Using arm_kinematics, Inverse Kinematics Calculations fail

asked 2011-07-11 07:20:56 -0600

755 ●24 ●34 ●45

I've been going through the PR2 Kinematics tutorials, but using our own arm ( http://www.ros.org/wiki/pr2_kinematic... ) and I've encountered a problem.

Using arm_kinematics ( http://www.ros.org/wiki/arm_kinematics ) in ROS C-Turtle, we're getting an Inverse Kinematics failed message.

I've also tried doing the forward-kinematics for an arbitrary robot pose, and then putting the result into the inverse kinematics request message. For some 'more complicated' poses (i.e. not very close to straight up) the inverse kinematics fails. For simple poses (close to straight up, all 0's) I can get a single inverse kinematic solution. The forward kinematics calculations look like they're all working correctly, however.

Any ideas of what I should look into?

edit retag flag offensive close merge delete

add a comment

2 Answers

Sort by » oldest newest most voted

answered 2011-07-11 08:02:29 -0600

David Lu

10932 ●123 ●219 ●250 http://MetroRobots.com/

Are you filling in anything for the ik_seed_state?

FWIW, arm_kinematics was written because no generic IK solver existed for anything but the PR2. I acknowledge that it doesn't work in all situations, and welcome patches for my naive implementation of IK.

edit flag offensive delete link

Comments

I am, and it was all Zeros, like in the tutorial. I'm able to find solutions to close to this, so I believe its probably a case of the solver timing out. However, I have a timeout of 10 seconds set, and its telling me no solution exists well before then.

John Hoare ( 2011-07-11 08:26:30 -0600 )edit

Changing the number of iterations didn't help either?

arebgun ( 2011-07-11 08:29:47 -0600 )edit

After changing the maxIterations, the search goes on for longer, but it still does not seem to find a solution.

John Hoare ( 2011-07-11 08:37:31 -0600 )edit

Try changing the seed state to not be all zeros. Use a random state (within joint limits). arm_kinematics uses a numerical solver unlike the pr2_arm_kinematics which uses a custom analytical solution. Numerical solvers can be very dependent on the initial seed choice.

Sachin Chitta ( 2011-07-11 08:54:46 -0600 )edit

Is the general approach to generate random configurations and try multiple times?

John Hoare ( 2011-07-11 09:25:17 -0600 )edit

For the kdl solver (which is the base of arm_kinematics), yes, you may need to start with different seed state positions, as local minima may prevent the solver from converging for certain seed states. Adding iterations should potentially help as well.

egiljones ( 2011-07-11 10:45:43 -0600 )edit

I find that seeding with the current joint_state rather than all zeros can also help find a solution more often.

Pi Robot ( 2011-07-11 11:15:50 -0600 )edit

Depending on the arm configuration all zeros is actually quite likely a very bad seed state as it is likely causing singularities.

dornhege ( 2011-07-11 11:38:39 -0600 )edit

see more comments

answered 2011-07-11 07:35:03 -0600

arebgun

2121 ●7 ●22 ●43 http://www.cs.arizona....

updated 2011-07-11 08:38:58 -0600

Did you try increasing the number of iterations or increasing the timeout value? It might be that it's timing out or reaching iteration limit before finding any IK solution.

To change the maximum number of iterations of the IK solver, set maxIterations parameter of arm_kinematics node to some value:

<node pkg="arm_kinematics" type="arm_kinematics" name="arm_kinematics" >
    <param name="maxIterations" value="5000" />
</node>

edit flag offensive delete link

Comments

I can't see an iteration limit in the request message. I do see a timeout, and I give it 10 seconds, but it returns no found positions before that.

John Hoare ( 2011-07-11 08:30:38 -0600 )edit

It's the maxIterations parameter of the arm_kinematics node, you have to do it when launching the node.

arebgun ( 2011-07-11 08:33:37 -0600 )edit

add a comment

Using arm_kinematics, Inverse Kinematics Calculations fail

2 Answers

Comments

Comments

Question Tools

Stats

Related questions

Using arm_kinematics, Inverse Kinematics Calculations fail edit

2 Answers

Comments

Comments

Question Tools

Stats

Related questions

Using arm_kinematics, Inverse Kinematics Calculations fail