Understeer and oversteer detector for a motor vehicle
A method for detecting a situation of loss of grip of a vehicle provided with a steering system operated by a steering wheel, the method including a step (a) of evaluating a first indicator of loss of grip (P 1 ) by calculating, as the first indicator of loss of grip (P 1 ), the partial derivative ( P 1 = ∂ ψ . ∂ α ) , relative to a variable (α) representative of the angular position of the steering wheel, of a driving parameter which is representative of the yaw rate ({dot over (ψ)}) of the vehicle.
1. A method for detecting loss of grip of a vehicle provided with a steering system operated by a steering wheel, the method comprising:
(a) calculating a first indicator of loss of grip (P 1 ) which is a partial derivative
(
P
1
=
∂
ψ
.
∂
α
)
of a driving parameter ({dot over (ψ)}) representative of yaw rate of the vehicle, relative to a variable (α) representative of angular position of the steering wheel;
(b) detecting loss of grip of the vehicle based on the calculated first indicator of loss of grip (P 1 ); and
(c) controlling the vehicle based on the detected loss of grip.
2. The method according to claim 1 , wherein the driving parameter ({dot over (ψ)}) is a measurement or an estimation of the yaw rate of the vehicle at a given instant or a theoretical angular position of the steering wheel (α theoretical ) obtained from an angle of Ackermann (α Ackermann ), which corresponds, in the absence of drift, to dynamic behavior of the vehicle observed at the given instant.
3. The method according to claim 1 , further comprising:
(d) comparing a value of the first indicator of loss of grip (P 1 ) with an understeer threshold (Threshold_US);
(e) detecting understeer if the value of the first indicator (P 1 ) is less than the understeer threshold (Threshold_US); and
(f) controlling the vehicle based on the detected understeer.
4. The method according to claim 1 , further comprising:
(d) comparing a value of the first indicator of loss of grip (P 1 ) with a first oversteer threshold (Threshold_OS_ 1 );
(e) detecting oversteer if the value of the first indicator (P 1 ) is greater than the first oversteer threshold (Threshold_OS_ 1 ); and
(f) controlling the vehicle based on the detected oversteer.
5. The method according to claim 1 , further comprising:
(d) comparing the value of the first indicator of loss of grip (P 1 ) with a second oversteer threshold (Threshold_OS_ 2 );
(e) calculating a second indicator of loss of grip (P 2 ) which is a partial derivative
(
P
2
=
∂
ψ
.
∂
γ
lat
)
of the driving parameter ({dot over (ψ)}), relative to lateral acceleration (γ lat ) of the vehicle;
(f) comparing a value of the second indicator (P 2 ) with a third oversteer threshold (Threshold_OS_ 3 );
(g) detecting oversteer with steering reduction (OS- 1 , α 2 ) if, cumulatively, the value of the first indicator is greater than the second oversteer threshold and the value of the second indicator is greater than the third oversteer threshold; and
(h) controlling the vehicle based on the detected oversteer with steering reduction (OS- 1 , α 2 ).
6. The method according to claim 1 , further comprising:
(d) comparing the value of the first indicator of loss of grip (P 1 ) with a second oversteer threshold (Threshold_OS_ 2 );
(e) calculating a second indicator of loss of grip (P 2 ) which is a partial derivative
(
P
2
=
∂
ψ
.
∂
γ
lat
)
of the driving parameter ({dot over (ψ)}) relative to lateral acceleration (γ lat ) of the vehicle;
(f) comparing a value of the second indicator (P 2 ) with a third oversteer threshold (Threshold_OS_ 3 );
(g) comparing respective signs of speed of rotation of the steering wheel ({dot over (α)}) and the driving parameter ({dot over (ψ)});
(h) detecting oversteer with countersteering (OS- 2 , α 3 ) if, cumulatively, the value of the first indicator is greater than the second oversteer threshold, the value of the second indicator is greater than the third oversteer threshold, and the signs of the speed of rotation of the steering wheel and of the driving parameter ({dot over (ψ)}) are opposite; and
(i) controlling the vehicle based on the detected oversteer with countersteering (OS- 2 , α 3 ).
7. The method according to claim 1 , wherein step (a) is inhibited while variation (∂α) of the variable (α) is zero.
8. The method according to claim 1 , wherein a signal which corresponds to the variable (α), a signal which corresponds to the driving parameter ({dot over (ψ)}), or a signal which corresponds to the first indicator (P 1 ), is filtered either temporally using a low-pass filter or using a validation filter which checks whether the signal remains less than an upper limit for a duration greater than a maintaining threshold.
9. The method according to claim 2 , wherein the driving parameter ({dot over (ψ)}) is provided by an electronic path stabilization system (ESP).