1
Reprinted From:
MODERN MACHINE SHOP Magazine Hole
Position Accuracy Assured By Laser Doppler Scale Bishop
Wisecarver Corporation builds linear motion systems complete
with guide wheel and track
for everything from large industrial copier machines where the carriage moves
back and forth to XY motion on photographic
stages. "Any kind of equipment that moves
back and forth on a track." said
Bud Wisecarver, president of Bishop Wisecarver, "Our niche is the generic
motion of getting something from A to B and
back again. It's the smallest part of a ten billion dollar market."
Until 1988, the company provided track
without mounting holes. Customers had to
drill their own.
To increase product value, it was
decided they would provide track with
pre-drilled mounting holes. Four drilling
machines were built, one for each size track made. The track is mounted on a
dolly, inserted and fed through an
automatic drilling head. A Berkeley processor is used to
program the layout of holes, whether
evenly or unevenly spaced. However, the chain drive
that moves the track dolly through the machine stretched and the
holes could not be accurately positioned.
The result would be track with predrilled holes that didn't line up with mounting
holes on customer's equipment. Obviously,
the situation was unacceptable and a method for
accurately locating holes on the track
had to be found immediately. "We're concerned
with a single axis measurement just
for length. We have to hold hole
position accuracy of
±0.002 inch in feet. We decided to
use a Laser Doppler Scale for
position accuracy, because it was smaller
and less complex than other types of laser systems," said Mr. Wisecarver. The Laser Doppler Scale
(LDS) is made by Optodyne, Inc. in Compton, California and is
based on its patented
technology. The LDS provides high speed,
high accuracy and long range positioning for one-, two-, and three-axis applications.
The LDS reflects a modulated laser
beam off of a movable target. The
beam is detected and processed for
displacement information used
by the control to determine position.
The system requires only three components, including laser
head, retroreflector and
processor. Additional
optical components
required by other laser systems, such
as tripods, and so forth, have been
eliminated. The LDS provides resolution of
25 microinches at a speed of 160 ips with a 2,000 inch range. In this application, the
LDS provides dynamic linear measurement for drilling operations on
track up to 20 feet
long. The track-carrying dolly has a
retroreflector (reflective mirror) mounted on it. The laser head is positioned so that its beam
reflects off the mirror. As the dolly
carries the track through the drilling machine the LDS encoded the reflected
beam and the control determines exactly how
far the track has moved. Whether the chain stretches or not is no longer a concern. Another potential problem
was averted by using a temperature compensation option which enables the software to use data
from the LDS's sensors to automatically
compensate for machine
temperature and air temperature. "Consider a 40°F rise in
temperature, as on a summer day, when it goes from 60°F in the
morning to 100°F degrees in the afternoon,
there will be expansion of about 0.090 inch in 20 feet. That means holes drilled at the end of the track would be
off by '/32 inch. If we supply the track
and the holes are not accurate, they won't line up with our customer's
equipment," said Mr. Wisecarver.
Click here to
download Wisecarver.pdf file
