SYSTEM AND METHOD FOR RECONNECTABLY COUPLING AN AFT VEHICLE TO A FORWARD VEHICLE IN FLIGHT
20220348346 · 2022-11-03
Inventors
- Gerald D. Budd (Lancaster, CA, US)
- Thomas Milton McNicholas (Laguna Niguel, CA, US)
- Angela Monheim (Rapid City, SD, US)
Cpc classification
B64D3/00
PERFORMING OPERATIONS; TRANSPORTING
B64D39/06
PERFORMING OPERATIONS; TRANSPORTING
International classification
B64D39/06
PERFORMING OPERATIONS; TRANSPORTING
B64D3/00
PERFORMING OPERATIONS; TRANSPORTING
Abstract
Disclosed are systems and methods for reconnectably coupling an aft vehicle to a forward vehicle in flight. A docking structure is affixed to the forward vehicle. A coupler has a line connection portion, a probe receiver port, and a plurality of aerodynamic controls. The probe receiving port receives and releasably retains a probe portion of the aft vehicle. A tow line has a proximal end, a distal end, and a line extension length. The proximal end may be attached to the tow actuation element. The distal end is connectable to the coupler. The tow actuation element is configured to adjust the line extension length. The aerodynamic control elements are configured to adjust lateral and vertical positioning of the coupler with respect to the probe portion when the system is in flight. Movement of the coupler is restrained with respect to the docking structure when docked thereto.
Claims
1. A system for reconnectably coupling an aft vehicle to a forward vehicle in flight, the system comprising: a docking structure configured to be affixed to the forward vehicle; a tow actuation element; a coupler element having a line connection portion, a probe receiver port, and a plurality of aerodynamic control elements, a coupler longitudinal axis being defined as extending from the line connection portion through the probe receiving port, the probe receiving port being configured to receive and releasably retain a probe portion of the aft vehicle, the coupler element being movable with respect to the docking structure between a docked position and an undocked position; and a tow line element having a proximal end, a distal end, and a line extension length, the proximal end being attached to the tow actuation element, the distal end being connectable to the coupler element, the line extension length being defined from the tow actuation element to the line connection portion; wherein (a) the tow actuation element is configured to increase and decrease the line extension length, (b) the aerodynamic control elements are configured to adjust lateral and vertical positioning of the coupler element with respect to the probe portion when the system is in flight, and (c) movement of the coupler element is restrained with respect to the docking structure when in the docked position.
2. The system as defined in claim 1, wherein the aerodynamic control elements include one or more actuatable control surfaces.
3. The system as defined in claim 1, wherein the aerodynamic control elements include one or more drag flaps.
4. The system as defined in claim 1, wherein the aerodynamic control elements are configured to control movement of the coupler element in radial directions with respect to the coupler longitudinal axis when the system is in flight.
5. The system as defined in claim 4, wherein the aerodynamic control elements are configured to control roll, yaw and pitch movement of the coupler element when the system is in flight.
6. The system as defined in claim 1, wherein the aerodynamic control elements are configured to control a line tension of the tow line element when the system is in flight
7. The system as defined in claim 1, wherein the coupler element is configured to have a higher aerodynamic frequency response than the aft vehicle.
8. The system as defined in claim 1, wherein the coupler element includes control electronics configured to (a) calculate a radial offset between the coupler longitudinal axis and the probe portion, and (b) command the aerodynamic control elements for reducing the radial offset
9. The system as defined in claim 8, wherein the control electronics is configured to (a) calculate a longitudinal distance between the coupler element and the probe portion, and (b) command the tow actuation element to adjust the line extension length for reducing the longitudinal distance.
10. The system as defined in claim 8, wherein the control electronics is configured to (a) calculate a longitudinal distance between the coupler element and the probe portion, and (b) command the forward vehicle or the aft vehicle to close the longitudinal distance.
11. The system as defined in claim 1, wherein the coupler element includes a rotary air turbine.
12. The system as defined in claim 11, wherein (a) the coupler element includes a rechargeable battery, and (b) the rotary air turbine is configured to charge the rechargeable battery when the system is in flight
13. The system as defined in claim 1, wherein the docking structure includes a dock stabilization ring for receiving the coupler element when in the docked position.
14. The system as defined in claim 13, wherein the dock stabilization ring includes a multiplicity of coupler guide plates configured to conformingly engage an outer surface of the coupler element.
15. The system as defined in claim 14, wherein the coupler guide plates are pivotably attached to the dock stabilization ring.
16. The system as defined in claim 14, wherein the docking structure includes a shuttle portion which (a) is movable with respect to the dock stabilization ring between a coupler capture position and a coupler release position, (b) when in the coupler release position, is configured to hold the coupler element out of the conforming engagement with the coupler guide plates, and (c) when in the coupler capture position, is configured to allow the coupler element to be in the conforming engagement by the coupler guide plates.
17. The system as defined in claim 16, wherein the shuttle portion is resiliently biased toward the coupler release position.
18. The system as defined in claim 17, wherein the resilient bias is configured to be overcome by way of the tow actuation element reducing the extension length after the coupler element has engaged the shuttle portion.
19. The system as defined in claim 1, wherein (a) the coupler element includes a tow pin engagement element having a pin barrel and a pin detent latch, the pin detent latch being movable into and out of a latch position, (b) the distal end of the tow line element is connectable to a tow pin, the tow pin being insertable into the pin barrel and configured to be engaged by the pin detent latch in the latch position, (c) when the tow pin is engaged by the pin detent latch, the tow pin is prevented from being removed from the pin barrel, and (d) when the tow pin is out of engagement with the pin detent latch, the tow pin is removable from the pin barrel.
20. The system as defined in claim 19, wherein (a) the pin detent latch is elastically biased toward the latch position, and (b) the tow pin engagement element includes a pin detent actuator configured to move the pin detent latch out of the latch position when commanded to do so.
21. The system as defined in claim 1, comprising a said aft vehicle having a said probe portion, wherein the probe portion includes a coupler securement portion securingly engageable with a probe securement portion in the probe receiver port for controllably locking the probe portion within the probe receiver port.
22. The system as defined in claim 21, wherein the coupler securement portion includes a lock mechanism configured to move the coupler securement portion into and out of said securing engagement with the probe securement portion.
23. The system as defined in claim 22, wherein (a) the coupler securement portion includes a plurality of radially-extendable tabs, and (b) the probe securement portion include one or more grooves configured to receive said radially-extendable tabs when the coupler securement portion is in the securing engagement.
24. The system as defined in claim 21, wherein the coupler element has a lower mass and higher control authority than the aft vehicle.
25. The system as defined in claim 1, wherein the tow line element is a refueling hose.
26. The system as defined in claim 1, further comprising a line sever element configured to sever the tow line element when commanded to do so.
27. The system as defined in claim 1, wherein the coupler element includes a pressure relief duct disposed in communication between the probe receiver port and an outer surface of the coupler element.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Further advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description of the preferred embodiments and upon reference to the accompanying drawings in which:
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0056] Referring now to the drawings, like reference numerals designate identical or corresponding features throughout the several views.
[0057] With reference to the several drawings, example embodiments of a system for reconnectably coupling an aft vehicle 102 to a forward vehicle 104 in flight are shown generally at 100. Moreover, a forward portion of the system 100 is shown generally at 100a, and an aft portion of the system 100 is shown generally at 100b.
[0058] Referring to
[0059] Referring to
[0060] Referring to
[0061] Referring to
[0062] Referring to
[0063] Referring to
[0064] Referring to
[0065] Referring to
[0066] Alternatively or in addition, in certain implementations of the system 100, the control electronics 158 may be configured to control relative movement between the forward vehicle 104 and aft vehicle 102 by sending commands to the forward vehicle 104, the aft vehicle 102, or both. For example, the control electronics 158 may be configured to send commands to the forward vehicle 104 to reduce its throttle or increase its drag, thereby reducing the airspeed of the forward vehicle 104 to facilitate closure with the aft vehicle 102. For further example, the control electronics 158 may be configured to send commands to the aft vehicle 102 to aerodynamically clean up (e.g., by retracting spoilers/drag flaps) to facilitate closure with the forward vehicle 104.
[0067] Referring to
[0068] Referring to
[0069] Referring to
[0070] Referring to
[0071] Referring to
[0072] Referring to
[0073] Referring to
[0074] Referring to
[0075] The pin detent latch 200 may be elastically biased (for example, by way of one or more pin detent springs 202) toward the latch position. The tow pin engagement element 196 may include a pin detent actuator 204 configured to move the pin detent latch 200 out of the latch position responsive to a command (e.g., an electrical or RF signal). Such commands may be transmittable from the forward vehicle 104. In certain implementations of the tow pin engagement element 196, the pin detent actuator 204 may be a motor, a solenoid, a pneumatic actuator, a pyrotechnic fastener, some combination thereof or the like.
[0076] Referring to
[0077] Referring to
[0078] Referring to
[0079] Disclosed below are specific features and steps of certain non-limiting example systems and methods in accordance with the present disclosure.
[0080] Forward Vehicle
[0081] A forward vehicle 104 may be, for example, an aircraft or other vehicle (manned or unmanned) utilized to provide propulsive force to tow an aft vehicle 102, through the use of a controlled coupling device 106 interface. Additionally or in the alternative, a forward vehicle 104 may be a tanker aircraft used to transfer fuel through a hose to the aft vehicle 102, through the controlled coupling device 106.
[0082] Controlled Coupling Device
[0083] The methods disclosed herein may utilize a controlled coupling device 106 located at the aft tip of the tow line element 108 extended from the forward vehicle 104, that autonomously, and actively, positions itself in front of an aft vehicle 102 (piloted or unmanned, powered or unpowered) using, for example, a combination of differential GPS and optical tracking systems to support closed-loop navigation between the controlled coupling device 106 and the aft vehicle 102 being connected (or refueled). Once the controlled coupling device 106 is positioned in front of the aft vehicle 102, the distance between the forward and aft vehicles is reduced until the vehicles make contact and connect with each other. This may be achieved, for example, by the forward vehicle (e.g., forward aircraft) that is towing the controlled coupling device 106 slowing to close the distance between the controlled coupling device 106 and the aft vehicle 102. Alternatively, the aft vehicle 102 could increase its speed relative to the controlled coupling device 106 to close the distance between itself and the controlled coupling device 106 to engage and connect with it. The relative distance between the controlled coupling device 106 and the aft vehicle 102 may also be reduced by extending the tow line element 108 further from the forward vehicle 104 (e.g., tow aircraft) until contact with the aft vehicle 102 is achieved, and a connection between the forward and aft vehicles is mechanically secured.
[0084] While the longitudinal distance between the controlled coupling device 106 and the aft vehicle 102 is decreasing, the controlled coupling device 106 would actively maintain its vertical and lateral alignment directly in front of the aft vehicle 102. Referring to
[0085] The controlled coupling device 106 may be a conical (or bi-conic) shape a similar body of rotation (not necessarily symmetric) for minimum aerodynamic drag. Alternatively, the outer mold line 212 of the controlled coupling device 106 may be shaped to mate up with the front nose section of the aft vehicle 102 such that when mated with the aft vehicle 102 it forms the completed nose section of the aft vehicle 102.
[0086] The controlled coupling device 106 may include maneuver control surfaces 126 (e.g., primary control surfaces like fins 126a). These control surfaces may be mounted on the surface of the controlled coupling device 106, and may be capable of independent pivotable or hinged movement, thereby allowing the controlled coupling device 106 to maneuver in an agile fashion while maintaining a desired position (e.g., vertical and lateral) or rotational orientation with respect to the probe portion 140 of the aft vehicle 102.
[0087] Deployable drag flaps 126b may be mounted on the surface of the controlled coupling device 106, and may be conformal to the outer surface 212 of the controlled coupling device 106 when retracted. The drag flaps 126b may be independently extendable to aid in maneuvering and positioning the controlled coupling device 106 horizontally, vertically, or in the yaw axis, while increasing drag to increase tow line (e.g., cable or hose) tension and dampen tow line (e.g., cable or hose) dynamics.
[0088] An open probe receiver port 130 in the aft end of the controlled coupling device 106 may be configured to receive a matching probe portion 140 of the aft vehicle 102. A circumferential locking device (e.g., coupler securement portion 182 and lock mechanism 184) may be provided to securely, mechanically mate the controlled coupling device 106 with the aft vehicle 102. In the case of towing, the probe portion 140 may be the front of a towed vehicle 102, and the mated pair may therefore form the completed nose section (or outer mold line) of the aft vehicle 102.
[0089] Alternate methods of attaching the controlled coupling device 106 to the aft vehicle 102 may include extendable locking pins, rings, or tabs, deployed mechanically, hydraulically, magnetically, pneumatically, or via electro-mechanical actuators.
[0090] Aft Vehicle
[0091] Depending upon the implementation of the system 100, the aft vehicle 102 could be a piloted or unmanned vehicle, powered or unpowered, or a refueling or connection probe of an existing aircraft. In one embodiment, once the controlled coupling device 106 connects to the aft vehicle 102, the mated pair constitute the nose of an aircraft designed to be towed in-flight by the forward vehicle (e.g., towing aircraft) 104.
[0092] The aft vehicle may be shaped to conform to the controlled coupling device 106 when mated or connected. Optionally, a connecting probe, not conformal to the controlled coupling device 106, may be utilized to attach a small aircraft or unmanned aerial vehicle to the controlled coupling device 106 for retrieval or recovery by the forward vehicle (e.g., towing aircraft) 104. Optionally, the probe portion 140 (e.g., a receptacle or probe) may be mounted on, or extend from, an aft vehicle 102 (e.g., aircraft) being refueled from a tanker aircraft connected to the controlled coupling device 106 via a hose designed to transfer fuel to the receiving vehicle (piloted or autonomously controlled) through the controlled coupling device 106.
[0093] Control of the Controlled Coupling Device
[0094] The controlled coupling device 106 would typically feature multiple control surfaces 126 that could be either deployable or fixed in place. Referring to
[0095] Secure Mechanical Connection
[0096] The controlled coupling device 106 may preferably connect to the aft vehicle 102 through a device specifically designed for the function being performed. For towing the aft vehicle 102, the mechanical connection system would be required to withstand the forces necessary to tow the aft vehicle 102 and may consist of a robust ring, extendable tabs, or probe on the aft vehicle 102 which attaches to a receiver port 130 within the rear, interior portion of the controlled coupling device 106. The receiver port 130 could contain a sensor 180 which triggers a locking mechanism 184 once the aft vehicle's nose (or probe) advances deep enough into the receiver port 130 of the controlled coupling device 106. For in-flight refueling, the mechanical connection could be designed to accommodate existing probes/booms in use on refuellable aircraft today and may be configured to meet break-away requirements for those aircraft. For drone mother ship functions, the securing mechanism could resemble a grappling hook designed to withstand the force required to pull the drone to the mothership after capture.
[0097] Tow Actuation Element
[0098] The tow line element 108 would preferably attach to the forward vehicle 104 via a tow actuation element 112 (such as a powered reel or winch) mounted to the aft underside of the forward vehicle 104. The tow actuation element 112 may therefore be capable of deploying and retracting 1000 feet or more of tow line incrementally, on the ground or in flight. The reel would be powered to deploy and retract the towline and may utilize a closed-loop control system to allow for precise, control of the tow line at any released increment. The reel may preferably have an aerodynamic fairing 128 around it to minimize aerodynamic drag and buffeting. The reel may also feature a line sever element 214 (e.g., a guillotine or hot-wire system) designed to cut or sever the tow line, thereby releasing it from the reel in the event of an in-flight emergency.
[0099] Alternate (Emergency) Method for Releasing the Towline from the Controlled Coupling Device:
[0100] The aft tip of the tow line element 108 could connect to the controlled coupling device 106 via one of several methods: a removable pin/latch system, or an openable loop/hook mechanism. The mechanism (such as that shown at 196) may allow for emergency, in-flight release of the tow line element 108 from the controlled coupling device 106 when mated to the aft vehicle 102. Additionally or in the alternative, referring to
[0101] The emergency release mechanism may be configured to engage with a tow pin attached to the tow line element via a clamping pin arrangement, where the tow line element is clamped in a cavity of a split tow pin, or via looping the towline through a cross hole in the forward portion of a solid tow pin. The release mechanism may employ spring-loaded latches, mounted on the aft side of the release bracket, that engage into a mating slot in the removable pin. This allows for the manual insertion and automatic securing of the tapered pin into the release mechanism without requiring access to the latching plates or powering up the release system. Release of the tow pin may be accomplished, for example, via rotation of a disk with pins that lift the spring-loaded latches out of the tow pin mating slot, allowing the tow pin to slide out of its guide tube, releasing the tow pin/tow line from the controlled coupling device. Rotation of the release disk may be via electro-mechanical, hydraulic or pneumatic actuators, motors, some combination thereof or the like. Mounting the latches on the aft side of the release bracket may allow for a small, lightweight, structurally robust, yet simple and reliable, emergency release mechanism.
[0102] Referring to
[0103] A swivelable mounting of the release mechanism to the controlled coupling device may also be employed to resolve the towline forces axially, or in-line, with the towline release mechanism (independent of the aft vehicle's orientation to the tow line element), to eliminate binding of the release mechanism due to lateral and/or vertical towline forces. One method of implementation may be the use of a two-axis gimbal mounted between the release mechanism and the controlled coupling device, thereby allowing the release mechanism to pivot freely, aligning the release mechanism axially with the tow line element.
[0104] Rotary Air Turbine
[0105] One or more rotary air turbines 134 may be employed on the controlled coupling device 106, and may comprise a rotating ring with small propeller blades 136. The air turbines 134 may be integrated into the external mold line of the vehicle and located at or toward the nose of the controlled coupling device 106. The rotary air turbine 134 may be configured to provide electrical power when in flight to continually charge internal batteries 156 that power movable fins, drag flaps, tow release mechanism, vehicle avionics systems, or the like. When not in use, the turbine blades 136 may be retractable via electromechanical, magnetic, pneumatic, or hydraulic systems to minimize aerodynamic drag and/or to eliminate the possibility of damage to the blades due to contact with external objects. Alternatively, the blades 136 may be of a hinged, folding design, that configures them to nest into conformal cavities in the outer mold line of the rotating ring. A shroud may optionally be used around the blades to provide protection for the blades and also for people working around the rotary air turbine. Another configuration with counter-rotating rings may be employed to eliminate any tendency for the controlled coupling device to roll when the blades are deployed.
[0106] Pressure-Relief Ducts
[0107] A series of small NACA air ducts (such as the duct 224 shown in
[0108] Docking Structure
[0109] A docking structure 110 is preferably located on or mounted to the forward vehicle 104, may be designed to securely capture and store the controlled coupling device 106 within the docking structure 110. Such configuration allows the forward vehicle 104 to land with the controlled coupling device 106 attached to the tow line element 108. The docking structure 110 would be configured to structurally support and stabilize the controlled coupling device 106, thereby preventing its movement once docked. The tow line element 108 may feed through an opening or guide 142 at the front of the docking structure 110 that would capture the nose of the controlled coupling device 106 as it is docked. Faired guides (e.g., coupler guide plates 146) would position the aft end of the controlled coupling device 106 within the docking structure 110.
[0110] The docking structure 110 may be of an open architecture, thereby allowing airflow around and through it to minimize weight, aerodynamic drag, and aerodynamic buffeting when the controlled coupling device 106 is deployed in towed flight. The tow line element 108 may feed through an opening (such as in forward towline guide 142) toward the front of the docking structure 110. The forward towline guide 142 would then capture the nose (e.g., line connection portion 132) of the controlled coupling device 106, thereby restricting its movement laterally and vertically. As the tow line element 108 is reeled in, the forward towline guide 142 would “steer” the nose of the controlled coupling device 106 into its captured, stowed position, while centering it in the docking structure 110. An aft faired ring 144, with guides 146 (e.g., conformal or protruded, low surface friction contact pads, or rollers) would center the aft end of the controlled coupling device 106 in place within the docking structure 110 once the controlled coupling device 106 enters the docking structure 110. Once docked, the guides 146 would be in contact with the outer mold line 212 of the controlled coupling device 106, positioned in front of the external control surfaces 126 of the controlled coupling device 106. The guides 146 may be configured to transfer the structural loads of carrying the controlled coupling device 106 through the faired ring 144 and adjacent mechanical components, to the mounting points on the forward vehicle 104. A brake on the reel 112 may then be engaged to prevent the tow line element 108 from being extended, thereby effectively locking the controlled coupling device 106 in place in the docking structure 110. For deployment of the controlled coupling device 106 from the docking structure 110, the process may be reversed.
[0111] Referring to
[0112] A set of rails or slide tubes (or equivalent, extendable, mechanical system) may be provided to limit the forward towline guide 142 motion to fore/aft movement along the shared axis of the forward towline guide 142 and aft faired ring 144. The forward towline guide 142 may be extended forward/aft via an electro-mechanical, pneumatic, hydraulic, or magnetic system.
[0113] Referring to
[0114] The following listing matches certain terminology used within this disclosure with corresponding reference numbers used in the non-limiting embodiments illustrated in the several figures. [0115] 100 system for reconnectably coupling an after vehicle to a forward vehicle in flight [0116] 100a forward portion (of the system 100) [0117] 100b aft portion (of the system 100) [0118] 102 aft vehicle (e.g., piloted or unmanned, powered or unpowered aircraft) [0119] 104 forward vehicle (e.g., tow or refueling aircraft) [0120] 106 coupler element (i.e., controlled coupling device) [0121] 108 tow line element (e.g., cable or fuel hose) [0122] 110 docking structure [0123] 112 tow actuation element (e.g., a powered reel or winch) [0124] 114 coupler longitudinal axis [0125] 116 probe longitudinal axis [0126] 118 radial offset (e.g., between coupler and probe longitudinal axes) [0127] 120 longitudinal distance (e.g., offset between coupler element and aft vehicle) [0128] 122 dock mounting bracket [0129] 124 winch mounting bracket [0130] 126 aerodynamic control elements (e.g., control surfaces, reaction control, etc.) [0131] 126a primary control surface (e.g., pivotable control fins) [0132] 126b drag flap (e.g., extendable and retractable) [0133] 128 winch fairing [0134] 130 probe receiver port [0135] 132 line connection portion [0136] 134 rotary air turbine [0137] 136 turbine blades [0138] 138 line guide [0139] 140 probe portion (e.g., nose cone or refueling probe of aft vehicle) [0140] 142 nose retention element (e.g., “forward towline guide”) [0141] 144 dock stabilization ring (e.g., “aft faired ring”) [0142] 146 coupler guide plates (e.g., pivotably attached to dock stabilizing ring) [0143] 148 shuttle portion (movable between a coupler capture and coupler release position) [0144] 150 shuttle spring [0145] 152 shuttle guide rail [0146] 154 shuttle guide tube [0147] 156 battery (e.g., rechargeable) [0148] 158 coupler control electronics [0149] 160 probe securement portion (e.g., a latch groove) [0150] 162 roll [0151] 164 pitch [0152] 166 yaw [0153] 168 upward vertical direction [0154] 170 downward vertical direction [0155] 172 left lateral direction [0156] 174 right lateral direction [0157] 176 forward direction [0158] 178 rearward direction [0159] 180 probe insert sensor [0160] 182 coupler securement portion (e.g., one or more extendable tabs) [0161] 184 lock mechanism (to lock the coupler element and probe portion) [0162] 186 tow pin (e.g., affixed to end of tow line element; e.g., solid pin or split pin) [0163] 188 latch interface portion (of tow pin; e.g., annular groove) [0164] 190 line interface portion (of tow pin; e.g., eyelet or clamp portion) [0165] 192 tapered end (of tow pin) [0166] 194 pin connection (whereby tow line connects to tow pin; e.g., tow line loop or hook) [0167] 196 tow pin engagement element (e.g., with release mechanism) [0168] 198 pin barrel [0169] 200 pin detent latch [0170] 202 pin detent spring [0171] 204 latch release actuator [0172] 206 proximal end (of tow line) [0173] 208 distal end (of tow line) [0174] 210 line extension length (of tow line) [0175] 212 outer surface of coupler element (e.g., outer mold line of the coupler element) [0176] 214 line sever element [0177] 216 primary clamp portion (of tow pin) [0178] 218 secondary clamp portion (of tow pin) [0179] 220 clamp fastener (e.g., screws, rivets or the like) [0180] 222 line tension [0181] 224 pressure-relief duct [0182] 226 ambient environment [0183] 228 attachment bolts (e.g., explosive bolts) [0184] 230 forward coupler bulkhead [0185] 232 frangible fairing
[0186] While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.