Abstract:
A hydrostatic continuously variable transmission includes a fixed capacity hydraulic pump and a variable capacity hydraulic motor, connected by a hydraulic closed circuit. A control system and method for the variable transmission provides a good deceleration feeling when going downhill. In accordance with the control system and method, a range of a selected gear is determined. Next, it is determined whether the vehicle is in an automatic gear shift mode. If these conditions are met, it is then determined whether or not certain criteria for descent control are satisfied. The certain criteria may include: (1) whether the throttle is closed; (2) a speed or acceleration of the vehicle; (3) whether the acceleration exceeds a threshold; and (4) whether simultaneous satisfaction of conditions (1)-(3) exist for a period of time exceeding a fixed time. If all of the criteria are satisfied, the gear ratio is shifted by a specified amount towards a LOW side. Therefore, when driving downhill, the vehicle is not subject to acceleration greater than a predetermined value.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a hydrostatic continuously variable transmission in which a fixed capacity hydraulic pump and a variable capacity hydraulic motor are connected by a hydraulic closed circuit.  
           [0003]    2. Description of the Relevant Art  
           [0004]    Hydrostatic continuously variable transmissions are well-known. Such transmissions are applied to various vehicles, such as motorcycles. Japanese Patent No. 2527199 describes a control method for a hydrostatic continuously variable transmission, in which an actual rotation speed (Ne) of a crankshaft, or the like, is compared with a target rotational speed (Ne), which has been determined in accordance with predetermined conditions, and an output is controlled by adjusting an inclination angle of a swash plate.  
           [0005]    Japanese Patent Laid-Open No. Hei 8-82354 discloses a control method for a continuously variable transmission in which a gear ratio is controlled in a step-by-step manner, as in a manual multi-step transmission (this method is called “multi-stage variable transmission control”).  
           [0006]    When a throttle is closed during downhill driving, for example, the foregoing continuously variable transmission holds the gear ratio at a value immediately prior to downhill driving in order that the vehicle can smoothly decelerate.  
           [0007]    The prior art suffers several drawbacks. When a vehicle provided with such a continuously variable transmission and pulling a heavy trailer or the like is driving downhill, it is pushed from behind. Although the throttle is closed in order to maintain the gear ratio, the vehicle is gradually accelerated during the downhill driving. Therefore, there is a need in the prior art for a control method for a continuously variable transmission, which enables a vehicle to reliably drive without acceleration even in the foregoing situation.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the present invention to solve one or more of the drawbacks associated with the background art.  
           [0009]    A continuously variable transmission of the present invention comprises a control method for controlling the continuously variable transmission, which continuously vanes a gear ratio, wherein the gear ratio is changed by a predetermined amount to a LOW side from a normal gear ratio and acceleration is regulated to be below a predetermined value when a throttle is closed and acceleration is above the predetermined value.  
           [0010]    In accordance with the present invention, when a throttle is closed and acceleration of a vehicle exceeds the prescribed value, control is performed to shift the gear ratio by a prescribed amount towards the LOW side from that immediately before, so that the acceleration is regulated to be equal to or less than the predetermined value. As a result, a feeling of natural deceleration is obtained when a throttle is closed. Therefore, this is especially effective when driving a vehicle pulling a trailer or the like downhill. Even if the trailer pushes the vehicle from behind, the vehicle can be driven without acceleration.  
           [0011]    Other objects and 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  
       [0012]    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:  
         [0013]    [0013]FIG. 1 illustrates a control system, in accordance with the present invention;  
         [0014]    [0014]FIG. 2 is a partial cross sectional view illustrating mechanical components for inclination angle control of a movable swash plate, in a hydrostatic continuously variable transmission;  
         [0015]    [0015]FIG. 3 is a flowchart of a continuously variable transmission control method;  
         [0016]    [0016]FIG. 4 is a graph used in determining RC (riding conditions);  
         [0017]    [0017]FIG. 5 is a graph illustrating a gear shift map;  
         [0018]    [0018]FIG. 6 is a flowchart of a multi-stage variable transmission control method;  
         [0019]    [0019]FIG. 7 is a diagram of various modes; and  
         [0020]    [0020]FIG. 8 is a flowchart of a downhill drive control method.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring to FIG. 1, a hydrostatic continuously variable transmission  1  comprises a fixed capacity hydraulic pump  2  and a variable capacity hydraulic motor  3  integrally provided on a drive shaft  4 , with the fixed capacity hydraulic pump  2  and the variable capacity hydraulic motor  3  being connected by a hydraulic closed circuit. A drive gear  7  provided on a crankshaft  6  of an engine  5  rotates a driven gear  8  of the fixed capacity hydraulic pump  2 , thereby providing hydraulic pressure thereto. This hydraulic pressure rotates the variable capacity hydraulic motor  3  at a variable speed, which transmits a changed output to the drive axle  4 . A gear ratio can be arbitrarily changed by the inclination angle control mechanism  10  that changes an inclination angle of the movable swash plate (to be described later) of the variable capacity hydraulic motor  3 .  
         [0022]    The inclination control mechanism  10  transmits the output of a control motor  11  via a linkage system, including a reduction gear  12 , and changes the inclination angle of the movable swash plate built into the variable capacity hydraulic motor  3  through a threaded rod and follower, such as a ball screw  13  and a slider  14 . The transmission output of the hydrostatic continuously variable transmission  1  is transmitted from an output gear  4   a  of the drive axle shaft  4  to a sub gear train  15 , being a secondary reduction gear train. A transmission output of sub gear train  15  is transmitted from an output gear  17  on a transmission shaft  16  to a final output gear  19  on a final output shaft  18 .  
         [0023]    The sub gear train  15  is switched by manually operating a sub-transmission lever  20  mounted on a drive range change-over switch  20   b  to drive a shifter  21 , so that each shift position of L or D on the forward side, reverse R, or neutral N can be selected. The range L is for driving at low speed, the range D is for driving at normal or ordinary speed, N is for neutral, and R is for reverse driving. When the shift position R is selected, the gear ratio is fixed at a LOW ratio (e.g., a ratio of transmission output/input is relatively lower).  
         [0024]    Each of the L and D shift positions on the forward side can be switched to various drive modes (to be described later) by a mode map switch  29  provided on a steering handle. The drive modes roughly comprise an automatic shift mode and a multi-stage variable transmission mode. When the variable transmission is selected, shift-up and shift-down can be manually performed by operating a shift switch  28  provided on the handle.  
         [0025]    [0025]FIG. 7 describes predetermined drive modes. When the L range is selected by the sub-transmission lever  20 , the mode map switch  29  is switched to D 1  or D 2 , and a mode is changed to an L range auto mode of the continuously variable transmission mode dedicated to the L range. Further, when the mode map switch  29  is switched to ESP, the mode is changed to an L range ESP mode of the manual mode dedicated to the L range, enabling manual change of 5 gears on the forward side.  
         [0026]    When the D range is selected, the mode map switch  29  is switched to D 1 , and a sports mode suitable for normal driving is selected. When the mode map switch  29  is switched to D 2 , a utility mode suitable for pulling trailers or cruising is selected. When switched to ESP, the manual mode for normal driving is adopted, in which it is possible to manually change 5 gears on the forward side.  
         [0027]    In the continuously variable transmission and multi-stage variable transmission, the gear ratio is actually changed by inclination angle control. The inclination angle control is performed by a control unit  22 , which controls the operation of the control motor  11  of the inclination angle control mechanism  10 , based on signals from various sensors. In addition, the control unit  22  outputs display signals to indicators of an instrument panel M, and is provided with power from a battery on the vehicle.  
         [0028]    As shown in the FIG. 1, the control unit  22  receives for the inclination control mechanism  10  the following signals, which are a throttle angle signal from a throttle sensor  23  mounted on the intake side of the engine  5 , an Ne signal from a rotation sensor  24  located near the crankshaft  6 , a vehicle speed signal from a speed sensor  25  located near the final output gear  19 , an inclination angle signal from an angle sensor  26  provided at the variable capacity hydraulic motor  3 , shift position signals from a shift sensor  27  integrated with a shift drum  21   a  of a shifter  21  so as to detect shift positions, and signals from the shift switch  28  and mode map switch  29  provided in the steering wheel. Further, the control unit  22  receives a signal from a lever switch  20   a  located at the lower part of a sub-transmission lever  20  of the drive range change-over switch  20   b.    
         [0029]    Next, a description is given of the inclination angle control mechanism  10  shown in FIG. 2. The control motor  11 , of the inclination angle control mechanism  10 , is supported by a housing  30  of the fixed capacity hydraulic pump  2 . An output of the control motor  11  is transmitted to a ball screw drive gear  35  from a gear  34  through an input gear  33  of a torque limiter  32 . The input gear  33  is driven by an output gear  31  of the control motor  11 . The ball screw drive gear  35  rotates together with the ball screw  13 . As the ball screw  13  rotates forward or backward, the slider  14  having a nut slides on the shaft in either direction. Both ends of the ball screw  13  are supported by a housing  36  of the hydraulic motor  3 .  
         [0030]    An arm  37  extending from the housing  36  has one end thereof rotatably attached to the slider  14 , and the other end thereof integrated with a swash plate holder  38  supported in the housing  36 . The swash plate holder  38  is rotatably supported on a concave surface  39  of the housing  36 . When the arm  37  rotates, the swash plate holder  38  also rotates on the concave surface  39  and changes the angle of the movable swash plate  40 .  
         [0031]    The movable swash plate  40  is rotatably held against the inner side of the swash plate holder  38  via bearings  41  and  42 , and by varying the angle of the swash plate holder  38 , the inclination angle formed between a rotation surface of the movable swash plate  40  and the axis of the drive shaft  4  is altered. FIG. 2 illustrates an angle of 90°, which is a TOP state where the gear ratio is 1.0.  
         [0032]    Hydraulic plungers  43  of the variable capacity hydraulic motor  3  are pressed against this movable swash plate  40 . A plurality of hydraulic plungers  43  are provided in a peripheral direction of a rotating body  44 , and are pushed out so as to press against the variable swash plate  40  side by the hydraulic pressure at the fixed capacity hydraulic pump  2  side. Rotational force is supplied to the rotating body  44  according to the inclination angle of the variable swash plate  40 . The rotating body  44  is fitted on the drive shaft  4  using a spline joint  45 , and the drive shaft  4  is driven to rotate by rotation of the rotating body  44 .  
         [0033]    Next, gear shift control for a continuously variable transmission in the control unit  22  will be described using FIG. 3. In step S 101 , RC (riding conditions) are created from a throttle signal sent from the throttle sensor  23 . RC basically increases or decreases in accordance with the value of the throttle signal:  
         [0034]    (1) When the throttle is opened, RC increases;  
         [0035]    (2) When the throttle is closed, RC decreases.  
         [0036]    In FIG. 4, TH stands for a throttle opening amount. The vertical axis represents the throttle opening amount and RC (each given as a %). The horizontal axis represents time.  
         [0037]    In step S 103 , vehicle speed is calculated separately from a vehicle speed signal sent from the throttle sensor  25 . Following, in step S 105 , a target Ne (e.g. rotation sped) is determined with reference to a previously built-in or stored gear shift map, based on the RC and the vehicle speed. One example of a gear shift map is shown in FIG. 5, in which a predetermined number of types have been prepared. For example, various modes such as L range exclusive, sports mode exclusive, utility mode exclusive etc. are stored, and these can be selected using the mode map switch  29 .  
         [0038]    In step S 107 , actual Ne is calculated from an Ne signal sent from the rotation sensor  24 . In step S 109 , the actual Ne and the target Ne are compared, and it is determined whether the rotation direction of the control motor  11  should be forward or reverse, and DUTY is determined. Specifically, a determination is made for the movement direction of the movable swash plate as described in the following:  
         [0039]    actual Ne&gt;target Ne→movable swash plate is moved toward the TOP side;  
         [0040]    actual Ne&lt;target Ne→movable swash plate is moved toward the LOW side.  
         [0041]    Duty is also determined from the following equation: 
         DUTY=K1×|actual Ne−target Ne|(where K1 is a coefficient) 
         [0042]    Here, duty represents a proportion or level of current flowing in the control motor  11 , and is used in speed control of the control motor  11 . With DUTY at 100% the control motor  11  is at maximum speed, while with DUTY at 0% the motor is stopped.  
         [0043]    In step S 113 , the control motor  11  is controlled based on a rotation direction of the motor and a movable swash plate angle calculated based on DUTY and an angle signal from the angle sensor  26  (as determined in step S 111 ). Specifically, the control motor  11  is driven using motor rotation direction and DUTY, each of the LOW and TOP ratios are measured using the movable swash plate angle, and at the time of disconnection from the TOP ratio, the control motor  11  is stopped.  
         [0044]    In this embodiment, stepped gear shift control is possible using a stepped gear shift mode. Stepped gear shift control means gear shift control that can manually shift a gear ratio, such as a many stepped transmission, as if it were a continuously variable transmission. This type of stepped gear shift control is carried out by controlling an inclination angle of the movable swash plate  40  under control of the control unit  22  in the same way as for the case described up to now, but in this case it is sufficient to only change the control method so as to carry out stepwise gear shifting.  
         [0045]    Switching between this type of stepped gear shift mode and the automatic gear shift mode is carried out by a mode switch  29 , and a stepped gear shift operation when in stepped gear shift mode is carried out by pressing the shift switch  28 . The shift switch  28  is provided with a shift up button and a shift down button, and each time one of these buttons is pressed the gears are shifted up or shifted down by one stage.  
         [0046]    [0046]FIG. 6 shows a control procedure of the control unit  22  for stepped gear shift control. In step S 201 , an inclination angle is calculated from the swash plate inclination angle signal from the angle sensor  26 . In step S 203 , a shift command making the operation content shift up or shift down is determined using a shift signal from the shift switch  28 . This determination sets a shift up command if the shift up button of the shift switch  28  is pressed, or sets a shift down command if the shift down button of the shift switch  28  is pressed.  
         [0047]    In step S 205 , a meter display determination and a target swash plate angle are determined based on the above described inclination angle and shift command. The meter display determines a number of gear stages, which corresponds to a number of shift stages of a manual transmission, using the inclination angle. Next, the meter display determines a display signal to go to an indicator of the meter M, and outputs this signal to the meter M. In step S 207 , the determined number of gear stages is displayed on the meter M.  
         [0048]    Determination of the target inclination angle for the swash plate  40  is established under the following conditions, with respect to a current gear display signal, when there is input of a shift command:  
         [0049]    shift up command→single stage shift up  
         [0050]    shift down command→single stage shift down  
         [0051]    Following that, in step S 209 , the target swash plate angle (determined in step S 205 ) is compared with the inclination angle (S 201 ), and based upon the comparison the forward or reverse rotation direction of the control motor  11  and DUTY are determined from the following:  
         [0052]    inclination angle&gt;target swash plate angle→shift movable swash plate  40  toward the LOW side;  
         [0053]    inclination angle&lt;target swash plate angle→shift movable swash plate  40  toward the TOP side.  
         [0054]    DUTY is determined from the following equation: 
         DUTY=K2×|inclination angle−target swash plate angle|(where K2 is a coefficient) 
         [0055]    Next, in step S 211 , the control motor  11  is drive controlled based on the motor rotation direction and the DUTY, to incline or decline the movable swash plate  40  to a specified angle. In this way, the hydrostatic continuously variable transmission  1  can carry out stepped gear shifting, which is corresponds to a stepped gear shifting of a manual multi-stepped transmission.  
         [0056]    This embodiment also has a descent control function. FIG. 8 is a flow chart showing this descent control method for the control unit  22 . If control is started, the first thing to occur is that it is judged whether or not a selection position of the travel range selection switch  20   b  set using the sub-transmission lever  20 , namely the gear being selected, is in the D range or the L range (step Si). If the result of this judgment is YES, processing proceeds to the next step S 2 . If the result is NO, processing transfers to step S 7 , where a reverse (R) position or a neutral (N) position is set and the gear ratio is fixed at LOW.  
         [0057]    In step S 2 , it is judged whether or not one of automatic mode (AT) or stepped transmission mode (MT) are set. If it is AT, processing proceeds to step S 3 . If it is MT, processing proceeds to step S 6  and MT control is carried out, which will be described later. In step S 3 , it is judged whether or not judgment conditions for descent judgment are satisfied. If the result is YES, the descent control of step S 4  is carried out. If the result is NO, processing transfers to step S 5 , and regular AT control is performed.  
         [0058]    The descent control of step S 4  involves control to shift a gear ratio to the LOW side by only a prescribed amount from the immediately preceding value, when the descent judgment conditions are met, so that acceleration above a specified value does not occur. The descent judgment conditions are brought about by satisfying all of the following conditions:  
         [0059]    Throttle closed. Specifically, it is determined that the throttle is closed when a throttle signal voltage from the throttle sensor  23  is lower than a predetermined threshold;  
         [0060]    Vehicle speed generated (using the signal of the speed sensor  25 );  
         [0061]    Acceleration exceeding a threshold;  
         [0062]    Simultaneous formation of conditions ( 1 )-( 3 ) for in excess of a fixed time.  
         [0063]    If these descent control conditions are satisfied, the current gear shift position is detected from the angle sensor  26 , a target gear shift position where the current position is shifted by a specified amount towards the LOW side is determined, and DUTY and rotation directions are determined for the control motor  11  of the inclination angle control mechanism  10  to change the gear ratio by driving the control motor  11 .  
         [0064]    By performing control in this way, there is no acceleration when descending which means that a good deceleration feeling is obtained. In particular, when descending while hauling a trailer or the like there is no acceleration due to the heavy trailer or the like pushing from behind, which is advantageous for downhill travel while hauling a heavy object, such as a trailer.  
         [0065]    The present invention is not limited to the above described embodiments, and various modifications and applications are possible. For example, it is possible for the present invention to apply not only to a hydrostatic continuously variable transmission system, but also to improved precision position detection system, such as a CTV system, or an electronically controlled belt conveyor.  
         [0066]    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.