Abstract:
A piezoelectric quartz accelerometer includes a sensitive element, a signal processing circuit, a base, an outer case, and a socket, wherein the sensitive element includes two round piezoelectric quartz wafers, and a supporting frame, wherein the two round piezoelectric quartz wafers are symmetrically mounted on both sides of the center axial line of the supporting frame; the sensitive element further includes an axial shock buffer unit and a transverse retaining unit for protecting overload of the two round piezoelectric quartz wafers; the signal processing circuit includes an oscillation circuit for obtaining frequency signal, a frequency differential forming circuit for extracting signal, a phase lock and times frequency circuit for amplifying signal, compensating zero phase, compensating non-linearization and compensating temperature, and an output circuit.

Description:
BACKGROUND OF THE PRESENT INVENTION 
   1. Field of Invention 
   The present invention relates to a piezoelectric quartz accelerometer in a sensitive electronics mainly applied in the attitude stabilized and control system of the aircraft, robot, vehicle, ship, oil drilling platform, construction, industrial automation equipment, comprising a sensitive element, signal processing circuit, base, outer case and socket. 
   2. Description of Related Arts 
   Since J MRaajski from IBM determines the property between force and frequency of piezoelectric quartz through experiment in the 1960&#39;s, the piezoelectric quartz is used for accelerometer. For example, Kearfott Company adopts double mass blocks and double to develop a piezoelectric quartz accelerometer that can detect the gravity changes caused by the moon. US Army Space and Missile Defense Command and Allied Signal Aerospace Instrument System adopt double mass blocks and frequency differential structure to develop a piezoelectric quartz accelerometer that can measure a range up to 1200 g, and has a proportion coefficient of 1.1 Hz/g. U.S. Pat. Nos. 5,578,755 and 5,962,786 disclosed different piezoelectric quartz accelerometer embodiments. ONERA adopts vibration beam structure to develop a piezoelectric quartz accelerometer. However, the resolution, linearization, stability and startup speed of the above piezoelectric quartz accelerometers can not meet the requirement for high performance device, and the piezoelectric quartz accelerometer can not resist shock. 
   SUMMARY OF THE PRESENT INVENTION 
   A main object of the present invention is to provide a piezoelectric quartz accelerometer that has high resolution, linearization, stability and startup speed, and can resist shock. 
   Accordingly, in order to accomplish the above object, the present invention detects the force changing on the piezoelectric quartz caused by an accelerated object, so as to adjust the resonate frequency of the quartz resonator. Therefore, the present invention adopts two symmetrical mounted piezoelectric quartz wafers with same performance index in the sensitive element, wherein two wafers are spacedly apart, and their lead-in wires are connected with the excitation circuit respectively. The sensitive direction of the accelerometer is the center line of the two piezoelectric quartz wafers. When acceleration is detected, the force exerted on the piezoelectric quartz wafers change, wherein one gets an increasing pressure, and the other gets an increasing tension, so that one increases the frequency, and the other decreases the frequency. And then a digital signal proportional to the acceleration is obtained by difference frequency, therefore the acceleration can be measured by detecting resonate frequency variation of the quartz resonator. 
   As shown in  FIG. 7 , the present invention symmetrically mounts two quartz wafers with same property between the sensitive block and the base in the sensitive element. At the same time, they are composed of oscillator with their respective excitation circuit. The sensitive block is used for transferring the force produced by the acceleration to the two round piezoelectric quartz wafers. When there is no acceleration, the two round piezoelectric quartz wafers stands the same force from the sensitive block, so that the force inside is equal and the output signal is equal too; when there is a vertical acceleration, the two round piezoelectric quartz wafers stands the different force from the sensitive block, so that the output signal is not equal. According to the force-frequency property of the piezoelectric wafer, the following formula can be derived. 
   
     
       
         
           
             
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   f 
                 
                 = 
                 
                   
                     
                       K 
                       f 
                     
                     · 
                     
                       
                         f 
                         2 
                       
                       
                         D 
                         · 
                         n 
                       
                     
                     · 
                     Δ 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   F 
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   In this formula, Δf is resonate frequency variation of the piezoelectric quartz resonator; ΔF is an inertia force of the piezoelectric quartz resonator; D is the sectional width of the inertia force; K f  is Ratajski coefficient of the piezoelectric quartz resonator; n is the harmonic times; and f is the resonate frequency of the piezoelectric quartz resonator. Known from the above formula, resonate frequency variation Δf of the piezoelectric quartz resonator has linear relationship with the inertia force ΔF. 
   When the acceleration of a is inputted, one of the two piezoelectric quartz wafers has an increasing pressure, and the other has an increasing tension. Suppose resonate frequency of the piezoelectric wafer is f 0  at balance state, resonate frequency of the wafer with increasing pressure becomes higher to f 1 =f 0 +Δf 1 ; resonate frequency of the wafer with increasing tension becomes smaller to f 2 =f 0 −Δf 2 . The differential frequency output of the two piezoelectric quartz wafer is
 
 f=f   1   −f   2 =( f   0   +Δf   1 )−( f   0   −Δf   2 )
 
 f=Δf   1   +Δf   2   (2)
 
   Because the thickness of the piezoelectric quartz wafer is far smaller than the diameter, the relationship among the inertia force F, F′, the structure parameter of the L that is the distance between the mass center of the sensitive block and the attaching surface, H that is the distance between the two piezoelectric quartz wafer, and the acceleration a is 
   
     
       
         
           
             
               
                 F 
                 = 
                 
                   
                     L 
                     H 
                   
                   ⁢ 
                   
                     m 
                     ⁡ 
                     
                       ( 
                       
                         a 
                         + 
                         g 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
           
             
               
                 
                   F 
                   ′ 
                 
                 = 
                 
                   
                     L 
                     H 
                   
                   ⁢ 
                   
                     m 
                     ⁡ 
                     
                       ( 
                       
                         a 
                         + 
                         g 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Take a =0 as a reference state, so 
               F   0     =       L   H     ⁢           ⁢   mg       ,         and   ⁢           ⁢     F   0   ′       =       L   H     ⁢           ⁢   mg       ;           
when a ≠0, the force variation of the two piezoelectric quartz wafer is
 
   
     
       
         
           
             
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     F 
                     1 
                   
                 
                 = 
                 
                   
                     F 
                     - 
                     
                       F 
                       0 
                     
                   
                   = 
                   
                     
                       L 
                       H 
                     
                     ⁢ 
                     ma 
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
           
             
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     F 
                     2 
                   
                 
                 = 
                 
                   
                     
                       F 
                       ′ 
                     
                     - 
                     
                       F 
                       0 
                       ′ 
                     
                   
                   = 
                   
                     
                       L 
                       H 
                     
                     ⁢ 
                     ma 
                   
                 
               
             
             
               
                 ( 
                 6 
                 ) 
               
             
           
         
       
     
   
   Know from the formula (1), (2), (5), (6), 
   
     
       
         
           
             
               
                 f 
                 = 
                 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         K 
                         f 
                       
                       · 
                       
                         
                           f 
                           0 
                           2 
                         
                         
                           D 
                           · 
                           n 
                         
                       
                       · 
                       
                         L 
                         H 
                       
                     
                     ⁢ 
                     
                       m 
                       · 
                       a 
                     
                   
                   = 
                   Ka 
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   In this formula, 
           K   =     2   ⁢           ⁢       K   f     ·       f   0   2       D   ·   n       ·     L   H       ⁢   m           
is the proportional coefficient, m is the mass of the sensitive block. Known from the formula (7), the acceleration can be determined by measuring the differential frequency f.
 
   The piezoelectric quartz accelerometer of the present invention comprises a sensitive element, a signal processing circuit, a base, an outer case, and a socket. The base for supporting and the outer case are set up to form a cavity. The sensitive element for testing and the signal processing circuit are mounted on the base. The signal processed and the power supply is led out through socket. The sensitive element comprising two round piezoelectric quartz wafers symmetrically mounted on both sides of the centre axial line of the column supporting frames. One supporting frame is mounted on the pallet, and the pallet is mounted on the base. The sensitive block is mounted on another supporting frame. The signals of two round piezoelectric quartz wafers are led out by wire, and are connected with respective excitation circuit forming an oscillation circuit. There are three preferred structures for mounting the two round piezoelectric quartz wafers of the sensitive element in the present invention: a structure with double beams and a single island, a structure with three beams and a signal island, and a structure with symmetrical attached pieces. 
   The sensitive element comprises an axial shock absorber and a transverse retaining unit for protecting overload of the two round piezoelectric quartz wafers. The axial level overload protective unit comprises a sensitive block and a shock pad between the supporting frame and the pallet. The sensitive block comprises three column segments with different diameter, wherein said segment with big diameter is block having a mass adjusting block, said segment with medium diameter is elastic block, preferable a spring for resist shock, and said segment with small diameter is mounting bolt for mounting another supporting frame. The sensitive block of the sensitive element has four preferred structures: a structure with signal spiral, a structure with double spiral, a structure of            -shape with two holes, and a structure of          -shape with three holes. The transverse retaining unit comprises a retaining frame, four adjustable retaining bolts, a retaining bolt, and a fastening bolt between said adjustable retaining bolts and said retaining bolt and said sensitive block.
   The sealed cover of the cavity of the sensitive element and the base are sealed by a sealed structure with a sealed gasket, and fastened by a bolt. There are two preferred structures: an engaged structure with a protruding ring and a concave ring, an engaged structure with a protruding wedge and a concave wedge. 
   As shown in  FIG. 8 , the signal processing circuit comprises an oscillation circuit, frequency differential forming circuit for extracting signal, phase lock and times frequency circuit for amplifying signal, compensating zero phases, compensating non-linearization and compensating temperature, and output circuit. When an acceleration a is input, the output frequency signal is acquired by the oscillation circuit, is extracted and transformed by a frequency differential and transforming circuit, and is amplified by a phase lock and times frequency circuit. A compensating and output compensates zero phases, non-linearization and temperature circuit, and outputs a digital signal having linearization relationship with the acceleration. The magnitude and polarization of the digital signal can precisely reflect the magnitude and direction of the acceleration. 
   The resolution rate of the piezoelectric quartz accelerometer is less than 5*10 −5  g; the non-linearization degree is less than 1*10 −5  g; the measurement range is from 10 −4  g to 10 2  g; the working temperature range is from −40° C. to 80° C.; the gradation factor temperature coefficient is less than 15 ppm/° C.; the start up time is less than 20 s; the strength resisting overload shock is bigger than 110 g. Comprising with ordinary piezoelectric accelerometer, the accelerometer of the present invention has advantage of high resolution rate, high stability, low non-linearization, quick start up time, wide measurement range and working temperature range, high strength for resisting overload shock, and digital output. 
   These and other objectives, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of the piezoelectric accelerometer of the present invention. 
       FIG. 2  is a sectional view of the sensitive element of the piezoelectric accelerometer of the present invention. 
       FIG. 3  is a sectional view of the piezoelectric quartz wafers of the present invention, wherein  FIG. 3   a  is a structure with two beams and one island,  FIG. 3   b  is a structure with three beams and island, and  3   c  is a structure with symmetrical attached pieces. 
       FIG. 4  is a structure diagram of sensitive block of the sensitive element of the present invention, wherein  FIG. 4   a  illustrates a structure with single spiral,  FIG. 4   b  illustrates a structure with double spiral,  FIG. 4   c  illustrates a structure of             shape with two holes, and  FIG. 4   d  illustrates a structure of           shape with three holes.
       FIG. 5  is a sectional view of the retaining and shock protective frame of the sensitive element of the present invention. 
       FIG. 6  is a sectional view of sealed cavity of the sensitive element of the present invention, wherein  FIG. 6   a  is a structure of a protruding ring and a concave ring, and  FIG. 6   b  is a structure of a protruding wedge and a concave wedge. 
       FIG. 7  is a schematic view of operational principle of the sensitive element of the present invention. 
       FIG. 8  is a block diagram of the signal processing circuit of the present invention 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The piezoelectric quartz accelerometer of the present invention comprises a sensitive element  3 , signal processing circuit  8 , base  1 , the outer case  5 , and the socket  11 . As shown in  FIG. 1 , the base  2  and the outer case  5  are closely attached to become a cavity. The sensitive element  3  is mounted on the base  1  by a bolt  4 , and a shock pad  2  is mounted under the sensitive element  3 . The signal processing circuit  8  and electronic element  9  are fastened to the sensitive element  3  by circuit board bolt  7 . There is an insulation pad  6  between the signal processing circuit  8  and the sensitive element  3 . The power and signal are in and out through the socket  11  by cable  10 . 
   The sensitive element  3  comprises two round piezoelectric quartz wafer  22  symmetrically mounted on the both sides of the center line between the first column frame  21  and the second column frame  23 . As shown in  FIG. 2 , the second frame  23  is mounted on a plate  15  by a bolt  29 . The plate  15  is fastened to the base  12  of the sensitive element  3  by a plate screw  26 . The sensitive block  20  presses on the supporting frame  21 . The sealing cover  14  of the sensitive element  3  covers on the base  12 . The base  12  comprises a first connector  28   a , a second connector  28   b , and a vacuum pipe  30  in the sealing cover  14 . The signal of the piezoelectric quartz wafer  22  is led out from the connector  28   a  and  28   b  through a connecting wire  27 , and is connected to the corresponding excitation circuit to become an oscillation circuit. 
   Referring to the  FIGS. 3   a ,  3   b , and  3   c , three mounting structures of the round piezoelectric quartz wafer  22  of the piezoelectric quartz accelerometer. 
   As shown in  FIG. 3   a , use an adhesive clip to fasten a first supporting frame  21   a  and a second supporting frame  23   a , apply adhesive agent on the adhesive cambered surface of the supporting frame  21   a  and  23   a , and imbed the quartz wafer  22   a   1  and  22   a   2  in the cambered surface, wherein the two piezoelectric quartz wafer  22   a   1  and  22   a   2  are parallel. The adhesive agent is made of resin, the supporting frame is made of 1Cr18Ni9Ti, the depth of the adhesive cambered surface is 0.2 mm-1 mm, and the space between the two piezoelectric quartz wafer is 1 mm-5 mm. The lead-in wires of the two piezoelectric quartz wafers are connected with the excitation circuit respectively. The sensitive direction of the accelerometer is the center connecting direction of the two piezoelectric quartz wafers. When acceleration is inputted, the forces exerted on the two piezoelectric quartz wafer change, one wafer increases in pressure, and the other increases in tension, so that one wafer increases its frequency, and the other decreases its frequency. A digital signal proportional to the acceleration can be obtained through the differential frequency. This structure has an advantage of high sensitivity, which is suitable to detect small acceleration signal. 
   As shown in  FIG. 3   b , the supporting frame  21   b  and  23   b  are connected by a flexibility beam  22   b   3 , wherein the supporting frame  21   b  and  23   b  and the flexibility beam  22   b   3  are made by one piece of blank. Apply adhesive agent on the adhesive cambered surface of the supporting frame  21   b  and  23   b , and imbed the quartz wafer  22   b   1  and  22   b   2  in the cambered surface, that is equal to add a connecting beam between the two piezoelectric quartz wafers  22   b   1  and  22   b   2 . The parallel space between the two piezoelectric quartz wafers is 3 mm-8 mm. The flexible hinges of flexible beam have a thickness of 0.2 mm-0.7 mm, and are made of 1Cr18Ni9Ti. The lead-in wires of the two piezoelectric quartz wafers are connected with the excitation circuit respectively. The sensitive direction of the accelerometer is the center connecting direction of the two piezoelectric quartz wafers. When acceleration is inputted, the forces exerted on the two piezoelectric quartz wafer change, one wafer increases in pressure, and the other increases in tension, so that one wafer increases its frequency, and the other decreases its frequency. A digital signal proportional to the acceleration can be obtained through the differential frequency. This structure has an advantage of resisting shock, which is suitable to detect high acceleration. 
   As shown in  FIG. 3   c , the two supporting frame  21   c  and  23   c  are integral structure, the supporting frame  21   c  is in the center, and the supporting frame  23   c  is symmetrically on the both sides. Apply adhesive agent on the adhesive cambered surface of the supporting frame  21   c  and  23   c , and imbed the quartz wafer  22   c   1  and  22   c   2  in the cambered surface. The two flexible hinges of the supporting frame have a thickness of 0.2 mm-0.7 mm, and are made of 1Cr18Ni9Ti. The lead-in wires of the two piezoelectric quartz wafers are connected with the excitation circuit respectively. The sensitive direction of the accelerometer is the center connecting direction of the two piezoelectric quartz wafers. When acceleration is inputted, the forces exerted on the two piezoelectric quartz wafer change, one wafer increases in pressure, and the other increases in tension, so that one wafer increases its frequency, and the other decreases its frequency. A digital signal proportional to the acceleration can be obtained through the differential frequency. This structure is suitable to detect high acceleration. 
   The sensitive element  3  comprises an axial shock absorber and a transverse retaining unit for protecting the two round piezoelectric quartz wafers  22  from overload. The axial shock absorber comprises a sensitive block  20  and a shock pad  31  between the supporting frame  23  and the plate  15  to protect sensitive element  3  from overload in the axial direction. The sensitive block  30 , shown in  FIG. 4   a , comprises three segments of column with different diameter, wherein the segment with big diameter is block  20   a  having a mass adjusting block, the segment with medium diameter is elastic block  20   b , and the segment with small diameter is  20   c  for mounting bolt. The elastic block  20   b  can be embodied as a spring. The retaining frame  16  is mounted outside the plate, and comprises adjustable retaining bolts  17   a ,  17   b ,  17   c , and  17   d  provided surrounding the mass block  20   a , and a retaining bolt  19  provided on the side of the mass block  20   a , so as to become a transverse retaining unit for protecting sensitive element  3  from transverse overload. As shown in  FIG. 2 , the retaining frame  16  further comprises a fastening bolt  25  to adjust the space between the retaining bolt  17   a ,  17   b ,  17   c , and  17   d  and the sensitive block, wherein the space determines the swing range of the sensitive block in the axial and transverse direction. The retaining frame  16  is made of the same material with the supporting frame  21 . As shown in  FIG. 5 , the retaining frame  16  has four retaining bolt  17   a ,  17   b ,  17   c , and  17   d  provided surrounding the mass block  20   a , a retaining bolt  19  provided on the side of the mass block  20   a , and a fastening bolt  25 . The retaining frame  16  is made of the same material with the supporting frame  21 . 
   The elastic block of the sensitive block  20  of the piezoelectric quartz accelerometer can be embodied as four alternatives. First, as shown in  FIG. 4   a , the sensitive block  20  comprises three columns with different diameter, a mass block  20   a , an elastic body  20   b  as a spring, and a bolt  20   c , wherein the three columns are manufactured as an integral part, and are made of spring steel, such as 1Cr18Ni9Ti. Single-spiral elastic body  20   b  has a spiral spacing 2 mm-6 mm, slot width 1 mm-2 mm, inner spiral diameter 5 mm-8 mm, and outer spiral diameter 10 mm-14 mm. The elastic body  20   b  can buffer the strong shock in the axial direction for protecting the piezoelectric quartz wafer of the accelerometer. 
   As shown in  FIG. 4   b , the sensitive block comprises mass block  20   d , double-spiral elastic body  20   e  and a bolt  20   f , which are manufactured as an integral part, and are made of same material mentioned above. The double-spiral elastic body has a spiral spacing 4 mm-8 mm, a slot width 1 mm-2 mm. 
   As shown in  FIG. 4   c , a            -shaped sensitive block with two holes comprises a mass block  20   m , a          -shaped elastic body  20   n  with two holes, and a bolt  20   s , which are manufactured as an integral part, and are made of same material mentioned above. The          -shaped elastic body has a spacing 0.8 mm-2 mm, slot width 0.3 mm-0.5 mm, column width 1 mm-4 mm.
   As shown in  FIG. 4   d , a            -shaped sensitive block with three holes comprises a mass block  20   x , a          -shaped elastic body  20   y  with three holes, and a bolt  20   z , which are manufactured as an integral part, and are made of same material mentioned above. The          -shaped elastic body with three holes has a spacing 0.8 mm-2 mm, slot width 0.3 mm-0.5 mm, column width 1 mm-4 mm.
   The sealing cover  14  and the base  12  are sealedly closes by a sealing gasket  13 , and fastened by a bolt  24 , as shown in  FIG. 6 . To achieve a better effect, sealing cover  14  and the base  12  are pushed to be sealed. Two sealing structures are given as below. 
   As shown in  FIG. 6 , the base  12  comprises a protruding sealing positioning ring  32   a , and the sealing cover  14  comprises a concave sealing positioning ring  32   b . Use a fastening clip to extrude the sealing cover  14  and the base  12 , the protruding sealing positioning ring  32   a  of the base  12  presses and distorts the sealing gasket  13 , and imbed into the concave sealing positioning ring  32   b  of the sealing cover  14 , which are fastened by a bolt  24 , so as to seal the cavity. The sealing cavity  14  and the base  12  are made of the same material with the sensitive block  20 , and the sealing gasket is made of oxygen-free copper. 
   The protruding sealing positioning ring  33   a  can be embodies as a wedge shape, and the tip of the wedge has a guiding curve; the concave sealing positioning ring  33   b  can be embodied as a wedge shape, and the tip of the wedge has a guiding curve, wherein the wedge angle is 45-90 degree, and the height is 0.5 mm-1.5 mm. Use a fastening clip to extrude the sealing cover  14  and the base  12 , the protruding sealing positioning ring  33   a  of the base  12  presses and distorts the sealing gasket  13 , and imbed into the concave sealing positioning ring  33   b  of the sealing cover  14 , which are fastened by a bolt  24 , so as to seal the cavity. 
   
     
       
             
           
             
             
             
           
             
             
             
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               the performance of the piezoelectric quartz accelerometer 
             
           
        
         
             
                 
               Technique index 
               Technique performance 
             
             
                 
                 
             
             
                 
               Measuring range (g) 
               10 −4  g~10 2  g 
             
             
                 
               Output format 
               Digital output 
             
             
                 
               Shift value(mg) 
               0.5 
             
             
                 
               Value range (g) 
               ≦5 × 10 −5   
             
             
                 
               Non-linearization degree 
               ≦1 × 10 −5   
             
           
        
         
             
                 
               misalignment 
               (δ p ) 
               &lt;|30″| 
             
             
                 
               angel 
               (δ 0 ) 
               &lt;|30″| 
             
           
        
         
             
                 
               Second order non- 
               ≦10 
             
             
                 
               linearization coefficient 
             
             
                 
               (μg/g 2 ) 
             
             
                 
               Gradation factor temperature 
               ≦15 
             
             
                 
               coefficient 
             
             
                 
               (ppm/□) 
             
             
                 
               Misalignment temperature 
               ≦20 
             
             
                 
               coefficient 
             
             
                 
               (μg/□) 
             
             
                 
               Misalignment monthly 
               ≦20 
             
             
                 
               stability (μg) 
             
             
                 
               Gradation factor monthly 
               ≦20 
             
             
                 
               stability (ppm) 
             
             
                 
               Temperature range (□) 
               −40~80 
             
             
                 
               Operation voltage (V) 
               ±15 
             
             
                 
               Operation current (mA) 
               ≦20 
             
             
                 
               Weight (g) 
               &lt;80 
             
             
                 
                 
             
           
        
       
     
   
   One skilled in the art will understand that the embodiment of the present invention as shown in the drawings and described above is exemplary only and not intended to be limiting. 
   It will thus be seen that the objects of the present invention have been fully and effectively accomplished. It embodiments have been shown and described for the purposes of illustrating the functional and structural principles of the present invention and is subject to change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.