On 05/17/2018 08:24 AM, andy pugh wrote:
It isn't clear from your description precisely when you
want this to happen, but I imagine that choosing the
conditions is also something easiest inside the custom
component.
Let me explain what mr. Lemonnier is trying to do.
He has a signal from an external device. He wants to record
the position of an encoder when there is a pulse from that
device. Now, the rub is he wants to be moving the axes
quickly, so that there are multiple encoder counts/servo
period. but, he wants the exact position recorded when the
pulse comes.
So, after stewing over his problem for a while, I realized
there was a way to do it. Feed the encoder to TWO encoder
counters. One works normally, and is for position control
under LinuxCNC. The other receives the external pulse on
the Z input. And, when the external pulse is expected, turn
on the index-enable pin of encoder. When the pulse comes,
the encoder counter will be zeroed, just like a spindle
encoder on a G33.1 or an axis when homing to the index
pulse. Now, motion continues, and then the servo thread
triggers. The axis will have the position when the servo
period triggered, and the extra encoder counter will have
the distance traveled between the pulse on Z and when the
servo period triggered. By calculating axis position MINUS
extra encoder, you get the axis position when the Z pulse
triggered the extra encoder to zero.
So, a component needs to set the encoder-index to true, and
then watch for it to go false. When it sees the
encoder-index go false, if does the subtraction and stores
the difference in a net. It then turns the index-enable
back to true.
Jon
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