End-point position control of a single-link arm using shape memory alloy actuators

Though the conventional pyrotechnic fastener mechanisms are widely used in spacecraft for their reliable releasable-fastening function, they still have several unavoidable problems: physical shock, high maintenance cost, to name a few. This paper introduces a new type of smart releasable mechanism based on a Shape Memory Alloy (SMA) spring and its corresponding heating apparatus. To obtain the high heating efficiency and increase the response speed, the SMA spring is transitionally fit with the heating apparatus shell instead of directly heating by electric current. As soon as the heating apparatus begins to work, the SMA spring will provide an un-locking force to release the fastening device within the standard time, which also realizes the similar releasable-fastening function comparing with the conventional pyrotechnic fastener mechanisms. In order to ensure the reliability of space products, the heating apparatus is composed of two identical ceramic heating elements which can be controlled independently or synchronously. Finally, the experimental results clearly show that, under the satellite power supply at the constant value of 28 V, the SMA spring can reach the desired 30 N unlocking force within 93 s and 51 s by single or dual heating elements, respectively. The maximal output force can even be increased as large as 40 N under the limited volume of the releasable mechanism.

This paper presents a gripper actuated by a pair of differential shape memory alloy (SMA) springs with the tactile feedback controller of the polyvinylidene fluoride (PVDF) films. The SMA actuator is heated by the electric current and cooled by the fans, and a proportional-integral (PI) controller is proposed to control the output position of the gripper. In order to handle soft, deformable or slippery objects very well, the PVDF sensor is added to the finger of the gripper to perform the tactile feedback when it grasps the objects. The hardware system is established including data acquisition and signal processing parts, the control system is programmed. Experiments of grasping a soft object are carried out, and the gentle grasp ability of the gripper is also tested. Experiments results show that the SMA gripper with the PVDF tactile sensor could grasp a soft object through controlling tactile forces.

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.

The use of Shape Memory Alloy (SMA) materials for actuation represents a technological opportunity for the development of innovative applications. The issue of using the SMA actuators is an enticing solution that has attracted the interest of specialists in recent years, due to their numerous advantages (i.e. reliability, compactness and flexibility, reduced cost, linear or angular movement etc.). In addition, their unmatched energy density, easy integration into various structures and the simplicity in terms of design, have all motivated the authors to develop a barrier structure model, actuated by three SMA helical springs. The actuator used in this structure works as a linear actuator, contracting with great strength and speed, thus exerting the necessary force to lift the barrier arm in the SMA springs heat-activated, austenitic state. The first part of this paper provides a description of the accomplished model and of its functioning principle. The second part of the study presented in this paper focuses on the authors' investigation on how the performances of the actuator inserted in the barrier structure—namely the actuator's heating time, cooling time, actuation time and stroke—are affected by the SMA spring activating direct current values. The tests were conducted while keeping the geometrical parameters of SMA springs and the weight of the barrier arm constant.

To unlock the lock of futuristic developments, nowadays shape memory alloys (SMAs) are acting as key by outnumbering the existing smart materials in view of new design competence, innovative techniques, enhanced technologies, and meeting the dire need according to the demand of ongoing scientific and industrial progress. Undoubtedly, it can be worthy to associate SMAs with the term polymorphism, as they possess unique capability to encapsulate multitudinous inherent characteristics under their name. Shape memory effect, superelasticity, and damping are there to name a few among many others, as prime indicators. The exhibition of these characteristics of SMAs is due to the transformation that takes place between austenite phase and martensite phase which is achieved either by temperature or stress variation. The fascinating characteristics they possess enable them to attract researchers and designers toward their potential applications in diverse domain as smart and multi-functional materials. The present work focuses on the review of research works accomplished in the recent 5 years pertaining to modeling and applications of shape memory alloys in distinct domains. Under the topic of modeling, a look into the research carried out on a variety of SMA structures—like functionally graded SMAs, SMA springs, porous SMAs, novel SMA actuators—is presented. In addition to this, the work carried out to represent SMAs with large strains and large rotations and to model polycrystalline SMAs is also covered. This is supplemented by application-oriented domains, each of which disseminates the attributes of SMAs that are self-sufficient to express what factors make SMAs more prominent. It is believed that the foregoing works comprehensively detailed in this review article will certainly lay a solid platform for the future investigations that are going to take place in time to come.

This paper describes the development of fuzzy systems for modeling the hysteresis behavior of shape memory alloy (SMA) actuators. Due to their simplicity and ease of actuation, SMA actuators are very attractive for applications such as miniature robots for micro manufacturing. However, SMAs have not been widely used for motion control applications due to their nonlinear behavior and control difficulties. One approach to design a position controller for SMA systems is to employ an inverse-model of the system in the control loop to compensate the hysteresis properties of the material. Fuzzy systems, due to their nonlinear learning and adaptation abilities, are good candidates for obtaining inverse-models. In this paper two fuzzy modeling approaches are employed and compared to develop a model for a SMA wire actuator. A set of experiments are conducted to generate the training data. The test stand includes a Nickel-Titanium (TiNi) SMA wire, a position sensor, a bias spring and a current amplifier. By comparing the performance of the two employed fuzzy modeling techniques, it is revealed that the approach based on fuzzy Gustafson-Kessel (GK) clustering shows a better performance in the modeling of the hysteresis in the SMA wire. Thus, GK clustering algorithm is employed to develop the inverse-model for the SMA. The reported results demonstrate the ability of the employed fuzzy algorithm for modeling the hysteresis in the system, and the merits of the introduced inverse-model in the control of the position of the SMA.

Shape memory alloys (SMA) can be used to create actuators for use in mechanical systems that carry pronounced benefits with their low weight, high strength, and low cost when coupled with advancements such as robust self-sensing. However, there exist drawbacks in the form of slow system response and complex material behavior. The design and implementation of controllers that drive SMA actuators successfully can pose a challenge, and accurate modelling of the material in software can help to optimize the system response time and power requirements. We have created a variety of tools to help implement these actuators into simulations that capture accurate thermal and mechanical responses of various SMA systems under a variety of control laws. In particular, thermo-electro-mechanical models of SMA behavior are implemented into software representations of mechanical systems such that the entire temperature, stress, and phase profiles of an SMA actuator can be accurately determined via simulation. Because analytical equations for modelling SMA behavior quickly become so complex to be untenable, we use stable numerical schemes to solve for these profiles. In particular, a finite difference scheme allows for spatial and temporally discretized temperature profiles along a shape memory actuator which can be solved for using empirically derived expressions for the heat transfer coefficient that dictated convective heat transfer. These types of models allow for a variety of boundary conditions to capture a number of SMA geometries, orientations, and applications. This paper presents the results of the numerical schemes for thermal cycling and a sliding mode controller used to drive a simple SMA actuator with varying boundary conditions and compares them to experimental results.

Development of a fuzzy precise controller for the continuum modules utilizing shape memory alloy actuators is the main focus of this study. To this end, two continuum and flexible shape memory alloy–actuated modules, containing shape memory alloy wires or shape memory alloy springs, are considered as the testbed of the control problem to be tackled. The fuzzy controllers in this application are developed using two strategies in this research. In the first technique, the position errors of the two motion variables of the system are considered in the controller design process. The resulted controller is referred as “error-based fuzzy controller.” In the second technique, which is called as “improved fuzzy controller,” the parameters of the desired configuration of the system, in addition to their errors, are considered in controller design process. This procedure makes it possible to overcome the error-based fuzzy controller drawbacks. In order to validate the simulation results, experimental tests are conducted. Both simulation and experimental results reveal the performance of the developed novel two-stage improved fuzzy controller against error-based fuzzy controller and traditional proportional–integral–derivative controller.

A helical spring shape memory alloy actuator has been investigated to be used in a mechanical circulatory support system. The design procedure to determine shape memory alloy and passive bias spring specifications was established. A model device was designed to provide 2 cm stroke by a 100 cm2 pusher plate against a maximum 200 mm Hg pressure load. The prototype device was also developed to verify the feasibility of the shape memory alloy actuator and to confirm the design procedure. In vitro water bath test results verified the feasibility of the shape memory alloy actuator for use in a mechanical circulatory support system.

Wireless Capsule Endoscopy (WCE) is a minimally invasive technology for imaging the gastrointestinal (GI) tract, particularly the small intestine, where conventional endoscopy faces accessibility limitations. Traditional capsule endoscopes rely on passive motion driven by natural peristalsis, which limits controllability and may increase the risk of capsule retention. To address these challenges, this study presents the design and experimental validation of a compliant active capsule endoscope actuated by four Shape Memory Alloy (SMA) spring actuators. A key feature of the proposed system is a steering mechanism that reuses the same SMA actuators responsible for locomotion, enabling control of the camera orientation without increasing system complexity, size, or weight. The capsule architecture consists of rigid polylactic acid (PLA) links connected through thermoplastic polyurethane (TPU) flexure hinges, fabricated using dual-material 3D printing. Nonlinear finite element analysis (FEA) was employed to optimize the flexure hinge geometry for maximum displacement while maintaining safe stress levels. To validate the concept, a 3.5× scaled prototype was fabricated and integrated with SMA actuators and an Arduino-based control system. The experimental results demonstrate effective locomotion and steering capabilities, achieving a maximum stroke of approximately 5.4 mm and a steering angle of 24° for the 3.5× scaled prototype, corresponding to an estimated stroke of approximately 1.98 mm (Based on the FEA) at the intended clinical scale. Thermal characterization of the SMA actuators was also conducted to identify suitable operating current ranges for future biomedical deployment. The results demonstrate the feasibility of integrating locomotion and steering within a compact compliant capsule architecture, representing a step toward next-generation capsule endoscopy systems with improved navigation and diagnostic capability.

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

We have developed a discretely actuated steerable probe for percutaneous procedures. We propose use of shape memory alloy (SMA) actuators in our design due to their small size and high power density. SMAs are attractive actuators when large forces or displacements are required and limited spaces are available. SMA actuators are shape-setted to an arc shape and mounted on the outer surface of the probe to generate bending action upon thermal actuation. SMA can recover large deformation on thermal activation and recovery strain of SMA is related to its temperature. Hence, we propose controlling the temperature of the SMA actuators for position control of the probe by heating up the SMA wires at each joint. Pulse width modulation (PWM) based control scheme is used to be able to control all SMA wires simultaneously. PWM is implemented via use of a switching circuit. Proposed controller is validated through an experiment to heat up the SMA wires to a desired temperature. Another experiment is carried out inside gelatin to mimic the motion of the probe inside soft tissue. PWM control was successfully implemented and we were able to demonstrate local actuation of the steerable probe.

This paper describes development of a motion controller for Shape Memory Alloy (SMA) actuators using a dynamic model generated by a neuro-fuzzy inference system. This kind of smart alloy is known to have a unique characteristic in that its shape can be controlled by temperature that can be varied by passing a current through it. Using SMA actuators, it would be possible to design miniature mechanisms for a variety of applications. Today SMA is used for valves, latches, and locks, which are automatically activated by heat. However it has not been used as a motion control device due to difficulty in the treatment of its highly non-linear strain-stress hysteresis characteristic which is further influenced by its temperature. In this project, a dynamic model of a SMA actuator is developed using ANFIS, a neuro-fuzzy inference system provided in MATLAB environment. Using neuro-fuzzy logic, the system identification of the dynamic system is performed by observing the change of state variables (displacement and velocity) responding to a known input (voltage across the SMA actuator). Then, using the dynamic model, the estimated input voltage required to follow a desired trajectory is calculated in an open-loop manner. The actual input voltage supplied to the SMA actuator is the sum of this open-loop input voltage and an input voltage calculated from an ordinary PD control scheme. This neuro- fuzzy logic-based control scheme is a very generalized scheme that can be used for a variety of SMA actuators. Experimental results are provided to demonstrate the potential for this type of controller to control the motion of the SMA actuator.

This article proposes a numerical model to predict actuation frequencies (strain-cycling frequencies) for 1-D shape memory alloy (SMA) actuators. The numerical model is designed by comprehensively considering the factors that significantly affect an SMA actuator's strain-cycling frequency, i.e., heat convection methods, constant or nonconstant bias loads, and Joule heating methods. For computing strain cycles of SMA actuators, we develop a computation framework that effectively integrates an SMA constitutive model, a phase transformation kinetic model, stress-strain models for different types of bias stress, and a heat transfer model of SMA wires. The strain and temperature rates are formulated by nonlinear ordinary differential equations. The strain and temperature cycles can be accurately approximated by numerical iterative methods. Experimental verification was conducted to investigate the effectiveness of the proposed model. By initially identifying the SMA characteristic parameters, we employed Latin hypercube sampling method to generate 30 sets of design parameters for comparing the predicted and measured actuation frequencies. In addition, we investigated how the uncertainty of premeasured SMA characteristic parameters affect the numerical prediction of actuation frequency by conducting sensitivity analysis. The experiments demonstrate 0.7%-9.03% and 2.65%-17.57% differences between predicted and measured actuation frequencies for constant bias loads and spring bias loads, respectively.

A new approach of designing and controlling a multi-axis actuator system using segmented binary control (SBC) is presented. In SBC, Shape Memory Alloy (SMA) actuator wire is divided into multiple segments and their thermal states are controlled in a binary manner using thermoelectric modules. Two-dimensional array architecture for multi-axis SMA actuators is created using SBC. Similar to the biological muscle network, multi-axis SMA actuator wires can be combined flexibly with a two-dimensional segmentation method in order to activate multiple links of a skeletal robot structure in a coordinated manner. Ten-axis actuator is built using this method. Thermoelectric modules are laid on a substrate made of PCB board and SMA actuators are placed in between two substrate layers with a gap to allow the movement of the actuators. The actuator creates coordinated gross motion as well as independent fine movements with reduced number of control loops. The new actuator architecture will provide a novel actuator system forming a network of array actuators connected to a vast number of bone-like linkages.