Method for control of an automatic transmission of a motor vehicle

A method for control of a preferably electro-hydraulically activated automatic transmission of a motor vehicle equipped with an internal combustion engine provides for prevention of upshifting, i.e. reduction in translation in and before a curve. The upshift prevention is initiated if pressure is removed quickly from the gas pedal in idle operation. The upshift prevention is cancelled if the vehicle does not drive through a curve after elapse of a certain time period after traction operation is recognized. If idle operation is again recognized during elapse of the time period, the upshift prevention is maintained for another time period. After termination of the upshift prevention, step by step adjustment of the gear level to the value determined by the shifting characteristic field takes place.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to a method of controlling an automatic transmission 
of a motor vehicle which prevents shifting of the transmission in response 
to defined operating conditions of the vehicle as known from DE-39 22 040 
A1. 
Conventional automatic transmission controls of motor vehicles powered by 
internal combustion engines generally initiate shifting into a higher gear 
(reduction in translation) if the amount of force exerted on the gas pedal 
is reduced. However, this is not always desirable when driving through a 
curve or when braking, since such changes in load could possibly result in 
unsafe driving conditions or one or more reductions in gear when the motor 
vehicle accelerates again, due to greater force on the gas pedal. 
From German patent document DE-33 41 662 C1, it has become known in 
connection to avoid this upshifting in a curve by determining the lateral 
acceleration of the motor vehicle. However, this measure only makes it 
possible to avoid upshifting in a curve. 
In order to prevent upshifting even when approaching a curve, in targeted 
manner, the method for control of an automatic transmission pursuant to 
DE-39 22 040 A1 determines the gas pedal change velocity, and a signal to 
prevent an upshift process is derived if idle mode is detected. Thereupon, 
upshifting is prevented until traction mode occurs again and a 
predetermined period of time has elapsed. 
In DE-39 22 051 A1, it is additionally provided that this period of time be 
made dependent on another parameter (driving activity), which is derived 
from one or a combination of several operational or driving parameters of 
a motor vehicle and evaluates a driving style of a driver or a prevailing 
traffic situation. 
Proceeding from this state of the art, it is the object of the invention to 
create a method for control of an automatic transmission of a combination 
of several operational or driving parameters of a motor vehicle and 
evaluates a driving style of a driver or a prevailing traffic situation. 
Based on this state of the art, it is the object of the invention to 
provide a method for control of an automatic transmission of a motor 
vehicle, which is further improved particularly with regard to shifting 
behavior before curves and while braking. 
The object is accomplished according to the invention by activating an 
upshift prevention mode of the vehicle transmission in response to the 
entry of the vehicle into an coasting mode. This mode is continued for a 
preset time period T1 during which if the vehicle again enters the 
coasting mode, the upshift preventing mode is continued until expiration 
of a second preset time period T2. 
The advantages of the invention primarily consist of the fact that a method 
for control of an automatic transmission of a motor vehicle is created, in 
which the shifting behavior, particularly before curves and while braking, 
is further improved. 
By having a renewed delay of upshifting after recognition of a traction 
mode during the first time period, within which renewed delay upshifting 
before curves is avoided, the driver of a motor vehicle having a 
transmission equipped with such a control can temporarily give gas even 
before a curve, without causing an undesirable upshifting. In this manner, 
incorrect assessments by the driver with regard to approaching a curve in 
traction or idle mode are tolerated without negative effects on the 
overall driving behavior of the motor vehicle. 
In a further embodiment of the invention, it is provided that after the end 
of the prevention of upshifting, large gear jumps (over several gear 
levels) are avoided, in that these gear levels are carried out step by 
step, with intermediate elapse of certain additional time periods. 
Likewise, automatic downshifting (increase in translation) can also be 
carried out when braking the motor vehicle, if certain conditions are 
present; adherence to the conditions thereby guarantees safe operation of 
the motor vehicle. Thus, particularly monitored factors are that the 
lateral acceleration is not too high, the motor vehicle does not 
decelerate too greatly and that the driving speed is not too high, in 
order to avoid a loss of the longitudinal and lateral guide forces of the 
wheels of the motor vehicle, in particular. 
The braking momentum of the drive (internal combustion) engine, which has a 
greater effect on the drive wheels after a downshift, can therefore not 
have a negative effect on the driving behavior of the motor vehicle. In 
this connection, downshifting takes place in steps, with intermediate 
elapse of a certain time period in each instance. 
Downshifting while braking is preferably initiated if a condition of 
preventing upshifting is active. The latter is activated in known manner, 
if the motor vehicle is approaching a curve and the driver no longer 
depresses the gas pedal. 
By downshifting while braking, the braking effect of the drive engine of 
the motor vehicle in idle mode is reinforced, on the one hand, so that the 
brake (operational brake) of the motor vehicle is under less stress. On 
the other hand, in connection with maintaining a certain gear, the driver 
always has a gear level available in and after a curve, which is the 
optimum for re-acceleration of the motor vehicle. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWING 
FIG. 1 shows an electro-hydraulic control 1 of an automatic transmission 2 
of a motor vehicle, such as that described in Bosch "Technische Berichte" 
["Technical Reports"], 7(1983)4 on pages 160 to 166 and in ATZ 85(1983)6 
on pages 401 to 405. 
In the following, those signals or variables, which change with time, are 
represented as functions of time f(t). 
A control device 3 receives a kick-down signal kd(t) of a kick-down 
transmitter 4 at the gas pedal of the motor vehicle, as well as of an idle 
signal ll(t) of a throttle valve switch 5, a throttle valve position 
alpha(t) of a throttle valve angle transmitter 6 (or an equivalent 
position transmitter for the position of an element, which influences the 
output of a drive engine of the motor vehicle, such as a gas pedal or an 
injection pump lever of a self-igniting diesel internal combustion 
engine), an engine speed nmot(t) of an engine speed transmitter 7 of an 
internal combustion engine, not shown, and a driving speed v(t) (gear 
output speed) of a gear output speed transmitter 8. As a function of these 
inputs, the control device 3 controls: 
a pressure regulator 9 for the pressure of a hydraulic fluid (signal output 
ds), 
a first electromagnetic valve 10 to control a converter i.e. a converter 
bridge coupling (signal output wk), 
a second electromagnetic valve 11 to control a gear level shift between 
gear levels I and II (signal output sI/II), 
a third electromagnetic valve 12 to control a gear level shift between gear 
levels II and III (signal output sII/III), 
a fourth electromagnetic valve 13 to control a gear level shift between 
gear levels III and IV (signal output sIII/IV). 
In this connection, the control can usually be influenced by the motor 
vehicle driver, via a selector lever 14 to preselect the gear positions P, 
R, N, D, 3, 2, 1. Here, the gear positions that can be used are P (park), 
R (reverse), N (neutral), D (automatic selection of all four gear levels 
IV, III, II and I), 3 (automatic selection of the three lowest gears III, 
II, I), 2 (automatic selection of the two lowest gears II, I) and 1 
(locking of the first gear I). 
In the transmission described above, a program selector switch 15 is 
furthermore provided, with which at least two shifting programs 
(fuel-efficiency shifting program "E" (SKF1), and high-performance 
shifting program "S" (SKFS) can be manually selected. The two shifting 
characteristic fields SKFj, control the four gear levels, which are 
automatically shifted in the driving positions D, III and II. A manual 
program "M" for direct selection of the four gear levels IV, III, II, I 
via the selector lever positions D, 3, 2, 1, is also provided. 
As an alternative to the program selector switch 15, a control method can 
also be implemented in the control device 3, which evaluates the driving 
style of a driver or his actions in response to the traffic situation with 
regard to control of the motor vehicle over a longer period of time, and 
derives a driving activity SK(t) (gas pedal activity) from one or more 
operational, after "driving" insert i.e. driving parameters, as disclosed 
for example, in German patent documents DE-33 48 652 C2 or DE-39 22 051 
A1. On the basis of this driving activity SK(t), one of several shifting 
programs, i.e. shifting characteristic fields SKFj can then be used to 
shift the gear levels IV, III, II and I, in accordance with the shifting 
position of the program selector switch 15. 
To implement the method, according to the invention, in addition to the 
transmitters 4 to 7, further sensors such as an air amount or air mass 
measuring device 16, which determines an air mass ml(t) supplied to the 
internal combustion engine, as well as a lateral acceleration transmitter 
17 (lateral acceleration aq(t)) and a brake signal transmitter 18 (brake 
signal b(t)), are also necessary, as is a reference signal transmitter 19, 
which determines the velocity of the wheels of a non-driven axle, or 
determines the true velocity of the vehicle relative to the road surface 
in other known manner (reference velocity vref(t)). 
It is particularly desirable that upshifting of such a transmission be 
avoided if the vehicle is approaching a curve, for example, and the driver 
is taking his foot off the gas pedal. 
As already shown in German patent documents DE-39 22 040 A1 and DE-39 22 
051 A1, such curve recognition can be performed, for example, by sensing 
the time change of the throttle valve position dalpha(t)/dt. (Normally, a 
driver takes his foot off the gas pedal before a curve--and therefore also 
lowers the position of the throttle valve--faster than he does under 
normal conditions, in order to reduce the driving speed v(t), for 
example.) 
Upshifting, which is normally performed by transmission controls of stepped 
transmissions if the gas pedal position is reduced, (i.e. if the gas pedal 
is not depressed, is prevented according to the method, as long as the 
condition of upshift prevention hsv is active, hsv=1. The condition of 
upshift prevention goes into the active state, hsv=1, if a time change 
dalpha(t)/dt of the throttle valve position alpha(t) goes below a negative 
limit -alphag and idle operation is recognized. The condition of upshift 
prevention hsv returns to the inactive state as soon as traction operation 
is recognized and after elapse of a first time period T1(SK(t)): hsv=0. 
The terms traction operation and idle operation depend on the system being 
considered. A differentiation can be made between: 
The motor vehicle as a total system: Traction operation is understood to be 
acceleration of the motor vehicle (time change of the driving speed) 
dv(t)/dt&gt;0, while coasting operation corresponds to deceleration of the 
motor vehicle dv(t)/dt&lt;0. 
The system of coupling (torque converter)/transmission: In traction 
operation, the input speed of the coupling (of the torque converter) is 
greater than its output speed/the transmission is tensed in the opposite 
direction, while in coasting operation, the input speed is less than the 
output speed/the transmission is tensed in the same direction. 
The system of the internal combustion engine: Traction operation means 
throttle valve position alpha(t)&gt;0 and time change of the engine speed 
dnmot(t)/dt&gt;0, while in coasting operation the throttle position is 
alpha(t)=0 or the time change of the engine speed is dnmot/(t)/dt&lt;0. 
With regard to the transmission control and thus also the overall behavior 
of the motor vehicle, it has proven to be practical to determine the terms 
traction operation and coasting operation as follows: 
Idle operation is recognized if the throttle valve position alpha(t) drops 
below a limit characteristic line azsg(nmot) dependent on the engine 
speed, as shown in FIG. 2: 
alpha(t)&lt;azsg(nmot). 
Traction operation is recognized if both the throttle valve position 
alpha(t) exceeds the limit characteristic line azsg(nmot) dependent on the 
engine speed according to FIG. 2, and the time change of the driving speed 
dv(t)/dt takes on positive values: 
alpha(t)&gt;azsg(nmot).andgate.dv(t)/dt&gt;0. 
In the entire patent application, reference is made to the definitions of 
traction and coasting operation determined in this way. 
In accordance with the invention, upshift prevention hsv now remains in the 
active state, hsv=1, as long as idle mode is again detected during elapse 
of the first time period T1(SK(t)); upshift prevention hsv=1 remains 
active until traction mode is again recognized and a time period T2(SK(t)) 
has again elapsed. 
In a further development of the invention, upshifting initiated after 
elapse of the first or second time period TI(SK(t)), T2(SK(t)) takes place 
in steps up to that gear level g, which is provided for the instantaneous 
operating point in the shifting characteristic field set at that time. 
This has the result that the transmission control does not upshift 
suddenly by as many as 3 gear levels (in the case of four-speed 
transmissions) after the upshift prevention has elapsed. 
Step by step upshifting always takes place by one gear level at a time, 
with at least a third time period T3(SK(t)) lying between two shifts. 
Furthermore, it can also be provided that step by step downshifting is made 
possible when braking, preferably starting from active upshifting hsv=1. 
Step by step downshifting only takes place, however, only if all of the 
following are true 
i) an operating brake of the motor vehicle is activated, the brake signal 
b(t)=1, or alternatively (or in addition), the time change in the driving 
speed dv(t)/dt is less than a first negative longitudinal acceleration 
limit albg(g, nmot, t), albg(g, nmot, t)&lt;0: 
dv(t)/dt&lt;albg(g, nmot, t), 
ii) the lateral acceleration detected by the lateral acceleration sensor 17 
aq(t) lies below a first defined lateral acceleration limit line 
aqg1(v(t)) dependent on driving speed: 
ag(t)&lt;aqg1(v(t)), 
iii) the time change in driving speed dv(t)/dt is greater than a second 
negative longitudinal acceleration limit albbg(nmot, g, SK(t), t)-k(g-1, 
SK(t))*dv/dt.vertline..sub.g-1 : 
dv(t)/dt&gt;albbg(nmot, g, SK(t), t); dv(t)/dt&gt;k(g-1, 
SK(t))*dv/dt.vertline..sub.g-1 albg(g, nmot, t), 
iv) the driving speed v(t) is less than a second driving speed limit vg(g, 
SK(t), t): 
v(5)&lt;vg (g, SK (t), t). 
Step by step downshifting always takes place by one gear level at a time, 
with at least a fourth time period T4(SK(t)) lying between two shifts. 
Step by step downshifting is carried out up to that gear level g, which is 
permissible at the instantaneous operating point of the motor vehicle in 
the shifting characteristic field (SKFj) set at that time (in order to 
avoid overly high speeds of the internal combustion engine). 
The first negative longitudinal acceleration limit albg(g, nmot, t) is 
dependent on the instantaneous values of the gear level g, which is 
engaged, and on the engine speed nmot(t), and hereby corresponds to the 
(negative) longitudinal acceleration dv/dt in each instance (and thus the 
deceleration) of the motor vehicle rolling on a level road surface in a 
defined condition (load, tire air pressure, ambient conditions, etc.) with 
a closed throttle valve alpha=0, at the value pairs of the gear level g, 
which is currently engaged, and the engine speed nmot(t) in each instance. 
The first negative longitudinal acceleration limit albg(g, nmot, t) is 
determined from the instantaneous values of these factors, preferably via 
a first characteristic field ALB(g, nmot): albg(g, nmot, t)=ALB(g, nmot). 
An example of such a first characteristic field ALB(g, nmot) is shown in 
FIG. 3. As an alternative to this, the determination of the longitudinal 
acceleration limits albg(g, nmot, t) can, of course, also take place via a 
corresponding functional relationship. 
The characteristic lines according to FIG. 3 clearly show the dependence of 
the deceleration values of a motor vehicle with an internal combustion 
engine on the gear level g and the engine speed nmot(t). In this 
connection, the individual characteristic lines for the four gear levels 
I, II, III, IV--without any restriction of the general applicability--each 
assign a certain value ALB plotted on the vertical axis in the unit g=9.81 
. . . meters per second.sup.2 (acceleration due to gravity) to a value of 
the engine speed nmot (in revolutions per minute) plotted on the 
horizontal axis. 
For increasing values of the engine speed nmot(t), the deceleration values 
become greater, due to the reinforced engine braking effect and the 
increased rolling resistance (air resistance) of the vehicle. Likewise, 
the deceleration values increase with a lower gear level g, since the 
braking momentum of the internal combustion engine has a greater effect on 
the deceleration rate of the motor vehicle, due to the greater 
translation. 
In this connection, the first lateral acceleration limit line aqg1(v(t)) is 
preferably dependent on driving speed and the corresponding limit values 
derived from it are reduced with an increasing driving speed v(t). A 
corresponding characteristic line is shown in FIG. 4. 
The second negative longitudinal acceleration limit albbg(nmot, g, 
SK(t))=k(g-1, SK(t))*dv/dt.vertline..sub.g-1 is determined according to a 
product of a factor k(g-1, SK(t)) dependent on the gear level, and a value 
of the longitudinal acceleration dv/dt.vertline..sub.g-1 to be expected in 
the next lower gear level g-1, calculated at the current operating 
conditions of the motor vehicle. 
To determine this longitudinal acceleration dv/dt.vertline..sub.g-1 to be 
expected in the next lower gear level g-1, the value of the current 
driving speed v(t) is first used, and the engine speed nmot(t).sub.g-1 
=i(g-1*v(t) to be expected in the next lower gear level g-1 is determined 
from it. For this, the product of the current value of the driving speed 
v(t) and the value of the gear translation i(g-1) in the next lower gear 
level g-1 is formed. 
The value of the longitudinal acceleration dv/dt.vertline..sub.g-1 to be 
expected in the next lower gear level g-1 is finally determined via the 
characteristic field ALB(g, nmot) from the next lower gear level g-1 and 
the value of the engine speed nmot(t).vertline..sub.g-1 to be expected in 
the next lower gear level g-1. 
The factor k(g-1, SK(t)), which is dependent on the gear level, is 
determined via a second characteristic field F(g, SK(t)), from the next 
lower gear level (g-1), k(g-1, SK(t))=F(g, Sk(t)). An example for the 
second characteristic field is shown in FIG. 5. 
The second driving speed limit vg(g, SK(t), t) depends on the gear level g 
and the driving activity SK(t). 
The effect of the individual method steps can be explained as follows: 
By monitoring the activation of the operating brake of the motor vehicle 
(brake signal b=1), or alternatively (or in addition, by checking whether 
the rate of change in the driving speed dv(t)/dt lies below the first 
negative longitudinal acceleration limit albg(g, nmot), dv(t)/dt&lt;albg(g, 
nmot), the driver's wish for greater deceleration, i.e. for downshifting, 
is determined. 
By checking whether the lateral acceleration aq(t) lies below the first 
defined lateral acceleration limit aqg1(v(t)), the method monitors whether 
the vehicle is already driving in a curve, with relatively high lateral 
acceleration aq(t). If such curve driving is present, then downshifting is 
prevented, so that the contact between wheel and road surface is not lost 
as a result of the braking effect, which would otherwise increase. 
A comparable safety function is represented by the monitoring of whether 
the value goes below the second negative longitudinal acceleration limit 
albbg(nmot, g, SK(t)): Here, it is determined whether the expected 
deceleration of the motor vehicle after a requested downshift would not 
result in exceeding the adhesion friction limit of the wheels. 
For this purpose, a maximum permissible deceleration at a particular time 
is determined from the deceleration to be expected after downshifting at 
the current driving conditions, by weighting (multiplication) by the 
factor k(g-1, SK)t)), which is dependent on gear level, and this is 
compared with the current vehicle deceleration dv(t)/dt; if the 
instantaneous deceleration is greater, downshifting is prevented. 
In this, the factor k(g-1, SK)t)), which is dependent on gear level, takes 
into consideration that the second negative longitudinal acceleration 
limit albbg is less than the first negative longitudinal acceleration 
limit albg(g, nmot), in other words, must be greater in amount 
(corresponding to a higher deceleration rate). 
By monitoring whether the driving speed limit vg(g, SK(t), t), which is 
dependent on gear level, is exceeded, additional safety criteria with 
regard to downshifting at overly high driving speed or prevention of 
exceeding limits for the speed of rotation of the driving internal 
combustion ending after downshifting can be fulfilled. These safety 
criteria are highly vehicle-specific and must therefore be adjusted to 
every vehicle, so that representation of such a characteristic field is 
not necessary. 
In order to avoid a change in gear level g after approaching a curve or 
after braking before a curve, the lateral acceleration aq(t) of the 
vehicle is monitored. The change in gear level g is avoided, or the time 
periods T1(SK(t)), T2(SK(t)), T3(SK(t)) are set to zero, if the amount of 
the lateral acceleration (.vertline.aq(t).vertline.) exceeds a second 
lateral acceleration limit line aqg2=f(v(t)) dependent on the driving 
speed, according to FIG. 4, i.e. as long as a fifth time period T5(SK(t)) 
has not yet elapsed after the value went below the second lateral 
acceleration limit line aqg2(v(t)). In this connection, it is evident that 
the second lateral acceleration limit line aqg2(v(t)) for curve detection 
lies clearly below the first lateral acceleration limit line aqg1(v(t)), 
which is used as a safety function. 
Furthermore, gear changes, but particularly downshifting can be avoided in 
coasting operation, and/or the time periods T1(SK(t)), T2(SK(t)), 
T3(SK(t)) or T5(SK(t)) can be set to zero, if excessive slip occurs at at 
least one of the wheels of the motor vehicle, or if the contact between at 
least one wheel of the motor vehicle and the road surface driven on is 
interrupted. 
In this connection, shifting is only permitted if a difference velocity 
Dv(t)=vref(t)-v(t) between the speed vref(t) of a non-driven axle and the 
driving speed v(t) detected at a driven axle does not exceed a permissible 
difference velocity value Dvzul(SK(t)): 
Dv(t)&lt;Dvzul(SK(t)). 
If the permissible difference velocity value Dvzul(SK(t)) is exceeded, in 
addition 
a converter bridge coupling of a transmission equipped with a torque 
converter can be opened, 
a holding time Th can be set, during which upshifting cannot be prevented, 
the gear level g, which is engaged, can be raised by one (upshifting) and 
downshifting can be prevented, 
where these functions are reset as soon as traction operation is recognized 
and positive values of the change in driving speed v(t) are present. 
The time periods T1(SK(t)), T2(SK(t)), T3(SK(t)), T4(SK(t)), T5(SK(t)) and 
Th(SK(t)) can be of the same or of different duration, and at least one of 
the time periods or the driving speed limit vg(g, SK(t), t) or the factor 
k(g-1, SK(t)), which is dependent on the gear level, or the permissible 
difference velocity value Dvzul(SK(t)) can be arbitrarily adjustable, and 
preferably can be set together with a setting of the shifting 
characteristic fields SKFj by means of the program selector switch 15 
(fuel-efficient shifting characteristic field SKF1), high-performance 
shifting characteristic field SKF5), in such a way that the time periods 
T1(SK(t)), T2(SK(t)), T3(SK(t)), T5(SK(t)) and the driving speed limit 
vg(g, (SK(t)) are greater for more high-performance shifting 
characteristic fields (driving programs), and the time periods T4(SK(t)), 
Th(SK(t)), the factor k(g-1, SK(t)), which is dependent on the gear level, 
and the permissible difference velocity value Dvzul(SK(t)) become smaller 
(see FIG. 5 and FIG. 6). 
If the transmission control provides for automatic adjustment of the 
shifting characteristic fields to the driving style of the driver or a 
traffic situation, then at least one of the time periods T1(SK(t)), 
T2(SK(t)), T3(SK(t)), T4(SK(t)), T5(SK(t)) or Th(SK(t)), or at least the 
driving speed limit vg(g, Sk(t)) or the factor k(g-1, SK(t)), which is 
dependent on the gear level, or the permissible difference velocity value 
Dvzul(SK(t)) can be dependent on the driving activity SK(t), which 
assesses the driving style of the driver or the traffic situation. With 
increasing, more performance-oriented driving activity SK(t), the time 
periods T1(SK(t)), T2(SK(t)), T3(SK(t)), T5(SK(t)) and the driving speed 
limit vg(g, SK(t)) become greater, and the time periods T4(SK(t)), 
Th(SK(t)), the factor k(g-1, SK(t)), which is dependent on the gear level, 
and the permissible difference velocity value Dvzul(SK(t)) become less 
(see FIG. 5 and FIG. 6). 
The driving activity is determined by means of a functional relationship 
assessed over a longer term by means of the driving style of the driver or 
his actions in response to the traffic situation, with regard to the 
control of the motor vehicle (formation of sliding mean from current and 
past values of a single operational characteristic), or from a single 
value composed of a number of different operational characteristics of a 
motor vehicle. This can be done, for example, analogous to the method 
shown in DE-39 22 051 A1 or DE-33 41 652 C2.