Calibrating an external kinematic system

  • Dear Friends

    In order to be able to move the robot with a mathematical coupling to a kinematic System, the robot must know the precise location of the kinematic system.

    This location is determined by means of root point calibration.The TCP of a tool that has already been calibrated is moved to a referencePoint on the kinematic system 4 times.

    So where is the point?

    I have tried to do this process according the KUKA documentation, but when I move the robot to the reference point (Second point) around my external kinematic system I see this error message:

    Measured point of external kinematic system too close to other measured point.

    You see in the photo that I have attached.

    I am surprising because I moved my external kinematic system more than minimum distance 50.00 mm.that I am sure that I have done.

    As you see in the picture!!!!I see this error messages with several time with different robot configuration and different distances of my external kinematic system.

    And finally I am confusing that where is the location of my fault?

    Does anybody has any experience to manipulate this fault and help me to remedy this error message?

  • first make sure your kinematics is setup correctly.

    this means show us your transforms, show reference mark you are using.


    blue arrow indicates axis of rotation (Z)

    red "X" is a "pin" or reference mark (normally this is on X axis)

    yellow line between them is a distance along that X axis to measure and enter in the transform (last of five transforms).

    note that pin or reference mark cannot be at the axis of rotation. it must be away (as far as possible for better accuracy).

    if making custom kinematics, pick a suitable point sufficiently far from axis of rotation, mark it (engrave an "X" for example) and make sure correct distance from center is specified.

    this should be accessible for measurement so of turntable has something mounted on it, it may be a good idea to put the mark on the side but... then you also need to measure Z.

    If i recall KUKA positioner DKP400 has this point some 210mm or so away from this center axis.

    then you can run root calibration routine. this means moving kinematic (jogging turntable) to some point, then bring calibrated robot tool (pointer) to the reference mark and teaching the point. This is repeated until all points are saved. if positioner has more than one axis, (DKP has two) move both of them before bring robot pointer to pin/reference mark.

    your screenshot shows that reference mark position is at the center of the flange - obviously that is never going to work.

    "Zwischen Messpoung und FL" = (transform) Between measuring point and center of the flange.

    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

  • Hello Panic mode

    I have prepared motor with gearbox (MGU3100-118,5_KSP64) form KUKA ,

    Then I have mounted on the structure as you see in picture .So my External kinematic type is rotary and also is connected to the robot controller, and I can jog the axis with KCP also.

    So according the explanation that you have mentioned I have to change the Value of X distance of this Variable ?


    X : Distance of surface of external axis to my reference point that should be sufficiently far from axis of rotation, this is correct ?

  • if you put point on the axis, and then rotate about that axis 4 times, all four points are in exactly the same place. if they are all in the same place, it is impossible to do any calculations such as axis direction or plane at which those points are.

    you still did not show us:

    a) where on the flange of your kinematic is the reference mark.

    b) how did you determine its distance from the center of rotation (center of flange)

    c) that you have entered that value into transform (either by editing DAT file or by specifying values on the plugin in your screenshot).

    in other words your kinematic is incomplete and that is why you cannot perform the measurement.

    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

  • Hello Panic Mode

    I have study KUKA documentation and according the documentation, I have to change the variable:

    FRAME $ET2_TA1KR= {X 0.0,Y 0.0,Z 1016.0,A 0.0,B 0.0,C 90.0}

    For my application, I have to insert my Z value !!!!

    I am right ? It's ok , what is your idea ?

    I don't have more experience regarding the external axis .


  • you are not following advice...

    your case is very simple - just one axis which is installed at the fixed location. all that robot need to know is where is that axis of rotation in space. it does not matter where the foot of your kinematics is since it is not changing and robot is not interacting with it (could be across the street and would not make a difference). so even if you do not enter these values (first one or first four transforms) ...but do enter one i mentioned before (the last transform), robot will still be able to perform measurement you need. it will just compute wrong point in space where the foot of the kinematics supposedly is (which does not matter here). and the values you see in manual are only examples. those are not exact numbers defining your kinematic. so if you are going to use the values, use correct ones.

    in other words, flange location will never change, so first four transforms are arbitrary. if you just measured the distance from your reference point to axis of rotation, entered it in the last of five transforms as already suggested, you could do the measurement and be on your happy way.

    but i would not do it like that. your choice of reference point is scary.

    who wants reference point on a workpiece? if you change workpiece or just reposition it, reference mark will be gone. besides one really wants reference mark and frame to be on something ... solid ... something that can be measured from... like a flange. then you can use measure tape and check if things match.

    if you pick some arbitrary point in open space (which is what you would get if workpiece is removed or replaced) then you would have no point to measure from.

    so you want to have it like this:

    you need to put the mark into the flange (like a crosshair). this mark must always be there for future use. so you need to engrave it or punch in a dot using center punch... this need to be a fine point since it is used for calibration... so do not try to make the mark using something crude like angle grinder, sledge hammer or blow torch.

    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

  • if you rotate previous example to match your setup, it would look like this:

    granted some things can be confusing. like the way words "pin" and "reference" are used.

    also your image seem to be part of series of slides describing the transforms one at a time and this was just the first one of examples. so check them all to learn more. hope this helps clear things up.

    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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