<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> <P> When the core is displaced toward the top, the voltage in the top secondary coil increases as the voltage in the bottom decreases . The resulting output voltage increases from zero . This voltage is in phase with the primary voltage . When the core moves in the other direction, the output voltage also increases from zero, but its phase is opposite to that of the primary . The phase of the output voltage determines the direction of the displacement (up or down) and amplitude indicates the amount of displacement . A synchronous detector can determine a signed output voltage that relates to the displacement . </P> <P> The LVDT is designed with long slender coils to make the output voltage essentially linear over displacement up to several inches (several hundred millimetres) long . </P>

When does the phase shift in lvdt output signal occurs
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