Parallel mechanism IK problem.

Typically: "How do I... ", "How can I... " questions
Post Reply
ag_raed
Posts: 14
Joined: 04 Jul 2017, 14:54

Parallel mechanism IK problem.

Post by ag_raed » 07 Dec 2017, 12:11

Hello,

I am analyzing the mechanism consisting of a Base, and 4 parallel legs, each leg is constituting of 4 revolutes joints, these legs end up parallel to a moving platform in which I am trying to control:

I am replicating this motion:
http://www.anthrobot.com/omni_III/

I got inspiration of the parallelForwardandInverseKinematics scene to build the chains and make the loop closures.

My question:
  • How do I make the IK linking the moving platform to the base, in the meaning, I want to give a specific orientation to the moving platform End-Effector(white color), and see the response of the first joints - joints1/2/3/4 - in the base (pink color)


Also:
  • Why sometimes my joints rotate unproperly? I don't want to add a spherical joint instead of revolute one because that would lead to unrealistic analysis ...
How would you suggest doing this, please ?

Scene file:

https://ufile.io/l8zz2

Thanks for cooperation.

coppelia
Site Admin
Posts: 6859
Joined: 14 Dec 2012, 00:25

Re: Parallel mechanism IK problem.

Post by coppelia » 10 Dec 2017, 21:17

Hello,

just quickly looked at your scene. From your question, you are trying to solve the IK problem of the mechanism. This is actually the easier of the two (i.e. IK or FK), in case of a parallel mechanism. You have however built your system in order to be able to solve the FK problem (i.e. if you set the appropriate 3 actuator joints into passive mode, you can set their respective positions, and the platform will move appropriately. But currently your model is not set-up for the IK problem. You have two possibilities:

1. If you just need to solve the IK problem, then you could rebuild your model like three equivalent branches from the base, that each are controlled via an individual IK chain (i.e. one IK group per chain), and solve the three IK groups sequentially. Each IK chain will have at its tip a tip dummy, that will try to move onto a target dummy attached to the platform (the platform will not be built on any of the chains.

2. If you want to be able to solve IK and FK with the same model, your approach is correct. You will have to add an additional IK chain that goes from the base to the center of the platform. Put that IK element in the same IK group as the 3 other IK elements. The target of that last IK element is a dummy that could be attached to the base of the model. Then you can simply move that target dummy, and the platform should follow. Make sure that in that case all joints are in IK mode. If at a later stage you want to be able to solve the FK problem, disable that last IK element, and turn the 3 first base joints into passive mode. This is hoe the following demo models work: Models/robots/non-mobile/ABB IRB 360.ttm and Models/robots/non-mobile/Adept Quattro 650HS.ttm

Cheers

ag_raed
Posts: 14
Joined: 04 Jul 2017, 14:54

Re: Parallel mechanism IK problem.

Post by ag_raed » 13 Dec 2017, 08:54

Thank you!

Post Reply