Abstract:
A speed ratio containment process limits the speed ratio of a variator for a CVT for a motor vehicle when rolling backward by commanding a speed ratio that is higher than the actual speed ratio in an overdrive direction. Accordingly, the actual speed ratio moves to a lowest limit, which provides maximum torque when a driver of the motor vehicle steps on the accelerator pedal to resume forward motion of the motor vehicle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/652,762, filed May 29, 2012, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to continuously variable transmissions. More specifically, the present disclosure relates to containment control for continuously variable transmissions. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
         [0004]    A continuously variable transmission (CVT) typically includes gearing that operatively couples a variator between a rotary power source, such as an engine or electric motor, and a final drive unit. The variator includes a rotary input disk and a rotary output disk which are able to steplessly or continuously vary the ratio of an input speed to an output speed (the “variator ratio”). The overall speed ratio provided by the CVT is a function of the variator ratio and the associated gearing. The output disc includes integrally formed gear teeth that are in mesh with and drive a corresponding gear. The gear in turn is functionally coupled to an output shaft or layshaft that is functionally coupled to the final drive unit. 
         [0005]    In typical CVT designs, when the variator disk changes its rotational direction, the ratio control system changes from negative feedback to positive feedback, such that the actual ratio runs away from a desired command value. Therefore, there is a need in the art for a CVT design that allows for containment of the overall speed ratio. 
       SUMMARY 
       [0006]    A speed ratio containment process limits the speed ratio of a variator for a CVT for a motor vehicle when rolling backward by commanding a speed ratio that is higher than the actual speed ratio in an overdrive direction. Accordingly, the actual speed ratio moves to a lowest limit, which provides maximum torque when a driver of the motor vehicle steps on the accelerator pedal to resume forward motion of the motor vehicle. 
         [0007]    Further features, advantages, and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0008]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the views. In the drawings: 
           [0009]      FIG. 1A  is a schematic view of a variator with a ratio control subsystem for a continuously variable transmission in accordance with the principles of the present invention; 
           [0010]      FIG. 1B  is a side view of the variator of  FIG. 1A ; 
           [0011]      FIG. 1C  is a close-up view of the region C in  FIG. 1A ; 
           [0012]      FIG. 2  is a block diagram illustrating a process for operating the variator control subsystem; and 
           [0013]      FIG. 3  illustrates a process of containing the overall speed ratio of the variator. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0015]    Referring now to  FIGS. 1A and 1B , a portion of a continuously variable transmission (CVT) for a motor vehicle is designated at  10 . The CVT  10  includes a variator  12  and a ratio control subsystem  14 . The variator  12  is generally coupled to an engine that provides input torque to the variator  12  and a drivetrain that supplies torque to the wheels of the motor vehicle. The engine may be a conventional internal combustion engine or an electric motor, or any other type of prime mover, without departing from the scope of the present disclosure. 
         [0016]    The CVT  10  includes a typically cast, metal housing which encloses and protects the various components of the CVT  10 . The housing includes a variety of apertures, passageways, shoulders and flanges which position and support these components. Generally speaking, the variator  12  includes an input shaft  16  and an output shaft  18 . The input shaft  16  is functionally interconnected with the engine and receives input torque or power from the engine. The output shaft  22  is preferably connected with a final drive unit which may include, for example, a gear box, a propshaft, a differential assembly, and drive axles connected to wheels, etc. The gearbox generally includes one or more gear sets, clutches and/or brakes, and shafts to provide various forward and reverse gear ratios. 
         [0017]    The variator  12  is illustrated as a toroidal race rolling type variator. However, it should be appreciated that various other types of variators may be employed without departing from the scope of the present invention. The variator  12  includes an input disc  20  and an output disc  22 . The input disk  20  includes a toroidal outer surface or input race  20 A and the output disk  22  includes a toroidal outer surface or output race  22 A. The input race  20 A and the output race  22 A cooperate to define a toroidal cavity  24 . Each of the disks  20  and  22  share a common rotational axis defined by a variator shaft  26 . The input disk  20  and the output disk  22  are rotationally coupled to the variator shaft  26  with a roller  28 . It should be appreciated that any number of rollers may be employed without departing from the scope of the present invention. 
         [0018]    The roller  28  is mounted to a trunnion  30  for rotation about a roller axis  32  and rolls upon the toroidal races  20 A and  22 A of its associated input and output disks  20  and  22 . Changes in variator torque ratio are achieved by precession of the roller  28  such that the roller&#39;s axis  32  is able to tilt about the trunnion axis  34  to change the inclination of the roller axis to the variator axis  26 . Precession of the roller  28  results in changes of the radii of the path traced upon the races  20 A and  22 A by the roller  28  and hence results in a change of variator drive ratio between the input disk  20  and the output disk  22 . 
         [0019]    The trunnion  30  of the variator  12  is connected to the ratio control subsystem  14  with a shaft  36 . The ratio control subsystem  14  further includes a piston  38  mounted about the shaft  36 . The piston  38  is disposed in a chamber  40  defined by the inner surface of a housing  42 . The shaft  36  is further connected to a cam  44 , which, in turn, is coupled to a spool through a link  48 . It should be appreciated, however, that various other types of ratio control subsystem may be employed without departing from the scope of the present invention. Accordingly, as the piston  38  and hence the cam  44  move upward in the, x, direction, the spool  46  moves toward the left as shown in  FIG. 1A . And as the cam  44  moves downward, the spool  46  moves to the right. The spool  46  reciprocates relative to a sleeve  50 , both of which are enclosed in a housing  52 . The sleeve  50  is coupled to a stepping motor  54  that receives command signals  56 , which instruct the stepping motor  54  to move the sleeve to the right or to the left. Hence, the sleeve  50  and the spool  46  move independently of each other. The housing  52  includes an inlet  57  for pressurized gas or fluid and is coupled to the housing  42  with a pair of conduits or lines  58  and  60 . The line  58  communicates with a subchamber  40 A, and the line  60  communicates with a subchamber  40 B. Note that  FIG. 1C  shows a close up region, C, between the  22 A disk and the roller  28 . Specifically,  FIG. 1C  shows slip  60  associated with a liquid, such as, for example, and oil film, disposed between the disk  22 A and the roller  28 . 
         [0020]    Referring also to  FIG. 2 , there is shown a block diagram of a process  100  for controlling the speed ratio of the variator  12 . In a step  102 , a command signal is delivered or transmitted to the stepping motor  54  which provides a desired ratio control gain  104  by moving the sleeve  50  to a particular position. The movement of the sleeve  50  closes or opens the lines  58  and  60  to adjust the pressure in the subchambers  40 A and  40 B so that the piston  38  moves up or down, which, in turn, moves the roller  28  relative to the disks  20  and  22  to provide an actual ratio  106 . Note that as the roller  28  moves along the axis  34 , the shaft  36  and hence the cam  44  move as well. Therefore, movement of the roller  28  moves the spool  46  to close or open the lines  58  and  60  so that the position of the spool  46  provides feedback  108  to the initial stepping motor position command  102 . 
         [0021]    In a normal operation, when the disks  20  and  22  are rotating in the indicated directions in  FIG. 1A , once a speed ratio is selected, the difference between P L  and P H  is balanced with the tractive force on the roller  28 , so that the axis  32  of the roller  28  stays in the center balanced position where x=0 and the roller is at a steady state tilt and a steady state ratio. 
         [0022]    When the driver of the motor vehicle desires to change the speed of the vehicle, the command signal  56  is sent to the stepping motor  54 . Hence, when the motor  54  move the sleeve  50  to the left, P H  increases and P L  decreases. This imbalance moves the roller  28  linearly upward along the axis  34  off the center position and causes the roller  28  to tilt about the trunnion axis  34  in a counterclockwise direction (that is, r 3  decreases and r 1  increases) because of the direction of the linear speed at the contact point so that the speed ratio ω 3 /ω 1  increases. As the roller  28  moves upward and tilts counterclockwise, the shaft  36  and hence the cam  44  move upward and turns as well. Therefore, the link  48  and the spool  46  move towards the left so that P H  decreases and P L  increases, thereby again achieving a balanced situation or configuration where the roller  28  moves back to the center position along the axis  34 . 
         [0023]    Similarly, when the motor  54  moves the sleeve  50  to the right. P H  decreases and P L  increases. This imbalance moves the roller  28  linearly downward along the axis  34  off the center position and causes the roller  28  to tilt about the trunnion axis  34  in a clockwise direction (that is, r 3  increases and r 1  decreases) because of the direction of the linear speed at the contact point so that the speed ratio ω 3 /ω 1  decreases. As the roller  28  moves downward and tilts clockwise, the shaft  36  and hence the cam  44  move downward and turns as well. Therefore, the link  48  and the spool  46  move towards the right so that P H  increases and P L  decreases, thereby again achieving a balanced situation where the roller  28  moves back to the center position along the axis  34 . Accordingly, for the normal operation of the CVT  10 , a higher position of the piston  38  increases the speed ratio and a lower position of the piston  38  decreases the speed ratio. 
         [0024]    In some CVTs, the disk  22  reverses direction when the motor vehicle comes to a stop on a hill and then begins to roll backwards. When the output disk  22  rotates in a reversed direction (that is, opposite of the direction indicated in  FIG. 1A ), a higher position of the piston  38  results in an undesired decrease in the speed ratio and a lower position results in an undesired increase in the speed ratio. Specifically, in the situation in which the stepping motor moves the sleeve  50  to the left as described above so that the roller  28  moves upwards, the speed ratio ω 3 /ω 1  actually decreases because the roller  28  rotates clockwise (r 3  increases and r 1  decreases) about the trunnion axis  34  when the disk  22  reverses direction; therefore, the link  48  and hence the spool  46  move to the right causing an even bigger difference between P H  and P L . And in the situation in which the stepping motor moves the sleeve  50  to the right as described above so that the roller  28  moves downwards, the speed ratio ω 3 /ω 1  actually increases because the roller  28  tilts counterclockwise (r 3  decreases and r 1  increases) about the trunnion axis  34  when the disk  22  reverses direction; therefore, the link  48  and hence the spool  46  move to the left causing an even bigger difference between P H  and P L . In either of these undesirable situations, with a some CVTs, the spool  46  and the sleeve  50  move away from each other so that the piston  38  stays at either the top position or the bottom position when the disk  22  reverses direction, which eventually changes the ratio to the most underdrive or overdrive value where there are physical limits to the ratio. 
         [0025]    Referring now to  FIG. 3 , there is shown a containment method to prevent the actual speed ratio from running away from a target ratio when the output disk reverses direction. In  FIG. 3(   a ), the motor vehicle comes to a stop on a grade, such as a hill. Here the target speed ratio and the actual speed ratio match up. In  FIG. 3(   b ) the vehicle rolls back. With a conventional CVT, the actual speed ratio runs away from the target ratio. When this occurs there is not enough torque to move the motor vehicle forward. 
         [0026]    In accordance with the principles of the present invention, with ratio containment, the actual speed ratio runs away to the underdrive condition by moving the target speed ratio toward the overdrive direction. This is accomplished by ensuring P H  is higher than P L  by some margin so that the piston  38  stays at the top position when the disc  22  rolls backwards. When this occurs the sleeve  50  moves in a direction opposite to the spool  46  providing containment of the speed ratio for the CVT  10 . Accordingly, when the motor vehicle moves forward on the grade with ratio containment, the actual speed ratio moves to the target ratio to achieve a balanced situation. 
         [0027]    Ratio containment can be triggered once the variator reverse rotation is detected. This detection can occur with various types of sensors that include, but are not limited to:
       1. Transmission output speed sensor: a directional sensor that can sense the vehicle rolling back by detecting the output shaft rotation direction, subject to the minimal speed detection limit.   2. Trunnion speed sensor: a directional speed sensor that can sense the vehicle rolling back by detecting the direction of the trunnion shaft rotation, subject to the minimal speed detection limit.   3. Variator trunnion pressure sensor: The change in difference between the piston high side pressure and the low side pressure can be employed to identify if the ratio control subsystem is behaving as a positive feedback system or a negative feedback system, which can indicate if the vehicle is rolling backwards.       
 
         [0031]    The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.