A relative torsional vibration monitoring method for intelligent ships and associated error analysis

Abstract

In this era of rapidly developing transport technology, shipping accounts for two thirds of the total international trade and transport industry. Nowadays, 'ship intelligence' is an important avenue to pursue in the development of the shipping industry and it requires ships and their working machinery to undergo real-time and high-precision inspection and diagnoses of their operational state. This paper develops a simple method for monitoring relative torsional vibration using multiple measuring points, which is suitable for the monitoring of intelligent ship rotating machinery. A proof-ofprinciple prototype is also designed to verify whether this calculational method can achieve clearer and more accurate physical significance and engineering practicability compared with the traditional single-point measuring method. According to the test results, under the condition of 100 pulses per revolution and at a speed of less than 3000 r/min, the relative error of speed measurement of the proof-of-principle prototype is within ±0.011%, which is close to the theoretical maximum accuracy of a 54 MHz sampling rate. The measurement result for the relative torsional angle shows that the maximum error is less than 0.17 mrad, the relative error is less than 1.75% and the maximum error in the amplitudes of the first and third harmonics is 9 mdeg. These accuracies are higher than those of the traditional method, which are 2.62% and 12 mdeg, respectively, in this study. Under the condition of speed fluctuations and rolling vibration, distortion occurs in the time-domain and frequency-domain torsional angle curves when the single-point measuring method is adopted. With the multi-point measuring method, much more accurate measurement values of the torsional angle are acquired compared with the single-point measuring method and accurate time-domain and frequency-domain curves are also obtained. Finally, the health status of the torsional angle is evaluated using an intelligent dynamic alarm system with a torsional vibration threshold, which has a better evaluation effect in line with the engineering practice and specification requirements. Therefore, this simple multi-point relative torsional vibration inspection method has universal applicability and wide application value.