PLCs come in various sizes, allowing for a wide range of control capabilities. Smaller PLCs may control a single device or even a component at a single site, while larger PLCs can manage multiple devices, an entire production line, or even an entire factory. It can be said that PLCs are indispensable in both small and large industrial control scenarios.
Initially, PLCs were primarily used for logical control of discrete signals. With advancements in technology, the application areas of PLCs have continuously expanded. Nowadays, they are not only used for discrete control but also for analog and digital signal control, data collection and storage, and monitoring of control systems. They can also be networked and communicate, enabling extensive control and management across large geographical areas.
PLCs have increasingly become an important member of the industrial control device family.
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01
Used for Discrete Control
PLCs have a strong capability for controlling discrete signals. The number of input and output points they can control ranges from a few dozen to several hundred, or even thousands, and due to their networking capabilities, the number of points is virtually unlimited. They can handle a variety of logical control problems: combinational, sequential, instantaneous, delayed, non-counting, counting, fixed sequence, random operation, etc.
The hardware structure of a PLC is variable, and the software program is programmable, making it very flexible for control purposes. Multiple sets or groups of programs can be written and called as needed, making it well-suited for the diverse conditions and state changes in industrial environments.
There are many examples of using PLCs for discrete control in industries such as metallurgy, machinery, light industry, chemical engineering, and textiles. Almost all industrial sectors require PLCs. Currently, the primary advantage of PLCs, which other controllers cannot match, is their convenience and reliability in controlling discrete signals.
0 2
Used for Analog Control
Analog signals, such as current, voltage, temperature, and pressure, vary continuously. In industrial production, especially in continuous production processes, it is often necessary to control these physical quantities.
As an industrial control electronic device, it is a significant drawback if a PLC cannot control these quantities. Therefore, PLC manufacturers have invested heavily in this area. Currently, not only large and medium-sized PLCs can perform analog control, but even small PLCs can do so. For PLCs to perform analog control, they must be equipped with A/D and D/A units for converting between analog and digital signals. These are also I/O units, but they are specialized I/O units.
The A/D unit converts external circuit analog signals into digital signals, which are then sent to the PLC; the D/A unit converts the PLC’s digital signals back into analog signals for the external circuit. As a special type of I/O unit, it still possesses characteristics such as anti-interference for I/O circuits, isolation between internal and external circuits, and information exchange with input/output relays (or internal relays, which are also a writable area of the PLC’s working memory).
In this context, the ‘A’ in A/D usually refers to current or voltage, and sometimes temperature. The ‘A’ in D/A typically refers to voltage or current. The voltage and current ranges are often 0-5V, 0-10V, or 4-20mA, with some capable of handling both positive and negative values. The ‘D’ in small PLCs is often an 8-bit binary number, while medium and large PLCs typically use 12-bit binary numbers. A/D and D/A units can be single-channel or multi-channel, with multi-channel units occupying more input/output relays. With A/D and D/A units, the remaining processing is all digital, which is not difficult for PLCs with information processing capabilities. Medium and large PLCs have even stronger processing capabilities, allowing them to perform addition, subtraction, multiplication, division, square root, interpolation, and even floating-point operations. Some also have PID instructions for proportional, integral, and derivative calculations, generating corresponding outputs, making them capable of performing almost any calculation a computer can do.
Thus, it is entirely feasible to achieve analog control using PLCs.
PLCs can also perform analog control with A/D and D/A units combined, and can use PID or fuzzy control algorithms to achieve high-quality control. The advantage of using PLCs for analog control is that while performing analog control, discrete signals can also be controlled simultaneously. This is an advantage that other controllers do not possess, or their implementation is not as convenient as with PLCs. Of course, if the system is purely for analog control, using a dedicated regulator may offer better performance-to-price ratios than using a PLC.
3Used for Motion Control
In addition to discrete and analog signals, actual physical quantities also include motion control, such as the displacement of machine tool components, which is often represented as digital signals. An effective method for motion control is NC, or numerical control technology, which originated in the United States in the 1950s based on computer control technology. Today, it is widely used and well-developed.
Currently, in advanced countries, the rate of numerical control in metal cutting machine tools has exceeded 40%-80%, with some even higher. PLCs are also based on computer technology and are continually improving. PLCs can receive counting pulses at frequencies ranging from a few kHz to several tens of kHz, and can receive these pulses in various ways, including multi-channel reception. Some PLCs also have pulse output functions, with pulse frequencies reaching tens of kHz. With these two functions, combined with the PLC’s data processing and calculation capabilities, and when equipped with appropriate sensors (such as rotary encoders) or pulse servo devices, it is entirely possible to implement various controls based on NC principles. High-end and mid-range PLCs have also developed NC units or motion units that can achieve point control. Motion units can also perform curve interpolation and control curved motion.
Therefore, if a PLC is equipped with such units, it can completely use NC methods for digital control. Newly developed motion units have even introduced programming languages for NC technology, providing convenience for better digital control using PLCs.
0 4
Used for Data Acquisition
With the development of PLC technology, their data storage capacity has increased significantly. For example, the PLCs from Devicenet can have a data storage area (DM area) of up to 9999 words. This large data storage area can hold a substantial amount of data. Data acquisition can be performed using counters to accumulate recorded pulse counts and periodically transfer them to the DM area. Data acquisition can also utilize A/D units, where analog signals converted to digital signals are periodically transferred to the DM area. PLCs can also be equipped with small printers to periodically print out the data from the DM area.
PLCs can also communicate with computers, allowing the computer to read data from the DM area and process it further. In this case, the PLC acts as a data terminal for the computer.
Electricity users have previously used PLCs to record real-time electricity consumption, enabling different pricing methods based on varying electricity usage times, encouraging users to consume more electricity during off-peak hours, thus achieving reasonable and energy-saving electricity usage.
05
Used for Signal Monitoring
PLCs have many self-check signals and numerous internal components, but most users do not fully utilize their capabilities. In fact, they can be used to monitor the PLC’s own operation or the controlled objects. For a complex control system, especially an automatic control system, monitoring and even self-diagnosis are very necessary. This can reduce system failures, facilitate troubleshooting, and improve the average cumulative mean time between failures, thereby enhancing system reliability.
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Used for Networking and Communication
PLCs have strong networking and communication capabilities, with new networking structures continuously being introduced.
PLCs can connect to personal computers for communication, allowing computers to participate in programming and managing PLC control, making PLCs more convenient to use.
To fully leverage the capabilities of computers, one computer can control and manage multiple PLCs, with the number reaching up to 32. A single PLC can also communicate with two or more computers, exchanging information to achieve greater monitoring of the PLC control system.
PLCs can also communicate with each other, allowing for one-to-one PLC communication, multiple PLC communications, and even dozens or hundreds of PLCs.
PLCs can also connect and communicate with intelligent instruments and smart actuators (such as frequency converters), exchanging data and operating together. They can be connected into remote control systems, with ranges extending up to 10 kilometers or more. They can form local networks, where not only PLCs but also high-end computers and various intelligent devices can be integrated. Bus networks and ring networks can be used, and networks can be nested. Networks can also be bridged. Networking can organize thousands of PLCs, computers, and intelligent devices into a single network. Nodes within the network can communicate and exchange information directly or indirectly.
Networking and communication are well-suited to the needs of today’s Computer Integrated Manufacturing Systems (CIMS) and the development of intelligent factories. This allows industrial control to evolve from point (Point) to line (Line) and then to area (Aero), integrating device-level control, production line control, and factory management control into a cohesive whole, thereby creating greater benefits. This limitless and promising future is becoming increasingly clear to our generation.
The above applications focus on qualitative aspects. Quantitatively, PLCs come in various sizes, allowing for a wide range of control capabilities. Smaller PLCs may control a single device or even a component at a single site, while larger PLCs can manage multiple devices, an entire production line, or even an entire factory. It can be said that PLCs are indispensable in both small and large industrial control scenarios.