MM--安全库存

安全库存只是一个笼统的说法，本文归纳了SAP四种安全库存（时间）的设置，用于增加计划过程中库存数量（时间）的弹性。为何安全库存能增加计划的弹性呢？因为在净需求计算（Net Requirements Calculation）中，Quantity available  = Fixed receipts + Stock available for planning - Safety stock - requirements，若可用数量小于零，则报缺料，视为短缺数量（Shortage quantity）或需求数量（Net requriements），将用于批量计算并得出最终的获取建议。

• Safety stock
• Safety stock partially available
• Dynamic safety stock
• Safety time

一、Safety stock

这就是“安全库存”了，安全库存的数量在计划中是不可用的。MRP中，若物料的数量低于安全库存水平，就会产生计划订单。安全库存在物料主数据MRP2视图设置。

二、Safety stock partially available

如果设置了安全库存，一低于该水平就会产生补货的请求，哪怕只是缺了一点，这增加了计划的工作量。为了弥补这个不足，我们可以在MRP Group中设置一个百分比，只有安全库存低于该百分比，补货请求方会产生。

三、Dynamic safety stock

动态安全库存就更加厉害了，它根据平均的日需求（Average daily requirements）数量，来确定未来几个时期的安全库存水平（数量等于若干个平均日需求）：最小库存、目标库存、最大库存。若小于最小库存，产生补货请求至目标库存；若大于最大库存，系统将提示例外信息。若同时设置了安全库存，将累加。该配置称为Range of Coverage Profile，在MRP2视图分配，配置在IMG: Production - MRP - Planning - MRP Calculation - Define Range of Coverage Profile (Dynamic Safety Stock)。

四、Safety time

前面三种方案都是对安全库存数量上的设置，Safety time则是设置时间上的余量。物料的补货提议时间将提前于实际的需求时间，提前日期在MRP2视图中配置。考虑到物料可能用在产品的不同层次，为了避免重复提前，可以在MRP2视图的Safety time indicator栏位设置安全时间有效于所有需求还是仅相关需求，或者干脆无效。而考虑到不同时期对安全时间的长度要求不同，可以在后台定义Period Profile，IMG: Production - MRP - Planning - MRP Calculation - Define Period Profile for Safety Time/Actual Range of Coverage，并在MRP2视图分配给物料。

注意： 百分比是根据MPR组设置的，在后台：Define Safety Stock Availability。

Dynamic Safety stock:

The dynamic safety stock is calculated in the planning run of the order proposal quantity. In the MRP evaluation, MD04 - you can check the parameters used to calculate the dynamic safety stock in the period totals display.To set the dynamic safety stock themaster data settingsrequired are,

1.      Maintain MRP view.

2.      In MRP 2 view enter the coverage profile which in configuration will carry the elements required as detailed below.

3.      Material master- you have MRP type PD.It is maintained in SPRO under the node->      

SPRO -Production-Material Requirements PlanningàPlanning-MRP CalculationàDefine Range of Coverage Profile (Dynamic Safety Stock).

It is maintained with respect to a plant as the calculation is plant specific.Calculation-
Thedynamic safety stockis calculated using the following formula: average daily requirements * range of coverage

The individual parameters are described below:

Average daily requirements
The calculation of the average daily requirements is carried out using the following formula: requirements in the specified periods/number of days in the total period lengthYou must maintain the following parameters for the calculation of the average daily requirements:

1.      Period indicator
You use this indicator to determine the calendar period for which you want to carry out the average daily requirements.
Here you can enter the following:

• weeks (W)
• months (M)
• periods according to the planned calendar (K)

2.      Type of period length -You use this indicator to determine the length of the individual periods in detail.
Here you can enter the following:

• workdays
• calendar days
• standard days- if you select standard days as a period length, you must determine the number of days per period in the field, "No. standard days"
  If you set standard days as the period length, you must enter the number of days per period in the "NO. standard days" field.
•  

3.      Number of periods
Here, you define the number of periods you use to calculate the average daily requirements.

Determining the range of coverage

You can determine the range of coverage for a maximum of three periods and you can maintain different coverage for each period.
To do this, you must define the ranges of coverage and the periods in the screen section entitled, "Determine range of coverage".
You also have the option of limiting the range of coverage in each of the three periods by entering a minimum and a maximum range of coverage. The system checks whether the actual range of coverage ( available quantity / average daily requirements) lies below the minimum range of coverage or above the maximum range of coverage. If this is the case, the dynamic safety stock level is recalculated using the information for the target range of coverage.

Example - 1
Coverage profile - 001, with following settings assign in material master - MRP2 view,Period type = M
No of period =2
Type of length = 3 and days/period = 20.

Range of coverage for first period (target) = 3, no of periods= 2
Lets us say requirements areas below -
12.11.2007 - 1000
19.11.2007 - 2000No of days = 2*20 = 40
average Daily requirement = (1000+2000)/40 = 3000/40 = 750

Safety stock requirement = 750*3(target) = 225.Run MRP, result is as below
12.11.2007 - pl order qty- 1225 available qty  - 1225
12.11.2007 - VSF 1000- available qty  - 225
19.11.2007 - pl order qty - 2000 balance - 2225
19.11.2007 - VSF 2000-  available qty  - 225.

Example - 2

Coverage profile - 002, withfollowing settings, assign in material master - MRP2 view
Period = M
No of period = 1
Type period= 1 i.e. working days as per factory calendar, please check your factory calendar. it is taking form no of days of factory calendar, remove the days which already passed, then remove holidays.
Example - In factory calendar -
 
2,3,9,10,16,17,23,24,30 are holiday.i,e 9 days in 30 days are holidays,
First, today is le us say 8th, hence from 30 days remove 7 days, ( as it is already passed), hence left with 23 days.
Holidays applicable are, 9,10,16,17,23,24,30 i.e. 7 days i.e. from 23 days I need to remove 7 days hence the working days are only 16 days.
range of coverage = target 7, no of period = 1.
now my requirement is 3000 in that month
daily average consumption is 3000/16 = 187.5
safety stock = 187.5*7 = 1313.
 
12.11.2007 - pl order - 1 qty 2313 available qty  - 2313
12.11.2007 - VSF 1000- available qty  - 1313
19.11.2007 - pl order - 2000 available qty  - 3313
19.11.2007 - VSF 2000- available qty  - 1313.
  
I hope the above 2 examples clears the concept of dynamic safety stock calculation.  

原文：

https://www.sdn.sap.com/irj/sdn/wiki?path=/display/ERPLO/Dynamic+safety+stock+calculation

http://www.mexp.net/wordpress/?p=138