Re: plastic strips for encoders

Gar Willis wrote:

> The SENSORs that US Digital relies on, namely those made by HP, are NOT
> made in a resolution that will accomodate 250 lines/inch, at present. NO
> bloody reason by HP couldn't make some at that res, but they don't, and
> although I DID talk to the Product Marketing Mgr. of this sensor line,
> for some reason I couldn't seem to convince him that us hobbyists should
> be considered in their overall product scheme. B)
>
> Bottom line is that the quadrature sensors with their arrays of leds &
> sensors, are NOT made by HP at the moment in such a resolution to allow
> US Digital to make the films/gratings work, and make us all happy. You
> CAN use gratings of slightly varying resolutions from the nominals that
> the sensors are rated at, but apparently 250 *IS* too far off from 200
> to just whip out a 250lpi grate, and get it to work properly with a
> 200lpi sensor from HP. Sad, but apparently we're toast on that hope.

If you can get the strip in a resolution that you like, it is really easy to
make the read head. What you do is take two small pieces of the
strip and place them over the photocells, and provide a means to move
one of the photocell/grating units up to 1/2 of a grating pitch. Another
way is to use one large piece of the strip that covers both photocells,
and allow the entire unit to be rotated relative to the normal axis of
the grating. The correct angle puts the photocells closer together
by 1/4 of a grating pitch. This is about all there is in the HP head,
an LED, 2 photocells and a dual comparator chip.

Jon