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Since the heavy truck industry offers a wide variety of vehicle layouts, the locations of many of the truck’s subsystems were made parametric for easy modification. For example, the front axle subassembly(wheels, axles, leaf springs, and shock absorbers) were linked to a variable defining the longitudinal position of the front axle. Using this technique, truck models with different front axle positions can be quickly developed by changing the value of this variable. This procedure was duplicated for the following subassemblies: rear suspension, cab ,engine ,hood, and fifth wheel and trailer .Tire to road contact is handled with the ADAMS built-in tire routines and includes models for tire handling and durability forces. In ADAMS road profiles are represented as a mesh of triangles similar to a finite element mesh. The geometry and mesh for the road profiles are generated with Pro/Engineer. A custom software program is
then used to translate the mesh into two files for ADAMS :a road file format for the solver to determine the tire/road interaction forces, and a graphics format to view the road during post-processing animation. These files are stored in a common directory for easy retrieval. Custom control algorithms were developed to control vehicle speed, steering, and drive torque.These functions can be quickly modified to execute different vehicle maneuvers such as roll stability, a high speed lane change, or durability bumps similar to a proving ground. After the simulations are run, the forces and torques acting on the frame are written to data files. A custom software program is then used to extract the loads at specific time steps and write them to an ANSYS load file. The load file is then read into ANSYS and applied to a finite element model of the frame. The frame stresses are then calculated using an inertial relief solution.
In summary, the model uses custom software routines and the existing links between the CAD and CAE codes to create a custom environment for evaluating the performance and durability of a heavy-duty truck. However, the model assumes that the truck frame is a rigid, under formable body. In reality, the truck frame contains a great deal of flexibility which can impact vehicle performance and stability. As a result, these effects must be captured in the multi-body system simulation.
CAE SOLUTION FOR FRAME FLEXIBILITY
PREVIOUS TECHNIQUES – In the past, several techniques have been employed to capture frame flexibility in a MSS model. Three popular methods are: bushings,mass beam elements, and FEA super element reduction. In the first method the frame is pided into two or more rigid bodies connected together with force elements having bushing-like properties: stiffness anddamping in three translational directions and three rotational directions. The bushing properties are adjusted to give the overall frame bending and torsional stiffness.
As can be expected, this method is cumbersome to use, and if properly tuned, it will be capable of capturing only the fundamental bending and torsional modes of the frame. In the second method the frame is pided into a large number of rigid bodies interconnected by massless beam elements. This is similar to the bushing method but many more rigid bodies are usually used, and they are connected with massless beam elements whose equations (Timoshenko beam theory) are better suited to modelling truck frame rails and cross-members.Nonetheless, it istime consuming to build a frame with this method and careful tuning of the beam elements is still required to capture the frame’s flexural response. The third method is the most accurate of the three methods and is based on a finite element representation of the frame. In this method the finite element model is reduced to a super element representation with the overall stiffness and mass properties condensed to a set of master nodes. The reduced model is checked against the original finite element model to ensure that the important frame dynamics are still captured. It is then imported into the MSS environment where the super elements and master nodes are converted to an equivalent representation of rigid bodies and force elements. Although this method is based on a finite element solution, it can still be difficult to achieve accurate results. For example, care must be taken in selecting the master nodes to ensure that the mass and stiffness condensation process is accurate.