Sunday, December 8, 2013

Eyeball Details: Mechanical

I said I would give some more details of how the mechanical eyeball works, so here we go.

The design uses two motors, enough to turn the eyeball forward or backward around two different axes of rotation. At first I considered driving it with a “reverse mouse wheel” setup, in which each motor turns a roller that touches one side of the ball. But when I sat down with my materials and actually started thinking about how I would mount the motors, I started liking this idea less and less. For this movement to work properly, the rollers need to be held against the ball with enough pressure that they won't slip, but not so much that the whole thing binds up and stops turning … not a big problem if you have a well-constructed housing to hold everything together, but I had cardboard and ice pop sticks to work with. You can only make things so precise with those materials. Okay, not very precise at all, usually.
Anatomy of the human eye.  By Patrick J. Lynch, medical
illustrator, and C. Carl Jaffe, MD, cardiologist, via
Wikimedia Commons.

Thinking about how my own eyeball works, I thought of connecting strings to four points around the ball to act as “muscles.” But muscles can only pull, not push, and I didn't want to be doubling the number of motors needed. I wound a pair of strings around each motor shaft so that when the shaft turns, one string loosens as the other tightens … so a single motor can drive a differential pair of “muscles.” The motor shafts are made from ballpoint pens, and the plastic is soft enough that I could poke holes in the shafts and thread the strings through them. This arrangement seems far more reliable than the reverse mouse wheel drive. If the shaft turns, the eyeball moves, even if things have (ahem) shifted around a bit inside the housing; there's no need to worry about rollers slipping. When the eyeball was undergoing its major demonstration (at work on Halloween), I discovered a couple of minor problems. One of the strings got snagged on a protruding piece of wire and had to be manually disentangled, and another frayed through because it was routed around one of the ice pop sticks and was rubbing against it constantly. I think I can avoid this in the next design iteration with more durable strings (perhaps a polymer monofilament line instead of embroidery thread) and more care given to string routing and possible snag points.

I actually went to several stores trying to get a ball that would work for this project. Originally I wanted one without holes. It also needed to be hollow so I could put a camera inside some future version, and needed a fairly smooth, rigid surface so it could turn easily in the cradle. The only ball I could find that met the other requirements and was about the right size was a wiffle ball, so I had to live with the holes. Then I decided to use the muscle drive, and the holes turned out to be very fortunate, since they made it easy to attach the strings to the ball. This wiffle ball was a one-dollar party favor that came with a little rubber ball that holds flashing LEDs inside … not useful for my purposes, but it doesn't hurt anything. The only problem was the seam where the two halves of the ball were joined together. It wanted to catch on the edge of the cradle when the ball was turned. I solved that problem by filling the crack at the seam with superglue, then vigorously sanding all around the circumference of the ball until the seam was smooth.

The motors I used are six-wire unipolar stepper motors, model #PF35T-48L4. They're fairly precise, easy to control, and have an appropriate torque-speed balance for this project. (I.e. no gearboxes were needed. Phew.)

Closeup of one of the motors and the ball cradle.
There isn't too much to say about the housing, which is really just a junky prototype until I get the tools to make something better. The cradle that holds the ball is made of two cardboard rings of different diameters, separated with pieces of foam. I covered them with part of a smooth plastic bag so that the rough edges of the cardboard would not impede the ball from turning. Wire hoops at the four corners of the cradle guide the strings so that they pull the ball in the right directions. More ball point pen pieces support the cradle above the base. The motors and the ends of the motor shafts are held in place with the ice pop sticks and tongue depressors, which go all the way through the base and the lid to hold everything together. I joined all the pieces with bent paperclip wire or pieces of narrow plastic acting like cotter pins, because I've learned that glue is not to be trusted. (Some strong glue is fine to keep things from wiggling around, but never expect it to bear a load.)

I'll go into the electronics and software in another post.

Eyeball budget (mechanical systems)
Stepper motor* (PF35T-48L4) X 2: $3.50
Wiffle ball X 1:                                 $1.00
Everything else: probably <                 $.25
Total:                                               $4.75

* This model doesn't seem to be available any more, at least not in hobbyist quantities. If you're looking for an extremely cheap stepper motor, the best replacement I can find is this.

Until the next cycle,

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