Smita Bharti

Aircraft structural morphing using tendon-actuated compliant cellular trusses

Recently, smoothly-deforming aircraft structures have been investigated for their ability to adapt to varying flight conditions. Researchers aim to achieve large changes in the shape of the wings: area changes of up to 50% and aspect-ratio changes of up to 200% are being pursued. The research described in this paper aims to develop a structural concept capable of achieving continuous stable deformations over a large range of aircraft shapes. The basic concept underlying the approach is a compliant cellular truss, with tendons used as active elements. The truss members of the unit cell are connected through compliant joints such that only modest bending moments may be transmitted from one member to another. Actuation is achieved by pulling on one set of cables while releasing another set. The tendonactuated compliant truss can be made to behave locally, and temporarily, as a nearmechanism, by releasing appropriate cables. As a result, in the absence of aerodynamic forces, the structure can be morphed using relatively low forces. The cables are reeled in or released in a controlled manner while the structure is loaded, hence, the stability of the structure can be maintained in any intermediate position. Highly-distributed actuation also enables the simultaneous achievement of global shape changes as the accumulation of local ones, while the use of compliant joints rather than true rotating joints eliminates binding as a significant concern. A six-noded octahedral cell with diagonal tendon actuation is developed for a bending type deformation in the wing. Initial cell geometry is determined by “strain matching” to the local morphing deformation required by the application. A finite element analysis is performed on a wing made of these unit cells and sized for a representative UAV weighing 3000 lbs. The areas of the individual truss members are sized so that they don’t fail or buckle under the air loads, while deflection at the wing tip is reduced. The octahedral unit cell is capable of achieving smooth deformations of the truss structure. The cell size is dictated by the available space and the morphing strain. The cell sizes are reasonable for strains on the order of 10% to 15% and get smaller for larger strains. Additional cell shapes are being investigated for larger area changes through a process of topology optimization using genetic algorithms. Numerous other technical challenges remain, including the details of actuation and a robust skin.

Tendon actuated cellular mechanisms for morphing aircraft wing

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

Optimal Design and Experimental Characterization of a Compliant Mechanism Piezoelectric Actuator for Inertially Stabilized Rifle

Compliant mechanism amplifiers are often used in conjunction with piezoelectric actuators since they do not incur displacement losses that frequently occur in pin-jointed mechanisms. In this paper the design of a compliant mechanism amplifier and piezoelectric stack actuator is presented. The application is an inertially stabilized rifle (INSTAR) in which the compliant actuator is used to make small adjustments in the position of the barrel. A topology optimization method is used to obtain the initial topology of the compliant mechanism, followed by detailed finite element analysis. The effect of geometry parameters, material selection, and epoxy bonding layers in the piezoelectric actuator are studied. A prototype actuator is fabricated and characterized experimentally. The force and displacement performance of the prototype actuator are shown to exceed the design specifications for the INSTAR application.

Compliant Mechanical Amplifier Design using Multiple Optimally Placed Actuators

This article discusses a methodology for designing compliant mechanisms with piezoelectric actuation to obtain maximum deflection and force at the output point. The focus is on design of compliant mechanisms with multiple piezoelectric actuators. The number, size, and position of the actuators within the compliant mechanism are optimized for the maximum output deflection. Predicted results demonstrate that compliant mechanisms with multiple, optimally placed actuators outperform those with a single actuator placed at a predetermined location.

Compliant Mechanical Amplifier Design Using Multiple Optimally Placed Actuators

The paper discusses a methodology for designing compliant mechanisms with piezoelectric actuation to obtain maximized deflection and force at output. The focus is on design of compliant mechanisms with multiple optimally placed and sized piezoelectric actuators. Thus the number, length and position of actuators in the body of the compliant mechanism are optimized in addition to maximizing the output deflection and force. Results demonstrate that compliant mechanisms with multiple, optimally placed actuators outperform those with a single actuator placed at a predetermined location.Copyright

Optimal design of tendon-actuated morphing structures: nonlinear analysis and parallel algorithm

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.

Optimal Morphing-Wing Design Using Parallel Nondominated Sorting Genetic Algorithm II

The focus of this paper is an optimal design ofmorphing aircraft wings employing a wing structure composed of an internal layout of cables and struts. Cables are used to provide actuation and stiffness, and struts provide stiffness without actuation. Topology optimization is used to place cables and struts in a bay or a section of the wing. Nonlinear finite element analysis is used to capture the large deformations of the structure, and the optimization is achieved using the Nondominated Sorting Genetic Algorithm II. The optimization procedure is illustrated using a morphingwing example. The effect of the upper limit on actuation forces is studied, and solutions are found with good agreement between the desired and obtained deflections under actuation and aerodynamic loads. The implemented parallelized optimization algorithm is successful in solving a computationally intense, multi-objective, multiconstraint problem with a large number of discrete and continuous design variables in a reasonable amount of time.

Parallel Genetic Algorithm for Design of Morphing Cellular Truss Structures

A parallel genetic algorithm is developed for the design of morphing aircraft structures using tendon actuated compliant truss. The wing structure in this concept is made of solid members and cables. The solid members are connected through compliant joints so that they can be deformed relatively easily without storing much strain energy in the structure. The structure is actuated using cables to deform into a required shape. Once the structure is deformed, the cables are locked and hence carry loads. Previously an octahedral unit cell made of cables and truss members was developed to achieve the required shape change of a morphing wing developed at NASA. It was observed that a continuously deformable truss structure with required morphing capability can be achieved by a cellular geometry tailored to local strain deformation. A wing structure made of these unit cells was sized for a representative aircraft and was found to be suitable. This paper describes the development of new unit cell designs that fit the morphing requirements using topology optimization. A ground structure approach is used to set up the problem. A predetermined set of points is selected and the members are connected in between the neighboring nodes. Each member in this ground structure has four possibilities, 1) a truss member, 2) a cable that morphs the structure into a required shape, 3) a cable that is antagonistic and brings it back to the original shape 4) a void, i.e., the member doesn’t exist in the structure. This choice is represented with a discrete variable. A parallel genetic algorithm is used as an optimization approach to optimize the variables in the ground structure to get the best structural layout. The parallelization is done using a master slave process. A fitness function is used to calculate how well a structural layout fits the design requirements. In general, a unit cell that has lesser deflection under external loads and higher deflection under actuation has a higher fitness value. Other requirements such as having fewer cables and achieving a required morphing shape are also included in the fitness function. The finite element calculations in the fitness function can be done using either linear or nonlinear analysis. The paper discusses the different ways of formulating the fitness function and the results thereof.Copyright

Active and passive material optimization in a tendon-actuated morphing aircraft structure

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.

Topology optimization and detailed finite element modeling of piezoelectric actuators: effect of external loads and detail geometry on actuator output

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.