lvdt compensation

[nuc_navt_et]

i was looking at a diagram of an lvdt with compensation for a change in primary voltage. there was a feedback from the output to the excitation source. this circuit some how compensated for the change in primary excitation and deliveded a vdiff.out of the secondaries =to the original beofre the change. does anyone know how this is done or where i can find a circuit diagram of what is happening here. thanks alot.