Carbon dioxide concentration sensor

• US 6,456,943 B1
• Filed: 05/14/1999
• Issued: 09/24/2002
• Est. Priority Date: 05/15/1998
• Status: Expired due to Fees

• Current Assignee: Riken Keiki Company Limited
• Original Assignee: Riken Keiki Company Limited
• Inventors: Yonemura, Shinichi, Kogure, Shinsuke
• Primary Examiner(s): Hoff, Marc S.
• Assistant Examiner(s): TSAI, CAROL S W
• Application Number:
  US09/311,578
• Time in Patent Office: 1,229 Days
• Field of Search: 702/22, 702/23, 702/24, 702/27, 702/30-32, 702/50, 702/85, 702/104, 702/107, 702/116, 702/127, 702/176, 702/178, 702/179, 702/181, 702/182, 702/183, 702/188, 702/189, 702/FOR.103, 702/FOR.104, 702/FOR-115, 702/FOR-127, 702/FOR-134, 702/FOR.139, 702/FOR.156, 702/FOR.154, 702/FOR-159, 702/FOR-170, 204/401, 204/406, 73/10.2, 73/10.6, 73/10.3, 73/10.7, 73/232, 73/233.1, 73/240.2
• US Class Current: 702/23
• CPC Class Codes: G01N 33/0006 Calibrating gas analysers
• CPC Class Codes: G01N 33/004 for CO, CO2
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Abstract

A carbon dioxide sensor has an arithmetic unit 12 that predicts the minimum value of carbon dioxide concentration that is regulated by the diffusion of the atmosphere by judging that a signal from a carbon dioxide concentration sensing unit 1 indicates a declining tendency and a calibration unit 13 that updates the sensitivity correction factor based on the predicted minimum value. This permits quick calibration by eliminating the need to wait until the carbon dioxide concentration in any given environment reaches the minimum value.

Claims

1. A carbon dioxide concentration sensor, comprising:

an arithmetic unit;

a carbon dioxide sensing unit having an output coupled to an input of the arithmetic unit; and

a calibration unit adapted to receive an output of the arithmetic unit and the output of the carbon dioxide sensing unit,

wherein the arithmetic unit predicts a minimum value of a carbon dioxide concentration in an environment when the carbon dioxide concentration is declining over a plurality of successive time intervals, and

wherein the calibration unit updates a sensitivity correction factor of the carbon dioxide sensing unit based on said minimum value of the carbon dioxide concentration predicted by the arithmetic unit.

2. A carbon dioxide concentration sensor according to claim 1, further comprising a memory arranged between the carbon dioxide sensing unit and the arithmetic unit,

wherein said arithmetic unit calculates a converging point of the carbon dioxide concentration by calculating a prediction curve based on a plurality of carbon dioxide concentration data points collected by said carbon dioxide sensing unit and stored in the memory.

3. A carbon dioxide concentration sensor according to claim 1, wherein said arithmetic unit calculates a converging point of the carbon dioxide concentration based on values of carbon dioxide concentration measured at each of three consecutive times spaced by one or more time intervals.

4. A carbon dioxide concentration sensor according to claim 1, wherein said arithmetic unit predicts said minimum value of the carbon dioxide concentration when a current elapsed time agrees with a preset time interval.

5. A carbon dioxide concentration sensor according to claim 1, wherein, when the carbon dioxide concentration does not exhibit a declining tendency,

said arithmetic unit does not predict the maximum value of the carbon dioxide concentration in the environment, and

said calibration unit corrects the sensitivity correction factor of the carbon dioxide sensing unit based on a preceding sensitivity correction factor.

6. A carbon dioxide concentration sensor according to claim 1, wherein said calibration unit updates the sensitivity correction factor of the carbon dioxide sensing unit so that the minimum value of the carbon dioxide concentration predicted by said arithmetic unit agrees with a reference value.

7. A method for compensating for changes in a sensitivity of a carbon dioxide concentration sensor, the method comprising:

detecting a carbon dioxide concentration in an environment during each of a plurality of consecutive time intervals;

determining a decreasing carbon dioxide concentration trend in the environment over the plurality of consecutive time intervals;

predicting a minimum carbon dioxide concentration value in the environment;

correcting at least one calibration factor of the carbon dioxide concentration sensor by using the predicted minimum carbon dioxide concentration value as a reference value if the decreasing carbon dioxide concentration trend is determined; and

outputting a calibrated carbon dioxide concentration value.

8. The method of claim 7, further comprising compensating for a temperature-dependent sensitivity of the carbon dioxide concentration sensor.

9. The method of claim 7, wherein said correcting step includes updating a sensitivity correction factor of the carbon dioxide concentration sensor.

10. The method of claim 7, wherein said correcting step includes updating a zero point of the carbon dioxide concentration sensor.

11. The method of claim 7, further comprising storing, in a memory, a plurality of carbon dioxide concentration values associated with the plurality of consecutive time intervals.

12. The method of claim 7, wherein said correcting step includes using a preceding calibration factor if the decreasing carbon dioxide concentration trend in the environment is not determined.

13. The method of claim 7, wherein said predicting step comprises:

measuring the carbon dioxide concentration at each of three consecutive times spaced by one or more time intervals; and

calculating the minimum carbon dioxide concentration value by predicting a convergence point of the carbon dioxide concentration based on values of carbon dioxide concentration at said each of three consecutive times.

14. The method of claim 7, wherein said predicting step comprises fitting associated carbon dioxide concentration values at each of the plurality of consecutive time intervals to a dampened exponential curve.

15. The method of claim 14, wherein said fitting step includes converging the associated carbon dioxide concentration values to predict the minimum carbon dioxide concentration value.

16. The method of claim 7, wherein said predicting step comprises fitting associated carbon dioxide concentration values at each of the plurality of consecutive time intervals to a gas diffusion equation.

17. The method of claim 7, wherein said predicting step predicts the minimum carbon dioxide concentration value in the environment before an actual carbon dioxide concentration value decreases to the minimum carbon dioxide concentration value.