Movement strategies are used in warehouse operations to comply with specified guidelines and at the same time achieve the best possible throughput performance. The respective movement strategy determines the arrangement in which storage and retrieval machines are used for storing, transferring or removing material.
Important: Movement strategies are not always compatible with possible allocation strategies.
Movement strategies include:
- Double play/double play strategy
- Single play
- Relocation strategy
- Aisle change strategy
- Feed strategy
- Remove strategy
Movement strategies: Double play/double play strategy
Double play or the double play strategy is a movement strategy within warehouse operations in which putting and picking are combined, thereby performing put and pick cycles.
Double play process
The double play strategy can be used for both individual orders and larger numbers of orders. For individual orders, the storage location close to the retrieval location is selected. By selecting the optimal storage location, the storage and retrieval machine can move on to retrieval immediately after the material has been stored, and vice versa. For collective orders, orders whose storage and retrieval locations are close to each other are combined.
An optimized double play strategy is, for example, storage in the forward travel area of the retrieval location or retrieval in the return travel area of the storage location. In the best case, this movement strategy saves the average travel time between the storage and retrieval points.
Advantages and disadvantages of double play
Advantage of double play is the travel route optimization of the operating device. The combined storage and retrieval increases efficiency throughout the entire value-added process of warehouse logistics. The strategic movement sequences ensure that the storage and retrieval machine avoids empty runs, repeatedly stores and retrieves items on its way, and improves throughput by up to 10%.
The disadvantage is that individual movement sequences take slightly longer and must be carefully calculated in advance.
Movement strategies: Single play / single play strategy
The single game strategy (single game) is a movement strategy in which only one item is stored or retrieved. Storage and retrieval do not take place simultaneously or in combination.
Single game strategy process
In the single game strategy, the storage and retrieval machine picks up a storage unit and moves it to the designated storage location. The storage and retrieval machine then returns to transport the next storage unit. Single games often begin and end at designated buffer locations. Depending on whether goods receipt or goods issue has priority, either putaways or picks take place in single games.
Calculation of the single-game strategy
The time required for the single game provides information about the handling performance of an automated warehouse, such as a high-bay warehouse. The single game time is calculated from the travel time of the storage and retrieval machine from the origin to the storage location, the travel time to pick up the storage unit at the storage location, and the travel time to deliver the container to the storage location. The travel time of the rack-boarding device from the point of origin to the storage location is multiplied by two, as this travel distance is included twice in the single cycle. Once when transporting the unit to the storage location and once when returning from the storage location to the point of origin.
The advantage of the single cycle is the increased performance in storage and retrieval.
The disadvantage is that single trips involve longer empty runs for the storage equipment.
Movement strategies: Relocation strategy, relocation
The relocation strategy (relocation) is another movement strategy within warehouse operations. Such a goods movement is usually triggered by a pending removal of hidden units.
The following characteristics of the storage equipment used are important for the transfer performance of a warehouse system:
- Design
- Driving behavior
- Aisle dependency
- Load handling equipment
- Capacity
The number of storage devices used also plays a role.
The more a multi-location warehouse fills up, the more storage channels are double occupied. This in turn increases the likelihood of necessary transfers.
There are four strategies possible:
- Random transfer
- Transfer near removal
- Combined transfer and removal
- Relocation to the next storage compartment
In random relocation, the location to which the goods are to be relocated is selected at random. However, this strategy usually takes more throughput time than the other relocation strategies. The random movement of inventory is therefore rarely used in practice.
Relocation near removal is generally used frequently. The load unit to be transferred should be stored as close as possible to the removal channel.
A combined transfer and removal is used when double-width load handling devices are available. The load unit to be transferred is picked up by the warehouse equipment and transported to the location where the load unit to be removed is to be picked up. The load unit that is picked up for removal blocks one side of the storage device. Therefore, the load unit to be relocated is stored opposite the unit to be removed.
Relocation to the next storage bin can be used when multiple storage devices are in operation. Free storage devices are used as temporary buffers and transport the units to be relocated to the next storage bin.
Movement strategies: Aisle change strategy
In the aisle change strategy, putaway and removal orders are collected and only executed after a certain period of time.
Aisle change strategy process
To fulfill a picking order, inventory must usually be picked from different storage bins, and the storage bins to be accessed for an order may be located in different aisles. The aisle change takes place when all positions for a picking order have been reached in an aisle. The warehouse equipment then moves to the next aisle where articles must be picked. This may be because the previous order has not yet been completed or because articles are being collected for the next order.
The aisle change strategy is primarily aimed at route-optimized picking. Since aisle changes take a relatively long time, the storage and retrieval performance is significantly reduced when the aisle change frequency is high.
The aisle change frequency is calculated using the formula vGW=1/h, where h stands for the maximum storage and retrieval time, i.e., the specified cycle time.
Automation
In order to operate a warehouse automatically, it is necessary for warehouse equipment to follow a precise track when changing aisles. This track guidance can be mechanical, optical, acoustic or electronic, among other things. The aisle dependency of the warehouse equipment is more or less dependent on the driving behavior, the type of track guidance and the design of the equipment. One of the following warehouse equipment is usually required for aisle changes:
- Aisle-changing storage equipment
- Aisle-independent storage equipment
Aisle-changing storage equipment includes, for example, rack feeders with transfer devices, while aisle-independent storage equipment includes lift trucks and forklift trucks.
For a certain cycle, all storage and retrieval orders are initially stored without being processed further. The orders are then sorted according to specific storage aisles. The storage equipment used in the aisles then executes the orders at regular intervals, i.e., after a specified cycle time.
Advantages of the aisle change strategy
Advantage of the aisle change strategy is continuous adaptation to the environment. Long storage and retrieval times, which would have a negative impact on throughput, should be avoided. The aisle change of the storage equipment aims to minimize performance loss.
Movement strategies: Feed and remove strategy
The feed and remove strategy is a movement strategy in intralogistics.
Feed strategy
The feed strategy can be implemented by using a so-called feed conveyor. The strategy can be implemented in two different ways:
- The loading units are assigned to the individual storage aisles according to specific cycles.
- The loading units are assigned in batches to the aisles between the racks where the feed conveyor has the lowest occupancy.
Discharge strategy
The discharge strategy uses a so-called discharge shuttle. Loading units are routed to the discharge line via this discharge shuttle. In accordance with the removal strategy, the units that are needed most urgently are given priority and have right of way.
Summary
Movement strategies are used within warehouse operations. Depending on which movement strategies are used in the warehouse and ultimately carried out by the warehouse equipment, the arrangement of the storage and retrieval machines for storing, transferring or removing material differs. The aim is to achieve the highest possible performance in storage, retrieval, and throughput while complying with specified guidelines. Movement strategies and allocation strategies cannot be combined arbitrarily.
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