TECHNICAL FIELD

[0001] Embodiments relate to spring-return actuators.

BACKGROUND

[0002] Various types of spring-return actuators are known in the art. They generally comprise a "load" piston housed in a first cylinder chamber, hereinafter also referred to as a "load chamber", and a return spring housed in a second, tandem, cylinder chamber hereinafter also referred to as a "return chamber". A flow valve introduces a gas or liquid fluid under pressure into the load chamber to generate a force during a "power stroke" of the actuator that moves the load piston in the load chamber from a first position in the load chamber to a second position. Motion of the load piston moves a load to which the load piston is coupled and in addition, simultaneously compresses the return spring. To move the load piston back to the first position, the flow valve releases the pressurized fluid from the load chamber so that the return spring, during a "return stroke" of the actuator, can expand and force the load piston back to the first position. In case of a malfunction resulting in loss of fluid pressure, the return spring operates as a safety spring to return the load piston back to the first position.

[0003] The first position, hereinafter also referred to as an "OFF position" is generally a position for which the load coupled to the load piston is considered to be in a corresponding "OFF", or "benign", position. The second position may be referred to as an ON or "working" position. By way of example, the load may be a ball valve to which the actuator is coupled to rotate a ball in the valve between a first position in which the valve is closed (in an OFF position) and a second position for which the valve is open (in an ON position). The ball valve may be in a safe position when the ball valve is closed or OFF, and in a working position to enable flow of fluid through the valve when the ball valve is open or ON.

SUMMARY

[0004] An aspect of an embodiment of the disclosure relates to providing a spring-return actuator, hereinafter also referred to as a double piston rod spring return scotch yoke actuator (DPR-actuator). The DPR actuator, comprises a load chamber in which a load piston moves and a return chamber housing a return piston and an elastic compressible element, such as a return spring, which motion of the return piston compresses. A scotch yoke transmission for coupling the actuator to a load is located between the load piston and return piston. A first piston rod, also referred to as a "load" piston rod, couples the load piston to the scotch yoke. A second piston rod, also referred to as a "return" piston rod, is coupled to the return piston and has an end aligned with and substantially facing an end of the load piston rod so that the end of the return piston rod may engage the end of the load piston rod.

[0005] During a power stroke of the DPR-actuator, pressurized fluid is introduced into to the load chamber and the return chamber. The pressurized fluid in the load chamber drives the load piston from an OFF position in the load chamber to an ON position in the load chamber to move a load attached to the scotch yoke between positions corresponding to the OFF and ON positions of the load piston. The pressurized fluid introduced into the return chamber drives the return piston to compress the return spring. During the power stroke the return piston rod is disengaged from the load piston rod, and compression of the return spring is independent of motion of the load piston. When the load piston reaches its ON position, the end of the load piston rod optionally engages the end of the return piston rod. Releasing pressurized fluid from the load chamber and the return chamber allows the return spring in the return chamber to expand and force the return piston and thereby the return piston rod and the load piston rod with which it is engaged, to move the load piston back to its OFF position.

[0006] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF FIGURES

[0007] Non-limiting examples of embodiments of the disclosure are described below with reference to figures attached hereto that are listed following this paragraph. Identical features that appear in more than one figure are generally labeled with a same label in all the figures in which they appear. A label labeling an icon representing a given feature of an embodiment of the disclosure in a figure may be used to reference the given feature. Dimensions of features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale.

[0008] Figs. 1A-1C show schematic snapshot images of a double piston rod spring return scotch yoke actuator (DPR-actuator) as it moves during a power stroke from a safe, OFF, position to a working, ON, position with its return piston rod disengaged from its load piston rod in accordance with an embodiment of the disclosure; [0009] Fig. ID-IE show schematic snapshot images of the DPR-actuator shown in Figs. 1A- 1C as it moves during a return stroke from the ON position to the OFF position, with the return piston rod engaged to the load piston rod in accordance with an embodiment of the disclosure;

[0010] Figs. 2A and 2B schematically show a DPR-actuator comprising a housing formed having a fluid flow manifold that provides pressurized fluid to a load chamber and a return chamber of the DPR-actuator and a one-way restriction valve that restricts fluid flow into the load chamber, in accordance with an embodiment of the disclosure; and

[0011] Figs. 3A and 3B schematically show a configuration of a one-way restriction valve that may be used to restrict rate of flow of pressurized fluid into the load chamber so that it is smaller than rate of flow of pressurized fluid into the return chamber, in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

[0012] In the detailed description below aspects of a DPR-actuator are described with reference to Figs. 1A-1E, which schematically show interaction of features of the actuator during different phases of its motion during power and return strokes. Figs 1A-1C schematically show snapshot images of The DPR-actuator at different times during a power stroke and Figs. ID and IE show snap shot images of the DPR actuator at different times during a return stroke. Figs. 2A and 2B show an implementation of a DPR-actuator comprising a common fluid flow manifold for simultaneously introducing pressurized fluid at a same pressure and into load and return chambers of the actuator. An example configuration of a oneway restriction valve comprised in the fluid flow manifold that may be used to control flow of pressurized fluid into and out from the load chamber is discussed with respect to Figs. 3A and 3B.

[0013] In the discussion, unless otherwise stated, adjectives such as "substantially" and "about" modifying a condition or relationship characteristic of a feature or features of an embodiment of the disclosure, are understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended. Wherever a general term in the disclosure is illustrated by reference to an example instance or a list of example instances, the instance or instances referred to, are by way of non-limiting example instances of the general term, and the general term is not intended to be limited to the specific example instance or instances referred to. Unless otherwise indicated, the word "or" in the description and claims is considered to be the inclusive "or" rather than the exclusive or, and indicates at least one of, or any combination of more than one of items it conjoins.

[0014] Fig. 1A shows a simplified schematic cutaway of a DPR-actuator 20 when the DPR actuator is OFF and immediately prior to beginning a power stroke in accordance with an embodiment of the disclosure.

[0015] DPR-actuator 20 comprises a load piston 30 housed in a load chamber 31 of a load cylinder 32, and a return cylinder 42 having a return chamber 41 housing a return spring 43 that seats on a return piston 40. Optionally, load chambers 31 and 41 have circular cross- section having substantially same diameters. A load piston rod 35 is attached to load piston 30 and extends out from load cylinder 32. A return piston rod 45 is attached to return piston 40 and extends out from return cylinder 42. The load piston rod is married to a slider 51 of a scotch yoke transmission 50 having a guide rod 52 along which the slider is free to move. Guide rod 52 optionally bridges and is supported by load and return cylinders 32 and 42. A coupling pin 53 extends from slider 51 into a slot 55 of a crank 56 that extends from a drive shaft 60 to couple the slider to the crank. Load and return piston rods 35 and 45 have ends 36 and 46 respectively that are optionally aligned along a mutual axis (not shown) and configured so that the ends may mate when in contact. In the OFF state of DPR-actuator 20 shown in Fig. 1A, load and spring return pistons 30 and 40 are in their OFF positions at the left ends of their respective chambers 31 and 41, and in accordance with an embodiment of the disclosure, end 46 of spring return rod 45 contacts end 36 of load piston rod 35. Optionally, as shown in Fig. 1A and figures that follow, end 46 of return piston rod 45 has a male shape and load piston rod 36 has a female shape configured to receive end 46 of return piston rod 45.

[0016] Motion of slider 51 towards and away from return cylinder 40 along guide rod 52 rotates crank 56 and drive shaft 60 to which it is attached clockwise and counterclockwise respectively about an axis of the shaft represented by a filled circle 61. Drive shaft 60 extends out from DPR-actuator 20 in a direction perpendicular to the plane of Fig. 1 A and is configured to be attached to a load that the DPR-actuator is intended to move. In an embodiment, DPR- actuator 20 is configured so that motion of load and return pistons 30 and 40 from their respective OFF positions to their respective ON positions during a power stroke and back towards their OFF positions during a return stroke of the DPR-actuator, rotates crank 56 and drive shaft 60 clockwise and counterclockwise respectively through a dynamic angular range of optionally about 90°. Clockwise rotation is indicated by a curved arrow 98 in Fig. 1A and figures IB and 1C that follow, and counterclockwise rotation in figures ID and IE is indicated by a curved arrow 99.

[0017] Motion of slider 51 to rotate drive shaft 60 is controlled by motion of load and return pistons 30 and 40 between their OFF and ON positions in their respective cylinder chambers 31 and 41. Flow of pressurized fluid into load and return chambers 31 and 41 through fluid flow power ports 37 and 47 respectively, and corresponding fluid flow out from the load and return chambers via fluid flow exhaust ports 38 and 48, control motion of pistons 30 and 40 from OFF positions to ON positions during a power stroke of DPR-actuator 20. Flow of pressurized fluid out from load and return chambers 31 and 41 through fluid flow power ports 37 and 47 respectively, and corresponding fluid flow into the load and return chambers via fluid flow exhaust ports 38 and 48, control motion of pistons 30 and 40 from ON positions to OFF positions during a return stroke of DPR-actuator 20. In Fig. 1A and figures that follow solid arrows are used to indicate direction of pressurized fluid flow into or out from load and return chambers 31 and 41 via their fluid flow power ports 37and 47 during power and return strokes. Dashed arrows are used to indicate fluid flow via fluid flow exhaust ports 38 and 48. Fluid flow into or out of chambers 31 and 42 via the exhaust ports is generally at ambient pressure.

[0018] In an embodiment, during a power stroke, pressurized fluid is substantially simultaneously provided to both the load chamber and the spring return chamber. However, flow of pressurized fluid into load chamber 31 is restricted relative to flow of pressurized fluid into return spring chamber 41 so that pressurized fluid enters the load chamber at a flow rate that is less than the flow rate at which pressurized fluid enters the return chamber. As a result, during a power stroke, return piston 40 moves away from its OFF position towards its ON position faster than load piston 30 moves away from its OFF position towards its ON position and disengages return piston rod 35 from load piston road 45. The load and return pistons are therefore decoupled during a power stroke of DPR-actuator 20 and move away from their respective safe positions independently of each other. Force that the pressurized fluid generates on load piston 32 is not applied to compressing return spring 43. During a return stroke of DPR-actuator 20, flow of pressurized fluid out from load chamber 31 through power port 37 is enabled at a flow rate that is greater than the restricted flow rate at which pressurized fluid enters the load chamber during a power stroke of the DPR-actuator. Optionally, during the return stroke, flow rate of pressurized fluid out of load chamber 31 through power port 37 is about equal to flow rate of pressurized fluid out of return chamber 41 through power port 47.

[0019] In an embodiment, flow rate of pressurized fluid into and out of load chamber 31 during power and return strokes is controlled by a one-way restriction valve 70. One-way restriction valve 70 restricts rate of fluid flow into the load chamber via power port 37 during a power stroke but allows a substantially greater flow rate of pressurized fluid out of the load chamber via the power port during a return stroke. In an embodiment, to enable an "emergency shutdown" (ESD) of DPR-actuator 20, during a return stroke the restriction valve allows exhaust of pressurized fluid from load chamber 31 via power port 37 at a flow rate that is about equal to a flow rate at which pressurized fluid exhausts from return chamber 41 via power port 47.

[0020] In Fig. 1A as noted above, DPR-actuator is OFF, load and return pistons are in their OFF positions and end 46 of return piston rod 45 is seated in end 36 of load piston rod 35. DPR-actuator 20 is at a beginning of a power stroke and pressurized fluid is being provided to both load and return chambers 31 and 41 as indicated by solid arrows 101. However, since flow rate of pressurized fluid into load chamber 31 via power port 37 is restricted by restriction valve 70, the pressurized fluid enters the load chamber at a smaller flow rate at which pressurized fluid enters return chamber 41. And corresponding outflow from the load chamber of fluid at substantially ambient pressure is smaller than fluid outflow from return chamber 41. Flow of pressurized fluid into load and return chambers 31 and 41 via their respective power ports 37 and 47 is represented by solid flow arrows 102 and 103. Flow of fluid at substantially ambient pressure out from load and return chambers 31 and 41 via their exhaust ports 37 and 47 is represented by dashed flow arrows 104 and 105 respectively. Arrows 102 and 104 are smaller than arrows 103 and 105 respectively to schematically represent the smaller flow rates into and out of load chamber 31 resulting from the functioning of one-way restriction valve 70.

[0021] As a result of the lower flow rate of pressurized fluid into load chamber 31 and the higher flow rate of pressurized fluid into return chamber 41, return piston 40 moves away from its OFF position toward its ON position at a higher speed than load piston 30 moves away from its OFF position toward its ON position. End 46 of return piston rod 45 therefore separates from end 36 of load piston rod 35 substantially upon initiation of the power stroke and remains separated from the end of the load piston rod throughout the power stroke. Force that the pressurized fluid entering load chamber 31 generates on load piston 32 is not applied to compressing return spring 43. For example, Fig. IB schematically shows DPR-actuator 20 at a time during the power stroke after the power stroke has begun. Load piston 30 is located in between its OFF and ON positions, return piston rod 45 is separated from load piston rod 35, crank 56 has been rotated clockwise partially through its full dynamic range of 90°, and return piston 40 is close to the ON position of the return piston.

[0022] As a result of the separation of the return piston rod 45 from the load piston rod 35 during a power stroke, for a given torque that a DPR-actuator, such as DPR actuator 20, in accordance with an embodiment of the disclosure is required to produce, the DPR-actuator may be smaller than a conventional spring return actuator or operate at lower pressures than a conventional spring return actuator. For example, for a same cross section load piston, a DPR- actuator, in accordance with an embodiment may operate at pressurized fluid pressures that, optionally is about half that at which a conventional spring return actuator operates.

[0023] In Fig. 1C DPR-actuator 20 is shown ON, after completion of the power stroke.

Pressurized fluid that has entered load chamber 31 and return chamber 41 during the power stroke has pushed load and return pistons 30 and 40 all the way to their respective ON positions, and crank 56 and drive shaft 60 have been rotated through about 90° to a maximum clockwise position. With load and return pistons in their ON positions, as when they are in their OFF positions, end 46 of return piston rod 45 contacts and is seated in end 36 of load piston rod 35.

[0024] Fig. ID schematically shows DPR-actuator 20 after being ON as shown in Fig. 1C and at a beginning of a return stroke when the DPR-actuator is to be turned OFF, in accordance with an embodiment of the disclosure. In Fig. ID pressurized gas that resides in load and return chambers 31 and 41 is being released through power ports 37 and 47 as schematically shown by flow arrows 111 and 112 respectively. As a result, pressure in return chamber 31 drops to allow return spring 43 to expand and force return piston 40 to move back to the OFF position of the return piston. As the return piston moves back to its OFF position, return piston rod end 46 remains in contact with load piston rod end 36 and applies force to the load piston rod end that pushes load piston 30 back towards its OFF position. As pressurized fluid leaves load and return chambers 31 and 41, fluid at ambient pressure enters the load and return pistons through their respective exhaust ports 38 and 48, as indicated by dashed flow arrows 114 and 115. Fig. IE schematically shows DPR 20 at a time during the return stroke at which the DPR-actuator 20 is moving toward its OFF state. Load piston 30 is between its ON and OFF states and crank 56 and drive shaft 60 are rotated counterclockwise by less than 90° from the maximum clockwise ON angle shown in Fig. 1C to the maximum counterclockwise OFF angle of the crank and drive shaft.

[0025] Figs 2A-2B schematically show a cutaway cross section of a DPR-actuator 120 that is an implementation of DPR-actuator 20 optionally comprising a housing 122 that houses components shown in Figs. 1A-1E comprised in DPR-actuator 20. In an embodiment, housing 122 is formed having a fluid flow manifold 124 through which pressurized fluid may be provided to and released from both load and return chambers 31 and 41 via power ports 37 and 38 respectively. Optionally, exhaust ports 47 and 48 of DPR-actuator 120 communicate with a common exhaust fluid channel 126 schematically shown in dashed lines. A one-way restriction valve represented by a shaded circle 150 seats in power port 37 of load chamber 31 in accordance with an embodiment. The figures show DPR-actuator 120 connected to and controlled by a 3/2 flow valve 200 in accordance with an embodiment of the disclosure. Fig. 2A shows DPR-actuator 120 at the beginning of a power stroke and Fig. 2B schematically shows DPR-actuator 120 at the end of a return stroke.

[0026] 3/2 flow valve 200 has an output port 201 connected to fluid flow manifold 124 and two input ports 202 and 203. Input port 202 is optionally connected to a source (not shown) of pressurized fluid for operating DPR-actuator 120. Input port 203 is optionally open to fluid at an ambient pressure. 3/2 valve is selectively controllable to connect output port 201 to input port 202 and thereby connect fluid flow manifold 124 to pressurized fluid to initiate and control a return stroke or to connect the output port and thereby the common flow channel to input port 203, to initiate and execute a return stroke. Fig. 2A schematically shows DPR- actuator 120 with fluid flow manifold 124 connected by 3/2 valve 200 to input port 202 to enable flow of pressurized fluid into load and return chambers 31 and 41 to initiate a power stroke. Fig. 2B schematically shows DPR-actuator 120 at an end of a return stroke, having output port 201 and thereby common flow channel 124 connected by 3/2 valve 200 to input port 203 to enable outflow of pressurized fluid from load and return chambers 31 and 41.

[0027] Fig. 3A and 3B schematically show an example embodiment of one-way restriction valve 150 that may be seated in power port 37 of DPR-actuator 20 and 120, to provide relatively restricted flow of pressurized fluid into load chamber 31 during a power stroke of DPR-actuator 20 and relatively enhanced outflow of pressurized fluid from the load chamber during a return stroke of the DPR-actuator, in accordance with an embodiment of the disclosure.

[0028] One-way restriction valve 150 optionally comprises a valve housing 160, shown very schematically in outline form, and a shuttle 180 that seats in a valve chamber 161 of the housing. In an embodiment, housing 160 has a front end 162 having a front end rim 163 surrounding a front end flow aperture 164 and a back end 166 having a back end rim 167 surrounding a back end flow aperture 168. Shuttle 180 optionally has a tapered "nose" 181 that is formed having at least one lateral flow aperture 182 that communicates with a lumen 183 of the shuttle. Optionally, the at least one lateral flow aperture 182 comprises a plurality of lateral flow apertures 182. In an embodiment, nose 181 is circularly symmetric about an axis 184 and the plurality of lateral flow apertures are symmetrically angularly distributed around axis 184. Nose 181 of the shuttle ends in a front wall 185 formed having a relatively small fluid flow aperture 186 that also communicates with lumen 183. A back wall 187 (shown clearly in Fig. 3B) of shuttle 180 is formed having a relatively large flow aperture 188 that communicates with lumen 183.

[0029] Shuttle 180 is shorter than valve chamber 161 along a direction of axis 184. The shuttle is configured to move freely in either direction in valve chamber 161 along axis 184, to meet and substantially seal front wall 185 of the shuttle against front end rim 163 of housing 160 or to meet and substantially seal back wall 187 of the shuttle against back end rim 167 of the housing. Optionally, valve housing 160 has an internal wall 169 that delineates a portion of valve chamber 161 and has a shape that is substantially a negative of the external surface of tapered nose 181. When shuttle 180 seals against front end rim 163 of valve housing 160 internal wall 169 seals lateral flow apertures 182 of shuttle 180 against fluid flow. When shuttle 180 seals against back end rim 167of valve housing 160 lateral flow apertures 182 of shuttle 180 are unsealed to allow fluid flow though the flow apertures.

[0030] One-way restriction valve 150 provides relatively restricted fluid flow for fluid that enters the valve through back end flow aperture 168 of housing 160. Fluid flowing into oneway restriction valve 150 through back end aperture 168, as schematically shown in Fig. 3B, pushes shuttle 180 to seal against front end rim 163 of housing 160. As result, back to front, "forward", fluid flow through the one-way valve is restricted by the relatively small size of small fluid flow aperture 186. Restricted forward flow of fluid through one-way restriction valve 150 is schematically represented by small block arrows 220. On the other hand, as schematically shown in Fig. 3A, fluid that flows into one-way restriction valve 150 through front end flow aperture 164 of housing 160 pushes shuttle 180 "backward" to seal against back end rim 167 of the housing. When sealed to the back end rim, fluid flows through one-way restriction valve via lateral apertures 182, as well as through small fluid aperture 186, to provide enhanced fluid flow, schematically represented by large block arrows 222, through the one-way valve. 031] In the above description and referenced figures, restricted flow of pressurized fluid into load chamber 31 is provided by a one-way restriction valve, and an example embodiment of a one-way restriction valve 150 is schematically shown in Figs. 3A and 3B. However, practice of an embodiment of the disclosure is not limited to use of a one-way restriction valve to restrict flow of pressurized fluid into the load chamber, nor is practice of an embodiment that comprises a one-way restriction valve limited to the particular one-way restriction valve 150 shown in Figs. 3 A and 3B. Restriction of fluid flow may for example be provided by a relatively small fluid flow power port 37 rather than a one-way restriction valve. Alternatively or additionally, restriction of pressurized fluid flow into load chamber 31 may be provided by a differential fluid flow manifold that provides pressurized fluid flow to fluid flow power port 37 at a restricted fluid flow rate. Optionally to provide the reduced fluid flow to power port 37 the differential fluid flow manifold may have a narrowing along a flow path of fluid flow through the manifold to power port 37. Alternatively or additionally restricted fluid flow may be provided by a one-way restriction valve configured differently than the example one-way restriction valve 150 shown in Figs. 3A and 3B.

[0032] It is further noted that restriction of fluid flow into the load chamber of a DPR-actuator in accordance with an embodiment of the disclosure decouples the piston rod attached to the return piston of the DPR-actuator from the piston rod attached to the load piston of the DPR- actuator. In an embodiment decoupling the piston rods may, alternatively or additionally be provided by having the return piston having a smaller cross-section area than the cross section area of the load piston and inflowing pressurized fluid at substantially same fluid flow rates into the cylinder chambers respectively housing the pistons.

[0033] There is therefore provided in accordance with an embodiment of the disclosures a spring-return actuator comprising: a first cylinder having a first cylinder chamber housing a first piston; a first piston rod coupled to the first piston; a scotch yoke transmission coupled to the first piston rod, a second cylinder having a second cylinder chamber housing a second piston and a return spring compressed by motion of the second piston; a second piston rod that is coupled to the second piston and faces the first piston rod; wherein the first and second piston rods may be brought into contact with or separated from each other by rates at which pressurized fluid is introduced into first and second cylinder chambers.

[0034] Optionally, the actuator comprises at least one first fluid flow port in communication with the first chamber and at least one second fluid flow port in communication with the second chamber. Optionally, the first piston is movable between first and second positions in the first chamber by flow of pressurized fluid into the first chamber via the at least one first fluid flow port and the second piston is moveable between third and fourth positions in the second chamber by flow of pressurized fluid into the second chamber and wherein in motion from the third to the fourth position, the second piston compresses the return spring. Optionally, when the first and second pistons are in the first and third positions respectively the ends of the first and second piston rods are in contact. Additionally or alternatively when the first and second pistons are in the second and fourth positions respectively, the ends of the first and second piston rods are in contact.

[0035] In an embodiment, the first at least one fluid flow port and the second at least one fluid flow port are configured so that when provided with pressurized fluid at a same pressure, rate of flow of the pressurized fluid through the second at least one fluid flow port is greater than flow of the pressurized fluid through the at least one first fluid flow port.

[0036] In an embodiment, the first at least one fluid flow port has a cross section area greater than a cross section area of the second at least one fluid flow port.

[0037] In an embodiment, the at least one first fluid flow port comprises a one-way restriction valve configured to restrict flow of pressurized fluid into the first chamber via the at least one first fluid flow port to a first flow rate and to allow flow of pressurized fluid out from the first chamber via the at least one first fluid flow port at a second flow rate greater than the first flow rate.

[0038] In an embodiment, the actuator comprises a housing in which the first and second cylinders are formed. Optionally, the housing is formed having a fluid flow manifold in communication with both the first at least one fluid flow port and the second at least one fluid flow port. The fluid flow manifold may be configured to reduce rate of fluid flow through the manifold to the first at least one fluid flow port relative to the rate of fluid flow through the manifold to the second at least one fluid flow port. [0039] In an embodiment, the actuator comprises a 3/2 flow valve connected to the fluid flow manifold that is selectively controllable to provide pressurized fluid to the manifold and exhaust pressurized fluid from the manifold. In the description and claims of the present application, each of the verbs, "comprise" "include" and "have", and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb.

[0040] Descriptions of embodiments of the invention in the present application are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments, will occur to persons of the art. The scope of the invention is limited only by the claims.