I. Overview
A servo system is an electrical drive automatic control system composed of a mechanical motion drive device, a motor as the controlled object, a controller as the core, and a power electronic power conversion device as the actuator, under the guidance of automatic control theory.
As the actuator of CNC machine tools, servo systems integrate power electronic devices, control, drive, and protection. With advancements in digital pulse width modulation technology, special motor materials technology, microelectronics technology, and modern control technology, they have evolved from stepper motors to DC motors, and then to AC motors. There are many types of servo systems in CNC machine tools. This article analyzes their structure and provides a simple classification, briefly discussing their current technological status and development trends.
II. Structure and Classification of Servo Systems
From a basic structural perspective, a servo system mainly consists of three parts: a controller, a power drive unit, and a feedback unit, and a motor (Figure 1). The controller adjusts the control quantity according to the difference between the given value of the CNC system and the actual operating value detected by the feedback unit; the power drive unit, as the main circuit of the system, on the one hand, applies electrical energy from the power grid to the motor according to the magnitude of the control quantity, adjusting the magnitude of the motor torque, and on the other hand, converts the constant voltage and constant frequency power supply from the power grid into the AC or DC power required by the motor according to the motor's requirements; the motor then drives the mechanical operation according to the power supply.
The main components in Figure 1 vary widely, and any change in any part can constitute different types of servo systems. For example, based on the type of drive motor, they can be divided into DC servos and AC servos; based on the different controller implementation methods, they can be divided into analog servos and digital servos; based on the number of closed loops in the controller, they can be divided into open-loop control systems, single-loop control systems, dual-loop control systems, and multi-loop control systems. Considering the application of servo systems in CNC machine tools, this paper first divides them into feed servos and spindle servos according to the different transmission mechanisms in the machine tool, and then discusses the technical characteristics of different servo systems based on other factors.
III. Current Status and Future Prospects of Feed Servo Systems
Feed servos control the axes of a CNC machine tool to generate the cutting feed motion. Therefore, feed servos are required to quickly adjust the movement speed of the axes and accurately control position. Specifically, they need a wide speed range, high displacement accuracy, good stability, and fast dynamic response. Depending on the motor used in the system, feed servos can be further classified into stepper servos, DC servos, AC servos, and linear servos.
(I) Stepper Servo System
A stepper servo motor is a control system that uses pulse signals for control and converts these pulses into corresponding angular displacements. The angular displacement is proportional to the number of pulses, and the rotational speed is proportional to the pulse frequency. The motor speed can be adjusted by changing the pulse frequency. If some windings remain energized after the motor stops, the system also has a self-locking capability. Each revolution of a stepper motor has a fixed number of steps, such as 500, 1000, or 50,000 steps, and theoretically, its step error will not accumulate.
Stepper servos have a simple structure, meeting the needs of digital system development, but they suffer from poor accuracy, high energy consumption, and low speed; the higher the power, the lower the movement speed. In particular, stepper servos are prone to step loss, limiting their use to economical CNC machine tools and the retrofitting of older equipment where speed and accuracy requirements are not high. However, recent advancements in constant frequency chopper drives, PWM drives, micro-stepping drives, ultra-micro-stepping drives, and hybrid servo technologies have significantly improved the high and low frequency characteristics of stepper motors. Especially with the development of intelligent ultra-micro-stepping drive technology, the performance of stepper servos will be elevated to a new level.
(ii) DC servo system
The working principle of a DC servo is based on the law of electromagnetic force. Related to the electromagnetic torque are two independent variables: the main magnetic flux and the armature current. These control the excitation current and armature current respectively, allowing for convenient torque and speed control. From a control perspective, a DC servo is a single-input, single-output, single-variable control system. Classical control theory is fully applicable to this type of system. Therefore, DC servo systems are simple to control and have excellent speed regulation performance, and they once dominated the feed drive of CNC machine tools.
However, from a practical operational perspective, DC servo motors incorporate mechanical commutation devices. These devices are costly, prone to failure, difficult to maintain, and frequently disrupt production due to sparks generated by carbon brushes, while also causing electromagnetic interference to other equipment. Furthermore, the commutation capability of the mechanical commutator limits the motor's capacity and speed. The armature being located on the rotor results in low motor efficiency and poor heat dissipation. To improve commutation capability and reduce armature leakage inductance, the rotor becomes shorter and thicker, impacting the system's dynamic performance.
(III) AC Servo System
To address the shortcomings of DC motors, a "turnaround" approach can be adopted, where the drive windings are assembled on the stator and the rotor is a permanent magnet component. An encoder on the rotor shaft measures the magnetic pole positions, creating a permanent magnet brushless motor. Simultaneously, the practical application of vector control methods has endowed AC servo systems with excellent servo characteristics. Their wide speed range, high speed stability, fast dynamic response, and four-quadrant operation make their dynamic and static characteristics fully comparable to DC servo systems. Furthermore, field weakening high-speed control can be achieved, broadening the system's speed range and meeting the requirements of high-performance servo drives.
Currently, permanent magnet synchronous AC servo systems are mainly used in machine tool feed servos, and there are three types: analog, digital, and software. Analog servos have a single purpose, only receiving analog signals, and position control is usually implemented by a host computer. Digital servos can be used for multiple purposes, such as speed, torque, and position control. They can receive analog and pulse commands, and all parameters are set digitally, offering good stability. They also have rich self-diagnostic and alarm functions. Software servos are microprocessor-based fully digital servo systems. They implement the monitoring programs for various control methods and servo motors of different specifications and power in software. During use, the user can set codes and related data, and the system automatically enters the working state. Equipped with a digital interface, changing the operating mode or replacing the motor specifications only requires resetting the code; therefore, they are also called universal servos.
AC servos have become the dominant force in machine tool feed servos and are constantly improving with the development of new technologies, specifically in three aspects. First, the power electronic devices in the system power drive device are constantly developing towards higher frequencies, and intelligent power modules are becoming more widespread and applied. Second, the maturity of microprocessor-based embedded platform technology will promote the application of advanced control algorithms. Third, the promotion of networked manufacturing models and the maturity of fieldbus technology will make network-based servo control possible.
(iv) Linear Servo System
Linear servo systems employ a direct drive method. Compared to traditional rotary transmission methods, their biggest advantage is the elimination of all intermediate mechanical transmission links between the motor and the worktable, effectively reducing the length of the machine tool's feed transmission chain to zero. This "zero-transmission" method delivers performance indicators unattainable by rotary drives, such as acceleration exceeding 3g, 10 to 20 times that of traditional drive systems, and feed speeds 4 to 5 times faster. In terms of motor operating principles, linear motors come in various types, including DC, AC, stepper, permanent magnet, electromagnetic, synchronous, and asynchronous. Structurally, they also vary, including moving-coil, moving-iron, flat-plate, and cylindrical types. Currently, the main types used in CNC machine tools are high-precision, high-frequency-response, short-stroke linear motors and high-thrust, long-stroke, high-precision linear motors.
Linear servo systems are the ideal drive mode for high-speed, high-precision CNC machine tools, attracting significant attention from machine tool manufacturers and experiencing rapid technological development. At the 2001 European Machine Tool Exhibition, dozens of companies showcased high-speed machine tools driven by linear motors, with rapid traverse speeds reaching 100–120 m/min and accelerations of 1.5–2 g . Among them, Germany's DMG and Japan's MAZAK were particularly representative. In 2000, DMG already had 28 models using linear motor drives, producing over 1,500 units annually, accounting for approximately one-third of its total output. MAZAK is also about to launch a supersonic machining center based on a linear servo system, with a cutting speed of Mach 8, a maximum spindle speed of 80,000 r/min, a rapid traverse speed of 500 m/min, and an acceleration of 6 g. All of this signifies that the second generation of high-speed machine tools, represented by linear motor drives, will replace the first generation of high-speed machine tools, represented by high-speed ball screw drives, and gradually dominate their use.
IV. Current Status and Future Prospects of Spindle Servo Systems
Spindle servos provide the cutting power required for machining various workpieces; therefore, only spindle speed regulation and forward/reverse rotation functions are needed. However, when machine tools are required to perform thread machining, precise stop, and constant surface speed machining, corresponding position control requirements are placed on the spindle. Therefore, high output power, constant torque and constant power ranges, precise stop control, and spindle-feed linkage are required. Similar to feed servos, spindle servos have evolved from ordinary three-phase asynchronous motor drives to DC spindle drives. With advancements in microprocessor and high-power transistor technology, we have now entered the era of AC spindle servo systems.
(I) AC Asynchronous Servo System
AC asynchronous servo motors generate a sinusoidal current with variable amplitude and frequency in the stator windings of a three-phase asynchronous motor. The rotating magnetic field generated by this sinusoidal current interacts with the induced current in the motor rotor to produce electromagnetic torque, thereby enabling the motor to rotate. The amplitude of the sinusoidal current can be decomposed into the vector sum of a given or adjustable excitation current and the equivalent rotor torque current; the frequency of the sinusoidal current can be decomposed into the sum of rotor speed and slip, thus achieving vectorized control.
AC asynchronous servos typically come in two types: analog and digital. Compared to analog servos, digital servos have near-linear acceleration characteristics, shorter acceleration times, and can improve the rigidity and accuracy of the system during spindle positioning control. They are also easier to operate, making them the primary form of machine tool spindle drive. However, AC asynchronous servos have two main problems: firstly, rotor heating leads to lower efficiency, lower torque density, and larger size; secondly, a lower power factor necessitates a larger inverter capacity to achieve a wider constant power speed range.
(II) AC Synchronous Servo System
In recent years, with the development and continuous improvement of the performance of high-energy, low-cost permanent magnets, the performance of AC synchronous servo systems using permanent magnet synchronous speed-regulating motors has become increasingly prominent, bringing hope for solving the problems existing in AC asynchronous servos. Compared with asynchronous servos using vector control, permanent magnet synchronous motors have lower rotor temperatures, higher axial connection position accuracy, lower cooling requirements, less impact on machine tool environment temperature, and can easily reach extremely low speed limits. Even at low speed limits, they can operate with constant torque, making them particularly suitable for heavy-duty cutting. Simultaneously, they have high torque density, low moment of inertia, and good dynamic response characteristics, making them particularly suitable for high-productivity operation. They can more easily achieve very high speed ratios, allowing the same machine tool spindle to have multiple machining capabilities, capable of machining both low-hardness materials like aluminum and very hard and brittle alloys, creating conditions for optimal cutting on the machine tool.
(III) Electric Spindle
An electric spindle is a product that integrates an electric motor and a spindle. It directly incorporates the stator and rotor of the spindle motor into the spindle assembly, with the motor's rotor being the rotating part of the spindle. By eliminating the transmission and connection between the gearbox and the motor, it achieves an integrated, "zero-transmission" spindle system. Therefore, it boasts advantages such as compact structure, light weight, low inertia, and excellent dynamic characteristics. It also improves the dynamic balance of machine tools, avoids vibration and noise, and has been widely used in ultra-high-speed cutting machine tools.
Theoretically, an electric spindle is a high-speed electric motor, which can be either an asynchronous AC induction motor or a permanent magnet synchronous motor. The drive of an electric spindle generally uses vector control frequency conversion technology, typically with a built-in pulse encoder to achieve precise position control and coordination with the feed. Due to the extremely high operating speed of the electric spindle, special requirements are placed on its heat dissipation, dynamic balancing, and lubrication. These must be properly addressed in application to ensure high-speed operation and precision machining of the electric spindle.
V. Conclusion
In recent years, various servo drive technologies have been developed to improve the dynamic and static characteristics of servo systems. It is foreseeable that with the development of advanced manufacturing technologies such as ultra-high-speed cutting, ultra-precision machining, and network manufacturing, fully digital servo systems with network interfaces, linear motors, and high-speed electric spindles will become the focus of the CNC machine tool industry and represent the future direction of servo system development.
