Re: [CAD_CAM_EDM_DRO] DRO ENCODER TOOL

"D.F.S." wrote:

> From: "D.F.S." <dfs@...>
>
> >
> > From: "jguenthe" <jguenthe@...>
> >
> > Ok, let me throw some of my ignorance in on this subject! When you
> say
> > absolute encoder are do you mean a strip that really has say 1000
> lines per
> > inch for example?
> We are comparing apples and oranges here.
>
> What I mean when I say "Absolute Encoder" is a strip that has the
> current position encoded directly on the strip.
> There needs to be a bunch of sensors, rather than 2 or 4.

True Gray-code absolute encoders make sense in a rotary mode, they
don't make any sense at all in linear encoders of moderate size and
resolution.
Let's figure it out. First, there is no quadrature x 4 multiplication
of counts, so
the number of vertical stripes of bits is equal to the resolution of the
encoder.
Let's say we want .0001" resolution, that means we need 1000 codes, or
just
under 10 full bits for one tenth of an inch. We need 10000 codes for
one inch,
or about 14 bits, and we need 120,000 codes for 12 inches, or about 17
bits.
For the X scale on a Bridgeport, then, we'd need 18 carefully aligned
optical
sensors, all reading a stripe only .0001" wide, which doesn't permit
much light
through. One advantage of the repeating pattern on an incremental
encoder
scale is that it uses many slots in the scale, which transmit a good
amount of
light when aligned with similar slots in the analyzer grating in the
read head.
Typically, just under 50% of the light gets through.

Keeping all this aligned and working without errors on a machine tool is

not practical. Commercial absolute scales for machine tools don't work
this
way. They add one additional track to the A/B quadrature signals, that
have
some pattern of ones and zeroes which uniquely identify locations on the

scale, often at about 1" intervals. So, you only have to move 1" to
pass one
of these indexing areas, and the readout knows where it is in absolute
coordinates.

I'm running with shaft encoders, and have resolution of .00005" on X and

Y, and .000025" on Z, and wish I could get more resolution in my
CNC system. It does work pretty well, though.

Jon