JP2012002757A - Infrared ray-absorbing type sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an infrared ray-absorbing type COsensor which has high responsiveness.SOLUTION: The infrared ray-absorbing type COsensor includes a light source and a light source driving device adapted to irradiate object gas with an infrared ray which is intensity-modulated at a modulation frequency fm, detection devices which receives infrared rays transmitted through the object gas by bolometers, and detect variations in resistance value of those bolometers, a differential amplifier which amplifies detection signals, and a synchronous detection device 60 which synchronously detect the amplified signals. The synchronous detection device 60 includes multipliers 62C, 62S which multiply the amplified signals by a reference signal having the same frequency as the modulation frequency fm and output detection signals, LPFs 64C, 64S which attenuate high-frequency components of the detection signals, and a degree-(n) synchronous IF filter 67 which processes a signal sampled at a sampling frequency fs. Vessel filters are used as the LPFs 64C, 64S, and the degree (n) of the synchronous IF filter, the modulation frequency fm, and the sampling frequency fs satisfy n/fs=1/fm.

Description

  The present invention relates to an infrared absorption sensor. In particular, the present invention relates to an infrared absorption sensor suitable for controlling the flow rate of EGR gas in an internal combustion engine.

In order to reduce the amount of NOx generated in an internal combustion engine or improve fuel efficiency, a technique for recirculating a part of the exhaust gas to the intake air as EGR gas is known. Here, in the control of the flow rate of the EGR gas, a CO ₂ sensor that detects the CO ₂ concentration in the exhaust gas may be used.

FIG. 11 is a schematic diagram showing a configuration of a conventional infrared absorption CO ₂ sensor 100.
The so-called infrared absorption CO ₂ sensor 100 irradiates the target gas with infrared light and measures the absorption spectrum in the 4.2 μm band, which is the vibration mode of the CO ₂ molecule, to thereby reduce the CO ₂ concentration in the target gas. taking measurement. More specifically, in the infrared absorption CO ₂ sensor 100, infrared light that has been intensity-modulated at a modulation frequency fm is irradiated from the light source 101 to the target gas G through the optical filter 102, and the infrared light that has passed through the target gas. Light is detected by the detection device 103. A minute detection signal output from the detection device 103 is amplified by the amplifier 104, synchronously detected by the synchronous detection device 105, and then extracted as a desired output signal.

The amount of infrared light in the 4.2 μm band irradiated from the light source 101 and passing through the target gas G varies according to the CO ₂ concentration in the target gas G. In the detection device 103, the transmitted light of the target gas G is received by the bolometers B1 and B2, so that the change in the amount of infrared light corresponding to the CO ₂ concentration in the target gas is changed as the change in the resistance value of the bolometers B1 and B2. To detect. As described above, as the detection device 103 that detects the CO ₂ concentration in the target gas G as a change in the resistance value of the bolometers B1 and B2, as shown in FIG. 11, bolometers B1 and B2 and fixed resistors R1 and R2 are used. It is composed of a bridge circuit as an element (see Patent Document 1).

  As shown in FIG. 11, the synchronous detector 105 includes a high-pass filter 106 that extracts an AC component from the amplified signal, and a multiplier that multiplies the signal that has passed through the high-pass filter 106 by a reference signal having a frequency equal to the modulation frequency fm. 107 and a low-pass filter 108 that removes unnecessary alternating current components from the signal multiplied by the reference signal. As the low-pass filter 108, a Butterworth filter, a Bessel filter, a Chebyshev filter, an FIR filter, or a combination of these (see Patent Document 2) is used.

Japanese Patent Publication No.61-6326 Japanese Patent No. 3730985

By the way, in order to control the flow rate of the EGR gas based on the output of the infrared absorption CO ₂ sensor as described above, the CO ₂ sensor is required to have high responsiveness. Here, the relationship between the responsiveness of the CO ₂ sensor and the characteristics of the low-pass filter 108 will be examined with reference to FIG.

FIG. 12 is a diagram showing the step response of the output signal, that is, the behavior of the output signal when the output of the amplifier is given in a step shape at time t = 0. FIG. 12 shows an example in which a third-order Butterworth filter is used as the low-pass filter. In FIG. 12, the cutoff frequency fc is changed to 0.2 / 0.5 / 1.0 / 2.0 / 5.0 [Hz] while the modulation frequency fm is fixed to 30 [Hz]. The case where it was made to show.
As shown in FIG. 12, when the cut-off frequency fc is increased, the time required for the output voltage to settle, that is, the settling time is shortened. This settling time serves as a standard indicating the response required for the sensor. Therefore, in order to shorten the settling time in order to improve the response of the sensor, it is generally necessary to sufficiently increase the cut-off frequency fc of the low-pass filter.

  However, when the cutoff frequency is set to 5.0 [Hz], for example, in order to achieve the response required for the sensor in the EGR gas flow rate control (for example, about 100 ms or less in the settling time), FIG. As shown in FIG. 4, the modulation frequency fm accompanying the modulation and the ripple having a frequency that is an integral multiple of the modulation frequency fm cannot be removed and are superimposed on the output signal. In order to solve such a problem, it is conceivable to increase the modulation frequency fm as well as the cut-off frequency fc. However, in the case where the bolometer is used as a detection element as described above, the modulation is required to ensure sufficient sensitivity. It is necessary to keep the frequency fm at about 10 to 30 [Hz]. For this reason, the cut-off frequency fc must also be smaller than a value corresponding to the limit of the modulation frequency fm. Therefore, there is a limit to improving the response by simply adjusting the cut-off frequency fc.

  For the purpose of simply improving responsiveness, it is possible to use a semiconductor sensor capable of high-speed detection instead of an infrared absorption sensor using the bolometer as described above as a detection element. As a vehicle-mounted sensor, the cost may increase significantly.

  An object of the present invention is to provide an infrared absorption sensor with high responsiveness.

In order to achieve the above object, the present invention provides an irradiation means (for example, a light source 10 and a light source driving device 20 described later) that irradiates a target gas (for example, a target gas G described later) with infrared light whose intensity is modulated at a modulation frequency fm. And detecting means (for example, a detection device 40 to be described later) that receives infrared light transmitted through the target gas with a bolometer (for example, bolometers B1 and B2 described later) and detects a change in resistance value of the bolometer. An infrared absorption sensor (for example, an infrared absorption CO ₂ sensor described later) that measures the concentration of a specific component (for example, CO ₂ described later) in the target gas based on a signal output from the detection means is provided. To do. The infrared absorption sensor amplifies the signal output from the detection means and outputs an amplified signal (for example, a differential amplifier 50 described later), and synchronous detection means (for example, synchronous detection of the amplified signal) A synchronous detection device 60) to be described later. The synchronous detection means includes multiplication means (for example, multipliers 62C and 62S described later) for multiplying the amplified signal by a reference signal having a frequency equal to the modulation frequency fm and outputs a detection signal, and high frequency components from the detection signal. A Bessel filter (for example, LPF 64C, 64S described later) and an FIR filter of order n (for example, a synchronous FIR filter 67 described later) for processing a signal sampled under the sampling frequency fs. The following equation, n / fs = 1 / fm, holds between the order n of the FIR filter, the modulation frequency fm, and the sampling frequency fs.

  According to this invention, the signal output from the detecting means and amplified by the amplifying means is multiplied by the reference signal having the modulation frequency fm by the multiplying means, and then an unnecessary AC component is generated by the Bessel filter and the FIR filter of order n. Removed. Here, the FIR filter of order n with respect to the signal sampled under the sampling frequency fs has a characteristic that the gain periodically drops to zero for each frequency of fs / n, while the detection signal has a modulation frequency. The ripple of fm and its integral multiple is included. Therefore, when the following equation, n / fs = 1 / fm, holds between the order n of the FIR filter, the modulation frequency fm, and the sampling frequency fs, the ripple contained in the detection signal is efficiently processed by the FIR filter. Can be removed. Further, by removing ripples with the FIR filter in this way, it is not necessary to set the Bessel filter so as to remove ripples excessively, so that the cutoff frequency can be increased and the responsiveness can be improved. In addition, as described above, by combining the Bessel filter and the FIR filter, ripples can be sufficiently removed while leaving a drift component smaller than the modulation frequency fm, so that the actual concentration in the target gas can be changed. It can follow and measure.

  By the way, when an infrared absorption sensor as described above is mounted on a vehicle, it is necessary to cooperate between this sensor and another device mounted on the vehicle, so the degree of freedom in setting the sampling frequency fs is reduced. . According to the present invention, even when the sampling frequency fs is determined in advance, the order n of the FIR filter and the modulation frequency fm are adjusted so that the following expression, n / fs = 1 / fm is established. Thus, it can be mounted on a vehicle without impairing the above effect.

  In this case, it is preferable that an optical filter (for example, an optical filter 30 described later) that allows infrared light in the 4.2 μm band to pass through is provided in the optical path from the light source of the irradiation unit to the bolometer.

According to the present invention, since the optical filter that passes infrared light in the 4.2 μm band, which is the vibration mode of the CO ₂ molecule, is provided in the optical path from the light source to the bolometer, the concentration of CO _{2 in} the target gas can be efficiently reduced. Can be detected.

  In this case, it is preferable that the following expression, fc = fm / 2, holds between the cutoff frequency fc of the Bessel filter and the modulation frequency fm.

  According to the present invention, by setting the cut-off frequency fc in the Bessel filter to a half value of the modulation frequency fm, it is possible to efficiently reduce unnecessary ripples accompanying modulation while ensuring high response. be able to.

  In this case, the order of the Bessel filter is preferably 3.

  In the present invention, by using the high-frequency component of the detection signal in combination with the third-order Bessel filter and the FIR filter, the response can be improved while removing the ripple more efficiently.

It is a schematic diagram which shows the structure of the infrared rays absorption type sensor which concerns on one Embodiment of this invention. It is a block diagram which shows the structure of the synchronous detection apparatus which concerns on the said embodiment. It is a figure which shows the step response of the output signal of LPF at the time of using a 3rd-order Bessel filter as LPF. It is a figure which shows the relationship between the settling time of the output signal of LPF, and a cutoff frequency. It is a figure which shows the relationship between the amplitude of the ripple of the output signal of LPF, and a cutoff frequency. It is a figure which shows the structure of the n-th order synchronous FIR filter which concerns on the said embodiment. It is a figure which shows the gain characteristic of the synchronous FIR filter which concerns on the said embodiment. It is a figure which shows the spectrum of the output signal which concerns on the said embodiment. It is a figure which shows the step response of the output signal which concerns on the said embodiment. It is an enlarged view of the part shown with the broken line in FIG. It is a schematic view showing a conventional infrared absorption of CO ₂ sensor configuration. It is a diagram illustrating a step response of a conventional infrared absorption CO ₂ output signal in the sensor.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram illustrating a configuration of an infrared absorption CO ₂ sensor 1 according to the present embodiment.
The infrared absorption CO ₂ sensor 1 includes a light source 10 that emits infrared light to a target gas G, a light source driving device 20 that drives the light source 10, a detection device 40 that detects transmitted light of the target gas G, and a detection signal. And a synchronous detector 60 for synchronously detecting the amplified signal.

The light source 10 is an infrared light source that emits infrared light having a spectral distribution centered around 4.2 μm. The light source driving device 20 drives the light source 10 in pulses and irradiates the target gas G to be detected with infrared light intensity-modulated at the modulation frequency fm. Of the optical path from the infrared light source 10 to the bolometers B1 and B2 (described later) of the detection device 40, between the infrared light source 10 and the target gas G, there is a window in the vicinity of 4.2 μm corresponding to the vibration mode of CO ₂ molecules. An optical filter 30 is provided. The modulation frequency fm is set to a value corresponding to the sensitivity of the bolometers B1 and B2. In the present embodiment, the modulation frequency fm is set to 30 [Hz] as a value corresponding to the sensitivity upper limit of the bolometers B1 and B2, but the present invention is not limited to this.

  The detection device 40 is configured by assembling a bridge circuit with the two bolometers B1 and B2 and the two fixed resistors R1 and R2 as resistance elements. That is, the detection device 40 receives the infrared light transmitted through the target gas G by the bolometers B1 and B2, and detects the resistance values of the bolometers B1 and B2 as changes in the potential difference between the output terminals o1 and o2. The differential amplifier 50 amplifies a minute detection signal detected by the detection device 40 by, for example, about 1000 times.

That is, when the CO ₂ concentration in the target gas G is low, the amount of light reaching the bolometers B1 and B2 increases, so that the resistance value of the bolometers B1 and B2 increases. On the other hand, when the CO ₂ concentration is high, the amount of light reaching the bolometers B1 and B2 is small, so that the increase in resistance values of the bolometers B1 and B2 is small. In the infrared absorption CO ₂ sensor 1, the detection device 40 detects changes in resistance values of the bolometers B 1 and B ₂ , and measures the concentration of CO _{2 in} the target gas G based on this detection signal.

FIG. 2 is a block diagram showing a configuration of the synchronous detection device 60.
The synchronous detector 60 includes a high-pass filter (hereinafter referred to as “HPF”) 61, multipliers 62C and 62S, a cos oscillator 63C, a sin oscillator 63S, and a low-pass filter (hereinafter referred to as “LPF”) 64C and 64S. A vector operation unit 65, an A / D converter 66, and a synchronous FIR filter 67.

The HPF 61 extracts an AC component from the signal amplified by the differential amplifier 50. The cos oscillator 63C outputs a reference signal having a frequency equal to the modulation frequency fm, and the sin oscillator 63S outputs a reference signal having a π / 2 phase different from that of the reference signal output from the cos oscillator 63C. The multiplier 62C multiplies the signal that has passed through the HPF 61 by the reference signal output from the cos oscillator 63C, and outputs the cos component V _C ′ of the detection signal to the LPF 64C. The multiplier 62S multiplies the signal that has passed through the HPF 61 by the reference signal output from the sin oscillator 63S, and outputs the sin component V _S ′ of the detection signal to the LPF 64S.

The LPFs 64C and 64S attenuate the high frequency components of the components V _C ′ and V _S ′ of the detection signal, respectively. Bessel filters are used for these LPFs 64C and 64S. Among these, a tertiary Bessel filter is particularly preferable. Further, when a third-order Bessel filter is used as the LPFs 64C and 64S, the cut-off frequency fc is preferably set so that the following expression (1) is established with respect to the modulation frequency fm.
fc = fm / 2 (1)

  FIG. 3 is a diagram showing a step response of the output signal of the LPF when a third-order Bessel filter is used as the LPF. FIG. 3 shows a case where the cutoff frequency fc is changed to 0.6 / 1.5 / 3.0 / 6.0 / 15.0 [Hz]. As shown in FIG. 3, when the cutoff frequency fc is brought close to the modulation frequency fm (= 30 [Hz]), the settling time of the output signal is shortened. Further, as can be seen from comparison with FIG. 12 described above, by using a third-order Bessel filter as the LPF, the settling time of the output signal is shortened compared to the case where a third-order Butterworth filter is used. Overshoot can also be suppressed.

FIG. 4 is a diagram showing the relationship between the settling time of the output signal of the LPF and the cutoff frequency fc. FIG. 5 is a diagram showing the relationship between the ripple amplitude of the output signal of the LPF and the cutoff frequency fc.
As shown in FIG. 4, the third-order Bessel filter has a shorter settling time for the same cutoff frequency fc than the second- and third-order Butterworth filters. Also, as shown in FIG. 5, the third-order Bessel filter has a smaller ripple amplitude for the same cutoff frequency than the second- and third-order Butterworth filters. Therefore, by using a third-order Bessel filter as the LPF, overshoot can be suppressed while shortening the settling time, and further ripples caused by modulation can be efficiently suppressed.

  As shown in FIGS. 3 and 5, even when a third-order Bessel filter is used, if the cut-off frequency fc is increased to 15 [Hz] (corresponding to fm / 2), the settling time is increased. On the other hand, ripples of a magnitude that cannot be ignored begin to appear in the output signal of the LPF. However, as will be described later in detail, this ripple can be efficiently removed by a subsequent synchronous FIR filter. Therefore, in anticipation that this ripple is removed by the subsequent-stage synchronous FIR filter, in the upper-stage Bessel filter, the above equation (1) is established in favor of shortening the settling time over the ripple removal. The cutoff frequency fc can be set to a value close to the modulation frequency fm.

Returning to FIG. 2, the vector calculation unit 65 calculates the square root V = (V _C ² + V _S ² ) ^{1 /} of the square sum of the components V _C and V _S of the detection signal that has passed through the LPFs 64C and 64S as described above. ² is calculated. The A / D converter 66 samples the detection signal V calculated by the vector calculation unit 65 at a period fs.

The synchronous FIR filter 67 processes the signal sampled at the sampling frequency fs by the A / D converter 66 and outputs a final signal.
FIG. 6 is a diagram showing the configuration of the nth-order synchronous FIR filter 67. As shown in FIG. 6, the nth-order synchronous FIR filter 67 delays the signal sampled by the A / D converter 66 under the sampling frequency fs by the n delay operators for each sampling period 1 / fs. Elements are generated, and further, n adders are added to each delay element under the weights A ₀ , A ₁ ,... A _n−1 , _An , and output.

FIG. 7 is a diagram illustrating gain characteristics of the synchronous FIR filter. FIG. 7 shows a case where the weights A ₀ = A ₁ =... = A _n = “1”. As shown in FIG. 7, the synchronous FIR filter has a characteristic that the gain drops to zero every fs / n.
Therefore, in the present embodiment, using the gain characteristic of the synchronous FIR filter, the order n of the synchronous FIR filter, the modulation frequency fm, and the modulation frequency fm are set in order to remove the modulation frequency fm and its integral multiple ripple as described above. The order n and the sampling frequency fs of the synchronous FIR filter 67 are synchronized with the modulation frequency fm so that the following expression (2) is established between the sampling frequency fs and the sampling frequency fs.
n / fs = 1 / fm (2)

FIG. 8 is a diagram showing the spectrum of the output signal. In FIG. 8, the spectrum when the synchronous FIR filter is provided in the latter stage of the LPF is indicated by a solid line, and the spectrum when the synchronous FIR filter is not provided is indicated by a one-dot chain line. Further, the cut-off frequency fc of the LPF is both 15 [Hz].
When the synchronous FIR filter is not provided, that is, when the cut-off frequency fc is 15 [Hz], a ripple of the modulation frequency fm and an integer multiple of the modulation frequency fm is superimposed on the output signal. On the other hand, ripples can be efficiently removed by providing a synchronous FIR filter synchronized with the modulation frequency fm in the subsequent stage of the LPF so as to satisfy the above equation (2). More specifically, as shown in FIG. 8, the ripple amplitude decreased by about −45 dB.

9 is a diagram showing a step response of the output signal, and FIG. 10 is an enlarged view of a portion indicated by a broken line in FIG. 9 and 10, the step response when the synchronous FIR filter is provided in the subsequent stage of the LPF (fc = 15 [Hz]) is shown by a solid line, and is synchronized with the subsequent stage of the LPF (fc = 8, 15 [Hz]). Step responses when no FIR filter is provided are indicated by a broken line and a one-dot chain line, respectively.
As shown in FIG. 9, by providing a synchronous FIR filter, ripples included when only LPF is used can be sufficiently removed. Further, when the settling time when the synchronous FIR filter is provided is compared with the settling time when the synchronous FIR filter is not provided, fc = 15 [Hz] is longer, but fc = 8 [Hz]. On the other hand, it becomes shorter. Also, as shown in FIG. 10, by providing a synchronous FIR filter in the subsequent stage of the Bessel filter, ripples can be sufficiently removed while leaving a drift component of about 20 [Hz] or less, so that the actual CO ₂ concentration It is possible to measure by following the change of.

According to this embodiment, the following effects can be obtained.
(1) According to the present embodiment, the order n of the FIR filter and the sampling frequency fs so that the above equation (2) is established among the order n of the FIR filter, the modulation frequency fm, and the sampling frequency fs. Is synchronized with the modulation frequency fm, ripples included in the detection signal can be efficiently removed. Further, by removing ripples with the FIR filter in this way, it is not necessary to set the Bessel filter so as to remove ripples excessively, so that the cut-off frequency fc can be increased and the responsiveness can be improved. . In addition, as described above, by combining the Bessel filter and the FIR filter, ripples can be sufficiently removed while leaving a drift component smaller than the modulation frequency fm, so that the actual CO ₂ concentration in the target gas can be reduced. Measurements can be made following changes.
Further, according to the present embodiment, even when the sampling frequency fs is determined in advance, such as when the infrared absorption CO ₂ sensor 1 is mounted on a vehicle, the modulation frequency so that the above equation (2) is established. By adjusting the fm and the order n of the FIR filter, it can be mounted on a vehicle without impairing the above effects.

(2) According to the present embodiment, since the optical filter 30 that passes infrared light in the 4.2 μm band that is the vibration mode of the CO ₂ molecule is provided in the optical path from the light source 10 to the bolometers B1 and B2, the target gas G The concentration of CO ₂ in it can be detected efficiently.

  (3) According to the present embodiment, by setting the cut-off frequency fc in the Bessel filter to a half value of the modulation frequency fm, it is possible to efficiently eliminate unnecessary ripples generated with modulation while ensuring high responsiveness. Can be reduced.

  (4) According to this embodiment, the high frequency component of the detection signal is used in combination with the third-order Bessel filter and the synchronous FIR filter, so that the response can be improved while removing the ripple more efficiently. Can be improved.

The present invention is not limited to the embodiment described above, and various modifications can be made.
The various calculations in the synchronous detection device 60 of the above embodiment are configured by analog circuits except for the synchronous FIR filter 67, but are not limited thereto, and may be configured by digital circuits.
In the above embodiment, a third-order Bessel filter has been described as an example. However, the order of the filter is not limited to this, and may be a second-order or fourth-order or higher.
Moreover, in the said embodiment, as shown to the said Formula (1), although the cutoff frequency fc was set to the half value of the modulation frequency fm, it is not restricted to this. The cut-off frequency fc may be set to a value larger than half the modulation frequency fm so that the responsiveness is further improved.

1 ... Infrared absorbing CO ₂ sensor (Infrared absorbing sensor)
10: Light source (irradiation means)
20: Light source driving device (irradiation means)
30 ... Optical filter 40 ... Detection device (detection means)
B1, B2 ... Bolometer 50 ... Differential amplifier (amplifying means)
60. Synchronous detection device (synchronous detection means)
62C, 62S... Multiplier (multiplication means)
64C, 64S ... LPF (Bessel filter)
66 ... A / D converter 67 ... Synchronous FIR filter

Claims (4)

Irradiation means for irradiating the target gas with infrared light intensity-modulated at a modulation frequency fm;
Detection means for detecting infrared light transmitted through the target gas with a bolometer and detecting a change in the resistance value of the bolometer, and determining the concentration of the specific component in the target gas based on the signal output from the detection means An infrared absorption sensor for measuring,
Amplifying means for amplifying the signal output from the detection means and outputting an amplified signal;
A synchronous detection means for synchronously detecting the amplified signal,
The synchronous detection means includes
Multiplication means for multiplying the amplified signal by a reference signal having a frequency equal to the modulation frequency fm and outputting a detection signal;
A Bessel filter that attenuates high frequency components from the detected signal;
An FIR filter of order n that processes a signal sampled under a sampling frequency fs;
Between the order n of the FIR filter, the modulation frequency fm, and the sampling frequency fs,
n / fs = 1 / fm,
An infrared absorption sensor characterized by the fact that   2. The infrared absorption sensor according to claim 1, wherein an optical filter that allows infrared light in a 4.2 μm band to pass is provided in an optical path from the light source of the irradiation unit to the bolometer. Between the cutoff frequency fc of the Bessel filter and the modulation frequency fm,
fc = fm / 2,
The infrared absorption type sensor according to claim 1, wherein:   The infrared absorption sensor according to claim 1, wherein the order of the Bessel filter is 3. 5.

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