Servo motors are classified into AC servo motors and DC servo motors. The principle of an AC servo motor is the same as that of a two-phase AC asynchronous motor, with two windings on the stator—an excitation winding and a control winding. The excitation winding and the control winding are spatially separated by 90°. Wiring: Wiring of the excitation winding; Wiring of the control winding; The purpose of connecting the capacitor C in series in the excitation winding is to generate a two-phase rotating magnetic field.
Servo motors primarily rely on pulses for positioning. Essentially, a servo motor receives one pulse and rotates by the angle corresponding to that pulse, thus achieving displacement.
Because servo motors have the function of emitting pulses, they emit a corresponding number of pulses for each rotation angle. This creates a feedback loop with the pulses received by the servo motor, or a closed loop. In this way, the system knows how many pulses were sent to the servo motor and how many pulses were received back. This allows for very precise control of the motor's rotation, thereby achieving accurate positioning down to 0.001mm.
Advantages of servo motors:
1. Precision: It achieves closed-loop control of position, speed, and torque; overcoming the problem of stepper motor step loss;
2. Speed: Excellent high-speed performance; the rated speed can generally reach 2000-3000 rpm.
3. Adaptability: It has strong overload resistance and can withstand loads three times the rated torque, making it particularly suitable for applications with instantaneous load fluctuations and those requiring rapid start-up;
4. Stability: Smooth operation at low speeds, without exhibiting the stepping motion characteristic of a stepper motor. Suitable for applications requiring high-speed response.
5. Timeliness: The dynamic response time of the motor's acceleration and deceleration is short, generally within tens of milliseconds;
6. Comfort: Heat generation and noise are significantly reduced.
Servo motors can control speed and position with extremely high accuracy. They convert voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, they are used as actuators and have characteristics such as a small electromechanical time constant and high linearity. They can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are divided into two main categories: DC and AC servo motors. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
The function of a servo motor: A servo motor enables very accurate control of speed and position.
Working principle of servo motor
1. A servo system is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value).
2. The rotor inside a servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's built-in encoder feeds back signals to the driver. The driver compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. The accuracy of a servo motor depends on the accuracy (line count) of the encoder.
A servo motor is also known as an "execution motor." The word "execution" in its name indicates that it acts as an executor. Execution requires instructions, which originate from the controller interpreted by the driver. Therefore, when discussing servo motors, we often cannot ignore the presence of a servo driver. Together, they constitute a motion control unit, commonly known as a "servo system."
Servo drives, also known as servo controllers or servo amplifiers, are controllers used to control servo motors. Their function is similar to that of a frequency converter for a regular AC motor. They are part of a servo system and are primarily used in high-precision positioning systems. Generally, they control the servo motor through position, speed, and torque to achieve high-precision positioning of the transmission system, representing a high-end product in transmission technology.
An encoder consists of a scale code disk, light-emitting diodes, infrared photodiodes, transistor amplifiers and shaping circuits, etc. Its working principle is shown in the figure below.
An encoder serves as a speed and position feedback element in a servo system. The speed U equals the number of pulses per revolution divided by the time of one revolution. The distance S is equal to the number of pulses output by the encoder plus the pulse equivalent. For example, the pulse equivalent is one micrometer. The number of pulses output multiplied by one micrometer is the distance the work platform moves. Therefore, an encoder can accurately measure the motor speed and the distance the worktable moves.
An AC servo motor mainly consists of three parts: a stator, a rotor, and an encoder. The stator of an AC servo motor is a three-phase winding. When a three-phase alternating current is applied, a rotating magnetic field is generated in the stator, and its working principle is the same as that of a three-phase motor. The rotor is a permanent magnet that rotates synchronously with the magnetic field under the influence of the stator's rotating magnetic field. Therefore, a servo motor can also be regarded as a synchronous motor.
Therefore, the encoder is coaxial with the rotor of the servo motor and rotates coaxially with the rotor. The encoder resolution is 131072 pulses/revolution, which means that the encoder outputs 131072 pulses for every one revolution of the servo motor. The encoder output pulses are fed back to the servo driver, forming a closed-loop control, so that the servo motor can move at high speed and with high precision.
