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SERVO MOTORS

servo_motor
SERVO MOTORS

A servo motor is a rotary electric motor that allows precise control of angular position, speed and acceleration. It consists of a motor coupled to a position sensor (for the feedback). It also requires a relatively sophisticated controller, often a dedicated module, specifically designed for use with servo motors.

Brushed DC Servo Motor

brush_servo_motor
Brushed DC Servo Motor

DC brush motors are the simplest form of a servo motor. The motor has copper coils on the rotor and permanent magnets on the stator. The windings are excited to create a rotational field which, in turn, turns the rotor windings. In order for a rotating field to occur on the motor, the rotor current must be switched to adjacent coils or windings. This is done by passing the brushes over the switching segments located on the motor rotor. As the rotor spins, different coils are powered by the brushes and switching segments. With the addition of an encoder, such as the KCD kit absolute encoder, on the rear shaft of the DC motor, it can be used in a servo control loop and becomes a DC servo motor.

Brushless DC Servo Motor

brushless_servo_motor
Brushless DC Servo Motor

As the name suggests, a brushless DC servo motor, BLDC, does not have brushes or switching segments to switch current through the windings to produce a rotating field. The design of the brushless DC motor is reversed 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 rotor. This type of design is advantageous because it displaces the heavy copper wire from the rotor and replaces it with much lighter permanent magnets. This reduces the rotational inertia of the rotor, allowing it to accelerate and decelerate much faster than a brushed motor. In the world of motion control, the ability to accelerate and decelerate faster results in a much better performing machine.

The downside of brushless DC motors is that they require some form of electrical switching signals produced by Hall sensors or by a “Hall track” encoder. These signals provide rotor position data to the motor drive, so that the drive can switch current through the stator windings to create a rotating magnetic field. With the advent of POSITAL’s high efficiency and low cost KCD multi-turn magnetic kit encoders, drives are moving away from traditional “switching tracks” to use the absolute position information of the encoder to switch the phase currents of the motor. brushless. By using absolute encoder feedback, the position information from the encoder can be used for motor switching and servo loop position control.

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