There are two main types of servo motors: rotary motors and linear motors.
Rotary Servo Motors
Rotary servo motors are what most people think of when they think of servo motors. There are several types of rotary servo motors: AC servo motors, stepper servo motors, brushed DC servo motors, permanent magnet synchronous servo motors, and brushless DC servo motors. The motion of a rotary servomotor is usually converted to linear motion by using threads (ball screws or leadscrews) or by using belts and pulleys.
Any motor that has a closed-loop feedback device that responds to disturbances in the motion profile is categorized as a servomotor.
Typically, a servo amplifier (or driver) is coupled to a permanent magnet synchronous motor and provides feedback of speed and position via an encoder mounted on the shaft. The amplifier may also receive feedback signals from encoders mounted at other locations in the application, such as on a screw or drive pulley, to close the loop on the motion profile.
The feedback signal changes the amount of current the motor receives from the amplifier so that more current is delivered when the motor needs more torque and less current is delivered when the motor does not. In torque control mode, the servo motor increases and decreases voltage to maintain speed and always deliver the specified torque.
Linear Servomotors
Linear servomotors are flat servomotors where the rotor is located inside and the coils are located on the outside of the removable U-shaped channel. Both of these servo motor types are becoming more popular as servo motor prices continue to drop.