Re: encoder head pitch, or, quit your griping.

Ian,
What you described reminds me of a shaft encoder design that utilizes two
discs with equal width opaque and transparent sections and the total number
of sections for each disc to be different by one opaque section. One disc
is stationary and the other is rotated. If the pair is illuminated from
one side, interference produces a pattern that is dark at one angle and
lighter as you progress across the diameter to the opposite (180 degrees)
side. This pattern rotates 360 degrees as one disc is advanced one
dark/light increment. So resolution is multiplied by the number of light
sensors. For instance, discs with 100 and 99 dark lines and 10 equally
spaced sensors looking through the discs, (parallel to the rotation axis),
would yield a resolution of 1000 states per revolution. Sorry if this
description is not too clear. I'm not that familiar with the terminology.
Bill

At 09:48 AM 6/8/99 +0100, you wrote:

>From: "Ian W. Wright" <Ian@...>
>
>
>
>Hi,
>
>A number of measuring systems I have seen use the Moire effect - is this
>a tecnique which might be used to overcome the lack of a suitable pitch
>grating? As I remember, two gratings are used mounted at a slight angle
>to each other. this has the effect of producing strong dark bands moving
>at right angles to the lines of the grating and allowing finer
>measurement from relatively coarse gratings. The angle at which the
>gratings are set affects the width of the bands and IIRC a smaller angle
>increases resolution - or have I got it all wrong?
>The kind of system I envisage would be easily made on the mechanical
>side and would consist of a tape with a number of straight black lines
>running the length of the machine - say ten at any even spacing - and a
>reading head incorporating another strip of the same kind of stuff and
>say ten IR phototransistors looking through the crossed gratings via
>small slits in a metal plate. I think that by choosing the number of
>lines, the grating angle and the number of phototransistors to give an
>accurate repeat rate, it should be possible to achieve good resolution
>by simple means. The grating could proboably be made by dragging a strip
>of acetate across the cutting adge of a screw tap in a suitable frame.
> Obviously, the electronics would have to discriminate
>which of the Phototrannies was dark and interpret this into a
>measurement but I'm sure this shouldn't be too difficult to achieve. I
>haven't tried this as I'm working too hard on my CNC
>mill but I'd be interested to hear your views.
>
>Ian
>
>> Mouse electronics count fringes. A serial port mouse sends the position
>> data to the computer when inquired or when state changes. I wonder if

these

>> could be adapted to DRO's?
>
>Best wishes
>
>Ian
>
>--
>
>Ian W. Wright LBHI
>Sheffield Branch Chairman of the British Horological Institute.
>Bandmaster and Euphonium player of the Hathersage Brass Band. UK.
>See our homepage at:- http://www.iw63.freeserve.co.uk or
>http://www.iw63.demon.co.uk/ or
>http://www.GeoCities.com/Hollywood/6067/index.html
>
>'Music is the filling of regular time intervals with harmonious
>oscillations.'
>
>
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