Effects of material type and surface roughness of the rotor on the electrostatic sensing based rotational speed measurement

Rotational speed is a key parameter in the condition monitoring of rotating machineries such as generators, electromotors, centrifugal and machine tool spindles. The fundamental principles and preliminary results of rotational speed measurement using electrostatic sensors have been reported previously. However, it is not known how the material type, size and surface roughness of the rotor affect the measurement technique in term of accuracy and reliability. This paper focusses on quantifying the effects of material type, size and surface roughness of the rotor on the performance of the rotational speed measurement system based on a single electrostatic sensor. Rotors made of different materials including Nylon, polyvinyl chloride (PVC) and polytetrafluoroethylene (PTFE) are assessed on a purposed-built test rig under a range of conditions. For each type of rotor, different sized rotors (60 mm and 120 mm in diameter) with two different surface roughnesses (Ra 3.2 and Ra 6.3) are evaluated. Experimental results suggest that the measurement system has a relative error of no greater than ±1.5% for all the tested rotors. Meanwhile, the system performs better on the PTFE rotors with a larger size and coarser surface for the same rotational speed.