If my external rotary axis rotates 1,000s of rotations, am I losing precision because of the limits of REAL values?

    I want to know if I am losing some precision if I let my rotary axis rotate 10,000s of revolutions.

    I noticed, when I show the $AXIS_ACT value, I only get a couple of decimal places on my rotary axis and it has many more integer values than any other axis.

    I don't know if there is a way to see the full precision of the REAL value, or if what I am seeing is all that can be known because of the limitations of 32-bit REAL.

    If I am losing precision, is there anything I can do to keep my external axis value low so I have better decimal precision? I don't need to know that I have rotated millions of degrees... Is there any way to set the value back to 0.0 without re-mastering the axis?

  • Go to Best Answer

    We have our external axis set so that we can go from +179 to -179 and it will only rotate the 2 degrees, so we don't have to specify really large numbers. I am wondering, if the external axis has rotated 10,000 revolutions for whatever reason, and we tell it to go to 120.456789123, will the the decimal values be truncated in some way to lose precision in the actual position of the axis?

    • Best Answer

    REAL type is 32-bit.

    of the 32-bits, one is sign, 8 are exponent and remaining 23 bits are significand (mantissa). when looking at values in decimal format, rounding happens at about seventh digit...

    in other words

    120.456789123 is not differentiable from

    120.456787

    try it...

    DECL REAL a,b

    a=120.456787

    b=120.456789123

    if a==b then...

    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

    Quote from mjnewsum

    Crazy... so is this loss of precision only in the display of the external axis position as a variable, or does it actually lose precision in the motion of the external axis as well?

    precision is determined by size of memory variable uses (number of bits). this does not change based on type of axis (robot axis or external axes).

    so... why would it be any different?

    if your external axis is used as a turntable, or carousel, and the precision is important to you, you probably do not want to keep running it one way endlessly. you may want to alternate direction as long as possible, in order to avoid or minimize accumulation error...

    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

    Quote from mjnewsum

    Crazy... so is this loss of precision only in the display of the external axis position as a variable, or does it actually lose precision in the motion of the external axis as well?

    Well, it's impossible to get a motor/axis motion ratio that's perfect, so some error is inevitable. An error that would be completely infinitesimal over a mere 360deg of rotation would accumulate to something much more substantial over 360000deg in a single direction. So "unwinding" your "endless" axis on a regular basis is generally considered to be "best practice".

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