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Scheduling
Within an organization, scheduling pertains to establishing the timing of the use of specific resources of that organization. It relates to the use of equipment, facilities, and human activities. Scheduling occurs in every organization, regardless of the nature of its activities. For example, manufacturers must schedule production, which means developing schedules for workers, equipment, purchases, maintenance, and so on. Hospitals must schedule admissions, surgery, nursing assignments, and support services such as meal preparation, security, maintenance, and cleaning. Educational institutions must schedule classrooms, instruction, and students. And lawyers, doctors, dentists, hairdressers, and auto repair shops must schedule appointments.
Scheduling
High-Volume Systems
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high-volume systems, intermediate-volume systems, and low-volume (job shop) scheduling.
High-volume systems are characterized by standardized equipment and activities that provide identical or highly similar operations on customers or products as they pass through the system. The goal is to obtain a smooth rate of flow of goods or customers through the system in order to get a high utilization of labor and equipment.
A major aspect in the design of flow systems is line balancing, which concerns allocating the required tasks to workstations so that they satisfy technical (sequencing) constraints and are balanced with respect to equal work times among stations. Highly balanced systems result in the maximum utilization of equipment and personnel as well as the highest possible rate of output.
High-Volume Systems (contd.)
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Work Center #1
Work Center #2
Output
Examples of high-volume products include autos, personal computers, radios and televisions, stereo equipment, toys, and appliances. In process industries, examples include petroleum refining, sugar refining, mining, waste treatment, and the manufacturing of fertilizers
Intermediate-Volume Systems
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it is more economical to process these items intermittently
The three basic issues in these systems are the run size of jobs, the timing of jobs, and the sequence in which jobs should be processed.
SCHEDULING IN LOW-VOLUME SYSTEMS
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Loading refers to the assignment of jobs to processing (work) centers. Loading decisions involve assigning specific jobs to work centers and to various machines in the work centers
Gantt Charts. Visual aids called Gantt charts are used for a variety of purposes related to loading and scheduling.
The purpose of Gantt charts is to organize and visually display the actual or intended use of resources in a time framework
The characteristics of low-volume systems (job shops) are considerably different from those of high- and intermediate-volume systems. Products are made to order, and orders usually differ considerably in terms of processing requirements, materials needed, processing time, and processing sequence and setups. Because of these circumstances, job-shop scheduling is usually fairly complex. This is compounded by the impossibility of establishing firm schedules prior to receiving the actual job orders. Job-shop processing gives rise to two basic issues for schedulers: how to distribute the workload among work centers and what job processing sequence to use.
Scheduling Low-Volume Systems
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There are a number of different types of Gantt charts. Two of the most commonly used are the load chart and the schedule chart.
A load chart depicts the loading and idle times for a group of machines or a list of departments
A manager often uses a schedule chart to monitor the progress of jobs. The vertical axis on this type of Gantt chart shows the orders or jobs in progress, and the horizontal axis shows time. The chart indicates which jobs are on schedule and which are behind or ahead
Loading
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Infinite loading assigns jobs to work centers without regard to the capacity of the work center
Finite loading projects actual job starting and stopping times at each work center, taking into account the capacities of each work center and the processing times of jobs, so that capacity is not exceeded.
Forward scheduling is used if the issue is “How long will it take to complete this job?” Backward scheduling would be used if the issue is “When is the latest the job can be started and still be completed by the due date?” Forward scheduling enables the scheduler to determine the earliest possible completion time for each job and, thus, the amount of lateness or the amount of slack can be determined. That information can be combined with information from other jobs in setting up a schedule for all current jobs.
Scheduling Low-Volume Systems
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Input/Output Control. Input/output (I/O) control refers to monitoring the work flow and queue lengths at work centers.
The purpose of I/O control is to manage work flow so that queues and waiting times are kept under control. Without I/O control, demand may exceed processing capacity, causing an overload at a center. Conversely, work may arrive slower than the rate a work center can handle, leaving the work center underutilized. Ideally, a balance can be struck between the input and output rates, thereby achieving effective use of work center capacities without experiencing excessive queues at the work centers
The deviations in each period are determined by subtracting “planned” from “actual.” For example, in the first period, subtracting the planned input of 100 hours from the actual input of 120 hours produces a deviation of 20 hours. Similarly, in the first period, the planned and actual outputs are equal, producing a deviation of 0 hours. The backlog for each period is determined by subtracting the “actual output” from the “actual input” and adjusting the backlog from the previous period by that amount. For example, in the second period actual output exceeds actual input by 10 hours. Hence, the previous backlog of 50 hours is reduced by 10 hours to 40 hours.
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Assignment Method of Linear Programming. The assignment model is a special purpose linear programming model that is useful in situations that call for assigning tasks or other work requirements to resources. Typical examples include assigning jobs to machines or workers, territories to salespeople, and repair jobs to repair crews. The idea is to obtain an optimum matching of tasks and resources. Commonly used criteria include costs, profits, efficiency, and performance.
If there are to be n matches, there are n! different possibilities. In this case, there are 4! 24 different matches. One approach is to investigate each match and select the one with the lowest cost. However, if there are 12 jobs, there would be 479 million different matches! A much simpler approach is to use a procedure called the Hungarian method to identify the lowest-cost solution.
The problem is arranged in a format that facilitates evaluation of assignments. The numbers in the body of the table represent the value or cost associated with each job–worker combination. In this case, the numbers represent costs. Thus, it would cost $8 for worker A to do job 1, $6 for worker B to do job 1, and so on. If the problem involved minimizing the cost for job 1 alone, it would clearly be assigned to worker C, since that combination has the lowest cost. However, that assignment does not take into account the other jobs and their costs, which is important since the lowest-cost assignment for any one job may not be consistent with a minimum-cost assignment when all jobs are considered.
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Sequencing
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Using the S/O rule, the designated job sequence may change after any given operation, so if that happened, it would be necessary to reevaluate the sequence after each operation. Note that any of the previously mentioned priority rules could be used on a station-by-station basis for this situation; the only difference is that the S/O approach incorporates downstream information in arriving at a job sequence
Slack per operation (S/O): Jobs are processed according to average slack time (time until due date minus remaining time to process). Compute by dividing slack time by number of remaining operations, including the current one.
Two Work Center Sequencing
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Sequencing Jobs When Setup Times Are Sequence-Dependent
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Scheduling Difficulties
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Minimizing Scheduling Difficulties
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Scheduling Services Considerations
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Scheduling Services
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Yield Management
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Cyclical Scheduling
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Service Operation Problems
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