Antagonistic Shape Memory Alloy Research Articles

Controlling the Deformation of the Antagonistic Shape Memory Alloy System by LSTM Deep Learning

The antagonistic system of two shape memory alloy wires is a great inspiration for the robotics field where it is applied as a linear actuator due to its shape memory effect. However, its control is still a challenge due to its hysteresis behavior. For that reason, a new controller is proposed in this paper for the displacement of the system’s effector. It is based on a Long Short-Term Memory neural network model. The aim is achieved by combining temperature-deformation data from an analytical model with voltage-temperature-deformation data from real experiments. Hence, these datasets are studied to overcome the nonlinearity obstacle of this system in order to be able to integrate it into robotic applications.

Performance analysis of position sensorless control of antagonistic shape memory alloy actuators

Abstract This paper presents a comprehensive performance analysis of position sensorless control of antagonistic shape memory alloy actuators (ASMAA), focusing on enhancing the sensorless position sensing capabilities and servo performance. The mechanisms and influencing factors of ASMAA self-sensing characteristics are comprehensively analyzed. A self-sensing model framework based on the constitutive model of ASMAA is proposed, and a self-sensing model based on differential resistance feedback is developed. An experimental platform based on the ASMAA motion mechanism is established to test and analyze SMA wires of two different materials under various pretension force conditions. The experimental results show that the antagonistic configuration mitigates hysteresis and improves linearity. A simple polynomial fitting is employed to establish the resistance-displacement relationship, achieving good accuracy with minimal residuals. The system identification is employed to avoid the parameter complexity and time-varying in the constitutive model. Based on the identified transfer function, a traditional PID controller is designed for position sensorless servo control. However, the inherent nonlinear hysteresis of ASMAA is difficult to suppress using a linear PID controller. Furthermore, a compound control strategy based on the Duhem model and PID controller is proposed, which utilizes the particle swarm optimization (PSO) algorithm to identify the Duhem inverse model controller. Experimental results demonstrate that the compound controller significantly enhances position tracking accuracy and response speed compared to a standalone PID controller.

Finite‐Element Analysis of an Antagonistic Bistable Shape Memory Alloy Beam Actuator

A finite‐element (FE) analysis of the active bistability of an antagonistic shape memory alloy (SMA) beam actuator of TiNiCu is presented. The actuator comprises two coupled SMA beams that are clamped at both ends and coupled in their center by a spacer having different memory shapes being deflected in opposite out‐of‐plane directions. The actuator is characterized by two equilibrium positions. To determine bistable behavior as a function of geometrical parameters, a force criterion is defined by the coupling force of the beams in austenitic and martensitic states. Bistable behavior is achieved, if the coupling force does not change sign in the entire displacement range. This implies that the austenitic beam dominates the opposing martensitic beam. Thus, selective heating of the SMA beams results in a snap‐through motion of the coupled SMA beams. Depending on which of the two beams is in austenitic state, either of the two equilibrium positions is reached without the need for an external force. It is demonstrated that geometrical parameters like initial predeflection and spacer length have a crucial effect on the bistable performance. Bistable regions as well as critical limits characterized by geometry‐dependent stability ratios, beyond which the actuator's performance becomes monostable, are identified.

Morphing flap driven by link with antagonistic shape memory alloy wires

Abstract This paper describes a study of a morphing flap using antagonistically arranged shape memory alloy (SMA) wires as actuators. The main focus of this study is to examine how existing aircraft structures and mechanisms can be modified to make the morphing airfoil more practical. The first step is to consider the drawbacks of existing morphing airfoils and then examine the advantages of the morphing airfoil proposed in this study. Then, a morphing airfoil is proposed to realize four significant technical issues: weight reduction, compatibility with existing structures, securing internal space, and seamless skin made with shape memory polymer (SMP) films. First, elemental experiments were conducted to examine the feasibility of the mechanical design, and the feasibility of the design of a large-morphing airfoil operating test model was confirmed. Next, aerodynamic and structural-mechanical analyses are conducted while designing the experimental operating model to verify whether it could operate up to the target flap angle under simulated flight conditions. After completing the experimental model, actuation tests are conducted as a morphing wing and confirm whether the wing could operate at the target flap angle in a realistic environment. As a result, the two-way morphing flap angle was experimentally observed. It is noted that only aerodynamic force was not considered in the experiment, which is further investigated in wind tunnel tests in future study. In addition, control rate of the proposed system seems improved than conventional model because of the reduction of the wire length of the actuation system. Thus, the results obtained in this study suggested the proposed link-driven SMA wire mechanism can be applied to make the morphing airfoil more practical.

A spray-cooling based antagonistic SMA actuator (SCASA) with low resistance consumption and high driving frequency

Owing to the large strain output and high power-to-weight ratio, using temperature-induced shape memory alloy (SMA) springs in the form of antagonistic actuators offers the opportunity to develop simple, lightweight, and multi-mode robotic systems. Currently, the capabilities and deep application of these robotic systems are hindered by the relatively large resistance consumption and limited driving frequency of the antagonistic SMA actuators, primarily attributed to the cooling rate of SMA. In this paper, a spray-cooling based antagonistic SMA actuator (SCASA) was proposed, aiming to address the existing challenges in antagonistic SMA actuators. Theoretical modeling of the SCASA was comprehensively investigated. Experimental findings highlight the superior cooling efficacy of the spray-cooling method, attaining a cooling rate surpassing 100 °C per second for a single SMA spring. Using the spray-cooling based method, the driving frequency of a single SMA spring is approximately twice that of the forced-air cooling method. Experimental results also demonstrate the superior performance of the SCASA using the spray-cooling method, resulting in a reduction of approximately 50% in resistance consumption and an increase of approximately 40% in driving frequency compared to the forced-air method. This work elucidates the promising application prospects of the spray-cooling method in SMA actuators.

Bi-direction and flexible multi-mode morphing wing based on antagonistic SMA wire actuators

This work evaluates the viability of a cutting-edge flexible wing prototype actuated by Shape Memory Alloy (SMA) wire actuators. Such flexible wings have garnered significant interest for their potential to enhance aerodynamic efficiency by mitigating noise and delaying flow separation. SMA actuators are particularly advantageous due to their superior power-to-weight ratio and adaptive response, making them increasingly favored in morphing aircraft applications. Our methodology begins with a detailed delineation of the fishbone camber morphing wing rib structure, followed by the construction of a multi-mode morphing wing segment through 3D-printed rib assembly. Comprehensive testing of the SMA wire actuators’ actuation capacity and efficiency was conducted to establish their operational parameters. Subsequent experimental analyses focused on the bi-directional and reciprocating morphing performance of the fishbone wing rib, which incorporates SMA wires on the upper and lower sides. These experiments confirmed the segment’s multi-mode morphing abilities. Aerodynamic assessments have demonstrated that our design substantially improves the Lift-to-Drag ratio (L/D) when compared to conventional rigid wings. Finally, two phases of flight tests demonstrated the feasibility of SMA as an aircraft actuator and the validity of flexible wing structures to adjust the aircraft attitude, respectively.

Systematic Methodology for an Optimized Design of Shape Memory Alloy‐Driven Continuum Robots

Continuum robots stand out due to their high dexterity, which allows them to effectively navigate in confined spaces and dynamic environments. At present, motor‐controlled tendons represent the most prominent actuation method used in continuum robots which require large drive units, increasing the overall system size and weight. A potential alternative technology to overcome those limitations is represented by shape memory alloy (SMA) wire actuators, which are characterized by extremely high energy density and flexibility, leading to a reduction of the size, weight, and design complexity of continuum robots. The complex thermomechanical behavior of SMA wires, however, makes the design of SMA‐based applications a challenging task, and systematic approaches to design SMA‐driven continuum robots are poorly understood. To overcome this issue, this article presents a novel systematic methodology for designing SMA‐driven continuum robots capable of motion in a three‐dimensional environment. First, the kinematic relationship between SMA wires and continuum robot deformation as well as the required actuator force in quasi‐static conditions, is mathematically described based on the assumption of a constant curvature deformation. Subsequently, the model is validated by in‐plane experiments for different design parameters. Based on the results, a fully integrated, antagonistic SMA continuum robot is built and validated.

Design and Control of Turtle Locomotion-Inspired Robots Actuated By Antagonistic Shape Memory Alloy Springs

Inspired by turtle locomotion, a novel type of robot with flippers actuated by antagonistic shape memory alloy (SMA) springs is presented that can crawl based on two different variable friction mechanisms. First, unlike robots incorporating a variable friction mechanism based on weight, the turtle-inspired robot bends its flippers through “ <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on-off”</small> control of the SMA springs in an open-loop system and achieves high friction based on the electromagnet. It can crawl on a vertical surface under a load with a load ratio of 78.5% without accurate locomotion control. To achieve fast and accurate displacement for each locomotion sequence, a radial basis function (RBF) compensator based on the minimum parameter algorithm and a mechanical model of the flippers is used to control the antagonistic SMA springs. The robot can regulate the bending shape of its flippers using an RBF compensator and climb a nonmagnetic surface with an inclination angle of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$72^\circ $</tex-math></inline-formula> when it utilizes suction units in the foot for its variable friction mechanism. The robot can crawl forward along both the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> - and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> -axis directions. In conclusion, antagonistic SMA actuation with an RBF compensator provides fast and accurate performance and is a feasible alternative to the disturbance of the control of the flippers. This article can be used to develop design guidelines for crawling robots.

Numerical simulation and experimental comparison of antagonistic shape memory alloy linear actuator for automotive applications

This paper introduces several commonly used constitutive models of SMA materials, expounds the relationship between material temperature, stress and strain from the perspective of one-dimensional model, and establishes the simulation model of reciprocating motion of SMA actuator based on the material parameters obtained from quasi-static mechanical property experiments in the previous paper. A double wire pull SMA actuator is designed and a prototype is made. The output displacement data obtained from the experiment is compared with the established simulation model to prove the accuracy of the simulation process.

Analyzing the effect of parametric variations on the performance of antagonistic SMA spring actuator

This paper presents a mathematical model of a shape memory alloy (SMA) spring actuator in an antagonistic configuration to determine the work generation potential. As SMAs are observed as configuration-dependent, traveled path history becomes an important parameter to be considered in the mathematical model. Moreover, in most real-world applications, loads are found to be unpredictable and fluctuating in nature, which results in an incomplete transformation of the material. Based on the above observations, for the first time in the published literature, the current work considers these two parameters - loading history and arbitrary loading in the mathematical model for SMA antagonistic springs. The model comprising of heat dynamics, constitutive model, and phase kinetics with loading history is implemented under normal as well as arbitrary loading conditions. Performance of antagonistic SMA spring actuator is investigated for its force generation capability and displacement, considering its material properties, geometrical parameters, and applied input voltage. At least three investigations are carried out for each parametric variation. Simulation results are found to be qualitatively in agreement with the published literature. Higher force generation capability of antagonistic SMA spring pair is reported under arbitrary thermo-mechanical loading conditions. It is concluded that the geometrical parameters of SMA spring such as the number of turns, diameter of spring wire, and spring index significantly affect the performance of the antagonistic spring pair than the material parameters.

High‐Speed Antagonistic Shape Memory Actuator for High Ambient Temperatures

This work presents the development of an innovative shape memory alloy (SMA) actuator principle, which allows high‐speed switching cycles through the decoupling of antagonistically arranged SMA wires. Being optimized for the use at high ambient temperatures up to 65 °C, a possible application area is the active venting of injection molds where it can be used to expel air, which is trapped during the injection mold process. The patented actuator principle is based on a decoupled agonist–antagonist SMA‐spring system and allows a high‐speed closing movement by a compact and lightweight design. Another innovation compared to conventional antagonistic SMA actuator systems is the integrated fail‐safe mechanism, which guarantees a defined closed state in case of power failure. Subsequently, in the motivation the need for active venting valves for injection molding is first described. Second, the novel actuator principle is introduced, and the development of an electronics concept is discussed. Finally, the design process, assembly, and validation of two iterations of the actuator prototype are presented. The final prototype validation measurements showcase high performance by valve strokes of 1 mm within 100 ms at ambient temperature of 65 °C.

Deep Neural Network-Based Physics-Inspired Model of Self-Sensing Displacement Estimation for Antagonistic Shape Memory Alloy Actuator

The paper introduces a self-sensing feature that utilizes differential resistance measurement of an antagonistic Shape Memory Alloy (SMA) actuator to estimate linear displacement. The external position sensor used for control feedback becomes extraneous while utilizing an electrical resistance as feedback. The self-sensing capability provides additional advantages such as the overall reduction in size, weight, and interface complexity of an actuator. SMA actuator wires were used in the antagonistic configuration for the bi-directional actuation of the targeted application. A Deep Neural Network (DNN) having Long Short Term Memory (LSTM) layers were used to estimate the dynamic relation of resistance and displacement. A novel DNN model was developed for estimation purposes inspired by the physics-based description of the SMA actuation phenomenon. Accordingly, DNN was developed using the first and last layers as LSTM and a middle layer of feedforward neural network. Estimation results are presented with performance evaluation in terms of the R-squared index and mean absolute error (MAE) of the proposed model; also, the model has been compared with the generic LSTM 2-layered model.

Performance analysis of constant current heated antagonistic shape memory alloy actuator using a differential resistance measurement technique

This work presents the development of a precise, constant current heating mechanism for an antagonistic shape memory alloy (SMA) actuator. The actuator was developed using a pair of SMA wires arranged in an antagonistic configuration. SMA possesses a unique phase-dependent, resistance variation property which is called self-sensing. This phenomenon is observed during thermal phase transition. A constant heating current was employed to measure combined differential resistance (ΔR) which provides insignificant hysteresis and linear relationship with displacement. ΔR eventually helps to determine the present position of the actuator for sensorless feedback control. The aim is to remove additional external sensors, reducing actuator footprint and interface complexity using the proposed study. The performance analysis of the actuator was evaluated under constant current by the tracking trajectory of reference signals. The tracking results confirmed the improvement in operating bandwidth by a reduction in displacement. The heating module mainly consisted of a low pass filter, operational amplifier with a current sense feedback mechanism that regulates the heating current in proportion to PWM signals. The result shows a significant 21% variation in the observed value of ΔR (1.200–0.254 Ω) between the major–minor loops. The study confirms linearity and maintains similarity by highest correlation 0.9508 during open-loop, which further improves to 0.9891 in close feedback reference tracking with an error band ±0.05 mm.

Differential resistance based self-sensing recurrent neural network model for position estimation and control of antagonistic Shape Memory Alloy actuator

The paper presents the development and experimental investigation of recurrent neural network (RNN) based self-sensing position estimation (SSPE) model for shape memory alloy actuator (SMA). RNN is used as an estimator in the position feedback control loop to replace the external additional position sensor. The model was inspired by the physics-based analogy of the Mass-Spring-Damper (MSD) system for the antagonistic SMA wire actuator. Actuator displacement presents the hysteresis and non-linear dynamic relationship with observed differential resistance (sensing signal) during phase transformation. The resistance variation causes due to dissimilar resistivity between primary phases of SMA material. The RNN based estimation model was considered because it consists of memory elements for storing the processed information and feedback connections for dynamic modeling of the system. RNN was trained with input sensing signal and target displacement datasets. The estimation accuracy of the model was real-time evaluated during trajectory tracking of reference signals in the feedback control loop. A quantitative performance analysis is assessed in terms of correlation (ℛ), mean absolute error (MEA), and root mean square error (RSME) of the learned model along with the developed actuator system. The tracking results confirm the close agreement between estimated and measured displacement at a reasonable accuracy.

A 4D printed active compliant hinge for potential space applications using shape memory alloys and polymers

This paper presents the proof-of-concept for a 4D printed active compliant hinge with a selectively variable stiffness for the deployment and reorientation of satellite appendages. We use 4D printing to create an active compliant hinge capable of bending to a given angular position, holding the position without consuming energy and reorienting itself multiple times in a slow and controlled manner without using rigid mechanisms and, therefore, requiring no lubrication. The deployment and the reorientation of the hinge are achieved by exploiting thermally induced stiffness modulation of one of the constituting materials and two antagonistic shape memory alloy actuators. The hinge is specifically designed for the case study of a 6U CubeSat with two orientable solar panels. In this work, we first explain the working principle of the hinge and propose three different actuation strategies to increase the energy collection of the considered CubeSat. Second, we describe the specific functional and geometric requirements of the hinge, the resulting design and the fabricated functional prototype. The latter is tested in a standard laboratory environment to measure the range of motion, the energy consumption and the actuation time. Finally, the feasibility of the three proposed actuation strategies is evaluated considering the corresponding net increase in collected energy. The results show that the hinge is compatible with the stowing requirements and capable of achieving maximum angular positions larger than 90° in both directions and holding any intermediate position with an accuracy of less than 3°. The three actuation strategies considered lead, in a standard laboratory environment, to an increase in energy generation between 54% and 72%.

Development of a biomimetic transradial prosthetic arm with shape memory alloy muscle wires

This paper presents a first concept of a new biomimetic transradial prosthetic arm design, called ‘MataPro-1,’ that features a 3D-printed hand bone structure that mimics the shape of human finger phalanges and palm bones, flexible elastic joints, artificial muscles, and silicone flesh that covers and protects the internal components, provides restoring force, enables better gripping capability, and appears cosmetically realistic. The artificial muscles that actuate MataPro-1 are shape memory alloy (SMA) wires, which ensure effective grip strength for many everyday objects, without causing any noise. In order to avoid the need to cool SMA wires in the small volume of the fingers, the SMA wires are not routed through the finger phalanges. The SMA wires are spooled in the forearm, cooled by a fan only during the finger restoration process, and are connected to steel wires that are routed through the finger phalanges. The finger restoring force provided by the flexible joints and silicone flesh acts as bias force for SMA wires, avoids the need for antagonistic SMA wires, and speeds up the finger restoration process. The control system of MataPro-1 is intuitive and non-invasive achieved by voice recognition phone application, or an EEG headset that monitors brainwaves and facial expressions. MataPro-1 was successful in gripping different objects of various shapes, weights and sizes in multiple different gripping positions.

A Mechanically Intelligent Crawling Robot Driven by Shape Memory Alloy and Compliant Bistable Mechanism

Abstract Mechanical components in a robotic system were used to provide body structure and mechanism to achieve physical motions following the commands from electronic controller. This kind of robotic system utilizes complex hardware and firmware for sensing and planning. To reduce computational cost and increase reliability for a robotic system, employing mechanical components to fully or partially take over control tasks is a promising way, which is also referred to as “mechanical intelligence” (MI). This paper proposes a shape memory alloy driven robot capable of using a reciprocating motion to crawl over a surface without any use of electronic controller. A mechanical logic switch is designed to determine the activation timing for a pair of antagonistic shape memory alloy (SMA) actuators. Meanwhile, a compliant pre-strain bistable mechanism is introduced to cooperate with the SMA actuators achieving reliable reciprocating motion between the two stable positions. The SMA actuator is modeled base on a static two-state theory while the bistable mechanism is described by combining a pseudo-rigid-body model (PRBM) with a Bi-beam constraint model (Bi-BCM). Following this, the design parameters of the bistable mechanism and SMA actuators are determined according to theoretical simulations. Finally, a robotic prototype is fabricated using anisotropic friction on its feet to convert the reciprocating motion of the actuator to uni-directional locomotion of the robot body over a surface. Experiments are carried out to validate the presented design concept and the modeling methods.

Design, Modeling, and Control of a Compact SMA-Actuated MR-Conditional Steerable Neurosurgical Robot

The letter presents a compact shape memory alloy (SMA)-actuated magnetic resonance (MR)-conditional neurosurgical robotic system. It consists of a 2-degree of freedom (DoF) cable-driven steerable end effector, an antagonistic SMA springs-based actuation module, and a quick-connect module, packaged into a single integrated device measuring 305 mm length × 76 mm diameter. The system is also highly adaptable, such that it could operate a cable-driven end effector up to a maximum of 4-DoFs and its length can be easily modified due to the acrylic plate-based modular construction. In addition to the kinematics of the robotic end effector and the SMA constitutive model, we also present the antagonistic SMA springs model under the known tension and cable displacement from the robotic end effector. We performed extensive characterization experiments to obtain SMA model parameters and integrated a feedforward component in our controller to achieve improved tracking of a sinusoidal reference up to 80° bending angle amplitude and 100 s period. Lastly, proof-of-concept robot demonstrations were performed in a gel phantom and in the MRI that confirmed the robot motion capability in the brain and MRI compatibility of the robot.

Towards the Development of A Steerable and MRI-Compatible Cardiac Catheter for Atrial Fibrillation Treatment.

To treat atrial fibrillation (AFib), radiofrequency ablation is usually implemented with a cardiac catheter to block abnormal electrical signals from pulmonary veins. However, it is challenging to manipulate a passive catheter. In addition, the radiation exposure to patients under fluoroscopic guidance has been a concern. To address these two major issues, this work presents the development of a steerable and MRI-compatible cardiac catheter by using shape memory alloy (SMA) as an alternative interventional tool for AFib. The developed cardiac catheter is comprised of a passive and flexible tubing and a steerable tip. The tip is composed of multiple bending modules and each module is actuated by a pair of antagonistic SMA wires. To improve the MRI-compatibility of SMA actuators, the electric current required for SMA actuation is significantly reduced by developing a technique of conductive heating actuation. In this paper, the design and fabrication of the cardiac catheter are presented, followed by modeling the SMA bending module. The working performance of the SMA bending module is experimentally studied. In the end, a proof-of-concept demonstration of the cardiac catheter is performed in an explanted heart.

Multifeedback Control of a Shape Memory Alloy Actuator and a Trial Application

Shape memory alloy (SMA) actuators exhibit high strain, high energy density, and self-sensing ability, which are promising characteristics for application in soft robot systems. This paper proposes a new SMA actuator with high accuracy that is driven by antagonistic SMA and super-elastic SMA (SSMA) wires or pairs of SMA wires that is intended to be used in miniature flexible structures. A multifeedback control methodology is presented for the actuator that uses feedback from both the SMA and SSMA wires. The prestress was chosen to minimize the gap between the heating and cooling paths of the strain-to-resistance (S-R) curve for both antagonistic SMA and SSMA. A radial basis function neural network was used to establish the S-R curve model for both self-sensing feedback and SSMA feedback. To achieve better system accuracy and stability, a data-fusion algorithm (based on the support function) was used to combine the multifeedback data. Accurate and stable motion control is demonstrated through multistep response, sinusoidal tracking, and force-disturbance tests. The experimental results show that the combination model using all the feedback has superior accuracy compared to the models that use self-sensing feedback or SSMA feedback. In addition, the maximum standard deviation within the SMA-SSMA antagonistic platform and the paired-SMA antagonistic platform can be reduced to 0.036% and 0.026%, respectively. A soft joint with two degrees of freedom is illustrated to demonstrate how the current research concept could potentially be implemented in bionic soft robots.