Cobacabana (کنترل موجودی های ناوبری مبتنی بر کارت): یک سیستم مبتنی بر کارت برای کنترل تولید کارگاهی

Existing card-based production control systems such as Kanban are mostly dedicated to repetitive production environments. Cards-based systems for job shop control are lacking, while particularly this industry segment shows a need for simple control systems. This paper aims at filling the gap by presenting a simple card-based system for job shop control, the Cobacabana (control of balance by card-based navigation) system. It is based on the concept of workload control (WLC), which has already proven its value in job shops. Developments towards more robust norms now allow for transforming the WLC concept into a simple card-based system, with loops of cards accompanying orders from release until completion at critical work centers. Cards returning from work centers authorize the planner to release new orders. A card-based display supports the planner with an overview of the shop floor situation, based on non-released cards. An additional loop of cards between sales and planning enables support of the order acceptance and due date promising function. A card-based acceptance display indicates what delivery dates are realistic, considering the current workload situation. Both the scientific roots of the system and practical implementation issues are discussed in this paper.

Particularly, the large industry segment of small make-to-order job shops is confronted with a lack of support for production planning and control. Industrial practice shows that hardly any job shop is able to use the planning and control modules provided in its ERP-package. Many solutions provided in ERP-packages focus on Gantt chart or Leitstand scheduling, which is generally doomed to fail in job shops because of the high data maintenance requirements and because of their high sensitivity to uncertainty, resulting in unstable schedules. Other ERP-packages only provide material-oriented planning solutions such as MRP, while capacity planning and control is critical in most job shops. To prevent from turning back to legacy systems, these companies often opt for a planning and control system which can be implemented with limited software support. This creates an obvious need for card-based systems in job shops. The popularity of card-based control systems has been rising since the introduction of Kanban as a material control system for repetitive manufacturing environments. During the last decennium new card-based systems such as POLCA (Suri, 1998) have been developed, which can be implemented in capacity-oriented control situations. The Generic POLCA system (Fernandes and do Carmo-Silva, 2006) can be seen as an important step to make POLCA principles suitable for the specific situations of job shops. It links POLCA card loops with capacity allocations at the order release decision. Still, the basic idea of POLCA to use card loops for each possible combination of successive work centers will reduce its practical applicability in shops with high routing mix variability. In a computerized system, POLCA principles have been adapted for use in a job shop (Vandaele et al., 2005). But, as also concluded by Stevenson et al. (2005), none of the currently available card-based systems will meet the dynamic requirements of job shop manufacturing. This paper aims at filling the gap in card-based systems by choosing a perspective, which is different from the existing POLCA variants. Instead it elaborates on the concept of workload control (WLC), a concept, which fits the typical characteristics of job shop manufacturing (Henrich et al., 2004a). The use of norms in this concept facilitates a translation into numbers of cards. Earlier research (Oosterman et al., 2000) shows that the workload norms used in classical methods of WLC are sensitive to changes in the order mix. But the developments towards more robust norms allow for transforming the concept into a simple card-based system. The theoretical foundations for the new card-based system are presented in Section 2. Section 2 briefly discusses the basic principles of the WLC approach to job shop control. The recent developments towards robust norms are reviewed and supplemented such that norms can be translated into numbers of cards. Section 3 discusses the developed Cobacabana (control of balance by card-based navigation). Order release based on workload norms is transformed into cards loops between the planner and critical workstations. Additional card loops between Planning and Sales are introduced to support delivery date promising. Before concluding in Section 5, Section 4 briefly assesses the implementation issues related to the introduction of Cobacabana systems.

In this paper a new card-based system for job shop control, Cobacabana, has been presented based on workload control theory. Recent advances in this theory have been extended to enable the translation of workload norms into a fixed number of cards. It has enabled the development of a system with release cards authorizing the planner to release new orders and acceptance cards supporting a sales department to quote realistic delivery times. According to WLC principles the Cobacabana system focuses on control of workstation throughput times. Only controlled throughput times allow for a good timing of order releases and for promising reliable delivery dates. The restricted number of cards allowed in the Cobacabana loops between release and workstations guarantees that the average workstation throughput times cannot exceed their planned values. The card-based release display of the Cobacabana systems shows the planner in one glance whether the released quantities are balanced. As a consequence of controlled throughput times on the shop floor, the waiting time before release is the only variable delivery time component. To estimate this time a card loop is created between acceptance and release of orders in the Cobacabana system. The numbers of cards in this loop indicate the actual length of the waiting time before release. Thus, the new system fills the gap in available card-based systems by supporting—the main decisions of job shop control: order acceptance and order release. It allows the use of simple priority rules once orders are on the shop floor, thus not requiring any computerized scheduling support. The implementation issues discussed in this paper suggest a number of topics, which require further research. Besides, the number of cards in a Cobacabana loop is based on the assumption of a constant capacity level. As such, the presented system is currently suitable for job shop environments with inflexible capacity. Future research will address the issue of utilizing available capacity flexibility. But as long as capacity flexibility is limited, adequate job shop control depends on acceptance and release functions that balance capacity requirements with available capacity in order to control throughput times. The developed Cobacabana system enables this control of balance by card-based navigation.