Forward/reverse drive and stop control system for hydrostatic -mechanical transmissions

A control system in which a judgment is made such that (i) a vehicle is in a forward drive control state, on condition that the forward/reverse drive lever is placed in the forward drive position and the actual speed ratio exceeds a specified small value in a forward drive direction, (ii) the vehicle is in a reverse drive control state on condition that the lever is placed in the reverse drive position and the actual speed ratio exceeds a specified small value in a reverse drive direction, (iii) the vehicle is in an engine brake control state, on condition that the lever is placed in the neutral position and the actual speed ratio exceeds the specified small value in the forward or reverse drive direction, (iv) the vehicle is in an FR shift control state, on condition that the lever is placed in the forward or reverse drive position and the actual speed ratio exceeds the specified small value in the reverse or forward drive direction, and (v) the vehicle is in a neutral control state, on condition that the lever is placed in the neutral position and the actual speed ratio does not exceed the specified small values in the forward and reverse drive directions.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The invention relates to a forward/reverse drive and stop control system 
for hydrostatic-mechanical transmissions, that is suited for use in 
tracklaying vehicles such as bulldozers, and more particularly, to a 
technique for controlling the forward drive, reverse drive and stoppage of 
such vehicles according to the position of the forward/reverse drive 
lever. 
(2) Description of the Prior Art 
One of such control systems for hydrostatic-mechanical transmissions is 
disclosed in Japanese Patent Publication No. 62-31660 (1987). According to 
the system taught by this publication, the angle of a swash plate for 
controlling the discharge of a pump in the hydrostatic transmission unit 
is adjusted according to the difference between a target engine revolution 
speed calculated from a throttle position and an actual engine revolution 
speed such that the actual engine revolution speed is made close to the 
target engine revolution speed. 
SUMMARY OF THE INVENTION 
The prior art control system described above, however, exhibits poor 
response because of its feedback control in which the difference between 
actual and target engine revolution speeds is used to obtain an amount 
that adjusts the swash plate for controlling the discharge of a pump in 
the hydrostatic transmission unit. In order to solve this problem, we have 
proposed a control system in Japanese Patent Application No. 2-323930 
(1990) (now published as Japanese Patent Publication Laid Open No. 
4-191558 (1992)). In this system, a target motor speed ratio (i.e., a 
target value for the ratio of the revolution speed of the motor in the 
hydrostatic transmission unit to the revolution speed of the power source) 
is first computed and then the angle of the discharge controlling swash 
plate for the pump in the hydrostatic transmission unit is adjusted by 
feed forward control using the target motor speed ratio. 
Generally, it is necessary in a vehicle incorporating an automatic 
transmission to accurately control the forward drive, reverse drive, start 
and stoppage of the vehicle according to the position of the 
forward/reverse drive lever operated by the operator. As the 
hydrostatic-mechanical transmission disclosed in the above publication 
(Japanese Patent Publication Laid Open No. 4-191558 (1992)) has a 
construction basically different from that of, for example, a transmission 
equipped with a hydraulic torque convertor, the conventional 
forward/reverse drive and stop control method cannot be applied to the 
vehicle incorporating the hydrostatic-mechanical transmission. 
The present invention has been made in view of the above problem and one of 
the objects of the invention is therefore to provide a system for 
accurately controlling the forward drive, reverse drive and stoppage of a 
vehicle provided with a hydrostatic-mechanical transmission, in accordance 
with lever operation performed by the operator. The foregoing object can 
be achieved by a system according to the invention, in which how the 
vehicle should be driven (i.e., the control state of the vehicle) is 
judged from the position of the forward/reverse drive lever and the 
driving condition of the vehicle, and the vehicle is then controlled 
according to the vehicle control state. More specifically, according to 
the invention, there is provided a forward/reverse drive and stop control 
system for a hydrostatic-mechanical transmission which is equipped with a 
mechanical transmission unit actuated through an input shaft connectable 
to a power source; a hydrostatic transmission unit which is connectable to 
the input shaft and comprises a pump and motor having their respective 
discharge controlling swash plates, the angle of at least either of the 
swash plates being variable; and a differential unit for actuating both 
the mechanical transmission unit and the hydrostatic transmission unit by 
connecting an output shaft thereto, the control system comprising, as 
shown in FIG. 1 which illustrates the principle of the invention, 
(a) speed ratio detecting means (1) for detecting an actual speed ratio 
that is the ratio of the revolution speed of said output shaft to the 
revolution speed of said power source; 
(b) lever position detecting means (2) for determining which of forward 
drive position, neutral position and reverse drive position a 
forward/reverse drive lever is placed in; and 
(c) vehicle control state judging means (3) for making a judgment based on 
the detections performed by the speed ratio detecting means (1) and the 
lever position detecting means (2), such that (i) a vehicle is in a 
forward drive control state in which the vehicle is required to be 
forwardly driven, on condition that the forward/reverse drive lever is 
placed in the forward drive position and the actual speed ratio exceeds a 
specified small value in a forward drive direction, (ii) the vehicle is in 
a reverse drive control state in which the vehicle is required to be 
reversely driven, on condition that the forward/reverse drive lever is 
placed in the reverse drive position and the actual speed ratio exceeds a 
specified small value in a reverse drive direction, (iii) the vehicle is 
in an engine brake control state in which the vehicle is required to be 
gradually stopped, on condition that the forward/reverse drive lever is 
placed in the neutral position and the actual speed ratio exceeds the 
specified small value in the forward drive direction or the specified 
small value in the reverse drive direction, (iv) the vehicle is in an: FR 
shift control state in which the vehicle is required to be once 
immediately stopped and then driven in an opposite direction, on condition 
that the forward/reverse drive lever is placed in the forward drive 
position or reverse drive position and the actual speed ratio exceeds the 
specified small value in the reverse drive direction or the specified 
small value in the forward drive direction, and (v) the vehicle is in a 
neutral control state in which the vehicle is required to be completely 
stopped, on condition that the forward/reverse drive lever is placed in 
the neutral position and the actual speed ratio does not exceed the 
specified small values in the forward drive direction and reverse drive 
direction. In the control system having the above construction, according 
to the actual speed ratio (i.e., the ratio of the revolution speed of the 
output shaft to the revolution speed of the power source) detected by the 
speed ratio detecting means (1) and the position of the forward/reverse 
drive lever detected by the lever position detecting means (2), the 
vehicle control state judging means (3) judges which of the five states, 
namely, the forward drive control state, reverse drive control state, 
engine brake control state, FR shift control state and neutral control 
state the vehicle is in. The position of the forward/reverse drive lever 
and the actual speed ratio which correspond to each control state are as 
follows: 
(1) Forward drive control state: the forward/reverse drive lever is placed 
in the forward drive position and the actual speed ratio exceeds the 
specified small value in the forward drive direction. 
(2) Reverse drive control state: the forward/reverse drive lever is placed 
in the reverse drive position and the actual speed ratio exceeds the 
specified small value in the reverse drive direction. 
(3) Engine brake control state: the forward/reverse drive lever is placed 
in the neutral position and the actual speed ratio exceeds the specified 
small value in the forward direction or the specified small value in the 
reverse drive direction. 
(4) FR shift control state: the forward/reverse drive lever is placed in 
the forward drive position or reverse drive position and the actual speed 
ratio exceeds the specified small value in the forward drive direction or 
the specified small value in the reverse drive direction. 
(5) Neutral control state: the forward/reverse drive lever is placed in the 
neutral position and the actual speed ratio does not exceed the specified 
small values in the forward drive direction and reverse drive direction. 
The control system of the invention further comprises: 
(a) target speed ratio computing means (4) for computing a target speed 
ratio, which is a target value for the ratio of the revolution speed of 
the output shaft to the revolution speed of the power source, according to 
the control state of the vehicle judged by the vehicle control state 
judging means (3); 
(b) target motor speed ratio computing means (5) for computing a target 
motor speed ratio, which is a target value for the ratio of the revolution 
speed of the motor in the hydrostatic transmission unit to the revolution 
speed of the power source, from the target speed ratio computed by the 
target speed ratio computing means (4); and 
(c) swash plate angle controlling means (6) for controlling the angle of at 
least either of the discharge controlling swash plates according to the 
target motor speed ratio computed by the target motor speed ratio 
computing means (5). 
In the control system having these means, a target speed ratio is computed 
according to the control state of the vehicle and a target motor speed 
ratio is computed based on the target speed ratio. As the target motor 
speed ratio corresponds to the volume ratio of the discharge of the motor 
to the discharge of the pump, the angle of at least either of the 
discharge controlling swash plates is directly controlled by the swash 
plate angle controlling means (6) using the target motor speed ratio as a 
control amount, so that the revolution speed of the power source can be 
readily adjusted to the target revolution speed. Thus, the forward drive, 
reverse drive and stop-page of the vehicle can be appropriately carried 
out in accordance with lever operation performed by the operator. 
Preferably, the target speed ratio computing means (4) computes the target 
speed ratio with a target engine revolution speed fixed at a value more 
than the revolution speed of the engine in a full-throttle state, when the 
vehicle control state judging means (3) judges that the vehicle is in the 
engine brake control state. This arrangement makes it possible to 
gradually reduce the value of the target speed ratio, thereby gradually 
decelerating the vehicle. In this case, the target speed ratio computing 
means (4) computes the target speed ratio, limiting the difference between 
the actual revolution speed of the engine and a target revolution speed 
for the engine according to a speed range in which the mechanical 
transmission unit is presently placed. With this arrangement, the degree 
of the deceleration can be limited according to the present speed range, 
so that more smooth, natural stoppage can be ensured. 
Preferably, the target speed ratio computing means (4) computes the target 
speed ratio such that it decreases a certain value at a time, when the 
vehicle control state judging means (3) judges that the vehicle is in the 
FR shift control state. With this arrangement, the vehicle can be 
immediately stopped by selecting an appropriate value as the certain 
value. 
Further, it is preferable that the target speed ratio computing means (4) 
fixes the target speed ratio at zero, when the vehicle control state 
judging means (3) judges that the vehicle is in the neutral control state. 
This enables it to stop the vehicle without fail. 
In this case, the target speed ratio computed by the target speed ratio 
computing means (4) is limited within a specified range which is set for 
every speed range for the mechanical transmission unit, using the 
characteristic function of the target motor speed ratio against the target 
speed ratio. 
Other objects of the present invention will become apparent from the 
detailed description given hereinafter. However, it should be understood 
that the detailed description and specific examples, while indicating a 
preferred embodiment of the invention, is given by way of illustration 
only, since various changes and modifications within the spirit and scope 
of the invention will become apparent to those skilled in the art from 
this detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now, reference will be made to the accompanying drawings for describing a 
preferred embodiment of a forward/reverse drive and stop control system 
for a hydrostatic-mechanical transmission according to the invention. 
In FIG. 2, there are provided a mechanical transmission unit 24 which 
includes a gear box for providing three forward speeds and three reverse 
speeds and a hydrostatic transmission unit 25 having a hydraulic 
pump-motor. These units 24 and 25 are connected to an input shaft 23 in 
such a manner that power transmitted from an engine 21 can be split, and 
the input shaft 23 is coaxially connected to an output shaft 22 of the 
engine 21 which is employed as one example of the power source of the 
invention. There is also provided a differential unit 27 that selectively 
connects an output shaft 26 to both the mechanical transmission unit 24 
and the hydrostatic transmission unit 25, or to the hydrostatic 
transmission unit 25 only, for driving. 
The mechanical transmission unit 24, hydrostatic transmission unit 25 and 
differential unit 27 will be hereinafter described in that order. 
(1) Mechanical transmission unit 24 
Referring to FIG. 2, the input shaft 23 is provided with a reverse 
planetary gear train 30 and a forward planetary gear train 31. The gear 
trains 30 and 31 are of the single planetary type and are aligned in an 
axial direction of the input shaft 23 in this order when enumerating from 
the left. The reverse planetary gear train 30 is composed of a sun gear 
30a fixedly attached to the input shaft 23; a ring gear 30b positioned 
outside the sun gear 30a; a planet gear 30c that is in mesh with the gears 
30a and 30b, being positioned therebetween; and a planet carrier 30d that 
is for the planet gear 30c and can be hydraulically braked by a reverse 
hydraulic clutch 32. Similarly, the forward planetary gear train 31 is 
composed of a sun gear 31a fixedly attached to the input shaft 23; a ring 
gear 31b that is positioned outside the sun gear 31a and can be 
hydraulically braked by a forward hydraulic clutch 33; a planet gear 31c 
that is in mesh with the gears 31a and 31b, being positioned therebetween; 
and a planet carrier 31d that is for the planet gear 31c and is integral 
with the ring gear 30b of the reverse planetary gear train 30. 
There is provided an intermediate shaft 35 positioned coaxially with and in 
an extending direction of the input shaft 23. In FIG. 2, the intermediate 
shaft 35 is provided, at the left end thereof, with a clutch plate 37 that 
is hydraulically connectable by a 2nd-speed hydraulic clutch 36. The 
2nd-speed hydraulic clutch 36 is formed integrally with the planet carrier 
31d of the forward planetary gear train 31. The intermediate shaft 35 is 
also provided with a first 3rd-speed planetary gear train 38 and a second 
3rd-speed planetary gear train 39. The gear trains 38 and 39 are of the 
single planetary type and are aligned in an axial direction of the 
intermediate shaft 35 in this order when enumerating from the left of FIG. 
2. 
The first 3rd-speed planetary gear train 38 is composed of a sun gear 38a 
rotatably supported by the intermediate shaft 35; a ring gear 38b that is 
positioned outside the sun gear 38a, being integral with the planet 
carrier 31d of the forward planetary gear train 31 and the 2nd-speed 
hydraulic clutch 36; a planet gear 38c that is in mesh with the gears 38a 
and 38b, being positioned therebetween; and a planet carrier 38d that is 
for the planet gear 38c and can be hydraulically braked by a 3rd-speed 
hydraulic clutch 40. Similarly, the second 3rd-speed planetary gear train 
39 is composed of a sun gear 39b that is rotatably supported by the 
intermediate shaft 35, being integral with a clutch plate 41; a ring gear 
39c that is positioned outside the sun gear 39b, being integral with the 
sun gear 38a of the first 3rd-speed planetary gear train 38; a planet gear 
39d that is in mesh with the gears 39b and 39c, being positioned 
therebetween; and a fixed planet carrier 39e that is for the planet gear 
39d and is integral with a 1st-speed hydraulic clutch 42 for hydraulically 
connecting the clutch plate 41. 
(2) Hydrostatic transmission unit 25 
The input shaft 23 is coupled through a gear train 51 to a variable 
displacement pump 50 having a discharge controlling variable-angle swash 
plate 50a which can be inclined both in the positive and negative 
directions. The variable displacement pump 50 is connected, through a pair 
of conduits 52 consisting of an outgoing path and a return path, to a 
variable displacement motor 53 having a discharge controlling 
variable-angle swash plate 53a which can be inclined in one direction. The 
variable displacement motor 53 has an output shaft 54 connected to a gear 
train 55. The discharge controlling variable-angle swash plates 50a and 
53a provided in the variable displacement pump 50 and the variable 
displacement motor 53 are designed such that the revolution speed of the 
variable displacement pump 50 and that of the variable displacement motor 
53 vary according to variations in the angles of the discharge controlling 
variable-angle swash plates 50a and 53a, as described bellow. 
The revolution speed of the variable displacement pump 50 is specified, and 
the discharge controlling variable-angle swash plate 53a of the variable 
displacement motor 53 is inclined at a maximum tilt angle. In the above 
condition, as the tilt angle of the discharge controlling variable-angle 
swash plate 50a of the variable displacement pump 50 is inclined from zero 
in the positive direction, the revolution speed of the variable 
displacement motor 53 increases from zero in the positive direction. Then, 
the tilt angle of the discharge controlling variable-angle swash plate 50a 
of the variable displacement pump 50 is set to a maximum positive value. 
In this condition, as the tilt angle of the discharge controlling 
variable-angle swash plate 53a of the variable displacement motor 53 is 
decreased, the revolution speed of the variable displacement motor 53 
further increases in the positive direction. 
On the other hand, as the tilt angle of the discharge controlling 
variable-angle swash plate 50a of the variable displacement pump 50 is 
inclined from zero in the negative direction with the discharge 
controlling variable-angle swash plate 53a of the variable displacement 
motor 53 being inclined at a maximum tilt angle, the revolution speed of 
the variable displacement motor 53 decreases from zero in the negative 
direction. Then, the tilt angle of the discharge controlling 
variable-angle swash plate 50a of the variable displacement pump 50 is set 
to a maximum negative value. In this condition, as the tilt angle of the 
discharge controlling variable-angle swash plate 53a of the variable 
displacement motor 53 is decreased, the revolution speed of the variable 
displacement motor 53 further decreases in the negative direction. 
(3) Differential unit 27 
Referring to FIG. 2, the intermediate shaft 35 is provided, at the right 
end thereof, with a first differential planetary gear train 60 of the 
double planetary type and a second differential planetary gear train 61 of 
the single planetary type. These gear trains 60 and 61 are aligned 
coaxially with and in an extending direction of the intermediate shaft 35 
in this order when enumerating from the left. The first differential 
planetary gear train 60 is composed of a sun gear 60a that is rotatably 
supported by the intermediate shaft 35, being integral with the sun gear 
39b of the second 3rd-speed planetary gear train 39 and the clutch plate 
41; a ring gear 60b positioned outside the sun gear 60a; a planet gear 60c 
that is in mesh with either of the gears 60a and 60b, being positioned 
therebetween; and a planet carrier 60d that is for the planet gear 60c and 
is integral with an input gear 62 connected through the gear train 55 to 
the output shaft 54 of the variable displacement motor 53 in the 
hydrostatic transmission unit 25. Similarly, the second differential 
planetary gear train 61 is composed of a sun gear 61a that is rotatably 
supported by the intermediate shaft 35, being integral with the planet 
carrier 60d of the first differential planetary gear train 60; a ring gear 
61b that is positioned outside the sun gear 61a, being integral with the 
output shaft 26 positioned (at the right hand in FIG. 2) coaxially with 
and in an extending direction of the intermediate shaft 35; a planet gear 
61c that is in mesh with the gears 61a and 61b, being positioned 
therebetween; and a planet carrier 61d that is for the planet gear 61c and 
is integral with the ring gear 6Ob of the first differential planetary 
gear train 60 and the intermediate shaft 35. 
There will be given an explanation on the mechanical operations, of the 
mechanical transmission unit 24, hydrostatic transmission unit 25 and 
differential unit 27. FIG. 4 shows the relationship between speed ratio 
and motor speed ratio in the respective speed ranges (i.e., 1st forward 
speed (F1); 2nd forward speed (F2); 3rd forward speed (F3); 1st reverse 
speed (R1); 2nd reverse speed (R2); and 3rd reverse speed (R3)). Note that 
the above speed ratio is the ratio of the revolution speed of the output 
shaft 26 to the revolution speed of the output shaft 22 of the engine 21 
(=the revolution speed of the engine) and the above motor speed ratio is 
the ratio of the revolution speed of the output shaft 54 of the variable 
displacement motor 53 (=the revolution speed of the motor) to the 
revolution speed of the output shaft 22 of the engine 21 (=the revolution 
speed of the engine). 
(i) 1st forward speed (F1) and 1st reverse speed (R1): Only the 1st-speed 
hydraulic clutch 42 is engaged. The engagement of the clutch 42 causes the 
sun gear 60a of the first differential planetary gear train 60 to be 
hydraulically braked through the: clutch plate 41 and causes the 
intermediate shaft 35 to be in a freely rotated state. Accordingly, only 
the torque of the variable displacement hydraulic motor 53 in the 
hydrostatic transmission unit 25 is transmitted to the output shaft 54 of 
the variable displacement hydraulic motor 53; the gear train 55; the input 
gear 62, the planet carrier 60d, the planet gear 60c and the ring gear 60b 
of the first differential planetary gear train 60, the planet carrier 61d, 
the planet gear 61c and the ring gear 61b of the second differential gear 
train 61 in the differential unit 27; and the output shaft 26 in that 
order. In short, the output shaft 26 is driven, being connected only to 
the hydrostatic transmission unit 25 by means of the differential unit 27. 
As the motor speed ratio is thus increased from zero in the positive 
direction, the revolution speed of the output shaft 26 increases from zero 
in the positive direction. On the other hand, as the motor speed ratio 
decreases from zero in the negative direction, the: revolution speed of 
the output shaft 26 also decreases from zero in the negative direction. 
Thus, the speed ratio is infinitely varied both in the positive and 
negative directions within a specified range. 
In 1st forward speed (F1) and 1st reverse speed (R1), the 2nd-speed 
hydraulic clutch 36 may be engaged, or disengaged. However, when taking 
into account the case where the vehicle may be shifted to 2nd forward 
speed (F2) or 2nd reverse speed (R2) by clutch operation, the 2nd-speed 
hydraulic clutch 36 is preferably engaged. 
In 1st speed, when the revolution speed of the output shaft 26 increases in 
the positive direction and the speed ratio is a specified positive value 
a, the relative revolution speed of the forward hydraulic clutch 33 in 
relation to the ring gear 31b of the forward planetary gear train 31 
becomes zero. In this condition, if the forward hydraulic clutch 33 is 
engaged and the 1st-speed hydraulic clutch 42 is disengaged, 2nd forward 
speed (F2) will be obtained. At that time, the 2nd-speed hydraulic clutch 
36 is engaged. 
In 1st speed, when the revolution speed of the output shaft 26 decreases in 
the negative direction and the speed ratio is a specified negative value 
b, the relative revolution speed of the reverse hydraulic clutch 32 in 
relation to the planet carrier 30d of the reverse planetary gear train 30 
becomes zero. In this condition, if the reverse hydraulic clutch 32 is 
engaged and the 1st-speed hydraulic clutch 42 is disengaged like the above 
case, 2nd reverse speed (R2) will be obtained. At that time, the 2nd-speed 
hydraulic clutch 36 is engaged. 
(ii) 2nd forward speed (F2) 
Since the clutch plate 37 is hydraulically connected by the engagement of 
the 2nd-speed hydraulic clutch 36 and the ring gear 31b of the forward 
planetary gear train 31 is hydraulically braked by the engagement of the 
forward hydraulic clutch 33, the torque of the input shaft 23 is 
transmitted to the forward planetary gear train 31, the 2nd-speed 
hydraulic clutch 36 and the intermediate shaft 35 in the mechanical 
transmission unit 24, and then to the second differential planetary gear 
train 61 in the differential unit 27. During the transmission, the 
revolution speed is reduced. The torque of the variable displacement motor 
53 in the hydrostatic transmission unit 25 is also transmitted to the 
output shaft 54 of the variable displacement motor 53, the gear train 55, 
the input gear 62, the planet carrier 60d of the first differential 
planetary gear train 60 and then to the second differential planetary gear 
train 61 in the differential unit 27, while the revolution speed being 
reduced. The second differential planetary gear train 61 connects the 
mechanical transmission unit 24 and the hydrostatic transmission unit 25, 
whereby their revolution speeds are combined to drive the output shaft 26. 
Thus, the motor speed ratio decreases thereby increasing the revolution 
speed of the output shaft 26 in the positive direction. When the motor 
speed ratio is positive in 2nd forward speed (F2), part of torque from the 
second differential planetary gear train 61 in the differential unit 27 
flows backwardly to the input gear 62 through the planet gear 61c and the 
sun gear 61a of the second differential planetary gear train 61 and the 
first differential planetary gear train 60 so that the variable 
displacement motor 53 performs its pumping operation. The pumping 
operation of the variable displacement motor 53 causes the variable 
displacement pump 50 to be driven, and the torque of the variable 
displacement pump 50 is transmitted through the gear train 51 to the input 
shaft 23 where the torque is combined with the torque of the engine 21. 
When the motor speed ratio is negative on the other hand, part of the 
torque of the input shaft 23 drives the variable displacement pump 50 
through the gear train 51. The actuation of the variable displacement pump 
50 actuates the variable displacement motor 53 whose torque is transmitted 
to the gear train 55, the input gear 62 etc. in the differential unit 27, 
and then to the second differential planetary gear train 61 in the 
differential unit 27. At the second differential planetary gear train 61, 
the transmitted torque is combined with torque from the mechanical 
transmission unit 24 to drive the output shaft 26. 
In 2nd forward speed (F2), when the speed ratio is increased to a specified 
value c, the relative revolution speed of the 3rd-speed hydraulic clutch 
40 in relation to the planet carrier 38d of the first 3rd-speed planetary 
gear train 38 becomes zero. In this condition, if the 3rd-speed hydraulic 
clutch 40 is engaged and the 2nd-speed hydraulic clutch 36 is disengaged, 
3rd forward speed (F3) will be obtained. 
In 2nd forward speed (F2), when the speed ratio decreases from a higher 
value to the specified value a, the relative revolution speed of the 
1st-speed hydraulic clutch 42 in relation to the clutch plate 41 becomes 
zero. In this condition, if the 1st-speed hydraulic clutch 42 is engaged 
and the forward hydraulic clutch 33 is disengaged, 1st forward speed (F1) 
will be obtained. 
(iii) 3rd forward speed (F3) 
Since the planet carrier 38d of the first 3rd-speed planetary gear train 38 
is hydraulically braked by the engagement of the 3rd-speed hydraulic 
clutch 40 and the ring gear 31b of the forward planetary gear train 31 is 
hydraulically braked by the engagement of the forward hydraulic clutch 33, 
the torque of the input shaft 23 is transmitted through the forward 
planetary gear train 31, the 2nd-speed hydraulic clutch 36, the first 
3rd-speed planetary gear train 38 and the second 3rd-speed planetary gear 
train 39 in the mechanical transmission unit 24 to the first and second 
differential planetary gear trains 60 and 61 in the differential unit 27, 
whilst the revolution speed being reduced. Also, the torque of the 
variable displacement motor 53 in the hydrostatic transmission unit 25 is 
transmitted through the output shaft 54 of the variable displacement motor 
53 and the gear train 55 to the first and second differential planetary 
gear trains 60 and 61 in the differential unit 27, whilst the revolution 
speed being reduced. The first and second differential planetary gear 
trains 60 and 61 connect the mechanical transmission unit 24 and the 
hydrostatic transmission unit 25, whereby their revolution speeds are 
combined to drive the output shaft 26. 
As the motor speed ratio is thus increased, the revolution speed of the 
output shaft 26 increases in the positive direction. 
When the motor speed ratio is negative in 3rd forward speed (F3), part of 
torque from the first and second differential planetary gear trains 60 and 
61 in the differential unit 27 flows backwardly to the input gear 62 so 
that the variable displacement motor 53 performs its pumping operation and 
the torque of the variable displacement motor 53 is transmitted, as 
described above, through the variable displacement pump 50 and the gear 
train 51 to the input shaft 23 where the transmitted torque is combined 
with the torque of the engine 21. 
When the motor speed ratio is positive on the other hand, part of the 
torque of the input shaft 23 drives the variable displacement pump 50 
through the gear train 51, and the torque of the variable displacement 
motor 53 is transmitted, as described above, through the, gear train 55 
and the input gear 62 etc. in the differential unit 27 to the first and 
second differential planetary gear trains 60 and 61 in the, differential 
unit 27. At the first and second differential planetary gear trains 60 and 
61, the torque is combined with torque from the., mechanical transmission 
unit 24 to drive the output shaft 26. In 3rd forward speed (F3), when the 
speed ratio decreases from a higher value to the specified value c, the 
relative revolution speed of the 2rd-speed hydraulic clutch 36 in relation 
to the clutch plate 37 becomes zero. In this condition, if the 2rd-speed 
hydraulic clutch 36 is engaged and the 3rd-speed hydraulic clutch 40 is 
disengaged, 2rd forward speed (F2) will be obtained. 
(iv) 2rd reverse speed (R2) 
Since the clutch plate 37 is hydraulically connected by the engagement of 
the 2nd-speed hydraulic clutch 36 and the planet carrier 30d of the 
reverse planetary gear train 30 is hydraulically braked by the engagement 
of the reverse hydraulic clutch 32, the torque of the input shaft 23 is 
transmitted through the reverse planetary gear train 30, the 2nd-speed 
hydraulic clutch 36 and the intermediate shaft 35 in the mechanical 
transmission unit 24 to the second differential planetary gear train 61 in 
the differential unit 27, whilst the revolution speed being reduced. The 
torque of the variable displacement motor 53 in the hydrostatic 
transmission unit 25 is transmitted, as described above, through the 
output shaft 54 of the output shaft 54 of the variable displacement motor 
53, the gear train 55, the input gear 62 and the planet carrier variable 
displacement motor 53, the gear train 55, the input gear 62 and the planet 
carrier 60d of the first differential planetary gear train 60 in the 
differential unit 27 to the second differential planetary gear train 61, 
whilst the revolution speed being reduced. The first differential 
planetary gear train 61 connects the mechanical transmission unit 24 and 
the hydrostatic transmission unit 25 thereby combining their revolution 
speeds to drive the output shaft 26. 
As the motor speed ratio is increased accordingly, the revolution speed of 
the output shaft 26 decreases in the negative direction. 
In 2nd reverse speed (R2), when the motor speed ratio is negative, part of 
torque from the second differential planetary gear train 61 in the 
differential unit 27 flows backwardly to the hydrostatic transmission unit 
25 so that the variable displacement motor 53 performs its pumping 
operation. When the motor speed ratio is positive, the operation to be 
carried out is the same as that described in the case of 2nd forward speed 
(F2), except that a partial flow of the torque of the input shaft 23 
toward the hydrostatic transmission unit 25 occurs. 
In 2nd reverse speed (R2), when the speed ratio decreases from a higher 
value to a specified value d, the relative revolution speed of the 
3rd-speed hydraulic clutch 40 in relation to the planet carrier 38d of the 
first 3rd-speed planetary gear train 38 becomes zero. In this condition, 
if the 3rd-speed hydraulic clutch 40 is engaged and the 2nd-speed 
hydraulic clutch 36 is disengaged, 3rd reverse speed (R3) will be 
obtained. 
When the speed ratio is increased to the specified value b in 2nd reverse 
speed (R2), the relative revolution speed of the 1st-speed hydraulic 
clutch 42 in relation to the clutch plate 41 becomes zero. In this 
condition, if the 1st-speed hydraulic clutch 42 is engaged and the reverse 
hydraulic clutch 32 is disengaged, 1st reverse speed (R1) will be 
obtained. 
(v) 3rd reverse speed (R3) 
Since the planet carrier 38d of the first 3rd-speed planetary gear train 38 
is hydraulically braked by the engagement of the 3rd-speed hydraulic 
clutch 40 and the planet carrier 30d of the reverse planetary gear train 
30 is hydraulically braked by the engagement of the reverse hydraulic 
clutch 32, the torque of the input shaft 23 is transmitted through the 
reverse planetary gear train 30, the 2nd-speed hydraulic clutch 36, the 
first 3rd-speed planetary gear train 38 and the second 3rd-speed planetary 
gear train 39 in the mechanical transmission unit 24 to the first and 
second differential planetary gear trains 60 and 61 in the differential 
unit 27, while the revolution speed being reduced. Also, the torque of the 
variable displacement motor 53 in the hydrostatic transmission unit 25 is 
transmitted, as described above, through the output shaft 54 of the 
variable displacement motor 53 and the gear train 55 to the first and 
second differential planetary gear trains 60 and 61 in the differential 
unit 27, while the revolution speed being reduced. The first and second 
differential planetary gear trains 60 and 61 connect the mechanical 
transmission unit 24 and the hydrostatic transmission unit 25 thereby 
combining their revolution speeds to drive the output shaft 26. 
As the motor speed ratio is decreased accordingly, the revolution speed of 
the output shaft 26 decreases in the negative direction. 
In 3rd reverse speed (R3), when the motor speed ratio is positive, part of 
torque from the first and second differential planetary gear trains 60 and 
61 in the differential unit 27 flows backwardly to the hydrostatic 
transmission unit 25 so that the variable displacement motor 53 performs 
its pumping operation. When the motor speed ratio is negative, the 
operation to be carried out is the same as that described in the case of 
3rd forward speed (F3), except that a partial flow of the torque of the 
input shaft 23 toward the hydrostatic transmission unit 25 occurs. 
In 3rd reverse speed (R3), when the revolution speed ratio is increased to 
the specified value d, the relative revolution speed of the 2nd-speed 
hydraulic clutch 36 in relation to the clutch plate 37 becomes zero. In 
this condition, if the 2nd-speed hydraulic clutch 36 is engaged and the 
3rd-speed hydraulic clutch 40 is disengaged, 2nd reverse speed (R2) will 
be obtained. 
The operation for controlling the mechanical transmission unit 24 and the 
hydrostatic transmission unit 25 will be explained below. 
In FIG. 2, the output shaft 22 of the engine 21 is provided with an engine 
revolution speed detector 70 for detecting the revolution speed of the 
output shaft 22 to detect the revolution speed n.sub.E of the engine 21, 
and the output shaft 54 of the variable displacement motor 53 in the 
hydrostatic transmission unit 25 is provided with a motor revolution speed 
detector 71 for detecting the revolution speed n.sub.m and revolution 
direction of the variable displacement motor 53. An engine throttle (not 
shown) is provided with a throttle position detector 72 for detecting the 
position X of the engine throttle manipulated. A change lever (not shown) 
is provided with a lever position detector 73 for detecting the lever 
position FNR (i.e., forward(F), neutral(N) or reverse(R)) of the change 
lever manipulated. The engine revolution speed detector 70, motor 
revolution speed detector 71, throttle position detector 72 and lever 
position detector 73 issue an engine revolution speed signal, motor 
revolution speed signal, throttle position signal and lever position 
signal respectively to a control unit 74. The control unit 74 is composed 
of a central processing unit (CPU) 74A for executing a specified program, 
a read only memory (ROM) 74B for storing the specified program and various 
tables, and a random access memory (RAM) 74C serving as a working memory 
necessary for executing the specified program. The control unit 74 
executes arithmetic processing by executing the specified program in 
accordance with the engine revolution speed signal, motor revolution speed 
signal, throttle position signal and lever position signal, and issues a 
shift control signal to a shift valve 75. In response to the shift control 
signal, the shift valve 75 executes the above-described 
engagement/disengagement of the reverse hydraulic clutch 32, forward 
hydraulic clutch 33, 2nd-speed hydraulic clutch 36, 3rd-speed hydraulic 
clutch 40 and 1st-speed hydraulic clutch 42. The control unit 74 also 
supplies an angle control signal to a valve 76 for changing the angle of 
the discharge controlling variable-angle swash plate 50a of the variable 
displacement pump 50 and to a valve 77 for changing the angle of the 
discharge controlling variable-angle swash plate 53a of the variable 
displacement motor 53, respectively. 
A target engine revolution speed N.sub.E for the engine 21 is obtained 
according to the position X of the engine throttle and a control direction 
for speed ratio is obtained according to the lever position FNR of the 
change lever. Hence, speed ratio control is performed in the control unit 
74 as shown in Table 1. This control is based on (i) the condition 
(positive, negative, or zero) of the actual speed ratio e; (ii) the 
relationship between the actual engine revolution speed n.sub.E that is 
obtained from the engine revolution speed signal from the engine 
revolution speed detector 70 and the target engine revolution speed 
N.sub.E that is obtained from the throttle position signal from the 
throttle position detector 72; and (iii) the lever position FNR obtained 
from the lever position signal from the lever position detector 73. 
TABLE 1 
______________________________________ 
Relationship 
Between actual 
Engine 
Actual 
Revolution 
Speed Speed N.sub.E Lever 
Ratio And Target Engine 
Position Speed Ratio 
e Revolution Speed N.sub.E 
FNR Control 
______________________________________ 
.gtoreq.0 
n.sub.E &gt; N.sub.E 
forward increase to positive 
&gt;0 n.sub.E &lt; N.sub.E decrease to zero 
=0 n.sub.E &lt; N.sub.E maintain at zero 
.gtoreq.0 
n.sub.E = N.sub.E maintain constant 
&lt;0 
##STR1## increase to zero 
&gt;0 
##STR2## neutral decrease to zero 
=0 
##STR3## maintain at zero 
&lt;0 
##STR4## increase to zero 
.ltoreq.0 
n.sub.E &gt; N.sub.E 
reverse decrease to 
negative 
&lt;0 n.sub. E &lt; N.sub.E increase to zero 
=0 n.sub.E &lt; N.sub.E maintain at zero 
.ltoreq.0 
n.sub.E = N.sub.E maintain constant 
&gt;0 
##STR5## decrease to zero 
______________________________________ 
*all situations 
With reference to the flow chart of FIG. 4 which shows a basic program, the 
engine revolution speed control that is performed by controlling speed 
ratio will be described in detail. 
A: According to the throttle position signal from the throttle position 
detector 72, the target engine revolution speed N.sub.E of the engine 21 
for the throttle position X is obtained through arithmetic operation which 
includes conversion and is performed using a preset characteristic 
function or table. The characteristic function or table is set based on 
the characteristic curve of the target engine revolution speed N.sub.E 
against the throttle position X. This characteristic curve is prepared 
from the characteristic curve of torque against the revolution speed of 
the engine 21. 
B-D: The speed range presently selected in the mechanical transmission unit 
24 which is controlled by the control unit 74 with the, help of the shift 
valve 75 is detected. From the present actual engine revolution speed 
n.sub.E indicated by the engine revolution speed signal from the engine 
revolution detector 70 and the present actual motor revolution speed 
n.sub.m indicated by the motor revolution speed signal from the motor 
revolution speed detector 71, an actual motor speed ratio e.sub.m (the 
ratio of the actual motor revolution speed n.sub.m to the, actual engine 
revolution speed n.sub.E (=n.sub.m /n.sub.E)) is obtained by arithmetic 
operation. The present, actual speed ratio e is obtained by converting the 
actual motor speed ratio e.sub.m in accordance with the, present speed 
range detected, by the use of the preliminarily stored characteristic 
function e=f(e.sub.m) or table. The characteristic function e=f(e.sub.m) 
is set in accordance with the characteristic curve (see FIG. 4) of the 
actual motor speed ratio e.sub.m plotted against the actual speed ratio e. 
E: Vehicle control judgment routine: In this routine, how the vehicle 
should be controlled is determined from the lever position signal from the 
lever position detector 73 and the present, actual speed ratio e. The 
details will be explained later with reference to FIG. 5 and the flow 
chart of FIG. 6. 
F: The value of a constant k (k represents the response of the transmission 
to a change in engine revolution and its unit is rpm) is obtained. This 
constant k is to be substituted in the following equation (1) for 
obtaining a target speed ratio E which is used for slowing down the 
response to a change in load during digging operation in order to free the 
operator from extra blade operation for vehicle speed control. 
G: Engine brake control routine: In this routine, the vehicle is gradually 
decelerated to stop the vehicle during the engine brake control. The 
details will be described later with reference to the flow chart of FIG. 
7. 
H: The target speed ratio E is obtained by substituting the above target 
engine revolution speed N.sub.E, actual speed ratio e and actual engine 
revolution speed n.sub.E in the following equation (1). 
EQU E=e.+-.k(n.sub.E -N.sub.E) (1) 
The sign .+-. in the equation (1) means that it should be "+" at the time 
of forward drive and should be "-" at the time of reverse drive. 
I: The rate of change in the target speed ratio E with respect to time is 
limited. This decreases vehicle acceleration at the primary stage of 
digging and therefore allows the operator to be free from extra blade 
operation for vehicle speed control, particularly when digging ground that 
is too hard to strike into by the blade. 
J: The target speed ratio E is limited by operating the speed control 
lever. The limitation of the target speed ratio E is carried out in order 
to reduce the turning radius of the vehicle, for example, when the 
steering lever is operated excessively beyond a specified displacement 
range at the time of high-speed vehicle moving. 
K: FR shift control routine: In this routine, the vehicle is immediately 
stopped during the FR shift control. The details will be described later 
with reference to the flow chart of FIG. 8. 
L: Neutral (N) control routine: In this routine, the vehicle is securely 
stopped during the neutral control. The details will be described later 
with reference to the flow chart of FIG. 9. 
M: E-value limitation routine: Since the target speed ratio E varies 
depending on speed ranges, the range of the target speed ratio E is 
limited for each speed range. The details will be described later with 
reference to the graph of FIG. 10 and the flow chart of FIG. 11. 
N: A target motor speed ratio E.sub.m is obtained by converting the target 
speed ratio E in accordance with the speed range presently selected by the 
mechanical transmission unit 24. This conversion is done by the use of a 
characteristic function E.sub.m =f(E) or table which is a characteristic 
curve slimier to FIG. 4 and has been preset according to the 
characteristic curve of the target motor speed ratio E.sub.m plotted 
against the target speed ratio E. 
O: An operation amount A is obtained from the target motor speed ratio 
E.sub.m and actual motor speed ratio e.sub.m. Concretely, a feed forward 
amount KE.sub.m (K: feed forward coefficient) which is proportional to the 
target motor speed ratio E.sub.m is added to the sum of the proportional 
components and integral components of the difference (=E.sub.m -e.sub.m) 
between the target motor speed ratio E.sub.m and the actual motor speed 
ratio e.sub.m, whereby the operation amount A is obtained. This operation 
amount A is released as an angle control signal to the angle changing 
valves 76 and 77. 
Accordingly, the actual motor speed ratio e.sub.m is adjusted to be equal 
to the target motor speed ratio E.sub.m and the actual speed ratio e is 
adjusted to be equal to the target speed ratio E, so that the actual 
engine revolution speed n.sub.E is adjusted to be equal to the target 
engine revolution speed N.sub.E corresponding to the throttle position X 
of the engine throttle. 
Next, the vehicle control judgment routine (Step E) will be described in 
detail. In the forward/reverse drive and stop control system for a 
hydrostatic-mechanical transmission of this embodiment, starting and 
stoppage of the vehicle is controlled according to the position of the 
forward/reverse drive lever operated by the operator. When the 
forward/reverse drive lever is shifted from the neutral (N) position into 
the forward (F) or reverse (R) position to start the vehicle from a 
standstill for example, the target speed ratio E(=e.+-.k(n.sub.E -N.sub.E) 
is increased from zero toward the forward or reverse drive direction so 
that the vehicle starts off (i.e., the forward drive control or reverse 
drive control). If the operator shifts the forward/reverse drive lever 
from the forward (F) or reverse (R) position into the neutral (N) 
position, this lever operation indicates that the operator intends to 
gradually stop the vehicle, and therefore, the vehicle is controlled to be 
stopped by means of the engine brake (i.e., the engine brake control). 
When the vehicle is moving, if the forward/reverse drive lever is shifted 
into a position for a drive direction opposite to the moving direction of 
the vehicle, this lever operation indicates that the operator intends to 
immediately change the moving direction of the vehicle, and therefore the 
vehicle immediately stops and starts again in the opposite direction 
(i.e., the FR shift control). 
In this embodiment, how the vehicle should be controlled is accordingly 
judged from the position of the change lever (i.e., the foward/reverse 
drive lever) and the actual speed ratio e, and the vehicle is controlled 
to start or stop based on the judgment. Referring to FIG. 5 which 
concretely illustrates the judgment, when the forward/reverse drive lever 
is in the forward drive (F) position and the actual speed ratio e is more 
than a specified small value -.epsilon. (.epsilon. is for example the 
speed ratio when vehicle speed is 0.04 km/h and the engine is in its 
full-throttle state), it is determined that the vehicle is in the forward 
drive control state. When the forward/reverse drive lever is in the 
forward drive (F) position with the actual speed ratio e being equal to 
-.epsilon. or less, it is determined that the vehicle is in the FR shift 
control state. When the forward/reverse drive lever is in the neutral (N) 
position and the actual speed ratio e is less than -.epsilon. or more than 
+.epsilon., it is determined that the vehicle is in the engine brake 
control state. When the forward/reverse drive lever is in the neutral (N) 
position with the: actual speed ratio e being between -.epsilon. and 
+.epsilon., it is determined that the vehicle is in the neutral control 
state. When the forward/reverse drive lever is in the reverse drive (R) 
position and the actual speed ratio e is less than +.epsilon., it is 
determined that the vehicle is in the reverse drive control state. When 
the forward/reverse drive lever is in the reverse drive (R) position and 
the actual speed ratio e is equal to +.epsilon. or more, it is determined 
the vehicle is in the FR shift control state. 
FIG. 6 shows the control flow of the vehicle control judgment illustrated 
in FIG. 5. This control flow will be described below. 
E-1: Judgment is made to check which of the positions F, N and R the 
forward/reverse drive lever is placed in. 
E-2 to E-9: If the forward/reverse drive lever is in the F position, the 
vehicle is in the forward drive control state on condition that the actual 
speed ratio e is more than -.epsilon. (e&gt;-.epsilon.) and is in the FR 
shift control state on condition that the actual speed ratio e is equal to 
-.epsilon. or less (e.ltoreq.-.epsilon.). If the forward/reverse drive 
lever is in the N position, the vehicle is in the neutral control state on 
condition that the absolute value of the actual speed ratio e is less than 
.epsilon. (.vertline.e.vertline.&lt;.epsilon.) and is in the engine brake 
control state on condition that the absolute value of the actual speed 
ratio e is equal to .epsilon. or more 
(.vertline.e.vertline..gtoreq..epsilon.). If the forward/reverse drive 
lever is in the R position, the vehicle is in the reverse drive control 
state on condition that the actual speed ratio e is less than .epsilon. 
(e&lt;.epsilon.) and is in the FR shift control state on condition that the 
actual speed ratio e is equal to .epsilon. or more (e.gtoreq..epsilon.). 
Reference is now made to FIGS. 7, 8 and 9 for describing the engine brake 
control routine (Step G), FR shift control routine (Step K) and neutral 
control routine (Step L). 
(a) Engine brake control (FIG. 7) 
G-1 to G-2: If the vehicle is in the engine brake control state, the, 
target engine revolution speed N.sub.E is fixed at a value (e.g., 2,200 
rpm) which is not less than the revolution speed (2,100 rpm) of the engine 
when it is in a full-throttle state. With N.sub.E fixed at 2,200 rpm, the 
value of k(n.sub.E -N.sub.E) in the equation (1) which is used to 
calculate the target speed ratio E always becomes minus, so that the value 
of the target speed ratio E gradually decreases, decelerating the vehicle. 
G-3 to G-5: To change the degree of limitation for deceleration according 
to the present speed range, judgment is made to check which of the speed 
ranges (1st speed, 2nd speed and 3rd speed) the transmission is placed in. 
If it is determined that the transmission is in 1st speed, a value mm is 
calculated by the following equation. 
EQU mm=a.sub.1 .times..vertline.preceding value E.vertline.+b.sub.1 
On the other hand, if the transmission is in 2nd speed or 3rd speed, the 
value mm is obtained from the following equation. 
EQU mm=a.sub.2 .times..vertline.preceding value E.vertline.+b.sub.2 
In these equations, a.sub.1, b.sub.1, a.sub.2 and b.sub.2 are all constants 
and the preceding value E is the preceding value of the target speed ratio 
E. 
G-6 to G-7: The value of k(n.sub.E -N.sub.E) is compared to -mm and if 
k(n.sub.E -N.sub.E)&lt;-mm, k(n.sub.E -N.sub.E) is set to -mm (i.e., 
k(n.sub.E -N.sub.E)=-mm). If k(n.sub.E -N.sub.E).gtoreq.-mm, the flow is 
terminated since there is no need to limit vehicle deceleration. 
If the vehicle is not in the engine brake control state, the flow is 
terminated without proceeding to Step G-2 onwards. 
Vehicle deceleration determined by the value of k(n.sub.E -N.sub.E) is thus 
limited, so that the vehicle can be smoothly stopped without being 
subjected to abrupt deceleration. 
(b) FR shift control (FIG. 8) 
In this control, the value of the target speed ratio E is forcibly made 
small irrespective of the equation (1) (E=e+k(n.sub.E -N.sub.E). However, 
if E is set to 0 in one step, vehicle deceleration cannot be controlled 
and therefore the target speed ratio E is reduced stepwise using the 
following equation. 
EQU E=preceding value E-.DELTA.E 
As understood from the flow chart of FIG. 8, judgment is first made to 
check whether the vehicle is in the FR shift control state (K-1). If the 
vehicle is in the FR shift control state, E is obtained from the above 
equation E=preceding value E-.DELTA.E (K-2). To adjust deceleration, the 
value of .DELTA.E is appropriately set in this calculation. For example, 
if rapid deceleration is required, 66 E is set to a large value. 
(c) Neutral control (FIG. 9) 
In this control, judgment is first made to check whether the vehicle is in 
the neutral control state (L-1). If the vehicle is in the neutral control 
state, the target speed ratio E is then fixed at zero (L-2). This permits 
the vehicle to stop without fail. 
Next, the E-value limitation routine for each speed range (Step M) will be 
described in detail. 
Since the target motor speed ratio E.sub.m is calculated (see Step N in the 
flow chart of FIG. 4) based on the target speed ratio E whose range varies 
depending on speed ranges as shown in FIG. 10, the range of the value E 
has to be specified for each speed range by this routine. Reference will 
now be made to the flow chart of 
FIG. 11 for explaining the E-value limitation routine. 
M-1: The speed range in which the transmission is presently placed is 
detected. 
M-2 to M-6: If the transmission is placed in 1st forward speed (F1), 
judgment is made to check whether or not the target speed ratio E is 
E.gtoreq.F.sub.12 E.sub.0. If E.gtoreq.F.sub.12 E.sub.0, E is then set to 
F.sub.12 E.sub.0, and if E&lt;F.sub.12 E.sub.0, it is then determined whether 
the target speed ratio is E.gtoreq.0. If E&lt;0, E is set to 0 and then, the 
flow is completed. If E.gtoreq.0, the flow is immediately completed. 
M-7 to M-11: If the transmission is placed in 1st reverse speed (R1), it is 
then determined whether or not the target speed ratio is E.ltoreq.R.sub.12 
E.sub.0. If E.ltoreq.R.sub.12 E.sub.0, E is set to R.sub.12 E.sub.0, and 
if E&gt;R.sub.12 E.sub.0, it is then determined whether the target speed 
ratio is E&gt;0. If E&gt;0, E is set to 0 and then, the flow is completed. If 
E&lt;0, the flow is immediately completed. 
M-12 to M-16: If the transmission is placed in 2nd forward speed (F2), it 
is then determined whether or not the target speed ratio is 
E.gtoreq.F.sub.23 E.sub.0. If E.gtoreq.F.sub.23 E.sub.0, E is set to 
F.sub.23 E.sub.0, and if E&lt;F.sub.23 E.sub.0, it is then determined whether 
the target speed ratio is E&lt;F.sub.12 E.sub.0. If E&lt;F.sub.12 E.sub.0, E is 
set to F.sub.12 E.sub.0 and then, the flow is completed. If 
E.gtoreq.F.sub.12 E.sub.0, the flow is immediately completed. 
M-17 to M-21: If the transmission is placed in 2nd reverse speed (R2), it 
is determined whether or not the target speed ratio is E.ltoreq.R.sub.23 
E.sub.0. If E.ltoreq.R.sub.23 E.sub.0, E is set to R.sub.23 E.sub.0, and 
if E&gt;R.sub.23 E.sub.0, it is then determined whether the target speed 
ratio is E&gt;R.sub.23 E.sub.0. If E&gt;R.sub.12 E.sub.0, E is set to R.sub.1 
E.sub.0 and then, the flow is completed. If E.ltoreq.R.sub.12 E.sub.0, the 
flow is immediately completed. 
M-22 to M-26: If the transmission is placed in 3rd forward speed (F3), it 
is determined whether or not the target speed ratio is E.gtoreq.F.sub.max. 
If E.gtoreq.F.sub.max, E is then set to F.sub.max, and if E&lt;F.sub.max, it 
is then determined whether the target speed ratio is E&lt;F.sub.23 E.sub.0. 
If E&lt;F.sub.23 E.sub.0, E is set to F.sub.23 E.sub.0 and then, the flow is 
completed. If E.gtoreq.F.sub.23 E.sub.0, the flow is immediately 
completed. 
M-27 to M-31: If the transmission is placed in 3rd reverse speed (R3), it 
is determined whether or not the target speed ratio is E&lt;R.sub.max. If 
E.ltoreq.R.sub.max, E is then set to R.sub.max, and if E&gt;R.sub.max, it is 
then determined whether the target speed ratio is E&gt;R.sub.23 E.sub.0. If 
E&gt;R.sub.23 E.sub.0, E is set to R.sub.23 E.sub.0 and then, the flow is 
completed. If E.ltoreq.R.sub.23 E.sub.0, the flow is immediately 
completed. 
M-32: If the transmission is not placed in any of F.sub.1, R.sub.1, 
F.sub.2, R.sub.2, F.sub.3 and R.sub.3, the target speed ratio E is set to 
0 and then the flow is completed. 
While the equation (1) is used for obtaining the target speed ratio E in 
the foregoing embodiment, the following equation may be used. 
EQU E=e.times.(n.sub.E /N.sub.E) 
Alternatively, the preceding value E' of the target speed ratio may be 
substituted in the following equation. 
EQU E=E'+k(n.sub.E -N.sub.E) 
The following equation may be also used. 
EQU E=E'.times.(n.sub.E /N.sub.E) 
In this case, there is no need to obtain the actual speed ratio e in order 
to obtain the target speed ratio E. 
Although the actual motor speed ratio e.sub.m is obtained directly from the 
ratio of the revolution speed of the motor to the revolution speed of the 
engine, it may be obtained in other ways. For example, the revolution 
speed of the input shaft 23 and that of the output shaft 26 are detected 
taking the reduction ratio of the engine 21 etc. into account, and the 
actual motor speed ratio e.sub.m is obtained from the ratio of the 
revolution speed of the output shaft 26 to the revolution speed of the 
input shaft 23. Another alternative is such that the revolution speed of 
the input shaft 23 and that of the output shaft 54 of the variable 
displacement motor 53 are detected and the actual motor speed ratio 
e.sub.m is obtained from the ratio of the revolution speed of the motor 53 
to the revolution speed of the input shaft 23. In these case, the target 
revolution speed of the input shaft 23 for the throttle position X may be 
obtained from a throttle position signal from the throttle position 
detector 72, and the target motor speed ratio E.sub.m may be obtained 
through the arithmetic operation in which the target speed ratio of the 
revolution speed of the output shaft 26 to the revolution speed of the 
input shaft 23 is obtained by converting the actual motor speed ratio 
e.sub.m into the speed ratio of the revolution speed of the input shaft 23 
to the revolution speed of the output shaft 26. In addition, the actual 
motor speed ratio may be obtained from the ratio of the revolution speed 
of the output shaft 26 to the revolution speed of the engine 21 or from 
the ratio of the revolution speed of the motor 53 to the revolution speed 
of the output shaft 26, taking the reduction ratio of the engine 21 etc. 
into account. 
In the foregoing embodiment, the angle of the discharge controlling 
variable-angle swash plate 50a for the variable displacement pump 50 and 
that of the discharge controlling variable-angle swash plate 53a for the 
variable displacement motor 53 are controlled in accordance with the 
operation amount A by means of the angle changing valves 76, 77 
respectively. However, the angle of either of the discharge controlling 
variable-angle swash plates 50a and 53a may be controlled. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.