Submitted to: Journal of Cotton Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/5/2003
Publication Date: 2/10/2004
Citation: Pelletier, Mathew G. 2004. Multipath interference investigation for cotton bale microwave moisture sensing. Journal of Cotton Science. 8:170-178.
Interpretive Summary: In the production and processing of cotton lint in cotton gins, the control over moisture plays a key role in maintaining quality and productivity levels. The latest developments in this industry have seen a rise in the use of cotton moisture restoration systems. To date, only a few very expensive units are available which can be used to determine an accurate moisture content. This research is part of an ongoing research effort to develop a low cost non-contact accurate moisture sensor. The sensor under investigation is a microwave based system that is based upon measuring the electrical permitivity of the cotton bale. This measurement has provided accurate results in a lab environment, but has yielded varying calibrations when placed into operation out in commercial cotton gins. This portion of the research was undertaken to explore the possible causes of this observed phenomenon. This paper presents an electro-magnetic model that was developed to determine the cause in order to provide insights into potential solutions. The results of the simulations provide good agreement with the closed form solution of Maxwell's Electro-Magnetic equations for plane-wave propagation. When the model was used to simulate the system in a typical cotton gin that is comprised of a metal clad structure, the model gave a clear picture that the cause of the varying calibrations is due to the effects of the signal following multiple paths toward the receiving and analyzing antenna. The model also indicated that this affect could be calibrated out of the system to yield an accurate prediction of the moisture content of cotton bales within a metal clad structure.
Technical Abstract: The effects of multi-path interference upon the quantitative measurement of the permitivity for use in microwave cotton moisture measurement is explored. A finite difference time domain model (FDTD) was developed to aid in the development of a microwave moisture sensor. The prediction of cotton bale, microwaves, antennas, and structure interactions were investigated utilizing this model. The model was shown to have a good correlation to the closed form solution of Maxwell's Equations for free space microwave propagation. When the model was used to determine the effects of a surrounding metal structure, such as are typically found in cotton gins, it indicated that multi-path reflections severely affect the transmitted signal. It also indicated that this affect could be calibrated out of the system to yield an accurate prediction of the moisture content of cotton bales within a metal clad structure.