I. Servo Motor Principle
A servo motor is a motor that controls the operation of mechanical components in a servo system. Servo motors can control speed and position with very high accuracy, 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, converting received electrical signals into angular displacement or angular velocity output on the motor shaft.
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. Therefore, a servo motor refers to a motor that follows commands at all times, and it is named after its working nature.
Servo motors primarily rely on pulses for positioning. When a servo motor receives a pulse, it rotates by the angle corresponding to that pulse, thus achieving displacement. The servo motor itself has an encoder that generates pulses, so it sends out a corresponding number of pulses for each angle the motor rotates. This essentially feeds back detailed information about the motor's rotation, forming a closed loop. In this way, the system knows how many pulses were sent to the motor and how many were received, allowing for very precise control of the motor's rotation and highly accurate positioning.
I. Classification of Servo Motors
1. DC servo
While the structure is simple and controllable, from a practical operation perspective, the introduction of a mechanical commutator in DC servo motors leads to high costs, frequent malfunctions, and difficult maintenance. Sparks from the carbon brushes often disrupt production and generate electromagnetic interference. Furthermore, the carbon brushes require maintenance and replacement. The commutation capability of the mechanical commutator also limits the motor's capacity and speed.
AC servo
They are divided into permanent magnet synchronous servo motors and asynchronous servo motors. Currently, synchronous motors are basically used for motion control.
The rotor inside a permanent magnet synchronous servo motor is a permanent magnet. The three-phase (U/V/W) 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, which 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. Its characteristics are as follows:
1. The control speed is very fast, taking only a few milliseconds from start-up to rated speed; while an asynchronous motor would take several seconds under the same conditions.
2. It has a large starting torque, which can drive objects with large inertia to move.
3. High power density: Compared to asynchronous motors, it can be made smaller and lighter within the same power range.
4. High operating efficiency.
5. It can support low-speed, long-term operation.
6. No self-rotation occurs when power is off; the action can be quickly controlled to stop.
7. Its control and response performance is much higher than that of asynchronous servo motors.
III. Servo Motor Selection
1. Motor torque
Motor torque, simply put, is the magnitude of the rotational force. In other words, it's how much force a motor can generate. Torque is a type of torque, and the definition of torque in physics is:
Torque = Force × Lever arm
The lever arm here can be considered as the radius of rotation of the object being driven by the motor. If the motor torque is too small, it cannot drive the object being driven, meaning the motor feels like it doesn't have enough "force".
Assuming we use a ball screw to make the workpiece move horizontally:
