Design and Analysis of a Relaxation Oscillator-Based Interface Circuit for LVDT

Abstract

This paper presents a simple, novel interfacing scheme for linear variable differential transformer (LVDT) that does not require a precise sine wave oscillator (with stable amplitude and frequency) for excitation or any phase matching circuit like traditional methods. The primary and the secondary coils of the LVDT are integral parts of a relaxation oscillator (RO). The RO injects a current waveform with a triangular shape through the primary coil which in turn induces a square-shaped voltage waveform in the secondary coils. The RO utilizes the amplitudes of those waveforms and generates a pulsewidth modulation (PWM) output signal whose duty cycle ratio is proportional to the measurand, core position. This quasi-digital PWM signal is passed through a low-pass filter to obtain an analog output. PWM time period measurement using a counter yields a digital output. This method guarantees good stability since the output is dependent only on a pair of dc reference voltages apart from the transformation constant of the sensor. A detailed analysis of the sources of error, their effect on the performance of the scheme and appropriate design choices for error mitigation are also presented. A prototype of the proposed circuit was built and tested in the laboratory. The nonlinearity error noted from the tests conducted was less than 0.16% for the analog output and 0.18% for the digital output. A resolution of up to 11 bits was achieved for the digital output.