Doppelspiel

What is dual cycling?

Dual cycling describes a control strategy in automated warehouse systems in which a storage and retrieval machine (SRM) combines two tasks within a single work cycle: it first stores a pallet or container at a target location and then, on the return journey – or in close proximity – retrieves a unit already reserved for dispatch. This combined cycle contrasts with single cycling, in which the machine returns to a defined home position after each storage or retrieval operation before starting the next order. Dual cycling is not a physical property of the machine, but the result of intelligent order planning by the warehouse management system or material flow controller.

How does dual cycling work in practice?

Effective dual cycling requires high-performance control software that combines storage and retrieval orders in such a way that the SRM travels the shortest possible total distance. To achieve this, the warehouse management system continuously analyzes the order pool and selects the order pairs whose target and source locations are spatially most favorable relative to each other. The optimization takes into account both horizontal and vertical travel time, since SRMs can generally travel simultaneously in the X and Z directions.

Typical systems that use dual cycling include:

• High-bay warehouses with rail-guided storage and retrieval machines
• Automated small-parts warehouses (AS/RS) with miniload systems
• Shuttle systems with multiple levels
• Pallet warehouses with single- or multiple-deep storage

It is important that the goods to be stored are already available at the transfer point before the SRM begins its journey. Otherwise, waiting times arise that negate the benefits of dual cycling. The synchronization between the inbound conveyor system and order assignment is therefore a critical factor in the efficiency actually achievable.

What matters when selecting and planning?

Whether dual cycling delivers a measurable advantage over single cycling depends heavily on the ratio of storage to retrieval volumes and on the warehouse structure. The potential can best be exploited when storage and retrieval volumes are balanced. If volumes are highly asymmetric, the combination rate falls and the control system more frequently reverts to single cycles.

Further planning considerations include:

• Slotting strategy: ABC-zoned warehouses facilitate order pair combination because frequently needed items are concentrated in a narrow rack area.
• Machine type: Single-mast SRMs and shuttle vehicles differ in their ability to transport multiple load units simultaneously, which influences the variants of dual cycling available.
• Software performance: The quality of the combination algorithm is a key factor in determining how close real-world throughput comes to the theoretical maximum. This aspect is vendor-dependent and should be specifically addressed in the requirements specification.
• Buffer capacities: Adequately dimensioned inbound and outbound buffers at the transfer stations are a prerequisite for the control system to plan a sufficient number of orders in advance.

In practice, dual cycling is now expected as standard in virtually all newly designed automated warehouses. However, the actual throughput improvement achievable over single cycling is system-specific and should be substantiated in simulation or in the functional specification using realistic order mix scenarios.