Scheduling

Learning Objectives

• Explain what scheduling involves and the importance of good scheduling.

• Discuss scheduling needs in high-volume and intermediate-volume systems.

• Discuss scheduling needs in job shops. Use and interpret Gantt charts, and use the assignment method for loading.

• Discuss and give examples of commonly used priority rules.

• Describe some of the unique problems encountered in service systems, and describe some of the approaches used for scheduling service systems.

Scheduling

Specifies when labor, equipment, Specifies when labor, equipment, facilities are needed to produce a facilities are needed to produce a product or provide a serviceproduct or provide a service

Last stage of planning before Last stage of planning before production occursproduction occurs

Scheduled Operations

• Process Industry– Linear programming– EOQ with non-

instantaneous replenishment

• Mass Production– Assembly line

balancing• Project– Project -scheduling

techniques (PERT, CPM)

• Batch Production– Aggregate planning–Master scheduling–Material

requirements planning (MRP)

– Capacity requirements planning (CRP)

Objectives in Scheduling

Meet customer due datesMeet customer due datesMinimize job latenessMinimize job latenessMinimize response timeMinimize response timeMinimize completion timeMinimize completion timeMinimize time in the systemMinimize time in the systemMinimize overtimeMinimize overtimeMaximize machine or labor utilizationMaximize machine or labor utilizationMinimize idle timeMinimize idle timeMinimize work-in-process inventoryMinimize work-in-process inventory

There is no single scheduling rule that optimizes all of these performance criteria. We consider the trade-offs.

Scheduling Low-Volume Systems

• Loading - assignment of jobs to process centers

• Sequencing - determining the order in which jobs will be processed

• Job-shop scheduling–Scheduling for low-volume systems

with many variations in requirements

Loading

• Assignment of jobs to process centers

• Perform work with most efficient resources

• Use assignment method of linear programming to determine allocation

• Infinite loading – jobs are assigned to work centers without regard for the capacity of the work center.

• Finite loading – jobs are assigned to work centers taking into account the work center capacity and job processing times

• Vertical loading / Horizontal loading

• Forward scheduling – scheduling ahead from some point in time.

• Backward scheduling – scheduling by working backwards in time from the due date(s).

• Schedule chart – a form of Gantt chart that shows the orders or jobs in progress and whether they are on schedule.

Loading

Sequencing

• Sequencing: Determine the order in which jobs at a work center will be processed.

• Workstation: An area where one person works, usually with special equipment, on a specialized job.

• Priority rules: Simple heuristics used to select the order in which jobs will be processed.

• Job time: Time needed for setup and processing of a job.

Priority Rules• FCFS - first come, first served

• SPT - shortest processing time

• EDD - earliest due date

• CR - critical ratio

• S/O - slack per operation

• Rush - emergency

If no order specified use first-come first-served (FCFS)

Performance Measures of Priorities

• Meeting due dates of customers or downstream operations.

• Minimizing the flow time (the time a job spends in the process).

• Minimizing work-in-process inventory.

• Minimizing idle time of machines or workers.

Assumptions of Priority Rules

• The setup of jobs is known• Setup time is independent pf

processing sequence• Setup time is deterministic• There will be no interruptions in

processing such as:–Machine breakdowns–Accidents–Worker illness

Two Work Center Sequencing

• Johnson’s Rule: technique for minimizing completion time for a group of jobs to be processed on two machines or at two work centers.

• Minimizes total idle time

• Several conditions must be satisfied

Johnson’s Rule Conditions

• Job time must be known and constant

• Job times must be independent of sequence

• Jobs must follow same two-step sequence

• Job priorities cannot be used

• All units must be completed at the first work center before moving to second

Johnson’s Rule Optimum Sequence

1. List the jobs and their times at each work center

2. Select the job with the shortest time

3. Eliminate the job from further consideration

4. Repeat steps 2 and 3 until all jobs have been scheduled

Sequencing Jobs Through Two Serial Process

Johnson’s RuleJohnson’s Rule1.1. List time required to process each job at each List time required to process each job at each

machine. Set up a one-dimensional matrix to machine. Set up a one-dimensional matrix to represent desired sequence with # of slots equal to # represent desired sequence with # of slots equal to # of jobs.of jobs.

2.2. Select smallest processing time at either machine. Select smallest processing time at either machine. If that time is on machine 1, put the job as near to If that time is on machine 1, put the job as near to beginning of sequence as possible.beginning of sequence as possible.

3.3. If smallest time occurs on machine 2, put the job as If smallest time occurs on machine 2, put the job as near to the end of the sequence as possible.near to the end of the sequence as possible.

4.4. Remove job from list.Remove job from list.5.5. Repeat steps 2-4 until all slots in matrix are filled Repeat steps 2-4 until all slots in matrix are filled

and all jobs are sequenced.and all jobs are sequenced.

Monitoring

• Work package–Shop paperwork that travels with a job

• Gantt Chart–Shows both planned and completed

activities against a time scale

• Input/Output Control–Monitors the input and output from

each work center

Scheduling Difficulties

• Variability in

–Setup times

–Processing times

– Interruptions

–Changes in the set of jobs

• No method for identifying optimal schedule

• Scheduling is not an exact science

• Ongoing task for a manager

Minimizing Scheduling Difficulties

• Set realistic due dates

• Focus on bottleneck operations

• Consider lot splitting of large jobs

Scheduling Services Considerations

• Scheduling services different from manufacturing–Inability to store or inventory services–Random nature of customer requests

for service• Point of customer contact–Back-office operations–Front-office operations

Scheduling Services

• Appointment systems– Controls customer arrivals for service

• Reservation systems– Estimates demand for service

• Scheduling the workforce– Manages capacity for service

• Scheduling multiple resources– Coordinates use of more than one

resource

Yield Management

• Yield Management – the application of pricing strategies to allocate capacity among various categories of demand.

• The goal is to maximize the revenue generated by the fixed capacity

• Fixed capacity–Hotel, motel rooms–Airline seats

• Unsold rooms or seats cannot be carried over

Cyclical Scheduling

• Hospitals, police/fire departments, restaurants, supermarkets

• Rotating schedules

–Set a scheduling horizon

–Identify the work pattern

–Develop a basic employee schedule

–Assign employees to the schedule

Employee Scheduling

Because:

• Labor is very flexible resource

• Scheduling workforce is complicated, repetitive task

• Heuristics are commonly used

There is a lot if industry specific software available.

Service Operation Problems

• Cannot store or inventory services

• Customer service requests are random

• Scheduling service involves– Customers– Workforce– Equipment