Design and implementation of direct torque control system based on DSP

    Abstract: A direct torque control system based on TMS320F240 DSP was developed to realize motor AC speed regulation. The system design scheme and software and hardware implementation methods were introduced. Experimental results show that the system has good performance.

    Keywords: digital signal processor, direct torque control, AC speed-regulated asynchronous motor

With the development of power electronics and computer technology, high-performance asynchronous motor speed control systems have been widely used. High-performance AC speed control systems are inseparable from digital signal processors. In the past, digital signal processing was fast but had poor control capabilities. The new F24X/C24X series DSP is a digital motor speed controller specially developed by TI for three-phase AC speed regulation. It not only has the speed of a general DSP, but also has the control function of three-phase AC speed regulation. This paper develops a high-performance AC speed control system based on TMS320F240 DSP based on the principle of direct torque control of asynchronous motors. Experimental results show that the control system using TMS320F240 DSP has the characteristics of simple hardware circuit and excellent performance.

1 System structure and principle

The principle of the direct torque control system uses the space vector analysis method and uses the stator magnetic field orientation to perform a 3/2 transformation of the stator current and voltage, thereby directly calculating the torque and flux linkage of the AC motor in the stationary α-β coordinate system, and then Compare with the given torque and flux linkage respectively, and find the optimal switching table based on the comparison results to generate a PWM signal to directly control the switching state of the inverter. The basic block diagram is shown in Figure 1.

The following space vector expressions of stator voltage and current can be obtained from the 3-phase/2-phase coordinate transformation:

According to the UI model, the expression of the stator magnetic flux is:

φs= ∫(us-Rsis)dt (3)

The electromagnetic torque equation is:

T=np( φsαisβ-φsβisα) (4)

After obtaining the stator magnetic flux and electromagnetic torque, compare it with the given value, determine the appropriate switching state according to the comparison result and the vector angle θ , generate a PWM inverter signal, and control the working state of the main circuit inverter, thereby real-time Control the torque of the motor.

2 Hardware system design

This AC speed control system takes TMS320F240 DSP as the core and consists of a rectifier, voltage-type main loop and control loop. The entire system is modularly designed according to the power circuit board, DPS control board, 15V switching power supply, etc.

2.1 Power circuit

The power circuit is composed of EMS, uncontrollable rectifier module and IGBT inverter module IPM. It also has overcurrent, overvoltage and other protection functions. Through the reactance coil, EMS eliminates electromagnetic interference from the power grid and improves the anti-interference ability of the entire system. The drive circuit uses the smart IGBT inverter module IPM (PM10RSH120) from Mitsubishi Corporation of Japan. It not only integrates the power switching device and the drive circuit, but also has overcurrent, overvoltage, undervoltage, temperature and other protection circuits, which can not only protect the IPM module, and also simplifies the off-chip drive circuit, reducing the system failure rate. The fault output signal FO of the IPM is connected to the PDPINT port of the DSP through the optocoupler. When the IPM fails, the DSP can promptly set all event manager output pins to a high-impedance state, prohibiting PWM signal output, and protecting the system. The drive circuit uses HP HCPL4504 as the PWM drive circuit and is isolated to prevent strong current from entering the DSP control loop. Figure 2 shows a drive circuit.

The inverter requires that one of the upper and lower switching tubes on the same bridge arm is always on and the other is off, and must be interlocked. Therefore, the PWM signal must have a dead zone for a certain period of time, otherwise the upper and lower switch tubes of the same bridge arm will be turned on at the same time, causing the IGBT module to burn out. TMS320F240 integrates a dead-time adjustment circuit, and the dead-time can be adjusted from 0 to 102 μs . The system dead time is set by software. The dead time of this system is 4.5 μs .

The detection of the two-phase stator current is completed using two magnetically balanced Hall current sensors, which are directly installed on the power board. The detected signal is sent to the TMS320F240 control board through the connector, and the on-chip ADC is used to obtain the real-time stator current signal. .

2.2 DSP control board

DPS control consists of TMS320F240 DSP, simulation debugging interface JTAG, 128K Words external SRAM, 10M crystal oscillator, hardware reset loop, etc. Its core is the TMS320F240 DSP chip, which is a digital motor regulator developed by the American TI company specifically for three-phase AC speed regulation. speed controller. The powerful processing capabilities of TMS320F240 enable motor control-oriented control algorithms such as vector control and direct torque control to be quickly implemented. The DSP control board mainly completes tasks such as algorithm processing, PWM output, analog/digital conversion, communication with the host computer, and display. In order to complete the above tasks, this system has been expanded on the basis of the DSP minimum system. The 16-channel 10-bit ADC interface is used to receive feedback signals such as current; the RS232 serial port is used to communicate with the host computer and display data and waveforms; 16V8 GAL is used to Combined control signals. In addition, an LCD display circuit, keyboard input circuit, three 34-pin peripheral interfaces (connected to power boards, sensors, keyboards, etc.) and circuit modules are also added.

2.3 Switching power supply module

IPM has high requirements on driving power supply. Each IGBT unit of the upper arm needs a separate isolated 15V voltage supply to enhance anti-interference ability and reduce power supply noise; all units of the lower arm are powered by a common 15V power supply. Therefore, the power module includes four 15V power supplies isolated from each other to provide energy for the drive circuit of the IPM module. The core of the switching power supply is the single-chip three-terminal switching power supply chip TOP227, which integrates all the functions of the PWM control system into the three-terminal chip, including PWM, power switching field effect transistor, self-bias circuit, protection circuit, and high-voltage starting circuit and loop compensation circuit.

3 System software design

The software of the entire system is written in C2000 DPS assembly language. TMS320F240 starts the ADC through the event manager, obtains current and voltage signals, and realizes control of magnetic flux and torque. The entire software includes three parts: initialization, fault diagnosis, and GP TIMER1 interrupt service routine. The system software module flow chart is shown in Figure 3. Among them, the initialization module mainly completes the setting of motor parameters, initial value assignment of variables, setting of timer time, setting of timer service subroutine address, etc. The initialization flow chart is shown in Figure 4.

The interrupt service program is the core of software design. It completes almost all control algorithms, such as ADC detection, 3/2 conversion, calculation of magnetic flux and torque, acquisition of angle θ , voltage vector selection, action time and dead time time Calculation and setting, PWM generation, etc., this process is completed within one sampling period (50 μs ). The interrupt service subroutine flow chart is shown in Figure 5.

The generation of PWM signals is completed by the time manager through the configuration of special registers. The symmetrical PWM register configuration code is as follows:

SPLK #2840H, T1CON; up/down count in 50ns steps

SPLK #PWM_PERIOD,T1PR ;PWM carrier frequency

;fpwm=50ns * 2 *

PWM_PERIOD

SPLK #1207H,COMCON ;CMPRx,T1PR reload at Timer 1

;ACTR reload at Timer1=0

;enable PWM1-6 outputs

SPLK #0207H ,COMCON ;SYMMETRICAL PWM

SPLK #0666H ,ACTR ;PWM 1,3,5 active high

;PWM 2,4,6 active low

SPLK #32E0H ,DBTCON ;dead band =2.5 μs */

SPLK #8000,COMCON ;enable compare unit */

4Experimental results

The above direct torque control system experiment uses a 4-pole asynchronous motor: 2.2kW; 380V; 2.5A; 1500r/min. A DC generator acts as a load for an asynchronous motor. Figure 6 shows the torque step response curve and stator flux waveform. Experimental results show that the system has better performance.

The high-performance direct torque control system based on TMS320F240 makes full use of the high-speed computing function and rich on-chip peripheral resources of TMS320F240, making the AC speed control system simple in structure and high in reliability. Experimental results show that the system proposed in this article has high control accuracy and fast dynamic response, and is a high-performance AC speed control system.