Control system for continuously variable transmission

[0002] The present invention relates to improvements in a control system of a continuously variable transmission (CVT) for a vehicle, and more particularly to a control system of a belt type CVT.

[0003] A Japanese Patent Provisional Publication No. 8-74959 discloses a line pressure control system for a belt type CVT. The conventional line pressure control system is arranged to control a duty valve by outputting a duty ratio indicative signal as a command value to the duty valve in order to control a belt holding force of the pulley of the CVT. The command value to the duty valve is limited within a range except dead zones so as to ensure the responsibility of the line pressure applied to the CVT.

[0004] However, if a limit value of the command value is simply set at a boundary value of the dead zone, the output pressure of the duty valve tends to fluctuate though the responsibility of the line pressure control is maintained. On the other hand, higher the rotation speed of the pulleys of the belt type CVT becomes, higher a centrifugal pressure of the working fluid in the pulleys becomes. When the rotation speed of the pulley becomes greater than a predetermined value, the belt is held by the centrifugal pressure through the pulley regardless the line pressure so as to prevent a shift control and a line pressure control from being accurately executed in such a centrifugal pressure dominant range. Therefore, it is necessary to omit the centrifugal pressure dominant range from a transmission ratio control range. Further, it is necessary to extend the centrifugal pressure dominant range upon taking account of the dispersion caused by the fluctuation of the output pressure. As a result, the transmission ratio control range is decreased by the margin for the dispersion due to the characteristic of the duty valve.

[0005] It is therefore an object of the present invention to provide a CVT control system which extends the controllable range of a CVT transmission ratio without degrading the responsibility of a line pressure for controlling the CVT.

[0006] A control system according to the present invention comprises a continuously variable transmission (CVT), a CVT pressure control valve, a centrifugal pressure detecting means, a centrifugal pressure deciding means, and a limit value switching means. The CVT comprises a driver pulley, a follower pulley and a belt intercoupling the pulleys. The CVT varies a transmission ratio by changing effective diameters of the pulleys. The CVT pressure control valve controls a pressure of working fluid to be supplied to the CVT according to a command signal. The centrifugal pressure detecting means detects a centrifugal pressure of the working fluid in the pulley. The centrifugal pressure deciding means decides that the centrifugal pressure is in a centrifugal pressure dominant range in which motive power is transmitted only by the centrifugal pressure. The limit value switching means switches a limit value of the command signal from a high response value to a high stability value when the centrifugal pressure detecting means detects the centrifugal pressure dominant range.

[0007] In the drawings, like reference numerals designate like parts and elements throughout all figures, in which:

[0008] FIG. 1 is a schematic view showing a CVT and a control system thereof according to an embodinment of the present invention;

[0009] FIG. 2 is a graph showing a control map for setting a duty ratio of a duty valve for controlling a line pressure according to an objective line pressure;

[0010] FIG. 3 is a flowchart showing a program executed by a transmission control unit of FIG. 1;

[0011] FIG. 4 is a graph showing a control map for setting an engine torque from an throttle opening and an engine revolution speed;

[0012] FIG. 5 is a graph showing a control map for setting a lockup vehicle speed and an unlockup vehicle speed from the vehicle speed and the throttle opening;

[0013] FIG. 6 is a graph showing a control map for setting a transmission ratio from the throttle opening and the vehicle speed;

[0014] FIG. 7 is a flowchart showing a program for executing a step 4 of FIG. 3:

[0015] FIG. 8 is a graph showing a control map for setting a torque ratio from the torque converter input and output speed ratio;

[0016] FIG. 9 is a graph showing a control map for setting a reference pressure from an input torque and the transmission ratio:

[0017] FIG. 10 is a graph for explaining the dispersion of the line pressure generated with respect to the line pressure duty ratio; and

[0018] FIG. 11 is a graph for explaining an expansion of a transmission ratio control range according to the present invention.

[0019] Referring to FIGS. 1 to 11, there is shown an embodiment of a control system of a continuously variable transmission (CVT) in accordance with the present invention.

[0020] FIG. 1 shows an embodiment of the CVT and the control system thereof according to the present invention. A power transmission mechanism of the CVT is basically the same as that of a power transmission mechanism disclosed in a U.S. Pat. No. 5,697,866 except that a torque converter 12 is employed instead of a fluid coupling. Therefore, the same parts and elements of the power transmission mechanism are designated by same reference numerals, and only a brief explanation thereof will be given hereinafter.

[0021] As shown In FIG. 1, an engine 10 is connected to a V-belt type CVT mechanism 29 through the torque converter 12 and a forward/reverse change over mechanism 15. An output shaft of the CVT mechanism 29 is connected to a differential unit 56. These elements 10, 12, 29 and 56 constitute a so-called drive system. The drive system transmits rotation of an output shaft 10aof the engine 10 to right and left drive shaft 66 and 68 at a selected speed ratio in a selected rotational direction to drive a vehicle.

[0022] A throttle valve 19 is disposed in an air intake passage 11 of the engine 10 and is arranged to change a degree of its opening in response to a depression degree of an accelerator pedal (no numeral). A throttle opening sensor 303 is installed to the throttle valve 19 to detect a throttle opening TVO of the throttle valve 19 and to output a signal indicative of the throttle opening TVO. An engine revolution speed sensor 301 is installed on the output shaft 10aof the engine 10 to detect an engine revolution speed N_Eand to output a signal indicative of the engine revolution speed N_E.

[0023] The engine 10 is connected to an engine control unit 200 by which various engine operation factors such as fuel injection amount and timing and ignition timing are controlled so as to put the engine 10 in an optimum operating condition according to the running condition of the vehicle and a driver's intent.

[0024] The throttle opening indicative signal TVO outputted from the throttle opening sensor 303 also corresponds to the magnitude of the depression degree of the accelerator pedal. Although the engine revolution speed sensor 301 is arranged to detect the rotation speed of the output shaft 10aof the engine 10 in this embodiment, it will be understood that it may be arranged to count the number of ignition pulses of the engine 10 as an engine revolution speed.

[0025] The torque converter 12 of this drive system is a lockup torque converter of a known type. This lockup torque converter 12 comprises a pump impeller (input member) 12c, a turbine runner or turbine (output member) 12d, a stator 12e, and a lockup facing member (or lockup clutch) 12ffor providing a direct mechanical drive by directly coupling the input and output members 12cand 12d. The lockup facing member 12fis connected with an output shaft (turbine shaft) 13. The lockup facing member 12dseparates an apply side fluid chamber 12aand a release side fluid chamber 12b. The release chamber 12bis formed between a cover 12gof a torque converter 12 and the lockup facing member 12f, and the apply chamber 12ais disposed on the opposite side of the lockup facing member 12f. When a fluid pressure is supplied into the apply chamber 12a, the lockup facing member 12fis pressed against the cover 12g, and the torque converter 12 is put in a lockup state in which the input member 12cand the output member 12dare directly connected. When the fluid is supplied sufficiently into the release chamber 12b, the lockup facing member 12fis disengaged from the cover 12g, and the torque converter 12 is held in a non-lockup state (or unlockup state). The fluid pressure supplied to the release chamber 12bis drained through the apply chamber 12a.

[0026] An input rotation speed sensor 305 is installed on an output shaft 13 (turbine output shaft) of the torque converter 12 in order to detect an input rotation speed of the CVT mechanism 29.

[0027] When the vehicle is put in a normal running condition, the forward clutch 40 is put in a full engagement state. Therefore, the rotation speed of the turbine output shaft 13 is used as an input rotation speed N_Priof the CVT mechanism 29. When a foot of a driver is released from the accelerator pedal, the forward/reverse change over mechanism 15 functions to control a creep running force by variably adjusting the engagement force of the forward crutch 40. The fluid supplied to the release chamber 12bis drained through the apply chamber 12ato a reservoir 130, and a drained fluid of the fluid supplied to the apply chamber 12cis supplied from the release chamber 12bto other cooling and lubricating systems. That is, the switching control between the lockup state and the unlockup state is executed by changing a flow direction of the fluid supplied to the lockup mechanism of the torque converter 12 as mentioned above without changing the fluid passage.

[0028] The forward/reverse drive direction change over mechanism 15 comprises a planetary gear system 17, the forward clutch 40 and a reverse brake 50. The planetary gear system 17 comprises multistage pinion trains and a pinion carrier supporting these pinion trains. The pinion carrier is connected to the driver pulley 16 of the CVT mechanism 29 through the driver pulley shaft 14, and a sun gear is connected to the turbine rotation shaft 13, The pinion carrier is engageable with the turbine rotation shaft 13 by means of the forward clutch 40. A ring gear of the planetary gear system 17 is engageable with a stationary housing by means of the reverse brake 50.

[0029] The reverse brake 50 is disposed between a ring gear of the planetary gear system 17 and a stationary housing to hold the ring gear. When the forward clutch 40 is put in the engaged state by the supply of the fluid pressure to a fluid chamber 40a, the driver pulley shaft 14 and the turbine output shaft 13 are rotated in the same direction with equal speed through a pinion carrier. When the reverse brake 50 is engaged by the supply of the fluid pressure to the fluid chamber 50a, the driver pulley shaft 14 and the turbine output shaft 13 are rotated with equal speed but in the opposite direction.

[0030] The V-belt CVT mechanism 29 comprises a driver pulley 16, a follower (or driven) pulley 26, and a V-belt 24 for transmitting power between the pulleys. The driver pulley 16 is mounted on the driver pulley shaft 14. The driver pulley 16 comprises an axially stationary fixed conical disk 18, and an axially movable conical disk 22, which confront each other and define a V-shaped pulley groove therebetween for receiving the V-belt 24. The fixed disk 18 rotates as a unit with the driver shaft 14. By a fluid pressure in a driver pulley cylinder chamber 20, the movable disk 22 is axially movable.

[0031] The follower pulley 26 is mounted on a follower pulley shaft 28. The follower pulley 26 comprises an axially stationary fixed conical disk 30, a follower pulley cylinder chamber 32, and an axially movable conical disk 34. The fixed and movable disks 30 and 34 confront each other and define a V-shaped pulley groove for receiving the V belt 24. The fixed disk 30 rotates as a unit with the follower shaft 28. The movable disk 34 is axially movable in dependence on a fluid pressure in the follower pulley cylinder chamber 32.

[0032] The V-belt CVT mechanism 29 further comprises a stepping motor 108 controlled by a transmission control unit 300. A pinion 108ais connected to a rotation shaft of the stepping motor 108 and is meshed with a rack 182. An end of the rack 182 and the movable conical disk 22 are interconnected through a lever 178. By the operation of the stepping motor 108 according to the drive signal D_S/Mfrom the transmission control unit 300, the movable conical disk 22 of the driver pulley 16 and the movable conical disk 34 of the follower pulley 26 are axially moved to vary the effective radius of the contact position of each pulley of the driver and follower pulleys 16 and 26 axially. By so doing, the CVT mechanism 29 can vary the speed ratio (transmission ratio or pulley ratio) between the driver pulley 16 and the follower pulley 26.

[0033] The shift control system (pulley ratio varying control) is generally arranged to vary the pulley groove width of one of the driver and follower pulleys and allow the groove width of the other to be adjusted automatically. This arrangement is achieved by the belt of a push type which transmits the driving force mainly in the pushing direction, The push type V-belt 24 comprises a set of plates which are overlappingly arranged in the longitudinal direction or wound direction of the belt.

[0034] A drive gear 46 fixed to the follower pulley shaft 28 is meshed with an idler gear 48 formed on an idler shaft 52. A pinion gear 54 formed on the idler shaft 52 is meshed with a final gear 44 with which right and left drive shafts 66 and 68 are interconnected through the differential unit 56. A vehicle speed sensor 302 is installed on this final output shaft to detect a vehicle speed V_SPand output a signal indicative of the vehicle speed V_SPto the transmission control unit 300.

[0035] Next, a construction of a hydraulic pressure control apparatus of the CVT will be discussed. The hydraulic pressure control apparatus comprises a pump 101 driven by the engine 10. The pump 101 draws a working fluid from a reservoir 130 and supplies the fluid to an actuator unit 100 while applying a sufficient pressure to the fluid. The construction of the actuator unit 100 is basically the same as that of the actuator unit disclosed in a U.S. Pat. No. 5,697,866. Therefore, the same parts and elements are designated by same reference numerals, and only a brief explanation thereof will be given hereinafter.

[0036] A manual valve 104 is directly operated by a selector lever 103 so as to switchingly control a clutch pressure P_CLto a cylinder chamber 40aof the forward clutch 40 and a brake pressure, P_BRKto a cylinder chamber 50aof the reverse brake 50.

[0037] An inhibitor switch 304 installed on the selector lever 103 detects a shift position selected by the selector lever 103 and outputs a shift range signal S_RANGEindicative of the selected shift position. More particularly, the shift range signal S_RANGEincludes signals corresponding to P, R, N, D, 2, L according to the actually selected shift position.

[0038] A shift control valve 106 lis controlled according to a relative displacement between the stepping motor 10 and the movable conical disk 22 of the driver pulley 16, more particularly it is controlled according to the movement of the lever 178. That is, the shift control valve 106 controls the fluid pressure (line pressure) P_L(Pri)supplied to the driver pulley 16 according to a relative relationship between the required transmission ratio and a groove width of the driver pulley 16.

[0039] A lockup control duty valve 128 controls a lockup mechanism of the torque converter 12 so as to to put the torque converter 12 in one of a lockup state and an unlockup state. A transmnission control unit 300 outputs a drive signal D_L/Uto the lockup control valve 128 to operate the lockup mechanism of the torque converter 12. More particularly, when the drive signal D_L/Uindicative a large duty ratio is outputted to the lockup control duty valve 128, the torque converter 12 is put in the lockup state. When the drive signal D_L/Uindicative of a small duty ratio is outputted to the lockup control valve, 128, the torque converter 12 is put in the unlockup (non-lockup) state.

[0040] A clutch engagement duty valve 129 controls an engagement force for one of the forward clutch 40 and the reverse brake 50 according to a drive signal D_CLfrom the transmission control unit 300. When the drive signal D_CLindicates a large duty ratio, one of the forward clutch 40 and the reverse brake 50 is engaged. When the drive signal D_CLindicates a small duty ratio, it is disengaged.

[0041] A line pressure control duty valve 120 controls the line pressure P_Lsupplied mainly to the follower pulley 26 and partly to the driver pulley 16 so as to hold the belt 24 by the pulleys 26, 16 according to a drive signal D_PLfrom the transmission control unit 300. This duty valve 120 is represented as a modifier duty valve in the above cited Patent document. The reason for this is the output pressure from the duty valve 120 once functions as a pilot pressure of a pilot pressure control valve named as a pressure modifier valve. As a result, an output pressure from the pressure modifier valve functions as a pilot pressure of the line pressure control valve so as to control the line pressure P_Lproduced in an upstream side of the line pressure control valve. That is, by controlling the duty ratio of the duty valve 120, the line pressure P_Lis indirectly controlled. P_L(0R)is linearly increased according to the increase of the control signal (command value) to the line pressure control duty valve 120 or duty ratio D/T_PLof the drive signal except for predetermined dead zones shown in FIG. 2. More particularly, a lower dead zone of FIG. 2 ranges from a minimum value D/T_PL-MIN0(=0%) of the line pressure duty ratio D/T_PLto a lower dead zone threshold D/T_PL-MIN1, and an upper dead zone of FIG. 2 ranges from an upper dead zone threshold D/T_PL-MAX1to a maximum value D/T_PL-MAX0(100%). Therefore, a linear operation zone ranges from the lower dead zone threshold D/T_PL-MIN1to the upper dead zone threshold D/T_PL-MAX1. Accordingly, when the output pressure of the pressure modifier valve is increased, the base (original) pressure of the clutch pressure and the base pressure of the lockup) pressure of the torque converter 12 are simultaneously increased although the gradients and intercepts thereof are different respectively from others.

[0042] The transmission control unit 300 comprises a microcomputer 310 functioning as a control means, and four drive circuits 311 to 314. The microcomputer 310 outputs control signals for controlling the CVT mechanism 29 and the actuator unit 100 by executing a process shown by a flowchart of FIG. 3. The first to fourth drive circuits 311 to 314 respectively convert the control signals outputted from the microcomputer 310 into drive signals for practically controlling the actuators such as the stepping motor 108 and the respective duty valves 120, 128 and 129.

[0043] The microcomputer 310 comprises an input interface circuit 310aincluding an A/D converter, a calculation processor 310bsuch as a microprocessor, a storage device 310csuch as ROM and RAM, and an output interface circuit 310dincluding a D/A converter.

[0044] The microcomputer 310 executes various operations such as obtaining a rotation angle of the stepping motor 108 for setting the actual transmission ratio, outputting a pulse signal S_S/Mfor achieving the rotation angle, obtaining the optimum line pressure for holding the belt 24, calculating the duty ratio D/T_PLof the line pressure control duty valve 120 for achieving the necessary line pressure P_L, obtaining the fluid pressure (torque converter pressure) P_T/Cfor controlling the lockup mechanism of the torque converter 12, calculating the duty ratio D/T_L/Uof the lockup control duty valve 128 for achieving the torque converter pressure P_T/C, outputting a lockup control signal S_L/Uaccording to the lockup control duty ratio D/T_L/U, obtaining the fluid pressure (clutch pressure) P_CLwhich is optimum to extecute a creep running of the vehicle under when the accelerator pedal is put in the off condition (a foot of an operator is released from the accelerator pedal), calculating a duty ratio D/T_CLof the clutch engagement control duty valve 129 necessary for achieving the required clutch pressure P_CL, outputting a clutch engagement control signal S_CLaccording to the clutch pressure control duty ratio D/T_CL, and so on.

[0045] The first drive circuit 311 converts the pulse control signal S_S/Minto a drive signal D_S/Mand outputs it to the stepping motor 108. The second drive circuit 312 converts the pulse control signal S_PLinto a drive signal D_PLand outputs it to the line pressure control duty valve 120. The third drive circuit 313 converts the pulse control signal S_L/Uinto a drive signal D_L/Uand outputs it to the lockup control duty valve 128. The fourth drive circuit 314 converts the pulse control signal S_CLinto a drive signal D_CLand outputs it to the clutch engagement control duty valve 129. If the control signals according to the duty ratio and the pulse control signals satisfactorily indicate the desired duty ratios and number of pulses, the drive circuits 311 to 314 simply execute the amplifications thereof.

[0046] The engine control unit 200 also comprises a microcomputer which is interconnected with the microcomputer 310 of the transmission control unit 300 such that the engine 10 and the CVT are controlled according to the vehicle running condition.

[0047] The transmission control of the present embodiment will be discussed with reference to a calculating process shown by a flowchart of FIG. 3 which is executed by the microcomputer 310. This calculating process is basically executed when the D-range is selected and when the engine control unit 200 requires no process to the transmission control unit 300.

[0048] Herein, a general flow of the shift control will be discussed. This calculating process is impletmented as a timer interruption process at predetermined sampling time ΔT such as 10 milliseconds (msec). Although the flowchart of FIG. 3 does not show a step for a communication with others, the transmission control unit 300 timely executes a process for reading programs, maps and data used in the processor 310bfrom the storage device 310cand properly updates data calculated at the processor 310band stores it in the storage device 310c.

[0049] At a step S1, the microcomputer 310 reads the signals indicative of the vehicle speed V_SPoutputted from the vehicle speed sensor 302, the engine revolution speed N_Eoutputted from the engine revolution sensor 301, the input rotation speed N_Prioutputted from the input rotation speed sensor 305, a throttle opening TVO outputted from the throttle opening sensor 303 and a shift range signal S_RANGEform the inhibitor switch 304.

[0050] At a step S2, the microcomputer 310 calculates a present transmission ratio C, according to the vehicle speed V_SPand the input rotation speed N_Priby executing an independently provided process (subroutine). More particularly, the output rotation speed N_Secof the CVT mechanism 29 is obtained by dividing the vehicle speed V_SPin proportion with the final output shaft rotation speed by a final reduction speed ratio n (N_Sec=V_SP/n), and the present transmission ratio C_Pis obtained by dividing the input rotation speed N_Priby the obtained output rotation speed N_Sec(C_P=N_Pri/N_Sec).

[0051] At a step S3, the microcomputer 310 calculates the engine torque T_Eon the basis of the throttle opening TVO and the engine revolution speed N_Eby executing an independently provided process (subroutine) such as a retrieval of a control map. More particularly, the present engine torque T_Eis obtained from the output characteristic map shown in FIG. 4 according to the throttle opening TVO and the engine revolution speed N_E.

[0052] At a step S4, the microcomputer 310 exacutes the control of the line pressure P_Laccording to the process shown by a flowchart of FIG. 7. The detailed explanation of this line pressure control will be done later with reference to the flowchart of FIG. 7.

[0053] At a step S5, the microcomputer 310 executes a lockup control by jumping the routine to an independently provided process (subroutine). More particularly, a lockup speed V_ONand am unlockup speed V_OFFare determined by using a control map shown in FIG. 5 according to the vehicle speed V_SPand the throttle opening TVO. Basically, when the vehicle speed V_SPis greater than the lockup speed V_ON, the microcomputer 310 generates and outputs the control signal S_L/Uincluding a lockup command for putting the torque converter 12 into the lockup state. When smaller than the unlockup speed V_OFF, the microcomputer 310 generates and outputs the control signal S_L/Uincluding an unlockup command for putting the torque converter 12 into the unlockup state. In particular, in case that the condition of the torque converter 12 is changed from the unlockup state to the lockup state, when a difference between the engine rotation speed N_Eand the input rotation speed N_Priis greater than a predetermined value, that is, when a difference between the engine revolution speed N_Eand the rotation speed of the turbine of the torque converter 12 is greater than the predetermined value, a gain employed for increasing the duty ratio D/T_L/Uis increased according to the magnitude of the difference. When the difference is smaller than a predetermined value, that is, when the torque converter 12 tends to be put in the lockup state, the gain for increasing the duty ratio D/T_L/Uis decreased so as to buffer shift shocks caused by the transition to the full lockup state.

[0054] At a step S6, the microcomputer 310 calculates a goal transmission ratio C_Dby executing an independently provided process (subroutine) such as the retrieval of a control map. The goal transmission ratio C_Dis the most ideal transmission ratio for achieving the present engine revolution speed N_Sfrom the vehicle speed and the throttle opening TVO. More particularly, as shown in FIG. 6, if a transmission ratio C, by which the vehicle speed V_SP, the throttle opening TVO and the engine revolution speed N_Sare completely matched, is set, it is possible to ensure an acceleration according to the depression degree of the accelerator pedal (the throttle opening TVO) while satisfying the vehicle speed V_SPand the engine revolution speed N_S. Herein, if it is assumed that the map shown in FIG. 6 is the control map for setting the goal transmission ratio C_P, a straight line crossing with an origin point and having a constant gradient is a constant transmission ratio. For example, a straight line having the largest gradient in the whole area of the shift pattern is the largest speed reduction ratio of the vehicle, that is, a maximum transmission ratio C_L0. In reverse, a straight line having the smallest gradient in the whole area of the shaft pattern is the smallest speed reduction ratio of the vehicle, that is, a minimum transmission ratio C_0H1. Herein, if the 2-range is selected through the select lever 103, the shift control is implemented within an area from the maximum transmission ratio C_L0to a 2-range minimum transmission ratio C_2H1.

[0055] At a step S7, the microcomputer 310 calculates the objective transmission ratio C_Raccording to an individually prepared process (subroutine). More particularly, when the goal transmission ratio C_Dis greater than the present transmission ratio C_P, the objective transmission ratio C_Ris set to execute the downshift. When smaller than the present transmission ratio C_P, the objective transmission ratio C_Ris set to execute the upshift. For example, the objective transmission ratio C_Ris set at a transmission ratio obtained at a moment elapsing a predetermined sampling time ΔT from when the shifting is executed from the present transmission ratio C_Pby the highest shift speed dC_R/dt or smallest time constant τ. Herein, if the throttle opening TVO is decreased from a nearly full open state, that is, when the depression degree of the accelerator pedal is decreased, the shift speed dC_R/dt is little decreased or the time constant τ is little increased. Further, when the throttle opening TVO is further quickly decreased, such that the depression of the accelerator pedal is cancelled, the shift speed dC_R/dt is further decreased or the time constant τ is further increased. That is to say, the objective transmission ratio C_Ris changed according to the closing change amount of the throttle valve 19.

[0056] in this embodiment, the time constant τ is employed in order to set the objective transmission ratio C_R, that is, to control the shift speed. Therefore, when the goal transmission ratio C_Dis set, the objective transmission ratio C_Ris determined as a curve which takes values gradually converging to the goal transmission ratio C_D.

[0057] At a step S8, the microcomputer 310 executes a clutch engagement control according to an individually provided process (subroutine). More particularly, as a basic manner, when the vehicle speed V_SPis greater than a creep control threshold, the forward clutch 40 is engaged. When the vehicle speed V_SPis smaller than the creep control threshold and when the throttle opening TVO is greater than a creep control full close threshold, the engagement of the forward clutch 40 is released. Such operations are executed by generating and outputting the corresponding signal S_CLfrom the microcomputer 310 to the fourth drive circuit 314. When the vehicle speed V_SPis smaller that the creep control threshold and when the throttle opening TVO is smaller than the full close threshold, the gain for changing the duty ratio D/T_CLis changed in inverse proportion to the difference between the engine revolution speed N_Eand the input rotation speed N_Pri(turbine output shaft rotation speed). By this arrangement, the engagement force of the clutch 40 is decreased if the vehicle tends to generate a creep running due to a road condition such as a up-slope road running condition, and the engagement force of the clutch 40 is increased if the vehicle does not tend to genarate a creep running.

[0058] At a step S9, the microcomputer 310 executes a transmission ratio control according to an individually prepared process (subroutine). More particularly, with respect to the objective transmission ratio C_Rthe total number of the pulses and the number of pulses per a unit time are determined. Then, the microcomputer 310 generates and outputs the pulse control signal S_S/Msatisfying the both numbers. After the execution of the step S9, the routine of this program returns to the main program.

[0059] Next, the line pressure control executed at the step S4 of the flowchart of FIG. 3 will be discussed in detail with reference to the flowchart of FIG. 7.

[0060] At a step S401, the microcomputer 310 calculates a torque ratio t of the torque converter 12 from the speed ratio N_E/N_Priby executing the individually provided calculation such as the retrieval of the control map. More particularly, the torque converter input and output speed ratio N_E/N_Priis obtained by dividing the engine revolution speed N_Eby the input rotation speed N_Priequal to the turbine output rotation speed. The microcomputer 310 determines according to the torque converter input and output speed ratio N_E/N_Priand the map of FIG. 8 whether the torque converter 12 is put in the unlockup (converter) state or the lockup state. Further, the microcomputer 310 computes the torque ratio t according to the torque converter input and output speed ratio and the map of FIG. 8 if the unlock state.

[0061] At a step S402, the microcomputer 310 calculates an input torque T_Priby multiplying the torque ratio t by the engine torque T_E.

[0062] At a step S493, the microcomputer 310 calculates a reference line pressure P_L0on the basis of the input torque T_Priand a control map of FIG. 9. That is, the reference line pressure P_L0is read from the control map of FIG. 9 according to the present transmission ratio C_Pupon treating the input torque T_Prias a parameter. Since the line pressure P_Lcorresponds to the belt holding force, it is preferable to possibly keep the line pressure P_Lsmall in view of the durability of the belt 24 and the energy efficiency of the CVT. On the other hand, since the belt 24 is required to transmit the torque between the driver and follower pulleys 16 and 26, it is necessary to control the line pressure P_Lso as to prevent the belt from slipping from the pulleys. That is, it is necessary to apply a necessary line pressure to the pulleys 16 and 26 to hold the belt 25 thereby. Since the torque to be transmitted by the CVT is increased according to the increase of the transmission ratio C_Por the increase of the input torque T_Pri, it is necessary to increase the line pressure P_Laccording to the increase of the transmission ratio C_Por the input torque T_Pri. In order to comply with the above-mentioned requirement, the reference line pressure P_L0is set only by the transmission ratio C_Rand the input torque T_Pri. The reference line pressure P_L0takes a value which is much smaller than a value which directly affecting the durability of the belt 24.

[0063] At a step S404, the microcomputer 310 calculates an output rotation speed N_Secby dividing the vehicle speed V_SPby a final reduction ratio n (N_Sec=V_SP/n.

[0064] At a step S405, the microcomputer 310 calculates a follower pulley centrifugal pressure P_L1by multiplying a predetermined coefficient k with a square (N_Sec)²of the output rotation speed N_Sec(P_L1=k·(N_Sec)²). When the working fluid in the cylinder chamber 32 of the follower pulley 26 is pushed toward an outer side in the cylinder chamber 32 by the centrifugal force, the pressure of the working fluid at the outer side in the chamber 32 is increased. Such an increased pressure due to the centrifugal force is defined as the centrifugal pressure P_L1in this embodiment. Since the centrifugal pressure P_L2is in proportion with the square (N_Sec)²of the output rotation speed as is similar to the centrifugal force, it is derived by multiplying the square (N_Sec)²by the proportion coefficient k which is determined upon taking account of the properties of the working fluid such as specific gravity and viscosity.

[0065] At a step S406, the microcomputer 310 determines a difference between the reference line pressure P_L0and the centrifugal pressure P_L1as an objective line pressure P_L0R(P_L0R=P_L0−P_L1).

[0066] At a step S407, the microcomputer 310 calculates a reference line pressure control duty ratio D/T_PL0for achieving the objective line pressure P_L0R, from a control map shown in FIG. 2. A conventional duty ratio control The control map for determining the reference line pressure control duty ratio D/T_PL0may be applied to the control map used in this step.

[0067] At a step S408, the microcomputer 310 determines whether or not the engine revolution speed N_Eis greater than or equal to a preset switching control value N_E0or not. When the determination at the step S408 is affirmative (N_E≧N_E0), the routine proceeds to a step S409. When it is negative (N_E<N_E0), the routine proceeds to a step S410.

[0068] At the step S410, the microcomputer 310 determines whether or not the vehicle speed V_SPis smaller than the switching control value V_SP0. When the determination at the step S410 is affirmative (V_SP<V_SP0), the routine proceeds to a step S411. When the determination at the step S410 is negative (V_SP≧V_SP0), the routine proceeds to the step 5409. The explanation of the switching control values N_E0and V_SP0will be given later.

[0069] At the step S409, the microcomputer 310 stores the minimum value D/T_PL-MIN0as a duty ratio lower limit D/T_PL-MIN. Then, the routine proceeds to a step S412.

[0070] At the step S411, the microcomputer 310 stores the lower dead zone threshold D/T_PL-MIN1as the duty ratio lower limit D/T_PL-MIN. Then, the routine proceeds to the step S412.

[0071] At the Stop S412, the microcomputer 310 determines whether or not the reference line pressure duty ratio D/T_PL0is greater than or equal to the duty ratio lower limit D/T_PL-MIN. When the determination at the step S412 is affirmative (D/T_PL0≧D/T_PL-MIN), the routine proceeds to a step S413. When the determination at the step S412 is negative (D/T_PL0<D/T_PL-MIN), the routine proceeds to a step 414.

[0072] At the step S413, the microcomputer 310 stores the reference line pressure duty ratio D/T_PL0as the line pressure control duty ration D/T_PL.

[0073] At the step S414, the microcomputer 310 stores the duty ratio lower limit D/T_PL-MINas the line pressure control duty ration D/T_PL.

[0074] Following to the execution of the step S413 or S414, the routine proceeds to a step S415 wherein the microcomputer 310 generates and outputs the line pressure control signal S_PLaccording to the line pressure control duty ration D/T_PL.

[0075] After the execution of the step S415, the routine returns to the step S5 of FIG. 3. Herein, the generation of the line pressure control signal S_PLcorresponding to the line pressure duty ratio D/T_PLmay employ a known PWM (Pulse Width Modulation) control, and therefore the explanation thereof will be omitted herein.

[0076] The function or the processing of FIG. 7 will be discussed in detail.

[0077] By the execution of the steps S401 to S403, the reference line pressure P_L0is determined according to the input torque T_Prito the CVT mechanism 29. The reference line pressure P_L0is a necessary pressure for holding the belt against the input torque T_Prito be transmitted.

[0078] By the execution of the steps S404 and S405, the ceintrifugal pressure P_L1calculated from the square of the output rotation speed N_Sec.

[0079] By the execution of the step S406, the objective line pressure P_L0Rto be generated by the CVT mechanism pressure control valve including the line pressure control duty valve 120 is obtained by subtracting the centrifugal pressure P_L1from the reference line pressure P_L0.

[0080] By the execution of the step S407, the reference line pressure duty ratio D/T_PL0necessary to achieve the objective line pressure P_L0Ris set.

[0081] By the execution of the steps S408 to S411, the duty ratio lower limit is determined. That is, when the engine revolution speed N_Eis greater than or equal to the switching control value N_E0or when the vehicle speed V_SPis greater than or equal to the switching control value V_SP0, the minimum value D/T_PL-MIN0is treated as the duty ratio lower limit D/T_PL-MIN. When the engine revolution speed N_Eis smaller than the switching control value N_E0and when the vehicle speed V_SPis smaller than the switching control value V_SP0, the lower dead zone threshold D/T_PL-MIN1is treated as the duty ratio lower limit D/T_PL-MIN.

[0082] By the execution of the steps S412 to S414, the line pressure duty ratio is determined. That is, when the reference line pressure duty ratio D/T_PL0is greater than or equal to the duty ratio lower limit D/T_PL-MIN, the reference line pressure duty ratio D/T_PL0is used as the line pressure duty ratio D/T_PL. When the reference line pressure duty ratio D/T_PL0is smaller than the duty ratio lower limit D/T_PL-MIN, the duty ratio lower limit D/T_PL-MINis used as the line pressure duty ratio D/T_PL.

[0083] On the basis of the determined line pressure duty ratio D/T_PL, the line pressure control signal S_PLis generated and outputted. That is to say, by the execution of the steps S408 to S414, a selecting control of the duty ratio lower limit D/T_PLis executed. More particularly, when the engine revolution speed N_Eis high or when the vehicle speed V_SPis high, the duty ratio lower limit D/T_PL-MINfunctioning as a limit value for finally limiting the line pressure duty ratio D/T_PLis switched from the lower dead zone threshold D/T_PL-MIN1to the minimum value D/T_PL-MIN0(duty ratio=0%).

[0084] The explanation of the switching control values N_E0and V_SP0will be given hereinafter.

[0085] The characteristic curve between the duty ratio and the line pressure output shown in FIG. 2 is a nominal characteristic for a control valve constituted by a common duty valve. Practically, the characteristic curve therebetween is represented as shown in FIG. 10 with a large dispersion such that the line pressure P_Lis vertically fluctuated with respect to the line pressure duty ratio D/T_PLwithin a range including the ON-OFF boundary of the line pressure control duty valve 120. This width of the fluctuation represents the dispersion of the output pressure of the working fluid. However, this dispersion is almost decreased to zero when the duty ratio D/T_PLunder the OFF condition is set at the minimum value D/T_PL-MIN0(=0%). Because the line pressure control duty valve 120 receiving the minimum value D/T_PL-MIN0(=0%) is put in a full close (or full open) state in that the line pressure control duty valve 120 receives no affection of the voltage of the duty ratio control signal, the temperature of the working fluid and babbles in the fluid.

[0086] When the line pressure control duty valve 120 is controlled to slightly change its state from the OFF state to the ON state, a spool or puppet of the valve 120 is put in a reciprocating state or floating state in the working fluid. Under this state of the valve 120, the various factors of the fluctuation such as the voltage of the duty ratio control signal, the temperature of the working fluid and babbles in the fluid affect the output pressure of the valve 120 to produce the dispersion by each valve. The line pressure P_Lof the CVT mechanism 29 changes with a large gain and widely ranges from the minimum value to the maximum value. Therefore, the width of the dispersion of the line pressure P_Lis represented largely as shown in FIG. 10.

[0087] On the other hand, all actual line pressure P_Lof a control valve including this kind of a duty valve is almost not changed within a dead zone smaller than the lower dead zone threshold D/T_PL-MIN1. Therefore, when the line pressure P_Lis quickly and largely increased from the minimum value of a dead-zone value, the line pressure P_Ltends to be short with respected to an objective value (to produce an undershoot of the line pressure). Such an undershoot will cause the degradation of the responsibility of the line pressure control. Therefore, the embodiment of the present invention is arranged such that the lower dead zone threshold D/T_PL-MIN1is used as the line pressure duty ratio lower limit D/T_PL-MINwhen the engine revolution speed N_Eis not high and when the vehicle speed V_SPis not high, so as to prevent the line pressure duty ratio D/T_PLfunctioning as a command value from taking a value smaller than the duty ratio corresponding to the lower dead zone threshold D/T_PL-MIN1. This arrangement ensures the responsibility of the line pressure control. That is, the lower dead zone threshold D/T_PL-MIN1is set as the command value which is a limit value attaching importance to the responsibility with respect to the line pressure duty ratio D/T_PL.

[0088] The centrifugal pressure P_Lis increased according to the increase of the rotation speed of the follower pulley 26. When the rotation speed of the follower pulley 26 becomes greater than a predetermined value, the pulleys holds the belt 24 so as to be able to transmit the maximum input torque from the engine regardless the magnitude of the line pressure P_L. Under this holding state, it is impossible to quickly and properly execute the transmission ratio control.

[0089] Accordingly, such a centrifugal pressure dominant range of the line pressure is to be omitted from the transmission control range. The range to be omitted from the transmission ratio control range is a range where the input load is transmitted only by the holding force due to the centrifugal pressure under a condition that the line pressure P_Lis zero, that is, the line pressure duty ratio D/T_PLis the minimum value D/T_PL-MIN0(=0%). This centrifugal pressure dominant range is shown by a hatching area B in FIG. 11. in FIG. 11, the maximum value N_EMAX0is the maximum value of the engine revolution speed N_Ewithin the transmission ratio control range except for the centrifugal pressure dominant range. Further, when the engine revolution speed N_Eis the maximum value N_EMAX0, the vehicle speed V_SPtakes a vehicle speed maximum value V_SPMAX0. In this embodiment, the transmission ratio control range is limited within a range excepting the vehicle speed maximum value V_SPMAX0corresponding to the engine speed maximum value N_EMAX0.

[0090] However, if the line pressure lower limit D/T_PL-MINis kept at the lower dead zone threshold D/T_PL-MIN1, the range to be omitted from the transmission ratio control range should be set upon taking account of the dispersion amount of the line pressure in addition to the centrifugal pressure P_L1. Therefore, this centrifugal pressure dominant range takes a hatching area A in FIG. 11. As a result, the controllable area of the transmission ratio is decreased. However, when the engine revolution speed N_Eis greater than a predetermined value or when the vehicle speed V_SPis greater than a predetermined value, the follower pulley 24 is rotated in high speed and therefore the centrifugal pressure is high according to tho high rotation speed of the follow pulley 26. Accordingly, under such condition even if the responsibility of the line pressure control is improved by setting the line pressure duty ratio lower limit at the minimum valnle D/T_PL-MIN1(in order to respond to the condition that the line pressure duty ratio D/T_PLis small, that is, the line pressure P_Lis small), such improvement does not effectively function. Therefore, when the vehicle operating condition is put in the range A of FIG. 11, the line pressure duty ratio lower limit D/T_PL-MINis switched to the minimum value D/T_PL-MIN0(=0%) so as to extend the transmission ratio control range to the boundary defined by the engine revolution speed maximum value N_EMX0and the vehicle speed maximum, value V_SPMAX0. More particularly, when the engine revolution speed N_Eis greater than or equal to the switching control value N_E0or when the vehicle speed V_SPis greater than or equal to the switching control value V_SPMAX0, the transmission ratio controllable range is extended to the boundary of the range B of FIG. 11. In view of the durability of the belt 24, it is effective to eliminate the dispersion of the line pressure P_Ldue to the lower dead zone threshold D/T_PL-MIN1under the high engine revolution state or high vehicle speed state.

[0091] Within the range A, the belt 24 is rotated in high speed and receives a large centrifugal pressure. If a large axial load due to the line pressure PL is applied to the belt 24 under the high centrifugal force receiving condition, the belt 24 may slip or be stretched so as affect the durability of the belt 24. Therefore, by setting the line pressure duty ratio lower limit D/T_PL-MINat the minimum value D/T_PL-MIN0, it is possible to eliminate the dispersion corresponding to added amount of the centrifugal pressure P_L1. This eliminated amount contributes to improve the durability of the belt 24.

[0092] The line pressure duty valve 120 constitutes a CVT mechanism control valve according to the present invention. The steps S408 and S410 of the flowchart of FIG. 7 constitute a centrifugal pressure detecting means. The steps S409 and S411 of FIG. 7 constitute a switching means.

[0093] The contents of Application No. TOKUGANHEI 9-191263, with a filing date Jul. 16, 1997 in Japan, are hereby incorporated by reference.

[0094] Although the preferred embodiment according to the present invention has been shown and described such that it is determined that the centrifugal pressure P_L1is high enough to transmit the input load by determining whether or not the engine revolution speed N_Eis high, or whether or not the vehicle speed V_SPis high, it will be understood that if a means for directly detecting the centrifugal pressure P_L1is installed in the control system, the above-mentioned determination will be further easily executed.

[0095] Although the preferred embodiment has been shown and described such that the control unit thereof is constituted by a microcomputer, it will be understood that a combination of electronic circuits such as calculating circuits may be employed in the control unit instead of the microcomputer.