It uses hydraulic oil as working medium, converts mechanical energy of prime mover into pressure energy of hydraulic oil through power element (oil pump), then passes through control element, and converts pressure energy into mechanical energy with help of actuator (cylinder or oil motor), drive load to realize linear or rotary motion, adjust force and speed of actuator through remote control of control element and adjustment of flow.
When outside world disturbs above system, output of actuator generally deviates from original setting value, resulting in a certain error hydraulic control: like hydraulic transmission, system also includes power components, control components and actuators, power is also transmitted through oil.
Difference between the two is that hydraulic control has a feedback device. Function of feedback device is to feed back output of actuator (displacement, speed, force and other mechanical quantities) and compare it with input (which can be variable or constant), use compared deviation to control system, so that output of actuator changes or remains constant with change of input. It is a hydraulic transmission system that constitutes a closed-loop circuit, also called hydraulic servo system or hydraulic servo system.
On-off or logic control elements are used in hydraulic transmission system, their control purpose is to maintain stability of set value or simply change direction, also called fixed value and sequence control elements.
Servo control elements are used in hydraulic control system, which has a feedback structure and is controlled by electrical devices. It has high control accuracy and response speed, controlled pressure and flow often change continuously. Output power can be amplified.
Proportional control is a kind of control between above two. Proportional control valve used is a new type of electro-hydraulic control element developed on basis of on-off control elements and servo control elements. Some characteristics of the two types of components can not meet requirements for manual on-off control, but also do not require strict pollution control requirements of servo valve for hydraulic system.

 

In current four major types of transmission methods (mechanical, electrical, hydraulic and pneumatic), no power transmission is perfect, and hydraulic transmission has following extremely obvious advantages:
From structural point of view, output power per unit weight and output power per unit size are stronger than those in four types of transmission modes, and have a large moment-to-inertia ratio. Under condition of transmitting same power, hydraulic transmission device is small in size, light in weight, small in inertia, compact in structure and flexible in layout.
In terms of work performance, speed, torque and power can be adjusted steplessly, with fast action response, quick reversal and speed change, wide speed regulation range, and speed regulation range can reach 100:1 to 2000:1; action is fast, control and adjustment are relatively simple, operation is more convenient and labor-saving, it is convenient to cooperate with electrical control, and connection with CPU (computer) is convenient to realize automation.
From point of view of use and maintenance, self-lubrication of components is good, it is easy to realize overload protection and pressure keeping, which is safe and reliable; components are easy to realize serialization, standardization and generalization. All equipment using hydraulic technology is safe and reliable.
From perspective of economic cost: hydraulic technology has strong plasticity and variability, which can increase flexibility of flexible production, it is easy to change and adjust production procedures. Manufacturing cost of hydraulic components is relatively low, and adaptability is relatively strong. Combining hydraulics with new technologies such as microcomputer control to form "mechanical-electrical-hydraulic-optical" integration has become trend of world development, which is convenient for digitalization.

Everything is divided into two, and hydraulic transmission is no exception:
Hydraulic transmission inevitably leaks due to relatively moving surface, and oil is not absolutely incompressible. In addition to elastic deformation of oil pipe, hydraulic transmission cannot obtain a strict transmission ratio, so it cannot be used in inline transmission chain of machine tools such as machining threaded gears.
There are edge loss, local loss and leakage loss in process of oil flow, and transmission efficiency is low, which is not suitable for long-distance transmission.
Under conditions of high temperature and low temperature, it is difficult to adopt hydraulic transmission.
In order to prevent oil leakage and meet certain performance requirements, manufacturing precision of hydraulic components is required to be high, which brings certain difficulties to use and maintenance.
Failure is not easy to check, especially in units with less popular hydraulic technology. This contradiction often hinders further promotion and application of hydraulic technology. Hydraulic equipment maintenance needs to rely on experience, and it takes a long time to train hydraulic technicians.