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.
Servo motors are divided into two main categories: AC servo motors and DC servo motors.
The basic structure of an AC servo motor is similar to that of an AC induction motor (asynchronous motor). It has two excitation windings, Wf and Wco, on the stator, with a phase spatial displacement of 90° electrical degrees. Both are connected to a constant AC voltage. The motor's operation is controlled by utilizing the AC voltage applied to Wc or changes in its phase. AC servo motors are characterized by stable operation, good controllability, fast response, high sensitivity, and strict requirements for the nonlinearity of their mechanical and adjustment characteristics (requiring less than 10%–15% and less than 15%–25%, respectively).
The basic structure of a DC servo motor is similar to that of a general DC motor. The motor speed n = E/K1j = (Ua - IaRa)/K1j, where E is the armature back electromotive force, K is a constant, j is the magnetic flux per pole, Ua and Ia are the armature voltage and armature current, and Ra is the armature resistance. Changing Ua or φ can control the speed of the DC servo motor, but generally, the armature voltage is controlled. In permanent magnet DC servo motors, the excitation winding is replaced by a permanent magnet, and the magnetic flux φ is constant. DC servo motors have excellent linear regulation characteristics and fast time response.
Advantages and disadvantages of DC servo motors
Advantages: Precise speed control, strong torque-speed characteristics, simple control principle, easy to use, and inexpensive.
Disadvantages: Brush commutation, speed limitation, additional resistance, generation of abrasive particles (unsuitable for dust-free or explosive environments).
Advantages and disadvantages of AC servo motors
Advantages: Excellent speed control characteristics, smooth control across the entire speed range with virtually no oscillation, high efficiency exceeding 90%, low heat generation, high-speed control, high-precision position control (depending on encoder accuracy), constant torque within the rated operating range, low inertia, low noise, no brush wear, and maintenance-free (suitable for clean and explosive environments).
Disadvantages: The control is relatively complex, the driver parameters need to be adjusted on-site to determine the PID parameters, and more wiring is required.
DC servo motors are divided into brushed and brushless motors.
Brushed motors are low in cost, simple in structure, have high starting torque, wide speed range, and are easy to control. They require maintenance, but maintenance is convenient (replacing carbon brushes). They generate electromagnetic interference and have requirements for the operating environment. They are usually used in ordinary industrial and civilian applications where cost is the minimum requirement.
Brushless motors are small in size and light in weight, with high output, fast response, high speed and low inertia, stable torque and smooth rotation. They are complex and intelligent in control, with flexible electronic commutation methods that can use square wave or sine wave commutation. The motors are maintenance-free, highly efficient and energy-saving, with low electromagnetic radiation, low temperature rise and long life, and are suitable for various environments.
AC servo motors are also brushless motors, and are divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control due to their wide power range, high power output, large inertia, and low speed, with the speed decreasing uniformly as power increases. They are suitable for low-speed, stable operation. A servo motor is an engine that controls the operation of mechanical components in a servo system; it is an auxiliary motor with indirect speed change. 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 has 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 there is no self-rotation when the signal voltage is zero, and the speed decreases uniformly as torque increases.
DC servo motors are divided into brushed motors and brushless motors. Brushed motors are simple in structure, low in cost, have a wide speed range, high starting torque, and are easy to control. They require maintenance, but maintenance is convenient (replacing carbon brushes), making them suitable for cost-sensitive general industrial and civilian applications. Brushless motors have advantages such as high output, light weight, small size, high speed, fast response, low inertia, smooth rotation, and stable torque. They are complex to control but easy to implement intelligent systems. Their electronic commutation methods are flexible, allowing for both square wave and sine wave commutation. These motors are highly efficient, maintenance-free, have low electromagnetic radiation, low operating temperature, and long service life, making them suitable for use in various environments. DC servo motors can be used in EDM machines, robotic arms, precision machine tools, etc., and can be equipped with a 2500P/R standard encoder and a high-resolution speedometer. They can also be equipped with a reducer, bringing reliable precision and high torque to mechanical equipment. DC servo motors offer good speed regulation, small unit weight and size, and high output power, exceeding that of AC motors and far surpassing stepper motors. Their multi-stage structure also results in less torque fluctuation.
AC servo motors are also brushless motors, and are divided into synchronous motors and asynchronous motors. Currently, motion control generally uses synchronous motors, which have a wide power range and can achieve very high power. AC servo motors have high inertia, high speed, and low speed, which decreases rapidly with increasing power, making them suitable for stable low-speed operation. 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 with a 90° phase difference: one is the excitation winding Rf, which is always connected to the AC voltage Uf; the other is the control winding L, which is connected to the control signal voltage Uc. Therefore, AC servo motors are also called dual servo motors. The rotor of an AC servo motor is usually a squirrel-cage type, but in order to make the servo motor have a wide speed range, linear mechanical characteristics, no "self-rotation" phenomenon, and fast response performance, it should have two characteristics compared with ordinary motors: high rotor resistance and low moment of inertia.
The rotor inside a servo motor is a magnet. The three-phase U/V/W power supply controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's 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 of the encoder.
In CNC machine tools, the commonly used DC servo motors in the feed system mainly include the following types:
(1) Small inertia DC servo motor
Small inertia DC servo motors are named for their small moment of inertia. These motors are generally of the water-magnetic type, with armature windings available in three types: phaseless armature, printed armature, and hollow cup armature. Because small inertia DC motors minimize the armature's moment of inertia, they achieve the fastest response speed. These servo motors were widely used in early CNC machine tools. Small inertia servo motors are still in use on CNC machine tools in some countries, such as France.
(2) DC torque motor
Also known as a high-inertia, wide-range DC servo motor. On one hand, due to its larger rotor diameter and increased number of coil winding turns, it has a larger torque and a larger turning radius compared to other types of motors. It can also operate for extended periods under significant overload torque. Therefore, it can be directly connected to a leadscrew without the need for an intermediate transmission device. On the other hand, because it has no excitation circuit losses, its overall size is smaller than other similar DC servo motors. Another outstanding feature is its ability to achieve smooth operation at low speeds, with the lowest speed reaching 1 r/min, or even 0.1 r/min. Therefore, this type of servo motor is widely used in CNC machine tools.
(3) Brushless DC servo motor
This type of servo motor is also called a commutatorless motor. It has no commutator. It consists of a synchronous motor and an inverter, which is controlled by a rotor position sensor mounted on the rotor. Essentially, it is an AC speed-regulating motor, and its speed regulation performance can reach the level of a DC servo generator. Furthermore, the elimination of the commutator and brush components greatly improves the motor's lifespan.
