Moire DRO

> I think what Jon might have been talking about was moire fringes - if you
> draw a series of parallel lines on two sheets of transparent stuff (glass,
> tracing paper etc) and tilt one of them a little with respect to the other
> and then move the tilted one in the direction of the straight lines on the
> other one, you will see a series of dark lines or bars moving at an angle to
> the motion. These effectively form a vernier type of readout and could be
> read with photocells. The degree of accuracy can be altered by changing the
> spacing of the lines and the angle between the two sheets of material. Have
> a play with it and you'll see how it works.

A number of years ago, I worked for a company that made joysticks for IBMPCs.
The company wanted to branch and make a Macintosh joystick. ( The principles
of the company were hard core gamers when they weren't being accountants and
investment bankers and whatnot. )

As Macs had no joystick input, the plan was to create a mouse in a stick-like
package, and use as much existing stick hardware as possible. ( The cost of
getting a high quality - high volume mold made up was quite substantial, so
reuse of parts was a necessity )

We ended up replacing the two pots with an optical setup as described above.
Each pot was replaced with homemade sensors - two LEDS and two phototransistors
on a PCB ( remarkably crude, come to think of it ) and two small strips of film.
In this scheme, ONE of the films moved. The electronics and one of the films
remained stationary.

Failing memory can't recall the lines/inch of the films, but the key was that
moire effect WAS used to increase the "resolution" of the films.

THERE WAS A PROBLEM, THOUGH and it almost sunk the company. ( Why these
Macheads tried so hard to fill so small a market remains a mystery. )

To get ANY moire effect, the films had to be placed emulsion to emulsion. This
meant that they'd wear out, and they did, far too quickly. At the big MacWorld
show in Boston that year, I had to fly in with all my tools and replace the
films in the back of the booth every half hour or so.

Placing the films mylar to mylar moved the lines so far apart that the moire
effect disappeared. We ended up shipping these rolls of film to Hollywood! for
a treatment applied to movies. This coating upped the life of the films to
about a year, in normal "gameplaying" use - about 100 hours per week, near as
I could tell. ( For a guy who's only ever played chess on a computer, these
game players were an eye-opener. )

Needless to say, I've considered this application for my own DRO, but have
discarded it.

Alan


--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta