Rotational Speed Measurement Using Single and Dual Electrostatic Sensors

Rotational speed is a key parameter for condition monitoring and control of rotating devices in many industrial processes. This paper presents a technique for the rotational speed measurement using a single or dual electrostatic sensors coupled with correlation signal processing algorithms. In the case of a single sensor, autocorrelation algorithm is applied to process the signal and measure the period of the rotational motion. In the case of dual sensors, cross correlation algorithm is applied to obtain the time delay between the two signals and hence the measurement of rotational speed. The fundamental characteristics of the sensing techniques with single or dual sensors for different sized rotors are studied through finite-element modeling. Experimental tests were conducted on a purpose-built test rig to assess the performance of both techniques over the speed range of 100-3000 r/min. The rotors used in the experimental tests are made of polyvinyl chloride with a diameter of 60 and 120 mm, respectively. Experimental results suggest that the measurement system using a single sensor is capable of producing repeatable rotational speed measurement with a maximum error of ±1.2% over the speed range of 600-3000 r/min. However, the measurement system with dual electrostatic sensors is capable of achieving valid measurements over a wider range of speeds (100-2000 r/min), although the measurement error is larger than that of the single-sensor system. Both techniques perform better with a larger rotor under a higher rotational speed.