Time–frequency analysis of tribological systems—part I: implementation and interpretation

A novel technique adapting the time–frequency analysis has been utilized to characterize stationary and non-stationary signals from tribological interactions. This representation displays time, frequency, and signal magnitude to decipher signals emanating from such interactions. Short-time Fourier transform, Wigner, Coi–Williams, and Zhao–Atlas–Marks distributions are suited to represent stationary and non-stationary signals. Some of the most complex tribological phenomena involve head–disk interactions in magnetic recording systems. Examples drawn from practical head–disk interface tests are analyzed by using the fast Fourier transform algorithm to illustrate the dynamic features of various distributions. Time–frequency representation of output spectrums of laser doppler vibrometer (LDV), strain gage sensor, and acoustic emission (AE) sensor obtained from head–disk experiments giving evidence of stationary and non-stationary behavior are investigated.     

Contact-induced off-track vibrations of air bearing-slider-suspension system in hard disk drives

Contact-induced vibration of air bearing-slider-suspension system is a crucial issue for slider flying stability and head positioning precision of 1 Tbit/in² hard disk drives. In this paper, the contact-induced off-track vibrations of air bearing-slider-suspension system are investigated by simulation. A dynamic simulator is developed to calculate the interactions between the air bearing dynamics and vibrations of slider-suspension assembly. The simulation model consists of a finite element model of suspension assembly, an air bearing model based on the generalized lubrication equation, and a slider–disk contact model based on the probability distributions of surface roughness. A sequential method is used to couple all these models and analyses. The time history of the slider and suspension motions, together with the time-varying forces including air bearing force, air shear forces, contact force and friction force can be obtained. The effects of different contact conditions, such as the contact intensity, friction coefficient, and disk surface waviness on off-track vibrations are investigated numerically in details. The results reveal some mechanisms on how these factors contribute to the off-track vibrations of suspension assembly.     

Time–frequency analysis: a tool to discriminate artefacts from near-field optical data

Near-field optical data are non-stationary, which means that their spectral content varies with the position of the tip, due to both the scanning-probe recording process and the variations of the optical signal. Therefore time–frequency representations are potentially powerful tools for local characterization as they distribute the energy of the analysed signal over the time and frequency variables, and faithfully depict the signal local behaviour. In this paper, the time–frequency distributions are shown to be appropriate tools to analyse near-field optical data by using it first on simulated data, and second on experimental near-field optical images. Within this context, we observe that time–frequency analysis allows a possible separation of relevant optical signals from artefacts, especially in the usual case where the near-field optical signal is lower band than the feedback data.     

Nanolubrication: Characterization of patterned lubricant films on magnetic hard disks

Patterned lubricant films on magnetic hard disks offer potential advantages in controlled bonding sites, higher average shear strength, and longer durability. However, since the lubricant film thickness is at 1 or 2 nm, characterization of the pattern is difficult. Normal atomic force microscopic techniques can only image very small area in the nanometer range and the sharp tip can potentially modify the pattern. A wide area optical technique is needed to characterize the patterns. This paper examines patterned lubricant film using an optical surface analyzer (OSA) to image the bonded phase and mobile phase of an alcohol functionalized perfluoropolyether (PFPE) on magnetic hard disks. The phase shift signal and reflectivity intensity of the polarized light spectra provide clear optical images of the lubricant film at nanometer thickness. Optical images were successfully obtained before and after the buffing process and the ramp load and unload (L/UL) testing. Results of 100% bonded, 100% mobile, and 20% zigzag patterned lubricant films confirm that the patterned lubricant films can control the bonded/mobile ratio of such films better.     

Effect of relative humidity and disk acceleration on tribocharge build-up at a slider–disk interface

High performance disk drives require high spindle speed. The spindle speed of typical hard disk drives has increased in recent years from 5400 to 15000 rpm and even higher speeds are anticipated in the near future. The increasing disk velocity leads to increasing disk acceleration and slider–disk interaction. As the head-to-disk spacing continues to decrease to facilitate increasing recording densities in disk drives, the slider–disk interaction has become much more severe due to the direct contact of head and disk surfaces in both start/stop and flying cases. The slider–disk interaction in contact-start-stop (CSS) mode is an important source of particle generation and tribocharge. Charge build-up in the slider–disk interface can cause electrostatic discharge (ESD) damage and lubricant decomposition. In turn, ESD can cause severe melting damage to MR or GMR heads. We measured the tribocurrent/voltage build-up generated at increasing disk acceleration. In addition, we examined the effects of relative humidity on the tribocharge build-up. We found that the tribocurrent/voltage was generated during pico-slider/disk interaction and that its level was below 250 pA and 0.5 V, respectively. Tribocurrent/voltage build-up was reduced with increasing disk acceleration. Higher humidity conditions (75–80%) yielded lower levels of tribovoltage/current. Therefore, a higher tribocharge is expected at a lower disk acceleration and lower relative humidity condition.     

蝶形弹簧特性随机有限元分析

基于ANSYS商用有限元分析软件,建立了一蝶形弹簧的轴对称有限元模型,并分析了该蝶形弹簧在大变形条件下的非线性载荷-位移特性。有限元结果与理论分析结果吻合较好。在该有限元模型基础上讨论了基于ANSYS的Monte Carlo随机有限元分析技术确定碟簧最大应力分布规律的基本原理和方法。最后表明在外部载荷不变的情况下,蝶形弹簧内径和外径结构参数对碟簧内的应力分布影响最大,分析结果对于今后蝶形弹簧的工程应用有一定的参考价值。     

Sync-period frequency analysis and its application to the diagnosis of multiple element defects of rolling bearings

A method for diagnosing multiple element defects in rolling bearings has been investigated. The method combines the time-synchronous averaging and envelope spectral analysis techniques to produce spectra of synchronously averaged envelope signals with a range of synchronous frequencies. The spectra are displayed in the synchronous period versus frequency domain, to result in the sync-period versus frequency distribution. The distribution separates the characteristic defect frequencies and their associated sidebands in the synchronous period axis. This analysis technique makes it possible to detect and diagnose multiple defects appearing in different elements of rolling bearings. Another main benefit of the method is the significant noise reduction by both the enveloping and the synchronous averaging processes. Results from both computer synthesised data and experimental simulated data are presented.     

Instantaneous frequency of an arbitrary signal

This paper defines the non-negative pointwise instantaneous frequency (pIF) and pointwise instantaneous amplitude (pIA) of an arbitrary time signal to be the circular frequency and radius of curvature of the signal’s instantaneous trajectory on the complex plane consisting of the signal and its conjugate part from the Hilbert transform. One analytical and three computational methods are derived to prove and validate this concept. The analytical method is derived based on the definition of pIF and circle fitting. A five-point frequency tracking method is developed to eliminate the incapability of the original four-point Teager–Kaiser algorithm (TKA) for obtaining pIF of signals with moving averages. A three-point conjugate-pair decomposition (CPD) method is derived based on circle fitting using a pair of conjugate harmonic functions for frequency tracking. Moreover, the Hilbert–Huang transform (HHT) uses the empirical mode decomposition (EMD) to sift a signal’s instantaneous dynamic component from its sectional moving average (sMA) as the first intrinsic mode function, and then Hilbert transform is used to compute the first IMF’s frequency and amplitude as the sectional instantaneous frequency (sIF) and sectional instantaneous amplitude (sIA). Because finite difference is used in the five-point TKA, its accuracy is easily destroyed by noise. On the other hand, because CPD uses a constant and a pair of windowed regular harmonics to fit data points and estimate pIF and pIA, noise filtering is an implicit capability of CPD and its accuracy increases with the number of processed data points. Numerical simulations confirm that pIF and pIA are non-negative and physically meaningful and can be used for frequency tracking and accurate characterization of complex signals. However, sIF and sIA from HHT are more useful for system identification because the IMFs sifted by EMD often correspond to actual vibration modes.     

基于FLUENT的吸叶机风盘数值模拟分析

吸叶机是用于城市环境落叶的清扫设备,其核心的结构是吸叶系统,基于FLUENT数值模拟分析软件对吸叶系统的流道和风盘进行数值模拟分析,从速度、压力和效率等方面进行分析、对比,从而确定吸叶系统的风盘型样和结构参数,以进一步提高吸叶系统的工作性能,为产品的研发提供帮助和支持.     

减压阀阀瓣的数控加工工艺分析与编程

介绍了减压阀阀瓣的结构特点,论述了数控加工工艺的制定方法,给出了在卧式数控车床上加工阀瓣的程序。     

Interface technology of ultra-low flying height and highly stable head–disk interface for perpendicular magnetic recording

This paper reports authors’ efforts in slider and interface technologies with extremely small and very high stability head–disk spacing. The dual shallow step strategy is proposed in the femto form-factor slider design. It is found that the dual shallow step design is very effective in reducing flying height modulation (FHM) caused by disk waviness and enhancing the cooling effects on the read/write elements. A simple geometric model is built to explain the schematic of the improvement in FHM.     

Theoretical analysis and experimental verification on the static properties of externally pressurized air-bearing pads with load compensation

Load compensation can lead to externally pressurized bearings with infinite stiffness at the operating conditions. Calculations and experiments carried out on these bearings are discussed in this paper for two types of circular bearing pads with compliant surfaces. The mathematics is amply treated in appendices, and the numerical procedures are illustrated in diagrams. The experimental set-up is explained.     

重复轨迹运动迭代学习控制的频域分析

应用频域分析方法讨论了一类闭环迭代学习算法的收敛条件和性能,指出其比Arimoto开环迭代学习算法具有明显的优越性,并在讨论迭代收敛条件的基础上给出了闭环迭代学习算法的频域设计方法.     

Resonant frequency characterization of MEMS based energy harvesters by harmonic sampling analysis method

This paper presents a new and accurate experimental method based on harmonic distortion analysis to determine the resonant frequency of MEMS devices to be used as energy scavengers or more generally in widespread MEMS-based applications. This technique uses the mechanical–electrical analogy of MEMS variable capacitor acting as a low-pass filter to give access to both resonant frequency and damping factor of the mechanical system through the determination of the filter parameters as the cut-off frequency. Resonant frequencies ranging from 0.8 kHz to 5 kHz of electrostatic actuated MEMS-based harvesters have been measured by this technique with an uncertainty as low as a few parts in 10³ in a good agreement with measurements carried out by using Deep Level Transient Spectroscopy system.     

The influence of variation of viscosity with temperature on the steady state characteristics of journal bearings — simplified analysis

Investigation of the influence of temperature on journal bearing characteristics, through the strong dependence of viscosity on temperature, is rendered difficult by the complicated nature of the thermal boundary conditions, the flow in the cavitated region, etc. and by the computational requirements for the simultaneous solution of the Reynolds equation and the energy equation. It was found that the judicious use of the experimentally observed fact that the variation of temperature in the axial direction is negligible can drastically simplify the investigation without impairing its accuracy. The analysis is presented briefly, in dimensionless form, together with a note of caution on the erroneous use of isoviscous bearing characteristics for the estimation of the effective temperature of lubricant film.     

All-digital image capture and whole-field analysis of ciliary beat frequency

We hypothesized that a high‐speed all‐digital video imaging system, with computerized analysis, would precisely capture and measure ciliary beat frequency (CBF) and would shorten the time from data capture to data analysis. We compared a conventional analog video system with a new high‐speed digital system we developed for CBF analysis. Using ciliated primary bovine bronchial epithelial cells we made simultaneous analog and digital CBF measurements of the same region of interest (ROI) while temperature was varied. This yielded nearly identical data over a wide range of frequencies (7–15 Hz) using either system. Unlike the digital system however, the analog system did not accurately detect CBF above 15 Hz (temperatures higher than 30 °C). We also compared ROI analysis with a new analysis algorithm we have named whole‐field analysis (WFA). WFA measurement of CBF agreed with ROI and reduced operator time required to analyse data by more than 90% compared with the analog system. We conclude that all‐digital computerized CBF analysis correlates closely with standard video methods, markedly speeds up data analysis and provides new ways, including WFA, to analyse entire fields of motile cilia simultaneously. We have termed this system ‘Sisson–Ammons Video Analysis’ (SAVA).     

A precise scheme for detection of current transformer saturation based on time frequency analysis

Current Transformers (CTs) are prone to saturation due to large amplitude of fault current and existence of decaying Direct Current (DC) offset. Since the CT saturation leads to mal-operation of protective relays, detection and correction of saturated currents is one of the most important challenges in the power systems protection. In this paper, a new algorithm is presented for detection of CT saturation time interval by using the Improved S-Transform (IST). Simultaneously, IST can yield a complete visualization of signal in both time and frequency domains. An index based on instantaneous power of the CT secondary current is calculated using the IST output matrix. Then, the saturation region is estimated by determining the maximum and minimum values of the proposed index in the time domain. Comprehensive simulations are implemented using PSCAD/EMTDC software. Main parameters which have direct effect on the saturation levels are considered in simulation studies. As extremum points of the proposed index are determined for detection of saturation regions, the proposed method is not affected by CT parameters. The obtained results show that the proposed algorithm can precisely detect CT saturation time intervals even in noisy conditions.     

A novel precision positioning table utilizing impact force of spring-mounted piezoelectric actuator—part II: theoretical analysis

A simple mechanical vibration model was formulated to analyze the dynamic characteristics of the precision positioning table utilizing a spring-mounted piezoelectric actuator. The formulated model was validated by comparing the results of numerical simulations with the experimental data of Part I. The performance of the precision positioning table was predicted by changing the parameters of mass ratio and stiffness of spring. It is found that the analytic method described in this paper can provide an effective means in designing the precision positioning table utilizing a spring-mounted piezoelectric actuator.     

Accuracy analysis of the normalized frequency estimation of a discrete-time sine-wave by the average-based interpolated DFT method

The normalized frequency of a sine-wave can be estimated by the average of the two estimates achieved using the Interpolated Discrete Fourier Transform (IpDFT) method. This approach is called average-based IpDFT method. The paper investigates the effect of the spectral interference from the image component and additive wideband noise on the accuracy of the normalized frequency estimator provided by the average-based IpDFT method with maximum sidelobe decay windows in the case of a discrete-time sine-wave. The expressions of the maximum of the normalized frequency estimator error due to the spectral interference and variance of the normalized frequency estimator due to the wideband noise are derived. Also, the expression of the combined standard uncertainty of the normalized frequency estimator is derived. The accuracies of all derived expressions are confirmed by means of computer simulations. Moreover, the performance of the average-based IpDFT method is compared with those of a state-of-the-art three-point IpDFT method and IpDFT method through both computer simulations and experimental results.     

气体管道泄漏模态声发射时频定位方法

针对声发射信号频散特性导致基于时延估计的气体管道泄漏定位误差大的问题,提出一种基于模态声发射时频分析的泄漏定位方法。该方法采用平滑伪Wigner-Ville时频分布对两泄漏信号的互相关函数进行时频分析,利用互相关函数的时频谱可同时提取泄漏信号的时间延迟和与之对应的频率;然后根据泄漏声发射信号的主导模态的频散曲线即可确定该频率对应的声速,利用实时确定的声速和时间延迟并根据两传感器之间的距离即可确定泄漏点的位置。实验结果表明,采用时频分析的气体管道泄漏定位误差与互相关相比减少了6倍。所提出的模态声发射时频定位方法能有效抑制泄漏信号的频散,提高泄漏信号的相关性,从而更适合用于声发射管道泄漏定位。