Rotational Speed Measurement of a Metallic Rotor through Electret-assisted Electrostatic Sensing

The rotational speed monitoring of rotating machinery is required in many industrial processes. Electret marker tracking provides a new approach to improving the performance of electrostatic sensors in rotational speed measurement of metallic rotors. This paper proposes a technique for monitoring the rotational speed of a metallic rotor using an electrostatic sensing system with electret markers on the rotor. The impact of the geometric shape, size, number, and arrangement of electret markers on the sensing characteristics of the sensor are analysed through numerical simulation, leading to the optimisation of the design of the markers. Experimental tests were conducted with a metallic rotor with attached markers made from five types of material, including fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), polyimide (PI), polyethylene terephthalate (PET), and polyvinyl chloride (PVC). The performance of the measurement system is assessed in terms of accuracy, reliability, and rangeability. Experimental results indicate that, FEP electret exhibits superior charge affinity as markers than the other materials. The lower end of the measurement range is as low as 1 rpm (revolution per minute). The measurement system yields a relative error within ±3% over the range of 1 to 3000 rpm with a repeatability of 4.8%. While ensuring the minimum measurable speed (1 rpm), the upper limit of the distance between the rotor surface and the electrodes is 4 mm.