Short-time scheduling

Types of short-term scheduling

Forward scheduling and backward scheduling

Forward scheduling

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

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

minimize completion time, maximise utilisation, minimise WIP inventory, minimise customer wait time

process-focused work centres

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

assigning jobs to work centres; approaches: input-output control, Gantt charts, assignment method

Input-output control

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

shows relative workload in facility, negatives: does not account for unexpected events, must be updated regularly

Assignment Method

occurs in many environments: resources-jobs, planes-gates, course-rooms); type of binary linear programming model

Priority rules for Dispatching Jobs (sequencing)

FCFS, EDD, SPT, LPT, CR

FCFS

First come first serve; the first job to arrive at a work centre is processed first

EDD

earliest due date; the job with the easiest due date is processed first

SPT

shortest processing time first

LPT

longets processing time first

CR

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

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