1、机电 工程学院 机械系机械设计制造及其自动化班 学生 毛祖庆 毕业设计 第 1 页 共 13 页 井冈山大学外文翻译用纸 Mechanical Systems and Signal Processing, 2007, 21: 11151126 A study of hydraulic seal integrity P. Chena, P.S.K. Chua, G.H. Lim Abstract: The work described in this paper involved on-line detection of seal defects in a water hydraulic cyl
2、inder. An obvious effect of seal defect is internal leakage. Therefore, the approach used was to detect the internal leakage using suitable technique. The technique used involved detecting the acoustic emission (AE) due to the internal leakage. This paper evaluated various parameters of AE signals i
3、n terms of their capability in estimating the internal leakage rate in a water hydraulic cylinder. Experiments were carried out to study the characteristics of AE parameters at different internal leakage rates, the parameters including the root-mean-square (rms) value, the count rate, the peak magni
4、tude of power spectral density and the energy. The correlations between these parameters and the internal leakage rate were analysed carefully. The results show that energy-based AE parameters, especially the rms value, are more suitable to interpret AE signals generated by internal leakage. Keyword
5、s: Acoustic emission； Water hydraulic cylinder； Internal leakage； AE count rate； Root mean square； Power spectral density； AE energy 1. Introduction Modern water hydraulics, using tap water as the hydraulic fluid, has gained much interest in the past decade due to its inherent advantages compared to
6、 oil hydraulics. These advantages include environment friendliness, good product compatibility and no fire hazards 1, 2. However, some problems with modern water hydraulics are still to be addressed. One of the most common problems is the relatively large internal leakage in water hydraulic componen
7、ts. For example, a water hydraulic cylinder could suffer from internal leakage across the piston seals. This is due to the very low viscosity of water in comparison with that of hydraulic oil 1, 3. Therefore, it is important to monitor the internal leakage to achieve optimal performance and reliable
8、 and safe operations of water hydraulic systems. The work presented in this paper is part of a project that aims to develop a quantitative model to estimate the internal leakage flow rate in a water hydraulic cylinder by means of AE. It is focused on the internal leakage smaller than 1.0 L/min. In o
9、rder to model the AE signal generated by the internal leakage, suitable parameters 机电 工程学院 机械系机械设计制造及其自动化班 学生 毛祖庆 毕业设计 第 2 页 共 13 页 井冈山大学外文翻译用纸 must first be selected to interpret the signal. Therefore, experiments were conducted to study the characteristics of various AE parameters in terms of thei
10、r effectiveness in estimating the internal leakage rate, as described in this paper. 2. Acoustic emission AE is defined as the transient elastic waves that are generated by the rapid release of energy from localised sources. It has been found that AE signals can be generated by fluid leakage. Polloc
11、k and Hsu 10 studied the physical origin of these signals in detail and Goodman et al. 12 reported a variety of AE source mechanisms associated with leakage from vessels, tanks and pipelines. In the case of internal leakage in water hydraulic cylinders, the generation of AE signals is largely attrib
12、uted to the turbulence induced by the internal leakage. AE signals can be categorised into two basic types. The burst-type AE refers to AE signals corresponding to individual AE events, while the continuous-type AE refers to an apparently sustained signal level from rapidly occurring AE events 16. A
13、E signals generated by internal leakage in water hydraulic cylinders are of continuous type, as shown in Fig. 1. AE counts are widely used as a practical measure of AE activity. This parameter is defined as the number of times the signal exceeds a counter threshold. For continuous-type AE, AE count
14、rate is often used to measure the variation of AE counts with time. The root-mean-square (rms) value is often used to measure the energy content of AE signals. For an AE signal consisting ofx 0， x 1, , x N1 , its rms value is 机电 工程学院 机械系机械设计制造及其自动化班 学生 毛祖庆 毕业设计 第 3 页 共 13 页 井冈山大学外文翻译用纸 The advantage
15、 of energy measurement is that the energy content of the AE signal can be directly related to important physical parameters associated with the energy release at the AE source 14. The above parameters have been used to describe AE signals in a variety of applications 11, 17, 18. The aforementioned p
16、arameters are measured in the time domain. Besides, parameters measured in the frequency domain are also of interest, such as the frequency and magnitude of the dominant frequency component and the energy contained within frequency bands. For the continuous-type AE, these parameters can be obtained
17、through spectral analysis using Fourier transform. The power spectral density (PSD) of AE signals can be computed using the following equation 19: where P k is the power spectral density, X k is the discrete Fourier transform (DFT) of an AE signal xn, andT is the sampling period. The PSD represents
18、the distribution of the signal power over frequencies. Some studies of AE signals in the frequency domain can be found in Refs. 10, 13, 20, 21. 3. Experimentation Due to the complexity of AE phenomena, analytical methods are not well established. Therefore, experimental methods are introduced to inv
19、estigate AE. In order to study the characteristics of AE signals generated by internal leakage in water hydraulic cylinders, experiments were deliberately designed, as described below. For each record of AE signal, the AE count rate, denoted as _N AE was calculated by dividing the AE counts by the s
20、ignal duration. Both a fixed threshold and a floating threshold were used for counting. Since there was no well-defined procedure to choose the threshold value, a wide range of values were tried. For the fixed threshold, a value of 0.04V yielded the best results, as shown in Fig. 6a. It is noted tha
21、t the AE count rate drops fast as the internal leakage rate decreases. For the floating threshold, the threshold value was set to be proportional to the rms value of the signal. The resulting AE count rate remained at a constant level, no provide a desirable simulation of the dynamic processes existing in a cylinder subject to internal leakage. Thus in the present work, efforts have been made to simulate the real internal
