The Application of PLC in Electromechanical Equipment
A Programmable Logic Controller (PLC) is an industrial control device that has developed rapidly and is widely used since the 1960s. It is the preferred product for modern industrial automatic control, standing alongside robotics and CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing) as one of the three pillars of production automation.
Before PLC
Before the advent of PLCs, production line control primarily used relays. Controlling a production line required numerous hardware devices and complex wiring, wasting hardware, delaying construction periods, and increasing product costs.
Application Areas of PLC
PLCs are widely used in both domestic and international industries, including steel, metallurgy, chemical, light industry, food, power, machinery, transportation, automotive manufacturing, construction, environmental protection, and public utilities.
Applications Based on Control Types
1. Sequential Control of Switching Values
The most common and fundamental function of PLCs is replacing relay control systems. This can be used for controlling single equipment, multi-machine control, and automated production lines, such as motor control, machine tool electrical control, elevator automation, automated production lines, CNC machines, and traffic lights.
2. Analog Process Control
Besides switching values, PLCs can control continuously changing analog quantities like pressure, speed, flow, liquid level, voltage, and current. Using various sensors, these analog quantities are converted to electrical signals, then transformed into digital quantities via A/D modules for PLC processing. The processed digital quantities are converted back to analog quantities through D/A modules for output control. This functionality is mainly used in systems like constant pressure water supply and boiler temperature control.
3. Motion Control
PLCs provide single-axis or multi-axis position control modules for driving stepper or servo motors. These modules enable linear or circular motion control, primarily used in machine tools, robots, and assembly machinery.
4. Data Processing
PLCs offer functions for data arithmetic operations, data transfer, data conversion, data sorting, and bit operations, facilitating data collection, analysis, and processing. These data can be transmitted to other intelligent devices via communication systems, compared with reference values in memory, or used to generate various reports. Data processing functions are generally employed in medium and large-scale control systems across various industries.
5. Communication Functions
To meet the needs of modern industrial automation systems for centralized and remote management, PLCs can communicate with other PLCs, microcontrollers, printers, and upper-level computers, exchanging information seamlessly.
Comparison of PLC Control with Other Control Methods
1. PLC vs. Relay Control
Traditional relay control can only handle switching values, whereas PLCs manage both switching and analog values, and can network with computers for hierarchical control.
Ladder diagrams in PLC programming resemble relay control circuits, making it easy to transition from relay to PLC control.
2. Differences between PLC and Relay Control Systems
2.1. Component Composition: Relay control circuits consist of many hardware relays, whereas PLCs use “soft relays” within the device.
2.2. Number of Contacts: Relays have limited contacts (4-8 pairs), while “soft relays” in PLCs can have virtually unlimited programmable contacts.
2.3. Control Method: Relay systems use hard-wired connections, making the control function fixed, whereas PLCs use software programming for flexible control.
2.4. Operation Method: In relay circuits, each relay is constantly engaged when powered. In PLCs, “soft relays” are cyclically scanned, with each relay engaged for only a brief period.
3. PLC vs. Microcontroller
The core of a PLC is a microcontroller, with corresponding interface circuits (hardware) and monitoring programs (software). However, PLCs differ from standalone microcontroller systems in several ways:
3.1. High Reliability: PLCs employ measures like optical isolation, digital filtering, and electromagnetic shielding, making them more reliable than microcontrollers, which are more susceptible to environmental interference.
3.2. Ease of Programming: PLCs use ladder diagrams that are intuitive and derived from actual wiring diagrams, unlike microcontrollers that require specialized programming languages.
3.3. Convenient Interface Connection: PLCs have easily configurable interfaces and standard outputs (relay, transistor, thyristor), along with A/D, D/A, and RS232 interfaces, eliminating the need for users to handle complex setup issues.
3.4. Application Scope: PLCs are widely used for process control, while microcontrollers are often utilized in real-time control systems.
Characteristics of PLC
PLC is widely used in modern automated production equipment due to its high interference resistance, reliability, cost-effectiveness, and simple programming. It acts as the “brain” of the production line, the microprocessing unit.
High Reliability: PLCs use relays or optocouplers for input/output ports, with added isolation and anti-interference measures, allowing them to operate reliably even in harsh environments.
Compact Size: By using large-scale integrated circuits and microprocessors, and replacing hard-wiring with software programming, PLCs are compact and easy to install.
Good Versatility: PLCs feature modular structures, including CPU modules, power modules, communication modules, PID modules, and analog input/output modules. These modules can be flexibly configured to form different control systems by simply selecting the appropriate modules and designing the corresponding programs.
Convenience and Flexibility: Once the control object and input/output hardware structure are selected, changes in control methods or adjustments to control objects can be easily made by modifying the program without extensive rewiring. This reduces on-site debugging workload and improves efficiency.
In summary, the application of PLCs in electromechanical equipment offers significant advantages in terms of flexibility, reliability, and ease of use, making them indispensable in modern industrial automation.
