A medical device assembly including a support member, a first arm rotatably connected to the support member at a first rotary axis, a second arm rotatably connected to the first arm at a second rotary axis, and a medical unit connected to an end of the second arm opposite the first arm. The first arm is prevented from rotating about the first rotary axis when the first arm and the second arm are substantially aligned. The first arm and/or the second arm can also be prevented from rotating to prevent the medical device assembly from abutting against an obstacle.