Development and Application of Servo System of CNC Machine Tools

Since the emergence of CNC machine tools in the 1950s, servo systems, which are an important part of CNC machine tools, have gone from stepper servo systems to DC servo systems to today's developments with the development of new materials, electronic power, control theory and other related technologies. The process of AC servo system. With the development of AC servo technology, AC servo system will gradually replace DC servo system.

Numerical Control is the abbreviation of Numerical Control Technology. It is a method of using digital information to control the machine tool and process. The numerical control system is an important part of the numerical control machine tool and it develops with the development of computer technology. Now the CNC system is completed by the computer before the work done by the previous hardware NC, for special emphasis, sometimes also called the computer digital control system. The computer numerical control (CNC) system is characterized by the microprocessor technology and it is an advanced processing and manufacturing system that has been continuously developed with the development of electronic technology, computer technology, numerical control technology, communication technology, and precision measurement technology. . CNC system block diagram shown in Figure 1, which consists of numerical control program, input and output devices, operation panel, CNC equipment, programmable logic controller (PLC), spindle servo system, feed servo system, testing equipment and some electrical aids and other components .

The servo system is an electric drive automatic control system composed of a drive device-motor as a control object, a controller as a core, a power electronic power conversion device as an executing mechanism, under the guidance of an automatic control theory, which includes a servo driver and a servo motor. . The role of the CNC machine tool servo system is to receive the command signal from the numerical control device, drive the moving part of the machine tool to follow the instruction pulse motion, and ensure the fast and accurate action. This requires high-quality speed and position servo. The technical specifications of CNC machine tools such as accuracy and speed often depend mainly on the servo system.

Development and Classification of Servo System of CNC Machine Tools

The servo system of CNC machine tools should meet the following basic requirements:

High precision

The CNC machine tool cannot use manual operation to adjust and compensate for various errors like traditional machine tools, so it requires a very high positioning accuracy and repeatability.

Fast response characteristics

Fast response is one of the hallmarks of the dynamic quality of servo systems. It requires the servo system to follow the command signal not only to follow the error, but also to respond quickly and have good stability. After the system gives a given input, it can reach a new balance after a short adjustment or it can quickly restore the original equilibrium state under the influence of outside interference.

Speed ​​range

Because of the different workpiece materials, tools and machining requirements, it is necessary to ensure that the CNC machine tool can obtain the best cutting conditions under any circumstances. The servo system must have enough speed range to meet the requirements of high-speed machining and to meet the requirements of low speed. Give request. The speed range is generally greater than 1:10000. In addition, a relatively stable torque output is required during low-speed cutting.

System reliability is better

The use rate of CNC machine tools is very high, and often 24 hours of continuous work can not be downtime, thus requiring reliable work.

The basic composition of the CNC machine tool servo system is shown in Figure 2. The servo system of CNC machine tools is divided into open-loop control systems and closed-loop control systems according to whether there are feedback detection components. The drive control unit is the signal required to translate the feed instruction into the actuator and the actuator turns the signal into a mechanical displacement. The open-loop control system does not feedback the detection elements and the comparison control link. These are the necessary parts of the closed-loop control system.

The servo system is divided into a feed drive system and a spindle drive system according to the purpose and function. According to whether the feedback detection components are divided into open-loop control systems and closed-loop control systems. According to the different actuators, it is divided into step servo system, DC servo system and AC servo system.

Stepping servo system

Prior to the 1960s, the stepper servo system was characterized by a stepper motor-driven hydraulic servomotor or direct drive by a power stepper motor, and the servo system was controlled by open loop.

The stepping servo system receives the pulse signal, and its rotation speed and rotation angle depend on the frequency or the number of command pulses. Because there is no detection and feedback link, the accuracy of the stepper motor depends on the accuracy of the step angle, gear transmission gap, and so on, so its accuracy is low. Moreover, the stepper motor is prone to vibration at low frequencies, and its output torque decreases with increasing speed. Because the stepping servo system is open-loop control, the stepping motor is prone to "lost step" or "blocked rotation" when the starting frequency is too high or the load is too large, and the overshoot is likely to occur when the speed is too high at the time of stopping. In addition, the time required for the stepping motor to accelerate from the standstill to the operating speed is longer and the speed response is slower. However, due to its simple structure, easy adjustment, reliable operation, and low price, it can be applied in many occasions with low requirements.

DC servo system

After the 60-70s, most of the CNC systems used DC servo systems. DC servo motor has a good wide speed performance. With large output torque and strong overload capacity, the servo system has also developed from open-loop control to closed-loop control, resulting in more extensive use in industrial and related fields. However, with the rapid development of modern industry, its corresponding equipment such as precision CNC machine tools, industrial robots and so on put forward higher and higher requirements for the electrical servo system, especially the accuracy, reliability and other performance. The traditional DC motor adopts a mechanical commutator, and faces many problems in the application process, such as brush and commutator wear, high maintenance workload and high cost; the commutator will produce spark when reversing, making The maximum speed and application environment of the motor are limited; the DC motor is complex in structure, high in cost, and easily interferes with other devices.

The existence of these problems has limited the application of DC servo systems in high-precision, high-performance servo drive applications.

AC servo system

For the shortcomings of DC motors, people have been striving to find ways to replace DC servo motors with mechanical commutators and brushes with AC servo motors to meet a variety of application fields, especially in the field of high-precision, high-performance servo drives. . However, due to the strong coupling, non-linear characteristics of the AC motor, and the complexity of control, high-performance applications have been limited. Since the 1980s, with the development of various technologies such as electronic power, especially the development of modern control theory, breakthroughs in vector control algorithms have previously plagued the problem of AC motors and the development of AC servos has become increasingly fast.

AC servo system features

In addition to the advantages of good stability, rapidity, and high accuracy, AC servo systems have a number of advantages over DC servo motor systems:

There is no limit to the commutator circumference speed regulation of the AC motor, and there is no numerical limitation of the reactance potential in the armature element. The speed limit can be designed to be higher than that of a DC motor of the same power.

The speed range is wide. At present, the speed ratio of most AC servo motors can reach 1:5000, and the transmission ratio of high-performance servo motors has reached 1:10000. Satisfy the requirements of wide range of transmission speed and small static error rate of CNC machine tools.

The torque frequency characteristics are good, and the AC motor is constant torque output, that is, the rated torque is output within its rated speed, and the constant power output is above the rated speed. And with torque overload capability, the moment of inertia of the inertial load at the moment of starting can be overcome. Satisfy the requirements of large output torque, dynamic response, and high positioning accuracy of the machine tool servo system.

Domestic Research on AC Servo

The AC servo system includes AC servo system based on asynchronous motor and AC servo system based on synchronous motor. The main machine currently used is a permanent magnet synchronous AC servo system. In the field of AC servo research, research in Japan, the United States, and Europe has been at the forefront of the world. Japan's Yaskawa Corporation successfully developed the world's first AC servo drive in the mid-1980s. Subsequently, companies such as FANUC, Mitsubishi, and Panasonic launched their own AC servo systems, most of which are based on asynchronous motors. Domestic research on AC servo systems based on asynchronous motors is relatively late, so far no products have been available. Many domestic scholars focus their research on permanent magnet synchronous motor servo systems. China's Huazhong University of Science and Technology, Beijing Machine Tool Research Institute, Xi'an Micro Motor Research Institute, Shenyang Institute of Automation, Chinese Academy of Sciences, Lanzhou Electric Motors and other units began research and introduced AC servo system. Among them, the DA98 all-digital AC servo drive produced by Guangzhou CNC Company has opened up the situation in China's high-precision CNC servo drive industry, breaking the monopoly of foreign companies and creating a new era of national brands.

There are three modes of AC servo signal and CNC system interface, and it is also the three stages that it experienced. Domestically speaking, Guangzhou CNC's DA98 is a first-generation epoch-making servo driver. It is the first all-digital AC servo drive in China, and it accepts direction command pulses. The second generation is the EDB series represented by Eston, which not only accepts pulse command signals, but also accepts analog inputs for speed control or torque control. The third generation is a networked AC servo system. The networked servo system is an organic combination of industrial fieldbus technology and all-digital AC servo. All-digital AC servo technology can enable users to adjust parameters according to the load conditions, and also eliminates the instability factors such as drift caused by some analog circuits. Using fieldbus-based network control technology, microprocessors and fieldbus interfaces are placed into all-digital AC motor servo drives to form an intelligent, independent, all-digital servo control unit that is directly connected to the industrial fieldbus. A new fieldbus based network control system. The number of hardware and connections are reduced. Each smart unit is independent in its structure and can share data with the outside world and each other. It can also add other on-site control devices and is easy to expand. So far, networked AC servers have not matured products in China. The Robot Research Institute of Beijing University of Aeronautics and Astronautics has developed and designed a networked AC servo control system based on DSP+FPGA+ASIPM. The prototype has been initially verified on the three-dimensional engraving machine.

Most current server drivers use high-speed DSP processors to advance the use of various advanced motion control algorithms on new types of drives. Most of the major servo system suppliers in the hardware structure adopt DSP + CPLD (FPGA) structure. Due to the repeatable programming of DSP and CPLD (FPGA), the modular reconfiguration of the AC servo system can be realized. As long as the system is configured with different software (including control algorithms), asynchronous motors, permanent magnet synchronous servo motors, and brushless DC motors can be controlled and driven, and DC motors and three-phase induction high voltages can also be driven by reconfiguring the FPGA. Stepper motor. This leaves a lot of space for the upgrading and innovation of CNC machine tools.

AC servo development trend

With the continuous development of productivity, AC servo systems are developing in the direction of integration, intelligence and networking.

Integrated

Using a single, multi-functional control unit, position control and speed control functions are implemented through software settings. The self-configured feedback unit can be used to form a semi-closed loop or a high-precision full-closed loop control system can be constructed through an external interface.

Intelligent

The intelligence of the server control mode, such as the internal pre-programmed to achieve a certain kind of movement trajectory, control the IO port around the site, with the following master-slave following mode adjustment, electronic cam and so on.

Networking

Network distributed control between servers. The server implements modular reconfigurability and saves costs.

Conclusion

Modern CNC machine tools are moving toward precision and speed. As the main part of the CNC machine tool servo system, more and more choices remain on the AC servo, it has the advantage of several other servo systems unmatched. With the development of AC servo technology, AC servo system will gradually replace the DC servo system.