EP3658239B1

EP3658239B1 - A mask and control method

Info

Publication number: EP3658239B1
Application number: EP18753063.9A
Authority: EP
Inventors: Peng Zhang; Weizhong Chen; Jun Shi; Shuang Chen; Wei Su; Qiushi ZHANG
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2017-07-28
Filing date: 2018-07-20
Publication date: 2021-02-24
Anticipated expiration: 2038-07-20
Also published as: CN111278512A; WO2019020503A1; CN111278512B; EP3658239A1

Description

FIELD OF THE INVENTION

This invention relates to a mask and control method, particularly to a mask containing a temperature and/or relative humidity sensor.

BACKGROUND OF THE INVENTION

Air pollution is a worldwide concern. 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. Nearly 300 smog-hit cities fail to meet national air quality standards.
Official outdoor air quality standards define particle matter concentration as mass concentration per unit volume (e.g. µg/m³). A particular concern is pollution with particles having a diameter less than 2.5 µm (termed "PM2.5") as they are able to penetrate into the gas exchange regions of the lung (alveoli), and very small particles (<100 nm) may pass through the lungs to affect other organs.
Since this problem will not improve significantly on a short time scale, a common way to deal with this problem is to wear a mask which provides cleaner air by filtration and the market for masks in China and elsewhere has seen a great surge in recent years. For example, it is estimated that by 2019, there will be 4.2 billion masks in China.
However, during use, the temperature and relative humidity inside the mask increases and combined with the pressure difference inside the mask relative to the outside, makes breathing uncomfortable. To improve comfort and effectiveness, a fan can be added to the mask which draws in air through a filter. For efficiency and longevity reasons these are normally electrically commutated brushless DC fans.
The benefit to the wearer of using a powered mask is that the lungs are 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 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 have been numerous approaches to improving the user experience when wearing a powered mask. The approaches have tended to focus on regulation of fan speeds, both to improve user comfort and to improve the electrical efficiency of the fan. For example, GB 2 032 284 discloses a respirator in which the pressure inside a mask is measured by a pressure sensor and the fan speed is varied in dependence on the sensor measurements.
Differential pressure sensors which determine a difference in pressure between air outside the device and air inside the device are thus often used in powered masks. Differential pressure sensors provide accurate monitoring of the breathing cycle of the user because there is a minimal time lag between the detected variation of differential pressure over time and the inhalation and exhalation timing of the user. This time lag is of the order of milliseconds or thousandths of a second. However, differential pressure sensors are expensive.
Temperature sensors are a cheaper alternative but there is a time lag when detecting the temperature such that the detected temperature is not a true reflection of the real-time temperature. The time lag is the difference between when a temperature value exists in real-time and when this temperature value is detected. This time lag is of the order of seconds such as 2 to 8 seconds based on different types of temperature sensor measurement principles, which is significant when compared to the duration of the breathing cycle (typically 3-5 seconds for a healthy adult at rest). This time lag also varies from person to person. Therefore, when using a temperature sensor to regulate the fan speed in masks, the time lag may cause the fans to act in opposition to the breathing cycle of the user. For example, an inlet fan may be turned on when the user is exhaling. This makes breathing in masks uncomfortable.
Therefore, there exists a need for cheaper alternative sensors for detecting the breathing cycle of a user in masks with a reduced time lag between the detected variation of the parameter over time and the inhalation and exhalation timing of the user.
WO 92/18201 A1 discloses a method for controlling, in respiration-synchronized manner, a portable air supply unit for a respirator covering at least the nose and/or mouth of the user, and having an air inlet and an air outlet.
WO 2016/157159 A1 discloses a user-wearable device incorporating a respirator or breathing air filter in combination with an electronic system for providing functionality to a wearing user.

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 mask comprising:

an air chamber;
a filter;
a fan arrangement for ventilating the mask;
at least one sensor for detecting the temperature and/or the relative humidity of air inhaled and exhaled over time by a user; and
a controller which is adapted to:
- calculate a first value which depends on a first time derivative of said temperature and/or on a first time derivative of said relative humidity;
- calculate a second value which depends on a second time derivative of said temperature and/or on a second time derivative of said relative humidity;
- determine a start of an inhalation and/or a start of an exhalation based on the first value and the second value; and
- control the fan arrangement based on the determined start of the inhalation and/or the determined start of the exhalation.

There is proposed a concept of using a first derivative with respect to time (i.e. the first time derivative, the rate of change, or the first order derivative) to accurately determine a start of an inhalation and/or a start of an exhalation. This is particularly advantageous when using temperature and/or relative humidity sensors as these sensors suffer from a sensing delay. For example, the temperature of the air detected using a temperature sensor having a time lag of 2 to 8 seconds is a measurement of the temperature of the air 2 to 8 seconds ago. However, by calculating instantaneous differences in the detected temperature and/or humidity values over time (e.g. a rate of change), the time lag becomes irrelevant and it is possible to accurately determine information relating to the breathing cycle of a user. In this way, the use of a value which depends on a first time derivative of the temperature and/or a first time derivative of the relative humidity compensates for a time lag between the detected variation of the temperature or relative humidity over time and the inhalation and exhalation timing of the user. With this information, a fan arrangement in a mask is controlled to assist the breathing of the user thus making breathing in the mask more comfortable.
The mask of the invention has the benefit of using a temperature sensor and/or a humidity sensor, which are cheap compared to differential pressure sensors. Therefore, the mask uses inexpensive components to monitor the breathing cycle of the user. The sensors perform the dual role of inhalation/exhalation detection as well as providing feedback information for use in controlling the comfort of the mask.
The mask monitors the breathing cycle of the user using a value which depends on the first time derivative of the temperature and/or relative humidity parameter rather than the parameter itself. This helps to reduce the effect of the time lag.
It will therefore be appreciated that the inventors have realized that temperature and/or humidity sensors can be used instead of differential pressure sensors to accurately determine the inhalation and/or exhalation cycle of a user and that this information can be used to control a fan arrangement in a mask to make breathing in a mask more comfortable.
Based on the determined start of inhalation and/or the determined start of exhalation, the fan arrangement is controlled by the controller. The fan may thus, for example, be controlled to blow air into the mask during inhalation. The fan may, for instance, blow less or no air into the mask during exhalation.
The controller may be adapted to calculate the first value by obtaining the first time derivative of the temperature and/or the first time derivative of the relative humidity parameter, and performing a low pass filtering. This low pass filtering may comprise a time averaging of samples, and it is used to reduce noise in the signal so that a predictable set of crossing points is obtained.
The controller is further adapted to calculate a second value which depends on a second time derivative of the temperature and/or on a second time derivative of the relative humidity, and determine the start of an inhalation and/or the start of an exhalation based on the first and second values.
This enables a remaining time lag which still exists when using the first time derivative for timing determination, to be compensated. In particular, a second time derivative value (i.e. the derivative of the first time derivative value) is used. Using the first and second values to determine the start of an inhalation and/or the start of an exhalation is more accurate than just using the first value alone. In this way, the first derivative with respect to time and the second derivative with respect to time (i.e. the derivative of the first derivative, the derivative of the rate of change, or the second order derivative) are used to accurately determine a start of an inhalation and/or a start of an exhalation. This allows the fan arrangement to more closely synchronize with the breathing of the user thus further improving the comfort of the user.
The start of the inhalation of the user may be determined when:

m>0 (or m≥0); and
m + t₁n < 0 (or m + t₁n ≤ 0), wherein m is the first value, t₁ is a time value representing a sensor time lag associated with the first value and n is the second value.

The time t₁ relates to the remaining time lag which is present even when using the first time derivative to obtain the breathing cycle timing. The time value t₁ may for example be obtained by a calibration process.
This is a simple check that allows accurate determination of the start time of the inhalation. m relates to a first time derivative and provides information on whether the user is currently inhaling, exhaling or undergoing a transition between inhalation and exhalation. t₁ can be adjusted and tailored to the sensing delay of the particular sensor being used to monitor the breathing cycle of the user. n relates to a second time derivative and provides information on how m is changing. The inequality allows the prediction of m at the sensing delay so that if the user is predicted to be inhaling, exhaling or undergoing a transition between inhalation and exhalation at the sensing delay, the event is representative of the current breathing cycle of the user because of the sensing delay.
Similarly, the start of the exhalation of the user may be determined when:

m<0 (or m≤0); and
m + t₁n > 0 (or m + t₁n ≥ 0), wherein m is the first value, t₁ is a time value representing a sensor time lag associated with the first value and n is the second value.

This is a simple check that allows accurate determination of the start time of the exhalation. The inequality is described above.
Preferably, the first value is an average first value which depends on the first time derivative of the temperature and/or on the first time derivative of the relative humidity. In this way, any noise in the variation of the first value over time is reduced and the signal to noise ratio is increased.
In another embodiment, the fan arrangement comprises an inlet fan and an outlet fan and the controller is adapted to determine the start of an inhalation and the start of an exhalation. In this way, ventilation is improved when the user is both inhaling and exhaling.
The controller may be adapted to operate the inlet fan at a first speed at the start of the inhalation and a second, lower, speed at the start of the exhalation, and operate the outlet fan at a third speed at the start of the exhalation and a fourth, lower, speed at the start of the inhalation. In this way, the breathing cycle is fully assisted. The inlet and outlet fans synchronize with the breathing cycle of the user; the inhalation cycle is assisted by the inlet fan and the exhalation cycle is assisted by the outlet fan.
The second speed and the fourth speed may be zero. This minimizes battery use when the breathing cycle is in the opposite phase to the respective fan.
Preferably, the mask further comprises a battery to power the at least one sensor, the controller and the fan arrangement.
In another aspect of the invention, there is provided a method of controlling a mask, which mask comprises an air chamber, a filter and a fan arrangement for ventilating the mask, wherein the method comprises:

detecting the temperature and/or the relative humidity of air inhaled and exhaled over time by a user;
calculating a first value which depends on a first time derivative of the temperature and/or on a first time derivative of the relative humidity;
determining a start of an inhalation and/or a start of an exhalation based on the first value; and
controlling the fan arrangement based on the determined start of the inhalation and/or the determined start of the exhalation.

The method may further comprise calculating the first value by obtaining the first time derivative of the temperature and/or the first time derivative of the relative humidity, and performing a low pass filtering.
The method further comprises:

calculating a second value which depends on a second time derivative of the temperature and/or on a second time derivative of the relative humidity; and
determining the start of an inhalation and/or the start of an exhalation of the user based on the first and second values.

In another aspect of the invention, there is provided a computer program comprising computer program code means which is adapted, when said computer program is run on the controller of the mask as defined above, to implement the method defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 shows the differential pressure versus time detected using a differential pressure sensor and the temperature versus time detected using a temperature sensor during breathing cycles;
Fig. 2 shows a mask containing a fan arrangement;
Fig. 3 shows one example of the components of a mask containing an inlet fan and an outlet fan;
Fig. 4 shows the differential pressure versus time and the differential temperature versus time;
Fig. 5 shows an enlarged part of the differential pressure versus time plot and differential temperature versus time plot;
Fig. 6 shows the differential pressure versus time and the differential temperature versus time of Fig. 4 with reduced noise based on a time averaging approach;
Fig. 7 shows a mask operating method of the invention;
Fig. 8 shows a preferred embodiment of the mask operating method of the invention for a mask containing an inlet fan and an outlet fan;
Fig. 9 shows the differential pressure versus time and the detected breathing cycle timing for a user walking;
Fig. 10 shows the differential pressure versus time and the detected breathing cycle timing for a user speaking; and
Fig. 11 shows the differential pressure versus time and the detected breathing cycle timing for a user at rest wearing a mask with an inlet fan and an outlet fan, which are run in synchronism with the start of the inhalation and the start of the exhalation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a mask which incorporates an air chamber, a filter, a fan arrangement, a sensor and a controller. The sensor detects a parameter relating to the temperature and/or relative humidity of air inhaled and exhaled over time by a user of the mask. The controller calculates a first value which depends on a first order time derivative of the parameter. Based on this calculation, the controller determines a start of an inhalation and/or a start of an exhalation of the user. Therefore, the controller is able to accurately determine the inhalation and/or exhalation cycle of the user, thereby compensating for a time lag in detected variation of the parameter over time. This time lag otherwise prevents accurate determination of the inhalation and exhalation timing of the user. With this information, the controller operates the fan arrangement in synchronism with the inhalation and/or exhalation cycle of the user.
Fig. 1 shows an exemplary variation 101 of differential pressure versus time for a breathing cycle (as detected using a differential pressure sensor) and an exemplary variation 102 of temperature versus time for the same breathing cycles (as detected using a temperature sensor). The x-axis is time/seconds. The left-hand y-axis is the differential pressure/Pa and the right-hand y-axis is the temperature/°C.
The breathing cycle is the pattern of inhalation and exhalation timings. There are a series of peaks and troughs in the breathing cycle corresponding to exhalation and inhalation, respectively. The breathing cycle may be represented by any suitable parameter which varies with inhalation/exhalation over time. Usually the breathing cycle is detected by monitoring differential pressure versus time, but other parameters may be used, such as temperature, relative humidity, oxygen and/or carbon dioxide concentration etc.
During inhalation, the differential pressure inside the mask decreases (negative pressure relative to the external ambient pressure) and the temperature inside the mask decreases. During exhalation, the differential pressure inside the mask increases (positive pressure) and the temperature inside the mask increases.
A differential pressure sensor provides accurate monitoring of the breathing cycle of the user because there is a minimal time lag in the detected variation of pressure over time, and hence the differential pressure closely follows the timing of inhalation and exhalation of the user. Therefore, the differential pressure versus time plot 101 depicted in Fig. 1 is an accurate representation of the breathing cycle of the user. Further, it is relatively straightforward to determine the fan control point using differential pressure sensors. The fan control point is the time when the fan is turned on or off. For a differential pressure sensor, this is when the differential pressure reaches 0 Pa inside the mask. For example, for a mask containing an inlet fan and an outlet fan, if the differential pressure increases from a negative value to a positive value and passes through 0 Pa, the outlet fan is turned on and the inlet fan is turned off. Similarly, if the differential pressure decreases from a positive value to a negative value and passes through 0 Pa, the inlet fan is turned on and the outlet fan is turned off.
However, differential pressure sensors are expensive. Further, differential pressure sensors cannot detect the temperature and/or relative humidity inside the mask and thus without additional sensors, the fan arrangement is not able to have different working speeds, e.g. low, medium and high, that are dependent on the temperature and/or relative humidity inside the mask. Therefore, a mask containing only a differential pressure sensor is unable to fully respond to the conditions inside the mask.
In contrast, the use of a temperature and/or relative humidity sensor is cheaper. Further, temperature and/or relative humidity sensors can of course detect the temperature and/or relative humidity inside the mask so that the fan working speed can be adjusted in response to the conditions inside the mask.
However, temperature and/or relative humidity sensors do not directly provide an accurate representation of the breathing cycle of the user because there is a time lag in detection of temperature and/or relative humidity over time, and hence a time lag with respect to the actual inhalation and exhalation timing of the user. This time lag is about 2 to 8 seconds for a temperature sensor, which is significant when compared to the duration of the breathing cycle (typically 3-5 seconds for a healthy adult at rest). Therefore, the temperature versus time plot 102 depicted in Fig. 1 is an inaccurate representation of the breathing cycle of the user. The time lag associated with using a temperature sensor can be seen in Fig. 1 by comparing the temperature versus time plot 102 with the differential pressure versus time plot 101. If the time lag between the detected variation of temperature over time and the inhalation and exhalation timing of the user was negligible, the temperature versus time plot would have the same timing as the differential pressure versus time plot. Therefore, when using a temperature sensor to regulate the fan speed, the time lag may cause the fans to act in opposition to the breathing cycle of the user. For example, an inlet fan may be turned on when the user is exhaling. This makes breathing in masks uncomfortable.
The invention is based on providing a mask which monitors the breathing cycle of the user using a value which depends on the first time derivative of the parameter instead of (or as well as) the parameter itself. This allows the fan arrangement to be controlled more accurately in synchronism with the inhalation and/or exhalation cycle of the user.
Fig. 2 shows a mask of the invention containing a fan arrangement.
A user 10 is shown wearing a face mask 11 which covers at least the nose and mouth of the user. The purpose of the mask is to filter air before it is breathed in by the user. For this purpose, in Fig. 1, the mask body itself acts as an air filter 12. Air is drawn into an air chamber 13 formed by the mask by inhalation. During inhalation, an outlet valve 15 such as a check valve is closed due to the low pressure in the air chamber 13.
When the subject breathes out, air is exhausted through the outlet valve 15. This valve is opened to enable easy exhalation, but is closed during inhalation. A fan arrangement 14 ventilates the mask and in the embodiment shown, assists in the removal of air through the outlet valve 15. 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 15 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 12. However, it may instead be an electronically controlled valve.
Fans generate a flow of air through the mask to reduce the temperature and relative humidity inside the mask and to regulate the pressure difference inside the mask relative to the outside. The fans are able to track the breathing cycle of the user, to make breathing in the mask more comfortable. For example, an inlet fan present in a mask may rotate during inhalation and an outlet fan may rotate during exhalation.
The fan arrangement 14 may comprise an inlet fan or an outlet fan to ventilate the mask. An inlet fan for example draws air through a filter (rather than through the wall of the face mask 11 which may then be gas impermeable) from outside the air chamber 13 into the air chamber. An inlet fan may be positioned before or after such a filter. An outlet fan draws air from inside the air chamber 13 to the outside. In this embodiment, the exhaust air would not need to pass through the filter, but it could be drawn through the filter by the outlet fan as well.
An inlet fan or an outlet fan assists the breathing of the user. For example, if an inlet fan is present, it may be switched on during inhalation and switched off during exhalation. Alternatively, if an outlet fan is present, it may be switched off during inhalation and switched on during exhalation.
The fan arrangement 14 may comprise an inlet fan and an outlet fan. In this way, the inhalation/exhalation cycle is fully assisted. The inlet and outlet fans synchronize with the breathing cycle of the user; inhalation is assisted by the inlet fan to bring fresh air into the mask and exhalation is assisted by the outlet fan to expel the air breathed out by the user from inside the mask to the outside.
When the mask is not in use, it may be switched off. In one design, the mask comprises a switch for starting and stopping the fan arrangement 14. This allows the user to have full control over when to start and stop the fan arrangement. For example, the user could ensure that the fan arrangement is switched off at all times when the mask is not in use. When the mask is switched on, the fan arrangement may start operating. Alternatively, there may instead be detection using a sensor arrangement of when the mask is being worn to provide automatic control of the fan arrangement. The mask may then go straight into its operating mode.
Fig. 3 shows one example of the components of a mask containing an inlet fan and an outlet fan. The same components as in Fig. 2 are given the same reference numbers.
In addition to the components shown in Fig. 2, Fig. 3 shows an inlet fan 16 having an inlet fan blade 16a and an inlet fan motor 16b, an outlet fan 17 having an outlet fan blade 17a and an outlet fan motor 17b, a controller 20, a local battery 21 and a sensor 22 for detecting a parameter relating to the temperature and/or relative humidity of air inhaled and exhaled by the user 10.
The sensor 22 detects a parameter relating to the temperature and/or relative humidity of air inhaled and exhaled over time by a user of the mask, for example generating the plot 102 as shown in Fig. 1.
A temperature sensor is for example used to detect the temperature inside the mask, and hence of the air inhaled and exhaled over time by the user. A suitable temperature sensor is a Sensirion (Trade Mark) STS3x sensor. A relative humidity sensor is for example used to detect the relative humidity inside the mask and hence of the air inhaled and exhaled over time by the user. A suitable relative humidity sensor is a Sensirion (Trade Mark) SHT3x sensor. SHT3x is a high-accuracy temperature and relative humidity sensor that can be used to detect both temperature and relative humidity.
Alternatively, the sensor 22 may be a temperature and relative humidity sensor for detecting the temperature and relative humidity of air inhaled and exhaled over time by a user.
Additional sensors may also be used to detect other parameters such as carbon dioxide and/or oxygen sensors or a combination of any of the above sensors may be used. The sensor 22 detects the parameter and this information is transmitted to the controller 20. The controller 20 then calculates a first value which depends on a first order time derivative of the parameter.
The fan control point using temperature and/or relative humidity sensors is when the temperature and/or relative humidity reaches a local maximum (at the end of an exhalation) or a local minimum (at the end of an inhalation). At the local maxima and minima, the rate of change of the parameter is zero, i.e. the first time derivative of the detected temperature and/or relative humidity is zero:
$\frac{dT}{dt} = 0 and / or \frac{dRH}{dt} = 0,$
where T is temperature, t is time and RH is relative humidity. t in fact represents the sample interval and thus the sample rate of the sensor.
By calculating a first value which depends on a first time derivative of the parameter, it is possible to determine the time when the rate of change of the parameter is zero and hence the start of an inhalation and/or the start of an exhalation.
The first value may be a first time derivative of the parameter. An approximation is simply a difference value in the parameter over a discrete time interval, and this enables a reduced number of calculations. For example, the first value may be ΔT_i = T_i - T_i-1 and/or ΔRH_i = RH_i - RH_i-1, where T_i and RH_i are the parameters at time i, and T_i-1 and RHi-i are the parameters at time i-1. i and i-1 are always shifting. ΔT_i and ΔRH_i are thus the differential temperature value and differential relative humidity value, respectively. Further, as t represents the sample rate of the sensor, a plot of the first time derivative of the parameter is effectively the same as a plot of the difference value in the parameter over time.
Fig. 4 shows the differential pressure versus time for a set of breathing cycles as plot 401 and the differential temperature versus time for the breathing cycles as plot 402. The x-axis is time/seconds. The left-hand y-axis is the differential pressure/Pa and the right-hand y-axis is the differential temperature in °C.
During inhalation, the differential temperature inside the mask decreases and is a negative value. During exhalation, the differential temperature inside the mask increases and is a positive value.
As can be seen from Fig. 4, the differential temperature versus time plot matches the differential pressure versus time plot much more closely than the temperature versus time plot does in Fig. 1. Therefore, the differential temperature versus time provides a more accurate representation of the breathing cycle than the temperature versus time. The device thus monitors the breathing cycle using the first value which depends on the first time derivative of the parameter.
The controller 20 determines a start of an inhalation and/or a start of an exhalation of the user based on the calculated first value. The first value indicates whether the user is inhaling, exhaling or is undergoing a transition between inhalation and exhalation. If the first value is negative, the user is inhaling until just before the first value becomes positive, when the user is undergoing a transition between inhalation and exhalation. If the first value is positive, the user is exhaling until just before the first value becomes negative, when the user is undergoing a transition between exhalation and inhalation.
In response, the controller controls the fan arrangement 14 in synchronism with the inhalation and/or exhalation cycle of the user.
For a fan arrangement 14 comprising an inlet fan 16 and an outlet fan 17, when the differential temperature increases from a negative value to a positive value (and passes through 0 °C, the baseline temperature), the outlet fan 17 is turned on and the inlet fan 16 is turned off. Similarly, when the differential temperature decreases from a positive value to a negative value (and passes through 0 °C, the baseline temperature), the inlet fan 16 is turned on and the outlet fan 17 is turned off.
In one example, the fan motors 16b and 17b are electronically commutated brushless motors. Electronically commutated brushless motors are preferred for efficiency and longevity reasons.
In use, the inlet fan 16 and the outlet fan 17 may be run such that the controller 20 is adapted to operate the inlet fan 16 at a first speed during inhalation and a second, lower, speed during exhalation, and operate the outlet fan 17 at a third speed during exhalation and a fourth, lower, speed during inhalation. The first and second speeds of the inlet fan 16 and the third and fourth speeds of the outlet fan 17 refer to rotation speeds.
When a transition from exhalation to inhalation is determined, the controller 20 sends a signal to the inlet fan motor 16b to increase the rotation speed of the inlet fan blade 16a from the second speed to the first speed. The controller 20 also sends a signal to the outlet fan motor 17b to decrease the rotation speed of the outlet fan blade 17a from the third speed to the fourth speed. In this way, during inhalation, the inlet fan 16 is run at the first speed and the outlet fan 17 is run at the fourth speed. This compensates for the decrease in differential pressure inside the mask during inhalation.
Conversely, if a transition from inhalation to exhalation is determined, the controller 20 sends a signal to the outlet fan motor 17b to increase the rotation speed of the outlet fan blade 17a from the fourth speed to the third speed. The controller 20 also sends a signal to the inlet fan motor 16b to decrease the rotation speed of the inlet fan blade 16a from the first speed to the second speed. In this way, during exhalation, the outlet fan 17 is run at the third speed and the inlet fan 16 is run at the second speed. This compensates for the increase in differential pressure inside the mask during exhalation.
When both an inlet fan 16 and an outlet fan 17 are present, the second speed of the inlet fan 16 is preferably the same as the fourth speed of the outlet fan 17. This provides a consistent user experience, in terms of feel and sound.
The first speed of the inlet fan 16 may be the same as or different to the third speed of the outlet fan 17, depending on the design of the inlet and outlet flow paths of the mask and the differential pressure inside the mask created by the inlet fan 16 and the outlet fan 17. For example, if air is drawn into the mask through the filter and drawn out of the mask through a valve, the inlet fan 16 would need to generate a higher pressure than the outlet fan 17. This could be achieved using a first speed for the inlet fan 16 that is higher than the third speed for the outlet fan 17.
The second speed and the fourth speed may be zero or a minimum non-zero speed. Switching off the fans minimizes battery use when the breathing cycle is in the phase for which the respective fan is not needed. Alternatively, the second speed and the fourth speed could be non-zero. One of the benefits of running the inlet fan 16 at a minimum non-zero second speed and the outlet fan 17 at a minimum non-zero fourth speed is that the fans are run at a low idling speed which uses minimal power, but reduces latency. Further, continuously running the inlet and outlet fans at least at a minimum level ensures that there is minimal delay when switching the operation of the inlet fan to the outlet fan during the transition between inhalation and exhalation, and when switching the operation of the outlet fan to the inlet fan during the transition between exhalation and inhalation. Thus, the air flow in the mask may be synchronized more easily with the breathing cycle of the user, ultimately making breathing in the mask more comfortable.
The fan speeds may be tailored to the breathing of the user (e.g. breath frequency and tidal volume) and may be adjusted to take into account different breathing scenarios (e.g. exertion like walking and running).
The speeds to be used may be determined during a calibration process or they may be provided by the fan manufacturer. The calibration process for example involves analyzing the fan speed information over a period during which the user is instructed to inhale and exhale regularly with normal breathing. The captured fan speed information can then be used to determine the appropriate fan speeds. The controller may also provide for settings for the user to regulate the higher first and third speeds, and the lower second and fourth speeds, and any intermediate speeds.
In a most simple example, the rotation speeds of the inlet fan 16 and the outlet fan 17 alternate between two set values, with the changes in rotation speed implemented at the detected transitions between inhalation and exhalation.
There may also be a number of intermediate rotation speeds at which the inlet and outlet fans may be run between the first and third speeds and between the second and fourth speeds. However, the second and fourth speeds normally set the minimum rotation speed. The minimum rotation speed ideally provides an optimum balance between lag time and power efficiency. The first and third speeds are typically dependent on the breathing of the user (e.g. breath frequency and tidal volume) and could be adjusted to take into account different breathing scenarios (e.g. exertion like walking and running). In one simple embodiment, the first and third speeds set the maximum rotation speed. In this way, the first and third speeds ideally provide an optimum balance between lag time and power efficiency on the one hand, and assistance given to the user on the other. The first and third speeds could also be adjusted to take into account the conditions inside the mask such as the temperature and/or relative humidity. For example, the first and third speeds may have three different settings such as low, medium and high, and the speed may increase proportionally with the temperature and/or relative humidity inside the mask.
The rotation speeds of the inlet and outlet fans are for example controlled by a pulse width modulation signal, whereby the duty cycle controls the rotation speed.
Monitoring the differential temperature over time as opposed to the temperature over time compensates for the time lag to a certain extent and thus the differential temperature plot provides a good approximation of the actual breathing cycle of the user. However, the differential temperature versus time plot is still not a fully accurate representation of the breathing cycle of the user and this is apparent if the differential temperature versus time plot is enlarged.
Fig. 5 shows an enlarged portion of the differential pressure versus time plot 501 and the differential temperature versus time plot 502. The x- and y-axes are the same as those for Fig. 4. It shows a smoothed version of the differential temperature plot, as explained further below.
As can be seen from Fig. 5, there is still a time lag between the differential temperature versus time plot and the differential pressure versus time plot and thus there is still a time lag between the differential temperature versus time and the actual inhalation and exhalation timing of the user. This time lag is shown as Δt, and it is for example in the range 0.2 s to 0.8 s.
The outlet fan will be turned on at time t2 based on the differential time plot, i.e. the detected start of exhalation, when the differential temperature passes through 0 °C. However, the outlet fan should have been turned on at time t0, the actual start of exhalation, when the differential pressure passes through 0 Pa. The differential temperature is not a fully accurate representation of the breathing cycle of the user and as such there is the time lag Δt between the detected start of inhalation and/or the detected start of exhalation based on the differential temperature and the actual start of inhalation and/or the actual start of exhalation.
The time lag Δt can be further compensated for by calculating a second value which depends on a second order time derivative of the parameter. The second value may simply be the second time derivative of the parameter, i.e. the derivative of the first time derivative of the parameter and hence the slope (or rate of change) of the first time derivative of the parameter. The second value may again be an approximation as a change in the difference value in the parameter over a discrete time interval in order to reduce the number of calculations. For example, the second value may be Δ²T_j = ΔT_j - ΔT_j-1 and/or Δ²RH_j = ΔRH_j - ARH_j-1, where T_j and RH_j are the parameters at time j, and T_j-1 and RH_j-1 are the parameters at time j-1. j and j-1 are always shifting. Δ²T_j and Δ²RH_j are thus the change in the differential temperature value and the change in the differential relative humidity value, respectively.
The controller is able to determine the start of an inhalation and/or the start of an exhalation based on the first and second values. Using the first and second values to determine the start of an inhalation and/or the start of an exhalation is more accurate than just using the first value alone. In this way, the first order derivative with respect to time and the second order derivative with respect to time are used to accurately determine a start of an inhalation and/or a start of an exhalation. This allows the fan arrangement to more closely synchronize with the breathing of the user thus further improving the comfort of the user.
The first value provides a first approximation as to whether the user is inhaling, exhaling or is undergoing a transition between inhalation and exhalation. If the first value is negative, the user is generally inhaling, but at the end of the negative period the user is undergoing a transition between inhalation and exhalation or is exhaling, e.g. between t1 and t2 as depicted in Fig. 5. If the first value is positive, the user is generally exhaling but at the end of the positive period the user is undergoing a transition between exhalation and inhalation or is inhaling, e.g. between t4 and t3 as depicted in Fig. 5. The second value helps to precisely determine the transitions between inhalation and exhalation. In this way, the first and second values may both be used to determine more accurately the transitions between inhalation and exhalation.
The first and second values are processed to determine a start of an inhalation and/or exhalation of the user. Processing of the first and second values leads to a determination of inhalation and/or exhalation occurring sooner than would otherwise be determined by just relying on the first value.
For example, a time lag Δt of 0.2 s is assumed, and the first and second values are obtained at time t1.
As a first approximation, ΔT>0 means the user is exhaling and ΔT<0 means the user is inhaling (where ΔT is the first i.e. differential temperature, value).
However, it may be seen in Fig. 4 that the differential temperature versus time plot has many fluctuations about zero at each inhalation-exhalation transition. Thus, using the crossing point of this plot does not give a single value to represent a change between inhalation and exhalation, at the fan control points. Specifically, the fluctuating signal may fail to trigger the fan control point signal. Thus, the fans may not be controlled appropriately.
The fluctuation in the differential temperature versus time plot is due to noise in the detected parameter values. The fluctuation is especially problematic where the sensor has a high sample rate in order to fully reflect the breathing cycle of the user. In these situations, the noise in the differential temperature versus time plot may be reduced by performing a time window averaging.
For example, an averaged value of the first value may be obtained by calculating: $Δ T_{j} = \frac{Δ T_{i} + Δ T_{i - 1} + Δ T_{i - 2}}{3} = m$
where ΔT is the averaged first value and ΔT_i, ΔT_i-1 and ΔT_i-2 are the first values associated with times i, i-1 and i-2. A similar calculation may be performed for relative humidity.
The averaging calculation is applied to further process the first value calculation. The results of applying the averaging calculation to the differential temperature versus time plot of Fig. 4 can be seen in Fig. 6.
As is apparent from Fig. 6, the differential temperature versus time plot 601 has fewer fluctuations than the corresponding plot in Fig. 4. Thus, the fan control points may be determined more easily.
This time averaging function is essentially a discrete (sample-based) low pass filtering function. Other low pass filtering approaches may be used to remove the noise in the first value.
As mentioned above, the second value may be defined as:
n = Δ²T_j (or n =Δ²RH_j), where the squaring represents a second order difference.
The start of the exhalation of the user may be determined when:

m<0; and
m + t₁n ≥ 0, wherein m is the first value, t₁ is a time value representing the time lag Δt and n is the second value.

By way of example the value t₁ may be selected as a fixed estimate of the time lag Δt or a value just below the time lag. Note that this is the time lag between the inhalation timing based on the first order derivative (ΔT) and the real cycle timing (rather than the larger time lag based on a temperature or relative humidity plot).
The value m<0 generally indicates that inhalation is taking place. However, if m + t₁n ≥ 0 exhalation may be determined.
With reference to Fig. 5, time t1 may be assumed to be 0.2 seconds before time t2. At time t1 the gradient of the plot 502 (i.e. wherein the gradient is n = d²T/dt²) is such that the plot itself, i.e. m = dT/dt, reaches zero by time point t2. Thus, assuming a linear plot between t1 and t2 with the gradient as determined at time t1, m + 0.2n = 0. The crossing point in m will be reached providing m + 0.2n ≥ 0.
Thus, by checking the second order derivative, it is possible to predict in advance when the time 0.2 seconds (in this example) before the crossing point in the first value has been reached.
Similarly, the start of the inhalation of the user may be determined when:

m>0; and
m + t₁n < 0.

The time lag Δt (between the crossing point of the derivative dT/dt and the actual breathing cycle) will be different for different sensors. It can be set as part of a calibration routine and it represents the typical time lag between the detection based only on the first value and the true timing of the inhalation and exhalation cycle.
The result of the use of the second value is that the fan control times may be improved from t2 and t3 to t1 and t4 as depicted in Fig. 5.
By way of example:

a typical range of values for the time lag between the temperature sensor signal T and the actual breathing cycle is 2s to 8s;
a typical range of values for the time lag between the relative humidity sensor signal RH and the actual breathing cycle is 6s to 10s;
a typical range of values for the time lag between the temperature sensor first derivative signal dT/dt and the actual breathing cycle is 0.2s to 0.8s;
a typical range of values for the time lag between the relative humidity first derivative signal dT/dt and the actual breathing cycle is 0.6s to 1.0s.

It will be appreciated that alternative inequalities and equations may be used to determine a start of an inhalation and/or a start of an exhalation based on the calculated first and second values. For example, the first and second values may be compared to a different set value and/or the first and second values may be scaled by an appropriate factor.
In one embodiment, the first value is an average first value which depends on the first time derivative of the parameter. Average in this context means the mean value.
Fig. 7 shows a device operating method of the invention. The method is for controlling a mask. The mask comprises an air chamber 13, a filter 12 and a fan arrangement 14 for ventilating the mask. The method comprises the following steps.
In step 701, a parameter is detected relating to the temperature and/or relative humidity of air inhaled and exhaled over time by a user 10.
In step 702, a first value is calculated which depends on a first time derivative of the parameter.
In step 703, a start of an inhalation and/or a start of an exhalation of the user is detected based on the first value. This compensates for a time lag between the detected variation of the parameter over time and the inhalation and exhalation timing of the user.
In step 704, the fan arrangement 14 is controlled in synchronism with the determined inhalation cycle and/or the determined exhalation cycle of the user.
The method may further comprise calculating a second value which depends on a second time derivative of the parameter; and determining the start of an inhalation and/or the start of an exhalation of the user based on the calculated first and second values.
Fig. 8 shows a preferred embodiment of the mask operating method of the invention for a mask containing an inlet fan and an outlet fan. This shows the basic idea of the algorithm that may be implemented by the controller.
In step 801, the software is initialized and the inlet fan 16 and the outlet fan 17 are started.
In step 802, the first three temperatures inside the mask are recorded, giving data T_i, T_i-1 and T_i-2.
In step 803, the first values are calculated for T_i, T_i-1 and T_i-2. For example, ΔT_i = T_i - T_i-1.
In step 804, the average first value is calculated for T_i, T_i-1 and T_i-2: $Δ T_{j} = \frac{Δ T_{i} + Δ T_{i - 1} + Δ T_{i - 2}}{3} = m .$
In step 805, the second value is calculated for T_j: $Δ^{2} T_{j} = {ΔT}_{j} - {ΔT}_{j - 1} = n .$
In step 806, if m ≥ 0, the user is currently exhaling and step 807 follows.
In step 807, if m + 0.2n ≤ 0, step 808 follows and the inlet fan is turned on and the outlet fan is turned off to assist inhalation. Thus it is determined that ΔT will shortly go negative and that exhalation has in reality ended.
In step 809, if m < 0, the user is currently inhaling and step 810 follows.
In step 810, if m + 0.2n ≥ 0, step 811 follows and the outlet fan is turned on and the inlet fan is turned off to assist exhalation. Thus it is determined that ΔT will shortly go positive and that inhalation has in reality ended.
The present invention also provides a computer program comprising computer program code means which is adapted, when said computer program is run on a computer, to implement the method of the present invention.
The method of the present invention makes 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.
Preferably, the device further comprises a battery to power the sensor 22, the controller 20 and the fan arrangement 14.
The mask may be for covering only the nose and mouth (as shown in Fig. 2) or it may be a full face mask. The example shown in Fig. 2 is a mask for filtering ambient air. However, the mask may be used with a breathing gas from an external supply, for example a breathing assistance device, such as a continuous positive air pressure (CPAP) system.
Further, the mask design described in Fig. 2 has the main air chamber formed by the filter material, through which the user breathes in air. The filter comprises a filter member in series with an inlet fan, when present. The outer wall of the air chamber may define the filter. Alternatively, a filter may be provided only at the location of the inlet fan, when present, in combination with a non-permeable outer housing. In this case, the inlet fan assists the user in drawing in air through the filter, thus reducing the breathing effort for the user. An inlet valve may be provided adjacent to the inlet fan, when present, and an outlet valve may be provided adjacent to the outlet fan, when present. In one embodiment, the mask further comprises a valve for exhausting air from inside the air chamber 13 to the outside.
It will be seen that the invention may be applied to medical ventilators and many different mask designs, with fan-assisted inhalation and exhalation, and with an air chamber formed by a filter membrane or with a sealed hermetic air chamber.

EXAMPLES

In the examples, the sensor used was a Sensirion (Trade Mark) SHT3x sensor, with 2.5 x 2.5 x 0.9 mm dimension size. The typical accuracy using this sensor is 2% relative humidity and 0.3°C temperature. The sensing range is 0 to 100% of relative humidity, and -40°C to 125°C of temperature. The sample rate was relatively high such as 10 Hz or higher. The sensor was controlled by a microcontroller through I2C interface. The sample data was stored in a microcontroller flash buffer.
In a first example, before applying the preferred method of Fig. 8 to the data of Fig. 6, the time lag ΔT (between the timing for the first value and the actual breathing cycle) based on ten breaths was 0.23 s. After applying the preferred method to this data, the time lag based on ten breaths was reduced to approximately 0.081 s, which is close to the sampling time (0.1 s) of the sensor.
In a second example, calculated first time derivatives of the temperature of air inhaled and exhaled over time by a user wearing a mask are shown in Figs. 9-11.
Fig. 9 shows the differential pressure versus time plot 901 and the breath rhythm detection plot 902 using the algorithm of Fig. 8 for a user walking;
Fig. 10 shows the differential pressure versus time plot 1001 and the breath rhythm detection plot 1002 using the algorithm of Fig. 8 for a user speaking; and
Fig. 11 shows the differential pressure versus time plot 1101 and the breath rhythm detection plot 1102 using the algorithm of Fig. 8 for a user at rest wearing a mask with an inlet fan and an outlet fan, which are run in synchronism with the start of the inhalation and the start of the exhalation
It is apparent that these detected breathing cycles match the differential pressure versus time plots (i.e. the true breathing cycle timings) well in a range of scenarios, namely walking in Fig. 9, speaking in Fig. 10 and sitting in Fig. 11. Thus, the mask of the invention may be used to accurately determine a start of an inhalation and/or a start of an exhalation of the user.
It is noted that use of first and second values as described above may be employed by a breathing cycle monitoring device such as a medical ventilator to accurately determine the start of an inhalation and/or the start of an exhalation of a user of the device. Thus, the invention may provide a breathing cycle monitoring device comprising:

a sensor for detecting a parameter relating to the temperature and/or relative humidity of air inhaled and exhaled over time by a user of the device;
a controller which is adapted to:
- calculate a first value which depends on a first time derivative of the parameter;
- calculate a second value which depends on a second time derivative of the parameter; and
- determine a start of an inhalation and/or a start of an exhalation of the user based on the calculated first and second values.

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 mask (11) comprising:
an air chamber (13);

a filter (12);

a fan arrangement (14) for ventilating the mask;

at least one sensor (22) for detecting the temperature and/or the relative humidity of air inhaled and exhaled over time by a user (10); and

a controller (20) which is adapted to:
calculate a first value which depends on a first time derivative of said temperature and/or on a first time derivative of said relative humidity;

calculate a second value which depends on a second time derivative of said temperature and/or on a second time derivative of said relative humidity;

determine a start of an inhalation and/or a start of an exhalation based on the first value and the second value; and

control the fan arrangement (14) based on the determined start of the inhalation and/or the determined start of the exhalation.
A mask as claimed in claim 1, wherein the controller is adapted to calculate the first value by obtaining the first time derivative of said temperature and/or the first time derivative of said relative humidity, and performing a low pass filtering.
A mask as claimed in claim 1, wherein the start of the inhalation is determined when:
the first value is greater than zero; and

m + t₁n < 0, wherein m is the first value, t₁ is a time value representing a sensor time lag associated with the first value and n is the second value.
A mask as claimed in claim 1 or 3, wherein the start of the exhalation is determined when:
the first value is less than zero; and

m + t₁n > 0, wherein m is the first value, t₁ is a time value representing a sensor time lag associated with the first value and n is the second value.
A mask as claimed in any preceding claim, wherein the first value is an average first value which depends on the first time derivative of said temperature and/or on the first time derivative of said relative humidity.
A mask as claimed in any preceding claim, wherein the fan arrangement (14) comprises an inlet fan (16) and an outlet fan (17).
A mask as claimed in claim 6, wherein the controller (20) is adapted to operate the inlet fan (16) at a first speed at the start of the inhalation and a second, lower, speed at the start of the exhalation, and operate the outlet fan (17) at a third speed at the start of the exhalation and a fourth, lower, speed at the start of the inhalation.
A mask as claimed in claim 7, wherein the second speed and the fourth speed are zero.
A mask as claimed in any preceding claim, wherein the mask further comprises a battery (21) to power the at least one sensor (22), the controller (20) and the fan arrangement (14).
A method of controlling a mask (11) according to claims 1-9, wherein the method comprises:
detecting the temperature and/or the relative humidity of air inhaled and exhaled over time by a user (10);

calculating a first value which depends on a first time derivative of said temperature and/or on a first time derivative of said relative humidity;

calculating a second value which depends on a second time derivative of said temperature and/or on a second time derivative of said relative humidity;

determining a start of an inhalation and/or a start of an exhalation based on the first value and the second value; and

controlling the fan arrangement (14) based on the determined start of the inhalation and/or the determined start of the exhalation.
A method as claimed in claim 10, comprising calculating the first value by obtaining the first time derivative of said temperature and/or the first time derivative of said relative humidity, and performing a low pass filtering.
A computer program comprising computer program code means which is adapted, when said computer program is run on the controller of the mask according to any one of claims 1 to 9, to implement the method according to claim 10 or claim 11.