Transmission ration control system for a continuously variable transmission

A control system for a continuously variable transmission for a motor vehicle has a line pressure control valve and a transmission ratio control valve, each having a spool for controlling oil supplied to a cylinder of a pulley. The transmission ratio control valve has chambers at both ends of the spool. The flow rate of oil supplied to the chambers is controlled by an on-off valve at duty ratio in accordance with a desired transmission ratio so that the spool is shifted, so that the transmission ratio changing speed is controlled. The duty ratio is corrected by line pressure, so that the transmission ratio is controlled in accordance with the line pressure.

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 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 dependency 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 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 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 the transmission ratio up to a desired transmission 
ratio can not be controlled in accordance with driving conditions. 
Accordingly, hunting or overshooting of the transmisson ratio occurs, 
which causes the driveability of the vehicle to reduce. 
EP-A-No. 207603 discloses a system wherein desired transmission ratio is 
decided by a throttle valve opening degree and an engine speed to control 
the 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. 
SUMMARY OF THE INVENTION 
The object of the present invention is 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, the system comprising 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 engaged with both 
pulleys, a first hydraulic circuit having a pump for supplying oil to both 
the hydraulic cylinders, a line pressure control valve provided in the 
first hydraulic circuit, and having a spool, a transmission ratio control 
valve provided in the first hydraulic circuit and having a spool for 
controlling the oil supplied to the cylinder of the drive pulley to change 
the transmission ratio to a desired transmission ratio. 
The system comprises first means for shifting the spool of each control 
valve, sensing means for sensing operating conditions of the engine and 
the transmission and for producing a first signal dependent on the 
conditions, second means responsive to the first signal for producing an 
actual transmission ratio signal, third means responsive to the first 
signal for producing a desired line pressure signal and a desired 
transmission ratio signal, fourth means responsive to the desired 
transmission ratio signal and the actual transmission ratio signal for 
producing a control signal for operating the first means to shift the 
spool of the transmission ratio control valve to provide a transmission 
ratio, fifth means responsive to the desired line pressure signal for 
producing a control signal for operating the first means to shift the 
spool of the line pressure control value to provide a line pressure, sixth 
means for producing a correcting signal dependent on the line pressure, 
seventh 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. 
In an aspect of the invention the transmission ratio control valve has 
chambers at both ends of the spool, the first means includes a second 
hydraulic circuit for supplying oil to the chambers, and control valve 
means provided with the second hydraulic circuit for controlling flow rate 
of control oil supplied to the chambers of transmission ratio control 
valve. 
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 EMBODIMENTS 
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 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. 2aand 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 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.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 signal N.sub.S and output signal 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.sub.S and .theta.. 
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 (rate) 
did/dt and coefficients K1 and K2 are applied to a transmission ratio 10 
changing speed calculator 78 to produce a transmission ratio changing 
speed (rate) di/dt from a formula di/dt=K1(id-i)+K2-did/dt. 
The speed (rate) 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 .theta. of throttle position sensor 74 and the 
output N.sub.e of engine speed sensor 73 are fed to an engine torque 
calculator 96, so that engine torque T.sub.0 is calculated based on 
throttle position 8 and engine speed N.sub.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.sub.LU per unit torque. The necessary line pressure 
P.sub.LU and the engine torque T.sub.0 are applied to a desired line 
pressure calculator 104 where a desired line pressure P.sub.L is 
calculated by equation P.sub.L =P.sub.LU .times.T.sub.0. 
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 desired 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. 
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 
##EQU1## 
where p.sub.p is the pressure in chamber 9, 
p.sub.L is the lone pressure, 
C is the coefficient for flow rate, 
g is the acceleration of gravity, 
.gamma. is the specific gravity of oil, 
S.sub.s is the opening area of supply port 51a, 
S.sub.d is the opening area of drain port 51c. 
Since line pressure P.sub.L is proportional to the transmitting torque 
T.sub.O, if a basic line pressure corresponding to a basic torque Tb is 
Pb, supply flow rate Qs is 
##EQU2## 
where Qsb is 
##EQU3## 
In the same manner, drain flow rate Qd is 
##EQU4## 
wherein Qdb is 
##EQU5## 
Average flow rate Q represented by the duty ratio D during one cycle of 
pulsis forming the duty ratio is 
If (where T is a correcting coefficient, namely a torque ratio factor), 
the equation is rewritten as follows. 
##EQU6## 
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).times.{Qdb/(Qsb+Qdb)} 
The duty ratio D determined by the above equation cam be corrected by 
changing values of Db, T and Qdb/Qsb+Qdb) term (Qdb/Qsb+Qdb) is determined 
by the construction of valves and the transmission ratio. A flow rate 
characteristic Qc which is defined as the above-mentioned term (wherein 
Qc+Qdb/(Qsb+Qdb)) can be previously set as Qc=F(i). 
Further, above-mentioned equation can be rewritten as follows 
##EQU7## 
As described above, 
EQU Db=(Q+Qdb)/(Qsb+Qdb). 
Further, Qdb/(Qsb+Qdb), namely Qcm 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 above equation is rewritten as 
follow 
EQU D=(Db-DN).multidot.1/T+DN 
FIG. 4 shows a duty table of the duty ratio Db dependent on changing speed 
(rate) di/dt and transmission ratio i. on the transmission ratio changing 
rate di/dt and the transmission ratio i. A solid line shows a duty ratio 
DN when the transmission ratio changing rate is zero (di/dt=). As shown 
for example by a dashed line, the duty ratio Db is obtained by the duty 
ratio table 79, FIGS. 3, 4 from the transmission ratio changing rate di/dt 
(calculated by the transmission ratio changing speed calculator 78) and 
the actual transmission ratio i (see FIG. 3). That is FIG. 4 represents a 
three-dimensional table, and the unknown value Db is derived from the 
known value of the transmission ratio changing rate di/dt (shown on the 
vertical axis) and the known value of the transmission ratio i (shown on 
the horizontal axis). The duty ratio D can be obtained by the duty ratios 
Db and DN, and the torque ratio factor T by using the above equation. 
In accordance with the above described principle duty ratio correcting 
section 84 is provided at the output Db 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 the a flow rate characteristic Qc. A torque ratio calculator 86 is 
applied with the engine torque T.sub.0 of the engine torque calculator 96 
to calculate torque ratio factor (correcting coefficient) T. The flow rate 
characteristic Qc, the duty ratio Db and the torque ratio factor 
(correcting coefficient T 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.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 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.sub.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 (rate) 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. 
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 rate 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 (rate) 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 ratio changing speed (rate) 
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, the torque T.sub.0 is obtained in 
accordance with throttle position .theta. and engine speed N.sub.3, which 
is applied to desired line pressure calculator 104. The calculator 
calculates the desired line pressure P.sub.L. 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 torque T.sub.0. Thus, power is transmitted through 
the transmission without slipping of the belt. 
The actual transmission ratio i increases in accordance mainly with the 
difference Kl (id-i) to the desired transmission ratio id. As the actual 
transmission ratio i approaches the desired ratio id, the transmission 
ratio changing rate di/dt is corrected by the rate of change of the 
desired transmission ratio (di/dt) to advance a phase of the control 
operation so as to eliminate a control delay. 
The duty ratio Db is corrected at correcting section 84 in accordance with 
the torque ratio T which is proportional to the line pressure and with the 
flow rate characteristic Qc. Thus, the transmission ratio is controlled in 
accordance with the variation of the lie 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.sub.L and 
produces a correcting coefficient Pc dependent on the pressure P.sub.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 claims.