Definiciones múltiples de la transitividad

In Bottleneck station control strategies there is one loop from the start of the line up to the bottleneck station that signals to call for a lot release. If a signal is generated due to a change in bottleneck station status, a lot is released to the production line. It should be mentioned that Prakash and Chin [34] considered this type of control strategies as a variation of CONWIP; as CONWIP does not take into account is the impact of the bottleneck station may have on the performance of a system [16]; however, in this work it is decided to consider them as an independent class.

Examples of bottleneck station control strategies considered in this work are: Drum-Buffer-Rope, Starvation Avoidance, Workload Regulating, and CONLOAD, which are reviewed in the following sections.

Drum-Buffer-Rope (DBR)

The DBR is a Theory of Constraint approach introduced by Goldratt, it has 3 main components. The “Drum” is the control point of this system, or main constraint. It is the process that has the least amount of excess capacity (bottleneck rate) and controls the total throughput rate of the system. To avoid the starvation of the bottleneck a protection time “Buffer” is maintained before the drum. Finally, the “Rope” signals the releasing of lots to the shop floor [35-43].

In DBR, when a lot completes processing by the bottleneck station, another lot with the bottleneck’s rate is released at the start of the production line [4, 16, 26]. Hence, it controls the WIP up to the bottleneck station [15, 44], as presented in Figure 2-9. It should be noted that DBR is more general than CONWIP in that it can be applied to pure job shop manufacturing systems whereas CONWIP cannot. However, when both applied to flow lines, they have nearly similar results [26].

Figure 2-9: DBR lot release control strategy.

DBR has produced excellent results across a wide variety of manufacturing environments. It has been implemented as a manual system and is quite successful in providing increased system throughput rate, significant reductions in cycle times and work-in-process inventories, and improved due date performance [4]. As a result DBR is became popular in many manufacturing industries, especially semiconductor manufacturing [44], although, its implementation is not straight forward in semiconductor manufacturing due to the presence of re-entrant flow [38, 43].

Starvation Avoidance

This is a lot release strategy, in which a new lot is released into the shop floor to avoid starvation of a bottleneck station. In this rule, a new lot is released when virtual inventory at the bottleneck station falls down to a predetermined value. This rule can only be directly applied to a system with single bottleneck station producing a single product type [4, 45, 46].

Workload Regulating

Workload Regulating (WR) or sometimes called CONWORK, is applied to take into consideration the system loading situation and how much work a single lot will create for a bottleneck. Where, the total of processing times at the bottleneck that is currently represented by the lots being processed in the production line is measured. A lot is released to the production line if the current workload plus the total amount of bottleneck processing times of this lot is less than a given limit. As soon as it is released the workload is increased by the sum of bottleneck processing times of this lot, and each time a lot leaves the bottleneck station the workload is decreased by its bottleneck processing time [47]. Briefly, a new lot is released to the system when the sum of the remaining processing times, at any bottleneck station, over all lots in the fab falls below a critical value [4, 45, 46, 48]. This strategy was compared to others, and showed significant impact on the performance of the cycle time [4]. Though, this strategy provides a better picture of the loading situation of the system, it does not reflect how the load is distributed over time [47].

CONLOAD

CONLOAD is a lot release control strategy developed by Rose to overcome some performance problems of traditional lot release control strategies like CONWIP and WR during product mix changes at semiconductor manufacturing. Where, it is stated that CONWIP and WR are not capable to avoid overload because of their lack in tracking the current load situation of the system accurately enough, and that CONLOAD takes into consideration how much load is added to a single

machine or a group of machines by a particular lot to decide on releasing this lot into the fab [47].

CONLOAD is a simple extension of WR, instead of considering the amount of work for the bottleneck station, the load for the bottleneck station is computed (the sum of bottleneck processing times of the lot divided by the average cycle time of lots of this product type). A new lot is allowed to enter the system if the current bottleneck load plus the load introduced by the new lot is less than a predetermined level. Each time a lots enters the line, the bottleneck load is increased by the lots load, and each time a lot leaves the line, it is decreased by the same amount [47].

CONLOAD varies from CONWIP, WR, and other lot release control strategies, in that the predetermined level that controls the releasing of lots is a natural constant of the system, while, that of the others are usually determined by simulation. The predetermined level is the target utilisation of the bottleneck station multiplied by the number of bottleneck machines. For instance, if the maximum bottleneck load should be 95% and the bottleneck consists of 4 machines, then the predetermined level is 3.8. The only parameters that have to be determined in advance by simulation or queuing analysis are the average cycle times for each lot [47].

In conclusion, CONLOAD out-performs CONWIP and WR with respect to keeping the bottleneck utilisation at a desired level and to provide a smooth evolution of the WIP. Also, it reduces the variations in cycle times and smooth’s the lot departure process of the system [47].