Measurement transducer for lengths or spacings in micrometer range

摘要

The length or spacing to be measured which is in the micrometre range is made the spacing of the plates of an electrical capacitor assembly. Subsequent measurement with the length-analogue capacitance gives the required parameter value. The basic relationship on which the transducer action depends is: C = k.A / [d + dx] where the measured distance and its variable component are d and dx, with A the area of the transducer capacitor, k the electric permittivity and C the measured capacitance. It follows that the measured distance is always smaller than the basic interplate spacing. In practical realisation the capacitor structure takes the form of two electrodes, (Es1, Es2), which are each an assembly of a multiplicity of parallel plates, (E1,E2), meshing with each other, mounted on plates of insulating material (e.g. PTFE) (Ts1,Ts2). The total capacitance results from the parallel connection of individual plates to form N pairs, with the capacitance calculated from the expression above by introduction of a multiplying factor N. In measurement of shaft displacement clamping rings are placed about the shaft with an intermediate insulating layer for electrical isolation. The capacitor sections (E1,E2) are mounted on the rings whose axial spacing determines the capacitor plate meshing while any peripheral displacement corresponds to the plate spacing.