U.S. patent application number 10/693848 was filed with the patent office on 2005-01-06 for resonant wingbeat tuning circuit using strain-rate feedback for ornithoptic micro aerial vehicles.
This patent application is currently assigned to Administrator of the National Aeronautics and Space Administration. Invention is credited to Raney, David L..
Application Number | 20050001091 10/693848 |
Document ID | / |
Family ID | 33513736 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050001091 |
Kind Code |
A1 |
Raney, David L. |
January 6, 2005 |
RESONANT WINGBEAT TUNING CIRCUIT USING STRAIN-RATE FEEDBACK FOR
ORNITHOPTIC MICRO AERIAL VEHICLES
Abstract
A resonant wingbeat tuning circuit automatically tunes the
frequency of an actuating input to the resonant frequency of a
flexible wing structure. Through the use of feedback control, the
circuit produces the maximum flapping amplitude of a mechanical
ornithoptic system, tracking the resonant frequency of the
vibratory flapping apparatus as it varies in response to changes in
flight condition, ambient pressure, or incurred wing damage.
Inventors: |
Raney, David L.; (Yorktown,
VA) |
Correspondence
Address: |
NATIONAL AERONAUTICS AND SPACE ADMINISTR
ATION LANGLEY RESEARCH CENTER
3 LANGLEY BOULEVARD
MAIL STOP 212
HAMPTON
VA
236812199
|
Assignee: |
Administrator of the National
Aeronautics and Space Administration
Washington
DC
|
Family ID: |
33513736 |
Appl. No.: |
10/693848 |
Filed: |
October 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60422181 |
Oct 15, 2002 |
|
|
|
Current U.S.
Class: |
244/11 ; 244/22;
244/72; 416/79 |
Current CPC
Class: |
B64C 33/02 20130101;
B64C 2201/025 20130101; B64C 39/028 20130101 |
Class at
Publication: |
244/011 ;
244/022; 244/072; 416/079 |
International
Class: |
B64C 033/02 |
Goverment Interests
[0002] The invention described herein was made by an employee of
the United States Government and may be manufactured and used by or
for the Government for any governmental purpose without payment of
any royalties thereon or therefor.
Claims
1. A tuning circuit for enabling the excitation of a flexible
structure at its resonant frequency, comprising a strain rate
sensor configured for attachment to the flexible structure and
capable of producing a feedback signal in response to the
excitation of the flexible structure, a tuning algorithm capable of
converting the feedback signal to a desired periodic waveform, and
a first actuator capable of receiving the converted waveform and
configured for exciting an oscillatory vibration of the flexible
structure.
2. A tuning circuit according to claim 1, further comprising an
amplifier capable of amplifying the desired periodic waveform.
3. A tuning circuit according to claim 2, further comprising a
filter capable of removing steady state drift from the feedback
signal.
4. A tuning circuit according to claim 3, further comprising a
capacitor configured for and capable of removing high frequency
noise from the feedback signal.
5. A tuning circuit according to claim 4, wherein the desired
periodic waveform is a square wave.
6. A tuning circuit according to claim 5, further comprising a
second actuator configured so as to provide vibratory input to the
flexible structure along an axis different from the oscillatory
vibration excited by the first actuator.
7. A tuning circuit according to claim 6, wherein the vibratory
input provided by the second actuator to the flexible structure
differs in phase and amplitude from the oscillatory vibration
excited by the first actuator.
8. A tuning circuit according to claim 4, further comprising a
second actuator configured so as to provide vibratory input to the
flexible structure along an axis different from the oscillatory
vibration excited by the first actuator.
9. A tuning circuit according to claim 8, wherein the vibratory
input provided by the second actuator to the flexible structure
differs in phase and amplitude from the oscillatory vibration
excited by the first actuator.
10. A tuning circuit according to claim 9, wherein the desired
periodic waveform is a square wave.
11. A tuning circuit according to claim 9, wherein the desired
periodic waveform is a sine wave.
12. A tuning circuit according to claim 9, wherein the desired
periodic waveform is a sawtooth.
13. A tuning circuit according to claim 9, wherein the desired
periodic waveform is a ramp.
14. A method for maximizing the vibrational amplitude of a flexible
structure, comprising the steps of exciting vibration of a flexible
structure instrumented with a strain rate sensor capable of
producing a feedback signal in response to the vibration of the
flexible structure, converting the feedback signal to a desired
periodic waveform, and re-exciting the flexible structure with a
first actuator driven by the desired periodic waveform.
15. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 14, further comprising the step of
amplifying the desired periodic waveform.
16. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 15, further comprising the step of
filtering steady state drift from the feedback signal.
17. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 16, further comprising the step of
removing high frequency noise from the feedback signal.
18. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 17, wherein the desired periodic
waveform is a square wave.
19. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 18, further comprising the step of
providing vibratory input from a second actuator to the flexible
structure along an axis different from the oscillatory vibration
excited by the first actuator, simultaneous to the step of
re-exciting the flexible structure with an actuator driven by the
desired periodic waveform.
20. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 19, wherein the vibratory input
provided by the second actuator to the flexible structure differs
in phase and amplitude from the oscillatory vibration excited by
the first actuator.
21. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 17, further comprising the step of
providing vibratory input from a second actuator to the flexible
structure along an axis different from the oscillatory vibration
excited by the first actuator, simultaneous to the step of
re-exciting the flexible structure with an actuator driven by the
desired periodic waveform.
22. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 21, wherein the vibratory input
provided by the second actuator to the flexible structure differs
in phase and amplitude from the oscillatory vibration excited by
the first actuator.
23. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 22, wherein the desired periodic
waveform is a square wave.
24. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 22, wherein the desired periodic
waveform is a sine wave.
25. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 22, wherein the desired periodic
waveform is a sawtooth.
26. A method for maximizing the vibrational amplitude of a flexible
structure according to claim 22, wherein the desired periodic
waveform is a ramp.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/422,181, filed Oct. 15, 2002.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates generally to micro aerial
vehicles. It relates more particularly to maximizing the flapping
amplitude of ornithoptic micro aerial vehicles.
[0005] 2. Description of the Related Art
[0006] The fundamental technology objective that motivates the
present invention is the creation of a highly agile micro aerial
vehicle (MAV), capable of rapidly performing reconnaissance
missions in highly congested environments, such as the interior of
a building, beneath a forest canopy, or within a network of tunnels
and caves. Several biologically inspired concepts for the design of
such a vehicle have been proposed that would employ an ornithoptic
(flapping wing) system to enable the types of agile maneuvers and
flight modes exhibited by insects and humming birds. Numerous
technical challenges currently prevent the creation of such a
device, including miniaturization of lightweight actuation
mechanisms, structures, electronics and power sources. These are
the basic building blocks that would be required to create an agile
ornithoptic MAV. But even beyond the technical challenge of
creating those building blocks lies the challenge of integrating
them into an efficient, controllable flight system.
[0007] Current research efforts in flapping flight systems
generally acknowledge the significance of operating at resonance,
but the current practice is simply to estimate the resonant
frequency of the flapping system based on observation, and then to
drive the system with an open-loop periodic excitation at this
frequency. Current and past research efforts using such an approach
have attempted to provide ornithoptic MAV designs employing
flexible wing structures, but to date no efforts have been made to
develop an effective closed-loop self-tuning circuit to enable the
flapping system to be driven at its resonant frequency.
[0008] The basic disadvantage of the prior art is that it assumes
that the resonant frequency has been appropriately identified
through a-priori testing and that it is invariant with flight mode,
ambient conditions, and mechanical wear or damage to the system.
Furthermore, if the significant effect of manufacturing variation
in a one-off fabrication type of environment is to be accounted
for, the resonant frequency of each article must be individually
identified via a-priori testing.
BRIEF SUMMARY OF THE INVENTION
[0009] By contrast, the present invention provides a closed-loop
excitation system that is continuously tuned to the resonant
frequency of the flexible wing structure. The invention represents
a fundamental basis for the control of a resonating wing structure
that would compose the flight apparatus of an agile ornithoptic
MAV. The concept has been developed and demonstrated using a
benchtop laboratory apparatus.
[0010] The key feature of the problem addressed by this invention
is the operation of the flapping wing mechanism at the resonant
frequency of the fundamental structural mode of the flexible
wing-and-actuator system. This is the frequency at which the
actuation energy that is provided to the system is most efficiently
converted to mechanical displacement of the wing structure; i.e.:
the amplitude of vibration of the flexible wing structure will be
maximized when periodic excitation of the structure is provided at
the resonant frequency.
[0011] In order to accomplish this objective, a piezoelectric
strain rate sensor is attached to the flexible wing structure. An
actuator system is provided to excite vibration of the wings.
Excitation of the wings causes a periodic vibratory signal to be
emitted from the sensor. The sensor's periodic signal is fed
through a software algorithm, which converts the signal to a square
wave of the same frequency. The resulting square wave is amplified
and fed back through the actuator to drive the flexible wing
structure at the frequency and amplitude provided by the square
wave input. After a very few cycles, the closed-loop, iterative
process causes the system to tune itself to resonance, thereby
driving the flexible wing structure at its resonant frequency, at
which the system operates most efficiently.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0012] FIG. 1 is an illustration of strain-rate-sensing flexible
wing components.
[0013] FIG. 2 is an illustration of a completed strain-rate-sensing
wing assembly.
[0014] FIG. 3 is a depiction of one embodiment of a resonant tuning
circuit algorithm.
[0015] FIG. 4 is an illustration of data resulting from strain rate
sensor output and vibratory actuator input from one embodiment of
the present invention.
[0016] FIG. 5 is a flowchart depicting components and operation of
one embodiment of the present invention.
[0017] FIG. 6 is a frequency sweep showing the maximum sensor
output at the fundamental resonant frequency of one flexible wing
structure.
[0018] FIG. 7 is an illustration of time-histories of actuator
excitation frequency showing convergence to resonant frequency at
24 Hz.
[0019] FIG. 8 is a depiction of changes in resonant frequency with
decreasing ambient pressure.
DETAILED DESCRIPTION OF THE INVENTION
[0020] Purpose
[0021] The purpose of the present invention is to automatically
tune the inputs that drive an aeroelastic flapping flight system to
the resonant frequency of the flexible wing structure, thereby
maximizing the amplitude of the resulting vibration, which
constitutes the flapping motion produced by the mechanism. The
technological innovation is a novel application of existing
components, namely, the application of a thin film polyvinylidene
fluoride (pvdf) strain rate sensor to a flexible wing structure
with the express purpose of using the resulting strain rate signal
to drive the wing structure at its fundamental vibratory frequency.
Although the innovation was developed with the use of a flexible
wing structure consisting of a latex membrane bonded to a
graphite-epoxy prepreg composite frame, the innovation is not
limited to this particular wing composition.
[0022] Components
[0023] Components of the innovation are shown in FIGS. 1 through 3.
Specifically, a thin film pvdf strain rate sensor 1, in this case a
metallized pvdf film measuring 28 .mu.m.times.15 mm.times.40 mm
manufactured by Measurement Systems, Inc. (MSI), along with an
example of a flexible wing structure having a geometry inspired by
a humming bird wing, is shown in FIG. 1. The wing structure,
fabricated at NASA Langley, consists of a graphite-epoxy frame 2
and a 4-mil latex membrane 3. The latex membrane 3 is bonded to the
composite frame using a spray adhesive. Leads are attached to the
sensor 1 using tape or spray adhesive, and the sensor is then
bonded to the flexible wing structure using spray adhesive. The
position and orientation of the sensor on the structure should be
selected to provide adequate sensitivity to the lowest frequency
bending and torsion modes of the wing. An example of a completed
assembly is shown in FIG. 2.
[0024] A capacitor should be placed in parallel with the sensor to
provide some effective attenuation of signal noise from the strain
rate sensor. In one embodiment of the present invention, a 0.047
.mu.F ceramic disk capacitor is used. Also in one embodiment, a
high-pass filter (washout) is provided in the software that
operates on the output of the strain rate sensor to eliminate any
steady-state bias or drift in the sensor signal. In the current
embodiment of the present invention, the washout filter has the
transfer function shown in equation (1) with Tau=0.125.
[0025] Equation 1, Transfer function of washout filter applied to
strain-rate sensor signal: 1 Num ( s ) Den ( s ) = s s + 1
[0026] Another component of the present invention is a feedback
algorithm that converts the strain rate sensor output into a signal
that drives the vibratory excitation actuator. In its simplest
form, the sign of the strain rate sensor output signal
(appropriately conditioned with the capacitor and washout filter
components that were previously described) may be multiplied by a
constant to produce a square wave of appropriate magnitude for
input to an amplifier that drives the excitation actuator. A
diagram describing this simple feedback algorithm is shown in FIG.
3. Such a system has been implemented and has performed quite
reliably in a prototype laboratory setup through which practical
application has been achieved.
[0027] Example time histories from the strain rate sensor and the
resulting square wave vibratory excitation signal are shown in FIG.
4, which illustrates the periodic vibratory strain rate sensor
output, and the resultant square wave actuator input.
[0028] A flowchart depicting the arrangement of the aforementioned
components is shown in FIG. 5. In FIG. 5, the strain rate sensor 1
is attached to the flexible wing structure 2,3. The flexible wing
structure is excited by the actuator 4, which can be attached to
the wing structure by any number of means, in this case a nylon
tendon 5. The strain rate feedback signal 6 passes back through the
filter 10 and the tuning algorithm 7, preferably in parallel with a
capacitor 8. After conversion by the tuning algorithm to a square
wave, the feedback signal passes through amplifiers 9 and then
drives the actuator(s) 4 to once again excite the flexible wing
structure 2,3, where the iterative, closed-loop process begins
again.
[0029] Functional Operation
[0030] In operation, the piezoelectric properties of the
strain-rate sensor cause it to generate a signal in response to
vibration of the flexible wing structure. This signal is used to
generate a periodic waveform that is amplified to drive an
actuation apparatus that excites further vibration of the wing
structure. In this way, a closed-loop system is produced that
undergoes a limit cycle oscillation at the resonant frequency of
the flexible wing structure. The signal that drives the excitation
actuators is automatically tuned to the resonant frequency of the
structure by virtue of the positive feedback of strain rate, which
is 90 degrees out of phase with the displacement of the
structure.
[0031] Alternate Embodiments
[0032] A fundamental extension to the tuning circuit is possible
for cases in which the flexible wing structure is driven with more
than one vibratory excitation actuator. By varying the phase and
relative amplitudes between the actuator feedback signals, it is
possible to achieve control over the resonant wingbeat pattern
produced by the apparatus. Such an arrangement was shown in FIG. 5.
By virtue of the dual-actuator arrangement, this test is able to
generate wingbeat patterns that approximate those of a humming bird
in various flight modes. Wingtip trajectories produced by the test
stand may be made apparent by the addition of LEDs to the tips of
the flexible wing structures. Various wingbeat patterns
approximating those of humming birds may be produced by altering
the phase and amplitude of the strain-rate feedback signal to the
individual actuators in the dual actuation system. Using this
technique in lab tests with the system illustrated in FIG. 5,
wingbeat patterns have been produced which closely model those of a
hummingbird in high-speed cruise, low-speed cruise, hover, and
reverse flight modes.
[0033] There are also many potential variations of the feedback
algorithm that converts the strain rate sensor output into the
signal that drives the vibratory excitation actuator. In its
simplest form, the sign of the strain rate sensor output signal
(appropriately conditioned with the capacitor and washout filter
components that were previously described) may simply be multiplied
by a constant to produce a signal of appropriate magnitude for
input to an amplifier that drives the excitation actuator. But this
algorithm generates an actuator input having the form of a square
wave. It may be desirable to generate an input having a different
form, such as a sine wave, a sawtooth, or a ramp. In such cases,
generally simple modifications may be made to the feedback
algorithm to generate these forms.
[0034] Peripheral Equipment
[0035] The peripheral equipment described in this section was used
in the operation of the resonant ornithoptic test stand shown in
FIG. 5, and therefore represents one embodiment of the present
invention. Although these devices were used to develop and
demonstrate the innovation in a laboratory setting, the innovation
concept itself is independent of these particular devices, and may
be implemented by other means or applied using a different
apparatus.
[0036] Actuators: Two Labworks Model ET-126A electro-dynamic shaker
actuators were used to provide the vibratory excitation inputs to
the resonant ornithoptic test-bed apparatus.
[0037] Amplifiers: Two Labworks Model PA-138-1 power amplifiers
were used to amplify the signals from the resonant tuning feedback
algorithm to drive the electro-dynamic shaker actuators.
[0038] Real-time control processor & I/O boards: The resonant
tuning control algorithm was implemented using a dSpace Model DS
1005 480 MHz Power PC 750 processor with a model DS2003 16-bit
A-to-D converter board, and a Model DS2103 14-bit D-to-A converter
board. The real time process was implemented to run at a frame rate
of 1 KHz.
[0039] Additional Features
[0040] One feature of the present invention is that it uses
feedback to continuously tune the periodic excitation of a flexible
flapping wing structure to the resonant frequency of that
structure, thereby maximizing the amplitude of the flapping motion
that is produced by the energy supplied to the system. The practice
of using strain rate feedback to intentionally destabilize an
aeroelastic system (in this case resulting in a limit cycle
oscillation at the resonant frequency of the system) is novel.
[0041] The advantages that result from the use of feedback in this
instance are the same as those provided by the use of closed-loop
control in general, namely the ability to maintain desired
performance in the presence of variation in the operating
environment or changes in system characteristics. But in this
instance there is the additional benefit of eliminating the need
for extensive a-priori characterization of the test article prior
to operation; i.e., the resonant flapping frequency need not be
identified for each article prior to operation, but rather is a
natural outcome that is reported as a result of closed-loop
operation of the device.
[0042] Test data are shown that illustrate two features key to the
present invention. The first is an open-loop frequency sweep of a
sinusoidal input to a flexible wing test-bed equipped with the
strain rate sensor film laminated to the wing structure as
previously described. The RMS (root-mean-square) output of the
strain rate sensor is plotted against input frequency in FIG. 6.
The sensor output reaches a maximum at the frequency at which the
amplitude of the flexible wing vibration is greatest. This maximum
defines the fundamental resonant frequency of the flexible wing
structure, shown on the FIG. 6 embodiment at 24 Hz.
[0043] In FIG. 7, two time histories of frequency of actuator
excitation are shown. In the first time history (lower line), the
system is started at an open-loop frequency of 12 Hz. When the
closed-loop resonant tuning system is turned on, the actuator input
rapidly converges to that frequency which was identified as the
resonant frequency as in FIG. 6. Likewise, in the second time
history (upper line), the system is started at an open-loop
frequency of 40 Hz, and converges to the resonant frequency within
approximately 1 second of activating the closed-loop resonant
tuning circuit.
[0044] Another feature of the present invention illustrated by test
data is its ability to track changes in the resonant frequency of
the mechanical system in response to variation in ambient pressure.
The time histories shown in FIG. 8 show the change in excitation
frequency of the closed-loop system as it was subjected to pressure
changes of approximately 18" and 20" of mercury (over a period of
20 seconds), while operating within a vacuum-chamber bell jar
apparatus. The closed-loop system tracks the changes in resonant
frequency due to the variation in ambient pressure.
[0045] Although the feedback control algorithm in this research
activity was developed using a Matlab control analysis software
package, and implemented using dSpace real-time
hardware-in-the-loop engineering tools, these tools are not part
of, or required by, the present invention. The specific application
of strain-rate sensor components to the ornithoptic system, and
design for the associated resonant tuning feedback control
algorithm, constitute the subject of the present invention and are
independent of the software tools that would be used to implement
them.
[0046] The descriptions of the present invention represent the
invention in its current embodiment as practical application has
been achieved in the lab environment. It should be understood that
additional changes in the details, materials, process steps, and
part arrangement may be made by those skilled in the art within the
principle and scope of the invention as expressed in the appended
claims.
[0047] Although the invention has been described relative to a
specific embodiment, there are numerous variations and
modifications that will be readily apparent to those skilled in the
art in light of the above teachings. It is therefore to be
understood that, within the scope of the appended claims, the
invention may be practiced other than as specifically
described.
* * * * *