Abstract:
A cell balancing circuit with a self-balancing function and a secondary battery with the cell balancing circuit, the cell balancing circuit includes a balancing unit provided for every two adjacent unit cells among the unit cells. The balancing unit includes a discharge unit and a voltage-dividing unit. The discharge unit sets a discharge path to discharge only the unit cell with the higher voltage among the two adjacent unit cells. The voltage-dividing unit uses the voltages of the two adjacent unit cells to provide an enable signal to the discharge unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0073741, filed Aug. 11, 2009, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Example embodiments relate to a cell balancing circuit and a secondary battery with the cell balancing circuit, and more particularly, to a cell balancing circuit with a self-balancing function and a secondary battery with the cell balancing circuit. 
     2. Description of the Related Art 
     Recently, the use of portable electronic devices is increasing with the rapid development of the electronic, communication and computer industries. Rechargeable secondary batteries are being widely used as power sources for portable electronic devices. 
     A secondary battery with a plurality of serially-connected unit cells is used for a high power source. Cell balancing is important when using a secondary battery with a plurality of serially-connected unit cells. The cell balancing is to maintain the voltage difference between the serially-connected unit cells to be within an allowable range. The cell balancing greatly influences the lifetime and output power of the secondary battery. If failing to be cell-balanced, the unit cell degrades, thus reducing the lifetime and output power of the secondary battery. 
     In a related art cell balancing method, a control IC compares the voltage of each unit cell with a reference voltage and discharges the unit cell exceeding the reference voltage, through a discharge path formed corresponding to each unit cell. 
     SUMMARY 
     Embodiments are directed to a cell balancing circuit and a secondary battery with the cell balancing circuit, which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art. 
     According to another embodiment of the present invention, there is provided a cell balancing circuit capable of performing a cell balancing operation even without using a control IC, and a secondary battery with the cell balancing circuit. 
     According to another embodiment of the present invention, there is provided a circuit for balancing a plurality of serially-connected unit cells, including: a balancing unit provided for every two adjacent unit cells among the unit cells, the balancing unit including: a discharge unit setting a discharge path to discharge only the unit cell with the higher voltage among the two adjacent unit cells; and a voltage-dividing unit using the voltages of the two adjacent unit cells to provide an enable signal to the discharge unit. 
     According to another embodiment of the present invention, the balancing unit may be configured to discharge only the unit cell with a voltage higher than the average voltage of the two adjacent unit cells, among the two adjacent unit cells. 
     According to another embodiment of the present invention, the discharge unit and the voltage-dividing unit may be connected in parallel to each other; the voltage-dividing unit may include two serially-connected resistors with the same resistance value; and the discharge unit may be connected to a node located between the two resistors of the voltage-dividing unit. 
     According to another embodiment of the present invention, the discharge unit of the balancing unit may include a switching unit having two switching elements connected in series to receive the enable signal of the voltage-dividing unit, and a node located between the two switching elements may be electrically connected to a node located between the two adjacent unit cells. 
     According to another embodiment of the present invention, one of the two switching elements may be a P-channel field effect transistor (FET), and the other may be an N-channel FET. Herein, the sources of the two switching elements may be connected to each other; the drain of the P-channel FET may be connected to the negative electrodes of the two adjacent unit cells; and the drain of the N-channel FET may be connected to the positive electrodes of the two adjacent unit cells. Also, the enable signal of the voltage-dividing unit may be transferred to the gate of the P-channel FET and the gate of the N-channel FET. 
     According to another embodiment of the present invention, the discharge unit may include the two or more switching units connected in parallel to each other. 
     According to another embodiment of the present invention, the balancing unit may further include a power-consuming resistor connected electrically to both ends of the discharge unit. 
     According to another embodiment of the present invention, at least one of the above and other features and advantages may be realized by providing a secondary battery with the above cell balancing circuit. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments with reference to the attached drawings, in which: 
         FIG. 1  is a circuit diagram of a secondary battery with a cell balancing circuit according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a circuit diagram of a secondary battery with a cell balancing circuit according to an embodiment. Referring to  FIG. 1 , a secondary battery  100  includes a cell module  200  and a cell balancing circuit  300 . 
     The cell module  200  includes a first unit cell  210 , a second unit cell  220  and a third unit cell  230  that are serially connected to store electrical energy and provide the same to an external device. Each of the unit cells  210 ,  220  and  230  may be a bare cell without a protective circuit. The first unit cell  210  has a positive electrode  211  and a negative electrode  212 . The second unit cell  220  has a positive electrode  221  and a negative electrode  222  connected to the positive electrode  211  of the first unit cell  210 . The third unit cell  230  has a positive electrode  231  and a negative electrode  232  connected to the positive electrode  221  of the second unit cell  220 . Although it has been described in this embodiment that the cell module  200  includes three serially-connected unit cells, the inventive concept is not limited thereto. The cell module  200  may include four or more serially-connected unit cells. 
     The cell balancing circuit  300  includes a first balancing unit  400  and a second balancing unit  500 . The cell balancing circuit  300  performs a cell balancing operation on the cell module  200 . Each of the balancing units  400  and  500  is provided for every two adjacent unit cells. If the cell module  200  includes four or more N serially-connected unit cells, (N−1) balancing units are provided for every two adjacent unit cells. 
     The first balancing unit  400  includes a first discharge unit  410 , a first voltage-dividing unit  450 , a first power-consuming resistor  460   a , and a second power-consuming resistor  460   b . The first balancing unit  400  performs a cell balancing operation on the first unit cell  210  and the second unit cell  220 . 
     The first discharge unit  410  includes a first switching unit  420 , a second switching unit  430  and a third switching unit  440  that are connected in parallel to each other. Both ends of the first discharge unit  410  are electrically connected to the negative electrode  212  of the first unit cell  210  and the positive electrode  221  of the second unit cell  220  to establish a discharge current path. 
     The first switching unit  420  includes a first switching element  421  and a second switching element  422  that are connected in series to each other. A node  423  located between the first switching element  421  and the second switching element  422  is electrically connected to a node  213  located between the first unit cell  210  and the second unit cell  220 , thereby forming a discharge path  214  of the first unit cell  210  and a discharge path  224  of the second unit cell  220 . The discharge path  214  of the first unit cell  210  is a path from the positive electrode  211  of the first unit cell  210 , through the node  213  located between the unit cells  210  and  220  and the node  423  located between the switching elements  421  and  422 , to the negative electrode  212  of the first unit cell  210 . The first switching element  421  is located on the discharge path  214  of the first unit cell  210 . The discharge path  224  of the second unit cell  220  is a path from the positive electrode  221  of the second unit cell  220 , through the node  423  located between the switching elements  421  and  422  and the node  213  located between the unit cells  210  and  220 , to the negative electrode  222  of the second unit cell  220 . The second switching element  422  is located on the discharge path  224  of the second unit cell  220 . When there is a voltage difference between the first unit cell  210  and the second unit cell  220 , the first switching unit  420  opens the discharge path of only the unit cell having the higher voltage among the unit cells  210  and  220 . 
     The first switching element  421  may be a P-channel field effect transistor (FET) to open/close the discharge path  214  of the first unit cell  210 . A source S of the first switching element  421  is electrically connected to the node  213  located between the unit cells  210  and  220 . Accordingly, the voltage of the first unit cell  210  is applied to the source S of the first switching element  421 . A gate G of the first switching element  421  is electrically connected to the first voltage-dividing unit  450 . Accordingly, the average voltage of the unit cells  210  and  220  is applied to the gate G of the first switching element  421 . A drain D of the first switching element  421  is electrically connected to the negative electrode  212  of the first unit cell  210 . Since the first switching element  421  is a P-channel FET, only when the voltage of the first unit cell  210  is higher than the average voltage of the unit cells  210  and  220  (i.e., when the voltage applied to the gate G of the first switching element  421  is lower than the voltage applied to the source S of the first switching element  421 ), the first switching element  421  is turned on to discharge the first unit cell  210 . 
     The second switching element  422  may be an N-channel FET to open/close the discharge path  224  of the second unit cell  220 . A source S of the second switching element  422  is electrically connected to the node  213  located between the unit cells  210  and  220 . Accordingly, the voltage of the first unit cell  210  is applied to the source S of the second switching element  422 . A gate G of the second switching element  422  is electrically connected to the first voltage-dividing unit  450 . Accordingly, the average voltage of the unit cells  210  and  220  is applied to the gate G of the second switching element  422 . A drain D of the second switching element  422  is electrically connected to the positive electrode  221  of the second unit cell  220 . Since the second switching element  422  is an N-channel FET, only when the voltage of the second unit cell  220  is higher than the average voltage of the unit cells  210  and  220  (i.e., when the voltage applied to the gate G of the second switching element  422  is higher than the voltage applied to the source S of the second switching element  422 ), the second switching element  422  is turned on to discharge the second unit cell  220 . 
     The second switching unit  430  and the third switching unit  440  have the same configuration as the first switching unit  420 , and thus a detailed description thereof will be omitted for conciseness. Although it has been described in this embodiment that the first balancing unit  400  includes three switching units  420 ,  430  and  440 , the inventive concept is not limited thereto. The first balancing unit  400  may include one, two, or four or more switching units. As the number of switching units increases, the number of discharge current paths increases, thus increasing a balancing current to reduce a cell balancing time. 
     The first voltage-dividing unit  450  is electrically connected to the first unit cell  210  and the second unit cell  220 , and is connected in parallel to the both ends of the first discharge unit  410 . The first voltage-dividing unit  450  includes a first resistor  451  and a second resistor  452  that are serially connected and have the same resistance value. A node  453  located between the resistors  451  and  452  is electrically connected to the gate G of each of the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442  of the first discharge unit  410 . Since the resistors  451  and  452  have the same resistance value, the average voltage of the first unit cell  210  and the second unit cell  220  is applied to the gate G of each of the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442  of the first discharge unit  410 . The resistors  451  and  452  may have a large resistance value of about several MO. This is to prevent the unit cells  210  and  220  from being discharged through a connection path of the resistors  451  and  452 . 
     The first power-consuming resistor  460   a  and the second power-consuming resistor  460   b  are electrically connected respectively to the both ends of the first discharge unit  410 . The first power-consuming resistor  460   a  is electrically connected to the negative electrode  212  of the first unit cell  210 . The first power-consuming resistor  460   a  serves to increase the power consumption by the discharge current of the first unit cell  210 , thereby reducing a balancing time. The second power-consuming resistor  460   b  is electrically connected to the positive electrode  221  of the second unit cell  220 . The second power-consuming resistor  460   b  serves to increase the power consumption by the discharge current of the second unit cell  220 , thereby reducing a balancing time. 
     The second balancing unit  500  includes a second discharge unit  510 , a second voltage-dividing unit  550 , a third power-consuming resistor  560   a , and a fourth power-consuming resistor  560   b . The second balancing unit  500  performs a cell balancing operation on the second unit cell  220  and the third unit cell  230 . 
     The second discharge unit  510  includes a fourth switching unit  520 , a fifth switching unit  530  and a sixth switching unit  540  that are connected in parallel to each other. Both ends of the second discharge unit  510  are electrically connected to the negative electrode  222  of the second unit cell  220  and the positive electrode  231  of the third unit cell  230  to establish a discharge current path. 
     The fourth switching unit  520  includes a third switching element  521  and a fourth switching element  522  that are connected in series to each other. A node  523  located between the third switching element  521  and the fourth switching element  522  is electrically connected to a node  223  located between the second unit cell  220  and the third unit cell  230 , thereby forming a discharge path  225  of the second unit cell  220  and a discharge path  234  of the third unit cell  230 . The discharge path  225  of the second unit cell  220  is a path from the positive electrode  221  of the second unit cell  220 , through the node  223  located between the unit cells  220  and  230  and the node  523  located between the switching elements  521  and  522 , to the negative electrode  222  of the second unit cell  220 . The third switching element  521  is located on the discharge path  225  of the second unit cell  220 . The discharge path  234  of the third unit cell  230  is a path from the positive electrode  231  of the third unit cell  230 , through the node  523  located between the switching elements  521  and  522  and the node  223  located between the unit cells  220  and  230 , to the negative electrode  232  of the third unit cell  230 . The fourth switching element  522  is located on the discharge path  234  of the third unit cell  230 . When there is a voltage difference between the second unit cell  220  and the third unit cell  230 , the fourth switching unit  520  opens the discharge path of only the unit cell having the higher voltage among the unit cells  220  and  230 . 
     The third switching element  521  may be a P-channel FET to open/close the discharge path  225  of the second unit cell  220 . A source S of the third switching element  521  is electrically connected to the node  223  located between the unit cells  220  and  230 . Accordingly, the voltage of the second unit cell  220  is applied to the source S of the third switching element  521 . A gate G of the third switching element  521  is electrically connected to the second voltage-dividing unit  550 . Accordingly, the average voltage of the unit cells  220  and  230  is applied to the gate G of the third switching element  521 . A drain D of the third switching element  521  is electrically connected to the negative electrode  222  of the second unit cell  220 . Since the third switching element  521  is a P-channel FET, only when the voltage of the second unit cell  220  is higher than the average voltage of the unit cells  220  and  230  (i.e., when the voltage applied to the gate G of the third switching element  521  is lower than the voltage applied to the source S of the third switching element  521 ), the third switching element  521  is turned on to discharge the second unit cell  220 . 
     The fourth switching element  522  may be an N-channel FET to open/close the discharge path  234  of the third unit cell  230 . A source S of the fourth switching element  522  is electrically connected to the node  223  located between the unit cells  220  and  230 . Accordingly, the voltage of the second unit cell  220  is applied to the source S of the fourth switching element  522 . A gate G of the fourth switching element  522  is electrically connected to the second voltage-dividing unit  550 . Accordingly, the average voltage of the unit cells  220  and  230  is applied to the gate G of the fourth switching element  522 . A drain D of the fourth switching element  522  is electrically connected to the positive electrode  231  of the third unit cell  230 . Since the fourth switching element  522  is an N-channel FET, only when the voltage of the third unit cell  230  is higher than the average voltage of the unit cells  220  and  230  (i.e., when the voltage applied to the gate G of the fourth switching element  522  is higher than the voltage applied to the source S of the fourth switching element  522 ), the fourth switching element  522  is turned on to discharge the third unit cell  230 . 
     The fifth switching unit  530  and the sixth switching unit  540  have the same configuration as the fourth switching unit  520 , and thus a detailed description thereof will be omitted for conciseness. Although it has been described in this embodiment that the second balancing unit  500  includes three switching units  520 ,  530  and  540 , the inventive concept is not limited thereto. The second balancing unit  500  may include one, two, or four or more switching units. As the number of switching units increases, the number of discharge current paths increases, thus increasing a balancing current to reduce a cell balancing time. 
     The second voltage-dividing unit  550  is electrically connected to the second unit cell  220  and the third unit cell  230 , and is connected in parallel to the both ends of the second discharge unit  510 . The second voltage-dividing unit  550  includes a third resistor  551  and a fourth resistor  552  that are serially connected and have the same resistance value. A node  553  located between the resistors  551  and  552  is electrically connected to the gate G of each of the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542  of the second discharge unit  510 . Since the resistors  551  and  552  have the same resistance value, the average voltage of the second unit cell  220  and the third unit cell  230  is applied to the gate G of each of the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542  of the second discharge unit  510 . The resistors  551  and  552  may have a large resistance value of about several MO. This is to prevent the unit cells  220  and  230  from being discharged through a connection path of the resistors  551  and  552 . 
     The third power-consuming resistor  560   a  and the fourth power-consuming resistor  560   b  are electrically connected respectively to both ends of the second discharge unit  510 . The third power-consuming resistor  560   a  is electrically connected to the negative electrode  222  of the second unit cell  220 . The third power-consuming resistor  560   a  serves to increase the power consumption by the discharge current of the second unit cell  220 , thereby reducing a balancing time. The fourth power-consuming resistor  560   b  is electrically connected to the positive electrode  231  of the third unit cell  230 . The fourth power-consuming resistor  560   b  serves to increase the power consumption by the discharge current of the third unit cell  230 , thereby reducing a balancing time. 
     Hereinafter, a cell balancing process according to the above embodiment will be described in detail with reference to  FIG. 1 . 
     First, a description will be given of a cell balancing operation when the voltage V 1  of the first unit cell  210  becomes higher than the voltage V 2  of the second unit cell  220 . The voltage V 1  of the first unit cell  210  is applied to the source S of each of the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442  of the first balancing unit  400 . Also, the average voltage ((V 1 +V 2 )/2) of the cells  210  and  220  is applied to the gate G of each of the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442  of the first balancing unit  400 . The voltage applied to the gate G of the second switching elements  422 ,  432  and  442  of the first balancing unit  400  becomes lower than the voltage applied to the source S of the second switching elements  422 ,  432  and  442 . Accordingly, the second switching elements  422 ,  432  and  442  of the first balancing unit  400  are turned off to interrupt the discharge of the second unit cell  220 . The voltage applied to the gate G of the first switching elements  421 ,  431  and  441  of the first balancing unit  400  becomes lower than the voltage applied to the source S of the first switching elements  421 ,  431  and  441 . Accordingly, the first switching elements  421 ,  431  and  441  of the first balancing unit  400  are turned on to discharge the first unit cell  210  until the voltage V 1  of the first unit cell  210  becomes equal to the voltage V 2  of the second unit cell  220 . When the voltage V 1  of the first unit cell  210  becomes equal to the voltage V 2  of the second unit cell  220 , the voltage applied to the source S of each of the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442  of the first balancing unit  400  becomes equal to the voltage applied to the gate G of each of the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442 . Accordingly, the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442  are all turned off to interrupt the cell balancing operation in the first balancing unit  400 . 
     A description will now be given of a cell balancing operation when the voltage V 1  of the first unit cell  210  becomes lower than the voltage V 2  of the second unit cell  220 . In this case, the voltage applied to the gate G of the first switching elements  421 ,  431  and  441  of the first balancing unit  400  becomes higher than the voltage applied to the source S of the first switching elements  421 ,  431  and  441 . Accordingly, the first switching elements  421 ,  431  and  441  of the first balancing unit  400  are turned off to interrupt the discharge of the first unit cell  210 . The voltage applied to the gate G of the second switching elements  422 ,  432  and  442  of The first balancing unit  400  becomes higher than the voltage applied to the source S of the second switching elements  422 ,  432  and  442 . Accordingly, the second switching elements  422 ,  432  and  442  of the first balancing unit  400  are turned on to discharge the second unit cell  220  until the voltage V 2  of the second unit cell  220  becomes equal to the voltage V 1  of the first unit cell  210 . When the voltage V 2  of the second unit cell  220  becomes equal to the voltage V 1  of the first unit cell  210 , the voltage applied to the source S of each of the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442  of the first balancing unit  400  becomes equal to the voltage applied to the gate G of each of the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442 . Accordingly, the switching elements  421 ,  422 ,  431 ,  432 ,  441  and  442  are all turned off to interrupt the cell balancing operation in the first balancing unit  400 . 
     Through the above process, the voltage V 1  of the first unit cell  210  becomes equal to the voltage V 2  of the second unit cell  220 . Hereinafter, a description will be given of a cell balancing operation between the second unit cell  220  and the third unit cell  230 . 
     First, a description will be given of a cell balancing operation when the voltage V 2  of the second unit cell  220  becomes higher than the voltage V 3  of the third unit cell  230 . The voltage V 2  of the second unit cell  220  is applied to the source S of each of the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542  of the second balancing unit  500 . Also, the average voltage ((V 1 +V 2 )/2) of the cells  220  and  230  is applied to the gate G of each of the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542  of the second balancing unit  500 . The voltage applied to the gate G of the fourth switching elements  522 ,  532  and  542  of the second balancing unit  500  becomes lower than the voltage applied to the source S of the fourth switching elements  522 ,  532  and  542 . Accordingly, the fourth switching elements  522 ,  532  and  542  of the second balancing unit  500  are turned off to interrupt the discharge of the third unit cell  230 . The voltage applied to the gate G of the third switching elements  521 ,  531  and  541  of the second balancing unit  500  becomes lower than the voltage applied to the source S of the third switching elements  521 ,  531  and  541 . Accordingly, the third switching elements  521 ,  531  and  541  of the second balancing unit  500  are turned on to discharge the second unit cell  220  until the voltage V 2  of the second unit cell  220  becomes equal to the voltage V 3  of the third unit cell  230 . When the voltage V 2  of the second unit cell  220  becomes equal to the voltage V 3  of the third unit cell  230 , the voltage applied to the source S of each of the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542  of the second balancing unit  500  becomes equal to the voltage applied to the gate G of each of the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542 . Accordingly, the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542  are all turned off to interrupt the cell balancing operation in the second balancing unit  500 . 
     A description will now be given of a cell balancing operation when the voltage V 2  of the second unit cell  220  becomes lower than the voltage V 3  of the third unit cell  230 . In this case, the voltage applied to the gate G of the third switching elements  521 ,  531  and  541  of the second balancing unit  500  becomes higher than the voltage applied to the source S of the third switching elements  521 ,  531  and  541 . Accordingly, the third switching elements  521 ,  531  and  541  of the second balancing unit  500  are turned off to interrupt the discharge of the second unit cell  220 . The voltage applied to the gate G of the fourth switching elements  522 ,  532  and  542  of the second balancing unit  500  becomes higher than the voltage applied to the source S of the fourth switching elements  522 ,  532  and  542 . Accordingly, the fourth switching elements  522 ,  532  and  542  of the second balancing unit  500  are turned on to discharge the third unit cell  230  until the voltage V 3  of the third unit cell  230  becomes equal to the voltage V 2  of the second unit cell  220 . When the voltage V 3  of the third unit cell  230  becomes equal to the voltage V 2  of the second unit cell  220 , the voltage applied to the source S of each of the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542  of the second balancing unit  500  becomes equal to the voltage applied to the gate G of each of the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542 . Accordingly, the switching elements  521 ,  522 ,  531 ,  532 ,  541  and  542  are all turned off to interrupt the cell balancing operation in the second balancing unit  500 . 
     The above cell balancing operation in each of the balancing units  400  and  500  repeats until the voltages V 1 , V 2  and V 3  of the unit cells  210 ,  220  and  230  become equal to each other. Although the cell balancing operation on three serially-connected unit cells  210 ,  220  and  230  has been described in the above embodiment, those skilled in the art will understand that a cell balancing operation on four or more serially-connected unit cells may be performed in the same manner, which is also included in the scope of the inventive concept. 
     According to the embodiments described above, cell balancing is possible even without using a control IC, thus making it possible to reduce the fabrication cost of a secondary battery. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.