This is a typical block diagram of a PLC control system:
1. Used for Switch Control
The PLC has a strong capability for controlling switch quantities. The number of controlled input and output points can range from a few to hundreds, thousands, or even tens of thousands. Because it can be networked, the number of points is virtually unlimited. No matter how many points there are, it can control them, and the logic problems it can handle are diverse: combinatory, sequential, instantaneous, delayed, non-counting, counting, fixed order, random operation, etc. The hardware structure of the 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 very suitable for the needs of industrial sites with multiple working conditions and state changes. There are many examples of using PLCs for switch control in industries such as metallurgy, machinery, light industry, chemical industry, textiles, etc. Almost all industrial sectors require it. Currently, the primary goal of PLCs, which other controllers cannot match, is their convenient and reliable use for switch control.
2. Used for Analog Control
Analog quantities, such as current, voltage, temperature, pressure, etc., change continuously. In industrial production, especially in continuous production processes, it is often necessary to control these physical quantities. As an industrial control electronic device, if a PLC cannot control these quantities, it is a significant shortcoming. Therefore, PLC manufacturers have conducted extensive development in this area. Currently, not only large and medium-sized machines can perform analog control, but even small machines can do so. To perform analog control with a PLC, it must be equipped with A/D and D/A units that convert between analog and digital quantities. These are also I/O units, but special I/O units. The A/D unit converts the external circuit’s analog quantity into a digital quantity and sends it to the PLC; the D/A unit converts the PLC’s digital quantity back into an analog quantity and sends it to 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/output relays (or internal relays, which are also a writable area of the PLC’s working memory). In the A/D unit, ‘A’ often refers to current or voltage, and sometimes temperature. In the D/A unit, ‘A’ often refers to voltage or current. The voltage and current ranges are usually from 0 to 5V, 0 to 10V, or 4 to 20mA, with some capable of handling both positive and negative values. The ‘D’ in small machines is mostly 8-bit binary, while medium and large machines are mostly 12-bit binary. A/D and D/A can be single-channel or multi-channel. Multi-channel occupies 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 for not only addition, subtraction, multiplication, and division of digital values but also square roots, interpolation, and floating-point calculations. Some even have PID instructions that can perform proportional, differential, and integral calculations on deviation measurements to produce corresponding outputs; in essence, they can compute almost anything a computer can. Thus, it is entirely feasible to achieve analog control using a PLC. The advantages of using a PLC for analog control are that while performing analog control, switch quantities can also be controlled simultaneously. This advantage is not available in other controllers, or its implementation is not as convenient as with PLCs. Of course, if the system is purely for analog quantities, using a PLC may not be as cost-effective as using a regulator.
3. Used for Motion Control
In addition to switch and analog quantities, actual physical quantities also include motion control, such as the displacement of machine tool components, often represented as digital quantities. Effective motion control is achieved through NC, or numerical control technology. This technology originated in the 1950s in the United States and is 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% to 80%, with some even higher. PLCs are also based on computer technology and are becoming increasingly sophisticated. PLCs can receive counting pulses with frequencies ranging from several kHz to tens of kHz, and they can receive these pulses in various ways, even from multiple sources. Some PLCs also have pulse output functions, with pulse frequencies also reaching tens of kHz. With these two functions, combined with the data processing and computational capabilities of PLCs, and when equipped with appropriate sensors (such as rotary encoders) or pulse servo devices, it is entirely possible to achieve various controls based on NC principles. High-end and mid-range PLCs also have developed NC units or motion units to achieve point control. Motion units can also implement curve interpolation and control curved movements. Therefore, if a PLC is equipped with such units, it can completely use NC methods for digital control. Newly developed motion units even include NC programming languages to facilitate better digital control using PLCs.
4. Used for Data Acquisition
With the development of PLC technology, the size of its data storage area has increased significantly. For example, the PLC from Devison Company has a data storage area (DM area) that can reach up to 9999 words. Such a large data storage area can hold a vast amount of data. Data acquisition can utilize counters to cumulatively record the number of pulses collected and periodically transfer them to the DM area. Data acquisition can also use A/D units; after the analog quantity is converted into a digital quantity, it can be periodically transferred to the DM area. PLCs can also be configured with small printers to regularly print the data from the DM area. PLCs can communicate with computers, allowing computers to read the data from the DM area and process it further. In this case, the PLC becomes a data terminal for the computer. Power users have previously used PLCs to record real-time electricity usage to implement different billing methods based on varying electricity usage times, encouraging users to consume more electricity during low-demand periods, thus achieving reasonable and energy-saving electricity usage.
5. Used for Signal Monitoring
PLCs have many self-checking signals and numerous internal components; most users do not fully utilize their capabilities. In fact, they can be used to monitor the PLC’s own operations or the controlled objects. For a complex control system, especially an automatic control system, monitoring and further self-diagnosis are very necessary, as they can reduce system failures, facilitate troubleshooting, enhance the average cumulative fault-free operating time, reduce fault repair time, and improve system reliability.
6. 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 operations, making PLCs more convenient to use. To fully leverage the computer’s capabilities, 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 to exchange information, enabling enhanced monitoring of the PLC control system. PLCs can also communicate with each other, allowing for one-to-one communication or multiple PLCs communicating simultaneously, potentially numbering in the dozens or hundreds.
PLCs can also connect to intelligent instruments and smart actuators (such as frequency converters) for networking and data exchange, enabling remote control systems that can cover 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 be integrated. Bus networks and ring networks can be used, and networks can be bridged. Networking can organize thousands of PLCs, computers, and smart devices into a single network. The nodes between networks can communicate and exchange information directly or indirectly.
Networking and communication align perfectly with the current needs of Computer Integrated Manufacturing Systems (CIMS) and the development of intelligent factories. It enables industrial control to evolve from point (Point) to line (Line) and then to surface (Aero), creating an integrated system for equipment-level control, production line control, and factory management, ultimately leading to greater efficiency. This promising future is becoming increasingly clear for our generation.
The above applications focus on qualitative aspects. Quantitatively, PLCs come in various sizes. Therefore, their control range can also vary. Smaller PLCs may control only one device, or even a component or a single station; larger PLCs can control 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.
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