COBACABANA - COntrol of BAlance by CArd BAsed NAvigation (Land, 2009; Thürer et al., 2014) - translates the information flow of Workload Control into cards providing a simple, visual control solution for both Customer Enquiry Management and Order Release. The planned workload is controlled by establishing a card loop between customer enquiry management and the pre-shop pool. The shop floor workload is controlled by establishing card loops between order release and each work center on the shop floor.
Key Definitions
Customer Enquiry Management
A basic assumption underlying Workload Control – and consequently COBACABANA – is that its order release mechanism stabilizes the time buffer on the shop floor. It does so by shifting part of the time buffer into the pool. As a consequence of this rational the pool waiting time is seen as only variable component of the time buffer of confirmed orders.
To determine a minimum allowance for the time buffer in the pool (the pool waiting time) COBACABANA uses a card loop between Customer Enquiry Management and the pre-shop pool. Cards appear in pairs made up of one acceptance card per operation and one floor card per job, representing acceptance cards different amounts of workload according to their size. While the acceptance card(s) stay with the sales person, the floor card travels with the order until the job has been released.
When an order arrives at Customer Enquiry Management a feasible due date is determined by summing the allowance for the pool waiting time derived from the sales person's display as depicted below, an allowance for the time buffer for each operation on the shop floor (which is stabilized by order release) and an external allowance to account for variability.
The allowance for the pool waiting time is given by the maximum pool workload among the work centers which are in the routing of the order.
The order contributes to the pool load of each work center in its routing. In other words, for each work center in the routing of an order, a card of the size corresponding to the ‘full’ workload of the order at the particular work center is added to the workload of that work center on the sales person's display, the floor card is attached to an order guidance form and the order moves into the pool to await release.
As for order release, only three different card sizes - to distinguish between small, medium and large operations - maintains most of the performance benefits.
The management of capacity adjustments is also supported by the sales person's display. The resource requirements posed on each work center by the jobs in the pool (the pool workload) gives a clear indication were to adjust capacity. Adjusting capacity in a timely manner before resource requirements posed on a work center are too high, allows for further stabilizing the time buffer.
COBACABANA vs. Kanban and ConWIP
From the above, it becomes clear that COBACABANA is distinctly different from other card-based systems you may be aware of – as Kanban and Constant Work-In-Process (ConWIP) - and overcomes the problems of other systems in job shops.
First, COBACABANA establishes card loops between each work center and the pre-shop pool of orders coordinated by the planner; hence, control remains centralized. This also means that cards are only used to control the release of jobs and not to control the flow of jobs on the shop floor, e.g. between work centers, which remains under the control of the dispatching rule.
In this sense, COBACABANA is distinctly different from Kanbans where cards typically operate between work centers and, hence, where control is decentralized.
A central release function facilitates a ‘global view’ of the shop floor, which better supports load balancing in job shops.
The centralized control provided by COBACABANA does have similarities with ConWIP; however, under ConWIP, only one card (not part number specific) is assigned to each job. ConWIP restricts the number of cards in operation between the pool and shop floor, thereby controlling the number of orders on the shop floor. Hence, although ConWIP controls the number of orders on the shop floor, it does not balance workloads across work centers.
In other words, order characteristics have to be aligned with shop characteristics if workloads are to be controlled using ConWIP. This severely limits the applicability of the method to high-variety job shops.
The fact that COBACABANA cards represent a workload contribution to a work center rather than a complete order avoids this problem.
Second, COBACABANA assigns a set of cards to a particular work center, and each card represents a defined amount of workload (e.g. in hours, though translated into percentages). Hence, unlike Kanban cards, COBACABANA cards are not part number specific. Part number specific card systems soon become unmanageable when there is high variety as they can quickly lead to hundreds if not thousands of cards being in operation.
Third, COBACABANA extends the use of cards to Customer Enquiry Management. This provides a simple, visual and effective solution to quote due dates which appropriately reflect a firms current operational capabilities.
Order Release Control
COBACABANA establishes card loops between the planner performing the release decision and each work center. Cards appear in pairs made up of one release card and one operation card per operation representing release cards different amounts of workload according to their size. While the release card stays with the planner, the operation card travel with the order until the operation has been completed. Different cards are maintained for each work center; for example, cards can be color coded, with a different color used for each work center.
Balancing Workloads:
The Periodic Release Element of COBACABANA
At fixed (periodic) time intervals, orders in the pool are sorted according to their planned release dates. Orders are then considered for release, beginning with the first order in the sequence. The workload of each operation in the routing of the order currently being considered for release contributes to the load of the corresponding work center(s) according to the corrected aggregate load (which will be explained in detail below).
In other words, for each work center in the routing of an order, a pair of release and operation cards is taken and the release card – of a size corresponding to the corrected workload of the order at the particular work center – is added to the workload of that work center on the planner's display. In the example depicted below a new order with two operations is considered for release: one operation at Work Center 3 and one at Work Center 1 (in gray).
Similar to a heijunka box or planning board (see, e.g. Marchwinski et al., 2008), this simple display provides a quick overview of the situation on the shop floor. But while the heijunka box typically levels the mix and volume of production by assigning capacity to product types, COBACABANA (as can be seen from the display) contributes the workload of individual jobs to the total workload allowed for release to each work center.
The released workload is represented by available cards and the release possibilities by available space on the display. Hence, the board provides a useful, visual tool for understanding both the workload on the shop floor and the release possibilities. For example, the set of release cards currently withdrawn for a specific order number provides information on the progress of that order on the shop floor.
Notice that the workload contributions in the Figure are standardized by setting the workload norms – i.e. the upper limit or bound on the workload released per work center – equal to 100%.
If, for any work center in the routing of an order, the workload represented by the release cards (the existing workload plus the workload of the new card) violates (i.e. exceeds) the 100% level, the order is retained in the pool and the release cards of the order are removed. Otherwise, the release cards are assigned to all of the work centers in the routing of the order, the corresponding operation cards are attached to the order guidance form of the order, and the order is released. In the example, the order is released (together with the corresponding operation cards) as the workload norms are not violated by the new workload contributions.
The operation card returns to the planner as soon as an operation is completed and the release card corresponding to the operation card is then removed from the display.
Starvation Avoidance:
The Continuous Release Element of COBACABANA
In addition to the above periodic release mechanism, COBACABANA incorporates a continuous workload trigger. If any work center runs idle, then the first order in the pool sequence with that work center as the first in its routing is released from the pre-shop pool, irrespective of whether its release would exceed any of the workload norms at other work centers in its routing. The corresponding release cards are assigned to the work centers in the routing of the order, the operation cards are attached to the order guidance form, and the order is released.
This avoids work center starvation or premature idleness caused by periodic release combined with the strict enforcement of workload norms (Kanet, 1988; Land & Gaalman, 1998). Premature work center idleness refers to the emergence of a capacity buffer at a transforming resource although there exists resource requirements. It emerges if a job is not released due to a high workload at another work center in its routing.
Hence, COBACABANA recognizes the need to temporarily violate the 100% workload limit at a certain work center to avoid preliminary idleness at another work center.
Simplifying Processing Time Estimation
Finally, at the time of order release, the cards can be cut to exactly the right size, representing the load contributions of the operations involved. However, we can also simplify the need for processing time estimations by limiting the number of card sizes. In this case, a card size represents a certain range of load contributions, rounded to the estimated average in that range. Thürer et al. (2014) recently showed that the use of only three card sizes – for small, medium and large workload contribution – maintains most of the performance benefits of Workload Control.