Speed ratio controller and control method of toroidal continuously variable transmission

A speed ratio of a toroidal continuously variable transmission is varied by a step motor (4). The step motor (4) is responsive to a command signal Astep which a controller (61) outputs corresponding to a target speed ratio. The controller (61) is programmed to initialize the command signal Astep when it is activated while the vehicle is running (S152) such that the command signal coincides with an actual operation position of the step motor (4). The controller (61) is further programmed to limit the command signal, after performing this initialization, within a first limiting range which is narrower than a second limiting range corresponding to a physical operation limit of the step motor (4) (S168, S170). By applying such a limitation to the command signal, the command signal is prevented from exceeding the operation limit of the step motor (4) even when a torque shift error is introduced into the command signal by the initialization while the vehicle is running.

FIELD OF THE INVENTION 
This invention relates to speed ratio control of a toroidal continuously 
variable transmission of a vehicle, specifically to speed ratio control 
when power supply to a controller is turned on while the vehicle is 
running. 
BACKGROUND OF THE INVENTION 
Speed ratio of a V-belt continuously variable transmission and a toroidal 
continuously variable transmission for a vehicle is generally controlled 
by an electronic controller via an actuator which is responsive to a 
command signal output by the controller. A step motor is an example of 
such an actuator. 
Tokkai Hei 8-178063 published by the Japanese Patent Office in 1996 
discloses an initializing method of the command signal so as to make the 
command signal coincide with the actual operation position of the step 
motor. The operation position of the step motor is considered to be 
equivalent to the real speed ratio of the transmission. 
The controller according to this prior art first determines if the vehicle 
is running, immediately after the power supply of the controller is 
started. If the vehicle is not running, the controller drives the step 
motor to an end position in the speed ratio increase direction which is 
equivalent to the maximum speed ratio of the transmission. At this 
position, the controller initializes the command signal such that a signal 
to drive the step motor to this position corresponds to a signal 
commanding the maximum speed ratio. 
It is also possible that the power supply to the controller instantaneously 
stops or the voltage becomes too low for the operation of the controller 
while the vehicle is running. On such an occasion, if the step motor is 
driven to the end position when the power supply is recovered, the speed 
ratio suddenly takes a maximum value and an undesirable downshift of the 
transmission occurs. When the vehicle is running, therefore, the 
controller initializes the command signal by a different method. That is, 
the current operation position of the step motor is estimated from the 
real speed ratio of the transmission as detected by sensors, and the 
command signal is modulated to coincide with the estimated operation 
position of the step motor. 
SUMMARY OF THE INVENTION 
However, a specific problem arises when the above initializing method is 
applied to the controller of a toroidal continuously variable 
transmission. In a toroidal continuously variable transmission, a real 
speed ratio of the transmission detected by rotation sensors and a command 
signal output from the controller do not necessarily correspond due to a 
so-called torque shift error which is specific to the toroidal 
continuously variable transmission. 
In the toroidal continuously variable transmission, an input torque is 
converted into an output torque by power rollers at an arbitrary speed 
ratio corresponding to their gyration angle. 
When an input torque varies, trunnions supporting the power rollers deform 
and a mechanical feedback mechanism which feeds back the movement of the 
trunnion to a control valve becomes erroneous. Herein, the control valve 
is driven by the step motor and provides oil pressure to vary the gyration 
angle of the power rollers. A deviation of the real speed ratio from the 
speed ratio designated by the command signal due to the error of the 
mechanical feedback mechanism is called a torque shift error. Since the 
transmission transmits no torque when the vehicle is not running, the 
torque shift error occurs only when the vehicle is running 
When the command signal is initialized while the vehicle is running, 
therefore, the accuracy of the initialization of the command signal is 
adversely affected by this torque shift error, and there is a possibility 
that the controller may output a command signal to the step motor which 
actually surpasses the operation limit of the step motor. 
It is therefore an object of this invention to prevent a command signal 
from surpassing the operation limit of a motor even when the command 
signal is initialized while the vehicle is running. 
In order to achieve the above object, this invention provides a speed ratio 
controller for a toroidal continuously variable transmission of a vehicle, 
wherein the transmission comprises a motor varying an operation position 
according to a command signal so as to vary a speed ratio of the 
transmission. The controller comprises a sensor for detecting a running 
condition of the vehicle, a sensor for detecting that the vehicle is 
running, a sensor for detecting a real speed ratio of the transmission, 
and a microprocessor programmed to perform, when the vehicle is running, 
an initialization of the command signal with respect to an actual 
operation position of the motor based on the real speed ratio of the 
transmission, determine a target speed ratio based on the running 
condition of the vehicle, determine the command signal based on the target 
speed ratio, limit the command signal within a first limiting range which 
is narrower than a second limiting range corresponding to a physical 
operation limit of the transmission, after the initialization is 
performed, and output the command signal after limiting, to the motor. 
This invention also provides a speed ratio control method of a toroidal 
continuously variable transmission of a vehicle, wherein the transmission 
comprises a motor varying an operation position according to a command 
signal so as to vary a speed ratio of the transmission. The method 
comprises detecting a running condition of the vehicle, detecting that the 
vehicle is running, detecting a real speed ratio of the transmission, 
performing, when the vehicle is running, an initialization of the command 
signal with respect to an actual operation position of the motor based on 
the real speed ratio of the transmission, determining a target speed ratio 
based on the running condition of the vehicle, determining the command 
signal based on the target speed ratio, limiting the command signal within 
a first limiting range which is narrower than a second limiting range 
corresponding to a physical operation limit of the transmission, after the 
initialization is performed, and outputting the command signal after 
limiting, to the motor. 
The details as well as other features and advantages of this invention are 
set forth in the remainder of the specification and are shown in the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 of the drawings, a toroidal continuously variable 
transmission to which this invention is applied comprises an input shaft 
20 and an output gear 29. 
The input shaft 20 is connected to an engine of a vehicle via a torque 
converter. The engine and the torque converter are located on the right 
side of FIG. 1, but not shown. The output gear 29 outputs a rotation 
torque for driving the vehicle. 
A cam flange 27 screws in to the tip of the input shaft 20. A nut 26 is 
tightened to the tip of the input shaft 20 so that the cam flange 27 is 
fixed to the input shaft 20. 
The cam flange 27 is inserted in a cylindrically shaped back side part of 
an input disk 1. The input shaft 20 passes through the center of the input 
disk 1 leaving a small clearance. By this arrangement, the input disk 1 is 
maintained coaxial with the rotation shaft 20. The cam flange 27 is 
supported in a case 21 via a bearing 22, and the base end of the input 
shaft 20 is supported by an angular bearing 32. 
Cam rollers 28 are disposed between the cam flange 27 and the input disk 1. 
The cam rollers 28 comprise cam surfaces which press the input disk 1 to 
the right of the figure according to the relative rotational displacement 
of the cam flange 27 and the input disk 1. 
An output disk 2 is attached free to rotate relative to the input disk 1 on 
the outer circumference of the rotation shaft 20. 
The input disk 1 and power output disk 2 comprise toroidal curved surfaces 
1A, 1B which face each other, and a pair of power rollers 3 is gripped 
between these curved surfaces 1A, 1B. 
The output disk 2 is spline jointed to a sleeve 25 supported on the outer 
circumference of the rotation shaft 20 via a needle bearing. A large 
diameter part 25A is formed in the sleeve 25 to support a thrust load 
which interacts on the power output disk 2 towards the right of FIG. 1. 
The sleeve 25 is supported by an intermediate wall 23 of the case 21 via a 
radial bearing 24, and is also supported by an angular bearing 30. The 
angular bearing 30 and an angular bearing 32 are engaged inside a 
cylindrically-shaped cover 31 fixed to the case 21. 
A spacer 33 which engages with the inside of the cover 31 is also gripped 
by the angular bearings 30, 32. 
The thrust force exerted by the input disk 1 on the rotation shaft 3 
towards the left of the drawing, and the thrust force exerted by the 
output disk 2 on the sleeve 25, therefore cancel each other out due to the 
spacers 33 gripped between the angular bearings 30, 32. Also, the load 
which acts on the angular bearings 30, 32 in the radial direction is 
supported by the cover 31. 
The output gear 29 is spline jointed to the outer circumference of the 
sleeve 25. The rotation of the output g ea r 29 is transferred to the 
outside of the case 21 via a gear unit, not shown. 
The power rollers 3 are supported by trunnions 41. 
By driving the trunnions 41 in a direction perpendicular to the rotation 
shaft 20, the contact positions of the power rollers 3 with the input disk 
1 and output disk 2 are changed. Due to this change of contact positions, 
a force is exerted on the power rollers 3 by the disks 1 and 2 so as to 
gyrate the power rollers 3 around an axis O.sub.3 which causes the 
gyration angle of the power rollers 3 to vary. As a result, the distance 
of the contact point between the power rollers 3 and the input disk 1 from 
a center axis O.sub.2 of the rotation shaft 20, and the distance of the 
contact point between the power rollers 3 and the output disk 2 from the 
axis O.sub.2, vary, and a speed ratio varies accordingly. Herein, the 
speed ratio denotes the rotation speed of the input disk 1 divided by the 
rotation speed of the output disk 2 
Referring to FIG. 2, the trunnions 41 support the power rollers 3 such that 
they are free to rotate about an axis O.sub.1 via a crank-shaped shaft 
41A, and such that they are free to swing within a small range around the 
base end of the shaft 41A. 
The upper end of each trunnion 41 is joined to an upper link 43 via a 
spherical joint 42, and a lower end is joined with a lower link 45 via a 
spherical joint 44. The upper link 43 and lower link 45 are supported in 
the case 21 via spherical joints 46 and 47, respectively. Due to these 
links, the pair of trunnions 41 always displace in reverse directions and 
by an equal distance along the axis O.sub.3 of each trunnion 41. 
A piston 6 is fixed to each of these trunnions 41. The piston 6 displaces 
the trunnion 41 along the axis O.sub.3 according to an oil pressure 
balance of oil chambers 51, 53 and oil chambers 52, 54 which are formed in 
the case 21. Oil pressure is supplied to these oil chambers 51, 52, 53, 
and 54 from an oil pressure control valve 5. 
The oil pressure control valve 5 comprises an outer sleeve 5C, inner sleeve 
5B and a spool 5A which slides on the inside of the inner sleeve 5B. A 
port 5D which draws the pressure of an oil pump 55, port 5E connected to 
the oil chambers 51, 54, and port 5F connected to the oil chambers 52, 53 
are formed in the outer sleeve 5C, respectively. The inner sleeve 5B is 
connected with a step motor 4 via a rack and pinion. Also, openings at the 
ends of the inner sleeve 5B are connected to drain passages, not shown. 
The spool 5A is joined to a link 8. The link 8 displaces the spool 5A 
according to a rotational displacement around the axis O.sub.3 and a 
displacement along the axis O.sub.3 of a precess cam 7 fixed to the lower 
end of one of the trunnions 41, and mechanically feeds back the gyration 
angle of the power roller 3 to the oil pressure control valve 5. 
The oil pressure control valve 5 changes the pressure supplied to the ports 
5E, 5F according to a command signal Astep input to the step motor 4 from 
the controller 61. 
For example, when the spool 5A, outer sleeve 5B and inner sleeve 5C are in 
the positions shown in FIG. 2, the oil chambers 52, 53 receive high 
pressure oil of an pressure pump 55 from the port 5F, and oil in the oil 
chambers 51, 54 is drained via the port 5E. 
As a result, the trunnion 41 on the left of the figure moves upwards along 
the axis O.sub.3, and the trunnion 41 on the right of the figure moves 
downwards along the axis O.sub.3. Hence, the rotation axis O.sub.1 of the 
power roller 3 displaces from a neutral position at which it intersects 
the rotation axis O.sub.2 of the input disk 1 and the output disk 2, in 
the direction shown by the arrow Y in the drawing. 
Due to this displacement, the input disk 1 and output disk 2 cause the 
power roller 3 together with the trunnions 41 to gyrate around the axis 
O.sub.3 and thereby continuously vary the speed ratio. 
At this time, the precess cam 7 fixed to the lower end of one trunnion 41 
feeds back the displacement amount in the direction of the axis O.sub.3 of 
the trunnion 41 and the rotational dispacement of the power roller 3 
around the axis O.sub.3, to the oil pressure control valve 5 via a link 8, 
and the spool 5A is displaced in the direction shown by the arrow X in the 
drawing. 
When a speed ratio corresponding to the above-mentioned command signal 
Astep is attained by this feedback operation, the positional relationship 
of the spool 5A and inner sleeve 5B is restored to the neutral position 
wherein inflow and outflow of oil to and from all the oil chambers is 
stopped. 
Hence, the trunnions 41 are maintained in a state where they are displaced 
in the direction of the axis O.sub.3. 
On the other hand, the power roller 3 which gyrated around the O.sub.3 axis 
swings around the base end of the shaft 41A while maintaining the new 
gyration angle, and returns to the neutral position at which the axis 
O.sub.1 and the axis O.sub.2 intersect. 
The reason why the precess cam 7 feeds back not only the rotational 
displacement around the axis O.sub.3 of the power roller 3, i.e., the 
gyration angle, but also the axial displacement of the trunnion 41, is 
that the feedback of the axial displacement of the trunnion 41 works as a 
damping element which prevents the speed ratio control from oscillating. 
The command signal Astep is determined by the controller 61. 
The controller 61 comprises a microprocessor comprising a central 
processing unit (CPU), random access memory (RAM), read-only memory (ROM) 
and input/output interface (I/O interface). 
Signals are input to the controller 61 from a throttle sensor 62 which 
detects a throttle opening TVO of the engine, vehicle speed sensor 63 
which detects a vehicle speed VSP, rotation speed sensor 64 which detects 
a rotational speed Ni of the input disk 1, rotation speed sensor 65 which 
detects a rotational speed No of the output disk 2, oil temperature sensor 
66 which detects a temperature TMP of the above-mentioned oil, line 
pressure sensor 67 which detects a line pressure PL, i.e., the oil 
pressure which the port 5D supplies from the oil pressure pump 55, engine 
speed sensor 68 which detects a rotation speed Ne of the engine, and a 
range sensor 69 which detects an operating mode of the transmission chosen 
by a selector lever, not shown. 
The controller 61 calculates the command signal Astep based on the 
above-mentioned signals and outputs it to the motor 4. 
For this purpose, the controller 61 comprises processing units shown in 
FIG. 3. These units are virtual units constructed from the functions of 
the above-mentioned CPU, read-only memory and random access memory. 
A speed ratio map selecting unit 71 selects a speed ratio map to use based 
on an oil temperature TMP detected by the oil temperature sensor 66, and 
other vehicle running conditions. In this speed ratio map, a final input 
rotation speed Ni* which is a final target rotation speed of the input 
disk 1, is defined according to the vehicle speed VSP and the throttle 
opening TVO. Plural maps of this kind are stored beforehand in the 
controller 61 with respect to various running conditions. 
A final input rotation speed calculation unit 72 calculates the final 
target input rotation speed Ni* of the transmission based on the speed 
ratio map that is currently effective. 
A final target speed ratio computing unit 73 divides the final input 
rotation speed Ni* by a rotation speed No of the output disk 2 detected by 
the rotation speed sensor 65, and calculates a final target speed ratio 
i*. 
A speed ratio variation time constant calculating unit 74 determines a time 
constant Tsft of a speed ratio variation based on the operating mode of 
the transmission detected by the range sensor 69, vehicle speed VSP, 
throttle opening TVO, and deviation between a real speed ratio and a 
transient target speed ratio which will be described later. 
The time constant Tsft is a constant specifying the rate of speed ratio 
variation until the final target speed ratio i* is attained, but as the 
time constant Tsft is varied dynamically in this embodiment as mentioned 
above, it is actually treated as a variable. The transient target speed 
ratio calculating unit 75 calculates a transient target speed ratio RatioO 
as a target value for every control cycle from the final target speed 
ratio i* and time constant Tsft. 
An input torque calculating unit 76 calculates an engine output torque from 
the throttle opening TVO and engine rotation speed Ne, and calculates a 
torque ratio t of the torque converter from the speed ratio of the input 
rotation speed and output rotation speed of the torque converter. The 
engine output torque is then multiplied by the torque ratio so as to 
calculate a transmission input torque Ti. 
A torque shift error correction unit 77 calculates a torque shift error 
correction value TSrto for correcting a torque shift error which is a 
phenomenon peculiar to a toroidal continuously variable transmission from 
the aforesaid transient target speed ratio RatioO and the transmission 
input torque Ti. This torque shift error will now be described. 
When the toroidal continuously variable transmission is operating, the 
input disk 1 and output disk 2 grip the power rollers 3. This grip 
pressure acts as force tending to keep the power rollers 3 away from the 
axis O.sub.1, and it deforms the trunnions 41 which support the power 
rollers 3. The deformation of the trunnions 41 introduces an error into 
the feedback operation of the precess cam 7, and produces a discrepancy 
between the command signal Astep input into the step motor 4 and the 
actual speed ratio realized by the command signal. This phenomenon is 
known as the torque shift error. The magnitude of the torque shift error 
varies according to the transient target speed ratio RatioO and 
transmission input torque Ti. 
The torque shift error correction unit 77 calculates the torque shift 
correction value TSrto from the transient target speed ratio RatioO and 
transmission input torque Ti by looking up a map stored beforehand in the 
controller 61. The torque shift error correction value TSrto is input into 
an adder 85 together with the transient target speed ratio RatioO and a 
speed ratio feedback correction amount FBrto which is output from a PID 
control unit 84. 
Next, the speed ratio feedback correction amount FBrto will be described. 
To make the real speed ratio follow a target value TSRatioO, the speed 
ratio feedback control performed by the controller 61 adds a correction to 
the signal output to the step motor 4. The correction is performed by 
software. The feedback control performed by the above-mentioned precess 
cam 7 is control performed with hardware so that the speed ratio of the 
continuously variable transmission coincides with the command signal 
Astep, and is therefore different from the feedback control performed by 
the controller 61. 
In order to perform this feedback correction, a real speed ratio 
calculating unit 78 computes the real speed ratio Ratio of the 
transmission by dividing the input rotation speed of the transmission, 
i.e., the rotation speed Ni of the input disk 1, by the output rotation 
speed, i.e., the rotation speed No of the output disk 2. A speed ratio 
deviation calculating unit 79 subtracts the real speed ratio Ratio from 
the transient target speed ratio RatioO to calculate the speed ratio 
deviation RtoERR. 
Based on the speed ratio deviation RtoERR, a first feedback gain 
calculating unit 80 sets a first feedback gain for feedback controlling 
the speed ratio on the basis of a proportional integral differential (PID) 
control known in the art. 
The parameters set here are a first proportional control feedback gain 
fbpDATA1, first integral control feedback gain fbiDATA1 and first 
differential control feedback gain fbdDATA1 which are set based on the 
transmission input rotation speed Ni and the vehicle speed VSP, 
respectively. 
To set these first feedback gains, a two-dimensional map of each first 
feedback gain with the transmission input rotation speed Ni and vehicle 
speed VSP as parameters is stored beforehand in the controller 61, and the 
first feedback gain computing unit 80 calculates these first feedback 
gains by looking up each map based on the transmission input rotation 
speed Ni and the vehicle speed VSP. 
The second feedback gain calculating unit 81 sets a second feedback gain 
based on the transmission oil temperature TMP and the line pressure PL. 
The parameters set here are a second proportional control feedback gain 
fbpDATA2, second integral control feedback gain fbiDATA2 and second 
differential control feedback gain fbdDATA2. These second feedback gains 
are also found by looking up maps stored beforehand in the controller 61. 
A feedback gain calculating unit 83 then calculates the proportional 
control feedback gain fbpDATA, the integral control feedback gain fbiDATA 
and the differential control feedback gain fbdDATA by multiplying the 
first feedback gains by corresponding second feedback gains. 
These feedback gains fbpDATA, fbiDATA and fbdDATA are input to the PID 
control unit 84 together with the speed ratio deviation RtoERR, calculated 
by the speed ratio deviation calculating unit 79. 
A PID control unit 84 calculates a speed ratio feedback correction amount 
FBrto using the speed ratio deviation RtoERR and these feedback gains. For 
this purpose, a speed ratio feedback correction amount due to proportional 
control is found by multiplying the speed ratio deviation RtoERR by the 
gain fbpDATA, a speed ratio feedback correction amount due to integral 
control is found by multiplying the speed ratio deviation RtoERR by the 
gain fbiDATA, and a speed ratio feedback correction amount due to 
proportional control is found by multiplying the speed ratio deviation 
RtoERR by the gain fbdDATA. These are then substituted into the following 
PID control equation known in the art to calculate the speed ratio 
feedback correction amount FBrto. 
##EQU2## 
The adder 85 adds the torque shift error correction value TSrto and the 
speed ratio feedback correction amount FBrto to the transient target speed 
ratio RatioO to calculate a compensated transient target speed ratio 
DsrRTO. 
A target step number calculating unit 86 calculates a target number of 
steps DsrSTP of the step motor 4 corresponding to the compensated 
transient target speed ratio DsrRTO by looking up a map stored beforehand 
in the controller 61. 
A step motor drive rate determining unit 88 determines a physical operating 
limit rate of the step motor 4 based on the oil temperature TMP of the 
transmission. 
A step motor drive position command limiting unit 89 determines an 
allowable range of the command signal Astep based on a physical operation 
limit of the step motor 4. The allowable range is different depending on 
the initialization condition of the command signal when the controller 61 
started its operation, i.e., if the vehicle was running when the command 
signal was initialized with respect to the actual operation position of 
the step motor 4. This function will be described later in detail. 
A step motor drive position command computing unit 87 determines whether or 
not the step motor 4 can attain a target number of steps DsrSTP in the 
aforesaid speed ratio control cycle based on this physical operating limit 
rate. A value obtained by correcting the target step number DsrSTP based 
on the physical operating limit rate is set as the command signal Astep. 
Further, it applies the limitation to the command signal Astep according 
to the allowable range defined by the step motor drive position command 
limiting unit 89 and finally outputs the signal Astep to the step motor 4. 
The command signal is therefore considered to correspond to the actual 
rotation position of the step motor 4. 
The above functions of the controller 61 are materialized by performing 
routines shown in the flowcharts of FIGS. 4-11. 
FIG. 4 show s the flow of a main routine and FIGS. 5-8 and 10 show the flow 
of subroutines. All of these routines are repeatedly performed, for 
example, at an interval of 10 milliseconds. 
FIG. 9 shows a special routing for the initialization of the command 
signal. This routine is performed only once immediately after power supply 
to the controller 61 is started. 
In a step S91 in the main routine of FIG. 4, the controller 61 calculates 
the transient target speed ratio RatioO. 
In order to perform this calculation, the calculation of the final target 
input rotation speed Ni* and the final speed ratio i* are previously 
calculated by using the active speed ratio map. This step S91 is therefore 
equivalent to the function of the speed ratio map selecting unit 71, final 
input rotation speed calculation unit 72, final target speed ratio 
computing unit 73, speed ratio variation time constant calculating unit 74 
and transient target speed ratio calculating unit 75 of the block diagram 
of FIG. 3. 
In a step S92, the subroutine shown in FIG. 5 is used to calculate the 
torque shift error correction value TSrto. 
This subroutine is equivalent to the function of the input torque 
calculating unit 76 and the torque shift error correction unit 77 of the 
block diagram of FIG. 3. 
Describing this subroutine, in a step S111, the engine output torque is 
first calculated from the throttle opening TVO and the engine speed Ne 
referring to an engine performance map stored beforehand in the controller 
61. 
In a step S112, the torque ratio t which is the ratio of the input rotation 
speed and output rotation speed of the torque converter is calculated. 
In a step S113, the engine output torque is multiplied by torque ratio t to 
calculate the transmission input torque Ti. 
In a step S114, the torque shift error correction value TSrto is calculated 
from the transmission input torque Ti and the transient target speed ratio 
RatioO which was found in the step S91 of the main routine, by looking up 
the map stored beforehand in the controller 61. 
After calculating the torque shift error correction value TSrto by the 
above subroutine, the main routine proceeds to a step S93 where the speed 
ratio feedback correction amount FBrto is calculated. 
This calculation is performed by the subroutines of FIGS. 6-9. 
FIG. 6 shows a subroutine for calculating the speed ratio deviation RtoERR. 
This subroutine is equivalent to the function of the real speed ratio 
calculating unit 78 and the speed ratio deviation calculating unit 79 in 
the block diagram of FIG. 3. 
First, the transient target speed ratio RatioO is read in a step S121. 
In a step S122, the rotation speed Ni of the input disk 1 is divided by the 
rotation speed No of the output disk 2 to calculate the real speed ratio 
Ratio of the continuously variable transmission. 
In a step S123, the real speed ratio Ratio is deducted from the transient 
target speed ratio RatioO to calculate the speed ratio deviation RtoERR. 
Further, in a step S124, a deviation between the speed ratio deviation 
RtoERR and the speed ratio deviation RtoERR(old) calculated on the 
immediately preceding occasion the routine was executed, i.e., 10 
milliseconds before, is calculated as a difeferetial value of speed ratio 
deviation, 
##EQU3## 
FIG. 7 shows a subroutine which calculates the PID control feedback gain. 
This subroutine is equivalent to the functions of the first feedback gain 
calculating unit 80, the second feedback gain calculating unit 81 and the 
feedback gain calculating unit 83 in the block diagram of FIG. 3. 
Describing this subroutine, first in a step S131, the vehicle speed VSP and 
a rotation speed Ni of the input disk 1 of the continuously variable 
transmission are read. 
In a step S1232, a first proportional control feedback gain fbpDATA1, first 
integral control feedback gain fbiDATA1 and first differential control 
feedback gain fbdDATA1 are calculated by looking up the maps stored 
beforehand in the controller 61 as mentioned above based on VSP and Ni. 
In a step S133, the oil temperature TMP and the line pressure PL are read. 
In a step S134, a second proportional control feedback gain fbpDATA2, 
second integral control feedback gain fbiDATA2 and second differential 
control feedback gain fbdDATA2 are calculated by looking up the maps 
stored beforehand in the controller 61 as mentioned above based on TMP and 
PL. 
In a step S135, the proportional control feedback gain fbpDATA, integral 
control feedback gain fbiDATA and differential control feedback gain 
fbdDATA are calculated by multiplying the first gains by corresponding 
second gains. 
FIG. 8 shows a subroutine for calculating the speed ratio feedback 
correction amount FBrto due to PID control, and the limited speed ratio 
feedback correction amount LmFBrto. 
This subroutine is equivalent to the functions of the PID control unit 84 
in the block diagram of FIG. 3. 
In this subroutine, in a step S141, the speed ratio deviation RtoERR and 
its differential value 
##EQU4## 
both of which were calculated by the subroutine of FIG. 6, are read. 
In a next step S142, the feedback gains fbpDATA, fbiDATA and fbdDATA which 
were found in the subroutine of FIG. 7, are read. 
In a step S143, the speed ratio feedback correction amount FBrto is 
calculated by the following equation. 
##EQU5## 
After calculating the speed ratio feedback correction amount FBrto in the 
step S93 by using the subroutines of FIGS. 6-9, the main routine proceeds 
to a step S94. 
Herein, the compensated transient target speed ratio DsrRTO is computed by 
the following equation. 
This is equivalent to the function of the adder 85 in the block diagram of 
FIG. 3. 
EQU DsrRTO=RatioO+TSrto+FBrto 
where, RatioO=transient target speed ratio, 
TSrto=torque shift error correction value, and 
FBrto=speed ratio feedback correction amount. 
In a following step S95, the target number of steps DsrSTP of the step 
motor 4 for attaining the compensated transient target speed ratio DsrRTO 
is calculated by looking up the map as mentioned above. This step is 
equivalent to the function of the target step number calculating unit 86 
in the block diagram of FIG. 3. 
In a following step S96, the physical operating limit rate of the step 
motor 4 is determined based on the oil temperature TMP of the 
transmission. This step is equivalent to the function of the step motor 
drive rate determining unit 88 in the block diagram of FIG. 3. 
In a final step S97, the command signal Astep is calculated by correcting 
the target step number DsrSTP calculated in the step S95 based on the 
physical operating limit rate determined in the step S96. 
Further, the command signal Astep is limited by upper and lower limiting 
values which define the aforesaid allowable range of the command signal 
Astep based on the physical operation limit of the step motor 4. These 
upper and lower limiting values are calculated by a subroutine shown in 
FIG. 10. 
The controller then outputs the command signal Astep after the limitation 
to the step motor 4 and the main routine is terminated. This step is 
equivalent to the function of the step motor drive position command 
calculating unit 87 and the step motor drive position command limiting 
unit 89 in the block diagram of FIG. 3. 
Next, the subroutine of FIG. 10 will be described by referring to a command 
signal initializing routine shown in FIG. 9. 
The command signal initializing routine of FIG. 9 is a routine for 
performing an initialization of the command signal such that it coincides 
with the actual operation position of the step motor 4. This routine is 
different from the other routine and subroutines in that it is performed 
only once immediately after power supply to the controller 61 is started. 
The routine determines in a step S151 if the vehicle is running by the 
following method. When either of the conditions that the vehicle speed VSP 
is not less than a predetermined vehicle speed and that the input rotation 
speed Ni detected by the rotation speed sensor 64 is not less than a 
predetermined rotation speed is satisfied, the routine determines that the 
vehicle is running. 
This routine is performed immediately after power supply to the controller 
61 is started before performing the other routine and subroutines. So the 
decision that the vehicle is running means that the power supply to the 
controller 61 is started while the vehicle is running. This situation 
occurs when the power supply to the controller 61 has stopped while the 
vehicle is running and is subsequently resumed. 
When the determination result in the step S151 is affirmative, the 
initialization of the command signal based on the actual operation 
position of the step motor 4 is performed in a step S152. As mentioned in 
the background of the invention, this initialization involves the process 
of estimating the actual operation position of the step motor 4 form the 
real speed ratio Ratio of the transmission and modulating the command 
signal so as to coincide with the actual operation position of the step 
motor 4. The initialization process is identical to that of the aforesaid 
prior art Tokkai Hei 8-178063. After performing this initialization, a 
flag FLAGINI is set equal to 1 and the routine is terminated. Since the 
flag FLAGINI is reset to 0 when power supply to the controller 61 is 
started, the initial value of the flag FLAGINI is 0. 
When the determination result in the step S151 is negative, the routine 
skips the step S151 and is immediately terminated. In this case, another 
initializing routine of the command signal specifically for the case where 
the vehicle is not running is performed before performing the routine and 
subroutines of FIGS. 4-8 and 10. Since this initialization routine is also 
known by the aforesaid prior art Tokkai Hei 8-178063, the explanation of 
the routine is omitted. 
The controller determines the upper limiting value and the lower limiting 
value of the command signal Astep according to the value of the flag 
FLAGINI by the subroutine of FIG. 10. 
The subroutine first reads the command signal Astep in a step S161. 
In a next step S162, it is determined if the FLAGIN is equal to 1. 
When the flag FLAGIN is equal to 1, the subroutine proceeds to a step S163, 
and when the flag FLAGIN is not equal to 1, the subroutine proceeds to a 
step S164. 
In the step S163, the command signal Astep is compared with an upper 
limiting value RTO1 and a lower limiting value RTO2 for the normal 
operation. These limiting values correspond to the physical operation 
limits of the step motor 4. 
When the command signal Astep is smaller than the lower limiting value 
RTO2, it is corrected to be equal to RTO2 in a step S165. When the command 
signal Astep is a value between the lower limiting value RTO2 and the 
upper limiting value RTO1, the subroutine does not correct the command 
signal Astep. When the command signal Astep is larger than the upper 
limiting value RTO1, it is corrected to be equal to RTO1. 
In the step S164, the command signal Astep is compared with an upper 
limiting value INITRTO1 and a lower limiting value INITRTO2 which are the 
limiting values specifically defined for the case where the initialization 
of the command signal is performed while the vehicle is running. The upper 
limiting value INITRTO1 is set smaller than the upper limiting value RTO1 
for the normal operation, and the lower limiting value INITRTO2 is set 
larger than the lower limiting value RTO2. 
When the command signal Astep is smaller than the lower limiting value 
INITRTO2, the command value Astep is corrected to be equal to INITRTO2 in 
a step S168. When the command value Astep is a value between the lower 
limiting value INITRTO2 and the upper limiting value INITRTO1, the 
subroutine does not correct the command signal Astep. When the command 
signal Astep is larger than the upper limiting value INITRTO1, it is 
corrected to be equal to INITRTO1 in a step S170. 
After the command value Astep is thus limited, the subroutine proceeds to a 
step S171. Herein, it is determined if the vehicle is running from the 
speed VSP. When the vehicle is not running, the flag FLAGIN is reset to 0 
in a step S172 and the subroutine is terminated. When the vehicle is 
running, the subroutine is immediately terminated without resetting the 
flag FLAGIN. 
When power supply to the controller 61 is once stopped and resumed while 
the vehicle is running, therefore, the allowable range of the command 
signal Astep is narrowed until the vehicle stops. When power supply is 
resumed, the controller first initialize the command signal Astep with 
respect to the actual operation position of the step motor 4, but since 
the vehicle is running, the initialization result may not be very accurate 
because the aforesaid torque shift error is introduced in the 
initialization. However, the allowable range of the command signal Astep 
is narrowed until the vehicle stops, the command signals Astep output to 
the step motor 4 will not command the operation of the step motor 4 beyond 
its physical operation limits. 
When the command signal Astep exceeds the physical operation limits of the 
step motor 4, a deviation of the actual operation position of the step 
motor 4 from the command signal Astep appears and a driver of the vehicle 
may experience some discomfort due to this deviation. Since the effect of 
this deviation is more conspicuous to the driver when the speed ratio is 
large than when it is small, it is also possible to set the upper limiting 
value INITRTO1 smaller than the upper limiting value RTO1 for the normal 
operation while the lower limiting value INITRTO2 is set equal to the 
lower limiting value RTO2 for the normal operation. 
The contents of Tokugan Hei 10-225892, with a filing date of Aug. 10, 1998 
in Japan, are hereby incorporated by reference. 
Although the invention has been described above by reference to certain 
embodiments of the invention, the invention is not limited to the 
embodiments described above. Modifications and variations of the 
embodiments described above will occur to those skilled in the art, in 
light of the above teachings. 
For example, in the above embodiments, the oil pressure control valve is 
driven by a rotational step motor, but this invention may be applied to 
the case where the oil pressure control valve is driven by a linear step 
motor or servo motor associated with a pulse encoder. 
The embodiments of this invention in which an exclusive property or 
privilege is claimed are defined as follows: