Publication

Abstract : This paper proposes an Inductance-to-Digital Converter (LDC) that measures the change in inductance ∆L of a Differential-type Inductive Sensor (DIS) independent of its coil resistance. The ∆L is measured using a ratio metric approach that eliminates error due to the tolerances of the components used in the circuit. The proposed LDC is realized using a simple dual-slope conversion technique and possesses advantages such as noise immunity and high accuracy. Existing schemes for measuring differential inductance assume the coil resistance is negligible or equally matched. This assumption may not be practical always, and hence a noticeable error in inductance measurement will be introduced in the existing architectures, while the proposed design does not suffer from this limitation. In addition, the final output provided by the proposed LDC will always be proportional to the measurand, irrespective of whether the measurand has a linear or inverse relationship with the sensor inductance. The prototype developed showed superior performance with a worst-case error of 0.72% when the series coil resistance was varied from 0 to 1000 Ω in steps of 100 Ω. Moreover, the developed architecture showed a good resolution of 3.5 µH and high linearity, with a worst-case error of 0.55% when ∆L was varied from –6 mH to +6mH.