Integrated capacitive measurement of air gap height in aerostatic bearings

Measurement of air gap height in aerostatic bearings is often necessary, for example, in closed-loop position control of precision stages. The air gap height can be measured directly with distance sensors, or indirectly from pressure in the bearing gap when the performance is well known.
The present study investigated an air gap height measurement method for aerostatic bearings using an integrated capacitive sensor. The method was investigated experimentally with a thrust bearing. The structure of the bearing was made from conductive material which was used for one of the electrodes for the capacitive sensor. The second electrode, in this case, was the steel guide
surface of the bearing. Thus, a plate capacitor was formed between the steel guide surface and the graphite restrictor, where the air gap is the dielectric medium. The distance between the two plates in a plate capacitor is inversely proportional to its capacitance. Therefore, measurement of the air gap between the bearing and the guide surface is possible.
The integrated capacitive sensor consisted of a modified aerostatic bearing and a measuring circuit. The circuit consisted of a Wien bridge oscillator and an LC-tank in which the aerostatic bearing acted as the capacitor. Current through the LC-tank was measured using a resistor and an amplifier. The measurement results of the proposed method were compared to measurements obtained using
an external gap-height sensor in a static test bench. The results show corroborative evidence on the feasibility of the proposed
method.