Permanent magnet synchronous servo motor drive system
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 great strides.
With the performance of permanent magnet AC servo systems improving and their prices becoming more reasonable, permanent magnet AC servo systems are replacing DC servo systems, especially in the field of servo drives with high precision and high performance requirements, making it a development trend in modern electric servo drive systems.
Advantages of permanent magnet AC servo systems
The electric motor has no brushes or commutator, making it reliable in operation and easy to maintain.
Stator windings dissipate heat quickly
Low inertia makes it easier to improve the system's speed.
Suitable for high-speed and high-torque operation
With the same power output, 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 analog, permanent magnet AC servo drive systems have now entered the era of fully digital. Fully 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, resulting in servo drives that are not only simpler in structure but also more reliable in performance.
Currently, most high-performance servo systems employ permanent magnet AC servo systems, which consist of two parts: permanent magnet synchronous AC servo motors and fully digital AC permanent magnet synchronous servo drives. Servo drives comprise two parts: drive hardware and control algorithms. The control algorithm is one of the key technologies determining the performance of an AC servo system, and it 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
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, a speed controller, a torque and current controller, etc.
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). IPMs integrate the drive circuitry and also feature 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.
Figure 1. Structure of AC permanent magnet synchronous servo drive
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.
Figure 2 Power Board
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 the 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 is then frequency-converted by a three-phase sinusoidal PWM voltage-source 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 drive circuits, protection circuits, and power switches. The main topology adopts a three-phase bridge circuit schematic shown in 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, that is, by changing the pulse width, the magnitude of the inverter output voltage is changed to achieve the purpose of power regulation.
Figure 3 Three-phase inverter circuit
The following rules apply to the switching states of each bridge arm in Figure 3: 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.
The three bridge arms have only two states, "0" and "1", thus forming eight switching modes: 000, 001, 010, 011, 100, 101, and 111. Among them, the 000 and 111 switching modes make the inverter output voltage zero, so this switching mode is called the zero state.
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.
Servo drives achieve 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 typically controls the stator current or voltage by detecting or estimating the position and amplitude of the rotor magnetic flux. In this way, the motor torque depends only on the magnetic flux and current, similar to the control method of DC motors, resulting in high control performance. https://shop213429207.taobao.com
For permanent magnet synchronous motors, the rotor flux position is the same as the rotor mechanical position. Thus, by detecting the actual position of the rotor, the magnetic flux position of the motor rotor can be determined, which simplifies the vector control of permanent magnet synchronous motors compared to asynchronous motors.
Servo drivers control AC permanent magnet servo motors (PMSMs). When controlling AC permanent magnet servo motors, servo drivers can operate in current (torque), speed, and position control modes respectively.
The control structure block diagram of the system is shown in Figure 4. Since the AC permanent magnet servo motor (PMSM) uses permanent magnet excitation, its magnetic field can be regarded as constant. At the same time, the motor speed of the AC permanent magnet servo motor is the synchronous speed, that is, its slip is zero.
These conditions significantly reduce the complexity of the mathematical model for AC servo drives when driving AC permanent magnet servo motors. As shown in Figure 4, the system is based on the feedback of the two-phase current of the motor and the motor position.
Figure 4 System control structure
The measured phase currents, combined with position information, are transformed using coordinates (from the a, b, c coordinate system to the rotor d, q coordinate system) to obtain components that are then fed into their respective current regulators. The outputs of the current regulators undergo 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, after inversion and delay, uses these three-phase voltage commands to generate six PWM waves, which are then output to the power devices to control the motor operation.
Under different command input methods, the system uses corresponding control regulators to obtain reference commands for the next level. In the current loop, the torque current components 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 (=0), but when the speed exceeds a certain limit, a higher speed value can be obtained by weakening the magnetic field (<0).
The transformation from coordinate system a, b, c to coordinate system d, q is achieved using the 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. The following are the two transformation formulas for the Clarke transformation:
Summarize
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.
