Patent Publication Number: US-2013247913-A1

Title: Respiratory gas supply circuit for an aircraft carrying passengers

Description:
The present invention relates to a respiratory gas supply circuit for protecting the passengers of an aircraft against the risks associated with depressurization at high altitude and/or the occurrence of smoke in the cockpit. 
     To ensure the safety of the passengers in case of a depressurization accident or the occurrence of smoke in the aircraft, aviation regulations require on board all airliners a safety oxygen supply circuit able to supply each passenger with an oxygen flow rate function of the aircraft altitude. 
     In other words, the source of gas under pressure must be capable of instantly delivering oxygen or air greatly enriched in oxygen at a pressure sufficient for feeding the passengers. 
     Current systems are mainly pneumatic systems, regulating the pressure of the supplied oxygen thanks to a reducing valve operating as a function of the cabin pressure, or cabin altitude. By cabin altitude, one may understand the altitude corresponding to the pressurized atmosphere maintained within the cabin. This value is different than the aircraft altitude which is its actual physical altitude. 
     Such a pneumatic system is known from FR2646780. The described supply circuit allows an altitude-dependent regulation of the flow of respiratory gas fed to passengers through an orifice provided on breathing masks and comprises high-pressure oxygen reservoirs, a pressure regulator, and a valve. The valve is an altitude-dependent valve with an on/off functioning and does not provide any regulating function. The regulation of the oxygen flow is ensured individually for each cluster of breathing masks thanks to regulation means comprising an altimetric cell acting on a movable leak proof membrane. 
     The known pneumatic supply circuits generally lack a feedback loop, and are oversized as far too much oxygen is supplied to the mask wearers to ensure that the oxygen flow rate matches the regulatory minimums. 
     An object of the present invention is to provide an improved respiratory gas supply circuit that is simple, reliable and does not present the drawbacks from the known systems. An additional object of the present invention is to provide a supply circuit with a feedback loop that optimizes the need in respiratory gas and thus limit the onboard mass of breathing gas. 
     To this end, there is provided a respiratory gas supply circuit for an aircraft carrying passengers as claimed in claim  1 . 
     The pulse width modulation (PWM) signal allows an easy piloting of the electro valve, which is a reliable regulating device. 
    
    
     
       The above features, and others, will be better understood on reading the following description of particular embodiments, given as non-limiting examples. The description refers to the accompanying drawing. 
         FIG. 1  is a simplified view of a respiratory gas supply circuit for an aircraft carrying passengers according to a first embodiment of the invention; 
         FIG. 2  is a simplified view of a respiratory gas supply circuit for an aircraft carrying passengers according to a second embodiment of the invention, and; 
         FIG. 3  is an example of a PWM signal. 
     
    
    
     As seen on  FIG. 1 , the supply circuit according to the invention comprises the hereafter elements. A source of pressurized respiratory or breathable gas, here a couple of oxygen tanks R 1  and R 2  each comprising a reducing valve on their respective outlet, is provided to deliver through a supply line  2  a respiratory gas to the passengers of the aircraft. Other sources of pressurized breathable gas may be used in the supply circuit according to the invention. A plurality of secondary feedlines  3  is connected between supply line  2  and clusters  4  of respiratory masks  9 . Each cluster  4  of masks  9  may be provided in an enclosure  5  placed over the passengers&#39; seats. The enclosure  5  may comprise a junction  11  of feedline  3  into said box, a door  6  articulated around hinge  7  (and seen closed in the central cluster, and open in the right hand side cluster), and a connecting casing  8  that connects feedline  3  with the respiratory masks  9  thanks to flexible pipes  10 . The breathable gas is generally supplied to its wearer through an orifice within said mask. 
     A regulating device  12  is further provided, for example within enclosure  5 , to control the supply in respiratory gas to the masks and the passengers. In the supply circuit according to the first implementation of the invention, the regulating device  12  comprises an electro-valve controlled by a pulse with modulation signal provided by an electronic unit. 
     Pulse width modulation (PWM) is a powerful technique for controlling analog circuits with a microprocessor&#39;s (CPU) digital outputs. PWM is employed in a wide variety of applications, ranging from measurement and communications to power control and conversion. Pulse-width modulation control works by switching the power supplied to the electro-valve on and off very rapidly and at a varying frequency. A DC voltage is converted to a square-wave signal, alternating between fully on (e.g. nearly 12V or 18V) and zero, giving the valve a series of power “kicks” of varying length. An example of such a signal is shown in  FIG. 3 . 
     To that effect an electronic unit  20 , or CPU, is provided to elaborate the PWM signal sent to electro-valve  12 , as seen in doted lines for both clusters  4  of masks. A first pressure sensor  25  is provided in the cabin of the aircraft to supply a first pressure signal to the CPU  20  for elaborating a set point to control the electro-valve  12 . Pressure sensor  25  measures the cabin pressure, and allows the supply in respiratory gas as a function of the cabin altitude, so that the regulations oxygen supply curves are ensured. The pressure sensor  25  may be one of the pressure sensors available in the aircraft, its value being available upon connection to the aircraft bus. In order to ensure a reliable reading of the pressure independent of the aircraft bus system, the circuit according to the invention may be provided with its own pressure sensor, i.e. a sensor  25  is provided for each electronic unit  20 . 
     A second pressure sensor  15  is provided on the supply line downstream the regulating device  12 , i.e. in the example of  FIG. 1  within the enclosure  5  between electro-valve  12  output and connecting casing  8 , to supply a second pressure signal to the CPU  20  that corresponds to the regulated pressure. Second pressure sensor  15  allows a feedback loop to ensure that the right supply in oxygen follows the demand from the passengers when wearing the masks. 
     To that effect, the electronic unit  20  compares the set point to the regulated pressure, i.e. the value of sensor  15  to elaborate the PWM signal. 
     A PID module (proportional, integral, derivative) may be comprised within electronic unit  20  to elaborate the PWM signal from the comparison of the set point and the regulated pressure. 
     In an additional embodiment, electro-valve  12  is a solenoid valve. More precisely, in a preferred embodiment, electro-valve  12  is a two position on/off solenoid valve, with a variable duty ratio. Such a valve is particularly suited to be driven by the PWM signal sent by CPU  20 . The valve may also be a piezo electric valve. In the first implementation of the supply circuit according to the invention, valve  12  is provided on the supply line, and directly opens and cuts off the supply in respiratory gas. More precisely, in the illustration of  FIG. 1 , valve  12  is provided within the box  5  between junction  11  and connecting casing  8 . 
     The first implementation of the invention is particularly well suited to drive a cluster of masks locally through the regulating device  12 . Each cluster  4  is attached to its own regulating device. This ensures that if for some reasons one cluster fails, its does not affect the other clusters that carry on the supply in respiratory gas. 
     In the first implementation, the electro-valve  12  directly drives the supply in breathable gas as valve  12  is located on supply line  3 . 
     The regulating means or the pressure sensor  15  may be advantageously located close to the cluster of masks. By a close location, one may understand a location on the supply line wherein the pressure loss between each mask and the regulating device, or the pressure sensor respectively, is negligible. 
     The second implementation of the supply circuit according to the invention is illustrated in  FIG. 2 . Unless written otherwise, the same numbers refer to the same parts. 
     The regulating device comprises a flow amplifier  30  provided on the supply line  2  connecting a source of pressurized breathable gas (not shown) to a plurality of respiratory masks  9  provided for example within an enclosure  5  as described for the previous embodiment. The flow amplifier  30  further comprises a piston  32 , e.g. an annular piston, subjected to the pressure difference between the ambient pressure and the pressure that exists inside a piston chamber  34 . An electro-valve  12 , e.g. specifically a solenoid valve, serves to connect the piston chamber  34  to the pressurized respiratory gas through pipe  122 . Chamber  34  may also be connected to the ambient pressure in the cabin through pipe  123 . 
     Electro-valve  12  thus serves to vary the pressure within chamber  34  so that piston  32  is movable between a first position wherein the supply line is open (piston  32  is kept away from supply line  2  inner section) and a second position wherein the supply line is closed (piston is pushed to close an inner section of supply line  2 ). Piston  32  is movable in response to the outlet pressure of the two positions on/off solenoid valve  12 , its inlet being connected to the source of pressurized respiratory gas. 
     When the piston chamber  34  is connected to the cabin ambient pressure, i.e. solenoid valve  12  is off, and the pressure in chamber  34  is maintained to the cabin ambient pressure thanks to pipe  123 , a spring  38  holds piston  32  in a position away from closing supply line  2 . When solenoid valve  12  is on, chamber  34  is connected to the pressurized source of respiratory gas through pipe  122 . A narrow section may be provided on pipe  123  so that its section is insufficient to lower the pressure in chamber  34  when solenoid valve  12  is on. 
     Electro-valve  12  is controlled through CPU  20  that sends a PWD signal that can be elaborated thanks to the first pressure sensor  25  provided in the cabin of the aircraft and/or thanks to the second pressure sensor  15  provided downstream the regulating device as described before. 
     The second implementation of the invention allows to drive a large number of masks through the regulating device thanks to the flow amplifier  30 . 
     In the second implementation, as the demand in breathable gas may be larger and the pressure loses along supply line  3  larger, a flow amplifier  30  is required. The supply in breathable gas is driven indirectly by valve  12  as a result of valve  12  piloting piston  32 . 
     The invention allows to control the volume of breathable gas supplied to the masks. The successive opening and closing cycles of the regulating means lead to a controlled average volume or “integrated” volume of breathable gas downstream the regulating means. The average volume creates a pressure P that is measured thanks to pressure sensor  15 . Based on the cabin altitude, a breathable gas must be fed to the mask at a pressure set point value. The PWM signal is elaborated by the electronic unit to pilot the regulating means to deliver said breathable gas at said pressure set point value. 
     The time between pulses and/or the length of each pulse may vary to ensure the right volume of breathable gas fed to the masks, based on the feedback loop and the set point. 
     The respiratory gas supply circuit according to the invention is particularly well suited to be associated to a rebreathing bag as known from US 2003,101,997. Such a document discloses a respiratory mask for protecting passengers of an airplane against depressurization of an airplane cabin at high altitude, the mask being provided on a respiratory supply circuit comprising a feed control unit for supplying an adjustable continuous flow rate to a general pipe from a source of respiratory gas under pressure. The masks are further connected to said general pipe via a flexible economizer bag. Furthermore, a flexible re-breathing bag is connected to each of said mask by means enabling gas to enter freely into the flexible re-breathing bag from the mask and retarding re-breathing from said flexible re-breathing bag after beginning of breathing in by one of said passengers bearing the mask. The re-breathing bag has preferable a volume when inflated such that it is capable to store only an initial fraction of the gas breathed out on each exhalation by the passenger wearing the mask. The control unit of US 2003,101,997 further has means for regulating the flow rate of additional oxygen delivered to said pipe responsive to ambient pressure to which the mask wearers are subjected in order to limit said flow rate to a fraction only of the flow rate that would be necessary in the absence of re-breathing.