Short-time scheduling

 0    16 flashcards    dklekowski
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Types of short-term scheduling
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Forward scheduling and backward scheduling
Forward scheduling
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begins the schedule as soon as the requirements are known; often builds up WIP; jobs performed to customer order, schedule can be accomplished even if due date is missed
Backward Scheduling
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begins with the due date of the final operation, schedules jobs in reverse order, used in many manufacturing environments; ex. catering
Goals of short-term scheduling
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minimize completion time, maximise utilisation, minimise WIP inventory, minimise customer wait time
process-focused work centres
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high variety, low volume, mate-to-order, different materials and processing for each product, complex production planning and control, production scheduling aspects
Loading Jobs in Work Centres
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assigning jobs to work centres; approaches: input-output control, Gantt charts, assignment method
Input-output control
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CONWIP - constant WIP; for given average output rate, try to keep L low and constant then W will be short and constant; Little's formula
Gantt Load chart
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shows relative workload in facility, negatives: does not account for unexpected events, must be updated regularly
Assignment Method
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occurs in many environments: resources-jobs, planes-gates, course-rooms); type of binary linear programming model
Priority rules for Dispatching Jobs (sequencing)
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FCFS, EDD, SPT, LPT, CR
FCFS
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First come first serve; the first job to arrive at a work centre is processed first
EDD
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earliest due date; the job with the easiest due date is processed first
SPT
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shortest processing time first
LPT
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longets processing time first
CR
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critical ratio; the ratio of time remaining to required work time remaining is calculated and jobs are schedules in order of increasing ratio; CR = time remaining/work days remaining -> process jobs with the smallest CR first
Criteria to evaluate priority rules
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Average Completion Time= Sum of flow times / # jobs; Utilisation = Sum process time / sum flow time; Average number of jobs in system Sum Flow time /Sum processing time; average jobs lateness = sum late times / number of jobs

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