Abstract:
A dual stage hybrid inflation device includes a pressure switch that prevents the second stage pyrotechnic gas generator from being initiated when the additional heat and gas would overpressurize the first stage pressure vessel. As the pressurized gas stored in the first stage pressure vessel escapes the first stage pressure vessel, the pressure in the first stage pressure vessel drops. If the pressure in the first stage pressure vessel drops below the threshold pressure of the pressure switch, the pressure switch closes allowing the voltage signal to reach the pyrotechnic gas generator thereby initiating the second stage gas source. In an alternative embodiment, the pressure transducer is replaced by a pressure transducer or a temperature transducer and timing circuit. By delaying the initiation of the second stage gas source until after the pressure in the first stage pressure vessel has dropped below the threshold level, the appropriate augmentation of the compressed first stage gas can be realized without the necessity of designing the first stage pressure vessel to withstand initiation of the pyrotechnic gas generator under maximum inflation pressure conditions.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to emergency evacuation equipment for aircraft. In particular, this invention relates to an inflation device for inflating an inflatable aircraft evacuation slide or other inflatable device. 
   The requirement for reliably evacuating airline passengers in the event of an emergency is well known. Emergencies at take-off and landing often demand swift removal of the passengers from the aircraft because of the potential for injuries from fire, explosion, or sinking in water. A conventional method of quickly evacuating a large number of passengers from an aircraft is to provide multiple emergency exits, each of which is equipped with an inflatable evacuation slide. Current state of the art emergency evacuation slide systems comprise an inflatable evacuation slide which is stored in an uninflated folded state together with a source of inflation gas. The source of inflation gas typically comprises a gas generator, stored compressed gas, or a combination thereof. Pyrotechnic gas generators have an advantage in that they are small, lightweight, and produce a high volume of gas, however, the high temperature gas produced by a gas generator alone can cause numerous problems including sagging of the evacuation slide as the inflation gas cools and, in some cases, melting of the fabric out of which the inflation slide if fabricated. Use of stored compressed gas by itself, although simple, implicates a weight penalty that must be paid for carrying a pressure vessel having sufficient capacity (in terms of volume and pressure) to inflate the evacuation slide over the wide operational temperature range specified for such slides. Additionally, where only a compressed gas is used to inflate the evacuation slide, a large drop in temperature occurs as the gases expand, often causing ice to form, which can block the flow of gas. Accordingly, state-of-the-art emergency evacuation slide evacuation systems typically comprise a hybrid inflator, which utilizes a stored compressed gas together with a pyrotechnic gas generator. The pyrotechnic gas generator augments the stored compressed gas by providing additional gas, as well as heat to counteract the effects of the expansion-induced cooling of the compressed gas as it expands out of the pressure vessel. 
   To further augment the volume of gas delivered to the evacuation slide, many evacuation systems utilize aspirators such as that disclosed in U.S. Pat. No. 4,368,009 to Heimovics, et al. As the compressed gas flows through the aspirator, a venturi is produced that causes the aspirator to pump about two to three times as much gas as is supplied by the compressed gas source alone. 
   Despite these advances, there still exist problems due to the wide ambient temperature range over which these inflation systems are required to operate, typically from −65° F. to +165° F. The amount of gas available must be enough to pressurize the evacuation slide at the coldest temperature. However, because of the relationship between pressure and temperature within a fixed volume, as the ambient temperature rises above the minimum, the pressure within the pressure vessel rises proportionately. Accordingly, in current state-of-the-art hybrid inflation systems, the storage vessel must be capable of withstanding the pressure of the compressed gas at 165° F. Not only this, but the storage vessel must withstand the overpressure at 165° F. caused by the pyrotechnic gas generator, even though use of the pyrotechnic gas generator at 165° F. causes the inflation system to produce substantially more gas than is necessary to inflate the evacuation slide. Additionally, the substantial overpressure caused at high temperature further increases the weight of the system, because additional Pressure Relief Valves (PRV&#39;s) must be incorporated into the evacuation slide to vent the excess gases. 
   Accordingly, what is needed is a system for tailoring initiation of the pyrotechnic gas generator in a hybrid compressed gas/pyrotechnic inflation device to prevent overpressurization of the compressed gas pressure vessel over a wide range of temperatures. 
   SUMMARY OF THE INVENTION 
   The present invention comprises a dual stage hybrid inflation device including a first stage gas source comprising a pressurized gas stored in a first stage pressure vessel and a second stage gas source having a gas output directed into the first stage pressure vessel. According to an embodiment of the present invention, the first stage pressure vessel is in fluid communication with an inflatable device such as an aircraft emergency evacuation slide via a combination valve/regulator and one or more conventional aspirators. A pressure transducer (or normally open pressure switch) senses the pressure in the first stage pressure vessel. The inflation system is initiated by a voltage signal, for example from a sensor mounted to detect opening of the aircraft door. The voltage signal opens the valve of the first stage pressure vessel and simultaneously sends a voltage signal to arm the firing circuit for the pyrotechnic gas generator comprising the second stage gas source. The pressure switch, which is normally open above a threshold pressure, interrupts a voltage signal to or from the firing circuit, which causes the firing circuit to delay initiation of the pyrotechnic gas generator if the pressure in the first stage pressure vessel is above the threshold level. This prevents the pyrotechnic gas generator from being initiated when the additional heat and gas would overpressurize the first stage pressure vessel. As the pressurized gas stored in the first stage pressure vessel escapes the first stage pressure vessel into the inflatable evacuation slide, the pressure in the first stage pressure vessel drops. If the pressure in the first stage pressure vessel drops below the threshold pressure of the pressure switch, the pressure switch closes, which permits the firing circuit to initiate the second stage gas source. By delaying the initiation of the second stage gas source until after the pressure in the first stage pressure vessel has dropped below the threshold level, the appropriate augmentation of the compressed first stage gas can be realized without the necessity of designing the first stage pressure vessel to withstand initiation of the pyrotechnic gas generator under maximum inflation pressure conditions. This enables the first stage pressure vessel to be of a lighter weight construction thereby saving substantially both in costs and fuel savings for the aircraft on which such systems are installed. 
   In an alternative embodiment the pressure switch is replaced with a pressure transducer. An external circuit monitors the pressure signal and closes a circuit or otherwise provides a signal to the second stage gas source when the pressure has dropped below the threshold level. In another alternative embodiment, the pressure transducer is replaced by a temperature transducer. An external timing circuit closes a circuit or otherwise sends a signal to initiate the second stage gas source based on known pressure versus time behavior of the inflation system. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a partially schematic sectional view of an inflator incorporating features of the present invention; 
       FIG. 2  is an illustrative projected pressure-time profile of the pressure output of the embodiment of  FIG. 1  operating at three different operating temperatures; 
       FIG. 3  is a partially schematic sectional view of an alternative embodiment of an inflator incorporating features of the present invention; and 
       FIG. 4  is a partially schematic sectional view of another alternative embodiment of an inflator incorporating features of the present invention. 
   

   DETAILED DESCRIPTION 
   The drawing figures are intended to illustrate the general manner of construction and are not necessary to scale. In the detailed description and in the drawing figures, specific illustrative examples are shown and herein described in detail. It should be understood, however, that the drawing figures and detailed description are not intended to limit the invention to the particular form disclosed, but are merely illustrative and intended to teach one of ordinary skill how to make and/or use the invention claimed herein and for setting forth the best mode for carrying out the invention. 
   The present invention is directed to a method and apparatus for inflating an inflatable device such as an aircraft emergency evacuation slide over a wide range of operating temperatures. An inflator incorporating features the present invention is shown in FIG.  1 . Inflator  100  comprises a first stage gas source  110  and a second stage gas source  112 . First stage gas source  110  comprises a pressure vessel  114  containing a mixture of compressed inflation gases  116 . In the illustrative example of  FIG. 1 , inflation gas mixture  116  comprises approximately a 2:1 ratio of carbon dioxide to argon. In general, however, the mixture of inflation gases, as well as the ratio of gases, can vary based on the particular application. Because carbon dioxide liquifies at ambient temperatures at a relatively modest pressure, depending on the ambient temperature, some or all of the carbon dioxide component of inflation gas mixture  116  will be in a liquid state. Accordingly, as used herein the term compressed gas means and refers to gas that is in a gaseous state under pressure as well as gas that has changed phase to a liquid state under pressure. Pressure vessel  114  has a gas exit opening  118  to which is attached a regulator valve  120 . Regulator valve  120  is normally closed but is remotely electrically actuated via an input terminal  122  which is adapted to receive a voltage signal that opens regulator valve  120  in a conventional manner. Regulator valve  120  is in fluid communication with an output duct  124  leading to the inflatable device  128  through one or more conventional aspirators  126 . 
   Although an argon/carbon dioxide mixture is disclosed in the embodiment of  FIG. 1 , any of the pressurized inflation gases well known in the art may be used for inflation gas mixture  116 . For example, gases that may be utilized in accordance with the present invention either alone or in combination include, but are not limited to carbon dioxide, nitrogen, chlorofluorocarbons, bromofluorocarbons, nitrous oxide and argon. The combination of carbon dioxide and argon is preferred, however, because of the ability of carbon dioxide to liquify, and thus require a minimum of storage volume, and the high specific heat of argon which enables it to absorb substantial heat from a pyrotechnic gas generator. 
   Second stage gas source  112  comprises a pyrotechnic gas generator  130  either alone or in combination with a compressed gas  132  (such as disclosed in U.S. Pat. No. 5,988,438 to Lewis, et al. and assigned to the Assignee of the present invention). Pyrotechnic gas generator  130  comprises a pyrotechnic material  134  which may either be in stick form or pressed into a container (not shown). Pyrotechnic material  134  is initiated by an initiator or squib  136  comprising a bridge wire  138  and an initiation composition  140 . Pyrotechnic material  134  may be any pyrotechnic gas generator material such as sodium-azide, or lithium-azide coupled with an oxidizer such as sodium nitrate, potassium nitrate, potassium perchlorate and the like but preferably comprises ammonium nitrate in combination with a secondary explosive such as cyclotrimethylene trinitramine (RDX); cyclotetramethylene tetranitramine (HMX); pentaerythritol tetranitrate (PETN), hexanitrohexaazaisowurtzitane (CL20) or similar energizers that produce a high volume of gaseous combustion products with little or no particulates. A most preferred gas generator material is UPCO 8043, a slow burning, relatively insensitive ammonium nitrate base propellant available from Universal Propulsion Company, Inc. of Phoenix, Ariz. The initiation composition  140  may be any heat sensitive primary explosive such as a mixture of zirconium or titanium with potassium perchlorate, boron calcium chromate, lead styphnate or similar primary explosive suitable for use in hot wire electro explosive devices. 
   In operation, upon an initiation event, such as the opening of an aircraft emergency exit door in the “armed” position, a voltage signal is transmitted along input conductor  142  to input terminal  122  of regulator valve  120 , input terminal  152  of pressure switch  150 , and input terminal  72  of firing circuit  70 . The signal received at input terminal  122  of regulator valve  120  causes regulator valve  120  to open immediately, beginning the flow of inflation gas mixture  116  through output duct  124  into inflatable device  128 . The signal received at input terminal  72  of firing circuit  70  simultaneously arms firing circuit  70 . Firing circuit  70  may be may be a safe and arming firing circuit such as disclosed in U.S. Pat. No. 5,335,598, the teaching of which is incorporated herein by reference to the extent necessary to supplement this specification. The valve portion of regulator valve  120  may be a conventional solenoid operated valve, explosively initiated valve or other conventional valve that may be remotely actuated. Additionally, although regulator valve  120  in the illustrative embodiment of  FIG. 1  is electrically actuated, regulator valve  120  may also be mechanically actuated (e.g. by a lanyard or other mechanical means) in which case other means for simultaneously applying a voltage to input terminal  152  of pressure switch  150  and input terminal  72  of firing circuit  70  (e.g. a switch applying a battery voltage) would be provided. Similarly, although the regulator portion of regulator valve  120  in the illustrative embodiment is a conventional sliding spool regulator, other regulation means, such as a simple orifice may be used in accordance with the present invention. In the illustrative embodiment of  FIG. 1 , the pressure within pressure vessel  114  may be from approximately 1,000 psi at −65° F. to 4,000 psi at 165° F. The aspirator  126 , however, performs most efficiently with an inlet pressure of 400 psi. Therefore, in addition to providing the opening function upon initiation, regulator valve  120  also operates to regulate the pressure within pressure vessel  114  down to 400 psi at the inlet to aspirator  126 . 
   Pressure switch  150  is normally open above a threshold pressure (e.g. 2,000 psi). Accordingly, as long as the pressure within pressure vessel  114  is above the threshold pressure of pressure switch  150 , the voltage signal at input terminal  152  of pressure switch  150  does not appear at output terminal  154  of pressure switch  150  and thus does not appear at trigger terminal  74  of firing circuit  70 . As a result, firing circuit  70  does not initiate pyrotechnic gas generator  130 . When the pressure in pressure vessel  114  drops below the threshold pressure of pressure switch  150 , however, pressure switch  150  closes and the voltage signal at input terminal  152  is transmitted to the trigger terminal  74  of firing circuit  70 , which in turn causes firing circuit  70  to initiate squib  136  of pyrotechnic gas generator  130 . Upon initiation of pyrotechnic gas generator  130  the pressure in second stage pressure vessel  144  rises until burst disk  148  ruptures allowing the high temperature gas from second stage gas source to enter first stage pressure vessel  114  thereby raising the pressure inside pressure vessel  114  and vaporizing any remaining liquified constituent of inflation gas mixture  116 . By delaying the initiation of pyrotechnic gas generator  130  until the pressure in pressure vessel  114  has dropped below the threshold level, inflatable device  128  is inflated rapidly, yet the peak pressure exerted on pressure vessel  114  does not exceed the maximum operating pressure of the pressure vessel irrespective of the operating temperature of the system. 
   This advantage of an inflator constructed in accordance with the teachings of the present invention can be best understood with reference to  FIG. 2 , which illustrates projected pressure-time profiles of the illustrative embodiment of  FIG. 1  operating at three different operating temperatures. The pressure-time profile at −65° F. is represented by the line having reference numeral  210 . At −65° F., the initial pressure in first stage pressure vessel  114  is 1,000 psi, which is below the 2,000 psi threshold pressure of pressure switch  150 . Accordingly, pressure switch  150  is closed. Therefore, when a voltage signal is received along input conductor  142 , squib  136  initiates pyrotechnic gas generator  130  simultaneous with the opening of regulator valve  120 . Similarly, at +70° F., represented by the line having reference numeral  212 , the initial pressure in first stage pressure vessel  114  is 2,000 psi. Accordingly, pressure switch  150  is just at the point of closing. Therefore, when a voltage signal is received along input conductor  142 , squib  136  initiates pyrotechnic gas generator  130  simultaneous with the opening of regulator valve  120 . 
   However, at 165° F., the initial pressure in first stage pressure vessel  114  is already 4,000 psi. If the pyrotechnic gas generator  130  were initiated simultaneous with the opening of valve  120 , as indicated by the line having reference numeral  214 , the pressure inside first stage pressure vessel  114  would rise rapidly to in excess of 5,000 psi, substantially above the 4,100 psi maximum operating pressure (MOP) of first stage pressure vessel  114 . With the incorporation of pressure switch  150 , however, as indicated by the line having reference numeral  216 , upon receipt of a voltage signal along input conductor  142 , regulator valve  120  opens and the pressure within first stage pressure vessel  114  begins to drop as gas expands out of first stage pressure vessel  114 . Only when the pressure in first stage pressure vessel  114  drops below the threshold pressure of pressure switch  150  is the voltage signal communicated to bridge wire  138  of squib  136  thereby initiating pyrotechnic gas generator  130 . Upon the initiation pyrotechnic gas generator  130 , the pressure in first stage pressure vessel  114  rises again, however, it does not exceed the 4,100 psi MOP of first stage pressure vessel  114 . 
   As is evident from the foregoing, there may be instances for systems with extremely wide operating temperature ranges that a hybrid gas generator/compressed gas inflator having sufficient capacity to inflate an inflatable device at the coldest temperature may not require the gas generator to function at all at the highest temperature range. Accordingly, a simple timing circuit or other device may be incorporated to preclude the initiation of the pyrotechnic gas generator if the pressure does not drop below the threshold pressure within a predetermined period of time (e.g. within the first four seconds after opening the regulator valve). In this case, the inflatable device will simply be inflated completely using the compressed gas without having to vent the excess inflation gas that would be produced by the gas generator. 
   An alternative embodiment of an inflator incorporating features of the present invention is shown in FIG.  3 . In the embodiment of  FIG. 3 , pressure switch  150  is replaced with a pressure transducer  350 . Initiation of pyrotechnic gas generator  130  is controlled by an external initiation circuit  310 , which may be a safe and arming firing circuit such as disclosed in U.S. Pat. No. 5,335,598, the teaching of which is incorporated herein by reference to the extent necessary to supplement this specification. In operation, a voltage signal is received along input conductor  342  from an external source such as a sensor mounted to an aircraft emergency evacuation door. As with the embodiment of  FIG. 1 , the voltage signal causes regulator valve  120  to open immediately allowing the inflation gas mixture  116  to begin to flow into inflatable device  128  via aspirator  126 . Simultaneously, the voltage signal is received at input terminal  312  of circuit  310 . Circuit  310  monitors the pressure signal from pressure transducer  350  via input  314  and produces a firing signal to initiator  136  via output  316  when the pressure signal from pressure transducer  350  indicates that the pressure in first stage pressure vessel  114  has dropped below the predetermined threshold. The advantage of using an external  310  over the simple series connection of the embodiment of  FIG. 1  is that circuit  310  can include additional circuitry for monitoring the status of pressure transducer  350  and provide an error signal in the event pressure transducer  350  shows an out-of-range reading or otherwise appears to be malfunctioning. 
     FIG. 4  depicts another alternative embodiment of an inflator incorporating features of the present invention. In the embodiment of  FIG. 4 , the pressure transducer  350  has been replaced with a temperature transducer  450  and circuit  310  has been replaced with timing and firing circuit  410 . In operation, a voltage signal is transmitted along input conductor  442  in response to the opening of an aircraft emergency evacuation door. As with the embodiment of  FIG. 1 , the voltage signal causes regulator valve  120  to open immediately to allow inflation gas mixture  116  to begin inflating inflatable device  128  via aspirator  126 . Simultaneously, the voltage signal is received at input  412  of timing/firing circuit  410 . Since the temperature pressure decay rate of inflation gas mixture  116  in first stage pressure vessel  114  can be characterized, timing/firing circuit  410  can be preprogrammed to initiate squib  136  after a predetermined delay based on the temperature signal received from temperature transducer  450  at input  414  of timing/firing circuit  410 . The advantage of using a timing/firing circuit  410  is that a temperature transducer such as a thermocouple may be less costly to incorporate into first stage pressure vessel  114  and may be more reliable since a thermocouple involves no moving parts. 
   Although certain illustrative embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the spirit and scope of the invention for example a pressure switch could be interposed between the firing circuit and the squib of the embodiment of  FIG. 1  to simply interrupt the firing signal. Accordingly, it is intended that the invention shall be limited only to the extent required by the appended claims and the rules and principals of applicable law.