Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-09-02
2002-05-21
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C073S504020, C073S504120, C073S504140, C073S514010, C073S514380, C250S231100
Reexamination Certificate
active
06392220
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to monolithically fabricated micromachined structures and, more particularly, to micromachined structures in which a first frame is coupled to a plate or to a second frame for rotation of the plate or second frame with respect to the first frame about an axis.
2. Description of the Prior Art
A fundamental micromachined structure having many diverse uses is a torsional oscillator formed by a first frame that is coupled to a plate or to a second frame by diametrically opposed torsion bars that extend between the first frame and the plate or second frame. The torsion bars permit rotation of the second frame or the plate with respect to the first frame about an axis established by the torsion bars. Practical uses for this basic micromachined structure include optical beam micromachined torsional scanners having a reflective surface, described in U.S. Pat. No. 5,629,790 (“the '790 patent”), that have uses in digital imaging, bar code reading and printing as described in U.S. Pat. No. 5,841,553 (“the '553 patent”), and in magneto-optical recording as described in Published Patent Cooperation Treaty (“PCT”) International Patent Application WO 98/09289 entitled “Optical head Using Micro-Machined Elements” (“the '09289 PCT patent application”). Other practical applications for the basic micromachined structure occur in various other scientific and industrial systems such as rate gyroscopes described in U.S. Pat. No. 5,488,862, micro-flow meters described in U.S. Pat. No. 5,895,866, and profilometer and/or atomic force microscope (“AFM”) heads described in U.S. Pat. No. 5,861,549 that are used in surface inspection systems.
Collectively, the preceding patents describe various techniques for applying electrostatic and electromagnetic forces to the plate and/or second frame to energize rotation about the axis established by the torsion bars. The usefulness of the basic micromachined structure is greatly enhanced by integrating a torsion sensor into at least one of the torsion bars as described in U.S. Pat. No. 5,648,618 (“the '618 patent”) for measuring rotation of the second frame or the plate with respect to the first frame about an axis established by the torsion bars.
FIG. 1
illustrates a torsional oscillator, i.e. a typical torsional scanner, such as that described in '790 patent which is referred to by the general reference character
52
. The torsional scanner
52
includes torsion bars
54
which extend inward from an encircling frame
56
to support a torsional scanner plate
58
and permit the plate
58
to rotate about an axis
62
established by the torsion bars
54
. The frame
56
rests upon an insulating substrate
64
which also carries a pair of electrically conductive electrodes
66
. A frame shaped spacer
68
, resting on the frame
56
, supports a membrane window
72
a short distance above the plate
58
. A light beam
74
, indicated by arrowed lines in
FIG. 1
, enters the torsional scanner
52
through the membrane window
72
, impinges upon and reflects from a mirror surface
76
on the plate
58
, and then exits the torsional scanner
52
through the membrane window
72
. A voltage V applied alternatively between the plate
58
and first one and then the other of the electrodes
66
that switches back and forth between the electrodes
66
at the frequency of the principal torsional vibrational mode of the plate
58
applies an electrostatic force to the plate
58
which urges it to rotate back and forth at that frequency about the axis
62
.
When using the basic micromachined structure for the optical beam torsional scanners
52
, a mirror surface
76
on the plate
58
or second frame deflects the light beam
74
, usually from a fixed light source, over an angle ranging from several degrees to tens of degrees. Such reflective torsional scanners
52
may be used for sweeping a beam of light back-and-forth at a frequency determined in part by a mechanical resonant frequency of the plate
58
or second frame. Alternatively, torsional scanners
52
may be used for moving or switching a point at which a beam of light impinges upon one or more other elements between two (2) or more alternative locations.
The '790 patent describes a critical mechanical vibrational mode spectrum which commercially practical torsional oscillators should possess. This mode spectrum is particularly desirable for sinusoidal oscillation of the torsional scanner
52
at video or even higher frequencies. The same mode spectrum is also advantageous when the torsional scanner
52
operates in a quasi-static mode such as when switching a point at which the light beam
74
impinges upon other elements. Operating in a quasi-static mode, the torsional scanner
52
rotates to and remains fixed in one orientation for some interval of time, and subsequently rotates swiftly through a relatively large angle to another orientation where it again remains fixed for some interval of time.
As illustrated in
FIG. 2
of U.S. Pat. No. 5,673,139 (“the '139 patent”), for applications in which torsional scanners
52
must rotate about one or two axes and must be packed very closely together it is often desirable to eliminate open space between the frame
56
and the plate
58
or second frame occupied by the length of the torsion bars
54
. This open space may be eliminated if the length of the torsion bars
54
is located within a “butterfly-shaped” frame as illustrated in the '139 patent, or within a butterfly-shaped plate
58
. However, since torsion bars
54
tend to be very long and slender even the butterfly-shaped plate
58
or frame such as that illustrated in the '139 patent may occupy too much space. Merely shortening the torsion bars
54
to reduce the space which they occupy can be disadvantageous because, in general, shortening the torsion bars
54
make them stiffer which raises the frequency of the principal torsional vibrational mode, or alternatively increases the force that must be applied to rotate either the plate
58
or the second frame about the axis
62
.
In many instances for various reasons energizing rotation of the plate
58
with low power electrostatic fields as described above is highly desirable. However, some applications for the torsional scanner
52
may require that the plate
58
rotate through large angles. Due to an electrostatic instability, using electrostatic force to energize rotation of the plate
58
, or a second frame, either statically or dynamically without feeding a signal that is proportional to angular rotation back to the circuit that generates the electrostatic drive signals generally limits the rotation angle of the plate
58
.
The electrostatic instability occurs because a restoring torque applied to the plate
58
by the torsion bars
54
increases linearly with rotation of the plate
58
while a driving torque generated by electrostatic attraction between the plate
58
and one of the electrodes
66
increases quadratically as the separation between them decreases. For sinusoidally oscillating electrostatically driven torsional scanners operating at the resonance frequency of their principal torsional vibrational mode, the electrostatic instability is of little concern because voltage applied between the electrodes
66
and the plate
58
is generally zero (0.0) when the plate
58
rotates nearest to the closest electrode
66
. That is, for sinusoidally oscillating electrostatically driven torsional scanners operating at the resonance frequency of their principal torsional vibrational mode, rotation of the plate
58
is out of phase with, i.e. lags, application of the alternating voltage V between the plate
58
and first one and then the other of the electrodes
66
. However, if for quasi-static operation a constant voltage V that exceeds some threshold value were applied across the plate
58
and one of the electrodes
66
, rotation of the plate
58
about the axis
62
becomes unstable. That is, if the voltage V applied betwe
Neukermans Armand P.
Slater Timothy G.
Lee John R.
Schreiber Esq. D. E.
XROS, Inc.
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