LVDTs operate on the principle of a transformer and consist of a stationary coil assembly and a moveable core. An LVDT measures displacement by associating a specific signal value for any given position of the core. LVDT signal conditioners generate a sine wave for the primary output signal and synchronously demodulate the secondary output signal. The demodulated output is passed through a lowpass filter to remove high-frequency ripple. The resulting output is a DC voltage proportional to core displacement. The sign of the DC voltage indicates whether the displacement is to the left or right.
LVDTs require special electronics designed for the sensor. LVDTs typically have a delay of approximately 10 ms caused by filtering in the signal conditioner.
LVDTs typically come in 4-wire, or open wire, and 5-wire, or ratiometric wire, configurations. Wires from the sensor connect to a signal conditioning circuit that translates the output of the LVDT to a measurable voltage. The method of signal conditioning used on the signals from the first and second secondaries differentiate the 4-wire and 5-wire configurations. In the 4-wire configuration, the sensor only measures the voltage difference between the two secondaries.
The benefit of using a 4-wire configuration is that you require a simpler signal conditioning system. However, temperature changes can alter the efficiency of the magnetic induction of the LVDT. Because the 4-wire scheme is also sensitive to phase changes between the primary and the resulting secondary voltage, long wires or a poor excitation source also can cause problems.
The 5-wire configuration is less sensitive to both temperature changes and phase differences between the primary and the secondaries. The device determines phase information at the signal conditioning circuitry without needing to reference the phase of the primary excitation source. Therefore, you can use longer wires between the LVDT and the signal conditioning circuitry.
LVDTs are extremely rugged, operate over wide temperature ranges, and are insensitive to moisture and dirt. LVDTs are a preferred sensor in harsh environments, where very long life is needed because there are no moving parts in contact or where very low friction is required. Also, LVDT technology lends itself well to applications requiring accurate measurements less than 0.1 in., such as measuring the thickness of sheet material. The main advantage of the LVDT transducer over other types of displacement transducer is the high degree of robustness. Because there is no physical contact across the sensing element, there is no wear in the sensing element.
Because the device relies on the coupling of magnetic flux, an LVDT can have infinite resolution. Therefore, suitable signal conditioning hardware can detect the smallest fraction of movement, and only the resolution of the data acquisition system determines the resolution of the transducer.