PLCs come in a wide variety of types, differing in structure, performance, capacity, instruction set, programming method, price, and application scenarios. Therefore, selecting the right PLC plays a crucial role in maximizing its application in control systems.
1. Model Selection
The basic principle for selecting a PLC model is to choose the most reliable, easiest to maintain and use, and most cost-effective model, provided that the functional requirements are met.
In situations where the process is relatively fixed and the environmental conditions are good (with less maintenance), it is recommended to choose an integrated PLC; otherwise, it is better to choose a modular PLC.
For projects involving switch control and those primarily using switch control with a small amount of analog control, control speed is generally not a concern. Therefore, a low-end machine with A/D conversion, D/A conversion, addition/subtraction, and data transmission functions can meet the requirements.
In engineering projects with complex control requirements and high-level control functions (such as PID calculations, closed-loop control, and communication networking), mid-range or high-end PLCs 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 PLC function options based on different applications.
For a large enterprise system, it is advisable to use standardized PLC models as much as possible. 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. In addition, because their external equipment is common and resources can be shared, multiple PLCs controlling independent systems can be connected into a multi-level distributed control system with a host computer, facilitating communication and centralized management.
2. Input/Output Selection
A PLC is an industrial control system that controls industrial production equipment or processes in an industrial production environment. Its connection to the industrial production process is achieved through I/O interface modules.
I/O interface modules can detect various parameters of the controlled production process and use this field data as control information to control the controlled object. Simultaneously, the I/O interface modules send the controller's processing results to the controlled equipment or industrial production process, thereby driving various actuators to achieve control. Information collected from the field and control signals output to external devices by the PLC must travel a certain distance. To ensure the accuracy of this information, the PLC's I/O interface modules have good anti-interference capabilities. Depending on actual needs, PLCs generally have many I/O interface modules, including digital input modules, digital output modules, analog input modules, analog output modules, and other special modules. The appropriate module should be selected based on its characteristics.
2.1 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 expertise.
Table 2 lists the number of I/O points required for typical transmission equipment and commonly used electrical components.
2.2 Digital Input/Output
Signals can be received from sensors and switches (such as pushbuttons, limit switches, etc.) and control (on/off) devices (such as indicator lights, alarms, motor starters, etc.) via standard input/output interfaces. 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. These include jitter circuits to eliminate erroneous signals and surge protection circuits to prevent damage from large transient overvoltages. Furthermore, most input circuits include optional isolation circuitry between the high-voltage power input and the control logic section of the interface circuit.
When evaluating discrete outputs, fuses, surge protection, and isolation circuitry between the power supply and logic circuitry should be considered. While fuse circuitry may initially cost more, it can be less expensive than externally mounted fuses.
2.3 Analog Input/Output
Analog input/output interfaces are typically 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 +10V, 4 to 20mA, or 10 to 50mA.
Some manufacturers design special analog interfaces on their PLCs, allowing them to 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.
2.4 Special Function Input/Output
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 provides special modules that help minimize control overhead. Some special interface modules can process some field data themselves, thus freeing the CPU from time-consuming task processing.
2.5 Intelligent Input/Output
Currently, PLC manufacturers have successively launched a number of intelligent input/output modules. Generally, intelligent input/output 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.
Intelligent input/output modules include high-speed counters (capable of addition or subtraction), cam simulators (used as absolute encoded input), cam simulators with speed compensation, single-loop or multi-loop PID controllers, ASCII/BASIC processors, RS-232C/422 interface modules, etc. Table 3 summarizes the general rules for selecting I/O modules.
3. Selection of PLC memory type and capacity
The memory used in PLC systems is basically composed of three types: PROM, E-PROM, and PAM. The storage capacity varies with the size of the machine. Generally, the maximum storage capacity of a small machine is less than 6kB, the maximum storage capacity of a medium-sized machine can reach 64kB, and the maximum storage capacity of a large machine can reach megabytes. When using it, 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.
4. Software Selection
In the implementation of a system, PLC programming is crucial. Users should understand the software capabilities of the chosen PLC product. Typically, a system's software is used to process the control hardware provided by the controller. However, some application systems require software functions beyond the control hardware components. For example, an application system might include special control or data acquisition functions requiring complex mathematical calculations and data processing operations. The choice of instruction set will determine the ease of implementing the software task. The available instruction set will directly affect the time required to implement the control program and the program execution time.
5. Considerations for supporting technical conditions
When selecting a PLC, the availability of supporting technical conditions is also an important selection criterion. Supporting technical conditions include the following:
(1) Programming methods
Portable simple programmers are mainly used for small PLCs, which have a small control scale and simple programs, and can be programmed using simple programmers.
CRT programmers are suitable for large and medium-sized PLCs. In addition to programming and inputting programs, they can also edit and print program text.
With the widespread adoption of IBM-PCs, IBM-PC and compatible programming software packages are excellent programming tools for PLCs. Currently, PLC manufacturers are dedicated to developing IBM-PC and compatible programming software packages suitable for their own PLC models, and have achieved success.
(2) Perform program text processing
Simple program text processing, as well as the processing of graphs, parameter states, and positions, including printing ladder logic;
Program annotations, including the assignment names of contacts and coils, network comments, etc., are very useful for users or software engineers to read and debug programs.
Graphics and text processing.
(3) Program storage method
For technical data archives and backup data, programs can be stored using methods such as magnetic tape, floppy disk, or EEPROM storage cartridges. The specific storage method chosen depends on the technical specifications of the selected model.
(4) Communication software package
For network control structures or control systems requiring management by a host computer, the availability of communication software is a primary factor in selecting a PLC. Communication software is often used in conjunction with communication hardware, such as modems.
6. Environmental adaptability of PLC
Since PLCs are typically used directly in industrial control, manufacturers design them to operate reliably in harsh environments. Nevertheless, each PLC has its own environmental specifications, and users must give full consideration to these environmental conditions when selecting one, especially when designing a control system.
Generally, PLCs and their external circuits (including I/O modules, auxiliary power supplies, etc.) can operate reliably under the environmental conditions listed in Table 5.
Although the article summarizes some methods for selecting PLCs, in actual work, appropriate adjustments must be made according to the actual situation in order to design a control system that meets expectations.
