Browse Prior Art Database

Beam Deflection Measurement Tip

IP.com Disclosure Number: IPCOM000117015D
Original Publication Date: 1995-Dec-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 4 page(s) / 131K

Publishing Venue

IBM

Related People

Johnson, AO: AUTHOR [+3]

Abstract

In developing a positional measurement system, a problem arises in determining whether or not, the probe tip of the measurement system is in contact with the object being measured. The application requires that this continuity be established within the following constraints.

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Beam Deflection Measurement Tip

      In developing a positional measurement system, a problem
arises in determining whether or not, the probe tip of the
measurement
system is in contact with the object being measured.  The application
requires that this continuity be established within the following
constraints.

      Electrical conduction through the object being measured is not
possible.

      Contact force must be continuously measured in the course of
operation such that loss of contact with the object being measured
will be immediately detected.  This allows the measurement system to
regard or discard the measurement data as appropriate.

      The contact direction must be resolvable in two dimensions
along with the ability to provide a signal which is directly
proportional to the amount of force exerted.  This in turn can be
mapped to provide positional correction for any tip deflection due to
excessive contract force.

      A device has been concepted which will provide the means to
achieve the requirements.

      Referring to Fig. 1, a bending beam is attached to the
measurement probe tip.  The deflection of the beam is measured by a
pair of thru-beam optical sensors mounted orthogonal to one another
in the vertical and lartal planes.  The optical sensors use a beam of
light that is produced by a Light Emitting Diode (LED) which is
directed at a photodiode.  The two devices are mounted and aligned in
the same package so that no external alignment adjustments are
required.  The bending beam would be inserted between the LED and
photodiode and the light beam would pass through a hole in the
bending beam.

      The photodiode produces an output current that is directly
proportional to the amount of light it is exposed to.  Referring to
Fig. 2, as the beam bends, more or less light will be blocked or
passed by the hole in the beam and the output current will change
accordingly.

      The through-beam pairs would be mounted to track the vertical
and horizontal deflections of the beam.  If the two deflections are
taken together, then a measure of rotation of the beam can be made.
It is also possible to map or calibrate the deflections by forcing a
known deflection or rotation, measuring the outputs, and then storing
these measurements in a look-up table in the computer application
which runs the measurement system.  The look-up table will eliminate
any non-linearity in the sensors, light path, or light beam shape.

      The photodiode will produce a bell shaped output curve shown in
Fig. 3.  The through-beam pair will be positioned relative to the
bending beam such that the photodiode output is halfway up the slope
of the curve.  This is not a critical set-up because in operation, it
is possible to hold the tip in free space and electrically zero its
output with no probe tip load.  The direction of deflection will be
referenced to that initial zero poi...