At my first test, 6 kg servo successfully managed to turn the lock cylinder and opened the door. But after a couple turns, the servo destroyed itself. As you may notice, the servo I've choose for this project has plastic gears on it. I've found out that the plastic gears are not suitable for this kind of applications. Servo arm and it's attachment point were also damaged during the tests. This happened since there was no sensor threshold detection algorithm yet. Therefore I have to find a threshold detection algorithm before continue testing.

Modified New Servo

I found a metal geared servo in my small junk yard. I removed the internal electronics of the destroyed servo and mounted them to the metal geared servo shell. I also bought a metal servo arm which has side screws to mount it to the servo shaft.