Embedded Shape Memory Alloy Actuators Research Articles

Safe Supervisory Control of Soft Robot Actuators.

Although soft robots show safer interactions with their environment than traditional robots, soft mechanisms and actuators still have significant potential for damage or degradation particularly during unmodeled contact. This article introduces a feedback strategy for safe soft actuator operation during control of a soft robot. To do so, a supervisory controller monitors actuator state and dynamically saturates control inputs to avoid conditions that could lead to physical damage. We prove that, under certain conditions, the supervisory controller is stable and verifiably safe. We then demonstrate completely onboard operation of the supervisory controller using a soft thermally actuated robot limb with embedded shape memory alloy actuators and sensing. Tests performed with the supervisor verify its theoretical properties and show stabilization of the robot limb's pose in free space. Finally, experiments show that our approach prevents overheating during contact, including environmental constraints and human touch, or when infeasible motions are commanded. This supervisory controller, and its ability to be executed with completely onboard sensing, has the potential to make soft robot actuators reliable enough for practical use.

Inexpensive monolithic additive manufacturing of silicone structures for bio-inspired soft robotic systems

In soft robotics, the fabrication of extremely soft structures capable of performing bio-inspired complex motion is a challenging task. This paper introduces an innovative 3D printing of soft silicone structures with embedded shape memory alloy (SMA) actuators, which is completed in a single printing cycle from CAD files. The proposed custom-made 3D printing setup, based on the material extrusion (MEX) method, was used in conjunction with a cartesian pick and place robot (CPPR) to completely automate the fabrication of thick silicone skins (7 mm) with embedded shape memory alloy actuators. These structures were fabricated monolithically without any assembly tasks and direct human intervention. Taking advantage of the capability to 3D print different geometries, three different patterns were fabricated over the silicone skin, resulting in remarkable dynamic motions: an out-of-plane deformation (jumping of the structure from the x-y plane to the x-z plane) was achieved for the first-time employing silicone skin, to the best of the author’s knowledge. In addition, two process parameters (printing speed and build plate temperature) and the extruded silicone curing mechanisms were investigated to enhance the printing quality. This paper aims to advance the role of additive manufacturing in the field of soft robotics by demonstrating all the benefits that a low-cost, custom-made silicone 3D printer can bring to the table in terms of manufacturing soft bio-inspired structures.

Design, manufacturing, and testing of an adaptive composite panel with embedded shape memory alloy actuators

This article presents a novel design procedure leading to fabrication of a composite adaptive panel prototype with an embedded array of linear shape memory alloy actuators. This procedure allows determination of the panel’s radius of curvature as a function of the shape memory alloy actuators’ actuation temperature and panel configuration (number and orientation of plies, number and location of actuators, the materials’ properties, and the geometric arrangement). This design procedure integrates properties of the host structure (composite laminate) and of the actuators (shape memory alloy wires) in the framework of a unique design diagram. The performances of shape memory alloy actuators are obtained by experimental manipulations, whereas the rigidity of the host structure is calculated using the finite element numerical modeling approach. To validate the developed design procedure, a [90/90/90/wire/90] 425 × 425 × 5-mm adaptive panel prototype is fabricated using the vacuum-assisted resin transfer molding method. The results obtained from experimental tests are used to monitor the global performance of the adaptive panel and to validate the developed design procedure.

Adaptive Composite Panel with Embedded SMA Actuators: Modeling and Validation

This article focuses on the development of a finite elements model of an adaptive composite panel with embedded shape memory alloy actuators. It is firstly shown that a combination of shell, beam and link elements could be employed to model the panel. A simplified version of the Likhachev model is then employed as the constitutive relation for the actuators. Several experimental tests were successfully carried out on a prototype to validate the model and to demonstrate the possibility of controlling the panel shape. Further works should consider improving the technique employed for reading the actuators’ temperature during experimental tests.

Special issue on smart materials and structures

Smart materials and structures are inspired in nature and try to mimic adaptive characteristics of natural systems. In brief, it is possible to say that smart materials have special properties that couple mechanical and non-mechanical fields, conferring adaptive characteristics. Smart systems and structures use this kind of material as sensors and actuators. In general there are four major groups of smart materials: piezoelectric, shape memory alloys, magnetostrictive materials, electro-magneto rheological fluids. Nowadays, several fields of science and technology are exploiting the remarkable properties of smart systems and structures including bioengineering, aerospace engineering, robotics and vibration/shape control. Therefore, several applications have been developed, giving the increasing importance to this kind of system. In order to consolidate this new design paradigm in Brazil, several research groups from different Brazilian Universities and also international partners created the National Institute of Science and Technology of Smart Structures in Engineering (INCT-EIE) . Moreover, the Brazilian community created the Committee of Smart Materials and Structures of the Brazilian Society of Mechanical Sciences and Engineering (M&EInt/ABCM) . In order to celebrate the consolidation of this area in Brazil, we are producing this Special Issue on Smart Materials and Structures of the Journal of the Brazilian Society of Mechanical Sciences and Engineering. This special issue presents a general overview of the activities of the Brazilian community together with the international partners. Applications of piezoelectric materials and shape memory alloys are mainly discussed in the contributions. Kalamkarov and Savi (2012) presented a review of micromechanical modeling of smart composite structures reinforced by a periodic grid that can exhibit piezoelectric behavior. The asymptotic homogenization is employed. Trindade and Benjeddou (2012) developed a parametric analysis of effective material properties of thickness-shear piezoelectric macro-fibre composites. Finite element homogenization method is employed evaluating the influence of several parameters on material properties. Medeiros et al. (2012) presented a procedure to evaluate effective properties on smart composite materials with piezoelectric fibers. Finite element method is applied for unidirectional periodic piezoelectric composites. Nitzsche (2012) discussed the realization of semi-active actuators employed for application in structural control. The general idea is applied for an actuator using piezoelectric material as adaptive material. Motter et al. (2012) discussed vibration-based energy harvesting employing piezoelectric materials. Different rectifier circuits are of concern, analyzing numerical and experimental results. Abreu et al. (2012) presented a discussion about active vibration control of a cantilever beam using a model-based digital controller and piezoelectric sensor and actuators. The main focus of the paper is the design of the controller, showing experimental results that assure its efficacy. Martins et al. (2012) treated the structural health monitoring for aeronautical applications by using piezoelectric materials. Electromechanical impedance measures are employed to perform nondestructive inspection. The paper discussed a case study of an aluminum aircraft window with satisfactory results. De Paula et al. (2012) investigated the nonlinear dynamics of large-scale space structures with embedded shape memory alloy actuators. The investigation considered an archetypal model composed of a single mass connected by SMA elements. This system has a complex behavior including chaos. The paper discussed some aspects of the system dynamics giving special attention for the influence of geometrical imperfections. Lima et al. (2012) discussed the control of a beam using shape memory alloys as actuators. A PI controller is employed to an experimental rig showing its applicability for the deformation control.

Model-Based Control of SMA Actuators with a Recurrent Neural Network in the Shape Control of an Airfoil

This paper describes the development of a neural network control for shape memory alloy (SMA) actuators as well as the control tests. Precise control of SMA actuators that are integrated in the composite structure is difficult because of the hysteretic behaviour of SMA and time delays in the control system. The weakness of the static model of SMA is that it does not take into account the actuator dynamics. The NARMA-L2 neural network model enables us to solve the inverse dynamic model of the nonlinear discrete-time dynamical system. In this study the NARMA-L2 model of the airfoil (a cross section of wind turbine blade) was created using Matlab Simulink software and neural networks. Data for network training was measured from the specimen. The performance of the constituted NARMA-L2 controller was tested in control tests. According to the control tests the NARMA-L2 controller is well suited for controlling embedded SMA actuators. The constituted controller enabled fast and stable actuation in step responses and it was also able to compensate for changes in cooling conditions.

Development of a Preliminary Model-Scale Adaptive Jet Engine Chevron

Reduction of jet noise continues to be an important research topic. Exhaust-nozzle chevrons have been shown to reduce jet noise, but parametric effects, including immersion amount and azimuthal distribution, are not well understood. Additionally, thrust loss due to static chevrons at cruise suggests a significant benefit from deployable chevrons. The focus of this study is the development of an adaptive-chevron concept for the primary purpose of parametric studies for jet noise reduction in the laboratory and secondarily for development of technology that can be leveraged for full-scale systems. The adaptive-chevron concept employed in this work consists of a laminated composite structure with embedded shape memory alloy actuators. The actuators are embedded on one side of the middle surface such that joule heating of the actuators causes them to attempt recovery of prestrain, thereby generating a moment and deflecting the structure. A brief description of the chevron design is given followed by details of the fabrication approach. Results from bench-top tests are presented and correlated with numerical predictions from a model for such structures that was previously implemented in MSC.Nastran and ABAQUS. Excellent performance and agreement with predictions is demonstrated. Results from tests in a representative flow environment are also presented. Excellent performance is again achieved for both open- and closed-loop tests, the latter demonstrating control of deflection to a specified immersion into the flow. The actuation authority and immersion performance is shown to be relatively insensitive to nozzle pressure ratio. Very repeatable immersion control with modest power requirements is demonstrated.

Modelling, simulation and experiments of novel planar bending embedded SMA actuators

The new type of structure named planar bending embedded shape memory alloy actuator is developed. Two SMA wires are embedded in parallel with the axis of the elastic rod. The recovering wire, which is superposed along rod’s axis, is set to obtain “U” memory shape and the restoring wire, with straight memory shape, is placed off-axially. The differential strain gauges are located at suitable position in corresponding to the actuator’s bending direction in order to measure the signal of displacement. The mathematic model obtains both for simulation and control system analysis of the actuator, inherently non-linear device exhibiting significant major and minor hysteresis loops. The model calculates martensite fraction using two exponential equations, one for heating and the other for cooling. By making use of continuity, common origin and common limit conditions and adjusting martensite fraction coefficients appropriately, the model can adequately account for the presence of minor hysteresis loops. Finally, the experimental and simulated results of step response using two-stage hybrid controller prove the validity of the non-linear model.

Research on novel shape memory alloy multi-fingered humanoid hand

Two new type actuators named planar bending embedded shape memory alloy actuator (ESMAA) and spacial bending ESMAA were presented. Using shape memory effect of SMA wires and elasticity of room temperature vulcanization silicone rubber, the ESMAAs could output force and bend displacement steadily. Furthermore, a novel three-fingered humanoid robot hand consisting of six ESMAAs was developed first. The structural parameters of the actuators, such as rod's radius, wire's radius, wire's recoverable curvature, and offset distance were optimized by combining analytical model with experimental results. On the basis of bionics, the lengths of hand's knuckles were determined through statistical research on the configuration of human beings' hands. The locations of fingers were carefully chosen through a volume optimal index. It is shown in the experiments that maximum angle between ends' tangents of each finger are 68.5°, 79°, and 79°, respectively. The tip of each finger could reach its final position approximately at the same time and by controlling the bending of each finger, the hand could accomplish fine manipulation like that of a human being.

Nonlinear model accounting for minor hysteresis of embedded SMA actuators

A quantitative index martensite fraction was used to describe the phase transformation degree of shape memory alloy (SMA). On the basis of the martensite fraction, a non-linear analysis model for major and minor hysteresis loops was developed. The model adopted two exponential equations to calculate the martensite fractions for cooling and heating, respectively. The martensite fractions were derived as the relative parameters were adjusted timely according to continuous, common initial and common limit constraints. By use of the linear relationship between the curvature of embedded SMA actuator and SMA’s martensite fraction, the curvature was determined. The results of the simulations and experiments prove the validity and veracity of the model.

Research and application of novel planar bending embedded shape memory alloy actuator

To overcome low-response speed and low-control precision in the existing traditional shape memory alloy (SMA) actuators, a new type of structure named planar bending embedded SMA actuator was developed. Two SMA wires were embedded in parallel with the axis of the elastic rod. The recovering wire, which was superposed along rod's axis, was set to obtain ‘U’ memory shape and the restoring wire, which was placed off-axially, got straight memory shape. The differential stain gauges were located at suitable position in corresponding to the actuator's bending direction in order to measure the signal of displacement. By making use of continuity, common origin and common limit conditions, and adjusting martensite fraction coefficients appropriately, the analytical model was deduced to adequately account for the presence of major and minor hysteresis loops. The structural parameters of 60 mm long actuator, such as rod's radius, wire's radius, wire's recoverable curvature, and offset distance, were optimized by combining analytical model with experimental results. The experimental results prove the merits in optimal prototype.

Smart composites with embedded shape memory alloy actuators and fibre Bragg gratingsensors: activation and control

This paper describes the production of an adaptive composite by embedding thinpre-strained shape memory alloy actuators into a Kevlar-epoxy host material.In order to combine the activation and sensing capabilities, fibre Bragg gratingsensors are also embedded into the specimens, and the strain measured in situduring activation. The effect of manufacturing conditions, and hence of the initialstress state in the composite before activation, on the magnitude of the measuredstrains is discussed. The results of stress and strain simulations are comparedwith experimental data, and guidelines are provided for the optimization of thecomposite. Finally, a pilot experiment is carried out to provide an example of how astrain-stabilizing feedback mechanism can be implemented in the smart structure.

Behavior of Concrete Driven by Uniaxially Embedded Shape Memory Alloy Actuators

The behavior of a concrete specimen uniaxially embedded with a shape memory alloy (SMA) wire actuated by electrical current is studied in this paper. The main factors influencing the axial strain of the concrete specimen were examined through a sophisticated experimental program. The experimental results indicate that the SMA actuator can be employed as a way of prestressing for axial concrete specimens and can be used for controlling axial strain of concrete specimens. Also the influences of initial prestrain of SMA wires, modes activating electrical current, the actuation times, etc., on the axial strain behavior of concrete specimen are significant.

A novel robot hand with embedded shape memory alloy actuators

This paper describes the development of an active robot hand, which allows smooth and lifelike motions for anthropomorphic grasping and fine manipulations. An active robot finger 10mm in outer diameter with a shape memory alloy (SMA) wire actuator embedded in the finger with a constant distance from the geometric centre of the finger was designed and fabricated. The practical specifications of the SMA wire and the flexible rod were determined on the basis of a series of formulae. The active finger consists of two bending parts, the SMA actuators and a connecting part. The mechanical properties of the bending part are investigated. The control system on the base of resistance feedback is also presented. Finally, a robot hand with three fingers was designed and the grasping experiment was carried out to demonstrate its performance.

Control of natural frequencies of a clamped–clamped composite beam with embedded shape memory alloy wires

In this study, an analytical model for the evaluation of natural frequencies of glass fibre composite beams with embedded shape memory alloy (SMA) wires, called “SMA composites” is presented. The beams were clamped at both ends and different numbers of SMA actuators were embedded at an interlayer of the composite beams. The changes of tensile modulus, internal recovery stress and strain, and stresses due to the thermal expansion of the beams and wires were considered in the study. The natural frequencies evaluated from the analytical model of the SMA composite beams compared well with experimental measurements. In the study, it was found that the natural frequencies of composite beams decreased with increasing the number of embedded SMA actuators at a temperature below martensite finish temperature, M f. Although the modulus of the SMA material was relatively higher than glass fibre composites, the decrease of the natural frequencies was due to the increase of the overall density of the SMA composite beams. However, at a temperature above austenite finish temperature, A f, the natural frequencies of the beams with low SMA wire fraction were initially decreased. The frequencies were then increased with continuously increasing the number of SMA wires. These phenomena agree well with the experimental observations.

Thermal Buckling of Laminated Composite Beams with Embedded Shape Memory Alloy Actuators

In this paper, the thermal buckling and postbuckling behaviours of a composite beam with embedded shape memory alloy (SMA) wires are investigated experimentally and analytically. For the purpose of enhancing the critical buckling temperature and reduction of the lateral deflection on thermal buckling and postbuckling, the characteristics of thermal buckling are investigated through the use of the shape recovery force. The results of thermal buckling tests using uniformly heated and clamped composite beam specimens with embedded SMA wire actuators are discussed. The temperature-load-deflection behaviour records present quantitatively how the shape recovery force affects the thermal buckling behaviour. For this experiment, we considered the initial geometric imperfections, the slenderness ratio of the beam and the embedding position of the SMA wire actuators. The experimental results show that the shape recovery force reduces the thermal expansion of the composite laminated beam. This results in an increase of the critical buckling temperature and a reduction of the lateral deflection of the beams.

Thermal buckling and postbuckling analysis of a laminated composite beam with embedded SMA actuators

In this paper, the thermal buckling and postbuckling behaviours of a composite beam with embedded shape memory alloy (SMA) wires are investigated analytically. For the purpose of enhancing the critical buckling temperature and reducing the lateral deflection for the thermal buckling, the characteristics of thermal buckling are investigated through the use of the shape recovery force associated with SMA wire actuators. The results of both thermal buckling and postbuckling behaviours present quantitatively how the shape recovery force affects the thermal buckling behaviour. The analytical results show that the shape recovery force reduces the thermal expansion of the composite laminated beam, which results in both an increment of the critical buckling temperature and also a reduction of the lateral deflection of postbuckling behaviours. A new formula is also proposed to describe the critical buckling temperature of the laminated composite beam with embedded SMA wire actuators.

Some Durability Issues Associated with the Use of Shapememory Alloy 'Smart' Composites

AbstractThis paper presents the results from an experimental study on the durability of embedded Shape-Memory Alloy (SMA) hybrid composites. It shows that the cyclic actuation of such composites can be degraded due to the accumulation of internal damage within the host composite structure. This damage results from thermal effects within the host composite, and from mismatch in the strain capacities of the actuators and structural composite. The implications of this data for the design of embedded SMA actuator 'smart' composites is discussed.

Formulation of a Mechanical Model for Composites With Embedded SMA Actuators

Shape memory alloy reinforced composites are an extremely versatile class of materials with adaptive characteristics that may be exploited for damage control, active structural acoustic control, dynamic tuning, and shape control. In order to design structures reliably with embedded shape memory alloy actuators, a fundamental mechanical model must be formulated to predict the materials’ variable stiffness, induced moments and loads, and the change of curvature that can result. The mechanical model described in this paper is formulated from the micromechanical behavior of the highly nonlinear shape memory alloy actuators and classical lamination theory. The behavior of typical shape memory alloys will be presented and then used in formulating a one- and two-dimensional model and the results will be applied to plate theory.