Slip control apparatus for motor vehicle lock-up clutch

An apparatus for controlling the amount of slip of a lock-up clutch between pump and turbine impeller of a torque converter or other fluid-filled power transmitting device in a motor vehicle, the lock-up clutch including a piston which is operated by a pressure difference between pressures in two oil chambers on the opposite sides of the piston, and the apparatus including a slip control device for controlling the lock-up clutch according to a slip control output such that the actual slip of the lock-up clutch coincides with a target slip speed, and an output oscillating device for oscillating the slip control output to oscillate the pressure difference without vibrating the piston. The slip control output is oscillated preferably when the pressure difference is relatively small, for example, when the torque of the lock-up clutch is smaller than a threshold or the vehicle deceleration rate is higher than a threshold, or when the target slip speed determined depending upon the vehicle running condition is almost equal to the actual slip speed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for controlling the amount of 
slip of a lock-up clutch provided in a power transmitting system of a 
motor vehicle. 
2. Discussion of the Related Art 
In a motor vehicle having a fluid-filled power transmitting device equipped 
with a lock-up clutch, such as a torque converter or fluid coupling 
incorporating such a lock-up clutch, it is proposed to control the lock-up 
clutch in a slip control mode (partially slipping mode) such that an 
actual amount of slip (slip speed) of the lock-up clutch, namely, a 
difference between the speeds of a pump impeller and a turbine impeller 
eventually coincides with a predetermined target slip speed, for the 
purpose of improving the fuel economy of the vehicle while minimizing the 
power loss due to slipping of the lock-up clutch. The slip control mode is 
established when the running condition of the vehicle is in a 
predetermined slip control area which is intermediate between a fully 
releasing area in which the lock-up clutch should be held in a fully 
released state, and a fully engaging area in which the lock-up clutch 
should be held in a fully engaged state. These fully releasing, fully 
engaging and slip control areas are defined by suitable parameters (e.g., 
throttle valve opening and vehicle running speed) indicative of the 
vehicle running condition. 
Generally, a lock-up clutch whose slip speed or amount is adjustable is 
provided with a piston which is operated by a hydraulic pressure source 
that permits full engagement of the lock-up clutch. Described in detail, 
the piston is moved depending upon a difference between pressures in two 
oil chambers, which are formed on the opposite sides of the piston. The 
amount of slip of the lock-up clutch is controlled by controlling the 
pressure difference of the two oil cheers to thereby change a thrust force 
acting on the piston and the resulting friction force of the clutch. Since 
the hydraulic pressure source that permits the full engagement of the 
clutch is utilized to control the lock-up clutch in the slip control mode, 
even a small amount of change in the pressure difference of the two oil 
chambers will result in a considerable amount of change in the slip amount 
of the lock-up clutch. That is, the slip amount of the clutch controlled 
in a feedback fashion tends to be excessively sensitive to a change in a 
slip control signal generated by a feedback control system. Further, the 
feedback control of the slip amount of the lock-up clutch suffers from 
comparatively low control stability, such as a variation in the speed of a 
vehicle engine due to a low control response upon initiation of the slip 
control of the lock-up clutch. To avoid engine speed variation, there is 
proposed a slip control apparatus wherein the slip control output prior to 
the initiation of the slip control is adjusted so that the pressure 
difference of the oil chambers is suitable for initiating the slip control 
without a variation in the engine speed due to the low control response. 
An example of such a slip control apparatus is disclosed in JP-A-5-99331. 
However, the conventional slip control apparatus indicated above suffers 
from a poor response of the clutch piston to a change in the slip control 
output during the slip control of the lock-up clutch while the vehicle is 
in a certain decelerating or accelerating condition. Described more 
particularly, the pressure difference and the engagement force of the 
lock-up clutch are small where the target slip speed determined depending 
upon the vehicle running condition is almost equal to a difference of the 
input and output speeds of the fluid-filled power transmitting device 
(=slip speed of the device), which difference would exist or be 
established when the the lock-up clutch is placed in its fully released 
state. In this condition, the target slip speed can be attained by a 
comparatively small change in the slip control output of the slip control 
apparatus. Where the pressure difference of the lock-up piston is small, 
therefore, the actual slip speed of the lock-up clutch is not highly 
responsive to a change in the slip control output, due to a sliding 
resistance of the clutch piston and an oil leakage from the hydraulic 
system. Therefore, the feedback control of the pressure difference of the 
clutch tends to cause a control hunting, leading to deteriorated control 
stability of the lock-up clutch in certain running condition of the 
vehicle as indicated above. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an apparatus 
for controlling the amount of slip of a lock-up clutch in a fluid-filled 
power transmitting system of a motor vehicle, which apparatus assures 
improved control stability even if the vehicle is in a running condition 
wherein the target slip speed determined by the running condition is 
almost equal to a speed difference or slip speed of the power transmitting 
device, which difference would be established when the lock-up clutch is 
placed in the fully released state. 
The above object may be achieved according to the principle of this 
invention, which provides an apparatus for controlling an amount of slip 
of a lock-up clutch disposed between a pump impeller and a turbine 
impeller of a fluid-filled power transmitting device of a motor vehicle, 
the lock-up clutch including a piston operated by a pressure difference 
between hydraulic pressures in two oil chambers formed on opposite sides 
of the piston, the apparatus comprising: (a) slip control means for 
effecting a slip control operation according to a slip control output to 
control the amount of slip of the lock-up clutch while a running condition 
of the vehicle is in a predetermined slip control area, such that an 
actual slip speed of the lock-up clutch coincides with a target slip speed 
determined depending upon the running condition; and (b) output 
oscillating means operable during the slip control operation of the slip 
control means, for oscillating the slip control output of the slip control 
means, to oscillate the pressure difference without vibrating the piston. 
In the slip control apparatus of the present invention constructed as 
described above, the slip control output of the slip control means for 
controlling the amount of slip of the lock-up clutch is oscillated by the 
output oscillating means to oscillate the pressure difference of the two 
oil chambers, without vibrating the piston. Accordingly, the thrust force 
which acts on the piston is oscillated so as to permit smooth movement of 
the piston with a high response to a change in the slip control output, 
even when the pressure difference acting on the piston is considerably 
small. In other words, the present slip control apparatus assures improved 
stability of control of the slip amount of the lock-up clutch even when 
the vehicle is in a running condition wherein the target slip speed 
determined by the running condition is almost or substantially equal to 
the speed difference or slip speed of the power transmitting device, which 
slip speed would be established when the lock-up clutch is placed in the 
fully released state. 
The output oscillating means may be adapted to oscillate the slip control 
output of the slip control means at a predetermined period. 
In one preferred form of this invention, the apparatus further comprises 
torque monitoring means which is operated during the slip control 
operation of the slip control means, for determining whether a torque 
transmitted through the lock-up clutch is smaller than a predetermined 
threshold. In this case, the output oscillating means oscillates the slip 
control output if the torque monitoring means determines that the torque 
is smaller than the predetermined threshold. The torque smaller than the 
threshold indicates that the vehicle is in a running condition wherein the 
target slip speed of the lock-up clutch is almost equal to the slip speed 
of the lock-up clutch which would be established when the lock-up clutch 
is placed in the fully released state. In this form of the invention, the 
slip control output of the slip control means is not oscillated during the 
entire period of the slip control operation, but is oscillated only while 
the torque transmitted through the lock-up clutch (transmission torque of 
the clutch) is smaller than the threshold value. Accordingly, the life 
expectancy of a control value used in the slip control means for 
controlling the pressure difference of the lock-up clutch is improved. 
In one advantageous arrangement of the above preferred form of the 
invention, the torque monitoring means comprises an idling position switch 
for detecting an engine idling position of a throttle valve for idling an 
engine of the vehicle, and the output oscillating means oscillates the 
slip control output for a predetermined length of time after the idling 
position switch has detected the engine idling position of the throttle 
valve. When the throttle valve is placed in the engine idling position, 
the transmission torque of the lock-up clutch is considerably small. In 
this condition, the slip control output is oscillated for the limited 
length of time after the throttle valve is brought to the engine idling 
position. Thus, the life expectancy of the control valve for controlling 
the pressure difference of the lock-up clutch is further improved. Where 
the slip control output of the slip control means includes a learning 
control value in addition to a feed-forward control value and a feedback 
control values which are normally used, the predetermined length of time 
during which the slip control output is oscillated is determined so that 
the learning control value can be determined within that length of time. 
In this case, the slip control of the lock-up clutch can be effected with 
high stability even after the predetermined length of time has passed. 
In another preferred form of the present invention, the apparatus further 
comprises deceleration monitoring means for determining whether a rate of 
deceleration of the vehicle is higher than a predetermined threshold. In 
this instance, the output oscillating means oscillates the slip control 
output if the deceleration monitoring means determines that the rate of 
deceleration is higher than the predetermined threshold. The vehicle 
running condition may enter the slip control area if the engine speed is 
suddenly lowered due to deceleration of the vehicle upon brake 
application, for example. In this condition, the target slip speed 
determined depending upon the vehicle running condition may be almost 
equal tot the slip speed of the lock-up clutch, and the pressure 
difference acting on the piston of the lock-up clutch is considerably 
small, whereby the piston is not highly responsive to a change in the slip 
control output. According to the present form of the invention wherein the 
slip control output is oscillated upon detection of the vehicle 
deceleration at a rate higher than the predetermined threshold, the thrust 
force acting on the clutch piston is oscillated, so as to permit smooth 
operation of the piston in response to the slip control output even when 
the pressure difference is considerably small. 
In a further preferred form of the instant invention, the apparatus further 
comprises: slip speed calculating means operable upon initiation of the 
slip control operation of the slip control means, for calculating an slip 
speed of the lock-up clutch which is a difference between rotating speeds 
of the pump and turbine impellers of the fluid-filled power transmitting 
device; target slip speed determining means operable upon initiation of 
the slip control operation, for determining a target slip speed of the 
lock-up clutch; and slip speed monitoring means for determining whether 
the actual slip speed calculated by the slip speed calculating means is 
almost equal to the target slip speed determined by the target slip speed 
determining means. In this case, the output oscillating means oscillates 
the slip control output of the slip control means if the slip speed 
monitoring means determines that the actual slip speed is almost equal to 
the target slip speed. In a certain running condition of the vehicle, the 
target slip speed of the lock-up clutch may be almost equal to the actual 
slip of the lock-up clutch (speed difference of the pump and turbine 
impellers of the fluid-filled power transmitting device), even when the 
throttle valve is not placed in its engine idling position. In the present 
arrangement, such running condition of the vehicle is detected by 
comparing the actual slip speed and the target slip speed with each other, 
and the slip control output is oscillated when such running condition is 
detected, namely, when the pressure difference of the lock-up clutch is 
considerably small. Thus, the oscillation of the slip control output 
permits smooth movement of the piston with high response t the slip 
control output even when the pressure difference acting on the clutch 
piston is small.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to the schematic view of FIG. 1, there is shown a part of a 
power transmitting system of a motor vehicle, wherein power generated by 
an engine 10 is transmitted to a differential gear device and drive wheels 
through a torque converter 12 equipped with a lock-up clutch 32, and an 
automatic transmission 14 which includes three planetary gear sets to 
selectively establish a plurality of operating positions (gear positions). 
The torque converter 12 includes; a pump impeller 18 connected to a 
crankshaft 16 of the engine 10; a turbine impeller 22 fixed to an input 
shaft of the automatic transmission 14 and rotatable by the pump impeller 
18; a stator impeller 28 fixed to a stationary member in the form of a 
housing 26 through a one-way clutch 24; and the above-indicated lock-up 
clutch 32 connected to the input shaft 20 through the turbine impeller 22. 
The pump impeller 18 includes a radially outer portion which is U-shaped 
in cross section, and a plurality of curved vanes which are arranged in 
the circumferential direction and formed so as to cause a flow of a 
working oil, which flow includes a component moving toward the turbine 
impeller 22 on the side of the engine 10. The turbine impeller 22 includes 
a plurality of curved vanes opposed to the vanes of the pump impeller 18. 
In operation of the torque converter 12, the turbine impeller 22 is 
rotated by the oil flow from the vanes of the pump impeller 18 rotated by 
the engine 10. The lock-up clutch 32 includes a piston 30 which engages a 
hub of the turbine impeller 22 such that the piston 30 is axially slidable 
relative to and rotatable with the turbine impeller 22. 
The piston 30 of the lock-up clutch 32 divides an interior of the torque 
converter 12 into two oil chambers 33 and 35. The lock-up clutch 32 is 
released and engaged by axial movements of the piston 32 depending upon a 
difference between oil pressures in these two oil chambers 33, 35, which 
will be hereinafter referred to as a releasing oil chamber 33 and an 
engaging oil chamber 35, respectively. Described more specifically, the 
piston 30 is retracted to its fully retracted position when the pressure 
in the releasing oil chamber 33 is increased while the engaging oil 
chamber 35 is drained. When the pressure in the engaging oil chamber 35 is 
increased while the releasing oil chamber 33 is held at the lowest level, 
the piston 30 is advanced to its fully advanced position. In the fully 
retracted position of the piston 30, the lock-up clutch 32 is placed in 
its fully released position in which the torque received by the pump 
impeller 18 is amplified or boosted at a ratio depending upon the ratio of 
the input and output speeds of the torque converter 12. In the fully 
advanced position of the piston 30, the lock-up clutch 32 is placed in the 
fully engaged position in which the frictional coupling portion of the 
clutch 32 is forced against the radially outer U-shaped portion of the 
pump impeller 18, whereby the pump impeller 18 is directly connected to 
the input shaft 20, that is, the crankshaft 16 as an input member of the 
torque converter 12 is directly connected to the input shaft 20 of the 
transmission 14, which serves as an output member of the torque converter 
12. When the pressure in the releasing oil chamber 33 is increased to a 
predetermined level while the pressure in the engaging oil chamber 35 is 
held at a higher level, the piston 30 is advanced to a predetermined 
position in which the frictional coupling portion of the lock-up clutch is 
located near the corresponding coupling portion (radially outer U-shaped 
portion) of the pump impeller 18. The predetermined level of the pressure 
in the releasing oil chamber 33 indicated above is determined by a second 
term ("feed forward term") of a right member of an equation (2) which will 
be described. 
The automatic transmission 14 includes: the input shaft 20, a first, a 
second and a third planetary gear set 34, 36, 38; an output gear 39 which 
rotates with a ring gear of the third planetary gear set 38; and an output 
shaft in the form of a counter shaft 40 which connects the output gear 39 
and the differential gear device. The planetary gear sets 34, 36, 38 
include components which are connected integrally with each other, and 
components which are connected to each other when three clutches C0, C1, 
C2 are selectively engaged. The planetary gear sets 34, 36, 38 also 
include components which are fixed or connected to the housing 26 and 
thereby inhibited from rotating when four brakes B0, B1, B2, B3 are 
selectively engaged. The planetary gear sets 34, 36, 38 further include 
components which are connected to each other or to the housing 26 through 
three one-way clutches F0, F1, F2, depending upon the rotating directions 
of the components. 
Each of the clutches C0, C1, C2 and brakes B0, B1, B2, B3 may consist of a 
multiple-disk clutch, or a band brake which uses two bands wound in 
opposite directions. These clutches and brakes are operated by respective 
hydraulically operated actuators, which are controlled by an electronic 
transmission controller 184 shown in FIG. 3 (which will be described), so 
as to selectively establish a plurality of operating positions of the 
automatic transmission 14. That is, the automatic transmission 14 has four 
forward drive positions, first-speed ("1st"), second-speed ("2nd"), 
third-speed ("3rd") and overdrive ("O/D") positions, and one backward 
drive position "R", as indicated in FIG. 2. The four forward drive 
positions "1st", "2nd", "3rd" and "O/D" have respective different speed 
ratios I which decrease in the order of description. The speed ratio I is 
defined as the speed of the input shaft 20 divided by the speed of the 
counter shaft (output shaft) 40. 
It is to be noted that the lower halves of the torque converter 12 and 
automatic transmission 14 and the upper half of the counter shaft 40 are 
not shown in FIG. 1 in the interest of simplification, since these 
elements 12, 14, 40 are symmetrical with respect to their axes of 
rotation. 
Referring next to the block diagram of FIG. 3, there will be described a 
control system provided to control the engine 10, lock-up clutch 32 and 
automatic transmission 14 of the motor vehicle. The control system 
includes the electronic transmission controller 184 indicated above, which 
is adapted to control a hydraulic control device 44. The hydraulic control 
device 44 includes a transmission control circuit for shifting the 
automatic transmission 14 to an appropriate one of the operating 
positions, and a lock-up clutch control circuit for controlling the 
operating state of the lock-up clutch 32. The transmission control circuit 
is provided with a first and a second solenoid-operated valve S1, S2, 
which have respective solenoid coils. The clutches C0, C1, C2 and brakes 
B0, B1, B2, B3 are selectively engaged to selectively establish the 
operating positions ("1st", "2nd", "3rd" and "O/D") of the transmission 
14, depending upon respective combinations of the operating states of the 
first and second solenoid-operated valves S1, S2, as indicated in FIG. 2. 
In this figure, ".smallcircle." indicates the energization of the solenoid 
coils of the valves S1, S2 or the engagement of the clutches and brakes. 
The lock-up clutch control circuit of the hydraulic control device 44 
includes a third solenoid-operated valve S3, a lock-up relay valve 52, a 
linear solenoid valve SLU, and a lock-up clutch control valve 56, as shown 
in FIG. 4. The third solenoid-operated valve S3 has a solenoid coil 48 
which is turned on and off. When the coil 48 is on, the valve 53 generates 
a LOCK-UP SWITCHING pressure P.sub.SW. The lock-up relay valve 52 has a 
releasing state and an engaging state for releasing and engaging the 
lock-up clutch 32, respectively, depending upon whether the pilot pressure 
P.sub.SW is generated by the valve S3. The linear solenoid valve SLU is 
adapted to generate a SLIP CONTROL pilot pressure P.sub.SLU corresponding 
to a SLIP CONTROL current I.sub.SLU supplied from the transmission 
controller 184. The lock-up clutch control valve 56 is adapted to regulate 
a pressure difference .DELTA.P between the pressures in the engaging and 
releasing oil chambers 35, 33 of the torque converter 12, according to the 
SLIP CONTROL pilot pressure P.sub.SLU received from the linear solenoid 
valve SLU, for thereby controlling an amount of slip of the lock-up clutch 
32. 
As shown in FIG. 4, the hydraulic control device 44 includes a pump 60 for 
pressuring a working oil sucked from a suitable reservoir through a 
strainer 58. The pump 60 is driven by the engine 10. The pressure of the 
oil delivered by the pump 60 is adjusted to a first line pressure Pl1 by a 
first pressure regulating valve 62 of an overflow type. The first pressure 
regulating valve 62 is arranged to receive a THROTTLE pilot pressure 
indicative of an opening TAP of a first throttle valve 166 (FIG. 3), and 
regulate the first line pressure Pl1 in a first pressure line 64 such that 
the pressure Pl1 increases with the THROTTLE pilot pressure. The hydraulic 
control device 44 further has a second pressure regulating valve 66 of an 
overflow type, which is adapted to regulate the pressure of the oil 
received from the first pressure regulating valve 62, to a second line 
pressure Pl2 according to the THROTTLE pressure, so that the second line 
pressure Pl.sup.2 corresponds to the output torque of the engine 10. The 
device 44 further has a third pressure regulating valve 68, which is a 
pressure reducing valve adapted to reduce the first line pressure Pl1 to a 
predetermined third line pressure Pl3. 
The motor vehicle has a shift lever 174 (FIG. 3) which has six operating 
positions "P" (KING), "R" (REVERSE), "N" (NEUTRAL), "D" (DRIVE), "S" 
(SECOND) and "L" (LOW), as indicated in FIG. 2. The hydraulic control 
device 44 includes a manual valve 70 (FIG. 4) adapted to generate a 
REVERSE pressure P.sub.R when the shift lever 174 is placed in the REVERSE 
position "R" (which is the backward drive position referred to above with 
respect to the automatic transmission 14). The device 44 also includes an 
OR valve 72 which is adapted to generate a higher one of a BRAKE B2 
pressure P.sub.B2 and the REVERSE pressure P.sub.R, which serves as the 
LOCK-UP SWITCHING pilot pressure P.sub.SW generated when the valve S3 is 
turned ON as explained below in detail. The BRAKE B2 pressure P.sub.B2 is 
generated to engage the brake B2 for establishing the second-speed 
("2nd"), third-speed ("3rd") and overdrive ("O/D") positions. 
The lock-up relay valve 52 has: a releasing port 80 communicating with the 
releasing oil chamber 33; an engaging port 82 communicating with the 
engaging oil chamber 35; an input port 84 adapted to receive the second 
line pressure Pl2; a first drain port 86 through which the oil in the 
engaging oil chamber 35 is discharged when the lock-up clutch 32 is 
released; a second drain port 88 through which the oil in the releasing 
oil chamber 33 is discharged when the lock-up clutch 32 is engaged; a 
supply port 90 adapted to receive the oil discharged from the second 
pressure regulating valve 66 so that the oil is cooled during engagement 
of the lock-up clutch 32; a spool 92 operable between an ON position and 
an OFF position, for switching the mutual communication or connection of 
the ports indicated above; a spring 94 for biasing the spool 92 toward the 
OFF position; a plunger 96 abuttable on the end of the spool 92 on the 
side of the spring 94; an oil chamber 98 defined between the 
above-indicated end of the spool 92 and the opposed end of the plunger 96, 
and adapted to receive the REVERSE pressure P.sub.R from the manual valve 
70; an oil chamber 100 partially defined by the other end of the plunger 
96 and adapted to receive the first line pressure Pl1; and an oil chamber 
102 partially defined by the other end of the spool 92 and adapted to 
receive the LOCK-UP SWITCHING pressure P.sub.SW from the third 
solenoid-operated valve S3, for generating a thrust force for moving the 
spool 92 toward the ON position. 
The third solenoid-operated valve S3 has a ball which is seated on a valve 
seat to disconnect a line communicating with the oil chamber 102 of the 
lock-up relay valve 52 and the OR valve 72 when the solenoid coil 48 is 
de-energized or OFF. In this state, the LOCK-UP SWITCHING pilot pressure 
P.sub.SW is not applied to the oil chamber 102. When the coil 48 is 
energized or ON, the ball is unseated to permit the communication between 
the OR valve 72 and the oil chamber 102, whereby the LOCK-UP SWITCHING 
pressure P.sub.SW is applied to the oil chamber 102. In the OFF state of 
the valve S3, therefore, the spool 92 of the lock-up relay valve 52 is 
moved to its OFF position by the biasing force of the spring 94 and a 
force based on the first line pressure Pl1 in the oil chamber 100, whereby 
the input port 84 communicates with the releasing port 80 while the first 
drain port 86 communicates with the engaging port 82. As a result, a 
pressure Poff in the releasing oil chamber 33 is made higher than a 
pressure Pon in the engaging oil chamber 35, to thereby release the 
lock-up clutch 32, while at the same time the engaging chamber 35 is 
drained through the first drain port 86, an oil cooler 104 and a check 
valve 106. 
In the ON state of the valve S3, on the other hand, the LOCK-UP SWITCHING 
pilot pressure P.sub.SW is applied to the oil chamber 102, and the spool 
92 is moved by a force based on the pressure P.sub.SW against the biasing 
force of the spring 94 and the force based on the first line pressure Pl1 
in the oil chamber 100, whereby the input port 84 communicates with the 
engaging port 82 while the first and second drain ports 86, 88 communicate 
with the supply and releasing ports 90, 80, respectively. As a result, the 
pressure Pon in the engaging oil chamber 35 is made higher than the 
pressure Poff in the releasing oil chamber 33, to thereby engage the 
lock-up clutch 32, while at the same time the releasing oil chamber 33 is 
drained through the second drain port 88 and the lock-up clutch control 
valve 56. 
The linear solenoid valve SLU is a reducing valve adapted to reduce the 
predetermined third line pressure Pl3 to the SLIP CONTROL pilot pressure 
P.sub.SLU, such that the pilot pressure P.sub.SLU increases with an 
increase in the SLIP CONTROL current I.sub.SLU supplied from the 
transmission controller 184, namely, increases with an increase in a duty 
ratio D.sub.SLU of the linear solenoid valve SLU. The thus controlled 
pilot pressure P.sub.SLU is applied to the lock-up clutch control valve 
56. The linear solenoid valve SLU has: a supply port 110 adapted to 
receive the third line pressure Pl3; an output port 112 from which the 
SLIP CONTROL pilot pressure P.sub.SLU is applied to the valve 56; a spool 
114 for closing and opening the ports 110, 112; a spring 115 for biasing 
the spool 114 in a valve closing direction; a spring 116 for biasing the 
spool 114 in a valve opening direction by a force smaller than that of the 
spring 115; a solenoid coil 118 for biasing the spool 114 in the valve 
opening direction by a force determined by the SLIP CONTROL current 
I.sub.SLU ; and an oil chamber 120 adapted to receive a feedback pressure 
(SLIP CONTROL pilot pressure P.sub.SLU) which biases the spool 114 in the 
valve closing direction. The spool 114 is moved to a position of 
equilibrium between a sum of the biasing forces of the solenoid coil 118 
and the spring 116 and a sum of the biasing force of the spring 115 and a 
force based on the feedback pressure P.sub.SLU. 
The lock-up clutch control valve 56 has: a line pressure port 130 adapted 
to receive the second line pressure Pl2; an input port 132 adapted to 
receive the oil discharged from the releasing oil chamber 33 through 
second drain port 88 of the valve 52; a drain port 134 through which the 
oil received by the input port 132 is discharged; a spool 136 operable 
between a first position (indicated at left in FIG. 4) and a second 
position (indicated at right in FIG. 4); a plunger 138 abuttable on the 
spool 136 for biasing the spool 136 toward the first position; an oil 
chamber 140 adapted to receive the SLIP CONTROL pilot pressure P.sub.SLU 
for biasing the plunger 138 so as to generate a thrust force which biases 
the spool 136 toward the first position; an oil chamber 142 adapted to 
receive the oil pressure Poff in the releasing oil chamber 33, for biasing 
the plunger 138 so as to generate a thrust force which biases the spool 
136 toward the first position; an oil chamber 144 adapted to receive the 
oil pressure Pon in the engaging oil chamber 35, for generating a thrust 
force for biasing the spool 136 toward the second position; and a spring 
146 received in the oil chamber 144, for biasing the spool 136 toward the 
second position. 
In the first position of the spool 136 of the lock-up clutch control valve 
56, the input port 132 communicates with the drain port 134 to cause the 
releasing oil chamber 33 to be drained, for thereby increasing the 
pressure difference .DELTA.P(=Pon-Poff) of the oil chambers 33, 35. In the 
second position of the spool 136, the input port 132 communicates with the 
line pressure port 130 to cause the second line pressure Pl2 to be applied 
to the releasing oil chamber 33, for thereby reducing the pressure 
difference .DELTA.P. 
The plunger 138 has a first land 148 adjacent to the oil chamber 142, and a 
second land 150 remote from the oil chamber 142. The first land 148 has a 
cross sectional area A1, and the second land 150 has a cross sectional 
area A2 larger than the area A1. The spool 136 has a third land 152 
adjacent to the pilot pressure oil cheer 140, and a fourth land 154 remote 
from the oil chamber 140. The third land 152 has a cross sectional area 
A3, and the fourth land 154 has a cross sectional area equal to the cross 
sectional area A1. In this arrangement of the lock-up clutch control valve 
56, the plunger 138 and the spool 136 are moved together as a unit with 
the plunger 138 held in abutting contact with the spool 136. With the 
movement of the plunger and spool 138, 136, the pressure difference 
.DELTA.P(=Pon-Poff) on the opposite sides of the piston 30 of the lock-up 
clutch 32 is controlled depending upon the SLIP CONTROL pilot pressure 
P.sub.SLU generated by the linear solenoid valve SLU. The pressure 
difference .DELTA.P changes with the pilot pressure P.sub.SLU as shown in 
FIG. 6, at a rate or gradient 10 represented by a value (A2-A1)/A1 
included in the following equation (1): 
EQU .DELTA.P=Pon-Poff=[(A2-A1)/A1]P.sub.SLU -Fs/A1 (1) 
where, 
Fs: biasing force of the spring 146. 
The graph of FIG. 6 indicates the output characteristic of the lock-up 
clutch control valve 56, namely, the relationship between the pressure 
difference .DELTA.P generated by the valve 56 and the SLIP CONTROL pilot 
pressure P.sub.SLU generated by the valve SLU. While the lock-up clutch 
control valve 56 is ON with the spool 136 placed in the first position, an 
increase in the pilot pressure P.sub.SLU results in an increase in the 
pressure difference .DELTA.P of the engaging and releasing oil chambers 
35, 33, and thereby causes a decrease in a slip speed N.sub.SLP of the 
lock-up clutch 32, while a decrease in the pilot pressure P.sub.SLU causes 
an increase in the slip speed N.sub.SLP. The slip speed N.sub.SLP is a 
difference (N.sub.P -N.sub.T) between a speed N.sub.P of the pump impeller 
18 (speed N.sub.E of the engine 10) and a speed N.sub.T of the turbine 
impeller 22 (speed Nin of the input shaft 20). 
Referring back to the block diagram of FIG. 3, the motor vehicle has 
various sensors and switches including: an engine speed sensor 160 for 
detecting the speed N.sub.E of the engine 10, that is, speed N.sub.P of 
the pump impeller 18; an intake air quantity sensor 162 for detecting a 
quantity Q of an intake air sucked into the engine 10 through an intake 
pipe; an intake air temperature sensor 164 for detecting a temperature 
T.sub.AIR of the intake air; a throttle sensor 167 for detecting the 
opening TAP of the first throttle valve 166 operated by an accelerator 
pedal 165, the throttle sensor 167 being equipped with an idling position 
switch for detecting the idling position of the throttle valve 166; a 
vehicle speed sensor 168 for detecting a running speed V of the vehicle on 
the basis of a speed Nout of the output shaft 40 of the automatic 
transmission 40; a water temperature sensor 170 for detecting a 
temperature T.sub.WA of a coolant water of the engine 10; a brake switch 
172 for detecting an operation of a brake pedal; a shift position sensor 
176 for detecting a currently selected operating position Ps of the 
automatic transmission 40, namely, a currently selected one of the 
operating positions "L", "S", "D", "N", "R" and "P" of the shift lever 
174; a turbine speed sensor 178 for detecting the speed N.sub.T of the 
turbine impeller 22, that is, the speed Nin of the input shaft 20 of the 
transmission 20; and an oil temperature sensor 180 for detecting a 
temperature T.sub.OIL of the working oil in the hydraulic control device 
44. The output signals generated by the above sensors and switch are 
applied directly or indirectly to an electronic engine controller 182 and 
the electronic transmission controller 184. The two controllers 182, 184 
are connected to each other by a communication interface, for applying the 
necessary signals to each other. 
The transmission controller 184 is comprised of a so-called microcomputer 
incorporating a central processing unit (CPU), a read-only memory (ROM), a 
random-access memory (RAM) and an interface. The CPU processes the input 
signals according to various control programs stored in the ROM, while 
utilizing a temporary data storage function of the RAM, for controlling 
the automatic transmission 14 and the lock-up clutch 32 by controlling the 
first, second and third solenoid-operated valves S1, S2, S3 and the linear 
solenoid valve SLU. 
For controlling the automatic transmission 14 so as to shift the 
transmission 14 to the appropriate operating position, a plurality of 
shift patterns are stored in the ROM, and one of the shift patterns which 
corresponds to the currently selected position of the transmission 14 is 
selected to determine the operating position (one of the four forward 
drive positions) to which the transmission 14 should be shifted down or 
up. For instance, each shift pattern consists of a shift-down boundary 
line and a shift-up boundary line which are relationships between the 
throttle valve opening TAP and the vehicle speed V. On the basis of the 
determined forward drive position to which the transmission 14 should be 
shifted, the solenoid-operated valves S1 and S2 are suitably controlled 
(with their solenoid coils being suitably energized or de-energized), so 
as to establish an appropriate combination of the operating states of the 
clutches and brakes C0, C1, C2, B0, B1, B2, B3, which combination 
corresponds to the determined forward drive position. 
The transmission controller 184 is adapted to control the lock-up clutch 32 
in the manner explained below, when the vehicle is running with the 
transmission 14 placed in the third-speed or fourth-speed or overdrive 
position ("3rd" or "O/D"), for example. For controlling the lock-up clutch 
32 differently depending upon the running condition of the vehicle, 
predetermined boundaries defining three different control areas as 
indicated in FIG. 7 are stored in the ROM. For instance, the boundaries 
are relationships between the throttle valve opening TAP and the output 
speed Nout of the output shaft 40 of the transmission 14 (vehicle speed 
V). Described more specifically, these boundaries define a fully releasing 
area in which the lock-up clutch 32 should be fully released, a fully 
engaging area in which the clutch 32 should be fully engaged, and a slip 
control area in which the amount of slip of the clutch 32 should be 
suitably controlled according to the principle of the present invention as 
described below in detail. Depending upon the currently detected throttle 
opening TAP and output speed Nout, one of the three control areas is 
determined or selected by the CPU of the transmission controller 184, 
according to the boundaries stored in the ROM. 
When the vehicle running condition (TAP and Nout) is in the slip control 
area, the lock-up clutch 32 is controlled to be held in a partially 
slipping state for transmitting power of the engine 10 to the automatic 
transmission 14 so as to maximize the fuel economy of the vehicle while 
absorbing a torque variation of the engine 10 to assure high drivability 
of the vehicle. The determination as to whether the vehicle running 
condition falls in the slip control area according to the boundaries of 
FIG. 7 stored in the ROM is effected while the vehicle is accelerating. In 
this respect, it is noted that the amount of slip of the lock-up clutch 32 
is also controlled while the vehicle is coasting or decelerating with the 
throttle valve 166 placed in the idling position. This slip control is 
effected to increase an effect of the fuel-cut control of the engine 10. 
In this case, however, the slip control area is determined on the basis of 
only the vehicle speed V, since the throttle opening TAP is zero during 
the coasting of the vehicle. 
If the CPU of the controller 184 determines that the vehicle running 
condition falls in the fully engaging area, the solenoid coil of the third 
solenoid-operated valve S3 is energized to turn ON the lock-up relay valve 
52, and the SLIP CONTROL current I.sub.SLU applied to the linear solenoid 
valve SLU is reduced to the minimum value, whereby the lock-up clutch 32 
is fully engaged. If the vehicle running condition is determined to be in 
the fully releasing area, the solenoid coil of the valve S3 is 
de-energized to turn OFF the lock-up relay valve 52, so that the lock-up 
clutch 32 is fully released irrespective of the SLIP CONTROL current 
I.sub.SLU. If the vehicle running condition is determined to be in the 
slip control area, the solenoid coil of the valve S3 is energized to turn 
ON the lock-up relay valve 52, and the SLIP CONTROL current I.sub.SLU to 
be applied to the valve SLU, namely, a duty ratio D.sub.SLU of the valve 
SLU is adjusted according to the following equation (2) to control the 
amount of slip of the lock-up clutch 32 in a slip control mode: 
EQU D.sub.SLU (=I.sub.SLU)=DFWD+KGD+DFB+tDITH (2) 
For instance, the duty ratio D.sub.SLU is calculated to zero an error 
.DELTA.E (=N.sub.SLP -TN.sub.SLP) between a target slip speed TN.sub.SLP 
and the actual slip speed N.sub.SLP (=N.sub.E -N.sub.T) of the lock-up 
clutch 32. The first term DFWD of the right member of the above equation 
(2) is a feed-forward control value, which varies as a function the output 
torque of the engine 10, for example. The second term KGD is a learning 
control value which changes so as to reflect the varying characteristics 
of the lock-up clutch 32. The third term DFB is a feedback control value 
consisting of a proportional value, a differential value and an integral 
value of the control error .DELTA.E. The fourth term tDITH is an 
oscillation value (which will be described) for eliminating a delay of the 
engagement force of the lock-up clutch 32 which would occur when the 
pressure difference .DELTA.P=Pon-Poff is relatively small. 
The feedback control value DFB is obtained according to the following 
equation (3): 
EQU DFB=K.sub.p [.DELTA.E+(1/T1).DELTA.Edt+T.sub.D (d.DELTA.E/dt)](3) 
The electronic engine controller 182 is comprised of a microcomputer 
similar to that of the transmission controller 184, which has a CPU 
adapted to process the input signals according to programs stored in a ROM 
while utilizing a temporary data storage function of a RAM, for 
controlling the engine 10, more specifically, for effecting a fuel 
injection control for controlling a fuel injection valve 186 so as to 
optimize the combustion condition of the engine 10, an ignition control 
for controlling an ignitor 188 so as to optimize the ignition timing, a 
traction control for controlling a second throttle valve 192 via a 
throttle actuator 190 so as to control the traction force of the vehicle 
while preventing slipping of the drive wheels on the road surface, and a 
fuel-cut control for holding the fuel injection valve 186 closed while the 
engine speed N.sub.E is higher than a predetermined fuel-cut threshold 
level N.sub.CUT during coasting of the vehicle, so that the fuel economy 
of the vehicle is improved. 
Referring next to the block diagram of FIG. 8, there will be described the 
functions of various functional means provided in a slip control 
apparatus, which is primarily constituted by the electronic transmission 
controller 184. The slip control apparatus incorporates slip control means 
196, output oscillating means 198, torque monitoring means 200, 
oscillation time determining means 202, deceleration monitoring means 204, 
slip speed calculating means 206, target slip speed determining means 208, 
and slip speed monitoring means 210. 
When the vehicle running condition is determined to fall in the slip 
control area explained above by reference to FIG. 7, the slip control 
means 196 provides a slip control output in the form of the SLIP CONTROL 
current I.sub.SLU (=duty ratio D.sub.SLU) calculated according to the 
above equation (2), to control the amount of slip of the lock-up clutch 32 
in the slip control mode, so that the actual slip speed N.sub.SLP (N.sub.E 
-N.sub.T) coincides with the target slip speed TN.sub.SLP. The output 
oscillating means 198 is adapted to oscillate the slip control output 
I.sub.SLU or D.sub.SLU at a predetermined period or interval, without 
oscillating the piston 30, while the slip control means 196 is in 
operation. The torque monitoring means 200 is adapted to determine whether 
the torque transmitted through the lock-up clutch 32 (hereinafter referred 
to as "transmission torque of the clutch 32) is smaller than a 
predetermined threshold during operation of the slip control means 196. 
The transmission torque of the lock-up clutch 32 smaller than the 
predetermined threshold indicates a vehicle running condition in which the 
target speed TN.sub.SLP determined by the running condition according to a 
predetermined relationship (described below) is almost equal to a speed 
difference (N.sub.P -P.sub.T) of the torque converter 12 (=slip speed 
N.sub.SLP of the clutch 32), which would be established or exist when the 
clutch 32 is placed in the fully released state. In this specific 
condition, the target slip speed TN.sub.SLP can be attained by a small 
change in the slip control output D.sub.SLU or I.sub.SLU of the slip 
control means 196. In other words, the pressure difference .DELTA.P and 
the engagement force of the lock-up clutch 32 are so small that the piston 
30 is not highly responsive to a change in the slip control output 
D.sub.SLU. This specific condition wherein the transmission torque of the 
clutch 32 is relatively small may be, for example: a condition in which 
the idling position switch of the throttle sensor 167 is in the ON 
position (the throttle valve 166 is in the engine idling position); a 
condition in which the amount of opening TAP of the throttle valve 166 is 
smaller than a predetermined threshold; and a condition in which the speed 
difference of the torque converter 12 (slip speed N.sub.SLP of the lock-up 
clutch 32) when the vehicle running state has just entered the slip 
control area of FIG. 7 is almost equal to the determined target slip speed 
TN.sub.SLP. 
If the torque monitoring means 200 determines that the transmission torque 
of the lock-up clutch 32 is smaller than the predetermined threshold, the 
output oscillating means 198 oscillates the slip control output of the 
slip control means 196 for a length of time determined by the oscillation 
time determining means 202. The deceleration monitoring means 204 
determines whether the deceleration of the vehicle is higher than a 
predetermined threshold. If the deceleration monitoring means 204 
determines that the vehicle deceleration is higher than the predetermined 
threshold, the output oscillating means 198 oscillates the slip control 
output of the slip control means 196 at the predetermined period. 
The slip speed calculating means 206 is adapted to calculate the slip speed 
N.sub.SLP (=N.sub.P -N.sub.T) of the torque converter 12 (speed difference 
between the pump and turbine impellers 18, 22) prior to the initiation of 
the slip control of the lock-up clutch 32 by the slip control means 196. 
The target slip speed determining means 208 determines the target slip 
speed TN.sub.SLP used in the slip control by the slip control means 196. 
The slip speed monitoring means 210 determines whether the target slip 
speed TN.sub.SLP determined by the target slip speed determining means 208 
is almost equal to the slip speed N.sub.SLP calculated by the slip speed 
calculating means 206. If the slip speed monitoring means 210 determines 
that the target slip speed TN.sub.SLP and the slip speed N.sub.SLP are 
almost equal to each other, the output oscillating means 198 oscillates 
the slip control output of the slip control means 196 at the predetermined 
period. 
Referring to the flow chart of FIG. 9, there will be described a routine 
executed by the slip control apparatus when the lock-up clutch 32 is 
placed in the fully released position. 
The routine is initiated with step SE1 to determine whether conditions for 
initiating the slip control of the lock-up clutch 32 by the slip control 
means 196 are satisfied. If a negative decision (NO) is obtained in step 
SE1, the control flow goes to step SE7 to inhibit the slip control of the 
lock-up clutch 32. If an affirmative decision (YES) is obtained in step 
SE1, the control flow goes to step SE2 corresponding to the torque 
monitoring means 200, to determine whether the idling position switch of 
the throttle sensor 167 is in the ON position, that is, whether the 
throttle valve 166 is placed in the engine idling position. 
If a negative decision (NO) is obtained in step SE2, it means that the 
transmission torque of the lock-up clutch 32 is not smaller than the 
predetermined threshold. In this case, step SE8 is implemented to reset a 
time counter CLLONX to "0", and step SE6 is then implemented to initiate 
the slip control of the clutch 32 by the slip control means 196. It will 
be understood that step SE6 corresponds to the slip control means 196. In 
the slip control in step SE6, the target slip speed TN.sub.SLP is 
determined on the basis of the detected throttle opening angle TAP and the 
detected input speed Nin of the automatic transmission 14 (detected 
turbine impeller speed N.sub.T of the torque converter 12), and according 
to a predetermined relationship between the target slip speed TN.sub.SLP 
and the detected throttle opening TAP and speed Nin (=N.sub.T), as 
indicated in the graph of FIG. 10. This relationship is represented by a 
data map stored in the ROM of the transmission controller 184. In step 
SE6, the control error .DELTA.E (=TN.sub.SLP -N.sub.SLP) is calculated, 
and then the slip control output in the form of the SLIP CONTROL current 
I.sub.SLU (=duty ratio D.sub.SLU) is calculated according to the above 
equation (2), so that the amount of slip of the lock-up clutch 32 is 
controlled according to the calculated slip control output I.sub.SLU, so 
as to zero the control error .DELTA.E. 
If an affirmative decision (YES) is obtained in step SE2, it means that the 
transmission torque of the lock-up clutch 32 is smaller than the 
predetermined threshold, and that the vehicle is in deceleration or is 
going to be in deceleration. In this case, the pressure difference and the 
engagement force of the lock-up clutch 32 are small, and the actual slip 
speed N.sub.SLP of the lock-up clutch 32 is not highly responsive to a 
change in the slip control output D.sub.SLU of the slip control means 196, 
due to a sliding resistance of the clutch piston 30 and an oil leakage 
from the hydraulic system, whereby the feedback control of the slip speed 
N.sub.SLP tends to suffer from a control hunting. In view of this 
drawback, step SE6 corresponding to the slip control means 198 is 
implemented to initiate the slip control of the lock-up clutch 32 after 
steps SE3-SE5 are implemented. Steps SE3 and SE4 correspond to the 
oscillation time determining means 202, and step SE5 corresponds to the 
output oscillating means 198. 
Described in detail, step SE3 is implemented to increment the time counter 
CLLONX to measure a time after the idling position switch is turned ON 
(after the affirmative decision is obtained in step SE2). Step SE3 is 
followed by step SE4 to determine whether the content of the time counter 
CLLONX is smaller than a predetermined threshold T, which is determined to 
be sufficient for the learning control value KGD in the equation (2) to be 
determined. For example, a threshold time corresponding to the 
predetermined threshold T is about six seconds. 
An affirmative decision (YES) is obtained in step SE4 until the time 
corresponding to the threshold value T has passed. In this case, step SE5 
is implemented to determine the oscillation value tDITH(t) included in the 
above equation (2). The oscillation value tDITH(t) is represented by a 
pulse which has a frequency of about 10 Hz and a predetermined amplitude 
tDALT, as indicated in the graph of FIG. 11. This amplitude tDALT is 
determined in step SE5, so as to be generally within a range of 0.3-3%, 
preferably 0.5-2%, and more preferably in the neighborhood of 1%. These 
percent values correspond to the percent value of the duty cycle 
D.sub.SLU. Step SE5 is followed by step SE6. With steps SE5 and SE6 
repeatedly implemented until a negative decision (NO) is obtained in step 
SE4, the slip control output I.sub.SLU or D.sub.SLU as calculated 
according to the above equation (2) which includes the oscillation value 
tDITH(t) determined in step SE5 is oscillated at a frequency of 5-20 Hz, 
preferably 7-15 Hz and more preferably about 10 Hz. When the predetermined 
time (about six seconds) has passed after the idling position switch is 
turned ON, the negative decision (NO) is obtained in step SE4, and step 
SE6 is implemented without implementation of step SE5. In this case, the 
time counter CLLONX is reset in step SE8. Thus, the amplitude tDALT of the 
oscillation value tDITH(t) in the equation (2) is set within the range 
indicated above for the predetermined length of time corresponding to the 
threshold value T, and is set to be zero after the predetermined time has 
passed. Accordingly, the overall slip control output D.sub.SLU is 
oscillated by the oscillation value tDITH within the predetermined time. 
Since the oscillation value tDITH(t) has a period TDITH of about 100 ms, 
the slip control output in the form of the duty ratio D.sub.SLU is 
oscillated at the predetermined frequency of about 10 Hz as indicated 
above. This frequency of oscillation of the slip control output is 
determined by experiments, so as to oscillate the pressure difference 
.DELTA.P of the piston 30 for smooth operation of the piston 30 
irrespective of the sliding resistance, and without vibrating the piston 
30 per se. In this respect, it is noted that the slip control output of 
the slip control means 196 may be oscillated for the purse of assuring 
smooth movement of the spool 114 of the linear solenoid valve SLU even in 
the presence of some foreign matters mixed in the working fluid. In other 
words, the slip control output is oscillated at a predetermined frequency 
of about 30 Hz, for oscillating the thrust force of the spool 114 without 
vibrating the SLIP CONTROL pilot pressure P.sub.SLU. Thus, the purpose and 
frequency of the oscillation of the thrust force of the spool 114 are 
different from those of the oscillation value tDITH(t) according to the 
principle of the present invention. The amplitude tDALT of the oscillation 
value tDITH(t) is also determined by experiments, so as to oscillate the 
pressure difference .DELTA.P of the piston for smooth operation of the 
piston 30 irrespective of the sliding resistance and without vibrating the 
piston 30. 
In the present embodiment illustrated in FIG. 9, step SE5 is implemented so 
that the output changing means 198 oscillates the slip control output 
D.sub.SLU of the slip control means 196 at the predetermined period, so as 
to oscillate the pressure difference .DELTA.P of the piston 30 without 
vibrating the piston 30. Consequently, the thrust force acting on the 
piston 30 is oscillated, and the piston 30 may be smoothly moved with 
improved response to a change in the slip control output D.sub.SLU, 
whereby the amount of slip of the lock-up clutch 32 can be controlled with 
high stability, even if the pressure difference is relatively small, that 
is, even when the target slip speed TN.sub.SLP determined depending upon 
the vehicle running condition is almost equal to the slip speed of the 
torque converter 12 which would be established when the lock-up clutch 32 
is placed in the fully released state. 
It is also noted that the step SE5 corresponding to the output oscillating 
means 198 to oscillate the slip control output D.sub.SLU of the slip 
control means 196 is implemented only when the affirmative decision (YES) 
is obtained in step SE2 corresponding to the torque monitoring means 200, 
that is, only when the torque monitoring means 200 determines that the 
transmission torque of the lock-up clutch 32 is relatively small. That is, 
it is not required to oscillate the slip control output D.sub.SLU during 
the entire period of the slip control means 196, which includes the linear 
solenoid valve SLU to provide the SLIP CONTROL pilot pressure P.sub.SLU 
for controlling the pressure difference .DELTA.P. Accordingly, the life 
expectancy of the valve SLU is increased. 
In the present embodiment of FIG. 9, the oscillation time determining means 
202 (steps SE3 and SE4) is provided to command the output oscillating 
means 198 to oscillate the slip control output D.sub.SLU of the slip 
control means 196, for only the predetermined time after the torque 
determining means 200 has determined in step SE2 that the idling position 
switch of the throttle sensor 167 is in the ON position, that is, for only 
the predetermined time (corresponding to the threshold value T) during 
which the transmission torque of the lock-up clutch 32 is relatively 
small. The limited period of the oscillation of the slip control output 
leads to further increased life expectancy of the linear solenoid valve 
SLU which is operated to control the pressure difference .DELTA.P of the 
lock-up clutch 32. It is also noted that the learning control value KGD in 
the above equation (2) can be determined within the predetermined time 
after the idling position switch is turned ON, so that the slip control of 
the lock-up clutch (2) according to the equation 32may be effected with 
high stability even after the predetermined time has passed, that is, even 
after the oscillation of the slip control output by the output oscillating 
means 198 is terminated. 
There will next be described another embodiment of this invention wherein a 
routine illustrated in FIG. 12 is executed by the transmission controller 
184, in place of or in parallel with the routine of FIG. 9. In the routine 
of FIG. 12, step SE9 is substituted for steps SE2-SE4 of FIG. 9. Step SE9 
corresponds to the deceleration monitoring means 204 indicated above. This 
step SE9 is provided to determine whether the deceleration rate of the 
vehicle is higher than a predetermined threshold. This determination may 
be effected on the basis of an output signal of a suitable sensor which 
represents the vehicle deceleration, for example: a sensor for detecting 
the accelerator or deceleration of the vehicle; a sensor for detecting the 
pressure of a brake fluid applied to a brake of the vehicle; and a sensor 
for detecting an amount of operation of the brake pedal. The threshold of 
the deceleration rate is determined so that the determination in step SE9 
makes it possible to determine whether the vehicle is in a decelerating 
state that causes a control hunting of the slip speed of the lock-up 
clutch 32 due to a sudden drop of the engine speed N.sub.E (=turbine 
impeller speed N.sub.T) when the vehicle running condition has entered the 
slip control area of FIG. 7. In a simple arrangement of the deceleration 
monitoring means 204, the brake switch 172 may be used as a sensor for 
detecting the vehicle deceleration in step SE9. 
If a negative decision (NO) is obtained in step SE9, the control flow goes 
to step SE6 to effect the slip control of the lock-up clutch 32 without 
activation of the output oscillating means 198. If an affirmative decision 
(YES) is obtained in step SE9, the control flow goes to step SE5 to 
determine the oscillation value tDITH to oscillate the overall slip 
control output D.sub.SLU in step SE6. Steps SE5 and SE6 are repeatedly 
implemented until the negative decision (NO) is obtained in step SE9. 
However, the oscillation of the slip control output may be effected for a 
predetermined length of time after the affirmative decision is obtained in 
step SE9. 
In the present second embodiment of the invention, the slip control output 
D.sub.SLU of the slip control means 196 is oscillated at the predetermined 
period TDITH by the output oscillating means 198 for a predetermined 
length of time after the deceleration monitoring means 204 has determined 
that the vehicle deceleration exceeds the predetermined threshold. This 
arrangement is effective to avoid control instability of the lock-up 
clutch 32 which would result from a low response of the piston 30 due to a 
small pressure difference .DELTA.P when the target slip speed TN.sub.SLP 
is almost equal to the actual slip speed N.sub.SLP, for example, when the 
vehicle running condition has entered the slip control area of FIG. 7 with 
the engine speed N.sub.E being suddenly lowered upon deceleration of the 
vehicle by brake application. The oscillation of the slip control output 
causes oscillation of the pressure difference .DELTA.P, and oscillation of 
the thrust force acting on the piston 30, which permits the piston 30 to 
be smoothly moved in response to a change in the slip control output, 
thereby assuring improved control stability of the lock-up clutch 32. 
A third embodiment of the invention will be described by reference to the 
flow chart of FIG. 13, which illustrates a routine to be executed in place 
of or in parallel with the routine of FIG. 9. In the routine of FIG. 13, 
steps SE10 through SE13 are substituted for steps SE2-SE4 of FIG. 9. 
Step SE10 is provided to determine whether the slip control means 196 is in 
operation. If an affirmative decision (YES) is obtained in step SE10, the 
control flow goes to step SE6 to continue the slip control of the lock-up 
clutch 32. If a negative decision (NO) is obtained in step SE10, the 
control flow goes to step SE11 corresponding to the slip speed calculating 
means 106, to calculate the slip speed N.sub.SLP (=N.sub.P -N.sub.T), 
which is a difference between the speeds N.sub.P and N.sub.T of the pump 
and turbine impellers 18, 22. Then, step SE12 corresponding to the target 
slip speed determining means 208 is implemented to determine the target 
slip speed TN.sub.SLP on the basis of the throttle valve opening TAP and 
the input speed Nin of the transmission 14 (turbine impeller speed 
N.sub.T), and according to the predetermined relationship as indicated in 
the graph of FIG. 10. 
Step SE12 is followed by step SE13 corresponding to the slip speed 
monitoring means 210, to determine whether the actual slip speed N.sub.SLP 
is almost equal to the target slip speed TN.sub.SLP. Step SE13 is provided 
to determine whether the pressure difference .DELTA.P of the piston 30 
when the vehicle running condition is in the slip control area of FIG. 7 
is considerably small. If an affirmative decision (YES) is obtained in 
step SE13, the control flow goes to step SE5 to determine the oscillation 
value tDITH, so that the slip control output D.sub.SLU is oscillated by 
the oscillation value tDITH in step SE6 which follows step SE5. Steps SE5 
and SE6 are repeatedly implemented until the slip control of the lock-up 
clutch 32 is terminated in step SE7 or until the negative decision (NO) is 
obtained in step SE13. 
In the present third embodiment, the output oscillating means 198 
oscillates the slip control output D.sub.SLU (=I.sub.SLU or P.sub.SLU) in 
step SE13 at the predetermined period, if the slip speed monitoring means 
210 determines in step SE13 that the actual and target slip speeds 
N.sub.SLP and TN.sub.SLP are almost equal to each other. As a result, the 
pressure difference .DELTA.P of the piston 30 is oscillated, and the 
thrust force acting on the piston 30 is accordingly oscillated, whereby 
the piston 30 is smoothly moved in response to a change in the slip 
control output D.sub.SLU even when the pressure difference .DELTA.l is 
considerably small. This arrangement is based on a finding that the 
response of the piston 30 tends to be low where the pressure difference 
.DELTA.P is small with the pump and turbine impeller speeds being 
substantially equal to each other at the time of initiation of the slip 
control of the lock-up clutch 32, even if the throttle opening TAP is not 
considerably small. 
While the present invention has been described in detail in its presently 
preferred embodiments, it is to be understood that the invention is not 
limited to the details of the illustrated embodiments, but may be 
otherwise embodied. 
In the first embodiment of FIG. 9, the determination by the torque 
monitoring means 200 as to whether the transmission torque of the lock-up 
clutch 32 is smaller than a predetermined threshold is effected by 
determining whether the idling position switch of the throttle sensor 167 
is in the ON position or not. However, the torque monitoring means 200 may 
rely on other suitable parameters which reflect a change in the 
transmission torque of the lock-up clutch 32. For example, the torque 
monitoring means 200 may use the engine speed N.sub.E, throttle opening 
TAP of the throttle valve 166, an amount of fuel injection into the engine 
10, an operating amount of the accelerator pedal 165, or the transmission 
torque as detected by a suitable torque sensor. 
The throttle opening TAP used to determine the target slip speed TN.sub.SLP 
may be replaced by other suitable parameters representative of a load 
acting on the engine 10, such as the operating amount of the accelerator 
pedal 165, or the amount of fuel injection or intake air quantity of the 
engine 10. 
In the third embodiment of FIG. 13, the output of the turbine speed sensor 
178 is directly used by the speed calculating means 206 to detect the 
speed N.sub.T (=speed Nin of the input shaft 20 of the automatic 
transmission 14) for calculating the actual slip speed N.sub.SLP (=N.sub.P 
-N.sub.T) in step SE12. However, the turbine impeller speed N.sub.T may be 
calculated by using other speed sensors such as the vehicle speed sensor 
168 adapted to detect the speed Nout of the output shaft 40 of the 
transmission 14, or a wheel speed sensor adapted to detect the rotating 
speed of a vehicle wheel. Where the vehicle speed sensor 168 is used, the 
turbine impeller speed N.sub.T may be calculated by multiplying the output 
shaft speed Nout of the transmission 14 by a currently selected speed 
ratio of the transmission 14. Where the wheel speed sensor is used, the 
turbine impeller speed N.sub.T may be calculated by multiplying the wheel 
speed by the speed ratio of the transmission 14 and the speed reduction 
ratio of the final gear device. In these cases where the turbine impeller 
speed N.sub.T is calculated from the speed Nout or wheel speed, the 
turbine speed sensor 178 is not necessary. 
Although the hydraulic control device 44 is arranged as illustrated in FIG. 
4, the construction of the device 44 may be modified as needed. For 
instance, the lock-up relay valve 52 and the lock-up clutch control valve 
56 may be combined into a unitary structure. 
While the automatic transmission 14 is connected to the torque converter 12 
equipped with the lock-up clutch 32, the torque converter 12 may be 
replaced by other fluid-filled power transmitting device equipped with a 
lock-up clutch, such as a fluid coupling equipped with a lock-up clutch. 
It is to be understood that the present invention may be embodied with 
various other changes, modifications and improvements, which may occur to 
those skilled in the art, without departing from the spirit and scope of 
the invention defined in the following claims.