Garrett Nelson

Bridge life extension using semiactive vibration control

This paper focuses on the use of a control system to extend the life of a highway bridge. The safe life of a bridge can be more than tripled if the peak strain levels it experiences are reduced by just 33%. As of 2012, over 5000 bridges in the country have been deemed to be structurally deficient. Hence, the use of a vibration control system to extend the lives of bridges can be of tremendous societal impact. This paper utilizes a dynamic model of the Cedar Avenue tied arch steel bridge in Minnesota to investigate avenues for peak strain reduction. Simulations show that the use of passive structural modification devices such as stiffeners and dampers is inadequate to reduce the key resonant peaks in the frequency response of the bridge. Both active and semiactive vibration control strategies are then pursued. Active vibration control can effectively reduce all resonant peaks of interest, but is practically difficult to implement on a bridge due to power, size, and cost considerations. Semiactive control with a variable orifice damper in which the damping coefficient is changed in realtime using bridge vibration feedback can be practically implemented. Simulation results show that, when employed with multiple devices, the proposed semiactive control system can reduce the response at all critical resonant frequencies. Further analysis reveals that the location and number of actuators on the bridge is critical for controlling these specific resonant frequencies.

6-DOF Shaker Test Input Derivation from Field Test

Six degree of freedom (6-DOF) subsystem/component testing is becoming a desirable method, for field test data and the stress environment can be better replicated with this technology. Unfortunately, it is a rare occasion where a field test can be sufficiently instrumented such that the subsystem/component 6-DOF inputs can be directly derived. However, a recent field test of a Sandia National Laboratory system was instrumented sufficiently such that the input could be directly derived for a particular subsystem. This input is compared to methods for deriving 6-DOF test inputs from field data with limited instrumentation. There are four methods in this study used for deriving 6-DOF input with limited instrumentation. In addition to input comparisons, response measurements during the flight are compared to the predicted response of each input derivation method. All these methods with limited instrumentation suffer from the need to inverse the transmissibility function.

Vibro-acoustic model of a piezoelectric-based stethoscope for chest sound measurements

This article focuses on the influence of noise and vibration on chest sound measurements with a piezoelectric stethoscope. Two types of vibrations, namely inputs through the patient chest and disturbances from the physician, influence the acoustic measurement. The goal of this work is to develop a model to understand the propagation of these vibrational noises through the stethoscope and to the piezoelectric sensing element. Using the model, methods to reduce the influence of disturbances acting on the stethoscope from the physician handling the device are explored.A multi-DOF rigid body vibration model consisting of discrete connected components is developed for the piezoelectric stethoscope. Using a two-port lumped parameter model, the mechanical vibrations are related to the resulting electrical signal. The parameterized state space model is experimentally validated and its parameters are identified by using a thorax simulator and vibration shaker. Based on predictions from the model, the introduction of vibration isolation to reduce the influence of physician noise on the transducer is then pursued. It is shown that direct vibration isolation between the transducer and the rest of the stethoscope structure leads to a reduction in coupling with the patients chest. However, if isolation is instead introduced between the transducer housing and the rest of the stethoscope, then vibration isolation from the physician is achieved with far less reduction in patient coupling. Experimental results are presented to study the influence of the proposed design changes and confirm the predicted model behavior.

Experimental Execution of 6DOF Tests Derived from Field Tests

Recent advances in 6DOF testing has made 6DOF subsystem/component testing a preferred method because field environments are inherently multidimensional and can be better replicated with this technology. Unfortunately, it is rare that there is sufficient instrumentation in a field test to derive 6DOF inputs. One of the most challenging aspects of the test inputs to derive is the cross spectra. Unfortunately, if cross spectra are poorly defined, it makes executing the tests on a shaker difficult. In this study, tests were carried out using the inputs derived by four different inverse methods, as described in a companion paper. The tests were run with all 6DOF as well with just the three translational degrees of freedom. To evaluate the best way to handle the cross spectra, three different sets of tests were run: with no cross terms defined, with only the coherence defined, and with the coherence and phase defined. All of the different tests were compared using a variety of metrics to assess the efficacy of the specification methods. The drive requirements for the different methods are also compared to evaluate how the specifications affect the shaker performance. A number of the inverse methods show great promise for being able to derive inputs to a 6DOF shaker to replicate the flight environments.

Accelerometer-Based Acoustic Control: Enabling Auscultation on a Black Hawk Helicopter

During aeromedical patient transport, acoustic noise levels in excess of 90 dB make auscultation with a stethoscope virtually impossible. Existing approaches to address this noise in literature have relied on the use of either passive shielding of the stethoscope sensing element or active noise cancellation (ANC) with the use of additional reference microphone sensors. These solutions have achieved inadequate performance. In this paper, custom instrumented stethoscopes are used to obtain structural vibration and acoustic data from a UH-60 Black Hawk helicopter. The data show that the structural vibrations contribute significantly more to stethoscope signals than air-borne acoustic noise. Therefore, a novel adaptive acoustic control system using a reference accelerometer is pursued. By varying the levels of acoustic and vibrational noise sources in a controlled laboratory environment, the benefits and limitations of microphone or accelerometer-based active noise control are investigated and compared. Experimentally simulating the field acoustic and vibration characteristics of the UH-60 Black Hawk helicopter, the accelerometer-based ANC system is shown to provide significantly superior performance. The accelerometer-based ANC system yields an average 25-dB reduction in ambient noise levels over the entire relevant frequency range, thereby enabling successful auscultation for the first time in this military aircraft environment.

Responses of Structures to SDoF vs. MDoF Vibration Testing

The vibration excitation mechanisms for structures in service are typically multi-directional. However, during product testing conducted in a lab setting the standard practice is to replicate these environments with three orthogonal single axis vibration tests. Recent advances in technology have made it possible to perform multi-axis simulations in the laboratory. Simultaneous multi-axis excitation can result in different stress states, rates of damage accumulation, and peak accelerations and strains than those resulting from sequential single axis testing. Accordingly, a series of experiments were run on a plate structure to investigate and quantify these differences. The experiments included single and multiple axis tests with different excitation amplitudes. The single axis tests were performed on both uniaxial and multiaxial shaker systems. The control levels, response energy, modal behavior, and peak accelerations were compared for each test condition. The data illustrates the differences between the structural response for single and multi-axis tests and enables an objective comparison between testing conducted on single and multiple axis shaker systems.

The Cross Spectrum in Multiple Input Multiple Response Vibration Testing

Random vibration tests have been conducted for over 5 decades using vibration machines which excite a test item in uniaxial motion. With the advent of multi shaker test systems, excitation in multiple axes and/or at multiple locations is feasible. For random vibration testing, both the auto spectrum of the individual controls and the cross spectrum, which defines the relationship between the controls, define the test environment. This is a striking contrast to uniaxial testing where only the control auto spectrum is defined.

A Systematic Evaluation of Test Specification Derivation Methods for Multi-axis Vibration Testing

In the past decade, multi-axis vibration testing has progressed from its early research stages towards becoming a viable technology which can be used to simulate more realistic environmental conditions. The benefits of multi-axis vibration simulation over traditional uniaxial testing methods have been demonstrated by numerous authors. However, many challenges still exist to best utilize this new technology. Specifically, methods to obtain accurate and reliable multi-axis vibration specifications based on data acquired from field tests is of great interest. Traditional single axis derivation approaches may be inadequate for multi-axis vibration as they may not constrain profiles to adhere to proper cross-axis relationships—they may introduce behavior that is neither controllable nor representative of the field environment. A variety of numerical procedures have been developed and studied by previous authors. The intent of this research is to benchmark the performance of these different methods in a well-controlled lab setting to provide guidance for their usage in a general context. Through a combination of experimental and analytical work, the primary questions investigated are as follows: (1) In the absence of part-to-part variability and changes to the boundary condition, which specification derivation method performs the best? (2) Is it possible to optimize the sensor selection from field data to maximize the quality/accuracy of derived multi-axis vibration specifications? (3) Does the presence of response energy in field data which did not originate due to rigid body motion degrade the accuracy of multi-axis vibration specifications obtained via these derivation methods?

Predicting Flight Environments with a Small-Scale, Direct-Field Acoustic Test Facility

In order to predict flight environments for ground support equipment, a small-scale, direct-field acoustic test (DFAT) laboratory was recently constructed at Sandia National Laboratories. This unique laboratory setup—consisting of 24 commercial off-the-shelf monitor speakers driven by a multi-input multi-output control system—was capable of exciting the component with an acoustic environment of 103 dB overall sound pressure level (OASPL). The resulting measured data was used to predict the flight environment response of the component and to derive vibration test specifications for future mechanical shaker testing. This paper describes the small-scale DFAT laboratory setup, the applied acoustic test method, and the process used to predict the flight environments for the given ground support equipment.

Observers with Dual Spatially Separated Sensors for Enhanced Estimation: Industrial, Automotive, and Biomedical Applications

The presence of two eyes, ears, and nostrils endows mammals with many benefits that go beyond just having a second (spare) sensory organ. The spatial separation between two similar sensory organs enables enhanced sensory perception. For instance, in the case of the eyes, it is well known that the presence of two spatially separated eyes enables stereopsis (three-dimensional depth perception) [1], which is important for tasks requiring spatial discrimination, such as threading a needle or judging the space between cars on the road, and also for many athletic activities. Other lesser-known benefits of laterally separated eyes include.