Dynamic storage systems "goods-to-person" principle (WzP)

Definition: What is the "goods-to-person" principle (WzP)?

The "goods-to-person" (WzP) principle, often referred to as "goods-to-person" (GTP), is a method of picking that reverses the traditional process. Instead of a warehouse employee (picker) walking along aisles to remove items from shelves (as in the "person-to-goods" principle, PzW), the required goods – typically in containers, cartons or on trays – are transported directly to a stationary or semi-stationary picking workstation by automated storage and conveyor technology.

The employee remains at this ergonomically optimized workstation, removes the items presented by the system and assembles them for the customer order. This entire process is controlled by a higher-level warehouse management system (WMS), which sequences the orders and coordinates the means of transport. The core goal is to drastically reduce unproductive travel times and increase picking performance.

Core components and technologies of WzP systems

WzP is not a single product, but a system that consists of several core technologies. The most common forms of realization are:

• Shuttle systems (AS/RS): These are automated small parts warehouses (AS/RS). Stacker cranes (SRMs) or, in the case of modern systems, more flexible shuttles (autonomous vehicles that move on rails in the rack levels) store and retrieve containers. They transport the containers to the pick stations via a connected conveyor system (lifter, vertical conveyor, roadway conveyor).
• Cubic storage systems (e.g. AutoStore): These systems offer the highest storage density. They consist of a self-supporting aluminium grid in which bins are stacked on top of each other. On the top of the grid, robots navigate to lift the bins, rearrange them and bring them to workstations (ports) located at the edges (or within) the grid.
• Vertical lift systems (VLM): Known as "storage lifts" (e.g. from Kardex, Hänel). They resemble an oversized drawer cabinet. Inside, an extractor moves vertically between two shelf shafts and retrieves the desired tray, which is then provided in an ergonomic opening (operating opening). They are ideal for small parts with very small space requirements (floor space).
• Carousel storage (paternoster): An older but proven WzP technology in which shelves rotate (horizontally or vertically) like a Ferris wheel and bring the goods to the retrieval opening.

The picking workstation is a crucial component. It is often equipped with pick-by-light, put-to-light, scanners, scales and screens to guide the employee through the picking and distribution process without errors and quickly.

WzP in Warehouse Logistics: Performance and Processes

From the perspective of pure warehouse logistics, the focus is on key performance indicators (KPIs) and process optimization. This is where WzP systems come into their own:

1. Throughput and picking performance: While picking capacities of 100 to 150 items per hour and employee are common in manual PzW picking, WzP workstations enable outputs of 400 to over 1,000 picks per hour and station. This results from the elimination of walking distances (often 60-70% of the working time at PzW).
2. Error reduction: The system-guided provision and verification at the workplace (e.g. scanning, light display) reduces the error rate (incorrect sampling) drastically, often to less than 0.1%. This significantly reduces the costs of returns and post-processing.
3. Ergonomics and employee deployment: The workstations are ergonomically designed (e.g. height-adjustable). Bending, stretching and lifting heavy loads is minimized. This reduces the physical strain and thus the disease rate. In times of a shortage of skilled workers, this is a decisive factor in retaining staff and deploying them efficiently.
4. Warehouse compaction: WzP systems, especially cubic warehouses and VLMs, make optimal use of the room height and do not require wide aisles for employees or industrial trucks. The storage density (SKUs per cubic meter) is second to none.

Q&A: Frequently Asked Questions about Dynamic WzP Systems

Question: Are WzP systems suitable for every product range (SKU)?

Answer: No, not unreservedly. WzP systems are optimized for small to medium-sized items that can be stored in standardized containers or on trays (typical AS/RS spectrum). They are ideal for assortments with medium to high access frequency (B and A turners). They are unsuitable for pallet goods or very bulky goods (bulky goods). Slow-moving items (C-parts) can be stored, but may block valuable, highly automated storage space.

Question: How flexible is a WzP system during order peaks (seasonality)?

Answer: Scalability varies greatly by technology. In the case of shuttle or robot-based systems (such as AutoStore or Exotec Skypod), the performance (throughput) can often be increased relatively easily by adding more vehicles (robots/shuttles). Expanding the storage capacity (volume) usually requires a major structural intervention. VLMs are hardly scalable in terms of performance (only by adding more devices) and in volume only by adding them.

Question: What happens in the event of a technical failure of a WzP system?

Answer: This is the "Achilles' heel" of high automation. While in decentralized systems (e.g. AutoStore) the failure of one robot is hardly significant (others take over), the failure of a central component (e.g. main lifter in a shuttle system) leads to a standstill in this area. Redundancy (e.g. double conveyor technology) is an important but expensive planning criterion. Manual access to the goods in the event of a malfunction is often impossible or extremely time-consuming in high-density systems.

The Role of WzP in Contract Logistics (3PL)

For contract logistics service providers (3PL), choosing a WzP system is a strategic balancing act.

• Investment (CapEx): WzP systems mean a high initial investment (CapEx). 3PL contracts often only run for 3-5 years. The return on investment (ROI) must pay off within this time or the system must be usable for a follow-up customer (or customers).
• Multi-client capability: The WMS that controls the WzP system must be multi-client capable. It must be able to physically manage and prioritize the inventories of different customers (clients) separately in the same automated warehouse.
• Flexibility vs. standardization: The 3PL customer often expects flexibility when changing the product range. However, a WzP system enforces a high level of standardization (container sizes, article dimensions). This can be a potential for conflict.
• Competitive advantage: On the other hand, WzP enables the service provider to guarantee service level agreements (SLAs) for e.g. same-day shipping or extremely high delivery quality, which are hardly achievable manually. This is increasingly demanded by e-commerce customers.

Models such as "Robotics as a Service" (RaaS), in which automation is rented instead of purchased, are gaining traction here to reduce CapEx risk for 3PLs.

Requirements for the logistics property (hall)

The use of WzP systems places specific, often high demands on the logistics property:

1. Soil conditions: This is the most critical point. Automation systems, especially high-bay warehouses (AS/RS) or systems with fast shuttles/robots, require an extremely high level of flatness of the floor (often according to DIN 18202 Table 3, line 4, or stricter). Deviations lead to vibrations, increased wear and tear and malfunctions.
2. Floor load: WzP systems are heavy. A distinction must be made between the distributed surface load and the extremely high point loads (e.g. under the supports of the racking systems or the conveyor technology). A standard hall with a surface load of 5 t/m² is often not sufficient or needs to be reinforced in a targeted manner.
3. Clear height (UKB): The usable hall height (lower edge of the truss) defines efficiency. Vertical lift systems or shuttle systems benefit from heights above 12 meters to make the most of the floor space. Cubic systems (e.g. AutoStore) are more flexible and can also be efficient in lower halls (from 6m) as they fill the height "from below".
4. Fire protection: High-density bearings (especially plastic container warehouses) pose a high risk of fire. Standard ESFR sprinkler systems are often not enough. In-rack sprinklers or, for ultimate protection (and to prevent water damage to the goods), oxygen reduction systems (inerting) are often required. These are expensive and TGA-intensive.
5. Technical building equipment (TGA): A stable and redundant power supply (UPS), high-performance IT cabling (network ports for workstations, WLAN for robots) and adequate lighting are basic requirements.

Conclusion: When is the use of WzP worthwhile?

The implementation of a "goods-to-person" system is a strategic long-term decision with a high investment requirement. The use is primarily worthwhile if several of the following criteria are met:

• High personnel costs or acute shortage of skilled workers in the warehouse.
• High to very high picking frequencies (e.g. in e-commerce, retail fulfillment, spare parts).
• A small to medium-sized, container-ready product range.
• Strict requirements for delivery quality and error prevention (e.g. pharmaceuticals, electronics).
• Limited warehouse footprint that requires high storage density and space utilization.

WzP systems transform the warehouse from a pure cost factor to a high-performance fulfilment center that takes efficiency, quality and ergonomics to a new level.