I. Overview PLC control cabinet is an electric cabinet with PLC as the core control unit, which can realize the control of fans, pumps, electric valves and other field equipment. According to the actual control scale, the automatic control of single cabinet can be realized, and the DCS control system can also be composed of multi cabinets through industrial Ethernet or industrial fieldbus network. It is widely used in electric power, metallurgy, chemical industry, paper making, environmental protection sewage treatment and other operations. It can realize remote assistant control, data acquisition, real-time data transmission, online data analysis, large data analysis and application, etc.   â?¡. Systematic Programmes and Characteristics 1. PLC + Relay Control System Suitable for small-scale control system. The relay hardware circuit is mainly used to control the system, and the simple automation control is mainly realized. Principle: In manual mode, the user can control the field equipment manually according to the button instructions on the operation panel of the control door; in automatic mode, the small PLC can output the contact signal of the intermediate relay through the logical operation of the program according to the received control conditions, and drive the control unit of the equipment (such as the controlled motor) with this signal. AC contactor, electric control valve, solenoid valve, frequency converter, soft starter, etc., to achieve the purpose of control. Advantages: Easy operation, low technical requirements for operators. The system runs safely and reliably, maintains conveniently and costs low.