3DOF robot in Moveit2

Question

I've successfully imported a 3 DOF SCARA bot via the Moveit2 setup assistant. While I can set a random valid pose in RViz, and plan path and execute, I can't move the end effector to a desired pose. The interactive marker exists and can be rotated, but it cannot be translated.

As a next step, when I was following this tutorial, I ran into a problem. In the tutorial, a pose is specified for the end effector, and a path is planned and executed. When I try this with my robot, I get the error:

/src/ompl/geometric/planners/rrt/src/RRTConnect.cpp:252  Unable to sample any valid states for goal tree

I've verified that the given poses were valid. When I set random joint values using this instead of pose, I get the robot working. But what I'd like is the ablility to input pose, instead of joints' values -- I want IK to be handled by Moveit.

Going through some of the answers, I encountered suggestions like it may be goal tolerance, or could be that the IK solvers aren't good at robots with < 6 DOFs. While I've tested with higher goal tolerances with no success, I'm skeptical that the solvers aren't good for robots < 6 DOFs.

I'd really appreciate if someone can point out the exact issue and direct me to a solution. If you need more specifics, please let me know. Thanks!

Answer 1

Unable to sample any valid states for goal tree

This error usually means that IK failed to find a valid solution for one of the intermediate points between start and goal. A 3DOF arm is going to have a huge number of EEF poses that it can never reach.

You may have more success asking for a "position only" trajectory (i.e. you're saying that you don't care what the EEF orientation is.) If this also fails, try giving the planner more execution time.