OSCM 373 sCh16Scheduling

Overview of Scheduling

  • Definition: Timing the use of human activities, equipment, and facilities.

  • Benefits of Effective Scheduling:

    • Cost savings

    • Increased productivity

    • Enhanced customer satisfaction and competitiveness

Scheduling Systems

1. High Volume Systems

  • Flow System: A repetitive process where all jobs follow the same sequence.

  • Flow System Scheduling Goals:

    • Ensure a smooth rate of flow for goods or customers through the system.

    • Achieve high utilization of labor and equipment, which involves line balancing.

2. Low Volume Systems

  • Job Shop Scheduling: Scheduling for systems with low volume but many variations in requirements.

    • Focus on make-to-order products.

    • Different processing requirements (materials, time, sequence) create a complex scheduling environment.

    • Firm schedules cannot be established until actual orders are received.

Scheduling Approaches

1. Forward Scheduling

  • Involves scheduling ahead from a specific point in time.

  • Example Question: "How long will it take to complete this job?"

2. Backward Scheduling

  • Scheduling backwards from a due date.

  • Example Question: "When is the latest this job can start and still be completed on time?"

Loading Strategies

1. Loading Definition

  • Assignment of jobs to processing centers.

2. Types of Loading

  • Infinite Loading: Jobs are assigned without regard for center capacity. May lead to overloading (requiring overtime, work shifts, or contracting out).

  • Finite Loading: Jobs are assigned considering center capacity and job processing times.

Gantt Charts

  • Definition: A visual aid for loading and scheduling, displaying actual or intended resource use in a time framework.

    • Managers use for trial and error schedule development

    • require constant updating

  • Types:

    • Gantt Load Chart: Shows loading and idle times for machines/departments.

    • Gantt Schedule Chart: Displays jobs in progress and their scheduling status.

Sequencing

  • Definition: Determines the order of jobs processed within a work center.

  • Priority Rules: Heuristics for selecting job processing order.

Priority Rules

  • FCFS (First Come, First Served): Jobs processed in arrival order.

  • SPT (Shortest Processing Time): Jobs with the shortest processing time processed first.

  • EDD (Earliest Due Date): Jobs with the earliest due dates processed first.

  • CR (Critical Ratio): CR = Time Remaining until due date / Remaining processing time. Lowest CR is processed first.

  • S/O (Slack per Operation): Average slack time is calculated. (time until due date - remaining processing time)/ remaining number of operations

  • RUSH (Emergency or rush jobs prioritized.

Sequencing Performance Metrics

  • Job Flow Time:

    • Time from job arrival to completion

    • Includes waiting time on top of processing time

  • Makespan:

    • Total time needed to complete a group of jobs from beg to end

  • Job Lateness:

    • Measures how much a job exceeds due date.

  • Average Number of Jobs:

    • Calculated as total flow time / makespan.

    • Jobs in shop considered WIP inventory

Application of Sequencing Methods

Examples

  • FCFS Example: Computed metrics such as total flow time, lateness, makespan, and average number of jobs.

  • SPT Example: Similar metrics calculated, demonstrating efficiency.

  • EDD Example: Analyzed to optimize lateness.

  • CR Example: Jobs sequenced using Critical Ratio with resulting metrics detailed.

Priority Rules Comparison

Two Work Center Sequencing

  • Defintion: Must follow same two - step sequence, all jobs must be completed at first work center before moving to second work center

    • Job time known and constant

    • Job time independent of sequence

  • Goal : Minimize makespan

Johnson's Rule

  • Definition: Technique to minimize makespan for jobs processed at two work centers with the same sequence.

  • Method:

  1. List jobs and times at each work center

  2. Select job with the shortest processing time for scheduling.

    • If shortest time is at first wrk center, schedule that job first

    • If shortest time is at scnd wrk center, schedule that job last

  3. Repeat steps working towards center of the sequence Scheduled until all jobs are processed.

Method Example

Assignment Model for Loading

  • Goal:

    • Optimal assignment of tasks to resources.

  • Hungarian Method:

    • Step-by-step method to find the lowest cost solution, including row/column reductions.

Hungarian Method Steps

  1. Row Reduction:

    • Subtract smallest number in each row from every number in row

  2. Column Reduction:

    • Subtract smallest numbr in each clmn from every numbr in clmn

  3. Evaluate whether optimum assignment can be made

    • Determing minimum num of lines to cross all zeros

    • if num of lines = num of rows go step 6, if not go step 4

  4. If num of line less than num of rows, modify table

    • subtract smallest uncovered num from every uncovered num

    • add smllest uncovered num to nums at intrsct of cross out lines

    • nums crossed out but not at intersection of cross out lines carrry over unchanged to next table

  5. Repeat steps 3 and 4 until optimal table is obtained

  6. Make assignments

    • Begin with rows or columns with only one zero

    • match items that have zeros, using only 1 match for each row and column

    • eliminate both the row and column after the match

Scheduling Challenges

  • Challenges include variability in setup times, processing times, interruptions, and job set changes.

  • No exact method for optimal schedules, making it a challenge for managers.

Strategies to Minimize Scheduling Challenges

  • Set realistic due dates.

  • Focus on bottleneck operations to increase capacity.

  • Schedule according to bottlenecks and consider splitting large jobs into smaller batches.