【正文】 l 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 continuoustype AE, AE count rate is often used to measure the variation of AE counts with time. The rootmeansquare (rms) value is often used to measure the energy content of AE signals. For an AE signal consisting ofx [0]， x [1], …… , x [N?1] , its rms value is 機電 工程學院 機械系機械設(shè)計制造及其自動化班 學生 毛祖慶 畢業(yè)設(shè)計 第 3頁 共 13頁 井岡山大學外文翻譯用紙 The 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 have 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 ponent and the energy contained within frequency bands. For the continuoustype AE, these parameters can be obtained through spectral analysis using Fourier transform. The power spectral density (PSD) of AE signals can be puted 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 x[n], 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. Experimentation Due to the plexity of AE phenomena, analytical 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, 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 welldefined procedure to choose the threshold value, a wide range of values were tried. For the fixed threshold, a value of 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 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 機電 工程學院 機械系機械設(shè)計制造及其自動化班 學生 毛祖慶 畢業(yè)設(shè)計 第 4頁 共 13頁 井岡山大學外文翻譯用紙 leakage in hydraulic cylinders. In the following, the leakage mechanism is first studied。 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 present work, to make scores on the piston seal surfaces of a water hydraulic cylinder. Fig. 2 shows the scored piston seals used in the experiments. These seals lead to an internal leakage smaller than L/min for the pressure range of 0–70 bar. Sixteen scores were equally distributed along the circumference of the seals. The dimensions of these scores were measured with a noncontact optical measurement system. shows the profile of a score taken by the measurement system. Along the