Abstract:
An infrared gas sensing apparatus for sensing gas includes an infrared source for emitting infrared light, a wavelength tunable filter for selectively allows passage of the infrared light at a specific wavelength, an infrared detector for detecting the amount of the filtered infrared light, and a control circuit for controlling the wavelength tunable filter. The control circuit has a check circuit that checks whether the wavelength tunable filter selects a collect wavelength. The check circuit controls the wavelength tunable filter so that the wavelength tunable filter allows passage of the infrared light at a wavelength within a wavelength range where atmospheric gases do not absorb the infrared light. The check circuit checks whether the wavelength tunable filter operates normally by comparing the amount of the filtered infrared light with a reference value.

Description:
CROSS CHECK TO RELATED APPLICATION 
       [0001]    This application is based on and incorporates herein by reference Japanese Patent Application No. 2006-254716 filed on Sep. 20, 2006. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an infrared gas sensing apparatus and method. 
       BACKGROUND OF THE INVENTION 
       [0003]    U.S. Pat. No. 6,590,710 corresponding to JP-A-2001-228326 discloses an infrared gas sensing apparatus for measuring the concentration of a target gas that absorbs a specific wavelength of infrared light. The gas sensing apparatus includes an infrared source that emits the infrared light, a wavelength tunable filter (i.e., Fabry-Perot filter) that selects the specific wavelength of the infrared light, and an infrared detector that detects the filtered infrared light. The gas sensing apparatus measures the concentration of the target gas based on the amount of the infrared light detected by the infrared detector. 
         [0004]    As shown in  FIG. 6 , the wavelength tunable filter includes a first mirror  3  and a second mirror  4 . The first mirror  3  is formed on a silicon substrate  1  through a first oxide film  2 . The second mirror  4  is formed on a second oxide film  5 , which is formed on the first mirror  3 . The first and second mirrors  3 ,  4  face each other. 
         [0005]    A gap H is formed between the first and second mirrors  3 ,  4  by etching the second oxide film  5  via an etching hole  6 . Therefore, the second mirror  4  is displaceable with respect to the first mirror  3  by application of an external force. The gap distance of the gap H is equal to the thickness of the second oxide film  5 . 
         [0006]    The first and second mirrors  3 ,  4  are made of polysilicon, for example. The first mirror  3  has a first electrode  7  on one surface. Also, the second mirror  4  has a second electrode  8  on one surface. The first and second electrodes  7 ,  8  are formed by a highly-concentrated impurity doping applied to the surfaces of the first and second mirrors  3 ,  4 , respectively. 
         [0007]    A first external electrode  9  is formed on the first electrode  7  and electrically coupled to the first electrode  7 . Also, a second external electrode  10  is formed on the second electrode  8  and electrically coupled to the second electrode  8 . 
         [0008]    The wavelength tunable filter has a center wavelength λ determined by the gap distance of the gap H, i.e., the thickness of the second oxide film  5 . For example, the center wavelength λ is 3100 nanometers (nm). Since the first mirror  3  serves as a lower mirror of the wavelength tunable filter, the optical thickness needs to be equal to a quarter of the center wavelength λ. For example, the second oxide film  5  has the thickness of 592 nm and the refractive index of 1.309. Each of the first and second mirrors  3 ,  4  has the thickness of 248 nm and the refractive index of 3.125. 
         [0009]    When a voltage is applied between the first and second electrodes  7 ,  8  through the first and external electrodes  9 ,  10 , electrostatic attraction force is produced between the first and second electrodes  7 ,  8 . The second electrode  8  is displaced with respect to the first electrode  7  by the electrostatic attraction force. As a result, the gap distance of the gap H is changed. The gap distance is adjusted by adjusting the voltage applied between the first and second electrodes  7 ,  8 . Therefore, the wavelength tunable filter can select the specific wavelength of the infrared light according to the target gas. 
         [0010]    In the wavelength tunable filter shown in  FIG. 6 , the gap distance of the gap H can be adjusted in three levels so that the wavelength tunable filter can select the specific wavelength from three different wavelengths of the infrared light. Thus, the infrared gas sensing apparatus can detect the concentrations of two components of the target gas with one filter. Therefore, the infrared gas sensing apparatus has a small size and is manufactured at low cost. 
         [0011]    However, if a foreign matter enters the gap H and is sandwiched between the first and second electrodes  7 ,  8 , the gap distance of the gap H cannot be adjusted. As a result, the infrared gas sensing apparatus incorrectly detects the concentration of the target gas, because the wavelength tunable filter cannot select the specific wavelength where the target gas absorbs the infrared light. 
       SUMMARY OF THE INVENTION 
       [0012]    In view of the above-described problem, it is an object of the present invention to provide an infrared gas sensing apparatus and method for accurately measuring the concentration of a target gas by checking whether a wavelength tunable filter selects a correct wavelength. 
         [0013]    A gas sensing apparatus for sensing gas includes an infrared source, a wavelength tunable filter, an infrared detector, a housing, and a control circuit. The gas absorbs infrared light at a first wavelength. The infrared source emits the infrared light. The wavelength tunable filter selectively allows passage of the infrared light at a specific wavelength. The control circuit includes a measurement circuit and a check circuit The measurement circuit controls the wavelength tunable filter, so that the wavelength tunable filter allows passage of the infrared light at the first wavelength and outputs a first filtered infrared light. The check circuit controls the wavelength tunable filter, so that the wavelength tunable filter allows passage of the infrared light at a second wavelength and outputs a second filtered infrared light. The infrared detector detects a first amount of the first filtered infrared light and a second amount of the second filtered light. The infrared light source, the wavelength tunable filter, and the infrared detector are accommodated in the housing. The housing has an inlet for introducing the gas therein. 
         [0014]    The measurement circuit calculates concentration of the gas based on the first amount of the first filtered infrared light. The check circuit checks whether the wavelength tunable filter operates normally by comparing the second amount of the second filtered infrared light with a reference value. The second wavelength is within a wavelength range where atmospheric gases do not absorb the infrared light. Due to the check circuit, the gas sensing apparatus can accurately detect the gas. The check circuit checks the wavelength tunable filter based on the transmittance of the infrared light through the atmospheric gases. In other words, the check circuit checks the wavelength tunable filter by using air, not special gas. In such an approach, structure of the gas sensing apparatus can be simplified. Further, because of the simple structure, the gas sensing apparatus can have a small size and be manufactured at low cost. 
         [0015]    A method for sensing gas using a wavelength tunable filter includes emitting an infrared light, setting the wavelength tunable filter so that the wavelength tunable filter allows passage of the infrared light at a first wavelength and outputs a first filtered infrared light, detecting a first amount of the first filtered infrared light, and checking whether the wavelength tunable filter operates normally by comparing the first amount with a reference value. The first wavelength is within a wavelength range where atmospheric gases do not absorb the infrared light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]    The above and other objectives, features and advantages of the present invention will become more apparent from the following detailed description made with check to the accompanying drawings. In the drawings: 
           [0017]      FIG. 1  is a cross-sectional view of an infrared gas sensor according to an embodiment of the present invention; 
           [0018]      FIG. 2  is a block diagram of the infrared gas sensor of  FIG. 1 ; 
           [0019]      FIG. 3  is a cross-sectional view of a wavelength tunable filter in the infrared gas sensor of  FIG. 1 ; 
           [0020]      FIG. 4  is a flow chart of the infrared gas sensor of  FIG. 1 ; 
           [0021]      FIGS. 5A-5I  are diagrams showing transmittance of infrared light through the atmospheric gases; and 
           [0022]      FIG. 6  is a cross-sectional view of a wavelength tunable filter in a conventional infrared gas sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    As shown in  FIGS. 1 ,  2 , an infrared gas sensor  100  according to an embodiment of the present invention includes an infrared (IR) source  20 , a wavelength tunable filter  30 , an infrared (IR) detector  40 , a housing  50 , and a control circuit  60 . The infrared source  20 , the wavelength tunable filter  30 , the infrared detector  40 , and the control circuit  60  are accommodated in the housing  50 . 
         [0024]    The infrared source  20  may be, for example, an incandescent lamp. As indicated by arrows in  FIG. 1 , the infrared source  20  emits infrared light with a continuous range of wavelengths from 2 micrometers (μm) to 10 μm. 
         [0025]    The wavelength tunable filter  30  is a Fabry-Perot interference filter. The wavelength tunable filter  30  selects a specific wavelength of the infrared light to be sent to the infrared detector  40  from the infrared source  20 . As shown in detail in  FIG. 3 , the wavelength tunable filter  30  includes a substrate  31 , an antireflective film  32  formed on the substrate  31 , a first mirror  33  formed on the substrate  31  through the antireflective film  32 , and a second mirror  34  formed on the first mirror  33  through a sacrificial layer. The first and second mirrors  33 ,  34  face each other. 
         [0026]    A gap  37  is formed between the first and second mirrors  33 ,  34  by etching the sacrificial layer via an etching hole  38 . The second mirror  34  can be displaced with respect to the first mirror  33 , when external force is applied to the second mirror  34 . The first mirror  33  has a first electrode  35  on one surface. The second mirror  34  has a second electrode  36  on one surface. The first and second electrodes  35 ,  36  may be, for example, formed by a highly-concentrated impurity doping applied to the surfaces of the first and second mirrors  33 ,  34 , respectively. The first and second electrodes  35 ,  36  face each other. 
         [0027]    The substrate  31  may be, for example, made of silicon, quartz, or the like. The first and second mirrors  33 ,  34  and the first and second electrodes  35 ,  36  are thin layers and may be, for example, made of molybdenum, silicon, germanium, silicon nitride, oxide silicon, or the like. 
         [0028]    Thus, the wavelength tunable filter  30  has a small size and can be manufactured easily by using micro-electro-mechanical systems (MEMS) technology. 
         [0029]    In the wavelength tunable filter  30 , the infrared light at the specific wavelength equal to a half or quarter of a gap distance D of the gap  37  is multiply reflected between the first and second mirrors  33 ,  34  so that interference occurs. As a result, the infrared light only at the specific wavelength passes through the wavelength tunable filter  30 . 
         [0030]    When a voltage is applied between the first and second electrodes  35 ,  36 , electrostatic attraction force is produced between the first and second electrodes  35 ,  36 . The second mirror  34  is displaced with respect to the first mirror  33  by the electrostatic attraction force. The gap distance D of the gap  37  can be steplessly adjusted by adjusting the voltage applied between the first and second electrodes  35 ,  36 . By adjusting the gap distance D, therefore, the wavelength tunable filter  30  can select the specific wavelength of the infrared light to be sent to the infrared detector  40 . 
         [0031]    The infrared detector  40  detects the filtered infrared light and outputs an electrical signal indicative of the amount of the detected infrared light. The infrared detector  40  may be, for example, a thermopile, a pyroelectric sensor, or the like. 
         [0032]    Referring again to  FIG. 1 , the wavelength tunable filter  30  and the infrared detector  40  are sealed in a container formed with a stem  51  and a case  52  with a transparent window  53 . The infrared light emitted by the infrared source  20  enters the wavelength tunable filter  30  through the transparent window  53 . 
         [0033]    The housing  50  has an inlet and an outlet for a target gas to be detected. The target gas is introduced in a light path between the infrared source  20  and the wavelength tunable filter  30 . The target gas at least partially absorbs the specific wavelength of the target gas. The wavelength tunable filter  30  allows the passage of the infrared light only at the specific wavelength. 
         [0034]    The filtered infrared light reaches the infrared detector  40  and is converted into the electrical signal. The electrical signal is transmitted to a processor in the control circuit  60 , and the processor calculates the concentration of the target gas based on the electrical signal. 
         [0035]    As described above, the infrared gas sensor  100  includes the infrared source  20  that emits the infrared light, the wavelength tunable filter  30  that selects the specific wavelength of the infrared light, and the infrared detector  40  that detects the filtered infrared light and converts the detected infrared light into the electrical signal. The specific wavelength selected by the wavelength tunable filter  30  can be steplessly changed by changing the gap distance D of the gap  37  between the first and second mirrors  33 ,  34 . Thus, the infrared gas sensor  100  can detect concentrations of various kinds of gases. 
         [0036]      FIGS. 5A-5I  are diagrams showing transmittance of infrared light through atmospheric gases. The diagrams are founded in a Japanese book “sekigaisen kougaku” (ISBN-13: 978-4885521225) published by Haruyoshi Hisano in April 1994. As can be seen from  FIGS. 5A-5I , the transmittance of infrared light is approximately 100 percent in wavelength ranges between 1.55 micrometers (μm) and 1.75 μm, between 2.05 μm and 2.33 μm, between 3.5 μm and 4.16 μm, and between 9.4 μm and 12.4 μm. The control circuit  60  may have filter check circuit for checking, based on the transmittance of infrared light through the atmospheric gases, whether the wavelength tunable filter  30  operates normally. The wavelength ranges, where the transmittance of infrared light is approximately 100 percent, are hereinafter called “non-absorption wavelength range”. Wavelength ranges outside the non-absorption wavelength range are hereinafter called “absorption wavelength range”. 
         [0037]    The infrared gas sensor  100  operates according to a flow chart of  FIG. 4 . At step S 1 , the infrared gas sensor  100  is powered on. Then, at step S 2 , the infrared source  20  is powered on by a driver in the control circuit  60  and emits infrared light. 
         [0038]    Then, at step S 3 , the gap distance D of the wavelength tunable filter  30  is adjusted so that the wavelength tunable filter  30  selects a first check wavelength λF 1  within the non-absorption wavelength range. For example, the first check wavelength λF 1  is 2.1 μm. Then, at step S 4 , the infrared detector  40  detects the filtered infrared light and converts the detected infrared light into a first check signal VF 1  indicative of the amount of the detected infrared light. Then, at step S 5 , the first check signal VF 1  is stored in a memory (not shown) in the control circuit  60 . 
         [0039]    The steps S 3 -S 5  are repeated one more time. At the second step S 3 , the gap distance D of the wavelength tunable filter  30  is adjusted so that the wavelength tunable filter  30  selects a second check wavelength λF 2  within the non-absorption wavelength range. The second check wavelength λF 2  is different from the first check wavelength λF 1 . For example, the second check wavelength λF 2  is 2.3 μm. In this case, the second check wavelength λF 2  is within the same non-absorption wavelength range as the first check wavelength λF 1 . Alternatively, the second check wavelength λF 2  may be within the different non-absorption wavelength range from the first check wavelength λF 1 . For example, the second check wavelength λF 2  may be within the wavelength range between 9.4 μm and 12.4 μm. Then, at the second step S 4 , the infrared detector  40  detects the filtered infrared light and converts the detected infrared light into a second check signal VF 2  indicative of the amount of the detected infrared light. Then, at the second step S 5 , the second check signal VF 2  is stored in the memory in the control circuit  60 . 
         [0040]    Then, at step S 6 , the control circuit  60  calculates a signal ratio VF 1 /VF 2  between the first check signal VF 1  and the second check signal VF 2 . Then, at step S 7 , the control circuit  60  determines whether the signal ratio VF 1 /VF 2  is approximately one. Since the first check wavelength λF 1  and the second check wavelength λF 2  are approximately the same and within the non-absorption wavelength, the first check signal VF 1  and the second check signal VF 2  are approximately the same. Therefore, the signal ratio VF 1 /VF 2  becomes approximately one, unless the wavelength tunable filter  30  malfunctions. 
         [0041]    At step S 7 , if the signal ratio VF 1 /VF 2  is not approximately one, the control circuit  60  determines that the wavelength tunable filter  30  malfunctions and selects an incorrect wavelength i.e., a wavelength other than the first check wavelength λF 1  and the second check wavelength λF 2 . Therefore, at step S 8 , a first error message, indicating that the wavelength tunable filter  30  malfunctions, appears on a screen shown in  FIG. 2 , and the process is stopped. 
         [0042]    In contrast, at step S 7 , if the signal ratio VF 1 /VF 2  is approximately one, the control circuit  60  determines that the wavelength tunable filter  30  operates normally. Therefore, the process is continued. 
         [0043]    Then, at step S 9 , the gap distance D of the wavelength tunable filter  30  is adjusted so that the wavelength tunable filter  30  selects a third check wavelength λL. Then, at step S 10 , the infrared detector  40  detects the filtered infrared light and converts the detected infrared light into a third check signal VL indicative of the amount of the detected infrared light. Then, at step S 11 , the control circuit  60  determines whether the third check signal VL exceeds a threshold value VT. 
         [0044]    At step S 11 , if the third check signal VL is less than the threshold value VT, the control circuit  60  determines that the amount of the infrared light emitted by the infrared source  20  is reduced due to, for example, age deterioration. Therefore, at step S 12 , a second error message, indicating that the infrared source  20  malfunctions, appears on the screen, and the process is stopped. 
         [0045]    In contrast, at step S 11 , if the third check signal VL exceeds the threshold value VT, the control circuit  60  determines that the infrared source  20  operates normally. Therefore, the process is continued. 
         [0046]    Then, at step S 13 , the gap distance D of the wavelength tunable filter  30  is adjusted so that the wavelength tunable filter  30  selects a target wavelength λS where the target gas to be sensed absorbs the infrared light. Then, at step S 14 , the infrared detector  40  detects the filtered infrared light and converts the detected infrared light into a detection signal VS indicative of the detected infrared light. Then, at step S 15 , the detection signal VS is stored in the memory in the control circuit  60 . Then, at step S 16 , the control circuit  60  calculates the concentration of the target gas based on the detection signal VS. Then, at step S 17 , the calculated concentration appears on the screen. If there is a need to detect concentrations of a plurality of components of the target gas, steps S 13 -S 17  are repeated. 
         [0047]    Then, at step S 18 , the infrared source  20  is powered off. Then, at step S 19 , the infrared gas sensor  100  is powered off. In the flow chart of  FIG. 4 , the steps S 3 -S 8  correspond to a filter check process for checking whether the wavelength tunable filter  30  operates normally, the steps S 9 -S 12  correspond to an infrared source check process for checking whether the infrared source  20  operates normally, and the steps S 13 -S 17  correspond to a measurement process for measuring the concentration of the target gas. 
         [0048]    As described above, according to the infrared gas sensor  100 , the filter check process is performed before the measurement process is performed. In the filter check process, the amount of the filtered infrared light at the first and second check wavelengths λF 1 , λF 2  within the non-absorption wavelength range is detected and converted into the first and second check signals VF 1 , VF 2 , respectively. The malfunction of the wavelength tunable filter  30  is checked based on the signal ratio VF 1 /VF 2 . In such an approach, the malfunction of the wavelength tunable filter  30 , can be detected, even if the amount of the infrared light emitted by the infrared source  20  is reduced due to the deterioration. 
         [0049]    The malfunction of the wavelength tunable filter  30  is checked based on the transmittance of the infrared light through the atmospheric gases. In other words, the malfunction of the wavelength tunable filter  30  is checked by using air, not special gas. Therefore, structure of the infrared gas sensor  100  can be simplified so that the wavelength tunable filter  30  can be easily checked in a short time. Further, because of the simple structure, the infrared gas sensor  100  can have a small size and be manufactured at low cost. 
         [0050]    It is preferable that the first and second check wavelengths λF 1 , λF 2  should be near the border of the non-absorption wavelength range. It is more preferable that the non-absorption range be bounded by the first and second check wavelengths λF 1 , λF 2 . In such an approach, even if the wavelength tunable filter  30  malfunctions slightly, the wavelength tunable filter  30  allows passage of the infrared light at a wavelength outside the non-absorption wavelength range, i.e., within the absorption wavelength range. As a result, although the wavelength tunable filter  30  malfunctions slightly, the signal ratio VF 1 /VF 2  significantly deviates from one. Therefore, the malfunction of the wavelength tunable filter  30  can be surely detected so that the concentration of the target gas can be accurately measured. 
         [0051]    Alternatively, a reference signal V 0  corresponding to a reference wavelength λF 0  within the non-absorption wavelength range may be prestored in the memory in the control circuit  60 . In this case, the wavelength tunable filter  30  is checked as follows. In the filter check process, the gap distance D of the wavelength tunable filter  30  is adjusted so that the wavelength tunable filter  30  selects the reference wavelength λF 0 . Then, the infrared detector  40  detects the filtered infrared light and converts the detected infrared light into a reference signal VF 0  indicative of the amount of the detected infrared light. The wavelength tunable filter  30  is checked based on a signal ratio VF 0 /V 0 . In such an approach, the filter check process can be simplified. 
         [0052]    In addition to the filter check process, the infrared light source check process is performed before the measurement process is performed. In such an approach, the malfunction of the infrared source  20  can be detected so that the concentration of the target gas can be measured more accurately. 
         [0053]    As described above, the infrared gas sensor  100  according to the embodiment has a small size and is manufactured at low cost. Further, the infrared gas sensor  100  can measure the concentrations of many components of the target gas by using the wavelength tunable filter  30 . Furthermore, the malfunctions of the infrared source  20  and the wavelength tunable filter  30  are detected so that the concentrations can be accurately measured. Therefore, the infrared gas sensor  100  can be used even under severe conditions. 
         [0054]    For example, the infrared gas sensor  100  may be mounted to a vehicle to measure an exhaust gas of the vehicle. The exhaust gas mainly contains COx, NOx, and SOx. These main components of the exhaust gas absorb infrared light at a wavelength between 3 μm and 8 μm. Therefore, when the infrared gas sensor  100  is used to measure the exhaust gas of the vehicle, it is preferable that the wavelength tunable filter  30  be checked by using the wavelength ranges between 2.05 μm and 2.33 μm, and between 9.4 μm and 12.4 μm, each of which is the non-absorption wavelength range and located next to the wavelength range between 3 μm and 8 μm, where the exhaust gas absorb infrared light. In such an approach, the infrared gas sensor  100  can accurately measure the concentrations of the components of the exhaust gas. 
         [0055]    (Modifications) 
         [0056]    The embodiment described above may be modified in various ways. For example, the steps S 3 -S 5  may be repeated two or more times so that the malfunction of the wavelength tunable filter  30  can be more surely detected. 
         [0057]    In the above described embodiment, the filter check process is performed each time the infrared gas sensor  100  is powered on, i.e., the wavelength tunable filter  30  is powered on. Alternatively, the filter check process may be performed each time step S 17  is completed, i.e., one component of the target gas is measured. In such an approach, the infrared gas sensor  100  can measure the concentration of the target gas more accurately. 
         [0058]    Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.