A Microwave Sensor for Evaluation of Plastic Wall Thickness

摘要

Non destructive measurement of the thickness and monitoring its variation of lossy dielectric slab products is of great interest in many facets of industry. Methods are based on measurements of transmitted and/or reflected electromagnetic power from a dielectric material under test by an incident electromagnetic wave. The recorded amplitude and phase of the transmitted or reflected wave is analyzed and processed to create appropriate correlations with physical and/or dimensional variations of a material. Many techniques (transmission/reflection line, free space, open ended coaxial or waveguide probe) have been developed to measure these properties such as techniques in time domain or frequency domain with one port or two ports, etc. Every technique is limited to specific frequencies, materials and applications [1-3]. However, in some industrial processes used materials have a low permittivity or high loss; opposite wall is not metal-backed and only one side of materials is accessible. Reflectivity signal from the opposite wall is low, these methods unsuitable in industry. Resonant measurements are the most accurate. To reduce the sensor size, it is appropriate to use the resonance properties of microstrip antennas [4]. The lightweight construction and the suitability for integration with MICs (Microwave Integrated Circuits) are two more of their numerous advantages. Microstrip printed antenna technology is suitable for low cost manufacturing. This is important, since MICs are much easier to handle and less expensive than the alternative waveguides. Suitability of microstrip antennas for low-frequency applications have been demonstrated in [5]. Although microstrip antennas are widely used, there is a little research on the design of near-field microstrip antennas for evaluation of plastic wall thickness and monitoring its variation for industrial applications.