Fishes are fast, agile and efficient creatures, they are fit to explore the enviroment where their evolution spent the last 500 milion years. Thus, many of them are now incredible animals inspiring several bio-mimiking robots developed for different missions, Nereid is going to be one of them.
Nereid is an humble attempt to grasp the endurance and efficiency of migratory species like cetacean and tunas, by mimicking the latter.
I am now developing this simple and modular tuna robot to keep on the work started from my completed bachelor thesis, so that I can put down a first brick for a more detailed project to carry on in the future.
Since I am currently studying for a master degree in off-shore engineering, I won't have a lot of spare time, so my keywords for this project are: short deadline, low budget, simple choices and modularity.
Studying steals me time from doing a full in depth showcase of the current progress, but so far I have designed the hull and bearing parts to allow Nereid to flex, discretely (pun intended). In fact, this robot will be divided in 4 parts to describe a waving/flapping motion: head, first half of the tail, second half and fin. This requires the parts to rotate around the z axis and to not block each other, so I worked on this and on the bearings to make this rotations possible under a certain loads.
Here a screen shot of the external result so far.
A big decision that made this 3d design possible is the use of 3d printed PETG and other assembling pieces made of special plastics. This will allow me to implement a big variety of complex surfaces that this bio-mimetic robot needs, meanwhile other sturdier plastic components will help me run from the problem of rust. However, plastic is a choise that I have to carefully ponder, because seaworthiness needs to go hand in hand with biocompatibility.
Selecting the right materials will be crucial, but it won't be hard. Plastic components can be very reliable and durable and I have easily found affordable components on several websites, some for offshore and others for medical use. An interesting intersection of markets.
I managed to make a rough estimate of the tail loads in order to consider a simple kinematic mechanism and a trasmission system. Those are going to be the next groups of components to be implemented on the cad.
This project will probably become a long one, so I will be thinking on how to give it a better purpose...maybe I will organize a crowdfunding to build Nereid and later opensourcing the entire project. But let's see what happens with each mile of swimming through this adventure.
Welcome to my first log, I will be coincise and straightforward as I hope to have the time to put more detailed explanations in the details section. But as I mentioned before, this project is more focused on a simple build from where to start a project with a more scientific and scrupulous approach.
As I want to build and efficient robot that mimicks the kinematic of a tuna, I will choose the geometry of a Thunnus thynnus whose plans are well documented in "The design of a flexible hull undersea vehicle propelled by an oscillating foil" (Barrett, 1994). It should be quite hydrodynamic, even though I simplified the geometry by sweeping the elliptical cross sections of costant ratios along parabolic curves.
Once I chose the geometry, I defined the lenghth using the strouhal number formula for caudal propelling fishes. I imposed a Strouahl number of 0.3 for best efficiency, a cruising frequency of 3Hz as it is in many species and a swimming speed of 0.5 m/s. By imposing these numbers and linking the peak to peak foil amplitude to the body length (15% as mentioned in "The Kinematics and Dynamics of Undulatory Motion, Masoomi, 2015), I got an overall length of 650mm.
Consequently, I divided the overall length in head (rigid body, 46%), tail (ondulating part, 33%) and caudal fin (foil, 21%). Those divisions are derived from "Tunabot flex, White, 2021" and other works of George Lauder. The latter has published several anatomical and kinematics studies about Thunnus Albacares.
The foil design is a big topic to discuss later on, but at least I have the lengths of the moving part of the robot and the rigid one.