# Beyond Kanban: POLCA-Constrained Scheduling for Job Shops
## 2 Literature Review  
Over the years, several PCS strategies have emerged: Drum Buffer Rope, Kanban, CONWIP, COBACABANA, etc. This study deals with POLCA, which is a card-based PCS developed by [PERSON] in 1998 for HMLV job shops organized into cells with multi-skilled workers [1]. This system establishes a loop between each pair of cells through which the so-called POLCA cards circulate, communicating upstream cells about capacity availability at the downstream cell. The card is a visual instrument that helps control the Work In Process (WIP) and guides workers in determining the next step of jobs routing. Compared to Kanban, POLCA is not item-specific, a feature that makes it suitable for HMLV systems [2]. Compared to CONWIP, jobs are authorized only if they can flow downstream without accumulating in the output buffer of the arriving cells. Compared to Drum Buffer Rope, POLCA can be used in job shops with shifting bottlenecks over time. Although COBACABANA is suitable for managing job shops with undirected routings and variable cycle times [3], the literature provides few contributions about real industrial cases [2,4]. The underlying mechanism is based on the principle of capacity reservation. Each loop has a limited number of cards assigned to jobs. A job can be authorized to be processed on a cell only if the following three conditions are met: (i) the corresponding part number has arrived in the input buffer of the cell; (ii) there is a card available for the loop formed with the subsequent cell required by the work cycle; (iii) the job's authorization date has passed for that cell. The last condition explains why POLCA is defined as a hybrid push-pull mechanism. POLCA cards are not sufficient to ensure that cell capacity is utilized efficiently and that production aligns with real-time demand fluctuations. Therefore, the system requires a high-level MRP that calculates authorization dates for each cell in a job's routing by working backward using cell lead times. Estimating the correct number of cards is a crucial aspect, because it determines the maximum level of WIP flowing between two cells in a fixed amount of time. In the literature, different approaches are employed: Little Law [5], mathematical programming models [6], and simulation-driven methods [7]. If each task requires a different amount of work, a minimum quantity is defined and called a quantum during the system design phase, serving as a unit of measure to determine the number of POLCA cards needed for authorization. The quantum represents the standardized minimum work quantity, which serves as the basis for sizing the control resources. The unit of measurement of workload can refer directly to the amount of material processed or to the working time. The systems that adopt the second option are called load-based (POLCA-LB) [8]. Ref. [9] provide the first comprehensive implementation roadmap for POLCA in HMLV systems. Their four-phase methodology-pre-POLCA assessment, loop design, system launch, and post implementation auditign turns POLCA from theory into a workable practice. Evidence from three North-American plants shows up to 60% WIP reduction, shorter lead-times and significant improvements in on-time delivery, confirming the suitability of POLCA as a hybrid push-pull control strategy in MTO settings. Recent empirical evidence further substantiates the industrial viability of POLCA integrated with Quick Response Manufacturing (QRM). Ref. [10] reports a full-scale implementation of a POLCA-QRM framework in a precision-parts job shop, achieving a 30-80% reduction in lead time and a commensurate decrease in WIP levels while simultaneously lowering profit-loss penalties to below 1% of sales. The study confirms the effectiveness of POLCA when embedded in an enterprise-wide QRM strategy, yet it relies on purely reactive control without an optimisation layer. Several studies have investigated how POLCA compares to alternative production control systems, particularly in terms of throughput time, WIP levels, and the percentage of tardy jobs.