Abstract:
A method for controlling a vehicle driveline includes using current conditions to estimate wheel slip probability and vehicle dynamics handling support requirements, producing two-wheel drive operation, if said slip probability and handling support requirement is low and a condition for forced driveline connection is absent, and producing four-wheel drive operation, if said slip probability and/or handling support requirement is high and a condition for forced driveline disconnection is absent.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a motor vehicle driveline, which in operation transmits power continually to a first wheel set and selectively to a second wheel set. 
         [0003]    2. Description of the Prior Art 
         [0004]    All-wheel-drive (AWD) systems tend to reduce vehicle fuel economy due to increased driveline parasitic losses, even when AWD is not activated. Driveline disconnect systems improve fuel economy by disconnecting as many of the driveline rotating parts as possible, as close to the transmission output and the secondary drive wheels as possible, when all-wheel-drive is not activated. 
         [0005]    In virtually all front-wheel-drive (FWD) vehicles and many rear-wheel-drive (RWD) vehicles that produce all-wheel drive (AWD) or four-wheel drive (4WD), operation in two-wheel-drive (2WD) is not provided. In such vehicles, 2WD operation is produced in response to being manually selected by the vehicle operator. But requirement that 2WD operation be manually selected creates an inconvenience for operators, who may expect fully automatic operation of the driveline. It also decreases fuel economy for operators who leave the vehicle in AWD/4WD mode, or in vehicles that provide no selectable 2WD operation. 
         [0006]    A need exists in the industry for a control method that automatically switches between 2WD and AWD or 4WD modes to save fuel while minimizing or eliminating any disruptions that the vehicle occupants might notice. 
       SUMMARY OF THE INVENTION 
       [0007]    A method for controlling a vehicle driveline includes using current conditions to estimate wheel slip probability and the likelihood that AWD torque transfer will be required to support vehicle handling performance, producing two-wheel drive operation, if said slip probability is low, handling support is not required and a condition for forced driveline connection is absent, and producing four-wheel drive operation, if said slip probability is high and/or handling support is required and a condition for forced driveline disconnection is absent. 
         [0008]    The control provides a method for automatically switching between 2WD and AWD/4WD modes to improve fuel economy while minimizing or eliminating any disruptions that the vehicle occupants might notice. 
         [0009]    The control monitors numerous vehicle signals and preemptively produces shifts from 2WD to AWD/4WD when wheels slip is likely to occur and/or handling support is required. The control uses a rule based or fuzzy logic control system to anticipate the likely occurrence of wheel slip, and performs the shift at a connect speed that is determined to not produce excessive noise, vibration or harshness. 
         [0010]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic diagram of a motor vehicle driveline having primary and secondary road wheels; 
           [0013]      FIG. 2  is a cross section showing a drive system that connects a power source continually to a primary wheel set and selectively to a secondary wheel set; and 
           [0014]      FIG. 3  is diagram showing information flow and method steps for engaging the driveline of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    The driveline  10  of  FIG. 1  includes a power source  12 , such as an internal combustion engine or an electric motor, and a transmission  14  that produces a variable ratio between the speed of its output  16 , which is continually driveably connected through a differential mechanism  18  to the primary road wheels  20 ,  22 , and the speed of the transmission input, which is driveably connected to the power source. 
         [0016]    The primary wheels  20 ,  22  are driven continually by the engine during torque transfer conditions. The secondary wheels  26 ,  28  are undriven road wheels, except that they are driven by the engine during torque transfer conditions when AWD is operating. 
         [0017]    A power transfer unit (PTU)  24  transmits power from the transmission output  16  selectively to the secondary road wheels  26 ,  28 . A driveshaft  30  transmits rotating power from the PTU  24  to a rear drive unit (RDU)  32 . 
         [0018]    The PTU  24  comprises a coupler  34 , such as a dog clutch or synchronizer, whose input is driveably connected to the transmission output  16 ; a bevel ring gear  36  connected to the output of the PTU coupler  34 , and a bevel pinion gear  38  meshing with the bevel ring gear  36  and connected to driveshaft  30 . The PTU coupler  34  disconnects the rotating components of the PTU and driveline components downstream of the PTU from the transmission output  16 . 
         [0019]    The RDU  32  includes a bevel pinion gear  40 , secured to driveshaft  30 ; a bevel ring gear  42 , meshing with pinion  40 , a differential mechanism  44 , and a low-drag coupling  46 . The secondary wheels  26 ,  28  are driven by halfshafts  48 ,  50  though coupling  46  and differential  44 . Coupling  46  alternately connects and disconnects halfshafts  48 ,  50  from the rotatable components of the RDU  32 . 
         [0020]      FIG. 2  illustrates details of the power path that connects the transmission output  16  continually to the halfshafts  60 ,  62  for the primary wheels  20 ,  22  through differential  18 , and to the PTU input shaft  64 , which is connected to bevel ring gear  36 . 
         [0021]    A compound planetary differential  18  includes a sun gear  72 , secured through a spline  74  to axle shaft  62 ; a carrier  76 , secured through a spline  78  to axle shaft  60 ; a ring gear  80 , engaged with an pinion  82  formed on the transmission output shaft  16 ; first planet pinions  84  supported on the carrier and meshing with the ring gear  80 ; and second planet pinions  85  supported on the carrier  76  and meshing with the sun gear  72  and the first planet pinions  84 . One side of ring gear  80  is secured to a disc  86  and supported at a bearing  88 ; the other side of ring gear  80  is secured to a disc  90  and supported at a bearing  92 . Disc  90  is formed with an internal spline  93 , which engages an external spline formed on a coupler sleeve  94 . 
         [0022]    Disc  90  forms a cylinder  96 , which contains a piston  98 , actuated by pressurized hydraulic fluid carried to cylinder  96  through a passage  100 . A compression return spring  102  restores piston  98  to the disengaged position shown in the  FIG. 2 . Piston  98  is secured to coupler sleeve  94  such that they move along an axis  103  and rotate about the axis as a unit. 
         [0023]    The volume  104  enclosed by piston  98  and spring retainer  106  forms a balance dam containing hydraulic fluid supplied from source of hydraulic lubricant  108  through a lube circuit, which includes passages  110 ,  112 ,  114 ,  116 . 
         [0024]    In operation, fluid from a source of line pressure is carried to a valve, which is controlled by a variable force solenoid. The valve opens and closes a connection between the line pressure source and passages  126 ,  128 , which carry piston-actuating pressure to cylinder  96  depending on the state of the solenoid. When passages  126  and  128  are pressurized, piston  98  and coupler sleeve  94   30  move leftward, causing frictional contact at the conical surface between a member  130  and a synchronizing ring  132 . Member  130  is rotatably secured by spline  134  to PTU input shaft  64 . As the speed of member  130  is synchronized with the speed of ring gear  80 , the internal spline of coupler sleeve  94  engages the dog teeth on synchronizing ring  132  and the clutch teeth  136  on the radial outer surface of connecting member  130 , thereby driveably connecting ring gear  80  and PTU input shaft  64 . 
         [0025]    When passages  126  and  128  are vented, piston  98  and sleeve  94  move rightward to their disengaged positions, causing connecting member  130  to disengage the ring gear  80 , thereby disconnecting ring gear  80  from PTU input shaft  64 . 
         [0026]    Although the description refers to the speed of connecting member  130  being synchronized with the speed of ring gear  80  using a synchronizer, a connection between ring gear  80  and PTU input shaft  64  can be completed using a coupler, such as a clutch, instead of a synchronizer. 
         [0027]    In the disconnected state, the RDU coupling  46  and PTU coupling  34  are open, causing the rotatable RDU components, driveshaft  30 , and rotatable PTU components to be disconnected from the secondary wheels  26 ,  28  and halfshafts  48 ,  50 . 
         [0028]    In the connected state, the PTU coupler  34  is closed, causing driveshaft  30  to rotate with the primary wheels  20 ,  22  and transmission output  16 . The RDU coupling  46  has a variable torque transmitting capacity, which may produce a fully engaged connection or a defined speed difference between driveshaft  30  and the secondary wheels  26 ,  28 , as required to produce AWD operation. 
         [0029]      FIG. 3  shows the method steps of a rule-based or fuzzy logic type control system for engaging and disengaging 2WD, and AWD/4WD in the driveline of  FIG. 1 . The probability of vehicle wheel slip occurring or handling support being required under current conditions is estimated with reference to the current vehicle, road and weather conditions, which may include without limitation, vehicle operator driving patterns evidenced by driver control of the vehicle, terrain detection, terrain response mode, temperature, GPS data, weather data, coefficient of friction detection methods such as comparing amount of tire rotation for driven vs. undriven wheels, difference in left-to-right tire rotation during turns vs. steering wheel angle, and actual yaw vs. intended yaw. 
         [0030]    At step  150  a controller reads various driveline sensors incorporated in software modules. The output for each module is in a range between 0 and 1, zero representing a low probability that wheel slip will occur due to the sensed variable corresponding to a respective module, unity representing a high probability that wheel slip will occur due to the current value of the sensed variable. For example, certain sensors indicate the degree to which the following current conditions indicating that wheel slip is probable or imminent and/or that handling support is required: (i) the vehicle is travelling on a rough road  152 , (ii) anti-lock brake system (ABS), brake traction control system (BTCS) or electronics stability control (ESC) intervention is currently active  154 ; (iii) wheel slip is occurring  156 ; (iv) vehicle handling is challenging  158 ; and (v) the vehicle is towing a trailer  160 . Other output signals  162  produced by vehicle sensors indicate the degree to which the following variables influence wheel slip and the weight attributed to the current value of the variable: the vehicle is turning on a road having a low coefficient of friction; vehicle speed is low; the status of BTCS/ESC over-ride switch, the status of the AWD terrain mode selector switch; detected gear in which the transmission is operating; estimated ambient temperature; weather conditions (either sensed directly or inferred from an external wireless data transmission such as a weather report); hill or incline detected; GPS vehicle location data; driving resistance; AWD torque transfer; estimation of tire to road friction; road curve detected; axle articulation (either measured directly via sensors located on the vehicle or inferred from calculation of various vehicle state conditions); radar sensor information. In this way a weighted sum is produced indicating the probability that wheel slip will occur under current conditions. 
         [0031]    “Terrain Mode” is the operating mode selected in a Terrain Management system. Different modes can be selected by the driver for different driving situations, e.g. Normal, Grass/Gravel/Snow, Mud/Ruts, Sand. Terrain Mode is independent of ESC, but can change ESC modes. Axle articulation is the movement of the suspension. When driving off-road, for example, the wheels might go from being at full droop, i.e., fully extended, to being at full compression. We look at all four wheels. Wheel travel sensors, fitted to all four wheels with active damping, would be used to measure wheel position, and it would be tracked over time to assess the road/ground conditions. 
         [0032]    A controller monitors these signals and changes preemptively the operating state of the driveline  10  between 2WD and AWD/4WD in accordance with the weighted sum. 
         [0033]    At step  166  the sum of the module outputs is determined. 
         [0034]    At step  170 , signals  152 ,  154 ,  156 ,  158  are used to evaluate noise, vibration and harshness (NVH) and vehicle body movement to determine whether the vehicle occupants will notice a fast engagement of AWD/4WD. Reconnecting the AWD system too quickly can result in audible clunks, tactile vibrations, or a drop in vehicle acceleration that can be felt or heard by vehicle occupants as objectionable NVH. This step recognizes that various NVH events, such as driving on a rough road, will mask what would normally be perceivable NVH resulting from the rapid engagement of the AWD system on a smooth road, thereby allowing a much faster engagement time than would normally be considered acceptable. 
         [0035]    At step  172 , signals  156 ,  158  are used to evaluate need for a fast engagement of AWD/4WD. The need for a fast engagement may result from the need for AWD to immediately reduce wheel slip or influence vehicle handling to maintain acceptable vehicle driveability. 
         [0036]    At step  174  a test is made to determine whether the sum determined at step  166  is equal to or greater than a reference, such as  1 . 0 . 
         [0037]    If the result of test  174  is logically false, at step  176 , the controller determines the history of changes in driveline state between 2WD and AWD/4WD. 
         [0038]    At step  178  a test is made to determine whether changes in driveline state between 2WD and AWD/4WD have occurred at a high rate, e.g., a rate that exceed a reference rate. 
         [0039]    If the result of test  178  is true indicating frequent changes in driveline state, or if test  174  is true indicating a high probability of wheel slip and/or that handling support is required, control advances to step  180  where the controller determines whether a condition is present that would require a disconnect regardless of connect input, i.e., require that the driveline produce 2WD. Conditions that would require a disconnect regardless of connect input may include an ESC event in progress, a reported failure mode, and the current gear produced by the transmission  14 . 
         [0040]    If the result of test  178  is false, at step  184  the controller determines whether there are special conditions that would force a connect, i.e., require that the driveline produce AWD/4WD. Conditions that would require a connect regardless of disconnect input may include a driver disabled ESC system or has selected a special Terrain Response mode, very low vehicle speed or a reported failure mode. 
         [0041]    If the result of test  180  is true, or the result of test  184  is false, at step  182  the controller disconnects the transmission output  16  from the RDU  32 , thereby producing 2WD. 
         [0042]    If the result of test  180  is false indicating that a disconnect would not be forced, or test  184  is true indicating that a connect would be forced, at step  186 , a test is made to determine whether a fast connect is possible or required. 
         [0043]    If the result of test  186  is true, at step  188  a fast connection is executed by connecting transmission output  16  through PTU input shaft  64  and bevel ring gear  36  to the RDU  32 , thereby producing AWD/4WD. 
         [0044]    If the result of test  186  is false, at step  190  a connection between the transmission output  16  and the RDU  32  is executed at normal speed, thereby producing AWD/4WD. 
         [0045]    The speed of connection can be continuously variable depending on current conditions as represented by the outputs signals of the sensors. For example, during a “normal connect,” the AWD system can be connected with “good NVH” in a much shorter time at lower speeds than if the vehicle is traveling at higher speeds. Similarly, at higher operating temperatures a “normal connect” can occur much quicker than at lower temperatures. A fast connect would be around 100 ms, and a slow connect about 400ms. 
         [0046]    If preemptive measures of  FIG. 3  fail, traction control and/or stability control would be used to maintain acceptable vehicle performance during the first AWD/4WD engagement. 
         [0047]    Powertrain controls can be used to increase torque during each 2WD to AWD/4WD shift to compensate for the loss of power due to inertia and spin resistance of the secondary drive path. 
         [0048]    The driveline system will disconnect, i.e., shift from AWD/4WD to 2WD, using similar inputs, potentially including ignition cycles and cruise control status. 
         [0049]    In a FWD-based application, a low loss clutch with limited capacity can be placed in front of the PTU  24  to synchronize the PTU, rear driveshaft  30  and front end of the AWD clutch under many conditions while assistance will be required from the AWD clutch will be required under more severe conditions such as operating at high vehicle speed or low temperature. Alternately, if vehicle packaging permits, a high capacity clutch may be placed in front of the PTU to synchronize the secondary driveline under all conditions while a simple dog clutch is utilized to lock and unlock the secondary driveline from the rear wheels. 
         [0050]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.