Transmission ratio control system for a continuously variable transmission

A control system 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. The transmission ratio control valve has chambers at both ends of the spool. The amount of oil supplied to the chambers is controlled by a control signal which is determined by a desired transmission ratio and the actual transmission ratio, so that the transmission ratio is controlled.

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. 
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 control ratio 
control valve. Each valve comprises a spool to control the oil supplied to 
the servo devices. 
The transmissio ratio control valve operates to determine the transmission 
ratio in accordance with a control signal dependent on 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 a control signal dependent on 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. 
When starting 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 rato is automatically and continuously reduced at a rate 
which is determined by line pressure, the pressure of oil supplied to the 
servo device of the drive pulley, and the actual transmission ratio. In 
such a system, the rate of changing of transmission ratio up to a desired 
transmission ratio can not be controlled in accordance with driving 
conditions. Accordingly, hunting or overshooting of the transmission ratio 
occurs, which decreases the driveability of the vehicle. 
European patent application No. 205257 discloses a transmission ratio 
control system in which a transmission ratio changing rate is di/dt is 
obtained by making calculation K(id-i), where id is desired transmission 
ratio and i is actual transmission ratio. In order to make the 
calculation, memories and calculator sections are provided. Accordingly, 
the system and program become complicated. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a system for continuously 
variable transmission, which is simple in construction and program. 
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 the 
shafts 5 and 6 respectively. A fixed conical disc 7b of the drive pulley 7 
is integral with the 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, the 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 the 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 oil reservoir 26 through passage 27. 
The transmission ratio control valve 50 comprises a valve body 51, 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 the spool 52. The port 51b communicates with the chamber 9 
through conduit 23, and the 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 the 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 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 the reduction of the 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 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 the pulses applied to the 
solenoid operated control valve 68 and the transmission ratio is explained 
hereinafter. 
The necessary volume V of oil in the chamber 9 is a function of the 
transmission ratio i, namely: 
EQU V=f(i) 
The flow rate Q is obtained by differentiating the volume V with respect to 
time and expressed as 
EQU Q=dV/dt=df(i)/di di/dt 
EQU di/dt=f(Q,i) 
The supply flow ratio Q.sub.s and drain flow rate Q.sub.d are presented as 
##EQU1## 
where P.sub.p is the pressure in chamber 9, 
Pl is the line pressure, 
C is the coefficient for the flow rate, 
g is the acceleration of gravity, 
.gamma. is the specific gravity of oil, 
S.sub.s is the opening area of the supply port 51a, and 
S.sub.d is the opening area of the drain port 51c. 
Designating by D the duty ratio of the 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 
##EQU2## 
Assuming a, S.sub.s and S.sub.d to be constants, 
EQU Q=f(D,Pl,P.sub.p) 
The line pressure Pl is determined by the transmission ratio i and engine 
torque, and the pressure P.sub.p in the chamber 9 is determined by the 
transmission ratio i and line pressure Pl. Accordingly, assuming the 
engine torque to be constant, 
EQU Q=f(D,i) 
EQU Since di/dt=f(Q,i) 
EQU di/dt=f(D,i) 
Therefore 
EQU D=f(di/dt,i) (1) 
Accordingly, the duty ratio is determined by the transmission ratio 
changing rate di/dt and the transmission ratio i. 
In a feedback control system, the transmission ratio changing rate di/dt 
can be determined by the diffence between the actual transmission ratio i 
and a a desired transmission ratio id, as follows. 
EQU di/dt=K(id-i) 
where K is a coefficient. 
Accordingly, the equation (1) can be expressed as follows. 
##EQU3## 
If K is fixed, 
EQU D=f(id,i) 
Therefore, the duty ratio D is determined in accordance with the desired 
transmission ratio id and actual transmission ratio i to the control 
transmission ratio changing rate di/dt. The duty ratio D can be derived 
from a look-up table having axes of id and i. 
FIG. 5a shows a duty ratio table. The region of id&gt;i is for downshifting of 
the transmission and the region of id&lt;i is for upshifting. 
In order to change the rate di/dt with respect to the same id and i, the 
desired transmission ratio id in the table is changed. For example, in 
order to increase the rate di/dt in the downshift direction, a correcting 
quantity .DELTA.id is added to id, and in the upshift direction .DELTA.id 
is subtracted from id as shown in FIG. 5b. In order to reduce the rate 
di/dt, the reverse operation is done. 
Referring to FIG. 3, the system is arranged to control the transmission 
ratio in accordance with the above described principle. In the system, a 
drive pulley speed sensor 71, a driven pulley speed sensor 72, an engine 
speed sensor 73 and a throttle position sensor (or intake manifold 
pressure sensor) 74 are provided. Output signals N.sub.P and N.sub.S of 
the sensors 71 and 72, which are dependent on the speed of the drive 
pulley and driven pulley, 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. The output singla N.sub.S and output signal .theta. 
(dependent on the throttle valve position) 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 signal .theta. is also applied to a 
throttle valve swinging speed detector 80 which produces a throttle speed 
signal .theta.. 
The desired transmission ratio id and throttle speed signal .theta. are fed 
to a desired transmission ratio correcting section 78 where the correction 
of desired transmission ratio id.+-..DELTA.id is made to produce a 
corrected desired transmission ratio id'. The actual transmission ratio i 
and corrected desired transmission ratio id' are applied to a duty ratio 
table 81 to derive the duty ratio D. The duty ratio D is supplied to the 
solenoid operated valve 68 through a 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 is calculated based on throttle 
position .theta. 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 are applied to a desired line pressure 
calculator 104 where a desired line pressure P.sub.L is calculated. 
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. 
In operation, while the vehicle is at a stop, the 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. and the duty ratio D are 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 drive belt 11 and driven pulley 8, and 
is further transmitted to the axles 18 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 corrected desired transmission ratio id' is produced from the 
section 78. The corrected desired transmission radio id' and actual 
transmission ratio i are fed to the duty ratio table 81, so that duty 
ratio D for valve 68 is obtained from the table 81. When the depression of 
the accelerator pedal stops, the transmission ratio changing speed di/dt 
becomes negative. Accordingly, the value of the duty ratio D becomes 
larger than the neutral value, so that the pressure in the chamber 51d of 
the control valve 50 is higher tha 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 at the rate di/dt. When the actual transmission ratio i 
reaches the desired transmission ratio id, the upshifting operation stops. 
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 to reduce the line pressure. 
As the difference between the desired ratio id and actual ratio i becomes 
large and the desired transmission ratio changing 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 ratio changing 
rate. When the opening degree of the throttle valve is reduced for 
deceleration, the duty ratio is reduced along a low engine speed line, 
thereby shifting the spool 52 to the right to drain the chamber 9. Thus, 
the transmission is downshifted. The transmission ratio changing rate at 
downshifting increases with reducing of the duty ratio. 
The control operation of line pressure will be described hereinafter with 
reference to FIG. 4. When the throttle speed .theta. is lower than a 
predetermined speed .DELTA..theta., the correcting quantity .DELTA.id is 
set to zero (id'=id). Accordingly, the duty ratio is derived from the 
table 81 in accordance with the actual transmission ratio i and desired 
ratio id. When .theta..gtoreq..DELTA..theta., the duty ratio is determined 
by quantities of id and i. For example, when id&lt;i , which means 
upshifting, the duty ratio is derived in accordance with id' 
(id-.DELTA.id) and i. Accordingly, the duty ratio D becomes large by a 
quantity corresponding to .DELTA.id, so that the transmission ratio 
changing rate is increased. 
The present invention is not limited to the above described embodiment. For 
example, the correcting quantity .DELTA.id may be changed in accordance 
with an operating condition such as the throttle valve swinging speed 
.theta.. The duty ratio can be obtained in accordance with the difference 
between the desired transmission ratio and the actual transmission ratio 
(id-i) and with the actual transmission ratio. It is possible to change 
the transmission ratio changing rate di/dt by multiplying id-i by a 
coefficient K' (K'(id-i)). 
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.