How to configure UR_Kinematics to prefer "elbow up" instead of "elbow down"?

[victorwhwu]

My current problem is the use of Descartes as the path planner for my UR3. First I did not know that the default IK solver KDL is not very good for Descartes. Then I found out that we should use IKFast but I cannot compile it for my UR3 with a torch at the tool0. Then I found that some one has already had something similar to IKFast, the UR_Kinematics. I used it then I encountered the following problem. It would be better illustrated by the attached Picture from RViz. It is obvious to us, human beings, that if the elbow, or the upper arm, is above the horizon then the tool0 can be moved to much closer to the origin of the manipulator. However Descartes (or UR_Kinematics) keep coming up with paths that have the elbow below horizon and hence the closest path that the tool0 can traverse is only about 0.31 m or 310mm from the origin of the manipulator.

How can I resolve the problem? One of the ways that I can think of is to constrain the joint value of the elbow since Descartes is supposed to be able to handle semi-constrained path. However, I do not know how to specify this constrain, ie the elbow has to be above the horizon? Or can I specify this requirement to UR_Kinematics?

Your help would be very much appreciated.

Best regards,
Victor Wu.