<P> The linear variable differential transformer has three solenoidal coils placed end - to - end around a tube . The center coil is the primary, and the two outer coils are the top and bottom secondaries . A cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides along the axis of the tube . An alternating current drives the primary and causes a voltage to be induced in each secondary proportional to the length of the core linking to the secondary . The frequency is usually in the range 1 to 10 kHz . </P> <P> As the core moves, the primary's linkage to the two secondary coils changes and causes the induced voltages to change . The coils are connected so that the output voltage is the difference (hence "differential") between the top secondary voltage and the bottom secondary voltage . When the core is in its central position, equidistant between the two secondaries, equal voltages are induced in the two secondary coils, but the two signals cancel, so the output voltage is theoretically zero . In practice minor variations in the way in which the primary is coupled to each secondary means that a small voltage is output when the core is central . </P> <P> This small residual voltage is due to phase shift and is often called quadrature error . It is a nuisance in closed loop control systems as it can result in oscillation about the null point, and may be unacceptable in simple measurement applications too . It is a consequence of using synchronous demodulation, with direct subtraction of the secondary voltages at AC . Modern systems, particularly those involving safety, require fault detection of the LVDT, and the normal method is to demodulate each secondary separately, using precision half wave or full wave rectifiers, based on op - amps, and compute the difference by subtracting the DC signals . Because, for constant excitation voltage, the sum of the two secondary voltages is almost constant throughout the operating stroke of the LVDT, its value remains within a small window and can be monitored such that any internal failures of the LVDT will cause the sum voltage to deviate from its limits and be rapidly detected, causing a fault to be indicated . There is no quadrature error with this scheme, and the position - dependent difference voltage passes smoothly through zero at the null point . </P> <P> Where digital processing in the form of a microprocessor or FPGA is available in the system, it is customary for the processing device to carry out the fault detection, and possibly ratiometric processing to improve accuracy, by dividing the difference in secondary voltages by the sum of the secondary voltages, to make the measurement independent of the exact amplitude of the excitation signal . If sufficient digital processing capacity is available, it is becoming commonplace to use this to generate the sinusoidal excitation via a DAC and possibly also perform the secondary demodulation via a multiplexed ADC . </P>

Principle and operation of lvdt and its application