Pseudoelastic SMA Spring Elements for Passive Vibration Isolation: Part II – Simulations and Experimental Correlations

Advances in smart materials and structures technology, especially in applications of Shape Memory Alloys (SMA) as actuators and vibration isolation devices require understanding of the nonlinear hysteretic response found in SMAs. SMA hysteresis can be modeled either through constitutive models based on physical material parameters or through models based on system identification. In this work, a simplified material model for the pseudoelastic response of SMAs is presented, suitable for vibration isolation applications. Response obtained from the simplified model is compared with the response obtained from an existing thermodynamic constitutive SMA model and the results from the two models are found to match well. The computation time required by the simplified model was approximately seven times faster compared with the thermodynamic constitutive model. The simplified model is utilized to simulate a single degree of freedom mass-SMA system where the SMA acts as a passive vibration isolation device, showing a substantial reduction in displacement transmissibility.

This paper presents the design, control and evaluation of a novel robotic finger actuated by shape memory alloy (SMA) tubes which intrinsically afford an internal conduit for fluidic cooling. The SMA tubes are thermomechanically programmed to flex the robotic finger when Joule heated. A superelastic SMA plate provides a spring return motion to extend the finger when cooling liquid is pumped through the internal channel of the SMA tube actuators. The mechanical design and nonlinear force controller are presented for this unique robotic finger. Sinusoidal and step response experiments demonstrate excellent error minimization when operated below the bandwidth which was empirically determined to be 6 rad s−1. Disturbance rejection experiments are also performed to demonstrate the potential to minimize externally applied forces. This method of internal liquid cooling of Joule heated SMA tubes simultaneously increases the system bandwidth and expands the potential uses of SMA actuators for robotic applications. The results show that this novel robotic finger is capable of precise force control and has a high strength to weight ratio. The finger can apply a force of 4.35 N and has a mass of 30 g. Implementing this design into wearable prosthetic devices could enable lightweight, high strength applications previously not achievable.

Shape memory alloys (SMAs) are adaptive materials that exhibit complex thermomechanical behaviors due to multiphysics coupling. The thermomechanical modeling of SMAs is a complex task due to several phenomena involved, and the Preisach model is an interesting alternative to describe the SMA hysteretic behavior based on experimental data. This paper deals with the description of the thermomechanical behavior of SMA using the Preisach model. Experimental tests are performed considering NiTi pseudoelastic wires subjected to different load conditions, establishing reference cases. Afterward, the Preisach model is employed to describe the SMA behavior. Numerical simulations are carried out and compared with the experimental data showing a good agreement. Other experimental data available in the literature are employed to investigate different macroscopic behaviors related to SMAs, including strain-temperature relations of wires and force-displacement relations of springs. Results show that the model is able to describe the thermomechanical behavior of SMAs, being in close agreement with experimental data. Preisach model has advantages such as a simple numerical implementation when compared to phenomenological and thermodynamic-based models, being an interesting approach useful for a wide range of applications that include different macroscopic behaviors.

This paper investigates the design of an iteration-based controller combined with feedback control to address the positioning error caused by hysteresis in a shape memory alloy (SMA) actuator. Without compensation, the positioning error due to hysteresis can be excessively large (greater than 20%), therefore limiting the performance of SMAs in positioning applications. An iterative controller based on the Preisach hysteresis model is used to compensate for the hysteresis. However, one of the major challenges with SMA at or near the phase-transition zones, from martensite to austenite and vice versa, is the output response is shown to be sensitive to small changes in the input. In practice, an iterative controller provides limited performance due to lack of robustness. It is demonstrated that combining a simple feedback controller with an iterative controller provides the needed robustness to enable high-precision tracking. Experimental results are presented that show maximum tracking error of 0.15% of the total displacement range—this value is approximately the noise level of the sensor measurement.

There are two different ways of using shape memory alloy (SMA) wire as an actuator for shape control of flexible structures: it can be either embedded within the composite laminate or externally attached to the structure. As the actuator can be placed at different offset distances from the beam, external actuators produce more bending moment and, consequently, considerabnle shape changes with the same magnitude of actuation force compared with the embedded type. Such a configuration also provides faster heat transfer rate owing to convection, which is very important in shape control applications that require a highfrequency response of SMA actuators. Although combination and physics-based modeling of externally attached SMA actuator wires and strips have been considered by many researchers, these studies have some drawbacks, which, if neglected, result in a number of errors in the theoretical results compared with experimental results. These shortcomings are considering the linear Euler-Bernoulli beam theory, not deriving the actuation force of the SMA from the constitutive equations, and taking into account the effect of only one SMA wire actuation force on, for example, the structure. These assumptions may lead to erroneous theoretical modeling results compared with experimental results. In this study, the aforementioned difficulties of attaching SMA actuators to the smart structures have been addressed. In other words, instead of linear beam theory, nonlinear beam theory is used in system modeling and, therefore, the proposed method and results are valid in large deflection and rotation behavior of beam. Also, in comparison to many other analyses that the effect of only one SMA wire is investigated, in the present research the effect of all active and inactive SMA wires is considered. Accordingly, the result of this paper can easily be generalized to the structure with several SMA wire actuations. Moreover, with the purpose of having practical applications in modeling and control, the heat transfer equations of all SMA wires are considered in the analysis and, as a result, the control inputs of the presented model are SMA wire electric currents rather than SMA temperatures. First, a flexible beam actuated by two active SMA actuators is modeled. Then, the Brinson constitutive equations and thermoelectric equations for SMA materials are coupled with the nonlinear beam behavior, and the coupled system of equations is numerically solved for some particular practical cases. Finally, the numerical results of the model simulation are verified against the experimental results using a test setup to validate the proposed model prediction. The implemented method used in this paper can be easily extended to the more complex smart structure with numerous externally attached SMA wires.

Two hysteresis models are investigated for modeling static hysteresis phenomenon in nonoriented magnetic steels. First, the static Preisach model is studied, and two types of experimental data, i.e., first-order reversal curves (FORCs) and concentric hysteresis loops, are employed to identify the weight function. Due to negative values in the obtained experimental weight functions, the resulting Preisach models show nonphysical behavior. To eliminate this nonphysical behavior, an optimization procedure is applied to remove these negative values. The nonphysical behavior is resolved in the modified Preisach model; however, the model still has poor accuracy for asymmetric loops. Alternatively, a new congruency-based static hysteresis model is proposed. In this model, third- or higher-order reversal curves are approximated using measured FORCs and second-order reversal curves (SORCs) by means of a congruency regularity. According to this regularity, any third- or higher-order reversal curve is congruent to SORCs with the same reversal point. A 3-D piecewise interpolation function is employed to interpolate the measured major loop, FORCs, and SORCs. The proposed model results are compared with quasi-static measurements. The comparison reveals that the model is capable of approximating the magnetization curves for the measurements containing complex-shaped B waveforms with a relative energy error of less than 4%.

Shape memory alloys (SMA) are more and more integrated in engineering applications. These materials with their shape memory effect permit to simplify mechanism and to reduce the size of actuators. SMA parts can easily be activated by Joule effect but their modelling and consequently their control remains difficult, it is principally due to their hysteretic thermomechanical behaviour. Another difficulty is that the characteristics of the material are time-varying, especially during cyclic loadings. So, most of successful control strategy applied to SMA actuator are particularly heavy and used the Preisach model or neural networks to model the hysteretic behaviour of these material but this kind of models are difficult to identify and to use in real time. That is why this study deals with an application of the new framework of model-free control and restricted model control applied to a SMA spring based actuator. This control strategy is based on new results on fast derivative s estimation of noisy signals, its main advantages are: its simplicity, its robustness and the fact that it is easy to compute. Experimental results and comparisons with PID control are exposed that demonstrate the efficiency of this new control strategy despite thermal perturbations.

Shape memory alloy (SMA) actuators’ unique features like compact design and high force to weight ratio attracts numerous position control applications. Due to the nonlinear behaviour and control difficulties SMA actuators are not widely employed in position control applications. In this work, a simple and easy to construct closing mode SMA wire actuated gripper is presented. The mechanical structure of the gripper includes a pair of jaws - one movable and other fixed, an SMA wire employed to close the jaw upon actuation and a torsion spring bias for the strain recovery in SMA wire. An experimental setup of the SMA gripper is established to perform the study. The SMA actuator is heated by an electric current. A non-contact laser displacement sensor is engaged to measure the position of the movable jaw. In order to control the position of the devised gripper, a model of the gripper is estimated from open loop experimental data using system identification technique. Experimental evaluation of manually tuned PI controller for SMA actuated closing mode gripper is compiled to track various stationary and variable (dynamic) trajectories to suit applications that can manipulate objects of varied dimensions. Control results demonstrate that the gripper is able to track the position rapidly and precisely.

Performing user experience (UX) and usability studies in VR to assess early product prototypes is common in the automotive and other sectors but has not yet been applied for the development of surgical instruments. For the development of an aspiration/irrigation instrument using a shape memory alloy (SMA) tube, we wanted to get an early user feedback of our prototype from medical staff. We conducted a study with 22 participants with a medical background and considered a potential presence bias of the UX and usability ratings. The results confirm the novelty and usefulness of a SMA tube and gave us valuable feedback for optimizing the shapeshifting mechanism.

Shape memory alloy (SMA) components under torsional loading have shown the ability to deliver near-constant forces over large working strokes, thus enabling them to find various engineering applications. In this work, a model to capture the torsional response of SMA components under both superelastic and shape memory effects is formulated. A three-species Gibbs-potential-based formulation is employed to separate the thermoelastic response of the SMA from its dissipative response. The dissipative part is then modeled with a discrete Preisach approach. The proposed approach combines the elegance of the thermodynamic-based approach with the algorithmic efficiency/simplicity of the Preisach model and thus providing an effective way in predicting complex SMA responses. The model is constructed directly in terms of experimentally measurable quantities: torque and angle of twist rather than estimating them by solving for non-homogeneous shear stresses across the specimen cross-section. The model is used to predict the torsional response of SMA components such as wires, rods and springs at different twists and temperatures. Models capable of predicting torsional responses directly in terms of torque and angle of twist at different temperatures or twists under superelastic and twinning response conditions would play a pivotal role in the design of many SMA devices from both structural and control systems standpoints.

Smart materials such as piezoceramics, magnetostrictive materials, and shape memory alloys exhibit significant hysteresis, especially when driven with large input signals. Hysteresis can lead to unwanted harmonics, inaccuracy in open loop control, and instability in closed loop control. The Preisach independent domain hysteresis model has been shown to capture the major features of hysteresis arising in ferromagnetic materials. Noting the similarity between the microscopic domain kinematics that generate static hysteresis effects in ferromagnetics, piezoceramics, and shape memory alloys, we apply the Preisach model for the hysteresis in piezoceramic and shape memory alloy materials. This paper reviews the basic properties of the Preisach model, discusses control-theoretic issues such as identification, simulation, and inversion, and presents experimental results for piezoceramic sheet actuators bonded to a flexible aluminum beam, and a Nitinol SMA wire muscle that applies a bending force to the end of a beam.

The ability of shape memory alloys (SMA) to respond to an external stimulus by modifying their dimensions can be used to generate motion or force in electromechanical devices and micro-machines. It has been often demonstrated that SMA-based devices are serious alternatives to conventional micrometric actuators. We have previously demonstrated that, using a high-quality position sensor, such as a linear variable differential transformer (LVDT), to provide the position feedback, accuracies about 3 μm in position control can be obtained. In this work, we present an actuator prototype based in a SMA wire, conceived to be used in lightweight applications, where the bulky position sensor previously used is replaced with a lighter alternative. The most convenient one, and also the most challenging, is to use the wire’s own resistance as a measure of its position, that is, to implement a sensorless control strategy. We propose to use a neural network to characterize the relation between the resistance of the wire and its strain and introduce this correspondence as the position feedback in a simple PID closed loop. The experimental results show that, in this way, accuracies about 70 μm can be obtained. The great advantage of this procedure is that the actuator is reduced to a single SMA element without any additional sensor, which is of great importance when the main goals are to reduce the overall weight, size, and cost of the actuator.

Innovative prestressing technique using curved shape memory alloy reinforcement

Shape Memory Alloy (SMA) based composites are unique because their static and dynamic characteristics can be controlled. Active modal modification and active strain energy tuning are two common techniques used to control the dynamics of SMA based composite materials. Using Active Modal Modification Technique (AMMT), the stiffness of the SMA composite can be modified by changing the temperature of the SMA. The stiffness change is a result of a reversible phase transformation, which is tempeature activated. The Active Strain Energy Tuning Method (ASETM) takes advantage of the SMA’s ability to impart forces to the structure thereby changing the stored strain energy within the composite. The result is a change in the modal characteristics of the composite material. This paper will present the results of the application of an AMMT used to modify the dynamic characteristics of an SMA composite. A rectangular example of quasi-isotropic SMA composite was used in this study. As a result of a temperature change, the stiffness of the composite changed. This change is stiffness altered the dynamic responses such as the resonant frequency of the composite. Time Average Opto-Electronic Holography (TAOEH) [6] was used to monitor the dynamic behavior of the structure. Analytical modeling was used along with the experimental results to obtain the temperature history of the composite as a result of the activation of the SMA.

This study investigates a novel timber dowel-type connection system using superelastic shape memory alloy (SMA) bar and tubes as dowels, in order to provide self-centering effect. Double-shear connections with SMA and mild steel dowels were tested under dynamic loadings at different displacement levels. The results showed that SMA dowel-type connections have good self-centering behaviours and can mitigate the residual deformation effectively compared with steel dowel-type connections after excessive deformation; although the steel dowel-type connections present higher strength. These tests reveal that the connection with tube dowels show higher equivalent viscous damping ratio than those use solid bar as tube would allow larger deformation to dissipate energy. To demonstrate application of the benefit of this system, an analytical model of a 3-storey timber framed structure was built for parametric study. The results showed that the structures with conventional dowel-type type connections exhibit large unrecoverable deformation after timber framed structures experience an earthquake. In comparison, those with the connections developed in this project show limited unrecoverable deformation due to the self-centering capacity of the connections.