A linear variable differential transformer is a special electrical transformer that has an output voltage and a phase that is determined by the linear position of its core. It uses two secondary windings that share a common core. When the core material is exactly between the two secondary windings, there is total cancellation between the outputs if the two windings are connected in a series-opposing mode.
The dot convention in transformers is a practice that marks same phase outputs. When there are two isolated and same-turns-count secondary windings, 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 will add up or cancel each other. To simplify the phase considerations in wiring, the dot convention is used. A dot and no dot for a winding will be all that is needed to wire the outputs as series aiding or series opposing. In series aiding, the voltages are double the output of one winding, while in series opposing the voltages cancel out and produce almost no output.
The linear variable differential transformer makes use of the linearly movable core to produce equal or non-equal outputs in the two secondary windings. When the core is positioned at the center, the outputs are equal and will cancel totally if the connection is series opposing. Once the core is moved away from the center, there will be a net greater voltage on one output winding. The result is an output voltage with a voltage and phase that is determined by the position of the core.
To monitor angular displacement, the rotary variable differential transformer (RVDT) may be used. The inside core may be rotated at an angle to produce a different output for the two windings. Linear displaced is tracked by the linear variable differential transformer, while the rotary variable differential transformer uses rotational or angular displacement. For instance, the angular position of a panning radar antenna may be generated using an RVDT.
The linear variable differential transformer works only with alternating current. It may be used as sensor for tracking linear position in certain applications. Other linear transducer designs are simplified by the use of digital techniques. For instance, the horizontal position of a printhead along the line on a page may be encoded by sensors referred to as optical-slotted transducers. These array-type transducers produce pulses of specific relation to each other so that a left-bound and right-bound displacement is detectable.