Dear sir When I follow the tutorial of the formula student physical training videos(Part 9: Building Mechanical Assemblies, Section 2), I have some problems. The model does not run well after I add the calculate force Subsystem and connect the Test Force system to the wheel subsystem. The problems shown by Matlab said the problems were caused by : Error using sm.sli.internal.sim (line 15) * ['chassis']: * Model not assembled: position violation * Resolve this issue in order to simulate the model. The model report shows that Test_force_subsystem.Planar_joint is unassembled and the Shock_absorber.Prismatic_Joint is also wrong.However, the model runs well before I added the force calculate subsystem. Hope you can help with my problem.I really want to know what problems I've made. I attached my model in the Zip file. Another problem is that the setting of the Shock absorber might have problems as well.The scope of the SA shows that the sping just extended without damping.What problem might that be?
MATLAB: How doI solve the problems(Model not assembled:position violation) with this quater car model
formulastudentSimscape Multibody
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Simscape is having a hard time calculating the forces because the model is over constrained. The moving tray (RAMA) has 6 degrees of freedom in a 3D space (3 translations, 3 rotations). If you connect it to the ground with 4 planar joints, each imposing 3 constraints, you impose a total of 4*3=12 constraints. As a result, the system as 12-6 = 6 geometric constraints to satisfy. In the real system, these constraints are satisfied by small elastic deformations of the parts.
To have correct calculation of the forces in Simscape, you want to remove the redundant constraints and allow for small elastic deformations at the contact points between the wheels and the rails.
I attached a corrected file to this post. I added some stiffness and damping to the prismatic and 6DOF joints. The system bounces for about 0.5 s before stabilizing. You might want to increase the stiffness to reduce the displacement along the z axis.
You might need the latest version of MATLAB (R2017b) to open this file.
Hello Shuo,
This is because, you have not defined the densities for the solids appropriately. Set the density of all the following solids to '8000 kg/(m^3)' so that it resembles steel.
- Cylinder (in the Shock absorber subsystem)
- Piston (in the Shock absorber subsystem)
- rim (in the Wheel subsystem)
You can find this parameter in the 'Inertia' dialogue:
Your tyre solid can remain at 1200 kg/(m^3) to resemble rubber if you want (the example model probably has an 8000 kg/(m^3) for that as well and could be considered a bug). In this case, my result looks as follows:
But if you want to replicate the exact same result as in the example, set the density of the tyre also to 8000 kg/(m^3).
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