Mismatch between DH matrix computation and visualization of robot on webots

[alecive]

Hello, I'm a relatively new user of Webots and I trying to create a simple exercise in which students can understand Denavit Hartenberg. I'm using UR5e, and a modified version of the UR demo in ENU coordinates (I used the scripts in here to modify the default world file to use ENU).

What I am doing is the following:

1. I imported DH parameters of the UR5e from here
2. I created my DH chain, computed the forward kinematics of the robot and checked that it matches with the excel file available from the link above and also here
3. I computed the end-effector pose by pre-multiplying the forward kinematics above with the base of the robot
4. I visualized the pose with a Transform object

Unfortunately, as you can see here the computed pose does not match with the end-effector pose. It seems to be an issue with rotations, however I do not understand how to debug this (primarily because I'm a new user):

• How can I get the position of the end-effector from within webots?
• Where can I see how webots rotates bodies and reference frames?
• Do you think there may be a hidden bug (for example, emerging from this PR Allow Z vertical axis #1643)?
• Is there a way to create a DH matrix from the proto file? I see there is a URDF-to-PROTO converter, but not the other way around.

Also, I'm trying not to share my code given that this is an assignment (but I can share bits of it if needed). Here is my world file: ure.zip

Thanks for the help!

[lukicdarkoo]

How can I get the position of the end-effector from within webots?

Select the gripper and click on the Position tab in the scene tree:

Where can I see how webots rotates bodies and reference frames?

You can right-click on the robot and Convert to Base Node(s) to display the full node tree in the scene tree. Then, in the scene tree, you can see a pose (relative to any other node), velocity, joint properties...

Do you think there may be a hidden bug (for example, emerging from this PR Allow Z vertical axis #1643)?

Probably not as we didn't touch anything UR specific.

Is there a way to create a DH matrix from the proto file? I see there is a URDF-to-PROTO converter, but not the other way around.

You can save it as a URDF. Right-click on the robot, Export, and save it as <something>.urdf.

[alecive]

Thanks for the feedback @lukicdarkoo . Here are my replies:

Select the gripper and click on the Position tab in the scene tree:

I am aware of that. But how can I do that programmatically? I tried to getField("Position").getSFVec3f() but it doesn't work. Also, I am not understanding how to get the list of attributes for a node/object.

You can right-click on the robot and Convert to Base Node(s) to display the full node tree in the scene tree. Then, in the scene tree, you can see a pose (relative to any other node), velocity, joint properties...

Very cool! Didn't know that.
However, my question was more generic: can you please point me to the code where webots computes transformations? It's a big codebase and I'm yet to fully grasp it.

[omichel]

I am aware of that. But how can I do that programmatically? I tried to getField("Position").getSFVec3f() but it doesn't work.

You should use node.getPosition() instead.

Also, I am not understanding how to get the list of attributes for a node/object.

You should use node.getNumberOfFields() and node.getFieldByIndex() to browse all the fields of a node. See also field.getName() and field.getType().

However, my question was more generic: can you please point me to the code where webots computes transformations? It's a big codebase and I'm yet to fully grasp it.

Maybe you can start here.

[alecive]

Wonderful, thanks!