How often do you master a KuKa robot

  • Hi all,


    For 3 years now I've been using a second-hand KuKa KR 100-2 P 2000 shelf robot, with a KRC2 control system. I use it to do various milling and trimming jobs on rather large panels. You can see a video here:

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    However, until recently, I never mastered this robot, not even after installation. In hindsight, this is probably a rather large mistake. However, I managed to get quite decent results with this setup. Throughout the years, the calibration asjustments I needed to make in the program grew larger and larger. In pursuit of more accurate milling results, I recently bought a dial gauge and an adapter to master the robot by hand, and was surprised by the results. I really should have done this much earlier.


    Now to my question: As i've said I noticed that the robot mastering slightly drifts. I had to readjust several points in my program about every 3 months. Is this normal? I'd like to remaster the robot instead of figuring out what adjustments have to be made in the future. How often do you remaster a KuKa robot?


    Could it be that this drift away from the mastered position is because I shut down the robot after each work day, and start it up in the morning? I've had the idea that the drift got way worse during the unusual hot summer we've had here last year.


    Thanks in advance for your replies.

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  • In general i only master my robots once a year during maintenance, but they also get remastered after big collisions or other malfunctions. Basically whenever i have someone from kuka over to fix anything i always ask if they can check the mastering. (i dont have a mastering tool myself).


    The drift you describe seems not normal to me. How much drift do you have in your points? Can see a significant difference between mastering positions of for example a2 en a3?


    are you sure that it is the mastering that drifts? not the mounting of the robot?

    Every problem has a solution, that isn't the problem. The problem is the solution.

  • Over the past two years it has been necessary to reteach the tools that have to be picked up by the toolchanger. This drift was about 2 to 3 mm in total.


    After mastering the robot, I could remove certain adjustments in my program that built up to 6mm in total.


    I assume the mastering drifts, but I will keep a close watch on it (by checking the mastering once a month the next few months) as I would assume a drifting mount would mean loose bolts, and loose bolts would certainly be noticed.


    Personally, I hypothesized that last summer, shutting down the robot at the hottest point of the day (5:00 pm, 40°C ambient, with all gears and motors hot) and starting it at the coldest point of the day (7:00 am, 25° ambient, with all gears and motors cold) on several hot days might have been the biggest problem. But keeping the robot powered on seems like a waste of electricity. Perhaps I should stop shutting it down at hot summer days?

  • I can't honestly say I've ever seen a KR's Mastering drift over time, in 20+ years of using the orange beasts. I've only seen Mastering become inaccurate due to mechanical issues (collision, worn-out parts, etc), or when an electrical issue causes the Mastering data to become lost or corrupted.


    The one time I had a robot where it really seemed like the Mastering was drifting, it turned out that the bolts holding the robot to the concrete flooring were actually a bit loose, and the robot's position on the floor was shifting over time.


    Are you mastering using an EMT, or some other method?


    One way to see what's going on is to perform a Check Mastering, instead of just re-mastering the axes. The Check Mastering will show you the difference between the last Mastering, and the current readings. Tracking this over time (without doing a full Mastering) should show you any trends in the Mastering, and which axes are drifting. And if the Mastering isn't drifting, then your root issue is probably somewhere else.

  • Yes, I am using a dial gauge. Before mastering, I've done a 'check mastering' by hand. I did this by finding the mastering position with the dial gauge, and simply wrote down the axis position on the teach pendant. Differences were:


    A1: + 0,10°
    A2: + 0,10°

    A3: + 0,07°

    A4: + 0,15°
    A5: + 0,15°
    A6: - 0,19°


    However, this was the first time I've mastered this robot myself. I assume the last time is was mastered was at the last overhaul at the company I bought it from (which sells used robots), before shipment.


    This is not an issue with parallax nor with hysteresis. The differences mentioned where clearly visible on my dial gauge (as much as half a turn, 0.5mm!), parallax on the dial gauge is at most 0.01mm and hysteresis seemed to be about 0.05mm (I was curious, so I checked)


    I plan to check mastering (by hand, as I don't have an EMT device) end of March, end of April, and when we've had a few hot days.

  • well this is an interesting one. Fortunately we can rule somethings out.

    In your case after remastering you could remove your offsets so a problem with the mount of the robot seams unlikely. And based on your mastering check data your mastering seems to be really drifting.


    I can only guess at what the cause of this is, but in the only way i know that mastering can drift is when the robot is moved with the kabinet powered down or not communicating. The only time a have heard about something like this was with a robot with a heavy AOT and the brakes on A2 slipping when the robot was left overnight. But in this case it was only 1 axis and the drift was between 1 or 2 degrees so it was pretty obvious.

    Every problem has a solution, that isn't the problem. The problem is the solution.

  • but in the only way i know that mastering can drift is when the robot is moved with the kabinet powered down or not communicating.

    That is why I think it might have to do with hot days: if the robot is hot (after running at high ambient temperature) and I power down the cabinet, the robot 'shrinks' because of cooling down without it being noticed. Perhaps the axis move that way.


    Are you sure it is the robot drifting and not fixtures used by robot...

    I will explain: The only 'fixtures'/fixed teach points I use are the tools that the toolchanger needs to pick up. The rest of all my programs are fully parametric, with a laser distance measurement on the robot that determines the root point of every product each time the program is run. This laser distance measurement is calibrated about once a month.


    Right after installing the robot, I used basic trigonometry to figure out the root point of each product with four distance measurements. However, over time, I had to add adjustments to account for some 'drift' of the measured root point, not so much for X, Y and Z, but mostly for the measured A. After I mastered the robot last week, I could remove all these adjustments, so everything is exactly as it should be theoretically.


    Here's another video I made just this morning, to show this procedure of finding the root point with distance measurements.
    0:00 - 0:19 Portal moves
    0:19 - 0:45 Robot does panel detection with long-range sensor
    0:45 - 0:59 Robot does edge detection and collision avoidance scan for first measurement
    1:00 - 1:06 Robot does first measurement
    1:07 - 1:30 Robot does edge detection, collision avoidance scan and second measurement
    1:30 - 2:10 Robot does edge detection, collision avoidance scan and third measurement

    Rest of video: Robot scans panel surface and saves measurements for use during milling and trimming


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  • One note: Mastering needs to be done in a fixed direction, to minimize the backlash effects of the axes. An EMT process always moves each axis in the negative direction at low speed and "captures" the event when the bottom of the gauge notch passes under the pin. So if you are using a dial gauge, you need to avoid "hunting" back and forth across the notch -- this will corrupt your data. You need to hit the lowest reading on the gauge without reversing direction. Also, any future Mastering or Check Mastering must be carried out in the same direction, or else you are injecting the backlash error directly into your Mastering.

  • One note: Mastering needs to be done in a fixed direction, to minimize the backlash effects of the axes.

    Yes, the manual is quite clear in this part, luckily. I mastered by moving to the pre-mastering position, finding the lowest point in the notch, adjusting my dial gauge so the lowest point would be at zero, moved all the way back to the pre-mastering position, and moved until the needle hit zero. If I was unsure, I would go back to the pre-mastering position and do it again. After pressing 'Master', I checked whether I actually hit the lowest position.


    Out of curiosity I did some hunting 'back and forth' just because I wanted to know how small this backlash is, and I was surprised how small the effect was. Anyway, the linearity of the robot has been hugely improved, discrepancies are now in the order of half a millimeter, instead of several millimeters.


    ...and always from same pose. good idea is to bring all axes to mastering position before doing mastering on any of them.

    Which I did, except for A1. The manual states that moving A1 away from the mastering position is not a problem. I did this, as it is very difficult to reach all points with A1 fixed at 0°.

  • Hi all,


    I've been keeping track of the mastering of my KuKa robot since I did a mastering in febuary. I think I can be fairly certain that the mastering indeed drifts slightly over time. I'll show why I think that.


    I mastered the robot in febuary. I did a mastering check (with a dial gauge) in april. My results then were

    Code
    Axis 1: +0.01°, +59 increments
    Axis 2: -0.01°, -66 increments
    Axis 3: +0.00°, +58 increments
    Axis 4: +0.01°, +46 increments
    Axis 5: +0.00°, +2  increments
    Axis 6: -0.01°, -51 increments

    These differences were too small to redo the mastering, so I left it at this. However, another 6 months later, last month, I did another check and my results were as follows

    Code
    Axis 1: +0.02°, +187 increments
    Axis 2: -0.01°, -135 increments
    Axis 3: +0.01°, +109 increments
    Axis 4: +0.02°, +122 increments
    Axis 5: +0.01°, +66  increments
    Axis 6: -0.03°, -153 increments

    While these differences are still quite small, they are significantly larger than they were in april. As I had some problems with accuracy, I decided to remaster the robot.


    I decided to post my results for the curious :smiling_face:

  • I have to agree with Panic. If you're trying to get accurate trend data over time with dial mastering, I would recommend that you do each Check Mastering at least 3 times in a row and average the results -- better if you do it 5 times, throw out the upper and lower outliers, and average the remainder. It's all about statistically nulling out the "noise" in the measurement.

  • I can't honestly say I've ever seen a KR's Mastering drift over time, in 20+ years of using the orange beasts.


    KUKA had troubles with the firmware from some RDW2-boards drifting symmetry- and offset values long times ago (around 2004 / 2005).

    Result was sometimes drifting mastering (only few increments) after switching ON/OFF.

    It was also dependent on the KSS version you used. (about V4.1. x - early V5.2.xx)
    Solution was update the Firmware-File RDW2.bin >= DEZ. 263 // Hex 0107.

    To Check in the RDW Table, Index 83



    ktmf say nothing about KSS-Version........maybe.....

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