Electronics Lab Philippines

Homemade LVDT

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Homemade LVDT
« on: April 05, 2008, 09:08:07 PM »


Robust Position Sensor

Linear variable differential transformers are linear position sensors that are used in harsh industrial or aerospace environments where reliability and/or performance requirements exceed the capabilities of potentiometers. LVDTs have no sliding electrical contacts to corrode or wear. The only moving part is a more or less inert chunk of iron. The unit can be sealed from harmful environmental elements.

Simple: 3 windings and a moveable core

An LVDT is a transformer with a single primary winding and two identical secondary windings. The transformer core is moveable. If the core is centered, it provides equal coupling between the primary winding and each of the secondary windings. As a result, each secondary produces the same voltage. As the core is moved, the coupling with one secondary grows and its voltage increases. The coupling with the other drops so its voltage decreases. The output windings are generally wired together so their voltages cancel. With the core centered, the net output voltage is nulled. The voltage increases  as the core is moved from the null position.

The “Linear” in the name refers to the linear motion of the core. Turns out,  an LVDT is electrically linear too. The difference in the two output voltages varies quite linearly with the movement of the core. Linearity better than 2% full scale comes without any particular effort in building an LVDT. When care is taken to construct the device symmetrically, linearity in the range of .1~.2% full scale results.

Because they are so simple, I decided to build an LVDT and measure its performance.