Four common servo control systems are as follows:

Hydraulic servo control system

Hydraulic servo control system is based on motor to provide power, the use of hydraulic pump mechanical energy into pressure, promote hydraulic oil. Through the control of various valves to change the flow of hydraulic oil, so as to promote the hydraulic cylinder to make different strokes, different directions of the action, to complete a variety of equipment for different action needs. The hydraulic servo control system can be divided into machine-hydraulic, electro-hydraulic, gas-hydraulic and so on according to the different ways of obtaining and transmitting the deviation signal, among which the machine-hydraulic and electro-hydraulic control system are more used.

Ac servo control system

Ac servo control system includes ac servo system based on asynchronous motor and ac servo system based on synchronous motor. In addition to the characteristics of good stability, good rapidity and high precision, it has a series of advantages. Its performance index can be measured from the speed range, positioning accuracy, steady speed accuracy, dynamic response and operation stability. 

Dc servo control system

The working principle of AC servo control system is based on the law of electromagnetic force.  Related to the electromagnetic torque are two independent variables, the main flux and the armature current, which control the excitation current and the armature current respectively, so that the torque and speed control can be easily carried out. On the other hand, from the control point of view, dc servo control is a single input single output single variable control system, the classical control theory is completely suitable for this system, therefore, it by virtue of simple control, speed control performance is excellent, has occupied the leading position in the CNC machine feed drive. 

Electro-hydraulic servo control system

It is a feedback control system composed of electrical signal processing device and hydraulic power mechanism. The most common are electro-hydraulic position servo system, electro-hydraulic speed control system and electro-hydraulic force or torque) control system.