Scan Path Research Articles

Template-Based Category-Agnostic Instance Detection for Robotic Manipulation

An intelligent robotic system is one of the key pillars of a smart factory that requires flexibility to handle a variety of tasks. Perception is a key enabling technology for robots. Most existing object detection studies have mainly focused on category-specific objects and have achieved impressive performance. However, robotic systems, particularly in industrial scenarios, typically interact with many category-agnostic objects, which the robot must detect instantly without pre-training. Therefore, in this study, we proposed a template-based detection and segmentation approach, which incorporated a multi-level correlation model and a similarity-refine module, for handling the category-agnostic instance. The proposed approach was then validated and demonstrated in an interactive and adaptive robotic application scenario designed for the typical pick-and-place task. Among them, the picking scan path and location were instructed through human guidance with hand tracking. The neural rendering technology was also introduced to render novel views of the template. The proposed approach was evaluated using a benchmark and verified through a real demonstration.

Analytical modeling of temperature distribution in laser powder bed fusion with different scan strategies

This study proposes an analytical model to predict the transient temperature distribution and melt pool size in laser powder bed fusion (LPBF) process with different scan strategies. The scan strategies are divided into discontinuous scan path and continuous scan path. The moving Gaussian surface heat source solution and the instantaneous Gaussian surface heat source transient solution are applied for these two different scan strategies. The residual temperature linear superposition is adopted to consider the multi-track effect on the temperature distribution. The virtual negative power heat source method is utilized to consider the heat diffusion for the tracks that has been scanned of the build part. The image heat source method is used to consider the zero-flux boundary condition at the lateral surface of the build part. The proposed model is validated by the experimental data of the melt pool in single-track LPBF process. The analytical models of these two different scan strategies are verified by the comparison of the melt pool size. Based on the validated analytical model, it is extended for the multiple scan strategies combination processes. The temperature and melt pool predicted by the analytical model show good consistency with the FEM results. The analytical model method is much more efficient than FEM, which is about 120 times faster than FEM for the calculation of round-corner part. Due to the satisfied efficiency and accuracy in the prediction of melt pool size and the thermal history, the proposed analytical model has great potential application in optimization of the scan path planning and avoidance of the undesired residual temperature accumulation in the part.

Mechanical characteristic comparison of additively manufactured Ti–6Al–4V lattice structures in biocompatible bone tissue growth

This study proposed the development of Ti–6Al–4V lattice structures suitable for the growth of bone cells using laser powder bed fusion (PBF) and heat treatment of vacuum furnace tubes. The designed and fabricated Diamond (DIA), Trifolium (TF), and Trishuriken (TS) lattice structures had high mechanical strength and Young's modulus similar to human cancellous bone. The pore size, porosity, compressive strength, and internal defect of the lattice structures are also discussed herein. According to the experimental observations, after the holding temperature at 950 °C for 1, 2, and 3 h, the grain sizes of TS500 samples were 3.59, 5.05 and 7.87 μm2, respectively. The compressive strength and ductility of these lattice structures changed significantly after the heat treatment. Two kinds of internal defects appeared in PBF production, namely, the powder that did not melt completely during printing and the micro-spherical pores caused by the residual gas. These were found concentrated at the middle location of the cross of the laser scanning path. Furthermore, the defect distribution can be detected through micro computed tomography.

Simulation Analysis of Nanosecond Laser Processing of Titanium Alloy Based on Helical Trepanning

Titanium alloy is a type of high-strength material that is difficult to process. In particular, in the aerospace field, the processing accuracy of titanium alloy is high. Recently, laser processing has emerged as a new technology with high processing precision. However, the laser processing methods have obvious differences in processing accuracy and effect. Among them, the laser spiral scanning method plays an important role in welding and drilling, but owing to the complexity of the laser molten pool behavior, there have been limited studies on the material removal mechanism based on laser spiral scanning. To understand the variable process of titanium alloy melt pool in laser spiral scanning processing, a light heat conduction model with mass transfer source term was simulated. The effects of laser power, scanning speed, and scanning path on the morphology were studied. The simulation results show that the unit energy density was the main factor for material removal, and the distribution of the material temperature affected the size of the recast layer. The experimental and simulation results were compared, and good agreement between them was observed. This study can provide a research foundation for the further application of laser spiral scanning technology.

Inspection of Cracks in the Piston Rod of a Hydraulic Cylinder Using Injected Alternating Current-Field Measurement

This paper proposes a method of injected alternating current-field measurement (IAC-FM) for detecting orthogonal cracks in the piston rod of a hydraulic cylinder in a gate hoist. Using this method, both longitudinal and transverse cracks can be detected at the same time. An alternating magnetic field is produced inside the steel rod by axially injecting an alternating current into the rod. The longitudinal crack perturbs the circumferential magnetic field, whereas the transverse crack perturbs the current in the axial direction. Analyses of the behaviors of the magnetic field in the vicinity of the cracks were proposed, using a three-dimensional finite element software. An experimental setup was built and validation experiments were performed. The effects of the operating frequency and scan path were also studied. The results verified the feasibility of the IAC-FM method and showed great potential for the inspection of in-service hydraulic cylinders of gate hoists.

Lap-shear strength and fracture behavior of CFRP/3D-printed titanium alloy adhesive joint prepared by hot-press-aided co-bonding

Multimaterial structures comprising carbon fiber reinforced plastics (CFRPs) and titanium alloys can be widely utilized in the aerospace and automotive industries to achieve optimal weight reduction and reliable structures. In this study, three-dimensional-printed titanium (3DP-Ti) adherends prepared using selective laser melting were co-bonded with woven-fabric carbon fiber reinforced phenolic matrix composite by hot pressing. Upon controlling the scanning speed and path, the 3DP-Ti adherends were prepared with various porosities and macrostructures on their surfaces, and the influences of these surface structures on the lap-shear strengths and fracture morphologies of the 3DP-Ti adherends were investigated. The lap-shear tests of the lap-shear joints with flat 3DP-Ti and pyramidal-macrostructured 3DP-Ti displayed the cohesive failure of the phenolic resin matrix. In contrast, the specimen with cylindrical macrostructure exhibited CFRP fracture and delivered the highest bond strength of 20.6 MPa, which was 18–64% greater than the strength of the lap-shear joint with the conventional Ti alloy plate and 3DP-Ti adherends with flat surface and pyramidal macrostructure. The major possible mechanisms behind the failure mode transition and the highest bond strength included the mechanical interlocking between the cylindrical macrostructure and CFRP.

Defect and Distortion in Powder Bed Fusion of Metal Additive Manufacturing Parts

One of the state-of-the-art technologies for metal fabrication is laser powder bed fusion, which includes the following printing techniques: selective laser melting, selective laser sintering, direct metal laser sintering, and electron beam melting. This work examines defect formation in laser powder bed fusion, predominately focusing on selective laser melting. It also explores recent research findings on defect formation and classification and analyzes various internal defects, such as porosity, lack of fusion, balling, and solidification cracking. The influence of process parameters on defect formation and the effect of defects on mechanical properties are analyzed. This review also discusses defect inspection technologies (melt pool, scan path, and slice monitoring), defect mitigation strategies (online detection, process parameters, and numerical simulation), and their applications in additive manufacturing, such as laser powder bed fusion. This review would aid manufacturers in determining the root cause of defect formation and developing inspection technologies and mitigation strategies.

Vision-Based Approach for Autism Diagnosis Using Eye Tracking based on Machine Learning and Deep Learning

Although the early diagnosis of autism spectrum disorder (ASD) is highly sought, it is still a challenging task that requires a battery of cognitive tests and hours of clinical exams to be properly accomplished. ASD may manifest itself in various ways, which makes diagnosing it even more challenging. However, although diagnostic tests are generally devised by specialists, human mistakes are still possible. Computer-assisted technologies can be quite useful in this respect for aiding in the selection procedure. The use of eye tracking as a crucial component of ASD screening evaluation based on the distinctive characteristics of the eye gaze is continued in this article. This research adds to the growing body of evidence that eye tracking technologies may help in ASD screening. Eye tracking, visualization, using machine learning and deep learning are part of the suggested solution. Eye-tracking scan paths were first converted into a visual representation in the form of a series of pictures. After that, an image classification job was taught to a convolutional neural network. The findings of the experiments showed that using a visual representation simplified the diagnostic process while also achieving excellent accuracy. The convolutional neural network model, in particular, can attain promising classification accuracy. This shows that visuals are capable of encoding information about gaze movements and its underlying dynamics. Based on the greatest information coefficient, we examined potential relationships between autism severity and eye movement dynamics. The results demonstrate that using the great potential in combining eye tracking, visualization, machine learning, and deep learning to create an impartial tool to help in ASD screening. Generally, the approach we propose may be used to identify various diseases, including neurodevelopmental problems.

A novel general-purpose three-dimensional continuously scanning laser Doppler vibrometer system for full-field vibration measurement of a structure with a curved surface

Three-dimensional (3D) full-field vibration measurement is significant to structures, especially those with curved surfaces. A triaxial accelerometer and a commercial non-contact 3D scanning laser Doppler vibrometer (SLDV) system are usually used in 3D vibration measurement of a structure. However, the triaxial accelerometer can lead to the mass-loading problem for a light-weight structure, and the 3D SLDV system can take a long time to complete scanning of a structure with a large surface in a step-wise scanning mode. This study proposes a novel general-purpose 3D continuously scanning laser Doppler vibrometer (CSLDV) system to measure 3D full-field vibration of a structure with a curved surface in a non-contact and fast way. The proposed 3D CSLDV system consists of three CSLDVs, a profile scanner, and an external controller, and is experimentally validated by measuring 3D full-field vibration of a turbine blade with a curved surface under sinusoidal excitation and identifying its operating deflection shapes (ODSs). A 3D zig-zag scan path is proposed for scanning the curved surface of the blade based on results from the profile scanner, and scan angles of mirrors in CSLDVs are adjusted based on relations among their laser beams to focus three laser spots at one location, and direct them to continuously and synchronously scan the proposed 3D scan path. A signal processing method that is referred to as the demodulation method is used to identify 3D ODSs of the blade. The first six ODSs from 3D CSLDV measurement have good agreement with those from a commercial 3D SLDV system with modal assurance criterion values larger than 95%. In the experiment, it took the 3D SLDV system about 900 s to scan 85 measurement points, and the 3D CSLDV system 115.5 s to scan 132,000 points, indicating that the 3D CSLDV system proposed in this study is much more efficient than the 3D SLDV system for measuring 3D full-field vibration of a structure with a curved surface.

Effect of Scanning Speed on Properties of Laser Surface Remelted 304 Stainless Steel

In order to study the microstructure and properties of stainless steel after laser surface remelting, based on the theory of laser surface remelting, a simulation model of nanosecond-pulsed laser surface remelted stainless steel was established to study the evolution law of the Marangoni force of the molten pool during laser surface remelting. A single-lane laser remelting experiment was performed to study the variation of the scanning speed on the remelting width, roughness, and layer microtopography. The “S” scanning path was used to remelt the stainless steel surface to investigate the bonding force between the remelted layer and the substrate, the hardness, microscopic morphology, and corrosion resistance. The results show that the viscosity of the liquid metal in the molten pool increases with the increase of the scanning speed. Larger liquid viscosity and smaller surface tension temperature gradients promote a weaker flow of liquid metal, which reduces the velocity of the liquid metal flow in the molten pool. With the increase of scanning speed, the remelting width gradually decreases, but the roughness gradually increases. When the element content of Cr increases, the element content of Fe and O decreases. The surface is covered with an oxide film, the main components of which are oxides of Cr and Fe, the remelted layer is greater than that of the substrate, and the corrosion resistance is improved. Laser surface remelting technology can improve the structure and properties of 304 stainless steel.

Development of an on-machine measurement system with chromatic confocal probe for measuring the profile error of off-axis biconical free-form optics in ultra-precision grinding

Ultra-precision grinding of large-scale free-form optics made of hard brittle material is faced with several thorny problems, such as the grinding process is complex, hard to estimate the wear of the grinding wheel and the on-machine measurement technique of profile error is immature, which seriously affects the rapid convergence of profile accuracy. The on-machine measurement of profile error can effectively avoid the re-installation error introduced into the compensation period, and then the effectiveness and machining efficiency of compensation grinding are enhanced. Therefore, we developed an on-machine measurement system with a chromatic confocal probe for measuring the profile error of off-axis biconical free-form optics in ultra-precision grinding. First, the on-machine measurement system was developed on a 4-axis ultra-precision machine tool, and the scanning principles and position calibration were studied. Then, the scanning strategy was systematically researched, which included the centering approach, scanning path planning, scanning parameter calculation, and the evaluation principle of profile error. Finally, the on-machine measurement experiments were carried out on the off-axis biconical free-form optics with a width of 220 mm and height of 100 mm. The sub-micron variation range of measurement results is achieved by measuring the full-aperture profile error under different sampling parameters. The developed on-machine measurement system and scanning strategy can significantly improve the profile accuracy and machining efficiency of the large-scale free-form optics made of hard brittle material, and reduce the production cost.

Widefield OCT Imaging for Quantifying Inner Retinal Thickness in the Nonhuman Primate.

PurposeTo determine the agreement and repeatability of inner retinal thickness measures from widefield imaging compared to standard scans in healthy nonhuman primates.MethodsOptical coherence tomography (OCT) scans were acquired from 30 healthy rhesus monkeys, with 11 animals scanned at multiple visits. The scan protocol included 20° × 20° raster scans centered on the macula and optic nerve head (ONH), a 12° diameter circular scan centered on the ONH, and a 55 × 45° widefield raster scan. Each scan was segmented using custom neural network–based algorithms. Bland–Altman analysis were used for comparing average circumpapillary retinal nerve fiber layer (RNFL) thickness and ganglion cell inner plexiform layer (GCIPL) thickness for a 16° diameter region. Comparisons were also made for similar 1° × 1° superpixels from the raster scans.ResultsAverage circumpapillary RNFL thickness from the circular scan was 114.2 ± 5.8 µm, and 113.2 ± 7.3 µm for an interpolated scan path from widefield imaging (bias = −1.03 µm, 95% limits of agreement [LOA] −8.6 to 6.5 µm). GCIPL thickness from standard raster scans was 72.7 ± 4.3 µm, and 73.7 ± 3.7 µm from widefield images (bias = 1.0 µm, 95% LOA −2.4 to 4.4 µm). Repeatability for both RNFL and GCIPL standard analysis was less than 5.2 µm. For 1° × 1° superpixels, the 95% limits of agreement were between −13.9 µm and 13.7 µm for RNFL thickness and −2.5 µm and 2.5 µm for GCIPL thickness.ConclusionsInner retinal thickness measures from widefield imaging have good repeatability and are comparable to those measured using standard scans.Translational RelevanceMonitoring retinal ganglion cell loss in the non-human primate experimental glaucoma model could be enhanced using widefield imaging.

A Local Enhancement Method for Large-Scale Building Facade Depth Images using Densely Matched Point Clouds

In recent years, laser scanning systems have been widely used to acquire multi-level three-dimensional spatial objects in real time. The laser scanning system is used to acquire the three-dimensional point cloud data of urban scenes. Due to the large-scale characteristics of urban scenes, and the problems of scanning occlusion, scanning path, and limited scanning laser range, the laser scanning system cannot scan every object in the scene comprehensively, multidirectionally and finely, so the corresponding three-dimensional point cloud data collected by many objects are incomplete, and the data images are relatively sparse and unevenly distributed. The existing point cloud denoising and enhancement algorithms, such as AMLS, RMLS, LOP, and WLOP, all use local information to enhance the missing or sparse parts of the point cloud. This point cloud enhancement method is only limited to a small range and cannot do anything for the larger missing area of the point cloud. Even if it is done reluctantly, the effect is not satisfactory. There are a lot of repetitive and similar features in urban buildings, such as the repetitive areas of floors and balconies in buildings. These repetitive areas are distributed in different positions of point clouds, so the repetitive information has non local characteristics. Based on the nonlocal characteristics of building point cloud data and the repetitive structure of buildings, this article proposes a nonlocal point cloud data enhancement algorithm, which organizes the point cloud data in the repeated area into a set of basic geometric elements (planes). The structures are registered in a unified coordinate system, and the point cloud is enhanced and denoised through two denoising processes, “out-of-plane” and “in-plane.”

Development and Validation of a Calculation Routine for the Precise Determination of Pulse Overlap and Accumulated Fluence in Pulsed Laser Surface Treatment

In laser material processing, a variety of parameters like pulse fluence, total dose, step size, and pulse‐to‐pulse overlap are used to define and compare laser processes. Of these parameters, the pulse‐to‐pulse overlap can be the hardest to access as it is not implemented directly but instead depends on the spot diameter, its shape, and the respective scanning path that is used to cover the surface. This article shows that existing calculation routes overestimate the actual overlap by up to 21%. A novel calculation route is developed that greatly facilitates the determination of the pulse overlap and thereby the average number of laser pulses that interact with a given point on the surface. This approach makes it possible to achieve more reliable and comparable laser processes, which in return leads to better control of the procedure as the effect of individual parameters on a given output can be determined with greater precision.

Study on stress measurement for steel bars inside RC beams based on self-magnetic flux leakage effect

The self-magnetic flux leakage (SMFL) effect has been widely studied because it not only can identify early damage evaluation of ferromagnetic materials but is highly sensitive to the stress state. To promote the application of the SMFL effect in the health detection of reinforced concrete (RC) bridge, the whole process failure experiment of the RC beams under four-point bending loading was carried out and a method for detecting and quantitatively evaluating reinforcement stress in RC beams was established. A three-axis automatic displacement device based on metal magnetic memory was used to collect the magnetic signals of the RC beams under different loads, the magnetic signals under different values (LOVs) and multiple scanning paths were analyzed. Based on the magnetic signals under different stress states, a dimensionless parameter Aσ was defined to quantitatively calculate the steel bars stress of RC beams. The experimental results show that in the pure bending section of RC beam, the steel bars stress, and the dimensionless parameter Aσ emerged with a linear relationship. Furthermore, the effect of parameter lift-off values (LOV) on the test results was discussed.

Micromachining of Alumina Using a High-Power Ultrashort-Pulsed Laser.

We report on a comprehensive study of laser ablation and micromachining of alumina using a high-power 1030 nm ultrashort-pulsed laser. By varying laser power up to 150 W, pulse duration between 900 fs and 10 ps, repetition rates between 200 kHz and 800 kHz), spatial pulse overlap between 70% and 80% and a layer-wise rotation of the scan direction, the ablation efficiency, ablation rate and surface roughness are determined and discussed with respect to an efficient and optimized process strategy. As a result, the combination of a high pulse repetition rate of 800 kHz and the longest evaluated pulse duration of 10 ps leads to the highest ablation efficiency of 0.76 mm/(W*min). However, the highest ablation rate of up to 57 mm/min is achieved at a smaller repetition rate of 200 kHz and the shortest evaluated pulse duration of 900 fs. The surface roughness is predominantly affected by the applied laser fluence. The application of a high repetition rate leads to a small surface roughness Ra below 2 m even for the usage of 150 W laser power. By an interlayer rotation of the scan path, optimization of the ablation characteristics can be achieved, while an interlayer rotation of 90 leads to increasing the ablation rate, the application of a rotation angle of 11 minimizes the surface roughness. The evaluation by scanning electron microscopy shows the formation of thin melt films on the surface but also reveals a minimized heat affected zone for the in-depth modification. Overall, the results of this study pave the way for high-power ultrashort-pulsed lasers to efficient, high-quality micromachining of ceramics.

Numerical simulation on molten pool flow of Inconel718 alloy based on VOF during additive manufacturing

In this paper, the discrete element method is used to establish the model of the powder bed. The numerical simulation model is established by the finite volume method. The results shows that the heat diffusion is relatively slow when the laser act on the powder bed, which result in a small heat-affected zone of the powder bed and the base metal. The molten pool is a semi-elliptical shape, and the cladding layer formed after the molten pool solidifies is flat. As the heat source get closer to a certain point on the scanning path, the flow direction of the molten pool became uniform gradually from random at the beginning. The initial temperature of the powder bed become higher when the second track is deposited, and the depth of the molten pool is deeper than that of the first track. The second cladding layer overlap the first one well. The flow direction of the molten pool at a fixed point gradually change from disorder to uniform with the approach of the heat source. The flow direction of the molten pool in the remelting part always remain uniform.

Extracting Skeletons of Two-Manifold Triangular Mesh Surface for Planning Skeleton-Guided Five-Axis Surface Inspection Path

Abstract Skeleton curve-guided five-axis sweep scanning is an emerging surface inspection technique with marvelous inspection efficiency. The precondition of applying such a technique to a complex surface is to extract the skeleton curves of a two-manifold surface associated with partitioning the surface into compact surfaces. However, the work on extracting skeletons of two-manifold triangle mesh surfaces is scarce, as existing skeletonization methods mostly focus on either 2D planar shapes or 3D solid shapes. In this paper, we present a new approach to extract the skeletons on a two-manifold triangle mesh with boundaries. The skeletons are formed in the most intuitive manner of wave front propagation which is based on computing the initial value problem of geodesics on a triangle mesh. The step-length of wave front propagation is adaptively controlled to guarantee the appropriate density of skeleton points for having good connectivity. Experiments show that, as a direct application of the proposed skeleton generation and its associated surface partitioning result, the five-axis sweep scanning path of complex free-form surface can be generated conveniently. Experimental results also validate that the computation of our proposed approach is simpler and faster than the state-of-the-art geodesic Voronoi diagram (GVD) method when most of the two-manifold triangle mesh is nearly planar. Additionally, the augmented parameter used for tracing the wave front’s geodesic propagation information is helpful for the skeleton-based surface partition, which is necessary for skeleton curve-guided five-axis sweep scanning.

A Study on the Residual Stress of the Co-Based Alloy Plasma Cladding Layer.

The distribution law of residual stress in multi-channel scanned plasma cladding of Co-based alloy under different process parameters was studied by means of simulation and tests, and the optimum process parameters were optimized. The simulation model of the plasma cladding stress field was established by ABAQUS software, and the influence law of the working current, scanning speed, and scanning mode on the residual stress of the Co-based alloy multi-channel scanning was studied. A set of optimal cladding process parameters were obtained. The residual stress of the cladding layer was measured by the blind hole method and compared with the stress value in the finite element model. The results show that there is residual tensile stress on the surface of the cladding layer. The residual stress along the direction of the scanning path is greater than that along the direction of the scan sequence. The residual stress increases with the increase of the working current. The scanning speed is greater, and the residual stress is smaller. The residual stress of the short-edge scanning is greater than that of the long-edge scanning. The residual stress of the successive scanning is greater than that of the reciprocating scanning. The long-edge reciprocating scanning is the best scanning mode. The best combination of process parameters is the working current of 90 A, the scanning speed of 100 mm/min, and the long-edge reciprocating scanning mode.

Investigation on laser forming of open cell aluminum foam

Open cell aluminum foam having high porosity has the potential to increase the efficiency of a heat exchanger and also to be used for diverse other functions. However, being prone to fail easily under tensile mechanical load, their thermal forming using a laser has been proposed in the literature. This work investigates the effect of laser parameters, orientation-position-curvature of scan path, the number of scans, and foam thickness on the bending angle achieved while forming 95% porous pure aluminum (99.7% aluminum) open cell foam plates using a diode laser. Furthermore, the capability of laser forming to produce developable and nondevelopable surfaces out of this foam has been demonstrated. Higher line energy gave a higher bending angle. Under the same line energy, the combination of higher power-higher scan speed produced a higher bending angle. In contradiction to laser forming of the sheet metal, no saturation or reduction in bending angle per scan pass was observed with an increase in scan pass number. This observation could be explained with the help of cell densification by previous scan passes leading to an increase in the coupling of more thermal energy for subsequent scan passes. Scan paths with increased curvature (or less radius) also produced higher bending due to a higher amount of cell collapse in the irradiated region.