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:

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.

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?

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.

Quote from Leon

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.

Quote from panic mode

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

Quote from SkyeFire

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.

Quote from panic mode

...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

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

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