Fig.1 and Fig.2 show the fundamental principle of operation. Rod consists of high magnetic permeability parts and low magnetic permeability parts which are alternately put in the steel pipe.
Coil unit consists of three pairs of primary coil and secondary coil which are rolled up concentrically,and which are arranged at one third of one rod pitch length interval. Three sinusoidal excitation signals whose differences of phase are 120 degrees each other are put on above mentioned three primary coils as the input signals, and inductive signals appropriate to the inputs are induced on each corresponding three secondary coils. As the value of induced signals depend on the magnetic permeability of the part of rod, they are changed according to the shift of relative position between rod and coil unit. Therefore output signal (Y) is obtained as is shown in equation below by adding above three induced signals based on the addition theorem of trigonometric function.
The phase of output signal (Y) changes from 0 to 2pi compared with the one of excitation signal when the displacement (X) changes from 0 to P as is shown in Fig.2, then X is calculated by counting the phase shift (T).