1Brake-by-Steer Concept
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Brake-by-Steer ConceptSteer-by-wire application with independently actuated wheels used for stopping a vehicle
Bas Jansen 25-03-2010
Master Thesis PresentationDepartment of Precision and Microsystems Engineering
2Brake-by-Steer Concept
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Content1. Introduction
- SKF
- Drive-by-wire
- Brake-by-steer concept
2. Modeling the Brake-by-Steer system- Tire model
- Vehicle model
- Brake-by-steer cases
3. Implementation on a Go-Kart- Go-kart introduction
- Design Implementations
4. Test Results - Braking performance
- Lateral behavior
5. Conclusion & Recommendations
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1.Introduction
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IntroductionSKF - Svenska Kullagerfabriken AB
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Introduction
SKF European Research CentreNieuwegein
SKF - Svenska Kullagerfabriken AB
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Introduction
Conventional steering system
Steer-by-wire with independently actuated wheels
What is Steer-by-WireSteering wheel
Steering shaft
Rack & Pinion
Steering ControllerSensor & actuator
Sensor & actuator
Data transport
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Introduction
Replace hydraulic brake system with an individually electrically actuated brake system
What is Brake-by-WireElectro mechanical braking actuators
Braking controller
Brake pedal & sensor
Data transport
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IntroductionWhy By-Wire
Modular design provides design freedom, reduces weight and requires less space
Personalized and adaptive driving experience by varying control settings
Increased safety potential in combination with intelligent vehicle safety systems
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Introduction Safety challenge for By-Wire
Increase safety level:• Implemented redundant components
• Assign secondary function to initial primary function of a sub system
• Steer by uneven distributed brake force• Brake-by-steer concept
Primary systems with redundant back-up systems
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Introduction
Research Question:
Is it possible to stop a vehicle with the brake-by-steer concept and how does
this influence the steering controllability?
Brake-by-Steer concept Position the front wheels such that
they generate a braking force
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2.Modeling the Brake-by-Steer
system
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Brake-by-Steer Modeling
Model build-up:• Tire model • Vehicle (kart) model• Brake-by-Steer cases
Model construction
tireF
Width
Lengthm, I
vehicleV
tireV
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Brake-by-Steer Modeling
Tire behavior:• No resistance force in longitudinal direction (x)• Resistance force in lateral direction (y)
Slip angle:The angle between tire’s direction of travel (V) and the direction towards which it is pointing (x)
Tire modeling
tanv
u
, ,
x y Tire coordinatesu v Tire velocitiesV Tire velocity vector
Slip angle
,y v
,x u
V
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0 5 10 15 20 25 300
0.2
0.4
0.6
0.8
1
Slip angle (deg)
Late
ral t
ire f
orce
(N
)Brake-by-Steer Modeling Tire modeling
Slip angle
Late
ral T
ire F
orc
e
, ,
Lat
x y Tire coordinatesu v Tire velocitiesV Tire velocity vector
Slip angleF Lateral tire force
C
1
, for , for
sin tan
saturateLat
saturate saturate
lat
CF
F
F d c b
lateral tireF
,y v
,x u
V
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Brake-by-Steer Modeling Vehicle Model
V
,y v
,x u
lateral tireF
X
Y
Z
Fu vr
mF
v urm
Mr
I
X
Y
Vehicle equations of motion
m, IV
u
v
cos
sinX lateral tire
Y lateral tire
F F
F F
X
Y
Tricycle model
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Brake-by-Steer Modeling Brake-by-Steer cases
Symmetric Asymmetric
Toe-in
Toe-
out
Steering angle Left S
teer
ing
ang
le R
ight
Toe-out
Toe-in
Toe-out
Toe-in
Steady state straight line driving brake force
Bra
ke f
orc
e [
N]
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Brake-by-Steer Modeling
tireF
X leftFtireF X rightF
tireF
X right X leftvehicle t F FM
Steering to the right results in vehicle moment to the left
Steering to the right results in vehicle moment to the right
Toe-in steer to the right Toe-out steer to the right
t
X right X leftvehicle t F FM
X leftF X rightF
t
Effect of longitudinal vehicle force for vehicle heading
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Brake-by-Steer Modeling
tireF
Y leftFtireF Y rightF
tireF
Y Y left Y rightF FF
Steering to the right results in lateral vehicle force to the left
Steering to the right results in lateral vehicle force to the left
Toe-in steer to the right Toe-out steer to the right
X leftF X rightF
Y Y right Y leftF FF
Effect of lateral vehicle force for vehicle heading
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Brake-by-Steer Modeling Theoretical Results – Lateral Behavior
Steering angle Left
Ste
erin
g an
gle
Rig
ht
Su
mm
atio
n L
ate
ral V
eh
icle
Fo
rce
[N
] Symmetric toe equilibrium
Asymmetric toe equilibrium
Toe-out
Toe-in
There is no asymmetric toe-out equilibrium line
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3.Implementation on a Go-Kart
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Implementation on a Go-Kart Go-Kart Introduction
Kart specific features:• No individual wheel suspension• Flexible tube frame acts as suspension• Fixed rear axle • Caster angle and kingpin inclination
Caster angle
Kingpin inclination
Caster angle
Left tire side view
Rotational path
Remove mechanical linkage
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Implementation on a Go-Kart
Steering WheelElectromechanical Modifications
Steering wheel actuator
Steering wheel angle sensorSteering shaft
Toe handle
• Absolute magnetic encoder measures steering angle• DC motor provides force feedback sense • Toe levers measure toe angle setpoints
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Implementation on a Go-Kart Electromechanical ModificationsWheels
Extension brackets
Motor + gear
Absolute angle sensor
Encoders
• DC motor positions the wheels• Encoder used as control position signal • Absolute angle sensor used homing during initialization
Stub axle
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Implementation on a Go-Kart Control algorithm
CController +/-
K
Feedback position control for wheel positions
Force feedback to steering wheel
Toe mode selection
Motor currentsForce
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Implementation on a Go-Kart Control algorithm
CController
K0
kartV
+/-
Mimic steering torque with speed dependent return to center torque
Feedback position control for wheel positions
Toe mode selection
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Implementation on a Go-Kart Implemented design
Electronics
Batteries
Left wheel actuation
Steering wheel actuation
Velocity sensor
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4.Test Cases and Results
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Test cases & Results
• Braking performance of the brake-by-steer concept • Lateral vehicle behavior during brake-by-steer maneuver
Test cases
Test track at SKF ERC Nieuwegein
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Test cases & Results Results – Braking Performance
0 20 40 60 80 100 120 140 160 180-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
Effective toe angle (deg)
Bra
ke f
orce
(kN
)
Toe in
Toe outOne wheel
Theoreticle data
,sin itire saturatebrake
i left rightF F
Theoretical maximum:
1.5 kN
0 1 2 3 4 5 6 7-6
-4
-2
0
2
4
6
8
10
Time (sec)
(Var
ious
)
Velocity (m/s)
Acceleration (m/s2)
Brake Force (kN)
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Test cases & Results Results – Lateral behavior
Calculated driven path for symmetric toe-in (30º) with steering offset of 2, 4, 6, 8 degrees to the right
0 0.5 1 1.5 2 2.5 3 3.5
-30
-20
-10
0
10
20
30
40
0 0.5 1 1.5 2 2.5 3 3.5-10
-5
0
5
Path for L: 38 (rad),
R: -22 (deg),
Velocity: 5 [m/s], C1: 850, C2: 1200
Time (sec)
var
ious
u (m/s)
v (m/s)r (deg/s)
0 1 2 3 4 5 6 7 8 9
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
X [m]
Y [
m]
Slip angles
Velocities
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0 0.2 0.4 0.6 0.8 1 1.2 1.4
-60
-40
-20
0
20
40
60
80
100
Time (sec)
Slip
ang
le (
deg)
Right
LeftRear
0 0.2 0.4 0.6 0.8 1 1.2 1.4-14
-12
-10
-8
-6
-4
-2
0
2
4
6
Path for L: -52 (rad),
R: 68 (deg),
Velocity: 5 [m/s], C1: 850, C2: 1200
Time (sec)
var
ious
u (m/s)
v (m/s)r (deg/s)
Test cases & Results Results – Lateral behavior
Calculated driven path for symmetric toe-out (60º) with steering offset of 2, 4, 6, 8 degrees to the right
Slip angles
Velocities
0 0.5 1 1.5 2 2.5 3
-0.25
-0.2
-0.15
-0.1
-0.05
0
X [m]
Y [
m]
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5.Conclusions & Recommendations
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Conclusions & Recommendations
Brake-by-steer concept can back-up failing brakes with a reduced braking performance (~50%).
Lateral behavior changes drastically and ranges of inverted steering occur. These make the vehicle uncontrollable for the driver.
Conclusions Brake-by-Steer Concept
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Brake-by-Steer Modeling
Symmetric Asymmetric
Toe-in
Good braking capability
Good steering capability
(although inverted)
Not effective braking
Good steering capability
Toe-out
Good braking capability
Good steering capability
(although partly inverted)
Not effective braking
Impossible to drive straight
Conclusions Toe-modes
Theory and practice differ on effectiveness of toe modes due to due to caster angle and kingpin inclination induced roll motion. The kart tire that is turned out the most gains vertical axle load and dictates the lateral behavior of the vehicle.
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Conclusions & Recommendations Recommendations Brake-by-Steer Concept
Before the brake-by-steer concept can be applied in cars, the relation between steering angle and vehicle heading must be restored. Calculate how to position the wheels to generate a brake force and follow expected steering input according toe strategy.
Controller Wheel
actuators
Steering angleBrake pedal
...
Velocity
Lateral accelerations
To create this model the presented conceptual model needs to be extended and validated on a car in stead of a go-kart
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Thank you for your attention
Questions?
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Implementation on a Go-Kart Steering system design requirements
Strain gages
Angle sensor
Velocity sensor
Performance requirements: • Wheel steering rate typical 80 º/s • Steering frequency typical 1 Hz (amp = ~10 deg)• Steering torque at wheels
• Nominal 8 Nm• Peak 50 Nm
Measured braking performance• Braking Force 1,2 kN
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Brake-by-Steer Modeling
-80 -60 -40 -20 0 20 40 60 80-1000
-800
-600
-400
-200
0
200
400
600
800
1000
Slip angle (deg)
Late
ral f
orce
(N
)
A0
A1
B0
B1
coslateral vehicle lateral tire
F F
Late
ral V
ehi
cle
For
ce [
N]
Slip angle
Inverted steering occurs at symmetric toe mode for > saturate
A1
Brake-by-Steer cases – Vehicle controllability
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The hub motor (2) is located inside the wheel rim (1).
The electronic wedge brake (3) uses pads driven by electric motors.
An active suspension (4) and electronic steering (5) replace conventional hydraulic systems.
Siemens VDO eCornerBACKUP
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tireF
X vehicle leftF
X vehicle leftF
tireF
tireF
tireF
X vehicle rightF
X vehicle rightF
BACKUP