“Before the plane takes off, the ground power supply usually supplies power to the onboard equipment of the aircraft, and after takeoff, the aircraft engine drives the generator to generate electricity to supply power to the onboard equipment. The quality of the power supply grid is directly related to the life of the airborne equipment and flight safety, and the airborne electrical box mainly monitors the voltage, frequency, differential current, phase sequence and other parameters of the aircraft generator network or ground power network in real time.
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1 Introduction
Before the plane takes off, the ground power supply usually supplies power to the onboard equipment of the aircraft, and after takeoff, the aircraft engine drives the generator to generate electricity to supply power to the onboard equipment. The quality of the power supply grid is directly related to the life of the airborne equipment and flight safety, and the airborne electrical box mainly monitors the voltage, frequency, differential current, phase sequence and other parameters of the aircraft generator network or ground power network in real time. Whether it is the ground power supply before takeoff or the generator power grid after takeoff, once there is overvoltage, overvoltage, undervoltage, underfrequency, under-under-frequency, over-frequency, over-over-frequency, reverse sequence and differential fault, it must be delayed. The generator or ground power supply is automatically disconnected within a certain time, and the backup power supply is used to supply power to the aircraft, so as to effectively protect the airborne equipment and flight safety. This paper proposes an airborne electrical box tester designed with AT89S52 microcontroller. The tester can test the protection function of the airborne electrical box and measure the delay protection time.
2 System composition and working principle
The hardware design of the airborne electrical box tester is composed of AT89S52 single-chip microcomputer, Display module, system reset module, clock oscillation module, power supply module, timing module, test selection module and interface circuit (Figure 1). The voltage and frequency of the program-controlled power supply , phase and phase sequence can be adjusted to simulate the power failure of the ground power supply or the generator grid. The working principle of the system: when the power supply grid fails, the interface circuit sends a control signal to introduce the faulty power supply into the electrical box, and the interface circuit sends a signal to the single-chip microcomputer. When the single-chip microcomputer receives the signal, the control timing module starts timing and displays the The module displays specific fault items such as overvoltage, overvoltage, undervoltage, underfrequency, underfrequency, overfrequency, overfrequency, reverse sequence and differential; after a period of time, the electrical box sends a control signal to cut off the power grid The power supply of the airborne equipment, while the control signal enters the interface circuit, the interface circuit controls the timer to stop timing; read the time of the timing module to determine whether the time is within the allowable range of the system, if it is not within the allowable range or the onboard electrical box does not send out If the timing stop signal (the control signal that cuts off the power supply from the power grid to the airborne equipment), the fault monitoring and protection function of the electrical box cannot be realized, indicating that the electrical box is faulty.
3. 1 AT89S52 microcontroller
AT89S52 microcontroller is a low-power, high-performance CMOS 8-bit microcontroller with 8KB system programmable flash flash memory, 256 bytes of internal RAM, 32 I/O port lines, watchdog timer (WDT) ), 2 data pointers, 3 16-bit timings, counters, a 6-vector 2-level interrupt structure, a full-duplex serial communication port, an on-chip crystal oscillator and a clock circuit. The powerful functions of AT89S52 can meet the design requirements of this tester.
3.2 System reset, clock oscillation module
The system reset module consists of S1, R1, R2 and C3, of which S1 is the reset button, C3 is an electrolytic capacitor, and R1 and R2 are precision resistors; and the clock oscillation module is composed of crystal oscillators Y1, C1 and C2.
3.3 Display module
The 9 LEDs in Figure 2 are used to indicate that the electrical box is testing for a specific fault. 74LS154 is a 4-16 decoder, the strobe pins G1, G2 are grounded, A, B, C, D are connected to the microcontroller P1.0, P1.1, P1.2 and P1.3 in turn, through the P1 port of the AT89S52 Write the corresponding data to determine the specific channel of the decoder strobe. The output of the decoder drives the Transistor in reverse after two TTL7404 NOT gates, and lights the corresponding LED. When writing 0 to 8 to the lower 4 bits of port P1 in sequence, the output of the decoder will drive the triode after reversed by the NOT gate to light up the undervoltage indicator light, overvoltage indicator light, overvoltage indicator light, and underfrequency indicator light respectively. Indicator light, underfrequency indicator light, overclocking indicator light, overclocking indicator light, reverse sequence indicator light and differential indicator light. When the input of the P1 port is high, all the indicators are off.
3.4 Timing Module
The timing module is composed of a timing control circuit and a timer. Among them, the timing control circuit is composed of 2 NPN transistors and a current limiting resistor; the timer is a 6-digit Display timer, and the timing can be accurate to the millisecond level. Pin 2 is the timing control end of the timer. When the pin is When it is low, the timer starts counting; when it is high, the timer stops counting. Therefore, when the power grid fails, the interface circuit sends a control signal to the AT89S52, and the single-chip microcomputer sets the P0.3 port to a low level, and the timer starts timing. When AT89S52 receives the timing stop signal sent by the electrical box and converted by the interface circuit, the single-chip microcomputer sets the P0.3 port to a high level, and the timer stops timing. At this time, it can be judged whether the delay protection time is within the allowable range according to the timer reading. The button switch S2 is the reset button of the timer. Press S2 to reset the timer.
3.5 Test Selection Module
3.6 Interface circuit
The interface circuit is shown in Figure 3. When the ground power supply fails, the timing stop signal 1 is grounded (0V) through the electrical box. At this time, the P0.1 port is at a low level, and the timer counts; when the electrical box cuts off the ground power supply to the machine When the power supply of the load device is connected, the timing stop signal 1 is connected to the power supply (24V) through the electrical box. At this time, the P0.1 port is at a high level, and the timer stops timing. When the power grid of the aircraft generator fails, the timing stop signal 2 is connected to the power supply (24V) through the electrical box. At this time, the P0.2 port is at a low level, and the timer counts; when the electrical box cuts off the power supply of the generator grid to the airborne equipment , the timing stop signal 2 is connected to the power supply (0V) through the electrical box, and the P0.2 port is at a high level at this time. The timer stops.
Taking the generator grid overvoltage fault as an example, combined with Figure 2 and Figure 3, when K8 is closed, the triode is turned on, and the electrical box pins are grounded through resistors, diodes and triodes. Power supply simulation), P0.4 port becomes low level, the microcontroller controls P0.3 to low level, the timer starts timing, and at the same time writes data to the P1 port, the overvoltage indicator lights up; the microcontroller keeps checking P0.2 After a certain delay, the electrical box cuts off the power supply of the generator grid to the airborne equipment. The timing stop signal 2 makes the P0.2 port become a high level. Once the microcontroller detects that the P0.2 port becomes a high level, it will write a high level to the P0.3 port, the timer will stop timing, and the timer will be read out. data to determine whether the time is within the allowable range. At this time, the single-chip microcomputer keeps checking the P0.4 port. Once K8 is disconnected, the P0.4 port becomes a high level, the single-chip computer writes data to the P1 port, and the overvoltage indicator light goes out. Complete the test of the electrical box for the overvoltage fault protection function of the generator grid.
Each interface module is connected with the port of AT89S52 to complete different test functions. Among them, connecting with P0.4 can be used to test the overvoltage fault protection function of the electrical box on the generator grid, connecting with P0.5 to test the undervoltage fault protection function of the electrical box on the generator grid, connecting with P0.6 for testing The electrical box is used to protect the generator grid over and over voltage fault. It is connected to P0.7 to test the electrical box to the generator grid under-frequency fault protection function, and it is connected to P2.7 to test the electrical box to the generator grid under-frequency. Fault protection function, connect with P2.6 to test the differential fault protection function of the electrical box to the generator grid; connect with P2.5 to test the undervoltage fault protection function of the electrical box to the ground power supply, connect with P2.4 to use Test the overvoltage fault protection function of the electrical box to the ground power supply, connect with P2.3 to test the electrical box to the ground power supply overvoltage fault protection function, connect with P2.2 to test the electrical box to the ground power supply underfrequency fault protection function , connect with P2.1 to test the overclocking fault protection function of the electrical box to the ground power supply, connect with P2.0 to test the overclocking fault protection function of the electrical box to the ground power supply, connect with P3.1 to test the electrical box to the ground Power reverse sequence fault protection function.
4 System software design
After power-on reset, the single-chip microcomputer first queries the P0.0 port. When P0.0 is high, it indicates that the electrical box is testing the generator grid fault protection function. When P0.0 is low, it indicates that the electrical box is being tested. Ground power failure protection. Because the I/O ports of the single-chip microcomputer are all high level after reset, the tester automatically enters the electrical box to test the fault protection function of the generator grid, and the single-chip microcomputer loops to query P0.0, P0.4, P0.5, P0 .6, P0.7, P2.7 and P2.6 When any one of P0.4, P0.5, P0.6, P0.7, P2.7 and P2.6 is detected to be low level, enter the corresponding fault Protection function test; when it is detected that the P0.0 port is low, the tester enters the electrical box to test the ground power failure protection function, and the single-chip microcomputer loops to query P0.0, P2.5, P2.4, P2.3, P2.2, P2.1, P2.0 and P3.1 When any one of P2.5, P2.4, P2.3, P2.2, P2.1, P2.0 and P3.1 is detected as When it is low, it will enter the corresponding fault protection function test.
Once the fault protection function test is entered, first determine that it is indeed a power grid fault, then light the corresponding indicator, and control the timer to start timing; when the timing stop signal arrives, control the timer to stop timing, the timer reading is the delay time, wait for After the fault disappears, the indicator light goes out, and the test exits; if the timing stop signal does not arrive or the delay time exceeds the allowable time, the protection function cannot be realized, disconnect the fault grid, the fault disappears, the indicator light goes out, and the test exits . Figure 4 shows the program flow of the electrical box tester.
5 Conclusion
Electromagnetic interference is the main problem encountered in the development process of the tester. The tester uses a metal box package to cut off the crosstalk caused by the electromagnetic interference propagation path, circuit board wiring and component layout, and uses data redundancy protection and error correction in the software. Solve the problem of interference, thereby greatly improving the anti-interference of the tester. The practical application shows that the tester has the advantages of accurate test, advanced performance, reliable operation, convenient operation and low cost, and plays an important role in daily maintenance and fault diagnosis.