Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
2002-02-04
2004-08-31
Decady, Albert (Department: 2133)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
Reexamination Certificate
active
06784671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention, the Moisture and Density Detector (MDD), relates to an apparatus and method for detecting the moisture content (MC) and/or density of dielectric materials.
2. Related Art
Moisture Estimation Using Radio Frequency Signals
Several devices have been developed to measure moisture in materials. These devices are based primarily on resistance and capacitance principles. Resistance is the opposition of a body or substance to a current passing through it. Capacitance is the property of a circuit element that permits it to store charge. For resistance devices, a direct current (DC) radio frequency signal is passed through a dielectric material (a material that does not conduct electricity) and the signal strength is measured as a function of the resistance of the material. This resistance measurement is then converted to a moisture content (MC) value using correction factors for temperature and species. Capacitance devices measure capacitance value or “power-loss” and estimate MC based on known correlation.
U.S. Pat. No. 4,259,633 to Rosenau describes a resistance MC estimation technique. The technique applied by Rosenau and others is limited in that it requires that metal pins be inserted into the wood sample being tested. In addition, the electrolytic polarization effects when using DC voltage can result in measurement error. Inserted-pin resistance devices are considered to provide inaccurate estimates when the wood MC is above the fiber saturation point of 24 to 30 percent.
U.S. Pat. No. 3,600,676 to Lugwig et al. teaches the capacitance technique whereby an alternating current (AC) radio frequency capacitance device was developed using adjacent electrodes and resonance to determine the MC of bulk materials (i.e., coal, chips, etc.). This device applies a range of frequencies to the dielectric material adjacent to the electrodes. The frequency with maximum signal strength is termed the resonant frequency and is a direct function of the MC of the dielectric material. The Lugwig et al. device determines the resonant frequency at which signal strength (amplitude) reaches a maximum. Applicant's invention also applies a range of frequencies to the dielectric material and measures the signal strength of each in terms of amplitude. However, Applicant's invention does not determine the resonant frequency but rather relates the measured amplitude of each frequency to predetermined values to determine the MC of the dielectric material. In addition, in contrast to Applicant's invention, the Lugwig et al. device does not use phase shift as additional information to estimate MC or density.
U.S. Pat. No. 4,616,425 to Burns describes an opposed electrode device based on resistance or capacitance controlled oscillator circuits. Whether based on resistance or capacitance, this device requires conversion to a frequency-dependent DC voltage. Signal strength of the DC voltage is related to predetermined voltage values for the dielectric material to allow MC estimation. Direct contact with the dielectric material is required. In contrast to Burns, Applicant's invention does not employ conversion from AC signal to DC signal. In addition, direct physical contact with the wood surface is possible, but not necessary. Furthermore, Burns does not use measurement of phase shift to improve the MC estimate. The Burns device also has no capability to estimate dielectric material density.
U.S. Pat. No. 3,430,357 to Perry discloses an opposed electrode device that measures capacitive impedance and associated MC in a stack of lumber in a dry kiln. The resistance between a capacitance probe inserted several courses of lumber above a ground electrode gives a measure of stack MC in the lumber between the electrodes. This method requires direct contact between the capacitance probe and the lumber. With the Perry device, an AC signal is converted to a DC signal prior to measurement of the signal strength as voltage. Perry differs from Applicant's invention in that Applicant directly measures the strength of the AC signal. Perry also does not employ a phase shift measurement to improve the MC estimate. In addition, the Perry device has no capability to estimate dielectric material density.
U.S. Pat. No. 4,580,233 to Parker et al. teaches an adjacent electrode AC moisture sensing device with two alternating frequencies that measures the imbalance in a capacitance bridge to estimate the MC of dielectric materials. Circuitry and methodology is incorporated to correct for potential wood temperature differences. As with the Lugwig et al., Burns, and Perry disclosures, the AC signal is converted to a DC signal prior to measurement of voltage to determine signal strength. This differs from Applicant's invention, which directly measures the strength of the AC signal. In addition, Parker et al. does not employ phase shift either to improve the MC estimate or to allow for estimation of dielectric material density.
U.S. Pat. No. 5,402,076 to Havener et al. recites a portable device, similar to Perry's device, that measures MC in a stack of lumber but with the AC radio frequency signal transmitted between adjacent electrodes. As with Perry, Applicant's invention differs because Applicant measures the phase shift and has the capability to estimate wood density.
U.S. Pat. No. 5,486,815 to Wagner discloses an in-line AC moisture meter employing opposed capacitance electrodes to sense MC in lumber moving between the electrodes. A single 4 MHz frequency is transmitted between electrodes and the received signal strength is measured to provide an estimate of the wood MC. The 4 MHz signal is applied to two pairs of electrodes with a 20-volt peak-to-peak amplitude signal applied to one pair and a 4.5 volt peak-to-peak amplitude signed to the other. The 4.5 volt signal is applied 180° out-of-phase with the higher 20-volt signal. Wagner teaches that analysis of the out-of-phase signal responses reduces the effects on the signal of electrical loading of the material. Wagner differs from Applicant's invention because Wagner does not improve the estimate of MC by adding phase shift information and Wagner has no described capability to estimate the dielectric material density. This device is also limited to detection of MC below 24 percent.
The teachings described above have employed measures of signal strength of both resistance and capacitance electrodes to estimate dielectric material MC. Both AC and DC devices have been developed. However, none of the described devices are reportedly accurate in measuring MC above the fiber saturation point of approximately 24 to 30 percent MC. In addition, none have employed measurement of signal phase shift to improve their estimate of MC. Furthermore, none report the capability of estimating the density of the dielectric material by combined analysis of amplitude and phase shift of a radio frequency signal.
U.S. Pat. No. 5,086,279 to Wochnowski discloses a means for estimating MC in a stream of materials by both reflecting and passing electrical energy through the stream in the form of infrared, microwave, or energy generated by a high-frequency oscillator circuit. For each of the electrical energy types, the energy is both reflected from and transmitted through the material stream. The transmitted energy from the high-frequency oscillator may be inferred to be in the same radio frequency range as Applicant's invention, although Wochnowski did not define the spectrum.
The Wochnowski MC estimate of the stream of materials depends on measures of signal strength and phase shift with each obtained by two methods. The two methods are to obtain a reflected signal detected by a sensor on the same side of the stream of materials and also a through signal such as is obtained by an opposed or adjacent electrode configuration. Therefore, the MC estimate provided by Wochnowski depends partially on the correction for the mass of the stream of materials by analysis of the “damping of oscillation
Cooper Jerome E.
Steele Philip H.
De'cady Albert
Kelber Steven B.
Kerveros J.
Mississippi State University
Piper Rudnick LLP
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