Design of dual closed-loop speed control system for three-phase asynchronous motor based on TMS320LF2407A and AT89S52

Abstract: Aiming at the speed regulation requirements of three-phase AC asynchronous motor in a certain equipment, a current and speed dual closed-loop speed regulation control system is designed with TMS320LF2407A and AT89S52 as the core and adopting the magnetic field oriented control strategy. The hardware principle block diagram, key components, design ideas and program flow chart are given. The experimental results show that the control system has the advantages of fast dynamic response, high control accuracy, real-time display, data storage, strong anti-interference, etc. Keywords: TMS320LF2407A ; AT89S52; asynchronous motor; magnetic field oriented control; real-time display

O Introduction
Three-phase AC asynchronous motors are widely used in weapons and equipment, feeding systems, CNC machine tools, flexible manufacturing technology, various automation equipment and other fields due to their simple structure, small size, light weight, low price and convenient maintenance. The performance of their speed control system directly determines the performance of the equipment. With the emergence of high-performance microprocessors and new power electronic devices, the frequency conversion speed regulation method using fully controlled power electronic devices and space vector (SVPWM) control technology has become the mainstream of AC motor speed control.
Compared with other microprocessors, DSP has the advantages of fast computing speed, can generate PWM output with dead time, can realize complex algorithms such as fuzzy control, and has few peripheral hardware, so it is widely used in digital control of motors. This paper designs a full digital speed control system for three-phase AC asynchronous motors with TMS320LF2407A DSP chip and AT89S52 single-chip microcomputer as the core. The experimental results show that the system has the advantages of real-time display, data storage, fast dynamic response, high control accuracy and strong anti-interference.

1 Introduction to TMS320LF2407A
TMS320LF2407A mainly includes arithmetic logic unit (CALU), register set, auxiliary arithmetic logic unit (ARAU), multiplier, multiplication shifter, accumulator, addition shifter, clock phase-locked loop circuit, two completely identical event managers A and B (including general timer, comparison unit, capture/orthogonal encoder pulse circuit), internal A/D converter, dual serial port, watchdog, CAN bus circuit unit, etc.
TMS320LF2407A adopts advanced Harvard structure and pipeline operation. At an internal clock frequency of 30 MHz, the instruction cycle is only 33 ns. Its internal memory contains two types of RAM blocks. One is DRAM and the other is SRAM. For DRAM, it is divided into three RAM blocks, namely B0, B1, and B2, with capacities of 256 words, 256 words, and 32 words respectively. All these RAMs are allowed to be accessed twice in one instruction cycle, so there is a significant increase in data processing capabilities. At the same time, the B0 block can also be dynamically configured as a data memory area or a program memory area through a program. If configured as a program area, the floating-point algorithm subroutine or data table can be loaded into this area from an external slow EPROM when powered on, thereby alleviating the contradiction between the high-speed processor and the slow peripherals, which is of great help in improving the dynamic performance of the control system. The TMS320LF2407A contains a 10-bit precision, high-speed A/D converter with built-in sampling and holding, and the shortest conversion time is 500 ns (sampling and holding + conversion time). In addition, the TMS320LF2407A also has a rich and powerful interrupt system and commonly used I/O interfaces, which simplify the hardware circuit when designing a speed control system.

2 System Hardware Design
The hardware block diagram of the dual closed-loop speed control system for three-phase AC asynchronous motor based on TMS320LF2407A is shown in Figure 1.

The main circuit of the system adopts AC-DC-AC voltage inverter, and the power device adopts intelligent power module IPM. The module contains 6 IGBTs and 6 freewheeling diodes connected in anti-parallel with the IGBTs. The control circuit part consists of AT89S52 single-chip control unit, TMS320LF2407A controller unit, current detection circuit, voltage detection circuit, speed detection circuit, overcurrent protection circuit, LCD display circuit and keyboard input interface circuit.

2.1 AT89S52 single-chip control unit
The AT89S52 control unit mainly completes the following functions: First, it completes the setting of the given speed through the keyboard input interface; second, it completes the display of the given speed, the speed when the motor starts, and the speed when it reaches steady state through the LCD display unit; third, it completes the reading of the data stored in the dual-port RAM, and imports the read data into the host computer through the USB interface circuit or inputs it into the analog device through the D/A output circuit. Among them, the LCD display unit adopts the Chinese graphic dual-purpose LCD display module OCMJ4X8B-2; the keyboard input adopts the matrix key keyboard, which can call the preset Chinese characters through the single-chip computer, and can input numbers (used to set the speed); the dual-port module is used to store the variable waveform data collected by TMS320LF2407 A. 2.2 TMS320LF2407A control unit The circuit schematic diagram of the TMS320LF2407A control unit is shown in Figure 2. The control unit circuit mainly includes an optocoupler isolation circuit, a speed detection circuit, a current detection circuit and a voltage detection circuit, which respectively complete the functions of IPM driving, speed detection and control, overcurrent protection, overvoltage and undervoltage protection.

The optocoupler isolation circuit is composed of 6 pieces of Toshiba's TLP127 and corresponding current limiting resistors. It mainly completes the electrical isolation between TMS320LF2407A and IPM intelligent power module, and amplifies the output PWM signal. The
speed detection circuit uses Omron 1024 original rotary linear encoder E6B2-CWZ6C. The pulse output by the encoder can achieve 4096 pulses per revolution after being quadrupled inside TMS320LF2407A, thus ensuring the accuracy of the speed. According to the comparison between the sampled data and the given data, the width of the DSP output drive pulse is adjusted to adjust the speed of the AC motor.
The current sampling circuit uses 3 pieces of Hall current sensors CN61M/TBC25C04. One way sends the instantaneous current value on the DC bus to the overcurrent protection circuit. When its value is greater than the overcurrent value, the corresponding overcurrent protection circuit generates a protection signal and shuts off the output of the PWM signal; the other two ways detect the current flowing through the motor, and change the drive pulse output by the DSP by transformation, thereby keeping the motor speed unchanged. In the control system designed in this paper, TMS320LF2407A uses three channels, ADCIN00, ADCIN01 and ADCIN02, to collect the current of motor phase A, phase B and DC bus.
The sampled voltage of the DC bus is input to DSP through ADCIN03 channel. According to the sampled data, when the voltage exceeds the set upper and lower limits, DSP shuts off the output of PWM pulses, thereby realizing overvoltage and undervoltage protection functions.

3 System software design
3.1 Principle of closed-loop speed control
The principle block diagram of the dual closed-loop speed control system designed in this paper is shown in Figure 3.

Among them, the given speed is input into the AT89S52 single-chip control system by the keyboard input interface circuit, and the speed PI regulation, current PI regulation, magnetic field position angle and speed feedback are calculated by TMS320 LF2407A. The measured motor speed is output to the LCD display unit through the AT89S52 control system for real-time display.
Assuming that the three-phase windings of the motor stator and rotor are completely symmetrical; the stator and rotor surfaces are smooth and have no cogging effect, and the air gap magnetomotive force of each phase of the stator and rotor is sinusoidally distributed in space; magnetic saturation, eddy current and core loss are all negligible, then the torque equation of the three-phase AC asynchronous motor is as follows:

Where: Lr, Lm are the rotor self-inductance and mutual inductance respectively; p is the differential operator; isq is the component of the stator current on the q axis; ψrd is the component of the rotor flux on the d axis.
It can be seen from formula (1) that the torque of the asynchronous motor is related to the stator current vector and the rotor magnetic field and the angle. Therefore, in order to control the torque, the magnetic flux must be detected and controlled first. When the dq coordinate system is on the synchronous rotating magnetic field and each AC quantity in the stationary coordinate system is converted into the corresponding DC quantity in the rotating coordinate system, the d-axis and the rotor magnetic field direction are made to coincide, and the field orientation control equation can be obtained as follows:

From equation (2), it can be seen that the rotor flux amplitude can be observed by detecting the d-axis component (excitation component) of the stator current; from equation (4), it can be seen that when ψrd is constant, the electromagnetic torque can be controlled by controlling the q-axis component (torque component) of the stator current. The specific working principle is as follows:
the stator current iA and iB output by the inverter are measured by the current sensor, converted into digital quantities by the A/D converter of the DSP, and ic is calculated using ic=-(iA+iB). The currents iA, iB, and iC are transformed by Clarke and Park to obtain the excitation feedback current isd and torque feedback current isq in the dq coordinate system. The difference between the given excitation current isdref and torque current isqref is adjusted by PI, and then the voltage in the αβ coordinate is output through Park inverse transformation. The DSP uses this voltage to generate the six drive signals required by the three-phase inverter. The real-time measured motor speed signal is used to compare with the given speed to generate isqref, and enters the current-position flux conversion model to find the position of the flux and is used for Clarke and Park inverse transformation.
3.2 Program Flowchart
After the AT89S52 single-chip microcomputer control system is powered on, the given speed is first input through the keyboard input interface. The single-chip microcomputer stores the given speed in the dual-port RAM. At the same time, the given speed is output to the drive control chip SED1520 of the liquid crystal display unit through the single-chip microcomputer P0 port. SED1520 drives OCMJ4X8B-2 to display the speed. The speed display range is 0~9999 r/min. Then the keyboard is used to determine whether to sample and store the relevant variables. The program flow chart of this part is shown in Figure 4(a).

Secondly, the TMS320LF2407 A DSP control system is powered on, and the initialization program is run to complete the initial state setting (including sampling and storing relevant variable data, etc.). The DSP control system samples the motor speed and armature current and compares them with the given values. If the set speed is reached, it will run in a loop. When the given speed changes, it will enter the interrupt processing subroutine. The main program flow chart and the interrupt processing subroutine flow chart are shown in Figure 4(b) and Figure 4(c), respectively.

4 Experimental results
The motor speed, stator current, magnetic flux and other variables were sampled, saved, and transferred to the host computer through the USB interface circuit; the experimental waveform obtained on the host computer is shown in Figure 5, and the data sampling points of the waveform are 2048.

As can be seen from Figure 5(a), the motor reaches the set speed value of 1000 r/min in a very short time. During the startup process, the stator current fluctuates due to PWM control. When the speed reaches the set value, the stator current quickly stabilizes and the dynamic response is very fast. As can be seen from Figure 5(b), when the speed changes, the q-axis component (torque component) of the stator current basically does not change, and the electromagnetic torque of the motor will not change. As can be seen from Figures 5(c) and (d), when the motor speed changes, the change amplitude of the motor's magnetic field can be ignored, so the motor speed will not change. That is, the designed control system has a high control accuracy for the speed and fully meets the design requirements.

5 Conclusion
This paper adopts a three-phase AC asynchronous motor double closed-loop speed control system with TMS320LF2407 A as the PWM control core and AT89S21 control unit as the motor speed control management core. It has functions such as data storage and real-time display. The experimental results show that the control system has the advantages of fast dynamic response and high control accuracy. Practice has proved that the system also has good anti-interference. The system also has a high guiding significance for the design and implementation of the motor speed control system.