how to let the end effector reach targetpose along an approach direction?

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Hi all, I 'm using moveit to plan a trajectory for the end effector to reach a target pose. I expect that in last part of the trajectory the end effector moves along a given direction.

Currently I set a waypoint B which is on the direction and plan in two stages. First from initial state A to waypoint B, use the normal setposetarget(B) and group.plan(plan). Secondly I try to use computeCartesianPath(C,arg) so that the end effector can move along the approach direction. But the second part always fails to be executed.

Is there another way to do this?

Thanks for any help!