1. Used for Switch Control
PLC has a strong capability to control switch signals. The number of controlled input and output points can range from a few dozen to several hundred, or even thousands, and it can control tens of thousands of points. As it can connect to virtually unlimited points, the logic problems it can handle are diverse: combinations, sequences, instantaneous, delayed, non-counting, counting, fixed order, random operations, etc., all of which can be managed.
The hardware structure of PLC is variable, and the software program is programmable, making it very flexible for control. If necessary, multiple sets or groups of programs can be written and called as needed. It is well-suited for the diverse conditions and state changes in industrial settings.
There are numerous examples of using PLC for switch control across various industries such as metallurgy, machinery, light industry, chemical engineering, textiles, etc. Almost all industrial sectors require its use. Currently, the primary advantage of PLC, which other controllers cannot match, is its convenience and reliability in switch control.
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 shortcoming if 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 PLC to perform analog control, it needs to be equipped with A/D and D/A units that convert analog signals to digital and vice versa. These are also I/O units, albeit special ones.
The A/D unit converts the external analog signal into a digital signal, which is then sent to the PLC; the D/A unit converts the PLC’s digital signal back into an analog signal for the external circuit. As a special 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 and output relays (or internal relays, which also serve as a writable area of PLC’s working memory).
In this context, the A in A/D mostly refers to current or voltage, and sometimes temperature. The A in D/A typically refers to voltage or current. The voltage or current range often spans from 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 not only basic arithmetic operations but also square roots, interpolation, and floating-point calculations. Some even include PID instructions for proportional, differential, and integral calculations to generate corresponding outputs. Essentially, PLCs can compute almost anything a computer can.
Thus, achieving analog control with PLC is entirely feasible.
PLC can also perform analog control using a combination of A/D and D/A units, employing PID or fuzzy control algorithms for high-quality control. The advantage of using PLC for analog control is that it can simultaneously control switch signals, a benefit that other controllers do not possess or cannot achieve as conveniently. However, if the system is purely for analog control, PLC may not offer the best cost-performance ratio compared to dedicated regulators.
3. Used for Motion Control
In addition to switch and analog signals, motion control is also a critical physical quantity. For instance, the displacement of machine tool components is often represented as a digital quantity. Effective motion control is achieved through NC, or numerical control technology, which originated in the 1950s in the USA based on computer technology. Today, it is widespread and well-developed. In advanced countries, the proportion of numerical control in metal-cutting machine tools has exceeded 40%-80%, with some even higher. PLC is also based on computer technology and is 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, even from multiple sources. Some PLCs also have pulse output capabilities, with pulse frequencies reaching tens of kHz. With these two functions, combined with PLC’s data processing and computational abilities, and when equipped with appropriate sensors (like rotary encoders) or pulse servo devices, it can fully implement various controls based on NC principles. High-end and mid-range PLCs have also developed NC units or motion units for point control. Motion units can also achieve curve interpolation and control curve motion. Therefore, if a PLC is equipped with such units, it can entirely use NC methods for digital control. Newly developed motion units even provide programming languages for NC technology, facilitating better digital control with PLCs.
4. Used for Data Acquisition
PLCs can communicate with computers, allowing computers to read data from the DM area and process this data. In this scenario, PLCs serve as data terminals for computers.
Electricity consumers have used PLCs to record real-time electricity usage, implementing different pricing strategies based on varying electricity usage times, encouraging users to consume more electricity during low-demand periods, thereby achieving reasonable and economical electricity use.
5. Used for Signal Monitoring
PLCs have many self-checking signals and 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 further self-diagnosis are very necessary. This can reduce system faults, facilitate troubleshooting, improve the average cumulative fault-free operation time, reduce fault repair time, and enhance system reliability.
6. Used for Networking and Communication
PLCs have strong networking and communication capabilities, with new networking structures being continuously introduced.
PLCs can connect with personal computers for communication, allowing computers to participate in programming and managing PLC control, making PLCs more user-friendly.
To maximize the computer’s role, one computer can control and manage multiple PLCs, with up to 32 units, or a single PLC can communicate with two or more computers to exchange information, enabling extensive 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 hundreds of PLCs communicating together.
PLCs can also network and communicate with intelligent instruments and smart actuators (like frequency converters), exchanging data and performing operations. They can be connected into remote control systems, with a range of up to 10 kilometers or more. They can form local networks where not only PLCs but also high-end computers and various smart devices can participate. Networking can use bus networks or ring networks, and networks can be nested. Networks can also bridge to connect multiple networks. Networking can organize thousands of PLCs, computers, and smart devices into a single network, where nodes can communicate and exchange information directly or indirectly.
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