Shape Memory Alloy Wire Actuators Research Articles

Dynamic tracking control of an SMA wire actuator based on model matching

Wires of shape memory alloys (SMAs) contract in response to heat, and are used as actuators because of this ability. However, as this deformation is accompanied by hysteresis, SMA actuators cannot be described as a linear time invariant (LTI) system. In this article, we demonstrate a method for modelling SMA wire actuators that is valid in the full displacement range, to be applied in linear control systems. To this end, we describe the response of the SMA actuators as a first-order lag system, using a model matching method. The time constant used in this first-order lag system is selected based on observations of the limited range below the austenite onset temperature. Analysis of the step response of the SMA actuator established that a PI controller alone is unable to approximate this first-order lag system properly. Hence, with our technique, a PI-controlled SMA actuator is considered as a plant, to which we apply H-infinity control design, which improves the approximation to the first-order lag system. This improvement was verified through a comparison of the transfer functions of the H-infinity-based system and a solely PI-controlled system. We also consider self-sensing by measuring the resistance of the SMA actuator directly, instead of using a load cell to observe the force induced by actuator deformation. Based on these results, we are able to demonstrate that the effect of self-sensing on the closed-loop characteristic is within the permissible range, suggesting that the creation of compact self-sensing actuators is viable.

An accurate dynamic model for polycrystalline shape memory alloy wire actuators and sensors

Thermal shape memory alloy (SMA) wires exhibit a mechanical hysteresis of which the shape depends on both temperature and loading rate. Commercially available actuator wires typically exhibit polycrystalline behavior, which also depends on training effects. Polycrystallinity may lead to complex hysteresis loops, differing substantially from standard box shapes often employed in modeling attempts. In addition, actuation often results in loading trajectories leading through the interior of the hysteresis, making accurate modeling and control of SMA systems a highly challenging task. In this paper, we present a novel dynamic model for polycrystalline SMA actuator wires based on a modified version of the Müller–Achenbach–Seelecke model. The model permits to predict time evolution of stress and resistance of a one-dimensional SMA wire under arbitrary input strain and Joule heating profiles. The constitutive equations are developed by properly exploiting the concept of a representative single-crystal, resulting in an optimal trade-off between physical interpretation and computational efficiency. After developing constitutive model equations, experimental validation is performed by means of two case studies, given by a superelastic NiTi wire and a quasi-plastic NiTi wire, respectively. The experiments are intended to illustrate the model capabilities in predicting internal hysteresis loops, loading rate effects, as well as actuation and sensing behavior at the same time. A remarkable accuracy is observed in all of the investigated experimental scenarios, making the model particularly suitable for high-precision control and self-sensing applications.

A novel variable-stiffness flexible manipulator actuated by shape memory alloy for minimally invasive surgery.

This article presents a novel variable-stiffness flexible manipulator for minimally invasive surgery. Each module of the proposed manipulator contains a variable-stiffness mechanism actuated by proactive deformation of shape memory alloy. Due to low driving current, apparent mechanical deformation, suitable phase transformation temperature and biocompatibility of shape memory alloy wire actuation, it is well suited for the manipulator applied in minimally invasive surgery, where variable stiffness is urgently required. In this article, the conceptual design, elastic modulus model, thermo-electric model, stiffness controlling method and finite element method simulation for a single module of the proposed variable-stiffness flexible manipulator are presented. Moreover, the memory shape setting experiment of shape memory alloy wire and fabrication of the single module are carried out. Finally, stiffness characterizations of the mechanism and the single module are studied separately, theoretically and experimentally.

Reducing the Power Consumption of a Shape Memory Alloy Wire Actuator Drive by Numerical Analysis and Experiment

The use of shape memory alloy (SMA) wire actuators for a moving body has some advantages over other conventional actuators because of the high specific power, small size, and simplicity of these actuators. However, the large power consumption of SMA wire actuators prevents its practical use. This paper shows how to optimize the input voltage of SMA wire actuators. First, a numerical actuator model is created from the measured data. Next, input voltage waves are optimized for three evaluation functions using multiobjective optimization: maximum input voltage, time-averaged actuators power consumption, and time-averaged bending moment (the output of actuator). The obtained numerical results show that a high maximum input voltage achieves efficient drive of SMA wire actuators. We also verified the tradeoff relations of the consumed power, maximum input voltage, and output from the SMA wire actuator under one condition of the multiobjective optimization. Experimental data also partially supported the tradeoff relations obtained by the multiobjective optimization and the numerical simulation.

High-speed and high-efficiency shape memory alloy actuation

When standard voltage levels commonly adopted in industry are used to activate thermal shape memory alloy (SMA) wire actuators, they often result in very high electrical currents which may eventually damage or destroy the actuators. To improve performance of SMA wire actuators operating in industrial environments, in this paper we investigate a novel, fast and energy-efficient actuation strategy based on short pulses in the millisecond range. The use of higher voltages leads to a highly dynamic activation process, in contrast to commonly used quasi-static activation based on low-voltage. A test setup is designed to examine the effects of the control parameters (i.e., supply voltage, activation pulse duration, SMA wire pre-tension and wire diameter) on the measured displacement and force output of the SMA wire. It is shown that actuation times in the range of 20 ms and strokes of more than 10% of the SMA wire length can be reached. Additionally, energy savings of up to 80% with respect to conventional quasi-static actuation are achieved. Possible applications for this activation method are release mechanisms, switches or safety applications.

Modeling SMA actuated systems based on Bouc-Wen hysteresis model and feed-forward neural network

Despite the fact that shape-memory alloy (SMA) has several mechanical advantages as it continues being used as an actuator in engineering applications, using it still remains as a challenge since it shows both non-linear and hysteretic behavior. To improve the efficiency of SMA application, it is required to do research not only on modeling it, but also on control hysteresis behavior of these materials which are the fundamentals of several research opportunities in this area. Having considered these requirements, we have introduced a mathematical model to describe the hysteresis behavior of a mechanical system attached to SMA wire actuators using Bouc-Wen hysteresis model and feed-forward neural network. Due to inability of linear mass-spring-damper equations of classic Bouc-wen model to explain the hysteresis behavior of SMA actuators, in this paper we have applied changes in the mentioned equations of classic Bouc-Wen model to describe hysteresis loops of model. We also have used flexibility of the neural network systems to describe Bouc-Wen output in the main equation. Parameters of the developed model have been trained for a real mechanical system using simulation data after selecting proper configuration for the selected neural network. Finally, we have checked the accuracy of our model by applying two different series of validation data. The result shows the acceptable accuracy of the developed model.

Stitched shape memory alloy wires enhance damage recovery in self-healing fibre-reinforced polymer composites

A major issue in composite technology is matrix micro-cracking due to low-velocity impact damage, which may severely limit service lifetimes of composite parts. In a novel approach, remarkable levels of healing of impact damage are obtained using shape memory alloy (SMA) wires to close longitudinal cracks in woven glass fibre-reinforced polymer plates with an epoxy-polycaprolactone (EP-PCL) matrix that shows dual-phase continuity. Thermal actuation of SMA wires stitched through the thickness of the stacked glass fibre plies introduces compressive loads to the cracks thanks to anchoring of the SMA loops at the fabric surfaces and debonding of the intervening threads, which prevents local deformation of the SMA, so that crack closure by about 200 μm is achievable. Concomitant expansion of the vascular network formed by the molten PCL fills the compressed cracks, resulting in highly effective healing on cooling, as demonstrated by C-scan images. Specimens stitched with SMA wires hence show almost complete healing, i.e. damage area recovery of 85%, after low-velocity impact at up to 17 J followed by heat treatment at 150 °C. This represents a 55% improvement over previous results for unstitched EP-PCL composites, and hence significantly greater degrees of healing than so far reported for this range of impact energies and this type of system.

Design and development of platform ankle rehabilitation robot with Shape Memory Alloy based actuator.

Various ankle rehabilitation robots have been developed in the recent decade to improve ankle strength and mobility and to alleviate the burdens on clinicians and physiotherapists. However, existing designs have stiff actuating mechanism, heavy and bulky electromagnetic or pneumatic actuators. This paper introduces ankle rehabilitation robot that utilizes Shape Memory Alloy (SMA) wire actuator to provide foot plantarflexion and dorsiflexion during rehabilitation exercise. SMA wire is a smart material that can contract when it is heated and expand back to its original shape when it is cooled. It also has high power to weight ratio. However, the SMA wire has low cooling rate thus limits its operating frequency. A periodical cooling mechanism is proposed here. Its efficacy is then evaluated against natural cooling and continuous forced convection. The experimental study shows promising results and demonstrates that periodical cooling has both the advantages of natural cooling and forced convection and can improve the cooling rate of SMA wire actuator.

Methods to Create and Control Shape Memory Alloy Wire Actuators as Reference Design for Makers

Methods to Create and Control Shape Memory Alloy Wire Actuators as Reference Design for Makers

Self-sensing Shape Memory Alloy wire actuator based on Unscented Kalman Filter

In this report, an Unscented Kalman Filter (UKF) model is developed for estimating the state of Shape Memory Alloy (SMA) wire actuator from its electrical resistance variation during phase transformation. The behavior of the spring biased SMA wire actuator is modeled using heat balance equation, constitutive behavior of SMA and the phase kinetics. Since the model comprises of non-linear differential equations, an Unscented Kalman Filter is developed to estimate the state of the SMA wire actuator from its noisy electrical resistance measurement. An experimental setup comprising of spring biased SMA wire actuator is used to evaluate the performance of the developed algorithm. The SMA wire actuator is heated electrically with different voltage profiles and the corresponding state of the SMA is estimated using UKF. Then the displacement of the spring is evaluated from the estimated state of SMA and is compared with the experimentally measured displacement. A qualitative agreement between the estimated and experimental displacement is observed. Furthermore, the performance of UKF is compared with the previously reported Extended Kalman Filter (EKF) model, which is based on the linearized version of the governing differential equations. It has been observed, that the UKF consumes lesser computational time than that of the EKF without any significant differences in the estimated responses.

Optimum placement of shape memory alloy wire actuator

Shape memory alloy wire actuators can be used in combination with compliant structures to attain desired force and displacement capabilities. The wires can be placed inside a matrix, as in composite, or outside the material connected at different points on the structure. In the latter case, the offset of the wire and the location of the points decides the overall deformation of the structure. In this article we study the effects of offset distance, and the number of points, called attachments, where the shape memory alloy wire is connected to a host beam. First the characteristic curve of the shape memory alloy wire actuator is derived from a constrained recovery model. Then the response of a beam model, undergoing large deflection due to follower forces, is superposed with the characteristic curve to obtain the maximum beam deformation. It is found that there exists a particular offset, called optimum offset, for which the deformation of the host is maximum. Moreover, the ratio of stress and change in strain in the shape memory alloy corresponding to the optimum offset, attains a particular value, irrespective of the flexural rigidities of the beam. Furthermore, it has been observed that for a set of beams that have flexural rigidity less than a particular value, the deformation increases with number of attachments. However, for the beams that have flexural rigidity more than that particular value, the deformation remains almost unaltered with number of attachments. These numerical results are also supported qualitatively by the experimental observations.

Characterization, modeling, and control of Ni-Ti shape memory alloy based on electrical resistance feedback

The use of shape memory alloy actuators has steadily increased within the fields of aerospace, robotics, and biomedical engineering due to their superior properties compared to other actuation systems. Position control of shape memory alloy actuators is difficult due to the highly non-linear behavior but has been well studied using numerous approaches. Electrical resistance can be used to estimate strain in shape memory alloy actuator wire due to a correlation between the two parameters. Previous models of this correlation are subject to one or more drawbacks such as being limited to a single applied load, not accounting for hysteresis effects, or applying only to a specific actuator size. This article presents a stress–strain–resistance model that accounts for varying applied load, major and minor hysteresis effects and is normalized in terms of actuator geometry. Results of simulation and a simple position control experiment are demonstrated, validating the performance of the model. Furthermore, a correlation between the model and an augmented version of the Liang and Rogers model is also presented.

Development of an extended Kalman filter for the self-sensing application of a spring-biased shape memory alloy wire actuator

This report presents the development of an extended Kalman filter (EKF) to harness the self-sensing capability of a shape memory alloy (SMA) wire, actuating a linear spring. The stress and temperature of the SMA wire, constituting the state of the system, are estimated using the EKF, from the measured change in electrical resistance (ER) of the SMA. The estimated stress is used to compute the change in length of the spring, eliminating the need for a displacement sensor. The system model used in the EKF comprises the heat balance equation and the constitutive relation of the SMA wire coupled with the force–displacement behavior of a spring. Both explicit and implicit approaches are adopted to evaluate the system model at each time-update step of the EKF. Next, in the measurement-update step, estimated states are updated based on the measured electrical resistance. It has been observed that for the same time step, the implicit approach consumes less computational time than the explicit method. To verify the implementation, EKF estimated states of the system are compared with those of an established model for different inputs to the SMA wire. An experimental setup is developed to measure the actual spring displacement and ER of the SMA, for any time-varying voltage applied to it. The process noise covariance is decided using a heuristic approach, whereas the measurement noise covariance is obtained experimentally. Finally, the EKF is used to estimate the spring displacement for a given input and the corresponding experimentally obtained ER of the SMA. The qualitative agreement between the EKF estimated displacement with that obtained experimentally reveals the true potential of this approach to harness the self-sensing capability of the SMA.

Self-sensing SMA Actuator Using Extended Kalman Filter and Artificial Neural Network

In this paper, self-sensing capability of Shape Memory Alloy wire actuator has been explored using Extended Kalman Filter assisted Artificial Neural Network. The change in length of a linear spring actuated using a Shape Memory Alloy wire is first estimated from the variation of its electrical resistance using Extended Kalman Filter. Though the estimation is qualitatively in agreement with the experiment, the quantitative mismatch makes it difficult to control the stretch of the spring solely based on the Extended Kalman Filter estimation. An Artificial Neural Network has been used to bridge the gap between the Extended Kalman Filter estimation and actual stretch of the spring. To evaluate the effectiveness of the Extended Kalman Filter based Artificial Neural Network model, the responses of the same are compared with that of the another Artificial Neural Network model, trained only using the experimental data. It has been observed that for the same number of neurons and same training data, Extended Kalman Filter based Artificial Neural Network model yields better result at higher frequencies.

A Measurement System for Shape Memory Alloy Wire Actuators

In this paper, an experimental set-up for the measurement of actuation properties of shape memory alloy wire actuators is presented. A test rig is designed and developed to characterise shape memory alloy wire actuators. The test rig is equipped with force, displacement and temperature sensors. The test rig is validated by measuring the strain of bi-atomic nickel–titanium shape memory alloy actuators by supplying various actuation currents. At 420 mA, the 0.15-mm-diameter nickel–titanium actuators underwent 3.25% strain which lies within the guideline range (3%-4%) reported by the manufacturer of the actuators.

A large-stroke shape memory alloy spring actuator using double-coil configuration

One way to increase the range of motion of shape memory alloy (SMA) actuators is to create displacements of the SMA associated with not only the deformation from straining but also rigid-body motion from translation and rotation. Rigid-body motion allows the SMA to create larger displacements without exceeding the maximum recovery strain so that the SMA actuators can have a larger shape recovery ratio. To improve the linear actuation stroke of SMA wire actuators, a novel SMA spring actuator is proposed that employs a double-coil geometry that allows the displacement of the SMA to be mainly induced by rigid-body motion. A double-coil SMA spring actuator is fabricated by coiling an SMA wire twice so that the double coiling results in a reduction of the initial length of the double-coil SMA spring actuator. The effects of the geometric parameters on the actuation characteristic of a double-coil SMA spring actuator are verified numerically by finite element analysis and experimentally according to a parametric study of the geometric parameters. The displacement-to-force profile of the double-coil SMA spring actuator is nonlinear, and the spring stiffness changes when the actuator transforms its configuration from a double-coil shape to a single-coil shape. According to the results of the parametric study, increasing the wire diameter increases both primary and secondary coil stiffness, and increasing the primary inner coil diameter decreases both primary and secondary coil stiffness, whereas increasing the secondary inner coil diameter decreases only the secondary coil stiffness. The result shows that one of the double-coil SMA spring actuators with an initial length of 8 mm has a recovery ratio of 1250%, while the recovery ratio of the single-coil SMA spring actuator with the same geometric parameters is 432%.

Time-delayed dynamic neural network-based model for hysteresis behavior of shape-memory alloys

Shape-memory alloys (SMAs) have received considerable amount of attentions for their engineering applications in recent years. The hysteresis in SMAs is sensitive to the state-varying tendency and frequency. Utilizing past information to estimate the hysteretic behavior gets increasing attention. In this paper, a time-delayed dynamic neural network (TDDNN) is proposed for modeling hysteresis of SMAs in online applications. By introducing a time delay between the input and output response, the TDDNN considers the time delay's effect on the hysteresis. This proposed network was applied to a SMA wire actuator. Experimental results demonstrate the effectiveness of TDDNN. The identified results obtained by TDDNN are better than those obtained by dynamic neural network without considering the delay information. It demonstrates the importance of introducing the time delay. The different values of time delay item can also affect TDDNN's identified results.

Feasibility of Shape Memory Alloy Wire Actuation for an Active Steerable Cannula

Needle insertion is used in many diagnostic and therapeutic percutaneous medical procedures such as brachytherapy, thermal ablations, and breast biopsy. Insufficient accuracy using conventional surgical cannulas motivated researchers to provide actuation forces to the cannula's body for compensating the possible errors of surgeons/physicians. In this study, we present the feasibility of using shape memory alloy (SMA) wires as actuators for an active steerable surgical cannula. A three-dimensional (3D) finite element (FE) model of the active steerable cannula was developed to demonstrate the feasibility of using SMA wires as actuators to bend the surgical cannula. The material characteristics of SMAs were simulated by defining multilinear elastic isothermal stress–strain curves that were generated through a matlab code based on the Brinson model. Rigorous experiments with SMA wires were done to determine the material properties as well as to show the capability of the code to predict a stabilized SMA transformation behavior with sufficient accuracy. In the FE simulation, birth and death method was used to achieve the prestrain condition on SMA wire prior to actuation. This numerical simulation was validated with cannula deflection experiments with developed prototypes of the active cannula. Several design parameters affecting the cannula's deflection such as the cannula's Young's modulus, the SMA's prestrain, and its offset from the neutral axis of the cannula were studied using the FE model. Real-time experiments with different prototypes showed that the quickest response and the maximum deflection were achieved by the cannula with two sections of actuation compared to a single section of actuation. The numerical and experimental studies showed that a highly maneuverable active cannulas can be achieved using the actuation of multiple SMA wires in series.

Design of an Optically Controlled MR-Compatible Active Needle.

An active needle is proposed for the development of magnetic resonance imaging (MRI)-guided percutaneous procedures. The needle uses a low-transition-temperature shape memory alloy (LT SMA) wire actuator to produce bending in the distal section of the needle. Actuation is achieved with internal optical heating using laser light transported via optical fibers and side coupled to the LT SMA. A prototype, with a size equivalent to a standard 16-gauge biopsy needle, exhibits significant bending, with a tip deflection of more than 14° in air and 5° in hard tissue. A single-ended optical sensor with a gold-coated tip is developed to measure the curvature independently of temperature. The experimental results in tissue phantoms show that human tissue causes fast heat dissipation from the wire actuator; however, the active needle can compensate for typical targeting errors during prostate biopsy.

Effect of stress on bandwidth of antagonistic shape memory alloy actuators

This article presents the experimental study on the influence of stress generated between the antagonistically connected shape memory alloy wire actuators over its bandwidth of operation. It is a well-known fact that behaviour of shape memory alloy is strongly coupled with temperature, stress and strain. An attempt is made to understand the effect of these factors on its bandwidth from control perspective through frequency response analysis. A phenomenological correlation is presented between the frequency response of antagonistic shape memory alloy and stress acting between the wires under different strains and temperatures. A clear demarcation between influence of temperature and stress over the operating frequency is discussed. Experimental result shows that apart from temperature, operating frequency of antagonistically connected shape memory alloy is also highly influenced by stress acting between the wires at higher temperatures. Also, the effect becomes much more significant under partial transformation strain. The result of the study is extended and demonstrated through tracking control of antagonistic shape memory alloy at a frequency of about 2 Hz.