Abstract: The TMS320F240 digital signal processing chip is a dedicated DSP controller for motor and motion control. This paper utilizes the TMS320F240 DSP controller to achieve a fully digital induction motor direct torque control system. The hardware of the control system, including peripheral interfaces, was designed, and the software was programmed using DSP assembly language. Experiments demonstrate the correctness of the system design. Experimental results show that the system has superior dynamic and static performance. Keywords: DSP; Induction Motor; Direct Torque Control; Digital Control ABSTRACT: Aiming at the control of induction motor, the digital direct torque control scheme is implemented with DSP (TMS320F240), which is specially developed for motor and motion control. The hardware is completely designed, including wealthy peripherals, and the software is programmed in DSP assembly language. All the above design has been confirmed by experiments. It indicates that the control system of induction motor has better dynamic and static characteristics. KEY WORDS: DSP; Induction Motor; Direct Torque Control; Digital Control 1 Introduction The TMS320F240 is a DSP specially designed for motor control. Due to its high-speed processing capability of 20MIPS and dedicated peripherals for motor control, it has been widely used in the all-digital control of motors. The chip mainly includes the F240 central processing unit (CPU); the peripherals of the TMS320F240 include 1) event manager; 2) analog-to-digital converter module; 3) serial interface module; 4) system protection module. Using microprocessors to control various motors not only facilitates the implementation of control circuits but also enables the execution of complex, high-performance control strategies. By controlling the motor's voltage, current, torque, speed, and angle, the microprocessor ensures the motor operates accurately according to given instructions, significantly improving its performance. This paper uses the F240 as the core, constructs the system's peripheral hardware, develops the system control software, and finally implements an algorithm for fully digital control of an induction motor through experiments. 2. Direct Torque Control Principle of Induction Motors Direct torque control treats the inverter's control mode and the motor's operating performance as a whole, controlling the stator flux linkage of the induction motor by controlling its input voltage. When the induction motor is powered by a three-phase inverter, the motor's input voltage depends entirely on the inverter's switching mode, and the waveform of the motor's flux depends on the input voltage mode. Therefore, the goal of direct torque control is to establish the relationship between the flux linkage and the inverter's switching mode, ensuring the correct switching of the inverter's switches to achieve an approximately circular magnetic field in the motor's air gap. In direct torque control, the rotational speed of the stator flux linkage is controlled by the space voltage vector to change the average rotational speed of the stator flux linkage, thereby changing the slip, i.e., the flux angle, to control the motor torque. The inverter switching signal is determined by the outputs of three functional modules: stator flux linkage range judgment, flux linkage hysteresis adjustment, and torque hysteresis adjustment, to achieve the purpose of controlling the motor speed and torque. The basic structure of the system control is shown in Figure 1. These represent stator voltage, current, stator flux linkage, and electromagnetic torque, respectively. [align=center] Figure 1 Block diagram of induction motor control system[/align] 3 System Hardware Structure The hardware circuit structure for implementing the control system includes an A/D conversion circuit for stator current measurement, a speed measurement circuit required for PI regulation, the main control circuit of the F240, and a rectifier-inverter main circuit controlled by the output PWM pulse. The inverter circuit uses the Mitsubishi PM50RSA120 intelligent power module. The DSP chip communicates with the host computer via an RS232 bus, and the F240 provides an SCI serial communication interface module. The system uses a MAX232C to convert TTL level (0V, 5V) to RS232 signal level (+12V, -12V). Opto-isolation is used between the F240 and the main circuit, and the output control signal is transmitted to the main circuit via an optocoupler isolation circuit. The control system configuration is shown in Figure 2. [align=center] Figure 2 Hardware Block Diagram of the Control System[/align] The motor stator phase current is detected using a LEM LA25-NP current sensor. The output of the current sensor is sent to the high-speed analog-to-digital converter port of the TMS32OF240 via an operational amplifier. The F240's dual A/D conversion circuit can convert two signals simultaneously, and can simultaneously measure the A and B phase currents to obtain the real-time stator current value. The sampling circuit is shown in Figure 3. The motor speed is measured using an incremental photoelectric encoder, which outputs two photoelectric pulses, A and B, staggered by 90°. These pulse signals are sent to the capture unit or quadrature encoder circuit (QEP) of the TMS32OF240 to detect the edge transition of two consecutive pulse signals, enabling the internal timer of the TMS32OF240 to count, and obtaining the motor speed by the M/T method. The speed measurement circuit is shown in Figure 4. [align=center] Figure 3 Sampling and Conditioning Circuit[/align] [align=center] Figure 4 Speed ​​Measurement Circuit[/align] 4 Control Algorithm Software Design The main program of the system software mainly includes the initialization of the variables used and the initialization of each module of the system. It waits for the timer T1 underflow interrupt and enters the interrupt service subroutine to implement the algorithm and control functions. The interrupt service routine is shown in Figure 5. When an interrupt occurs, the CPU points to the corresponding address in the interrupt vector table and sets the interrupt flags IFR and EVIFRA. After the CPU responds to the interrupt, it jumps to the specified general interrupt service routine. The interrupt flag IFR is automatically cleared, and the interrupt mode bit INTM is set, disabling all other maskable interrupts. [align=center]Figure 5 Interrupt Service Subroutine Flowchart[/align] The subroutines to be completed in the interrupt service routine include sampling and conversion of feedback signals (current, voltage, and speed), 3/2 coordinate transformation, flux linkage and torque calculation, hysteresis adjustment, switch state judgment, and pulse signal output, etc., to implement the direct torque control algorithm. The output signal controls the motor. 5 Experimental Results The three-phase induction motor adopts a Y-connection, with a rated power of 0.2KW, a stator rated voltage of 380V, a frequency of 50Hz, and a rated speed of 1420r/m. The correctness of the current and voltage sampling, phase voltage calculation, coordinate transformation, and stator flux linkage calculation procedures was verified through experiments. When the stator flux linkage is set to a certain value, Figure 6 shows the measured approximately circular magnetic field; when the reference rotor speed is given as 400r/m, the speed response can stabilize at the set value in a short time, at which time the motor output torque is 4.5Nm. Figure 7 shows the actual rotor speed and motor output torque response curves. [align=center] Figure 6 Approximately circular magnetic field Figure 7 Speed ​​response and output torque curve[/align] 6 Conclusion The author's innovation is that a fully digital AC speed control system, including the design of peripheral circuits for control, was realized through the DSP chip TMS320F240. The direct torque control algorithm was written using assembly instructions. The real-time computing capability realizes the efficient control algorithm. The experimental results verify that the digital control system has excellent reliability and flexibility. 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