Variable control device for a continuously variable transmission

A variable control device which controls the rotations of a step motor in a continuously variable transmission and maintains the variable ratio is provided with a feed back control system and a feed forward control system. These control systems are switched on the basis of the driving conditions. The feed back control is provided with an actual pulley compensator 108b which calculates the external disturbance compensation from the actual pulley ratio (Aip) and a command value external disturbance compensator 108a which calculates the external disturbance compensation from the command pulley ratio. When switching from open loop control to feed back control, the actual pulley ratio compensator 108b is initialized to the actual variable ratio (Aip). On the other hand, the command value compensator 108a is initialized to the command pulley ratio (ip.sub.R). Hence it is possible to prevent the command pulley ratio from varying greatly immediately after switching and control the generation of variable shocks when transferring from open loop control to feed back control.

FIELD OF THE INVENTION 
The present invention relates to a variable control device for a 
continuously variable transmission used in a vehicle or the like. 
BACKGROUND OF THE INVENTION 
V-belt or toroidal continuously variable transmissions are known in the 
prior art as continuously variable transmissions used in vehicles. 
Variable control valves are driven by actuators such as step motors and 
the variable ratio is continuously varied on the basis of oil pressure. 
It is not possible to correctly perform feed back control in this kind of 
variable control device when the oil temperature of the working oil is 
extremely low because the viscosity of the working oil increases and the 
response characteristics of the control valves are greatly reduced or when 
the actual pulley ratio i.e. the variable ratio, can not be detected due 
to extremely low speed running. In this event, open loop control exhibits 
greater accuracy. Hence JPA-9-199524 filed by the present applicant shows 
the use of open loop control at times of low speed running or low 
temperatures, such use reverting to feed back control when conditions 
return to normal. 
JP-A-9-329229, JP-A-9-89494 and JP-A-9-89496, filed by the present 
applicant, are examples of devices provided with an external disturbance 
compensator to improve normal response characteristics and control 
external disturbances to the variable mechanism, the influence of age 
softening and lags in the response time of the step motor. 
However convention variable control devices for a continuously variable 
transmissions initialize the external disturbance compensator of the 
control device and the like to the actual pulley ratio (Aip) which equals 
the actual variable ratio, when the transition is made from open loop 
control to feed back control. In other words, when feed back control is 
commenced, adjustment is made so that the deviation of the command pulley 
ratio (ip.sub.R) from the actual pulley ratio (Aip) is equal to 0. 
However at this time, it is not certain that the command pulley ratio 
(ip.sub.R), that is to say the feed back command value, and the actual 
pulley ratio (Aip) will agree. For example, if the variable ratio is 
undergoing extreme variation, the deviation easily increases. When there 
is a large deviation, the command pulley ratio (ip.sub.R) will rapidly 
change on initialization from the value during open loop control. 
Furthermore if the command value during open loop control is in agreement 
with the value during feed back control, no rapid change will occur in the 
command value when switching. 
When the command pulley ratio (ip.sub.R) rapidly changes, the actuator 
controlling the variable ratio operates rapidly and there is the 
possibility that a shock will be generated due to the large variation in 
th e variable ratio. 
SUMMARY OF THE INVENTION 
The object of the present invention is to suppress the generation of 
variable shocks associated with transferring from open loop control to 
feed back control. 
In order to achieve the above object, the variable control device of a 
continuously variable transmission of the present invention is provided 
with a target variable ratio setting circuit which sets the target 
variable ratio in response to the driving conditions of the vehicle, a 
feed back compensation calculation circuit which calculates the feed back 
command value so that the actual variable ratio follows the target 
variable ratio, a control switch circuit which switches open loop control 
in response to the target variable ratio and feed back control in response 
to the feed back command value based on the driving conditions. 
The feed back compensator calculating circuit is initialized when said 
control switching circuit switching from open loop control to feed back 
control so as to maintain the deviation of the actual variable ratio and 
the feed back command value immediately before the control switching. 
The details as well as other features and advantages of the invention are 
set forth in the remainder of the specification and shown in the 
accompanying drawings.

PREFERRED EMBODIMENTS OF THE INVENTION 
The preferred embodiments of the invention will be explained below with 
reference to the accompanying drawings. 
FIG. 1 shows a continuously variable transmission 17 which is provided with 
a primary pulley 16 connected to the engine (not shown) and a secondary 
pulley 26 connected to the drive shaft which act as a pair of variable 
pulleys. This pair of variable pulleys is connected by a V-belt 24. 
The drive ratio of the continuously variable transmission 17 (hereafter 
pulley ratio) is controlled by an oil pressure control device 3. In order 
to control the oil pressure, a step motor 64 or the like is provided as an 
actuator which drives the variable control valve 63 in response to the 
target pulley ratio from the CVT control unit 1 as shown in FIG. 1 and the 
line pressure control valve (not shown) which adjusts the line pressure. 
The CVT control unit 1 reads the signals from the primary pulley rotation 
speed sensor 6 which detects the rotation speed (Npri) of the primary 
pulley 16 of the continuously variable transmission 17, the signal from 
the secondary pulley rotation speed sensor 7 which detects the rotation 
speed (Nsec) of the secondary pulley 26, the select position and the 
variable mode (Mode) from the inhibitor switch 8, and the degree of 
throttle aperture (TVO) (hereafter degree of accelerator depression) from 
the throttle aperture sensor 5 in response to the degree of depression of 
the accelerator pedal operated by the driver. Furthermore the vehicle 
speed (VSP) and oil temperature (Tf) of the continuously variable 
transmission 17 from the temperature sensor (not shown) are read. The 
target pulley ratio (ip) is variably controlled in response to the driving 
conditions of the vehicle or the demands of the driver. (Below this will 
be termed the achievable pulley ratio, however the value represents the 
ultimate target value of the pulley ratio in transferring from the current 
pulley ratio. In other words it represents the target variable ratio.) 
In the present embodiment, vehicle speed VSP is read as a multiple of a 
fixed number with respect to the secondary rotation speed (Nsec). 
A torque converter 12, which is provided with a lock up clutch 11, is 
interposed between the V-belt continuously variable transmission 17 and 
the engine (not shown). The output of the torque converter 12 is 
transmitted to the primary pulley 16 which acts as a input shaft. 
The primary pulley 16 is formed as a unit with a rotating fixed conical 
plate 18 and a variable conical plate 22 which is disposed facing the 
fixed conical plate 18, forms the V shaped pulley groove and displaces in 
the axial direction due to the oil pressure (primary pulley oil pressure) 
moving towards the primary pulley cylinder chamber 20. 
The secondary pulley 26 is connected to the drive shaft and is comprised of 
a rotating fixed conical plate 30 and a variable conical plate 34 which is 
disposed facing the fixed conical plate 30, forms the V shaped pulley 
groove and displaces in the axial direction due to the oil pressure (line 
pressure) moving towards the secondary pulley cylinder chamber 32. These 
two are formed as a unit on the same axis as the secondary pulley 26. 
The pulley ratio of the primary and secondary pulley s 16, 26, that is to 
say the pulley ratio (ip) varies in response to the variation in the 
radius of contact with the V belt 24 as a result of the displacement of 
the variable conical plate 34 of the secondary pulley 26 and the variable 
conical plate 22 of the primary pulley 16 in the axial direction. 
For example if the width of the V shaped pulley groove of the primary 
pulley 16 decreases, the contact radius of the V belt 24 on the secondary 
pulley 26 side decreases and it is possible to reduce the pulley ratio 
(the variable ratio on the High side). If the variable conical plate 22 
displaced in the opposite direction, the pulley ratio (the variable ratio 
on the Low side) will increase. 
In such a way, variable control which varies the width of the V shaped 
pulley groove of the primary pulley 16 and the secondary pulley 26 is 
performed by the control of oil pressure towards the primary pulley 
cylinder chamber 20. As shown in FIGS. 2 and 3, control is performed by a 
step motor 64 which drives the variable control valve 63 of the oil 
pressure control device 3. 
The step motor 64 drives the variable control valve 63 in response to 
commands from the CVT control unit 1 through the variable link 67. By 
adjusting the oil pressure supplied to the cylinder chamber 20 of the 
primary pulley 16, the step motor 64 controls the actual pulley ratio 
(Aip), that is to say, the actual variable ratio so that it agrees with 
the achievable pulley ratio (ip). 
The mechanism for controlling oil pressure feedback is the same as that in 
the conventional device above. The step motor 64 is engages with the rack 
65 in a meshing fashion through the pinion 66. The rack 65 is connected to 
one end of the variable link 67 of the fixed lever ratio. The spool of the 
variable control valve 63 is connected along the variable link 67. A feed 
back member 158 which displaces in the axial direction of the variable 
conical plate 22 which comprises the primary pulley 16 is connected to the 
other end of the link 67. 
One end of the feed back member 158 is connected in the axial direction to 
the outside periphery of the variable conical plate 22 and is connected to 
the rod 60a of the line pressure control valve 60 at determined points. 
The variable control valve 63 and the line pressure control valve 60 are 
driven by the oscillation of the variable link 67 in response to the 
displacement of the step motor 64 and the relative displacement of the 
feed back member 158. 
The variable control valve 63 controls the supply of oil pressure to the 
cylinder chamber 20 of the primary pulley 16 in response to the 
displacement of the variable link 67 as a result of the impelling level of 
the step motor 64. The line pressure is normally supplied to the cylinder 
chamber 32 of the secondary pulley 26 from the line pressure valve 60. 
Therefore when the actual pulley ratio (Aip) agrees with the achievable 
pulley ratio (ip) based on the displacement of the step motor 64, a fixed 
achievable pulley ratio (ip) is maintained as the variable link 67 
connected to the variable conical plate 22 displaces the spool of the 
variable control valve 63 and maintains the oil pressure of the cylinder 
chamber 20. 
In FIG. 2, reference numeral 78 denotes a manual valve displacing in 
response to the shift lever, 76 is a negative pressure diaphragm, 77 is a 
throttle valve displacing in response to negative pressure diaphragm 76, 
95 is a Low switch which is placed in the ON position by the displacement 
of the rack 65 to the minimum Low pulley ratio (minimum Low variable 
ratio). 
Next the variable control performed by the CVT control unit will be 
explained with reference to FIGS. 4 and 5. 
From a map (not shown), the achievable pulley ratio calculation part 100 
searches for the achievable pulley ratio (ip) based on driving conditions 
such as the vehicle speed (VSP) and the degree of throttle aperture (TVO). 
The actual pulley ratio calculation part 101 computes the actual pulley 
ratio (Aip) from the rotation speed (Npri) of the primary pulley 16 and 
the rotation speed (N sec) of the secondary pulley 26. 
After the achievable pulley ratio (ip) is smoothed out on the basis of 
driving conditions such as the degree of throttle aperture (TVO) at the 
filter 102, the result is inputted into the feed forward (F/F) compensator 
103. 
The feed forward (F/F) compensator 103 calculates the target pulley ratio 
(ip.sub.T) which can follow the achievable pulley ratio (ip) at a time 
constant (Tt) from the present actual pulley ratio (Aip) on the basis of 
the time constant (Tt) set by the target time constant calculation part 
104 based on the actual pulley ratio (Aip). 
Next the feed forward back (F/B) compensator 103 searches for the time 
constant (Tp) which the control object model 106 of the variable mechanism 
and the step motor 64 calculate on the basis of the actual pulley ratio 
(Aip), and the command pulley ratio (ip.sub.R), i.e. the feedback command 
value, considering the motion characteristics of the control object model 
106 based on the deviation of the target pulley ratio (ip.sub.T) and the 
actual pulley ratio (Aip). The command pulley ratio (ip.sub.R) is input 
into the command value limiter 107. 
After the external disturbance compensation amount from the external 
disturbance compensator 108 as shown in FIG. 5 is input into the command 
value limiter 107 and the command pulley ratio (ip.sub.R) is adjusted on 
the basis of the external disturbance compensation amount, the result is 
input into the step command part 109 and is converted to the target 
position (DsrSTP) of the step motor 64 corresponding to the command pulley 
ratio (ip.sub.R) based on the characteristics of the predetermined map. 
The target position (DsrSTP) corresponds to the degree of target 
displacement of the rack 65 as shown in FIG. 2. 
The target position (DsrSTP) is set in the step motor motive part 110 as a 
step number (STP) and motive speed (PPS) i.e. pulse rate corresponding to 
the oil temperature (Tf) of the continuously variable transmission and is 
output to the step motor 64 in order to control the present pulley ratio. 
Therefore the target pulley ratio (ip.sub.T) on which the current actual 
pulley ratio (Aip) can follow is searched for corresponding to the time 
constant (Tt) set from conditions such as driving conditions and on the 
basis of the achievable pulley ratio (ip) calculated from the driving 
conditions. Feed back compensation is applied into the target pulley ratio 
(ip.sub.T), the command pulley ratio (ip.sub.R) is calculated, and the 
step number of the step motor 64 and the motive speed (PPS) are calculated 
from the two above values to which external disturbance has been added. 
As shown in FIG. 5, the external disturbance compensator is comprised of a 
command value compensator 108a (first signal processing part) which 
performs external disturbance compensation based on the command pulley 
ratio (ip.sub.R), a delay block 108c which delays the output of the 
command value compensator 108a, and an actual pulley ratio compensator 
108b (second signal processing part) which performs external disturbance 
compensation based on the actual pulley ratio (Aip). 
The external disturbance compensator 108 is provided with an inverse 
characteristic control object model 106 and calculates the time constant 
(Tt) corresponding to the control object fixed time (Tp) of the control 
object model 106. The external disturbance compensator 108 performs 
external disturbance compensation calculations after compensating for the 
fixed time (T.sub.H) corresponding to the variable direction (upshift or 
downshift). This function is the same as motion characteristic 
compensation control containing external disturbance compensation 
disclosed by the present applicant in JP-A-9-89494 and JP-A-9-89496. 
The degree of external disturbance compensation calculated by the external 
disturbance compensator 108 is output to the command value limiter 107 in 
FIG. 4. 
The command pulley ratio (ip.sub.R) to which external disturbance 
compensation is applied in the command value limiter 107, is input into 
the step command part 109 from the control switching part 120 and ratio is 
switched to the target position (DsrSTP) of the step motor 64. 
The control switching part 120 for example performs open-loop control in a 
similar way to the conventional device above when the oil temperature (Tf) 
of the continuously variable transmission 17 or the vehicle speed does not 
reach a fixed value. As a result if the achievable pulley ratio (ip) is 
selected and is output to the step command part 109, the step motor 64 
will undergo open-loop control. Once a determined feed back condition is 
established, the command value limiter 107 is selected, the value is 
output to the step command part 109 and the step motor 64 undergoes feed 
back control. 
After the step command part 109 converts the command pulley ratio 
(ip.sub.R) or the achievable pulley ratio (ip) to the step number (step) 
based on the map not shown in the figure, the hysteresis corresponding to 
the variable direction is set and the target position (DsrSTP) is output. 
Based on the target position (DsrSTP), the step motor motive part 110 
outputs the command step number (STP) to the step motor 64 at a motive 
speed (PPS) which corresponds to the oil temperature (Tf). When the 
frictional force of the oil increases with its decreasing temperature, a 
large motive force is necessary, the corresponding speed is set low and 
the motor torque increases. 
The output command step number (STP) is calculated by subtracting the 
actual step number from the target position (DsrSTP). 
The motive speed (PPS) is set corresponding to the oil temperature (Tf) of 
the continuously variable transmission 17 based on the predetermined map 
or the like. In other words when the oil temperature (Tf) is low, the 
motive speed (PPS) is set low and the motive force of the step motor 64 is 
maintained. On the other hand, when the oil temperature (Tf) is high, the 
motive speed (PPS) is set high and the response is raised. After the 
determined motive speed (PPS) is limited to within the maximum motive 
speed range corresponding to the motive characteristics of the step motor 
64, the calculated command step number (STP) is output to the step motor 
64 at a motive speed (PPS). 
The feed back system is illustrated in FIGS. 5 and 6. 
In FIG. 5, the feed back compensator 105 is comprised of the feed forward 
compensator part 105a on the achievable pulley ratio (ip) side and the 
feed back compensator part 105b on the actual pulley ratio (Aip) side. In 
FIG. 5, the S/M driver corresponds to the step command part 109 and the 
step motor motive part 110 above. In FIG. 5, each symbol is defined as set 
out below. 
T.sub.F/B : Fixed Time for Feed Back 
Tplant: Fixed Time for Control Object Model 
T.sub.H : Fixed Time Compensation Characteristic for External Disturbance 
Compensator 
T.sub.S : Sample Period 
Z.sup.-1 : One Cycle Delay 
Z.sup.-n : Delay Corresponding to Continuous Time of the Control Object 
Each control element such as the feed foward compensator 103, the feed back 
compensator 105 and the external disturbance compensator 108 are divided 
into those elements which process signals with respect to command pulley 
ratio (ip.sub.R) and those elements which process signals with respect to 
the actual pulley ratio (Aip). The command value compensator 108a and the 
delay block 108c process signals with respect to command pulley ratio 
(ip.sub.R). The actual pulley ratio compensator 108b processes signals 
with respect to the actual pulley ratio (Aip). 
When the control switch part 120 changes from open loop control to feed 
back control, it is necessary to perform initialization of each control 
element. This initialization is performed according to the flowchart in 
FIG. 7. 
Firstly the switch from open loop control to feed back control is detected 
in a Step 1. In a Step 2, the actual pulley ratio compensator 108b which 
performs signal processing with respect to the actual pulley ratio (Aip) 
is initialized at the actual pulley ratio (Aip) of the control switch 
point. Also the feed foward compensator 103 is initialized at the actual 
pulley ratio (Aip). 
Then in a Step 3, the command compensator 108a and the delay block 108c 
which perform signal processing with respect to the achievable pulley 
ratio ip are initialized and the initialization process is completed. 
The schematic diagram of the control elements such as the external 
disturbance compensator 108 and the feed forward back compensator 103 are 
divided into FIG. 6 (A)-(C). Initialization is performed on the delay 
block Z.sup.-1 of each control element and set to the set initialization 
value. 
This corresponds to the command value compensator 108a in FIG. 6(A), the 
actual pulley ratio compensator 108b in FIG. 6(B) and the delay block 108c 
in FIG. 6(C). 
During open loop control, if the achievable pulley ratio=2.0 and the actual 
pulley ratio=2.3, and the transition is made from open loop control to 
feed back control and initialization is performed as above, each output, 
that is to say, the pulley ratios A-F shown in FIG. 5 are initialized as 
shown below. 
The actual pulley ratio compensator 108b is initialized on the basis of an 
actual pulley ratio of 2.3, the external disturbance compensator delay 
block 108c is initialized on the basis of an achievable pulley ratio of 
2.0, and the feed forward compensator is initialized on the basis of an 
actual pulley ratio of 2.3. As a result, the output of all components is 
shown as below. 
A; Actual Pulley Ratio Compensator 108b Output Pulley Ratio=2.3 
B; External Disturbance Compensator Delay Block 108c Output Pulley 
Ratio=2.0 
C; Feed Forward Compensator Output Pulley Ratio=2.3 (Target Pulley Ratio 
Ip.sub.T) 
D; Feed Back Compensator 105 Ouput Pulley Ratio=2.3 
As a result the limiter output value which represents the addition of 
output of the external disturbance compensator, which is the deviation of 
(A) and (B), and the feed back value is set out as below. 
E; External Disturbance Compensator Output Pulley Ratio Deviance=+0.3 
F; Command Value Limiter Output Pulley Ratio=2.0 (command pulley ratio 
ip.sub.R) 
Hence the command pulley ratio which corresponds to the feed back command 
value becomes 2.0. This value is the same as the achievable pulley ratio 
during open loop control immediately before initialization. 
Hence as shown in FIG. 8, even if there is a switch from open loop control 
to feed back control, it is possible to maintain the command pulley ratio 
(ip.sub.R) at 2.0: the value immediately prior to switching. The deviation 
of the command pulley ratio (ip.sub.R) and the actual pulley ratio (Aip) 
is maintained at the deviation before the control switch. Hence it is 
possible to avoid rapid fluctuations in the command value unlike the 
conventional device around the switch from open loop control to feed back 
control. This enables the prevention of excessive changes in the present 
pulley ratio which result from the rapid change in the command value and 
the prevention of variable shock during control switching. 
Hence by feed back control, the achievable value is modified to correspond 
to the deviation which existed between the command pulley ratio and the 
actual pulley ratio during open loop control. The actual pulley ratio 
varies in response to this, but modifying the command value in this way is 
not performed in the rapid way as in initialization during control 
switching. In other words the modification of the command value is 
performed in the range of the responses during normal feed back actual. 
Hence the control pulley ratio does not generate variable shocks and 
varies at a suitable response speed. 
The present invention is not limited to the embodiments described in the 
specification and obviously extends to modifications by the person skilled 
in the art within the scope of the claims.