Working principle of servo motor

The operating principle of a servo motor is relatively simple, but its efficiency is high. The servo circuit is built into the motor unit, using a flexible shaft, often equipped with gears. Electrical signals control the motor and determine the amount of shaft movement. The internal structure of a servo motor is simple: a small DC motor, a control circuit, and a potentiometer. The DC motor is connected to the control wheel via gears. When the motor rotates, the resistance of the potentiometer changes, and the control circuit accurately adjusts the movement and direction.

When the shaft is in the correct (ideal) position, the motor stops receiving power. If the shaft does not stop at the target position, the motor continues to run until it reaches the correct direction. The target position is transmitted using a signal line of electrical pulses. Therefore, the speed of the motor is proportional to the actual and ideal positions. As the motor approaches the required position, it begins to rotate slowly, but when the motor is further away, the speed is faster. In other words, servo motors only need to complete the task as quickly as possible, making them efficient devices.

In an AC servo motor, without control voltage, only the pulsating magnetic field generated by the excitation winding exists in the air gap, and the rotor remains stationary without starting torque. When there is a control voltage and the control winding current and excitation winding current are out of phase, a rotating magnetic field is generated in the air gap, producing an electromagnetic torque that causes the rotor to rotate in the direction of the rotating magnetic field. However, servo motors are required not only to start under the action of control voltage but also to stop immediately after the voltage disappears. If the servo motor continues to rotate like a general single-phase asynchronous motor after the control voltage disappears, it will result in loss of control. This phenomenon of self-rotation due to loss of control is called self-rotation.

The basic working principle of a traditional DC servo motor is exactly the same as that of an ordinary DC motor. It relies on the interaction between the armature current and the air gap magnetic flux to generate electromagnetic torque, driving the servo motor to rotate. An armature control method is usually adopted, that is, under the condition that the excitation voltage remains unchanged, the speed is adjusted by changing the armature voltage. The lower the armature voltage, the lower the speed; when the armature voltage is zero, the motor stops. Since the armature current is also zero when the armature voltage is zero, the motor does not produce electromagnetic torque and will not self-rotate.