Browse Prior Art Database

Interferometric Scale

IP.com Disclosure Number: IPCOM000107289D
Original Publication Date: 1992-Feb-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 3 page(s) / 111K

Publishing Venue

IBM

Related People

Cato, RT: AUTHOR [+4]

Abstract

The disclosed scale provides an accurate measure of weight in a low profile and inexpensive package that is well suited for use in a scanner/scale. Current scale technologies use methods that require the summation of multiple load cells or large mechanisms that concentrate the platen force through a single load cell. These load cells are traditionally analog devices with expensive circuits that must be extremely stable. The disclosed scale offers a digital technology in an inexpensive embodiment that can accommodate a scanner window in its center without significantly changing the size of the scanner.

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This is the abbreviated version, containing approximately 52% of the total text.

Interferometric Scale

       The disclosed scale provides an accurate measure of
weight in a low profile and inexpensive package that is well suited
for use in a scanner/scale.  Current scale technologies use methods
that require the summation of multiple load cells or large mechanisms
that concentrate the platen force through a single load cell.  These
load cells are traditionally analog devices with expensive circuits
that must be extremely stable.  The disclosed scale offers a digital
technology in an inexpensive embodiment that can accommodate a
scanner window in its center without significantly changing the size
of the scanner.

      The scale is shown in Fig. 1.  The platen is supported on four
pivot links which are set at an angle of about 45 degrees.  This
imparts a horizontal displacement to a vertical platen weight.  This
horizontal displacement is resisted by two springs and bias springs
secure the assembly and insure that the horizontal displacement is
directly proportional to the weight on the platen.  This mechanism
provides the same horizontal displacement regardless of where the
item is placed on the platen.

      The horizontal displacement is measured by an interferometer
shown in Fig. 2.  The light source is a laser diode or an LED which
is filtered with a narrow band filter. A laser diode of the type in
compact disc (CD) players can be used.  The laser beam is reflected
by a collimating mirror which collimates only one axis.  The beam is
reflected to the beam splitter which reflects half of the beam
towards a mirror which moves with the platen; and the other half
passes through to the reference path mirror. Both the beams are
reflected back to the beam splitter where they are recombined to form
a fringe pattern as shown at the observation window.

      Motion of the platen mirror changes the beam path length
through one branch of the interferometer system.  A movement of 1/4
wavelength will cause photo detector 1 to change states.  For
example, a CD laser with a wavelength of 780 nanometers produces a
fringe for every 7.6 microinches of platen movement.  A resolution of
0.01 pounds and a 30     weight will cause a 0.023 inch platen
movement. If parallellism of the platen mirror becomes a problem, a
c...