“A springless animatronic tentacle mechanism that can move in all directions and is strong enough to pick things up.
because I can. When has a lack of good reasons ever stopped anyone.
Also, the original spark for this project was this hackaday article. In this article the construction of a cable driven animatronic tentacle mechanism was described. While this construction used has been known for decades, and the article gave a great example, I was captivated by an idea.
Most cable driven tentacles use springs. These springs make sure that the cable tension evenly distributes itself over the length of each joint. This is a simple solution, but it presents a problem. If you want to carry 1kg, your spring needs to support this. Any load you apply will interfere with the spring distributing the load. The more weight you want to carry, the stiffer the spring needs to be. The winches are fighting these springs, so this becomes inefficient quickly. These mechanisms are only ever used to make something move tentacle like. Also, keeping the tentacle torsionally rigid is a great challenge. The tentacle in the article only moves it’s own weight, and it tends to flop over when oriented poorly. It is not load carrying. A common material used for the core of a tentacle mechanism is a speedometer cable. This cable is flexible and torsionally (fairly) rigid, but only until a certain point.
Trying to kill two birds with one stone, I wondered if it would be possible for a mechanism to distribute a force without the spring. Can you make a tentacle curve evenly without a spring, thus not being influence by any external load. Any mechanism was bound to also be torsionally rigid. This question led me down a 2 week rabbit hole of mechanisms, ultimately leading to this tentacles design.
Length of tentacle: 60cm
Maximum load: 0.5kg (with difficulty)
3 sections with in total 6 Degrees of freedom in the tentacle
Total range of motion: 270 degrees in each direction
6x Nema17 stepper motors for the movement
2x Corona DS339HV servo in the head
Power usage: 120W
Teensy 3.5 in the controller
Arduino Nano in the remote
How it works
The tentacle consists of 3 sections each being able to bend in 2 directions. Each section has 4 cables. When 1 cable contracts, and the opposite cable expands, the section bends evenly. In the base are 6 winches that contract and expand each set of cables. Bicycle gear outer cable is used to bring the cable from the winch to the joints.
The mechanism in each section is a gear linked virtual rolling joint contact based on the motion of an anti-parallelogram. This mechanism is explained in more detail further down on this page.
The head uses 2 hobby servos, one to rotate and one to open and close the gripper.