Fire detection system using modulation ratiometrics

• US 5,838,242 A
• Filed: 10/10/1997
• Issued: 11/17/1998
• Est. Priority Date: 10/10/1997
• Status: Expired due to Term

• Current Assignee: Meggitt Safety Systems, Inc. (Meggitt PLC)
• Original Assignee: Whittaker Corp (Parker-Hannifin Corporation)
• Inventors: Marsden, Gary C.
• Primary Examiner(s): Hofsass, Jeffery A.
• Assistant Examiner(s): LA, ANH V
• Application Number:
  US08/947,501
• Time in Patent Office: 403 Days
• Field of Search: 340/628, 340/629, 340/630, 340/577, 340/578, 340/579, 340/508, 340/511, 340/522, 340/584, 250/554, 250/575
• US Class Current: 340/628
• CPC Class Codes: G08B 17/10 Actuation by presence of smoke or gases , e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means
• CPC Class Codes: G08B 29/26 by updating and storing reference thresholds

Abstract

The present invention utilizes the modulation ratio between the flicker of a sensor signal and the absolute signal average to detect fire conditions. The system of the present invention requires that the signal on a sensor channel be above a certain threshold and the ratio of the flickering portion of the signal to the absolute signal average be within a certain range. The system may be applied to any sensor signal in response to any source, including, but not limited to, radiation, acoustic or optical signals including ultraviolet, visible or infrared radiation. Signals may be filtered with a median filter to remove noise. A least-mean-square curve-fit is made to the data to account for any growth or decay in the fire signal. The flicker can be calculated using any of several metrics such as standard deviation, p-norms, or maximum deviation, but mean deviation seems to provide optimal performance. The modulation ratio system can be augmented with a separate detection scheme for large-scale fires. The system can also be modified to allow for increased sensitivity in the case of a previous alarm condition. Multi-channel modulation ratio systems may be configured such that each channel'"'"'s corresponding ratio and absolute signal average must meet the corresponding requirements before a fire alarm is declared. Multi-channel systems may include a form of cross-thresholding wherein the threshold of one channel is dependent upon the signal levels of the other remaining channels.

Claims

1. A method of establishing a detected fire condition from a source of incident radiation, said method comprising the steps of:

generating a sensor signal from a sensor responsive to incident radiation;

generating an absolute signal from said sensor signal;

determining an absolute signal average from said absolute signal;

generating a flicker signal from said absolute signal;

determining a flicker value from said flicker signal;

calculating a modulation ratio from said flicker value and said absolute signal average; and

providing an indication of a detected fire condition when said absolute signal average exceeds a predetermined threshold value and said modulation ratio is within a selected range of values.

2. The method of claim 1, wherein said sensor is responsive to acoustic radiation.

3. The method of claim 1, wherein said sensor is responsive to electromagnetic radiation.

4. The method of claim 3, wherein said sensor is responsive to a restricted bandwidth of electromagnetic radiation.

5. The method of claim 4, including the step of using an optical filter to restrict said bandwidth of electromagnetic radiation.

6. The method of claim 1, wherein said sensor is a thermopile.

7. The method of claim 5, including the step of using an optically filtered thermopile to restrict said bandwidth to include 4.4 um. infrared radiation.

8. The method of claim 1, wherein said sensor is a photodiode.

9. The method of claim 4, including the step of using a photodiode to restrict said bandwidth to include 940 nm. ultraviolet radiation.

10. The method of claim 1, wherein the step of generating said absolute signal includes using a DC filter.

11. The method of claim 1, wherein the step of generating said absolute signal includes generating said absolute signal devoid of bias from background radiation.

12. The method of claim 1, wherein the step of generating said absolute signal includes the steps of sampling an interval of said sensor signal at a selected rate and collecting a plurality of samples derived from said sampling.

13. The method of claim 1, wherein the step of generating said absolute signal includes using a median filter to remove noise.

14. The method of claim 1, wherein the step of generating said flicker signal includes using an AC filter.

15. The method of claim 1, wherein the step of generating said flicker signal includes using a three-point median filter to remove noise.

16. The method of claim 1, wherein the step of generating said flicker signal includes the steps of sampling an interval of said sensor signal at a selected rate and collecting a plurality of samples derived from said sampling;

and further including the step of generating a baseline determined by said flicker signal.

17. The method of claim 16, wherein the step of generating said baseline includes determining an average of said flicker signal.

18. The method of claim 16, wherein the step of generating said baseline includes determining a least-mean-square curve-fit of said collected samples.

19. The method of claim 16, wherein the step of determining said flicker value includes determining a p-norm of the deviation of said collected samples about said baseline.

20. The method of claim 16, wherein the step of determining said flicker value includes determining a mean deviation of said collected samples about said baseline.

21. The method of claim 16, wherein the step of determining said flicker value includes determining the maximum deviation of said collected samples about said baseline.

22. The method of claim 1, further including the steps of establishing a preselected large fire threshold value, comparing said absolute signal to said large fire threshold value in a large fire augmentation scheme, and providing an indication of a detected fire condition when said absolute signal exceeds said large fire threshold value.

23. The method of claim 1, further including the steps of establishing a preselected large fire threshold value, comparing said absolute signal average to said large fire threshold value in a large fire augmentation scheme, and providing an indication of a detected fire condition when said absolute signal average exceeds said large fire threshold value.

24. The method of claim 1, further including the steps of maintaining a record of previously detected fire conditions and using said record in determining said predetermined threshold value and said selected range of values.

25. The method of claim 1, further including using a plurality of sensors in a corresponding plurality of channels in a multi-channel system to establish individual detected fire conditions, each sensor being used as set forth in claim 1.

26. The method of claim 25, wherein establishing a detected fire condition from said multi-channel system includes the further step of receiving an indication of detected fire condition from each different channel.

27. The method of claim 26, wherein, for each of said channels, said predetermined threshold value is dependent on the absolute signal averages of the other channels of said multi-channel system.

28. The method of claim 26, wherein the predetermined threshold value of each individual channel is subject to modification by cross-thresholding from the sensor signals of sensors in the other channels of said multi-channel system.