Axis accelaration error

    Hello all,


    I have a similar problem to the one described in an old thread: kr200 odd movement behavior

    its difficult to explain indeed, as uberdoom said. So i will try my best... Also attached a video link:

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    The robot i use for wood milling and foam wire cutting:

    -kr200 L140 comp 2,

    -krc2 ed05 Daimler version,

    -windows XP embedded 2.0, KSS 5.6.6.

    -Manufactured 2007-11

    -RDC said 15.400h when bought and i put another 600h


    Problem appears in this example when Jogging the robot in T1-global using the A +/- also appears in the beginning of all moves in global or tool specific and at change of direction in LIN and CIRC motions in T2 and AUT. The exception is the Axis specific which works fine although i get a slow rotation of the A5 servo after releasing the green button. Its like comming back to the correct position. The error is repeatable and speed proportional. Also i think it worth to mention that the error was there since commissioning but it seams to have intensified. I would like to give it a shoot in fixing it as much as possible before going for the expensive service with kuka.


    I already checked:

    -Mechanical wear and nothing obvious,

    -DSE-RDW tool for errors and got 00C0 in the error box of the "RDC - Check communications"

    -Tool load data but only tested in jog and did not have any effect (not sure if it needs to run in a program to have effect)

    -Mastering with dial gauge

    -tool calibration

    -Mada file as prescribed in the attached thread


    Any ideas?


    I have to agree with Panic here -- this really seems like a wear issue in the robot.


    If you jog in axis mode, can the error be isolated to one axis? Or is it extremely pronounced on one axis?


    To measure backlash, I would set up a dial gauge on each axis, and jog in minimum speed, or in one of the smaller increment modes, and try to establish the backlash on each axis.


    You could also try testing incremental jog in Cartesian mode. Once the backlash is fully taken up by jogging in one direction, you should get very accurate (like, ~10um) accuracy in incremental steps in the same direction, over a short distance. If you can't achieve this, then there may be something more than just backlash issues.


    Another thing that might be worth trying: take some O-Scope traces of the axis following errors when doing that test you made the video of. It almost looked to me as if the robot was over-correcting for some reason, but I'm not sure how that would happen without generating faults. But if that is happening, it should show up as spikes in the following error of one (or more) axes.

    First of all tnx for your answers! :beerchug:


    I was a complete newbie when i bought the bot and there is a lot of thinks that i regret now (e.g. not to have the manuals delivered with robot)...

    But nevertheless its a nice adventure and this forum was the greatest help so far:respect:


    Have been said all that, i am also suspecting backlash since its an old bot but din't know how to measure it.

    I will aply your methods as well the ones mentioned in this thread:

    Backlash Determination


    Also spend some time in the forum and found out that tool load data was set correctly (or at least this is what i think) but the robot is seriously under loaded (20kg). Skyfire stretched it enough in many threads "at least 30% of max weight"... So i will scrap some lead and built a couple of weights. Is there something like KUKA.LoadDataDetermination for kss 5,6? Or is somewhere a good explanation on how to do it manualy? So i can compare it with what i did so far.


    Now concerning the O-scope traces definitely worth trying. I know where it exists but have no idea how to use it. I will need to spend some time with the Fault-Analysis-Diagnosis manual. It has an extended explanation about it.


    I will come back to you once i will acquire all this valuable data. Until then Farewell!

    Thanks for the clarification! Got some time with the bot today and here is what i got so far:


    -Concerning the backlash when you say "set up a dial gauge on each axis" do you mean in the mastering plugs or externally attached to measure against something fixed (Sorry if this sounds stupid)? Because when i test in Cartesian in the same spot as the test in the first video at X and Y directions i get between 0.3mm and 0.4mm of lost motion (I have no idea what should i expect... but as long as i get an average accuracy of around 1mm on my millings i would be satisfied)


    -Opened the belt cap and took a video of the "visible backlash" by moving the belt manually. I would like to know if this is looking nominal to you.


    -Another thing that i forgot to mention before... Every time after start-up the robot jumps in its first move as shown in the beginning of the following video. Don't know if its related to my problem but it feeds my curiosity.


    -Run also the precision test that you suggested (included as well in the following video) and everything looked inside the tolerance.


    -Now the most interesting thing is the o-scope traces (while performing the same test as in the beginning of the thread) and A1 following error Spiked.




    Video order:

    0:03 Robot jumps in first move after start -up
    0:08 Cartesian move in X 0.1 increments jog speed 100%
    0:23 Cartesian move in Y
    0:56 Cartesian move in Z
    1:14 Backlash on A5 and A6


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    No, I meant attach a dial indicator where it can touch the moving part of the axis, and measure the axis motion. Because you're probably stuck touching the castings, at non-ideal locations relative to the axis of rotation, these will only be approximate, but they might be indicative.


    Can't judge by eyeball, sorry.


    The "jump" at first motor-on suggests the brakes are worn, letting the arm "sag" some when the power is off -- not enough to generate a system fault, but enough to create a "jump" when the robot switches between brake holding and servo holding.


    .3-.4mm of lost Cartesian motion seems a bit high, but not really excessive -- it depends a lot on payload, robot model, robot pose, and other factors.


    I'm not seeing an obvious "smoking gun" here -- it's entirely possible that the motion issues you're seeing are the accumulation of lots of very small wear issues in the robot, from A1 to A6.


    One other thing I would suggest: create a program to do these tests, and execute it, instead of using jog mode. There are some (usually insignificant) differences in how the robot moves between Playback and Jog. I'd be especially interested to see if the O-Scope following error trace can be reduced by using lower acceleration values.

    Just start-up the controller to proceed with further testing and got 1223 error saying ackn.under voltage A1 (it was maybe overlooked all this time and if that's the case i deeply apologize). Once acknowledged it doesn't reappear until next start-up. I will proceed with the further testing tonight, but thought that would be wise to let you know.

    Sorry for coming back to you only now. But I measured the backlash on every axis and found out you were right about it. Its the accumulation of many wear issues in all of the axis. I didn't mean to doubt you since i only have a tiny fraction of your experience. But since mechanical issues are the most laborious and risky to fix i thought it would be wise to eliminate any other possibility. Now we decided with a dear friend of me, who has very good mechanical knowledge and with the help of some manuals to take the arm apart in order access the gearboxes and did so with success. Finally its crystal clear where the problems are. The big question now is:


    -Can you refurbish the gearboxes (by third parties or is kuka the only way to go)?


    -If i have to buy new ones would you have a good reference for me or again kuka is the only way to go?


    -Would it worth? Considering the purchase cost of the robot including transport and VAT was 12.5k euros (From Germany to Switzerland)


    I know Nabtesco and Harmonics AG are making gearboxes for Kuka and other robot manufacturers but haven't tried yet to contact them.

    Now my next questions are forum related.


    -Should i change the title of the thread to make it better relate with the actual problem? If yes would you have any suggestions?


    -Shall i mark it as resolved and open a new one concerning the repairs?


    Thank you for your understanding and help!

    i am not aware of gearbox refurbishing service. new gearbox (single) for one of main axis would cost you as much as you paid for used robot. one option is to consider getting another robot arm or entire system for spare parts.

    1) read pinned topic: READ FIRST...

    2) if you have an issue with robot, post question in the correct forum section... do NOT contact me directly

    3) read 1 and 2

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