Antenna pattern measurements require discipline and precise positioning of antenna measurement probes, especially at millimeter-wave frequencies and higher. In their configurable robotic millimeter-wave antenna (CROMMA) facility, members of the National Institute of Standards and Technology (NIST, Boulder, Colo.) developed a cost-effective antenna-pattern measurement system using robotic motion control to achieve antenna positioning within 25 μm RMS for precision near-field measurements. With the aid of a commercial VNA for measuring amplitude and phase, the robotic test system captured near-field and far-field data for a standard gain horn at 183 GHz.
Antenna pattern measurements can be quite challenging at millimeter-wave frequencies, because of the small wavelengths and the need to perform probe positioning with precision equal to a fraction of a wavelength for achieving pattern measurements with high fidelity. By combining six-axis robotics, optical spatial metrology, and a coordinated metrology approach, the NIST researchers created the CROMMA system for scanning antenna patterns in multiple configurations from 75 to 500 GHz. The positioning resolution goal for the system is better than 15 μm in support of antenna pattern measurements at 500 GHz. The system incorporates a six-axis industrial robotic arm and controller from MOTOMAN.
The system’s motion is simulated by means of computer modeling, using a kinematic model to determine the Denavit and Hartenberg (DH) parameters. The robot controller uses these parameters to define the kinematic activity of the robot arm and its antenna measurement probe, allowing the antenna and arm to be positioned with six degrees of freedom and precise resolution. The motion of the robotic arm is refined through a form of “teaching process” that allows it to adapt to almost arbitrary probe shapes and sizes for precise positioning of the antenna measurement probe.
This robotic arm is augmented by a second type of robot, a hexapod based on a parallel network of six prismatic actuators, as part of the CROMMA system. The hexapod actuators feature individual positioning accuracy of 500 nm for a combined positioning accuracy of 1 μm. In contrast to the larger robotic arm, this robotic positioner provides extremely high precision, but for a much smaller total volume.
Measurements were performed with a commercial four-port, 50-GHz VNA and frequency extenders for antenna pattern measurements at 183 GHz. The system covers a near-field radius of 100 mm and a far-field radius of 1000 mm and is currently being evaluated through 300 GHz, with the expectation of being capable of performing robotic antenna pattern measurements through 500 GHz. See “Millimeter-Wave Near-Field Measurements Using Coordinated Robots,” IEEE Transactions on Antennas and Propagation, December 2015, p. 5351.
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