Conwip
Wallace J. Hopp and Mark L. Spearman call a method of production control ConWIP ( Constant Work-In-Process ). Production systems are principally controlled according to a pull or push system. Hopp and Spearman define push systems as production systems in which production orders are triggered again according to demand, while in pull systems the order is triggered on the basis of a system state. In push systems, a plan outside of the production system controls the triggering of orders. In pull systems, the trigger is a state in the system itself. In CONWIP systems, the trigger is when an order leaves the system. A new order is only added to the system when an order leaves the system. This means that the number of orders in the system and thus WIP is relatively constant.
In contrast to other procedures, dates are not primarily controlled, but the amount of material in production . This material inventory is kept largely constant. The average lead times thus remain largely stable. This production control is very simple. The process is similar to Kanban, which controls replenishment processes. In contrast to this, however, a stock is not tracked in a chain from station to station, but from defined start and end points across several stations.
Control principle
In a logistic system, e.g. B. in an assembly line, the lead time of a production order increases or decreases proportionally to the amount of inventory currently in the system ( Little's Law ). ConWIP uses this connection. With ConWIP, the inventory is kept constant within a logistic system. Similar to the Kanban procedure, cards (ConWIP cards) are used, either physically or electronically. These cards represent stocks and are assigned to the individual orders in the system.
Typical units of measurement for ConWIP cards are the number of products, the number of orders or even capacity hours. It is important to establish a high degree of comparability between the production orders. Are z. If, for example, different products with different work content are produced in a production area, the unit capacity hour is better suited than the unit piece.
In a process section that is controlled with ConWIP, a new job is only started when another job has been completed and the corresponding ConWIP card (s) has (have) been free. This ensures - taking into account a few other boundary conditions and rules of the game, such as B. FIFO - largely stable order processing times. The lower the inventory level (up to a practicable lower limit) in the system, the shorter the processing time of an order. The transfer of the principles of flow production to workshop production is somewhat idealized .
function
Complex production processes can usually be broken down into manageable, semi-autonomous sections. Production sections that represent potential bottlenecks, especially those with changing bottleneck capacities (machines or workstations), are critical for the order flow. Designed as a ConWIP control loop , these situations can be mastered. ConWIP control loops can be arranged in parallel or coupled one behind the other.
Combined with supplementary procedures, for example priority control for customer orders, the ConWIP control can be ideally suited for manufacturing companies with discrete production in order to ensure a very high delivery service for customers.
literature
- H. Lödding: Process of production control. Springer-Verlag, Berlin 2005, ISBN 3-540-20232-3 .
- W. Hopp, M. Spearman: Factory Physics. McGraw-Hill Higher Education, New York 2001, ISBN 0-256-24795-1 .
- Herbert Jodlbauer: Production optimization, value creation and customer-oriented planning and control. 2nd Edition. Vienna / New York 2008, ISBN 978-3-211-78140-1 .
swell
- ^ A b Wallace J. Hopp, Marl L. Spearman: Factory Physics: foundations of manufacturing management. 2nd Edition. McGraw-Hill Higher Education, 2000, ISBN 0-256-24795-1 .