Advanced methods for repeatable runout compensation [disc drives]

This paper presents and compares experimental results from two types of periodic disturbance compensation methods. The repeatable runout (RRO) cancellation techniques studied in this paper are adaptive feedforward cancellation (AFC) and repetitive control. Two modifications (phase advance and a feedthrough term) to the basic AFC structure are also studied experimentally. Of the AFC methods, the feedthrough technique is superior, but the repetitive controller provides better RRO rejection. Overall it is found that the removal of repeatable runout improved the tracking precision by as much as 53%     

Phase measurement of distorted periodic signals based onnonsynchronous digital filtering

Measurement of the phase angle between fundamental components of distorted periodic signals at power frequencies is described. It is based on a nonsynchronous multirate digital filtering algorithm, which is applied to the voltage and/or current signals to be measured. The digital filters implemented are designed to suppress the higher harmonics and ensure the accuracy of measurement. The measurement method is insensitive to frequency variations in a wide frequency band around the nominal frequency. For a frequency band of ±10% around the nominal frequency, this method can resolve phase differences of less than 5 μrad with an uncertainty of less than ±25 μrad     

Adaptive feedforward cancellation of sinusoidal disturbances in superconducting RF cavities

A control method, known as adaptive feedforward cancellation (AFC), is applied to damp sinusoidal disturbances due to microphonics in superconducting radio frequency (SRF) cavities. AFC provides a method for damping internal and external sinusoidal disturbances with known frequencies. It is preferred over other schemes because it uses rudimentary information about the frequency response at the disturbance frequencies, without the necessity for an analytic model (transfer function) of the system. It estimates the magnitude and phase of the sinusoidal disturbance inputs and generates a control signal to cancel their effect. AFC, along with a frequency estimation process, is shown to be very successful in the cancellation of sinusoidal signals from different sources. The results of this research may significantly reduce the power requirements and increase the stability for lightly loaded continuous-wave SRF systems.     

Multirate asynchronous sampling of sparse multiband signals

Because optical systems have a huge bandwidth and are capable of generating low-noise short pulses, they are ideal for undersampling multiband signals that are located within a very broad frequency range. We propose a new scheme for reconstructing multiband signals that occupy a small part of a given broad frequency range under the constraint of a small number of sampling channels. The scheme, which we call multirate sampling (MRS), entails gathering samples at several different rates whose sum is significantly lower than the Nyquist sampling rate. The number of channels does not depend on any characteristics of a signal. In order to be implemented with simplified hardware, the reconstruction method does not rely on the synchronization between different sampling channels. Also, because the method does not solve a system of linear equations, it avoids one source of lack of robustness of previously published undersampling schemes. Our simulations indicate that our MRS scheme is robust both to different signal types and to relatively high noise levels. The scheme can be implemented easily with optical sampling systems.     

Generation of carrier-envelope phase-controlled isolated attosecond pulses using chirped femtosecond excitation lasers

ABSTRACT

We present an efficient approach for producing a carrier-envelope phase controlled isolated attosecond pulse by an optimized intense driving laser pulse. High-order harmonics are produced by numerically solving the time-dependent Schrödinger equation for the one-dimensional hydrogen atom in an ultrashort laser pulse. We define an efficient cost function to optimize the laser pulse by a genetic algorithm scheme. Our approach produces single attosecond pulses with desired properties, including the carrier-envelope phase, central frequency, and duration. Also, we analyze the time–frequency profiles of the attosecond emissions to gain a deeper insight into the underlying physical mechanism.     

Reduced harmonic representation for continuous wave, shock-producing focused beams

An extension to the frequency domain solution to Burgers' equation (FDSBE) is presented in the context of a continuous wave, nonlinear beam propagation model. The extension adds a specified number of harmonics to each FDSBE nonlinear substep in the shock-bearing region. The extension harmonics have amplitudes given by a 1/n amplitude decay assumption. The corresponding phases are given by linear extrapolation. The extension scheme allows for stable and accurate, reduced harmonic focal beam modeling. An algorithm is also presented that allows this scheme to provide accurate heating rate and radiation force output.     

Measurement of higher harmonics in periodic vibrations using phase-modulated TV holography with digital image processing

We separately measure the higher harmonics vibration patterns of a periodic vibrating object by using time-average TV holography and phase modulation. During measurements the frequency of the phase modulation is adjusted to each harmonic component while the excitation of the object is set low enough to record all components on the linear part of the fringe function. Using acoustical phase stepping and calibration of the fringe function, we compute the amplitude and phase distributions of the frequency component. We measure components up to the 65th harmonic by using square-wave excitation.     

Demodulation of Fiber-Optic Sensors for Frequency Response Measurement

The neural-network-based processing of extrinsic Fabry-Perot interferometric (EFPI) strain sensors was investigated for the special case of sinusoidal strain. The application area is modal or cyclic testing of structures in which the frequency response to periodic actuation must be demodulated. The nonlinear modulation characteristic of EFPI sensors produces well-defined harmonics of the actuation frequency. Relationships between peak strain and harmonic content were analyzed theoretically. A two-stage demodulator was implemented with a Fourier series neural network to separate the harmonic components of an EFPI signal and a backpropagation neural network to predict the peak-to-peak strain from the harmonics. The system performance was tested using theoretical and experimental data. The error for high-strain cases was less than about 10% if at least 12 harmonics were used. The frequency response of an instrumented cantilever beam provided the experimental data. The demodulator processing closely matched the actual strain levels     

A Time-Domain Approach for the Analysis of Nonstationary Signals in Power Systems

Voltage and current waveforms produced by several power systems are often characterized by a wide spectrum that contains numerous components, namely the fundamental, harmonics, and interharmonics. They can be properly analyzed through discrete Fourier transform (DFT)-based methods only if they remain stationary in time; in fact, in the presence of nonstationary components, DFT-based approaches do not grant reliable results because of the absence of a well-defined periodicity interval. The digital signal processing method proposed in this paper aims at analyzing a large category of nonstationary signals observed in power systems. It is capable of analyzing waveforms produced by power systems that seemingly operate in steady-state conditions, in which time-varying components represent anomalies, as well as waveforms generated by some power apparatuses, which supply nonstationary waveforms by design. The wide range of applicability of the proposed method is granted by a suitable processing scheme, including distributed filtering and phase unwrapping operations. Distributed filtering operations are performed to distinguish and separately analyze the major components contained in complex waveforms. Phase unwrapping operations are needed in the presence of nonstationary components characterized by heavy drifts and/or large swings in frequency.      

Design of multisine excitations to characterize the nonlineardistortions during FRF-measurements

Multisine excitation signals are designed which will be used to detect and qualify the nonlinear distortions on frequency response function (FRF) measurements, and this without making additional experiments. The main idea in this method is to apply well chosen periodic excitations where not all harmonics are excited. The nonexcited frequency lines are used to detect, qualify and quantify the nonlinear distortions, while the FRF is measured at the same time at the excited harmonics. The selection of the multisine that will be used depends on the characteristics of the system that are studied. For that reason, not every multisine will produce in every case accurate, robust, and reliable results for every system. This paper presents some signals by which the following aims are accomplished: simplicity, robustness, and a reliable nonlinear characterization     

考虑齿距累积误差的人字齿轮系统动态特性分析

基于Timoshenko梁理论和有限单元法,引入时变啮合刚度和综合啮合误差,建立了人字齿轮系统动力学模型,研究了齿距累积误差对人字齿轮系统动态特性的影响。研究表明:齿距累积误差使动态传递误差出现显著的轴频成分和调制边频带。当负载扭矩较小时,边频成分大于啮合频率及其倍频成分,随着负载扭矩的增加,啮合频率及其倍频成分逐渐增强。当齿距累积误差相位不同时,人字齿轮系统将出现明显的轴向窜动现象。同时,齿距累积误差相位差对系统振动影响显著,通过调整相位差可以显著降低系统振动。研究结果可为人字齿轮系统低噪声设计加工与装配提供理论依据。     

Propagation of longitudinal leaky surface waves under periodic metal grating structure on lithium tetraborate

The dispersion properties of longitudinal leaky surface waves propagating under the periodic Al strip grating on lithium tetraborate (Li(2)B(4)O(7); LBO) are described theoretically and experimentally for applications of the mode to high frequency SAW devices. A theoretical method developed here is based on Floquet's theorem using space harmonics as an orthogonal function set and real boundary integral equations derived from the method of weighted residuals for a period of each region, i.e., substrate, metal, and free space. The boundary integral equations are solved by using the Galerkin procedure. The periodic strip gratings with both single-electrodes and double-electrodes are investigated, considering the convergency of the numerical computation for the number of the space harmonics. As a result, the propagation loss for shorted gratings was found to be relatively low in the thickness range of the Al strip below about 1% for the single-electrodes and 2% for the double-electrodes, although it greatly increases for a thickness over 2% for the single-electrodes and 3% for the double-electrodes.     

Dynamic behavior of a digital phase estimator

A frequency-domain estimator for the phase of a sinewave with respect to an arbitrary time reference is presented. In particular, the behavior of the estimate is analyzed when the unknown phase is a time-varying function. No information regarding the signal frequency is required, and highly accurate results are provided even when the signal is distorted by other spectral components, both harmonics and spurious tones. Modern digital signal processors allow the realization of digital instrumentation that achieves a real-time bandwidth of some hundreds of kilohertz     

二次非线性粘弹性圆板的2/1超谐解

计及材料的非线性弹性和粘性性质,研究了圆板在简谐载荷作用下的2/1超谐解,导出了相应的非线性动力方程。提出一类强非线性动力系统的叠加迭代谐波平衡法。将描述动力系统的二阶常微分方程,化为基本解为未知函数的基本微分方程;及分岔解为未知函数的增量微分方程。通过叠加迭代谐波平衡法得出了圆板的2/1超谐解。同时,对叠加迭代谐波平衡法和数值积分法的精度进行了比较。并且讨论了2/1超谐解的渐近稳定性。     

A comparison of multirate robust track-following control synthesis techniques for dual-stage and multisensing servo systems in hard disk drives

This paper presents the system modeling, design, and analysis of multirate robust track-following controllers for a dual-stage servo system with a microelectromechanical systems (MEMS) microactuator (MA) and an instrumented suspension. A generalized model is constructed which includes a nominal plant, disturbances, uncertainties, and multirate sensing and control. Two major categories of controller design methodologies are considered. The first includes synthesis methodologies that are based on single-input single-output (SISO) design techniques, and includes the sensitivity decoupling (SD) and the PQ methods. In this case, a high sampling-rate inner loop damping control is first implemented using the auxiliary sensor signals. Subsequently, a low-rate outer loop controller is designed for the damped plant using either the SD or PQ design methods. The second category of design methodologies includes those based on multirate, multi-input multi-output (MIMO) design techniques, including mixed H/sub 2//H/sub /spl infin//, mixed H/sub 2///spl mu/, and robust H/sub 2/ synthesis. In this case, a set of controllers, which is periodically time-varying due to multirateness, is designed by explicitly considering plant uncertainty and hence robust stability. Comparisons are made between all the design techniques in terms of nominal H/sub 2/ performance, robust stability, and robust performance between these controllers, when the feedback controller is closed around the full order, perturbed plant. The advantages and disadvantages of each of these methods are discussed, as well as guidelines for their practical implementation.     

An improved time-varying mesh stiffness algorithm and dynamic modeling of gear-rotor system with tooth root crack

When a tooth crack failure occurs, the vibration response characteristics caused by the change of time-varying mesh stiffness play an important role in crack fault diagnosis. In this paper, an improved time-varying mesh stiffness algorithm is presented. A coupled lateral and torsional vibration dynamic model is used to simulate the vibration response of gear-rotor system with tooth crack. The effects of geometric transmission error (GTE), bearing stiffness, and gear mesh stiffness on the dynamic model are analyzed. The simulation results show that the gear dynamic response is periodic impulses due to the periodic sudden change of time varying mesh stiffness. When the cracked tooth comes in contact, the impulse amplitude will increase as a result of reductions of mesh stiffness. Amplitude modulation phenomenon caused by GTE can be found in the simulation signal. The lateral–torsional coupling frequency increases greatly within certain limits and thereafter reaches a constant while the lateral natural frequency nearly remains constant as the gear mesh stiffness increases. Finally, an experiment was conducted on a test bench with 2 mm root crack fault. The results of experiment agree well with those obtained by simulation. The proposed method improves the accuracy of using potential energy method to calculate the time-varying mesh stiffness and expounds the vibration mechanism of gear-rotor system with tooth crack failure.     

Harmonic Analyzer/Generator

A method of measuring the phase angle and magnitude of "n" harmonics corresponding to a periodic signal is described. The method includes the generation of sinusoidal bursts to reproduce significant components of distorted and complex signals; burst amplitude, burst initial phase, and burst frequency correspond, respectively, to harmonic amplitude, harmonic phase angle, and harmonic frequency.     

A spectral representation based model for Monte Carlo simulation

A new model is proposed for generating samples of real-valued stationary Gaussian processes. The model is based on the spectral representation theorem stating that a weakly stationary process can be viewed as a superposition of harmonics with random properties. The classical use of this theorem for Monte Carlo simulation is based on models consisting of a superposition of harmonics with fixed frequencies but random amplitude and phase. The resulting samples have the same period depending on the discretization of the frequency band. In contrast, the proposed model consists of a superposition of harmonics with random amplitude, phase, and frequency so that different samples have different periods depending on the particular sample values of the harmonic frequencies.

A band limited Gaussian white noise process is used to illustrate the proposed Monte Carlo simulation algorithm and demonstrate that the estimates of the covariance function based on the samples of the proposed model are not periodic.     

Frequency spectrum and vibration analysis of high speed gear-rotor system with tooth root crack considering transmission error excitation

A finite element node dynamic model of gear-rotor-bearing system with different lengths of crack by taking the time-varying mesh stiffness, backlash, transmission error excitation, flexible shaft and supporting bearing into account is proposed. The time-varying mesh stiffness of gear-pair with cracked tooth is obtained by applying the improved potential energy method. Due to the periodic features of the dynamic responses of the system when the tooth is cracked, the short term component (tooth profile error and tooth pitch error) and long term component (accumulative pitch error) transmission error excitations are introduced, the RMS values, kurtosis values, and frequency spectrum diagrams of the dynamic response with respect to input speed considering different forms of transmission error excitation are gained. The influences of transmission error excitation and crack length on the dynamic responses are investigated. The effectiveness of the RMS value, kurtosis value and frequency spectrum in the judgment of the crack length is analyzed.