Design of vector control frequency conversion speed regulation system based on DSP

Abstract: By using the powerful computing power and fast real-time processing capability of TI's digital signal processing chip TMS320F2808, the problem of the difficulty in implementing the complex control algorithm of vector control is solved, and the hardware and software design of the vector control variable frequency speed regulation system is completed. The experimental results show that the system has good stability, dynamic characteristics, and fast fault handling function.
Keywords: vector control; asynchronous motor; TMS320F2808

Introduction
Vector control is a high-performance control technology that appeared in the 1970s, which improves the static and dynamic performance of the AC speed regulation system. Vector control has the characteristics of wide speed regulation range and fast torque response, but it involves many asynchronous motor parameters and requires complex rotation transformation calculations. With the rapid development of power electronic devices and computer technology, the emergence of various high-performance chips has solved the cumbersome calculation problems of high-performance vector control methods, making vector control technology more widely used and developed.
According to the basic principle of vector control, this paper adopts the digital processing chip TMS320F2808 with powerful computing power and fast real-time processing capability of TI as the control chip, designs a fully digital vector control hardware system, and gives the protection circuit, current detection circuit, speed detection circuit, and part of the program flow.

1 Basic principle of vector control
The basic theory of vector control is to simulate the law of torque control of DC motor on three-phase AC motor, apply the coordinate transformation Clarke transformation to convert the three-phase AC system into a two-phase AC system, and then convert the two-phase AC system into a rotating DC system through Park transformation. In the rotor magnetic field orientation coordinate, the stator current vector is decomposed into the excitation component that generates magnetic flux and the torque component that generates electromagnetic torque, and the two components are made perpendicular to each other, realizing the decoupling of the stator current excitation component and the torque component, achieving the purpose of controlling the magnetic flux and torque of the asynchronous motor separately, thereby obtaining the same excellent static and dynamic speed regulation performance as the DC motor speed regulation system. Its basic principle is shown in Figure 1.

2 System hardware circuit design
2.1 Main circuit
This system adopts a voltage-type "AC-DC-AC" frequency conversion structure as the main circuit, which is mainly composed of a rectifier circuit, a filter circuit and an inverter circuit. In order to make the main circuit structure simple and easy to replace and repair components, this design adopts a modular structural design scheme. Figure 2 is a block diagram of the asynchronous motor vector control system based on TMS320F2808.

This design uses Mitsubishi's rectifier inverter brake module CP10TD1-24A. Its features are: using LPT-CSTBTTM silicon chip technology and integrated injection molded small package for rectification, inverter, brake, and NTC temperature detection, low saturation voltage drop, small module thermal resistance, built-in NTC temperature sensor, etc.
2.2 Control loop
Since the vector control system has a large amount of calculation, the control loop uses TI's DSP chip TMS320F2808 and its peripheral circuits to implement the vector control core algorithm, related voltage and current detection and processing functions.

2.2.1 Power supply circuit
Figure 3 shows the power supply circuit. In order to improve the stability of the control system and extend the service life of the device, this design uses a high-performance voltage regulator chip and a low-dropout voltage regulator LM1117 to provide a reliable power supply for the TMS 320F2808. 3.3 V and 1.8 V fixed voltage output chips are selected to power the DSP, respectively. The output current can reach 800 mA, the output voltage accuracy is within ±1%, and it has current limiting and thermal protection functions.

2.2.2 JTAG simulation debugging interface circuit
In order to facilitate the connection between the speed control system and the host computer, realize the access of the simulator to the DSP and simulate and debug the vector control speed control program, the JTAG simulation debugging interface circuit is indispensable in the variable frequency speed control design. The specific circuit diagram is shown in Figure 4.

2.2.3 Overvoltage and undervoltage protection circuit
In order to improve the reliability of the system and better protect the inverter components and asynchronous motors, the speed control system should be equipped with a set of accurate protection measures to prevent various faults. This paper adopts the overvoltage and undervoltage protection circuit of the DC bus voltage, as shown in Figure 5. When the detected DC bus voltage exceeds or is lower than the preset voltage, all control signals will be turned off, thereby playing a protective role. Among them, LM393 is a dual voltage comparator.

2.3 System detection circuit
The detection circuit is an important part of the speed control system. Its function is to convert the detected signal into a DSP recognizable signal after analog/digital conversion, and then output the required signals of each part of the circuit through a certain algorithm, so as to realize the predetermined function and provide necessary protection for the system and the motor. Therefore, whether the detected signal is reasonable and accurate is directly related to the reliability and control accuracy of the entire system. According to the needs of the vector control system, this paper carries out stator current detection and motor speed detection.
2.3.1 Stator current detection circuit
In the vector control system, the accuracy and real-time performance of the stator current are an important factor affecting the control accuracy of the speed control system. This paper uses the Hall current sensor TBC-05SY with high accuracy, good linearity, wide bandwidth, fast response, strong overload capacity and no loss of measurement circuit to detect the stator current. Figure 6 shows the stator current detection circuit. Among them, LMC6464 is a low-power, rail-to-rail input and output CMOS operational amplifier.

2.3.2 Motor speed detection circuit
The accuracy of motor speed detection is also an important factor affecting the control accuracy and stability of the speed regulation system. In order to expand the speed regulation range and improve the low-speed stability, the speed measuring element is required to have stable low-speed output, small ripple and good linearity. The usual speed measuring element will not only affect the application circuit, but also generate electromagnetic interference. This paper uses the HEDSS company's low-inertia, low-noise, high-resolution incremental photoelectric encoder HKT5617-301G1024BZ3/5E to detect the actual speed of the motor. The interface circuit between the encoder and TMS320F2808 is shown in Figure 7.

3 System software design
The system software mainly consists of two parts: the main program and the interrupt program. The main program mainly completes the initialization of hardware and various variables, and sets the initial value for each register; the interrupt program includes the PWM interrupt program, various fault protection interrupt programs, etc. The corresponding process is shown in Figure 8.

4 Experimental results
In this design, a load experiment was conducted. Different loads were added under the condition of a given speed of 1400 r/min to measure the actual speed of the motor. The experimental results when the speed given value is 1400 r/min are listed in Table 1. Among them, TN is the rated load torque.
It can be seen from Table 1 that the experimental results of this design are within the estimated allowable error range, and the system can work stably. This shows that this design is feasible.

Conclusion
This paper adopts the modular idea and designs a vector control variable frequency speed regulation system based on TMS320F2808, as well as a good fault protection circuit. The system has the advantages of simple structure, high speed regulation accuracy, wide speed regulation range, low power consumption, strong anti-interference ability and reliable performance.