We'll make a two-axis gimbal to enable the eye to look left and right, as well as up and down, and a mix of these positions.
There are many interesting, clever, and sometimes complicated ways to build animatronic eyes. You can use fixed servos with linkages, and even mulitply the linkage layout to drive two eyes simultaneously. But for this project, we'll keep it simple.
We'll connect the eyeball directly to the micro servo used for left/right movement, and then connect the micro-servo to the full-size servo. This means we don't need any complex or fiddly linkages. The only trick is to get the center of rotation of each servo to match the pivot of the eyeball on both axes.
We can do this by connecting the micro servo to the full sized servo via a small bracket made of cardboard! You can see how this works in the diagram below.
First, we'll prepare the eyeball. You can start with a white ping pong ball and use markers to add a pupil and iris. A white paint marker makes the sclera look a bit more realistic. You can even add some veins with a thin red marker!
- Start with a black marker for the pupil
- Add a lighter colored iris
- You can add subtle streaks in the iris
- White paint or paint marker can be used for extra detail in the sclera
You can use hot glue to attach the top of the ping pong ball to the micro servo. Pick a servo horn and glue it in place, then place the horn on the splined shaft when it is dry.
- Choose a point at the top pole of eyeball and place a dab of hot glue or white glue -- don't use too much glue or it will fill up the horn's center and it won't fit over the servo shaft
- Set the servo horn on the glue so that the larger, open splined part will still be able to fit over the servo shaft
- Hold it in place as the glue cures
- Place the horn onto the micro servo's shaft
Add a small cardboard bracket to hold the micro servo in place and connected to an arm on the full sized servo. The important part here to to place the pivot of the eye at the same center as the full size servo's shaft.
You will glue on a corner brace to keep the 90° bracket stable.
- Cut a small strip of cardboard and trace the micro servo as shown
- Cut out the slot for the micro servo
- Press fit the servo into place as shown
- With the eyeball in place, bend down the cardboard into an 'L' bracket so it reaches a bit below the center of the eyeball -- this is where the bracket will attach to the full size servo for up/down rotation
- Cut and glue a small triangle onto the bracket for support as shown
The full size servo will rotate the bracket, micro servo, and eyeball around the up/down pivot point.
- Mark the up/down center of the eyeball on the bracket
- Glue a small servo horn to the pivot point of the bracket
- Once the glue cures, press the horn onto the servo shaft as shown
Test out the motion by rotating the servo shafts (this only works when they aren't plugged in and powered up). You can turn them each about 180°.
You may find that the shafts weren't centered when we attached the servo horns. That's no problem -- we'll find the center positions and re-connect the bracket and eyeball later to match.
Your eyeball gimbal is complete! Time to program it.