Moving Laser Spot Research Articles

Experimental study of laser spot tracking for underwater optical wireless communication.

In this paper, a novel laser spot tracking algorithm that incorporates the Kalman filter with the continuously adaptive Meanshift algorithm (Cam-Kalm) is proposed and employed in an underwater optical wireless communication (UOWC) system. Since the Kalman filter has the advantage of predicting the state information of the target spot based on its spatial motion features, the proposed algorithm can improve the accuracy and stability of the moving laser spot tracking. A 2 m optical wireless communication experimental system with auto-tracking based on a green laser diode (LD) is built to evaluate the tracking performance of different algorithms. Experimental results verify that the proposed algorithm outperforms conventional tracking algorithms in aspects of tracking accuracy, interference resistance, and response time. With the proposed Cam-Kalm algorithm, the experimental system can establish an effective communication link, while the maximum tracking speed is 20 mm/s given the forward-error-correction (FEC) threshold.

A close look at temperature profiles during laser powder bed fusion using operando X-ray diffraction and finite element simulations

In laser powder bed fusion (LPBF), complex components are manufactured layer-by-layer via scanning the cross-sections of a 3D CAD model using a high intensity laser. Throughout this process, the material is exposed to temperature profiles that significantly differ from conventional manufacturing methods, and result in development of a unique and inhomogeneous microstructure and high levels of residual stresses in additively fabricated parts. The large temperature gradients and rapid cooling rates around the moving laser spot, and the overall heterogeneity of the temperature field need to be better understood in order to optimize the process parameters for increased production quality. In this study, operando X-ray diffraction (XRD) was employed to measure and compare temperature histories on the laser path under various processing conditions for Hastelloy X. Finite element thermal simulations were validated based on the acquired XRD data and then used as a supplementary tool to discuss the cooling behaviour and thermal heterogeneities across the geometry. The increase in the deposited energy density was qualitatively linked with higher temperature levels and slower cooling rates during LPBF. The melt-pool lengths showed strong sensitivity to the laser power and little variation with the scanning speed. Furthermore, even for a single set of parameters, large variations in the temperature field within the build were observed such that the cross-section edges located at higher build layers were exposed to markedly higher temperature levels.

Analysis of the remote laser cutting process induced damage in carbon fibre reinforced polymers

AbstractIn machining technologies for fibre reinforced polymers, remote laser cutting plays a special role as it can minimize the drawbacks of conventional laser cutting methods, in particular thermally induced damage such as charred edges and matrix evaporation. In this contribution, a thermal simulation model for the remote laser cutting process is presented. With the aim of a universally applicable model that works independently of the selected carbon fibres, matrix system, and fibre content, the effective thermal material parameters are calculated with a rule of mixture from the microscopic parameters for fibre, matrix, and process gas. The evaporation of matrix and fibre material is modelled with phase transitions. The moving laser spot is modelled with a Gaussian distribution as a heat input on the upper side. The emerging process gas is assumed to absorb a part of the laser power according to the Lambert‐Beer law.

ANALYSIS OF TEMPERATURE DISTRIBUTION IN LASER ALLOYING OF PURE COPPER

In many cases, the use of copper is limited by the unsatisfactory properties of its surface layer, i.e., low hardness and wear resistance. Laser surface-layer treatment may be a better alternative to other techniques used in surface engineering intended for the elements, whose high conductivity, combined with high functional properties, is required. In the present work, laser alloying of pure copper with Ni powder is performed. Thermographic measurements during the process and measurements of the melt-pool dimensions after the alloying are performed. A 3-D model of a cylindrical specimen is developed. The enthalpy-based material model involving the phase change is applied. The nickel powder is taken into account with an appropriate value of the workpiece absorptance in the heat flux boundary condition imposed in the moving laser spot area. This study utilized the ANSYS-based Simulation software. Results of the temperature simulation show acceptable agreement with the experiment. The developed model can be used to predict the temperature distribution and identify the workpiece absorptance.

Epitaxial re-solidification of laser-melted Ni-Mn-Ga single crystal

Additive manufacturing (AM) of magnetic shape-memory alloys (MSMAs) allows fuller use of geometry in the design of MSMA parts and avoids the segregation and high cost associated with single crystal production. While most research effort in AM of MSMAs pursues functional foams or polycrystals, epitaxial growth during liquid-phase AM may enable fully-dense single-crystalline MSMA parts, with associated availability of the full blocking stress. We melted a Ni51Mn24.4Ga24.6 single crystal with a moving laser spot under several process parameter combinations of laser power and velocity. While tracks created with lower laser travel velocity were almost entirely epitaxial, the track created with highest velocity (10 mm/s) included non-epitaxial columnar grains and grains at the top of the track. Synchrotron-based high-energy diffraction microscopy (HEDM) experiments revealed that mosaic spread of epitaxial material was slightly higher than that of surrounding non-re-solidified material. Our results demonstrate epitaxial growth of Ni-Mn-Ga with minimal grain content using full-melting laser processing.

Path planning for moving target tracking by fixed-wing UAV

For the automatic tracking of unknown moving targets on the ground, most of the commonly used methods involve circling above the target. With such a tracking mode, there is a moving laser spot on the target, which will bring trouble for cooperative manned helicopters. In this paper, we propose a new way of tracking, where an unmanned aerial vehicle (UAV) circles on one side of the tracked target. A circular path algorithm is developed for monitoring the relative position between the UAV and the target considering the real-time range and the bearing angle. This can determine the center of the new circular path if the predicted range between the UAV and the target does not meet the monitoring requirements. A transition path algorithm is presented for planning the transition path between circular paths that constrain the turning radius of the UAV. The transition path algorithm can generate waypoints that meet the flight ability. In this paper, we analyze the entire method and detail the scope of applications. We formulate an observation angle as an evaluation index. A series of simulations and evaluation index comparisons verify the effectiveness of the proposed algorithms.

A semi-analytical thermal modelling approach for selective laser melting

Selective laser melting (SLM) wherein a metal part is built in a layer-by-layer manner in a powder bed is a promising and versatile way for manufacturing components with complex geometry. However, components built by SLM suffer from substantial deformation of the part and residual stresses. Residual stresses arise due to temperature gradients inherent to the process and the accompanying deformation. It is well known that the SLM process parameters and the laser scanning strategy have a substantial effect on the temperature transients of the part and henceforth on the degree of deformations and residual stresses. In order to provide a tool to investigate this relation, a semi-analytical thermal model of the SLM process is presented which determines the temperature evolution in a 3D part by way of representing the moving laser spot with a finite number of point heat sources. The solution of the thermal problem is constructed from the superposition of analytical solutions for point sources which are known in semi-infinite space and complimentary numerical/analytical fields to impose the boundary conditions. The unique property of the formulation is that numerical discretisation of the problem domain is decoupled from the steep gradients in the temperature field associated with localised laser heat input. This enables accurate and numerically tractable simulation of the process. The predictions of this semi-analytical model are validated by experiments and the exact solution known for a simple thermal problem. Simulations for building a complete layer using two different scanning patterns and subsequently building of multiple layers with constant and rotating scanning patterns in successive layers are performed. The computational efficiency of the semi-analytical tool is assessed which demonstrates its potential to gain physical insight in the full SLM process with acceptable computational costs.

Vertical cracks characterization using lock-in thermography: I infinite cracks

Early detection of cracks is a challenging task to prevent failures in working structures. In the last decades the ‘flying spot’ method, based on heating the sample with a moving laser spot and detecting the surface temperature with an infrared detector, has been developed to detect cracks in a fast manner. The aim of this work is to measure the width of an infinite vertical crack using lock-in thermography. An analytical solution for the surface temperature of a sample containing such a crack when the surface is illuminated by a modulated laser beam focused at a fixed spot close to the crack is obtained. Measurements on samples containing calibrated cracks have been performed using an infrared camera. A least square fit of the amplitude and phase of the surface temperature is used to retrieve the thickness of the crack. A very good agreement between the nominal and retrieved thicknesses of fissure is found, even for widths down to 1 µm, confirming the validity of the model.

A high-speed, flexible-scanning chemical imaging system using a light-addressable potentiometric sensor integrated with an analog micromirror

A semiconductor-based chemical imaging sensor is a type of field-effect, label-free sensing system that can visualize a two-dimensional distribution of concentrations for specific chemical species on a sensor surface. This report presents the development of a high-speed, flexible chemical imaging sensor system using an analog micromirror as a light-addressing setup to scan the light-addressable potentiometric sensor (LAPS) surface from the underside. In the proposed system, a two-axis electrostatic comb-driven micromirror is used to control a modulated laser beam with rapid and tunable scanning capabilities. The position, X- and Y-axis step, direction of movement and speed of the moving laser spot can be arbitrarily defined using the programmed control on the angular rotation of the micromirror. A high-speed spatiotemporal recording of the change in pH at a rate of approximately 16fps (frames per second) using backside illumination has been achieved by the current setup. In addition, a high-resolution chemical image with 200k pixels of a test pattern in a sensor area of 14.5×10.5mm2 was achieved within 40s. The frequency-dependent photovoltage, photovoltage-reference bias voltage characteristics, and pH sensitivity are also demonstrated and discussed systematically for optimization.

Numerical Simulation of Multi-Track Laser Cladding Process

Multi-track laser cladding is necessary in the forming of parts. The overlap between neighbor tracks plays an important role, which determines the quality and surface smoothness of the clad layer. In this paper, numerical simulation of the multi-track laser cladding process is studied. The heat source of laser spot is applied by updating the profile of clad region and the location of the moving laser spot. The clad profile of overlapped track was obtained, from which the surface roughness was analyzed by the variation of overlap ratio. Meanwhile, experiments were carried out to evaluate the effect of overlap ratio. The simulated and the experimental results are in good agreement; both show that there is an optimal overlap ratio to achieve best surface roughness.

Enhancement of surface activity in CO oxidation on Pt(110) through spatiotemporal laser actuation

We explore the effect of spatiotemporally varying substrate temperature profiles on the dynamics and the resulting reaction rate enhancement of the catalytic oxidation of CO on Pt(110). The catalytic surface is "addressed" by a focused laser beam whose motion is computer controlled. The averaged reaction rate is observed to undergo a characteristic maximum as a function of the speed of this moving laser spot. Experiments as well as modeling are used to explore and rationalize the existence of such an optimal laser speed.

CMOS image sensor for spatiotemporal image acquisition

We present a 64-×64-pixel CMOS image sensor chip that can acquire a 16-gray-level image containing both spatial and temporal information of the moving luminous object under observation. The image can next be processed using any image processing software in order to determine the speed, trajectory, and direction of motion of the moving object. The architecture of the sensor and an example of its application to the determination of the speed of a moving laser spot are described.

Modeling of thermal processes in laser sintering of reactive powder compositions

A two-dimensional mathematical model describing a combination of the processes of self-propagating high-temperature synthesis and selective laser sintering of powder compositions (it is controlled by the laser-radiation energy) has been proposed. The model makes it possible to evaluate the correspondence of the geometric characteristics of the system to the values of the velocities of a moving laser spot at which the layer poured completely reacts in the vicinity of the spot.

Photoelectrochemically induced periodical deposition patterns of platinum on p-InP.

A kinetic model is presented which rationalizes the previous observation of spatially periodical platinum deposition induced photocathodically by a moving laser spot on p-InP. The model is similar to that of the Liesegang phenomenon. It includes the following processes: nucleation of ad-atoms, growth of nuclei, formation of a depletion zone, limitation of mass transport by diffusion, shift of the photocathodic reaction zone along with the moving laser spot. Computer simulations of the model equations reproduce the experimentally found periodical platinum deposition and demonstrate the spatiotemporal evolution of the number of nuclei, the deposition peaks and the corresponding hexachloroplatinate(IV) concentration profile. The influence of the model parameters is discussed.

Up-down asymmetry in vertical induced motion.

Induced motion (IM) is the illusory movement of an object in the direction opposite to the real motion of adjacent detail. One theory of IM suggests that it results, in part, from suppression of optokinetic nystagmus (OKN) by fixational (smooth-pursuit) effort. In several studies an asymmetry in human vertical OKN has been reported, with upward optokinetic stimulation eliciting higher OKN gain than downward motion. This provides a test of the nystagmus-suppression theory of IM. If suppression of OKN contributes significantly to IM, upward inducing stimuli should result in a greater magnitude of the illusion than should downward stimulus motion. Additionally, the asymmetry of vertical OKN should become more pronounced at higher stimulus velocities. Therefore, the asymmetry of vertical IM should be greater at higher inducing-stimulus velocities. Twelve subjects viewed a large, random-dot stimulus, which moved either upward or downward at a velocity of 10, 40, or 70 deg s-1. Subjects fixated a horizontally moving laser spot and adjusted a rod to match the apparent slope of the motion path of the spot. IM magnitude was derived from these measures. Mean IM velocity was significantly higher with upward than with downward stimulation, and the difference was maximal at velocities of 40 and 70 deg s-1. The results are discussed within the context of the nystagmus-suppression theory and other theories of IM.

Helium-Neon Laser: Thermal High-Resolution Recording

Scan-line recording by means of a moving laser spot has been achieved on metallic and organic thin films Recording rates of the order of one million spots per second were obtained with a laser beam power of 38 milliwatts at the recording surface. Typical recorded line widths were of the order of 2 microns.