Different structure. DC servo motor has carbon brushes inside, brushless servo motor has no carbon brushes inside.
Different principles. DC servo motor is based on the principle of rotating the energized coil in the magnetic field by force, while AC servo motor is to change the distribution of the stator magnetic field to realize the control of the motor.
Brushless servo motors rely on Hall elements to detect the position of the magnetic poles and control the corresponding current to drive the motor.
Different advantages and disadvantages. DC servo motor maintenance is convenient, but electromagnetic interference, environmental requirements, and brushless servo motor small size and light weight, large force response, high speed, small inertia, smooth rotation torque stability,
Control complexity and high degree of intelligence, electronic phase change flexible, energy efficient, small electromagnetic radiation, long life and other advantages.