Whereas capacity planning determines in advance the capacities required to implement a production program, capacity control determines the actual capacities implemented shortly beforehand. The capacity control generally decides about the work hours and about which workstation cross-trained operators are allocated to. In particular it thus determines when overtime, shortened work hours and special measures related to the capacity flexibility are implemented.
Figure 1 illustrates how the capacity control fits into the manufacturing control model; the tasks and corresponding actuating variable are outlined in bold. Via the actual output, the capacity control impacts the backlog and consequently the schedule reliability of a production. Ensuring high schedule reliability is the primary logistic target of the capacity control. Furthermore, one of the essential economical aims is to efficiently implement the capacity flexibility.
Characteristics for Classifying Capacity Controls
Three characteristics for classifying capacity controls can be distinguished:
- The capacity control’s criterion determines the characteristic according to which the enterprise makes decisions about adjusting the capacities.
- The capacity control’s degree of detail (Sect. 26.1.2) determines whether the capacities are adjusted for the entire production (low degree of detail) or for only part of the production (high degree of detail).
- Periodic and an event oriented capacity control are differentiated from one another according to the trigger logic. The first makes decisions about adjusting capacities at defined intervals, while the latter makes decisions following the occurrence of a specific event e.g., exceeding a backlog limit.
The capacity control determines the work hours of the production workers and therefore can directly give rise to costs. Consequently, it impacts the social and financial aspects more immediately than the remaining manufacturing control tasks.
As a result, it is all the more important to define suitable criteria that can trigger the capacities to be adjusted on short notice. Two types of criteria can be roughly differentiated: Criteria 1–5 are aimed at fulfilling customer demand or planning values. In comparison, criteria 6 and 7 should lead to the alignment of the capacities of a production’s workstations. In many cases, an enterprise has to consider a number of criteria in a capacity control in order to attain both the logistic objective of high schedule reliability as well as the economical target of efficiently implementing the capacity flexibility.
1. Fulfilling Customer Demand: The basic task of capacity planning and control is to orient the capacities on the customer demand. The more successful they are, the more punctual orders can be completed (make-to-order productions) or the less stock is required to cover the demand (make-to-stock productions). In (almost) every case, orienting the capacities on the customer demand is a component of capacity planning. Especially when the customer demand is difficult to forecast and fluctuates it can also be the main criteria for the capacity control. A simple example here would be the proprietor of a banquet hall who flexibly adjusts the work hours of the personnel to the duration of the celebration.
2. Stock: In a make-to-stock production the stock of a product or semi-finished products can serve as a criterion for the capacity control. Enterprises increase the capacities when the stock has fallen below a lower limit and reduce the capacities if it exceeds an upper limit. The capacity control is thus indirectly coupled to the customer demand through the stock. In many cases, enterprises do not orient the capacity control directly on the customer demand, but rather on the production planning. Criteria for the capacity control are then deviations from the planned output (backlog) or from the planned capacities. The implicit assumption of such a method, is that the production planning ensures the coupling with the customer demand.
3. Backlog: With a backlog control, enterprises increase the capacities when the actual output falls below the planned output (backlog > 0). In comparison, if the production’s actual output surpasses the planned output the capacities are decreased (backlog < 0). The aim of the backlog control is to realize the due dates and output level provided by production planning as precisely as possible even when there are disruptions. Due to the significance of the backlog control in the industry an entire chapter is dedicated to it.
4. Deviations from the Planned Capacity: With this form of the capacity control, enterprises increase the actual capacity if it is lower than the planned capacity (the reverse case of a capacity reduction when the planned capacities are exceeded rarely occurs on the shop floor). A simple example is when one of the machines in a machine group breaks down and the production management extends the operating hours of the remaining machines. Unlike with backlog control, the capacity measure can already be introduced before a backlog develops. However, since other reasons for the development of a backlog remain unconsidered, the deviation from the planned capacity generally does not suffice as the only criterion for the capacity control. If the deviation from the planned capacity only impacts part of the workstations, it inevitably results in different capacity profiles in the production. Thus, taking into consideration this criterion contributes to a better alignment of the capacities.
5. Probable Lateness of Orders: Begemann designed a method for controlling capacities that increases the capacities when one or more orders would otherwise be late. In order to do so, the method continually (re)calculates the probable completion date of the orders via forwards scheduling. Unlike the backlog control, by increasing capacities the method can also reduce lateness resulting from sequence interchanges or a late input.
A number of criteria for controlling capacities are aimed at aligning the capacities of a production’s workstations. When the capacities are not aligned there is a risk of breaks in the material flow on workstations with higher capacities and a WIP buildup on workstations with lower capacities. It is the task of capacity planning to keep the unavoidable capacity differences to a minimum with a well adjusted work hours and personnel planning. Frequently though, it is also necessary to implement additional control measures as a supplement. Possible criteria for such a capacity control include:
6. WIP Before or After the Workstation: The basic idea here is to reduce capacities when the WIP builds-up after a workstation and exceeds an upper limit. This indicates that the capacities on the following workstations are lower. Increasing the capacities is reasonable when the WIP falls below a lower limit after a workstation and the following workstations are threatened by a break in the material flow. On the other hand, WIP before the workstation should be sufficient for protecting the workstation from breaks in the material flow once the capacities are increased. Especially with manual work places, there is the possibility that differences in capacities are compensated not by changing the work hours, but rather by adjusting the work speed. Reducing the work speed causes losses of productivity when operators are paid hourly. Conversely, increasing the work speed beyond a reasonable degree can lead to stress and exhaustion as well as a lack of quality.
7. Active and Passive Blockings: If the WIP before or after a workstation is limited (e.g. due to the number of storage spaces), the resulting blockings are a possible criteria for the capacity control. A passive blocking arises when a workstation is unable to produce anymore because the upper limit after the workstation is exceeded (the workstation is blocked). With an active blocking WIP exceeds the upper stock limit before the workstation and thus blocks the preceding workstation. Similar to the comments about the WIP criterion, it is reasonable to reduce the capacities for a workstation that is blocked. In comparison, it is possible to increase the capacities for those workstations that are actively blocking other workstations, without themselves being blocked. According to the criterion of the capacity control, the methods for controlling capacities that are described in the following chapters can be classified as follows:
REFERENCE: H. Lödding, Handbook of Manufacturing Control.