Abstract:
A combined hybrid drive and brake system for use with a rotatably driven mechanism includes a hybrid drive system that is adapted to decelerate a rotatably driven mechanism, accumulate the energy resulting from such deceleration, and use the accumulated energy to subsequently accelerate the rotatably driven mechanism. A brake system is adapted to decelerate the rotatably driven mechanism. A control apparatus is responsive to a request for braking torque for decelerating the rotatably driven mechanism by either (1) the hybrid drive system operating alone, (2) the brake system operating alone, or (3) both the hybrid drive system and the brake system operating in combination.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/968,102, filed Aug. 27, 2007, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates in general to hybrid drive systems, such as are used in conjunction with drive train systems for vehicles. In particular, this invention relates to an improved control apparatus and method for operating a combined hybrid drive and brake system in such a manner that transitions between various operating modes occur in a smooth and unnoticeable manner. 
         [0003]    Drive train systems are widely used for generating power from a source and for transferring such power from the source to a driven mechanism. Frequently, the source generates rotational power, and such rotational power is transferred from the source of rotational power to a rotatably driven mechanism. For example, in most land vehicles in use today, an engine generates rotational power, and such rotational power is transferred from an output shaft of the engine through a driveshaft to an input shaft of an axle assembly so as to rotatably drive the wheels of the vehicle. 
         [0004]    In some of these land vehicles and other mechanisms, a hybrid drive system (also known as an energy recovery system) is provided in conjunction with the drive train system to decelerate the rotatably driven mechanism, accumulate the energy resulting from such deceleration, and use the accumulated energy to subsequently accelerate the rotatably driven mechanism. To accomplish this, a typical hybrid drive system includes a reversible energy transfer machine that is coupled to the drive train system and an energy storage device that communicates with the reversible energy transfer machine. To decelerate the vehicle, the hybrid drive system is operated in a retarding mode, wherein the reversible energy transfer machine slows the rotation of the rotatably driven mechanism and stores the kinetic energy of the vehicle in the energy storage device as potential energy. To subsequently accelerate the vehicle, the hybrid drive system is operated in a driving mode, wherein the potential energy stored in the energy storage device is supplied to the reversible energy transfer machine to rotatably drive the rotatably driven mechanism. 
         [0005]    Although hybrid drive systems of this general type function in an energy-efficient manner, it is often necessary or desirable to provide a separate brake system to affirmatively slow or stop the rotation of the rotatably driven mechanism in certain situations. For example, when used in conjunction with the drive train system of a vehicle that is relatively heavy or moving relatively fast, the hybrid drive system may not always have the capacity to adequately retard the rotation of the rotatably driven mechanism as quickly as requested by a driver. Additionally, when used in conjunction with the drive train system of a vehicle that is stopped on an inclined surface, the hybrid drive system cannot positively stop the rotatably driven mechanism to prevent any movement of the vehicle. To address these and other situations, the separate brake system (which can be embodied as a conventional pneumatically or hydraulically actuated friction brake system) is often provided in conjunction with the hybrid drive system. In such a combined hybrid drive and brake system, the hybrid drive system can be actuated to normally retard the rotation of the rotatably driven mechanism in the energy-efficient manner described above, and the brake system can be actuated when otherwise necessary. 
         [0006]    In a combined hybrid drive and brake system such as described above, deceleration of the rotatably driven mechanism can be accomplished by either (1) the hybrid drive system operating alone, (2) the brake system operating alone, or (3) both the hybrid drive system and the brake system operating in combination. The selection of which of these three operating modes is appropriate can be determined by a control apparatus in accordance with a variety of parameters. Because these parameters can (and typically do) change during the deceleration of the rotatably driven mechanism, the control apparatus will frequently transition between two or more of the three operating modes of the combined hybrid drive and brake system. Thus, it would be desirable to provide an improved control apparatus and method for operating a combined hybrid drive and brake system in such a manner that the transitions between these various operating modes occur in a smooth and unnoticeable manner. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention relates to an improved control apparatus and method for operating a combined hybrid drive and brake system in such a manner that transitions between various operating modes occur in a smooth and unnoticeable manner. The combined hybrid drive and brake system includes a hybrid drive system that is adapted to decelerate a rotatably driven mechanism, accumulate the energy resulting from such deceleration, and use the accumulated energy to subsequently accelerate the rotatably driven mechanism. A brake system is adapted to decelerate the rotatably driven mechanism. A control apparatus is responsive to a request for braking torque for decelerating the rotatably driven mechanism by either (1) the hybrid drive system operating alone, (2) the brake system operating alone, or (3) both the hybrid drive system and the brake system operating in combination. 
         [0008]    Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a drive train system including a combined hybrid drive and brake system in accordance with this invention. 
           [0010]      FIG. 2  is a block diagram of a control apparatus for operating the combined hybrid drive and brake system illustrated in  FIG. 1 . 
           [0011]      FIG. 3  is a flowchart of a method for operating the control apparatus illustrated in  FIG. 2  in accordance with this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    Referring now to the drawings, there is illustrated in  FIG. 1  a drive train system, indicated generally at  10 , for generating power from a source and for transferring such power from the source to a driven mechanism. The illustrated drive train system  10  is a vehicular drive train system that includes an engine  11  that generates rotational power to an axle assembly  12  by means of a combined hybrid drive and brake system, indicated generally at  20 . However, the illustrated vehicle drive train system  10  is intended merely to illustrate one environment in which this invention may be used. Thus, the scope of this invention is not intended to be limited for use with the specific structure for the vehicular drive train system  10  illustrated in  FIG. 1  or with vehicle drive train systems in general. On the contrary, as will become apparent below, this invention may be used in any desired environment for the purposes described below. 
         [0013]    The illustrated combined hybrid drive and brake system  20  includes a power drive unit  21  that is connected between the engine  11  and the axle assembly  12 . The illustrated power drive unit  21  is, in large measure, conventional in the art and is intended merely to illustrate one environment in which this invention may be used. Thus, the scope of this invention is not intended to be limited for use with the specific structure for the power drive unit  21  illustrated in  FIG. 1 . The illustrated power drive unit  21  includes an input shaft  22  that is rotatably driven by the engine  11 . An input gear  23  is supported on the input shaft  22  for rotation therewith. The input gear  23  is connected for rotation with a primary pump drive gear  24  that, in turn, is connected for rotation with an input shaft of a primary pump  25 . Thus, the primary pump  25  is rotatably driven whenever the engine  11  is operated. The purpose of the primary pump  25  will be explained below. 
         [0014]    The illustrated power drive unit  21  also includes a main drive clutch  26  that selectively connects the input shaft  22  to an output shaft  27 . When the main drive clutch  26  is engaged, the input shaft  22  is connected for rotation with the output shaft  27 . When the main drive clutch  26  is disengaged, the input shaft  22  is not connected for rotation with the output shaft  27 . The output shaft  27  is connected for rotation with an input shaft of the axle assembly  12 . Thus, the axle assembly  12  is rotatably driven by the engine  11  whenever the main drive clutch  26  is engaged. 
         [0015]    The illustrated power drive unit  21  further includes a low drive clutch  30  that selectively connects the output shaft  27  to a low drive clutch gear  31 . The low drive clutch output gear  31  is connected for rotation with both a first low drive output gear  32  and a second low drive output gear  33 . The first low drive output gear  32  is connected for rotation with a first shaft  32   a  that, in turn, is connected for rotation with an input shaft of a first pump/motor  34 . Similarly, the second low drive output gear  33  is connected for rotation with a second shaft  33   a  that, in turn, is connected for rotation with an input shaft of a second pump/motor  35 . Thus, when both the main drive clutch  26  and the low drive clutch  30  are engaged, the output shaft  27  rotatably drives both the first pump/motor  34  and the second pump motor  35 . The purpose for both the first pump/motor  34  and the second pump motor  35  will be explained below. 
         [0016]    Similarly, the illustrated power drive unit  21  further includes a high drive clutch  36  that selectively connects the output shaft  27  to a high drive clutch gear  37 . The high drive clutch output gear  37  is connected for rotation with both a first high drive output gear  38  and a second high drive output gear  39 . The first high drive output gear  38  is connected for rotation with the first shaft  32   a  that, as mentioned above, is connected for rotation with the input shaft of the first pump/motor  34 . Similarly, the second high drive output gear  39  is connected for rotation with the second shaft  33   a  that, as also mentioned above, is connected for rotation with the input shaft of the second pump/motor  35 . Thus, when both the main drive clutch  26  and the high drive clutch  36  are engaged, the output shaft  27  rotatably drives both the first pump/motor  34  and the second pump motor  35 . The low drive gears  31 ,  32 , and  33  are selected to provide a relatively low gear ratio when the main drive clutch  26  and the low drive clutch  30  are engaged, in comparison with the relatively high gear ratio provided by the high drive gears  37 ,  28 , and  39  when the main drive clutch  26  and the high drive clutch  36  are engaged. 
         [0017]    The illustrated power drive unit  21  also includes an accumulator  40  or similar relatively high fluid pressure storage device. The accumulator  40  selectively communicates with a first port of the primary pump  25  through a primary pump valve  41 . The primary pump valve  41  is conventional in the art and can be operated in a first position (shown in  FIG. 1 ), wherein fluid communication from the accumulator  40  to the first port of the primary pump  25  is prevented and fluid communication from the first port of the primary pump  25  to the accumulator  40  is permitted. However, the primary pump valve  41  can be operated in a second position (to the right when viewing  FIG. 1 ), wherein fluid communication from the accumulator  40  to the first port of the primary pump  25  is permitted and fluid communication from the first port of the primary pump  25  to the accumulator  40  is permitted. For the purposes of this invention, the primary pump valve  41  is always maintained in the illustrated first position, wherein fluid communication from the accumulator  40  to the first port of the primary pump  25  is prevented and fluid communication from the first port of the primary pump  25  to the accumulator  40  is permitted. 
         [0018]    The accumulator  40  also selectively communicates with a first port of the first pump/motor  34  through a first control valve  42 . The first control valve  42  is conventional in the art and can be operated in a first position (shown in  FIG. 1 ), wherein fluid communication from the first port of the first pump/motor  34  to the accumulator  40  is permitted and fluid communication from the accumulator  40  to the first port of the first pump/motor  34  is prevented. However, the first control valve  42  can be operated in a second position (to the right when viewing  FIG. 1 ), wherein fluid communication from the first port of the first pump/motor  34  to the accumulator  40  is permitted and fluid communication from the accumulator  40  to the first port of the first pump/motor  34  is permitted. 
         [0019]    The accumulator  40  further selectively communicates with a first port of the second pump/motor  35  through a second control valve  43 . The second control valve  43  is conventional in the art and can be operated in a first position (shown in  FIG. 1 ), wherein fluid communication from the first port of the second pump/motor  35  to the accumulator  40  is permitted and fluid communication from the accumulator  40  to the first port of the second pump/motor  35  is prevented. However, the second control valve  43  can be operated in a second position (to the right when viewing  FIG. 1 ), wherein fluid communication from the first port of the second pump/motor  35  to the accumulator  40  is permitted and fluid communication from the accumulator  40  to the first port of the second pump/motor  35  is permitted. 
         [0020]    The illustrated power drive unit  21  further includes a reservoir  44  or similar relatively low fluid pressure storage device. Each of the primary pump  25 , the first pump/motor  34 , and the second pump/motor  35  includes a second port, and all of such second ports communicate with the reservoir  44  to draw fluid therefrom when necessary, as described below. 
         [0021]    The basic operation of the drive train system  10  will now be described. When the engine  11  of the drive train system  10  is initially started, the main drive clutch  26 , the low drive clutch  30 , and the high drive clutch  36  are all disengaged, and the valves  41 ,  42 , and  43  are all in their first positions illustrated in  FIG. 1 . In this initial condition, the engine  11  rotatably drives the primary pump  25  through the input shaft, the input gear  23 , and the primary pump drive gear  24 , as described above. As a result, the primary pump  25  draws fluid from the reservoir  44  through the second port thereof, and further supplies such fluid under pressure from the first port of the primary pump  25  through the primary pump valve  41  to the accumulator  40 . As discussed above, the first and second control valves  42  and  43  prevent the pressurized fluid from the primary pump  25  or the accumulator  40  from being supplied to the first ports of the first and second pump/motors  34  and  35 , respectively. Such initially operation continues until a sufficient amount of such pressurized fluid has been supplied to the accumulator  40 . Because the main drive clutch  26 , the low drive clutch  30 , and the high drive clutch  36  are all disengaged, the engine  11  does not rotatably drive the output shaft  27  or the axle assembly  12  in this initial operation of the drive train system  10 . 
         [0022]    When it is desired to move the vehicle, the low drive clutch  30  is engaged, while the main drive clutch  26  and the high drive clutch  36  remain disengaged. As a result, the output shaft  27  is connected to the low drive clutch gear  31  for concurrent rotation. At the same time, the first control valve  42  and the second control valve  43  are each moved to their second positions. This permits pressurized fluid from the accumulator  40  to flow to the first ports of both the first pump/motor  34  and the second pump/motor  35 . Lastly, the first and second pump/motors  34  and  35  are each placed in a positive displacement mode, wherein they function as motors to use the pressurized fluid supplied by the accumulator  40  to rotatably drive the first and second shafts  32   a  and  33   a . In turn, this causes the low drive gears  31 ,  32 , and  33  and the output shaft  27  to be rotatably driven. As a result, the axle assembly  12  is rotatably driven at the relatively low gear ratio provided by the low drive gears  31 ,  32 , and  33 . Such a relatively low gear ratio is well suited for providing the relatively high torque needed to accelerate the vehicle from a standstill. 
         [0023]    Once it has begun to move, it may be desirable to move the vehicle at a higher speed that is suitable for the relatively low gear ratio provided by the low drive gears  31 ,  32 , and  33 . In this instance, the power drive unit  21  can be operated to disengage the low drive clutch  30  and engage the high drive clutch  36 , while maintaining the main drive clutch  26  disengaged. As a result, the output shaft  27  is connected to the high drive clutch output gear  37  for concurrent rotation. The first control valve  42  and the second control valve  43  are each moved to (or maintained in) their second positions. As described above, this permits pressurized fluid from the accumulator  40  to flow to the first ports of both the first pump/motor  34  and the second pump/motor  35 . As also described above, the first and second pump/motors  34  and  35  are each placed (or maintained) in a positive displacement mode, 
         [0024]    wherein they function as motors to use the pressurized fluid supplied by the accumulator  40  to rotatably drive the first and second shafts  32   a  and  33   a . In turn, this causes the high drive gears  37 ,  38 , and  39  and the output shaft  27  to be rotatably driven. As a result, the axle assembly  12  is rotatably driven at the relatively low gear ratio provided by the high drive gears  37 ,  38 , and  39 . Such a relatively high gear ratio is well suited for providing the relatively low torque needed to accelerate the vehicle to a relatively high speed. 
         [0025]    If it is desired to operate the vehicle at a further higher speed, the power drive unit  21  can be operated to disengage the high drive clutch  36  and engage the main drive clutch  26 , while the low drive clutch  30  remains disengaged. As a result, the output shaft  27  is connected to the input shaft  22  for concurrent rotation. At the same time, the first control valve  42  and the second control valve  43  are each moved to their first positions. As described above, this prevents pressurized fluid from the accumulator  40  from flowing to the outputs of both the first pump/motor  34  and the second pump/motor  35 . As a result, the first and second pump/motors  34  and  35  are isolated from the drive train system  10 . 
         [0026]    Under certain circumstances, the above-described components of the combined hybrid drive and brake system  20  can also be used to slow or stop the movement of the vehicle. To accomplish this, the main drive clutch  26  and the low drive clutch  30  are disengaged, while the high drive clutch  36  is engaged (in some instances, it may be preferable that the main drive clutch  26  and the high drive clutch  36  be disengaged, while the low drive clutch  30  is engaged). Regardless, the first control valve  42  and the second control valve  43  are each moved to (or maintained in) their second positions. This permits pressurized fluid from the first ports of both the first pump/motor  34  and the second pump/motor  35  to flow to the accumulator  40 . Lastly, the first and second pump/motors  34  and  35  are each placed in a negative displacement mode, wherein they function as pumps to use the rotational energy of the rotating output shaft  27  to supply pressurized fluid to the accumulator  40 . As a result, the output shaft  27  rotates the high drive gears  37 ,  38 , and  39 , which causes the first pump/motor  34  and the second pump/motor  35  to be rotatably driven. Consequently, the rotation of the axle assembly  12  is decelerated as the kinetic energy thereof is stored as fluid pressure in the accumulator  40 . 
         [0027]    As discussed above, however, it is often necessary or desirable to provide a separate brake system to affirmatively slow or stop the rotation of the axle assembly  12 . As shown in  FIG. 1 , such a separate brake system is provided within the axle assembly  12  of the illustrated drive train system  10  as a pair of friction brakes  45  associated with respective wheels of the vehicle. The friction brakes  45  are conventional in the art and may be actuated in any desired manner, such as pneumatically or hydraulically. The details regarding how the various components of the combined hybrid drive and brake system  20  are used to decelerate the vehicle will be explained in detail below. 
         [0028]    In the illustrated combined hybrid drive and brake system  20 , pressurized fluid is used as the actuating mechanism. In such a hydraulic hybrid drive system, the accumulator  40  functions as the energy storage device, and the pump/motors  34  and  35  function as reversible hydraulic machines. Another commonly known hybrid drive system uses electricity as the actuating mechanism. In such an electric hybrid drive system, an electrical energy storage device (such as a capacitor or a battery) and a reversible electrical machine (such as generator/motor) are provided and function in a similar manner as described above. This invention is not intended to be limited to the specific structure of the hybrid drive and brake system, but rather is intended to cover any similar structures. 
         [0029]      FIG. 2  is a block diagram of a control apparatus, indicated generally at  50 , for operating the combined hybrid drive and brake system  20  illustrated in  FIG. 1 . The illustrated control apparatus  50  includes a controller  51 , which may be embodied as a conventional microprocessor or any other programmable control device. The controller  51  receives a first input signal from a brake pedal sensor  52  or other conventional device that generates a signal that is representative of the amount of braking torque that is requested by an operator of the drive train system  10 . The controller  51  receives a second input signal from an actual speed sensor  53  or other conventional device that generates a signal that is representative of the actual speed of the drive train system  10 . The controller  51  receives a third input signal from one or more fault condition sensors  54  or other conventional device that generates a signal that is representative of any desired operating condition of the drive train system  10  that is desired to be monitored. If desired, the controller  51  may receive one or more additional input signals representing any other condition or group of conditions that would be helpful in controlling the operation of the combined hybrid drive and brake system  20 . 
         [0030]    The controller  51  generates a first output signal to a pump/motor displacement control circuit  55  in response to one or more of the first, second, and third input signals. The pump/motor displacement control circuit  55  is conventional in the art and is adapted to vary the displacement of either or both of the pump/motors  34  and  35  in response to the first output signal. The controller  51  generates a second output signal to a control valve control circuit  56  in response to one or more of the first, second, and third input signals. The control valve control circuit  56  is conventional in the art and is adapted to control the movements of the first and second control valves  42  and  43  in response to the second output signal. The controller  51  generates a third output signal to a friction brake control circuit  57  in response to one or more of the first, second, and third input signals. The friction brake control circuit  57  is conventional in the art and is adapted to control the operation of the friction brakes  45  in response to the third output signal. If desired, the controller  51  may generate one or more additional output signals representing any other portion or portions of the combined hybrid drive and brake system  20  that is desired to be controlled. 
         [0031]      FIG. 3  is a flowchart of a method, indicated generally at  60 , for operating the control apparatus illustrated in  FIG. 2  in accordance with this invention. In an initial decision point  61  of the method  60 , it is determined whether a request for braking torque has been made by an operator of the drive train system  10 . This determination can be made from the first input signal generated by the brake pedal sensor  52  discussed above. The method  60  repeats this initial decision point  61  until it is determined that such braking torque request has been made. 
         [0032]    When a braking torque request has been made, the method  60  branches from the initial decision point  61  to a decision point  62 , wherein it is determined whether the actual speed of the drive train system  10  is greater than a first predetermined threshold. Generally speaking, the first predetermined threshold is selected to represent a speed above which it is considered to be desirable to slow the rotation of the drive train system  10  solely by means of the friction brakes  45  described above and not by means of the first and second pump/motors  34  and  35  operating in the negative displacement mode, as described above. The first predetermined threshold can be characterized in any desired manner. For example, if the drive train system  10  is used in a vehicle, then the first predetermined threshold can be characterized as an actual speed of the vehicle. Also, the magnitude of this first predetermined threshold can be determined in accordance with specific parameters of the drive train system  10 . For example, if the drive train system  10  is used in a relatively heavy vehicle, such as a garbage truck, then the magnitude of this first predetermined threshold can be set at seven miles per hour. 
         [0033]    If the actual speed of the drive train system  10  is greater than the first predetermined threshold, then the method  60  branches from the decision point  62  to an instruction  63 , wherein the controller  51  causes only the friction brakes  45  to be engaged to slow the rotation of the drive train system  10 , as described above. Then, the method  60  returns to the initial decision point  61 , wherein it is again determined whether a request for braking torque has been made by the operator of the drive train system  10 . 
         [0034]    If, on the other hand, the actual speed of the drive train system  10  is not greater than the first predetermined threshold, then the method  60  branches from the decision point  62  to a decision point  64 , wherein it is determined whether the actual speed of the drive train system  10  is greater than a second predetermined threshold. Generally speaking, the second predetermined threshold is less than the first predetermined threshold and is selected to represent a speed above which it is desirable to slow the rotation of the drive train system  10  by means of the first and second pump/motors  34  and  35  operating in the negative displacement mode, as described above. The characterization and magnitude of the second predetermined threshold can be characterized in any desired manner, such as described above. For example, if the drive train system  10  is used in a relatively heavy vehicle, such as a garbage truck, then the magnitude of this second predetermined threshold can be set at five miles per hour. 
         [0035]    If the actual speed of the drive train system  10  is greater than the second predetermined threshold, then the method  60  branches from the decision point  64  to an instruction  65 , wherein the controller  51  causes the first and second pump/motors  34  and  35  to operate in the negative displacement mode, as described above. Then, the method  60  enters a decision point  66 , wherein it is determined whether the friction brakes  45  are currently engaged. If not, then the method  60  returns to the initial decision point  61 , wherein it is again determined whether a request for braking torque has been made by an operator of the drive train system  10 . If, however, it is determined that the friction brakes  45  are currently engaged, then the method  60  branches from the decision point  66  to a further decision point  67 , wherein it is determined whether the first and second pump/motors  34  and  35  (which are currently operating in the negative displacement mode, as described above) are applying sufficient braking torque to slow the rotation of the drive train system  10  as requested by the operator. This determination can be made in any desired manner including, for example, calculating the rate of deceleration from the actual speed signal as a function of time. 
         [0036]    If it is determined that the first and second pump/motors  34  and  35  are applying sufficient torque to slow the rotation of the drive train system  10  as requested by the operator, then the method  60  branches from the decision point  67  to an instruction  68 , wherein the friction brakes  45  are disengaged. In this transition from deceleration as a result of both the first and second pump/motors  34  and  35  and the friction brakes  45  to deceleration as a result of only the first and second pump/motors  34  and  35 , it may be desirable to insure that deceleration occurs for a predetermined minimum amount of time as a result of both the first and second pump/motors  34  and  35  and the friction brakes  45 . The amount of this predetermined minimum amount of time can be determined in any desired manner and may be based upon a variety of factors, including the weight of the vehicle and the like. The controller  51  can be easily programmed to accomplish this predetermined minimum amount of time before disengaging the friction brakes  45  in the instruction  68 . Following this predetermined minimum amount of time, the deceleration of the drive train system  10  occurs solely as a result of the first and second pump/motors  34  and  35 . This situation is desirable because it provides for maximum recovery of kinetic energy in the form of pressurized fluid in the accumulator  40 . 
         [0037]    If, on the other hand, it is determined that the first and second pump/motors  34  and  35  are not applying sufficient torque to slow the rotation of the drive train system  10  as requested by the operator, then the method  60  leaves the friction brakes  45  engaged and returns from the decision point  67  to the initial decision point  61 , wherein it is again determined whether a request for braking torque has been made by the operator of the drive train system  10 . Although less desirable from an energy recovery standpoint, this branch of the method  60  is necessary to insure that sufficient braking torque is applied to slow the rotation of the drive train system  10  as requested by the operator. 
         [0038]    Referring back to the decision point  64 , if it is determined that the actual speed of the drive train system  10  is not greater than a second predetermined threshold, then the method  60  branches from the decision point  64  to an instruction  69 , wherein the controller  51  causes the friction brakes  45  to be engaged to slow the rotation of the drive train system  10 , as described above. As mentioned above, the second predetermined threshold is less than the first predetermined threshold and is selected to represent a speed above which it is desirable to slow the rotation of the drive train system  10  by means of the first and second pump/motors  34  and  35  to operate in the negative displacement mode, as described above. The second predetermined threshold can also represent a speed below which it is desirable to slow the rotation of the drive train system  10  by means of the friction brakes  45  only, as also described above. Thus, the method  60  next enters a decision point  70 , wherein it is determined whether the first and second pump/motors  34  and  35  are applying braking torque to slow the rotation of the drive train system  10 . If not, then the method  60  returns to the initial decision point  61 , wherein it is again determined whether a request for braking torque has been made by the operator of the drive train system  10 . 
         [0039]    If, however, it is determined that the first and second pump/motors  34  and  35  were previously engaged to apply braking torque to slow the rotation of the drive train system  10 , then the method  60  branches from the decision point  70  to an instruction  71 , wherein the controller  51  causes the first and second pump/motors  34  and  35  to be disengaged at a predetermined rate or over a predetermined period of time. This “ramp down” operation of the first and second pump/motors  34  and  35  can be accomplished in any desired manner including, for example, adjusting the displacement of the first and second pump/motors  34  and  35  from a negative magnitude to zero. The purpose of this “ramp down” disengagement of the first and second pump/motors  34  and  35  functions to gradually phase out the influence thereof on the deceleration of the drive train system  10  over a period of time and, as a result, avoid any relatively sudden change in the rate of deceleration that might be considered annoying to the operator. The amount of time that the first and second pump/motors  34  and  35  is to gradually phase out can be determined in any desired manner including, for example, in accordance with the weight of the vehicle on which the drive train system  10  is used. Thereafter, the method  60  returns to the initial decision point  61 , wherein it is again determined whether a request for braking torque has been made by the operator of the drive train system  10 . 
         [0040]    The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.