Abstract:
A downhole sensor including a body configured for attachment to a downhole pump. The body having a fluid receiving portion; and, an ultrasonic transducer supported by the body; wherein ultrasonic pulses transmitted by the ultrasonic transducer are directed towards the fluid receiving portion, and reflected waves receivable by the ultrasonic transducer are indicative of a liquid fluid level and type of fluid within the fluid receiving portion. A method of determining a liquid fluid level and a type of fluid adjacent a downhole pump.

Description:
BACKGROUND 
       [0001]    In the drilling and completion industry, the formation of boreholes for the purpose of production or injection of fluid is common. The boreholes are used for exploration or extraction of natural resources such as hydrocarbons, oil, gas, water, and alternatively for CO2 sequestration. 
         [0002]    Downhole fluid level is an important parameter in the use of progressive cavity pumps (“PCPs”), electric submersible pumps (“ESPs”), and other artificial lift tools for oil and gas production. A successful well operation requires the balance between well production and equipment protection. Because not only has the optimized well production required a proper fluid level, the operation of downhole equipment also demands minimum fluid level to provide enough cooling and lubrication for equipment&#39;s performance and longevity. Most of the current available technologies, e.g. gas gun echometer and floater type level sensor, cannot provide continual measurement and perform poorly in foamy fluid situation. 
         [0003]    The art would be receptive to alternative devices and methods for downhole fluid level sensing and detecting. 
       BRIEF DESCRIPTION 
       [0004]    A downhole sensor including a body configured for attachment to a downhole pump, the body having a fluid receiving portion; and, an ultrasonic transducer supported by the body; wherein ultrasonic pulses transmitted by the ultrasonic transducer are directed towards the fluid receiving portion, and reflected waves receivable by the ultrasonic transducer are indicative of a liquid fluid level and type of fluid within the fluid receiving portion. 
         [0005]    An ultrasonic level sensing assembly includes at least one downhole sensor, each sensor including a body configured for attachment to a downhole pump, the body having a fluid receiving portion; and, an ultrasonic transducer supported by the body; a pulser/receiver; and, a diagnostic/control unit; wherein ultrasonic pulses transmitted by the ultrasonic transducer are directed towards the fluid receiving portion, and reflected waves receivable by the ultrasonic transducer are indicative of a liquid fluid level and type of fluid within the fluid receiving portion. 
         [0006]    A method of determining a liquid fluid level and a type of fluid adjacent a downhole pump, the method includes attaching a downhole sensor to the downhole pump, the downhole sensor including a body having a fluid receiving portion, and an ultrasonic transducer supported by the body, a liquid fluid level within the fluid receiving portion being substantially same as a liquid fluid level exterior to the fluid receiving portion; pulsing the ultrasonic transducer towards the fluid receiving portion; receiving reflected waves at the ultrasonic transducer; and, determining the liquid fluid level and type of fluid adjacent the downhole pump based on the reflected waves at the ultrasonic transducer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0008]      FIG. 1  depicts a system diagram of an exemplary embodiment of a downhole system having an exemplary embodiment of an ultrasonic sensor assembly associated with a pump of the downhole system; 
           [0009]      FIG. 2  depicts a schematic cross-sectional view of the exemplary pump and ultrasonic sensor assembly of  FIG. 1 ; 
           [0010]      FIG. 3  depicts a perspective view of an exemplary embodiment of an ultrasonic level sensor for the ultrasonic sensor assembly of  FIG. 2 ; 
           [0011]      FIG. 4  depicts a side plan view of the exemplary ultrasonic level sensor of  FIG. 3 ; 
           [0012]      FIG. 5  depicts a graph of signals from three paths within the exemplary ultrasonic level sensor of  FIG. 4 ; 
           [0013]      FIG. 6  depicts a perspective view of an exemplary embodiment of a test set-up for the exemplary ultrasonic sensor assembly; 
           [0014]      FIGS. 7A-7C  depict graphs for signals at various fluid levels within the test set-up of  FIG. 6 ; 
           [0015]      FIG. 8  depicts a schematic cross-sectional view of the exemplary pump of  FIG. 1  and another exemplary embodiment of an ultrasonic sensor assembly; 
           [0016]      FIG. 9  depicts a schematic cross-sectional view of the exemplary ultrasonic sensor assembly of  FIG. 8 ; 
           [0017]      FIG. 10  depicts a schematic cross-sectional view of the exemplary ultrasonic sensor assembly of  FIG. 9  with an exemplary embodiment of an auxiliary fluid receiving portion; and, 
           [0018]      FIG. 11  depicts a schematic cross-sectional view of the exemplary pump of  FIG. 1  and the exemplary ultrasonic sensor assembly of  FIG. 9  with another exemplary embodiment of an auxiliary fluid receiving portion. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
         [0020]    An exemplary embodiment of a downhole system  10  for fluid-type detecting and fluid level sensing is shown in  FIG. 1 . The downhole system  10  includes a pump system  12  having downhole pump  14 , such as the illustrated progressive cavity pump (“PCP”), within a borehole  16  through a formation  18  that may be uncased (also known as open) or cased using a perforated casing for allowing the formations fluids to flow within the interior of the casing. The pump system  12  assists in the production of a variety of fluids from the borehole  16  to the surface  20  in an uphole direction. For the purposes of this description, it should be understood that a first feature may be said to be “downhole” of a second feature when the first feature is located further in the borehole  16  (such as from the surface  20 ). Likewise, the second feature may be said to be located “uphole” of the first feature when the second feature is not as far in the borehole  16  as the first feature (such as located closer to the surface  20 ). Even if the borehole  16  deviates horizontally, the path through the borehole  16  determines whether one feature is uphole or downhole the other feature. Other pumps and downhole pumps may also be included in the downhole system  10 , such as, but not limited to, an electric submersible pump (“ESP”) and other artificial lifts. The pump system  12  further may further include a motor  22 , seal section  24 , gear reducing unit  26 , power cable  28 , and any other component known in downhole pump systems  12 . It has been determined herein that having knowledge of the fluid level (of both wellbore liquids and gas) in the borehole  16  within an annulus  30  between the borehole wall  32  or casing and the pump  14 , and having knowledge of the type of fluid in the annulus  30  surrounding the pump  14 , would be of value to an operator. Types of fluid may include, but are not limited to, liquids such as oil and water, and gases such as air. 
         [0021]    Thus, in addition to the pump system  12 , the downhole system  10  further includes an ultrasonic level sensing assembly  34  that provides an indication of fluid level within the borehole  16  with respect to the pump  14  as well as an indication of the type of fluid that is within the borehole  16  surrounding the pump  14 . The ultrasonic level sensing assembly  34  includes a body  36  that is attached or otherwise configured for connection to, or at least adjacent to, the pump  14 . The ultrasonic level sensing assembly  34  is connected via a cable  38  to a pulser/receiver  40 , which is locally mounted or a ground surface unit. The cable  38  may be part of the same cable  28  for the motor  22  of the pump  14 . At the surface  20 , or at a remote location, signals from the receiver  40  are sent to a diagnostic/control unit  42 , exemplarily including a processor  44 , memory  46 , and programs  48 . The diagnostic/control unit  42  can also control the pulser  40  to control the frequency of pulses emitted by the ultrasonic level sensing assembly  34 . A display  50  is connected to the diagnostic/control unit  42  for viewing the signals processed by the diagnostic/control unit  42 . 
         [0022]    In the illustrated embodiment of  FIG. 2 , the ultrasonic level sensing assembly  34  is shown to include one or more ultrasonic level sensors  52  employed and positioned at different longitudinally spaced areas relative to a longitudinal axis  54  of the downhole pump  14 . The bodies  36  of the sensors  52  are mounted on the housing  56  of pump  14  with minimum or no requirements to change the original design of the pump  14 . It should be understood that the relative sizes between the pump  14  and the sensors  52  are exaggerated for clarity, and that the sensors  52  may be significantly smaller than the pump  14 . That is, the attached sensors  52  do not increase the diameter of the pump  14  or only slightly increase the diameter. The level sensors  52  may utilize the same cable  28  as that of the pump  14  or sensor section for power, although may include its own cable  38 . Also, while depicted exteriorly of the pump  14 , the pump  14  may alternatively include pockets or indentations (not shown) in the pump housing  56  for mounting the sensors  52  therein. Further, the ultrasonic level sensing assembly  34  may include a screened or apertured housing  58  protecting all of the ultrasonic level sensors  52  therein, and/or the ultrasonic level sensors  52  may individually include screened or apertured housings  60  ( FIG. 4 ) for protecting individual sensors  52 . In addition to protecting the ultrasonic level sensors  52 , the screened or apertured housings  58 ,  60  can assist in preventing debris and solid particulates from interfering with the sensor operations. 
         [0023]    For applications to monitor downhole fluid level relative to the pump  14 , a set of sensors  52  can provide level measurements at different locations along the pump  14 . For example, three ultrasonic level sensors  52  are depicted at three longitudinally distinct areas of the pump  14 . A first ultrasonic level sensor  62  is provided at an upholemost location of the ultrasonic level sensing assembly  34  relative to the pump  14 . A second ultrasonic level sensor  64  is positioned longitudinally between the first ultrasonic level sensor  62  and a third ultrasonic level sensor  66 . If fluid (in particular, liquid) is detected by the first, second, and third ultrasonic level sensors  62 ,  64 ,  66 , this indicates a normal liquid fluid level relative to the pump  14 . This information will assure an operator that the downhole system  10 , and in particular the pump  14 , has the proper liquid fluid level to function normally. While a borehole  16  may be filled with liquid to a proper liquid fluid level, the liquid may not be the type of liquid required for proper pump function, and therefore it is also a feature of the downhole sensors  52  to assess the type of fluid adjacent the pump, in addition to providing liquid fluid level information. If no liquid is detected by the first ultrasonic level sensor  62 , but liquid is detected by the second and third ultrasonic level sensors  64 ,  66 , an operator may be provided with a warning that the fluid level relative to the pump  14  is not at a normal level, and a more urgent warning may be provided if no liquid is detected by the first and second ultrasonic fluid level sensors  62 ,  64 , but liquid is detected by the third fluid level sensor  66 . Depending on the fluid level, appropriate investigations may be performed to determine issue and follow-up as needed. If no liquid is detected by the first, second, and third ultrasonic level sensors  62 ,  64 ,  66 , the diagnostic/control unit  42  may be programmed to provide a signal to an operator to shut off the pump  14 , or the pump  14  may be automatically stopped to prevent damage to the pump  14 . The number of level sensors  52  and the spacing there between may be altered, as well as the response associated with each level sensor  52 . For example, the responses may also include warnings and/or shutting off the pump  14  if it is determined that the liquids in which the pump  14  is immersed are not suitable for proper pump function or if the wellbore liquids are undesirable for production. 
         [0024]    Turning now to  FIGS. 3 and 4 , an exemplary embodiment of the ultrasonic level sensor  52  is shown in more detail. The ultrasonic level sensor  52  includes a rugged design based on the characteristics of ultrasound propagating through different surrounding media. The ultrasonic level sensor  52  can provide both in situ and continuous fluid level monitoring for downhole pumps  14 . Furthermore, the same sensors  52  can provide qualitative measurement on the fluid properties, such as density, of the fluid within the fluid receiving portion  68  of the sensors  52 . The level sensors  52  may be designed for robustness for long-term fluid level monitoring, and advantageously the sensors  52  enable extra fluid information to be obtained using the same sensor  52 . 
         [0025]    The sensor  52  includes a body  36  supporting an ultrasonic transducer  70  and a wave guide  72 . The size of the ultrasonic transducer  70  and wave guide  72  can be easily adjusted to fit different applications. The ultrasonic transducer  70  is capable of sending an ultrasound (pulse wave) as well as detecting the reflected sound (reflected wave) and converting the reflected wave to an electrical signal. To produce the ultrasound, a piezoelectric crystal  74  has an alternating current applied across it, which causes the piezoelectric crystal  74  to vibrate at high speed and produce an ultrasound (“the piezoelectric effect”). Reflected sounds hit the piezoelectric crystal  74  causing the mechanical energy produced from the sound vibrating the crystal  74  to be converted into electrical energy. A measurement of the time between when the sound was sent and received is indicative of the type of fluid within the wave guide  72 . The wave guide  72  includes the fluid receiving portion  68 , a reflection portion  76 , and a reference portion  78 . The reflection portion  76  of the wave guide  72  is attached to the ultrasonic transducer  70  by the reference portion  78 . The space between the ultrasonic transducer  70  and the reflection portion  76  and uphole of the reference portion  78  is identified as the fluid receiving portion  68 . Because the fluid receiving portion  68  is an open space, it is capable of accurately indicating a fluid level within the annulus  30  by not trapping any fluid therein. Thus, if the fluid level receiving portion  68  includes any side walls, they are screened or at least perforated at a lower portion thereof so that the fluid receiving portion  68  is incapable of holding a fluid therein. 
         [0026]    The diagnostic/control unit  42  of the ultrasonic sensing assembly  34  can control when and how often the ultrasonic transducer produces an ultrasound. During a measurement, the ultrasonic transducer  70  excites ultrasonic signal that propagates along the wave guide  72 . Based on the design of wave guide  72 , there are at least two propagating paths for the signal: one is through the reference portion  78  of the wave guide  72  (Path  1 ), and the other one is through the fluid receiving portion  68  (Path  2 ). Additional paths may be defined through the fluid receiving portion  68 , such as Path  3 . Identifying a third path may be helpful when more than one type of fluid is within the fluid receiving portion  68 . For example, a layer of oil may be detectable on top of a layer of water. The travel time and intensity of the reflected ultrasonic signals through Path  2  (and Path  3 ) is a function of the fluid level (if there is even liquid within the fluid receiving section  68 ) and fluid type, e.g. gas, oil, water, or oil-water mixture. When the liquid fluid level doesn&#39;t reach the sensor, only one ultrasonic signal (a reference signal through the reference portion) is observed though Path  1 . As the liquid fluid level rises and gradually fills the fluid receiving portion  68 , the ultrasonic signal through Path  2  begins to appear and its intensity becomes stronger until the fluid receiving portion  68  is totally filled with liquid. In the exemplary design of the wave guide  72 , the signal from Path  1  always provides a reference measurement, which ensures accurate level monitoring using the signal from Path  2 , and Path  3 . The material of the wave guide  72  does not change during use and therefore a constant signal is always detected through Path  1 , regardless of liquid fluid level and type of fluid in which the pump is employed. Meanwhile, the signal speed (sound velocity) through Path  2  and Path  3  is a function of fluid type of the fluid that fills the open space of the fluid receiving portion, and generally signal speed varies with fluid density. Therefore the same sensors can be used to provide the information about the fluid type. An exemplary chart showing three signals read through the ultrasonic level sensor  52  is shown in  FIG. 5 , where the reference signal is labeled Path  1 , the signal through a first fluid  80  within the fluid receiving portion  68  is labeled Path  2 , and the signal through a second fluid  82  within the fluid receiving portion  68  is labeled Path  3 . Because the signals through Path  2  and Path  3  are different, an operator will know that there are two different types of fluid  80 ,  82  within Paths  2  and  3  of the ultrasonic level sensor  52 . Also, based on known values of ultrasonic sound velocity through various materials, an operator will further be able to identify the different materials that are within the Paths  2  and  3  of the ultrasonic level sensor  52 . 
         [0027]    Turning to FIGS.  6  and  7 A- 7 C, a test conducted to demonstrate the effect of fluid level on an exemplary ultrasonic level sensing assembly  34  having two sensors  52 , where fluid level within an annulus  30  is demonstrated by changing liquid fluid levels within a flask  88 . With reference to  FIG. 7A , when the liquid fluid is below both sensors  52 , where a first sensor  84  is positioned lower than a second sensor  86 , such as at level “A” as noted in  FIG. 6 , only first and second reference signals  90 ,  92  are noted in the time of flight vs. signal intensity chart. With reference to  FIG. 7B , when the liquid fluid level is above the first sensor  84 , but below the second sensor  86 , such as at level “B” as noted in  FIG. 6 , both reference signals  90 ,  92  are still noted in the chart, a first fluid-detecting signal  94  next to the first reference signal  90  appears, but no fluid-detecting signal next to the second reference signal  92  is shown. This would indicate to an operator that a liquid fluid level higher than the first sensor  84  but lower than the second sensor  86  is present. And, with reference to  FIG. 7C , when the liquid fluid level is over both sensors  84 ,  86 , such as at level “C” as noted in  FIG. 6 , both reference signals  90 ,  92  are still noted in the chart, the first fluid-detecting signal  94  next to the first reference signal  90  still appears, and an additional second fluid-detecting signal  96  next to the second reference signal  92  appears, indicative of liquid fluid at a level at least higher than the second sensor  86 . The strength of the signals  94 ,  96  can further be used to identify the type of liquid fluid sensed by the sensors  84 ,  86 , by comparing signal strength with known values (or a range of values) of ultrasound propagating through various fluids. 
         [0028]    Because the ultrasonic transducer body  36  and wave guide  72  are made of metal (such as Stainless Steel 316), the proposed level sensor  52  has a broad range with respect to working environments. The screened or apertured housing  58 ,  60  that surrounds the ultrasonic level sensing assembly  34  and/or the individual ultrasonic level sensors  52  allows for fluids to enter in an uninterrupted fashion to the fluid receiving portion  68  of the waveguide  72 , however the housings  58 ,  60  prohibit gravel, sand, and other particulates from lodging themselves into the fluid receiving portion  68 , as such particulates may reflect the propagated ultrasonic waves and provide incorrect signal readings of the fluids in the vicinity of the pump  14 . 
         [0029]    Turning now to  FIGS. 8 and 9 , in another exemplary embodiment of an ultrasonic level sensing assembly  100 , the assembly  100  also has an operating property based on the characteristics of ultrasound propagating through different paths as liquid fluid level varies. The ultrasonic level sensor  102  used in the ultrasonic level sensing assembly  100  can provide both in situ and continuous liquid fluid level monitoring for a downhole pump  14 , such as, but not limited to, PCPs, ESPs, and other artificial lift tools. The ultrasonic level sensing assembly  100  also includes a rugged design for robustness of the device for long-term and continuous liquid fluid level monitoring. Further, the ultrasonic level sensing assembly  100  also has the ability to obtain extra fluid information using the same sensor  102 . The dimensions of the ultrasonic level sensor  102  are adjustable for different applications. 
         [0030]    As shown in  FIG. 8 , the ultrasonic level sensing assembly  100  includes a single ultrasonic level sensor  102  which is capable of providing liquid fluid level information and fluid type information throughout the entire longitudinal length of the annulus  30  within a selected area  104  of the annulus  30 . While only one ultrasonic level sensing assembly  100  is necessary, an operator may choose to attach a second ultrasonic level sensing assembly  100  (not shown) to another radial location of the downhole pump  14  for redundancy. 
         [0031]    As more clearly shown in  FIG. 9 , the ultrasonic level sensor  102  includes a body  110  supporting an ultrasonic transducer  106  and having a fluid receiving portion  112 . The body  110  includes a transducer housing  108 , which forms the fluid receiving portion  112 . The fluid receiving portion  112  has a longitudinal axis  134  that may be substantially parallel to the longitudinal axis  54  of the pump  14 . As with the fluid receiving portion  68  of the ultrasonic sensor  52 , the fluid receiving portion  112  is incapable of retaining fluid therein, and is dependent upon liquid fluid level exterior of the transducer housing  108  to fill the fluid receiving portion  112  with liquid. That is, as the liquid fluid level exterior  118  of the transducer housing  108  changes, so does the liquid fluid level within the interior  120  of the transducer housing  108  such that the liquid fluid level within the interior  120  of the transducer housing  108  is indicative of the liquid fluid level exterior  118  of the transducer housing  108 . The ultrasonic transducer  106  excites ultrasonic signal that probes the liquid fluid level change continuously inside the transducer housing  108 , and is capable of working at high pressures, such as up to 20,000 psi or more, and up to high temperatures, such as up to 175° C. (283° F.) or higher. The ultrasonic transducer  106  is hermetically sealed at the bottom of the transducer housing  108 . As noted above, the transducer housing  108  provides a controlled environment for accurate liquid fluid level measurement. In an exemplary embodiment of the transducer housing  108 , the transducer housing  108  includes a multi-layer structure, such as the illustrated two layer structure. An inner layer  114  of the transducer housing  108  may be made of stainless steel with small holes (an apertured tubular) along the housing  108  to allow fluid to enter the interior  120  of the transducer housing  108 . An outer layer  116  of the transducer housing  108  may be a wrapping screen, such as one made of porous sponge and fine metal screen, to prevent small formation debris and bubbles to enter the interior  120  of the transducer housing  108 . The housings  58 ,  60  for the ultrasonic level sensing assembly  34  may use a similar multi-layered structure with inner and outer layers  114 ,  116 . The size and length of the ultrasonic transducer  106  and transducer housing  108  are adjustable to fit different applications. The ultrasonic level sensing assembly  100  is attachable to the downhole pump  14 , such as by welding or using a securement device, such as, but not limited to clips, clamps, straps, or the like. Alternatively, the downhole pump  14  may be configured with a housing specially designed to incorporate the ultrasonic level sensing assembly  100  thereon. 
         [0032]    During a measurement, the ultrasonic transducer  106  excites ultrasonic signal that propagates along the fluid volume inside the transducer housing  108 , as indicated by pulse waves  122 . The signal is reflected, as depicted by reflected waves  124 , at the interface  126  between the wellbore fluid  128  and air  130  inside the housing  108 . If wellbore liquid fluid  128  completely fills the transducer housing  108 , then the signal will be reflected at the reflection portion  132  of the transducer housing  108 . The travel time of the signal is proportional to the liquid fluid level. As the liquid fluid level rises gradually, the signal takes a correspondingly longer time to travel back to the ultrasonic transducer  106  in a linear fashion. Furthermore, since the signal is excited and probes the liquid fluid level from below (downhole) the liquid fluid rather than above the liquid fluid, the ultrasonic level sensor  102  is more immune to the effect of a possible foamy layer in the borehole  16 , which usually stays right above the real liquid fluid level. 
         [0033]    For the applications to monitor the downhole fluid level for downhole pumps  14 , PCPs, ESPs, and artificial lift tools, the ultrasonic level sensing assembly  100  can be mounted on the housing of the tools with minimum requirements to change the original design. The same ultrasonic level sensor  102  can also provide qualitative measurement on the fluid properties, such as density, passing through the sensor. As with the ultrasonic level sensor  52 , the speed in which the reflected signal returns to the ultrasonic transducer  106  will provide an indication as to the type of fluid contained within the fluid receiving portion, such as oil or water. To further avoid the interference with gas bubbles, changes can be made to the housing  108  as shown in  FIGS. 10 and 11 . With reference to  FIG. 10 , a “U” shape transducer housing  140  includes the housing  108  with an auxiliary fluid receiving portion  142  having a connecting portion  144  fluidically connected to the transducer housing  108 , such as at a location between the ultrasonic transducer  106  and the fluid receiving portion  112  or along the fluid receiving portion  112 . The auxiliary fluid receiving portion  142  may extend substantially parallel with the transducer housing  108 , and may also include a porous and/or aperture structure, such as that of, or similar to, the multi-layer structure  114 ,  116  shown in  FIG. 9 . With reference to  FIG. 11 , a dip tube type transducer housing  150  includes the housing  108  with an auxiliary fluid receiving portion  152  having a connecting portion  154  fluidically connected to the transducer housing  108 , such as at a location between the ultrasonic transducer  106  and the fluid receiving portion  112  or along the fluid receiving portion  112 . Different from the embodiment shown in  FIG. 10 , the auxiliary fluid receiving portion  152  includes an imperforate or solid-walled structure, where the bottom opening  158  into the auxiliary fluid receiving portion  152  is lower than casing perforations  156 . In addition, with further reference to  FIGS. 8 and 9 , the ultrasonic level sensor  102  can utilize the same cable  28  as that of the downhole pump  14  or sensor section for power supply and signal transmission, or use its own cable  38 . The ultrasonic level sensing assembly  100  may further incorporate a similar surface set-up as is shown in  FIG. 1  for monitoring and control. 
         [0034]    While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.