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What is w in orientation doing ?

asked 2017-08-11 20:40:43 -0500

rozoalex gravatar image

Hi Guys, I am trying to send a goal to navigation stack, however I just if I set the goal pose as Location x=5 y=5 z=0 orientation x=0 y=0 z=0 w=0 It wouldn't generate a path.

If I put z as 1 and w as 0, it works sometimes and doesn't work sometime. The weirdest is that sometimes it's spinning to the goal like a fresbee...

My understanding about the goal pose is that z is just the yaw, since we are moving in 2d space there is no pitch and roll. So the question is : is my understanding right? And what is w? What it is doing?

Thanks!

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answered 2017-08-12 01:01:32 -0500

jayess gravatar image

updated 2017-08-12 11:09:58 -0500

The way that ROS defines orientation is with a mathematical concept called a quaternion. They're a good way to represent orientation as they're less ambiguous than roll, pitch, and yaw. But, they have the drawback of being a little difficult to understand. Here's a tutorial that someone wrote to help you understand them.

Now, on to the meat of your question. The reason that you can't generate a path with those components is because if you want to send an orientation equivalent of roll = 0, pitch = 0, and yaw = 0 you need to have the w = 1, not 0. The way that you have the quaternion is not valid.

Many people choose to generate an orientation in roll, pitch, and yaw and convert it to a quaternion. That tutorial will show you how to do this.

Here's another question that should help.

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The way that ROS defines orientation is with something called a quaternion.

This phrasing seemed a little ambiguous to me: it could almost be understood to mean that we 'invented' something new to represent orientation. Quaternions are obviously just a generic mathematical concept.

gvdhoorn gravatar image gvdhoorn  ( 2017-08-12 02:57:46 -0500 )edit

@gvdhoorn I updated the answer with your comment in mind.

jayess gravatar image jayess  ( 2017-08-12 11:10:57 -0500 )edit

Thank you so much! But even after I always set the quaternion to 0 0 0 1, the robot is still spinning toward the goal. I If I use the 2D nav goal on rviz, sometimes it still spins. Why is that? How can I avoid that?

rozoalex gravatar image rozoalex  ( 2017-08-12 13:36:36 -0500 )edit

@rozoalex, no problem, happy to help. You should create a new question that includes (preformatted) terminal output, what you've done, what you expect, and any (preformatted) parameters/configurations that you have for help with your other questions.

jayess gravatar image jayess  ( 2017-08-12 14:48:06 -0500 )edit

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Asked: 2017-08-11 20:40:43 -0500

Seen: 4,976 times

Last updated: Aug 12 '17