Rack force disturbance rejection
A road vehicle steering system which is adapted to use measurements and estimates characterising the motion of and forces acting on the vehicle and the steering system such as to derive an estimate of steering system disturbance force and to then use the latter estimate to establish a torque demand for the vehicle electronic power steering (EPS) which substantially compensates for this disturbance force.
1. A road vehicle steering system comprising a controller structured and configured to receive and use signals from one of a vehicle stability control (VSC) system and a vehicle electric power steering (EPS) system, said signals defining measurements and estimates of the motion of, and forces acting on, a vehicle and a steering system,
wherein a control algorithm uses said signals to derive an estimate of steering system disturbance force,
wherein said estimate of steering system disturbance force is used to establish a torque demand for said vehicle EPS system which substantially compensates for said disturbance force, and
wherein, said controller is further structured and configured to estimate an expected force on a steering rack within said vehicle steering system when an undisturbed vehicle undergoes an actual measured motion of said vehicle, said estimate of the expected force on said steering rack used to estimate the steering system disturbance force.
2. A steering system as claimed in claim 1 , which uses an estimate of steering system disturbance based upon estimated undisturbed steering rack force in order to establish said compensating torque demand.
3. A steering system as claimed in claim 2 , wherein undisturbed steering rack force ({circumflex over (M)} kp ) is established from
M
^
kp
=
l
3
F
^
yf
+
M
^
zf
=
(
l
3
+
C
mzf
C
f
)
F
^
yf
where 1 3 =caster offset
F yf =estimated lateral force at front axle
M zf =total kingpin movement
c mzf =self alinging moment coefficient
Cf=lateral tire force coefficient.
4. A steering system as claimed in claim 3 , wherein estimated steering rack force ({circumflex over (F)} rk ) is established from
F
^
rk
=
M
^
kp
l
2
where l 2 =perpendicular distance from steering rack to kingpin axis.
5. A steering system as claimed in claim 1 , wherein the disturbance force is calculated using a measurement of the total force on the steering rack.
6. A steering system as claimed in claim 1 , wherein the total force on the steering rack is estimated based just on an estimation of the lateral force at the front axle.
7. A steering system as claimed in claim 1 , wherein an estimate ({circumflex over (F)} yf ) of the lateral force at the front axle can be calculated using a bicycle model in an observer of the lateral force, and wherein the estimate (F yf ) of the lateral force at the front axle is calculated using:
F
^
yf
=
mbA
y
+
I
r
′
a
+
b
where a=distance from front axle to vehicle centre of gravity
b=distance from rear axle to vehicle centre of gravity
A y =lateral acceleration of vehicle
I=yaw inertia of vehicle
r′=yaw acceleration of vehicle.
8. A steering system as claimed in claim 7 , wherein the bicycle model is provided with two input signals, one being an estimate of the steer angle of the front wheels and the other being a scaled error between the lateral acceleration and/or yaw rate measured on the vehicle and those predicted by the bicycle model.