building a real-size functional alien egg - help me please

vykos

Unblooded
Hello all

My husband built a colonial marine costume and won the qualifier round for a cosplay contest. Now we need to improve said costume for the finals.He wants to recreate a scene where the alien egg opens and a facehugger pops out. so...
I have been searching all over for help, I don't know if anyone has seen/made what we are trying to do:

we are building a real-size alien egg, and I would like it to be a functional egg, meaning it would have to open the "petals" just like in the movies. The egg is built in pepakura+fibreglass up to 3/4 of the height, and I will be using EVA foam to make the "petals" (I'm sorry, I'm having a mental blockage and can't think of a better word for the egg's upper part).
I was thinking about using cables attached to each individual "petal" and pull those cables using a servo, possibly controlled by an arduino (or something similar). Probably I am overcomplicating, but I am lost and running out of time.
Has anyone done or seen this? I have seen tutorials, but for smaller eggs, the size of a party balloon, that open manually.
Thank you for your help and I apologize if this post is in the wrong thread

cheers from portugal
 

Wreav

Elite Hunter
Community Staff
I have seen a silicone egg before. Which opened with electronics. Looked so real and so cringy. I'm going to bump this since it looks like you need help. Hopefully someone understands electronics - anematronics (auto correct)
 

Honus

Blooded
I haven't been here in a long time so I'm probably too late but you're on the right track using cable control and microcontroller. You could use an Arduino or something like a Pololu Maestro if you're not comfortable doing programming- the Micro Maestro will get the job done and it'll allow you to get a nice slow controlled motion. The best way to get that with an Arduino is to use digital servos that allow you to program their speed. You're definitely going to want to use strong servos to move those four large petals. Use NiMH or NiCD batteries for power as alkaline batteries won't handle the current draw of the more powerful servos. You could trigger it using a foot switch or operate it remotely.
 

vykos

Unblooded
thank you for your input :) I tried a basic servo to roll the cables, it was a 13,2kg.m torque at 6V using a NiMH battery and a servo tester, and the petals didn't budge. I then cut the cables and tried to use the servo to pull just one, and it worked (sort of...), but it was still difficult for the servo to do the job. I think that if i place an articulation system inside the petals it might lower the strength needed to fold the foam (I hope). I have a video of the egg opening manually here:
If all else fails, I will get a stronger servo.
I also want to thank you for telling me about the pololu maestro, I had no idea it even exhisted (robotics isn't my main field), it looks really interesting, I will surely look into it
cheers
 

Honus

Blooded
Wow- that looks awesome!

I'm guessing that is the stall torque rating of those servos? How long is the servo arm? Just looking at that it would appear you're going to need something a whole lot more powerful. I'd recommend using a servo with a gearbox like this-
https://www.servocity.com/html/spg805a_standard_rotation.html

One thing you might try is adding an extension spring to the servo arm to counterbalance the force of the petal and reduce the load on the servo. Just get a long strip of metal and mount it to the servo arm. Drill a hole in one end so you can attach an extension spring. Make sure the egg petals are closed and then pull on the other end of the spring just until the egg petals begin to move- that will tell you where to begin mounting the other end of the spring. You may have to play around positioning the spring until you can get a good balancing force.

The Maestro controllers are really neat and very easy to set up. They're perfect for a job like this.
 
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