- Used for switch control
The ability of PLC to control switch values is very strong. The number of input and output points controlled can range from a few dozen or dozens to hundreds, thousands, or even tens of thousands. Due to its ability to connect to the internet, the number of points is almost unlimited, and any number of points can be controlled. The logic problems controlled can be diverse: combinatorial, sequential, real-time, delayed, uncountable, countable, fixed order, random work, and so on. The hardware structure of PLC is variable, and the software program is programmable, making it very flexible when used for control. When necessary, multiple sets or groups of programs can be written and called as needed. It is very suitable for the needs of multiple working conditions and state transitions in industrial sites. There are many examples of using PLC for switch control, such as metallurgy, machinery, light industry, chemical industry, textile, etc., almost all industrial industries need it. At present, the first goal of using PLC, which cannot be compared to other controllers, is that it can be conveniently and reliably used for switch control.
- Used for analog control
Analog quantities, such as current, voltage, temperature, pressure, etc., have a continuously changing magnitude. Industrial production, especially continuous production processes, often requires control over these physical quantities. As an industrial control electronic device, if PLC cannot control these quantities, it is a major drawback. Therefore, various PLC manufacturers have conducted extensive development in this area. At present, not only large and medium-sized computers can perform analog control, but also small computers can perform such control. PLC for analog control should be equipped with A/D and D/A units that convert analog and digital signals to each other. It is also an I/O unit, but a special type of I/O unit. The A/D unit converts analog signals from external circuits into digital signals and sends them to the PLC; The D/A unit converts the digital signals of the PLC into analog signals and sends them to the external circuit. As a special type of I/O unit, it still has the characteristics of anti-interference of I/O circuits, isolation of internal and external circuits, and exchange of information with input/output relays (or internal relays, which are also an area of PLC working memory that can be read and written). The A in the A/D here mostly refers to current or voltage, as well as temperature. The A in D/A is mostly voltage or current. The voltage and current variation range is mostly 0-5V, 0-10V, 4-20mA, and some can also handle positive and negative values. Here, D is mostly an 8-bit binary number for small computers, and a 12 bit binary number for medium and large computers. A/D and D/A have both single and multiple channels. Multiple input and output relays occupy multiple channels. With A/D and D/A units, the remaining processing is digital, which is not difficult for PLCs with information processing capabilities. Medium and large PLCs have stronger processing capabilities, not only capable of adding, subtracting, multiplying, and dividing numbers, but also of square root and interpolation, as well as floating-point operations. Some even have PID instructions, which can perform proportional, differential, and integral operations on deviation control quantities, thereby generating corresponding outputs. They can calculate almost everything that computers can. In this way, it is entirely possible to implement analog control using PLC. PLC is used for analog control, and there are units that combine A/D and D/A, which can be controlled using PID or fuzzy control algorithms to achieve high control quality. The advantage of using PLC for analog control is that while performing analog control, switch values can also be controlled. This advantage is not possessed by other controllers, or the implementation of control is not as convenient as PLC. Of course, if the system is purely analog, using a PLC may not be as cost-effective as using a regulator.
- Used for motion control
The actual physical quantities, in addition to switch values and analog values, also include motion control. The displacement of machine tool components is often represented by numerical quantities. The effective method for motion control is NC, which stands for digital control technology. This is a computer-based control technology born in the 1950s in the United States. It is now very popular and well-established.
At present, the CNC rate of metal cutting machine tools in advanced countries has exceeded 40% to 80%, and some are even higher. PLC is also based on computer technology and is becoming increasingly sophisticated. PLC can receive counting pulses with frequencies ranging from several kilohertz to tens of kilohertz. It can receive these pulses in multiple ways and can also receive them in multiple channels. Some PLCs also have pulse output function, and the pulse frequency can reach tens of k. With these two functions, coupled with the data processing and computing capabilities of PLCs, if equipped with corresponding sensors (such as rotary encoders) or pulse servo devices, various controls can be fully realized according to the principle of NC.
High and mid-range PLCs are also developed with NC units or motion units, which can achieve point control. The motion unit can also achieve curve interpolation and control curve motion. So, if the PLC is configured with this type of unit, it is entirely possible to use NC to control digital quantities. The newly developed motion unit has even released a programming language for NC technology, providing convenience for better digital control using PLC.
- Used for data collection
With the development of PLC technology, its data storage area is becoming larger and larger. The data storage area (DM area) of PLC from Devison Company can reach 9999 words. Such a massive data storage area can store a large amount of data. Data collection can use a counter to accumulate and record the number of pulses collected, and periodically transfer them to the DM area. Data acquisition can also be done using an A/D unit, which converts analog signals into digital signals and then periodically stores them in the DM area. PLC can also be equipped with a small printer to regularly print out data from the DM area. PLC can also communicate with computers, where the data in the DM area is read out and processed by the computer. At this point, PLC becomes the data terminal of the computer. Power users have used PLCs to record their electricity usage in real time, in order to implement different charging methods for different electricity usage times and pricing. This encourages users to use more electricity during low electricity consumption periods, achieving the goal of reasonable and energy-saving electricity consumption.
- Used for signal monitoring
There are many self-test signals and internal components in PLC, and most users have not fully utilized their functions. In fact, it can be used to monitor the operation of the PLC itself or to monitor the controlled objects. Monitoring and further self diagnosis are essential for a complex control system, especially an automatic control system. It can reduce system failures, facilitate troubleshooting, improve cumulative mean time between failures, reduce fault repair time, and enhance system reliability.
- Used for networking and communication
PLC has strong networking and communication capabilities, and new networking structures are constantly being introduced. PLC can be connected to personal computers for communication, and computers can be used to participate in programming and control the management of PLC, making it more convenient to use. In order to fully utilize the role of computers, one computer can control and manage multiple PLCs, up to 32 of which can be used. One PLC can also communicate and exchange information with two or more computers to achieve multiple monitoring of the PLC control system. PLCs can also communicate with each other, one-on-one communication with PLCs, communication with several PLCs, and up to tens or hundreds of PLCs. PLC can also communicate with intelligent instruments and intelligent execution devices (such as frequency converters) through networking, exchange data, and operate with each other. Can be connected to form a remote control system, with a system range of up to 10 kilometers or more. It can form a local network, not only PLC, but also high-end computers and various intelligent devices can enter the network. Bus network or ring network can be used. The internet can also be nested. Networks can also be bridged. Networking can organize thousands of PLCs, computers, and intelligent devices into one network. Nodes between networks can communicate and exchange information directly or indirectly.
Networking and communication are meeting the needs of the development of computer integrated manufacturing systems (CIMS) and intelligent factories today. It can enable industrial control from point to line and then to surface, connecting equipment level control, production line control, and factory management control as a whole, thereby creating higher efficiency. This infinitely beautiful prospect has become increasingly clear in front of our generation.
