Load Monitoring of Aerospace Structures Using Micro-Electro-Mechanical Systems (MEMS)

The National Research Council of Canada has developed Structural Health Monitoring (SHM) test platforms for load and damage monitoring, sensor system testing and validation. One of the SHM platform consists of two 2.25 meter long, simple cantilever aluminium beams that provide a perfect scenario for evaluating the capability of a load monitoring system to measure bending, torsion and shear loads. In addition to static and quasi-static loading procedures, these structures can be fatigue loaded using a realistic aircraft usage spectrum while SHM and load monitoring systems are assessed for their performance and accuracy. In this study, Micro-Electro-Mechanical Systems (MEMS), consisting of triads of gyroscopes, accelerometers and magnetometers, were used to compute changes in angles at discrete stations along the structure. A Least Squares based algorithm was developed for polynomial fitting of the different data obtained from the MEMS installed in several spatial locations of the structure. The angles obtained from the MEMS sensors were fitted with a second, third and/or fourth order degree polynomial surface, enabling the calculation of displacements at every point. The use of a novel Kalman filter architecture was evaluated for an accurate angle and subsequent displacement estimation. The outputs of the newly developed algorithms were then compared to the displacements obtained from the Linear Variable Displacement Transducers (LVDT) connected to the structures. The determination of the best Least Squares based polynomial fit order enabled the application of derivative operators with enough accuracy to permit the calculation of strains along the structure. The calculated strain values were subsequently compared to the measurements obtained from reference strain gauges installed at different locations on the structure. This new approach for load monitoring was able to provide accurate estimates of applied strains and loads.

Analytical and Numerical Modelling of Shape Memory Alloy Negator Springs for Long-Stroke Constant-Force Actuators

This paper explores the merits of shape memory Negator springs as powering elements for solid state actuators. A Negator spring is a spiral spring made of strip of metal wound on the flat with an inherent curvature such that, in repose, each coil wraps tightly on its inner neighbour. The unique characteristic of Negator springs is the nearly-constant force needed to unwind the strip for very large, theoretically infinite deflections. Moreover the flat shape, having a high area over volume ratio, grants improved bandwidth compared to any solution with solid wires or helical springs. The SMA material is modelled as elastic in austenitic range while an exponential continuum law is used to describe the martensitic behaviour. The mathematical model of the mechanical behaviour of SMA Negator springs is provided and their performances as active elements in constant-force, long-stroke actuators are assessed. The SMA Negator spring is also simulated in a commercial finite element software, ABAQUS, and its mechanical behaviour is estimated through FE analyses. The analytical and the numerical prediction are in good agreement, both in martensitic and in austenitic range.

Modelling of Mechanical Nonlinearity in an Electromagnetic Vibration Energy Harvester Using a Forced Duffing Equation

This work reports on the modelling and experimental validation of a bi-axial vibration energy harvesting approach that uses a permanent-magnet/ball-bearing arrangement and a wire-coil transducer. The harvester’s behaviour is modelled using a forced Duffing oscillator, and the primary first order steady state resonant solutions are found using the homotopy analysis method (or HAM). Solutions found are shown to compare well with measured bearing displacements and harvested output power, and are used to predict the wideband frequency response of this type of vibration energy harvester. A prototype harvesting arrangement produced a maximum output power of 12.9 mW from a 12 Hz, 500 milli-g (or 4.9 m/s2) rms excitation.

A Hybrid Master-Slave Genetic Algorithm-Neural Network Approach for Modeling a Piezoelectric Actuator

This work presents an approach for developing the model of a smart fin dynamics that is activated by a fully-enclosed piezoelectric (PZT) bimorph actuator, which is created by bonding two Macro Fiber Composites (MFCs). Observing the dynamics of the fin indicates that the use of a linear dynamic model does not adequately describe its behavior. An earlier work proposed incorporating a proportional damping matrix as well as Bouc-Wen hysteresis model and backlash operators to create a more accurate model. However, the number of parameters describing the expanded model is large, which limits its use. Therefore, there is a need for a different approach for developing an alternative model of the fin. In this work, a hybrid master-slave Genetic Algorithm (GA)-Neural Network (NN) model is proposed to identify the optimal set of parameters for the damping matrix constants, the Bouc-Wen hysteresis model and the backlash operators. A total of nine sinusoidal input voltage cases that resemble a grid of three different amplitudes excited at three different frequencies are used to train and validate the model. Three input cases are considered for training the NN architecture, connection weights, bias weights and learning rules using GA. The NN consists of three layers: an input layer that has two nodes for the amplitude and the frequency of the input voltage, an output layer that has seven nodes for the backlash, hysteresis, and damping operators, and a hidden layer that is free to have any number of nodes between two and nine. The GA constantly performs natural selection of chromosomes that propagate best compilation of NN parameters. Simulation results show that the proposed model can predict the damping, hysteresis and backlash of the smart fin–actuator system under various operational conditions.

Tunable Band-Pass Filters Employing Stretchable Electronic Components

This paper describes some of the recent results of an ongoing U.S. Army research program examining the electronic behavior of hyperelastic stretchable capacitor, resistor, and inductor networks for which the conductor material employed is stretchable. As with traditional rigid analog components, stretchable electronic components exhibit frequency-dependant behavior. Unlike their rigid counterparts, stretchable electronic components may also exhibit dramatic strain-dependent behavior. In this way stretchable circuit networks may be viewed as controllable spatio-temporal filters. Resistance, capacitance, and inductance all change to varying degrees depending on the specific set of spatio-temporal inputs. These variations may be harnessed to create an adaptive circuit element that is controllable. This paper describes the results of integrating stretchable components into a tunable band-pass filter. Center frequency, bandwidth, and gain can be varied in a controllable way by varying the capacitance or resistance of specific circuit elements by stretching them. Biaxially stretchable components are described that are subjected to equibiaxial strain-states as high as 100% area strain. We examine the influence that the type of compliant conductor has on tunable circuit properties and on control authority.

Clarifying the Carbon Nanotube Actuation Mechanism: An Experimental Approach

Experimental investigations of different architectures made of pure, as produced carbon nanotubes (CNTs) are the main focus of this presented article. Different types of experimental setups are used to analyze the free strain of the CNT-based architectures. According to their build-up different experimental setups like actuated tensile tests, in-plane and out-of-plane strain measurements are realized to investigate the actuation mechanism and possible dependencies. The first analyzed architecture can be characterized as a 2D paper of randomly oriented, entangled single walled CNTs, also called Bucky-paper. In contrast the second investigated architecture consists of highly oriented, vertically aligned multi walled CNTs grown on a substrate of glassy carbon. The results are evaluated according to findings of various other material quality tests in order to find a significant statement for their possible actuation mechanisms.

Design, Development, and Assembly of Space Flight Structural Health Monitoring Experiment

The paper presents the design, development, and assembly of Structural Health Monitoring (SHM) experiments intended to be launch in space on a sub-orbital rocket flight as well as a high altitude balloon flight. The experiments designed investigate the use of both piezoelectric sensing hardware in a wave propagation experiment and piezoelectric wafer active sensors (PWAS) in an electromechanical impedance experiment as active elements of spacecraft SHM systems. The list of PWAS experiments includes a bolted-joint test and an experiment to monitor PWAS condition during spaceflight. Electromechanical impedances of piezoelectric sensors will be recorded in-flight at varying input frequencies using an onboard data acquisition system. The wave propagation experiment will utilize the sensing hardware of the Metis Design MD7 Digital SHM system. The payload will employ a triggering system that will begin experiment data acquisition upon sufficient saturation of g-loading. The experiment designs must be able to withstand the harsh environment of space, intense vibrations from the rocket launch, and large shock loading upon re-entry. The paper discusses issues encountered during design, development, and assembly of the payload and aspects central to successful demonstration of the SHM system during both the sub-orbital space flight and balloon launch.

A Minimally Invasive Cage for Spinal Fusion Surgery Utilizing Superelastic Hinges

An innovative shape memory alloy actuated cage has been developed for spinal fusion surgery. Spinal fusion surgery is performed on people suffering from low back pain. The viscoelastic spinal disc between the two vertebrae can degenerate in some fashion, such as herniation, and the space needs to be restored to relive the pressure on the nerves within the lower back. There are two main parts to a spinal disc, the annulus fibrosis and the nucleolus. The annulus fibrosis is a cartilaginous structure and is of interest to preserve. Therefore a minimally invasive cage utilizing superelastic elements has been developed. Furthermore, the cage safety and efficacy has been proven and will be presented here. Within this work, the efficacy and longevity of the cage will be presented. To this end, ASTM testing for spinal implants has been conducted on an electromechanical test system capable of inducing simultaneous axial and torsional forces.

The Quantification of Uncertainty in SHM Features Derived From Transmissibility Estimation

This paper develops uncertainty models for various an estimator used in computing the frequency-domain transmissibility function for single-input-multiple-output (SIMO) linear systems. The uncertainty is assumed to come from both internal (estimator) and external (environmental, noise, etc.) sources, and it is propagated through the estimation process to arrive at closed-form probability density functions for the estimation magnitude. The models is validated on a bolted plate in the laboratory, and receiver operating characteristics (ROCs) are computed to evaluate performance in terms of meaningful metrics such as probability of detecting a structural change within the context of structural health monitoring.

Wave Propagation in Periodic Piezoelectric Elastic Waveguides

The prorogation of electro-magneto-elastic coupled shear-horizontal waves in one dimensional infinite periodic piezoelectric waveguides is considered within a full system of the Maxwell’s equations. Such setting of the problem allows to investigate the Bloch-Floquet waves in a wide range of frequencies. Two different conditions along the guide walls and three kinds of transmission conditions at the interfaces between the laminae of waveguides have been studied. Stop band structures have been identified for Bloch-Floquet waves both at acoustic and optical frequencies. The results demonstrate the significant effect of piezoelectricity on the widths of band gaps at acoustic frequencies and confirm that it does not affect the band structure at optical frequencies. The results show that under electrically shorted transmission conditions Bloch-Floquet waves exist only at acoustic frequencies. For electrically open interfaces the dynamic setting provides solutions only for photonic crystals. In this case the piezoelectricity has no effect on band gaps.