Temperature compensation for displacement transducer

Claim 1

An improved displacement measurement apparatus having a main current conducting coil for attachment to a first one of two relatively movable bodies, an electrically conducting, non-ferromagnetic first wall in telescoping relationship to said coil and mounted to the second body for axial displacement relative to the main coil for varying their relative overlap and thereby vary the inductance of the coil; an AC electrical energy source which is connected to apply an AC signal to the coil at a frequency which is at least high enough that the skin depth of the first wall is less than the physical depth of the wall; and a detector circuit means connected to the coil for detecting a signal which is proportional to coil inductance, wherein the improvement comprises: a bobbin wall interposed between the coil and said electrically conducting, non-ferromagnetic first wall, the bobbin wall formed of an electrically conducting, non-ferromagnetic material having a thickness which is small compared to the skin depth of the bobbin wall material at said frequency, said bobbin having a combination of wall thickness and a rate of change of skin depth as a function of temperature which combination causes the magnetic field intensity within the coil to vary as a function of temperature in inverse proportion to the change in the effective cross-sectional area of said first wall that results from changes in the skin depth of the first wall due to changes in temperature, said change in magnetic intensity reducing the net charge, as a function of temperature, in said detected signal.