Since the air is compressible, the stability of the pneumatic transmission system is poor, which has a great impact on the accuracy of position control and speed control. Therefore, the positioning signal of the pneumatic actuator usually produces an execution error with a resolution of about 3~5%. This leads to inaccurate valve positioning correction, the valve should be closed but not closed, and at the same time, it cannot be positioned due to the execution error caused by the resolution, so that the position feedback of the positioner continuously corrects the position, and the entire control loop swings and oscillates or loses control. In addition, because the working medium air itself has no lubricity, an oil lubricating device must be installed in the air circuit.

Compared with gas, the compressibility of hydraulic oil is small, so the hydraulic transmission works smoothly, the reversing impact is small, it is easy to realize quick start, braking and frequent reversing, the operation and control are convenient, and a wide range of stepless speed regulation can be realized. Realize automatic control of large load, high precision and complex motion.

Generally, mineral oil is used as the working medium, the relative moving surface can be self-lubricated, and the service life is long.