Method and apparatus for detecting a component gas in a sample

• US 5,184,017 A
• Filed: 08/19/1991
• Issued: 02/02/1993
• Est. Priority Date: 09/12/1989
• Status: Expired due to Fees

• Current Assignee: Sensors, Inc.
• Original Assignee: Sensors, Inc.
• Inventors: Johnson, Ross E., Kaste, Keith, Danielson, David O., Tury, Edward L.
• Primary Examiner(s): NOLAND, THOMAS
• Application Number:
  US07/746,939
• Time in Patent Office: 533 Days
• Field of Search: 73/1 G, 73/23.21, 73/23.22-23.29, 250/343, 250/340, 250/341, 250/344, 250/345, 250/346, 250/339, 250/349, 250/252.1 R, 250/252.1 A
• US Class Current: 250/343
• CPC Class Codes: G01N 2021/3129 Determining multicomponents by multiwavelength light
• CPC Class Codes: G01N 2021/3174 Filter wheel
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 2201/1285 Standard cuvette

Abstract

A non-dispersive infrared gas analyzer especially adapted for measuring the concentrations of HC, CO and CO₂ in a vehicle exhaust includes a sample chamber for holding a sample gas, a radiation emitter for directing a beam through the chamber and a detector for indicating the amount of radiation absorbed. A plurality of filters each of which transmit radiation at an absorption band of a gas component to be detected are alternatingly positioned in the radiation path to produce a time-multiplexed signal having concentration information for all gases. The time-multiplexed detector output is corrected for gain and offset errors "on-the-fly", without requiring demultiplexing, through the use of an analog multiplier circuit. The correction factors are themselves determined without demultiplexing the detector output signal.

Claims

1. An apparatus for detecting a component as in a sample comprising:

a sample chamber for containing a sample gas including the component gas to be detected;

source means for directing infrared radiation through the sample chamber in a preselected spectral band;

detector means responsive to radiation incident thereon in the preselected spectral band passing through the sample chamber from the source means for producing a detector output;

signal processing means responsive to said detector output for indicating the relative concentration of the component gas to be detected, said signal processing means including

a circuit having first and second inputs and adapted to producing a difference output that is the difference between a signal provided to said first input and a signal provided to said second input;

first means for concurrently applying said detector output to said first input and a zero-value signal to said second input with substantially nonabsorbent gas filling the sample chamber to produce an offset value on said difference output;

second means for concurrently applying both said detector output to said first input and said offset value to said second input with a sample gas including a component gas to be detected filling said sample chamber thereby producing a difference output that indicates the relative concentration of said component gas.

2. The apparatus in claim 1 in which said circuit further includes a third input and in which said difference output is the sum of a signal applied to said third input and said difference between a signal provided to said first input and a signal provided to said second input.

3. An apparatus for detecting a component gas in a sample comprising:

a sample chamber for containing a sample gas including the component gas to be detected;

source means for directing infrared radiation through the sample chamber in a preselected spectral band;

detector means responsive to radiation incident thereon in the preselected spectral band passing through the sample chamber from the source means for producing a detector output;

signal processing means responsive to said detector output for indicating the relative concentration of the component gas to be detected, said signal processing means including

a circuit having first, second and third inputs and adapted to producing a difference output that is the sum of a signal applied to said third input and the difference between a signal provided to said first input and a signal provided to said second input;

first means for applying said detector output to said first output and a zero-value signal to said second input with substantially nonabsorbent gas filling the sample chamber to produce an offset value on said difference output;

second means for applying said detector output to said first input and said offset value to said second input with a sample gas including a component gas to be detected filling said sample chamber thereby producing a difference output that indicates the relative concentration of said component gas;

third means for applying said detector output to said first input and said offset value to said second input with a substantially nonabsorbent gas filling the sample chamber to produce a trim value on said difference output; and

wherein said second means includes means for applying said trim value to said third input.

4. An apparatus for detecting at least one component gas in a sample comprising:

a sample chamber for containing a sample gas including the component gas to be detected;

source means for directing infrared radiation through the sample chamber in a preselected spectral band;

detector means responsive to radiation incident thereon in the preselected spectral band passing through the sample chamber from the source means for producing a detector output;

multiplexing means for applying a plurality of filters during distinct time intervals between said source means and said detector means to time-multiplex said detector output, one of said filters having radiation transmitting characteristics that will provide an indication of the concentration of the component gas to be detected; and

a correction circuit adapted to applying at least one correction factor to said detector output to provide a corrected output, said correction circuit being synchronized with said multiplexing means in order to apply a particular said correction factor to said detector output for said component gas to be detected during the interval of time that said one of said filters is between said source means and said detector means.

5. The apparatus in claim 4 in which said correction factor includes values for correcting said output for offset and gain errors.

6. The apparatus in claim 4 in which said correction circuit is responsive to said detector output for providing said corrected output as a difference between said detector output and a first correction factor and as a quotient between said detector output and a second correction factor.

7. The apparatus in claim 6 in which said first correction factor is an offset correction factor and said second correction factor is a gain correction factor.

8. The apparatus in claim 4 in which said correction circuit is further adapted to applying a second particular said correction factor to said detector output for another component gas to be detected during another interval of time that another one of said filters is between said source means and said detector means, said another one of said filters having radiation transmitting characteristics that will provide an indication of the concentration of the another component gas to be detected.

9. A method for detecting at least one component gas in a sample including the steps of:

providing a sample chamber for containing a sample gas including the component gas to be detected;

directing infrared radiation through the sample chamber in a preselected spectral band;

detecting radiation in the preselected spectral band passing through the sample chamber and producing a detector signal;

imposing a plurality of filters during distinct time intervals in the path of radiation to time-multiplex said detector signal, one of said filters having radiation transmitting characteristics that will provide an indication of the concentration of the component gas to be detected; and

applying a correction signal to said detector signal in synchronism with said imposing to apply a correction factor to said detector output for said component gas to be detected during the interval of time that said one of said filters is in said path of radiation and thereby providing a corrected detector value.

10. The method in claim 9 in which said step of applying a correction signal includes producing said corrected detector signal as a difference between said detector signal and a first correction factor and as a quotient between said detector signal and a second correction factor.

11. The method in claim 9 in which said first correction factor is an offset correction factor and said second correction factor is a gain correction factor.

12. The method in claim 9 in which said step of applying a correction signal includes:

producing a difference output that is the difference between

said detector signal and an offset value whereby said difference output indicates the relative concentration of said component gas.

13. The method in claim 12 further including the step of producing said offset value by obtaining the difference between said detector signal and a zero-value signal with a substantially nonabsorbent gas filling the sample chamber.

14. The method in claim 12 further including the step of summing a trim value with said difference output.

15. The method in claim 14 further including the step of:

producing said trim value by obtaining the difference between said detector signal and said offset value with a substantially nonabsorbent gas filling the sample chamber.

16. The method in claim 9 in which said step of applying a correction signal includes applying a second correction factor to said detector signal for another component gas to be detected during another interval of time that another one of said filters is in the path of radiation source, said another one of said filters having radiation transmitting characteristics that will provide an indication of the concentration of the another component gas to be detected.