Abstract: Contact resistance is one of the standards used to indicate the quality of contact points between components, so the measurement of contact resistance is essential. This article describes the development of a contact resistance tester, detailing the composition of each part of the system and the functions performed by each part.
Keywords: contact resistance, four-point method, analog-to-digital conversion, serial communication
1. Introduction
In electronic, communication or power systems, reliable electronic connections are needed not only between components, circuits, and devices, but also within components [1]. However, due to the different atmospheric environments at the conductor interfaces, they are often covered with oxide layers or electrochemical corrosion layers of different gases, which complicates the contact state [2], increases the contact resistance between the contact surfaces, and affects the contact performance. Therefore, in order to study the reliability of the contacts, the measurement of contact resistance is essential. The hardware circuit of the micro-resistance tester mainly uses chips such as LM1458 and ICL7135 to complete the measurement of contact resistance. The accuracy of the resistance tester is four and a half digits, the range is 2V/2Ω, and the accuracy is 0.1mV/0.1mΩ. LM1458 is a dual operational amplifier. Its function is to amplify the actual measured voltage value and output it to the ICL7135 chip. The ICL7135 chip is used to complete the A/D conversion, converting the transmitted data from analog to digital, and dynamically displaying the measured resistance value by a five-digit LED display.
2. Research on Contact Resistance Testing System
2.1 Measurement Principle
The method for measuring the contact resistance of precious metal electrical contact materials refers to the national standard GB/T 15078-1994 Method for measuring the contact resistance of precious metal electrical contact materials [3]. The four-point method is used. Since the four-point method uses a constant current source of 20mA/100mA to measure the contact resistance, the essence of measuring the contact resistance is to measure the micro-motion contact voltage. The measurement principle of the four-point method for contact resistance is shown in Figure 1.
Figure 1. Schematic diagram of the four-point method for measuring contact resistance.
The resistance measured in Figure 1 is the resistance when the contactor and the sample are in contact. The constant current source provides current IC to the contact area, and the voltmeter measures the voltage VC between P+ and P-. Since the internal resistance of the voltmeter is relatively large compared to the contact resistance being measured (a contact resistance greater than 100mΩ is generally considered high resistance), the current diverted to the voltmeter is negligible. Therefore, the voltage VC measured by the voltmeter can be considered the voltage value between P+ and P-, and the ratio of voltage VC to current IC is the resistance value. However, because the contact area between the contactor and the sample is very small, the wiring shown in the figure yields the resistance value between P+ and P-. To make the measured data as close as possible to the true contact resistance value, the P+ terminal should be placed as close as possible to the contact area between the contactor and the sample.
2.2 Hardware Structure
2.2.1 Amplifier Circuit:
The four-point method is used to measure contact resistance, but in reality, the resistance value is the ratio of the measured voltage to the current. Therefore, if you directly read the voltmeter reading, you still need to calculate the resistance value separately, which is quite cumbersome. Since the current is constant, you can find the relationship between voltage and resistance so that the measured voltage reading is the resistance value, reducing the calculation trouble. When the constant current source is 20mA, amplifying the output voltage by 50 times will display the resistance value, in mΩ; if the constant current source is 100mA, amplifying the output voltage by 10 times will also display the resistance value, in mΩ.
The circuit diagram of the amplifier section is shown in Figure 2.
Figure 2. Amplification circuit of the contact resistance measurement system
The circuit diagram contains two LM1458 chips, with P+ and P- connected to the two ends of the resistor being measured (as shown in Figure 1). The output voltage after passing through the amplifier circuit in Figure 2 is: Since R1 and R2 are fixed values (R1 = 2kΩ, R2 = 1kΩ), to amplify the output voltage by 50 times or 10 times, the value of the adjustable resistor R3 must be changed to control the amplification factor. Calculations show that when R3 ≈ 85Ω, the output voltage is amplified by 50 times, and when R3 ≈ 470Ω, the output voltage is amplified by 10 times.
2.2.2 Analog-to-digital conversion and digital tube driver circuit
The analog-to-digital conversion (A/D) section is the core component. This system uses the ICL7135 chip for A/D conversion, displaying the measured voltage value in digital form after the A/D conversion. The ICL7135 chip requires a 120kHz clock input. We selected a 3.840MHz crystal oscillator, and the 3.840MHz frequency of the crystal oscillator was divided by 32 using an HCF4060BE chip to obtain a 120kHz frequency.
The wiring diagram for the ICL7135 chip to perform A/D conversion is shown in Figure 3.
Figure 3 Wiring diagram of ICL7135 chip A/D conversion [5]
The digital tube uses dynamic display, which can significantly reduce hardware costs and power consumption. The ICL7135 chip has five digit selection signal outputs (D1 (LSD) to D5 (MSD)) and four BCD code outputs (B1, B2, B4, B8). The five digit selection signal outputs are connected to the driver (MC1413) to drive the five digital tubes to select a specific digit. The four BCD code outputs are connected to the BCD-7-segment decimal latch (MC14513) to make the digital tube display the data of the selected digit.
Meanwhile, the ICL7135 chip's strobe signal, busy signal, and five digital drive signals D1 (LSD) to D5 (MSD), as well as four BCD codes B1, B2, B4, and B8, can be sent to the serial interface to connect to a computer for real-time data storage and processing.
This millivolt-milliohm meter has a precision of four and a half digits, therefore it has five readings, accurate to one decimal place. A parallel LED digital tube dynamic scanning display circuit (common cathode) is used to drive the digital tube.
2.2.3 Rectifier and Voltage Regulator Section
The rectification and voltage regulation section provides +5V and -5V reference voltages for the A/D converter chip ICL7135. The schematic diagram is shown in Figure 4.
Figure 4 Schematic diagram of rectifier and voltage regulator
The 220V AC power is adjusted to 10V AC power through a transformer, and then rectified by a bridge rectifier to supply a voltage regulator. The +10V power is supplied to the MC7805, which outputs +5V after level conversion; the -10V power is supplied to the MC7905, which outputs -5V after level conversion.
3. Hardware system and PC interface
This article uses the RS232 protocol and implements serial communication through the uP51C microcontroller learning board. The core microcontroller is the AT89S51.
Data transmission between computers or between a computer and a terminal can be achieved using either serial or parallel communication. Serial communication is widely used due to its advantages of requiring fewer lines, lower cost, and, especially, avoiding inconsistencies in the characteristics of multiple lines, particularly in long-distance transmission. Serial communication requires both communicating parties to use a standard interface, allowing different devices to be easily connected and communicate.
The RS-232-C interface (also known as EIA RS-232-C) is currently the most commonly used serial communication interface.
The AT89S51 is a low-power, high-performance CMOS 8-bit microcontroller with 4k Bytes of ISP (In-system programmable) Flash read-only memory that can be repeatedly erased and rewritten 1000 times. The device is manufactured using ATMEL's high-density, non-volatile memory technology and is compatible with the standard MCS-51 instruction set and 80C51 pin structure. The chip integrates a general-purpose 8-bit central processing unit and an ISP Flash memory unit [6].
3.1 Introduction to uP51C Microcontroller
The main features of the uP51C microcontroller include: RS232 serial interface; programming of AT89S5x microcontrollers using a download cable; USB power supply; multi-functional expansion interfaces with all resources brought out; all resources can be selected using jumpers for convenience and clarity; sufficient space is provided for the main chip mounting, allowing for the installation of either a chip socket or a ZIF socket (which, when used with a download cable, allows the board to be used as a programmer).
The pinout diagram of the AT89S51 core component of the uP51C microcontroller is shown in Figure 5.
Figure 5 AT89S51 pin diagram
The ICL7135's STROBE pin is connected to port P3.2, triggered by a low level. The BUSY pin is connected to port P2.0. The four BCD signals are connected to P1.0, P1.1, P1.2, and P1.3 respectively, and the four bit select signals are connected to P1.4, P1.5, P1.6, and P1.7 respectively. The microcontroller and the millivoltmeter share a common ground.
3.2 Custom format for transmitted data
A computer byte has only 8 bits, but we need to transmit 9 effective bits: 4 BCD codes and 5 bit select signals. Therefore, we use software encoding to convert the 5 bit select signals into 4 BCD codes, which are placed in the high 4 bits of the transmitted byte, and the low 4 bits are used to store the displayed value.
The truth table for converting 5 bit select signals into 4 BCD codes is shown in Table 1.
Table 1. Correspondence between the 5 bit selection signals and BCD codes
4. PC software development
This paper uses VB6.0 to implement the software programming of the PC interface. The serial communication object of Visual Basic encapsulates the low-level operation of RS232, and users can use the high-level BASIC syntax to communicate with the outside world using RS232[7]. The software programming flowchart is shown in Figure 6.
Figure 6 Software programming flowchart
5. Test the developed system.
The actual contact resistance of the developed system was tested, and the test results are shown in Figure 7. (The tested sample was a pure copper sample after SO2 etching.)
Figure 7 Actual contact resistance test
According to the image analysis, the corroded copper sheet will wear away the SO2 corrosion film through a certain period of micro-motion process, and will always remain at a few milliohms. Combined with some past results of electrical contact theory [8], it can be considered that the test results are correct and the test system can complete a certain accuracy test task.
6. Conclusion
This paper details a contact resistance measurement system, including its hardware, interface, and PC software programming components. It achieves real-time measurement of contact resistance, data recording, and post-processing. The system was tested in practice, verifying its feasibility and providing a valuable tool for studying connector contact performance.
