CONTROL SYSTEM FOR AN INFINITELY VARIABLE TRANSMISSION

Abstract: A control system for an infinitely variable transmission has a transmission ratio control valve having a spool for controlling oil supplied to a cylinder of a drive pulley to change the transmission ratio. Acceleration of depression of an accelerator pedal is detected to produce an acceleration signal. By controlling the pressure of oil supplied to the transmission ratio control valve in accordance with engine operating conditions and the acceleration signal, the spool is shifted, so that the transmission ratio changing speed is controlled.

Description: BACKGROUND OF THE INVENTION The present invention relates to a control system for an infinitely variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the speed of changing the transmission ratio in accordance with driving conditions of the vehicle. A known control system for an infinitely variable belt-drive transmission disclosed in United States Patent No. 4,369,675 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 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. Output of an engine is transmitted to the drive pulley through a clutch. 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). At that time if the engine speed is kept constant, 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. 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. Japanese Patent Laid Open 59-159456 discloses a system provided with a first valve for changing the direction of the transmission ratio change and a second valve for controlling the transmission ratio changing speed. By controlling the spool of the second valve, the transmission ratio changing speed is controlled. However, the system is complicated in construction, since two control valves are provided in addition to the conventional system. Japanese Patent Laid Open 59-217048 shows a system which operates to vary a desired transmission ratio in accordance with the deviation of an actual transmission ratio from the desired ratio. However, such a system causes overshoot of the control operation. SUMMARY OF THE INVENTION An object of the present invention is to provide a system which may control the transmission ratio changing speed by a single control valve. Another object of the present invention is to provide a system which may correct the transmission ratio changing speed in accordance with load on the engine, thereby preventing the overshoot of the control and improving the response of the control operation. In accordance with the present invention, the transmission ratio control value has chambers at both ends the spool, and a solenoid operated on-off control valve is provided for controlling the amount of oil supplied to the chambers of the transmission ratio control valve. The system is provided with sensing means for sensing operating conditions of the engine and the transmission and for producing signals dependent on the conditions, an engine load detecting means for producing a load signal, a control unit responsive to the signals from the sensing means and the load signal for producing an output signal for operating the control valve, so that the shifting of the spool is controlled to control the transmission ratio changing speed. The other objects and features of this invention will be apparently understood from the following description with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic illustration of an infinitely variable belt-drive transmission; Figs. 2a, 2b are schematic diagram showing a control system according to the present invention; Fig. 3 is a block diagram showing a control unit; Fig. 4a shows various transmission ratios; Figs. 4b and 4c are tables storing desired transmission ratio (id) and duty ratio (D); and Fig. 5 is a graph showing the relationship between changes of transmission ratio and torque. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, the infinitely variable belt-drive automatic transmission for a motor vehicle to which the present invention is applied, comprises an electromagnetic powder clutch 2 for transmitting the power of an engine 1 to transmission 4 through a selector mechanism 3. The belt-drive transmission 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 the movable 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 the vehicle driving wheels 19 through a differential 17. Referring to Figs. 2a and 2b, chamber 9 of the drive pulley 7 is applied 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 pressure of a spring 43 provided between the end of the spool and a retainer 45 the position of which is adjustable by a screw 44. The port 41a is communicated with a port 41b of a drain passage 27 in accordance with the position of a land of the spool 42. The transmission ratio control valve 50 comprises a valve body 51, spool 52, spring 53 for urging the spool 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 a conduit 23, and port 51a communicates with line pressure control valve 40 through conduit 22a. The drain port 51c is communicated with the oil reservoir 26 through a check valve 25. The drain port 41b communicates with oil reservoir 26 through conduit 27. The system of the present invention is provided with a regulator supply valve 55, regulator valve 60, solenoid operated on-off control valves 66 and 68. The regulator supply valve 55 comprises a valve body 56, spool 57, spring 58 for urging the spool in a direction, port 56a connected to line pressure conduit 22 through passage 35, port 56c connected to drain passage 27 through a passage 36, and end chamber 56d which is communicated with drain passage 27 to be applied with the drain oil pressure opposite the spring 58. When the line pressure is at a high level, the pressure of drain oil is at a low level because of closing the port 41b of the line pressure control valve 40. In such a state, spool 57 is shifted to the right to communicate port 56a with an output port 56b. On the other hand, when the pressure of the drain oil becomes higher than a set value, the spool 57 is shifted to the left, causing port 56c to communicate with port 56b. Thus, sufficient amount of oil is supplied to regulator valve 60 through an orifice 59 and passage 37. The regulator valve 60 comprises a valve body 61, spool 62, end chamber 61c, spring 63 urging the spool 62 to the chamber 61c. When the pressure of supplied oil 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 a passage 34, orifice 67, and solenoid operated on-off valve 68. The solenoid operated 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 orifice 65. The solenoid operated valve 68 is the same as valve 66 in construction and operation. The control valves 66 and 68 are operated by signals from a control unit 70. Thus, pressure controlled by the control valves 66 and 68 is applied to chambers 41d, 51d and 51e. In the line pressure control valve, the relationship between spring load F S and line pressure Pl, line pressure receiving area S a of the spool, control pressure P d in the chamber 41d, and control pressure receiving area S d is as follows: F s = Pl S a + P d ·S d Pl = (F s - P d ·S d )/S a Accordingly, the line pressure Pl is inverse proportion to the control pressure P d . In the transmission ratio control valve, 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 control 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 increases with decreasing 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 relationship between the duty ratio of pulses applied to the solenoid operated control valve 68 and the transmission ratio is explained hereinafter. Necessary volume V of oil in the chamber 9 is a function of transmission ratio i, namely: V = f(i) Flow rate Q is obtained by differentiating the volume V with time and expressed as Q = dV/dt = df(i)/dixdi/dt di/dt = f(Q,i) Supply flow rate Q s and drain flow rate Q d are presented as Q d = C·S d ((2g P p )/γ)/2 = a·S d (P p )/2 Q s = a·S s (Pl-P p )/2 a = c(2g / γ )/2 where P p is the pressure in chamber 9, Pl 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. The duty ratio D of pulses applied to the control valve, that is the ratio of ON/OFF of the valve, average flow rate Q in one cycle (oil supply state is positive ) is Q = a (D·S s (Pl-P p )/2 - (1-D) × S d (P p )/2) Assuming a, S s and S d to be constants, Q = f(D·Pl·P p ) The line pressure Pl is decided by the transmission ratio i and engine torque, and the pressure P p in the chamber 9 is decided by the transmission ratio i and the line pressure Pl. Accordingly, assuming the engine torque to be constant, Q=f(D,i) Since di/dt =f(Q, i) di/dt = f(D,i) D = f(di/dt,i) Accordingly, the duty ratio is decided by the transmission ratio changing speed di/dt and the transmission ratio i. On the other hand, the transmission ratio changing speed di/dt is dependent on the difference between the actual transmission ratio i and a desired transmission ratio id, di/dt = K (id-i), where K is a coefficient Accordingly, if the transmission ratio changing speed di/dt is decided, the duty ratio D can be obtained from the speed. When the actual transmission ratio i is larger than the desired transmission ratio id (i>id), the value of di/dt is negative. In such a state, the duty ratio D is increased to reduce the pressure in the chamber 51e so as to upshift the transmission. The downshift is performed in the reverse manner. Fig. 5 shows constant torque lines and typical three transmission ratio lines wherein LOW designates a maximum transmission ratio (low speed) and OD shows an overdrive transmission ratio line. When an accelerator pedal is depressed and throttle valve opening degree ϑ increases during driving of the motor vehicle at a transmission ratio at a point A, a desired transmission ratio is set, for example, to a ratio at a point B. Up to the desired ratio, torque of the engine is increased. The increase rate ▵ Te of the torque is proportional to the difference (id-i). Accordingly, the coefficient K is function of ▵ Te. The rate ▵ Te is proportional to the changing rate ϑ of opening degree ϑ of the throttle valve. Accordingly, the coefficient K should be corrected by the rate ϑ or in accordance with the change of engine load. Referring to Fig. 3, the system is arranged to control the transmission ratio in accordance with above described principle. In the system, a drive pulley speed sensor 71, driven pulley speed sensor 72, engine speed sensor 73 and throttle valve position sensor 74 are provided. Output signals N p and N s 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 signals N s and ϑ of the throttle valve position sensor 74 are fed to a desired transmission ratio table 76. Fig. 4a shows various actual transmission ratios i and Fig. 4b shows the table 76. The desired transmission ratio id is fetched from the table in accordance with the signals N s and ϑ. On the other hand, the output 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 so as to correct the coefficient to produce a corrected coefficient K. The actual ratio i, desired 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 table 79 to derive the duty ratio D. Fig. 4c shows the duty ratio table in which the duty ratio decreases with increases of speed di/dt and ratio i. The duty ratio D is supplied to the solenoid operated valve 68 through a driver 80. On the other hand, an output signal Ne of the engine speed sensor 73, throttle position signal ϑ and actual ratio i are fed to a desired line pressure calculator 91 to produce the desired line pressure Pl. The desired line pressure Pl is applied to a duty ratio table 92 to derive a duty ratio dependent on the desired line pressure. The duty ratio is applied to the solenoid operated valve 66 through a driver 93. The duty ratio is set to increase with increase of the line pressure. In operation, while the vehicle is at a stop, chamber 10 of the driven pulley is supplied with line pressure through conduit 22b, and the chamber 9 of the drive pulley is drained, since the N p , N s , ϑ are zero and duty ratio is zero, so that 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 infinitely 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 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. At that time 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, 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 table 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 control 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. As the difference between the desired ratio id and actual ratio i becomes large, the duty ratio for the control valve 68 becomes large, thereby increasing the transmission changing speed di/dt. When the opening degree of the throttle valve is reduced for deceleration, the duty ratio is reduced, 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. When the vehicle is accelerated at steady state, acceleration ϑ increases to increase the coefficient K. Accordingly, the transmission is quickly downshifted, thereby promoting the acceleration. In accordance with the present invention, since a single transmission ratio control valve is operated to control the transmission ratio changing speed, the construction and operation are simplified. Since the transmission ratio changing speed is controlled in accordance with load on the engine, response of the control operation at acceleration is improved. While the presently referred 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 scope of the invention as set forth in the appended claim.

Claims: 1. An improved control system for an infinitely variable transmission for transmitting the power of an internal combustion engine, the system comprising a drive pulley having a hydraulically shiftable disc and a hydraulic cylinder for shifting the disc, a driven pulley having a hydraulically shiftable disc and a hydraulic cylinder for operating the disc, a belt engaged with both pulleys, a first hydraulic circuit having a pump for supplying oil, a transmission ratio control valve having a spool for controlling the oil supplied to the cylinder of the drive pulley to change the transmission ratio, wherein the improvement comprises: the transmission ratio control valve being provided to shift the spool by pressure of oil; a second hydraulic circuit for supplying oil to the transmission ratio control valve so as to shift the spool; control valve means provided in the second hydraulic circuit for controlling the amount of oil supplied to the transmission ratio control valve; sensing means for sensing operating conditions of the engine and the transmission and for producing operating condition signals dependent on the conditions; means for detecting load on the engine and for producing a load signal; a control unit responsive to the operating condition signals from the sensing means and to the load signal for producing an output signal for operating the control valve, so that the shifting of the spool is controlled to control the transmission ratio changing speed. 2. The control system according to claim 1 wherein the control valve means is a solenoid operated on-off valve, and the output signal of the control unit is pulses the duty ratio of which is changed so as to control the transmission ratio changing speed. 3. The control system according to claim 1 wherein the second hydraulic circuit is provided with a regulator valve for maintaining the pressure of oil at a constant value.

Inventors: MORIMOTO, YOSHIHIKO, NAKAMURA, HIROYUKI
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: F16H11
IPC codes: F16H61, B60W10, F16H9