U.S. patent application number 12/761966 was filed with the patent office on 2010-08-05 for slippage detection system and method for continuously variable transmissions.
This patent application is currently assigned to Toyota Jidosha Kabushiki Kaisha. Invention is credited to Kazumi Hoshiya, Kunihiro IWATSUKI, Yasunori Nakawaki, Hiroyuki Nishizawa, Masataka Osawa, Yasuhiro Oshiumi, Hideyuki Suzuki, Hiroyuki Yamaguchi.
Application Number | 20100197454 12/761966 |
Document ID | / |
Family ID | 19122472 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100197454 |
Kind Code |
A1 |
IWATSUKI; Kunihiro ; et
al. |
August 5, 2010 |
SLIPPAGE DETECTION SYSTEM AND METHOD FOR CONTINUOUSLY VARIABLE
TRANSMISSIONS
Abstract
A slippage detection system for a continuously variable
transmission capable of continuously changing a gear ratio between
an input rotation speed of an input member and an output rotation
speed of an output member is provided. The slippage detection
system compares a difference between an actual gear ratio
calculated from measurement values of the input rotation speed and
the output rotation speed and a target gear ratio with a
predetermined reference value, and determines slippage in the
continuously variable transmission when the difference exceeds the
reference value at least a predetermined number of times in a
predetermined period of time.
Inventors: |
IWATSUKI; Kunihiro;
(Toyota-shi, JP) ; Hoshiya; Kazumi; (Gotenba-shi,
JP) ; Oshiumi; Yasuhiro; (Susono-shi, JP) ;
Nakawaki; Yasunori; (Nishikamo-gun, JP) ; Yamaguchi;
Hiroyuki; (Aichi-ken, JP) ; Nishizawa; Hiroyuki;
(Aichi-ken, JP) ; Suzuki; Hideyuki; (Aichi-ken,
JP) ; Osawa; Masataka; (Aichi-ken, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Toyota Jidosha Kabushiki
Kaisha
Toyota-shi
JP
|
Family ID: |
19122472 |
Appl. No.: |
12/761966 |
Filed: |
April 16, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10491042 |
Aug 11, 2005 |
|
|
|
PCT/IB02/04001 |
Sep 30, 2002 |
|
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12761966 |
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Current U.S.
Class: |
477/44 |
Current CPC
Class: |
Y10T 477/755 20150115;
F16H 2059/465 20130101; Y10T 477/6939 20150115; Y10T 477/6237
20150115; F16H 61/66272 20130101; F16H 61/6649 20130101; Y10T
477/69367 20150115; Y10T 477/816 20150115; Y10T 477/624
20150115 |
Class at
Publication: |
477/44 |
International
Class: |
F16H 61/662 20060101
F16H061/662 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 2001 |
JP |
2001-302181 |
Claims
1. A slippage detection system for a continuously variable
transmission capable of continuously changing a gear ratio between
an input rotation speed of an input member and an output rotation
speed of an output member, comprising: a comparator configured to
compare a difference between an actual gear ratio calculated from
measurement values of the input rotation speed and the output
rotation speed and a target gear ratio with a predetermined
reference value; and a slippage determining portion configured to
determine slippage in the continuously variable transmission when
the difference exceeds the reference value at least a predetermined
number of times in a predetermined period of time.
2. The slippage detection system according to claim 1, wherein: the
continuously variable transmission includes the input member, the
output member, and a torque transmitting member for transmitting
torque between the input member and the output member; and the
slippage determining portion determines slippage of the torque
transmitting member in the continuously variable transmission.
3. The slippage detection system according to claim 1, wherein the
target gear ratio is determined based on a value obtained as a
result of a smoothing operation performed on a target input
rotation speed that is determined based on an output requirement of
a vehicle in which the continuously variable transmission is
installed.
4. The slippage detection system according to claim 1, further
comprising a slippage determination canceling portion configured to
inhibit determination of slippage when the continuously variable
transmission is determined to be in the process of a shifting
action based on a change in a running state of a vehicle in which
the continuously variable transmission is installed.
5. A method of detecting slippage in a continuously variable
transmission capable of continuously changing a gear ratio between
an input rotation speed of an input member and an output rotation
speed of an output member, comprising the steps of: comparing a
difference between an actual gear ratio calculated from measurement
values of the input rotation speed and the output rotation speed
and a target gear ratio with a predetermined reference value; and
determining slippage in the continuously variable transmission when
the difference exceeds the reference value at least a predetermined
number of times in a predetermined period of time.
6. The method according to claim 5, wherein: the continuously
variable transmission includes the input member, the output member,
and a torque transmitting member for transmitting torque between
the input member and the output member; and the determining step
determines slippage of the torque transmitting member in the
continuously variable transmission.
7. The method according to claim 5, wherein the target gear ratio
is determined based on a value obtained as a result of a smoothing
operation performed on a target input rotation speed that is
determined based on an output requirement of a vehicle in which the
continuously variable transmission is installed.
8. The method according to claim 5, further comprising the step of
inhibiting determination of slippage when the continuously variable
transmission is determined to be in the process of a shifting
action based on a change in a running state of a vehicle in which
the continuously variable transmission is installed.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a division of application Ser.
No. 10/491,042, filed on Mar. 26, 2004, which is a National Stage
application of PCT/IB02/04001, filed Sep. 30, 2002 and claims
benefit of priority from JP 2001-302181, filed Sep. 28, 2001, the
entire contents of each of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] The invention relates to a slippage detection system and
method for use in a continuously variable transmission capable of
continuously changing the gear ratio that is a ratio between an
input rotation speed and an output rotation speed of the CVT.
[0004] 2. Description of Related Art
[0005] As a continuously variable transmission capable of changing
the gear ratio continuously, a belt-type continuously variable
transmission and a toroidal type (traction-type) continuously
variable transmission are known in the art. The belt-type
continuously variable transmission is adapted to transmit torque
and change the gear ratio by using a belt, and the toroidal type
continuously variable transmission is adapted to transmit torque
and change the gear ratio by using a power roller. In the belt-type
continuously variable transmission, the belt is wound around drive
and driven pulleys each capable of changing a groove width, and
torque is transmitted by use of frictional force between contact
surfaces of the pulleys and the belt. With this arrangement, the
gear ratio of the CVT is changed by changing the groove width of
the drive pulley so as to change an effective radius of the belt
wound around the pulley.
[0006] In a toroidal continuously variable transmission, on the
other hand, a power roller is sandwiched between an input disk and
an output disk, and torque is transmitted by use of shearing force
of traction oil present between the power roller and each of the
disks. With this arrangement, the gear ratio of the CVT is changed
by slanting or inclining the rotating power roller to thereby
change the radius of the position at which torque is transmitted
between the power roller and each disk. In the continuously
variable transmission of the above types, a torque transmission
portion takes the form of a surface, namely, torque is transmitted
via surfaces of mutually facing members, so that the gear ratio can
be continuously changed.
[0007] As a power transmitting mechanism that transmits torque via
surfaces, a friction clutch, a friction brake, and the like are
known. Such a friction clutch or a friction brake is constructed
such that the entire areas of frictional surfaces come in contact
with and are spaced apart from each other, with the frictional
surfaces being designed in view of wears. A continuously variable
transmission, on the other hand, is constructed so as to transmit
torque by bringing a belt or a power roller into contact with a
portion of a torque transmitting surface of each pulley or disk
while continuously changing the torque transmitting portion. In
such a continuously variable transmission, the torque transmitting
surface is designed without substantially allowing for wear, and
therefore a local wear of the torque transmitting surface may
result in poor torque transmission or a damage to the continuously
variable transmission.
[0008] Besides, there is a limit to the strength of constituent
members or elements of the CVT, such as the belt, pulleys, disks,
and traction oil. Therefore, the contact pressure between the
corresponding members cannot be increased without limit to avoid
slippage of the continuously variable transmission. Furthermore,
when the contact pressure is increased to a certain level, the
efficiency of power transmission and the durability of the
continuously variable transmission may be undesirably reduced.
[0009] In continuously variable transmissions, therefore, the
clamping force for clamping the belt or the power roller (or the
load applied to clamp the belt or the power roller) is desired to
be set to the minimum value in a range that ensures that excessive
slippage (so-called macro-slip) does not occur between the belt and
the corresponding pulley or between the power roller and the
corresponding disk. Nevertheless, in general, the torque applied to
the continuously variable transmission continuously changes.
Especially when a vehicle in which a continuously variable
transmission is used goes through a sudden acceleration or brake,
or is brought into a complicated operating state, such as temporary
idling or slippage of drive wheels, a sudden and temporarily large
torque may be applied to the continuously variable
transmission.
[0010] If the clamping force is set to be a greater value in
preparation for such temporarily large torque, the power
transmitting efficiency and the fuel efficiency may deteriorate
while the vehicle is running in normal or steady-state conditions.
Accordingly, it is preferable to perform a control to increase the
clamping force or reduce the torque applied to the CVT when
slippage due to large torque as described above is actually
detected.
[0011] In the meantime, a system adapted for detecting a condition
caused by slippage in a continuously variable transmission has been
proposed in Japanese Laid-open Patent Publication No. 62-2059. The
system disclosed in this publication is arranged to determine a
failure or problem in the continuously variable transmission. In
this system, rotation speeds of a main pulley and a sub-pulley are
measured using sensors to calculate a gear ratio. If the gear ratio
thus measured or the rate of change in the gear ratio exhibits an
extreme value that is not obtained in normal state, the system is
determined to be at faulty.
[0012] The sensors used in the system disclosed in the above
publication are the same as or equivalent to sensors generally used
for a gear ratio control of a continuously variable transmission.
With the above system thus constructed, therefore, a failure or a
problem in the continuously variable transmission can be detected
without using other sensor(s) newly provided for this purpose.
Thus, the system disclosed in the above publication is designed for
determining a failure of the continuously variable transmission,
but is not provided, by nature, with any function of dealing with
excessive slippage of the belt.
[0013] That is, the system disclosed in the above publication is
constructed so as to detect a failure of the continuously variable
transmission for the first time when the gear ratio or the rate of
change in the gear ratio takes an abnormal value as a result of
excessive slippage of the belt. The system, therefore, cannot be
used for the purpose of avoiding a problem caused by excessive
slippage of the belt. In other words, the system disclosed in the
above publication is not able to detect, with sufficiently high
speed and accuracy, the beginning of a so-called macro-slip (i.e.,
considerably large slip) of the belt or a state that may lead to a
macro-slip. Consequently, the above-described conventional system
cannot be used as a slippage detection system for performing a
control for dealing with temporary slippage of the belt in the
continuously variable transmission.
SUMMARY OF THE INVENTION
[0014] It is therefore an object of the invention to provide a
system capable of immediately and accurately detecting slippage of
a belt, or the like, in a continuously variable transmission, or
detecting a start of slippage or a state that may lead to slippage,
without a dedicated sensor or sensors.
[0015] To accomplish the above object, there is provided according
to the invention a slippage detection system for a continuously
variable transmission having an input member, an output member and
a torque transmitting member for transmitting torque between the
input member and the output member, the continuously variable
transmission being capable of continuously changing a ratio between
an input rotation speed of the input member and an output rotation
speed of the output member. The slippage detection system includes
(a) correlation coefficient calculating means for calculating a
correlation coefficient relating to the input rotation speed and
the output rotation speed, based on a plurality of measurement
values of the input rotation speed and a plurality of measurement
values of the output rotation speed, and (b) slippage determining
means for determining slippage of the torque transmitting member in
the continuously variable transmission based on the correlation
coefficient calculated by the correlation coefficient calculating
means.
[0016] The slippage in the continuously variable transmission may
be considered as a state or condition in which the relationship
between the input-side rotation speed and the output-side rotation
speed deviates from a predetermined one that corresponds to a gear
ratio (i.e., the ratio of the output-side rotation speed to the
input-side rotation speed) to be established. The slippage
detection system as described above calculates a correlation
coefficients that represents the relationship between the input
rotation speed and the output rotation speed, and is therefore able
to immediately and accurately determine a slipping state of the
CVT, or a state that leads to excessive slippage, or a start of
excessive slippage.
[0017] In one preferred embodiment of the invention, the slippage
determining means determines slippage of the torque transmitting
member in the continuously variable transmission when the
correlation coefficient calculated by the correlation coefficient
to calculating means is smaller than a predetermined reference
value. The reference value may be set based on an operating state
of a vehicle in which the continuously variable transmission is
installed.
[0018] By setting the reference value to an appropriate value, the
slippage detection system is able to determine even a small degree
of slippage as well as large slippage, and perform suitable control
to deal with the slippage. At the same time, the slippage detection
system can avoid excessively sensitive determination of slippage
which would lead to unnecessary control for dealing with the
slippage.
[0019] In another embodiment of the invention, the correlation
coefficient calculating means calculates the correlation
coefficient when the operating state of the vehicle in which the
continuously variable transmission is installed satisfies at least
one predetermined condition. With this arrangement, even if the
running state of the vehicle changes in a complicated manner,
calculation of the correction coefficient is carried out only when
the vehicle is in a suitable running state. It is thus possible to
accurately determine a slipping state in the continuously variable
transmission.
[0020] In a further embodiment of the invention, the correlation
coefficient calculating means sets the number of the measurement
values of each of the input rotation speed and the output rotation
speed used for calculating the correlation coefficient, based on an
operating state of a vehicle in which the continuously variable
transmission is installed.
[0021] The above arrangement makes it possible to avoid erroneous
determination of slippage in the continuously variable
transmission, or avoid a situation in which calculation of the
correlation coefficient is unnecessarily repeated even if the
running state of the vehicle does not change.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The foregoing and/or further objects, features and
advantages of the invention will become more apparent from the
following description of preferred embodiments with reference to
the accompanying drawings, in which like numerals are used to
represent like elements and wherein:
[0023] FIG. 1 is a flowchart for explaining one example of control
performed by a controller of a slippage detection system according
to one embodiment of the invention;
[0024] FIG. 2 is a graph showing changes in a correlation
coefficient with time;
[0025] FIG. 3 is a graph schematically showing the trend or
tendency of changes of reference values in accordance with the
operating conditions of the vehicle;
[0026] FIG. 4 is a graph schematically showing the trend or
tendency of changes of the number of sampling points for obtaining
the correlation coefficient in accordance with the operating
conditions of the vehicle;
[0027] FIG. 5 is a flowchart for explaining another example of
control performed by the slippage detection system according to the
invention;
[0028] FIG. 6 is a graph showing changes in a band-pass value
obtained by subjecting an input-shaft rotation speed to a filtering
process;
[0029] FIG. 7 is a flowchart for explaining another example of
control performed by the slippage detection system according to the
invention;
[0030] FIG. 8 is a graph showing changes in the accumulated
band-pass value;
[0031] FIG. 9 is a flowchart for explaining another example of
control performed by the slippage detection system according to the
invention;
[0032] FIG. 10 is a graph showing changes in a difference between
an actual gear ratio and a target gear ratio of a CVT;
[0033] FIG. 11 is another flowchart obtained by partially modifying
the flowchart of FIG. 9;
[0034] FIG. 12 is a view showing changes in the sum of the
differences;
[0035] FIG. 13 is another flowchart obtained by partially modifying
the flowchart of FIG. 11; and
[0036] FIG. 14 is a view schematically showing a drive system and a
control system of a vehicle with a continuously variable
transmission in which a slippage detection system according to the
invention is employed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0037] Some exemplary embodiments of the invention will be
described in detail. First, a drive system and a control system of
a motor vehicle to which the invention is applied will be described
with reference to FIG. 14. FIG. 14 schematically shows a drive
system including a belt-type continuously variable transmission
(CVT) 1 as a transmission. The CVT 1 is coupled to a power source 3
via a forward/reverse-drive switching mechanism 2.
[0038] The power source 3 is a drive unit for generating power to
run the vehicle, and is provided by an internal combustion engine,
a combination of an internal combustion engine and an electric
motor, an electric motor, or the like. In this embodiment, the
power source 3 takes the form of an engine. The
forward/reverse-drive switching mechanism 2 is employed since the
engine 3 can rotate only in one direction, and is arranged to
output the input torque as it is or in a reverse direction.
[0039] In the example shown in FIG. 14, a double-pinion type
planetary gear mechanism is used as the forward/reverse-drive
switching mechanism 2. In this mechanism, a ring gear 5 is disposed
concentrically with a sun gear 4, and a pinion gear 6 that engages
with the sun gear 4 and another pinion gear 7 that engages with the
pinion gear 6 and the ring gear 5 are disposed between the sun gear
4 and the ring gear 5. The pinion gears 6, 7 are supported by a
carrier 8 such that these gears 6, 7 are freely rotatable about
their center axes and about the center axis of the planetary gear
mechanism. A forward-drive clutch 9 is provided for coupling two
rotating elements (i.e., sun gear 4 and carrier 8) into one unit.
Also, a reverse-drive brake 10 is provided for reversing the
direction of torque output from the forward/reverse-drive switching
mechanism 2 by fixing the ring gear 5 selectively.
[0040] The construction of the CVT 1 is the same as or equivalent
to that of a known belt-type continuously variable transmission.
The CVT 1 is provided with a drive pulley 11 and a driven pulley
12, which are arranged in parallel to each other. Each of the
pulleys 11 and 12 principally consists of a stationary sheave and a
movable sheave that is adapted to be moved forward and backward in
an axial direction thereof by a hydraulic actuator 13 or 14. With
this arrangement, the groove width of each of the pulleys 11, 12
changes as the movable sheave of the pulley is moved in the axial
direction thereof, thus continuously changing the winding radius of
the belt 15 wound around the pulleys 11, 12 (i.e., the effective
radius of each pulley 11, 12) thereby to continuously change the
gear ratio of the CVT 1. The drive pulley 11 is connected to the
carrier 8 serving as an output element of the forward/reverse-drive
switching mechanism 2.
[0041] A hydraulic pressure (a line pressure or its corrected
pressure) is applied to the hydraulic actuator 14 for the driven
pulley 12 via a hydraulic pump and a hydraulic control device or
system (not shown). The level of the hydraulic pressure applied to
the hydraulic actuator 14 is controlled to be commensurate with the
magnitude of the torque received by the CVT 1. With this
arrangement, the belt 15 is clamped or gripped between the sheaves
of the driven pulley 12 and is thus provided with suitable tensile
force, so that a suitable clamping force (or contact pressure)
surely appears between each of the pulleys 11, 12 and the belt 15.
On the other hand, the hydraulic actuator 14 of the drive pulley 11
is supplied with an oil pressure that depends on a desired gear
ratio, thereby setting the groove width (or pitch diameter) of the
pulley 11 to a target value.
[0042] The driven pulley 12 is connected to a differential gear
unit 17 via a pair of gears 16, and is adapted to output a torque
to drive wheels 18 via the differential gear unit 17.
[0043] Various sensors are provided for detecting operating
conditions (or running conditions) of the vehicle including the CVT
1 and the engine 3. More specifically, there are provided an engine
speed sensor 19 for measuring the rotation speed of the engine 3
and generating a signal indicative of the engine speed, an input
rotation speed sensor 20 for measuring the rotation speed of the
drive pulley 11 and generating a signal indicative of the input
rotation speed, and an output rotation speed sensor 21 for
measuring the rotation speed of the driven pulley 12 and generating
a signal indicative of the output rotation speed. In addition, an
accelerator position sensor, a throttle opening sensor, a brake
sensor, and other sensors are provided, though not shown in the
figure. The accelerator position sensor is arranged to measure the
amount of depression of an accelerator pedal and output a signal
indicative of the accelerator pedal position. The throttle opening
sensor is arranged to measure the opening amount of the throttle
valve and output a signal indicative of the throttle opening. The
brake sensor is arranged to output a signal when a brake pedal is
depressed.
[0044] Also, an electronic control unit (CVT-ECU) 22 for
transmission is provided for controlling engagement and release of
each of the forward-drive clutch 9 and the reverse-drive brake 10,
the clamping force applied to the belt 15, and the gear ratio of
the CVT 1. The electronic control unit 22 for transmission
includes, for example, a microcomputer as its main component, and
is arranged to perform calculations based on input data and data
stored in advance, thereby to perform controls such as
establishment of a selected operating mode, such as forward-drive,
revere-drive or neutral mode, setting of the required clamping
pressure, and setting of the gear ratio of the CVT 1.
[0045] Input data (or signals) received by the electronic control
unit 22 for transmission may include, for example, signals
indicative of input-shaft rotation speed Nin and output rotation
speed No of the CVT 1 received from corresponding sensors (not
shown). In addition, the electronic control unit 22 for
transmission receives signals indicative of engine speed Ne, engine
(E/G) load, throttle opening, accelerator position that represents
the amount of depression of the accelerator pedal (not shown) and
so on, from an electronic control unit (E/G-ECU) 23 for controlling
the engine 3.
[0046] The CVT 1 is capable of continuously or steplessly
controlling the engine speed as the input rotation speed as
described above. When the CVT 1 is installed on a motor vehicle,
therefore, the fuel efficiency of the vehicle is improved. For
example, a target driving force is determined based on the required
driving amount as represented by the accelerator pedal position, or
the like, and the vehicle speed. Then, a target output of the CVT 1
needed for achieving the target driving force is determined based
on the target driving force and the vehicle speed. Then, an engine
speed for achieving the target output with the optimum fuel
efficiency is determined using a predetermined map. Finally, the
gear ratio of the CVT 1 is controlled so as to achieve the
determined engine speed.
[0047] To make advantage of the improvement in the fuel efficiency,
the power transmitting efficiency of the CVT 1 is controlled to a
desirably high level. More specifically, the torque capacity or the
belt clamping pressure of the CVT 1 is controlled to be the minimum
value in a range in which the CVT 1 can transmit the target torque
determined based on the engine torque without causing slippage of
the belt 15. This control is normally performed in a steady state
in which the vehicle speed and the output requirement hardly change
or in an almost steady state in which one or both of these
parameters slightly changes.
[0048] Meanwhile, if the vehicle is suddenly braked or accelerated
or if the vehicle runs upon a dropped object or a step, the torque
applied to the drive system including the CVT 1 suddenly changes.
In this case, the torque capacity of the CVT 1 may become
relatively insufficient, thus increasing the possibility of
slippage of the belt 15. In such a case, therefore, the control
system of the embodiment performs so-called reactive control to
temporarily increase the belt clamping force or temporarily reduce
the engine torque. The control system of the embodiment is arranged
to perform the following control so as to judge or determine
occurrence of a situation (i.e., macro-slip) that requires the
reactive control as described above.
[0049] FIG. 1 is a flowchart showing one example of the control for
determining a macro-slip of the belt 15 of the CVT 1. In this
control, a correlation coefficient obtained based on the input and
output rotation speeds is used. As shown in FIG. 1, it is first
determined in step S1 whether the running state of the vehicle is
within a calculation range of the correlation coefficient. The
correlation coefficient used in this control is a coefficient
calculated based on the input-shaft rotation speed (xi) and the
output-shaft rotation speed (yi). When each of the input and output
rotation speeds has a value other than 0 and the gear ratio is kept
almost constant, the running state of the vehicle is determined to
be within the calculation range of the correlation coefficient.
That is, the correlation coefficient is within the calculation
range when the vehicle is running while the gear ratio is kept
almost constant (i.e., the speed ratio, which is the inverse of the
gear ratio, is kept almost constant).
[0050] If a negative determination is made in step S1, a flag F is
reset to 0 in step S2 and the control returns. If a positive
determination is made in step S1, on the other hand, the flag F is
set to 1 in step S3 and the input-shaft rotation speed (xi) and the
output-shaft rotation speed (yi) are read in steps S4 and S5,
respectively. These rotation speeds (xi, yi) are respectively
measured by the input rotation speed sensor 20 and the output
rotation speed sensor 21 shown in FIG. 14. In step S6, a
correlation coefficient S is obtained using N sets of the rotation
speeds (xi, yi) that have been read so far.
[0051] The correlation coefficient S is represented by the
expression (1) below:
correlation coefficient S = x 1 y 1 + x 2 y 2 + + x n y n x 1 2 + x
2 2 + + x n 2 y 1 2 + y 2 2 + + y n 2 ( 1 ) ##EQU00001##
In the above expression (1), each suffix (1, 2 . . . n) represents
a sampling point at which the rotation speed (xi or yi) was
measured and n represents the present time.
[0052] An occurrence or possibility of slippage (macro-slip) of the
belt is determined by using the correlation coefficient S in the
following manner. In the expression (1), the power of the rotation
speeds of the input and output members (i.e., the input and output
shafts of the CVT 1) is normalized by the square root of the powers
of the input rotation speed and the output rotation speed.
According to the expression (1), when the power of the input and
output rotation speeds decreases, the normalized value decreases.
More specifically, when slippage of the belt 15 does not occur, the
correlation coefficient S is equal to 1. When slippage of the belt
15 occurs, conversely, the value becomes smaller than 1.
[0053] Thus, while the belt 15 is not slipping but is being gripped
by the drive and driven pulleys 11, 12, the relationship as
represented by expression (2) below is true:
yi=.gamma.xi (2)
Here, .gamma. represents the speed ratio (which is the inverse of
the gear ratio).
[0054] When the expression (2) is assigned to the above expression
(1), the correlation coefficient S is represented by expression
(3), and its value becomes 1.
correlation coefficient S = .gamma. ( x 1 x 1 + x 2 x 2 + + x n x n
) x 1 2 + x 2 2 + + x n 2 .gamma. 2 ( x 1 2 + x 2 2 + x n 2 ) =
.gamma. ( x 1 2 + x 2 2 + + x n 2 ) .gamma. ( x 1 2 + x 2 2 + + x n
2 ) = 1.0 ( 3 ) ##EQU00002##
[0055] As described above, one of the conditions for the
calculation is that the speed ratio .gamma. be almost constant in
order to put the speed ratio .gamma. out of the parentheses. It is
thus not preferable to measure the input-shaft and output-shaft
rotation speeds at long time intervals or sampling time.
[0056] Next, there will be described the case where the belt 15 is
not being sufficiently gripped by the drive and driven pulleys 11,
12 and is slipping. While the belt 15 is slipping, the relationship
between the input-shaft rotation speed (xi) and the output-shaft
rotation speed (yi) with respect to the speed ratio .gamma. that is
currently set becomes untrue. The relationship between these
rotation speeds is then represented in expression (4) below:
yi=ki.gamma.xi (4)
Here, ki, which is a real number larger than "0", is a coefficient
representing rotational fluctuations or variations.
[0057] In this case, the expression (4) is assigned to the above
expression (1), and the correlation coefficient S is then
represented by the following expression (5):
correlation coefficient S = .gamma. ( k 1 x 1 x 1 + k 2 x 2 x 2 + +
k n x n x n ) x 1 2 + x 2 2 + + x n 2 .gamma. 2 ( k 1 2 x 1 2 + k 2
2 x 2 2 + k n 2 x n 2 ) = k 1 x 1 2 k 2 x 2 2 + + k n x n 2 x 1 2 +
x 2 2 + + x n 2 ( k 1 2 x 1 2 + k 2 2 x 2 2 + k n 2 x n 2 ) ( 5 )
##EQU00003##
When the coefficient ki is not constant due to the rotational
fluctuations caused by slippage of the belt 15, the correlation
coefficient S becomes smaller than 1. Namely, the expression (5) is
transformed into the following expression (6).
correlation coefficient S = ( k 1 x 1 2 + k 2 x 2 2 + + k n x n 2 )
2 x 1 2 + x 2 2 + + x n 2 ( k 1 2 x 1 2 + k 2 2 x 2 2 + + k n 2 x n
2 ) ( 6 ) ##EQU00004##
When the numerator and denominator of the expression (6) are
expanded, the following expressions (7) and (8) will be provided
respectively:
( k 1 x 1 2 + k 2 x 2 2 + + k n x n 2 ) ( k 1 x 1 2 + k 2 x 2 2 + +
k n x n 2 ) = k 1 x 1 2 ( k 1 x 1 2 + k 2 x 2 2 + + k n x n 2 ) + k
2 x 2 2 ( k 1 x 1 2 + k 2 x 2 2 + + k n x n 2 ) + k n x n 2 ( k 1 x
1 2 + k 2 x 2 2 + + k n x n 2 ) = k 1 2 x 1 4 + k 2 2 x 2 4 + + k n
2 x n 4 + x 1 2 ( k 1 k 2 x 2 2 + + k 1 k n x n 2 ) + x 2 2 ( k 2 k
1 x 1 2 + + k 2 k n x n 2 ) + x n 2 ( k n k 1 x 1 2 + + k n k n - 1
x n - 1 2 ) ( 7 ) ( x 1 2 + x 2 2 + + x n 2 ) ( k 1 2 x 1 2 + k 2 2
x 2 2 + + k n 2 x n 2 ) = x 1 2 ( k 1 2 x 1 2 + k 2 2 x 2 2 + + k n
2 x n 2 ) + x 2 2 ( k 1 2 x 1 2 + k 2 2 x 2 2 + + k n 2 x n 2 ) + x
n 2 ( k 1 2 x 1 2 + k 2 2 x 2 2 + + k n 2 x n 2 ) = k 1 2 x 1 4 + k
2 2 x 2 4 + + k n 2 x n 4 + x 1 2 ( k 2 2 x 2 2 + + k n 2 x n 2 ) +
x 2 2 ( k 1 2 x 1 2 + + k n 2 x n 2 ) + x n 2 ( k 1 2 x 1 2 + k n -
1 2 x n - 1 2 ) ( 8 ) ##EQU00005##
[0058] If the sampling time n is 3, the expressions (7) and (8) are
rewritten into the following expressions (9) and (10),
respectively:
k 1 2 x 1 4 + k 2 2 x 2 4 + k 3 2 x 3 4 + x 1 2 ( k 1 k 2 x 2 2 + k
1 k 3 x 3 2 ) + x 2 2 ( k 2 k 1 x 1 2 + k 2 k 3 x 3 2 ) + x 3 2 ( k
3 k 1 x 1 2 + k 3 k 2 x 2 2 ) = k 1 2 x 1 4 + k 2 2 x 2 4 + k 3 2 x
3 4 + ( 2 k 1 k 2 x 1 2 x 2 2 + 2 k 1 k 3 x 1 2 x 3 2 + 2 k 2 k 3 x
2 2 x 3 2 ) ( 9 ) k 1 2 x 1 4 + k 2 2 x 2 4 + k 3 2 x 3 4 + x 1 2 (
k 2 2 x 2 2 + k 3 2 x 3 2 ) + x 2 2 ( k 1 2 x 1 2 + k 3 2 x 3 2 ) +
x 3 2 ( k 1 2 x 1 2 + k 2 2 x 2 2 ) = k 1 2 x 1 4 + k 2 2 x 2 4 + k
3 2 x 3 4 + ( x 1 2 x 2 2 ( k 1 2 + k 2 2 ) + x 1 2 x 3 2 ( k 1 2 +
k 3 2 ) + x 2 2 x 3 2 ( k 2 2 + k 3 2 ) ) ( 10 ) ##EQU00006##
[0059] When the respective coefficients affixed to
x.sub.1.sup.2x.sub.2.sup.2, or the like, in the expressions (9) and
(10) are compared, the relationship as represented by the following
expression (11) is found to be true:
k.sub.j.sup.2+k.sub.m.sup.2.gtoreq.2k.sub.jk.sub.m (11)
Here, j and m are suffixes such as 1 and 2.
[0060] The expression (11) may be rewritten into the following
expression (12):
(k.sub.j-k.sub.m).sup.2.gtoreq.0 (12)
Here, since Ki and Km are real numbers, the relationship of the
expression (12) is always true and therefore the relationship of
the expression (11) is also true. Also, when the sampling number n
is larger than 3, the relationships of the expressions (11) and
(12) are true. When the input/output rotation speeds start varying,
the value of the denominator as described above becomes larger than
that of the numerator. As a result, the correlation coefficient S
becomes smaller than 1. Accordingly, it is possible to determine an
occurrence of slippage of the belt 15 based on the correlation
coefficient S.
[0061] In step S6 of FIG. 1, the correlation coefficient S is
determined through calculations. In step S7, it is determined
whether the correlation coefficient S is equal to or smaller than a
first reference value S1 determined in advance. The first reference
value S1 is smaller than 1 and is determined in advance as a value
corresponding to a state in which a macro-slip is occurring or a
slipping state which may lead to a macro-slip.
[0062] If the correlation coefficient S is equal to or smaller than
the first reference value S1 and a positive determination is
therefore made in step S7, an occurrence or a possibility of a
macro-slip is then determined (macro-slip determination is made) in
step S8. In the next step S9, a responsive control is performed in
response to the macro-slip determination made in step S8. In short,
the responsive control is performed to avoid or suppress
macro-slips. As the responsive control, for example, the clamping
force applied to the belt 15 is increased or the engine torque is
reduced. In addition, if a clutch is used for transmitting torque
to the CVT 1, for example, the capacity of the clutch is reduced
under the responsive control. The degree of the responsive control
is set in accordance with the degree of the macro-slip, namely, the
value of the correlation coefficient S.
[0063] Conversely, when the correlation coefficient S is larger
than the first reference value S1 described above, and a negative
determination is made in step S7, it is then determined in step S10
whether the correlation coefficient S is equal to or larger than a
second reference value S2. The second reference value S2 is a value
larger than the first reference value S1 but smaller than 1 and is
determined in advance as a value that corresponds to a state where
a relatively small macro-slip is occurring or a state that may lead
to such a relatively small macro-slip.
[0064] When a negative determination is made in step S10, namely,
when the correlation coefficient S is smaller the second reference
value S2, it indicates that a relatively small macro-slip is
occurring or is highly likely to occur. In such a case, therefore,
the control proceeds to step S8 to make a macro-slip determination.
Subsequently, the responsive control is performed in step S9.
[0065] When a negative determination is made in step S10,
conversely, a non-macro-slip determination is made in step S11. The
non-macro-slip determination is made when a macro-slip is not
occurring or is not likely to occur, or when the belt 15 is
slipping but the degree of slipping is within a permissible range.
In this case, responsive control is performed as needed in step S12
in response to the non-macro-slip determination made in step S11.
One example of the responsive control is a control for reducing the
clamping force applied to the belt 15. This control is intended to
improve the power transmitting efficiency of the CVT 1 and to
minimize the hydraulic pressure supplied to the CVT 1 to thereby
reduce power loss at the hydraulic pump, thus assuring improved
fuel efficiency.
[0066] FIG. 2 is a graph showing changes in the correlation
coefficient S during a transition from a state where a macro-slip
of the belt 15 is not occurring to a state where a macro-slip of
the belt 15 is occurring. When torque is being transmitted via the
CVT 1, slippage, which can be denominated "micro-slip" in contrast
to "macro-slip", unavoidably occurs. Thus, the torque is
transmitted via the CVT 1 with the correlation coefficient S
changing extremely slightly. When an operating state of the vehicle
that leads to a macro-slip of the belt 15 takes place for some
reason, the correlation coefficient S starts decreasing to some
extent. When a macro-slip subsequently starts, the correlation
coefficient S starts decreasing rapidly. For example, the reference
values S1 and S2 are respectively set to the values as indicated in
FIG. 2.
[0067] In the meantime, the correlation coefficient S is determined
based on the input-shaft and output-shaft rotation speeds. The
relationship between these rotation speeds change not only due to
slippage of the belt 15 but also due to changes in the engine
torque, acceleration/deceleration of the vehicle, and the like,
which are caused when the accelerator operation amount changes.
While the correlation coefficient S may decrease due to these
changes, such a decrease in the correlation coefficient S does not
indicate an occurrence or a possibility of a macro-slip. In this
case, therefore, there is a need to avoid making the macro-slip
determination. To meet this need, each of the reference values S1
and S2 may be changed in accordance with the operating conditions
of the vehicle, such as the rate of change of the accelerator
operation amount, acceleration/deceleration of the vehicle, and so
on.
[0068] FIG. 3 is a graph showing one example of the tendency of
changes in the reference values S1, S2. As shown in FIG. 3, the
respective reference values S1, S2 are reduced as the rate of
change of the accelerator operation amount .DELTA.ACC or the
acceleration/deceleration .DELTA.V increases. Thus, if the
correlation coefficient S decreases due to a factor or factors
other than a macro-slip, an occurrence or a possibility of a
macro-slip will not be determined by mistake, and a responsive
control will not be performed by mistake in response to the
decrease in the correlation coefficient S. Thus, erroneous
determination of a macro-slip is avoided, and unnecessary
responsive control can also be avoided or suppressed. Furthermore,
a delay in determining an occurrence or a possibility of slippage
is prevented.
[0069] Also, the correlation coefficient S is calculated using a
plurality of detection values representing the input and output
rotation speeds. The number of sets of the detected values (which
will be referred to as "set number") is preferably determined in
accordance with the operating state of the vehicle. FIG. 4 is a
graph schematically showing one example of the tendency or trend in
determining the set number N. As shown in FIG. 4, the set number N
is reduced as the vehicle speed V, the acceleration/deceleration
.DELTA.V, the rate of change in the accelerator operation amount
.DELTA.ACC, the gear ratio .gamma., or the like, increases. When
the rate of change of the accelerator operation amount is large,
for example, the degree or magnitude of a corresponding change in
the gear ratio is supposed to be large. In this case, the set
number is reduced in order to prevent the correlation coefficient S
from being calculated based on the rotation speeds at largely
different gear ratios and thereby prevent erroneous determination
of a macro-slip and a delay in determining an occurrence or a
possibility of slippage of the belt 15.
[0070] As described above, the slippage detection system of the
embodiment is arranged to determine macro-slips based on the
correlation coefficient by measuring the input and output rotation
speeds by means of the sensors 21, 22 that are normally used for
determining the gear ratio of the CVT 1. With this arrangement,
slippage of the belt 15 can be immediately determined with
sufficiently high accuracy without using other sensor or sensors
dedicated to this function. Moreover, since the slippage detection
system is able to carry out necessary responsive control based on
the determination of macro-slips, a damage to the CVT 1 as a result
of excessive slippage of the belt 15 can be prevented or
suppressed.
[0071] Meanwhile, the input-shaft rotation speed Nin of the CVT 1
changes due to various factors, such as the gear ratio control,
slippage of the belt 15, or periodical variations in the input
torque. Therefore, by determining the amount of change of the input
rotation speed that is caused by slippage of the belt 15, out of
the overall change amount, it is possible to determine an
occurrence or a possibility of a macro-slip of the belt 15 based on
the determined value (change amount). One example of such control
will be described in the following.
[0072] FIG. 5 is a flowchart showing one example of the control. In
this control, the input-shaft rotation speed Nin detected by the
input rotation speed sensor 20 is first read in step S21. Next, a
vibration component Nin.cndot.vib contained in the input-shaft
rotation speed Nin and resulting from slippage of the belt 15 is
obtained. Here, the vibration component Nin.cndot.vib can be
obtained, for example, by carrying out a band-pass filtering
process or on the basis of a deviation of the actual input-shaft
rotation speed Nin from a command value of the input-shaft rotation
speed. The command value is determined so as to achieve the desired
gear ratio. During the band-pass filtering process, measurement
noises are also removed.
[0073] FIG. 6 is a graph showing one example of changes in the
band-pass value with time when the input-shaft rotation speed Nin
is subjected to band-pass filtering (20-30 Hz). When a macro-slip
does not occur as shown in FIG. 6, the band-pass value stays in a
relatively small range. When a macro-slip takes place, on the other
hand, the band-pass value rapidly increases. In view of this, a
reference value Nin.cndot.vib1 is determined in advance as an index
or criteria for determining an occurrence of a macro-slip, and it
is determined in step S23 whether the vibration component
Nin.cndot.vib obtained in step S22 is equal to or larger than the
reference value Nin.cndot.vib1. Meanwhile, the reference value
Nin.cndot.vib1 may be changed in accordance with the operating
state of the vehicle, rather than being constant, so as to prevent
erroneous determinations from being made and also avoid a delay in
determining an occurrence of a macro-slip.
[0074] If the vibration component Nin.cndot.vib is equal to or
larger than the reference value Nin.cndot.vib1 and a positive
determination is therefore made in step S23, the macro-slip
determination is made in step S24, and a responsive control is
performed in step S25. The operations in step S24 and step S25 are
the same as or equivalent to those in step S8 and step S9 of FIG.
1, respectively.
[0075] If the vibration component Nin.cndot.vib is smaller than the
reference value Nin.cndot.vib1 and a negative determination is
therefore made in step S23, it indicates that a macro-slip is not
occurring as is understood from FIG. 6. In this case, a
non-macro-slip determination is made in step S26. Subsequently,
normal control is performed in step S27. In this normal control,
the belt clamping force is set in accordance with, for example, the
engine torque or the amount of depression of the accelerator pedal
(i.e., accelerator operation amount).
[0076] In order to perform the control as illustrated above with
reference to FIGS. 5 and 6, the slippage detection system of the
embodiment uses only the input rotation speed sensor 20 as a sensor
so as to immediately and accurately determine macro-slips of the
belt 15 without requiring other sensor or sensors for this purpose.
Moreover, since the slippage detection system is able to perform
required responsive control upon detection of a macro-slip, an
otherwise possible damage to the CVT 1 as a result of excessive
slippage of the belt 15 can be prevented.
[0077] While an occurrence of a macro-slip is determined based on
the band-pass values in the control illustrated in FIG. 5, the
slippage detection system according to another embodiment of the
invention is constructed so as to determine an occurrence of a
macro-slip based on the accumulated value of the vibration
components due to slippage of the belt during a period from a
previous point of time to the current point of time. One example of
such control will be described in the following.
[0078] In this control, as shown in FIG. 7, the input-shaft
rotation speed Nin is read in step S31 and the vibration component
Nin.cndot.vib is determined in step S32 in the same manner as in
steps S21 and step S22 of FIG. 5, respectively. Subsequently, it is
determined in step S33 whether it is possible to carry out
accumulation of the vibration components Nin.cndot.vib. More
specifically, it is determined in step S33 whether i sets of data
required for executing a time-window accumulation of the vibration
components have been obtained.
[0079] If a negative determination is made in step S33, the control
returns, and waits for a required number of data sets to be
obtained. If a positive determination is made in step S33,
conversely, step S34 is executed to calculate a time-window
accumulation value S-vib(N) of the vibration components obtained
during a period between the present time (N time point) and a
previous point of time (N-1 time point) that is a predetermined
time prior to the present time. Here, the number of data sets to be
accumulated or the time period during which the data are
accumulated may be changed depending upon the operating state of
the vehicle.
[0080] FIG. 8 is a graph showing changes in the time-window
accumulation value of the vibration components Nin-vib resulting
from slippage of the belt 15. When a macro-slip does not occur, the
accumulated band-pass value S-vib(N) is held within a relatively
small range, as shown in FIG. 8. When a macro-slip occurs, on the
other hand, the accumulated band-pass value S-vib(N) increases
rapidly. In view of this, a reference value Sa, which is used as a
criteria or threshold for determining an occurrence of a
macro-slip, is set in advance, and it is determined in step S35
whether the accumulated band-pass value S-vib(N) obtained in step
S34 is equal to or larger than the reference value Sa. Meanwhile,
the reference value Sa may be changed in accordance with the
operating state of the vehicle, rather than being constant, so as
to prevent an erroneous determination of a macro-slip and also
avoid a delay in determining an occurrence of a macro-slip.
[0081] If the accumulated band-pass value S-vib(N) is equal to or
larger than the reference value Sa and a positive determination is
made in step S35, the macro-slip determination is made in step S36
and a responsive control is then performed in step S37. These
operations in steps S36 and S37 are the same as or equivalent to
those in step S24 and step S25 of FIG. 5, or those in step S8 and
step S9 of FIG. 1, respectively.
[0082] If the accumulated band-pass value S-vib(N) is smaller than
the reference value Sa and a negative determination is made in step
S35, on the other hand, it indicates that a macro-slip is not
occurring as is understood from FIG. 8. In this case, therefore, a
non-macro-slip determination is made in step S38. Subsequently,
normal control is performed in step S37. These operations in step
S38 and step S39 are the same as or equivalent to those in step S26
and step S27 of FIG. 5, respectively.
[0083] In order to perform the control as illustrated above with
reference to FIGS. 7 and 8, the slippage detection system of the
embodiment uses only the input rotation speed sensor 20 as a sensor
to immediately and accurately determine macro-slips of the belt 15
without requiring other sensor or sensors for this purpose.
Moreover, since the slippage detection system is able to perform
required responsive control upon detection of a macro-slip, an
otherwise possible damage to the CVT 1 as a result of excessive
slippage of the belt 15 can be prevented.
[0084] As described above, slipping of the belt 15 causes changes
in the input and output rotation speeds. With the rotation speeds
thus changed, the actual gear ratio, which is obtained as a ratio
between the input-shaft rotation speed and the output-shaft
rotation speed, deviates from a gear ratio (i.e., a target gear
ratio) established immediately before the occurrence of slippage of
the belt 15, resulting in a difference between the actual gear
ratio and the target gear ratio. According to another embodiment of
the invention, an occurrence of a macro-slip is determined on the
basis of the above-described difference between the actual gear
ratio and the target gear ratio.
[0085] FIG. 9 is a flowchart showing one example of control for
determining a macro-slip in the above-described manner. In this
control, it is first determined in step S41 whether the gear ratio
is being changed, namely, the CVT 1 is in the middle of a shifting
action. The target gear ratio is generally set on the basis of the
output requirement (e.g., accelerator operation amount) and the
vehicle speed or engine speed, for example. When the CVT 1 is in
the middle of a shifting action, however, the target gear ratio or
the target input rotation speed corresponding to the target gear
ratio may be set as a value with a first-order lag with respect to
a finally set value. Accordingly, the varying target gear ratio
cannot be used as a basis for determining an occurrence or a
possibility of slippage of the belt 15. Accordingly, in step S41,
it is determined whether the CVT 1 is in the middle of a shifting
action, and if a positive determination is made in step S41, the
control returns without performing any particular control.
[0086] If the CVT 1 is not in the middle of a shifting action and a
negative determination is made in step S41, the actual gear ratio
.gamma. is calculated in step S42 as a ratio between the input
rotation speed Nin and the output rotation speed Nout, both
obtained through actual measurements. Subsequently, a target gear
ratio .gamma.tag is calculated in step S43 as a ratio between the
target input rotation speed Nint and the output rotation speed Nout
obtained through an actual measurement. Then, it is determined in
step S44 whether an absolute value of a difference between the
actual gear ratio .gamma. and the target gear ratio .gamma.tag is
larger than a reference value .DELTA..gamma.a that has been
determined in advance.
[0087] FIG. 10 is a graph showing one example of a situation where
the difference between the actual gear ratio .gamma. and the target
gear ratio .gamma.tag changes. Since the input rotation speed
changes due to various factors while the CVT 1 is being operated,
as described above, the gear ratio difference between the actual
and target gear ratios keeps varying slightly in the positive and
negative directions with respect to zero as shown in FIG. 10. When
a macro-slip is not occurring, the gear ratio difference is
maintained in a small range. When a macro-slip occurs, however, the
input rotation speed starts deviating largely from the target
value, resulting in an increase in the gear ratio difference.
Accordingly, it is possible to determine an occurrence or a
possibility of a macro-slip by determining whether the gear ratio
difference is smaller or larger than a threshold value established
for determination of macro-slips.
[0088] More specifically, in the example as illustrated in FIG. 9,
the number of times the gear ratio difference exceeds the reference
value .DELTA..gamma.a is counted in step S45. Subsequently, it is
determined in step S46 whether the above number of times the above
condition (|.gamma.-.gamma.tag|>.DELTA..gamma.a) of step S44 is
satisfied has reached a predetermined number within a predetermined
period of time. This determination is made so as to prevent an
erroneous determination due to disturbances such as noise.
[0089] If a positive determination is made in step S46, an
occurrence or a possibility of slippage, or a macro-slip, of the
belt 15 is determined in step S47. In this case, the slippage
detection system performs a control in response to the detected
macro-slip, such as increasing the belt clamping force or reducing
the engine torque, as in the respective examples of control as
described above. If a negative determination is made in step S46,
conversely, the control returns.
[0090] On the other hand, if the gear ratio difference is equal to
or smaller than the reference value .DELTA..gamma.a and a negative
determination is made in step S44, it is then determined in step
S48 whether this state has lasted for a predetermined period of
time. If a negative determination is made in step S48, the control
returns, thus waiting for time to pass. When a positive
determination is made in step S48, on the other hand, it indicates
that the actual gear ratio .gamma. is not largely different from
the target gear ratio .gamma.tag and this situation has lasted for
the predetermined period. In this case, the slippage determination
is canceled in step S49.
[0091] In order to perform the control as illustrated above with
reference to FIG. 9, the slippage detection system of the
embodiment uses only the input rotation speed sensor 20 as a sensor
to immediately and accurately determine macro-slips of the belt 15
without requiring other sensor or sensors for this purpose.
Moreover, since the slippage detection system is able to perform
required responsive control upon detection of a macro-slip, an
otherwise possible damage to the CVT 1 as a result of excessive
slippage of the belt 15 can be prevented.
[0092] In the control as illustrated in FIG. 9, the number of times
the gear ratio difference exceeds the reference value
.DELTA..gamma.a is counted for determining an occurrence or a
possibility of slippage as described above. Instead, the sum of
gear ratio differences that have been accumulated for a
predetermined period of time or at a predetermined number of
sampling points may be used for determining an occurrence or a
possibility of slippage. More specifically, the number of times the
sum of the gear ratio differences accumulated as described above
exceeds a predetermined reference value sum.gamma. is counted. If
the number of times has reached a predetermined number within a
predetermined period, it is determined that slippage is occurring.
FIG. 11 is a flowchart showing one example of control for
determining an occurrence or a possibility of a macro-slip in such
a manner. The operations in respective steps of the flowchart
illustrated in FIG. 11 are the same as those in the flowchart
illustrated in FIG. 9 except that step S44 of FIG. 9 is replaced by
to step S44A of FIG. 11. Meanwhile, the reference value sum.gamma.
may be changed in accordance with the operating state of the
vehicle, rather than being constant, so as to prevent an erroneous
determination or a delay in determining an occurrence or a
possibility of a macro-slip.
[0093] FIG. 12 is a graph showing changes in the sum of the gear
ratio differences that are added up with respect to a predetermined
number of (e.g., ten) sampling points shown in FIG. 10. As is
apparent from FIG. 12, when a macro-slip does not occur, the sum of
the gear ratio differences is maintained at relatively small
values. When a macro-slip occurs, on the other hand, the sum begins
to increase rapidly. Accordingly, it is possible to determine an
occurrence or a possibility of a macro-slip when the sum exceeds a
predetermined threshold value. Alternatively, it is also possible
to determine an occurrence or a possibility of a macro-slip based
on the number of times the sum exceeds the reference value
sum.gamma., rather than merely comparing the sum with the threshold
value, so that an erroneous determination due to some type of
disturbance can be prevented or avoided.
[0094] In order to perform the control as illustrated above with
reference to FIG. 11, the slippage detection system of the
embodiment uses only the input rotation speed sensor 20 as a sensor
to immediately and accurately determine macro-slips of the belt 15
without requiring other sensor or sensors for this purpose.
Moreover, since the slippage detection system is able to perform
required responsive control upon detection of a macro-slip, an
otherwise possible damage to the CVT 1 as a result of excessive
slippage of the belt 15 can be prevented.
[0095] As is apparent from FIG. 10 or 12 showing changes in the
gear ratio difference or the sum of gear ratio differences, once
the belt 15 starts slipping (i.e., a macro-slip appears), these
values continue to increase progressively, and are maintained at
large values until, for example, the CVT 1 breaks and stops
operating, or the belt clamping force is extremely increased, or
the engine torque is extremely reduced. Accordingly, an occurrence
or a possibility of a macro-slip may be determined based on a time
duration for which the sum of gear ratio differences is kept larger
than the reference value sum.gamma., instead of counting the number
of times the sum exceeds the reference value sum.gamma..
[0096] FIG. 13 is a flowchart showing one example of control for
determining an occurrence or a possibility of a macro-slip in such
a manner. The operations in respective steps of the flowchart
illustrated in FIG. 13 are the same as those in the flowchart
illustrated in FIG. 11, except that steps S45 and S46 in the
flowchart of FIG. 11 are replaced by step S45A of the flowchart of
FIG. 13. In step S45A, it is determined whether the condition
determined in step S44A has been continuously satisfied for a
predetermined period of time.
[0097] In order to perform the control as illustrated above, the
slippage detection system of the embodiment uses only the input
rotation speed sensor 20 as a sensor to immediately and accurately
determine macro-slips of the belt 15 without requiring other sensor
or sensors for this purpose, as in the case of control of FIG. 11.
Moreover, since the slippage detection system is able to perform
required responsive control upon detection of a macro-slip, an
otherwise possible damage to the CVT 1 as a result of excessive
slippage of the belt 15 can be prevented.
[0098] While the determination of belt slippage (macro-slip) is not
performed during a shifting action of the CVT 1 in the control
routines as illustrated in FIGS. 9, 11, and 13, the slippage
detection system may be constructed so as to perform the
determination of belt slippage even when the CVT 1 is in the middle
of a shifting action. In this case, however, the target input
rotation speed is set to a large value and the difference between
the actual gear ratio and the target gear ratio becomes large,
which may result in a deterioration in the accuracy in determining
belt slippage. When performing the determination of belt slippage
by using the gear ratio difference during a shifting action of the
CVT 1, therefore, it is preferable to subject the target input
rotation speed to a smoothing operation, and calculate the gear
ratio difference using a target gear ratio determined based on the
smoothened target input rotation speed. In this manner, an
erroneous determination on an occurrence or a possibility of a
macro-slip can be suppressed or avoided.
[0099] While the slippage detection systems of the illustrated
embodiments of the invention are adapted for use in belt-type CVTs,
the invention may be applied to slippage detection systems for use
in toroidal-type (traction-type) continuously variable
transmissions. Furthermore, the input rotation speed is not limited
to the rotation speed of the input shaft. More specifically, the
input rotation speed may be defined as the rotation speed of any
member of the continuously variable transmission that is arranged
to rotate by receiving torque from a power source, or the rotation
speed of any member provided as a unit with that member. In the
same way, the output rotation speed is not limited to the rotation
speed of the output shaft. More specifically, the output rotation
speed may be defined as the rotation speed of any member of the
continuously variable transmission that is arranged to rotate with
torque transmitted from an input-side member or the rotation speed
of any member provided as a unit with that member. Still further,
the slippage detection system according to the invention may be
constructed so as to perform a plurality of the above-described
slippage determination controls in combination.
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