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When measuring small distances on the order of millimeters with submillimeter resolution, one typically uses a so-called linear voltage differential transformer (LVDT), which is an electromechanical device with three solenoid coils placed in series around a hollow core. The center coil is the primary, and the two outer coils are the secondaries. A cylindrical ferromagnetic core, attached to the measuring rod, slides inside the tube. An alternating voltage drives the primary and causes a voltage signal induced in each secondary proportional to the core displacement. The AC-output induced voltages contain, after demodulation, the information of the distance displaced by the measuring rod. The LVDT range varies from a few millimeters up to 200 mm, with resolutions that can reach 1 mm. The demodulation process is a simple electronic system that produces in its output a linear voltage or digital signal proportional to the displacement of the measuring rod. Although very precise, simple to use, and relatively cheap, the LVDT has the disadvantage of needing a nearby electronic box for each unit that connects to the transducer by a multi-conductor cable. For a multi-transducer system, each unit needs its own electronic box, and each box has to be connected to the control system.
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