Laser Triangulation Systems Research Articles

Real-time uncertainty reduction in laser triangulation via dynamic speckle correlation

The speckle-induced measurement uncertainty is one of the major bottlenecks to achieve high measurement accuracy in laser triangulation sensors (LTS). Conventional methods for improving uncertainty focus on suppressing speckles and refining centroid extraction algorithms, overlooking the potential information about object motion that speckles inherently contain. To address this deficiency, a dynamic speckle correlation (DSC) method is proposed to reduce speckle-induced measurement uncertainty in laser triangulation systems. This method uses the correlation between speckle patterns and the movement of the object being measured, thereby enabling real-time error compensation without any speckle reduction mechanisms. To verify the effectiveness of the DSC method, we have designed a compact LTS based on oblique incident laser. Extensive experiments demonstrate that, for displacement measurement of objects with surface roughness ranging from Ra0.4 to Ra3.2, the DSC method can effectively reduce the root mean square (RMS) measurement error to 3∼5μm. Compared with using the traditional grayscale centroid method alone, the measurement accuracy after DSC compensation is improved by at least 20%.

Enriching Existing 3D Pavement Condition Survey Datasets to Support Paving Project Cost Estimates, Project Planning, Designs, and Automated Machine Guidance

State departments of transportation (DOTs) typically perform annual pavement condition inspections, which serve as an important input into pavement management systems (PMS) software. Road surface defects (cracking, rutting, smoothness, etc.) are analyzed by PMS software to model the deterioration of pavements and to make budget and performance-based recommendations about which roads to maintain and how and when to maintain them. Increasingly at the state DOT level, these data are captured using high-speed 3D lasers (laser triangulation systems) that acquire the 3D shape of the road surface to evaluate its condition. Traditionally the capture of road elevation data relied entirely on the use of survey crews. Although accuracy can be quite high, the process of capturing elevations can require a lot of manpower, is time-consuming, requires lane closures, and results in a relatively small number of points per kilometer of road with which to perform all of the tasks from early project planning through construction. This paper explores an alternate approach that leverages existing 3D laser technology utilized by DOTs to measure the condition of in-service pavements. Typically, these laser systems capture “relatively referenced” 3D profiles of the roadway to evaluate pavement condition based on surface distortion. However, there is often no connection between these “relative” 3D profiles and real-world locations. This new approach involves the addition of high-accuracy blended global navigation satellite system + inertial navigation system positioning systems, as well as specialized software, to map the absolute position of 3D profiles in real-world coordinates.

Digitalization of directed energy deposition process through a multidirectional height monitoring sensor system

Industrialization of directed energy deposition (DED) process relies on the development and implementation of a quality assurance system that makes the process reliable and repetitive. One of the geometrical characteristics that needs constant monitoring and control during any DED process is the height of the deposited layer. In this paper, a multidirectional layer height monitoring system relying on the laser triangulation principle that has been developed for real-time direct measurement, in contrast to the majority of the sensor systems that derive layer height based on melt pool temperature, is presented. Existing commercial laser triangulation systems are unidirectional, which hinder the flexibility of the material handling system, like industrial robot, in most of the cases. An additional challenge lies on the integration with the deposition head due to its form, size, and extreme working conditions. Therefore, a novel configuration of laser triangulation sensors that enable multidirectionality and limit shadowing effect has been developed at Fraunhofer IAPT. In this paper, the development of such a sensor with a focus on the mechanical design has been presented. The sensor is designed for operation at high temperature and harsh environmental conditions close to the build zone. The developed sensor has been successfully tested for the powder-based laser metal deposition system. In-process layer height measurement and control are possible with this sensor, thereby enabling digitalization and quality assurance during the build process. 3D point cloud generated using this sensor can be used for the dimensional deviation measurement and also for downstream processes like machining. Such an in-process quality measurement and assurance system negates a postmeasurement step, thereby saving cost and time and ensuring quality.

A new static method of calibration for low-cost laser triangulation systems

As a non-contact high-efficiency method, laser triangulation has long been studied by many scholars. Building such a system usually requires calibration where system parameters are found or amended to achieve the targeted precision. Traditional calibration methods require using either precise movement or high precision gauge object that are accompanied with extra manufacturing cost and auxiliary equipment/devices and hence increase the overall cost as well as prevent widespread applications, for instance, due to required environment conditions to be fulfilled. A new static method was, therefore, proposed to calibrate a system without need of a motion or a high precision gauge object to be deployed. The method relies on preparation of the data on a scalar compound target with a geometric estimator as quality indicator. Based on this method, three versions of a low-cost prototype were built, which achieved sub-millimeter precision. Although the precision is sufficient for a measurement often used manufacturing large panels, higher precision is achievable with additional work which is to be undertaken in the future. The proposed method makes tradeoff between precision and cost. This was followed by testing on the measurement of a small panel part as application-case study. The proven feasibility of the method proposed suggested potential of developing low-cost, inline industrial system (large or small) for inspecting sheet parts during manufacturing, especially panel component manufacturing, including that from metal sheets and composite materials.

Geometrical Verification based on a Laser Triangulation System in Industrial Environment. Effect of the Image Noise in the Measurement Results

The use of laser triangulation systems is widely spread in industrial applications, especially in industrial metrology. These applications are usually affected by external perturbations, in particular, image noise and reflections can be an important error source for these kinds of sensors when high precision is required or when the scanning task is developed in an industrial environment. This research is focused in the improvement of the behavior of a laser triangulation sensor working with high noise images. The aim of the image analysis technique is to avoid or reduce the effect of the background noise on the measurement results (flatness). The analysis technique is tested with images captured in the industrial environment of the measurement system. The results show a proper behavior of the algorithm with high noise component images and the feasibility of the technique for use in the inspection of 100% of the production.

3D Geometrical Inspection of Complex Geometry Parts Using a Novel Laser Triangulation Sensor and a Robot

This article discusses different non contact 3D measuring strategies and presents a model for measuring complex geometry parts, manipulated through a robot arm, using a novel vision system consisting of a laser triangulation sensor and a motorized linear stage. First, the geometric model incorporating an automatic simple module for long term stability improvement will be outlined in the article. The new method used in the automatic module allows the sensor set up, including the motorized linear stage, for the scanning avoiding external measurement devices. In the measurement model the robot is just a positioning of parts with high repeatability. Its position and orientation data are not used for the measurement and therefore it is not directly “coupled” as an active component in the model. The function of the robot is to present the various surfaces of the workpiece along the measurement range of the vision system, which is responsible for the measurement. Thus, the whole system is not affected by the robot own errors following a trajectory, except those due to the lack of static repeatability. For the indirect link between the vision system and the robot, the original model developed needs only one first piece measuring as a “zero” or master piece, known by its accurate measurement using, for example, a Coordinate Measurement Machine. The strategy proposed presents a different approach to traditional laser triangulation systems on board the robot in order to improve the measurement accuracy, and several important cues for self-recalibration are explored using only a master piece. Experimental results are also presented to demonstrate the technique and the final 3D measurement accuracy.

3D CAD model generation of mechanical parts using coded‐pattern projection and laser triangulation systems

PurposeInvestigate the use of two imaging‐based methods – coded pattern projection and laser‐based triangulation – to generate 3D models as input to a rapid prototyping pipeline.Design/methodology/approachDiscusses structured lighting technologies as suitable imaging‐based methods. Two approaches, coded‐pattern projection and laser‐based triangulation, are specifically identified and discussed in detail. Two commercial systems are used to generate experimental results. These systems include the Genex Technologies 3D FaceCam and the Integrated Vision Products Ranger System.FindingsPresents 3D reconstructions of objects from each of the commercial systems.Research limitations/implicationsProvides background in imaging‐based methods for 3D data collection and model generation. A practical limitation is that imaging‐based systems do not currently meet accuracy requirements, but continued improvements in imaging systems will minimize this limitation.Practical implicationsImaging‐based approaches to 3D model generation offer potential to increase scanning time and reduce scanning complexity.Originality/valueIntroduces imaging‐based concepts to the rapid prototyping pipeline.

The Fourier slice theorem for range data reconstruction

This paper proposes a new approach to resolve the ambiguity problem in multistriping laser triangulation systems [1]. The application of this new method to circularly symmetrical objects can achieve time and space complexity of order one. No optical shutter is required in this new method. The new solution is based on the Fourier slice theorem, which forms the basis of X-ray CT (computed tomography) reconstruction. This paper demonstrates how this theorem can also be used to resolve multistripe laser ambiguity effectively.