System for controlling the pressure of oil in a system for a continuously variable transmission

A control system for a motor vehicle provided with a clutch and a continuously variable transmission. The transmission has a line pressure control valve having ports and a spool for controlling the line pressure of oil supplied to a cylinder of a pulley device to change the transmission ratio. The transmitting torque in an engine power transmission system is detected and a clutch torque dependent on the transmitting torque is obtained. The line pressure is set to a minimum value sufficient for holding a belt of the pulley device for transmitting the clutch torque.

BACKGROUND OF THE INVENTION 
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 line pressure in a hydraulic 
circuit for the transmission. 
A known control system for a continuously variable belt-drive transmission 
disclosed in U.S. Pat. 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 dependency on 
driving conditions. The system is provided with a hydraulic circuit 
including a pump, 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 determine 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 the pulleys in order to transmit the output of the 
engine. 
Japanese Patent Laid Open No. 58-214054 discloses a hydraulic control 
system for controlling the line pressure to minimum value, while the belt 
is prevented from slipping by detecting the slipping thereof. Since the 
control of the line pressure is performed by detecting the slipping of the 
belt, the response of this control operation is liable to delay. Further, 
if engine torque changes rapidly, the belt will slip on the pulleys 
because of the delay of the control operation. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a transmission control 
system which controls line pressure to a minimum value in accordance with 
engine torque, so that the belt of a transmission is prevented from 
slipping on the pulleys, and in which if the engine torque changes 
quickly, the change of the engine torque is not transmitted to the 
transmission. In the system of the present invention, the transmitting 
torque in an engine power transmission system is detected and a clutch 
torque dependent on the transmitting torque is obtained. The line pressure 
is set to a minimum value sufficient for holding the belt of the pulley 
device for transmitting the clutch torque. 
The other objects and features of this invention will become understood 
from the following description with reference to the accompanying drawings 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 which 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 
the hydraulic 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, a transmission ratio control valve 50, and a 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 the 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 the oil reservoir 26 through passage 27. 
The transmission ratio control valve 50 comprises a valve body 51, a spool 
52, and a 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, inlet port 61a connected 
to the pump 21 through passages 37, 22, spool 62, end chamber 61c 
connected to the passage 37, and a spring 63 urging the spool 62 to toward 
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, the 
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 the solenoid 
operated on-off valve 68. 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 pulsations of the pressure of oil in the passage 32 is smoothed 
by the accumulator 32a. The solenoid operated valve on-off 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 and 51e. 
In the transmission ratio control valve 50, the 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 the 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 the control pressure in the 
chamber 51e. Further, the speed of the movement of the spool 52 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. 
Referring to FIGS. 3a and 3b, 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.sub.p and N.sub.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.sub.p /N.sub.s. Output signals 
N.sub.s and .theta. of the throttle valve position sensor 74 are fed to a 
desired transmission ratio table 76. The desired transmission ratio id is 
fetched from the table 76 in accordance with the signals N.sub.s and 
.theta.. On the other hand, the output signal .theta. is fed to an 
acceleration calculator 82 to obtain acceleration .theta.. The signal of 
the acceleration 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/d=K(id-i). 
The speed di/dt and the actual transmission ratio i are applied to a duty 
ratio table 79 to derive the duty ratio D. The duty ratio D is supplied to 
the solenoid operated valve 68 through a driver 80. 
On the other hand, the output signal .theta. of throttle position sensor 74 
is fed to an acceleration/steady state detector 95. The output of the 
detector 95 and the output N.sub.e of engine speed sensor 73 are fed to a 
driving torque table 96, so that driving torque T.sub.D is derived from 
the table based on throttle position .theta. and engine speed Ne. 
Further, the output .theta. is applied to a closed throttle detector 97 
which produces an output when the output .theta. is zero. The output of 
the detector 97 and output N.sub.s of driven pulley speed sensor 72 are 
supplied to an acceleration/deceleration calculator 98 which calculates 
magnitudes of acceleration and deceleration and produces an 
acceleration/decleration signal. The output of the calculator 98 at 
deceleration is applied to a transmitting torque calculator 100 which 
produces a torque signal T.sub.B dependent on the weight of the vehicle 
and the magnitude of deceleration. The output of calculator 98 at 
acceleration (for example, at downhill coasting) is applied to a 
transmitting torque calculator 101 which produces a torque signal T.sub.M 
dependent on engine speed. 
Torque signals T.sub.D, T.sub.B and T.sub.M are applied to a desired clutch 
torque setting section 102 which produces a desired torque T.sub.C in 
accordance with the torque signals. 
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.sub.LU per unit torque. The necessary line pressure 
P.sub.LU and the clutch torque T.sub.C are applied to a desired line 
pressure calculator 104 which calculates a desired line pressure P.sub.L 
which is slightly higher by a coefficient than a line pressure for 
transmitting the clutch torque. The desired line pressure P.sub.L is 
expressed as follows: 
EQU P.sub.L =P.sub.LU .times.T.sub.C +.alpha. 
The desired line pressure P.sub.L is applied to a duty ratio table 105 to 
derive a duty ratio D.sub.L corresponding to the line pressure P.sub.L. 
The duty ratio D.sub.L is supplied to a driver 106 which operates the 
solenoid operated on-off valve 66 at the duty ratio. 
Further, the outputs Ne and .theta. of engine speed sensor 73 and throttle 
position sensor 74 are supplied to a vehicle start detector 107 and the 
output N.sub.S of driven pulley speed sensor 72 is applied to a clutch 
engage detector 108. Outputs of both detectors 107 and 108 are fed to a 
clutch torque calculator 110 to produce a clutch current signal which is 
supplied to a clutch torque controller 111. A clutch current is corrected 
to a value corresponding to the desired clutch torque dependent on the 
clutch torque T.sub.C from the section 102. The corrected clutch current 
is applied to a clutch coil 2a of the clutch 2 through a driver 112 to 
control the clutch. 
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 N.sub.p, N.sub.s, 
.theta. 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, so that 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 18 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 line pressure 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, the 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, the line pressure reduces, and the transmission is 
upshifted, since oil is still supplied to the chamber 9 through the 
control valve 50. During the driving, the clutch torque is set to a value 
approximately equal to the driving torque T.sub.D by the desired clutch 
torque T.sub.C. 
The control operation of the line pressure will be described hereinafter 
with reference to FIGS. 2a, 2b, 3a, 3b, 4a and 4b. From the table 96, the 
driving torque T.sub.D is obtained in accordance with throttle position 
.theta. and engine speed N.sub.e, which is applied to the desired clutch 
torque setting section 102. The section 102 produces desired clutch torque 
T.sub.C which is applied to clutch torque controller 111 to engage the 
clutch 2. 
On the other hand, desired line pressure calculator 104 calculates a 
desired line pressure P.sub.L which is slightly higher than a line 
pressure necessary for transmitting the clutch torque T.sub.C. The 
solenoid operated on-off valve 66 is operated at a duty ratio 
corresponding to the desired line pressure P.sub.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 torques T.sub.D 
and T.sub.C respectively. Thus, power is transmitted through the 
transmission without slipping of the belt. When the transmitting torque 
changes rapidly, the clutch 2 slips, since the transmitting torque at the 
belt 11 is larger than the clutch torque. Accordingly, the change of 
transmitting torque is not transmitted to the transmission. 
When the throttle valve is closed, transmitting torque calculator 100 or 
101 produces a torque signal T.sub.B or T.sub.M in accordance with the 
acceleration/deceleration signal output of acceleration/deceleration 
calculator 98. Accordingly, the desired torque T.sub.C is corrected by the 
torque signal T.sub.B or T.sub.M, so that the clutch torque is corrected 
and the line pressure is set to such a value that the transmitting torque 
at the belt becomes slightly larger than the transmitting torque T.sub.B 
or T.sub.M. 
Although an electromagnetic clutch is employed in the above described 
system, other clutches or torque converters can be used. 
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 scope of the invention as set forth in 
the appended claims.