Abstract:
A system and algorithm-based method of determining engine health and assuring available propulsion power based on historical data reflecting the individual engine&#39;s unique performance “fingerprint.”

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with Government support under contract N00019-09-D-0008 awarded by the Naval Air Systems Command. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The application generally relates to monitoring and predicting engine health and, more particularly, to a system and algorithm-based method of determining engine health and assuring available propulsion power. 
       BACKGROUND OF THE INVENTION 
       [0003]    Aircraft engine health is typically monitored using a Power Assurance Check (“PAC”). PACS generally measure and calculate engine horsepower as a function of measured gas temperature (“MGT”), corrected measured gas temperature (“MGTc”), or corrected gas generator speed (“NGc”) under steady operating conditions, before comparing that performance level to a baseline specification. The comparison to the baseline specification provides an indication of engine health level. PACs, however, have several limitations. 
         [0004]    As one example, a PAC requires several minutes of stable, steady-state operation—i.e., in the case of a rotorcraft, several minutes of hover—in order to be performed accurately. To perform an automated PAC, a rotorcraft might have to hover for up to five minutes in relatively stable, wind-free conditions. If such conditions are not maintained for an adequate time period, the PAC routine will abort. Alternatively, a manual PAC can be performed. While a manually performed PAC will not abort because of a shorter period of stable, steady-state operation, accuracy will suffer if the period of steady operation is significantly shorter. Manually performed PACs also introduce human error in plotting performance data points, charting a line or curve on a graph, and interpreting the graph to predict available power and determine engine health. And manual performance of a PAC can be burdensome for a flight crew, or require additional personnel. For instance, for an aircraft requiring two pilots, one pilot must fly the aircraft while the other records the data and interprets the results. Alternatively, the flight crew could include a flight engineer to perform the PAC analysis, but the additional crew member diminishes the aircraft&#39;s capacity. 
         [0005]    Another limitation of PACs is that it generally assumes that if, for example, an engine is providing 100% of baseline performance at one load level, it will provide 100% performance at higher loads as well. But engine health often differs at different engine load levels. As a result, for example, a PAC can indicate 100% engine health, only to have the pilots find that only 94% of the predicted power level is actually available when they reach higher altitude, attempt to lift heavier loads, or enter worse operating environments. Thus, the flight crew might find that the engine is marginal when its performance is most crucial. 
         [0006]    Yet another limitation of PACs is that, for best accuracy, a PAC should be performed at high engine loads, e.g., within about 100° F. of the engine&#39;s maximum MGT rating. But reaching such a high engine load can require high altitudes, heavier aircraft loads, and/or high outside air temperatures. These can be difficult or impossible to acquire, depending on conditions. 
         [0007]    Consequently, a need exists for an engine health assessment system and method that: (1) does not require several minutes of stable, steady-state operation; (2) does not place a significant burden on the flight crew; (3) accurately predicts engine health and performance levels at all operating loads and power levels, based on historical data reflecting the individual engine&#39;s unique performance “fingerprint”; and (4) does not require operation at high power levels and loads to obtain accuracy. These and other advantages of the present invention will become apparent to one skilled in the art. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect, the invention includes a method for collecting engine data for use in determining the health of an engine on an aircraft, the method comprising: receiving engine operating information; determining from the engine operating information a power parameter and a critical parameter; recording as a data point the power parameter and the critical parameter; and merging the data point with a stored data set, the stored data set including previously recorded power parameter and critical parameter data points. 
         [0009]    In another aspect, the invention includes a method of determining the health of an engine on an aircraft, the method comprising: selecting an engine performance curve to use in creating an engine health model; executing a curve-fitting process to obtain a fitted curve, the fitted curve being based on the selected engine performance curve and a stored data set; evaluating engine health by comparing the fitted curve to a 100% specification-level performance curve for the engine; and outputting engine health information. 
         [0010]    In a third aspect, the invention includes a method of determining the health of an engine on an aircraft, the method comprising: receiving engine operating information; determining from the engine operating information a power parameter and a critical parameter; recording as a data point the power parameter and the critical parameter; merging the data point with a stored data set, the stored data set including previously recorded power parameter and critical parameter data points; selecting an engine performance curve to use in creating an engine health model; obtaining a fitted curve by translating and rotating the selected engine performance curve to achieve a low error with respect to the stored data set; evaluating engine health by comparing the fitted curve to a 100% specification-level performance curve for the engine; and outputting engine health information. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Preferred features of certain embodiments of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein: 
           [0012]      FIG. 1  shows a rotorcraft according to one embodiment; 
           [0013]      FIG. 2  is a block diagram of an engine health system according to one embodiment; 
           [0014]      FIG. 3  is a flow chart of representing a data collection process according to one embodiment; 
           [0015]      FIG. 4  is a flow chart of representing a process for determining engine health according to one embodiment; 
           [0016]      FIG. 5  is a chart of collected data for use in a process for determining engine health according to one embodiment; 
           [0017]      FIG. 6  is the chart of collected data according to the embodiment of  FIG. 5 , with a baseline engine performance curve introduced; 
           [0018]      FIG. 7  is the chart of collected data according to the embodiment of  FIG. 6 , with the baseline engine performance curve translated to achieve the lowest error with respect to the collected data points; 
           [0019]      FIG. 8  is the chart of collected data according to the embodiment of  FIG. 7 , with the baseline engine performance curve rotated to achieve the lowest error with respect to the collected data points; and 
           [0020]      FIGS. 9 and 10  depict the chart of collected data according to the embodiment of  FIG. 8 , with the baseline engine performance curve reintroduced in its original position. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The embodiments of the present invention will now be described more fully, with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, the illustrated embodiments are provided so that this disclosure will be thorough and complete and will convey the scope of the invention to those skilled in the art. 
         [0022]    In the interest of clarity and brevity, all features of an embodiment may not be described. In the development of any actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. While such a development effort might be complex and time-consuming, it would, nevertheless, be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0023]    The invention is described, in part, with the help of functional and/or logical block components, and with reference to symbolic representations of operations, processing tasks, and functions that may be performed by various computing components or devices. Such operations, tasks, and functions are sometimes referred to as being computer-executed, computerized, software-implemented, or computer-implemented. In practice, one or more processor devices can carry out the described operations, tasks, and functions by manipulating electrical signals representing data bits at memory locations in the system memory, as well as other processing of signals. The memory locations where data bits are maintained are physical locations that have particular electrical, magnetic, optical, or organic properties corresponding to the data bits. It should be appreciated that the various block components shown in the figures may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of a system or a component may employ various integrated circuit components (e.g., memory elements, digital signal processing elements, logic elements, look-up tables, or the like), which may carry out a variety of functions under the control of one or more microprocessors or other control devices. Furthermore, the connecting lines shown in the various figures are intended to represent exemplary functional relationships and/or physical couplings between the various elements. However, one skilled in the art will appreciate that many alternative or additional functional relationships or physical connections may be present in an embodiment. 
         [0024]      FIG. 1  shows a rotorcraft  100  according to one example embodiment. Rotorcraft  100  features a rotor system  110 , blades  120 , a fuselage  130 , a landing gear  140 , and an empennage  150 . Rotor system  110  may rotate blades  120 . Rotor system  110  may include a control system for selectively controlling the pitch of each blade  120  in order to selectively control direction, thrust, and lift of rotorcraft  100 . Fuselage  130  represents the body of rotorcraft  100  and may be coupled to rotor system  110  such that rotor system  110  and blades  120  may move fuselage  130  through the air. Landing gear  140  supports rotorcraft  100  when rotorcraft  100  is landing and/or when rotorcraft  100  is at rest on the ground. Empennage  150  represents the tail section of the aircraft and features components of a rotor system  110  and blades  120 ′. Blades  120 ′ may provide thrust in the same direction as the rotation of blades  120  so as to counter the torque effect created by rotor system  110  and blades  120 . It should also be appreciated that teachings regarding rotorcraft  100  may apply to aircraft and vehicles other than rotorcraft, such as airplanes and unmanned aircraft, to name a few examples. 
         [0025]    Referring now also to  FIG. 2 , an exemplary engine health system  200  includes an engine  201 , which provides power to rotor system  110 . One skilled in the art will appreciate that the engine health system  200  may, alternatively, include multiple engines  201 . The engine health system  200  further includes one or more engine sensor devices  202 , which are configured to transmit engine operating information  220 . For example, engine sensor devices  202  may measure, calculate, estimate, and/or transmit information such as MGT, MGTc, NGc, torque output, horsepower output, etc. The engine sensor devices  202  provide the engine operating information  220  via an aircraft data bus  203 . Aircraft data bus  203  may be a conductive-wire serial data bus, such as a data bus compliant with MIL-STD-1553, or a fiber optic data bus, such as a data bus compliant with MIL-STD-1773. An engine health monitoring device  204  collects the engine operating information  220  via the aircraft data bus  203  and processes the engine operating information  220  as described below with reference to  FIGS. 3-10 . 
         [0026]    After processing the engine operating information  220  collected from the aircraft data bus  203 , the engine health monitoring device  204  provides engine health information  230  to one or more output devices. In the embodiment of  FIG. 2 , the output devices include: data storage device  205 , flight crew interface device  206 , and transceiver device  207 . While engine health monitoring device  204 , data storage device  205 , flight crew interface device  206 , and transceiver device  207  are depicted as being separate devices, one of ordinary skill in the art will appreciate that two or more of these devices may be combined into a single component. For instance, in an alternative embodiment, all of these devices  204 ,  205 ,  206 ,  207  may be integrated into a single component. Additionally,  FIG. 2  depicts a data storage device  205 , a flight crew interface device  206 , and a transceiver device  207  each connected directly to the engine health monitoring device  204 . However, a variety of physical connections among these devices is possible. For example, in an alternative embodiment, each of these devices may be interconnected via the aircraft data bus  203 . 
         [0027]    Data storage device  205  is configured to store the engine health information  230  for retrieval. As one example, data storage device  205  may include a data port that allows the engine health information  230  to be downloaded and analyzed at regular intervals. Data storage device  205  may be queried, either by on-board systems that provide periodic or on-demand queries, or by an off-aircraft maintenance and diagnostic tool  209 . More specifically, such queries may be generated by—or communicated through—aircraft data bus  203 , the engine health monitoring device  204 , flight crew interface device  206 , off-aircraft maintenance and diagnostic tool  209 , and/or other components and systems. When responding to a query, data storage device  205  communicates engine health information  230  to one or more of the aircraft data bus  203 , engine health monitoring device  204 , flight crew interface device  206 , transceiver device  207 , base station  208 , off-aircraft maintenance and diagnostic tool  209 , and/or other components and systems. Data storage device  205  can also serve as a repository for the stored data set  330  (see  FIGS. 5-10 ). 
         [0028]    Flight crew interface device  206  is configured to provide the engine health information  230  to the flight crew. As one example, the flight crew interface device  206  may include a gauge or display screen that indicates engine health information  230 . Such gauges or display screens may continuously indicate the engine health information  230 , or such information  230  may be available on-demand. For example, a button pressed in the cockpit may prompt engine health information  230  to become displayed on a screen. As another example, the flight crew interface device  206  may include warning indicators, lights, tactile alerts, or audible alarms that alert the flight crew in the event that the engine health information  230  falls outside of a predetermined range. 
         [0029]    Transceiver device  207  is configured to transmit engine health information  230  off-aircraft. For example, transceiver device  207  may transmit engine health information  230  to a base station  208  while the aircraft is in operation. Base station  208  may be a maintenance database, grounds crew, mobile command center, chase vehicle or aircraft, etc. One skilled in the art will appreciate that transceiver device  207  need not be a dedicated transceiver operable to transmit and receive only engine health  230  and related information. Rather, transceiver device  207  may be a multipurpose transceiver configured to transmit and receive other information in addition to engine health information  230 , including for example, voice data, GPS data, HUMS data, control commands in the case of an unmanned vehicle, etc. 
         [0030]    Referring still to the embodiment of  FIG. 2 , other aircraft systems  210  are in communication with aircraft data bus  203 . More specifically, other sensors and processors make available via the aircraft data bus  203  additional information relating to the aircraft and/or the operating environment  240 , for instance: (1) altitude; (2) outside air temperature; (3) whether there is weight on the landing gear  140 ; (4) whether the aircraft is flying in helicopter mode, airplane mode, or somewhere in between (in the case of a tiltrotor or tiltwing aircraft); (5) engine inlet configuration or restriction information; and/or (6) exhaust configuration or restriction information. The engine health monitoring device  204  may optionally process this additional information  240 , along with the engine operating information  220 , in order to determine the engine health information  230 . 
         [0031]      FIGS. 3 and 4  depict flowcharts for a data collection process  300  and a process for determining engine health  400 , respectively. These processes  300 ,  400  may be performed by software, hardware, firmware, or any combination thereof. In one example, engine health monitoring device  204  (see  FIG. 2 ) performs the data collection process  300  and process for determining engine health  400 . In another example, portions of these processes  300 ,  400  relating to the storage of the stored data set  330  (see  FIGS. 5-10 ) are performed by data storage device  205 . Accordingly, one of ordinary skill will appreciate that various elements and devices can perform all or portions of the processes  300 ,  400  without departing from the scope of the invented method. 
         [0032]      FIG. 3  depicts an exemplary data collection process  300 . The data collection process  300  includes the steps of receiving  301  engine operating information  220  and receiving  302  additional information relating to the aircraft and/or the operating environment  240 . The engine operating information  220  and additional information  240  may optionally be received from aircraft data bus  203  (see  FIG. 2 ). 
         [0033]    The data collection process  300  further includes determining  303  whether the engine operating information  220  represents a stable data point  320  (see  FIGS. 5-10 ). That is, one or more mathematical data noise-reduction processes, which are generally known in the art, are applied to the engine operating information  220 . As one example, one or more lag filters may be applied to the engine operating information  220 . If the noise-reduction process reveals that the engine operating information  220  does not represent a stable data point  320 , then the engine operating information  220  is discarded, and the data collection process  300  restarts  304 . 
         [0034]    The data collection process  300  further includes determining  305  whether the engine operating information  220  represents a valid data point  320  (see  FIGS. 5-10 ). Various validity criteria may be selected and verified in order to determine whether the engine operating information  220  represents a valid data point  320 . Such validity criteria may require reference to engine operating information  220  and/or additional information relating to the aircraft and/or the operating environment  240 . For instance, one validity criteria may be that there is no weight on the landing gear  140  (see  FIG. 1 ). Another exemplary validity criteria may be that MGT is greater than 1000° F. If all of the validity criteria are not met, then the engine operating information  220  is discarded, and the data collection process  300  restarts  304 . 
         [0035]    The data collection process  300  depicted in  FIG. 3  includes the further step of determining and recording  307  a power parameter (e.g., torque value, power value, torque margin, or power margin) and a critical parameter (e.g., MGT, MGTc, or NGc value), from the engine operating information  220 , as a data point  320  (see  FIGS. 5-10 ). More specifically, in the embodiment of  FIG. 3 , the power margin, or “delta” between the 100% specification-level performance power and the actual measured power, for the given MGT, is determined. Once determined, the power margin and MGT data is then recorded as a data point  320  for further processing. One of ordinary skill in the art will appreciate that, alternatively, MGTc or NGc data might be recorded as a critical parameter in addition to, or in place of, MGT. Preferentially, MGT/MGTc data will be recorded if the engine ordinarily reaches its MGT limit before it reaches its NGc limit. And, conversely, NGc data will be recorded if the engine ordinarily reaches its NGc limit before it reaches its MGT limit. 
         [0036]    In the embodiment of  FIG. 3 , the determining and recording step  307  also includes “correcting” the power parameter and the critical parameter based on additional information relating to the aircraft and/or the operating environment  240 . For instance, if the outside air temperature is high, then a correction factor may be applied to the power margin data and MGT data, before the data point  320  is recorded. In this scenario, the data point  320  is corrected or normalized to eliminate variations in performance attributable to current installation losses, operating configurations, and environmental conditions. The resulting “corrected” data point  320 , therefore, represents the engine&#39;s power if it were performing on a test stand under ideal conditions or standard conditions, at the given MGT, rather than installed on an aircraft and subjected to varying operating configurations and environmental conditions. Such a correction may be necessary, for instance, if the power parameter is MGT because the stored data set  330  will otherwise contain scattered data points  320 , which may not be reduced to a fitted curve  440  (see  FIGS. 5-10 ) due to the influence of other variables. It is, therefore, preferable that the stored data set  330  include corrected data points  320  (i.e., corrected power margin and MGTc values) rather than observed values (i.e., observed power margin and MGT values). 
         [0037]    In an alternative embodiment, the raw values (i.e., the actual measured power—rather than a power margin—and the measured MGT) is recorded for further processing. However, it may be advantageous to record as a data point  320  only the power margin and MGTc data in order to optimize data handling and processing. 
         [0038]    The data collection process  300 , as depicted in  FIG. 3 , also includes merging  308  the recorded data point  320  with a stored data set  330 . The stored data  330  set includes historical data points  320 , based on previous power margin data and MGTc data. Merging  308  the newly recorded data point  320  with the stored data set  330  may include grouping the new data point  320  with statistically similar historical data points in the stored data set  330 , and merging those statistically similar data points into a single, representative data point. This merging step  308  may further include purging one or more older data points from the stored data set  330 . In this way, the stored data set  330  may change over time as new data is merged and old data is purged. The stored data set  330 , therefore, is constantly updated to reflect the recent historical power margin (i.e., the “performance fingerprint”) for the individual engine. 
         [0039]    Referring still to  FIG. 3 , the process  300  includes the additional step of determining  309  whether to invoke the process for determining engine health  400 . The process for determining engine health  400 , which is used to determine engine health information  230 , can be set to occur, for example, based on any combination of the following: (1) periodically based on time (e.g., every minute); (2) periodically based on flight hours (e.g., every fifteen minutes of flight time); (3) on demand (e.g., when the flight crew queries engine health information  230 ); (4) when engine health information  230  is transmitted off of the aircraft (e.g., when retrieved by a maintenance or diagnostic tool  209 , or when transmitted to a base station  208  by a transceiver device  207 ); and/or (5) when recent power margin and MGTc data points  320  depart from historical data in the stored data set  330 , potentially indicating a sudden or rapid degradation of engine health (e.g., when an engine suffers damage ingests foreign material). If, under one or more of the potential criteria, updating the engine health information  230  is due, then the process  400  (described in detail below, with reference to  FIGS. 4-10 ) occurs. If, however, updating the engine health information  230  is not yet due to occur, then the data collection process  300  restarts  304 . 
         [0040]      FIG. 4  depicts an exemplary process for determining engine health  400 . The process for determining engine health  400  includes selecting  401  an engine performance curve  430  to use in creating an engine health model. For instance, the selected performance curve  430  could be the engine&#39;s 100% specification-level performance curve  450  (see  FIGS. 9-10 ). Alternatively, the selected performance curve  430  could be the engine&#39;s fully-deteriorated performance curve. As yet another example, the process  400  could select both the 100% specification-level performance curve  450  and the fully-deteriorated performance curve. In such an example, the analysis would proceed using both performance curves, and the curve that best fits the data would become the selected performance curve  430  used in creating an engine health model. 
         [0041]    The process for determining engine health  400  further includes a curve-fitting process  402 . The curve-fitting process  402  includes translating and rotating the selected performance curve  430  to best fit the stored data set  330 . The process for determining engine health further includes the step of evaluating engine health  403 . In this step, the fitted curve  440  resulting from the curve-fitting process  402  is compared to the 100% specification-level performance curve  450 , to obtain an estimate for overall engine health. In addition, or as an alternative, the fitted curve  440  can also be evaluated at an extrapolation point  460  in order to determine an expected power margin, or performance level, at a given load (i.e., at a given MGT or MGTc). The engine health information  230  discussed above with reference to  FIG. 2  may comprise an overall engine health estimate and/or an expected power margin at a given load. The curve-fitting process  402  and evaluation step  403  are described in greater detail below, with reference to  FIGS. 5-10 . 
         [0042]    The process for determining engine health  400  further includes outputting  404  the resulting engine health information  230 . Exemplary output devices (i.e.,  205 ,  206 ,  207 ) and associated methods are discussed above, with reference to  FIG. 2 . 
         [0043]    Referring to  FIG. 5 , the curve-fitting process  402  of the depicted embodiment includes charting the stored data set  330 , which comprises a constantly updating set of data points  320  representing historical power margin (y-axis) as a function of MGTc data (x-axis). As discussed above, the illustrated embodiment includes the steps of determining whether each data point is stable  303  and valid  305  as part of the data collection process  300  (see  FIG. 3 ). However, one of ordinary skill will readily appreciate that these data-filtering steps can, alternatively, be taken as part of the curve-fitting process  402 . That is, all data points  320  might be recorded as part of the stored data set  330 . But when the curve-fitting process  402  begins with charting the stored data set  330 , each data point  320  that is not both stable and valid may be ignored. 
         [0044]      FIGS. 6-8  illustrate additional steps in the curve-fitting process  402 . Referring to  FIG. 6 , the curve-fitting process  402  includes introducing the selected engine performance curve  430 , which was selected as part of step  401  (see  FIG. 4 ). As best shown in  FIG. 7 , the selected engine performance curve  430  is then translated (i.e., shifted without rotation) to achieve the lowest error with respect to the stored data set  330 . This results in a translated curve  435 . As illustrated in  FIG. 8 , the translated curve  435  is then rotated to achieve the lowest error with respect to the stored data set  330 . This results in a fitted curve  440 . One of ordinary skill in the art will appreciate that, alternatively, the rotation step ( FIG. 8 ) could occur before translation ( FIG. 7 ). 
         [0045]    The translation and rotation of the selected engine performance curve  430  is preferably achieved based on an “optical fit.” That is, the curve is translated and rotated so as to minimize the apparent error, or average distance, between the fitted curve  440  and the collection of data points  320 . In alternative embodiments, a polynomial least square method can be used to as a fitting routine to create the fitted curve  440 . However, extrapolations from a fitted curve  440  that results from a polynomial least squares fitting method may be less reliable. 
         [0046]      FIGS. 9 and 10  illustrate the step of evaluating engine health  403 . As best shown in  FIG. 9 , the 100% specification-level performance curve  450  is introduced. The margins  455  between the fitted curve  440  and the 100% specification-level performance curve  450 , are evaluated at various points to determine an overall estimate of engine health. As one skilled in the art will appreciate, this can be accomplished using well-known statistical methods. 
         [0047]    Additionally, or as an alternative,  FIG. 10  illustrates that the fitted curve  440  can be compared to the 100% specification-level performance curve  450 , at a specific point  460 , to determine the expected power margin  465  at a given load (i.e., at a given MGTc or MGT). 
         [0048]    Whether the engine health information  230  takes the form of an overall estimate of engine health ( FIG. 9 ), or an expected power margin at a given load ( FIG. 10 ), the step of evaluating engine health  403  may include “correcting” the margin data  455 ,  465  based on the additional information relating to the aircraft and/or the operating environment  240 . For instance, if the outside air temperature is high, then a correction factor may be applied to the power margin data  455 ,  465  before it is used to determine the engine health information  230  that will be output. That is, the margin data  455 ,  465  is corrected or normalized to eliminate variations in performance attributable to current installation losses, operating configurations, and environmental conditions. The resulting “corrected” engine health information  230 , therefore, may represent the engine&#39;s power under real-world, installed conditions, rather than theoretical performance levels. 
         [0049]    Modifications, additions, or omissions may be made to the systems and apparatuses described herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. 
         [0050]    Although several embodiments have been illustrated and described in detail, it will be recognized that substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the appended claims. 
         [0051]    To aid the Patent Office, and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims to invoke 35 U.S.C. §112(f) as it exists on the date of filing hereof unless the words “means for” or “step for” are explicitly used in the particular claim.