Temperature compensation of an integrated low power inductive proximity microsensor

Abstract

A simple temperature compensation method for inductive proximity microsensors based on the differential relaxation oscillator has been developed and successfully tested. With this compensation and for the temperature range from −20°C to +80°C, an accuracy better than ±10 μm at 1 mm distance to an aluminum target has been measured. The microsensor has been integrated with a 3-V, 1-μm CMOS read-out circuit using a gold bumping layer to form a 3.8-mm side flat coil. The power consumption of the whole compensated microsystem is lower than 10 mW. To achieve this, the temperature behaviors of the whole microsensor and of its building elements, namely the sensing coil (nearby a target) and the read-out circuit, have been studied and a compensation method has been developed. The inductance of the integrated coil is temperature-independent in the frequency range up to 12 MHz, whereas its resistance depends mainly on the temperature coefficient of the conductor resistivity. The resonance frequency of the coil is not affected by temperature. In its principle, the electronic circuit has a temperature-dependent drift in the sensing distance range. This drift can, however, be compensated using a negative temperature coefficient resistor. Analytical derivations and simulation tools have been used for the choice of the optimal coefficient for a specific sensing distance range.