C4tfish – the MINDSTORMS fish

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In this article I share the building instructions for a LEGO MINDSTORMS Catfish. I designed it as part of the #RobotRemix5 challenge. I share some of the thinking behind the design and give some hints for programming the fish with a little YouTube tutorial at the end. Enjoy!

With this creation I wanted to simulate a very lifelike fish in LEGO MINDSTORMS. Realistic robot fish have been a fascination of mine for some time now. And recently at LEGO World 2018 in Copenhagen I was invited to partake in the RobotRemix #5 challenge. That was the ideal moment to finally realise my fish dream: the challenge was to build new model based on the LEGO MINDSTORMS retail set and the LEGO Technic Hovercraft. This article is a build log and at the end you will find the building and programming instructions to make your own fish.

Technic Hovercraft 42076
MINDSTORMS 31313

I first entered the challenge with some sketches to explain my idea. I had looked the parts list of the hovercraft online and tried to combine them into a fish. The 31313 was known territory for me, I didn’t have to look op these parts.

Initial sketch of the Technic Catfish
Sketching out all views for motor and brain placement

For the propulsion I had the idea of using a skating movement, so no forward driving motor, but generating a forward force by pushing a wheel sideways. My first prototypes focused on getting this movement working.

In this phase I just built sketches, without limiting myself to the parts list. The challenge here was to find a working setup and the right weight distribution in the model. Ultimately I succeeded in a reasonable movement.

Forward motor

Still I decided it wasn’t fishy enough. The friction of the floor and the wheels didn’t have the lazy, easy feel of fish under water. So sadly I had to give up the innovative propulsion and add a forward motor anyway.

Detailing the shape

When the movement was ok, I could finally start detailing the shape.

Mouth version 8
Mouth version 8
Mouth version 14 and eyes verion 3
Mouth version 14 and eyes version 3
Mouth version 14 and eyes version 5
Mouth version 14 and eyes version 3
Mouth version 18 and eyes version 2
Total restart. Back to the drawing board to find the right expression.
Total restart. Back to the drawing board to find the right expression.
Final version
Final version

I ended up adding an Ultrasonic Sensor to the catfish. If you don’t have one, buy one or use the color sensor in it’s place. With the US sensor the fish can swim around the room like in a fish bowl. It is very quieting to watch.

Build your own Catfish

Make sure to plug in a cable when the building instructions show a motor or sensor. The first motor in the instructions is the Large motor for the wheels. The cabling for the fish is as follows:

  • Port A: 25 cm – Large motor with wheels
  • Port B: 35 cm – Medium motor last tail segment
  • Port C: 25 cm – Large motor in first tail segment
  • Port 1: 25 cm – Touch sensor
  • Port 3: 35 cm – Ultrasonic sensor / Color sensor if you have no US Sensor
  • Port 4: 25 cm – IR Sensor
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Can you figure out how to program the fish? Here’s a little YouTube video about motor synchronization that might come in handy:

Technic Hovercraft 42076
MINDSTORMS 31313
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