This invention was made with United States government support awarded by the following agency: ARMY Grant No. DAAH04-94-G-0328. The United States has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to braking systems for electric vehicles or hybrid electric vehicles, and in particular to braking systems which comprise a regenerative braking system in conjunction with a conventional, mechanical friction braking system. As the term is used herein, “electric” vehicles are those which are propelled solely by an electric motor which receives energy from an on-board storage battery; “hybrid electric” vehicles are those which may be propelled, generally at different times, by either an electric motor, which receives energy from either an on-board storage battery or from a generator operatively coupled to an internal combustion engine (a “series hybrid electric vehicle”), or by an internal combustion engine selectively operatively coupled to the ground-engaging wheels (a “parallel hybrid electric vehicle”). Thus, while both electric and hybrid electric vehicles comprise a rechargeable on-board storage battery, electric vehicles are propelled by an electric motor alone, whereas hybrid electric vehicles are propelled by an electric motor and also comprise an internal combustion engine which is also used for propulsion or, alternatively, for generating electrical power via a generator to recharge the battery. 
     Generally, electrical power is connected to the motor only when driving propulsion is demanded by the operator (e.g., by pressing on the accelerator or “gas” pedal). At other times (during coasting or braking) the power feeding the motor is disconnected. The inertia of the moving vehicle, however, continues to rotate the rotor of the motor, which is coupled to a ground-engaging wheel of the vehicle. Regenerative braking systems use the rotating motor as a generator which works against an electrical load placed in communication with the motor/generator upon actuation of a vehicle braking control device, such as a brake pedal. The electrical load comprises the partially-depleted battery, which is at least partially recharged by the motor acting as a generator powered by the still-moving vehicle&#39;s inertia. The electrical load on the motor/generator slows the rotational speed thereof, thereby braking the vehicle. 
     Previous electric or hybrid electric vehicles have employed hybrid brake systems comprising both an electrical, regenerative brake system and a conventional, mechanical friction braking system. Generally, such vehicles couple the two brake systems so that they work together, with the regenerative braking system being first actuated upon initial depression of the brake pedal to slow the vehicle and provide a charge to the battery. Further depression of the brake pedal then additionally actuates, or completely switches to, the conventional mechanical braking system for stopping the vehicle. A problem encountered with prior hybrid brake systems, however, is that the transition from only regenerative braking to mechanical braking, alone or in combination with regenerative braking, has been abrupt, resulting in poor brake pedal “feel” and impairing the driveability the vehicle. 
     For example, some prior vehicles employing hybrid braking systems used only regenerative braking until the brake pedal was depressed a considerable distance toward the floorboard. Drivers operating these vehicles felt very little pedal resistance and encountered sluggish transition from regenerative braking to conventional braking because there was little or no pressure in the brake lines. Drivers of these vehicles overcorrected by stomping on the brakes, abruptly applying the conventional mechanical brake system, thereby causing erratic handling of the vehicle. 
     A means for providing a smooth transition between regenerative and conventional braking in an electric or hybrid electric vehicle is desired. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the shortcomings of prior hybrid braking systems in electric or hybrid electric vehicles by providing a smooth transition between regenerative and conventional friction braking. 
     The present invention provides a hybrid brake system for a vehicle propelled at least by a rotating electric motor powered by a storage battery and in communication with at least one ground-engaging wheel, and includes an electrical brake system comprising the electric motor and the battery, and a vehicle braking control device. The battery provides an electrical load on the motor during times when the vehicle braking control device is actuated, the rotation of the motor during actuation of the vehicle braking control device providing electrical power to the battery, whereby the battery receives an electrical charge. The motor rotation slowed by the electrical load and the vehicle is thus braked by the motor. The hybrid brake system of the present invention also includes a mechanical brake system comprising a hydraulic cylinder with a piston sealably and slidably disposed therein. One side of the piston and the hydraulic cylinder partially define a chamber of variable volume, the pressure of the fluid therein varying with movement of the piston in the cylinder. A mechanical brake arrangement is in fluid communication with the chamber and is operatively coupled to at least one ground-engaging wheel for slowing the rotation thereof. The mechanical brake arrangement is variably actuated in response to changes in the pressure of the fluid during times when the vehicle braking control device is actuated, whereby the vehicle is braked by the mechanical brake system. The inventive hybrid brake system further includes means for deferring substantial actuation of the mechanical brake arrangement during actuation of the vehicle braking control device until after the electrical brake system has been actuated. The deferring means includes means for expanding the chamber volume in response to an increase in pressure of the fluid in the chamber. 
     In certain embodiments of the present invention, the above-described means for deferring substantial actuation of the mechanical brake arrangement includes partially defining the chamber with a displaceable wall which moves between a first position and second position. The wall is biased into the first position and is urged toward the second position by an increase in the pressure of the fluid in the chamber below a threshold pressure. Substantial application of the mechanical brake system during actuation of the vehicle brake control device is deferred until the threshold pressure is reached. 
     The present invention also provides a method for braking a vehicle propelled at least by a rotating electric motor powered by a battery, which includes the steps of: actuating a vehicle braking control device; placing the motor in mechanical communication with a rotating ground-engaging wheel and an electrical load comprising the battery; generating electrical energy with the motor; absorbing at least a portion of the electrical energy generated by the motor with the battery, thereby slowing the rotation of the motor, whereby the motor slows the vehicle; increasing the pressure of a fluid in a chamber between a first pressure and a threshold pressure during actuation of the vehicle braking control device, during which time the motor is slowing the vehicle; delaying application of a substantial fluid pressure to a mechanical brake arrangement in fluid communication with the chamber and in operative communication with a rotating ground-engaging wheel during actuation of the vehicle braking control device, until after the pressure of the fluid in the chamber has been increased to at least the threshold pressure; and applying a substantial fluid pressure above the threshold pressure to the mechanical brake arrangement during actuation of the vehicle braking control device, during which time the rotation of the ground-engaging wheel is slowed, whereby the mechanical brake arrangement further slows the vehicle. 
     The herein-described hybrid brake system and method has proven to be simple, and the results provided thereby closely approximate the feel of conventional braking alone. The present design can be implemented into production with little additional cost to the product and has no additional maintenance requirements. The present design is a low cost solution which can be retrofitted into existing electric or hybrid electric vehicles, or easily integrated into new models. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a plan view schematic of a hybrid electric vehicle including the hybrid brake system of the present invention; 
     FIG. 2 is a perspective schematic view of the conventional braking system of the vehicle of FIG. 1; 
     FIG. 3 is a partial sectional side view of a conventional master cylinder and its fluid reservoir, to which the present invention may be adapted; 
     FIG. 4 is a sectional side view of the master cylinder and reservoir of FIG. 3, modified with an embodiment of the present invention attached thereto; 
     FIG. 5A is a sectional side view of the master cylinder of FIG. 4 with the adapted components of the present invention assembled thereto; 
     FIG. 5B is a sectional side view of the master cylinder of FIG. 4 with adapted alternative components of the present invention assembled thereinto; 
     FIG. 6 is an exploded sectional side view of the assembly of FIG. 5A; 
     FIG. 7 is a graph of master cylinder piston position versus brake line pressure for an embodiment of the present invention; 
     FIG. 8A is a sectional side view of the modified master cylinder of FIG. 4 showing the pistons thereof in a first position; 
     FIG. 8B is a the modified master cylinder of FIG. 8A in a successive second position; 
     FIG. 8C is a the modified master cylinder of FIG. 8A in a successive third position; 
     FIG. 8D is a the modified master cylinder of FIG. 8A in a successive fourth position; 
     FIG. 9 is a side view of a brake pedal and anti-lock braking system (ABS) switch of a vehicle to which the present invention may be adapted; 
     FIG. 10 is a side view of the brake pedal and ABS switch of FIG. 9, with the rotary rheostat of one embodiment of the present invention adapted thereto; 
     FIG. 11 is a flowchart describing the logic for applying the hybrid brake system of the present invention; and 
     FIG. 12 is a graph of the conventional hydraulic brake system and the regenerative braking system separately showing there respective relationships between brake pedal travel and percent of the respective brake system delivered or applied. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplification set out herein illustrates embodiments of the invention, in several forms, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 there is shown a plan view schematic of a hybrid electric vehicle which employs one embodiment of a hybrid brake system according to the present invention. Although the depicted embodiment is shown adapted to one type of hybrid electric vehicle, it is be understood that the inventive hybrid brake system may be adapted to other types of hybrid electric vehicles or electric vehicles. 
     Vehicle  20  comprises internal combustion engine  22 , which in the depicted example is a diesel engine. Engine  22  is operatively coupled by known means to transaxle  24 , from which extend axle shafts  26  drivingly coupled to ground-engaging front wheels  28  and  30 . Vehicle  20  further comprises ground-engaging rear wheels  32  and  34 . 
     Electric motor  36 , which may be, for example, an AC induction motor, is mechanically coupled to engine  22  and transaxle  24 . When electrically powered, motor  36  is used for propulsion, driving transaxle  24  and thus wheels  28 ,  30 . Motor  36  is electrically powered through DC-to-AC inverter  38 , which is electrically connected via cables  40  to battery  42  disposed at the rear of vehicle  20 , behind fuel tank  44 . Tank  44  provides a source of fuel for engine  22 . In the depicted embodiment, battery  42  is a 7.5 Ampere-hour, 240 VDC (1.8 kW-hour) Nickel Cadmium battery. 
     Vehicle  20  is also provided with computer  46  which controls application of the inventive hybrid brake system&#39;s regenerative braking portion. Computer  46  may also be used to control other aspects of the vehicle&#39;s operation beyond the scope of the present invention. Indeed, it is envisioned that computer  46  may be integrated into a single controller (not shown) for the entire vehicle. As shown by the dashed lines in FIG. 1, computer  46  communicates with driver interface  48 , rheostat  50 , which is in mechanical communication with brake pedal assembly  52  and comprises part of the vehicle braking control device, battery  42 , engine  22 , transaxle  24  and inverter  38 . 
     In the conventional manner, brake pedal assembly  52  is in mechanical communication with hydraulic brake master cylinder  54 , which is of the ordinary type, but modified in accordance with the present invention. In the present embodiment, as will be described further hereinbelow, hybrid brake cylinder  56  is in fluid communication with master cylinder  54 . As usual in conventional mechanical braking systems, extending from master cylinder  54  is a plurality of brake lines  58 ,  60 ,  62  and  64  which extend from a fluid chamber within master cylinder  54  to mechanical brake devices  66 ,  68 ,  70  and  72  at each wheel  28 ,  30 ,  32  and  34 , respectively. Mechanical brake devices  66 ,  68 ,  70  and  72 , which together comprise a mechanical brake arrangement, are of conventional friction type such as disc brakes or drum brakes, and are hydraulically linked to master cylinder  54  through brake lines  58 ,  60 ,  62  and  64 . In the conventional manner, as the pressure of the hydraulic fluid within the master cylinder increases, each of the mechanical brake devices is increasingly actuated, and frictionally brakes its associated, ground-engaging wheel. FIG. 2 best shows the hydraulic communication of the mechanical brake arrangement with master cylinder  54 . Also shown in FIG. 2 is hybrid brake cylinder  56  attached to master cylinder  54 . 
     Referring now to FIG. 3, there is shown master cylinder  54 ′ which is entirely conventional in design and well known in the art. Master cylinder  54 ′ is provided with a supply of hydraulic brake fluid (not shown) from reservoir  74  disposed atop the master cylinder through ports  75 . Ports  75  are sealed against external leakage with o-rings (not shown). The fluid from reservoir  74  is provided to fluid chamber  76 ′ disposed within master cylinder  54 ′ and which is partially defined by cylinder  77  and piston  83  slidably and sealably disposed therein. In the well know manner, piston  82  is mechanically linked to brake pedal assembly  52  and moves within cylinder  77  in response to depression of the pedal. Generally, primary piston  82  provides fluid pressure to one pair of the four brake lines while secondary piston  83  provides fluid pressure to the other pair of brake lines. The pressure of the fluid in space  79  between pistons  82  and  83  is, for the most part, influenced by the pressure in chamber  76 ′. Secondary piston  83  is primarily in fluid communication with primary piston  82  and moves therewith by means of fluid pressure therebetween. Piston  83  thus moves in response to depression of the brake pedal. As piston  83  is moved with depression of the brake pedal, the volume of chamber  76 ′ is reduced and the pressure of the fluid therein, and in the brake lines communicating therewith, increases. The mechanical brake arrangement of the vehicle is actuated in response to the changing hydraulic pressure level within the master cylinder. The brake lines are connected to the master cylinder by conventional means such as fittings  78  and  80 . 
     Referring now to FIG. 4 there is shown ordinary master cylinder  54 ′ of FIG. 3 adapted with hybrid cylinder  56  of the present invention. End wall  84  of modified master cylinder  54  is provided with threaded hole  86  into which fitting  88  of hybrid cylinder  56  is received. Fitting  88  has passageway  89  extending from its terminal end into the interior of hybrid cylinder  56 , thereby placing master cylinder  54  and hybrid cylinder  56  in fluid communication. 
     Referring now to FIGS. 5A and 6, there is shown a sectional view of the assembly of master cylinder  54  and hybrid cylinder  56 . Pistons  82 ,  83  are not shown in FIGS. 5A and 6; nor is reservoir  74 . Hybrid cylinder  56  comprises housing  90  within which are provided small diameter cylinder  92  and coaxially adjacent larger diameter cylinder  94 . Small diameter portion  98  of piston  96  is slidably and sealably disposed within small diameter cylinder  92 , large diameter portion  100  of piston  96  slidably disposed in larger diameter cylinder  94 . In the circumferential surface of small diameter piston portion  98  is O-ring groove  102  within which is disposed O-ring seal  104  which seals the circumferential surface of small diameter piston portion  98  against the wall of cylinder  92 . 
     Opposite axial sides of large diameter portion  100  of piston  96  provide first and second annular shoulders  106 ,  108 , respectively. First shoulder  106  abuts annular shoulder  110  formed at the junction of cylinders  92  and  94  when piston  96  is in a first position. Piston  96  is biased into its first position by spring  112 , the end of which abuts second annular surface  108 . The opposite end of spring  112  abuts first annular surface  114  of stop member  116 . On the opposite axial side of stop member  116  is second annular surface  118  which abuts snap ring  120  disposed in snap ring groove  122  provided in the surface of cylinder  94 , near its open end, thereby retaining piston  96  and spring  112  within housing  90 . 
     Stop member  116  is provided with central threaded hole  124  within which is received the threaded portion of stop bolt  126 . The exteriorly-located end of stop bolt  126  is provided with knurled knob  128  for adjusting the advancement of stop bolt  126  within threaded hole  124 . The interface of cylinder  94  and stop member  116  may be provided with means (not shown) for preventing rotation of stop member  116 . The adjustment of bolt  126  establishes the second position which piston  96  will achieve when its axial end surface  132  abuts interiorly-located terminal end  130  of stop bolt  126 . Because piston  96  is moveable between its first and second positions, axial end surface  134  of piston  96 , in effect, serves as a displaceable wall of chamber  76 . Chamber  76  is thus contractible and expandible and is defined by fluidly connected cylinders  77  and  92 , the end surface of piston  83  and axial end surface  134  of piston  96 . As will be further discussed below, the volume of chamber  76  decreases as pistons  82 ,  83  move with depression of the brake pedal, and the pressure of the fluid in chamber  76  increases in response thereto, moving piston  96  away from its first position towards its second position against the biasing force of spring  112 . The rate of contraction of the volume of space  76  as piston  96  moves between its first and second positions is substantially reduced vis-a-vis a corresponding movement of piston  83  in ordinary master cylinder  54 ′, and no substantial actuation of the mechanical brake arrangement occurs until surface  132  of piston  96  abuts terminal end  130  of stop bolt  126 . Once the second position of piston  96  is achieved, the mechanical brake system operates in the normal fashion. 
     The movement of piston  96  is dictated by a simple force balance equation: 
     
       
           P   fluid   ×A   134   =K   spring   ×d   96   (equation 1)  
       
     
     wherein P fluid  is the hydraulic pressure (pounds per square inch) of the fluid in chamber  76  and the brake lines leading therefrom to the mechanical brake arrangement; A 134  is the area (square inches) of axial piston surface  134 , against which the fluid acts; K spring  is the spring rate (pounds per inch) of spring  112 ; and d 96  is the distance (inches) piston  96  has moved from its first position. 
     Referring now to FIG. 7, there is shown a graphical comparison of the relationship between master cylinder piston position (i.e., the position of piston  83 ) and brake line fluid pressure for an ordinary or normal master cylinder (e.g., master cylinder  54 ′ of FIG. 3) and for a master cylinder modified in accordance with the present invention (e.g., master cylinder  54  of FIG.  4 ). 
     Referring first to a normal master cylinder, line  136  extends between points  138  and  140  as piston  83  moves from its rest position (point  138 ) to the position where ports  75  of the master cylinder are covered (point  140 ). Along line  136 , changes in brake line pressure are negligible, and no appreciable mechanical braking is effected. Once ports  75  are covered, however, at point  140 , the fluid pressure increases substantially, following line  142  to the maximum, terminal brake line pressure at point  144 . As shown, line  142  is substantially linear, although it may be instead be somewhat curvilinear. 
     Referring now to a master cylinder modified in accordance with the present invention, again piston  83  moves from its rest position (point  138 ) along line  136  until ports  75  are covered (point  140 ). Following line  145 , which extends between points  140  and  146 , changes in brake line pressure are insubstantial, and no appreciable mechanical braking is effected, as in the case of the normal master cylinder. At point  140 , ports  75  are covered and piston  96  is in its first position. Proceeding along line  145 , piston  96  is moving in response to the slight increase in fluid pressure in chamber  76  towards its second position, which is achieved at point  146 . Here, the slight increase in fluid pressure brings the brake pads or shoes of the mechanical brake arrangement into light contact with their respective discs or drums, but no substantial mechanical braking is effected. During the portion of master cylinder piston travel along lines  136  and  145 , only regenerative braking is occurring, as will be discussed further hereinbelow. Between points  146  and  148 , along line  149 , the fluid pressure in chamber  76  and the brake lines increases substantially and the brake arrangement is actuated as along line  142 . In the modified master cylinder case, terminal brake line pressure (point  148 ) is the same as the pressure at point  144 . As shown, lines  145  and  149  are substantially linear, although it may instead be somewhat curvilinear. For example, hybrid cylinder  56  may comprise, instead of spring  112 , a nitrogen-filled cylinder  151  (FIG.  5 B). Alternatively, the liquid in chamber  76  may compress a nitrogen-filled bladder, as in a hydraulic accumulator. As piston  96  compresses the nitrogen gas, a non-linear response curve would result. Further, lines  142  and  149  are shown to be approximately parallel, although it is envisioned that there may slight differences in the rate of fluid pressure increases with piston travel between the normal and modified master cylinders. It should be noted that although regenerative braking is occurring along lines  136  and  145 , in certain embodiments of the present invention regenerative braking also continues along line  149 , during frictional braking, as discussed further hereinbelow. 
     Referring to FIGS. 8A-8D, there is shown a succession of views of master cylinder  54  and hybrid cylinder  56  during brake operation, illustrating how the volume of chamber  76 , and thus the fluid pressure acting on the mechanical brake arrangement, varies with travel of master cylinder piston  83 . FIG. 8A illustrates the rest position, corresponding to point  138  of FIG. 7, wherein piston  96  is at its first position. FIG. 8B illustrates a position in which ports  75  have just been closed by master cylinder pistons  82 ,  83 , corresponding to point  140  of FIG. 7; piston  96  is still at its first position. 
     FIG. 8C illustrates a position at which piston  96  has been moved from its first position towards its second position, against the force of spring  112 . This position corresponds to a point between points  140  and  146 , along line  145  of FIG.  7 . The force necessary to overcome spring  112  accounts for the positive slope of line  145 , but this increase in pressure is insufficient to provide any substantial mechanical braking effect, and while piston  96  is between its first and second positions only the regenerative braking system acts to brake the vehicle. 
     FIG. 8D illustrates a position at which piston  96  is in its second position, its axial surface  132  abutting terminal end  130  of stop bolt  126 . This position corresponds to a point between points  146  and  148 , along line  149  of FIG.  7 . Once piston  96  has reached its second position, the mechanical braking system is operational, and substantial hydraulic pressure is provided to the mechanical brake arrangement. As mentioned above, continued regenerative braking may also be applied during mechanical braking 
     Referring now to FIG. 9, there is shown a portion of an ordinary vehicle braking control device for one embodiment of an electric or hybrid electric vehicle to which the present invention may be adapted. Brake pedal assembly  52 , mechanically linked to master cylinder piston  82 , pivots about pivot pin  150  and includes pin  152  which is attached to linearly-moving arm  154  of anti-lock braking system (ABS) switch  156 . Switch  156  is electrically connected to an anti-lock braking control module (not shown) through wire harness  158 , and operates in a conventional, well known way. It is not necessary for a vehicle adapted with the inventive hybrid brake system to include an ABS braking system or any sort of switch attached to the brake pedal as shown. The ABS switch of the depicted embodiment merely provides a structure which is convenient for adaptation of the below-described rheostat by which the electrical load on the motor/generator may be varied. Indeed, although convenient, it is not necessary for actuation of the electrical load of the regenerative brake system to be directly linked to the brake pedal assembly at all. Alternatively, for example, a pressure transducer may be provided in fluid communication with chamber  76 , the transducer providing a variable voltage in response to fluid pressure changes therein which is communicated to computer  46 . Computer  46  would then vary the load on motor/generator  36  during regenerative braking in a manner similar to that described below. 
     FIG. 10 shows the brake pedal assembly of FIG. 9 with rheostat  50  attached to the outer surface of ABS switch  156 . Rheostat  50  is of a rotary-type, providing varying amounts of resistance to a current flowing therethrough as its crankarm  158  is rotated about its center  160 . Link  162  is pivotally attached to the end of crankarm  158  and to linearly-movable arm  154  of ABS switch  156 . As arm  154  is moved linearly toward or into switch  156  in response to depression of the brake pedal, link  162  causes rotation of crankarm  158  about center  160  of rheostat  50 . By this means, a current i 1 , flowing through wire  164  to rheostat  50  is variably reduced to a lower current level i 2  which flows through wire  166  to computer  46 . Those skilled in the art will recognize that instead of rotary rheostat  50 , a linear type may be used. Computer  46 , as will be discussed further below, interprets the difference in current values i 1  and i 2  to vary the load on motor/generator  36  through control adjustments at inverter  38  and/or battery  42 . Those skilled in the art will appreciate that computer  46  may be adapted to sense voltage values in lieu of current values, the logic of the computer as described below accordingly modified. 
     Referring now to FIG. 11, there is shown the basic control logic employed by computer  46  in varying the amount of electrical load on motor/generator  36  in response to changes in i 2  during regenerative braking. As discussed above, the inertial energy of the vehicle is converted into electrical energy by motor/generator  36 , which is operatively coupled to ground-engaging wheels  28 ,  30 . The electrical load on the motor/generator comprises battery  42  which, if not already fully charged, is at least partially recharged by the motor/generator during regenerative braking. As the load is continued or increased, the rotational speed of the motor is lowered, thereby braking the vehicle. Those skilled in the art will appreciate that where a very low resistance level is provided by rheostat  50 , only a very small part of the electrical energy generated by motor/generator  36  is absorbed by the load. As the resistance level of rheostat  50  increases, a greater demand on the electrical energy generated by motor/generator  36  is effected, more heavily loading motor/generator  36  and causing its rotation to slow down, whereby the vehicle is increasingly braked by the motor with increased depression of the brake pedal. 
     The logic of FIG. 11 first inquires (symbol  168 ) as to whether the brake switch is on. The brake switch referred to here is ABS switch  156 , although other means may be employed for determining whether the brakes have been activated. For example, a signal may be provided to the computer from a tail lamp brake lamp switch which will provide the necessary yes or no answer to the inquiry at symbol  168 . If the brake switch is not on, the logic continues to loop through symbol  168  until the brakes are activated. Once the brake switch is on, indicating that the brakes have been actuated, an inquiry is made (symbol  170 ) as to whether the battery currently has a full electrical charge. If so, the logic loops back to and through symbol  168 . If the battery is not fully charged, the logic proceeds to assess input value i 2  from the brake sensor (symbol  172 ). In the present embodiment, the brake sensor is rheostat  50 . The input brake sensor value is then compared with a maximum brake sensor value, or i 1  (symbol  174 ). Based on the proportion of i 1  represented by i 2 , a percentage value of the maximum regenerative braking level, or electrical load to be applied to the motor/generator, is then obtained (symbol  176 ). An electric motor controller (not shown), which may be an integral part of computer  46  or inverter  38 , adjusts the load on motor/generator  36  to the appropriate level. The electrical load placed on the motor/generator inversely correlates to the value of i 2 /i 1 . That is, for i 2 /i 1 =1, where rheostat  50  provides no resistance, there is no load placed on motor/generator  36 . For values of i 2 /i 1  which are extremely small, the maximum electrical load is placed on the motor/generator. Thus, with increased depression of the brake pedal, the electrical load on the motor/generator is proportionally increased, thereby providing smooth and predictable application of the regenerative braking system. 
     Referring now to FIG. 12, the relationships between brake pedal travel (in inches) and the percent of the individual regenerative and mechanical brake systems applied are shown for the embodiment of vehicle  20 . Line  178  shows the relationship between brake pedal travel and the percentage of the maximum regenerative braking system effect applied through motor/generator  36  in response to the varying resistance level provided by rheostat  50 . Line  180  shows the relationship between brake pedal travel and the percentage of the maximum mechanical braking system effect applied through master cylinder  54  and hybrid cylinder  56  in response to the movements of piston  83  within cylinder  77  and piston  96  within cylinders  92 ,  94 . Referring to the lefthand side of FIG. 12, point A represents the point where brake pedal assembly  52  is at rest in its non-braking position (i.e., zero pedal travel). In this position, which corresponds to point  138  of FIG. 7, the vehicle braking control device is not actuated. 
     As the brake pedal is depressed ⅛ inch from point A to point B, regenerative braking begins at point  182  (FIG. 12) along line  178 . Although line  178  is shown as being linear, those skilled in the art will appreciate that regenerative braking need not be applied in such a fashion. Point  182  represents the situation where current i 2  divided by current i 1  yields a value substantially equal to 1, and no load is placed on motor/generator  36 . Here, virtually no electrical energy being generated by motor/generator  36  is being provided to battery  42 . As the brake pedal further travels from point B to point C, where the brake pedal has traveled ¼ inch from its rest position, ports  75  communicating fluid reservoir  74  and chamber  76  are closed. Here, piston  96  is still at its first position. At point  186  (FIG.  12 ), which corresponds to point  140  of FIG. 7, only nominal and insubstantial loading of the mechanical brake arrangement begins. Meanwhile, regenerative braking continues along line  178 . 
     As the brake pedal further travels from point C toward point D, the fluid within chamber  76  is being compressed at a very slow rate, for spring  112  is being compressed by the movement of piston  96 . Those skilled in the art will appreciate that the amount of fluid pressure rise between the first and second positions of piston  96  may be refined by changing the spring rate (K) of spring  112 . Between points C and D, no substantial frictional braking is effected, for the pressure of the fluid in chamber  76  is still too low. Meanwhile, regenerative braking continues along line  178 . 
     At point D, the brake pedal has traveled ¾ inch from its rest position, and the pads or shoes of the mechanical brake arrangement are in light contact with their respective discs or drums. Point  188  of FIG. 12 corresponds to point  146  of FIG.  7 . At point  188  a threshold pressure in chamber  76  is reached, above which substantial braking is effected by the mechanical braking system, and frictional braking begins. 
     Point  190 , at which regenerative brake line  178  reaches its maximum value, also occurs at position D. Hereafter, the regenerative braking system maintains 100% of its effect. That is to say, although arm  154  of brake switch  156  may travel linearly further, and crankarm  158  of rheostat  150  may rotate further, no further increase in the electrical load provided on motor/generator  36  will be attained. In the present embodiment of the invention, once position D has been reached, the regenerative braking system maintains 100% of its effect during operation of the mechanical braking system. Those skilled in the art will appreciate, however, that embodiments of the present invention may provide further increases in regenerative braking between point D and the end of brake pedal travel, at point E. 
     Returning to the present example, however, with the regenerative braking system reaching its 100% delivered effect at position D and maintaining that level through further brake pedal travel, piston  96  has, at position D, achieved its second position, with its face  132  abutting terminal end  130  of stop screw  126 . At this point, piston  96  plays no further role in delaying the rise in pressure of the fluid in chamber  76 . As shown in FIG. 12, with further brake pedal travel from point D to point E, curve  180  progresses from point  188  to point  192 , applying increasing levels of mechanical friction braking while regenerative braking is maintained at 100%. Point  192  corresponds to point  148  of FIG. 7, at which terminal brake line pressure is reached. 
     To provide a smooth transition between regenerative and mechanical braking, it should be understood that, with reference to FIG. 12, 100% of regenerative braking should be approximately equivalent in braking force to the braking force provided along line  180  near point  188 . Adjustments to the regenerative braking load and the rate of fluid pressure rise in chamber  76  may be fine-tuned to provide a smooth transition between regenerative and mechanical braking to suit a wide variety of vehicle characteristics. 
     The electrical load on the motor/generator may, for the most part, be adjusted through software changed in computer  46 . As mentioned above, the rate of pressure increase in chamber  76  may be adjusted by changing spring  112  to one with a different spring rate or by changing the location of the second position of piston  96  by adjusting stop bolt  126 . Once an appropriate design has been developed, stop member  116  and stop bolt  126  may be replaced with a stop member (not shown) which provides an annular surface similar to surface  114  against which spring  112  abuts, and a central stop surface against which surface  132  of piston  96  abuts in its second position. 
     Further, it is envisioned that a master cylinder assembly according to the present invention need not be comprised of assembled parts such as master cylinder  54  and hybrid adapted cylinder  56 . Rather, it will be appreciated by those skilled in the art that hybrid cylinder  56  and master cylinder  45 , although in hydraulic communication, may be remotely located from each other, or that a single master cylinder assembly (not shown) which essentially comprises the structure provided by the assembled components (FIG. 5) may be easily manufactured and will deliver the desired results which the depicted embodiment provides. 
     While this invention has been described as having an exemplary design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
A hybrid brake system for a vehicle propelled at least by a rotating electric motor powered by a storage battery and in communication with at least one ground-engaging wheel. The hybrid brake system includes a vehicle braking control device and an electrical brake system including the electric motor and the battery, the battery providing an electrical load on the motor during times when the vehicle braking control device is actuated. Rotation of the motor during actuation of the vehicle braking control device provides electrical power to the battery, whereby the battery receives an electrical charge. The motor rotation is slowed by the electrical load, whereby the vehicle is braked by the motor. The hybrid brake system also includes a mechanical brake system including a hydraulic cylinder, a piston sealably and slidably disposed therein and partially defining with the cylinder a chamber of variable volume, the pressure of the fluid in the chamber varying with movement of the piston in the cylinder. A mechanical brake arrangement is in fluid communication with the chamber and is operatively coupled to at least one ground-engaging wheel for slowing the rotation thereof. The mechanical brake arrangement is variably actuated in response to changes in the pressure of the fluid during times when the vehicle braking control device is actuated, whereby the vehicle is braked by the mechanical brake system. The hybrid brake system also defers substantial actuation of the mechanical brake arrangement during actuation of the vehicle braking control device until after the electrical brake system has been actuated, by expanding the chamber volume in response to an increase in pressure of the fluid in the chamber.