Abstract:
A vehicle includes a motive power source, a transmission, and a clutch system. The vehicle further includes a controller that causes the clutch system to generate a generally constant clutch pressure to mechanically couple the motive power source and transmission as a line pressure associated with the transmission varies.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the control of mechanical coupling devices, such as clutches, for shafts or other elements of a vehicle driveline. 
       BACKGROUND 
       [0002]    Certain road vehicles may be driven by an internal combustion engine and/or an electric machine. An electric machine, for example, may be used to drive a vehicle at low speeds. An internal combustion engine and the electric machine may be used to drive the vehicle at high speeds. If the internal combustion engine and electric machine are arranged along a common driveline so as to share a common input shaft to a transmission, a clutch may be used to isolate the internal combustion engine from the electric machine. A clutch may also be used to isolate the internal combustion engine and electric machine from the transmission. Certain clutch control strategies may ensure a smooth transition from one source of motive power to another. 
       SUMMARY 
       [0003]    A vehicle may include an electric machine, a transmission, and a clutch system. The clutch system may receive a current having a magnitude that depends on a line pressure associated with the transmission and may control a clutch pressure to mechanically couple the electric machine and transmission based on the magnitude. The vehicle may further include a controller that alters the magnitude based on the clutch pressure such that the clutch pressure remains generally equal to a target as the line pressure varies. The vehicle may include a mechanical oil pump and electric oil pump each arranged to provide oil to the transmission to generate the line pressure. The line pressure may vary due to activation of the mechanical oil pump while the electric oil pump is on. The vehicle may include a pressure sensor in communication with the controller that senses the clutch pressure. 
         [0004]    A vehicle may include a motive power source, a transmission, and a mechanical oil pump and electric oil pump each arranged to generate at least a portion of a line pressure associated with the transmission. The vehicle may further include a clutch system and a controller that causes the clutch system to generate a generally constant clutch pressure to mechanically couple the motive power source and transmission as the line pressure varies due to activation of the mechanical oil pump while the electric oil pump is on. Causing the clutch system to generate a generally constant clutch pressure to mechanically couple the motive power source and transmission as the line pressure varies due to activation of the mechanical oil pump while the electric oil pump is on may include causing a non-constant current to be output to the clutch system. A magnitude of the current may be based on the clutch pressure. The vehicle may include a pressure sensor in communication with the controller that senses the clutch pressure. The motive power source may be an electric machine or internal combustion engine. 
         [0005]    A clutch system may be controlled by providing a current to the clutch system to mechanically couple an electric machine and transmission based on a target clutch pressure and a line pressure associated with the transmission, receiving data about an actual clutch pressure, and altering the current based on the data to keep the actual clutch pressure substantially equal to the target clutch pressure as the line pressure varies. The line pressure may vary due to activation of a mechanical oil pump while an electric oil pump is on. The clutch system may further be controlled by selecting the target clutch pressure based on the line pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a block diagram of a vehicle. 
           [0007]      FIG. 2  is a plot of transmission line pressure versus time. 
           [0008]      FIG. 3  is a plot of launch clutch pressure versus time. 
           [0009]      FIG. 4  is a plot of input speed for a mechanical oil pump versus time. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Embodiments of the present disclosure are described herein; however, it is to be understood that the disclosed embodiments are merely examples and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, may be desired for particular applications or implementations. 
         [0011]    Referring to  FIG. 1 , an automotive vehicle  10  may include an engine  12 , electric machine  14 , transmission  16 , and wheels  18 . The engine  12  may include an output shaft  20 , the electric machine  14  may include input and output shafts  22 ,  24 , and the transmission  16  may include input and output shafts  26 ,  28 . A disconnect clutch system  30  is arranged to mechanically couple the shafts  20 ,  22  so as to mechanically couple the engine  12  and electric machine  14 . A launch clutch system  32  is arranged to mechanically couple the shafts  24 ,  26  so as to mechanically couple the electric machine  14  and transmission  16 . The launch clutch system  32 , in other embodiments, may include a torque converter and lock up clutch in parallel with each other. Other arrangements are also contemplated. 
         [0012]    The engine  12  and/or electric machine  14  may be used to drive the wheels  18  via the transmission  16 . Beginning from a stop, for example, the disconnect clutch system  30  may be disabled to isolate the shafts  20 ,  22  from each other, the launch clutch system  32  may be enabled to lock the shafts  24 ,  26  together, and the electric machine  14  may be activated to cause the wheels  18  to move. As a demand for acceleration increases, the launch clutch system  32  may be caused to slip and the disconnect clutch system  30  may be enabled to lock the shafts  20 ,  22  together. The engine  12  may then be started and brought up to a desired speed. The amount of slip experienced by the launch clutch system  32  may then be reduced as the speed of the shafts  20 ,  22 ,  24  approaches the speed of the shaft  26 . 
         [0013]    The transmission  16  is serviced by an electric oil pump  34  and a mechanical oil pump  36 . The electric machine  14  and electric oil pump  34  are powered by electrical energy stored in a battery  38 . The mechanical oil pump  36  is powered by mechanical energy obtained from the rotation of the output shaft  24 . The electric oil pump  34  is intended to provide oil to the transmission  16  under circumstances in which a rotative speed of the output shaft  24  is not sufficient to provide threshold mechanical energy for proper operation of the mechanical oil pump  36 . 
         [0014]    A check valve  39  may regulate the flow of oil from the electric and mechanical oil pumps  34 ,  36 . In the example of  FIG. 1 , the check valve  39  is configured such that oil from the electric oil pump  34  will flow to the transmission  16  if a pressure of oil from the mechanical oil pump  36  is less than that of the oil from the electric oil pump  34  (assuming the electric oil pump  34  is active). If the pressure of the oil from the mechanical oil pump  36  is greater than that of the oil from the electric oil pump  34  (assuming the electric oil pump  34  is active), the oil from the electric oil pump  34  will not flow to the transmission  16 . 
         [0015]    A controller system  40  is operatively arranged with the transmission  16  and launch clutch system  32 . That is, the control system  40  may read information associated with the transmission  16  and operate the launch clutch system  32  on the basis thereof. For example, a solenoid and valve (not shown) of the launch clutch system  32  that control clutch pressure to mechanically couple the shafts  24 ,  26  may be controlled based on a line pressure from the transmission  16  to the launch clutch system  32 . 
         [0016]    The line pressure is proportional to the oil pressure supplied to the transmission  16  by the electric and/or mechanical oil pumps  34 ,  36 . Transitioning from the electric oil pump  34  to the mechanical oil pump  36  as described above (and/or changes in a state of the transmission  16 , such as a concurrent gear shift, etc.) may cause fluctuations in the oil pressure to the transmission  16  and hence, in the line pressure that influences operation of the solenoid and valve (not shown). Drivability issues may occur if these transitory events take place during a zero speed launch of the vehicle  10  that includes a pull up of the engine  12  as the actual line pressure may deviate from its target. Such deviations may cause a torque being transmitted to the wheels  18  to be altered in an undesirable fashion. 
         [0017]    Referring now to  FIGS. 2 ,  3  and  4 , the line pressure associated with the transmission  16 , launch clutch pressure of the launch clutch system  32 , and input speed to the mechanical oil pump  36  are plotted versus time. (The time axis is not necessarily to scale.) Prior to the vehicle movement initiated event, it is assumed that the engine  12  is off, the line pressure results from operation of the electric oil pump  34 , and a demand for vehicle movement is initiated (e.g., a driver steps on an accelerator pedal (not shown)). In response to the demand for vehicle movement, the controller system  40  generates a current for the solenoid (not shown) of the launch clutch system  32  based on the line pressure. The pressure generated by the launch clutch system  32  increases toward a target as a result. 
         [0018]    As vehicle movement begins, the input speed to the mechanical oil pump  36  increases because the shaft  24  begins to rotate. The mechanical oil pump  36  may begin to operate and thus influence the line pressure. Certain mechanical oil pumps may require some minimum threshold input speed to provide steady output. Such a threshold input speed for the mechanical oil pump  36  is indicated in  FIGS. 2 ,  3  and  4 . Below this threshold, the mechanical oil pump  36  may not provide steady output. As a result, the line pressure may deviate considerably from its target between the time when vehicle movement is initiated and the threshold input speed for the mechanical pump  36  is achieved. This may result, as mentioned above, in undesirable variations in torque transmitted to the wheels  18 . 
         [0019]    Variations in the line pressure may result in variations in the launch clutch pressure (as indicated by dashed line) because the current provided to the solenoid (not shown) of the launch clutch system  32  is based on the line pressure. That is, a magnitude of the current provided to the solenoid depends on a magnitude of the line pressure. It has been discovered, however, that information about the launch clutch pressure may be used to modify/alter/select the magnitude of the current to the solenoid of the launch clutch system  32  to minimize variations in the launch clutch pressure caused by variations in the line pressure. 
         [0020]    Referring again to  FIG. 1 , a pressure sensor  42  is operatively arranged with the launch clutch system  32  so as to be able to detect the launch clutch pressure and is in communication with the controller system  40 . The controller system  40  may thus use this pressure information as a basis for which to further control the launch clutch system  32 . The demand for vehicle movement, for example, may cause the controller system  40  to select a desired launch clutch system pressure and line pressure (and read associated data such as temperature, etc.) as known in the art. On the basis of this information, the controller system  40  may select a corresponding magnitude for current to be supplied to the solenoid (not shown) of the launch clutch system  32 . In certain examples, lookup tables mapping the desired clutch pressure, temperature data, and current magnitude may be used to facilitate the selection. Each of the lookup tables may correspond with a range of desired line pressures. That is, a first lookup table associated with desired line pressures less than a minimum threshold line pressure for non-electric vehicle mode may map values for the desired clutch pressure, temperature data, and current magnitude with each other. A second lookup table associated with desired line pressures greater than the minimum threshold and less than a feed pressure for the valve (not shown) of the launch clutch system  32  may map values for the desired clutch pressure, temperature data, and current magnitude with each other. A third lookup table associated with desired line pressures greater than the feed pressure may map values for the desired clutch pressure, temperature data, and current magnitude with each other. Hence, the desired line pressure determines which of the lookup tables to use. A value for the current magnitude, in other examples, may be obtained by aggregating data between the lookup tables. For a desired line pressure below the minimum threshold and any given desired clutch pressure and temperature, for example, the current magnitude may be found by aggregating (e.g., interpolating) the selected values from the first and second tables. Any suitable/known technique, however, may be used to determine the magnitude for the current. 
         [0021]    The controller system  40  may determine (periodically, continuously, etc.) whether the actual launch clutch pressure deviates from the desired launch clutch pressure. If the actual deviates from the desired, the controller system  40  may alter the current magnitude selected, in this example, from the lookup tables described above. The controller system  40  may reduce the selected current if the actual launch clutch pressure is greater than the desired launch clutch pressure until the actual and desired pressures are substantially equal. Referring to  FIG. 2 , an example of such a corrected current magnitude and corresponding corrected actual pressure are illustrated in phantom and solid lines respectively. If the actual launch clutch pressure is less than the desired launch clutch pressure, the controller system  40  may increase the selected current until the actual and desired pressures are substantially equal, etc. 
         [0022]    The controller system  40  may thus learn via the above algorithm the amount by which to alter the selected current based on the amount by which the actual launch clutch pressure deviates from its target. If subsequent variations in launch clutch pressure are observed as the threshold input speed to the mechanical pump  36  continues to increase, the controller system  40  may correct the current magnitude based on the information learned without having to perform closed loop control based on data from the pressure sensor  42 . This learned information, however, may only be valid for a particular instance of a transition from electric vehicle mode to non-electric vehicle mode. Subsequent vehicle launches and/or transitions may each require the closed loop control to be performed to correctly calibrate the system for that launch and/or transition. 
         [0023]    Once the threshold input speed for the mechanical pump  36  is achieved, the line pressure should take on values at least equal to the minimum threshold line pressure for non-electric vehicle mode, the launch clutch pressure should continue to increase towards its final target (to lock the shafts  24 ,  26 ), and the input speed to the mechanical pump  36  should continue to increase and allow the launch clutch system  32  to reach maximum torque capacity as the speed of the shaft  24  increases with the engine speed. 
         [0024]    The processes, methods, or algorithms disclosed herein may be deliverable to/implemented by a processing device, controller, or computer, which may include any existing programmable electronic control unit or dedicated electronic control unit. Similarly, the processes, methods, or algorithms may be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The processes, methods, or algorithms may also be implemented in a software executable object. Alternatively, the algorithms may be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, or other hardware components or devices, or a combination of hardware, software and firmware components. 
         [0025]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. 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 disclosure and claims. As previously described, the features of various embodiments may be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments may have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes may include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.