[Al Skierkiewicz]

Mikhail,
The simple answer is the Fisher Price even when used with the supplied transmission cannot hold itself in place when the power is removed especially with the weight of the arm and whatever it might be holding. You can couple to a worm gear system as some have discussed or you can use a pneumatic operated brake to hold the arm in place. You can even use a servo motor to operate a mechanical stopper, the choice is up to you.