Experimental Platform for Obtaining Electrical Resistance

of a Shape Memory Alloy Actuator

Wislayne Dayanne P. da Silva, Tony Carlos Moura Cavalcanti, Jaidilson J

o da Silva

and Angelo Perkusich

Department of Eletrical Engineer, Federal University of Campina Grande, Campina Grande, Paraiba, Brazil

Keywords:

Deformation, Electronic Instrumentation, Electrical Resistance, Shape Memory Alloy Actuator, Strain-Gauge.

Abstract:

Shape Memory Alloys (SMA) are unique metallic materials with the Shape Memory Effect (SME), which

refers to a material’s capacity to recover its original shape through temperature variation subjected to defor-

mations. These alloys are commonly used as actuators to control vibration, deformation, position and have

been used in several sectors in the last decades. Therefore, in order to enable a study about the behavior of

the electrical resistance of an SMA actuator and the deformation measurement using strain-gauge sensors, an

experiment is proposed in this work to be applied to the course of Electronic Instrumentation Laboratory of

Electrical Engineering Degree at the Federal University of Campina Grande, Brazil. We used an experimental

platform composed of a steel beam and an SMA actuator, and we also developed the necessary electronic

system and Human Machine Interface. The experiment consists of activating the actuator by applying electric

current and obtaining data corresponding to its electrical resistance and beam deformation. With the experi-

ment’s realization, the students will be able to draw the behavior curves and prepare a report with the analyses.

1 INTRODUCTION

Shape Memory Alloys (SMA) are metal alloys that,

when submitted to deformations, return to their orig-

inal shape after a thermal cycle. This effect is known

as the Shape Memory Effect (SME), and it was dis-

covered in 1963 by Willian J. Buehler from research

developed with nickel-titanium alloys (NiTi) at the

Naval Ordinance Laboratory (NOL) in Maryland,

USA. In the 1960s, SME was observed in other alloys

besides nickel-titanium, Cobalt (Co), and Iron (Fe).

The addition of these two elements allowed a reduc-

tion in the transformation temperatures of the alloys

(Lagoudas, 2008).

Later on, titanium-palladium (TiPd), titanium-

platinum (TiPt), and titanium-gold (TiAu) alloys were

used for high-temperature applications. However, it

was only in 1980 that SMA started to be used more

widely since the effect was better understood. Cur-

rently, SMA comprises a unique class of materials,

which presents the capacity to recover from the orig-

inal form when its temperature is increased and has

the characteristic of superelasticity. When increasing

the temperature, the form can be recovered from the

https://orcid.org/0000-0002-1273-7865

application of high loads, which cause high densities

of actuation energy (Lagoudas, 2008).

SMA actuators are widely used in sensing, impact

absorption, and vibration damping applications in this

context. Of the different industrial sectors that are ap-

plied, stand out: aerospace, automotive, biomedical,

dental, orthopedic, robotic, and oil.

Thus, in this work, we propose an experiment to

obtain an SMA wire actuator’s electrical resistance

behavior. Therefore, we developed an experimental

platform for which a mechanical structure was used,

composed of a steel beam and an SMA wire actuator.

Also, we developed an electronic system and a user

interface. The platform makes it possible to carry out

experiments to obtain the SMA’s electrical resistance

and measure the beam’s deformation caused by elec-

trical current application in the actuator.

The proposed experiment is to be applied in the

course of Electronic Instrumentation Laboratory of

the Electrical Engineering Degree at UFCG Brazil for

studies of sensors and actuators. The work’s main

motivation was the importance of carrying out experi-

ments in the laboratory as a learning tool in Engineer-

ing courses, allowing students to apply the concepts

presented in the classroom.

The rest of the paper is organized as follows. In

P. da Silva, W., Cavalcanti, T., Jó da Silva, J. and Perkusich, A.

Experimental Platform for Obtaining Electrical Resistance of a Shape Memory Alloy Actuator.

DOI: 10.5220/0010438203150322

In Proceedings of the 13th International Conference on Computer Supported Education (CSEDU 2021) - Volume 1, pages 315-322

ISBN: 978-989-758-502-9

315

Section 2 we present the theoretical foundation. In

Section 3 we present the developed experimental plat-

form, and in Section 4 we describe the proposed ex-

periment. In Section 5 we present the results and dis-

cussions, and in Section 6 we present conclusions and

future work.

2 THEORETICAL FOUNDATION

This section presents the main concepts that underlie

this work: the fundamentals of SMA, the strain-gauge

sensors, and their use in deformation measurement.

2.1 Shape Memory Alloys

SME refers to a material’s ability to recover its orig-

inal shape, through temperature variation, after be-

ing submitted to mechanical deformation. This phe-

nomenon can be perceived in special metallic mate-

rials called Shape Memory Alloys (SMA) (Elahinia,

2016).

SMA has two distinct phases, each with speciﬁc

physical properties. These phases are named austen-

ite and martensite. Austenite normally presents a

cubic crystalline structure with a centered face or

body-centered at high temperatures. The martensite

phase can present a tetragonal, orthorhombic, or mon-

oclinic structure at low temperatures. In the marten-

site phase, the crystals can be oriented in different di-

rections; each direction is called a variant. The vari-

ants can have two directions of orientation: twinned

martensite and detwinned or reoriented martensite.

Self-accommodating variants form the ﬁrst, and the

last has a variant that is dominant in the grouping

(Lagoudas, 2008).

The transformation from one phase to another can

occur directly or inversely. The direct transformation

refers to the change from the austenite to the marten-

site phase and occurs when cooling the SMA in the

absence of an applied load. In the reverse transforma-

tion, in turn, when heating the same SMA, the crys-

talline structure undergoes a reverse transformation,

returning from the martensite to austenite phase (Ku-

mar et al., 2020).

The transformations are shown in Figure 1, in

which the structures in the form of twinned marten-

site and austenite are presented. As can be seen, in

the direct transformation, without mechanical load,

the austenite phase begins the transformation to the

twined martensite phase at the initial temperature of

martensite (M

) and the transition is completed at the

ﬁnal temperature of martensite (M

). At this tempera-

ture, the alloy will be completely transformed into the

twinned martensite form, thus completing the direct

transformation. When heating the alloy, the process

of reverse transformation begins at the initial temper-

ature of austenite (A

). Furthermore, the transition is

completed at the ﬁnal temperature of austenite (A

Figure 1: Representation of the Phase Transformation of an

SMA (Lagoudas, 2008).

2.2 Deformation Measurement with

Strain-Gauge

The physical deformation of a material occurs

through the application of forces, which causes

changes and can be deﬁned as the relation between

the variation in the length of a material and its orig-

inal length. A common type is ﬂexion deformation,

which is caused by applying a linear force in the ver-

tical direction, resulting in a deformation of one side

of the material by contraction of the opposite side (In-

struments, 2019).

Deformation measurement is essential for the cor-

rect understanding of how a given object reacts to dif-

ferent applied forces. For this purpose, strain-gauge

sensors can be used, which are devices that vary their

electrical resistance when they undergo deformation.

They consist of a small grid formed by metal sheets

that can be ﬁxed to the surface of a component or

structure to be measured, with a layer of adhesive that

serves to transmit the structure’s deformations to the

sensor.

The strain-gauge is capable of measuring small

deformations in the range of 0 to 50 µm/m with an ac-

curacy of ±0.15%, typically presenting nominal elec-

trical resistance of 120 Ω, 350 Ω and 1000 Ω.

The electrical resistance of a strain-gauge is mea-

sured using a Wheatstone Bridge circuit. A generic

Wheatstone Bridge, illustrated in Figure 2, consists

of a network formed by resistive arms and an excita-

tion voltage, V

that is applied at the entrance to the

bridge.

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316

Figure 2: Wheatstone Bridge Circuit Diagram (National In-

struments, 2019).

Thus, the deformation ε of the device is proportional

to the output voltage of the circuit V

, as described by

Equation 1.



+ R

−

+ R



∗V

(1)

In turn, the Wheatstone bridge is considered electron-

ically balanced, V

= 0, when it has the following re-

lations:

. Therefore, a variation in one of the

resistors (∆R) causes a change in the output voltage

value. For example, if replacing R4 with an active

strain gauge, any change in its resistance unbalances

the bridge and produces a nonzero output voltage,

which is a function of deformation (National Instru-

ments, 2019).

Depending on the application, it is possible to

have one, two, three, or four bridge elements whose

resistance varies depending on the measured physical

quantity. For the application presented in this work,

the resistance variation occurs in two opposing active

elements: two strain-gauge. Thus, the output voltage

at the bridge is given by:



∆R



(2)

An essential parameter in the characterization of a

strain-gauge is the Gauge Factor (GF), which refers to

measuring the relative variation of the device’s elec-

trical resistance when it undergoes deformation. For

magnetic strain-gauges, this parameter is around 2.

The GF is calculated according to the relation pre-

sented in Equation 3.

GF =

∆R/R

∆L/L

∆R

R.ε

(3)

where: R is the electrical resistance, ∆R is the varia-

tion in resistance , L is the length of the strain-gauge,

∆L is the variation in length and ε is deformation.

3 EXPERIMENTAL PLATFORM

For the development of this work, an experimental

platform developed by (Lima et al., 2012) and opti-

mized by (Patriota et al., 2018) was used. This plat-

form has an SMA wire actuator ﬁxed to the free end of

a beam so that in the initial condition, the beam is de-

formed by its own balance weight, thereby stretching

the actuator. An electric current is applied to activate

it, and when doing so, the SMA recovers the original

shape by heating by joule effect. Consequently, the

actuator applies sufﬁcient force to the beam to return

it to the desired position.

For the proposed experiment, we used the plat-

form to analyze the electrical resistance behavior in

the SMA wire and the beam’s deformation by varying

the electric current in the actuator. Therefore, this sec-

tion will present the mechanical and electronic sys-

tems and HMI that make up the platform.

3.1 Mechanical System

Figure 3 illustrates the platform’s mechanical struc-

ture by the isometric view. It consists of a rectangular

iron base; a support column with a rectangular area; a

steel beam, with one end attached and the other free;

an SMA wire attached to the top of the support col-

umn and the free end of the beam; and a support for

possible application of external load weights.

Figure 3: Representation of the Mechanical System (Patri-

ota et al., 2018).

3.2 Eletronic System

The platform’s electronic system, adapted and imple-

mented for this work, consists of two strain-gauge

sensors, Wheatstone bridge and gain adjustment cir-

Experimental Platform for Obtaining Electrical Resistance of a Shape Memory Alloy Actuator

317

cuits, an SMA wire actuator, NI DAQ USB-6212

data acquisition system from National Instruments,

the current driver for actuating the SMA wire actu-

ator, and circuit for measuring the voltage in the ac-

tuator. The block diagram of the electronic system is

shown in Figure 4, and each subsystem is described

as follows.

Figure 4: Electronic System Block Diagram.

3.2.1 Strain-Gauge and Wheatstone Bridge

At the ﬁxed end of the beam, two strain-gauge were

ﬁxed, one located at the top and the other at the bot-

tom, to measure the ﬂexion deformation at that point.

The two strain-gauge were positioned at the same dis-

tance from the platform support column and under the

same axis to obtain a correct measurement. When one

strain-gauge is tensioned, the other is compressed fol-

lowing the same proportion so that the resistance vari-

ation of the two sensors has the same module, with

different signals, as shown in Figure 5.

The strain-gauge used are the model PA-06-125-

BA-350-LEN from Excel Sensors, of the collatable

type and recommended for steel surfaces. It has sim-

ple unidirectional resistance with a traditional shape

of 350 Ω and a gauge factor of 2.1.

Figure 5: Representation of the Deformation Measurement

Scheme Used. (National Instruments, 2019).

As the measurement of ﬂexion deformation involves

the resistance variation of two opposing elements, a

Wheatstone bridge circuit was used in the half-bridge

conﬁguration and the output voltage given by Equa-

tion 2. Performing the appropriate mathematical ma-

nipulations with Equations 3 and 2, Equation 4 was

obtained for the deformation.

ε =

2.V

.GF

(4)

The variation in the resistance of the strain-gauge is

less than 1%. Therefore, as the bridge’s output volt-

age is directly proportional to this variation, it will

also be practically negligible. Thus, it is necessary to

apply a high gain (G) to amplify the value of V

and

enable the data acquisition board’s digital-analog con-

verter to carry out the signal acquisition. Therefore,

the acquired voltage is V

acquired

= G.V

. Rewriting

Equation 4 we get that:

ε =

2.V

acquired

.GF.G

(5)

3.2.2 SMA Wire Actuator

The SMA wire used as an actuator is a NiTiNOL al-

loy of the type FLEXINOL TCF1140, manufactured

by Dynalloy, Inc. It has the two-way shape memory

effect and was thermomechanically trained by Patri-

ota (2018). According to the manufacturer manual, it

has the characteristics listed in Table 1.

Table 1: Features of SMA Wire Actuator.

Parameter Value

Diameter 0.2mm

Density 6,450 kg/m

Speciﬁc heat 837J/kg.K

Thermal conductivity 1,800W /(m.K)

Resistance 29Ω/m

Cooling time between 2.7s and 3.2 s

3.2.3 Data Acquisition System

We used the NI DAQ USB-6212 multifunctional de-

vice from National Instruments (see Figure 6) as the

data acquisition system (Instruments, 2009). The de-

vice performs the acquisition of the analog readings

and sends them to the LabVIEW software, in which

they will be processed and made available to the user.

It is also responsible for sending the activation com-

mands to the actuator driver.

3.2.4 Actuator Driver and Voltage Measurement

Circuits

The NI DAQ USB-6212 sends the SMA wire actua-

tor’s activation signal to the current driver input. The

driver receives a DC signal ranging from 250 mV a

5 V. The circuit is powered using a 12 V DC source,

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Figure 6: Image of the NI DAQ USB-6212 Data Acquisition

Device (National Instruments).

capable of supplying up to 1 A, sufﬁcient current to

drive the actuator.

In order to measure the voltage on the SMA wire

actuator, was implemented a differential measure-

ment circuit. The circuit consists of two stages. The

ﬁrst corresponds to the preampliﬁer with unit gain,

consisting of two voltage followers that isolate the in-

put voltage signals, thus increasing the circuit’s input

impedance. The second stage consists of a differential

ampliﬁer designed to have a gain of 1/3.

3.3 Human Machine Interface

To allow the student to interact with the platform

system and visualize the process outputs numerically

and graphically, we developed a Human-Machine

Interface (HMI) using the LabVIEW software (an

acronym for Laboratory Virtual Instrument Engineer-

ing Workbench).

Figure 7 shows the front panel of the VI developed

in LabVIEW.

Figure 7: Representation of the HMI.

The developed software performs the acquisition of

the measurements corresponding to current and volt-

age in the SMA actuator and the acquired voltage

to calculate the beam deformation. These values are

mathematically treated to be displayed in the user in-

terface.

With the acquisition of the voltage value at the

output of the Wheatstone Bridge gain adjustment cir-

cuit, the deformation calculation is performed using

Equation 3. Furthermore, with the SMA’s voltage and

current values, the corresponding electrical resistance

value is calculated. According to the percentage of

DC voltage entered by the user, the SMA activation

signal is sent to an analog output of the DAQ NI USB-

6212. This sending is done using the DAQ Assistant

block, and the analog output is connected to the cur-

rent driver circuit.

The HMI also allows the student to select his class

and record a txt ﬁle with the measurements to analyze

later.

4 PROPOSED EXPERIMENT

In Engineering courses, the execution of experiments

in laboratories is an essential tool for student learning.

Speciﬁcally, for the Electrical Engineer training, it is

required to understand how the measurement systems

interact with the environment and provide informa-

tion, which can be treated and applied in solving the

most varied problems.

In this context, the course of Electronic Instru-

mentation Laboratory offered by the Electrical Engi-

neering Degree at UFCG, Brazil, addresses the con-

tent of great importance for the formation of the future

professional of Electrical Engineering, since it intro-

duces the basic techniques and the main methods to

perform measurements using sensors or transducers.

The course has activities in the laboratory, where stu-

dents perform practical experiments using experimen-

tal platforms and data acquisition systems. With that,

the student can analyze several types of conditioning

circuits and know the operation of different types of

sensors.

Thus, in this work, an experiment is proposed to

be applied to the Electronic Instrumentation Labora-

tory course to enable a study about the electrical be-

havior of SMA wire actuator and deformation mea-

surement using strain-gauge sensors. For this, the ex-

perimental platform presented in Section 3 is used.

Figure 8 shows a photograph of the mechanical and

electronic systems.

The proposed experiment consists of varying from

5% to 100% the actuator signal’s duty cycle value,

which is the percentage of the DC voltage signal ap-

Experimental Platform for Obtaining Electrical Resistance of a Shape Memory Alloy Actuator

319

Figure 8: Experimental Platform Photography.

plied at the current driver input. The deformation,

current, voltage, and resistance values corresponding

to each measurement must be noted or saved in a txt

ﬁle using the ”Record Measurements” button present

on the HMI. However, it is necessary that before sav-

ing the data, the student must select his class.

When it reaches 100% of the actuator’s activation

signal, the experiment should be ended. With the data

collected, the student will be able to draw the Resis-

tance versus Current and Deformation versus Current

behavior curves and prepare a report with the results

and analyses achieved.

5 RESULTS AND DISCUSSIONS

Table 2 presents real measurement data using the de-

veloped system to exemplify the analyses that can be

performed with the proposed experiment. This table

is the values stored in the txt ﬁle for the twenty mea-

surements made during the experiment. The values

correspond to the measurements of beam deformation

(D), current (I), and voltage (V) measured in the SMA

and the resistance (R) calculated from the SMA.

The electrical resistance of an SMA sample in-

creases as the current increases until it reaches the

current value at which the phase transformation be-

gins. From that current value, the resistance starts to

decrease. Therefore, analyzing the last column of Ta-

ble 2, it can be seen that the resistance value changed

during the experiment, resulting in a variation of ap-

proximately 1.85 Ω. The resistance variation is di-

rectly related to the current applied to the alloy, so

an increase in resistance is observed as the current

increases, up to a current value of 240.63 mA. From

that value, the resistance starts to decrease with the in-

crease of the current. Thus, one can observe that the

phase transformation began when the current applied

Table 2: Experimental Data.

(%)

(µm/m)

I (mA) V (V) R (Ω)

5 0.4064 23.0087 0.5689 24.7254

10 0.4099 42.7998 1.0631 24.8389

15 0.4264 62.5595 1.5586 24.9139

20 0.4488 82.3349 2.0558 24.9688

25 0.4653 102.1104 2.5584 25.0552

30 0.5194 121.9485 3.0586 25.0811

35 0.5548 141.6613 3.5643 25.1607

40 0.5830 161.4368 4.0686 25.2024

45 0.6125 181.2437 4.5791 25.2649

50 0.6831 201.0035 5.0735 25.2409

55 0.7149 220.8104 5.5814 25.2769

60 0.7561 240.6330 6.0715 25.2314

65 0.7773 260.4086 6.4891 24.9189

70 0.8021 280.2312 6.8421 24.4159

75 0.8480 299.9912 7.1618 23.8734

80 0.9834 319.7513 7.5629 23.6524

85 1.8831 339.5270 7.9162 23.3154

90 3.0784 359.2714 8.3166 23.1485

95 5.1204 379.0629 8.7003 22.9521

100 7.7866 398.9639 9.1261 22.8745

in the alloy is greater than 240 mA. Using the MAT-

LAB software, the resistance versus current curve was

constructed, which describer this behavior (see Fig-

ure 9).

Figure 9: Representation of Resistance versus Current Be-

havior.

To apply the experiment in the Electronic Instru-

mentation Laboratory course, the student may also

determine an approximate mathematical model that

best describes this behavior. The best approximation

found for the Resistance versus Current curve was a

tenth-degree polynomial for the results presented. As

described by Equation 6, in which f (x) corresponds

to resistance and x corresponds to current.

CSEDU 2021 - 13th International Conference on Computer Supported Education

320

f (x) = p1 ∗ x

+ p2 ∗ x

+ p3 ∗ x

+ p4 ∗ x

+ p5 ∗ x

p6 ∗ x

+ p7 ∗ x

+ p8 ∗ x

+ p9 ∗ x

+ p10 ∗ x + p11

(6)

where: p1 = 2.1572 ∗ 10

−22

; p2 = −4.0457 ∗ 10

−19

;

p3 = 3.1871 ∗ 10

−16

; p4 = −1.3718 ∗ 10

−13

; p5 =

3.5183 ∗ 10

−11

; p6 = −5.5046 ∗ 10

−09

; p7 = 5.1505 ∗

−07

; p8 = −2.6684 ∗ 10

−05

; p9 = 0.00059919; p10

= 0.004991; p11 = 24.504.

With this information, the polynomial approxima-

tion graph was plotted, shown in Figure 10.

Figure 10: Representation of Resistance versus Current -

Approximate Model.

The analysis of the behavior of the electrical resis-

tance of an SMA is fundamental when we want to

use it as an actuator. This behavior can be considered

to study the performance criteria before implement-

ing the actuator. It is possible to determine the best

SMA operating region, analyzing which currents pro-

vide the phase transformation. Thus, one can choose

which type of SMA best attends to the needs of the

desired application.

Regarding the deformation measurement behavior

in the beam, we observed that when lower currents are

applied to the SMA actuator, not enough force was

applied to the beam to vary its position since the actu-

ator has not started to contract yet. Consequently, the

measurement of the strain-gauge in this range does

not show very signiﬁcant variations. However, for

higher current values, above 300 mA, the alloy starts

to contract and has enough force to lift the beam from

the initial position. Thus, the deformation measure-

ment begins to increase according to the applied cur-

rent, as shown in Figure 11.

The best approximation of the deformation ver-

sus current behavior was a ninth degree polynomial,

which is described in Equation 7.

f (x) = p1∗x

+ p2∗x

+ p3∗x

+ p4∗x

+ p5∗x

p6 ∗ x

+ p7 ∗ x

+ p8 ∗ x

+ p9 ∗ x + p10 (7)

where: p1 = 6.1361∗10

−21

; p2 = −1.4801∗10

−17

; p3

= 1.4118 ∗ 10

−14

; p4 = −7.04 ∗ 10

−12

; p5 = 2.0258 ∗

Figure 11: Representation of Deformation versus Current

Behavior.

−09

; p6 = −3.4738 ∗ 10−07; p7 = 3.5114 ∗ 10

−05

;

p8 = −0.0019751; p9 = 0.055351; p10 = −0.1668.

With this information, the polynomial approxima-

tion graph was plotted, shown in Figure 12.

Figure 12: Representation of Deformation versus Current

Behavior - Approximate Model.

6 CONCLUSIONS

In this work, we present an experimental platform for

studying an SMA wire actuator. The platform allows

experiments to be executed to obtain the characteris-

tic of the SMA’s electrical resistance and deformation

of a beam. For this, we ﬁrst described the concepts

and characteristics of the SMA and the strain-gauge

sensor. Then, we presented the main tools and proce-

dures performed to develop and use the experimental

platform. In this way, we explained the components

that make up the mechanical system, the electronic

system, and the HMI. After, the experiment proposal

was presented to be applied in the Electronic Instru-

mentation Laboratory course.

With the analysis of the results obtained experi-

mentally, we conclude that the platform satisﬁes the

Experimental Platform for Obtaining Electrical Resistance of a Shape Memory Alloy Actuator

321

requirements that it was intended to achieve. The

SMA resistance behavior analysis through the appli-

cation of current in the alloy is a study of great rele-

vance for using material as an actuator since it makes

it possible to determine the best operating region, ac-

cording to the phase transformation.

Besides, we found a relationship between the

SMA’s performance and the beam deformation, mea-

sured by strain-gauge sensors. We veriﬁed that when

low currents are applied to the SMA wire actuator,

not enough force is applied to the beam to vary its po-

sition. However, for higher current values, the alloy

starts to contract and consequently has enough force

to lift the beam from its initial position.

Thus, the proposed experiment can be used in the

Electronic Instrumentation Laboratory course to com-

plement the experiments carried out on sensors and

actuators.

We plan the use the experimental platform de-

veloped to investigate the phase transformation and

the SMA wire actuator hysteresis behavior during the

heating and cooling process. For this, we intend to es-

timate the SMA wire temperature from the electrical

parameters data resulting from the proposed experi-

ment.

ACKNOWLEDGEMENTS

The authors thank the support of the Coordenac¸

de Aperfeic¸oamento de Pessoal de N

ıvel Superior -

Brazil (CAPES), Electronic Instrumentation and Con-

trol Laboratory and Programa de P

os-Graduac¸

ao em

Engenharia El

etrica (COPELE), UFCG.

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