How to perform touch sensing (rough calibration)

    Hello everyone, I've recently begun learning about welding robots and have encountered some uncertainties that I hope those with more experience can help clarify.

    I was discussing with a colleague who mentioned that when welding large parts, it's common practice to conduct touch-sensing initially to establish the part's position (similar to a global calibration). Then, for each subsequent weld, touch sensing is performed again (referred to as local calibration).

    My question is, is this a typical procedure in the industry? And if so, how does the robot manage nested calibration?

    Thanks and have a nice day :smiling_face:

    Touch-sensing calibration for MIG-welding isn't unusual. Nested calibration is less typical, but completely doable, most often used as a sort of "rough then fine" calibration.

    The details differ by brand and software version. For an advanced programmer it's possible to write the entire software stack from scratch, but generally everyone buys the appropriate option package for their specific robot and lets the robot mfgr do most of the heavy lifting.

    The same underlying methods also apply to things calibration using vision or non-contact sensors. At the end of the day, it's all matrix algebra.

    As for how the robot handles nested calibration, again, it depends on the brand, but often the "rough" calibration is an offset from a fixed coordinate system, and then the "fine" calibration is an offset from the coordinate system that was moved by the "rough" calibration. Basically, it's stacking offsets on top of offsets.

    Thank you for the help. I will investigate rough calibration.

    I understand the concept of chaining matrices, but I'm not sure if it is commonly used in practice.

    I would like to see some examples of "robot code" applying this concept. Do you know where I can find it?

  • jorgefernandes April 9, 2024 at 10:08 AM

    Changed the title of the thread from “How to perform touch sensing (global calibration)” to “How to perform touch sensing (rough calibration)”.
    Quote from jorgefernandes

    I would like to see some examples of "robot code" applying this concept. Do you know where I can find it?

    That's very, very brand-dependent. While matrix algebra is universal, every brand's implementation is unique.

    The simplest example I can think of offhand would be ABB RAPID's use of WorkObjects -- each WorkObject has a UFrame and an OFrame. The UFrame is a 6DOF offset from the robot's World coordinate system, and the OFrame is an offset from the UFrame. So often, the UFrame would be "rough" calibrated, then the touchsense "search" program for the "fine" calibration would be run in that UFrame, with the results applied to the OFrame. But this is all handled implicitly in ABB, none of the math is exposed.

    The mathematically simplest example I can think of is in KUKA KRL, where offsets are arbitrary matrix multiplications, using the Geometric Operator ":". Something like:

    Code
    $BASE = BASE_DATA[1] ; activate non-moving base frame
RoughSearchProgram(); call the rough-search touchsense program
$BASE = $BASE_DATA[1] : RoughSearchResults ; shift $BASE by Base 1 * Search Results
FineSearchProgram() ; call the fine-search program.  The search moves will execute with the rough search results already applied
$BASE = $BASE : FineSearchResults ; Add the Fine results to the already-shifted $BASE
$BASE = BASE_DATA[1] : RoughSearchResults : FineSearchResults ; this line does the same thing as the previous line

; do your welding or whatever here, using the doubly-shifted $BASE.

    Where $BASE is the 6DOF FRAME (X, Y, Z, A, B, C) that the robot moves in, BASE_DATA[whatever] is an array of user-modified Bases to use in programs, and the Search programs return a Result variable that is also a 6DOF FRAME. The : simply performs a plain matrix multiplication of two FRAME variables.

    Thanks for all the help and suggestions.

    I investigated more about the topic and think I understand it a bit more.

    Although I understood the code you posted, my experience is with Fanuc robots. Do you have any examples you could share using Fanuc code?

    Quote from jorgefernandes

    Although I understood the code you posted, my experience is with Fanuc robots. Do you have any examples you could share using Fanuc code?

    I haven't done it in Fanuc -- from what I've seen, Fanuc doesn't make matrix math openly available to do the offsets the way KUKA does. Instead, Fanuc has purchasable options that do it all for you invisibly.

    That said, if you check out the Fanuc sub-forum here, our member Nation has posted several tools for doing matrix algebra "the hard way" in TP code, which should be closely equivalent to the KRL code I posted upthread.

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