Abstract:
A converter-regulator of a D.C. voltage into a D.C. voltage intended to connect a fuel cell to a filter capable of being connected to means of electrochemical storage of electric power in a charge operation of the storage means. The converter-regulator includes means capable of maintaining, during the charge operation, the voltage across the fuel cell at a given working voltage.

Description:
PRIORITY CLAIM 
       [0001]    This is a continuation-in-part application which claims priority from International Application No. PCT/FR2006/050726, published in French, filed Jul. 18, 2006, based on French patent Application No. 05/52226, filed Jul. 18, 2005, which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    Embodiments of the present invention relate to a converter-regulator of a D.C. voltage into a D.C. voltage, or D.C./D.C. converter regulator, used for the charge of a battery, for example, of a cellular phone battery, via a fuel cell. 
       BACKGROUND 
       [0003]    In the following description, “battery” will be used to designate an assembly of accumulators coupled to act simultaneously, an accumulator being an electrolytic element which is charged by a D.C. current and which can then discharge, that is, give back, in the form of a D.C. current of reverse direction, part of the power built up in chemical form. There exist different types of batteries, including nickel-cadmium batteries, nickel-metal-hydride batteries, lead batteries, and lithium batteries. The electronic components of a cellular phone are generally supplied by a battery capable of being charged several times. 
         [0004]    The charge of the battery of a portable phone may be performed at constant current with a minimum charge voltage or at constant voltage with a limited current according to the battery type. In a charge operation, the battery is generally connected to a generator providing an adapted charge voltage and charge current. The generator may include a converter of an A.C. voltage into a D.C. voltage receiving the A.C. voltage. It may also include a cell-supplied converter of a D.C. voltage into a D.C. voltage. 
         [0005]    A fuel cell is a system for providing electric power in which the electricity is obtained by oxidation on an electrode of the cell of a reductant fuel coupled with the reduction on the other electrode of an oxidant, such as oxygen. The fuel may be hydrogen or methanol which is turned into hydrogen for the oxidation reaction. A fuel cell has the advantage of not being polluting since it only rejects water. The fuel of the fuel cell may be stored in a tank feeding the fuel cell. The performances and dimensions of currently-available fuel cells enable considering their use for the charge of a battery, especially of a cellular phone battery. 
         [0006]      FIG. 1  shows an example of a curve  5  of variation of the voltage VFC across a fuel cell according to the current IFC provided by the fuel cell. Voltage VFC decreases from a maximum voltage VFCmax when no load is connected to the fuel cell down to a zero voltage for which the fuel cell provides a maximum current IFCmax. As an example, for a fuel cell likely to be used to supply a cellular phone battery, maximum voltage VFCmax may be on the order of 8 V and maximum current IFCmax may be on the order of from 400 to 500 mA.  FIG. 1  also shows a curve  6  of variation of power PFC provided by the fuel cell. Curve  6  has a bell shape exhibiting a maximum for a given voltage VFC and current IFC. 
         [0007]    To use a fuel cell for the charge of a battery, especially of a cellular phone battery, it is necessary to take into account the following constraints: the power provided by the fuel cell must be sufficiently high for the battery charge not to be excessively long, and the fuel cell efficiency must be high enough to avoid excessive consumption of the fuel of the fuel cell, which would translate as the impossibility to perform several successive charge operations without feeding again the fuel tank of the fuel cell. 
         [0008]    Such constraints result in the inability to directly connect a fuel cell to a battery. Indeed, the battery would require provision of a high current by the fuel cell. An overconsumption of fuel by the fuel cell may then result, thus requiring frequent change of the fuel cell tank. 
       SUMMARY 
       [0009]    An embodiment of the present invention is a D.C./D.C. converter-regulator enabling use of a fuel cell to charge a battery, for example, a cellular phone battery. 
         [0010]    According to another embodiment of the present invention, the regulator-converter has a high efficiency throughout an entire charge operation. 
         [0011]    According to another embodiment of the present invention, the converter-regulator has a simple structure. 
         [0012]    Another embodiment of the present invention is a method for converting the voltage provided by a fuel cell for the charge of a battery. 
         [0013]    For this purpose, one embodiment of the present invention provides a converter-regulator of a D.C. voltage into a D.C. voltage intended to connect a fuel cell to a filter capable of being connected to means of electrochemical storage of electric power in a charge operation of the storage means. The converter-regulator includes means capable of maintaining, during the charge operation, the voltage across the fuel cell at a given working voltage. 
         [0014]    According to an embodiment of the present invention, the converter-regulator includes means for providing an error signal representative of the difference between the voltage across the fuel cell and the given working voltage; and a voltage step-down or step-up circuit which drives the filter with an average voltage corresponding to the voltage across the fuel cell multiplied by a factor which depends on the error signal, whereby, when the voltage across the fuel cell is greater than the given working voltage, the current provided to the battery is increased and that, when the voltage across the fuel cell is lower than the given working voltage, the current provided to the battery is decreased. 
         [0015]    According to an embodiment of the present invention, the converter-regulator includes means for setting the given working voltage. 
         [0016]    According to an embodiment of the present invention, the converter-regulator includes a capacitor connected across the fuel cell. 
         [0017]    According to an embodiment of the present invention, the voltage step-down or step-up circuit is a chopper circuit controlled by a cyclic rectangular signal having a duty cycle which depends on the error signal. 
         [0018]    Another embodiment of the present invention provides a power supply system, intended to be connected to means of electrochemical storage of electric power in a charge operation of the storage means. The power supply system includes a fuel cell; a filter intended to be connected to the storage means during the charge operation; and a converter-regulator such as defined previously connecting the fuel cell to the filter. 
         [0019]    According to an embodiment of the present invention, the filter includes an inductance intended to be series-connected to the storage means. 
         [0020]    A further embodiment of the present invention provides an electronic system, especially a cellular phone, comprising means of electrochemical storage of electric power and a system for supplying said storage means such as previously defined. 
         [0021]    A still further embodiment of the present invention provides a method for converting the voltage across a fuel cell into a supply voltage of a filter connected to means of electrochemical storage of electric power, in a charge operation of the storage means, comprising the maintaining, during the charge operation, of the voltage across the fuel cell at a given working voltage. 
         [0022]    According to an embodiment of the present invention, the method includes the steps of providing an error signal representative of the difference between the voltage across the fuel cell and the given working voltage; and providing the filter with an average voltage corresponding to the voltage across the fuel cell multiplied by a factor which depends on the error signal, whereby, when the voltage across the fuel cell is greater than the given working voltage, the current provided to the battery is increased and, when the voltage across the fuel cell is smaller than the given working voltage, the current provided to the battery is decreased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The foregoing features and advantages of the present invention, as well as others, will be discussed in detail in the following non-limiting description of specific embodiments thereof in connection with the accompanying drawings. 
           [0024]      FIG. 1 , previously described, shows the variation of the voltage across a fuel cell and of the power provided by the fuel cell versus the current provided by the fuel cell; 
           [0025]      FIG. 2  schematically shows a cellular phone connected to a fuel cell via a converter-regulator according to an embodiment of the present invention; 
           [0026]      FIG. 3  schematically shows an example of a converter-regulator according to an embodiment of the present invention; 
           [0027]      FIG. 4  shows a more detailed embodiment of the converter-regulator of  FIG. 3 ; 
           [0028]      FIG. 5  shows the variation of characteristic voltages of the converter-regulator of  FIG. 4  in operation; 
           [0029]      FIG. 6  shows for the converter-regulator of  FIG. 4  the variation of the voltage across the fuel cell, of the voltage across the battery, and of the current provided to the battery during a battery charge operation; and 
           [0030]      FIG. 7  shows the efficiency variation of a converter-regulator according to an embodiment of the present invention according to the current provided by the fuel cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. For clarity, same elements have generally been designated with same reference numerals in the different drawings. 
         [0032]      FIG. 2  schematically shows a cellular phone  10  including a battery  11  connected to a charge control unit  12 . Battery  11  is, for example, a battery of lithium-ion type. The charge of battery  11  is performed via an electric power source  13  including a fuel cell  14  using, for the provision of electric power, fuel stored in a tank  15 . The cell may, for example, be a hydrogen or methanol fuel cell. Fuel cell  14  is connected to cellular phone  10  via a converter-regulator  16  and a filter  17 . Charge control unit  12  is capable of detecting a connection between telephone  10  and power source  13  to trigger a charge operation of battery  11 , for example, by detecting that a current greater than a determined current is supplied to battery  11 . Charge control unit  12  is also adapted to detecting whether battery  11  is sufficiently charged to interrupt the charge operation. 
         [0033]    During a charge operation, the operation of the fuel cell  14  is at a determined operating point, that is, at a determined couple of values (VFCopt, IFCopt) of voltage VFC and of current IFC. Such an operating point is called the optimum operating point and enables fast charge of the battery while avoiding too high a fuel consumption by the fuel cell. More specifically, embodiments of the present invention include maintaining of voltage VFC across fuel cell  14  at the voltage of optimum operating point VFCopt of fuel cell  14  in a charge operation. Thereby, fuel cell  14  provides a substantially constant operating current IFCopt to enable performing a charge at constant current. 
         [0034]      FIG. 3  schematically shows an example of the converter-regulator  16  according to an embodiment of the present invention. Converter-regulator  16  includes an error amplifier  22  which compares voltage VFC across fuel cell  14  with a reference voltage VREF provided by a reference voltage generator  26 . Error amplifier  22  provides an error voltage VERROR, representative of the difference between voltages VFC and VREF, to a PWM pulse-width modulator  28 . Modulator  28  provides a pulse-width modulated square voltage VPWM to a regulation unit  30 , which may correspond to a voltage step-down circuit or to a voltage step-up circuit. Unit  30  provides a voltage VL to filter  17  which drives battery  11  with a charge current IBAT. Charge control unit  12  is not shown in  FIG. 3 . 
         [0035]      FIG. 4  shows a more detailed example of embodiment of converter-regulator  16  of  FIG. 3 . Fuel cell  14  is shown as a constant voltage generator  34 , series-assembled with a resistor  36 , representing the internal resistance of fuel cell  14 . Fuel cell  14  is connected between a source of a reference voltage  38 , generally the ground, and a node F. To avoid any excessive load of fuel cell  14 , converter-regulator  16  includes a capacitor  40  connected between node F and the ground. 
         [0036]    Error amplifier  22  includes an operational amplifier  42  having its inverting input (−) connected to the output of a generator  43  of a constant voltage VCOMP via a resistor  44 . Further, the inverting input (−) is connected to the output of amplifier  42  via a capacitor  46 . The non-inverting input (+) of amplifier  42  is connected to node F via a resistor  48 . A variable resistor  49  is provided between the non-inverting input (+) and the ground. 
         [0037]    Pulse-width modulator  28  includes an oscillator  50  providing a triangular voltage VOSC of constant frequency and an operational amplifier  51  having its non-inverting input (+) receiving error voltage VERROR and having its inverting input (−) receiving triangular voltage VOSC. Amplifier  51  is assembled as a comparator and provides a rectangular voltage VPWM. In the present example embodiment, voltage VFCopt of the optimum operating point of fuel cell  14  is on the order of 5 V, which corresponds to the provision of a current IFCopt on the order of from 200 to 300 mA, and battery  11  is a lithium-ion battery having a capacity on the order of from 600 to 800 mA·h (that is, from 2,160 coulombs to 2,880 coulombs). Regulation unit  30  then corresponds to a voltage step-down circuit which includes a control unit  52  receiving voltage VPWM and which provides two control voltages S 1  and S 2 . Regulation unit  30  includes a P-type MOS transistor  54 , having its source connected to node F and its drain connected to an intermediary node O, and an N-type MOS transistor  56  having its drain connected to node O and having its source connected to ground. The gate of transistor  54  is controlled by voltage S 1  and the gate of transistor  56  is controlled by voltage S 2 . Filter  17  includes an inductance  58  connected between node O and an output terminal OUT of power source  13  and a capacitor  59  connected between output terminal OUT and the ground. The battery is shown as a capacitor  11  connected between output terminal OUT and the ground, the grounds of cellular phone  10  and of power source  13  being put in common on connection of cellular phone  10  to power source  13 . 
         [0038]    The supply of the components of error amplifier  22  and of pulse-width modulator  28  is performed via a stabilized voltage source, not shown, receiving, for example, voltage VFC. 
         [0039]      FIG. 5  shows the variation of characteristic voltages of converter-regulator  16  during operation. Error amplifier  22  performs an operation of amplification of the difference between voltage VFC and a reference voltage and a filtering operation. The reference voltage may be adjusted by modifying the value of variable resistor  49 . In the present example embodiment, error amplifier  22  corresponds to an assembly of subtractor-integrator type. Voltage VERROR is equal to the sum of a constant voltage VERROR 0 , or bias voltage, and of a variable voltage verror. The expression of variable voltage verror in the Laplace plane is the following: 
         [0000]    
       
         
           
             
               
                 
                   
                     v 
                     error 
                   
                   = 
                   
                     
                       
                         V 
                         FC 
                       
                       · 
                       
                         
                           R 
                           49 
                         
                         
                           
                             R 
                             49 
                           
                           + 
                           
                             R 
                             48 
                           
                         
                       
                       · 
                       
                         
                           
                             A 
                             42 
                           
                            
                           
                             ( 
                             
                               1 
                               + 
                               
                                 
                                   R 
                                   44 
                                 
                                  
                                 
                                   C 
                                   46 
                                 
                                  
                                 p 
                               
                             
                             ) 
                           
                         
                         
                           1 
                           + 
                           
                             
                               ( 
                               
                                 1 
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                                   A 
                                   42 
                                 
                               
                               ) 
                             
                              
                             
                               R 
                               44 
                             
                              
                             
                               C 
                               46 
                             
                              
                             p 
                           
                         
                       
                     
                     - 
                     
                       
                         V 
                         COMP 
                       
                        
                       
                         
                           A 
                           42 
                         
                         
                           1 
                           + 
                           
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   A 
                                   42 
                                 
                               
                               ) 
                             
                              
                             
                               R 
                               44 
                             
                              
                             
                               C 
                               46 
                             
                              
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                   ( 
                   1 
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         [0000]    where A 42  is the open loop gain of operational amplifier  42 , R 44 , R 48 , and R 49  are the respective values of resistors  44 ,  48 , and  49 , and C 46  is the capacitance of capacitor  46 . 
         [0040]    Gain A 42  being very large as compared with unity, equation (1) may be simplified as: 
         [0000]    
       
         
           
             
               
                 
                   
                     v 
                     error 
                   
                   = 
                   
                     
                       
                         V 
                         FC 
                       
                       · 
                       
                         
                           R 
                           49 
                         
                         
                           
                             R 
                             49 
                           
                           + 
                           
                             R 
                             48 
                           
                         
                       
                       · 
                       
                         
                           1 
                           + 
                           
                             
                               R 
                               44 
                             
                              
                             
                               C 
                               46 
                             
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                             44 
                           
                            
                           
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                             46 
                           
                            
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                     - 
                     
                       
                         V 
                         COMP 
                       
                        
                       
                         1 
                         
                           
                             R 
                             44 
                           
                            
                           
                             C 
                             46 
                           
                            
                           p 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0041]    At low frequencies, equation (2) becomes: 
         [0000]    
       
         
           
             
               
                 
                   
                     v 
                     error 
                   
                   = 
                   
                     
                       1 
                       
                         
                           R 
                           44 
                         
                          
                         
                           C 
                           46 
                         
                          
                         p 
                       
                     
                      
                     
                       ( 
                       
                         
                           
                             V 
                             FC 
                           
                           · 
                           
                             
                               R 
                               49 
                             
                             
                               
                                 R 
                                 49 
                               
                               + 
                               
                                 R 
                                 48 
                               
                             
                           
                         
                         - 
                         
                           V 
                           COMP 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0042]    The control of converter-regulator  16  tending to cancel variable voltage verror, voltage VFCopt towards which voltage VFC tends is thus given by the following relation: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     FCopt 
                   
                   = 
                   
                     
                       V 
                       COMP 
                     
                      
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             R 
                             48 
                           
                           
                             R 
                             49 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0043]    Voltage VPWM is obtained from the comparison between voltages VERROR and VOSC, shown to be superposed in  FIG. 5 . Voltage VPWM is a cyclic rectangular voltage having a duty cycle α equal to the ratio between time T 1  for which voltage VPWM is in a high state during a cycle and duration T 2  of a cycle. Duty cycle α depends on the value of voltage VERROR. Control voltages S 1  and S 2  are rectangular voltages obtained from voltage VPWM. When voltage S 1  is low, transistor  54  is on and when voltage S 1  is high, transistor  54  is off. When voltage S 2  is high, transistor  56  is on and when voltage S 2  is low, transistor  56  is off. Control voltages S 1  and S 2  are defined so that the rising and falling edges of voltages S 1  and S 2  are not simultaneous to avoid for transistors  54  and  56  to be simultaneously partially conductive. In the present example embodiment, voltage S 1  substantially corresponds to the inverse of voltage VPWM, voltage S 1  being however, for each cycle, in the low state for a time slightly shorter than T 1 , and voltage S 2  substantially corresponds to the inverse of voltage VPWM, voltage S 2  being, however, for each cycle, low for a time slightly longer than T 1 . 
         [0044]    When voltages S 1  and S 2  are low, transistor  54  is on and transistor  56  is off. Node O is then directly connected to node F and voltage VL is equal to voltage VFC decreased by the source-drain voltage of transistor  54 . The intensity of the current flowing through inductance  58  then tends to increase. When voltages S 1  and S 2  are high, transistor  54  is off and transistor  56  is on. Node O is then grounded. Voltage VL is substantially equal to the drain-source voltage of transistor  56  and the intensity of the current flowing through inductance  58  tends to decrease. The average of voltage VL is thus substantially equal to αVFC and the average of the current flowing through inductance  58  depends on duty cycle α and corresponds to the supply of a current IFC by fuel cell  14  which also depends on duty cycle α. Current IFC required by inductance  58  imposes the voltage across fuel cell  14 , that is, voltage VFC at node F. 
         [0045]    In steady state, voltage VFC is equal to voltage VFCopt of the optimum operating point of fuel cell  14  so that error voltage VERROR is equal to bias voltage VERROR 0 . The voltage VERROR 0  corresponds to a steady-state voltage VPWM having a determined duty cycle α 0 . As an example, voltage VERROR 0  can be selected so that duty cycle α 0  is equal to 0.5. In this case, bias voltage VERROR 0  is equal to half the sum of the maximum and minimum voltages provided by oscillator  50 . 
         [0046]    If voltage VFC is greater than VFCopt, a voltage VERROR greater than VERROR 0  is obtained. Voltage VPWM then has a duty cycle α greater than α 0 . An increase in the average time for which transistor  54  is on, and thus an increase in the average current flowing through inductance  58 , that is, an increase in the current IFC provided by fuel cell  14 , are thus obtained. This results in a decrease in voltage VFC. Conversely, if voltage VFC is smaller than VFCopt, error voltage VERROR is smaller than VERROR 0 . Voltage VPWM then has a duty cycle α smaller than α 0 . A decrease in the average time for which transistor  54  is on, and thus a decrease in the average current flowing through inductance  58 , that is, a decrease in current IFC provided by fuel cell  14 , are thus obtained. This results in an increase in voltage VFC. 
         [0047]      FIG. 6  illustrates the steps of a complete charge operation of battery  11  by fuel cell  14 . 
         [0048]    At step I, cellular phone  10  is not connected to output terminal OUT of power source  13 . Current IBAT provided to output terminal OUT is thus zero. Battery  11  is discharged and voltage VBAT is equal to a minimum voltage VBATmin. Further, fuel cell  14  is deactivated, fuel tank  15  being, for example, disconnected from fuel cell  14 . Voltage VFC is thus zero. 
         [0049]    At step II, fuel cell  14  is activated, battery  11  being still unconnected to output terminal OUT. This is obtained, for example, by supplying fuel cell  14  with fuel. Fuel cell  14  then reaches a steady operation state, which translates as an increase in voltage VFC up to a voltage VFCmax of no charge. 
         [0050]    At step III, battery  11  is connected to terminal OUT. Converter-regulator  16  then operates to maintain voltage VFC across fuel cell  14  at VFCopt, causing the provision of a substantially constant current IBAT to battery  11  and an increase in voltage VBAT. 
         [0051]    At step IV, battery  11  is considered as being charged. Such a detection of the charge state of battery  11  may be performed by charge control unit  12 . Battery  11  is then electrically disconnected from terminal OUT by charge control unit  12 , cellular phone  10  remaining mechanically connected to electric power source  13 . Converter-regulator  16  then no longer regulates voltage VFC, which rises back up to voltage VFCmax, while current IBAT becomes zero. Voltage VBAT decreases as battery  11  supplies the loads of cellular phone  10  to which it is connected. 
         [0052]    At step V, cellular phone  10  is disconnected from terminal OUT. At step VI, fuel cell  14  is deactivated, for example, by cutting off the fuel supply of fuel cell  14 . 
         [0053]      FIG. 7  shows two curves  60 ,  62  of variation of the efficiency of converter-regulator  16  according to an embodiment of the present invention according to the current IFC provided by fuel cell  14 . Curve  60  corresponds to a 3.6-V battery voltage VBAT which corresponds to an example of average voltage across battery  11  during a charge, and curve  62  corresponds to a 2.7-V battery voltage VBAT which corresponds to an example of the voltage across battery  11  at the beginning of a charge. The efficiency corresponds to the ratio between the power supplied to battery  11  and the power supplied by fuel cell  14  (that is, the sum of the power supplied to battery  11  and of losses). According to an embodiment of the present invention, the current provided to the battery being substantially constant and within a well-defined range, for example, from 150 mA to 290 mA, the efficiency of converter-regulator  16  is greater than 85% all along the charge. 
         [0054]    In the previously-described example embodiment, a regulation unit  30  corresponding to a voltage step-down circuit has been considered. However, if the optimum working voltage VFCopt of fuel cell  14  is smaller than the average voltage driving filter  17 , regulation unit  30  corresponds to a voltage step-up circuit, for example, controlled similarly to what has been previously described for the control of step-down circuit  30 . 
         [0055]    In the previously-described example, it has been considered that for a given VFC voltage, current IFC provided by fuel cell  14  is substantially constant. In practice, with a constant VFC, current IFC tends to slightly decrease along time. 
         [0056]    According to another embodiment of the present invention, electric power source  13  may be directly provided at the level of cellular phone  10  and permanently mechanically connected to battery  11 . A charge operation of battery  11  is then performed as described previously by the activation of fuel cell  14  of electric power source  13 . 
         [0057]    Of course, the present invention and embodiments thereof are likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, for example, the filtering operation performed by error amplifier  22  in the above-described embodiments may be more complex than what has been previously described. 
         [0058]    Embodiments of the present invention may be contained in a variety of different types of electronic devices and systems, such as cellular telephones, computer systems, portable devices such as personal digital assistants (PDAs) and MP3 players, and so on. 
         [0059]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.