1 push and pull systems: lecture 12 mrp and push systems kanban and conwip push and pull
Post on 21-Dec-2015
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1
Push and Pull Systems: Lecture 12
• MRP and push systems
• Kanban and CONWIP
• Push and pull
2
MRP and Push Systems
• Demands for chairs are given as follows
Week 1 2 3 4 5 6
chair 200 0 250 220 300 500
• How do we schedule the production, given its BOM? chair
4 legsframe seat
3
Information
Part Chair Leg Seat Frame On-hand 340 900 300 500Leadtime 2 1 1 2
Lot size L4L 800 400 400
Safety stock 50 200 50 100
4
MRP Mechanism
week 1 2 3 4 5 6
Chair (340) 200 0 250 220 300 500
Net demand 110 220 300 500
Leg (900) 440 880 1200 2000
order 800 1020 2000
Seat (300) 110 220 300 500
order 400 480
Frame (400) 110 220 300 500
order 400 400
Netting demands Offsetting leadtimes
5
MRP Record for the Leg
week 1 2 3 4 5 6 7 8
Gross Requirements 440 880 1200 2000 800 1200 1880 360
Scheduled. Receipt Project balance
(900)460 380 200 200 200 200 200 440
Planned order release
800 1020 2000 800 1200 1880 800
MRP record for the Leg
LS = 800, LT = 1 week, SS = 200
6
MRP Record for the Seat
week 1 2 3 4 5 6 7 8
Gross Requirements 110 220 300 500 200 300 470 90
Scheduled. Receipt Project balance
(300)190 370 70 50 250 350 280 190
Planned order release
400 480 400 400 400
MRP record for the Seat
LS = 400, LT = 1 weeks, SS = 50
7
MRP Record for the Frame
week 1 2 3 4 5 6 7 8
Gross Requirements 110 220 300 500 200 300 470 90
Scheduled. Receipt 400Project balance
(100)390 170 270 170 370 470 400 310
Planned order release
400 400 400 400 400
MRP record for the Frame
LS = 400, LT = 2 weeks, SS = 100
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MRP Terms• Gross requirement is the anticipated future usage
during a period
• Scheduled receipt is order placed before and will arrive at the beginning of a period
• Projected available balance is the inventory status at the end of a period
• Planned order release is order to be released at the beginning of a period
• Net demand of the end product is the amount that should be available at the beginning of the period
9
Connecting Records• Now that each frame needs 4 rivets to put
together. For rivets, leadtime= 1 wk, safety stock = 400, lot size = 1600. Current on-hand is 412
chair
4 legsframe seat
rivet (4)
10
frame/week 1 2 3 4 5 6 7 8
Gross Requirements 110 220 300 500 200 300 470 90
Scheduled. Receipt 400Project balance
(100)390 170 270 170 370 470 400 310
Planned order release
400 400 400 400 400
Gross Requirements
Scheduled. Receipt 1600
Balance (412)
Planned order release
MRP record for rivet
LS=1600, LT=1, SS=400
11
Rolling-Horizon: Week 1
week 1 2 3 4 5 6 7 8 9 10 11
1 25 36 27 25 26 28 30 25
2
3
4
5
6
12
At the Beginning of Week 2
week 1 2 3 4 5 6 7 8 9 10 11
1 25 36 27 25 26 28 30 25
2 36 27 25 26 28 30 25 23
3
4
5
6
13
At the Beginning of Week 3
week 1 2 3 4 5 6 7 8 9 10 11
1 25 36 27 25 26 28 30 25
2 36 27 25 26 28 30 25 23
3 27 30 28 30 31 28 28 28
4
5
6
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At the Beginning of Week 4
week 1 2 3 4 5 6 7 8 9 10 11
1 25 36 27 25 26 28 30 25
2 36 27 25 26 28 30 25 23
3 27 30 28 30 31 28 28 28
4 30 30 30 30 31 28 28 28
5
6
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Exercise 1• Fill the following MRP table
week .
Part A 1 2 3 4 5 6 .
Gross req.5 15 18 8 12 22
Scheduled rec. .
Project availablebalance (21) .Planned order release .Q=20; LT=1; SS=0
During week 1, the following occur: find 12 additional units in inventory;actual requirement is 15 units; wk 3 and wk 4 requirements reduced to 16and increased to 14, respectively; wk 7 requirement is 15
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Exercise 1 Continues• Fill the following MRP table
week .
Part A 2 3 4 5 6 7 .
Gross req. .
Scheduled rec. .
Project availablebalance ( ) .Planned order release .Q=20; LT=1; SS=0
During week 1, the following occur: find 12 additional units in inventory;actual requirement is 15 units; wk 3 and wk 4 requirements reduced to 16and increased to 14, respectively; wk 7 requirement is 15
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MRP Based Control
FG
Partsmachine buffer demand
Planned orders
Forecast demandgenerate time phased plans
18
Push Production Systems
• Forecast or anticipate demand• Translate forecasts into production plan• Convert production plan into schedules of lower
level components in a top-down fashion• Executing schedules in a bottom-up fashion
19
Comments on MRP• MRP started a new way for factory management and
is one of the most important manufacturing management paradigms
• MRP process can be easily computerized, this led to the earliest MRP system by an IBM team led by Orlicky
• MRP was the first major industry application of computer and has been the core (engine) of the information backbone systems
• MRP logic is flawed. The fundamental weakness is the constant leadtime assumption, which amounts to the assumption of infinite processing capacity and no uncertainty in the system
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MRPII: A Push Planning Framework
Resourceplanning
Rough-cutcapacityplanning
capacity requirementplanning
Finite loading
Input/outputanalysis
Shop-floorsystems
Vendorsystems
MRP
MPS
Production
planning
Demand
management
BOM. Routing
InventoryStatus
Low level planning
Execution
Figure 1
21
Kanban and CONWIP
• Toyota production system– Continuous improvement– Smooth production– Eliminate wastes
• How do we create a smooth (stable) production flow so that production matches demand and wastes in production process are minimized?
27
Card Count in CONWIP• Stable line speed and WIP level • Characterize the line with
- Practical production rate, RP
- Minimum practical leadtime, TP
• By Little’s law, the CONWIP level IC is
IC= RPTP (2)
• TP includes all (reasonable) detractors include setups, failures, etc.
28
Kanban Versus CONWIP
• CONWIP– MPS generates a backlog in front of a line– backlog can be sequenced– a card releases the first job to the line– requires a stable volume but can tolerate
more of the product mix changes-- shifting bottleneck
• Kanban– card for each station and part number– requires a stable product mix and volume
29
Variations of CONWIP
• Tandem CONWIP: a long line separated to two or more sections
• CONWIP with shared resource: two CONWIP controlled lines share a common machine
• Pull from bottleneck: for a line with a stable bottleneck, control only the upstream section from the bottleneck
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Push and Pull
• Push: schedules the release of works based on (forecasted) external demands
• Pull: authorizes the release of work based on system status
demandsPush
Pull
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CONWIP and MRP: Observability
• Control throughput and observe WIP
demand --- throughput
fluctuation --- congestion• Control WIP and observe throughput• Observability
- WIP is easy to observe and easy to control
- TH is tied with capacity, when effective capacity changes, it is hard to control
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CONWIP and MRP: Efficiency
• For a given level of throughput, a push system will have more WIP than an equivalent CONWIP
• A CONWIP system can be modeled as a closed queueing network in which the number of customers is a constant
• A MRP control system can be approximately seen as an open queueing network in which each machine has fixed workload
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Efficiency Example• A line with 5 identical exponential single-machine
stations with µ=1 job/hour
• For CONWIP control with a fixed w (WIP):
Throughput rate TH = w µ /(5/µ+ w – 1)=w/(4+w)
• For MRP, no control after release. The line behaves like 5 independent stations
ρ = TH/ µ for every station
• The total
)1/()1/()( THTHTHWIPi
wwTHTHWIP 25.14/5)1(/5
34
Hierarchical Planning in a Pull System
PersonnelPlan
FORECASTING
CAPACITY/FACILITYPLANNING
WORKFORCE PLANNING
MarketingParameters
Product/ProcessParameters
LaborPolicies
CapacityPlan
AGGREGATEPLANNING
AggregatePlan Strategy
WorkSchedule
WIP/QUOTASETTING
DEMANDMANAGEMENT
SEQUENCING & SCHEDULING
CustomerDemands
MasterProductionSchedule
SHOP FLOORCONTROL
WIPPosition Tactics
REAL-TIMESIMULATION
PRODUCTIONTRACKING
WorkForecast
Control
35
Push and Pull in Supply Chains• Extend the idea of pull beyond production
control to supply chain: produce or make product available only when demand is received
• Make-to-order instead of make-to-stock• The overall leadtime is often too long under
the current competitive environment• Assemble-to-order make-to-stock for
some upstream supply chain and make-to-order for the lower stream supply chain
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Item 1
Assembly(pull/push)
InboundLogistics
(pull/push)
Item n
delivery
supplierssupermarket
Mixed Layout in Supermarket Manufacturing
A Hybrid Push-Pull System
37
Today’s Takeaways
• MRP is an important manufacturing paradigm and provides the basis for push systems
• Kanban and CONWIP are pull control with a theoretical basis in queueing theory, i.e., equations (1) and (2)
• Push schedule job release while pull control the workload, and is more efficient and robust
• MRP is easily computerized