Actually robot hand and robot arm should be connected end-to-start, so i set robot hand as the child of robot arm, and spatially position them together. But when starting the game, i found funny movements because of collision between two rigidbodies. How can i fix??
You should use a fixed joint to connect the two. Select one of the rigidbodies. Then, under Add Component\Physics, add a fixed joint. Drag the other object onto the Connected Rigidbody slot, and you should be set to go.