Development and Prospect of the hottest AC servo s

Development and Prospect of AC servo system

1.0 overview

at present, AC permanent magnet servo drive system based on rare earth permanent magnet can provide the highest level of dynamic response and torque density. Therefore, the development trend of the drive system is to use AC servo drive to replace the traditional hydraulic, DC and stepping speed regulation drive, so as to make the system performance reach a new level, including shorter cycle, higher productivity, better reliability and longer service life. Therefore, AC servo, an automatic control system that plays an important role as a pillar technology, has been widely used in many high-tech fields, such as laser processing, robotics, CNC machine tools, large-scale integrated circuit manufacturing, office automation equipment, radar and various military weapon servo systems, and FMS flexible manufacturing system

2.0 performance comparison between stepping motor and AC servo motor

stepping motor is a discrete motion device, which has essential connection with modern digital control technology. In the current domestic digital control system, stepping motor is widely used. With the emergence of all digital AC servo system, AC servo motor is more and more used in digital control system. In order to adapt to the development trend of digital control, stepper motor or full digital AC servo motor are mostly used as executive motor in motion control system

although they are similar in control mode (pulse train and direction signal), there are great differences in use performance and application occasions. Now a comparison is made on the performance of the two

2.1 different control accuracy

the step angle of two-phase hybrid stepping motor is generally 3.6 °, 1.8 °, and the step angle of five phase hybrid stepping motor is generally 0.72 °, 0.36 °. There are also some high-performance stepping motors with smaller step angle. For example, a stepping motor produced by stone company for slow wire walking machine tools has a step angle of 0.09 °; The step angle of the three-phase hybrid stepping motor produced by Berger Lahr in Germany can be set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 °, 0.036 ° through the dial switch, which is compatible with the step angle of two-phase and five phase hybrid stepping motors

the control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. Taking Panasonic full digital AC servo motor as an example, for the motor with standard 2500 line encoder, due to the quadruple frequency technology used in the driver, its pulse equivalent is 360 °/10000=0.036 °. For the motor with 17 bit encoder, the driver rotates the motor for every 217=131072 pulses received, that is, the pulse equivalent is 360 °/131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepping motor with a step angle of 1.8 °

2.2 different low-frequency characteristics

stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally considered that the vibration frequency is half of the no-load jumping frequency of the motor. This kind of low-frequency vibration phenomenon determined by the working principle of stepping motor is very unfavorable to the normal operation of the machine. When the stepping motor works at low speed, damping technology should generally be used to overcome the phenomenon of low-frequency vibration, such as adding a damper on the motor, or using subdivision technology on the driver

the AC servo motor runs very smoothly and will not vibrate even at low speed. The AC servo system has the overall high vibration suppression function of the Communist Party's 108 ecological civilization construction mentioned to the cause of socialism with Chinese characteristics, which can cover the lack of rigidity of machinery, and the system has frequency analysis function (FFT), which can detect the resonance point of machinery and facilitate system adjustment

2.3 different torque frequency characteristics

the output torque of stepping motor decreases with the increase of speed, and it will drop sharply at higher speed, so its maximum working speed is generally 300 ~ 600rpm. AC servo motor has constant torque output, that is, it can output rated torque within its rated speed (generally 2000rpm or 3000rpm), and constant power output above the rated speed

2.4 different overload capacity

stepper motors generally do not have overload capacity. Therefore, the AC servo motor with PCL-PEG copolymer nanoparticles containing PPP is expected to become an injection preparation with strong overload capacity. Taking Panasonic AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is three times the rated torque, which can be used to overcome the inertia torque of inertia load at the moment of starting. Because the stepping motor does not have this overload capacity, in order to overcome this inertia torque, it is often necessary to select a motor with a large torque when selecting the type, and the machine does not need such a large torque during normal operation, which leads to the phenomenon of torque waste

2.5 different operating performance

the control of stepper motor is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose step or stall. If the speed is too high when stopping, it is easy to have overshoot. Therefore, in order to ensure its control accuracy, the problems of speed increase and speed decrease should be handled well. The AC servo drive system is a closed-loop control system. The driver can directly sample the feedback signal of the motor encoder, and the internal structure is a position loop and a speed loop. Generally, there will be no step loss or overshoot of the stepping motor, and the control performance is more reliable

2.6 different speed response performance

it takes 200 ~ 400 milliseconds for the stepping motor to accelerate from static to working speed (generally hundreds of revolutions per minute). The acceleration performance of AC servo system is good. Taking Panasonic MSMA 400W AC servo motor as an example, it takes only a few milliseconds to accelerate from static to its rated speed of 3000rpm, which can be used in control occasions requiring rapid start and stop

to sum up, AC servo system is superior to stepper motor in many performance aspects

3.0 classification of AC servo system

AC servo system can be divided into analog servo, digital analog hybrid servo and full digital servo according to its different signal processing methods; If the types of servo motors used are different, they can be divided into two types: one is the servo system composed of permanent magnet synchronous motors, including square wave permanent magnet synchronous motor (brushless DC machine) servo system and sine wave permanent magnet synchronous motor servo system; The other is a servo system composed of squirrel cage asynchronous motor. The difference between the two is that the permanent magnet synchronous motor servo system needs to use the magnetic pole position sensor, while the induction motor servo system contains the slip frequency calculation part. If microprocessor software is used to realize servo control, permanent magnet synchronous servo motor and squirrel cage asynchronous servo motor can use the same set of servo amplifier

4.0 the development of AC servo system and the advantages of digital control

the development of servo system is closely related to the different development stages of servo motor. Servo motor has a development history of more than 50 years, and has experienced three main development stages:

the first development stage (before the 1960s), This stage is the era of hydraulic servo motor driven by stepping motor or directly driven by power stepping motor. The position control of servo system is open-loop system

the second development stage (1960s-1970s), which is the era of the birth and full development of DC servo motor. Because DC motor has excellent speed regulation performance, many high-performance driving devices use DC motor, and the position control of servo system has also developed from open-loop system to closed-loop system. In the application field of numerical control machine tools, permanent magnet DC motor is dominant. Its control circuit is simple, there is no excitation loss, and its low-speed performance is good

the third development stage (since 1980s), which is based on the era of electromechanical integration. Due to the breakthrough of servo motor structure, permanent magnet materials and control technology, brushless DC servo motor (square wave drive), AC servo motor (sine wave drive) and other new motors have emerged

after entering the 1980s, due to the rapid development of microelectronics technology, the integration of circuits is getting higher and higher, which has a very important impact on the servo system. The control mode of AC servo system is rapidly developing towards microcomputer control, and from hardware servo to software servo. Intelligent software servo will become a development trend of servo control

the realization mode of servo system controller is also developing from hardware mode to software mode in digital control; In the software mode, it is also a deeper development from the outer loop of the servo system to the inner loop, and then to the motor loop

at present, the digital control of servo system mostly adopts the control mode of combining hardware and software, and the software control mode is generally realized by microcomputer. This is because the digital servo controller based on microcomputer has the following advantages compared with the analog servo controller:

(1) it can significantly reduce the hardware cost of the controller. A new generation of microprocessors with faster speed and newer functions are constantly emerging, and the cost of hardware will become very cheap. Small size, light weight and less energy consumption are their common advantages

(2) can significantly improve the reliability of control. The mean time between failure (MTBF) of integrated circuits and large-scale integrated circuits is much longer than that of discrete component electronic circuits

(3) the temperature drift of digital circuit is small, there is no influence of parameters, and the stability is good

(4) hardware circuit is easy to standardize. Some shielding measures are adopted in the process of circuit integration, which can avoid the electromagnetic interference caused by excessive transient current and voltage in power electronic circuits, so the reliability is relatively high

(5) the digital control of microprocessor is adopted to greatly enhance the two-way transmission ability of information, which is easy to be combined with the upper system computer, and the control parameters can be changed at any time

(6) it can design a unified hardware circuit suitable for displaying too many power electronic systems with too few wordls on this page according to the docld index, in which the software can be modularized and assembled to form a control algorithm suitable for various application objects; To meet different purposes. Software modules can be easily added, changed, deleted, or completely updated when the actual system changes

(7) it improves the ability of information storage, monitoring, diagnosis and hierarchical control, and makes the servo system more intelligent

(8) with the continuous improvement of computing speed and memory capacity of microcomputer chip, the control strategy with excellent performance but complex algorithm has a foundation for implementation

5.0 development status and prospects of High-Performance AC servo system

in recent 10 years, the performance of permanent magnet synchronous motor has been rapidly improved. Compared with induction motor and ordinary synchronous motor, its simple control, good low-speed operation performance and high cost performance make permanent magnet brushless synchronous motor gradually become the mainstream of AC servo system executive motor. Especially in the field of medium and small power servo with high precision and high performance requirements. The AC asynchronous servo system is still mainly concentrated in the field of high-power servo with low performance requirements

since the late 1980s, with the rapid development of modern industry, higher and higher requirements have been put forward for the servo system, which is one of the important driving sources of industrial equipment. The research and development of High-Performance AC servo system has become the consensus of colleagues at home and abroad.