LVDTS VS. DIGITAL GAUGES

LVDTs (Linear Variable Differential Transformer) are small analog devices that measure position. Their principle of measurement is based on magnetic transfer. An LVDT transducer comprises a coil former or bobbin onto which three coils are wound. The first coil, the primary, is excited with an AC current, normally in the region of 1kHz to 10kHz at 0.5V rms to 10V rms. The other two coils, the secondaries, are wound so that when a ferritic core is in the central linear position, an equal voltage is induced into each coil. However, the secondaries are connected in opposition so that in the central position the outputs of the two secondaries cancel each other. The electronics of this component combine information on the phase of the output with information on the magnitude of the output.

Because of this type of technology, these devices have a natural significant co-efficient of their measurement over time and temperature, and must be recalibrated often, usually once or twice each year in an extensometer application. These typically work like thermometers; the high measurement readings will vary as the room temperature changes. Also, their range of measuring is limited to 1mm or 2mm, which causes most companies who use them to stock many different lengths of LVDTs. When Virtech used the LVDTs, they had a stroke of about 1/10 in. to 2/10 in.

Digital gauges work by an optical principle. There is a small glass/ceramic scale that has an incremental track, (normal graduation) as well as a reference mark. There is also a tiny optical scanning unit (reader). This type of technology allows a longer range of measurement, from 12mm up to 100mm on some gauges, thereby reducing the number of masters required. The scales are also much more accurate over longer distances, and when made of ceramics, have no thermal growth at all. This increases the application spectrum where there are heated/cooled measurements.