1. Introduction
In recent years, it is evident that people's living standards are improving day by day, and consumption is also rising continuously. As awareness and the demand for quality of life are increasingly placed on the agenda, consumers not only demand the quality of the intrinsic materials, but also the corresponding requirements for packaging. This has forced manufacturers to upgrade their production equipment to accelerate production and meet people's higher demands. As a result, automated numerical control equipment has been introduced into production, and servo motors have been widely used.
Based on recent observations and development trends, servo motor control technology is moving towards digitalization, intelligence, and communication. So, as a component of CNC machine tools and an actuator, how can servo motors be better utilized in various CNC machine tools? With the continuous advancement of digital pulse width modulation technology, microelectronics technology, and current control technology, this paper will briefly discuss the technology, current status, and development trends of servo motors.
2. Introduction to Servo Motors
Currently, in production, we usually use AC servo motors. Let's take AC servo motors as an example to analyze their structural composition.
AC servo motors can be broadly divided into two parts: the rotor and the stator. The commonly used squirrel-cage rotor and non-magnetic cup rotor are the rotor parts. What about the stator? The common stator structure is similar to that of a rotary transformer; both consist of two-phase windings spaced 90 degrees apart within the stator core. Therefore, servo motors can also be called two-phase AC motors.
In the two commonly used rotor structures, the squirrel-cage rotor AC servo motor consists of a rotor core, a shaft, and rotor windings. The outer stator of the non-magnetic cup-shaped rotor AC servo motor is exactly the same as that of the squirrel-cage stator, but the inner stator is different. It is made of stacked annular steel sheets and serves as part of the motor's magnetic circuit. The inner stator usually does not contain windings; it simply replaces the core of the squirrel-cage rotor.
However, based on the current market situation, squirrel-cage rotor servo motors are generally more widely used because the non-magnetic cup-shaped rotor has low inertia and low bearing friction torque. Also, because there are no teeth between the rotor and stator, there is no tooth adhesion between the stator and rotor, and vibration does not occur during constant speed rotation. However, within a certain power range and for the same size and weight, cup-shaped rotor servo motors produce lower starting torque and output power than squirrel-cage rotor servo motors. At the same time, the construction and manufacturing process of cup-shaped rotor servo motors are relatively more complex. Therefore, cup-shaped rotor servo motors are only used in certain applications requiring extremely smooth operation (such as integrator circuits).
3. Application of servo motors in CNC
3.1 Application Characteristics of Servo Motors in CNC Systems
AC servo motors are a type of brushless motor, but they are divided into synchronous and asynchronous motors. Synchronous motors are more common in motion control because they can achieve very high power. At their highest rotational speed, the speed is low, and it decreases rapidly as the power increases, making them suitable for low-speed, stable operation applications. Therefore, their greatest characteristics are high precision, wide speed range, ability to output large torque at low speeds, fast response, and no overshoot.
3.2 What are the advantages of servo motors compared to other types of motors?
AC servo motors outperform stepper motors in many aspects. Although stepper motors are often used as actuators in some special or less demanding applications, AC servo motors remain the most popular and widely used motors. So, what are the differences between AC servo motors and stepper motors?
Firstly, there is a difference in control precision. The control precision of an AC servo motor is ensured by a rotary encoder at the rear end of the motor shaft, while the angle of a two-phase stepper motor is generally 3.6 degrees or 18 degrees, and the angle of a five-phase hybrid stepper motor is 0.72 degrees or 0.36 degrees.
Secondly, the difference lies in low-frequency characteristics. Stepper motors are prone to low-frequency vibration at low speeds. This low-frequency vibration is inherent in the working principle of stepper motors and is detrimental to normal machine operation. In contrast, AC servo motors operate very smoothly, maintaining stability even at very low speeds without vibration. This is because AC servo systems have resonance suppression capabilities; they possess an internal frequency analysis function that adjusts the system promptly when a resonance point is detected.
There are also differences in torque-frequency characteristics, overload capacity, and operational and speed response performance. In the design of a control system, it is necessary to comprehensively consider control requirements, cost, and other factors to select the most appropriate control motor at the right time.
4. Development Trends of Servo Motors
As China transforms from a manufacturing giant into a manufacturing powerhouse, and with the increasing performance-price ratio of digital AC servo systems, the market share of AC servo systems, as high-end precision components for controlling motors, is steadily rising. As a crucial component of CNC machine tools, the servo system has always been one of the key indicators affecting the system's machining performance.
In recent years, various servo drive technologies have been developed and emerged to improve the dynamic and static characteristics of servo systems. The key factor determining the performance of an AC servo system remains servo control technology. However, due to the advanced control principles, low cost, and maintenance-free nature of AC servo systems, and the fact that their control characteristics are comprehensively surpassing those of DC servo systems, they are bound to largely, or even completely, replace DC servo systems in the future. Based on current operating models, their future development will be towards higher efficiency, higher speed, higher precision, and higher performance.
Furthermore, with the widespread adoption of intelligent technologies and the prevalence of networking and modularization, modern AC servo drive devices also possess parameter memory capabilities, as well as the ability to diagnose and analyze their own faults. Some servo motors even have the ability to identify parameters and automatically suppress vibrations, automatically measure and zero the encoder. These represent the development trend of servo motors towards intelligentization. The key development direction for networking is how to adapt to the real-time, synchronous, and reliable data transmission requirements of high-performance motion control. The successful development of high-end CNC systems also indicates that the development of networked digital servo systems has become an urgent priority. Servo motor drivers, power supplies, regenerative braking, and communication between motors are all continuously developing towards modularization.
