Intended learning outcomes: Describe the problem of calculating capacity per load period. Explain the problem of load assignment for one operation during the load periods. Present the issue of calculating the load in a given time period when various operations occur only partly within the time period.
A load profile calculation is not more than an approximation and must be interpreted as such. Thus, if interoperation times fluctuate widely (see Section 13.2), it will be difficult if not impossible to execute the operations as scheduled.
Further inaccuracies arise from the quality of the algorithms employed by ERP software or electronic control boards. Figure 14.2.2.1 shows a first problem associated with algorithms: assigning capacities to each time period on the planning horizon.
Fig. 14.2.2.1 Calculating capacity per load period.
The length of the load periods may vary. Also, we need flexible selection of the start date for the first and the end date for the last period under consideration. Then, loads that lie in the past cannot be compared against capacities, since the capacities are available only from “today” onward. “Today” may also fall within one of the time periods, in which case capacity is available only for the time remaining from “today” to the end of the time period.
Another problem is that a simple but imprecise method will assign the load to the time period containing the start date for the operation. An operation can extend across several load periods, however. Figure 14.2.2.2 shows this problem and offers an improved procedure.
Fig. 14.2.2.2 Load assignment for one operation during the load periods.
The possible load per time period is obtained from the theoretical capacity (Fig. 14.1.1.1) and from the operation (splitting factor). The start date falls within a given period i. From this we can determine the time remaining until the end of period i and calculate the possible load Bi. The partial sum Si is the sum of all the operation loads Bj, with j £ i of the operations that have already been assigned to the periods up to and including i. The load for the last period is the remaining load for the operation. At this point, Si represents the entire load.
A third problem is how to determine all the operations that occur in a given time period [start, end]. Figure 14.2.2.3 shows that various operations occur only partly within the time period.
Fig. 14.2.2.3 Operations to be included in the load for a work center.
In practice, we arrange the open (or open and planned) operations by start date. We consider any operation whose start date is earlier than the end date of the time period and whose completion date is later than the start date of the time period. The load of the operation occurring within the time period is assumed to be proportional to the lead time occurring within the time period.
Course section 14.2: Subsections and their intended learning outcomes
14.2 Infinite Loading
Intended learning outcomes: Present load profile calculation and problems associated with algorithms for load profile calculation. Explain methods of balancing capacity and load. Describe order-wise infinite loading.
14.2.1 Load Profile Calculation
Intended learning outcomes: Explain an example of a work-center-load profile. Present an example of a load profile known as an overload or underload curve along the time axis.
14.2.2 Problems Associated with Algorithms for Load Profile Calculation
Intended learning outcomes: Describe the problem of calculating capacity per load period. Explain the problem of load assignment for one operation during the load periods. Present the issue of calculating the load in a given time period when various operations occur only partly within the time period.
14.2.3 The Cumulative Load Profile
Intended learning outcomes: Describe the analysis of the load profile and the cumulative load profile.
14.2.3b Adapting Capacity to Load Rather Than Load Leveling
Intended learning outcomes: Explain possible strategies for capacity planning, in case of a trend toward persistent overload or underload, or in case of frequent and infrequent self-compensating fluctuations. Differentiate between adapting capacity to load and load leveling. Present an evaluation of the technique. Identify its limitations and typical areas of application.
14.2.4 Order-Wise Infinite Loading
Intended learning outcomes: Describe order-wise infinite loading. Identify the suitability of order-wise infinite loading.
