object following a target : pb if rigidbody activated

i have a script that moves an objectIA to a target. - target is player and has CharacterControler and Third personn controler - objectIA has RigidBody and spherecollider

when playing on a terrain :

First pb: when UseGravity is checked on ObjectIA, it can't follow target when it moving on some elevation. When UseGravity is unchecked, it flies and no follows the terrain anymore so it's not realistic.

Second : when target collide with objectIA, later isn't following target anymore (strange moving in fact) . It seems that a force has been applyed to objectIA, discarding it moves controlled by script ....

thank you for you help