Traction control method for stabilizing motor vehicle motion in the event of increased driving wheel slip

A method controls the driving stability in the event of increased slip at the driving wheels of a motor vehicle having an electronic traction system with braking intervention at the driving wheels. With excessive high-adhesion wheel slip, synchronous braking intervention on both sides is undertaken at the driving wheels when the vehicle is travelling around a curve. A reduction takes place in the brake pressure at the low-adhesion wheel, as used for controlling drive slip or increasing traction, when the vehicle is travelling in a straight line, the high-adhesion wheel slip is used as the control parameter in either situation. This method permits braking interventions at the driving wheels for the purpose of controlling drive slip and/or for increasing traction while still maintaining a high level of driving stability.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is related to application Ser. No. 08/453,002 filed May 
30, 1995, pending for "MOTOR VEHICLE TRACTION SYSTEM CONTROL OSCILLATION 
DAMPING METHOD USING LOW-ADHESION WHEEL BRAKE INTERVENTION" in the name of 
Gerhard FISCHLE et al.; application Ser. No. 08/449,660 filed May 24, 1995 
now U.S. Pat. No. 5,479,811 for "PROCEDURE FOR CALIBRATING THE WHEEL 
SPEEDS FOR A MOTOR VEHICLE" in the name of Matthias BAUMANN et al.; 
application Ser. No. 08/452,532 filed May 30, 1995, pending, for "METHOD 
FOR CONTROLLING VEHICLE BRAKE PRESSURE AS A FUNCTION OF THE DEVIATION OF 
THE ACTUAL SLIP OF WHEELS RELATIVE TO A DESIRED SLIP" in the name of Peter 
BOSCH et al.; and application Ser. No. 08/452,994 filed May 30, 1995 
pending, for "METHOD FOR INCREASING DRIVE TORQUE WITH CONTROLLED BRAKE 
ENGAGEMENT" in the name of Gerhard FISCHLE et al. 
BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a method of traction control to stabilize 
the driving of a motor vehicle in the case of increased slip at the 
driving wheels, and more particularly, to a method which uses braking 
interventions at the driving wheels in the event of increased slip at 
driving wheels as a function of whether the motor vehicle is travelling 
around a curve or in a straight line and as a function of the vehicle 
speed. 
It is known that the spinning of the driving wheels, that is to say the 
appearance of an undesirably high driving wheel slip which reduces 
traction, can be prevented by braking interventions of an acceleration 
skid control system (ASR). One driving wheel, specifically that with the 
lower adhesion (the so-called low-adhesion wheel) frequently begins to 
spin first, particularly in the case of different coefficients of 
friction, with the result that there is a loss of driving torque. In order 
to combat this driving torque loss, an electronic traction system can be 
provided which undertakes a one-sided braking intervention at the 
low-adhesion wheel. This active braking on the low-adhesion wheel not only 
brings it back into a generally more favorable slip range, but also acts 
simultaneously as a limited-slip torque to increase the traction at the 
opposite driving wheel, i.e. the wheel with the greater adhesion (the 
so-called high-adhesion wheel). When the adhesion is no longer sufficient, 
the wheel slip at the high-adhesion wheel can also become excessive so 
that the driving stability can be impaired by such braking interventions 
to increase traction, particularly in the case of high vehicle speeds and 
on curves. 
The article "Ausgebremst" (translation, "Fully Braked") in the 
"Auto-Motor-Sport" Journal, p. 34 (2.8.1986), describes an electronic 
differential lock, in which the effect of increasing the traction at the 
high-adhesion wheel, caused by the effects of braking at the low-adhesion 
wheel, is used to increase traction. 
The difficulty of adequate driving stability, such as is also known from 
ASR systems, occurs in such electronic traction systems, and for this 
reason the known electronic differential lock mentioned above, is used 
only as a pull-away aid and is automatically switched off at vehicle 
speeds above 40 km/h. 
DE 31 27 301 A1 describes a method of traction control to stabilize driving 
which operates on the so-called select-low principle. That is, an 
intervention controlling traction takes place on condition that the 
low-adhesion wheel slip is above a specified limit. As a further measure 
to stabilize driving, such a traction-control intervention takes place as 
soon as the system recognizes that the vehicle is travelling round a curve 
at a speed already in excess of a lower limiting speed of, for example, 40 
km/h whereas, when the vehicle is travelling in a straight line, such an 
intervention is only undertaken when a higher limiting speed of, for 
example, 100 km/h is exceeded. In the case of vehicle speeds below the 
lower limiting speed, no traction control intervention to reduce the 
driving torque and stabilize driving takes place. The traction control 
interventions provided include subjecting the low-adhesion wheel to brake 
pressure, with an attempt being made to set a brake pressure which leads 
to maximum traction torque at the high-adhesion wheel. If the brake disc 
temperature exceeds a specified threshold value due to these braking 
interventions, the traction control device is switched off for a specified 
time interval. 
DE 35 18 221 C2 describes a brake system in which measures are taken to 
stabilize driving during braking procedures when travelling around curves. 
These measures include the determination of a yaw angle reference value as 
a function of the vehicle transverse forces, of the vehicle speed and of 
the distance travelled around the curve as well as the activation of the 
front and rear wheel brakes, when braking is demanded during travelling 
around a curve, so that the actual yaw angle value derived from the 
vehicle transverse forces recorded is adjusted to the yaw angle reference 
value. This takes place automatically by way of an electronic control 
unit. 
DT 2 319 862 describes an anti-lock system in which, to increase driving 
stability, two electronic analysis circuits of the wheels of one axle are 
configured in such a way and are associated with one another such that the 
brake pressure of one wheel is retained, built up or lowered in order to 
avoid large brake force differences at the wheels of one axle and/or to 
achieve a common control cycle variation from an output signal of the 
other wheel or, during the individual occurrence of the output signal, on 
the wheel which has to be influenced. In the case of a pair of signals 
which are time displaced relative to one another, the leading or trailing 
signal can be used for individual occurrence of a signal can be used for 
control. 
It is an object of the present invention to provide a traction control 
method to stabilize driving so that a high level of driving stability can 
be maintained in continuous driving operation even in the case of vehicles 
in which braking interventions to regulate acceleration skid, and in 
particular one-sided braking interventions to increase traction, are 
undertaken at the driving wheels. 
This object has been achieved in accordance with the present invention by a 
method involving monitoring whether high-adhesion wheel slip is above a 
specified limit and whether vehicle speed is in a range between a 
specified lower limiting speed and a specified upper limiting speed and, 
if the response is positive, using the high-adhesion wheel slip as the 
control parameter; effecting brake pressure at the driving wheels on both 
sides of the motor vehicle with synchronous brake pressure control upon 
recognizing that the motor vehicle is travelling around a curve, and 
reducing brake pressure which increases traction at the low-adhesion wheel 
upon recognizing that the motor vehicle is travelling in a straight line. 
In the event of excessive high-adhesion wheel slip, the method reacts in a 
different manner to stabilize driving, depending on whether the vehicle is 
travelling around a curve or in a straight line. 
When the vehicle is travelling around a curve, the brake pressure is 
controlled synchronously on both sides to stabilize the driving on the 
curve, with the high-adhesion wheel slip being the control parameter which 
is controlled within a range which ensures sufficient cornering force for 
the vehicle. The cornering force at the high-adhesion wheel can be 
maintained by control based on the high-adhesion wheel slip and not on the 
low-adhesion wheel slip. The low-adhesion wheel is spinning and no longer 
possesses any cornering force. 
Driving instabilities can be prevented by also specifically increasing the 
brake pressure at the high-adhesion wheel. These driving instabilities are 
particularly due to the yaw velocity present when the vehicle is 
travelling around a curve where one-sided braking intervention controlling 
traction takes place on a condition that the low-adhesion wheel slip is 
above a low-adhesion wheel when the vehicle is travelling in a straight 
line, the low-adhesion wheel can be prevented from accelerating by the 
driver reducing the driving torque, should this be desired. 
The present invention has the advantage that the braking intervention which 
places a load on the brakes on both sides during travel around a curve, 
remains activated for at most a specified period, with the result that 
excessive brake heating is prevented. In order to avoid disturbing control 
oscillations, furthermore, such a braking intervention is deactivated 
whenever the amount of curvature, or the high-adhesion wheel slip, falls 
below respectively specified deactivation limits, which are preferably 
smaller than the activation limits which, when exceeded, activates the 
braking intervention. 
A further feature of the present invention is that brake pressure at the 
low-adhesion wheel to increase traction is retained when the vehicle speed 
is below a specified minimum speed even if the vehicle is travelling in a 
straight line and there is increased high-adhesion wheel slip. This brake 
pressure retention permits a certain increased high-adhesion wheel slip in 
the low speed range in order to achieve maximum traction by way of a 
scraping effect, the driving stability not being noticeably impaired in 
this range of low speeds. 
It has also been found beneficial for the speed range with active stability 
control to be from 15 km/h to 80 km/h. In this speed range, travel around 
a curve can be recorded very reliably by wheel rotational speed sensor 
techniques alone without a transverse acceleration sensor, steering angle 
sensor or yaw velocity sensor. Below 15 km/h, the driver can still react 
sufficiently rapidly, it being the case, in addition, that fairly large 
skidding movements, for example escape turns, remain possible.

DETAILED DESCRIPTION OF THE DRAWINGS 
The flow diagram is part of a more comprehensive, cyclically operated 
control method of an electronic traction system already present in a motor 
vehicle. Those parts of the program which are not of more detailed 
interest in the present context are reproduced in a simplified manner as 
the program block (A). The method of stabilizing driving in this case 
forms a functional module within the electronic traction system and takes 
priority over the actual braking interventions to increase traction at the 
low-adhesion wheel. 
The activation of the part of the program which stabilizes the driving 
initially assumes that the high-adhesion wheel slip (HS) exceeds a 
specified slip limit (HS.sub.L) which is fixed as a function of a 
reference wheel speed which represents the average wheel speeds of the 
non-driven wheels, and also that the vehicle speed is between a lower 
limiting speed (v.sub.l) and an upper limiting speed (v.sub.u), something 
which is determined by interrogation in an initial Step 1. The lower speed 
limit (v.sub.l) is set, in the present example, to 15 km/h and the upper 
speed limit (v.sub.u) is set to 80 km/h. The vehicle speed is determined 
as the average of the wheel speeds, corrected for curve and wheel 
adjustments, of the non-driven wheels. The high-adhesion wheel is 
recognized as the driving wheel which is rotating more slowly and the 
low-adhesion wheel is recognized as the driving wheel which is rotating 
more rapidly. The driving wheel speeds corrected for curve and wheel 
adjustment are compared with one another for this purpose. It may, 
therefore, be seen that recording the wheel rotational speeds is 
sufficient for the determination of these parameters. The high-adhesion 
wheel slip (HS) is then evaluated in the form of the difference between 
the high-adhesion wheel speed and the reference wheel speed. 
If the response to the interrogation in Step 1 is negative, the procedure 
is continued with other parts of the traction control program, for example 
with an actual braking intervention to increase traction. If, on the other 
hand, the response to the interrogation is positive, the next 
interrogation step, i.e. Step 2, checks whether the vehicle is travelling 
around a curve. The conclusion that the vehicle is starting to travel 
around a curve is drawn if the wheel speed difference of the front wheels 
is greater than a specified limit, which is set to 1 km/h in the present 
illustration. If the fact that this limit is being exceeded, and therefore 
that the vehicle is starting to travel around a curve, is recognized, the 
method of controlling the driving stability provides for stability control 
as follows while the curve is being negotiated as provided in Step 3. 
The control system is activated each time the high-adhesion wheel slip (HS) 
exceeds the specified slip limit (HS.sub.L) plus a percentage curve offset 
value which is used to prevent premature initiation of the control in the 
case of extremely tight curve radii. As an example, the curve offset value 
is a tenth of the difference between the wheel speeds of the non-driven 
front wheels. 
After activation, braking intervention at the driving wheels takes place on 
both sides in order to remove the excess torque by braking action. The 
intervention is carried out synchronously so that no limited-slip torque, 
and therefore no yawing torque, is generated. The high-adhesion wheel is 
brought back into the stable range by specific phases of pressure 
build-up, pressure retention and pressure reduction which depend on the 
high-adhesion wheel slip and the high-adhesion wheel slip acceleration, 
with the difference between the high-adhesion wheel speed and the 
reference wheel speed being used as the control parameter. 
In order to protect the brakes from thermal overload, the braking 
intervention on both sides is only permitted for a fixed maximum duration 
which is, for example, 2 s in the present example. This duration is 
normally sufficient because a maneuver involving a turn does not, on 
average, last any longer. 
A time counter is always set whenever the system concludes that the vehicle 
is starting to travel around a curve to recognize how long the vehicle has 
been travelling around the curve. If the high-adhesion wheel slip ceases 
while the vehicle is travelling around a curve, the time counter remains 
at its instantaneous value. If the high-adhesion wheel slip reappears 
while the vehicle is travelling around the same curve, the braking 
intervention can be continued for the period still remaining. If the fixed 
permissible maximum duration has elapsed but the requirement for 
high-adhesion wheel slip control is still present, the brake pressure is 
slowly reduced on both sides with the result that the driver is left with 
sufficient time to react to the onset of the vehicle yawing motion. 
If the driving wheel brakes have not been activated when braking 
intervention is started on both sides in order to stabilize the vehicle's 
motion around a curve, this braking intervention takes place, as above 
noted, synchronously at both driving wheels by a simultaneous and equally 
large brake pressure change. 
The braking intervention to stabilize the vehicle's motion around a curve 
can also have been activated by a previous, one-sided braking intervention 
on the low-adhesion wheel in order to increase traction, in situations 
where this braking intervention at the low-adhesion wheel has led to an 
excessive drive slip at the high-adhesion wheel because of the 
limited-slip torque placed on the high-adhesion wheel. In these 
situations, a certain brake pressure is already present at the 
low-adhesion wheel at the beginning of the method for stabilizing the 
vehicle's motion around the curve. The brake pressure control to stabilize 
the vehicle's motion around the curve then takes place by initially 
retaining the brake pressure at the low-adhesion wheel and increasing the 
brake pressure at the high-adhesion wheel. If the wheel slip to be 
adjusted at the high-adhesion wheel has already been reached before its 
pressure is equal to that at the low-adhesion wheel, the brake pressure at 
the low-adhesion wheel also continues to be retained throughout a 
subsequent high-adhesion wheel brake pressure retention phase. If, 
however, the desired high-adhesion wheel slip has not yet been reached 
when the high-adhesion wheel brake pressure is the same as that of the 
low-adhesion wheel, the brake pressures of both driving wheels are 
subsequently further increased synchronously during the same period. 
An analogous procedure is adopted during brake pressure reduction phases. 
If, on one hand, the same brake pressures are initially present, both of 
them are reduced synchronously. If, on the other hand, the high-adhesion 
wheel brake pressure is still lower than the low-adhesion wheel brake 
pressure at the beginning of a brake pressure reduction phase, the 
low-adhesion wheel brake pressure is first reduced to the pressure level 
of the high-adhesion wheel, after which both brake pressures are then, 
once again, further controlled in a synchronous manner. This mode of 
operation has the advantage that when the vehicle is travelling around a 
curve, the limited-slip torque is initially reduced because the pressures 
at the two driving wheels are brought into balance. 
In addition, the excess driving torque caused by the driver and endangering 
stability is eliminated by the build-up of pressure at the high-adhesion 
wheel so that the high-adhesion wheel is again given maximum cornering 
force. When the vehicle has been restabilized, one-sided braking 
intervention to increase traction is only initiated afresh if both driving 
wheels are no longer subjected to brake pressure. 
Apart from the situation where the maximum control duration has been 
exceeded, as described above, the control phase to stabilize the vehicle's 
motion around a curve is terminated when the vehicle is again travelling 
in a straight line or when the vehicle speed is no longer in the 
previously described range between 15 km/h and 80 km/h or when the 
high-adhesion wheel slip has fallen below a specified deactivation limit. 
Depending on whether the vehicle is travelling around a curve, this 
deactivation limit is preferably selected in this case to be somewhat 
smaller than the slip limit (HS.sub.L), smaller in the present example by 
1/15 of the difference between the wheel speeds of the front wheels. 
Selecting a switch-off threshold which is somewhat lower than the 
switch-on threshold avoids control oscillations. 
A system conclusion is drawn that the vehicle is beginning to travel in a 
straight line, i.e. the vehicle has stopped travelling around a curve, 
when the wheel speed difference at the front wheels is smaller than a 
limit which, in the present case, is set to 0.5 km/h. Here again, 
selecting a limit for recognizing that the vehicle has stopped travelling 
around a curve which is lower than that for recognizing that it has 
started to travel around a curve is once again intended to avoid 
undesirable control oscillations. The control for stabilizing the 
vehicle's motion around a curve is terminated in each instance by setting 
a pressure reduction gradient to correspond with the wheel slip variation. 
Furthermore, the stability control for a vehicle travelling around a curve 
is terminated immediately if a braking procedure to decelerate the vehicle 
is initiated and/or the anti-lock system announces priority. 
If, after a previous positive response to the interrogation in Step 1, the 
interrogation Step 2 recognizes that the vehicle is not travelling around 
a curve but is travelling in a straight line, the method controls the 
stability of the vehicle's motion for straight line motion in Step 4 as 
follows. The system first determines whether the vehicle speed has, in the 
meantime, dropped below the lower range limit of 15 km/h. A comparatively 
high high-adhesion wheel slip is permitted in this low speed range in 
order to reach the maximum traction by a scraping effect because, in this 
speed range, the driving stability is not essentially impaired. For this 
purpose, the brake pressure in the low-adhesion wheel is initially 
retained and the system waits to see whether the high-adhesion wheel has 
already begun to adhere again after a very short period, as is the case 
when the vehicle drives over spots with a low coefficient of friction. If 
the high-adhesion wheel slip now increases further and if, in fact, this 
wheel should possibly rotate even more rapidly than the low-adhesion 
wheel, the system recognizes that increased traction is no longer 
attainable and it starts to reduce the pressure at the low-adhesion wheel 
until the high-adhesion wheel slip is again below the limit (HS.sub.L). 
Thereafter, actual traction control can again be undertaken. 
If, however, the vehicle is travelling in a straight line at a speed above 
the lower limit of 15 km/h, the driving stability control immediately 
reduces the brake pressure present at the low-adhesion wheel for the 
purpose of increasing traction. This reduces the driving torque at the 
high-adhesion wheel and the high-adhesion wheel can again rapidly 
undertake the transmission of side forces. During this stabilization 
phase, the low-adhesion wheel can only be prevented from accelerating by a 
reduction in driving torque, for example by the driver, if this should 
appear expedient. As soon as the high-adhesion wheel slip again falls 
below the specified limit (HS.sub.L), the actual one-sided, 
traction-increasing brake pressure control at the low-adhesion wheel can 
continue. 
The above-described method permits the maintenance of a high level of 
driving stability both when the vehicle is travelling around a curve and 
when it is travelling in a straight line, particularly in those cases 
where the driving stability could be otherwise impaired because of 
one-sided braking interventions at the driving wheels for the purpose of 
increasing traction. This makes it possible to employ an electronic 
traction system which fulfills the function of an automatic differential 
lock but without endangering driving stability over a wide range of 
possible driving conditions and without the system having to be limited, 
for example, to the function of a pull-away aid. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. For example, the 
numerical data described above only represents one possible selection and 
can be respectively modified in a suitable manner for other applications. 
The method according to the present invention for driving stability 
control can, of course, also be usefully employed in association with a 
conventional ASR system. The spirit and scope of the present invention 
are, therefore, to be limited only by the terms of the appended claims.