EP3915645A1

EP3915645A1 - A pollution mask and control method

Info

Publication number: EP3915645A1
Application number: EP20176217.6A
Authority: EP
Inventors: Tao Kong; Weizhong Chen
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2020-05-25
Filing date: 2020-05-25
Publication date: 2021-12-01

Abstract

A pollution mask has a fan for drawing air from outside the mask chamber into the air chamber and/or drawing air from inside the mask chamber to the outside. The mask has an auto mode for automatic fan speed control and also manually selectable fan rotation speeds. A depth of breathing and a rate of breathing of the user are monitored. Based on monitoring during time periods with manually selected fan speeds, a fan speed strategy is derived for use in the auto mode. This provides a personalized fan speed setting based on a self-learning algorithm.

Description

FIELD OF THE INVENTION

This invention relates to a pollution mask, for providing filtered air to the wearer of the mask, with the flow assisted by a fan.

BACKGROUND OF THE INVENTION

The World Health Organization (WHO) estimates that 4 million people die from air pollution every year. Part of this problem is the outdoor air quality in cities. The worst in class are Indian cities like Delhi that have an annual pollution level more than 10 times the recommended level. Well known is Beijing with an annual average 8.5 times the recommended safe levels. However, even in European cities like London, Paris and Berlin, the levels are higher than recommended by the WHO.
Since this problem will not improve significantly on a short time scale, the only way to deal with this problem is to wear a mask which provides cleaner air by filtration. To improve comfort and effectiveness one or two fans can be added to the mask. These fans are switched on during use and are typically used at a constant voltage. For efficiency and longevity reasons these are normally electrically commutated brushless DC fans.
A possible benefit to the wearer of using a powered mask is that the lungs can be relieved of the slight strain caused by inhalation against the resistance of the filters in a conventional non-powered mask.
Furthermore, in a conventional non-powered mask, inhalation also causes a slight negative pressure within the mask which leads to leakage of the contaminants into the mask, which leakage could prove dangerous if these are toxic substances.
A powered mask which uses an inhaling fan delivers a steady stream of air to the face and may for example provide a slight positive pressure, which may be determined by the resistance of an exhale valve, to ensure that any leakage is outward rather than inward.
There are several advantages if the fan operation or speed is regulated. This can be used to improve comfort by more appropriate ventilation during the inhalation and exhalation sequence or it can be used to improve the electrical efficiency. The latter translates into longer battery life or increased ventilation. Both of these aspects need improvement in current designs.
To regulate the fan speed, the pressure inside the mask can be measured and both pressure as well as pressure variation can be used to control the fan.
For example, the pressure inside a mask can be measured by a pressure sensor and the fan speed can be varied in dependence on the sensor measurements. A pressure sensor is costly so it would be desirable to provide an alternative method of monitoring pressure inside a mask.
The applicant has proposed a solution in which a rotation speed of the fan is used as a proxy for pressure measurement. A pressure or a pressure change is determined based on the rotation speed of the fan. Using this pressure information, the breathing pattern of the user can be tracked it can also be determined whether the mask is worn or not. This approach is described in WO 2018/215225 .
Current active masks commonly provide several fan speed levels for users and let them select their preferred fan speed settings. However, this is not a very convenient user experience.
First, if multiple fan speed settings are provided to users, it is highly likely that many users don't know which air flows exactly fit their breathing needs under different activities. Second, during use, users need to pay extra attention to their breathing comfort and keep pressing buttons to locate the proper air flow settings. People change their breathing patterns from time to time, for example during the commute there may be slow or fast waking on the road, sitting in the bus/subway or bicycling from the subway station to work.
The manual control function will bring an extra burden to users. Users may typically just choose one largest air flow to meet all activities. Additionally, if the user interface of the fan module is not well designed, users may find it difficult to locate and press the buttons.
The applicant has also proposed, but not yet published (in European patent application number 19150499.2 ), an approach by which a fan speed is set in dependence on a first value representing the depth of breathing and second value representing the rate of breathing, as well as based on the ambient temperature. The fan speed is set to a selected one of a plurality of fan speeds.
This approach provides automatic adjustment of the fan speed based on monitoring the fan rotation speed (to determine the user's breathing pattern) and the external temperature. The fan speed adjustment is able to provide different air flows under different environmental conditions (temperature) and for different levels of activity of the user.
This approach considers both breathing rates and depth for the development of a control algorithm. The control algorithm is then pre-embedded into the main control unit (MCU) of fan module. The algorithm is generic rather than related to the individual users' own preferences.
A personalized fan adaption approach would be desired. There is therefore a need for a fan speed control method and apparatus which can be implemented at low cost and which can provide fan speed control which takes account of a user's individual characteristics and requirements.

SUMMARY OF THE INVENTION

The invention is defined by the claims.
According to examples in accordance with an aspect of the invention, there is provided a pollution mask comprising:

an air chamber;
a filter which forms a boundary between the air chamber and the ambient surroundings outside the air chamber;
a fan for drawing air from outside the air chamber into the air chamber and/or drawing air from inside the air chamber to the outside;
a user interface for allowing a user to manually select fan rotation speeds, the pollution mask further having an auto mode for automatic fan speed control;
a means for determining a first value which relates to a depth of breathing and a second value which relates to a rate of breathing;
a controller which is adapted to:
- monitor the first and second values during time periods with manually selected fan speeds; and
- derive, from the monitoring during the time periods with manually selected fan speeds, a fan speed strategy for use in the auto mode which relates the derived first and second values to a fan speed setting.

In order to achieve personalized air flow delivery, a process is provided for monitoring users' manually selected settings, then provide an optimized auto adaption control algorithm for users. The optimization of the algorithm is thus based on a self-learning procedure.
The first and second values enable user activities to be taken into account. The first value for example relates to a depth of breathing, and by this is meant there is a positive correlation between the first value and the depth of breathing. More generally, the first value may for example relate to (i.e. correlate with) a magnitude of a pressure fluctuation across the fan. The pressure fluctuations are caused by breathing when the mask is worn and in normal use. The second value has a positive correlation with the rate of breathing. This may be used as another indicator of the activity level of the user, in addition to the depth of breathing as indicated by the first value.
The invention relates to a pollution mask.
A first set of examples relates to a device which has the primary purpose of filtering ambient air to be breathed by the user. The mask does not then perform any form of patient treatment. In particular, the pressure levels and flows resulting from the fan operation are intended solely to assist in providing comfort (by influencing the temperature or relative humidity in the air chamber) and/or to assist in providing a flow across a filter without requiring significant additional breathing effort by the user. The mask does not provide overall breathing assistance compared to a condition in which the user does not wear the mask.
A second set of examples relates to a medical mask which may perform other functions such as breathing assistance as well as pollution filtering.
The user interface for example also allows the use to select the auto mode. However, the mask may instead use the auto mode as a default setting, and it may revert to the auto mode automatically some time after a user has input a manual fan speed setting.
Various sensors may be used for determining the breathing characteristics. However, in one example, the means for determining first and second values is for example adapted to monitor a rotation speed of the fan and the controller is adapted to derive the first and second values from the determined fan rotation speed or change in fan rotation speed.
In this version, fan speed monitoring is used instead of having pressure measurement. To measure the fan speed, the fan itself may be used so that no additional sensors are required. The chamber may be closed in normal use, so that pressure fluctuations in the chamber have an influence on the load conditions of the fan and hence alter the fan electrical characteristics. Similarly, the fan electrical characteristics may determine the nature of the chamber, for example its volume, and if it is an open or closed volume.
The first value is for example based on a maximum swing in fan rotation speed during a sampling window. This swing is representative of the degree of pressure fluctuation, and hence it relates to the depth of breathing. The second value is for example a frequency based on the time between a consecutive maxima and minima in the fan rotation speed. This time period corresponds to half the breathing period, hence a frequency directly derived from this value corresponds to double the breathing rate (i.e. frequency).
The sampling window is chosen to be sufficient to capture at least one full breathing cycle, for example 6 seconds to capture a full breathing cycle at the lowest breathing rate of 10 breaths per minute.
The means for determining first and second values may instead comprise a differential pressure sensor. A stress sensor may also be used.
The filter for example forms a boundary directly between the air chamber and the ambient surroundings outside the air chamber. This provides a compact arrangement which avoids the need for flow transport passageways. It means the user is able to breathe in through the filter. The filter may have multiple layers. For example, an outer layer may form the body of the mask (for example a fabric layer), and an inner layer may be for removing finer pollutants. The inner layer may then be removable for cleaning or replacement, but both layers may together be considered to constitute the filter, in that air is able to pass through the structure and the structure performs a filtering function.
The filter thus preferably comprises an outer wall of the air chamber and optionally one or more further filter layers. This provides a particularly compact arrangement and enables a large filter area, because the mask body performs the filtering function. The ambient air is thus provided directly to the user, when the user breathes in, through the filter.
For an exhaust fan, the pressure may be positive or negative (compared to the ambient pressure), for example if the fan is off there will be negative pressure caused by inhalation and a positive pressure caused by exhalation. If an exhaust fan is on, negative or positive pressures are possible during breathing depending on the fan speed and breathing characteristics. A negative pressure will result during periods of no breathing.
If the fan is for providing an increased pressure in the air chamber (e.g. a flow into the air chamber during inhalation), it is only required to provide a small increased pressure, for example for assisting inhalation of the user.
In all cases, a maximum pressure in the air chamber in use is for example below 4 cmH₂O, for example below 2 cmH₂O, for example below 1 cmH₂O, higher than the pressure outside the air chamber.
The controller is for example adapted to perform the monitoring during an initial period of usage of the pollution mask during which the fan rotation speed is manually selected. Thus, the user can manually select the most appropriate fan speed setting during an initial period, from which the mask will learn the user's personal preferences. The auto mode may be disabled during this initial calibration and learning period, or else the user can be instructed to make manual selections whenever the fan speed (set by a default auto mode) is not considered appropriate. In the latter case, a default relationship is used, and the user may choose to maintain the default setting if it suits them.
The controller is preferably further adapted to update the speed strategy over time after the initial period of usage based on periods when the rotation speed is manually selected. Thus, the user can always make a correction to the fan speed setting, and the pollution mask will learn from the user inputs.
The controller may be adapted, for each fan speed setting, to:

group the first and second values and a third value representing the time during which the fan speed setting was selected with those first and second values present;
derive a measure of the distribution of the first value in that fan speed setting;
derive a measure of the distribution of the second value in that fan speed setting; and
from the measures of distribution, deriving a mapping between the first and second values and the fan speed level to be applied in the auto mode.

The mapping thus takes account of the range of breathing depths and range of breathing rates (i.e. the first and second values) which are suitable for each fan speed setting for that particular user.
The mask may further comprise a temperature sensor for measuring an ambient temperature outside the air chamber, wherein the controller is adapted to set a fan speed further in dependence on the temperature level. Different fan speed settings may be appropriate at different temperatures. Taking fast walking for example, users may need a large air flow to help to manage the climate in the mask during the summer, whereas relatively less air flow is needed in the winter in order to avoid making the wearer cold. The use of ambient temperature measurement, i.e. outside the mask chamber, avoids water condensation issues.
The mask may further comprise a humidity sensor for measuring an ambient humidity level outside the air chamber, wherein the controller is adapted to set a fan speed further in dependence on the humidity level. Thus, both the external ambient temperature and humidity may be taken into account. This provides increased accuracy in the determination of the environmental conditions.
For example, the controller may be adapted to:
derive from the ambient temperature and the ambient humidity level a heat index value which relates to a measure of comfort.
This heat index value is representative of the general ambient environmental conditions and can be used so that the fan speed is set taking into account the expected user comfort in those particular conditions.
The fan may be driven by an electronically commutated brushless motor, and the means for determining rotation speed comprises an internal sensor of the motor. Alternatively, the means for determining rotation speed may comprise a circuit for detecting a ripple on the electrical supply to a motor which drives the fan.
An internal sensor is already provided in such motors to enable rotation of the motor. The motor may even have an output port on which the internal sensor output is provided. Thus, there is a port which carries a signal suitable for determining the rotation speed. Alternatively, a ripple is detected which results from switching current through the motor coils, which cause induced changes in the supply voltage as a result of the finite impedance of the input voltage source.
The fan may be a two-wire fan and the circuit for detecting a ripple comprises a high pass filter. The additional circuitry needed for a motor which does not already have a suitable fan speed output can be kept to a minimum.
The controller may be adapted to:

determine a respiration cycle (e.g. from the fan rotation speed or change in fan rotation speed); and
to control an outlet valve in dependence on the phase of the respiration cycle and/or to turn off the fan during an inhalation time.

The determination of the respiration phase, for example again using the fan rotation monitoring, may be used to control the timing of a venting valve of the mask or to determine whether or not the mask is worn and hence in use.
The controller may be adapted to turn off the fan during an inhalation time. This may be used to save power. Shutting down the fan during inhalation may be desirable for a user who does not have difficulty breathing through the filter, to save power if configured in such a way.
The fan may be only for drawing air from inside the air chamber to the outside. In this way, it may at the same promote a supply of fresh filtered air to the air chamber even during exhalation, which improves user comfort. In this case, the pressure in the air chamber may be below the outside (atmospheric) pressure at all times so that fresh air is always supplied to the face.
The outlet valve may comprise a passive pressure-regulated check valve or an actively driven electrically controllable valve. This may be used to make the mask more comfortable. During inhalation, by closing the valve (actively or passively), it is prevented that unfiltered air is drawn in. During exhalation, the valve is opened so that breathed out air is expelled.
The invention also provides a non-therapeutic method of controlling a pollution mask, the method comprising:

drawing gas into and/or out of an air chamber of the mask using a fan which forms a boundary between the air chamber and the ambient surroundings outside the air chamber;
deriving a first value which relates to a depth of breathing and a second value which relates to a rate of breathing;
monitoring the first and second values during time periods with manually selected fan speeds; and
deriving from the monitoring during the time periods with manually selected fan speeds a fan speed strategy for use in an auto mode for automatic fan speed control, which relates the derived first and second values to a fan speed setting.

The pollution mask is not a mask for delivering therapy to a patient.
The method provides self learning of the most appropriate mapping between the first and second values (representing breathing depth and breathing rate) and the fan speed setting for a particular user.
The method may comprise performing the monitoring during an initial period of usage of the pollution mask during which the fan rotation speed is manually selected. The speed strategy may be updated over time after the initial period of usage based on periods when the rotation speed is manually selected.
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

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

Figure 1 shows a pressure monitoring system implemented as part of a face mask;
Figure 2 shows one example of the components of the pressure monitoring system;
Figure 3 shows an example of a combined temperature and humidity sensor;
Figure 4A shows a rotation signal during inhalation and during exhalation and Figure 4B shows how a fan rotation speed varies over time; and
Figure 5 shows a circuit for controlling the current through one of the stators of a brushless DC motor;
Figure 6 shows a fan operating method in accordance with the invention;
Figure 7 shows an example of how to achieve the self-learning auto adaption process;
Figure 8 shows in the top image the fan rotation signal and in bottom image how the breathing rate and breathing depth are determined from the fan rotation signal;
Figure 9 shows the collection of data into different usage patterns each for a different fan speed setting;
Figure 10 shows a change caused by optimization from a pre-set algorithm (left image) to a personalized algorithm (right image); and
Figure 11 shows a chart of relative humidity (%RH) versus temperature (degrees Celsius).

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described with reference to the Figures.
It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.
The invention provides a pollution mask which has a fan for drawing air from outside the mask chamber into the air chamber and/or drawing air from inside the mask chamber to the outside. The mask has an auto mode for automatic fan speed control and also manually selectable fan rotation speeds. A depth of breathing and a rate of breathing of the user are monitored. Based on monitoring during time periods with manually selected fan speeds, a fan speed strategy is derived for use in the auto mode. This provides a personalized fan speed setting based on a self-learning algorithm.
Figure 1 shows a face mask with automatic fan speed control.
A subject 10 is shown wearing a face mask 12 which covers the nose and mouth of the subject. The purpose of the mask is to filter air before it is breathed in the subject. For this purpose, the mask body itself acts as an air filter 16. Air is drawn in to an air chamber 18 formed by the mask by inhalation. During inhalation, an outlet valve 22 such as a check valve is closed due to the low pressure in the air chamber 18.
Optionally, a sensing arrangement 24 is provided for measuring the ambient temperature or even both the ambient temperature and humidity (e.g. relative or absolute humidity) outside the mask chamber 18.
The filter 16 may be formed only by the body of the mask, or else there may be multiple layers. For example, the mask body may comprise an external cover formed from a porous textile material, which functions as a pre-filter. Inside the external cover, a finer filter layer is reversibly attached to the external cover. The finer filter layer may then be removed for cleaning and replacement, whereas the external cover may for example be cleaned by wiping. The external cover also performs a filtering function, for example protecting the finer filter from large debris (e.g. mud), whereas the finer filter performs the filtering of fine particulate matter. There may be more than two layers. Together, the multiple layers function as the overall filter of the mask.
When the subject breathes out, air is exhausted through the outlet valve 22. This valve is opened to enable easy exhalation, but is closed during inhalation. A fan 20 assists in the removal of air through the outlet valve 22. Preferably, more air is removed than exhaled so that additional air is supplied to the face. This increases comfort due to lowering relative humidity and cooling. During inhalation, by closing the valve, it is prevented that unfiltered air is drawn in. The timing of the outlet valve 22 is thus dependent on the breathing cycle of the subject. The outlet valve may be a simple passive check valve operated by the pressure difference across the filter 16. However, it may instead be an electronically controlled valve.
There will be a varied pressure inside the chamber if the mask is worn and the user is breathing. In particular the chamber is closed by the face of the user. The pressure inside the closed chamber when the mask is worn will also vary as a function of the breathing cycle of the subject. When the subject breathes out, there will be a slight pressure increase and when the subject breathes in there will be a slight pressure reduction.
If the fan is driven with a constant drive level (i.e. voltage), the different prevailing pressure will manifest itself as a different load to the fan, since there is a different pressure drop across the fan. This altered load will then result in a different fan speed. The rotation speed of the fan may thus be used as a proxy for a measurement of pressure across the fan.
For a known pressure (e.g. atmospheric pressure) at one side of the fan, the pressure monitoring enables determination of a pressure, or at least a pressure change, on the other side of the fan. This other side is for example a closed chamber which thus has a pressure different to atmospheric pressure.
The pressure variation, as detected based on monitoring the fan rotation speed, may be used to obtain information about the breathing of the user. In particular, a first value may represent the depth of breathing and a second value may represent the rate of breathing. The first and second values, and optionally as well as the ambient temperature and/or the ambient humidity level, are used to set the fan speed. Furthermore, by detecting an equal pressure on each side the fan (or other conditions relating to the first and second values), it can also be determined that the chamber is not closed but is connected to atmospheric pressure on both sides.
This will result in a variation of the fan speed which falls below a threshold. This situation may thus be used to determine that the mask is not worn and hence not in use. This information can be used to switch off the fan to save power.
The means for determining a rotation speed may comprise an already existing output signal from the fan motor or a separate simple sensing circuit may be provided as an additional part of the fan. However, in either case the fan itself is used so that no additional sensors are required.
Figure 2 shows one example of the components of the system. The same components as in Figure 1 are given the same reference numbers. The optional sensing arrangement is shown as a separate temperature sensor 24a and humidity sensor 24b.
In addition to the components shown in Figure 1, Figure 2 shows a controller 30, a local battery 32 and a means 36 for determining the fan rotation speed.
The fan 20 comprises a fan blade 20a and a fan motor 20b. In one example, the fan motor 20b is an electronically commutated brushless motor, and the means for determining rotation speed comprises an internal sensor of the motor. Electronically commutated brushless DC fans have internal sensors that measure the position of the rotor and switch the current through the coils in such a way that the rotor rotates. The internal sensor is thus already provided in such motors to enable feedback control of the motor speed.
The motor may have an output port on which the internal sensor output 34 is provided. Thus, there is a port which carries a signal suitable for determining the rotation speed.
Alternatively, the means for determining the rotation speed may comprise a circuit 36 for detecting a ripple on the electrical supply to the motor 20b. The ripple results from switching current through the motor coils, which cause induced changes in the supply voltage as a result of the finite impedance on the battery 32. The circuit 36 for example comprises a high pass filter so that only the signals in the frequency band of the fan rotation are processed. This provides an extremely simple additional circuit, and of much lower cost than a conventional pressure sensor.
This means the motor can be of any design, including a two-wire fan with no in-built sensor output terminal. It will also work with a DC motor with brushes.
If the outlet valve 22 is an electronically switched value, the respiration cycle timing information may then be used to control the outlet valve 22 in dependence on the phase of the respiration cycle. The fan speed monitoring thus provides a simple way to determine inhalation phases, which may then be used to control the timing of the outlet valve 22 of the mask.
In addition to controlling the outlet valve, the controller may turn off the fan during an inhalation time or an exhalation time. This gives the mask different operating modes, which may be used to save power.
For a given drive level (i.e. voltage) the fan speed increases at lower pressure across the fan because of the reduced load on the fan blades. This gives rise to an increased flow. Thus, there is an inverse relationship between the fan speed and the pressure difference.
This inverse relationship may be obtained during a calibration process or it may be provided by the fan manufacturer. The calibration process for example involves analyzing the fan speed information over a period during which the subject is instructed to inhale and exhale regularly with normal breathing. The captured fan speed information can then be matched to the breathing cycle, from which threshold values can then be set for discriminating between inhalation and exhalation.
Figure 3 shows one possible design of a module incorporating the fan, check valve and the sensing arrangement 24. The module comprises a printed circuit board 34 which carries the battery 32 and sensor arrangement 24 on one side, and the fan 20 and valve 22 is mounted on the other side. A cover 36 is provided over the top side. The sensing arrangement faces outside the mask.
Figure 4A shows schematically the rotor position (as a measured sensor voltage) against time.
The rotational speed may be measured from the frequency of the AC component (caused by the switching events in the motor) of the DC voltage to the fan. This AC component originates from the current variation that the fan draws, imposed on the impedance of the power supply.
Figure 4A shows the signal during inhalation as plot 40 and during exhalation as plot 42. There is a frequency reduction during exhalation caused by an increased load on the fan by the increased pressure gradient. The observed frequency changes thus results from the different fan performance during the breathing cycle.
Figure 4B shows the frequency variation over time, by plotting the fan rotation speed versus time. There is a maximum difference in fan rotation speed Δfan between successive maxima and minima, and this correlates with the depth of breathing. This is the first value derived from the fan rotation signal. The time between these points is used to derive the second value, for example the frequency corresponding to this time period (which is then twice the breathing rate).
Note that the first value may be obtained from the raw fan rotation signal or there may be smoothing carried out first. Thus, there are at least two different two ways to calculate the maximum swing, based on untreated real-time speeds or treated speeds. In practice, there is noise or other fluctuations added on the real-time signals. A smoothing algorithm may be used to treat the real-time signal and calculate the first value from the smoothed signal.
During the exhalation, fan operation forces air out of the area between face and mask. This enhances comfort because exhalation is made easier. It can also draw additional air onto the face which lowers the temperature and relative humidity. Between inhalation and exhalation, the fan operation increases comfort because fresh air is sucked into the space between the face and the mask thereby cooling that space.
During inhalation, the outlet valve is closed (either actively or passively) and the fan can be switched off to save power. This provides a mode of operation which is based on detecting the respiration cycle.
The precise timing of the inhalation and exhalation phases can be inferred from previous respiration cycles, if the fan is turned off for parts of the respiration cycle, and hence not giving pressure information.
For the fan assisted exhalation, power needs to be restored just before the exit valve opens again. This also makes sure that the next inhale-exhale cycle remains properly timed and sufficient pressure and flow are made available.
Around 30% power savings are easily achievable using this approach, resulting in prolonged battery life. Alternatively, the power to the fan can be increased by 30% for enhanced effectiveness.
With different fan and valve configurations the measurement of the fan rotation speed enables control to achieve increased comfort.
As mentioned above, a fan using an electronically commutated brushless DC motor has internal sensors that measure the position of the rotor and switch the current through the coils in such a way that the rotor rotates.
Figure 5 shows an H-bridge circuit which functions as an inverter to generate an alternating voltage to the stator coils 50 of the motor from a DC supply VDD, GND. The inverter has a set of switches S1 to S4 to generate an alternating voltage across the coil 50. The switches are controlled by signals which depend on the rotor position, and these rotor position signals may be used to monitor the fan rotation.
To the extent described above, the mask system has already been proposed, but not yet published, by the applicant. The fan speed is controlled by increasing the fan speed, decreasing the fan speed, or keeping the fan speed the same, in response to analysis of the breathing rate and breathing depth over time as well as temperature and humidity information.
As outlined above, the invention provides a fan speed adjustment approach which is tailored to a user's preferences, and which evolves over time, rather than following a pre-set algorithm.
Figure 6 shows a fan operating method in accordance with the invention, which may be applied to the mask design described above.
In step 60, the fan is off.
The user interface allows the user to select an auto mode or to select their own fan speed setting. For example, the user interface may have a single button for making the selection, and the system toggles between values, as shown by the cyclic settings in Figure 6.
As shown, the mode 62 is the auto mode. The next three modes 64,66,68 are the fan settings for levels L1, L2 and L3. Thus, the user may select a desired fan speed setting by pressing the button multiple times. Of course, the use of three manual fan speed settings is merely an example.
The auto mode 62 realizes an auto adaption feature, by which the fan speed is adapted according to a personalized preference of the user, based on a self-learning approach.
The manual modes may be used for training of the auto mode, but also to cater for specific non-breathing comfort related factors, for example to decrease noise in subway or train, users may want to switch to Level 1, although Level 1 is not a proper setting for the best breathing comfort.
The controller continuously receives and processes the fan signal data in order to implement the self-learning and optimization processes. The invention may be implemented using a conventional fan module and controller, so the improvement may be implemented by software or firmware.
One example of how to achieve the self-learning auto adaption process is described below with reference to Figure 7.
This process may for example be performed during initial use of the pollution mask. During this time, there may be no auto mode, so the user must select the fan speed setting they find most comfortable. This gives a rapid learning process. It is instead possible to perform the learning only during normal use, by starting with a default algorithm and updating it over time.
In step 70, the fan is turned on.
In step 72, a data collection process takes place. The controller collects the fan rotation signals in real time and calculates the breathing rate and breathing depth based on the fan signals.
In particular, as explained above, a first value, representing the depth of breathing, and a second value, representing the depth of breathing, are continuously monitored. The normal breathing rate for an adult at rest is 12-18 bpm (breath per minute), and it increases when people are performing activities, e.g., running, fast walking or bicycling. The sampling window should contain at least 1 breathing cycle, otherwise the breathing rate cannot be acquired.
The first value, representing the depth of breathing, may be based on the maximum swing in rotation speed captured within a sampling window (of at least one breath, for example 5 seconds) as shown in Figure 4B: $Δfan = Fan (\max) - Fan (\min)$
The second value, representing the rate of breathing, may be given by the frequency corresponding to the timing difference for those maximum and minimum values: $f = 1 / |t_{Fan (\max)} - t_{Fan (\min)}|$
The second value is thus a frequency based on the time between a consecutive maxima and minima in the fan rotation speed. Again, the second value may be obtained from the raw fan rotation signal or there may be smoothing carried out first.
A breathing rate may be defined as 0.5f. For the calculation of f, another method is to identify the inflection points from descending (ascending) to ascending (descending).
In step 74, a dataset is generated representing the usage patterns, containing the above acquired breathing patterns, for the fan speed settings (Level 1, Level 2 or Level 3) and also recording the usage time of each of those fan speed settings (e.g., 3 min for Level 1, 5 min for Level 2 or 10 min for Level 3).
In step 76, the control algorithm is optimized. Thus, the fan control algorithm is based on the personal usage patterns. As more data are collected, the algorithm will evolve gradually.
In step 78 it is detected if the fan is switch has been switched to off. If so, the fan is switched off in step 80, otherwise the collection and processing of data continues.
Figure 8 shows shows in the top image the fan rotation signal 80 (left y-axis, rpm) for different breathing volumes (liters per minute, slm). The differential pressure 82 is also shown (right y-axis, Pa).
The bottom image shows how the breathing rate and breathing depth are determined from the fan rotation signal.

The acquired breathing pattern includes the breathing rate, the breathing depth and usage time.

Table 1 below shows raw data. For each fan usage, the data relating to the breathing rate, breathing volume and duration is collected for each fan speed setting.

Usage number	Fan Level	Breathing rate	Breathing volume	Usage duration
1	1	BR11	V11	t11
		BR12	V12	t12
		...	...	...
	2	BR21	V21	t21
		BR22	V22	t22
		...	...	...
	3	BR31	V31	t31
		BR32	V32	t32
		...	...	...
2	1	BR11	V11	t11
		BR12	V12	t12
		...	...	...
	2	BR21	V21	t21
		BR22	V22	t22
		...	...	...
	3	BR31	V31	t31
		BR32	V32	t32
		...	...	...
...	...	...	...	...

Thus, each usage may be a period of time between turning the fan on and turning it off. This may for example be a period of hours. Within that period, the user may selected any of the fan speed settings (of which there are three in this example). For each fan speed setting, the breathing rate and breathing volume are recorded and the time during which each combination of breathing rate and breathing volumes occurred.
The breathing rate and breathing volume are for example binned into a set of ranges (e.g. breathing rates 1 to 5 and breathing volumes 1 to 5) to simplify the data processing.
For better visualization, the data can be categorized into usage patterns. A usage pattern is an indication of which fan speed settings are preferred by a particular user for different breathing patterns (for example corresponding to different activities).
Figure 9 shows the collection of data into different usage patterns 90, 92, 94 each for a different fan speed setting.
An algorithm optimization is based on the usage patterns. From the usage patterns, the distribution of breathing rate and breathing depth may be calculated for each fan speed setting. This distribution may then be weighted by usage time. The usage time is useful for the breathing rate and depth distribution calculation because if a user feels uncomfortable with the current fan speed setting they will change it to a more suitable fan speed setting after a short time. Thus, periods with the wrong fan speed setting will result in a lower weighting.
Taking the breathing rate distribution calculation for example, the distribution can be defined as: $Breathing rate distribution in Level x = \frac{\sum {Br}_{x} \times t_{x}}{\sum t_{x}} \pm σ_{Brx}$
In this equation:

Br_x is the breathing rates in each fan speed setting x (x=1, 2 or 3 for Level 1, Level 2 and Level 3).
t_x is the usage time in Fan speed setting x for certain breathing rate Br_x
σ_Brx is the standard deviation of Br_x

This defines a range of values of breathing rate (within one standard deviation of the weighted mean).
The breathing depth distribution follows a similar calculation. $Breathing depth distribution in Level x = \frac{\sum V_{x} \times t_{x}}{\sum t_{x}} \pm σ_{Vx}$
In this equation:

V_x is the breathing depths in each Fan speed setting x (x=1, 2 or 3 for Level 1, Level 2 and Level 3).
t_x is the usage time in Fan speed setting x for certain breathing depth V_x
σ_Vx is the standard deviation of V_x

This also defines a range of values of breathing depth (within one standard deviation of the weighted mean).
The combination of breathing rate and breathing depth distributions can be used to map which fan speed setting should be used during mask usage.
Figure 10 shows a change caused by optimization from a pre-set algorithm (left image) to a personalized algorithm (right image).
One clear advantage of the personalized algorithm is that it considers the users' own usage patterns. It is an evolving and optimized algorithm.
In practice, after the user receives the mask product, the user may be instructed to perform a calibration cycle during which they are required to wear and actively adjust the fan setting during a first period, such as the first 1 to 2 weeks. This will allow the fan module to start to record and learn the usage patterns. The more the user changes the fan speed settings during different activities, the more usage patterns are then collected. In this way a more optimized algorithm will be obtained.
After the initial calibration cycle, the user may choose when to apply manual settings, in particular when the auto mode is not creating the desired comfort. They will be motivated to do so, in the knowledge that the self learning function will then improve the personalization of the mask to their preferences.
The system may additionally monitor ambient temperature and/or humidity and further take these into account in the fan speed setting algorithm. These two measurements may for example be combined to provide a measure of a comfort level, such as a measure known as a heat index. Figure 11 shows a chart of relative humidity (%RH) versus temperature (degrees Celsius). Different regions are shaded differently to show different heat index values.
The heat index is a function of the ambient temperature (T), the square of the ambient temperature, the ambient humidity level (rh) and the square of the ambient humidity level. An example is: ${Index}_{heat} = - 42.379 + (2.04901523 \times T) + (10.14333127 \times rh) - (0.22475541 \times T \times rh) - (6.83783 \times 10^{- 3} \times T^{2}) - (5.481717 \times 10^{- 2} \times {rh}^{2}) + (1.22874 \times 10^{- 3} \times T^{2} \times {rh}^{2}) + (8.5282 \times 10^{- 4} \times T \times {rh}^{2}) - (1.99 \times 10^{- 6} \times T^{2} \times {rh}^{2})$
The heat index gives a measure of the degree of comfort under different temperature and relative humidity conditions. For example, less than 29 is categorized as no discomfort, 29 to 34.5 is categorized as acceptable, 34.5 to 39 is categorized as some discomfort, 39 to 45 is categorized as great discomfort, 45 to 54 is categorized as dangerous and over 54 corresponds to imminent heat stroke. Thus, the higher the index value, the more discomfort is felt. The heat index is applicable when T>10 °C. For T<10 °C other chilling parameters should be taken into account.
The controller may then set a default fan speed rotation based on this heat index as shown in Table 2 below: Table 2

Heat Index <29 29-34.5 34.5-39 39-45 >45

T>10 deg. 4000rpm 5000rpm 6000rpm 7000rpm 8000rpm

T<10 deg. 4000rpm
When T>10 °C, the initial fan rotation speed increases with an increase of heat index. While T<10 °C, a relatively low fan rotation is provided. The low fan rotation makes sure that user doesn't feel too cold. The rotation speeds in the table are of course just examples.
The algorithm then has more parameters than as shown above in Figure 10, and instead derives the fan speed setting from the breathing rate and depth but also the temperature and/or humidity.
The mask may be for covering only the nose and mouth (as shown in Figure 1) or it may be a full face mask. The mask is for filtering ambient air.
The mask design described above has the main air chamber formed by the filter material, through which the user breathes in air. An alternative mask design has the filter in series with the fan as also mentioned above. In this case, the fan assists the user in drawing in air through the filter, thus reducing the breathing effort for the user. An outlet valve enables breathed out air to be expelled and an inlet valve may be provided at the inlet.
The invention may again be applied for detecting the pressure variations caused by breathing for controlling the inlet valve and/or the outlet valve. The fan in this example needs to be turned on during inhalation, to assist the user in drawing air through the series filter, but it may be turned off during exhalation when the outlet valve is open. Thus, the pressure information derived may again be used to control the fan to save power when the fan operation is not needed. The detection of whether the mask is worn or not may also be implemented.
It will be seen that the invention may be applied to many different mask designs, with fan-assisted inhalation or exhalation, and with an air chamber formed by a filter membrane or with a sealed hermetic air chamber.
One option as discussed above is thus the use of the fan only for drawing air from inside the air chamber to the outside, for example when an exhaust valve is open. In such a case, the pressure inside the mask volume may be maintained by the fan below the external atmospheric pressure so that there is a net flow of clean filtered air into the mask volume during exhalation. Thus, low pressure may be caused by the fan by during exhalation and by the user during inhalation (when the fan may be turned off).
An alternative option is the use of the fan only for drawing air from the ambient surroundings to inside the air chamber. In such a case, the fan operates to increase the pressure in the air chamber, but the maximum pressure in the air chamber in use remains below 4 cmH₂O higher than the pressure outside the air chamber, in particular because no high pressure assisted breathing is intended. Thus, a low power fan may be used.
For consumer (non-medical) masks, the pressure inside the air chamber preferably remains below 2 cmH₂O, or even below 1 cmH₂O or even below 0.5 cmH₂O, above the external atmospheric pressure. The pollution mask in such as case is not for use in providing a continuous positive airway pressure, and is not a mask for delivering therapy to a patient.
In such consumer applications, the mask is preferably battery operated so the low power operation is of particular interest.
The invention may however be applied to other applications in which therapy is delivered to a patient or in which a continuous positive airway pressure is provided.
In the examples above, a first value is derived which relates to a depth of breathing based on a maximum swing in fan rotation speed during a sampling window. Other analysis of the fan rotation speed may be used. For example a maximum positive deviation from an average fan speed or a maximum negative deviation from an average fan speed may be used. The swing in fan speed may be the maximum for any one breathing cycle within a sampling window or it may be the difference between the lowest and highest fan speed observed over the full sampling window. Other statistical measures may first be taken, such as averages or moving averages, before the first value is derived. Extreme values may be filtered out, for example to exclude events such as sneezing or coughing.
Thus, there are different ways to derive the first value and more complex signal processing may be carried out than described above. However, generally there is a correlation between the amplitude of the variations in fan rotation speed and the breathing depth.
In the examples above, a second value is derived which relates to a rate of breathing based on the time between a consecutive maxima and minima in the fan rotation speed. Other analysis of the fan rotation speed may be used. For example, the time may be between consecutive maxima (so not using the timing of the minima), or between consecutive minima (so not using the time of the maxima). There may also be averaging over a time window, and other processing may be performed for example to exclude events such as sneezing or coughing as mentioned above. Thus, more complicated signal processing may be carried out to determine the rate of breathing, again such as averages or moving averages.
Thus, there are different ways to derive the second value. However, generally there is a correlation between the timing between variations in fan rotation speed and the breathing rate.
The fan speed is controlled with multiple possible fan speed values. Thus, the fan speed is not only controlled between an on fan speed and a zero (off) fan speed. There may be 3, 4, 5 or more distinct fan speed settings.
The fan speed is thus set to a selected one of a plurality of non-zero fan speeds. The mask has the capability to drive the fan to any one of these fan speeds. There may be a discrete set of fan speeds as mentioned above to which the fan can be set, or there may be a continuum of fan speeds within a range, and an algorithm may then select any desired fan speed within that possible range.
The example above is based on mapping a distribution of breathing depths and volumes to each fan speed setting. Other statistical approaches may of course be used. For example, periods with usage time below a threshold may be removed from the analysis altogether rather than being give a low weighting.
Temperature and humidity related values, such as heat index, can be also integrated in the distribution analysis.
The determination method for breathing rate and volume is not limited to fan rotation speed only, sensors such as differential pressure sensor or stress sensors can also achieve breathing rate and volume determination. Taking differential pressure sensors for example, the breathing rate and volume can be derived from the patterns of the pressure difference between inside and outside mask chamber. In these patterns, the amplitude of pressure change relates to the breathing volume, while the period of the pressure change relates to the breathing rate.
As discussed above, embodiments make use of a controller, which can be implemented in numerous ways, with software and/or hardware, to perform the various functions required. A processor is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform the required functions. A controller may however be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions.
Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).
In various implementations, a processor or controller may be associated with one or more storage media such as volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM. The storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform the required functions. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller.
Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

A pollution mask comprising:
an air chamber (18);

a filter (16) which forms a boundary between the air chamber and the ambient surroundings outside the air chamber;

a fan (20) for drawing air from outside the air chamber (18) into the air chamber and/or drawing air from inside the air chamber to the outside;

a user interface for allowing a user to manually select fan rotation speeds, the pollution mask further having an auto mode for automatic fan speed control;

a means (34, 36) for determining a first value which relates to a depth of breathing and a second value which relates to a rate of breathing;

a controller (30) which is adapted to:
monitor the first and second values during time periods with manually selected fan speeds; and

derive from the monitoring during the time periods with manually selected fan speeds a fan speed strategy for use in the auto mode which relates the derived first and second values to a fan speed setting.
The mask according to claim 1, wherein the means (34, 36) for determining first and second values is adapted to monitor a rotation speed of the fan and the controller is adapted to derive the first and second values from the determined fan rotation speed or change in fan rotation speed.
The mask according to claim 2, wherein the first value is based on a maximum swing in fan rotation speed during a sampling window and wherein the second value is a frequency based on the time between a consecutive maxima and minima in the fan rotation speed.
The mask according to claim 1, wherein the means for determining first and second values comprises a differential pressure sensor.
The mask according to any one of claims 1 to 4, wherein the controller is adapted to perform the monitoring during an initial period of usage of the pollution mask during which the fan rotation speed is manually selected.
The mask according to claim 5, wherein the controller is further adapted to update the speed strategy over time after the initial period of usage based on periods when the rotation speed is manually selected.
The mask according to any one of claims 1 to 6, wherein the controller is adapted, for each fan speed setting, to:
group the first and second values and a third value representing the time during which the fan speed setting was selected with those first and second values present;

derive a measure of the distribution of the first value in that fan speed setting;

derive a measure of the distribution of the second value in that fan speed setting; and

from the measures of distribution, deriving a mapping between the first and second values and the fan speed level to be applied in the auto mode.
The mask according to any one of claims 1 to 7, further comprising a temperature sensor (24a) for measuring an ambient temperature outside the air chamber, wherein the controller is adapted to set a fan speed further in dependence on the temperature level.
The mask according to claim 8, further comprising a humidity sensor (24b) for measuring an ambient humidity level outside the air chamber, wherein the controller is adapted to set a fan speed further in dependence on the humidity level.
A mask according to claim 9, wherein the controller is adapted to:
derive from the ambient temperature and the ambient humidity level a heat index value which relates to a measure of comfort.
The mask according to any one of claims 1 to 10, wherein:
the fan (20) is driven by an electronically commutated brushless motor, and the means for determining rotation speed comprises an internal sensor of the motor; or

the means (36) for determining the first and second values comprises a circuit for detecting a ripple on the electrical supply to a motor which drives the fan.
The mask according to any one of claims 1 to 11, wherein the controller (30) is adapted to:
determine a respiration cycle; and

to control an outlet valve (22) in dependence on the phase of the respiration cycle and/or to turn off the fan during an inhalation time.
A non-therapeutic method of controlling a pollution mask, the method comprising:
drawing gas into and/or out of an air chamber of the mask using a fan which forms a boundary between the air chamber and the ambient surroundings outside the air chamber;

deriving a first value which relates to a depth of breathing and a second value which relates to a rate of breathing;

monitoring the first and second values during time periods with manually selected fan speeds; and

deriving from the monitoring during the time periods with manually selected fan speeds a fan speed strategy for use in an auto mode for automatic fan speed control, which relates the derived first and second values to a fan speed setting.
The method according to claim 13, comprising performing the monitoring during an initial period of usage of the pollution mask during which the fan rotation speed is manually selected.
The method according to claim 13 or 14, comprising updating the speed strategy over time after the initial period of usage based on periods when the rotation speed is manually selected.