Simultaneous measurements for the interlink of electro-thermo-mechano-electro characteristics in shape memory springs

Highlights

• Experimental facility for the measurement of multiphysics behavior of SMA spring.

• Synchronized measurement of thermo-mechanical and thermo-electrical features.

• Mathematical models: electromechanical (thermoelectrical) for actuation (sensing).

• Equivalent circuit: SMA spring is validated as a series resistance with inductance.

Abstract

This experimental study aimed to obtain the actuation properties of Shape Memory Alloy (SMA) spring actuators available under the commercial names of Biometal® NiTiCu and KRL® NiTi NiTiCu, NiTiFe. An objective is to characterize the electro-thermo-mechano-electro behavior of shape memory spring through experimentation via simultaneous measurement techniques. These measurements are synchronized by embedding an impedance analyzer and data acquisition module through a unique program. Moreover, to obtain the mathematical model by parts of the SMA spring and equivalent circuit analysis of active spring through their electro-thermo-mechano-electro characteristics exhibited during shape memory effect. Based on the experimental results, the SMA spring’s equivalent circuit is considered a series resistance – inductance circuit. It was found from the experimental results that the Biometal® actuator produced more resistance, inductance variation and offered more displacement as compared to KRL® actuators.

Introduction

Shape memory alloys (SMAs) belong to a class of intelligent material, experiences a unique functional behavior of solid–solid phase transformation. SMAs are available in several shapes — bars, wires, strips, and springs. Its wire form is more general; it is readily available and provides a significant recovery force; easy to model. Its spring form gives a more powerful actuation stroke, but its retrieval force decreases as the stress focuses on the wire’s perimeter [1]. SMA element-based actuator’s being able to function against restoring elements (constant load or passive spring) because, at low temperatures, this element overwhelms the resistance of the effortlessly deformable SMA element. Because of the exceptional electro-thermo-mechano-electro characteristics, the SMAs found applications in broad domains. SMAs play a vital role in medical and aerospace actuation systems requiring large deformations and forces compared with other functional materials. Shape memory effect exhibits in numerous alloys — AuCd, CuZnX (X $=$ Si, Al, Sn) but NiTi and NiTiCu based SMA has verified to be more efficient [2]. The shape memory material is a designed innovative material, also called responsive material, being able to alter its properties in a precise manner with the effect of external stimuli temperature and stress. This material remembers its original shape(s) at transition temperature(s); that phenomenon is called the shape memory effect (SME). SMA exhibits two essential behavior — shape memory effect (SME) during temperature-induced phase transformation and pseudoelastic effect due to stress-induced phase transformations. SMA embedded actuator, activated by controlled Joule heating thus its temperature increases from ambient up to its transition temperature. Two Phases of transformation occur within this temperature range; reverse (endothermic) and forward (exothermic) transformation. SME results with temperature-induced phase transformation due to complete heating and cooling phase. SMA undergoes solid–solid phase transformation during actuation and exhibits variation in its electrical, thermal, mechanical, and magnetic properties. Hysteresis effects with a difference in transition temperatures between the heating and cooling phase. These materials have attracted increasing attention due to their intrinsic capacity to sense and actuate. SMA spring is designed to obtain required stiffness, an actuating force by proper selection of spring index, number of coils, and pitch angles [3].

The unique functional behavior of SMA finds enormous applications in industrial and non-industrial fields. Ample of literature available under modeling and control of SMA wires; hence utmost controllers are also designed for SMA wires. However, for SMA springs, only a few models and controllers were developed. An adaptive controller is employed to control the displacement of active springs during actuation [4]. In SMA springs modeling, three variables like input (temperature or voltage) and output (stroke or strain) and (force or stress) play vital roles in describing springs’ non-linear behavior; based on this, an inclusive model for SMA spring is proposed [5]. Due to the complex behavior of SMA elements, an exact and straightforward model has not yet been developed. Complete details of electro-thermo-mechano-electro characteristics and properties of intelligent material are mandatory to explore innovative design in various industrial and non-industrial applications, including rehabilitation systems, soft robotics, and alike wherever the mandate arises for smart actuation and sensing. The electrical resistance variation offered in SMA wire during temperature-induced phase transformations is significant; hence, it can be considered as a sensing signal. Likewise, the SMA spring exhibits electrical inductance besides resistance; hence, the electrical impedance offered during actuation can also be considered a sensing signal. Contextual works mainly describe self-sensing based on electrical resistance. Single literature is found, as of date, directly related to inductance-based sensing. In [6], Based on the experimental result of phase transformations, the inductance and resistance offered by SMA are determined from the time constant. However, it is reported that the hardware complexity increases as the SMA helical spring diameter decreases below 3 mm for inductance measurement [7].

This work focuses on capturing the functional behavior of the SMA spring revealed during the shape memory effect (SME) as a result of phase transformation. Factors to be considered for selecting SMA spring include transition temperature effects with SME and material composition effects with a transition temperature. Furthermore, its functional behavior depends on its spring index and no. of active turns; hence these two factors are also considered for the selection of SMA spring. Innovative materials are used in complex devices that enhance the coupling of multiple physical effects. Therefore, to optimize the design of these devices, the study of intelligent materials requires careful consideration of these interactions. In general, the electrical resistance and inductance are determined from displacement due to SME and vice versa. However, in this work, the synchronized measurement technique is realized by embedding impedance analyzer with data acquisition module through a Visual Basics (VB) macro program developed in a PC for the simultaneous measurement of electro-thermo-mechano-electro characteristics of shape memory spring. Thereby obtaining the measurements simultaneously can significantly improve test throughput, and also the data validated soon.