Deep Comparison and Case Analysis of PLC Control and Relay Control

Aug 29, 2023

1、 Logic control method

PLC control

PLC is a controller designed specifically for industrial environments based on digital computing electronic systems. It uses memory logic to achieve control by executing pre written instructions. The control logic of PLC adopts relay symbol diagram programming, which is easy to understand. For example, PLC can achieve logical control relationships such as AND, OR, and NOT, making it very suitable for complex logical control needs.

Relay control

Relay control is a traditional control method that achieves logical control through physical relays. The relay control circuit is connected through wires and utilizes the normally open and normally closed states of the relay contacts to achieve logical functions. The advantages of this control method are intuitive and easy to understand, but in complex logic control, the wiring is complex and difficult to maintain.

2、 Sequential control method

PLC control

PLC has powerful sequential control function, which can easily achieve complex sequential control. By using state diagrams or step-by-step instructions, PLC can easily achieve sequential control of equipment startup, operation, and stop. In addition, PLC can also achieve parallel or concurrent control to meet the needs of complex industrial control.

Relay control

Relay control mainly achieves the start, operation, and stop of equipment through manual operation of buttons, travel switches, etc. In simple control systems, relay control can achieve basic sequential control, but in complex industrial environments, relay control has poor flexibility and is difficult to meet requirements.

3、 Control speed

PLC control

PLC adopts digital operation method, and its processing speed is faster than relay control. Especially in real-time control, PLC can quickly respond to external input signals and adjust the equipment operation status in a timely manner. For example, in a temperature control system, PLC can quickly adjust the operating status of heating or cooling equipment to reach the set temperature.

Relay control

The relay itself in the relay control system is a mechanical structure, and its response speed is relatively slow. In complex industrial control systems, the response speed of relay control may not meet the requirements. For example, in traffic signal control, if the response speed of relay control is too slow, it may lead to traffic congestion or accidents.

4、 Flexibility and scalability

PLC control

PLC has high flexibility and scalability, making it easy to upgrade or expand the system. PLC can be connected to other devices through a serial communication interface to achieve remote monitoring and control. In addition, PLC can also achieve different control requirements and adapt to different industrial environments by writing programs. For example, in the food processing industry, PLC can easily adjust control programs according to different production process requirements.

Relay control

The flexibility of the relay control system is poor, making it difficult to upgrade or expand the system. When it is necessary to adjust the control program, it is necessary to rewire or change hardware components, which is a significant workload. For example, in the power system, if new control functions need to be added, new relays and circuits may need to be redesigned and installed, resulting in higher costs.

5、 Counting function

PLC control

PLC has powerful counting function, which can achieve high-speed counting and precise counting. By using special counter modules, PLC can achieve the expansion and optimization of counting functions. For example, in production line counting, PLC can accurately count the quantity and position of products, improving production efficiency.

Relay control

The counting function of relay control is weak, making it difficult to achieve high-speed counting and precise counting. When counting operations are required, it may be necessary to use mechanical counters or other specialized counting equipment, which is costly. For example, in the textile industry, if it is necessary to accurately count the quantity and quality of yarn, the accuracy and speed of relay control may not meet the requirements.

6、 Reliability and Maintainability

PLC control

The PLC adopts a modular design with high reliability. The hardware and software of PLC have undergone strict testing and verification, and have high reliability. In addition, PLC has self diagnostic function, which can quickly detect and locate faults, improving maintainability. For example, in the chemical industry, PLC control systems can operate stably for a long time, ensuring the stability and safety of the production process.

Relay control

The relays in relay control systems are susceptible to mechanical wear and electrical noise, resulting in relatively low reliability. In addition, the wiring in the relay control system is relatively complex, prone to problems such as poor connections or incorrect connections, making maintenance difficult. For example, in the power system, relay control systems may experience malfunctions or refusal to operate, which affects the stable operation of the power system.

7、 Fault handling

PLC control

PLC has powerful fault diagnosis and processing capabilities. PLC can detect faults during operation and take corresponding measures to handle them. For example, when the PLC detects abnormal input signals, it can trigger fault alarms or safety shutdown measures to ensure equipment safety. In addition, PLC can also record fault information for easy troubleshooting and maintenance.

Relay control

Fault detection and handling in relay control systems are relatively simple. When a malfunction occurs in a relay control system, it is usually triggered by sound, light, and other means. When troubleshooting, manual troubleshooting and maintenance are required. For example, in traffic signal light control, if a relay malfunctions, it may cause the signal lights to not switch normally and require manual troubleshooting and maintenance.

8、 Summary

In summary, there are certain differences between PLC control and relay control in logic control mode, sequence control mode, control speed, flexibility, scalability, counting function, reliability and maintainability, as well as fault handling. PLC control is widely used in modern industrial control, suitable for complex logic control, sequential control, and high-speed control scenarios, with high reliability and maintainability. As a traditional control method, relay control still has certain applications in simple industrial control scenarios, but in complex industrial control, its flexibility and reliability may not meet the requirements.

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