Abstract:
A method for controlling operation of a transfer case in a motor vehicle driveline that includes by an engine controlled by a throttle having a variable position, and a transmission driveably connected to the engine for producing multiple ratios of the speed of a transmission input relative to the speed of a transmission output. The transfer case transmitting rotating power in response to an electric signal applied to a clutch. The method includes determining that the engine throttle position is less than a first reference throttle position during a period of predetermined length; determining that a speed of the vehicle speed is in a predetermined range; determining that the transmission is operating in a speed ratio greater than a reference speed ratio; determining that the engine throttle position is greater than a first reference throttle position; and increasing the torque capacity of the clutch for a predetermined period.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The preferred embodiment relates generally to a strategy for controlling a motor vehicle. More particularly, it pertains to electronically detecting potential inertial transfer case engagements in two wheel drive (2WD) mode and manipulating a transfer case clutch to eliminate harshness. 
         [0002]    Some four wheel drive (4WD) systems use a ball ramp mechanism to transmit rotary power to a secondary driveline while continuing to transmit power to a primary driveline. Such drive systems are intended to be activated electronically, but sudden accelerations of the input shaft can activate a torque transfer mechanism due to inertial effects. If the system is operating in 2WD mode, this activation can cause a rapid acceleration of the secondary driveline, resulting in a sharp, objectionable noise, called clunk. 
         [0003]    During tip-ins from a coasting state at higher speeds while operating in 2WD mode, the transfer case clutch may inadvertently engage and cause a significant clunk. Addressing the clunk through mechanical methods can decrease torque capacity of the 4WD system, increase parastic losses, and/or make the system slower and more difficult to control. Alternatively, increasing the amount of drag in the transfer case clutch system to prevent the secondary driveline from slowing enough to produce a clunk necessarily increases parasitic losses and decreases vehicle fuel economy. 
         [0004]    There is a need, therefore, to anticipate the likely occurrence of a clunk in a transfer case due to this cause, to detect electronically the conditions under which such a clunk is likely to occur, and to take corrective action to avoid its occurrence without compromising vehicle fuel economy, diminishing control responsiveness, or increasing parastic losses. 
       SUMMARY OF THE INVENTION 
       [0005]    A control strategy for this purpose monitors the operating modes, 2WD and 4WD, engine throttle position, the time rate of change of throttle position, vehicle speed and the current transmission gear to determine if a transfer case clunk is likely to occur. It then activates a transfer case clutch at a minimum level and duration necessary to eliminate the clunk while having minimal effect on parasitic losses. 
         [0006]    When the required conditions are detected, the control delays for a period of predetermined length, and then produce a step increase to a predefined duty cycle that is applied to the clutch for a brief period of predetermined length. This action results in a transient increase in torque applied to the secondary power path through the clutch, which prevents inadvertent engagement of the clutch when a tip-in occurs shortly after a coast condition. 
         [0007]    According to a preferred embodiment, a method for controlling operation of a transfer case in a motor vehicle driveline that includes an engine controlled by a throttle having a variable position, and a transmission driveably connected to the engine for producing multiple ratios of the speed of a transmission input relative to the speed of a transmission output. The transfer case transmits rotating power in response to an electric signal applied to a clutch. The method includes determining that the engine throttle position is less than a first reference throttle position during a period of predetermined length; determining that a speed of the vehicle speed is in a predetermined range; determining that the transmission is operating in a speed ratio greater than a reference speed ratio; determining that the engine throttle position is greater than a first reference throttle position; and increasing the torque capacity of the clutch for a predetermined period. 
         [0008]    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 
         [0009]    These and other advantages will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
           [0010]      FIG. 1  is a top view of a motor vehicle driveline that includes a transmission, transfer case, front and rear drive shafts, and shafts extending to front wheels and rear wheels. 
           [0011]      FIG. 2  is a cross section of a mechanism for transmitting torque in the transfer case; 
           [0012]      FIG. 3  is a logic diagram of the method for controlling the actuating mechanism of the transfer case to avoid driveline clunk; 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    With reference now to the drawings and particularly to  FIG. 1 , the powertrain of a motor vehicle, to which the present invention can be applied, includes front and rear wheels  10 ,  12 , a power transmission  14  for producing multiple forward and reverse speed ratios driven by an engine (not shown), and a transfer case  16  for continuously driveably connecting the transmission output to a rear drive shaft  18 . The transfer case  16  selectively connects the transmission output to both the front drive shaft  20  and rear drive shaft  18  when a four-wheel drive mode of operation is selected, either manually or electronically. Shaft  18  transmits power to a rear wheel differential mechanism  22 , from which power is transmitted differentially to the rear wheels  12  through axle shafts  24 ,  26 , which are contained within a differential housing. The front wheels are driveably connected to right-hand and left-hand halfshafts  32 ,  34 , to which power is transmitted from the front drive shaft  20  through a front differential mechanism  36 . 
         [0014]    The transfer case assembly  16  continually transmits rotating power to the rear driveshaft  18  and rear wheels  12 , which is the primary power path. The transfer case  16  intermittently transmits rotating power to the front driveshaft  20  and the front wheels  10 , which is the secondary power path, when a clutch, located in the transfer case, is actuated. 
         [0015]      FIG. 2 , a cross section of the transfer case  16 , shows a transmission output shaft  37 , a main shaft  38 , driveably coupled to shaft  37  through a speed reduction gear set  39 , a first sprocket wheel  40  supported on shaft  38 , and a second sprocket wheel  41 , driveably connected by a chain to the first sprocket wheel. Main shaft  38  is driveably connected by a spline to the rear driveshaft  18 . Sprocket wheel  41  is driveably connected to the front drive shaft  20 . 
         [0016]    The housing and an outer set of clutch plates of a multi-plate friction clutch  42  are splined to sprocket wheel  40 . An inner set of friction plates, interleaved with the plates of the outer set, are splined to main shaft  38 . Therefore, the inner set of plates rotate with the rear driveshaft  18 , and the outer set of plates rotate with the front driveshaft  20 . 
         [0017]    One half  43  of a ball-cam mechanism is secured to the housing of an electric coil  44 , which is concentric with the central axis and encircles the main shaft  38 . The other half  45  of the ball-cam mechanism is splined to the main shaft  38 . 
         [0018]    When the coil  44  is deenergized, a clutch apply plate  46 , located adjacent the clutch  42 , is spaced from the ball-cam mechanism, and no torque is transmitted from main shaft  38  to the front drive shaft  20  through clutch  42  and the sprocket wheels  40 ,  41 . All of the torque is transmitted to the rear drive shaft  18  from main shaft  38 . But when coil  44  is energized, the ball-cam mechanism  43 ,  45  forces the apply plate  46  against the clutch, forcing the inner and outer plates into frictional engagement and transmitting torque to the front driveshaft  20  through clutch  42  and the sprocket wheels  40 ,  41 . 
         [0019]    When the directional sense of torque carried by the main shaft is reversed, as occurs when the vehicle operator tips-in, i.e., abruptly depresses the accelerator pedal, after a coast condition, driveline lash causes clunk in the transfer case mechanism. The driveshaft lash causes the ball-cam mechanism to index which causes clunk. 
         [0020]    Refer now to  FIG. 3  where a method for controlling the actuating mechanism of the transfer case  16  to avoid driveline clunk is set forth. Preferably, the data communications occurs with CAN messages using a universal bus protocol. 
         [0021]    At step  50 , the control strategy is entered provided the ignition key is ON and the transfer case mode selector is in the 4×2 position, i.e. power is transmitted to the rear wheels only. The throttle position TP is determined at step  52 . At step  54 , a test is made to determine whether the current throttle position is less than a first predetermined throttle position (about 5% of the full throttle pedal range of displacement) for a first predetermined period length (about 100 ms). If the test at step  54  is logically true, a coast condition is detected and a coast flag is set at step  56 . Numbers cited here and representing the magnitudes of specific variables and parameters for a particular application are calibratable and subject to wide variation in other applications from the magnitudes mentioned here. 
         [0022]    If Coast=True, control passes to step  58  where the current vehicle speed, preferably represented by the speed of the rear driveshaft  18 , is compared to a minimum vehicle speed (about 35 kph) and to a maximum speed (about 60 kph) to determine whether the vehicle speed in the range between the minimum and maximum speeds. If the test at  58  is true, the current gear in which the transmission is operating is compared at step  60  to a reference gear to determine whether the current gear is higher than the reference gear. The reference gear is preferably second gear. If the test at  60  is true, the engine throttle position, preferably as represented by the duty cycle applied to the solenoid that actuated the engine throttle, is compared at  62  to a reference throttle duty cycle. The reference duty cycle is about 60%. 
         [0023]    If a coast condition is detected at step  56  and the tests at steps  58 ,  60 ,  62  are true, control passes to step  64  where the time rate of change of throttle position is determined over several intervals, preferably about 20 ms, 40 ms and 80 ms. At step  66 , the strategy compares the time rate of change of throttle position over one or all of the intervals to a corresponding a reference TP rate. Preferably, the reference TP rate of change for the 20 ms interval is about 4%; for the 40 ms interval, about 6%; and for the 80 ms interval, about 8%. 
         [0024]    If the test at step  66  is true, control passes to step  68  where a delay occurs for a predetermined period of about 100 ms to minimize the length of the transfer clutch actuation. Then, at step  70 , a duty cycle impulse is applied to coil  44  by increasing in a step the coil duty cycle to about 14% for a period of about 400 ms, after which the duty cycle is reduced in a step to 0%. Alternatively, if a coast condition is detected at step  56  and the tests at steps  58  and  60  are true, the test at step  66  may be deleted, whereupon control passes to steps  64  and  66 , as described above. In this case, the duty cycle impulse is applied to coil  44  after comparing the time rate of change of the engine throttle duty cycle to a reference rate of change, but without comparing the engine throttle position to reference throttle position. In either case, the control algorithm is exited at step  72 . 
         [0025]    The control, therefore, produces a transient increase in torque applied to the secondary power path through the clutch, which prevents inadvertent engagement of the clutch and avoids clunk when a tip-in occurs shortly after a coast condition. 
         [0026]    Any yaw-control, roll-control or anti-lock brake system event, preferably, would supersede the output of this control strategy. If a shift into the 4×4 mode is commanded, this strategy can be aborted at any point after the clutch activation part of the 4×4 shift occurs. However, this strategy should not be aborted during the switch debounce period, which is a brief period (usually a few tenths of a second) after which a switch is moved and during which the system takes no action. The switch debounce period is provided in case the vehicle operator has a change-of-mind or is still in the process of moving the switch to another state. 
         [0027]    References throughout the description of the control strategy and the claims to engine throttle position indicates that the motor vehicle is equipped with an electronic throttle system, in which a microprocessor controls the engine throttle opening or position as a function of vehicle speed, accelerator pedal position, the time rate of change of accelerator pedal position, and other variables, rather than by accelerator pedal position alone. However, the control strategy is applicable also to vehicles in which the engine throttle position is mechanically connected directly to the accelerator pedal. Therefore, references to “engine throttle position” are interchangeable with “accelerator pedal position.” 
         [0028]    Although the powertrain of the vehicle is described with reference to one in which the rear wheels are in a primary power path and the front wheels are in a secondary power path, the control strategy is also applicable to a powertrain in which the front rear wheels are in the primary power path and the rear wheels are in the secondary power path. Although the control strategy is described here for operation in the 4×2 mode to prevent clunk due to lash in a rear wheel power path, the strategy can be applied also to prevent clunk due to lash in either the front or rear wheel power paths. 
         [0029]    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.