1、Mechanical Systems and Signal Processing, 2007, 21: 11151126A study of hydraulic seal integrityP. Chena, P.S.K. Chua, G.H. LimAbstract:The work described in this paper involved on-line detection of seal defects in a water hydraulic cylinder. An obvious effect of seal defect is internal leakage.There

2、fore,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 in terms of their capability in estimating the internal leakage rate

3、 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 magnitude of power spectral density and the energy.The correlations betwe

4、en 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.Keywords: Acoustic emission; Water hydraulic cylinder; Internal leakage; AE co

5、unt rate; Root mean square; Power spectral density; AE energy1. IntroductionModern water hydraulics, using tap water as the hydraulic fluid, has gained much interest in the past decade due to its inherent advantages compared to oil hydraulics. These advantages include environment friendliness,good p

6、roduct 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 components. For example, a water hydraulic cylinder could suffer from internal leak

7、age 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 and safe operations of water hydraulic systems.The work presented in this p

8、aper 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 order to model the AE signal generated by the internal leakage, suitable param

9、eters must first be selected to interpret the signal. Therefore, experiments were conducted to study the characteristics of various AE parameters in terms of their effectiveness in estimating the internal leakage rate,as described in this paper.2. Acoustic emissionAE is defined as the transient elas

10、tic 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. Pollock and Hsu 10 studied the physical origin of these signals in detail and Goodman et al. 12 reported a variety of AE source mechanisms associat

11、ed with leakage from vessels, tanks and pipelines. In the case of internal leakage in water hydraulic cylinders, the generation of AE signals is largely attributed to the turbulence induced by the internal leakage. AE signals can be categorised into two basic types. The burst-type AE refers to AE si

12、gnals corresponding to individual AE events, while the continuous-type AE refers to an apparently sustained signal level from rapidly occurring AE events 16. AE signals generated by internal leakage in water hydraulic cylinders are of continuous type, as shown in Fig. 1. AE counts are widely used as

13、 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 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 cont

14、ent of AE signals. For an AE signal consisting ofx 0， x 1, , x N1 , its rms value isThe advantage 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 ha

15、ve been used to describe AE signals in a variety of applications 11,17,18.The aforementioned parameters 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 c

16、ontained within frequency bands. For the continuous-type AE, these parameters can be obtained 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

17、 Fourier transform (DFT) of an AE signal xn, andT is the sampling period. The PSD represents 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. ExperimentationDue to the complexity of AE phenomena, analytical

18、 methods are not well established. Therefore,experimental methods are introduced to investigate 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,

19、 the AE count rate, denoted as _N AE was calculated by dividing the AE counts by the signal 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 thre

20、shold, a value of 0.04V yielded the best results, as shown in Fig. 6a. It is noted that 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

21、 at a constant level, no provide a desirable simulation of the dynamic processes existing in a cylinder subject to internal leakage. Thusin the present work, efforts have been made to simulate the real internal leakage in hydraulic cylinders. In the following, the leakage mechanism is first studied;

22、 then, the simulation of the leakage is presented.In order to simulate scores created by the abrasive action of solid particulates, a file was used, in the presentwork, to make scores on the piston seal surfaces of a water hydraulic cylinder. Fig. 2 shows the scored pistonseals used in the experimen

23、ts. These seals lead to an internal leakage smaller than 1.0 L/min for the pressure range of 070 bar. Sixteen scores were equally distributed along the circumference of the seals. The dimensions of these scores were measured with a non-contact optical measurement system. Fig.3shows the profile of a

24、score taken by the measurement system. Along the edge of the score, five key points were selected and their coordinates were measured. The width and depth of the score were then measured. In addition, a circular arc fit to these five points was calculated. Thus, an approximate radius of the score co

25、uld be obtained.Fig. 2. The 16-score piston seals.Fig. 3. The profile of a score.4. Experimental resultsIn the experiment, 100 sets of data were acquired at different internal leakage rates, with each set consisting of 40 records of AE signals measured at a certain leakage rate. Each record of AE si

26、gnal contained 4096 points sampled at 5 MHz, from which AE parameters were calculated. For each AE parameter, results obtained from the 40 records were then averaged. In the following, all the results are the average values.For each record of AE signal, the AE count rate, denoted as _N AE, was calcu

27、lated by dividing the AE counts by the signal 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 resul

28、ts, as shown in Fig. 4a. It is noted that the AE count rate drops fast as the internal leakage rate decreases. Fig. 4. AE count rate versus internal leakage rateFor the floating threshold, the threshold value was set to be proportional to the rms value of the signal. The resulting AE count rate rema

29、ined at a constant level, nom atter how the leakage rate varied. This is shown in Fig. 4b, where the AE count rate was obtained with the threshold equal to the rms value of the signal. It can be seen that there is no desirable trend in the AE count rate with respect to the leakage rate.5. Predict th

30、e internal leakage rateAs has been shown in the above, the energy content of AE signal is closely related to the internal leakage rate in the water hydraulic cylinder. Therefore, it may be used to predict the internal leakage rate. The error of prediction, then, is of interest. In the following, an

31、empirical model is built to predict the internal leakage rate based on measured AE signals and the error of prediction is analysed with statistical methods. Due to the simplicity in calculation, the rms value Vrms is chosen instead of the energy Ef to characterise AE signals. From the previous exper

32、imental data, the relationship between the AE rms value Vrms and the internal leakage rate Qi is obtained using the least squares method, given byQi=7.86Vrms+0.14.For a measured AE rms value, the internal leakage rate may be predicted with Eq.(7). Suppose the measured AE rms value is Vrms0. A 95% pr

33、ediction interval for the true value of the internal leakage rate,denoted asQi0, is given bywhere Qi is the internal leakage rate predicted by Eq. (7) based on the measured Vrms0 and d is a measure of the width of the prediction interval. Note that d is not a constant but varies with the measured AE

34、 rms value Vrms0. For the range of the internal leakage rates smaller than 1.0 L/min, d is about 0.078 L/min. Eq. (8) means that for the measured AE rms value Vrms0, the true value of the internal leakage rate Qi0 lies inside the intervale Qi d; Qi t dT with 95% confidence. 6. ConclusionsThis paper

35、analysed the characteristics of AE signals generated by internal leakage in a water hydraulic cylinder. Experiments were carefully designed, including the simulation of the internal leakage across the piston seals in a water hydraulic cylinder and the measurement of the internal leakage rate. AE sig

36、nals obtained from the experiments were analysed, in which several AE parameters were extracted from the AE signals and their effectiveness for predicting the internal leakage rate were studied.From the analysis results,some conclusions can be made, as follows:(1) AE signals are sensitive to small i

37、nternal leakage in a water hydraulic cylinder and AE-based methods are able to predict the internal leakage that is smaller than 1.0 L/min.(2) Energy-based AE parameters, whether measured in the time domain or in the frequency domain, are more suitable than the AE count rate and the peak PSD magnitu

38、de to interpret AE signals generated by the internal leakage.References1 G.W. Krutz, P.S.K. Chua, Water hydraulicstheory and applications 2004, in: Proceedings of the Workshop on Water Hydraulics,Agricultural Equipment Technology Conference (AETC 04), Louisville, KY, USA, February 810, 2004.2 E. Tro

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40、t Diagnosis in Fluid Power Systems, Ellis Horwood, New York, USA, 1992.5 T.T. Le, J. Watton, D.T. Pham, An artificial neural network based approach to fault diagnosis and classification of fluid powersystems, Proceedings of the Institution of Mechanical Engineers, Part I, Journal of Systems and Cont

41、rol Engineering 211 (1997)307317.6 T.T. Le, J. Watton, D.T. Pham, Fault classification of fluid power system using a dynamics feature extraction technique and neuralnetworks, Proceedings of the Institution of Mechanical Engineers, Part I, Journal of Systems and Control Engineering 212 (1998)8797.7 G

42、. Thompson, G. Zolkiewski, An experimental investigation into the detection of internal leakage of gases through valves byvibration analysis, Proceedings of the Institution of Mechanical Engineers, Part E, Journal of Process Mechanical Engineering 211(1997) 195207.8 M. Pietola, R. Ma kinen, P. Va yr

43、ynen, S. Kesanto, J. Varrio, Using a high resolution thermograph in predictive maintenance andfault diagnosis of fluid power components and systems, in: Proceedings of the Fourth Scandinavian International Conference onFluid Power, Tampere, Finland, September 2629, 1995, pp. 719725.機械系統與信號處理, 2007,

44、21: 11151126液壓密封完整性調查研究P. Chena, P.S.K. Chua, G.H. Lim摘要：本文中所涉及在液壓缸的上線檢測密封缺陷. 一個明顯的影響密封的缺陷是內部泄漏。因此，所采用的辦法是使用合適的技術探測內部泄漏. 所采用的技術涉及由于檢測聲發射（ AE ）內部泄漏.本文評估了AE信號的各種各樣的參量,根據他們估計液壓缸內部漏出率。實驗分析了AE參量不同的內部漏出率，參量包括根均方(rms)值，計數率、繁忙程度功率譜密度和能量的特征。分析了這些參量和內部漏出率之間的交互關系。結果表示，基于能量的AE參量，特別是均方根值，是更加適當解釋內部漏出引起的AE信號。關鍵詞：聲

45、發射；液壓缸；內部泄漏；聲發射計數率；均方根功率譜密度；AE能量1 引言現代水利使用自來水作為液壓油，在過去幾十年中，由于液壓油其固有的優勢相比。這些優勢包括環境，友善，良好的產品兼容性，并沒有發生火警的危險。但是，一些問題，與現代水力仍有待解決。其中最常見的問題是比較大的內部滲漏水液壓元件。舉例來說，一個水上液壓缸可能遭受橫跨活塞封印的內部漏出。這歸結于非常低粘度的水力液壓油。因此，重要的是要監測內部漏出以達到最佳性能和水液壓機構的可靠和安全。本文提出的是打算開發一個定量模型通過AE估計在水液壓缸的內部漏出流速項目的一部分。它集中于內部漏出小于1.0 L /min。為了塑造AE發信號引起由內

46、部漏出，適當的參量必須首先選擇解釋信號。所以，試驗根據他們的在估計內部漏出率的有效率做了各種各樣的AE參量的特征，正如本文所描述。2. 聲發射AE被定義作為由迅速能量的釋放從局部性引起的瞬變彈性波。AE信號產生液體泄漏。可。波洛克和古德曼等人詳細研究了這些信號的物理起源，并且Goodman等報告了各種各樣的AE來源機制與從船、坦克和管道的漏出相關。在液壓缸的內部漏出情況下，AE信號主要歸因于內部漏出導致的動蕩。聲發射信號可分為兩種基本類型。爆裂型聲發射是指聲發射信號所對應的個人聲發射事件，而連續型聲發射指的顯然是持續的信號水平迅速發生聲發射活動。聲發射信號的產生是由內部滲漏水液壓缸連續型，如圖

47、1所示。圖1：AE信號所產生的內部滲漏水液壓缸一般來說，直接觀察的連續聲發射信號很少關于AE信號源資料。為了提取更多有用的信息，由聲發射信號，首先應該適當地解釋信號，通常涉及描繪他們與有些參量。為定量AE調查，任何一個數學模型進行之前參數必須加以界定，。各項參數已用于AE信號特性，無論是在時域和頻域。以下簡單地描述有些用途廣泛的參量連續式AE發信號。AE計數用途廣泛，是作為AE活動一項實用措施。當次數信號超出逆門限，這個參量被定義。對于連續式AE， AE計數率是常用的測量AE計數的變異與時間的。均方根(rms)是常用的測量AE信號能量內含。對于包括N樣品的AE信號，它的rms值 x 0， x

48、1， x N1其有效值為（1）能量測量的優點是AE信號的能量內含可以直接地與重要物理參量釋放能量。上述參量被廣泛應用于描述AE信號。上述的參量在時間界域被測量。其外，在頻域測量的參量也是利益，例如在頻帶內和巨大包含的統治頻率組分和能量的頻率。使用傅立葉變換，對于連續式AE，這些參量可以通過光譜分析得到。功率譜密度（ PSD ）的聲發射信號，可使用以下公式連續計算(2)其中P k 是功率譜密度，X k是分離傅立葉變換(DFT) AE信號x n，T是取樣周期。PSD的代表分布的信號功率超過頻率。AE信號的有些研究在頻域的可以在Refs找到。3. 實驗由于AE現象的復雜，沒有固定得分析方法。所以，介

49、紹實驗法調查AE。為了學習AE信號的特征在液壓缸的內部漏出引起的，實驗設計，如下所述：在液壓缸的內部漏出通過連接流量控制閥模仿了與圓筒平行。人工介紹的內部漏出流經了閥門而不是液壓缸。這種的優點是模仿漏出率可能容易地是受控的。然而，模仿的有效性要求進一步調查。相信這種方法不可能提供存在于圓筒的動態過程的中意的模仿受內部漏出支配。因此，在當前工作，努力被做了模仿在液壓缸的真正的內部漏出。首先研究下面漏出機制然后，然后提出漏出的模仿。為了模仿堅實微粒物質的磨蝕行動創造的比分，文件在當前工作在活塞被用于，做密封水液壓缸的表面。圖2顯示用于實驗的被計分的活塞封印。這些封印帶領內部漏出070酒吧的壓力范圍的小于1.0升/分鐘。16個得分沿封印的圓周平等地被分布了。維度這些比分測量了與一個沒有接觸的光學測量系統。圖3顯示測量系統采取的比分的外形。沿比分的邊緣，五個關鍵被選擇了，并且測量了他們的座標。然后測量了得分