Re: More questions !!

"Arne Chr. Jorgensen" wrote:

> From: "Arne Chr. Jorgensen" <instel@...>
>
> Sorry - something more popped into my mind:
>
> Something is nagging me ! A while back - sorry I don't remember
> who said it - they said something about linear and rotary encoders.
> Something like - there is a problem, you can't replace a linear
> encoder with the rotary on the motor. Why not ?

If the motor drives the leadscrew directly with a STIFF coupling, then you can
probably do this. But, if you drive the leadscrew with a toothed reduction
belt, you will introduce belt slippage (should be very small with new belts,
but creeps up on you as the belt wears) and cyclical errors due to variations
in belt pitch and eccentricity in the belt sprockets. These can add up to
VERY substantial errors in an otherwise precise machine. What good is
it to put a .001"/FOOT anti-backlash leadscrew into a machine when a sloppy
sprocket will reduce accuracy to .005"/INCH?

> And to Jon Elson, - I remember your laser for PCB artwork, and
> have wanted to ask what kind of resolution you have on this ?

The photoplotter has resolution of 1000 x 1000 pixels per inch.
It can do a 20 x 22" film. The small-scale resolution is not too bad,
but I can't make a 500 line/inch grating with much accuracy. Those
lines would have an accuracy of +/- 20% or so.

> The thing is: Is 8 or 16 mm movie films still available ? You
> could make miles of linear encoders ! You could add index pulse,
> gray or V-scan areas if you like. As far as I know, - the frames on
> the film, is made by the shutter. That is, the film is in fact
> blank. With index, you could correct some with lookup tables. (
> gray coding may be useful here )
> Put on a reflective bar, then you could also use reflective opto
> sensors. The area available is large, so there should bee plenty of
> room for them.

Well, to get resolution similar to what I've got with my good leadscrew and
rotary encoders, I'd need a film with 5000 lines/inch, meaning I'd need
a photoplotter which could put down 10000 pixels/inch, with an accuracy
of a few percent in the location of each pixel. The big metrology
companies, Sony, Teledyne-Gurley, Heidenhain, etc. have the very
exotic optical gear to do these, I sure don't.

I DID have a crazy idea of how to do this with a low resolution optical
scale, which is affordable. Let's say I had an optical scale with 100 THIN Lines/inch.
This is what my antique Bridgeport optical readout had.
Illuminate the scale, and project a focused, enlarged image on a linear
CCD array, from a scanner. Have a CPU read the image from the scanner
very fast, and keep track of where the lines show up. The lines would be
covering several pixels, so you could interpolate to find the center.
You could produce a VERY high linear resolution, and if the scale was
accurately made, get a very high accuracy, too. But, I don't know
how fast you could make this thing respond.

Jon