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Mean Error before calibration is over 30mm

Hello we have a question regarding the calibration procedure.
When calibrating the robot, the base measurement threw an error and the nominal accuracy was at 30mm.

When calibrating the robot in 6 DOF using the T-Mac the base calibration was fine. 

How can we explain this large error during calibration? Is this due to an error in the uncalibrated kinematic model? 

Thank you for your help. Kind regards

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The large nominal error is probably due to some wrong measurements when taking measurements of the base setup.

You can see that nominal distance errors are reasonable (around 2-10 mm). This means there is a large pose offset at the robot base.

Can you attach the RoboDK calibration project file? We can better take a look.
Yes, here it is.
Thank you.

Attached Files
.rdk   RoboDKProjekt.rdk (Size: 160.49 KB / Downloads: 149)
Great, the calibration project looks good. What robot payload did you use? By looking at the 3D model I assume you took all measurements with a high robot payload.

When you use the default Base setup scheduled measurements and a high payload you can see a warning saying the values do not look accurate (WARNING: these distances seem too innacurate). For example, the distance found between axes 1 and 2 does not match the nominal distance. It is a good indicator that the circular/planar errors fit to an arc. It means the measurements were taken properly and there are no measurement outliars. 

It would probably help you get a better base setup if you removed the robot payload only for these measurements. You could also expand the range of motion for joint 1 to get a better arc (from -90 to -90 deg) and/or change joints 4, 5 or 6 to bring the payload closer to the robot axis 1 (for example, by setting joint 5 at 75 deg).

It issue is not a big deal though. It look like the calibration was successful and average distance accuracy can be improved from 2.2 mm to 0.3 mm (distance errors isolate the robot base errors).

If you click on the Parameters button (robot panel) and expand more options at the bottom you'll see the base pose is:
XYZWPR (mm/deg) = [ 23.7,     1.0,    -7.8,     0.0,    -0.1,     0.0 ]

Which means that most of the error is coming from the base setup (23 mm along the X axis). In short, the base is offset by 23 mm towards the back of the robot as axis 1 was somehow tilted.

I assume you had to change this setting:
  1. Select Tools-Options
  2. Select the Accuracy tab
  3. Set the Position tolerance to exclude measurements from 20mm to 100mm or so.
This is important to take the measurements into account for calibration and validation.

By improving the base setup measurements you can make sure you aim to the right target faster as the first guess for calibration will be more accurate.
Hi Albert,

thank you. The Calibration was done with just 2kg payload. And yes, I had to expand the tolerance range.

We will just try to expand the movement range of axis 1.

Kind regards 

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