PLC has big and small, so its control range can also be big and small. Small only control a device, or even a component, a site; large can control multiple devices, a production line, to the entire factory. It can be said that the size of the industrial control occasions are inseparable from the PLC.
Initially, PLC is mainly used for switching logic control. With technological advances, PLC applications continue to expand. Nowadays, not only for switching control, but also for analog and digital control, can collect and store data, but also to monitor the control system; can also be networked, communication, to achieve a wide range of cross-regional control and management.
PLC has increasingly become an important role in the family of industrial control devices.
1, for switching volume control
PLC control switching capacity is very strong. The control of the number of points in and out, less than a dozen points, dozens of points, more can be hundreds, thousands, or even tens of thousands of points, because it can network, the number of points is almost unlimited, no matter how many points can be controlled by the control of the logic of the problem can be a variety of things: combinations, timing, immediate, delayed, do not need to count, the need to count, fixed order, random work, etc., can be carried out.
The hardware structure of PLC is variable and the software program is programmable and very flexible when used for control. If necessary, it can write multiple sets or groups of programs and call them as needed. It is well adapted to the industrial field of multi-operating conditions, multi-state transformation needs.
Examples of switching control with PLC is a lot of metallurgy, machinery, light industry, chemical industry, textiles, etc., almost all industrial sectors need to use it. At present, the PLC first goal, but also other controllers can not be compared with it, is that it can be easily and reliably used for switching control.
2,For analog control
Analog quantities, such as current, voltage, temperature, pressure, etc., its size is continuously changing. Industrial production, especially continuous production processes, often to control these physical quantities.
As an industrial control electronics, PLC if these quantities can not be controlled, that is a major shortfall, for this reason, the PLC manufacturers are in this area to carry out a lot of development. At present, not only large, medium-sized machine can be analog control, is a small machine, but also for such control.PLC analog control, to be configured with analog and digital conversion of A/D, D / A unit. It is also an I/O unit, but it is a special I/O unit.
A/D unit is the external circuit of the analog, converted to digital, and then sent to the PLC; D / A unit, is the PLC's digital into analog, and then sent to the external circuit. As a special I/O unit, it still has the I/O circuit anti-interference, internal and external circuit isolation, and input and output relays (or internal relays, which is also an area of the PLC's working memory, can be read and written) to exchange information and so on.
The A in A/D here is mostly current, or voltage, and also temperature.The A in D/A is mostly voltage, or current. Voltage, current range of 0 ~ 5V, 0 ~ 10V, 4 ~ 20mA, some can also handle positive and negative values. Here D, small machines are mostly 8-bit binary number, medium and large-scale more than 12-bit binary number.A/D, D/A has a single circuit, there are also multi-channel. Multi-way accounted for more input and output relays. With A/D, D / A unit, the rest of the processing are digital quantities, which has information processing capabilities of the PLC is not difficult. Medium and large PLC processing capacity is stronger, not only for digital addition, subtraction, multiplication, division, but also square, interpolation, but also floating-point operations, some have PID instructions, can be deviated from the amount of proportionality, differentiation, integration, and then produce the corresponding output, the computer can calculate it can calculate almost all.
In this way, the PLC to achieve analog control is completely possible.
PLC for analog control, there are A/D, D / A combined unit, and available PID or fuzzy control algorithms to achieve control, you can get a very high quality of control. The advantage of analog control with PLC is that while analog control is performed, switching can also be controlled. This advantage is not available in other controllers, or the realization of control is not as convenient as PLC. Of course, if the system is purely analog, using a PLC may not be as good as using a regulator in terms of performance to price ratio.
3, for motion control
The actual physical quantities, in addition to switching and analog, and motion control. Such as the displacement of machine components, often expressed in digital quantities. Motion control, the effective way is NC, that is, digital control technology. This is a computer-based control technology born in the United States in the 1950s. Today it is very popular and also very perfect.
At present, the advanced countries of the metal cutting machine tools, the ratio of numerical control has more than 40% ~ 80%, and some even higher.PLC is also based on computer technology, and increasingly improved.PLC can receive counting pulse, the frequency can be as high as a few k to tens of kilohertz, can be used in a variety of ways to receive the pulse, but also can be received in multiple ways. Some PLC also has a pulse output function, the pulse frequency can also be up to tens of k, with these two functions, coupled with the PLC has the data processing and arithmetic capabilities, if equipped with the appropriate sensors (such as rotary encoders) or pulse servo device, it can be fully in accordance with the principle of NC to achieve a variety of control. High- and mid-range PLC, but also the development of NC units, or motion units, can be realized point control. Motion unit can also realize curve interpolation, can control the curve motion.
Therefore, if the PLC is configured with such a unit, it is possible to use the NC method for digital control. Newly developed motion unit, and even issued the NC technology programming language, for better digital control with PLC provides a convenient.
4, for data acquisition
With the development of PLC technology, its data storage area is getting bigger and bigger. Such as the PLC of Devison, its data storage area (DM area) can reach 9999 words. Such a large data storage area can store a large amount of data. Data acquisition can be done with a counter, which cumulatively records the number of pulses captured and regularly dumps them into the DM area. Data acquisition can also be used A/D unit, when the analog quantity is converted to digital quantity, and then regularly transferred to the DM area, PLC can also be configured with a small printer, the DM area of the data regularly typed out.
PLC can also communicate with the computer, the computer to read out the data in the DM area, and then by the computer to process these data. At this point, the PLC becomes the data terminal of the computer.
Power users have used PLC to record real-time user electricity consumption, in order to realize the different time of electricity consumption, different pricing charging methods, to encourage users to use more electricity in the trough of electricity consumption, to achieve the purpose of rational use of electricity and saving electricity.
5,For signal monitoring
PLC self-test signals are many, there are many internal devices, most users do not give full play to its role. In fact, it can be fully utilized to monitor the work of the PLC itself, or to monitor the control object. For a complex control system, especially automatic control systems, monitoring and further self-diagnostics is very necessary, it can reduce system failures, faults or to find, can improve the cumulative average trouble-free operation time, reduce the time to repair the faults, and improve the reliability of the system.
6, for networking, communication
PLC networking, communication capabilities are very strong, there are constantly new networking structure launched.
PLC can be connected to a personal computer for communication, the computer can be used to participate in the programming and management of the PLC control, so that the PLC with more convenient.
In order to give full play to the role of the computer, a computer can be implemented to control and manage more than one PLC, up to 32 units. Can also be a PLC and two or more computers to communicate, exchange information to achieve more on the PLC control system monitoring.
PLC and PLC can also communicate, can be one-to-one PLC communication, can be several PLC communication, can be as many as dozens, hundreds.
PLC and intelligent instrumentation, intelligent actuators (such as inverters), can also be networked communication, exchange of data, mutual operation. Can be connected to a remote control system, the system range can be as large as 10 kilometers or more. Can form a local network, not only PLC, but also high-grade computers, all kinds of intelligent devices can also enter the network. Available bus network, also available ring network. The network can also set the network. Network and network can also be bridged. Networking can be thousands of PLCs, computers, intelligent devices organized in a network. The nodes between the network can communicate and exchange information directly or indirectly.
Networking, communication, is adapted to today's computer integrated manufacturing system (CIMS) and intelligent factory development needs. It can make industrial control from the point (Point), to the line (Line) and then to the surface (Aero), so that the equipment level control, production line control, factory management control into a whole, which can create higher efficiency. This infinitely bright future has been more and more clearly shown in front of our generation.