With built-in electronic oscillator and demodulator producing an dc output proportional to core position
DFg series; have separate coil/electronic assemblies with a free core fitted with a polyacetal homopolymer bearing which can rub on the inside coil assembly to ease guiding requirements
DG2.5; This has a non-rotating spring-loaded armature running in precision linear ball bearings

Linear Variable Differential Transformers (LVDT)

These devices comprise a movable nickel iron core and three coils - one for energisation and two for pick-up. They need to be driven by a sine wave to produce an output amplitude and phase directly proportional to the position of the core with respect to the pick-up coils. A phase sensitive detector is required to demodulate the output signal. The main advantages of LVDTs are;

No wear problems as with potentiometers
Infinite resolution, high accuracy and linearity
No friction introduced into the measured system
Available in ranges from ±1mm to ±50mm