The word "servo" originates from the Greek word for "slave." A "servo motor" can be understood as a motor that absolutely obeys the command of a control signal: before the control signal is issued, the rotor remains stationary; when the control signal is issued, the rotor immediately rotates; and when the control signal disappears, the rotor stops immediately. A servo motor is a micro-motor used as an actuator in automatic control devices, and its function is to convert electrical signals into angular displacement or angular velocity of the shaft.
Working principle
A servo system 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). Servo systems 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 angle it rotates. This creates 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 and AC servo motors
1. 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 can be used in cost-sensitive general industrial and civilian applications. Brushless motors are small in size, light in weight, have high output, fast response, high speed, low inertia, smooth rotation, and stable torque. They are complex to control but easy to implement intelligently. Their electronic commutation method is flexible, allowing for square wave or sine wave commutation. The motors are maintenance-free, highly efficient, operate at low temperatures, have very low electromagnetic radiation, and have a long lifespan, making them suitable for various environments. 2. AC servo motors are also brushless motors, divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control. They have a wide power range and can achieve very high power. They have high inertia, a low maximum rotational speed, and their speed decreases rapidly with increasing power. Therefore, they are suitable for low-speed, stable operation applications. 3. The rotor inside a servo motor is a permanent magnet. The three-phase electricity (U/V/W) 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. The accuracy of a servo motor depends on the accuracy (line count) of the encoder. 3. Functional differences between AC servo motors and brushless DC servo motors: AC servos are generally better because they use sinusoidal wave control, resulting in less torque ripple. DC servos use trapezoidal waves. However, DC servos are simpler and cheaper.
Permanent magnet AC servo motor
The main advantages of permanent magnet AC servo motors compared to DC servo motors are: (1) No brushes or commutator, therefore reliable operation and low maintenance requirements. (2) Easier stator winding heat dissipation. (3) Low inertia, making it easier to improve system responsiveness. (4) Suitable for high-speed, high-torque operation. (5) Smaller size and weight for the same power output.
