Servo system has an important position in electromechanical equipment, and high-performance servo system can provide flexible, convenient, accurate and fast drive. With the advancement of technology and the continuous development of the entire industry, the development trend of the traction system is to replace the traditional hydraulic, DC, stepper and AC variable frequency speed control drives with AC servo drives, in order to bring the system performance to a whole new level, including shorter cycles, higher productivity, better reliability and longer life.
First, the development process of the servo system
The development of servo systems has gone through a process from hydraulic to electrical. Electrical servo systems are divided into direct current (DC) and alternating current (AC) servo systems according to the type of motor being driven. In the 50s of the 20th century, brushless motors and DC motors were commercialized and widely used in computer peripherals and mechanical equipment, and in the 70s of the 20th century, DC servo motors were the most widely used era. However, the DC servo motor has the shortcomings of complex mechanical structure and large maintenance workload, and the rotor is easy to heat up during operation, which affects the accuracy of other mechanical equipment connected to it, and is difficult to be applied to high-speed and large-capacity occasions, and the mechanical commutator has become the bottleneck of the development of DC servo drive technology.
From the late 70s to the early 80s of the 20th century, with the development of microprocessor technology, high-power and high-performance semiconductor power device technology and the manufacturing process of motor permanent magnet materials and the increasing performance-price ratio, AC servo technology - AC servo motor and AC servo control system have gradually become the leading products. The AC servo motor overcomes the shortcomings of the brushes, commutators and other mechanical components of the DC servo motor, especially the overload characteristics and low inertia of the AC servo motor reflect the superiority of the AC servo system.
AC servo system is divided into two main categories according to the type of drive motor it uses: permanent magnet synchronous (SM type) motor AC servo system and inductive asynchronous (IM type) motor AC servo system. Among them, the AC servo system of permanent magnet synchronous motor has become fully mature in technology, has excellent low-speed performance, and can realize high-speed control of weak magnetism, broadens the speed regulation range of the system, and adapts to the requirements of high-performance servo drive. With the substantial improvement of the performance of permanent magnet materials and the reduction of prices, their application in the field of industrial production automation will become more and more extensive, and it has become the mainstream of AC servo system. The induction asynchronous motor AC servo system has a good development prospect due to the strong structure, easy manufacturing and low price of the induction asynchronous motor, which represents the direction of servo technology in the future. However, due to the vector transformation control of the system, the control is more complex than the permanent magnet synchronous motor servo system, and there are still technical problems to be overcome such as low efficiency and serious heating when the motor runs at low speed, so it has not been widely used at present.
The actuator of the system is generally an ordinary three-phase squirrel cage asynchronous motor, and the power conversion device usually adopts an intelligent power module IPM. To further improve the dynamic and static performance of the system, closed-loop control of position and speed can be employed. The follow-up control of three-phase AC current can effectively improve the current response speed of the inverter and limit the transient current, which is conducive to the safe operation of IPM. The speed and position loops can be controlled using a microcontroller to enable the control strategy to achieve higher control performance. If the current regulator is in proportional form, the three AC current loops are controlled by a large enough proportional regulator, and its proportional coefficient should be selected as large as possible under the premise of ensuring that the system does not produce oscillation, so that the amplitude, phase and frequency of the three-phase AC current of the controlled asynchronous motor change rapidly with the given value, so as to realize the rapid current control of the voltage inverter. The current is proportionally adjusted, which has many advantages, such as simple structure, good current following performance, and fast and reliable restriction of motor braking current.
From the current application of servo drive products, DC servo products are gradually decreasing, AC servo products are increasing, and the market share is gradually expanding. In practical applications, AC servo products with higher precision, faster speed and more convenient use have become mainstream products in various fields such as factory automation.
Second, the servo drive product overview
Due to the wide range of servo drive products in industrial production, there are many types of related products on the market, from ordinary motors, variable frequency motors, servo motors, frequency converters, servo control to motion controllers, single-axis controllers, multi-axis controllers, programmable controllers, upper control units and even workshop and factory-level monitoring workstations.
(1) Servo motor
With the continuous improvement of the manufacturing process of permanent magnet materials, the new generation of servo motors mostly use the latest Nd2Fe14b1 (rubidium-iron-boron) material, the remaining magnetic density, coercivity and maximum magnetic energy product of the material are better than other permanent magnet materials, coupled with the reasonable design of magnetic pole, magnetic circuit and motor structure, which greatly improves the performance of the motor, and at the same time reduces the external size of the motor. Most of the new generation of servo motors use a new type of position encoder, the number of signal lines of this position encoder is reduced from 9 to 5, and supports two types of incremental and absolute type, the communication rate is up to 4M/s, the communication cycle is 62.5μs, the data length is 12 bits, and the encoder resolution is 20bit/rev, that is, 1 million pulses are generated per revolution, the maximum speed is 6000r/min, and the encoder power supply current is only 16μA. Servo motors can be divided into ultra-small (MINI type), small capacity type, medium capacity type and large capacity type according to their capacity. The power range of the ultra-small capacity type is 10W to 20W, the power range of the small capacity type is 30W-750W, the power range of the medium capacity type is 300W-15KW, and the power range of the large capacity type is 22KW-55KW. The supply voltage of the servo motor ranges from 100V to 400V (single and three-phase).
(2) Servo control unit
Although the traditional analog control has the advantages of good continuity, fast response speed and low cost, it also has the disadvantages that are difficult to overcome, such as the difficulty of system debugging, the drift caused by the influence of environmental temperature changes, the difficulty of realizing flexible design, the lack of the ability to realize complex calculations, and the inability to realize the control algorithm under the guidance of modern control theory. Therefore, modern servo control adopts a fully digital structure, and the main theory of the servo control system also adopts the idea of modern vector control, which realizes the amplitude control and phase control of the current vector. In order to improve the performance of the product, the new generation of servo controller adopts a variety of new technologies and new processes, which are mainly reflected in the following aspects:
1. The d-q axis conversion current unit is used in the current loop, and in the new control mode, the amount of operation of the main CPU is reduced, and the current loop is controlled through hardware, that is, the control algorithm is solidified in the LSI special hardware loop. By adopting a high-speed D-Q axis conversion current unit, the torque control accuracy of the current loop is further improved, and good performance can be maintained in both steady-state and transient operation.
2. The pulse encoder multiplication function is adopted, and the new control algorithm shortens the setting time of position control to one-third of the original.
3. The speed control loop adopts the speed real-time detection and control algorithm, which further improves the low-speed performance of the motor, and minimizes the speed fluctuation and torque fluctuation. The online automatic locking function is adopted, which shortens the commissioning time of the servo system and simplifies the operation.
4. The structure of electrical isolation between the main circuit and the control loop is adopted, which makes the operation and fault detection more convenient and safe, and the power supply voltage is extended from 100V to 400V (single-phase/three-phase).
5. Servo control generally adopts the position signal collected from the position encoder at the end of the motor shaft for feedback, and there is no feedback sampling signal in the controlled execution of the mechanical part, that is, the semi-closed-loop control mode. At present, the new product adopts a full closed-loop control mode, so that the influence caused by errors such as machining error, gear clearance, and structural elastic deformation is corrected through calculation in the servo controller.
6. Using RICS (Reduced Instruction Computer System) technology, the data processing capacity of the CPU is increased from 8 bits and 16 bits to 32 bits, and the main frequency of the microprocessor is increased to more than 100 megabits.
(3) Higher-level control
With the improvement of industrial mechanization equipment for high-speed, high-precision and miniaturization, as well as multi-variety and low-batch, high reliability and maintenance-free performance, the upper computer control group has been widely used. From the upper level of the programmable controller (PLC), motion controller, machine tool CNC controller, all the way to the bottom of the universal input/output (I/O) control unit and vision sensing system. Programming languages include ladder diagram, NC language, SFC language, motion control language, etc., which can be flexibly configured according to user requirements. The system can control the number of axes from a single axis to support up to 44 axes, and the controller can connect various signal types from analog signals to network signals, which can be widely used in semiconductor manufacturing equipment, processing machinery, handling machinery, winch machinery, etc., with a high performance-to-price ratio.
Third, the development trend of servo system
From the previous discussion, it can be seen that the application of digital AC servo system is becoming more and more extensive, and the user's requirements for servo drive technology are also getting higher and higher. In general, the development trend of servo system can be summarized as follows:
(1) Communicative
Servo technology will continue to rapidly shift from DC servos to AC servos. Judging from the current situation in the international market, almost all new products are AC servo systems. In industrially developed countries, the market share of AC servo motors has exceeded 80%, and there are more and more manufacturers of AC servo motors in China, which are gradually surpassing the manufacturers of DC servo motors. It is foreseeable that in the near future, AC servo motors will completely replace DC servo motors, except in the field of some micro motors.
(2) Full digitalization
Servo control units with new high-speed microprocessors and dedicated digital signal processors (DSPs) will completely replace the servo control units dominated by analog electronics, thus realizing a fully digital servo system. The realization of full digitalization has changed the original hardware servo control into software servo control, which makes it possible to apply advanced methods of modern control theory in servo systems.
(3) Highly integrated
The new servo system product changes the practice of dividing the servo system into two modules, the speed servo unit and the position servo unit, and replaces it with a single, highly integrated, multi-functional control unit. The performance of the same control unit can be changed as long as the system parameters are set by software, and the semi-closed-loop adjustment system can be formed by using the sensors configured by the motor itself, or the high-precision full-closed-loop adjustment system can be formed with external position or speed or torque sensors through the interface.
(4) Intelligence
Intelligence is the current trend of all industrial control equipment, and the servo drive system is no exception as an advanced industrial control device. The latest digital servo control units are usually designed as intelligent products, and their intelligent characteristics are reflected in the following aspects:
1) It has the function of parameter memory. All the parameters of the system can be set by the software through the man-machine dialogue, saved inside the servo unit, through the communication interface, these parameters can even be modified by the upper computer during the operation;
2) It has the function of fault self-diagnosis and analysis. Whenever a system fails, the type of failure and the possible cause of the failure are clearly displayed through the user panel, which simplifies the complexity of maintenance and commissioning; (3) It has the function of parameter self-tuning. As we all know, the parameter setting of the closed-loop adjustment system is an important link to ensure the performance index of the system, and the servo unit with the self-tuning function can automatically set the parameters of the system through several trial runs, and automatically realize its optimization.
(5) Modularization and networking
In foreign countries, the factory automation (FactoryAutomation abbreviated as FA) engineering technology based on industrial LAN technology has made great progress in the past decade and has shown a good momentum of development. In order to adapt to this development trend, the latest servo systems are equipped with standard serial communication interfaces (such as RS-232C interfaces, etc.) and dedicated LAN interfaces. The setting of these interfaces significantly enhances the interconnection ability of the servo unit with other control equipment, so that the connection with the CNC system is also very simple, only a cable or fiber optic cable is required, and several or even dozens of servo units can be connected to the host computer to form the entire CNC system.
To sum up, the servo system will develop in two directions: one is to meet the requirements of general industrial applications, applications with low performance index requirements, and drive products with the characteristics of low cost, less maintenance, and simple use, such as frequency conversion motors, frequency converters, etc.; The other is the leading products that represent the development level of servo system - servo motor and servo controller, which pursues high-performance, high-speed, digital, intelligent and networked drive control to meet the higher requirements of users.
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