Laser Interferometer Measuring System
Measurement
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The
laser interferometer measurement system is
a highly precise instrument used to measure changes in the height of the
product surface in a UST. The laser interferometer system consists of two
elements: a laser interferometer mounted on a rugged adjustable stand, and two
brass lubes inside of which are positioned floats containing comer-cube beam
reflectors. The signal from the two interferometer channels is shown on two
local displays which are in rum connected to a data acquisition computer so
that long-term testing can be conducted. In operation, the laser measures
changes in float position with respect to the interferometers mounted on the
stand. The reference standard for these distance measurements is the wavelength
of the laser light, which is very tightly reguluied.
Product-level changes are measured
simultaneously in both tubes, with one opened and one closed to the product in
the tank. Product-level changes in the tube open to the tank are identical to
the product-level changes in the tank, and thus are affected by all sources of
noise and simulated leaks. Product-level changes in the closed rube ire
theoretically affected only by the thermal expansion or contraction of the
product and by evaporation or condensation of the product.
Because
the hear generated by the laser head and electronics is separated from the
interferometers, the height measurements are very precise. The resolution and
precision of the laser interferometer (according to its manufacturer's
specification) are 0.025 and 0.043 u,m. respectively. The laser is re-zeroed
each time the power is turned on or off, or each lime the laser display units
are reset. As a consequence, the system is used to measure product-level
changes but is not used to measure the depth of product in the tank. The
product-level data are sampled at 200 Hz (200 samples/s), with a
special-purpose data acquisition program (LIDAS) (261. written in
Hewlett-Packard (HP) Basic, on an HP 9836 computer system that collects,
downsamples. stores, and displays the data tn real time. The data are
transferred between the laser display units and the computer system via two
binary-coded decimal (BCD) interface cards and two specifically designed 64-pin
connecting cables.
(Volumetric leak
detection methods for underground fuel storage tanks, J. W. Maresca, Pg:142)
Previous:
Presently
lasers are used as length measuring devices. They are commonly used for
positional accuracy measurements. They are also used as length measuring
machines of high accuracy (accuracy of the order of 0.01 micrometer). The
feedback of this can be used for positioning of the machine and also for
computation of measurements. Nowadays it has become a common practice to use laser-measuring
system for the calibration of CNC machines.
Using
laser-measuring system the measurements performed are reliable, accurate and
faster compared to conventional methods. The laser interferometer can be
directly interfaced with a computer. This makes it easy for the operator to
evaluate the results as per the evaluation procedures mentioned in various
standards like AMT, AFNOR, VDI, MTTA, and JIS etc. Using different attachments
laser interferometer is also used for other measurements like straightness,
flatness, squareness, velocity, pitch, yaw etc.
(Radhakrishnan P.,
Subrahmanyan S., Raju V., CAD/CAM/CIM, 3rd Edition, p. 513 )
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