Abstract:
A photoacoustic detector wherein control circuits compensate for long term variations of components therein including a light source and sensing microphones. The control circuits intermittently energize the source to evaluate changes in at least source resistance. The control circuits intermittently energize an acoustic generator to evaluate changes in one or more generator responsive microphones.

Description:
FIELD 
     The application pertains to photoacoustic detectors. More particularly, the application pertains to such detectors which include circuitry for carrying out long term drift compensation. 
     BACKGROUND 
     Various types of photoacoustic sensors are known to detect gases. These include, Fritz et al., US Patent Application No. 2009/0320561, published Dec. 31, 2009 and entitled “Photoacoustic Cell”; Fritz et al., US Patent Application No. 2010/0027012, published Feb. 4, 2010 and entitled, “Photoacoustic Spectroscopy System”; Fritz et al., US Patent Application No. 2010/0045998, published Feb. 25, 2010 and entitled “Photoacoustic Sensor”; and Tobias, US Patent Application No. 2010/0147051, published Jun. 17, 2010 and entitled, “Apparatus and Method for Using the Speed of Sound in Photoacoustic Gas Sensor Measurements. The above noted published applications have been assigned to the assignee hereof, and are incorporated herein by reference. 
     Precise and repeatable performance of photoacoustic detectors is preferred. Changes in detector response over a period of time can result in measurements exhibiting variances from those initially determined during manufacture and initial calibration. In some instances this can result in the respective detector drifting out of specification. 
     User calibration is a partial solution to detector drift. However, not all users have the ability or interest required to carry out field calibration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a detector in accordance herewith; and 
         FIGS. 2A ,  2 B are flow diagrams of compensation methods usable with the detector of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     While disclosed embodiments can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles thereof as well as the best mode of practicing same, and is not intended to limit the application or claims to the specific embodiment illustrated. 
     The long term drift in a photoacoustic (PA) sensor is primarily due to changes in elements that can wear or degrade over time, for example, the light source (lamp), and the PA microphones. Methods are described below for detecting and compensating for long term changes in the light source and one or more microphones of a photoacoustic sensor. An acoustic generator operating at multiple frequencies to monitor the long-term drift of the sensor&#39;s microphones as well as the photoacoustic cavity&#39;s membrane and pressure sealing elements. The long term drift of the light source can be monitored and corrected for by measuring source electrical resistance and optical output. 
       FIG. 1  is a block diagram of an exemplary detector  10  in accordance herewith. It will be understood that the detector  10  is exemplary only. Other detectors come within the spirit and scope hereof. 
     The detector  10  can monitor concentrations of one or more airborne gases in an adjacent region R. Detector  10  includes a housing  12  which can carry a photoacoustic sensing chamber or cell  14 . 
     Detector  10  includes a radiant energy emitting and control system  16  and an acoustic generator  18 . Dual microphones  22   a, b  are carried by or adjacent to the chamber  14  and respond to inputs from generator  18 . The microphones  22   a, b  also respond to audio generated by radiant energy, or light L, from a source  26   a.    
     The source  26   a  injects light into the chamber  14  as would be understood by those of skill in the art to produce a photoacoustic audio signal, and need not be discussed further. The source  26   a  can emit infra-red radiant energy. 
     Feedback is provided in system  16  by a photodetector  26   b  which couples a signal, indicative of the output of source  26   a  through an amplifier and filter  26   c , via an analog-to-digital converter  26   d  to drive and drift compensation circuits  28 . 
     Dual channel output signals on lines  32   a, b  from the microphones  22   a, b  can be coupled via amplifiers  34   a, b  to analog-to-digital converters  36   a, b  to lock-in detection circuits  38   a, b . Output signals on lines  40   a, b  from the detection circuits  38   a, b  can be coupled to ambient noise correction processing circuits  42 . Processing circuits  42  can be implemented with one or more programmable processors  42   a  which execute software or control programs  42   b  pre-stored on computer readable media such as semiconductor memory chips. 
     The corrected outputs can be coupled to control and processing circuits  46  which can carry out gas concentration detection. Circuits  46  can be implemented with one or more programmable processors  46   a  which execute software or control programs  46   b  pre-stored on computer readable media such as semiconductor memory chips. Using pre-stored instructions, such as  42   b , baseline and span corrections can be carried out as explained below. 
     Interface circuits  46   c , also coupled to the control circuits  46  provide for bidirectional communication with a docking station, or, a displaced monitoring system via a wired or wireless medium  46   d . Environmental sensors  50   a, b, c  can detect ambient temperature, pressure or humidity in the vicinity of the housing  12 . Signals from the sensors  50   a, b, c  can be digitized in analog-to-digital converters  52   a, b, c  and the coupled to the control circuits  46  as discussed above. 
     Further, closed loop control system  16 , which can include the infra-red emitter of radiant energy, source  26   a , can sense emitted radiant energy intensity, or amplitude, via detector  26   b . The system  16  compensates for drift in output of the radiant energy source  26   a.    
     The detector  10  can be calibrated at manufacture. A detector response transfer function can be established at initial calibration. Characteristics of the initial transfer function can be stored by control circuits  46  for subsequent use. 
     Subsequently, in one embodiment, the intensity of the infra-red source  26   a  can be varied while measuring the cell output signal. An updated transfer function can be established and compared to the stored transfer function. Span and baseline correction values can be determined using the original and current transfer functions. The correction values can also be stored by control circuits  46  for subsequent use in compensating gas concentration values. 
     A method  100 , of lamp degradation monitoring and compensation, is illustrated in  FIG. 2A . The radiant energy output of the source  26   a  can be monitored, constantly or intermittently using a silicon photodiode, or other optical sensor, such as  26   b , and by measuring the lamp filament resistance as at  102 . Since the change in resistance of a tungsten filament with temperature is well known, measuring the filament resistance will enable the filament temperature to be determined if the filament resistance at any given temperature is known. The latter condition can be satisfied by simply measuring the filament resistance after turning the lamp off for several seconds and using a local ambient temperature sensor, such as sensor  50   a.    
     The lamp monitoring photodiode signal, from sensor  26   b , and the filament resistance will vary as the lamp power is modulated at the operating frequency of the detector  10  (typically 7 or 11 Hz), so their values at several points in the modulation cycle will need to be acquired. In order to assure that the calibration of the PA detector  10  remains valid, the drive power to the filament of the source  26   b  can be slowly adjusted as to keep the filament temperature as stable as possible, and thus maintain the same radiance spectrum at  104 . 
     The cold resistance (lamp off for several seconds) of the filament of source  26   b  can also be periodically measured as at  106 , to monitor tungsten loss from the filament. This provides monitoring of any IR output loss due to tungsten deposition on the glass bulb of source  26   b . This measurement can be made in combination with the output of photodiode  26   b  to determine the extent of the tungsten deposition on the glass bulb. Any loss of IR signal at a given filament temperature would be compensated for, as at  108 , by increasing the processed (ambient noise corrected) PA microphone signal value just prior to the control circuits  42  using that value to compute a current CO2 concentration. 
     An onboard acoustic generator  18  can be used to track the absolute signal levels  32   a,b  of the two PA microphones  22   a,b  over time and at several frequencies selected based on the fundamental performance characteristic of microphones. Therefore, drift in the PA microphones  22   a,b  can be detected and compensated for, as in  FIG. 2B , method  120 . 
     The acoustic generator  18  can be driven at a predetermined target frequency, as at  122 , and allowed to stabilize prior to measuring the signal amplitude and phase of both microphones  22   a,b  using lock-in detection  38   a,b  over several complete cycles (the PA lamp  26   a  would not be modulated during this measurement), as at  124 . Several frequencies could be sequentially measured in this manner (for example 7, 14, 28, 55, and 110 Hz) and then the detector  10  would return to normal operation. 
     The response of microphone to all of these frequency stimulants can be measured and compared by the circuits  42 , as at  126 , to the initial performance of the microphones  22   a,b . The CO2 concentration before and after this measurement and the environmental conditions during it (pressure, temperature, and relative humidity) could be monitored, as at  128 , to assure that nothing had changed significantly during the calibration measurements. If significant changes were found, the calibration data could be discarded, as at  130 . The time points selected for this procedure could be selectively chosen to occur at certain times of the day or night or when the environmental conditions were especially appropriate. 
     The data obtained from the calibration procedure  120  could be accumulated and stored for an extended period, for example in flash memory-type storage, and could be used to detect changes over time in the response of both microphones  22   a,b , their respective membranes, leaks in the PA chamber  14  due to the failure of sealing elements, or changes in output of the acoustic generator  18  used for the calibration. In general, changes in amplitude or phase common to both microphones  22   a,b  would be assumed to be due to changes in the output of the acoustic generator  18 . 
     In the acoustic generator  18 , where amplitude changes at all frequencies used in the calibration can be expected to track together the average of each microphone over all frequencies tested could be used to compute the change in acoustic generator amplitude. Further, leaks in the PA chamber  14  or associated membranes would cause a frequency dependent change in microphone amplitude. Slow leaks would cause the low frequency calibration response to increase, but not the high frequency response. 
     The magnitude of the PA microphone signals  32   a,b  in response to increases at the normal PA operating frequency could be used to compensate for the corresponding increase in PA cavity loss at that frequency. Larger leaks in the PA chamber  14  would lead to substantial increases at all frequencies, but would also probably render the PA detector  10  inoperable (the leaks would be too large to compensate). Changes in the microphone response over time could also be detected using this technique. Given that all the frequencies typically track together, the high frequency response can be used to track microphone gain changes and the low frequency response can be used to track PA cavity leaks. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.