Evolution of Electronic Control Systems for Improving the Vehicle

Evolution of Electronic Control Systems for Improving the Vehicle
Evolution of Electronic Control Systems for
Improving the Vehicle Dynamic Behavior
Anton T. van Zanten
Robert Bosch GmbH
CS-AS/ESF
P.O. Box 30 02 40
70442 Stuttgart
GERMANY
Phone: (+49) 711 811 1860
Fax: (+49) 711 811 1862
E-mail: [email protected]
Starting with ABS (Antilock Brake System) the steps towards integrated active safety
systems dealing with vehicle dynamics is shown. While ABS and TCS were initially designed as open loop controllers for the lateral vehicle motion a first approach towards
closed loop control of the lateral vehicle motion for active safety systems was realized by
ESP (Electronic Stability Program. Estimation algorithms and model following control are
used with ESP to compensate for the lack of sensors. Since 2001 ESP is available for
cars with an electro hydraulic brake system. The extension of ESP in combination with
active front steering is expected to enter the market in 2003.
Keywords: Safety, Handling, Observers, Yaw moment, Brakes, Steering, Suspension
1. INTRODUCTION
The loss of yaw response of the vehicle to
steering inputs during full braking while the wheels
are locked has lead to very early investigations (in
th
the beginning of the 20 century) to prevent wheel
lock. ABS, which can prevent this can preserve a
high level of handling performance during full
braking [1]. Similarly, if the driven wheels spin due
to excess engine torque handling becomes difficult,
particularly if the driven and steered wheels are
identical. This has lead then to the introduction of
traction control systems which preserve also a high
level of handling performance during driving with
excess engine torque. Since neither the steering
angle nor the yaw moment on the car were available, even a feed forward handling control by control of the brake and engine was not possible. With
the introduction of ESP these important quantities
became available. Together with the measurement
of the yaw velocity, a feed back control of the vehicle handling could be introduced [2], [3], [4].
The difficulty of handling of a car at the physical
limit can be explained by the research contribution
of Shibahata [5]. In this paper the β-method was
developed to analyze the influence of the slip angle
of the vehicle on its maneuverability. One important
result is, that the sensitivity of the yaw moment on
the vehicle w.r.t. changes in the steering angle
decreases rapidly as the slip angle of the vehicle
increases (Fig. 1). Large slip angles mean here
values at which the µ-slip angle curve of the tire
has its maximum. Therefore, on dry surfaces vehicle maneuverability is lost at vehicle slip angle values larger than approximately 15°, whereas on
packed snow this value is approximately 4°. Inagaki [6] has shown the inherent instability of the
vehicle motion for certain combinations of the slip
angle and its velocity (Fig. 2, dark areas). For other
combinations, the slip angle velocity will return to
zero and lead to a stable value of the slip angle
(white area). However, for increasing steering angles the stability area shrinks to become eventually
zero: the vehicle motion is unstable for all combinations of slip angle and slip angle velocity.
Fig. 1: Yaw moment in dependence of the slip angle for
different steering angles (source: [5])
The design of vehicles should therefore center
around the normal driver. Professional testers, test
engineers and endurance testers are not at all
typical for the real population of normal drivers, and
therefore it is not unrealistic that they judge vehicle
behavior according to criteria that are not relevant
to the large number of average drivers.
Accidents are often said to be in 90% of all
cases the result of driver errors. Käppler [8] however, notes that these statements must be taken
very carefully since they originate from the police
jargon. According to Brown [9] drivers are only in
19% of all cases responsible for the accidents.
Vehicles are in 31% and the environment in 50% of
all cases responsible for the accidents. Rompe et
al [10] investigated the activities of drivers in critical
driving situations just before the accidents happened. He found that steering was most often
(50%) involved. Similarly Edwards et al [11] found
that evasive maneuvers took place just ahead of
48% of all accidents, 50% just ahead of all collisions and 64% just ahead of all accidents in which
the vehicle left the road.
Fig. 2: Phase plane of the vehicle slip angle and its
angular velocity at zero steering angle (source: [6])
Another reason for the problems normal drivers
have in these situations is, that their driving experience is limited largely to driving well within the
physical limit of adhesion. Förster [7] has analyzed
this situation and set up some important rules.
First, the driver can never recognize the coefficient
of friction between the tires and the road and he
has no idea of the vehicle's lateral stability margin.
Second, if the limit of adhesion is reached the
driver is caught by surprise and very often reacts in
a wrong way and usually steers too much. This, he
notes, is the real weak point in the system vehicledriver-environment. Third, in traffic situations the
need for the driver to act thoughtfully has to be
minimized. Förster therefore comes to the conclusion that the concept of the vehicle including the
tires and the suspension should very strongly account for the normal human behavior. Deviations
from normal vehicle behavior that are inherent to
the vehicle design must be controlled and reduced
to negligible differences. Unexpected vehicle motions may lead to panic reactions of normal drivers.
Fig. 3: Typical severe accident resulting from car spin
(source: [12])
Statistics from the German Association of Insurance Companies (GdV) [12] show that severe
accidents typically involve spinning cars (Fig. 3).
Furthermore, while the number of people killed
decreases continuously, the total number of people
injured and killed remained approximately constant
during an 8-year period (Fig. 4).
Contrary to the common belief that spinning of
cars mainly occurs on slippery roads and at high
speeds the statistics show, that by far most severe
accidents occur on dry roads and at speeds between 60 km/h and 100 km/h.
A first approach to solve the problem of handling at the physical limit is given by van Zanten in
[13]. Here the brake slip distribution is investigated
during full braking while cornering which results in
a reduced deviation of the vehicle motion from a
desired nominal motion and simultaneously in a
minimum stopping distance. Optimal control theory
was used to get an idea of the best tuning of the
brake control. It is shown, that given the momentary slip angle of a tire the brake slip of that tire
must not necessarily be optimized to get the largest
possible brake force.
Fig. 5: Typical µ-slip curve
Fig. 4: Number of people injured and killed in Germany (source: [12])
An industrial approach is given by Heess [14].
He describes ways in which available control systems like Antilock Brake Systems (ABS) and traction control systems (ASR), suspension control
systems and steering control systems can be used
as subsystems for a superimposed vehicle dynamics control system. His suggestion applied to a
full four wheel ABS/ASR led to the development of
the Vehicle Dynamics Control system ESP of
Bosch which controls the motion of the vehicle not
only during full braking but in all situations like partial braking, coasting, acceleration and engine drag
on the driven wheels. Its area of operation is
therefore extended well beyond that of ABS (full
braking control) and TCS (traction control).
The question how much the wheel brake pressures should be reduced is solved by the control
algorithm which monitors the wheel speeds but not
the brake pressures (Fig. 6). If a wheel decelerates
too fast (Phase 2), then its brake pressure is reduced and if because of the pressure reduction the
wheel accelerates again then the pressure is increased again. The increase will be done in a
stepwise manner in order to reduce the influence of
the transients in the wheel behavior (phase 7) and
the pressure may be reduced immediately if the
deceleration becomes large (phase 8). Thus the
average slip value is kept close to λ k (Fig 5).
2. ANTILOCK BRAKE SYSTEM
Locked wheels generate forces on the car which
are in a direction opposite to the lineal wheel motion. Changing the steering angle has virtually no
effect on the force vectors on the wheels. If the
brake pressure induced by the driver is such that
the wheels lock, then the brake pressure must be
reduced to regain steerability.
For this task ABS uses a hydraulic unit which
has electromagnetic valves to keep the pressure in
the wheel brakes below the level induced by the
driver
The main task of the control algorithm is to keep
a high level of braking force while at the same time
keeping a sufficient level of lateral force generation
by steering to preserve a high level of handling
performance. This information is not readily available and thus ABS relies on assumptions about the
shape of the µ-slip curve and the wheel behavior
during braking and cornering (Fig. 5).
Fig. 6: ABS control concept
4. ELECTRONIC STABILITY PROGRAM
3. TRACTION CONTROL SYSTEM
As a first approach to control the driving forces
on the driven wheels one may try to control the tire
slip to the value λ k of the µ-slip curve (Fig. 5) by
the same concept chosen for ABS. This however
fails, since not only the rotating inertia of the driven
wheels is too large for a significant change in the
wheel acceleration (because of the engaged engine and transmission) but also the engine torque
is nonlinearly dependent on the engine speed, and
thus also on the wheel speed. Therefore during
traction control it is not possible to clearly differentiate between the stable and the unstable region of
the µ-slip curve.
Feedback control of the vehicle motion is possible by extending the traction control system with
four additional sensors: steering wheel angle,
brake pressure, yaw rate and lateral acceleration.
Since the nominal trajectory desired by the driver
is unknown, the driver's inputs are taken to obtain
nominal state variables that describe the intended
vehicle motion instead. These inputs are the
steering wheel angle, the engine drive torque as
derived from the accelerator pedal position and the
brake pressure.
The handling performance of the car can be improved if in dependence of the steering wheel angle the yaw moment on the car can be controlled.
The main task of ESP as an active safety system
is, however, to limit the slip angle of the vehicle β
in order to prevent vehicle spin.
Fig. 7: Fundamental blocks of the traction control system. 1: Wheel Speed Sensor, 2: Hydraulic Unit,
3: TCS-ECU, 4: Throttle-ECU, 5 Engine-ECU
Fortunately, for single axle driven cars, the
speeds of the non driven wheels may be used to
compute the free rolling speed of the driven
wheels. Therefore slip control of the driven wheels
becomes now possible. The only unknown variable
then is the value of λ k which may vary with the
road surface and the tire type and state. Traction
control uses a mean value of λ k and increases it
for low car speeds. For four wheel driven cars, the
determination of the free rolling speed is not easily
possible and only estimates can be made which
depend on the distribution of the engine torque
between the front and the rear axle.
Drive slip can be reduced by reducing the engine
torque or additionally by applying the brakes at the
driven wheels (Fig. 7). By closing the throttle valve,
reducing the spark advance or inhibiting fuel injection the engine torque may be reduced. Because of
engine and exhaust emission regulations, all three
interventions are not always and at the same time
available. Furthermore, the time constants of these
three different interventions are mutually different
and vary with the state of the engine (cold start, low
ambient temperature, state of the catalyst etc.).
Similarly the time constant of the brake pressure
varies, in particular with the temperature of the
hydraulic unit.
Fig. 8: Yaw moment change by slip control
ESP can control the yaw moment on the car by
controlling the value of the slip at each wheel. This
can be shown by the influence of some brake slip
value λ0 at the left front tire of a free rolling car in a
right turn (Fig. 8). FR(λ=0) is the lateral force on the
free rolling tire. Because of the brake slip λ0 the
lateral force will be reduced to FS(λ0) where it is
assumed, that neither the normal force FN nor the
tire slip angle α0 are changed. As a result of the
brake slip the brake force FB(λ0) is generated.
FR(λ0) is the resultant force on the tire, which is the
vectorial sum of FS(λ0) and FB(λ0). If the tire friction
limit is reached, the magnitudes of FR(λ=0) and
FR(λ0) are approximately equal.
The influence of brake slip λ is now obvious: a
change in the brake slip value results in a rotation
of the resultant force on the tire. As a result of the
rotation the yaw moment on the car is changed.
However, simultaneously the lateral force and the
longitudinal force on the car are influenced. The
control concept determines by what amount the slip
at each tire shall be changed to generate the required change in the yaw moment. Usually it is
required that the driver must not have the impression that with ESP the car is slower than without
ESP.
its slip angle is small then the estimate can be
readily obtained by a simple time integration
t
a

&
β(t ) = β 0 + ∫ βdt = β 0 + ∫  y − ψ& dt
v

0
0 v
t
Offset and other errors in the sensor and estimated signals may quickly lead to large errors in
the estimate. Furthermore, during full braking the
car deceleration can not be neglected. Therefore,
during full braking an alternative estimate of the slip
angle based on an observer is used.
The observer is based on a full four wheel model
of the car and uses two dynamic equations, one for
the yaw velocity and the other for the lateral velocity of the car ([15]). These equations are rearranged and discretized to be used as the model for
a Kalman filter. Since the yaw velocity is measured, the solution of the differential equation of the
yaw velocity is used to derive the measurement
equation.
For these equations the longitudinal force FB at
any wheel is required and can be estimated by the
following generic equation
FB = c p ⋅
Here
p whl M CaHalf J whl d
−
+ 2 ⋅ v whl
R
R
dt
R
c p denotes a known brake constant, p whl
denotes the brake fluid pressure in the brake wheel
cylinder, R denotes the known tire radius,
M CaHalf denotes half of the engine torque at the
Fig. 9: Simplified block diagram of the ESP control
The vehicle dynamics controller part of ESP
(Fig. 9) constitutes the upper part of a hierarchical
control. Output are the nominal tire slips λ Noi . In
the lower part the slip values of the tires are controlled. The vehicle dynamics controller part consists of several processing blocks. On the top left
the motion desired by the driver is derived from his
inputs by a linear bicycle model (which uses a linear relationship between the slip angle and the
lateral force of the tire). On the top right the motion
of the car is measured and missing state variables
are estimated.
As a first approach to estimate the slip angle of
the car the derivative of the slip angle is used:
1
(a y ⋅ cos β − a x ⋅ sin β )
β& = −ψ& +
vv
This estimate is valid if the pitch and roll angles
of the car are neglected and furthermore, if the car
moves on a horizontal plane. In this equation a y is
a x is its
&
longitudinal acceleration, v v is its velocity and ψ
the lateral acceleration of the car and
is its yaw velocity. If the car velocity is constant and
axle,
J whl denotes the known moment of inertia of
the
wheel
about
its
axis
of
rotation
and
v whl denotes the wheel speed which is the product
of the wheel angular velocity and the tire radius.
The engine torque value can be obtained from the
engine management system, while the rotational
wheel velocity is measured by the wheel speed
sensor. Finally by modeling the hydraulic unit the
wheel brake pressure is estimated at each wheel.
The side forces are not readily available.
Therefore a tire model is used. Specifically, the
HSRI tire model as described in [15] is used which
allows for a simple relation between the lateral and
the longitudinal force.
FS =
C α ⋅ tan α
⋅ FB
Cλ ⋅ λ
C λ and C α are the slip and
cornering stiffness of the tire and λ and α are the
In these equations,
tire slip and slip angle respectively.
The estimate of the lateral velocity by the Kalman filter is robust to tire changes as only the ratio
of the lateral and longitudinal tire stiffness is used.
For winter tires the ratio is nearly the same as for
summer tires. The same is true for new and worn
tires, conventional and wide tires etc. Thus both
evaluations of the slip angle are more or less insensitive to changes in the tire properties.
Unfortunately the vehicle slip angle estimation is
not always sufficiently accurate and the confidence
level of its value is sometimes low. Therefore, the
vehicle dynamics controller uses additionally a
model following control for the yaw velocity of the
car, for which the already mentioned linear bicycle
model is taken. Output of the linear bicycle model
& No . Thus a
is the nominal value of the yaw rate ψ
the front axle worn and on the rear axle new tires
are mounted (Fig. 12). In such cases the vehicle
& No is obfirst value for the nominal yaw velocity ψ
tained (Fig. 10).
& No =
ψ
vx ⋅ δw
 v
l ⋅ 1 +  x
  v ch



2



The wheel base l is a simple geometric parameter while the vehicle forward velocity v x is
estimated by the brake slip controller.
Fig. 11: Nominal yaw velocity from the full four wheel
model with nonlinear new and worn summer and winter
tires (steering wheel angle 60°)
behavior deviates significantly from the behavior of
the linear bicycle model (Fig. 10) and ESP interventions can be expected for vehicle maneuvers
which are well within the physical limit.
Fig. 10: Nominal yaw velocity from the linear bicycle
model
The characteristic speed
on the lateral tire stiffness
v ch depends mainly
C α of the tires. There-
fore, the nominal yaw velocity changes with the tire
type, make and state (new or worn). This change
may occur suddenly if new tires are mounted. The
model following control is thus sensitive to changes
in the tire stiffness and ESP may suddenly change
its behavior. This will be shown below. ESP must
therefore be checked to correctly perform with all
released tires.
Since the lateral acceleration of the car can not
exceed the maximum coefficient of friction between
the tire and the road µ , the nominal yaw velocity
must be limited to a second value by the following
relation (see the hyperbola in Fig. 10)
ψ& No ≤ µ ⋅ g v v
For summer tires the nominal yaw velocity is
different from that of winter tires (Fig. 11). Similarly,
for worn tires the yaw velocity is different from that
of new tires. The vehicle becomes oversteer if on
Fig. 12: Nominal yaw velocity from the full four wheel
model with nonlinear summer and winter tires, with
worn tires at the front axle and new tires at the rear axle
(steering wheel angle 60°)
A first nominal limit value for the slip angle of the
car (Fig. 9) is chosen as discussed using the Beta
method in dependence of the coefficient of friction
between the tires and the road. This value is reduced in dependence of the velocity of the car to a
second value β No , in order to improve the support
for the driver at higher speeds.
If the state of the car as described by its yaw
velocity and its slip angle differs from its nominal
state, then the vehicle dynamics controller checks
if this difference is within some tolerable dead
zone. If not, a yaw moment is generated to reduce
this difference to within this tolerable dead zone.
Slip is controlled by the brake slip and traction slip
controllers as described in [15].
5. ELECTROHYDRAULIC BRAKE SYSTEM
The Electro Hydraulic Brake System (EHB, also
called SBC: Sensotronic Brake Control) is a brake
by wire system, where the brake pressure in the
wheel brakes are controlled in accordance with the
brake master cylinder pressure using proportional
valves.
Not only during maneuvers at the physical limit,
but also during all brake maneuvers EHB improves
the handling performance of the car. EHB allows
for complete flexibility in choosing the brake force
distribution in any situation, so that the brake force
distribution can be chosen in dependence of the
braking situation as a feed forward control. Thus
almost neutral handling performance can be obtained during any combined braking and steering
maneuver. Thus ESP is extended to control the
vehicle handling well within the physical limit (Fig.
15).
Fig. 13: Block diagram of the Electro Hydraulic
Brake System
Fig. 13 shows the concept of EHB. ESP (here
called VDC) is housed in the block named “Brake
functions”. Essential for the control of the vehicle
dynamics is, that the wheel brake cylinder pressures are measured. Furthermore, pressure can be
very rapidly increased resulting in very fast active
brake slip interventions.
Fig. 15: Improvement of cornering stability by improved brake force distribution (Source: [16])
Although the ESP control performance is improved, limits on the improvement are set by the
safety concept. It is not possible to immediately
react on rapid changes in the sensor signals since
the possibility of signal failures can not be disregarded. Internal tests like that of the yaw rate sensor triggered by the ECU introduce delays in the
signal analysis like the estimation of its time derivative (Fig. 16). Thus the limits in the reliability of
the signals limit the performance of the EHB.
Fig. 14: Improvement of slip angle control of ESP
during a double lane change maneuver by SBC
(Source: [16])
Since the estimation of the brake pressure at the
wheels is no longer required, the confidence level
of the brake force estimation is higher than before.
This improves the estimation and control of the
vehicle slip angle as can be seen from the results
of a double lane change maneuver (Fig. 14). Early
interventions are possible so that they are less
vicious and more comfortable.
Fig. 16: Yaw rate signal with superimposed test
signal
To partially compensate this drawback a new
concept “suspicion of failure” was introduced. Only
if a signal is suspicious of a possible failure the
intervention of EHB is delayed by increasing the
dead zone in the yaw rate (Fig. 9) and by reducing
the pressure gradient. The general rule for the
safety concept is to better not intervene where it
would be beneficial for the driver then to surprise
him with an intervention resulting from a corrupted
sensor signal.
important performance potential is lost. Identification, adaptation and learning control is not feasible.
6. FUTURE SYSTEMS
If an active steering system is available, ESP
can also use this system to control the handling
behavior of the car, not only in limit situations but
also during normal driving. Fig. 17 shows the concept of a first realization of such a system with
which ESP can modify the steering angle [17]. Another system is shown in Fig. 18, in which ESP can
modify the normal force distribution on the tires by
control of special anti roll bars [17].
Fig. 19: Coexistence approach using the brakes,
active steering and active suspension to control
vehicle handling
Later proposals use a central yaw rate controller
which takes the different properties of the actuators
in consideration (Fig. 20). This is particularly important if the brake actuation is no more much
slower than the steering actuation as is the case
with EHB. The superimposed central yaw rate control uses then the subsystems as intelligent actuators.
Fig. 17: Active steering system with incremental
steering angle
Fig. 18: Active suspension with split anti roll bar
Each of these systems can influence the yaw
moment on the car. The question is now, how to
implement the total yaw rate control. Early proposals were based on the observation of the different
intervention bandwidths of the brake, engine, active
steering and active suspension actuators. Each
actuator had its own yaw rate controller (Fig. 19).
The controller gains had to be tuned such that the
interference between the interventions did not lead
to undesirable or even unstable vehicle behavior.
The charm of this approach is, that to some extend
the development of the controllers can be done
independent of each other. Obviously, if the controller gains must be drastically reduced to guarantee the peaceful coexistence of the controllers,
Fig. 20: Integration approach using the brakes active
steering and active suspension to control vehicle
handling
EHB and the future systems can influence the
handling behavior of the car also in situations
where the physical limit is not reached. If the passive car handling changes, e.g. because of
changes in the tire properties, than these systems
can restore normal vehicle behavior by interventions which are not noticed by the driver. These
systems therefore have the potential of making
handling robust against parameter changes. However, in order to fully exploit the high speed of the
interventions, the test cycle of the yaw rate signal
(Fig. 16) must be avoided which results in the requirement for redundant yaw rate signals.
[9]
7. CONCLUSION
ESP has extended active safety systems from a
wheel behavior control (ABS, TCS) to a vehicle
behavior control resulting in an increased active
safety of the car. A practical system solution requires a model following yaw rate control which
introduces robustness problems with changing
vehicle parameters like the tire properties. Future
systems may compensate these changes to a robust vehicle behavior but the safety concept must
be changed to fully exploit the fast actuator interventions.
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