A relaxation oscillator based interface circuit for LVDT

Abstract

This paper presents a simple relaxation oscillator based read-out circuit for Linear Variable Differential Transformer (LVDT). The new scheme does not require precise sine wave oscillator for excitation and the associated phase matching circuits like the traditional methods. The scheme is designed to perform ratiometric computation of the sensing variables and provides an output that is proportional to the measurand, displacement. The primary and the secondary coils are integral parts of a Relaxation Oscillator (RO). The RO injects a current waveform with triangular shape through the primary. This induces a square shape waveform in the secondary coils. The RO utilizes the amplitudes of those waveforms to derive the core position and represents it as a change in duty cycle of the output signal. The PWM output lends itself to easy digitization of the displacement using an up/down counter. Or, if analog output is required, the PWM output can be passed through a low pass filter and the same can be obtained. A prototype of the proposed read-out circuit has been built and tested (a) using standard variable inductors emulating the change in the value of the inductances of an LVDT and (b) a commercially available LVDT. The worst case non-linearity error noted from the tests conducted was less than 0.3 %.