Shape Memory Alloy Wire Actuators Research Articles

Design and investigation of a shape memory alloy actuated gripper

This paper proposes a new design of shape memory alloy (SMA) wire actuated gripper for open mode operation. SMA can generate smooth muscle movements during actuation which make them potentially good contenders in designing grippers. The principle of the shape memory alloy gripper is to convert the linear displacement of the SMA wire actuator into the angular displacement of the gripping jaw. Steady state analysis is performed to design the wire diameter of the bias spring for a known SMA wire. The gripper is designed to open about an angle of 22.5 when actuated using pulsating electric current from a constant current source. The safe operating power range of the gripper is determined and verified theoretically. Experimental evaluation for the uncontrolled gripper showed a rotation of 19.97. Forced cooling techniques were employed to speed up the cooling process. The gripper is simple and robust in design (single movable jaw), easy to fabricate, low cost, and exhibits wide handling capabilities like longer object handling time and handling wide sizes of objects with minimum utilization of power since power is required only to grasp and release operations.

Sensorless resistive-based control of shape memory alloy actuators in locking mechanism

This article presents a sensorless resistive-based control method to control shape memory alloy wire actuators to be used in the development of a laparoscopic surgical locking system. Shape memory alloy wire electrical resistance is measured to control the martensite phase fraction of shape memory alloy wire. The martensite phase fraction is calculated based on mathematical heating model, resistance models, and the measured resistance. Experiments are conducted to evaluate the validity and performance of the method in the control of the locking system angle. The results show that the sensorless resistive-based control method accurately controls the wire without using any position sensors, resulting in the lower cost, size, and weight of the system. The results show very small steady-state errors, verifying the practicability of this method. The sensorless resistive-based control method has the potential to be used in many applications in which cost and space are limited and the use of an external sensor is impossible or costly.

A coupled thermomechanical beam finite element for the simulation of shape memory alloy actuators

The proposed article describes the development of a new beam finite element for the coupled thermomechanical analysis of shape memory alloy actuators. The element is formulated, assuming coupled equilibrium equations for the thermoelastic stresses and thermal loads. Displacements and temperature are treated as internal degrees of freedom giving the ability to predict the coupled thermal–displacement response of a shape memory alloy beam. The constitutive shape memory alloy model of Lagoudas and coworkers is implemented in the formulation. A generalized beam theory is formulated assuming shear deformation with a cubic temperature field through the thickness. The new element is capable to simulate heat transfer phenomena, electric Joule heating as direct input, and heat convection effects. The coupling between mechanical and thermal equilibrium equations due to endothermic/exothermic martensitic transformation procedures is also included. Numerical results and evaluations of the developed beam element are presented for the thermomechanical response of shape memory alloy wire actuators and an adaptive strip subject to various types of applied thermal loading and heat convection conditions. The effect of coupling terms on the prediction of shape memory alloy actuator response is also evaluated.

Adaptive-length pendulum smart tuned mass damper using shape-memory-alloy wire for tuning period in real time

Due to the shift in paradigm from passive control to adaptive control, smart tuned mass dampers (STMDs) have received considerable attention for vibration control in tall buildings and bridges. STMDs are superior to tuned mass dampers (TMDs) in reducing the response of the primary structure. Unlike TMDs, STMDs are capable of accommodating the changes in primary structure properties, due to damage or deterioration, by tuning in real time based on a local feedback. In this paper, a novel adaptive-length pendulum (ALP) damper is developed and experimentally verified. Length of the pendulum is adjusted in real time using a shape memory alloy (SMA) wire actuator. This can be achieved in two ways i) by changing the amount of current in the SMA wire actuator or ii) by changing the effective length of current carrying SMA wire. Using an instantaneous frequency tracking algorithm, the dominant frequency of the structure can be tracked from a local feedback signal, then the length of pendulum is adjusted to match the dominant frequency. Effectiveness of the proposed ALP-STMD mechanism, combined with the STFT frequency tracking control algorithm, is verified experimentally on a prototype two-storey shear frame. It has been observed through experimental studies that the ALP-STMD absorbs most of the input energy associated in the vicinity of tuned frequency of the pendulum damper. The reduction of storey displacements up to 80 % when subjected to forced excitation (harmonic and chirp-signal) and a faster decay rate during free vibration is observed in the experiments.

Development of a remote shape memory alloy experiment for engineering education

In this paper, we present and compare two different implementation approaches for a remote shape memory alloy (SMA) experiment development. The remote SMA experiment aims to offer students a hands-on experience in testing and control of an SMA wire actuator via remote laboratory platform. One approach is based on the LabVIEW remote panel and other one is based on a novel unified remote laboratory framework. The new remote SMA experiment based on a novel unified framework is implemented at Texas A&M University at Qatar (TAMUQ) for mechanical engineering teaching. With this new remote experiment, the user can remotely conduct the SMA experiment by using any portable device without installing any plug-in.

Modulated adaptive fuzzy controller for position control of SMA wire actuator

Conventional control depends on the mathematical model of the shape memory alloy (SMA) actuator being controlled. When this model is uncertain, intelligent controllers, like fuzzy logic controllers (FLC) promise better performance. This paper presents an experimental investigation of position control of a shape memory alloy wire actuator with fuzzy, fuzzy-GSPI (fuzzy gain scheduled proportional integral), fuzzy-PWM (fuzzy pulse width modulation) and fuzzy GSPI-PWM controllers. The performance of the controllers is experimentally evaluated with and without thermal disturbance. The results demonstrates that the performance of fuzzy GSPI-PWM surpass fuzzy, fuzzy-GSPI, fuzzy-PWM.

Precise position control of shape memory alloy actuator using inverse hysteresis model and model reference adaptive control system

Position control of Shape Memory Alloy (SMA) actuators has been a challenging topic during the last years due to their nonlinearities in the governing physical equations as well as their hysteresis behaviors. Using the inverse of phenomenological hysteresis model in order to compensate the input–output hysteresis behavior of these actuators shows the effectiveness of this approach. In this paper, in order to control the tip deflection of a large deformation flexible beam actuated by an SMA actuator wire, a feedforward–feedback controller is proposed. The feedforward part of the proposed control system, maps the beam deflection into SMA temperature, is based on the inverse of the generalized Prandtl–Ishlinskii model. An adaptive model reference temperature control system is cascaded to the inverse hysteresis model in order to estimate the SMA electrical current for tracking the reference signal. In addition, a closed-loop proportional–integral controller with position feedback is added to the feedforward controller to increase the accuracy as well as eliminate the steady state error in position control process. Experimental results indicate that the proposed controller has great accuracy in tracking some square wave signals. It is also experimentally shown that the suggested controller has precise tracking performance in presence of environmental disturbances.

Performance range of SMA actuator wires and SMA–FRP structure in terms of manufacturing, modeling and actuation

In this contribution we present a comprehensive theoretical and experimental description of an active shape memory alloy (SMA) fiber reinforced composite (FRP) hybrid structure. The major influences on actuation performance arising from variations in the design and manufacturing process are discussed, utilizing a new phenomenological model to describe the actuating SMA material. The different material properties for the activated, respective the unactivated, SMA as well as the influence of different loading conditions or pre-treatment of the material are taken into account in this model. To validate our material model we performed new actuation experiments with an exemplary SMA–FRP structure, which we compared to finite element (FE) simulation results. Our FE-model is based on a material model for the actuating SMA elements derived from experiments and data on the actual microscopic geometry of the hybrid composite. Therefore it is able to predict very precisely the actuation behavior of a typical FRP structure for industrial use cases: a thin walled CFRP sheet with SMA wires attached to the top for performing a bending motion with a maximum deflection of approx. 25% of its length.

Thermo-electric behaviour of NiTi shape memory alloy

The thermo-electric behavior of shape memory alloy (SMA) wire was presented. When the wire was electrically heated above its transformation temperature by current, a large mechanical force is exerted due to transformation in its phases. In order to make use of SMA wire as an actuator, different parameters and their relationships were investigated. These parameters are recoverable strain (displacement), temperature hysteresis and electrical resistance variation under different stress levels. Optimum safe heating current was assessed and phase transformation temperatures were estimated by heat transfer model. The wire was heated and cooled by 680 mA of current for 796 s under natural air convection. The strain recovered is 4.33% and corresponding change in resistance is 11.2% at 43 MPa of stress. The resistance variation shows linearly with displacement and current during heating and cooling cycle respectively. This study will be useful in precisely controlling of SMA wire actuator with and without external sensor feedback.

A two-degrees-of-freedom miniature manipulator actuated by antagonistic shape memory alloys

This paper presents a miniature manipulator that can provide rotations around two perpendicularly intersecting axes. Each axis is actuated by a pair of shape memory alloy (SMA) wires. SMA wire actuators are known for their large energy density and ease of actuation. These advantages make them ideal for applications that have stringent size and weight constraints. SMA actuators can be temperature-controlled to contract and relax like muscles. When correctly designed, antagonistic SMA actuators have a faster response and larger range of motion than bias-type SMA actuators. This paper proposes an antagonistic actuation model to determine the manipulator parameters that are required to generate sufficient workspace. Effects of SMA prestrain and spring stiffness on the manipulator are investigated. Taking advantage of proper prestrain, the actuator size can be made much smaller while maintaining the same motion. The use of springs in series with SMA can effectively reduce actuator stress. A controller and an anti-slack algorithm are developed to ensure fast and accurate motion. Speed, stress, and loading experiments are conducted to demonstrate the performance of the manipulator.

Use of load generated by a shape memory alloy for its position control with a neural network estimator

This paper presents the position control of a shape memory alloy (SMA) wire actuator by measuring the load generated by the SMA and using a neural network as an estimator for displacement. The experimentation facility is designed and developed for the position control of the SMA. Gain scheduled proportional integral (GSPI) and pulse width modulated GSPI controllers are designed and implemented. The closed-loop performance of the position control system using the displacement estimated from the load measurement is the same as the performance obtained with the direct position feedback. The position control of the SMA actuator using the SMA load with the neural network estimator is an easily implementable method that does not affect the frequency of operation of the SMA actuator.

Stress, strain, and resistance behavior of two opposing shape memory alloy actuator wires for resistance-based self-sensing applications

Shape memory alloy actuator wires undergo a significant (∼4%) contraction and a corresponding change in resistance because of a temperature- and load-induced phase transformation. When a restoring force such as a pre-stretched bias spring is placed in series with a shape memory alloy wire, the system becomes an actuator that can generate a repeatable force. Simultaneously, the resistance of the wire can be correlated to strain and enable self-sensing, eliminating the need for external feedback sensors. The self-sensing task, however, is complicated in applications requiring multiple coupled wires, for example, advanced two-dimensional or three-dimensional positioning. The presence of coupled (passive or active) actuator wires with nonlinear, hysteretic force–displacement characteristics has a strong impact on an individual wire’s resistance behavior that has not been systematically studied to date. This article expands upon previous work that studied a single-shape memory alloy–spring system by adding a second opposing shape memory alloy wire and focusing on the resistance to strain mapping that is crucial for self-sensing applications. Systematic stress–strain and resistance–strain experiments are presented alongside physics-based modeling results that help to identify several sources of hysteresis in the resistance–strain behavior and facilitate intelligent calibration schemes for multifunctional self-sensing and actuation applications.

Quantification of the Effectiveness of Various Adhesives in Holding an SMA Actuator Wire During Joule Heating

Shape memory alloy (SMA) actuator wires undergo a large contraction (∼4%) and resistance change (∼7%) in response to a material phase change induced by heating. However, a major impediment to using SMA wires as embedded actuators or sensors in practical applications is the challenge of attaching the small wires to the surrounding structure without adding significant bulk to the system. This work systematically determines the bond strength of different adhesives that could be used to attach small (<100-µm diameter) SMA wires to a structure by exposing the adhesive bond to the thermal and mechanical loading expected during typical embedded SMA applications. Results show that for all adhesives tested, bonding strength is reduced as heating current is increased; however, some two-part epoxies and contact adhesives are still able to maintain 6 MPa of bonding strength to a heated SMA wire. This number can be used to determine how much adhesive is needed to hold different diameter wires for a range of applications.

Design and construction of an artificial finger based on SMA actuators

Shape Memory Alloy (SMA) wires are currently employed in robotics, as actuators of prosthetic limbs and medical equipment due to advantages such as reducing the size in the application, high power-to-weight ratio and elimination of complex transmission systems. In this paper, a new prosthetic finger is presented using SMA wire actuators. This robotic finger has been designed and modeled with three revolute joints and three SMA wires as the tendon in order to adduction each phalange of the finger and torsional springs to restore themto their original positions. The dynamic model of the finger has been simulated and validated in MATLAB/Simulation under different input voltages. By using this simulation, kinematic and kinetic of this finger have been analyzed. Also, by attention of the design goals, the mechanism of artificial finger is defined and has been developed in the Solid Works software. Based on simulation results, optimal choice of parameters and system features has been obtained and a prototype of finger has been built and tested.The results of practical experiments show that the designed artificial finger can achieve to the desired positions with high accuracy. Also, the results of practical tests confirm validity, high accuracy and authenticity of simulation model.

Design and Construction of an Artificial Finger based on SMA Actuators

Shape Memory Alloy (SMA) wires are currently employed in robotics, as actuators of prosthetic limbs and medical equipment due to advantages such as reducing the size in the application, high power-to-weight ratio and elimination of complex transmission systems. In this paper, a new prosthetic finger is presented using SMA wire actuators. This robotic finger has been designed and modeled with three revolute joints and three SMA wires as the tendon in order to adduction each phalange of the finger and torsional springs to restore themto their original positions. The dynamic model of the finger has been simulated and validated in MATLAB/Simulation under different input voltages. By using this simulation, kinematic and kinetic of this finger have been analyzed. Also, by attention of the design goals, the mechanism of artificial finger is defined and has been developed in the Solid Works software. Based on simulation results, optimal choice of parameters and system features has been obtained and a prototype of finger has been built and tested.The results of practical experiments show that the designed artificial finger can achieve to the desired positions with high accuracy. Also, the results of practical tests confirm validity, high accuracy and authenticity of simulation model.

Adaptive online inverse control of a shape memory alloy wire actuator using a dynamic neural network

Shape memory alloy (SMA) actuators exhibit severe hysteresis, a nonlinear behavior, which complicates control strategies and limits their applications. This paper presents a new approach to controlling an SMA actuator through an adaptive inverse model based controller that consists of a dynamic neural network (DNN) identifier, a copy dynamic neural network (CDNN) feedforward term and a proportional (P) feedback action. Unlike fixed hysteresis models used in most inverse controllers, the proposed one uses a DNN to identify online the relationship between the applied voltage to the actuator and the displacement (the inverse model). Even without a priori knowledge of the SMA hysteresis and without pre-training, the proposed controller can precisely control the SMA wire actuator in various tracking tasks by identifying online the inverse model of the SMA actuator. Experiments were conducted, and experimental results demonstrated real-time modeling capabilities of DNN and the performance of the adaptive inverse controller.

Simulation of shape memory alloy wire actuator behavior under arbitrary thermo-mechanical loading

Shape memory alloy (SMA) wires are becoming potential candidates as actuators due to their high stress-generating and strain-recovery capabilities. As actuators, these wires undergo arbitrary thermo-mechanical loading. For an efficient design, one requires a model to simulate their behavior under any stress–temperature situation. In this study, based on an existing phase kinetics dependent phenomenological model, an algorithm capable of simulating SMA wire behavior under an arbitrary loading situation has been developed and implemented. To demonstrate this, two cases have been simulated. In the first case, the behavior of an SMA wire undergoing varying stress and temperature while actuating a single degree of freedom manipulator has been simulated. The results are found to illustrate the discrepancies reported in the literature. The behavior of two antagonistically placed and independently activated SMA wires is modeled and simulated in the second case, where the results qualitatively agree with the existing experimental results.

Numerical simulation and verification of a curved morphing composite structure with embedded shape memory alloy wire actuators

Shape memory alloys have been actively studied in various fields in an attempt to utilize their high energy density. In particular, shape memory alloy wire-embedded composites can be used as load-bearing smart actuators without any additional manipulation, in which they act like a hinge joint. A shape memory alloy wire-embedded composite is able to generate various deformation behaviors via the combination of its shape memory alloy and matrix materials. Accordingly, a study of the various design parameters of shape memory alloy wire-embedded composites is required to facilitate the practical application of smart structures. In this research, a numerical simulation of a shape memory alloy wire-embedded composite is used to investigate the deformation behavior of a composite panel as a function of the composite width per shape memory alloy wire, composite thickness, and the eccentricity of the shape memory alloy wire. A curved morphing composite structure is fabricated to confirm the results of the numerical simulation. The deformation of the shape memory alloy wire-embedded composite panel is determined by measuring its radius of curvature. The simulated deformation behaviors are verified with the experimental results. In addition, an analysis of the deformation and internal stress of the composites is carried out. It can be used to obtain guidelines for the mechanical design of shape memory alloy wire-embedded composite panels.

Robust actuation of silicon MEMS using SMA wires integrated at wafer-level by nickel electroplating

This paper reports on both the wafer-level fixation and electrical connection of pre-strained SMA wires to silicon MEMS using electroplating, and on the fabrication of the first Joule-heated Shape memory alloy (SMA) wire actuators on silicon. The integration method provides both high bond strength and electrical connections in one processing step, and it allows mass production of microactuators having high work density. SEM observation showed an intimate interconnection between the SMA wires and the silicon substrate. The variation of the actuators’ performance across the wafer was evaluated on three 4.5mm×1.8mm footprint devices, proving repeatable results. The actuators showed a mean hot state deflection of 536μm and a mean stroke of 354μm at a low power consumption (less than 70mW). One actuator was tested for m150×103 cycles, and it demonstrated a highly reliable long-term performance, showing neither material degradation, nor failure of the nickel anchors.

Position control of shape memory alloy actuator based on the generalized Prandtl–Ishlinskii inverse model

Hysteresis and significant nonlinearities in the behavior of Shape Memory Alloy (SMA) actuators encumber effective utilization of these actuator. Due to these effects, the position control of SMA actuators has been a great challenge in recent years. Literature review of the research conducted in this area shows that using the inverse of the phenomenological hysteresis models can compensate the hysteresis of these actuators effectively. But, inverting some of these models, such as Preisach model, is numerically a complex task. However, the generalized Prandtl–Ishlinskii model is analytically invertible, and therefore can be implemented conveniently as a feedforward controller for compensating hysteresis nonlinearities effects in SMA actuators. In this paper a feedforward–feedback controller is used to control the tip deflection of a large deflected flexible beam actuated by an SMA actuator wire. The feedforward part of the control system is based on the generalized Prandtl–Ishlinskii inverse model while a conventional proportional–integral feedback controller is added to the feedforward controller to increase the accuracy together with eliminating the steady state error in position control process. Experimental results show that the proposed controller performs well in terms of achieving small overshoot and undershoot for square wave tracking as well as small tracking errors for sinusoidal trajectory. It has also great capability for tracking hysteresis minor loops.