Bài giảng môn Quản trị kinh doanh - Chapter 15: Constraint management: simplifying complex systems

Chapter 15Constraint Management:Simplifying Complex Systems1Learning ObjectivesDefine a constraint.Compare utilization and activation from a constraint management perspective.Describe the global measures of throughput, inventory, and operating expense, and compare them to traditional global measures.Explain the five-step focusing process of constraint management.Explain how systems can be protected from disruptions by using time buffers.Describe how time buffers function and how they affect the exploitation of a constraint.Explain what is meant by a constraint buffer, a shipping buffer, and an assembly buffer.Differentiate between a production batch and a transfer batch.Compute the appropriate product mix for a constrained production system.Compare the kanban system to the buffering system used in constraint management.2Introduction: Maximizing System OutputThe early development of constraint management (CM) can be credited to one individual, Eliahu Goldratt.The 1984 book The Goal brought Constraint Management, or as it is sometimes known “The Theory of Constraints” to the forefront.Numerous extensions, applications, and associated techniques have been developed since.3Defining a ConstraintA constraint is anything that inhibits a system’s progress toward its goals.It could be a resource (labor, machine capacity, warehouse space, etc.)It could be a policy (no deliveries on Sunday, no more than 6 hours of overtime per employee, etc.)It could be inputs (raw material availability, electricity availability, etc.)It could be an external force (demand) Constraint Management: A framework for managing the constraints of a system in a way that maximizes the system’s accomplishment of its goals.4Defining a ConstraintWork center 3 is a constraint. The system cannot produce at a rate faster than 9 minutes per unit. If all the other work centers slowed down to match work center 3, the system would not get any slowerExhibit 15.1 Simple Productive System with Constraint5Defining a ConstraintUtilization (constraint management definition)The time a resource is used and contributing to throughput divided by the time the resource was available.Activation (constraint management definition)Running a machine or resource when it doesn’t contribute to throughput.Exhibit 15.2 Constraint’s Determination of Work Center Utilization6Defining a ConstraintWork center 3 determines the utilization rates of all the other work centers.Increasing utilization rates for any of the other work centers will not increase system performance. It would only cause inventory to build up.Speeding up work center 3 from 9 minutes to 8 minutes would increase output. Making work center 3 go any faster than 8 minutes would not help, because work center 3 would no longer be the constraint. Work centers 3 and 4 would be the constraints.Exhibit 15.2 Constraint’s Determination of Work Center Utilization7Global Performance MeasuresConstraint management uses a unique set of global performance measures.ThroughputCM definition: Dollars generated by salesTraditional definition: The rate at which products are produced, regardless of whether or not they are soldInventoryCM definition: Money invested in things the system intends to sell. Includes equipment and facilities.Traditional definition: Does not include equipment and facilitiesOperating ExpensesCM definition: Money the system spends turning inventory into throughput8Global Performance MeasuresThe global measures of constraint management can be equated to traditional business performance measures:Exhibit 15.3 Constraint Management Global Measures and Traditional Measures9The Constraint Management Focusing ProcessThe five-step focusing process is:Step 1: Identify the constraintStep 2: Exploit the constraintStep 3: Subordinate all other decisions to step 2 and step 3Step 4: Elevate the constraintStep 5: If, in steps 2 through 4, the constraint is eliminated, go back to step 110The Constraint Management Focusing Process: Step 1. IdentificationSome constraints are obvious.In more complex environments, may need an analysis.Compare demand for a resource with available capacity.Any resource that does not have sufficient capacity to meet demand would be considered a constraintNonresource constraints can be more difficult to identify.IdentifyExploitSubordinateElevateRepeat11IdentifyExploitSubordinateElevateRepeatThe Constraint Management Focusing Process: Step 2. ExploitationThe exploitation of a constraint means it should be used to its fullest extent.Prevent it from ever being idleMake sure it never runs out of materials to processDon’t waste capacity of the constraint on products that already have quality problemsPolicy constraints differ- a constraining policy must be eliminated or modified.12The Constraint Management Focusing Process: Step 3. SubordinationThe subordination step means that any system change should be analyzed to make sure it doesn’t inhibit the exploitation done in Step 2.All decisions should be checked to make sure they do not conflict with efforts to exploitIdentifyExploitSubordinateElevateRepeat13The Constraint Management Focusing Process: Step 4. ElevationIf exploitation does not remove the constraint, the constraint must be elevated by actually increasing the capacity of the resource.Buy additional capacityIf cash is the constraint, take out a loanIf inputs are a constraint, get a new supplierIdentifyExploitSubordinateElevateRepeat14The Constraint Management Focusing Process: Step 5. RepeatCheck if the resource is still the constraint. If it is no longer a constraint, stop exploiting and elevating it, go back to Step 1 and identify the new constraintIdentifyExploitSubordinateElevateRepeat15The Role of Disruptions in Productive Systems: Protecting the SystemNonconstraints, because they have excess capacity, can experience a level of disruption without reducing system output.Constraints have no excess capacity, and so they must be protected from disruptions.In constraint management, this is done by decoupling the constrained resource: reducing the direct dependency of a process step on its predecessor using inventory16The Role of Disruptions in Productive SystemsRandom fluctuation is present in many processes. Variability can cause disruptions because of statistical fluctuation among dependent eventsWhen processes depend on each other, variability in processing times accumulatesExhibit 15.4 Simple System with Processing Variability17The Role of Disruptions in Productive Systems: Buffering to Protect ConstraintsDisruptions to a system can generally be measured by their duration. So protection must also be measured in time.Time bufferA buffer of inventory that will keep a resource busy for a specified amount of time.How much protection is needed?The size of the buffer (the amount of time it protects) should be proportional to the size of potential disruptions18The Role of Disruptions in Productive Systems: Buffering to Protect ConstraintsIf work center 1 broke down for eight hours, it would create an eight hour disruption at work center 3 (the constraint). WC3 would be idle for 8 hours.We would need an eight hour buffer of inventory in front of the constraint for protection8 hours = 480 minutes. 480/9 (nine minutes to process a unit) = 53.3, or 54. So inventory of 54 units would protect for eight hours.Exhibit 15.6 Inventory Flow In and Out of a Time Buffer19The Role of Disruptions in Productive Systems: Buffering to Protect ConstraintsIf we have specific orders with due dates, buffers make it necessary to start orders earlier.If an order of 27 units were due on day 14, it would be started on day 11 without a buffer: It takes half a working day (4 hours) for each work center to process the order, for a total of 3 days.With a buffer, the order must wait in queue for 8 hours. So it would be started on day 10 insteadExhibit 15.7 Order Processing in System with Time Buffer20The Role of Disruptions in Productive Systems: Buffering to Protect ConstraintsTime buffers placed immediately prior to a constraint are called constraint buffers.If there are never disruptions, constraint buffers would always be full. That would mean that they really aren’t needed.If sized appropriately, a time buffer should be, on average, two-thirds full. Large enough to cover serious disruptions but not so large that most of it is never usedAn assembly buffer is a time buffer placed immediately prior to an assembly for nonconstrained components. It ensures that the parts needed to be assembled with a part that has gone through the constraint are ready so that the part that has gone through the constraint is not delayed.21The Role of Disruptions in Productive Systems: Buffering to Protect ConstraintsThe constraint buffer protects orders from being late because of disruptions prior to work center 3. Another buffer at the end of the line, the shipping buffer, will protect against disruptions after the constraint.Exhibit 15.8 System with Shipping Buffer AddedProtects from disruptions hereProtects from disruptions here22Constraint Management and Batch SizesProduction batch: The quantity produced at a workcenter before changing over to produce something else.Larger production batches increase utilizationThis is a form of exploitation. Constraints should use larger production batchesFor nonconstraints, smaller production batches are more desirable (to lower inventory, improve quality...) All nonconstraint idle time should be used for changeovers, under the best scenarioTransfer batch: The quantity produced at a workcenter before transferring the products to the next step in the process.Transfer batches should always be as small as possible23The Role of the Constraint: A Product Mix ExampleA manufacturer makes two products, P and Q:Exhibit 15.9 Production System24The Role of the Constraint: A Product Mix ExampleThere are four work centers (A,B,C, &D).Each work center must accomplish two tasks.There are no changeovers required when switching tasks.Exhibit 15.9 Production System25The Role of the Constraint: A Product Mix ExampleWe have the following production system data:Exhibit 15.10 Production System Data26The Role of the Constraint: A Product Mix ExampleWhat combination of Ps and Qs should be produced to maximize profit?Step 1. Identify the constraint.Compute demand requirements on each resource and compare them to availabilityExhibit 15.11 Identification of ConstraintIdentifyExploitSubordinateElevateRepeat27The Role of the Constraint: A Product Mix ExampleWork center B is the constraint: We need 3,000 minutes to meet demand and we only have 2,400.Step 2. Exploit the constraintThe goal of this system is to maximize profitWhat is the best use of the constrained resource: Making Ps or making Qs?Exhibit 15.12 Calculation of Dollar Return per Constraint MinuteMaking Ps is a better use of the constrained resourceIdentifyExploitSubordinateRepeatWhich is best?Elevate28The Role of the Constraint: A Product Mix ExampleThe solution is to keep making Ps until demand for them is met. Whatever amount of time is left over on the constrained work center can then be used to make Qs.Produce 100 Ps at 15 minutes each = 1,500 minutes on the constraint2,400 – 1,500 = 900 minutes remaining for Q900 minutes, at 30 minutes to make one Q = 30 QsExhibit 15.13 Profit Calculation29The Role of the Constraint: A Product Mix ExampleExhibit 15.13 Profit CalculationA product mix of 100P and 30Q yields a profit of $30030The Role of the Constraint: A Product Mix ExampleBasing our decision strictly on contribution margin would have resulted in producing all of the Qs and then using the remaining capacity to make P.A solution based on contribution margins would actually have resulted in a loss of $300Commissions based on selling price would similarly lead to selling more Qs than Ps, which is the wrong thing to do31Lean Systems and Constraint ManagementLean systems and constraint management frameworks do not necessarily conflict.CM buffering is similar to the kanban systemKanban provides small buffers between work centers, however, while CM buffers only at critical pointsLean systems provide a larger buffer at the finished-goods point, CM has a shipping bufferA key difference is that lean systems focus on elimination of waste, while constraint management focuses on maximizing throughput.32