Control system for continuously variable transmission

Abstract: A control system controls a line pressure applied to a CVT upon taking account of a centrifugal pressure caused by a rotation of pulleys. The control system determines whether the centrifugal pressure is greater than a predetermined value only by which the pulley transmits an input torque from an engine. When the determination is affirmative, a lower limit of a duty ratio for operating a line pressure control valve is switched from a lower limit of a linear range to 0% duty ratio. Therefore, an actual transmission ratio control range is expanded while preventing an undershoot of the line pressure.

Description: BACKGROUND OF THE INVENTION 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, according to the preamble of claim 1. 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. SUMMARY OF THE INVENTION 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. EP-A-0 440 422 according to the precharacterizing part of claim 1 discloses a control system for a continuously variable transmission comprising a driver pulley, a follower pulley and a belt intercoupling the pulleys. Said continuously variable transmission varies a transmission ratio by changing effective diameters of the pulleys. A pressure control valve controls line pressure supplied to the continuously variable transmission according to a command value. A centrifugal pressure of working fluid in the pulley is detected and a control device decides that the centrifugal pressure in a centrifugal pressure dominant range in which motive power is transmitted only by the centrifugal pressure. The command value for controlling the pressure control valve is limited by a limit value limiting a range taken by the command value. 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. This object is solved by the features of claim 1. Further embodiments are claimed in the sub-claims. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, like reference numerals designate like parts and elements throughout all figures, in which: Fig. 1 is a schematic view showing a CVT and a control system thereof according to an embodiment of the present invention; 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; Fig. 3 is a flowchart showing a program executed by a transmission control unit of Fig. 1; Fig. 4 is a graph showing a control map for setting an engine torque from an throttle opening and an engine revolution speed; 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; Fig. 6 is a graph showing a control map for setting a transmission ratio from the throttle opening and the vehicle speed; Fig. 7 is a flowchart showing a program for executing a step 4 of Fig. 3; Fig. 8 is a graph showing a control map for setting a torque ratio from the torque converter input and output speed ratio; Fig. 9 is a graph showing a control map for setting a reference pressure from an input torque and the transmission ratio; Fig. 10 is a graph for explaining the dispersion of the line pressure generated with respect to the line pressure duty ratio; and Fig. 11 is a graph for explaining an expansion of a transmission ratio control range according to the present invention. DETAILED DESCRIPTION OF THE INVENTION 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. 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. Patent 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. 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 10a of 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. 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 10a of the engine 10 to detect an engine revolution speed N E and to output a signal indicative of the engine revolution speed N E . 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. 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 10a of 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. 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) 12f for providing a direct mechanical drive by directly coupling the input and output members 12c and 12d. The lockup facing member 12f is connected with an output shaft (turbine shaft) 13. The lockup facing member 12d separates an apply side fluid chamber 12a and a release side fluid chamber 12b. The release chamber 12b is formed between a cover 12g of a torque converter 12 and the lockup facing member 12f, and the apply chamber 12a is 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 12f is pressed against the cover 12g, and the torque converter 12 is put in a lockup state in which the input member 12c and the output member 12d are directly connected. When the fluid is supplied sufficiently into the release chamber 12b, the lockup facing member 12f is 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 12b is drained through the apply chamber 12a. 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. 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 Pri of 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 clutch 40. The fluid supplied to the release chamber 12b is drained through the apply chamber 12a to a reservoir 130, and a drained fluid of the fluid supplied to the apply chamber 12c is supplied from the release chamber 12b to 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. 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. 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. 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. 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. The V-belt CVT mechanism 29 further comprises a stepping motor 108 controlled by a transmission control unit 300. A pinion 108a is 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/M from 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. 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. 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 SP and output a signal indicative of the vehicle speed V SP to the transmission control unit 300. 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. Patent 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. A manual valve 104 is directly operated by a selector lever 103 so as to switchingly control a clutch pressure P CL to a cylinder chamber 40a of the forward clutch 40 and a brake pressure P BRX to a cylinder chamber 50a of the reverse brake 50. 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 RANGE indicative of the selected shift position. More particularly, the shift range signal S RANGE includes signals corresponding to P, R, N, D, 2, L according to the actually selected shift position. A shift control valve 106 is controlled according to a relative displacement between the stepping motor 108 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. A lockup control duty valve 128 controls a lockup mechanism of the torque converter 12 so as to put the torque converter 12 in one of a lockup state and an unlockup state. A transmission control unit 300 outputs a drive signal D L/U to the lockup control valve 128 to operate the lockup mechanism of the torque converter 12. More particularly, when the drive signal D L/U indicative 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/U indicative 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. 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 CL from the transmission control unit 300. When the drive signal D CL indicates a large duty ratio, one of the forward clutch 40 and the reverse brake 50 is engaged. When the drive signal D CL indicates a small duty ratio, it is disengaged. A line pressure control duty valve 120 controls the line pressure P L supplied 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 PL from 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 vale 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 L produced 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 L is indirectly controlled. In this embodiment, the objective line pressure P L(OR) 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 PL of 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 PL to 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-MAX1 to a maximum value D/T PL-MAX0 (100%). Therefore, a linear operation zone ranges from the lower dead zone threshold D/T PL-MIN1 to 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. 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. The microcomputer 310 comprises an input interface circuit 310a including an A/D converter, a calculation processor 310b such as a microprocessor, a storage device 310c such as ROM and RAM, and an output interface circuit 310d including a D/A converter. 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/M for achieving the rotation angle, obtaining the optimum line pressure for holding the belt 24, calculating the duty ratio D/T PL of the line pressure control duty valve 120 for achieving the necessary line pressure P L , obtaining the fluid pressure (torque converter pressure) P T/C for controlling the lockup mechanism of the torque converter 12, calculating the duty ratio D/T L/U of the lockup control duty valve 128 for achieving the torque converter pressure P T/C , outputting a lockup control signal S L/U according to the lockup control duty ratio D/T L/U , obtaining the fluid pressure (clutch pressure) P CL which is optimum to execute 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 CL of the clutch engagement control duty valve 129 necessary for achieving the required clutch pressure P CL , outputting a clutch engagement control signal S CL according to the clutch pressure control duty ratio D/T CL , and so on. The first drive circuit 311 converts the pulse control signal S S/N into a drive signal D S/M and outputs it to the stepping motor 108. The second drive circuit 312 converts the pulse control signal S PL into a drive signal D PL and outputs it to the line pressure control duty valve 120. The third drive circuit 313 converts the pulse control signal S L/U into a drive signal D L/U and outputs it to the lockup control duty valve 128. The forth drive circuit 314 converts the pulse control signal S CL into a drive signal D CL and 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. 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. 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. Herein, a general flow of the shift control will be discussed. This calculating process is implemented 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 310b from the storage device 310c and properly updates data calculated at the processor 310b and stores it in the storage device 310c. At a step S1, the microcomputer 310 reads the signals indicative of the vehicle speed V SP outputted from the vehicle speed sensor 302, the engine revolution speed N E outputted from the engine revolution sensor 301, the input rotation speed N Pri outputted from the input rotation speed sensor 305, a throttle opening TVO outputted from the throttle opening sensor 303 and a shift range signal S RANGE form the inhibitor switch 304. At a step S2, the microcomputer 310 calculates a present transmission ratio C P according to the vehicle speed V SP and the input rotation speed N Pri by executing an independently provided process (sub routine). More particularly, the output rotation speed N Sec of the CVT mechanism 29 is obtained by dividing the vehicle speed V SP in 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 P is obtained by dividing the input rotation speed N Pri by the obtained output rotation speed N Sec (C P =N Pri /N Sec ). At a step S3, the microcomputer 310 calculates the engine torque T E on the basis of the throttle opening TVO and the engine revolution speed N E by executing an independently provided process (sub routine) such as a retrieval of a control map. More particularly, the present engine torque T E is obtained from the output characteristic map shown in Fig. 4 according to the throttle opening TVO and the engine revolution speed N E . At a step S4, the microcomputer 310 executes the control of the line pressure P L according 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. 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 ON and an unlockup speed V OFF are determined by using a control map shown in Fig. 5 according to the vehicle speed V SP and the throttle opening TVO. Basically, when the vehicle speed V SP is greater than the lockup speed V ON , the microcomputer 310 generates and outputs the control signal S L/U including 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/U including 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 E and the input rotation speed N Pri is greater than a predetermined value, that is, when a difference between the engine revolution speed N E and 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/U is 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/U is decreased so as to buffer shift shocks caused by the transition to the full lockup state. At a step S6, the microcomputer 310 calculates a goal transmission ratio C D by executing an independently provided process (subroutine) such as the retrieval of a control map. The goal transmission ratio C D is the most ideal transmission ratio for achieving the present engine revolution speed N E from 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 E are 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 SP and the engine revolution speed N E . Herein, if it is assumed that the map shown in Fig. 6 is the control map for setting the goal transmission ratio C D , 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 LO . In reverse, a straight line having the smallest gradient in the whole area of the shift pattern is the smallest speed reduction ratio of the vehicle, that is, a minimum transmission ratio C DHi . 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 L0 to a 2-range minimum transmission ratio C ZHi . At a step S7, the microcomputer 310 calculates the objective transmission ratio C R according to an individually prepared process (subroutine). More particularly, when the goal transmission ratio C D is greater than the present transmission ratio C P , the objective transmission ratio C R is set to execute the downshift. When smaller than the present transmission ratio C P , the objective transmission ratio C R is set to execute the upshift. For example, the objective transmission ratio C R is 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 P by 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 R is changed according to the closing change amount of the throttle valve 19. 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 D is set, the objective transmission ratio C R is determined as a curve which takes values gradually converging to the goal transmission ratio C D . 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 SP is greater than a creep control threshold, the forward clutch 40 is engaged. When the vehicle speed V SP is 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 CL from the microcomputer 310 to the fourth drive circuit 314. When the vehicle speed V SP is 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 CL is changed in inverse proportion to the difference between the engine revolution speed N E and 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 generate a creep running. 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 R , the 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/M satisfying the both numbers. After the execution of the step S9, the routine of this program returns to the main program. 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. At a step S401, the microcomputer 310 calculates a torque ratio t of the torque converter 12 from the speed ratio N E /N Pri by 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 Pri is obtained by dividing the engine revolution speed N E by the input rotation speed N Pri equal to the turbine output rotation speed. The microcomputer 310 determines according to the torque converter input and output speed ratio N E /N Pri and 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. At a step S402, the microcomputer 310 . calculates an input torque T Pri by multiplying the torque ratio t by the engine torque N E . At a step S403, the microcomputer 310 calculates a reference line pressure P L0 on the basis of the input torque T Pri and a control map of Fig. 9. That is, the reference line pressure P L0 is read from the control map of Fig. 9 according to the present transmission ratio C P upon treating the input torque T Pri as a parameter. Since the line pressure P L corresponds to the belt holding force, it is preferable to possibly keep the line pressure P L small 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 L so 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 P or the increase of the input torque T Pri , it is necessary to increase the line pressure P L according to the increase of the transmission ratio C P or the input torque T Pri . In order to comply with the above-mentioned requirement, the reference line pressure P L0 is set only by the transmission ratio C R and the input torque T Pri . The reference line pressure P L0 takes a value which is much smaller than a value which directly affecting the durability of the belt 24. At a step S404, the microcomputer 310 calculates an output rotation speed N Sec by dividing the vehicle speed V SP by a final reduction ratio n (N Soc = V SP /n). At a step S405, the microcomputer 310 calculates a follower pulley centrifugal pressure P L1 by multiplying a predetermined coefficient k with a square (N Sec ) 2 of 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 L1 in this embodiment. Since the centrifugal pressure P L1 is in proportion with the square (N Sec ) 2 of the output rotation speed as is similar to the centrifugal force, it is derived by multiplying the square (N Sec ) 2 by the proportion coefficient k which is determined upon taking account of the properties of the working fluid such as specific gravity and viscosity. At a step S406, the microcomputer 310 determines a difference between the reference line pressure P L0 and the centrifugal pressure P L1 as an objective line pressure P L0R (P L0R = P L0 -P L1 ). At a step S407, the microcomputer 310 calculates a reference line pressure control duty ratio D/T PL0 for 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 PL0 may be applied to the control map used in this step. At a step S408, the microcomputer 310 determines whether or not the engine revolution speed N E is greater than or equal to a preset switching control value N E0 or 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. At the step S410, the microcomputer 310 determines whether or not the vehicle speed V SP is 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 S409. The explanation of the switching control values N E0 and V SP0 will be given later. At the step S409, the microcomputer 310 stores the minimum value D/T PL-MIN0 as a duty ratio lower limit D/T PL-MIN . Then, the routine proceeds to a step S412. At the step S411, the microcomputer 310 stores the lower dead zone threshold D/T PL-MIN1 as the duty ratio lower limit D/T PL-MIN . Then, the routine proceeds to the step S412. At the step S412, the microcomputer 310 determines whether or not the reference line pressure duty ratio D/T PL0 is 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. At the step S413, the microcomputer 310 stores the reference line pressure duty ratio D/T PL0 as the line pressure control duty ratio D/T PL . At the step S414, the microcomputer 310 stores the duty ratio lower limit D/T PL-MIN as the line pressure control duty ratio D/T PL . 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 PL according to the line pressure control duty ratio D/T PL . 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 PL corresponding to the line pressure duty ratio D/T PL may employ a known PWM (Pulse Width Modulation) control, and therefore the explanation thereof will be omitted herein. The function of the processing of Fig. 7 will be discussed in detail. By the execution of the steps S401 to S403, the reference line pressure P L0 is determined according to the input torque T Pri to the CVT mechanism 29. The reference line pressure P L0 is a necessary pressure for holding the belt against the input torque T Pri to be transmitted. By the execution of the steps S404 and S405, the centrifugal pressure P L1 is calculated from the square of the output rotation speed N Sec . By the execution of the step S406, the objective line pressure P L0R to 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 L1 from the reference line pressure P L0 . By the execution of the step S407, the reference line pressure duty ratio D/T PL0 necessary to achieve the objective line pressure P L0R is set. By the execution of the steps S408 to S411, the duty ratio lower limit is determined. That is, when the engine revolution speed N E is greater than or equal to the switching control value N E0 or when the vehicle speed V SP is greater than or equal to the switching control value V SP0 , the minimum value D/T PL-MIN0 is treated as the duty ratio lower limit D/T PL-MIN . When the engine revolution speed N E is smaller than the switching control value N E0 and when the vehicle speed V SP is smaller than the switching control value V SP0 , the lower dead zone threshold D/T PL-MIN1 is treated as the duty ratio lower limit D/T PL-MIN . 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 PL0 is greater than or equal to the duty ratio lower limit D/T PL-MIN , the reference line pressure duty ratio D/T PL0 is used as the line pressure duty ratio D/T PL . When the reference line pressure duty ratio D/T PL0 is smaller than the duty ratio lower limit D/T PL-MIN , the duty ratio lower limit D/T PL-MIN is used as the line pressure duty ratio D/T PL . On the basis of the determined line pressure duty ratio D/T PL , the line pressure control signal S PL is 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 PL is executed. More particularly, when the engine revolution speed N E is high or when the vehicle speed V SP is high, the duty ratio lower limit D/T PL-MIN functioning as a limit value for finally limiting the line pressure duty ratio D/T PL is switched from the lower dead zone threshold D/T PL-MIN1 to the minimum value D/T PL-MIN0 (duty ratio =0%). The explanation of the switching control values N E0 and V SP0 will be given hereinafter. 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 L is vertically fluctuated with respect to the line pressure duty ratio D/T PL within 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 PL under 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. 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 L of 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 L is represented largely as shown in Fig. 10. On the other hand, an actual line pressure P L of 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 L is quickly and largely increased from the minimum value of a dead-zone value, the line pressure P L tends 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-MIN1 is used as the line pressure duty ratio lower limit D/T PL-MIN when the engine revolution speed N E is not high and when the vehicle speed V SP is not high, so as to prevent the line pressure duty ratio D/T PL functioning 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-MIN1 is 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 . The centrifugal pressure P L is 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. 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 L is zero, that is, the line pressure duty ratio D/T PL is 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 EMAX0 is the maximum value of the engine revolution speed N E within the transmission ratio control range except for the centrifugal pressure dominant range. Further, when the engine revolution speed N E is the maximum value N EMAX0 , the vehicle speed V SP takes 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 SPMAX0 corresponding to the engine speed maximum value M EMAX0 . However, if the line pressure lower limit D/T PL-MIN is 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 E is greater than a predetermined value or when the vehicle speed V SP is greater than a predetermined value, the follower pulley 24 is rotated in high speed and therefore the centrifugal pressure is high according to the 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 value D/T PL-MIN1 (in order to respond to the condition that the line pressure duty ratio D/T PL is small, that is, the line pressure P L is 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-MIN is 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 EMX0 and the vehicle speed maximum value V SPMAX0 . More particularly, when the engine revolution speed N E is greater than or equal to the switching control value N EO or when the vehicle speed V SP is 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 L due to the lower dead zone threshold D/T PL-MIN1 under the high engine revolution state or high vehicle speed state. 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-MIN at 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. 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. Although the preferred embodiment according to the present invention has been shown and described such that it is determined that the centrifugal pressure P L1 is high enough to transmit the input load by determining whether or not the engine revolution speed N E is high, or whether or not the vehicle speed V SP is high, it will be understood that if a means for directly detecting the centrifugal pressure P L1 is installed in the control system, the above-mentioned determination will be further easily executed. 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.

Claims: A control system comprising: a continuously variable transmission (CVT) having a defined linear operation range, comprising a driver pulley (16), a follower pulley (26) and a belt (24) intercoupling the pulleys (16, 26), said CVT varying a transmission ratio by changing effective diameters of the pulleys (16, 26); a CVT pressure control valve controlling a pressure (P L ) of working fluid to be supplied to said CVT according to a command value; a centrifugal pressure detecting means (310) for detecting a centrifugal pressure (P L1 ) of the working fluid in the pulley (26); a centrifugal pressure deciding means (310) deciding that the centrifugal pressure (P L1 ) is in a centrifugal pressure dominant range in which motive power is transmitted only by the centrifugal pressure (P L1 ); and a command value outputting means (310) outputting the command value (D/T PL ) to the CVT pressure control valve, a limit value (D/T PL-MIN ) limiting a range taken by the command value (DT PL ); characterized in that a limit value switching means (310) switching the limit value D/T PL-MIN ) of the command value from a first value (D/T PL-MIN1 ) which is a limit value of the linear operation range to a second value (D/T PL-MIN0 ) which is a minimum value of the command value (D/T PL ). A control system as claimed in claim 1, wherein said centrifugal pressure detecting means includes a calculating means for calculating the centrifugal pressure (P L1 ) from a follower pulley rotation speed. A control system as claimed in claim 1 or 2, wherein said CVT pressure control valve includes a duty valve (120) to which the command value (D/T PL ) corresponding to the duty ratio indicative signal is applied. A control system as claimed in one of claims 1 to 3, wherein the command value (D/T PL ) applied to said CVT pressure control valve is calculated according to an objective line pressure (P L0R ) obtained by subtracting the centrifugal pressure (P L1 ) from a reference line pressure (P L0 ). A control system as claimed in claim 4, wherein the reference line pressure (P L0 ) is calculated from the input torque (T Pri ) and a transmission ratio (C p ) of said CVT. A control system as claimed in claim 5, wherein the reference line pressure (P LO ) is calculated on the basis of a predetermined map showing a relationship of the reference pressure to the input torque (T Pri ) and the transmission ratio (C p ). A control system as claimed in one of claims 1 to 6, wherein a controller comprises said centrifugal pressure detecting means, said centrifugal pressure deciding means, said command value outputting means and said limit value switching means, said controller including a microcomputer (310) which stores a plurality of control maps for computing parameters indicative of vehicle operating condition. A control system as claimed in claim 7, wherein the microcomputer (310) outputs the command value (D/T PL ) to said CVT pressure control valve. A control system as claimed in claim 7, wherein the controller is arranged to perform determining that the centrifugal pressure (P L1 ) is greater than a road-transmitting value enough to transmit an input load from the engine (10) to the CVT, and to perform switching the limit value of the command value (D/T PL ) from the first value to the second value when the centrifugal pressure (P L1 ) is greater than the load-transmitting value. A control system as claimed in claim 7, wherein the CVT pressure control valve is of a duty valve (120) which varies a line pressure (P L ) applied to the CVT according to a duty ratio, the controller selecting the second value when an engine revolution speed (N E ) is higher than the predetermined switching revolution speed (N EO ) or when the vehicle speed (V SP ) is higher than the predetermined switching vehicle speed (V SPo ).

Inventors: OKAHARA HIROFUMI, SAWADA MAKOTO, OKAHARA, HIROFUMI, SAWADA, MAKOTO
Applicants: NISSAN MOTOR, NISSAN MOTOR COMPANY, LIMITED
Application year: 1998
Main classification: F16H61
IPC codes: F16H59, F16H61, F16H9