publications

2025

Nanometric Surficial Reflectors: Achieving High-Performance and High-Throughput Structural Coloration Through Modulation-Assisted Machining

Yaoke Wang , Malachi Landis , and Ping Guo

Advanced Science, 2025

Abstract PDF Video

Structural coloration, which generates color through physical structures, has broad applications but often faces challenges in achieving high color performance while being scalable for mass production. In this study, a novel single-layer wavelength-selective reflector capable of producing a broad color gamut with local angle independence is introduced, fabricated using a modulation-assisted ultra-precision machining process. The structure features multi-level submicron steps that sequentially filter light of different wavelengths, overcoming the limitations of conventional periodic structures that suffer from narrow color gamut and angle-dependent color shifts. This approach integrates a purely mechanical machining process, shaping reflectors in a single step using a precisely controlled modulation trajectory. This enables efficient, scalable fabrication while preserving the desired structural characteristics at the nanoscale. Color performance comparable to the sRGB space is demonstrated and full-color images are successfully rendered on metallic surfaces. The proposed reflector design provides a new pathway to scalable production of vibrant, angle-independent structural colors with potential applications in anti-counterfeiting, data storage, and high-quality coloration.
Graph Neural Networks for patterned welds detection on point clouds

Kojo Welbeck , Yahui Zhang , Ru Yang , Guangze Li , Hui-Ping Wang , Blair Carlson , and Ping Guo

Journal of Manufacturing Processes, 2025

Abstract PDF

Remote laser welding is an increasingly common process in automotive assembly, particularly in body-in-white and battery manufacturing. While 3D vision cameras enable quick-turnaround inspections of the welds, traditional machine vision algorithms to detect welds, namely template-matching raster-search implementations, often struggle with the variability in weld appearance and quality. Consequently, these implementations may fail to detect partial welds, distorted welds, or welds with any significant deviation from the templates used within the algorithms. Also, the raster scan sometimes detects false positives, welds that are, in fact, not present. To address these challenges, this study introduces a weld surface detection system that uses Graph Neural Networks (GNNs) to detect the presence and locations of welds in 3D point cloud data, implicitly incorporating the variability inherent in the training data samples. The proposed deep-learning algorithm comprises two GNN models chained together: one for segmenting welds by the direction and identity, and another for locating the center of welds. This approach also operates directly on point clouds, offering a computationally efficient alternative to the typical 2D normal map projection or 3D voxelization pre-processing operations on point clouds. In the experimental results, the proposed algorithm identified all welds present, including those with shape deviations, demonstrating a relative strength compared to a template-matching baseline currently used in production.
Large-scale functional patterning using mobile robot swarms and ergodic control

Malachi Landis , Muye Jia , Annalisa T. Taylor , Todd D. Murphey , and Ping Guo

CIRP Annals, 2025

Abstract PDF Video

Large-scale surfaces, such as ship hulls, face challenges in applying functional structures due to manufacturing constraints. This work presents a scalable approach using low-cost, autonomous robot swarms guided by an ergodic control framework to pattern micro-scale features on meter-scale metallic surfaces. The robots operate collaboratively to achieve density-specific coverage, optimizing surface functionalities like friction reduction and hydrodynamic performance. Experimental validation demonstrates the critical role of feature density in tribological performance, and an example application highlights the creation of gradient-density patterns. This innovative method unlocks new possibilities for functional surface patterning on large-scale structures with imprecise robots.
Multiscale optical surface integrating multifocal imaging and wavelength filtering for compact snapshot spectral imaging

Xinquan Zhang , Yaoke Wang , Hao Wu , Limin Zhu , and Ping Guo

CIRP Annals, 2025

Abstract PDF

Snapshot spectral imaging enables single-exposure acquisition of spectral images but is constrained by bulky designs and compromised spatial resolutions. This study presents a multiscale optical surface that integrates macro-scale structures for multifocal imaging and micro-scale staircases for wavelength filtering into a unified ultra-compact form. The design employs a multiscale framework grounded in Fourier optics and is fabricated using ultra-precision diamond turning. Validation experiments demonstrate the system’s ability to capture discrete spectral images using only a single optical component, with measured spectral characteristics closely matching theoretical predictions. This new design provides a compact and cost-effective solution for diverse spectral imaging applications.
Confidence-aware photometric stereo networks enabling end-to-end normal and depth estimation for smart metrology

Yahui Zhang , Ru Yang , and Ping Guo

IEEE/ASME Transactions on Mechatronics, 2025

Abstract PDF Video Code

The acquisition of geometric 3-D information is crucial for ensuring quality standards and monitoring procedures in various manufacturing applications. Photometric stereo is an established technique in computer vision to recover 3-D surfaces of objects. However, existing photometric stereo methods mainly focus on normal estimation of objects, without considering the depth estimation. On the other hand, current methods tend to prioritize accuracy while overlooking the confidence of predictions, which holds valuable information within the industry. In this article, we propose a deep learning-based photometric stereo system, consisting of hardware implementation, dataset generation, and algorithm design, to reconstruct 3-D information of physical objects, represented by normal and depth maps. In terms of the proposed algorithm, a coarse-to-fine network is introduced to improve the performance by exploiting the relationship between initial normal and depth predictions. Furthermore, the pixel-wise confidence associated with predictions is also estimated without requiring the ground truth, making a contribution to enhancing both performance and practicality. The experimental results on our synthetic dataset and real samples demonstrate the effectiveness of the proposed method on both normal/depth and confidence estimation.

2024

Wearable network for multilevel physical fatigue prediction in manufacturing workers

Payal Mohapatra , Vasudev Aravind , Marisa Bisram , Young-Joong Lee , Hyoyoung Jeong , Katherine Jinkins , Richard Gardner , Jill Streamer , Brent Bowers , Lora Cavuoto , Anthony Banks , Shuai Xu , John Rogers , Jian Cao , Qi Zhu , and Ping Guo

PNAS Nexus, Oct 2024

Abstract MEDIA PDF Video

Manufacturing workers face prolonged strenuous physical activities, impacting both financial aspects and their health due to work-related fatigue. Continuously monitoring physical fatigue and providing meaningful feedback is crucial to mitigating human and monetary losses in manufacturing workplaces. This study introduces a novel application of multimodal wearable sensors and machine learning techniques to quantify physical fatigue and tackle the challenges of real-time monitoring on the factory floor. Unlike past studies that view fatigue as a dichotomous variable, our central formulation revolves around the ability to predict multilevel fatigue, providing a more nuanced understanding of the subject’s physical state. Our multimodal sensing framework is designed for continuous monitoring of vital signs, including heart rate, heart rate variability, skin temperature, and more, as well as locomotive signs by employing inertial motion units strategically placed at six locations on the upper body. This comprehensive sensor placement allows us to capture detailed data from both the torso and arms, surpassing the capabilities of single-point data collection methods. We developed an innovative asymmetric loss function for our machine learning model, which enhances prediction accuracy for numerical fatigue levels and supports real-time inference. We collected data on 43 subjects following an authentic manufacturing protocol and logged their self-reported fatigue. Based on the analysis, we provide insights into our multilevel fatigue monitoring system and discuss results from an in-the-wild evaluation of actual operators on the factory floor. This study demonstrates our system’s practical applicability and contributes a valuable open-access database for future research.
In-process part tracking and shape measurement using vision-based motion capture for automated English wheeling

Yahui Zhang , Derick Suarez , Kornel Ehmann , Jian Cao , and Ping Guo

Manufacturing Letters, Oct 2024

52nd SME North American Manufacturing Research Conference (NAMRC 52)

Abstract PDF Video

An English wheel is an exceedingly adaptable instrument in traditional metalworking. It is a manual manufacturing technique, enabling skilled craftsmen and blacksmiths to shape complex compound curves in sheet metal panels. Accurate measurements and precise adjustments are essential when operating an English wheel to ensure that the metal is shaped with the desired curvature. An automated method to form English wheeled panels through robot forming has recently been proposed. For such a method to be successful, accurate tracking of sheet information including positions, orientations, and deformation is important for error compensation and the design of the subsequent tool paths. In this study, a Vicon motion capture system is employed to monitor the position and shape of the sheet metal during the English wheeling process. The initial experimental results demonstrate the potential of such an in-process metrology system, along with possible avenues for future work.
Image to patterning: density-specified patterning micro-structured surfaces with a mobile robot

Annalisa T. Taylor , Malachi Landis , Yaoke Wang , Todd D. Murphey , and Ping Guo

In 2024 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) , Oct 2024

Abstract PDF Video

Micro-structured surfaces possess useful properties such as friction modification, anti-fouling, and hydrophobicity. However, manufacturing these surfaces in an affordable, scalable, and efficient manner remains challenging. Standard coverage methods for surface patterning require precise placement of micro-scale features over meter-scale surfaces with expensive tooling for support. In this work, we address the scalability challenge in surface patterning by designing a mobile robot with a credit-card-sized footprint to generate micro-scale divots using a modulated tool tip. We provide a control architecture with a target feature density to specify surface coverage, eliminating the dependence on individual indentation locations. Our robot produces high-fidelity surface patterns and achieves automatic coverage of a surface from sophisticated target images. We validate an exemplary application of such micro-structured surfaces by controlling the friction coefficients at different locations according to the density of indentations. These results show the potential for compact robots to perform scalable manufacturing of functional surfaces, switching the focus from precision machines to small-footprint devices tasked with matching only the density of features.
Digital fringe projection for interlayer print defect characterization in directed energy deposition

Guanzhong Hu , Rujing Zha , Yaoke Wang , Jian Cao , and Ping Guo

Jul 2024

Abstract PDF

Directed Energy Deposition (DED) is one of the main additive manufacturing (AM) families, enabling the fabrication of multi-material parts with high material addition rates. However, the incremental nature of DED fabrication makes it prone to local defect formation due to process condition fluctuations. Known for its rapid and precise 3D surface measurement capabilities, digital fringe projection (DFP) was previously demonstrated in process monitoring for powder bed AM. This study brings DFP to the DED process through development of a custom motor stage system and validates its effectiveness in assessing surface topography and build height measurement. Measurements were taken on both correctly deposited builds and builds with off-nominal deposition conditions, where the system was able to detect pitting as small as 0.425 mm in the lateral size and 0.154 mm in depth in the case of reduced laser energy. This work paves the way for future machine learning-enabled interlayer defect identification, classification, and healing via altering subsequent processing settings.
Solid-state production of uniform metal powders for additive manufacturing by ultrasonic vibration machining

Yaoke Wang , Malachi Landis , Clement Ekaputra , Valeria Vita , and Ping Guo

Additive Manufacturing, Jul 2024

Abstract PDF

This work presents a new technique to generate uniform and micron-sized metal powders for additive manufacturing. By collecting discrete chips resulting from ultrasonic vibration machining, we demonstrate the feasibility of all solid-state production consistent powders with tight dimensional tolerance, the ability to control powder geometry, and good efficiency. The technique offers a new route for sustainable and low-cost manufacturing of high-quality metal powders. The powder generation mechanism is analyzed with a special tool path design to ensure consistent dimensions over multiple cuts. An analytical model to predict the dimensions of produced powders under different cutting parameters is introduced. Aluminum and brass powders of different dimensions are produced, and the overall shear ratio that governs the deformation during the machining process is calibrated with the experimental results. The morphology consistency of produced powders is investigated over multiple hours of production, illuminating the role of tool wear on final powder shape. A high-efficiency powder collection system and a scalable solution for parallel production are proposed for the introduced technique. Additive manufacturing experiments (laser powder bed fusion) are conducted using produced A356 aluminum powders, demonstrating the printability of produced powders in additive manufacturing. The microhardness of the printed parts for five different process parameters is measured to be 45% higher than the raw material on average.

2023

Generic fabrication solution of freeform Fresnel optics using ultra-precision turning

Yaoke Wang , Jianjian Wang , and Ping Guo

Optics Express, Jul 2023

Abstract PDF Video

Freeform Fresnel optics represent an emerging category of modern optics that reproduces powerful optical functionalities while maintaining an ultra-compact volume. The existing ultra-precision machining (UPM) technique faces technical challenges in meeting the fabrication requirements for freeform Fresnel optics because of the absence of appropriate geometry definition and corresponding tool path planning strategy to overcome the extreme asymmetry and discontinuity. This study proposes a new scheme for ultra-precision machining using four axes (X, Y, Z, C) to fabricate freeform Fresnel optics, including a general geometry description for freeform Fresnel optics, the quasi-spiral tool path generation strategy to overcome the lack of rotary symmetry, and the adaptive tool pose manipulation method for avoiding tool interference. In addition, the tool edge compensation and the adaptive timestep determination are also introduced to enhance the performance and efficiency of the proposed scheme. The machining of two exemplary freeform Fresnel lenses is successfully demonstrated. Overall, this study introduces a comprehensive routine for the fabrication of freeform Fresnel optics and proposes the adaptive tool pose manipulation scheme, which has the potential for broader applications in the ultra-precision machining of complex or discontinuous surfaces.
Fabrication of fiber-reinforced composites via immersed electrohydrodynamic direct writing in polymer gels

Yunzhi Xu , Nathanial Buettner , and Ping Guo

MRS Communications, Jul 2023

Abstract PDF Video

Fiber-reinforced composites have provided tremendous opportunities in advanced engineering materials, but the fiber generation and spatial distribution are the most challenging aspects. This paper proposes a novel fabrication approach for fiber-reinforced composites with spatially resolved fiber distribution by combining immersion and near-field electrospinning. The new Immersed Electrohydrodynamic Direct-writing (I-EHD) process makes use of an electrostatic force to draw ultrafine fibers and allows the freestanding of electrospun fibers all inside a liquid matrix. This novel approach enables the dynamic control of fiber morphology and 3D spatial distribution inside the composites, which may lead to future scalable 3D printing of multifunctional composites.
DPPS: A deep-learning based point-light photometric stereo method for 3D reconstruction of metallic surfaces

Ru Yang , Yaoke Wang , Shuheng Liao , and Ping Guo

Measurement, Jul 2023

Abstract PDF Video Code

Three-dimensional (3D) measurement provides essential geometric information for quality control and process monitoring in many manufacturing applications. Photometric stereo is one of the potential solutions for in-process metrology and active geometry compensation, which takes multiple images of an object under different illuminations as inputs and recovers its surface normal map based on a reflectance model. Deep learning approaches have shown their potential in solving the highly nonlinear problem for photometric stereo, but the main challenge preventing their practical application in process metrology lies in the difficulties in the generation of a comprehensive dataset for training the deep learning model. This paper presents a new Deep-learning based Point-light Photometric Stereo method, DPPS, which utilizes a multi-channel deep convolutional neural network (CNN) to achieve end-to-end prediction for both the surface normal and height maps in a semi-calibrated fashion. The key contribution is a new dataset generation method combining both physics-based and data-driven approaches, which minimizes the training cost and enables DPPS to handle reflective metal surfaces with unknown surface roughness. Even trained only with fully synthetic and high-fidelity dataset, our DPPS surpasses the state-of-the-art with an accuracy better than 0.15 cm over a 10 cm × 10 cm area and its real-life experimental results are on par with commercial 3D scanners. The demonstrated results provide guidance on improving the generalizability and robustness of deep-learning based computer vision metrology with minimized training cost as well as show the potential for in-process 3D metrology in advanced manufacturing processes.

2022

One step machining of hierarchical optical structures for autostereoscopic images

Yaoke Wang , and Ping Guo

CIRP Annals, Jul 2022

Abstract PDF Video

This work presents a new optical design and the corresponding novel machining strategy to achieve the one step generation of hierarchical optical structures on a flat surface for autostereoscopic effects. The hierarchical structures combine V-groove arrays as a parallax barrier for stereopsis and integrated diffraction gratings with variable grating spacing for motion parallax. The multi-scale surface structures are machined simultaneously using the tool geometry to form V-grooves while utilizing elliptical tool vibration to generate blazed gratings concurrently. An autostereoscopic image with strong depth perception is fabricated with a pixel density exceeding 1,660 pixels per inch.
Deep DIC: Deep learning-based digital image correlation for end-to-end displacement and strain measurement

Ru Yang , Yang Li , Danielle Zeng , and Ping Guo

Journal of Materials Processing Technology, Jul 2022

Abstract MEDIA PDF Video Code

Digital image correlation (DIC) has become an industry standard to retrieve accurate displacement and strain measurement in tensile testing and other material characterization. Though traditional DIC offers a high precision estimation of deformation for general tensile testing cases, the prediction becomes unstable at large deformation or when the speckle patterns start to tear. In addition, traditional DIC requires a long computation time and often produces a low spatial resolution output affected by filtering and speckle pattern quality. To address these challenges, we propose a new deep learning-based DIC approach – Deep DIC, in which two convolutional neural networks, DisplacementNet and StrainNet, are designed to work together for end-to-end prediction of displacements and strains. DisplacementNet predicts the displacement field and adaptively tracks a region of interest. StrainNet predicts the strain field directly from the image input without relying on the displacement prediction, which significantly improves the strain prediction accuracy. A new dataset generation method is developed to synthesize a realistic and comprehensive dataset, including the generation of speckle patterns and the deformation of the speckle image with synthetic displacement fields. Though trained on synthetic datasets only, Deep DIC gives highly consistent and comparable predictions of displacement and strain with those obtained from commercial DIC software for real experiments, while it outperforms commercial software with very robust strain prediction even at large and localized deformation and varied pattern qualities. In addition, Deep DIC is capable of real-time prediction of deformation with a calculation time down to milliseconds.
Blazed grating enables highly decoupled optically variable devices fabricated by vibration-assisted diamond texturing

Jianjian Wang , Yaoke Wang , Jianfu Zhang , Volker Schulze , and Ping Guo

Optics Express, Jul 2022

Abstract PDF Video

Optically variable devices (OVDs) are well received for anti-counterfeiting and decorative applications. In this study, new strategies to develop highly decoupled OVDs were proposed and demonstrated based on the fast patterning of blazed gratings by vibration-assisted diamond texturing. A unique surface generation mechanism was revealed as a combined cutting and forming process. One facet of blazed grating is generated by the cutting motion defined by the tool tip trajectory. The other facet is formed by the tool flank face, which establishes the blaze angle. This process is able to generate high-resolution, structurally colored graphics by modulating cutting velocity to control the grating distribution. Due to the unique surface generation mechanism, the orientation of the created blazed gratings is intrinsically perpendicular to the cutting direction. Thus, it enables the flexible control of concentration directions of diffracted light by tuning the orientation of blazed gratings. We designed and demonstrated two types of highly decoupled OVDs based on vibration-induced blazed gratings. The orthogonal-type OVD utilizes the azimuth angle dependence of blazed gratings to encode two images in orthogonal cutting directions. The in-plane-type OVD utilizes the optimized diffraction efficiency of blazed gratings in a given diffraction order to encode two images in opposite cutting directions. The fabricated OVDs are presented and compared with optical simulation results based on an extended scalar diffraction theory.
Fabrication of fiber-reinforced polymer ceramic composites by wet electrospinning

Yunzhi Xu , Junior Ndayikengurukiye , Ange-Therese Akono , and Ping Guo

Manufacturing Letters, Jul 2022

Abstract PDF

We propose a novel approach of wet electrospinning to yield fiber-reinforced polymer ceramic composites, where a reactive ceramic precursor gel is used as a collector. We illustrate our approach by generating polyethylene oxide (PEO) fibers in a potassium silicate gel; the gel is later activated using metakaolin to yield a ceramic-0.5 wt% PEO fiber composite. An increase of 29% and 22% is recorded for the fabricated polymer ceramic composites in terms of indentation modulus and indentation hardness respectively. Our initial findings demonstrate the process viability and might lead to a potentially scalable manufacturing approach for fiber-reinforced polymer ceramic composites.

2021

Structural coloration of non-metallic surfaces using ductile-regime vibration-assisted ultraprecision texturing

Jianjian Wang , Yaoke Wang , Jianfu Zhang , Yang Yang , and Ping Guo

Jul 2021

Abstract PDF

Structural coloration stemming from microstructure-induced light interference has been recognized as a promising surface colorizing technology, based on its potential in a wide array of applications, including high-definition displays, anti-counterfeiting, refractive index sensing, and photonic gas and vapor sensing. Vibration-assisted ultraprecision texturing using diamond tools has emerged as a high-efficiency and cost-effective machining method for colorizing metallic and ductile surfaces by creating near-wavelength microstructures. Although theoretically possible, it is extremely challenging to apply the vibration-assisted texturing technique directly to colorize non-metallic and brittle materials (e.g., silicon and acrylic polymers) with high-quality, crack-free microstructures owing to the intrinsic brittleness of these materials. This study demonstrates the feasibility of direct texturing near-wavelength-scale gratings on brittle surfaces in the ductile regime to fabricate crack-free micro/nanostructures. The effects of tool vibration trajectories on the ductile-to-brittle transition phenomena were investigated to reveal the cutting mechanism of ductile-regime texturing and optimize the processing windows. Structural coloration on silicon and acrylic surfaces was successfully demonstrated by creating programmable and pixelated diffraction gratings with spacing values ranging from 0.75 to 4 μm.
Stiffness modeling for near field acoustic levitation bearings

Yaoke Wang , and Ping Guo

Applied Physics Letters, Jul 2021

Abstract PDF

The dynamic characteristics of near field levitation bearings have been investigated in this study. Through theoretical analysis, two different types of system stiffness are defined and derived analytically. The dynamic stiffness relates the excitation amplitude to the dynamic force amplitude, while the effective stiffness governs the time-averaged force–displacement relationship. The results indicate two non-linear and asymmetric spring constants that can effectively predict levitation force and height. The models are verified with a carefully designed experimental setup to eliminate the structural resonance effect. Besides, some unique dynamic behaviors are investigated and predicted based on the proposed stiffness model.

2020

On ductile-regime elliptical vibration cutting of silicon with identifying the lower bound of practicable nominal cutting velocity

Jianjian Wang , Yang Yang , Zhiwei Zhu , Yaoke Wang , Wei-Hsin Liao , and Ping Guo

Journal of Materials Processing Technology, Jul 2020

Abstract PDF

This study investigates the ductile-to-brittle transition behavior in elliptical vibration cutting (EVC) of silicon and identifies the practical process window for ductile-regime cutting. EVC has been reported to increase the critical depth of ductile-regime cutting of silicon. This study demonstrates that the enhanced ductile cutting performance, however, is only optimal in a carefully-determined process window. The vibration amplitudes and nominal cutting velocity have significant impacts on the ductile-to-brittle transition behaviors. Systematic experiments covering a wide span of vibration amplitudes and cutting velocity have been conducted to investigate their effects. Two quantitative performance indices, the critical depth and ductile ratio, are utilized to analyze cutting performance by considering two unique characteristics of elliptical vibration cutting, i.e., the time-varying undeformed chip thickness and effective cutting direction angle. The results show that there exists a lower bound for the nominal cutting velocity to ensure the ductile-regime material removal, besides the well-known upper bound. Besides, the increases of vibration amplitudes in both the cutting and depth-of-cut (DOC) directions first enhance but then deteriorate the cutting performance. Based on the theoretical analysis and experimental results, the optimal process parameters have been recommended for the elliptical vibration cutting of silicon.
Fabrication of structurally colored basso-relievo with modulated elliptical vibration texturing

Jianjian Wang , Yaoke Wang , Yang Yang , Ru Yang , Wei-Hsin Liao , and Ping Guo

Precision Engineering, Jul 2020

Abstract PDF Video

Elliptical vibration texturing is a newly arising method for the fast and cost-effective generation of near-subwavelength micro-structures on metal surfaces. This study proposes a fabrication method for structurally colored basso-relievo by combining the surface sculpturing and elliptical vibration texturing in one-step machining. Synergistic modulations of both nominal cutting speed and depth-of-cut (DOC) in elliptical vibration texturing are applied to provide concave-convex topography and structural coloration. A new rendering strategy for the face turning configuration with an equidistant spiral tool path has been presented to improve the image rendering efficiency over the raster scan tool path. In addition, different from the conventional elliptical vibration texturing, the nominal DOC dynamically changes pixel by pixel in the proposed new process. In order to identify the effects of DOC and elliptical vibration trajectories on the surface quality and fidelity of generated gratings, grooving experiments have been conducted on brass samples to further the understanding of nonlinear tool-workpiece interactions. Finally, both raster scan and face turning tests have been performed on an ultraprecision platform with optimized process parameters. Structurally colored basso-relievos with high quality have been successfully demonstrated using the proposed fabrication method.
Structural coloration using face turning and variable tool vibration frequency

Yaoke Wang , JianJian Wang , Alyssa Chen , Nicholas Kwok , and Ping Guo

Journal of Manufacturing Processes, Jul 2020

Abstract PDF

The technique of manufacturing surface micro-/nano- structures to colorize metal surfaces by using elliptical vibrating cutting has been continuously developed and has good industrial potential. However, the image rendering is based on a raster scan path and constant variation of cutting velocity. The process efficiency and stability are significantly restricted. In this study, a new approach is proposed to implement the elliptical vibration texturing in a face turning setup, where the image rendering is achieved by an equidistant spiral tool path and modulation of the tool vibration frequency. The basic principle is that, while moving at a constant surface velocity, the tool follows a variable frequency elliptical vibration to machine gratings with different spacings on the workpiece surface. In order to ensure the uniformity in the radial direction and the consistency of the color under the same frequency excitation, a smoothly accelerated equidistant spiral tool path is designed. In addition, since the running speed of the machine tool is not completely synchronized with the signal of the tool vibration, a compensation method through the pre-experimental calibration test is proposed. The machining test on the ultra-precision lathe is performed. It is found that on the compensated machined image, cumulative errors caused by the asynchronous operation of the lathe and vibration tool are eliminated, which validates the feasibility and effectiveness of the method.
Modulated vibration texturing of hierarchical microchannels with controllable profiles and orientations

Jianjian Wang , Ru Yang , Shiming Gao , Fei Weng , Yaoke Wang , Wei-Hsin Liao , and Ping Guo

CIRP Journal of Manufacturing Science and Technology, Jul 2020

Abstract PDF

Hierarchical microchannels, which consist of the primary channel formation and superimposed secondary nanostructures, are attracting ever-increasing attention due to their unique capacity to enhance and modify the surface characteristics and functional performance. The mechanical machining methods for microchannel fabrication, such as micro-milling and diamond turning, can achieve high material removal rates without changing material properties. However, they have limited capacity to control the channel cross-section profiles and shapes due to the relative size between the channel dimension and tool geometry. This study proposes a new cutting-based approach for the fast and cost-effective fabrication of hierarchical microchannels with controllable profiles and orientations by utilizing modulated elliptical vibration texturing. The modulation motion is adopted to form the primary channel in an incremental approach, while the elliptical vibration texturing is utilized to create micro/nano-scale secondary textures. By controlling the tool modulation trajectory, hierarchical dimple arrays with controllable cross-section profiles are first demonstrated. Then, by programming the layout of dimples to adjust the overlapping ratio between each cut, channels can be formed with arbitrary cross-section profiles and orientations. The efficacy of the proposed process has been demonstrated through numerical simulation and experimental results. Hierarchical microchannels with straight and curving shapes, as well as different cross-section profiles (sinusoidal, triangular, trapezoidal), have been presented.
Noncontact Planar Stage Based on Near-Field Acoustic Transportation

Yang Yang , Keyu Chen , and Ping Guo

Journal of Micro and Nano-Manufacturing, Jul 2020

Abstract PDF Video

Acoustic radiation force in the near-field of a vibrating source can be utilized to lift and transport objects, which provides a noncontact driving technology in addition to maglev. This paper presents a novel design of a self-levitated planar stage based on near-field acoustic transportation. A closed-loop system is proposed to design a capacitance surface encoder to provide direct two-dimensional (2D) position feedback. A dynamic model based on the Reynolds equation is established to study its driving mechanism. A prototype including the levitation stage, encoder, and controller is implemented to demonstrate the potential of arbitrary trajectory tracking in two-dimensional space.
Fabrication of Microscale Polymeric Wavy and Coiling Structures via Side-Electrode-Assisted Near-Field Electrospinning: Modeling and Experiments

Xiangyu You , Yang Yang , and Ping Guo

Journal of Manufacturing Science and Engineering, Jul 2020

Abstract PDF Video

It is challenging for the existing fabrication strategies to generate microscale wavy and coiling structures with low cost and high efficiency. In this work, we develop a novel and simple method that allows the fabrication of microscale wavy and coiling fiber arrays via near-field electrospinning (NFES). In addition to the main vertical electric potential for polymer jet generation, additional electrostatic signals are applied to the side-auxiliary electrodes to dynamically control the fiber deposition. Compared with traditional electrospinning based on the buckling instability or mechanical collector movement, the proposed method shows advantages in terms of the controllability, stability, accuracy, and minimal feature size. A theoretical model to describe the polymer jet behaviors has been proposed to simulate the fabrication process by considering the momentum balance of viscoelastic, charge repulsive, and electric forces. The model has been directly verified through the comparison with experimental results. The effects of different process parameters on the fiber deposition patterns are analyzed and discussed. The processing capability has been further demonstrated by fabricating two-dimensional wavy and coiling patterns as well as three-dimensional wavy structures with the radius of curvature less than 100 µm.
Modulated ultrasonic elliptical vibration cutting for ductile-regime texturing of brittle materials with 2-D combined resonant and non-resonant vibrations

Jianjian Wang , Wei-Hsin Liao , and Ping Guo

International Journal of Mechanical Sciences, Jul 2020

Abstract PDF

In this study, modulated ultrasonic elliptical vibration cutting (modulated UEVC) is proposed to generate micro-structured surfaces on brittle materials in ductile-regime. A novel tool is first developed to generate 2-D combined resonant and non-resonant vibrations in a single compact structure. The ultrasonic elliptical vibration is excited at the coupled resonant frequency of 20 kHz to enhance the ductile-to-brittle transition depth, while the simultaneously generated non-resonant modulation motion (up to 2 kHz) is used to adjust the tool center to generate surface structures dynamically. A theoretical model is established to analyze the instantaneous uncut chip thickness in modulated UEVC by considering a more general case of an inclined elliptical vibration trajectory. The analysis indicates that the orientation angle of 135° is optimal to achieve the maximal critical depth-of-cut in ductile-regime cutting. Experimental results are provided to demonstrate the process capability and to verify the proposed theoretical model. Micro dimple arrays have been successfully generated using the proposed modulated UEVC for a depth-of-cut up to 700 nm in ductile-regime, and an extended depth-of-cut up to 1 µm with minimal surface damage.

2019

An ultrafast 2-D non-resonant cutting tool for texturing micro-structured surfaces

Jianjian Wang , Hanheng Du , Shiming Gao , Yang Yang , Zhiwei Zhu , and Ping Guo

Journal of Manufacturing Processes, Jul 2019

Abstract PDF

Surface texturing using two-dimensional non-resonant vibration cutting tools (2-D NRVCT) has emerged as a promising method to fabricate micro-structured surfaces. The non-resonant vibration tool is able to work in a continuous frequency range with precise control of the tool trajectories compared with their resonant counterparts. The existing designs, however, usually suffer from either small vibration amplitudes or a low bandwidth due to a set of contradictory design requirements. This study presents a tool design that features a high bandwidth of up to 6 kHz without extra cooling, a working space of 9.3 μm × 16 μm, and a small cross-axis coupling within 5%. In addition, this study attempts to generalize the design requirements for 2-D non-resonant vibration cutting tools in five key functional specifications, including the stroke, output stiffness, resonant frequency, drive current, and coupling coefficient. Analytical modeling of the working performance on structural stiffness and output strokes using the compliance matrix method is presented and verified by finite element analysis. Dimensions of the proposed design have been optimized to achieve a balance between the amplification ratio and the bandwidth. The tool performance is then experimentally evaluated along with surface texturing results using the elliptical, quadrate and modulated elliptical tool trajectories.
A novel realization of diffractive optically variable devices using ultrasonic modulation cutting

Ping Guo , and Yang Yang

CIRP Annals, Jul 2019

Abstract PDF Video

Optically variable devices (OVDs) enable the angle-dependent optical effect for anti-counterfeiting. This paper proposes a multi-image encoding strategy and demonstrates single-, two-, and three-layer OVDs using ultrasonic modulation cutting. An analytical model that incorporates the diffractive intensity spectrum, resonance characteristics of diffraction efficiency, and grating geometry is presented to accurately predict the apparent color information in hue, saturation and brightness. Ultrasonic modulation cutting is proposed for ultrafast pixel-level rendering by tuning the grating spacing. Two-layer image encoding is achieved by interlacing gratings with spatially separated diffractive spectra, while a layer of relief image is added for three-layer image encoding.
Global tool path optimization of high-resolution image reproduction in ultrasonic modulation cutting for structural coloration

Yang Yang , and Ping Guo

International Journal of Machine Tools and Manufacture, Jul 2019

Abstract PDF Video

Diffraction gratings are capable of splitting and diffracting parallel white light into a wide diffractive spectrum containing light with different wavelengths travelling in different directions. The apparent angle-dependent color effect is a form of structural coloration. In this paper, a rendering strategy for grating-induced high-resolution image reproduction has been demonstrated based on tool path optimization. The grating structures are generated by ultrasonic modulation cutting due to the cutting depth modulation. The high-resolution image rendering is accomplished by tailoring the interaction between visible light and grating structures through the design and optimization of the distribution of machined grating arrays. Fabricating the desired grating arrays requires a complex objective tool path with ever-changing discontinuous step velocity profiles, which is not achievable for any existing machine tools considering the limited acceleration capability. Unlike conventional toolpath optimization methods, the proposed research aims to relate the cutting process to high-resolution image rendering performance and optimize the quality of machined images through optimally approximating the objective tool velocity-location curve with a series of parametric splines in terms of minimum overall velocity error. A recursive optimization method has been developed to ensure the global optimum of the tool velocity profile considering the physical limitation of motion axes. The fitting performance of proposed method is quantitatively analyzed and evaluated through the simulation and experiment by comparing the real-time velocity-location profiles to the objective curves. In addition, the reproduction quality of machined images is evaluated by measuring the similarity between the reproduced and original images with both simulation and experimental results.
A novel strategy to fabricate thin 316L stainless steel rods by continuous directed energy deposition in Z direction

Fei Weng , Shiming Gao , Jingchao Jiang , JianJian Wang , and Ping Guo

Additive Manufacturing, Jul 2019

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Thin 316L stainless steel rods were fabricated by continuous directed energy deposition in Z direction. The process parameters (laser power, scan velocity, and powder feeding rate) were carefully selected to obtain a stable deposition process and the effects of powder feeding rate and scan velocity were studied. A preliminary study on microstructure and tensile properties of the specimens was carried out. Results indicated that the specimen showed superior austenite/ferrite (γ/δ) dual phase microstructure, high strength (608.24 MPa), and good plastic deformation capacity (65.08% shrinkage rate) when setting the laser power at 45.2 W, powder feeding rate at 2.81 g/min, and scan velocity at 0.5 mm/s. The technique reported in this paper is expected to lay the foundation for the deposition of wire or frame structures more efficiently than traditional layer-by-layer directed energy deposition.
A support interface method for easy part removal in directed energy deposition

Jingchao Jiang , Fei Weng , Shiming Gao , Jonathan Stringer , Xun Xu , and Ping Guo

Manufacturing Letters, Jul 2019

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In traditional directed energy deposition (DED) processes, post-processing involving laser cutting or wire electrical discharge machining is necessary for removing printed parts from the substrate, which is time consuming and labor intensive. In this letter, a support interface method for DED is proposed for direct part removal without additional machining operations. A strut array is first printed as a sacrificial layer, upon which the actual part is then deposited. This letter demonstrates the method feasibility with a customized DED setup. It is expected that the proposed strategy will be beneficial in various DED processes to enhance process efficiency and automation.
On the validity of compliance-based matrix method in output compliance modeling of flexure-hinge mechanism

Jianjian Wang , Yang Yang , Ru Yang , Pingfa Feng , and Ping Guo

Precision Engineering, Jul 2019

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Compliance-based matrix method (CMM) has been regarded as an efficient technique for output compliance modeling of the flexure hinge-based compliant mechanism, owing to its simplicity and high accuracy. However, this study demonstrates that CMM is not always valid due to the intrinsic ill-condition of the compliance matrix of right circular flexure hinge (RCFH). Inversion of compliance matrix can result in numerical instability in the calculation of its stiffness matrix. It is shown in this study that CMM can be effectively applied to serial compliant mechanism, while its adoption in modeling parallel compliant mechanism needs to be carefully examined due to the matrix inversion involved. The validity of CMM is highly dependent on the spatial configuration, degree of freedom, and singularity of the parallel mechanism. The validity criteria of CMM are discussed in detail with exemplary configurations of 3RRR, 2RR, and bridge-type compliant mechanisms.
Friction reduction performance of microstructured surfaces generated by nonresonant modulation cutting

Chi Shing Yeung , Yang Yang , Hanheng Du , Jianjian Wang , and Ping Guo

Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, Jul 2019

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Microstructured surfaces can reduce friction force between two contact pairs in relative motion under hydrodynamic contact. In this study, we evaluate the friction reduction performance of microdimpled surfaces with different cross-sectional profiles generated by nonresonant modulation cutting. Computational fluid dynamics simulation was conducted to simulate the friction reduction effect between surfaces with lubricants. An elliptical modulation texturing method is introduced based on a two-dimensional nonresonant vibration tool, which could generate adjustable high-frequency elliptical tool trajectories. Different dimpled surfaces were generated using three types of tool trajectories. Their influence on the friction reduction and load-carrying capacity were experimentally studied and evaluated by comparing the simulation and experimental results.

2018

An active non-contact journal bearing with bi-directional driving capability utilizing coupled resonant mode

Ping Guo , and Han Gao

CIRP Annals, Jul 2018

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Non-contact journal bearings are conventionally based on pressurized air or magnetic levitation. This paper reports on a new design of an active non-contact journal bearing with bi-direction driving capabilities. It combines the functions of an axis positioner, non-contact journal bearing, and rotary motor utilizing coupled vibration modes. The shaft levitation is achieved by creating a stable air film using near-field acoustic force; while the non-contact rotation is realized by controlling the pressure distribution within the air film using coupled resonant mode. The essential design methodology and theoretical principles are presented along with the performance evaluation of the functional prototypes.

2017

Electric field manipulation for deposition control in near-field electrospinning

Xiangyu You , Chengcong Ye , and Ping Guo

Journal of Manufacturing Processes, Jul 2017

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Electrospinning is recognized as an efficient and versatile technique that has been widely used in nanoscale fiber fabrication. Electric field manipulation is one of the efficient ways to precisely control an electrospinning jet. While there have been several studies of the electric field manipulation effect on nanofiber deposition control, these works are limited to the control of a far-field electrospinning (FFES) jet to deposit in one dimension or to suppress the chaotic whipping mode to some extent. Few work has been done to control a near-field electrospinning (NFES) jet for deposition of complex patterns using electric field manipulation. To this end, we propose a novel design by adding a moving sharp-pin electrode beneath the plane collector. The sharp pin electrode is charged with a positive voltage and moved to redistribute the electric field for jet trajectory control, while the plane collector is kept stationary. The focusing of the electric field and the guiding effect of the jet trajectory due to the additional sharp-pin electrode are studied and demonstrated. Various parameters (voltage, electrode translation speed, and collection mechanism) are analyzed to experimentally study their effects on the fiber deposition control. It is demonstrated in the current work the feasibility of controlling a single fiber for deposition of complex patterns in near-field electrospinning by manipulating the electric field distribution.
Study of Microscale Three-Dimensional Printing Using Near-Field Melt Electrospinning

Xiangyu You , Chengcong Ye , and Ping Guo

Journal of Micro and Nano-Manufacturing, Jul 2017

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Three-dimensional (3D) printing of microscale structures with high-resolution (submicron) and low-cost is still a challenging work for the existing 3D printing techniques. Here, we report a direct writing process via near-field melt electrospinning (NFME) to achieve microscale printing of single filament wall structures. The process allows continuous direct writing due to the linear and stable jet trajectory in the electric near field. The layer-by-layer stacking of fibers, or self-assembly effect, is attributed to the attraction force from the molten deposited fibers and accumulated negative charges. We demonstrated successful printing of various 3D thin-wall structures with a minimal wall thickness less than 5 μm. By optimizing the process parameters of NFME, ultrafine poly (ε-caprolactone) (PCL) fibers have been stably generated and precisely stacked and fused into 3D thin-wall structures with an aspect ratio of more than 60. It is envisioned that the NFME can be transformed into a viable high-resolution and low-cost microscale 3D printing technology.
Vibration analysis and development of an ultrasonic elliptical vibration tool based on a portal frame structure

Yang Yang , Shiming Gao , Keyu Chen , Yayue Pan , and Ping Guo

Precision Engineering, Jul 2017

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An ultrasonic elliptical vibration tool utilizing the coupled resonant vibration modes is the key component in the elliptical vibration cutting/texturing process, which has been successfully applied to ultra-precision machining and surface texturing. In this paper, an analytical approach is proposed to analyze the resonant frequency and mode shapes of the ultrasonic elliptical vibration tool. A new design of ultrasonic elliptical vibration tool based on a portal frame structure is presented and analyzed using the proposed model. The model assumes Euler-Bernoulli beams and utilizes the transfer matrix technique to reduce the order of the system to only six variables. The model can be utilized to provide a systematic approach for an optimal design and be extended to dynamic analysis of the tool for study of machine-tool dynamics. Finite element simulation results as well as experimental data based on the prototype design are presented to verify the model. An application of the proposed tool is also demonstrated in machining micro/nano-structured surfaces for structural coloration.
Structural coloration of metallic surfaces with micro/nano-structures induced by elliptical vibration texturing

Yang Yang , Yayue Pan , and Ping Guo

Applied Surface Science, Jul 2017

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Creating orderly periodic micro/nano-structures on metallic surfaces, or structural coloration, for control of surface apparent color and optical reflectivity has been an exciting research topic over the years. The direct applications of structural coloration include color marking, display devices, and invisibility cloak. This paper presents an efficient method to colorize metallic surfaces with periodic micro/nano-gratings using elliptical vibration texturing. When the tool vibration is coupled with a constant cutting velocity, controlled periodic ripples can be generated due to the overlapping tool trajectory. These periodic ripples with a wavelength near visible spectrum can act as micro-gratings to introduce iridescent colors. The proposed technique also provides a flexible method for color marking of metallic surfaces with arbitrary patterns and images by precise control of the spacing distance and orientation of induced micro/nano-ripples. Theoretical analysis and experimental results are given to demonstrate structural coloration of metals by a direct mechanical machining technique.

2016

Self-running and self-floating two-dimensional actuator using near-field acoustic levitation

Keyu Chen , Shiming Gao , Yayue Pan , and Ping Guo

Applied Physics Letters, Jul 2016

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Non-contact actuators are promising technologies in metrology, machine-tools, and hovercars, but have been suffering from low energy efficiency, complex design, and low controllability. Here we report a new design of a self-running and self-floating actuator capable of two-dimensional motion with an unlimited travel range. The proposed design exploits near-field acoustic levitation for heavy object lifting, and coupled resonant vibration for generation of acoustic streaming for non-contact motion in designated directions. The device utilizes resonant vibration of the structure for high energy efficiency, and adopts a single piezo element to achieve both levitation and non-contact motion for a compact and simple design. Experiments demonstrate that the proposed actuator can reach a 1.65 cm/s or faster moving speed and is capable of transporting a total weight of 80 g under 1.2 W power consumption.

2014

Generation of hierarchical micro-structures for anisotropic wetting by elliptical vibration cutting

Ping Guo , Yong Lu , Kornel F. Ehmann , and Jian Cao

CIRP Annals, Jul 2014

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This paper reports enhanced anisotropic wettability using two-level hierarchical micro-structures. First-order micro-channels with superimposed second-order micro-textures were machined on aluminum surfaces using ultrasonic elliptical vibration cutting (EVC). Controllable sinusoidal micro-textures with a wavelength one order of magnitude smaller than the widths of the first-order micro-channels were applied to the surface. Anisotropic wettability was evaluated by water contact angle measurements in two orthogonal directions. The results have shown that the generated hierarchical micro-structures have nearly doubled the anisotropic contact angle in comparison to one-level structures. A relationship between anisotropic wetting characteristics and process parameters is also presented.
Fast Generation of Micro-Channels on Cylindrical Surfaces by Elliptical Vibration Texturing

Ping Guo , Yong Lu , Pucheng Pei , and Kornel F. Ehmann

Journal of Manufacturing Science and Engineering, May 2014

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Micro-structured surfaces are assuming an ever-increasing role since they define the ultimate performance of many industrial components and products. Micro-channels, in particular, have many potential applications in micro-fluidic devices, micro heat exchangers, and friction control. This paper proposes an innovative vibration-assisted machining method to generate micro-channels on the external surface of a cylinder. This method, referred to as elliptical vibration texturing, was originally developed by the authors to generate dimple patterns. It uses the modulation of the depth-of-cut by tool vibrations to create surface textures. The most promising features of the proposed method are its high efficiency, low cost, and scalability for mass production. It is shown that with proper combinations of the process parameters the created dimples start to overlap and form channels. An analytical model is established to predict channel formation with respect to the overlapping ratios of the dimples. Channel formation criteria and expressions for channel geometries are given along with a channel generation map that relates channel geometry to the process parameters. Experimental results are given to verify the model. A further example of micro-pattern generation is also given to showcase the flexibility of the process.

2013

An analysis of the surface generation mechanics of the elliptical vibration texturing process

Ping Guo , and Kornel F. Ehmann

International Journal of Machine Tools and Manufacture, May 2013

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The elliptical vibration texturing process is a vibration assisted machining method for the fast generation of micro structured surfaces. It adds a higher order motion component to the cutting tool that leads to periodic changes in the cutting depth during the machining process. This results in the creation of micro-dimples on the machined surface, whose shape is a function of the tool geometry and trajectory. This paper studies the surface generation mechanics of the elliptical vibration texturing process through experimentation and modeling. A surface generation algorithm is presented for this newly developed process. The model fully describes the motion and the 3D geometry of the cutting tool including its rake face, flank face, and the cutting edge, since all these tool features influence the topography of the generated surface. Since the process takes place in the micro/meso-scale cutting regime, the model includes the minimum chip thickness and elastic recovery effects. The experimental results are shown to validate the simulation model. The simulation model is used to characterize the influences of the process parameters on the texture patterns. The effects of the tool geometry on the process, including the cutting edge radius, are also analyzed.
Development of a tertiary motion generator for elliptical vibration texturing

Ping Guo , and Kornel F. Ehmann

Precision Engineering, May 2013

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The elliptical vibration texturing process is an innovative machining method for the fast generation of textured surfaces. It adds a tertiary motion component to the tool tip, which introduces deliberate elliptical vibrations between the cutting tool and the workpiece. The elliptical locus lies in the plane that is defined by the cutting direction and the radial direction in the turning operation. This paper proposes a new design for a resonant mode 2D tertiary motion generator (TMG) that can deliver the required elliptical trajectory at an ultrasonic frequency. The device works in the resonant mode, with tangential and normal vibrations at a nearly identical resonant frequency. Simulation and experiments were carried out to perform a modal analysis of the system. Different design parameters were adjusted to achieve large vibration amplitudes in both tangential and normal directions. The elliptical vibration texturing process was implemented by integrating the newly developed TMG into a turning operation. Preliminary test results of dimple array patterns are presented that validate the performance and principle of the proposed design.