A servo motor is a motor that controls the operation of mechanical components in a servo system; it is an auxiliary motor with indirect speed control. Servo motors enable highly accurate speed and position control, converting 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, it is used as an actuator and possesses characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. It 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. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
Working principle
1. A servo mechanism is an automatic control system that enables the output controlled variables, such as position, orientation, and state, of an object to follow any changes in the input target (or given value). Servos 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 the servo motor itself has the function of generating pulses, it generates a corresponding number of pulses for each rotation angle. This forms a feedback loop, or closed loop, between the pulses sent to and received by the servo motor. In this way, the system knows how many pulses were sent to and received by the servo motor, allowing for very precise control of the motor's rotation and achieving precise positioning down to 0.001mm. DC servo motors are divided into brushed and brushless motors. Brushed motors are low-cost, simple in structure, have high starting torque, wide speed range, and are easy to control. However, they require maintenance, which is inconvenient (replacing carbon brushes), generates electromagnetic interference, and has environmental requirements. Therefore, they are suitable for cost-sensitive general industrial and civilian applications.
Brushless motors are small in size, lightweight, powerful, fast-responding, high-speed, low-inertia, smooth-rotating, and stable in torque. While their control is complex, they are easily made intelligent. Their electronic commutation is flexible, allowing for either square wave or sine wave commutation. The motors are maintenance-free, highly efficient, operate at low temperatures, have minimal electromagnetic radiation, and a long lifespan, making them suitable for various environments.
2. AC servo motors are also brushless motors, and they are divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control because they have a wide power range and can achieve very high power. They have high inertia, low maximum rotational speed, and their speed decreases rapidly as power increases. Therefore, they are suitable for applications requiring low-speed, stable operation.
3. The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field, and the rotor rotates under the influence of this magnetic field. At the same time, the encoder built into the motor 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 the servo motor depends on the accuracy (line count) of the encoder.
The functional differences between AC servo motors and brushless DC servo motors: AC servo motors are generally better because they use sinusoidal wave control, resulting in less torque ripple. DC servo motors use trapezoidal wave control. However, DC servo motors are simpler and cheaper.
Major brand manufacturers include Siemens (Germany), Kollmorgen (USA), Panasonic (Japan), and Yaskawa (Japan).
