Abstract:
A continuously variable transmission is mounted on a vehicle capable of selecting plural drive modes and changes a target engine speed between drive modes. A pulley ratio is controlled by a motor. An ECU for transmission control includes a calculation section which outputs a target engine speed as a function of throttle opening and vehicle speed, and a motor control value determination section which outputs a control value for controlling the motor based on the target engine speed and an actual engine speed. In a case where, when a drive mode is selected, the current target engine speed changes by an amount exceeding a predetermined judgment value according to vehicle speed, the motor control value determination section updates the current target engine speed in a stepwise manner. The resulting configuration reduces a shift shock caused by a drive mode change.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2005-191626, filed Jun. 30, 2006, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a continuously variable transmission controller, and more particularly, to a continuously variable transmission controller suitable for reducing a shift shock generated when a drive mode is changed causing a shift characteristic to largely change.  
         [0004]     2. Description of Background Art  
         [0005]     A controller (See, for example, JP-A No. 123351/1994) for a belt-type continuously variable transmission connected to an internal-combustion engine (hereinafter referred to as an “engine”) has been known in which a pulley ratio is controlled by, using a motor, sliding a movable side of a drive pulley along an output shaft of the engine. In the controller, a target ratio is determined by searching a map based on a throttle opening and a vehicle speed, and the motor is driven such as to realize the target ratio. Furthermore, when the difference between the target ratio and the actual pulley ratio is larger, the duty ratio of the motor is made larger. At the same time, the duty is controlled according to whether an accelerator is on or off. The transmission controlled in the above manner is expected to operate smoothly meeting driving-condition-dependent requests for gear shifting.  
         [0006]     For a vehicle carrying a continuously variable transmission, use of a control method in which plural drive modes are set for the vehicle and the shift characteristic is changed between drive modes has been under study. In such a vehicle, changing the shift characteristic at a time of a drive mode change causes, in a case where there is a large difference between a target engine speed and the actual engine speed, the transmission gear ratio to be changed sharply to cope with the large difference between the target and actual engine speeds. Such an arrangement causes a shift shock which may impair the riding comfort of the vehicle and affect the durability of the transmission.  
         [0007]     Controlling the duty of a motor based on the difference between a target value and an actual value as in the controller described in JP-A No. 123351/1994 may be considered to solve the above problem. A shift shock caused by a change in shift characteristic resulting from a drive mode change, however, cannot be adequately reduced just by adjusting the duty of the motor according to a difference between pulley ratios. Hence, a further study has been considered necessary.  
       SUMMARY AND OBJECTS OF THE INVENTION  
       [0008]     An object of the present invention is to provide a continuously variable transmission controller which can reduce a shift shock generated when a shift characteristic is changed in a continuously variable transmission capable of changing the shift characteristic between drive modes of a vehicle.  
         [0009]     According to a first aspect of the present invention having the above object, a controller for a continuously variable transmission capable of selecting plural drive modes includes an actuator for changing a transmission gear ratio of the continuously variable transmission. The controller also includes target value output means which outputs a target engine speed as a function of throttle opening and vehicle speed, and shift characteristic determination means which determines a driving direction and a driving speed of the actuator based on a difference between the target engine speed and the actual engine speed. Furthermore, the shift characteristic determination means includes shift characteristic changing means that updates stepwise a current target engine speed to change to a value for use after a drive mode change, in case where it is judged that the current target engine speed is changed exceeding a predetermined judgment value according to vehicle speed when a drive mode is selected.  
         [0010]     According to a second aspect of the present invention, the controller includes means for deciding a throttle opening judgment value set as a function of drive mode and vehicle speed. Furthermore, the shift characteristic changing means is configured such that, when a current throttle opening is larger than the throttle opening judgment value, the target engine speed is updated according to a drive mode, and when a current throttle opening is smaller than the throttle opening judgment value, the target engine speed is updated according to the drive mode and the vehicle speed.  
         [0011]     According to a third aspect of the present invention, an updating speed of the target engine speed differs depending on whether the drive mode change causes the target engine speed to increase or decrease.  
         [0012]     Furthermore, according to a fourth aspect of the present invention, the updating speed of the target engine speed is made lower when the engine speed is higher at a time of the drive mode change.  
         [0013]     According to the first aspect of the present, a target engine speed can be changed in a stepwise manner. Therefore, when a change of shift characteristic which causes the engine speed to change by a large amount is requested following a drive mode change and the variation required of the engine speed is larger than a judgment value that takes into account the vehicle speed at that time, the target engine speed can be changed gradually. As the target engine speed is changed gradually, an actuator operates in small steps. In this way, a shift shock can be reduced to prevent a vehicle driver from feeling discomfort and to enhance the durability of the transmission.  
         [0014]     According to the second aspect of the present invention, when the current throttle opening is larger than a judgment value corresponding to the current drive mode and vehicle speed, that is, when it is determined that the vehicle is accelerating, the shift characteristic is changed by a variation set according to the drive mode regardless of the vehicle speed. On the other hand, when the current throttle opening is smaller than the judgment value, that is, when it is determined that the vehicle is decelerating, the shift characteristic is changed by a variation set according to both of the drive mode and the vehicle speed. This is because, during deceleration, the shift shock easily changes depending on the vehicle speed. In this way, it is possible to change the shift characteristic in small steps corresponding to the vehicle speed.  
         [0015]     According to the third aspect of the present invention, the speed of changing an engine speed can be made different depending on whether the engine speed is increasing, or whether the engine speed is decreasing. This is convenient as a shift shock caused by a change of an engine speed differs depending on whether the engine speed is increasing or decreasing.  
         [0016]     According to the fourth aspect of the present invention, when the current engine speed is higher, a longer time can be taken for transition to a target engine speed so as to reduce the shift shock caused by the change of the engine speed. This is convenient as a larger shift shock results when the current engine speed is higher.  
         [0017]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0019]      FIG. 1  is a block diagram showing functions of essential parts of a continuously variable transmission controller according to an embodiment of the present invention;  
         [0020]      FIG. 2  is a diagram showing a system configuration of a continuously variable transmission controller according to an embodiment of the present invention;  
         [0021]      FIG. 3  is a diagram showing how a target engine speed Netgt changes;  
         [0022]      FIG. 4  is a diagram showing how a target engine speed Netgt changes;  
         [0023]      FIG. 5  is a flowchart of a process for changing a characteristic of a continuously variable transmission;  
         [0024]      FIG. 6  is a diagram showing an example of a map of judgment values for judging a difference between target engine speeds;  
         [0025]      FIG. 7  is a flowchart showing an example of calculation of a variation per unit time of a target engine speed Netgt;  
         [0026]      FIG. 8  is a diagram showing an example of a map of target engine speed variations per unit time ΔNetgt dependent on vehicle speed;  
         [0027]      FIG. 9  is a diagram showing an example of a map of throttle opening comparison values THv dependent on vehicle speed;  
         [0028]      FIG. 10  is a diagram showing an example of a shift characteristic in a fuel-efficient drive mode; and  
         [0029]      FIG. 11  is a diagram showing an example of a shift characteristic in a sport drive mode.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     An embodiment of the present invention will be described in the following with reference to the accompanying drawings.  FIG. 2  is a diagram showing a system configuration of a continuously variable transmission controller according to an embodiment of the present invention. A continuously variable transmission  1  is connected to a crankshaft, that is, an output shaft  2 , of an engine (not shown) used as a drive source, for example, for an all terrain vehicle (ATV). A drive pulley  3  includes a fixed pulley piece  31  and a movable pulley piece  32  which is mounted on the output shaft  2  to be slidable along the axial direction of the output shaft  2 . A slider  5  is supported on a periphery of a hub of the movable pulley piece  32  via a bearing  4 . A gear  51  is formed on a periphery of the slider  5 . The gear  51  engages with a final stage gear  64  of a speed reducer  6  comprising four gears  61  to  64 . The first stage gear  61  of the speed reducer  6  engages with an output gear  71  of a motor  7 . A female screw formed on an inner periphery of the slider  5  is engaged with a male screw formed on an outer periphery of a cylindrical part  9  fixed to a case  8 .  
         [0031]     When the motor  7  rotates and causes the slider  5  to rotate, the female screw formed on the slider  5  rotates about the male screw formed on the cylindrical part  9 , and the slider  5  moves along the output shaft  2  due to a feeding function of the screws. As the slider  5  moves, the distance between the fixed pulley piece  31  and the movable pulley piece  32  making up the drive pulley  3  changes.  
         [0032]     A passive pulley  10  of the continuously variable transmission  1  is supported by a passive shaft  11 . The passive pulley  10  includes a movable pulley piece  101  and a fixed pulley piece  102 , both of which are rotatable about the passive shaft  11 . The movable pulley piece  101  is slidable along the passive shaft  11 . It is pushed toward the fixed pulley piece  102  by a coil spring  103 . The passive shaft  11  is provided with a centrifugal clutch  12 . The movable pulley piece  101  is connected to the passive shaft  11  via the clutch  12 . The passive shaft  11  is connected to a drive shaft of an ATV via a speed reducer which includes a gear  13 . A V-belt  14  is wound around the drive pulley  3  and the passive pulley  10 .  
         [0033]     A position sensor  15  which detects a reset position of the movable pulley piece  32  of the drive pulley  3  is provided in proximity to a periphery of the movable pulley piece  32 . A passive pulley rotation speed sensor  16  which detects a rotation speed of the passive pulley  11  is disposed to oppose a magnetic material (not shown) which rotates together with the passive pulley  10 .  
         [0034]     An ECU  17  for transmission control which drives the motor  7  is provided as a controller for the continuously variable transmission  1 . The ECU  17  for transmission control has a microcomputer and is powered by a battery  18 .  
         [0035]     The vehicle is provided with a mode switch  19  for selecting a drive mode. When a drive mode is selected out of a manual mode and plural automatic modes, a corresponding shift characteristic is selected. A change switch  20  outputs a change signal CH for switching the direction of changing a shift position between upward and downward. The change switch  20  becomes effective when the manual mode is selected, and causes a shift position to be selected according to the change signal CH. In the manual mode, the motor  7  is driven such that a predetermined pulley ratio is set for each shift position. The pulley ratio is a ratio of a rotation speed N 0  of the drive pulley  3  to a rotation speed N 1  of the passive pulley  10 , that is, (N 1 /N 0 ). A throttle sensor  21  detects an opening of a throttle valve of the engine (not shown) and outputs opening information TH. An engine speed sensor  22  detects a reluctor provided on a rotor of a generator (ACG) (not shown) connected to the output shaft of the engine and outputs a rotation speed of the ACG, that is, a rotation speed Ne of the engine.  
         [0036]     Next, drive modes will be described. Plural drive modes have been set, and each of the plural drive modes has a corresponding shift characteristic. The present embodiment uses one manual mode and two automatic modes, that is, a sport drive mode and a fuel-efficient drive mode.  
         [0037]     Plural transmission gear ratios have been set for use in the manual mode. Of the plural transmission gear ratios, one specified by the change switch  20  is selected. The vehicle can then be driven using the selected transmission gear ratio as a fixed transmission gear ratio.  
         [0038]     In the sport drive mode, more powerful driving is possible at a higher engine speed than in the manual mode. In the fuel-efficient drive mode, contrary to the sport drive mode, driving at an engine speed lower than in the manual mode is realized.  FIGS. 10 and 11  show examples of shift characteristics corresponding to the fuel-efficient drive mode and the sport drive mode, respectively.  
         [0039]      FIG. 1  is a block diagram showing functions of essential parts of the transmission controller (ECU  17 ). A target engine speed calculation section  23  calculates a target engine speed Netgt based on the throttle opening TH and a vehicle speed V. The target engine speed calculation section  23  can be made up of, for example, a map which can output a target engine speed Netgt as a function of the throttle opening TH and the vehicle speed V. Such a map is prepared for each drive mode. The rotation speed of the passive pulley  10  that is detected by the passive pulley rotation speed sensor  16  may be substituted for the vehicle speed V.  
         [0040]     A motor control value determination section  24  constitutes means of determining a shift characteristic. It determines the direction of rotation of the motor  7  and the duty of the motor  7 , that is, the rotation speed of the motor  7  based on a difference between the target engine speed Netgt calculated by the target engine rotation calculation section  23  and an actual engine speed Ne obtained by the engine speed sensor  22 .  
         [0041]     When the target engine speed Netgt is higher than the actual engine speed Ne, the motor  7  is driven in the direction for widening the distance between the fixed pulley  31  and the movable pulley  32  to increase the pulley ratio. When the difference between the target engine speed Netgt and the actual engine speed Ne is larger than the judgment value corresponding to the vehicle speed, the target engine speed Netgt is changed gradually so as to extend the time used to change the pulley ratio. This will be described in more detail later. The motor  7  is driven in accordance with control values outputted from the motor control value determination section  24 , that is, in the direction of rotation and at the duty of the motor  7  specified by the control values so as to meet the target engine speed Netgt. Consequently, the pulley ratio is changed.  
         [0042]     In a conventional apparatus, when the drive mode is changed, the target engine speed Netgt changes sharply generating a large shift shock. In the present embodiment on the other hand, when the drive mode is changed, the target engine speed Netgt is changed gradually, so that the shift shock is reduced.  
         [0043]      FIG. 3  is a diagram showing how the target engine speed Netgt changes when no measure to make the target engine speed Netgt change gradually is taken. Lines A and B represent characteristics of the target engine speed Netgt before and after a drive mode change, respectively.  
         [0044]      FIG. 4  is a diagram showing an example effect of a measure taken to reduce the shift shock. As shown, when the drive mode is changed, the target engine speed Netgt is not changed instantly to the line B. It is updated gradually from the line A to the line B over a transition period. In this way, the shift shock can be reduced. The transition period is varied according to the target drive mode, the current drive mode, and the vehicle speed at the time when the current drive mode is changed (see  FIG. 7 ).  
         [0045]      FIG. 5  is a flowchart of a characteristic changing process for gradually changing the target engine speed Netgt. In step S 1 , whether or not the mode switch  19  has been operated, that is, whether or not the drive mode has been changed is determined. When the drive mode is determined to have been changed, the process advances to step S 2 . In step S 2 , whether or not the difference between the target engine speed in the drive mode to be changed (current drive mode) and the target engine speed in the drive mode to be changed to (target drive mode) exceeds a threshold value (judgment value) is determined. At this time, the judgment value is read out from a prepared map which corresponds to the vehicle speed V.  FIG. 6  shows an example of a map showing a relationship between vehicle speed and judgment value.  
         [0046]     When the difference between the two target engine speeds is determined to be larger than the judgment value, the process advances to step S 3 , in which it is determined whether a transition mode flag is on or off. As the transition mode flag is initially off, it is determined to be off when checked for the first time, so that the process advances to step S 4  in which the transition mode flag is turned on. In step S 5 , the current target engine speed added to by a variation per unit time is set as a target engine speed Netgt for the transition period.  
         [0047]     When the transition mode flag is determined to be on in step S 3 , the process advances to step S 6  in which the previous target engine speed Netgt added to by a variation per unit time is set as a target engine speed Netgt for the transition period. In step S 7 , transmission control is performed based on the target engine speed Netgt calculated in step S 6  or S 7  and using functions described with reference to  FIG. 2 .  
         [0048]     When the outcome of step S 1  or S 2  is negative, the process advances to step S 8  in which the transition mode flag is turned off. In step S 9 , transmission control is performed using the normal target engine speed Netgt (for example, as represented by the foregoing line B), not the target engine speed for the transition period.  
         [0049]     Even though, in steps S 5  and S 6  described above, the current or previous target engine speed Netgt was added to by a variation per unit time, the current or previous target engine speed Netgt is subtracted by the variation per unit time in cases where the target engine speed Netgt for after the drive mode change is lower than the current or previous target engine speed Netgt.  
         [0050]      FIG. 7  is a flowchart showing an example of a calculation of a variation per unit time of the target engine speed Netgt. The variation determines a transition period. In the calculation example, the variation of the target engine speed Netgt is determined by the drive mode and vehicle speed V, or by the drive mode only, depending on the magnitude of the throttle opening TH.  
         [0051]     In step S 11  shown in  FIG. 7 , a comparison value THv to be compared with the throttle opening TH is calculated. The comparison value THv is calculated based on the drive mode and the vehicle speed V. It is possible, for example, to prepare a map showing a relationship between the throttle comparison value THv and the vehicle speed V for each drive mode and determine, when required, a value of the comparison value THv by referring to the corresponding map.  
         [0052]     In step S 12 , the current throttle opening TH is compared with the comparison value THv. When the throttle opening TH is smaller than the comparison value THv, the process advances to step S 13 . In step S 13 , the target engine speed variation per unit time ΔNetgt corresponding to the drive mode and vehicle speed V is calculated. It is possible, for example, to prepare a map showing a relationship between the target engine speed variation per unit time ΔNetgt and the vehicle speed V for each drive mode and determine, when required, a value of the target engine speed variation per unit time ΔNetgt by referring to the corresponding map.  
         [0053]     When the outcome of step S 12  is negative, that is, when the throttle opening TH is larger than the comparison value THv, the process advances to step S 14 . In step S 14 , the target engine speed variation per unit time ΔNetgt preset for the drive mode is read out.  
         [0054]      FIG. 8  is a diagram showing an example of a map showing the target engine speed variation per unit time ΔNetgt dependent on the vehicle speed. Different maps like this are prepared for different drive modes. It is preferable that different maps corresponding to each drive mode be prepared for use in addition and for use in subtraction, respectively. When different maps are used for addition and for subtraction, the transition period becomes different between when addition is made and when subtraction is made. For example, the target engine speed variation per unit time ΔNetgt corresponding to a vehicle speed is made smaller for use in addition than for use in subtraction.  
         [0055]      FIG. 9  is a diagram showing an example of a map which can be used in step S 11  and which shows the throttle opening comparison value THv dependent on the vehicle speed. Different maps like this are prepared for different drive modes.  
         [0056]     Also, when the engine speed Ne is higher at the time of a drive mode change, the transition period may be made longer, that is, the target engine speed variation per unit time ΔNetgt may be made smaller.  
         [0057]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.