Control system for a continuously variable transmission

A control system for a continuously 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. The tansmission ratio control valve has chambers at both ends of the spool. By controlling flow rate of oil supplied to the chambers with a control signal in accordance with a desired transmission ratio, the spool is shifted, so that the transmission ratio is controlled to a desired transmission ratio. When the actual transmission ratio does not coincide with the desired ratio in a steady state of an engine, the control signal is corrected so as to reduce the difference between the desired transmission ratio and the actual ratio.

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 dependence 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 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. 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 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 is not 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 No. 59-217048 discloses a system for controlling 
the transmission ratio changing speed e. Various values of the speed are 
stored in a look-up table and proper speed is derived from the table in 
accordance with engine speed and vehicle speed. The derived transmission 
ratio is used for deciding the control signal for the transmission ratio. 
However, if the viscosity of oil varies with change of temperature thereof 
or the variances of control valves caused by manufacturing tolerance 
and/or by changes with time, transmission ratio can not be properly 
controlled to a desired transmission ratio. As a result a deviation 
between the desired ratio and the actual ratio remains in a steady driving 
state. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a system which may 
control the transmission ratio to a desired ratio without deviation by 
correcting the control signal. 
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, 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 transmission ratio 
control valve having a spool for controlling the oil supplied to the 
cylinders of the drive pulley to change the transmission ratio. 
The system comprises first means for shifting the spool, 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 a desired transmission ratio, 
control means responsive to the desired transmission ratio for producing a 
control signal for operating the first means to shift the spool to control 
the transmission ratio, third means for detecting a steady state of the 
transmission and for producing a steady state signal, fourth means for 
detecting deviation between the desired transmission ratio and an actual 
transmission ratio and for producing a deviation signal, fifth means 
responsive to the steady state signal and to the deviation signal for 
correcting the control signal so as to reduce the deviation. 
The other objects and features of this invention will be apparently 
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. 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 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, 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 
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 by increasing the duty ratio, 
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: 
EQU V=f(i) 
Flow rate Q is obtained by differentiating the volume V with time and 
expressed as 
##EQU1## 
Supply flow rate Q.sub.s and drain flow rate Q.sub.d are presented as 
##EQU2## 
where P.sub.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, 
.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. 
Designating by D the duty ratio 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 
##EQU3## 
Assuming a, S.sub.s and S.sub.d to be constants, 
EQU Q=f(D, Pl, P.sub.p) 
The line pressure P1 is decided by the transmission ratio i and engine 
torque, and the pressure P.sub.p in the chamber 9 is decided by the 
transmission ratio i and the line pressure P1. Accordingly, assuming the 
engine torque to be constant, 
EQU Q=f(D,i) 
Since di/dt=f(Q, i) 
EQU di/dt=f(d,i) 
Therefore 
EQU D=f(di/dt,i) 
Accordingly, the duty ratio is decided by the transmission ratio changing 
speed rate di/dt and the transmission ratio i. In a feedback control 
system, the transmission ratio changing speed rate di/dt can be decided by 
the difference between the actual transmission ratio i and a desired 
transmission ratio id, as follows. 
EQU di/dt=K1 (id-i) 
where K1 is a coefficient. 
Further, if the transmission ratio changing speed rate 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&gt;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. 
However, the response of the transmission control system including various 
mechanisms is slow, which means that the conversion of the actual 
transmission ratio to the desired transmission ratio delays. 
In order to eliminate the control delay, the transmission ratio changing 
speed rate di/dt is corrected by the speed of change of the desired 
transmission ratio (did/dt), as follows. 
EQU di/dt=K1 (id-i)+K2.multidot.did/dt 
where K2 is a coefficient. 
The desired transmission ratio changing speed rate did/dt is to advance the 
phase of the control operation. Thus, the response of the system can be 
improved. The speed rate did/dt is obtained by the amount (.DELTA.id) of 
change of the desired transmission ratio at a predetermined intervals 
(.DELTA.t), that is .DELTA.id/.DELTA.t. 
The coefficient K1 may be changed in accordance with the opening degree of 
the throttle valve, and the coefficient K2 may also be changed in 
accordance with physical conditions of the system, such as viscosity of 
oil used in the system. 
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, 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. The 
output signal N.sub.S and output signal .theta. 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 the speed N.sub.S and signal .theta.. 
The desired transmission ratio id is fed to a desired transmission ratio 
changing speed calculator 78 which produces a desired transmission ratio 
changing speed rate did/dt. A coefficient setting section 79 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 a transmission ratio 
changing speed calculator 80 to produce a transmission ratio changing 
speed rate di/dt from the formula di/dt=K1(id-i)+K2.multidot.did/dt. 
However, the formula does not represent the transmission ratio changing 
direction, that is the upshift direction or downshift direction. In order 
to decide the direction, the formula is rewritten as follows. 
EQU di/dt=K1.multidot.{(id+K.sub.2 /K.sub.1 .multidot.did/dt)-i} 
When (id+K.sub.2 /K.sub.1 .multidot.did/dt)&gt;i and di/dt&gt;0, the transmission 
is downshifted, and when (id+K.sub.2 /K.sub.1 .multidot.did/dt)&lt;i and 
di/dt&lt;0, upshifted. 
The speed di/dt and actual ratio i are applied to a duty ratio table 81 to 
derive the duty ratio D. The duty ratio D is supplied to the solenoid 
operated on-off valve 68 through a duty ratio correcting section 112 and a 
driver 82. 
Further, the output signal 0 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 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 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. 
When the control system is properly operated, the actual transmission ratio 
converges on the desired transmission ratio in a steady driving condition. 
The fact that there is a deviation between actual and desired transmission 
ratios means failures of the system. Accordingly, in the system of the 
invention, the detecting means for detecting the deviation in a steady 
state is provided, as described hereinafter. 
The throttle valve opening degree signal 0 is applied to a steady state 
detector 110 in which opening degree changing speed dO/dt is calculated. 
When the speed d8/dt is below a small value, the vehicle driving condition 
is regarded as steady. The detector 110 produces a steady state signal in 
such a steady state and the signal is fed to a correcting quantity 
calculator 111. As the method for the detection of the steady state, at 
least one of changing rates of vehicle speed, desired transmission ratio 
id, and actual transmission ratio i may be used. 
The correcting quantity calculator 111 is applied with signals i, id and 
N.sub.P. Deviation .DELTA.i is obtained by absolute value 
.vertline.id-i.vertline. in the calculator 111. Correcting quantity 
.DELTA.D, which is applied to the correcting section 112 to correct the 
duty ratio D, is decided as an increasing function of the deviation 
.DELTA.i (.DELTA.D=f (.DELTA.i) ). For example, the function for 
correcting quantity .DELTA.D is composed by proportional (P) and integral 
(I) correcting terms, as follows. 
EQU .DELTA.D=f (.DELTA.i)=f (.DELTA.D.sub.I, .DELTA.D.sub.P) 
FIG. 4c shows the deviation and proportional and integral constituents 
under a downshift condition (id&gt;i). 
On the other hand, necessary flow rate of oil for providing a speed di/dt 
becomes large as the transmission ratio i becomes small (high vehicle 
speed). Accordingly, correcting quantity .DELTA.d at a time becomes large 
with reduce of the transmission ratio i as shown in FIG. 4a. As described 
hereinafter, the duty ratio D is reduced, the transmission is downshifted. 
Accordingly, in order to correct the duty ratio in the downshift 
direction, correcting quantity is set to -.DELTA.d. 
Further, considering relationship between the change of transmission ratio 
and the speed of the drive pulley (engine speed), the speed of the drive 
pulley must be largely changed in a high speed range when the difference 
(.DELTA.i) between transmission ratios is constant. FIG. 4b shows various 
transmission ratio lines. When the transmission ratio is downshifted from 
a line L.sub.1 to line L.sub.2 by .DELTA.i, the increment .DELTA.N.sub.P 
of drive pulley speed increases with increase of the speed. Accordingly, 
when the drive pulley speed N.sub.P is higher than a predetermined value 
N.sub.Pl, the correcting quantity is corrected by a calculation 
.DELTA.d.multidot..alpha.or -.DELTA.d.multidot..alpha. (where, .alpha.&gt;1). 
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 id, desired transmission ratio 
changing speed did/dt and transmission ratio changing speed di/dt are 
calculated at calculators 77, 78, 80. The transmission ratio changing 
speed di/dt is 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 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 di/dt becomes zero, so that 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. Thus, line pressure reduces, and the 
transmission is upshifted to the desired transmission ratio id at the 
speed di/dt. 
As the difference between the desired ratio id and actual ratio i becomes 
large and the desired transmission ratio changing speed 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 opening degree of the throttle valve is reduced for deceleration 
at a small transmission ratio (overdrive), 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 changing speed 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, a torque T is obtained in accordance with 
throttle position .theta. and engine speed N.sub.e, which is applied to 
desired line pressure calculator 104. The calculator calculates a 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. Thus, power is transmitted through the transmission without 
slipping of the belt. 
Correcting operation is described hereinafter with reference to FIG. 5. At 
a steady state, the correcting quantity .DELTA.D is calculated in 
accordance with the deviation .DELTA.i=.vertline.id-i.vertline.. When 
id&lt;i, correcting quantity .DELTA.D is calculated by .DELTA.D.sub.I =f 
(.DELTA.D.sub.I -.DELTA.d) and .DELTA.D.sub.P =-.DELTA.d.multidot..alpha.. 
The duty ratio correcting section 112 corrects the duty ratio D by 
calculation D+.DELTA.D to reduce the duty ratio to downshift the 
transmission. When id&lt;i, the duty ratio is increased to upshift. FIG. 6 
shows the converging operation of the actual transmission ratio i. 
Referring to FIG. 7 showing another embodiment of the invention, actual 
transmission ratio i and output signal .theta. representing the opening 
degree of the throttle position sensor 74 are fed to a desired drive 
pulley speed table 115. The desired drive pulley speed N.sub.P d is 
derived from the table 115 in accordance with the ratio i and signal 
.theta.. FIG. 8 shows a look-up table for the desired drive pulley speed 
N.sub.P d. 
The desired drive pulley speed N.sub.P d and driven pulley speed N.sub.S 
are fed to a desired transmission ratio calculator 116 where the 
calculation of desired transmission ratio (id), id=N.sub.p d/N.sub.S is 
made. The desired transmission ratio id is fed to desired transmission 
ratio changing speed calculator 78 which produces a desired transmission 
ratio changing speed did/dt. 
Other parts of the system are the same as the first embodiment and 
identified with the same references. The operation of the second 
embodiment is substantially same as the first embodiment. 
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 spirit and scope of the invention as 
set forth in the appended claims.