PARTICLE SENSOR AND METHOD

Abstract

A particle sensor is provided for sensing the number or mass concentration of particles within a particular particle size range, the particles having a particle size distribution. The sensor comprises a light source (14) for providing light which is scattered by the particles to generate scattered light; a light detector (16, 18) for detecting the scattered light to provide a light detector signal; and a controller (24) for analyzing the light detector signal to determine information relating to the particle size distribution. Based on that information relating to the particle size distribution, the controller selects a mode of operation of the particle sensor to sense the particles only within the particular size range.

Claims

1. A particle sensor for sensing particles within a particular particle size range, comprising: a light source, collimated by collimator, for providing light which is scattered by the particles to generate scattered light; a light detector for detecting the scattered light to provide a light detector signal; and a controller, wherein the particle sensor has a plurality of particle size modes of operation, characterized in that the controller is adapted to analyze the light detector signal to determine the particle size distribution by performing a sweep of particle size modes, and based on that information to select one of said swept particle size modes of operation of the particle sensor to sense the particles only within the particular size range.

2. The particle sensor according to claim 1, wherein the light source has an intensity that is variable and the controller is for selecting a mode of operation by varying the intensity.

3. The particle sensor as claimed in claim 1, wherein the light source is pulsed to provide pulses of light having a particular duration and the controller is for selecting a mode of operation by varying the pulse duration.

4. The particle sensor as claimed in claim 1, wherein the controller is for selecting a mode of operation by applying a threshold setting to the light detector signal.

5. The particle sensor as claimed in claim 1, wherein the controller is adapted to select a mode of operation by selecting light source and/or light detector control settings such that: for a particle of maximum size within the particular size range, the light detector output has just reached a clipped saturated output; and for a particle of minimum size within the particular size range, the light detector output has just exceeded a set minimum sensing threshold.

6. The particle sensor as claimed in claim 1, wherein said particle size modes of operation together form a continuous size range.

7. The particle sensor as claimed in claim 1, wherein the light source is a laser and the light detector comprises a photodiode.

8. The particle sensor as claimed in claim 1, wherein the sensor is for sensing particles in air.

9. The particle sensor as claimed in claim 1, wherein the sensor is for sensing particles which are pollen.

10. The particle sensor according to claim 1, wherein the sensor comprises a user interface for selecting manually the mode of operation of the particle sensor to sense the particles only within the particular size range.

11. The particle sensor according to claim 1, wherein the size range is selected from: >10 μm; 2.5 μm-10 μm; 1 μm-2.5 μm; <1 μm; and a user-specified size range.

12. A particle sensing method for sensing the number or mass of particles within a particular particle size range using a particle sensor which has a plurality of particle size modes of operation, the particles having a particle size distribution, comprising: irradiating the particles with a light source, collimated by a collimator, for providing light which is scattered by the particles to generate scattered light; and detecting the scattered light with a light detector to provide a light detector signal, characterized in that the method comprises analyzing the light detector signal to determine the particle size distribution by performing a sweep of the particle size modes, and based on that information selecting an optimal one of said swept particle size modes of operation of the particle sensor to sense the particles only within the particular size range.

13. The method according to claim 12, wherein selecting an optimal mode of operation comprises one or more of: selecting a light source intensity; selecting a light source pulse duration; and selecting a threshold setting for the light detector signal analysis.

14. The method according to claim 12, comprising selecting a mode of operation by selecting light source and/or light detector control settings such that: for a particle of maximum size within the particular size range, the light detector output has just reached a clipped saturated output; and for a particle of minimum size within the particular size range, the light detector output has just exceeded a minimum sensing threshold.

15. The method according to claim 12 wherein said particle size modes together form a continuous size range.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

[0059] FIG. 1 shows particle sensor according to the invention;

[0060] FIG. 2 shows how the variations of the properties of the light source and threshold setting affect the pulse width modulation (PWM) output;

[0061] FIG. 3 shows how the different sized particles provide a different pulse width modulation (PWM) output;

[0062] FIG. 4 shows a plot of pulse width versus particle size in a coarse sensing mode; and

[0063] FIG. 5 shows a flow chart demonstrating a particular use of the system of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0064] This invention provides a particle sensor for sensing the number or mass concentration of particles within a particular particle size range in a sample of particles having a particular particle size distribution.

[0065] The particle sensor comprises a light source for providing light which is scattered by the particles to generate scattered light and a light detector for detecting the scattered light to provide a light detector signal. The particle sensor has a controller for analyzing the light detector signal to determine information relating to the particle size distribution and based on that information selecting a mode of operation of the particle sensor to sense the particles only within the particular size range.

[0066] Particle sensors which physically pre-separate particles in advance of sensing, thereby measuring only particles within a particular size range, are known. This invention is based on a particle sensor which has a controller to analyze the light detector signal generated by the light scattered by the particles, which allows the signal provided only by particles within a particular size range to be analyzed, and therefore provides a method of detecting particles within that range without first separating those particles from particles having other sizes.

[0067] FIG. 1 shows an example of a particle sensor according to the invention. There is a fluid (gas) flow 10 from an inlet 11 of a flow channel 13 to an outlet 12 of the flow channel 13. The flow channel 13 is formed by a conduit which has a length between the inlet 11 and outlet 12. The particles pass through a region which is irradiated by a light source 14 for providing light which is scattered by the particles to generate scattered light. The scattered light is detected by a light detector 16. A collimator 14′ focuses the incident light from the light source 14 into a small measurement zone 15, in which only one particle is present at any moment to realize individual particle detection.

[0068] A flow control device 22, shown schematically in FIG. 1, is used for inducing flow through the particle sensor. It may comprise a fan, or a heater to create a convective heat flow. In a system using heating, the resulting buoyancy causes air to flow towards the top of the detector, carrying the particles through the flow channel. In such a case, the flow channel may be vertically upwards.

[0069] The light source is to one side of the flow channel 13 and the light detector 16 is on the opposite side. An alternative design may make use of the reflection of light. The light source may be a laser diode (e.g. pulsed laser) or an infrared LED.

[0070] The particles are irradiated in the measurement zone 15 at transparent portions of the conduit that defines the flow channel 13, which allow the light to pass through the conduit. The conduit may be part of a housing which is placed on a printed circuit board with the electronics to convert the signal due to the particles into a count. Leakage of incident light directly towards the photodiode light detector, which would give a background signal, is minimized.

[0071] The light detector 16 comprises a photodiode sensor 18 and a focusing lens 20 at which scattered light is detected thereby generating a light detector signal.

[0072] The controller 24 controls the operation of the flow control device and light source.

[0073] In addition, the controller 24 is for analyzing the light detector signal to determine information relating to the particle size distribution and based on that information selecting a mode of operation of the particle sensor to sense the particles only within a particular size range.

[0074] Most low cost sensors include signal processing electronics which contains factory settings and calibration parameters. In addition, the sensor may have ports for user inputs to override the default settings. The mode of operation may be varied for example by varying the intensity of the light source, varying the duration of a pulsed light source, or varying the threshold setting applied to the light detector signal.

[0075] FIG. 2 shows how varying these parameters alters the signal output for a particular particle size in an embodiment in which the light source is a pulsed laser and the light detector is a photodiode.

[0076] Signals I-IV represent respectively the laser diode driving voltage, the photocurrent detected by the photodiode, the signal after amplification and filtering, and the pulse width modulation (PWM) output.

[0077] The laser diode emits laser pulses (Signal I) having a particular duration or pulse width 30 (for example around 20-40 ms) and intensity 32 (controlled by the laser driving voltage) that illuminate the particles. If a particle is present during a pulse, the photodiode will detect the light scattered by that particle and converts its intensity to a photocurrent (Signal II, μA). This analogue scattered light detector signal can be processed to give a digital output. The photocurrent is filtered and amplified by the controller's signal processing electronics to provide Signal III. This signal is converted to a digital PWM output by a comparator threshold 34 to give the PWM Signal IV. This is a Boolean signal with either high or low voltage. A low voltage pulse corresponds to the presence of a particle, and the width of the low voltage pulse is calibrated to the particle size.

[0078] As shown, the PWM signal is low when the amplified signal (Signal III) exceeds the threshold 34. The threshold 34 thus implements a band pass filtering function. The threshold is for example implemented as a threshold voltage applied to a comparator which controls the particle size sensitivity of the sensor system. By adjusting the threshold, particle size bins may be defined to enable a particle size distribution information to be obtained. The length of a low voltage pulse in the PWM signal (Signal IV) is equal to the time during which Signal III exceeds the comparator threshold.

[0079] The low voltage pulse length 35 of the PWM output is determined by the previous values as follows:

(i) The pulse width 35 of a PWM output is determined by the time period for which the height of Signal III is greater than the comparator threshold 34;
(ii) The height of Signal III is determined by the height and width of Signal II
(iii) The width of Signal II is determined by the width 30 of Signal I; and
(iv) The height of Signal II is determined by the height 32 of Signal I and the size of the particle in the measurement zone.

[0080] Thus when measuring particles with fixed size, the PWM output will be influenced by the width 30 of Signal I, i.e. the laser pulse duration, the height 32 of the Signal I, i.e. the intensity of the laser output; and the threshold 34 of Signal III, i.e. the threshold setting applied to the light detector signal. Thus, there are three control variables which determine the size range for which the sensor operation is tuned.

[0081] In order to make a measurement of particle size distribution, the controller alternates between the different size ranges by operating with different sets of values for the three control variables defined above. Within the different size ranges, different mass concentrations are obtained or different particle number concentrations.

[0082] The light detector for example requires 20 seconds to obtain a stable particle concentration result in each mode. The sweep of all modes, in order to obtain a particle size distribution, is for example repeated every hour to obtain a renewed size distribution measurement and then determine whether the “optimal” mode will remain the same or will need to be switched. The sweeping of three particle size modes (coarse, fine and ultrafine) as described above may thus last one minute, and this may be repeated a number of times to ensure sufficient data collection. For example, there may be 5 to 10 repetitions.

[0083] In this way, the particle sensor makes a measurement of the particle size distribution. Then, having determined information relating to particle size, the controller selects a mode of operation of the particle sensor to sense the particles only within a particular size range by varying the three control variables, thereby sensing the number of particles only within a particular particle size range, without pre-separation of the particles.

[0084] Thus, for example, after analyzing the light detector signal to determine that the majority of particles are within 2.5-10 μm, the controller can select a mode of operation to sense only the particles within this particular size range.

[0085] This is achieved by making Signal III generated by the largest particle in the size range (i.e. 10 μm) reach the maximum value of Signal III (i.e. 1.4 V in this example) that does not trigger clipping (clipping being shown as 36 in FIG. 3), and making Signal III generated by the smallest particle in the size range (i.e. 2.5 μm) just exceed the comparator threshold (i.e. 0.3 V in this example).

[0086] Thus, for a particle size range, for a particle of maximum size within the particular size range, the light detector output has just reached a clipped saturated output whereas for a particle of minimum size within the particular size range, the light detector output has just exceeded a minimum sensing threshold.

[0087] The duration and intensity of the laser pulse (Signal I) correlate with the height of Signal III, and so variation of these parameters determine the upper limit of the active size range (i.e. 10 μm), thereby ensuring that the largest particle in this range does not trigger clipping of Signal III. The comparator threshold is determined so that all particles larger than the lower size limit (i.e. d.sub.p-min=2.5 μm) are recorded in the PWM output (Signal IV).

[0088] Thus, the particle sensor therefore adjusts to a mode of operation to sense the particles only within the particular size range.

[0089] In particular, the controller can select a mode of operation to sense only the particles within the following particular five size ranges:

[0090] pollen mode (>10 μm or user-defined cut-off size);

[0091] general mode (user-defined active size range);

[0092] coarse mode (2.5 μm-10 μm);

[0093] fine mode (1 μm-2.5 μm);

[0094] ultrafine mode (<1 μm)

[0095] FIG. 3 shows the particle sensor in operating in a coarse mode (PM.sub.2.5-10). Different laser and/or threshold settings may be used for other modes. As explained further below, the same laser pulse used in the coarse mode (Signal I) may also be used for pollen detection (>10 μm) but the way the light detector signal is interpreted differs depending on whether the coarse mode is being used or the pollen sensing mode.

[0096] In the coarse mode, the width and height of the laser pulse (Signal I) are set to make upper size limit to 10 μm, and the comparator threshold is set to the lower size limit of 2.5 μm. With these settings, the sensor only reports particles in the PWM output with sizes within this range (Signal IV). For the coarse mode, information for particles below 2.5 μm is not needed.

[0097] The width of each low voltage pulse 35 in the PWM output is related to the particle size. As shown in FIG. 3, a larger particle 44 (e.g. 8 μm) will induce a wider low voltage pulse in Signal IV compared to a smaller particle 42 (e.g. 3 μm). By analyzing the width of each low voltage pulse, the sensor can further differentiate particle size and yield a size resolved particle number distribution spectrum within the active size range of the said mode.

[0098] The three control variables mentioned above can further be adjusted to define additional operation modes with user specific upper and lower size limits. For instance, with the lower size limit set as 1 μm and the upper limit as 2.5 μm, the sensor defines a fine mode where number/mass concentration and size resolved number distribution of particles having size from 1 μm to 2.5 μm will be obtained.

[0099] In addition to particle number or mass concentration within the coarse size range, the particle sensor can also determine particle mass concentration below the lower size limit (<2.5 μm or <1 μm) using the background signal (Signal III below the threshold level). As small particles 46 are dominant in population across the whole size spectrum and usually appear in clusters in the measurement zone of the particle sensor (instead of individually as larger particles), the sensor is not able to tell particle number concentration but only mass concentration from the aggregated scattering signal by particle ensembles. In course mode, the threshold 34 is set to exclude these small particles.

[0100] The mass concentration of particles <2.5 μm (or <1 μm) can be determined by using the background in Signal III when no particle of 2.5 μm-10 μm (or no particle of 1 to 2.5 μm) is present in the measurement zone. The background signal arises from scattering by clusters of fine particles, but can also be contaminated by stray light from undesired scattering in the sensor chamber or even electronics noise. If stray light and electronic noise are suppressed to an optimized level, e.g. coating the sensor optics chamber with light absorbing material, or providing additional filtering stage in the signal processing electronics, the sensor will be able to interpret mass concentration of particles <2.5 μm or <1 μm from the background signal.

[0101] An operation mode with a smaller upper size limit requires a laser pulse with greater duration 30 and increased intensity 32 (Signal I), and an operation mode with a smaller lower size limit requires a lower threshold 34 (Signal III). Specific values for the three parameters appropriate for particular size limits can be provided to users in a look-up table from factory calibrations.

[0102] There are thus different size modes, such as a coarse mode, a fine mode and an ultrafine mode.

[0103] Some of these size modes have upper and lower size limits. The lowest size mode has only an upper limit (<1 μm).

[0104] No threshold value is required for the lowest size mode. In most instances, the light detector sees more than one ultrafine particle at a time, so the particle sensor uses signal III to directly interpret PM.sub.1 total mass concentration with no particle number count via the PWM output at signal IV.

[0105] In contrast to these modes, for which particles which cause clipping are excluded from detection, the pollen mode counts a clipping event 36 in Signal III as the presence of pollen 48 (i.e. particles>10 μm, defined as the cut-off size) or another large particle type. This clipping event is detected as a saturated light detector output.

[0106] The output signal (e.g. Signal III) is for example digitized and analyzed in software to detect the clipping events.

[0107] FIG. 3 shows one such clipping event 36 in Signal III when the sensor records the presence of one pollen particle. The final output in pollen mode is the particle (pollen) count.

[0108] However, when the signal is clipped, the low voltage width in the PWM output (Signal IV) is no longer sensitive to particle size. Thus pollen mode cannot report an accurate size resolved pollen distribution spectrum, but only a pollen number concentration instead.

[0109] The pulse width of the PWM signal after the comparator (Signal IV) is most responsive to particle size when Signal III lies below clipping level. Above the clipping level, a large increase in particle size will only result in a minor increase in pulse width of Signal IV, so that size resolution is not accurate but the number count is valid.

[0110] This issue is shown in FIG. 4 which plots the pulse width versus the particle size when in the coarse mode. The upper size cutoff is at 10 μm and the lower boundary is at 2.5 μm. It can be seen that after clipping, the pulse width is no longer highly sensitive to particle size.

[0111] The cut-off size of pollen mode is a lower size limit, only above which particles will be recorded. The cut-off limit for the pollen mode is adjusted by modifying the intensity of the laser and the duration of the laser pulse such that only particles of the size of pollen cause clipping. If the lower size limit for the pollen mode is the same as the upper size limit for the coarse mode, the same laser pulse may be used, as mentioned above. However, with a different laser duration and intensity, the sensor will record the presence of pollen with a different cut-off size such as 20 μm (i.e. particles>20 μm).

[0112] As mentioned above, the pollen mode detects the presence of all particles above the cut-off size but cannot reliably distinguish particle size. Thus, an error may be introduced in the reported pollen number count by an unwanted large particle (e.g. sand). A sensible choice of cut-off size in specific sensing scenarios can minimize this adverse effect.

[0113] The number of pollen detected by the sensor during a specific interval (for example 2 minutes) will satisfy a Poisson distribution:

[00001]$P\left(n.Math..Math.\mathrm{pollens}.Math..Math.\mathrm{in}.Math..Math.\mathrm{interval}\right)=\frac{{\lambda }^n.Math.e^{-\lambda }}{n!}$

[0114] where:

[0115] λ is the average pollen count during the interval

[0116] n takes on values 1, 2, 3

[0117] λ is calculated from the sensor measurement and can be used to estimate the probability of inhaling a certain number of pollen within a certain breath time or volume (e.g. the probability of a person inhaling 5 and 10 pollens per 2 minutes of normal breathe is 0.453 and 0.132 respectively). This measurement may be used to provide evidence for allergy severity analysis and for providing a pollen alert to a user. The sensor may have an output to provide a user alert based on the pollen reading.

[0118] FIG. 5 shows a flow chart describing a possible use of the system and method.

[0119] In step 50 the sensor is started.

[0120] In step 52, the user selects whether the machine should operate in pollen mode (“PM”). Thus, in this example, the pollen mode is selected manually by the user whereas the method is able to automatically select between different size modes.

[0121] If yes, the sensor operates in a first mode (M1) in step 53, wherein the sensor only detects particles over a certain size i.e. which are pollen or mold spores. Smaller particles are neglected.

[0122] In step 54, the user may input a lower cut-off size for the pollen mode. For example, the user can instruct the sensor to detect only particles above 20 μm. This user input is provided in step 56. Alternatively, the sensor can operate in its default pollen mode and measure particles above a default cut-off size e.g. >10 μm as set in step 58. The detector then provides an output in the form of a number of pollen particles (pollen count “PC”) and pollen concentration in step 59.

[0123] Alternatively, the user can instruct the sensor not to operate in pollen mode, and indeed in the absence of user input the normal particle mode is selected. This is a second mode (M2) which is selected in step 60.

[0124] In that case, in step 61 the user can input a particle size range to be measured,

[0125] In the absence of user input in step 61, there is the option in step 62 of the user selecting a fixed mode (fine, ultrafine, coarse or full scanning mode). If a specific mode is selected, the particle sensing takes place and the results are output in step 64, for example as a histogram of size bins (e.g. 5) within the specific size range.

[0126] In the coarse mode and fine mode, the particle sensor outputs a particle number and mass concentration based on signal IV. In the ultrafine mode, the particle sensor outputs the particle mass concentration based on signal III. This is because it is difficult for the sensor to see ultrafine particles individually and thereby retrieve a particle count.

[0127] It is possible to provide an additional flow control mechanism to make individual particle sensing in the ultrafine mode possible. The particle sensor would then also be able to output a particle number concentration in the ultrafine mode but this would add cost to the particle sensor.

[0128] In the full scanning mode, the particle sensor cycles continuously through all of the three pre-set modes and outputs a size resolved particle number concentration across the whole size spectrum

[0129] In the absence of a selected individual mode, the sensor enters a self-learning mode 65. In this case, the sensor operates in an ultrafine mode (<1 μm), a fine mode (1 μm-2.5 μm) and a coarse mode (2.5 μm-10 μm), consecutively for a set period of time. After operating in each mode, the sensor determine which size range includes the largest mass concentration of particles, and operates in that mode.

[0130] The ultrafine mode is operated in step 66 followed by a mass concentration measurement in step 67. The fine mode is operated in step 68 followed by a number count and mass concentration measurement in step 70. The coarse mode is operated in step 72 followed by a number count and mass concentration measurement in step 74.

[0131] The sensor then determines which mode is appropriate based on which size range is dominant for a particular sample, in step 76. The sensor then chooses that mode as the optimal operation mode, and measures particles within that size range to provide a mass concentration or indeed a size resolved particle distribution within the range. The results are output in step 77, for example as a histogram of particle count for a set of size bins within the specific size range, and an overall mass concentration for that full size range. Of course, it may be that only particle count information is needed or that only a mass concentration is needed. The particle size distribution is obtained based on the analysis of the widths of the low pulses in Signal IV.

[0132] Referring back to step 61, if there is user input, a size range is input in step 78 and the sensor configures its operation to that mode and measures particles within that size range to provide a mass concentration or indeed a size resolved particle distribution within the range. The results are output in step 79.

[0133] In the fine mode, coarse mode, and user-defined mode, a particle number count is output with respect to size (derived based on the number and width of low voltage pulses in Signal IV). The particle number count may for example be illustrated as a particle size distribution in a histogram with a set of size bins within the entire size range of each mode. There may for example be 3 to 8 size bins, for example 5 size bins within the size range of each mode.

[0134] The particle number count can be converted to a mass concentration after assuming a representative particle density.

[0135] The controller can include seasonal, geographical, and weather forecast information. For example, if the seasonal information denotes spring i.e. the pollination season and the geographical information denotes Japan, the sensor will switch its primary mode to pollen mode. Alternatively, if the seasonal information denotes summer i.e. high temperature and humidity and the geographical information denotes Los Angeles or Shanghai, the sensor will switch the primary mode to fine mode and thereby measure secondary aerosol particles resulting from photochemical reactions.

[0136] The additional information may be obtained locally or from a remote data source such as over the Internet.

[0137] In applications where the operation of the sensor needs to be highly automated (e.g. in a sensor box, air purifier, vacuum cleaner, etc.), the request for user input values can be omitted so the sensor operation is determined by the default settings or be realized by buttons (e.g. button instructing sensor to work in pollen mode).

[0138] In addition to single mode operation, the sensor can also work in multiple modes alternately. For example, the sensor may operate with five cycles in the fine mode, five cycles in the coarse mode, five cycles in the pollen mode, five cycles in the fine mode, five cycles in the coarse mode, five cycles in the pollen mode and so on. The alteration among multiple operation modes enables the sensor to perform high-resolution particle sensing over broader active size ranges.

[0139] Particular applications of the particle sensor and method include particle sensing, for obtaining the number or mass concentration in one dominant size range or in multiple size ranges, or pollen sensing: to provide a pollen alert or an indication of likely pollen allergy severity. The sensor may also be used for mold spore or other biological airborne particle sensing. The sensor may be used in a sensor box to determine a wide range of airborne particle concentrations with high resolution; and in air purifying for filtration mode selection (for example fine particle filtration, coarse particle filtration, pollen filtration).

[0140] In the example above, the pollen mode is selected manually by the user whereas the method is able to automatically select between three different size modes by cycling through those three modes to provide an initial size distribution determination. In other examples, the pollen mode may also be an automatically selected mode of operation based on the detection of pollen as the dominant airborne particle. In this case, the self-learning mode operates the coarse mode in both an LPO % mode and in a clipping count mode.

[0141] The example above has three different size modes. There may of course be a different number of size ranges in addition to the pollen mode. The pollen mode is a mode which detects particles with a minimum size, and may thus be used for detecting any type of particle above a minimum size. Thus, this mode is not restricted to the detection of pollen.

[0142] As discussed above, embodiments make use of a controller. The controller can be implemented in numerous ways, with software and/or hardware, to perform the various functions required. A processor is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform the required functions. A controller may however be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions.

[0143] Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).

[0144] In various implementations, a processor or controller may be associated with one or more storage media such as volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM. The storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform the required functions. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller.

[0145] As explained above, one concept of the invention is to provide automatic mode selection based on a particle size distribution analysis. Another concept of the invention is to provide selectable upper and lower particle size limits, by adapting the light source intensity and pulse width.

[0146] In this aspect, there is provided a particle sensor for sensing particles within a particular particle size range, comprising:

[0147] a light source (14), collimated by collimator, for providing light which is scattered by the particles to generate scattered light;

[0148] a light detector (16, 18) for detecting the scattered light to provide a light detector signal; and

[0149] a controller (24),

[0150] wherein the light source provides light pulses of controllable pulse duration and intensity, wherein the controller is adapted to select the pulse duration and intensity thereby to define a particular size range over which particles are detected, having a non-zero lower size limit and an upper size limit.

[0151] Preferably, the light detector control settings are also adjustable, in particular so that the detection thresholds may be set.

[0152] In this aspect, a mode of operation is defined by selecting light source and/or light detector control settings such that:

[0153] for a particle of maximum size within the particular size range, the light detector output has just reached a clipped saturated output; and

[0154] for a particle of minimum size within the particular size range, the light detector output has just exceeded a set minimum sensing threshold.

[0155] Preferably, the sensor has a user input for enabling user selection of the lower size limit and the upper size limit.

[0156] Preferably, the sensor has a user input for enabling user selection of a set of predefined size ranges, each having a pre-set lower size limit and a pre-set upper size limit.

[0157] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.