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Manufacturing Resource Planning (MRP II)
• Sometime called MRP, in contrast with mrp (“little”
mrp); more recent implementations are called ERP
(Enterprise Resource Planning).
• Extended MRP into:
– Master Production Scheduling (MPS)
– Rough Cut Capacity Planning (RCCP)
– Capacity Requirements Planning (CRP)
– Production Activity Control (PAC)
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MRP II Planning Hierarchy
Long-term
Forecast
Resource
Planning
Aggregate Production
Planning
Rough-Cut Capacity
Planning
Master Production
Scheduling
Bills of
Material
Inventory
Status
Short-term
Forecast
Demand
management
Material Requirements
Planning
Job
Pool
Job
Release
Capacity Requirements
Planning
Routing
Data
Job
Dispatching
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Long-Range Planning
• Forecasting
– Long-range forecasting determines the capacity, tooling, and
personnel requirements.
– Short-term forecasting converts a long-range forecasts of part
families to short-term forecasts of individual end items.
– Both kinds of forecasts are input to the intermediate-level
function of demand management.
• Resource planning
– Determines capacity requirements over the long term.
– Provides projected available capacity over the long-term planning
horizon.
• Aggregate planning
– Determines levels of production, staffing, inventory, overtime,and
so on over the long term.
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000
http://www.factory-physics.com
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000
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Master Production Scheduling (MPS)
• Generates an anticipated build scheduling at the highest
level of planning details. (contains an order quantity in
each bucket for every end item)
• MPS drives MRP
• Should be accurate in near term (firm orders)
• May be inaccurate in long term (forecasts)
• Software supports
– forecasting
– order entry
– netting against inventory
• Frequently establishes a “frozen zone” in MPS
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000
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Rough Cut Capacity Planning (RCCP)
• Quick check on capacity of critical resources
• Use bill of resources (BOR) for each end item in MPS
• Bill of resources give the number of hours required at each
critical resource to build a particular end item
• Generates usage of critical resources by exploding MPS
against BOR
• Insufficient or excess capacity addressed by adjusting
MPS (changing due dates) or adjusting capacity (adding or
taking away resources, using overtime, or subcontracting)
• Does not perform any offsetting and netting, and thus is
difficult to gauge its behavior.
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000
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Capacity Requirements Planning (CRP)
• Provides a more detailed capacity check on MRP-generated
production plan.
• Uses all planned order releases, existing WIP positions,
routing data, as well as capacity and lead times for all
process centers.
• Performs infinite forward loading.
• Generates job completion times for each process center,
using given fixed lead times, and then computes a predicted
loading over time.
• No correction is made for an overloaded situation, and it
offers nothing about what caused an overloaded situation.
• There are enormous data requirements, and the output is
voluminous and tedious.
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Short-term Control
• Job release, job dispatching, and input/output control.
• Job release converts planned order releases to scheduled
receipts.
• Allocation: the job release function rationalize conflicts by
allocating parts to one job or another.
• Once a job is released, some control must be maintained to
make sure it is completed on time with correct quantity
and specification.
• Shop floor control performs two main functions: job
dispatching and input/output control.
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000
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Shop Floor Control (SFC)
•
•
•
•
•
Sometimes called production activity control (PAC)
Provides routing/standard time information
Sets planned start times
Can be used for prioritizing/expediting
Can perform input-output control (compare planned with
actual throughput)
• Modern term is MES (Manufacturing Execution System),
which represents functions between Planning and Control.
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000
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Job Dispatching
• Develop a rule for arranging the queue in front of each
workstation that will maintain due date integrity while
keeping machine utilization high and manufacturing times
low.
• Shortest process time (SPT)
– Jobs at the process center queue are sorted with the shortest
jobs first in line. Thus, the job in the queue having the shortest
processing time will always be performed next.
– Use of SPT typically decreases average manufacturing times
and increases machine utilization.
– While average due date performance of SPT is good, the
variance of the lateness can be quite high.
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000
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Job Dispatching (cont.)
• Earliest due date (EDD)
– The job closest to its due date is worked on next.
– EDD works better than SPT if jobs are all approximately the
same size or particularly long jobs and routing are fairly
consistent.
• Least slack
– The slack for a job is its due date minus the remaining
processing time (including setups) minus the current time.
– The highest priority is the job with the lowest slack time.
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Job Dispatching (cont.)
• Least slack per remaining operation
– Divide the slack by the number of operations remaining on the
routing.
– The highest-priority job has the smallest value.
• Critical ratio
– Jobs are sorted according to an index computed by dividing
the time remaining (I.e., due date minus the current time) by
the number of hours of time remaining.
– The highest-priority job has the smallest value of critical ratio.
– The idea is to provide a balance between SPT and EDD.
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Input/Output Control
• Keeps lead times under control
– Monitor the WIP level in each process center.
– If the WIP goes above a certain level, then the current
release rate is too high, so reduce it.
– If the WIP goes below a specific lower level, then the
current release rate is too low, so increase it.
– It it stays between these control levels, the release rate is
correct for the current conditions.
• These actions (reduce and increase) must be done by
changing the MPS.
• Provides an easy way to check releases against available
capacity.
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000
http://www.factory-physics.com
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