APPARATUS AND METHOD FOR GENERATING LIGHT PULSES FROM LEDS IN OPTICAL ABSORPTION GAS SENSORS

• US 20130271751A1
• Filed: 10/28/2011
• Published: 10/17/2013
• Est. Priority Date: 11/01/2010
• Status: Active Grant

• Current Assignee: Gas Sensing Solutions, Ltd.
• Original Assignee: Gas Sensing Solutions, Ltd.
• Inventors: Macgegor, Calum John, Gibson, Desmond Robert
• Granted Patent: US 9,410,886 B2
• Time in Patent Office: Days
• US Class Current: 356/72
• CPC Class Codes: G01J 11/00 Measuring the characteristics of individual optical pulses or of optical pulse trains
• 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 21/61 Non-dispersive gas analysers G01N21/3504 takes precedence
• CPC Class Codes: G01N 2201/0696 Pulsed
• CPC Class Codes: G01N 2201/1211 for temperature
• CPC Class Codes: G01N 2201/127 Calibration; base line adjustment; drift compensation

Abstract

An optical absorption gas sensor for detecting an analyte gas comprises a gas sample receiving chamber, at least one light emitting diode (LED) and a photodiode or other photosensor. A plurality of light pulses are generated by passing pulses of current through the at least one LED. The current through the at least one LED is measured a plurality of times during each pulse and taken into account when generating a compensated output signal. The transfer ratio between LED current and photodiode output signal is calculated a plurality of times during each pulse. An ADC measures the LED and photodiode currents alternately. The LED pulses are generated by inductor discharge flyback and the period of time for which current is supplied to the inductor prior to each pulse is selected so that the photodiode output current is at an optimal region within the input range of the ADC. At least the temperature of the at least one LED is measured and taken into account when generating the compensated output signal. Thus, rather than providing especially careful control of the LED pulses, the pulses are measured, enabling a simpler, lower power circuit which is tolerant of variations in temperature to be provided.

Claims

1. A method of operating an optical absorption gas sensor for detecting an analyte gas, the sensor comprising a gas sample receiving chamber, at least one light emitting diode (LED) and a photosensor having an output signal sensitive to the amount of light received by the photosensor, the method comprising generating a plurality of pulses of light by passing a plurality of pulses of current through the at least one LED, and measuring the current through the at least one LED a plurality of times during each said pulse, and generating a compensated signal indicative of the concentration of an analyte gas in the gas sample receiving chamber taking into account both the photosensor output signal and the plurality of measurements of the current through the at least one LED.

2. A method according to claim 1, wherein the photosensor output signal is also measured a plurality of times during each said pulse.

3. A method according to claim 2, wherein the plurality of measurements of the photosensor output signal are also taken into account when generating the compensated signal.

4. A method according to claim 1, wherein the current through the at least one LED and the photosensor output signal are measured alternately during the plurality of pulses.

5. A method according to claim 1, wherein the current through the at least one LED is measured using an analogue to digital converter (ADC).

6. A method according to claim 1, where in an ADC is used to measure the current through the at least one LED and the photosensor output signal.

7. A method according to claim 1, wherein the temperature of the at least one LED is measured and the measured temperature is taken into account when generating the compensated signal.

8. A method according to claim 1, wherein the method comprises calculating the ratio of the measured current through the at least one LED to the photosensor output signal, at least once during a pulse.

9. A method according to claim 1, wherein the method comprises carrying out a frequency domain analysis of a plurality of measurement of the measured current through the at least one LED and the photosensor output signal.

10. A method according to claim 1, wherein the method comprises generating the pulses with a known frequency and calculating a parameter related to the energy of the measured current through the at least one LED and/or the photosensor output signal in one or more frequency bands.

11. A method according to claim 1, wherein the pulses of current through the at least one LED are generated by switching off a current source to an inductor and directing current from the inductor through the at least one LED.

12. A method according to claim 1, wherein one or more properties of an LED current pulse are selected depending on one or more of the measurements of the current through the at least one LED carried out during an earlier LED current pulse.

13. A method according to 12, wherein the said one or more properties are selected by determining the period of time for which current is supplied by a switching device to an inductor operatively connected to the at least one LED before the supply of current to the inductor is restricted by the switching device.

14. A method according to claim 12, wherein one or more properties of an LED current pulse is selected depending on one or more measurements of the photosensor output signal.

15. A method according to claim 12, wherein the one or more properties of an LED current pulse are selected to compensate for a measured property of the at least one LED or the photosensor.

16. A method according to claim 1, wherein the current through the at least one LED is measured by passing the current through a resistor and measuring the potential difference across the resistor.

25. An optical absorption gas sensor comprising a gas sample receiving chamber, at least one light emitting diode (LED) and a light detector having an output signal sensitive to the amount of light received by the photosensor, and a control circuit configured to cause the optical absorption gas sensor to operate according to the method of claim 1.

17. An optical absorption gas sensor comprising a gas sample receiving chamber, at least one light emitting diode (LED) and a light detector having an output signal sensitive to the amount of light received by the photosensor, an LED control circuit configured to generate a plurality of pulses of light in a measurement mode of the optical absorption gas sensor by passing a plurality of pulses of current through the at least one LED, a measuring device configured to measure the current through the at least one LED a plurality of times during each said pulse, and a compensation module configured to output a compensated signal indicative of the concentration of an analyte gas in the gas sample receiving chamber, taking into account both the light detector output signal and the plurality of measurements of the current through the at least one LED.

18. An optical absorption gas sensor according to claim 17, wherein the gas sensor comprises a measuring device to measure the photosensor output signal a plurality of times during each pulse.

19. An optical absorption gas sensor according to claim 17, comprising an ADC to measure both the current through the at least one LED and the photosensor output signal.

20. An optical absorption gas sensor according to claim 17, wherein the optical absorption gas sensor also comprises one or more temperature sensors and the compensation module takes into account the one or more sensed temperatures.

21. An optical absorption gas sensor according to claim 17, wherein the LED control circuit is configured to determine one or more properties of the LED pulses controlled by the at least one LED control circuit responsive to one or more of the measurements of the current through the at least one LED and/or one of the more of the measurement of the photosensor output signal carried out during an earlier LED current pulse.

22. An optical absorption gas sensor according to claim 19, wherein the LED control circuit comprises an inductor operatively connected to the at least one LED and is operable to supply current to the inductor and then to restrict the supply of current to the inductor, thereby causing current to flow through the at least one LED.

23. An optical absorption gas sensor according to claim 22, wherein the LED control circuit is configured to select the current supplied to the inductor and/or the period of time for which the current is supplied to the inductor responsive to one or more of the measurement of the current through the at least one LED and/or one of the more of the measurement of the photosensor output signal carried out during an earlier LED current pulse.

24. An optical absorption gas sensor according to claim 17, comprising a frequency domain analysis module operable to analyse a plurality of measurement of the measured current through the at least one LED and the photosensor output signal