Design of signal source for aviation AC power supply test system

Aiming at the research on cold platform debugging technology of aviation AC power supply system equipment, this paper discusses the design of generator AC signal source, one of the main components of the test system, in detail, and mainly introduces its basic hardware composition. This system adopts modular design and bus structure, with good scalability and maintainability. It can be adjusted and applied to other test systems as needed. It has good use and development space.

1. Introduction

With the development of aircraft power supply systems, AC power supply has become the main power supply method for most aircraft. At present, various countries still continue the traditional "inverter-generator-drag station" hot stage debugging technology. Its disadvantages are large investment, high repair cost, high energy consumption, and complicated operation. In view of the above shortcomings of hot stage debugging technology, we have conducted research on the cold stage debugging technology of aviation AC power supply system. The signal source introduced in this article is the link for powering the cold stage debugging system and providing detection signals.

2. Signal source main simulation signal During the research process, according to the requirements of aviation AC power supply system and debugging process, the main simulation signals of the system are determined as follows:

The generator outputs three-phase voltage signal: three-phase AC signal with adjustable frequency and voltage: 360-440Hz/100-300V, power <200VA, adjustment accuracy: frequency step ≤0.1Hz, voltage 0.1V.

Online voltage signal: single-phase AC signal with adjustable frequency and voltage: 360-440Hz/0-120V, power <100VA, adjustment accuracy: frequency step ≤0.1Hz, voltage 0.1V.

Auxiliary exciter signal: three-phase AC signal with adjustable frequency and voltage: 700-800Hz/0-60V, power <300VA, adjustment accuracy: frequency step ≤0.2Hz, voltage 0.1V.

Reactive current balancing signal: same or opposite to one phase of the generator output voltage signal. 0-30V, power <30VA, accuracy: voltage regulation ≥0.1V.

Active current balancing signal: same or opposite to one phase of the generator output voltage signal. Accuracy: voltage regulation ≥ 0.1V.

Generator and feeder short circuit signal: same or opposite to one phase of the generator output voltage signal. Power <30VA.

3. System Hardware Composition

1. Design Concept

In the power supply system test, the system output power is required to have a small waveform distortion compared with the actual output of the generator, and the system can quickly stabilize when the frequency changes. From the perspective of design requirements, this system uses 512 sample points to form a periodic waveform of the sine signal; class A and B complementary power amplification is used in the power amplifier link; phase-locked technology is used in the frequency synthesis part to ensure frequency stability. The system flow chart is shown in Figure 1.

The single-chip microcomputer is the core component of the system. It communicates with the industrial computer through the serial port , receives the frequency modulation, phase modulation and amplitude modulation information sent by the industrial computer, and controls the frequency divider to generate a 51.2Hz frequency reference signal after data processing. After being sent to the frequency synthesizer, it is phase-locked and frequency-locked to obtain a stable frequency signal with a frequency of F=n×51.2Hz. This signal is input into the cycle counter to generate an address signal, which is synthesized with the phase control signal in the adder to make the signal phase adjustable. The sinusoidal waveform data is taken out through the waveform memory. It is converted into a sinusoidal AC power supply signal by the D/A conversion circuit . The output signal amplitude is adjustable through the multiplier, and then the amplitude and current are amplified by the power amplifier to generate a three-phase AC signal with adjustable frequency, phase and amplitude.

2. Frequency Synthesizer

The frequency synthesizer is composed of 8254 timer/counter and 4046 phase-locked loop. The circuit is shown in Figure 2.

4046 has 2 phase comparison channels, channel 1 is an OR gate structure network, channel 2 is an edge-triggered digital memory network. Channel 1 is used for signal comparison with a duty cycle of 50%, while channel 2 has no such limitation. This system uses 2 channels of 4046. The parameters of the low-pass filter composed of RL1, RL2 and capacitor C determine the frequency range generated by the VCO. In this system, RL1, RL2 and C are 150K, 16K and 1μ respectively. The frequency range generated by the VCO is 180~620Hz.

The 51.2Hz frequency signal F-in generated by 36000 frequency division of the 1.8432MHz crystal oscillator is compared with the frequency signal after 1-channel frequency division of 8254. The output frequency difference voltage signal is led out from the PC2 pin, passed through the low-pass filter to the VCO, and an oscillation signal with a center frequency of F-out is generated. F-out is divided by n by 8254 to generate a comparison signal. After the frequency is stabilized, F-out is output to the cyclic address generator to obtain the waveform address.

From the principle of 4046, we can know that after frequency locking, F-out=51.2×n.

3. Circular address generator and waveform memory

The cyclic address generator is implemented by a 4526 binary counter. The signal of the F-out frequency is connected to the CLK terminal of the 4526. The frequency of F-out is the counting frequency of the counter. The 12-bit counting data generated by the counter is added to the phase control signal sent by the microcontroller in the 4008. The resulting 12-bit data is sent as the waveform address to the waveform memory composed of 27256EPROM to read the waveform data. This signal system uses a sine waveform, and the waveform data of one cycle is divided into 512 points for storage. Therefore, the frequency of the output AC signal is:

F = F-out/512 = n × 51.2/512 = 0.1n

By changing the value of n, you can get signals of different frequencies. Changing the waveform data in the waveform memory can generate waveforms of different shapes, and you can also improve the waveform of the signal by increasing the stored waveform data.

4. D/A conversion and multiplication circuit

The D/A circuit unit consists of two parts: DA conversion and analog multiplier, both of which are implemented by AD7541 and opa27 operational amplifier. The specific circuit is shown in Figure 3.

In the circuit, a single-stage binary operation connection is used between AD7541 and op amp OPA27. The left half is the D/A conversion circuit, and the right half is the analog multiplication circuit. The D/A circuit converts the waveform data into a single-stage sinusoidal analog signal with a DC component. According to the calculation formula of this connection mode of the AD7541 chip, the signal after D/A conversion is:

Vout1=-VREF×(1-BX/212)

BX is the waveform digital signal output by the waveform memory.

In this system, VREF is set to 10V, so the sinusoidal signal obtained after D/A conversion is Vout1=10sin(wt). This signal is directly connected to the reference end of the multiplier and multiplied by the control signal AM of the microcontroller to obtain an AC sinusoidal signal with adjustable amplitude. The conversion formula is:

Vout=VREF× (1-BX/212)×(1-AM/212)

Therefore, the amplitude of the output AC signal can be adjusted by changing the value of AM, that is, Vout = AM × 10sin (wt).

5. Power amplifier system

The power amplifier system is divided into two parts: amplitude amplification and current amplification, which are implemented by operational amplifiers and class AB complementary power amplifiers respectively. The circuit connection is shown in Figure 4.

Because the output signal of the D/A circuit is a sinusoidal signal with a DC component, in order to obtain a bipolar sinusoidal signal, capacitor C1 is used at the input end of the amplifier to filter out the DC component. According to the principle of the operational amplifier:

Uoutput/Uinput=(R2+R3)/R2

Uinput is the amplitude of the sinusoidal signal with the DC component filtered. Therefore, by adjusting the resistance values ​​of R3 and R2, the signal amplitude can be amplified and the amplification factor can be changed as needed.

Class AB complementary power amplifiers are used in the current amplification part to reduce the crossover distortion of the signal and ensure the integrity of the amplified signal waveform. Darlington tubes are used as single amplification units to increase input impedance. When the power is not enough to meet the system requirements, they can be used in parallel to further increase the power amplification factor.

4. System Software Introduction

This system is used in conjunction with the supporting industrial computer, which controls the signal source system through a serial interface . The program of the industrial computer is written in Visual Basic, and the signal system program is written in assembly language using WAV6000 as the development platform.

The command sent from the industrial computer to the microcontroller is a binary number. One command consists of 3 bytes and is sent twice in succession. After the microcontroller receives and verifies the command correctly, it returns 00H, otherwise it returns FFH. If it is not received correctly, it can be resent after 0.5 seconds. The data format is shown in Table 1.

From the data format, we can see that the microcontroller determines the processing action based on the first byte of the command. The second and third bytes are specific control data. According to this format, the function of the system can be easily expanded.

V. Conclusion

The power supply voltage, frequency, phase and waveform shape of this signal source system can be adjusted by the single chip microcomputer according to the actual situation. It has the advantages of small size, light weight and easy use. During use, the system runs reliably, and the parameters of the output power signal meet the system design requirements. It has passed the technical appraisal and has been well applied in other test fields. Good scalability and cost performance increase its use space.