The Direction Sensitive Locking Differential (DSLD)

Transcription

THE DIRECTION SENSITIVE LOCKING DIFFERENTIAL (DSLD)
- EXPERIMENTAL EVALUATION OF
A PROTOTYPE FOR A FWD SAAB 93 Aero V6
Jonas Alfredsona, Mathias Lidbergb
atechno model tm AB, SWEDEN
bVehicle
Dynamics, Chalmers University of Technology, SWEDEN
Informationsmöte
Volvo Cars
040824
DSLD@Vehicle Dynamics
Expo
2010p. 11(29)
(40)
Outline

Introduction
– Background
– The limitations of the open differential
– The Cross-over point
– The Direction Sensitive Locking Differential (DSLD)

Basic DSLD Control Strategies
– Cornering Performance Mode
– Yaw Stability Mode

Transient Cornering Results
– Step Steer in Throttle On
(Cornering Performance)
– Sine with Dwell
(Yaw Stability)

Experimental Results

Conclusions and Future Work
DSLD@ Vehicle Dynamics Expo2010
2 (40)
Background
Steering kinematics and lateral force distribution for an axle:

The lateral load transfer during cornering increases
(decreases) the normal force of the corner outer (inner)
wheel.

Due to the steering kinematics, the slip angles of the wheels
of one axle will be almost equal side to side.

The lateral force of each tire is then proportional to the normal
force of that tire.
3 (40)
Background
Steering kinematics and lateral force distribution for an axle
Conclusion: The kinematics of the steering system causes the
wheels of an axle to contribute to the total lateral force of the
axle in accordance to their ability.
Front axle in cornering, with no longitudinal forces
4 (40)
Background
Ideal differential for longitudinal force distribution for an axle:

Taking the traction circle into account it would be beneficial to
distribute the longitudinal forces in a similar way.

To get equal longitudinal slip at both drive wheels of a driven
axle requires a forced differentiation equal to the theoretical
differentiation (track width/cornering radius).

Forced differentiation in general can be accomplished by a
torque vectoring system, e.g. redistributing drive force from a
slower to a faster rotating wheel.
5 (40)
Background
Ideal differential for longitudinal force distribution for an axle.
Front axles in cornering, with different amounts of longitudinal forces
6 (40)
The Limitations of the Open Differential

The majority of cars are equipped with open axle differentials.

The open differential divides the incoming torque evenly
between the two output shafts by letting them differentiate
freely.

Individual slip rate of each drive wheel can vary almost
infinitely.
7 (40)

The open differential only give the drive wheels the same
individual rotational speed as the previously mentioned ideal
differential when there is no incoming torque to the
differential.

For a positive torque the differentiation will be less than the
theoretical differentiation.

For negative engine torque or braking the differentiation will
exceed the theoretical differentiation.
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Conclusion: The open differential forces both drive wheels to
contribute equally to the generation of longitudinal forces without
taking their individual capability into consideration.
t
9 (40)
The Cross-over point

The differentiation is changing direction when the difference
of the slip of the inner wheel and the outer wheel is as big as
the theoretical differentiation.

The point where the differentiation is changing direction can
be called the cross-over point, because at this point the
locked differential changes from producing an understeeringto an oversteering yaw moment.

At the cross-over point the drive forces of the axle are equal.

A differential that is open below the cross-over point and
locked above it can avoid many of the inherent weaknesses
of the open differential.
10 (40)
The DSLD is designed to react to the cross-over point
by switching between the open or locked position.
This gives the DSLD the ability to give the same basic
advantages that electronically controlled limited slip
differentials (eLSD) can give. But with less need for advanced
control, less complicated and hence cheaper mechanical design.
2
Working Prototype
1
1
5
3
4 2
53
64
5
of DSLD:
9
2
1
0
1
4
12
3
1
21
01
1
6
4
9
7
68
7
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By default, the DSLD works like an open differential below the
Cross-over point. Above the cross-over point the DSLD locks
and thereby lets both wheels contribute to overall force
generation in close accordance to their individual capability.
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
The DSLD can also be controlled to not allow differentiation
below the cross-over point in order to give yaw damping
for example in collision avoidance maneuvers.

The DSLD can be used more preemptively because it gives
yaw damping without the unwanted net speed decrease
associated with ESC interventions.

Additionally, the yaw damping from a locked differential (such
as the DSLD or the eLSD) is mechanically self regulating (the
more yaw motion the more yaw damping).
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Basic DSLD Control Strategies
The main control of the DSLD is divided into two control modes:

Cornering Performance Mode (default)
Self locking above the cross-over point gives a yaw moment
that counteracts the inherent understeer tendency.

Yaw Stability Mode
Gives a yaw moment that counteracts the yaw motion.
The yaw stability control mode can be entered for negative
engine torque indicated by throttle position or steering
reversals.
Special Control Mode:

Brake Based DSLD Control
Unlocking of the DSLD from its locked state in yaw stability
mode can be accomplished by a brief brake pulse at the
corner outer wheel.
14 (40)
Transient Cornering Simulations

The performance of the DSLD with various control strategies
is illustrated for transient cornering maneuvers.

All the maneuvers are simulated with open loop inputs with
the FWD SAAB 93 in third gear on a high µ surface with ESC
inactive.

The maneuvers are performed with an open differential as a
reference (indicated by dashed curves) and the DSLD in
various control modes (indicated by solid curves).
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Step Steer in Throttle On
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17 (40)
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Step Steer in Throttle Off
19 (40)
20 (40)
21 (40)
Sine with Dwell
22 (40)
Sine with Dwell
23 (40)
Sine with Dwell
24 (40)
Experimental Results

Maneuvere: Throttle On in a right hand curve

Cornerning Radius: ~30m

Test input: High but modulated throttle in 2nd gear
starting at ~4m/s2 latreral acceleration.

Objective: Maximazing speed with maintained
cornering radius.

Main results: The DSLD gives better and more
consistent cornering performance.

Test vehicle: Saab 93 Aero V6 FWD

Test Site: Stora Holm outside of Gothenburg

Instrumentation: CAN, DL1 (Mathworks XPC Target)
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r
[
Open Differential
d
e
e 900
p
S
800
l
a 700
n
o
i 600
t
a
t 500
o
R
400
l
e 300
e
h
W 200
Left
Right
100
0
0
2
4
6
8
Time [s]
10
12
14
16
29 (40)
r
[
DSLD Cornering Performance Mode
d
e
e 900
p
S
800
l
a 700
n
o
i 600
t
a
t 500
o
R
400
l
e 300
e
h
W 200
Left
Right
100
0
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Time [s]
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30 (40)
/
m
[
n
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c
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A
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a
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a
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Open Differential
2
CAN Bus
DL1
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-2
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Time [s]
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31 (40)
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CAN Bus
DL1
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Time [s]
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32 (40)
e
d
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n
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P
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Open Differential
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Time [s]
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33 (40)
e
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Time [s]
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34 (40)
Open Differential
0
]
s
/
g
e
d
[
e
t
a
R
w
a
Y
-5
-10
-15
-20
-25
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-40
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Time [s]
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35 (40)
0
]
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Time [s]
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36 (40)
37 (40)
38 (40)
Conclusions and Future Work

The DSLD opens up the possibility to significantly increase
the cornering performance of front wheel drive passenger
cars.

To verify the performance and the stability of the DSLD in
more detail it is needed to continue the experimental work
with more maneuvers as well as deeper analysis of existing
results.
39 (40)
Thank you for your attention!
40 (40)

The Direction Sensitive Locking Differential (DSLD)

Transcription

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