Design of permanent magnet motor propulsion system based on DSP

With the development of control theory, power electronics and permanent magnet materials, permanent magnet propulsion motors are widely used in various variable speed drive occasions. This is mainly because permanent magnet propulsion motors have the advantages of simple structure, reliable operation, small size, light weight, high efficiency and power factor.

The traditional permanent magnet synchronous motor speed control system generally adopts a double closed-loop system. The speed control of the outer loop can generally be digitally controlled, while the current control of the inner loop is generally not easy to digitally control. This is mainly because the electrical time constant of the motor is relatively small, and the real-time requirements for current control are very high, which is difficult for general microprocessors to meet. However, with the development of power electronics technology and microprocessor technology, especially the DSP-F240 launched by TI for motor control, it provides a more realistic means to achieve full digital control. DSP-F240 is mainly composed of CPU, on-chip RAM and programmable FLASH ROM, event manager, on-chip peripheral interface and other parts. Its operating frequency is relatively high, generally greater than 20MIPS, and many peripheral devices for motor control are integrated on the chip, making the implementation of the entire system relatively easy. This article introduces the hardware structure and software flow of the permanent magnet synchronous motor propulsion system with TMS320LF2407 ADSP as the core, and conducts Matlab/Simulink simulation and low-speed operation experiments on this solution.

1 Vector control strategy of permanent magnet synchronous motor

The vector control theory was proposed by F. Blaschke in 1971. Its basic principle is: in the rotor flux dqO rotating coordinate system, the stator current is decomposed into two mutually orthogonal components id and iq, where id is in the same direction as the flux, representing the stator current excitation component, and iq is orthogonal to the flux direction, representing the stator current torque component. The armature reaction magnetic field generated by these two current components is used to equivalently replace the armature reaction magnetic field generated by the original stator three-phase winding current ia, ib, ic, that is, Park transformation is performed:

Where: γ is the rotor position angle, that is, the electrical angle of the rotor d-axis leading the center line of the stator a-phase winding. Then, by independently controlling id and io, good dynamic characteristics like a DC motor can be obtained. The d-axis and q-axis inductances of the permanent magnet synchronous motor with a surface protruding rotor structure are basically the same, so its electromagnetic torque equation is:

Where: pn is the number of rotor pole pairs, Ψf is the effective value of the fundamental flux generated by the permanent magnet.

In order to make the stator unit current produce maximum torque and improve the working efficiency of the motor, this paper selects the maximum torque/current vector control. It can be seen from formula (2) that for the permanent magnet synchronous motor with a surface protruding rotor structure, id=0 can be set, and the torque control can be achieved by adjusting iq. As shown in Figure 1, the entire servo system consists of three control loops.

1) Position loop: collects the pulse signal output by the motor rotary encoder, performs calculation after phase detection and frequency multiplication, and provides the rotor position information required for coordinate transformation;

2) Speed ​​loop: The difference between the actual speed n and the set speed nref is adjusted by PI and used as the q-axis current reference value iqr, which is then adjusted by the current loop;

3) Current loop: Compare the actual current values ​​id and iq with the reference values ​​idr and iqr, and generate the d-axis and g-axis voltage reference values ​​udr and uqr after PI adjustment. Convert them to the stationary coordinate system to obtain uαr and uβr, and generate the inverter trigger signal according to the SVPWM method to drive the motor.

2 System Hardware Structure

The hardware structure of the permanent magnet synchronous motor propulsion system is shown in Figure 2. It mainly provides the following three functions: implementation of motor control strategy, detection and sampling of control quantity, and power drive.

2.1 TMS320LF2407A DSP

The entire system control strategy is implemented by the TMS320LF2407A DSP, which has the characteristics of low power consumption and high speed, and its single instruction cycle can be as short as 25 ns. The two event managers (EVA and EVB) on the chip each have two general-purpose timers, one external hardware interrupt pin, three capture units (CAP) and one orthogonal encoding unit (QEP). These functions, together with modules such as the serial peripheral interface (SPI), facilitate data processing, strategy execution and decision output in the motor control process.

2.2 Control quantity detection part

The acquisition of the motor's mechanical quantity is completed by an incremental photoelectric encoder, and its output includes two groups of pulse signals: A, B, Z and U, V, W. Their connection with the DSP is shown in Figure 3. Among them, the A and B signals are orthogonal. The orthogonal encoding unit quadruples them and sends them to the corresponding counter for counting. The counting direction is determined by the phase sequence of the A and B signals. The Z signal outputs a pulse every time the rotor rotates one circle. According to their different states, the 360° electrical angle plane can be divided into 6 parts to determine the initial rotor position angle of the motor.

The motor current state quantity is collected by the Hall current sensor. Its sampling circuit is shown in Figure 3. The input-output relationship is:

In order to ensure the sampling accuracy when the current is small and improve the operation of the motor at low speed and light load, the 12b dual A/D converter ADS7862 is used to replace the 10b analog/digital conversion module inside the DSP. Through the external memory expansion interface of the DSP, the analog current of formula (3) is converted into a digital result and input into the DSP.

2.3 Power drive part

The power drive of the permanent magnet synchronous motor is AC-DC-AC PWM mode, in which the rectifier part adopts single-phase bridge uncontrolled rectifier, and the inverter part adopts intelligent power module PS21869, which integrates 6 insulated gate bipolar transistors and their drive and protection circuits. The trigger signal is provided by the PWM1~6 pins of the DSP. When an overcurrent or undervoltage fault occurs, the IGBT drive circuit can be turned off, and a fault signal is output from the corresponding fault pin to the PDPINTA pin of the DSP, blocking the PWM pulse output through hardware interrupt.

3 System Software Design

The software of the permanent magnet motor propulsion system mainly consists of three parts: initialization program, main program and interrupt service subroutine. When the system is reset, the initialization program is executed first to detect and set the working mode and initial state of each module inside the DSP. The main program is responsible for collecting a series of real-time operation information such as motor current and speed; the timing interrupt subroutine is the core program for realizing the motor vector control strategy, which mainly completes the two functions of PI regulation and SVPWM waveform generation. Its flow chart is shown in Figure 4.