Express the coordinates of one point relative to another, in a specific frame

    Hello,

    I think that I need something like :
    trsf compose-1(point p1, point pRobot, frame F1)
    This instruction returns the trTransformation from p1 to obtain pRobot relative to F1

    I don't know if my question is clear at all, but I'll try to explain what I'm trying to achieve:

    I have a point p1 in the world.
    The robot is located at a position pRobot in the world.

    In a frame F1,
    I want to express the coordinates of p2 in F1 frame, which are the coordinates to add to p1 to obtain pRobot. ,relative to F1.

    In other words, adding p2 to p1  in F1 should give me pRobot.

    I tried :

    Code
    tmpP1.trsf = position(p1, F1)
tmpProbot.trsf = position(pRobot, F1)

p2.trsf.x = tmpProbot.trsf.x - tmpP1.trsf.x
p2.trsf.y = tmpProbot.trsf.y - tmpP1.trsf.y
p2.trsf.z = tmpProbot.trsf.z - tmpP1.trsf.z
p2.trsf.rx = tmpProbot.trsf.rx - tmpP1.trsf.rx
p2.trsf.ry = tmpProbot.trsf.ry - tmpP1.trsf.ry
p2.trsf.rz = tmpProbot.trsf.rz - tmpP1.trsf.rz

    Seems to work with x,y,z but not for the orientation rx,ry,rz.

    Am I on the right track or completely off?

    Edited once, last by Kelidor (March 18, 2025 at 2:55 PM).

    While searching the documentation, I came across the * and ! operators with the trsf.

    So I modified my code like this:

    Code
    p1.trsf = position(p1, F1)  
pRobot.trsf = position(pRobot, F1)  

p2.trsf = !T1 * T2

    But same result, seems to work with x,y,z but not for the orientation rx,ry,rz.

    Thank for your reply

    You are totally right in my exemple, but I have an exemple that I don't hunderstand :

    All point relative to F1 Frame:

    p1={0,0,0}{0,0,-90}

    pRobot={0,0,0}{15,18,-65}

    Code
    p2.trsf= ! p1.trsf * pRobot.trsf

    here p2= {0,0,0}{-18.59,15.25,29.73} when I expect something like p2= {0,0,0}{15,18,25}

    to verify :

    Code
    pRobot = compose(p1,F1,{0,0,0,-18.59,15.25,29.73}) -> WRONG
pRobot = compose(p1,F1,{0,0,0,15,18,25}) -> TRUE


    I don't hunderstand p2 values, what i'm missing here ?

    Hi Kelidor,

    The orientation of a point is given with Euler angles. You may shortly check wikipedia to have full description.

    In the expression p2.trsf= ! p1.trsf * pRobot.trsf,

    let's consider p2 as a starting frame (a frame and a point are both a location in the space from pure mathematical point of view)

    the 15° are applied around the x direction of p1 orientation (which is indeed rotated by RZ=-90° compared to frame1) .

    P1 is then newly oriented in the space. The 18° are applied around y direction of this new p1 orientation

    P1 is again newly oriented in the space. The -65° are applied around z direction of this again new p1 orientation

    whereas in the expression:

    pRobot = compose(p1,F1,{0,0,0,-18.59,15.25,29.73})

    Rotations are applied in the F1 frame, which is originally not oriented like P1. This is the reason why the result of this calculation is not the original pRobot.


    Last but not least, you first assumption p2= {0,0,0}{15,18,25} is wrong. Calculation with oriented positions in the workspace are mathematically matrix multiplication -> do not simply add rx's,ry's,rz's

    regards

    • New

    Hello Klausi,

    Thank you for taking the time to answer me.


    Quote from klausi

    Last but not least, you first assumption p2= {0,0,0}{15,18,25} is wrong. Calculation with oriented positions in the workspace are mathematically matrix multiplication -> do not simply add rx's,ry's,rz's

    I was not trying to add rx's,ry's,rz's here.

    I determined p2 because:

    pRobot = compose(p1,F1,p2)

    with p2 = {0,0,0,15,18,25}, it gives me the good result for pRobot -> {0,0,0,15,18,-65}


    So if p2 is unknown here : pRobot = compose(p1,F1,p2)

    There is no simple way to determine p2 without going into complicated things like matrix calculations?


    regards

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