Abstract : This article mainly introduces the technological innovations of Estun PRONET series all-digital AC servo systems in response to the high-speed and high-precision requirements of five-axis CNC machine tools for AC servo systems.
Keywords : AC servo system, five-axis linkage, technological innovation, high speed and high precision
I. Introduction
Equipment manufacturing is the cornerstone of a nation's industry, providing crucial means for the development of new technologies and products and modern industrial production. It is an indispensable strategic industry, highly valued even by developed industrialized nations. In recent years, with the rapid development of China's national economy and the needs of national defense, there has been an urgent and substantial demand for high-end CNC machine tools. Machine tools symbolize a nation's manufacturing level, and five-axis CNC machine tools represent the pinnacle of machine tool manufacturing. Five-axis CNC machine tools have a significant impact on a nation's aviation, aerospace, military, scientific research, precision instruments, and high-precision medical equipment industries. Currently, it is widely believed that five-axis CNC machine tools are the only means of machining impellers, blades, marine propellers, heavy-duty generator rotors, steam turbine rotors, and large diesel engine crankshafts. Servo drive technology, as one of the key technologies of five-axis CNC machine tools, is still in its early stages of development in China. Currently, domestically developed servo products with independent intellectual property rights can only be used on ordinary CNC machine tools, with positioning accuracy generally within a few micrometers. Neither its performance nor its reliability can meet the requirements of a five-axis CNC machine tool.
Estun, a professional manufacturer of AC servo systems, has successfully secured a key research project from the Jiangsu Provincial Department of Science and Technology: "Development of Key Common Technologies for High-End CNC Systems, High-Speed and High-Precision Servo Control Systems and Typical Mainstream Machine Tools." This project specifically focused on in-depth research into the high-speed and high-precision requirements of AC servo systems for five-axis CNC machine tools. Now, Estun has finally developed its new generation of fully digital AC servo systems—the PRONET series. This series offers power ranging from 0.5kW to 15kW, comprehensively covering various mechanical automation control applications, including CNC machine tools.
II. Technological Innovations in AC Servo Systems
The Estun PRONET series of all-digital AC servo systems adopts several new technologies and processes, including the following:
1. The PRONET series servo drives utilize the latest digital signal processors, increasing data processing capability from 16-bit to 32-bit and boosting the clock frequency to over 100 MHz. Current feedforward control has been added to the digital current loop. Leveraging the high-speed computing power of the 32-bit digital signal processor, the current loop's runtime is significantly reduced to just tens of microseconds, comparable to foreign products that embed the control algorithm into a dedicated hardware loop. By improving the current detection accuracy to 16-bit, the torque control accuracy of the current loop is further enhanced, maintaining excellent performance in both steady-state and transient operations. With a new software and hardware platform specifically designed for high-speed, high-precision control requirements, the PRONET series servo drives achieve optimal control and significantly reduced settling time, reducing it to only 1/5 of our previous products, resulting in outstanding responsiveness.
2. The new generation of servo motors adopts a new high-resolution serial encoder, which reduces the number of signal lines to 5. It supports both incremental and absolute encoder types, with a communication rate of 5 Mbps, a maximum communication cycle of 16 µs, and an encoder resolution of 17 bits/rev, generating 131,072 pulses per revolution. Future expansion will extend this to 22 bits/rev, generating 4,194,304 pulses per revolution, thus laying the foundation for nanometer-level positioning control. The servo driver integrates the independently developed serial encoder data decoding function and real-time speed detection function into a single unit using a large-scale programmable device. This further improves the low-speed performance of the motor, minimizing speed and torque fluctuations; it also enhances positioning accuracy, and the new control algorithm reduces the position control settling time to 1/5 of the original.
3. To enable users to utilize servo systems more flexibly, the PRONET series servo drives have pioneered a design that emphasizes scalability, flexibility, and openness. Various communication and feedback interfaces can be expanded through optional modules. 1) PROFIBUS-DP Bus Module: Supports the PROFIBUS-DP communication network. Servo information (position, speed, torque, input/output, alarms, etc.) can be read and various commands can be modified in real-time. A single host controller can achieve multi-axis motion control at up to 31 stations. 2) Full Closed-Loop Optional Module (Under Development): Servo control typically uses position signals acquired from a position encoder at the motor shaft end for feedback, i.e., a semi-closed-loop control method; there is no feedback sampling signal in the controlled mechanical part. The PRONET series servo drives will provide users with full closed-loop control functionality through the full closed-loop optional module, allowing the effects of machining errors, gear backlash, and structural stress-induced elastic deformation to be corrected within the servo drive through calculation.
4. In a fully digital environment, the PRONET series servo drives achieve software-based servo control and employ several new algorithms. The main control algorithms include current feedforward control, speed feedforward control, position feedforward control, variable gain control, active disturbance rejection control, mode tracking control, vibration damping control, and online automatic inertia identification control. These functional algorithms achieve a high level of responsiveness, stability, accuracy, and operability, while enhancing overshoot and vibration suppression. Due to the expanded new control algorithms, even for low-rigidity machinery, the positioning and settling time can be reduced by one-third (compared to our previous products).
5. The PRONET series servo drives prioritize ease of use, automatically determining mechanical characteristics and setting the required servo gain, achieving "online automatic adjustment." This automatic adjustment to match the servo system's mechanical characteristics shortens setup time and simplifies operation. Even first-time users can quickly achieve optimal settings.
6. The PRONET series servo drives adopt an electrical isolation structure between the main circuit and the control circuit, making operation and fault detection more convenient and safer. Furthermore, the power has been expanded from the original 5kW to 15kW, with 0.5-5kW using three-phase 200V power supply and 7.5-15kW using three-phase 400V power supply.
7. With the continuous improvement of permanent magnet material manufacturing processes, Estun's new generation of servo motors utilizes the latest Nd2FeI4B1 (rubidium iron boron) material. This material boasts superior remanent magnetic flux density, coercivity, and maximum energy product compared to other permanent magnet materials. Combined with a rational design of the magnetic poles, magnetic circuit, and motor structure, this significantly improves motor performance while simultaneously reducing the motor's overall size. The power range of the servo motors is from 0.5kW to 15kW. The power supply voltage range is from 200V to 400V.
III. Future Outlook
The application fields of fully digital AC servo systems are becoming increasingly wide, and users' requirements for servo system technology are also becoming increasingly demanding. Therefore, AC servo systems will inevitably undergo further development and improvement in the future. The following is an introduction to the future development direction of Estun's servo systems.
1. High-Performance Future Servo Systems: Performance improvements will primarily focus on two aspects. First, enhancing the performance of the servo motor itself through new processes and structures, reducing size while increasing output torque and optimizing output efficiency. Second, improving the performance of the servo driver. This involves employing high-speed DSPs, ASICs, and IPM intelligent power devices in the hardware, and adopting new control theories in the software to improve processing speed and current control accuracy. The detection and feedback mechanism will see significant improvements in resolution and multifunctionality. In the near future, even more miniaturized and multifunctional position detection devices will also be available.
2. Miniaturization: High-performance permanent magnetic materials are used in servo motors, the magnetic field structure is improved, and winding and cooling technologies are optimized.
3. Software-based servo drives utilize very large-scale integrated circuits, making more of the hardware structure software-based.
4. Networking: Networking technology connects the drive unit, motion control, host control, and main computer (central processing unit) through a network to form a unified system solution. It enables high-speed data transmission through the network and unified overall management, making mechanical adjustment and fault diagnosis simpler and more reliable. It can even be used for remote management, control, and maintenance via the Internet.
5. Environmental Adaptability: Future servo systems will be improved in terms of anti-interference and low noise to reduce environmental requirements and interference. In summary, Estun's AC servo system technology will evolve towards higher performance, faster speed, stronger fully digital, and highly intelligent drive control.
References :
[1] Li Chengtong, Zhang Jingbin, Ding Yuanyan, Current Status and Prospect of New Generation Servo Systems, Mechatronics Technology at Home and Abroad, 1999.4
