Large machines, especially those having working envelopes in excess of fifteen
feet, exhibit unacceptable errors because of thermal expansion and mechanical misalignments
between the axes. The errors have traditionally been minimized by enclosing the
machine in a thermal enclosure, by careful calibration, or by mounting a laser
interferometer on each axis. These solutions are costly, may require frequent recalibration,
and do not correct for small rotations of one axis relative to another axis due
to wear etc. The present invention uses an interferometric laser tracker or a comparable
3D position sensor to measure the position of a retroreflector attached to the
end effector, e.g. a machine head when the machine comes to rest. A computer compares
the measured position to the desired position according to the machine media, and
adds the appropriate correction with trickle feed media statements to move the
machine to the correct position prior to further machining.