June 17, 2019, 05:34:53 AM
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 Accuracy/Repeatability of the EMT mastering device ?

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May 21, 2019, 01:19:31 PM
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lionpeloux


Hello,

I am working on KR500-2-MT with KRC2 ed2005 / KSS 5.6.

I would like to know what is the specified accuracy/repetability of the EMT mastering system ?

If doing twice the mastering on a given axis, what would be the expected maximum relative difference between the notch and the spike ? Is there a technical sheet I missed describing this somewhere ?

Behind this, I am thinking of reseting an infinite axis to zero (a turntable in my case) in place of unwinding the axis back to the zero after each milling program.

Today at 05:34:53 AM
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May 21, 2019, 04:33:24 PM
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SkyeFire

Global Moderator
There isn't a strict number for the repeatability of the EMT, that I recall.  But the expectation is, as long as conditions are kept constant (same payload, same pose, etc) the EMT should be extremely repeatable.

If you want to look into this experimentally, you can do a Check Mastering on your axes several times, and record the deltas each time.  That should give you a decent statistical average.

As for zeroing an infinite External Axis... what does that have to do with the EMT?  You can't run an axis with the EMT attached.  Unless you're thinking of deliberately re-mastering the turntable between every program run.  In which case... hm.  An infinite axis is going to amplify any small errors n $ROT_MOT_AX, so if you don't "unwind" the axis physically, then simply re-zeroing the infinite axis in software might be less accurate than re-mastering it.  It would probably depend on just how many rotations the turntable has done.

May 22, 2019, 06:32:39 AM
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lionpeloux


Yes, this is exactly what I was thinking about.

Our milling programs are quite long (from 1 hour to a full day). And the turntable can make lots of turns. It is thus a lot of time to unwind the turntable (the max speed is 8rpm) and some permanent hardware like the EMT or something similar that can help zeroing the table and avoid unwinding it would be great.

=> Is there any type of material/sensor you would recommend to do this ?

I understand that simply programing a motion to the "nearest Zero" and getting ride of the full turns like this :
Quote
PTP {E6POS:E2 0}

;FOLD RAZ Axe E2
$ASYNC_AXIS='B00000'
WAIT SEC 0.5
$ASYNC_EX_AX_DECOUPLE= 'B000010'
WAIT SEC 0.5
$AXIS_ACT.E2=0
WAIT SEC 0.5
$ASYNC_EX_AX_DECOUPLE= 'B000000'
WAIT SEC 0.5
$ASYNC_AXIS='B000010'
WAIT SEC 0.5
$ASYNC_AXIS='B00000'

... is not a good approach as it will cumulate any errors on $ROT_MOT_AX.

May 22, 2019, 10:02:11 AM
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panic mode

Global Moderator
Repeatability depends on more than zeroing motor position. For example backlash, gear ratio, mastering cartridge condition etc all play a role


You can do check mastering to compare results.
under 0 002 deg for axis is perfect, that is close to mastering tool limit
Under 0.020 is good for most applications
Under 0.1 may work in some applications but it is poor result. 
1) http://www.robot-forum.com/robotforum/kuka-robot-forum/read-first/
2) if you want reply about robot, post it in forum
3) read 1 and 2

May 22, 2019, 11:30:57 AM
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SkyeFire

Global Moderator
Permanently mounting an EMT is a bad idea -- it's not made for that, and the EMT is pretty expensive to subject to that kind of damage risk.

As Panic says, backlash plays a part -- that's one reason that KRCs always move an axis through Mastering position in the same direction.  So whatever means you use to re-zero the turntable, you'll want to ensure that you start on the + side of the zero position, and approach it using - motion.  If you do mount a permanent sensor of some kind, you'll need to ensure that you approach it from a consistent direction, at low speed, and stop on the edge of contact (probably using an interrupt).

Rather than trying to constantly re-zero the axis, you might be able to tune $RAT_MOT_AX more precisely.  Master the turntable using the EMT, then rotate it in one direction multiple rotations -- however many your worst-case milling program might do it.  You'll need a way to precisely measure the position of the turntable -- you might be able to do this with the EMT using Check Mastering, but I've never tried this trick with an infinite axis.  However you measure the turntable's position, the ratio between where it physically is, and where the robot reports it to be, should allow you to tweak $RAT_MOT_AX.  You would need to repeat the test to confirm that the tweak worked properly.


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