Based on its characteristics, PLC has been widely used. Currently, its main applications are:
1. Used for sequence control
Sequential control generates the required switching outputs based on the current and historical status of relevant input switching quantities, enabling the system to operate in a specific sequence. This is the most basic control mechanism for system operation and the most commonly used control mechanism in discrete manufacturing processes.
Commonly used sequence control methods include:
Random control, which implements control based on randomly occurring conditions;
Motion control, which involves implementing control based on the completion status of the action;
Time control, implementing control based on the progress of time;
Count control, which implements control based on the cumulative count;
Hybrid control, which includes a combination of the above control methods;
There are other controls besides the ones mentioned above, and so on.
Using a PLC to implement sequential control is the original purpose of the PLC and also its strength. In the field of sequential control, no other controller has been able to replace it to date.
2. Used for process control
Process control utilizes analog signals. Analog signals generally refer to continuously changing quantities, such as physical quantities like current, voltage, temperature, and pressure. The purpose of process control is to generate the required switching or analog outputs based on the current and historical inputs of relevant analog signals, so that the system operating parameters can function according to certain requirements. It is the most commonly used control method in continuous production processes.
There are many types of process control.
The use of PLCs for process control has become a trend. This is because PLCs are cheaper than other control methods, and they can easily perform other controls while controlling analog signals. Furthermore, the development and application of various process control modules, along with the availability of related software, have made PLC-based process control much easier, and programming is also simple. Therefore, some manufacturers now use PLCs for analog control more than for sequential control.
3. Used for motion control
Motion control mainly refers to the control of the position, velocity, and acceleration of a work object. It can be single-axis, controlling the object's linear motion; or multi-axis, controlling the object's planar, three-dimensional, or even angular transformation motions. Sometimes, multiple objects can be controlled, and the motions of these objects may need to be coordinated.
Numerical control (NC) technology, which originated in the United States in the 1950s, is a motion control technology that has been continuously developed and improved based on electronic computers and the application of pulse quantities. Today, it has reached a very sophisticated stage and has become an important pillar of modern automation technology.
PLCs also have the capability to handle pulse signals. PLCs have pulse signal input points or modules that can receive pulse signal inputs (PI). PLCs also have pulse signal output points or modules that can output pulse signals (PO). With these two functions—PI/PO processing—and the PLC's existing data processing and calculation capabilities, motion control can be performed entirely based on the principles of NC (Non-Controlled Logic).
Implementing this control using a PLC is significantly cheaper than using an NC. Furthermore, it can perform other controls simultaneously with motion control. With the development and application of various motion control modules for PLCs, along with the availability and use of related software, performing various motion controls with PLCs has become very easy, and programming can be done using NC languages, making it very simple.
In recent years, PLCs specifically designed for motion control have emerged, namely P (Programmable) M (Motion) C (Controller), or PMC for short. This provides an excellent platform for PLCs to be used in motion control with higher precision, longer travel distances, more controlled coordinates, and easier operation. Therefore, using PLCs for motion control can, to a considerable extent, replace more expensive CNC systems.
4. Used for information control
Information control, also known as data processing, refers to data acquisition, storage, retrieval, transformation, transmission, and data table processing.
With the development of technology, PLCs can be used not only for system operation control, but also for system information control.
There are two types of PLCs used for information control: dedicated and dual-purpose.
Dedicated: The PLC is only used for data acquisition, processing, storage and transmission.
Dual-purpose: While implementing control via PLC, it can also implement information control.
PLCs are used for information control, or perform information control as a secondary function. This is not only an important aspect of PLC applications, but also the foundation of information technology.
5. For remote control
Remote control refers to the detection and control of the behavior and effects of a remote part of a system. PLCs have multiple communication interfaces, strong networking and communication capabilities, and new networking modules and structures are constantly being introduced. Therefore, PLC remote control is very convenient.
PLCs can form a control network. They can communicate, exchange data, and operate interoperably. Dozens or even hundreds of PLCs can participate in the communication. Networks can also be interconnected. Thus, the number of PLCs participating in the communication is virtually unlimited.
PLCs can also be connected to intelligent sensors and intelligent actuators (such as frequency converters) to form an equipment network. They can also communicate, exchange data, and operate interoperably. They can be connected to form a remote control system with a range of tens, hundreds, or even larger. This remote control not only improves control capabilities but also simplifies hardware wiring and maintenance.
PLCs and programmable terminals can also be networked and communicated. PLC data can be displayed on it, and data can also be written to the PLC through it, making it the interface for people to operate the PLC.
PLCs can communicate with computers and be integrated into information networks. Utilizing the powerful information processing and display capabilities of computers, SCADA (Supervisory Control and Data Acquisition) can be implemented for computer-controlled systems. Furthermore, computers can be used for PLC programming, monitoring, and management.
PLCs also have Ethernet modules, which allow them to connect to the internet. They can also be configured with their own website address and webpage. Some factories controlled by such PLCs are called transparent factories. Any computer on Earth with internet access, provided it has the necessary permissions, can directly access them.
Remote control is incredibly powerful. It can enhance the control capabilities of PLCs, expand the control area, and improve control efficiency. In short, remote control has become an important aspect of PLC applications.
The five control methods introduced above, the first three of which are designed to enable automation in different systems. Information control aims to achieve informatization, with the goal of building automation on an information-based foundation, combining management and control, and ultimately achieving seamless connection between supply, production, and sales to ensure the benefits of automation.
Remote control enables automation built on information technology to be performed remotely. It allows for the aggregation of information from all corners, ensuring information integrity and facilitating comprehensive information use. It also expands automation from the local device level to the global production line level, workshop level, and even the factory and regional level, making it possible to establish automated factories and digital cities. Clearly, this large-scale, wide-ranging automation and informatization will have greater power and yield greater benefits.
However, with the advancement of automation, informatization, and remote operation, the system will become increasingly complex. Therefore, it is essential to implement controls to manage these systems. Otherwise, if circumstances change or malfunctions occur, and a timely response is not possible, all the benefits of these controls will be lost.
PLCs, ironically, possess the capability to control these systems. They implement control by processing information and also have numerous self-diagnostic functions. By fully utilizing these two advantages, enabling PLCs to possess a certain degree of self-adaptation and self-diagnostic ability when implementing the aforementioned controls, it is possible to achieve intelligent control after achieving automation, informatization, and remote control. This is also the inevitable trend in the development of these control systems.
Of course, PLCs are not the only ones capable of performing so many control functions, but it is a recognized trend that PLCs are becoming the main players in this field.
