Flame detector using a discrete fourier transform to process amplitude samples from a flame signal

• US 5,073,769 A
• Filed: 10/31/1990
• Issued: 12/17/1991
• Est. Priority Date: 10/31/1990
• Status: Expired due to Fees

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Kompelien, Arlon D.
• Primary Examiner(s): Ng, Jin F.
• Assistant Examiner(s): ODA, CHRISTINE K
• Application Number:
  US07/608,054
• Time in Patent Office: 412 Days
• Field of Search: 340/578, 250/554, 364/576, 364/550
• US Class Current: 340/578
• CPC Class Codes: F23N 2223/08 Microprocessor; Microcomputer
• CPC Class Codes: F23N 2223/10 Correlation
• CPC Class Codes: F23N 2229/08 detecting flame flicker
• CPC Class Codes: F23N 5/082 using electronic means
• CPC Class Codes: G08B 17/12 Actuation by presence of radiation or particles, e.g. of infrared radiation or of ions

Abstract

A circuit for detecting presence of a flame receives a flame signal from a standard photocell positioned to receive radiation from the flame and digitally processes the amplitude variations in the photocell'"'"'s output to sense for the presence of frequencies near a frequency which is characteristic of a flame. The frequencies substantially higher than the characteristic frequency are filtered from the signal, and the remaining signal is sampled at a frequency which is preferably four times the characteristic frequency. The samples are converted to digital values and processed using a discrete Fourier transform. If the value resulting from the transform operation exceeds a preselected value, presence of a flame is essentially certain. Such digital processing allows use of a dedicated microcircuit or a microprocessor for the flame sensing function and avoids the need for many large discrete components.

Claims

1. A flame sensing system for providing a flame sense signal responsive to receiving a flame signal from a photocell receiving radiative energy from a flame having in its energy output intensity a flicker whose frequency is within a predetermined frequency range, said flame signal instantaneously representative of the intensity of the energy emanating from the flame, the system comprising

a) a low pass filter receiving the flame signal and providing a filtered flame signal from which a substantial percentage of the amplitude of frequencies above the predetermined frequency range has been removed;

b) a clock circuit providing a clock signal having individual pulses at four times the frequency of the midpoint of the predetermined frequency range;

c) an analog to digital converter receiving the filtered flame signal and the clock signal, sampling he filtered flame signal responsive to each clock signal pulse, and providing a digital flame signal having a plurality of successive discrete, ordinary designated, digital values each encoding the amplitude of the filtered flame signal at successive sampling instants;

d) a fist accumulator register receiving the digital flame signal and forming from the plurality of digital values in the flame signal, the sum of the difference of successive pairs of digital values having even oridinal designations, and providing after a sampling instant a first intermediate digital transform signal ascending he current contents o the fist accumulator register;

e) a second accumulator register receiving the digital flame signal and forming from the plurality of digital values in the flame signal, the sum of he difference of successive pairs of digital values having odd ordinal designations, and providing after a sampling instant a second intermediate digital transform signal encoding the current contents of the second accumulator register;

f) a calculator means receiving the first ad second intermediate digital transform signals, and providing a transform signal at lest approximately equal o the square root of the sum of the squares of the digital value encoded in each of the fist and second intermediate digital transform signals; and

g) a comparator means receiving he transom signal for comparing the digital value encoded in the transform signal with a predetermined transform constant value, and if generator than the transform constant value, issuing a flame sense signal signifying presence of flame.

2. The sensing system of claim 1 adapted for detecting a flame having a flicker frequency range of approximately 13 to 17 hz., where the clock circuit has a pluse rate of 60 hz. and wherein the low pass filter has a cutoff frequency of approximately 20 hz.

3. The sensing system of claim 1 wherein the clock circuit comprises a signal generator receiving a 60 hz. AC signal input and providing a 60 hz. square wave logic level signal whose frequency is that of the AC signal input.

4. The sensor of claim 1 wherein the calculator means comprises means for comparing the digital value encoded in the first intermediate digital transform signal with that encoded in the second intermediate digital transform signal, selecting the larger of the two intermediate digital transform signals, and forming the sum of the larger of the digital values encoded in the intermediate digital transform signals and one fourth of the smaller of the digital values encoded in the intermediate digital transform signals, and encoding this value in the transform signal.

5. A flame sensing system for providing a flame sense signal responsive to receiving a flame signal from a photocell receiving radiative energy from a flame having a flicker in its energy intensity whose frequency is within a predetermined frequency range, said flame signal instantaneously representative of the intensity of the energy emanating from the flame, the system comprising

a) a low pass filter receiving the flame signal and providing a filtered flame signal from which a substantial percentage of the amplitude of frequencies above the predetermined frequency range has been removed;

b) a clock circuit providing a clock signal having individual pulses at four times the frequency of the midpoint of the predetermined frequency range;

c) an analog to digital converter receiving the filtered flame signal and the clock signal, sampling the filtered flame signal responsive to each clock signal pulse, and providing a digital flame signal having a plurality of successive discrete, ordinally designated, digital values each encoding the amplitude of the filtered flame signal at successive sampling instants; and

d) a microprocessor for executing instructions and including data registers, an instruction memory containing a plurality of instructions, and a digital input channel, said microprocessor instruction memory including instructions for causing the microprocessor to function as;

i) a first accumulator register receiving through the input channel the digital flame signal from the analog to digital converter and forming from the plurality of digital values in the flame signal, the sum of the difference of successive pairs o f digital values having even ordinal designations, and providing at the end of each sampling interval a first intermediate digital transform signal encoding the current contents of the first accumulator register;

ii) a second accumulator register receiving through the input channel the digital flame signal from the analog to digital converter and forming from the plurality of digital values in the flame signal, the sum of the difference of successive pairs of digital values having odd ordinal designations, and providing at the end of each sampling interval a second intermediate digital transform signal encoding the current contents of the second accumulator register;

iii) a calculator means receiving the first and second intermediate digital transform signals and providing a transform signal approximating the square root of sum of the squares of the digital values encoded in each of the first and second intermediate digital transform signals; and

iv) a comparator means receiving the transform signal for comparing the digital value encoded in the transform signal with a predetermined transform constant value, and if greater than the transform constant value, issuing a flame sense signal signifying presence of flame.

6. The flame sensing system of claim 5 adapted for detecting a flame having a flicker frequency range of approximately 13 to 17 hz., where the clock circuit has a pulse rate of 60 hz. and wherein the low pass filter has a cutoff frequency of approximately 20 hz.

7. The sensor of claim 5 wherein the calculator means comprises means for comparing the digital value encoded in the first intermediate digital transform signal with that encoded in the second intermediate digital transform signal, selecting the larger of the two intermediate digital transform signals, and forming the sum of the larger of the digital values encoded in the intermediate digital transform signals and one fourth of the smaller of the digital values encoded in the intermediate digital transform signals, and encoding this value in the transform signal.