Structure and working principle of DC and AC servo motors
I. Origin and Definition of Servo Motor
Servo: The word originates from the Greek word for "slave." People wanted to use "servo mechanisms" as skillful, docile tools that obeyed the commands of control signals.
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). The main task of a servo system is to amplify, transform, and regulate power according to the requirements of control commands, making the torque, speed, and position control of the drive device highly flexible and convenient.
It is named a servo motor because of its "servo" performance. Its function is to convert the input voltage control signal into angular displacement and angular velocity output on the shaft to drive the controlled object.
Servo motors are generally divided into two main categories: DC servo motors and AC servo motors.
II. AC servo motor
1. Structure of AC servo motor
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. The rotor rotates under the action of this magnetic field. At the same time, the encoder built into the motor feeds back a signal to the driver. The driver compares the feedback value with the target value and adjusts the rotation angle of the rotor.
The stator structure of an AC servo motor is basically similar to that of a capacitor-split-phase single-phase asynchronous motor. Its stator has two windings positioned 90° apart: an excitation winding RF, always connected to the AC voltage Uf; and a control winding L, connected to the control signal voltage Uc. The rotor of an AC servo motor is usually made in a squirrel-cage configuration. However, to achieve a wide speed range, linear mechanical characteristics, no "self-rotation," and rapid response, the servo motor should have high rotor resistance and low moment of inertia compared to ordinary motors. Currently, two rotor structures are commonly used: one is a squirrel-cage rotor with high-resistivity conductor bars made of high-resistivity conductive material, which is made slender to reduce the rotor's moment of inertia; the other is a hollow cup-shaped rotor made of aluminum alloy with very thin walls (only 0.2-0.3 mm). To reduce the magnetic reluctance of the magnetic circuit, a fixed inner stator is placed inside the hollow cup-shaped rotor. The hollow cup-shaped rotor has very low moment of inertia, rapid response, and smooth operation, and is therefore widely used.
2. Working principle of AC servo motor
When there is no control voltage, the stator of an AC servo motor only has a pulsating magnetic field generated by the excitation winding, and the rotor remains stationary. When a control voltage is applied, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under constant load, the motor speed varies with the magnitude of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.
3. Compared with single-machine asynchronous motors, servo motors have three significant characteristics:
1. High starting torque
Due to its high rotor resistance, the torque characteristic curve of this motor differs significantly from that of a typical asynchronous motor. This allows for a more precise control over the critical slip, resulting in a torque characteristic (mechanical characteristic) that is closer to linear and also provides a larger starting torque. Therefore, the rotor rotates immediately upon the application of control voltage to the stator, exhibiting characteristics of rapid starting and high sensitivity.
2. Wide operating range
3. No rotation phenomenon
A normally functioning servo motor will immediately stop operating if the control voltage is lost. When the servo motor loses the control voltage, it operates in a single-phase state. Due to the high rotor resistance, the two torque characteristics and the resultant torque characteristics generated by the interaction of the two rotating magnetic fields rotating in opposite directions in the stator with the rotor are as follows.
The output power of an AC servo motor is typically 0.1-100W. When the power supply frequency is 50Hz, the voltage is available in 36V, 110V, 220V, and 380V; when the power supply frequency is 400Hz, the voltage is available in various types such as 20V, 26V, 36V, and 115V.
AC servo motors operate smoothly and with low noise. However, their control characteristics are nonlinear, and due to high rotor resistance, losses are high and efficiency is low. Therefore, compared with DC servo motors of the same capacity, they are larger and heavier, and are only suitable for low-power control systems of 0.5-100W.
III. DC Servo Motor
1. Structure of DC servo motor
The earliest servo motors were ordinary DC motors. They were used as servo motors only when high control precision was not required. Structurally, current DC servo motors are essentially low-power DC motors. Their excitation often employs armature control and field control, but armature control is more common.
2. The principle of DC servo motors
The working principle of a DC servo motor is basically the same as that of a regular DC motor. It relies on the interaction between the armature airflow and the air gap magnetic flux to generate electromagnetic torque, causing the servo motor to rotate. Armature control is typically used, where the speed is changed by altering the voltage while maintaining a constant excitation voltage. The lower the voltage, the lower the speed; when the voltage is zero, rotation stops. Because the current is also zero when the voltage is zero, the motor does not generate electromagnetic torque and therefore does not exhibit self-rotation.
3. Characteristics of DC servo motors
A rotating electric motor whose input or output is DC power. Its analog speed control system typically consists of two closed loops: a speed loop and a current loop. To ensure their coordination and effectiveness, two regulators are used in the system to regulate the speed and current respectively. The two feedback loops are nested, forming a double-loop speed control system, which offers advantages such as fast dynamic response and strong anti-interference capabilities, thus gaining widespread application. Typically, analog operational amplifiers form PI or PID circuits; signal conditioning mainly involves filtering and amplifying the feedback signal. Considering the mathematical model of a DC motor, the dynamic transfer function relationship of the analog speed control system is crucial. During the debugging process, due to significant differences between the motor parameters or load mechanical characteristics and theoretical values, it is often necessary to frequently replace components such as resistors and capacitors to change circuit parameters and obtain the expected dynamic performance indicators. This is very cumbersome. However, if programmable analog devices are used to construct the regulator circuit, system parameters such as gain, bandwidth, and even circuit structure can be modified through software, making debugging much more convenient.
