Direct inductive sensor-to-microcontroller interface circuit

This paper proposes and analyses a microcontroller-based interface circuit for inductive sensors with a variable self-inductance. Besides the microcontroller (μC) and the sensor, the circuit just requires an external resistor and a reference inductor so that two RL circuits are formed. The μC appropriately excites such RL circuits in order to measure the discharging time of the voltage across each inductor (i.e. sensing and reference) and then it uses such discharging times to estimate the sensor inductance. Experimental tests using different commercial μCs at different clock frequencies show the limitations (especially, due to parasitic resistances and quantisation) and the performance of the proposed circuit when measuring inductances in the millihenry range. A non-linearity error lower than 0.3% full-scale span (FSS) and a resolution of 10 bits are achieved, which are remarkable values considering the simplicity of the circuit.