Control system for vehicle automatic transmission

A control system of a lockup clutch of a torque converter of a vehicle automatic transmission. A basic manipulated variable is determined in response to the vehicle operating condition in accordance with a predetermined characteristic, and the lockup clutch engaging force is controlled in response to the variable. In the system, fuzzy reasoning is carried out using the detected vehicle operating parameters to correct the basic manipulated variable, and the engaging force is controlled in response to the corrected manipulated variable, when the control condition is met. The corrected manipulated variable is gradually decreased with respect to time when the vehicle driving state has shifted from a region in which the engaging force is controlled in response to the corrected manipulated variable to a region in which the lockup clutch is disengaged. In addition, the corrected manipulated variable is gradually increased when the vehicle driving state has shifted from a region in which the lockup clutch is disengaged to a region in which the engaging force is controlled in response to the corrected manipulated variable.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a control system for a vehicle automatic 
transmission, more particularly to a control system for the lockup clutch 
of a torque converter of a vehicle automatic transmission. 
2. Description of the Prior Art 
Automatic transmissions are usually designed to automatically select the 
optimum gear by detecting vehicle or engine speed and the degree of 
throttle opening or some other such parameter indicating engine load and 
then retrieving a gear by the detected values of these parameters from 
gear shift characteristics (a shift map) determined in advance based on 
the same parameters. 
In the automatic transmission of this type, a hydraulic torque converter 
equipped with a lockup clutch is installed between the internal combustion 
engine (power source) and the transmission unit. The torque increase 
characteristic of the torque converter is utilized when the vehicle is 
accelerating, as during drive-away or overtaking, while the lockup clutch 
completely or partially engages the input and output sides of the 
hydraulic torque converter during cruising. The ON (operative) and OFF 
(inoperative) regions of the lockup clutch are defined in advance in terms 
of the gear (gear ratio) and other driving conditions and the lockup 
clutch is controlled based thereon to prevent transmission efficiency 
loss. 
Because a type of surging known as body vibration may occur in the 
acceleration region when the operator steps down on the accelerator pedal, 
drivability considerations make it extremely difficult to optimize the 
ON/OFF characteristics of the lockup clutch. The prior art practice has 
therefore been to keep the lockup clutch normally OFF when in the 
acceleration region is entered owing to depression of the accelerator 
pedal or, more precisely, to define this region as a weak lockup region in 
which only a weak engagement determined from the characteristics of the 
hydraulic circuit is imparted. A description of this prior art technology 
can be found, for example, in Japanese Laid-Open Patent Application No. 
Sho 63(1988)-180,757. 
The need to expand the ON (operative) region of the lockup clutch has 
heightened in recent years, however, owing to increased demand for lesser 
fuel consumption. The prior art cannot sufficiently respond to this 
requirement. On the other hand, the ON (operative) region cannot be 
indiscriminately broadened because, as just pointed out, this would lead 
to surging and degrade drivability. 
An object of this invention is therefore to overcome the aforesaid problems 
by providing a control system for a vehicle automatic transmission which 
expands the ON (operative) region of the lockup clutch and achieves 
improved fuel efficiency while avoiding the occurrence of surging. 
SUMMARY OF THE INVENTION 
Another object of this invention is to provide a control system for a 
vehicle automatic transmission which expands the ON (operative) region of 
the lockup clutch and achieves improved fuel efficiency and which, 
contrary to what might be expected, simultaneously provides an improvement 
in drivability with regards to direct control feel and the response of 
vehicle speed to accelerator pedal depression in the acceleration region. 
This invention achieves this object by providing a system for controlling 
an automatic transmission of a vehicle, comprising, coupling means (clutch 
means) having an input connected to an internal combustion engine mounted 
on the vehicle and an output connected to a gear system in the 
transmission, said coupling means passing engine power to the gear system, 
gear ratio establishing means for establishing a gear ratio of the gear 
system in response to a gear shift command to transmit the engine power to 
a vehicle wheel through the established gear ratio, engaging force control 
means for controlling the engaging force of the coupling means including 
at least an engaging state and a disengaging state, vehicle operating 
condition detecting means for detecting operating conditions of the 
vehicle, and basic manipulated variable determining means for determining 
a basic manipulated variable to be applied to said engaging means in 
response to a parameter of the detected vehicle operating conditions in 
accordance with a predetermined characteristic. In the system, said 
engaging force control means includes, fuzzy reasoning means for carrying 
out fuzzy reasoning using a parameter of the detected operating conditions 
of the vehicle to determine a correction value of the basic manipulated 
variable, and manipulated variable correcting means for correcting the 
basic manipulated variable based on the correction value. And in the 
system, said engaging force control means controls the engaging force of 
the coupling means in response to the corrected manipulated variable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the invention will now be explained with reference to the 
attached drawings. 
FIG. 1 is an overall view of a control system for a vehicle automatic 
transmission according to the invention. 
As shown in FIG. 1, a crankshaft 10 of an internal combustion engine E is 
connected with a main shaft MS, through a hydraulic torque converter 
(hydraulic coupling means) 12 having a lockup clutch L (coupling means), 
of a vehicle automatic transmission T. The automatic transmission T has a 
gear system made up of the main shaft MS, a countershaft CS and a 
secondary shaft SS which are arranged in parallel and support gears 
thereon. 
More specifically, the main shaft MS supports a main first-speed gear 14, a 
main third-speed gear 16, a main fourth-speed gear 18 and a main reverse 
gear 20. The countershaft CS supports a counter first-speed gear 22 
engaged with the main first-speed gear 14, a counter third-speed gear 24 
engaged with the main third-speed gear 16, a counter fourth-speed gear 26 
engaged with the main fourth-speed gear 18 and a counter reverse gear 30 
engaged with the main reverse gear 20 through a reverse idle gear 28. The 
secondary shaft SS supports a first secondary second-speed gear 32 and a 
second secondary second-speed gear 34. 
The first gear is established when the main first-speed gear 14 rotatably 
supported on the main shaft MS is connected with the main shaft MS by a 
first-speed hydraulic clutch C1. Since the first-speed hydraulic clutch C1 
is also maintained in the engaged state during establishment of the 
second-fourth gears, the counter first-speed gear 22 is supported by a 
one-way clutch COW. A hydraulic clutch CLH is provided for holding the 
first gear so as to ensure that the driven wheels W drive the engine E, 
i.e., to ensure the engine braking effect when 1st or 2nd range (explained 
later) is selected. 
The second gear is established, through the main third-speed gear 16, the 
counter third-speed gear 24 and the first secondary second-speed gear 32, 
when the second secondary second-speed gear 34 rotatably supported on the 
secondary shaft SS is connected with the secondary shaft SS by a 
second-speed hydraulic clutch C2. The third gear is established when the 
counter third-speed gear 24 rotatably supported on the countershaft CS is 
connected with the countershaft CS by a third-speed hydraulic clutch C3. 
The fourth gear is established when the counter fourth-speed gear 26 
rotatably supported on the countershaft CS is connected with the 
countershaft CS by a selector gear SG and with this state maintained the 
main fourth-speed gear 18 rotatably supported on the main shaft MS is 
connected with the main shaft MS by a fourth-speed/reverse hydraulic 
clutch C4R. The reverse gear is established when the counter reverse gear 
30 rotatably supported on the countershaft CS is connected with the 
countershaft CS by the selector gear SG and with this state maintained the 
main reverse gear 20 rotatably supported on the main shaft MS is connected 
with the main shaft MS by the fourth-speed/reverse hydraulic clutch C4R. 
The rotation of the countershaft CS is transmitted through a final drive 
gear 36 and a final driven gear 38, engaged therewith, to a differential 
D, from where it is transmitted to the driven wheels W through left and 
right drive shafts 40 (only one shown). 
A throttle position sensor S1 is provided in an air intake pipe (not shown) 
of the engine E at a point in the vicinity of a throttle valve (not shown) 
for detecting the degree of opening or position .theta.TH of the throttle 
valve. A vehicle speed sensor S2 for detecting the vehicle traveling speed 
V from the rotational speed of the final driven gear 38 is provided in the 
vicinity of the final driven gear 38. A transmission input shaft speed 
sensor S3 is provided in the vicinity of the main shaft MS for detecting 
the rotational speed NM of the transmission input shaft from the rotation 
of the main shaft MS, and a transmission output shaft speed sensor S4 is 
provided in the vicinity of the countershaft CS for detecting the 
rotational speed NC of the transmission output shaft from the rotation of 
the countershaft CS. 
A shift lever position sensor S5 is provided in the vicinity of a shift 
lever (not shown) installed on the vehicle floor near the driver's seat. 
The shift lever position sensor S5 detects which of the seven ranges or 
positions P, R, N, D4, D3, 2, 1 has been selected by the driver. A crank 
angle sensor S6 is provided in the vicinity of the crankshaft 10 of the 
engine E for detecting the engine speed Ne from the rotation of the 
crankshaft 10, and a coolant temperature sensor S7 for detecting the 
engine coolant temperature Tw is provided at an appropriate location on a 
cylinder block (not shown) of the engine E. Outputs of the sensors S1, 
etc., are sent to an ECU (electronic control unit). 
The ECU is constituted as a microcomputer comprising a CPU (central 
processing unit) 50, a ROM (read-only memory) 52, a RAM (random access 
memory) 54, an input circuit 56 and an output circuit 58. The outputs of 
the sensors S1, etc., are input to the microcomputer through the input 
circuit 56. The CPU 50 of the microcomputer conducts gear shift control 
including the lockup clutch control and issues a command to a hydraulic 
control circuit O via the output circuit 58. 
The hydraulic control circuit O has shift solenoids SL1, SL2, an ON/OFF 
control solenoid SL3 and a capacity (engaging force) control solenoid SL4 
for the lockup clutch L, and a linear solenoid SL5 for regulating clutch 
oil pressure. More specifically, the CPU 50 determines the shift position 
(gear ratio) based on the outputs of the sensors and energizes/deenergizes 
the shift solenoids SL1, SL2 of the hydraulic control circuit O via the 
output circuit 58 so as to switch shift valves (not shown) and thereby 
engage/disengage the hydraulic clutches of prescribed gears. As will be 
explained later, it also controls the lockup clutch in the ON (fully 
engaged or operative) state, or in the OFF (fully disengaged or 
inoperative) state, or in a partially engaged or slip-controlled state 
between these two states such that the torque converter 12 is 
slip-engaged. 
The torque converter 12 comprises a pump 12a connected to the crankshaft 
10, a turbine 12b connected to the main shaft MS, a stator 12c and the 
lockup clutch L. The lockup clutch L is of the known configuration 
comprising a lockup piston, a damper spring, etc. Depending on the amount 
of oil pressure supplied to its left and right chambers, the lockup clutch 
assumes the ON state (shown by solid lines in FIG. 2 discussed below), the 
OFF state (shown by phantom lines in FIG. 2) or the slip-controlled state. 
When the lockup clutch is ON, the power of the engine E is transmitted to 
the main shaft MS through a drive plate, a torque converter cover and the 
lockup clutch L. When the lockup clutch is OFF, the engine power is 
transmitted to the main shaft MS through the drive plate, the torque 
converter cover, the pump 12a and the turbine 12b. 
FIG. 2 is a block diagram functionally illustrating the hydraulic operation 
of the lockup clutch L. The lockup clutch is turned ON and OFF by 
supplying or not supplying line pressure from a manual valve to a lockup 
shift valve which receives modulator pressure from a modulator valve 
through the solenoid SL3. An L/C (lockup) control valve which receives the 
modulator pressure (through the solenoid SL4) controls the engaging force 
of the lockup clutch L by regulating the oil pressure supplied to the 
right chamber of the clutch. The torque converter 12 is thus controlled to 
the aforesaid completely locked-up (fully-engaged or ON) state by an L/C 
(lockup) timing valve which receives throttle pressure through the linear 
solenoid (throttle valve) SL5 and the modulator pressure (through the 
solenoid SL4). 
FIG. 3 is a flowchart of the operations of a control system for a vehicle 
transmission according to this invention. Prior to going into the details 
of this flowchart, however, a background explanation will first be given 
with reference to FIG. 4 regarding the lockup clutch operating 
characteristics (hereinafter referred to as the LC map) in the invention 
system. The characteristics shown in FIG. 4 are defined relative to 
throttle opening .theta.TH and vehicle speed V. 
The reference symbols 1, 2 and 3 in this figure designate the switchover 
lines of the aforesaid solenoid SL3 and solenoid SL4. To the right of line 
3 in this figure, the solenoid SL3, which conducts two position 
(engagement/disengagement) control of the lockup clutch, and the solenoid 
SL4, which controls the engaging force therebetween based on the desired 
slip ratio of the torque converter, are both ON. In the control of the 
engaging force, the duty value (PWM duty value or ratio) applied to the 
solenoid SL4 as the manipulated variable is calculated based on the 
desired slip ratio (amount) of the torque converter. 
More specifically, on the right side of the figure the region (a) is a 
completely locked-up region in which a duty value of 100% is applied to 
the solenoid SL4 for directly connecting the input and output sides of the 
torque converter. The region (b) is a strong lockup region in which the 
duty value of the solenoid SL4 is increased from the value in the adjacent 
region toward 100% in prescribed increments so as to gradually increase 
the engaging force and decrease the slip ratio. The region (c) is a region 
of relatively stable driving conditions involving little fluctuation in 
engine speed. In this region, the duty value for obtaining the desired 
slip ratio is learned by using a PI controller to conduct feedback control 
based on the error between the desired slip ratio and the actual slip 
ratio. 
The region (d) comprises a hatched portion .alpha. and a region .beta. 
lying outside the hatched portion. In the prior art systems, the solenoid 
SL4 is turned OFF in the region .beta. outside the hatched portion and the 
lockup clutch is turned OFF in hatched portion .alpha.. In the invention, 
on the other hand, the solenoid SL4 is left ON in the region (d) and the 
engaging force is duty-controlled (PWM controlled). As a specific example 
of prior-art system operation, take the case where, as indicated by the 
arrows a in FIG. 4, the operating state moves from the region (c) or the 
like, across the line 3 and into the region .beta. outside the hatched 
portion of the region (d) owing to a decrease in the vehicle speed V or an 
increase in the throttle opening .theta.TH. In this case, the solenoid SL3 
remains ON, but since the solenoid SL4 is turned OFF, the lockup clutch is 
applied with only the minimum capacity determined from the characteristics 
of the hydraulic circuit, so that only the weak engaging force at this 
lower limit is applied. Then when the operating state moves across the 
line 2, the solenoid SL3 is turned OFF and the lockup clutch becomes 
inoperative. 
Thus the prior art systems cope with the risk that even a slight variation 
in throttle opening may lead to surging in the region (d) by limiting the 
engaging force to not more than that dictated by the system hardware. In 
contrast, this invention enables the engaging force to be increased by 
determining the duty value through fuzzy reasoning in the manner described 
hereinafter. 
The region (e) is a deceleration lockup region. Since variation in the 
torque from the engine E is not a problem in this region, feedback control 
employing a PI controller is controlled for obtaining the duty value 
needed to secure the desired slip ratio of the torque converter 
(=NM/Ne.times.100%; explained later), namely, a slip ratio in the range of 
102 to 103%, thereby ensuring a good engine braking effect. In the region 
(f), the lockup clutch is disengaged, in both the prior art and the 
invention. 
The operation of the invention system will now be explained with reference 
to the flowchart of FIG. 3, taking as an example the case where the 
operating state passes over the line 3 from the right as indicated by the 
arrows a in FIG. 4. The routine of FIG. 3 is actuated at appropriate time 
intervals of, for example, 20 ms. 
First, in S10, it is checked whether the ordinary lockup clutch (LC) 
operating conditions are met. Specifically, it is checked whether the 
engine coolant temperature Tw, engine speed Ne, vehicle speed V and 
throttle opening .theta.TH are within prescribed ranges and that system 
failure has not occurred. 
When the result in S10 is NO, the routine is immediately terminated. When 
it is YES, the program goes to S12, in which it is checked whether the 
selected range is D. When it is not, the program goes to S14, in which 
control for the selected range is conducted, and when it is, it goes to 
S16, in which it is checked whether the lockup clutch is in operation (ON 
state). In other words, it is confirmed whether the operating state is 
moving in the direction of the arrows a in FIG. 4, not in the direction of 
the arrows b. 
This step is necessary because structural (hardware) differences between 
different lockup clutches, including their hydraulic circuits, make it 
impossible to achieve the desired torque converter slip ratio even if fine 
engaging force control is started immediately after the lockup clutch has 
been disengaged. Conversely, good control performance can be obtained when 
the lockup clutch is in operation, namely, when the solenoids SL3 and SL4 
are ON. 
More specifically, the general tendency is for the engaging force of the 
lockup clutch and the slip ratio of the torque converter to decrease as 
the operating state moves from right to left in FIG. 4, and the control 
performance is better in the direction of decreasing engaging force and 
slip ratio. When the result in S16 is NO, therefore, the program goes to 
S18, in which ordinary D-range lockup clutch control is conducted. 
Specifically, SL4 is turned OFF and weak lockup control determined by the 
hardware is conducted. 
On the other hand, when the result in S16 is YES, the program moves to S20, 
in which it is checked whether the vehicle is currently traveling over 
level ground. This step is conducted because the driven wheels W may drive 
the engine E depending on the slope of an uphill or downhill grade, and it 
affects the torque converter slip ratio and the lockup clutch engaging 
force and makes the probability of surging occurrence high. 
Discrimination of whether the road is level can be conducted by using an 
inclination sensor mounted at an appropriate location on the vehicle or by 
adopting the technique taught by the assignee's Japanese Laid-Open Patent 
Application No. Hei 6(1994)-109,122 (filed in the United States and 
patented under the number of U.S. Pat. No. 5,317,937) of using an index 
indicative of the running resistance calculated from the vehicle 
acceleration to select from multiple maps for level-road driving, 
hill-climbing, etc., prepared in advance and making the discrimination 
based on whether or not an LC map for level-road driving is selected. 
When the result in S20 is NO, the program goes to S18, in which control is 
conducted in the same manner as in the prior art, and when it is YES, the 
program goes to S22, in which it is checked whether the current gear 
(speed) is fourth gear. This step is conducted because a margin for 
surging is greater for a higher gear (smaller gear ratio) in the D range. 
When the result in S22 is NO, the program goes to S18, in which control is 
conducted in the same manner as in the prior art, and when it is YES, the 
program goes to S24, in which it is checked whether the current vehicle 
speed V is at or below a predetermined speed of, for instance, 50 km/h. 
When the result in S24 is YES, the program goes to S26, in which it is 
checked whether any auxiliary equipment is in operation. By "auxiliary 
equipment" is meant an air conditioner or other such equipment driven by 
the output (power) of the engine E. 
When the result in S26 is YES, the program goes to S18, in which control is 
conducted in the same manner as in the prior art, and when it is NO, the 
program goes to S28, in which lockup clutch control is conducted using 
fuzzy reasoning (approximate reasoning). This method is adopted because 
the operation of auxiliary equipment at or below the predetermined speed 
strongly affects the engine output and the rotational speed on the input 
side of the torque converter 12, making appropriate slip control of the 
lockup clutch difficult. When S24 finds that the current vehicle speed is 
greater than the predetermined speed, the risk of fluctuation in the 
engine output and the rotational speed on the input side of the torque 
converter 12 can be assumed to be low. Since the slip control is therefore 
not likely to be affected, the program skips S26 and goes directly to S28. 
FIG. 5 is the flowchart of a subroutine for determining the manipulated 
variable for the lockup clutch control using fuzzy reasoning, and FIGS. 6 
to 10 are diagrams for illustrating fuzzy production rules used in the 
fuzzy reasoning. 
As shown in FIGS. 6 to 10, the fuzzy reasoning uses ten rules whose 
antecedents include as parameters the vehicle speed V, the throttle 
opening .theta.TH and the torque converter slip ratio ETR. As shown in the 
figures, membership functions are defined within the ranges of a vehicle 
speed V between 0 and 255 km/h, a throttle opening .theta.TH between fully 
closed and wide open, and a torque converter slip ratio ETR between 18 and 
120%. 
The slip ratio ETR of a torque converter is ordinarily calculated as 
(rotational speed of turbine input shaft)/(rotational speed of pump input 
shaft). In this embodiment, however, it is calculated as (rotational speed 
of main shaft NM)/(rotational speed of engine Ne).times.100%. (The upper 
limit of 120% is set in consideration of the engine braking effect.) 
The parameter of the conclusion is a correction coefficient (correction 
coefficient LFK) for correcting the basic duty value (basic manipulated 
variable). As illustrated, the membership function is set between 0 and 
1.0. The basic duty value (indicated as BDUTY in FIG. 5 etc.) is 
established as a table (based on the characteristic curve shown in FIG. 
11) and is a value defining the upper limit of the duty value output to 
the solenoid SL4 as a function of engine load (throttle opening 
.theta.TH). In other words, the correction coefficient LFK is obtained by 
fuzzy reasoning using the vehicle speed V or the like and the basic duty 
value BDUTY is multiplied thereby to obtain the corrected duty value (duty 
value FBDY) to be output to the solenoid SL4 as the control input. 
As shown by the characteristic curve of FIG. 11, the basic duty value is 
defined to decrease with increasing engine load (throttle opening 
.theta.TH). Needless to say, the purpose of this is to counteract the 
higher probability of surging with increasing throttle opening by reducing 
the duty value and thus lowering the engaging force of the lockup clutch. 
Among the rules shown in FIGS. 6 to 10, rules 1 and 2, 3 and 4, 5 and 6, 7 
and 8, and 9 and 10 are established in connection with the slip ratio ETR 
(of the torque converter) so that the membership functions of the slip 
ratio ETR complement each other. In order to prevent hunting, however, the 
membership function of the correction coefficient LFK of the conclusion is 
given hysteresis. 
As driving conditions, rules 1 and 2 contemplate low vehicle speed and 
large slip ratio, rules 3 and 4 contemplate rather low vehicle speed and 
rather large slip ratio, rules 5 and 6 contemplate medium vehicle speed 
and medium slip ratio, rules 7 and 8 contemplate rather high vehicle speed 
and rather small slip ratio, and rules 9 and 10 contemplate high vehicle 
speed and small slip ratio. Since the basic duty value is set with respect 
to the throttle opening .theta.TH, the membership function of the throttle 
opening is set at 1.0 over the whole throttle opening range, meaning that 
the throttle opening is not actually used in the fuzzy reasoning. It 
should be understood, however, that it can be set as desired if the 
necessity arises. 
This fuzzy reasoning will now be explained with reference to the flowchart 
of FIG. 5. First, in S100 and S102, computation tables LHIGH and LAREA 
(explained later) are initialized to zero, whereafter the value of a 
counter LNRULE (which counts the number of rules) is initialized to zero 
in S104. 
The program then advances to S106, in which the value of the rule counter 
LNRULE is incremented, to S108, in which the counter value is set to n 
(initial value 1), to S110, in which the value of the antecedent of rule n 
(in this case rule 1) is calculated, to S112, in which the value of the 
conclusion is calculated, and to S114, in which the value of the rule 
counter is compared with 10, whereafter S106 to S112 are looped until the 
counter value reaches 10 and all rules have been similarly processed. 
The reasoning used is illustrated in FIG. 12. The minimum value among the 
three antecedent membership functions, the value of slip ratio ETR in the 
illustrated case, is used to determine the conclusion membership function 
grade (the height Yn' on the y-axis; corresponding to the aforesaid LHIGH) 
and the area LAREA is then calculated as shown in the figure. The y-axis 
height LHIGH of the conclusion and the area LAREA are calculated for each 
loop of the procedure up to S114 and the results are totaled. Then, in 
S116 and the following steps, the inferred value (the correction 
coefficient LFK) is calculated by dividing the total value of LAREA by the 
total value of LHIGH to obtain the center of gravity. 
More specifically, the program advances to S116, in which it is checked 
whether the y-axis height LHIGH is zero and, when it is, to S118, in which 
correction coefficient LFK is set to zero to avoid division by zero. When 
the result in S116 is NO, the program goes to S120, in which the center of 
gravity is calculated as explained and the so-calculated value in the 
x-axis (universe of discourse) is defined as the correction coefficient. 
The program then goes to S122, in which it is checked whether the 
calculated correction coefficient LFK is in overflow and, when it is, to 
S124, in which the calculated correction coefficient LFK is rewritten to 
its upper limit of 1.0. 
On the other hand, when the result in S122 is NO, the program goes to S126, 
in which the detected throttle opening .theta.TH is used as address data 
to retrieve the basic duty value BDUTY from a table corresponding to the 
characteristic curve of FIG. 11 and then to S128, in which the retrieved 
basic duty value BDUTY is multiplied by the correction coefficient LFK 
obtained by fuzzy reasoning to thereby obtain the output duty value FBDY 
(corrected manipulated variable). This output duty value FBDY is then 
output to the solenoid SL4 by another routine not shown in the figures. 
The foregoing is the control conducted in the region (d) of FIG. 4 when, 
owing to depression of the accelerator pedal or the like, the operating 
state moves across the line 2 from one in which the solenoids SL3 and SL4 
are both ON and the lockup clutch is engaged. The prior art response to 
this situation is to turn the solenoid SL4 OFF at the point of crossing 
the line 2 and conducted only weak lockup control in the region .beta. 
outside the hatched portion of the region (d). 
As explained in the foregoing, however, in this invention the solenoid SL4 
is kept on until the line 1 is crossed. As a result, an improvement in 
fuel economy is achieved owing to the increased engaging force in the 
region (d) (including both the hatched portion .alpha. and the region 
.beta. outside the hatched portion, as defined earlier). In addition, 
since the degree of increase in the engaging force, i.e., the degree of 
increase in the duty value, is decided through fuzzy reasoning, the 
improvement in fuel economy can be achieved without giving rise to 
surging. Moreover, the drivability can be simultaneously improved in 
regard to direct control feel and the response of vehicle speed to 
accelerator pedal depression in the acceleration region. 
In addition, the manipulated variable is obtained by correcting the basic 
manipulated variable by multiplying the correction coefficient obtained 
through fuzzy reasoning. It is thus easy to introduce the result of the 
fuzzy reasoning in the manipulated variable and to adjust the value with 
the manipulated variable obtained without conducting fuzzy reasoning, 
making the system configuration simple. 
The foregoing describes the control when the operating state moves in the 
direction of the arrows a in FIG. 4. 
Next, the control when the operation state moves in the direction of the 
arrows c in FIG. 4 will be explained, as a second embodiment of the 
invention. 
This is the control conducted at the time of moving from the region (d) 
into the region (f), namely, at the time of shifting from engaging force 
control using fuzzy reasoning (i.e., control based on the corrected 
manipulated variable) to control with the lockup clutch inoperative. 
One case in which this state arises is when the vehicle speed V decreases 
or the engine load increases (accelerator pedal depression). Another is 
when the technique taught by the assignee's earlier mentioned Japanese 
Laid-Open Patent Application No. Hei 6(1994)-109,122 (U.S. Pat. No. 
5,317,937) is adopted and an LC map other than one for level-road driving 
is selected. More specifically, control for increasing the engaging force 
by fuzzy reasoning is conducted only when the LC map for level-road 
driving is in use. If during driving in the region (d) using a map for 
level-road driving, the vehicle should begin hill-climbing or 
hill-descent, for example, the LC map is switched to the ordinary LC map 
for the D range. In the case of the ordinary LC map at this time, the 
hatched portion .alpha. of the region (d) is a region in which the lockup 
clutch is inoperative. The switching of the LC map thus causes a shift 
from engaging force control using fuzzy reasoning to control with the 
lockup clutch inoperative. In this embodiment, when a control shift occurs 
in either of these two ways and, in addition, the operating state stays in 
the region (f) without a gear shift (with the transmission kept in fourth 
gear), the output duty value is gradually reduced to prevent the shock 
that would be caused by suddenly disengaging the lockup clutch. 
This control will now be explained with reference to the flowchart of FIG. 
13. Like the routine of FIG. 3, this routine is also actuated at 
appropriate time intervals of, for example, 20 ms. 
First, in S200, it is checked whether the operating state is in a lockup 
clutch disengaged region, namely, whether it is in the region (f). When 
the result is NO, the routine is immediately terminated. When it is YES, 
the program goes to S202, in which it is checked whether the operating 
state was in a fuzzy control region, i.e., the region (d), in the 
preceding cycle. If the result is NO, the routine is immediately 
terminated. If it is YES, the program goes to S204, in which it is checked 
whether gear shift has occurred. This check is made because if gear shift 
has occurred the lockup clutch has to be promptly disengaged in order to 
prevent an accompanying shock. 
If the result in S204 is NO, the program goes to S206, in which it is 
checked whether the output duty value FBDY is zero. If the result is YES, 
this means that engaging force decrease control need not be conducted and 
the routine is immediately terminated. If the result is NO, the program 
goes to S208, in which a duty value reduction amount Delta d is 
calculated. The value of the reduction amount Delta d increases with 
increase in the slip ratio ETR of the torque converter or with the rate of 
change in the engine load (specifically the difference value Delta 
.theta.TH in the detected throttle opening .theta.TH between the preceding 
and current cycles). This relationship is established because of the need 
to rapidly disengage the lockup clutch to prevent shock when the slip 
ratio is large. The program then goes to S210, in which, as shown in FIG. 
14, the calculated reduction amount Delta d is subtracted from the output 
duty value FBDY (corrected manipulated variable) and the value obtained is 
defined as output duty value DUTY. (This duty value is then output to the 
solenoid SL4 by another routine not shown in the figures.) The aforesaid 
processing is repeated until S206 finds that the output duty value FBDY 
has become zero. 
This control according to the flowchart of FIG. 13 imparts an engaging 
force in the region (d) that is greater than the weak lockup and then 
enables a smooth, shock-free transition to the lockup clutch disengaged 
region (f). 
An explanation will now be made regarding a third embodiment of the 
invention that relates to a separate auxiliary control, namely, to a 
control for increasing the engaging force by use of fuzzy reasoning when 
the operating state moves from the region (f) to the region (d) in FIG. 4. 
Since, as mentioned earlier, engaging force increase control using fuzzy 
reasoning is not conducted at the time of a direct shift from region (f) 
to the region (d), owing to control performance considerations, this 
control is, more precisely speaking, that in the case where the operating 
state moves from the region (f) through the region (d) to the region (c) 
and then immediately back to the region (d). 
This control will now be explained with reference to the flowchart of FIG. 
15. This routine is also actuated at appropriate time intervals of, for 
example, 20 ms. 
First, in S300, it is checked whether the operating state is in a fuzzy 
control region, specifically, the region in which the corrected 
manipulated variable is used in the control, more specifically the region 
(d). If the result is NO, the routine is immediately terminated. If the 
result is YES, the program goes to S302, in which it is checked whether 
the elapsed time period T from the establishment of a lockup clutch 
operative state in which the control using fuzzy reasoning is not 
conducted, such as in the region (c), exceeds a prescribed time period T1. 
The reason for this is that owing to response delay of the hydraulic 
circuit, etc., of the lockup clutch the lockup clutch does not 
instantaneously follow the anticipated state when an output duty value is 
calculated by fuzzy reasoning and sent to the solenoid SL4 as a command. 
In other words, there is a risk that the feedback control of the engaging 
force based on the desired slip ratio will not be accurately conducted. 
If, for instance, the hydraulic circuit cannot actually follow a command 
to increase the engaging force, the engaging force will remain unchanged 
and engine revving may occur. If feedback control is conducted under this 
condition, the engaging force is increased even further, making it likely 
that the lockup clutch will suddenly engage and produce shock. In the case 
of moving from the inoperative (disengaged) state of the lockup clutch to 
the operative (engaged state) thereof (except in the region (d)), it is 
checked whether the time elapsed since entry into the region (c) is equal 
to or greater than the prescribed time period T1. (See FIG. 16.) 
When the result in S302 is YES, the program goes to S304, in which it is 
checked whether the slip ratio ETR is between an upper limit value ETRH 
(110%, for example) and a lower limit value ETRL (80%, for example). This 
check is made to avoid inaccurate control and engine revving which may 
occur owing to the fact that the control performance is poor even after 
the elapse of the prescribed time period T1 if the slip ratio ETR of the 
torque converter is not in the prescribed range (80-110%). When the result 
in S304 is YES, since it can be concluded that the prescribed time period 
T1 has passed and the control performance is good, the program goes to 
S306, in which the output duty value FBDY calculated by fuzzy reasoning is 
defined as the output duty value DUTY. 
On the other hand, if S302 finds that the prescribed time period T1 has not 
passed, the program goes to S308, in which a timer (down counter) T2 is 
set and started, to S310, in which it is checked whether a second time 
period T2 has passed, i.e., whether the time value has reached zero. Since 
the timer was just set in the preceding step, the result here is naturally 
NO and the program goes to S312, in which the output duty value DUTY is 
set to 0%. 
The reason for setting the output duty value to 0% instead of to the duty 
value calculated just before crossing the line 3 in FIG. 4 is that if the 
duty value calculated just before crossing the line 3 should be output at 
the point of entering the fuzzy reasoning region, the duty value would 
thereafter actually be decreased to a lower value calculated by fuzzy 
reasoning. During rapid change of the throttle opening in the relatively 
low vehicle speed region where surging is a particular problem, the 
probability of surging occurrence is high when the duty value calculated 
just before crossing the line 3 is output. 
In this embodiment, therefore, the duty value is first set to 0% and then 
gradually increased to the value calculated by fuzzy reasoning. Rather 
than setting the duty value to 0%, it is also possible when control 
response is a concern to set it to a low value smaller than the duty value 
calculated just before crossing the line 3. 
When S304 in FIG. 15 finds that the slip ratio ETR is not in the prescribed 
range, the program goes to S310 and if the result here is NO, the same 
processing as just described is conducted. Then when S310 finds that the 
time period T2 has passed, the program goes to S314, in which the duty 
value is gradually increased from 0% (or a low value) to the output duty 
value FBDY calculated by fuzzy reasoning. 
In other words, as shown in FIG. 17, when the line 3 is crossed from the 
inoperative (disengaged) state of the lockup clutch before the first time 
period T1 has passed, or when a control performance degrades because the 
slip ratio is not in the prescribed range, the duty value of 0% (or a low 
value) is maintained for the second time period T2 and is thereafter 
gradually increased to the output duty value FBDY calculated by fuzzy 
reasoning. As a result, accurate feedback control can be achieved. 
Since this embodiment is configured in the foregoing manner, the solenoid 
SL4 is not turned OFF until the line 1 is crossed. As a result, an 
engaging force greater than that of weak lockup conducted in the prior art 
is applied in the region (d), thereby enabling an improvement in fuel 
economy over the prior art. 
In addition, since the degree of increase in the engaging force, i.e., the 
degree of increase in the duty value, is determined through fuzzy 
reasoning, the improvement in fuel economy can be achieved without giving 
rise to surging and thus without degrading drivability. To the contrary, 
the increase in engaging force improves the drivability by enhancing 
direct control feel and the response of vehicle speed to accelerator pedal 
depression in the acceleration region. 
Further, since the duty value is gradually raised/decreased and the 
engaging force is gradually increased/decreased at movement from the fuzzy 
reasoning region (d) into the lockup clutch disengaged region (f) and at 
movement from the lockup clutch disengaged region (f) immediately into the 
fuzzy reasoning region (d) via the region (c) or the like, no sudden 
engagement/disengagement of the lockup clutch occurs. The transition can 
therefore be made smoothly without producing shock. 
Moreover, at movement from the lockup clutch disengaged region (f) into the 
fuzzy reasoning region (d) via the region (c) or the like, a duty value 
calculated by fuzzy reasoning is used only after elapse of the time period 
T1 and when the slip ratio is within a predetermined range, and in other 
cases the duty value of 0% (or the low duty value) is held until the 
second time period T2 has passed and is thereafter gradually raised to the 
duty value calculated by fuzzy reasoning. As a result, problems such as 
engine revving do not occur. And since accurate feedback control is 
conducted, shock and other problems caused by sudden increase in engaging 
force are also nonexistent. 
While the foregoing description assumes the slip state of the torque 
converter to be ascertained in terms of slip ratio, it can instead be 
ascertained in terms of slip amount. 
While the invention has thus been shown and described with reference to the 
specific embodiments, it should be noted that the invention is in no way 
limited to the details of the described arrangements, and changes and 
modifications may be made without departing from the scope of the appended 
claims.