PARTICULATE MATTER MEASUREMENT SYSTEM BACKGROUND OF THE INVENTION

[0001] This invention relates generally to systems and sensors for measuring particulate matter, and more particularly to systems and methods for measuring particulate matter emitted in the exhaust gas of internal combustion engines, such as, but not limited to, diesel engines and/or gasoline engines.

[0002] The measurement and control of particulate matter (PM) is an increasing concern with regards to health-related issues. Consequently, emission regulators world-wide are both introducing new lower-emission standards as well as looking for cost-effective solutions for testing and enforcing those lower-emission standards.

[0003] In recent years, opacimeters have been widely deployed as a measurement technique for the control and monitoring of smoke from diesel engines. These devices have provided a low- cost technique for regulators to deploy in high- volume demanding control environments; for example used in "inspection and maintenance" programs in garages. Opacimeters work under the principle of light absorption and utilize the Lambert-Beer Law for the calculation of PM concentration.

[0004] I = I ₀ e ^"kL

Where:

Io is the Light intensity when there is no smoke,

I is the Light intensity when smoke exists,

L is the cell length (m), and

k is the light absorption coefficient (m ^"1 ).

[0005] The particulate matter concentration result is normally expressed as the light absorption coefficient. Opacimeter systems such as these can be easily calibrated in the field by inserting optical filters of known attenuation (absorption coefficient) into the optics path and thus negate the difficult and time-consuming experimentally-based calibration techniques that are often employed with other types of particulate matter analyzers. However, with the increasing reductions in PM levels being emitted from engines, the detection limit of traditional opacimeters is being significantly challenged. For example, optics contamination may cause drift at low levels of particulate matter. Further, restrictions on path length may result in signal-to-noise issues. Also, the small size of particulate matter in ultra-clean engines reduces the interaction of the particulate matter with the light source, thereby impairing the operation of the opacimeter at low levels of particulate matter.

SUMMARY OF THE INVENTION

[0006] The various aspects of the present invention address these opacimeter issues and limitations by augmenting the opacity measurement with a laser-light scattering detection system. The opacimeter system provides both the traditional smoke measurements as well as a reference system for the in-situ calibration of the light scattering system. The combination of the opacimeter and laser light scattering detector in the same system enables the system to detect relatively higher levels of particulate matter with the opacimeter and relatively lower levels of particulate matter with the light scattering detector, thereby achieving not only a broader range of detection, but also a better ability to detect low particulate matter levels due to the laser light scattering detector. Still further, the opacimeter can serve as a reference system enabling the calibration of the laser light scattering detector, thereby avoiding the heretofore time consuming and difficult calibration process normally associated with conventional laser light scattering detectors.

[0007] According to one embodiment, a system for measuring particulate matter in a gas is provided. The system includes an opacimeter, a laser light scattering detector, and a controller. The opacimeter detects particulate matter in the gas and outputs an opacimeter measurement. The laser light scattering detector detects particulate matter in the gas and outputs a scattering measurement. The controller uses the opacimeter measurement and the scattering measurement to determine a level of particulate matter in the gas.

[0008] According to another embodiment, a system for measuring particulate matter in a gas is provided. The system includes an opacimeter, a laser light scattering device, and a controller. The opacimeter detects particulate matter in the gas and outputs an opacimeter measurement. The laser light scattering detector detects particulate matter in the gas and outputs a scattering measurement. The controller uses the opacimeter measurement to calibrate the light scattering detector.

[0009] According to another embodiment, a method of determining a level of particulate matter in a gas is provided. The method includes measuring particulate matter in the gas using an opacimeter; measuring particulate matter in the gas using a laser light scattering device; using only the opacimeter measurement to determine the level of particulate matter if the opacimeter measurement exceeds a lower threshold; and using only the scattering measurement to determine the level of particulate matter if the scattering measurement is lower than an upper threshold.

[0010] According to another embodiment, a method of determining a level of particulate matter in a gas is provided. The method includes measuring particulate matter in the gas using an opacimeter; measuring particulate matter in the gas using a laser light scattering device; and using both the opacimeter measurement and the scattering measurement to determine the level of particulate matter in the gas.

[0011] According to still another embodiment, a method of determining a level of particulate matter in a gas is provided. The method includes measuring particulate matter in the gas using an opacimeter; measuring particulate matter in the gas using a laser light scattering device; and calibrating the laser light scattering device using the opacimeter measurement.

[0012] According to still other embodiments, the lower threshold for the system may lie within a range of absorption coefficients of between approximately ten and twenty percent, although coefficients outside this range may also define the lower threshold. The upper threshold for the system may also lie within a range of absorption coefficients of between ten and twenty percent. If both the opacimeter and scattering measurements lie within a specified range, the controller may use both measurements to determine the level of particulate matter. The use of both measurements to determine the level of particulate matter may involve a splining operation, or some other suitable mathematical operation for combining the different

measurements. The use of the opacimeter to calibrate the laser light scattering device may only occur when the opacimeter measurement lies within a predetermined range, such as, but not limited to, an absorption coefficient of between five to twenty five percent, or between ten and twenty percent, or between other ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a diagram of a particulate measuring device according to an embodiment;

[0014] FIG. 2 is a flowchart of steps that may be taken by the controller of the particulate measuring device of FIG. 1;

[0015] FIG. 3 is an example graph illustrating a relationship between different light absorption coefficients and a particulate matter sensor output; and [0016] FIG. 4 is an example graph illustrating a possible amount of particulate matter detected over time from a vehicle experiencing a sudden acceleration.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] An illustrative particulate matter (PM) sensor 20 according to an embodiment is depicted in FIG. 1. PM sensor 20 may be used to detect particulate matter that is being exhausted from an engine of a car, a truck, or any other device that employs an internal combustion engine. In some embodiments, PM sensor 20 may be particularly suited for measuring particulate matter levels in the exhaust of diesel engines, although it will be understood by those skilled in the art PM sensor is not necessarily limited to use in conjunction with diesel engines.

[0018] PM sensor 20 includes an opacimeter 22 and a laser light scattering detector 24. Both the laser light scattering detector 24 and the opacimeter 22 may be conventional, commercially available devices. For example, opacimeter 22 may be a model LCS 2400 opacimeter marketed by Sensors, Inc. of Saline, Michigan, although other types of opacimeters may be used. The laser light scattering detector may be any suitable commercially available, or non-commercially available, laser light scattering device.

[0019] Opacimeter 22 operates generally as follows. A gas, such as air, that may contain particulate matter is drawn by way of one or more fans 26 into a chamber 28. Inside chamber 28, a mirror 30 is placed at an end of chamber 28 opposite a light source 32. The light source 28 emits light, or other suitable electromagnetic waves, that pass through a half mirror 34 before traveling through a lens 36. After passing through lens 36, the light travels through chamber 28 until it reaches mirror 30, where it is reflected back through lens 9. The reflected light that passes through lens 9 bounces off of half mirror 34 where it is directed to, and detected by, a light detector 38. During the travel of the light through chamber 28, any particulate matter that is present in the gas will absorb a certain amount of the light. The amount of absorbed light will provide any indication of the level of particulate matter in the air, according to known criteria, including, but not limited to, the Lambert-Beer law referred to earlier. The amount of light detected by detector 28 may be forwarded via a line 31 to a controller 40 that uses this reading to generate an opacimeter measurement that indicates a level of particulate matter within the air inside chamber 28. The manners in which controller 40 may determines the opacimeter measurement of the level of particulate matter is generally within the knowledge of one of ordinary skill in the art, and therefore will not be repeated here. For purposes of description herein, the term "opacimeter measurement" refers to a measurement of the level of particulate matter made by opacimeter 22.

[0020] Laser light scattering detector 24 operates generally as follows. The air, or other gas containing particulate matter, that is passed through chamber 28 is also diverted through a channel 42 by way of an inlet 44. The air drawn into inlet 44 comes from the same sample of air that is drawn into chamber 28. Thus, at any given time, opacimeter 22 and detector 24 should be measuring particulate matter in essential the same sample of air, or other gas. A laser diode 46 emits a beam of laser light that passes through the air traveling through channel 42. The particulate matter within the air inside channel 42 will scatter a certain amount of this laser light before it is detected by a laser light detector 48. The amount of laser light detected by detector 48 is forwarded onto controller 40 via a line 50 wherein controller 40 calculates an amount of particulate matter within the air based upon the amount of light scattering and other factors. Except as discussed below with respect to calibration, the manner in which controller 40 calculates the level of particulate matter from the data generated by detector 48 is generally known by those of skill in the art, and therefore won't be discussed further.

[0021] Controller 40 may be one or more microprocessors programmed to carry out the mathematical algorithms described herein in a manner that would be readily within the skills of a person of ordinary skill in the art. Controller 40 may alternatively be any electronic circuitry that is capable of carrying out the algorithms described herein, as would also be known to one of ordinary skill in the art.

[0022] Controller 40 uses the data from scattering detector 48 to generate a scattering measurement. For purposes of description herein, the term "scattering measurement" refers to a measurement of the particulate matter level that is made by laser light scattering device 24 (as opposed to opacimeter 22). Controller 40 may use the data from detectors 38 and 48 to determine a level of particulate matter in any of a variety of manners. One such manner is illustrated in flowchart form in FIG. 2.

[0023] As noted, the measurements made by opacimeter 22 and scattering detector 24 are fed to controller 40 on lines 31 and 50, respectively. As shown in FIG. 2, line 31 may feed directly into a step 52, or, alternatively, the electrical voltage or other signals on line 31 may be processed further before feeding into step 52. Such processing may include conversion of an analog signal to a digital signal, as well as any other suitable processing. Similarly, line 50 may be fed directly into step 54, or, alternatively, the electrical voltage or other signals on line 50 may be processed further before feeding into step 54. Such processing may include conversion of an analog signal to a digital signal, as well as any other suitable processing.

[0024] At step 52, calibration data is stored for opacimeter 22. Opacimeter 22 may be calibrated in any known manner. One such manner includes the insertion of optical filters into chamber 28 that have known attenuation or absorption coefficients. Such known optical filters may correspond to several different absorption coefficients, such as, but not limited to, 25%, 50%, and/or 75%. By using these known optical filters, controller 40 is able to determine and save calibration data at step 52. Such calibration data essentially defines the slope of a line 56 (FIG. 3) that relates the amount of light absorption to the opacimeter 22 's output.

[0025] At step 58, controller 40 calculates an absorption coefficient K based upon the data supplied from line 31. After calculating K in step 58, controller 40 may undertake one or more mathematical filtering operations illustrated generally by reference numeral 60. Such filtering operations may include Butterworth filtering, as well as, or in lieu of, other types of

mathematical filtering. Such mathematical filtering is known in the art, and therefore will not be described in greater detail. One general purpose of the filtering is to smooth out and account for multiple measurements of particulate matter over a period of time so that an accurate

measurement of particulate matter is made despite variations that may occur in

pressing depressing a throttle of an internal combustion engine. For example, as shown in FIG. 4, during an emission test of a vehicle in which the gas pedal, or throttle of the engine, is forcefully pushed all the way to the floor for a set period of time— such as, but not limited to, two seconds— the amount of particulate matter detected over time will vary. FIG. 4 illustrates an arbitrary example of particulate matter that may be detected by a PM sensor during such a test. Line 62 illustrates the amount of particulate matter with respect to time during such a test.

Filtered line 64 illustrates the detected amount of particulate matter after mathematical processing by filtering operations 60. Filtering operations 60 thus help prevent substantial variations between emission tests that might otherwise occur when the individual depressing the gas pedal doesn't consistently depress the gas pedal in the same manner.

[0026] After completing filtering operations 60, controller 40 calculates an opacimeter measurement at step 66. If this opacimeter measurement lies within a certain range— such as, but not limited to, ten to twenty percent, as an example— this opacimeter measurement may be passed to block 68 where it is used for calibrating light scattering device 24. Additionally, regardless of the value of the opacimeter measurement calculated at step 66, the opacimeter measurement is forwarded to block 70 for use in a manner discussed in greater detail below.

[0027] The signals coming from laser light detector 50 are used by controller 40, after appropriate calibration is applied at step 54, to calculate an absorption coefficient K at step 72. The first time laser light scattering system 24 is used, the calibration data may be pre-stored. Thereafter, the pre-stored data may be replaced at block 68 by calibration data derived from opacimeter 22 every time opacimeter 22 detects a particulate matter level that falls with a specified range. As shown in FIG. 2, the specified range may be between 0.1 and 0.2 (ten to twenty percent). Other ranges may also be used.

[0028] Controller 40 may apply filtering operations 74 to the absorption coefficient calculated at step 72 in a similar manner that it applies filtering operations 60 to the coefficient calculated at step 58. After applying the filtering operations, a scattering measurement is generated at step 76. The scattering measurement of step 76 is forwarded onto block 70 for processing.

[0029] At block 70, controller 40 determines what to output as a reading indicating the level of particulate matter that it has detected. One suitable algorithm by which controller 40 determines what particulate level reading to output is as follows. If opacimeter 22 detects a particulate matter above a lower threshold, regardless of what detector 24 detects, controller 40 outputs opacimeter 22 's reading. If opacimeter 22 detects a particular matter below an upper threshold, controller 40 will output detector 24' s reading. If opacimeter detects a particulate matter that lies between the upper and lower threshold, then it will combine both the opacimeter 22 measurement and the detector 24 measurement in a suitable manner and output the combined measurement. One such suitable manner for combining the measurements is a mathematical splining operation. Other methods may alternatively be used.

[0030] In one example, the lower threshold may be twenty percent (K = 0.2), while the upper threshold may be ten percent (K = 0.1). Other thresholds may be used. Generally speaking, controller 40 will use the measurements made by opacimeter 22 when the particulate matter readings are high enough such that opacimeter 22 may reliably and accurately detect such particulate matter levels. When the particulate matter readings fall below this reading, controller 40 will use the light scattering detector 24 to determine the particulate matter level because of the greater ability of the light scattering detector 24 to detect lower levels of particulate matter. An overlap region may be defined wherein controller 40 combines the two measurements of detectors 22 and 24, such as by splining, when the particulate matter levels are such that both readings are reliably accurate. Alternatively, in some embodiments, no overlap region may be defined and controller 40 may simply choose to output either opacimeter 22 's readings or detector 24 's readings, depending upon the level of particulate matter that is measured.

[0031] As noted earlier, the particulate matter reading made at step 66 may be forwarded onto block 68 where it is used to calibrate detector 24. This automatic calibration of light scattering detector 24 alleviates the otherwise long and involved calibration of light scattering device 24 that would otherwise be necessary. By using the measurements of opacimeter 22 to calibrate detector 24, the use of detector 24 becomes substantially less burdensome.

[0032] While the foregoing description describes several embodiments of the present invention, it will be understood by those skilled in the art that variations and modifications to these embodiments may be made without departing from the spirit and scope of the invention, as defined in the claims below.