Design of vector control system of AC permanent magnet linear synchronous motor based on DSP

Abstract: Based on the comparison between rotary motors and linear motors, the structural characteristics of AC permanent magnet synchronous linear motors are analyzed. Based on the particularity of linear motors, a vector conversion control method for AC permanent magnet synchronous linear motor speed regulation is given, and the hardware and software design of the control system based on DSP is made.

1 Introduction

The traditional method of providing the linear driving force required in the manufacturing industry is to use a rotary motor plus a ball screw. Practice has shown that this type of drive has shown its shortcomings in many high-precision, high-speed applications. In this case, linear motors came into being. Linear motors directly generate linear motion without any intermediate conversion link. The power is directly generated in the air gap magnetic field, which can achieve positioning accuracy and fast response speed several times higher than traditional drive mechanisms [1]. At present, the United States, Japan, Germany, Switzerland and other countries are relatively high-level countries in the research of linear direct drive systems. Products of companies such as SIEMENS and Kollmorgen have been commercialized [2]. China attaches great importance to the research and development of linear motors. Many scientific research institutes have carried out experimental research, but have not achieved industrialization. This paper designs and applies a drive system based on vector conversion control based on the AC permanent magnet synchronous linear motor developed by our department.

2. Working principle of AC permanent magnet synchronous linear motor

The working principle of a linear motor is equivalent to a rotating motor that is expanded radially. When a three-phase AC current is applied to an AC permanent magnet synchronous linear motor, a magnetic field is generated in the air gap. If the end effect is not considered, the magnetic field is sinusoidally distributed in the linear direction. The traveling magnetic field interacts with the secondary to generate electromagnetic thrust, causing relative motion between the primary and secondary. Figure 1 shows the developed and designed AC permanent magnet synchronous linear motor.

3. Vector control principle of permanent magnet linear synchronous motor

Compared with scalar control, vector control can achieve steady-state operation in a shorter time due to its fast dynamic response. After more than 30 years of industrial practice, improvement and enhancement, it has reached a mature stage [3] and has become the preferred method for AC servo motor control. Therefore, linear motors use the AC vector control drive method.

The three-phase voltage (U, V, W phase) of the primary of the linear motor constitutes a three-phase primary coordinate system (a, b, c axis system), in which the phase angle of the three-phase winding differs by 120°, that is, the pole pitch differs by 1/3 in the horizontal direction. Referring to the theory of vector transformation of rotating motors, a two-phase primary coordinate system (α-β axis system) is set. The transformation from the three-phase primary coordinate system to the rectangular coordinate system is called Clark transformation, see formula (1).

The transformation from a stationary coordinate system to a rotating coordinate system is called Park transformation, see equation (2). The reverse is called inverse Park transformation.

θ is the angle between the d-axis and the axis. According to the Park transformation theory of rotating motors and the comparison of the two motor structures. Due to the difference in the moving parts of the motors, the linear motor mover is equivalent to the rotating motor stator, and the linear motor stator is equivalent to the rotating motor mover. Therefore, in the rotating motor, the rotating coordinate system is fixed on the mover, and the rotating coordinate system rotates synchronously with the motor rotor. In the linear motor, according to the principle of relativity of motion, the linear motion of the mover can be understood as the stator moving in the opposite direction relative to the mover. Therefore, the "rotating coordinate system" (in fact, this coordinate system is a linear motion, which should be called a linear motion coordinate system) is fixed on the stator and moves linearly with the stator relative to the mover, as shown in Figure 3. At this time, the linear motor mover moves linearly to the right, and its stator moves linearly to the left relative to the mover. The coordinate system fixed on the stator also moves to the left relative to the mover together with the stator. The traveling magnetic field inside the mover moves to the left relative to the mover itself, so the traveling magnetic field of this synchronous motor is stationary when standing on the coordinate system fixed on the stator. So let the d-axis be on the N-pole axis of the secondary permanent magnet, and the q-axis be ahead of the d-axis by 90°, which is 1/4 of the pole pitch. θ is determined by the position of the mover when the linear motor moves.

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4. Design of permanent magnet synchronous linear motor control system

According to the working principle of linear motor, vector transformation is used to design its control drive system.

The controller uses a DSP processor, and the TMS320F2812 DSP from TI is selected . It is the latest 32-bit fixed-point high-speed digital signal processor launched by TI. The execution speed of 150MIPS shortens the instruction cycle to 6.67ns. It has a built-in 12-bit AD converter with a minimum conversion time of 80ns[4]. The power drive part uses an IPM module, and the PWM frequency can reach up to 20K.

The structural block diagram of the permanent magnet synchronous linear motor drive control system is shown in Figure 3.

5. Software Structure

The system software consists of five parts: hardware and software initialization program, main program, initial positioning subroutine, control process display program and interrupt service subroutine. After the system is reset, the initialization program is first executed to set the working mode of each functional module inside the DSP and detect the initial state; then the main program is executed to enable the timing interrupt, external protection interrupt and initial positioning subroutine; after obtaining the accurate position information of the mover, it enters the running state and executes the interrupt service subroutine [5]. The main functions of the system, including the calculation of current magnitude, speed position information and vector transformation, are completed by the interrupt service subroutine. The software structure is designed according to the system operation principle. Figure 5 is a diagram of the system operation program:

The system interrupt subroutine diagram is shown in Figure 4:

6. Conclusion

The algorithm program designed in this paper has been successfully debugged and can achieve basic operation, proving the correctness of the software and hardware design. Figure 5 shows some debugging results.

Due to the end effect of linear motors and the direct loading of external loads, the stability of linear motor control systems is required to be high. In order to improve the robustness of its control, its algorithm needs to be further improved. Appropriate control algorithms and control strategies must be adopted to make the system have fast dynamic response, strong anti-interference ability, and high steady-state tracking accuracy. Therefore, the control theory of linear motors needs further in-depth discussion and research.

The author's innovation is to analyze the difference between the vector transformation theory of permanent magnet linear synchronous motor and rotary motor, and on this basis, implement its control system based on TMS320F2812 DSP.