Automated Twin-Tower Sheet ASRS Storage Tower Explained

Automated storage and retrieval systems — ASRS — have moved well beyond the pallet-handling cranes that defined the technology thirty years ago. In modern sheet metal fabrication, an automated twin-tower sheet ASRS storage tower is a purpose-built vertical machine designed around one material: flat metal plate. It stores it, retrieves it, tracks it, and delivers it to a downstream cutting or forming cell without a forklift entering the storage volume. The twin-tower layout is the configuration that makes this throughput viable for multi-machine plants.

Herochu builds its twin-tower ASRS around a central elevator and shuttle that travels vertically between two parallel storage structures. The mechanism pulls a drawer-type cassette from a designated layer on either tower, lowers it to the discharge height, and presents it to an outbound manipulator or operator station. The cycle then reverses, returning the cassette — and any unused plate or offcut — to its assigned storage level. The entire sequence runs under PLC control from a touchscreen interface.

The Retrieval Cycle

Understanding the value of an ASRS storage tower means understanding the retrieval cycle. A call comes in — typically from the production scheduling system or an operator entry at the touchscreen — for a specific material, thickness, and quantity. The PLC identifies the storage layer assigned to that material. The central elevator, driven by a heavy-duty chain and linear guide rail assembly, travels to that layer height. The shuttle mechanism engages the cassette on whichever tower holds the target material, extracts it, and lowers it to the discharge position.

Transfer speed across the vertical travel ranges from 5 to 15 meters per minute, depending on the model and load. Repeatability of positioning holds at ±2 mm — the CNC system uses laser closed-loop position detection to measure and compensate for thermal expansion in the lifting chain and mechanical elongation under load. Without that compensation, a system running through daily temperature swings in an unclimate-controlled plant would drift out of position within weeks. The laser feedback loop keeps the cassette engagement accurate over the long term, which is what allows the drawer rails to align with the storage slots cycle after cycle without mechanical interference.

This is the engineering distinction between an ASRS and a motorized rack. A motorized rack moves material. An ASRS knows where the material is, positions the extraction mechanism to within two millimeters, and repeats that accuracy indefinitely. The repeatability spec is not a marketing figure — it is the tolerance that keeps the drawer rails from crashing into the rack frame, and it is the reason the system can run unattended through overnight shifts.

PLC Touchscreen Control

The operator interface is a PLC touchscreen mounted at the discharge station. From it, the operator selects a material, requests a quantity, monitors the active retrieval cycle, and reviews the real-time inventory state of both towers. The interface shows material dimensions, weight, and layer assignment — not a static label, but a live readout that updates as material is withdrawn and as offcuts are returned.

This is where automated operation meets inventory discipline. A manual rack requires an operator to remember where a specific plate grade was stored, walk to it, extract it, and then record the withdrawal somewhere — assuming the record gets made at all. The ASRS storage tower eliminates the walk and eliminates the reliance on memory. The layer assignment is in the control system. Retrieval is one touch. Inventory updates automatically.

Drawer Cassettes and Material Protection

The storage medium inside each tower is the drawer-type cassette. These are heavy-duty removable trays that slide on guided rails into designated rack slots. A steel plate, whether it is 3 mm stainless for a food-grade enclosure or 20 mm carbon plate for a structural fabrication, sits on the cassette — not on the rack frame, not on another plate. The cassette is the interface between the material and the storage structure.

This matters for material protection. Manual forklift extraction from a conventional rack stack means forks sliding between plates, edges contacting steel, and surfaces picking up scratches and dings. In a twin-tower ASRS, the cassette extracts horizontally on guided rails with the plate seated on its support surface. No forks. No edge contact. No sliding friction between metal surfaces. For fabricators shipping cosmetic stainless or painted aluminum downstream, this is the difference between a clean blank and a rejected blank.

Models and Capacity Range

Herochu’s ASRS tower lineup scales across four standard configurations. The HC-S3015-2-63 handles 3000×1500 mm sheets across six layers at 3000 kg per level. The HC-S4015-2-83 moves to 4000×1500 mm across eight layers. The HC-S4020-2-105 covers 4000×2000 mm across ten layers at 5000 kg per level. The HC-S6020-2-155 is the largest standard configuration — 6000×2000 mm plates across fifteen layers, each tier rated at 5000 kg. Clear space between layers is 240 mm standard, customizable for thicker plate inventories.

The two-tower count is standard; the layer count is customizable. A plant with heavy plate inventory but lower SKU variety may opt for fewer, higher-capacity layers. A plant running many grades and thicknesses needs more layers for finer segregation. The ASRS architecture accommodates both because the layer count and cassette specification scale with the inventory profile, not the other way around.

Closed-Loop Positioning Technology

The laser closed-loop positioning system deserves attention because it is the component that makes long-term unattended operation viable. Chain-driven elevators elongate under load and expand with heat. Over a 15-meter vertical travel, the cumulative elongation across a long retrieval cycle can exceed the engagement tolerance of the drawer rails. Without compensation, the system would mis-engage and stall.

Herochu’s CNC control measures actual position via laser feedback and commands the elevator to the compensated target height on every cycle. Thermal expansion, chain wear, and load-induced stretch are all corrected in real time. The ±2 mm repeatability figure holds over the service life of the system, not just on the day of commissioning. For a plant running the ASRS through three shifts, that consistency is what keeps the retrieval cycle time predictable and the throughput stable.

Connecting the Tower to Production

An ASRS storage tower that feeds a laser cutting cell does its best work when it is not isolated from that cell. Herochu’s systems integrate with overhead gantry truss manipulators that pick raw sheet or plate from the discharge position and transfer it directly to a laser cutting bed. A single central manipulator can serve both towers and multiple downstream machines, which is the architectural reason the twin-tower layout out-produces a single-tower system in a multi-machine cell.

ERP and MES connectivity completes the integration. The tower reports stock levels, material dimensions, and retrieval counts to the factory inventory system. Offcuts returned to storage are logged automatically — no manual data entry, no spreadsheet reconciliation. When the scheduling system needs to know whether a specific grade and thickness is available for an upcoming job, the answer comes from the ASRS control, not from a walk to the rack.