Abstract:
A particle detection device includes a scattered light detector detecting an intensity of light scattered by a particle irradiated with a laser, an incandescent light detector detecting an intensity of incandescent light from the particle being irradiated with the laser, and a signal processor including: a first peak hold circuit holding a peak in the intensity of the light scattered by the particle; a second peak hold circuit holding a peak in the intensity of the incandescent light from the particle; and a threshold value comparison circuit comparing the peak in the first peak hold circuit to a threshold and, when the peak in the first peak hold circuit exceeds the threshold, outputs a reset signal to the second peak hold circuit immediately thereafter so the peak previously in the second peak hold circuit is reset immediately after the peak in the first peak hold circuit exceeds the threshold.

Description:
BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to a particle detection device for measuring properties such as the number, size, or mass concentration of particles contained in the atmosphere or in the air in a cleanroom, for example. 
     Background Art 
     In one well-known class of devices for detecting particles suspended in a gas, sample air that contains the particles is input to the detection device and irradiated with laser light, and then properties such as the number, size, and mass concentration of the particles are measured by detecting the scattered light or incandescent light produced when the particles cross a region that is irradiated with the laser light. 
     Exhaust gas from diesel engines and exhaust gases produced from burning fuels that are composed primarily of carbon (such as coal, firewood, or biomass fuels, as well as gas produced by forest fires) contains primarily black carbon. When black carbon is momentarily heated by irradiating it with strong laser light such as that in a laser cavity or that from a pulse laser, the black carbon emits incandescent light due to the resulting black-body radiation. Detecting this incandescent light makes it possible to measure the number and size of black carbon particles. This method of detecting the incandescent light produced by black carbon is known as the laser-induced incandescence (LII) method (see Patent Document 1). 
       FIG. 10  is a block diagram of a signal processor in a conventional particle detection device. As illustrated in  FIG. 11 , scattered light and incandescent light signals received by a scattered light detector  129  and an incandescent light detector  130  are pulse waves, for example. 
     The threshold value comparison circuit  133  illustrated in  FIG. 10  sets an appropriate threshold value for the received signals, which is used to determine which pulse waves to record. Next, the pulse waves to record are converted from analog values to digital values by AD converters  131  and  132 . The digital pulse waves are then input to and recorded on a personal computer (PC)  134  or the like. 
     However, recording the pulse waves as-is as described above produces an extremely large amount of data, which results in longer signal processing times and a high load on the signal processor. A method such as the following offers a simpler alternative. 
       FIG. 12  is a block diagram of a signal processor for calculating particle size in a conventional particle detection device. In  FIG. 12 , components with the same reference characters as components in  FIG. 10  are the same components as in  FIG. 10 . As illustrated in  FIG. 12 , the peak values of the received pulse waves are held by peak hold circuits  141  and  142 . Then, the stored peak values are compared to a threshold value set in a threshold value comparison circuit  145 , and the stored peak values that are larger than the threshold value are converted from analog values to digital values by the AD converters  143  and  144 . 
     Here, assume that the peak values to compare are from the scattered light signals. There are two reasons for making this assumption. First, in most cases the particles will always produce scattered light but may not necessarily produce incandescent light. Second, if the scattered light and the incandescent light signals are both used for comparison purposes, then when the particle concentration increases, the amount of time occupied by the AD conversion process while getting the signals increases, which increases the amount of time during which particles cannot be detected (dead time). 
     Next, the digital scattered light and incandescent light signals are input to a CPU  147 , and reset circuits  149  and  150  send reset signals to the respective peak hold circuits  141  and  142 . Then the CPU  147  takes the input digital signals and converts the scattered light signals to particle size and the incandescent light signals to black carbon particle size according to peak value-particle size relationships configured in advance in a particle size setting circuit  146 . Finally, the calculated particle size values are displayed on a display device  148 . 
     The method described above makes it possible to get just the particle sizes (a small amount of data) from the large amount of data constituted by the original pulse waves, thereby making it possible to shorten processing time and reduce the load on the signal processor. 
     RELATED ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2012-88178 
     SUMMARY OF THE INVENTION 
     However, the conventional signal processing method described above has the following problems.  FIG. 13  is a timing chart illustrating the operation of the signal processor in the conventional particle detection device. 
     Assume that as illustrated in  FIG. 13 , during a certain particle detection event, a scattered light signal S 3  and an incandescent light signal S C  are obtained (also assume that as illustrated in  FIG. 11 , the incandescent light is detected slightly after the scattered light). The peak values of the scattered light signal S 3  and the incandescent light signal S C  are then respectively held by the peak hold circuits  141  and  142 . In this case, because the scattered light signal S 3  is not greater than the threshold value, the AD converters  143  and  144  do not perform the AD conversion process. Therefore, as illustrated in  FIG. 13 , no reset signals are output to the peak hold circuits  141  and  142 , and the current peak values L 3  and L C  remain stored as-is. 
     Then, during the next event, a scattered light signal S 4  and an incandescent light signal S D  are obtained. In this case, the scattered light signal S 4  is greater than the threshold value, and therefore the AD converter  143  converts the associated peak value L 4  to a digital signal. 
     However, because the new incandescent light signal S D  is less than the incandescent light signal S C  from the previous event, the peak vale L C  from the previous incandescent light signal S C  gets converted to a digital signal. As a result, particles that did not produce incandescent light or only produced weak incandescent light are recorded as particles that produced strong incandescent light, and the number of incandescent light-producing particles will be overestimated. Moreover, the scattered light and incandescent light signals no longer correspond uniquely to individual particles, and therefore characteristics of the particle mixture state can no longer be measured. 
     The present invention was made in view of the abovementioned problems and aims to provide a particle detection device that maintains a unique correspondence between individual particles and the resulting scattered light and incandescent light signals and does not overestimate the number of particles. Accordingly, the present invention is directed to a scheme that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     Additional or separate features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect, the present disclosure provides a particle detection device, including: a scattered light detector that detects an intensity of light scattered by a particle as a result of being irradiated with a laser beam; an incandescent light detector that detects an intensity of incandescent light generated by the particle as a result of being irradiated with the laser beam; and a signal processing part that includes: a first peak hold circuit that holds a peak value in the intensity of the light scattered by the particle detected by the scattered light detector; a second peak hold circuit that holds a peak value in the intensity of the incandescent light generated by the particle detected by the incandescent light detector; and a threshold value comparison circuit that compares the peak value held by the first peak hold circuit to a prescribed threshold value and, when the peak value held by the first peak hold circuit exceeds the prescribed threshold value, outputs a reset signal to the second peak hold circuit immediately thereafter so that the peak value previously held by the second peak hold circuit is reset immediately after the peak value held by the first peak hold circuit exceeds the prescribed threshold value. 
     The particle detection device of the present invention makes it possible to accurately detect particles even when using a relatively simple signal processing scheme that utilizes peak hold circuits without losing the unique correspondence between individual particles and the resulting scattered light and incandescent light signals and without overestimating the number of particles. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a signal processor for a particle detection device according to an embodiment of the present invention. 
         FIG. 2  is a timing chart illustrating the operation of the signal processor in the particle detection device according to the embodiment. 
         FIG. 3  illustrates the overall configuration of a particle detection device that utilizes the laser-induced incandescence (LII) method. 
         FIG. 4  is an expanded partial view of a particle detection device that includes a particle beam formation unit. 
         FIG. 5  illustrates the configuration of a laser cavity. 
         FIG. 6  illustrates the configuration of a detector. 
         FIG. 7  is a graph showing an example of the passband of an optical filter for detecting scattered light. 
         FIG. 8  is a graph showing the relationship between emission wavelength and color temperature. 
         FIG. 9  is a graph showing the passband of an optical filter for detecting incandescent light. 
         FIG. 10  is a block diagram of a signal processor in a conventional particle detection device. 
         FIG. 11  is a graph showing an example of a scattered light pulse wave and an incandescent light pulse wave. 
         FIG. 12  is a block diagram of a signal processor for calculating particle size in a conventional particle detection device. 
         FIG. 13  is a timing chart illustrating the operation of the signal processor in the conventional particle detection device. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Next, an embodiment of the present invention (hereinafter, simply “the present embodiment”) will be described in detail. Note, however, that the present invention is not limited to the following embodiment, and various modifications may be made without departing from the spirit of the present invention. 
     A signal processor of a particle detection device according to the present embodiment includes a number of characteristic features. However, first the overall configuration of the particle detection device will be described with reference to  FIG. 3 .  FIG. 3  illustrates the overall configuration of the particle detection device, which utilizes the laser-induced incandescence (LII) method. 
     As illustrated in  FIG. 3 , this laser-induced incandescence particle detection device includes a detection chamber  101 , a particle input unit  102  that inputs particles to the detection chamber  101 , a laser emitter  103  (a laser cavity), a detector  104  that detects scattered light and incandescent light, and a signal processor  105  that processes the signals corresponding to the detected light. 
     Next, each component of the configuration will be described in more detail. First, the particle input unit  102  will be described. In this particle detection device, sample air is irradiated with laser light that is focused to increase the irradiation energy density thereof and thereby make it possible to measure the particles with higher sensitivity. Due to the cross-sectional strength distribution of the laser light, there tends to be a significant difference in the strength of signals from particles that pass through the center region of the laser light and the signals from particles that pass through the peripheral regions of the laser light, even for particles of the same type and shape. To reduce this difference in signal strength, the laser light irradiation region can be expanded, or a particle beam can be formed in order to reduce the size of the region through which the particles cross. Of these methods, the former tends to reduce the power density of the laser light and result in decreased detection sensitivity, and therefore it is preferable that the latter method of forming a particle beam be used. 
     One method of forming a particle beam involves using a sample flow and a sheath flow.  FIG. 4  is an expanded partial view of a particle detection device that includes a particle beam formation unit. 
     As illustrated in  FIG. 4 , a discharge nozzle (particle beam formation unit)  40  has a dual tube structure that includes an internal nozzle  41  and an external nozzle  42 . Sample air  43  is input to the internal nozzle  41 , and clean sheath air  44  is input to the external nozzle  42 . Enveloping the outermost layer of the sample air  43  with the sheath air  44  and discharging the resulting flow towards a detection chamber  45  at a relatively high velocity of several dozen m/s makes it possible to form a particle beam  46 . Moreover, appropriately adjusting the flow rates of the sample air  43  and the sheath air  44  focuses the particle beam  46  to a diameter of approximately 0.1 mm at a position approximately 2 to 5 mm from the discharge nozzle  40 , and the particle beam  46  then passes through a prescribed detection region in the detection chamber  45 . For example, the flow rate of the sheath air  44  is set to a value approximately 5 to 10 times the flow rate of the sample air  43 . 
     Next, the laser emitter  103  will be described.  FIG. 5  illustrates the configuration of the laser cavity. As illustrated in  FIGS. 3 and 5 , the laser cavity includes a pump laser  111 , a collimating lens  112 , an imaging lens  113 , a laser crystal (such as an Nd:YAG crystal)  114  for converting the wavelength of the laser light, and a high-reflectivity concave mirror (HR mirror)  115 . 
     The pump laser  111  emits laser light with a wavelength of 808 nm, for example, which is then focused by the collimating lens  112 , the imaging lens  113 , and the laser crystal  114 . Moreover, it is preferable that both surfaces of the collimating lens  112  and the imaging lens  113  have an anti-reflective (AR) coating in order to prevent optical feedback to the pump laser  111 . The laser crystal  114  converts the focused laser light from a wavelength of 808 nm to a wavelength of 1064 nm. Moreover, an 808 nm AR coating and a 1064 nm high-reflectivity (HR) coating are applied to the surface of the laser crystal  114  through which the 808 nm laser light enters. Furthermore, a 1064 nm AR coating is applied to the surface of the laser crystal  114  that emits the 1064 nm laser light. Together, the 1064 nm HR coating surface of the laser crystal  114  and the high-reflectivity concave mirror  115  form a 1064 nm laser light intracavity. The beam waist of the 1064 nm laser light in the intracavity (the diameter ∅ illustrated in  FIG. 5 ) is approximately 0.3 mm, for example. Note that the configuration of the laser emitter  103  described above is only an example, and the laser emitter  103  is not limited to this configuration. 
     Next, the configuration of the detector  104  will be described.  FIG. 6  illustrates the configuration of the detector. As illustrated in  FIG. 6 , the detector  104  includes an avalanche photodiode (APD)  121 , a photomultiplier tube (PMT)  122 , lenses  123  and  124 , and optical filters  125  and  126 , for example. The scattered light from the particles is received by a scattered light detector  1 , which includes the lens  123 , the optical filter  125 , and the APD  121 . 
     The scattered light has the same wavelength as the laser light used to irradiate the particles. Moreover, an optical filter having pass-through characteristics such as those illustrated in  FIG. 7 , for example, is used for the optical filter  125  so that the scattered light detector  1  does not detect incandescent light. This makes it possible to ensure that the scattered light detector  1  only detects the scattered light. 
     Furthermore, the incandescent light is received by an incandescent light detector  2 , which includes the lens  124 , the optical filter  126 , and the PMT  122 . The incandescent light is black-body radiation (of temperature 4000-5000K), and therefore as illustrated in  FIG. 8 , the emission wavelengths exhibit a peak near approximately 500 to 600 nm. Therefore, an optical filter with pass-through characteristics such as those illustrated in  FIG. 9 , for example, is used for the optical filter  126  so that the passband of the optical filter  126  includes the emission wavelengths of the incandescent light but will not pass light of the same wavelength as the laser light. 
     Next, the signal processor  105  will be described.  FIG. 1  is a block diagram of the signal processor of the particle detection device according to the present embodiment. 
     As illustrated in  FIG. 1 , the signal processor  105  includes a first peak hold circuit  3 , a second peak hold circuit  4 , AD converters  5  and  6 , a threshold value comparison circuit  7  (threshold comparator circuit), a CPU  8 , a particle size setting circuit  9 , and reset circuits  10  and  11 . 
     As illustrated in  FIG. 1 , once scattered light is produced, the scattered light detector  1  converts that scattered light to an electrical signal, yielding a scattered light waveform such as that illustrated in  FIG. 11 . Similarly, the incandescent light detector  2  converts any incandescent light that is produced to an electrical signal, yielding an incandescent light waveform such as that illustrated in  FIG. 11 . As illustrated in  FIG. 11 , the incandescent light signal is obtained slightly after the scattered light signal. This is because the incandescent light is produced when black carbon is momentarily heated due to being irradiated with the strong laser light. A non-zero absorption time is required for the black carbon to absorb thermal energy, and therefore the incandescent light is always produced after the scattered light. The present embodiment takes advantage of this fact to improve the configuration of the signal processor  105  in comparison with conventional signal processors. 
     As illustrated in  FIG. 1 , the electrical signal from the scattered light detector  1  is sent to the first peak hold circuit  3 , and the first peak hold circuit  3  stores the peak value of the intensity of the scattered light as represented by that electrical signal. Similarly, the electrical signal from the incandescent light detector  2  is sent to the second peak hold circuit  4 , and the second peak hold circuit  4  stores the peak value of the intensity of the incandescent light as represented by that electrical signal. 
     Next, a specific example of the operation of the signal processor will be described with reference to  FIGS. 1 and 2 .  FIG. 2  is a timing chart illustrating the operation of the signal processor in the particle detection device according to the present embodiment. 
     Assume that as illustrated in  FIG. 2 , during a certain particle detection event, a scattered light signal S 1  and an incandescent light signal S A  are obtained. The peak values of the scattered light signal S 1  and the incandescent light signal S A  are then respectively held by the peak hold circuits  3  and  4 . 
     A threshold value is set in advance to the threshold value comparison circuit  7  illustrated in  FIG. 1 . As illustrated in  FIG. 1 , the threshold value comparison circuit  7  compares the scattered light signal S 1  as obtained from the first peak hold circuit  3  to the threshold value. As illustrated in  FIG. 2 , in this case the scattered light signal S 1  is less than the threshold value, and therefore the AD converters  5  and  6  do not perform the AD conversion process. Moreover, the reset circuits  10  and  11  do not output reset signals to the peak hold circuits  3  and  4 , and as illustrated in  FIG. 2 , the peak values L 1  and L A  remain stored as-is. 
     Next, assume that as illustrated in  FIG. 2 , a scattered light signal S 2  and an incandescent light signal S B  are obtained during the next particle detection event. As illustrated in  FIG. 2 , in this case the scattered light signal S 2  is greater than the threshold value, and therefore this event is treated as a true particle detection event. The threshold value comparison circuit  7  sends a reset signal to the second peak hold circuit  4  in order to reset the second peak hold circuit  4 . It is preferable that this reset be performed at substantially the same time as it is detected that the scattered light signal S 2  is greater than the threshold value. However, as illustrated in  FIG. 2 , the incandescent light signal S B  is obtained slightly after the scattered light signal S 2 , and therefore the reset may be performed after a small time lag equal in duration to this delay. 
     As illustrated in the “Incandescent light peak hold reset” timing chart in  FIG. 2 , a reset signal is output once the scattered light signal S 2  exceeds the threshold value, thereby resetting the currently stored incandescent light peak value L A . 
     As illustrated in  FIG. 2 , after the reset, the second peak hold circuit  4  holds the peak value of the new incandescent light signal S B  and stores this value as the peak value L B . Moreover, the first peak hold circuit  3  holds the peak value of the new scattered light signal S 2  and stores this value as the peak value L 2 . 
     Furthermore, as illustrated in  FIG. 2 , after a prescribed period of time T elapses, the AD converters  5  and  6  convert the (analog) peak values L 2  and L B  from the peak hold circuits  3  and  4  to digital values and outputs those digital values to the CPU  8 . At the same time, the reset circuits  10  and  11  output reset signals to the peak hold circuits  3  and  4  (see the “Scattered light peak hold reset” and “Incandescent light peak hold reset” charts in  FIG. 2 ) in order to reset the peak hold circuits  3  and  4 . Note that the prescribed period of time T from once it is detected that the scattered light signal S 2  is greater than the threshold value until when the AD conversion process is implemented provides a delay that allows the scattered light signal S 2  and the incandescent light signal S B  to reach their respective peak values. 
     A table that defines the correspondence between particle size and the intensity of the scattered light and the incandescent light is stored in advance in the particle size setting circuit  9 . The CPU  8  illustrated in  FIG. 1  converts the digital values from the AD converters  5  and  6  to particle sizes according to this correspondence table. Then, the calculated particle sizes are displayed on a display device  12  or the like. Moreover, multiplying the particle sizes by an appropriate density makes it possible to calculate the mass concentration per unit time as well. Furthermore, the number of particles for which the intensity of the resulting scattered light was greater than the threshold value can be detected. In this way, after the prescribed period of time T elapses, the CPU (processing unit)  8  can calculate one or more of the number of particles, the particle size, and the particle mass from the intensity of the scattered light and the incandescent light produced thereby. 
     As described above, in the present embodiment, the threshold value comparison circuit  7  sends a reset signal to the second peak hold circuit  4  when the signal from the first peak hold circuit  3  is greater than the threshold value. Therefore, the present embodiment makes it possible to accurately detect particles even when using a relatively simple signal processing scheme that utilizes the peak hold circuits  3  and  4  without losing the unique correspondence between individual particles and the resulting scattered light and incandescent light signals and without overestimating the number of particles. 
     INDUSTRIAL APPLICABILITY 
     The particle detection device of the present invention makes it possible to accurately detect particles and can be used to effectively measure the number, size, and mass concentration of particles contained in gases such as the atmosphere or the air in a cleanroom. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents. In particular, it is explicitly contemplated that any part or whole of any two or more of the embodiments and their modifications described above can be combined and regarded within the scope of the present invention.