How to make parallel construction stable?

Typically: "How do I... ", "How can I... " questions
Post Reply
mkt
Posts: 2
Joined: 20 May 2018, 22:15

How to make parallel construction stable?

Post by mkt » 21 May 2018, 09:26

Hello!
I have been trying to design a model of this parallel scissor robot:
https://www.youtube.com/watch?v=F7uzU95NOyY (it is genious btw).

I created two pairs of scissors and joint them to plate at the top.
Only pistons at the bottom should be actuated (2 prismatic joints for each scissors pair), all other
joints should just... be (not actuating, but enabling dofs).

I switched all the joints in to dynamic/torque mode.
Weights of all links and plate are about the same - 1~2kg.

I closed loops using dummies - construction is glued in many places - every scissor has
two arms, which must be connected to each other, every arm connects also to the plate at the top.
For dummy connections I used dynamics/overlap mode (I think it is ok?)

This is my scene:
https://drive.google.com/file/d/1ZH-t3_ ... sp=sharing

My problem is that construction is breaking on the joints - it is not standing stable, or even
falling slowly and gracefuly - it just snaps (on the revolute joints in scissor arms).
Why is this happening? And if You can be so kind - is the path I taken to develop this
model a proper path? Or have I made some critical mistake?

coppelia
Site Admin
Posts: 6617
Joined: 14 Dec 2012, 00:25

Re: How to make parallel construction stable?

Post by coppelia » 24 May 2018, 12:23

Hello,

you have several problems with your scene:
  • B_PlateJointDummy should be the child of a spherical joint, itself child of B_TopArm (actually in the exact same way as you do with the other scissor branch)
  • Same as above for the other side (i.e. D_PlateJointDummy)
  • Spherical joints B_ArmsJoint and D_ArmsJoint should be revolute joints instead
  • Each dummy-dummy pair in a dynamic overlap constraint link should have the exact same position and orientation (otherwise, at simulation start, they will try to overlap in order to reach the same position and orientation)
  • The prismatic joints do probably not have enough force.
  • The links have a very small inertia value. Try increasing it a bit.
Try also using the Vortex engine, you will get overall better results.

Cheers

mkt
Posts: 2
Joined: 20 May 2018, 22:15

Re: How to make parallel construction stable?

Post by mkt » 25 May 2018, 14:57

Hi!
Thanks for tips.
I've done following:

I fixed structure,
changed spherical joints to revolute joints,
fixed dummies positions,
set larger max forces on prismatic joints,
increased intertia on links and reduced their weights.
I tested model on Vortex, Bullet and Newton engines,
and Newton get me most proper results (but still very bad).

There is something wrong - joints are breaking, like there was some massive force
applied to them. In reality whole construction is very light, I dont understand from where this force
is coming. Im out of ideas how to fix this thing - everything seems fine...

Please, take a look:
https://drive.google.com/file/d/1OVw_NX ... sp=sharing

coppelia
Site Admin
Posts: 6617
Joined: 14 Dec 2012, 00:25

Re: How to make parallel construction stable?

Post by coppelia » 29 May 2018, 06:03

you need to check more carefully your construction:

Linked dummies D_ArmsJointDummy and C_ArmsJointDummy do not have the same orientation. Same for the other similar dummy-pairs located at the scissor crossing. Then switch to Vortex. It gives most stable and precise results. If you still want to use a different engine, you'll have to fiddle around more with the masses and inertias.

Cheers

Post Reply