Linear Variable Differential Transformer
Background
A Linear Variable Differential Transformer (LVDT) is used to measure linear displacement. An LVDT is comprised of three solenoidal coils placed end to end wrapped around a tube. The center solenoid is the primary coil and the two on the edges are secondary coils. A ferromagnetic core slides along the inside of the tube. An AC current is run through the primary coil and causes a voltage to be induced in each secondary coil. As the ferromagnetic core slides up and down, the primary coil’s magnetic linkage to the two secondary coils changes and causes the induced voltages in the secondary coils to vary. When the core is in its central position, equidistant between the two secondaries, the output voltage will be theoretically zero. As the core moves up and down the voltage should change as a linear function of displacement.
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Design and Construction
There were two major steps in the construction of the LVDT. First was the mechanical assembly of the LVDT which involved wrapping copper wire around a tube and creating a ferromagnetic core.The second step in the construction of the LVDT was to make a signal modification system. A circuit to convert the induced voltage to a linear displacement including an Arduino Uno had to be built and configured. Finally, the data would be presented through an LCD display.
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Three primary materials had to be bought in the construction of the LVDT, a ferromagnetic core, a hollow tube and copper wire. The core that was found was galvanized steel in a cylindrical shape, with a 1.5 cm diameter and a 10 cm length. To match the ferromagnetic core a 2 cm diameter hollow PBC tube was used. The copper wire bought was 18 gauge, so it could wrap 100 times around 10 cm of the outer tube (the length of a solenoid).
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Once the three materials were bought it was a matter of manually wrapping copper wire around the outside tube in 3 different solenoids. The wire was taped down with electrical tape and a dowel was attached to the end of the ferromagnetic core, so it could slide in and out of the tube easily. Running an AC current through the LVDT and moving the ferromagnetic core through the tube produced an induced voltage and confirmed that the physical setup of the LVDT had been built correctly. The picture below shows what the ferromagnetic core looks like getting inserted into the hollow tube.
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Once it had been confirmed that an induced voltage was running through the LVDT, the voltage had to be processed into displacement. This was done through an Arduino Uno. An Arduino Uno can take in a DC signal from 0-5 volts and convert it to an integer value between 0-1023. This integer is then placed into a function that represents linear displacement and measures the distance the ferromagnetic core travels
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There were several corrections that had to be made to the initial signal output, before an acceptable value could be read on the Arduino Uno. First and foremost, the AC wave had to be converted into a DC signal. This was solved by passing the AC output of the solenoids through a full wave rectifier with a smoothing capacitor to produce a proportionate DC voltage. The picture below shows the full wave rectifier circuit, it is four diodes soldered together.
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Finally, a relationship between the voltage output of the LVDT and the Arduino was determined by moving the ferromagnetic core between the centers of the two secondary solenoids. Every 0.5 centimetres a measurement was taken. A picture of the full wave rectifier circuit and the relationship between voltage and displacement can be seen below.
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