setPoseTarget succeeded while setPositionTarget failed

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Hi all,

I'm using TRAC_IKKinematicsPlugin to solve the ik problem of a 5DOF arm. If I set the position and orientation of eef_link using move_group.setPoseTarget(), the plan succeeded. But if I only set the position of eef_link using move_group.setPositionTarget(), the plan failed, which makes no sense since less constraints are set. Or does it randomly generate an orientation then plan?

To test it, I use setPoseTarget() to solve 6DOF arm, it indeed gives different orientations of eef_link when I plan several times... For a 5DOF, if a random orientation is set, it is more likely to be unreachable.

Is there any way to let setPositionTarget() have a high success rate?

Thanks for any help!