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. The transmission ratio changing speed is corrected by the line pressure, so that the transmission ratio is controlled in accordance with the line pressure.

Description: The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at a minimum transmission ratio. A known control system for a continuously variable belt-drive transmission comprises 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 control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. 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 pressure of oil supplied to the servo device of the drive pulley, and actual transmission ratio. In such a system, the speed of changing of transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshoot of transmission ratio occurs, which causes the driveability of the vehicle to reduce. EP-A-207603 discloses a system wherein desired transmission ratio is decided by a throttle valve opening degree and an engine speed to control transmission ratio changing speed. Further, in an oil pressure control system, the line pressure is directly applied to a cylinder of the driven pulley and controlled line pressure is supplied to a cylinder of the drive pulley. In the system, the flow rate to the drive pulley cylinder is calculated, assuming that transmitting torque in the transmission is constant. However, during the operation in accordance with the desired transmission ratio, the transmitting torque always varies. Thus, the line pressure is controlled in response to the variation of the transmitting torque. Since the line pressure is supplied to the drive pulley cylinder in order to control the transmission ratio, the variation of the line pressure affects necessarily the transmission ratio changing speed. As a result, actual transmission ratio does not coincide with desired transmission ratio. The present invention seeks to provide a system which may correct the transmission ratio changing speed in accordance with the variation of the line pressure so as to approach the desired transmission ratio changing speed. 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 having a hydraulically shiftable disc and a hydraulic cylinder for operating the disc, a belt engaging with both pulleys, the system comprising: a hydraulic circuit having a pump for supplying oil to both the hydraulic cylinders, a spool type valve in the hydraulic circuit for controlling the pressure; a spool type valve in the hydraulic circuit for controlling a transmission ratio by varying the oil supply to the cylinder of the drive pulley means for controlling the valves; means for sensing operating conditions of the engine and the transmission and for producing an output signal dependent on the conditions; means responsive to the output signal for producing an actual transmission ratio signal; means responsive to the output signal for producing a desired line pressure signal and a desired transmission ratio signal; means responsive to the desired transmission ratio signal and the actual transmission ratio signal for producing a control signal for operating the valve control means to shift the spool of the transmission ratio control valve to provide a transmission ratio; means responsive to the desired line pressure signal for producing a control signal for operating the valve control means to shift the spool of the line pressure control valve to provide a line pressure; means for producing a correcting signal dependent on the line pressure; means responsive to the correcting signal for correcting the control signal for the transmission ratio control valve so as to control the transmission ratio in accordance with the line pressure. 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; Figures 2a and 2b show a hydraulic circuit for the transmission; Figures 3a and 3b show a schematic diagram of a control system according to the present invention; Figure 4 is a graph showing a duty ratio; and Figure 5 is a diagram showing another embodiment of the invention. Referring to Figure 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 control 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. Referring to Fig. 3, 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 desired transmission ratio changing speed calculator 80 which 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 id, desired transmission ratio changing speed did/dt and coefficients K1 and K2 are applied to a transmission ratio changing speed calculator 78 to produce a transmission ratio changing speed di/dt from a 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 to derive the duty ratio Db. The duty ratio Db is supplied to the solenoid operated on off valve 68 through a duty ratio correcting section 84 and driver 82. 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 T₀ 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 T₀ are applied to a desired line pressure calculator 104 where a desired line pressure P L is calculated by equation P L = P LU × T₀. 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. The correcting of the duty ratio Db in accordance with the change of the line pressure will be described hereinafter. Since the line pressure is determined by the engine torque, the duty ratio Db for the transmission ratio is corrected by the engine torque in the first embodiment of the invention. Explaining the principle of the control, supply flow rate Qs to the chamber 9 and drain flow rate Qd from chamber 9 are represented as follows where P p is the pressure in chamber 9, P L is the line pressure, C is the coefficient for flow rate, g is the acceleration of gravity, γ is the specific gravity of oil, S s is the opening area of supply port 51a, S d is the opening area of drain port 51c. Since line pressure P L is proportional to the transmitting torque T₀, if a basic line pressure corresponding to a basic torque Tb is Pb, supply flow rate Qs is Average flow rate Q represented by the duty ratio D during one cycle is Q = D · Qs - ( 1 - D ) Qd If the equation is rewritten as follows. D = ( Q + Qd ) / (Qs + Qd) = ( Q + Qdb · T ) /{( T (Qsb + Qdb)} = {( Q + Qdb ) / (Qsb + Qdb ) + (T - 1 ) Qdb / (Qsb + Qdb )} / T The duty ratio D is rewritten by duty ratio Db at the basic torque Tb as follows. Since Db = ( Q + Qdb ) / (Qsb + Qdb ), the duty ratio D is D = (Db/T) + (1-1/T) × { Qdb/(Qsb + Qdb)} The formula Qdb/(Qsb + Qdb) is determined by the construction of valves and the transmission ratio. The flow rate characteristic Qc can be previously set as Qc = f(i). Further, above-mentioned equation can be rewritten as follows. D = (Q + QdbT ) / { ( Qsb + Qdb ) T } = ( Q + Qdb ) / ( Qsb + Qdb ) - Qdb / ( Qsb + Qdb ) · 1 / T + Qdb / ( Qsb + Qdb ) As described above, Db = (Q + Qdb ) / (Qsb + Qdb ). Further, Qdb / (Qsb + Qdb) can be transformed into a duty ratio DN when the transmission ratio changing speed di/dt is zero, that is the average flow rate Q is zero. Thus, the first term of the right side of the above equation is rewritten as follows. D = ( Db - DN ) · 1 / T + DN Fig. 4 shows a duty table of the duty ratio Db dependent on changing speed di/dt and transmission ratio i. By a derived duty ratio Db and torque T, duty ratio D can be obtained. In accordance with the above described principle duty ratio correcting section 84 is provided at the output of the duty ratio table 79. A flow rate characteristic providing section 85 is applied with the actual transmission ratio i of the actual transmission ratio calculator 75 to provide a flow rate characteristic Qc. A torque ratio calculator 86 is applied with the engine torque T₀ of the engine torque calculator 96 to calculate torque ratio Tr. The flow rate characteristic Qc and the torque ratio Tr are applied to the duty ratio correcting section 84. 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 Db for valve 68 is obtained from the table 79. The duty ratio Db is corrected by the correcting section 84. When the value of the duty ratio Db is larger than the neutral value, so that the pressure in the chamber 51d of the control valve 50 becomes 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 becomes large, the duty ratio for the valve 68 becomes large, thereby increasing the shifting speed of the spool 52 to increase the actual transmission changing speed. When the duty ratio is smaller than the neutral value, the spool 52 is shifted 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 T₀ 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 T₀. Thus, power is transmitted through the transmission without slipping of the belt. 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 duty ratio Db is corrected at correcting section 84 in accordance with the torque ratio Tr which is proportional to the the line pressure and with the flow rate characteristic Qc. Thus, the transmission ratio is controlled in accordance with the variation of the line pressure. Accordingly, an actual transmission ratio can be controlled to a desired transmission ratio. Fig. 5 shows another embodiment of the present invention. The system is provided with a duty ratio correcting coefficient calculator 108. The calculator 108 is supplied with the desired line pressure P L and produces a correcting coefficient Pc dependent on the pressure P L . The coefficient Pc is applied to the duty ratio correcting section 84. Thus, in the system, the transmission ratio is directly controlled in accordance with the line pressure. While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claim.

Claims: 1. 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 and a hydraulic cylinder for operating the disc, a driven pulley (8) having a hydraulically shiftable disc and a hydraulic cylinder for operating the disc, a belt (11) engaging with both pulleys, the system comprising: a hydraulic circuit (Figure 2a, 2b) having a pump (21) for supplying oil to both the hydraulic cylinders, a spool type valve (40) in the hydraulic circuit for controlling the pressure; a spool type valve in the hydraulic circuit for controlling a transmission ratio by varying the oil supply to the cylinder of the drive pulley (7); means (66, 68, 70) for controlling the valves; means (71-73) for sensing operating conditions of the engine and the transmission and for producing an output signal dependent on the conditions; means (75) responsive to the output signal for producing an actual transmission ratio signal; means (76, 103, 104) responsive to the output signal for producing a desired line pressure signal and a desired transmission ratio signal; means (78, 79, 82) responsive to the desired transmission ratio signal and the actual transmisson ratio signal for producing a control signal for operating the valve control means (68) to shift the spool of the transmission ratio control valve (50) to provide a transmission ratio; means (105, 106) responsive to the desired line pressure signal for producing a control signal for operating the valve control means (66) to shift the spool of the line pressure control valve (40) to provide a line pressure; means (86) for producing a correcting signal dependent on the line pressure; means (84) responsive to the correcting signal for correcting the control signal for the transmission ratio control valve (50) so as to control the transmission ratio in accordance with the line pressure. 2. A control system as claimed in claim 1, wherein the transmission ratio control valve (50) has a chamber (51d, 51c) at both ends of the spool, the valve control means includes a hydraulic circuit (37, 38, 60) for supplying oil to the chambers, and control valve means (68) provided in the hydraulic circuit for controlling flow rate of control oil supplied to the chambers of the transmission ratio control valve. 3. A control system as claimed in claim 2, wherein the control valve means (68) is a solenoid operated on-off valve, and the control signal is a pulse train the duty ratio of which is varied to control the transmission ratio changing speed. 4. A control system as claimed in claim 2 or 3, wherein the second hydraulic circuit is provided with a regulator valve (60) for maintaining the pressure of oil at a constant value. 5. A control system as claimed in claim 1, wherein the sensing means comprises a drive pulley speed sensor, driven pulley speed sensor and throttle valve position sensor. 6. A continuously variable transmission characterised by a control system as described in any preceding claim.

Inventors: MORIMOTO, YOSHIHIKO
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: F16H59
IPC codes: B60W10, F16H61