A Non-Contact Micro-Ampere DC Current Digital Sensor Based on the Open-Loop Structure

The non-contact accurate measurement of weak direct current (dc) is very important for various fields, such as the online insulation monitoring of power cables. Compared with other dc current sensors, the magnetic modulator is more suitable for dc measurement with its high sensitivity, low noise, and stable working. Most magnetic modulators adopt the closed-loop structure to achieve accurate measurement of large current. And the magnetic modulator only realized the accurate measurement of mA current in the filed of weak dc current, which is difficult to meet the $\mu \text{A}$ current measurement requirements of the online insulation monitoring. Therefore, this article proposes a new structure of open-loop digital magnetic modulator for precision measurement of $\mu \text{A}$ dc current. It adopts an annular shielding layer to weaken the zero offset and stray magnetic field. And a low-frequency excitation source, which can be adjusted with the ambient temperature, is used to improve the temperature stability. Instead of the traditional analog demodulation method, a digital demodulation algorithm is designed to reduce the measurement error caused by the zero offset and excitation source fluctuation. Measurement results show that the new sensor with the absolute error of 0.5 $\mu \text{A}$ can measure 5-1000 $\mu \text{A}$ dc current with a relative error of 0.05% in the full scale. And the sensor can effectively suppress the measurement error caused by the fluctuation of the excitation source and the ambient temperature to less than 0.5%.