Patent Document

CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 10/864,869, filed Jun. 10, 2004, now U.S. Pat. No. 8,517,012, which is continuation-in-part of U.S. application Ser. No. 10/360,757, which was filed on Dec. 10, 2001, now U.S. Pat. No. 6,910,483, and is hereby incorporated in its entirety by reference. This application also claims the benefit of U.S. Provisional Application No. 60/477,063, filed Jun. 10, 2003, and U.S. Provisional Application No. 60/477,063 filed Jun. 11, 2003, each of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an apparatus for supplying breathable gas to a human, used in, for example, Continuous Positive Airway Pressure (CPAP) treatment of Obstructive Sleep Apnea (OSA), other respiratory diseases and disorders such as emphysema, or the application of assisted ventilation. 
         [0004]    2. Description of Related Art 
         [0005]    CPAP treatment of OSA, a form of Noninvasive Positive Pressure Ventilation (NIPPY), involves the delivery of a pressurized breathable gas, usually air, to a patient&#39;s airways using a conduit and mask. Gas pressures employed for CPAP can range, e.g., from 4 cm H 2 O to 28 cm H 2 O, at flow rates of up to 180 L/min (measured at the mask), depending on patient requirements. The pressurized gas acts as a pneumatic splint for the patient&#39;s airway, preventing airway collapse, especially during the inspiratory phase of respiration. 
         [0006]    Typically, the pressure at which a patient is ventilated during CPAP is varied according to the phase of the patient&#39;s breathing cycle. For example, the ventilation apparatus may be pre-set, e.g., using control algorithms, to deliver two pressures, an inspiratory positive airway pressure (IPAP) during the inspiration phase of the respiratory cycle, and an expiratory positive airway pressure (EPAP) during the expiration phase of the respiratory cycle. An ideal system for CPAP is able to switch between IPAP and EPAP pressures quickly, efficiently, and quietly, while providing maximum pressure support to the patient during the early part of the inspiratory phase. 
         [0007]    In a traditional CPAP system, the air supply to the patient is pressurized by a blower having a single impeller. The impeller is enclosed in a volute, or housing, in which the entering gas is trapped while pressurized by the spinning impeller. The pressurized gas gradually leaves the volute and travels to the patient&#39;s mask, e.g., via an air delivery path typically including an air delivery tube. 
         [0008]    There are currently two common ways in which the blower and impeller can be configured to produce the two different pressures, IPAP and EPAP, that are required in an ideal CPAP system. A first method is to set the motor/impeller to produce a constant high pressure and then employ a diverter valve arrangement that modulates the high pressure to achieve the required IPAP and EPAP pressures. CPAP systems according to the first method are called single-speed bi-level systems with diverters. A second method is to accelerate the motor that drives the impeller to directly produce IPAP and EPAP pressures. CPAP systems according to the second method are called variable-speed hi-level systems. 
         [0009]    Variable-speed bi-level CPAP systems have a number of particular disadvantages. A first disadvantage is that in order to switch rapidly between IPAP and EPAP, the impeller must be accelerated and decelerated rapidly. This causes excessive stress on the impeller, motor, and bearings. However, if the impeller is accelerated slowly, the pressure rise may be unsatisfactorily slow, and thus, the patient may not receive adequate treatment. 
         [0010]    Rapid acceleration and deceleration of the motor and impeller also result in excessive heat generation and undesirable acoustic noise. (“Undesirable” acoustic noise, as the term is used here, refers to acoustic noise that is overly loud, as well as acoustic noise which occurs at a frequency that is irritating to the user, regardless of its volume.) In addition, design engineers are often forced to make a compromise, sacrificing optimal pressure and flow characteristics in favor of achieving a desired peak pressure. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention, in one aspect, relates to variable speed blowers providing faster pressure rise time with increased reliability and less acoustic noise. Blowers according to an embodiment of the present invention comprise a gas flow path between a gas inlet and a gas outlet, a motor, and an impeller assembly. 
         [0012]    Preferably, the impeller assembly may include a shaft in communication with the motor for rotational motion about a first axis and first and second impellers coupled, e.g., fixedly secured, to the shaft. The impellers are placed in fluid communication with one another by the gas flow path such that both impellers are disposed between the gas inlet and the gas outlet to cooperatively pressurize gas flowing from the gas inlet to the gas outlet. 
         [0013]    In one embodiment, the impellers are disposed in series between the gas inlet and the gas outlet. The blower may also comprise a housing, portions of the housing being disposed around each of the first and second impellers. In particular, the housing may include first and second volutes, the first volute containing gas flow around the first impeller and the second volute containing gas flow around the second impeller. The gas inlet may be located in the first volute and the gas outlet may be located in the second volute. 
         [0014]    The impellers may be arranged such that they are vertically spaced from one another along the first axis. In particular, they may be disposed at opposite ends, respectively, of the blower housing. 
         [0015]    A blower according to an embodiment of the present invention may have varying configurations. In one embodiment, the two impellers are designed to rotate in the same direction. In another embodiment, the two impellers are designed to rotate in opposite directions. 
         [0016]    Another aspect of the invention relates to an in-plane transitional scroll volute for use in either a double- or single-ended blower. The in-plane transitional scroll volute gradually directs pressurized air away from a spinning impeller. 
         [0017]    A further aspect of the invention involves a method and apparatus for minimizing blower-induced turbulence presented to a flow meter for measuring the air flow. In one embodiment, the flow meter is positioned upstream from the blower. 
         [0018]    In yet another aspect, a blower has a motor provided with opposed first and second shafts. First and second stage impellers are provided to the first and second shafts, respectfully. An inner casing supports the motor and an outer casing supports the inner casing. In addition, a substantially annular channel is provided between the inner and outer casings. In operation, gas is directed from the first stage impeller towards the second stage impeller via the substantially annular channel. 
         [0019]    Additional aspects, advantages and features of the present invention are set forth in this specification, and in part will become apparent to those skilled in the art on examination of the following, or may be learned by practice of the invention. The inventions disclosed in this application are not limited to any particular set of or combination of aspects, advantages and features. It is contemplated that various combinations of the stated aspects, advantages and features make up the inventions disclosed in this application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Various exemplary embodiments will be described with reference to the following drawings, in which like reference characters represent like features, wherein: 
           [0021]      FIG. 1  is a perspective view of a double-ended blower according to a first embodiment of the present invention; 
           [0022]      FIG. 2  is a partially sectional perspective view of the double-ended blower of  FIG. 1 ; 
           [0023]      FIG. 3  is an exploded, perspective view of an in-plane transitional scroll volute suitable for use in blowers according to the present invention; 
           [0024]      FIG. 4  is a perspective view of a double-ended blower according to a second embodiment of the present invention; 
           [0025]      FIG. 4A  is a rear perspective view of the double-ended blower of  FIG. 4 , illustrating the flow therethrough; 
           [0026]      FIG. 5  is a sectional perspective view of the double-ended blower of  FIG. 4 ; 
           [0027]      FIGS. 6A and 6B  are a perspective view of an impeller having scalloped edges; 
           [0028]      FIG. 7  is an exploded perspective view of a double-ended blower according to another embodiment of the present invention; 
           [0029]      FIG. 7A  is a view of the press-fit connection between the motor and the contoured plate in  FIG. 7 ; 
           [0030]      FIG. 7B  is a cross-sectional view of an alternative embodiment of the circular plate in  FIG. 7A ; 
           [0031]      FIG. 8  is an assembled perspective view of the double-ended blower of  FIG. 7  from one side; 
           [0032]      FIG. 9  is an assembled perspective view of the double-ended blower of  FIG. 7  from another side; 
           [0033]      FIG. 10  is an exploded perspective view of a double-ended blower according to a further embodiment of the present invention. 
           [0034]      FIG. 11A  is a side view of a first damping sleeve fitted into a casing of the blower represented in  FIG. 10 ; 
           [0035]      FIG. 11B  is a side view of a second damping sleeve fitted into a casing of the blower represented in  FIG. 10 ; 
           [0036]      FIG. 12  is a perspective view of the press-fit connection between stationary flow guidance vanes and the contoured plate in  FIG. 10 ; 
           [0037]      FIG. 13  represents an assembled view of the blower of  FIG. 10 ; 
           [0038]      FIG. 13A  is a partial cross-sectional view of a blower according to another aspect of the technology; 
           [0039]      FIG. 14  is an exploded perspective view of an enclosure for a blower according to the present invention; 
           [0040]      FIG. 15  is a further exploded perspective view of an enclosure for a blower according to the present invention; 
           [0041]      FIG. 16  is a top perspective view of the enclosure of  FIG. 14 ; 
           [0042]      FIG. 17  represents an assembled view of the enclosure of  FIG. 14 ; 
           [0043]      FIG. 18  is a perspective view of a protrusion of a blower according to the present invention provided with a rubber suspension bush; 
           [0044]      FIG. 19  is a top perspective view of the main seal of the enclosure of  FIG. 14 ; 
           [0045]      FIG. 19A  is a detailed view taken from  FIG. 19 ; 
           [0046]      FIG. 20  is a top perspective view of the enclosure base of the enclosure of  FIG. 14 ; 
           [0047]      FIG. 21  is a bottom perspective view of the enclosure lid of the enclosure of  FIG. 14 ; 
           [0048]      FIGS. 22A and 22B  are perspective views of the flow meter of the enclosure of  FIG. 14 ; 
           [0049]      FIG. 23  is a perspective view of the inlet connector of the enclosure of  FIG. 14 ; and 
           [0050]      FIG. 24  is a perspective view of a filter retainer for the enclosure of  FIG. 14 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0051]      FIG. 1  is a perspective view of a double-ended blower  100  according to a first embodiment of the present invention. Blower  100  has a generally cylindrical shape with impeller housings, or volutes  112 ,  113 , disposed at each end. Thus, blower  100  accommodates two impellers  114 ,  115 , which are best seen in the cutaway perspective view of  FIG. 2 . 
         [0052]    As shown in  FIGS. 1 and 2 , the two impellers  114 ,  115  are placed in fluid communication with one another by an airpath  116 . The airpath  116  of blower  100  is comprised of piping that extends from the first volute  112  to the second volute  113 , the terminal ends of the airpath  116  being contoured around, and gradually fusing with, the body of blower  100  proximate to the volutes  112 ,  113  to form a single, integral structure. The airpath  116  may be comprised of substantially rigid piping that is, e.g., integrally molded with the other components of the blower  100 , or it may be separately provided and joined to the blower  100  at each volute  112 ,  113 . 
         [0053]    Blower  100  has a single air intake  118  positioned such that air, or another suitable gas, flows directly into the first volute  112  and can be drawn in by the turning impeller  114  inside the first volute  112 . Once drawn into the air intake  118 , the air is circulated and pressurized by the motion of the impeller  114  before gradually exiting the volute  112  and entering the airpath  116 . Once in the airpath  116 , the air travels to the second volute  113 , where it is further circulated and pressurized by the impeller  115  of the second volute  113  before exiting the blower  100  through the outflow conduit  120 . The path of the air in blower  100  is indicated by the arrows in  FIG. 1 . As shown, in blower  100 , air from the first volute  112  travels along a relatively straight section of the airpath  116  and enters the second volute  113  through an intake cavity just above the second volute  113  (not shown in  FIG. 1 ). 
         [0054]    Blower  100  could have, e.g., two air intakes  118 , one for each volute  112 ,  113 , if the impellers  114 ,  115  are designed to work in parallel, rather than in series. This type of parallel impeller arrangement may be beneficial if installed in a low-pressure CPAP device requiring high flow rates. 
         [0055]    The design of the airpath  116  can affect the overall performance of the blower  100 . In general, several design considerations influence the design of an airpath for use in blowers according to the present invention. First, airpaths to be used in blowers according to one embodiment of the present invention are most advantageously configured to provide low flow resistance, because low flow resistance in the airpath minimizes the pressure drop between the two volutes  112 ,  113  in the blower. Second, airpaths according to one embodiment of the present invention are best configured such that the air entering the second volute  113  enters from a direction for which the blades of the impeller  115  were designed. (As will be described in more detail below, the two impellers of a blower according to the present invention may be designed to spin in the same or different directions.) Third, airpaths according to one embodiment of the present invention are most advantageously of a compact design. 
         [0056]    The design considerations set forth above may be embodied in an airpath having long, sweeping bends to minimize the pressure drop around the bends. It is also beneficial to have a relatively straight section after a bend in the airpath, because a relatively straight section after a bend aids in allowing the gas flow to become more fully developed before entering a volute. An appropriate length for a straight airpath section following a bend is, e.g., about three times the diameter of the airpath. The relatively straight section also aids in the flow entering the second volute  113  being axial, the flow orientation for which many impellers are designed. If additional flow shaping is desired, stator vanes or other similar flow directing structures may be added to the blower, however, stator vanes may be costly in terms of flow impedance and pressure drops. 
         [0057]    In view of the three major airpath design considerations set forth above, the airpath  116  of the embodiment depicted in  FIG. 1  has a long, relatively straight section because the relatively straight section is one of the shortest possible paths between the two volutes  112 ,  113 . Those skilled in the art will realize that the airpath  116  need not be straight at all. 
         [0058]    Blowers according to the invention may be designed manually, using prototypes and experimental measurements of air flows and pressures in those prototypes to optimize the design of the airpath  116  and other components. Alternatively, they may be designed, either as a whole or in part, by using computational fluid dynamics computer simulation programs. A variety of computational fluid dynamics programs are known in the art. Computational fluid dynamics programs particularly suited for the design of blowers according to the invention include, e.g., FLOWORKS (NIKA GmbH, Sottrum, Germany), ANSYS/FLOTRAN (Ansys, Inc., Canonsburg, Pa., USA), and CFX (AEA Technology Engineering Software, Inc., El Dorado Hills, Calif., USA). Such simulation programs give the user the ability to see the effects of airpath design changes on a simulated gas flow. 
         [0059]    Many different types of configurations for the two volutes  112 ,  113  and airpath  116  are possible in a double-ended blower according to the present invention. In general, each volute is designed to retain the gas around the impeller for a short period of time, and to permit a gradual exit of gas into the airpath. The exact configuration of the airpath may depend on many factors, including the configuration of the volutes and the “handedness,” or direction of airflow, around each impeller. 
         [0060]    The design of the volutes is an art unto itself, as improperly designed volutes may cause a noise, or may interfere with the generation of the desired pressure and flow characteristics. The computational fluid dynamics computer programs described above may also be useful in designing the volutes, although the number of variables involved in volute design usually precludes the volute from being entirely computer-designed. 
         [0061]    The type and direction of flow into each volute  112 ,  113  may influence the performance and noise characteristics of the impellers  114 ,  115 . For this reason, a bell-shaped intake, rounded intake edges, stator vanes, or other flow directing/enhancing structures may be used at the entrance to either or both of the volutes  112 ,  113 . However, the use of these types of flow enhancing/directing structure may increase the flow resistance. 
         [0062]    One common problem with volutes  112 ,  113  is that they may provide too abrupt of a transition into the airpath  116 . An abrupt transition between the volute  112 ,  113  and the airpath  116  usually leaves a forked path or “lip” around the opening. When the impeller blades pass by this lip, a noise called “blade passing frequency” is created. Double-ended blowers according to the present invention are particularly suited for, e.g., use with volutes that are constructed to reduce the occurrence of “blade passing frequency” and other noise. See  FIG. 3 , for instance, which is a perspective view of an in-plane transitional scroll volute  300  suitable for use in a blower according to the present invention. Additionally, the volute  300  may be employed in any conventional blower apparatus. In the view of  FIG. 3 , the volute  300  is provided with its own motor  302 , although it may be adapted for use in a double-ended blower having a single motor driving the impellers in two volutes. As shown, the volute  300  is comprised of two halves  304 ,  306 , the two halves defining upper and lower portions of the volute  300 , respectively. The air intake of the volute  308  is located at the center of the top half  304 . The two halves  304 ,  306  define a path which slowly “peels” away from the air rotating with the impeller. In the path defined by the two halves, there is no sudden “lip” or “split” as in conventional volutes, therefore, “blade passing frequency” is reduced or eliminated entirely. 
         [0063]    Alternatively, any common type of volute may be used, depending on the dimensions of the motor installed in the blower. Another suitable type of volute is the axial volute disclosed in U.S. patent application Ser. No. 09/600,738, filed on Jul. 21, 2000, the contents of which are hereby incorporated by reference herein in their entirety. 
         [0064]    One design consideration for a double-ended blower according to the present invention is the “handedness,” or direction of airflow, around each impeller. This “handedness” may be determined by the direction in which the impeller spins, or it may be determined by the orientation and configuration of the individual blades or vanes of the impeller. For example, one impeller may be spun or the blades oriented to drive the air in a clockwise direction, and the other impeller may be spun or the blades oriented to drive the air in a counterclockwise direction, resulting in a “opposing-handed” double-ended blower. Alternatively, both impellers could be driven in the same direction, resulting in a “same-handed” double-ended blower. Blower  100  of  FIG. 1  is an example of an “opposite-handed” type of double-ended blower. 
         [0065]    A “same-handed” blower is advantageous because the two impellers can be identical, reducing the part count and cost of the blower. However, it should be noted that a designer may choose to design a “same-handed” blower in which the two impellers are each designed and optimized separately for the air flow in their respective volutes. 
         [0066]    An “opposing-handed” blower permits the designer to reduce the length of the shaft on which the impellers arc mounted. This may increase the stability of the shaft itself, because it reduces the problems associated with having an imbalance on a long, cantilevered shaft rotating at high speed. 
         [0067]      FIGS. 4 ,  4 A, and  5  illustrate a “same-handed” blower  200  according to the present invention. Blower  200  also has two volutes  212 ,  213 , an airpath  216 , an air intake  118  and an air outlet  220 . However, as is shown in  FIGS. 4 ,  4 A, the airpath  216  has the shape of a spiral. That is, airpath  216  transitions away from the first volute  212  and then slopes downward as it follows the circumference of the blower  200 , before bending and gradually fusing with an intake cavity located between the motor  150  and the arcuate flange  160  (See  FIG. 5 ), which acts as an air intake in blower  200 . The airflow through the blower  200  is illustrated by the arrows in  FIGS. 4 ,  4 A. 
         [0068]    The internal configuration of blower  200  is shown in the partially sectional perspective view of  FIG. 5 . The internal arrangements of blowers  100  ( FIGS. 1 ,  2 ) and  200  ( FIGS. 4 ,  4 A,  5 ) are substantially similar, and will be described below with respect to components of both blowers, where applicable. As shown in  FIG. 5 , a double-shafted electric motor  150  is installed in the center of the blower  200 . Although only one motor  150  is shown, two motors  150 , one for each impeller, may be used. Various types of known brackets and mountings may be used to support the motor and to secure it to the interior of the blower  200 , although for simplicity, these are not shown in  FIG. 5 . 
         [0069]    The motor  150  drives the double shaft  152  to rotate at speeds up to, e.g., about 30,000 RPM, depending on the configuration of the impellers  114 ,  115 ,  214  and the desired pressures. The shaft  152  traverses substantially the entire length of the blower  100 ,  200  along its center, and is secured to an impeller  114 ,  115 ,  214  at each end. The shaft may be round, square, keyed, or otherwise shaped to transmit power to the two impellers  114 ,  115 ,  214 . The diameter of the shaft may be in the order of, e.g., 3-5 mm, with graduations in diameter along the length of the shaft  152 . For example, the shaft  152  may have a smaller diameter (e.g., 3 mm) on the end closest to the air intake to assist with air intake and a diameter of about 4.5 mm at the end that is cantilevered. The connection between the impellers  114 ,  115 ,  214  and the shaft  152  may be created by an interference fit between the two parts, a weld, an adhesive, or fasteners, such as set screws. In blowers  100  and  200 , the connection between the shaft  152  and the impellers  114 ,  115 ,  214  is by means of a vertically oriented (i.e., oriented along the axis of the shaft  152 ) annular flange  154  formed in the center of the impellers  114 ,  115 ,  214 . In  FIG. 5 , the connection between the impeller  214  and the shaft is shown as an interference fit. 
         [0070]    The impeller  114 ,  115 ,  214  is substantially annular in shape. The center section  156  of the impeller  114 ,  115 ,  214 , is a thin plate which extends radially outward from the shaft  152  to the blades  158 , and is upswept, gradually curving downward as it extends outward from the shaft  152  towards the blades  158 . The actual diameter of each impeller  114 ,  115 ,  214  may be smaller than that of a conventional blower with a single impeller. Fast pressure rise time in a blower requires a low rotational inertia, which varies as the diameter to the fourth power. Because impellers  114  and  214  of blowers  100  and  200  are smaller in diameter, they have less rotational inertia, and thus, are able to provide a faster pressure rise time. In addition to diameter, other design parameters of the impellers  114 ,  214  may be modified to achieve a lower rotational inertia. Other techniques to reduce rotational inertia include “scalloping” the shrouds to produce a “starfish-shaped” impeller, using an internal rotor motor, and using materials, such as liquid crystal polymer, that can be molded into thinner wall sections, so that impeller blades can be hollowed out and strengthened by ribs. The scalloping of the impellers may also advantageously result in a weight reduction of the impeller, therewith allowing faster rise times. See also  FIGS. 6A and 6B  (starfish shaped impeller  214  with aerofoil blades  258  and scalloped edges  259 ). Liquid crystal polymer impeller blades may have wall sections as low as 0.3 mm. 
         [0071]    In embodiments of the invention, the impellers  114 ,  115 ,  214  would typically have an outer diameter in the order of, e.g., 40-50 mm, for example 42.5 mm or 45 mm. The inner diameter of the impellers  114 ,  115 ,  214  may be in the order of, e.g., 18-25 mm. Blade height may be in the range of, e.g., 6-10 mm, although stresses on the impeller blades  158  increases with taller blades. In general, if the blades  158  are taller, the diameter of the impeller may be reduced. The impeller blades  158  themselves may be aerofoils of standard dimensions, such as the NACA 6512, the NASA 66-221, and the NASA 66-010. If the blades  158  are aerofoils, it may be advantageous to select aerofoil profiles that produce good lift at a variety of angles of attack. The impellers  114 ,  115 ,  214  are preferably designed and/or selected so that, in cooperation with the motor, the blower  100 ,  200  can generate a pressure at the mask of about 25 cm H 2 O at 180 L/min and about 30 cm H 2 O at 150 L/min. Given that the airpath  116  will cause pressure drops from the blower  100 ,  200  to the mask, the impellers  114 ,  115 ,  214  are preferably capable of producing about 46 cm H 2 O at 150 L/min and about 43 cm H 2 O at 180 L/min. 
         [0072]    The top of the first volute  112 ,  212  is open, forming the air intake  118 . At the air intake  118 , the top surface  120  of the blower  100 ,  200  curves arcuately inward, forming a lip  122  over the top of the impeller  114 ,  214 . The upswept shape of the impeller center section  156  and the lip  122  of the top surface  120  confine the incoming air to the blower volume inside the first volute  112 ,  212  and help to prevent air leakage during operation. An arcuate flange  160  similar to the arcuate top surface  120  extends from the lower interior surface of the blower  200 , forming the top of the second volute  213 . A contoured bottom plate  162 ,  262  forms the bottom of the second volute  113 ,  213  of each blower  100 ,  200 . The bottom plate  162  of blower  100  has a hole in its center, allowing the airpath  116  to enter, while the bottom plate  262  of blower  200  has no such hole. As described above, the arcuate flange  160  acts as the air intake for the second volute  213  of blower  200 . In blower  200 , stator vanes and additional flow shaping components may be added to the cavity between the motor  150  and the arcuate flange  160  to assist in distributing the incoming air so that it enters the second volute  213  from all sides, rather than preferentially from one side. 
         [0073]    As is evident from  FIGS. 1 ,  2 ,  4 A, and  5 , blowers according to the present invention may have many intricate and contoured surfaces. Such contours are used, as in the case of the arcuate top surface  120  and arcuate flange  160 , to direct gas flow and prevent gas leakage The no-leak feature is particularly beneficial when the gas flowing through the blower  100 ,  200  has a high concentration of oxygen gas, If high-concentration oxygen is used, gas leakage may pose a safety hazard. Also, apart from any safety considerations, leaking gas may produce unwanted noise, and may reduce blower performance. 
         [0074]    The number of intricate, contoured surfaces present in blowers in embodiments according to the present invention makes a production method such as investment casting particularly suitable. Investment casting can produce a single part with many hidden and re-entrant features, whereas other methods of production may require that a design be split into many parts to achieve equivalent function. However, a large number of parts is generally undesirable—in order to minimize the potential for gas leaks, the number of parts is best kept to a minimum and the number of joints between parts is also best kept to a minimum. 
         [0075]    There are also a number of materials considerations for blowers according to the present invention. Metals are typically used in investment casting, but some metals are particularly sensitive to oxidation, which is a concern because medical grade oxygen gas may be used in blowers according to the present invention. One particularly suitable material for the blowers  100 ,  200  is aluminum. Whereas steel may rust on exposure to high concentrations of oxygen, aluminum oxidizes quickly, the oxide forming an impervious seal over the metal. Whichever metal or other material is used, it is generally advantageous that the material has a high thermal conductivity and is able to draw heat away from the airpath, to prevent any heat-related ignition of oxygen. 
         [0076]    While the use of aluminum has many advantages, it does have a tendency to “ring,” or resonate, during blower operation. Therefore, damping materials may be installed in an aluminum blower to reduce the intensity of the vibration of the aluminum components. 
         [0077]    In blowers  100  and  200 , the electric motor  150  may be driven at variable speeds to achieve the desired IPAP and EPAP pressures. The double-ended (i.e., two-stage) design of the blowers means that the range of motor speeds traversed to achieve the two pressures is reduced. The narrower range of motor speeds results in a faster pressure response time than that provided by a single-stage blower having similar motor power and drive characteristics. In addition, the narrower variation in speed applies less stress to the rotating system components, resulting in increased reliability with less acoustic noise. 
         [0078]    The performance of blowers  100  and  200  is approximately equal to the combined performance of the two impeller/volute combinations, minus the pressure/flow curve of the airpath  116 ,  216  between the two volutes  112 ,  113 ,  212 ,  213 . For a variety of reasons that are well known in the art, the actual performance of the blowers  100 ,  200  will depend upon the instantaneous flow rate of the particular blower  100 ,  200 , as well as a number of factors. At higher flow rates, the pressure drop in the airpath  116 ,  216  is generally more significant. 
         [0079]    Double-ended blowers according to the present invention may be placed in a CPAP apparatus in the same manner as a conventional blower. The blower is typically mounted on springs, or another shock-absorbing structure, to reduce vibrations. 
       A Further Embodiment 
       [0080]    A further embodiment of the present invention is illustrated in  FIG. 7 , an exploded perspective view of a double-ended blower  400  according to the present invention. The motor and stator blade portion  402 , located in the center of the exploded view, is investment cast from aluminum in this embodiment, although other manufacturing methods are possible and will be described below. The aluminum, as a good conductor of heat, facilitates the dissipation of heat generated by the accelerating and decelerating motor. Each end  404 A and  404 B of the shaft  404  is shown in  FIG. 7 , but the motor windings, bearing and cover are not shown. The motor power cord  406  protrudes from the motor and stator blade portion  402 . The motor and stator blade portion  402  includes, at its top, a bottom portion of the upper volute  408 . 
         [0081]    As a variation of the design illustrated in  FIG. 7 , the motor and stator blade portion  402  may be made separately from the bottom portion of the upper volute  408 . If the two components are made separately, investment casting would not be required. For example, the motor body may be die cast, while the bottom portion of the upper volute  408  may be injection molded. 
         [0082]    Secured to the motor and stator blade portion  402  by bolts or other fasteners is a circular plate  410 , in which a hole  412  is provided for the passage of the shaft  404 . An impeller  414  rests atop the circular plate. The impeller  414  is scalloped along its circumference to reduce its rotational inertia, giving it a “starfish” look (see also  FIGS. 6A and 6B ). As depicted in more detail in  FIG. 7A , the contoured plate has a side  411  that extends perpendicular to the annular surface  413 . In another embodiment, schematically shown in  FIG. 7B , the side  411 A extends more gradually from the annular surface. Having side  411 A extend more gradually facilitates, relative to the perpendicular side  411 , the air flow created by impeller  414  and therewith aids in noise suppression. Hole  412  is depicted in  FIG. 7B  as being of constant radius. In one embodiment, hole  412  may neck down or have a diameter of non-constant cross-section. 
         [0083]    Referring back to  FIG. 7 , an upper endcap  416  is secured above the impeller  414 , and provides the top portion of the upper volute. The upper and lower volutes in this embodiment are versions of the in-plane transitional scroll volute  300  illustrated in  FIG. 3 . An aperture  418  in the center of the upper endcap  416  serves as the air intake of the blower  400 . 
         [0084]    On the lower end of the blower  400 , a contoured plate  420  forms the top portion of the lower volute. As depicted in more detail in  FIG. 7A , the motor and stator blade portion  402  may comprise feet  462  that can be connected to contoured plate  420  via press-fit recesses  464 . The motor  402  and contoured plate may also be connected instead or in addition via, e.g., adhesives, screws etc. or, alternatively, the motor  402  and contoured plate  420  may be cast as a single piece. 
         [0085]    The top of the contoured plate  420  is raised and curves arcuately downward toward a hole  422 . As was explained above, the contoured plate  420  helps to shape the airflow and to ensure that it enters the impeller cavity from all sides, rather than preferentially from a single direction. Beneath the contoured plate  420 , a lower impeller  414  rotates proximate to a lower endcap  428 . The two endcaps,  416 ,  428  may be die cast (e.g., from aluminum or magnesium alloy) or they may be injection molded from an appropriate metal. 
         [0086]    The outer sidewalls of the airpaths in the upper and lower volutes are essentially defined by the damping sleeves  438  and  440 . The damping sleeves are inserted into left side casing  424  and right side casing  426 . The left side casing  424  provides the air outlet  442  for the blower  400 . The left  424  and right  426  side casings are secured together with, e.g., bolts or other removable fasteners. On the top surface of the side casings  424 ,  426  are square flanges  430 ,  432  having protrusions  434 ,  436  that allow the blower  400  to be mounted on springs inside a CPAP apparatus. In  FIG. 7 , the protrusions  434 ,  436  are shown as having different sizes and shapes, however, in  FIGS. 8 and 9 , the protrusions  434  are shown as having the same shape. It will be realized that the protrusions  434 ,  436  may take either of the depicted shapes, or any other shape, depending on the properties and arrangement of the springs onto which the blower  400  is mounted. 
         [0087]    In one embodiment, the damping sleeves  438 ,  440  are rubber or foam rubber components that are, e.g., injection molded to match the internal contours of the left  424  and right  426  side casings, respectively. In one implementation, the damping sleeves  438 ,  440  are 40 Shore A hardness polyurethane formed from a rapid prototype silicone mold. Alternatively, the damping sleeves  438 ,  440  could be silicone, or another elastomer that is stable at the high temperatures generated by the motor. The damping sleeves  438 ,  440  serve three major purposes in blower  400 : (i) they define (part of) the airpaths in the upper and lower volutes, (ii) they provide a seal between the other components, and (iii) they dampen the vibrations of the other parts. 
         [0088]      FIG. 8  is an assembled perspective view of blower  400  from one side. The assembled air outlet  442  is shown in  FIG. 8 , as is the seam  444  between the left  424  and right  426  side casings. As shown in  FIG. 8 , flanges  446 ,  448  protrude laterally from the edge of each side casing  424 ,  426  and abut to form the seam  444 . As shown in  FIG. 9 , the two side casings  424 ,  426  are secured together by bolts  452  that pass through the flange  446  provided in the right side casing  426  and into threaded holes provided in the flange  448  of the left side casing  424 . Furthermore, the power cord  406  exits the assembled blower through a sealed orifice  450  (see  FIG. 9 ) 
         [0089]    Blower  400  has several advantages. First, investment casting is not required to produce blower  400 , which reduces the cost of the blower. Additionally, because the components of blower  400  have fewer hidden and intricate parts, the castings can be inspected and cleaned easily. Finally, blower  400  is easier to assemble than the other embodiments because the components are clamped together using the two side casings  424 ,  426 , which can be done with simple fasteners. 
       Another Embodiment 
       [0090]    Another embodiment of the present invention is illustrated in  FIG. 10 , an exploded perspective view of a double-ended blower  500  according to the present invention. The motor  502 , located in the center of the exploded view, is investment cast from aluminum in this embodiment, although other manufacturing methods are possible and will be described below. The aluminum, as a good conductor of heat, facilitates the dissipation of heat generated by the accelerating and decelerating motor. Examples of suitable motors are described, for instance, in U.S. provisional application 60/452,756, filed Mar. 7, 2003, which is hereby incorporated in its entirety by reference. The shaft  504  has two ends (only one end  504 B is shown in  FIG. 10 , but compare end  404 A in  FIG. 7 ) to which the impellers  514 ,  515  can be functionally connected. The motor power cord  506  protrudes from the motor  502  and exits the blower  500  through recess  550  (see also  FIG. 11A ) in damping sleeve  540 . Damping sleeve  538  comprises a substantially corresponding protrusion  552  (See  FIG. 11B ) to minimize or avoid airflow leaks and to reduce the risk of pulling forces on the power cord being transferred to the power cord/motor connection. In one embodiment, shown in  FIGS. 11A and 11B , protrusion  552  comprises ribs  554  that substantially interlock with ribs  556  in recess  550  to further minimize airflow leaks. Also, in one embodiment the wires in the motor power cord are silicon rubber covered wires (allowing increased flexibility and noise suppression). 
         [0091]    The motor  502  comprises stationary flow guidance vanes  560 , which may be aerofoil shaped. The vanes  560  are capable of changing the direction of the airflow arriving at the vanes  560  through the spiral airpath defined by damping sleeves  538 ,  540  from tangential to radial, i.e. towards the hole  522 . As depicted in more detail in  FIG. 12 , the motor  502  can be connected to contoured plate  520  via press-fit recesses  564  in contoured plate  520  for some of the vanes  560 . Other ways to connect motor  502  to contoured plate  520  may also be used (e.g. screws or adhesives). 
         [0092]    In one embodiment, the motor  502  includes, at its top, a portion  508  of the upper volute. As a variation of the design illustrated in  FIG. 10 , the motor  502  may be made separately from the portion  508  of the upper volute. If the two components are made separately, the motor body may, for instance, be die cast, while the portion  508  of the upper volute may be, for instance, injection molded. 
         [0093]    Secured to the motor  502  by bolts or other fasteners is a circular plate  510 , in which a hole is provided (not shown, but compare hole  412  in  FIG. 7 ) for the passage of the shaft  504 . 
         [0094]    The impellers  514 ,  515 , connected to the ends of the shaft  504 , are scalloped along their circumference to reduce rotational inertia, giving them a “starfish” look. 
         [0095]    An upper endcap  516  is secured above impeller  514 , and provides the top portion of the upper volute. An aperture  518  in the center of the upper endcap  516  serves as the air intake of the blower  500 . 
         [0096]    On the lower end of the blower  500  in  FIG. 10 , a contoured plate  520  forms the top portion of the lower volute. The bottom of the contoured plate  520  is curved arcuately upward toward a hole  522 . Part of the bottom of contoured plate  520  is ribbed. Beneath the contoured plate  520 , an impeller  515  rotates proximate to a lower endcap  528 , which comprises two protrusions  537 . The two endcaps,  516 ,  528  may be die cast (e.g., from aluminum or magnesium alloy) or they may be injection molded from an appropriate metal. 
         [0097]    The side casing  524  defines air outlet  542  for the blower  500 . The side casings  524  and  526  are secured together with bolts or other removable fasteners. On the top surface of the side casings  524 ,  526  are protrusions  534 ,  536  that allow the blower  500  to be mounted on springs inside a CPAP apparatus. It will be realized that the protrusions  534 ,  536  may take any shape depending on the properties and arrangement of the springs onto which the blower  500  is mounted. 
         [0098]    The double-ended blower  500  includes two damping sleeves  538 ,  540 . The damping sleeves  538 ,  540  are, e.g., rubber or foam rubber components that are, e.g., injection molded to match the internal contours of the side casings  524 ,  526 , respectively. In one implementation, the damping sleeves  538 ,  540  are formed from a rapid prototype silicone mold. Alternatively, the damping sleeves  538 ,  540  may be, for instance, silicone or another elastomer that is stable at the temperatures generated by the motor. 
         [0099]    As is evident from  FIGS. 10 ,  11 A and  11 B. the combination of damping sleeves  538 ,  540  defines, along with the components (e.g. motor  502 ) positioned between the sleeves, a spiral airpath/conduit. The portion of the spiral conduit defined by damping sleeve  540  has a decreasing cross-sectional area in the direction of airflow. 
         [0100]      FIG. 13  is an assembled perspective view of blower  500  (180° rotated with respect to  FIG. 10 ). 
         [0101]    In operation, blower  500  takes in air at aperture (external inlet)  518  through rotation of impeller  514 . The air is transported through the spiral conduit defined by damping sleeves  538 ,  540  to the stationary flow guidance vanes  560 , which substantially change the velocity vector of the arriving air from primarily tangential to primarily radial, i.e. toward internal inlet  522 . Rotation of impeller  515  then transports the air arriving through hole (internal inlet)  522  via a second airpath (defined primarily by the space between lower endcap  528  and contoured plate  520 ) to external air outlet  542 . 
         [0102]      FIG. 13A  illustrates a partial cross-sectional view of a blower  600  according to another embodiment of the present invention. Blower  600  includes a motor  602  having a pair of opposed shafts  604  and  606  that connect to respective first and second stage impellers  608  and  610 , respectively. Motor  602  is supported by an inner casing  612  that includes an aperture  614  leading to the second stage impeller  610  to allow for passage of shaft  606 . A lid  616  is provided to the first stage end of casing  612 , and includes an aperture to accommodate passage of shaft  604 . 
         [0103]    An outer casing  618  is provided to support inner casing  612  via one or more support members  620 , two of which are shown in  FIG. 13A . The inner and outer casings  612 ,  618  are spaced from one another by a gap G, which defines a channel adapted for the passage of pressurized gas from the first stage to the second stage. The gap G is defined by a generally annular chamber between adjacent side walls  636 ,  638  of the inner and outer casings. The channel is also formed between bottom walls  628 ,  630  of the inner and outer casings. 
         [0104]    In operation, gas, e.g., air, is directed through blower  600  as indicated by the arrows. In particular, gas is drawn in towards the first stage impeller  608  through an aperture  634  provided in cap  622 . First stage impeller  608  forces the air radially outwards, such that the air follows a path along the inside domed surface  632  of the cap  622 . Air then proceeds along the gap G provided between inner and outer casings  612 ,  618 , passing along support members  620 . Air moves radially inwardly between bottom walls  628 ,  620  and then proceeds through aperture  614  towards second stage impeller  610 . Second stage impeller forces the air radially outwards and into an inlet  624  of conduit  626 , whereby the now pressurized gas is directed to outlet  628 , for delivery to a patient interface (e.g., mask) via an air delivery conduit (not shown). 
         [0105]      FIGS. 14-17  show an embodiment wherein blower  500  is placed in an enclosure  700 . The blower  500  is mounted in the enclosure on springs  702  that are provided over all six protrusions  534 ,  536 , and  537  (only the springs provided over protrusions  537  are shown). The springs aid in reducing vibration and noise. In another embodiment, suspension bushes (e.g. rubber suspension bushes) are provided over the protrusions instead of springs  702  to reduce vibration and noise. An example of a rubber suspension bush  703 . 1  provided over a protrusion  703  is shown in  FIG. 18 . 
         [0106]    The enclosure  700  comprises a main seal  720 . See also  FIG. 19 . Outlet  722  of main seal  720  is connected to outlet  542  of blower  500  and securely fastened with a spring clip  724  (outlet  542  is shown in  FIG. 10 ). Main seal  720  is positioned between enclosure base  710  and enclosure lid  730 , which are connected using screws  732 . In one embodiment, the enclosure base  710  and the enclosure lid  730  are made of metal, e.g. aluminum. For example, the enclosure base  710  and enclosure lid  730  are made form die cast aluminum. One of the advantages of aluminum is its good corrosion/bum resistance, even in oxygen rich environments. The aluminum has sufficient mass to resist movement and therefore serves to attenuate noise generated by the working of the blower. However if the aluminum resonates and thereby generates a ringing noise, that ringing noise can be attenuated/eliminated by the use of the main seal  720 , e.g., a silicone gasket. Seal  720  also works well with the enclosure&#39;s aluminum casing sections to achieve the desired leak free seal. In this embodiment only three holding points (which use screws) are required to apply the force necessary to achieve the leak free joining of the seal between the two aluminum-casing sections. 
         [0107]    In one embodiment, the main seal  720  is made from rubber, e.g. silicone rubber. A main seal construed from rubber may aid in reducing noise that can be created by vibrations of enclosure base  710  and enclosure lid  730 . Main seal  720  allows for a plurality of blower wires  720 . 1  to pass therethrough. For example, seal includes a plurality of fingers  720 . 2  that are resiliently flexible, as shown in  FIG. 19A . Adjacent pairs of fingers  720 . 2  define an aperture, e.g., a round hole, to accommodate the cross-sectional shape of wires  720 . 1 . Main seal  720  also includes a relatively thinner and/or more flexible portion  720 . 3  to facilitate alignment and coupling with blower outlet. In the illustrated embodiment, the seal gasket includes apertures for allowing the passage of the eight wires that form the blower motor power and control leads. The typically bunched wires would not readily lend themselves to cooperating with a compression silicon gasket in order to achieve the desired sealing. The emergence point of the wires from the enclosure is designed so as not to compromise the enclosure&#39;s seal. In this embodiment eight apertures are formed in the seal gasket, each one intended to receive one of the motor wires. Each aperture is in the form of a circular orifice intersected with a ‘V’ split leading up to the top of the silicone gasket. The ‘V’ split facilitates the easy locating of the wire into the circular orifice. On assembly of the enclosure, each wire is located in its allocated circular orifice, and the seal is positioned between the two aluminum-casing sections. The force imposed when the screws as tightened cause silicone to fill the space of each circular orifice and around each wire and thereby achieve the seal. 
         [0108]    In addition, the main seal  720  aids in minimizing leaks. See also  FIG. 20  for an individual representation of the enclosure base  710  and  FIG. 21  for an individual representation of enclosure lid  730 . As shown in  FIG. 20 , base  710  includes a blower chamber  710 . 1  and a muffling chamber  710 . 2 . Base  710  includes a secondary expansion or muffling chamber  710 . 2  to muffle noise as pressurized gas passes through straight section  722 . 1  out of outlet  722 . Lid  730  includes a channel forming member  730 . 1  which allows incoming air to travel from muffling chamber  710 . 1  to blower chamber  710 . 2 . See the directional arrows in  FIGS. 15 and 16 . 
         [0109]    The resulting structure is an enclosure that is completely sealed i.e., has only known, characterized air paths. By contrast, uncharacterized air paths or leaks have undesirable consequences: 
         [0110]    A. Inappropriate flow generator performance due to the processing of any inaccurate flow signal. Inappropriate flow generator performance may compromise patient treatment. The control circuit corrects the filtered flow signal to estimate the flow at identified points of the breathing circuit, e.g., at the blower outlet or at the patient interface. The corrected flow signal is used by the treatment algorithm or by other systems such as a flow generator, a fault diagnosis system, etc., and the control circuit responds accordingly. An example of a flow generator fault diagnosis system that can use a corrected flow signal embodied within blowers commercially available from ResMed. The control circuit&#39;s performance is dependent upon the flow sensor providing a signal that maintains a known relationship with the downstream flow. The known relationship will not be applicable; or will be less accurate, where the enclosure seal is compromised. Accordingly the corrected flow signal will not be accurate where the enclosure leak is unpredictable in occurrence, in magnitude or otherwise not recognizable as being inaccurate by the control circuit. Therefore to maximize performance of system that places the flow sensor upstream of the blower it is preferable to eliminate the opportunity for the occurrence of unintended leaks in the flow generator. 
         [0111]    B. A sealed enclosure will prevent contamination of the breathable gas flowing through the enclosure. 
         [0112]    C. A sealed enclosure will prevent the breathable gas escaping from the air path. This is a particularly desirable when oxygen or other treatment gas is added to the flow through the flow generator. 
         [0113]    D. A sealed enclosure will maximize the effect of the enclosure&#39;s noise attenuating characteristics. 
         [0114]    The silicone pathway connected to the blower outlet is preferably molded in one piece with the seal. This configuration means that there is no need for the sealing gasket to assume the shape and degree of precision that would otherwise be required to property fit around an enclosed rigid outlet pipe or to achieve a seal should the rigid outlet pipe be formed of two or more separable parts. 
         [0115]    A flow meter  740  is sealingly connected to main seal  720 . See also  FIGS. 22A and 22B  for an individual representation of a flow meter. In one embodiment, the flow meter is designed to measure air flows in the range of 0-200 LPM, and preferably in the range of 150-180 LPM. In a further embodiment, the flow meter is designed to be safe for even 100% oxygen flows. As evident from  FIGS. 14-17 , the flow meter may be positioned upstream from the blower inlet. Positioning the flow meter upstream instead of downstream can be helpful in improving the accuracy of air flow measurement as it reduces/minimizes blower-induced turbulence in the air presented to the flow meter. This, in turn, provides an improved signal to the control algorithm, which signal does not require complex filtering of turbulence or noise to provide a useful signal. 
         [0116]    An inlet connector  750  is sealingly connected to flow meter  740 . The inlet connector ensures that the air intake is supplied from outside the flow generator. See also  FIG. 23  for an individual representation of the inlet connector. In one embodiment, the inlet connector is made from plastic and/or rubber, e.g. silicone rubber. The inlet connector  750  provides location for filter retainer  755 . See  FIG. 24 . The filter retainer  755  can be sealingly inserted in the opening  752  of the inlet connector  750  and serves to receive a filter. For example, filter retainer includes a flange  755 . 1  that is received within a groove  750 . 1  of the inlet connector  750 , upon assembly. In accordance with the depicted embodiment, the filter retainer  755  may be construed asymmetrically to conveniently and safely give a user only one correct way of placing the filter. Furthermore, the filter retainer  755  prevents the inlet connector  750  from sagging. Filter retainer  755  also includes one or more cross bars  755 . 2  that prevent the filter from being sucked into the inlet connector  750 . Filter retainer  755  also includes a pair of receiving apertures  755 . 3  to receive an inlet cap with resilient arms. 
         [0117]    Also, the inlet connector  750  provides a barrier for water being able to reach the blower. First, in combination with the filter retainer  755  and filter cover (not shown) it forms a water barrier at the entry of the enclosure. Second, with the enclosure being positioned horizontal, the upward slope  753  of the inlet connector (See  FIG. 17 ) provides an obstacle for water being spilled into the inlet connector  750  to travel further into the system. 
         [0118]    Further, inlet connector  750  provides a relatively linear flow of air to flow meter  740 , which helps decrease turbulence and the creation of “noise” that would otherwise need to be filtered before providing a useful signal to the control algorithm. Moreover, there is no need to maintain a linear path downstream of the flow meter  740 , which opens further design options. 
         [0119]    The illustrated embodiments utilize this freedom of configuration by placing the flow sensor generally parallel with blower. This configuration reduces the overall length of the flow generator as it allows for the desired linear (i.e., turbulence minimizing) pathway between the flow generator air-from-atmosphere inlet and the flow sensor inlet while eliminating the length adding placement of the flow sensor and connecting turbulence-reducing linear pathway at the blower outlet. This configuration has the air travel around a corner (i.e., a typically turbulence inducing maneuver) into muffler chamber which is situated forward of the blower chamber. From there the air enters the blower chamber and then enters the blower inlet. The turbulent air emerging from the blower outlet travels a short distance through a silicone pathway to the flow generator outlet. The linear component connecting the flow generator air-from-atmosphere inlet to the flow sensor inlet may be conveniently located in any position relative to the blower because of the irrelevance of avoiding the development of turbulence after the flow sensor outlet. Furthermore there is avoided the need to perform flow signal filtering to eliminate the remnant blower-induced turbulence. 
         [0120]    Each of the described embodiments provides for a modular construction having relatively few, self-aligning components that may be readily assembled and disassembled for maintenance. The inner sides of the aluminum-casing sections include locating feature buckets to facilitate the positioning and retention of internal components such as the blower suspension springs, or alternatively, substitute silicone suspension bushes. 
         [0121]    Another feature relates to a safety measure. If motor bearing wear reaches a predetermined limit, the consequent shaft movement will position a shaft mounted blade so as to cut something on or protruding from the motor internal circuit board and thereby cause the motor to stop (say due to a loss of power). The amount of shaft movement required to give effect to this would be something less than the amount of movement required to have the shaft mounted impeller make contact with the volute wall. In this way the system stops before impeller/volute wall scraping or collision would lead to denegation of either or both components and cause particles to contaminate the air path or friction that would cause ignition to occur—especially in an oxygen rich environment (i.e., where oxygen is being added to the breathing gas). 
         [0122]    While the invention has been described by way of example embodiments, it is understood that the words which have been used herein are words of description, rather than words of limitation. Changes may be made without departing from the scope and spirit of the invention in its broader aspects. Although the invention has been described herein with reference to particular embodiments, it is understood that the invention is not limited to the particulars disclosed. The invention extends to all appropriate equivalent structures, uses and mechanisms.

Technology Category: 2