U.S. patent application number 09/756234 was filed with the patent office on 2001-08-02 for shift control apparatus for continuously variable transmission of motor vehicle.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Hattori, Yuji, Inoue, Takashi, Kono, Katsumi, Matsuo, Kenji, Taniguchi, Hiroji.
Application Number | 20010011051 09/756234 |
Document ID | / |
Family ID | 18545502 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010011051 |
Kind Code |
A1 |
Hattori, Yuji ; et
al. |
August 2, 2001 |
Shift control apparatus for continuously variable transmission of
motor vehicle
Abstract
A shift control apparatus for a motor vehicle including an
Internal combustion engine and a continuously variable transmission
(CVT) is provided for controlling the speed ratio of the CVT
depending upon operating conditions of the vehicle. The control
apparatus determines whether an electronic throttle valve of the
engine is at fault or not, and restricts the speed ratio of the CVT
when the throttle valve is at fault such that the input-side
rotation speed of the transmission is variable within a range that
is narrower than a range in which the rotation speed is variable
during normal running of the vehicle. The range of the input-side
rotation speed may be defined by the upper limit, or the lower
limit, or both the upper limit and the lower limit.
Inventors: |
Hattori, Yuji; (Bisai-shi,
JP) ; Matsuo, Kenji; (Toyota-shi, JP) ;
Taniguchi, Hiroji; (Okazaki-shi, JP) ; Inoue,
Takashi; (Nissin-shi, JP) ; Kono, Katsumi;
(Toyota-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, McCLELLAND, MAIER & NEUSTADT, P.C.
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-cho,
Toyota-shi
JP
|
Family ID: |
18545502 |
Appl. No.: |
09/756234 |
Filed: |
January 9, 2001 |
Current U.S.
Class: |
477/121 ;
477/125; 477/49 |
Current CPC
Class: |
F16H 61/66254 20130101;
F16H 61/12 20130101; Y10S 477/906 20130101; F16H 2061/124 20130101;
F16H 2061/1208 20130101; F16H 59/24 20130101; F16H 2061/166
20130101; F16H 61/66259 20130101 |
Class at
Publication: |
477/121 ;
477/125; 477/49 |
International
Class: |
B60K 041/12; F16H
059/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 27, 2000 |
JP |
2000-018688 |
Claims
What is claimed is:
1. A shift control apparatus for a motor vehicle including an
internal combustion engine whose output is controlled by an
electronic throttle valve for adjusting the flow rate of intake
air, and a continuously variable transmission disposed in a power
transmission path between the internal combustion engine and drive
wheels, and operable to change a speed ratio thereof, comprising: a
shift control device configured and connected to control the speed
ratio of the continuously variable transmission depending upon
operating conditions of the vehicle, such that an input-side
rotation speed of the transmission is variable within a first range
during normal running of the vehicle; a fault detecting unit
configured and connected to determine whether the electronic
throttle valve is at fault or not; and a restricting unit
configured and connected to restrict the speed ratio of the
continuously variable transmission such that the input-side
rotation speed of the transmission is variable within a second
range when the fault detecting unit determines that the electronic
throttle valve is at fault, said second range being set to be
narrower than said first range.
2. A shift control apparatus according to claim 1, wherein said
restricting unit comprises an upper-limit setting unit that sets an
upper limit of the input-side rotation speed of the continuously
variable transmission, and restricts the speed ratio of the
transmission so that the input-side rotation speed does not exceed
the upper limit when the electronic throttle valve is faulty.
3. A shift control apparatus according to claim 2, wherein the
upper limit of the input-side rotation speed at the time of a
failure in the electronic throttle valve is approximately equal to
a rotation speed at which the output of the internal combustion
engine reaches a maximum thereof at a throttle opening of the
electronic throttle valve that is established when the electronic
throttle valve is faulty.
4. A shift control apparatus according to claim 1, wherein said
restricting unit comprises a lower-limit setting unit that sets a
lower limit of the input-side rotation speed of the continuously
variable transmission, and restricts the speed ratio of the
transmission so that the input-side rotation speed does not fall
below the lower limit when the electronic throttle valve is
faulty.
5. A shift control apparatus according to claim 4, wherein the
vehicle further includes a brake booster that utilizes an intake
manifold negative pressure of the internal combustion engine so as
to assist in a braking operation, and wherein the lower limit of
the input-side rotation speed at the time of a failure in the
electronic throttle valve is approximately equal to a rotation
speed at which a certain intake manifold negative pressure required
for operating the brake booster is obtained by an operation of the
engine at a throttle opening of the electronic throttle valve that
is established when the electronic throttle valve is faulty.
6. A shift control apparatus according to claim 1, wherein said
restricting unit comprises an upper-limit and lower-limit setting
unit that sets an upper limit and a lower limit of the input-side
rotation speed of the continuously variable transmission, and
restricts the speed ratio of the transmission so that the
input-side rotation speed is held between the upper limit and the
lower limit when the electronic throttle valve is faulty.
7. A shift control apparatus according to claim 6, wherein the
upper limit of the input-side rotation speed at the time of a
failure in the electronic throttle valve is approximately equal to
a rotation speed at which the output of the internal combustion
engine reaches a maximum thereof at a throttle opening of the
electronic throttle valve that is established when the electronic
throttle valve is faulty.
8. A shift control apparatus according to claim 6, wherein the
vehicle further includes a brake booster that utilizes an intake
manifold negative pressure of the internal combustion engine so as
to assist in a braking operation, and wherein the lower limit of
the input-side rotation speed at the time of a failure in the
electronic throttle valve is approximately equal to a rotation
speed at which a certain intake manifold negative pressure required
for operating the brake booster is obtained by an operation of the
engine at a throttle opening of the electronic throttle valve that
is established when the electronic throttle valve is faulty.
9. A shift control apparatus according to claim 1, further
comprising: a target speed setting unit that sets a target
input-side rotation speed of the continuously variable transmission
based on operating conditions of the vehicle, said target
input-side rotation speed being variable within the first range
during normal running of the vehicle; wherein said shift control
device controls the speed ratio of the continuously variable
transmission so that the input-side rotation speed of the
continuously variable transmission becomes substantially equal to
the target input-side rotation speed of the continuously variable
transmission; wherein said restricting unit restricts the speed
ratio of the continuously variable transmission such that the
target input-side rotation speed of the transmission is variable
within the second range when the fault detecting unit determines
that the electronic throttle valve is faulty.
10. A shift control apparatus according to claim 9, wherein said
restricting unit comprises an upper-limit setting unit that sets an
upper limit of the target input-side rotation speed of the
continuously variable transmission, and restricts the speed ratio
of the transmission so that the target input-side rotation speed
does not exceed the upper limit when the electronic throttle valve
is faulty.
11. A shift control apparatus according to claim 9, wherein said
restricting unit comprises a lower-limit setting unit that sets a
lower limit of the target input-side rotation speed of the
continuously variable transmission, and restricts the speed ratio
of the transmission so that the target input-side rotation speed
does not fall below the lower limit when the electronic throttle
valve is faulty.
12. A shift control apparatus according to claim 9, wherein said
restricting unit comprises an upper-limit and lower-limit setting
unit that establishes an upper limit and a lower limit of the
target input-side rotation speed of the continuously variable
transmission, and restricts the speed ratio of the transmission so
that the target input-side rotation speed is held between the upper
limit and the lower limit when the electronic throttle valve is
faulty.
13. A method of controlling a continuously variable transmission of
a motor vehicle which includes an internal combustion engine whose
output is controlled by an electronic throttle valve for adjusting
the flow rate of intake air, and a continuously variable
transmission disposed in a power transmission path between the
internal combustion engine and drive wheels and operable to change
a speed ratio thereof, comprising the steps of: controlling the
speed ratio of the continuously variable transmission depending
upon operating conditions of the vehicle, such that an input-side
rotation speed of the transmission is variable within a first range
during normal running of the vehicle; determining whether the
electronic throttle valve is faulty or not; and restricting the
speed ratio of the continuously variable transmission such that the
input-side rotation speed of the transmission is variable within a
second range when the electronic throttle valve is faulty, said
second range being set to be narrower than said first range.
14. A method according to claim 13, wherein said step of
restricting the speed ratio comprises a sub-step of setting an
upper limit of the input-side rotation speed of the continuously
variable transmission, and a sub-step of restricting the speed
ratio of the transmission so that the input-side rotation speed
does not exceed the upper limit when the electronic throttle valve
is faulty.
15. A method according to claim 14, wherein the upper limit of the
input-side rotation speed at the time of a failure in the
electronic throttle valve is approximately equal to a rotation
speed at which the output of the internal combustion engine reaches
a maximum thereof at a throttle opening of the electronic throttle
valve that is established when the electronic throttle valve is
faulty.
16. A method according to claim 13, wherein said step of
restricting the speed ratio comprises a sub-step of setting a lower
limit of the input-side rotation speed of the continuously variable
transmission, and a sub-step of restricting the speed ratio of the
transmission so that the input-side rotation speed does not fall
below the lower limit when the electronic throttle valve is
faulty.
17. A method according to claim 16, wherein the vehicle further
includes a brake booster that utilizes an intake manifold negative
pressure of the internal combustion engine so as to assist in a
braking operation, and wherein the lower limit of the input-side
rotation speed at the time of a failure in the electronic throttle
valve is approximately equal to a rotation speed at which a certain
intake manifold negative pressure required for operating the brake
booster is obtained by an operation of the engine at a throttle
opening of the electronic throttle valve that is established when
the electronic throttle valve is faulty.
18. A method according to claim 13, wherein said step of
restricting the speed ratio comprises a sub-step of setting an
upper limit and a lower limit of the input-side rotation speed of
the continuously variable transmission, and a sub-step of
restricting the speed ratio of the transmission so that the
input-side rotation speed is held between the upper limit and the
lower limit when the electronic throttle valve is faulty.
19. A method according to claim 18, wherein the upper limit of the
input-side rotation speed at the time of a failure in the
electronic throttle valve is approximately equal to a rotation
speed at which the output of the internal combustion engine reaches
a maximum thereof at a throttle opening of the electronic throttle
valve that is established when the electronic throttle valve is
faulty.
20. A method according to claim 18, wherein the vehicle further
includes a brake booster that utilizes an intake manifold negative
pressure of the internal combustion engine so as to assist in a
braking operation, and wherein the lower limit of the input-side
rotation speed at the time of a failure in the electronic throttle
valve is approximately equal to a rotation speed at which a certain
intake manifold negative pressure required for operating the brake
booster is obtained by an operation of the engine at a throttle
opening of the electronic throttle valve that is established when
the electronic throttle valve is faulty.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2000-018688 filed on Jan. 27, 2000 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a shift control apparatus for a
continuously variable transmission of a motor vehicle. In
particular, the invention is concerned with shift control of a
continuously variable transmission when an electronic throttle
valve is faulty.
[0004] 2. Description of the Related Art
[0005] There is known a shift control apparatus for a motor vehicle
including (a) an internal combustion engine whose output is
controlled by an electronic throttle valve for adjusting the flow
rate of intake air, and (b) a continuously variable transmission
disposed in a power transmission path between the engine and drive
wheels, for changing the speed ratio. The shift control apparatus
is adapted to control the speed ratio of the continuously variable
transmission in accordance with operating or driving conditions of
the vehicle. In general, shift control of the continuously variable
transmission is performed according to predetermined shift
conditions in the form of, for example, a map, using the output
requirement indicated by the driver, such as an accelerator
operation amount, and the vehicle speed as parameters. The shift
conditions are usually determined such that the speed ratio (=the
input-side rotation speed/the output-side rotation speed) increases
with an increase in the output requirement and with a reduction in
the vehicle speed.
[0006] When the electronic throttle valve fails, the output of the
engine cannot be controlled with the opening of the throttle valve
being maintained at a certain degree, but it has been proposed to
perform special shift control that enables "limp-home", namely,
permits the vehicle to limp or travel to a nearby repair shop, or
the like. For example, an apparatus as disclosed in Japanese Patent
No. 2616154 is adapted to perform shift control solely based on the
output requirement made by the driver (i.e., the power demanded by
the driver) when the throttle valve is faulty.
[0007] If shift control is performed solely based on the output
requirement, and if the output requirement is kept large when the
driver continues to depress the accelerator pedal in an attempt to
increase the vehicle speed, for example, the rotation speed of the
engine increases as a result of an increase in the speed ratio of
the continuously variable transmission. As a result, the torque,
and accordingly the output, are reduced, which produces a reduction
or deterioration in the running performance (driving torque).
Namely, even if the valve opening of the electronic throttle valve
is kept constant, the output of the engine changes in accordance
with the rotation speed, as shown in FIG. 7 by way of example, such
that the engine output is gradually reduced after it reaches its
maximum at a certain engine speed (NE.sub.P in FIG. 7). Thus, even
if the torque is amplified in accordance with the speed ratio of
the continuously variable transmission, a sufficiently large
driving torque may not be obtained.
[0008] In the meantime, where a brake booster is provided for
assisting the braking force by use of the intake manifold negative
pressure of the engine, the intake manifold negative pressure is
reduced when the electronic throttle valve fails and is maintained
at a predetermined valve opening (e.g., when
.theta..sub.th=.theta..sub.thF), as compared with the case where
the electronic throttle valve is completely closed
(.theta..sub.th=0%), as shown in FIG. 6. Also, in this case, the
intake manifold negative pressure tends to be reduced with a
reduction in the engine speed (NE). Thus, if the rotation speed of
the engine is reduced in accordance with a reduction in the vehicle
speed when the vehicle is stopped, for example, a required booster
pressure (e.g., the minimum booster pressure PB) cannot be
obtained, with the result that the brake assist force is reduced,
and the driver feels uncomfortable when applying a brake to the
vehicle.
[0009] Here, it is to be understood that the magnitude of the
negative pressure is considered being opposite to that of the
positive pressure. Namely, the reduction in the negative pressure
means that the pressure increases to be close to the atmospheric
pressure (at which the negative pressure is equal to zero).
SUMMARY OF THE INVENTION
[0010] It is an object of the invention to provide a shift control
apparatus that performs shift control on a continuously variable
transmission, while avoiding reductions in the driving torque or
the assist force of a brake booster due to a failure in an
electronic throttle valve, and thus preventing the driver from
feeling uncomfortable due to the reduction in the torque or brake
assist force.
[0011] To accomplish the above and other objects, the invention
provides a shift control apparatus for a motor vehicle including an
internal combustion engine whose output is controlled by an
electronic throttle valve for adjusting the flow rate of intake
air, and a continuously variable transmission disposed in a power
transmission path between the internal combustion engine and drive
wheels and operable to change a speed ratio thereof, comprising a
shift control device that controls the speed ratio of the
continuously variable transmission depending upon operating
conditions of the vehicle such that an input-side rotation speed of
the transmission is variable within a first range during normal
running of the vehicle; a fault detecting unit that determines
whether the electronic throttle valve is faulty or not; and a
restricting unit that restricts the speed ratio of the continuously
variable transmission such that the input-side rotation speed of
the transmission is variable within a second range when the fault
detecting unit determines that the electronic throttle valve is
faulty, the second range being set to be narrower than the first
range.
[0012] In one preferred form of the invention, the restricting unit
comprises an upper-limit setting unit that sets an upper limit of
the input-side rotation speed of the continuously variable
transmission and restricts the speed ratio of the transmission so
that the input-side rotation speed does not exceed the upper limit
when the electronic throttle valve is faulty.
[0013] In the above form of the invention, since the speed ratio is
restricted so that the input-side rotation speed does not exceed
the upper limit when the electronic throttle valve is faulty, the
rotation speed of the engine is also controlled to be equal to or
lower than a predetermined value that corresponds to the upper
limit of the input-side rotation speed. If the upper limit of the
input-side rotation speed is set in the vicinity of a rotation
speed at which the output of the internal combustion engine reaches
its maximum, an excessive increase in the engine speed and a
resulting reduction in the output can be advantageously prevented.
Thus, the upper limit of the input-side rotation speed can be
suitably determined in view of the problems associated with a
failure in the throttle valve, thereby to cause the engine to
operate at desired operating conditions.
[0014] In another preferred form of the invention, the restricting
unit comprises a lower-limit setting unit that sets a lower limit
of the input-side rotation speed of the continuously variable
transmission, and restricts the speed ratio of the transmission so
that the input-side rotation speed does not fall below the lower
limit when the electronic throttle valve is faulty.
[0015] In the above form of the invention, since the speed ratio of
the transmission is restricted so that the input-side rotation
speed does not fall below the lower limit when the electronic
throttle valve is faulty, the engine speed is also controlled to be
equal to or higher than a predetermined value corresponding to the
lower limit of the input-side rotation speed. If the lower limit of
the input-side rotation speed is set in the vicinity of a rotation
speed at which a certain intake manifold negative pressure required
for operating the brake booster is obtained by an operation of the
engine, an excessive reduction in the engine speed and a resulting
reduction in the booster pressure can be advantageously prevented.
Thus, the lower limit of the input-side rotation speed can be
suitably determined in view of the problems associated with a
failure in the throttle valve, thereby to cause the engine to
operate at desired operating conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic view of a drive unit of a vehicle to
which the invention is applied.
[0017] FIG. 2 is a block diagram useful for explaining a shift
control apparatus for a continuously variable transmission in the
drive unit shown in FIG. 1.
[0018] FIG. 3 is a hydraulic circuit diagram showing an example of
the shift control circuit as shown in FIG. 2.
[0019] FIG. 4 is a graph showing an example of shift conditions
that are used when an NINT calculating unit of FIG. 2 calculates a
target rotation speed NINT.
[0020] FIG. 5 is a flowchart useful for explaining the operations
to be performed by the restricting unit of FIG. 2.
[0021] FIG. 6 is a graph showing an example of torque
characteristics of an engine when an electronic throttle valve is
faulty.
[0022] FIG. 7 is a graph showing an example of output
characteristics of the engine when the electronic throttle valve is
faulty.
[0023] FIG. 8 shows an example of negative pressure characteristics
of the intake manifold of the engine when the electronic throttle
valve is faulty.
[0024] FIG. 9 shows an example of upper limit NINTUG and lower
limit NINTLG between which the target rotation speed NINT is
restricted by the restricting unit of FIG. 2.
[0025] FIG. 10 is a graph showing, by way of example, the
relationship between the throttle command value S.sub.th and the
accelerator operation amount .theta..sub.ACC.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] In principle, when an electronic throttle valve fails, a
shift control apparatus according to the invention performs shift
control on a continuously variable transmission based on an output
requirement made by the driver, such as an operation amount of an
accelerator pedal, or an operating status of a brake, or the like,
as disclosed, for example, in Japanese Patent No. 2616154. However,
the shift control apparatus may perform shift control in various
other fashions; for example, even in the event of a failure in an
electronic throttle valve, the apparatus may perform shift control
using shift conditions (such as a map) for normal running, in which
vehicle operating conditions, such as an output requirement and a
vehicle speed, are used as parameters. More specifically, the shift
control apparatus may calculate a target speed ratio and control an
actual speed ratio to be equal to the target speed ratio. In
another method, the shift control apparatus may calculate a target
rotation speed of the input shaft of the transmission based on a
vehicle speed, actual rotation speed of the output shaft of the
transmission and others, and control an actual rotation speed of
the input shaft to be equal to the target rotation speed. Since the
target input-shaft speed corresponds to the target speed ratio,
there is no need to obtain the target speed ratio itself.
[0027] A failure in the electronic throttle valve may be a
mechanical failure in the throttle valve itself or a drive unit
(such as an electric motor), or may be an electric failure in a
control system for controlling opening/closing of the throttle
valve. In short, the failure in the throttle valve may be of any
type, provided the internal combustion engine may operate with a
certain throttle opening (a certain degree of opening of the
valve), though the throttle opening cannot be controlled. In the
event of an electric failure, for example, it is desirable to fix
or set the throttle opening to a predetermined degree (provided for
the occasion of a failure), by means of a spring or the like.
However, the invention can also be applied to the case where the
electronic throttle valve becomes uncontrollable at an arbitrary
throttle opening. In this case, the throttle opening at the time of
a failure may be detected using a sensor or the like. The
predetermined throttle opening for the occasion of a failure, in
other words the "limp home" position of the throttle valve, is
desirably set, for example, within a range of 5% to 20%, depending
upon the output characteristics of the engine and others, so as to
provide certain running performance required for limp-home without
significantly affecting braking operations.
[0028] The upper and lower limits of the input-side rotation speed
at the time of a failure are suitably set based on the output
characteristics, torque characteristics and the intake manifold
negative-pressure characteristics of the engine, which are
determined in accordance with the throttle opening of the
electronic throttle valve at the time of the failure. Where the
throttle valve is fixed at a predetermined throttle opening (limp
home position), the upper and lower limits may be set to
predetermined values. If the electronic throttle valve becomes
uncontrollable at an arbitrary throttle opening, the upper and
lower limits may be set in accordance with the actual throttle
opening at the time of failure, based on data maps or arithmetic
expressions using the throttle opening as a parameter. The upper
and lower limits may also be changed as needed, using various
operating conditions, such as a vehicle speed, as parameters.
[0029] Where the upper limit of the input-side rotation speed at
the time of a failure is to be defined, the upper limit may be set
to a speed that allows the engine to produce approximately the
maximum output. However, where the output of the engine changes at
a relatively low rate in the vicinity of its maximum or peak, with
respect to the revolution speed of the engine, for example, the
upper limit is not necessarily to be set to the speed at which the
output reaches its maximum. Rather, the upper limit may be set in
accordance with the output characteristics, for example, within
.+-.10%, preferably .+-.5%, of the rotation speed at which the
maximum output is obtained. The upper limit may also be set within
a speed range in which 80% or more, or 90% or more, of the maximum
output can be obtained. The upper limit may be set using operating
conditions such as a vehicle speed, as parameters, within a certain
range in which the output almost reaches its maximum. In another
form of the invention, the upper limit of the input-side rotation
speed is not necessarily set to the speed at which the output of
the engine almost reaches its maximum. For example, the upper limit
may be set to a speed that allows the engine to provide an output
required for limp-home. The upper limit of the input-side rotation
speed at the time of a failure may also be defined in the case
where any problem arises from an increase in the engine speed for
any reason other than a reduction in the engine output.
[0030] Where the lower limit of the input-side rotation speed at
the time of a failure is to be defined, the lower limit may be set
to a speed that can provide the lowest or minimum booster pressure
required for causing a brake booster to produce brake assist force.
However, the lower limit may be set to a speed that can provide a
certain booster pressure that results in a certain brake assist
force. As in the case of the upper limit, the lower limit may be
set within a suitable range using operating conditions such as a
vehicle speed as parameters. In another form of the invention, the
lower limit of the input-side rotation speed is not necessarily set
in terms of the brake booster, but may be set in terms of other
boosters or devices utilizing the intake manifold negative pressure
or vacuum. The lower limit of the input-side rotation speed at the
time of a failure may also be defined in the case where any problem
arises from a reduction in the engine speed, for any reason other
than a reduction in the intake manifold negative pressure.
[0031] The internal combustion engine may be a gasoline engine or a
diesel engine that operates through combustion of fuel. In order to
ensure that the engine can continue to operate even if the vehicle
stops, a fluid coupling such as a torque converter, a friction-type
clutch capable of continuously controlling torque to be
transmitted, or the like, is disposed between the engine and a
continuously variable transmission. However, these components are
not necessarily provided in a hybrid vehicle that is able to start
with an electric motor. As the continuously variable transmission
to be controlled by the shift control apparatus of the invention,
various types of transmissions, such as a belt-type CVT and a
toroidal-type CVT, may be employed.
[0032] A presently preferred embodiment of the invention will be
described in detail with reference to the drawings.
[0033] FIG. 1 is a schematic view of a vehicular drive unit 10 to
which the invention is applied. The vehicular drive unit 10, which
is of the transversely-mounted type, is favorably employed in a
front-engine front-drive vehicle. The drive unit 10 includes an
engine 12, such as an internal combustion engine, used as a driving
source for running. The output of the engine 12 is transmitted from
a torque converter 14 to a differential gear device 22 via a
forward/backward drive switching device 16, a belt-type
continuously variable transmission (CVT) 18 and reduction gears 20.
The output transmitted to the differential gear device 22 is then
distributed to left and right drive wheels 24L, 24R.
[0034] The output of the engine 12 is controlled by an electronic
throttle valve 26 for adjusting the flow rate of intake air. The
throttle opening .theta..sub.th of the electronic throttle valve 26
is adjusted by means of an electric motor according to a throttle
command value S.sub.th generated by a controller in accordance with
an operation amount .theta..sub.ACC of the accelerator pedal, as
shown in FIG. 10 by way of example. The operation amount
.theta..sub.ACC of the accelerator pedal represents the output
requirement indicated by the driver, or the power required by the
driver. If the electric motor stops operating due to a failure in
the control system, the electronic throttle valve 26 is fixed to a
predetermined throttle opening .theta..sub.thF provided for the
occasion of a failure by a mechanical device, such as a spring. The
throttle opening .theta..sub.thF at the time of a failure is set,
for example, to about 10%, depending upon the output
characteristics of the engine 12, so that the vehicle exhibits
suitable running performance that enables "limp-home" without
greatly affecting braking operations.
[0035] A brake booster 30 is connected to an intake manifold 28 of
the engine 12. With this arrangement, the force applied to a brake
pedal 32 (the braking force) is assisted by a negative pressure in
the intake manifold 28.
[0036] The forward/backward drive switching device 16 consists of a
planetary gear unit of the double-pinion type. A turbine shaft 34
of the torque converter 14 is connected to a sun gear 16s, and an
input shaft 36 of the continuously variable transmission 18 is
connected to a carrier 16c. If a clutch 38 disposed between the
carrier 16c and the sun gear 16s is engaged, the forward/backward
drive switching device 16 is rotated as a unit, so that the turbine
shaft 34 is directly connected to the input shaft 36 and the
forward driving force is transmitted to the drive wheels 24R, 24L.
If a brake 40 disposed between a ring gear 16r and a housing is
engaged while the clutch 38 is released, the input shaft 36 is
rotated in a reverse direction with respect to the turbine shaft
34, so that a backward driving force is transmitted to the drive
wheels 24R, 24L.
[0037] The continuously variable transmission 18 includes an
input-side variable pulley 42, an output-side variable pulley 46
and a drive belt 48 that is wound around the variable pulleys 42,
46. The input-side variable pulley 42 is mounted on the input shaft
36 and has a variable effective diameter. The output-side variable
pulley 46 is mounted on the output shaft 44 and has a variable
effective diameter. The variable pulleys 42, 46 have variable
V-groove widths. The hydraulic pressure to be applied to the
input-side variable pulley 42 is controlled, for example, by a
shift control circuit 50 as shown in FIG. 3. With the hydraulic
pressure thus controlled, the V-groove widths of the variable
pulleys 42. 46 and the winding diameter (the effective diameter) of
the drive belt 48 are changed. As a result, the speed ratio .gamma.
(=the input-side rotation speed NIN/the output-side rotation speed
NOUT) is continuously changed.
[0038] The shift control circuit 50 as shown in FIG. 3 includes an
upshift solenoid-operated valve 52, a flow control valve 54, a
downshift solenoid-operated valve 56 and a flow control valve 58.
The upshift solenoid-operated valve 52 and the flow control valve
54 are operable to reduce the speed ratio .gamma.. The downshift
solenoid-operated valve 56 and the flow control valve 58 are
operable to increase the speed ratio .gamma.. The shift control
circuit 50 operates in a similar manner to that as disclosed in
Japanese Patent Laid-Open Publication No. HEI 11-182657. If the
duty ratio of the upshift solenoid-operated valve 52 is controlled
by a shift controller 60 (see FIG. 2), a certain control pressure,
to which the modulator pressure PM is reduced, is transmitted to
the flow control valve 54. The line pressure PL is regulated in
response to the control pressure and then supplied to the
input-side variable pulley 42, which results in a reduction in the
V-groove width of the input-side variable pulley 42, and a
reduction in the speed ratio .gamma.. If the duty ratio of the
downshift solenoid-operated valve 56 is controlled by the shift
controller 60, a certain control pressure, to which the modulator
pressure PM is reduced, is transmitted to the flow control valve
58. A drain port is opened in response to the control pressure,
whereby working fluid in the input-side variable pulley 42 is
drained at a suitable flow rate. As a result, the V-groove width is
increased, resulting in an increase in the speed ratio .gamma.. The
hydraulic pressure applied to the output-side variable pulley 46 is
regulated, for example, in accordance with the torque to be
transmitted by the CVT, so that a desired belt tension is
obtained.
[0039] The shift controller 60 shown in FIG. 2 includes a
microcomputer. The shift controller 60 performs signal processing
according to programs prestored in a ROM, utilizing the temporary
storage function of a RAM, so as to perform shift control on the
continuously variable transmission 18. The shift controller 60 is
functionally provided with an NINT calculating unit 62, a
restricting unit 64, a comparing unit 65 and a feedback control
unit 66. The shift controller 60 constitutes a principal part of
the shift control apparatus.
[0040] Signals indicative of the operation amount .theta..sub.ACC
of the accelerator pedal and the vehicle speed V (more
specifically, the rotation speed NOUT of the output shaft 44) are
supplied to the NINT calculating unit 62 from an accelerator sensor
68 and a vehicle speed sensor 70, respectively. The NINT
calculating unit 62 calculates a target rotation speed NINT, which
is a target value of the input-side rotation speed NIN, according
to shift conditions that are determined using operation states of
the vehicle, such as the accelerator operation amount
.theta..sub.ACC and the vehicle speed V, as parameters. The shift
conditions are defined by data maps, arithmetic expressions, or the
like, such that the target rotation speed NINT is set in such a
manner that the speed ratio .gamma. increases as the vehicle speed
V decreases and the accelerator operation amount .theta..sub.ACC
increases, as shown in FIG. 4 by way of example. The shift
conditions are stored in advance in a storage device such as ROM.
Because the vehicle speed V corresponds to the output-side rotation
speed NOUT, the target rotation speed NINT, which is a target value
of the input-side rotation speed NIN, corresponds to the target
speed ratio, and is set within a range from the minimum speed ratio
.gamma.min to the maximum speed ratio .gamma.max of the
continuously variable transmission 18. The accelerator operation
amount .theta..sub.ACC corresponds to the output requirement
indicated by the driver (the power demanded by the driver).
[0041] As shown in FIG. 4 by way of example, during normal running
of the vehicle (i.e., when the electronic throttle valve 26 is not
faulty), the speed ratio of the continuously variable transmission
18 is suitably controlled to be within a region that is defined by
the maximum speed ratio .gamma.max and the minimum speed ratio
.gamma.min. Furthermore, the target rotation speed NINT of the
input shaft of the CVT 18 is held in a range defined between the
upper limit corresponding to the maximum output requirement (where
the accelerator operation amount .theta..sub.ACC is 100%) and the
lower limit corresponding to the minimum output requirement (where
the accelerator operation amount .theta..sub.ACC is 0%).
[0042] The restricting unit 64 sets the upper and lower limits of
the target rotation speed NINT when the electronic throttle valve
26 fails, and performs signal processing according to, for example,
a flowchart as shown in FIG. 5. As will be understood later, the
upper and lower limits set by the restricting unit 64 define a
narrower or smaller range than that defined by the upper and lower
limits established for normal running of the vehicle as described
above. In step S1 of FIG. 5, it is judged whether the electronic
throttle valve 26 is faulty, for example, based on a deviation of
an actual throttle opening .theta..sub.th detected by a throttle
opening sensor 74 (see FIG. 1) from a throttle command value
S.sub.th, or depending on whether the throttle opening
.theta..sub.th substantially coincides with the preset throttle
opening .theta..sub.thF for the occasion of a failure. If the
electronic throttle valve 26 is not faulty, the routine is
immediately terminated so that the target rotation speed NINT is
outputted as it is to the comparing unit 65. However, if the
throttle opening .theta..sub.th has been fixed to the failure-time
throttle opening .theta..sub.thF due to a failure in the control
system, for example, step S2 and subsequent steps are then
executed.
[0043] In step S2, the upper limit NINTUG is set. The upper limit
NINTUG may be set, for example, to a predetermined rotation speed
NE.sub.P at which the engine output reaches its maximum, based on
the torque characteristics of the engine 12 when the throttle
opening with .theta..sub.th equal to the failure-time valve opening
.theta..sub.thF as shown in FIG. 6. In this embodiment, however,
the upper limit NINTUG is calculated from a predetermined map or
arithmetic expression(s) using a vehicle speed V and other
parameters, so as to be set within .+-.5% of the rotation speed
NE.sub.P. FIG. 7 shows a relationship between the engine output and
the engine speed NE. The engine output does not change at a
considerably high rate in the vicinity of the rotation speed
NE.sub.P (within the range of .+-.5%). If the engine speed NE
increases to be higher than .+-.5% of NE.sub.P, the rate of change
(reduction) of the engine output increases. In this embodiment,
since the engine speed NE is approximately equal to the input-side
rotation speed NIN during forward running of the vehicle (the
engine speed NE is exactly equal to the input-side rotation speed
NIN when a lock-up clutch of the torque converter 14 is ON), the
engine speed NE can be used as the input-side speed NIN. However,
if any shift operation is performed between the engine and the
continuously variable transmission 18, the engine speed NE needs to
be corrected by taking account of the speed ratio or change gear
ratio, so as to obtain the input-side rotation speed NIN. The map
and/or arithmetic expressions(s) used for calculating the upper
limit NINTUG are stored in a storage unit such as a ROM.
[0044] In step S3, the lower limit NINTLG is set. The lower limit
NINTLG may be set, for example, to a predetermined rotation speed
NE.sub.B at which the minimum booster pressure P.sub.B required for
an operation of the brake booster 30 is obtained, based on the
intake manifold negative pressure characteristics of the engine 12
(which are substantially the same as the torque characteristics)
when the throttle opening .theta..sub.th is equal to the
failure-time valve opening .theta..sub.thF as shown in FIG. 6. In
this embodiment, while the lower limit NINTLG is set to the
predetermined rotation speed NE.sub.B within the shift range of the
continuously variable transmission 18 as shown in FIG. 9, the lower
limit NINTLG increases with an increase in the vehicle speed V in a
high vehicle-speed region (FIG. 9) in which the lower limit NINTLG,
if it is kept constant, would fall below the minimum speed ratio
.gamma.min. Thus, in the high vehicle-speed region, the lower limit
NINTLG corresponds to the minimum speed ratio .gamma.min.
[0045] FIG. 8 shows a relationship between the intake manifold
negative pressure and the engine speeds NE (which is substantially
the same as FIG. 6). As the engine speed NE decreases, the intake
manifold negative pressure is reduced. Also in this case, if any
shift operation is performed between the engine 12 and the
continuously variable transmission 18, the lower limit NINTLG is
set by taking account of the speed ratio or change gear ratio. A
map and/or arithmetic expression(s) used for calculating the lower
limit NINTLG are stored in a storage unit such as a ROM. While the
upper limit NINTUG may be basically set within the range of .+-.5%
of the rotation speed NE.sub.P, it is desirable to increase the
upper limit NINTUG in accordance with an increase in the lower
limit NINTLG in the high vehicle-speed region in which the lower
limit NINTLG increases, as shown in FIG. 9.
[0046] In the next step S4, it is judged whether or not the target
rotation speed NINT is within the range from the lower limit NINTLG
to the upper limit NINTUG. If the target rotation speed NINT is
within this range, the routine is immediately terminated, and the
target rotation speed NINT is outputted as it is to the comparing
unit 65. However, if the target rotation speed NINT is below the
lower limit NINTLG or above the upper limit NINTUG, a restricting
operation is performed in step S5. More specifically, if the target
rotation speed NINT is below the lower limit value NINTLG, the
lower limit NINTLG replaces the target rotation speed NINT, namely,
the lower limit NINTLG is defined as the updated target rotation
speed NINT. Similarly, if the target rotation speed NINT is below
the upper limit NINTUG, the upper limit NINTUG replaces the target
rotation speed NINT, namely, the upper limit NINTUG is defined as
the updated target rotation speed NINT. Thus, when the electronic
throttle valve 26 is faulty, the target rotation speed NINT is
restricted to the range between the lower limit NINTLG and the
upper limit NINTUG.
[0047] Referring again to FIG. 2, the comparing unit 65 receives a
signal indicative of the actual input-side rotation speed NIN from
the input-side rotation speed sensor 72, to calculate a speed
deviation .DELTA.NIN of the actual input-side rotation speed NIN
from the target rotation speed NINT supplied from the restricting
unit 64. The comparing unit 65 then outputs the speed deviation
.DELTA.NIN to the feedback control unit 66. The input-side rotation
speed sensor 72 is disposed to detect, for example, the rotation
speed of the input-side variable pulley 42. However, as a matter of
course, the sensor 72 may also be adapted to detect the rotation
speed of the input shaft 36, or detect the rotation speed of the
turbine shaft 34, or the like, which has a predetermined
relationship with the rotation speed of the input shaft.
[0048] The feedback control unit 66 performs feedback control on
the solenoid-operated valves 52, 56 of the shift control circuit 50
so that the speed deviation .DELTA.NIN becomes equal to 0. As a
result, the actual input-side rotation speed NIN is made
approximately equal to the target rotation speed NINT.
Consequently, the speed ratio .gamma. of the continuously variable
transmission 18 is suitably controlled in accordance with the
accelerator operation amount .theta..sub.ACC and the vehicle speed
V. When the electronic throttle valve 26 is faulty, shift control
is restricted such that the input-side rotation speed NIN (the
target rotation speed NINT) of the continuously variable
transmission 18 falls within the range between the upper limit
NINTUG and the lower limit NINTLG as indicated by the hatched area
in FIG. 9. Accordingly, the engine speed NE is maintained in the
range between the upper limit NINTUG and the lower limit
NINTLG.
[0049] The upper limit NINTUG is approximately equal to the
rotation speed NE.sub.P at which the output of the engine 12
reaches its maximum at the failure-time throttle opening
.theta..sub.thF. In the shift control of the continuously variable
transmission 18, therefore, the engine speed NE of the engine 12 is
prevented from becoming excessively high with a result of a
reduction in the engine output, thus eliminating such a problem
that the driving torque of the vehicle is reduced in spite of an
increase in the accelerator operation amount .theta..sub.ACC.
Suppose that shift control is performed according to the shift
conditions as shown in FIG. 4 when the electronic throttle valve 26
is fixed to the failure-time throttle opening .theta..sub.thF in
the event of a failure. In this case, if the accelerator operation
amount .theta..sub.ACC is kept substantially equal to 100% since
the vehicle speed V does not increase, the speed ratio .gamma. of
the continuously variable transmission 18 increases, and the engine
speed NE increases and exceeds NE.sub.P, with a result of a
reduction in the engine output as is apparent from FIG. 7. As a
result, even if the torque is amplified in accordance with the
speed ratio .gamma. of the continuously variable transmission 18,
the running performance (the driving torque) may deteriorate.
[0050] The lower limit NINTLG is set on the basis of the rotation
speed NE.sub.B at which the minimum booster pressure P.sub.B is
obtained while the engine 12 is operating with the failure-time
throttle opening .theta..sub.thF. Therefore, when the vehicle is
stopped, for example, the engine speed NE of the engine 12 is
prevented from becoming too low to provide the required booster
pressure. This also avoids such a situation that the brake assist
force is reduced due to insufficient booster pressure, and the
driver feels uncomfortable when applying a brake to the vehicle. In
this embodiment, in the event of a failure in the electronic
throttle valve 26, the opening of the throttle valve 26 is fixed to
the failure-time throttle opening .theta..sub.thF (e.g.
approximately equal to 10%) which provides suitable running
performance that enables limp-home. As is apparent from FIG. 6, the
intake manifold negative pressure (the booster pressure) is smaller
at the failure-time throttle opening .theta..sub.thF as compared
with the case where the throttle opening .theta..sub.th is equal to
0, and the minimum booster pressure P.sub.B cannot be obtained when
the engine speed NE is lower than NE.sub.B.
[0051] The NINT calculating unit 62 calculates the target rotation
speed NINT according to the predetermined or fixed shift conditions
as shown in FIG. 4, regardless of whether there is a failure in the
electronic throttle valve 26 or not. However, the NINT calculating
unit 62 may also be designed to calculate the target rotation speed
NINT according to shift conditions exclusively established for the
occasion of a failure. For example, when the electronic throttle
valve 26 is faulty, shift control may be performed solely based on
the accelerator operation amount .theta..sub.ACC, as disclosed in
Japanese Patent No. 2616154.
[0052] In the aforementioned embodiment, the target rotation speed
NINT is limited to the range between the upper limit NINTUG and the
lower limit NINTLG. It is, however, possible to define a range
using only one of the upper limit NINTUG and the lower limit
NINTLG, and restrict the target rotation speed NINT to this
range.
[0053] Although the embodiment of the invention has been described
in detail with reference to the drawings, it is to be understood
that the embodiment is nothing more than one mode of implementing
the invention. That is, the invention can be embodied with various
changes or improvements, based on the knowledge of those skilled in
the art.
* * * * *