2006 Prentice Hall, Inc Prentice Hall, Inc Prentice Hall, Inc. 15 4

Transcription

1 Operations Management hapter 5 Short-Term Scheduling Outline The Strategic Importance Of Short- Term Scheduling Scheduling Issues Forward and ackward Scheduling Scheduling riteria PowerPoint presentation to accompany Heizer/Render Principles of Operations Management, 6e Operations Management, 8e 2006 Prentice Prentice Hall, Inc. Hall, Inc Prentice Hall, Inc Outline ontinued Scheduling Process-Focused Facilities Loading s Gantt harts ssignment Method Outline ontinued Sequencing s Priority Rules for Dispatching s Sequencing N s on Two Machines: Johnson s s Rule Limitations Of Rule-ased Dispatching Systems 2006 Prentice Hall, Inc Prentice Hall, Inc. 15 4

2 Strategic Importance of Short-Term Scheduling Effective and efficient scheduling can be a competitive advantage Faster movement of goods through a facility means better use of assets and lower costs dditional capacity resulting from faster throughput improves customer service through faster delivery Good schedules result in more reliable deliveries 2006 Prentice Hall, Inc Scheduling Issues Scheduling deals with the timing of operations The task is the allocation and prioritization iti of demand d Significant issues are The type of scheduling, forward or backward The criteria for priorities 2006 Prentice Hall, Inc Scheduling Issues Figure Prentice Hall, Inc Scheduling Decisions Organization rnold Palmer Hospital University of Missouri Lockheed-Martin factory Hard Rock afe Delta irlines Table 15.1 Managers Must Schedule the Following Operating room use Patient admissions Nursing, security, maintenance staffs Outpatient treatments lassrooms and audiovisual equipment Student and instructor schedules Graduate and undergraduate courses Production of goods Purchases of materials Workers hef, waiters, bartenders Delivery of fresh foods Entertainers Opening of dining areas Maintenance of aircraft Departure timetables Flight crews, catering, gate, ticketing personnel 2006 Prentice Hall, Inc. 15 8

3 Forward and ackward Scheduling Forward scheduling starts as soon as the requirements are known Produces a feasible schedule though it may not meet due dates Frequently results in excessive e work-in in- process inventory Now Due 2006 Prentice Hall, Inc Forward and ackward Scheduling ackward scheduling begins with the due date and schedules the final operation first Schedule is produced by working backwards though h the processes Resources may not be available to accomplish the schedule Now Due 2006 Prentice Hall, Inc Scheduling riteria i 1. Minimize completion time 2. Maximize utilization of facilities 3. Minimize work-in in-process (WIP) inventory 4. Minimize customer waiting time Optimize the use of resources so that production objectives are met 2006 Prentice Hall, Inc Scheduling Process- Focused Facilities 1. Schedule incoming orders without violating capacity constraints 2. heck availability of tools and materials before releasing an order 3. Establish due dates for each job and check progress 4. heck work in progress 5. Provide feedback 6. Provide work efficiency statistics and monitor times 2006 Prentice Hall, Inc

4 Loading s ssign jobs so that costs, idle time, or completion time are minimized Two forms of loading apacity oriented ssigning specific jobs to work centers Gantt harts Load chart shows the loading and idle times of departments, machines, or facilities Displays relative workloads over time Schedule chart monitors jobs in process ll Gantt charts need to be updated frequently 2006 Prentice Hall, Inc Prentice Hall, Inc Gantt Load hart Example Gantt Schedule hart Example Work enter Day Metalworks Mechanical Monday Tuesday Wednesday Thursday Friday Electronics Painting Processing Unscheduled enter not available Figure 15.3 Figure 15.4 Day Day Day Day Day Day Day Day Maintenance Now Start of an activity End of an activity Scheduled activity time allowed ctual work progress Nonproduction time Point in time when chart is reviewed 2006 Prentice Hall, Inc Prentice Hall, Inc

5 ssignment Method special class of linear programming models that assign tasks or jobs to resources Objective is to minimize cost or time Only one job (or worker) is assigned to one machine (or project) ssignment Method uild a table of costs or time associated with particular assignments $11 $14 $ 6 $ 8 $10 $11 $ 9 $12 $ Prentice Hall, Inc Prentice Hall, Inc ssignment Method 1. reate zero opportunity costs by repeatedly subtracting the lowest costs from each row and column 2. Draw the minimum number of vertical and horizontal lines necessary to cover all the zeros in the table. If the number of lines equals either the number of rows or the number of columns, proceed to step 4. Otherwise proceed to step 3. ssignment Method 3. Subtract the smallest number not covered by a line from all other uncovered numbers. dd the same number to any number at the intersection of two lines. Return to step Optimal assignments are at zero locations in the table. Select one, draw lines through h the row and column involved, and continue to the next assignment Prentice Hall, Inc Prentice Hall, Inc

6 ssignment Example Step 1a - Rows $11 $14 $ 6 $ 8 $10 $11 $ 9 $12 $ 7 Step 1b - olumns $ 5 $ 8 $ 0 $ 5 $ 6 $ 0 $ 0 $ 2 $ 3 $ 0 $ 0 $ 3 $ 2 $ 5 $ 0 $ 2 $ 3 $ 0 ssignment Example Step 2 - Lines The smallest uncovered number is 2 so this is subtracted from all other uncovered numbers and added to numbers at the $ 5 $ 6 $ 0 intersection of lines $ 0 $ 0 $ 3 $ 2 $ 3 $ 0 Step 3 - Subtraction ecause only two lines are needed to cover all the zeros, the solution is not optimal $ 3 $ 4 $ 0 $ 0 $ 0 $ 5 $ 0 $ 1 $ Prentice Hall, Inc Prentice Hall, Inc ssignment Example Step 2 - Lines Start by assigning to worker as this is the only possible assignment for worker. must $ 3 $ 4 $ 0 go to worker as worker is already assigned. This $ 0 $ 0 $ 5 leaves for worker. $ 0 $ 1 $ 0 Step 4 - ssignments ssignment Example From the original cost table Minimum cost = $6 + $10 + $9 = $25 Step 4 - ssignments ecause three lines are needed, the solution is optimal and assignments can be made $ 3 $ 4 $ 0 $ 0 $ 0 $ 5 $ 0 $ 1 $ 0 $11 $14 $ 6 $ 8 $10 $11 $ 9 $12 $ 7 $ 3 $ 4 $ 0 $ 0 $ 0 $ 5 $ 0 $ 1 $ Prentice Hall, Inc Prentice Hall, Inc

7 Sequencing s Specifies the order in which jobs should be performed at work centers Priority rules are used to dispatch or sequence jobs FFS: First come, first served SPT: Shortest processing time EDD: Earliest due date LPT: Longest processing time pply the four popular sequencing rules to these five jobs Work (Processing) Due (Days) (Days) D 3 15 E Prentice Hall, Inc Prentice Hall, Inc FFS: ---D-E Work (Processing) Flow Due D E FFS: ---D-E Work verage completion time = = 77/5 = 15.4 days (Processing) Number Flow of jobs Due Total job work time Utilization i = 6 6 = 28/77 8 = 36.4% verage number of jobs in the system = = 77/28 = 2.75 jobs 8Total job work 16 time 18 0 D 3 Total late 19 days 15 4 verage job lateness = 2 E 9 Number 28of jobs = 11/5 = 2.2 days Prentice Hall, Inc Prentice Hall, Inc

8 SPT: -D---E Work (Processing) Flow Due D E SPT: -D---E Work verage completion time = = 65/5 = 13 days (Processing) Number Flow of jobs Due Total job work time Utilization i = 2 2 = 28/65 6 = 43.1% 0 D verage number of jobs in the system = = 65/28 = 2.32 jobs 6Total job work 11 time Total late 19 days 18 1 verage job lateness = E 9 Number 28of jobs = 9/5 = 1.8 days Prentice Hall, Inc Prentice Hall, Inc EDD: --D--E Work (Processing) Flow Due D E EDD: --D--E Work verage completion time = = 68/5 = 13.6 days (Processing) Number Flow of jobs Due Total job work time Utilization i = 2 2 = 28/68 6 = 41.2% verage number of jobs Din the system = = 68/28 = 2.43 jobs 3Total job work 11 time Total late 19 days 18 1 verage job lateness = E 9 Number 28of jobs = 6/5 = 1.2 days Prentice Hall, Inc Prentice Hall, Inc

9 LPT: E---D- Work (Processing) Flow Due E D LPT: E---D- Work verage completion time = = 103/5 = 20.6 days (Processing) Number Flow of jobs Due Total job work time E Utilization i = 9 9 = 28/ = 27.2% 2% verage number of jobs in the system = = 103/28 = 3.68 jobs 6Total job work 23 time 8 15 D 3 Total late 26 days verage job lateness = 2 Number 28of jobs = 48/5 = 9.6 days Prentice Hall, Inc Prentice Hall, Inc Summary of Rules Rule verage verage Number verage ompletion Utilization of s in (Days) (%) System (Days) FFS SPT EDD LPT Prentice Hall, Inc omparison of Sequencing Rules No one sequencing rule excels on all criteria SPT does well on minimizing flow time and number of jobs in the system ut SPT moves long jobs to the end which may result in dissatisfied d customers FFS does not do especially well (or poorly) on any criteria but is perceived as fair by customers EDD minimizes lateness 2006 Prentice Hall, Inc

10 Sequencing N s on Two Machines: Johnson s s Rule Works with two or more jobs that pass through the same two machines or work centers Minimizes total production time and idle time Johnson s Rule 1. List all jobs and times for each work center 2. hoose the job with the shortest activity time. If that time is in the first work center, schedule the job first. If it is in the second work center, schedule the job last. 3. Once a job is scheduled, it is eliminated from the list 4. Repeat steps 2 and 3 working toward the center of the sequence 2006 Prentice Hall, Inc Prentice Hall, Inc Johnson s s Rule Example Johnson s s Rule Example Work enter 1 Work enter 2 (Drill Press) (Lathe) D 10 7 Work enter 1 Work enter 2 (Drill Press) (Lathe) D 10 7 E D E 7 12 E Prentice Hall, Inc Prentice Hall, Inc

11 Johnson s s Rule Example Johnson s s Rule Example Work enter 1 Work enter 2 (Drill Press) (Lathe) D 10 7 E D Work enter 1 Work enter 2 (Drill Press) (Lathe) D 10 7 E D E 7 12 E W 1 E D W 1 E D W 2 W 2 E D Prentice Hall, Inc E D 2006 Prentice Hall, Inc Limitations of Rule-ased Dispatching Systems 1. Scheduling is dynamic and rules need to be revised to adjust to changes 2. Rules do not look upstream or downstream 3. Rules do not look beyond due dates Finite it apacity Scheduling Overcomes disadvantages of rule-based systems by providing an interactive, computer-based graphical system May include rules and expert systems or simulation to allow real-time response to system changes Initial data often from an MRP system FS allows the balancing of delivery needs and efficiency 2006 Prentice Hall, Inc Prentice Hall, Inc

12 Finite it apacity Scheduling Finite apacity Scheduling MRP Data Master schedule OM Inventory Priority rules Expert systems Simulation models Interactive Finite apacity Scheduling Routing files Work center information Tooling and other resources Setups and run time Figure Prentice Hall, Inc Prentice Hall, Inc Scheduling Services Service systems differ from manufacturing Manufacturing Services Schedules machines Schedule staff and materials Inventories used to smooth demand Seldom maintain inventories Machine-intensive intensive i and Labor-intensive i and demand may be smooth demand may be variable Scheduling may be bound Legal issues may constrain by union contracts flexible scheduling Few social or behavioral Social and behavioral issues issues may be quite important 2006 Prentice Hall, Inc Scheduling Services Hospitals have complex scheduling system to handle complex processes and material requirements anks use a cross-trained and flexible workforce and part-time time workers irlines must meet complex F and union regulations and often use linear programming to develop optimal schedules 24/7 Operations use flexible workers and variable schedules 2006 Prentice Hall, Inc

13 Demand Management ppointment or reservation systems FFS sequencing rules Discounts or other promotional schemes When demand management is not feasible, managing capacity through staffing flexibility may be used 2006 Prentice Hall, Inc