Abstract:
A method of monitoring a backup battery for a magnetic bearing centrifugal compressor. The method includes: establishing threshold voltage values applicable to the battery backup battery level; monitoring the actual voltage of the battery backup; comparing the actual voltage of the battery backup to the threshold voltage; initiating a soft shutdown of the magnetic bearing centrifugal compressor if the magnetic bearing centrifugal compressor is operating and the actual voltage is below the threshold voltage for a defined period of time; preventing the magnetic bearing centrifugal compressor from starting if the magnetic bearing centrifugal compressor is not operating and the actual voltage is below the threshold voltage; or allowing the magnetic bearing centrifugal compressor to continue operation or begin operation if the actual voltage is greater than the threshold voltage.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a system for monitoring a battery backup which supplies power to the magnetic bearing in the event of the failure of an electrical supply voltage. In particular, the invention is directed to a system of monitoring a battery backup and monitoring system for use with a centrifugal chiller which possesses a magnetic bearing centrifugal compressor. 
       BACKGROUND OF THE INVENTION 
       [0002]    In general terms, a centrifugal compressor for a chiller typically consists of the following components: inlet guide vanes, one or more impellers within a housing surrounded by one or more diffusers with collectors driven by some mechanical shaft means, such as for example, an electric motor. The mechanical shaft means is supported by one or more bearings of the rolling element, journal, or magnetic bearing type which accommodate both radial and axial loads. In variable speed electric chillers, the centrifugal compressor is supplied with electrical power through an adjustable speed motor drive which alters the frequency and/or voltage of the power to the motor to modulate the speed of the compressor. 
         [0003]    Rolling element bearings are generally passive devices and, during normal operation, operate without the requirement of active control. The chiller control system does not typically provide active control of the rolling element bearings where, in this context, active implies continual adjustment of some bearing feature. Chiller control systems for centrifugal chillers which use rolling element bearings in the compressor may monitor the bearing temperature, at periodic intervals, as an indication of whether the machine is operating properly. An elevated temperature is used as an indication of a potential mechanical problem with the bearings. If the measured bearing temperature exceeds a predefined set point, the chiller control system may be programmed to stop the machine and alert the user. 
         [0004]    In magnetic bearing centrifugal compressors, the compressor rotor is suspended on a magnetic field generated in the magnetic bearings. For definitional purposes, “magnetic bearings” are electromagnetic devices used for suspending a rotating body in a magnetic field without mechanical contact. The bearings can be further classified as active, indicating that some type of active control system is necessary to ensure stable levitation of the rotating body. 
         [0005]    Distinct from other compressor types, a magnetic bearing centrifugal compressor uses magnetic bearings as the primary means for supporting the rotor structure. There is a clearance gap between the rotating and stationary components of the bearing that is measurable and controllable. For the magnetic bearings to operate properly, electrical power and proper operation of the magnetic bearing control electronics are required. 
         [0006]    In contrast to conventional rolling body bearings, magnet bearings need to be continuously supplied with electrical energy in order to ensure a bearing arrangement in accordance with regulations. If the supply voltage of the magnet bearing fails, for example as a result of an electrical failure of the power supply system, not only will the motor run down, but the functionality of the magnet bearing will no longer be provided. In order to avoid damage to the magnet bearing the shaft and/or other components in the event of failure of the supply voltage when the motor is still rotating, so-called emergency operation conditions may be implemented mechanically. However, such emergency operations are only effective for a limited number of failures of the supply voltage of the magnet bearing without the magnet bearing, the shaft or other components being damaged. Therefore, applications of magnet bearings in fields with insufficiently stable electrical power supply systems are still problematic. In such circumstances, the maximum permissible number of emergency operation rundowns of the magnet bearing is quickly reached, with the result that the abovementioned components need to be replaced even after a relatively short period of time. This replacement is generally time-consuming and associated with high costs. 
         [0007]    Therefore, backup systems, such as batteries, are often used for safeguarding the magnet bearing in the event of failure of the electrical supply voltage. Such backup systems maintain the functionality of the magnet bearing arrangement until the rotating shaft is braked to a sufficient extent such that no damage or wear occurs to the bearings or shaft. 
         [0008]    As the battery/batteries provide the necessary backup power during an electrical utility power outage, it is important that the battery/batteries be operative when such power outage occurs. Therefore, the batteries must be periodically tested to insure that a proper charge is maintained during operation. 
         [0009]    It would be beneficial to provide an improved battery monitoring system which monitors the condition and state of the battery when the battery is under load and communicates such status to a control unit, thereby allowing a missing, disconnected, weak or dead battery to be detected and appropriate action taken. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an object of the present invention to provide a monitoring system for a battery backup system for magnetic bearing centrifugal compressors which can simply and reliably determine the battery level while the compressor has power and the batteries are charging. 
         [0011]    It is an object of the present invention to provide a monitoring system which prevents operation of the compressor in the event of a problem with the battery backup system, thereby preventing damage to the compressor and prolonging magnetic bearing operating life. 
         [0012]    It is an object of the present invention to provide a monitoring system which gradually powers down the operation of the compressor in the event that a problem occurs with the battery backup system as the compressor is operating, thereby preventing damage to the compressor and prolonging magnetic bearing operating life. 
         [0013]    It is an object of the present invention to monitor the battery to allow for the operation of the chiller to be inhibited when there is not adequate available power in the battery to protect the system. 
         [0014]    It is an object of the present invention to provide a monitoring system which tests the system at known loads and compare the results to determined limits, allowing flat faults and warnings to be displayed when a missing, disconnected, weak or dead battery is detected. 
         [0015]    An embodiment is directed to a method of monitoring a backup battery for a magnetic bearing centrifugal compressor. The method includes: establishing threshold voltage values applicable to the battery backup battery level; monitoring the actual voltage of the battery backup; comparing the actual voltage of the battery backup to the threshold voltage; initiating a soft shutdown of the magnetic bearing centrifugal compressor if the magnetic bearing centrifugal compressor is operating and the actual voltage is below the threshold voltage for a defined period of time; preventing the magnetic bearing centrifugal compressor from starting if the magnetic bearing centrifugal compressor is not operating and the actual voltage is below the threshold voltage; or allowing the magnetic bearing centrifugal compressor to continue operation or begin operation if the actual voltage is greater than the threshold voltage. 
         [0016]    An embodiment is directed to a method of monitoring a backup battery for a magnetic bearing centrifugal compressor. The method comprising: monitoring voltage amplitude over a period of time at the input to a universal power source of the magnetic bearing centrifugal compressor to determine if the battery backup is connected or disconnected, whereby if the difference between the minimum and maximum voltage is greater than a defined variance, the sample is recorded and saved; displaying a fault if the defined variance is exceeded for a defined amount of consecutive samples indicating a disconnected battery backup; initiating a soft shutdown of the magnetic bearing centrifugal compressor if the magnetic bearing centrifugal compressor is operating and the defined variance is exceeded for a defined amount of consecutive samples indicating a disconnected battery backup; preventing the magnetic bearing centrifugal compressor from starting if the magnetic bearing centrifugal compressor is not operating and the defined variance is exceeded for a defined amount of consecutive samples indicating a disconnected battery backup; or allowing the magnetic bearing centrifugal compressor to continue operation or begin operation if the defined variance is not exceeded for a defined amount of consecutive samples indicating a disconnected battery backup. 
         [0017]    An embodiment is directed to a method of monitoring a backup battery for a magnetic bearing centrifugal compressor. The method comprising: monitoring the number of times bearings drop a shaft of the magnetic bearing centrifugal compressor while the shaft is rotating during a power loss event; counting the number of times bearings drop a shaft of the magnetic bearing centrifugal compressor; comparing the counted the number of times bearings drop a shaft of the magnetic bearing centrifugal compressor to a defined threshold number of times bearings drop a shaft of the magnetic bearing centrifugal compressor; displaying a fault if the counted number of times bearings drop a shaft of the magnetic bearing centrifugal compressor is greater than the defined threshold the number of times bearings drop a shaft of the magnetic bearing centrifugal compressor; and preventing the magnetic bearing centrifugal compressor from starting until such fault is cleared. 
         [0018]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a diagramatic view of a an illustrative apparatus for supplying electric power to a motor. 
           [0020]      FIG. 2  is a graph showing the relationship between battery voltage and battery level at various temperatures. 
           [0021]      FIG. 3  is a graph showing the relationship between battery voltage threshold and elapsed time at various temperatures. 
           [0022]      FIG. 4  is a graph showing the relationship between batter voltage and battery level. 
           [0023]      FIG. 5  is a graph showing the relationship between battery voltage and ambient temperature. 
           [0024]      FIG. 6  is a graph showing the relationship between peak battery voltage and elapsed time. 
           [0025]      FIG. 7  is a graph showing the relationship between battery voltage and battery level remaining during a power loss event. 
           [0026]      FIG. 8  is a graph showing the results of testing as the power was cycled from a levitated load to a delevitated load to a no load, allowing the battery to charge between load cycles. 
           [0027]      FIG. 9  is a graph showing the timeline during the test of  FIG. 8  as the low battery voltage limit is approached. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The description of illustrative embodiments according to principles of the present invention is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description of embodiments of the invention disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation unless explicitly indicated as such. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. Moreover, the features and benefits of the invention are illustrated by reference to the preferred embodiments. Accordingly, the invention expressly should not be limited to such preferred embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features; the scope of the invention being defined by the claims appended hereto. 
         [0029]      FIG. 1  illustrates a diagrammatic representation of an embodiment of an illustrative apparatus  2  for supplying electric power to a motor  4  which has magnetic bearings. As shown in  FIG. 1 , a magnetic bearing cable  8  and a motor cable  10  are connected the induction motor  4  whose rotor is rotatably levitated and supported by magnetic bearings. The magnetic bearing cable  8  and the motor cable  10  are connected through a control console or panel  14  to a three-phase, 200-V commercial AC power supply  12 . The control panel  14  has a magnetic bearing controller  16 , connected to the commercial AC power supply, which supplies a magnetic bearing control current through the magnetic bearing cable  8  to control the magnetic bearings  6  which levitate and support the rotor out of contact therewith at a target position. The control panel  14  also has a motor driver  18  which comprises a frequency/voltage converter such as a general-purpose inverter or the like that is connected to the commercial AC power supply. The motor driver  18  supplies electric power having a predetermined frequency/voltage through the motor cable  10  for operating the motor  4  at a predetermined speed. The magnetic bearing controller  16  is backed up by a battery power supply  20  such as, but not limited to, a 48-V, 2.2-Ahr battery. When the commercial AC power supply  12  fails, the battery power supply  20  supplies power to the magnetic bearing controller  16  to enable the magnetic bearing controller  16  to continuously operate the magnetic bearings  6 . 
         [0030]    If the commercial AC power supply is subjected to a failure while the motor  4  is in operation, the magnetic bearings may be continuously operated by the power supplied from the battery power supply  20  or a soft shut down may be initiated. However, since the motor driver  18  is no longer supplied with the AC power from the commercial AC power supply, the rotor of the induction motor  4  rotates in a free-running condition. The rotor which is rotatably supported by the magnetic bearings usually rotates at a high speed of at least several thousand rpm. Because the rotor is magnetically kept out of contact with the magnetic bearings, the rotor does not suffer any decelerating friction force. Consequently, it takes a long period of time until the rotor comes to a stop. Alternatively, a braking system may be included to cause the rotor to stop more quickly. 
         [0031]    In order for the magnetic bearings to be properly operated when a power supply failure occurs, the batteries must be properly charged to provide sufficient power to properly maintain the magnetic field as required. The invention is, therefore, directed to a system and method to monitor and communicate the state of the battery that is used as a backup for an uninterruptible power supply (UPS) system for the magnetic bearings and control panels, thereby preventing damage to the magnetic bearings when the AC power is lost. 
         [0032]    In operation, if the apparatus is powered and the battery is charging, the battery voltage of the battery power supply  20  is monitored while the apparatus  2  has power and the battery is charging to determine the battery level. The battery level is communicated to a control panel  30  which controls the apparatus  2 . Based on observed values, the relationship between battery voltage and battery level (Ah) is defined at different temperatures. An illustrative example of the relationship between battery voltage and battery level (Ah) at different temperatures is shown in  FIG. 2 . The information provided in  FIG. 2  is meant to be illustrative, i.e. directed to particular equipment, it is not meant to limit the scope of the invention to the specific relationship shown. Line  202  represents a 4 degree temperature change, line  204  represents a 12 degree temperature change, line  206  represents a 24 degree temperature change, and line  208  represents a 50 degree temperature change. 
         [0033]    For example, in the illustrative embodiment shown, when properly operating, after the battery is fully charged the voltage will display and hold a trickle charge voltage above the threshold, thereby preventing any warnings and/or faults from occurring. In the embodiment shown the trickle charge voltage is approximately 13.3 v±0.06 v for over 5 minutes (as shown in  FIG. 3 ). In contrast if the battery voltage is below a defined threshold (12.7 v for the illustrative embodiment) and remains below the defined threshold for a fixed period of time (12.7 v for 5 minutes for the illustrative embodiment), warnings and/or faults will be sent indicating a faulty battery. 
         [0034]    Regardless of the particular relative values detected by the monitoring system, if the battery voltage is below a defined threshold (e.g. 12.7 v for 5 seconds) and the apparatus  2  is not running, the system will inhibit or prevent the apparatus from starting until the battery voltage is above the threshold. In the illustrative embodiment shown, the apparatus  2  will be prevented from starting until the battery voltage exceeds the threshold by at least 0.1 volts. (12.8 v for the illustrative embodiment). 
         [0035]    If the battery voltage is below a defined threshold and the apparatus  2  is running, a warning is displayed indicating low battery voltage. After the warning has been displayed for a specific period of time, a low battery voltage fault shall initiate a soft shutdown of the apparatus  2 . In the illustrative embodiment, the specific period of time is sixty minutes as shown in the elapsed time of  FIG. 3 . In other words, if the battery is not properly charged within 60 minutes, a shutdown of the apparatus  2  will be initiated. For the illustrative embodiment shown, line  402  represents a 4 degree temperature change, line  404  represents a 12 degree temperature change, line  406  represents a 24 degree temperature change, and line  408  represents a 50 degree temperature change. 
         [0036]    Alternatively, if the battery voltage rises above the threshold during the specific period of time, the warning is removed and the fault is cleared. For the illustrative embodiment shown, line  412  represents a 4 degree temperature change, line  414  represents a 12 degree temperature change, line  416  represents a 24 degree temperature change, and line  418  represents a 50 degree temperature change. 
         [0037]    The battery voltage threshold is influenced by the temperature of the battery. However, if no temperature sensor is provided proximate the battery, the control panel temperature can be used. In such instances, it is assumed that when the apparatus is not running the temperature inside of the control panel and the battery will equalize to nearly the same temperature. Therefore, this threshold is used as a start-inhibit monitoring voltage while the unit is not running. However, while running, the battery will charge above the lowest threshold before the control panel temperature has time to drift enough to cause nuisance warnings. 
         [0038]    Referring to  FIG. 5 , in the illustrative embodiment, the battery voltage threshold for the expected ambient temperature range is shown. A linear interpolation or curve fit is used to determine the threshold at other temperatures. 
         [0039]    The battery voltage threshold may be adjustable within the measured voltage range of the battery with service level access to better meet the needs of sites that may experience nuisance issues. For example, an offset, multiplier, or a programmed threshold that moves with temperature (similar to the stall voltage limit moving with Mach number) may be used. 
         [0040]    The monitoring system must be able to detect a battery which is disconnected from the apparatus. Such disconnection may result from, but is not limited to, an open disconnect, blown fuse, disconnected wire, or other battery issues. The monitoring system measures the voltage at the input to the control panel. Therefore, in the event that the battery is disconnected from the apparatus, the voltage measured by the monitoring system will be only the charging voltage output by the UPS. 
         [0041]    The charging voltage output from the UPS is noticeably different when the battery is disconnected in comparison to when a battery is connected and is charging. With a battery connected, the charging voltage is steady over time. When disconnected, the voltage bounces up and down several times a minute with a defined or noticeable amplitude. In the illustrative embodiment shown, the amplitude is approximately 1 volt, as shown  FIG. 6 . In order to properly determine whether the battery is connected or disconnected, the monitoring system monitors or samples the amplitude readings over a defined period of time. In the illustrative embodiment shown, the defined period of time is 10 seconds. During the defined time period, if the difference between the minimum and maximum voltage is greater than a defined variance, the sample is recorded and saved. In the embodiment shown, the defined variance is 0.5 volts. If the defined variance is exceeded for 4 consecutive samples a fault is displayed indicating a disconnected battery. 
         [0042]    If the fault occurs while the apparatus is not running, the system will inhibit or prevent the apparatus from starting up until the fault clears or is cleared. The fault can be cleared manually with service level access if the charging voltage is stable within the expected range for a defined period of time. In the illustrative embodiment shown, the defined period of time is 60 seconds. 
         [0043]    If the fault occurs while the apparatus is running, a warning is displayed indicating a disconnected battery. After the warning has been displayed for specific designated period of time, a battery disconnect fault shall initiate a soft shutdown of the apparatus  2 . In the illustrative embodiment, 10 minutes is the designated period of time. The fault can be cleared manually with service level access if the charging voltage is stable within the expected range for a defined period of time. In the illustrative embodiment shown, the defined period of time is 60 seconds. 
         [0044]    In addition, whether the apparatus is running or not running, when the battery is disconnected there is commonly a voltage peak above typical charging voltage range. If the battery voltage goes above a threshold level, i.e. 16 volts for the illustrative embodiment shown, a warning is displayed to check the battery connection. This warning is counted as the first sample toward the 4 consecutive samples that lead to the disconnected battery fault. 
         [0045]    In the event of a power loss to the apparatus, the monitoring system monitors the battery voltage to determine the health of the battery. In the illustrative embodiment shown, the load on the battery has been determined to be 500-600 W while levitated and around 200 W while delevitated and powering only the control panels, as shown in  FIG. 6  which illustrates the relationship between battery voltage and battery level (% Ah remaining). It is assumed that the power draw from the video graphic display of the control panel is small in comparison to the magnetic bearing compressor and only lasts a couple of seconds. As a result the effect of video graphic display power consumption is negligible. 
         [0046]      FIG. 7  illustrates cycling between the levitated and delevitated loads without allowing the battery to charge between cycles. This test was done first on a apparatus at room temperature and second on a bench test at a minimum specified ambient temperature, i.e. 4 degrees Celsius for the embodiment shown. The results show that the temperature does not have as much influence while the battery is under load as it does while the battery is charging. This supports the use of a low voltage limit that does not change with temperature. For the illustrative embodiment shown, line  702  represents the apparatus at room temperature and line  704  represents the apparatus at 4 degrees Celsius. 
         [0047]    As shown in  FIG. 8 , for the illustrative embodiment, nearly 100 load cycles are possible before the battery voltage drops below the low battery voltage limit (10.5 v in the illustrative embodiment) and no longer supports the entire load duration for the embodiment shown. For the illustrative embodiment shown, line  802  represents power, line  804  represents cycle count and line  806  represents CH1 V. Although the change in battery load changes the battery voltage, a single low voltage limit is used to simplify the control design. The worst case of the levitated and delevitated voltage limits will be used. In general, the low battery voltage limit is set where the battery still has enough charge to withstand a defined number of load cycles, such as, but not limited to, at least 4 load cycles. This ensures that the unit is protected even with variation between batteries and conditions. 
         [0048]      FIG. 9  illustrates a timeline as the low battery voltage limit of  FIG. 8  is approached. Based on this data, a low battery voltage limit is set, such as, but not limited to 11.0 v for the illustrative embodiment shown. However, this limit can be adjustable through administration level access, such as, but not limited to, from 10.5 v to 12.0 v for the illustrative embodiment shown. For the illustrative embodiment shown, line  902  represents CH1 V, line  904  represents value, line  906  represents power, line  908  represents cycle count and line  910  represents elapsed time. 
         [0049]    In general, if the battery voltage is below the defined threshold while the battery is supplying power to the apparatus a fault is displayed indicating low battery voltage. 
         [0050]    Once unit power is applied and battery is charging the fault can be manually cleared only if the battery voltage is above the threshold as recited above. Alternatively, the fault may be cleared subject to a battery health test being successfully performed. 
         [0051]    If battery faults or warnings occur a battery health test can be performed to determine if the battery is functional or if the battery should be replaced. This test is a manual procedure initiated through the video graphic control panel to monitor battery voltage while under a known load to ensure that the battery has enough health left to withstand a power failure event. The test is available through the service access level. 
         [0052]    A user initiates the battery health test by pressing a button on a screen of the video display of the control panel. In order to initiate this test, the apparatus must not be running and a safety stop switch must be open. The user is directed through the video display to disconnect main power to the apparatus, forcing the battery back-up to power the apparatus as it would during a power failure. The user must manually disconnect the power. Once the control panel detects that power is supplied by the battery, it commands the magnetic bearing compressor to levitate for a defined period, i.e. for the illustrative embodiment shown, 60 seconds. Throughout the test the monitoring system monitors the battery voltage in the same manner as the monitoring system monitors the battery voltage during a power failure event to make sure it does not drop below the low battery voltage limit. 
         [0053]    For this test the power loss holdup time must be increased (to 10 minutes for the illustrative embodiment) to allow time for the test to take place. 
         [0054]    If the voltage drops below the low limit the test fails and the battery should be replaced or inspected. If a shut down occurs due to voltage, fuses, disconnect, or any other reason the test fails and a fault is displayed on the next power up stating that the battery health test has failed. This fault can be cleared by running a successful battery health test. 
         [0055]    Alternatively, if the voltage remains above the low limit during the test, the control panel displays a message stating the battery passed the test. 
         [0056]    The control panel commands the magnetic bearing compressor to delevitate the shaft for the remainder of the test. Upon completion, the control panel instructs the user to manually reconnect main power to the apparatus. Once power is connected the control panel can then display the message that the battery health test is complete. 
         [0057]    If the main power is not reconnected within a set period of time (5 minutes for the illustrative embodiment), a warning is displayed indicating that the test failed and needs to be redone. If the power loss holdup timer runs out the control panel displays the same fault that is used for the shut down, which can be cleared by running a successful battery test. If the battery voltage drops below the low battery voltage limit after delevitating and while waiting for the main power to be reconnected a warning is displayed indicating that the test failed. 
         [0058]    If the bearings drop the shaft while rotating during a power loss event it is an indication that there is something wrong with the bearings, magnetic bearing compressor, control panel or, wiring. A method for counting these landings is provided. In the event that the landing count increments over acceptable amounts, a landing fault should be displayed and require a manual clear before the apparatus will start again. This fault should not be allowed to clear if battery voltage is below charging threshold and such issue is not remedied as described above. 
         [0059]    In the event of reoccurring landing counts a fault appears stating that there is an magnetic bearing compressor power panel system failure that requires administration level access to clear. The fault should not be allowed to clear if the battery voltage is below charging threshold and if other criteria have been encountered, such as but not limited to, 3 landings are counted within the last 24 run hours or 10 landings are counted within the last 3000 run hours. 
         [0060]    This invention is beneficial when the UPS does not monitor or provide battery status to the control panel. The present invention provides a reliable and cost effective method of monitoring the status of the battery backup at known loads through determined limits which are specific to the apparatus, thereby communicating flag faults and warnings to the control panel. In so doing, the apparatus is inhibited or prevented from continued operation when there is insufficient backup power to protect the apparatus when a power failure occurs, thereby preventing excess wear to the touchdown bearings and shaft of the apparatus. 
         [0061]    While the invention has been described with reference to a preferred embodiment, 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 spirit and scope of the invention of the invention as defined in the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other specific forms, structures, arrangements, proportions, sizes, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. One skilled in the art will appreciate that the invention may be used with many modifications of structure, arrangement, proportions, sizes, materials, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being defined by the appended claims, and not limited to the foregoing description or embodiments.