WO2009019467A1 - Temperature compensation for gas detection - Google Patents

Temperature compensation for gas detection Download PDF

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Publication number
WO2009019467A1
WO2009019467A1 PCT/GB2008/002661 GB2008002661W WO2009019467A1 WO 2009019467 A1 WO2009019467 A1 WO 2009019467A1 GB 2008002661 W GB2008002661 W GB 2008002661W WO 2009019467 A1 WO2009019467 A1 WO 2009019467A1
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WO
WIPO (PCT)
Prior art keywords
measurement
diode
power
gas
gas concentration
Prior art date
Application number
PCT/GB2008/002661
Other languages
French (fr)
Inventor
Michael J. Smith
Original Assignee
Gas Sensing Solutions Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Gas Sensing Solutions Limited filed Critical Gas Sensing Solutions Limited
Publication of WO2009019467A1 publication Critical patent/WO2009019467A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/06—Illumination; Optics
    • G01N2201/062—LED's
    • G01N2201/0624—Compensating variation in output of LED source
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/12—Circuits of general importance; Signal processing
    • G01N2201/121—Correction signals
    • G01N2201/1211—Correction signals for temperature

Definitions

  • the present invention relates to temperature compensation in gas detectors, in particular gas detectors having diodes.
  • Non Dispersive InfraRed (NDIR) gas detectors in which radiation is passed through a gas, a Light Emitting Diode (LED) may be used as the radiation source and a photodiode may be used as the radiation detector.
  • the LED and photodiode are both sensitive to temperature. There is therefore a problem in differentiating a temperature induced change from a gas induced change in the sensor output.
  • a particular problem with LEDs and photodiodes in NDIR detectors is that both devices have series and parallel parasitic resistances which vary with temperature.
  • the band-gap voltage will also vary with temperature.
  • the temperature dependent frequency shift of the detected infrared light with respect to the filter frequency can lead to an offset between the measured signals of the reference and measurement photodiodes. In all it is typical for at least 5 different parameters effecting operating efficiency, light output and spectral shift to be affected by temperature change.
  • multiple diodes on a single common substrate to simplify temperature measurement. Additionally, mounting multiple diodes on a common substrate by direct bonding in a close coupled or separated configuration to a heatsink or copper tracked hybrid circuit arrangement also aids stability and measurement.
  • the relative effect of temperature variation is larger.
  • the temperature sensitivity of LEDs and photodiodes can combine with this increased effect of temperature variation to cause poor accuracy in compact NDIR gas sensors.
  • Temperature compensation methods are known in NDIR detectors. However, poor measurement of temperature can result in incorrect compensation, therefore the accuracy of gas detection is compromised, for example the sensor may not accurately detect the concentration of gas.
  • the precision of temperature measurement required depends on the application, but may be as precise as +/- 0.02 0 C. As the sensitive region of the diode which causes the performance change is within its internal junctions, at the core of the device, it is very difficult to measure to the accuracy required, the temperature of this region with an external measuring system or detector as the temperature varies from the external ambient air and changes in ⁇ s as a result of each driving pulse.
  • a method of temperature compensation for a gas sensor comprising a diode, the method comprising the steps of: measurement of a temperature sensitive characteristic of the diode; measurement of a gas concentration; and compensation of the measurement of the gas concentration using the measured temperature sensitive characteristic of the diode.
  • the step of the measurement of the gas concentration uses the measured diode.
  • the step of the measurement of the gas concentration uses a further diode on the same chip as the measured diode.
  • the step of the measurement of the gas concentration uses a further diode in thermal communication with the measured diode.
  • the temperature sensitive characteristic comprises an electrical characteristic.
  • the electrical characteristic comprises a forward voltage.
  • the diode comprises a radiation emitting diode and the step of measuring the gas concentration comprises the step of driving the radiation emitting diode so as to emit radiation.
  • the step of measurement of the temperature sensitive characteristic comprises supplying a first power to the radiation emitting diode and the step of driving the radiation emitting diode so as to emit radiation comprises the step of supplying a second power to the radiation emitting diode.
  • the second power is larger than the first power.
  • the second power is at least ten times larger than the first power.
  • the step of compensation of the measurement of the gas concentration comprises altering the first power to the radiation emitting diode responsive to the measured temperature sensitive characteristic.
  • the method further comprises the step of measurement of a temperature of the components supplying the first power and the step of compensation of the measurement of the gas concentration further uses the measured temperature of the components supplying the first power.
  • the measured temperature of the components supplying the first power is used to correct the measurement of temperature sensitive characteristic.
  • the method further comprises the step of measurement of a temperature of the components supplying the second power and the step of compensation of the measurement of the gas concentration further comprises using the measured temperature of the components supplying the second power.
  • the measured temperature of the components supplying the second power is used to correct the measurement of the gas concentration.
  • the step of compensation of the measurement of the gas concentration comprises force heating or cooling the radiation emitting diode to a predetermined level responsive to the measured temperature sensitive characteristic.
  • the diode comprises a photodiode and the step of measurement of the gas concentration comprises measurement of a signal from the photodiode so as to detect radiation.
  • the diode comprises a reference photodiode and the step of measurement of the gas concentration comprises measuring a signal from the reference photodiode so as to detect radiation.
  • the step of compensation of the measurement of the gas concentration comprises altering the gain of an amplifier of a signal from the photodiode responsive to the measured temperature sensitive characteristic.
  • the step of measurement of the temperature sensitive characteristic comprises supplying the diode with a predetermined voltage or current.
  • the step of compensation of the measurement of the gas concentration comprises using a look-up table to correct the gas concentration responsive to the measured temperature sensitive characteristic.
  • a gas sensor comprising: a diode; a diode measurement means for measurement of a temperature sensitive characteristic of the diode; a gas measurement means for measurement of a gas concentration; and a compensation means for compensation of the measurement of the gas concentration using the measured temperature sensitive characteristic of the diode.
  • the gas measurement means is operable to use the measured diode for the measurement of the gas concentration.
  • the gas measurement means is operable to use a further diode on the same chip as the measured diode for the measurement of the gas concentration.
  • the gas measurement means is operable to use a further diode in thermal communication with measured diode for the measurement of the gas concentration.
  • the temperature sensitive characteristic comprises an electrical characteristic.
  • the electrical characteristic comprises a forward voltage.
  • the diode comprises a radiation emitting diode and the gas measurement means is operable to drive the radiation emitting diode so as to emit radiation for the measurement of the gas concentration.
  • the diode measurement means is operable to supply a first power to the radiation emitting diode for measurement of the temperature sensitive characteristic of the diode and the gas measurement means is operable to drive the radiation emitting diode so as to emit radiation by supplying a second power to the radiation emitting diode.
  • the second power is larger than the first power.
  • the second power is at least ten times larger than the first power.
  • the compensation means is operable to alter the first drive power to the radiation emitting diode responsive to the measured temperature sensitive characteristic.
  • the gas sensor further comprises a first temperature measurement means for measurement of a temperature of the components supplying the first power and the compensation means is further operable to use the measured temperature of the components supplying the first power for the compensation.
  • the first temperature measurement means is operable to use the measured temperature of the components supplying the first power to correct the measurement of temperature sensitive characteristic.
  • the gas sensor further comprises a second temperature measurement means for measurement of a temperature of the components supplying the second power and the compensation means is further operable to use the measured temperature of the components supplying the second power for the compensation.
  • the second temperature measurement means is operable to use the measured temperature of the components supplying the second power to correct the measurement of the gas concentration.
  • the compensation means is operable to force heat or cool the radiation emitting diode to a predetermined level responsive to the measured temperature sensitive characteristic.
  • the diode comprises a photodiode and the gas measurement means is operable to measure a signal from the photodiode so as to detect radiation.
  • the diode comprises a reference photodiode and the gas measurement means is operable to measure a signal from the reference photodiode so as to detect radiation.
  • the compensation means is operable to alter the gain of an amplifier of a signal from the photodiode responsive to the measured temperature sensitive characteristic.
  • the diode measurement means is operable to supply the diode with a predetermined voltage or current.
  • the compensation means is operable to use a look-up table to correct the gas concentration responsive to the measured temperature sensitive characteristic.
  • Figure 1 illustrates in schematic form an apparatus for temperature measurement in accordance with an embodiment of the present invention.
  • Figures 2 to 4 illustrate in schematic form a circuit for the forward voltage measurement of an LED in accordance with an embodiment of the present invention.
  • Figure 2 illustrates in schematic form the Pulse Wave Modulated to DC level conversion section of the circuit.
  • Figure 3 illustrates in schematic form the main and Vf LED drive section of the circuit.
  • Figure 4 illustrates in schematic form the Vf value amplification section of the circuit.
  • This embodiment of the present invention is a CO 2 NDIR gas sensor.
  • the LED 1 is driven with two 500 ⁇ S pulses of between 1 mA and 5mA.
  • the LED is driven by a constant current driver 2, the current being monitored by a microprocessor.
  • the voltage across the LED is measured via a precision voltage measurement circuit 3.
  • other diode measurement means can be used to measure any temperature sensitive characteristic of the diode, electrical or otherwise.
  • the resulting voltage pulse across the LED is fed into a tuned buffer amplifier which produces two pulses (one positive and one negative) for each pulse into the LED. Both the negative and positive pulses are fed into an A/D converter, the negative pulse being inverted in firmware and added to the positive pulse.
  • Vf forward voltage
  • the Vf measurement may be measured on a diode on the same chip as either the LED or photodiode, or by a diode in thermal communication with either the LED or photodiode.
  • the multiple diodes could be mounted on a TO can or similar package, or in an assembly, which could be constructed using bonded, moulded of cast techniques such that the Vf sensing diode is within the combined thermal mass of the assembly. This still has an advantage of measuring compensating for the temperature of the diode being used for gas concentration measurement.
  • the high power LED pulse train is now triggered resulting in a CO2 measurement being captured by the microprocessor, although other digital or analogue circuits could be used as a gas measurement means.
  • the value of the combined Vf reading is then used in a look-up table by the microprocessor to compensate the CO 2 measurement for the LED temperature. This is done by the microprocessor, although other digital or analogue circuits could be used as a compensation means.
  • Vf may be measured during the CO 2 measurement pulse.
  • a non-constant current source may be used.
  • the voltage may be fixed constant with a measurement of the resulting current being used for compensation.
  • a Pulse Width Modulated square wave of frequency 2 kHz is presented to R70. This is smoothed by C34 to become a DC voltage A, B output to the mail LED drive and Vf LED drive sections respectively, shown in Figure 3.
  • the upper section of Figure 3 is the precision driver for the high power drive to the LED D19.
  • the lower section of Figure 3 relates to the low power Vf measurement.
  • the DC voltage at the output of IC8A is passed through R81 and, in the normal state, is shorted by Q2.
  • a short 4.5 ms active low pulse enables the DC voltage through to the precision current source formed by IC7C and Q5.
  • a square pulse of a set current is driven through the LED D19 and a voltage drop is created across it.
  • This square pulse C is output from the LED drive sections of Figure 3 into the Vf Value Amplification section shown in Figure 4. differentiated by C36 and attenuated by the combination of resistor R83 with resistors R79 and R82.
  • the differentiated pulse is buffered by IC7D, low-pass filtered by R13 and C4 then passed D to the processor for A/D measurement.
  • the processor Since it is the processor that initiates the drive pulse, it is able to coherently oversample the differentiated pulse 1200 times.
  • the resultant number, scaled to a 16 bit integer value gives an accuracy of 12 bits of resolution from the processor's 10 bit A/D system.
  • the Vf measurement is linear over a decade of measurement range.
  • the level of the drive pulse can be adjusted by controlling the PWM value to arrange that the resultant measurement can be scaled for best accuracy.
  • the operation of the LED used is very temperature dependant and accurate measurement of its forward voltage correlates directly with its local temperature. By knowing the Vf and therefore the temperature, we can correct the output value of the system using the measured LED forward voltage.
  • the photodiode is positioned at the start of a high gain (10OdB) amplifier chain.
  • the chain is tuned to a known frequency at which the diode is pulsed.
  • Options for detecting the voltage drop across the photodiode include:
  • the photodiode is driven with a known current from a constant current source and the voltage drop across it is measured.
  • the current source must be well isolated from the photodiode when gas measurements are being taken. This can be achieved using a device such as a FET.
  • the photodiode is driven with a known AC (Alternating Current) signal through a series resistor. This will form a potential divider with a potential across the photodiode varying with temperature.
  • the resulting signal will be amplified by the photodiode amplifier chain, and the resulting signal can be used to derive the temperature.
  • This has the advantage of measuring the temperature response of the entire amplifier chain. As a refinement, this signal can be scaled to suit the amplifier chain by changing its frequency away from the centre of the band-pass frequency of the amplifier chain. This will reduce the gain in the chain, allowing the signal to be scaled to a suitable level.
  • the compensation can be achieved by analogue processing, or by digitising the temperature signal and using a microprocessor to carry out the compensation.
  • the LED is driven from a constant current source with a known current. The voltage across the LED is then measured. Alternatively, the LED is driven from a constant voltage source with a known voltage. The current across the LED is then measured.
  • the performance of the LED and Photodiode are highly dependent on their device temperatures.
  • Using accurate measurement of the forward voltage of the LED gives a very accurate indication of its operating temperature.
  • Unfortunately there is a temperature induced error in the precision current driver that is used to drive 1 milliamp into the LED so that its forward voltage can be measured. This is a very small error but is enough to skew the forward voltage measurement and the gas concentration measurement that is dependent upon its accuracy.
  • the microprocessor on the control board has a temperature sensing thermistor, or first temperature measurement means, connected to it that allows the system to know the temperature of the control board and therefore the temperature of the precision current driver components.
  • control board is ramped between the extremes of its operating temperatures in a thermal chamber while the LED and
  • Photodiode are kept at static room temperature of 20 Degrees Centigrade. While the temperature is being ramped, the control board temperature measurement is recorded along with the LED forward voltage.
  • a lookup table is created that is an error correction for the LED forward voltage cross referenced to control board temperature.
  • the LED forward voltage is measured and the numerical value is corrected by using the control board temperature to reference the correction lookup table held on an on-board ROM.
  • the main precision current driver that drives the LED for CO2 measurement is affected by the control board temperature.
  • the value of the measured CO2 requires correction.
  • the CO 2 measurement is corrected by taking the present value of the control board temperature, using a second temperature measurement means such as a thermistor, and using it to refer to a lookup table of correction factors that have been collected during test.
  • the drive current is typically at least 10 times smaller than the current used to illuminate the LED for measurements. Furthermore, the device is only driven for 32 off 1 millisecond pulses per second. This equates to a 3.2% duty cycle so self heating of the LED is reduced to a minimum. Any heating that is brought about may be corrected for by the procedure detailed above.
  • the raw measurement can be passed through suitable signal conditioning such as filtering or moving average measurements.

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Abstract

A method and gas sensor for compensation of the measurement of gas concentration using a measured temperature sensitive characteristic. A LED (1 ) is driven by a constant current driver (2), the current being monitored by a microprocessor. The voltage across the LED is measured by a precision voltage measurement circuit (3). A derived Vf (forward voltage) is used for temperature compensation (4). The arrangement can compensate for temperature in the core of the diode being used for gas concentration measurement. Alternatively, the Vf measurement may be measured on a diode on the same chip as either the LED or photodiode.

Description

Temperature compensation for gas detection
The present invention relates to temperature compensation in gas detectors, in particular gas detectors having diodes.
In Non Dispersive InfraRed (NDIR) gas detectors in which radiation is passed through a gas, a Light Emitting Diode (LED) may be used as the radiation source and a photodiode may be used as the radiation detector. The LED and photodiode are both sensitive to temperature. There is therefore a problem in differentiating a temperature induced change from a gas induced change in the sensor output.
A particular problem with LEDs and photodiodes in NDIR detectors is that both devices have series and parallel parasitic resistances which vary with temperature. The band-gap voltage will also vary with temperature. Furthermore, where a reference photodiode is used having a filter, the temperature dependent frequency shift of the detected infrared light with respect to the filter frequency can lead to an offset between the measured signals of the reference and measurement photodiodes. In all it is typical for at least 5 different parameters effecting operating efficiency, light output and spectral shift to be affected by temperature change.
These factors contribute to a large temperature variation effect on measured gas concentration in LED/photodiode based NDIR detectors.
To assist with thermal stability it is preferable to form multiple diodes on a single common substrate to simplify temperature measurement. Additionally, mounting multiple diodes on a common substrate by direct bonding in a close coupled or separated configuration to a heatsink or copper tracked hybrid circuit arrangement also aids stability and measurement.
In miniaturised gas sensors having a shorter optical path length through the gas, the relative effect of temperature variation is larger. The temperature sensitivity of LEDs and photodiodes can combine with this increased effect of temperature variation to cause poor accuracy in compact NDIR gas sensors.
Temperature compensation methods are known in NDIR detectors. However, poor measurement of temperature can result in incorrect compensation, therefore the accuracy of gas detection is compromised, for example the sensor may not accurately detect the concentration of gas. The precision of temperature measurement required depends on the application, but may be as precise as +/- 0.020C. As the sensitive region of the diode which causes the performance change is within its internal junctions, at the core of the device, it is very difficult to measure to the accuracy required, the temperature of this region with an external measuring system or detector as the temperature varies from the external ambient air and changes in μs as a result of each driving pulse.
It is an object of an aspect of the present invention to provide improved compensation for temperature in gas detection.
According to a first aspect of the present invention there is provided a method of temperature compensation for a gas sensor comprising a diode, the method comprising the steps of: measurement of a temperature sensitive characteristic of the diode; measurement of a gas concentration; and compensation of the measurement of the gas concentration using the measured temperature sensitive characteristic of the diode.
Preferably, the step of the measurement of the gas concentration uses the measured diode.
Preferably, the step of the measurement of the gas concentration uses a further diode on the same chip as the measured diode.
Preferably, the step of the measurement of the gas concentration uses a further diode in thermal communication with the measured diode.
Preferably, the temperature sensitive characteristic comprises an electrical characteristic.
Preferably, the electrical characteristic comprises a forward voltage.
Preferably, the diode comprises a radiation emitting diode and the step of measuring the gas concentration comprises the step of driving the radiation emitting diode so as to emit radiation.
Preferably, the step of measurement of the temperature sensitive characteristic comprises supplying a first power to the radiation emitting diode and the step of driving the radiation emitting diode so as to emit radiation comprises the step of supplying a second power to the radiation emitting diode.
Preferably, the second power is larger than the first power. Preferably, the second power is at least ten times larger than the first power.
Preferably, the step of compensation of the measurement of the gas concentration comprises altering the first power to the radiation emitting diode responsive to the measured temperature sensitive characteristic.
Preferably, the method further comprises the step of measurement of a temperature of the components supplying the first power and the step of compensation of the measurement of the gas concentration further uses the measured temperature of the components supplying the first power.
Preferably, the measured temperature of the components supplying the first power is used to correct the measurement of temperature sensitive characteristic.
Preferably, the method further comprises the step of measurement of a temperature of the components supplying the second power and the step of compensation of the measurement of the gas concentration further comprises using the measured temperature of the components supplying the second power.
Preferably, the measured temperature of the components supplying the second power is used to correct the measurement of the gas concentration.
Preferably, the step of compensation of the measurement of the gas concentration comprises force heating or cooling the radiation emitting diode to a predetermined level responsive to the measured temperature sensitive characteristic. Preferably, the diode comprises a photodiode and the step of measurement of the gas concentration comprises measurement of a signal from the photodiode so as to detect radiation.
Preferably, the diode comprises a reference photodiode and the step of measurement of the gas concentration comprises measuring a signal from the reference photodiode so as to detect radiation.
Preferably, the step of compensation of the measurement of the gas concentration comprises altering the gain of an amplifier of a signal from the photodiode responsive to the measured temperature sensitive characteristic.
Preferably, the step of measurement of the temperature sensitive characteristic comprises supplying the diode with a predetermined voltage or current.
Preferably, the step of compensation of the measurement of the gas concentration comprises using a look-up table to correct the gas concentration responsive to the measured temperature sensitive characteristic.
According to a second aspect of the present invention there is provided a gas sensor comprising: a diode; a diode measurement means for measurement of a temperature sensitive characteristic of the diode; a gas measurement means for measurement of a gas concentration; and a compensation means for compensation of the measurement of the gas concentration using the measured temperature sensitive characteristic of the diode.
Preferably, the gas measurement means is operable to use the measured diode for the measurement of the gas concentration.
Preferably, the gas measurement means is operable to use a further diode on the same chip as the measured diode for the measurement of the gas concentration.
Preferably, the gas measurement means is operable to use a further diode in thermal communication with measured diode for the measurement of the gas concentration.
Preferably, the temperature sensitive characteristic comprises an electrical characteristic.
Preferably, the electrical characteristic comprises a forward voltage.
Preferably, the diode comprises a radiation emitting diode and the gas measurement means is operable to drive the radiation emitting diode so as to emit radiation for the measurement of the gas concentration.
Preferably, the diode measurement means is operable to supply a first power to the radiation emitting diode for measurement of the temperature sensitive characteristic of the diode and the gas measurement means is operable to drive the radiation emitting diode so as to emit radiation by supplying a second power to the radiation emitting diode. Preferably, the second power is larger than the first power.
Preferably, the second power is at least ten times larger than the first power.
Preferably, the compensation means is operable to alter the first drive power to the radiation emitting diode responsive to the measured temperature sensitive characteristic.
Preferably, the gas sensor further comprises a first temperature measurement means for measurement of a temperature of the components supplying the first power and the compensation means is further operable to use the measured temperature of the components supplying the first power for the compensation.
Preferably, the first temperature measurement means is operable to use the measured temperature of the components supplying the first power to correct the measurement of temperature sensitive characteristic.
Preferably, the gas sensor further comprises a second temperature measurement means for measurement of a temperature of the components supplying the second power and the compensation means is further operable to use the measured temperature of the components supplying the second power for the compensation.
Preferably, the second temperature measurement means is operable to use the measured temperature of the components supplying the second power to correct the measurement of the gas concentration. Preferably, the compensation means is operable to force heat or cool the radiation emitting diode to a predetermined level responsive to the measured temperature sensitive characteristic.
Preferably, the diode comprises a photodiode and the gas measurement means is operable to measure a signal from the photodiode so as to detect radiation.
Preferably, the diode comprises a reference photodiode and the gas measurement means is operable to measure a signal from the reference photodiode so as to detect radiation.
Preferably, the compensation means is operable to alter the gain of an amplifier of a signal from the photodiode responsive to the measured temperature sensitive characteristic.
Preferably, the diode measurement means is operable to supply the diode with a predetermined voltage or current.
Preferably, the compensation means is operable to use a look-up table to correct the gas concentration responsive to the measured temperature sensitive characteristic.
The present invention will now be described by way of example only with reference to the following figures in which:
Figure 1 illustrates in schematic form an apparatus for temperature measurement in accordance with an embodiment of the present invention. Figures 2 to 4 illustrate in schematic form a circuit for the forward voltage measurement of an LED in accordance with an embodiment of the present invention.
In particular, Figure 2 illustrates in schematic form the Pulse Wave Modulated to DC level conversion section of the circuit.
Figure 3 illustrates in schematic form the main and Vf LED drive section of the circuit.
Figure 4 illustrates in schematic form the Vf value amplification section of the circuit.
This embodiment of the present invention is a CO2 NDIR gas sensor.
With reference to Figure 1 , the LED 1 is driven with two 500μS pulses of between 1 mA and 5mA. The LED is driven by a constant current driver 2, the current being monitored by a microprocessor. The voltage across the LED is measured via a precision voltage measurement circuit 3. In other embodiments, other diode measurement means can be used to measure any temperature sensitive characteristic of the diode, electrical or otherwise. The resulting voltage pulse across the LED is fed into a tuned buffer amplifier which produces two pulses (one positive and one negative) for each pulse into the LED. Both the negative and positive pulses are fed into an A/D converter, the negative pulse being inverted in firmware and added to the positive pulse. The A/D converter then takes a series of eight readings on each half of the pulse to create a total reading for each LED pulse. The data from the two LED pulses is added together to create a combined total Vf (forward voltage) reading. Because Vf is a temperature sensitive characteristic, it can be used for temperature compensation by the compensation means 4. This has the advantage of compensating for temperature where it matters, in the core of the diode being used for gas concentration measurement.
Alternatively, the Vf measurement may be measured on a diode on the same chip as either the LED or photodiode, or by a diode in thermal communication with either the LED or photodiode. For example in the latter case, the multiple diodes could be mounted on a TO can or similar package, or in an assembly, which could be constructed using bonded, moulded of cast techniques such that the Vf sensing diode is within the combined thermal mass of the assembly. This still has an advantage of measuring compensating for the temperature of the diode being used for gas concentration measurement.
The high power LED pulse train is now triggered resulting in a CO2 measurement being captured by the microprocessor, although other digital or analogue circuits could be used as a gas measurement means.
The value of the combined Vf reading is then used in a look-up table by the microprocessor to compensate the CO2 measurement for the LED temperature. This is done by the microprocessor, although other digital or analogue circuits could be used as a compensation means.
Alternatively, Vf may be measured during the CO2 measurement pulse.
As an alternative to using a constant current source, a non-constant current source may be used. For example, the voltage may be fixed constant with a measurement of the resulting current being used for compensation. With reference to Figure 2, a Pulse Width Modulated square wave of frequency 2 kHz is presented to R70. This is smoothed by C34 to become a DC voltage A, B output to the mail LED drive and Vf LED drive sections respectively, shown in Figure 3.
The DC Voltage from the PWN to DC Level Conversion section of Figure 2 is buffered by IC2A (in Figure 2) and IC8A and IC8B (in Figure 3).
The upper section of Figure 3 is the precision driver for the high power drive to the LED D19.
The lower section of Figure 3 relates to the low power Vf measurement. The DC voltage at the output of IC8A is passed through R81 and, in the normal state, is shorted by Q2. A short 4.5 ms active low pulse enables the DC voltage through to the precision current source formed by IC7C and Q5. A square pulse of a set current is driven through the LED D19 and a voltage drop is created across it.
This square pulse C is output from the LED drive sections of Figure 3 into the Vf Value Amplification section shown in Figure 4. differentiated by C36 and attenuated by the combination of resistor R83 with resistors R79 and R82.
The differentiated pulse is buffered by IC7D, low-pass filtered by R13 and C4 then passed D to the processor for A/D measurement.
Since it is the processor that initiates the drive pulse, it is able to coherently oversample the differentiated pulse 1200 times. The resultant number, scaled to a 16 bit integer value gives an accuracy of 12 bits of resolution from the processor's 10 bit A/D system. The Vf measurement is linear over a decade of measurement range. The level of the drive pulse can be adjusted by controlling the PWM value to arrange that the resultant measurement can be scaled for best accuracy.
The operation of the LED used is very temperature dependant and accurate measurement of its forward voltage correlates directly with its local temperature. By knowing the Vf and therefore the temperature, we can correct the output value of the system using the measured LED forward voltage.
The photodiode is positioned at the start of a high gain (10OdB) amplifier chain. The chain is tuned to a known frequency at which the diode is pulsed.
Options for detecting the voltage drop across the photodiode include:
1 ) As with the LED, the photodiode is driven with a known current from a constant current source and the voltage drop across it is measured. In this case, the current source must be well isolated from the photodiode when gas measurements are being taken. This can be achieved using a device such as a FET.
2) The photodiode is driven with a known AC (Alternating Current) signal through a series resistor. This will form a potential divider with a potential across the photodiode varying with temperature. The resulting signal will be amplified by the photodiode amplifier chain, and the resulting signal can be used to derive the temperature. This has the advantage of measuring the temperature response of the entire amplifier chain. As a refinement, this signal can be scaled to suit the amplifier chain by changing its frequency away from the centre of the band-pass frequency of the amplifier chain. This will reduce the gain in the chain, allowing the signal to be scaled to a suitable level.
Once the temperature sensitive characteristic of the diode is known, it can be used to compensate the readings by:
a) Altering the drive current to the LED; b) Force heating or cooling the LED and Photodiode to a predetermined level; c) Altering the gain of the photodiode amplifier; d) Using a look-up table to correct the ultimate reading for temperature; or e) A combination of the above.
The compensation can be achieved by analogue processing, or by digitising the temperature signal and using a microprocessor to carry out the compensation.
The LED is driven from a constant current source with a known current. The voltage across the LED is then measured. Alternatively, the LED is driven from a constant voltage source with a known voltage. The current across the LED is then measured.
As described above, the performance of the LED and Photodiode are highly dependent on their device temperatures. Using accurate measurement of the forward voltage of the LED gives a very accurate indication of its operating temperature. Unfortunately, there is a temperature induced error in the precision current driver that is used to drive 1 milliamp into the LED so that its forward voltage can be measured. This is a very small error but is enough to skew the forward voltage measurement and the gas concentration measurement that is dependent upon its accuracy.
Rather than build temperature correction in to the precision driver itself, in this embodiment it was decided to correct for this error using correction lookup tables. The microprocessor on the control board has a temperature sensing thermistor, or first temperature measurement means, connected to it that allows the system to know the temperature of the control board and therefore the temperature of the precision current driver components.
During production test, the control board is ramped between the extremes of its operating temperatures in a thermal chamber while the LED and
Photodiode are kept at static room temperature of 20 Degrees Centigrade. While the temperature is being ramped, the control board temperature measurement is recorded along with the LED forward voltage.
From this data, a lookup table is created that is an error correction for the LED forward voltage cross referenced to control board temperature.
In operation, the LED forward voltage is measured and the numerical value is corrected by using the control board temperature to reference the correction lookup table held on an on-board ROM.
To a lesser extent, the main precision current driver that drives the LED for CO2 measurement is affected by the control board temperature. As a result of this error, the value of the measured CO2 requires correction. In a similar manner to the above method, the CO2 measurement is corrected by taking the present value of the control board temperature, using a second temperature measurement means such as a thermistor, and using it to refer to a lookup table of correction factors that have been collected during test.
To avoid self heating, the drive current is typically at least 10 times smaller than the current used to illuminate the LED for measurements. Furthermore, the device is only driven for 32 off 1 millisecond pulses per second. This equates to a 3.2% duty cycle so self heating of the LED is reduced to a minimum. Any heating that is brought about may be corrected for by the procedure detailed above.
The raw measurement can be passed through suitable signal conditioning such as filtering or moving average measurements.
Further modifications and improvements may be added without departing from the scope of the invention described by the claims.

Claims

Claims
1. A method of temperature compensation for a gas sensor comprising a diode, the method comprising the steps of: measurement of a temperature sensitive characteristic of the diode; measurement of a gas concentration; and compensation of the measurement of the gas concentration using the measured temperature sensitive characteristic of the diode.
2. The method of claim 1 , wherein the step of the measurement of the gas concentration uses the measured diode.
3. The method of claim 1 or claim 2, wherein the step of the measurement of the gas concentration uses a further diode on the same chip as the measured diode.
4. The method of any previous claim, wherein the step of the measurement of the gas concentration uses a further diode in thermal communication with the measured diode.
5. The method of any previous claim, wherein the temperature sensitive characteristic comprises an electrical characteristic.
6. The method of claim 5, wherein the electrical characteristic comprises a forward voltage.
7. The method of any previous claim, wherein the diode comprises a radiation emitting diode and the step of measuring the gas concentration comprises the step of driving the radiation emitting diode so as to emit radiation.
8. The method of claim 7, wherein the step of measurement of the temperature sensitive characteristic comprises supplying a first power to the radiation emitting diode and the step of driving the radiation emitting diode so as to emit radiation comprises the step of supplying a second power to the radiation emitting diode.
9. The method of claim 8, wherein the second power is larger than the first power.
10. The method of claim 9, wherein the second power is at least ten times larger than the first power.
11. The method of any of claims 8 to 10, wherein the step of compensation of the measurement of the gas concentration comprises altering the first power to the radiation emitting diode responsive to the measured temperature sensitive characteristic.
12. The method of any of claims 8 to 11 , further comprising the step of measurement of a temperature of the components supplying the first power and the step of compensation of the measurement of the gas concentration further uses the measured temperature of the components supplying the first power.
13. The method of claim 12, wherein the measured temperature of the components supplying the first power is used to correct the measurement of temperature sensitive characteristic.
14. The method of any of claims 8 to 13, further comprising the step of measurement of a temperature of the components supplying the second power and the step of compensation of the measurement of the gas concentration further comprises using the measured temperature of the components supplying the second power.
15. The method of claim 14, wherein the measured temperature of the components supplying the second power is used to correct the measurement of the gas concentration.
16. The method of any of claims 7 to 15, wherein the step of compensation of the measurement of the gas concentration comprises force heating or cooling the radiation emitting diode to a predetermined level responsive to the measured temperature sensitive characteristic.
17. The method of any of claims 1 to 6, wherein the diode comprises a photodiode and the step of measurement of the gas concentration comprises measurement of a signal from the photodiode so as to detect radiation.
18. The method of claim 17, wherein the diode comprises a reference photodiode and the step of measurement of the gas concentration comprises measuring a signal from the reference photodiode so as to detect radiation.
19. The method of claim 17 or claim 18, wherein the step of compensation of the measurement of the gas concentration comprises altering the gain of an amplifier of a signal from the photodiode responsive to the measured temperature sensitive characteristic.
20. The method of any previous claim, wherein the step of measurement of the temperature sensitive characteristic comprises supplying the diode with a predetermined voltage or current.
21. The method of any previous claim, wherein the step of compensation of the measurement of the gas concentration comprises using a look-up table to correct the gas concentration responsive to the measured temperature sensitive characteristic.
22. A gas sensor comprising: a diode; a diode measurement means for measurement of a temperature sensitive characteristic of the diode; a gas measurement means for measurement of a gas concentration; and a compensation means for compensation of the measurement of the gas concentration using the measured temperature sensitive characteristic of the diode.
23. The gas sensor of claim 22, wherein the gas measurement means is operable to use the measured diode for the measurement of the gas concentration.
24. The gas sensor of claim 22 or claim 23, wherein the gas measurement means is operable to use a further diode on the same chip as the measured diode for the measurement of the gas concentration.
25. The gas sensor of any of claims 22 to 24, wherein the gas measurement means is operable to use a further diode in thermal communication with measured diode for the measurement of the gas concentration.
26. The gas sensor of any of claims 22 to 25, wherein the temperature sensitive characteristic comprises an electrical characteristic.
27. The gas sensor of claim 26, wherein the electrical characteristic comprises a forward voltage.
28. The gas sensor of any of claims 22 to 27, wherein the diode comprises a radiation emitting diode and the gas measurement means is operable to drive the radiation emitting diode so as to emit radiation for the measurement of the gas concentration.
29. The gas sensor of claim 28, wherein the diode measurement means is operable to supply a first power to the radiation emitting diode for measurement of the temperature sensitive characteristic of the diode and the gas measurement means is operable to drive the radiation emitting diode so as to emit radiation by supplying a second power to the radiation emitting diode.
30. The gas sensor of claim 29, wherein the second power is larger than the first power.
31. The gas sensor of claim 30, wherein the second power is at least ten times larger than the first power.
32. The gas sensor of any of claims 29 to 31 , wherein the compensation means is operable to alter the first drive power to the radiation emitting diode responsive to the measured temperature sensitive characteristic.
33. The gas sensor of any of claims 29 to 32, further comprising a first temperature measurement means for measurement of a temperature of the components supplying the first power and the compensation means is further operable to use the measured temperature of the components supplying the first power for the compensation.
34. The gas sensor of claim 33, wherein the first temperature measurement means is operable to use the measured temperature of the components supplying the first power to correct the measurement of temperature sensitive characteristic.
35. The gas sensor of any of claims 29 to 34, further comprising a second temperature measurement means for measurement of a temperature of the components supplying the second power and the compensation means is further operable to use the measured temperature of the components supplying the second power for the compensation.
36. The gas sensor of claim 35, wherein the second temperature measurement means is operable to use the measured temperature of the components supplying the second power to correct the measurement of the gas concentration.
37. The gas sensor of any of claims 28 to 36, wherein the compensation means is operable to force heat or cool the radiation emitting diode to a predetermined level responsive to the measured temperature sensitive characteristic.
38. The gas sensor of any of claims 22 to 27, wherein the diode comprises a photodiode and the gas measurement means is operable to measure a signal from the photodiode so as to detect radiation.
39. The gas sensor of claim 38, wherein the diode comprises a reference photodiode and the gas measurement means is operable to measure a signal from the reference photodiode so as to detect radiation.
40. The gas sensor of claim 38 or claim 39, wherein the compensation means is operable to alter the gain of an amplifier of a signal from the photodiode responsive to the measured temperature sensitive characteristic.
41. The gas sensor of any of claims 22 to 40, wherein the diode measurement means is operable to supply the diode with a predetermined voltage or current.
42. The gas sensor of any of claims 22 to 41 , wherein the compensation means is operable to use a look-up table to correct the gas concentration responsive to the measured temperature sensitive characteristic.
PCT/GB2008/002661 2007-08-06 2008-08-05 Temperature compensation for gas detection WO2009019467A1 (en)

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