GB2089972A - Noise filter for combustion gas analyzer - Google Patents

Abstract

A method and apparatus for removing noise from measurement signals from a sensor 12 used particularly in analyzing combustion gases, the apparatus including a rotating wheel 21 having a plurality of openings. As the wheel rotates, openings permit electromagnetic radiation from a source 11 to pass through the wheel 21 and through the combustion gas 7 to strike the sensor 12. The radiation 40 causes the sensor to generate an output signal indicative of the radiation and the background noise. At least one other time during the rotation of the wheel, a solid portion of the wheel prevents radiation emanating from the source from striking the sensor. At such time the sensor output signal will be solely a function of the noise in the system. By subtracting the sensor output signal corresponding to the obscured source from that corresponding to the exposed source, a more accurate analysis of the combustion gas(es) is achieved. <IMAGE>

Description

SPECIFICATION Noise filter for combustion gas analyzer This invention relates to method and apparatus for removing noise from measurement signals such as those produced by a sensor used in analyzing combustion gases.

Gas analyzers for measuring or detecting components in combustion gases are usually mounted in a smokestack or flue and typically consist of a source unit which supplies electromagnetic radiation of desired wavelengths (for example, infrared, ultraviolet, and/or visible light) and a detector or sensor unit for analyzing the electromagnetic radiation after the radiation has passed through the combustion gases. The source may be mounted on one side of the smokestack while the sensor is mounted on the opposite side. Alternatively, a mirror may be used to enable mounting of the source and sensor on the same side of the stack.

The source and sensor measure qualitatively and quantitatively the flue gas constituents by utilizing measurements of the absorption of specific frequencies of radiation by the flue gas.

The measurement process, however, is complicated by background radiation from the flue gases and the stack and by noise inherent within the sensor itself. Various techniques have been employed in prior art combustion gas sensors and detectors to remove this noise from the measurement signal. In one prior technique the background radiation was assumed to change at a low frequency, and the background radiation noise was removed using a high band pass filter.

Unfortunately the assumption that the background radiation changes at low frequencies is not correct in high temperature operations because both the frequency and the magnitude of the background radiation noise are high relative to the desired measurement signal.

This invention therefore provides a method for removing noise from measurement signals generated by a sensor in response to both noise and a source of electromagnetic radiation and comprises: during a selected time interval measuring an output signal from the sensor while the sensor is exposed to both noise and the source of electromagnetic radiation, during at least one time interval adjacent the selected time interval measuring the output signal from the sensor while the sensor is not exposed to the source of electromagnetic radiation, and subtracting the output signal measured during the at least one time interval from the output signal measured during the selected time interval to thereby produce an output signal representative of the source of electromagnetic radiation and not the noise.The time intervals are generated in one embodiment of the invention by moving an object having a desired number of openings through the path of the radiation emanating from the source. During intervals when an opening in the object permits the radiation to reach the sensor, the sensor output signal will supply a signal indicative of both the noise and the source of electromagnetic radiation. During intervals when the object prevents the radiation from the source from reaching the sensor, the sensor output signal will be indicative of only the noise. In other embodiments of the invention the at least one time interval adjacent the selected time interval comprises a time interval preceding the selected time interval and a time interval following the selected time interval.The sensor output signal is measured during both the preceding and the following time intervals, then the average output signal is determined and subtracted from the output signal during the selected time interval.

There is further provided apparatus for removing noise from measurement signals generated by a sensor in response to both noise and a source of electromagnetic radiation comprising: a source of electromagnetic radiation; a sensor for generating output signals in response to electromagnetic radiation to which the sensor is exposed; means for periodically preventing said sensor from being exposed to radiation from the source of electromagnetic radiation; and signal processing means for subtacting the output signal generated by the sensor when the sensor is not exposed to the source of electromagnetic radiation from the output signal generated by the sensor when the sensor is exposed to the source of electromagnetic radiation to thereby provide an output signal indicative of only the source of electromagnetic radiation and not the noise.

In a preferred embodiment the means for preventing radiation from the source from reaching the sensor is a rotating wheel having a plurality of openings disposed to periodically block the source. The openings in the rotating wheel permit radiation from the source to reach the sensor, thereby causing the sensor to emit an output signal. The solid portions of the rotating wheel prevent the source from supplying radiation to the sensor and cause the sensor output signal to be indicative of all radiation other than from the source. By increasing the speed of rotation of the wheel higher frequencies of noise may be removed from the sensor output signal.Typically the signal processing means includes an analog to digital converter for converting the sensor output signal from analog form to digital form, and includes a data processing apparatus for storing the sensor output signals and subtracting the signal indicative of noise from the signal indicative of the source and the noise.

In the accompanying drawings: Fig. 1 is a schematic view of apparatus in accordance with this invention for removing noise from a combustion gas analyzer.

Fig. 2 is a front view of the rotating wheel shown in Fig. 1.

Fig. 3A is a hypothetical output signal from the sensor shown in Fig. 1.

Fig. 3B shows the output signal from an indexing detector.

Fig. 3C shows the output signal from the read detector.

A schematic representation of the method and apparatus for removing noise from combustion gas analyzer measurements is shown in Fig. 1. As shown, a sensor 12 for detecting incident electromagnetic radiation from source 11 and background noise is connected to supply output signals on lines 35 to an analog to digital converter 15, which is in turn connected to data processing means 1 8. Processor 1 8 and analog to digital converter 15 are each of well known design. The design of source 11, sensor 12 and the manner of their operation are discussed in further detail in United States Patent Application Serial No. 070,744 filed August 29, 1979, and entitled "Electrn-optical Flue Gas Analyzer", which is assigned to the same assignee as this application.

The noise filter comprises a moving object in the form of a rotating wheel 21 (shown in crosssection), having a first plurality of sensor openings 23, and a second plurality of read and/or index openings 25 and 28. Electro-optical means, for example, a light source 30 and light detectors or photosensors 31 and 32 operate in conjunction with read 25 and index openings 28. A beam of electromagnetic radiation 40 is shown emanating from source 11, passing through an opening 23 in wheel 21, through smokestack 5 containing flue gases 7 and striking sensor 12. As is well known, some of the particular characteristics of the electromagnetic radiation 40 emitted by source 11 will be altered by the gases 7 in flue 5.For example, if source 11 emits infrared radiation and the gases 7 include carbon monoxide, certain narrow bands of frequencies of the infrared will tend to be absorbed by the carbon monoxide. The extent to which this phenomena occurs is reflected by the output signal of sensor 12 thereby providing a measure of the amount of carbon monoxide in gases 7.

In operation of the noise filter, wheel 21 is rotated, for example, at 1,500 revolutions per minute by a motor 22. As shown by Fig. 2, radiation 40 will pass through the openings 23 in wheel 21 six times for each revolution of the wheel. Between openings 23 the source 11 is blocked from sensor 12. If desired, selective filters may be placed in openings 23 to filter unwanted incoming radiation 40. For example, if sensor 1 2 is to detect infrared light, an infrared filter may be inserted into one or more of openings 23 to remove other types of incident light.

As wheel 21 rotates, index opening 28 and read openings 25 pass between photosensors 31 and 32 and photosource 30. Source 30 and sensors 31 and 32 may be of any well-known design, for example, source 30 may supply visible light while sensors 31 and 32 are photodiodes or other light sensitive apparatus. As will be explained in conjunction with Figs. 3A, B and C, index opening 28 and read openings 25 serve to supply information to processor 1 8 with respect to the presence of absence of an opening 23 between source 11 and sensor 12.

In a preferred embodiment index opening 28 is used to supply a pulse of visible light to detector 31 which, by a signal on line 33 notifies processor 18 that the start of a revolution of wheel 21 has begun. At the same time the read opening 28 supplies a signal on line 34 which indicates to processor 1 8 whether sensor 12 is exposed to and therefore receiving radiation 40 from source 11, or whether wheel 21 is blocking radiation 40 from being supplied by source 11. The radiation striking sensor 12, whether from source 11 or background noise, causes analog signals to be supplied on lines 35 to analog to digital converter 1 5.

Converter 1 5 changes the analog information on lines 35 to digital information and supplies this information to processor 1 8. Processor 1 8 correlates the information supplied by the sensor 12 when an opening permits radiation 40 from source 11 to generate a signal with information from sensor 12 when wheel 21 prevents radiation from source 11 from reaching sensor 12. Processor 18 subtracts the digital value of the output signal from sensor 12 when the source 11 is obscured from the digital value of the signal when the source 11 is not obscured. In a preferred embodiment the digital value of the output signal both before and after the source 11 is exposed to the sensor 1 2 are computed and averaged.This average value is then subtracted from the sensor output with the source exposed. Read openings "tell" processor 18 when the source 11 is and is not supplying radiation 40 to sensor 12. The output signal from processor 1 8 is supplied on line 38 after the noise signal has been removed from the radiation signal generated by source 11.

Fig. 2 is a front view of one embodiment of the rotating wheel 21 used to interrupt the radiation from source 11. In the illustration six openings 23 permit tranmission of electromagnetic radiation from source 11 through the flue gases 7 to sensor 12 with each revolution of the wheel. If desired particular optical, or other, filters may be fitted into openings 23 to selectively filter the radiation from source 11.

Fig. 2 also shows the read openings 25 and index opening 28 in wheel 21. Because the output signal from sensor 12 is measured both when source 11 is obscured and exposed, there are twice as many read openings 25 as openings 23.

Of course any desired number of either type openings 23 and 25 may be provided.

Fig. 3A is a hypothetical output signal as supplied by sensor 12 on lines 35. In fig. 3A the detected radiation D(t) is shown as a function of time t. The detected radiation D(t) may be written as the sum of the true measurement signal, that is, as generated by radition 40, plus all other sources of noise, for example, from nearby structure and gases, and from sensor 23 itself.

Thus, the detected signal is equal to the measurement signal S(t) plus the noise signal N(t) or: D(t) = S(t) +N(t) Signal S(t) will vary with time depending upon the specific gases being measured and the performance of the equipment generating the gases.

The method of determining the true signal strength from a signal frequency, or other measured parameter, involves the simultaneous determination of two functions: D1(t) = S(t) + N(t) D2(t) = N(t) By subracting D2 from D, the desired function S(t) may be obtained. The determination of D1 and D2 by the apparatus shown in Fig. 1 is performed by alternately measuring D1 and D2 as shown in Fig.

3A. Thus at a first time t1 corresponding to the presence of an opening 23 to permit radiation 40 from source 11 to strike sensor 12, D(t) is shown in Fig. 3A. At a subsequent time t2 a solid portion of wheel 21 prevents radiation 40 emitted by source 11 from striking sensor 12. The measured signal strength, frequency, or other parameter at time t2 is shown in Fig. 3A. Similarly, at times t3, tx, and t7 the total radiation plus noise (D,) is detected, while at times t4, t6 and t8 only the noise (D2) is detected.Before the noise signal D2 can be subracted from the radiation plus noise signal D at a particular time t,, the value of D2 must be determined at time tn Processor 1 8 accomplishes this by averaging the strength of signal N(t) on each "side" of each measurement of signal plus noise D1.Thus:

Therefore, the average value of the noise before and after the measurement of the specific parameter is determined by processor 1 8. The signal at time tn is given by: S(t) = D1(tn)D2(tn) where D1(tn) is measured by sensor 1 2 and D2(tn) is computed as explained above. the timing signals supplied to processor 1 8 on lines 33 and 34 are shown in Figs. 3B and 3C, respectively.

Once each revolution of wheel 21, index opening 28 permits light from source 30 to strike detector 31. This creates the index signal shown in Fig. 3B which informs processor 1 8 that a new revolution of wheel 21 is beginning. The index signal enables processor 1 8 to correlate the presence or absence of filters in openings 23, if desired.

Read openings 25, by permitting light to travel from source 30 to detector 32, supply a signal on line 34 to processor 1 8 indicating when measurements D1 and D2 are to be taken. This "read" signal is shown in Fig. 3C.

As has been described, this invention provides an improved method and apparatus for determining and removing background noise from a detected signal. Removal of the background noise enables a more accurate determination of the detected signal 40. By increasing the rotational speed of wheel 21 higher frequencies of noise may be removed from the detected signal caused by radiation 40.

Although the invention has been described with reference to a preferred embodiment, various modifications may be made for example, wheels with different numbers of openings, different sensore positions, or different indexing techniques may be used. Alternatively other types of shutters may be used to control the radiation supplied by source 11 to sensor 12, or source 11 itself may be cyclically switched on and off.

Claims (11)

1. A method for removing noise from measurement signals generated by a sensor in response to both noise and source of electromagnetic radiation comprising: during a selected time interval measuring an output signal from the sensor while the sensor is exposed to both noise and the source of electromagnetic radiation; during at least one time interval adjacent the selected time interval measuring the output signal from the sensore while the sensor is not exposed to the source of electromagnetic radiation; and subtracting the output signal measured during the at least one time interval from the output signal measured during the selected time interval to thereby produce an output signal representative of the source of electromagnetic radiation and not the noise.

2. A method as in claim 1 wherein the selected time interval and the at least one time interval are determined by alternately permiting the radiation from the source to reach the sensor and preventing the radiation from the source from reaching the sensor.

3. A method as in Claim 1 or Claim 2 wherein the time intervals are determined by a moving object which includes a first plurality of openings which periodically permit the radiation from the source to reach the sensor.

4. A method as in Claim 1,2 or 3 wherein an additional step of converting all the output signals from the sensor from analog to digital form is accomplished before the step of subtracting.

5. Apparatus for removing noise from measurement signals generated by a sensor in response to both noise and a source of electromagnetic radiation comprising: a source of electromagnetic radiation; a sensor for generating output signals in response to electromagnetic radiation to which the sensor is exposed; means for periodically preventing said sensor from being exposed to radiation from the source of electromagnetic radiation; and signal processing means for subtracting the output signal generated by the sensor when the sensor is not exposed to the source of electromagnetic radiation from the output signal generated by the sensor when the sensor is exposed to the source of electromagnetic radiation to thereby provide an output signal indicative of only the source of electromagnetic radiation and not the noise.

6. Apparatus as in Claim 5 wherein the means for preventing the sensor from being exposed to radiation from said source includes a moving object having a first plurality of openings for alternately permitting exposure of said sensor to radiation from said source.

7. Apparatus as in Claim 5 or Claim 6 wherein the signal processing means includes: analog to digital converter means for converting the output signals from the sensor from analog form to digital form; and data processing means for processing the output signals from the analog to digital converter means.

8. Apparatus as in Claim 7 when appendant to Claim 6 wherein the moving object includes a second plurality of openings for controlling the signal processing means.

9. Apparatus as in Claim 8 wherein the second plurality of openings control the data processing means.

10. A method of removing noise from measurement signals substantially as described herein with reference to the accompanying drawings.

11. Apparatus for removing noise from measurement signals substantially as described herein with reference to the accompanying drawings.