The physics simulation of the actor is invalid when set the position of the actor directly.

My robot model has no wheel,so I remove the Friction of the ground.But the robot can not keep a certain position, then I send the position signal to the model in a period way.However, in this way the robot’s physics simulation is invalid and I can not control the robot to grasp anything.
My question is how to keep the physics simulation on while I set the position of the robot? Do you know how the handle the problem?
Thanks sincerely.