If you’ve been following along with Will Cogley’s fantastic work on YouTube, you’ll know that he has managed to make a really incredible robotic/animatronic humanoid head. It has an absurd number of servos crammed into the skull and is capable of very lifelike facial expressions. But until now, it was just a head and that limited its potential. In his newest video, Cogley expanded his design to add a neck. He engineered the neck with inspiration from human anatomy and a Stewart platform.
A Stewart platform is an interesting type of manipulator consisting of six actuators (usually hydraulic arms or linear actuators) linking two plates. The actuators are in a kind of “zig zag” arrangement that enables six degrees of freedom between the two plates — meaning the top plate can be set to any angle (relative to the bottom plate) and also move in all three axes.
While studying a human anatomy textbook for inspiration, Cogley realized that the muscles connecting the neck to the torso have an arrangement similar to a Stewart platform, if you squint hard enough. That led him to this design, which is a neck that connects the head to the shoulders (a stationary base, for now) through six miniature linear actuators. That makes it so Cogley can program very lifelike movements.
He did his programming in Python and implemented inverse kinematics algorithms, which let him enter a desired position and the program will figure out how to move the linear actuators to reach that position. A PC controls the linear actuators through boards of Cogley’s own design. One is for power distribution and it can output common voltages, as the neck motors have different requirements than the face motors. The second board is for motor data control and it is also configurable for different voltages.
But there was a problem: the geometry wasn’t ideal for a Stewart platform, as Cogley wanted to prioritize a human appearance over stability. That resulted in somewhat jerky movement. The solution was to integrate a spine that mirrors a human spine — providing support while still being flexible. Cogley 3D-printed the individual vertebrae in rigid plastic, then connected those with flexible discs 3D-printed in TPU. It took some trial and error to get the flexibility of the discs just right, but the artificial spine turned out very well.
Now Cogley has a fully actuated neck to match the rest of the head, which will dramatically increase the sophistication of the robot. And at this rate, we can only assume that he’ll move on to the torso next.
