Clutch pressure control device for continuously variable transmission

A continuously variable transmission has a fixed pulley piece and a movable pulley piece supported on the fixed pulley piece so that the movable pulley piece can be moved toward and away from the fixed pulley piece, the width of a groove between the pieces being increased or decreased by changing hydraulic pressure so that the rotational radius of a belt looped around the pulleys is increased or decreased to vary a belt ratio. A clutch pressure control device includes a hydraulic clutch for which engagement and disengagement is controlled according to various control modes, and a control arrangement which calculates a target clutch pressure with reference to a slip amount of the hydraulic clutch and which feedback controls the target clutch pressure so that the slip amount of the clutch is maintained substantially constant.

FIELD OF THE INVENTION 
This invention relates to a clutch pressure control arrangement for a 
continuously variable transmission and, more particularly, to a clutch 
pressure control arrangement for a continuously variable transmission 
where clutch pressure is electronically controlled in accord with a 
driving state of a vehicle and a clutch slip amount defined as a 
difference of rotational speeds between an input shaft and an output shaft 
of the clutch is maintained constant so that engine braking works well 
when the vehicle is coasting at a low speed, so that accelerating 
performance is enhanced when the vehicle is required to be accelerated, 
and so that a satisfactory driving feeling is obtained. 
BACKGROUND OF THE INVENTION 
In a vehicle, a transmission is interposed between an internal combustion 
engine and a driving wheel. This transmission changes the driving power 
provided to the driving wheel and the traveling speed in accordance with 
traveling conditions which are widely changed, so that the internal 
combustion engine can exhibit excellent performance. As an example of one 
kind of such transmission, there is a continuously variable transmission 
in which a transmission gear ratio (belt ratio) for transmitting power is 
changed by increasing and decreasing a rotational radius of a belt looped 
around a pulley having a fixed pulley piece fixed to a rotational shaft 
and a movable pulley piece movably supported on the rotational shaft so 
that the movable pulley piece can be moved toward and away from the fixed 
pulley piece to increase and decrease the width of a groove formed between 
the two pulleys by means of variations in hydraulic oil pressure. Such a 
continuously variable transmission is disclosed, for example, in Japanese 
Patent Early Laid-open Publication No. Sho 57-186656, Japanese Patent 
Early Laid-open Publication No. Sho 59-43249, Japanese Patent Early 
Laid-open Publication No. Sho 59-77159 and Japanese Patent Early Laid-open 
Publication No. Sho 61-233256. 
Also, as such a continuously transmission, there is one which has a single 
plate type hydraulic clutch and selectively supplies power under hydraulic 
oil pressure control. This single plate type hydraulic clutch is 
controlled in various control modes in accord with a signal representing 
an engine speed, such as an opening degree of a throttle valve of a 
carburetor or the like. 
In a conventional clutch pressure control device for a hydraulic clutch, 
there is a method for controlling clutch pressure to a target clutch 
pressure determined by an engine speed in a hold state of a half-clutch 
state, a method for controlling pressure to realize a target number of 
rotations determined by a throttle opening degree obtained by stepping on 
an accelerating pedal in a start mode, and a method for gradually 
increasing clutch pressure in accord with passage of time or determining 
clutch pressure as a minimum or maximum. 
However, the hydraulic clutch is normally locked up when a vehicle is 
coasting at a medium or low speed where an accelerating pedal is not 
significantly pressed, that is, where a throttle opening degree is almost 
zero and a condition of traveling speed less than 5 km/hr is satisfied. 
Therefore, there arises an inconvenience in that, when the accelerating 
pedal is stepped on at that time, the engine torque generated is directly 
transmitted to the transmission and a vehicle body and gives vibrations or 
a shock to passengers. 
Also, when accelerating from a reduced speed or a very low speed traveling 
when the hydraulic clutch is locked up, the engine speed is restrained 
immediately after the accelerating pedal is stepped on. As a result, an 
acceleration response is delayed and the state of the clutch is radically 
changed to generate a great shock. Consequently, the driving feeling is 
badly spoiled. 
It is therefore an object of the present invention to realize a clutch 
pressure control in which, in order to eliminate the above-mentioned 
inconveniences, an engine braking works well while a vehicle is coasting 
at a medium or low speed by calculating a target clutch pressure with 
reference to a slip amount of a hydraulic clutch and feed-back controlling 
the target pressure so that the slip amount of the clutch is maintained 
substantially constant. When accelerating, a response to acceleration is 
enhanced to improve acceleration performance, and a radical change of the 
state of the clutch is prevented to improve the driving feeling. 
SUMMARY OF THE INVENTION 
In order to achieve the above-mentioned object, the present invention 
provides a clutch pressure control device for a continuously variable 
transmission having a fixed pulley piece and a movable pulley piece 
supported on the fixed pulley piece so that the movable pulley piece can 
be moved toward and away from the fixed pulley piece, the width of a 
groove between the pieces being increased or decreased by changing 
hydraulic pressure so that the rotational radius of a belt looped around 
the pulleys is increased or decreased to vary a belt ratio, the clutch 
pressure control device including a hydraulic clutch for which engagement 
and disengagement is controlled according to various control modes, and a 
control arrangement for calculating a target clutch pressure with 
reference to a slip amount of the hydraulic clutch and feed-back 
controlling the target clutch pressure so that the slip amount of the 
clutch is maintained substantially constant. 
According to the present invention, the control arrangement calculates a 
target clutch pressure with reference to a slip amount of the hydraulic 
clutch and feed-back controls the target pressure so that the slip amount 
of the clutch is maintained substantially constant. By this, since the 
slip amount of the clutch can be maintained substantially constant, the 
hydraulic clutch achieves a generally synchronous state during a coasting 
operation at medium and low speeds and thus the engine braking works well. 
Moreover, engine speed is increased when accelerating and a response to 
acceleration is enhanced to improve acceleration performance, and a 
radical change of the state of the clutch is prevented from occurring. 
Thus, a driving feeling is improved.

FIGS. 1 through 4 show a preferred embodiment of the present invention. In 
FIG. 1, reference numeral 2 denotes a continuously variable transmission, 
4 denotes a drive belt, 6 denotes a driving side pulley, 8 denotes a 
driving side fixed pulley piece, 10 denotes a driving side movable pulley 
piece, 12 denotes a driven side pulley, 14 denotes a driven side fixed 
pulley piece, and 16 denotes a driven side movable pulley piece. The 
driving side pulley 6, as shown in FIG. 1, includes a driving side fixed 
pulley piece 10 fixed to a rotational shaft 18 which is rotated by a prime 
mover such as an engine, and a driving side movable pulley 12 mounted on 
the rotational shaft 18 in a manner so that the pulley 12 can move in the 
axial direction relative to the rotational shaft 18 but is incapable of 
rotation. Also, the driven side pulley 12 is of the same construction as 
the driving side pulley 6. The driven side pulley 12 includes a driven 
side fixed pulley piece 14 and a driven side movable pulley 16. 
The driving side movable pulley piece 10 and the driven side movable pulley 
piece 12 respectively are cooperable with first and second housings 20 and 
22 to thereby form first and second hydraulic chambers 24 and 26, 
respectively. The driven side second hydraulic chamber 26 has therein a 
spring 28 adapted to energize the driven side movable pulley 16 so that 
the pulley 16 is urged toward the driven side fixed pulley 14. 
The rotational shaft 18 is connected at one end thereof to an oil pump 30. 
This oil pump 30 is adapted to feed oil from an oil pan 32 into the first 
and second hydraulic chambers 24 and 26 via an oil filter 34 and through 
first and second oil paths 38 and 40 which form a hydraulic circuit 36. 
The first oil path 38 communicates with a primary pressure, control valve 
44 serving as a change speed control valve of pressure control means 42 in 
order to control a primary pressure as an input shaft sheave pressure. 
Also, a third oil path 46 communicates with the second oil path 40 at the 
side of the oil pump 30 with respect to the primary pressure control valve 
44 and communicates with a constant pressure control valve 48 for 
controlling a line pressure (5 to 25 kg/cm.sup.2) in general to a constant 
pressure (for example, 3 to 4 kg/cm.sup.2). Furthermore, the primary 
pressure control valve 44 is continuously in communication with a primary 
pressure controlling first three-way electromagnetic valve 52 through a 
fourth oil path 50. 
Also, the second oil path 40 is continuously in communication at its 
midpoint through a fifth oil path 56 with a line pressure control valve 54 
serving as an escape valve for controlling a line pressure as a pump 
pressure. The line pressure control valve 54 is continuously in 
communication with a line pressure controlling second three-way 
electromagnetic valve 60 through a sixth oil path 58. 
Furthermore, the second oil path 40 communicates on the side of the second 
hydraulic chamber 26 with a clutch pressure control valve 62 for 
controlling a clutch pressure through a seventh oil path 64. This clutch 
control valve 62 is continuously in communication with a clutch pressure 
controlling third three-way electromagnetic valve 68 through an eighth oil 
path 66. 
Also, the primary pressure control valve 44, the primary pressure 
controlling first electromagnetic valve 52, the constant pressure control 
valve 48, the line pressure control valve 54, the line pressure 
controlling second three-way electromagnetic valve 60, the clutch pressure 
control valve 62, and the clutch pressure controlling three-way 
electromagnetic valve 68 each communicate with a ninth oil path 70 
controlled by valve 48. 
The clutch pressure control valve 62 communicates with a hydraulic clutch 
74 through a tenth oil path 72, which communicates with the seventh oil 
path 64. This tenth oil path 72 communicates at one end with a pressure 
converter or detector 78 through an eleventh oil path 76. This pressure 
converter 78 can directly detect hydraulic pressure when a clutch pressure 
is controlled to be in hold or start modes, etc. The converter 78 has as a 
function to facilitate bringing the detected hydraulic pressure to a 
target clutch pressure. Also, when in a drive mode, the clutch pressure is 
generally equal to the line pressure, and so the pressure converter 78 
also contributes to control of the line pressure. 
The hydraulic clutch 74 comprises a piston 80, a ring-like spring 82, a 
first pressure plate 84, a friction plate 86, a second pressure plate 88, 
etc. 
Also, there is provided an electronic control unit (ECU) 90 for performing 
control of a change of speed in response to various input conditions such 
as an opening degree of a throttle of a carburetor (not shown) of a 
vehicle, the engine speed thereof, the pressure detected by detector 78, 
etc., in particular by varying a duty ratio of control signals in order to 
control opening and closing of the primary pressure controlling first 
three-way electromagnetic valve 52, the line pressure controlling second 
three-way electromagnetic valve 60, and the clutch pressure controlling 
third three-way electromagnetic valve 68. 
Now, there will be described in detail various signals input into the 
control unit 90, and the function of these input signals. 
(1) Signal for indicating the position of a shift lever. 
This signal facilitates control of a line pressure, a belt ratio, and a 
clutch pressure required for various speed ranges according to various 
range signals representing shift lever positions such as P, R, N, D, L, 
etc. 
(2) Signal for indicating the opening degree of a carburetor throttle. 
This signal facilitates an estimate of engine torque from a memory loaded 
by a program beforehand and facilitates determination of a target belt 
ratio or a target engine speed. 
(3) Signal for indicating the state of a carburetor idle sensor. 
This signal improves accuracy in correction and control of a sensor for the 
opening degree of a throttle of a carburetor. 
(4) Signal for indicating the position of an accelerating pedal. 
This signal represents a driver's will according to the state of an 
accelerating pedal the driver has stepped on, and facilitates determining 
a way of control when a vehicle is running or when a vehicle is starting. 
(5) Signal for indicating the state of a brake. 
This signal indicates whether a step-down action on a brake pedal has been 
carried out and facilitates determination of a way of control such as 
disengagement of a clutch. 
(6) Signal for indicating selection of a power mode option. 
This signal may optionally be used to facilitate determining the 
performance of a vehicle either in a sport mode or in an economy mode. 
The control unit 90 calculates a target clutch pressure from a slip amount 
of a clutch determined as a rotational difference between an input shaft 
and output shaft of the hydraulic clutch 74, and effects feed-back control 
to actuate the various electromagnetic valves in order to maintain this 
target clutch pressure and thus the slip amount of the clutch. More 
specifically, as shown in FIG. 3, in a clutch slip mode (hereinafter 
simply referred to as the "coast" mode) a difference between a current 
slip amount (CLUSLP) and a target slip rotation (SLPSP, normally 50 rpm) 
is subjected to integral processing (XCSLP), and the result is added with 
a product obtained by multiplying a proportional gain (KPSLP) from the 
clutch slip amount (CLUSLP). As a result, a target value (PCSLP) of the 
coast mode can be obtained. 
A calculating equation for this target value (PCSLP) is shown hereunder: 
EQU PCSLP (n+1).vertline.CLUSLP .vertline.x 
EQU KPSLP+(.vertline.CLUSLP.vertline.- SLPSP) x 
EQU KISLP+XCSLP (n) 
EQU where XCSLP(0)=0 
That is, in the coast mode, by performing a proportional and integral 
control with respect to the clutch slip amount, a control for maintaining 
the clutch slip amount to the target slip rotation is also carried out. In 
the clutch control, a closed control is carried out under a pressure based 
on the coast mode target value (PCSLP) like other control modes (see FIG. 
3). 
This coast mode, as shown in the state diagram of FIG. 4, is functionally 
situated in an intermediate position between the hold mode, the two start 
modes, and the drive mode. Changes between these control modes, as shown 
in FIG. 4, are effected as shown by arrows, and changes are not possible 
between modes which are not connected by an appropriate arrow. For 
example, although a direct change from neutral mode to hold mode is 
possible, no direct change from neutral mode to coast mode is possible. 
Also, conditions of this coast mode include 5 km/h .ltoreq.vehicle speed 
.ltoreq.25 km/h, engine speed &gt;700 rpm, accelerating pedal signal off or 
on, and opening degree of throttle .ltoreq.5%. This means that if the 
accelerating pedal is pressed further down during a coasting operation at 
a medium speed or low speed, the system goes from a special start mode to 
a drive mode, or if a vehicle speed is lowered, it goes to a normal start 
mode and a hold mode. 
On the other hand, in the coast mode, the slip amount of a clutch is about 
50 rpm and the hydraulic clutch 74 is in synchronism although it is not 
completely connected. 
Also, as shown in FIG. 1, an input shaft rotation detecting gear 102 is 
disposed outside the first housing 20 and an input shaft side first 
rotation detector 104 is disposed in the vicinity of an outer peripheral 
portion of this input shaft rotation detecting gear 102. Also, an output 
shaft rotation detecting gear 106 is disposed outside the second housing 
22 and an output side second rotation detector 108 is disposed in the 
vicinity of an outer peripheral portion of this output shaft rotation 
detecting gear 106. Detecting signals from the first and second rotation 
detectors 104 and 108 are routed to the control unit 90 and utilized for 
determining an engine speed and a belt ratio. 
The hydraulic clutch 74 is provided with an output power transmitting gear 
110. A third rotation detector 112 for detecting the rotational speed of a 
final output shaft is disposed in the vicinity of an outer peripheral 
portion of this output power transmitting gear 110. That is, the third 
rotation detector 112 is adapted to detect the rotational speed of a final 
output shaft which is directly connected with a reduction gear and a 
differential gear, a driving shaft and a tire, and is thus capable of 
detecting vehicle speed. Also, the second and the third rotation detectors 
108 and 112 make it possible to respectively detect the rotational speeds 
of the input shaft and output shaft of the hydraulic clutch 74, and are 
thus capable of detecting a slip amount of the clutch. 
The operation of this embodiment will now be described. 
In the continuously variable transmission 2 shown in FIG. 1, the oil pump 
30 situated on the rotational shaft 18 is driven by rotation of the 
rotational shaft 18, and oil from the oil pan 32 is drawn through the oil 
filter 34. A line pressure as a pump pressure is controlled by the line 
pressure control valve 54. If a leakage amount from this line pressure 
control valve 54, that is an escape amount of the line pressure control 
valve 54, is large, the line pressure becomes low. To the contrary, if the 
leakage amount from this line pressure control valve 54, that is if the 
escape amount of the line pressure control valve 54, is small, the line 
pressure be comes high. 
The action of the line pressure control valve 54 is controlled by the 
exclusive second three-way electromagnetic valve 60, and the line pressure 
control valve 54 is activated following the action of this second 
three-way electromagnetic valve 60. This second three-way electromagnetic 
valve 60 is controlled by controlling a duty ratio of a constant frequency 
signal. That is, a condition that the duty ratio is 0% indicates a state 
where the second three-way electromagnetic valve 60 is not activated at 
all and the output side communicates with the atmosphere, thus making the 
output hydraulic pressure zero. At the other end of the spectrum, a 
condition that the duty ratio is 100% indicates that the second three-way 
electromagnetic valve 60 is continuously activated and the output side 
communicates continuously with the input side, thus making the output 
pressure the maximum output hydraulic pressure, which is equal to the 
control pressure. That is, the output hydraulic pressure is variable 
according to variation of the duty ratio of the signal supplied to the 
second three-way electromagnetic valve 60. Accordingly, the 
characteristics of the second three-way electromagnetic valve 60 make it 
possible to activate the line pressure control valve 54 analogously, and 
thus the line pressure can be controlled by valve 54 by appropriately 
varying the duty ratio of the second three-way electromagnetic valve 60. 
The action of this second three-way electromagnetic valve 60 is controlled 
by the control unit 90. 
The primary pressure for controlling the change of speed is controlled by 
the primary pressure control valve 44, and this primary pressure control 
valve 44, similar to the line pressure control valve 54, is controlled in 
its action by the exclusive first three-way electromagnetic valve 52. This 
first three-way electromagnetic valve 52 is used to connect the primary 
pressure at 38 to either the line pressure at 40 or to the atmosphere. 
That is, the first three-way electromagnetic valve 52 connects the primary 
pressure to the line pressure in order to shift the gear ratio to a full 
overdrive state or it connects the primary pressure to the atmosphere in 
order to shift the gear ratio to a full low state. 
The clutch pressure control valve 62 for controlling a clutch pressure 
effects a connection of line 72 with the line pressure at 40 when the 
maximum clutch pressure is required and with the atmosphere when the 
minimum clutch pressure is required. This clutch pressure control valve 
62, similar to the line pressure control valve 54 and primary pressure 
control valve 44, is controlled by the action of the exclusive third 
three-way electromagnetic valve 68. Therefore, a repeated description of 
this control will be omitted here. The clutch pressure varies in a range 
from the lowest or zero value (atmosphere) to the highest value (line 
pressure). 
The approach for controlling the clutch pressure has six modes, as shown in 
FIG. 5. 
(1) Neutral Mode 
When the shift position is N or P and the hydraulic clutch is completely 
disconnected, the clutch pressure is the lowest (zero) and the hydraulic 
clutch is off. 
(2) Hold Mode 
When the shift position is D or R and the throttle is off because the 
driver has no will for driving the vehicle, or in the case where the 
driver wants to reduce the vehicle speed and cuts off the engine torque 
during his driving operation, the clutch pressure is low but sufficient so 
that the clutch would be contacted, and in particular the clutch pressure 
is 3.5 to 4.0 kg/cm.sup.2 and the clutch is in a half-clutch state (creep 
state). 
(3) Normal Mode 
The clutch pressure is 5.0 to 15 kg/cm.sup.2 and an engine torque is 
transmitted to the wheel. 
(4) Special Start Mode 
The clutch pressure is 5.0 to 15 kg/cm.sup.2 and an engine torque is 
transmitted to the wheel. 
(5) Coast Mode 
The condition of clutch slip amount being about 50 rpm is satisfied and the 
clutch input shaft and output shaft are synchronized in rotation with 
respect to each other. 
(6) Drive Mode 
For a complete traveling state the clutch is completely engaged (clutch 
locked-up state), or else after it goes from the start mode to the 
traveling mode the clutch is almost locked up and the clutch pressure is 
in a sufficiently high level for bearing the engine torque. 
The neutral mode (1) of this pattern is carried out by an exclusive switch 
valve (not shown) interlocked with the shifting operation. The other modes 
(2), (3), (4), (5), and (6) are carried out by appropriate controlling 
duty ratios of control signals which are for the first, second, and third 
three-way electromagnetic valves 52, 60 and 68 and which are generated by 
the control unit 90. Particularly, when in the drive mode (6), the seventh 
oil path 64 and the tenth oil path 72 are in communication with the second 
oil path 40 through the clutch pressure control valve 62, thus bringing 
about a maximum pressure generating state. The clutch pressure and the 
line pressure become the same. 
The primary pressure control valve 44, the line pressure control valve 54, 
and the clutch pressure control valve 62 respectively are controlled by 
output hydraulic pressure from the first, second and third three-way 
electromagnetic valves 52, 60 and 68. The control hydraulic pressure for 
controlling the first, second and third three-way electromagnetic valves 
52, 60 and 68 is a constant hydraulic pressure regulated by the constant 
pressure control valve 48. This control hydraulic pressure is always lower 
than the line pressure, and it is a stable constant pressure. The control 
hydraulic pressure is also introduced into the various control valves 44, 
54 and 62 in order to stabilize these control valves 44, 54 and 62. 
Electronic control of the continuously variable transmission 2 will be 
described next. 
The continuously variable transmission 2 is hydraulically controlled, but 
the proper line pressure for holding the belt and transmitting the torque, 
the primary pressure for changing the gear ratio, and the clutch pressure 
for effectively engaging the hydraulic clutch 74 are controlled according 
to control signals from the control unit 90. 
A control mode of a clutch pressure of the hydraulic clutch 74 will be 
described with reference to the flowchart of FIG. 2. 
When the program starts, it is judged whether or not it is a neutral mode 
first (step 201). 
If it is a neutral mode and is therefore judged as YES in step 201, a 
neutral mode controlling is carried out in step 202. In this case, the 
clutch pressure to the hydraulic clutch 74 is 0 kg/cm.sup.2. 
On the other hand, in step 201, in the case that it is in a mode other than 
the neutral mode, it goes to step 203. 
In this step 203, it is judged whether or not it is a hold mode. If it is 
the hold mode and is therefore judged as YES in step 203, it is then 
judged whether or not the accelerating pedal signal for detecting the 
accelerating operation is on in step 204. 
In the case that the accelerating pedal signal is off and it is therefore 
judged as NO in step 204, it is then judged in step 205 whether the 
vehicle speed is 5 km/h or less or is more than 5 km/h. In the case that 
it is judged as vehicle speed .ltoreq.5 km/h in this step 205, the hold 
mode controlling is carried out in step 206. On the other hand, in the 
case that it is judged as vehicle speed &gt;5 km/h in step 205, it is then 
judged in step 207 whether the engine speed is less than 700 rpm, or 700 
rpm or more. 
In this step 207, in the case that it is judged as engine speed &lt;700 rpm, 
the hold mode controlling is carried out in the preceding step 206. On the 
other hand, in the case that it is judged as engine speed .gtoreq. 700 rpm 
in step 207, then it goes to step 228 and, as will be described, a slip 
timer setting is carried out. 
In the above-mentioned step 204, in the case that it is judged as the 
accelerating pedal signal being on and therefor YES, it is then judged in 
step 208 whether the vehicle speed is 6 km/h or less or is more than 6 
km/h. 
In this step 28, in the case that it is judged as vehicle speed &lt;6 km/h, 
then it goes to step 214 and, as will be described, a normal start mode 
controlling is carried out. On the other hand, in the case that it is 
judged as vehicle speed .gtoreq.6 km/h in step 208, then it goes to step 
220 and, as will be described, a special start mode controlling is carried 
out. 
On the other hand, in the case that it is in a mode other than the hold 
mode in above-mentioned step 203, it is then judged in step 209 whether it 
is a normal start mode. 
In the case that it is judged as a normal start mode and therefore YES in 
this step 209, it is then judged whether the accelerating pedal signal is 
on in step 210. In the case that the accelerating pedal signal is judged 
as off and therefore NO in step 210, it is then judged in step 211 whether 
the vehicle speed is 5 km/h or less or is more than 5 km/h. In the case 
that it is judged as vehicle speed &gt;5 km/h in this step 211, the hold mode 
control is carried out in the above-mentioned step 206. On the other hand, 
in the case that it is judged as vehicle speed .ltoreq.5 km/h in step 211, 
a slip timer setting is carried out in step 228 as will be described. 
In the case that the accelerating pedal signal is on and therefore YES in 
the above-mentioned step 210, it is then judged in step 212 whether the 
engine speed is 1200 rpm or less or is more than 1200 rpm. In the case 
that it is judged as engine speed &gt;1200 rpm in this step 212, it is then 
judged whether the clutch is engaged in step 213. 
In the case that the clutch is not engaged and NO in this step 21 3, a 
normal start mode controlling is carried out in step 214. 
On the other hand, in the case that it is judged as engine speed 
.ltoreq.1200 rpm in the above-mentioned step 212, a normal start mode 
controlling is carried out in step 214. Also, in the case that the clutch 
is engaged and YES in step 213, it goes to step 238 and, as will be 
described, a drive mode controlling is carried out. 
In the case that it is a mode other than the normal start mode and 
therefore NO in the above-mentioned step 209, it is then judged in step 
215 whether it is a special start mode. 
In the case that it is judged as a special start mode and therefore YES in 
this step 215, it is then judged whether in step 216 the vehicle speed is 
less than 5 km/h, or 5 km/h or more. 
In the case that it is judged as vehicle speed .gtoreq.5 km/h in this step 
216, it is then judged in step 217 whether the engine speed is 700 rpm or 
less or is more than 700 rpm. In the case that it is judged as engine 
speed &gt;700 rpm in step 217, it is then judged in step 218 whether the 
throttle opening degree is 5% or less or is more than 5%. 
On the other hand, in the case that it is judged as vehicle speed &lt;5 km/h 
in the above-mentioned step 216 or as engine speed .ltoreq.700 rpm in the 
above-mentioned step 217, then it goes to the above-mentioned step 206 and 
a hold mode controlling is carried out. 
In the case that it is judged as throttle opening degree &gt;5% in the 
above-mentioned step 218, it is then judged whether or not the clutch is 
engaged in step 219. In the case that the clutch is engaged and therefore 
YES in this step 219, then it goes to step 238 and, as will be described, 
a drive mode control is carried out. In the case that the clutch is not 
engaged and therefore NO in step 219, then a special start mode 
controlling is carried out in step 220. 
On the other hand, in the case that it is judged as throttle opening degree 
.ltoreq.5% in the above-mentioned step 218, it is then judged in step 221 
whether the accelerating pedal signal is on. In the case that the 
accelerating pedal is on and therefore YES in this step 221, it is then 
judged whether or not the clutch is engaged in the above-mentioned step 
219. On the other hand, in the case that the accelerating pedal signal is 
off and therefore NO in step 221, then it goes to step 228 and, as will be 
described, a slip timer setting is carried out. 
In the case that it is a mode other than the special start mode and 
therefore NO in above-mentioned step 215, it is then judged whether or not 
it is a coast mode in step 222. In the case that it is a coast mode and 
therefore YES in this step 222, it is then judged whether or not the 
vehicle speed is less than 5 km/h, or 5 km/h or more in step 223. In the 
case that it is judged as vehicle speed .gtoreq.5 km/h in this step 223, 
it is then judged in step 224 whether the engine speed is 700 rpm or less 
or is more than 700 rpm. 
In the case that it is judged as vehicle speed &lt;5 km/h in the 
above-mentioned step 223 or as engine speed .ltoreq.700 rpm in the 
above-mentioned step 224, then a hold mode controlling is carried out in 
the above-mentioned step 206. 
In the case that it is judged as engine speed &gt;700 rpm in the 
above-mentioned step 224, it is then judged in step 225 whether the 
accelerating pedal signal is on. In the case that the accelerating pedal 
signal is on and therefore YES in step 225, it is then judged in step 226 
whether the throttle opening degree is 5% or less or is more than 5%. In 
the case that the accelerating pedal signal is off and therefore NO in 
step 225, then it goes to step 227. 
In the case that it is judged as throttle opening degree &gt;5% in step 226, 
then a special start mode controlling is carried out in the 
above-mentioned step 220. 
In the case that it is judged as throttle opening degree .ltoreq.5% in step 
226, it is then judged in step 227 whether the vehicle speed is less than 
25 km/h, or is 25 km/h or more. 
In the case that it is judged as vehicle speed &lt;25 km/h in this step 227, 
then a slip timer setting is carried out in step 228 and a coast mode 
controlling is carried out in step 229. In the case that it is judged as 
vehicle speed .gtoreq.25 km/h in the above-mentioned step 227, it is then 
judged whether or not the slip timer is zero in step 230. 
In the case that the slip timer is zero and therefore YES in this step 230, 
then a drive mode controlling is carried out in step 238, as will be 
described. 
On the other hand, in the case that the slip timer is not zero and 
therefore NO in step 230, then the slip timer is decremented in step 231 
and a coast mode controlling is carried out in step 229. 
This coast mode controlling is carried out in the manner shown in FIG. 3. 
That is, in the coast mode, a target slip rotation (SLPSP: normally 50 
rpm) is compared (302) with a current or actual clutch slip amount 
(CLUSLP) determined using a first table (301). The thus obtained 
difference is multiplied (303) by an integral gain (KISLP), the result is 
added (305) with a previous value Z.sup.-1 (304) of a value XCSLP, and the 
result is then processed (306) to obtain an integral value (XCSLP). This 
integral value (XCSLP) is added (308) with a product (307) obtained by 
multiplying a proportional gain (KPSLP) with the clutch slip amount 
(CLUSLP). Then, a target value (PCSLP) of the coast mode is found using a 
second table (309). 
##EQU1## 
wherein XCSLP (O)=0. 
In this coast mode, by performing a proportional and integral control with 
respect to the clutch slip amount (CLUSLP), a feed-back controlling is 
carried out in order to maintain the clutch slip amount at a target clutch 
slip rotation, that is, at a target clutch slip amount (SLPSP). 
In the clutch controlling after the coast mode controlling, a close 
controlling is carried out under pressure, like other control mode, with 
reference to such calculated coast mode target value (PCSLP). That is, in 
the above-mentioned controlling, one of the normal start mode control, 
special start mode control, coast mode control and hold mode control is 
selected in a first switch portion (310), a target value (Pcc) of a creep 
(half-clutched state) is found, then a clutch engage pressure (PCE) and a 
clutch pressure (PCLU) are compared (311) with each other, then such 
obtained difference is multiplied (312) by a proportional gain (KAPC), and 
then such proportional gain value is multiplied (313) by an advance/delay 
gain (LEADLAG). 
Then, the result is multiplied (314) by an integral gain (DPIC) and the 
resulting value is added (316) with a previous upper/lower limit processed 
value Z.sup.-1 (315) and then upper and lower limit processed (317). 
Thereafter, such advance/delay gained value from 313 is added (318) with 
such upper and lower limit processed (317) and integral gained value, the 
result being upper and lower limit processed (319), then the value from 
319 and a clutch solenoid duty value (NPC) are added (320) with each 
other, and then the result is further upper and lower limit processed 
(321). 
In a second switch portion (322), one of the integral value from 321, the 
neutral mode control and the drive mode control is selected, and a signal 
of a clutch duty value is output. 
On the other hand, referring again to FIG. 2, in the case that it is a mode 
other than the coast mode and therefore NO in the above-mentioned step 
222, it is then judged in step 232 whether it is the drive mode. 
In the case that it is the drive mode and therefore YES in step 232, it is 
then judged in step 233 whether the vehicle speed is 5 km/h or less or is 
more than 5 km/h. In the case that it is judged as vehicle speed &gt;5 km/h 
in this step 233, it is then judged in step 234 whether the engine speed 
is less than 900 rpm, or is 900 rpm or more in step 234. 
In the case that it is other than a drive mode and therefore NO in the 
above-mentioned step 232, that it is judged as vehicle speed .ltoreq.5 
km/h in the above-mentioned step 233, or that it is judged as engine speed 
&lt;900 rpm in the above-mentioned step 234, then a hold mode controlling is 
carried out in the above-mentioned step 206. 
In the case that it is judged as engine speed .gtoreq.900 rpm in the 
above-mentioned step 234, it is then judged in step 235 whether the 
vehicle speed is 23 km/h or less or is more than 23 km/h. In the case that 
it is judged as vehicle speed &gt;23 km/h in this step 235, then it goes to 
step 238 as will be described and a drive mode controlling is carried out. 
In the case that it is judged as vehicle speed .ltoreq.23 km/h in the 
above-mentioned step 235, it is then judged whether the accelerating pedal 
signal is on in step 236. In the case that the accelerating pedal signal 
is on and therefore YES in this step 236, it is then judged in step 237 
whether the throttle opening degree is 5% or less or is more than 5%. 
In the case that the accelerating pedal signal is off and therefore NO in 
the above-mentioned step 236 or that it is judged as throttle opening 
degree .ltoreq.5% in the above-mentioned step 237, then a slip timer 
setting is carried out in the above-mentioned step 228. 
In the case that it is judged as throttle opening degree &gt;5in the 
above-mentioned step 237, then a drive mode controlling is carried out in 
step 238. 
As a result, when the accelerating pedal is stepped on to open the throttle 
valve during a coasting operation at a medium or low speed, it is shifted 
from a special start mode to a drive mode, or if the vehicle speed is 
lowered, it is shifted to a normal start mode or a hold mode. Therefore, 
in the coast mode, the clutch slip amount is about 50 rpm and the 
hydraulic clutch is in a synchronous state even if it is not completely 
engaged. 
By this, during the coasting operation at a medium and low speed, the 
workability of the engine braking can be improved. Furthermore, during a 
coast mode, even when the throttle valve is greatly opened to generate an 
engine torque, the clutch control is rapidly shifted to an engine rotation 
controlling. Therefore, responsiveness of an engine rotation is not 
jeopardized, an accelerating performance is enhanced, and in particular 
the generation of shocks, etc. can be reduced when the vehicle is 
accelerated from its reduced speed or a very low speed running. 
Also, as no radical on or off of the hydraulic clutch occurs when shifting 
from the coast mode to another control mode, the state of the hydraulic 
clutch 74 is not radically changed. Therefore, no shocks, etc. are 
generated and a satisfactory drive feeling can be obtained. 
Furthermore, the conventional clutch control system can be modified by 
adding a feed-back loop according to a clutch slip amount of the hydraulic 
clutch 74. Accordingly, there can be attained an effective clutch pressure 
controlling. As a practical matter this is very useful indeed. 
As apparent from the foregoing detailed description, according to this 
invention there is provided a control arrangement for calculating a target 
clutch pressure according to a clutch slip amount of hydraulic clutch and 
for using feed-back control in order to maintain the clutch slip amount 
substantially constant at said target clutch pressure. Accordingly, 
workability of the engine brake during a coasting operation at a medium 
and low speed can be improved and an acceleration responsiveness in 
improved when the vehicle is accelerated, thereby improving accelerating 
performance. Furthermore, when it is shifted from a control mode to 
another mode, the state of the clutch is not radically changed. 
Accordingly, no shocks, etc. are generated and a satisfactory drive 
feeling can be obtained. 
Although a particular preferred embodiment of the invention has been 
disclosed in detail for illustrative purposes, it will be recognized that 
variations or modifications of the disclosed apparatus, including the 
rearrangement of parts, lie within the scope of the present invention.