Robot movements smoothness

  • My KR210 + KRC2 with KR 5.10.6 robot, only when axes are streched is not moving smoothly manually at 100% but in small shaking steps increasing in amplitude as it streches.


    Reducing to 30% or better 10% is the only way to minimize it. This is an annoyance when milling with some degree of precision and very slow time consuming.


    Since this only happens away from a 1.5m radius approx. I believe that is not gears related.


    Any advice as to what is or could be the cause ?


    Thanks.

    Edited once, last by Ceta11 ().

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  • This happens only in manual moving, in cartesian ?
    What if move the axis rotating each one, still same shaking? How many working hours has your robot ?


    Have you made changes in machine.dat parameters ? I could suspect some parameters in machine.dat that could make one or more axis to pulse if they are not well setted.

  • OK, my question is 2 fold, is Hardware or Software ?


    Software
    This happens only in manual moving, in cartesian ?
    A- on both manual and Auto. So it happens in Manual at 30% and up and under a program if I run at the same speed. So I have been forced to run at very slow speed for milling.


    Have you made changes in machine.dat parameters ?
    A- None. It is as it was. The MADA and machine.dat files has been KUKA checked. Robot is mastered and tools calibrate better than 0.8mm


    Hardware
    How many working hours has your robot ?
    A- 15k plus. was given a written clean bill by Kuka 200 hrs ago.


    What if move the axis rotating each one, still same shaking?
    A- Each axis individually move sillky smooth at any position thru its whole range.


    I could suspect some parameters in machine.dat that could make one or more axis to pulse if they are not well setted.
    A- Pulsing when moving in Y or X direction is the best description on what seem to happen, amplitude is directly related to speed.


    I don't know how to check the values on machine.dat other than replacing it with original file along with the MADA.


    Any advice? Thanks a lot.

    Edited once, last by Ceta11 ().

  • I have same robot type KR210 KRC2 ed 2005 from 2007 and I can send you my machine.dat file to compare my values with yours.
    I think is the simplest way to check if any parameter is the cause.
    If you change something note the old values just in case you need to put it back.


    give me an e-mail and I'll send you my machine.dat for comparison.


    At 15 k working hours I dont think to be a mechanical problem with any gearbox.

  • are you sure your loads are correct?
    A- forgive my ignorance, what are 'loads' ? electrical ? ...my 'loads' inputs in soft and real are very benign for the foam I am cutting, low engagement, low linear speed, etc. Besides, this X-Y pulsing is happening even without running spindle.


    are you sure your programs are using them?
    A- I can see the speeds are correlated to soft input, ex: 15, 20 or 30 mm/s with 4mm depth and cut over 12mm dia tool.


    If other please explain. Thanks a lot.

  • [size=2]LOAD (for the robot) is information about ANYTHING you added to it. whatever you attach to robot arm is more load.(tool, cables, junction boxes, material feeder, brackets, valve banks... ANYTHING)



    if you mount tool at the end of the arm - that is load (it has weight, center of gravity, inertia etc.).
    such info must be communicated to robot, it is part of commissioning process.



    jumping at programming (without doing proper commissioning) is a good way to punish and wear out robot... [/size]
    [size=2]it will do millions of cycles stressing entire system (motors, cables, drives...). :grinning-smiley: [/size]

    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

  • and it is not enough to specify loads... they need to be actively selected and used in programs.
    otherwise robot will think it is using wrong payload (which is at least what robot is rated for, sometimes more).



    one client used KR2150 to do some arcwelding... did not specify load and within a year they actually had failure (after many fault messages).
    robot was thinking that it was supposed to use default payload while weaving (mig torch was maybe 1kg, robot was assuming 175kg on flange and another 50kg on A3).

    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

  • Thanks. I did not rushed into programming at all, tried to learn and follow Kuka recommendations on everything.


    OK, yes I had specified Loads, Mass and Inertia are well calculated. I had done that well for many years in aerospace engineering. Besides It is 0.00kg mass with a fresh load of KSS. I had recently replaced the RDC card that died and configured loads (10kg spindle and plate on 3, 0.0 on 1 and 2kg on 2 at proper CG).


    I am not aware of any milling software inserting any Loads, it uses what is on the configuration... but I checked to make sure. So Loads are what is configured after running a program.


    It seems to be more than that... I suspect. It is like a 2-4Hz pulse that changes amplitude with speed.

    Edited once, last by Ceta11 ().

  • Velichko, Thanks.


    The pulsing happens not because of software but under higher speeds, i.e. running with pendant or with soft, on the X-Y axis only, not Z. I already corroborated this at lengths.


    Individually all axis are silky smooth as well and Loads have been checked correctly.


    I can email you a program but I am sure is not the problem. Just let me know if you have other ideas.

  • you can compare current MADA with clean copy on D:\
    you can check if robot is positionally accurate ("absolut vermessen" sticker next to robot nameplate) and if this is active (hmi, robot info)
    there may be additional controllers active (elasticity etc.) but to check or deactivate those you need to contact Kuka

    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

  • There is no sticker "absolut vermessen"...


    I did replace the R1 contents (machine + robot dats) with the ones stored on D: and the pulsing is gone apparently up to 75% TCP... which is a noticeable improvement. They are the same files that were supposed to be, but somehow were not in good shape.


    I also contacted KUKA and awaiting what they suggest.


    At least things run better so far. Thanks !

    Edited once, last by Ceta11 ().

  • Well the response from the manufacturer support was less than satisfactory, to put it in a nice way.


    * "your robot will never run smooth because is designed for higher load and you just have 9 kg of load".
    * positionally accurate option software and installation, calibration and comissioning will cost $10K +
    * "but your robot still needs to work with added weight to be smooth, even with that option".
    * you are running too slow (20 mm/s) you should run at 60 mm/s (this in spite of having provided information to them and clear explanation that amplitude is larger at 40 mm/s)
    * "your load is only active in PTP not in LINEAR, this only is activated with the software option"

    I said to him that this type of robots are used in Europe and the US without adding ANY additional load, I have seen it and there are videos everywhere.
    I tried to ask several times what the Control variables are available to adjust for light load... He could not answer and would deviate to other subject.
    Load IS active in LINEAR instructions, I can see that the robot acts differently if I use different load parameters.


    Maybe you can have a good lough at this... but the fact is that the support guy was not very knowledgeable of Control's Engineering, to me he was just an installer/integrator with "a lot of milling installation experience"...


    Maybe his Engineering background was not there. Period.


    It would be nice to get some comment from a knowledgeable european KUKA Engineer. I asked for that: Could you please consult with KUKA Germany...?


    This is were things are so far.

    Edited once, last by Ceta11 ().

  • From experience, I know that Abs-Acc only works for robots running at least 50% (more like 75%) of their rated load. Part of the issue is simply that AA calibration is done in the factory using a nearly maximum payload, and the further your actual payload is from the calibration payload, the less precise the corrective math becomes. And even without AA, running a robot so far under its rated load can still give the gravity compensation fits.


    The root issue here is that no one really has a reliable model for all the ways that a load effects the robot's accuracy, across wide ranges of loads and distances. This is a problem that has been keeping a lot of researchers busy for a long time, without a satisfactory practical answer. The best results so far that I've seen have been from New River Kinematics' SARCA module, which gets its performance by building a volumetric correction table across the robot's working volume, calibrating in the actual working conditions with the actual working payload. But I've never seen anyone come up with a predictive model that works -- the working solutions all depend on individually calibrating the robot under working conditions.


    I do have one quick suggestion: try turning AA off. It sounds counterintuitive, but under conditions like this, AA can actually have negative effects.

  • Unfortunately, my KR210 doesn't have AA option ...as far as I know... don't see any sticker. New River Kinematics' product is for position accuracy, not what we talk here, a dynamic behavior of the robot+controller's control software.


    I am aware of the Control may have been calibrated for 50-75% load, I see that as a logical way of setting up for basic use of the rated Load. I lieu of this reasoning, I tried loading a parameter of load of 75kg mass at 3... you guess it right the pulsing was worse. I also tried negative values and although message says 'illegal' on screen I see behavior changing... but as you know it doesn't seem to have an even behavior across the distances. I think is interesting.


    What I believe, because I have seen it done, is that the Control algorithms have gains parameters that can be changed. This is probably the most influential one that they tune at factory.... AND I suspect they may have a combo that can provide me...albeit not perfect but far away better than the pulsing I see at certain speeds.


    I also can see videos posted by member Mr. V. Velikov in Youtube that show a smooth running milling path X and Y ... i.e. no pulsing at similar speeds where I get it.


    I just have to find the right people in Engineering at KUKA or elsewhere to get me the set up. If as you mention there are parameters for elastic constants, there are also many structural and system damping, etc in the Control that are adjustable. We know that is what a Control Engineer does when designing a system.


    I will keep you posted. Regards.

    Edited once, last by Ceta11 ().

  • Whoops. Lost track of the discussion -- I thought you did have AA. Well, so much for that theory.


    Setting a negative value into the M element of $LOAD causes the robot to assume "default load", which is rated maximum. If you look at the LOAD_DATA array in $CONFIG.DAT, any entries that have an M of -1 are still at the factory default. Which is one of the reasons that setting correct load data on a Kukabot will generally make it go faster -- if you use the default loads, the robot assumes it's carrying its max payload, and de-rates all motions accordingly.


    PID tuning of servo parameters can be done, but normally only for external axes -- the settings for the arm are not editable (or rather, you can edit the file, but the changes will be overwritten by the "correct" data when you save). But since KMCs exist, it should be possible to unlock those settings -- you'll just have to get KUKA to tell you that, and acknowledge that you're voiding whatever warranties this robot might still have.


    The big issue, though, is that tuning individual servos is only a small part of the overall problem. Tuning the Cartesian motion model is... mathematically hellacious.


    One thing that I think might potentially be helpful: make an O-Scope trace of the robot's position and velocity while this "shaky" motion is happening. The O-scope is one of the KRC2's greatest features when trying to deal with issues like this, and I wish every robot manufacturer would add it.


  • ...
    Which is one of the reasons that setting correct load data on a Kukabot will generally make it go faster -- if you use the default loads, the robot assumes it's carrying its max payload, and de-rates all motions accordingly.
    ...


    Long time ago I had a robot that was faster with maximum load than with correct load! :hmmm:???
    He moved nearly exclusively in Axis 1, the gripper was symmetric and perpendicular under Axis 6, Axis 6 looked straight down to the floor. Kuka Hotline told me to switch off the optimization.

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