Manufacturing Resource Planning (MRP II)

Transcription

1 P.135 Manufacturing Resource (MRP II) Sometime called MRP, in contrast with mrp ( little mrp); more recent implementations are called ERP (Enterprise Resource ). Extended MRP into: Master Production Scheduling (MPS) Rough Cut Capacity (RCCP) Capacity Requirements (CRP) Production Activity Control (PAC) 1 P.136 MRP II Hierarchy Resource Long-term Forecast Aggregate Production Short-term Forecast Rough-Cut Capacity Bills of Material Inventory Status Master Production Scheduling Material Requirements Job Pool Job Release Capacity Requirements Routing Data Demand management Job Dispatching 2 1

2 Long-Range P.136 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 Wallace J. Hopp, Mark L. Spearman, 1996, P.137 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 4 2

3 P.138 Rough Cut Capacity (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. 5 P.139 Capacity Requirements (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. Wallace J. Hopp, Mark L. Spearman, 1996,

4 P.141 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. 7 P.141 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 and Control. 8 4

5 P.141 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. 9 P.142 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. 10 5

6 P.142 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. 11 P.142 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. 12 6