Potentiometers owe their reliability primarily to a simple design. They consist mainly of three components; the resistance element, the wiper, and the input shaft. The input shaft moves the wiper, which changes its position on the resistance element acting as a variable voltage divider picks off a voltage that is proportional to the linear or angular position. The analog voltage output signal of the potentiometer is absolute and can track high speed motion in real time.
The name LVDT is an acronym for Linear Variable Differential Transformer, an electromechanical transducer that converts rectilinear motion of its core relative to the zero position into a corresponding electrical signal. The LVDT’s primary winding, P, is energized by a constant amplitude AC source (carrier). The magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1 and S2, at midway core position between S1 and S2, equal flux is coupled to each secondary so the voltages, E1 and E2, induced in the windings S1 and S2 respectively, are equal. This is known as the null point, the differential voltage output, (E1 – E2), is zero and the movement of the core will result into a change in the amplitude and the direction of the core movement from the null point will result in a 180 degree phase shift.
Usually the LVDT sensors require a signal conditioner modulator-demodulator that is either built in the unit or external to provide the carrier excitation and to convert the output to DC voltage. The high resolution of the LVDT allows it to measure movements as small as a few millionths of an inch, but LVDT-s can have ranges of up to ±20 inches (±0.5 m) as well.
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