SYSTEM FOR CONTROLLING TRANSMISSION RATIO OF A CONTINUOUSLY VARIABLE TRANSMISSION FOR A MOTOR VEHICLE

Abstract: A control system for a motor vehicle provided with a continuously variable transmission. The transmission has a drive pulley (7), a driven pulley (8), a spool valve for transmission ratio control (50), and a hydraulic circuit (37,38,39,68) for supplying oil to the transmission ratio control valve (50) so as to control the transmission ratio. When a speed sensor (71,72) of one of the pulleys (7,8) fails to sense the speed of the pulley, the transmission ratio control valve (50) is operated by fail-safe operation (111) so as to maintain the ratio to a proper value.

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 fail-safe system for a failure of sensors such as speed sensors of a pulley of the transmission. A known continuously variable belt-drive transmission (hereinafter called CVT) for a motor vehicle 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 very 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 of supplying oil to servo devices, and respective spool valves for line pressure control and transmission ratio control. The transmission ratio control valve operates to set the transmission ratio in accordance with the opening degree of the throttle valve of the engine and the speed of the engine. One known type of control system as described in EP-A 0182616 includes a drive pulley speed (engine speed) sensor, a driven pulley speed sensor, and a throttle position sensor. The actual transmission ratio (i) is calculated on the basis of the output signals (N p ) of the drive pulley speed sensor and the output signal (N S ) of the driven pulley speed sensor in accordance with a formula (i = N P /N S ). The desired transmission ratio (id) is calculated from the output signal (N S ) of the driven pulley speed sensor and the output signal (ϑ) of the throttle position sensor. The actual transmission ratio i is controlled to the desired transmission ratio id by shifting the spool of the transmission ratio control valve. When failures occur in the sensing system, such as failure of one of sensors, or disconnection of a connector of a sensor, the transmission ratio cannot be controlled. For example, if a connector of the driven pulley speed sensor disengages, the output signal N P of the sensor becomes zero. Accordingly, the output (i = N P /N S ) of the calculator becomes a minimum value i min. In most cases, the desired transmission ratio id is larger than the minimum value i min. Therefore, the transmission is downshifted so that the actual ratio becomes the desired ratio id. As a result, the transmission ratio i becomes a maximum value i max. If the transmission is rapidly downshifted during operation of the vehicle, a rapid increase of engine speed to an excessive value may occur, which can cause damage to the engine. On the other hand, if the output signal N S of the driven pulley speed sensor becomes zero, the output (i = N p /N S ) of the calculator reaches a maximum value i max. Accordingly, the transmission is upshifted so as to try to provide the desired value. As a result, the transmission ratio reaches a minimum value i min, so that the driving torque is reduced to a very low value, which in turn causes a reduction, in driveability of the vehicle. The present invention seeks to provide a fail-safe system which may avoid the above described problems upon failure of the pulley speed sensor. According to the present invention there is provided a continuously variable transmission for a motor vehicle, the transmission including a drive pulley including a hydraulically shiftable disc and a first hydraulic cylinder for shifting the disc, a driven pulley including a hydraulically shiftable disc and a second hydraulic cylinder for operating the disc, a belt engaged with both pulleys, a spool valve for transmission ratio control, a hydraulic circuit having a pump for supplying oil to the first and second cylinders through the transmission ratio control valve, and first means for shifting the spool of the transmission ratio control valve in dependence on operating conditions of the engine; the control system comprising a drive pulley speed sensor for producing a drive pulley speed signal, a driven pulley speed sensor for producing a driven pulley speed signal dependent on the speed of the driven pulley, means for detecting an abnormal output from either of the pulley speed sensors and for producing a first abnormality signal upon failure of the drive pulley speed sensor and a second abnormality signal upon failure of the driven pulley speed sensor, second means responsive to the first abnormality signal for producing a fail-safe transmission ratio signal dependent on the driven pulley speed signal, third means responsive to the fail-safe transmission ration signal for controlling the first means so as to control the transmission ratio to the fail-safe transmission ration, fourth means responsive to the second abnormality signal for producing a fail-safe driven pulley speed signal, and fifth means responsive to the fail-safe drive pulley speed signal and connected to the first means so as to control the drive pulley speed to the fail-safe drive pulley speed. One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a continuously variable belt-drive transmission; Figures 2a and 2b are schematic diagrams showing a control system according to the present invention; Figure 3 is a block diagram showing a control unit; Figs. 4 and 5 are graphs showing transmission characteristics in fail-safe operations; and Figs. 6 and 7 are flowcharts showing the operation of the control system. 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 arranged 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 control 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 chamber 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 rate of change 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 of the throttle position sensor 74 are fed to a desired transmission ratio calculator 76. The desired transmission ratio id is calculated by the calculator 76 in accordance with the signals N and ϑ. On the other hand, the output signal ϑ is fed to an acceleration calculator 82 to obtain acceleration ϑ. The signal of the acceleration ϑ is supplied to a coefficient setting section 77 to produce a coefficient K. The actual transmission ratio i, desired transmission ratio id and coefficient K from the coefficient setting section 77 are applied to a transmission ratio changing speed calculator 78 to produce a transmission ratio changing speed di/dt from the formula di/dt = K(id - i). The speed di/dt and actual ratio i are applied to a duty ratio signal generator 79 to generate the duty ratio D. The duty ratio D is supplied to the solenoid operated valve 68 through a driver 80. 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 Ne. 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 the system of the present invention, the signals N P and N S of pulley speed sensors 71 and 72 are supplied to abnormality detectors 107 and 108 so as to detect failures of drive and driven pulley speed sensor systems. Each of detectors 107 and 108 produces an abnormality signal, when the output of each sensor is zero. The abnormality signal of the detector 107 is applied to a first fail-safe controller 110, and the abnormality signal of the detector 108 is fed to a second fail-safe controller 111. Output signals of the fail-safe controllers 110 and 111 are applied to the duty ratio signal generator 79. In operation, while the vehicle is stationary, 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 hand 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 set into motion. At that time the line pressure is at the highest value set 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 hand end position. When the throttle valve is opened for acceleration, the desired transmission ratio id and transmission ratio changing speed di/dt are calculated by calculators 76, 78, and duty ratio D is obtained from the duty ratio signal generator 79. The value of the duty ratio D is 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. On the other hand, duty ratio for the valve 66 is reduced, thereby shifting the spool 42 of the valve 40 to the left. The port 41a communicates with the port 41b of the drain passage 27. Thus, line pressure reduces, and the transmission is upshifted, since oil is still supplied to the chamber 9 through the control valve 50. when the vehicle speed (output signal N s ) exceeds a predetermined value, the clutch 2 is fully engaged. The control operation of line pressure will be described hereinafter with reference to Figs. 2a, 2b, 3. 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 slippage of the belt. In the fail-safe system, if the drive pulley sensor fails the transmission ratio control is achieved by an open loop control sequence in dependence on the output signal N S of the driven pulley speed sensor 72, and if the driven pulley sensor fails, the control is achieved by a closed loop control sequence in dependence on the output signal N p of the drive pulley speed sensor 71. This fail-safe operation will now be described with reference to Figures 6 and 7. At steps S₁, S₂ and S₃ in Figure 6, output signals ϑ, N p , N S are read out. When the output signal N P = 0 at a step S₄, an abnormality flag is set at a step S₁₃. When N P is not zero, an abnormality flag (if it is set at a previous program) is reset at a step S₅. At a step S₆, when the output signal N S is not zero, the above described control operation is performed at steps S₇ to S₁₀. The fail-safe operation at the set of the abnormality flag at the step S₁₃ is performed by an interrupt routine shown in Figure 7. The interrupt routine is executed at predetermined intervals of time. When the flag is set (step S₂₁) and when the output signal N S of the driven pulley speed sensor is lower than a predetermined low value N O at a step S₂₂, the duty ratio is set to zero at a step S₂₇. Therefore, primary pressure P in the chamber 9 of the drive pulley 7 becomes a minimum value P O at a step S₂₈. Thus, the transmission ratio is at maximum value i max as shown in Figure 4. When N S is larger than N₀, a desired (fail-safe) transmission ratio iFAIL for the fail-safe operation is derived from a look-up table in accordance with output signal N S or signals N S and ϑ at a step S₂₃. At a step S₂₄, a desired primary pressure P P for the chamber 9 is obtained from a look-up table in accordance with the desired transmission ratio iFAIL. Further, at a step S₂₅, a duty ratio D for the pressure P P is calculated. The relationship between the duty ratio D and the pressure P P after a short time Δt is as follows. where β is the compression rate of oil, Q is the oil flow rate for the chamber 9, V is the capacity of the chamber 9, a is a coefficient characteristic of the oil, P L is the line pressure, SI is the sectional area of the inlet port of the transmission ratio control valve 50, SD is the sectional area of the drain port of the transmission ratio control valve 50. Thus, by operation the valve 68 at the calculated duty ratio D, the pressure P becomes P P (step S₂₆), so that the transmission ratio is controlled to the ratio iFAIL. Line ℓ of Fig. 4 shows a variation of the transmission ratio by the fail-safe operation. When the output signal N S is zero at step S₆ of Fig. 6, a desired (fail-safe) drive pulley speed N P D is derived from a look-up table in accordance with the output signal ϑ of the throttle position sensor 74 at a step S₁₁. At a step S₁₂, a duty ratio D is derived from a look-up table in accordance with the signals N P and N P D. The transmission ratio is controlled by the derived duty ratio D, so that the signal N P may converge to the desired speed NpD by a feedback control. Fig. 5 shows variations of transmission ratios at various opening degrees of the throttle valve. While the presently preferred embodiment of the 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 within the scope of the appended claims.

Claims: 1. A control system for a continuously variable transmission for a motor vehicle, the transmission comprising a drive pulley including a hydraulically shiftable disc and a first hydraulic cylinder for shifting the disc, a driven pulley incuding a hydraulically shiftable disc and a second hydraulic cylinder for operating the disc, a belt engaged with both pulleys, a spool valve for transmission ratio control, and a hydraulic circuit having a pump for supplying oil to the first and second cylinders through the transmission ratio control valve, the control system comprising: first means for shifting the spool of the transmission ratio control valve in dependence on the operating conditions of the engine so as to provide a suitable transmission ratio; a drive pulley speed sensor for producing a drive pulley speed signal (Np); a driven pulley speed sensor for producing a driven pulley speed signal (NS); means (S₄,S₆) for detecting an abnormal output from either of the pulley speed sensors and for producing a first abnormality signal upon failure of the drive pulley speed sensor and a second abnormality signal upon failure of the driven pulley speed sensor; second means (S₂₃) responsive to the first abnormality signal for producing a fail-safe transmission ratio signal dependent on the driven pulley speed signal (NS); third means (S₂₄, S₂₆) responsive to the fail-safe transmission ratio signal for controlling the first means so as to control the transmission ratio to a fail-safe transmission ratio; fourth means (S₁₁) responsive to the second abnormality signal for producing a fail-safe drive pulley speed signal (NPD); fifth means (S₁₂) responsive to the fail-safe drive pulley speed signal for controlling the first means so as to control the drive pulley speed to the fail-safe drive pulley speed. 2. A system according to claim 1 wherein the first means comprises a hydraulic circuit for applying oil to the transmission ratio control valve so as to shift the spool and a control valve for controlling the supplied oil.

Inventors: OHKUMO, HIROYA
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: F16H59
IPC codes: F16H61, B60W10, F16H9