The LVDT: A Simple and Accurate Position SensorAugust 1, 2005 By: David S. Nyce, Revolution Sensor Sensors
The venerable linear variable differential transformer is still a viable solution to many position sensing problems.
The linear variable differential transformer (LVDT) has been around for many years, and remains a popular sensing technology for absolute position measurement. It is relatively simple, operates over a wide temperature range, has extremely fine resolution, never wears out, and has high reliability. It is well suited for linear measurements over ranges from microns to several inches, but becomes less cost-effective at stroke lengths greater than ±3 in.
Differential transformers have been used in various forms since the 1930s, and the LVDT became widely known in the 1940s when Herman Schaevitz published his paper, "The Linear Variable Differential Transformer," . The LVDT became more prevalent as an industrial sensor in the 1960s with the advent of solid-state electronics. It is still popular today, having undergone many improvements in performance and having been adapted for the miniaturization of the associated electronics.
The basic LVDT (see Figure 1) comprises three axially aligned stationary coils having a central bore, and a core that is movable within the bore. There is enough clearance between the core and the bore to prevent them from contacting each other. The center coil is the primary of the transformer, and is driven by an AC waveform at a constant frequency of 50 Hz to >10 kHz. The most popular operating frequency is 2.5 kHz. The two secondary coils are wired in series-bucking, so that their voltages subtract.
Figure 1. The basic configuration of an LVDT comprises three coils and a movable core.
When the core is centered, an equal voltage amplitude is induced across each secondary by transformer action. But since the secondaries are wired in series-bucking, the phases of the two voltages are opposite, thus producing a theoretical output of zero V. When the core is centered, it is called the null position (see Figure 2).
Figure 2. An LVDT is shown with the core positioned at null (A) and at full scale (B).
Although the output voltage at null is theoretically zero, there remains a small AC null voltage. The exact position of the null is where the sum of the outputs of the two secondaries is at its lowest value . The null voltage, however, is insignificant after demodulation. When the core moves to one side of null, the voltage across that coil increases as the other decreases. This results in a steadily increasing voltage across the output leads. This AC voltage is usually rectified or demodulated to produce a DC output voltage that increases with the distance of the core from null, and with a polarity (positive or negative) that indicates the direction of travel from null. So, for example, an LVDT with a range of ±1.000 in. could be demodulated to provide a DC output signal of ±1.000 V. Then the output would linearly change from +1 V at +full scale of +1.000 in., down to zero V at null, and then continue to –1.000 V when it reaches negative full scale.
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