Nitinol Animatronic Eye

I first constructed this rough mockup of the nitinol control circuit using electronics components that I had on hand. This was mostly a proof of concept, and was definitely not optimized.

Once I felt confident that the circuit could work, I started locking in design variables, such as actuation voltage and nitinol wire length. I then sourced the necessary components for a more permanent prototype online, and soldered it together on perfboard. After testing and validating this more robust version, I moved on to modeling the circuit using Fusion 360.

Using Fusion 360's ECAD features, I was able to develop a PCB of my custom circuit. I began by selecting my components and defining the electrical schematic.

After defining the schematic, I moved on to the PCB layout, including component placement, defining traces and ground fills, and silkscreen text.

Although not strictly necessary, I also created a CAD model of my PCB in case I ever want to integrate it into a larger assembly project.

Once I confirmed that my PCB design conformed to the design rule checks (DRC) of my chosen fabrication service, I sent off my files. A few weeks later, I received a set of manufactured PCBs. After integrating and testing the nitinol actuator, I concluded that the position feedback signal needed significant refinement to be useful for closed-loop control.

I updated my circuit to include additional components to refine the position feedback signal. While I was at it, I also converted the existing components to SMT (surface mount) versions to increase the compactness of the design - except the resistor R1, which I want to be able to easily desolder and swap for different values to accommodate different lengths of nitinol wire. In addition to the PWM input and position feedback output, it also features a small potentiometer that can be used to adjust the “zero” value of the position feedback signal.

After testing my updated PCB prototype, I've seen remarkable improvements in the position feedback signal. I also discovered some areas that needed more fine-tuning, as shown in my v2.2 PCB layout.

In the most recent iteration of my design, I added a new input pin for an external reference voltage and some more components to smooth out the linear voltage regulator and feedback signal.

I’m troubleshooting the latest iteration of my PCB prototype, since the position feedback signal is experiencing interference from the PWM input. I think the high current passing through the nitinol circuit creates a voltage differential between the PCB and microcontroller ground planes, since I can measure the correct signal when referencing the PCB ground plane. The current PCB v2.2 is approaching a finished product, as demonstrated in the video below.

This design concept was heavily inspired by existing mechanisms available on the internet, such as Will Cogley's 3D-printed animatronic eye assembly. I quickly realized that this mechanism is better suited to rigid linkages and servo motors than flexible nitinol actuators. Reflecting this, I decided to pursue concepts that better account for nitinol's advantages and disadvantages.

My second design concept for the eyeball mechanism consisted of a hollow eyeball encased in a socket. The nitinol wires attached to a smaller sphere within the eyeball, magnifying the wires’ relatively small range of motion. The idea was to exploit the simplicity of nitinol as an actuator by minimizing linkages or other moving parts.

Upon testing my initial design solution, it became clear that the biggest drawback was friction between the 3D-printed parts. After iterating through a dozen or so prototypes - varying tolerances, surface finishes, and lubrication methods - with little success, I decided to return to the ideation process.

After returning to the drawing board, I brainstormed as many novel joint concepts as I could, even if I wasn't sure they would work. I made rough initial prototypes for a few, including this one that uses four cords in tension.

This concept uses a much smaller internal ball joint - the idea was that the much smaller surface area would reduce friction considerably, but the design was also (predictably) much too weak and easily broken.

This concept uses a cord in tension, running axially through the cross-shaped rocker that holds the eyeball. A knot in the center of the cord kept the rocker in place. Instead of friction, maintaining tension in the cord became the driving issue with this design. Plus, it was really hard to assemble.

This version uses a pseudo-ball joint inspired by the mechanism inside a joystick. I was struck by how similar the motion of a joystick is to the desired motion of an animatronic eye. This solution was reliable and easy to assemble, making it a top contender. I continued tinkering with similar solutions to see whether I could improve it.

This steel-bead ball joint was the easiest to print and assemble. Plus, it had a smooth, consistent motion across its full range. It just had one drawback - I couldn’t find a good way to attach the wires such that the small range of actuation was converted to a large range of motion.

After much ideation and prototyping, I finally settled on a winning design. This mechanism uses a rolling contact joint to leverage surface friction into a design feature rather than a constraint. In addition, it has a good range of motion, is easy to print and assemble, and has a geometry that enables straightforward wire attachment.