Patent Description:
Poor air quality due to chemical and particulate pollutants is a health hazard in urban areas. According to the World Health Organization, WHO, exposure to air pollutants has contributed to seven million deaths in <NUM>, that being one in eight of total global deaths. In addition to the effect of air pollutants on respiratory systems of humans, strong links between exposure to air pollution and, among many other medical conditions, cardiovascular diseases and cancer have been established.

Negative health effects from airborne pollutants are manifold and depend on their composition and state, for example, gaseous or solid state. Monitoring of various air pollutants, their concentrations and space-time distribution is, therefore, important not only on the global scale, but on a more localized basis within regions and localities for localization of pollution sources and a geographical extent of the pollution. In order to measure transport of pollutants and to forecast evolution of pollution spread, the measurements may be conducted frequently and preferably over a dense spatial grid.

Filter-based monitoring of air pollutants comprises using filters with selectivity for particulate sizes of interest. Once the filters have been exposed to air traversing them, they may be assessed for particulate matter caught therein, to estimate concentrations of particles in the air, or, more generally, a gas.

Particulate pollutants come in a range of sizes. Smog particles may range from <NUM>,<NUM> to <NUM> micrometre, fly ash particles from <NUM> to <NUM> micrometres, pollen particles from <NUM> to <NUM> micrometres, heavy dust from <NUM> to <NUM> micrometres and cat allergens from <NUM>,<NUM> to <NUM> micrometres, for example. Consequently, using filters, a bank of filters of differing selectivity may be used to obtain an estimate of a distribution of particle sizes of particles in the gas, such as air. The distribution of particle sizes may comprise plural estimates of particle concentrations of specific particle size, in the gas.

Document <CIT> discloses a MEMS capacitor configured to measure particulate matter flowing between the capacitor plates, and to infer particle sizes using capacitance change as a function of time. Document <CIT> discloses a capacitor configured to measure particulate matter accumulating between the capacitor plates by detecting variation in capacitance.

According to some aspects, there is provided the subject-matter of the independent claims. Some embodiments are defined in the dependent claims.

According to a first aspect of the present invention, there is provided an apparatus comprising a microelectromechanical, MEMS, capacitor comprising two plates and a gap between the plates, a gas conveyor configured to cause gas to flow through the gap, and readout circuitry configured to measure a capacitance of the MEMS capacitor, wherein the readout circuitry is configured to detect a change in the capacitance of the MEMS capacitor caused by a particle flowing through the gap with the gas, wherein in that a width of the gap is adjustable.

According to a second aspect of the present invention, there is provided an apparatus comprising means for directing a gas conveyor configured to cause gas to flow through an gap between plates of a microelectromechanical, MEMS, capacitor to convey the gas through the gap, means for receiving inputs from readout circuitry configured to measure a capacitance of the MEMS capacitor, a change in the capacitance of the MEMS capacitor being caused by a particle flowing through the gap with the gas, and means for deriving, from the inputs, a particle concentration in the gas, wherein the apparatus is further configured to cause a width of the gap to be adjusted.

Various embodiments of the second aspect may comprise at least one feature from the following bulleted list:.

According to a third aspect of the present invention, there is provided a method, comprising directing a gas conveyor to cause gas to flow through an gap between plates of a microelectromechanical, MEMS capacitor, receiving inputs from readout circuitry configured to measure a capacitance of the MEMS capacitor, a change in the capacitance of the MEMS capacitor being caused by a particle flowing through the gap with the gas and deriving, from the inputs, a particle concentration in the gas, and causing a width of the gap to be adjusted.

Various embodiments of the third aspect may comprise at least one feature corresponding to a feature from the preceding bulleted list laid out in connection with the second aspect.

According to a fourth aspect of the present invention, there is provided a computer program configured to cause a method in accordance with the third aspect to be performed.

A microelectromechanical, MEMS, capacitor may be employed to detect particles in a gas which flows between the plates of the capacitor. A particle flowing through a gap between the plates of the capacitor causes a transient change in capacitance of the capacitor, which may be detected with suitable readout circuitry.

A thickness of the gap in the MEMS capacitor is adjustable, to thereby obtain selectivity as to the particle size that may pass through the gap. In detail, particles with a diameter larger than the gap width cannot fit through. By modifying the thickness of the gap, a distribution of particle sizes may be determined.

<FIG> illustrates an example system in accordance with at least some embodiments of the present invention. A MEMS capacitor <NUM> comprises two plates <NUM>, <NUM>, which may be made of, or coated with, a metallic substance, for example. The plates <NUM>, <NUM> need not be of the simple shape which is illustrated in <FIG> for the sake of simplicity and clarity of the figure. MEMS capacitor <NUM> has a housing <NUM> onto which other elements of the MEMS capacitor <NUM> are mounted. Plate <NUM> is mounted on housing <NUM> using a spring mounting <NUM>, such that the distance between plates <NUM> and <NUM> is adjustable, for example by applying a selectable bias voltage to the plates <NUM> and <NUM> to thereby generate an electrostatic attractive force of selectable strength. The spring mounting <NUM> is illustrated in <FIG> in a schematic manner, and many mechanical variations of the spring mechanism may be employed, or, additionally or alternatively, other ways to enable adjusting the distance between plates <NUM> and <NUM>. The distance between the plates <NUM> and <NUM> defines a width of the gap.

While discussed herein primarily in terms of one gap and two plates, in general the MEMS capacitor <NUM> may comprise a plurality of gaps connected in parallel, each gap having its own readout electronics, each gap being between two plates. As such, in this general form the number of plates may be more than two.

Readout circuitry <NUM> is configured to measure a capacitance of MEMS capacitor <NUM>. Readout circuitry <NUM> is enabled to detect relatively quick changes in the capacitance, as will be described herein below. Readout circuitry <NUM> is operatively coupled, via connection <NUM>, with a control device <NUM>. In some embodiments, connection <NUM> traverses housing <NUM>. Readout circuitry <NUM> may be configured to measure the capacitance of MEMS capacitor <NUM> by determining its response to a square wave, or by a resonance measurement, for example, as is known in the art.

Control device <NUM> may be configured to record capacitance measurement signals from readout circuitry <NUM>. Control device <NUM> is configured to cause the gap width between the plates to change, for example by causing a bias voltage to change. Control device <NUM> may be configured to perform a series of measurements using MEMS capacitor <NUM>, the series of measurements comprising a plurality of measurements with different gap widths. Connection <NUM> may connect control device <NUM> to further nodes, for example via the Internet, the Internet of Things or a sensor network. Connection <NUM> may be wire-line or at least in part wireless.

Gas conveyor <NUM> is configured to cause gas, such as air, to flow between plates <NUM>, <NUM> through the gap. For example, gas conveyor may be arranged to generate a pressure gradient across the length of the gap. A pressure gradient may be generated by at least one fan installed to create under-pressure between gas conveyor <NUM> and the gap, as illustrated in <FIG>, and/or to create over-pressure between gas conveyor <NUM> and the gap.

Another possibility is to use a thermophoretic force, also known as thermodiffusion, whereby a temperature gradient is caused across the length of the gap, such that the gas is caused to flow through the gap. While the width of the gap is the distance between plates <NUM> and <NUM>, the length of the gap is perpendicular to this, such that as the gas flows along the length of the gap, the gas flows from one side of the MEMS capacitor <NUM> to another side, through the gap separating plates <NUM>, <NUM>. A temperature gradient may be caused across the length of the gap by using a heatable grid or plate, for example.

Existing fine particle detection schemes are typically bulky, that is, not portable, and expensive, their prices ranging in the tens of thousands of euros, while on-chip solutions would have several advantages over existing solutions, such as their small size, low cost and low power consumption. A miniaturized particle sensor platform is a key enabler for sensor networks for air quality monitoring that can be formed either by embedding sensors in basic infrastructure or even in mobile devices. Air quality data together with pressure information may be collected and reported to a cloud service, for example, and utilized for air quality forecasting and/or monitoring. Forecasting may further enable an early warning system for air pollution levels. Also, a mobile fine particle sensor could work as a personal dosimeter to measure accumulated exposure to fine particle hazards. Such a sensor network cold have a significant societal and economic impact, due to reduction in mortality rates and healthcare costs. For example, a user might react to an alarm concerning particulate pollution by donning a protective mask.

In use, gas conveyor <NUM> pushes or pulls gas <NUM>, such as air, through the gap between plates <NUM> and <NUM>, while readout circuitry <NUM> measures the capacitance of MEMS capacitor <NUM>. In case a particle is conveyed through the gap, the capacitance of the MEMS capacitor <NUM> changes, in other words, the capacitance of the MEMS capacitor <NUM> is different depending on whether there is only gas, or gas and a particle, in the gap. A transient change in the capacitance may be counted as a particle that has flown through the gap. In practice, the capacitance will transiently increase when a particle is between the plates, since relative permittivity of a particle is, in general, greater than that of air.

Readout circuitry <NUM> or control device <NUM> may be configured to assign an estimated size to the particle passing through the gap, based on a size of the transient effect on the capacitance. The width of the gap defines an upper limit for a diameter of a particle passing through. A mapping may be prepared from the size of the transient change in capacitance to an estimate of particle size. The mapping may be prepared, before measurements are conducted, experimentally or from first principles. Since the gap in a practical MEMS capacitor <NUM> may be relatively narrow, of the order of <NUM>,<NUM> to a few micrometres, it may be relatively unlikely that two particles would be in the gap simultaneously, which enables more reliable counting of individual particles.

During measurement, control device <NUM> may compile statistics of the number and size of particles passing through the gap. As described above, the size of each particle may be estimated based on the gap width and the size of the change in capacitance.

Control device <NUM> may be arranged to conduct a series of measurements with different gap widths, for example starting from a narrow gap and progressing to wider gap widths. Alternatively, a measurement series may start with a wider gap and proceed to a narrower gap width. Since the gap width acts as a natural cut-off for particle diameter, this manner of measurement may be useful in deriving a particle size distribution of particles present in the gas, which may be air, for example.

To conduct the series of measurements, control device <NUM> may direct the gap between plates <NUM> and <NUM> to first assume an initial value, and then gather measurement results sufficient to characterize the concentration in the gas of particles capable of fitting through the gap when the gap is at the initial value width. Control device <NUM> may then cause the gap to assume a second width, for example wider than the initial value, and to gather measurement results sufficient to characterize the concentration in the gas of particles capable of fitting through the gap when the gap is in the second width.

When increasing the gap width, it may be possible to identify transient changes in capacitance caused by particles too large to have been present in an earlier measurement, performed when the gap was narrower. This is so, since the change in capacitance is the larger the larger is the particle. Thus particles already accounted for in measurements using narrower gap widths may be eliminated from statistics when using a wider gap width, which may assist in deriving a size distribution for the particles.

Control device <NUM> may be configured to dynamically determine, when to conclude a measurement using a specific gap width. For example, once a preconfigured number of particles have been detected passing through the gap, control device <NUM> may decide that enough data has been collected to characterize particles using a specific gap width. The gap width may then be changed, or, if the measurement series only has one measurement with one gap width, the measurement series may be concluded. For example, <NUM> or <NUM> particles may be sufficient. Alternatively or in addition, a measurement may be concluded after a pre-configured time has elapsed during which the measurement has been active. For example, in case no particles, or very few particles, are present in the gas, detecting the preconfigured number of particles may be difficult or even impossible.

To determine a concentration of particles, control device <NUM> may have an estimate of how much gas passes through the gap. This may be known beforehand, using a table of gas flow rates, using gas conveyor <NUM>, as a function of the gap width.

<FIG> comprises two plots in accordance with at least some embodiments of the present invention. In the upper plot, a relative change in capacitance is plotted against particle diameter. The relative change in diameter is on the vertical axis, and the particle diameter is on the horizontal axis. The gap between the plates has a width, G, of <NUM> micrometre. A height, H, of the plates is <NUM> micrometres and a width, W, of the plates is ten micrometres. Both axes are logarithmic. For example, a particle of diameter of <NUM>,<NUM> micrometres causes a relative change in capacitance of <NUM>. <NUM> × <NUM>-<NUM> while a particle of diameter <NUM>,<NUM> micrometres causes a relative change in capacitance of <NUM> × <NUM>-<NUM>. <NUM> × <NUM>-<NUM>.

In the lower plot, a frequency of particles is plotted against particle concentration. On the vertical axis, the frequency is presented in a logarithmic scale, and the particle concentration is, likewise in a logarithmic scale, presented on the horizontal axis, in micrograms per square meter. The topmost curve represents particles of diameter <NUM>,<NUM> micrometres, the middle curve represents particles of diameter <NUM>,<NUM> micrometres and the lowest curve represents particles of diameter <NUM> micrometre. The dimensions of the plates and the gap are the same as in the upper plot, and a velocity of gas is one metre per second. As the plot indicates, mostly less than one particle per second is expected, with only the smallest particles at highest concentrations presenting about five particles per second. Taking into account the size of the MEMS capacitor and speed of gas <NUM>/s, even in these conditions it is expected to be rare that two particles would be present in the gap at the same time.

<FIG> illustrates an example apparatus capable of supporting at least some embodiments of the present invention. Illustrated is device <NUM>, which may comprise, for example, a control device <NUM> of <FIG>. Comprised in device <NUM> is processor <NUM>, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multi-core processor comprises more than one processing core. Processor <NUM> may comprise, in general, a control device. Processor <NUM> may comprise more than one processor. Processor <NUM> may be a control device. A processing core may comprise, for example, a Cortex-A8 processing core manufactured by ARM Holdings or a Steamroller processing core produced by Advanced Micro Devices Corporation. Processor <NUM> may comprise at least one Qualcomm Snapdragon and/or Intel Atom processor. Processor <NUM> may comprise at least one application-specific integrated circuit, ASIC. Processor <NUM> may comprise at least one field-programmable gate array, FPGA. Processor <NUM> may be means for performing method steps in device <NUM>. Processor <NUM> may be configured, at least in part by computer instructions, to perform actions.

Device <NUM> may comprise a transmitter <NUM>. Device <NUM> may comprise a receiver <NUM>. Transmitter <NUM> and receiver <NUM> may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. Transmitter <NUM> may comprise more than one transmitter. Receiver <NUM> may comprise more than one receiver. Transmitter <NUM> and/or receiver <NUM> may be configured to operate in accordance with global system for mobile communication, GSM, wideband code division multiple access, WCDMA, <NUM>, long term evolution, LTE, IS-<NUM>, wireless local area network, WLAN, Ethernet and/or worldwide interoperability for microwave access, WiMAX, standards, for example.

Device <NUM> may comprise user interface, UI, <NUM>. UI <NUM> may comprise at least one of a display, a keyboard, a touchscreen, a vibrator arranged to signal to a user by causing device <NUM> to vibrate, a speaker and a microphone. A user may be able to operate device <NUM> via UI <NUM>, for example to configure particle detection measurements.

Device <NUM> may comprise further devices not illustrated in <FIG>. For example, where device <NUM> comprises a smartphone, it may comprise at least one digital camera. Some devices <NUM> may comprise a back-facing camera and a front-facing camera, wherein the back-facing camera may be intended for digital photography and the front-facing camera for video telephony. Device <NUM> may comprise a fingerprint sensor arranged to authenticate, at least in part, a user of device <NUM>. In some embodiments, device <NUM> lacks at least one device described above.

Processor <NUM>, memory <NUM>, transmitter <NUM>, receiver <NUM> and/or UI <NUM> may be interconnected by electrical leads internal to device <NUM> in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to device <NUM>, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the embodiment various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the present invention.

<FIG> illustrates signalling in accordance with at least some embodiments of the present invention. On the vertical axes are disposed, on the left, MEMS capacitor <NUM> of <FIG>, in the centre, control device <NUM> of <FIG> and on the right, a separate controlling entity CTRL, which may correspond to a meteorological institute or civil defence control centre, for example. Time advances from the top toward the bottom.

In phase <NUM>, the controlling entity CTRL requests a measurement from control device <NUM>. Phase <NUM> may comprise instructing control device <NUM> concerning a kind of measurement series that is requested. In response, control device <NUM> instructs MEMS capacitor <NUM> to perform a measurement with a first gap width between plates <NUM>, <NUM>. The gap width may be identified in phase <NUM>, or phase <NUM> may comprise control device <NUM> controlling MEMS capacitor <NUM> to assume the desired gap width between the capacitor plates. Phase <NUM> comprises MEMS capacitor <NUM> performing the measurement requested in phase <NUM>. Once MEMS capacitor <NUM> has the measurement result with the first gap width, it returns the result to control device <NUM> in phase <NUM>. Subsequently, in phase <NUM>, control device <NUM> instructs MEMS capacitor <NUM> to perform a measurement with a second gap width between plates <NUM>, <NUM>. Phase <NUM> comprises MEMS capacitor <NUM> performing the measurement requested in phase <NUM>. Once MEMS capacitor <NUM> has the measurement result with the second gap width, it returns the result to control device <NUM> in phase <NUM>. More than two measurements may be requested and performed, although two are illustrated in <FIG>. Once control device <NUM> has the results of the measurement series, it informs the controlling entity CTRL of them, phase <NUM>.

<FIG> is a flow graph of a method in accordance with at least some embodiments of the present invention. The phases of the illustrated method may be performed in device <NUM>, an auxiliary device or a personal computer, for example, or in a control device configured to control the functioning thereof, when installed therein.

Phase <NUM> comprises directing a gas conveyor to cause gas to flow through a gap between plates of a microelectromechanical, MEMS capacitor. Phase <NUM> comprises receiving inputs from readout circuitry configured to measure a capacitance of the MEMS capacitor. Finally, phase <NUM> comprises deriving, from the inputs, a particle concentration in the gas.

Concerning capacitance of an air gap:
The detector in <FIG> consists of two electrodes and air, or another gas, is blown through the gap between them. A perspective representation of the detector, without a particle in the gap, is presented in the upper part of the figure and a diagram of the detector, with a particle of diameter d in the gap, is presented in the lower part of the figure. A particle in the gap changes the capacitance. This is calculated for a cubic particle in the device of <FIG>. If the particle permittivity εr is close to <NUM> and the gap G is narrow the model particularly accurate.

This is plotted in the upper part of <FIG>. The magnitude of the capacitance change is roughly proportional to ΔC ~ d<NUM>. <NUM> and thus gives an indication of the particle size. If relative resolution of the capacitance change is ΔClC<NUM>= <NUM> ppm the smallest detectable particle size is d ≈ <NUM>.

A feature of the air gap detector is the slow operation. An average frequency of transport of particle through the channel is
<MAT>
where gas flow velocity is v, mass density of particles ρ, volume flow of air dV/dt= vWG, particle mass concentration m, particle mass M=ρd<NUM>,and particle number density N=m/M. This is plotted in the lower part of <FIG>. There, the following conditions may apply: ρ= <NUM>/m<NUM>, dV/dt = <NUM>-<NUM> m<NUM>/s.

One way to increase the frequency of signal pulses is to increase the pressure difference across the detector. This increases the gas velocity but also shortens the signal pulse. In the conditions the lower part of <FIG>, the pulse duration may be τ = H/v = <NUM>, which benefits from high speed capability for the measurement electronics.

Because the pulse caused by the particle is relatively short, a wide bandwidth may be used. With reasonable values U= <NUM> V, T= <NUM>, B= <NUM> centered at driving frequency <NUM>, C= <NUM> fF, Q= <NUM> we get noise limited ΔC= <NUM> aF or ΔC/C= <NUM> ppm.

If several gaps are connected in parallel, the total capacitance increases and this lowers the relative capacitance resolution. Every gap may be furnished with its own readout electronics to maintain the relative resolution. This may be accomplished using, for example, integrated read-out electronics.

At least some embodiments of the present invention find industrial application in particle detection.

Claim 1:
An apparatus comprising:
- a microelectromechanical, MEMS, capacitor (<NUM>) comprising two plates (<NUM>, <NUM>) and a gap between the plates (<NUM>, <NUM>);
- a gas conveyor (<NUM>) configured to cause gas to flow through the gap, and
readout circuitry (<NUM>) configured to measure a capacitance of the MEMS capacitor (<NUM>), wherein the readout circuitry (<NUM>) is configured to detect a change in the capacitance of the MEMS capacitor (<NUM>) caused by a particle flowing through the gap with the gas, characterized in that
a width of the gap is adjustable.