FIELD OF THE INVENTION

The present invention relates to particle sensors. In particular, the invention relates to highly accurate low cost particle sensors for use in, for example, air purification systems.

BACKGROUND OF THE INVENTION

Low-cost optical particle sensors typically use a heater resistor for driving airflow through the detection volume by convection. However, the low flow rate yields a very limited particle counting frequency, compromising the sensor's accuracy and/or response time. For increasing the flow rate, other types of particle sensors use fans to drive air at a high flow rate. However, these come at the cost of acoustical noise and larger sensor housing.

Further, prior art optical particle sensors suffer from contamination of the optical sensors which decreases detection accuracy and life-time.

There is a need for a low cost, silent, long-lifetime, accurate and compact particle sensor that is capable of sensing particles at a high frequency.

US 2010/0229658 Al discloses chemical fluidic sensors with a sniffing functionality to deliver sample gases or liquid media to the sensors and induce small-scale fluid motions near the surface of the sensors to overcome diffusion-limited mass transfer at the surface of the sensing elements.

US 2007/0097372 Al discloses a particle detection apparatus and a detection method which can detect dust, pollen and smoke particles. The apparatus has a light source and a detection device for detecting light from the light source, and detects particles floating in the air at a position through which the light passes. The apparatus further has an airflow generation/control device capable of controlling an airflow of the air present in the light passing position so as to keep the airflow constant or nearly zero, and an image sensor is used as a photodetector of the detection device.

WO 2016/094007 Al discloses techniques for using synthetic jet technology as an air delivery device for sensing applications. SUMMARY OF THE INVENTION

In a first aspect of the invention a particle sensor is presented, comprising: a housing having an inlet and an outlet, wherein a detection space is located in between the inlet and the outlet; a detector for detecting particles present in the detection space; an airflow generator for generating an air flow through the detection space, the air flow generator is a synthetic jet generator. The particle sensor further comprises a filter adapted/positioned such that air entering the particle sensor via the filter creates an air flow in front of the detector thereby protecting the detector from contamination.

According to an embodiment of the invention, the air flow generator is located in between the outlet and the detection space, and the synthetic jet generator is positioned to direct a generated air flow towards the outlet.

According to an embodiment of the invention, the synthetic jet generator is sealing or closing the outlet, the synthetic jet generator comprises an opening for drawing air into the synthetic jet generator and the synthetic jet generator is positioned to: 1) draw in air present inside the housing via the opening, and 2) to direct a generated air flow out of the housing. For example, the synthetic jet generator is air-tightly sealing the outlet.

According to an embodiment of the invention, the synthetic jet generator is located in between the inlet and the detection space and the synthetic jet generator is positioned to direct a generated air flow towards the detection space.

According to an embodiment of the invention, the synthetic jet generator is sealing or closing the inlet, the synthetic jet generator comprises an opening for drawing air into the synthetic jet generator, and the synthetic jet generator is positioned to: 1) draw in air external to the housing via the opening, and 2) to direct a generated air flow towards the detection space. For example, the synthetic jet generator is air-tightly sealing the inlet.

According to an embodiment of the invention, the filter is located in the housing of the particle sensor. According to an embodiment of the invention, the filter is located in a housing of the detector.

According to an embodiment of the invention, the detector is an optical detector system.

According to an embodiment of the invention, the synthetic jet generator is a micro-blower. According to an embodiment of the invention, the particle sensor further comprises a tube. One end of the tube is coupled to the synthetic jet generator, for example to the orifice of the synthetic jet generator, such that generated air exits the generator via the tube. The other end of the tube is located near the detection space for delivering generated air to the detection space.

According to an embodiment of the invention, the synthetic jet generator comprises: a cavity having an orifice or exhaust for exiting a generated air flow from the cavity, and a vibrating component functioning as a wall of the cavity, for generating the air flow.

According to an embodiment of the invention, the orifice is the only opening of the cavity.

According to a second aspect of the invention, an air quality detection system is presented comprising a particle sensor according to the first aspect of the invention.

According to a third aspect of the invention, an air purification system is presented comprising a particle sensor according to the first aspect of the invention.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG 1 illustrates a particle sensor according to an embodiment of the invention FIG 2A-C illustrates the working principle of a synthetic air flow generator according to an embodiment of the invention

FIG 3 illustrates a particle sensor according to an embodiment of the invention

FIG 4 illustrates an embodiment of a synthetic jet generator

FIG 5 illustrates a particle sensor according to an embodiment of the invention FIG 6 illustrates a particle sensor according to an embodiment of the invention

FIG 7 illustrates a particle sensor according to an embodiment of the invention FIG 8 illustrates a particle sensor according to an embodiment of the invention FIG 9 illustrates a particle sensor according to an embodiment of the invention FIG 10 illustrates flow velocity inside an embodiment of a particle sensor FIG 11 illustrates streamlines inside an embodiment of a particle sensor FIG 12 illustrates flow velocity inside an embodiment of a particle sensor FIG 13 illustrates streamlines inside an embodiment of a particle sensor FIG 14 illustrates flow velocity inside an embodiment of a particle sensor FIG 15 illustrates streamlines inside an embodiment of a particle sensor.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Any reference signs in the claims shall not be construed as limiting the scope. In the different drawings, the same reference signs refer to the same or analogous elements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Throughout the description reference is made to a "synthetic jet generator". This refers to a device capable of generating a concentrated high speed air flow having a flow velocity of at least O.lm/s. Traditionally, a synthetic jet generator comprises a cavity having a single opening (orifice) and a vibrating member being part of the cavity for the intake and the expelling of air via the opening. For example, the width of the generated air flow at the orifice or the exhaust of the synthetic jet generator may be between 0.1 mm and 5 mm or, for example, between 0.1mm and 1mm. Thus, the width of the orifice or exhaust itself may be between 0.1 mm and 5 mm or, for example, between 0.1mm and 1mm

The problems as described above are solved by using a synthetic air flow generator that creates an air flow through the detection space/volume of the particle sensor. The synthetic air flow generator is capable of creating a high speed air flow. The high speed air flow is at least 0.1 liter/min. Preferably, the high speed air flow is above 1 liter/min. By providing such a high air flow through the detection space, the particle count per time in the detection space performed by the detector of the particle sensor can be increased with high accuracy. It is an advantage of the invention that a high aerosol particle counting frequency can be achieved.

In contrast, at a low PM2.5 of 10 microgram/m3 convection-based solutions have a particle count in the order of 0.1 particle count/sec. The use of a synthetic jet generator allows the particle count to be increased by at least a factor 10.

In contrast to convection-based devices the sensor orientation is of no influence. As an advantage the design of the particle sensor is less constrained and allows the device to be smaller or manufactured in simpler manner.

In contrast to fan-based solutions, the synthetic air flow generator generates a confined or concentrated jet of air with aerosols through the detection space as opposed to a large flow area generated by a fan. As an advantage, high rate particle detection per unit time can be performed, at the same flowrate. Furthermore, this also enables derivation of aerosols size distribution more easily. Using a smaller part around the focus of the light beam as detection volume, implies that the light intensity in the detection volume is more uniform. In that case there is a stronger one-to-one relation between, for example, pulse height measured at the photodiode and the size of the aerosol particle. This means that individual aerosol particles sizes and therefore size distribution of many particles can be measured better.

It is an advantage of the invention that the synthetic air flow generator is a very low noise device and much more silent than fan-based devices.

It is an advantage of invention that the synthetic air flow generator provides a constant performance over its lifetime. The constant air flow results in good detection and a low detection error rate. This in contrast to fan-based solutions in which the flow rate is not constant leading to a high detection error rate.

The use of a synthetic air flow generator ensures a small footprint and flexible form factor. This allows using a smaller housing. As an advantage, the particle sensor can be more compact. This decreases cost-to-manufacture and allows the sensor to be used for a wider range of applications.

It is advantage of the invention that by using a synthetic air flow generator, the generation of aerosols by the generator itself is low. Hence, accuracy of the sensing is higher compared to prior art devices.

The invention will now be described in detail in the paragraphs below.

In a first aspect of the invention, a particle sensor is presented. The particle sensor comprises a housing having an inlet and an outlet. Inside the housing, a detection space is present. This space is located in between the inlet and the outlet. The inlet of the particle sensor is an opening in the housing allowing air to enter or exit the particle sensor and the detection space. The detection space is a particular volume/location inside the particle sensor where detection of particles is performed. The volume/location of the detection space is defined by the type and position of the detector used to detect particles. The outlet is an opening in the housing which allows air to enter or exit the inside of the housing. The outlet functions as the exhaust of the particle sensor.

The particle sensor further comprises a detector for detecting particles present in or passing through the detection space. The detector may be an optical based detector, e.g. an optical scattering detector. The detector may comprise multiple components such as a light source and a sensor. The light source may be an LED. The sensor may be a component capable of recording light such as a photodiode or a plurality of photodiodes. The sensor may be an image sensor. The detector may be fixed within the housing and positioned to allow detection of particles in the detection space. For example, the light source is positioned to illuminate particles flowing through the detection space while the sensor is positioned to allow recording of light patterns caused by illuminating particles and originating from the detection space. The detector may be contained in a dedicated detector housing which in turn is fixed in the housing of the particle sensor.

The particle sensor further comprises an airflow generator capable of generating a high speed air flow. The air flow generator is a synthetic jet generator. Such a synthetic jet generator is not a convection-based air flow generator such as resistor based air flow generators. Such a synthetic jet generator is not a fan-based air flow generator.

FIG 1 illustrates an embodiment of the particle sensor (100). The particle sensor has a housing (101). The housing has an inlet (102) and an outlet (103). A detector (105', 105") is fixed inside the housing and is positioned such that particles present in or passing through the detection space (104) can be detected. The detector comprises a light source (105") and a sensor (105'). A synthetic jet generator (106) is located at the inlet (102) of the particle sensor (100) such that air external to the housing (101) can be sucked into the synthetic jet generator (106). The synthetic jet generator (106) is a cavity (109) with an orifice (107) and a vibrating member (112) functioning as a wall of the cavity. The orifice (107) may feature a nozzle for directing the generated air flow. The orifice (107) is directed towards the detection space (104) such that a generated air flow passes through the detection space (104). The synthetic jet generator (106) is fixed at the location of the inlet (102). The synthetic jet generator (106) does not block the inlet (102) such that air can still enter or exit the housing (101) via the inlet (102).

When the particle sensor is activated, the synthetic jet generator (106) creates an air flow from the inlet (102) to the outlet (103), through the detection space (104). By activating the vibrating member (112), air comprising particles is sucked into the housing (101) via the inlet (102) and into the cavity (109). During vibration of the vibrating member (112), the air comprising particles is then expelled via the orifice (107) through the detection space (104) towards the outlet (103). During this air flow generation, the detector (105', 105") continuously performs detection of particles in the detection space (104).

It is an advantage of the invention that the use of a synthetic jet generator enables a high form factor. For example, tubing or ducting may be used to place the synthetic jet generator further away from the detection space. For example, one end of a tube is connected to the orifice of the synthetic jet generator. The other end of the tube is located near the detection space. In contrast to prior art techniques, in this arrangement the synthetic jet generator may be placed anywhere as long as external air can be drawn in. This allows a considerable design freedom which in turn allows to make more cost efficient design choices regarding manufacturability. Because of this freedom, the particle sensor can also be very compact. This reduces costs and widens the applicability of the sensor in other applications. It is a further advantage of the invention that the other end of the tube can be placed very close to the detection space leading to a more accurate particle detection because the air flow inside the tube cannot be contaminated by particles outside of the tube.

According to embodiments of the invention, the orifice of the synthetic jet generator features a nozzle for directing the generated air flow.

According to a particular embodiment of the invention, the synthetic jet generator is a focusing synthetic jet generator. Thus, the synthetic jet generator is designed to focus a generated air flow in the detection space. For example, the orifice of the synthetic jet generator is shaped such that a generated air flow focuses in the detection space of the device.

According to an embodiment of the invention, the synthetic jet generator comprises a cavity featuring an orifice. A wall of the cavity is a vibrating component. The vibrating component is capable of vibrating back and forth. Due to the vibration of the vibrating component air is alternately drawn into the cavity via the orifice and expelled again via the same orifice. Air is drawn in from all directions, while jet formation occurs when air is expelled from the cavity via the orifice. Due to this jet formation net flow momentum is injected into the surrounding fluid, and by entrainment, the synthetic jet generator is able to establish a net air flow.

FIG 2A-C illustrates the working principle of an embodiment of the synthetic jet generator. FIG 2 A illustrates an embodiment of the synthetic jet generator comprising a vibrating membrane (112), a cavity (109) and an orifice (107). FIG 2B illustrates the situation where the membrane (112) is actuated away (black arrow) from the cavity (109). This causes air present in the vicinity of the orifice (107) to be drawn into the cavity (109). In FIG 2C the membrane (112) is actuated towards (black arrow) the cavity (109). This causes the drawn in air in the cavity (109) to be expelled from the cavity (109). The continuous vibration of the membrane (112) creates an air flow. A suitable air flow can be determined by adapting size of the cavity, size of the orifice, size of the membrane and frequency of the vibration. These adaptations are well known to a person skilled in the art. Reference is made to international application PCT/IB2011/054138 for the design of such a synthetic jet generator.

According to an embodiment of the invention, the vibrating component may be an actuated membrane. The actuated membrane can be of various types: for instance an electrodynamic loudspeaker, or a piezoelectric actuator may be used. Considering typical sensor dimensioning and sound production, preferably ultrasound vibration frequency (>~20 kHz) may be used for the membrane.

According to an embodiment of the invention, the air flow generator is located in between the outlet and the detection space. For example, the air flow generator is fixed inside the housing between the outlet and the detection space. The synthetic jet generator is positioned to generate air towards the outlet. The size/shape/dimensions of the outlet are adapted for allowing a suitable generation of the air flow through the detection space. It is an advantage of the invention that by placing the synthetic air flow generator in between the outlet and the detection space, contamination of the detection space with particles generated by the synthetic air flow generator itself can be minimized.

FIG 3 illustrates an embodiment of a particle sensor (100). Housing (101) has an air inlet (102) and an air outlet (103). The detector comprises a light source (105') and a sensor (105") and is positioned in the housing (101) such that particles in the detection space (104) in between the inlet (102) and the outlet (103) can be detected. The synthetic flow generator (106) is located at the outlet (103). Its orifice (107) is directed towards the outlet (103) such that a generated air flow exits the housing (101) via the outlet (103). The housing (101), the synthetic air flow generator and the outlet (103) are adapted such that a suitable air flow is generated through the detection space (104).

According to an embodiment of the invention, the synthetic jet generator itself comprises a dedicated opening for drawing in air and an air flow channel interconnecting the dedicated opening with the orifice of the cavity for providing air to the cavity. This allows air to be drawn into the cavity of the synthetic jet generator via the opening and the connecting air flow channel. In other words, in such an embodiment the air flow generating component (cavity, membrane and orifice) is integrated with an enclosure for guiding and directing the air flow. The ensemble being the enclosure and the air flow generating component forms the synthetic jet generator. The enclosure comprises a further opening for exiting and directing generated air.

FIG 4 illustrates the internal structure of an embodiment of a synthetic jet generator. The dashed arrows represent the air flow in the device. The synthetic jet generator features an air generating component (106). The air generating component (106) comprises a cavity (109), a member (112) that can vibrate when actuated and functioning as a wall of the cavity and a component (108) for vibrating the member (112). The cavity (109) has a single opening (107) for drawing in and expelling air caused by vibration of the member (112). The member (112) may be a membrane. The component (108) may be a piezoelectric element. An enclosure (113) is covering or enclosing the air generating component (106). The enclosure

(113) has an opening (114) on the side (115) covering the side (116) of the cavity (109) featuring the orifice (107). The enclosure opening (114) is aligned with the orifice (107) such that air exiting the orifice (107) can easily exit the enclosure (113) via the enclosure opening

(114) . The side (117) of the enclosure (113) opposite to the enclosure opening (114) features openings (110) for drawing in air. The enclosure (113) is enclosing the air generating component (106) such that a spacing between the enclosure (113) and the air generating component (106) is present. This spacing functions as an air flow channel (118). Openings (110) in the enclosure (113) provide access of air to the air flow channel (118). When activated, the member (112) starts to vibrate and the air generating component (106) will draw in and expel air. This creates a pressure difference in the air flow channel (118), resulting in air being drawn in via the openings (110). It is an advantage of the invention that by using such a synthetic air jet generator, a very confined air stream through the detection space can be produced. Such a synthetic jet generator may be an off-the-shelf piezo micro- blower. For example, a Murata microblower (htt :// www .murata . com/ en- us/products/mechatronics/fluid/microblower). Using off-the-shelf components reduces costs of the overall particle sensor. Typical dimensions of such a synthetic jet generator are 20mmx20mm with an enclosure opening of 2mm. A typical operating frequency is around 25 kHz.

In particular, by confining the outlet from such a microblower, more particles per unit time can be detected than when using a fan with a comparable flow rate, as more particles will pass near the optical focus where they can be detected. Furthermore, as the particles pass the optical detection volume at a well-defined position compared to detectors in which the air flow is not confined, extraction of e.g. particle size from the detector signal for individual aerosols can be more readily achieved.

FIG 5 illustrates an embodiment of a particle sensor (100) featuring a synthetic air jet generator (106) as illustrated in FIG 4. The detector, being a light source (105") and a sensor (105'), is fixed in the housing (101). The detector (105', 105") is positioned to detect particles in the detection space (104). The synthetic air jet generator (106) is fixed in an opening of the housing (101) thereby closing that opening. Whereas in the particle sensor (100) illustrated in FIG 1 inlets (102) of the housing (101) are used to draw in air; in the particle sensor (100) embodiment illustrated in FIG 5 the synthetic air jet generator (106) itself draws in the air via its dedicated opening(s) (110) and air channel(s). Air drawn in by the synthetic air jet generator (106) is expelled via opening (114) and passes through the detection space (104). Because all the air is drawn in by the synthetic air jet generator (106) and not supplied via other openings in the housing to the synthetic air jet generator (106), it is an important advantage of the invention that contamination of the detector (105', 105") can be minimized. Further, this also increases the accuracy of the particle detection.

According to an embodiment of the invention, each dedicated opening (110) for drawing in air into the synthetic jet generator comprises a filter for filtering out particles which are not of interest. For example, for filtering particles outside of the detection range of the detector. For example, for a PM2.5 sensor, the filter filters all material above 2.5 micrometer.

To minimize the detection of particles generated by the synthetic air jet generator, the synthetic air jet generator may also be fixed at the outlet of the housing of the particle sensor thereby closing the outlet. The generated air flow is directed to the outlet of the particle sensor. Such an embodiment is illustrated in FIG 6.

In embodiments where the synthetic air jet generator is located at the outlet or in between the outlet and the detection space, the generated air flow is directed towards the outlet of the particle sensor. Hence, the generated air flow does not pass through the detection space. However, the housing, the synthetic air flow generator and the outlet are adapted such that a suitable air flow through the detection space is generated caused by the drawing in of air by the synthetic jet generator.

For a configuration as illustrated in FIG 6, the air flow rate through the sensor is determined by the air flow rate generated by the micro-blower. For achieving a certain air flow rate a correct choice of the micro-blower including its driving voltage would be sufficient.

An air tight fitting between the housing and the synthetic air jet generator may be achieved using one or more gaskets present in between the housing and the synthetic air jet generator. Such a fitting is well known to a person skilled in the art.

According to an embodiment of the invention, the synthetic air jet generator is adapted such that, apart from generating an air flow which passes through the detection space, another air flow is created which passes in front of the detector, for example in front of a light source and/or sensor of the detector. Thus, the synthetic air jet generator generates two or more air flows depending on the number of components of the detectors that must be protected. It is an advantage of the invention that by creating an air flow in front of the detector, contamination of the detector by particles present in the housing can be prevented. This prevention leads to a longer life-time of the device and an accurate particle detection for a longer period of time. To create these extra air flows, an airflow may be split up using branches in a flow duct/nozzle. Each branch directs a part of the air flow into a different direction.

FIG 7 illustrates an embodiment of a particle sensor (100) featuring air flows that protect the detector (105', 105") from contamination. Enclosure opening (114), being the exhaust of the synthetic jet generator (106), is designed to create three air flows (119, 120, 121) in the housing (101). Enclosure opening (114) features a nozzle with branches. A first branch is adapted to direct a part of the air flow (119) in front of the sensor (105') for preventing contamination of the sensor (105'). The first branch comprises a filter (111 ') for filtering particles such that the air flow (119) generated in front of sensor (105') does not contain particles that could contaminate the sensor (105'). A second branch is adapted to direct a part of the air flow (120) towards the detection space. A third branch is adapted to direct a part of the air flow (121) in front of the light source (105") for preventing

contamination of the light source (105"). The third branch comprises a filter (111 ") for filtering particles such that air flow (121) generated in front of light source 105" does not contain particles that could contaminate the light source 105". The filters (11 , 111 ") are filters that filter out particles outside of the detection range of the detector (105', 105"). For example, for a PM2.5 sensor, the filters (111 ', 111 ") filter all material above 2.5 micrometer.

According to an embodiment of the invention, the particle sensor comprises a filter. The filter is positioned such that external air can flow into the housing via the filter. The filter's position is further determined such that the incoming air forms an air front/layer in front of the detector. The air flow prevents particles from attaching to the detector. It is an advantage of the invention that by creating such an air flow, contamination of the detector by particles present in the housing can be prevented. This prevention leads to a longer life-time of the device and an accurate particle detection for a longer period of time.

According to an embodiment of the invention, the filter is located in the housing. This is illustrated in FIG 8. In a wall of the housing (101), filters (111 ', 111 ") are present and placed near the detector (105', 105"). Each filter (111 ', 111 ") is adapted to 1) allow external air to enter the housing (101) and 2) to create an air flow in front of the detector (105', 105"). A first filter (11 ) creates an air flow, indicated by the dashed arrow line, in front of the sensor (105') of the detector. A second filter (111 ") creates an air flow, indicated by the dashed arrow line, in front of the light sensor (105").

According to an embodiment of the invention the filter is located in the detector. For example, the filter is located in the housing that holds/features the detector. This is illustrated in FIG 9. Sensor (105') of the detector features a filter (111 ') in the housing that holds the sensor (105'). The filter (111 ') is positioned such that an air flow is created in front of the sensor (105') from the air flowing into the sensor housing via the filter (111 '). In this embodiment, another filter (111 ") is present in the housing that holds the light source (105"). The filter (111 ") is positioned such that an air flow is created in front of the light source (105") from air flowing into the light source housing via the filter (11 Γ). According to a particular embodiment, a filter is located in the housing (101) of the particle sensor (100) and in the housing of the detector (105', 105"). According to a particular embodiment, multiple filters are located in the housing (101) and/or in the detector housing.

The particle sensor as presented in this disclosure may be used in an air quality detection system or in an air purification system.

According to an embodiment of the invention, openings of the particle sensor may comprises a filter for retaining larger dust particles, e.g. particles having a size larger than 10 um. Such particles can contaminate the detector or have an adverse effect on the functioning of the synthetic jet generator.

According to an embodiment of the invention, a typical flowrate of the synthetic jet generator is in the order of 1 liter/minute. For the effective detection volume largely covering the air jet, this yields a particle counting frequency in the order of 10 to 100 counts/second, already for a low PM2.5 level of 10 ug/m3.

According to an embodiment of the invention, the flow velocity close to the synthetic jet generator outlet is in the order of lOm/s to 50m/s. With the optical focus positioned close to the blower outlet, pulse lengths of aerosol detection events are in the order of microseconds (depending on focus parameters).

Compared to heater convection based optical particle sensors, the higher flow velocity yields much shorter detection pulses. This puts different requirements on the sensor electronics. This is mostly identical to known fan-based solutions, and is realizable at comparatively low cost.

Further, synthetic jet generators based on other vibrational actuators than described above may be used. For example, MEMS devices such as Capacitive

Micromachined Ultrasonic Transducers (CMUT) may be considered.

Simulation results:

Computational fluid dynamics (CFD) simulations have been carried out to get some insight into preferred geometrical configurations. For these flow simulations, the synthetic jet generator was located at lower part of sensor (at the inlet) and air is entrained into sensor from the bottom. In the experiments, the earlier mentioned Murata microblower was used.

A 20mmx20mmx50mm channel was considered as the interior of the sensor. This is also a rough representative of prior art sensors. Jet flow is introduced at the bottom of the channel (nozzle diameter 0.5mm, flow velocity lOm/s). Three different inlet

configurations for entrained air are considered:

Configuration 1 : 1mm slits at the sides (horizontal inlet)

Configuration 2: 3mm slits at the sides (horizontal inlet)

Configuration 3 : 1 mm slits at the bottom of the sensor (vertical inlet)

FIGs 10-15 shows results for flow velocity and stream lines. FIG 10 shows flow velocity for configuration 1. FIG 11 shows stream lines for configuration 1. FIG 12 shows flow velocity for configuration 2. FIG 13 shows stream lines for configuration 2.

FIG 14 shows flow velocity for configuration 3. FIG 15 shows stream lines for configuration 3.

For all cases recirculation can be observed between the jet and walls (most clear for configuration 1), which can lead to contamination of the sensor optics. For less recirculation a narrower and shorter channel will be preferred. For example, the channel is narrower than 20 mm. Furthermore, for configuration 1 the horizontal inflow at the inlet causes strong circulation and unwanted inflow at the top of the channel. In that sense vertical inflow at the inlet, like in configuration 3 is better. In general, for minimizing inflow at the top of the channel, and directing the flow through the channel as much as possible from bottom to top, it is preferred to have a narrower channel as well as unhampered inflow at the bottom inlet. The latter can be achieved by larger inlets (see configuration 2) and/or guiding the flow (e.g. having a vertically orientated inlet like configuration 3).

In an embodiment of the invention, the width of the inlets is 1mm or larger. Preferably, the width of the inlets are 3mm or larger.

Preferably and as proven by the simulations, the inlets are located at the bottom of the synthetic jet generator wherein the bottom is defined as the side opposite to the opening that is used to exit generated air. These inlets are preferably oriented in the same direction as the direction of the generated air.