0 Introduction:
Traditional asynchronous motor control systems often employ photoelectric digital pulse encoders as measuring devices. However, these encoders are susceptible to interference, reducing system reliability and making them unsuitable for harsh operating environments. To address these shortcomings, this paper proposes a sensorless control method using spatial pulse width modulation (SVPWM). Utilizing modern digital signal processing technology, complex flux linkage and speed control can be achieved. Furthermore, sensorless vector control of the asynchronous motor is implemented based on the DSP TMS320F2812.
1. Spatial Pulse Width Modulation Principle
For asynchronous motors, the three-phase AC current applied to the stator generates a rotating magnetic field, which interacts with the induced magnetic field of the rotor to generate torque and cause the rotor to rotate. Space pulse width modulation converts the three-phase current vector of the stator into two equivalent and orthogonal components, one of which is equivalent to the magnetic field current and the other is equivalent to the torque current. Space vector control controls the magnitude, frequency and phase of the three-phase current of the stator to keep its magnetic field component at the maximum allowable value and adjusts the torque current component to control the magnitude of the torque. By controlling the switching mode of the inverter, the space vector of the stator voltage of the motor moves along a circular trajectory, thereby significantly reducing torque ripple [1]. The commonly used three-phase voltage source inverter main circuit structure is shown in Figure 1.
Figure 1. Three-phase voltage source inverter circuit structure diagram
The three-phase inverter has eight switching states (upper bridge arm conducting switch state is 1, lower bridge arm conducting switch state is 0), corresponding to eight basic space voltage vectors. Two of these are zero voltage vectors (O0, O111), and the other six basic voltage space vectors (U0, U60, U120, U180, U240, U300) are spatially separated by 60 degrees, and their amplitudes are all equal.
The voltage space vector (Ux, UX+60) and two zero voltage space vectors (O0, O111) are synthesized according to the parallelogram law, as shown in Figure 2. Uref is as shown in equation (1):
(1) Sensorless control principle
Sensorless vector control detects the phase current and phase voltage of the asynchronous motor and uses certain observation techniques to determine the motor's speed, which serves as the speed feedback for the speed closed loop in the vector control system. (See Figure 3.)
Figure 3. Control principle system block diagram
2.1 Rotor flux estimation
In a rotor magnetic field oriented vector control system, accurate estimation and control of rotor flux linkage is one of the key factors affecting motor control performance. There are two types of rotor flux linkage estimation: voltage type and current type. The traditional voltage model algorithm is simple and less affected by changes in motor parameters, but its observation accuracy is low at low speeds and the error accumulation and drift problems of pure integral terms are serious. The traditional current model does not involve pure integral terms, and its observation performance at low speeds is worse than that of the voltage model method, but it is not as good as the latter at high speeds, and it is greatly affected by the rotor time constant [2].
This paper combines voltage and current models to estimate rotor flux linkage. The flux linkage calculated by the current model is subjected to PI calculation, and the result of the PI calculation is used to compensate for the flux linkage of the voltage model. By adjusting the value of the PI parameter, the voltage model plays a major role at high speed and the current model plays a major role at low speed, thus overcoming their shortcomings and improving the accuracy of estimation.
In rotor field-oriented control, the current model rotor flux linkage equations in the two-phase rotating coordinate system and the two-phase stationary coordinate system are shown in equations (2) and (3), respectively:
The rotor flux linkage can be calculated from the voltage model stator flux linkage:
2.2 Speed Estimation Principle
The rotational speed of the sensorless vector control system is estimated based on the rotor flux output from the flux estimation model. The flux vector relationship is shown in equation (9):
3 Control System Design
Based on the principle of sensorless vector control, the TMS320F2812 was selected as the core controller to design the hardware of the control system, and the software program was written on the CCS2000 compilation platform.
3.1 Hardware Design
The sensorless vector control system is also composed of a main circuit and a control circuit. The system uses IGBT power devices to form a three-phase inverter circuit. The main circuit of the AC-DC-AC voltage type general-purpose frequency converter is composed of rectifier circuit, filter circuit, drive protection circuit and IGBT. The TMS320F2812 is used as the core to form the control core. The DSP is responsible for sampling the three-phase current of the motor, realizing the algorithm of sensorless vector control, and finally outputting PWM to drive the three-phase inverter bridge to work [3]. The basic structure of the system hardware is shown in Figure 4.
The high-power IGBT parallel structure is adopted, and the IR21363S is used as the PWM driver chip. It has three independent high-voltage side and low-voltage side output signals, and can output six PWM signals at the same time. The PWM working frequency can reach 500kHz. It has undervoltage and overcurrent protection functions [4]. Hall effect and magnetic ring are used to detect the two-phase current of the asynchronous motor. The DC bus voltage is detected by resistor voltage division and filtered by RC filter circuit, which improves the accuracy of AD sampling and the reliability of system operation.
Figure 4 Overall Hardware Framework
3.2 Software Design
The system software is written in C language and mainly includes the main program and the timer underflow terminal subroutine. The specific program flowcharts are shown in Figures 5 and 6.
4. Conclusion
With the development of various control theories, digital signal processors (DSPs), and their widespread application in motor control, motor control technology has entered a new stage. This study demonstrates that the brushless DC motor control system based on the TMS320F2812 core exhibits high control precision, strong real-time performance, low power consumption, and a rich array of control functions—features unmatched by traditional control systems, fully showcasing the superiority of DSP control.
About the author:
Li Lanlan, born in 1986, is a female master's student at the School of Information Science and Engineering, Wuhan University of Science and Technology. Her research interests include control theory and control engineering.
Mailing Address: P.O. Box 244, School of Information Science and Technology, Wuhan University of Science and Technology, Qingshan District, Wuhan, Hubei Province
Telephone number: 15972158067
