Abstract:
A swash plate type compressor comprises a cylinder block; a rotation shaft rotatably held in the cylinder block; a swash plate swingably connected to the rotation shaft to rotate therewith; a plurality of piston bores circumferentially arranged about the rotation shaft; a plurality of pistons operatively received in the piston bores respectively, each piston having a holding portion that slidably holds a peripheral portion of the swash plate, so that when the rotation shaft is rotated about its axis, the swash plate pulls and pushes the pistons thereby to reciprocate the same; a valve plate connected to a rear end of the cylinder block, the valve plate having a group of inlet openings connected to the piston bores respectively and another group of outlet openings connected to the piston bores respectively; a rear head connected to the valve plate, the rear head having an intake chamber exposed to the inlet openings and a discharge chamber exposed to the outlet openings, the intake chamber surrounding the discharge chamber, the rear head having an intake port connected to the intake chamber and a discharge port connected to the discharge chamber. An obstruction plate is installed in the intake chamber to obstruct a direct flow of a refrigerant gas from the intake port to a given group of the inlet openings.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to swash plate type compressors employed in an automotive air conditioning system, and more particularly to the swash plate type compressors of a type having a pulsation damping structure. 
     2. Description of the Related Art 
     In order to clarify the task of the present invention, a known swash plate type compressor will be briefly described with reference to FIGS. 16 and 17 of the accompanying drawings. 
     In FIG. 16, there is shown the known swash plate type compressor for use in an automotive air conditioner system, which comprises a cylinder block  2  in which a rotation shaft  10  is rotatably held. A rear head  6  is attached to a rear end of the cylinder block  2  through a valve plate  9 . A swash plate  15  is pivotally held on the rotation shaft  10  through a holder arm  15   a  fixed to the rotation shaft  10 . Designated by numeral  5  is a crank chamber defined in the cylinder block  2 . Six cylindrical piston bores  3  are circumferentially arranged around the right end of the rotation shaft  10 , each having a piston  18  axially slidably received therein. Each piston  18  has a holding portion that slidably holds a peripheral portion of the swash plate  15 . Thus, when, with the swash plate  15  kept inclined relative to the rotation shaft  10  as shown, the rotation shaft  10  is rotated about its axis, the swash plate  15  rotates therewith thereby to pull and push (viz., reciprocate) the pistons  18  in the associated piston bores  3  one after another. Due to the reciprocating movement of each piston  18 , a refrigerant gas is led into each piston bore  3  from a refrigerant intake chamber  7  through an inlet opening  3 A, compressed in the piston bore  3  and then discharged to a refrigerant discharge chamber  8  through an outlet opening  3 B. The outlet opening  3 B is equipped with a valve plate  3   b  that permits only a discharge flow of the refrigerant from the piston bore  8  to the discharge chamber  8 . The inlet and outlet openings  3 A and  3 B are formed in the valve plate  9 , as shown. The refrigerant intake and discharge chambers  7  and  8  are defined by a generally annular partition wall  11  formed in an inner side of the rear head  6 . That is, the refrigerant intake chamber  7  extends circumferentially around the annular partition wall  11 . As is seen from FIG. 17, the refrigerant intake chamber  7  is connected with a refrigerant intake port  12 , and the refrigerant discharge chamber  8  is connected with a refrigerant discharge port (not shown). 
     When the rotation shaft  10  is rotated about its axis and thus the pistons  18  are forced to reciprocate in the corresponding piston bores  3 , a refrigerant gas from an evaporator (not shown) is led into the refrigerant intake chamber  7  through the refrigerant intake port  12 , and led into the piston bores  3  and compressed one after another by the corresponding pistons  18 . The compressed refrigerant is then led to the refrigerant discharge chamber  8  through the respective outlet openings  3 B and led to a condenser (not shown). 
     SUMMARY OF THE INVENTION 
     However, due to inherent construction of the above-mentioned compressor, under operation, a certain pressure difference tends to occur between a position near the refrigerant intake port  12  (see FIG. 17) and a position remote from the intake port  12 . It has been revealed that such pressure difference is caused by a pressure loss inevitably produced when the refrigerant gas flows from the intake port  12  toward the inside of the refrigerant intake chamber  7 . However, when, under appearance of such pressure difference, the refrigerant gas is led into the piston bores  3  through the inlet openings  3 A, the flow of the refrigerant gas tends to produce undesirable pulsation in accordance with the pressure difference and/or the unevenness of the pressure in the refrigerant intake chamber  7 . In addition to this, since the rear head  6  is commonly equipped with both a flow control valve (not shown) for the refrigerant gas and an actuating mechanism (not shown) for the flow control valve, the refrigerant intake chamber  7  is compelled to have a complicated shape, which promotes creation of the undesired pressure difference in the chamber  7 . 
     The above-mentioned undesirable phenomenon may be much clarified from the following description with the aid of FIG.  17 . That is, under operation of the compressor  6 , the pressure at the portion B 1  for a first piston bore  3  is kept higher than that at the portion B 2  and/or B 3 , which causes the pressure difference in the intake chamber  7  and thus generation of pulsation of the refrigerant gas flow. As is known, such pulsation causes generation of vibration and/or noises of the compressor. Although enlargement of the refrigerant intake chamber  7  may reduce or dampen the pressure difference, the same causes enlargement of the entire construction of the compressor. 
     Accordingly, an object of the present invention is to provide a swash plate type compressor which is free of the above-mentioned drawbacks. 
     That is, according to the present invention, there is provided a swash plate type compressor which can dampen the undesirable pulsation of a refrigerant flow thereinto irrespective of its simple and compact construction. 
     According to a first aspect of the present invention, there is provided a compressor which comprises a cylinder block; compressing means installed in the cylinder block to compress a refrigerant gas led thereinto; a valve plate connected to a rear end of the cylinder block, the valve plate having a group of inlet openings which are connected to the compressing means to introduce a refrigerant gas into the compressing means and another group of outlet openings which are connected to the compressing means to discharge the refrigerant gas thus compressed from the compressing means; a rear head connected to the valve plate, the rear head having an intake chamber exposed to the inlet openings and a discharge chamber exposed to the outlet openings, the intake chamber surrounding the charge chamber, the rear head having an intake port connected to the annular intake chamber and a discharge port connected to the circular discharge chamber; and a baffle plate installed in the intake chamber to obstruct a direct flow of the refrigerant gas from the intake port to the inlet openings. 
     According to a second aspect of the present invention, there is provided a swash plate type compressor which comprises a cylinder block; a rotation shaft rotatably held in the cylinder block; a swash plate swingably connected to the rotation shaft to rotate therewith; a plurality of piston bores circumferentially arranged about the rotation shaft; a plurality of pistons operatively received in the piston bores respectively, each piston having a holding portion that slidably holds a peripheral portion of the swash plate, so that when the rotation shaft is rotated about its axis, the swash plate pulls and pushes the pistons thereby to reciprocate the same; a valve plate connected to a rear end of the cylinder block, the valve plate having a group of inlet openings connected to the piston bores respectively and another group of outlet openings connected to the piston bores respectively; a rear head connected to the valve plate, the rear head having an intake chamber exposed to the inlet openings and a discharge chamber exposed to the outlet openings, the intake chamber surrounding the discharge chamber, the rear head having an intake port connected to the intake chamber and a discharge port connected to the discharge chamber; and a baffle plate installed in the annular intake chamber to obstruct a direct flow of a refrigerant gas from the intake port to the inlet openings. 
     According to a third aspect of the present invention, there is provided a compressor which comprises a cylinder block; compressing means installed in the cylinder block to compress a refrigerant gas led thereinto; a valve plate connected to a rear end of the cylinder block, the valve plate having a group of inlet openings connected to the piston bores respectively and another group of outlet openings connected to the piston bores respectively, each outlet opening having a valve plate that permits only a discharge flow of the refrigerant gas from the piston bore; a rear head connected to the valve plate, the rear head having a generally annular intake chamber exposed to the inlet openings and a generally circular discharge chamber exposed to the outlet openings, the rear head having an intake port connected to the annular intake chamber and a discharge port connected to the circular discharge chamber; and an arcuate baffle plate installed in the generally annular intake chamber in a manner to obstruct a direct flow the refrigerant gas from the intake port to a given group of the inlet openings. 
     According to a fourth aspect of the present invention, there is provided a swash plate type compressor which comprises a cylinder block; a rotation shaft rotatably held in the cylinder block; a swash plate swingably connected to the rotation shaft to rotate therewith; a plurality of piston bores defined in the cylinder block and circumferentially arranged about the rotation shaft; a plurality of pistons operatively received in the piston bores respectively, each piston having a holding portion that slidably holds a peripheral portion of the swash plate, so that when the rotation plate is rotated about its axis, the swash plate pulls and pushes the pistons thereby to reciprocate the same; a valve plate connected to a rear end of the cylinder block, the valve plate having a group of inlet openings connected to the piston bores respectively and another group of outlet openings connected to the piston bores respectively, each outlet opening having a valve plate that permits only a discharge flow of a refrigerant gas from the piston bore; a rear head connected to the valve plate, the rear head having a generally annular intake chamber exposed to the inlet openings and a generally circular discharge chamber exposed to the outlet openings, the rear head having an intake port connected to the annular intake chamber and a discharge port connected to the circular discharge chamber; and an arcuate baffle plate installed in the generally annular intake chamber in a manner to obstruct a direct flow the refrigerant gas from the intake port to a given group of the inlet openings. 
     SUMMARY OF THE DRAWINGS 
    
    
     Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a plan view of a pulsation reducing structure employed in a swash plate type compressor of a first embodiment of the present invention; 
     FIG. 2 is a sectional view taken along the line II—II of FIG. 1; 
     FIG. 3 is a view similar to FIG. 1, but showing a pulsation reducing structure employed in a swash plate type compressor of a second embodiment of the present invention; 
     FIG. 4 is a view similar to FIG. 3, but showing a first modification of the second embodiment of the present invention; 
     FIG. 5 is a view also similar to FIG. 3, but showing a second modification of the second embodiment of the present invention; 
     FIG. 6 is a sectional view taken along the line VI—VI of FIG. 5; 
     FIG. 7 is a view similar to FIG. 1, but showing a pulsation reducing structure employed in a swash plate type compressor of a third embodiment of the present invention; 
     FIG. 8 is a perspective view of an partition member employed in the third embodiment; 
     FIG. 9 is a plan view of a rear head employed in the swash plate type compressor of the third embodiment; 
     FIG. 10 is a sectional view taken along the line X—X of FIG. 7; 
     FIG. 11 is a sectional view taken along the line XI—XI of FIG. 7; 
     FIG. 12 is a sectional view taken along the line XII—XII of FIG. 7; 
     FIG. 13 is a sectional view taken along the line XIII—XIII of FIG. 7; 
     FIG. 14 is a sectional view taken along the line XIV—XIV of FIG. 9; 
     FIG. 15 is a view similar to FIG. 9, but showing a modification of the third embodiment of the present invention; 
     FIG. 16 is a sectional view of a known swash plate type compressor; and 
     FIG. 17 is a plan view of a rear head employed in the known swash plate type compressor. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, since the embodiments of the invention are substantially same as the above-mentioned known swash plate type compressor of FIGS. 16 and 17 except the rear head  6 , only the rear heads for the embodiments will be described in the following. 
     Referring to FIGS. 1 and 2, there is shown a rear head  6 A which is employed in the swash plate type compressor of a first embodiment of the present invention. The rear head  6 A is constructed to have a pulsation reducing structure in its inner side, as will be described in the following. As has been mentioned hereinabove, the rear head  6 A is a member tightly attached to the rear end of the cylinder block  2  (see FIG. 16) through the valve plate  9 . For the tight attaching to the cylinder block  2 , six column portions  23  are integrally formed in the inner side of the rear head  6 A, each having a threaded bore for engaging with a bolt (not shown) extending from the cylinder block  2 . 
     As is seen from FIGS. 1 and 2, like in the known rear head  6  of FIG. 16, the rear head  6 A is constructed to incorporate with a compressor having six pistons  18 . That is, six inlet openings  3 A for the respective piston bores  3  are formed in the valve plate  9  at equally spaced intervals. 
     It is to be noted that portions of the rear head  6 A that face the six inlet openings  3 A are denoted by references B 1 , B 2 , B 3 , B 4 , B 5  and B 6  respectively. It is further to be noted that these portions B 1 , B 2 , B 3 , B 4 , B 5  and B 6  correspond to first, second, third, fourth, fifth and sixth piston bores  3  with respect to a normal rotation direction of the rotation shaft  10 , which is indicated by an arrow “D” in FIG.  1 . 
     The rear head  6 A is formed at the inner side thereof with refrigerant intake and discharge chambers  7 A and  8 A which are partitioned by a generally annular partition wall  11 . That is, the intake chamber  7 A is shaped generally annular and arranged to surround the annular partition wall  11  which is generally circular. More specifically, the annular intake chamber  7 A is defined between an outer surface of the annular partition wall  11  and an inner surface of a cylindrical outer wall of the rear head  6 A. The intake chamber  7 A is connected with a refrigerant intake port  12 A, and the discharge chamber  8 A is connected with a refrigerant discharge port  12 B which is provided at a diametrically opposite position of the intake port  12 A. The intake port  12 A is positioned between the portions B 1  and B 6 , as shown. 
     In this first embodiment  6 A, as is understood from the drawings, a baffle plate  13 A is arranged in the refrigerant intake chamber  7 A. That is, the baffle plate  13 A is generally arcuate in shape and extends from a position near the refrigerant intake port  12  to a position corresponding to the portion B 3 . More specifically, the arcuate baffle plate  13 A extends from the position near the intake port  12  to the portion B 3  through the portions B 1  and B 2 . 
     As is seen from FIG. 2, due to provision of the arcuate baffle plate  13 A, the refrigerant intake chamber  7 A at the portions B 1 , B 2  and B 3  is divided into first and second sections S 1  and S 2 . The baffle plate  13 A is so positioned that the refrigerant intake port  12  is exposed to the first section S 1  of the intake chamber  7 A. That is, the baffle plate  13 A is so arranged as to obstruct a direct flow of the refrigerant gas from the intake port  12 A to the inlet openings  3 A of the portions B 1 , B 2  and B 3 , that is, of the first, second and third piston bores  3 . 
     As is seen from FIG. 1, bolts  50  are used for fixing the baffle plate  13 A to the rear head  6 A. A flow control valve “FCV” is integrally installed in the inner side of the rear head  6 A within an area occupied by the portions B 4 , B 5  and B 6 . Designated by reference P 1  is a first passage which connects an inlet port of the flow control valve “FCV” with a crank chamber of the cylinder block  2 , and designated by reference P 2  is a second passage which connects an outlet port of the control valve “FCV” with the crank chamber of the cylinder block  2 . Thus, by handling the control valve “FCV”, a bypass connection between the intake and discharge chambers  7 A and  8 A is adjusted. 
     Designated by reference “a” is a first baffle rib raised from a bottom of the intake chamber  7 A at a position between the refrigerant intake port  12 A and the portion B 6 , and designated by reference “b” is a second baffle rib raised from the bottom of the intake chamber  7 A at a position between the portions B 3  and B 4  and near the portion B 4 . 
     In operation, the refrigerant gas is led into the intake chamber  7 A from the intake port  12 A. However, due to provision of the baffle plate  13 A and first and second baffle ribs “a” and “b” which are arranged in the above-mentioned manner, distribution of the refrigerant gas to the six inlet openings  3 A of the first to sixth piston bores  3  is evenly and equally carried out. 
     That is, at the portions B 1 , B 2  and B 3 , major part of the refrigerant gas from the intake port  12 A is forced to flow in the first section S 1  of the intake chamber  7 A, being obstructed from directly flowing to the inlet openings  3 A of the first, second and third piston bores  3 . In other words, the inlet openings  3 A of these first, second and third piston bores  3  are forced to have a longer intake passage for the refrigerant gas. Thus, the corresponding portions B 1 , B 2  and B 3 , particularly the portion B 1  can show a relatively low pressure due to a larger pressure loss produced at those portions. Of course, part of the refrigerant gas from the intake port  12 A is directly led into the inlet openings  3 A of the portions B 1 , B 2  and B 3 . 
     While, at the portions B 4 , B 5  and B 6  where no baffle plate is arranged, the refrigerant gas flow into the inlet openings  3 A of the fourth, fifth and sixth piston bores  3 , that is, of the portions B 4 , B 5  and B 6  substantially consists of a first gas flow which runs counterclockwise (in FIG. 1) from the intake port  12 A getting over the first baffle rib “a” and a second gas flow which runs clockwise (in FIG. 1) from the intake port  12 A passing along the baffle plate  13 A and getting over the second baffle rib “b”. This flow causes the refrigerant gas pressure at such portions B 4 , B 5  and B 6  to show a controlled value. 
     Accordingly, the portions B 1  to B 6  of the refrigerant intake chamber  7 A have a generally even pressure therethroughout, and thus undesirable intake pulsation of the refrigerant gas is suppressed or at least minimized. 
     Referring to FIG. 3, there is shown a rear head  6 B which is employed in the swash plate type compressor of a second embodiment of the present invention. Since the rear head  6 B of the second embodiment is similar in construction to that of the above-mentioned first embodiment  6 A, only portions different form those of the first embodiment  6 A will be described in detail in the following, and substantially same parts and portions as those of the first embodiment  6 A are denoted by the same numerals. 
     In this second embodiment, the rear head  6 B is constructed to incorporate with a compressor having seven pistons  18 . That is, seven inlet openings  3 A for the respective piston bores  3  are formed in the valve plate  9 . It is to be noted that portions of the rear head  6 B that face the seven inlet openings  3 A are denoted by references B 1 , B 2 , B 3 , B 4 , B 5 , B 6  and B 7  respectively. It is further to be noted that these portions B 1  to B 7  correspond to first to seventh piston bores  3  with respect to a normal rotation direction of the rotation shaft  10 , which is indicated by an arrow “D” in FIG.  3 . 
     As shown in the drawing, in this second embodiment  6 B, a generally arcuate baffle plate  13 B is arranged in the refrigerant intake chamber  7 B within an area occupied by the portions B 6 , B 7  and B 1 . That is, the baffle plate  13 B covers the area near the refrigerant intake port  12 A. 
     Thus, in this second embodiment, the direct flow of the refrigerant gas from the intake port  12 A to the inlet openings  3 A of the portions B 6 , B 7  and B 1 , that is, of the sixth, seventh and first piston bores  3  is obstructed by the baffle plate  13 B. Thus, for the reasons as mentioned in the first embodiment  6 A, the portions B 6 , B 7  and B 1  can show a relatively low pressure due to a larger pressure loss produced at those portions. 
     While, at the portions B 2  and B 5  where no baffle plate is arranged, the distance from the intake port  12 A causes the portions B 2  and B 5  to show a controlled pressure which is generally the same as that produced at the portions B 6 , B 7  and B 1 . At the portions B 3  and B 4  where no baffle plate is arranged, the refrigerant gas flow into the inlet openings  3 A of the third and fourth piston bores  3 , that is, of the portions B 3  and B 4  substantially consists of a first gas flow which runs counterclockwise (in FIG. 3) from the intake port  12 A while being obstructed by the first baffle rib “a” and a second gas flow which runs clockwise (in FIG. 3) from the intake port  12 A while being obstructed by the second baffle rib “b”. This flow causes the refrigerant gas pressure at such portions B 3  and B 4  to show a controlled value. 
     Accordingly, the portions B 1  to B 7  of the refrigerant intake chamber  7 B have a generally even pressure therethroughout, and thus undesirable intake pulsation of the refrigerant gas is suppressed or at least minimized. 
     Referring to FIG. 4, there is shown a first modification  6 B′ of the rear head  6 B of the above-mentioned second embodiment. 
     As shown, in this modification  6 B′, the arcuate baffle plate  13 B′ is slightly longer than the baffle plate  13 B of the second embodiment. That is, both ends of the baffle plate  13 B′ are slightly enlarged for enhancing the partitioning effect to the refrigerant gas flow. 
     Referring to FIGS. 5 and 6, particularly FIG. 5, there is shown a second modification  6 B″ of the rear head  6 B of the above-mentioned second embodiment. 
     In this second modification  6 B″, an apertured arcuate baffle plate  13 B″ is employed in place of the baffle plate  13 B of the second embodiment. That is, a plurality of small circular openings  20  are formed in the baffle plate  13 B″, which are arranged to make a line as shown in FIG.  5 . As is seen from FIG.  6 , due to provision of the small openings  20 , part of the refrigerant gas flowing in the first section S 1  of the refrigerant intake chamber  7 B can flow into the second section S 2  through the openings  20 , which enhances pressure controlling at the portions B 6 , B 7  and B 1 . 
     Referring to FIGS. 7 to  14 , there is shown a rear head  6 C which is employed in the swash plate type compressor of a third embodiment of the present invention. Since the rear head  6 C of the third embodiment is similar in construction to that of the above-mentioned first embodiment  6 A, only portions different from those of the first embodiment will be described in detail in the following, and substantially same parts and portions as those of the first embodiment  6 A are denoted by the same numerals. 
     As is seen from FIGS. 7 and 9, in the third embodiment  6 C, a refrigerant discharge port  12 B communicated with the refrigerant discharge chamber  8 C is provided at a generally opposite position of the refrigerant intake port  12 A, like in the above-mentioned first and second embodiments  6 A and  6 B. 
     As is seen from FIG. 7, a generally arcuate baffle plate  13 C is arranged in the refrigerant intake chamber  7 C within an area occupied by the portions B 1 , B 2  and a half of the portion B 3 . 
     As is seen from FIG. 9, two column portions  24  are integrally formed in the inner side of the rear head  6 C, each having a threaded bore for receiving the above-mentioned bolt  50 . That is, the arcuate baffle plate  13 C is put on the column portions  24  and secured thereto by the bolts  50  engaged with the threaded bores. Designated by numeral  25  is a projection for supporting the arcuate baffle plate  13 C. 
     FIG. 8 shows in detail the arcuate baffle plate  13 C. As shown, the baffle plate  13 C has a raised left end  13   a  which is to be positioned at the refrigerant intake port  12 A. As is seen from FIG. 11, the raised left end  13   a  is positioned above the refrigerant intake port  12 A not to extend across the intake port  12 A, and thus the flow of the refrigerant gas from the port  12 A into the first section S 1  is smoothly carried out. 
     Referring back to FIG. 8, the baffle plate  13 C has two rounded cut portions  13   b  for intimately receiving therein corresponding two of the column portions  23  and two bolt openings  13   c  through which the bolts  50  pass. 
     The arrangement of the arcuate baffle plate  13 C in the refrigerant intake chamber  7 C is well understood from FIGS. 10,  11 ,  12  and  13 , which are sectional views taken along the line X—X, line XI—XI, line XII—XII and line XIII—XIII of FIG.  7 . 
     As is seen from FIG. 11, the raised left end  13   a  of the baffle plate  13 C is arranged not to obstruct the intake port  12 A. As is seen from FIG. 10, the other end of the baffle plate  13 C is positioned near a baffle rib “c” positioned in the portion B 3 . As is seen from FIG. 12, the baffle plate  13 C is secured to the column portion  24  by the bolt  50 , and as is seen from FIG. 13, the refrigerant discharge port  12 B is formed in an enlarged lower portion of the annular partition wall  11 . 
     FIG. 14 is a sectional view taken along the line XIV—XIV of FIG. 9, showing the flow of the refrigerant gas led from the intake port  12 A to the intake chamber  7 C. As is seen from this drawing, due to provision of the baffle plate  13 C, direct flow of the refrigerant gas from the intake port  12 A to the inlet openings  3 A of the partitions B 1 , B 2  and B 3  is blocked, which brings about an even pressurizing throughout the portions B 1  to B 6 . 
     Referring to FIG. 15, there is shown a modification  6 C′ of the rear head  6 C of the above-mentioned third embodiment. 
     As shown, in this modification  6 C′, a plurality of small circular openings  20  are formed in the baffle plate  13 C′. As has been mentioned hereinbefore, due to provision of the openings  20 , the pressure at the portions B 1 , B 2  and B 3  is much finely controlled. 
     If desired, the following modifications may be further carried out in the invention. That is, in case wherein the intake pressure at the position where the baffle plate is located is relatively low, the baffle plate may be formed with one or several small openings. With this measure, the inlet openings  3 A of the valve plate  9  show even pressure therethroughout. 
     Although the above-description is directed to the swash plate type compressor, the concept of the present invention is applicable to other type compressors, that is, swing type compressor, rotary type compressor, scroll type compressor and the like. 
     The entire contents of Japanese Patent Applications 2000-267555 (filed Sep. 4, 2000) and 2000-391183 (filed Dec. 22, 2000) are incorporated herein by reference. 
     Although the invention has been described above with reference to the embodiments of the invention, the invention is not limited to such embodiments as described above. Various modifications and variations of such embodiments may be carried out by those skilled in the art, in light of the above description.