| In order to detect the quality and performance of equipment in engineering practices, micro-resistance measurement is needed. However, this can’t be done perfectly by the digital multimeter, especially when the value is very small with milliohm or micro-ohm scalar. The accuracy of existing micro-ohmmeters is also incompetent due to its high test current and long measuring time. According to the need of accurate micro-ohm measurement, a micro-ohmmeter based on four-line method is proposed and designed.The design consists of hardware system and software system. The hardware system includes power supply circuit, constant-current source, A/D converter and display with LCD. The software system is mainly composed of initialization subroutine, data acquisition subroutine, data processing subroutine and some corresponding interruption subroutines. Based on a thorough analysis of the factors which mainly restrict the precision of micro-ohm measurement, feasible solutions have been put forward accordingly. The adoption of four-line method can effectively eliminate the effects resulting from lead resistance and contact resistance. A wide-range high-precision constant-current source is designed to improve the accuracy and stability of the test current. LMS is used to filter noises in the measurement of micro-resistance. The design based on CPLD greatly simplifies the circuit structure and improves the data transfer rate. On account of the demand of portability, a dual-use power which can provide power supply with both AC mains and accumulator is designed. It is concluded that the device can realize the resistance measurement within 1μΩ to 2MΩ automatically with the precision of 1‰. Moreover, the system has the function of resistance separation, which can be connected to automated production lines for the resistance parameter inspection. |
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