introduction
With the rapid increase in cars, the problem of urban parking difficulties has continued to worsen. Automated multi-level parking systems, which are designed for high-rise and high-space applications, are gaining popularity due to their advantages such as small footprint, high parking capacity, flexible layout, high efficiency and low cost, high cost-effectiveness, and safety and reliability. Currently, the most common mechanical multi-level parking systems on the market include eight types: lift-and-slide, vertical circulation, multi-level circulation, horizontal circulation, planar movement, aisle stacking, vertical lift, and simple lift. Among these, the lift-and-slide type holds a dominant market share in the domestic parking garage market due to its simple structure, convenient operation, safety and reliability, and low cost.
Operating principle of lifting and traversing automated parking garage
Lift-and-slide mechanical parking garages utilize pallet movement to create vertical channels, enabling the lifting and retrieval of vehicles in multi-level parking spaces. Their parking space structure is a 2D matrix, which can be designed with multiple layers and columns. Due to limitations imposed by the chain-retracting device and vehicle entry/exit times, they are generally 2-4 stories (national regulations stipulate a maximum of 4 stories), with 2- and 3-story designs being the most common. The operating principle of this parking garage will be explained using a typical above-ground 3×3 lift-and-slide type as an example.
The structural characteristics of a multi-level parking garage are: the bottom level can only move horizontally, the top level can only move vertically, and the middle levels can both move horizontally and move vertically. Except for the top level, both the middle and bottom levels must have an empty parking space reserved for vehicle entry and exit. When a vehicle enters or exits a bottom-level parking space, it can do so directly without moving other pallets. When entering or exiting a middle or top-level parking space, it must first be checked whether the corresponding space below is empty. If it is not empty, it must be moved horizontally until the space below is empty before descending. After entering or exiting, it rises back to its original position. The general principle of its movement is: vertical movement resets, horizontal movement does not reset.
Determination of control system scheme
Lift-and-slide automated parking systems primarily accommodate cars, making them relatively expensive. Furthermore, the operation of these systems involves the safety of people and vehicles, thus requiring extremely high levels of safety and reliability from the equipment. PLCs, employing computer-based general-purpose automatic control devices, integrate microcomputer technology, automation technology, and communication technology. They offer high reliability, cost-effectiveness, compact design, good scalability, and ease of operation, making them suitable for frequent starts and harsh environments. Therefore, PLCs are typically used as the core of the electrical control system in automated parking system control systems.
As the underlying network layer, fieldbus technology, with its simple structure, demonstrates significant advantages in the design, installation, operation, and maintenance of control systems. Therefore, this paper adopts Profibus-FMS and Profibus-DP to construct a two-layer control network. Profibus-FMS mainly handles medium-speed cyclic and non-cyclic communication tasks, typically used for communication between PLCs and PCs, and between PLCs themselves. The underlying network is Profibus-DP, primarily because it is an optimized high-speed communication connection used for communication between device-level distributed I/O, forming a single-master system with the shortest overall cycle time. This system utilizes fieldbus technology to achieve open, digital, multi-node communication between field intelligent devices and automated control equipment, improving the reliability and flexibility of system operation.
Meanwhile, this system uses a host computer as the monitoring unit, utilizing its data communication capabilities, data processing capabilities, and graphic display and multimedia technology. Through the fieldbus, it receives and processes various status, control, and alarm signals collected from the field by the lower-level PLC. These signals are then used to drive various graphics on the PC control interface, displaying various on-site conditions in real time. This creates a visual and intuitive interface between the operator and the parking garage, providing prompts and alarms for operation and faults.
Design of automated parking system
Composition of the control system
The automated parking system control system consists of a host computer monitoring system and a slave PLC control system, as shown in Figure 1. The monitoring system comprises a host computer, a Profibus fieldbus, a PLC, and field operators, with a PC at its core, and is equipped with printers, audio equipment, a cash register, and monitors. If the parking garage consists of multiple 3x3 units, one PLC can control one unit, and multiple PLCs can form a multi-point local area network. If the parking garage is large enough, operators, touchscreens, and IC card readers can be added to achieve intelligent automatic control.
Figure 1 Control system structure diagram
The garage control system has three levels: manual, semi-automatic, and fully automatic. Manual control involves using a hand-operated device to jog each pallet, applicable in four situations: garage maintenance, sudden power outage, emergency shutdown, and garage malfunction. Semi-automatic control uses buttons on the PLC control panel for automatic logic control via the PLC. Fully automatic control involves the PLC executing tasks based on access commands issued by the computer (requiring an operator). Manual mode has the highest priority, while semi-automatic and fully automatic modes are used for normal vehicle entry and exit, with semi-automatic having higher priority than fully automatic. In offline mode, the PLC control panel can handle all vehicle access operations, and this design requires interlocking between manual, semi-automatic, and fully automatic modes.
PLC control system design
The PLC is the core of the parking garage control system. Its operation can be broadly divided into three categories: operations primarily for fault diagnosis and handling; data I/O operations related to on-site conditions; and operations executing user programs and responding to commands from external devices connected to the PLC. When a parking or retrieval operation occurs, the PLC receives and analyzes the instructions input by the operator via the control panel buttons or the host computer, making appropriate industrial control arrangements: determining the status of detection elements and reading information from the parking garage's mechanical drive system; then, feeding this information back to the actuators, dragging the parking space panels to move their positions, completing the vehicle parking or retrieval operation and displaying signals (indicator lights). The entire operating area is equipped with photoelectric detection and multiple safety systems to prevent abnormal situations from occurring.
In this system, the PLC mainly performs the detection of the position and running status of the pallets and trays, and the operation of storing and retrieving vehicles. Various photoelectric switches and limit switches are used to detect the position status, and contactors and relays are used to execute the start and stop control of the drive motor.
The operation of the parking spaces involves controlling the small lateral movement motor and the large lifting motor, enabling them to rotate in both directions at different times. Furthermore, the lifting action of the upper level and the lateral movement of the lower levels must be interlocked; that is, when an upper-level parking space is being lifted or lowered, lower-level parking spaces cannot move, and vice versa. Only one upper-level parking space can be lifted or lowered at a time.
To ensure reliable and safe vehicle storage and retrieval, the system requires precise positioning. Limit switches ensure the tray can move horizontally to the predetermined position and rise or fall to the accurate position; however, the limit switch logic must be strictly interlocked. For example, only one of the level 1 and 2 limit switches should be open in a static state; if more than one switch is closed, it indicates the tray is not in position. When the garage is stationary, all hook signals on levels 2 and 3 should be disconnected (negative logic), the level 2 upper limit switch should be open, and the level 3 upper limit switch should be closed.
In addition, to ensure the safety of the pallet during operation, the transmission system must be designed with a self-locking safety mechanism and a safety hook safety mechanism. For example, the chain drive uses a brake motor, which is in a self-protected state no matter what happens. The electromagnet that controls the movement of the safety hook must have a feedback signal to indicate whether the hook has properly hung the pallet.
Photoelectric switches have different functions depending on their location: photoelectric switches installed on the left and right sides of the bottom layer of the pallet can detect whether the car is parked properly on the pallet; photoelectric switches placed diagonally on the pallet can detect whether there is a car on the pallet; photoelectric switches installed on the left and right sides of the vehicle entrance of the parking garage can also be used to detect external malfunctions and abnormal situations when the parking space is moved, such as the vehicle not being parked properly, the presence of people or objects in the action area, or an unexpected situation where a car tries to drive in during operation. In such cases, the light of the photoelectric switch is blocked, which will send a level change signal to the PLC, thereby changing the PLC input, sounding an alarm, and the equipment not running or stopping.
The garage also utilizes various sensors, such as smoke and temperature sensors, displacement sensors that detect rope breakage, slack rope, or chain breakage alarms, as well as warning devices, emergency stop switches, manual buttons, and reset switches.
PLC control system programming
Control program flowchart
This system controls vehicle access only for upper-level (second, third, and fourth-level) parking spaces. For lower-level parking spaces, vehicles simply need to drive in and out. The control software is written in ladder logic. The program flowchart is shown in Figure 2.
Figure 2 Flowchart of the upper-level control program
The software employs a "parallel branching and merging" technique when designing the entry and exit procedures for different levels. Parallel branching refers to the simultaneous execution of each branch's process. After all processes are completed, the merged state actions are determined based on the corresponding execution conditions. For example, if the third-level pallet entry/exit is selected, the first and second levels can move simultaneously (left or right). This allows the control system to automatically handle interlocking or dual outputs between equipment actions, and greatly facilitates trial operation and fault diagnosis, saving significant time and improving work efficiency.
Control program optimization
Since the lifting and lowering of the upper pallet can only be carried out after the lower parking space is empty, taking the movement of three parking spaces on the ground as an example, there are N possible empty parking spaces on the first floor, N2 possible movement modes for the lifting and lowering of the second floor pallet, and N3 possible movement modes for the third floor pallet. As the number of parking spaces and floors increases, the program will expand dramatically. Therefore, finding a simple method to optimize the program will be the difficulty in designing the program for this system. Taking the second level as an example, the variable Dm stores the parking space number to be accessed on the second level, which ranges from 1 to N. For example, if parking space X (1 ≤ X ≤ N) on the upper level is accessed, then Dm = X. The variable Dn stores the empty parking space number on the lower level; let's say the empty parking space is Y, then Dn = Y. When accessing a parking space, the values in Dm and Dn are compared. If the result is zero, the tray for the upper level parking space can be moved down directly. If the result is greater than zero, it means the empty parking space is on the left. In this case, the first tray to the right of the empty parking space is moved to the left to fill the empty space. This process is repeated until the empty parking space is directly below the parking space to be accessed on the upper level, at which point the tray for the upper level parking space can be raised or lowered. The handling method for accessing parking spaces on the third and fourth levels is similar to that of the second level.
Modular programming
The PLC control program adopts a modular programming approach. During parking space operation, only subroutine modules need to be called, which greatly reduces the complexity of the program, facilitates program modification, and provides convenient conditions for the expansion of parking spaces. The entire program includes a main program module, a manual button subroutine module, an emergency stop button subroutine module, an initialization program module, a parking space number storage and retrieval assignment program module, an empty parking space number and moving parking space number assignment program module, a pallet translation movement program module, a photoelectric switch subroutine module, a pallet lifting movement program module, and a fault alarm subroutine module.
Handling key issues in software design
The state elements, timers, and data memory used in the program are all selected with power-off protection. When the system loses power, the elements retain their state before the power failure to save the field information and continue to complete the interrupted actions after power is restored. In case of an accident, pressing the emergency stop button will stop the system operation and save the field breakpoint information. When electrical or mechanical faults such as motor overload or overheating occur, the system operation will be automatically stopped and an audible and visual alarm will be issued. At the same time, the system will switch to manual mode for fault handling.
Conclusion
The control system of the lift-and-slide automated parking system utilizes PLC and Profibus fieldbus control, significantly improving the reliability of the entire system and meeting the control function and performance requirements of the parking garage. It fully realizes intelligent control of vehicle entry and exit. The system also employs manual, semi-automatic, and fully automatic multi-level redundant control methods in its hardware design, coupled with software/hardware interlocking protection, greatly enhancing system reliability. Furthermore, optimized PLC software design simplifies the expansion of parking spaces. In addition, the software design utilizes parallel branching and merging techniques, significantly shortening entry and exit times and improving work efficiency.
