第四章生产运作管理

10
Introduction
Dependent demand and independent demand
独立需求: Finished Goods
A
B(4)
C(2)
相关需求: Raw Materials, Component parts, Sub-assemblies, etc.
D(2)
E(1)
D(3)
Review time
Continuous review : e.g., supermarket Periodic review : e.g., warehouse
Excess demand
demand that cannot be filled immediately from stock. backordered or lost.
17
Motivation for Holding Inventories
作为缓冲的库存
运作管理
库存 质量 调整时间长 材料搬运 计划太糟糕 交货
设备故障 旷工
18
Motivation for Holding Inventories
被隐问题
通过降低库存暴露问题，然后解决问题，再降低库存、 通过降低库存暴露问题，然后解决问题，再降低库存、 暴露问题、 暴露问题、解决问题 ……..
The complexity of the resulting model depends on the
assumptions about the various parameters of the system the major distinction is between models for known demand and random demand.
Inventory in different places
8
Introduction
The classical inventory results: are central to more
modern techniques of manufacturing management, such as MRP, JIT,TOC.
2
Introduction
Question:
库存是一个必要的恶魔
3
Introduction
The current investment in inventories in USA is enormous; It amounted up to $1.37 trillion in the last quarter of 1999; It accounts for 20-25% of the total annual GNP (general net product); There exists enormous potential for improving the efficiency of economy by scientifically controlling inventories;
Breakdown of total investment in inventories
4
Introduction
Definition: Inventory is the stock of any item or resource used in an organization. An inventory system is the set of policies and controls that monitors levels of inventory or determines what levels should be maintained. Generally, inventory is being acquired or produced to meet the need of customers; The fundamental problem of inventory management :
16
Motivation for Holding Inventories
Control costs
The cost of maintaining the inventory control systems It can be less costly to the firm in the long run to maintain large inventories of inexpensive items than to expend worker time to keep detailed records.
Chapter 4 Inventory Control Subject to Known Demand
1
Contents
Introduction Types of Inventories Motivation for Holding Inventories; Characteristics of Inventory Systems; Relevant Costs; The EOQ Model; EOQ Model with Finite Production Rate
6
Introduction
Inventory: plays a key role in the logistical behavior of virtually all manufacturing systems.
工厂
采购 资材 仓库
验收 在制品
(供应商)
出货
在制品
出库 (完成)
成品 仓库
供应
7
Introduction
12
Types of Inventories
A natural classification is based on the value added from manufacturing operations
Raw materials: Resources required in the production or processing activity of the firm. Components: Includes parts and subassemblies. Work-in-process (WIP): the inventory either waiting in the system for processing or being processed.
13
Contents
Introduction Types of Inventories Motivation for Holding Inventories; Characteristics of Inventory Systems; Relevant Costs; The EOQ Model; EOQ Model with Finite Production Rate
The level of WIP is taken as a measure of the efficiency of a production scheduling system. JIT aims at reducing WIP to zero.
Finished good: also known as end items or the final products.
14
Motivation for Holding Inventories
For economies of scale
It may be economical to produce a relatively large number of items in each production run and store them for future use.
20
Characteristics of Inventory Systems
Demand (patterns and characteristics)
Constant versus variable Known versus random
Lead Time
Ordered from the outside: time that elapse from the instant that an order is placed until it arrives. Produced internally: the amount of time required to produce a batch of items.
Smoothing
Producing and storing inventory in anticipation of peak demand helps to alleviate the disruptions caused by changing production rates and workforce level.
19
Contents
Introduction Types of Inventories Motivation for Holding Inventories; Characteristics of Inventory Systems; Relevant Costs; The EOQ Model; EOQ Model with Finite Production Rate
Coping with uncertainties
Uncertainty in demand Uncertainty in lead time Uncertainty in supply
For speculation
Purchase large quantities at current low prices and store them for future use. Cope with labor strike
15
Motivation for Holding Inventories
Transportation
In-transit inventories is the inventory moving from point to point, e.g., materials moving from suppliers to a plant, from one operation to the next in a plant.
F(2)
11
Contents
Introduction Types of Inventories Motivation for Holding Inventories; Characteristics of Inventory Systems; Relevant Costs; The EOQ Model; EOQ Model with Finite Production Rate
Logistics
To cope with constraints in purchasing, production, or distribution of items, this may force a system maintain inventory. For example, an item must be purchased in minimum quantities.
When to place order for replenishing the stock ?定货点 How much to order?订货批量
5
Introduction
企业物流
输入
订货 原材料
分销中心
转化
输出
用 户
供 应 商
接 收
成品 1 2 3
用 户
…
N
在制品
分销中心
用 户
生产企业的物料流
9
Introduction
Dependent demand and independent demand Independent demand system 独立需求： 独立需求：由市场决定的，不可控的需求，每个品种之间 的需求是不相关的 Dependent demand system - the demand of components and subassemblies (lower levels depend on higher level) -MRP; 相关需求： 相关需求：由其它产品或品种的需求决定的，可以直接 计算出来的需求