Resolution Enhancement in Directly Interfaced System for Inductive Sensors

In this paper, a resolution enhancement method for direct interfacing of inductive sensors, without employing analog-to-digital converter and/or external driver circuit, is presented. In order to achieve better resolution with minimum relative error and current consumption factors, different schemes are implemented which vary in terms of: 1) step-input voltage provision methodology, i.e., through microcontroller (μC) pin or through external supply; 2) number of switches used; and 3) method of voltage detection, i.e., either using capture module or employing built-in comparator only. The maximum achievable resolution limits are explored by reducing the lower reference voltage (VRefL) in case of detection through built-in analog comparator and/or reducing the series resistance. The one-point calibration technique is used to determine the accuracy. The experiments are performed at 16-MHz clock frequency in the inductance range of: 1) 10-100 μH and 2) 100-1000 μH. The results show that in the limits of a maximum current sourcing/sinking capacity of a typical μC, i.e., 40 mA, the proposed methodology enhances the resolution to 4 μH with a relative error of 1.2 in the percentage of full-scale span (%FSS). The maximum nonlinearity error in %FSS for inductance range 10-100 μH and 100-1000 μH is 1.1% and c0.4%, respectively.