Prem K. Rachakonda

Methods and Considerations to Determine Sphere Center from Terrestrial Laser Scanner Point Cloud Data

The Dimensional Metrology Group (DMG) at the National Institute of Standards and Technology (NIST) is performing research to support the development of documentary standards within ASTM E57 committee. This committee is addressing the point-to-point performance evaluation of a subclass of 3D imaging systems called Terrestrial Laser Scanners (TLSs) which are laser-based and use spherical coordinate system. This paper discusses the usage of sphere targets for this effort and methods to minimize the errors due to the determination of their centers. The key contributions of this paper include the methods to segment sphere data from TLS point cloud and the study of some of the factors that influence the determination of sphere centers.

Determining geometric error model parameters of a terrestrial laser scanner through two-face, length-consistency, and network methods

Terrestrial laser scanners (TLS) are increasingly used in large-scale manufacturing and assembly where required measurement uncertainties are on the order of few tenths of a millimeter or smaller. In order to meet these stringent requirements, systematic errors within a TLS are compensated in-situ through self-calibration. In the Network method of self-calibration, numerous targets distributed in the work-volume are measured from multiple locations with the TLS to determine parameters of the TLS error model. In this paper, we propose two new self-calibration methods, the Two-face method and the Length-consistency method. The Length-consistency method is proposed as a more efficient way of realizing the Network method where the length between any pair of targets from multiple TLS positions are compared to determine TLS model parameters. The Two-face method is a two-step process. In the first step, many model parameters are determined directly from the difference between front-face and back-face measurements of targets distributed in the work volume. In the second step, all remaining model parameters are determined through the Length-consistency method. We compare the Two-face method, the Length-consistency method, and the Network method in terms of the uncertainties in the model parameters, and demonstrate the validity of our techniques using a calibrated scale bar and front-face back-face target measurements. The clear advantage of these self-calibration methods is that a reference instrument or calibrated artifacts are not required, thus significantly lowering the cost involved in the calibration process.

Metrological Evaluation of Contrast Target Center Algorithm for Terrestrial Laser Scanners

Terrestrial Laser Scanners (TLSs) are used in a variety of large scale scanning applications such as reverse engineering, assembly of aircraft or ships and surveying. Contrast targets are used with such instruments for enabling scene registration or to establish a scale when used on a scale bar. Currently, the algorithms to calculate the centers of contrast targets (CCT) are either proprietary to the original equipment manufacturers (OEMs) or not precise and accurate. Some of these algorithms may also operate only on OEMs proprietary data format. To overcome these limitations, a novel algorithm was developed at the National Institute of Standards and Technology (NIST) to calculate the center of contrast targets. Several targets in various orientations were scanned by one TLS and their centers were calculated by both the NIST algorithm and one OEM software. The results show that the NIST algorithm is robust, addresses many data quality issues and has better precision than the OEM software in most cases.

Relative range error evaluation of terrestrial laser scanners using a plate, a sphere, and a novel dual-sphere-plate target

Terrestrial laser scanners (TLS) are a class of 3D imaging systems that produce a 3D point cloud by measuring the range and two angles to a point. The fundamental measurement of a TLS is range. Relative range error is one component of the overall range error of TLS and its estimation is therefore an important aspect in establishing metrological traceability of measurements performed using these systems. Target geometry is an important aspect to consider when realizing the relative range tests. The recently published ASTM E2938-15 mandates the use of a plate target for the relative range tests. While a plate target may reasonably be expected to produce distortion free data even at far distances, the target itself needs careful alignment at each of the relative range test positions. In this paper, we discuss relative range experiments performed using a plate target and then address the advantages and limitations of using a sphere target. We then present a novel dual-sphere-plate target that draws from the advantages of the sphere and the plate without the associated limitations. The spheres in the dual-sphere-plate target are used simply as fiducials to identify a point on the surface of the plate that is common to both the scanner and the reference instrument, thus overcoming the need to carefully align the target.

In-situ Temperature Calibration Capability for Dimensional Metrology

Abstract: The Dimensional Metrology Group (DMG) at the National Institute of Standards and Technology (NIST) has developed a new in-situ temperature calibration system. This paper discusses the system components, an in-situ calibration procedure, and the uncertainty sources involved in the calibration process. It also presents an uncertainty budget, and examines it with a Monte Carlo simulation. This system enables the DMG to perform quicker in-situ temperature calibration, at frequent intervals with minimal downtime, and provides lower uncertainties for the dimensional measurements.