MATLAB: The problem with measuring the force in a planar joint

force sensingplanar jointSimscape Multibody

Hello! I want to ask you for help. I am a post-graduate student of the machine-building academy from Ukraine. I use Matlab to build a model for the transportation mechanism of slabs. I built the model but there were difficulties. I use a planar connection for 4 wheels. All is well. But I need to analyze the forces at the points of contact of the wheels with the rails. If the planar connection is replaced by a welded joint, then the reactions of the supports are calculated correctly. If I use planar connections, then the reactions of the supports are not calculated correctly. Please tell me what I'm wrong. Thank you in advance.
My MODEL
Model with planar joint – MODEL_PLANAR.zip
Model with weld joint – MODEL_WELD.zip
Frame step model – SBORKA.zip
Wheel step model – KOLESO.zip
FOR WELD JOINT
FOR PLANAR JOINT

Best Answer

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.
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