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  Midpoint Scheduling

Purpose

This process describes how scheduling is executed for the order activities in Model Mix Planning.

In midpoint scheduling, the system only calculates the exact start and finish times for the planning segment. For the lines before or after the planning segment, the system uses either forward or backward scheduling to calculate the start and finish times of the order activities. This is clarified in the following graphic:

Therefore, the following disadvantages of midpoint scheduling:

Midpoint scheduling may result in too many restriction hits (orders which apply to the object dependency defined in the restriction) in a period on a line or on a group of alternative lines which does not belong to the planning segment. This is caused by forward and backward scheduling as not all orders lie in one period but continue into the following period. This means that no even capacity load of 100 % can be achieved in a period for lines that are not planning segments. However, on average, the load over neighboring periods is usually correct again.

The optimization using the LP procedure provides an optimal distribution of orders with restriction hits and, if desired, loads the periods to 100% capacity. However, the results cannot be represented accordingly in planning. Therefore, the disadvantages mentioned above. See also: Displaying the Restriction Hits

Prerequisites

  • You have maintained all the line resource data. See also: Defining Line Resource Data Relevant to Scheduling

  • You have defined the planning segment in iPPE Line Design.

  • You have defined the number of takts for the lines in iPPE Line Design.

  • You have defined the average retention period for the buffer in iPPE Line Design, if necessary.

  • You do not have to maintain a strategy profile because, in Model Mix Planning, planning is carried out using a separate, pre-defined strategy profile (forward, infinite).

Process Flow

The following process describes the scheduling of a line network consisting of lines that all work at the same pace (have the same rate) or where the fastest line is situated at the beginning of the line network:

  1. The system dispatches the orders to the planning segment. The system then calculates the start and end times of the planned orders for the order activity executed on the planning segment using the following formula:

    Start time = period start (the same for all period packages)

    End time= start time + duration of the order activity

    Duration of the order activity = order quantity x takt time

  2. In a next step, the system calculates the lead time for the planning segment. The lead time is the time the period packages have to be moved using forward scheduling to calculate the start time on the next line resource. The lead time is calculated using the following formula:

    Lead time per piece = number of takts on line resource x takt time + average retention period in the buffer.

    In the following example, the line consists of 100 takts. The takt time is 0.6 mins. The order quantity is 200 pieces. There is no buffer. The shift starts at 6.00 a.m.

    The lead time per piece is 100 takts x 0.6 mins = 60 mins.

    The duration of the order activity is therefore 200 pieces x 0.6 mins = 120 mins.

    From 6.00 a.m. to 8.00 a.m. one piece is placed on the line every 0.6 mins. Between 7.00 a.m. and 9.00 a.m., an assembly is completed every 0.6 mins and leaves the line.

    The system displays 6.00 a.m. as the start time of the order activity. 8.00 a.m. is displayed as the end time of the activity.

  3. The system then calculates the start time of the planned orders on the following line resource using the following formula:

    Start time of order activity 2 = start time of order activity 1 + lead time on line resource 1.

  4. The system dispatches the planned orders to the next line resource at this start time and then calculates the end time on this line resource:

    End time of order activity 2 = start time + duration of order activity 2.

    Duration of order activity 2 = order quantity x takt time of line resource 2.

  5. The system then calculates the start and end times for order activity 3 on the third line resource the same way.

  6. Finally, the system calculates the start and end times of the planned orders using the start time of order activity 1 and the end time of order activity 3.

If in a line network, the second line works faster (higher rate) than the first, the system uses other formulas than the ones specified above. Otherwise, activity 2 would seem to be completed before activity 1 which is not the case.

The end time of activity 2 is calculated using the following formula:

End time of order activity 2 = end time of order activity 1 + lead time on line resource 2 + average retention period in the buffer.

Start time of order activity 2 = end time of order activity 2 – duration of order activity 2.

Here, scheduling is determined by the underlying start-start and end-end relationship of the two order activities – that is, activity 2 cannot start until the first part has passed through line 1 and, if necessary, the buffer. This procedure is clarified in the following graphic: