Surface Profilometry Based on Fringing Capacitance Measurement

A rugged, compact sensor for in-process measurement of surface finish profiles is proposed. The concept is based on the detection of variations in the fringe electric field generated between the surface and a perpendicular, ceramic-encased electrode. An analysis of both static and spatial dynamic characteristics of this “capacitive profilometer” is presented. For some surfaces, the device acts as a low-pass spatial filter, attenuating the higher spatial frequencies. A method of dynamic compensation, based on Fourier transform techniques, is developed. Experimental results demonstrate the validity of the analysis and dynamic compensation. Direct comparisons of surface profiles measured with the capacitive and standard stylus-type are presented and show good agreement.