Optimization of electrostatic sensors for rotational speed measurement of a metallic rotor

Previous studies have demonstrated that it is feasible to apply the electrostatic sensing technique for speed monitoring of non-metallic rotating machinery. The attachment of electrostatic markers makes it possible to measure the rotational speed of metallic rotors with electrostatic sensors. The geometric shape and size of the electrodes and their spacing and distance to the rotor surface have a significant influence on the performance of electrostatic sensors. This paper presents a scheme for the optimization of electrostatic sensors applied in the rotational speed measurement of a metallic rotor. Through computational modelling, fundamental characteristics of the electrostatic sensor including spatial sensitivity, output response and frequency property are analyzed, then the optimal range of electrode parameters is obtained. An optimized sensor with double strip-shaped electrodes, is used to measure the rotational speed of a metallic rotor with a triboelectric marker attached. Experimental results indicate that, the electrostatic sensor coupled with correlation signal processing algorithms enables repeatable speed measurement of a metallic rotor, and the rangeability has been significantly extended. The system is capable of measuring the rotational speed as low as 30 rpm (revolution per minute) with a relative error within ±3.4% over the range of 30 to 120 rpm and within ±0.12% over the range of 120 to 3000 rpm.