安全庫存

[@more@]

安全庫存只是一個籠統的說法，本文歸納了SAP四種安全庫存（時間）的設定，用於增加計劃過程中庫存數量（時間）的彈性。為何安全庫存能增加計劃的彈性呢？因為在淨需求計算（Net Requirements Calculation）中，Quantity available = Fixed receipts + Stock available for planning - Safety stock - requirements，若可用數量小於零，則報缺料，視為短缺數量（Shortage quantity）或需求數量（Net requriements），將用於批次計算並得出最終的獲取建議。

Safety stock
Safety stock partially available
Dynamic safety stock
Safety time
一、Safety stock

這就是“安全庫存”了，安全庫存的數量在計劃中是不可用的。MRP中，若物料的數量低於安全庫存水平，就會產生計劃訂單。安全庫存在物料主資料MRP2檢視設定。

二、Safety stock partially available

如果設定了安全庫存，一低於該水平就會產生補貨的請求，哪怕只是缺了一點，這增加了計劃的工作量。為了彌補這個不足，我們可以在MRP Group中設定一個百分比，只有安全庫存低於該百分比，補貨請求方會產生。

三、Dynamic safety stock

動態安全庫存就更加厲害了，它根據平均的日需求（Average daily requirements）數量，來確定未來幾個時期的安全庫存水平（數量等於若干個平均日需求）：最小庫存、目標庫存、最大庫存。若小於最小庫存，產生補貨請求至目標庫存；若大於最大庫存，系統將提示例外資訊。若同時設定了安全庫存，將累加。該配置稱為Range of Coverage Profile，在MRP2檢視分配，配置在IMG: Production - MRP - Planning - MRP Calculation - Define Range of Coverage Profile (Dynamic Safety Stock)。

四、Safety time

前面三種方案都是對安全庫存數量上的設定，Safety time則是設定時間上的餘量。物料的補貨提議時間將提前於實際的需求時間，提前日期在MRP2檢視中配置。考慮到物料可能用在產品的不同層次，為了避免重複提前，可以在MRP2檢視的Safety time indicator欄位設定安全時間有效於所有需求還是僅相關需求，或者乾脆無效。而考慮到不同時期對安全時間的長度要求不同，可以在後臺定義Period Profile，IMG: Production - MRP - Planning - MRP Calculation - Define Period Profile for Safety Time/Actual Range of Coverage，並在MRP2檢視分配給物料。

注意： 百分比是根據MPR組設定的，在後臺：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.