Michael Hays

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.

A modeling and uncertainty quantification framework for a flexible structure with macrofiber composite actuators operating in hysteretic regimes

Macrofiber composites are low cost, durable, and flexible piezoceramic devices that are presently being considered for applications that include shape control of airfoils for improved flight performance, vibration, and noise suppression and energy harvesting. However, macrofiber composites also exhibit hysteresis and constitutive nonlinearities that need to be incorporated in models and model-based control designs to achieve their full capability. In this article, we combine constitutive relations, constructed using the homogenized energy model for ferroelectric hysteresis, with Euler–Bernoulli theory to construct a dynamic macrofiber composite model that quantifies a range of rate-dependent hysteretic behavior of macrofiber composites. Using homogenizing strategies, the macrofiber composite patch is treated as a monolithic material with effective parameters. We initially calibrate the model by estimating parameters through a least squares fit to a subset of the measured data. We find that the estimated parameters yield very accurate fits for quasi-static hysteresis. The estimated parameters also provide reasonably accurate predictions for a range of frequencies that include the first two harmonics. Second, we employ an adaptive Markov chain Monte Carlo algorithm to construct densities and analyze the correlation between parameters. The kernel density estimates derived from the Markov chain Monte Carlo chains imply that most of the model parameters exhibit non-Gaussian distributions.

Uncertainty quantification and stochastic-based viscoelastic modeling of finite deformation elastomers

Material parameter uncertainty is a key aspect of model development. Here we quantify parameter uncertainty of a viscoelastic model through validation on rate dependent deformation of a dielectric elastomer that undergoes finite deformation. These materials are known for there large field induced deformation and applications in smart structures, although the rate dependent viscoelastic effects are not well understood. To address this issue, we first quantify hyperelastic and viscoelastic model uncertainty using Bayesian statistics by comparing a linear viscoelastic model to uniaxial rate dependent experiments. The probability densities, obtained from the Bayesian statistics, are then used to formulate a refined model that incorporates the probability densities directly within the model using homogenization methods. We focus on the uncertainty of the viscoelastic aspect of the model to show under what regimes does the stochastic homogenization framework provides improvements in predicting viscoelastic constitutive behavior. It is show that VHB has a relatively narrow probability distribution on the viscoelastic time constants. This supports use of a discrete viscoelastic model over the homogenized model.

Modeling the Nonlinear Behavior of Macro Fiber Composite Actuators

Macro Fiber Composites (MFC) are planar actuators comprised of PZT fibers embedded in an epoxy matrix that is sandwiched between electrodes. Due to their construction, they exhibit significant durability and flexibility in addition to being lightweight and providing broadband inputs. They are presently being considered for a range of applications including positioning and control of membrane mirrors and configurable aerospace structures. However, they also exhibit hysteresis and constitutive nonlinearities that must be incorporated in models to achieve the full potential of the devices. In this paper, we discuss the development of a model that quantifies the hysteresis and constitutive nonlinearities in a manner that promotes subsequent control design. The constitutive model is constructed using the homogenized energy framework for ferroelectric hysteresis and used to develop resulting system models. The performance of the models is validated with experimental data.

Statistical parameter estimation and uncertainty quantification for macro fiber composite actuators operating in nonlinear and hysteretic regimes

Macro Fiber Composites (MFC) are planar actuators comprised of PZT fibers embedded in an epoxy matrix that is sandwiched between electrodes. Due to their construction, they exhibit significant durability and flexibility in addition to being lightweight and providing broadband inputs. They are presently being considered for a range of applications including positioning and control of membrane mirrors and configurable aerospace structures. However, due to the noncentrosymmetric nature of PZT, MFC also exhibit hysteresis and constitutive nonlinearities that must be incorporated in models and control designs to achieve their full potential. In this paper, we discuss issues associated with the estimation of parameters and uncertainty quantification (UQ) for a distributed model that quantifies the hysteretic dynamics of the devices. Statistical parameter estimation techniques are used to construct densities for model parameters. These uncertainties are subsequently propagated though the model to construct error bounds.

Statistical parameter estimation for macro fiber composite actuators using the homogenized energy model

Macro Fiber Composites (MFC) are planar actuators comprised of PZT fibers embedded in an epoxy matrix that is sandwiched between electrodes. Due to their construction, they exhibit significant durability and flexibility in addition to being lightweight and providing broadband inputs. They are presently being considered for a range of applications including positioning and control of membrane mirrors and configurable aerospace structures. However, they also exhibit hysteresis and constitutive nonlinearities that must be incorporated in models to achieve the full potential of the devices. In this paper, we discuss the development of a model that quantifies the hysteresis and constitutive nonlinearities in a manner that promotes subsequent control design. The constitutive model is constructed using the homogenized energy framework for ferroelectric hysteresis and used to develop resulting system models. The performance of the models is validated with experimental data.

Flow Sensory Actuators for MAVs

A macro fiber piezoelectric composite has been studied for boundary layer management in Micro-Air Vehicles (MAVs). Specifically, self-sensing piezoelectric composite structures have been designed, fabricated, and tested in wind tunnel studies to quantify performance characteristics, such as the velocity field response to actuation, relevant for actively managing boundary layers (laminar and transition flow control). The dynamic properties of these Flow Sensory Actuators (FSA) were also evaluated in-situ. Results based on velocity field measurements and unsteady pressure measurements show that these macro fiber composites can sense the state of flow above the surface and have sufficient authority to manipulate the flow conditions. Control authority using open loop and closed loop control designs have also been investigated in trade-off studies to quantify performance enhancements versus power input and weight requirements (a critical driver in MAVs) relevant to this application.

Nonlinear Bending Mechanics of Hygroscopic Liquid Crystal Polymer Networks

A chemically responsive liquid crystal polymer network is experimentally characterized and compared to a nonlinear constitutive model and integrated into a finite element shel l model. The constitutive model and large deformation shell model are used to understand water vapor induced bending. This class of materials is hygroscopic and can exhibit large bending as water vapor is absorbed into one side of the liquid crystal network (LCN) film. This gives rise to deflection away from the water vapor source which provides unique sensing and actuation characteristics for chemical and biomedical applications. The constitutive behavior is modeled by coupling chemical absorption with nonlinear continuum mechanics to predict how water vapor absorption affects bending deformation. In order to correlate the mode l with experiments, a micro-Newton measuring device was designed and tested to quantify bending forces generated by the LCN. Forces that range between 1-8 μN were measured as a function of the distance between the water vapor source and the LCN. The experiments and model comparisons provide important insight into linear and nonlinear chemicall y induced bending for a number of applications such as microfluidic chemical and biological sensors.

Broadband pulsed flow using piezoelectric microjets

A piezohydraulic microjet design and experimental results are presented to demonstrate broadband active flow control for applications on various aircraft structures including impinging jets, rotor blades, cavity bays, etc. The microjet actuator includes a piezoelectric stack actuator and hydraulic circuit that is used to throttle a 400 μm diameter microjet using hydraulic amplification of the piezoelectric stack actuator. This system is shown to provide broadband pulsed flow actuation up to 800 Hz. Unsteady pressure measurements of the microjets exit flow are coupled with high-speed phase imagery using micro-Schlieren techniques to quantify the flow field. These results are compared with in situ stack actuator displacements using strain gauge measurements.

Fluid–structural dynamic characterization of an electroactive membrane wing

The interactions between low-Reynolds-number fluid flow and an electroactive membrane wing is characterized to illustrate changes in harmonic and transient behavior from electric field excitation of the membrane wing. The wing is constructed of a dielectric elastomer material that changes its tension as a function of the applied field. The field excitation leads to changes in the shape of the wing under aerodynamic loads and subsequently, increased lift and delay of stall. Prior work in time-averaged lift and drag characterization is extended to better understand dynamic characteristics. Benchtop membrane structural dynamics are compared with visual image correlation, hotwire measurements, and particle image velocimetry within a low-Reynolds-number wind tunnel. An applied electric field leads to a 13.7% reduction in structural resonance of the membrane under ambient conditions while wind tunnel measurements illustrate a 5.1% reduction in resonance under the same applied field. Despite these differences, the structural and fluid dynamic harmonics are closely correlated for low-Reynolds-number flow at 10 m/s.