Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Patent
1991-06-19
1993-01-05
Harvey, Jack B.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
324639, 324642, 324663, 324690, G01N 2200, G01R 2726
Patent
active
051774453
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the detection of non-metallic materials, and in particular to a method and apparatus for determining the nature of non-metallic materials by examining their electrical properties.
It is a general characteristic of non-metallic materials that their electrical properties vary with the frequency of the electrical signal used to measure them. This is described, for example, in sections 11.9 and 11.10 of the book "Electricity and Magnetism" by W. J. Duffin (3rd edition, McGraw-Hill) in the chapter 11 on Dielectric Materials. There it is explained that the variation with frequency of the dielectric constant (or relative permittivity) .epsilon..sub.6 of a dielectric material stems from polarisation at the molecular level; electronic, ionic and orientational polarisation. Because the variation in electrical properties is related to the non-metallic material's molecular structure it is possible to identify a material by examining its electrical properties at different frequencies
For non-metallic materials which are composed of molecules having a significant dipole moment the variation in dielectric constant at low frequencies is mainly due to orientational polarisation. Materials of this nature tend to have a high dielectric constant at low frequencies and this falls in discrete steps as the frequency is increased. Each fall in the dielectric constant is accompanied by a peak in the electrical conductivity of the material. Thus it is possible to test an item for the presence of a particular material composed of molecules having a significant dipole moment by subjecting the item to several different specific frequencies and checking whether or not the characteristic peaks in conductivity occur.
Similarly, non-metallic materials may be identified and detected by using applied fields at higher frequencies and checking for characteristic effects in the variation in electrical properties due to ionic and/or electronic polarisation at the molecular level. It is to be understood that the embodiments of the invention described below with reference to detection of the effects of orientational polarisation may be modified so as to enable predetermined materials to be identified and detected through detection of the effects of the ionic and/or electronic polarisation.
These principles may be incorporated into devices for a number of different applications such as examining whether individuals are carrying drugs, testing freight for the presence of explosives, or analysing the composition of an object.
The present invention provides a method of detecting a pre-determined non-metallic material in a detection region, comprising the steps of:
subjecting the detection region to an alternating electric field at a plurality of frequencies;
measuring the response of the contents of the detection region to the alternating electric field;
comparing the measured response with stored information indicative of the frequency response of the predetermined non-metallic material; and
indicating the result of the comparison.
The present invention further provides apparatus for detecting a predetermined non-metallic material, comprising:
means for subjecting the detection region to an alternating electric field at a plurality of different frequencies;
means for measuring the response of the contents of the detection region to the alternating electric field;
means for storing information indicative of the frequency response of the predetermined non-metallic material;
means for comparing the measured response with stored information indicative of the frequency response of the predetermined non-metallic material; and
means for indicating the result of the comparison.
From a first aspect the method and apparatus may involve determining the frequency response of a material to applied electrical signals.
From a second aspect the method and apparatus may involve applying a large electromagnetic pulse to the non-metallic material and examining the energy re-emitted as a result by the non-metallic material.
Features and
REFERENCES:
patent: 3255410 (1966-06-01), Norwich
patent: 4058766 (1977-11-01), Vogel et al.
patent: 4322678 (1982-03-01), Capots et al.
patent: 4370611 (1983-01-01), Gregory et al.
patent: 4710550 (1987-12-01), Kranbuehl
Harvey Jack B.
Zetetic International Limited
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