Conwip

CONWIP (Constant Work-In - Process) call Wallace J. Hopp and Mark L. Spearman a method of production control. Production systems are controlled principally by a pull or a push system. Hopp and Spearman define push systems as production systems in which production orders in accordance with the demand to be re-triggered while in pull systems the order is triggered on the basis of a system state (page 340). In push systems thus controls a plan outside the production system to the release of orders. In pull systems, the trigger is a state in the system itself in CONWIP systems is the trigger when a job leaves the system. It is a new order is therefore only be included in the system when a job leaves the system. This reduces the number of jobs in the system and thus WIP is relatively constant.

In contrast to other methods, appointments are not primarily controlled, but the amount of material stock in production. This material inventory is maintained substantially constant. The average processing times remain largely stable. This production control system is very simple. The process is similar Kanban controls which Nachfüllprozesse. In contrast to this is not followed in a chain from station to station an inventory, but defined starting and ending points across several stations.

Control principle

In a logistic system, such as in an assembly line, increases or decreases the processing time of a production order proportional to the amount of inventory that is currently in the system is located ( Little's Law). This connection uses CONWIP. With CONWIP inventory within a logistics system is kept constant. Similar to the Kanban method to map ( CONWIP cards) used, either physically or electronically. These cards represent stocks and are assigned to the individual jobs in the system.

Typical units for CONWIP cards are the number of products, the number of orders or capacity hours. It is important to establish a high degree of comparability between production orders is. Be produced different products with different work content, eg in a production area, so for example, the unit capacity hour is better than the unit piece.

In a section of the process is controlled with CONWIP, a new job is always only start when another is finished and the corresponding CONWIP card (s) was free ( n ). This guarantees - in compliance with certain other conditions and rules of the game, such as FIFO - largely stable order lead times. The lower the inventory level is (up to a practical lower limit) in the system, the shorter the cycle time of a job. Something idealized that is the transfer of the principles of flow production to the production workshops.

Function

Complex production processes can usually be broken down into manageable, semi-autonomous sections. Critical to the order cycle are production stages, which represent potential bottlenecks, especially those with varying bottleneck capacity (machines or workstations ). As a CONWIP loop designed, these situations can be mastered. CONWIP control loops can be arranged in parallel or sequentially coupled.

Combined with complementary methods, such as a scheduler for customer orders, the CONWIP control for manufacturing companies can with discrete manufacturing ideally suited to ensure a very high delivery service to customers.