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 (<inline-formula> <tex-math notation="LaTeX">$\Delta L$ </tex-math></inline-formula>) independent of offset inductance (<inline-formula> <tex-math notation="LaTeX">$L_{01}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$L_{02}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$L_{03} $ </tex-math></inline-formula>), mismatch in offset inductance (<inline-formula> <tex-math notation="LaTeX">$L_{01} \neq L_{02}$ </tex-math></inline-formula>), and series coil resistance (<inline-formula> <tex-math notation="LaTeX">$R_{C1}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$R_{C2}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$R_{C3}$ </tex-math></inline-formula>) is presented in this work. In most cases, the change in sensor inductance <inline-formula> <tex-math notation="LaTeX">$\Delta L$ </tex-math></inline-formula> due to the measurand will be very small as compared with <inline-formula> <tex-math notation="LaTeX">$L_{0}$ </tex-math></inline-formula>. Thus, the existing architectures that measure the sensor inductance <inline-formula> <tex-math notation="LaTeX">$L_{Xi} (=\! \! L_{0i} \pm \Delta L; i=1,2,3$ </tex-math></inline-formula>) leads to underutilization of the output range. The proposed scheme is effective in measuring <inline-formula> <tex-math notation="LaTeX">$\Delta L$ </tex-math></inline-formula> insensitive to <inline-formula> <tex-math notation="LaTeX">$L_{0i}$ </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">$L_{01}$ </tex-math></inline-formula> = <inline-formula> <tex-math notation="LaTeX">$L_{02}$ </tex-math></inline-formula> for the differential pair <inline-formula> <tex-math notation="LaTeX">$L_{X1}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$L_{X2}$ </tex-math></inline-formula>. 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 <inline-formula> <tex-math notation="LaTeX">$L_{03}$ </tex-math></inline-formula> (for SIS) and associated mismatch <inline-formula> <tex-math notation="LaTeX">$L_{01} \neq L_{02}$ </tex-math></inline-formula> (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 <inline-formula> <tex-math notation="LaTeX">$\Delta L$ </tex-math></inline-formula> in the range of ±9 mH and a linearity error of 0.48% for coil resistance measured in the range of 100–<inline-formula> <tex-math notation="LaTeX">$1000~\Omega $ </tex-math></inline-formula>. The proposed approach paves path toward the use of a single readout in measuring <inline-formula> <tex-math notation="LaTeX">$\Delta L$ </tex-math></inline-formula> for both SIS and DIS, even with large nominal inductance values and coil resistance.