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URDF transmission linkage question (MeArm)

asked 2017-06-29 10:48:46 -0500

Elecia gravatar image

I have a MeArm with a hip joint connected to a torso connected to a shoulder joint -> upper arm -> elbow joint -> lower arm -> claw joint.

I created a URDF to describe it and it works ok. (It doesn't have a gripper, just a resolute wrist joint for now, that isn't really my question.)

I have it working Gazebo and rviz, even MoveIt!. Here you can see the physical arm, in the position command by MoveIt and shown in the rviz:

Works ok... here

However, my URDF is too simple! When the shoulder goes past 90, the elbow joint is affected and my position is wrong. The transmission linkages are way more complex on the MeArm than in my URDF. Note: I am an embedded software engineer, new to ROS, and not mechanically inclined.

Here you can see the error where the shoulder pulls back, not up.

Shoulder pulls back not up

So, how do I fix the URDF? Is it in the transmission types section of the URDF? I found the transmission interface page but is this a four bar transmission? How do I figure out what parameters to use? What mechanical engineering term do I search for to find other similar arms I can crib from? (I mentioned "not an ME", right?)

Also, does anyone have a super simple gripper/claw in URDF? As demonstrated by my rods-and-boxes approach, I'm fine with the simulation being an 80% approximation instead of a truly accurate description. I'd rather write the software or play with hardware than spend all my time in sim.

Thank you!

A few more images:

Good: Good

Not good: Not good

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1 Answer

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answered 2018-07-19 21:45:59 -0500

matrhint gravatar image

I have added a mimic element with a few small modifications to the URDF to get the arm to move as I think you want it to. I forked your git with the modifications to the URDF made. I believe your issue is that when you change the angle of the shoulder, it also changes the angle of the elbow. Please let me know if it fixed it. Sorry it took a year for someone to try to help you.

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Asked: 2017-06-29 10:48:46 -0500

Seen: 1,045 times

Last updated: Jul 19 '18