The lvdt transducer (Linear Variable Differential Transformer) is an absolute position sensor that associates specific signals with any displacement of its cylindrical core, via electromagnetic coupling of AC excitation. The LVDT can be used to detect force, pressure, weight and also can operate at high temperatures. For instance, a linear LVDT transducer can be built into a gauging rig that ensures the thickness of a manufactured product stays within specifications. Another example of a LVDT application is in the counting of bank notes and similar sheet items that require absolute numerical accuracy. Here, a gauging rig with a series of LVDTs can monitor each individual note, at high speed and ensure that they are counted accurately.
The basic construction of an LVDT consists of a cylindrical soft iron core passing through the center of a hollow coil assembly. Within the coil assembly there are three transformer windings: a primary located in the center, and two identical secondary windings on either side of the core. When AC excitation is applied to the primary, inductance currents are induced in both of the secondary windings. When the core is at dead center (equidistant from both secondary outputs), a zero output voltage appears, which is called the null position. When the core is moved, a corresponding change in the secondary output voltages occurs.
The difference in the amplitude and phase of the induced secondary output voltages is proportional to the core's position, and can be determined by measuring the output signal from the LVDT's internal electronics. The LVDT's design allows for very linear operation over a range of core positions, although the resolution does degrade as the core moves closer to the end of its specified displacement range.