Abstract:
A photoacoustic sensor that measures carbon black particles emitted in the exhaust gas of a vehicle traveling on a road or being tested on a dynamometer or engine stand. The sensor includes an acoustic waveguide and a pump mounted to an outlet of the waveguide. The pump pulls the gas through the waveguide. A critical orifice is mounted between the pump and the outlet. The critical orifice prevents noise generated by the pump from entering the waveguide. Optical windows are mounted at opposite ends of the waveguide. A modulated source of light is located adjacent one of the windows and irradiates the waveguide. A microphone is attached to the waveguide. The microphone detects an acoustic signal generated by absorption of the light by the particles in the gas. The acoustic signal is proportional to the mass concentration of particles in the gas. The microphone generates an electrical signal proportional to the acoustic signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to a photoacoustic instrument or sensor for measuring particulate emissions from vehicles and other combustors. It can be placed at a roadside location. The instrument is also used as a primary method for quantitatively measuring aerosol light absorption in the atmosphere. 
     2. Description of Related Art 
     Internal combustion engines used to power vehicles generate both gaseous and particulate emissions as they travel on roads and expressways. The gaseous emissions have been extensively sampled and tested. Particulate emissions have proven to be more difficult to sample and test at a roadside location. Photo-acoustic gas sensors have been used to detect concentrations of gases such as carbon monoxide and other hydrocarbons. Photo-acoustic gas sensors generate an acoustic pressure wave when the gas is irradiated with a modulated light source in a sample chamber. The radiation absorbed by the gas results in pressure variations in a given volume of gas. The pressure variation is proportional to the amount of energy absorbed. A microphone can detect the pressure wave. The magnitude of the wave is proportional to the concentration of the gas. The acoustic pressure wave arises as the gas absorbs the optical radiation and is heated. Periodic thermal expansion and pressure fluctuations result, corresponding to the modulation of the optical radiation. Measurement of the acoustic pressure then permits inferring the gas concentration. Different gases are characterized by the use of light waves of different wavelengths corresponding to the absorption wavelength of the gas being tested. 
     Photoacoustic sensors have a high degree of measurement sensitivity and have to be carefully designed in order to prevent external noise from generating erroneous results. One problem that occurs with photoacoustic sensors is that they can receive acoustic signals (noise) from outside the sample chamber. This noise enters the sensor through the same entrance as the sample gas. The external acoustic noise causes false readings. 
     Another problem with photoacoustic sensors is that they can drift out of calibration due to changing pressure on the microphone. Typical microphones have an electrically conducting membrane and a fixed back plate. If air is moving in the test chamber, the pressure on the membrane can be different than the pressure on the back plate causing the sensor to go out of calibration. 
     Another problem with photoacoustic sensors is that the light source has to be aligned with the sample chamber to obtain accurate readings. If the light source is an infrared laser, it cannot be visually aligned. For safety reasons, it is desirable to enclose the laser so that the laser cannot accidentally contact an eye. 
     A current unmet need exists for a roadside particulate emission sensor that is not influenced by external noise, that is readily calibrated, that stays in calibration and that has an easily alignable light source. 
     SUMMARY OF INVENTION 
     1. Advantages of the Invention 
     An advantage of the present invention is that it provides a sensor for detecting particulate emissions at a roadside location. 
     Another advantage of the present invention is that it provides a photoacoustic instrument that can measure black carbon particles. 
     A further advantage of the present invention is that it provides a photoacoustic sensor that uses cameras to align a laser beam. 
     An additional advantage of the present invention is that it provides a photoacoustic sensor that prevents external noise from generating erroneous results. 
     Yet another advantage of the present invention is that it provides a photoacoustic sensor that prevents calibration drift due to changing pressure on a microphone. 
     A further advantage of the present invention is that it provides a photoacoustic sensor that is compact and easily transported. 
     A further advantage of the present invention is that it provides a photoacoustic sensor that is easily calibrated. 
     These and other advantages of the present invention may be realized by reference to the remaining portions of the specification, claims, and abstract. 
     2. Brief Description of the Invention 
     The present invention comprises a photoacoustic sensor for measuring light absorbing particles in a gas. The most common particles sampled are black carbon or soot, though the choice of light wavelength allows other particles to be sampled. The photoacoustic sensor comprises an acoustic waveguide and a modulated source of light located in proximity to the waveguide. The modulated light irradiates the particle-laden gas in the waveguide. A microphone is attached to the waveguide. The microphone detects an acoustic signal generated by absorption of the light by the particles in the gas. The acoustic signal is proportional to the mass concentration of particles in the gas. A pump is mounted to the waveguide. The pump pulls the gas through the waveguide. A piezoelectric calibrator is mounted perpendicular to the waveguide. The piezoelectric calibrator provides a known acoustic signal for calibrating the sensor. A critical orifice is mounted between the pump and the waveguide. The critical orifice prevents noise generated by the pump from entering the waveguide. Helmholtz resonators are mounted to the sample inlet of the waveguide. The [helmholtz]Helmholtz resonator prevents unwanted noise frequencies from entering the waveguide. A pressure equalizer is mounted between the critical orifice and the microphone. The pressure equalizer equalizes the pressure in proximity of the microphone. 
     The above description sets forth, rather broadly, the more important features of the present invention so that the detailed description of the preferred embodiment that follows may be better understood and contributions of the present invention to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and will form the subject matter of claims. In this respect, before explaining at least one preferred embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of the construction and to the arrangement of the components set forth in the following description or as illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is substantially a schematic diagram of a photoacoustic sensor in accordance with the present invention. 
     FIG. 2 is substantially an exploded perspective view of a photoacoustic sensor in accordance with the present invention. 
     FIG. 3 is substantially an assembled side cross-sectional view of the photoacoustic sensor of FIG.  2 . 
     FIG. 4 is substantially a flowchart showing the operation of the photoacoustic sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part of this application. The drawings show, by way it of illustration, specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made with out departing from the scope of the present invention. 
     Photoacoustic Sensor 
     Referring to FIG. 1 schematic diagram of a photoacoustic instrument or sensor  10  for measuring particles in a gas is shown. A vehicle  13  using a fossil fuel generates an exhaust gas  15  that is diluted by the surrounding air. The exhaust gas contains entrained particulates such as black carbon particles. Sensor  10  is used to measure the particles in the air. Sensor  10  comprises an acoustic waveguide  20 , an air pump  74 , an air inlet  60 , laser  90 , a microphone assembly or acoustic sensor  110 , a piezoelectric calibrator  120 , a photodiode  130  and a computer  150 . The gas  15  is pulled into the waveguide  20  by pump  74 . The laser  90  passes a laser beam through the waveguide  20  where it is received by photodiode  130 . When the laser impinges on a particle, it generates an acoustical pressure that can be measured by microphone  110 . A computer  150  receives an electrical signal from the microphone and can calculate the quantity of particles in the gas. The computer is attached to the photodiode and the laser. The computer controls the pulse rate of the laser. A calibrator  120  is attached to the waveguide and is controlled by the computer. The calibrator generates a known acoustic signal. 
     With reference to FIGS. 2 and 3, more details of the photoacoustic instrument or sensor  10  for measuring particles in a gas are shown. Sensor  10  further comprises an acoustic waveguide  20 , left and right coupling sections  40  and  50 , a pump  74 , a low frequency acoustic bandstop filter  62 , a high frequency acoustic bandstop filter  64 , a critical orifice  76 , a microphone assembly  110 , a laser  90 , windows  94 , a pair of cameras  100 , and a piezoelectric calibrator  120 . 
     Waveguide  20  has a waveguide cavity  22  that passes through waveguide  20 . In the preferred embodiment, a microphone bore  24  intersects waveguide cavity  22  at a right angle or is perpendicular to the waveguide cavity. A calibrator bore  26  also preferably intersects waveguide cavity  22  at a right angle. Bores  24  and  26  are located at pressure nodes PN 1  and PN 2 . The pressure nodes allow the bores to have a minimal effect on the resonator or cavity acoustics. 
     First coupling section  40  has a bore  42  passing through and a de-tuning bore  44  that intersects bore  42  at a right angle. Similarly, second coupling section  50  has a bore  52  passing through and a de-tuning bore  54  that intersects bore  52  at a right angle. First and second coupling sections  40  and  50  are attached to waveguide  20  using alignment pins  59 . One half of alignment pins  59  rests in an aperture in the coupling section and the other half in an aperture in the waveguide. The coupling sections are affixed to the waveguide by bolts or other attachment devices (not shown). The waveguide and coupling sections are preferably formed out of aluminum. A block of aluminum can be milled and bored to form the waveguide and coupling sections. Other metals and manufacturing techniques could also be used. Several tubes could also be joined together to form the waveguide and coupling sections. 
     De-tuning cavities  66  are attached to de-tuning bores  44  and  54 . The de-tuning cavities  66  serve to reduce the coupling of acoustic noise into the waveguide cavity at the acoustic resonance frequency of the cavity. 
     A pump  74  is attached to an air outlet  72 . An outlet pipe  70  connects pump  74  to a critical orifice  76 . Critical orifice  76  comprises a hole  77  in a disc  78  mounted in the path of airflow. The speed of air flowing through the orifice approaches the speed of sound in the gas. The orifice is deemed critical when the pump pressure is sufficient to make the flow rate approach the speed of sound. Sound from the pump that would normally travel into the waveguide is reflected at the critical orifice back toward the pump, thereby preventing the sound from interfering with measurements in cavity  22 . The critical orifice is commercially available from Okeefe Controls. The critical orifice is attached to bore  42  by pipe  81 . 
     A microphone assembly  110  is mounted to waveguide  20 . Microphone assembly  110  includes a microphone  109  contained in a microphone enclosure  111 . Enclosure  111  is attached by a microphone mount  112  to waveguide  20 . A seal  122  seals the microphone. The microphone comprises a microphone cavity  113 , a flexible membrane  114  that faces into bore  24  and a fixed metallic back plate  117 . Membrane  114  and back plate  117  form a parallel plate capacitor. A wire  116  provides an electrical signal from the microphone. The microphone assembly is commercially available from ACO Pacific. 
     An equalizer tube  80  is connected above the critical orifice by an equalizer pipe  79 . Equalizer tube  80  is connected to microphone cavity  113 . Since the microphone has a movable electrically conducting membrane  114  facing into the acoustical waveguide, and a fixed metallic back plate  117 , to form a parallel plate capacitor, acoustical signals produced by the resonator force the movable membrane to change position with respect to the back plate thereby changing the capacitance. Since the waveguide has a continuous air sample being pulled through the waveguide, the static pressure on the movable membrane could be different from that on the backplate causing a drift of the microphone calibration and false measurements. The equalization tube and line prevents this pressure change and accompanying calibration drift. The equalization tube may contain a drying agent (not shown) in drying section  82 . The drying agent removes water vapor in the air. The water vapor could condense between the back plate and movable membrane of the microphone causing it to short out or the water vapor could condense on the preamplifier electronics in the microphone causing them to fail. The preferred drying agent is Dryerite available from Cole-Palmer. 
     An air sample inlet  60  admits sample air into the waveguide. Inlet  60  is at the end of inlet pipe  61 . Inlet pipe  61  is connected to bore  52 . A low frequency acoustic band stop filter or resonator  62  and a high frequency filter  64  are attached to inlet pipe  61 . Low frequency filter  62  is a Helmholtz resonator. A [helmholtz]Helmholtz resonator is an enclosed volume such as a metal sphere with a tube extending from the sphere to the inlet pipe. Helmholtz resonators reflect sound of certain wavelengths depending upon the dimensions of the resonator. Helmholtz resonator  62  rejects frequencies well below the resonant frequency of the acoustic waveguide. Resonator  62  rejects unwanted noise from outside the instrument that enters through the inlet  60 . Similarly, filter  64  rejects higher frequency noise from outside the instrument. 
     A laser  90  is aligned with windows  94 . A seal  96  seals window  94  to the coupling sections. Laser  90  generates a laser beam  92 . Laser  90  is preferably a 532 nm or 1047 nm unit available from Crystalaser. Laser beam  92  passes through window  94  and bore  42  into waveguide cavity  22  and irradiates the gas containing particles therein. The laser beam then passes through bore  52  and window  94 . Photodetector  130  receives the laser beam at the other end of waveguide  20 . A pair of cameras  100  is mounted to view each of the windows  94 . Cameras  100  are miniature pinhole black and white cameras commercially available from RadioShack. Cameras  100  are focused on windows  94 . Since laser  90  can be an infrared laser, aligning the laser visually is impossible. Since laser safety concerns may make it necessary to completely enclose the laser beam in an enclosure, the laser beam height, lateral orientation and tilt can be monitored and adjusted by the operator while safely viewing the laser beam position on a monitor. The laser is aligned when it is in the center of the window. 
     A piezoelectric calibrator  120  is mounted to waveguide  20  above calibrator bore  26 . A seal  122  is used to seal calibrator  120  to the waveguide. Piezoelectric calibrator  120  has a piezoelectric element  124  adjacent bore  26 . Piezoelectric calibrator element  124  is commercially available from RadioShack. The piezoelectric element is turned on in an alternating fashion between measurements. The piezoelectric element sends a known acoustic signal into the waveguide in order to obtain acoustic calibration of the resonator cavity (resonance frequency and quality factor). Other calibration sources such as a speaker can be used to generate a known acoustic signal 
     Operation 
     The waveguide cavity  22  is a full wavelength resonator with pressure nodes PN 1  and PN 2  at the ends of cavity  22  and a pressure antinode PA 1  at the center. A pressure node is a region of very low acoustic pressure. A pressure antinode is a region of highest acoustic pressure in the acoustic standing wave in the waveguide cavity. Where the coupling sections  40  and  50  join the waveguide  20 , the mating surfaces are angled or sloped at 45 degrees to accommodate insertion of the laser beam. The pressure nodes PN 1  and PN 2  are located where the laser enters the waveguide cavity  22 . The diameter of bores  42  and  52  is much smaller than the acoustic wavelength and along with being located at the pressure nodes allows the bores to have a minimal effect on the resonator or cavity acoustics. Laser beam  92  is modulated at the acoustic resonance frequency of the waveguide so that sound produced by light absorption is amplified by the waveguide resonance. This arrangement also helps to reduce the coupling of noise into the resonator at the acoustic frequency since holes are at the pressure nodes. The microphone element  114  is located at a pressure antinode PA 2  at the end of microphone bore  24 . The piezoelectric calibrator is located at a pressure antinode PA 3  at the end of calibrator bore  26 . 
     Referring to FIG. 4, a flowchart  200  of the operation of photoacoustic instrument is shown. First at step  202 , sample gas containing black carbon particles is continuously pulled through the waveguide cavity  22  by pump  74 . The air flows into inlet  60  and is expelled from the pump. Next, at step  204 , the laser passes through window  94 , through bore  42 , waveguide cavity  22 , bore  52 , window  94  and is received by photodetector  130 . The laser beam power amplitude and phase at the modulation frequency are obtained from photodetector  130 . The modulation frequency is the resonance frequency of the acoustic waveguide. 
     As the gas containing black carbon particles is irradiated with the laser, the heat from light absorption transfers from the particles to the surrounding air and an acoustic pressure wave or sound wave is generated. The sound is generated near the center of waveguide cavity  22  in the region of the pressure antinode PA 1 . The radiation absorbed by the black carbon particles results in pressure variations in a given volume. The pressure variation is proportional to the amount of energy absorbed. The pressure wave is detected by microphone  109  at step  206 . The magnitude of the wave is proportional to the amount of light absorption and amount of laser power. From the amount of light absorption, the concentration of particles can be determined. The microphone  109  generates an electrical signal that is proportional to the mass concentration of particles at step  208 . The electrical signal is provided to an amplifier (not shown) and computer  150  through wire  116 . The computer calculates, at step  210 , the concentration of the carbon black particles as well as the amount of light that is being absorbed or blocked by the carbon black particles in the air. The piezoelectric element  124  generates a known acoustic signal to calibrate the sensor. Calibration is performed between light absorption measurements. 
     Phase sensitive detection is used for processing of the electronic signals that control the laser beam power modulation and the microphone and photodiode signals. Phase sensitive detection can be accomplished with an appropriately configured lock-in amplifier. It can also be accomplished using an analog to digital converter with hardware trigger and laser beam power modulation controlled by an on-board counter/timer. The noise floor of the measurement can be determined from the weighted average of the electrical signal within the resonance response curve, excluding the signal exactly at the modulation frequency (resonance frequency of the waveguide). 
     The computer uses an algorithm to process the electrical signal. The algorithm calculates light absorption that is proportional to the quantity of particles. The light absorption is calculated using the acoustic pressure measured with the microphone, the resonator quality factor, the resonant frequency, the Fourier component of the laser beam power and the cross sectional area of the waveguide. 
     Photoacoustic sensor  10  is readily packaged such that it is easily transportable to a roadside location. Sensor  10  only needs to be connected to a standard source of ac power in order to operate. At a roadside location, sensor  10  is able to monitor the amount of soot in the air. Sensor  10  measures light absorption and the mass concentration of black carbon entrained in air. 
     Typical uses of the photoacoustic sensor are to measure the absorption component of viewing range and clarity degradation in cities or National Parks. The climatic influence of black carbon on the absorption of sunlight can be studied with the sensor. The mass concentration measurement can be used to quantify the black carbon produced by the burning of engine fuels and diesel fuel as an example. 
     Conclusion 
     The present invention solves many of the problems associated with the prior art. The present invention provides a sensor for detecting particulate emissions at a roadside location and in particular black carbon particles. The present invention also provides a photoacoustic sensor that uses cameras to align a laser beam and that prevents external noise from generating erroneous results. The present invention further provides a photoacoustic sensor that prevents calibration drift due to changing pressure on the microphone. 
     Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents rather than by the examples given.