Servo systems play a vital role in electromechanical equipment, and high-performance servo systems can provide flexible, convenient, accurate, and fast driving. With technological advancements and the continuous development of industry, the trend in drive systems is to replace traditional hydraulic, DC, stepper, and AC frequency converter drives with AC servo drives, in order to achieve a new level of system performance, including shorter cycle times, higher productivity, better reliability, and longer lifespan.
I. The Development Process of Servo Systems
The development of servo systems has evolved from hydraulic to electrical. Electrical servo systems are categorized into DC (direct current) and AC (alternating current) servo systems based on the type of motor they drive. In the 1950s, brushless motors and DC motors were commercialized and widely used in computer peripherals and mechanical equipment. The 1970s saw the most widespread application of DC servo motors. However, DC servo motors suffer from drawbacks such as complex mechanical structures and high maintenance workload. During operation, the rotor is prone to overheating, affecting the accuracy of other connected mechanical equipment and hindering their application in high-speed and high-capacity applications. The mechanical commutator became a bottleneck in the development of DC servo drive technology.
From the late 1970s to the early 1980s, with the development of microprocessor technology, high-power, high-performance semiconductor power device technology, and the manufacturing process of permanent magnet materials for motors, as well as their increasingly higher performance-price ratio, AC servo technology—AC servo motors and AC servo control systems—gradually became the dominant products. AC servo motors overcome the various shortcomings of DC servo motors caused by mechanical components such as brushes and commutators. In particular, the overload characteristics and low inertia of AC servo motors demonstrate the superiority of AC servo systems.
AC servo systems can be broadly categorized into two types based on the type of drive motor used: permanent magnet synchronous (SM) motor AC servo systems and induction asynchronous (IM) motor AC servo systems. Among these, permanent magnet synchronous motor AC servo systems are technically mature, possessing excellent low-speed performance and enabling high-speed control with field weakening, thus broadening the system's speed range and meeting the requirements of high-performance servo drives. With significant improvements in the performance and reduction in the price of permanent magnet materials, their application in industrial automation is becoming increasingly widespread, and they have become the mainstream of AC servo systems. Induction asynchronous motor AC servo systems, due to the robust structure, ease of manufacturing, and low cost of induction asynchronous motors, have excellent development prospects and represent the future direction of servo technology. However, because this system uses vector transformation control, it is more complex to control compared to permanent magnet synchronous motor servo systems. Furthermore, low efficiency and severe heat generation during low-speed motor operation remain technical challenges that need to be overcome, preventing its widespread application.
The system's actuators are typically ordinary three-phase squirrel-cage induction motors, and the power conversion devices usually employ intelligent power modules (IPMs). To further improve the system's dynamic and static performance, position and speed closed-loop control can be used. Three-phase AC current following control effectively improves the inverter's current response speed and limits transient current, thus benefiting the safe operation of the IPM. The speed and position loops can be controlled using a microcontroller to achieve higher control performance. If the current regulator is proportional, all three AC current loops should be controlled by sufficiently large proportional regulators. The proportional coefficient should be chosen as large as possible while ensuring the system does not oscillate, so that the amplitude, phase, and frequency of the controlled induction motor's three-phase AC current rapidly change with the given value, thereby achieving fast current control of the voltage-source inverter. Proportional current regulation has many advantages, including simple structure, good current following performance, and fast and reliable limitation of motor starting and braking current.
Looking at the current applications of servo drive products, DC servo products are gradually decreasing, while AC servo products are increasing and their market share is gradually expanding. In practical applications, AC servo products, which offer higher precision, faster speed, and easier operation, have become the mainstream products in various fields such as factory automation.
II. Overview of Servo Drive Products
Because servo drive products are widely used in industrial production, there are many types of related products on the market, ranging from ordinary motors, variable frequency motors, servo motors, frequency converters, servo controllers to motion controllers, single-axis controllers, multi-axis controllers, programmable controllers, upper control units, and even workshop and factory-level monitoring workstations.
(a) Servo motor
With the continuous improvement of permanent magnet material manufacturing technology, most new-generation servo motors adopt the latest Nd2Fe14b1 (rubidium iron boron) material. This material has better remanent magnetic flux density, coercivity, and maximum energy product than other permanent magnet materials. Combined with a reasonable magnetic pole, magnetic circuit, and motor structure design, this significantly improves motor performance while reducing the motor's size. Most new-generation servo motors also use a new type of position encoder. This encoder reduces the number of signal lines from 9 to 5, supports both incremental and absolute types, has a communication rate of 4M/s, a communication cycle of 62.5μs, a data length of 12 bits, an encoder resolution of 20bit/rev (i.e., 1 million pulses per revolution), a maximum speed of 6000r/min, and an encoder power supply current of only 16μA. Servo motors can be classified by capacity into ultra-small (MINI), small-capacity, medium-capacity, and large-capacity types. The power range for ultra-small capacity models is 10W to 20W, for small capacity models it is 30W to 750W, for medium capacity models it is 300W to 15KW, and for large capacity models it is 22KW to 55KW. The power supply voltage range for servo motors is 100V to 400V (single or three-phase).
( ii) Servo Control Unit
While traditional analog control boasts advantages such as good continuity, fast response, and low cost, it also has insurmountable drawbacks, including difficulties in system debugging, susceptibility to drift due to environmental temperature changes, limitations in flexible design, lack of capacity for complex calculations, and inability to implement control algorithms guided by modern control theory. Therefore, modern servo control systems employ a fully digital structure, and their core theory utilizes modern vector control principles, enabling amplitude and phase control of the current vector. To improve product performance, the new generation of servo controllers incorporates various new technologies and processes, primarily in the following aspects:
1. A dq-axis current transformer unit is used in the current loop. In this new control method, the computational load on the main CPU is reduced, and the current loop control is performed in hardware, meaning the control algorithm is embedded in the LSI's dedicated hardware loop. By employing a high-speed dq-axis current transformer unit, the torque control accuracy of the current loop is further improved, achieving good performance in both steady-state and transient operation.
2. The pulse encoder multiplication function is adopted, and the new control algorithm reduces the settling time of position control to one-third of the original.
3. The speed control loop employs a real-time speed detection and control algorithm, further improving the low-speed performance of the motor and minimizing speed and torque fluctuations. The online automatic locking function shortens the servo system's debugging time and simplifies operation.
4. The main circuit and control circuit are electrically isolated, making operation and fault detection more convenient and safer. The power supply voltage is extended from 100V to 400V (single-phase/three-phase).
5. Servo control typically uses a position encoder at the motor shaft end to collect position signals for feedback, with no feedback sampling signal from the controlled mechanical parts, i.e., a semi-closed-loop control method. Current new products employ a fully closed-loop control method, allowing the servo controller to calculate and correct for the effects of machining errors, gear backlash, and structural stress-induced elastic deformation.
6. Using RICS (Reduced Instruction Set Computer System) technology, the data processing capability of the CPU was increased from 8-bit and 16-bit to 32-bit, and the clock frequency of the microprocessor was increased to over 100 MHz.
(III) Higher-level control
With the increasing demands for high speed, high precision, miniaturization, multi-variety small-batch production, high reliability, and maintenance-free performance in industrial mechanized equipment, supervisory control systems (SCADA) have become widely used. From the upper-level programmable logic controllers (PLCs), motion controllers, and machine tool CNC controllers, the system can connect all the way down to the lower-level general-purpose input/output (I/O) control units and vision sensing systems. Programming languages include ladder logic, NC language, SFC language, and motion control language, all of which can be flexibly configured according to user requirements. The system can control up to 44 axes, and the controller can connect to various signal types, from analog signals to network signals. It can be widely used in semiconductor manufacturing equipment, processing machinery, material handling machinery, hoisting machinery, etc., offering a high performance-price ratio.
III. Development Trends of Servo Systems
As the preceding discussion has shown, the application of digital AC servo systems is becoming increasingly widespread, and users' requirements for servo drive technology are also rising. In general, the development trends of servo systems can be summarized in the following aspects:
(a) Communication
Servo technology will continue its rapid shift from DC servo systems to AC servo systems. Currently, almost all new products in the international market are AC servo systems. In industrialized countries, the market share of AC servo motors has exceeded 80%, and the number of domestic manufacturers producing AC servo motors is also increasing, gradually surpassing the number of manufacturers producing DC servo motors. It is foreseeable that in the near future, except in certain micro-motor applications, AC servo motors will completely replace DC servo motors.
(II) Fully Digitalized
Servo control units employing new high-speed microprocessors and dedicated digital signal processors (DSPs) will completely replace those based primarily on analog electronic devices, thus realizing a fully digital servo system. This full digitalization transforms traditional hardware servo control into software servo control, making it possible to apply advanced methods of modern control theory in servo systems.
(III) High integration
The new servo system products have changed the practice of dividing servo systems into two modules: speed servo units and position servo units. Instead, they use a single, highly integrated, and multifunctional control unit. The performance of this same control unit can be changed by setting system parameters through software. It can be used to form a semi-closed-loop control system using the sensors built into the motor itself, or it can be connected to external position, speed, or torque sensors through an interface to form a high-precision fully closed-loop control system.
(iv) Intelligentization
Intelligentization is the current trend in all industrial control equipment, and servo drive systems, as advanced industrial control devices, are no exception. The latest digital servo control units are typically designed as intelligent products, and their intelligent features are manifested in the following aspects:
1) It has parameter memory function. All system parameters can be set by software through human-computer interaction and stored inside the servo unit. Through the communication interface, these parameters can even be modified by the host computer during operation;
2) It has fault self-diagnosis and analysis functions. Whenever a fault occurs in the system, the type of fault and the possible causes of the fault will be clearly displayed on the user panel, which simplifies the complexity of maintenance and debugging; (3) It has parameter self-tuning function. As we all know, parameter tuning of the closed-loop control system is an important link to ensure the system performance indicators. The servo unit with self-tuning function can automatically tune the system parameters through several trial runs and automatically achieve its optimization.
(v) Modularization and networking
Abroad, factory automation (FA) engineering technology based on industrial local area network (LAN) technology has made significant progress in the last decade and shows a strong growth momentum. To adapt to this trend, the latest servo systems are equipped with standard serial communication interfaces (such as RS-232C interfaces) and dedicated LAN interfaces. These interfaces significantly enhance the interconnectivity of servo units with other control devices, thus simplifying the connection to CNC systems. Only a single cable or fiber optic cable is needed to connect several, or even dozens, of servo units to a host computer to form an entire CNC system.
In summary, servo systems will develop in two directions: one is to meet the requirements of general industrial applications, where performance requirements are not very high, and drive products with characteristics such as low cost, low maintenance, and ease of use are pursued, such as variable frequency motors and frequency converters; the other is the leading products representing the development level of servo systems—servo motors and servo controllers, which pursue high performance, high speed, digitalization, intelligence, and networking drive control to meet users' higher requirements.
