Development of Multi-core Cable Tester

1 Introduction

A wide variety of communication cables and control cables are widely used in various instruments and control equipment. Whether the cable is well-conducted and whether the insulation resistance between wires meets the requirements directly affects the normal operation of electrical equipment. Withstand voltage insulation resistance is an important indicator for measuring the performance of electrical insulation materials. The main disadvantages of the traditional method of measuring insulation resistance with an ohmmeter are: large measurement error, and the accuracy of the high-voltage power supply used for withstand voltage test cannot be guaranteed; the measurement results cannot be automatically saved and printed out; in the measurement of insulation resistance between core wires of multi-core cables , wiring replacement is cumbersome, easy to connect wrongly, and the manual labor is large. The multi-core cable tester introduced in this article can realize the automatic measurement of the continuity and withstand voltage insulation resistance of cables with a maximum of 48 cores, which not only ensures the accuracy of the test results, but also can output the test results from the LCD display and printer. It avoids the error of manual measurement and greatly improves work efficiency.

2 System Introduction

2.1 System functions

The system has relatively complete functions, the specific functions are as follows:

(1) Function of detecting the continuity and insulation resistance of the cable; (2) Function of self-diagnosis at startup; (3) Function of real-time display of test results; (4) Function of automatic switching of cable core wires. After the detection is started, it automatically switches to detect the continuity of each core wire of the cable and the insulation resistance between each core wire and automatically judges whether the insulation resistance value meets the requirements; (5) Function of setting the number of core wires of the cable to be tested; (6) Function of setting the insulation resistance over-limit value, which can be used to judge whether the insulation resistance values ​​of different levels are qualified or not.

2.2 System composition

The working principle of the multi-core cable tester is shown in Figure 1. The system mainly consists of the following three parts: input circuit: including keyboard circuit, insulation test circuit, A/D conversion circuit and continuity test circuit; output circuit: composed of CPLD system circuit, LCD liquid crystal display module, printer, relay group and 500V DC high voltage circuit; control circuit: composed of 8051 single-chip system circuit.

This system uses a 500V 直流电\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> DC power source generated by a high-voltage circuit as the test voltage source for insulation resistance. The measurement of insulation resistance adopts the balanced voltage method stipulated by the state. During the test, the core value of the cable to be tested is first set through the keyboard. The single-chip microcomputer controls the closing and opening of the relay group through the CPLD system, and then the A/D conversion or on-off test circuit converts the signal into a digital signal. The single-chip microcomputer collects and calculates the signal, and the results of the calculation processing are displayed, output and saved through the LCD screen, printer, and data storage.

3 System Circuit Design

3.1 Cable insulation resistance detection design

The principle of insulation resistance detection is shown in Figure 2. The test circuit consists of three parts: sampling, 运放\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> operational amplifier and AD7705. The core device of insulation resistance test is AD7705, which is a 16-bit Σ-△A/D converter launched by AD company , which can be used to measure low-frequency analog signals. AD7705 has a gain programmable amplifier , which can directly measure various tiny signals output by sensors through software programming . AD7705 has the characteristics of high resolution, wide dynamic range and self-calibration, so it is very suitable for high-precision detection and measurement. AD7705 has two fully differential input channels, and its main features are as follows: 16 bits without missing code; nonlinearity is 0.0003%; self-calibration and system calibration can be performed; with a three-wire SPI serial interface; low power consumption, etc.

The specific test process of insulation resistance is as follows: In Figure 2, Rx is the insulation resistance of the cable under test. During the test, 500V DC is first applied, and then relays S1 and S2 are closed. 500V DC is divided by R2 and R3 and sent to the op amp. After being conditioned by the op amp, it is sent to the AD ref terminal of AD7705 as the test reference voltage of AD7705 to eliminate the influence of 500V DC power supply fluctuation on the test results. The voltage after Rx and R1 is divided as the insulation test sampling voltage and sent to the op amp for conditioning and then to the AD in terminal of AD7705. The voltage at the AD in terminal is the actual sampled voltage, which ranges from 0 to 2.5V. The smaller the insulation resistance value, the higher the corresponding sampling voltage.

3.2 Cable continuity detection design

The principle of cable continuity detection is shown in Figure 3. Before starting the continuity detection, all the core wires at one end of the cable should be short-circuited with a short-circuit ring, and the other end should be connected to the relay array. The relay array is used to switch the core wires of the cable. All core wires are grounded through the normally closed contacts of the corresponding relays via resistors R1 and R2. When detecting the 1# core wire, close the S1 relay to close the normally open contact of the relay S1, and +15V is added to the resistors R1 and R2 through the core wire, and then sent to the microcontroller after the voltage is divided by R1 and R2. If the measured core wire is broken, Vo is 0V, otherwise Vo is +5V.

3.3 CPLD system circuit

The maximum number of detection cable cores in the test system is 48. The 51 single-chip microcomputer has 24 IO ports. If the single-chip microcomputer is used directly to control the relay group, the single-chip microcomputer must use an expansion chip to expand the IO port to meet the system requirements, such as 8255, which will increase the complexity of the system. Therefore, it is decided to use CPLD to control the relay group. CPLD (Complex Programmable Logic Device) is a complex user-programmable logic device. CPLD is a standard large-scale integrated circuit product that can be used in the design of various digital logic systems. [page]

In recent years, due to the use of advanced integration technology and mass production, the cost of CPLD devices has continued to decline, and the integration density, speed and performance have been greatly improved. One chip can realize a complex digital circuit system. Coupled with easy-to-use development tools, the use of CPLD devices can greatly shorten the product development cycle and bring great convenience to design modifications. This article uses ALTERA's MAX7000s, which is a high-precision, high-performance, in-system programmable CPLD chip based on the second-generation MAX structure. It is processed using advanced CMOS technology and contains an electrically erasable read-only memory EPROM. It can provide 600~5000 available selection pins, ISP, a delay of only 5ns, and a high-speed counter with a frequency of up to 175.4MHz.

After the relay on/off program is compiled, it is burned into the CPLD through a dedicated download line. The CPLD and the microcontroller are connected in an analog serial manner, which greatly simplifies the system circuit. After the system uses CPLD for IO port expansion, the microcontroller only needs to send the core wire number of the cable to be tested to the CPLD, and the CPLD completes the selection of the relay group. This greatly facilitates programming, making the main program structure compact and flexible to control.

3.4 Other circuits

In addition to the above circuits, the system also has circuits such as power supply, keyboard and system reset.

In addition to providing power to the DC high-voltage circuit, the power supply circuit also provides power to the microcontroller, LCD display module, relay group, detection circuit, etc.

The working voltage of the keyboard circuit is +5V. There are 7 buttons in total. Through these 7 buttons, the system parameter settings can be completed, such as the number of cores of the cable to be tested, the setting of the insulation resistance limit value, and the viewing of test results.

4 System Programming

The software part of this system is written in assembly language. The executable code generated by assembly language is fast and compact, and its running efficiency is better than that of the code obtained by C language program. The main program flow of the system is shown in Figure 4. The system software mainly includes system self-check program, cable parameter setting program, program for viewing the last test results, cable continuity test program, continuity test result display program, cable insulation test program, insulation test result display program, etc.

The self-check program is initialized when the system is powered on, and performs a self-check on the instrument to ensure the correctness of the instrument's working status.

The parameter setting program is used to set the cable insulation resistance over-limit value, ranging from 1 to 20M, as well as the total number of cable cores and the number of cable branch cores, ranging from 2 to 48 cores.

The program for viewing test results is used to view the last cable continuity and insulation resistance test results. The continuity test and insulation test programs are used to perform continuity test and insulation test on the cable respectively. The test results are displayed by the continuity test result display program and the insulation test result display program respectively.

5. Test Results Analysis

First, the accuracy of the system is tested. 1MΩ, 2MΩ, 5MΩ, 10MΩ, and 20MΩ high-precision resistors are selected for testing. The unit of the results is MΩ. The measured data is shown in Table 1:

It can be seen from Table 1 that the relative errors of the test results are all within 3.5%, achieving a high accuracy and meeting the design requirements.

In addition, the test speed of the tester was tested. Taking a 48-core cable as an example, it takes 30 minutes to test the insulation resistance between the core wires of the cable plug using a megohmmeter, and it takes 1 minute and 20 seconds to use this tester; the time for the continuity test is within 30 seconds. Using this system, a continuity and insulation resistance test can be completed within 2 minutes. Therefore, the test speed of this instrument is relatively high.

6 Conclusion

The author's innovation: The tester uses an organic combination of 51 single-chip microcomputer and CPLD chip to design a test system with fast test speed, flexible control, adaptability to continuity test and insulation test requirements of cables with different core numbers, and a relatively high degree of intelligence and automation.

The product has strong environmental adaptability due to anti-interference measures. The instrument can be used for various cable tests in industries such as power, communications, railways and national defense.

References

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