On Mon, 06 Feb 2006 23:54:59 +0000, (DoN.
Nichols) wrote:

According to Don Foreman :
On Sat, 04 Feb 2006 00:04:24 -0600, Richard J Kinch
wrote:

Don Foreman writes:

Gray code is relevant to absolute encoders, not to incremental
encoders. An absolute encoder will give correct position even if
the encoder was moved during a power-down situation.

Quadrature encoders are both incremental and absolute by virtue of the
index pulse, if you have cheap electronics, and you can tolerate homing in
the application.

A once-per-rev index pulse is still ambiguous if the encoder goes thru
more than one revolution in the travel range of the moving part, not
an uncommon situation. A rotary encoder might well be geared up for
higher resolution and accuracy.

That may be so -- but I think that he may have been referring to
the linear glass scales, which have one index pulse at one end, and
quadrature encoding throughout the range.

"Absolute" generally connotes being able to sense absolute position
even after a power outage during which movement occurred one way or
another. The absolute encoder knows immediately where it is without
having to hunt for an index, a fiducial mark or a limit sense
condition.

I agree. And this makes it easy to recover from an interrupted
CNC machining run.

Enjoy,
DoN.
Yet many machine tools use incremental encoders with an index along
with limit switches. To recover from lost position the machine will
move each axis to the limit switch and then reverse direction until it
senses the index. It works, but is slow. Looking at the price
difference between absolute and incremental encoders it's easy to see
why machine tool builders like the incremental encoders.
ERS