An Enhanced Inductance-to-Digital Converter for Single-Element and Differential-Type Inductive Sensors With Automatic Offset Eliminator

A novel inductance-to-digital converter (LDC) that outputs a digital value corresponding to the change in inductance (
$\Delta L$
) independent of offset inductance (
$L_{01}$
,
$L_{02}$
,
$L_{03} $
), mismatch in offset inductance (
$L_{01} \neq L_{02}$
), and series coil resistance (
$R_{C1}$
,
$R_{C2}$
,
$R_{C3}$
) is presented in this work. In most cases, the change in sensor inductance
$\Delta L$
due to the measurand will be very small as compared with
$L_{0}$
. Thus, the existing architectures that measure the sensor inductance
$L_{Xi} (=\!\!L_{0i} \pm \Delta L; i=1,2,3$
) leads to underutilization of the output range. The proposed scheme is effective in measuring
$\Delta L$
insensitive to
$L_{0i}$
for both single-element type inductive sensor (SIS) and differential-type inductive sensor (DIS). For most of the DIS, in practice, it is very difficult to obtain the matched offset inductance
$L_{01}$
=
$L_{02}$
for the differential pair
$L_{X1}$
and
$L_{X2}$
. This will introduce a noticeable error in the output for existing DIS readout schemes. Whereas, for the proposed LDC’s automatic offset elimination (AOE) mechanism ensure that digital output is made independent of the offset inductance
$L_{03}$
(for SIS) and associated mismatch
$L_{01} \neq L_{02}$
(for DIS). The LDC presented here uses dual-slope conversion technique and employs dc source for digitization. This eliminates the need for ac excitation source and possesses high measurement accuracy and noise immunity. The proposed LDC also measures coil resistance with negligible cross-sensitivity for SIS and DIS, which further enhances the practical efficacy of the proposed scheme. The developed prototype demonstrated a worst-case linearity error of 0.59% for
$\Delta L$
in the range of ±9 mH and a linearity error of 0.48% for coil resistance measured in the range of 100–
$1000~\Omega $
. The proposed approach paves path toward the use of a single readout in measuring
$\Delta L$
for both SIS and DIS, even with large nominal inductance values and coil resistance.