VehicleDynamics

Package for vehicle dynamics modelling

VehicleDynamics.Chassis VehicleDynamics.Wheels VehicleDynamics.Drivers VehicleDynamics.Utilities VehicleDynamics.Environments VehicleDynamics.PowerTrain VehicleDynamics.Aerodynamics

Information


VehicleDynamics Library

This library is used to model vehicle chassis. The library is divided into suspensions and components. Suspensions are complete front and rear suspensions ready to use, e.g. MacPherson, Double wishbone and Multilink. The components are used to build these suspensions, e.g. struts, wishbones, and anti-roll linkages. Below, the model structure, its hierarchical levels and the parametrisation are explained. There are also some predefined chassis that serves as examples of how the parts can be gathered, in figure 1 and figure 2, a BMW 3-series and a Dallara F3 are shown, respecitvely.

BMW

Figure 1 A BMW 3-series sedan.

Dallara

Figure 2 A Dallara formula 3 chassis.

"Modelling grammar"

The structure of a chassis model

The Chassis library is based on the ModelicaAdditions.MultiBody library and thus, the Car models must follow the tree structure requirements presented above. To handle this the direction of the car modelling is chosen to go from chassis towards the ground via the wheels as illustrated in figure 3. Since the model must start with an inertial system, in this case the ground, there must be an additional connection from ground to chassis. This is achieved with a free motion joint.

This way of modelling may seem awkward but has some important advantages:

Hierarchical levels

The library is based on a hierarchical structure within the total model. This is done to give a clear overview of the model and simplify reusage of components and suspensions. The hierarchy contains the following levels that can bee seen in figure 4:

A. Vehicle level.

B. Chassis level.

C. Suspension level.

D. Component level.

hierarchy

Figure 4 The hierarchical levels of a vehicle model. a, the vehicle level, b, the chassis level, c, the suspension level, c the component level and d, the elementary level.

Vehicle level The total model of the vehicle/car contains typically a chassis model. Additionally, models for drivers, power-trains, aerodynamics etc. can be added.

Chassis level Within the chassis level, a complete chassis is built up, typically with a front and a rear suspension, wheels and a body

Suspension level The idea with the suspension level is to make it easy to reconfigure a car by just swapping suspension and therefore, all suspension should share the same basic interface, i.e. one MBS-cut for the connection to the body. There should also be an MBS cut for each wheel (normally two) that is to be connected to the suspension. Additionally, there may be some extra connectors depending on the suspension. For example will a steerable suspension also have a connector for a steering wheel. All availible Suspensions are gathered under Chassis.Suspensions.

Component level With the component level, the foundation for efficient reuse of vehicle models is laid. Components like a-arms, bushings, MacPherson struts, trailing arms, multilinks, anti roll linkages, rack steerings etc. are available. These components are based on the Modelica and ModelicaAdditions libraries as well as some special models found under Utilities.

Parameterisation

To specify a linkage, the geometry has to be defined. Additionally, the mass and inertia properties of the parts within the linkage can be defined. For an understandable parameterisation of these properties, a systematic definition of the parameter names is necessary.

Nomenclature

The geometry is mainly defied by the connection joint locations, connection points. Additionally, the direction(s) of a joints' degree(s) of freedom must be given if it is (are) not defined by the connection joint points. The geometry parameters are named according to

  [geometry parameter]=[property][connection point]_[wheel no]
  [connection point]=[part 1][part 2][part n]
while the mass and inertia properties are component specific and are thus named according to
   [component parameter]=[property][part]_[wheel no]

The properties are named according to

  [property]
   r    - location
   n    - direction of rotation or translation
   m    - mass
   rcm  - location of centre of mass
   c    - stiffness
   d    - damping
   f    - force
   t    - torque
   i    - inertia element, (gear) ratio
   q0   - Relative offset of the unaffected state for spring/damper/strut,
          compared to the state given by the geometrical parameters. 
          Typically, if a strut is mounted in at rA and rB, then it Is 
          unaffected length is |rA-rB|+q0. 
   qInit- Initial value
and the parts are named according to
 
  [part]
   C - chassis 
   R - steering (rack) 
   U - upright, part that holds the wheel 
   P - pivot element,used for example in Watt's linkage 
   S - strut, 1D force element 
   L - link or rod
   B - body or bushing
   A - antiroll 
   X - Undefined part/General part, for instance an anti-roll linkage 
       could be attached to links as well as uprights.
   W - wheel
When there is more then one part of the same type, a number is added to the character. For example if there are more than one link, as in a double-wishbone, they are numbered L1, L2, etc., starting at the front upper link.

The wheels are numbered from front left towards right and rear. For a four wheel car this yields:

  [wheel no] 
   1 - front left wheel
   2 - front right wheel 
   3 - rear left wheel 
   4 - rear right wheel
Some examples of how parameters are named:
   rCL1_2  - Location of connection joint between chassis and link 1 at
             front right wheel. 
   i22L1_3 - Inertia element i22 of link 1 at rear left wheel. 
   nCL1_4  - Direction of revolution of the joint that connects link 1 to 
             the chassis at the right rear wheel. This could for example be the rotation
             axis of a swing axle. 
   rUL1L2_1- Location of connection joint between the upright and link 1 
            and 2 at the front left wheel. This could for example be the upper spindle
             joint at a double wishbone suspension

The frames are named after the part they are attached to. The frames are normally located at the parts outermost point referred to the tree structure. Examples:

   frame_L1L2- A frame attached to links 1 and 2 at their outer end. For a 
               MacPherson linkage this would mean that the frame is attached to 
               the link at the location of the spindle. 
   frame_U_2 - The frame attached at the outermost point of the right upright. 
               Typically this is the location of the wheel centre.

Coordinate system

The coordinate systems used within the library refers to the DIN standard, the x,y and z axes point forward, left and upward respectively, see figure 5.

coordinate system

Figure 5 The coordinate system used to define the suspension geometries.

Scaling

In many cases it is convenient to mirror components in a car, for example left and right suspensions. To handle this there is a three-dimensional scaleFactor. This can be used to rescale and mirror objects, for example

   scaleFactor={1,-1,1}
mirrors the model around the xz-plane

Main Author:
Johan Andreasson
Div. of Vehicle Dynamics
Royal Institute of Technology
Teknikringen 8c
SE-10044 Stockholm, Sweden
email: johan@fkt.kth.se

Release Notes:

Acknowledgements:
Some components of this library, such as the base wheel, the Rill tyre model and the aggregation joints for analytically solving kinematic loops, have been developed by Martin Otter, from DLR - Institute of Robotics and Mechatronics, Germany.
Part of this library was developed with financial support from Dynasim AB, Sweden and DLR - Institute of Robotics and Mechatronics, Germany. Some of the models are originally developed for the Driving Dynamics project within the "The Green Vehicle"/FCHEV Programme.

Copyright © 2003, Modelica Association, Johan Andreasson, DLR and Dynasim

NameDescription
Examples  
Chassis Modelica library to model vehicle chassis.
Wheels Wheel, tyre and road models
Drivers  
Utilities  
Environments  
PowerTrain  
Aerodynamics  


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