Derek D. Lichti

Structural Deflection Measurement with a Range Camera

AbstractRange cameras offer great potential for the measurement of structural deformations because of their ability to directly measure video sequences of three-dimensional coordinates of entire surfaces, their compactness, and their relatively low cost compared with other active imaging technologies such as terrestrial laser scanners. Identified limitations of range cameras for high-precision metrology applications such as deformation measurement include the high (centimeter level) noise level and scene-dependent errors. This paper proposes models and methodologies to overcome these limitations and reports on the use of a SwissRanger SR4000 range camera for the measurement of deflections in concrete beams subjected to flexural load-testing. Results from three separate tests show that submillimeter precision and accuracy—assessed by comparison with estimates derived from terrestrial laser scanner data—can be achieved. The high-accuracy range camera results were realized by eliminating the systematic, scen...

IMU and Multiple RGB-D Camera Fusion for Assisting Indoor Stop-and-Go 3D Terrestrial Laser Scanning

Autonomous Simultaneous Localization and Mapping (SLAM) is an important topic in many engineering fields. Since stop-and-go systems are typically slow and full-kinematic systems may lack accuracy and integrity, this paper presents a novel hybrid “continuous stop-and-go” mobile mapping system called Scannect. A 3D terrestrial LiDAR system is integrated with a MEMS IMU and two Microsoft Kinect sensors to map indoor urban environments. The Kinects’ depth maps were processed using a new point-to-plane ICP that minimizes the reprojection error of the infrared camera and projector pair in an implicit iterative extended Kalman filter (IEKF). A new formulation of the 5-point visual odometry method is tightly coupled in the implicit IEKF without increasing the dimensions of the state space. The Scannect can map and navigate in areas with textureless walls and provides an effective means for mapping large areas with lots of occlusions. Mapping long corridors (total travel distance of 120 m) took approximately 30 minutes and achieved a Mean Radial Spherical Error of 17 cm before smoothing or global optimization.

Measurement of Deflection in Concrete Beams During Fatigue Loading Test Using the Microsoft Kinect 2.0

Abstract This study focuses on 3 Hz fatigue load testing of a reinforced concrete beam in laboratory conditions. Three-dimensional (3D) image time series of the beam’s top surface were captured with the Microsoft time-of-flight Kinect 2.0 sensor. To estimate the beam deflection, the imagery was first segmented to extract the top surface of the beam. The centre line was then modeled using third-order B-splines. The deflection of the beam as a function of time was estimated from the modeled centre line and, following past practice, also at several witness plates attached to the side of the beam. Subsequent correlation of the peak displacement with the applied loading cycles permitted estimation of fatigue in the beam. The accuracy of the deflections was evaluated by comparison with the measurements obtained using a Keyence LK-G407 laser displacement sensors. The results indicate that the deflections can be recovered with sub-millimetre accuracy using the centreline profile modelling method.

New Models for Scattering Bias Compensation in Time-of-Flight Range Camera Self-Calibration

AbstractThe geometric calibration of time-of-flight range cameras is a necessary quality assurance measure performed to estimate the interior orientation parameters. Self-calibration from a network of range imagery of an array of signalized targets arranged in one or two planes can be used for this purpose. The latter configuration requires the addition of a parametric model for internal light scattering biases in the range observations to the background plane due to the presence of the foreground plane. In a previous study of MESA Imaging SwissRanger range cameras, such a model was developed and shown to be effective. A new parametric model is proposed here because the scattering error behavior is camera model dependent. The new model was tested on two pmdtechnologies range cameras, the CamCube 3.0 and CamBoard nano, and its effectiveness was demonstrated both graphically and statistically. The improvement gained in the root-mean square of the self-calibration range residuals of 22 and 32%, respectively,...

Ray-Tracing Method for Deriving Terrestrial Laser Scanner Systematic Errors

Abstract A new ray-tracing method for deriving the effect of instrumental systematic errors in terrestrial laser scanner (TLS) instruments is proposed. Three cases of mirror inclination error for the panoramic scanner architecture are used as examples to demonstrate the applicability of the method. The results show that all three errors can be effectively compensated for by model variables already inherent to the augmented TLS positioning equations. Specifically, the effects can be compensated for by the collimation axis and trunnion axis error terms and the tertiary rotation angle of the exterior orientation parameters.

New angular resolution measure for 3-D laser scanners

Laser scanner angular resolution greatly depends on both the spatial sampling interval and laser beamwidth, though often the former is emphasized and the latter overlooked. Given the widespread use of 3-D laser scanners, a rigorous metric that unifies both factors is necessary to accurately model system resolution. A new resolution measure that incorporates both sampling and beamwidth is derived using an ensemble average linear system theory. Analysis of two commercially available scanning systems demonstrates the need for the new measure.

Robust Segmentation of Planar and Linear Features of Terrestrial Laser Scanner Point Clouds Acquired from Construction Sites

Automated segmentation of planar and linear features of point clouds acquired from construction sites is essential for the automatic extraction of building construction elements such as columns, beams and slabs. However, many planar and linear segmentation methods use scene-dependent similarity thresholds that may not provide generalizable solutions for all environments. In addition, outliers exist in construction site point clouds due to data artefacts caused by moving objects, occlusions and dust. To address these concerns, a novel method for robust classification and segmentation of planar and linear features is proposed. First, coplanar and collinear points are classified through a robust principal components analysis procedure. The classified points are then grouped using a new robust clustering method, the robust complete linkage method. A robust method is also proposed to extract the points of flat-slab floors and/or ceilings independent of the aforementioned stages to improve computational efficiency. The applicability of the proposed method is evaluated in eight datasets acquired from a complex laboratory environment and two construction sites at the University of Calgary. The precision, recall, and accuracy of the segmentation at both construction sites were 96.8%, 97.7% and 95%, respectively. These results demonstrate the suitability of the proposed method for robust segmentation of planar and linear features of contaminated datasets, such as those collected from construction sites.

An Egg Volume Measurement System Based on the Microsoft Kinect

Measuring the volume of bird eggs is a very important task for the poultry industry and ornithological research due to the high revenue generated by the industry. In this paper, we describe a prototype of a new metrological system comprising a 3D range camera, Microsoft Kinect (Version 2) and a point cloud post-processing algorithm for the estimation of the egg volume. The system calculates the egg volume directly from the egg shape parameters estimated from the least-squares method in which the point clouds of eggs captured by the Kinect are fitted to novel geometric models of an egg in a 3D space. Using the models, the shape parameters of an egg are estimated along with the egg’s position and orientation simultaneously under the least-squares criterion. Four sets of experiments were performed to verify the functionality and the performance of the system, while volumes estimated from the conventional water displacement method and the point cloud captured by a survey-grade laser scanner serve as references. The results suggest that the method is straightforward, feasible and reliable with an average egg volume estimation accuracy 93.3% when compared to the reference volumes. As a prototype, the software part of the system was implemented in a post-processing mode. However, as the proposed processing techniques is computationally efficient, the prototype can be readily transformed into a real-time egg volume system.

Analysis of Interpolation Methods for Kinematic DGPS Control in Aerial Photogrammetry

Kinematic differential Global Positioning System (DGPS) positioning is routinely used in industry for directly observing an aircrafts position at each instant of photographic exposure during a photogammetric survey. A critical aspect of the subsequent data processing is estimation of the aircraft position at the exact time of exposure. GPS measurements are acquired at a uniform sampling rate, typically 1 Hz. The exposure times, however, do not generally coincide with these times. As a result, the exposure station positions must be interpolated from the adjacent GPS positions. This is typically done using a low-order polynomial, expressed as a function of time, for each coordinate dimension. However, trajectory perturbations induced by atmospheric turbulence can render such interpolation methods ineffective.This article will convey the results of an investigation into the use of several different interpolation models with airborne GPS data during straight, level flight. The fundamental task of time series reconstruction will first be addressed, in which several possible interpolation models are described. Two 10-Hz, airborne GPS data sets were collected to test the accuracy of each model. The error properties resulting from the application of each model to these data will be presented and analyzed in terms of time-domain statistics and frequency-domain characteristids. It will be demonstrated that interpolation error can be significantly reduced, especially in the height dimension, through judicious choice of an interpolator.