Our goal is to create a realistic looking arm snap due to a malevolent force consuming the owner of the arm’s body. Now of course visual effects and a green screen could do wonders for us, but we want whoever is watching our effect to feel so uncomfortable it’s as if their own arm is breaking. We plan on achieving this by having a silicone arm cast rip open and snap caused by an animatronic hidden beneath the skin of our arm cast to achieve a more lifelike snap.
The animatronic will simulate two bones being snapped in any direction we choose. The four blue oval shaped objects in the figure above, or the bones as I will refer to them, will be able to move in any direction (North, South, East, West, Northeast, Northwest, Southeast, and Southwest). This will be achieved by having each of he two oval shaped bone pieces closer to the elbow attached to one side of the left and right universal joints (described in figure below). This side will be move freely whereas the other side of the universal joint will be fixed to something like a wooden block. The side that can move freely has an L shaped 3-D printed object with a hole on each side of the L. One end of a rod is loosely attached to each hole and the other end of the rod is loosely attached to a servo motor. The rods are loosely attached so that the servo motor can push the rod more effectively. Each bone will have two servo motors each. One motor will push/pull left/right and the other motor will push/pull up/down. When both motors are moving at the same time, the bone can move in any direction besides backwards.
The other side of the blue oval shape closer to the elbow is loosely tied to one side of the other blue oval on the same side. This is so that the oval controlled by the servo motors can control the other oval not connected to the servos. For example, when the servo oval is pulled up, the attached side of the other oval will move up as well. The other side of the oval not controlled by the servo motors is attached to one side of a universal joint so that it too can move freely. The other side of that universal joint will stay fixed to another wooden block or something.
Our chosen material for the bones was originally ABS plastic from a 3-D printer but it would have too much of our time to wait for each oval to finish printing. Instead, we are going with a wooden dowel we shaped to be round to roll along the attached bone piece that is also rounded. This saves us so much time while also keeping the bone strong. The universal joints will be 3-D printed to save us some money. I designed them myself and I used them in the past so I know they work. The servo motors will be standard high torque servos so they can push the thick silicone skin. The servo motors will be controlled by an Arduino. For my next post, I will show how to use an Arduino to control servo motors, sensors, LEDs, sound, etc.