AU1552792A

AU1552792A - Method for controlling an air supply unit for respirators

Info

Publication number: AU1552792A
Application number: AU15527/92A
Authority: AU
Inventors: Thomas Arvidsson; Anders Nilsson
Original assignee: Sundstroem Safety AB
Current assignee: Sundstroem Safety AB
Priority date: 1991-04-12
Filing date: 1992-04-10
Publication date: 1992-11-17
Anticipated expiration: 2012-04-10
Also published as: DE4291092T1; SE9101097D0; GB2271286B; GB2271286A; SE467041B; WO1992018201A1; SE9101097L; AU650740B2; GB9320606D0

Description

METHOD FOR CONTROLLING AN AIR SUPPLY UNIT FOR RESPIRATORS

The present invention relates to a method for con¬ trolling, in respiration-synchronised manner, a portable air supply unit for a respirator covering at least the no and/or mouth of the user and having an air inlet and an a outlet, said air supply unit comprising an air inlet, an air filter, an air outlet communicating with the air inle of the respirator, a fan driven by an electric motor and supplying the respirator with air that has passed through the filter, a battery for driving the air supply unit, an an electrically operable valve for controlling the air fl to the respirator.

Though prior-art respirators with fan-driven air supply in some respects. are superior to conventional resp rators, they also have several shortcomings.

One superior point is that the filter presents no re sistance to the inhalation, and effective filters thus ma be used also for extremely heavy work without causing the user any discomfort. Another advantage is that the fan makes it possible to maintain at all times a certain posi tive pressure in the respirator, thereby preventing un- filtered air from penetrating into the respirator.

However, one disadvantage is that the service life o the filter is reduced by the considerable increase in the amount of air that has to be filtered. This increase is mainly caused by a lack of respiration synchronism im¬ pairing prior-art air supply units which often continue unabated during exhalation to supply air through the filt and into the respirator, where the air obstructs exhalati and finally escapes unused to the ambient atmosphere via the respirator outlet. Another disadvantage is that prior art respirators with fan-driven air supply are comparativ ly heavy. This is mainly due to the battery which drives the fan and thus has to be dimensioned also for all the unnecessary work the fan has to perform when there is no respiration synchronism. Efforts have long been made to obviate the above dis advantages, but no overall solution has been found. For instance, there are some constructions in which attempts have been made to adjust the air flow according to the respiration by controlling the fan speed. This is not, however, practically feasible, since the fans employed ha large, high-speed impellers with considerable inertia of rotation. Thus, even when the current supply to the fan motor is completely cut off, there is only a limited redu tion of the fan speed during the short time an exhalation lasts (1-3 s) and, consequently, an unnecessary large air flow through the filter. In addition, the limited speed reduction achieved entails energy losses when the speed, view of the next inhalation, is to be increased to the initial speed.

Thus, the solution seems to be some sort of valve arrangement for controlling the air flow to the respirato However, prior-art constructions of this type are impaire by a number of inconveniences, e.g. a large consumption o batteries due to the fan being not adjusted or adjusted to inexactly, and a strong respiration resistance caused by the use of a simple non-return valve in the air inlet of the respirator, this valve being designed to be closed by the pressure in the respirator against the pressure gene- rated by the fan i the air supply unit.

The object of the present invention is, therefore, to provide an air supply unit which in respiration-synchro¬ nised manner throttles the air flow to the respirator during exhalation, thus sparing the filter and reducing th exhalation resistance, and also controls the fan in a manner that minimises energy consumption.

According to the invention, this object is achieved b a method which is of the type stated in the introduction t this specification and which is characterised by sensing a pressure that is proportionate to the air pressure in the respirator; closing the valve when the pressure in the respirator, at the beginning of each exhalation phase, reaches an upper set limit value; opening the valve when the pressure in the respirator at the end of each exhala¬ tion phase reaches a lower set limit value; and so setting the operating voltage of the electric motor for each in- halation phase that a sufficiently high speed is imparted to the fan to maintain the pressure in the respirator on a level ensuring that the user has access to a surplus of filtered air during the inhalation phase, the setting operation being performed such that the operating voltage of the electric motor is increased/decreased if the lowest pressure sensed during at least one preceding inhalation phase or during the current inhalation phase goes below/ exceeds a third pressure value lying between said limit values. By throttling, according to the invention, the air flow to the respirator during exhalation by means of a valve, optimum saving in filter and minimum respiration resistance are achieved.

The method according to the invention will be de- scribed in more detail below with the aid of the illus¬ trated embodiments of air supply units operating according to the above method. In the drawings,

Fig. 1 is a schematic view illustrating an air supply unit with associated respirator, Fig. 2 is a schematic view illustrating an alternativ embodiment of the air supply unit of Fig. 1,

Fig. 3 is a diagram illustrating the volume flow rate during respiration, and

Fig. 4 is a diagram illustrating the variation in pressure during respiration.

Fig. 1 shows a respirator in the form of a full face mask 1 having an exhalation valve 2 and an air inlet 3 which, via a flexible bellows-type hose 4, communicates with a filter 5. This filter forms part of an air supply unit which in addition comprises a fan 6, a valve 7, an electronic control unit 8, a pressure-sensing duct 9, and battery (not shown) for driving the fan 6, the valve 7 and the control unit 8.

In the embodiment shown, the fan 6 is a radial fan, but might e.g. also be an axial fan, and is driven by a d.c. motor (not shown). The fan draws air from the atmoε- phere, and then feeds the air up to the valve 7 which is a electrically operated throttle valve for controlling the air flow to the filter 5. The throttle valve 7 is operated by a step motor which, in economical fashion and with high precision, opens and closes the throttle valve 7. Both in open and in closed position, the valve can be operated without much force owing to the fact that the forces actin on the throttle disc substantially counterbalance one an¬ other also with very large air flows. After the valve 7, the air from the fan 6 reaches the filter 5 which is of gas-,liquid- or dust-separating type and which cleans the air before this reaches the mask 1 via the flexible bellows-type hose 4 and the air inlet 3. Here, a fair amount of air is taken in by the user before being exhaled via the exhalation valve 2. The amount of air supplied to the mask, as well as the supply rate, is determined by the electronic control unit 8 controlling both the fan speed and the opening and closing of the throttle valve 7. The control unit 8 includes a pressure transducer which, via the pressure-sensing duct 9, senses the pressure at the inlet 3 of the mask 1, this pressure being proportionate t the pressure in the mask 1 proper.

Fig. 2 illustrates an alternative embodiment of the air supply unit of Fig. 1. The fan 6 and the filter 5 have, however, changed places, which does not affect the air supply function. Also the pressure-sensing duct 9 to the pressure transducer of the control unit 8 has been moved, and is here connected to the outlet of the fan 6, but the transducer still senses a pressure that is proportionate t the pressure in the mask 1. In Fig. 2, a dash-dot line has been drawn about the air supply unit to emphasise that it is a unit which in itself contains everything needed for its operation and which may be designed e.g. as a box carried on the stomach. The only requirement on the mask is that it must comprise an air inlet 3, for connection- the flexible bellows-type hose 4, and some sort of throt ling means for the air to be discharged, e.g. an air out 2 equipped with a non-return valve which opens at a cert positive pressure in the mask 1 so as to discharge the u air.

Fig. 3 is a diagram illustrating different air volu and volume flow rates of air occurring in the mask 1 in fixed-step adjustment. The continuous curve indicating t volume flow rate of air e.g. through the nose illustrate the respiration cycle of a user. This flow is regarded as positive when inhaled and negative when exhaled. The int ral I. for the positive part of the curve thus is a meas of the amount of filtered air that has to be supplied to the mask 1 with the aid of the fan 6.

The volume flow rate of air supplied by the fan 6 i illustrated by the discontinous curve in the Figure. As shown; the volume flow rate of air supplied by the fan 6 largely constant, and alternates very rapidly when the valve 7 is opened and closed between zero (or close to zero, since a certain leakage cannot be avoided) and a fl exceeding by a certain margin the maximum volume flow rate caused by the user when inhaling. This margin is me to act as a buffer to sudden changes in the respiration cycle and is further required to enable the maintaining all times in the mask 1 of a certain positive pressure required according to different standards and intended t prevent unfiltered air from penetrating into the mask 1, e.g. at the;,.edges thereof. The size of this margin depen on the fan speed, which in turn is dependent on the oper¬ ating voltage of the fan motor, this voltage being set, when the valve is open, at a value based on the pressure measured during at least one preceding inhalation phase, and being maintained during the entire inhalation phase. When the valve is closed, the operating voltage is tempo¬ rarily reduced in a fixed step, e.g. by 50%, to maintain even fan speed, thereby restricting the energy consumption of the fan motor. As appears from the integral I₂, "the air volume thus generated by the fan 6 covers the necessary ai requirement I. by a considerable margin. The saving of the filter achieved by the air supply unit according to the invention is illustrated, in the diagram, by means of the obliquely lined field I₃ showing the air volume that would be consumed unless the air current from the fan 6 into the mask 1 were controlled by the valve 7. Fig. 4 is a diagram whose time axis coincides with that of the diagram in Fig. 3 and which schematically illustrates alterations in pressure in the mask 1 during the above fixed-step adjustment. The diagram indicates two pressure levels on which the control of the valve 7 is based, namely P_hi, which is the limit value for the pres¬ sure at which the valve in the air supply unit is closed when exhalation has begun, and P_lo, which is slightly abov atmospheric pressure and is the limit value for the pres¬ sure at which the valve in the air supply unit is opened when exhalation is coming to an end. Also indicated in the diagram, P-_, _mJ _Tϊr is a desired value slightly exceeding the standard values stated above. The control unit 8 con¬ trols the fan speed towards this desired value by calcu¬ lated voltage variation. The method of the invention, comprising fixed-step control of the fan speed and operation of the valve, en¬ sures minimal energy consumption, the fan speed being essentially maintained constant during the respiration cycle, as well as minimal filter consumption, the flow of air through the filter being throttled during the major part of the exhalation phase.

In an alternative method according to the invention for controlling the fan speed and the valve, use is made o so-called PID control instead of fixed-step control for adjusting the fan speed. Thus, the operating voltage of the electric motor can, at the beginning of the inhalation phase, be determined with the aid of the pressure in the mask 1 and the deriva¬ tive of this pressure, to obtain rapid response at pressur drops, thereby further preventing leakage caused by too lo a pressure. Otherwise, the operating voltage can be deter¬ mined both at the end of the inhalation phase and during the exhalation phase with the aid of the pressure and its integral, to achieve slow alterations in speed and, conse- quently, a more even operation and a lower energy consump¬ tion. Also the proportionate part of the PID control can b used, either separately or combined with the above deriva¬ tive and/or integral, for controlling the operating voltag of the electric motor during the inhalation and the exhala tion phases.

I^*t goes without saying that the method according to the invention is not restricted to the two air supply unit illustrated in Figs 1 and 2, these being but Examples of possible applications of the invention. Thus, the inventio might also be used in a device in which the air supply function is completely or partly integrated with the respi rator. Also, the respirator, illustrated in the form of a full face mask, may be of another type and consist e.g. of a hood or a helmet.

Claims

1. Method for controlling, in respiration-synchronis manner, a portable air supply unit (5-9) for a respirator (1) covering at least the nose and/or mouth of the user an having an air inlet (3) and an air outlet (2), said air supply unit comprising an air inlet, an air filter (5), an air outlet communicating with the air inlet (3) of the respirator (1), a fan (6) driven by an electric motor and supplying the respirator (1) with air that has passed through the filter (5), a battery for driving the air supply unit, and an electrically operable valve (7) for controlling the air flow to the respirator, c h a r a c - t e r i s e d by sensing a pressure that is proportionate to the air pressure in the respirator (1); closing the valve (7) when the pressure in the respirator (1), at the beginning of each exhalation phase, reaches an upper set limit value (P.. ); opening the valve (7) when the pressure in the respirator (1) at the end of each exhalation phase reaches a lower set limit value (P_lo)? and so setting the operating voltage of the electric motor for each inhalatio phase that a sufficiently high speed is imparted to the fa (6) to maintain the pressure in the respirator (1) on a level ensuring that'the user has access to a surplus of filtered air during the inhalation phase, the setting oper ation being performed such that the operating voltage of the electric motor is increased/decreased if the lowest pressure sensed during at least one preceding inhalation phase or during the current inhalation phase goes below/ exceeds a third pressure value (P i.n, m„m,._^) lying between sai limit values (P_lor ^P _hi^ *

2. The method of claim 1, c h a r a c t e r i s ¬ e d by temporarily reducing the operating voltage of the motor when closing the valve.

3. The method of claim 1 or 2, c h a r a c t e r - i s e d by adjusting the operating voltage of the electr motor for each inhalation phase with the aid of the pres¬ sure measured during the inhalation phase. . The method of any one of the preceding claims, c h a r a c t e r i s e d by adjusting the operating voltage of the electric motor for each inhalation phase with the aid of the pressure measured at the initial stag of the inhalation phase, and its derivative. 5. The method of any one of the preceding claims, c h a r a c t e r i s e d by adjusting the operating vol age of the electric motor for each inhalation phase with the aid of the pressure measured at the final stage of th inhalation phase, and its integral. 6. The method of claim 1, c h a r a c t e r i s ¬ e d by setting, when opening the valve, the operating voltage of the electric motor for each inhalation phase a a value determined with the aid of the lowest pressure sensed during the immediately preceding inhalation phase. 7. The method of claim 6, c h a r a c t e r i s ¬ e d by reducing the operating voltage of the electric motor in one step when closing the valve.