Abstract : Permanent magnet AC servo systems are increasingly widely used in robotics, CNC, and other fields due to their superior performance. This paper provides a simple description of the functional implementation of its driver, including the rectification process of the rectifier section, the implementation of pulse width modulation (PWM) technology in the inverter section, and the corresponding algorithm of the control unit. Keywords : DSP, Rectification, Inverter, PWM, Vector Control 1 Introduction 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 is improving, and their prices are becoming more reasonable, making permanent magnet AC servo systems a development trend in modern electric servo drive systems, especially in the field of servo drives requiring high precision and high performance, replacing DC servo systems. Permanent magnet AC servo systems have the following advantages: (1) The motor has no brushes and commutator, making it reliable and easy to maintain; (2) The stator windings dissipate heat quickly; (3) The inertia is small, making it easy to improve the system's speed; (4) It is suitable for high-speed and high-torque operation; (5) Under 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, etc., meeting the development needs of the transmission field. After the development of analog and hybrid modes, permanent magnet AC servo system drivers have now entered the era of full digital. Full digital servo drivers not only overcome the large dispersion, zero drift, and low reliability of analog servo systems, but also give full play to the advantages of digital control in control accuracy and the flexibility of control methods, making the servo driver not only simple in structure, but also more reliable in performance. At present, most high-performance servo systems adopt permanent magnet AC servo systems, which include permanent magnet synchronous AC servo motors and full digital AC permanent magnet synchronous servo drivers. Servo drivers consist of two parts: driver hardware and control algorithm. Control algorithms are one of the key technologies determining the performance of AC servo systems, and are a major part of the foreign AC servo technology blockade, representing the core of technological monopoly. 2. Basic Structure of AC Permanent Magnet Servo Systems An 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, mainstream servo drives use digital signal processors (DSPs) as the control core. Their advantages include the ability to implement complex control algorithms, enabling digitization, networking, and intelligence. Power devices generally employ drive circuits designed around intelligent power modules (IPMs). The IPM integrates the drive circuit and features 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. [align=center]Figure 1. Structure of AC Permanent Magnet Synchronous Servo Driver[/align] The servo driver can be broadly divided into two modules: a power board and a control board, which are functionally independent. As shown in Figure 2, the power board (drive board) is the high-voltage section, comprising two units: a power drive unit (IPM) for driving the motor, and a switching power supply unit providing digital and analog power to the entire system. The control board is the low-voltage section, the core of motor control, and the platform for the core control algorithm of the servo driver technology. The control board outputs PWM signals through corresponding algorithms as drive signals for the drive circuit, thereby changing the inverter's output power to control the three-phase permanent magnet synchronous AC servo motor. [align=center]Figure 2. Power Board[/align] 3. 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. After rectification, the three-phase power or mains power is then frequency-converted by a three-phase sinusoidal PWM voltage-type inverter to drive the three-phase permanent magnet synchronous AC servo motor. 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 the power device drive circuit, protection circuit, and power switch. The main topology adopts a three-phase bridge 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, changing the on/off time ratio of the transistors in each half-cycle. In other words, by changing the pulse width, the magnitude of the inverter output voltage is changed to achieve the purpose of power regulation. [align=center] Figure 3 Three-phase inverter circuit[/align] In Figure 3, VT[sub]1[/sub]～VT[sub]6[/sub] are six power switches, and S[sub]1[/sub], S[sub]2[/sub], and S[sub]3[/sub] represent three bridge arms respectively. The switching states of each bridge arm are defined as follows: when the upper bridge arm switch is in the "on" state (at which time the lower bridge arm switch must be in the "off" state), the switching state is 1; when the lower bridge arm switch is in the "on" state (at which time the lower bridge arm switch must be in the "off" state), the switching state is 0. Since the three bridge arms only have two states, "0" and "1", S[sub]1[/sub], S[sub]2[/sub], and S[sub]3[/sub] form eight switching modes: 000, 001, 010, 011, 100, 101, and 111. Among these, the 000 and 111 switching modes result in zero inverter output voltage, so these switching modes are called the zero state. The output line voltages are UAB, UBC, and UCA, and the phase voltages are UA, UB, and UC, where UDC is the DC power supply voltage (bus voltage). Based on the above analysis, we can obtain the summary in Table 1. [align=center]Table 1 Three-Phase Inverter Circuit Analysis - 2 / 3[/align] 4 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, in addition to its fast data processing capabilities, 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. Servo drivers achieve vector control (VC) by employing field-oriented control (FOC) and coordinate transformation, while simultaneously using sinusoidal pulse width modulation (SPWM) control to control the motor. Vector control of permanent magnet synchronous motors (PMSMs) typically controls stator current or voltage by detecting or estimating the position and amplitude of the rotor's magnetic flux. Thus, the motor torque depends only on the magnetic flux and current, similar to the control method of DC motors, resulting in high control performance. For PMSMs, the rotor's magnetic flux position is the same as its mechanical position. Therefore, by detecting the actual rotor position, the rotor's magnetic flux position can be determined, simplifying vector control compared to asynchronous motors. Servo drivers controlling AC permanent magnet servo motors (PMSMs) 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 significantly reduce the complexity of the mathematical model for AC servo drives driving AC permanent magnet servo motors. As shown in Figure 4, the system is based on the measurement of the two-phase current feedback (I_a, I_b) and the motor position. The measured phase currents (I_a, I_b) are combined with the position information, and after coordinate transformation (from the a, b, c coordinate system to the rotor d, q coordinate system), I_d and I_q 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 three-phase voltage commands. The control chip uses these three-phase voltage commands, after inversion and delay, to obtain six PWM waves output to the power devices to control the motor operation. Under different command input methods, the system obtains the reference command for the next level through the corresponding control regulator. In the current loop, the torque current components (I_q) of the d and q axes are the output of the speed control regulator or an external given value. Under normal circumstances, the magnetic flux component is zero (I_d = 0), but when the speed is greater than the limit value, a higher speed value can be obtained by weakening the field (I_d < 0). [align=center]Figure 4 System Control Structure[/align] The transformation from the a, b, c coordinate system to the d, q coordinate system is achieved by Clarke and Park transformations; the transformation from the d, q coordinate system to the a, b, c coordinate system is achieved by the inverse Clarke and Park transformations. The following are two transformation formulas: Clarke Transform and Park Transform: 5. Conclusion This article provides a brief introduction to the implementation and principles of several main functional modules of a servo driver, intended to help readers gain a deeper understanding of servo drivers. For a more in-depth understanding of servo driver design principles, please refer to other literature. Due to the author's limited expertise, shortcomings are inevitable; readers are kindly requested to offer their criticism and corrections.