U.S. patent application number 09/809995 was filed with the patent office on 2001-11-15 for biased rotatable combdrive sensor methods.
Invention is credited to Behin, Behrang, Pannu, Satinderpall.
Application Number | 20010040419 09/809995 |
Document ID | / |
Family ID | 22707176 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040419 |
Kind Code |
A1 |
Behin, Behrang ; et
al. |
November 15, 2001 |
Biased rotatable combdrive sensor methods
Abstract
A method for rotating a combdriven device uses application of
bias force to controllably rotate the device about one or two axis
in response to position measurements of the rotating element. The
present invention can be employed with prior-art staggered
combdrives, single layer self-aligned combdriven devices, and in a
broad range of applications in optical telecommunication switching,
video, biomedical, inertial sensors, and in storage magnetic disk
drives.
Inventors: |
Behin, Behrang; (Berkeley,
CA) ; Pannu, Satinderpall; (Berkeley, CA) |
Correspondence
Address: |
JOSHUA D. ISENBERG
204 CASTRO LANE
FREMONT
CA
94539
US
|
Family ID: |
22707176 |
Appl. No.: |
09/809995 |
Filed: |
March 14, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60191856 |
Mar 24, 2000 |
|
|
|
Current U.S.
Class: |
310/309 |
Current CPC
Class: |
G02B 6/357 20130101;
G02B 26/0866 20130101; G02B 26/0833 20130101; G02B 6/359 20130101;
G02B 6/3518 20130101; G02B 6/3584 20130101; H02N 1/008 20130101;
G02B 26/0858 20130101; G02B 26/085 20130101; G02B 6/3524 20130101;
G02B 6/3512 20130101 |
Class at
Publication: |
310/309 |
International
Class: |
H02N 001/00 |
Claims
What is claimed is:
1. A method of operating a rotating comb-drive position sensor,
comprising: a) providing a plurality of first comb fingers and a
plurality of second comb fingers, wherein said second comb fingers
are interdigitated with said first comb fingers in an engagement;
b) mechanically coupling a rotating element to said first comb
fingers; c) coupling a biasing element to said rotating element; d)
exerting a biasing force with said biasing element, thereby causing
said first comb fingers along with said rotating element to undergo
a controlled angular displacement from said engagement about said
axis; and e) monitoring a position of said rotating element.
2. The method of claim 1 wherein step e) includes measuring a
capacitance between said second comb fingers and said first comb
fingers.
3. The method of claim 1 wherein step e) includes feeding back a
signal from a position sensor to the biasing element.
4. The method of claim 3, wherein the position sensor includes one
or more of the following: one or more gap closing electrodes, a
second plurality of first comb fingers coupled to the rotating
element and a second plurality of second comb fingers that
interdigitate with the first comb fingers in the second plurality,
a capacitance sensor coupled between the plurality of first comb
fingers and the plurality of second comb fingers a piezoresistive
strain gauge, a piezoelectric sensor, or an optical sensor.
5. The method of claim 1, wherein said biasing element includes one
or more of the following: a magnetic material attached to the
rotating element, a current carrying coil attached to the rotating
element, one or more gap-closing electrodes attached to the
rotating element, a piezoelectric mechanism coupled to said
rotating element, a thermal bimorph actuator coupled to the
rotating element, a spring-loaded element coupled to the rotating
element, a stress-bearing material carrying a residual stress
gradient, or a second plurality of first comb fingers coupled to
the rotating element and a second plurality of second comb fingers
that interdigitate with the first comb fingers in the second
plurality.
6. The method of claim 1, wherein the biasing element of step C
applies a fixed force.
7. The method of claim 1, wherein the biasing element of step C
applies a variable force.
8. The method of step 8 wherein the biasing element operates on
magnetic principals.
9. The method of step 8 wherein the biasing element operates on
electrostatic principals.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the benefit of Provisional
Application 60/191,856 filed Mar. 24, 2000, which is herein
incorporated by reference for all purposes.
FIELD OF THE INVENTION
[0002] This invention relates generally to
Micro-Electro-Mechanical-System- s (MEMS). More particularly, it is
related to a novel class of vertical combdrive devices serving as
rotating actuators and/or position sensors and methods for
operating any combdrive.
BACKGROUND ART
[0003] The advent of silicon fabrication technologies has made
possible a line of integrated devices in which micro-actuators and
micro-mechanical structures are fabricated using processing
technology similar to that used in the integrated-circuit industry.
These integrated actuators have been employed in a variety of
applications, such as fiber-optic switching, optical tracking for
applications such as free-space communications, inertial sensors,
and magnetic disk drives. They offer small size, low cost, high
reliability, and superior performance. Furthermore, micro-machined
structures may be integrated with Integrated Circuits (ICs)
fabricated on the same substrate.
[0004] Various actuation methods can be employed in these
integrated actuators, including electrostatic, electromagnetic,
thermal, and thermo-pneumatic means. Electrostatic actuation
becomes particularly attractive on a small size scale, since the
electrostatic force increases as the gap between two charged
elements decreases. Combdrive electrodes are widely used for
generating electrostatic driving forces.
[0005] It is often desirable to create out-of-plane actuation of
various microstructures, such as rotation of mirrors about an axis
parallel to a substrate. These rotating mirrors can be used
individually or in array form for applications such as adaptive
optics, visual displays, or fiber-optic switching. Vertical
combdrive actuators provide rotational motion or translational
motion perpendicular to a substrate. A micromachined electrostatic
vertical actuator is disclosed in U.S. Pat. No. 5,969,848, issued
to Lee et al. The device of Lee et al. contains a set of vertical
combdrives, with each drive capable of deflecting one edge of a
square mirror. By relying on an asymmetric distribution of
electrical fields when a bias voltage is applied between stationary
and movable comb fingers, the device of Lee et al. allows a small
vertical (i.e. out of the plane of the comb fingers) motion of each
mirror edge, at most 1.5 .mu.m.
[0006] Larger movements and more simplified fabrication techniques
are provided by staggered vertical combdrives, in which the
stationary and moving combdrives are positioned parallel to one
another, but with the plane of the moving comb above the plane of
the stationary comb. The stationary comb fingers are an integral
part of the substrate, while the moving comb is fixed to the
substrate only through flexures. Applying a voltage between the two
comb layers causes the moving comb teeth to be attracted to the
stationary teeth and move to increase the overlap area, thereby
exerting a force on the moving comb. Conventional fabrication
techniques for vertical combdrives using standard photolithography
processes require multiple steps for patterning the comb fingers.
First, one set of comb teeth is fabricated on a first wafer layer.
A second wafer layer is then bonded on top of the first wafer
layer, followed by patterning and etching of a second layer to form
the second set of comb teeth. The two wafer layers must be aligned
to a very high precision; typical applications require comb fingers
of 2 .mu.m wide with a 6 .mu.m separation distance, so that
adjacent overlapped fingers are separated by only 2 .mu.m. Vertical
combdrives fabricated using this technique are prone to alignment
problems. The steppers used to align the individual die on a wafer
typically have a lateral resolution of .+-.0.25 .mu.m. This
resolution places a lower limit on the gap between adjacent comb
fingers of about 2 .mu.m. Because two adjacent fingers are at
different potentials during operation, they cannot contact each
other. At high actuation voltages, errors in alignment of the
fingers can cause sideways motion and instability in the combdrive.
As a result, conventional fabrication techniques typically have low
production yields.
[0007] FIGS. 1A-1B depict a prior art rotating actuator employing a
two-layer vertical combdrive. FIG. 1A shows rotating actuator 100
in a nominal state. A plurality of movable comb fingers 10,
extending from a first micro-machined structure 11, are suspended
above a plurality of stationary comb fingers 12, which extend from
a second micro-machined structure 13. A rotating element 14,
attached to a flexure 15, is mechanically engaged with first
micro-machined structure 11 and therefore movable comb fingers 10.
Rotating element 14 may carry a reflective surface so as to provide
a scanning mirror for a given application. It is worth noting that
stationary comb fingers 12 and movable comb fingers 10 are
fabricated in two different layers of a substrate (not shown in
FIG. 1A). FIG. 1B depicts a rotating state of rotating actuator 100
of FIG. 1A. The rotation can be generated by an electrostatic
means, e.g., by applying a voltage between stationary comb fingers
12 and movable comb fingers 10. The capacitance between movable
comb fingers 10 and stationary comb fingers 12 may be measured and
resolved to determine and control the angular position of movable
comb fingers 10.
[0008] In a combdrive actuator, it is desirable for the angular
position of the movable comb fingers to vary with the applied
voltage in a linear fashion; and it is also desirable for the
stationary comb fingers and movable comb fingers to be aligned with
respect to each other in a precise lateral alignment. This is owing
to the fact that if the stationary and movable comb fingers are not
well aligned, such that each of the movable comb fingers is
centered within the gap between its respective neighboring
stationary fingers, there arises a net lateral force upon
application of a voltage between the stationary and movable comb
fingers. Such a lateral force can cause non-linear and unstable
behaviors in the motion of movable comb fingers. For example,
sufficient lateral force can cause the movable comb fingers to snap
into contact with the stationary comb fingers.
[0009] In the prior art combdrive system of FIGS. 1A-1B, however,
because stationary comb fingers 12 and movable comb fingers 10 are
not coplanar and therefore not substantially engaged in their
initial positions, the motion of the combdrive thus constructed is
significantly nonlinear, unless a sufficient force is exerted on
the combdrive to engage stationary comb fingers 12 and movable comb
fingers 10. Moreover, precise lateral alignment between stationary
comb fingers 12 and movable comb fingers 10 is also inherently
difficult to achieve in the above prior art combdrive system,
because stationary comb fingers 12 and movable comb fingers 10 are
fabricated in two different layers of the substrate. This can
further result in non-linear and unstable behavior.
[0010] For example, in the prior art combdrive system of Conant et
al. titled "Staggered Torsional Electrostatic Combdrive
Micromirror", U.S. pending application Ser. No. 09/584,835, a set
of stationary combdrives is fabricated in one lithographic masking
and etching step, and a set of moving combdrives is fabricated in a
subsequent lithographic masking and etching step. A precise lateral
alignment of these two sets of combdrives would dictate aligning
the second lithographic masking step to the pattern of the first
lithographic masking step during the manufacturing process. Hence,
any error in this alignment translates directly to a lateral
misalignment between the movable and stationary combdrives, which
results in nonlinear and unstable behaviors in the combdrive
actuator thus produced.
[0011] Hence, there is a need in the art for a new type of rotating
combdrive actuators and position sensors that overcome the
shortcomings of the prior art systems, while providing a wide range
of angular rotation along with versatile actuation and sensing
capabilities.
SUMMARY OF THE INVENTION
[0012] The disadvantages associated with the prior art are overcome
by a rotating device having one or more comb structures and biasing
element to apply torsion force thereto and in response to position
sensing of the rotating device. The device generally comprises a
plurality of first comb fingers that interdigitate with a plurality
of second comb fingers. In one embodiment, both pluralities may be
fabricated from a single layer of a substrate such that they are
self-aligned. The design of the combdrive device is such that in a
nominal state, the two sets of comb fingers are substantially
interdigitated according to a predetermined engagement. A rotating
element, attached to a rotatable flexure disposed along an axis, is
mechanically engaged with the first comb fingers. A biasing element
(e.g., a magnetic material) is attached to the rotating element.
When subject to a biasing force (e.g., a magnetic force), the
biasing element causes the rotating element along with the first
comb fingers to undergo a controlled angular displacement from the
initial engagement. In one embodiment of the present invention, the
combdrive device serves as a rotating actuator. This is
accomplished by an application of a voltage between the second and
first comb fingers, which causes rotation of the first comb fingers
along with the rotating element back towards their initial
position. The biasing force may be kept constant in this case. The
capacitance between the second and first comb fingers is measured
and used to monitor the angular motion of the rotating element. The
measured capacitance can be further utilized in a feedback loop to
control the angular position of the rotating element.
[0013] In another embodiment of the present invention, the
combdrive device provides for a position sensor. A capacitance
sensor measures a capacitance between the first and second comb
fingers to monitor the angular position of the rotating element by
way of the measured capacitance. A time-varying biasing force may
be applied in this case to generate further rotation of the
rotating element along with the first comb fingers in a
predetermined manner. The angular position signal can be further
fed to a feedback loop, so as to control the biasing force and
hence the angular motion of the rotating element.
[0014] The present invention accommodates alternative position
sensors comprised of gap closing electrodes, additional comb
fingers, piezoresistive strain gauges, coils, magnets,
piezoelectric sensors, optical sensors and combinations
thereof.
[0015] The rotatable flexure may be a torsional flexure with
cross-sections including a rectangular, I-shaped, or T-shaped
cross-section, a cantilever-like flexure, serpentine flexure, a
pin-and-staple type hinge, or any flexure, as one skilled in the
art is capable of applying, to achieve rotation. A variety of
biasing mechanisms may be employed to generate an initial angular
displacement between the first and second comb fingers, including
those that operate via pneumatic, thermal, magnetic principals,
including coils that interact with an external magnetic field,
electrostatic elements, such as gap closing electrodes,
piezoelectric actuators and thermal bimorph actuators.
Alternatively, the biasing element may be a mechanical, e.g.,
spring-loaded element, which may be incorporated into the rotatable
flexure.
[0016] In an alternative embodiment of the invention two sets of
biased comb structures may be incorporated into a two-dimensional
scanner. The scanner generally includes a rotatable gimbaled
structure having a base, an outer frame, and an inner part. The
outer frame may be attached to the base by a first pair of
torsional flexures that allow the outer frame to rotate about a
first axis. The inner part, hereby also known as the rotating
element, may be attached to the outer frame by a second pair of
torsional flexures that allow the inner part rotate about a second
axis. The inner part may include a reflective surface such as a
mirror. The scanner may include one or more electrostatic
combdrives: a first combdrive positioned between the outer frame
and the base, and a second combdrive positioned between the inner
part and the outer frame. A biasing element, coupled to the outer
frame, the inner part, or to both, causes initial angular
displacements in the first and second combdrives. Either or both of
the first and second combdrives may include one or more
self-aligned comb structures.
[0017] Applying a voltage to either of the first and second
combdrives causes the rotating element to undergo further rotation
about either of the first and second axes. The biasing element may
exert one or more constant forces on the rotating element.
Moreover, the capacitances of the first and second combdrives may
be measured to monitor and control the angular positions of the
rotating element about the first and second axes respectively. Such
a device may constitute a two-dimensional rotating actuator and use
feedback from an alternative position sensor (e.g. gap closing
electrodes, additional comb fingers, piezoresistive strain gauges,
coils, magnets, piezoelectric sensors, optical sensors and
combinations thereof) to control the angular position of the
rotating element.
[0018] Alternatively, the biasing element may to exert one or more
time-varying forces on the rotating element, causing it to undergo
further rotation about either of the first and second axes. The
capacitance between the first and second combdrives may be measured
to monitor and control the angular positions of the rotating
element about the first and second axes respectively. Such a device
may provide for a two-dimensional actuator with a biaxial combdrive
position sensor and use feedback from an alternative position
sensor (e.g. gap closing electrodes, additional comb fingers,
piezoresistive strain gauges, coils, magnets, piezoelectric
sensors, optical sensors and combinations thereof) to control the
angular position of the rotating element.
[0019] The first and second combdrives may be fabricated in a
single layer of a substrate material which may comprise, but not be
limited to, a combination of one or more of the following
materials: single-crystalline silicon, poly-crystalline silicon,
amorphous silicon, ceramic, silicon-oxide, silicon-nitride,
single-crystalline silicon-germanium, polycrystalline
silicon-germanium, or a metal (such as nickel, chromium, aluminum
or gold). The rotating element may be made of the same materials.
The rotating element may comprise a reflective or light deflective
surface, thereby providing a bi-axial steering or scanning mirror.
The light deflective surface may include a surface coating to allow
light deflection along more than one simultaneous path. Either of
the first and second flexures can be a torsional flexure with a
cross-section including, but not limited to, rectangular, I-shaped,
or T-shaped cross-section, a cantilever-like flexure, serpentine
flexure, pin-and-staple type hinge flexure and mechanical or
non-mechanical torsion providing means as one skilled in the art
would be capable of applying. First and second axes may be
typically orthogonal to each other, though they can also be
oriented in other ways as dictated by practical applications. The
biasing element may comprise, but not limited to one or more
biasing elements of magnetic, thermal, electrostatic, or mechanical
types.
[0020] One embodiment of the invention utilizes self-aligned
combdrives. An important advantage of self-aligned rotating
combdrive devices is that by fabricating the comb fingers in a
single layer of a substrate, the first and second comb fingers may
start from a substantially co-planar and interdigitated engagement,
thereby substantially diminishing non-linear rotational effects
that are often inherent in the prior art vertical combdrive
actuators. Furthermore, if the first and second combdrives are
defined by a single lithographic step, their alignment can be held
to much tighter tolerances than in the prior art, providing for
much more stable behavior than vertical combdrive actuators of the
prior art. The performance of the rotating actuators and position
sensors thus constructed is therefore more predictable than, and
superior to, the prior art vertical combdrive devices.
[0021] It must be stated that the uni-axial and bi-axial rotating
actuators and position sensors of the present invention can be
employed in a broad range of applications, including, but not
limited to, biomedical devices, optical devices for tracking and
display, telecommunication devices such as fiber-optic switches,
inertial sensors, and magnetic disk drives. For example, uni-axial
and/or bi-axial rotating actuators employing reflective rotating
elements can be used as steering mirrors to switch light between
optical fibers in telecommunication applications. Arrays of such
steering mirrors can be utilized to provide fiber-optic switches
with very large port-counts. Use of the combdrives for sensing the
angle of the mirrors in these applications is crucial since
position sensing is needed for active and accurate control of the
mirror angles, and pointing accuracy is the key to achieving low
insertion losses in a fiber-optic switch. The novel features of
this invention, as well as the invention itself, will be best
understood from the following drawings and detailed
description.
BRIEF DESCRIPTION OF THE FIGURES
[0022] FIGS. 1A-1B show a prior art rotating actuator employing a
two-layer combdrive in nominal and rotating states
respectively;
[0023] FIG. 2 depicts an exemplary embodiment of a uni-axial
rotating device employing a biased combdrive in a nominal state
according to an embodiment of the present invention;
[0024] FIG. 3 shows an exemplary embodiment of a uni-axial rotating
actuator employing a biased combdrive in a rotating state according
to an embodiment of the present invention;
[0025] FIG. 4 illustrates an exemplary embodiment of a uni-axial,
self-aligned rotating device in operation according to an
embodiment of the present invention;
[0026] FIG. 5 depicts an exemplary embodiment of a bi-axial
rotating device employing two biased combdrives according to an
embodiment of the present invention;
[0027] FIG. 6 shows simplified cross-sectional schematic diagram of
an exemplary embodiment of a fiber-optic switch employing rotating
devices of the present invention as beam steering mirrors; and
[0028] FIG. 7 depicts a simplified cross-sectional schematic
diagram of a rotating device according to an embodiment of the
present invention.
DETAILED DESCRIPTION
[0029] Although the following detailed description contains many
specific details for the purposes of illustration, anyone of
ordinary skill in the art will appreciate that many variations and
alterations to the following details are within the scope of the
invention. Accordingly, the exemplary embodiments of the invention
described below are set forth without any loss of generality to,
and without imposing limitations upon, the claimed invention.
[0030] FIG. 2 shows an exemplary embodiment of a uni-axial biased
rotating actuator 200 employing a biased combdrive in a nominal
state according to the present invention. In biased combdrive
rotating actuator 200, a plurality of first comb fingers 20,
extending from a first micro-machined structure 21, are
substantially co-planar with a plurality of second comb fingers 22
extending from a second micro-machined structure 23. According to
one embodiment of the present invention, second comb fingers 22 and
first comb fingers 20 may be fabricated from a single layer of a
substrate 27, thereby substantially interdigitated and self-aligned
according to a predetermined engagement. Although much of the
following discussion relates to self-aligned electrostatic
combdrives, the advantages of biased actuation may be applied to
other types of cob drives that are not self-aligned.
[0031] The substrate 27 may be, for example from a single-layer
structure or multi-layer structure, such as a silicon-on-insulator
(SOI) substrate having two conductive layers separated by an
insulating layer. Both sets of comb fingers 20, 22 may be defined
in the same step by etching a pattern in the substrate 27. The
first and second comb fingers 20, 22 may be substantially
co-planar. Alternatively, one of the sets of comb fingers may be
selectively etched to remove portions of one of the structure so
that the two sets of comb fingers are vertically offset with
respect to each other, although they are still self-aligned.
[0032] By way of example, the first and second comb fingers 20, 22
may be fabricated in a single layer of a substrate material
comprising a combination of one or more of the following materials:
single-crystalline silicon, poly-crystalline silicon, amorphous
silicon, ceramic, silicon-oxide, silicon-nitride,
single-crystalline silicon-germanium, polycrystalline
silicon-germanium, metal (such as nickel, chromium, aluminum, or
gold), or a combination of these materials.
[0033] An example of a suitable SOI substrate for this application
a silicon (Si) substrate layer approximately 400 .mu.m thick, a
silicon oxide (or glass) insulator layer approximately 2 .mu.m
thick, and a Si device layer approximately 50 .mu.m thick. In this
case, the rotating element, second comb fingers, and first comb
fingers are made from the device layer, and the substrate layer is
removed underneath the location of the rotating element and moving
comb fingers to allow them clearance to rotate. The rotating
element may comprise a reflective surface, so as to serve as a beam
steering or scanning mirror.
[0034] In the embodiment shown in FIG. 2, the second comb fingers
22 may be electrically isolated from first comb fingers 20. A
rotating element 24, comprising a micro-machined structure, may be
attached to the substrate 27 by a rotatable flexure 25 disposed
along an axis 30. The rotating element 24 may be further
mechanically coupled to the micro-machined structure 21 along with
first comb fingers 20. A biasing element 26, may be coupled to the
rotating element 24. By way of example, the biasing element may be
in the form of a magnetic material deposited on rotating element
24. The magnetic material may interact with an externally applied
magnetic field to cause the first comb fingers 20 along with the
rotating element 24 to undergo an angular displacement about the
axis 30 from the initial interdigitated engagement. The angular
position of the first comb fingers 20 may be measured, for example,
by a capacitance sensor 29 coupled between the first comb fingers
20 and the second comb fingers 22.
[0035] In the above embodiment, biasing element 26 may employ a
variety of biasing mechanisms to generate an initial angular
displacement between first comb fingers 20 and second comb fingers
22 in the present invention. For instance, a current element can be
attached to rotating element 24. When disposed in a magnetic field,
a magnetic biasing force may exert on the current element, causing
rotating element 24 along with first comb fingers 20 to undergo an
angular displacement. Alternatively, a first electrode can be
attached to rotating element 24 and a second electrode is placed in
close proximity to the first electrode. When disposed in an
electrical field, the electrical force between the two electrodes
produces an angular displacement of rotating element 24 along with
first comb fingers 20. Furthermore, a material with a residual
stress gradient, termed a stress-bearing material hereinafter, can
be coupled to rotating element 24, such that a releasing of the
stress gradient causes rotating element 24 along with first comb
fingers 20 to undergo an angular displacement. Rotatable flexure 25
may alternatively carry a stress-bearing material, to achieve the
same purpose. Additionally, a spring-loading mechanism can be
coupled to rotating element 24, such that upon unleashing the
spring-loading mechanism may cause rotating element 24 along with
first comb fingers 20 to undergo an angular displacement.
[0036] Alternatively, the biasing element 26 may include one or
more of the following: stress-bearing material carrying a residual
stress gradient, a thermal bimorph actuator, one or more
gap-closing electrodes, or a second set of interdigitating comb
fingers that are separate from the first and second comb fingers
22, 24. Other suitable biasing mechanisms may be used for
generating an angular displacement in a biased combdrive of the
embodiments of the present invention, for a given application.
[0037] FIG. 3 shows an exemplary embodiment of a method of the
present invention for generating an angular displacement in a
biased combdrive rotating actuator 200' of the type shown FIG. 2.
An external magnetic field B, may exert a magnetic biasing force on
the magnetic material of biasing element 26'. This magnetic biasing
force causes rotating element 24' and first comb fingers 20' to
undergo an angular displacement about axis 30' from an initial
interdigitated engagement with second comb fingers 22'. A rotatable
flexure 25', e.g. coupled to a substrate 27' such as a torsional
flexure, facilitates such a rotational motion. The angular position
of the first comb fingers 20' may be measured, for example, by a
capacitance sensor 29' coupled between the first comb fingers 20'
and the second comb fingers 22'.
[0038] FIG. 4 illustrates an exemplary embodiment of an operation
of a biased combdrive rotating actuator 200" of the type shown in
FIG. 2. A voltage source 28 may be coupled between first comb
fingers 20" and second comb fingers 22". The voltage source 28
applies a voltage between the first and second comb fingers 20".
The electrostatic force thus generated overcomes the magnetic
biasing force owing to external magnetic field B, causing first
comb fingers 20" along with rotating element 24" to rotate about
axis 30" towards an initial interdigitated engagement with the
second comb fingers 22". A rotatable flexure 25", coupled, e.g., to
a substrate 27", may further facilitate the rotational motion of
rotating element 24" along with first comb fingers 20".
[0039] A capacitance sensor 29" may be coupled between the second
comb fingers 22" and the first comb fingers 20". The capacitance
sensor can be used to measure a capacitance between the first and
second comb fingers 20", 22". The measured capacitance may be used
to monitor the angular position of rotating element 24". A feedback
mechanism FB may be coupled to the capacitance sensor 29" and the
voltage source 28 in a feedback loop. In this configuration, the
capacitance measured by the sensor 29" can be used to control the
rotational motion of rotating element 24" and first comb fingers
20". In an alternative embodiment, the comb fingers 20", 22" may
provide a driving force for rotating the rotating element 24" and a
separate sensing means measures the angular position of the
rotating element 24. Such a sensing means may include, but not be
limited to, one or more of the following: one or more gap closing
electrodes, a second plurality of first comb fingers coupled to the
rotating element and a second plurality of second comb fingers that
interdigitate with the first comb fingers in the second plurality,
a piezoresistive strain gauge, coil, magnet, piezoelectric sensor,
or an optical sensor. It must be stated that it is presently known
that the output of a first sensor can be used to tune a second
sensor to achieve higher accuracy telemetry than two sensors not
linked.
[0040] In the exemplary embodiments of FIGS. 2-4, the biased
combdrive structure may be fabricated in a single layer of a
substrate material that may comprise, but not be limited to,
silicon, ceramic, glass, single-crystalline silicon-germanium,
poly-crystalline silicon-germanium, or a metal (such as nickel or
gold). Surface and/or bulk micro-machining techniques as known in
the art can be employed to fabricate the substrate material. The
rotating elements 24, 24', 24" may be made of silicon, glass,
nickel, nickel-alloy, gold, aluminum, chromium, or other materials
having similar properties as one skilled in the art would be
capable of applying. The rotating elements 24, 24', 24" may include
reflective or light deflective surfaces, so as to provide a beam
steering or scanning mirror. Rotatable flexures 25, 25', 25" can
be, but should not be limited to, one or more of the following:
torsional flexures, a cantilever-like flexures, serpentine
flexures, or a pin-and-staple type hinges. Furthermore, the
rotatable flexures 25, 25', 25" may have rectangular, I-shaped, or
T-shaped cross-sections. A skilled artisan can implement a
rotatable flexure in accordance with the present invention for a
given application.
[0041] Although the comb fingers 20", 22" are depicted and
described as providing both the capacitive sensing and a rotational
driving force, the invention is in no way limited to this
configuration. The comb fingers 20", 22" may provide purely a
sensing function while the biasing mechanism 26" provides the
driving force that rotates the rotating element 24". In such a
case, the biasing mechanism may include, but not be limited to, one
or more of the following: one or more gap closing electrodes, e.g.
one electrode coupled to the rotating element 24" and one coupled
to an underlying substrate (not shown), a piezoelectric mechanism
coupled to the rotating element 24, a thermal bimorph actuator
coupled to the rotating element 24, a spring loaded element coupled
to the rotating element 24, a stress-bearing material carrying a
residual stress gradient, a magnetic material with, e.g., a
constant magnetic biasing field, or a second set of interdigitating
comb fingers that are separate from comb fingers 20", 22". It must
be stated that it is presently known that the telemetry from a
first biasing mechanism can be used to configure a second biasing
mechanism to achieve higher accuracy control than two biasing
mechanisms not otherwise linked.
[0042] The present invention further provides a bi-axial rotating
device comprising two biased combdrives arranged in a gimbaled
structure and a rotating element mechanically coupled to both of
the combdrives.
[0043] FIG. 5 depicts an exemplary embodiment of a bi-axial biased
combdrive rotating device of the present invention. In biaxial
rotating actuator 500, a plurality of first comb fingers 505,
extending from a first micro-machined structure 506, may be
substantially co-planar with a plurality of second comb fingers 507
extending from a second micro-machined structure 508, thus
constituting the first combdrive. Second comb fingers 507 and first
comb fingers 505 may be fabricated from a single layer of a
substrate (not shown in FIG. 5), thereby substantially
interdigitated in a first predetermined engagement. In the
embodiment of FIG. 5, second comb fingers 507 may be electrically
isolated from first comb fingers 505.
[0044] A rotating element 509 may be attached to a first rotatable
flexure 510 disposed along a first axis 552. Rotating element 509
may also be mechanically engaged with first micro-machined
structure 506 along with first comb fingers 505. First rotatable
flexure 510 may be further attached to a frame 511, which is in
turn mechanically engaged with a second rotatable flexure 512
disposed along a second axis 551 and also with a third
micro-structure 502. A plurality of third comb fingers 501,
extending from third micro-machined structure 502, may be
substantially co-planar with a plurality of fourth comb fingers 503
extending from a fourth micro-machined structure 504, constituting
the second combdrive. The third comb fingers 501 and fourth comb
fingers 503 may be fabricated in the same layer of the substrate as
first and second comb fingers 505, 507 in the first combdrive are,
thereby substantially interdigitated in a second predetermined
engagement. Fourth comb fingers 503 may likewise be electrically
isolated from third comb fingers 501. Moreover, second comb fingers
507 can be made to be electrically isolated from fourth comb
fingers 503. As such, the first and second combdrives may be
coupled by way of frame 511. First axis 552 may be configured to be
substantially orthogonal to second axis 551 in this embodiment. It
should be noted that first and second rotatable flexures 510, 512,
frame 511, rotating element 509, together with the first and second
combdrives, may be substantially co-planar.
[0045] A biasing element 513 may be coupled to the rotating element
509, serving to cause first comb fingers 505 and/or third comb
fingers 501, along with rotating element 509, to undergo angular
displacements from their respective initial engagements about axes
552, 551 respectively. The "cross-like" shape of biasing element
513 illustrated in FIG. 5 is shown only to symbolize that the
biasing element 513 is capable of generating angular displacements
about both axes 551, 552 respectively. It must be stated that the
shape and form of an actual biasing element in a given application
may be different from that depicted in FIG. 5. First and second
rotatable flexures 510, 512 may serve to facilitate the respective
angular displacements about axes 552, 551 respectively. Biasing
element 513 may be in the form of a single biasing element, such as
a magnetic material coupled to an external magnetic field, so as to
cause two angular displacements about two axes 552, 551
respectively. Biasing element 513 may also be in the form of two
(or more) biasing elements, such as a magnetic material in a
constant magnetic field, a magnetic coil, a stress-bearing
material, a piezoelectric element, a thermal bimorph actuator one
or more gap closing electrodes, comb finger electrodes, and the
like providing two angular displacements about axes 552, 551
respectively. Furthermore, the biasing element 513 may be coupled
to either or both of the Frame 511 and the fourth micro-machined
structure 504. In all the above cases, the comb fingers 501, 503
and/or 505, 507 may be used as sensors to determine an angular
position of the rotatable element with respect to the first and
second axes 551, 552.
[0046] The biasing element 513 may alternatively include one or
more of the following: one or more gap closing electrodes, e.g. one
electrode coupled to the rotating element 509 and one coupled to an
underlying substrate (not shown), a piezoelectric mechanism coupled
to the rotating element, a thermal bimorph actuator coupled to the
rotating element, a spring loaded element coupled to the rotating
element, a stress-bearing material carrying a residual stress
gradient, or a second set of interdigitating comb fingers that are
separate from comb fingers 505, 507, 501, 503. It must be stated
that it is presently known that the telemetry from a first biasing
mechanism can be used to configure a second biasing mechanism to
achieve higher accuracy control than two biasing mechanisms not
otherwise linked.
[0047] A voltage applied by a voltage source 516 may be coupled
between first comb fingers 505 and second comb fingers 507 to cause
first comb fingers 505, along with rotating element 509, to rotate
about axis 552 towards the first initial engagement. A capacitance
sensor 517 may be coupled between the second comb fingers 507 and
first comb fingers 505 to measure the capacitance therebetween. The
capacitance measurement may be utilized to monitor and/or control
the corresponding angular position. This measured capacitance can
be further used in a feedback loop to control the rotational motion
of first comb fingers 505 along with rotating element 509.
Likewise, applying a voltage source 514 coupled between third comb
fingers 501 and fourth comb fingers 503 may apply a voltage that
causes third comb fingers 501, along with rotating element 509, to
rotate about axis 551 towards the second initial engagement. The
first and second rotatable flexures 510, 512 facilitate respective
rotational motions. A capacitance sensor 515 may be coupled between
the fourth comb fingers 503 and the third comb fingers 501. The
capacitance sensor 515 measures the capacitance between the fourth
comb fingers 503 and the third comb fingers 501. The measured
capacitance may be used as to monitor and control the angular
position of third comb fingers 501 along with rotating element 509,
e.g. by use of a feedback mechanism coupled between the capacitance
sensor 515 and the voltage source 514.
[0048] Alternatively, the comb fingers 501, 503, 505, 507 may be
used for driving the rotating element 509 and the angular position
of the rotating element 509 with respect to the first and second
axes 551, 552 may be measured by a position-sensing means separate
from the comb fingers 501, 503, 505, 507. Such a position sensing
means may comprise, for example a second set comb fingers coupled
to the rotating element 509 and frame 511. Alternatively the
sensing means may include a piezoresistive strain gauge, a
piezoelectric sensor, a pair of gap closing electrodes, or an
optical sensor. Furthermore, the comb fingers 501, 503, 505, 507
may be used to sense the angular position of the rotating element
509 (e.g., by appropriate capacitance measurement) and the biasing
element 513 may provide the force for driving the rotation.
[0049] The use of two self aligned rotating combdrives configured
as shown in the embodiment of FIG. 5 enables the rotating element
to rotate bi-axially. Moreover, if one of the combdrives is
disabled, the system may effectively act as a uni-axial rotating
actuator, in which the rotating element can selectively rotate
about one of the two axes, such as axes 552, 551 in FIG. 5. It must
be stated that it is presently known that the telemetry from a
first axis sensor can be used with a second axis sensor to achieve
higher sensing and control accuracy.
[0050] In the above embodiment, the first and second combdrives can
be fabricated from a single layer of a substrate material
comprising silicon, ceramic, glass, single-crystalline
silicon-germanium, poly-crystalline silicon-germanium, metal (such
as nickel or gold), or materials with similar properties as one
skilled in the art is capable of applying. Surface and/or bulk
micro-machining techniques as known in the art can be employed.
Rotating element 509 may be made of silicon, glass, nickel,
nickel-alloy, gold, aluminum, chromium, or materials with similar
properties as one skilled in the art is capable of applying.
Rotating element 509 may comprise a reflective surface, thereby
providing a bi-axial steering or scanning mirror. Either of first
and second rotatable flexures 510, 512 can be a torsional flexure,
a cantilever-like flexure, serpentine flexure, or a pin-and-staple
type hinge. The rotatable flexures 510, 512 may have rectangular,
I-shaped, or T-shaped cross-sections. First and second axes 552,
551 may be typically orthogonal to each other; however, they can
also be oriented in other ways as dictated by practical
applications. Furthermore, biasing element 513 may comprise one or
more biasing elements, each of magnetic, electrostatic, or
mechanical means.
[0051] According to an embodiment of the invention, the first and
second comb fingers 501, 503, and/or the third and fourth comb
fingers 505, 507 may be formed in a single etching step such that
the resulting combdrives are self-aligned. An important advantage
of the self-aligned rotating combdrive actuators of this embodiment
of the present invention is that by fabricating the comb fingers
from a single layer of a substrate, the first and second comb
fingers start from a substantially co-planar and interdigitated
engagement, thereby diminishing non-linear rotational effects and
instability that are often inherent in the prior art vertical
combdrive actuators. The performance of the rotating actuators of
the present invention is therefore more predictable. Further
advantages of the self-aligned rotating combdrive actuators of the
present invention are manifest in their simple design, compact
size, low cost, and versatile performance.
[0052] The uni-axial and bi-axial rotating actuators of the present
invention can be employed in a broad range of applications,
including biomedical devices, optical devices for tracking and
display, telecommunication devices such as fiber-optic switches,
inertial sensors, and magnetic disk drives. For example, a
uni-axial rotating actuator employing a reflective rotating element
can be used as a scanning mirror for scanning a laser beam for
bar-code reading applications. Uni-axial and/or bi-axial rotating
actuators employing reflective rotating elements can be used as
steering mirrors to switch light between optical fibers in
telecommunication applications. Moreover, arrays of such steering
mirrors can be utilized to provide fiber-optic switches with very
large port-counts. A skilled artisan will know how to make use of
uni-axial and bi-axial rotating actuators of the present invention
for a given application.
[0053] As a way of example, FIG. 6 depicts an exemplary embodiment
of a fiber-optic switch employing arrays of rotating actuators of
the present invention. This is provided for example purposes only.
Fiber-optic switch 600 comprises an array of input fibers 601
optically coupled to a first plurality of micro-lenses 602, a first
array of steering mirrors 603 in the form of arrayed rotating
actuators of the present invention, a second array of steering
mirrors 604 in the form of arrayed rotating actuators of the
present invention, and an array of output fibers 605 optically
coupled to a second plurality of micro-lenses 606. The first array
of steering mirrors 603 is positioned to intercept a plurality of
input light beams, such as input beams 607, 608, emerging from the
input fibers 601 and the microlenses 602, and steer these light
beams towards the second array of steering mirrors 604. Each mirror
in second array of steering mirrors 604 may likewise act
independently to steer the light beams on an individual basis. The
first and second arrays of steering mirrors 603, 604 may include
uni-axial or bi-axial biased rotating combdrive actuators of the
types described herein with respect to FIGS. 2-5 and 7, or a
combination of these types.
[0054] Each mirror in the first array of steering mirrors 603 may
act independently, such that the input light beams are steered on
an individual basis. The second array of steering mirrors 604 may
be positioned to receive a plurality of steered light beams, such
as beams 609, 610, from the first array of steering mirrors 603.
The second array of steering mirrors 604 in turn direct a plurality
of output light beams, such as beams 611, 612, towards output
fibers 605 in such a way that each output light beam is eventually
coupled into one output fiber. By way of example, output beam 611
is coupled into output fiber 605A via its corresponding micro-lens,
and output beam 612 is coupled into output fiber 605D via its
corresponding micro-lens.
[0055] FIG. 7 is provided to show a schematic example of how the
sensing and actuating schemes described above are deployed to track
feedback for a rotating device. Any or all of these features may be
incorporated into a fiber optic switch such as that shown in FIG.
6, and those fiber optic switches that achieve beam steering that
use two single axis mirrors to steer the beam. It should be stated
that this FIG. 7 is provided for example only to demonstrate how a
rotating device 700 may include a first set of comb fingers 712
coupled to a rotating element 720. In this example, the first set
of comb fingers 712 may be interdigitating and, optionally, in
self-alignment with a second set of comb fingers 702, which may be
coupled to a substrate 701. The rotating element may be
mechanically coupled to the substrate 701 by a rotational flexure
711.
[0056] A voltage source V may be coupled between the first and
second sets of comb fingers 712, 702 to provide a driving voltage.
A capacitance sensor C may be coupled between the first and second
sets of comb fingers 712, 702 measure an angular position of the
rotating element 720. The first and second sets of comb fingers
712, 702 are shown offset from each other in FIG. 7 for the sake of
clarity. Alternatively the first and second sets of comb fingers
712, 702 may be in a substantially co-planar interdigitating
engagement in the absence of biasing force.
[0057] Several different types of biasing elements are depicted
schematically in FIG. 7 for the sake of example. The rotating
device 700 may include any or all of them or combinations where the
biasing elements are linked. Such biasing elements may exert a
constant biasing force to pull the first and second sets of comb
fingers out of alignment. Alternatively, the biasing element may
exert a time varying force that rotates the rotatable element. By
way of example, the biasing element may include a magnetic material
731 and/or a current carrying coil 732 formed on the rotating
element 720. A current source 742 may provide electric current to
the coil 732. The magnetic material 731 and/or coil 732 may
interact with an external magnetic field B produced by an external
coil 744 and/or magnetic material 746. A current source 748 may
provide electric current to the external coil 744.
[0058] Alternatively the biasing element may include a pair of
gap-closing electrodes 752, 754 coupled respectively to the
rotating element and the substrate 701. A voltage source V' coupled
between the two gap closing electrodes 752, 754 may provide a
voltage for driving the rotating element 720. In another
embodiment, the biasing element may include third and fourth sets
of comb fingers 714, 704 coupled respectively to the rotating
element and the substrate 701. A voltage source V" may be coupled
between the third and fourth sets of comb fingers to provide a
voltage for driving the third and fourth sets of comb fingers 714,
704.
[0059] Finally, the biasing element may be an actuator 760 coupled
to the rotating element 720 and the substrate 701. The actuator
760, which is represented schematically in FIG. 7, may be a
mechanical actuator such as a spring loaded element, a stress
bearing material carrying a residual stress gradient, a
piezoelectric element or a thermal bimorph actuator.
[0060] Alternatively, the rotational flexure 711 may provide a
torsional bias and thus act as the biasing element.
[0061] It must be stated that the torsion biasing force can be time
varied with application of combdrive electrostatics to modify
device damping ratio and resonant frequency device
characteristics.
[0062] The rotating device 700 may optionally include a position
sensing means other than the first and second sets of comb fingers
712, 702. For example, the sensing means may be the gap closing
electrodes 752, 754, which may optionally be coupled to a
capacitance sensor C' to provide a means for sensing the angular
position of the rotating element 720. Furthermore, the sensing
means may include the third and fourth sets of comb fingers 714,
704. A capacitance sensor C" may be coupled between the third and
fourth sets of comb fingers 714, 704 to provide a means for sensing
the angular position of the rotational element 720. Finally the
device 700 may include a sensor element 770, shown schematically in
FIG. 7, such as a piezoresistive strain gauge or piezoelectric
sensor mechanically coupled between the rotational element and the
substrate 701. Alternatively, the device 700 may include an optical
sensor 772, e.g., that senses a change in an optical signal from an
optical source 774 to sense a change in the angular position of the
rotating element 720.
[0063] Where the first and second sets of biased comb fingers are
used to drive the rotational element 720, the position sensing
means, e.g., any or all of the capacitance sensors C, C', C" or the
sensor element 770 may be coupled via a feedback element 780 to the
voltage source V that drives the first and second sets of biased
comb fingers 712, 702. Where the first and second sets of biased
comb are used to sense the angular position of the rotating element
720 the capacitance sensor C may be coupled via the feedback
element 780 to the biasing means, e.g., either of the current
sources 742, 744, either of the voltage sources V', V", or the
actuator 760. Of course, if the first and second sets of biased
comb fingers 712, 704 are used to both drive the rotational element
720 and sense its angular position, the capacitance sensor C may be
coupled to the voltage source V via the feedback element 780 in a
feedback control loop. The feed back control element 780 may be
implemented in hardware, software, firmware, or some combination
thereof.
[0064] The various embodiments of the present invention provide a
novel class of biased combdrive actuators and position sensors that
employ self-aligned combdrives and a biasing means for generating a
constant and/or time-varying angular displacement. Such
self-aligned combdrive devices can rotate uni-axially or
bi-axially, so as to provide for one-dimensional and
two-dimensional scanning devices.
[0065] An important advantage of the self-aligned rotating
combdrive and position sensor devices according to embodiments of
the present invention is that the first and second comb fingers may
be fabricated in a single layer of a substrate and hence
substantially interdigitated according to a predetermined
engagement. This significantly simplifies the underlying
fabrication process. Another important advantage of the present
invention is that because the first and second comb fingers begin
from a substantially co-planar and interdigitated engagement,
non-linear rotational effects that are inherent in the prior art
combdrive actuators are substantially reduced. Furthermore, since
the rotating devices of the present invention may be self-aligned,
instabilities that arise from misalignment between the first and
second combs in the prior art can be avoided. The self-aligned
rotating combdrive actuators and position sensors thus produced
provide more predictable and reliable performance. Further
advantages of the self-aligned combdrive rotating actuators and
position sensors of the present invention are manifest in their
simple design, compact size, low cost, and versatile
performance.
[0066] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions, and alterations can be made herein without departing
from the principle and the scope of the invention. Specifically, it
must be stated that the methods of operating a combdriven actuator
by application of a torsion force, sensing the position of the
actuator and controlling torsion force and/or comb-finger voltage
in response thereto, may apply to any combdrive as well as the
self-aligned combdrive of the present invention. It must also be
stated that the position sensor element may operate on principals
other than capacitance and that the bias force may be applied
mechanically, non-mechanically and through manipulation of
electromagnetic energy. Accordingly, the scope of the present
invention should be determined by the following claims and their
legal equivalents.
* * * * *