Estimation of Measurement Uncertainty of the Real-Time Location System (RTLS) with Ultra-Wideband (UWB) Technology

Abstract

The need to control the real-time location of assets is increasingly relevant worldwide. The Ultra-wideband (UWB) technology is an IoT solution for real-time locating systems (RTLS). The location of the asset is obtained by the signal exchange between a wireless tag (asset) and fixed anchors. The tag interacts with the fixed anchors, defining its position through the distances obtained by trilateration. This data is sent to the server through the gateway. It is well-known that this process has several sources of errors. However, the measurement uncertainty assessment of UWB technology is an important topic regarding its scope of use. This paper presents a task-specific measurement uncertainty evaluation for the UWB positioning system, according to the ISO GUM. It aims to propose a method to support decision-making regarding the possible uses of UWB technology. The position provided by the UWB is compared with reference points using Cartesian coordinates that are measured with a total station, providing metrological reliability. Using the information from the estimated uncertainty, one can define the minimum tolerance interval associated with UWB technology for a given use. A case study demonstrates the method.

Comparison of Dimensional Accuracy between a Laser Scanner and a Laser Tracker with Handheld Scan in a Laboratory Setting

“…The results of dimensional precision and accuracy of the FARO Focus S 150 laser scanner, using the Leica T-Scan as a reference measurement method and sphere centers as reference points, are presented in this section. Primarily, the accuracy and precision results of coplanar and non-coplanar distances (up to 10 m) obtained in a laboratory environment with controlled climatic conditions are reported, along with the respective estimates of measurement uncertainties calculated according to the ISO GUM method [26][27][28].…”

“…For Tables 4 and 5, the largest standard deviation value of each method was used for the calculations. These sources were considered Type A contributions to measurement uncertainty [26][27][28], with a Student's t probability distribution [31,32], using a divisor of √ n, where n is equal to six measurements, and degrees of freedom (ν i ) equal to n − 1 [26][27][28]. The values used in the calculations are presented in Tables 4 and 5.…”

“…Another source of uncertainty was the sphere form error, which was measured with the GOM 3D optical scanner for each sphere, with the largest error being 0.716 mm obtained for sphere 6. This sphere form error was considered a Type B contribution to measurement uncertainty in both measurements with the Leica T-Scan and the Focus laser scanner, and it was considered as a rectangular probability distribution with a divisor √ 12, and infinite degrees of freedom, according to ISO GUM [26][27][28].…”

“…The calculations to estimate the measurement uncertainty (Tables 4 and 5) were performed following the ISO GUM methodology [26][27][28]. In this methodology, the expression of measurement uncertainty was formulated with two significant figures, and the measurement of the measurand (sample mean) was adjusted and rounded to the same number of decimal places as the already rounded measurement uncertainty [28]. Equation ( 1) was employed in the calculations, in accordance with the section "F.2.4.5 Uncertainty when corrections from a calibration curve are not applied" of the GUM [26].…”

“…The use of a laboratory environment with controlled climatic conditions prevents their influences from significantly interfering with the precision, accuracy, and measurement uncertainty determinations in this study. Therefore, one of the results of this study was the comparison between the precision results of coplanar and non-coplanar measurements, along with their measurement uncertainties, calculated according to the ISO GUM (guide to the expression of uncertainty in measurement) method [26][27][28]. The determination of precision, accuracy, and measurement uncertainty in coplanar and non-coplanar distances, using the FARO Focus S 150 laser scanner, provides a better understanding of the metrological behavior of this measurement system when used to determine the dimensions of randomly positioned objects within a volume in a scanning scene at distances up to 10 m.…”