Measuring modifying impedance transducers

Abstract

Modifying impedance transducers are often used in hydroelectric building stability monitoring. Structurally, this transducer type includes an LC electronic oscillator, with inductance as variable parameter. The measured physical quantity (pressure, displacement, force, etc.) modifies a geometrical distance, changing the equivalent inductance of the oscillator circuit. In most cases, they are placed far away from the measuring/supply end. As result, the self oscillation frequency is highly influenced by environmental conditions. Long lines add their own inductance to the oscillator equivalent one. The line is noise susceptible, eventually mixing to the oscillator frequency. Important voltage drop reduces the local supply voltage. The useful signal uses the same wire pair as the supply current, so discriminating circuits are required, for both the transducer and the measuring/supply ends. The paper analyzes the power supply, long wire and discriminating circuitry induced errors as well as measuring methods used to minimize them.