On Friday 14 February 2020 02:01:08 andrew beck wrote:
> Chris. I'm still learning electronics. Could you expand a bit on
> this please.
>
> Maybe draw a napkin sketch of it. Sorry to be a bit slow on the
> uptake
I'm not Chris, but the word quadrature means two signals that change
state like this at a steady speed
---- ---- ---- ---- ---- logic +
____ ____ ____ ____ ____signal A, logic -
---- ---- ---- ---- ---- logic +
__ ____ ____ ____ ____ __signal B. logic -
time is left to right if clockwise, or right to left if counterclockwise.
poor ascii image of oscilloscope trace sorry.
>From this train of pulses coming out of the encoder the edges per time
unit can be very accurately turned into an rpm scale to be displayed.
Also the direction its moving is available at the output, freshly
determined at the instant of any change in status of these two signals.
Now add a 3rd signal, call it I for index, which has only a narrow pulse
that probably is identical to just one of the above but is only sent by
the encoder once per revolution. This tells the decoder, called an
encoder for some reason I don't control, that it has moved|turned one
full turn. linear scales may or may not have this last signal but
rotary encoders nearly always do. If present, this allows a scale to be
measured and established with which you can the measure how far
something has moved down to sub-micron accuracy. Applied to a spindle,
it allows linuxcnc to know exactly where in its rotation the spindle is
and that in turn allows linuxcnc to drive a tap into a predrilled hole,
and back out at the exact pitch of the tap. We call that rigid tapping
and taps last a loooong time time because they are not being bent or
otherwise stressed by hand tapping.
There are conditions in driveing a stepper motor faster than it can track
that same format of driving signal to the coils of a stepper, and
keeping that slow enough the stepper can keep up becomes that axis's
speed limit. But thats a whole different critter and subject.
This type of encoder signal never skips a count even after millions of
direction changes. And its cheap to build in hardware or software but
the software version will have a speed limit much slower than the
hardware version, but even that can work on a smallish mill at up to 30
or so inches a minute, with only a parport breakout board driving the
machines motors.
I hope this helps explain it. Introducing the element of time often
causes some confusion to the idea of drawing a vertical line thru the
above accii drawing to mark the instant of time to what happens next.
Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/gene>
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