U.S. patent application number 12/029591 was filed with the patent office on 2008-06-19 for mems scanner driven by two or more signal lines.
This patent application is currently assigned to Microvision Inc.. Invention is credited to Chancellor W. Brown, Dean R. Brown, Wyatt O. Davis, John R. Lewis, Thomas W. Montague, Randall B. Sprague, Jason B. Tauscher, Jun Yan.
Application Number | 20080143196 12/029591 |
Document ID | / |
Family ID | 35308510 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080143196 |
Kind Code |
A1 |
Sprague; Randall B. ; et
al. |
June 19, 2008 |
MEMS scanner driven by two or more signal lines
Abstract
A MEMS oscillator, such as a MEMS scanner, has an improved and
simplified drive scheme and structure. Drive impulses may be
transmitted to an oscillating mass via torque through the support
arms. For multi-axis oscillators drive signals for two or more axes
may be superimposed by a driver circuit and transmitted to the MEMS
oscillator. The oscillator responds in each axis according to its
resonance frequency in that axis. The oscillator may be driven
resonantly in some or all axes. Improved load distribution results
in reduced deformation. A simplified structure offers multi-axis
oscillation using a single moving body. Another structure directly
drives a plurality of moving bodies. Another structure eliminates
actuators from one or more moving bodies, those bodies being driven
by their support arms.
Inventors: |
Sprague; Randall B.;
(Carnation, WA) ; Yan; Jun; (Cincinnati, OH)
; Tauscher; Jason B.; (Sammamish, WA) ; Davis;
Wyatt O.; (Bothell, WA) ; Lewis; John R.;
(Bellevue, WA) ; Brown; Dean R.; (Lynnwood,
WA) ; Montague; Thomas W.; (Mercer Island, WA)
; Brown; Chancellor W.; (Everett, WA) |
Correspondence
Address: |
MICROVISION, INC.
6222 185TH AVENUE NE
REDMOND
WA
98052
US
|
Assignee: |
Microvision Inc.
Redmond
WA
|
Family ID: |
35308510 |
Appl. No.: |
12/029591 |
Filed: |
February 12, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10984327 |
Nov 9, 2004 |
|
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|
12029591 |
|
|
|
|
60571133 |
May 14, 2004 |
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Current U.S.
Class: |
310/36 ;
310/40R |
Current CPC
Class: |
G02B 26/0858 20130101;
G02B 26/101 20130101; G02B 26/085 20130101 |
Class at
Publication: |
310/36 ;
310/40.R |
International
Class: |
H02K 33/00 20060101
H02K033/00 |
Claims
1-39. (canceled)
40. A MEMS scanner, comprising: a frame; a scan plate coupled to
the frame, the scan plate being able to rotate around at least two
axes of rotation; one or more actuators coupled to the scan plate;
and two or more drive signal lines coupled to the one or more
actuators, the number of drive signal lines being less than twice
the number of axes of rotation; whereby rotation of the scan plate
around at least one of the axes of rotation is driven through a
pair of drive signal lines that also carries a signal for driving
the scan plate to rotate around at least a second axis of
rotation.
41. The MEMS scanner of claim 40, wherein the number of actuators
is less than the number of axes of rotation.
42. The MEMS scanner of claim 41, wherein the scan plate comprises
a single body coupled to the frame by a pair of arms that allow
rotation in a plurality of axes.
43. The MEMS scanner of claim 42, wherein the one or more actuators
consists of a single actuator.
44. The MEMS scanner of claim 41, wherein the scan plate further
comprises: a gimbal coupled to rotate around a first axis; a pair
of torsion arms coupled to the gimbal; and an inner scan plate
coupled to the pair of torsion arms and allowed to oscillate around
a second axis by the pair of torsion arms; whereby the inner scan
plate receives a drive signal through torque transmitted from the
gimbal through the pair of torsion arms.
45. The MEMS scanner of claim 41, wherein the scan plate further
comprises: a gimbal coupled to rotate around a first axis; a pair
of torsion arms coupled to the gimbal; and an inner scan plate
coupled to the pair of torsion arms and allowed to rotate around a
second axis by the pair of torsion arms; whereby the gimbal
receives a drive signal through torque transmitted from the inner
scan plate through the pair of torsion arms.
46. The MEMS scanner of claim 40, wherein: the one or more
actuators includes a plurality of actuators wired in series; the
scan plate is characterized by a resonance frequency range and
mechanical amplification factor in at least one axis of rotation;
and at least one of the plurality of actuators is responsive to a
drive signal having a frequency component corresponding to the
resonance frequency range of the scan plate.
47-58. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/984,327 filed Nov. 9, 2004, which claims
priority under 35 U.S.C. 119(e) to U.S. Provisional Application
Ser. No. 60/571,133, filed on May 14, 2004.
FIELD OF THE INVENTION
[0002] The present invention relates to MEMS devices and scanners
and scanned beam systems that use MEMS scanners, and more
particularly to MEMS oscillators having actuators with multiplexed
drive signals.
BACKGROUND OF THE INVENTION
[0003] MEMS (Micro electro mechanical system) devices may be used
in many applications including rear and front projection scanned
beam displays, scanned beam image capture devices, optical
gyroscopes, accelerometers, and other applications. In addition to
displays that project an image onto a conventional opaque or
translucent viewing screen, scanned beam displays can include
retinal scanning displays (RSDs) and heads-up displays (HUDs).
Scanned beam image capture applications include one-dimensional
(1D) or linear scanning devices such as linear bar code scanners
and two-dimensional (2D) image capture devices such as 2D bar code
or omnidirectional linear bar code imagers, 2D bar code scanners,
confocal microscopes, microprobes, medical imaging systems, and
others.
[0004] For cases where the MEMS device is used to scan a beam of
light, it is frequently called a MEMS scanner or beam deflector.
MEMS scanners may operate resonantly or non-resonantly, and may
scan in one or a plurality of axes.
[0005] MEMS devices may carry light emitters directly or
alternatively may deflect a beam through a scan angle. In beam
deflection applications, one or more scan plates have a reflective
surface that is used to scan an impinging beam over a field of
view. The reflective surface may include a plated reflective metal
such as gold or aluminum, a dielectric stack, bare silicon, or
other materials depending upon wavelength and other application
issues.
[0006] 2D scanning may be achieved by arranging a pair of 1D
scanners with their axes of rotation at substantially right angles
to one another. Alternatively, 2D scanners may use a single mirror
that is driven to rotate around both scanning axes. When a single
mirror is used to scan in two axes, a gimbal ring may be used to
allow appropriate rotation. Frequently, 2D scanners include an
inner scan plate carrying a mirror that performs a fast scan with
an outer gimbal ring performing a slow scan. Conventionally, the
fast scan sweeps back and forth horizontally across the field of
view (FOV) while the slow scan indexes down the FOV by one or two
lines. Such systems may be termed progressive scan systems. In such
systems the fast scan operates at a relatively high scan rate while
the slow scan operates at a scan rate equal to the video frame
rate. In some applications, the fast scan operates resonantly while
the slow scan provides a substantially sawtooth pattern, scanning
progressively down the frame for a (large) portion of the frame
time and then flying back to the top of the frame to start over. In
other applications, interleaved sawtooth scanning, triangular wave
scanning, sinusoidal scanning and other waveforms are used to drive
one or both axes.
[0007] Although this document frequently refers to a fast scan
direction as horizontal (rotating about a vertical scan axis) and a
slow scan direction as vertical (rotating about a horizontal scan
axis), it must be realized that such a convention is not limiting.
The teaching applies similarly to systems with fast and slow scans
in the vertical and horizontal directions, respectively, as well as
other directions.
[0008] In progressive scan systems, the beam may be scanned
unidirectionally or bidirectionally depending upon the desired
resolution, frame rate, and scanner capabilities. Bi-directionally
scanned systems may suffer from raster pinch as described by
Gerhard et al in U.S. Pat. No. 6,140,979 entitled Scanned Display
with Pinch, Timing, and Distortion Correction. One approach to
compensating for raster pinch is to add a correction mirror that
corrects the beam path to more nearly approximate an ideal raster
pattern.
[0009] More recently, work by the applicant has focused on
alternative scan patterns that scan the beam in a Lissajous scan
pattern over the FOV. Lissajous scan patterns have an advantage in
being able to operate the MEMS scanner resonantly in both axes,
thus reducing power consumption. Such systems may also have reduced
torque requirements and may thus be made smaller and have other
advantages.
[0010] Various actuation technologies for MEMS scanners have been
disclosed. Electrocapacitive drive scanners include both rear drive
pad and comb drive architectures. Magnetic drive scanners include
moving coil and moving magnet types. Other technologies include
thermal, piezoelectric, and impact motor drives. Rotation may be
constrained by torsion arms, bending flexures and other
arrangements. Electrocapacitive drive systems are sometimes
referred to as electrostatic in the literature. Bending flexures
are popularly referred to as cantilever arms.
[0011] Frequently, two or more drive schemes are combined to
provide independent drive in two or more axes. For example, the
Gerhard et al patent listed above shows a MEMS scanner with a fast
scan axis that is powered electrocapacitively and a slow scan axis
that is powered magnetically. The need to provide independent drive
actuators for each axis has heretofore limited size reductions as
well as the number of axes.
[0012] Another aspect of MEMS oscillator requirements frequently
includes the need to monitor device motion or angle. Various
schemas have been proposed and used including piezo-resistive and
optical feedback.
OVERVIEW
[0013] According to aspects of the present invention, a superior
actuator design may be applied to MEMS devices. Additionally,
according to other aspects, structures, functionality, performance
and cost may be improved.
[0014] According to one embodiment, a plurality of actuator
mechanisms may be coupled in series or in parallel. Each actuator
mechanism may be paired with an oscillator component having a
characteristic resonance frequency and amplification factor. A
single composite signal containing drive signals for each of the
actuator mechanisms may be used to actuate the actuators. The
actuator responds to one or more specific drive signal components
based on the resonance frequency and amplification factor
characteristics of its paired oscillator component.
[0015] According to some embodiments, the plurality of actuator
mechanisms may be electrically coupled through wires. In other
embodiments, the plurality of actuator mechanisms may be coupled
wirelessly through, for example, an electromagnetic or acoustic
interface. Electromagnetic interfaces may include RF, microwave,
infrared light, visible light, ultraviolet light, or other forms of
radiation.
[0016] According to other embodiments, various stationary magnet
designs may be used to improve coupling of a moving coil scan plate
to the magnetic field. The stationary magnets may be permanent
magnets or electromagnets.
[0017] According to another embodiment, a single axis magnetic
field may be used to drive scanning in two or more non-parallel
axes. The magnetic field may be oriented to be transverse to each
of the axes. The angle of the magnetic field may be optimized
according to the system requirements. Response variables include
minimization of peak current, minimization of power consumption,
maximization of torque in one or more of the axes, minimization of
size of one or more of the drive coils, minimization of response
time to a signal input, matching of oscillation amplitudes,
selection of phase relationships between frequency components of
the drive signal, and matching of resonant and non-resonant drive
schemas.
[0018] According to another embodiment, 2D scanning may be realized
using a structure having one or more flexures that allow rotation
in two or more axes to eliminate the gimbal ring.
[0019] According to another embodiment, a sensing coil may be used
to determine component position and movement. The sensing coil may
be formed using no additional mask layers by forming crossovers and
crossunders in the coil layers. The sensing coil is made continuous
by using a crossunder in the actuator coil conductor layer. The
actuator coil is made continuous by using a crossover in the
sensing coil conductor layer. A dielectric layer separates the
conductor layers.
[0020] According to another embodiment, portions of the MEMS
scanner are mechanically coupled to be driven in sympathetic
resonance. In this document, the term sympathetic resonance is to
be understood to refer to the phenomenon whereby slight movement by
one element of a MEMS system is mechanically communicated to a
second element of the system, the second element being thus driven
to relatively greater amplitude movement by virtue of its resonant
behavior. Such movement may be driven on-resonance or
off-resonance, as will be explained herein. Vibrations in one
portion of the scanner get transmitted and amplified by the
resonance and amplification factor of a second portion of the
scanner. The motion of first portion of the scanner receives
negligible input from the portion of the signal intended for the
second portion of the scanner. The second portion of the scanner
may be driven to substantial amplitude in sympathetic
oscillation.
[0021] According to another embodiment, attaching the surface to
flexures through a suspension may minimize deformation of an active
surface. In some embodiments, MEMS scan plates or portions thereof
are driven to rotate through torsion arms. The torsion arms undergo
significant strain. Spreading the torsional load over a torque
distribution member termed a suspension reduces strain in the
active surface. The active surface may comprise a mirror, one or
more emitters, or other features that benefit from maintaining a
predetermined shape.
[0022] According to another embodiment, a MEMS scanner may be
driven entirely sympathetically with little motion of the actuator
in the axis of oscillation of the actuator. One or more actuators
may be affixed to stationary surfaces. Periodic impulses of the
actuator are mechanically transmitted across the MEMS structure.
Portions of the MEMS device are thus driven to oscillate to a
desired amplitude via mechanical coupling through the device.
[0023] The terms oscillator and scan plate are used herein somewhat
interchangeably. Either term generally refers to a structure of a
MEMS device that may be driven through a periodic motion. A scan
plate may be driven with a sinusoidal periodicity that may be
referred to as oscillation. In addition, structures such as gimbal
rings that impart freedom of motion in a plurality of axes,
combined with the additional structures suspended therefrom, may be
thought of as oscillating assemblies. In other embodiments
according to the invention, structures such as gimbal rings and
scan plates may be driven in motions that are not simply
sinusoidal, but rather contain higher order sinusoidal components
that cooperate to confer motion approximating a sawtooth, square,
triangular, or other waveform.
[0024] According to another embodiment, drive signals for various
dimensions of movement by a MEMS device may be combined into a
single composite drive signal having a plurality of frequency
components. The composite drive signal is transmitted to the MEMS
device via a single pair of drive leads. The MEMS device is
designed such that each dimension of movement responds to one or
more intended frequency components according to its resonant
frequency and amplification factor, while minimizing response to
other frequency components.
[0025] According to another embodiment, two or more sinusoidal
signals may be combined in a drive circuit and transmitted to a
MEMS device as a single drive signal.
[0026] According to another embodiment, one or more resonant
signals may be combined with a non-resonant signal such as an
approximately sawtooth waveform, for example. For such an
embodiment, a non-resonant member may be directly driven while one
or more resonant drive signals are propagated through the
structure. To prevent the non-resonant signal from exciting the
resonant body, frequency components near the resonant frequency of
the resonant body are eliminated from the non-resonant signal. This
may be conveniently accomplished, for example, by including only
lower order harmonics in the non-resonant signal.
[0027] According to another embodiment, a MEMS drive signal
generator includes provision for generating and combining a
plurality of frequency components.
[0028] According to another embodiment, a moving magnet MEMS
actuator includes a magnet mounted on suspension elements. The
suspension elements spread the torque load across the active
surface of the device and thus limit distortion.
[0029] According to another embodiment, a moving magnet MEMS
actuator may include a moving system having antiparallel magnetic
fields. A single electromagnet may induce rotation about one or
more axes by simultaneously attracting one field while repelling
the other.
[0030] According to another embodiment, an improved MEMS scanner
may be used in a scanned beam imager.
[0031] According to another embodiment, an improved MEMS scanner
may be used in a scanned beam display.
[0032] According to another embodiment, an improved MEMS scanner
may be used to produce a corrected scan path. The scanner may
include three or more degrees of freedom, each of which responds to
a drive signal according to its resonant frequency and
amplification factor. Thus, it is practical to drive three or more
scan axes (some of which may be substantially parallel) without the
complication of providing three sets of drive leads and three
actuators. The drive signals are propagated through the scanner
using the off-resonance response of intermediate structures.
[0033] Other aspects of the invention will become apparent
according to the appended drawings and description, to be limited
only according to the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a top view of a magnetic drive MEMS scanner having
two scanning axes driven by moving coils wired in series.
[0035] FIG. 2A is a sectional view of the MEMS scanner of FIG. 1
showing magnets for generating a magnetic field.
[0036] FIG. 2B is a sectional view of the MEMS scanner of FIG. 1
showing another embodiment of magnets.
[0037] FIG. 2C is a sectional view of the MEMS scanner of FIG. 1
showing another embodiment of magnets.
[0038] FIG. 2D is a sectional view of the MEMS scanner of FIG. 1
showing another embodiment of magnets.
[0039] FIG. 2E is a sectional view of the MEMS scanner of FIG. 1
showing another embodiment of magnets.
[0040] FIG. 3A is a view of a magnetic drive MEMS scanner having
two scanning axes driven by a single moving coil and having a
single pair of biaxially compliant support arms.
[0041] FIG. 3B is a detailed view of the drive coil and sense coil
pass over and passunder of FIG. 3A.
[0042] FIG. 3C is a detailed view of the outer drive coil and sense
coil leads of FIG. 3A.
[0043] FIG. 4A is a view of a magnetic drive MEMS scanner having
two scanning axes driven by a single moving coil on the gimbal
ring.
[0044] FIG. 4B is a side view of a MEMS scanner illustrating
dynamic deformation of an unsuspended scan plate.
[0045] FIG. 4C is a side view of a MEMS scanner showing reduced
dynamic deformation in the scan plate achieved by using a
suspension.
[0046] FIG. 4D illustrates an embodiment wherein a suspension forms
a continuous structure around an inner scan plate.
[0047] FIG. 5 is a view of a single axis MEMS scanner driven
through sympathetic resonance from a piezoelectric actuator.
[0048] FIG. 6 shows individual and multiplexed waveforms for
driving a two-axis MEMS scanner.
[0049] FIG. 7A is a block diagram of a driving circuit for driving
a single or series actuator to induce movement in two axes.
[0050] FIG. 7B is a block diagram of a MEMS controller that
includes provision for position feedback.
[0051] FIG. 8 is a view of a single axis moving magnet scanner.
[0052] FIG. 9 is a view of a two-axis moving magnet scanner having
opposed fixed magnetic fields.
[0053] FIG. 10 is a block diagram of a scanned beam imager with a
MEMS scanner.
[0054] FIG. 11 is a diagram of a scanned beam display.
[0055] FIG. 12A is a diagram of information presented to the user
of the scanned beam display of FIG. 11 when used in a see-through
mode.
[0056] FIG. 12B is a diagram of information presented to the user
of the scanned beam display of FIG. 11 when used in an occluded
mode.
[0057] FIG. 13 is a beam position diagram showing the path followed
by the scanned beam in response to a ramped vertical scan
exemplified by individual waveforms 602 and 608 and combined
waveform 610 of FIG. 6.
[0058] FIG. 14 is a Lissajous scan pattern that may be created by a
correction mirror resonating at twice the frequency of a fast scan
mirror.
[0059] FIG. 15 is a beam position diagram showing a corrected scan
path formed by superimposing the pattern of FIG. 14 over a linear
vertical scan.
[0060] FIG. 16 is a response curve of a simple resonant body.
[0061] FIG. 17 shows response curves for two modes of a resonant
body.
[0062] FIG. 18 shows response curves for coupled modes between two
resonant bodies. The resonant frequencies are widely separated and
there is minimal perturbation of the curve shapes.
[0063] FIG. 19 is a mechanical model for a MEMS device having two
oscillatory masses.
[0064] FIG. 20 shows response curves for coupled modes between two
resonant bodies. The resonant frequencies are relatively similar
and the curves induce perturbations in one another.
[0065] FIG. 21A shows differential equations for describing the
dynamic movements of the indicated system, a simplification of the
system represented by FIG. 19.
[0066] FIG. 21B is a plot of response curves for coupled modes
between two resonant bodies of a real MEMS device. The resonant
bodies have resonant frequencies that are relatively close together
and the bodies induce perturbations in the response of one
another.
DETAILED DESCRIPTION OF THE INVENTION
[0067] FIG. 1 illustrates an embodiment according to the invention
having a series actuator that carries drive signals for a plurality
of axes. Mechanical motion for the various axes is determined by
the matching of drive signal frequency components to the mechanical
resonance of each axis.
[0068] MEMS scanner 102, here embodied as a beam scanner or beam
director, comprises various structures etched or formed in a
silicon die. Outer support structure 104 acts as a frame to anchor
the scanner to other mounting features (not shown) and includes
pads (not shown) for receiving drive signals and traces for
transmitting the drive signals to the actuator(s). Support
structure 104 may further include traces and pads for providing
drive current to sensors and transmitting position-sensing signals
to a controller.
[0069] Outer support frame 104 supports gimbal 106 on torsion arms
108a and 108b. As is conventional, the terms "gimbal" and "gimbal
ring" are used interchangeably herein. It should be understood that
a variety of specific structures may act as a gimbal including open
and closed-end rings, and other non-ring type structures that allow
controlled movement about selected axes.
[0070] Torsion arms 108a and 108b allow gimbal ring 106 to rotate
about axis 110 as indicated by arrow 111. Suspended within gimbal
106 is oscillator or scan plate 112, which may for example take the
form of a plate that has a mirror 113 formed thereon. In the
description herein, the terms "oscillator" and "scan plate" may be
used interchangeably for many purposes. Torsion arms 114a and 114b
couple scan plate 112 to gimbal ring 106, and allow the scan plate
to rotate about axis 116 as indicated by arrow 117. As is apparent,
axis 116 is fixed relative to the gimbal ring 106 and rotates along
with the gimbal ring relative to support frame 104
[0071] As an alternative or in addition to mirror 113, scan plate
112 may have one or more electromagnetic energy sources formed
thereon, the movement produced by scan plate 112 thus directly
scanning one or more beams of electromagnetic energy. Such
electromagnetic energy sources may emit any or several of a broad
range of wavelengths including gamma, x-ray, ultraviolet, visible,
infrared, microwave, or radio. For ultraviolet, visible, and
infrared emissions, the electromagnetic energy source (now termed a
light source) may include one or more laser diode light sources,
such as conventional edge-emitting or vertical cavity emitting
lasers for example, one or more LEDs, one or more fluorescent
sources, or other types of emitters.
[0072] The mass and distribution of mass within scan plate 112 and
the stiffness of torsion arms 114a and 114b determine a resonant
frequency and amplification factor for the rotation of scan plate
112 about axis 116. Similarly, the combined mass of the assembly
comprising gimbal ring 106, torsion arms 114a and 114b, and scan
plate 112 (and their mass distribution); and the stiffness of
torsion arms 108a and 108b determine a resonant frequency and
mechanical amplification factor (also called simply "amplification
factor") for the rotation of the scan plate and gimbal assembly
about axis 110. In general, the designer has wide latitude in
choosing a resonant frequency and amplification factor for each of
the two axes. For a two-axis (and by analogy, multi-axis) MEMS
scanner 102, the resonant frequency of scan plate 112 rotation
about axis 116 may be selected to be significantly higher than the
resonant frequency for the assembly's rotation about axis 110.
[0073] Actuator 118, embodied as a coil in the example of FIG. 1,
may be driven to produce rotation of gimbal ring 106 and suspended
scan plate 112 about axis 110. Similarly, actuator 120, also
embodied as a coil, may be driven to produce rotation of scan plate
112 about axis 116. Coils 118 and 120 will act as actuators when
MEMS scanner 102 is held in a magnetic field, such as that
indicated as 124, that is transverse to both axes 110 and 116. When
coil 120 receives a signal that is periodically driven at a rate
corresponding to the resonance frequency (or any frequency that
produces a suitable response) of scan plate 112, the amplitude of
the rotation of scan plate 112 will be increased proportionally to
its amplification factor. In a similar manner, when coil 118
receives a signal that is periodically driven at a rate
corresponding to the resonance frequency of the assembly comprising
scan plate 112, torsion arms 114a and 114b, and gimbal ring 106;
the assembly will oscillate about axis 110 with enhanced amplitude
owing to the mechanical amplification factor. By analogy, each
resonance frequency acts as a receiver tuned to receive a
respective signal.
[0074] In alternative embodiments it may be preferable to provide a
drive signal corresponding to one or more harmonics of a MEMS
member. Additionally, more complex waveforms may be used to achieve
a desired velocity profile as the MEMS member sweeps through its
range.
[0075] In the MEMS scanner 102 of FIG. 1, coils 118 and 120 are
wired in series. Alternatively, coils 118 and 120 could be wired in
parallel. In either event, according to one embodiment, each coil
responds to drive its respective element according to the resonance
characteristics of its associated member. Thus a single signal to
drive both axes is fed to the coils via leads 122a and 122b. When
the single signal contains frequency components equal to each of
the resonant frequencies of the system, each of the gimbal 106 and
scan plate 112 will respond preferentially to their individual
characteristic resonant frequencies. In some systems, the actuators
may respond with a characteristic 6 dB per octave roll-off or
higher. Thus, with sufficient resonant frequency separation between
the axes, each axis of rotation will substantially be driven only
at its resonant frequency.
[0076] In still other embodiments according to the invention, a
scan plate, gimbal ring, etc. may be driven off-resonance. As will
be explained in more detail with respect to FIGS. 16-20, a suitable
amount of movement in a member may be induced over a broad range of
frequencies. Thus, the term resonance, as used herein, is a
shorthand way of referring to a resonant response that occurs over
a range of frequencies, typically peaking at a single resonance
frequency.
[0077] The actuators of FIG. 1 operate to generate a variable
magnetic field. They are suspended in a magnetic field that is
transverse to each of the rotation axes 110 and 116. The example of
FIG. 1 shows the transverse B field substantially in the nominal
plane of the device. A magnetic B field, whose axis is indicated by
arrows 124, may be generated using various arrangements of
electromagnets or permanent magnets. FIGS. 2A through 2E illustrate
some of the possible arrangements of magnets, each illustrated as a
sectional view taken along section A-A' of FIG. 1. The orientation
of the B field, as indicated by the direction of arrows 124, may be
varied to achieve desirable operating characteristics, such as
setting the desired response to various drive signals, or to
suppress undesirable characteristics such as minimizing response in
an undesirable axis or mode.
[0078] In FIG. 2A, magnets 202a and 202b are oriented with opposing
magnetic poles facing one another across MEMS die 102. In FIG. 2B,
the far poles of magnets 202a and 202b are joined by a keeper 204.
The keeper 204 is optimally constructed of a high magnetic
permeability, high saturation material such as steel, for example.
A high permeability, high saturation keeper can help to concentrate
the magnetic field between the facing poles of magnets 202a and
202b by reducing the fringing field around each magnet to its
opposite face. In FIG. 2C, the magnetic field is generated by a
single magnet 202. The magnetic field is concentrated across MEMS
die 102 by opposing pole pieces 206a and 206b. Pole pieces 206a and
206b are again optimally formed of a high magnetic permeability,
high saturation material such as steel. FIG. 2D illustrates use of
a single magnet 202 that directs a fringing field across MEMS die
102. FIG. 2E illustrates a single magnet formed on the back of a
MEMS assembly. MEMS die 102 is joined to a spacer 208, forming a
cavity 210 that allows for rotation of rotating parts out of plane.
Magnet 202 is formed on the back of spacer 208. Spacer 208 may be
formed of several materials including ferromagnetic materials such
as steel and non-ferromagnetic materials such as silicon or
glass.
[0079] FIG. 3A illustrates an alternative embodiment of a MEMS
scanner 102 having two rotation axes 110 and 116. In the MEMS
scanner of FIG. 3, a single pair of biaxially compliant support
arms 302a and 302b support scan plate 112 and replace the separate
pairs of torsion arms 108a,b and 114a,b. The need for a separate
gimbal ring 106 is thus eliminated.
[0080] The resonant frequency and amplification factor of scan
plate 112 may be selected independently in each of the axes 110 and
116 by distributing its mass differently about each of the axes and
by designing the support arms 302a and 302b to have different
torsional stiffness in each axis. For the example of FIG. 3, scan
plate 112 may have a relatively high resonant frequency for
rotation about axis 116 and a somewhat lower resonant frequency for
rotation about axis 110.
[0081] Mirror 113 is shown as a dotted line because for the
particular embodiment of FIG. 3A, bare silicon is used as the
reflective surface. Thus there is no mirror edge per se, but rather
a mirror region that is defined by the extent of a beam impinging
upon the silicon surface. This defined edge may vary depending upon
particular beam alignment, shape, and size and depending upon the
instantaneous angle of mirror 112 relative to the beam. The
effective mirror surface becomes more circular for those instances
when the mirror rotates toward the beam forming a more normal
angle, and becomes more elliptical when the mirror rotates away
from the beam.
[0082] Scan plate 112 includes a single drive coil 202 positioned
peripherally around a mirror 113. The drive coil 202 is energized
by leads 122a and 122b. Although leads 122a and 122b are shown
carried on different support arms, they may alternatively be
carried on a single arm. Leads 122a and 122b may be connected to a
drive signal having a plurality of frequency components. Drive coil
202 then receives each of the frequency components. When drive coil
202 receives a frequency component equal to the resonant frequency
of axis 110, it drives scan plate 112 to rotate about axis 110 at
its resonant frequency. Similarly, when drive coil 202 receives a
frequency component equal to the resonant frequency of axis 116, it
drives scan plate 112 to oscillate about axis 116 at its resonant
frequency. Thus scan plate 112 may be driven substantially
independently to rotate about two axes at different
frequencies.
[0083] For the particular embodiment of FIG. 3a, the resonant
(horizontal) scan frequency around axis 116 is 2.6 KHz. The
resonant (vertical) scan frequency around axis 110 is 0.8 kHz. The
respective horizontal and vertical scan angles are 9.4.degree. and
0.85.degree.. There may be a small amount of crosstalk between the
vertical and horizontal scan. For example, for respective
amplification factors of 500 to 1500, the vertical drive may couple
into the horizontal drive, resulting in a horizontal pixel offset
of approximately 3 pixels top-to-bottom. Varying the pixel clock or
remapping the image may accommodate this where desired. Similarly,
the horizontal drive may couple into the vertical drive resulting
into a vertical pixel offset of approximately 0.7 pixel. This may
be accommodated by maintaining flexure symmetry, producing an
asymmetry to counteract the offset, or by image remapping. As noted
below, introducing a correction mirror may further compensate
vertical motion. The correction mirror may be designed to
accommodate horizontal-to-vertical drive coupling as well as raster
pinch.
[0084] The MEMS scanner of FIG. 3A further includes a sense coil
303 upon which drive coil 202 is partially superimposed. The sense
coil is connected to leads 304a and 304b. The sense coil is formed
over or beneath drive coil 202, as shown in FIGS. 3B and 3C, from a
first metal layer separated from the second metal layer of the
drive coil by a dielectric layer. The first metal layer
additionally forms a pass-under for the second metal layer and the
second metal layer forms a pass-over for the first metal layer. The
sense coil undergoes induced current flow caused by its movement
through the magnetic field. The current or voltage may be sensed
and the velocity or position of the scan plate and mirror
determined therefrom.
[0085] Referring to detail sections 306 and 308 for the particular
embodiment shown in FIGS. 3B and 3C, respectively, metal layer 1,
indicated by the darker traces, is comprised of deposited metal
consisting of 1000 angstrom TiW, 2400 ang Gold, 200 Ang TiW on 300
micron thick silicon, shown as the light gray region. The scan
plate and support arm silicon is selectively backside etched to 80
micron thickness by timed deep reactive ion etching to reduce
weight while forming reinforcing ribs to maintain stiffness. A
dielectric layer (not shown) is formed over metal layer 1. Metal 1
to Metal 2 connections are formed by leaving holes in the
dielectric layer at appropriate locations. Metal layer 2, indicated
by the lighter traces, is comprised of 10 micron thick gold. Metal
layer 2 is plated over the dielectric layer.
[0086] The sense coil 303, which lies under drive coil 202 for much
of its path, is formed in metal 1 (gold) and comprises 21-1/2
turns. The sense coil trace is approximately 12.5 microns wide with
10 micron spacing, yielding a coil resistance of 1.5 kilo-ohms.
Sense coil enters scan plate 112 through trace 304b, which
terminates in a passunder 310 as shown in FIG. 3B. Metal 2 jumper
312 connects passunder 310 to the inner end 314 of sense coil 303.
Sense coil spirals out in a counter-clockwise direction and exits
scan plate 112 at trace 304a as shown in FIG. 3C. The choice of a
counterclockwise-out spiral is arbitrary and could be substituted
by a clockwise-out spiral, resulting in a 180.degree. difference in
sensed phase.
[0087] The drive coil 202, which lies over sense coil 303 for much
of its path, is formed in metal 2 and comprises 9-1/2 turns. The
drive coil trace is approximately 28 microns wide with 10 micron
spacing, yielding a coil resistance of 12 Ohms. The drive coil
enters scan plate 112 through trace 122b as shown in FIG. 3A. Trace
122b connects to metal 1 passunder 316, which connects to the inner
end 318 of drive coil 202. The drive coil spirals out in a
clockwise direction and exits scan plate 112 at trace 122a as shown
in FIG. 3C. As with the sense coil, the choice of a clockwise-out
spiral is arbitrary.
[0088] In one particular embodiment, scan plate 112 is suspended in
a magnetic field oriented 30.degree. to the right of axis 116 with
a field strength of 0.21 Tesla. Under these conditions, the sense
coil produces a horizontal sense electro-motive force (EMF) of 80
mV peak and a vertical sense EMF of 2 mV peak when the scan plate
is driven at its designed angles and frequencies. Other magnetic
field angles may be used in some cases, depending upon the desired
vector components of the magnetic field according to the
application.
[0089] While the sense coil of the MEMS scanner of FIGS. 3A, 3B,
and 3C could be used to sense motion in both axes, it may be
desirable for some applications to add piezo-resistive,
photodetector, or other sensors to sense motion.
[0090] While FIG. 1 illustrates the case of a pair of series-wired
drive coils formed on both an inner scan plate and a gimbal ring
and FIG. 3A illustrates a single drive coil on an inner scan plate,
FIG. 4A illustrates the case of a single drive coil 202 on a gimbal
ring 106 with an inner scan plate 112 being induced to "ring"
through mechanical coupling across its torsion arms 114a and 114b.
Drive coil 202 rotates the assembly comprising gimbal ring 106 and
inner scan plate 112 about axis 110 directly. In this form, the
drive signal for the resultant slow scan may be either resonant or
may have another arbitrary shape. In some embodiments, the slow
scan may be of a modified sawtooth form with progressive movement
around the axis alternating with a rapid fly-back to the starting
position. When the drive signal also includes a component modulated
at the resonant frequency of the inner scan plate 112, the very
slight mechanical response of the gimbal 106 gets transmitted
across torsion arms 114a and 114b, through suspension elements 402a
and 402b, to scan plate 112. Owing to the resonant response of the
inner scan plate, the transmitted movement amplified by the system
and result in resonant rotation of the inner scan plate about fast
scan axis 116. When a mirror 113 is formed on inner scan plate 112,
the resultant rotational movements may be used to direct a beam of
light across a two-dimensional field of view.
[0091] Axes 110 and 116 may be placed at arbitrary angles to one
another. While the example of FIG. 4A (and other examples below)
are shown having "nested" scanning masses oriented at 90.degree. to
one another, other angles between 0.degree. and 90.degree. may be
used. The inventors have discovered that drive impulses at the
resonant frequency of inner scan plate 112 couple quite efficiently
at various angles.
[0092] While gimbal support arms 108a and 108b are indicated as
having a serpentine shape, straight, split, multiple and many other
shapes of torsion arms may alternatively be used. The scanner of
FIG. 4A may include piezo-resistive sensors in some or all of its
torsion arms to measure position.
[0093] The inner scan plate 112 of FIG. 4A is illustrated supported
by a suspension. The suspension transmits rotational torque between
torsion arms 114a and 114b and suspended structure 112 while
imposing a controlled dynamic deformation on the suspended
structure. In some applications, and particularly some applications
where the inner scan plate forms a base for a mirror 113, it is
useful to impose a minimal amount of dynamic deformation on inner
scan plate 112, thus keeping the mirror as flat as possible for
minimum optical distortion.
[0094] In the particular embodiments represented by FIG. 4A, the
suspension 402 includes a pair of suspension elements, or torque
distribution members, 402a and 402b, each connected to inner scan
plate 112 at three locations. As indicated in the Figure,
suspension element 402a includes an axial connection 404a and two
lateral connections, 405a and 405a', through which torque is
communicated with the inner scan plate 112. Similarly, suspension
element 402b includes an axial connection 404b and two lateral
connections, 405b and 405b', through which torque is communicated
with the inner scan plate 113. In the case of one particular
embodiment of FIG. 4A, axial connections 404a and 404b are
respectively smaller in cross section than torsion arms 114a and
114b. This limits the amount of torque concentration at the point
where axial connections 404a and 404b join inner scan plate 112
while eliminating lateral or pumping modes of motion.
[0095] While the particular arrangement illustrated by FIG. 4A
includes separate suspension elements with three discrete
connections to inner scan plate 112, a range of embodiments may be
useful according to the application. For example, axial connections
404 could be increased in size or eliminated entirely. The number
of discrete connections may be increased. Alternatively, the
connections between the suspension could be made continuous with
compliance determined by the amount of thinning between the outer
extent of the suspension and the outer extent of the inner scan
plate. In continuous suspension connections, variable compliance
may be created by forming grooves of variable width or variable
spacing between the outer extent of the suspension and outer extent
of the inner scan plate. The number of discontinuous suspension
elements may be increased above the two shown. Alternatively, the
suspension may form a continuous structure around inner scan plate
112.
[0096] FIG. 4B is a side view of dynamic deformation of a
conventional MEMS scan plate driven by torque applied to the center
of the scan plate. Scan plate 112 is shown at maximum deformation,
the amount of deformation being exaggerated for ease of
understanding. Torque 408 is applied in a counterclockwise
direction as shown, primarily by the torsional spring 114 (not
shown). At maximum deformation, torque from the torsion arm at axis
116 causes the scan plate to rotate counterclockwise, while
distributed inertial loads cause the ends of the scan plate to lag
the center of the scan plate. It may be noted that for applications
where the scan plate is being driven through the torsion arm by one
or more actuators, such as the example of FIG. 4A, torque 408 is
increased slightly relative to applications where the scan plate or
suspension itself is being driven; but for resonant scanning, the
vast majority of driving force is generated by energy stored in the
springs (torsion arms).
[0097] FIG. 4C is a side view of a MEMS scanner showing reduced
dynamic deformation in the scan plate achieved by using a
suspension. Scan plate 112 is shown at maximum deformation, being
driven counterclockwise about axis 116. Lateral connections 405
(not shown) drive the scan plate counterclockwise as illustrated by
tangential forces 410a and 410b. Additionally, axial connection 404
(not shown) drives the scan plate counterclockwise at axis 116 as
shown by torque 408. Because suspension members 402 themselves (not
shown) are dynamically deformed such that both left and right ends
are rotated clockwise relative to the scan plate (in a manner akin
to the deformation of the un-suspended scan plate 112 of FIG. 4B),
torques 412a and 412b are additionally applied to the ends of the
scan plate through respective lateral connections 405a and 405b
(not shown). The combined effects of torques 408 and 412a tend to
drive the left side of scan plate 112 downward while the combined
effects of torques 408 and 412b tend to drive the right side of
scan plate 112 upward, the effect of which helps keep the
respective intermediate portions of scan plate 112 flat. Thus the
use of a suspension partly or substantially overcomes the
deformation related to inertial lag exhibited by the significantly
deformed scan plate of FIG. 4B.
[0098] FIG. 4D illustrates an embodiment wherein the suspension 404
forms a continuous structure around inner scan plate 112. As shown
in FIG. 4D, the suspension 404 extends to substantially surround
the oscillator body 112.
[0099] As implied above, because a large majority of the driving
force in a resonant system comes from the stored energy in the
torsion arms, the use of a suspension may be used to help maintain
scan plate flatness for plates that are driven directly as well as
for plates that are driven through torsion arms.
[0100] While the examples shown heretofore have used moving-coil
magnetically driven actuators, other types of actuation
technologies; including moving-magnet, electrocapacitive,
piezoelectric, impact motor, fluid, and others; may be similarly
multiplexed to generate movement in multiple axes. Additionally,
the principles taught herein may be applied to driving single axis
scanners through mechanical coupling across torsion arms. FIG. 5 is
an example of a multi-axis scanner mechanically coupled across a
torsion arm to stacked piezoelectric actuators. Gimbal 106 is
suspended by torsion arms 108a and 108b. Torsion arm 108a
terminates at an anchor pad 502a that is, in turn, attached to a
fixed substrate 504. Torsion arm 108b terminates at a drive pad
502b that is coupled to piezoelectric stacks 506 and 506'.
Piezoelectric stacks 506 and 506' are mounted on fixed substrate
508 and are coupled to a drive signal respectively by electrical
traces 510 and 510' at their lower ends and are coupled in series
by an electrical trace on their upper ends (trace not shown).
[0101] As an alternative to the example of FIG. 5, anchor pad 502a
could be made into a drive pad by coupling it to a second pair of
piezoelectric drive stacks, thus driving the assembly through both
torsion arms 108a and 108b.
[0102] Piezoelectric stacks 506 and 506' may be such that when
trace 510 is set to a higher voltage than trace 510', the potential
causes stack 506 to extend and 506' to compress. When trace 510' is
set to a higher voltage than trace 510, the opposite potential
causes stack 506' to extend and stack 506 to compress. By
energizing traces 510 and 510' with an alternating periodic signal,
piezoelectric stacks 506 and 506' alternately extend and compress
in opposition to one another, causing a slight twisting motion of
drive pad 502b. In an alternative arrangement, piezoelectric stacks
506 and 506' may be driven independently, each through a pair of
leads.
[0103] The slight twisting motion of drive pad 502b is transmitted
as torsional stress through torsion arm 114b to gimbal 106. For a
given drive frequency, the amplitude of movement of gimbal ring 106
(and other structures suspended therefrom) will be proportional to
the voltage of the drive signal and to the mechanical amplification
factor of the rotating mass at the drive frequency (although not
necessarily linearly proportional). For drive frequency components
at or near the resonance frequency of the gimbal ring (and
suspended structures), the rotation of drive pad 502b will be
amplified, a small amount of drive pad rotation corresponding to a
relatively larger rotation of gimbal ring 106. For off resonance
drive frequency components, the amplitude of rotation of the gimbal
ring is reduced and, at certain frequency ranges, inverted.
[0104] Gimbal ring 112 may be caused to oscillate periodically by
introducing an asymmetry to the system. Such an asymmetry may
include a mass asymmetry about one or more axes of rotation (thus
introducing a slight bending mode in the respective plate or
gimbal), a rotation axis asymmetry (e.g. axis 116 not being at a
perfectly right angle to axis 110), or a drive asymmetry.
[0105] A drive asymmetry may be introduced by superimposing one or
more in-phase frequency components to piezo stacks 506 and 506'.
Such a drive asymmetry results in a slight upward-downward periodic
motion of the drive pad 506. This slight upward-downward periodic
motion (which may be of the same or opposite sign compared to the
upward-downward motion of the drive pad 506) is communicated
through gimbal ring 106 as a slight rotation about axis 116. The
rotation of gimbal ring 106 about axis 116 is then amplified as a
function of the mechanical amplification factor of gimbal ring 112
(with carried components including torsion arms 514a,b and inner
scan plate 512), resulting in an intended rotation of gimbal ring
112 about axis 116.
[0106] As may be seen, the mechanical coupling may be extended to
additional scan plates. Scan plate 112 acts as a gimbal ring for
inner scan plate 512, which is suspended from scan plate 112 by
torsion arms 514a and 514b. In the example of FIG. 5, inner scan
plate 512 is formed to rotate about axis 110. When the drive signal
energizing traces 510 and 510' further comprises a frequency
component equal to the resonant frequency of scan plate 512, the
slight twisting of drive pad 506 arising therefrom is transmitted
through torsion arm 108b, gimbal ring 106, torsion arms 114a and
114b, scan plate 112, and torsion arms 514a and 514b to scan plate
512 so as to drive scan plate 512 to rotate about axis 110 at a
transmitted frequency where the mechanical amplification factor of
inner scan plate 512 results in rotation.
[0107] In some embodiments, scan plate 512 may include a mirror 113
formed thereon. One application for such a device is to create a
raster pinch correction mirror in a 2D beam scanning system. The
phase relationships between and amplitudes of the various frequency
components of the drive signal may be controlled. In a raster pinch
correcting system, inner scan plate 512 may be designed to have a
resonant frequency twice that of scan plate 112. Its phase and
amplitude may be selected to create a vertical scan moving in
opposition to and substantially equal to the vertical scan motion
of gimbal 106 while scan plate 112 is traversing across its scan
range, and in the same direction as gimbal 106 while scan plate 112
is at the end of its travel. Thus, the mirror 113 may deflect a
beam comprising substantially parallel paths in both left-to-right
and right-to-left scanning directions, substantially eliminating
raster pinch.
[0108] As may be seen, additional levels of scan plates may be
nested and driven without incurring the additional expense, yield
loss, and electrical loss associated with the formation of
additional nested actuators. One consideration is that successive
scan plates are driven via at least minimal resonant response of
intermediate plates, expressed in the primary axis of motion of the
finally driven plate.
[0109] Alternatively, scan plates 112 and 512 could be eliminated
and the system used to drive a single-axis scan plate 106. As may
be appreciated, various combinations of the embodiments of FIGS. 1,
3A-3C, 4A-4D, and 5 could be constructed within the scope of the
invention.
[0110] As described above, the drive signals for actuating many of
the embodiments according to the invention involve combinations of
waveforms. By selecting mechanical amplification factors, resonant
frequencies, and drive signals acting on various portions of the
MEMS apparatus, a broad range of design freedom may be enjoyed.
FIG. 6 shows an example of waveforms for driving a plurality of
oscillating elements. Waveform 602 is a high frequency signal for
driving a first oscillator component at a corresponding high
resonant frequency. Waveform 604 is a lower frequency signal for
driving a second oscillator component at a corresponding lower
resonant frequency. Waveform 606 combines the signals of waveforms
602 and 604. A signal corresponding to waveform 606 may be
transmitted to the actuator or actuators of MEMS scanners
constructed according to the invention. Each frequency component
602 and 604 will thus actuate a particular oscillating element in
accordance with its resonant frequency and amplification
factor.
[0111] Waveform 608 is a non-resonant signal for driving a scanner
component in a non-resonant manner. Waveform 610 combines the
signals of waveforms 602 and 608. A signal corresponding to
waveform 610 may be transmitted to MEMS scanners constructed
according to the invention. When the amplification factor of a
scanner component having a resonant frequency corresponding to
signal 602 is sufficiently high, the component will reject signals
of different frequencies. Conversely, scanner components that have
low amplification factor will tend to receive a broad range of
signals. Signal component 608 of waveform 610 may, for example,
result in a progressive scan and flyback of a low amplification
factor gimbal ring having a relatively large number of actuator
coil windings while signal component 602 of waveform 610 drives its
nested high amplification factor inner scan plate.
[0112] FIG. 7A is a block diagram of a signal generator that
combines individual signal components into a drive signal for
driving a MEMS scanner having components preferentially responsive
to each of the signal components. X-axis waveform generator 702a
and y-axis waveform generator 702b each generate a respective
signal for driving a MEMS scanner to move about the x and y-axes.
Such movement may be rotational, translational, or other modes as
appropriate for the application. Waveforms such as those shown in
FIG. 6 may be used for example. If waveform generator 702a
generates waveform 602 and waveform generator 702b generates
waveform 604, they may be combined in multiplexer (MUX) 706 to
produce combined waveform 606. Alternatively waveform generator
702b may generate a non-sinusoidal signal such as waveform 608. In
that case MUX 706 may combine the waveforms generated in waveform
generator 702a and 702b to produce a signal such as waveform 610.
The combined waveform is transmitted to a MEMS actuator 708, which
may be of many forms including series coils 118 and 120 of FIG. 1,
combined drive coil 202 FIGS. 3A, 3B, 3C and 4, piezoelectric
stacks 506 and 506' of FIG. 5, or other types of actuators. As an
alternative to discrete waveform generators 702a and 702b, an
integrated device may produce drive waveforms such that individual
components (for example waveforms 602 and 604) are not exposed or
literally present.
[0113] FIG. 7B illustrates a MEMS drive block diagram having an
integrated x-y waveform generator 702 and a motion/position
detector 710 connected to a controller 712. Controller 712 issues
waveform parameters to x-y waveform generator 702. X-Y waveform
generator 702 creates drive waveforms and transmits them to a MEMS
scanner 102 via drive traces 122. The physical position and/or
motion is sensed and transmitted from the MEMS scanner 102 to a
motion/position receiver 710 via sense traces 304. Motion position
receiver 710 informs controller 712 of the motion and/or position
of the MEMS scanner. The controller may then maintain or modify the
waveform parameters sent to the x-y waveform generator depending
upon whether or not the MEMS scanner is performing the desired
motion. Controller 712 may instruct the light source drive 714 to
vary the sequential emission pattern of light source 716 to perform
image remapping to take into account the actual position of the
mirror on MEMS scanner 102. Light source 716 emits a beam 718,
which is deflected by the mirror on MEMS scan plate 112 onto a
field-of-view corresponding to the sensed position of the scan
plate.
[0114] While magnetic drive designs shown heretofore have been
moving coil types, it is also possible to apply the principles
described herein to moving magnet MEMS designs. FIG. 8 shows a
single axis moving magnet MEMS scanner having suspension elements
for reducing mirror distortion. Scan plate 112 has mirror 113 on
its surface. Scan plate 112 is suspended from torsion arms 114a and
114b, which, in turn, terminate at anchor pads 502a and 502b,
respectively. Anchor pads 502a and 502b are attached respectively
to substrates 504a (not shown) and 504b. Drive magnet 802, which
may be a permanent magnet or an electromagnet, is attached to the
scanner assembly at attachment points 804a and 804b as indicated in
the Figure. North and south poles of drive magnet 802 are aligned
respectively to the right and left sides of magnet as indicated in
the Figure.
[0115] Actuator coil 806 is be placed below scanner 102 on its
centerline as indicated. Alternatively, actuator coil 806 may be
placed at a different location such as in-plane or above scanner
102. Actuator coil 806 is energized by leads 122a and 122b to
create a variable magnetic B fields 808. When electromagnet 806 is
energized to produce a magnetic be field 808 oriented north up, the
south pole of drive magnet 802 is attracted thereto and the north
pole of drive magnet 802 repelled therefrom, causing scanner 102 to
rotate counterclockwise about axis 116. Conversely, when
electromagnet 806 drives magnetic field 808 south up, the south
pole of drive magnet 802 is repelled and the north pole attracted,
causing scanner 102 to rotate clockwise about axis 116.
[0116] Drive magnet 802 is affixed to the scanner assembly at
attachment points 804a and 804b as indicated. A moving magnet
actuator and torsion spring energy storage can cooperate to
generate a significant amount of torque, which could distort mirror
113 if drive magnet 802 and torsion arms 114a and 114b were affixed
directly thereto. Instead, attachment of drive magnet 802 and
torsion arms 114a and 114b to respective suspension elements 402a
and 402b confines distortion to the suspension elements, keeping
mirror surface 113 flat as illustrated in FIGS. 4B and 4C.
Suspension elements 402a and 402b may be attached to oscillating
mass 112 in various arrangements. In some embodiments it may be
optimal to attach suspension elements 402 to oscillating mass 112
at three points as indicated. Finite element analysis can aid the
designer in selecting optimum attachment points.
[0117] As an alternative to suspending oscillating mass 112 from a
pair of torsion arms, various cantilevered or other designs may be
substituted.
[0118] FIG. 9 shows a moving magnet embodiment of a two-axis MEMS
scanner 102. As with the MEMS scanners of FIG. 8 and FIG. 5, anchor
pads 502a and 502b attach the assembly to mounting points. Torsion
arms 108a and 108b extending therefrom support gimbal ring 106.
Gimbal ring 106, in turn, serves as an anchor for torsion arms 114a
and 114b, which connect to suspension elements 402a and 402b,
respectively. Suspension elements 402a and 402b connect to
oscillating mass 112 which has a mirror surface 113 disposed
thereon.
[0119] Two drive magnets 802 and 802' may be affixed to gimbal ring
106 as shown to provide actuation force. Drive magnet 802 is
attached to gimbal ring 106 at attachment points 804a and 804b
where the dotted lines represent locations on the bottom surface of
gimbal ring 106. Similarly, drive magnet 802' is attached to gimbal
ring 106 at attachment points 804a' and 804b'. In some embodiments,
it may be desirable to arrange the north and south poles of drive
magnets 802 and 802' to be anti-parallel to one another as
indicated. Such an arrangement allows a single actuator to create
opposing forces in the drive magnets for rotating the assembly
around axis 110.
[0120] The moving magnet oscillating assembly of FIG. 9 is driven
by an electromagnetic actuator 806, which, for example, may be
disposed below the plane of the MEMS scanner as indicated. As may
be appreciated by those having skill in the art, other positions
for electromagnet 806 are also possible. Drive magnet 806 is driven
to produce a variable magnetic B field 808 via leads 304a and 304b.
When electromagnet 806 is driven to produce a variable magnetic
field 808 with north up, drive magnet 802 is attracted thereto
while drive magnet 802' is repulsed therefrom. This produces
torsional force in the counterclockwise direction about axis 110
and drives the scan plate to rotate counterclockwise. Conversely,
when variable magnetic field 808 is driven south up, Drive magnet
802 is repulsed and drive magnet 802' is attracted, producing
rotation in the clockwise direction about axis 110.
[0121] Various waveforms may be used to drive rotation about axis
110. For example, a sinusoidal waveform such as waveform 604 of
FIG. 6 may, with proper frequency selection, produce resonant
rotation about axis 110. In other embodiments, a ramped waveform
approximating a sawtooth waveform, such as waveform 608 of FIG. 6
may be used to produce non-sinusoidal, non-resonant motion.
[0122] As indicated above, one scan plate 112 is suspended from the
gimbal ring 106 on torsional arms 114a and 114b. As with other
examples, the mass distribution of scan plate 112 and the stiffness
of torsion arms 114a and 114b determine a resonant frequency and
mechanical amplification factor for scan plate 112. In a manner
similar to other examples, electromagnet 806 may be driven with a
composite waveform comprising a plurality of frequency
components.
[0123] Of note in the example of FIG. 9, is the asymmetry of the
placement of the drive magnets 802 and 802'. Drive magnet 802 is
mounted at its far end at outer position 804b and its near end
(respectively, as pictured) at inner position 804a. The asymmetry
of drive magnet 802' is reversed, with its near end being mounted
in an outer position 804a' and its far end mounted at an inner
position 804b'. This drive asymmetry results in a slight rotation
of gimbal ring 106 about axis 116.
[0124] When variable B field 808 is driven with a frequency
component having a frequency equal or near to the resonant
frequency of scan plate 112, scan plate 112 will be sympathetically
driven to oscillate about axis 116. In a manner similar to the MEMS
devices of FIGS. 4B and 5, slight twisting of gimbal ring 106 about
axis 116 is amplified by the resonant system of scan plate 112,
creating torsional force through torsion arms 114a and 114b. Though
the overall twisting of gimbal ring 106 maybe slight, the amount of
torque transmitted to scan plate 112, arising both from driving
torque and energy stored in torsion arms 114a and 114b, may be
sizable. To reduce the tendency of this torsional force to distort
scan plate 112, and hence mirror surface 113, suspension elements
402a and 402b are interposed between scan plate 112 and torsion
arms 114a 114b, respectively.
[0125] Thus a composite drive signal such as waveforms 606 or 610
may be fed through leads 304a and 304b to produce movement in two
or more axes according to the resonant properties of the
oscillating components.
[0126] As described above, one or more other asymmetries including
rotation axis asymmetry and/or mass distribution asymmetry, could
alternatively or additionally be used to drive rotation of scan
plate 112.
[0127] As an alternative to the sympathetic drive system of FIG. 9,
each of the various oscillating components could be driven directly
by placement of drive magnets thereon.
[0128] As with other examples presented herein, a DC bias current
in coil 806 in either direction will tend to shift the amplitude of
rotation of the driven member (i.e. gimbal ring 106 in the example
of FIG. 9) to one side or the other depending upon the direction of
the DC bias. This effect is especially pronounced with magnetic
drive, owing to the relatively high drive torque of such systems.
Such a DC bias current may be used to vary or adjust the nominal
direction in which mirror 103 is aimed. This may be useful, for
example, to change the exit pupil position of a scanned beam
display or to pan the field-of-view for a scanned beam image
capture device.
[0129] While FIG. 9 shows a two axis, two body scanner (i.e. a
single scan plate and a single gimbal ring), it may be appreciated
that the moving magnet design may be readily applied to
single-axis, greater than two axes, single-body, or greater than
two bodies applications as well.
[0130] As mentioned earlier, the principles described herein may be
applied to the various MEMS Drive systems, magnetic or
non-magnetic. For example, electrocapacitive drive pads or
electrocapacitive interdigitated arms could be substituted for the
drive magnets and electromagnets used in the previous examples.
Alternatively, thermal, fluidic, or other actuation mechanisms
could be substituted and remain within the intended scope.
[0131] Various embodiments of MEMS scanners described herein may be
employed to scan beams of light in a scanned beam image capture
apparatus, scanned beam displays, laser printer imaging systems, or
other applications. A simplified block diagram of a scanned beam
image capture apparatus 1002 is shown in FIG. 10. An illuminator
716 creates a first beam of light 718. A scanner 102, having a
mirror formed thereon, deflects the first beam of light across a
field-of-view (FOV) to produce a second scanned beam of light 1010.
Taken together, the illuminator 716 and scanner 102 comprise a
variable illuminator 1009. Instantaneous positions of scanned beam
of light 1010 may be designated as 1010a, 1010b, etc. The scanned
beam of light 1010 sequentially illuminates spots 1012 in the FOV.
The scanned beam 1010 at positions 1010a and 1010b illuminates
spots 1012a and 1012b in the FOV, respectively. While the beam 1010
illuminates the spots, a portion of the illuminating light beam
1014 is reflected according to the properties of the object or
material at the spots to produce scattering or reflecting the light
energy. A portion of the scattered light energy travels to one or
more detectors 1016 that receive the light and produce electrical
signals corresponding to the amount of light energy received. The
electrical signals drive a controller 1018 that builds up a digital
representation and transmits it for further processing, decoding,
archiving, printing, display, or other treatment or use via
interface 1020.
[0132] The light source 716 may include multiple emitters such as,
for instance, light emitting diodes (LEDs), lasers, thermal
sources, arc sources, fluorescent sources, gas discharge sources,
or other types of illuminators. In one embodiment, illuminator 716
comprises a red laser diode having a wavelength of approximately
635 to 670 nanometers (nm). In another embodiment, illuminator 716
comprises three lasers; a red diode laser, a green diode-pumped
solid state (DPSS) laser, and a blue DPSS laser at approximately
635 nm, 532 nm, and 473 nm, respectively. While laser diodes may be
directly modulated, DPSS lasers generally require external
modulation such as an acousto-optic modulator (AOM) for instance.
In the case where an external modulator is used, it is typically
considered part of light source 716. Light source 716 may include,
in the case of multiple emitters, beam combining optics to combine
some or all of the emitters into a single beam. Light source 716
may also include beam-shaping optics such as one or more
collimating lenses and/or apertures. Additionally, while the
wavelengths described in the previous embodiments have been in the
optically visible range, other wavelengths are within the scope of
the invention.
[0133] As mentioned earlier, embodiments according to the invention
may be applied not only to scanning mirrors, but to other types of
MEMS devices as well. For example, a scan plate may have light
emitters or a fiber optic termination thereon in place of a mirror.
Such devices may be used to directly move the light beam in one or
more axes in place of or auxiliary to a scanning mirror 103.
[0134] Light beam 718, while illustrated as a single beam, may
comprise a plurality of beams converging on a single scanner 102 or
onto separate scanners 102.
[0135] A 2D MEMS scanner 102 scans one or more light beams at high
speed in a pattern that covers an entire 2D FOV or a selected
region of a 2D FOV within a frame period. A typical frame rate may
be 60 Hz, for example. Often, it is advantageous to run one or both
scan axes resonantly. In one embodiment, one axis is run resonantly
at about 19 KHz while the other axis is run non-resonantly in a
sawtooth pattern to create a progressive scan pattern. A
progressively scanned bi-directional approach with a single beam,
scanning horizontally at scan frequency of approximately 19 KHz and
scanning vertically in sawtooth pattern at 60 Hz can approximate
SVGA resolution. In one such system, the horizontal scan motion is
driven electrocapacitiveally and the vertical scan motion is driven
magnetically. Alternatively, both the horizontal and vertical scan
may be driven magnetically or capacitively. Electrocapacitive
driving may include electrocapacitive plates, comb drives or
similar approaches. In various embodiments, both axes may be driven
sinusoidally or resonantly. Other driving methodologies as
described above or as may be clear to one having skill in the art
may alternatively be used.
[0136] Several types of detectors may be appropriate, depending
upon the application or configuration. For example, in one
embodiment, the detector may include a PIN photodiode connected to
an amplifier and digitizer. In this configuration, beam position
information is retrieved from the scanner or, alternatively, from
optical mechanisms, and image resolution is determined by the size
and shape of scanning spot 1012. In the case of multi-color
imaging, the detector 1016 may comprise more sophisticated
splitting and filtering to separate the scattered light into its
component parts prior to detection. As alternatives to PIN
photodiodes, avalanche photodiodes (APDs) or photomultiplier tubes
(PMTs) may be preferred for certain applications, particularly low
light applications.
[0137] In various approaches, photodetectors such as PIN
photodiodes, APDs, and PMTs may be arranged to stare at the entire
FOV, stare at a portion of the FOV, collect light
retro-collectively, or collect light confocally, depending upon the
application. In some embodiments, the photodetector 1016 collects
light through filters to eliminate much of the ambient light.
[0138] The scanned beam image capture device may be embodied as
monochrome, as full-color, and even as a hyper-spectral. In some
embodiments, it may also be desirable to add color channels between
the conventional RGB channels used for many color cameras. Herein,
the term grayscale and related discussion shall be understood to
refer to each of these embodiments as well as other methods or
applications within the scope of the invention. Pixel gray levels
may comprise a single value in the case of a monochrome system, or
may comprise an RGB triad or greater in the case of color or
hyper-spectral systems. Control may be applied individually to the
output power of particular channels (for instance red, green, and
blue channels) or may be applied universally to all channels, for
instance as luminance modulation.
[0139] Other applications for the MEMS scanners and actuation
mechanisms described herein include scanned beam displays such as
that described in U.S. Pat. No. 5,467,104 of Furness et al.,
entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by
reference. As shown diagrammatically in FIG. 11, in one embodiment
of a scanned beam display 1102, a scanning source 1009 outputs a
scanned beam of light that is coupled to a viewer's eye 1104 by a
beam combiner 1106. In some scanned displays, the scanning source
1009 includes a MEMS scanner with a mirror, as described elsewhere
in this document, that scans a modulated light beam onto a viewer's
retina. In other embodiments, the scanning source may include one
or more light emitters that are rotated through an angular
sweep.
[0140] The scanned light enters the eye 1104 through the viewer's
pupil 1108 and is imaged onto the retina 1109 by the cornea. In
response to the scanned light the viewer perceives an image. In
another embodiment, the scanned source 1009 scans the modulated
light beam onto a screen that the viewer observes. One example of
such a scanner suitable for either type of display is described in
U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATURE
OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is
incorporated herein by reference.
[0141] Sometimes such displays are used for partial or augmented
view applications. In such applications, a portion of the display
is positioned in the user's field of view and presents an image
that occupies a region 43 of the user's field of view 1204, as
shown in FIG. 12A. The user can thus see both a displayed virtual
image 1206 and background information 1208. If the background light
is occluded, the viewer perceives only the virtual image 1206, as
shown in FIG. 12B. Applications for see-through and occluded
displays include head-mounted displays and camera electronic
viewfinders, for example.
[0142] As mentioned above in conjunction with the description of
FIG. 5, one use of various embodiments of the MEMS scanners
described herein is as raster pinch correcting scanners. FIG. 13
illustrates a scan path 1302 followed by a scanned beam emitted by
various devices including scanned beam image capture devices as
exemplified in FIG. 10 and scanned beam displays as exemplified in
FIG. 11. Though FIG. 13 shows only eleven lines of image, one
skilled in the art will recognize that the number of lines in an
actual display or imager will typically be much larger than eleven.
As can be seen by comparing the actual scan pattern 1302 to a
desired raster scan pattern 1304, the actual scanned beam 1302 is
"pinched" at the outer edges of the field of view. That is, in
successive forward and reverse sweeps of the beam, the pixels near
the edge of the scan pattern are unevenly spaced. This uneven
spacing can cause the pixels to overlap or can leave a gap between
adjacent rows of pixels. Moreover, because the image information is
typically provided as an array of data, where each location in the
array corresponds to a respective position in the ideal raster
pattern 1304, the displaced pixel locations can cause image
distortion.
[0143] To improve the quality of the image displayed or captured,
it is desirable to correct the "pinched" scan path 1302 to more
nearly approximate the ideal raster pattern 1304. One way to do
this is to provide a separate beam path correction mirror as
described in some of the patents cited and incorporated by
reference near the end of this detailed description section.
However, a separate correction mirror can have undesirable cost,
size, and complexity impacts. For many applications, it may be
desirable to instead use a scanner assembly 102 that includes a
correction feature.
[0144] Referring back to FIG. 5, one may note that inner scan plate
512 with mirrored surface 113 may be driven to scan around axis 110
in common with outer gimbal 106. If inner scan plate 512 is driven
at a frequency twice that of horizontal scan plate 112 at a proper
phase relationship, it may be appreciated (absent any motion by
gimbal 106) that a scanned beam reflected from mirror 113 could
trace a "bow tie" or Lissajous pattern as shown by FIG. 14.
Combining the Lissajous pattern of FIG. 14 with vertical,
substantially constant rotational velocity scanning by gimbal 106
produces the corrected scan pattern indicated by FIG. 15. FIG. 15
shows correction of the "pinched" scan path with a sinusoidal
motion of the correction mirror where the horizontal field of view
encompasses 90 percent of the overall horizontal scan angle. One
skilled in the art will recognize that the error in position of the
beam can be reduced further if the field of view is a smaller
percentage of the overall horizontal scan angle.
[0145] Correction scanners may be sympathetically or directly
driven. Of course, it is not necessary to use the stacked
piezoelectric drive mechanism of FIG. 5. Other drive mechanisms
including moving coil magnetic, moving magnet magnetic,
electrocapacitive, differential thermal expansion, etc. may be
used.
[0146] Further reductions in the scan error can be realized by
adding one or more additional correction mirrors to scanner 102.
Such scan plates may be added in a nested fashion as indicated in
FIG. 5 or, with the use of a "double bounce" or other beam path
that returns the beam to the plane of the substrate of scanner 102,
may be positioned laterally in either axis to first scan mirror
113. Another approach to reducing the error is to add one or more
higher order harmonics to the scanner drive signal so that the
scanning pattern of the inner scan plate 512, here acting as a
resonant correction scanner, shifts from a sinusoidal scan closer
to a sawtooth wave that approximates more precisely the movement of
gimbal 106.
[0147] Other uses for various embodiments of the MEMS scanner
described herein will be apparent to one having skill in the
art.
[0148] Various embodiments of the MEMS scanner described herein may
be integrated into systems and/or combined with embodiments
described in U.S. Pat. No. 6,140,979, entitled SCANNED DISPLAY WITH
PINCH, TIMING, AND DISTORTION CORRECTION; 6,245,590, entitled
FREQUENCY TUNABLE RESONANT SCANNER AND METHOD OF MAKING; 6,285,489,
entitled FREQUENCY TUNABLE RESONANT SCANNER WITH AUXILIARY ARMS;
6,331,909, entitled FREQUENCY TUNABLE RESONANT SCANNER; 6,362,912,
entitled SCANNED IMAGING APPARATUS WITH SWITCHED FEEDS; 6,384,406,
entitled ACTIVE TUNING OF A TORSIONAL RESONANT STRUCTURE;
6,433,907, entitled SCANNED DISPLAY WITH PLURALITY OF SCANNING
ASSEMBLIES; 6,512,622, entitled ACTIVE TUNING OF A TORSIONAL
RESONANT STRUCTURE; 6,515,278, entitled FREQUENCY TUNABLE RESONANT
SCANNER AND METHOD OF MAKING; 6,515,781, entitled SCANNED IMAGING
APPARATUS WITH SWITCHED FEEDS; and/or 6,525,310, entitled FREQUENCY
TUNABLE RESONANT SCANNER; for example; all commonly assigned
herewith and all hereby incorporated by reference.
[0149] Alternatively, illuminator 104, scanner 102, and/or detector
116 may comprise an integrated beam scanning assembly as is
described in U.S. Pat. No. 5,714,750, BAR CODE SCANNING AND READING
APPARATUS AND DIFFRACTIVE LIGHT COLLECTION DEVICE SUITABLE FOR USE
THEREIN which is incorporated herein by reference.
[0150] As indicated above in conjunction with the discussion of
FIGS. 1, 3A, 4A, 5, 6, 7A, 7B, and 9, rotation or other movement
about various axes is determined according to the physical response
of the moving body. In those discussions, there was assumed to be
little interaction between the various axes of rotation or other
movement. In real-world scenarios however the interaction between
modes and moving bodies may be significant.
[0151] FIG. 16 illustrates a simplified response curve 1602 for
rotation of a scan plate having a resonant frequency f.sub.R. In
this figure the vertical axis is denoted displacement amplitude,
and indicates a physical response. While a rotational response is
plotted by the curve 1602 of FIG. 16, other response modes may be
similarly represented. For example, a "pumping mode" would involve
up-down translational movement. The coupling between portions of
scanners may thus be used to drive modes other than rotation.
[0152] As is characteristic of many response curves, the
displacement amplitude of the oscillating body increases
monotonically with frequency until it nears its resonant frequency,
at which point the response climbs rapidly to a finite level
corresponding to mechanical amplification factor of the body at its
resonant frequency. As frequency is increased further the curve
drops, sometimes precipitously, as the body is no longer able to
respond at the rate of the drive signal. It is frequently
convenient to design systems to drive the MEMS device at or near
its resonant frequency to conserve energy and reduce power
consumption.
[0153] Where the response curve of FIG. 16 indicates response of a
single oscillating body around a single axis, the response curves
of FIG. 17 show the response for a body around multiple axes. Here
again, frequency is plotted along the horizontal axis increasing to
the right and the displacement response amplitude is plotted on the
vertical axis with larger displacement higher on the axis. The
primary oscillatory response curve 1602 has a resonant frequency at
f.sub.R1 and is similar in shape to the response curve 1602 shown
in FIG. 16. Also shown in FIG. 17 is a secondary oscillatory
response curve 1702. Response curve 1702 indicates the response of
the body along some other movement axis and represents a second
excitation mode. For the present discussion it is assumed the
primary oscillatory response 1602 measures rotation around an axis
defined by a pair of torsion arms. The secondary oscillatory
response curve 1702 represents a tilting response for rotation
about an axis in plane and orthogonal to the primary axis of
rotation. The primary oscillatory response may be envisioned as
rotation about a pair of torsion arms. The secondary oscillatory
response can be envisioned as tilting with the pair of torsion arms
alternately bending up and bending down out of the plane of the
device.
[0154] It should be noted that the secondary response curve 1702
displays a second resonant frequency f.sub.R2 that may be the same
as or different from the primary mode resonance frequency f.sub.R1.
For the present discussion it is assumed that f.sub.R2 is somewhat
higher in frequency that f.sub.R1 and that the maximum displacement
response amplitude in the secondary axis (the mechanical
amplification factor) is lower than the amount of response of the
primary axis.
[0155] Referring now to FIG. 18, there is a relationship between
various modes of motion of the various bodies of a MEMS device.
Curve 1702 represents the secondary resonant response of a first
moving body. In this case the first moving body is a gimbal ring.
As indicated in the earlier figures, the secondary response
(corresponding to tilting about an axis 116, orthogonal to the
gimbal ring support arm axis 110) increases monotonically as one
increases frequency to f.sub.1R2 (i.e. the resonance frequency of
the first body in mode 2) corresponding to a single resonance
frequency of the system in the second movement response, and then
decreases monotonically as the drive frequency is raised further.
For one real system the resonance frequency of curve 1702 is equal
to approximately 1500 hertz.
[0156] Superimposed over curve 1702, which represents tilting of
the outer plate, is response curve 1802, representing rotation of
an inner plate. For examples of physical embodiments one may refer
to inner plate 112 and gimbal ring 106 of FIG. 1 or 4. Thus, curve
1702 represents tilting of the gimbal ring 106 about axis 116, and
response curve 1802 represents rotation of inner plate 112 also
about axis 116. In the example of FIG. 18, response curve 1802
increases monotonically with frequency until it reaches a resonance
frequency f.sub.2R1 (i.e. the resonant frequency of the second
plate in rotation about axis 116). As frequency is increased
further the response of the inner plate 112 decreases. For one real
system the resonance frequency f.sub.2R1 of curve 1802 is about 20
kHz.
[0157] It is notable that the resonant frequencies of the two
response curves shown in FIG. 18 are relatively widely separated,
at 1500 hertz and 20 kHz respectively for curves 1702 and 1802.
This may be seen from inspection of point 1804 on curve 1802,
corresponding to the resonance frequency of curve 1702, and
inspection of point 1806 on curve 1702, corresponding to the
resonance frequency of curve 1802. In each case the shape of the
curve is relatively unaffected by the resonance of the other plate,
each curve instead resembling the "pure" responses of FIGS. 16 and
17.
[0158] Thus, when the outer plate is driven at f.sub.2R1 of 20 KHz,
it tilts very slightly according to the displacement amplitude of
curve 1702 at that frequency, but induces a sizable displacement in
the inner plate, which rotates significantly according to the
displacement amplitude of curve 1802 at that frequency. This energy
transfer is indicated by arrow 1808. This situation corresponds to
the cases described earlier, which referred to very slight
displacement of one member inducing sizable displacement in another
member.
[0159] When resonance frequencies of various components of the MEMS
system are closer together, other interactions may occur, with each
resonance mode affecting the response of the other modes. Before
looking at the shapes of curves for such interactions, we refer to
FIG. 19, which shows one way to model the mechanical system.
[0160] In the model of FIG. 19, a base 504, corresponding to a
mounting point of the system, is elastically coupled to a first
mass M1 106 (corresponding for example to a gimbal ring) through a
spring with stiffness k.sub.1 and with energy dissipation
(non-elastic response) represented by coefficient c.sub.1. These
are modeled respectively as a spring 108 and a dash pot 1902. A
force F.sub.1 (not to be confused with lowercase f, used to
designate frequency), corresponding to an actuator, may act to
displace mass M1 from its rest position. Spring 108 will act to
restore mass M1 to its rest position, as modified by the damping
action of dash pot 1902.
[0161] In this example, spring 108 is numbered to correspond to
torsion arms 108a and 108b of FIGS. 1 and 4. Correspondingly mass
M1 106 is numbered to correspond to gimbal ring 106 of the same
figures. While the primary displacement of mass M1 106 in response
to force F.sub.1 is, at many frequencies, rotation about axis 110,
one may recognize that mass M1 106 may also be displaced in an
orthogonal axis of rotation, i.e. in a tilting mode about axis 116.
For the present analysis, the main mode of interest is the
secondary mode of tilting about axis 116.
[0162] Connected to mass M1 106 is a second mass M2 112. Mass M2
may be modeled as being connected to mass M1 via a spring 114
having a spring constant k.sub.12 and a dash pot 1904 having a
damping coefficient C.sub.3. Taken in the context of FIGS. 1 and 4,
mass M2 112 may be seen to correspond to inner scan plate 112 and
spring 114 may be seen to correspond to torsion arms 114a and 114b.
Mass M2 112 further interacts with base 504 through dash pot 1906
having a damping coefficient C.sub.2. Damping coefficients C.sub.1,
C.sub.2 and C.sub.3 correspond to energy dissipation mechanisms of
the system. In particular, C.sub.1 corresponds to two primary
effects: energy dissipation the mounting between the MEMS die 102
and base structure 504, and gas damping acting on the gimbal ring.
C.sub.2 corresponds primarily to gas damping acting on the inner
scan plate 112. C.sub.3, which corresponds primarily to energy
dissipation in the torsion arms due to the relative motion of M1
and M2, is usually negligible and is therefore ignored when
modeling the system.
[0163] As with displacement of mass M1 106, mass M2 112 may be
displaced by a force F2. Upon such displacement, spring 114 tends
to restore mass M2 to its resting position with respect to mass M1
as a function of its spring constant k.sub.12 as modified by the
damping coefficient C.sub.2 of dash pot 1906. It can be appreciated
that a force F2 acting on mass M2 112 may distend not only spring
114 but also spring 108, depending on the ratio of their respective
spring constants k.sub.12 and k.sub.1. Under static conditions,
force F1 acts only on spring 108 but not on spring 114. Rather the
combined inertia of masses M1 106 and M2 112 tend to oppose force
F1 under dynamic conditions. Also under dynamic conditions, it can
be appreciated that interactions between the various components of
the system may produce complex relationships between the movement
of mass M1 and mass M2.
[0164] Several simplifying assumptions may be made to ease
modeling. These include linear behavior of springs and damping
(including no hysteresis), massless springs, linear behavior of the
drive forces, and constants that remain constant with various
environmental changes including temperature. For some systems,
especially systems that undergo large displacements, such
simplifying assumptions may not be appropriate, as is known to
those having skill in the art. Using the listed simplifying
assumptions, dynamic movements of the system represented by FIG. 19
are governed by the differential equations 2102 and 2104 given in
FIG. 21A.
[0165] According to the differential equations shown above, when
the system corresponding to FIG. 19 is driven periodically by force
F1, motion of both masses M1 106 and M2 112 will result. For
example when F2 is set to zero (i.e. F2=0) and F1 is driven in a
sine wave (F2=F0*sin(2.pi.f*t)), where F0 is the load amplitude, f
is frequency, and t is time, the motion of the two masses may
respond as shown by curves 1702 and 1802 of FIG. 20.
[0166] As with FIG. 18, curve 1702 represents the tilting mode of
the gimbal ring while curve 1802 represents rotation of the inner
scan plate. With respect to the model of FIG. 19, curve 1702 also
corresponds to displacement of mass M1 106 and curve 1802
corresponds to displacement of mass M2 112 on common displacement
axes. In accordance with the structures of FIGS. 1 and 4, the
common displacement axis 116 is exhibited as tilting of the gimbal
ring in the case of curve 1702 and rotation of the inner scan plate
in the case of curve 1802.
[0167] Curve 1702 rises monotonically until it reaches the resonant
frequency f.sub.1R2 of the tilting mode of the gimbal ring. It then
decreases with further increases in frequency. It does not,
however, decrease monotonically as with the system of FIG. 18.
Rather, its shape is affected by the response of curve 1802. In
other words, the dynamic response of mass M1 106 is affected by the
dynamic response of mass M2 112 as dictated by the spring
constants, damping coefficients, and masses of the model of FIG.
19. It should be emphasized that the values used in the model
correspond to real characteristics of the MEMS device.
[0168] Curve 1802 exhibits corresponding interaction with curve
1702. In particular, rather than the curve monotonically increasing
until mass M2 reaches its resonant frequency f.sub.2R1, curve 1802
shows a peak in response 1804 corresponding to the resonant
frequency f.sub.1R2 of the gimbal ring in tilting mode, as
represented by curve 1702. In a physical system, the corresponding
peaks at f.sub.1R2 represent in-phase movement of the scan plate
112 and the gimbal ring 106 about axis 116.
[0169] As indicated by arrow 2002 the tilting of the gimbal ring
transfers energy to the inner scan plate in the form of rotation.
Thus, at f.sub.1R2, an actuator physically coupled to the gimbal
ring may be used to drive rotation of the scan plate.
[0170] As can be seen from FIG. 20, in coupled systems having
relatively similar resonant frequencies, the scan plate may be
driven at a resonant frequency of the gimbal ring. For this
example, the resonant frequency is that of tilting about axis 116.
In principal, however, other modes and other resonant frequencies
may be similarly used.
[0171] At a higher frequency f.sub.2R1 near the resonant frequency
of inner scan plate 112 in rotation, a corresponding phenomenon may
be observed as may be seen from inspection of curve 1702 of FIG.
20. At a frequency below but approaching the resonance frequency of
the inner scan plate 112 in rotation, a local minimum is observed
in curve 1702. Continuing higher in frequency, the tilting response
of the gimbal ring then increases to reach a local maximum near the
resonant frequency of rotation of the inner scan plate 112,
f.sub.2R1. At this point, as indicated by arrow 1808, energy is
transferred to the inner plate to create a maximum in its
displacement amplitude. In the case of present example, this
displacement amplitude is exhibited as rotation about axis 116 of
the inner scan plate 112 relative to the gimbal ring 106.
[0172] Whereas the coupling between curves 1702 and 1804 was in
phase at resonance frequency f.sub.1R2, the coupling at f.sub.2R1
is out of phase. That is, when scan plate 112 is rotated clockwise
by an amount corresponding to the displacement amplitude of curve
1802, the gimbal ring is tilted counterclockwise by an amount
corresponding to the displacement amplitude of curve 1702. Thus,
while the curves 1702 and 1802 resulted in additive displacement of
the mirror surface at frequency f.sub.1R2, the direction of
displacement of curve 1702 at frequency f.sub.2R1 is in opposition
to the direction of displacement of curve 1802.
[0173] FIG. 21A shows differential equations 2102 and 2104 for
describing the dynamic movements of indicated system, a
simplification of the system represented by FIG. 19. FIG. 21B shows
the theoretical frequency response curves on a dB scale of a real
system according to the differential equations describing the
system of FIG. 19 shown above. The amplitudes are absolute values
calculated relative to an external fixed reference frame. At
frequency f.sub.2R1, the scan plate scan amplitude is 7, while the
gimbal ring oscillation amplitude is 2. The angles of the two
bodies have approximately 180 relative phase at f.sub.2R1.
[0174] While driving the system at a point corresponding to the
local minimum of curve 1702 would result in a higher effective
ratio of (mirror amplitude response to gimbal ring amplitude
response) mechanical amplification factors, he response of the
inner scan plate was not sufficient to generate an acceptable scan
angle.
[0175] Another candidate frequency for driving the system
corresponds to f.sub.1R2, where the responses of the gimbal ring
and the inner scan plate are approximately in-phase relative to one
another. At this frequency (about 1700 hertz) the system exhibited
its highest response of 25.degree. and 20.degree. respectively for
curves 1802 and 1702. However, the horizontal scan rate (i.e. about
1700 hertz) was not sufficient to meet other system
requirements.
[0176] While the examples discussed herein have related to scanning
phenomena, and particularly rotation of an inner scan plate
suspended by an outer gimbal ring exhibiting tilting, other types
of motion may be similarly coupled. Various modes of oscillation as
are known to the art may be useful in a variety of applications.
For example vertical translation may be used by a variety of
systems, including optical focusing applications, range finding
applications, or other embodiments were such motion is desired.
Similarly, in-plane rotation, plate (vibrational) modes, and
in-plane translation may be mechanically coupled to drive a scan
plate through resonance. Additionally, similar phenomena may be
noted with respect to coupled actuators such as the example of FIG.
1, multiply coupled bodies such as the example of FIG. 5, and with
respect to parallel primary oscillatory axes such as the example of
FIG. 5 (where induced bodies 106 and 512 both rotate about axis
110).
[0177] The preceding overview of the invention, brief description
of the drawings, and detailed description describe exemplary
embodiments of the present invention in a manner intended to foster
ease of understanding by the reader. Other structures, methods, and
equivalents may be within the scope of the invention. As such, the
scope of the invention described herein shall be limited only by
the claims.
* * * * *