Patent Publication Number: US-2007114852-A1

Title: Parallel uninterruptible power supply system

Description:
FIELD OF THE INVENTION  
      The present invention is related to an uninterruptible power supply system, and more particularly to a parallel uninterruptible power supply system.  
     BACKGROUND OF THE INVENTION  
      Uninterruptible power supply (UPS) is an emergent power supply device connected between a power source and a load, in which the power source can be a commercial AC power supply or any other AC power source. The uninterruptible power supply is configured to supply the power required for powering a load in an emergent condition so as to ensure the normal operation for the load when the power source becomes abnormal.  
      In order to protect important electronic equipment with more efficiency and safety, UPS has been extensively employed to ensure the normal operation for various electronic devices. More particularly, a parallel UPS system has been testified to be the best choice for providing high quality, high continuity electric power to electronic devices that are vulnerable to the problem of power outage.  
       FIG. 1  shows a circuit block diagram of a parallel UPS system according to the prior art. As shown in  FIG. 1 , a parallel UPS system  10  is connected between a power source  11  and a load  16  for continuously and stably supplying the electric power required for powering the load  16 . The parallel UPS system  10  is made up of a first uninterruptible power supply (first UPS)  12 , a second uninterruptible power supply (second UPS)  13 , a connector  14 , and a distribution box  15 . The first UPS  12  and the second UPS  13  receive AC power from the power source  11  and perform rectification, filtration and conversion to the received AC power, and output electric power respectively. The distribution box  15  is configured to receive the power outputted from the first UPS  12  and the second UPS  13 , and supply an operative power for the load  16  in accordance with the requirements of the load  16 . Besides, each of the first UPS  12  and the second UPS  13  include an external communication port  126 ,  136  for transmitting the system status information and associated data signal to the other uninterruptible power supply, so that the first UPS  12  and the second UPS  13  can communicate with each other to equally share the operative power for the load  16 . If one of the uninterruptible power supplies is malfunctioned, the other uninterruptible power supply can adjust its share of the operative power by means of the signal transmission between the communication ports  126 ,  136 . The connector  14  is configured as a signal transmission medium for the external transmission ports  126 ,  136 .  
       FIG. 2  shows a circuit block diagram detailing the internal circuitry of the parallel UPS system of  FIG. 1 , in which the circuit architecture of the first UPS  12  and the circuit architecture of the second UPS  13  are alike. As shown in  FIG. 2 , the first UPS  12  is made up of a switch device  121 , a power supply module  122 , a microprocessor  123 , a battery module  124 , a control module  125 , an external communication port  126 , and a display  127 , in which the external communication port  126  includes an internal communication chip  1261 . Likewise, the second UPS  13  is made up of a switch device  131 , a power supply module  132 , a microprocessor  133 , a battery module  134 , a control module  135 , an external communication port  136 , and a display  137 , in which the external communication port  136  includes an internal communication chip  1361 .  
      In the circuit block diagram of  FIG. 2 , the solid arrowheaded lines represent power transmission lines and the dashed arrowheaded lines represent signal transmission lines. As shown in  FIG. 2 , the power source  11  supplies the electric power required for powering the switch devices  121 ,  131 , the power supply modules  122 ,  132 , and control modules  125 ,  135  of the first uninterruptible power supply  12  and the second uninterruptible power supply  13 . The control modules  125 ,  135  are configured to perform signal transmission by means of the switch devices  121 , 131 , the microprocessors  122 ,  132  of the power modules  122 ,  132 , battery modules  124 ,  134 , and voltage output terminals (not shown).  
      The power supply modules  122 ,  132  of the first UPS  12  and the second UPS  13  are configured to process the AC power received from the power source  11  and output the processed electric power to the distribution box  15  through the switch devices  121 ,  131 , respectively. Also, the power supply modules  122 ,  132  are configured to convert the AC power into DC power and store the DC power in the battery modules  124 ,  134 , respectively. The microprocessors  123 , 133  are configured to transmit the status information of the power supply modules  122 ,  132  to the control modules  125 ,  135 . Also, the battery modules  124 , 134  can transmit their status information to the control modules  125 ,  135 .  
      The switch devices  121 ,  132  are under the control of the control modules  125 ,  135 , respectively. When the control modules  125 ,  135  detect that the output voltages S 1 , S 2  of the power supply modules  122 ,  132  are abnormal, i.e. the power supply modules  122 ,  132  are malfunctioned or the AC power derived by converting the DC power supplied from the battery modules  124 ,  134  can not meet the requirements of the load  16 , a control signal is issued to the switch devices  121 ,  131  to allow the power source  11  to provide the output voltages S 1 , S 2 . The displays  127 ,  137  are configured to display the system status information of the first UPS  12  and the second UPS  13 .  
      Because the first uninterruptible power supply  12  and the second uninterruptible power supply  13  are necessary to communicate with each other, the control module  125  and the control module  135   13  are configured to conduct signal transmission through respective external communication ports  126 ,  136 . That is, the signals outputted from the microprocessors  123 ,  133  and the signals outputted from the battery modules  124 ,  134  are first transmitted to the control modules  125 ,  135  and then respectively transmitted to the external transmission port of the other uninterruptible power supply. This ensures that the uninterruptible power supplies  12  and  13  share the system status information with each another. The uninterruptible power supplies can communicate with each other by means of the signal transmission and can further equally share the operative power for the load  16 .  
      As stated above, the signals outputted from the microprocessors  123 ,  133  and the battery modules  124 ,  134  are first transmitted to the control modules  125 ,  135  and then respectively transmitted to the other uninterruptible power supply through the external communication ports  126 ,  136 . Such communication mode would result in a long communication path and cause the slowness of the signal transmission, which would in turn prolong the response time of the entire uninterruptible power supply system  10 . More disadvantageously, both of the external communication ports  126 ,  136  have complicated circuit architecture and require an internal communication chip  1261 ,  1361  to conduct signal transmission, which would increase the manufacturing cost and deteriorate the product competitiveness.  
      There is a need to develop a parallel uninterruptible power supply system in which the internal uninterruptible power supplies can communicate with each other without the need of an external communication port.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a parallel uninterruptible power supply system that can conduct signal transmission between the internal uninterruptible power supplies without the need of a communication port. Therefore, the inventive parallel uninterruptible power supply system can provide a faster signal transmission rate and a shorter response time, and can further reduce the manufacturing cost by eliminating the internal communication chip located within of the communication port.  
      Another object of the present invention is to provide a parallel uninterruptible power supply system that can conduct signal transmission between the internal uninterruptible power supplies by means of signal transmission lines, so that each internal uninterruptible power supply can receive the data signal outputted from the power supply module of the other uninterruptible power supply to achieve multilateral communication between the internal uninterruptible power supplies.  
      To this end, a broader aspect of the present invention provides a parallel uninterruptible power supply system, which at least includes a first uninterruptible power supply, a second uninterruptible power supply, and a connector. The first uninterruptible power supply includes a first control module, a first power supply module, a first battery module, and a first signal transmission line connected to the first control module, the first power supply module, and the first battery module for transmitting data signal outputted from the first power supply module and the first battery module. In addition, a second uninterruptible power supply is connected in parallel with the first uninterruptible power supply and includes a second control module, a second power supply module, a second battery module, and a second signal transmission line connected to the second control module, the second power supply module, and the second battery module for transmitting data signal outputted from the second power supply module and the second battery module. The connector is connected with the first signal transmission line and the second signal transmission line for allowing the first control module to receive data signal directly from the second power supply module and the second battery module and allowing the second control module to receive data signal directly from the first power supply module and the first battery module, so that the first uninterruptible power supply and the second uninterruptible power supply can communicate directly with each other.  
      Now the foregoing and other features and advantages of the present invention will be best understood through the following descriptions with reference to the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a circuit block diagram of a parallel UPS system according to the prior art;  
       FIG. 2  is a circuit block diagram detailing the parallel UPS system of  FIG. 1 ;  
       FIG. 3  is a circuit block diagram of a parallel UPS system according to a first preferred embodiment of the present invention;  
       FIG. 4  is a circuit block diagram of a parallel UPS system according to a second preferred embodiment of the present invention;  
       FIG. 5  is a circuit block diagram detailing the internal circuitry of the parallel UPS system of  FIG. 4 ; and  
       FIG. 6  is an elevation view of a parallel UPS system according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Several preferred embodiments embodying the features and advantages of the present invention will be expounded in following paragraphs of descriptions. It is to be realized that the present invention is allowed to have various modification in different respects, all of which are without departing from the scope of the present invention, and the description herein and the drawings are to be taken as illustrative in nature, but not to be taken as limitative.  
       FIG. 3  illustrates a detailed circuit block diagram of a parallel UPS system according to a first preferred embodiment of the present invention. As shown in  FIG. 2 , a parallel uninterruptible power supply system  20  is configured to receive an input AC power from a power source  21  at one end and connect to a load  26  at the other end. The parallel UPS system  20  is made up of a first UPS  22 , a second UPS  23 , a connector  24 , and a distribution box  25 . The first UPS  22  and the second UPS  23  are configured to perform rectification, filtration and conversion to the input AC power and provide an output power respectively. The output power is distributed to the load  26  through the distribution box  25 .  
      The first UPS  22  and the second UPS  23  are connected in parallel with each other, and both are configured to receive the input AC power from the power source  21 . The first UPS  22  is made up of a first switch device  221 , a first power supply module  222 , a first microprocessor  223 , a first battery module  224 , a first control module  225 , a first display  226 , and a first signal transmission line  227 . In a preferred aspect of the present invention, the first signal transmission line  227  is implemented with a bus device for transmitting data signal.  
      In the circuit block diagram of  FIG. 3 , the solid arrowheaded lines represent power transmission lines and the dashed arrowheaded lines represent signal transmission lines. As depicted in  FIG. 3 , the first switch device  221  is connected to the power source  21  and the first power supply module  222  by means of power transmission lines, and connected to the first control module  225  by means of signal transmission lines. The first switch device  221  is under the control of the first control module  225 . When the first control module  225  detects that the output voltage of the first power supply module  222  is abnormal, i.e. the first power supply module  222  is malfunctioned or the AC power derived by converting the DC power received from the first battery modules  224  can not meet the requirements of the load  26 , the first control module  225  issues a control signal to allow the power source  21  to provide the output voltage S 3  directly by means of the first switch device  221 .  
      The first power supply modules  222  is configured to process the AC power received from the power source  21  and supply power to the distribution box  25  through the first switch device  221 , and convert the received AC power into DC power for storage in the first battery module  224 . The first microprocessor  223  of the first power supply module  222  is configured to transmit the data signal associated with the system status information of the first power supply module  222  to the first control module  225 . Also, the first battery module  224  is configured to transmit the data signal associated with the status information of the battery module  224  to the first control module  225 .  
      The first control module  225  is configured to receive the data signal associated with the system status information of the first power supply module  222  and the data signal associated with the system status information of the battery module  224 , and perform a corresponding control function accordingly. The first display  226  is under the control of the first control module  225  for displaying the system status information of the first UPS  22 .  
      The first control module  225 , the first microprocessor  223 , and the first battery module  224  are configured to conduct data signal transmission by means of the first signal transmission line  227 , so that the first control module  225 , the first microprocessor  223 , and the first battery module  224  can communicate with each other.  
      As shown in  FIG. 3 , the second uninterruptible power supply  23  includes a second switch device  231 , a second power supply module  232 , a second microprocessor  233 , a second battery module  234 , a second control module  235 , a second display  236 , and a second signal transmission line  237 . In a preferred aspect of the present invention, the second signal transmission line  237  is implemented with a bus device for transmitting data signal.  
      It should be noted that the principle and efficacy of the second switch device  231 , the second power supply module  232 , the second microprocessor  233 , the second battery module  234 , the second control module  235 , the second display  236 , and the second signal transmission line  237  are similar to those of the counterpart components located within the first UPS  22 , and it is not intended to give details herein.  
      The first signal transmission line  227  is connected to the first control module  225 , the first microprocessor  223  and the first battery module  224  by means of signal transmission lines at one end, and also connected to the connector  24  by means of signal transmission lines at the other end. Likewise, the second signal transmission line  237  is connected to the second control module  235 , the second microprocessor  233 , and the second battery module  234  by means of signal transmission lines at one end, and also connected to the connector  24  by means of signal transmission lines at the other end. Therefore, by connecting the first signal transmission line  227  and the second signal transmission line  237  with the connector  24 , the first control module  225  can receive data signal outputted from the second power supply module  232  and the second battery module  234 , and the second control module  235  can receive data signal outputted from the first power supply module  222  and the first battery module  224 . This would enable the first UPS  22  and the second UPS  23  to communicate directly with each other, and thereby reduce the manufacturing cost of the parallel UPS system  20 . Moreover, the signal transmission rate of the data signal can be accelerated because of the direction communication between the first UPS  22  and the second UPS  23  through the first signal transmission line  227  and second signal transmission line  237 , and thus the response time of the parallel UPS system  20  can be shortened.  
      In a second preferred embodiment of the present invention, a power transmission line  29  can be connected to the first UPS  22  and the second UPS  23  to achieve a parallel redundant configuration for a single-load condition or a multi-load condition, or achieve a power distribution system with capacity upgrade mode. In such embodiment, the distribution box  25  can be eliminated so that the manufacturing cost of the parallel UPS system  20  can be further reduced.  FIG. 4  is a circuit block diagram of a parallel UPS system according to a second preferred embodiment of the present invention. As depicted in  FIG. 4 , the parallel UPS system  20  is configured to receive an input AC power from a power source  21  at one end and connect to a first load  27  and a second load  28  at the other end for supplying an operative power for the first load  27  and the second load  28  respectively. The parallel UPS system  20  includes a first UPS  22 , a second UPS  23 , a first signal transmission line  227 , a second signal transmission line  237 , a connector  24 , and a power transmission line  29 .  
      As depicted in  FIG. 4 , the first UPS  22  and the second UPS  23  are connected in parallel with each other. The first UPS  22  is connected to the power source  21  and the first load  27  for receiving an input AC power from the power source  21  and outputting an operative power to the first load  27 . The second UPS  23  is connected to the power source  21  and the second load  28  for receiving an input AC power from the power source  21  and outputting an operative power to the second load  28 . The first UPS  22  and the second UPS  23  are configured to communicate with each other by means of the first signal transmission line  227 , the second signal transmission line  237 , and the connector  24 . It is to be noted that the design rule of the UPS system  20  according to the present embodiment is the same as the design rule of the UPS system  20  according to the previous embodiment, and it is not intended to give details herein. Besides, the power transmission line  29  serves as a medium for first UPS  22  and the second UPS  23  to transmit and distribute the output power.  
       FIG. 5  is a circuit block diagram detailing the internal circuitry of the UPS system of  FIG. 4 . In this embodiment, the internal circuitry of the first uninterruptible power supply  22  and the internal circuitry of the second uninterruptible power supply  23  are similar to those described in the previous embodiment, and it is not intended to give details herein. One end of the power transmission line  29  is connected between a first switch device  221  and a first voltage output terminal  228 , and the other end of the power transmission line  29  is connected between a second switch device  231  and a second voltage output terminal  238 . In order to prolong the lifetime of the uninterruptible power supplies mounted within the parallel UPS system  20  and enhance the reliability of the output power of the uninterruptible power supply system, the communication between the first uninterruptible power supply  22  and the second uninterruptible power supply  23  is carried out by means of the first signal transmission line  227  located within the first UPS  22 , the second signal transmission line  237  located within the second UPS  23 , and the connector  24 , so that the operative power for the first load  27  and the second load  28  can be equally shared by the first UPS  22  and the second UPS  23  to achieve balance on load sharing. After the communication between the first UPS  22  and the second UPS  23  is completed, the first uninterruptible power supply  22  or the second uninterruptible power supply  23  can supply a portion of its output power to the other uninterruptible power supply by means of the power transmission line  29  according to the result of communication, and thus the uninterruptible power supply at the power receiving end can supply the sum of its output power plus the received output power to the load connected therewith. For example, the first load  27  requires an operative power of 3 kva and the second load  28  requires an operative power of 2 kva. If the communication between the first UPS  22  and the second UPS  23  through the first signal transmission line  227 , the second signal transmission line  237  and the connector  24  determines that the first UPS  22  and the second UPS  23  equally share the operative power for the first load  27  and the second load  28 , each of the first UPS  22  and the second UPS  23  provides an output power of (3 kva+2 kva)/2=2.5 kva. Because the second load  28  to which the second UPS  23  is connected requires an operative power of 2 kva, 2 kva out of the output power of the second UPS  23  is provided to the second load  28  and 0.5 kva out of the output power of the second UPS  23  is delivered to the first UPS  22  through the power transmission line  29 . Thus, the first UPS  22  can provide the sum of its output power plus the output power delivered from the second UPS  23  to the first load  27 , that is, 2.5 kva+0.5 kva=3 kva, as the operative power for the first load  27 .  
      In the present embodiment, the power transmission line  29  can be a hard wire for transmitting electric power or any hard wires with sufficient capacity, all of which are without departing the scope of the present invention.  
      Certainly, the parallel UPS system  20  according to a preferred embodiment of the present invention has a parallel redundant mode and a capacity upgrade mode, and their operating principles are described as follows:  
      1. Parallel Redundant Mode:  
      The principle of a parallel redundant mode operation is described as follows: If one of the uninterruptible power supplies  22  and  23  can not operate due to an aged battery or faulty electronic parts, the other uninterruptible power supply is required to increase its share of the operative power for the load and transmit a portion of its output power to the malfunctioned uninterruptible power supply through the power transmission line  29  to provide the operative power to the load instead of the malfunctioned uninterruptible power supply. On the other hand, the output power S 3  of the first UPS  22  is limited to the rated output capacity P 1  for the output end  228 , like a socket element of the first UPS  22 . Also, the output power S 4  of the second UPS  23  is limited to the rated output capacity P 2  for the output end  229 , like a socket element of the second UPS  23 . Therefore, in order to enable the first UPS  22  and the second UPS  23  to operate under a parallel redundant mode, the relationship between the output power S 3 , S 4  and the sum S 5  of the output power S 3  and the output power S 4  should satisfy the following condition:  
      S 3 ≦P 1 , S 4 ≦P 2  and S 5 ≦(P 1 +P 2 )/2  
      For example, both of the rated output capacities P 1 , P 2  are set to 5 kva. In order to comply with the requirements for parallel redundant operation, the output power S 3 , S 4  and the sum S 5  of the output power S 3  and the output power S 4  must satisfy the condition of S 3 ,S 4 ≦5 kva and S 5 ≦(5+5)/2 kva. As an example, the first load  27  to which the first UPS  22  is connected requires an operative power of 3 kva, and the second load  28  to which the second UPS  23  is connected requires an operative power of 2 kva. That is, the output power S 3  should be 3 kva and the output power S 4  should be 2 kva, and the total output power S 5  should be 2 kva+3 kva=5 kva.  
      When the first UPS  22  and the second UPS  23  are operating under a normal condition, the first UPS  22  and the second UPS  23  communicate with each other by means of the first signal transmission line  227 , the second first signal transmission line  237 , and the connector  24 . If the result of the communication indicate that the first UPS  22  and the second UPS  23  equally share the operative power for the first load  27  and the second load  28 , i.e. each of the first UPS  22  and the second UPS  23  is required to supply an output power of (3 kva+2 kva)/2=2.5 kva. Because the second load  28  to which the second uninterruptible power supply  23  is connected requires an operative power of 2 kva only, 2 kva out of the output power of the second UPS  23  is outputted to the second load  28  and 0.5 kva out of the output power of the second UPS  23  is delivered to the first UPS  22  through the power transmission line  29  to achieve balance on load sharing. Under this condition, the sum of the output power generated by the first UPS  22  and the output power delivered from the second UPS  23 , i.e. 2.5 kva+0.5 kva=3 kva, is supplied to the first load  27  as the operative power for first load  27 .  
      If one of the first UPS  22  and the second UPS  23  is malfunctioned, the other one is obliged to increase its share of operative power for the load and replace the malfunctioned uninterruptible power supply for supplying the operative power for the load. For example, if the second UPS  23  is malfunctioned and becomes inoperable, the first UPS  22  is obliged to increase its output power from 2.5 kva to 5 kva. That is, the total operative power for the first load  27  and the second load  28  is entirely supplied from the first UPS  22 , in which 3 kva out of the output power of the first UPS  22  is outputted to the first load  27  and 2 kva out of the output power of the first UPS  22  is delivered to the second UPS  23  through the power transmission line  29  for output to the second load  28 , and thereby accomplish a parallel redundant operation.  
      2. Capacity Upgrade Mode:  
      Under this mode, the first UPS  22  and the second UPS  23  do not possess the parallel redundant functionality as stated above. However, the total output power of the first UPS  22  and the second UPS  23  can be maximized to the sum of the rated output capacity P 1  of the first UPS  22  and the rated output capacity P 2  of the second UPS  23 , i.e. S 5 ≦(P 1 +P 2 ). In this manner, the output power of the parallel UPS system  20  can be upgraded significantly. Besides, the output power S 3  of the first UPS  22  and the output power S 4  of the second UPS  23  are respectively limited by the rated output capacity P 1 , P 2  for the voltage output terminal  228 ,  238 , like socket elements, and thus the output power S 3  and S 4  should satisfy the condition of S 3 ≦P 1 , S 4 ≦P 2 . For example, both of the rated output capacities P 1  and P 2  for the first voltage output terminal  228  and the second voltage output terminal  238  of the first UPS  22  and the second UPS  23  are 5 kva. In order to comply with the requirements for the capacity upgrade mode, the output power S 3  and the output power S 4  have to comply with the condition of S 3 , S 4 ≦5 kva. As an example, the first load  27  to which the first UPS  22  is connected requires an operative power of 5 kva and the second load  28  to which the second UPS  23  is connected requires an operative power of 3 kva. That is, the output power S 3  is 5 kva and the output power S 4  is 3 kva, and the total output power S 5  is 5 kva+3 kva=8 kva.  
      When the first UPS  22  and the second UPS  23  are operating normally, the first UPS  22  and the second UPS  23  can communicate with each other by means of the first signal transmission line  227 , the second signal transmission line  237 , and the connector  24 . If the result of communication determines that the uninterruptible power supplies equally share the operative power for the first load  27  and the second load  28 , each of the first uninterruptible power supply  22  and the second uninterruptible power supply  23  is set to provide an output power of (5 kva+3 kva)/2=4 kva. However, the second load  28  to which the second uninterruptible power supply  23  is connected requires an operative power of 3 kva only, so that 3 kva out of the output power of the second UPS  23  is outputted to the second load  28  and 1 kva out of the output power of the second UPS  23  is delivered to the first UPS  22 . The first uninterruptible power supply  22  can supply a total output power of its output power plus the output powered delivered from the second UPS  23  to the first load  27  for powering the first load  27 .  
      When one of the first UPS  22  and the second UPS  23  is malfunctioned, the first UPS  22  and the second UPS  23  do not possess the parallel redundant functionality under the capacity upgrade mode. Hence, the operative power required for powering the first load  27  is supplied from the power source  21  through a bypass route  229  located within the first UPS  22 , and the operative power required for operating the second load  28  is supplied from the power source through a bypass route  239  located within the second UPS  23 .  
      By connecting the first UPS  22  and the second UPS  23  with the power transmission line  29 , the parallel UPS system  20  can provide a parallel redundant mode or capacity upgrade mode to improve the power distribution for the load. Also, the distribution box can be further removed from the parallel UPS system  20  so as to reduce the manufacturing cost of the UPS system  20  and avoid the inconvenience of assembling the distribution box.  
       FIG. 6  is an elevation view of a parallel UPS system according to a preferred embodiment of the present invention. As shown in  FIG. 6 , the first UPS  22  and the second UPS  23  are placed inside a first housing  3  and a second housing  4 , respectively. The first power supply module  222 , the second power supply module  232 , the first battery module  224 , and the second battery module  234  are hot swappable modules. Therefore, the power modules and the battery modules of the uninterruptible power supplies are interchangeable and replaceable to reduce the cost of maintenance and repair.  
      In conclusion, the internal uninterruptible power supply of the inventive parallel uninterruptible power supply system can perform signal transmission directly by signal transmission lines, so that the control module of each uninterruptible power supply can receive data signal outputted from the power supply modules and battery modules of other uninterruptible power supplies. Therefore, the internal uninterruptible power supplies can communicate with each other without the need of an external communication port, and thus the manufacturing cost of the parallel uninterruptible power supply system can be reduced, so that the signal transmission rate of the parallel uninterruptible power supplies can be accelerated and the response time of the entire uninterruptible power supply system can be shortened.  
      While the present invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention need not be restricted to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the present invention which is defined by the appended claims.