The laser/ball bar is a two-dimensional
measurement device that uses a laser
displacement meter and a flat-mirror target for
the measurement. This system uses a
modulated laser to reflect a beam off a
moving target. The control detects the
beam and processes it for
displacement
information to determine correct position.
This noncontact system is highly
accurate, typically 1
ppm, and traceable to NIST. Its measurements provide more information,
such as feed rate or
tangential velocity and acceleration. Also, the toolpaths are not limited to a
circular path. For
example, a toolpath with decreasing radius or a spiral path, is entirely
possible. A circular
path with many revolutions can easily be measured, and the radius of the
circular path can be
continuously varied. Motion
errors due to the mistuning of the CNC servocontrol system are often easier to
observe on
noncircular toolpaths. By comparing noncircular
contouring-error profiles at different feed
rates, motion errors
due to mechanical structures, acceleration/deceleration, and those due to the
CNC servocontrol
system can be identified. The laser/ball bar system compares contouring-error
profiles at
different feed rates to identify motion errors due to mechanical structures and
acc/dec, and those
due to the CNC servo control system. Volume
analysis. One technique that is said to cover the failings of
the interferometer and ball bar is volume analysis. Instead of checking a
limited amount of axis motion, this process looks at the entire operating volume of the machine
tool. Volumetric analysis uses a laser Doppler displacement meter in
conjunction with a vector measurement
technique. It measures linear position errors, vertical straightness errors
and horizontal straightness errors for all three linear
axes and
three squareness errors. By measuring
volumetric errors, the machine tool can
be compensated
over its whole volume, which improves the accuracy of the machine tool, not just
linear displacement.
