12 Short term scheduling

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Information about 12 Short term scheduling
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Published on December 12, 2007

Author: Natalya

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Slide1:  Operations Management Short-Term Scheduling Outline:  Outline Global Company Profile: Delta Airlines The Strategic Importance Of Short-Term Scheduling Scheduling Issues Forward and Backward Scheduling Scheduling Criteria Outline – Continued:  Outline – Continued Scheduling Process-Focused Facilities Loading Jobs Input-Output Control Gantt Charts Assignment Method Outline – Continued:  Outline – Continued Sequencing Jobs Priority Rules for Dispatching Jobs Critical Ratio Sequencing N Jobs on Two Machines: Johnson’s Rule Limitations Of Rule-Based Dispatching Systems Finite Capacity Scheduling (FCS) Outline – Continued:  Outline – Continued Theory Of Constraints Bottlenecks Drum, Buffer, Rope Scheduling Repetitive Facilities Scheduling Services Scheduling Service Employees with Cyclical Scheduling Learning Objectives:  Learning Objectives When you complete this chapter, you should be able to: Identify or Define: Gantt charts Assignment method Sequencing rules Johnson’s rule Bottlenecks Learning Objectives:  Learning Objectives When you complete this chapter, you should be able to: Describe or Explain: Scheduling Sequencing Shop loading Theory of constraints Delta Airlines:  Delta Airlines About 10% of Delta’s flights are disrupted per year, half because of weather Cost is $440 million in lost revenue, overtime pay, food and lodging vouchers The $33 million Operations Control Center adjusts to changes and keeps flights flowing Saves Delta $35 million per year Strategic Importance of Short-Term Scheduling:  Strategic Importance of Short-Term Scheduling Effective and efficient scheduling can be a competitive advantage Faster movement of goods through a facility means better use of assets and lower costs Additional capacity resulting from faster throughput improves customer service through faster delivery Good schedules result in more reliable deliveries Scheduling Issues:  Scheduling Issues Scheduling deals with the timing of operations The task is the allocation and prioritization of demand Significant issues are The type of scheduling, forward or backward The criteria for priorities Scheduling Issues:  Figure 15.1 Scheduling Issues Scheduling Decisions:  Scheduling Decisions Table 15.1 Forward and Backward Scheduling:  Forward and Backward Scheduling Forward scheduling starts as soon as the requirements are known Produces a feasible schedule though it may not meet due dates Frequently results in excessive work-in- process inventory Forward and Backward Scheduling:  Forward and Backward Scheduling Backward scheduling begins with the due date and schedules the final operation first Schedule is produced by working backwards though the processes Resources may not be available to accomplish the schedule Scheduling Criteria:  Scheduling Criteria Minimize completion time Maximize utilization of facilities Minimize work-in-process (WIP) inventory Minimize customer waiting time Optimize the use of resources so that production objectives are met Different Processes/ Different Approaches:  Different Processes/ Different Approaches Table 15.2 Scheduling Process-Focused Facilities:  Scheduling Process-Focused Facilities Schedule incoming orders without violating capacity constraints Check availability of tools and materials before releasing an order Establish due dates for each job and check progress Check work in progress Provide feedback Provide work efficiency statistics and monitor times Planning and Control Files:  Planning and Control Files Loading Jobs:  Loading Jobs Assign jobs so that costs, idle time, or completion time are minimized Two forms of loading Capacity oriented Assigning specific jobs to work centers Input-Output Control:  Input-Output Control Identifies overloading and underloading conditions Prompts managerial action to resolve scheduling problems Can be maintained using ConWIP cards that control the scheduling of batches Input-Output Control Example:  Input-Output Control Example Figure 15.2 Input-Output Control Example:  Input-Output Control Example Figure 15.2 Input-Output Control Example:  Input-Output Control Example Options available to operations personnel include: Correcting performances Increasing capacity Increasing or reducing input to the work center Gantt Charts:  Gantt Charts Load chart shows the loading and idle times of departments, machines, or facilities Displays relative workloads over time Schedule chart monitors jobs in process All Gantt charts need to be updated frequently Gantt Load Chart Example:  Gantt Load Chart Example Figure 15.3 Gantt Schedule Chart Example:  Gantt Schedule Chart Example Figure 15.4 Assignment Method:  Assignment Method A special class of linear programming models that assign tasks or jobs to resources Objective is to minimize cost or time Only one job (or worker) is assigned to one machine (or project) Assignment Method:  Assignment Method Build a table of costs or time associated with particular assignments Assignment Method:  Assignment Method Create zero opportunity costs by repeatedly subtracting the lowest costs from each row and column Draw the minimum number of vertical and horizontal lines necessary to cover all the zeros in the table. If the number of lines equals either the number of rows or the number of columns, proceed to step 4. Otherwise proceed to step 3. Assignment Method:  Assignment Method Subtract the smallest number not covered by a line from all other uncovered numbers. Add the same number to any number at the intersection of two lines. Return to step 2. Optimal assignments are at zero locations in the table. Select one, draw lines through the row and column involved, and continue to the next assignment. Assignment Example:  Assignment Example Assignment Example:  Assignment Example Because only two lines are needed to cover all the zeros, the solution is not optimal The smallest uncovered number is 2 so this is subtracted from all other uncovered numbers and added to numbers at the intersection of lines Assignment Example:  Assignment Example Because three lines are needed, the solution is optimal and assignments can be made Start by assigning R-34 to worker C as this is the only possible assignment for worker C. Job T-50 must go to worker A as worker C is already assigned. This leaves S-66 for worker B. Assignment Example:  Assignment Example Sequencing Jobs:  Sequencing Jobs Specifies the order in which jobs should be performed at work centers Priority rules are used to dispatch or sequence jobs FCFS: First come, first served SPT: Shortest processing time EDD: Earliest due date LPT: Longest processing time Sequencing Example:  Sequencing Example Apply the four popular sequencing rules to these five jobs Sequencing Example:  Sequencing Example FCFS: Sequence A-B-C-D-E Sequencing Example:  Sequencing Example FCFS: Sequence A-B-C-D-E Sequencing Example:  Sequencing Example SPT: Sequence B-D-A-C-E Sequencing Example:  Sequencing Example SPT: Sequence B-D-A-C-E Sequencing Example:  Sequencing Example EDD: Sequence B-A-D-C-E Sequencing Example:  Sequencing Example EDD: Sequence B-A-D-C-E Sequencing Example:  Sequencing Example LPT: Sequence E-C-A-D-B Sequencing Example:  Sequencing Example LPT: Sequence E-C-A-D-B Sequencing Example:  Sequencing Example Summary of Rules Comparison of Sequencing Rules:  Comparison of Sequencing Rules No one sequencing rule excels on all criteria SPT does well on minimizing flow time and number of jobs in the system But SPT moves long jobs to the end which may result in dissatisfied customers FCFS does not do especially well (or poorly) on any criteria but is perceived as fair by customers EDD minimizes lateness Critical Ratio (CR):  Critical Ratio (CR) An index number found by dividing the time remaining until the due date by the work time remaining on the job Jobs with low critical ratios are scheduled ahead of jobs with higher critical ratios Performs well on average job lateness criteria Critical Ratio Example:  Critical Ratio Example Currently Day 25 With CR < 1, Job B is late. Job C is just on schedule and Job A has some slack time. Critical Ratio Technique:  Critical Ratio Technique Helps determine the status of specific jobs Establishes relative priorities among jobs on a common basis Relates both stock and make-to-order jobs on a common basis Adjusts priorities automatically for changes in both demand and job progress Dynamically tracks job progress Sequencing N Jobs on Two Machines: Johnson’s Rule:  Sequencing N Jobs on Two Machines: Johnson’s Rule Works with two or more jobs that pass through the same two machines or work centers Minimizes total production time and idle time Johnson’s Rule:  Johnson’s Rule List all jobs and times for each work center Choose the job with the shortest activity time. If that time is in the first work center, schedule the job first. If it is in the second work center, schedule the job last. Once a job is scheduled, it is eliminated from the list Repeat steps 2 and 3 working toward the center of the sequence Johnson’s Rule Example:  Johnson’s Rule Example Johnson’s Rule Example:  Johnson’s Rule Example D A B C E Johnson’s Rule Example:  Johnson’s Rule Example B A C D E Time 0 3 10 20 28 33 Johnson’s Rule Example:  Johnson’s Rule Example B A C D E Time 0 3 10 20 28 33 Time 0 1 3 5 7 9 10 11 12 13 17 19 21 22 23 25 27 29 31 33 35 Limitations of Rule-Based Dispatching Systems:  Limitations of Rule-Based Dispatching Systems Scheduling is dynamic and rules need to be revised to adjust to changes Rules do not look upstream or downstream Rules do not look beyond due dates Finite Capacity Scheduling:  Finite Capacity Scheduling Overcomes disadvantages of rule-based systems by providing an interactive, computer-based graphical system May include rules and expert systems or simulation to allow real-time response to system changes Initial data often from an MRP system FCS allows the balancing of delivery needs and efficiency Finite Capacity Scheduling:  Finite Capacity Scheduling Figure 15.5 Interactive Finite Capacity Scheduling Finite Capacity Scheduling:  Finite Capacity Scheduling Theory of Constraints:  Theory of Constraints Throughput is the number of units processed through the facility and sold TOC deals with the limits an organization faces in achieving its goals Identify the constraints Develop a plan for overcoming the constraints Focus resources on accomplishing the plan Reduce the effects of constraints by off-loading work or increasing capacity Once successful, return to step 1 and identify new constraints Bottlenecks:  Bottlenecks Bottleneck work centers are constraints that limit output Common occurrence due to frequent changes Management techniques include: Increasing the capacity of the constraint Cross-trained employees and maintenance Alternative routings Moving inspection and test Scheduling throughput to match bottleneck capacity Drum, Buffer, Rope:  Drum, Buffer, Rope The drum is the beat of the system and provides the schedule or pace of production The buffer is the inventory necessary to keep constraints operating at capacity The rope provides the synchronization necessary to pull units through the system Scheduling Repetitive Facilities:  Scheduling Repetitive Facilities Level material use can help repetitive facilities Better satisfy customer demand Lower inventory investment Reduce batch size Better utilize equipment and facilities Scheduling Repetitive Facilities:  Scheduling Repetitive Facilities Advantages include: Lower inventory levels Faster product throughput Improved component quality Reduced floor-space requirements Improved communications Smoother production process Scheduling Services:  Scheduling Services Service systems differ from manufacturing Scheduling Services:  Scheduling Services Hospitals have complex scheduling system to handle complex processes and material requirements Banks use a cross-trained and flexible workforce and part-time workers Airlines must meet complex FAA and union regulations and often use linear programming to develop optimal schedules 24/7 Operations use flexible workers and variable schedules Demand Management:  Demand Management Appointment or reservation systems FCFS sequencing rules Discounts or other promotional schemes When demand management is not feasible, managing capacity through staffing flexibility may be used Scheduling Service Employees With Cyclical Scheduling:  Scheduling Service Employees With Cyclical Scheduling Objective is to meet staffing requirements with the minimum number of workers Schedules need to be smooth and keep personnel happy Many techniques exist from simple algorithms to complex linear programming solutions Cyclical Scheduling Example:  Cyclical Scheduling Example Determine the staffing requirements Identify two consecutive days with the lowest total requirements and assign these as days off Make a new set of requirements subtracting the days worked by the first employee Apply step 2 to the new row Repeat steps 3 and 4 until all requirements have been met Cyclical Scheduling Example:  Cyclical Scheduling Example Cyclical Scheduling Example:  Cyclical Scheduling Example Cyclical Scheduling Example:  Cyclical Scheduling Example Cyclical Scheduling Example:  Cyclical Scheduling Example Cyclical Scheduling Example:  Cyclical Scheduling Example Cyclical Scheduling Example:  Cyclical Scheduling Example Cyclical Scheduling Example:  Cyclical Scheduling Example Cyclical Scheduling Example:  Cyclical Scheduling Example

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