Patent Publication Number: US-2009236906-A1

Title: Hydraulic Regenerative Braking System For A Vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 10/535,354, filed Dec. 16, 2003, which in turn claims the benefit of U.S. provisional application Ser. Nos. 60/433,566 filed Dec. 16, 2002; 60/441,194 filed Jan. 21, 2003; 60/452,714 filed Mar. 10, 2003; 60/514,983 filed Oct. 29, 2003; and 60/523,337 filed Nov. 20, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention The present invention relates to a hydraulic regenerative braking system for a vehicle. 
     2. Background Art 
     It is well known that hydraulic regenerative systems promise improved efficiency over electric regenerative systems incorporating a battery. Hydraulic regeneration involves using a pump, connected in the vehicle drive train, as a retarding device, and then storing the resulting high pressure fluid in an accumulator. On the subsequent vehicle acceleration, the high pressure fluid from the accumulator is routed to a hydraulic motor and the stored energy is recovered in the form of mechanical work which drives the vehicle forward. A low pressure accumulator acts as a reservoir to make up for fluid volume variations within the high pressure accumulator, and also provides a charge pressure to the inlet side of the pump. 
     Since the accumulator pressure is determined by a gas precharge level in the accumulator, and by the volume change from additional fluid added, the current method of modulating braking and driving forces in hydraulic regenerative systems has been to incorporate a variable displacement device to operate in concert with the fixed pressure accumulator. Variable displacement hydraulic devices can be efficient, but they are typically bulky, heavy and expensive, and do not package easily in automotive passenger vehicles. In addition, space is often limited in the front of a vehicle, yet, because the front wheels of a vehicle typically support 60% of the vehicle mass, plus whatever weight transfer takes place as a result of the vehicle deceleration, an effective regenerative braking system must incorporate braking on the front wheels. 
     Thus, the packaging problem is further compounded, since the drive train of front wheel drive vehicles is typically very tightly packaged. This leaves little room to add a variable displacement hydraulic device which can operate as a hydraulic pump during braking and a hydraulic motor during acceleration. Fixed displacement pump-motors, or pump-motors having limited variable displacement, may require less space, but may not provide the functionality required of a regenerative braking system. Therefore, a need exists for a hydraulic regenerative braking system that can conserve space by using fixed displacement, or limited variable displacement, pump-motors. 
     In addition to the packaging problems discussed above, hydraulic pump motors are often undesirably large as a result of their design. In particular, a hydraulic pump-motor that uses pistons, and has a cam that is disposed outside the pistons, may be too large to incorporate into a regenerative braking system on many vehicles. Therefore, a need exists for a hydraulic pump-motor that includes a cam for actuating the pistons, where the cam is disposed inboard of the pistons, thereby conserving space by providing a smaller pump-motor. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a hydraulic regenerative braking system that can conserve space by using fixed displacement, or limited variable displacement, pump-motors. 
     The invention also provides a hydraulic regenerative braking system that uses a hydraulic transformer to effect changes in fluid pressure within the system, thereby eliminating the need for a variable displacement pump-motor. 
     The invention further provides a hydraulic pump-motor that includes a cam for actuating the pistons, where the cam is disposed inboard of the pistons, thereby conserving space by providing a smaller pump-motor. 
     The invention also provides a hydraulic regenerative braking system for a vehicle. The system includes at least one hydraulic machine operable as a pump configured to be driven by energy received from at least one vehicle wheel when the vehicle is braking, thereby facilitating storage of vehicle braking energy. The at least one hydraulic machine is further operable as a motor configured to be driven by stored braking energy, thereby providing torque to at least one vehicle wheel. A first accumulator is configured to receive fluid from the at least one hydraulic machine, and to store the fluid under pressure. The first accumulator is further configured to provide pressurized fluid to the at least one hydraulic machine, thereby facilitating use of the hydraulic machine as a motor. A second accumulator is configured to store pressurized fluid and to provide a charge pressure to an inlet of the at least one hydraulic machine. A variable ratio transformer is in communication with the first and second accumulators and the at least one hydraulic machine. The transformer is operable to vary the pressure of the pressurized fluid provided to the at least one hydraulic machine, thereby facilitating variation in the torque provided to the at least one vehicle wheel. The transformer is further operable to vary the pressure of the fluid received by the first accumulator. A control module is configured to receive inputs related to operation of the vehicle, and to control operation of the transformer. The inputs include an acceleration request and a braking request. 
     The invention further provides a hydraulic machine operable as a pump configured to be driven by a rotating shaft, thereby increasing the pressure of fluid flowing through the pump. The hydraulic machine is further operable as a motor configured to be driven by pressurized fluid, thereby providing torque to a shaft. The hydraulic machine includes a housing, including a high pressure fluid port and a low pressure fluid port. The hydraulic machine also includes a plurality of radial pistons. Each of the pistons is configured to reciprocate within a corresponding cylinder in the housing, thereby pumping fluid when the hydraulic machine is operating as a pump, and providing torque when the hydraulic machine is operating as a motor. Each of the pistons includes a corresponding cam follower. A cam is disposed within the housing, and has a plurality of external lobes configured to cooperate with the cam followers to translate rotational motion of the cam into linear motion of the pistons when the hydraulic machine is operating as a pump, and to translate linear motion of the pistons into rotational motion of the cam when the hydraulic machine is operating as a motor. The cam includes an aperture therethrough for receiving a rotatable shaft. A rotatable valve plate has a plurality of apertures therethrough; at least some of the apertures communicate with the high pressure fluid port and at least some of the apertures communicate with the low pressure fluid port. The valve plate is configured to provide a fluid path between the cylinders and the high pressure fluid port when corresponding pistons are in a power stroke and between the cylinders and the low pressure fluid port when corresponding pistons are in an exhaust stroke, thereby facilitating operation of the hydraulic machine as a motor. The valve plate is further configured to provide a fluid path between the cylinders and the high pressure fluid port when corresponding pistons are in an exhaust stroke and between the cylinders and the low pressure fluid port when corresponding pistons are in a power stroke, thereby facilitating operation of the hydraulic machine as a pump. 
     The invention also provides a variable pressure ratio hydraulic transformer for modifying the pressure, flow rate, or a combination thereof, of fluid flowing through the transformer. The transformer includes a housing having at least three housing ports. Each of the housing ports is configured to operate as a fluid inlet or as a fluid outlet. A rotor is rotatably disposed within the housing. A plurality of pistons is attached to the rotor. Each of the pistons include a shaft having a generally spherical end, and a head configured to cooperate with the generally spherical end of the shaft, thereby allowing the head to pivot relative to the shaft. The transformer further includes a plurality of cylinders. Each of the cylinders is configured to receive a corresponding piston, and has a cylinder axis non-parallel to a corresponding piston shaft. A first plate is configured to be rotatably driven by the rotor, and has a first surface configured to contact one end of each of the cylinders and to allow each of the contacting cylinder ends to slide relative to the first surface. The first plate includes a plurality of apertures therethrough, at least some of which are configured to facilitate fluid flow to and from the cylinders. A second plate has at least three plate ports therein. Each of the plate ports is configured to cooperate with at least one aperture in the first plate and one housing port, thereby facilitating fluid flow between a housing port and at least one cylinder. The second plate is rotatable relative to the housing ports to modify the transformer pressure ratio. 
     The invention further provides a compact hydraulic machine operable as a pump and a motor, and configured to be disposed on a vehicle driving shaft proximate a vehicle wheel. The hydraulic machine includes a housing which has a first housing portion, a second housing portion, and an outer ring. The first housing portion includes a high pressure fluid port and a low pressure fluid port. The second housing portion includes a plurality of radially oriented cylinders disposed therein, and the outer ring includes a tapered bore to facilitate sealing of each of the cylinders. The hydraulic machine also includes a plurality of pistons, each of which includes a cam follower, and each of which is configured to reciprocate within a corresponding cylinder. A cam is disposed within the housing, and has a plurality of external lobes configured to cooperate with the cam followers to translate rotational motion of the cam into linear motion of the pistons when the hydraulic machine is operating as a pump, and to translate linear motion of the pistons into rotational motion of the cam when the hydraulic machine is operating as a motor. The cam includes an aperture therethrough for receiving a rotatable shaft. A rotatable valve plate has a plurality of apertures therethrough, and is configured to selectively connect the cylinders with the low and high pressure fluid ports, thereby alternately facilitating operation of the hydraulic machine as a pump and a motor. 
     The invention also provides a method for operating a vehicle having a hydraulic regenerative braking system. The regenerative braking system includes at least one hydraulic machine operable as a pump and a motor, and operable to receive energy from, and provide energy to, at least one vehicle wheel. The regenerative braking system also includes first and second accumulators for storing and providing pressurized fluid, and a variable ratio transformer operable to vary the pressure of fluid provided to the at least one hydraulic machine and to vary the pressure of fluid provided to the first accumulator. The method includes operating the at least one hydraulic machine as a pump during a vehicle braking event. During the braking event, the at least one hydraulic machine is driven by energy received from the at least one vehicle wheel, thereby providing pressurized fluid to at least the first accumulator to store the pressurized fluid. The transformer is selectively operated to vary the pressure of the fluid provided to the first accumulator during the vehicle braking event. The at least one hydraulic machine is operated as a motor during a vehicle driving event; it is driven by pressurized fluid provided from at least the first accumulator, thereby providing torque to the at least one vehicle wheel. The transformer is selectively operated to vary the pressure of the fluid provided to the at least one hydraulic machine during the vehicle driving event. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a system in accordance with a first embodiment of the present invention; 
         FIGS. 2A and 2B  are control logic diagrams illustrating control logic that can be used with the system shown in  FIG. 1 ; 
         FIG. 3  is a schematic representation of a system in accordance with a second embodiment of the present invention; 
         FIG. 4  is a schematic representation of a system in accordance with a third embodiment of the present invention; 
         FIG. 5  is a cross-sectional view of a transformer shown in  FIG. 1 ; 
         FIG. 6  is a side view of the transformer shown in  FIG. 5 ; 
         FIG. 7  is a front view of a rotor and pistons shown in  FIG. 5 ; 
         FIGS. 8A and 8B  are front and side views of a barrel and a floating cup shown in  FIG. 5 ; 
         FIG. 9  is a side view of a weighted belt shown in  FIG. 5 ; 
         FIG. 10  is a side view of a port plate shown in  FIG. 5 ; 
         FIG. 11  is a sectional view of an alternative embodiment of the port plate shown in  FIG. 10 ; 
         FIGS. 12A and 12B  show projections of ports in a port plate and apertures in a barrel which can be used in a hydraulic transformer; 
         FIG. 13  is a side view of the rotor shown in  FIG. 7 , illustrating shuttle valves in the rotor; 
         FIG. 14  is a detail view of a shuttle valve shown in  FIG. 13 ; 
         FIGS. 15A and 15B  are plan views of a pump-motor shown in  FIG. 1 ; 
         FIG. 16  is a plan view of a cam shown in  FIG. 15B ; 
         FIGS. 17A and 17B  are plan views of a valve plate shown in  FIG. 15A ; and 
         FIGS. 18A and 18B  are plan views of an axial piston shown in  FIG. 15A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
       FIG. 1  shows a schematic representation of a hydraulic regenerative braking system  10  for a vehicle in accordance with the present invention. The vehicle, not shown in its entirety, includes an engine  12  and four wheels  14 ,  16 ,  18 ,  20 . The regenerative braking system  10  includes two hydraulic machines, or pump-motors  22 ,  24 . Each of the pump-motors  22 ,  24 , is mounted on a respective driving shaft  26 ,  28  of the vehicle. Each pump-motor  22 ,  24  includes a torque arm  29 —see FIG.  15 B—which attaches to a ball-ended link to a secure mounting point on the vehicle chassis. As explained more fully below, each of the pump-motors  22 ,  24  is operable as a pump, driven by energy received from the respective vehicle wheels  14 ,  16  while the vehicle is braking. 
     The pump-motors  22 ,  24  pump fluid into a first, or high pressure accumulator  30 , where the high pressure fluid is stored for later use. The pump-motors  22 ,  24  are also operable as motors, driven by fluid from the high pressure accumulator  30 . Thus, the braking energy stored in the high pressure accumulator  30  is used to operate the pump-motors  22 ,  24  as motors to provide torque to the wheels  14 ,  16 . In addition to the regenerative braking system  10 , the vehicle also includes a friction braking system, illustrated in  FIG. 1  by calipers  25 ,  27 ,  29 ,  31 , at respective wheels  14 ,  16 ,  18 ,  20 . 
     The regenerative braking system  10  also includes a variable ratio hydraulic transformer  32  which is configured to vary the pressure of the pressurized fluid provided to the pump-motors  22 ,  24  or received by the high pressure accumulator  30 . By using a variable ratio transformer, such as the transformer  32 , the pump-motors  22 ,  24  can be fixed displacement, which provides for significant space reduction near the wheels, when compared to traditional hydraulic regenerative systems, which employ variable displacement pump-motors. The transformer  32  communicates with the pump-motors  22 ,  24 , the high pressure accumulator  30 , and with a second, or low pressure accumulator  34 . The low pressure accumulator  34  is used as a reservoir, which can either provide fluid to the transformer  32  to increase the pressure of the fluid provided to the pump-motors  22 ,  24 , or the low pressure accumulator  34  can receive fluid from the transformer  32  when it is in a step-down mode. 
     The low pressure accumulator  34  also provides a charge pressure—i.e., a relatively low pressure—to the pump-motors  22 ,  24 , through a charge pressure fluid line  35 . The charge pressure helps to ensure that there is always some liquid supplied to the pump-motors  22 ,  24 , thereby avoiding cavitation. The low pressure accumulator  34  includes two parts: a liquid/gas container  36 , and a gas only container  38 . Similarly, the high pressure accumulator  30  includes two parts: a liquid/gas container  40 , and a gas only container  42 . Configuring each of the accumulators  30 ,  34  with two containers facilitates packaging by reducing the size of the liquid/gas container. Of course, high and low pressure accumulators, such as the high and low pressure accumulators  30 ,  34 , may include a single liquid/gas container, rather than the two-part configuration shown in  FIG. 1 . 
     The two container arrangement takes advantage of residual volume for gas in the accumulator which is available after the accumulator piston (not shown) is at the end of its stroke. The gas only containers  38 ,  42  may be approximately 30% of the total respective accumulator volume, though different sizes of gas only containers may be used. To increase efficiency of the accumulators  30 ,  34 , the gas side of each liquid/gas container  36 ,  40 , and the gas only containers  38 ,  42 , may be filled with an open cell foam, such as polyester, to help to ensure that compression and expansion of the gas occurs at constant temperature. 
     The regenerative braking system  10  also includes a control module  44 , which controls operation of the transformer  32 . The control module  44  receives inputs related to operation of the vehicle. Such inputs include driver initiated acceleration requests and braking requests. The control module  44  uses the inputs to effect operation of the transformer suitable to the vehicle operation. In addition to electronic inputs, the control module  44  also uses fluid lines  46 ,  48 ,  50 ,  52 ,  54 ,  56 ,  58 ,  60  to detect various fluid pressures in the system  10 , and to control operation of the transformer  32 . The control module  44  also controls operation of a small pump  62  that resides in a sump tank  64 . The sump tank  64  receives fluid from various parts of the system  10 , as a result of, for example, fluid leakage. The pump  62  is configured to pump scavenged fluid from the sump tank  64 , to a small low pressure accumulator  66 . A filter  68  is provided on fluid line  70  to help ensure that debris from the scavenged fluid is not pumped to the control module  44 . 
     The control module  44  is programmed with appropriate control logic to facilitate functionality of the system  10 .  FIGS. 2A and 2B  respectively show one possible control logic scheme for when the vehicle is in a braking mode and when it is in a driving mode. Some of the control logic illustrated in  FIGS. 2A and 2B  may reside in another controller—e.g., a vehicle system controller (VSC)  71 —which communicates with the control module  44 . For example, in the braking mode, the VSC  71  receives inputs from a high pressure accumulator pressure sensor, a vehicle brake pedal force sensor, and a vehicle speed sensor. Of course, additional inputs may also be used. Using the inputs, the VSC  71  determines if the pedal force is greater than a first predetermined force, for example, 1 lb; if not, it assumes the vehicle is not in a braking mode. If the pedal force is greater than one pound, the control module  44  then determines if it is less than a second predetermined force, for example, 40 lb. If the braking force is not less than 40 lb., the friction brakes are used to stop the vehicle. If, however, the braking force is between 1 lb. and 40 lb., the VSC  71  signals the control module  44 , and regenerative braking is employed. 
     In addition to the brake pedal force, the control logic also uses the vehicle speed to determine whether regenerative braking should be used. For example, at high speeds—e.g., 50 miles per hour (mph) or more—the friction brakes are used, since the pump rotational speed is high, and the flow rate may exceed the maximum flow capacity of the system  10 . Moreover, at very low speeds—e.g., less than 3 mph—the friction brakes are also used. This is because regenerative braking provides negative torque to the vehicle wheels, and once the vehicle speed reaches zero, continued application of regenerative braking torque could cause the vehicle to move in reverse. 
     Once the control module  44  is signaled to use regenerative braking, it effects opening of valves on the high and low pressure accumulators  30 ,  34 , sends control pressure to the pump-motors  22 ,  24 , and sets the pressure ratio of the transformer  32 . If the pressure ratio of the transformer  32  is at a maximum, or the pressure of the pump-motors  22 ,  24  is at a maximum, the friction brakes are used in addition to regenerative braking. If, however, the pressure ratio is not at a maximum, and the pressure of the pump-motors  22 ,  24  is not at a maximum, the transformer  32  is used to generate the pressure required by the pump-motors  22 ,  24  to comply with the stopping rate commanded by the vehicle operator. 
     The control logic for the driving mode is shown in  FIG. 2B . A vehicle speed sensor and a throttle position sensor provide inputs into the VSC  71 . The VSC  71  then determines whether the throttle position is greater than zero. If it is not, it may be possible to shutdown the engine to save fuel. If the throttle position is greater than zero, a determination is then made whether the vehicle speed is less than a predetermined speed, for example, 35 mph. If it is not, the VSC  71  commands the engine to a conventional driving mode. If, however, the vehicle speed is less than 35 mph, the pressure from the high pressure accumulator  30  is checked. If it is not above a minimum threshold, the VSC  71  commands the engine to a conventional driving mode. If the pressure of the high pressure accumulator is above the minimum threshold, the VSC  71  then signals the control module  44  to operate the pump-motors  22 ,  24  as motors to provide torque to the vehicle wheels  14 ,  16 . 
     To facilitate use of the pump-motors  22 ,  24  as motors, the control module opens valves on the high and low pressure accumulators  30 ,  34 , sends a control pressure to the pump-motors  22 ,  24  to operate in the motor mode, and sets the pressure ratio of the transformer  32 . As in the braking mode, it is then determined whether the pressure ratio of the transformer is at a maximum, or if the pressure of the pump-motors  22 ,  24  is at a maximum. If it is, the engine is operated in conjunction with the hydraulic motors  22 ,  24 . If the pressure ratio is not at a maximum, and if the pressure of the pump-motors  22 ,  24  is not at a maximum, the transformer  32  is used to generate the pressure required by the pump-motors  22 ,  24 . 
     The embodiment of the present invention shown in  FIG. 1  includes two pump-motors  22 ,  24 . It is not required to use two pump-motors, as a single pump-motor could be used to receive braking energy from, and provide torque to, two vehicle wheels, such as the wheels  14 ,  16 . In addition, the present invention also contemplates the use of more than two pump-motors, for example, in a four wheel drive assist configuration, as shown in  FIG. 3 .  FIG. 3  shows a system  72 , configured with similar components as the system  10 , shown in  FIG. 1 . Thus, the system  72  includes a transformer  74 , a control module  76 , and high and low pressure accumulators  78 ,  80 . Unlike the system  10 , the system  72  includes four pump-motors  82 ,  84 ,  86 ,  88 , mounted proximate four corresponding wheels  90 ,  92 ,  94 ,  96 . Such a configuration allows the system  72  to store braking energy from the front and rear wheels, and also provides an option to supply torque to all four wheels  90 ,  92 ,  94 ,  96  in an otherwise two wheel drive vehicle. 
     It is worth noting that a four wheel regenerative system, such as the system  72 , can be configured to operate in a four wheel drive assist mode as well as a regenerative mode. As described above, the high pressure accumulator  78  can provide fluid to all four pump-motors  82 ,  84 ,  86 ,  88 , so that each operates as a motor in the regenerative mode. Alternatively, in a four wheel drive assist mode, an engine  97  can be used to provide all of the torque to the front wheels  90 ,  92  (or to the rear wheels in a rear wheel drive vehicle). This allows the front pump-motors  82 ,  84  to operate as pumps to provide pressurized fluid to the rear pump-motors  86 ,  88 , which operate as motors. This facilitates the application of driving torque to the rear wheels  94 ,  96  on a continuous basis, since there is no dependency on the use of stored energy from the high pressure accumulator  78 . The control module  76  can be programmed to implement this drive strategy, so that a regenerative braking system, such as the system  72 , needs little additional hardware to implement a four wheel drive assist operation. Including a four wheel drive assist option on a regenerative system, such as the system  72 , takes advantage of shared components, and provides a cost effective, mass effective, vehicle drive train with features that appeal to a large segment of vehicle owners. 
     In  FIGS. 1 and 3 , the high and low pressure accumulators are containers separate from the vehicle frame. As an additional space saving measure, a hydraulic regenerative braking system, such as the system  10 , can be configured with accumulators that are hydraformed components which makeup some of the vehicle frame. For example,  FIG. 4  shows a hydraulic regenerative braking system  98 , similar to the systems  10 ,  72  shown respectively in  FIGS. 1 and 3 . Though configured somewhat differently from the other two systems, the system  98  includes two hydraulic pump-motors  100 ,  102 . The system  98  also includes a hydraulic transformer  104 , and a control module  106 . In addition, the system  98  also includes a high pressure accumulator  108 , and a low pressure accumulator  110 , both of which are hydraformed, tubular members. 
     The accumulators  108 ,  110  not only contain pressurized fluid, used by the system  98  as described above with reference to the system  10 , but also form a part of the vehicle frame. Although the accumulators  108 ,  110  are each shown as a single container, one or both could be split into separate liquid/gas and gas only containers, as shown in  FIGS. 1 and 3 . Moreover, where an accumulator is configured with two containers as described above, it may be desirable to make only one of the containers—e.g., the gas only container—a hydraformed vehicle frame member, while making the other container—e.g., the liquid/gas container—a conventional storage tank. 
       FIG. 5  shows a cross-sectional view of the transformer  32 , shown schematically in  FIG. 1 . The transformer includes a housing  112 , which itself includes two end plates  114 ,  116 , and an outer housing  118 . The housing  112  includes high pressure ports  120 ,  122  and low pressure ports  124 ,  126 .  FIG. 6 , which shows a right side view of the transformer  32 , shows the high and low pressure ports  122 ,  124 , and also shows a machine port  128 , which connects to the pump-motors  22 ,  24 . There is another machine port on the left side of the transformer  32 , which is not visible in the figures. Thus, the transformer  32  provides a three-way junction between the pump-motors  22 ,  24 , the high pressure accumulator  30 , and the low pressure accumulator  34 . Also shown in  FIG. 6  is a relief valve  130 , which provides an outlet for the fluid if the pressure at the pump-motors  22 ,  24  exceeds a predetermined value. 
     Returning to  FIG. 5 , it is shown that the transformer  32  includes a rotor  132 , having a plurality of pistons  134  attached to it. Each side of the transformer  32  includes nine pistons  134 , only four of which are shown in  FIG. 5 . Although the embodiment shown in  FIG. 5  includes nine pistons  134 , a transformer, such as the transformer  32 , may include more or less than nine pistons, with numbers of pistons that are multiples of three being particularly desirable. As best shown in  FIG. 7 , each of the pistons  134  has a two-piece configuration, including a shaft  136  and a head  138 . Each of the shafts  136  includes a generally spherical end  140 , which cooperates with a corresponding head  138 . This configuration allows the piston heads  138  to pivot relative to their corresponding piston shafts  136 , which, as described below, provides an advantage over one-piece piston designs. 
     As shown in  FIG. 5 , the transformer  32  also includes a plurality of cylinders, or floating cups  142 . Each of the floating cups  142  is configured to receive a corresponding piston  134 , and as shown in  FIG. 5 , each of the floating cups  142  has a cylinder axis that is non-parallel to its corresponding piston shaft  136 . Because of this non-parallel orientation, a one-piece piston, or any piston with a head rigidly mounted to its shaft, may be difficult to seal. For example, when the piston head is parallel to its shaft, and therefore non-parallel to its corresponding cylinder, the piston head is subject to a radial force. This means that a piston head seal must be configured to seal the interface between the piston head and the cylinder, and to withstand the radial force caused by the non-parallel orientation. This puts great demands on a seal, and may lead to short seal life. 
     In contrast, the present invention, with its pivoting piston head design, reduces or eliminates the radial forces on the piston head seal. This means that the piston head seal can be designed to perform a single function: seal; it does not need to be designed to also withstand radial forces. In addition, because the contact area at the interface of the piston head and cylinder is greater when the head is parallel to the cylinder, a second seal can be added to the piston head.  FIG. 7  shows that each piston head  138  includes two seals  144 . 
     As shown in  FIG. 5 , the transformer  32  also includes first plates, or barrels  146 .  FIGS. 8A and 8B  show one of the barrels  146 , with a floating cup  142  in contact with it. Each of the barrels  146  includes a plate portion  148  and a hub portion  150 , which is attachable to the plate portion  148  using a snap ring  152 . Providing the barrels  146  with a two-piece configuration allows the plate portion  148  to be lapped so that a first surface  154  receives a very smooth finish prior to assembling the plate and hub portions  148 ,  150 . Such a smooth finish is important to maintain a good seal between each of the floating cups  142  and their corresponding barrel  146 . 
     The piston cylinders, or floating cups  142 , are said to be floating since they move with respect to their corresponding barrels  146 , and are not rigidly attached to it. There are some floating cup designs known in the art, so a full explanation of their basic operation is not provided here. Although the floating cups  142  of the present invention do move relative to their respective barrels  146 , they are somewhat restrained by the use of a spring clip  156 . This helps to maintain contact between the cups  142  and the barrels when there is little or no fluid pressure to maintain the contact, for example, when the system  10  is deactivated. 
     As shown in  FIG. 9 , the barrel  146  includes a plurality of apertures  158  disposed therethrough. In particular, there are nine apertures  158 , one for each of the cups  142 . The apertures  158  are tapered to allow for the movement of the cups  142  relative to the barrel  146 . As explained below, some of the apertures are configured to facilitate fluid flow into corresponding cups  142 , and others are configured to facilitate fluid flow out of corresponding cups  142 . The barrels  146  are rotatably driven by the rotor  132 . The rotor includes a number of pins  160 —see FIG.  7 —which cooperate with slots  162  within the barrel hubs  150  to facilitate near synchronous rotation of the barrels  146  by the rotor  132 . 
     Because the rotor  132 , and therefore the cups  142 , may be rotated at relatively high speeds, another constraint, in addition to the spring clips  156 , is provided. Specifically, the cups may have a tendency to move radially outward and a second constraint can help inhibit this movement. Each of the sets of cups  142  is therefore provided with a retainer  164  which is circumferentially disposed around the cups  142 .  FIG. 9  shows a retainer  164 , which includes a belt  166  having a plurality of weights  168 , equally spaced between links  170 . The weights  168  and the links  170  are attached to the belt  166  with fasteners  172 . Centrifugal force causes the weights  168  to exert an outward force on the retainer  164 , which thereby exerts an inward force on the cups  142  through links  170 . 
     Returning to  FIG. 5 , it is shown that the transformer  32  includes second plates, or port plates  174 . Although the port plates  174  may have any of a number of different configurations, one configuration is shown in  FIG. 9 . One port plate  174  is shown in  FIG. 10 . For illustration purposes, it is assumed that the port plate shown in  FIG. 10  is on the right side of the transformer  32  shown in  FIG. 5 . It is understood that a similar description applies to the port plate  174  on the left side of the transformer  32 . Returning to  FIG. 10 , it is shown that the port plate  174  includes three plate ports  176 ,  178 ,  180 . The port  176  is configured to cooperate with the high pressure port  120  in the transformer housing  112 , and three of the apertures  158  in a barrel  146 . This facilitates fluid flow between the high pressure accumulator  30  and three of the cups  142 . 
     Similarly, the port  178  is configured to cooperate with the machine port  128  and three different cups  142 , to facilitate fluid flow between the pump-motors  22 ,  24  and three of the cups  142 . Finally, port  180 , which includes two openings  182 ,  184  and a partition  186 , is configured to cooperate with the low pressure port  124  and the three remaining cups  142 . This facilitates fluid flow between the low pressure accumulator  34  and three of the cups  142 . 
     The port plates  174  are rotatable relative to the ports  120 ,  124 ,  128  in the transformer housing  112 . This allows the pressure ratio of the transformer  32  to be varied. Although a number of mechanisms may be used to rotate the port plates  174 , the embodiment shown in the drawing figures uses a sprocket  188  and chain  190 —see  FIG. 6 . The sprocket  188  is driven by a small electric motor  192 , which is controlled by the control module  44 . The rotatable port plate  174  is designed such that one third of the cups  142  at any one instant connect with the port  176 , one third connect with the port  178 , and one third connect with port  180 . As the cups  142  and barrel  146  rotate past the port plate  174 , transitions are such that three cups  142  communicate with each port  176 ,  178 ,  180 . With the angle between the rotor  132  and the barrel  146 , which may be approximately 9°, 180° of rotation of the rotor  132  causes the contained fluid volume inside a cup  132  to increase and the remaining 180° of rotation causes the volume to decrease. 
     Superimposed on these two 180° segments on the barrel  146  are three 120° segments on the port plate  174 . Two-thirds of the increasing portion of the rotation typically connect with input pressure, causing it to act as a hydraulic motor and two-thirds of the decreasing portion typically connect with output pressure, causing it to act as a hydraulic pump. The remaining portion of rotation connecting with the low pressure accumulator  34  allows fluid to discharge from and/or fill the cup volume as required. By rotating the three segment port plate  174  in one direction from symmetry relative to the angled barrel  146 , the displacement of the input (motor) can be increased while the output (pump) displacement is decreased. This decreases the output flow relative to input flow, allowing output pressure to increase (step up) based on the conservation of power principle. The difference between output flow and input flow is made up by flow to the low pressure accumulator  34 . Rotating the port plate  174  in the opposite direction, input (motor) displacement is decreased, output (pump) displacement is increased, and flow difference is made up by flow from the low pressure accumulator  34  to the transformer  32 . This is the step down mode of the transformer  32  in which output pressure is less than input pressure and output flow is increased proportionately. 
     As shown in  FIG. 10 , each of the ports  176 ,  178 ,  180  is generally arcuate, and is disposed a corresponding radius from the center of the port plate  174 . The port  180 , which connects with the low pressure accumulator  34 , is at a larger radius than the ports  176 ,  178 . This allows the port  180  to radially overlap with the ports  176 ,  178 , thereby providing an increase in the pressure ratios provided by the transformer  32 . In addition, as noted above, the port  180  includes a partition  186 . This allows fluid flow to and from the low pressure accumulator  34  to be completely blocked, thereby improving the efficiency of the transformer  32  when a 1:1 ratio is desired. Because the partition may not completely block the fluid flow to and from the low pressure accumulator  34 , a small channel  194  is provided between the openings  182 ,  184 , thereby allowing some fluid to flow between them. Similar channels can be provided in the end plates  114 ,  116  of the transformer housing  112 , to allow a small amount of fluid to flow between the ports  176 ,  178 . 
     Other port plate designs are contemplated by the present invention. For example,  FIG. 11  shows a cross section of a port plate  196 . Shown in the cross section are two ports  198 ,  200 , with a third not visible. One side  202  of the port plate  196  is configured to cooperate with an end cap, such as the end plate  114 , shown in  FIG. 5 . The other side  204  of the port plate  196  is configured to cooperate with a barrel, such as the barrel  146 . As shown in  FIG. 11 , each of the ports  198 ,  200  is located at a radial distance from center axis  206  that is different for the two different sides  202 ,  204 . This configuration allows for increasing the size of the ports in the transformer housing  112 , thereby increasing the flow capability of the transformer  32 . 
     Regardless of the particular port design, a port plate can be configured to cooperate with corresponding apertures in a barrel, such as the apertures  158  in the barrel  146 , to maintain a generally constant contact area no matter what the relative positions of the port plate and the barrel. For example,  FIGS. 12A and 12  B shows a projection of three ports  208 ,  210 ,  212  and nine barrel apertures  214 . In  FIG. 12  A, each port  208 ,  210 ,  212  contains two apertures  214 , leaving a total of three of the apertures  214  completely outside a port. In  FIG. 12B , the barrel has rotated, changing the position of the apertures  214  relative to the ports  208 ,  210 ,  212 . Despite this change in relative position, the projected area of the apertures outside the ports  208 ,  210 ,  212 , still totals that of three apertures—the same as in  FIG. 12A . The design of the ports  208 ,  210 ,  212  is such that this relationship is maintained, regardless of the relative positions of the port plate and the barrel. 
     The transformer  32  is designed to inhibit pressure spikes, thereby reducing the forces on the transformer components, and increasing component life. For example,  FIG. 13  shows a side view of the rotor  132 , including a plurality of shuttle valves  216 , each of which communicates with a pair of ports  217 . Each of the shuttle valves  216  are configured to provide a fluid path between a corresponding pair of cups  142 . As shown in  FIG. 7 , each piston head  138  includes a channel  218  that communicates with a channel  220  in corresponding piston shafts  136 . This allows fluid to flow between the cups  142  and the shuttle valves  216 , thereby inhibiting pressure spikes, particularly when the fluid changes pressure in the transformer  32 . 
     A detail view of a shuttle valve  216  is shown in  FIG. 14 . Each shuttle valve  216  includes a piston  222 . The pistons  222  are specifically designed to inhibit impact which might otherwise reduce component life. For example, each piston  222  is configured with radii  224 , which reduce or eliminate metal to metal impact by trapping fluid when the piston  222  approaches the end of a stroke. This provides additional reliability for a transformer, such as the transformer  32 . 
     Turning now to the pump-motors,  FIGS. 15A and 15B  show two views of the pump-motor  22 ; although, the pump-motor  24  is constructed the same as the pump-motor  22 . The pump-motor  22  includes a housing that includes a high pressure fluid port  228 , and a low pressure fluid port  230 . The housing  226  includes first and second housing portions  232 ,  234 , and an outer ring  236  having a tapered bore  238 . The first housing portion  232  includes the high and low pressure ports  228 ,  230 . The second housing portion includes eight radial pistons  240 , three of which are shown in  FIG. 15B . Each of the pistons  240  are configured to reciprocate within a corresponding cylinder  242 , thereby pumping fluid when the pump-motor  22  is operating as a pump, and providing torque when the pump-motor  22  is operating as a motor. The housing  226  is also configured with a plurality of shuttle valves  243 , configured similarly, and provided for the same purpose, as the shuttle valves  216  in the transformer  32 . 
     A portion of a cam  244  is shown in  FIG. 15B , and shown in its entirety in  FIG. 16 . The cam  244  is disposed in the housing  226 , and is inboard of the pistons  240 . This conserves space and provides a more compact packaging arrangement for the pump-motor  22 , especially when compared to pump-motors having a cam circumferentially disposed outside the pistons  240 . Moreover, by having the cam  244  inboard of the pistons  240 , the pistons  240  can be configured to move radially outward away from the cam  244 , thereby disengaging the pistons  240  from the cam  244 , and conserving energy when the pump-motor  22  is not needed. This may be accomplished, for example, by providing a slight pressure differential to move the pistons outward when the system is disengaged. Another advantage of the inboard cam  244  is the reduced rolling velocity of the cam followers  248 . Noise produced by hydraulic machinery is largely speed related, so reducing rolling velocity, as the present invention does, reduces noise. 
     As shown in  FIG. 16 , the cam  244  includes six external lobes  246 . Of course, a cam, such as the cam  244 , may include more or less than six lobes. The cam  244  cooperates with cam followers  248  on each of the pistons. The cam includes an aperture  250  therethrough, which is configured to be keyed or splined to the driving shaft  26  of the vehicle—see  FIG. 1 . Thus, the driving shaft  26  turns the cam  244  which operates the pistons  240  to pump fluid to the high pressure accumulator  30  when the pump-motor  22  is operating as a pump—i.e., during vehicle braking. Conversely, when the pump-motor  22  is operating as a motor, the high pressure accumulator  30  provides fluid to the pump-motor  22  to operate the pistons  240 , which in turn rotate the cam  244  to provide torque to the driving shaft  26 , and thus, the vehicle wheel  14 . 
     Returning to  FIG. 15A , it is shown that the first housing portion  232  includes a scavenge port  252  and a mode port  254 . The scavenge port  252  provides a flow path back to the transformer  32  for fluid that does not exit via the high or low pressure ports  228 ,  230 . This may include fluid that leaks past the pistons  240 , or virtually anywhere else in the pump-motor  22 . In order to inhibit such leaks, each piston  240  is configured with a pair of seals  256  to prevent leaks between the piston  240  and the corresponding cylinder  242 . In addition, each cylinder head is sealed with a pair of seals  258  which are securely held in place by the tapered ring  236 . Thus, the present invention provides for redundant seals not only on the pistons  134  in the transformer  32 , but also on the pistons  240  in the pump-motor  22 , and at the heads of the cylinders  242  in the pump-motor  22 . The pump-motor  22  also has a number of redundant seals in the first housing portion—see  FIG. 15A . 
     The pump-motor  22  also includes a rotating valve plate  260 , shown assembled with the pump-motor  22  in  FIG. 15A  and isolated in  FIGS. 17A and 17B . The valve plate  260  alternatingly includes six apertures  262 , which communicate with the high pressure port  228 , and six apertures  264 , which communicate with the low pressure port  230 . The valve plate  260  provides a fluid path between the cylinders  242  and the high pressure port  228  when corresponding pistons  240  are in a power stroke, and between the cylinders  242  and the low pressure port  230  when corresponding pistons are in an exhaust stroke. This facilitates operation of the pump-motor  22  as a motor. 
     The valve plate  260  can also provide a fluid path between the cylinders  242  and the high pressure port  228  when corresponding pistons  240  are in an exhaust stroke, and between the cylinders  242  and the low pressure port  230  when corresponding pistons are in a power stroke. This facilitates operation of the pump-motor  22  as a pump. Fluid paths  263 ,  265  are shown in  FIG. 15A  in the second housing portion  234 . The fluid paths  263 ,  265 , and others not shown, facilitate the fluid transfer between the cylinders  242  and the apertures  262 ,  264  of the valve plate  260 . 
     In order to facilitate operation of the pump-motor  22  as both a pump and a motor, the valve plate  260  is indexable relative to the cam  244 . To effect the indexing, the pump-motor  22  includes an axial piston  266 , which, for illustrative purposes, is shown in  FIG. 15A  split into two parts, with each of the parts representing the piston  266  in a different position.  FIGS. 18A and 18B  show the piston  266  in its entirety. The piston  266  may be keyed or splined to the driving shaft  26 . The piston  266  drives the valve plate  260  via two or more links  268 , which cause the valve plate  260  to rotate as the piston  266  rotates. 
     In addition, the links  268  translate linear motion of the piston  266  into rotational motion of the valve plate  260 , to index the valve plate  260  relative to the cam  244 . Movement of the piston  266  in one direction is effected by fluid entering the mode port  254 . If necessary, a spring (not shown) can be provided to return the piston  266  to its previous position when the fluid pressure from the mode port  254  is exhausted. When the piston  266  is moved axially, the valve plate  260  rotates such that the apertures  262 ,  264  change their positions relative to the cam  244 . Thus, the pump-motor  22  can be operated as a pump and a motor without rerouting feed lines. 
     In order to inhibit movement of the piston  266  when the pump-motor  22  is operating at high speed, the pump-motor  22  includes a plurality of weights  270  disposed proximate the piston  266 . In particular, each of the weights  270  are constrained by a two-stage spring apparatus  272 . The two stage spring apparatus includes a first stage, which maintains the position of the piston  266  relative to the cam  244 , such that fluid flow is not prohibited, but it is reduced. This occurs when the apertures  262 ,  264  provide fluid to the cylinders  242  when the pistons  240  are not at top or bottom dead center. In the second stage, the position of the piston  266  is maintained relative to the cam  244  to prohibit operation of the pump-motor  22 . In order to inhibit movement of the piston  266 , the weights  270  are configured to mate with two steps  274 ,  276  on the piston  266 —see  FIG. 18B . Thus, when the weights  270  engage the first step  274 , flow through the pump-motor  22  is reduced, and when the weights  270  engage the second step  276 , external flow to and from the pump-motor  22  is prohibited. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.