Bài giảng Operations Management - Chapter 15: Short-Term Scheduling

Operations ManagementShort-Term SchedulingChapter 151OutlineGLOBAL COMPANY PROFILE: DELTA AIRLINESTHE STRATEGIC IMPORTANCE OF SHORT-TERM SCHEDULINGSCHEDULING ISSUESForward and Backward SchedulingScheduling CriteriaSCHEDULING PROCESS-FOCUSED WORK CENTERS2Outline - ContinuedLOADING JOBS IN WORK CENTERSInput-Output ControlGantt ChartsAssignment Method SEQUENCING JOBS IN WORK CENTERSPriority Rules for Dispatching JobsCritical RatioSequencing N Jobs on Two Machines: Johnson’s Rule3Outline - ContinuedLIMITATIONS OF RULE-BASED DISPATCHING SYSTEMS FINITE SCHEDULING THEORY OF CONSTRAINTSBOTTLENECK WORK CENTERSREPETITIVE MANUFACTURINGSCHEDULING FOR SERVICESScheduling Service Employees with Cyclical Scheduling4Learning ObjectivesWhen you complete this chapter, you should be able to :Identify or Define:Gantt chartsAssignment methodSequencing rulesJohnson’s rulesBottlenecks5Learning Objectives - continuedWhen you complete this chapter, you should be able to :Describe or Explain:SchedulingSequencingShop loadingTheory of constraints6Delta Airlines10% of Delta’s flights disrupted per year - half of those by weatherCost: $440-million in: lost revenueovertime payfood and lodging vouchers$33-million hi-tech nerve center18 staff$35-million savings7Strategic Implications of Short-Term SchedulingBy scheduling effectively, companies use assets more effectively and create greater capacity per dollar invested, which, in turn, lowers costThis added capacity and related flexibility provides faster delivery and therefore better customer serviceGood scheduling is a competitive advantage which contributes to dependable delivery8Deals with timing of operationsShort run focus: Hourly, daily, weeklyTypesToday Due DateBEForward SchedulingToday Due DateBEBackward SchedulingShort-Term Scheduling9HospitalOutpatient treatmentsOperating roomsUniversityInstructorsClassroomsFactoryProductionPurchases© 1984-1994 T/Maker Co.Short-Term Scheduling Examples10Organization Managers Must ScheduleOperating room usePatient admissionNursing, security, maintenance staffsOutpatient treatmentsClassrooms and audiovisual equipmentStudent and instructor schedulesGraduate and undergraduate coursesProduction of goodsPurchase of materialsWorkersScheduling DecisionsMount Sinai HospitalIndiana UniversityLockheed-Martin Factory11 Organization Managers Must ScheduleChefs, waiters,bartendersDelivery of fresh foodsEntertainersOpening of dining areasMaintenance of aircraftDeparture timetablesFlight crews, catering, gate, and ticketing personnelScheduling DecisionsHard Rock CafeDelta Airlines12Capacity Planning, Aggregate Scheduling, Master Schedule, and Short-Term SchedulingCapacity Planning 1. Facility size 2. Equipment procurementAggregate Scheduling 1. Facility utilization 2. Personnel needs 3. SubcontractingMaster Schedule 1. MRP 2. Disaggregation of master planLong-termIntermediate-termShort-termIntermediate-termShort-term Scheduling 1. Work center loading 2. Job sequencing13Forward and Backward SchedulingForward scheduling: begins the schedule as soon as the requirements are knownjobs performed to customer orderschedule can be accomplished even if due date is missedoften causes buildup of WIPBackward scheduling: begins with the due date of the final operation; schedules jobs in reverse orderused in many manufacturing environments, catering, scheduling surgery14The Goals of Short-Term SchedulingMinimize completion timeMaximize utilization (make effective use of personnel and equipment)Minimize WIP inventory (keep inventory levels low)Minimize customer wait time15Qualitative factorsNumber and variety of jobsComplexity of jobsNature of operationsQuantitative criteriaAverage completion timeUtilization (% of time facility is used)WIP inventory (average # jobs in system)Customer waiting time (average lateness)Choosing a Scheduling Method16Level Use MethodsRepetitive-FocusedProduct-FocusedProcess-FocusedVariety of MethodsScheduling Methods Differ by Process17High variety, low volume systemsProducts made to orderProducts need different materials and processingComplex production planning and controlProduction planning aspectsShop loadingJob sequencingProcess-Focused Work Centers18Requirements for Scheduling Process-Focused Work CentersSchedule incoming orders without violating capacity constraints of individual work centersCheck availability of tools and materials before releasing an order to a departmentEstablish due dates for each job and check progress against need dates and order lead timesCheck work-in-progress as jobs move through the shopProvide feedback on plant and production activitiesProvide work-efficiency statistics and monitor operator times for payroll and labor distribution analyses19Types of Planning FilesItem master file - contains information about each component the firm produces or purchasesRouting file - indicates each component’s flow through the shopWork-center master file - contains information about the work center such as capacity and efficiency20Process-Focused Planning SystemForecast &Firm OrdersMaterialRequirementsPlanningAggregateProductionPlanningResourceAvailabilityMasterProductionSchedulingShopFloorSchedulesCapacityRequirementsPlanningRealistic?YesNo, modify CRP, MRP, or MPS21Assigning jobs to work centersConsiderationsJob priority (e.g., due date)CapacityWork center hours availableHours needed for jobApproachesGantt charts (load & scheduling) - capacityAssignment method - job to specific work centerLoading Jobs in Work Centers22Week Ending6/66/136/206/277/47/11Planned input280280280280280Actual input270250280285280Cumulative Deviation-10-40-40-35Planned Output320320320320Actual Output270270270270Cumulative Deviation-50-100-150-200Cumulative Change in Backlog0-20-10-+5Input-Output ControlExplanation: 270 input, 270 output, implies 0 changeExplanation: 250 input, 270 output implies –20 change (20 standard hours less in the work center)23Order Release Begins Shop LoadingNoRelease OrderHold ReleasePriority & Capacity OK?Gross RequirementsScheduled ReceiptsAvailableNet RequirementsPlanned Order ReceiptPlanned Order ReleaseYes12100100324Options for Managing Facility Work FlowCorrecting performanceIncreasing capacityIncreasing or reducing input to the work center by:routing work to or from other work centersincreasing or decreasing subcontractingproducing less (or more)25Shows relative workload in facilityDisadvantagesDoes not account for unexpected eventsMust be updated regularlyWork CenterMTWThFMetal WorksJob 349Job 350MechanicalJob DJob GElectronicsJob BJob HPaintingJob CJob EJob IGantt Load Chart26Gantt Load Chart for Week of March 6Shows relative workload in facilityDisadvantagesDoes not account for unexpected eventsMust be updated regularlyDayWork Center MondayTuesdayWednesdayThursdayFridayMetalworksJob 349Job 350MechanicalJob 349Job 406ElectronicsJob 408Job 349PaintingJob 295Job 408Job 349ProcessingUnscheduledCenter not available (for example, maintenance time, repairs, shortages)27Gantt Scheduling ChartStart of an activityScheduled activity time allowedPoint in time when chart is reviewedSTWTFSDayJobJob AJob BJob CMaintenanceNowEnd of an activityActual work progressNon-production timeDay 1Day 2Day 3Day 4Day 5Day 6Day 728Assigns tasks or jobs to resourcesType of linear programming modelObjectiveMinimize total cost, time etc.Constraints1 job per resource (e.g., machine)1 resource (e.g., machine) per jobAssignment Method29Assignment Method - Four StepsSubtract the smallest number in each row from every number in that row; then subtract the smallest number in every column from every number in that columnDraw the minimum number of vertical and horizontal straight lines necessary to cover all zeros in the tableIf the number of lines equals either the number of rows or the number of columns, then you can make an optimal assignment (Step 4)Otherwise:Subtract the smallest number not covered by a line from every other uncovered number. Add the same number to any number(s) lying at the intersection of any two lines. Return to Step 2Optimal assignments will always be at the zero locations of the table 30Assignment Method – Type Setter ExampleTypesetterJobABCR-34$11$14$ 6S-66$ 8$10$11T-50$9$12$7Initial set-up31Step 1a & 1bTypesetterJobABCR-34580S-66023T-50250TypesetterJobABCR-34560S-66003T-50230Step 1aStep 1b32Step 2TypesetterJobABCR-34560S-66003T-50230Smallest uncovered number33Step 3TypesetterJobABCR-34340S-66005T-50010Make assignments34Production ControlProductionWhich job do I run next?Dispatch ListOrder Part Due QtyXYZ 6014 123 100ABC 6020 124 50Job PacketJob XYZOrder releaseSequencing Challenge35Specifies order jobs will be workedSequencing rulesFirst come, first served (FCFS)Shortest processing time (SPT)Earliest due date (EDD)Longest processing time (LPT)Critical ratio (CR)Johnson’s ruleSequencing36Priority Rules for Dispatching JobsFirst come, first servedThe first job to arrive at a work center is processed firstEarliest due dateThe job with the earliest due date is processed firstShortest processing timeThe job with the shortest processing time is processed firstLongest processing timeThe job with the longest processing time is processed firstCritical ratioThe ratio of time remaining to required work time remaining is calculated, and jobs are scheduled in order of increasing ratio.FCFSEDDSPTLPTCR37Process first job to arrive at a work center firstAverage performance on most scheduling criteriaAppears ‘fair’ & reasonable to customersImportant for service organizationsExample: RestaurantsFirst Come, First Served Rule38Shortest Processing Time RuleProcess job with shortest processing time first.Usually best at minimizing job flow and minimizing the number of jobs in the systemMajor disadvantage is that long jobs may be continuously pushed back in the queue.39Longest Processing Time RuleProcess job with longest processing time first.Usually the least effective method of sequencing.40Process job with earliest due date firstWidely used by many companiesIf due dates importantIf MRP usedDue dates updated by each MRP runPerforms poorly on many scheduling criteriaEarliest Due Date Rule41Ratio of time remaining to work time remainingProcess job with smallest CR firstPerforms well on average latenessCRTime remainingWork days remainingDue date - Today's dateWork (lead) time remaining==Critical Ratio (CR)42Advantages of the Critical RatioScheduling RuleUse of the critical ratio can help to:determine the status of a specific jobestablish a relative priority among jobs on a common basisrelate both stock and make-to-order jobs on a common basisadjust priorities and revise schedules automatically for changes in both demand and job progressdynamically track job progress and location43Criteria to Evaluate Priority Rules44Job Sequencing ExampleJobJob WorkProcessing time in daysJob Due Date (day)A68B26C818D315E92345FCFSParameterValueAverage completion time15.4 daysUtilization36.4%Average number of jobs in the system2.75 jobsAverage job lateness2.2 daysSequenceABCDE46SPTParameterValueAverage completion time13 daysUtilization43.1%Average number of jobs in the system2.32 jobsAverage job lateness1.8 daysSequenceBDACE47EDDParameterValueAverage completion time13.6 daysUtilization41.2%Average number of jobs in the system2.43 jobsAverage job lateness1.2 daysSequenceBADCE48LPTParameterValueAverage completion time20.6 daysUtilization27.2%Average number of jobs in the system3.68 jobsAverage job lateness9.6 daysSequenceECADB49SummaryRuleAverage Completion Time (days)Utilization (%)Average Number of Jobs in the SystemAverage Lateness (Days)FCFS15.436.42.752.2SPT13.043.12.321.8EDD13.641.22.431.2LPT20.627.23.689.650Critical Ratio (CR)JobJob WorkProcessing time in daysJob Due Date (day)Critical RatioA680.75B260.33C8180.44D3150.20E9230.39SequenceACEBD51Used to sequence N jobs through 2 machines in the same order© 1995 Corel Corp.© 1995 Corel Corp.SawDrillJob AJob BJob CJobs (N = 3)Johnson’s Rule52Johnson's Rule - Scheduling N Jobs on Two MachinesAll jobs are to be listed, and the time each requires on a machine shown.Select the job with the shortest activity time. If the shortest time lies with the first machine, the job is scheduled first; if with the second machine, the job is scheduled last.Once a job is scheduled, eliminate it.Apply steps 2-3 to the remaining jobs, working toward the center of the sequence.53List jobs &activity timesSelect job withshortest timeMachine?ScheduleFIRSTScheduleLASTEliminate jobfrom listJobs left?BreakarbitrarilyTies?Yes12YesStopNoNoJohnson’s Rule Steps54Johnson’s Rule - ExampleJobWork Center 1(Drill Press)Work Center 2(Lathe)A52B36C84D107E71255Johnson’s Rule - ExampleAStep 1BAStep 2BCAStep 3BDCAStep 4BEDCAStep 556Graphical Depiction of Job FlowBEDCABEDCAWork center 1Work center 20 3 10 20 28 330 3 9 10 20 22 28 29 33 35Time =>Time =>BEDCA= Job completed= Idle57Limitations of Rule-Based Dispatching SystemsScheduling 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 recognizedRules do not look beyond due dates58Finite Scheduling System59Deals with factors limiting company’s ability to achieve goalsTypes of constraintsPhysicalExample: Machines, raw materialsNon-physicalExample: Morale, trainingLimits throughput in operations Theory of Constraints60Theory of ConstraintsA Five Step ProcessIdentify the constraintsDevelop a plan for overcoming the identified constraintsFocus resources on accomplishing the constraints identified in step 2Reduce the effects of the constraints by off-loading work or by expanding capabilityOnce one set of constraints is overcome, return to the first step and identify new constraints61Bottleneck work centers have less capacity than prior or following work centersThey limit production output© 1995 Corel Corp.Bottleneck Work Centers62Techniques for Dealing With BottlenecksIncrease the capacity of the constraintEnsure well-trained and cross-trained employees are available to operate and maintain the work center causing the constraintDevelop alternate routings, processing procedures, or subcontractorsMove inspections and tests to a position just before the constraintSchedule throughput to match the capacity of the bottleneck63The 10 Commandments for Correct SchedulingUtilization of a non-bottleneck resource is determined not by its own capacity but by some other constraint in the systemActivating a resource is not synonymous with utilizing a resourceAn hour lost at a bottleneck is an hour lost of the whole systemAn hour saved at a non-bottleneck is a mirageThe transfer batch may not, and many times should not, be equal to the process batch64The 10 Commandments for Correct SchedulingThe amount processed should be verifiable and not fixedCapacity and priority need to be considered simultaneously, not sequentiallyDamage from unforeseen problems can be isolated and minimizedPlant capacity should not be balancedThe sum of the local optimums is not equal to the global optimum65Repetitive Manufacturing - Advantages of Level Material UseLower inventory levels, releasing capital for other usesFaster product throughputImproved component quality and hence improved product qualityReduced floor space requirementsImproved communication among employees because they are closer togetherSmoother production process because large lots have not “hidden” the problems66Scheduling for ServicesAppointment systems - doctor’s officeReservations systems - restaurant, car rentalFirst come, first served - deliMost critical first - hospital trauma room67Cyclical SchedulingPlan a schedule equal in weeks to the number of people being scheduledDetermine how many of each of the least desirable off-shifts must be covered each weekBegin the schedule for one worker by scheduling the days off during the planning cycle (at a rate of 2 days per week on average)68Cyclical Scheduling - ContinuedAssign off-shifts for the first workerRepeat this pattern for each other worker, but offset by one week from the previousAllow each worker to pick his/her “slot” or “line” in order of seniorityMandate that any changes from the chosen schedule are strictly between the personnel wanting to switch69