Zone-to-zone picking systems (Riegler, AKF, Winkler ...) frequently offer the possibility building a circular flow or loop. The purpose of the loop is the fault tolerance, i.e. to eliminate problems by shifting a request temporally. Jams in the single zones are the main problem in loops: feeding out containers or boxes is not possible. A loop offers for containers or cardboard boxes the possibility to start one or several attempts. As a rule, the order totes will approach further zones to process the stock removals on site and then attach the overloaded zone to the end of the transport route. Thanks to predefined order picking sequences other concepts can be selected as well.
Another application of loop controls is the waiting loop for feeding out containers. If no automatic consolidation buffer is integrated, orders consisting of several totes cannot be collected until the last tote has been completed to be then transported together to the shipping department. A loop offers the possibility to keep already completed totes waiting to ensure feeding out all totes upon completion in a timely manner. The negative aspect of loops is the capacity consumption of the loop - a circulating tote results in an immense performance loss which even the best plant control system cannot compensate. Simple calculation example: a loop with a length of 100 m, a speed of 0.5 m/s, a segment length of 700 mm and a tote length of 600 mm. To ensure an unhindered transport one segment is left empty between two totes. Without taking into account corner transfer units etc. (which are very important in practice as they are slower in most cases) this results in a theoretical throughput of 1278 totes per hour (the system can accommodate 71 totes; 18 circulations). Each circulation of a tote moving around costs a non-circulation-tote, i.e. a tote permanently going in circles costs 18 productive totes of throughput; its costs are 18 times as high as without circulation. In practice, the situation is even worse as even the theoretical values are lower by around 25 %. Thus, avoiding such totes is high up on the "to do" list at the order start which considers target capacities and controls dispatching accordingly. Hence, a loop as buffer option shall be avoided as far as possible.
