Rotational Speed Measurement Based On Electrostatic Sensors

Rotational speed is a key parameter to identify the health status, make fault diagnosis and establish a necessary maintenance strategy for rotating machineries. Rotating devices are widely seen in many industrial sectors, such as energy, chemical, electric and metallurgical industries. In order to monitor the conditions of these devices, it is essential to obtain rotational speed measurement in real time with high accuracy. However, existing instruments for such purposes have limitations in terms of installation, accuracy, cost, reliability and so on. Therefore better instruments operating on new principles should be developed. Recent successful applications of electrostatic detection techniques to gas-solid two-phase flow measurement and exhaust gas monitoring in aero-engines have led to the rapid development of this technique for rotational speed measurement.This thesis describes the recent advances in rotational speed measurement based on strip-shaped electrostatic sensors. Signals that are used for the measurement are derived by detecting the electrostatic charge on the surface of the moving mechanical parts. Mathematical modelling, computer simulation, system design optimization and experimental studies are conducted to establish the sensing mechanism and quantify the characteristics of the electrostatic sensors. The novelty of the research programme that has been undertaken in this study includes the following aspects:(a) The electrostatic sensing technique is applied for the first time in the field of rotational speed measurement. Three different configurations of the rotational speed measurement system as per the number of electrostatic sensors and signal processing techniques are proposed in this research. These configurations include a single sensor, dual sensors and quadruple sensors in conjunction with autocorrelation and/or cross-correlation signal processing algorithms.(b) According to the physical model of the rotational speed measurement system, the mathematical model of the strip-shaped electrostatic sensor is established. The sensing characteristics, including spatial sensitivity, frequency response, spatial filtering length and signal bandwidth are investigated, which provide theoretical foundation for the performance analysis and optimal design of the electrostatic sensors. Experimental tests were conducted by simulating the spatial electric field due to a rotating ideal point charge. Through a direct comparison between the modelling results and experimental data, the validity of the mathematical model is verified and the sensor design is optimized.(c) The electrostatic sensing unit, signal conditioning circuit, correlation signal processing algorithm and data fusion algorithm of the measurement system are designed, implemented and evaluated. The measurement system presents and records the rotational speed, electrostatic signal amplitude and correlation coefficient in real time. Such data are used to analyze the reliability and stability of the measurement system.(d) The performance of the measurement system in three different configurations (based on a single sensor, dual sensors and quadruple sensors) is assessed respectively in terms of measurement range, accuracy, repeatability and stability. The effects of the distance between the electrode and the rotor surface, rotational speed, material type, rotor size and surface roughness of the rotor on the performance of the measurement system are studied experimentally.