The present invention relates to a traction control system (TCS) and a corresponding method.
When a vehicle drives onto or accelerates on a slick road surface, its driven wheels relatively quickly begin to slip. If the driven wheels slip so much that they exceed a defined slip threshold, the TCS automatically intervenes in vehicle operation and adapts the engine torque, via an adjustment of the throttle valve, to the respective drive torque that may be transferred to the road. Optionally, the slipping driven wheel or wheels may also be decelerated by manner of a braking intervention. Once the slip has fallen below the defined threshold, the engine torque is raised again.
FIG. 1 shows a typical profile for an engine torque curve during a TCS control cycle. Firstly, the driver increases engine torque relatively sharply (see segment 7 of the curve). At time t1, at least one of the driven wheels exceeds a defined slip threshold, whereupon the TCS greatly reduces engine torque (see downturn 3). Lastly, once slip has fallen below the threshold, the engine torque is increased again in steps (see upper branch 4 of curve) until at time t2, one driven wheel once again exceeds the slip threshold and a new TCS control action begins. The time span between times t1 and t2 is referred to as a TCS control cycle.
At present, the period of one TCS control cycle is approximately 1 second, or the control frequency is, on average, approximately 1 Hz. An addition routine responsible for increasing the engine torque is set so that the drive wheels enter the slip phase approximately once a second. This ensures that after a torque reduction (segment 3), sufficient traction may be built up and hazardous driving situations due to inadequate acceleration may be prevented. This control frequency of approximately 1 Hz is moreover perceived as pleasant by the driver.
The aforementioned settings contribute to relatively comfortable vehicle behavior when traveling straight ahead. When cornering on a slick road surface, however, the drive slip effected deliberately at a frequency of 1 Hz results in poor lateral stability, with the result that the vehicle departs from its track. With rear-wheel-drive vehicles, wheel slip may cause the rear end to break away, and may result in uncontrollable driving situations.
It is an object of the present invention to improve the behavior of a vehicle during a TCS control action when cornering on a slick road surface, and thereby to improve driving safety.
The present invention provides a traction control system in such a manner that upon detection of a driving situation in which the vehicle is cornering on a road surface having a low coefficient of friction, the gradient of the engine torque increase during the TCS control action is decreased, and is set to a lower value than in a different driving situation, differently than, for example, when driving straight ahead. This may provide that the engine torque rises less quickly, and the moment at which the driven wheel begins to slip and the slip threshold is exceeded occurs considerably later. The duration of a TCS control cycle is thus lengthened.
For detection of the driving situation involving cornering on a surface having a low coefficient of friction, the TCS includes an arrangement for detecting cornering and the coefficient of friction. Detection of this driving situation may occur when defined threshold values are exceeded.
The increase in engine torque after it is reduced by the TCS may be accomplished either in stepped fashion or continuously. With a stepwise increase in engine torque, the initial jump, i.e., the first sharp increase in torque, is selected to be smaller than, for example, when driving straight ahead or at a high coefficient of friction.
The step height of the individual torque increases is preferably also comparatively lower when cornering at a low coefficient of friction than in other driving situations.
Optionally, the gradient of the engine torque increase may also be lowered by lengthening the dwell times between the increase steps.
According to an example embodiment of the present invention, the TCS control action with smaller gradients is performed only when the number of TCS control cycles at a high gradient has reached a defined value.
In addition, the TCS control action according to the present invention is not active when the vehicle acceleration exceeds a defined value. High vehicle accelerations are an indication of a high coefficient of friction, so that in such a case a low coefficient of friction may be ruled out. If, however, only low vehicle acceleration values are identified simultaneously with drive slip, it may be concluded that the TCS control action according to the present invention is necessary.
According to an example embodiment of the present invention, the gradient of the engine torque increase is set as a function of the (at least qualitatively) identified coefficient of friction, the gradient being flatter, the lower the coefficient of friction that is identified.
The gradient of the engine torque increase may also be set as a function of the vehicle speed; the gradient should be flatter for higher vehicle speeds.
The period of one TCS control cycle when cornering at a low coefficient of friction should be at least 1.5 seconds, in particular at least 2 seconds, and should be at least 3 seconds with low coefficients of friction.
A further improvement in vehicle stability may be achieved by the fact that the slip thresholds of the drive wheels are lowered upon detection of cornering at a low coefficient of friction.
If the conditions for activation of the slow engine torque increase according to the present invention are no longer present (i.e., cornering, low coefficient of friction, optionally number of previous control cycles, low vehicle acceleration), the control action according to the present invention may be maintained for a defined follow-on period. It is thereby possible to ensure that even if cornering or the coefficient of friction is detected incorrectly (but the vehicle is still in fact cornering at a low coefficient of friction), vehicle safety is guaranteed.