Scheduling (production processes)
Encyclopedia
Scheduling is an important tool for manufacturing
and engineering
, where it can have a major impact on the productivity of a process. In manufacturing, the purpose of scheduling is to minimize the production time and costs, by telling a production facility when to make, with which staff, and on which equipment. Production scheduling aims to maximize the efficiency of the operation and reduce costs.
Production scheduling tools greatly outperform older manual scheduling methods. These provide the production scheduler with powerful graphical interfaces which can be used to visually optimize real-time work loads in various stages of production, and pattern recognition allows the software to automatically create scheduling opportunities which might not be apparent without this view into the data. For example, an airline might wish to minimize the number of airport gates required for its aircraft, in order to reduce costs, and scheduling software can allow the planners to see how this can be done, by analyzing time tables, aircraft usage, or the flow of passengers.
Companies use backward and forward scheduling to allocate plant and machinery resources, plan human resources, plan production processes and purchase materials.
Forward scheduling is planning the tasks from the date resources become available to determine the shipping date or the due date.
Backward scheduling is planning the tasks from the due date or required-by date to determine the start date and/or any changes in capacity required.
The benefits of production scheduling include:
.
. Although, scheduling may apply to traditionally continuous processes, such as refining , it is especially important for batch processes such as those for pharmaceutical active ingredients, biotechnology processes and many specialty chemical processes . Batch production scheduling shares some concepts and techniques with finite capacity scheduling which has been applied to many manufacturing problems . The specific issues of scheduling batch manufacturing processes have generated considerable industrial and academic interest.
The following text-book recipe illustrates the organization.
A simplified S88-style procedural organization of the recipe might appear as follows:
Note that the organization here is intended to capture the entire process for scheduling. A recipe for process-control purposes may have a more narrow scope.
Most of the constraints and restrictions described by Pinedo are applicable in batch processing. The various operations in a recipe are subject to timing or precedence constraints that describe when they start and or end with respect to each other. Furthermore, because materials may be perishable or unstable, waiting between successive operations may be limited or impossible. Operation durations may be fixed or they may depend on the durations of other operations.
In addition to process equipment, batch process activities may require labor, materials, utilities and extra equipment.
Where CTmin is the shortest possible cycle time for a process with M unit-procedures and τj is the total duration for the jth unit-procedure. The unit-procedure with the maximum duration is sometimes referred to as the bottleneck. This relationship applies when each unit-procedure has a single dedicated equipment unit.
If redundant equipment units are available for at least one unit-procedure, the minimum cycle-time becomes:
Where Nj is the number of redundant equipment for unit procedure j.
If equipment is reused within a process, the minimum cycle-time becomes more dependent on particular process details. For example, if the drying procedure in the current example is replaced with another reaction in the reactor, the minimum cycle time depends on the operating policy and on the relative durations of other procedures. In the cases below, and increase in the hold time in the tote can decrease the average minimum cycle time.
Another time chart which also sometime called a Gantt chart shows the time during which key resources, e.g. equipment, are occupied. The previous figures show this occupancy-style Gantt chart.
Resources that are consumed on a rate basis, e.g. electrical power, steam or labor, are generally displayed as consumption rate vs time plots.
A wide variety of algorithms and approaches have been applied to batch process scheduling. Early methods, which were implemented in some MRP systems assumed infinite capacity and depended only on the batch time. Such methods did not account for any resources would produce infeasible schedules.
Mathematical programming methods involve formulating the scheduling problem as an optimization problem where some objective, e.g. total duration, must be minimized (or maximized) subject to a series of constraints which are generally stated as a set of inequalities and equalities. The objective and constraints may involve zero-or-one (integer) variables as well as nonlinear relationships. An appropriate solver is applied for the resulting mixed-integer linear or nonlinear programming (MILP/MINLP) problem. The approach is theoretically guaranteed to find an optimal solution if one exists. The disadvantage is that the solver algorithm may take an unreasonable amount of time. Practitioners may use problem-specific simplifications in the formulation to get faster solutions without eliminating critical components of the scheduling model.
Constraint programming is a similar approach except that the problem is formulated only as a set of constraints and the goal is to arrive at a feasible solution rapidly. Multiple solutions are possible with this method.
Manufacturing
Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...
and engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
, where it can have a major impact on the productivity of a process. In manufacturing, the purpose of scheduling is to minimize the production time and costs, by telling a production facility when to make, with which staff, and on which equipment. Production scheduling aims to maximize the efficiency of the operation and reduce costs.
Production scheduling tools greatly outperform older manual scheduling methods. These provide the production scheduler with powerful graphical interfaces which can be used to visually optimize real-time work loads in various stages of production, and pattern recognition allows the software to automatically create scheduling opportunities which might not be apparent without this view into the data. For example, an airline might wish to minimize the number of airport gates required for its aircraft, in order to reduce costs, and scheduling software can allow the planners to see how this can be done, by analyzing time tables, aircraft usage, or the flow of passengers.
Companies use backward and forward scheduling to allocate plant and machinery resources, plan human resources, plan production processes and purchase materials.
Forward scheduling is planning the tasks from the date resources become available to determine the shipping date or the due date.
Backward scheduling is planning the tasks from the due date or required-by date to determine the start date and/or any changes in capacity required.
The benefits of production scheduling include:
- Process change-over reduction
- Inventory reduction, leveling
- Reduced scheduling effort
- Increased production efficiency
- Labor load leveling
- Accurate delivery date quotes
- Real time information
Outputs
Outputs are the products produced in factories either for other factories or for the end buyer. The extent to which any one product is produced within any one factory is governed by transaction costTransaction cost
In economics and related disciplines, a transaction cost is a cost incurred in making an economic exchange . For example, most people, when buying or selling a stock, must pay a commission to their broker; that commission is a transaction cost of doing the stock deal...
.
Within the factory
The output of any one work area within the factory is an input to the next work area in that factory according to the manufacturing process. For example the output of the cutting room is an input to the sewing room.For the next factory
By way of example, the output of a paper mill is an input to a print factory. The output of a petrochemicals plant is an input to an asphalt plant, a cosmetics factory and a plastics factory.For the end buyer
Factory output goes to the consumer via a service business such as a retailer or an asphalt paving company.Resource allocation
Resource allocation is assigning inputs to produce output. The aim is to maximize output with given inputs or to minimize quantity of inputs to produce required output.Scheduling Algorithms
Production scheduling can take a significant amount of computing power if there are a large number of tasks. Therefore a range of short-cut algorithms (heuristics) (a.k.a. dispatching rules) are used:Heuristic Algorithms
- Modified due date scheduling heuristicModified due date scheduling heuristicThe modified due-date scheduling heuristic is used in scheduling tasks to resources ....
- Shifting bottleneck heuristicShifting bottleneck heuristicThe Shifting Bottleneck Heuristic is a procedure intended to minimize the time it takes to do work, or specifically, the makespan in a job shop. The makespan is defined as the amount of time, from start to finish, to complete a set of multi-machine jobs where machine order is pre-set for each job...
Background
Batch production scheduling is the practice of planning and scheduling of batch manufacturing processes. See Batch productionBatch production
Batch production is a technique used in manufacturing, in which the object in question is created stage by stage over a series of workstations. Batch production is common in bakeries and in the manufacture of sports shoes, pharmaceutical ingredients , inks, paints and adhesives. In the manufacture...
. Although, scheduling may apply to traditionally continuous processes, such as refining , it is especially important for batch processes such as those for pharmaceutical active ingredients, biotechnology processes and many specialty chemical processes . Batch production scheduling shares some concepts and techniques with finite capacity scheduling which has been applied to many manufacturing problems . The specific issues of scheduling batch manufacturing processes have generated considerable industrial and academic interest.
Scheduling in the Batch Processing Environment
A batch process can be described in terms of a recipe which comprises a bill of materials and operating instructions which describe how to make the product. The ISA S88 batch process control standard provides a framework for describing a batch process recipe. The standard provides a procedural hierarchy for a recipe. A recipe may be organized into a series of unit-procedures or major steps. Unit-procedures are organized into operations, and operations may be further organized into phases.The following text-book recipe illustrates the organization.
- Charge and Mix materials A and B in a heated reactor, heat to 80C and react 4 hours to form C.
- Transfer to blending tank, add solvent D, Blend 1hour. Solid C precipitates.
- Centrifuge for 2 hours to separate C.
- Dry in a tray dryer for 1 hour.
A simplified S88-style procedural organization of the recipe might appear as follows:
- Unit Procedure 1: Reaction
- Operation 1: Charge A & B (0.5 hours)
- Operation 3: Blend / Heat (1 hour)
- Operation 4: Hold at 80C for 4 hours
- Operation 5: Pump solution through cooler to blend tank (0.5 hours)
- Operation 5: Clean (1 hour)
- Unit Procedure 2: Blending Precipitation
- Operation 1: Receive solution from reactor
- Operation 2: Add solvent, D (0.5 hours)
- Operation 3: Blend for 2 hours
- Operation 4: Pump to centrifuge for 2 hours
- Operation 5: Clean up (1 hour)
- Unit Procedure 3: Centrifugation
- Operation 1: Centrifuge solution for 2 hours
- Operation 2: Clean
- Unit Procedure 4: Tote
- Operation 1: Receive material from centrifuge
- Operation 2: Load dryer (15 min)
- Unit Procedure 5: Dry
- Operation 1: Load
- Operation 2: Dry (1 hour)
Note that the organization here is intended to capture the entire process for scheduling. A recipe for process-control purposes may have a more narrow scope.
Most of the constraints and restrictions described by Pinedo are applicable in batch processing. The various operations in a recipe are subject to timing or precedence constraints that describe when they start and or end with respect to each other. Furthermore, because materials may be perishable or unstable, waiting between successive operations may be limited or impossible. Operation durations may be fixed or they may depend on the durations of other operations.
In addition to process equipment, batch process activities may require labor, materials, utilities and extra equipment.
Cycle-Time Analysis
In some simple cases, an analysis of the recipe can reveal the maximum production rate and the rate limiting unit. In the process example above if a number of batches or lots of Product C are to be produced, it is useful to calculate the minimum time between consecutive batch starts (cycle-time). If a batch is allowed to start before the end of the prior batch the minimum cycle-time is given by the following relationship :Where CTmin is the shortest possible cycle time for a process with M unit-procedures and τj is the total duration for the jth unit-procedure. The unit-procedure with the maximum duration is sometimes referred to as the bottleneck. This relationship applies when each unit-procedure has a single dedicated equipment unit.
If redundant equipment units are available for at least one unit-procedure, the minimum cycle-time becomes:
Where Nj is the number of redundant equipment for unit procedure j.
If equipment is reused within a process, the minimum cycle-time becomes more dependent on particular process details. For example, if the drying procedure in the current example is replaced with another reaction in the reactor, the minimum cycle time depends on the operating policy and on the relative durations of other procedures. In the cases below, and increase in the hold time in the tote can decrease the average minimum cycle time.
Visualization
Various charts are used to help schedulers visually manage schedules and constraints. The Gantt chart is a display that shows activities on a horizontal bar graph in which the bars represent the time of the activity. Below is an example of a Gantt chart for the process in the example described above.Another time chart which also sometime called a Gantt chart shows the time during which key resources, e.g. equipment, are occupied. The previous figures show this occupancy-style Gantt chart.
Resources that are consumed on a rate basis, e.g. electrical power, steam or labor, are generally displayed as consumption rate vs time plots.
Algorithmic Methods
When scheduling situations become more complicated, for example when two or more processes share resources, it may be difficult to find the best schedule. A number of common scheduling problems, including variations on the example described above, fall into a class of problems that become very difficult to solve as their size (number of procedures and operations) grows.A wide variety of algorithms and approaches have been applied to batch process scheduling. Early methods, which were implemented in some MRP systems assumed infinite capacity and depended only on the batch time. Such methods did not account for any resources would produce infeasible schedules.
Mathematical programming methods involve formulating the scheduling problem as an optimization problem where some objective, e.g. total duration, must be minimized (or maximized) subject to a series of constraints which are generally stated as a set of inequalities and equalities. The objective and constraints may involve zero-or-one (integer) variables as well as nonlinear relationships. An appropriate solver is applied for the resulting mixed-integer linear or nonlinear programming (MILP/MINLP) problem. The approach is theoretically guaranteed to find an optimal solution if one exists. The disadvantage is that the solver algorithm may take an unreasonable amount of time. Practitioners may use problem-specific simplifications in the formulation to get faster solutions without eliminating critical components of the scheduling model.
Constraint programming is a similar approach except that the problem is formulated only as a set of constraints and the goal is to arrive at a feasible solution rapidly. Multiple solutions are possible with this method.
Additional References
- Blazewicz, J., Ecker, K.H., Pesch, E., Schmidt, G. und J. Weglarz, Scheduling Computer and Manufacturing Processes, Berlin (Springer) 2001, ISBN 3-540-41931-4
- Herrmann, Jeffrey W., editor, 2006, Handbook of Production Scheduling, Springer, New York.
- McKay, K.N., and Wiers, V.C.S., 2004, Practical Production Control: a Survival Guide for Planners and Schedulers, J. Ross Publishing, Boca Raton, Florida. Co-published with APICS.
- Pinedo, Michael L. 2005. Planning and Scheduling in Manufacturing and Services, Springer, New York.
See also
- Manufacturing process managementManufacturing Process ManagementManufacturing process management is a collection of technologies and methods used to define how products are to be manufactured. MPM differs from ERP/MRP which is used to plan the ordering of materials and other resources, set manufacturing schedules, and compile cost data.A cornerstone of MPM is...
- Advanced planning and scheduling
- Timebar SchedulingTimebar schedulingTimebar scheduling is a method of staff scheduling that may be adopted by organizations with highly variable staff scheduling requirements.The primary advantage of timebar scheduling is that it makes it visually clear how many employees will be working or on break at any given time.To implement...
External links
- University of Nottingham. Inter-disciplinary Scheduling Network Research groups, scheduling link directories and a directory of scheduling suppliers.
- TORSCHE Scheduling Toolbox for Matlab is a freely available toolbox of scheduling and graph algorithms.
- frePPLe is a toolkit for building production planning solutions.
- Scheduling Software Orchestrate scheduling software for Advanced Planning and Scheduling solutions