CONTROL SYSTEM FOR A CONTINUOUSLY VARIABLE TRANSMISSION

Abstract: A control system for a continuously variable transmission (CVT) for a motor vehicle has a transmission ratio controlled in accordance with a desired transmission ratio, by adjusting the transmission ratio changing speed. This is achieved by a control circuit (70) adjusting the oil supply to a hydraulic circuit (Figure 2a, 2b) of the CVT through the transmission ratio control valve 50. At kickdown of the transmission, two desired control values are selected and applied to correct the desired transmission ratio and desired transmission changing speed, so as to increase the transmission ratio quickly to a value smaller than the desired transmission ratio at the kickdown.

Description: The present invention relates to a control system for a continouosly variable belt-drive automatic transmission for a motor veicle, and more particularly to a system for controlling the transmission ratio at quick downshifting for rapid acceleration, such as kickdown. A known control system for a continuously variable belt-drive transmission, as defined in the pre-characterising part of claim 1 of this application, is known from EP-A-181786 and includes an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependence on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices. The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine. At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed which is decided by line pressure, pressure of oil supplied to the servo device of the drive pulley, and actual transmission ratio. When command of rapid acceleration of the vehicle, which is dependent on driver's operation of an accelerator pedal, is inputted in the control system, it is necessary to downshift the transmission or to control the transmission ratio to meet the requirement. Various further systems for controlling the transmission ratio at transient state are known. JP-A-59-208253 discloses a system wherein a transmission ratio is fixed to a predetermined value at an initial stage of acceleration. Further, JP-A-60-88259 discloses a system which determines changing speed of desired transmission ratio in dependence on the operating speed of an accelerator pedal. However, in accordance with the systems of the prior art, the actual transmission ratio does not change to provide rapid acceleration of the vehicle at quick depression of an accelerator pedal so as to meet driver's demand. The present invention seeks to provide a system which can control the transmission ratio so as to meet the requirement of a driver's wishes for acceleration of a vehicle. According to the present invention there is provided a control system for a continuously variable transmission for transmitting the power of an internal combustion engine, having a drive pulley having a hydraulically shiftable disc and a hydraulic cylinder for operating the disc, a driven pulley with an hydraulically shiftable disk and a hydraulic cylinder for operating the disc and a belt engaged with both pulleys, the system comprising: a hydraulic circuit including a pump for supplying oil to both the hydraulic cylinders, a spool type valve for controlling the transmission ratio by varying the oil supply to the cylinder of the drive pulley; a solenoid operated control valve means provided in the hydraulic circuit for controlling an amount of oil to be supplied to the transmission ratio control valve; sensing means for producing a drive pulley speed signal, a driven pulley speed signal and a throttle position signal; means responsive to the driven pulley speed signal and the load signal for detecting rapid acceleration of the engine and for producing an acceleration signal; a calculator responsive to both the pulley speed signals for producing an actual transmission ratio signal; a calculator responsive to the driven pulley speed signal and the throttle position signal for producing a desired transmission ratio signal; a calculator means responsive to both said actual and desired transmission ratio signals for producing a transmission ratio changing speed signal; means responsive to the transmission ratio changing speed signal for producing a control signal for operating the control valve means to shift the spool type transmission ratio control valve at a speed dependent on the transmission ratio changing speed signal; characterised by means responsive to the acceleration signal to produce; a first value signal and a second value signal, said first value signal having a value indicative of a transmission ratio less than the desired transmission ratio and said second value signal having a value indicative of a transmission ratio greater than said first value signal and less than said desired transmission ratio; and correcting means acting at the incidence of the acceleration signal to replace the desired transmission ratio signal initially with the first value signal, and when a predetermined condition of the first value signal and the actual transmission ratio is reached, with the second value signal and, when a predetermined condition of the second value signal and the actual transmission ratio is reached, reverting to use of the desired transmission ratio; and a changing speed correction section for correcting the transmission ratio changing speed signal according to the first value signal. A preferred embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a schematic illustration of a continuously variable belt-drive transmission; Figs. 2a and 2b are schematic diagrams showing a control system according to the present invention; Fig. 3 is a block diagram showing a control unit; Figs. 4a and 4b are flowcharts showing the operation of the system; and Figs. 5 and 6 are graphs showing acceleration characteristics. Referring to Fig. 1, a motor vehicle is provided with an engine 1, an electromagnetic powder clutch 2 for transmitting the power of the engine to a continuously variable belt-drive transmission 4 through a selector mechanism 3. The belt-drive transmission 4 has a main shaft 5 and an output shaft 6 provided in parallel with the main shaft 5. A drive pulley (primary pulley) 7 and a driven pulley (secondary pulley) 8 are mounted on shafts 5 and 6 respectively. A fixed conical disc 7b of the drive pulley 7 is integral with main shaft 5 and an axially movable conical disc 7a is axially slidably mounted on the main shaft 5. The movable conical disc 7a also slides in a cylinder 9a formed on the main shaft 5 to provide a servo device. A chamber 9 of the servo device communicates with a hydraulic circuit 20. A fixed conical disc 8b of the driven pulley 8 is formed on the output shaft 6 opposite a movable conical disc 8a. The conical disc 8a has a cylindrical portion which is slidably engaged in a cylinder 6a of the output shaft 6 to form a servo device. A chamber 10 of the servo device is also communicated with circuit 20. A drive belt 11 engages with the drive pulley 7 and the driven pulley 8. Secured to the output shaft 6 is a drive gear 12 which engages with an intermediate reduction gear 13 on an intermediate shaft 14. An intermediate gear 15 on the shaft 14 engages with a final gear 16. The rotation of the final gear 16 is transmitted to axles 18 of vehicle driving wheels 19 through a differential 17. Referring to Figs. 2a and 2b, chamber 9 of the drive pulley 7 is supplied with pressurized oil by an oil pump 21 from an oil reservoir 26 passing through a line pressure conduit 22, ports 41a and 41e of a line pressure control valve 40, transmission ratio control valve 50, and conduit 23. The chamber 10 of driven pulley 8 is applied with pressurized oil through a passage 22b without passing through valves 40 and 50. The movable conical disc 7a of the drive pulley 7 is so designed that the pressure receiving area thereof is larger than that of movable conical disc 8a of the driven pulley 8. The line pressure control valve 40 comprises a valve body 41, spool 42, and chambers 41c and 41d. The spool 42 is applied with pressure of the pressurized oil in the chamber 41c supplied through a conduit 31. The other end of the spool 42 is applied with the force of a spring 43 provided between the end of the spool 42 and a retainer 45 the position of which is adjustable by a screw 44. The port 41a is communicated with a drain port 41b for a drain passage 27 in accordance with the position of a land of the spool 42. The drain port 41b communicates with oil reservoir 26 through passage 27. The transmission ratio control valve 50 comprises a valve body 51, spool 52, spring 53 for urging the spool 52 in the downshift direction. A port 51b of the valve body 51 is selectively communicated with a pressure oil supply port 51a or a drain port 51c in accordance with the position of lands of spool 52. Port 51b communicates with chamber 9 through conduit 23, and port 51a communicates with port 41e of line pressure control valve 40 through conduit 22a. The drain port 51c is communicated with the oil reservoir 26 through a conduit 24 and a check valve 25. The system is provided with a regulator valve 60, and solenoid operated on-off valves 66 and 68. The regulator valve 60 comprises a valve body 61, an inlet port 61a connected to the pump 21 through passages 37 and 22, a spool 62, an end chamber 61c connected to the passage 37, and a spring 63 urging the spool 62 to the chamber 61c. When the pressure of oil in the chamber 61c becomes higher than a set value, the spool 62 is shifted to the left, so that an inlet port 61a communicates with a drain port 61b to drain the oil. Thus, a constant pressure of oil is provided in the passage 37. The passage 37 is communicated with the chamber 41d of line pressure control valve 40 through a constant pressure passage 38, orifice 65, solenoid operated on-off valve 66, and passage 32 having an accumulator 32a. Further, the passage 38 is communicated with an end chamber 51d of the transmission ratio control valve 50 through a passage 33, and with another end chamber 51e through an orifice 67, solenoid operated on-off valve 68 and passage 34. The solenoid operated on-off valve 66 is adapted to be operated by pulses. When energized, a valve 66a opens a drain port 66b. The pulsation of the pressure of oil in the passage 32 is smoothed by accumulator 32a. The solenoid operated on-off valve 68 is the same as valve 66 in construction and operation. The valves 66 and 68 are operated by signals from a control unit 70. Thus, pressure controlled by the valves 66 and 68 is applied to chambers 41d and 51e. In the transmission ratio control valve 50, pressure receiving area of the spool 52 at chamber 51e is set to a value larger than the area at the chamber 51d. On the other hand, the control pressure in the chamber 51e can be changed between a maximum value, which is the same as the constant pressure in the chamber 51d, when the duty ratio is 0% and zero by controlling the duty ratio of pulses for operating the valve 68. The transmission ratio control valve 50 is so arranged that the spool 52 is at a neutral position at a middle duty ratio (for example 50%) and is located in an oil supply position by increasing the duty ratio from the middle duty ratio because of reduction of control pressure in the chamber 51e. Further, the speed of the movement of the spool 52 changes with the magnitude of changing of the duty ratio. The spool 52 is shifted to an oil drain position by decreasing the duty ratio. It will be understood that when the oil is supplied to the chamber 9, the transmission is upshifted. The duty ratio is decided by the transmission ratio changing speed and the transmission ratio. When the transmission is upshifted, the duty ratio D is a decreasing function of the actual transmission ratio i and an increasing function of the transmission ratio changing speed di/dt. When the transmission is downshifted, the duty ratio D is an increasing function of the actual transmission ratio i and a decreasing function of the transmission ratio changing speed di/dt. In a feedback control system, the transmission ratio changing speed di/dt can be decided by the difference between the actual transmission ratio i and a desired transmission ratio id, as follows. di/dt - K1 (id - i) where K1 is a coefficient. However, the response of the transmission control system including various mechanisms is slow, which means that the convergence of the actual transmission ratio to the desired transmission ratio delays. In order to eliminate the control delay, the transmission ratio changing speed di/dt is corrected by the speed of change of the desired transmission ratio (did/dt), as follows. di/dt = K1 (id-i) + K2 · did/dt where K2 is a coefficient. The desired transmission ratio changing speed did/dt is to advance the phase of the control operation. Thus, the response of the system can be improved. In order to decide the upshift or the downshift, the equation is rewritten as follows. di/dt = K1 { (id + K2/K1 · did/dt) - i } The transmission is downshifted when (id + K2/K1 · did/dt) > i, and di/dt > 0. The transmission is upshifted when (id + K2/K1 · did/dt) < i, and di/dt < 0. The speed did/dt is obtained by the amount (Δid) of change of the desired transmission ratio at a predetermined intervals (Δt), that is Δ id/Δt. The coefficient K1 may be changed in accordance with the opening degree of the throttle valve, and the coefficient K2 may also be changed in accordance with physical conditions of the system, such as viscosity of oil used in the system. Referring to Fig. 3, the system is arranged to control the transmission ratio in accordance with the above described principle. In the system, a drive pulley speed sensor 71, driven pulley speed sensor 72, engine speed sensor 73 and throttle position sensor (or intake manifold pressure sensor) 74 are provided. Output signals N P and N S of sensors 71, 72 are fed to an actual transmission ratio calculator 75 to produce an actual transmission ratio i in accordance with i = N p / N S . Output signal N S and output signal Θ representing the opening degree of the throttle position sensor 74 are fed to a desired transmission ratio table 76. The desired transmission ratio id is derived from the table 76 in accordance with signals N S and Θ. The desired transmission ratio id is fed to a transmission changing speed calculator 78 and to a desired transmission ratio changing speed calculator 80 through a desired ratio correcting section 94. The calculator 80 produces a desired transmission ratio changing speed did/dt. A coefficient setting section 77 produces coefficients K1 and K2. The actual transmission ratio i, desired transmission ratio changing speed did/dt and coefficients K1 and K2 are applied to transmission ratio changing speed calculator 78 to produce a transmission ratio changing speed di/dt from the formula di/dt = K1 (id - i) + K2 · did/dt. The speed di/dt and actual ratio i are applied to a duty ratio table 79 through a changing speed correcting section 95 to derive the duty ratio D. The duty ratio D is supplied to the solenoid operated on-off valve 68 through a driver 82. The system is provided with a control system for quick change of transmission ratio such as kickdown as described hereinafter. The throttle opening degree δ of the throttle position sensor 74 is fed to a throttle opening speed calculator 84 which produces opening speed dΘ/dt. The actual transmission ratio i, desired transmission ratio id, throttle opening degree Θ, and throttle opening speed dΘ/dt are applied to a kickdown detecting section 85, where a kickdown is detected in accordance with difference between the desired transmission ratio id and the actual transmission ratio i, the throttle opening speed dΘ/dt at the time when the throttle valve opening degree Θ exceeds a predetermined value. Output signal of the section 85 is applied to a first desired value providing section 86 and a second desired value providing section 87. A constant providing section 88 is provided for producing a constant m which is applied to the first desired value providing section 86 and for producing constant n applied to the second desired value providing section 87. The first desired value providing section 86 produces a first desired value id₁ for quick downshifting the transmission. The first desired value id₁ is obtained by id₁ = (id 0 - i 0 ) · m + i 0 , where i 0 is a transmission ratio before the accelerator pedal is depressed, and id 0 is a desired transmission ratio at the kickdown. The first desired value id₁ is fed to the correcting section 94 and to correcting section 95. The correcting section 94 produces a desired transmission ratio signal representing the value id₁ and the correcting section 95 corrects the transmission ratio changing speed di/dt to a high speed. The first desired value id₁ is cleared when the actual transmission ratio i becomes larger than the first desired value id₁ (i > id₁). The second value providing section 87 produces a second desired value id₂ for providing a desired transmission ratio id. The second desired value id₂ is id₂ = (id 0 - i 0 ) · n + i 0 . The second desired value id₂ is fed to the correcting section 94 after the first desired value id₁ is cleared. When the actual transmission ratio i becomes larger than the desired transmission ratio id₂ (i > id₂), the second desired value id₂ is cleared. In the constant providing section 88, the constant m is determined such that the first desired value id₁ is smaller than the second desired value id₂ (id₁ < id₂), and the constant n is determined such that the second desired value id₂ is smaller than the desired transmission ratio id 0 (id₂ < id 0 ). Further, the output signal Θ of throttle position sensor 74 and the output N e of engine speed sensor 73 are fed to an engine torque calculator 96, so that engine torque T is calculated based on throttle position Θ and engine speed N e . On the other hand, the actual transmission ratio i from the calculator 75 is applied to a necessary line pressure table 103 to derive a necessary line pressure P LU per unit torque. The necessary line pressure P LU and the engine torque T are applied to a desired line pressure calculator 104 where a desired line pressure P L is calculated. The desired line pressure P L is applied to a duty ratio table 105 to derive a duty ratio D L corresponding to the desired line pressure P L . The duty ratio D L is supplied to a driver 106 which operates the solenoid operated on-off valve 66 at the duty ratio. In operation, while the vehicle is at a stop, chamber 10 of the driven pulley 8 is supplied with line pressure through passage 22b, and the chamber 9 of the drive pulley 7 is drained, since the N P , N s , Θ are zero and duty ratio D is zero, and the spool 52 is at the right end position and the drain port 51c communicates with the chamber 9 through the conduit 23 as shown in Figs. 2a and 2b. Thus, in the pulley and belt device of the continuously variable belt-drive transmission, the driving belt 11 engages with the driven pulley 8 at a maximum running diameter to provide the largest transmission ratio (low speed stage). When the accelerator pedal is depressed, the clutch current increases progressively with increase of engine speed. The electromagnetic clutch 2 is gradually engaged, transmitting the engine power to the drive pulley 7. The power of the engine is transmitted to the output shaft 6 at the largest transmission ratio by the driving belt 11 and driven pulley 8, and further transmitted to axles of the driving wheels 19. Thus, the vehicle is started. When the vehicle speed (output signal N s ) exceeds a predetermined value, the clutch 2 is entirely engaged. At the start of the vehicle, the line pressure is at the highest value by the pressure control valve 40, since the duty ratio for the valve 66 is large, and the spool 42 of the control valve 40 is at the right end position. When the throttle valve is opened for acceleration of the vehicle, the desired transmission ratio changing speed did/dt and transmission ratio changing speed di/dt are calculated at calculators 80 and 78. The transmission ratio changing speed di/dt is fed to the duty ratio table 79, so that duty ratio D for valve 68 is obtained from the table 79. When the depression of the accelerator pedal stops, the transmission ratio changing speed di/dt becomes negative. Accordingly the value of the duty ratio D becomes larger than the neutral value, so that the pressure in the chamber 51d of the control valve 50 is higher than the chamber 51e. Thus, the spool 52 is shifted to the left to communicate the port 51a with port 51b, so that oil is supplied to the chamber 9 through the conduit 23 to upshift the transmission. When the actual transmission ratio i reaches the desired transmission ratio id, the changing speed di/dt becomes zero, so that the upshifting operation stops. As the difference between the desired ratio id and actual ratio i becomes large and the desired transmission ratio changing speed di/dt becomes large, the duty ratio D for the valve 68 becomes large, thereby increasing the shifting speed of the spool 52 to increase the actual transmission changing speed. When the opening degree of the throttle valve is reduced for deceleration, the duty ratio is reduced along a low engine speed line, thereby shifting the spool 52 to the right to drain the chamber 9. Thus, the transmission is downshifted. The transmission changing speed at downshifting increases with reducing of the duty ratio. The control operation of line pressure will be described hereinafter. From the engine torque calculator 96, a torque T is obtained in accordance with throttle position Θ and engine speed N e , which is applied to desired line pressure calculator 104. The calculator calculates a desired line pressure P L . The solenoid operated on-off valve 66 is operated at a duty ratio corresponding to the desired line pressure P L . The line pressure is applied to chamber 10 to hold the belt 11 at a necessary minimum force, the transmitting torque at which is slightly larger than torque T. Thus, power is transmitted through the transmission without slipping of the belt. Fig. 5 shows an acceleration characteristic in steady state. When the accelerator pedal is depressed, the actual transmission ratio i increases in accordance mainly with the difference K1 (id-i) to the desired transmission ratio id. As the actual ratio i approaches the desired ratio id, the ratio i gradually reduces so as to smoothly coincide with ratio id in accordance with the change of K₂ · did/dt. The transmission ratio control at the kickdown is described hereinafter with reference to Figs. 4a, 4b and 6. When the accelerator pedal is quickly and largely depressed, the kickdown detecting section 85 detects the kickdown. The first and second desired value id₁ and id₂, which are different from the desired transmission ratio in steady state, are derived from the first and second value providing sections 86 and 87. The first desired value id₁ is fed to the correcting sections 94 and 95 to set the desired transmission ratio to the value id₁ at section 94, and to increase transmission ratio changing speed di/dt at the section 95. Thus, the actual transmission ratio i is rapidly increased. As shown in Fig. 6, in the range I (i₀ to id₁), the actual transmission ratio i is quickly downshifted. When the actual ratio i becomes larger than the first desired value id₁ (i > id₁), the first desired value id₁ becomes zero (id₁ = 0). Then, the second desired value id₂ is fed to the correcting section 94 to set the desired transmission ratio to the value id₂. In the range II (id₁ to id₂), the actual ratio is downshifted in accordance with the second desired value id₂, so that the actual ratio i coincides with the second desired value id₂ at a point A. At the point A, the acceleration approaches a maximum acceleration Gmax because of the large transmission ratio. Thereafter, the ratio i converges to the desired transmission ratio id. In the range III, the second desired value id₂ becomes zero, so that the actual ratio i is controlled in accordance with the desired transmission ratio id. Each of constants m and n can be changed with a predetermined parameter. For example, during high speed driving, the constant n is set to a large value to provide a large driving power. When the throttle opening ratio dΘ/dt is large, constant m is set to a large value, so that the actual ratio i is quickly converged to the desired value. In accordance with the present invention, a transient state distinct from a steady state is provided at full throttle acceleration such as kickdown, in which the transmission ratio is quickly increased to a large value, so that a maximum acceleration is obtained in accordance with driver's demand.

Claims: A control system for a continuously variable transmission for transmitting the power of an internal combustion engine, having a drive pulley (7) having a hydraulically shiftable disc (7a) and a hydraulic cylinder (9a) for operating the disc, a driven pulley (8) with an hydraulically shiftable disk (8a) and a hydraulic cylinder (6a) for operating the disc (8a) and a belt (11) engaged with both pulleys (7, 8), the system comprising: a hydraulic circuit including a pump (21) for supplying oil to both the hydraulic cylinders (9a, 6a), a spool type valve (50) for controlling the transmission ratio by varying the oil supply to the cylinder (9a) of the drive pulley (7); a solenoid operated control valve means (68, 82) provided in the hydraulic circuit for controlling an amount of oil to be supplied to the transmission ratio control valve (50); sensing means (71,72, 74) for producing a drive pulley speed signal (Np), a driven pulley speed signal (Ns) and a throttle position signal (Θ); means (76, 84, 85) responsive to the driven pulley speed signal (Ns) and the load signal (Θ) for detecting rapid acceleration of the engine and for producing an acceleration signal; a calculator (75) responsive to both the pulley speed signals (Np, Ns) for producing an actual transmission ratio signal (i); a calculator (76) responsive to the driven pulley speed signal (Ns) and the throttle position signal (Θ) for producing a desired transmission ratio signal (id); a calculator means (78) responsive to both said actual (i) and desired (id) transmission ratio signals for producing a transmission ratio changing speed signal (di/dt); means (79) responsive to the transmission ratio changing speed signal (di/dt) for producing a control signal for operating the control valve means (68, 82) to shift the spool type transmission ratio control valve (50) at a speed dependent on the transmission ratio changing speed signal (di/dt); characterised by means (86, 87) responsive to the acceleration signal to produce; a first value signal (id1) and a second value signal (id2), said first value signal (id1) having a value indicative of a transmission ratio less than the desired transmission ratio (id) and said second value signal (id2) having a value indicative of a transmission ratio greater than said first value signal (id1) and less than said desired transmission ratio (id); and correcting means (94) acting at the incidence of the acceleration signal to replace the desired transmission ratio signal (id) initially with the first value signal (id1), and when a predetermined condition of the first value signal (id1) and the actual transmission ratio (i) is reached, with the second value signal (id2) and, when a predetermined condition of the second value signal (id2) and the actual transmission ratio (i) is reached, reverting to use of the desired transmission ratio (id);and a changing speed correction section (95) for correcting the transmission ratio changing speed signal (di/dt) according to the first value signal (id1). A control system as claimed in claim 1, wherein the transmission ratio control valve (50) has two chambers (51d, 51c) provided at opposite ends of the spool. A control system as claimed in claim 2, wherein the control signal for the control valve means (68) is a pulse train the duty ratio of which is varied to control the transmission ratio changing speed. A control system as claimed in claim 2 or 3, wherein the hydraulic circuit is provided with a regulator valve (60) for maintaining the pressure of oil at a constant value. A continuously variable transmission characterised by a control system as claimed in any preceding claim.

Inventors: TANAKA, HIROSHI
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: F16H59
IPC codes: F16H61, F16H63, F16H9, B60W10, B60W30