Giovanni Scirè Mammano

Modeling of Wire-on-Drum Shape Memory Actuators for Linear and Rotary Motion

The article presents the analytical model of a linear/rotary solid-state actuator formed by a shape memory wire wound over a cylindrical drum. The model assumes a bilinear stress-strain behavior of the wire in the martensitic state (low temperature) and a linear elastic response in the austenitic state (high temperature). Based on simple equilibrium conditions, the model calculates the stress and strain distributions in the wire when subjected to a constant external backup force and undergoing frictional sliding forces at the contact with the drum. Closed-form expressions are supplied for the stroke produced by whatever actuator geometry and are validated numerically against finite element results. For a particular actuator configuration, the analytical forecasts are also checked experimentally on a proof-of-concept prototype. The analytical model shows that large strokes (up to one-half of the drum’s diameter) are achieved if the frictional coefficient is kept below 0.01. Rolling-contact architectures or sonic-pulse excitations of the drum are discussed as technical solutions to obtain such low friction values.

Effect of Stress, Heating Rate, and Degree of Transformation on the Functional Fatigue of Ni-Ti Shape Memory Wires

Shape memory alloys, particularly in the form of thin wires, are becoming increasingly attractive in the industrial field for the construction of compact actuators with high-power density. The structural and functional fatigue behavior of shape memory alloys undergoing thermomechanical cycling has been investigated only partially in the technical literature. In particular, the effects of operating parameters like the degree of martensite-austenite transformation and the heating rate on the fatigue life of the alloy have received very little attention so far. This paper explores the effect of these two parameters on the fatigue response of commercial SMA wires exposed to two linear stress-strain profiles during cycling. The results show the beneficial effects of partial transformation on the structural and functional life of the wires, with negligible loss of performance in terms of useful stroke. Though less markedly, the heating rate also has an effect on the structural and functional response, with the sine waveform supply performing better than the square profile.

Modelling, simulation and characterization of a linear shape memory actuator with compliant bow-like architecture

The design of shape memory alloy actuators typically compromises between force and stroke, the two properties being inversely proportional to one another for any given shape memory alloy element. This article presents a bow-like compliant actuator aimed at improving the specific performance of shape memory wires on both accounts. Conceptually, the actuator is formed by two straight elastic beams mutually hinged at the ends with a pre-stretched shape memory alloy wire in between. Heating of the alloy shortens the wire, which in turn makes the beams to buckle outwards in a symmetric double-arched configuration. The transverse displacement of the beams amplifies the contraction of the wire while producing a favourable output force. This article develops a simple, though accurate, analytical model of the actuator upon which a step-by-step design procedure is built. The theoretical findings are compared with the outcome of a finite element simulation for a case study and with the test data gathered from a physical prototype actuator.

Elastic compensation of linear shape memory alloy actuators using compliant mechanisms

This article presents a modular architecture for shape memory alloy actuators elastically compensated by thin beams loaded axially beyond their buckling limit. Starting from the exact equations for the elastic curve of the beams, an approximate procedure is developed for the engineering design of the entire compensating system. The theory of the compensator is validated successfully against a finite element model and experimental results. The experimental characterization of a complete prototype actuator shows that the forces generated by the compensated actuator are constant for both instroke and outstroke over the full range of displacements. The actuator concept proposed lends itself to modular assembly to multiply either the stroke covered (series combination) or the force generated (parallel combination).

Design of a Dielectric Elastomer Cylindrical Actuator With Quasi-Constant Available Thrust: Modeling Procedure and Experimental Validation

A novel design for a dielectric elastomer (DE) actuator is presented. The actuator is obtained by coupling a cylindrical DE film with a series of slender beams axially loaded beyond their buckling limit. Similar to previous published solutions, where different actuator geometries were coupled with compliant mechanisms of various topologies, the elastic beams are designed so as to provide a suitable compensating force that allows obtaining a quasi-constant available thrust along the entire actuator stroke. Whilst the elastic beam are sized on the basis of an analytical procedure, the overall system performance is evaluated by means of multiphysics finite element (FE) analysis, accounting for the large deflection of the buckled-beam springs (BBSs) and for the DE material hyperelasticity. Numerical and experimental results are finally provided, which demonstrate the prediction capabilities of the proposed modeling method and confirm that well-behaved cylindrical actuators can be conceived and produced.

Design and testing of an enhanced shape memory actuator elastically compensated by a bistable rocker arm

This article presents the design, the prototype construction, and the experimental testing of a shape memory actuator implementing the concept of elastic compensation put forward in a previous publication by the authors. A two-shape memory alloy actuator, compensated by a spring-assisted bistable rocker arm, is designed theoretically to provide nearly constant output forces and then it is built and characterized experimentally under laboratory conditions. The test results closely agree with the theoretical predictions and show that for given output force, the compensated actuator produces net strokes from 2.5 to 22 times greater than a twin uncompensated actuator. The stroke improvement increases dramatically with the generated output force. Weaknesses of the compensated design are the heavier average stress sustained by the shape memory alloy springs, which could impair the fatigue life, and a higher response time.

Functional Fatigue of NiTi Shape Memory Wires under Assorted Loading Conditions

The rational design of shape memory alloy (SMA) actuators requires reliable data on the fatigue strength of the material under cyclic thermal activation (functional fatigue). Test results on SMAs under functional fatigue are scarce in the technical literature and the few data available are mainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased by elastic springs or by another SMA element, their stress state is far from constant in operation. The mismatch between actual working conditions and laboratory arrangements leads to suboptimal designs and undermines the prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality. Four test procedures are planned, covering most of the typical situations occurring in practice: constant-stress, constant-strain, constant-stress with limited maximum strain and linear stress-strain variation. The paper describes the experimental apparatus specifically designed to implement the four loading conditions and presents fatigue results obtained from commercial NiTi wires tested under three of the four protocols.

Modelling and validation of a continuous rotary motor combining shape memory wires and overrunning clutches

This article presents the conceptual design, modelling, prototyping and testing of a novel rotary motor featuring shape memory alloy wires and overrunning clutches. The device comprises a shape memory alloy wire wound around a low-friction cylindrical drum contrasted by a backup beam spring and fitted to the output shaft through an overrunning clutch. Electrical heating produces a contraction of the wire, hence a rotation of the drum which is transferred to the shaft. Thanks to the overrunning clutch, during the recoiling phase, the drum rotates backward while the shaft does not move. Spurious backward movements of the shaft are contrasted by a second overrunning clutch linking the shaft to the frame. This article develops a model for the quasi-static simulation of the motor and the experimental characterization of a prototype device featuring three active drums, a rotary sensor and an angular brake to apply the external load. Despite the low degree of optimization, the tested motor performs well in terms of specific stroke, specific output torque and specific output work per cycle. Winding of the wire on the drum impairs somewhat the fatigue life with respect to publish data on straight wires, a drawback which calls for further design refinements.

Modelling and simulation of a compact push-pull rubber actuator energized by an outer coil of shape memory wire

ABSTRACT This paper introduces the concept and develops the theory for a push–pull actuator made by winding a thin shape memory wire on a solid rubber cylinder. The incompressibility of the rubber converts the thermomechanical contraction of the heated wire in twice as much axial strain available in the rubber core. The intrinsic elastic backup provided by the core allows push–pull action of the device. Based on the assumption of both material and geometric linearity, a simple theoretical model is developed, which culminates in a simple closed-form equation for the output stroke of the actuator. The theoretical predictions closely agree with refined finite element simulations, anticipating a net stroke of about 5–6% of the overall actuator length, depending on the outer load applied. The working principle of the actuator and the accuracy of the model are validated by tests on a proof-of-concept prototype.

Mechanical design of buckled beams for low-stiffness elastic suspensions: Theory and application

Axially compressed buckled beams have been used for several decades as elastic suspensions characterized by high static stiffness and low dynamic stiffness. The most comprehensive mathematical modelling of buckled beams is based on the elastica theory, a rational framework that seeks the equilibrium configuration for arbitrarily large deflections and rotations. The use of the elastica model is straightforward for analysis purposes but is rather awkward for design tasks because it requires handling of elliptic functions. This paper presents approximate equations developed from the elastica solution to facilitate the structural synthesis of buckled-beam suspensions starting from high-level engineering specifications. The step-by-step design procedure is illustrated by means of a case study and the theoretical predictions are validated against test data and finite element results.