In this paper the design of a linear long-stroke quasi-constant force compliant mechanism (CM) is presented and discussed. Starting from a flexure-based slider-crank mechanism, providing the required constant force within a rather limited deflection range, the paper reports about the shape optimization carried out with the specific aim of extending the available CM operative range. The proposed device is suitable in several precision manipulation systems, which require to maintain a constant-force at their contact interface with the manipulated object. Force regulation is generally achieved by means of complex control algorithms and related sensory apparatus, resulting in a flexible behavior but also in high costs. A valid alternative may be the use of a purposely designed CM, namely a purely mechanical system whose shape and dimensions are optimized so as to provide a force-deflection behavior characterized by zero stiffness. In the first design step, the Pseudo-Rigid Body (PRB) method is exploited to synthesize the sub-optimal compliant configuration, i.e. the one characterized by lumped compliance. Secondly, an improved design alternative is evaluated resorting to an integrated software framework, comprising Matlab and ANSYS APDL, and capable of performing non-linear structural optimizations. The new embodiment makes use of a variable thickness beam, whose shape and dimensions have been optimized so as to provide a constant reaction force in an extended range. Finally, a physical prototype of the beam-based configuration is produced and tested, experimentally validating the proposed design method.

Industrial robots are commonly designed to be very fast and stiff in order to achieve extremely precise position control capabilities. Nonetheless, high speeds and power do not allow for a safe physical interaction between robots and humans. With the exception of the latest generation lightweight arms, purposely design for human-robot collaborative tasks, safety devices shall be employed when workers enter the robots workspace, in order to reduce the chances of injuries. In this context, Variable Stiffness Actuators (VSA) potentially represent an effective solution for increasing robot safety. In light of this consideration, the present paper describes the design optimization of a VSA architecture previously proposed by the authors. In this novel embodiment, the VSA can achieve stiffness modulation via the use of a pair of compliant mechanisms with distributed compliance, which act as nonlinear springs with proper torque-deflection characteristic. Such elastic elements are composed of slender beams whose neutral axis is described by a spline curve with non-trivial shape. The beam geometry is determined by leveraging on a CAD/CAE framework allowing for the shape optimization of complex flexures. The design method makes use of the modeling and simulation capabilities of a parametric CAD software seamlessly connected to a FEM tool (i.e. Ansys Workbench). For validation purposes, proof-concept 3D printed prototypes of both non-linear elastic element and overall VSA are finally produced and tested. Experimental results fully confirm that the compliant mechanism behaves as expected.

Origami provides inspiration and solutions to the fabrication and functionality of various structures. Origami design methods in the literature are limited to the idealization of the folds as creases of zeroth-order geometric continuity. This idealization is not proper for origami structures having non-negligible fold thickness or maximum curvature at the folds restricted by material or structural limitations. For these structures, the folds are not accurately represented as creases but instead as bent regions of higher-order geometric continuity. These fold regions of arbitrary order of continuity are denoted in this work as smooth folds. A method for the design of a single planar sheet and its associated pattern of smooth folds that morphs into a given three-dimensional goal shape represented as a polygonal mesh is proposed. The parameterization of the planar sheet and the constraints allowing for a valid smooth fold pattern and matching of the goal shape in a folded configuration are presented. The folding deformation of the determined sheet designs is simulated using a previously derived kinematic model for origami with smooth folds. Various testing examples considering diverse goal shapes are presented. The results demonstrate that each considered sheet design matches its corresponding goal shape in a known folded configuration having fold angles determined from the geometry of the goal mesh. The proposed method can be used for the design of origami structures having folds of arbitrary order of geometric continuity such as origami-inspired active structures.

The modeling and simulation of smart structures and systems involves coupled field calculations which cause currently high computational costs. Especially time and frequency analyses of sensor or actuator constructions described by equation systems with 10,000 to several 100,000 degrees of freedom demand efficient design methods. A successful approach to solve this problem is to increase the abstraction in a model hierarchy by switching to macro models. In this paper the merits of multi-physics network models applied as macro models are discussed. The main advantage is the significant reduction of the degree of freedom and the transition from PDEs to ODEs. Steady state problems can be efficiently solved with algebraic equations in the complex domain. Another important aspect appreciated by design engineers is the structural graphical system representation offered by networks. Particularly, feedback mechanisms, e.g. from an acoustic subsystem into the mechanical subsystem, can be overviewed. Moreover, linear time-invariant (LTI) multi-physics networks allow the transformation of network elements from one physical domain into another domain. As a consequence transducers can be eliminated and thus the network be simplified. Besides this, passive LTI systems are reversible. This property is the basis of precise measurement techniques.

Shape Memory Alloy (SMA) wire ratchet actuators overcome SMA wire strain limitations by accumulating actuation stroke over multiple cycles. The underlying architecture is effective for producing large strokes from a small package, creating continuous rotation or extended displacement, and precise. It also provides discrete positioning with zero-power hold. While there have been several successful implementations of SMA ratchet actuators, most are designed ad-hoc since limited models exist to predict the stroke and force interaction during actuation cycles. Since the SMA wire actuation is highly dependent on the forces experienced through the ratchet mechanism, a model requires the prediction of the force interaction between the rack and pawl teeth along with friction in the device, and of the external force variation over actuation cycles due to the relative position change between the external system and the SMA wire. This paper presents a model-based systematic design methodology for SMA ratchet actuator which actuates position-dependent external systems. A generalized ratchet mechanism and operation sequence is introduced along with a force balance model for both austenite and martensite equilibrium to address the mechanical coupling changes. Analytical kinematic and kineto-static rack and pawl interaction models are reviewed, which feed into the force balance models. The effective stroke is evaluated by subtracting backlash from the SMA wire stroke, found through equilibrium with the mechanism and external system. This effective stroke accumulates to produce the overall actuator motion. A design methodology is suggested along with visualization methods to aid design decisions. Parametric studies expose the effects of design parameters on the SMA ratchet actuator to gain further design insight. This model-based design foundation and parametric understanding enable the synthesis of SMA wire ratchet actuators.

SMAs have been demonstrated for many actuation applications and found a few commercial applications, generally for small components with ambient temperature actuation. A study into their use for an adaptive geometry exit nozzle for a large aero gas turbine started around 2001. SMA mechanisms offer significant advantages in weight, complexity and aerodynamic form which can be achieved. It quickly became clear however that virtually all aspects of the material, form availability, mechanism, design methods, actuation and application assessment would need significant further development. This paper will describe the work which has been done on those aspects which have been necessary to move the concept towards a practical reality and wind tunnel demonstration of a prototype component. A relatively simple structure has been pursued, but this has led to the unusual need for large sheet materials at higher temperatures than the materials which were available at the time. Unique processing and characterisation methods have been developed in parallel with the materials to provide a practical process which can be applied to provide predictable stable operation within a structure where the loading is predominantly in tension. The structure also allows more rapid cooling of the SMA which has been identified as an issue in many studies, although our solution does require increased heat input for actuation. The design of any engineering component is a series of compromises around parameters such as weight, efficiency and cost. SMA seems to offer a major opportunity in all of these areas. Safety is clearly paramount in any aircraft application and the compromises must be made without any detrimental effect on this. All of these issues are covered by standard industry procedures. The characteristic of SMA which are different to conventional actuation offer major advantages, but also significant challenges in achieving acceptance.

In support of the Department of Energy Sunshot initiative target of $1.25 per watt photovoltaics systems for commercial applications, whole system designs were pursued using the analogical design methodology, an essential step in the bio inspired design approach. A functional decomposition of solar panel systems was conducted, and then key functions critical to system integrity and cost reduction were identified. Three sources of bio-inspiration were mainly used: hierarchical structures as a common design dimension exploited in natural systems, and leaves’ ability to maintain position through changes in shape and angle of attack when triggered by wind flow, and limpet shells’ reduction of hydrodynamic forces by way of natural geometrical features. The design team developed concepts with varying degrees of abstraction then attempted to reconcile them with other functional requirements. Variants that descended from the leaf concept were generally found to be biophilic and offer aesthetic value; however, presented shortcomings in electrical design and installation procedure (Kellert 2008). Alternatively, concepts inspired by hierarchical structures and limpet shells were found to have greater variability, enabling reconciliation with other functional requirements, resulting in a complete system solution capable of meeting the cost reduction objective. From the analysis of these design variants, we may conclude that transferring solution principles directly from nature is best done when there is small set of functional requirements that must be fulfilled and value in a biophilic design. However, in cases of significant system complexity, abstracted lessons from nature may be found to be more flexible and easily reconciled with multiple requirements.

Ornithopters or flapping wing Unmanned Aerial Vehicles (UAVs) have potential applications in civil and military sectors. Amongst the UAVs, ornithopters have a unique ability to fly in low Reynolds number regions and also have the agility and maneuverability of a rotary wing aircraft. In nature, birds achieve such special characteristics by morphing their wings. The compliant spine (CS) design concept presented here represents a novel method of achieving wing morphing passively. In this paper, an optimal design method is developed that incorporates dynamic finite element analysis. To solve the CS design problem a new multi-objective optimization problem is formulated with three objective functions. The first objective function seeks to minimize the mass of the compliant spine. The second objective function seeks to maximize the deflection of the compliant spine for a particular dynamic loading condition. Finally, the third objective function seeks to minimize the stress in the design observed under the dynamic loading conditions experienced during flight. The deflections and stresses in the CS design are based on measured wing loads and are calculated by applying a sinusoidal forcing function at a prescribed forcing frequency. The optimization, performed via a controlled elitist genetic algorithm which is a variant of NSGA-II, is used to design CSs operating under dynamic conditions. Modal analysis and frequency response of an optimal compliant spine during the upstroke are also shown.

The high energy density actuation potential of SMA wire is tempered by conservative design guidelines set to mitigate complex factors such as functional fatigue (shakedown). Shakedown causes problems of stroke loss and interface position drift between the system and the SMA wire under higher stress levels if the wire does not undergo a pre-installation shakedown procedure. Limiting actuation strain has been reported as reducing shakedown as well as increasing fatigue life. One approach to limit actuation strain is using a mechanical strain limiter which sets a fixed Martensite strain position — useful for the development of in-device shakedown procedures which eliminates time consuming pre-installation shakedown procedures. This paper presents a new graphical design approach for SMA wire actuators which accounts for shakedown with the use of mechanical strain limiters to enable higher stress designs to maximize actuator performance. Experimental data on the effect of strain limiters along with stroke and work density contours form the basis for the new graphical design method. For each independent mechanical strain limiter, the maximum of the individual post-shakedown austenite curves at a range of applied stress are combined into a conglomerate stabilization design curve. These curves over a set of mechanical strain limiters provide steady state performance prediction for SMA actuation, effectively decoupling the shakedown material performance from design variables that affect the shakedown. The use and benefits of this new design approach are demonstrated with a common constant force actuator design example. This new design approach, which accounts for shakedown, supports design of SMA actuators at higher stresses with more economical use of material/power, and enables the utilization of strain limiters for cost saving in-device shakedown procedures.

This work describes a design and optimization method for developing hybrid, multi-material, compliant instruments which are expected to be useful in mini-laparoscopy and natural orifice translumenal endoscopic surgery. These two-material devices are designed specifically for Penn State’s lost mold rapid infiltration process, which is capable of fabricating hundreds of freestanding meso-scale parts in parallel. New narrow-gauge surgical procedures impose severe geometric constraints that challenge traditional compliant mechanism design methods. Since narrow-gauge constraints leave geometry optimization ineffective, new design methods are explored to improve the performance of a 1 mm diameter contact-aided compliant forceps. By considering hybrid designs, new design possibilities are enabled through material variation. The hybrid forceps has desired regions of flexibility and stiffness that can be isolated to improve tool performance. For instance, a hybrid forceps can be designed with greater flexibility in some regions to provide larger jaw openings while maintaining high stiffness in other regions to obtain large grasping forces, both vital features in a surgical forceps. Using ANSYS to model large deformation and contact, an optimization problem is formulated to maximize tool performance and to determine optimal segregation of hybrid materials considering a range of modulus ratios. Materials under consideration include nanoparticulate 3 mol% yttria partially stabilized zirconia (3YSZ) and austenitic (300 series) stainless steel. All results are compared to previously optimized homogeneous designs.

A new scheme to design morphing ornithopter wings using a passive compliant spine is presented in this paper. The objective of this work is to optimize steady level flight performance of an ornithopter by passively implementing the Continuous Vortex Gait (CVG) which requires bending, twist and sweep coupling during the upstroke. An optimization problem is formulated to design a compliant spine for pre-specified bending, sweep, and twist deflections. As a first step to achieving these 3 DOF kinematics, a 1 DOF compliant spine is considered to produce a specified bending deflection during the upstroke for drag reduction while remaining stiff during the downstroke for increased lift. The effect of the relevant geometric design parameters, namely contact gap, angle, and hinge geometry, are considered and optimized to achieve the aforementioned kinematics for both single and multiple joints, which make up a compliant spine. Results presented include the spine design optimization procedure, as well as a complete analysis for a 1DOF compliant spine to illustrate the efficacy of the methodology. This compliant spine design methodology and optimization procedure will be used, in the future, to design the 3-DOF compliant spine for the passively morphing ornithopter.