Helical Spring Actuators Research Articles

Performance simulation of metal hydride based helical spring actuators during hydrogen sorption

Self sensing/actuation materials are known as smart/intelligent materials due to their changes in structure and functionality based on external stimuli. Even though, metal hydrides are studied extensively as potential materials for hydrogen storage, their applicability becomes limited due to low gravimetric storage capacity. However their significant volumetric dilatation upon hydrogenation can make them potential candidates for sensors/actuators. As hydrogenation performance of these alloys is controlled by heat transfer as the major factor, devices based on this can be employed as thermal sensors/actuators. However response characteristics of such devices need detailed investigation. A numerical study is conducted on the performance of these actuator elements with LaNi5 as the hydrogen storage alloy. Effects of different operational and geometric parameters on hydrogenation and actuator displacement are studied.

Thermomechanical performance analysis and experiment of electrothermal shape memory alloy helical spring actuator

In this paper, an electrothermal shape memory alloy helical spring actuator constructed from shape memory alloy with copper-cored enameled wire is presented and fabricated. Based on the shear constitutive model of a shape memory alloy, the Thermo equilibrium equation and the geometrical equation of helical spring establish the thermomechanical theoretical model of helical spring actuator with electrothermal shape memory alloys under different scenarios. The thermomechanical behaviors of the actuator were verified by numerical simulation with experimental tests, and the actuator thermomechanical properties were derived from the analysis with current, temperature, response time, restoring force, and axial displacement as parameters. The experimental results show that the actuator produces a maximum recovery force of 70.2 N and a maximum output displacement of 7.7 mm at 100°C. The actuator response time is 26 s at a current of 3A. It is also demonstrated that the theoretical model can effectively characterize the complex thermo-mechanical properties of the actuator due to the strong nonlinearity of the shape memory alloy. The experimental temperature-force response and temperature-displacement response, as well as the force-displacement response at different temperatures, provide references for the design and fabrication of electrothermal shape-memory alloy coil spring actuators.

A novel model-based approach for resistance estimation using rise time and sensorless position control of sub-millimetre shape memory alloy helical spring actuator

Shape memory alloy shows considerable strain during heating and cooling. This effect is due to its phase transformation with temperature. Due to this property, shape memory alloys can be deployed for physical actuation in place of conventional actuators in bio-medical and bio-mimicking robots. Sub-millimetre diameter shape memory alloy wires wound as helical springs are also used for this purpose. Due to their small size, it is difficult to use sensors for temperature or displacement measurements of shape memory alloy springs. This article attempts to demonstrate that the rise time of the current through a sub-millimetre diameter shape memory alloy helical spring is directly proportional to its displacement. To characterize the rise time–displacement hysteresis, a constant current drive with overcurrent protection is developed. The data generated are utilized to implement an open-loop sensorless control. A method to estimate the resistance from the rise time is proposed with which the temperature of the shape memory alloy during actuation can be obtained. The design avoids using an analogue-to-digital converter for the direct measurement of voltage and current for measuring the resistance variation in the shape memory alloy under actuation. This helps in the development of a new sensorless control using only the digital Input/Output pins of a microcontroller/microprocessor.

A thermomechanically coupled finite deformation constitutive model for shape memory alloys based on Hencky strain

This paper presents a new thermomechanically coupled constitutive model for polycrystalline shape memory alloys (SMAs) undergoing finite deformation. Three important characteristics of SMA behavior are considered in the development of the model, namely the effect of coexistence between austenite and two martensite variants, the variation of hysteresis size with temperature, and the smooth material response at initiation and completion of phase transformation. The formulation of the model is based on a multi-tier decomposition of the deformation kinematics comprising, a multiplicative decomposition of the deformation gradient into thermal, elastic and transformation parts, an additive decomposition of the Hencky strain into spherical and deviatoric parts, and an additive decomposition of the transformation stretching tensor into phase transformation and martensite reorientation parts. A thermodynamically consistent framework is developed, and a Helmholtz free energy function consisting of elastic, thermal, interaction and constraint components is introduced. Constitutive and heat equations are then derived from this energy in compliance with thermodynamic principles. Time-discrete formulations of the constitutive equations and a Hencky-strain return-mapping integration algorithm are presented. The algorithm is then implemented in Abaqus/Explicit by means of a user-defined material subroutine (VUMAT). Numerical results are validated against experimental data obtained under various thermomechanical loading conditions. The robustness and efficiency of the proposed framework are illustrated by simulating a SMA helical spring actuator.

Modeling of mechanical response of NiTi shape memory alloy subjected to combined thermal and non-proportional mechanical loading: a case study on helical spring actuator

Textured polycrystals of NiTi-based shape memory alloys (SMA) exhibit pronounced anisotropic properties which significantly influence their response to mechanical and thermal loading. In this work, a constitutive model tailored for non-proportional multi-axial loading of NiTi SMA exhibiting two-stage phase transformation via R-phase is enhanced so that the anisotropy of martensitic structure is captured. Numerical simulations of the mechanical response of a NiTi SMA helical spring subjected to thermal cycling at a constant applied force are performed and compared with experimental data. Quantitative correspondence between experiments and simulations demonstrates the predictive potential of the model. Simulations also provide detailed information on the evolution of distributions of phase fractions and stress within a cross-section of the wire forming the spring. Because the loading is non-proportional, the evolution is rather complex and intriguing.

A room-temperature processed parylene-patterned helical ionic polymer–metal composite spring actuator with selectable active region

This paper presents a novel helical ionic polymer metal composite (IPMC) spring actuator. A room-temperature fabrication process is developed to selectively grow in situ nickel metal electrode on a coiled Nafion strip, which is patterned with parylene as a masking layer to delineate the inactive region. This method allows the helical spring actuator to be shaped in a near-constant temperature environment and minimizes the residual stress effect on the Nafion polymer, compared to other processes using thermal treatment. The developed process ensures that short-circuiting never occurs between the outer and inner electrodes of the actuator, and the active region can be designed via parylene patterning. The motion of the fabricated spring actuator is measured, and the maximal displacement reaches 1 mm at the endpoint of the spring under a 0.1 Hz 6 V square wave actuation. A microtensile experiment is performed for characterizing the stress and strain relation. The resulting Young’s moduli of the Nafion and fabricated IPMC strips are 183 and 227 MPa, respectively. Analysis based on Castigliano’s theorem is executed to derive equations related to the moment, force output, strain energy, and displacement of an arbitrary point of the fabricated spring actuator. Results show the produced moment, force, and strain energy of the operated spring actuator are in the level of 1.5 μN m, 300 mN, and 20−100 μJ, respectively.

Large scale simulation of NiTi helical spring actuators under repeated thermomechanical cycles

As typically utilized in applications, a shape memory alloy (SMA) actuator operates under a large number of thermomechanical cycles, hence the importance of accounting for the cyclic behavior characteristics in modeling and numerical simulation of these actuators. To this end, the present work is focused on the characterization of the cyclic, evolutionary behavior of binary 55NiTi using a newly developed, multi-axial, material-modeling framework and its finite element analysis (FEA) implementation for use in the simulations of SMA actuators. In particular, two different geometric configurations of four- and two-coil helical springs subjected to axial end-forces are investigated under the effect of a large number of thermal cycles leading to the saturated deformation state of the coils. In addition, two different boundary conditions were examined, corresponding to: (a) the loading end cross section assumed to be free-to-twist, and (b) the loading end cross section assumed to be restrained against twist rotation. The study has led to the following five important conclusions: (i) the states of stresses and strains in the coils exhibited marked spatial non-homogeneities, both along the length as well as the cross section of the wires; (ii) the cyclic deformation response of the coils exhibits a similar evolutionary character to that of the 55NiTi material when tested under simple isobaric tensile stress conditions; (iii) the end boundary conditions affect the evolution of the deformation response; (iv) the magnitudes of the evolving nonlinear deformation states (i.e., axial displacements on the martensite and austenite sides, as well as the actuation displacement) were found to be proportional to the number of coils in an essentially linear manner, and (v) the change in coil diameter, while maintaining the pitch height, wire diameter and the number of coils fixed, has a significant effect on the response of the helical spring, both with regard to the resulting stress state and the evolutionary axial displacement behavior during the thermal cycles.

A New Approach to the Design of Helical Shape Memory Alloy Spring Actuators

Shape memory alloys (SMAs) are smart materials that have the ability to recover their original shape by eliminating residual deformations, when subjected to adequate temperature rise (Shape memory effect). This special behavior attracts the use of SMAs as efficient stroke/force actuators. Most of the engineering applications require helical springs as actuators and proper design of SMA helical spring actuators is very important. In the traditional design approach of SMA spring (Waram, 1993), only strain between linear zones was considered in order to simplify the design and to improve the fatigue life. Only modulus difference between high-temperature and low-temperature phase was utilized, and the transformation strain was not considered as the total transformation strain will be more and will degrade the performance of actuator. In the present design, we have shown that transformation strain can be restricted by using hard stops and the partial transformation strain can be used to improving the capacity of SMA spring actuator. A comparison of the traditional design approach of SMA spring and the proposed design procedure has been made to give an idea of its effect on the design and the related parameters.

Thermoelastic properties on Cu-Zn-Al shape memory springs

This paper present a thermomechanical study of actuators in form of helical springs made from shape memory alloy wires that can work as actuator and/or as sensor. These abilities are due to the martensitic transformation. This transformation is a diffusionless phase transition that occurs by a cooperative atomic rearrange mechanism. In this work, helical spring actuators were manufactured from Cu-Zn-Al shape memory alloy wires. The springs were submitted to constant tensile loads and thermal cycles. This procedure allows to determine thermoelastic properties of the shape memory springs. Thermomechanical properties were analyzed during 50 thermal cycles in the temperature range from 20 to 130 °C. Results of variations in critical transformation temperatures, thermoelastic strain and thermal hysteresis are discussed based on defects rearrangement and martensitic transformation theory.

Magneto-Superelastic Analysis of Shape Memory Alloy Helical Spring Actuators Controlled by Magnetic Force

A method of coupled magneto-superelastic analysis by the sequential approach is proposed for shape memory alloy (SMA) helical spring actuators controlled by magnetic force. The commercial finite element software ANSYS/Emag is used for the magnetic field analysis, while the one-dimensional finite element program developed by the authors is used for the analysis of superelastic behaviors of SMA helical springs. The validity of the proposed method is verified by applying the method to the analysis of actuator models utilizing SMA composite or ferromagnetic SMA helical springs and comparing the calculated results with the experimental results.

Magneto-Superelastic Analysis of Shape Memory Alloy Helical Spring Actuators Controlled by Magnetic Force

A method of coupled magneto-superelastic analysis by the sequential approach is proposed for shape memory alloy (SMA) helical spring actuators controlled by magnetic force. The commercial finite element software ANSYS/Emag is used for the magnetic field analysis, while the one-dimensional finite element program developed by the authors is used for the analysis of superelastic behaviors of SMA helical springs. The validity of the proposed method is verified by applying the method to the analysis of actuator models utilizing SMA composite or ferromagnetic SMA helical springs and comparing the calculated results with the experimental results.