1 Introduction Induction motors possess advantages such as simple structure, robustness, high speed, large capacity, and reliable operation. However, as an induction motor is a high-order, nonlinear, and strongly coupled multivariable system, magnetic flux and torque are coupled together, unlike DC motors where magnetic flux and torque can be controlled separately. Therefore, high-performance induction motor speed control systems were not available until the 1980s. In recent years, with the development of power electronics technology, modern control theory, and other related technologies, induction motors have found increasingly widespread applications in adjustable drives. The proposed vector control strategy has further achieved decoupling control of magnetic flux and torque, with control effects comparable to DC motors. This paper analyzes the vector control principle of induction motors and establishes a simulation model of an induction motor slip-type vector control system based on MATLAB/Simulink. Simulation results demonstrate the rationality of the model. Based on this, the system's software and hardware are designed, and the correctness of the control strategy is verified through experiments. 2 Basic Principles of Vector Control For a long time, DC motors have had excellent operating and control characteristics, and torque can be easily controlled by adjusting the excitation current and armature current. Because its torque is linearly related to the armature current when the main pole excitation flux remains constant, torque control can be achieved quickly and accurately through the armature current loop, giving the system good steady-state and dynamic performance. However, due to the commutator and brushes, DC motors have inherent disadvantages, such as complex manufacturing, high cost, need for regular maintenance, limited operating speed, and difficulty in applications with special requirements for corrosion and explosion protection. Vector control aims to simulate the control characteristics of DC motors for AC motor control. Based on the dynamic model of the AC motor, through vector coordinate transformation and rotor flux orientation, an equivalent mathematical model of the DC motor is obtained, simplifying the dynamic model of the AC motor and decoupling flux linkage and torque. Then, a control system designed according to the DC motor model can achieve excellent static and dynamic performance. In a two-phase synchronous rotating coordinate system oriented by rotor flux linkage, the control equation of the induction motor vector control system is: [img=142,121]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-gs1.jpg[/img] (1) It can be seen from equation (1) that the rotor flux linkage ψr is generated only by the stator current excitation current ism and is independent of the stator current torque component ist. The electromagnetic torque te is proportional to the product of the rotor flux linkage and the stator current torque component. This fully demonstrates that the induction motor vector control system oriented by rotor flux linkage can achieve complete decoupling of flux and torque. The key to the vector control system oriented by rotor flux linkage is accurate orientation. However, direct detection of rotor flux linkage is very difficult, and the method of indirectly estimating flux linkage using flux linkage model is affected by changes in motor parameters, resulting in inaccurate control. Therefore, rather than using flux linkage closed-loop control with inaccurate feedback, it is better to use flux linkage open-loop control, which makes the system simple and reliable. The open-loop control method of flux linkage does not require the amplitude of rotor flux linkage, but the position signal of rotor flux linkage is still required for vector coordinate transformation. It can be seen that the calculation of rotor flux linkage is still unavoidable. If the position of rotor flux linkage is indirectly calculated using the given value, the system structure can be simplified. This method is called indirect orientation. The indirect orientation vector control system uses the slip formula in the vector control equation to form a slip-type vector control system [1]. This paper designs a slip-type vector control system. Its control idea is: in the control process, the rotor flux linkage of the motor remains unchanged, so that the torque of the motor can be the same as in steady-state operation, mainly determined by the slip rate. According to this idea, the given value of the stator current m-axis component can be obtained directly from the rotor flux linkage, and the closed-loop control of the flux can be avoided by effectively controlling the stator current. This control method uses the slip rate and the measured speed to estimate the position of rotor flux linkage by integration. The structure is relatively simple, and the dynamic performance that can be obtained can basically reach the level of DC double closed-loop control system. Its system model is shown in Figure 1. [align=center][img=567,265]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-01.jpg[/img] Figure 1. Schematic diagram of slip-type vector control system[/align] 3. System Simulation Based on the above principle analysis, a slip-type vector control system model for induction motors was built, and the system was simulated and analyzed using the MATLAB software Simulink. The system simulation model is shown in Figure 2. [align=center][img=567,274]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-02.jpg[/img] Figure 2. Simulation model of induction motor slip-type vector control system[/align] In the simulation system, the speed regulator, torque regulator, and flux regulator all adopt output-limited pi regulation. The simulation waveforms are shown in Figures 3 to 5. The simulation results show that the open-loop indirect vector control system with flux has good control performance. [align=center][img=425,280]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-03.jpg[/img] Figure 3 Speed ​​Response [img=425,277]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-04.jpg[/img] Figure 4 Three-Phase Current Waveform [img=425,297]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-05.jpg[/img] Figure 5 Output Torque[/align] 4 System Hardware Circuit Design Due to the advantages of digital signal processors (DSPs) such as simple hardware circuits, flexible control algorithms, strong anti-interference capabilities, no drift, and good compatibility, they are now widely used in AC motor control systems. Therefore, this design adopts a digital control system with DSP as the control core. The system employs an AC-DC-AC voltage-frequency conversion circuit, inputting single-phase 220V AC power and outputting three-phase AC power to control the induction motor. The control circuit uses the DSP chip TMS320LF2407 as its core, forming a fully functional, all-digital slip-type vector control system. The entire system mainly consists of two parts: the main circuit and the control circuit. 4.1 Main Circuit The main circuit is the power conversion actuator, including a rectifier circuit, a filter circuit, an energy dissipation circuit, and an inverter circuit. This system uses an AC-DC-AC voltage-type main circuit, first rectifying the fixed-frequency AC power into DC power, and then inverting the DC power into continuously adjustable-frequency three-phase AC power. The inverter circuit uses an IPM module of model IR16UP60A, which includes a gate drive circuit, a logic control circuit, and protection circuits for undervoltage, overcurrent, short circuit, and overheating. The application of this intelligent module reduces the size of the device and improves the system's performance and reliability. 4.2 Control Circuit The control circuit of the system uses TMS320LF2407 as the control core to complete the detection of current and speed signals, the implementation of the control algorithm, and the corresponding PWM signal output. The detection circuit is divided into two parts: current detection and speed detection. 4.2.1 Current Detection The result of current signal detection is used for coordinate transformation in vector control to achieve decoupling of flux linkage and torque. Since the sum of the instantaneous values ​​of the three-phase currents in the Y-connected winding is 0, i.e., ia + ib + ic = 0, only two phase currents need to be detected. The third phase can be obtained by adding and inverting the signals of the other two phases. This system uses a CHB-25NP current Hall sensor (see Figure 6) to output the detected current on the secondary side at a transformation ratio of 200:1. Since the allowable input of the TMS320LF2407 on-chip A/D converter is a unipolar signal of 0-3.3V, the acquired current signal needs to pass through a voltage offset circuit and a limiting circuit before entering the A/D conversion input channel of the DSP. [align=center][img=425,129]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-06.jpg[/img] Figure 6 Current Sampling Circuit[/align] 4.2.2 Speed ​​Detection Speed ​​detection is the key to the speed closed-loop control system, and its accuracy directly affects the control accuracy and stability of the speed regulation system. This system uses an incremental photoelectric encoder with a photoelectric code disk that outputs 2048 pulses. It is powered by 5V and has six outputs: a+, a-, b+, b-, z+, and z-. Among them, a and b are used for speed measurement, and they are 90° out of phase, outputting 2048 pulses per revolution; while the z-axis outputs one pulse per revolution to determine the spatial position of the rotor. The speed acquisition circuit is shown in Figure 7. [align=center][img=567,360]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-07.jpg[/img] Figure 7 Speed ​​Acquisition Circuit[/align] The signals a+, a-, b+, b-, z+, z- output from the incremental photoelectric encoder are input to the DS3486M. The DS3486M has anti-interference capabilities, which can improve the transmission accuracy and enable the speed signal to be transmitted over long distances. The output signal is shaped by a set of inverters and then input to the quadrature encoder pulse circuit (QEP circuit) in the DSP. The rotor position and speed information of the induction motor can be obtained through the quadrature encoder pulse circuit. 5 System Software Design The software of this system consists of two parts: the main program and the PWM interrupt service subroutine (see Figure 8). The main program initializes the hardware and variables, sets the initial values ​​of each control register, and allocates addresses and sets the corresponding initial values ​​for various variables used in the operation. The initialization module is executed only once after the DSP is powered on and reset, and then enters the loop waiting period. The interrupt service subroutine is the core part of the system, responsible for A/D conversion, speed calculation, coordinate transformation, PI adjustment, generation of PWM signal, etc. [2]. [align=center][img=425,1097]http://www.ca800.com/uploadfile/maga/inv2008-4/xuqiwei-08.jpg[/img] Figure 8 PWM interrupt program flowchart[/align] 6 Experimental study After completing the hardware design and software programming debugging of the control system, the operating performance of the system was experimentally studied. Figure 9 shows the steady-state current waveform of the induction motor, and Figure 10 shows the speed response curve, which follows the speed command very well. 7 Conclusion This paper uses TMS320LF2407 to design an induction motor slip vector control system. Through theoretical analysis, simulation research and experimental results, it is confirmed that the magnetic flux open-loop indirect vector control system has good static and dynamic performance. Meanwhile, it provides a foundation for realizing more complex control algorithms and also provides ideas for the design and debugging of actual induction motor vector control systems. About the author Xu Qiwei (1983-) Male Master student in the Department of Electrical Engineering, Harbin Institute of Technology. Research direction is motor drive control. References [1] Ruan Yi, Chen Weijun. Motion Control System. Beijing: Tsinghua University Press, 2006 [2] Wang Xiaoming, Wang Ling. DSP Control of Electric Motor. Beijing: Aerospace University Press, 2004