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.
Figure 1 A BMW 3-series sedan.
Figure 2 A Dallara formula 3 chassis.
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:
Figure 3 A complete chassis model. The tree structure starts with the ground element. Via the freeMotion element, the Body Geometric Reference (BGR) of the car's coordinate system is reached. From BGR, the tree is defined towards the wheels and the road.
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.
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.
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.
NomenclatureThe 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 valueand 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 - wheelWhen 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 wheelSome 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.
Figure 5 The coordinate system used to define the suspension geometries.
ScalingIn 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
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
Name | Description |
---|---|
Examples | |
Chassis | Modelica library to model vehicle chassis. |
Wheels | Wheel, tyre and road models |
Drivers | |
Utilities | |
Environments | |
PowerTrain | |
Aerodynamics |