Brief Analysis of the Distributor of Aviation Cable Test System Based on CAN Bus

　　Aviation cables are the nervous system of aircraft, connecting the aircraft electrical, avionics, fire control, control and other systems, providing power supply, control signals and data information for various aircraft components. Limited by the space of the aircraft fuselage, the cable system wiring is generally concentrated in the narrow walls of the aircraft, and the aircraft walls are almost full of wires. Therefore, the aviation cable system requires high reliability, high integration and high lightness; due to the wide variety of wires, various power lines, high and low frequency signal lines, and data lines are mixed together, with a length of up to hundreds of kilometers. The electrical environment is very complex, which increases its potential faults. Many air crashes and aircraft failures are directly or indirectly related to cable system failures. Therefore, the "health" of aviation cables is an important guarantee for the safe operation of aircraft, and cable safety issues are particularly important. However, the research and development of aviation cable testing technology for the entire aircraft in China is relatively lacking, far behind the current situation of conventional application abroad.

　　According to the test requirements of long cable distribution distance (nearly 100 meters) and many points (more than 30,000 points) of the whole aircraft, this paper proposes a distributed flexible cable test system based on CAN bus. The system has the advantages of scalability of test points, test flexibility based on intelligent cable identification, and multi-point excitation of distributed test terminals, which can meet the testing needs of assembly and maintenance departments of large passenger and large-scale aircraft.

　　1 Test system structure based on CAN bus

　　CAN bus is a serial communication bus that effectively supports distributed control. It has a simple structure, high reliability and data transmission rate. The number of nodes in the network is theoretically unlimited, and free communication can be achieved between nodes.

　　Figure 1 shows a distributed cable test system based on CAN bus, which consists of a host, a CAN bus adapter card, a CAN bus and a distribution machine.

　　The host generates a test program based on the cable connection information and connector information in the database, publishes information and monitors the working conditions of each distribution machine, and provides a human-computer interaction interface through the test software. The schematic diagram of the host structure is shown in Figure 2. The host communicates with the CAN bus through the CAN bus adapter card. The CAN bus adapter card uses the RS 232-CAN interface and consists of a MAX232 level conversion chip, an AT 89S52 microcontroller, a CAN bus control chip SJA1000, and a transceiver TJA1050. SJA1000 is an independent controller used in controller area networks in automobiles and general industrial environments. TJA1050 is the interface between the CAN protocol controller and the physical bus. It is a standard high-speed CAN transceiver that can provide differential transmission function for the bus. The basic working principle of the bus adapter card is: the host sends data to the microcontroller through RS 232, and the microcontroller forwards it to the CAN bus through the CAN bus controller SJA1000 and the CAN transmitter TJA1050 according to the specific CAN application protocol. The distribution machine realizes communication with the host and between distribution machines through the CAN bus, and completes functions such as cable continuity, resistance, insulation withstand voltage and capacitance testing and fault location. Under the unified deployment of the host, it can perform automatic cable identification and non-redundant multi-point excitation testing, thereby improving the test speed.

　　 

　　2 Hardware Structure of Distributed Machine

　　The system is based on modular design. Each distribution machine has the same transfer connector and test function. Since the system is designed with automatic cable feature recognition circuit and establishes corresponding data information, it can realize the blind plug function of the transfer cable, so it can meet the interchangeability of distribution machines for large-distance distribution measurement points. The hardware structure of the distribution machine is shown in Figure 3.

　　It consists of an ARM microcontroller module, a CPLD module, a relay matrix switch module and a test module. Each distribution machine has a test point capacity of 3,000 points, and the test point selection circuit is composed of a large-scale relay matrix switch controlled by a complex programmable device (CPLD), which can realize cable detection between multiple channels at the same time.

　　2.1 ARM Microcontroller

　　The distribution machine uses LPC2290ARM chip as the main controller. LPC2290 has rich on-chip resources, which can meet the control requirements of the system for the test circuit, reduce the complexity of the system hardware design, support JTAG real-time simulation, and facilitate development and debugging. At the same time, the LPC2290 internally integrates 2-way CAN controllers, which can be used as nodes of the CAN bus, eliminating the design of the peripheral circuits of the CAN controller and reducing interference. Its built-in CAN controller complies with CAN 2.0B and ISO11898-1 bus specifications. The data baud rate of the bus can reach 1 Mb/s, and 32-bit registers and RAM can be accessed. The global acceptance filter can recognize the 11-bit and 29-bit R identifiers of almost all buses. The acceptance filter provides FullCAN-style automatic reception for the selectable standard identifiers. The drive circuit of the CAN controller still selects the transceiver TJA1050, and adds an isolation circuit consisting of a DC-DC power isolation module and 2-way high-speed optocouplers 6N137 between the ARM and the transceiver to ensure that the controller can work normally when the CAN bus is severely interfered.

　　2.2 Test circuit module

　　The test circuit module is composed of a high-precision multimeter board to complete the functions of related cable tests such as continuity test, insulation test, capacitance test and fault location. The main controller LPC2290 of the distribution machine loads the test circuit to the required excitation cable test port through the control switch switching system according to the test command, and collects the information of the response port.

　　2.3 Address Strobe Control CPLD Module Design

　　Programmable logic devices have been widely used, bringing great flexibility to the design of digital systems. The hardware structure and working mode can be reconstructed through software programming, making hardware design as convenient and fast as software design. CPLD has a large number of gate circuits inside, which is suitable for realizing complex combinational logic.

　　2.3.1 CPLD Function Implementation

　　The distribution machine uses CPLD to realize the I/O port expansion of the main controller LPC2290. The main controller only needs to send the address of the cable to be tested to CPID through the serial interface SPI, and the CPLD controls the relay matrix switch to select it. The CPLD adopts the EPM570ZM256C6 of the MAXⅡ series of Altera, which has 160 general I/O ports. The development tool uses the comprehensive PLD development software QuartusⅡ launched by Altera. According to the measurement point capacity requirements of the distribution machine, the CPLD is designed into a 150-bit serial input and parallel output shift register and a 150-bit output latch. The control of the register and the output latch are independent of each other. The integrated functional module is shown in Figure 4. Among them, CLK is the clock input of the shift register, and the data is read in at the rising edge; SI is the serial input port; SO is the serial output, which is used for cascading; LAT is the output latch control signal, and the data is output at its rising edge; EN is the enable terminal, a high level enables the output, and a low level makes the output high impedance. Cascading 20 CPLDs can enable the distribution machine to achieve a test capacity of 3,000 points.

　　　2.3.2 LPC2290 Control of CPLD

　　LPC2290 controls CPLD via SPI interface, and the connection schematic is shown in Figure 5.

　　The flow chart of the SPI bus operation in this system is shown in Figure 6. The setting of the SPI interface data transmission format of LPC2290 must be consistent with the CPLD data transmission format, that is, SPCR = 0x30. The SPI interface sends one byte of data each time. In this system, 375 cycles are required to achieve 3,000 bits of data serial input and parallel output.

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　　The key program codes are as follows:

　　2.4 Relay Matrix Switch Design

　　The main function of the matrix switch is to select the conduction of the test points and switch the test circuit to the corresponding channel. Corresponding to the CPLD, the matrix switch is based on 150 points.

　　As the actuator of the measuring point on and off, the reliability and reaction speed of the relay directly affect the performance of the whole system. The 12 V relay EC2-12 with high reliability, high signal isolation and stable performance is selected as the switching control relay. EC2-12 is a single coil self-locking relay. When the +12 V excitation is input, the relay is closed and the state will remain until the -12 V excitation arrives. The relay will open. L298N is selected to form a relay drive circuit, which can convert the TTL logic level signal into the ±12 V voltage required by this system to realize the setting and resetting of the relay. The two normally open points of each EC2-12 (K1 and K2 are the normally open points of relay 1 and relay 2) are used to control two (for example, 0 and 1) measuring points. Each pair of relays controls the positions of two measuring points (relay 1 controls the output end, and relay 2 controls the input end). The schematic diagram is shown in Figure 7. The test circuit is connected to the input and output ends through the switching switch to realize the test of the cable to be tested.

　　3 Conclusion

　　Based on the CAN bus working mode, the system adopts modular design for the hardware of the distribution machine; for the control mode of large-scale matrix switch modules, a design scheme based on CPLD is proposed, and the hardware implementation method and part of the communication program flow are given. The system can expand the test capacity according to the object to be tested, is easy to use, can realize multi-point excitation under the unified deployment of the host, improve the test speed and test flexibility, and can be applied to large equipment occasions with complex cable networks such as aviation cables.