1, the servo system (servomechanism) is that the object position, orientation, state, etc. to follow the output of the controlled quantity target input (or reference value) of the automatic control system of any changes. Mainly by servo pulse positioned substantially can be understood, the servo motor receives a pulse, the pulse will correspond to a rotation angle, thereby achieving displacement, because the servo motor itself includes issuing a pulse, so that each of the servo motor a rotation angle, a corresponding number of pulses will be issued, so that the servo motor and a pulse forming received echo, or called closed loop, this way, the system knows the number of pulses sent to the servo motor, while the number of pulses received back so, it is possible to very accurately control the rotation of the motor to achieve precise positioning can be achieved 0.001mm. DC servo motors are brush and brushless motors. Brush motor is low cost, simple structure, a large starting torque, wide speed range, the control is easy, requiring maintenance, but it is not convenient to maintain (replace the brushes), electromagnetic interference, there are demands on the environment. So it can be used in cost-sensitive general industrial and civil applications. 
The brushless motor of small size, light weight, large output, fast response, high speed, small inertia, smooth rotation, torque stability. Control complexity, easy to implement intelligent, flexible manner which is electronically commutated and to be a sine wave or square wave commutation commutation. Motor maintenance, high efficiency, low operating temperature, electromagnetic radiation is very small, long life, can be used in various environments. 
2, AC servo motor is a brushless motor is divided into synchronous and asynchronous motors, motion control currently in use are generally synchronous motor, its power range, can do a lot of power. Large inertia, the maximum rotational speed is low, and as the power increases rapidly decreased. Therefore suitable for low speed applications running smoothly. 
3, internal rotor of the servo motor is a permanent magnet, the drive control of the U / V / W three-phase power electromagnetic field is formed, the rotor is rotated under the action of this magnetic field, while the motor comes encoder feedback signal to the drive, the drive according to the feedback value is compared with a target value, adjusting the angle of rotation of the rotor. Accuracy depends on the accuracy of the servo motor encoder (line number). 
The difference between AC servo motors and brushless DC servo motor in function: AC servo better because the control is a sine wave, the torque pulsation small. DC servo trapezoidal wave. But DC servo is relatively simple and inexpensive.