Patent Abstract:
A shift control system for a V-belt type continuously variable transmission is provided. The shift control system comprises a controller programmed to store an actual transmission ratio of the continuously variable transmission at stop of an associated vehicle drive source, and inhibit, at restart of the vehicle drive source, an initializing operation for returning an operational position of the shift actuator to a standard position when the actual transmission ratio is more on a high-speed side than a predetermined transmission ratio. A shift control method is also provided.

Full Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a shift control system and method for a V-belt type continuously variable transmission and particularly to a control of a shift actuator at start of an engine, which shift actuator is used in the shift control system for actuating a shift control valve. 
     Generally, in such a continuously variable transmission, an oil pressure (hereinafter referred to as a primary pulley pressure) that is obtained by regulating a line pressure serving as a base pressure is regulated by means of a shift control valve, is supplied to a primary pulley chamber. The primary pulley pressure is increased or decreased by means of the shift control valve for thereby varying a groove width of the primary pulley. By varying the groove width of the primary pulley, a diameter ratio between the primary pulley and a secondary pulley is varied, thus enabling the transmission ratio of the transmission to vary continuously. The shift control valve is connected to a middle portion of a shift link that constitutes a mechanical feedback mechanism. To the opposite ends of the shift link are connected a movable sheave of the primary pulley and a step motor serving as a shift actuator, respectively. In order to prevent a discrepancy between a rotational position of the step motor that causes the shift control valve to stroke into a position corresponding to a command value for attaining a desired transmission ratio and the command value supplied to the step motor, the step motor is initialized during stop of a vehicle or at start of the engine. 
     Initialization performed by a conventional system is for causing the command value supplied to the step motor that operates the shift control valve to exactly correspond to the rotational position of the step motor. An example of such initialization is that when an ignition switch is turned on a motor initializing operation means causes the step motor to move in one direction into a hardware limit position and thereafter return to a standard position through movement in the opposite direction. When an initializing operation of the step motor is finished, a motor command value initializing means initializes the motor command value so that the motor command value corresponds to the standard position. By this, the motor command value can exactly corresponds to the rotational position of the step motor and therefore an accurate shift control can be attained (refer to Unexamined Japanese Patent Publication No. 8-178063). 
     SUMMARY OF THE INVENTION 
     However, if a vehicle is stopped after rapid deceleration, there is a possibility that the transmission ratio of the automatic transmission cannot return to a low-speed side limit value (max. low value) to which the transmission ratio should return at stop of the engine but the operation of the transmission is stopped with the transmission ratio being held more on the high-speed side than the max. low value. 
     If the engine is restarted under such a condition and the initialization of the step motor is performed in the usual manner, the step motor is moved in one direction to cause the shift link to turn on the primary pulley that is not positioned at the max. low position and the shift control valve to move in the downshift direction. This causes the primary pulley pressure side to be communicated with a drain side, thus causing the primary pulley pressure to drop. If at this time the driver depresses an accelerator pedal, slippage of the belt occurs due to the insufficient primary pulley pressure, thus possibly lowering the durability of the belt. 
     It is accordingly an object of the present invention to provide a shift control system and method for a continuously variable transmission that is free from the above noted problem, i.e., that can assuredly prevent slippage of a V-belt at start of an engine and thereby improve the durability of the V-belt. 
     To achieve the above object, there is provided according to an aspect of the present invention a shift control system for a V-belt type continuously variable transmission having a primary pulley, a secondary pulley, a V-belt wound around the primary pulley and the secondary pulley, and a shift actuator for variably controlling V-shaped groove widths of the primary pulley and the secondary pulley through variable control of a difference between a primary pulley pressure and a secondary pulley pressure so that an actual transmission ratio attained by a rotational speed ratio between the primary pulley and the secondary pulley becomes equal to a target transmission ratio corresponding to an operational position of the shift actuator, the shift control system comprises a controller programmed to store an actual transmission ratio of the continuously variable transmission at stop of an associated vehicle drive source, and inhibit, at restart of the vehicle drive source, an initializing operation for returning an operational position of the shift actuator to a standard position when the actual transmission ratio is more on a high-speed side than a predetermined transmission ratio. 
     According to another aspect of the present invention, there is provided a shift control method for a V-belt type continuously variable transmission having a primary pulley, a secondary pulley, a V-belt wound around the primary pulley and the secondary pulley, and a shift actuator for variably controlling V-shaped groove widths of the primary pulley and the secondary pulley through variable control of a difference between a primary pulley pressure and a secondary pulley pressure so that an actual transmission ratio attained by a rotational speed ratio between the primary pulley and the secondary pulley becomes equal to a target transmission ratio corresponding to an operational position of the shift actuator, the shift control method comprises storing an actual transmission ratio of the continuously variable transmission at stop of an associated vehicle drive source, and inhibiting, at restart of the vehicle drive source, an initializing operation for returning an operational position of the shift actuator to a standard position when the actual transmission ratio is more on a high-speed side than a predetermined transmission ratio. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a shift control system for use with a V-belt type continuously variable transmission according to an embodiment of the present invention; 
         FIG. 2  is a block diagram showing the details of the shift control system of  FIG. 1 ; 
         FIG. 3  is a flowchart of a control process at start of an engine that is executed by the control system of  FIG. 1 ; and 
         FIG. 4  is a time chart of variations of a transmission ratio and a step motor operational position resulting when the control process of  FIG. 3  is executed. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIG. 1 , a V-belt type continuously variable transmission is generally indicated by  1  and includes a primary pulley  2  and a secondary pulley  3  that are disposed so as to have V-shaped grooves (no numeral) arranged in line. Around the V-shaped grooves of the pulleys  2 ,  3  is wound a V-belt  4 . An engine  5  that serves as a vehicle drive source is disposed coaxially with the primary pulley  2 . Between the engine  5  and the primary pulley  2  are disposed in order from the engine  5  side a lockup torque converter  6  and a forward/backward switching mechanism  7 . 
     The forward/backward switching mechanism  7  includes as a major component a double pinion planetary gear assembly  7   a  with a sun gear (no numeral) being connected to the engine  5  by way of a torque converter  6  and a carrier (no numeral) being connected to the primary pulley  2 . The forward/backward switching mechanism  7  further includes a forward clutch  7   b  directly connecting between the sun gear and the carrier and a backward brake  7   c  for fixing a ring gear (no numeral) of the double pinion planetary gear assembly  7   a . The forward/backward switching mechanism  7  transmits, upon engagement of the forward clutch  7   b , an input rotation transmitted thereto from the engine  5  by way of the torque converter  6  to the primary pulley  2  as it is and upon engagement of the backward clutch  7   c , to the primary pulley  2  while changing the rotational direction of the input rotation and reducing the rotational speed of the same. 
     Rotation of the primary pulley  2  is transmitted to the secondary pulley  3  by way of the V-belt  4 . Rotation of the secondary pulley  3  is thereafter transmitted to vehicle wheels (not shown) by way of an output shaft  8 , a gear train  9  and a differential gear assembly  10 . In order that a transmission ratio (transmission ratio) between the primary pulley  2  and the secondary pulley  3  can be varied during the above-described transmission of power, sheaves of the primary pulley  2  and the secondary pulley  3  that define the V-shaped grooves include fixed sheaves  2   a ,  3   a  and movable sheaves  2   b ,  3   b  that are movable in the axial direction. The movable sheaves  2   b ,  3   b  are urged against the respective fixed sheaves  2   a ,  3   a  by a primary pulley pressure Ppri and a secondary pulley pressure Psec that are produced by using the line pressure that is controlled in the manner as will be described in detail later, as a base pressure and supplied a primary pulley chamber  2   c  and a secondary pulley chamber  3   c . By this, the V-belt  4  is frictionally engaged with the sheaves  2   a ,  2   b ,  3   a ,  3   b  of the pulleys  2 ,  3  so as to enable the above-described transmission of power between the primary pulley  2  and the secondary pulley  3 . In the meantime, particularly in this embodiment, the pressure receiving areas of the primary pulley chamber  2   c  and the secondary pulley chamber  3   c  are made the same, thus not causing one of the pulleys  2 ,  3  to be larger in diameter and thereby enabling the V-belt type continuously variable transmission to be compact in size. 
     Upon shifting, a target transmission ratio can be realized by varying the V-shaped groove widths of the primary and secondary pulleys  2 ,  3  by the difference between the primary pulley pressure Ppri and the secondary pulley pressure Psec that are produced so as to correspond to a target transmission ratio as will be described later and thereby varying radii of circular arcs along which the V-belt  4  is wound around the pulleys  2 ,  3 . 
     The output of the primary pulley pressure Ppri and the secondary pulley pressure Psec is controlled by a hydraulic shift control circuit  11  together with the output of oil pressure for engaging a forward brake  7   b  upon selection of a forward running range and the output of oil pressure for engaging a backward brake  7   c  upon selection of a backward running range. The hydraulic control circuit  11  executes the above-described control in response to the signal from the transmission controller  12 . To this end, inputted to the transmission controller  12  are a signal from a primary pulley rotation sensor  13  for detecting a primary pulley rotational speed Npri, a signal from a secondary pulley rotation sensor  14  for detecting a secondary pulley rotational speed Nsec, a signal from a secondary pulley pressure sensor  15  for detecting a secondary pulley pressure Psec, a signal from an accelerator opening degree sensor  16  for detecting an accelerator pedal depression amount APO, a selected range signal from an inhibitor switch  17 , a signal from an oil temperature sensor  18  for detecting a transmission working oil temperature TMP, and a signal from an engine controller  19  for controlling the engine  5  (engine speed and fuel injection time). 
     The hydraulic shift control circuit  11  and the transmission controller  12  are structured as diagrammatically shown in FIG.  2 . Firstly, the hydraulic shift control circuit  11  will be described. The hydraulic shift control circuit  11  is provided with an oil pump  21  that is driven by the engine  5 . The working oil supplied from the oil pump  21  to an oil passage  22  is used as a medium and the pressure of the working oil is regulated by a pressure regulator valve  23  to a predetermined line pressure P L . The line pressure P L  of the oil passage  22  is regulated on one hand to a secondary pulley pressure Psec by means of a pressure reducing valve  24  and supplied to the secondary pulley chamber  3   c  and regulated on the other hand to a primary pulley pressure Ppri by means of a shift control valve  25  and supplied to the primary pulley chamber  2   c . In the meantime, the pressure regulator valve  23  controls the line pressure P L  in accordance with a drive duty of a solenoid  23   a , and the pressure reducing valve  24  controls the secondary pulley pressure Psec in accordance with a drive duty of a solenoid  24   a.    
     The shift control valve  25  has a neutral position  25   a , a pressure increasing position  25   b  and a pressure reducing position  25   c . To change the valve position, the shift control valve  25  is connected to a middle portion of a shift link  26  which is in turn connected at an end thereof with a step motor  27  serving as a shift actuator and at the other end thereof with the movable sheave  2   b  of the primary pulley  2 . The step motor  27  is driven from the standard position into an operational position advanced by a step number Astep corresponding to a target transmission ratio. By such an operation of the step motor  27 , the shift link  26  is caused to oscillate about a connecting portion at which it is connected to the movable sheave  2   b  and thereby the shift control valve  25  is caused to vary in the valve position from the neutral position  25   a  to the pressure increasing position  25   b  or the pressure reducing position  25   c . As a result, in case the target transmission ratio is on a high-speed side (upshift side), the line pressure P L  side is communicated with the primary pulley pressure Ppri side. On the other hand, in case the target transmission ratio is on the low-speed side (downshift side), the primary pulley pressure side is communicated with the drain side. By this, the primary pulley pressure Ppri is increased by the line pressure PL serving as a base pressure or reduced by being drained, thus causing a variation in the difference between the primary pulley pressure Ppri and the secondary pulley pressure Psec and causing upshift to the high-speed side transmission ratio or downshift to the low-speed side transmission ratio for thereby performing a shifting operation toward the target transmission ratio. 
     Progress of such shifting is fed back to the corresponding end of the shift link  26  by way of the movable flange  2   c  of the primary pulley  2 , and the shift link  26  is caused to oscillate about the connecting portion at which it is connected to the step motor  27 , thus causing the shift control valve  25  to return from the pressure increasing position  25   b  or the pressure reducing position  25   c  to the neutral position  25   a . By this, the shift control valve  25  is returned to the neutral position  25   a  at the time the target transmission ratio is attained, thus making it possible to keep the target transmission ratio. In the meantime, in case the primary pulley  2  is at a position where it is in contact with a mechanical stopper  2   d , the mechanical stopper  2   d  is adapted to apply a reaction force to the primary pulley  2  so that a capacity of a V-belt transmission torque is retained. 
     The solenoid drive duty of the pressure regulator valve  23 , the solenoid drive duty of the pressure reducing valve  24  and the shift command to the step motor  27  (drive step number Astep) are determined by the transmission controller  12  together with the control of whether oil pressure is supplied to the forward clutch  7   b  and the backward clutch  7   c  shown in  FIG. 1  for engagement thereof. The transmission controller  12  is made up of a pressure control section  12   a  and a shift control section  12   b  as shown in FIG.  2 . The pressure control section  12   a  determines the solenoid drive duty of the pressure regulator valve  23  and the solenoid drive duty of the pressure reducing valve  24 . The shift control section  12   b  determines the drive step number Astep of the step motor  27  so as to attain a gear ratio ranging from a max. low gear ratio to a max. high gear ratio in the following manner. In the meantime, the max. low gear ratio (i.e., second predetermined transmission ratio) is smaller (i.e., more on the high-speed side) than the gear ratio (i.e., first predetermined transmission ration) that is attained when the movable sheave  2   b  of the primary pulley  2  is in contact with a stopper  2   d  of the V-belt type continuously variable transmission  1  (refer to FIG.  1 ). 
     Namely, the shift control section  12   b  firstly finds a target transmission ratio corresponding to a driving condition (vehicle speed and accelerator pedal depression amount APO) by finding a target rotational input speed based on a predetermined shift map and by using a vehicle speed that can be found from the secondary pulley rotational speed Nsec and the accelerator pedal depression amount APO, and dividing the target input rotational speed by the secondary pulley rotational speed Nsec. Then, the actual transmission ratio (transmission ratio reached) is calculated by dividing the primary pulley rotational speed Npri by the secondary pulley rotational speed Nsec, and it is found a shift command for allowing the actual transmission ratio to get gradually closer to the target transmission ratio at the target shift speed while compensating for a disturbance according to a deviation of the actual transmission ratio with respect to the above-described target transmission ratio. Then, the step number Astep of the step motor  27  (the operational position of the step motor  27 ) for realizing the shift command is found and supplied to the step motor  27 . By this, the target transmission ratio can be by the above-described shifting operation. 
     Generally, at start of the engine  5 , the step motor  27  is initialized. The initialization is generally performed by driving the step motor  27  toward the low-speed side, i.e., the downshift side. In contrast to this, according to the present invention, in order to prevent slip of the V-belt  4  due to the initialization performed when the transmission ratio at stop of an engine is more on the high-speed side than a proper transmission ratio as described herein before, a process based on a control routine shown in  FIG. 3  is performed at start of the engine  5 . The control routine will be described in the following. 
     Firstly, before performing the routine, the transmission controller  25  stores beforehand the actual transmission ratio at stop of the engine  5 , i.e., the value ipe of the transmission ratio obtained from the speed Npri of the primary pulley  2  and the speed Nsec of the secondary pulley  3  at stop of the engine  5 . 
     In step S 101 , the transmission ratio ipe stored beforehand at previous stop of the engine  5  is read and is compared with a predetermined value ipp. If ipe is larger than the predetermined value ipp, i.e., more on the low-speed side, the process is ended and usual initialization of the step motor  27  is performed. On the other hand, if ipe is smaller than the predetermined value ipp, i.e., more on the high-speed side, the program proceeds to step S 102 . In the meantime, the predetermined value ipp is for example set in the following manner. In the V-belt type continuously variable transmission  1 , even when the transmission ratio does not return to the max. low ratio at stop of the engine  5 , it is actually returned to the max. low ratio if it ranges from the max. low ratio to a predetermined gear ratio due to the fact that an oil pressure produced by the oil pump  21  at restart of the engine  5  flows into the secondary pulley chamber  3   c  at a dash and resultantly causes the movable sheave  2   b  of the primary pulley  2  to move into the position where the movable sheave  2   b  is in contact with the mechanical stopper  2   d . Thus, a minimum (high-speed side ratio) of transmission ratios that allow the movable sheave  2   b  of the primary pulley  2  to move into the position where the movable sheave  2   b  is in contact with the mechanical stopper  2   d  due to the oil pressure produced at restart of the engine  5  is found experimentally, and the predetermined value ipp is set at a value nearly equal to the minimum of the transmission ratio with consideration of an error. 
     In step s 102 , initialization of the step motor  27  is inhibited, and in next step  103  the actual transmission ratio a predetermined time (e.g., 100 msec) before stop of the engine  5  is compared with the transmission ratio corresponding to the operational position Astep at stop of the engine  5  and a smaller value (high-speed side) is set to be a target transmission ratio and the step motor is driven into the position corresponding to the target transmission ratio and fixed thereat and then the program is ended. 
       FIG. 4  is a time chart showing time variations of the ignition key ON/OFF operation, the transmission ratio ip and the step motor step number (operational position) Astep. 
     Usually, when, as described above, the vehicle is decelerated and stopped and then the engine  5  is stopped, the transmission  1  is operated so as to return the transmission ratio ip to the max. low ratio. However, when the vehicle is stopped after rapid deceleration and the engine  5  is stopped, there can possibly occur such a case in which the transmission  1  is stopped with the transmission ratio ip that cannot return to the max. low ratio but is held in a state of ipe. 
     Thus, according to the present invention, the transmission ratio ipe at stop of the engine  5  is compared with a predetermined transmission ratio ipp, and when ipe is more on the high-speed side than ipp, the control process shown in  FIG. 3  is executed. In  FIG. 4 , since ipe is more on the high-speed side than ipp, the control process of  FIG. 3  is executed. 
     When the engine  5  is restarted at the time Ts, initialization of the step motor  27  is usually performed for thereby causing the step motor operational position Astep to move to the hardware limit value once as indicated by the dotted line and thereafter move to the standard position (max. low position). 
     However, since in this instance the transmission ratio ipp is of the value ipe that is more on the high-speed side than the max. low value and further than the value ipe that is more on the high-speed side than ipe, the step motor operational position Astep is of the value Ae corresponding to ipe. When usual initialization is performed under this condition, there is a possibility of causing slippage of the belt  4 , that is, the step motor  27  is moved in one direction to cause the shift link  26  to turn on the primary pulley  2  that is not positioned at the max. low position and the shift control valve  25  to move in the downshift direction. This causes the primary pulley pressure Ppri side to be communicated with a drain side, thus causing the primary pulley pressure Ppri to drop. If at this time the driver depresses the accelerator pedal (not shown), slippage of the belt  4  occurs due to the insufficient primary pulley pressure Ppri, thus possibly lowering the durability of the belt  4 . Thus, initialization of the step motor  27  is not performed but a comparison between the actual transmission ratio a predetermined time (e.g., 100 msec) before stop of the engine  5  and the actual transmission ratio corresponding to the step motor operational position Astep at stop of the engine  5  is made and smaller one is set to be a target transmission ratio. In this instance, the actual transmission ratio a predetermined time before stop of the engine  5  is usually smaller, so that the step motor  27  is driven in the upshift direction and is fixed at the position corresponding to the target transmission ratio. As a result, the line pressure P L  side and the primary pulley pressure Ppri side are communicated with each other, thus preventing the primary pulley pressure Ppri from reducing. Further, the transmission ratio ip is fixed at ipe since the vehicle is not moving. 
     As having been described above, in the shift control system for the continuously variable transmission  1 , the transmission ratio at stop of the engine  5  is stored beforehand, and when the transmission ratio is more on the high-speed side than a predetermined value at restart of the engine  5 , initialization of the shift actuator  27  is not performed. By this, the shift actuator  27  is not moved in the downshift direction by an initializing operation at restart of the engine  5 , and reduction of the primary pulley pressure Ppri accompanied by the initializing operation can be prevented. Thus, even when the transmission  1  is stopped with a transmission ratio more on the high-speed side than a predetermined transmission ratio, it becomes possible to prevent the durability of the belt  4  from being lowered due to occurrence of slippage of the belt  4  that is caused by the insufficient primary pressure Ppri. Further, in case the transmission ratio is more on the low-speed side than the predetermined transmission ratio at restart of the vehicle drive source, an initializing operation is assured. Thus, the initializing operation is not inhibited excessively but it becomes possible to make it as fewer as possible to perform a control under the condition where there is a difference between the operational position of the shift actuator  27  and the command position and it becomes possible to prevent the fuel consumption from being lowered. 
     Further, the actual transmission ratio a predetermined time before stop of the engine  5  and the shift actuator operational position Astep are stored beforehand. A comparison between the actual transmission ratio a predetermined time before stop of the engine  5  and the transmission ratio corresponding to the shift actuator operational position Astep at stop of the engine  5  is made at restart of the engine  5 . High-speed side one of the transmission ratios is set to be a target transmission ratio and the shift actuator  27  is operated so as to attain the target transmission ratio. Thus, in case the initializing operation is inhibited at restart after stop of the engine  5 , the shift actuator  27  is usually caused to maintain the operational position Astep at stop of the engine  5  or driven in the upshift direction. As a result, the operation of the shift actuator  27  in the downshift direction is inhibited assuredly, thus making it possible to prevent slippage of the belt  4  due to reduction of the primary pulley pressure Ppri assuredly. Further, by setting the predetermined transmission ratio ipp at a value nearly equal to the minimum of the transmission ratios that allow the movable sheave  2   b  of the primary pulley  2  to move into the position where the movable sheave  2   b  is in contact with the mechanical stopper  2   d , the more suitable control can be attained. 
     The entire contents of Japanese Patent Applications P2002-275306(filed Sep. 20, 2002) are incorporated herein by reference. 
     Although the invention has been described above by reference to a certain embodiment of the invention, the invention is not limited to the embodiment described above. Modifications and variations of the embodiment described above will occur to those skilled in the art, in light of the above teachings. For example, judgment on initialization of the step motor  27  is made based on the comparison between the predetermined value ipp and the transmission ratio at stop of the engine  5  that is calculated from the primary pulley rotational speed Npri and the secondary pulley rotational speed Nsec, this is not for the purpose of limitation. Namely, the “term transmission ratio at stop of engine” may encompass an actual transmission ratio or a target transmission ratio a predetermined time before stop of the engine  5 . Further, while the engine  5  has been described shown as a vehicle drive source, this is not for the purpose of limitation but can be replaced by a motor/generator. The scope of the invention is defined with reference to the following claims.

Technology Classification (CPC): 1