Abstract: A simulation study of the PID control characteristics of a compact and energy-saving variable frequency closed-loop hydraulic power unit was conducted. The results show that PID control can improve the system's response characteristics and enable the system speed and pressure to achieve the predetermined control effect. Keywords: variable frequency speed regulation; hydraulic power unit; closed oil circuit; PID control[b][align=center]The Response Characteristic of a Variable Frequency Close Hydraulic Power Un it Using PID Control HUANG Fang2p ing1, 2. State Key Laboratory of Fluid Power Transmission and Control, Zhejiang University, Hangzhou 310027, China)[/align][/b] Abstract: In this article, a compact structure and energy - saving variable frequency close hydraulic power unit is in2 troduced. The control character of this unit using P ID has studied. The response characteristic of the system has imp roved using P ID control. The pre lanned control effect of the speed and pressure has speed reached. Key words: variable speed control; hydraulic power unit; closed oil circuit; P ID control Variable frequency hydraulic technology is a new type of energy-saving transmission technology, widely used in machine tools, hydraulic lifting machinery, injection molding machines and many other mechanical equipment. This technology uses a variable frequency drive to change the input speed of the hydraulic pump, thereby adjusting the output flow rate of the hydraulic pump to regulate the speed of the system's actuators. A fixed displacement pump can replace the original variable displacement pump. Variable frequency hydraulic transmission systems have advantages such as fewer components, easy system integration, small size, light weight, and high efficiency. This transmission system is a global energy-saving system; the electric motor and the hydraulic cylinder or motor of the actuator can achieve a relatively ideal power match, resulting in significant energy savings. [b]1 Composition Principle of Variable Frequency Closed-Loop Hydraulic Power Unit 1.1 Hydraulic Principle Structure of the System[/b] The hydraulic structure principle diagram of the variable frequency closed-loop hydraulic power unit is shown in Figure 1. This power unit mainly consists of two parts: a main hydraulic circuit and a replenishing oil circuit. The main oil circuit uses a bidirectional hydraulic pump 2, and the rotation direction of the bidirectional hydraulic pump 2 is adjusted by controlling the speed of the electric motor 1 through a frequency converter. Oil leakage in the main hydraulic circuit is replenished by the replenishment circuit through shuttle valve 3. The relief valve 5 in the main hydraulic circuit provides safety protection for the system. The replenishment circuit mainly consists of a replenishment motor, a replenishment pump, an accumulator, and a pressure relay. The pressure relay 7 replenishes oil to the main hydraulic circuit. [b][align=center]For more details, please click: Research on PID Control of Variable Frequency Closed-Circuit Hydraulic Power Unit[/align][/b]