Pyrotechnic linear inflator

An inflator (10) for use in an inflatable vehicle occupant protection system. The inflator (10) includes a longitudinal enclosure (22) and a first gas generant (24) positioned along the enclosure (22). Enclosure gas exit apertures (40) are arranged along the enclosure (22) to enable fluid communication between the enclosure (22) and an exterior of the enclosure. And to provide controlled venting of combustion gases from the enclosure (22) as the first gas generant composition (24) burns, thereby controlling the speed and direction of combustion propagation of the first gas generant. Combustion of the first gas generant (24) results in an inflator housing internal pressure of at least a first predetermined minimum internal pressure. The second gas generant composition (91) is also configured to combust to generate inflation gas providing at least a second predetermined minimum internal pressure in the housing, the second predetermined minimum internal pressure being higher than the first predetermined minimum internal pressure. As combustion of first gas generant (24) nears completion, flame from combustion of the first gas generant composition (24) reaches the end (22b) of the enclosure (22) and contacts the second gas generant composition (91), igniting the second gas generant (91) and producing a burst of gas and a resulting in an increase in inflator housing internal pressure. Gas exit apertures (20) along the housing (12) are sized such that the housing internal pressure is maintained at at least the first predetermined minimum level after passage of a predetermined time period after inflator activation.

BACKGROUND OF THE INVENTION

The present invention relates to inflators for vehicle airbags and, more particularly, to a linear inflator which discharges inflation gas along the length of the inflator for use in side impact or head curtain airbag systems.

In certain applications, such as a side-curtain for rollover protection, it is desirable to have an airbag that remains inflated for up to 5-10 seconds. In addition, the same airbag must protect the occupant during the “first impact” stage of an accident, which is typically 0-100 msec. Prior art inventions have solved this problem by using a stored gas inflator with a sealed air bag. In this case, the gas bottle produces relatively cool gas in a short period of time-generating enough pressure to protect the occupant from 0-100 msec. Since the bag is sealed and the gas is cool, the gas pressure in the bag at 5 seconds after impact typically drops to about 50% of the pressure generated during the first impact. The pressure drops primarily because the gas temperature decreases due to heat transfer.

Due to certain disadvantages of a stored gas inflator (size, cost, weight, and reliability for example), it is desirable to use a pyrotechnic inflator for airbag inflation. However, a typical pyrotechnic inflator produces gas that is relatively very hot thereby introducing other engineering concerns. In this case, the first impact pressure is in the desired range, but the gas pressure drops quickly due to heat transfer and the bag pressure at 5 seconds is too low to provide any protection. The pyrotechnic linear inflators described in co-owned and co-pending U.S. application Ser. Nos. 09/846,004, 10/662,771, 60/520,956, and 60/536,134, herein incorporated by reference, provide a significantly cooler gas than typical pyrotechnic inflators and can meet the same requirements as a stored gas inflator in a sealed bag.

Because it is expensive to produce an airbag that is completely sealed, it is desirable to have an inflator that will produce gas for longer than 100 msec., more preferably for longer than 1 second. U.S. application Ser. Nos. 09/846,004, 10/662,771, 60/520,956, and 60/536,134 also describe linear pyrotechnic inflators that will burn for an extended period of time. This is useful, but because some of the propellant burns at a very low pressure, the gaseous effluents may in some circumstances not meet required specifications.

SUMMARY OF THE INVENTION

The present invention describes an inflator that burns for an extended period of time similarly to the aforementioned applications, but also has a secondary charge of propellant that burns quickly beginning 0.5-1 sec. after inflator activation.

The inflator comprises a longitudinal inflator housing including a perforated section to enable fluid communication between an interior of the housing and an exterior of the housing. A longitudinal enclosure extends along a portion of the housing interior, the enclosure having a substantially uniform cross-sectional area along at least a portion of the enclosure. The enclosure includes at least first and second perforated sections to enable fluid communication between the enclosure and an exterior of the enclosure. The first perforated section has a first total gas exit aperture area and the second perforated section having a second total gas exit aperture area, the second total gas exit aperture area being preferably greater than the first total gas exit aperture area. Combustion of the first gas generant results in an inflator housing internal pressure of at least a first predetermined minimum internal pressure.

A first gas generant composition is positioned within the enclosure, the first gas generant composition being distributed substantially uniformly within the enclosure along a first length so as to provide a quantity of first gas generant composition having a first mass of gas generant per unit length of the first length.

A second gas generant composition is positioned so as to enable combustion of the second gas generant composition in response to combustion of the first gas generant composition. The second gas generant preferably extends along a second length which is shorter than the first length along which first gas generant extends. The second gas generant composition is arranged so as to provide a quantity of second gas generant composition having a second mass of gas generant per unit length of the second length, the second mass of gas generant per unit length being greater than the first mass of gas generant per unit length. The second gas generant composition is also physically arranged within the inflator to generate, upon combustion, inflation gas providing at least a second predetermined minimum internal pressure in the housing, the second predetermined minimum internal pressure being higher than the first predetermined minimum internal pressure. As combustion of first gas generant nears completion, flame from combustion of the first gas generant composition ignites the second gas generant, producing a burst of gas resulting in an increase in inflator housing internal pressure to at least the second predetermined minimum internal pressure. Gas exit apertures along the housing are sized such that the housing internal pressure is maintained at at least the first predetermined minimum level after passage of a predetermined time period after inflator activation.

In sum, the present invention includes a gas generator that contains a housing defining a longitudinal enclosure having a first propellant reservoir in fluid communication with a plurality of gas exit orifices, the first propellant reservoir containing a relatively smaller diameter as compared to a second propellant reservoir and the first propellant reservoir being first ignitable; the second propellant reservoir containing a relatively greater diameter and in fluid communication with the first propellant reservoir. Additional propellant reservoirs arranged in alternating smaller diameter and then larger diameter orientation may also be provided in fluid communication with the first and second propellant reservoirs first described. Exemplary gas generant systems include a vehicle occupant protection system containing the gas generator described herein.

DETAILED DESCRIPTION

FIG. 1shows a cross-sectional view of an inflator10in accordance with the present invention. Inflator10is preferably constructed of components made from a durable metal such as carbon steel or iron, but may also include components made from tough and impact-resistant polymers, for example. One of ordinary skill in the art will appreciate various methods of construction for the various components of the inflator. U.S. Pat. Nos. 5,035,757, 6,062,143, 6,347,566, U.S. Patent Application Ser. No. 2001/0045735, WO 01/08936, and WO 01/08937 exemplify typical designs for the various inflator components, and are incorporated herein by reference in their entirety, but not by way of limitation.

Referring toFIG. 1, inflator10includes a tubular housing12having a pair of opposed ends14,16and a housing wall18extending along a longitudinal axis L. Housing12may be cast, stamped, extruded, or otherwise metal-formed. A plurality of gas exit apertures20are formed along housing wall18to permit fluid communication between an interior of the housing and an airbag (not shown).

A longitudinal gas generant enclosure22is inwardly radially spaced from housing wall18and is preferably oriented coaxially with housing wall18along longitudinal axis L. Enclosure22has an elongate, substantially cylindrical body defining a first end22aproximate end14of housing12, a second end22b, and an interior cavity for containing a quantity of a first gas generant composition24therein. Enclosure first end22ais positioned to enable fluid communication between an igniter26and the enclosure interior cavity. Enclosure22extends along longitudinal axis L from housing end14toward housing end16and terminates prior to reaching housing end16, thereby forming a cavity90for receiving therein a quantity of a second gas generant composition91, described in greater detail below. Enclosure22is configured to facilitate propagation of a combustion reaction of first gas generant composition24along the enclosure, in a manner described in greater detail below. Enclosure22may be environmentally sealed at first end22awith an aluminum tape29or any other effective seal.

An annular divider92is provided intermediate housing ends14and16for positioning and securing enclosure second end22bwithin housing12. Divider92may be cast, stamped, or otherwise metal-formed.

A first plurality of gas generant tablets24are preferably stacked side by side substantially uniformly within enclosure22along a first length L1so as to provide a quantity of first gas generant composition having a first mass of gas generant per unit length of the first length L1. Each tablet24preferably has substantially the same dimensions. In one embodiment, each gas generant tablet24has an outer diameter of ¼″ and a pair of opposing, generally dome-shaped faces27, providing a maximum tablet width of approximately 0.165″ between faces. As seen inFIG. 1, tablets24are preferably shaped or configured to advantageously create a cavity25between adjacent tablets24. These cavities25provide a volume of air space relative within enclosure22, thereby enhancing the burn characteristics of tablets24when they are ignited. An alternative arrangement of the gas generant along the length of the enclosure may be provided. However, any arrangement of gas generant along the enclosure preferably provides a substantially uniform average distribution of gas generant along the length of the enclosure. Examples of gas generant compositions suitable for use in the present invention are disclosed in U.S. Pat. Nos. 5,035,757, 5,872,329, 6,074,502, 6,287,400, 6,306,232 and 6,475,312, each incorporated herein by reference. Other suitable compositions are set forth in U.S. patent application Ser. Nos. 10/407,300 and 60/369,775, incorporated by reference herein. The range of suitable gas generants is not limited to those described in the cited patents.

A quantity of a known auto-ignition composition28may be positioned at an end of the stack of gas generant material24, proximate enclosure first end22aand in ignitable communication with first gas generant24.

An igniter26is secured to inflator10such that the igniter is in communication with an interior of gas generant enclosure22, for igniting gas generant24upon occurrence of a crash event. In the embodiment shown, igniter26is positioned within an annular bore of an igniter closure30. Igniter26may be formed as known in the art. One exemplary igniter construction is described in U.S. Pat. No. 6,009,809, herein incorporated by reference.

Igniter closure30is crimped or otherwise fixed to a first end14of housing12. A first endcap32is coaxially juxtaposed adjacent igniter closure30to form, in conjunction with igniter closure30, an inner housing for igniter26. First endcap32also provides a closure for gas generant enclosure22. A second endcap34is crimped or otherwise fixed to a second end16of housing12. An O-ring or other compressive seal37may be provided along surfaces of either (or both) of endcaps32and34residing opposite respective ends of housing12, for providing a gas tight seal to prevent migration of inflation gases through the ends of the inflator. Endcaps32and34and igniter closure30may be cast, stamped, extruded, or otherwise metal-formed. Alternatively, endcaps32and34may be molded from a suitable polymer.

A filter36may be incorporated into the inflator design for cooling gases generated by combustion of gas generant24and for filtering particulates from the gases. In general, filter36is positioned between gas generant24and apertures20formed along inflator housing wall18. In the embodiment shown inFIG. 1, filter36is positioned exterior of gas generant enclosure22intermediate enclosure22and housing wall18and extends between first endcap32and divider92. Filter36substantially occupies the annular space between gas generant enclosure22and housing wall18. The filter may be formed from any of a variety of materials (for example, a carbon fiber mesh, wire or sheet) known in the art for filtering gas generant combustion products.

Cavity90is preferably formed radially inward of housing wall18and is juxtaposed at one end to enclosure second end22band, at an opposite end, to housing end16. Cavity90preferably has a greater diameter than enclosure22and therefore preferably accommodates a greater amount of propellant per unit length than first enclosure22. A second gas generant composition91, of equivalent or different composition of the first gas generant24, is preferably randomly packed and housed within cavity90. Second gas generant91extends along a second length L2which is shorter that the first length L1along which first gas generant24extends. Second gas generant composition91is arranged so as to provide a quantity of second gas generant composition having a second mass of gas generant per unit length of the second length L2, the second mass of gas generant per unit length being greater than the first mass of gas generant per unit length. A quantity of a known auto-ignition composition28may be positioned at end16of housing12, in communication with cavity90and in ignitable communication with second gas generant91.

In accordance with the present invention, a plurality of gas exit apertures40is formed along enclosure22to tailor the rate of propagation of a combustion reaction of gas generant24along the enclosure. Apertures40are spaced apart along enclosure22as described in greater detail below and are preferably formed 180° opposite housing gas exit apertures20, as shown inFIG. 1. Enclosure22may be roll formed from sheet metal and then perforated to produce apertures40. Enclosure apertures40are environmentally sealed with an aluminum tape42or any other effective seal.

Many delay mechanisms for controlling combustion propagation rate are recognized in pyrotechnics and explosives and may consist of a compressed composition delay that is designed to burn over a specific period of time. This approach is not sufficiently effective as employed in the present airbag inflator because of the large pressure difference between the relatively smaller diameter of first chamber24and the relatively larger diameter of second chamber34. In accordance with the present invention, the combustion propagation rate of gas generant24in enclosure22is controlled by venting the enclosure so that first gas generant24will first burn at a relatively slower rate prior to burning of the second gas generant91. In this way, no physical separation between the first gas generant and second gas generant is necessary. When the flame reaches the secondary gas generant, it burns very quickly and provides a burst of gas to keep the bag inflated for up to 5 seconds.FIG. 2shows the inflator described in a previous disclosure whileFIG. 1shows the inflator described in the present invention.

As stated above, control of the combustion propagation rate along enclosure22is achieved by controlled venting of combustion gases generated in enclosure22during combustion of first gas generant24, as per design criteria. The controlled venting may be accomplished by providing multiple groups of gas exit apertures, with the aperture sizes and the spacing between apertures varying between the groups being determined in the manner disclosed in co-owned and co-pending patent application Ser. No. 11/034,892, incorporated herein by reference. Application Ser. No. 11/034,892 describes how the sizes of and/or spacings between gas exit apertures within a given group of apertures may be varied between groups of apertures as iteratively determined based on such factors as design criteria and the length of the inflator, to achieve a predetermined combustion propagation rate within a gas generant enclosure.

Two sample inflators were constructed to illustrate the principles of the present invention.

Example 1: An inflator was assembled as shown inFIG. 2. Enclosure22was a 48″ long tube with an outer diameter of ⅜″ and a wall thickness of 0.035″. One row of collinear apertures40was drilled in the enclosure as follows: 12× diam. 4.0 mm holes 1″ on center (OC) at 1-12″ from one end, then 23× diam 4.0 mm holes ½″ on center at 13-24″ from the same end, and than 91× diam. 5.0 mm holes ¼″ on center at 24.5-47″ from the same end. Filter36was a 30 mesh/0.012″ screen wrapped around the enclosure 10 times. The housing12had a diameter of 1″ and a wall thickness of 0.035″. 97 gas exit apertures of diameter ¼″ were drilled in one row ½″ on center along inflator housing12. The apertures20of 12 were positioned 180° from the apertures40along enclosure22. Enclosure22was loaded with 66 g of ¼″ diameter by 0.165″ thick propellant tablets with a dome on each side of 0.040″. The inflator was in a 60 liter tank. The results are shown inFIGS. 3 and 4.

Example 2: An inflator was assembled similarly to that shown inFIG. 1. The enclosure22was the same as that used in Example 1 and was loaded with the same mass and type of propellant. The filter36was the same as used in Example 1. Housing12was the same as in Example 1 except that it was 6″ longer to accommodate cavity90containing second gas generant91. Cavity90was loaded with 19 g of diameter ⅜″ by 0.145″ thick propellant. The inflator was fired in a 60 liter tank. The results are shown inFIGS. 3 and 4.

The sum of the areas of the apertures in the first grouping of apertures (12 holes spaced 1″ on center) defines a first total gas exit aperture area. The sum of the areas of the apertures in the second grouping of apertures (23 holes spaced ½″ on center) defines a second total gas exit aperture area. The sum of the areas of the apertures in the third grouping of apertures (91 holes spaced ¼″ on center) defines a third total gas exit aperture area. The sum of the areas of all of the apertures40formed along enclosure22defines a total gas exit aperture area. The sum of the areas of all of the apertures20formed along housing12defines a total housing gas exit aperture area.

The term “on center” (OC) is defined as the distance from the center point of one aperture to the center point of an adjacent aperture. The sizes of the holes or gas exit apertures preferably ranges from about one millimeter to about one-half the diameter of the propellant tube. Holes smaller than one millimeter are often difficult to manufacture with consistent size and with the desired efficiency. Holes or gas exit apertures larger than half the diameter of the propellant tube weaken the structure of the tube and are therefore relatively difficult to produce.

The gas exit apertures are preferably spaced about six millimeters to twenty-six millimeters on center. A spacing less than about six mm. may weaken the structure, and presents a further structural concern if the local or associated gas exit aperture size is relatively large or close to the diameter of the propellant tube. Spacing larger than twenty-six mm. may be employed although the efficiency of the cooling screen may consequently be reduced.

FIGS. 3 and 4show the effects of controlled venting of combustion gases from enclosure22using the predefined pattern of gas exit apertures described in detail above.FIGS. 3 and 4also show the effects of incorporating a second combustion chamber (i.e., cavity90) into the inflator design, as described herein.

Combustion of the first gas generant24produces an inflation gas resulting in a tank pressure of at least a first predetermined minimum internal pressure, whereby the tank pressure is directly related to a corresponding desired airbag pressure. It will be appreciated that tailoring the composition of the gas generant, the length of enclosure22, or both, as iteratively determined, provides tailoring of the associated airbag inflation pressure over a unit length of time. The size of enclosure22and the size of cavity90may be tailored to modify the respective propellant capacity and the associated airbag inflation profile. As shown inFIG. 1, second gas generant91extends along a second length L2which is shorter than the first length L1along which first gas generant24extends. Second gas generant composition91is positioned in a housing cavity90adjacent the enclosure22so as to enable combustion of the second gas generant composition91in response to combustion of the first gas generant composition24. The second gas generant composition (91) is arranged so as to provide a quantity of second gas generant composition having a second mass of gas generant per unit length of the second length L2, the second mass of gas generant per unit length being greater than the first mass of gas generant per unit length. The second gas generant composition (91) is also configured to combust to generate inflation gas providing at least a second predetermined minimum internal pressure in the housing, the second predetermined minimum internal pressure being higher than the first predetermined minimum internal pressure.

In certain applications, such as a side-curtain for rollover protection, it is desirable to have an airbag that remains inflated for up to 5-10 seconds. In addition, the same airbag must protect the occupant during the “first impact” stage of an accident, which is typically 0-100 msec. Prior art devices have addressed this problem by using a stored gas inflator with a sealed air bag. In this case, the gas bottle produces relatively cool gas in a short period of time-generating enough pressure to protect the occupant from 0-100 msec. Since the bag is sealed and the gas is cool, the gas pressure in the bag at 5 seconds after impact typically drops to about 50% of the pressure generated during the first impact. The pressure drops primarily because the gas temperature decreases due to heat transfer. As described below, in the present invention a pressure surge provided by the combustion of second gas generant91in cavity90delays the pressure drop in an airbag fluidly coupled to the inflator, enabling the inflator to remain inflated for a relatively extended period.

As combustion of first gas generant24nears completion (after which inflator housing internal pressure will begin to drop), flame from combustion of the first gas generant composition24reaches the end22bof the enclosure22and contacts the second gas generant composition91, igniting the second gas generant91and producing a burst of gas, resulting in an increase in inflator housing internal pressure. Gas exit apertures20along the housing12are sized such that the housing internal pressure is maintained at at least the first predetermined minimum level during passage of a predetermined time period after inflator activation.

As seen inFIG. 3(0-1.0 sec. time frame) andFIG. 4(0-5 sec. time frame), the “first impact” inflator tank pressure in the inflator of Example 1 is constant at 60-75 kPa from 0-100 msec. This is the time period in which the occupant first contacts the bag from an initial impact. In Example 1, the gas generant along about ⅔ of the length of enclosure22has been consumed in approximately 100 msec. The gas generant in the remaining ⅓ of the length of the enclosure burns more slowly from 100 msec to 900 msec. At about 900 msec. after igniter activation, the combustion of the first gas generant in Example 1 is complete and the inflator internal pressure begins to decrease as the gas cools.

As seen inFIG. 3(0-1.0 sec. time frame) andFIG. 4(0-5 sec. time frame), the “first impact” tank pressure relative to the inflator of Example 2 is constant at 60-75 kPa from 0-100 msec. This is the time period in which the occupant first contacts the bag from an initial impact. The gas generant along about ⅔ of the length of enclosure22has been consumed in approximately 100 msec. The gas generant in the remaining ⅓ of the length of the enclosure burns more slowly from 100 msec to 900 msec. In Example 2, at about 900 msec. after igniter activation, the flame from combustion of first gas generant24reaches cavity90and ignites second gas generant91, producing a burst of gas for about 200 msec. At approximately 5 seconds after igniter activation, the tank pressure from example 1 is about 70 kPa while that from Example 2 is about 90 kPa. The pressure surge provided by the combustion of second gas generant91in cavity90delays the pressure decrease over time of an airbag fluidly coupled to the inflator, thereby enabling the airbag to remain inflated for a relatively extended period.

Venting of combustion gases in enclosure22is believed to operate as follows. It is believed that after igniter26is activated, the propagation rate of the combustion reaction along the enclosure is dependent upon the number of apertures40and the spacing between the apertures along enclosure22. More specifically, it is believed that, along the sections of the enclosure where the aperture spacing is 1″ OC, the combustion reaction propagates via hot gases because the pressure inside this portion of the enclosure is relatively high due to the relative shortage of apertures to relieve the pressure; thus, there is a driving pressure force urging the hot gases further down the enclosure. In the sections where the aperture spacing is ½″ OC, the combustion reaction still propagates via hot gases but at a slower rate because the internal pressure is relatively lower, due to the shorter distance between apertures. In the sections where the aperture spacing is ¼″ OC, apertures40are relatively numerous, permitting the enclosure internal pressure to be more easily relieved; thus, there is minimal driving pressure force urging the hot gases further down the length of the enclosure. In this case, the combustion reaction continues to propagate at a relatively slower rate as each tablet24ignites the next adjacent tablet as it burns.

Along portions of the enclosure having a relatively greater spacing between enclosure apertures40, the more rapid propagation of the combustion reaction results in a more rapid burning of the gas generant and, thus, a more rapid generation of inflation gas, and more rapid inflation of an associated airbag, for example. Therefore, to affect the propagation rate of a combustion reaction along a portion of the enclosure, the apertures along the portion of the enclosure may be spaced apart a distance proportional to a desired rate of propagation of a combustion reaction of gas generant positioned between the apertures. The combustion propagation rate may be tailored using an appropriate arrangement of enclosure apertures, to accommodate greater or lesser desired airbag inflation rates, and also to accommodate desired shorter or longer inflation durations.

Because propagation of the combustion reaction within enclosure22is controlled by venting, no physical separation or barrier is required between first gas generant24and second gas generant91.

It is noted that the stacking of substantially uniform gas generant tablets24adjacent each other along enclosure22provides for a relatively constant average density of gas generant along the enclosure. Also, the use of an enclosure having a substantially constant cross-sectional area along the length of the enclosure provides for a substantially constant volume per unit length of the enclosure. These features aid in minimizing pressure variations within the enclosure due to such factors as variations in enclosure volume, and localized hot spots and higher pressure regions resulting from disparities in gas generant distribution along the enclosure. The dome-shaped faces of each propellant tablet further facilitates an ease of assembly in that each dome-shaped face provides a pivot point at its apex that physically communicates with the apex of an adjacent tablet's propellant face. Accordingly, by virtue of the pivot point created on each dome-shaped face, the same juxtaposed orientation of each propellant tablet is assured without undue complication.

In alternative embodiments as exemplified inFIG. 6, inflator10may be provided with an arrangement of multiple alternating enclosures22and cavities90as described above, all in fluid communication with each other and with housing gas exit apertures20. Enclosures22having a relatively smaller diameter may be arranged in alternating fashion with cavities90to provide a longitudinal, fluidly contiguous combustion chamber having a diameter alternating between the relatively smaller diameter of enclosures22and the relatively larger diameters of cavities90. The desired arrangement of alternating enclosures22and cavities90is designed to provide a predetermined inflation profile according to design requirements. It will be appreciated that tailoring the composition of gas generants24and91, the sizes of enclosures22, and/or the sizes of cavities91may be varied as iteratively determined to provide tailoring of the associated airbag inflation pressure over a unit length of time.

Referring now toFIG. 5, a gas generator constructed in accordance with the principles outlined above may be incorporated into an exemplary gas generating system such as a vehicle occupant restraint system200. Vehicle occupant restraint system200includes at least one airbag202and an inflator10constructed in accordance with the present invention and coupled to airbag202so as to enable fluid communication with an interior of the airbag. Vehicle occupant restraint system200may be in operative communication with a crash event sensor211which communicates with a known crash sensor algorithm that signals activation of vehicle occupant restraint system200via, for example, activation of airbag inflator10in the event of a collision.

It should be understood that the preceding is merely a detailed description of one embodiment of this invention and that numerous changes to the disclosed embodiment can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. For example, the relative amounts of gas generant in enclosure22and in cavity90may be different from the amounts disclosed in the above examples, according to the desired inflation profile for the inflator. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.