Abstract:
The torque of a vehicular air-conditioning compressor is predicted by the steps of starting a vehicle air-conditioning system having an engine and engine control module for controlling multiple vehicle functions, calculating a steady state torque value using an rpm value from the engine, calculating an engine RPM transient torque value using the rpm value from the engine, calculating a electronic control valve current transient torque value using a current value applied to an electronic control valve, selecting a final torque value from a group consisting of the steady state torque value, the engine RPM transient torque value, and the electronic control valve transient torque value, and providing a final torque value to the vehicle engine control module to control a predetermined vehicle function.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is related and claims priority to application U.S. Ser. No. 61/740,666 filed 21 Dec. 2012 to Joseph M. Bona et al., entitled Method of Torque Prediction for Automotive Air Conditioning Compressor. 
     
    
     TECHNICAL FIELD OF INVENTION 
       [0002]    The present invention relates to a method of predicting torque for an automotive air conditioning compressor. 
       BACKGROUND OF INVENTION 
       [0003]    Today&#39;s automobiles have increasingly stringent government-mandated fuel consumption and exhaust emission standards. Manufacturers are utilizing more powerful engine controllers and control algorithms to optimize the engine combustion process. In order to precisely meter fuel-to-air ratio, control idle speed, and provide good vehicle acceleration, a good estimate of engine load is needed. 
         [0004]    A belt-driven automotive air conditioning compressor, which utilizes power originating from the vehicle engine to cool a vehicle&#39;s interior compartment, is a significant load requiring an accurate estimation. 
         [0005]    Compressor power is a function of compressor speed and torque. While compressor speed can be easily derived from measured engine speed with a known constant pulley ratio, compressor torque cannot be measured cost-effectively in a production vehicle and thus requires estimation by an algorithm in the Engine Controller. 
         [0006]    Historically, adjustments in vehicle engine performance are made by the vehicle engine controller from a calculation of compressor torque or compressor power. One typical method of implementing the load calculation is to use an equation or look up table which is a function of compressor speed and discharge pressure. This equation is appropriate for steady-state operation of a fixed-displacement compressor or for steady-state operation of a variable-displacement compressor at ambient conditions requiring full capacity. 
         [0007]    A typical torque equation has limitations predicting torque during transient conditions or when the variable compressor is at partial capacity. The typical equation sometimes over predicts the compressor torque during transients and at partial capacity which results in decreased fuel consumption. 
         [0008]    Power estimation equations for fixed and variable displacement compressors are known in the art. Equation 1 below is appropriate for steady state conditions and full capacity although the steady state accuracy may be improved. Another known method utilizes a torque prediction look-up table as a function of discharge pressure and valve current. Again, the look-up table is appropriate for steady state and full capacity conditions. Steady-state accuracy is improved, but there is still a need for greater steady-state accuracy. 
         [0000]      Power= A+B *Pressure+ C *Pressure 2   +D *Pressure 3   −E *CRPM+ F *Pressure*CRPM,  Equation 1
 
         [0009]    where
       Pressure is in Bar Gage   Power is in Watts   CRPM is compressor revolutions per minute   The applicable range of this equation is from 100 to 6100 Watts.       
 
         [0014]    For compressor 1: A=10
       B=88   C=−17.375   D=0.921875   E=0.37390136   F=0.07336425       
 
         [0020]    For compressor 2: A=0
       B=46.9   C=0   D=0   E=0.4881   F=0.1056       
 
         [0026]    U.S. Patent Application Publication US 2010/0236265 A1 published 23 Sep. 2010 to Higuchi et al. entitled Air Conditioner for Vehicle, describes a torque estimation strategy with a set of inputs materially different from the present invention. U.S. Publication US 2010/0236265 A1 and its related foreign counterparts are hereby incorporated herein by reference. 
         [0027]    As described by U.S. Publication US 2010/0236265 A1, an air conditioner for a vehicle comprises a refrigeration cycle including a variable displacement compressor for refrigerant which uses an engine as a drive source, a condenser, an evaporator, a displacement adjuster means for outputting an adjustment control signal to the compressor, and a compressor torque calculation means for calculating the torque of the compressor. The compressor torque calculation means includes at least two torque estimation means of a saturation region torque estimation means corresponding to a case where the compressor is driven at a maximum discharge displacement and a displacement control region torque estimation means corresponding to a case where it is driven at a discharge displacement other than the maximum discharge displacement, and also includes a correction means for correcting the calculation of the torque of the compressor when a change in engine rotational speed greater than a set value is detected. Purportedly, even when the engine rotational speed changes rapidly, the torque of the compressor in the refrigeration cycle can be calculated. 
         [0028]    The methodology described in U.S. Publication US 2010/0236265 A1 is based on a torque calculation utilizing the following inputs:
       Displacement control signal   Outside air temperature   Compressor rotational speed   Vehicle running speed   Condenser fan voltage   High side refrigerant pressure   Blower voltage       
 
         [0036]    The methodology described in U.S. Publication US 2010/0236265 A1 is highly theoretical and fails to adequately instruct an expert in the art how to practice the invention. 
       SUMMARY OF THE INVENTION 
       [0037]    These and other features and advantages of this invention will become apparent upon reading the following specification, which, along with the drawings, describes a preferred embodiment of the invention in detail. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    Although the drawings represent a single embodiment of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplification set forth herein illustrates an embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
           [0039]      FIG. 1 , is a schematic diagram of an air conditioning system configured for automotive application embodying the present invention; 
           [0040]      FIG. 2 , is a graphical depiction of empirical bench test data and calculated compressor torque data of the air conditioning system of  FIG. 1  under steady-state conditions wherein accuracy is ±2 Nm or less; 
           [0041]      FIG. 3 , is a graphical depiction of empirical bench test data illustrating transient compressor torque when the electronic control valve (ECV) is transitioned from “off” to “on”; 
           [0042]      FIG. 4 , is a graphical depiction of empirical bench test data illustrating transient compressor torque when the electronic control valve (ECV) is transitioned from “on” to “off”; 
           [0043]      FIG. 5 , is a graphical depiction of empirical bench test data illustrating transient compressor torque during acceleration and de-acceleration; 
           [0044]      FIG. 6 , is a master flow diagram for the compressor torque prediction system embodied in the present invention; 
           [0045]      FIG. 7 , is a flow diagram for calculating steady state compressor torque in practicing the present invention; 
           [0046]      FIG. 8 , is a flow diagram for calculating electronic control valve current (Ievc) transient torque in practicing the present invention; 
           [0047]      FIG. 9 , is a flow diagram for calculating engine revolutions per minute (ERPM) transient torque in practicing the present invention; 
           [0048]      FIG. 10 , is a flow diagram for selecting a final torque (T F ) value in practicing the present invention; 
           [0049]      FIG. 11 , is a glossary of definitions and associated instrumentation terminology employed in describing and practicing the present invention; 
           [0050]      FIG. 12 , is Table 1: Coefficients for Base Equation; and 
           [0051]      FIG. 13 , is Table 2: Coefficients for Maximum and Minimum Torque Equations. 
       
    
    
       [0052]    Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to illustrate and explain the present invention. The exemplification set forth herein illustrates an embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION OF INVENTION 
       [0053]    Referring to  FIG. 1 , the present invention is preferably implemented or practiced in controlling an air conditioning system  10  installed within a ground vehicle such as an automobile  12 . The prime mover of the air conditioning system  10  is a compressor  14  which is mechanically driven via a belt drive and an electromagnetic clutch (not illustrated) by a motor/engine  16  of the automobile  12 . The compressor  14  compresses hot refrigerant vapor and pumps it to a condenser  18  where it is condensed ultimately to a liquid form. The still pressurized liquid refrigerant from condenser  18  is forced through an expansion valve  20 , after which, in its expanded, cooled state it enters an evaporator  22  disposed within an airflow passage formed by a ventilation module  24  within an automobile passenger space. The heated refrigerant vapor continues on to re-enter compressor  14  to renew the cycle. 
         [0054]    The low-pressure refrigerant flowing through the evaporator  22  exchanges heat with air flowing through ventilation module  24 , which is influenced by a blower  26 . Blower  26  is disposed in an air duct  28  through which air for air conditioning passes, and air drawn from an air suction port for air conditioning  30  is directed to evaporator  22  by the blower  26 . A portion of the air having passed through the evaporator  22  is directed to a heater unit  32  disposed at a downstream position, and the rate between the amount of air to be passed through the heater unit and the amount of air to be bypassed is adjusted by an air mixing damper  34 . In this embodiment, an air temperature sensor  36  at the evaporator  22  exit is provided at the exit side of the evaporator  22  for detecting the temperature of the air after passing through the evaporator  22 , and the detected signal is inputted into an air conditioning control unit  38  performing the air conditioning. Signals from various other system sensors  40  are also inputted into the air conditioning control unit  38 . An engine control unit  42  (ECU) associated with engine  16  is also electrically interfaced with the air conditioning control unit  38 . Both the air conditioning control unit  38  and the ECU  42  contain control circuitry, such as a microprocessor, and volatile and/or non-volatile memory operable to practice the torque prediction method described herein below as well as to affect control of the overall air condition system  10 , such as by outputting a displacement control signal to the compressor  14 . 
         [0055]    An embodiment of the invention is an apparatus and methodology used to calculate or predict the torque of an automotive air conditioner in real time for steady state and transient conditions. This methodology was derived to utilize data acquired from sensors readily available on a standard production vehicle. Thus, no additional expense or costs would be incurred to implement this technique. The torque is predicted utilizing the following inputs:
       Control valve current   Outside air temperature   Evaporator air outlet temperature   Engine speed   High side pressure   Evaporator blower voltage       
 
         [0000]      Torque, estimated= B 1* Vb*Teao *( Toa−Teao )*(( Pd/Ps )̂ K 1−1)/CRPM,
 
         [0062]    where:
       CRPM=Compressor RPM   B1=Correlation Constant   Vb=Blower Voltage   Teao=Evaporator Air Out Temperature   Toa=Outside Air Temperature   Pd=High Side Pressure   Ps=Saturation Pressure of Refrigerant at Teao (estimated suction pressure)   K1=(k−1)/k   k=specific heat ratio for the superheated refrigerant.       
 
         [0072]    An improved relationship, is: Torque, correlated=2.7*SQRT(Torque, estimated) 
         [0073]    This relationship is acceptable to estimate torque at a quasi-steady condition, but may not predict change in torque when, for example, ECV current is changing. 
         [0074]    Base Equation 
         [0075]    A base equation was derived from the above relationship based on empirical observations and inclusion of an ECV current parameter. Additional equations were derived empirically and are used for unique conditions to improve the accuracy of the calculation for these unique conditions. 
         [0076]    The base equation is: 
         [0077]               Compressor Torque is calculated as: 
         [0000]      Torque= A 0 +A 1 *P *( V ) A2   *ΔT *(( P/T ) A3 −1)*( I ) A4 /Rpm
 
         [0078]    Where:
       P=High Side Pressure   V=Evaporator Blower Voltage   ΔT=Evaporator Air Temperature Differential   T=Evaporator Air Outlet Temperature   I=Control Valve Current   Rpm=Engine Speed   A0=Torque Offset Constant   A1=Torque Scaling Constant   A2=Evaporator Scaling Constant   A3=Pressure Temperature Scaling Constant   A4=Current Scaling Constant       
 
         [0090]    Steady state bench test results for this equation are shown in  FIG. 2 . 
         [0091]    For the steady state conditions tested, the accuracy is ±2 Nm or less. 
         [0092]    Additional steady state accuracy can be achieved by establishing a set of constants for the based equation for a given rpm range. An example is shown below. 
         [0093]    The coefficient values used for a 6CVCc on a T3000 system are shown in Table 1, depicted in  FIG. 12 . 
         [0094]    Advanced Torque Prediction Method 
         [0095]    Transient bench test results are shown in  FIGS. 2-5 . As shown in the transient plots, the predictive accuracy of the base equation, shown as TP, is not equivalent to the steady state results. Additional equations were empirically derived for transient events as well as when Iecv (electronic control valve current) is below a calibratable threshold or the compressor is at full stroke. The results for the additional equations are shown on the transient plots as Adv TP. As shown on the transient plots, the predictive accuracy is greatly improved by utilizing the additional equations. 
         [0096]    Transient Conditions 
         [0097]    Both compressor rpm and ECV current can change rapidly enough to adversely affect the predicted torque during the transient event. To improve prediction accuracy it is possible to make note of a change in compressor rpm or ECV current and to mathematically compensate for the transient. 
         [0098]    RPM 
         [0099]    In the event of a rapid change in compressor RPM, the following strategy is utilized:
       Detect an RPM change above a predetermined threshold, called the trigger limit.   Calculate torque with the base equation and maintain this value for three seconds.   After 3 seconds, restart torque calculation using the Table 1 coefficients from the initial       
 
         [0103]    RPM conditions and the P, T, ΔT, V, Iecv, and RPM values stored in computer memory for 3 seconds (i.e. P, T, ΔT, V, Iecv, and RPM are updated every second, but are 3 seconds old.)
       Verify that RPM variation has stabilized below the trigger limit.   Change to the coefficients for the new RPM and remove the 3 second calculation delay.       
 
         [0106]    The trigger limit for RPM change is calibratable and dependent on compressor and system type. 
         [0107]    Iecv 
         [0108]    In the event of a rapid change in Iecv, calculate torque based on a linear relationship of Pd with an offset and return to the base equation when Pd stabilizes. 
       Example 
       [0109]    6CVCc on T3000 system. Iecv starts at 0 A and step changes to 1.0 A. 
         [0000]      Torque=0.045 Pd− 36 until  Pd 2 −Pd 1&lt;5 
         [0110]    Predictive accuracy was also improved by adding equations to establish a minimum and maximum compressor torque value for a given set of conditions. The following equations were established to achieve this objective: 
         [0000]      Minimum Torque= C 0 +C 1 *E  rpm 
         [0000]      Maximum Torque= D 0 +D 1 *Pd    
         [0111]    Where: 
         [0000]    C0=min torque offset coefficient. Is calibratable based on compressor, system and RPM.
 
C1=min torque coefficient. Is calibratable based on compressor, system and RPM.
 
D0=max torque offset coefficient. Is calibratable based on compressor, system and RPM.
 
D1=max torque coefficient. Is calibratable based on compressor, system and RPM.
 
         [0112]    The minimum torque equation is used when Iecv is less than min Iecv, a calibratable term. 
         [0113]    The maximum torque equation is used when Iecv is greater than max Iecv, a calibratable term, or the predicted torque is greater than the maximum calculated torque. 
         [0114]    The coefficient values, min Iecv and max Iecv used for a 6CVCc on a T3000 system are shown in Table 2 depicted in  FIG. 13 . 
         [0115]    This predictive system is intended for real time vehicle use and as such is dependent on the communication speed or frequency of information update between the HVAC module and the ECM. 
         [0116]    Note that the calibratable values shown in the preceding are for a specific compressor type on a specific AC system. A different combination of compressor type and AC system may require different calibratable values. It is expected that such values would be unchanged for a given compressor type and AC system combination. That is to say, once the calibratable values are established for a type of compressor on a vehicle AC system, they would be applicable for the entire production run of that compressor/system combination. 
         [0117]    Referring to  FIG. 6 , a master flow diagram  44  for the compressor torque prediction system embodied in the present invention is illustrated wherein a calculation logic block  46  is initialized by an AC start-up step  48 , and which, in turn provides a final torque T f  output logic signal to an engine control module (ECM)  58 . AC start-up step  48  feeds a logic step  50  for calculating steady state torque (T ss ) which, in turn, feeds a first input of a logic step  52  wherein final torque (T f ) is selected. Logic step  50  also feeds a T ss  signal to a logic step  54  which calculates engine revolutions per minute (ERPM) transient torque, which feeds an engine RPM from a host vehicle engine control module (speed sensor or a calculation based upon vehicle speed) to a second input of logic step  52  as T erpmt . Finally, logic step  56  calculates IECV transient torque, feeding an electronic control valve transient torque signal (T it ) to a third input of logic step  52 . 
         [0118]    Referring to  FIG. 7 , a flow diagram  60  for calculating steady state compressor torque in practicing the present invention expands on a step  62  of calculating steady state torque T corresponding with step  50  of  FIG. 6 . An engine speed (ERPM) transducer  64  feeds an input to a logic block  66 , which operates to select constants based on ERPM, and outputs selected constants D 0  and D 1  to a first input of a following logic block  70 . A discharge pressure signal (Pd) from discharge pressure transducer  68  feeds a second input of logic block  70 . Logic block  70  operates to calculate maximum steady state torque (T max ). Logic block  70  outputs a T max  signal to a first input of a yes/no logic block  72 . 
         [0119]    An ERPM transducer  74  feeds an input to a logic block  76 , which operates to select constants based on ERPM, and outputs selected constants A 0 , A 1 , A 2 , A 3  and A 4  to a first input of a following logic block  78 . A discharge pressure signal (Pd), an evaporator blower fan voltage signal (V), a thermister ambient temperature signal (Ta), an electronic control valve current signal from measurement of current applied to ECV (I) and a ERPM signal, from transducers collectively illustrated as  80 , provide a second input to logic block  78 , which are also employed in calculating a base steady state torque torque (T base ) output from logic block  78  which is fed to a second input of yes/no logic block  72 . 
         [0120]    An ERPM transducer signal and a thermistor ambient temperature (Ta) signal from transducers collectively illustrated as  82  feed a logic block  84  which operates to select constants based on ERPM, and output selected constants C 0  and C 1  to a first input of a following logic block  86 . A EPRM transducer  88  feeds a signal to a second input of logic block  86 , which are employed in calculating a minimum calculated steady state torque (T min ) as an output from logic block  86 . 
         [0121]    If T base &gt;T max , a first output of yes/no logic block  72  feeds a logic block  90  which equates T ss  with T max , and, in turn, feeds a T ss  output logic block  92 . If T base ≦T max , a second output of yes/no logic block  72  feeds a second yes/no logic block  94 . The T min  output of logic block  86  feeds a second input of yes/no logic block  94 . Yes/no logic blocks  72  and  94  are logically joined by common logic as set forth in phantom logic block  100 . If T base &lt;T min , a first output of yes/no logic block  94  feeds a logic block  98  which equates T ss  with T max , and, in turn, feeds T ss  output logic block  92 . If T base ≧T min , a second output of yes/no logic block  94  feeds a logic block  96  which equates T ss  with T base  and, in turn, feeds T ss  output block  92 . 
         [0122]    Referring to  FIG. 8 , a flow diagram  102  including a calculation logic block  104  for calculating electronic control valve current (Ievc) transient torque in practicing the present invention expands on a step  106  of calculating Ievc corresponding with step  56  of  FIG. 6 . An electronic control valve current (from measurement of current applied to ECV) transducer (I)  108  feeds an input to a logic block  110  which operates to calculate Iecv rate of change (ROC), and outputs a rate of control valve current change (ΔI) to an input of a yes/no logic block  114 . A maximum threshold rate of change of ECV current sensor (ΔI Lim )  112  feeds a second input to yes/no logic block  114 . Logic block further functions, at logic block  120  (in phantom) to check if Iecv is charging. If ΔI&lt;ΔI Lim , a first output to a logic block  116  equates electronic control valve transient torque (T IT ) to second input of logic block  1300 , wherein there is no I evc  transient which, in turn, feeds a T IT  output logic block  118 . If ΔI≧ΔI Lim , a second output of yes/o logic block  114  feeds a first input of a following logic block  122 . Logic block operates to calculate Pd rate of change (ROC). A Pd pressure transducer  124  feeds a second input of logic block  122 , which outputs a rate of discharge pressure change (ΔPd) to a first input of a yes/no logic block  126 . A minimum threshold rate of discharge pressure (ΔPd liml ) transducer  128  feeds a second input of yes/no logic block  126 . If ΔPd&gt;ΔPd liml , a first output of yes/no logic block  126  feeds a first input of another yes/no logic block  134 . Yes/no logic block  126  and  134  mutually check if Pd is charging. If ΔPd≦ΔPd Liml , a second output of yes/no logic block  126  feeds a first input of a logic block  130 . A maximum threshold rate of discharge pressure (ΔPd Limb ) transducer  136  feeds a second input of logic block  134 . If ΔPd&gt;ΔPd Limb , a first output of yes/no logic block  134  feeds a logic block  138  which equates electronic control valve transient torque (T IT ) with zero, i.e. no T IT  transient. which, in turn, provides an output to output logic block  118 . If ΔPd≦ΔPd Limb , a second output of yes/no logic block  134  feeds the input of logic block  130 . A transducer  132  feeds calibratable parameters F 0  and F 1  to a second input of logic block  130  which, in turn, feeds the output logic block  118 . A common logic block  140  (in phantom) checks yes/no logic blocks  126  and  134  to determine if Pd is charging. 
         [0123]    Referring to  FIG. 9 , a flow diagram  142  for calculating engine revolutions per minute (ERPM) transient torque (T) in practicing the present invention expends on a step  146  of calculating ERPM transient torque corresponding with logic step  54  of  FIG. 6 . An electronic control valve current (from measurement of current applied to ECV) transducer (I)  148  feeds an input to a logic block  150  which operates to calculate ERPM rate of change (ROC), and outputs a rate of engine RPM change (ΔERPM) to an input of a yes/no logic block  152 . A maximum threshold rate of ERPM (ΔERPM lim ) transducer  156  feeds a second input to yes/no logic block  152 . If ΔERPM&lt;ΔERPM Lim , yes/no logic block  152  feeds a logic block  154  which equates engine RPM transient torque (T ERPMT ) to zero which, in turn, outputs a feed to an output logic block  166 . A phantom logic block  158  coupled to yes/no logic block  152  checks if ERPM is changing. If ΔERPM≧ΔERPM Lim , yes/no logic block  152  feeds a first input to a present time interval start timer logic block  160 , which also receives a T SS  second input. Logic block  160  provides an output to a following logic block  162  to save T SS  at a present time interval (t i ), which, in turn, feeds a first input of a logic block  164  in which T ERPMT =T ss  when the timer is started and incrementally updated. The output of logic block  164  feeds the output logic block  166 . A timer value for a T ERPMT  calculation logic block  168  feeds logic block  164 . The output of logic block  160  also feeds a first input of a yes/no logic block  170 . A T Lim  transducer  172  feeds a second input of the yes/no logic block  170 . If t&lt;t Lim , yes/no logic block  170  feeds a first input of a no ERPM transient logic block  176  which, in turn, feeds the output logic block  166 . A T SS  input feeds a second input of logic block  176 . Yes/no logic block  170  is controlled by phantom logic block  174  to determine if the timer has expired. 
         [0124]    Referring to  FIG. 10 , a flow diagram  178  including a calculation block  180  for selecting a final torque (T F ) value in practicing the present invention expands on a step  182  of selecting a final torque (T F ) corresponding with step  52  of  FIG. 6 . Separate inputs comprising T SS , T IT  and T ERPMT  are fed into serially coupled yes/no logic blocks  184 ,  190 ,  194  and  198 . If T ERPMT =0 and T N =0, yes/no logic block  184  feeds a logic block  186  which equates T F  and T SS  which, in turn feeds an output to an output logic block  188 . If T ERPMT ≠0 or T n ≠0, a second output of yes/no logic block  184  feeds second yes/no logic block  190 . If T ERPMT =0 and T n ≠0, yes/no logic block  190  feeds a logic block  192  which equates T F  and T IT  which, in turn, feeds an output to output logic block  188 . If T ERPMT ≠0 or T n =0, a second output of yes/no logic block  190  feeds a third yes/no logic block  194 . If T ERPMT ≠0 and T n =0, yes/no logic block  194  feeds a logic block  196  which equates T F  with T ERPM  which, in turn, feeds an output to output logic block  196 . If T ERPMT =0 or T n ≠0, a second output of yes/no logic block  194  feeds a fourth yes/no logic block  198 . If T ERPMT ≠0 and T n ≠0, yes/no logic block  198  feeds a logic block  200  which equates T F  and T IT  which, in turn, feeds an output to output logic block  188 . 
         [0125]    Referring to  FIG. 11 , a schedule of definitions and associated instrumentation, including definitions of terms, calibration parameters and sensor inputs, is provided to assure a clear understanding of the forgoing description of the invention. 
         [0126]    While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow. 
         [0127]    It is to be understood that the invention has been described with reference to a specific embodiment with variations to provide the features and advantages previously described and that the embodiment is susceptible of modification as will be apparent to those skilled in the art. 
         [0128]    Furthermore, it is contemplated that many alternative, common inexpensive materials can be employed to construct the basic constituent components. Accordingly, the forgoing is not to be construed in a limiting sense. 
         [0129]    The invention has been described in an illustrative manner, and it is to be understood that the terminology, which has been used is intended to be in the nature of words of description rather than of limitation. 
         [0130]    Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be under stood that within the scope of the appended claims, wherein reference numerals are merely for illustrative purposes and convenience and are not in any way limiting, the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents, may be practiced otherwise than is specifically described.