An Overview of Activities at NIST Toward the Proposed ASTM E57 3D Imaging System Point-to-Point Distance Standard

Publication Details

Abstract

Terrestrial laser scanners (TLSs) are a class of 3D imaging systems that measure the 3D coordinates of an object in their work volume. They capture the 3D coordinates using a ranging unit mounted on two rotation stages that are orthogonal to each other as illustrated in Figure 1. They are used for a variety of applications, e.g., large scale assembly, surveying, forensics, reverse engineering etc. The Dimensional Metrology Group (DMG) at the National Institute of Standards & Technology (NIST), along with various other organizations has been involved in the development of a documentary standard for 3D imaging instruments that acquire data in spherical coordinate system. NIST led this effort and possesses unique expertise for this task. This expertise comes from NIST’s prior experience on standardization activities related to laser trackers which also use spherical coordinate system and have very similar error sources. TLSs were being used at NIST in several research projects starting in the early 2000s. It soon became clear that there was need to evaluate the instruments and determine the uncertainty of the measurements obtained. In this context, NIST organized three workshops between 2003 and 2006 which convened instrument manufacturers, end users and organizations like NIST to determine the needs of all the stake holders. During these workshops the participants agreed upon the need for standard terminology, artifacts and standardized protocols that are needed for evaluating TLSs. Based on these workshops, an ASTM subcommittee on 3D imaging systems was established in 2006, and a working group started work on evaluating the fundamental measurement of 3D imaging systems, their range. In 2015, this working group published the ASTM E2938-15 standard for 3D imaging systems to evaluate relative-range. The scope of the ASTM E2938-15 standard was limited to the evaluation of the relative-range of 3D imaging systems that acquire data in both the spherical and non-spherical coordinate systems. As this standard was being balloted, another working group was established in 2013 within the E57.02 sub-committee that started addressing the performance evaluation of these instruments over their entire work volume. The proposed standard titled “Standard test method for evaluating the point-to-point distance measurement performance of spherical coordinate 3D imaging systems in the medium range” was submitted for balloting in the spring of 2017. Challenges in developing the proposed standard included the lack of commercially available high quality targets, methods to obtain ground truth measurements and information about the sources of error. Other challenges included target mounting methods, data collection and data post-processing algorithms and to develop the performance criteria. The methods in the proposed standard were realized and tested at NIST before being incorporated into the document. This paper will present an overview of the work, the procedures and recommendations for evaluating TLSs that found consensus among the members of the working group and were included in the proposed standard.