Simulation of a Basic Automobile Model
OBJECTIVE :
To Build the suspension and the steering system mechanism in an automobile model with the help of the systems, attachments, and datasets and simulate the system for varying values of :
- Suspension Spring Stiffness
- Suspension Spring Damping Coefficient
- Coupler ratio between The steering wheel and Tyre
THEORY :
1) The Basic Automobile system :
The basic automobile system to be simulated consists of the following systems :
- The Vehicle Body : which consists of the Chasis and BIW
- The Steering system : which consists of the Steering wheel and Steering Shaft
- The Front suspension system : which consists of the wheel, tyre, knuckle, spring damper and is also coupled to the motion of the steering system
- The Rear suspension system : which consists of the wheel, tyre, knuckle & spring damper.
Furthermore, to simulate the motion of the above system, a Road system is required which will be connected to the above system by 3D contacts.
2) 3D contact Simulation :
Bodies can be constrained using joints and contacts to simulate the mechanism. In this case the relationship between the Road And the vehicle is defined by Contacts.
MotionSolve monitors the proximity between the geometries specified and a force based on the contact properties is generated. When onset of collisions is detected at certain time step, the collision detection algorith returns the set of interferring polygons, from which MotionSolve computes :
- Point of contact and surface normal vecotr
- Relative tangential slip Velocity
- The magnitude and direction of normal and frictional forces.
To provide the normal contact forces, in this case the Impact method is to be chosen as it is a non-linear spring-damper used in case of penetration. The parameters required are :
- Stiffness
- Stiffness exponent
- Maximum damping
- Penetraion at max. damping
And to provide the Frictional forces, for this simulation it is set to dynamic + Static. The parameter required are :
- MU Static
- MU Dynamic
- Striction Transition Velocity
- Friction Transition Velocity
For this simulation, the above parameters can be left as default.
3) Systems, Attachments and Datasets :
Sytem : Motionview encourages a modular model building approach with different entiites aggregated into containers, these containers can be a child of another system or other container entiites and can contain useful modelling entities like points, bodies, joints, forces etc.
Advantages of Using Systems :
- Provides clear understanding
- Easy to activate and deactivate
- can be exported and used for different model.
Importing and Exporting Systems :
By clicking on the parent system, it can be noted that there is a tab " Import/Export" , by selecting the import option and specifying the created Sub-System, it can be imported.
Likewise, to export a created subsystem, click on the said subsystem, navigate to Import/Export Tab and by specifying the location to import to, it can be exported.
Attachments : When assembling models with different sub assemblies, it is necessary to connect entities in one contained to entities in another container.
To retain modularity, the connections to the external entities are defined as attachments.
Datasets : It is a group of design variables that can be of different types.
These variables exist in design time for the purpose of helping to build the model and it can also be used by HyperStudy for DOE, optimization and stochastic analysis.
4) Initial Conditions :
The initial conditions tab on a system/assembly allows to specify the following values
* Translation Velocity along the axes : Vx, Vy, Vz
* Rotational Velocity about the axes : Wx, Wy, Wz
Once an inital condition is set up, it is applied to all subparts as well, unless the inital conditions are declared independently.
SETTING UP THE SYSTEM :
- The Vehicle Body :
The vehicle body was constructed using a 3D solid modelling software as shown below :
The created CAD data was saved as .STP file and is brought into the Motionview user interface using the "Import CAD or FE" option.
Once imported, the following hardpoints are added to the system :
The graphics, the body and the vehicle cg point will be brought into the UI when importing with mass and inertia, the other 4 points need to be created using the "Add points option". It is to be noted that while creating points, the radio button is to be changed from Single to pair for the wheel co-ordinates.
Description of the created points :
- Vehicle cg - Center of gravity of the vehicle
- Wheel center front - Point about which the left and right front wheels rotate.
- Wheel center rear - Point about which the left and right Rear wheels rotate.
- Steering wheel center - Point about which the steering wheel rotates.
- Steering Shaft - Point at which the steering shaft and vehicle body coincide.
The final setup of the Vehicle body system before exporting is shown below, once exported it can be called into the Main simulation.
- The Steering System :
As mentioned earlier, the steering system consists of a steering wheel and a Steering shaft, In this case, the steering wheel was designed using a 3D cad model software like the Vehicle body before being imported and processed in MotionView UI.
The steering shaft is created as a primitive geometry in MotionView and is attached to the wheel after importing. The hardpoints created in the steering system are :
Description of the Data :
- Steering wheel Body : It is created automatically when importing with mass and inertia. This body is common for both the steering wheel and shaft.
- Couplers : For now, the couplers are created but are not defined as these require data from the suspension system.
- Graphics : The steering wheel is created automatically when importing and the shaft is created by primitive geometry with origin point as center of steering wheel.
- Joint - The revolute joint at the center of the steering wheel connects the vehicle body and the steering wheel, it is created at the center point for now.
- Motion - The motion of the steering wheel is provided at the Revolute joint with expression : "20*Sin(2*pi*time)*DTOR"
- Steering wheel center - Point about which the steering wheel rotates.
The final setup of the Steering system before exporting is shown below, once exported it can be called into the Main simulation.
- The Suspension System :
This system is modelled entirely in MotionView UI, constructed using only primitive geometries.
Both the front and the rear suspension systems are made of the same hardpoints with the addition of a motion controlling the movement of the wheel on the rear suspension system.
Description of the Data : (All datas are created as Pairs)
- Bodies : Two bodies are created, one defining the wheel and tyre and other the knuckle.
- Joint - A revolute joint at the center of the wheel connecting the Wheel to the Knuckle and a Cylindrical joint connecting the Knuckle to the ground body for now.
- Points - The Wheel center point is assigned a random value for now and the other points are parametrized to this point
- Spring Damper - A spring is present between the vehicle body and the knuckle.
The properties of the spring damper are set as follows :
Damping coeff : 10N
Stiffness : 100 N/mm
Once these datas are created, it can be exported as the Front suspension system and also the rear suspension system after adding the motion constraint. The final subsystem with both suspension system is shown below.
- The Road :
It was modelled in 3D Cad modelling software and imported into MotionView UI but without a mass and inertia.
Description of the Data:
- 4 Contacts are created but since they require inputs from the suspension systems, they are not defined.
- The graphics will be brought in when importing.
- The outputs are created when contacts are defined.
- The Point is created to locate the geometry right below the Tyres.
The final Road System is as shown below :
SIMULATION :
- To start off, the datas for the gravity and units are checked to see if the gravity is applied in negative z direction with the units being millimeter, kilogram, second and newton for length, weight, time and force respectively.
- Now, all the created systems are imported.
- To define the relationship between the systems, the following attachments are created for each system :
Attachments for each system :
Vehicle Body - No attachments.
Suspension System - vehicle body and the wheel center points (as shown below) from Vehicle body system.
For the front suspension system, the wheel points front is selected and for the rear suspension system, the wheel points rear is selected.
Steering Sytem - The following attachments are created :
* Steering wheel center from vehicle body system
* Steering shaft center from vehicle body system
* Vehicle Body from vehicle body system
* Knuckle To Body - Left Joint from suspension system front
* Knuckle To Body - Right Joint from suspension system front
Road Sytem - The following attachments are created :
* Vehicle cg Point from vehicle body system
* Wheel and Tyre Body pair from suspension system front
* Wheel and Tyre Body pair from suspension system rear.
- Once the attachments are created, they have to be referenced within the systems. Hence the following changes are made to the system before exporting the systems again (to save the changes)
Changes to each system after attachment creation :
Vehicle Body - No Changes.
Suspension System :
- The vehicle body is used in places where the ground body was used initally. (Damper graphics, Spring Damper and Knuckle to Body Joint)
- The wheel center point in the suspension system was changed to wheel center point from Vehicle Body System.
Steering Sytem :
- The couplers created earlier are resolved now. With Coupler 1 being defined between the Steering motion and the Front Left wheel and Coupler 2 being degined between both the front wheels.
The coupler ratios are as follows :
The ratio of steering wheel to Left tyre is 1:8 and between the Two front tyres is 1:1.
- The Steering shaft is connected to the Steering shaft center point from vehicle body system.
- The Steering wheel is connected to the Steering Wheel center point from vehicle body system.
- The Vehicle body is used to define the joint in system.
Road Sytem :
- The created contacts are resolved, with each contact being defined between the ground body and the corresponding wheel body.
- The Locating point being parametrized w.r.t to the Vehicle cg Point from vehicle body system.
As discussed ealier, an initial velocity of 3000mm/s was provided to the parent system and an output to monitor the displacement of the vehicle body was created using the "Add Output" option.
The final setup is shown Below :
Now the parameters to be varied are noted and classified as different setups. Each setup is run individually and the output graphs are compared.
RESULTS :
The animation showing the motion of the system for the initial case (as defined above) is attached herewith
The various cases setup are as follows :
Case (i) : To monitor the effect of change in Spring Stiffness
Setup 1 : Spring Stiffness : 100 N/mm
Setup 2 : Spring Stiffness : 1000 N/mm
Case (ii) : To monitor the effect of change in Spring Damping Coefficient
Setup 1 : Spring Damping coefficient : 10 N
Setup 2 : Spring Damping coefficient : 100 N
Case (iii) : To monitor the effect of change in Coupler Ratio
Setup 1 : Coupler Ratio Between Steering wheel and Tyre : 1:8
Setup 2 : Coupler Ratio Between Steering wheel and Tyre : 1:3
Graphs :
The Graphs associated with each case is shown below :
Case (i) : To monitor the effect of change in Spring Stiffness
Blue - Setup 1
Red - Setup 2
Case (ii) : To monitor the effect of change in Spring Damping Coefficient
Red - Setup 1
Blue - Setup 2
Case (iii) : To monitor the effect of change in Coupler Ratio
Red - Setup 1
Blue - Setup 2
CONCLUSION :
Thus using Altair MotionView the suspension and the steering system mechanism in an automobile model was built and the effect of :
- Suspension Spring Stiffness
- Suspension Spring Damping Coefficient
- Coupler ratio between The steering wheel and Tyre
was Studied as Displacement along either Y-axis or Z-axis with respect to Time.
The corresponding files are attached herewith.