Periodic Ripples Research Articles

High Precision Low-Speed Control for Permanent Magnet Synchronous Motor.

Due to the process defects and imperfection of drivers, permanent magnet synchronous motors (PMSM) are problematic to control. There is still a lack of effective high-performance control methods for inertial stabilized platforms based on PMSM currently. At present, the most frequently used method is sliding mode control (SMC), but traditional sliding mode control cannot overcome the contradiction between high performance and system chattering. In order to solve this problem and improve the system reliability and pointing accuracy, a new approach law for the sliding mode controller is proposed in this paper. In view of the large periodic torque ripple in PMSM, an iterative learning controller (ILC) is introduced to compensate for the disturbance. Based on these, aimed at suppressing all kinds of real-time disturbances in the working environment of the system, the extended state observer (ESO) is brought into the servo system to observe the lumped disturbance of the system, and the total disturbance observed is compensated into the sliding mode controller, so as to better suppress the system chattering and enhance the system’s ability of resisting external disturbance. Experiments are carried out on an inertial stabilization platform based on DSP + CPLD. The final experiments verify that the SMC with the new approach, combined with ILC and ESO, is of outstanding performance when compared with the traditional proportional integral (PI) + disturbance observer (DOB) control scheme.

Investigation on the material removal behavior of single crystal diamond by infrared nanosecond pulsed laser ablation

In this paper, the material removal behavior of the single crystal diamond (SCD) in the multi-passes infrared nanosecond pulse laser ablation is investigated, focusing on the formation mechanism of the surface cracks, the periodic ripples, the graphitization and the shape evolution of the groove. The simulation of the temperature field in the SCD pulse laser ablation process is firstly performed to show the possibly achieved peak temperature which is closely related to the material removal mechanism, and the experimental results indicate that the diamond materials are mainly removed by gasification, oxidation, delamination and fragmentation. The typical surface characteristic of the ablated SCD is dependent on the pulse duration, where graphitization, cracking and chipping appeared as the main form at the pulse duration of 8 ns, 50 ns, and 120 ns, respectively. In addition, the periodic ripples of 500–1100 nm on the side walls of the machined groove were induced when the laser energy density is lower than 40 J/cm2. The obtained critical values of the laser fluence is of great significance for the optimization of the laser parameters in the surface microstructuring of SCD. On the whole, five characterized areas in the multi-passes infrared nanosecond pulse laser ablation surface are identified in the machined grooves, including the bottom smooth area, the side wall ripples area, the side wall non-ripples area, the edge transition area and the deposited metamorphic area due to the condensation of deposited carbon atoms and clusters, the formation mechanism of which, mainly including the thermal stress and the interaction between the laser pulse and the material, are discussed.

Laser beam self-focusing in collisional plasma with periodical density ripple

AbstractIn the paper, we applied the paraxial region theory and Wentzel–-Kramers–-Brillouin approximation to study laser beam self-focusing in the interaction of laser and collisional plasma with periodical density ripple. The results have shown that, under the influence of collision nonlinear effect, laser presents stable self-focusing, self-defocusing, and oscillational self-focusing in the plasma. Besides, the parameters of plasma with periodical density ripple have a greater impact on the effect of self-defocusing and oscillational self-focusing than stable self-focusing. In certain conditions, beam self-defocusing and oscillational self-focusing would decline and even disappear, and stable self-focusing would further be strengthened. Hence, selecting a suitable periodic plasma system is advantageous for separating self-defocusing and oscillational self-focusing, and for the formation of a more stable collisional self-focusing.

Tuning wettability of Si surface by ion beam induced silicon nitride formation and nanopatterning

The transformation of a hydrophilic surface to a hydrophobic surface due to chemical modification and periodic nano pattern formation on silicon surface by 5–12 keV N+ ion bombardment is reported. The reactive N+ ion bombardment at oblique angle incidence leads to form silicon nitride (Si3N4) on the surface. Also, a periodic wave-like ripple pattern is formed on Si surface at nanoscale range whose wavelength and surface roughness can be regulated by varying the ion energy (5–12 keV). It is found that the contact angle of water droplet on such patterned surface increases with the increase of ripple wavelength and surface roughness. In contrast, the contact angle does not change for the same energy Ar+ bombarded Si surfaces, where such type of surface nano patterns are absent and also surface roughness remains almost the same. The morphologies of the irradiated and un-irradiated Si surfaces are examined by Atomic Force Microscopy (AFM), whereas the surface chemical composition is investigated by X-ray photoelectron spectroscopy (XPS). The chemical modification in terms of Si3N4 formation on the surface, periodic wave-like ripple pattern formation and high increase of root mean square (rms) roughness for N+ bombarded Si surfaces result in hydrophilic to hydrophobic transformation. The tuning of wettability in terms of ion beam parameters for the patterned and chemically modified Si surfaces is discussed in detail.

Error Compensation Algorithm for SRF-PLL in Three-Phase Grid-Connected Converters

This paper proposes an error compensation algorithm based on synchronous reference frame phase-locked loop for accurate phase estimation of the distorted three-phase voltages including asymmetrical phase voltages, harmonics, and dc offset. These errors cause undesirable periodic ripples with grid frequency in the traditional PLL and significantly degrade the overall performance of the grid-connected converters. In this paper, the effects of the errors are comprehensively analyzed in the stationary and synchronous reference frames. In particular, the errors are estimated and compensated by controlling the $\alpha $ -axis and $\beta $ -axis components in a PLL to be balanced. The proposed compensation algorithm can be implemented by some integral operations followed by simple Integral-controllers. The performance and robustness of proposed compensation technique are investigated under distorted grid conditions. The proposed technique is numerically and experimentally verified using Matlab/Simulink and Dspace1202 control board platform.

Regular uniform large-area subwavelength nanogratings fabricated by the interference of two femtosecond laser beams via cylindrical lens

Inhomogeneity and low efficiency are two important factors that hinder the wide application of laser-induced periodic surface structures. Two-beam interference is commonly used to fabricate gratings with interference periods. This study reports regular and uniform periodic ripples fabricated efficiently by the interference of two femtosecond laser beams via a cylindrical lens. The interference period is adjusted to be an integer multiple of the wavelength of a surface plasmon polariton. Regular and uniform subwavelength nanogratings (RUSNGs) on a silicon wafer of a diameter of 100 mm are fabricated with a scanning velocity of 6–9 mm/s. Bright and pure colors (including purple, blue, and red) are demonstrated on different patterns covered with RUSNGs.

A Simple Approach to Reject DC Offset for Single-Phase Synchronous Reference Frame PLL in Grid-Tied Converters

Synchronous reference frame phase-locked loop (SRF-PLL) is widely used for grid voltage synchronization in single-phase grid-connected power converters. However, in the actual situation, dc offset component may be introduced in the input of the PLL, due to the transient fault of the power grid, sampling and measurement error, or A/D signal processing. A simple yet effective approach with additional all-pass filter based dc rejecter is presented for SRF-PLL, in this paper. Thereby, correct estimation and undesirable periodic ripple free can be achieved in SRF-PLL, when the input signal contains dc offset. The second order generalized integrator based PLL (SOGI-PLL) is first introduced, followed by the analysis on influence of the input dc offsets in SRF-PLL. The structure of enhanced-SOGI (ESOGI) with its analysis of dc offset rejection effects and performance have been then formulated in detail. Finally, experimental results are presented to demonstrate the effectiveness of the proposed ESOGI based PLL.

Picosecond Laser-Induced Hierarchical Periodic Near- and Deep-Subwavelength Ripples on Stainless-Steel Surfaces.

Ultrafast laser-induced periodic surface subwavelength ripples, categorized based on the ripple period into near-subwavelength ripples (NSRs) and deep-subwavelength ripples (DSRs), are increasingly found in the variety of materials such as metals, semiconductors and dielectrics. The fabrication of hierarchical periodic NSRs and DSRs on the same laser-irradiated area is still a challenge since the connection between the two remains a puzzle. Here we present an experimental study of linearly polarized picosecond laser-induced hierarchical periodic NSRs and DSRs on stainless-steel surfaces. While experiencing peak power density higher than a threshold value of 91.9 GW/cm2, in the laser-scanned area appear the hierarchical periodic NSRs and DSRs (in particular, the DSRs are vertically located in the valley of parallel NSRs). A large area of the uniformly hierarchical periodic NSRs and DSRs, with the spatial periods 356 ± 17 nm and 58 ± 15 nm, respectively, is fabricated by a set of optimized laser-scanning parameters. A qualitative explanation based on the surface plasmon polariton (SPP) modulated periodic coulomb explosion is proposed for unified interpretation of the formation mechanism of hierarchical periodic NSRs and DSRs, which includes lattice orientation of grains as a factor at low peak power density, so that the initial DSRs formed have a clear conformance with the metallic grains.

Improved virtual quadrature-coordinate EPLL considering input harmonics and DC offset

The two-phase quadrature-coordinate enhanced phase-locked loop (QC-EPLL) possesses a better transient performance than the standard single-phase EPLL. In this paper, a virtual QC-EPLL (VQC-EPLL) based on signal delay method is proposed to realize using QC-EPLL to estimate the synchronization information of the single-phase grid. Furthermore, the effect of the input harmonics and DC offset on the VQC-EPLL is originally derived and it reveals these nonideal conditions causes the unexpected periodical ripple in the estimated amplitude, phase angle and frequency. Furthermore, the improved approach, including the DC offset estimator and finite impulse response filter is proposed to eliminate these effect. Detailed experimental results verify the correctness and feasibility of the theoretical analysis and proposed improved approach.

Ultrafast dynamics of the thin surface plasma layer and the periodic ripples formation on GaP crystal irradiated by a single femtosecond laser pulse.

Ultrafast dynamic of thin surface plasma layer plays a crucial role in the formation of periodic surface ripples after laser pulse irradiation. Using the pump-probe imaging technique, a complete scenario of the periodic ripples formation on a GaP surface is demonstrated after being irradiated by single femtosecond laser pulse. The ripples firstly emerge at delay time of several tens of picoseconds, and disappear completely at several hundreds of picoseconds, resulting in a transient overheating solid state ablation crater. It's interesting that new ripples appear and gradually become deep and clear after hundreds of picoseconds. A part of these ripples remain after the ablation crater is solidified. The period of the remained ripples is measured and approximately equal to the periods of the two transient ripples. The thin surface plasma model with multi-layer is introduced to study the formation of periodic ripples. The dynamics of the carrier excitation, carrier and lattice temperature, transient dielectric constant, and other factors are obtained by the two-temperature model and the Drude model. The results show that the periods of electric field distributions at different depths of the plasma layer are the same. The formation of the two transient ripples and the remained ripples are all related to the periodic energy deposition due to the SPP excitation at the air-plasma interface.

Femtosecond Laser-Induced Periodical Nanomodification of Surface Composition

Nanoscale periodical chemical composition modification of different two-component compound materials GaAs, CdTe, W0.5C and TiC was realized for the first time through multi-shot femtosecond laser irradiation of their surfaces. The resulting surface modification represents periodical nanoscale ripples with different chemical composition in valleys and ridges. The underlying mechanism is related to a sequence of elementary processes, such as laser excitation surface plasmon-polariton, their interference with the incident laser radiation, inhomogeneous surface heating and incongruent cavitation-like material removal. The latter mechanism is responsible for higher evaporation rate of the components with higher volatility, resulting in growth of surface concentration of low-volatility components.

Surface nanomechanical properties of bio-modified reclaimed asphalt binder

In this work, three different bio-based rejuvenators were added to the reclaimed asphalt pavement (RAP) binder: a vegetable oil, a cashew nut shell oil and a tall oil. To better understand the microstructural morphology, atomic force microscopy (AFM) was used before and after aging. Quantitative nanomechanical mapping (QNM) showed how the addition of rejuvenators affected the surface elastic moduli of RAP. Changes of the microstructure in modified RAP binders as a function of aging treatments were seen. Generally, the microstructures show four domains topographically, while two distinct domains are present in the phase image. It has been observed that these two different domains correspond to zones with different mechanical properties. Comparison between the “bees” surface’s elastic moduli and the bulk elastic moduli for a virgin bitumen showed that surface’s moduli were consistently higher than the bulk moduli contributing to the formation of the “bee” structures. “Bees” were observed to be composed of periodic ripples with a wavelength in the order of 400 nm for the aged virgin binder and 600 nm for the unaged one while amplitude equal to 8 and 15 nm for the aged and unaged case respectively. The results presented in this paper contribute to an improved understanding of the link between surface and bulk properties of bitumen.

An adaptive dynamic programming method for torque ripple minimization of PMSM

<p style='text-indent:20px;'>The imperfect sinusoidal flux distribution, cogging torque, and current measurement errors can cause periodic torque ripple in the permanent magnet synchronous motor (PMSM). These ripples are reflected in the periodic oscillation of the motor speed and torque, causing vibration at low speeds and noise at high speeds. As a high-precision tracking application, ripple degrades the application performance of PMSM. In this paper, an adaptive dynamic programming (ADP) scheme is proposed to reduce the periodic torque ripples. An optimal controller is designed by iterative control algorithm using robust adaptive dynamic programming theory and strategic iterative technique. ADP is combined with the existing Proportional-Integral (PI) current controller and generates compensated reference current iteratively from cycle to cycle so as to minimize the mean square torque error. As a result, an optimization problem is constructed and an optimal controller is obtained. The simulation results show that the robust adaptive dynamic programming achieves lower torque ripple and shorter dynamic adjustment time during steady-state operation, thus meeting the requirements of steady speed state and the dynamic performance of the regulation system.

Dynamic Performance Enhancement of Single-Phase and Two-Phase Enhanced Phase-Locked Loops by Using In-Loop Multiple Delayed Signal Cancellation Filters

The estimation of phase-angle, frequency, and amplitude are vital for the control of grid-connected power electronic systems in both single-phase and three-phase systems. The single-phase enhanced phase-locked loop (1P-EPLL) and two-phase enhanced phase-locked loop (2P-EPLL) techniques can be used to estimate single-phase and three-phase grid voltage quantities, respectively. However, under the presence of harmonics, unbalance, and dc offsets, the estimated quantities by these enhanced phase-locked loops (EPLLs) may contain periodic ripples. These periodic ripples are eliminated by using multiple delayed signal cancellation (MDSC) filters in the phase-angle estimation loop and amplitude estimation loop of these EPLLs. To improve the dynamic responses of MDSC-based EPLLs, proportional-integral (PI) controller is replaced with a proportional-integral-derivative (PID) controller. The design guidelines for choosing the control parameters are provided. The MDSC filters in the EPLLs are realized both in direct-form and recursive-form structures. The proposed EPLLs are verified experimentally by using real-time system and compared with recent advanced EPLLs.

Explosive instability due to flow over a rippled bottom

Abstract. In this paper, we study Bragg resonance, i.e., the triad interaction between surface and/or interfacial waves with a bottom ripple, in the presence of background velocity. We show that when one of the constituent waves of the triad has negative energy, the amplitudes of all the waves grow exponentially. This is very different from classic Bragg resonance in which one wave decays to cause the growth of the other. The instabilities we observe are “explosive” and are different from normal mode shear instabilities since our velocity profiles are linearly stable. Our work may explain the existence of large-amplitude internal waves over periodic bottom ripples in the presence of tidal flow observed in oceans and estuaries.

SURFACE ABLATION OF 52100 BEARING STEEL USING FEMTOSECOND LASER IRRADIATION

Laser ablation is a rapid-material removal technique with potential application in remanufacturing of bearings to clean damaged surfaces. This study reports the ablation morphology and change in surface composition of 52100 bearing steel irradiated with a femtosecond laser. A smooth ablated surface was achieved by operating the laser at fluence just above the ablation threshold, but higher fluences resulted in roughened surfaces with recast formation. Such coarse morphology is believed to result from higher energy deposition and surface melting. Minimal oxidation occurred at lower fluences. Periodic ripple patterns formed in the ablated area at low fluence; their morphology and possible formation mechanism is discussed.

Motor Speed Signature Analysis for Local Bearing Fault Detection With Noise Cancellation Based on Improved Drive Algorithm

Motor speed signature analysis provides a noninvasive method for bearing fault detection. However, for the vector-controlled ac motors, periodic speed ripples related to fundamental frequency fe and its twice harmonic 2fe, which are caused by current measurement errors, are difficult to be attenuated by motor inertia or bandwidth of speed loop under low-speed conditions. The unwanted components would reduce the signal-to-noise ratio of motor speed and increase the difficulty of bearing fault detection. To solve the problem, this paper proposes a new noise cancellation strategy, which applies the improved drive algorithm instead of conventional signal processing schemes to cancel out the noise component before the data acquisition. Specifically, resonance controllers are introduced and set in parallel with the existed proportional-integral controller to suppress the speed ripples. Moreover, the envelope spectrum analysis is carried out to detect fault characteristic. The effectiveness of the proposed method is validated through simulation and experimental tests. Besides, its superiority under low-speed conditions is also demonstrated, compared with the spectral kurtosis of speed signal and three current-based methods.

Photo-induced birefringence and surface ripples structures in As-S-Ag chalcogenide thin-films

Photo-induced phenomena are investigated in new metastable Agx(As20S80)100-x chalcogenide thin-films prepared by the co-evaporation technique. Under irradiation using an Ar laser at 488 nm, periodic ripples structures formation are observed inside of the irradiated spot. With an increasing irradiation time or power density, a characteristic evolution of ripples is observed from a random structure to a series of generally aligned peaks-and-valleys periodic structures until a complete break-up of the film (laser dewetting). The scanning electron microscopy (SEM) of irradiated areas shows periodic ripples are aligned perpendicularly to the electric field vector of the incident light. Moreover, photo-induced birefringence (PIB) is observed during ripples formation. The kinetics of structural changes during the irradiation of the films is analysed using Raman spectroscopy. The molecular rearrangement from sulphur-cis toward sulphur-trans through irradiation is proposed to explain the experimentally observed PIB.

Force Ripple Modeling and Minimizing of an Ironless Permanent-Magnet Linear Synchronous Motor

A force ripple model considering both non-ideal magnetic field and windings was developed to study methods for the suppression of the force ripple in an ironless permanent-magnet linear synchronous motor (PMLSM) to improve its low speed stability and position tracking performance. With this new model, which uses the d-q reference frame, the force ripple information could be calculated at discrete positions using the winding features and magnetic flux density (MFD). A PMLSM with two movers and one stator was designed to measure the air gap MFD and force ripple at different positions. The reliability of the model was validated through a comparison of the force ripple data obtained with the new model and practical measurements. The error ratio between the theoretical calculations and measured forces was less than ± 4%. Force constants were available from the model using the measured MFD, and these force constants were then used as force compensation coefficients to adjust the current in terms of the mover’s position. The force ripple, particularly the periodic force ripple causing resonance of the mechanical system, was significantly reduced with compensation. The theoretical analysis and experimental results show the effectiveness of the improved model.

Torque Ripple Reduction of PMSMs Using a Novel Angle-Based Repetitive Observer

The angle-based repetitive controller (RC) has been applied successfully in the literature for torque ripple suppression in variable speed permanent magnet synchronous machines (PMSMs). The RC is preferred because of its learning capability, which allows the cancelation of all periodic ripples from multiple sources. However, its tuning method has not yet been proposed. Since the structure of the RC is found similar to the disturbance observer (DOB), which is convenient to tune through pole placement, this paper merges the angle-based RC and DOB into a novel angle-based repetitive observer (ARO) for the first time for the torque ripple reduction in PMSMs. ARO takes advantages of its DOB structure that it can be designed separately and can easily be added into an existing control loop. Taking advantage of its RC nature, ARO can tackle a wide frequency range of torque ripple even in the presence of measurement noise. The experimental test results have confirmed the robust performance of ARO under five conditions, including the ideal integral delay, the fractional delay, the load transient, the speed transient, and the detuned mechanical parameters conditions. The execution time of the ARO is less than 10 $\,\mu$ s.