Automatic transmission control apparatus

A control apparatus for an automatic transmission calculates the running resistance of a vehicle based on the vehicle acceleration and the torque generated by the engine. When the running resistance exceeds a first prescribed value, it is determined that the vehicle is travelling on an uphill slope on which upshifting should not take place, so the transmission is prevented from upshifting. When the running resistance falls below a second prescribed value, it is determined that the vehicle is travelling on a downhill slope, so the transmission is made to downshift to a gear in which the engine of the vehicle will perform engine braking. When the running resistance is between the first and second prescribed values, the control apparatus controls the transmission based on the vehicle speed and the throttle opening.

BACKGROUND OF THE INVENTION 
This invention relates to a control apparatus for an automatic transmission 
of an automotive vehicle. More particularly, it relates to a control 
apparatus which can improve the performance of an automatic transmission 
when the vehicle is ascending or descending a hill, thereby increasing the 
safety and comfort of the vehicle. 
The gear setting of a conventional automatic transmission for an automotive 
vehicle is controlled in accordance with the engine load, as indicated by 
the degree of opening of the throttle valve, and the vehicle speed. While 
this manner of control is satisfactory under many driving conditions, it 
can cause problems when the vehicle is travelling on a hill. 
For example, when a vehicle is entering a curve in a road on an uphill 
slope, the driver of the vehicle may decide to let up on the accelerator 
pedal in order to decrease the vehicle speed. As a result of his doing so, 
the throttle valve opening will decrease, and this decrease may cause a 
conventional transmission control apparatus to control the transmission so 
as to shift up into a gear which is unsuitable for hill climbing, and the 
vehicle will have difficulty ascending the slope. In addition, the driver 
experiences an unpleasant sensation when the transmission upshifts 
contrary to his expectations. When the vehicle is coming out of the same 
uphill curve in the road and the driver increases the depression of the 
accelerator pedal in order to again increase the vehicle speed, since the 
transmission is in too high a gear, the vehicle can not be accelerated as 
quickly as desired. In this case, the increase in the throttle valve 
opening when the driver depresses the accelerator pedal as he comes out of 
the curve may cause the transmission to shift down into a lower gear for 
acceleration. The downshifting by the transmission produces a sudden 
change in the torque applied to the drive wheels, and this sudden change 
greatly decreases the stability of the vehicle. 
A conventional transmission control apparatus also produces problems when a 
vehicle is travelling downhill. The transmission of a vehicle may be in 
high gear when it starts to enter a downhill slope. Unless the driver 
manually downshifts the transmission, it will remain in high gear, and the 
engine will tend to accelerate the vehicle. On a long slope, it is 
preferable to downshift the transmission to a gear in which the engine is 
performing engine braking, which refers to a state in which the engine is 
actually braking the vehicle rather than driving it. However, many drivers 
of vehicles equipped with automatic transmissions have a tendency not to 
perform any manual shifting and leave the transmission in the Drive 
setting under all forward driving conditions. Such drivers rely totally on 
the brakes to decelerate the vehicle on downhill slopes, but on very long 
slopes, prolonged and continuous use of the brakes may cause the brakes to 
overheat and fail. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a control 
apparatus for an automatic transmission of an automotive vehicle which can 
prevent the transmission from upshifting to an unsuitable gear when the 
vehicle is travelling on an uphill slope. 
It is another object of the present invention to provide a control 
apparatus for an automatic transmission which can automatically perform 
engine braking of a vehicle travelling on a downhill slope. 
It is still another object of the present invention to provide a control 
apparatus for an automatic transmission for an automotive vehicle which 
can increase the safety and riding comfort of the vehicle. 
It is yet another object of the present invention to provide a control 
apparatus for an automatic transmission which can be applied to existing 
automatic transmissions. 
A control apparatus for an automatic transmission according to the present 
invention calculates the running resistance of a vehicle based on the 
vehicle acceleration and the torque generated by the engine. In one form 
of the present invention, when the running resistance exceeds a first 
prescribed value, the control apparatus determines that the vehicle is 
travelling on an uphill slope on which upshifting should not take place, 
so the transmission is prevented from upshifting. In another form of the 
present invention, when the running resistance falls below a second 
prescribed value, the control apparatus determines that the vehicle is 
travelling on a downhill slope, so the transmission is made to downshift 
to a gear in which the engine will perform engine braking. When the 
running resistance is between the first and second prescribed values, the 
control apparatus controls the transmission based on the vehicle speed and 
the throttle opening in a conventional manner. 
Since the control apparatus prevents a transmission from upshifting to an 
inappropriate gear on an uphill slope, the hill climbing performance of 
the vehicle is improved, and since the transmission does not upshift 
unexpectedly, the comfort of the ride is improved. Further, a sudden 
change in drive force due to sudden upshifting or downshifting on a curve 
is prevented, so the stability and safety of the vehicle are increased. On 
a downhill slope, the control apparatus can automatically control the 
transmission so as to carry out engine braking, so the driver of the 
vehicle does not need to apply the brakes, and the danger of brake failure 
due to overheating from prolonged use on a downhill slope can be 
eliminated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of a transmission control apparatus according to the 
present invention will now be described while referring to the 
accompanying drawings, FIG. 1 of which is a block diagram of this 
embodiment as applied to a conventional automatic transmission 1. The 
transmission 1 is of the type having a torque converter drivingly 
connected to a gear box, both of which are actuated by a hydraulic fluid 
controlled by solenoid valves 2. The transmission 1 is driven by an engine 
3, which may be equipped with a conventional electronic control unit (ECU) 
15 for controlling the ignition timing and fuel supply of the engine 3. An 
engine rotation sensor 6 mounted on the transmission 1 detects the 
rotation of the input shaft of the transmission 1 and generates a 
corresponding electrical output signal which indicates the engine 
rotational speed. A vehicle speed sensor 7 detects the rotation of the 
output shaft of the transmission 1 and generates a corresponding output 
signal which corresponds to the vehicle speed. The output signals from 
sensors 6 and 7 are input to an electronic control unit (ECU) 10 for the 
transmission. 
An inhibitor switch 8 which is mounted on the transmission 1 senses when 
the transmission 1 is in the Neutral or Park position and sends a 
corresponding output signal to the transmission ECU 10. 
A throttle valve 5 which is opened and closed in response to movement of an 
unillustrated accelerator pedal is pivotally mounted in a suction pipe 4 
for the engine 3. A throttle opening sensor 9 mounted on the suction pipe 
4 senses the degree of opening of the throttle valve 5 and provides the 
transmission ECU 10 with a corresponding input signal. 
The transmission ECU 10 receives a speed range signal 11 from a selector 
mechanism which indicates the position (1, 2, Drive, etc.) to which the 
driver of the vehicle has set an unillustrated gear shift lever of the 
vehicle. The transmission ECU 10 also receives an ignition signal 12 from 
an unillustrated ignition switch. 
The transmission ECU 10 selects the gear at which the transmission 1 is to 
operate based on the vehicle speed and the throttle opening as indicated 
by the input signals from the vehicle speed sensor 7 and the throttle 
opening sensor 9, respectively, and controls the operation of the solenoid 
valves 2 so that the transmission 1 runs in the selected gear. 
Furthermore, the transmission ECU 10 calculates the running resistance of 
the vehicle based on the torque generated by the engine and the vehicle 
acceleration. In a first mode of operation, when the running resistance is 
greater than a first prescribed value, the transmission ECU 10 fixes the 
transmission in its present gear and prevents it from upshifting. In a 
second mode of operation, when the running resistance is less than a 
second prescribed value, the transmission ECU 10 controls the transmission 
1 so as to downshift to a gear in which engine braking is performed. 
FIG. 2 conceptually illustrates the structure of the transmission ECU 10. 
It includes a gear change pattern selector 10a which receives the input 
signals from the vehicle speed sensor 7 and the throttle opening sensor 9 
and selects a suitable gear change pattern based on the vehicle speed and 
the throttle opening. The gear change pattern selector 10a may include a 
memory containing a memory table in which are stored a plurality of gear 
change patterns corresponding to different values of the vehicle speed and 
throttle opening, and the gear change pattern selector 10a can determine 
the appropriate gear change pattern by a table look-up operation using the 
vehicle speed and throttle opening as input variables. An acceleration and 
torque calculator 10b receives the input signals from the engine rotation 
sensor 6, the vehicle speed sensor 7, and the throttle opening sensor 9 
and calculates the acceleration of the vehicle and the torque being 
generated by the engine 3. The vehicle acceleration can be calculated from 
the time rate of change of the vehicle speed as indicated by the vehicle 
speed sensor 7. The engine torque is experimentally determined from the 
engine rotational speed indicated by the engine rotation sensor 6 and the 
throttle opening indicated by the throttle opening sensor 9. The engine 
torque can be determined by use of a memory table in an unillustrated 
memory of the transmission ECU 10 in which the relationship of the engine 
torque to the engine rotational speed and the throttle opening is stored. 
Based on the vehicle acceleration and engine torque calculated by the 
calculator 10b, a running resistance calculator 10c calculates the running 
resistance of the vehicle. Running resistance here refers to the total 
resistance to movement by the vehicle. The calculated running resistance 
is then provided to a gear change command unit 10d which generates 
auxiliary commands which can override the gear change pattern selected by 
the gear change pattern selector 10a. When the running resistance is 
greater than a first prescribed value, the gear change command unit 10d 
determines that the vehicle is running uphill, so it generates a command 
to the gear change pattern selector 10a to fix the transmission 1 in its 
present gear and prevent the transmission 1 from shifting to a higher gear 
than the present gear. On the other hand, when the gear change command 
unit 10d determines that the running resistance is less than a second 
prescribed value, it determines that the vehicle is running downhill, so 
it generates a command to the gear change pattern selector 10a to make the 
transmission 1 shift down to a lower gear in which engine braking is 
performed. 
Electronic control units equipped with gear change pattern selectors which 
automatically select a gear change pattern for an automatic transmission 
based on throttle opening and vehicle speed are well known to those 
skilled in the art. The transmission ECU 10 of FIG. 2 differs from a 
conventional transmission ECU by the provision of elements 10b-10d in 
addition to the gear change pattern selector 10a. Elements 10a-10d of the 
transmission ECU 10 can be separate electrical components, or they can 
comprise a microcomputer or the like which performs the above-described 
functions of these elements by executing a program. It is also possible to 
incorporate the transmission ECU 10 into the engine ECU 15. 
If the engine ECU 15 is of the type which, as part of its normal operation, 
calculates the engine torque based on engine operating conditions such as 
the air intake rate into the engine 3, it is not necessary for the 
transmission ECU 10 to separately calculate the engine torque. In this 
case, the torque calculated by the engine ECU 15 can be provided to the 
transmission ECU 10 as an input signal, resulting in a simplification of 
the transmission ECU 10. 
FIG. 3 is a flow chart illustrating the first mode of operation of the 
embodiment of FIG. 1. Processing starts when the ignition switch is turned 
on. In Step S1, the transmission ECU 10 is initialized, after which a loop 
comprising Steps S2 through S11 is commenced. 
In Step S2, the transmission ECU 10 reads in information from the various 
sensors 6-9 and receives input signals 11 and 12. In Step S3, based on the 
throttle opening sensed by the throttle opening sensor 9 and the engine 
rotational speed indicated by the output signal from the engine rotation 
sensor 6, the acceleration and torque calculator 10b calculates the engine 
torque, as well known in the art. 
In Step S4, the transmission ECU 10 determines whether the vehicle speed as 
indicated by the vehicle speed sensor 7 is greater than 0. If the vehicle 
speed is 0, in Step S5, the running resistance is reset to a value 
corresponding to running conditions on a flat road, which is intrinsic to 
the vehicle, and Step S8 is proceeded to. On the other hand, if the 
vehicle speed is greater than 0, then in Step S6 the acceleration and 
torque calculator 10b calculates the vehicle acceleration based on the 
signal from the vehicle speed sensor 7. Next, in Step S7, the running 
resistance calculator 10c calculates the running resistance of the 
vehicle. An example of an equation that can be used to calculate the 
running resistance R is 
EQU R=(T.times.G.times.1/r)-.alpha..times.m (1) 
wherein T is the engine torque calculated in Step S3, G is the gear ratio 
of the transmission 1, r is the radius of the tires on the drive wheels of 
the vehicle, .alpha. is the vehicle acceleration calculated in Step S6, 
and m is the vehicle mass. 
In Step S8, the gear change command unit 10d compares the calculated 
running resistance R with a first prescribed value K1 which is determined 
by the present gear setting of the transmission 1. If the running 
resistance R is smaller than the first prescribed value K1, the gear 
change command unit 10d determines that it is not necessary to inhibit 
upshifting by the transmission 1, so in Step S10 the gear change pattern 
selector 10a controls the solenoid valves 2 of the transmission in the 
usual manner based on the throttle opening and the vehicle speed. 
On the other hand, if in Step S8 the running resistance R is greater than 
the first prescribed value K1, the gear change command unit 10d determines 
that the vehicle is travelling on an uphill slope, so in Step S9, the gear 
change command unit 10d generates a command to the gear change pattern 
selector 10a to fix the transmission 1 in its present gear, thereby 
preventing the transmission 1 from upshifting into a gear in which hill 
climbing is impossible. 
In Step S11, the solenoid valves 2 are driven by the gear change pattern 
selector 10a to operate the transmission 1 according to the gear change 
pattern selected by the gear change pattern selector 10a, after which Step 
S2 is returned to. 
It can be seen that in the mode of operation illustrated in FIG. 3, the 
transmission 1 is automatically prevented from shifting up to a higher 
gear than the present gear when the vehicle is climbing a hill of more 
than a certain grade. As a result, even if the driver lets up on the 
accelerator pedal on an uphill slope in order to reduce the vehicle speed, 
such as when entering a curve, the transmission 1 is prevented from 
shifting into a gear in which hill climbing becomes impossible, and when 
the driver again increases the depression of the accelerator pedal, the 
vehicle will immediately accelerate. Therefore, not only is the hill 
climbing performance of the vehicle improved, but the safety of the 
vehicle is increased since the wheels will not undergo a sudden change in 
drive force on a curve due to sudden shifting or downshifting by the 
transmission. Furthermore, because the transmission 1 is prevented from 
unexpectedly upshifting on a hill, the comfort of the ride experienced by 
the passengers of the vehicle is increased. 
In Step S9 of FIG. 3, the gear change command unit 10d issues a command to 
the gear change pattern selector 10a to fix the transmission 1 in its 
present gear. Alternatively, the gear change command unit 10d can be made 
to issue a command which permits downshifting but prevents upshifting by 
the transmission 1. 
FIG. 4 is a flow chart illustrating another mode of operation of the 
embodiment of FIG. 1 in which the transmission ECU 10 controls the 
transmission 1 so as to downshift and thereby perform engine braking when 
the vehicle is travelling downhill. The overall flow of operations is 
similar to that in the operating mode illustrated in FIG. 3, and Steps 
S21-S27 of FIG. 4 are identical to Steps S1-S7, respectively, of FIG. 3. 
Therefore, the operating mode illustrated in FIG. 4 will be explained 
beginning with Step S28. In this step, the running resistance calculated 
by the running resistance calculator 10c in Step S27 is compared with a 
second prescribed value K2 based on the gear in which the transmission 1 
is presently running. When the vehicle is running downhill, the running 
resistance given by Equation (1) is negative, so the second prescribed 
value K2 is usually a negative number. If the running resistance is 
greater than the second prescribed value K2, then it is determined that it 
is not necessary to perform engine braking, so in Step S30 the gear change 
pattern selector 10a selects a gear change pattern for the transmission 1 
in the usual manner based on the throttle opening and the vehicle speed. 
On the other hand, if in Step S28 it is determined that the running 
resistance is less than the second prescribed value K2, the gear change 
command unit 10d determines that it is necessary to perform engine 
braking, and in Step S29 it sends a command to the gear change pattern 
selector 10a to select a gear suitable for performing engine braking, the 
gear being determined by the vehicle speed and the running resistance. 
FIG. 5 illustrates an example of the relationship of the gear selected by 
the gear change pattern selector 10a when engine braking is to be 
performed to the running resistance and the vehicle speed. The more 
negative the running resistance R, the lower the gear that is selected for 
engine braking. On the other hand, the higher the vehicle speed, the 
higher the gear that is selected for engine braking, and when the vehicle 
speed exceeds a certain level, the gear change pattern selector 10a 
selects a gear change pattern in its usual manner based on the vehicle 
speed and the throttle opening, regardless of how negative the running 
resistance is. A relationship like the one shown in FIG. 5 can be stored 
in a memory table in a memory of the transmission ECU 10, and the gear 
change pattern selector 10a can perform a look-up operation of the table 
to determine the appropriate gear using the running resistance and the 
vehicle speed as input variables. 
In Step S31, the solenoid valves 2 are driven by the gear change pattern 
selector 10a to operate the transmission 1 according to the gear change 
pattern selected in Step S30 or Step S31, and then Step S22 is returned 
to. 
Thus, in the mode of operation illustrated in FIG. 4, engine braking is 
automatically performed when the vehicle is travelling downhill and the 
running resistance becomes more than a certain amount negative, so the 
vehicle is limited to a safe speed by the engine 3 without the driver 
having to apply the brakes. As a result, wear on the brakes is decreased, 
and the danger of brake failure due to overheating on long downhill slopes 
is eliminated. A transmission control apparatus according to the present 
invention is therefore particularly advantageous when the vehicle is 
travelling on mountain roads. 
In order to obtain much stronger engine braking, it is possible to cut off 
the fuel supply to the engine at the same time that the transmission 1 is 
made to downshift. Furthermore, if the transmission 1 is equipped with a 
lockup clutch, the effectiveness of engine braking can be increased by 
engaging the lockup clutch when downshifting is performed so as to 
directly connect the gear box of the transmission 1 to the engine 3. 
In the mode of operation illustrated in FIG. 4, the transmission 1 is 
automatically made to downshift when the running resistance becomes 
negative by a certain amount. However, the driver of the vehicle may not 
wish to slow down the vehicle at this point and may feel comfortable 
travelling downhill without applying the brakes. Thus, it is not necessary 
to perform engine braking until the driver of the vehicle actually desires 
to slow down the vehicle. To prevent engine braking before the driver 
wishes to decelerate, the transmission ECU 10 can be made responsive to 
the brakes of the vehicle so as to delay downshifting until the driver 
applies the brakes, whereby deceleration of the vehicle due to 
downshifting will coincide with the driver's desire to decelerate. One 
method of detecting the application of the brakes is shown in FIG. 1, in 
which a switch 14 is connected to the brake pedal 13 of the vehicle, and 
the switch 14 provides the transmission ECU 10 with an output signal when 
the switch 14 is actuated by depression of the brake pedal 13. Other 
methods can be used to detect the actuation of the brakes, such as sensing 
when current is flowing through the brake lights of the vehicle. 
As an alternative control method, instead of making the transmission 1 
downshift, the transmission ECU 10 can prevent the transmission 1 for 
upshifting on a downhill slope when the running resistance falls below the 
second prescribed value K2. 
A control apparatus according to the present invention can be constructed 
to operate in either one of the operating modes illustrated in FIG. 3 and 
FIG. 4, or it can be constructed to operate in both operating modes. FIG. 
6 is a flow chart of an operating mode which combines the operating modes 
of FIGS. 3 and 4. In FIG. 6, Steps S41-S47 are identical to Steps S1-S7, 
respectively, of FIG. 3, so the operating mode illustrated in FIG. 4 will 
be explained beginning with Step S48. In this step, the running resistance 
R calculated by the running resistance calculator 10c in Step S47 is 
compared with the first prescribed value K1. If the running resistance R 
is greater than or equal to K1, then the gear change command unit 10d 
determines that the vehicle is travelling on an uphill slope, so in Step 
S49, the gear change command unit 10d generates a command to the gear 
change pattern selector 10a to fix the transmission 1 in its present gear 
to prevent upshifting. 
On the other hand, if the running resistance is smaller than K1, the gear 
change command unit 10d determines that it is not necessary to inhibit 
upshifting, and in Step S50, the running resistance R is compared with the 
second prescribed value K2. If the rushing resistance is greater than K2, 
the gear change command unit 10d determines that it is not necessary to 
perform engine braking, so in Step S52, the gear change pattern selector 
10a selects a gear change pattern for the transmission 1 in the usual 
manner based on the throttle opening and the vehicle speed. 
If, however, the running resistance R is less than or equal to K2, the gear 
change command unit 10d determines that it is necessary to perform engine 
braking, and in Step S51 it sends a command to the gear change pattern 
selector 10a to select a gear suitable for performing engine braking using 
a relationship like that illustrated in FIG. 5. 
Next, in Step S53, the solenoid valves 2 are driven by the gear change 
pattern selector 10a to operate the transmission 1 according to the gear 
change pattern selected in Step S48, S51, or S52, and then Step S42 is 
returned to. 
The operating mode illustrated in FIG. 6 combines the benefits of the 
operating modes of FIGS. 3 and 4 and provides both good hill climbing 
performance on uphill slopes and automatic engine braking on downhill 
slopes.