Non-intrusive Detection of Current Flow Within a Power Cable

Abstract

The non-invasive current sensor is a device theoretically able to measure the current flowing through a power cable made of one, two or three wires.

At first, a theoretical approach has been set-up with several simulations by a former student "Mario Varandas" to check if this is feasible. And different algorithms have been implemented to deduce the current. After that a prototype is created to acquire all the necessary measurements to run the algorithms. A first prototype has been realized by the former student but there were several problems and a new prototype has been designed

Both prototypes are based on magnetoresistive sensors made of a nickel-iron and patterned as resistive strip elements. Since these elements are arranged in a Wheatstone bridge configuration, their variations produce a specific output voltage. This voltage is then amplified and read by an analog-to-digital converter and finally sent to a micro-controller. These different stages have to be well designed because the output voltage variation is very small.

Three amplification stages are discussed : the instrumentation amplifier, the fully differential amplifier and the analog-to-digital using analog front-end technology. The last one is chosen for the second prototype because it is more flexible and could theoretically reach a high resolution.

Once the schematics and the board are done, the prototype is built and the measurements are done. After checking the supply stage, the SPI communication and the S/R circuits, the output voltage of the sensors are measured and commented. After that these measurements are processed into the algorithms deducing the current. These results indicated that the acquisition process work but need to be more precise.

Finally, the non-invasive current sensor works but there are still challenges to solve to have an accurate device and several improvements are suggested.