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There have been other classes of production control strategies which cannot be directly classified into any of the groups above. Unlike the push or pull strategies that have set rules for transmitting demands and authorising the processing of parts, these strategies have been developed to work flexibly in synchronisation with the state of the system. Some of them are briefly described in the following sub-sections under wide classification that are based on their production control concepts.

2.3.1 Bottleneck and Workload Oriented Concepts

The main aim of the bottleneck oriented concept is to achieve the best utilisation of the bottleneck station without accumulating excess amount of inventory before it or starving the station after it of work to do. It regulates the release of materials into the system in synchronisation with the capacity of the bottleneck station. It was first proposed through the Theory of Constraints [41-43] before it found production scheduling application in the form of the Drum-Buffer-Rope [43, 44]. A technique called Starvation Avoidance similarly seeks to avoid starvation of the bottleneck station through a regulated release of new jobs into the system to ensure that the bottleneck station is kept running without an accumulation of inventory before it [45].

The workload oriented concepts on the other hand seek to regulate the amount of workload in a manufacturing system by always taking into account the current level of workload on the system – particularly at the bottleneck station, before releasing new jobs into it. They are similar to the CONWIP’s regulation of the number of items in the system, but they go further to consider the amount of load each item’s processing requirement translates to on the system’s capacity. There have been many variations of the workload oriented concepts in research in terms of how they measure the load resulting from the WIP items at the bottleneck station and in the system [46, 47]. Examples of such variations are the CONLOAD [48], the Workload Regulation or CONWORK [49, 50], the WIPLOAD [51], the Pull-From-Bottleneck (PFB) [46] and the CONFLOW [47]. The applications of the workload oriented concepts have been mostly in semiconductor fabrication environments.

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There have been comparisons between the two concepts and the push or the pull control strategies [45, 47, 48, 52]. One of such comparisons is that which reports that the PFB concept is able to protect and keep the bottleneck station running even if a station downstream of it was broken down; unlike the KCS and CONWIP which would at some point under such situation cease to have Kanbans to release items into the upstream stage [43, 46, 47].

2.3.2 Product Grouping Concepts

An approach that has gained popularity in managing manufacturing resources between multiple products is Cellular manufacturing whereby similar products are grouped into families and assigned to cells of machines for the processing of one or more families of products [53-56]. In addition to machining requirements and part design features, which are mostly used to group products, other external factors such as demand attributes have also been used recently [57]. However, there are situations whereby products only have minor differences in design features which would only require slightly different processing requirements through the same production route. In such situations, a cellular design may not be a viable option because it would possibly require a duplication of all the machines. Therefore, a line manager who is consigned to a situation of having to share manufacturing resources will have to determine the best way to setup the system to ensure a balanced delegation of production capacity between the products. The allocation of Kanbans and the setting of basestock levels for products have been shown to influence the performance of individual products [13, 14, 58], most especially because they have impact on how much access a product will have to the manufacturing resources. As a result, focussing on the management of such aspects might be the only option for such managers. Moreover, within the shared or dedicated cells of the cellular manufacturing, there is still need to manage the access to the manufacturing resources within and between the product families.

The Paired-cell Overlapping Loops of Cards with Authorisation (POLCA) [59, 60] is another category of multiproduct control strategy which uses signal cards to control the flow of parts between cells in a shared configuration. However, it does not implement Kanban control to coordinate the flow between the workstations that are within a cell. POLCA is mostly used for signalling when capacity becomes available downstream

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while Kanban is an inventory signal to control the replenishment of inventory for a specific product [60].

2.3.3 Flexible or Adaptive Control Strategies

Another category of production control strategies that can be found in literature are the Flexible or Adaptive KCS which operate like the traditional KCS, except that they propose a dynamic review of the initial Kanban settings in response to the state of the system [61-64]. They adjust the number of available Kanbans in accordance to a rise or fall in the demand arrival rate, inventory level or demand backlog level. Another study however suggests that adjusting the number of Kanbans may not be sufficient in some system variability situations, and that observing inventory level as a means of knowing when to adjust the number of Kanbans might not give prompt indication of when the system is undergoing instabilities [65]. The study thus proposes a robust Kanban design that would instead involve making adjustments to operational, tactical and strategic system settings to offset instabilities [65]. Such adjustments would be made to the average service time of machines, the number of machines and the materials supplier. However, this might be difficult and costly to achieve for production line designers whose desire is to be able to sustain a particular system design for a long time without the need for constant re-evaluation. Moreover, as a result of cost, time and some technical considerations, organisations cannot always afford to make such continuous changes for their manufacturing system to be adaptable to running a particular pull strategy. Therefore, instead of trying to continuously fine tune a single strategy, some studies have proposed the combination of desirable traits from different strategies into a single hybrid strategy.