Like my last Festo-based project Bionic NXTPod 3.0, my next project will use the Fin Ray Effect® (see bottom of page). Since it’s kind of difficult to find out what exactly the Fin Ray Effect® is online, I’ve tried to scramble together as much information as possible online to get a good understanding of it, and how it might work with LEGO MindStorms, Technic and pneumatic pieces (quite a mouthful:)). In the end, I hope to use this information to create a fast color sorter for LEGO balls or small bricks.
The Basics
The Fin Ray Effect® is based on the basic principle of how fish’s, especially rays’, fins work. In other words, “the Fin Ray Effect® is a design based on the functional anatomy of a fish’s fin”. A part using the Fin Ray Effect® is made up of two struts or rods, which are joined at their tips forming an acute triangle. It can also be done using three, four or six rods, to add the third dimension, like in the Bionic Tripod (the same goes for versions 3.0 and 2.0). Oftentimes, there are also supports in between the rods to keep them apart and make the structure steadier and more flexible.
Basic structure of the Fin Ray effect®. Image source
To move the structure, you simply move the ends of the rods up or down, and the movement gets translated into a smooth circular sideways motion.
Two lineal forces applied to the rods make the end curve sideways. Image source
Applications of the Fin Ray Effect®
Festo has already used the Fin Ray Effect for several robots in their Bionic Learning Network. They include:
The Bionic Tripod series
- Bionic Tripod 1.0 (vertical, tripod hangs below)
- Bionic Tripod 2.0 (horizontal)
- Bionic Tripod 3.0 (vertical, tripod stands on top; LEGO replica: Bionic NXTPod 3.0)
Underwater robots
Airborne robots
Miscellaneous
- InteractiveWall
- Fingripper (as seen on Bionic Tripod 1.0)
The last one I found out about is Festo’s bionic material sorting gate, a colored ball sorter using the Fin Ray Effect®, which is going to be the inspiration for my next project. The goal is to go for at least 50 successful sorts per minute (the Festo one does 3 per second). The above websites were also the sources I used for writing up most of the rest of this article.
Connecting with LEGO
To recreate the Fin Ray Effect® in LEGO, you would need to find LEGO pieces to replace all of the following:
- Two actuators
- The support beams
- The struts/ rods
For the actuators, there are a few possibilities. The “best” option would be to use Firgelli linear actuators (L12NXT), to control how far the actuators move. The problem is the price tag: they’re currently sold for 55 USD each. The next option would be the LEGO ones, but since those are mostly controlled with power functions, they wouldn’t be too precise. They’d be very big and clumpy with a NXT servo attached.
Firgelli’s Mindstorms based linear actuator. Image source
Stepping away from motorized/ electric actuators, next would be pneumatics. They’ve got quite some advantages: speed (if they’re modified), as much strength as you pump into it, and they’re relatively cheap. The only problem is that they’re not directly controllable from the NXT, and virtually impossible to keep or even get to a specific place using LEGO alone. The only options would be open or closed.
Simple NXT servo motor based linear actuator; better version here
As a last resort, you could build your own linear actuators, like I did. It isn’t going to be very compact and probably not as strong as a “real” one, but it’s the cheapest option by far (given that you have a NXT :))
The connection beams aren’t much of a problem either. You could just use regular rounded technic beams. They’d have to become smaller towards the end, though. It’d also be best if they went down in size gradually, so don’t use something like 11, 11, 9, 9, 7, 7, etc. because that’d compromise the flexibility. Something like 11, 9, 7, 5, etc. would work much better.
It’s also important to make the supports slightly turnable, so the beams don’t always have to be at a 90° angle from the struts. Remember the support beams are only there to keep the rods apart, not make them immovable.
Most difficult to figure out are the struts; there are pretty much only two options available. The first are the hollow hoses found in some Technic sets, usually in silver or black. These are very hard to get, so aren’t really usable for bigger constructions.
A yellow soft axle hose; usually seen in black. image source
The other ones are very good though. They’re thin, you can connect them using a simple pin in between, and they’re flexible but try to get back to their original position as much as they can. And you can get them for less than half a dollar each, over here! The problem is that these soft axle hoses aren’t very strong by themselves, so it’s a good idea to use two of them in parallel.
So there you have it: using the Fin Ray Effect® in LEGO is possible, and, with the right materials, can provide an interesting new way of building robotic arms and such.
Fin Ray Effect® is a trademark/brand of EvoLogics Gmb
The solution came in the form of a differential, which contributes the input power applied to it to one of two outputs depending on which one has a smaller resistance (check 
