Electricity: measuring and testing – Electrostatic field
Patent
1994-01-31
1996-01-16
Wieder, Kenneth A.
Electricity: measuring and testing
Electrostatic field
324663, 324671, 324672, G01N 2760
Patent
active
054850924
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and device capable of electrostatically investigating a surface and/or sub-surface structure. In particular but not exclusively, the method and device in accordance with the present invention are capable of sensing variations in the composition and density of material structures, and the presence of an internal mass in a building structure.
2. Brief Description the Prior Art:
A known method, capable of carrying out scientific measurements of dielectric constants in wood and materials containing wood, uses a pair of electrically conductive plates of the same dimensions applied on opposite sides of the wooden sample to fill all the space between the plates with this sample. An alternating current is then applied to the capacitor formed by the two plates and the sample. Theoretically, the capacitor may be applied to several points of a piece of wood to detect changes in the dielectric constant thereof. However, this procedure presents numerous disadvantages: of the wooden sample, which is often impracticable; an internal member of an already erected building structure should be inspected; the structures under investigation; and calibration.
An adaptation of the above method is the application of a pair of capacitive plates on the same face of a tested material. As the intensity of the electric field is inversely proportional to the square of the distance between electrically conductive plates charged with opposite polarities, the sensitivity of such capacitive sensors and the applications thereof are severely restricted.
An "electronic wall stud sensor" is described in U.S. Pat. No. 4,099,118 granted to Franklin et al. on Jul. 4, 1978. This sensor includes two sets of electrically conductive plates applied on the surface of a wall to be investigated. The plates of the two sets are charged at different rates and the sensor measures the difference in the time for the plate sets to reach a given voltage threshold, this time difference varying with the density of the sub-surface density. Although the stud sensor of Franklin et al. may be effective in some applications, it however presents the following drawbacks: point of high density corresponding to the position of a stud, the stud sensor will not detect the studs; it will not detect the points of high density as long as the calibration is made over a point of low density through trial and error; wall where the density is higher than that of the point of the last calibration; indications in the presence of thicker surface material, as is often the case in wood lath and plaster walls and in gypsum board walls and ceilings where thick layers of plaster are present to level off joints; the thickness of such a finish and the generally significant variations in density from one board or plank to the other; fasteners used in walls and ceilings, and more generally in the case of wood board finish floors; electrically conductive materials, without depth discrimination;
U.S. Pat. No. 4,992,741 granted to Douglas et al. on Feb. 12, 1991, proposes a device for detecting objects behind a wall. This detecting device comprises a plurality of capacitor plates at spaced intervals and a circuit for sensing alterations in the dielectric constant of a region of the surface being inspected close to each capacitor plate. Display elements are respectively associated to the capacitor plates to display the signals detected through these plates and image the object being detected. Although this detecting device may be efficient in some applications, its sensitivity is limited since, as discussed hereinabove, the intensity of the electric field is inversely proportional to the square of the distance between the plates charged with opposite polarities. The device of Douglas et al. is also susceptible of performing erroneous readings due to the user's position with respect to the detecting device. Finally, it is complex and expensive.
OBJECT OF THE INVENTION
The object of the-present invent
REFERENCES:
patent: 4065715 (1977-12-01), Jaffe et al.
patent: 4099118 (1978-07-01), Franklin et al.
patent: 4145619 (1979-03-01), Tseng
patent: 4208695 (1980-06-01), Noda et al.
patent: 4310797 (1982-01-01), Butler
patent: 4464622 (1984-08-01), Franklin
patent: 4686454 (1987-08-01), Pecukonis
patent: 4767940 (1988-08-01), Tuttle
patent: 4992741 (1991-02-01), Douglas et al.
patent: 5315884 (1994-05-01), Kronberg
Brown Glenn W.
Wieder Kenneth A.
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