Overview
For the ping pong project, it’s important to know precisely where the ping pong ball is and at what time, so that we can predict its future position. Let’s look over how we can do that.
ART
The ART system has cameras that emit IR light. They sell a passive marker that can then reflect that light back to the camera. A passive marker is just a sphere that has been made retroreflective by ART, with a threaded rod to attach to your part. With many of these passive markers, you can get a 6DOF position for your part, but with one all we can get is a 3DOF position. 3DOF is all we need, so that’s fine.
Retroreflection
If you think of a street sign at night, you may remember that it is sharply defined, as long as your headlights illuminate it. Trucks and vehicles have similar clarity, as well as any road workers in hi-viz. This is because of a property known as retroreflection. Retroreflective coatings normally achieve their properties by embedding or attaching microscopic glass spheres or prisms, that work to reflect light back to its source.
Possible methods
Let’s go over a few of the pros and cons of a couple possible methods:
Retroreflective tape
Apply a commercial off the shelf retroreflective tape to the ping pong ball.
| Pros | Cons |
|---|---|
| + Easy to acquire | - Imperfect fit |
| + Lightweight | |
| + Durable |
Retroreflective paint or coating
Either with an adhesive and glass powder, or with a premixed paint. Coat the ping pong ball somehow.
| Pros | Cons |
|---|---|
| + Easy to acquire | - May be fragile |
| + Lightweight |
Retroreflective glass sphere embedding
Somehow embed the glass balls directly into the ping pong ball. Maybe heat to “bake” the glass beads into the ABS?
| Pros | Cons |
|---|---|
| + Lightest weight | - Very hard to make |
| + Retain ball properties | - May be fragile |
Retroreflective fabric
Create a sphere around the ping pong ball with hi-viz fabric.
| Pros | Cons |
|---|---|
| + Durable | - Imprecise fit |
| + Easy to acquire | - Hard to make |
Retroreflective 3D print
Use something like Reflect-o-Lay to 3D print a ping pong ball analogue.
| Pros | Cons |
|---|---|
| + Easy to make | - Not a ping pong ball |
| + Heavy |
LED Active marker
Embed an IR LED inside the ping pong ball.
| Pros | Cons |
|---|---|
| + Most visible to ART | - Hard to make |
| - Heavy |
Prototyping
I decided to go with the tape. It seemed easy enough to do, and inexpensive to get started with. All I needed was some time with a craft knife and a template. This took around an hour, but worked pretty well.
Testing it in the ART system yields somewhat promising results:
