A Transistorized Displacement Meter using a Variable Frequency Oscillator

Abstract

In this paper the details of a transistorized displacement meter using a variable frequency oscillator are presented. The normal method of measurement of displacement or pressure using the linear variable differential transformer (LVDT) involves the initial balancing with regards to amplitude and phase. This necessitates the adjustment of the core critically to a particular position. In the method that has been described in this paper, an inductive transducer replaces the LVDT as the sensing element. The amplitude and phase balancing are altogether avoided and the transducer may be used anywhere in its linear range. The transducer itself forms part of an oscillator whose frequency varies when its inductance changes due to the displacement of the core. A frequency mixing circuit with a local oscillator is employed to make available the difference in frequency. This difference frequency signal is transmitted to the distant receiving station where this signal is regenerated and read with a suitable frequency meter or a decade counter. This method of data transmission and reception does not suffer from error arising due to long lines, noise, etc. It also affords digital measurement of displacement. With suitable modifications of the sensor, the system can also be employed to measure pressure.