom preferences • Course constraints • Need for labs or tiered rooms or tables • Required courses at conflicting times • Facility constraints • Limited rooms and limited lab availability • Insufficient facilities for high-demand times.

35. LABOR-LIMITED ENVIRONMENTS • The limiting resource thus far has been the number of machines or workstations available. A more typical constraint is the amount of labor available. • Labor-limited environmentis an environment in which the constraint is the amount of human labor available, rather than the number of machines or workstations. • Four possible approaches: • Assign people to the job that has been in the system longest. • Assign people to the workstation with the most jobs. • Assign people to the workstation having jobs that require the most work. • Assign people to the workstation with the job that has the earliest due date.

36. Sophisticated Scheduling Techniques • Expert Systems are a popular way to schedule complex work flows. • Artificial-Intelligent software emulates the decision-making of human experts. • Optimized Production Technology(OPT) and Q-Control are two very sophisticated computer programs for production scheduling in job shop environments. • They combine simulation and heuristics