With the increasing adoption of PLCs in industrial control, the variety of PLC products is growing, differing in structure, performance, capacity, instruction set, programming methods, and application scenarios. Therefore, selecting the right PLC is crucial for improving its application in control systems.
I. Model Selection
The following are some of the most popular PLC products on the Chinese market:
1. Schneider Electric Company, including products introduced by Modicon to Tianjin Instrument Factory in the early days, currently offers products such as Quantum, Premium, and Momentum;
2. Rockwell Automation (including AB) PLC products, currently including SLC, Micro Logix, Control Logix, etc.;
3. Siemens' products currently include the SIMATIC S7-400/300/200 series;
4. Products from GE;
5. Products from Japanese companies such as Omron, Mitsubishi, Fuji, and Panasonic, with Mitsubishi's F1, F2, and FX2 series being the most commonly used.
Basic principles for selecting PLC models:
Provided the functional requirements are met, select the most reliable, easiest to maintain and use, and best performance-price ratio PLC model. Generally, in situations with relatively fixed processes and favorable environmental conditions, it is recommended to choose an integrated PLC; otherwise, a modular PLC is preferable. For projects involving switch control and those primarily based on switch control with a small amount of analog control, control speed is generally not a concern; therefore, a low-end PLC with A/D conversion, D/A conversion, addition/subtraction, and data transmission functions is sufficient. However, in projects with more complex control requirements and higher functional demands (such as PID calculations, closed-loop control, and network communication), a mid-range or high-end PLC can be selected based on the scale and complexity of the control (high-end PLCs are mainly used for large-scale process control, fully PLC-based distributed control systems, and factory automation). Table 1 lists several functional options for PLCs based on different applications.
It should be noted that companies should strive to use the same PLC models. This allows PLC modules of the same model to serve as backups for each other, facilitating the procurement and management of spare parts. Furthermore, standardized functions and programming methods benefit technical personnel training, skill improvement, and functional development. Additionally, because their external devices are common and resources can be shared, multiple PLCs controlling independent systems can be connected into a single DCS system with a host computer, facilitating communication and centralized management.
II. I/O Selection
By the 1990s, PLCs had evolved into various types, including micro, small, medium, large, and giant. Based on the number of I/O points, they can be categorized into five types: micro PLC (32 I/O), small PLC (256 I/O), medium PLC (1024 I/O), large PLC (4.69 I/O), and giant PLC (8195 I/O).
The connection between a PLC and the industrial production process is achieved through I/O interface modules. PLCs have many I/O interface modules, including digital input modules, digital output modules, analog input modules, analog output modules, and some other special modules. The appropriate module should be selected based on its characteristics.
(a) Determine the number of I/O points
When determining the required number of I/O points based on the requirements of the control system, an additional 10% to 20% reserve should be added to allow for the addition of control functions as needed. For a single controlled object, the number of I/O points may vary depending on the control method used or the level of programming. Table 2 lists the number of I/O points required for typical drive equipment and commonly used electrical components.
(ii) Switch I/O
Digital I/O interfaces receive signals from sensors and switches (such as pushbuttons, limit switches, etc.) and control devices (such as indicator lights, alarms, motor starters, etc.). Typical AC input/output signals are 24–240V, and DC input/output signals are 5–240V. Although input circuits vary between manufacturers, some characteristics are the same, such as jitter circuits to eliminate erroneous signals. Furthermore, most input circuits include optional isolation between the high-voltage power input and the control logic section of the interface circuitry. When evaluating discrete outputs, fuses, surge protection, and isolation between the power supply and logic circuitry should be considered. Fuse circuits may cost more initially, but may be less expensive than externally installed fuses.
(iii) Analog I/O
Analog input/output interfaces are generally used to sense signals generated by sensors. These interfaces can be used to measure flow rate, temperature, and pressure, and to control voltage or current output devices. Typical ranges for these interfaces are -10 to +10V, 0 to +11V, 4 to 20mA, or 10 to 50mA. Some manufacturers design special analog interfaces on PLCs that can receive low-level signals, such as RTDs and thermocouples. Generally, these interface modules can be used to receive mixed signals from different types of thermocouples or RTDs on the same module.
(iv) Special Function I/O
When selecting a PLC, users may encounter special I/O limitations that cannot be implemented using standard I/O (such as positioning, fast input, frequency, etc.). In such cases, users should consider whether the supplier offers special modules that help minimize control workload. Some special interface modules can process some field data themselves, thus freeing the CPU from heavy processing tasks.
(v) Intelligent I/O
Currently, PLC manufacturers have successively launched a number of intelligent I/O modules. Generally, intelligent I/O modules have their own processors, which can perform pre-defined processing on input or output signals and send the processing results to the CPU or output them directly. This can improve the processing speed of the PLC and save memory capacity.
In summary, Table 3 outlines the general rules for selecting I/O modules.
III. Selection of Memory Type and Capacity
The memory used in PLC systems is basically composed of three types: PROM, EPROM, and RAM. The storage capacity varies with the size of the machine. Generally, the maximum storage capacity of a small machine is less than 6kB, that of a medium-sized machine can reach 64kB, and that of a large machine can reach megabytes. When using a PLC, the appropriate model can be selected according to the storage needs of the program and data. If necessary, the memory can also be specially designed for expansion.
The first method for selecting and calculating PLC memory capacity is to accurately calculate the actual memory usage based on the number of nodes used in the programming. The second method is estimation; users can estimate the capacity according to the formulas in Table 4, based on the control scale and application purpose. For ease of use, a margin of 25% to 30% should generally be allowed. The best way to obtain memory capacity is to generate a program, i.e., determine how many words are used. Knowing the number of words used for each instruction allows the user to determine the accurate memory capacity. Table 4 also provides methods for estimating memory capacity.
IV. Selection of Programmer and External Devices
PLC programming is crucial during system implementation. Users should understand the software functions of the chosen PLC product and the appropriate programmer. Typically, small control systems use inexpensive, simple programmers, while larger systems or systems using multiple PLCs require more powerful and user-friendly graphical programmers. If a personal computer is available, a programming software package that runs on a personal computer can be used. Furthermore, to prevent interference, lithium battery voltage drops, or other factors from corrupting the user program in RAM, an EEPROM module can be used as an external device.
V. Examples
(a) Using Mitsubishi PLC to achieve precise control of the printing press
The electrical design of the printing press is a system design. To ensure stable product performance and ease of maintenance, a control scheme based on a PLC as the main controller is adopted. The printing press requires ease of operation, high precision, and numerous input/output points; therefore, a dual-machine communication system is used. The upper-level computer uses a high-performance Mitsubishi FX2N-80MR, which has a built-in I/O interface, allowing for 40 inputs and 40 outputs. It is primarily responsible for controlling the main drive, the clutches of each unit, and the air pump and valves. The lower-level computer uses a Mitsubishi FX2N-64MR, which can connect to 32 inputs and 32 outputs. It is primarily responsible for controlling the damping roller motor, adjusting the speed of the main drive, and acquiring data from the plate-setting motor. The upper and lower-level computers communicate via RS485, ensuring convenient and reliable communication. A Mitsubishi 5.7-inch touchscreen is also selected, primarily responsible for displaying the damping roller motor speed, plate-setting display, and overall machine fault display. This system is reliable, easy to maintain, and simple and intuitive to operate, significantly enhancing the quality of the offset printing press.
(II) Application of OMRON PLC in the petroleum processing industry
In the petroleum processing industry, large rotating units are an important component of the equipment. The interlocking-self-protection system of the heavy oil catalytic cracking gas compressor unit, designed to meet process production needs while considering factors such as safety, reliability, economy, and scalability, utilizes the CPM2AH PLC manufactured by OMRON. The CPM2AH has a built-in I/O interface, supporting 36 inputs and 24 outputs in relay form. It communicates with a PC via an RS232C serial port, ensuring stable production performance and reliable operation. In the event of an accident, it provides self-protection for the unit and production equipment, preventing the escalation and spread of serious accidents, achieving significant results.
VI. Conclusion
With the continuous advancement of technology, PLCs are becoming increasingly diverse in type and their functions are gradually being enhanced. Although this article summarizes some methods for selecting PLCs, in actual work, it is still necessary to make appropriate adjustments based on the actual situation in order to design an industrial control system that meets expectations.
