Abstract: As a crucial functional component of CNC machine tools, the characteristics of servo systems have always been an important indicator affecting the system's machining performance. There are many types of servo systems in CNC machine tools. This article analyzes their structure and provides a simple classification, while also briefly discussing their current technological status and development trends.
1. Introduction
A servo system is an electrical drive automatic control system composed of a driving device for mechanical motion, a motor as the controlled object, a controller as the core, and a power electronic power conversion device as the actuator, guided by automatic control theory. This type of system controls the torque, speed, and angle of the motor, converting electrical energy into mechanical energy to achieve the motion requirements of the moving machinery. Specifically, in CNC machine tools, the servo system receives displacement and speed commands from the CNC system, transforms, amplifies, and adjusts them, and then drives the machine tool's coordinate axes and spindle through the motor and mechanical transmission mechanism. This drives the worktable and tool holder, and through the linkage of the axes, the tool produces various complex mechanical movements relative to the workpiece, thereby machining workpieces with complex shapes required by the user. As the actuator of a CNC machine tool, the servo system integrates power electronic devices, control, drive, and protection, and has undergone a development process from stepper motors to DC motors and then to AC motors with the advancement of digital pulse width modulation technology, special motor material technology, microelectronics technology, and modern control technology.
2. Current Status of Development at Home and Abroad
In the 1990s, the continuous improvement of microelectronics manufacturing processes led to a geometric increase in DSP processing speed, meeting the requirements of high-speed real-time control of servo loops. Some motion control chip manufacturers even integrated the peripheral circuits necessary for motor control (such as A/D converters, position/speed detection frequency multipliers, PWM generators, etc.) with the DSP core, enabling servo control loop sampling times to reach within 100μs. This allowed for automatic acceleration and deceleration control, electronic gear synchronization control, and digital compensation control of the position, speed, and current loops all to be achieved on a single chip. New control algorithms such as speed feedforward, acceleration feedforward, low-pass filtering, and dip filtering were also implemented. On the other hand, the development of power electronics technology increased the switching frequency of power components in the main circuit of servo systems from 2–5kHz to 15–20kHz. IGBTs (Insulated Gate Bipolar Transistors) and IPMs (Intelligent Power Modules) are products of this era, improving system stability and reducing system noise. These two aspects not only form the basis for the digitization of AC servos but also contribute to the miniaturization of AC servos. Currently, most servo system manufacturers in industrialized countries can provide fully digital AC servo systems or systems compatible with their own CNC systems. These include companies such as FANUC, Mitsubishi Electric, Yaskawa Electric, Panasonic, Sanyo Electric, Siemens, Rexroth Indramat, Lenze, AB, Kollmorgen, Relliance, Baldor, and Pacific Scientific.
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.
2.1 Stepper Servo System
A stepper servo is a control system that uses pulse signals for control and converts these pulse signals into corresponding angular displacements. Its angular displacement is proportional to the number of pulses, and its 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 machine 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, making them mainly used in economical CNC machine tools and the retrofitting of old equipment where speed and accuracy requirements are not high. However, the development of constant chopper drives, PWM drives, micro-stepping drives, ultra-micro-stepping drives, and hybrid servo technologies in recent years has greatly 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.
2.2 DC Servo System
The working principle of a DC servo motor is based on the law of electromagnetic force. Related to electromagnetic torque are two independent variables: the main magnetic flux and the armature current. These control the excitation current and armature current respectively, facilitating torque and speed control. From a control perspective, a DC servo motor is a single-input, single-output, single-variable control system. Classical control theory is fully applicable to this 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 operation perspective, the DC servo motor introduces a mechanical commutation device. This device is costly, prone to failure, difficult to maintain, and often affects production due to sparks generated by carbon brushes, and also causes electromagnetic interference to other equipment. Furthermore, the commutation capability of the mechanical commutator limits the motor's capacity and speed. The armature is located on the rotor, resulting in low motor efficiency and poor heat dissipation. To improve commutation capability and reduce armature leakage inductance, the rotor becomes shorter and thicker, affecting the system's dynamic performance.
2.3 AC Servo System
To address the shortcomings of DC motors, a "turnaround" process can be implemented, 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, with the practical application of vector control methods, AC servo systems possess 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 those of DC servo systems.
3 Applications on CNC machine tools
With the advancement of microprocessor technology and high-power transistor technology, we have now entered the era of AC spindle servo systems.
3.1 AC Asynchronous Servo System
AC asynchronous servo drives 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, producing electromagnetic torque and thus rotating the motor. 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, achieving vectorized control. AC asynchronous servos are typically available in analog and digital versions. 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.
3.2 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.
3.3 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, avoiding 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 use 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 incorporating a pulse encoder to achieve position control and accurate 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 the high-speed operation and precision machining of the electric spindle.
4. Development Trends of CNC Machine Tools
4.1 High precision
Improving the machining accuracy of CNC machine tools can generally be achieved by reducing CNC system errors and employing machine tool error compensation techniques. To reduce CNC system control errors, common methods include increasing the CNC system's resolution, improving position detection accuracy, and using feedforward and nonlinear control in the position servo system. Regarding machine tool error compensation techniques, in addition to backlash compensation, leadscrew pitch error compensation, and tool compensation, errors caused by thermal deformation of the equipment can also be compensated for. Furthermore, the quality of the servo system directly affects the machining accuracy of the CNC machine tool. Modern CNC machine tools utilize AC digital servo systems and employ new control theories to achieve high-speed response servo systems.
4.2 High-speed
To achieve high-speed CNC equipment, the CNC system must first be able to process machining programs composed of tiny program segments at high speed to calculate the movement of the servo motors. Simultaneously, the servo motors must be able to respond at high speed. Utilizing 32-bit and 64-bit microprocessors is an effective means of improving the high-speed processing capability of the CNC system. The key to achieving high-speed CNC equipment lies in increasing cutting speed, feed rate, and reducing auxiliary time.
4.3 High flexibility
Employing flexible automated equipment or systems is an effective means to improve machining accuracy and efficiency, shorten production cycles, adapt to changing market demands, and enhance competitiveness. While improving the flexibility of individual CNC machine tools, the industry is also developing towards unit flexibility and system flexibility. This has led to the emergence of flexible and efficient machining equipment such as programmable logic controller (PLC) controlled adjustable combination machine tools, CNC multi-axis machining centers, tool-changing and box-changing machining centers, and CNC three-coordinate power units; flexible machining units (FMC); flexible manufacturing systems (FMS); and flexible manufacturing units (FTUs) that fall between traditional automated lines and FMS.
4.4 Intelligentization
To adapt to the needs of flexible and automated manufacturing, intelligentization is becoming a hot topic in the research and development of CNC equipment. It not only runs through the entire production and processing process (such as intelligent programming, intelligent database, and intelligent monitoring), but also through the after-sales service and maintenance of products.
① Adaptive control technology: Adaptive control can automatically adjust working parameters according to changes in cutting conditions, so that the machining system can maintain the best working state, thereby obtaining higher machining accuracy and lower surface roughness. At the same time, it can also improve the service life of tools and the production efficiency of equipment, thus achieving the purpose of improving the system's operating state.
② Expert system technology stores expert experience and general and specific rules of cutting processes in a computer, and establishes an expert system with artificial intelligence, supported by a machining process parameter database, to provide optimized cutting parameters.
③ Fault self-diagnosis and self-repair technology: Throughout the entire working process, the system continuously performs self-diagnosis and inspection on the CNC system itself and various connected devices.
④ Pattern recognition technology applies image recognition and voice control technology, enabling machines to recognize patterns and process them according to natural voice commands.
4.5 Composite
As component integration increases and component quantities decrease, the shapes of processed products become increasingly complex. Furthermore, shorter product cycles require machine tools to be readily adaptable and adjust to new changes to meet the processing needs of diverse products. This necessitates a single machine tool handling processes that previously required several. The goal is to combine multiple different processing steps onto a single machine tool, while maintaining process concentration and minimizing workpiece repositioning. This reduces floor space, parts transfer and inventory, and ensures processing accuracy while achieving energy efficiency and cost reduction.
4.6 High Reliability
To improve the reliability of CNC machine tools, CNC systems adopt more integrated circuit chips and utilize large-scale or ultra-large-scale dedicated and hybrid integrated circuits to reduce the number of components and improve reliability. Hardware functions are software-based to adapt to the requirements of various control functions. At the same time, the modularization, standardization, generalization and serialization of the hardware structure of the machine tool body are adopted, which not only increases the hardware production volume, but also facilitates production and quality control.
4.7 Networking
Networking of CNC machine tools mainly refers to the network connection and control of machine tools with other external control systems or host computers through their equipped CNC systems. With the widespread adoption of information technology in CNC machine tools, more and more domestic users are demanding remote communication services and other functions from imported CNC machine tools.
4.8 Open Architecture
Open architecture can extensively utilize advanced technologies of general-purpose microcomputers, such as multimedia technology, to achieve voice-controlled automatic programming, graphic scanning automatic programming, etc. The hardware, software and bus specifications of its new generation of CNC systems are all open to the outside world. With ample software and hardware resources available, it not only provides strong support for CNC system manufacturers and users to integrate systems, but also greatly facilitates users' secondary development, promoting the development and widespread application of CNC systems at multiple levels and in various varieties.
5. Conclusion
As a crucial functional component of CNC machine tools, the characteristics of servo systems have always been a key indicator affecting system machining performance. 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 a focus of attention in the CNC machine tool industry and represent the future direction of servo system development.
