Automatic stability control system for a vehicle using an invariant characterizing any tire
A system is presented for controlling the stability of a vehicle. The system includes a controller using a slip parameter G OPT corresponding to a predetermined value of the coefficient of friction μ or a controller maintaining the functioning of the tire at an optimal value δ OPT of the drift angle corresponding to the maximum value of the draft thrust F target . The controller performs the following operations (where X is either the slip G or the drift angle δ, and Y is either μ or F). Estimations or measurements are determined for at least one pair of values (X i , Y i ). The corresponding values of the slope α i are determined for the straight line passing through the origin (X i , Y i ). Coefficients A p are calculated by direct calculation or by a regression from a sufficient number of pairs with (α i , X i ) so as to model a variation curve α i =f(X i , A p ). A target value X Target is calculated by using a predetermined invariant “Invt”.
1. A system for controlling the stability of a vehicle having wheels, including means for imparting a longitudinal force to a tire intended to roll on the ground, means for modulating the longitudinal force, and comprising a controller using at least a slip parameter G Opt corresponding to a predetermined value of a coefficient of friction μ, the system including means for performing the following operations:
(a) on each activation of the means for imparting a longitudinal force to the tire, for at least two different levels “i” of the longitudinal force each corresponding to one slip G i , on condition that there is no loss of grip, determining values of the coefficient of friction μ i ;
(b) determining a slope α i of a straight line passing through the origin and through (CT i , μ i );
(c) calculating coefficients A p by direct calculation or by a regression from a selected number of pairs with (α i , G i ) so as to model a variation curve α i =f(G i , A p );
(d) calculating the optimum slip G Opt by using a predetermined Invariant “Invt”; and
(e) acting on the means for imparting a longitudinal force to the tire so as to maintain the slip at optimum value G Opt .
2. A system for controlling the stability of a vehicle according to claim 1 , in which the Invariant is determined as follows:
Invt
=
μ
G
(
G
max
)
μ
G
(
p
·
G
max
)
,
with p having a positive value less than 1.
3. A system for controlling the stability of a vehicle according to claim 1 , in which the means for modulating the longitudinal force acts on a brake control.
4. A system for controlling the stability of a vehicle according to claim 1 , in which the means for modulating the longitudinal force acts on a driving torque at the wheels.
5. A system for controlling the stability of a vehicle having wheels and at least one tire intended to roll on the ground, the vehicle being equipped with a system for controlling a selected parameter λ depending on commands imparted by a vehicle driver to a control means and depending on commands delivered by a path controller aimed at maintaining functioning of the tire at a predetermined target value of drift thrust F target , the path controller using at least one optimum value δ Opt of drift angle corresponding to a maximum value of the drift thrust F target , the system for controlling the stability of a vehicle including means for performing the following operations:
(a) on each activation of the system controlling said parameter λ for at least two different levels “i” of drift angle δ, recording various values of F Yi , and the estimated or measured drift angle δ i ;
(b) determining a slope α i of a straight line passing through an origin and through (δ i , F Yi );
(c) calculating coefficients A p by direct calculation or by a regression from a selected number of pairs with (α i , δ i ) so as to model a variation curve α i =f(δ i , A p );
(d) calculating the optimum drift angle value δ Opt associated with the maximum value of the drift thrust F target by using a predetermined Invariant “Invt”; and
(e) producing a warning signal when the drift angle δ is close to δ Opt .
6. A system for controlling the stability of a vehicle according to claim 5 , in which the Invariant is determined as follows:
Invt
=
F
δ
(
δ
Opt
)
F
δ
(
p
·
δ
Opt
)
,
with p having a positive value less than 1.
7. A system for controlling the stability of a vehicle according to claim 5 , in which the parameter λ relates to steering of the wheels and, in the event of the occurrence of the warning signal, the system for controlling the parameter λ is acted upon so as to maintain the drift angle δ at the optimum value δ Opt .
8. A system for controlling the stability of a vehicle according to claim 5 , in which, in the event of the occurrence of the warning signal, a speed of the vehicle is limited or reduced.
9. A system for controlling the stability of a vehicle according to claim 1 or 5 , in which two particular coefficients A p , the coefficients A and B, are calculated by a regression chosen from the group consisting of a linear regression and an exponential regression.
10. A system for controlling the stability of a vehicle according to claim 2 or 6 , in which the value of p is between 0.25 and 0.75.
11. A system for controlling the stability of a vehicle according to claim 10 , in which p is 0.5.
12. A system for controlling the stability of a vehicle having wheels and at least one tire intended to roll on the ground and capable of functioning when subject to drift, including means for predicting a value of a drift angle δ of the tire where a lateral force is maximal, the means for predicting comprising means for performing the following operations:
(a) determining estimations (δi, F i ) for at least one pair “i” of values;
(b) determining the corresponding values of a slope α i of a straight line passing through an origin and through (δ i , F i );
(c) calculating coefficients A p by direct calculation or by a regression from a selected number of pairs with (α i , δ i ) so as to model a variation curve α i =f(δ i , A p );
(d) calculating a value of drift angle δ Opt by using a predetermined Invariant “Invt”;
(e) producing a warning signal when the drift angle δ is close to δ Opt ; and
(f) in the event of the occurrence of the warning signal, limiting or reducing automatically the vehicle speed.
13. A system for controlling the stability of a vehicle according to any one of claims 1 , 5 and 12 , in which the means for performing operation (d) includes means for using the Invariant “Invt” as an adjustment variable.
14. A system for controlling the stability of a vehicle according to claim 2 , in which the value of “Invt” is about 0.58.
15. A system for controlling the stability of a vehicle according to claim 1 , in which the slope α i is determined by direct calculation according to: α i =μ i /G i .
16. A system for controlling the stability of a vehicle according to claim 1 , in which the slope α i is determined by carrying out a regression.
17. A system for controlling the stability of a vehicle according to claim 1 , in which a linear regression is carried out according to:
∑
GG
=
∑
G
j
2
,
∑
G
μ
=
∑
G
j
·
μ
j
,
α
i
=
∑
G
μ
∑
GG
.
18. A system for controlling the stability of a vehicle according to claim 1 , in which the two particular coefficients A p , the coefficients A and B, are calculated by the following linear regression, applied to “n” measured or estimated points:
A
Lin
=
n
·
∑
G
·
α
-
∑
G
·
∑
α
n
·
∑
G
2
-
(
∑
G
)
2
,
B
Lin
=
∑
α
·
∑
G
2
-
∑
G
·
α
·
∑
G
n
·
∑
G
2
-
(
∑
G
)
2
.
19. A system for controlling the stability of a vehicle according to claim 18 , in which G Opt is calculated as follows:
G
Opt
=
B
Lin
A
Lin
·
1
-
Invt
1
-
p
·
Invt
,
with α=A Lin ·G+B Lin .
20. A system for controlling the stability of a vehicle according to claim 19 , in which in addition the value of μ corresponding to G Opt is determined as follows:
μ=μ coeff — lin ·G Opt ·( A Lin ·G Opt +B Lin ).
21. A system for controlling the stability of a vehicle according to claim 1 , in which the two particular coefficients A p , the coefficients A and B, are calculated by the following exponential regression, applied to “n” measured or estimated points:
A
Exp
=
n
·
∑
G
·
L
n
(
α
)
-
∑
G
·
∑
L
n
(
α
)
n
·
∑
G
2
-
(
∑
G
)
2
,
B
Exp
=
∑
L
n
(
α
)
·
∑
G
2
-
∑
G
·
L
n
(
α
)
·
∑
G
n
·
∑
G
2
-
(
∑
G
)
2
.
22. A system for controlling the stability of a vehicle according to claim 21 , in which G Opt is calculated as follows:
G
Opt
=
L
n
(
Invt
)
p
·
A
Exp
,
with
α
=
ⅇ
A
Exp
·
G
+
B
Exp
.
23. A system for controlling the stability of a vehicle according to claim 22 , in which μ max is determined as follows:
μ max =μ coeff — exp ·G Opt ·e/ A exp ·G Opt +B Exp .
24. A system for controlling the stability of a vehicle according to claim 5 , in which the slope α i is determined by direct calculation according to: α i =F i /δ i .
25. A system for controlling the stability of a vehicle according to claim 5 , in which the slope α i is determined by carrying out a regression.
26. A system for controlling the stability of a vehicle according to claim 5 , in which a linear regression is carried out according to:
∑
δδ
=
∑
δ
j
2
,
∑
δ
F
=
∑
δ
j
·
F
j
,
α
i
=
∑
δ
F
∑
δδ
.
27. A system for controlling the stability of a vehicle according to claim 5 , in which the two particular coefficients A p , the coefficients A and B, are calculated by the following linear regression, applied to “n” measured or estimated points:
A
Lin
=
n
·
∑
δ
·
α
-
∑
δ
·
∑
α
n
·
∑
δ
2
-
(
∑
δ
)
2
,
B
Lin
=
∑
α
·
∑
δ
2
-
∑
G
·
α
·
∑
δ
n
·
∑
δ
2
-
(
∑
δ
)
2
.
28. A system for controlling the stability of a vehicle according to claim 27 , in which δ Opt is calculated as follows:
δ
Opt
=
B
Lin
A
Lin
·
1
-
Invt
1
-
p
·
Invt
,
with α=A Lin ·δ+B Lin .
29. A system for controlling the stability of a vehicle according to claim 28 , in which in addition F Target is determined as follows:
F Target =F coeff Lin ·δ Opt ·(A Lin ·δ Opt +B Lin ).
30. A system for controlling the stability of a vehicle according to claim 5 , in which the two particular coefficients A p , the coefficients A and B, are calculated by the following exponential regression:
A
Exp
=
n
·
∑
δ
·
L
n
(
α
)
-
∑
δ
·
∑
L
n
(
α
)
n
·
∑
δ
2
-
(
∑
δ
)
2
,
B
Exp
=
∑
L
n
(
α
)
·
∑
δ
2
-
∑
δ
·
L
n
(
α
)
·
∑
δ
n
·
∑
δ
2
-
(
∑
δ
)
2
.
31. A system for controlling the stability of a vehicle according to claim 30 , in which δ Opt is calculated as follows:
δ
Opt
=
L
n
(
Invt
)
p
·
A
Exp
,
with
α
=
ⅇ
A
Exp
·
δ
+
B
Exp
.
32. A system for controlling the stability of a vehicle according to claim 30 , in which in addition F Target is determined as follows:
F Target =F coeff — exp·δ Opt ·e a exp ·δ Opt +A target .