A Novel Proximity Measurement System Using Microwave Antennas

In this paper, a novel approach for making proximity measurements using a microwave antenna system in an industrial setting is presented. The proposed technique is demonstrated by designing and building two differently sized antennas. Each antenna was placed inside its own mechanical housing in order to match the form and fit of a standard industrial eddy current sensor head. Matching the form and fit of currently accepted industrial dimensions will reduce the amount of redesign of the sensor mounts to accommodate the new system. Both of the differently sized antennas were designed to electrically resonate at a specific frequency. The first antenna was designed to resonate at similar to 3 GHz and the second antenna was designed to resonate at similar to 5.8 GHz. A vector network analyzer (VNA) is used to create the excitation signals and to monitor return loss changes as the observed target's displacement to the antennas is varied. Additionally, a linear stage from Thor Labs was used to carefully and repeatedly step a flat metal target away from the sensor head with the embedded antenna. This new system showed an extended measurement range compared to the conventional proximity measurement technologies. The microwave antenna system was able to measure a distance equal to at least 1 x the diameter of the probe tip. Also, good sensitivity, i.e., significant return loss change (> 1 dB/mm), was obtained as the observed the target displacement is varied. Furthermore, it showed great immunity to variations in permeability across a selection of metal targets. For example, at a fixed target displacement, the Sit variation between the different target materials was < 5%. A commercial software package, Ansys' HFSS, was utilized for the design, modeling, and analysis of the overall microwave antenna system. The modeling results matched the performance of the prototypes to within 3%. Future work in the areas of temperature stability, materials, and signal processing methodologies is also presented.