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The linear variable differential transformer (LVDT) uses a core’s position to determine output voltage and phase. The dot convention simplifies phase considerations in transformers. The LVDT only works with AC and can be used to track linear position. The rotary variable differential transformer (RVDT) monitors angular displacement. Other linear transducer designs use digital techniques.
A linear variable differential transformer is a special electrical transformer that has an output voltage and phase determined by the linear position of its core. Uses two secondary windings sharing a common core. When the core material is exactly between the two secondary windings, total cancellation occurs between the outputs if the two windings are connected oppositely in series.
The dot convention in transformers is a practice that marks the same phase outputs. When there are two secondary windings isolated and with the same number of turns, the output voltages will be the same and if the two secondary windings are connected in series, there is a possibility that the two phases add or cancel each other out. To simplify phase considerations in wiring, the dot convention is used. A dot and no dot for one winding will be all that is needed to wire the outputs as series help or series opposition. In series helping, the voltages are twice the output of one winding, while in opposite series the voltages cancel out and produce almost no output.
The linear variable differential transformer uses the plunger linearly to produce equal or unequal outputs in the two secondary windings. When the core is positioned in the center, the outputs are equal and will totally cancel if connected in series opposite. Once the core is pulled away from the center, there will be a higher net voltage across one output winding. The result is an output voltage with a voltage and phase determined by the position of the core.
To monitor the angular displacement, the rotary variable differential transformer (RVDT) can be used. The inner core can be angled to produce a different output for the two windings. Linear displacement is followed by linear variable differential transformer, while rotary variable differential transformer uses rotational or angular displacement. For example, the angular position of a panoramic radar antenna can be generated using a RVDT.
The linear variable differential transformer only works with alternating current. It can be used as a sensor to track linear position in certain applications. Other linear transducer designs are simplified by the use of digital techniques. For example, the horizontal position of a print head along a line on a page can be encoded by sensors called optical slit transducers. These matrix-type transducers produce pulses of specific relationship to each other so that a related left and right shift is detectable.
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