Michael Allman, David Novotny, Scott Sandwith, Alexandra Curtin, Josh Gordon, October 2017
The accurate alignment of antennas and field probes is a critical aspect of modern antenna metrology systems, particularly in the millimeter-wave region of the spectrum.Commercial off-the-shelf robotic arms provide a sufficient level of positional accuracy for many industrial applications.The Antenna Metrology Project in the Communications Technology Laboratory at the National Institute of Standards and Technology has shown that path-corrected commercial robotic arms, both in hardware and software analysis, can be used to achieve sufficient positioning and alignment accuracies (positioning error ~ /50) for antenna characterization measurements such as gain extrapolation and near-field pattern out to 183 GHz [1].
Position correction is achieved using a laser tracker with a 6 degree of freedom sensor attached to the robot end effector.The end effector’s actual position, measured using the laser tracker, is compared to its commanded position and a path correction is iteratively applied to the robot until the desired level of accuracy is achieved in the frequency range of interest.At lower frequency ranges (< 40 GHz), sufficient positional accuracy can be achieved, without path correction, using a using a calibrated kinematic model of the robot alone [2].This kinematic model is based on knowledge of the link frame transformations between adjacent links and captures deviations due to gravitational loading on the joints and small mechanical offsets between the joints.Additionally, the calibration procedure locates the robot’s base frame in the coordinate system of the robot’s end effector.Each link frame is described by four physical quantities, known as Denavit-Hartenberg (DH) parameters [3].
We performed calibration measurements of our CROMMA system’s DH parameters over a working volume of ~1 m3.We then use the laser tracker to compare the robot’s positional accuracy over this working volume with and without the calibrated kinematic model applied.The path errors for the calibrated case set an upper frequency limit for uncorrected antenna characterization measurements.
[1]D. R. Novotny, J.A. Gordon, J.R. Guerrieri, “Antenna Alignment and Positional Validation of a mm Wave Antenna System Using 6D Coordinate Metrology, ” Proceedings of the Antenna Measurements Techniques Association, pp 247-252, 2014
[2]R.Swanson, G. Balandran, S. Sandwith, “50-micron Hole Position Drilling Using Laser Tracker Controlled Robots, ” Journal of the CMSC, Vol 9, No 1, Spring 2014
[3].J.J. Craig, “Introduction to Robotics: Mechanics and Control, 3rd ed.,” New Jersey, Prentice Hall, 2004, pp. 62-69