Electromagnetic interference (EMI) immune and light-weight, fiber-optic sensor based Structural Health Monitoring
(SHM) will find increasing application in aerospace structures ranging from aircraft wings to jet engine vanes. Intelligent
Fiber Optic Systems Corporation (IFOS) has been developing multi-functional fiber Bragg grating (FBG) sensor systems
including parallel processing FBG interrogators combined with advanced signal processing for SHM, structural state
sensing and load monitoring applications. This paper reports work with Auburn University on embedding and testing
FBG sensor arrays in a quarter scale model of a T38 composite wing. The wing was designed and manufactured using
fabric reinforced polymer matrix composites. FBG sensors were embedded under the top layer of the composite. Their
positions were chosen based on strain maps determined by finite element analysis. Static and dynamic testing confirmed
expected response from the FBGs. The demonstrated technology has the potential to be further developed into an
autonomous onboard system to perform load monitoring, SHM and Non-Destructive Evaluation (NDE) of composite
aerospace structures (wings and rotorcraft blades). This platform technology could also be applied to flight testing of
morphing and aero-elastic control surfaces.
MEMS gyroscopes are used in many applications including harsh environments such as high-power, high-frequency
acoustic noise. If the latter is at the natural frequency of the gyroscope, the proof mass will be overexcited giving rise to
a corrupted gyroscope output. To mitigate the effect of the
high-power, high-frequency acoustic noise, it is proposed to
use nickel microfibrous sheets as an acoustic damper. For this purpose, the characterization of vibration damping in
Nickel microfibrous sheets was examined in the present research effort. The sheets were made from nickel fibers with
cellulose as a binding agent using a wet-lay papermaking technique. Sintering was done at 1000 °C to remove all the
cellulose giving rise to a porous material. Square sheets of 20 cm were made from three diameters of nickel fibers
namely 4, 8, and 12 microns. The sheets were cut into smaller pieces to fit the requirements of a fixture specially
designed for this study. The fixture was attached to a LDS V408 shaker with a mass resting on a stack of the
microfibrous sheets to simulate transmitted vibration by base motion with the sheet stack acting as a damper. A series of
experiments was conducted using these 3 fiber diameters, different number of layers of microfibrous sheets and varying
the vibration amplitude. From the collected vibration data, the stiffness and damping ratio of the microfibrous material
was characterized.
The response of a MEMS device that is exposed to a harsh environment may range from an increased noise floor to a
completely erroneous output to temporary or even permanent device failure. One such harsh environment is high power
acoustic energy possessing high frequency components. This type of environment sometimes occurs in small aerospace
vehicles. In this type of operating environment, high frequency acoustic energy can be transferred to a MEMS
gyroscope die through the device packaging. If the acoustic noise possesses a sufficiently strong component at the
resonant frequency of the gyroscope, it will overexcite the motion of the proof mass, resulting in the deleterious effect of
corrupted angular rate measurement. Therefore if the device or system packaging can be improved to sufficiently isolate
the gyroscope die from environmental acoustic energy, the sensor may find new applications in this type of harsh
environment. This research effort explored the use of microfibrous metallic cloth for isolating the gyroscope die from
environmental acoustic excitation. Microfibrous cloth is a composite of fused, intermingled metal fibers and has a
variety of typical uses involving chemical processing applications and filtering. Specifically, this research consisted of
experimental evaluations of multiple layers of packed microfibrous cloth composed of sintered nickel material. The
packed cloth was used to provide acoustic isolation for a test MEMS gyroscope, the Analog Devices ADXRS300. The
results of this investigation revealed that the intermingling of the various fibers of the metallic cloth provided a
significant contact area between the fiber strands and voids, which enhanced the acoustic damping of the material. As a
result, the nickel cloth was discovered to be an effective acoustic isolation material for this particular MEMS gyroscope.
As a result of the relatively low intrinsic damping in printed circuit boards, vibration and acoustic energy present in the
operating environment may excite vibration modes in the PCB that lead to deleterious effects in attached vibration
sensitive components, such as MEMS gyroscopes. An investigation of the use of sandwiched layers of microfibrous
metallic cloth in contact with the PCB to increase damping was investigated. Tests were performed for both vibration excitation and acoustic excitation. The initial results indicate that mechanical damping can be increased through this approach.
Some harsh environments contain high frequency, high amplitude mechanical vibrations. Unfortunately some very
useful components, such as MEMS gyroscopes, can be very sensitive to these high frequency mechanical vibrations.
Passive micromachined silicon lowpass filter structures (spring-mass-damper) have been demonstrated in recent years.
However, the performance of these filter structures is typically limited by low damping. This is especially true if
operated in low pressure environments, which is often the optimal operating environment for the attached device that
requires vibration isolation. An active micromachined vibration isolation filter can be realized by combining a state
sensor, and electrostatic actuator and feedback electronics with the passive filter structure. Using this approach, a
prototype active micromachined vibration isolation filter is realized and used to decrease the filter Q from approximately
180 to approximately 50, when evaluated in a low pressure environment. The physical size of these active filters is
suitable for use in or as packaging for sensitive electronic and MEMS devices, such as MEMS vibratory gyroscope chips.
Missiles, rockets and certain types of industrial machinery are exposed extreme vibration environments, with high frequency/amplitude mechanical vibrations which may be detrimental to components that are sensitive to these high frequency mechanical vibrations, such as MEMS gyroscopes and resonators, oscillators and some micro optics. Exposure to high frequency mechanical vibrations can lead to a variety of problems, from reduced sensitivity and an increased noise floor to the outright mechanical failure of the device. One approach to mitigate such effects is to package the sensitive device on a micromachined vibration isolator tuned to the frequency range of concern. In this regard, passive micromachined silicon lowpass filter structures (spring-mass-damper) have been developed and demonstrated. However, low damping (especially if operated in near-vacuum environments) and a lack of tunability after fabrication has limited the effectiveness and general applicability of such systems. Through the integration of a electrostatic actuator, a relative velocity sensor and the passive filter structure, an active micromachined mechanical lowpass vibration isolation filter can be realized where the damping and resonant frequency can be tuned. This paper presents the development and validation of a key component of the micromachined active filter, a sensor for measuring the relative velocity between micromachined structures.
Some harsh environments, such as those encountered by aerospace vehicles and various types of industrial machinery, contain high frequency/amplitude mechanical vibrations. Unfortunately, some very useful components are sensitive to these high frequency mechanical vibrations. Examples include MEMS gyroscopes and resonators, oscillators and some micro optics. Exposure of these components to high frequency mechanical vibrations present in the operating environment can result in problems ranging from an increased noise floor to component failure. Passive micromachined silicon lowpass filter structures (spring-mass-damper) have been demonstrated in recent years. However, the performance of these filter structures is typically limited by low damping (especially if operated in near-vacuum environments) and a lack of tunability after fabrication. Active filter topologies, such as piezoelectric, electrostrictive-polymer-film and SMA have also been investigated in recent years. Electrostatic actuators, however, are utilized in many micromachined silicon devices to generate mechanical motion. They offer a number of advantages, including low power, fast response time, compatibility with silicon micromachining, capacitive position measurement and relative simplicity of fabrication. This paper presents an approach for realizing active micromachined mechanical lowpass vibration isolation filters by integrating an electrostatic actuator with the micromachined passive filter structure to realize an active mechanical lowpass filter. Although the electrostatic actuator can be used to adjust the filter resonant frequency, the primary application is for increasing the damping to an acceptable level. The physical size of these active filters is suitable for use in or as packaging for sensitive electronic and MEMS devices, such as MEMS vibratory gyroscope chips.
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