Custom 3
LVDT For the Hobbyist
An LVDT or Linear Variable Differential Transducer is a non-contact position sensor. It functions exactly as you would expect a transformer might, with the exception of having a movable core.
Basic operation in plain english
An AC excitation is feed in to the primary coil, producing a magnetic field. This field is induced by the secondary coil and has a magnitude proportional to the area of core material between the two (and of course, the excitation) . The more core the greater the output signal.
Having two secondaries helps to extent the physical range of the transducer. But this also presents a problem; Lets say the core is 10mm to the left of center and the output is 0.125 volts. Well if the core were 10mm to the right it would have the same output of 0.125 volts. You wouldn't be able to determine which side of center you were on.
The solution to this is to wire the secondaries to electrically oppose each other. When one secondary is, the other one is also. This way they “buck” one another. On the dual trace oscilloscope, you can see the difference between sides. One side will be in phase with the excitation and the other will be out of phase by 180 degrees.
Basic four lead LVDT wiring
There are a variety of wiring schemes for LVDTs. This is an example of the typical four lead version. Other variants include running the center tap out as in the five lead. Six lead versions have every coil end attached to a lead. Some Kavlico transducers manufactured for Boeing even have multiple LVDTs sharing the same core.