Design Principle and Engineering Application of Intelligent Cable Testing System

　　The intelligent cable test system uses a combination of single-chip microcomputer and industrial computer to realize the test of the conduction and insulation relationship between 1536 test points. The hardware circuit design principle and engineering application scheme based on single-chip microcomputer are described in detail. After actual testing, the cable test system meets the design requirements and greatly improves the efficiency and accuracy of continental testing.

　　This paper proposes a new scheme for aviation multi-core cable fault detection and explains the system composition and test principle.

　　1 Test system composition

　　The cable test system is mainly composed of three parts: industrial computer system, single-chip computer system and relay array. The industrial computer is responsible for human-computer interaction and data processing, the single-chip computer system controls the hardware circuit to complete the corresponding action, and the relay array is responsible for responding to the request of the decoding circuit to connect the external cable to the test system. The single-chip computer and the industrial computer communicate through USB. As shown in Figure 1.

　　Among them: the hardware circuit system uses AT80C52 microcontroller as the control core, which mainly includes conduction test circuit, insulation test circuit, decoding circuit, relay array, A/D sampling circuit, high voltage generation circuit and USB communication circuit; the microcontroller software can control the hardware equipment according to the received commands to complete the performance test of each cable.

　　The industrial computer uses Advantech's IPC-610 industrial control computer as the terminal, which is responsible for human-machine interaction and data exchange. The software of the cable test system is mainly composed of the human-machine interface and the data processing part. The human-machine interface converts the information input by the operator into corresponding commands to control the microcontroller to perform different operations; the data processing part compares and corrects the test data, establishes a database of test data and its port information, and finally generates the conduction and insulation relationship of the port of the device under test, and provides display and printing functions.

　　The relay array consists of 3168 relays to achieve the conduction/insulation test between 1536 test points and the isolation between relay groups. The 1536 test points are distributed on 12 boards, each with 128 test points. The boards are divided into 16 rows, each with 8 columns, i.e. 12×16×8=1536. Each test point is controlled by two relays, namely the input relay (Kat) and the output relay (Kab). Each board corresponds to an external 128-core aviation plug, which is responsible for the connection with the product under test. The principle is shown in Figure 2.

　　2 Principle of the test system

　　The hardware part of the cable test system is controlled by a single-chip microcomputer and is mainly divided into three parts: conduction test circuit, insulation test circuit and relay decoding circuit. The working principles of each part are described as follows.

　　2.1 Continuity test section

　　Since the on-resistance is very small, generally in the ohm level, it is easily affected by external interference. The two arms of the Wheatstone bridge respond to the slight change of the power supply at the same time and send the output signal to the differential amplifier, thereby eliminating the common-mode interference and improving the accuracy of the test. The principle is shown in Figure 3.

　　In Figure 3: R1, R2 and R3 form a reference circuit; R4, R5 and Rx are connected in series to form the main test circuit. When the resistance Rx to be tested is zero, adjust R1 to make the bridge in a balanced state, that is, U1=U2, the circuit output is approximately zero, and the reference comparison voltage U1 is generated at the same time. Under normal working conditions of the circuit, after Rx is connected in series into the circuit, the balance of the bridge is broken, U2 becomes smaller, U1 and U2 are isolated by the operational amplifier OP497 and sent to the differential amplifier INA145 for amplification, and the amplified voltage signal is sent to the 12-bit precision MAX197 for sampling.

　　2.2 Insulation test section

　　For the insulation test circuit, since the input test voltage is 500-1000 V and is not very sensitive to interference, the insulation test circuit adopts a relatively simple resistance voltage division method to achieve it. [page]

　　In Figure 4: Rx is the insulation resistance between the two wires under test; Kat and Kab are the input control relay and output control relay of Rx respectively, which are selected by the decoding circuit, and the diode D1 protects the power supply; R1, R2 and R3 form a voltage divider test circuit, R4 is a current limiting resistor, C1 is used to filter out the interference of clutter, and the voltage divider value of the test circuit is input into the amplifier circuit after passing through the operational amplifier; MAX6176 is a high-precision, low-noise reference power supply, which provides a reference comparison voltage for the amplifier circuit INA145 after passing through the voltage divider circuit and the follower, and INA145 sends the amplified signal to MAX197 for sampling.

　　2.3 Relay decoding circuit

　　The function of the relay decoding circuit is to connect two of the 1536 test points to the corresponding test circuit under the control of the single chip microcomputer. For example, the decoding circuit selects the input relay Kat of test point 1 and the output relay Kab of test point 2, and the external cable under test is connected to the corresponding test circuit through these two test points, thereby realizing the conduction or insulation test. In order to realize such a function, the decoding circuit can be divided into an address latch circuit, an input relay decoding circuit and an output relay decoding circuit. Taking the input address latch circuit as an example, its principle is shown in Figures 5 and 6.

　　The microcontroller P0 port is used as the data bus to send the address signal to the latch 74HC573. At the same time, P2.4, P2.5, P2.6, and P2.7 drive the HC138 decoder to form a latch valid signal, so that the address signal is latched in 74HC573. Since the address signal is 11 bits, the single machine needs to send the address information twice.

　　When the 11-bit address is ready, the microcontroller sends an address valid signal to the decoding circuit. The principle of the decoding circuit is shown in Figure 7.

　　The input relay decoding circuit and the output relay decoding circuit have the same circuit structure. Taking the input relay decoding circuit as an example, it can be divided into three levels of decoding circuits. Each level of decoding circuit consists of a bus isolation chip 74HC245, a 3-8 line decoder 74HC138 and other logic control circuits. The first level decoding circuit consists of AT10, AT09, AT08, and AT07 in the 11-bit address signal, which is responsible for selecting one of the 12 boards; the second level decoding circuit consists of AT06, AT05, AT04, and AT03, which is responsible for selecting a row in a board; the third level decoding circuit consists of AT06, AT05, AT04, and AT03, which is responsible for selecting a row in a board; the third level decoding circuit consists of AT06, AT05, AT04, and AT03, which is responsible for selecting a row in a board; the fourth level decoding circuit consists of AT06, AT05, AT04, and AT03, which is responsible for selecting a row in a board; the fifth ...

　　AT02, AT01, AT00 are responsible for selecting a column in a single board. When the rows and columns are crossed, the input relay driving circuit of a test point is selected, thereby connecting the test point to the test circuit. The address signal is isolated by 74HC245 between the single boards to prevent the driving ability from decreasing. The selection process of the output relay is exactly the same and will not be repeated.

　　3. Research on engineering application solutions

　　3.1 Cable test system workflow

　　The cable test system is realized by the joint action of the single-chip microcomputer and the industrial computer. The human-computer interaction interface of the industrial computer sends the information input by the operator to the single-chip microcomputer through USB. The single-chip microcomputer starts the corresponding peripherals according to these commands to realize the corresponding test functions. The cable test system can realize system self-check, all conduction/insulation tests, conduction/insulation tests between two points, and conduction/insulation tests between two points in a specified range. The work flow of the single-chip microcomputer is shown in Figure 8. [page]

　　The following uses the on-resistance test as an example to illustrate the workflow of the cable test system: When the product under test is connected to the cable test system through the adapter cable, the operator selects the model of the product under test by setting parameters. When the single-chip microcomputer receives the command for all the on-resistance tests, it first closes the input relay of test point 1, then the output relay of the second test point, the output relay of the third test point to the output relay of the mth test point (m∈[2,1 536]); after closing the input relay of test point n (n∈[2,1 535]), the output relay of test point m is closed in turn (m∈[n+1,1 536]). Each time the output relay is closed, the output voltage value is sampled and uploaded to the industrial computer. The data processing part of the industrial computer generates a database with this voltage value and the corresponding address information. At the end of the test, the industrial computer compares the generated database with the standard database to determine which channels in the product under test are on. The workflow of the on-resistance test part is shown in Figure 9.

　　The workflow of the insulation test is similar to that of the continuity test, but it should be noted that when executing the insulation test command, the system will first perform a continuity test on the points within the test interval and record the channel numbers that are turned on. In this way, these channels will be automatically skipped when the industrial computer sends address instructions to the microcontroller, thereby ensuring the safety of the operator and the product under test and preventing catastrophic consequences that may be caused by high-voltage short circuits.

　　It should be noted that the insulation test voltage is realized by a switching power supply using the SG3524 chip. The 15V DC voltage is used as the input voltage. The PWM pulse generated by the SG3524 is driven by the MOSFET to drive the step-up transformer. After voltage doubling, rectification and filtering, a stable high-voltage output is obtained. The high-voltage output is fed back to the comparison input of the SG3524, and the duty cycle of the PWM waveform is controlled by the digital potentiometer, so that the output voltage is constant.

　　3.2 Cable test system self-test process

　　In order to ensure the normal operation of the equipment itself, the cable test system provides a self-test function. The self-test process is divided into two parts. The first part is to self-test the USB channel. The USB self-test process is shown in Figure 10. After the system is powered on, the industrial computer sends a check command, and the microcontroller determines whether the check code is correct. If the check code is correct, the reply code is returned according to the protocol. If the industrial computer determines that the reply code is correct, the USB communication is established normally. The second part is to self-test the relay array, and its process is similar to the conduction test. Taking test point 1 and test point 2 as examples, first, close the input relay of test point 1 and sample the test voltage. If the system determines that it is disconnected, it can be said that the output relay of test point 1 is not normally closed, otherwise it is a normally closed fault. Replace the input relay of test point 1 with the output relay. The same method can be used to determine whether the input relay of test point 1 is a normally closed fault. Secondly, open the input and output relays of test point 1 respectively. If the system determines that it is conductive, it can be said that both relays are normal based on the first step; if the system determines that it is disconnected, at least one of the two relays is a normally open fault. Finally, connect the self-test plug to the external port, select the input relay of test point 1 and the output relay of test point 2 respectively. If the system judges that it is turned on, it means that the internal relay and the lead from the board to the port are normal. Otherwise, it means that there is a problem with the lead from the relay to the test end.

　　4 Conclusion

　　The cable test system realizes the organic combination of the embedded sub-test system and the main control computer, and has good scalability and versatility. After actual testing, it can realize the test of the internal cable connection relationship of various products, greatly improving the test efficiency and accuracy.