This project aims to create an animatronic alien creature from the Crab nebula. The “Crabster” has a crab-lobster body and a pacumpkin head with lollipop eyeballs.

This animatronic machine turned into a giant mechanical design challenge with forty-four 3D printed parts, 20 of which were moving parts.

The project was intended to have real engineering challenges as well as make an interesting Halloween machine. Some of the challenges were:

  • to design the machine to snap together with no screws or metal fasteners, a bit like Leggo - using carefully designed 3D printed parts
  • to design the mechanisms involving 20 moving parts to be driven by a single small motor – I didn't want to buy a bunch of expensive motors for a low usage project
  • to design for minimum friction so the motor wouldn't stall
  • to make the machine as big as my printer would allow
  • to design to use minimum plastic while still being strong enough to perform its tasks
  • to design the machine to work off a 5 volt battery module
  • to make the system respond to the presence of people to trigger its animation

Figuring out the mechanism with all of its clearances and tolerances was quite challenging because my rudimentary CAD tool can't easily rotate parts to examine clearances. I had to draw each part in multiple orientations to figure it out. It was important to do all the math up front as I did not want to print multiple iterations of each component. The design took so much effort, I really didn't spend any time video documenting the process.

The motor and lights are controlled by a passive infrared sensor that detects the presence of warm bodies. It has adjustable sensitivity and the animation has adjustable duration. I ended up adding a level shifter in the driving circuit because the FETs I ordered did not arrive in time and the only substitute I had did not turn on fully with the 3.3V output of the sensor. I had to add a few diodes and a resistor to boost the output signal to 4.3V.

Here is a video showing the final build and Crabster in operation:

This project was my most ambitious 3D printing project yet and it involved tons of work, but the engineering objectives were successfully met and I am very happy with what I learned in the process.


Link to the Halloween area:

Halloween - Projects and Videos

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