Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2002-10-23
2004-08-31
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S370010
Reexamination Certificate
active
06784430
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a sensor and system for controlling emissions in an internal combustion engine and, more particularly, to use of a silicon carbide sensor to measure flame temperature of an internal combustion engine for controlling fuel injection.
Unnecessarily high turbine engine combustion temperatures can compromise fuel efficiency and increase emission pollution. For example, in a gas turbine designed to emit nine nitrogen oxide (NO
x
) particles per million (ppm), an increase from 2730° F. (1499° C.) to 2740° F. (1504° C.) reduces turbine efficiency by about two percent and increases NO
x
emissions by about two ppm. On an annual basis, this can amount to millions of dollars of lost revenue and to several tons increase in NO
x
emission.
A flame detector can be used in the turbine engine to detect initial flame during start up and then to control engine combustion and consequently to control NO
x
emissions. Typically, the flame detector is based on a silicon carbide (SiC) photodiode that senses electromagnetic radiation having a wavelength in the range of from about 190-400 nanometers, which is within the ultraviolet (UV) range. The photodiode generates a current (or “photocurrent”) proportional to the ultraviolet light intensity to which it is exposed. The output of the photodiode is amplified by signal conditioning circuitry to produce an electrical signal (either a voltage or a current) indicative of the intensity of a flame.
A SiC based photodiode is particularly advantageous for sensing flame temperature of an engine. The SiC exists in a relatively large number of different crystalline structures or forms, of which the 6H form and the 4H form (with “H” representing hexagonal crystalline packing) are the most readily available. Advantageously, the 6H form and the 4H form exhibit relatively wide band gaps of approximately 3 electron volts (eV). The wide band gap provides sensitivity to ultraviolet radiation, as well as low leakage current. A typical silicon diode operating at 300° C. exhibits leakage current on the order of 10 mA/cm
2
, while a 6H or 4H silicon carbide diode operating at the same temperature exhibits leakage current on the order of 1-10 pA/cm
2
, which is advantageously some six orders of magnitude less. This lower current leakage leads to lower noise, lower amplifier offset, and larger dynamic range (or operating range), thereby providing for the use, for example, of larger amplifier gains for the detection of a small number of UV photons required for initial flame detection during startup.
A 6H or a 4H silicon carbide photodiode begins to exhibit optical sensitivity to applied light at wavelengths of approximately 400 nm and exhibits a peak response at approximately 270 nm, depending on the thickness of the photodiode epitaxial layers. Hence, a 6H or 4H silicon carbide photodiode is substantially transparent to both infrared and visible light. Accordingly, a SiC detector can readily discern ultraviolet radiation that exists in the presence of a strong background of intense infrared and/or visible light. Hence, a SiC detector is a suitable high temperature turbine engine flame sensor, readily responding to ultraviolet emission produced in a jet engine while effectively ignoring all the infrared and visible radiation emitted by the heated parts of the engine.
The OH molecules in combustion flames have emission characteristics that reflect combustion flame temperature. The OH emission band is essentially isolated from weaker emission lines produced by other excited radicals. The SiC photodiode responsivity overlaps a hydrocarbon flame's strong OH band at 310 nm. Hence, a SiC diode can be used to detect even dim flames by detecting the strong OH band without being blinded by black body radiation from hot combustor walls.
Typically, a silicon carbide diode is manufactured using epitaxial growth beginning with either a 6H n or p type conductive substrate. If an n type conductive substrate is used, then a heavily doped p+ layer is epitaxially grown over the substrate. A lightly aluminum doped p type layer is then grown over the p+ layer. Alternatively, if a p type substrate is used, then the p− layer is grown directly over the substrate. Owing to the relatively low optical coefficient of absorption of silicon carbide, a relatively thick p− layer can be used to increase photodiode sensitivity to long ultraviolet wavelengths or the p− layer can be made relatively thin to decrease diode sensitivity to long ultraviolet wavelengths and thus to decrease its sensitivity to solar radiation occurring between 300 and 400 nm. Next, a nitrogen doped n+ layer can be epitaxially grown at a uniform thickness over the p− layer. A metallic contact can be formed on top of the n+ layer. Also, a metallic contact can be made to the back side of the p type substrate, or if an n type substrate is used, to an exposed portion of the top of the p+ layer.
Commonly assigned Brown, U.S. Pat. No. 6,350,988 patent teaches an optical spectrometer for combustion flame temperature determination. The spectrometer comprises at least two photodetectors positioned for receiving light from a combustion flame and having different overlapping optical bandwidths for producing respective output signals. The spectometer includes a computer for obtaining a difference between a first respective output signal of a first one of the at least two photodetectors and a second respective output signal of a second one of the at least two photodetectors, dividing the difference by one of the first and second respective output signals to obtain a normalized output signal and using the normalized output signal to determine the combustion flame temperature.
Copending commonly assigned Brown et al., U.S. application Ser. No. 09/793,432, filed Feb. 1, 2001, Publication No. 20010009268 teaches a dual diode SiC flame temperature sensor for combustion control systems with low dark current. The sensor has a flame temperature accuracy within about 20° F. (11° C.) in the temperature range of about 2500° F. (1371° C.) to about 3500° F. (1927° C.). The sensor comprises: a first photodiode device for obtaining a first photodiode signal, the first photodiode device comprising a silicon carbide photodiode and having a range of optical responsivity within an OH band; a second photodiode device for obtaining a second photodiode signal, the second photodiode device comprising a silicon carbide photodiode and a filter, the second photodiode device having a range of optical responsivity in a different and overlapping portion of the OH band than the first photodiode device; and a computer for obtaining a ratio using the first and second photodiode signals and using the ratio to determine the combustion flame temperature.
While the dual diode sensors of the Brown patent and Brown et al. application are improved, there is always a need for a photodiode with further improved light collection and sensitivity. Additionally, there is a need for a dual photodiode with minimum cross talk.
BRIEF DESCRIPTION OF THE INVENTION
The present invention provides a dual diode type sensor with the desired improved light collection and measurement accuracy and minimum cross talk. In a first embodiment, the invention is a flame sensor for combustion flame temperature determination, comprising a first photodiode formed with elongated extending digits, and having a range of optical responsivity within an OH band for producing a first output signal; and a second photodiode formed with elongated extending digits, and having a range of optical responsivity in a different and overlapping portion of the OH band than the first photodiode device OH band for producing a second output signal; wherein the elongated extensions are positioned with parallel interdigitated longitudinal axis with respect to one another. The invention also includes an optical spectrometer that comprises the sensor and a system that comprises the sensor.
In another embodiment, the invention relate
Brown Dale M.
Fedison Jeffrey B.
Kretchmer James W.
Matocha Kevin S.
Sandvik Peter M.
Cusick Ernest G.
Freedman Philip D.
Gabor Otilia
General Electric Company
Hannaher Constantine
LandOfFree
Interdigitated flame sensor, system and method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Interdigitated flame sensor, system and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Interdigitated flame sensor, system and method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3360617