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 can stop rotating instantly.
Servo motors are micro motors used as actuators in automatic control devices. Their function is to convert electrical signals into angular displacement or angular velocity of the rotating shaft.
Working principle
1. A servo mechanism 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). 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 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. This allows for very precise control of the motor's rotation, thereby achieving precise positioning, which can reach 0.001mm .
DC and AC servo motors
1. DC servo motors are divided into brushed motors 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, 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 servos are 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) It has no brushes or commutator, so it is reliable in operation and requires little maintenance.
(2) Stator windings are relatively easy to dissipate heat.
(3) Low inertia makes it easy to improve the speed of the system.
(4) Suitable for high-speed and high-torque working conditions.
(5) It has a smaller size and weight for the same power.
