The present invention relates to a sensor device comprising at least one environmental sensor for determining at least one environmental parameter associated with a sensor gas flow through the sensor device, to a gas flow system comprising such a sensor device, and to a method of determining at least one environmental parameter associated with a sensor gas flow. The environmental sensor may, in particular, be a particulate matter sensor for determining at least one property of particulate matter in the sensor gas flow.
In many situations, it is desired to determine one or more properties of particulate matter (PM) in a gas, e.g. a number or mass concentration in a certain size range or a size or mass distribution of PM. Applications include air quality monitoring of environmental air outdoors and HVAC systems in buildings and vehicles.
From the prior art, various devices for detecting and characterizing PM are known. An example is disclosed in WO 2008/140816 A1. A fan creates an air flow through a flow channel. A light source shines light into the flow channel. The light is scattered by particles entrained in the air flow. The scattered light is registered by a photodetector. The photodetector converts the scattered light into an electrical signal. The electrical signal is typically a pulse whose amplitude correlates with particle size and whose duration correlates with the transit time of the particle through the light beam. By counting the pulses, a number concentration of the PM may be determined. By analyzing the intensities of the pulses, a size distribution of the PM may be determined. By analyzing the duration of the pulses, the flow rate of the air flow may be calculated. Another example of a sensor device for optically detecting PM is disclosed in WO 2017/054098 A1.
Sometimes, PM sensors are used in environments in which fog may occur. The presence of fog may negatively affect the accuracy of a PM sensor. Fog is composed of microscopic water droplets suspended in the air. As PM sensors typically are not able to distinguish between solid particles and liquid droplets, fog may be mistaken for an increased PM load unless measures are taken to prevent the fog from reaching the PM sensor. In the prior art, measures have therefore been taken to evaporate all droplets before they can reach the PM sensor. However, this may require much energy, making this approach unsuitable for low-energy applications, as in solar- or battery-powered devices.
Another typical problem with PM sensors is contamination. Particles in the air flow tend to precipitate on the light source and the detector and thereby degrade the performance of the PM sensor. WO 2018/100209 A2 discloses various measures that may reduce contamination of the optical components. However, it would be desirable to prevent the most problematic sources of contamination from entering the PM sensor in the first place. This includes relatively large particles, e.g., pollen, fragments of leaves, insect wings, hairs etc., having a size that is typically far beyond the size range of interest, e.g., above 10 μm. Specifically, in gas flow systems comprising a duct, such particles often adhere to the duct wall, where they slowly move under the action of the gas flow. In the prior art, it has been proposed to provide sensor devices with impactors to separate large particles from small particles so as to prevent large particles from reaching the optical components. However, the available space in the impactor may fill up over time, requiring cleaning or replacement of the impactor. It would therefore be desirable to design a gas flow system in such a manner that large particles are prevented from entering the PM sensor device in the first place.
Similar problems also exist for other types of environmental sensors. For instance, gas concentration sensors may also show a cross-correlation to fog and may be sensitive to contamination by large particles.
In a first aspect, it is an object of the present invention to provide a sensor device comprising an environmental sensor, the sensor device exhibiting a reduced sensitivity to fog or other kinds of evaporable droplets in the sensor gas flow.
This object is achieved by a sensor device according to claim 1. Further embodiments of the invention are laid down in the dependent claims.
Accordingly, a sensor device is provided, the sensor device comprising at least one environmental sensor for determining at least one environmental parameter associated with a sensor gas flow through the sensor device. The sensor device is configured to preheat at least a portion of the sensor gas flow upstream of the environmental sensor, i.e., at a location where the sensor gas flow has not yet passed the environmental sensor, using waste heat generated by the environmental sensor itself and/or by another sensor that is comprised in the sensor device upstream of the environmental sensor. Waste heat from other electrically operated components of the sensor device may be used for preheating as well, for instance, waste heat from a flow-generating device such as a fan and/or from a control device for controlling operation of the sensor device. Essentially, waste heat generated by any component of the sensor device may be used. In this manner, the number and/or size of evaporable droplets in the sensor gas flow is reduced before the sensor gas flow arrives at the environmental sensor, thereby reducing potential adverse effects of the droplets on the operation and/or accuracy of the environmental sensor. Since waste heat is used for preheating, a separate heating element may not be required. However, the present invention does not exclude that an additional separate heating element is provided to additionally assist with preheating.
In the context of the present disclosure, the term “environmental sensor” is to be understood as relating to a sensor that is capable of determining at least one environmental parameter of the sensor gas flow. The term “environmental parameter” is to be understood as relating to any parameter that may characterize the composition of the sensor gas flow. In particular, the environmental sensor may be a particulate matter (PM) sensor, and the at least one environmental parameter may accordingly be at least one parameter that characterizes particulate matter in the gas flow, such as a number and/or mass concentration of the particulate matter, possibly restricted to at least one size or mass range (e.g., a concentration of PM that is smaller than a certain cutoff), and/or at least one parameter that is indicative of a size or mass distribution of the particulate matter. The term “particulate matter” (PM) is to be understood as relating to microscopic solid particles and/or liquid droplets suspended in the gas flow. In particular, a PM sensor may be an optical PM sensor, comprising a light source for shining light into the flow channel and a photodetector for registering light that has been scattered by particles entrained in the sensor gas flow. The photodetector may output an electrical signal that is indicative of the scattered light. For instance, the photodetector signal may comprise pulses corresponding to single scattering events in a comparatively small detection volume defined by the light, the amplitude of the pulses correlating with particle size and their duration correlating with the transit time of the particle through the detection volume, or it may be a DC signal whose amplitude correlates with the intensity of the scattered light in a comparatively large detection volume, thus directly correlating with the PM concentration in the detection volume. In other embodiments, the environmental sensor may be a gas concentration sensor, and the environmental parameter may be a concentration of one or more types of gas in the gas flow, in particular, a concentration of at least one of water vapor, carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), and one or more volatile organic compounds (VOC). The sensor gas flow may be an air flow or a flow of any other gas, e.g., a fuel gas or a medical gas.
In some embodiments, the sensor device may comprise a heat exchanger for exchanging heat between at least a first portion of the sensor gas flow upstream of the environmental sensor and at least a second portion of the sensor gas flow downstream of the environmental sensor. In this manner, the first portion of the sensor gas flow may be preheated using waste heat generated by the environmental sensor itself and possibly by other components of the sensor device that contribute to heating the second portion of the sensor gas flow before it enters the heat exchanger.
The terms “at least a first portion of the sensor gas flow” and “at least a second portion of the sensor gas flow” are to be understood as follows: The heat exchanger may be configured to preheat the entire sensor gas flow upstream of the environmental sensor by transferring heat from the entire sensor gas flow downstream of the environmental sensor. However, it is also conceivable that not the entire sensor gas flow upstream of the environmental sensor is preheated. For instance, if the sensor gas flow is formed by combining a core flow comprising the gas of interest and a sheath flow for protecting the optical components from contamination, it may be sufficient to preheat only the core flow or only the sheath flow. Accordingly, it is conceivable that only a first portion of the sensor gas flow upstream of the environmental sensor is preheated. In a similar spirit, it is also conceivable that not all of the sensor gas flow downstream of the environmental sensor is fed to the heat exchanger. For instance, a portion of the sensor gas flow may be diverted to some additional sensor elements, may be used for some other purpose, or may simply be lost through leaks of the sensor device. Accordingly, it is conceivable that only a second portion of the sensor gas flow downstream of the environmental sensor is fed to the heat exchanger. It is further conceivable that the sensor gas flow downstream of the environmental sensor or the second portion thereof is combined with another gas flow, e.g., a gas flow from another sensor in the sensor device, before being passed to the heat exchanger.
Many designs of heat exchangers are known, and the present invention is not limited to any particular design. In some embodiments, the heat exchanger may have a crossflow design, where the flow to be heated flows in a direction that is transverse to the flow direction of the gas from which heat is to be extracted. In other embodiments, the heat exchanger may have a counterflow design, wherein the gas flows between which heat is transferred have opposite flow directions. In a particularly simple configuration, the heat exchanger may have a concentric tube design, wherein the heat exchanger comprises an inner tube which is surrounded by an outer tube, leaving an annular space between the inner and outer tubes. The inner tube may then carry the flow that is to be heated, and the outer tube may carry the flow whose heat is to be extracted. The term “concentric tube design” is to be understood as not strictly requiring the tubes to be concentric in a mathematical sense; it suffices that the outer tube surrounds the inner tube.
In some embodiments, heat is exchanged between an inlet gas flow into the sensor device and an outlet gas flow that exits the sensor device. Accordingly, the sensor device may comprise:
The heat exchanger may then be configured to exchange heat between the inlet gas flow and the outlet gas flow. In a particularly simple, yet effective concentric tube design, the heat exchanger may comprise an inlet tubing section arranged between the inlet and the environmental sensor and an outlet tubing section arranged between the environmental sensor and the outlet, the outlet tubing section surrounding the inlet tubing section.
For generating the sensor gas flow, the sensor device may comprise a flow-generating device, e.g., a fan or a heating device that causes convective flow in the sensor device. Waste heat from the flow-generating device may contribute to the heating of the second portion of the sensor gas flow.
If the environmental sensor is a particulate matter (PM) sensor, fog may be detected by monitoring sensor signals of the PM sensor, in particular, sensor signals that are indicative of a number or mass concentration of PM and/or of its size or mass distribution, while the flow rate through the sensor device is varied. Since at higher flow rates the droplets spend less time in the zone where the sensor gas flow is preheated (in particular, in the heat exchanger), and since waste heat is carried away by a larger gas volume per unit of time at higher flow rates, leading to a lower temperature rise of the sensor gas flow due to the waste heat, less droplets are expected to be evaporated upstream of the PM sensor at higher flow rates. Therefore the sensor signals will depend on the flow rate more strongly when droplets are present than when droplets are absent. In this manner, the presence of droplets may be detected. A compensated particulate matter signal may be derived, which is compensated for the effect of droplets.
Accordingly, the sensor device may comprise:
In some embodiments, the sensor device may comprise not only a single environmental sensor, but two or more environmental sensors, and use waste heat from one or more of these environmental sensors and possibly other components of the sensor device to preheat the sensor gas flow for the other environmental sensor. In particular, the sensor device may comprise:
In this manner, the second sensor gas flow is preheated using waste heat generated by the first environmental sensor and any other components upstream of the second environmental sensor.
The terms “first sensor gas flow” and “second sensor gas flow” are to be understood as follows: The “first sensor gas flow” is the gas flow on which the first environmental sensor carries out its measurements for determining the first environmental parameter. The “second sensor gas flow” is the gas flow on which the second environmental sensor carries out its measurements for determining the second environmental parameter. These gas flows are preferably identical, i.e., the entire first sensor gas flow downstream of the first environmental sensor may be fed to the second environmental sensor. In this case, it is possible to define a single “sensor gas flow” without distinguishing between a “first” and a “second” sensor gas flow. However, it is also conceivable that the second sensor gas flow comprises only a portion of the first sensor gas flow, another portion being diverted from the first sensor gas flow to some other destination or being lost by leaks, or that an additional gas flow, e.g., a sheath flow, is added to the first sensor gas flows downstream of the first environmental sensor and upstream of the second environmental sensor. In such situations, it may be necessary to distinguish between the first and second sensor gas flows.
It goes without saying that more than one environmental sensor may be present upstream of the second environmental sensor. Accordingly, the second sensor gas flow may be preheated using waste heat generated by more than only one environmental sensor and its associated components.
In embodiments with two or more environmental sensors, in addition or as an alternative to using the waste heat of a first environmental sensor for preheating, also the waste heat of the second environmental sensor may be used for preheating. In particular, these two sources of waste heat may be combined for preheating. To this end, the sensor device may comprise a heat exchanger for exchanging heat between at least a first portion of the second sensor gas flow upstream of the second environmental sensor and at least a second portion of the second sensor gas flow downstream of the second environmental sensor. In particular, the heat exchanger may be configured to preheat the first portion of the second sensor gas flow downstream of the first environmental sensor, but still upstream of the second environmental sensor. In other embodiments, the heat exchanger may be configured to preheat at least a portion of the first sensor gas flow upstream of the first environmental sensor. Since the second sensor gas flow comprises at least a portion of the first sensor gas flow, this causes the second sensor gas flow to be preheated as well.
Both environmental sensors are preferably configured to determine the same type of environmental parameter. In some embodiments, the first environmental sensor is a first particulate matter (PM) sensor for detecting particulate matter in the first sensor gas flow, and the second environmental sensor is a second PM sensor for detecting particulate matter in the second sensor gas flow. In other embodiments, the first and second environmental sensors may be, e.g., gas concentration sensors for detecting a concentration of the same type of gas.
The sensor device may comprise a control device configured to receive first sensor signals from the first environmental sensor (in particular, from the first PM sensor) and second sensor signals from the second environmental sensor (in particular, from the second PM sensor) and to derive an output signal using the first and second sensor signals. In particular, the first and second sensor signals may be indicative of a number or mass concentration of PM in the respective gas flow, possibly restricted to at least one size or mass range of the PM (e.g., PM concentration below a certain size cutoff). The output signal may comprise at least one of:
In particular, the control device may be configured to form a (possibly weighted) difference of concentration values obtained from the first and second sensor signals to derive the droplet indicator signal and/or to form a (possibly weighted) sum of such concentration values to derive the compensated particulate matter signal.
In more sophisticated embodiments, the first and second sensor signals may be indicative of a size or mass distribution of the particulate matter, and accordingly also the droplet indicator signal and/or the compensated particulate matter signal may be indicative of a size or mass distribution.
The sensor device may comprise at least one flow-generating device configured to generate the first and second sensor gas flows. In particular, a single flow-generating device may be provided for generating both the first and second sensor gas flows. Waste heat of the at least one flow-generating device may optionally be used for additionally preheating the second sensor gas flow.
The control device may be configured to operate the at least one flow-generating device to cause a flow rate variation of at least one of the first and second sensor gas flows and to determine a response of the first and/or second sensor signals to the flow rate variation, and the control device may be configured to take said response to the flow rate variation into account when deriving the output signal. For instance, as discussed above, increasing the flow rate may cause less efficient preheating, which in turn may cause an increase in the difference between concentration values obtained from the first and second PM signals in the presence of droplets. The dependence of this difference on the flow rate may provide important information on the presence, concentration and/or size or mass distribution of the droplets.
In a similar spirit, the sensor device may comprise an additional heating device configured to preheat the second sensor gas flow upstream of the second particulate matter sensor using externally supplied power. Preferably, the second heating device is arranged downstream of the first particulate matter sensor. The control device may be configured to operate the heating device to cause a variation of heating power and to determine a response of the first and/or second sensor signals to the variation of heating power, and the control device may be configured to take said response to the variation of heating power into account when deriving the output signal. For instance, the control device may be configured to determine a heating power threshold above which the difference between the first and second PM signals does not change any more. This heating power threshold may provide important information on the concentration and/or size or mass distribution of the droplets.
In preferred embodiments, the first particulate matter sensor is an optical particulate matter sensor comprising a first light source and a first light detector, and the second particulate matter sensor is an optical particulate matter sensor comprising a second light source and a second light detector.
In some embodiments, the first and second light sources and, optionally, the first and second light detectors may be mounted on a common circuit board, in particular, on opposite sides thereof, and the circuit board may be arranged such that the second sensor gas flow is in thermal contact with the circuit board downstream of the first particulate matter sensor and upstream of the second particulate matter sensor. Thermal contact may be established, e.g., by establishing direct, physical contact between the second sensor gas flow and the circuit board, or by flowing the second sensor gas flow over a heat sink that is attached to the circuit board, the heat sink having larger surface area than the surface area portion of the circuit board to which it is attached. Preferably, the second sensor gas flow is brought into thermal contact with both a first side and a second side of the circuit board, wherein the second sensor gas flow may have a different flow direction while being in thermal contact with the second side as compared to the flow direction while being in thermal contact with the first side. The circuit board thus acts as a heat exchanger for heating the second sensor gas flow upstream of the second PM sensor, using the waste heat of both particulate matter sensors.
In some embodiments, the sensor device may comprise a valve that is movable between a first and a second state. In the first state, the valve feeds at least a portion of the first sensor gas flow downstream of the first environmental sensor to the second environmental sensor, while in the second state, the valve disconnects the first and second sensor gas flows from one another, enabling the first and second environmental sensors to be operated independently.
In a second aspect, the present invention provides a sensor device comprising:
According to the second aspect, the idea of varying the heating power of a heating device for preheating the sensor gas flow and of determining a response of the PM signal to the heating power variation in order to derive the output signal is not restricted to situations where two PM sensors are present, but may also be used in a sensor device that comprises only a single PM sensor. According to the second aspect, it is not required that waste heat of this PM sensor is used for preheating.
The sensor device of the first and second aspects may comprise at least one auxiliary sensor, and the control device may be configured to receive at least one auxiliary signal from the at least one auxiliary sensor and to take the at least one auxiliary signal into account when deriving the output signal. The at least one auxiliary sensor may comprising at least one of:
Temperature, humidity and flow rate may all be used to compute a more precise value of the fog indicator signal and/or of the compensated PM signal. For instance, the higher the difference between the temperatures of the first and second sensor gas flows, or the higher the absolute temperature of the second sensor gas flow, the higher the fraction of droplets that is likely to have been evaporated between the first and second PM sensors. Taking the temperature signals into account may therefore improve computation of the fog indicator signal and/or of the compensated PM signal. Likewise, also humidity and flow rate signals for the first and second sensor gas flows may be used to derive a more precise value of the fog indicator signal and/or of the compensated PM signal. For instance, increased humidity of the second sensor gas flow may indicate that droplets have been evaporated between the first and second PM sensors.
The sensor device of the first and second aspects may comprise a heat-insulating housing, the at least one environmental sensor being arranged inside the housing. The housing preferably comprises at least one housing wall section made of a material having a thermal conductivity of less than 0.2 W/(m*K), said housing wall section preferably defining at least 80%, more preferably at least 90%, of the surface area of the housing, most preferably essentially the entire surface area except necessary holes for the inlet, outlet and wiring. In this manner, it is ensured that as little as possible of the available heat for preheating the sensor gas flow (waste heat and/or heat generated by a heating device) is lost.
In a third aspect, the present invention addresses the problem of contamination by comparatively large particles such as pollen, fragments of leaves, insect wings, hairs etc., moving along a duct wall. According to the third aspect, the present invention provides a gas flow system comprising:
By creating a flow barrier using the outlet gas flow, large particles that slowly migrate along the duct wall are deflected away from the inlet and are thus prevented from entering the sensor device.
In order to create the flow barrier, the outlet preferably opens out into the duct in such a manner that at least a portion of the outlet gas flow enters the duct upstream of the inlet with respect to the duct gas flow, said portion of the outlet gas flow having a flow component perpendicular to the duct direction.
In particular, the outlet may radially surround the inlet with respect to the inlet flow direction. To this end, the inlet may be formed by an inlet tubing section, and the outlet may be formed by an outlet tubing section that surrounds the inlet tubing section, an annular space being formed between the inlet tubing section and the outlet tubing section. The inlet tubing section may then carry the inlet gas flow, and the annular space may carry the outlet gas flow. In particular, the inlet tubing section and the outlet tubing section may form a heat exchanger for preheating the inlet gas flow by heat transfer from the outlet gas flow, as discussed in more detail above in connection with the first aspect of the invention.
The outlet may surround the inlet in such a manner that the outlet gas flow has a flow component perpendicular to the duct direction, said component having a velocity maximum upstream of the inlet with respect to the duct gas flow. To this end, the arrangement of the inlet and the outlet may be non-symmetric, the annular space between the inlet and the outlet being narrower upstream of the inlet than downstream of the inlet, whereby the flow component perpendicular to the duct direction will have higher velocity upstream of the inlet than downstream of the inlet.
Formation of the flow barrier may be improved by configuring the outlet to form a nozzle, such that the nozzle accelerates the outlet gas flow along a direction that has an appreciable component perpendicular to the duct direction.
In some embodiments, the inlet is formed by inlet tubing that protrudes into the duct beyond the outlet (or, to be more precise, beyond an open end of outlet tubing, the open end defining the outlet), in particular, along a direction that is transverse to the duct direction, i.e., along a direction that is perpendicular to the duct direction or has an appreciable component perpendicular to the duct direction. Thereby, particles that travel along the duct wall are less likely to enter the inlet. The open end of the outlet tubing may be flush with the duct wall, or it may also protrude into the duct.
The gas flow system may additionally comprise a deflection element arranged in the duct, the deflection element being configured to direct particles moving along the duct wall away from the inlet. The deflection element may deflect the particles in at least one lateral direction in a plane that is parallel to the flow direction of the duct flow and perpendicular to the flow direction of the outlet gas flow, and/or in a radial direction with respect to the duct flow, away from the duct wall toward the interior of the duct.
For ensuring efficient deflection of particles by the flow barrier, it may be desirable to operate the flow-generating device in such a manner that a ratio of the flow velocity of the outlet gas flow and the flow velocity of the duct flow is kept essentially constant, or that the flow velocity of the outlet gas flow at least exhibits a positive correlation with the flow velocity of the duct flow. To this end, the gas flow system may comprise a duct gas flow sensor for determining a flow rate or flow velocity of the duct gas flow, and the sensor device may comprise:
In a fourth aspect, the present invention provides a method for determining at least one environmental parameter associated with a sensor gas flow using a sensor device comprising at least one environmental sensor. The sensor device is preferably a sensor device according to the first aspect of the present invention. The method comprises:
As discussed above, preheating may comprise exchanging heat between at least a first portion of the sensor gas flow upstream of the environmental sensor and at least a second portion of the sensor gas flow downstream of the environmental sensor, such that the sensor gas flow is preheated using waste heat generated by the environmental sensor itself.
As discussed above, the environmental sensor may be a particulate matter sensor, and the environmental parameter may accordingly be at least one parameter associated with particulate matter in the sensor gas flow.
As discussed above, the method may comprise:
As discussed above, in some embodiments the sensor device may comprise:
The method may then comprise feeding at least a portion of the first sensor gas flow to the second environmental sensor such that the second sensor gas flow comprises at least a portion of the first sensor gas flow downstream of the first environmental sensor, whereby the second sensor gas flow is preheated using waste heat generated by the first environmental sensor.
The method may comprise exchanging heat between at least a first portion of the second sensor gas flow upstream of the second environmental sensor and at least a second portion of the second sensor gas flow downstream of the second environmental sensor, whereby the second sensor gas flow is additionally preheated using waste heat generated by the second environmental sensor.
The first and second environmental sensor may be particulate matter sensors for detecting particulate matter in the first sensor gas flow. The method may comprise:
In some embodiments, the method may comprise:
In some embodiments, the method may comprise:
In some embodiments, the method may comprise:
In a fifth aspect, the present invention provides a method for detecting particulate matter in a sensor gas flow using a sensor device comprising a particulate matter sensor. The sensor device is preferably a sensor device according to the second aspect of the present invention. The method comprises:
Preferred embodiments of the invention are described in the following with reference to the drawings, which are provided for illustrating the present preferred embodiments of the invention and not for limiting the same. In the drawings,
In the drawings, components having the same or similar function are always designated by the same reference signs, unless indicated otherwise.
Inlet tubing 42 guides an inlet gas flow Fin from an inlet 41 into the sensor device 10. Outlet tubing 44 allows an outlet gas flow Fout to exit the sensor device through an outlet 43. In the present example, the inlet gas flow Fin, the sensor gas flow F and the outlet gas flow Fout are the same, i.e., no gas is added to or removed from the inlet gas flow Fin before it is probed by the PM sensor 12, and no gas is added to or removed from the sensor gas flow F before it forms the outlet gas flow Fout. However, it is also conceivable that the gas flows Fin, F and Fout are not identical. For instance, it is conceivable that a filtered sheath gas for sheathing the optical components of the PM sensor 12 is added to the inlet gas flow Fin to form the sensor gas flow F, it is conceivable that sensor gas flows of two or more different sensors are combined to form the outlet gas flow Fout, or it is conceivable that some of the sensor gas flow F is lost to the environment during its passage through the sensor device due to leaks.
The inlet tubing 42 and the outlet tubing 44 are arranged to form a heat exchanger 40. To this end, a section of the inlet tubing 42 is arranged inside a section of the outlet tubing 44, such that the outlet tubing section surrounds the inlet tubing section, defining an annular space between the inlet tubing 42 and the outlet tubing 44. The inlet gas flow Fin is passed through the inlet tubing 42, while the outlet gas flow Fout is passed through the annular space between the inlet tubing 42 and the outlet tubing 44 in counterflow with the inlet gas flow Fin. For achieving high efficiency of the heat exchanger 40, the material of the inlet tubing 42 may be a material having high heat conductivity, e.g., a metal. However, for low-cost applications, it may be sufficient to use a material having lower heat conductivity, such as common plastics materials, as long as the inlet tubing has a wall thickness that is not too high.
The control device 50 communicates with the PM sensor 12. In particular, the control device causes the light source and light detector of the PM sensor 12 to be activated, and the control device registers electrical signals from the light detector, for instance, individual pulses corresponding to individual scattering events or a DC signal reflecting overall PM concentration. In more general terms, the PM sensor 12 outputs sensor signals that characterize PM in the sensor gas flow F, more precisely, sensor signals that are indicative of at least one parameter associated with PM in the sensor gas flow F. This parameter may be, in particular, a number or mass concentration CS of the PM. In addition, the sensor signals may be indicative of a size or mass distribution of the PM and/or of the velocity of the PM. The control device 50 receives these sensor signals, analyzes them and derives output signals based on the sensor signals. The output signals may directly reflect the number or mass concentration CS, the size or mass distribution of the PM, and/or of the velocity of the PM. In addition, in some embodiments, the output signals may indicate the presence or absence of fog, as will be explained below.
The sensor device 10 may be used, in particular, to monitor environmental air or air in an HVAC system of a building or vehicle. In such applications, fog may enter the sensor device 10 through inlet 41. Fog comprises microscopic water droplets (schematically illustrated as dotted circles in
In operation, the PM sensor 12, the fan 13, the control device 50, and all other electrically operated components inside the housing 30 produce waste heat. This causes the outlet gas flow Fout to have higher temperature than the inlet gas flow Fin. In the heat exchanger 40, heat is transferred from the outlet gas flow Fout to the inlet gas flow. Fin. In this manner, the inlet gas flow Fin is preheated before it forms the sensor gas flow F and reaches the PM sensor 12. This preheating causes at least some of the water droplets in the inlet gas flow Fin to evaporate. In this manner, the negative impact of fog on the PM signals is reduced. No additional heating power is required for evaporating the water droplets. Only waste heat is used. This makes the design particularly suitable for applications where only little energy is available for operating the sensor device, as in solar- or battery-powered devices.
To maximize the preheating effect, the housing 30 is preferably heat-insulating. The material and thickness of the housing walls are chosen such that sufficient heat insulation is achieved. Preferably, the housing walls are made of a material having a thermal conductivity of less than 0.2 W/(m*K) and have a thickness of at least 5 mm, and the housing is essentially completely closed except at the inlet and outlet. In the present example, the heat exchanger 40 is arranged in the insulating top part 32 of the housing. However, any other housing design that reduces heat loss from the sensor device 10 may be used.
In some embodiments, the housing may comprise an inner housing subassembly surrounded by an outer housing subassembly. The inner housing subassembly does not need to be heat-insulating. For instance, the inner housing subassembly may be made of metal or of metal-coated plastics for achieving good electromagnetic compatibility. In particular, the inner housing subassembly, together with the components received inside it, may form a self-contained sensor module. This sensor module may then be provided with the heat exchanger 40 and may be received in the outer housing subassembly for the purpose of thermal insulation. In particular, the sensor module may be an existing, commercially available standard sensor module, being “retrofitted” with the heat exchanger 40 and with thermal insulation to better deal with fog.
The sensor device 10 may optionally comprise at least one auxiliary sensor 14, e.g., a temperature sensor, a humidity sensor or a flow sensor. While in
The control device 50 may optionally detect fog in the sensor gas flow F by the following method: The control device 50 may vary the flow rate of the sensor gas flow F by varying the power supplied to the fan 13. When fog is present, the PM concentration CS is expected to depend more strongly on the flow rate than when fog is absent. The main reasons are that at higher flow rates, the droplets spend less time in the heat exchanger, and the waste heat is carried away by a larger volume of gas per unit of time, causing the temperature difference between the inlet gas flow and the outlet gas flow to decrease. Therefore, less droplets will be evaporated at higher flow rates than at lower flow rates. The response of the PM concentration CS to the variation of flow rate thus enables the presence and, optionally, concentration of fog to be detected. The control device 50 may thus derive a droplet indicator signal (or, equivalently, fog indicator signal) Sfog, which indicates the presence of fog. In simple embodiments, this may be done in a binary manner, e.g., by comparing the dependence of the PM concentration on the flow rate to a reference dependence and setting a fog indicator signal to TRUE if the measured dependence deviates from the reference dependence by more than a predefined threshold. A reference dependence may be readily determined by appropriate calibration procedures.
While in many applications it may be sufficient to provide a binary indicator whether or not fog is present at all, in more sophisticated embodiments, the droplet indicator signal may include information about the characteristics of the fog, in particular, on droplet concentration. By subtracting the thus determined droplet concentration from the PM concentration, the control device may derive a compensated particulate matter signal SPM that characterizes the solid particulate matter in the sensor gas flow, excluding evaporable droplets.
The sensor device 10 of
The fans 13, 23 generate a gas flow through each sensor module 11, 21 and thus through the sensor device 10 from the inlet 41 to the outlet 43. The gas flow forms an inlet gas flow Fin in the inlet tubing, a first sensor gas flow F1 at the first PM sensor 12, a second sensor gas flow F2 at the second PM sensor 22, and an outlet gas flow Fout in the outlet tubing 44. Since no gas is added or removed anywhere, the inlet gas flow Fin, the first and second sensor gas flows F1 and F2, and the outlet gas flows Fout are the same. However, as explained in connection with the first embodiment, it is also conceivable that gas is added or removed somewhere between the inlet 41 and the outlet 43, e.g., due to leakage. Importantly, however, the second sensor gas flow F2 comprises at least a portion of the first sensor gas flow F1.
In contrast to the first embodiment, no heat exchanger is formed by the inlet tubing 42 and the outlet tubing 44. Instead, the second sensor gas flow F2 is preheated by waste heat generated in the first sensor module 11, in particular, by the first PM sensor 12, the fan 13 and any other electrically operated components of the first sensor module 11. Thereby, water droplets are evaporated on the way between the first PM sensor 12 and the second PM sensor 22. Again, no dedicated source of additional heating power is required, making the design suitable for applications having low energy requirements.
The control device 50 derives, inter alia, first and second PM concentrations CS1 and CS2 from the signals recorded by the first and second PM sensors 12, 22, respectively, and compares these concentrations. If no fog is present, the concentrations CS1 and CS2 are expected to be essentially the same, unless some of the PM in the gas flow (typically some of the larger particles) precipitates somewhere between the first and second PM sensors 12, 22. The resulting “baseline difference” between the concentrations CS1 and CS2 in the absence of fog may easily be measured by carrying out, from time to time, a baseline calibration measurement under conditions for which it is known or assumed that no fog is present. For instance, the control device may trigger an automatic baseline calibration measurement based on predefined criteria, e.g., based on the time of operation since the last baseline calibration measurement and based on temperature and/or humidity of the sensor gas flows, assuming that no fog will be present if the gas flow is warm and dry.
For the following considerations, it is assumed without loss of generality that the sensor device 10 is calibrated in such a manner that it obtains the same PM concentration values CS1 and CS2 in the absence of fog, and that therefore any difference between the first and second PM concentration values CS1 and CS2 indicates the presence of fog.
The control device 50 may calculate a droplet or fog indicator signal Sfog by comparing the first and second PM concentration signals:
S
fog
=F
fog(CS1,CS2).
Here, Ffog indicates a function of two variables that links the first and second PM concentration values CS1 and CS2 to a concentration of evaporable droplets. Various functional forms are conceivable for the function Ffog. To a good first approximation, a linear function may be used:
S
fog=α(CS1−CS2).
Here, a is a constant of proportionality, which may be determined by calibration measurements at one or more known droplet concentrations.
The control device 50 may further calculate a compensated particulate matter signal SPM based on the first and second PM concentration signals:
S
PM
=F
PM(CS1,CS2).
Here, FPM indicates a function of two variables that links the first and second PM concentration signals CS1 and CS2 to a concentration of PM, excluding evaporable droplets. Again, various functional forms are conceivable. A particularly simple functional form is the following linear form:
S
PM
=C
S2−β(CS1−CS2),
where β is again a constant of proportionality, which may be determined by calibration measurements at one or more known droplet concentrations. Another possible functional form is the following exponential form:
S
PM
=C
S1
e
k|C
−C
|
+C
S2(1−ek|C
where the parameter k may again be determined by calibration measurements at one or more known droplet concentrations.
As in the first embodiment, auxiliary sensors 14, 24 may be present, and the control device 50 may take their readings into account when analyzing the first and second PM signals, in particular, when deriving the fog indicator signal and/or the compensated PM signal.
While in
The control device 50 may vary the heating power of the heating element 60 to determine the droplet concentration more reliably. The underlying principle is illustrated in
A similar analysis may also be carried out by varying the flow rate through the sensor device, even in the absence of an additional heating element. At higher flow rates, the droplets have less time to evaporate than at lower flow rates, and the waste heat of the first PM sensor 12 is carried away by a larger volume of gas per unit of time, causing the gas flow to be heated to a lower temperature than at lower flow rates. Accordingly, the waste heat is able to evaporate less droplet volume than at lower flow rates. The control device 50 may vary the power supplied to fan 33 and monitor the difference signal S between the first and second PM concentration signals CS1 and CS2. Below a threshold flow rate, the difference signal will remain essentially constant if the fog is not too thick. The threshold flow rate may again be correlated with the total mass concentration of evaporable droplets in the flow, and the difference signal at flow rates below the threshold flow rate may be correlated with the total number concentration of evaporable droplets.
A similar analysis may even be carried out if only one single PM sensor is present and a heating element is provided upstream of that single PM sensor. For instance, in
In order to increase the efficiency of heat exchange with the circuit board 70 and/or the heating element 60 with the gas flow F, one or more heat sinks, which have an increased surface area as it is well known in the art, may be connected to the circuit board 70 and/or to the heating element 60 such that the gas flow F is in contact with the heat sinks instead of the circuit board or heating element itself.
The sensor device 10 exchanges gas with the duct gas flow FD through one or more openings in a duct wall 91. To this end, the sensor device 10 comprises inlet tubing 42 forming an inlet 41 for allowing an inlet gas flow Fin to enter the sensor device 10 from the duct 90 and outlet tubing 44 forming an outlet 43 for allowing an outlet gas flow Fout to exit the sensor device 10 into the duct 90. The inlet gas flow Fin is thus sampled from the duct gas flow FD, and all or part of the sampled gas is returned to the duct gas flow FD as outlet gas flow Fout. In order minimize flow resistance and disturbances in the duct 90, the inlet tubing 42 and outlet tubing 44 of the sensor device 10 do not appreciably protrude into the interior of duct 90.
Particulate matter may be entrained in the duct gas flow FD, including relatively large particles such as pollen, fragments of leaves, insect wings, hairs etc. Some of these large particles may slowly move along the duct wall 91, as symbolized by a wall flow FW. In order to prevent such slowly moving, large particles from entering the sensor device 10, the inlet tubing 42 and the outlet tubing 44 are arranged in such a manner that the outlet gas flow Fout has at least one portion that flows into the duct upstream of the inlet 41 with respect to the duct gas flow FD, this portion of the outlet gas flow Fout having an appreciable component perpendicular to the duct gas flow, thus forming a flow barrier (or gas barrier) that redirects large, slowly moving particles away from the inlet 41. In this manner, large particles near the duct wall 91 are less likely to enter the sensor device 10 through the inlet 41. Specifically, in the embodiment of
The flow barrier not only prevents at least some of the large particles near the duct wall from entering the sensor device 10, but it also reduces short-term variations (“noise”) of the output signals of the sensor device due to fluctuations of a property associated with the gas flow that is measured by the sensor device, e.g., fluctuations of the PM load. This is so because an arrangement of the inlet and outlet that leads to the formation of a flow barrier may also create a partial short circuit between the outlet gas flow and the inlet gas flow, causing part of the outlet gas flow to be recycled into the inlet gas flow. This partial short circuit has similar effects as calculating a moving average for the environmental parameter determined by the sensor device, e.g., for the PM concentration. This is in turn equivalent to applying a low-pass filter to the output signals of the sensor device. By causing a partial short circuit between the outlet gas flow and the inlet gas flow, noise is reduced, and the requirements for filtering during signal processing may thus be relaxed.
Yet another advantage of the proposed arrangement of the inlet and outlet is that the pressure difference between the inlet and the outlet in the presence of the duct flow is reduced. As a consequence, the influence of the flow rate of the duct flow on the flow rate through the sensor device is reduced, enabling these flow rates to be regulated separately.
In advantageous embodiments, the inlet tubing 42 and the outlet tubing 44 may form a heat exchanger 40, as explained in more detail in connection with the first, fifth and sixth embodiments.
As illustrated in
In the embodiment of
In the embodiment of
In the embodiments of
In the simulation that resulted in
Further simulations show that it is advantageous to keep the flow velocity of the outlet gas flow in a similar range as the flow velocity of the duct gas flow near the duct wall. It may therefore be advantageous to regulate the flow velocity of the outlet gas flow to correlate with the flow velocity of the duct gas flow. To this end, the gas flow system may comprise a duct gas flow sensor 92 (see
An exemplary method of operation is illustrated in
As will have become apparent by the present description, various modifications are possible without leaving the scope of the present invention, and it is to be understood that the invention is not limited to the specific embodiments described herein.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/EP2021/059323 | 4/9/2021 | WO |