3.2 Kinetics
3.2.4 Application Example
The equations derived in the previous sections are used in a spatial double pendulum example to demonstrate the functionality of the quaternion deriva-tive formulation. Two identical slender beam-shaped bodies of the dimensions
2mm×5mm×0.9m are meshed with 1200 brick elements and model order reduced to 4 eigenmodes with a tangent reference frame attached to the first interface node. The upper beam is attached to the environment by a hinge joint, allowing rotation around the y-axis. The second beam is attached to the first beam by a ball joint. The time integration is started with a horizontal first beam and the second beam hanging vertically, see figure 3.1. The system is stated on index-1 level and the MATLAB ode23t solver for stiff systems is used to perform the time integration. The system lies in the x−z plane, it is loaded by gravity and no other external forces are applied.
−2 −1.5 −1 −0.5 0 0.5 1 1.5 2
−2
−1.5
−1
−0.5 0 0.5
x-position [m]
z-position[m]
0s 1s
2s 3s
4s 5s
Figure 3.1: Double pendulum at 0s – 5s.
The trajectories of the hinge and ball joint of the first five seconds of simulation time are shown in fig. 3.1 as well as the pendulum positions and elastic defor-mations in 1s intervals. A double pendulum is a chaotic system, which can be seen at the trajectory of the second pendulum, following no predictable path.
The circular path of the ball joint indicate that no relevant constraint drift is present. No visible deflection at the ball joint is present either.
In fig. 3.2 and fig. 3.3 the corresponding quaternion velocities are shown. Since the system is located in the x−z plane, there is obviously no rotation around any axes other than the y-axis. When recalling eq. (2.17) and eq. (2.18), it is obvious that q˙1 = ˙q3 = 0 during the simulation.
Quaternion derivatives feature an elegant way to state the EOM of flexible bodies and Jacobian calculation is simplified significantly. A drawback of this method
0 1 2 3 4 5 6 7 8 9 10
−2 0 2
time [s]
quaternionvelocity[
1 s] q˙0
˙ q1
˙ q2
˙ q3
Figure 3.2: Quaternion velocity of the first beam.
0 1 2 3 4 5 6 7 8 9 10
−5 0 5
time [s]
quaternionvelocity[
1 s] q˙0
˙ q1
˙ q2
˙ q3
Figure 3.3: Quaternion velocity of the second beam.
is the higher system dimension and the lack of interpretability for the quaternion derivatives. Therefore in the following chapters this approach is not used for RT applications.
CHAPTER VEHICLE DYNAMICS WITH STRUCTURAL
FLEXIBILITY
The field of vehicle dynamics covers the spatial motion of a vehicle on a road or off-road surface. In addition, it may as well include the motion and behavior of occupants and the vehicle’s impact on road, air and other vehicles. If only a certain area of vehicle motion is of interest, reduced models can be used to gain sufficient information. Simple vehicle dynamics investigations can be done by using only vertical or lateral models as shown in figure 4.1.
x mw
mc δ
S yw β
yc
v
x y
Figure 4.1: Quarter car model (left) and single-track model (right).
A quarter car model only has two DOF in vertical direction for the car body motion yc and the wheel motion yw. It is sufficient for basic analysis of the ver-tical behavior of a car body and can be used for a first estimation of suspension setup parameters and comfort investigations. The single-track model [43] allows basic investigations of steady cornering and can, for example, be used to inves-tigate the understeering and oversteering behavior of the vehicle or for simple control algorithms. For vertical models structural flexibility can be introduced with additional sprung masses, e.g. to analyze the vertical vibration behavior of an internal combustion engine.
However, although such simple models allow basic vehicle dynamics investiga-tions and are of course RT-capable, their value for detailed description of vehicles in versatile environments is limited. Two-track models allow the simulation of all common vehicle conditions and the calculation of vehicle rotations around all
three axes, called pitch, roll and yaw. Figure 4.2 shows the most basic two-track model with a linear suspension setup, allowing the suspension to move along a linear axis relative to the car body. Considering the car body as a free body in space with 6 DOF, having one translational DOF at each suspension and one rotational DOF at each wheel, this vehicle model yields 14 DOF in total. The connection to the environment is then realized with generally nonlinear force elements at the wheels, representing the tire forces.
β yw
x z
y
x0 z0
y0
β yw
β yw
ρ,α
β yw
F
F
Figure 4.2: Two-track model without suspension kinematics.
Two-track models with simplified suspension kinematics have been proven to be suitable for RT simulation as well [30]. Advantages of such models are the ab-sence of kinematic loops and hence the ability to use ODE solvers for numerical solution. To some extend these models can be used to simulate structural flex-ibility within the car body or other vehicle components like suspension parts.
The following sections cover the integration of structural flexibility in vehicle dynamics.
4.1 Aspects on Vehicle Dynamics with Structural Flexibility
Several parts of a state-of-the-art road vehicle are prone to elastic deformation that may influence the handling, comfort or durability of the vehicle. While durability and some aspects of comfort like acoustic effects are hardly relevant for real-time applications, structural flexibility may be relevant when influencing the dynamic behavior of the vehicle [34]. The lower eigenfrequencies of the struc-tural assembly of a modern passenger car with significant influence on vehicle dynamics are the torsional and bending mode. These typically lie in the range of 20-40 Hz, depending on the body design of the car. Figure 4.3 shows the torsional eigenmode of a passenger car which e.g. may get induced by driving on a cobblestone road.
Flexibility of other vehicle components may as well have a great influence on the dynamic vehicle behavior and hence modeled as a flexible body. Lightweight
Figure 4.3: Torsional eigenmode of a Ford Taurus from [33] at 24.5 Hz.
design is advantageous especially for suspension components since it reduces the unsprung masses of the vehicle. Thus for most low-priced vehicles the front suspension wishbone is designed as a sheet-metal part that may undergo signif-icant elastic deformation [15]. Other suspension setups like torsion-beam sus-pensions or leaf-spring sussus-pensions can be simulated as flexible bodies as well [20, 64]. Half-rigid suspension setups like the torsion-beam axle are intention-ally designed to behave elasticintention-ally in driving maneuvers. These designs are of particular interest for flexible multibody simulations.