A servo motor is a rotary electric motor that allows for precise control of angular position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servo motors.
Brush DC Servo Motor
Brush DC motors are the simplest form of a servo motor. The construction of the motor has copper windings on the rotor and permanent magnets on the stator. The windings are energized on the rotor to set up a rotating field which in turn causes the windings on the rotor to rotate. In order to get a rotating field to occur on the motor, rotor current must be switched to adjacent coils or windings. This is done through brushes ridding on commutation segments located on the motor rotor. As the rotor rotates, different coils are energized through the brushes and commutation segments. With the addition of an encoder, such as the KCD Absolute Kit encoder, to the tail shaft of the DC motor, it can be used in a servo control loop and becomes a DC servo motor.
Brushless DC Servo Motor
As the name implies a brushless DC, BLDC, servo motor does not have any brushes or commutation segments to switch current through windings to produce a rotating field. The brushless DC motor design is essentially inverted from that of the DC servo motor in that the motor windings are located in the stator and the permanent magnets are located on the motor’s rotor. This type of design is beneficial as it moves the heavy copper wire from the rotor and replaces it with much lighter permanent magnets. This reduces the rotational inertia of the motor rotor which allows the motor rotor to accelerate and decelerate significantly quicker than a brush DC motor. In the world of motion control, the ability to accelerate and decelerate a motor quicker yields a much higher performance machine.
The draw back to brushless DC motors is that they require some form of electrical commutation signals produced by either hall effect sensors or an encoder with “hall tracks”. These signals provide rotor position data back to the motor drive such that the drive can switch current through the stator windings to cause a rotating magnetic field. With the advent of high performance, low cost Magnetic Absolute Multiturn kit encoder such as POSITAL's KCD series , drives are moving away from the traditional “commutation tracks” to using the absolute encoder position information to switch the brushless motor’s phase currents. By using absolute encoder feedback the encoder position information can be used for both motor commutation and servo loop position control.