Arm Movement
Image taken from HDE Walls
Design Development
Follow our design process to see how we got from rough sketches of octopus arms, to four moving animatronic tentacles and four stationary ones.
Our eight octopus arms attached to the top of the box
Above: set of initial design sketches
Below: basic prototype of left most sketch
Early Designs
Towards the beginning of ideating designs for our octopus’s arms, we had two possibilities of scope in mind. The minimum viable product for our arms were solid structures, made of a solid material like paper-mache or 3D print filament, that would be shaped to look like tentacles. These tentacles would rest atop rotating platforms and slowly spin back and forth while the octopus danced, and would stop spinning when someone approached the octopus.
Our more ambitious goal was to have arms that bent back and forth in a wave motion to look like a waving tentacle. We began pursuing this with sketches of different designs for achieving this movement. We prototyped our most promising sketch with cardboard, duct tape, and string. Each ring of cardboard had a hinge joint connecting to he ring below, the joints alternated sides to allow the “tentacle” to bend both ways. However, this design only allowed left/right movement along one line (no forward and backwards movement) and the movement from left to right looked jerky and unnatural.
Animatronics research
Researching animatronics led us to a blog, Poorman’s Guide to Animatronics, that detailed processes of designing and building different animatronic features. A series of articles on tentacle design and assembly inspired and informed our design: a scaled down version of the tentacle designs commonly used in the animatronics industry.
Poorman’s Guide details the materials used for each part of the arm, and how to create those parts and materials. Aluminum disks shaped by mill and lathe, as well as many of the blog's other suggestions, were beyond what our scope and budget would allow. The mechanical designs described on Poorman’s Guide include upper and lower segments, where each segment uses its own control wires to be able to move in a different direction from the other segment. We did not need this extra complex movement for our octopus’s arms, as we simply wanted them to wave back and forth, and we anticipated that the extra control wires and mechanical complexity would be too much given our time, budget, and space restrictions. The biggest concern was space, as all the control cables would need to be housed and controlled within the box, “tank”, of our octopus.
The Wider Plan
Our plan was to make 4 simpler and scaled down versions of an animatronic tentacle with 3D printed disks, and materials available in the lab: electrical wire for the control cables and central stability cable, and pieces of rubber tubing to place between the disks along the stability cable. Each moving tentacle would be attached to the top of the box with the control cables housed inside, connected to servo motors that would rotate to pull the tentacle to wave back and forth.
Proof of Concept
To test that our planned materials would work for this type of design, we printed just 5 disks and a base from the CAD model provided on Poorman’s Guide. This was a low stakes way to test our materials without spending the time to CAD, print, and assemble a full sized tentacle. Once assembled, we tested the movement by pulling the control wires by hand.
Below see the range of motion on our mini arm:
CAD model of all arm disks and base used for one tentacle
Scaling Up
Satisfied with the results of our scaled down materials test, our next step was to create a full scale tentacle. We decided on a size of about 16 to 20 cm in height, knowing that we would not get an exact height as the rubber tubing between the disks would be squished down as the tentacle moved. We CADed each arm disk size in Onshape, along with a base to be screwed onto the box to connect the arm. We choose to CAD 6 disk sizes and use 20 disks in total, to be arranged in a size gradient on the stability cable to give the impression of a tentacle getting thinner towards its end. The rubber tubing and wire cables are not included in the CAD model.
After 3D printing the disks and base, we assembled the arm using electrical wire for the control cables, and a piece of electrical wire, folded over and twisted, as the stability cable. This was to make the stability cable thicker and stronger to better support the weight of the up-sized arm. We soldered small bulbs on the ends of the wires as stoppers to stop the wire from sliding out of the arm disks.
As with the mini-prototype, we tested the movement of the arm by pulling the wires by hand. Even with the thicker stability cable, the arm’s movement was floppier than we had hoped. After some thought, we decided to continue with the same arm design and stability cable material. We believed that once we attached the arm to the motors that would control its movement, we could tension the control wires in a way that would have the arm stand up higher.
At this time, we also made the choice to pivot to laser cut arm disks due to time constraints. It was a lengthy print for 20 arm disks and a base, and we would need to print 7 more arms. We were also struggling to find enough available 3D printers in the lab spaces to have our arms printed in a reasonable time frame.
Laser Cutting
We continued forward with our same arm design apart from the switch to laser cut disks.
We painted the sides of the disks orange, and after assembling our first arm with laser cut disks, we found the disks worked just as well as the 3D printed pieces.
A birds eye view of our box with the arms all attached
The Arms Within the Box
The box, or “tank” housing includes 8 holes on the surface designed for the octopus’s tentacles to reach out from. We choose for the four corner arms to move as the octopus dances, and the other 4 to remain stationary just for aesthetics. The lower part of the bases of the arms are screwed to the underside of the holes, and the upper part of the bases stick out, with the rest of the arm attached on top.
The Motors
Below each moving arm's location, within the box, are two ET2045 servo motors, configured as shown in the leftmost image to the right. In this image, the black circle on the top is the base of the arm bolted to the top of the box. The motors are attached to the inside walls of the box with 3D printed brackets that our team designed, they are the green and blue pieces that the motors are bolted onto. Each motor takes two opposite control cables (the orange wires) from the same arm. The cables are tensioned securely and with some room for movement to the blades on the servo motors. When the motor rotates back and forth, the control cables are pulled to make the arm wave back and forth.
After first installing a few of the moving arms and testing their movements with the motors running, we had issues with the servo brackets not being strong enough. A few snapped and others bent. We redesigned them with a triangle support beneath the bracket, as shown in the image, and reinstalled them.
Servo motor with control cables tensioned to the blade.
This bracket change fixed the issue, and we successfully installed the rest of the arms. We attached the control cables to the servo motors by bringing the wire through the small holes in the servos' blades, pulling it to the desired tension, and tying stevedore stopper knots to prevent the wire from slipping back through.
In the image to the left, you can see the stopper knots tied at the ends of the orange wire.
Adding Support
After tensioning all the moving arms on the motors and getting them running, we saw that the arms were still floppier than we had hoped. Through some experimentation we found that a band of duct tape around the bottom four disks of an arm helped the arm to stand up taller, without affecting the movement of the upper part of the arm, which could still wave freely. We applied duct tape around the bottom four disks of each arm, including the stationary ones, and hot glued a band of red felt over-top to hide the tape and match the colors of our octopus
The arms on the box with no duct tape support
The arms on the box with duct tape support
The Final Design
Our Arms are each made up of one 3D printed base, 20 laser cut disks of 6 sizes, electrical wires as control cables, and doubled up and twisted electrical wire as the stabilizing cable. Duct tape and red felt are used a support bands around the bottom four disks of each arm. The control cable and stabilizing wires are secured at the top of the arm with soldered bulbs acting like stopper knots, preventing them from sliding back through the holes in the disks. The control cable wires are secured to the servo motors inside the box by stevedore stopper knots
With the arms all attached, and the code to direct the motors completed, our octopus could now dance her three hearts out! Some final touches and she's all done. Our team were all very happy with the motion and personality these tentacles conveyed. You can find the video of our dancing octopus in action on the homepage.
