Inverse kinematics of hybrid robot

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gloria
Posts: 7
Joined: 27 Jan 2016, 08:34

Inverse kinematics of hybrid robot

Postby gloria » 20 Apr 2017, 15:36

Hello,

I'm figuring out a problem of inverse kinematics of a hybrid robot. It has five degrees of freedom, among which three for translation and two for rotation. Also, its topology is hybrid, which means there are both parallel and serial chains, just similar to the robot shown in the user manual-Inverse Kinematics.
http://www.coppeliarobotics.com/helpFiles/en/inverseKinematicsModule.htm
Now I have already made connection between links and joints. Also, I've made three tip-target links for loop closures and one tip-target link for my task( I intend to let the tip of the robot move to any positions and orientations in its work space or follow a path.)
I've read http://www.coppeliarobotics.com/helpFiles/en/basicsOnIkGroupsAndIkElements.htmBut I don't know how to set ik groups if I want to do IK rather than FK. Should it be in the same ik group or not? I tried both, but ik all failed. There are always some tips not following their target. So I figure that if there would be necessary steps that I ignored?

Thank you!

gloria

coppelia
Site Admin
Posts: 5614
Joined: 14 Dec 2012, 00:25

Re: Inverse kinematics of hybrid robot

Postby coppelia » 21 Apr 2017, 08:02

Hello Gloria,

when doing FK for a parallel kinematic machine, you basically still do IK, but you will set the motor joint into passive mode, so that they stay fixed during ik calculation.
But what you are trying to achieve is pretty difficult. Have a look how things were done in the demo model Models/robots/non-mobile/ABB IRB 360.ttm

Basically, if you just need IK for your PKM, then it is pretty straightforward, since you simply have IK branches that need to follow the platform (the tip or end-effector).
But if you also need FK for your PKM, then you need to rethink the how hierarchy of your model, since loop closures need to by done at the base of the robot, not on a moving item.

Cheers

gloria
Posts: 7
Joined: 27 Jan 2016, 08:34

Re: Inverse kinematics of hybrid robot

Postby gloria » 21 Apr 2017, 15:09

Hi,

Thanks for your reply!

I just want to do IK. Now, if I use Pseudo method, by the time simulation begin, the robot collapse, while when I use DLS method, the tip follows the target well. However, value of sim_ikresult_fail is true during the whole simulation. Does it mean ik fails all the time? Why does this happen?

Gloria

coppelia
Site Admin
Posts: 5614
Joined: 14 Dec 2012, 00:25

Re: Inverse kinematics of hybrid robot

Postby coppelia » 24 Apr 2017, 07:29

The reason is probably that your ik chains are over-constrained. This is also the reason my only a damped resolution method is stable. You will have to carefully look at each kinematic chain and analyze what constraints can be activated and what constraints cannot be activated (e.g. you cannot constraint a 2 DoF kinematic chain to a 3D position).

Cheers


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