Evaluation of the Characteristics of a Shape Memory Alloy Spring Actuator

Abstract

The article presents the thermal characteristics evaluation of a typical intelligent system actuator using as active element the Shape Memory Alloy (SMA) helical spring. In order to determine the transformation temperatures and other thermal parameters of the studied element, the attention was concentrated on thermal analysis experiments. For the actuator configuration, a comprehensive graphical interface has been developed, to run in Visual Basic, with respect to the results of performed thermal analyses. [PUBLICATION ABSTRACT]

Headnote

Abstract - The article presents the thermal characteristics evaluation of a typical intelligent system actuator using as active element the Shape Memory Alloy (SMA) helical spring. In order to determine the transformation temperatures and other thermal parameters of the studied element, the attention was concentrated on thermal analysis experiments. For the actuator configuration, a comprehensive graphical interface has been developed, to run in Visual Basic, with respect to the results of performed thermal analyses.

Keywords: SMA, austenite phase, martensite phase, transformation temperatures, SMA spring.

I. INTRODUCTION

Shape Memory Alloys (SMAs) are smart materials which may undergo mechanical shape changes at low temperatures and retain them until heated, then coming back to the initial shape [1], [2].

Specimens of these materials exhibit two unique properties, the Shape Memory Effect - the ability of SMAs to be severely deformed and then returned to their original shape simply by heating them, and the PseudoElasticity - hysteresis behavior with total strain recovery during a mechanical loading-unloading cycle [3], [4]. The cause is a martensitic phase transformation between a high temperature parent phase, austenite (A), and a low temperature phase, martensite (M).

In order to distinguish between the two, it is important to know that the martensite phase has a twinned structure with relatively high deformability; austenite is the SMA's stronger phase, with an ordered structure, usually a cubic one.

In absence of stress, the start and finish transformation temperatures are typically denoted Ms, Mf (martensite start and finish) and As, Af (austenite start and finish).

The aforementioned two main properties are responsible for the exceptional characteristics that SMAs possess such as significant internal damping, extremely high yield stresses and large nonlinear elastic ranges [2], [3], and [5].

Due to their unique properties and behavior, SMAs play an increasingly important role in the intelligent systems performances. Recent applications in structural actuation and sensing demand increased material capabilities, and SMAs possess a great potential for use in this field [6], [7] and [8].

The paper presents the characteristics of a typical actuator of intelligent systems, using as active element SMA helical spring (of Ni-Ti composition) working against a conventional steel spring (referred here as the 'biasing' spring).

NiTi, known commercially as Nitinol, is the material used for the studied SMA element, due to its several advantages: very large recoverable motion, great ductility, excellent corrosion resistance, stable transformation temperatures, high biocompatibility and the ability to be electrically heated for shape recovery [2], [5], [6] and [9].

In order to determine the transformation temperatures and other thermal parameters of the studied element, the attention was concentrated on thermal analysis experiments.

For design optimization, a comprehensive graphical interface (based on the thermal analysis results), which runs under Visual Basic environment, has been developed for the SMA helical spring with biasing spring configuration. It provides a user friendly environment that allows intelligent system parameters configuration as well as the choice of the most adapted analysis methods and data display.

II. THERMAL ANALYSIS EXPERIMENTS

The force that a spring of any material produces at a given deflection depends linearly on the shear modulus (rigidity) of the material. SMAs exhibit a large temperature dependence on the material shear modulus, which increases from low to high temperature. Therefore, as the temperature is increased the force exerted by a shape memory element increases dramatically [1], [2]. Consequently, the determination of the transformation temperatures is necessary to establish the real shear modulus values at these functional temperatures for a high-quality design of intelligent systems [2], [8].

To characterize the transformations of the NiTi SMA spring material, during heating-cooling regimes, it is necessary to establish the start and finish transformation temperatures, under zero stress, and heat transfer of each process [1], [10].

The SMA spring was purchased from the Jameco Electronics Company.

Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) methods were used to determine the required transformation temperatures of SMA element, and Thermogravimetric Analysis (TGA) was used to prove the stability of the alloy. These methods are the most comprehensive and popular instrumental techniques used in thermal characterization of materials [11], [12].

During the tests, both isothermal and non-isothermal regimes combined with heating-cooling experiments, were used in order to characterize SMA samples.

The measurements were carried out on a horizontal Diamond Differential/Thermogravimetric Analyzer from Perkin-Elmer Instruments in dynamic air atmosphere (150 mL/min), in aluminum crucible, using as reference similar amounts of inert α-A2O3 powder.

Initial, the phase transitions of the test samples were identified by analyzing their behavior at programmed heating up to 180°C and cooling to ambient temperature, using a linear non-isothermal regime of 10 C/min. According to the characteristics observed of the sample, specific temperature program was used in order to enhance the quality of the observations. It was noticed that the samples' mass does not undergo any changes at heating and cooling. In consequence, the TGA curves are ignored in further measurements and in the present paper.

III. RESULTS AND DISCUSSION

Thermal Analysis measurements (DTA and DSC) of Ni-Ti SMA spring material were carried out in dynamic air atmosphere.

The heating/cooling sequences to determine Ni-Ti SMA spring material transformation temperatures are: heating from 30°C to 100°C by 1°C/min, isothermally holding for 10 min at 100°C, cooling from 100°C to 15°C by 1 °C/min.

Figure 1 shows the Thermal Analysis (DTA and DSC) results, during heating-cooling regime, of 6.82 mg Ni-Ti SMA spring material, in dynamic air atmosphere.

The DTA and DSC curves, presented in the Figure 1, exhibit two phase transitions, the first one occurs during the heating process while the second one appears during cooling.

The details of these thermal effects are presented in Figures 2 and 3 (reported from the DSC curve).

The synthesis of the figures 2 and 3, with the DSC parameters for the thermal analysis of Ni-Ti SMA spring material, in dynamic air atmosphere, is presented in Table 1.

For the studied sample the choice of the heatingcooling rate was made so that all transition thermal effects to be emphasized.

The Af and Mf transformation temperatures, obtained in this section, will be further used for the determination of the real shear modulus values for a high-quality design of SMA helical spring with biasing spring configurations.

IV. VISUAL BASIC APPLICATION

For the design of SMA helical spring with biasing steel spring configuration we have implemented a Visual Basic application [16].

The use of SMA spring as actuator provides the following advantages: reasonable force/motion characteristics, a compact size, a high work output, silent operation, design simplicity, and near step function operation [2], [3], [5].

The application presented in this section uses a work production operating mode [1] - [5]. In this kind of operating mode a shape memory element, works against a constant or varying force to perform work. The element therefore generates force and motion upon heating. In nearly all practical applications, force and deflection vary simultaneously as temperature changes.

In our configuration the SMA helical spring works against a varying force caused by the steel spring. The force that the SMA spring must now work against varies with deflection. At low temperatures, the steel spring is able to completely deflect the SMA spring to its compressed length. When increasing the temperature of the SMA spring, it expands, compressing the steel spring and moving, for example, a push rod.

This method of 'biasing' provides a convenient way to obtain two-way motion form from a SMA spring, and is the most common method used in actuator applications [2], [5], [8].

In the analyzed system the friction effect is neglected and a linear stress-strain behavior is assumed in order to simplify the analysis [2], [13] and [14].

The computation algorithms, which allowed developing this application, are entirely presented in [15].

Figure 4 shows the parameters involved in the design of a total system made up of a SMA spring and a biasing steel spring.

The basic problem here is to design a SMA actuator spring of the smallest force output possible, which will generate the required net, output force (Fn). In order to minimize the force that the SMA spring should provide to deflect the biasing spring at high temperatures, the spring rate of the biasing spring (Kb) must be as lower as possible. The minimum spring rate of the biasing spring will usually be imposed by spatial constraints (envelope length and diameter), since spring rate is inversely proportional to the mathematical cube of the average spring diameter [2], [5].

Below, a numerical example is given illustrating the abilities of the Visual Basic application. For the present design example, assume the following requirements: a Ni-Ti spring/biasing spring combination is required providing a net force Fn=3 N with an 8 mm stroke, the maximum cavity length and diameter are 38 mm and 5.5 mm respectively must be accomplished.

Assuming that the force exerted by the biasing spring Fh=2 N, the maximum corrected shear stress Tc=175 MPa, the SMA spring index c=6 and the low temperature shear strain γl = 0.015 (in order to ensure a good cyclic life of 50000 cycles).

The transformation temperatures, at heating and cooling, are those presented in Table 1, that are Af=56.19°C and Mf=33.08°C respectively. For these temperatures the experimental determined values of shear modulus are respectively Gh=16890 MPa and Gl=3759 MPa. Also the two springs are separated by a plug of thickness 2.5 mm.

Using standard steel spring design procedure, we assumed that the maximum shear stress for the wire is T = 1500 MPa. The bias spring shear modulus is G = 79300 MPa.

When the Visual Basic project for SMA spring with biasing spring design is run, a user interface is displayed, Figure 4.

First the user has to provide the initial parameters in the dialogue boxes in the lower part of the interface. The actuator system (SMA spring and biasing) is presented in the middle of the interface (Figure 4). The upper part of the interface contains boxes for design parameters (for both SMA spring and biasing).

The application displays warning pop-ups for two reasons:

- the difference between the cavity length and SMA's outer diameter becomes smaller than 0.5 mm.

- the full compression biasing spring length has higher values than the length of the high-temperature biasing spring.

The analyzed configuration is frequently used for SMA Latching Mechanisms, for SMA Bell Crank Mechanisms [2] [5], and for SMA Controlled Valves developed in our laboratory and used in the robotic field [6], [17].

V. CONCLUSIONS

The present paper evaluated the thermal characteristics of a typical actuator of intelligent systems, where Ni-Ti SMA helical spring with biasing steel spring was used as active element.

Thermal analysis of Ni-Ti SMA spring material, exhibiting its transformations during heating-cooling regimes, was performed in dynamic air atmosphere.

By using Thermal Analysis Methods, the experimental start and finish transformation temperatures for the studied sample were determined.

These experimental transformation temperatures were necessary to precisely establish the real shear modulus values of SMA spring material, for a high-quality design of the analyzed system.

In addition, for the SMA helical spring with biasing steel spring configuration, a Visual Basic application was developed, providing:

- adequate dialogue boxes for fast and easy initial parameters configuration;

- fast computation and display of all required information for a complete SMA element design;

- warning popup when the maximum imposed value of a parameter is exceeded;

- remarkable facilities to analyze results and choose an optimal solution.

This Visual Basic application is already used by ICMET-Craiova, Romania for engineering purposes and by the Faculty of Electromechanical Engineering of Craiova, Romania for didactical ones.

ACKNOWLEDGMENTS

This work was supported by the National University Research Council (CNCSIS) of the Romanian Minister of National Education. It is part of a project covering theoretical and applicative researches on SMA actuators used in the robotic field.

References

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