On Thu, 19 Mar 2020, John Dammeyer wrote:
Date: Thu, 19 Mar 2020 20:08:52 -0700
From: John Dammeyer <[email protected]>
Reply-To: "Enhanced Machine Controller (EMC)"
<[email protected]>
To: "'Enhanced Machine Controller (EMC)'" <[email protected]>
Subject: Re: [Emc-users] A hal question?
-----Original Message-----
From: Jon Elson [mailto:[email protected]]
Sent: March-19-20 4:13 PM
To: Enhanced Machine Controller (EMC)
Subject: Re: [Emc-users] A hal question?
On 03/19/2020 04:02 PM, Gene Heskett wrote:
Greetings; I am trying to reduce the f/r error in an ATS667 based
encoder.
The index pulse is turned upside down by a reversal of spindel
direction. Tts the nature of the beast to remain in this stable state
until the approach of the triggering ferrous metal causes the first
reversal which the edge ignores, and then goes back true or false as the
ferrous piece crosses the center.
Does anyone see any problems with that?
The index pulse is only used during the initial synch to the
spindle, then the encoder is counted
up and down during reversals, without looking at the index
again. So, rigid tapping doesn't care
that the index signal from the encoder does something
different when the spindle is running in
reverse. It has been this way since I did the first ATS667
encoder hack some years ago.
Jon
So can you tell me something more about that? I could see that first encoder
index edge clearing a a quadrature counter attached to the A/B pins. After
that the index is ignored.
But it's here where I become a bit confused. A 60 tooth gear at 1200 RPM
produces 7200 edges per second or 1 about every 139 microseconds. Say we're
rigid tapping 8 threads which means a total of 57600 pulses. If the hardware
counter is 16 bit signed that's larger than the 32767 before the counter rolls
over. If it's a 32 bit counter it's not an issue.
But what happens when someone has a 2500 line encoder on the spindle creating
10,000 edges per rev. or 200,000 edges per second at 1200 RPM. Now that 8
threads deep hole is 1.6million. Again, not an issue with a 32 bit signed
counter but a problem with a 16 bit.
Not an issue, the 16 bit hardware count is extended to 64 bits in the driver
(every servo thread) this does limit you to a 16 MHz maximum count rate at a 1
KHz servo thread but thats way above the capabilities of most encoders...
Finally, at that speed, and an edge every 5 micro seconds, how does the
software ever figure out that the spindle is varying in speed? Or is one of
the real time threads running maybe only 1 kHz actually look at the spindle
counter and compare it to what they expect every 1mS and then adjust the speed
of the Z axis?
Yes, the Z axis is "geared" to the spindle position
Thanks
John
Peter Wallace
Mesa Electronics
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