Abstract: This paper details the design scheme of automated storage and retrieval systems (AS/RS) commonly used in modern large enterprises. Starting with the characteristics and structure of stacker cranes, the stress on the uprights is analyzed, and the relationship between bending moment and deflection is derived. Design data is then provided for verification, with a focus on the structural design of the fork extension mechanism. A design scheme for a stacker crane safety mechanism that is compact, highly sensitive, reliable, and equipped with a flexible device is presented. This design achieves the desired results by selecting a suitable PLC and designing, programming, running, and debugging a corresponding control system.
I. Introduction
Automated Storage and Retrieval Systems (AS/RS) are an upgraded version of high-rise rack warehouses. They are the core of modern logistics management technology, primarily composed of high-rise racks, aisle stacker cranes, communication systems, fieldbus systems, electrical control systems, computer management and monitoring systems, handling trolleys, and pallets. Employing technologies such as variable frequency speed control, inbound and outbound conveyor information systems, 3D barcode recognition, infrared communication, carrier communication, radio frequency digital communication, photoelectric detection, and human-machine interfaces, they achieve manual or automatic control, centralized computer monitoring and control, and can be networked with a higher-level computer to form an internal local area network, thus realizing a fully automated warehouse logistics management system integrating management, monitoring, and execution.
An automated storage and retrieval system (AS/RS) consists of the following parts:
High-rise racking: A steel frame structure used for storing goods. Currently, there are mainly two types: welded racking and modular racking.
Pallets (cargo boxes): Tools used to carry goods, also known as workstation equipment.
Stacker cranes: Equipment used for automated storage and retrieval of goods. They are classified into two types based on their structure: single-column and double-column; and into three types based on their service mode: straight-track, curved-track, and transfer vehicle.
Conveyor system: The main peripheral equipment of an automated warehouse, responsible for transporting goods to or from stacker cranes. There are many types of conveyors, the most common being roller conveyors, chain conveyors, lifting platforms, distribution carts, hoists, belt conveyors, etc.
AGV system: also known as automated guided vehicle. Based on its guiding method, it can be divided into sensor-guided AGVs and laser-guided AGVs.
Automatic control system: The automatic control system that drives all equipment in the automated storage and retrieval system. Currently, it mainly uses fieldbus technology.
Inventory Information Management System: Also known as Central Computer Management System. It is the control core of a fully automated storage and retrieval system (AS/RS). Currently, typical AS/RS systems use large database systems (such as Oracle, Sybase, etc.) to build a typical client/server architecture, which can be networked or integrated with other systems (such as ERP systems).
The advantages of automated storage and retrieval systems (AS/RS) are multifaceted, mainly including the following aspects.
(1) Three-dimensional design improves space utilization
The basic starting point of the early concept of automated warehouses was to improve space utilization, and saving land has been linked to energy conservation, environmental protection, and many other aspects. The space utilization rate of a warehouse is closely related to its planning; generally speaking, the space utilization rate of an automated warehouse is 2 to 5 times that of a conventional warehouse.
(2) Advanced warehousing systems improve enterprise production management.
Traditional warehouses are merely storage locations for goods, serving only the function of preserving goods and representing static storage. Automated storage and retrieval systems (AS/RS), on the other hand, utilize advanced automated goods handling equipment. This not only enables automatic storage and retrieval of goods within the warehouse as needed but also allows for seamless integration with external production processes. Through advanced computer management systems and automated material handling equipment, the warehouse becomes a crucial component of enterprise production management.
(3) Accelerate the storage and retrieval of goods, reduce labor input, and improve productivity.
The advantages of establishing a logistics system centered on automated storage and retrieval systems (AS/RS) are also reflected in the AS/RS's rapid inbound and outbound capabilities, enabling the proper storage of goods and the timely and automated delivery of necessary parts and raw materials to the production line. At the same time, the AS/RS system reduces the overall labor input required by workers.
(4) Effectively reduce the accumulation of inventory funds.
To meet expected production capacity and requirements, sufficient raw materials and components must be prepared, leading to significant inventory backlogs. How to reduce inventory capital tied up while fully meeting production needs has become a crucial issue for large enterprises. Automated storage and retrieval systems (AS/RS) can effectively solve this problem.
II. Hardware Structure Design
The stacker crane is the core of the hardware structure. During the design process, it is necessary to conduct stress analysis for strength and stiffness. One of the most important aspects is that its metal frame structure bears all forces, inertial forces, and loads during both static and operational periods, making it the component that deforms the most. As the physical structure supporting the stacker crane's operation, its design quality is crucial in determining the stacker crane's service life and operational stability.
2.1 Rack Empirical Formula Calculation
2.1.1 Frame Structure Analysis
(1) External load calculation
Establish a three-dimensional coordinate system in the tunnel:
X-axis: The forks of the loading platform perform loading and unloading operations on goods.
Y-axis: The direction of travel of the stacker crane along the tunnel.
Z-axis: The loading platform on the column moves up and down along the guide rail.
When the load position is the highest, the column is in the most unfavorable stress condition. The position of the force and load of each part of the structure at this time is shown in Figure 1.
Figure 1. Simplified diagram of force analysis on the loading platform
(2) Along the longitudinal plane of the tunnel (i.e., the YOZ plane)
The column experiences the most unfavorable stress when the loading platform is fully loaded, at its highest position, and starts or brakes with maximum acceleration.
Figure 2. Simplified diagram of the forces acting on the structure in the YOZ plane.
In the diagram: H — Total height of the stacker crane
B – Total width of the stacker crane
P H — Horizontal inertial force
h — Distance between the upper roller and the top of the column
b1 and b2 — Distances from the neutral axis of the column section in the X direction to the two supports of the lower crossbeam.
F – Pressure at the top of the column
Calculation of lateral moment of column:
2.1.2 Structural strength calculation
For compression-bending members where the maximum internal force is at the support, the ultimate strength is reached when a plastic hinge appears in the cross-section. The utilization of material plasticity should be subject to certain limitations. These limitations are addressed by introducing a plasticity development coefficient into structural strength calculations, i.e., using the elastoplastic working stage where plasticity penetrates locally into the cross-section as a design criterion.
(1) Strength calculation formula
In steel structure design, the limit state method is applied to derive the strength calculation formula for biaxial compression-bending members:
Where: N — Axial pressure (N)
A – Cross-sectional area ( cm² )
M <sub>x </sub> and My<sub> y </sub> — Maximum bending moments (N*mm) along the X and Y axes, respectively.
M <sub>nx</sub> and M<sub> ny </sub> — Net section modulus of bending along the X and Y axes
U <sub>x </sub> and U<sub> y </sub> — Plastic development coefficients in the X and Y directions (they are related to the cross-sectional shape, depth of plastic development, ratio of flange to web cross-sectional area, and stress state).
2.2 Calculation of the strength of the lower crossbeam
The lower crossbeam in the YOZ plane can be simplified as shown in the calculation diagram.
Figure 3. Simplified calculation diagram of the lower beam of the shelf.
Where q is the uniformly distributed self-weight load of the lower crossbeam.
M_max — The maximum bending moment at the bottom section of the column, M_max = 705 N*mm
P <sub>total</sub> — the sum of rated lifting capacity and the self-weight of the column structure.
Clearly, the section at the connection between the lower beam and the column is the critical section. Therefore:
In the formula, Mmax is the bending moment at the critical section, and Wyx is the bending modulus of the lower crossbeam in the X-axis direction. Therefore, the allowable stress requirement is met.
2.3 Strength and Verification of Rack and Gear in Traveling Mechanism
Gear drives are the most commonly used in hoisting mechanisms, worm gear drives are the least used, and chain drives are only used in specific cases.
Gear drives are divided into closed-loop transmission and open-loop transmission. All mechanisms of electric cranes use closed-loop gear drives (reducers). Open-loop gear drives are only used in certain special cases (such as the arrangement of slewing mechanisms, which requires the final stage transmission to be an open-loop gear drive; or when no suitable reducer is available for selection during mechanism design). When used, the circumferential speed of the gears generally does not exceed 1.5 m/s.
Gear materials are primarily steel, followed by cast iron, copper alloys, and other materials. The choice of gear material and heat treatment method depends on the working level, working conditions, and manufacturing process possibilities of the mechanism. To improve the load-bearing capacity of gears and reduce their size and weight, high-quality carbon steel or alloy steel is widely used in gear manufacturing. To reduce costs, gears and shafts are preferably made of different materials, with the gear assembled on the shaft. To ensure the strength of the tooth surface and teeth, the tooth surface should have sufficient hardness, while the core should have sufficient rigidity and toughness; therefore, gears must undergo heat treatment.
III. Hardware Selection and Analysis
Choosing the right stacker crane motor directly affects its operation. Stacker cranes use motors for travel, forklifts, and lifting. If the lifting motor is too powerful, it will cause vibration and excessive noise throughout the system, including the lifting platform; conversely, if the lifting motor is too small, its lifting capacity will be insufficient to achieve the necessary lifting movements. Therefore, the capacity of the stacker crane motor should be selected appropriately.
The following issues should be considered when selecting the motor for a stacker crane.
(1) It should have sufficient durability in both electrical and mechanical aspects for repeated start-stop operations.
(2) The speed should be well controllable.
(3) The moment of inertia should be relatively small, the volume should be small and the weight should be light.
(4) The speed of the output shaft should be compatible with the structure of the speed reduction device.
(5) It should have a reliable outer shell protection structure.
With very few special cases, stacker cranes mostly use common AC power, and the most commonly used motors are three-phase squirrel-cage induction motors, which are robust in structure and inexpensive.
3.1 Selection of Stepper Motor Driver
The frequency of the pulse signal from the stepper motor driver is directly proportional to the speed of the stepper motor. Its output signal has the following two types:
(1) Initial phase signal: Each time the driver is powered on, the stepper motor will start at a fixed phase, which is called the initial phase. The initial phase signal is output as a high level whenever the stepper motor reaches the initial phase, and as a low level otherwise. This signal, when used in conjunction with the control system, also has a phase memory function.
(2) Alarm Output Signal: All drives have multiple protection measures (e.g., over-temperature, over-voltage, over-current, etc.). When a protection measure is activated, the drive enters an offline state, causing the motor to lose power, but the control system is unaware of this. To notify the system, an "alarm output signal" is used. This signal occupies two terminals; these two ends are normally open contacts of a relay, which close immediately upon alarm. Under normal drive conditions, the contacts are normally open. Contact specifications: DC24V/1A or AC110V/0.3A
For a two-phase, four-wire motor, it can generally be directly connected to a driver.
Therefore, this design uses a stepper motor driver of model SH-2H057. The SH-2H057 stepper motor driver uses a cast aluminum structure and is commonly used in low-power devices; its housing also serves as a heat sink. Therefore, it must be fixed on a thick and large metal plate or in a thick cabinet, and thermal grease must be applied between the contact surfaces. A fan should also be added nearby for good heat dissipation.
IV. Software Programming
4.1 PLC Control Flowchart
Based on the system control requirements, a basic flowchart can be summarized, as shown in Figure 4.
Figure 4 System Flowchart
4.2 Automated Warehouse PLC Master Program
Figure 5. Overall Program Diagram of Automated Warehouse PLC
V. Summary
This paper first introduces the history and development of automated storage and retrieval systems (AS/RS), and then describes the design of both hardware and software aspects.
On the hardware side, the main design included the stacker crane's electrical control system and the overall warehouse structure, such as the warehouse racking. The electrical control system primarily utilizes laser rangefinders for horizontal addressing, enhancing the stacker crane's accuracy and the reliability of the operating system, laying the foundation for future optimization. Closed-loop control methods were also employed in both horizontal and vertical directions, achieving accurate low-speed stopping, smooth speed changes, and the ability to operate at high speeds.
In summary, the system has excellent hardware performance, a simple structure, a user-friendly graphical interface, and powerful software functions, basically meeting the requirements of this design.
