Calibration-Less Direct Inductor-to-Microcontroller Interface

A sensor-to-microcontroller interface circuit for inductive sensors that does not require a calibration inductor is presented. Three digital ports of a microcontroller, two transistors, and an embedded timer were used to measure the charging times of the inductor through forward- and backward-flow currents using two different initial conditions. The difference between the charging times was used to estimate the sensor inductance accurately without the uncertainty due to the tolerance of threshold voltage of the input port, which delimits these time length measurements. Experimental results show that a conventional low-power microcontroller with an embedded 16 MHz clock and 22 $\Omega $ charge resistance can be used to achieve 2 $\mu \text{H}$ resolution and a systematic error below 2% for a measuring range from 100 $\mu \text{H}$ to 10 mH. The advantages of the proposed technique include the reduction of cost and space as well as avoiding complex calibration processes caused by the temperature drift of the reference inductors.