Benjamin T. Dickinson

Bioinspired carbon nanotube fuzzy fiber hair sensor for air-flow detection.

Artificial hair sensors consisting of a piezoresistive carbon-nanotube-coated glass fiber embedded in a microcapillary are assembled and characterized. Individual sensors resemble a hair plug that may be integrated in a wide range of host materials. The sensors demonstrate an air-flow detection threshold of less than 1 m/s with a piezoresistive sensitivity of 1.3% per m/s air-flow change.

Aerodynamic control of micro air vehicle wings using electroactive membranes

Dielectric elastomer materials are ideal candidates for developing high-agility micro air vehicles due to their electric field–induced deformation. Consequently, the aero-structural response and control authority of the dielectric elastomer material, VHB 4910, are characterized on an elliptical membrane wing. An experimental membrane wing platform was constructed by stretching VHB 4910 over a rigid elliptical wing-frame. The low Reynolds number (chord Reynolds number < 106) and aerodynamics of the elliptical wing were characterized when different electrostatic fields were applied to the membrane. We observe an overall increase in lift with maximum gains of 20% at an applied voltage of 4.5 kV and demonstrate the ability to delay stall. The time-averaged aerodynamic surface pressure is also investigated by comparing sting balance data and membrane deformation measured using visual image correlation. The experimental results are compared to a nonlinear finite element membrane model to further understand the effects of aerodynamic load and electric fields on membrane displacements. Model predictions of surface pressure provide insight into how the electrostrictive constitutive relations influence the fluid–structure interactions of the membrane. This is validated by comparing lift predictions from the model with time-averaged wind tunnel lift measurements near stall.

Variable deflection response of sensitive CNT-on-fiber artificial hair sensors from CNT synthesis in high aspect ratio microcavities

Crickets, locusts, bats, and many other animals detect changes in their environment with distributed arrays of flow-sensitive hairs. Here we discuss the fabrication and characterization of a relatively new class of pore-based, artificial hair sensors that take advantage of the mechanical properties of structural microfibers and the electromechanical properties of self-aligned carbon nanotube arrays to rapidly transduce changes in low speed air flow. The radially aligned nanotubes are able to be synthesized along the length of the fibers inside the high aspect ratio cavity between the fiber surface and the wall of a microcapillary pore. The growth self-positions the fibers within the capillary and forms a conductive path between detection electrodes. As the hair is deflected, nanotubes are compressed to produce a typical resistance change of 1-5% per m/s of air speed which we believe are the highest sensitivities reported for air velocities less than 10 m/s. The quasi-static response of the sensors to point loads is compared to that from the distributed loads of air flow. A plane wave tube is used to measure their dynamic response when perturbed at acoustic frequencies. Correlation of the nanotube height profile inside the capillary to a diffusion transport model suggests that the nanotube arrays can be controllably tapered along the fiber. Like their biological counterparts, many applications can be envisioned for artificial hair sensors by tailoring their individual response and incorporating them into arrays for detecting spatio-temporal flow patterns over rigid surfaces such as aircraft.

Robust and adaptive control of a rocket boosted missile

This paper is an exposition on the design of robust observer-based adaptive autopilots for aerospace systems. Using a rocket boosted missile as an example, we will discuss systematic design principles to meet closed-loop autopilot robustness and performance criteria. The controller consists of decoupled lateral and longitudinal linear gain scheduled optimal baseline designs with servomechanism acceleration tracking and observer-based adaptive augmentation. We will demonstrate improved robustness with observer-based adaption to various uncertainties including significant discrepancies in aerodynamic coefficients, center of gravity shifts, and actuator failures. These results support the practicality of observer-based adaptive output feedback laws for the control design of uncertain aerodynamic systems.

Artificial Hair Sensors: Electro-Mechanical Characterization

Performance demands of future unmanned air vehicles will require rapid autonomous responses to changes in environment. Towards this goal, we expect that the next generation flight control systems will include advanced sensors beyond the contemporary array. One promising scenario correlates measurements of flow footprints over aircraft surfaces with aerodynamic data to aid navigation and feedback control algorithms. As a sensor for this concept, we construct artificial hair sensors (AHSs) based on glass microfibers enveloped in an annular, radially-aligned piezoresistive carbon nanotube (CNT) forest to measure air flow in boundary layers. This study includes an analysis of the sensitivity based on laboratory scale electromechanical testing.The sensors in this work utilize nine micron diameter S2 glass fibers as the sensing mechanism for coupling to boundary layer air flows. The annular CNT forest resides in a fused silica microcapillary with electrodes at the entrance. The sensor electrical transduction mechanism relies on the resistance change of the CNT forest due to changes in both the bulk and contact resistance as a function of mechanical loading on the fiber. For the electromechanical analysis, the sensors are controllably loaded to measure both the force and moment acting at the base of the hair and the resulting deflection of the CNT forest inside of the microcapillary is measured to estimate the stress on the forest and the pressure between the forest and the electrode. The electrical responses of the sensors are compared to the mechanical state of the CNT forest. This work represents the development of a characterization tool to better understand and control the response of CNT based AHSs.Copyright

Aerodynamic Control of Micro Air Vehicle Wings Using Electroactive Membranes

Electrically controlled adaptive materials are ideal candidates for developing high agility micro-air-vehicles (MAV) due to their intrinsic multi-functionality. The dielectric elastomer VHB 4910 is one such material, where deformation occurs with an applied electric field. Here, we study the aerostructural response and control authority of a VHB 4910 membrane wing. An experimental membrane-wing platform was constructed by stretching VHB 4910 over a rigid elliptical wing-frame. The low Reynolds number (chord Reynolds number < 106 ) aerodynamics of the elliptical wing were characterized with different electrostatic fields applied. We observe an overall increase in lift with maximum gains of 20% at 4.5 kV, and demonstrate the ability to delay stall. Aerodynamic effects are investigated with membrane displacement and strain data obtained through visual image correlation (VIC). The VIC data is compared to a finite deforming finite element shell model to help understand structural shape changes under electrostatic fields and low Reynolds number aerodynamic flows. The model is formulated to directly input three dimensional membrane displacements to quantify aerodynamic loads on the electroactive membrane surface.© 2011 ASME

Dynamic Problem of Coupled Thermoelasticity for a Thin Composite Structure

A dynamic problem of coupled thermoelasticity for a composite body is analyzed. The body is modeled as a composite beam in response to elastic deformation. It consists of two parts of different densities chosen such that the center of mass is the junction point. The beam is clamped at the center of mass and free at its ends. The body acts as a two-dimensional structure in response to the heat flow. The problem reduces to three partial integro-differential equations, and the two beam equations are coupled by the heat equation. A method based on integral transformations and expansion of the transforms of the beam deflection in terms of shape and vibration modes is proposed. Ultimately, it leads to an infinite system of linear algebraic equations with respect to the Laplace transforms of the vibration modes solved by the method of successive approximations. In addition, expressions for the generalized free energy and the dissipation function are derived. By employing the variational principle it is shown that this formulation deduces the governing differential equations of thermoelastic vibration of the thin structure that follow from dynamic thermoelasticity and beam theory.

Bat-inspired integrally actuated membrane wings with leading-edge sensing

This paper presents a numerical investigation on the closed-loop performance of a two-dimensional actuated membrane wing with fixed supports. The proposed concept mimics aerodynamic sensing and actuation mechanisms found in bat wings to achieve robust outdoor flight: firstly, variable membrane tension, which is obtained in bats through skeleton articulation, is introduced through a dielectric-elastomer construction; secondly, leading-edge airflow sensing is achieved with bioinspired hair-like sensors. Numerical results from a coupled aero-electromechanical model show that this configuration can allow for the tracking of prescribed lift coefficient signals in the presence of disturbances from atmospheric gusts. In particular, disturbance measurements through the hair sensor (a feedforward control strategy) are seen to provide substantial advantage with respect to a reactive (feedback) control strategy determining a reduction of the oscillations of the lift coefficient.

Automated gain schedule synthesis for missile systems

Gain schedule synthesis typically involves significant manual tuning to meet various time and frequency domain design specifications. This work explores how optimization may achieve automatic tuning for missile systems. Here, vertical acceleration command tracking is considered with closed loop control of the short period dynamics. Tracking is achieved with robust servomechanism design and linear quadratic regulation. Tuning the controller is posed as an optimization problem to meet command tracking requirements subject to multiple design constraints. Sequential quadratic programming is applied to the optimization problem and provides automated tuning for individual and batch runs of controllers. A successful strategy for initial guess selection in batch runs is proposed and its importance for efficient automation is shown. These results support the viability of optimization methods for automated gain schedule synthesis in missile systems.

Dynamic Response of Biomimetic Hair Receptors in Both Steady and Unsteady Flow Environment

The high performance of nature’s creations and biological assemblies has inspired the development of engineered counter parts that may outperform or provide new capabilities to conventional systems. In particular, the wings of bats contain distributed arrays of micro-scaled flow sensitive hair receptors over their surface, which inspires artificial hair sensors (AHS) development in aerodynamic feedback control designs using the micro-electro-mechanical systems (MEMS). One approach investigates the possibility of installing AHS on the leading edges of the wings of small-scaled unmanned aerial vehicles (UAVs) to improve the aerodynamic control. Our major motivation for the present study is that current mathematical models have limited relevance to aerodynamic situations because they are analyzed in steady or purely oscillatory flows. Our overall objective is to understand AHS fluid-structure interaction (FSI) in flow regimes relevant to small-scaled UAVs, for which we speculate a steady baseline flow perturbed by an oscillatory component is an appropriate flow reference condition. Towards understanding the AHS in this situation, we investigate the dynamic response of a hair receptor in a creeping flow environment with a steady and oscillatory component. We present time varying deflection and bending moment of the artificial hair sensors at different freestream velocities. For this, a three-dimensional FSI model is developed for the flexible hair-structure in the airflow, which is coupled with a finite element model using the computational fluid dynamics (CFD). The Navier-Stokes equations including continuity equation are solved numerically for the CFD model. To describe the dynamic response of the hair receptors, the natural frequencies and mode shapes of the hair receptors, computed from the FSI model, are compared with the excitation frequencies of the surrounding airflow. This model also describes both the boundary layer effects and effects of inertial forces due to FSI of the hair receptors. For supporting the FSI model, the dynamic response of the hair receptor is also validated considering the Euler-Bernoulli beam theory including the steady and unsteady airflow.Copyright