Very interesting stuff! I have some ideas for how to implement a more
EDM-appropriate reverse run mechanism based on what you describe. Here's a
1) "Retract-to-setpoint-on-abort" modal code. If a retract point is set,
the machine would automatically retract to that position on abort. Abort
would also clear this set point.
auto-retract mode). Now, when a program is aborted early, the machine will
rapid to this position after stopping it's motion along the path.
command in task. I'd have to take a closer look at the logic surrounding
change anything at the TP level.
2) WEDM-style "jitter" feeding via HAL velocity input of some sort.
I agree that process control is important to address. If you had to write
motion path would be useful for this. It would take a bit of plumbing in
motion to implement, but I think this is doable without major disruption.
Post by TJoseph Powderlyresending, no attachment
using url instead
https://www.dropbox.com/s/xbmd0cij2ml9knn/edmGapSignal-vs-XpositionOverTime.jpg?dl=0
-------- Original Message --------
Subject: Re: [Emc-developers] Reverse Run
Date: Thu, 09 Apr 2015 13:07:06 -0500
Robert
thanks for looking into this
I'd like you to look at a real EDM cut
I've included a plot, but if you could actually watch the motor
during a real cut you would observe important info.
You may find that the adaptive feedrate idea is very wrong for edm.
The velocity is constantly changing sign during a real cut.
I include a plot of a real cut , and it is an extremely stable cut.
Its a sinker EDM plunging a .025x.375 rib into hastalloy.
notice the position proceeds in a single direction
but at any one servo update period, the direction is fwd or reverse
according the the process. The directional change is the jiggle of the line.
The direction of the last period has nothing to do with the value of the
next period.
so, imo the adaptive feed is incorrect
imo adaptive feed reduces the velocity to below the potential velocity
(the efficient target point of the control loop)
and thus forces inefficiency.
Adaptive feed ignores the process and 'works' by timidly staying way
behind any efficient/aggressive seeking of the control point.
Yes i know i saw the edm maple leaf cut once, ask the author how it
compared to the other machines in his edm repair shop.
But even with this reduction, the work you've done is very interesting.
Please remember there are 2 different EDM retracts/reversals
short term and long term
the immediate reversals are constant and of small dimension
see the picture, the jiggly advance is _necessary_ for the process.
The short term are controlled by the small fluctuations of the process
the efficiency of the cut is determined by following the process closely.
The long term reversals are after near fatal errors,
these attempt to clear the gap when the process control
has failed badly.
( the better the process control, the lower the need for
long term reversals )
I'd also suggest that there are 4 common forms of edm
Examining them shows their different TP strategies.
They are graduated in difficulty of control as follows
1) easiest, HOLE DRILLING, moves in 1 axis between 2 limits
this can be done in hal w/o tp changes
2) middle difficulty, SINK EROSION, including orbiting. this requires
a 'sky blue' safe position to retreat to.
During a roughing operation ( creates the net shape )
the tool simply can retract to the point where edm power was turned on.
IF the toll achieves the end of roughing,
i call its position ( duh ) the 'roughing point'.
All finishing operation move FROM the roughing point
and return TO the roughing point.
Several excursions (orbits) occur,
each with slightly lower power,
and slightly larger excursions
( the range of motion is 0 to 0.5mm in my experience )
Each excursion begins and ends at the roughing point
During each excursion the tool expands the eroded volume according to
a prescribed pattern.
DURING the excursion, the tool can retreat towards
the roughing point. ( think retreats to high ground )
Example: a 'circular orbit' is really a trace over a conical surface
the tip of the cone is the roughing point. The tool follows the
slope of the cone towards its base, then travels around the base.
IF there is a problem, the retract is towards the cone tip NOT
along the previous path. ( sky blue remember? )
theres more to this, i've written corner picking, square ,circular ,
spherical, barrel and other orbits but they all follow the sky blue rules.
3) WEDM this is the only EDM that needs to retrace the old path.
And the progress is still true EDM, not
adaptive-slowed-down-slower-than-the-process-wants strategy. So the
short term is constantly changing. The long term allows for retracting
thru previous geometric commands.
BTW, once this ( retract thru prev geometry) is needed, the process is
pretty much out of control and the action is NOT a strategy to make the
cut better.
4) EDM Grinding
i've only worked on a few edm grinders, and they really fall into
category 1. they really dont have a long term backup, as they have
pretty perfect flushing. I just never saw one back up far.
Yes they could use hal for stepover and drop,
like the hydro-mechanical trips and levers common on any wet grinder.
anyway
please look at the picture
the top trace is the immediate gap error value
( summary, its 0-5V with 2.5V= null velocity)
Post by Robert EllenbergOk, based on what everyone has said, I'm imagining the following
* a HAL pin to allow / disallow reverse run
the hal pin would need to be on the servo thread
and expect it to get banged a lot
Post by Robert Ellenberg* a HAL pin to command reverse /forward direction (with safe state change
handled by TP)
my elox agie and heidenhain used M3 or similar
Some used the 'spindle' to turn on the cutting power
Some had delay till the power was sensed before allowing 'edm motion'
If the power was turned on while the machine was shorted, the current
simply shunts thru the work ( no damage usually and a noobie mistake)
and the system see it has already backed up to the start point, and
stops ( it began there and didnt have to move to get to fatal condition)
Post by Robert Ellenberg* adaptive feeds of -1.0 to 0 trigger reverse run at the scaled feed rate
(with safe state change handled by TP)
i disagree with adaptive feed and will move on
the velocity can be fed into a 'velocity curve' comp.
with the error signal as input, two simple curves meet at
the 0 velocity point ( no error = no motion )
the 2 curves are infeed and outfeed
as the error moves away from the null point there can be
a 'dont care zone' with minimal velocity changes.
the dampens an overactive system
further away there are 'pay attention' zones.
I set mine up to run away fast
and to approach cautiously.
Post by Robert Ellenberg* motion commands to switch direction (as implemented now)
Am I missing anything?
i hope you best of luck in this work
i hope you dont work a lot on the TP side
without considering the process control side.
because driving down the street should not
be primarily avoiding the telephone poles.
that works but it aint good driving.
regards TomP tjtr33
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