In this study, a magneto-active elastomer (MAE) unimorph actuator is optimally designed for shape programming. An optimization design approach for the MAE unimorph actuator is developed to design the geometry and material properties of the structure. Within the design optimization, a previously developed and validated analytical model is applied to predict the actuation performance of MAE unimorph actuator under a user-specified external magnetic field. This model considers the unimorph as a segmented beam with large deflections and approximates the response of the material to the magnetic field as segment-wise applied torques. Then a single objective function representing the shape error is minimized using a genetic algorithm (GA). The multi-objective non-dominated sorting genetic algorithm II (NSGA-II) is also implemented to maximize both normalized free deflection and blocked force. A Pareto set of optimal solutions is obtained and the best design can be selected based on application requirements. This work has the potential to provide tailorable shape change in medical device applications that require adjustments over time such as changing patient anatomy as treatment progresses. The devices can be actuated under an external magnetic field without wires or human interference. Additive manufacturing can realize the feasibility of fabrication for the designed MAE unimorph actuator by printing complex geometry and spatially tailored magnetic and mechanical properties.
The field of adaptive structures and materials systems is multi-faceted and multidisciplinary, and therefore is poised to play a prominent role in the future of engineering research and technology development. In fact, many of our community members are already comfortable working across disciplines and institutional boundaries in convergent research teams. This presentation will highlight some my own work and experiences in collaborative research teams, specifically in the optimal design of adaptive structures targeted at applications ranging from aerospace systems to medical devices. Some projections and suggestions for future research directions to continue the positive trajectory of our research community will be addressed as well.
Smart materials and structures are enabling complex shape change in applications from self-folding origami to metamaterials. This presentation will focus on recent work on modeling and design optimization of self-folding origami actuated by electrostrictive terpolymer and magneto active elastomer materials. Since finite element analysis of these soft materials with large deformations due to multi-field actuation can be computationally intensive, the problem is decomposed into two stages. The first stage optimizes a computationally efficient reduced order rigid body model, while the second stage implements the full finite element analysis only to refine the features. Ongoing work in optimal design of metamaterials enabled by functionally graded superelastic NiTi will also be highlighted.
Electroactive polymer (EAPs)-based technologies have shown promise in areas such as artificial muscles, aerospace, medical devices and soft robotics because of large electromechanical actuation at relatively high speed. The promises of EAPs have led us to study EAP-based grippers. The in-plane actuation of P(VDF-TrFE-CTFE) is converted into bending actuation using unimorph configurations, where one passive substrate layer is attached to the active polymer. On-demand segmented folding is harnessed from this pure bending actuation by creating notch samples with an aim to implement them for applications like soft robotics gripper. In this paper, we studied the effect of various design parameters of notched folding actuators to establish a design reference and maximize the actuation performance of EAP based devices. Both finite element analysis (FEA) and micromechanics based analytical study is performed to investigate the effect of actuator parameters on the folding actuation of notched samples. The notched configuration has been analyzed via FEA for the non-uniform deformations and stress-fields. FEA analysis shows the importance of notch positioning to maximize the electromechanical performance. On the other hand, analytical study has proposed a design curve for the selection of proper notch parameters (e.g. notch length and Young’s Modulus) to maximize the actuation performance. Finally, based on the FEA and analytical analysis, a human finger inspired ‘finger-like’ biomimetic actuator is realized by assigning multiple notches to the structure.
Electroactive polymer (EAPs) based technologies have shown promise in areas such as artificial muscles, aerospace, medical and soft robotics. In this work, we demonstrate ways to harness on-demand segmented folding actuation from pure bending of relaxor-ferroelectric P(VDF-TrFE-CTFE) based films, using various design approaches, such as ‘stiffener’ and ‘notch’ based approaches. The in-plane actuation of the P(VDF-TrFE-CTFE) is converted into bending actuation using unimorph configurations, where one passive substrate layer is attached to the active polymer. First, we experimentally show that placement of thin metal strips as stiffener in between active EAPs and passive substrates leads to segmented actuation as opposed to pure bending actuation; stiffeners made of different materials, such as nickel, copper and aluminum, are studied which reveals that a higher Young’s modulus favors more pronounced segmented actuation. Second, notched samples are prepared by mounting passive substrate patches of various materials on top of the passive layers of the unimorph EAP actuators. Effect of notch materials, size of the notches and position of the notches on the folding actuation are studied. The motion of the human finger inspires a finger-like biomimetic actuator, which is realized by assigning multiple notches on the structure; finite element analysis (FEA) is also performed using COMSOL Multiphysics software for the notched finger actuator. Finally, a versatile soft-gripper is developed using the notched approach to demonstrate the capability of a properly designed EAP actuator to hold objects of various sizes and shapes.
Contact-Aided Compliant Cellular Mechanisms (C3M) are compliant cellular structures with integrated contact
mechanisms. The focus of the paper is on the design, fabrication, and testing of C3M with curved walls for high strain
applications. It is shown that global strains were increased by replacing straight walls with curved walls in the traditional
honeycomb structure, while the addition of contact mechanisms increased cell performance via stress relief in some
cases. Furthermore, curved walls are beneficial for fabrication at the meso-scale. The basic curved honeycomb cell
geometry is defined by a set of variables. These variables were optimized using Matlab and finite element analysis to
find the best non-contact and contact-aided curved cell geometries as well as the cell geometry that provides the greatest
stress relief. Currently, the most effective contact-aided curved honeycomb cell can withstand global strains
approximately 160% greater than the most effective contact-aided, non-curved cell. Four different designs were
fabricated via the Lost Mold-Rapid Infiltration Forming (LM-RIF) process. An array of the contact-aided optimized
curved cell was then mechanically tested using a custom designed test rig, and the results were found to have a higher
modulus of elasticity and lower global strain than the predictions. Despite these discrepancies, a high-strength highstrain
cellular structure was developed, for potential use in morphing aircraft applications.
The concept proposed in thei work for chord extension is the use of a bistable arch and thin plate system. There are two foci of this paper: (1) Design of the arch and (2)Model validation via experiment. Results show that bistability and symmetric deformation can be achieved when there are flexible hinges at the boundary and input. In addition, the presented finite element model provides good agreement with experimental results.
KEYWORDS: Finite element methods, Systems modeling, Aerodynamics, Motion models, Performance modeling, Composites, Actuators, Smart structures, System integration, Current controlled current source
Research efforts have shown that helicopter rotor blade morphing is an effective means to improve flight performance.
Previous example of rotor blade morphing include using smart-materials for trailing deflection and rotor blade twist and
tip twist, the development of a comfortable airfoil using compliant mechanisms, the use of a Gurney flap for air-flow
deflection and centrifugal force actuated device to increase the span of the blade. In this paper we explore the use of a
bistable mechanism for rotor morphing, specifically, blade chord extension using a bistable arc. Increasing the chord of
the rotor blade is expected to generate more lift-load and improve helicopter performance. Bistable or "snap through"
mechanisms have multiple stable equilibrium states and are a novel way to achieve large actuation output stroke.
Bistable mechanisms do not require energy input to maintain a stable equilibrium state as both states do not require
locking. In this work, we introduce a methodology for the design of bistable arcs for chord morphing using the finite
element analysis and pseudo-rigid body model, to study the effect of different arc types, applied loads and rigidity on arc
performance.
In this paper we explore the use of bistable mechanisms for rotor morphing, specifically, blade tip twist. The optimal
blade twist distributions for hover and high-speed forward flight are very different, and the ability of the rotor to change
effective twist is expected to be advantageous. Bistable or "snap-through" mechanisms have multiple stable equilibrium
states and are a novel way to achieve large actuation output stroke at relatively modest effort for gross rotor morphing
applications. This is because in addition to the large actuation stroke associated with the snap-through (relative to
conventional actuator/ amplification systems) coming at relatively low actuation effort, no locking is required in either
equilibrium state (since they are both stable). In this work, the performance of a bistable twisting device is evaluated
under an aerodynamic lift load. The device is analyzed using finite element analysis to predict the device's load carrying
capability and bistable behavior.
Sandwich structures consisting of contact-aided compliant mechanisms are presented for morphing aircraft skin.
A contact mechanism is used to alleviate stresses and to decrease the out-of-plane deflection. A methodology to
design such mechanisms, which takes into account the aerodynamic loads, is presented. The method is applied
to a small UAV and results are compared with those of honeycomb structures in terms of structural mass, global
strain and maximum stresses. Different material models such as linearly elastic and multi-linear elastic are
considered. For linearly elastic materials, contact-induced stress-relief is advantageous and for nonlinear elastic
materials, reduction of transverse deflection due to contact is useful. In either case, the structural mass of the
contact-aided structures is less than that of the corresponding non-contact structures.
Morphing aircraft wings offer great potential benefits of achieving multi mission capability as well as high
maneuverability under different flight conditions. However, they present many design challenges in the form of
conflicting design requirements. The current research aims to develop design methodologies for the design of a
morphing aircraft wing. Focus of this work is on developing an internal mechanism of the wing that can produce the
desired wing shape change.
This paper presents a design methodology that employs planar unit cells of pre-determined shape and layout as the
internal wing structure for achieving the desired wing shape change. This method is particularly useful in cases where
the desired morphing is two-dimensional in nature. In such cases, intuitive cell designs such as diamond or hexagonal
shaped cells may be used in layouts that achieve desired wing morphing. The shape change depends on the cell shape as
well as cell arrangement in the design domain.
In this paper, a design based on the TSCh wing (NextGen Aeronautics Inc.) using cellular mechanisms to achieve a two-dimensional
wing shape change is discussed. Additionally, a reeling mechanism for achieving cable actuation is presented
In this paper, the optimal location of a distributed network of actuators within a scissor wing mechanism is investigated. The analysis begins by developing a mechanical understanding of a single cell representation of the mechanism. This cell contains four linkages connected by pin joints, a single actuator, two springs to represent the bidirectional behavior of a flexible skin, and an external load. Equilibrium equations are developed using static analysis and the principle of virtual work equations. An objective function is developed to maximize the efficiency of the unit cell model. It is defined as useful work over input work. There are two constraints imposed on this problem. The first is placed on force transferred from the external source to the actuator. It should be less than the blocked actuator force. The other is to require the ratio of output displacement over input displacement, i.e., geometrical advantage (GA), of the cell to be larger than a prescribed value. Sequential quadratic programming is used to solve the optimization problem. This process suggests a systematic approach to identify an optimum location of an actuator and to avoid the selection of location by trial and error. Preliminary results show that optimum locations of an actuator can be selected out of feasible regions according to the requirements of the problem such as a higher GA, a higher efficiency, or a smaller transferred force from external force. Results include analysis of single and multiple cell wing structures and some experimental comparisons.
Achieving multi-mission capability with a single aircraft through in-flight morphing of the wing is highly beneficial due
to its efficiency under different flight conditions such as cruise and dash. In addition higher maneuverability is possible
from using such a vehicle. As opposed to traditional wing morphing where discrete surfaces such as hinged flaps and
ailerons are used, the current research focus is directed towards achieving continuous morphing in order to reduce drag
from geometric discontinuities.
The present research aims to achieve continuous wing morphing by employing a wing structure comprised of an
optimized internal layout of cables and struts. Cables are used as actuators while struts provide rigidity to the wing. In
addition to achieving continuous morphing by changing cable length, this structure has the advantage of being light in
weight. Also, distributed actuation may be achieved from this scheme.
Topology optimization is utilized to optimally place cables and struts in a "bay" or a section of the wing. The
optimization is achieved by using Genetic Algorithm. A Generic Algorithm, the Non-dominated Sorting Genetic
Algorithm II (NSGA II) has been used in this work. The current paper gives an overview of the algorithm and discusses
obtained results.
In this paper the general-purpose finite element (FE) software ABAQUS is used to develop models to simulate large nonlinear viscoelastic response of non-axisymmetric dielectric elastomer actuators. The FE models assume the material to be a simple homogenous, isotropic, and incompressible material. The hyperelastic and viscoelastic material constants are determined using results from constant load uniaxial tensile tests and a constant load uniaxial creep test, respectively. Actuator with elliptical and rectangular cavities fabricated and tested at 5.5, 6.0, and 6.5 kV. The FE models are validated using experimental results obtained after 90 seconds.
The idea of a morphing aircraft wing has generated considerable interest in recent years. Such a structure has inherent advantages of possessing high maneuverability and efficiency under different flight conditions such as take off, cruise and loiter. The current focus is on achieving continuous wing shape change, as opposed to discrete, in order to help reduce drag. This research aims to achieve continuous wing morphing by employing a wing structure comprising of an optimized internal layout of cables and struts. Cables are employed as actuators while struts provide rigidity to the wing. In addition to achieving continuous morphing by changing cable length, this structure has the advantage of being light in weight.
The focus of this paper is on obtaining an optimized cable and strut layout in the body of the wing. Non-linear Finite Element Analysis (FEA) has been performed to account for the large deflection requirements. An objective function that considers deflection under actuation and air loads has been incorporated. Results comparing linear and non-linear FEA are presented for a particular wing design. The nonlinear finite element is found to be effective when using large actuation forces.
This paper presents quasi-static and dynamic results for dielectric elastomer actuators subject to a uniform mechanical pressure and an applied voltage. The numerical quasi-static results are compared to experimental data for actuators made from 3M VHB material. It is shown that the theoretical model for the active inflation of hyperelastic membranes is sensitive to the explicit form of the assumed strain energy function. The optimal constants of 2-Term and 3-Term Ogden models are determined from uniaxial and biaxial stress experimental data. Using the best overall values for the material constants, the electro-elastic model is used to predict the voltage-dependent behavior for the inflation of dielectric elastomer actuators. The correlation between the numerical results and the experimental data is good. In previous work, inertial effects have been neglected and a quasi-static approach employed. The method is presently expanded to include the dynamic response of dielectric elastomer actuators. In this case inertial effects become increasingly important as different equilibria modes are obtained during dynamic operation. The results show the potential for voltage-controlled bifurcations during the inflation of spherical dielectric elastomer actuators.
A non-linear viscoelastic model for finite deformations of dielectric elastomer membranes using Christensen's theory of viscoelasticity is developed. For a time efficient numerical solution, the constitutive integral equations with a time dependent kernel (relaxation modulus) are reformulated into a recurrence form using Feng's recurrence formula and solution for the principal stretches are obtained. Uniaxial constant load tensile tests are conducted and compared with theoretical predictions. The model is also valid for small linear deformations.
A dielectric elastomer diaphragm is to be designed for potential use in a prosthetic blood pump. Application of an electric field deforms the membrane such that it moves from an initially flat configuration to an inflated state. This motion creates positive displacement of blood from the cardiac chambers thus mimicking the pump-like behavior of the natural heart. A comprehensive large deformation model accounting for the combined dielectric and elastic effect has been formulated. This paper presents recent developments in the model to further incorporate the entire nonlinear range of material elastic behavior and to more accurately represent the applied electric field by keeping the voltage constant as the membrane thickness decreases. The updated model is used to calculate the effects of varying system parameters such as pressure, voltage, prestretch, material constants, and membrane geometry. Analytical results are obtained for biaxially stretched 3M VHB 4905 polyacrylate films.
Previous research has demonstrated promise for the use of dielectric elastomer (DE) films in diaphragm pump applications. Because the films tend to be quite thin, single layers operate at very low pressures. To make this technology suitable for practical applications, the films may be organized into laminates which will operate at increased pressures. Radially stretched circular diaphragms of two materials were tested: 3M VHB 4905 polyacrylate and spin-cast Nusil CF19-2186 silicone. The diaphragms were stacked, each layer sharing an electrode with the adjacent layer. The stack was mounted on a sealed chamber and energized at varied electric fields while regulated pressure was applied to the interior chamber, displacing the diaphragm. The pressure-volume properties of the stacks were recorded for each activation state.
Continuously morphing aircraft wings are currently a focus of considerable research. Efforts are being made to achieve effective and optimal wing shape change under different flight conditions such as take off, cruise, dash, and loiter. The present research aims to achieve wing morphing by using an internal structure consisting of actuated tendons and passive struts. An important aspect of this approach is determining the optimal layout of tendons and struts. In this paper a genetic algorithm is developed to optimize the three-dimensional tendon-strut layout for a prescribed wing geometry and shape change. The method is applied to two morphing wing applications, the NASA HECS wing and NextGen TSCh wing.
Actuators based on smart materials generally exhibit a tradeoff between force and stroke. Researchers have surrounded piezoelectric materials (PZT’s) with complaint structures to magnify either their geometric or mechanical advantage. Most of these designs are literally built around a particular piezoelectric device, so the design space consists of only the compliant mechanism. Materials scientists researchers have demonstrated the ability to pole a PZT in an arbitrary direction, and some engineers have taken advantage of this to build “shear mode” actuators.
The goal of this work is to determine if the performance of compliant mechanisms improves by the inclusion of the piezoelectric polarization as a design variable. The polarization vector is varied via transformation matrixes, and the compliant actuator is modeled using the SIMP (Solid Isotropic Material with Penalization) or “power-law method.” The concept of mutual potential energy is used to form an objective function to measure the piezoelectric actuator’s performance. The optimal topology of the compliant mechanism and orientation of the polarization method are determined using a sequential linear programming algorithm. This paper presents a demonstration problem that shows small changes in the polarization vector have a marginal effect on the optimum topology of the mechanism, but improves actuation.
An analytical model of a thin annulus made of a dielectric elastomer was derived in our earlier work. It is the focus of this paper to fabricate and test the electroactive polymer actuators presented in the model. Radial displacement measurements were obtained using a digital image correlation technique that involved cross-correlating images of the actuator before and after a change in the electric field. Results showed that the two-term Ogden material model provided a good agreement with experimental data. Hysteresis of the dielectric actuators was observed. Radial displacements of material points on the actuator varied between two consecutive tests which may indicate that strain has history-dependent characteristics.
In the use of piezoelectric actuators, it is a clear choice to use stack (or d33 mode) architectures when very high force is required or benders (or d31 mode) architectures when very high displacements are needed. However, the choice isn't as clear for applications that need simultaneously a moderate force and displacement. This paper presents one such application, INSTAR that is posed with this dilemma. INSTAR is a novel rifle system that has an inertially stabilized barrel via an active suspension based on piezoelectric actuation. While the frequency required for this application was low (~10Hz), the displacement (± 200 to 400 microns) and the force (22-45 N) are moderate. Two very different actuation approaches were developed, modeled, fabricated and experimentally validated within the INSTAR demonstration platform: 1) a d31 approach based on the Recurve architecture with focus on generating higher forces than is common for d31 actuators and 2) a d33 approach based upon a compliant mechanism designed using topology optimization with focus on providing more amplified strain than is common for d33 actuators. Both approaches were successful in meeting the INSTAR requirements, but each had its on advantages and disadvantages.
The electromechanical behavior of dielectric elastomers is to be exploited for medical application in artificial blood pumps. It is required that the pump diaphragm achieves a swept volume increase of 70 cc into a systolic pressure of 120 mmHg with the main design objective being volumetric efficiency. As such, a model that accommodates large deformation behavior is used. In order to design prosthetic blood pumps that closely mimic the natural pumping chambers of the heart, a dielectric elastomer diaphragm design is proposed. The elastomer's change in shape in response to the applied electric field will permit it to be the active element of the pump just as the ventricular walls are in the natural heart. A comprehensive analytical model that accounts for the combined elastic and dielectric behavior of the membrane is used to compute the stresses and deformations of the inflated membrane. Dielectric elastomers are often pre-strained in order to obtain optimal electromechanical performance. The resulting model incorporates pre-strain and shows how system parameters such as pre-strain, pressure, electric field, and edge constraints affect membrane deformation. The model predicts more than adequate volume displacement for moderate pre-strain of the elastomer.
Compliant mechanical amplifiers are often used to amplify small motions such as those of PZT actuators, since they do not incur displacement losses that frequently occur in pin-jointed mechanisms. Their optimal design is key to maximizing actuator performance. Our previous work was focused on developing a topology optimization methodology wherein the size of the design domain and the location of the PZT actuator were pre-defined. The resultant solution was one that maximized stroke amplification. In this paper we study the effects of stack and structural properties on resultant topology and output stroke with focus on quantitative performance for practical application. The motivating example is an actuator-design problem where +/- 400micrometers stroke and 45 N force is required. The problem is solved using topology design methodology and the results obtained are verified using finite element analysis. We demonstrate that magnitude of output displacement is extremely sensitive to preload on the compliant mechanical amplifier, amplifier and actuator material, topology interpretation while converting it into a solid model, and magnitude of applied voltage. We discuss effects of asymmetric placement of the PZT stack, multiple stacks, and increased stack length on resultant displacement.
Much of the recent and past work in the area of smart materials and structures has focused on analysis of actuators and actively controlled systems. Although many sophisticated analysis models have been developed, they are often coupled with ad-hoc design methods or informal optimization procedures. A subset of the work done by the smart structures community has focused on development of formal design methodologies and optimization methods specifically for smart actuators and structures. The objective of this paper is to review the current work in development of design methodologies and application of formal optimization methods to the design of smart structures and actuators. In a related paper, optimization strategies for sensor and actuator placement were reviewed by a researcher at NASA Langley in 1999. The current paper reviews the recent work done in this area since 1999, in addition to optimization strategies for topology design of actuators, actively controlled structures, and drive electronics design. The main focus is on piezoelectric ceramic actuators, but relevant work in shape memory alloys and magnetostrictive actuation are included as well. Future directions for research in optimization are also recommended.
A model has been developed in MATLAB to design a new active, steerable end-effector. The end-effector design consists of a number of bimorph actuator sections in series with each active layer being individually controlled. Each section may behave as either a bimorph or a unimorph actuator, where in the case of unimorph one of the active layers is passive. By varying the strength and direction of the electric field across each section, a prescribed overall shape can be achieved to allow the user to steer the device. The focus of ths paper is on the model of the end-effector using electroactive polymer (EAP) materials. In the EAP model, the experimental data for the electrostrictive P(VDF-TrFE) copolymer is used to model the non-linear relationship between the electric field and the induced strain. Due to the large deflections achievable with the EAP, a pseudo rigid-body model for large deflections beams is also used. The behavior of piezoelectric ceramic is compared to that of electro-active polymer (EAP). The target application for this steerable device is a small-scale smart surgical instrument for minimally invasive surgery.
A metamodel-based presented is developed to optimize the force and displacement performance of a piezoceramic bimorph actuator. A segmented design with a variable piezoceramic layer thickness is proposed, where the thicknesses of discrete piezoceramic segments are used as the design variables. Design of experiments and metamodeling techniques are employed to construct computationally inexpensive approximations of finite element simulations of the PZT bimorph actuator. The metamodels are then used in lieu of the actual FEM for optimization. Design objectives include maximum tip deflection, maximum grasping force, and maximum work available at the tip. The metamodels are also used to rapidly generate the design space and identify the Pareto frontier for the competing design objectives of maximum deflection and maximum force. The accuracy and efficacy of two types of metamodels -- response surfaces and kriging models -- are compared in this study. By optimizing the thickness of the piezoceramic layers, and by allowing the voltage applied to each segment to vary, dramatic improvements in deflection and force are obtained when compared to a standard straight bimorph actuator. The motivation for this design is the need in the field of minimally invasive surgery for improved grasping tools, where a pair of optimized bimorph actuators can be used as a simple grasping device.
Recent advances in robotics, tele-robotics, smart material actuators, and mechatronics raise new possibilities for innovative developments in millimeter-scale robotics capable of manipulating objects only fractions of a millimeter in size. These advances can have a wide range of applications in the biomedical community. A potential application of this technology is in minimally invasive surgery (MIS). The focus of this paper is the development of a single degree of freedom prototype to demonstrate the viability of smart materials, force feedback and compliant mechanisms for minimally invasive surgery. The prototype is a compliant gripper that is 7-mm by 17-mm, made from a single piece of titanium that is designed to function as a needle driver for small scale suturing. A custom designed piezoelectric `inchworm' actuator drives the gripper. The integrated system is computer controlled providing a user interface device capable of force feedback. The design methodology described draws from recent advances in three emerging fields in engineering: design of innovative tools for MIS, design of compliant mechanisms, and design of smart materials and actuators. The focus of this paper is on the design of a millimeter-scale inchworm actuator for use with a compliant end effector in MIS.
Compliant mechanisms are currently used in conjunction with induced-strain actuators to provide stroke amplification. These compliant or flexure mechanisms are preferred over conventional rigid-link amplifiers because they avoid problems with clearances in mechanical joints. Many of the current compliant amplifiers are designed using ad-hoc or intuitive methods, however. In this paper, a topology optimization algorithm developed for systematic design of compliant mechanisms is applied to the design of compliant stroke amplifiers for induced-strain actuators. The underlying formulation for the optimization method is presented, and is then illustrated by two design examples. The functionality of the optimal solutions are verified by finite element analyses.
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