PLCs typically operate using a cyclic scanning mode, with some large and medium-sized PLCs incorporating an interrupt mode. After the user has debugged the user program, it is written into the PLC's memory using a programmer. Simultaneously, the field input signals and the controlled actuators are connected to the input terminals of the input modules and the output terminals of the output modules. The PLC is then set to run mode, and the subsequent tasks are performed by the PLC according to the user program. The overview diagram is a block diagram of the PLC execution process. During operation, the PLC primarily processes six modules.
In the industrial control industry, learning PLC programming is essential, but simply focusing on syntax and instructions is far from sufficient. Instead, we'll start with some typical small-scale automation systems, beginning with process functions, then I/O point allocation, followed by PLC program writing, and finally, simulation and debugging with a host computer. This process will make PLC programming more intuitive and faster. We've prepared three typical PLC examples below, and analyzed the PLC program's operation process using color-coded diagrams—very intuitive!
This paper presents a design scheme for a 50MW pulse modulator power control system, introducing a "host computer-PLC power control system" developed by modifying the DC high-voltage power supply system of a linear accelerator modulator using a programmable logic controller (PLC). The overall design method and structural characteristics of this control system are discussed. After being put into operation, the control system resolved the impact of large fluctuations in the power grid voltage on the machine, providing a prerequisite guarantee for the reliable operation of the de-Qing circuit, and thus ensuring the beam stability of the linear accelerator.
The main equipment of the Hefei National Synchrotron Radiation Laboratory (NSRL), the synchrotron radiation accelerator, consists primarily of a 200 MeV electron linear accelerator and an 800 MeV electron storage ring. The 200 MeV electron linear accelerator not only acts as the injector for the 800 MeV electron storage ring but also serves as an electron accelerator providing electron beams and other secondary radiation for research and applications in nuclear physics and other fields. Five klystron-modulator systems are a crucial component of the linear accelerator's microwave power source. Powered by a 21 kV DC high-voltage power supply, the modulator outputs a pulsed square wave with a repetition frequency of 50 Hz and an amplitude of approximately 250 kV. Under the action of the klystron, the pulsed electric field is converted into a microwave electromagnetic field, and the energy is fed into the accelerating cavity of the linear accelerator through waveguides and coupling devices to accelerate electrons. To stabilize the microwave output power, reduce pulse amplitude drift, and thus improve the stability of the electron beam of the linear accelerator, the DC high-voltage power supply of the modulator must maintain high stability under conditions of grid voltage fluctuations.
Since the mains voltage often fluctuates within ±5% or greater of AC380V, while the existing deQ'ing circuit can only operate within a very narrow range of mains voltage fluctuations (<1%), we must provide a means to maintain high stability of DC high voltage under large-scale mains voltage fluctuations. On the other hand, the existing six voltage regulators are 50kVA three-phase induction power regulators driven by stepper motors; in order to improve the voltage regulation accuracy and achieve fine voltage adjustment, it is necessary to modify them.
To address this, we developed a host computer-PLC power control system using an OMRON C200HE PLC. After the system was put into operation, testing showed that the DC high-voltage power supply stability was better than 0.5%. Furthermore, this computer system enables real-time monitoring and automatic recording of key power supply parameters, as well as graphical display of voltage trajectory data, providing operators with a user-friendly software control interface.
1. System Composition and Basic Working Principle
An industrial computer located in the linear accelerator control room serves as the host control computer, while a PLC located in the voltage regulating room acts as the field control device, directly controlling the five AC voltage regulating circuits. To eliminate the impact of strong electromagnetic interference from the klystron corridor on communication between the host computer and the PLC, optical fiber is used as the communication medium for long-distance digital signal transmission; the conversion between optical and electrical signals is completed through a Link adapter and the host connection unit on the PLC rack. The PLC directly controls the modulator's AC voltage regulating control circuit, as well as data acquisition, digital filtering and anti-interference processing, high-voltage protection and alarm at the DC high-voltage end, and responds in real time to command frames issued by the host computer (the format of which is defined by the Host Link protocol), processing the host computer's instructions. The overall data flow diagram of the system can be seen in Figure 2. In this way, a digital computer open-loop/closed-loop control system is formed with the PLC as the center. At the host computer end, the program flow within the PLC can be controlled to select between "open-loop voltage regulation" or "closed-loop voltage regulation and stabilization" modes. When operating in "closed-loop regulation and stabilization" mode, the system sets the required DC high-voltage value through a host computer. The PLC processes the sampled signal and adjusts the digital control voltage with an accuracy of 1/4000 to stabilize the output of the AC voltage regulation circuit. The core component of the AC voltage regulation circuit is a 3×150A SCR intelligent voltage regulation module.
In this system, the PLC performs the main real-time control tasks, while the host computer mainly provides the operator with a software interface with a virtual instrument style, allowing the operator to intuitively understand the operating status of the DC high-voltage power supply and perform various operations.
Corresponding to this computer control system, a manual circuit control system was also developed during the project's progress. If the computer control system malfunctions, the system can be switched to manual control by using the PLC's output to control the system switching circuit breaker within the AC voltage regulation control circuit.
2 PLC software design
The programming language used for PLCs is ladder diagrams. Because the klystron corridor experiences very strong electromagnetic interference, often causing control equipment to malfunction, the key issue to be addressed in this part of the programming is the anti-interference preprocessing of the sampled data.
The measured waveform shows that the electromagnetic interference is a strong pulse electromagnetic interference with a repetition frequency of 50Hz. The waveform after pulse broadening is shown in Figure 3.
For the low-amplitude high-frequency interference components in Figure 3, a multi-stage π-type RC filter network can be used to process them. For interference signals whose voltage amplitude is outside the normal signal range but are still acquired, a ladder diagram-based amplitude limiting algorithm can be used to eliminate them. Simultaneously, eliminating sampled signals exceeding the upper limit of the normal signal range results in the control signal no longer increasing, the output of the SCR intelligent voltage regulation module no longer increasing, and correspondingly, the DC high-voltage power supply will not exceed the modulator's normal operating limit, and the modulator hardware will not be damaged.
How can we eliminate interference signals that are within the normal signal range? Two software algorithms were designed for this purpose: an outlier removal algorithm and an averaging algorithm. When performing averaging, the number of sampling points should not be too large; 3 to 5 points are more suitable. Otherwise, the system will be less sensitive to voltage ripple and jitter, leading to a decrease in voltage stability.
3. Upper Computer Software Design
The host computer software design utilizes National Instruments' LabWindow-CVI 3.0.1 programming platform. Compared to Visual Basic and Visual C++, LabWindow-CVI is a platform specifically designed for EDA (Electronic Design Automatic) software development. Programming is done in C language using object-oriented programming (OOP). The host computer's software control panel is divided into four parts according to its functions: system configuration panel, high-voltage setting panel, voltage display panel, and system help menu. Furthermore, the host computer software system can be easily and conveniently installed using the Setup.exe installation file, with an installation process typical of Windows.
To monitor and control a programmable logic controller (PLC) using a host computer, a data transmission channel must be established between the two. First, the data transmission format between them must be defined, and the command format formation and response format decoding must be programmed. Omron defines all command frames and response frames as the Host Link Protocol. The Host Link Protocol contains as many as 39 identification code instructions, capable of performing various tasks such as reading and writing to the PLC's various memory areas (IR, SR, DM, AR, EM, LR, HR), initializing the PLC, setting bits, and generating I/O tables. The first row of Figure 4 shows a command frame from a host computer reading the PLC's internal relay area (IR area), and the second row shows the PLC's response frame to this command.
As can be seen from the Host link frame format in Figure 4, in the communication programming of the host computer, not only is it necessary to encode the form of the command frame, but also to decode the response frame of the programmable controller, extract the useful data, and then perform a number system conversion to transform it into the data format we want.
To accommodate system expansion, a 1:n host computer connection is configured, meaning one host computer can simultaneously control a network of N field-located programmable controllers (PLCs). This interconnects the PLCs with the other interlocking protection PLCs of the linear accelerator.
The software on the host computer sets the communication port parameters between the host computer and the PLC, sends the set high voltage value to the PLC to indirectly control the hardware circuit to complete voltage regulation, select the total stroke of voltage boosting, shut down the high voltage in an emergency, and automatically record the current high voltage setting value and time, as well as the list of on-duty personnel, in the duty log in the form of an Excel spreadsheet.
In summary, this system implements all three major functional blocks of traditional instruments through a computer: signal acquisition and processing are handled by the field control computer—a programmable logic controller (PLC); result output is handled by the host computer. The "voltage display panel" primarily performs the tasks of the "result output" function block of a traditional instrument. The computer screen can visually and conveniently simulate various instrument control panels, displaying test results and power supply operating status in various formats.
A PLC control system is an automation control system based on a programmable logic controller (PLC) used to control and monitor various mechanical, industrial, and production processes. PLC control systems utilize digital electronic technology to achieve highly reliable control and monitoring, improving production efficiency and quality. The working principle of a PLC control system is to transmit input signals (such as those from sensors and buttons) to the PLC, which then controls output signals (such as those from motors and valves) through a program to achieve control of the production process.
A PLC control system typically consists of the following parts:
1. Input module: Used to receive input signals from sensors, buttons, etc.
2. Central Processing Unit (CPU): Responsible for processing input signals and executing control programs.
3. Output module: Used to control the output signals of motors, valves, etc.
4. Communication module: Used to communicate with other devices, such as computers, HMIs, etc.
The advantages of a PLC control system include:
1. Stable and reliable: The PLC control system adopts digital electronic technology, which has high stability and reliability and can operate for a long time in harsh environments.
2. Flexible programmability: PLC control systems can be programmed as needed to achieve various complex control and monitoring functions.
3. Easy to maintain: Both the hardware and software of the PLC control system are easy to maintain, and fault diagnosis and repair can be carried out quickly.
4. High scalability: The PLC control system can expand input and output modules as needed to achieve more control and monitoring functions.
PLC control systems are widely used in various industrial and production processes, such as automated production lines, robot control, water treatment, and food processing. With the continuous development of technology, PLC control systems are also constantly being updated and upgraded, for example, by adopting technologies such as cloud computing and the Internet of Things to achieve more intelligent control and monitoring.
