Gyro power supply design

    The power supply of the gyro is generally a single-phase or three-phase variable frequency power supply with a medium frequency (commonly used is 400Hz). At the same time, this type of power supply is widely used in aerospace, ships, locomotives, induction heating, radar, communication switches and other equipment. Therefore, the research on this type of power supply has a high engineering practical value. This paper takes the semi-liquid floating rate gyro in a certain type of missile as the research object, and uses SPWM technology to provide a solution for the power supply of the semi-liquid floating rate gyro. At the same time, a method based on timing to achieve arbitrary angle difference of the three phases of the inverter power supply is proposed, which greatly simplifies the realization of the angle difference.

    l Semi-liquid floating rate gyro power supply design

    The specific technical indicators of the semi-liquid floating gyro power supply studied in this paper are listed in Table 1.

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    1.1 SPWM principle and basic scheme

    SPWM technology is an advanced modulation technology. Its connotation is to obtain a set of rectangular pulse waveforms with equal amplitude and unequal width by controlling the on and off of the switching devices according to a certain rule, which is used to generate a sinusoidal voltage wave of the required frequency. The 400Hz medium frequency power supply developed by this technology is advanced in technology, and its electrical performance indicators and reliability will be greatly improved.

It is much better than the previous multi-link medium frequency power supply. There are usually two ways to implement SPWM: one is analog method and the other is digital method. The analog method circuit is more complex and has temperature drift phenomenon, which affects the accuracy and limits the performance of the system; the digital method uses a computer to calculate each switching point according to different digital models, stores it in the memory, and then generates SPWM waves through table lookup and necessary calculations. However, the digital method is greatly affected by the memory and cannot guarantee the accuracy of the system. The SA series PWM waveform generator produced by Mitel has the advantages of high accuracy, strong anti-interference ability, and simple peripheral circuits. Among them, the SA4828 series SPWM generator can be connected to the microprocessor to complete the peripheral control function and make the system intelligent. The microprocessor only uses a small amount of machine time to control it, so it has the ability to detect, protect, and control the entire system. Based on the above reasons, we decided to use SA4828 as the core to develop the SPWM medium frequency power supply.

    1.2 Block diagram of the power system design for a semi-liquid rate gyro

    The power supply of the semi-liquid floating rate gyro is shown in Figure 1.

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    The basic working principle of the system is: the DC power supply is output after SPWM full-bridge inversion, filtering and isolation. The system uses amplitude feedback and frequency feedback to achieve the stability of the amplitude and frequency of the gyro power supply. The CPU uses the 89S52 microcontroller produced by Intel; the SPWM signal generator uses SA4828; the hybrid integrated drive circuit EXB841 constitutes an isolation drive protection circuit; the inverter main circuit uses an IGBT module.

    2 Hardware Circuit Design

    2.1 SPWM waveform generator

    SA4828 is a SPWM waveform generator designed by Mitel for three-phase inverter circuits. It can generate pure sine waves, especially digital operation of the system composed of CPU, strong anti-interference ability, simple interface, good versatility, and can be connected to microcontrollers with different interfaces. Under the condition that the circuit remains unchanged, the performance indicators of the inverter can be changed through software modification, which greatly improves the debugging efficiency and reduces the product cost. Its main features are as follows.

    1) The connection between the fully digital SA4828 and the microprocessor is applicable to a variety of microprocessors, and the interface circuit is simple. The interface circuit between SA4828 and the single-chip microcomputer 89S52 is shown in Figure 2.

    2) Flexible working mode SA4828 has 6 standard TEL level outputs to drive the 6 power switch devices of the inverter. The carrier frequency, modulation frequency, modulation ratio, minimum pulse width, dead time, amplitude and other working parameters can be set directly by software writing registers R0, R1, R2, R3, R4, R14, R15 without any external circuit.

    3) Wide operating frequency range and high precision The triangular carrier frequency is adjustable, the carrier frequency can reach up to 24kHz, the output modulation frequency can reach up to 4kHz, and the output frequency resolution can reach 16-bit word length; the fully digital pulse output has high precision and temperature stability.

    2.2 Isolation drive circuit

    There are many gate drive methods for IGBT. This solution uses the hybrid integrated IGBT dedicated drive module EXB841 launched by Fuji Corporation of Japan. It is one of the high-speed and large-capacity devices in the EXB series of drivers. It integrates isolation, drive and protection, has excellent performance, and fully embodies the two principles of "optimal drive and decentralized protection". The specific application circuit is shown in Figure 3.

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    2.3 MCU minimum system and peripheral expansion chip

    The control circuit of this system is composed of the minimum system of 89S52 single-chip microcomputer, a small number of extended peripheral chips and SAZ4828 three-phase PWM generator. The single-chip microcomputer completes the initialization, output pulse width control and frequency control of SA4828, and at the same time completes the calculation and data processing of open-loop and closed-loop control algorithms, the detection of analog and digital signals, protection functions and logical judgment, etc. The 89S52 single-chip microcomputer has 40 pins, which are divided into three categories: port lines, power lines and control lines. Among them, the port lines are divided into 4 groups, namely P0, P1, P2, and P3. Only P0 and P1 ports are used for data exchange in the system. P0 port is mainly used for data exchange between ADC0809 to form voltage feedback. P1 port is mainly used to transmit initialization data to SA4828 and adjust the output of SA4828 according to the result of the calculation. P3, 4 ports and pin 21 of SA4828 form frequency feedback.

    3 SA4828 working principle and system software design

    3.1 SA4828 internal structure and working principle

    The internal structure and working principle block diagram of SA4828 are shown in Figure 4

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    It generates SPWM waveform according to X(t)=Asinωt. Since the main working principle of SA4828 is similar to that of SA8282, it will not be described here. The special principle of SA4828 is explained as follows.

    1) The selection of three different waveforms is mainly through the transmission of commands to the initialization register and the control register to set the three-phase waveform ROM. They are sine, enhanced, and high efficiency.

Waveform, so that it can be used in various special occasions.

    2) When the "watchdog" circuit SA4828 receives a command from the microcontroller, if a problem occurs, the bus control will send a reset "watchdog" signal to delay the "watchdog" from shutting down the output drive signal.

    3) 8 register units In order to improve frequency accuracy and independently control the amplitude of the three-phase waveform, SA4828 adds 8 register units. The unit address and description are shown in Table 2.

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    When transmitting the initialization command, write R5 as the "watchdog" delay control word. When transmitting the control command, write R0 and R1 as the 16-bit frequency control word, and write R3, R4, and R5 as the three-phase output waveform amplitude control word. The above settings and adjustments are completed through the address/data bus, register unit, and initialization register and control register.

    3.2 Software Design

    Software programming is the core of the entire inverter control, which determines the output characteristics of the inverter, such as voltage, frequency range and stability, harmonic content, protection function perfection, reliability, etc. Figure 5 is the program flow chart of this system.

    In the main program, the parameters of SA4828 initialization command and control command are calculated and set, which are mainly used to determine the frequency adjustment range, dead time, output voltage amplitude, center frequency, amplitude, etc.

    From the technical requirements of gyro power supply (Table 1), we know that the gyro motor power supply requires a 90-degree phase difference between phases A and B. Here, the software delay method is used to achieve this. That is, after a delay of 0.625 ms after the output of phase A, phase B is output, achieving a 90-degree phase difference between phases A and B.

    Here, a method based on timing method is proposed to realize arbitrary angle difference between any two phases in three-phase power supply. It solves the problem that it is difficult to realize arbitrary angle difference between any two phases in inverter power supply. It greatly simplifies the tedious work and a large number of peripheral circuits of realizing arbitrary angle difference between any two phases in inverter power supply. However, while bringing some benefits, it also brings some problems. For example, in this system, 2 SA4828s are required to realize 90-degree phase difference between phases A and B, which increases the cost of the system to a certain extent. However, due to the wide application of SA4828 at present, its market price is also relatively low. Taking all factors into consideration, using SA4828 software delay function to realize arbitrary angle difference between any two phases in inverter power supply is still the best option.

    4 Experimental results and conclusion

    The experimental results show that the system design has achieved the power supply requirements of the semi-liquid floating rate gyro. A high-precision intermediate frequency power supply with a three-phase frequency stability of 0.1% and an amplitude stability of 0.1% has been achieved. By using the software delay method, a 90-degree phase difference between phases A and B has been achieved. At the same time, after the system adopts the 89S52 single-chip microcomputer and SA8282 signal generator, the sine wave control circuit is greatly simplified, the components are reduced, the structure is compact, the cost is reduced, and the reliability is improved. Through actual testing, relatively ideal results have been achieved, which fully meet the requirements of the actual system.