July 2013 6.5 Tilting and swivel axes 751
Backlash and
positioning errors
You can activate backlash measurement for all rotary axes in the input
parameter Q432 during programming. You must enter an angular value that is
greater than the actual backlash of the rotary axes.
Since the accuracy of this information depends on the measuring circle radius,
the text log indicates the measurement uncertainty in degrees per 1 µm of
system uncertainty:
The system uncertainty includes at least the position error of the linear axes
and the scatter of the touch probe. However, this data is not available to the
measuring cycle, and must therefore be estimated by the operator (see
example below).
Additionally, the positioning error of the rotary axis is always determined. The
calculated positioning error is also subject to this measurement uncertainty. In
order to realistically measure the positioning accuracy, it is recommended that
the angle increment when measuring the rotary axis is defined to be clearly
below 90° (recommendation: less than 45°). If the angle increment is too large,
then the positioning error is not calculated.
As a rule, the measurement uncertainty increases as the measuring circle
radius increases.
Example for calculating the measurement uncertainty (the greater the
calculated value of the measurement uncertainty, the less informative the
measurement is):
Position uncertainty of each linear axis: 10 µm
Uncertainty of touch probe: 2 µm
Measurement uncertainty: 0.0002 degrees/µm
System uncertainty = SQRT (102+102+102+102) = 17.44 µm
Measurement uncertainty = 0.0002 °/µm * 17.44 = 0.0034°
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