With the rapid development of supporting technologies such as modern motor technology, modern power electronics technology, microelectronics technology, permanent magnet material technology, AC adjustable speed technology, and control technology, permanent magnet AC servo technology has made significant progress. The performance of permanent magnet AC servo systems has been continuously improving, and their prices have become more reasonable, making them a growing trend in modern electric servo drive systems , especially in the field of high-precision, high-performance servo drives, and a replacement for DC servo systems. Permanent magnet AC servo systems have the following advantages:
The motor has no brushes or commutator, making it reliable in operation and easy to maintain; the stator windings dissipate heat quickly; it has low inertia, which easily improves the system's speed; and it is suitable for high-speed, high-torque operation.
With the same power, it has a smaller size and weight, and is widely used in machine tools, mechanical equipment, handling mechanisms, printing equipment, assembly robots, processing machinery, high-speed winding machines, textile machinery and other applications, meeting the development needs of the transmission field.
After evolving from analog to hybrid analog to fully digital, permanent magnet AC servo system drives have now entered the era of all-digital. All-digital servo drives not only overcome the limitations of analog servos, such as high dispersion, zero drift, and low reliability, but also fully leverage the advantages of digital control in terms of control precision and flexibility. This results in servo drives that are not only simpler in structure but also more reliable in performance. Currently, most high-performance servo systems utilize permanent magnet AC servo systems, which consist of a permanent magnet synchronous AC servo motor and a fully digital AC permanent magnet synchronous servo drive. A servo drive comprises two parts: the drive hardware and the control algorithm. The control algorithm is one of the key technologies determining the performance of an AC servo system and is a major area where foreign AC servo technology is restricted, representing the core of technological monopoly.
Basic structure of AC permanent magnet servo system
The AC permanent magnet synchronous servo drive mainly consists of a servo control unit, a power drive unit, a communication interface unit, a servo motor, and corresponding feedback detection devices, as shown in Figure 1. The servo control unit includes a position controller, speed controller, torque and current controller, etc. Our AC permanent magnet synchronous drive integrates advanced control technology and strategies, making it highly suitable for servo drive applications requiring high precision and performance. It also exhibits powerful intelligence and flexibility unmatched by traditional drive systems.
Currently, most mainstream servo drives use digital signal processors (DSPs) as their control core. Their advantage lies in their ability to implement complex control algorithms, enabling digitization, networking, and intelligent operation. Power devices generally employ drive circuits designed around intelligent power modules (IPMs). The IPM integrates the drive circuitry and includes fault detection and protection circuits for overvoltage, overcurrent, overheating, and undervoltage. A soft-start circuit is also added to the main circuit to reduce the impact on the drive during startup.
Servo drivers can be broadly divided into two modules: a power board and a control board, which are relatively independent in function. As shown in Figure 2, the power board (driver board) is the high-voltage section, which includes two units: one is the power drive unit (IPM) for driving the motor, and the other is the switching power supply unit for providing digital and analog power to the entire system.
The control board is the low-voltage component, the core of the motor control, and the platform for the core control algorithms of the servo drive technology. The control board outputs PWM signals through corresponding algorithms, which serve as drive signals for the drive circuit, thereby modifying the inverter's output power to control the three-phase permanent magnet synchronous AC servo motor.
Power drive unit
The power drive unit first rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding DC power. The rectified three-phase power or mains power then drives the three-phase permanent magnet synchronous AC servo motor through a three-phase sinusoidal PWM voltage-source inverter. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The main topology of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.
The inverter section (DC-AC) uses an intelligent power module (IPM) that integrates drive circuits, protection circuits, and power switches. The main topology adopts a three-phase inverter circuit schematic diagram (see Figure 3). It utilizes pulse width modulation (PWM) technology to change the frequency of the inverter output waveform by altering the alternating conduction time of the power transistors. This changes the on/off time ratio of the transistors in each half-cycle, which means that the output voltage of the inverter is changed by changing the pulse width to achieve the purpose of power regulation.
In Figure 3, VT1 to VT6 are six power switches, and S1, S2, and S3 represent three bridge arms. The switching states of each bridge arm are defined as follows: when the upper bridge arm switch is "on" (at which time the lower bridge arm switch must be "off"), the switching state is 1; when the lower bridge arm switch is "on" (at which time the lower bridge arm switch must be "off"), the switching state is 0. Since the three bridge arms only have two states, "0" and "1", S1, S2, and S3 form eight switching modes: 000, 001, 010, 011, 100, 101, and 111. The 000 and 111 switching modes result in zero inverter output voltage, hence these are called zero-state switching modes. The output line voltages are UAB, UBC, and UCA, and the phase voltages are UA, UB, and UC, where UBC is the DC power supply voltage. The attached table can be obtained based on the above analysis.
Control Unit
The control unit is the core of the entire AC servo system, realizing system position control, speed control, torque and current control. The digital signal processor (DSP) used not only has fast data processing capabilities but also integrates a wealth of dedicated integrated circuits for motor control, such as A/D converters, PWM generators, timer/counter circuits, asynchronous communication circuits, CAN bus transceivers, high-speed programmable static RAM, and large-capacity program memory. The servo driver achieves vector control (VC) by employing field-oriented control (FOC) and coordinate transformation, combined with sinusoidal pulse width modulation (SPWM) control mode to control the motor. Vector control of permanent magnet synchronous motors generally controls the stator current or voltage by detecting or estimating the position and amplitude of the rotor magnetic flux. Thus, the motor torque is only related to the magnetic flux and current, similar to the control method of DC motors, resulting in high control performance. For permanent magnet synchronous motors, the rotor magnetic flux position is the same as the rotor mechanical position. Therefore, by detecting the actual rotor position, the rotor magnetic flux position can be determined, simplifying the vector control of permanent magnet synchronous motors compared to asynchronous motors.
A servo driver controls an AC permanent magnet servo motor (PMSM). When controlling an AC permanent magnet servo motor, the servo driver can operate in current (torque), speed, and position control modes. The system control structure block diagram is shown in Figure 4. Since the AC permanent magnet servo motor (PMSM) uses permanent magnet excitation, its magnetic field can be considered constant; simultaneously, the motor speed of the AC permanent magnet servo motor is the synchronous speed, meaning its slip is zero. These conditions greatly reduce the complexity of the mathematical model for driving the AC permanent magnet servo motor by the AC servo driver. As shown in Figure 4, the system is based on the feedback of the two-phase current (ia, ib) of the motor and the motor position. The measured phase currents (ia, ib) are combined with the position information, and after coordinate transformation (from the a, b, c coordinate system to the rotor d, q coordinate system), the id and iq components are obtained, which are then fed into their respective current regulators. The output of the current regulator undergoes a reverse coordinate transformation (from the d, q coordinate system to the a, b, c coordinate system) to obtain the three-phase voltage command. The control chip, after inverting and delaying the three-phase voltage commands, outputs six PWM waves to the power devices to control the motor's operation. Under different command input methods, the commands and feedback are processed by the corresponding control regulators to obtain the reference commands for the next stage. In the current loop, the torque current components (iq) of the d and q axes are the output of the speed control regulator or an external input. Normally, the magnetic flux component is zero (id=0), but when the speed exceeds a certain limit, a higher speed value can be obtained through field weakening (id<0).
The transformation from coordinate system a, b, c to coordinate system d, q is achieved using Clarke and Park transformations; the transformation from coordinate system d, q to coordinate system a, b, c is achieved using the inverse Clarke and Park transformations.
Conclusion
This article provides a brief introduction to the implementation and principles of several main functional modules of a servo drive, intended to help readers gain a deeper understanding of servo drives. For a more in-depth understanding of the design principles of servo drives, please refer to other literature.
