Abstract: This paper takes the 2×500KN bidirectional gantry crane of the Nam Ou River I Hydropower Station as the research object, introduces the characteristics of the electrical system of the bidirectional gantry crane, describes the design of each component of the control system, elaborates on the selection of the system PLC controller and frequency converter , and gives the debugging parameters of the hoisting mechanism based on the frequency converter main circuit diagram. Finally, the system software logic design flow is listed.
Keywords: Gantry crane; Control system; Software design; Hoisting mechanism; Debugging parameters;
1 Introduction
Our company independently developed and designed a 2×500kN bidirectional gantry crane for the second phase of the Nam Ou River hydropower project. Installed at the intake of the Nam Ou River I hydropower station, the 2×500kN hoisting mechanism operates with dual lifting points, opening and closing the intake gate of the power generation system via a hydraulic grab beam. The bidirectional gantry crane also serves as the installation and hoisting mechanism for water turbines and electrical equipment. The crane's electrical control system adopts a "touchscreen + PLC + frequency converter + lifting machinery safety monitoring and management system" control scheme, offering the following advantages:
① The hoisting mechanism adopts a double-point lifting mechanism, consisting of two sets of drive devices and two sets of zigzag drum devices, to achieve multi-layer winding of the hoisting mechanism;
② Each transmission mechanism adopts ABB ACS880 series variable frequency speed control to ensure smooth starting and braking of each mechanism and accurate positioning; the hoisting mechanism adopts the D2D function of ABB series frequency converter to ensure the speed synchronization and torque consistency of the two hoisting mechanisms.
③ The whole machine adopts advanced touch screen monitoring technology, which enables the whole machine to have good human-computer interaction and high-quality information processing functions;
④ The whole machine uses Siemens PLC as the control center. The system collects various signals through PLC and uses the virtual contacts inside PLC to realize the timing logic control of the whole machine, reducing hardware failures of electrical system and improving the reliability of system control.
⑤ The machine has complete safety protection functions. In addition to the conventional protections required by the electrical control system, such as short circuit protection, overcurrent protection, undervoltage protection, phase loss protection, grounding protection, zero position protection, limit protection, overload protection, main disconnect switch, and emergency switch to disconnect the main power supply, this machine also has load protection, lifting height limit and overspeed protection.
⑥ The entire machine is equipped with a crane safety monitoring and management system, which can display, manage and record various data of the gantry crane in real time, greatly improving the operational safety of the equipment;
⑦ The gantry crane is equipped with a video monitoring system, allowing the operator to clearly view the real-time status of the engagement and disengagement pins in all operating areas of the two-way gantry crane on the water and at the underwater grab beam in the driver's cab.
2. Electrical System Design
The electrical system of this gantry crane mainly includes a power supply system, a hoisting mechanism control system, a trolley traveling mechanism control system, a gantry traveling mechanism control system, a hydraulic grab beam control system, a crane safety management system, a safety protection system, and a lighting and grounding lightning protection system.
2.1 Power Supply System
The machine is powered by a hysteresis cable reel, with a three-phase four-wire AC380V, 50Hz power supply. The control power supply is AC220V, provided by a dedicated transformer. The main power supply is equipped with an isolating knife switch, capable of cutting off all power to the machine, providing a clear disconnect point for safe maintenance. A main circuit breaker is also included, providing only short-circuit protection and generally not operating. The main power supply is switched on and off using a main contactor. The main circuit breaker includes a shunt trip coil, controlled by an emergency stop button on the power cabinet or the operator's cab control panel.
2.2 Lifting Mechanism Control System
The hoisting mechanism carries a typical potential energy load, driven by two YZPF225M-8C-TH30KW variable frequency motors, each driven by a separate ABB ACS880 series frequency converter with closed-loop control. The two frequency converters utilize the D2D function of the ABB ACS880 series converters to ensure consistent torque and speed between the two motors. The hoisting mechanism features various fault self-diagnosis and alarm functions, including instantaneous overcurrent protection, instantaneous power failure and undervoltage protection, grounding protection, cooling fan malfunction protection, overfrequency (overspeed) protection, phase loss protection, and overload protection, among others.
(1) Speed specification: 0.2～2m/min
(2) Speed regulation method:
The speed is determined by the position of the control switch on the control panel. There are four positions in both directions: 10%, 30%, 70%, and 100% of the rated speed, providing constant torque speed regulation. The system uses the control position switch input to the PLC, which processes the data and then outputs a switch signal to the frequency converter. The speed can be arbitrarily set within the frequency converter to meet different speed requirements.
(3) Operation and Protection
① Each hoisting mechanism consists of a working brake and a safety brake.
② When the frequency converter fails, a pair of normally open contacts of each output fault inside the device will be activated to stop the circuit and trigger an alarm.
③ The hoisting mechanism is equipped with a comprehensive protection device based on load and height. When the load at the lifting point reaches 90% of the rated lifting capacity, an automatic warning alarm signal is issued; when it reaches 105% of the rated lifting capacity, a delayed alarm and power cut-off occur; when it reaches 110% of the rated lifting capacity, an immediate alarm and power cut-off occur (operation continues even during descent); when the load on the lifting device is zero, an underload warning signal is given; when the hoisting mechanism reaches its limit position, the power supply in the corresponding direction is automatically cut off and an alarm is triggered. The hoisting mechanism is equipped with mechanical limit switches, which automatically cut off the power supply in the corresponding direction when the hook reaches its upper or lower limit position, stopping the mechanism's operation. This, together with the comprehensive protection device, constitutes a dual electromechanical protection system.
2.3 Control System for Trolley Traveling Mechanism
The trolley mechanism has a typical displacement load, driven by two 2.2kW variable frequency motors, using ABB ACS880 series frequency converters for transmission and open-loop control.
(1) Speed specification: 0.5~5m/min
(2) Speed Control Method: The speed is determined by the position of the master switch on the driver's cab control panel. There are four speed settings each for forward and backward, representing 10%, 30%, 70%, and 100% of the rated speed, respectively. The system uses a master switch input to the PLC, which processes the data and then outputs a switch signal to the frequency converter. The speed setting can be arbitrarily set within the frequency converter to meet different speed requirements.
(3) Operation and Protection
① The mechanism is equipped with two hydraulic electric rail clamps. The trolley mechanism can only operate when the rail clamps are open and the external wind speed does not exceed 20m/s.
② Each motor is equipped with thermal overload protection. In the event of a frequency converter failure, a pair of normally open contacts on each output fault inside the device will activate the fault stop and trigger an alarm.
2.4 Control System for the Tractor Traveling Mechanism
The trolley mechanism has a typical displacement load, driven by four 4kW variable frequency motors, using ABB ACS880 series frequency converters for transmission and open-loop control.
(1) Speed range: 2-20 m/min
(2) Speed Control Method: The speed is determined by the position of the master switch on the driver's cab control panel. There are four positions on the left and four on the right, representing 10%, 30%, 70%, and 100% of the rated speed, respectively. The system uses a master position switch input to the PLC, which processes the data and then outputs a switch signal to the frequency converter. The speed position can be arbitrarily set within the frequency converter to meet different speed requirements.
(3) Operation and Protection
① The mechanism is equipped with two hydraulic electric rail clamps, one anemometer and two sets of anchoring devices. The trolley mechanism can only operate when the rail clamps are open, the external wind speed is less than 20m/s and the anchoring devices are open.
② Each motor is equipped with thermal overload protection. In the event of a frequency converter failure, a pair of normally open contacts on each output fault inside the device will activate the fault stop and trigger an alarm.
2.5 Hydraulic grab beam control system
The hydraulic automatic beam attachment and release system utilizes a magnetically coupled cable reel for power supply and signal detection. The cable winding and unwinding speed is synchronized with the gantry crane's lifting speed. The automatic beam attachment and release mechanism is controlled by an oil pump motor and a solenoid directional valve installed in the hydraulic pump station. Detection signals are provided by six proximity switches distributed across various locations on the beam. Two are for alignment, two for pin insertion, and two for pin release.
A stop button is connected in series in the pin insertion circuit and the pin retraction circuit. If the oil pump motor and solenoid valve are not de-energized after the pin insertion or retraction is in place, the stop button can be used to stop the pin insertion and retraction actions.
Four underwater cameras are installed near the hydraulic grab beam pin insertion device. The signal is transmitted to the LCD screen in the driver's cab via a cable reel, allowing the operator in the driver's cab to observe the pin insertion and withdrawal status of the hydraulic grab beam in real time.
2.6 Lifting Machinery Safety Management System
The crane is equipped with a hoisting machinery safety management and monitoring system, which consists of video monitoring and data monitoring, and is installed in the operator's cab. The monitoring data required for the entire machine is collected by a PLC and transmitted via Ethernet to the data monitoring center in the operator's cab, and then uploaded to a remote server, greatly increasing the operational safety of the crane. A total of twelve cameras are installed in the gantry crane's traveling area and on the underwater grab beam. The signals collected by each camera are transmitted to the operator's cab via Ethernet communication. Real-time images of each area are then displayed online in real-time on the LCD screen in the operator's cab, allowing the operator to easily see all working areas of the crane. The overall network topology diagram is shown in Figure 1.
Figure 1 Network topology diagram
2.7 Lighting and Grounding Lightning Protection System
This machine is equipped with a separate lighting system, powered by a dedicated lighting transformer. Regarding the grounding system, it has a dedicated grounding wire, with a grounding resistance at the power supply end not exceeding 4 Ω . The steel structure of the driver's cab is reliably connected to the main body of the machine, ensuring that the grounding resistance at any point on the steel structure does not exceed 4 Ω . The machine is also equipped with a surge arrester to effectively protect personnel and the gantry's electronic equipment from lightning strikes.
3. System Hardware Selection
3.1 Selection of PLC Controller
The Siemens S7-200SMART series PLC from Germany has the following highlights:
A wide variety of models are available: different types of CPU modules with a high number of I/O points are offered, with a maximum of 60 I/O points per unit, which can meet the control needs of most small automated equipment.
Expandable Options: The innovative signal board design allows for the expansion of communication ports, digital channels, and analog channels. Without requiring additional space in the control cabinet, signal board expansion can better suit the user's actual configuration, improving product utilization while reducing expansion costs.
High-performance high-speed chip: Equipped with Siemens' dedicated high-speed processor chip, the basic instruction execution time can reach 0.15μs, which is far ahead of other small PLCs in the same class.
Ethernet connectivity, economical and convenient: The CPU module comes standard with an Ethernet interface, integrating powerful Ethernet communication capabilities. A single ordinary network cable is sufficient to download the program to the PLC, making it quick and easy, eliminating the need for a dedicated programming cable. The Ethernet interface also allows communication with other CPU modules, touchscreens, and computers, facilitating easy network formation.
Universal SD card for easy download: The machine integrates a MicroSD card slot, which can be used with a common MicroSD card to update the program and upgrade the PLC firmware. This greatly facilitates customer engineers in providing service support to end users and saves the inconvenience of returning the PLC to the factory for firmware upgrades.
User-friendly software with efficient programming: Building upon the powerful functions of Siemens programming software, it incorporates more user-friendly designs, such as innovative ribbon menus,
The PLC features a fully mobile programming interface window, convenient program commenting capabilities, and robust password protection. (Figure 2: PLC Hardware Components)
Based on the above advantages of S7-200SMART PLC, this bidirectional gantry crane uses S7-200SMART PLC as the control core, which fully meets the requirements of gantry crane use. The PLC hardware composition is shown in Figure 2 above.
3.2 Selection of Frequency Converter
Due to its expertise in the crane industry and its many successful applications in the sector, ABB frequency converters have become the preferred choice for motor and electrical drives. The ABB ACS8800 series frequency converters utilize ABB's unique Direct Torque Control technology, providing precise speed and torque control for any AC motor. The hardware schematic diagram of the gantry crane's hoisting mechanism's frequency conversion speed regulation is shown in Figure 3 below.
Figure 3. Schematic diagram of the variable frequency drive hardware of the hoisting mechanism
3.3 Main parameters of lifting frequency converter
Inverter optimization identification, also known as inverter self-learning, involves the inverter automatically testing various parameters of the motor through identification to fully understand its characteristics, thus enabling better control of the motor's operation. To ensure maximum control accuracy and achieve optimal results, the motor should be decoupled from the load and placed in a completely unloaded state before identification. After successful inverter optimization identification, before performing D2D function debugging, the synchronous rigid shafts of the two lifting mechanisms need to be connected. The parameters of the lifting mechanism inverters are shown in Table 1 below.
Table 1 Main parameters of the hoisting mechanism frequency converter
4 Software Design
Since this gantry crane uses S7-200SMART as the core of its control system, the system is programmed using STEP7-MicroWINSMARTV2.2 programming software. The specific program logic control flow is shown in Figure 4 below.
Figure 4 Program control flowchart
5. Conclusion
After debugging and testing, the system demonstrated excellent performance, stable operation, and consistent real-time video transmission. The gantry crane operated smoothly overall, with minimal start-up and braking impact, receiving unanimous praise from the client. Figure 5 shows the on-site application of the 2×500KN bidirectional gantry crane at the Nam Ou River I Hydropower Station in Laos.
Figure 52×500kN bidirectional gantry crane general layout
