Robot-based techniques for spherical near-field antenna measurements

RWTH Publications (RWTH Aachen)

Abstract

The introduction of robot-based positioning systems in antenna measurements represents a significant technological advancement. The primary advantage of these systems is their flexibility, which enables the integration of various measurement techniques, including planar, cylindrical, spherical and irregular measurements, within a single antenna measurement facility. However, the application of robot-based systems in antenna measurement technology poses challenges due to the optimization for their initial application in the industrial environment of production lines. Specifically, the system-inherent interlinking of six axes results in greater tolerances of the absolute accuracy compared to conventional systems. Additionally, the precise triggering of the measurement instrumentation for a large amount of sampling points is associated with greater deviations. This dissertation investigates the specific technical aspects of robot-based systems in spherical near-field measurement technology as well as the potentials for compensating these impacts using the underlying theory of the spherical wave expansion (SWE). Therefore, a robot-based measurement system in the millimeter-wave range has been set up at the Institute of High Frequency Technology at the RWTH Aachen University. An error model for the evaluation of the most important impact factors is presented specifically for this system, along with a calibration procedure that enables the isolated identification of the individual error terms. This allows the compensation of mechanical tolerances in the applied setup by adapting the robot's measurement path and, thus, contributes to more precise measurement results. Furthermore, the actual position and orientation at the respective sampling points are provided by an internal feedback loop of the robot controller. Therefore, the non-ideal measurement positions can be considered in the transformation to the far field based on the SWE. In order to compensate for inaccurate orientations, a pointwise correction scheme using the spherical mode spectrum of the probe antenna is implemented within the SWE. Further analyses focus on the advantages of spline-based measurements. These allow for a significantly improved precision in triggering the measurement instrumentation when reaching a sampling point. In comparison to the conventional trigger implementation, the resulting errors can be reduced by a factor of ten. In addition, spline paths enable the efficient implementation of irregular sampling grids. This is demonstrated by the example of a mathematically optimized, non-redundant distribution of sampling points. Such sampling grids cannot be implemented with most spherical positioners or only in a stepwise and, thus, inefficient scheme. The application of spline-based measurements using robotic systems allow to perform the irregular measurement in only 56% of the duration of the corresponding equiangular sampling grid. Therefore, the combination of the versatile applicable principles of the SWE with the flexibility of robot-based systems provides new perspectives in antenna measurement technology which were previously mainly of theoretical interest.

Authors 1

1. R. Moch corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen