Capacitance type non-contact displacement and vibration measuring device

Abstract

A new capacitance type non-contact displacement and/or vibration measurement instrument sensing head is described together with a novel method of maintaining calibration of the instrument using such sensing head. The invention takes the form of a central probe member having a first guard ring disposed in coaxial, electrically insulated relationship about the central probe member and driven at the same potential and frequency as the probe member. A secondary driven guard ring coaxially surrounds the first guard ring and is electrically isolated from it and is driven at the same alternating current frequency but at a higher voltage of the order of 1.5 to 1 times higher. The additional secondary guard ring, driven at a suitably boosted voltage level and the same frequency as the central probe member, serves to focus the otherwise divergent electric field, thus producing a very significant improvement in the output linearity when operated at relatively large displacements. Experiments have shown that a probe in accordance with the invention of 1 inch overall diameter can now be used to measure displacements up to approximately 1 inch, whereas previously a probe of the similar outside diameter could be used to measure displacements of only approximately 1/10 of an inch, unless some form of electronic linearization of its output was employed. With the improved sensing head assembly of this invention calibration of the instrument can be more easily maintained by simply removing the probe from a measurement position, applying a ground potential to the secondary guard ring and adjusting the output of the instrument to a fixed reference value to thereby recalibrate the instrument.