how to avoid deformable or bending links

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fferri
Posts: 681
Joined: 09 Sep 2013, 19:28

how to avoid deformable or bending links

Post by fferri »

Hello,

Again I'm trying to design a dynamic simulation for our robot, starting from the most complex part which is the tracked flipper-bogie-flipper body.

I believe I followed the recommendations for designing a dynamics simulation and here's the model tree during the simulation:

Image

it consists only of pure non-static shapes. I carefully created and placed every shape/joint numerically, following a reference cad design. Every joint is revolute.

This is the model at design time:

Image

Wheels weigh from 0.09 (smallest) to 3.7 (largest) and have the same density.

However, at simulation time, the links and/or the joints bend so badly:

Image

Every shape is pure, non-static and respondable with complementary mask. Color indicates the mask, blue is:

Image

and green is:

Image

Is there some parameter to control how much a link will bend? Or how much a joint will bend along an axis other than its rotation axis?
Or more probably, I've done some mistake I'm not aware of.
I can send scene file if needed.

coppelia
Site Admin
Posts: 8206
Joined: 14 Dec 2012, 00:25

Re: how to avoid deformable or bending links

Post by coppelia »

Hello,

you can try to check several things:
  • turn on contact point display (in the general dynamics properties), to see if some objects that are not supposed to collide, collide anyway.
  • during simulation, switch to the dynamic content visualization and verification mode (click the 14th toolbar button from the left). You could detect something suspect. Refer to the design consideration 3.
  • you probably already made sure that design considerations 7 and design consideration 8 are respected.
  • try to run the simulation with the Vortex engine. Do you notice the problems there too?
You can also post your scene.

Cheers

fferri
Posts: 681
Joined: 09 Sep 2013, 19:28

Re: how to avoid deformable or bending links

Post by fferri »

This is the dynamic content during simulation:

Image

I cannot try with Vortex, but with ODE the bending effect is less prominent.

Here is the scene file

fferri
Posts: 681
Joined: 09 Sep 2013, 19:28

Re: how to avoid deformable or bending links

Post by fferri »

Is it relevant that joints do not physically touch the link object?

To me the link object for the bogie and the two flippers it was not relevant (I could have used a dummy if only it allowed maintaining dynamically enabled joints), so I created a 5cm x 5cm pure plane with mass 0.3 Kg.

fferri
Posts: 681
Joined: 09 Sep 2013, 19:28

Re: how to avoid deformable or bending links

Post by fferri »

Also, lowering the masses (I've changed them to 0.1, 0.15, and 0.2 respectivelly) seems to help a bit.
However the effect never goes away (both in Bullet and ODE).

coppelia
Site Admin
Posts: 8206
Joined: 14 Dec 2012, 00:25

Re: how to avoid deformable or bending links

Post by coppelia »

If you increase the inertia of objects bogie_link, fli_1_link and fli_2_link, you can reach a much more stable situation. Vortex works fine also without those adjustments.

Cheers

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