Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Patent
1994-03-04
1996-03-05
Wieder, Kenneth A.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
324639, G01R 2732
Patent
active
054970997
DESCRIPTION:
BRIEF SUMMARY
The present invention relates, in general, to an apparatus for detecting the concentration or level of accumulation of RF susceptible, particulate material on a filter medium and, more specifically, to an antenna system for use therein.
BACKGROUND OF THE INVENTION
As is well known, a filter is placed in the exhaust system of diesel engines to remove soot from the exhaust gases of the engine. The filter must be changed or cleaned from time to time to ensure that soot accumulations do not adversely affect engine operation. It is known to remove or incinerate the soot particles by subjecting the filter, in situ, to heat from a fuel burner or other heat generating device, or from suitable running of the engine. Incineration is to be performed when the accumulation has reached a level where further accumulation would adversely affect engine performance or before that incineration would produce excessive temperatures and possibly damage the filter. There is a need, therefore, for a method and apparatus which monitors the level of soot accumulation and provides a signal when the accumulation reaches a predetermined level.
Soot accumulations exhibit dielectric properties. Accordingly, it is possible to monitor the level of soot accumulation on a diesel engine filter medium by detecting changes in the effective dielectric properties of the filter medium. The complex permittivity of a material is comprised of two components: a real component called the "dielectric constant" and an imaginary component called the "dielectric loss factor". Changes in either of these components can be detected using RF interrogation methods. It should be mentioned at this point that the dielectric constant and loss factor of soot increases with increasing temperature. This affects both transmission and reflection (resonance) type of measurements.
One method of applying this concept to monitoring soot levels in diesel filters is to construct the filter housing or containment in the form of a RF waveguide and then periodically excite the waveguide with RF energy at a fixed frequency and measure the reflected power. The reflected power will be a function of soot accumulation on the filter. More specifically, for any RF system, it is usually possible to determine a frequency at which the electrical load, i.e. the filter medium, the diesel soot and the filter containment, represents a matched impedance with respect to the power source. In other words, the equivalent electrical resistance, capacitance and/or inductance of the load are matched to the RF power source. When the load impedance is perfectly matched to the power source, all emitted RF power is absorbed by the load. If the impedance is not matched to the RF source, some of the RF power will be reflected from the load. The degree of load mismatch determines the mount of reflected power and, hence, reflected power can be used to measure the change in the effective dielectric constant. This method can be generally referred to as a reflectance or resonance type of measurement.
U. S. Pat. No. 4,477,771 granted to the General Motors Corporation on Oct. 16, 1984 describes a method of detecting soot content in a particulate trap using this method. The method detects changes in the effective dielectric constant only. The patent provides a metal filter housing constructed as cylindrical waveguide which defines a closed, RF resonance cavity for receiving a ceramic filter. A single probe is positioned at one end of the cavity and behaves as both a transmitting and a receiving antenna. A reflective screen is positioned at the opposite end of the cavity. All connecting exhaust pipe diameters are below the cutoff diameter of a circular waveguide needed to transmit the RF energy at the frequencies used in the device. The probe is connected to a RF source through a directional coupler and an isolator. A detector is also connected to the probe through the directional coupler. In one mode of operation of the device, the RF source is operated at the resonant frequency of the cavity when the filter i
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Brown Glenn W.
Engine Control Systems Ltd.
Wieder Kenneth A.
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