Shape memory alloy actuators and their reliability

O. Tohyama∗ , S. Maeda, K. Abe and M. MurayamaTelecommunication & Photonics Laboratory, Mitsubishi Cable Industries, Ltd.

ABSTRACT

We have developed two types of shape memory alloy (SMA) actuator and estimated the long-term reliability of SMAmicrocoils. A tube type tip articulator consists of 4 sets of SMA microcoil (wire diameter: 0.125 mm, coil diameter: 0.5mm) for driving source and super elastic alloy (SEA) microcoils (wire diameter: 0.1 mm, coil diameter: 0.5 mm) forbias springs, support plates and flexible outer tube. The tube type tip articulator was bent approximately 90 degrees inany directions when a 200 mA current was applied. The joint mechanism consists of base plate, universal joint,reflection plate (diameter: 170 mm), SMA microcoil springs (wire diameter: 0.1 mm, coil diameter: 0.9 mm) and biasspring. The joint mechanism showed good response for control values with maximum tilt angle of 3.2 degrees. We alsoestimated the thermal cycling behavior of SMA microcoil actuators with the resistance monitoring method. SMAmicrocoil actuators (wire diameter: 0.1 mm, coil diameter: 0.5 mm) were given 3.0% shear stress and heated by current.For 10,000 cycles, the force of SMA microcoil actuators was approximately constant and showed good long-termreliability. Since the results show good characteristics and reliability, SMA microcoil actuators can be used in a widerange of industrial and medical applications.

Keywords: shape memory alloy, micro actuator, micro coil, tip articulator, thermal cycling behavior

1. INTRODUCTION

When a micromachine works in narrow spaces inside tubules and equipment, micro actuators with large stroke andoutput power are required. SMA coil spring actuators are well suited to this purpose because of their large displacementcompared with conventional actuators. In addition, because of their large output power per unit cross section andreduction of heat capacity due to micronization, response is improved. As a flexible actuator that performs expansion-contraction operation by heating and cooling, this device will be suitable for micro mechanisms.

In the fabrication of SMA coil springs, we examined fabrication conditions such as the tension and pitch of the SMAwire. As a result, we have successfully fabricated coil springs with a minimum outer diameter of 76 µm. SMA wire of25 µm diameter and stainless steel wire of 30 µm diameter were used1). As an application using SMA actuators, wedeveloped an electrically and optically driven tip articulation mechanism for microfactories2) in our previous works 3-5).

To put the SMA actuators to practical use, the heat cycling behavior may be important. Especially, R-phase of Ti-Nialloy has very small hysteresis between the martensite-phase and austenite-phase. In R-phase transformation, we canmonitor the state of the transformation of SMA from electric resistance because the electric resistance of SMA isproportional to temperature6). Also, Ti-Ni is suited for electrical driving because of its comparatively high electricresistance. Therefore, it is very important to know the heat cycling behavior when the actuator is driven by an electriccurrent.

In this paper, we present the design and characteristics of two types of SMA microcoil actuator: the tube type tiparticulator and the joint mechanism. We also discuss the long-term reliability of SMA microcoil actuators withreference to thermal cycling behavior up to 10,000 heat cycles.

2. TIP ARTICULATION MECHANISM

In our previous works3-5), we developed a tip articulation mechanism that consists of a tip articulator and SMA coilspring actuator. The actuator has two kinds of SMA coil spring: helical compressive springs used in the tip articulator

and close wound coil springs used in the SMA coilspring actuator. The schematic diagram of the tiparticulation mechanism is shown in Fig. 1. The closewound coil springs are stretched, one end being fixed tothe support plates, and the other edge fixed to the wires.The helical compressive springs are compressed betweensupport plates. The principle of the mechanism is asfollows: At room temperature, the two kinds of SMAcoil spring are balanced and apply no force to the pullwire. When one SMA close wound coil spring on theactuator is heated with the backside SMA compressivecoil springs on the tip articulator to restore their originalshape, the displacement of the wire and force of thecompressive coil springs bend the tip articulator. Thesame occurs in the opposite direction. As a result, the tiparticulation mechanism was bent approximately 60degrees and returned to the straight position when acurrent was applied to each of the SMA coil springs. Butthe part of the SMA coil spring actuator was comparatively long and rigid; for applying the mechanism to flexibledevices, it was necessary to avoid making rigid parts.

3 FIBER-OPTIC ACTUATOR FOR TIP ARTICULATION

A fiberscope tip articulator is necessary to move the tip to theobservation point. Although we have been developing a tiparticulator using a shape memory alloy (SMA) coil spring3),electric current drove it. For observing in humidity atmosphereand EMI immunity, it was necessary to improve the heatingmethod.

For heating SMA coil spring, we have developed step-etchedfiber (see Fig.2). The fiber irradiates light in direction of not onlystraight but also axis. The step-etched fiber is fabricated bychemical etching of hydrofluoric acid. Schematic diagram offiber-optic actuator for tip articulation is shown in Fig.3. Laserdiode (wavelength: 810nm, 1W) was used as light source. For thetransmitting source, a laser fiber 400/500µm (core/clad) in

Figure 1: Tip articulation mechanism

Bending

Heat

Heat

wireSMA close wound coil springs

Fiberscope

SMA coil springs

Tip articulator SMA coil spring actuator

Taper angle θ

Figure 2: Step etched fiber

Tip articulator

SMA coil

Super Elastic Alloy coil

A

APull wire

SMA coil A-A

Step etched fiber

Support plate

Super ElasticAlloy coil Fiberscope

RingSMA coil

Ring

Figure 3: Fiber-optic articulator Figure 4: Motion of the fiber-optic tip articulator

diameter was used. The light from step-etched fiber can heat SMA actuator more than Af. Using the optically drivenSMA actuator, we have developed tip articulator for bending the tip of the environmental recognition device. Fig.4shows that optically driven tip articulator was bent approximately 60 degrees and returned to the straight position whenlaser light was turned off.

4.LATCH MECHANISM AND FIBER-OPTIC SENSOR FOR MICROSCOPIC OBSERVATION

Latch mechanism wants holding rigid state a tip-articulator to measuring the dimensions of microparts. Heating allSMA close round coil springs and pulling all wires locks the tip-articulator (See Fig.5). Using the latch mechanism, wehave developed microscopic observation system for fiberscope.Also a tactile sensor was developed for microscopic observation andmeasurement system. A fiber-optic tactile microsensor was attachedto the tip of the latch mechanism. A cross-sectional view of thetactile sensor we designed is shown in Fig.6. The tactile sensorconsists of optical fiber, rubber and steel ball/rod arranged in a SUSpipe of 0.6/0.36 mm outer diameter. The light through the opticalfiber is reflected at the steel ball/rod surface. If the steel ball/rodcontacts the object, the optical reflection intensity is modulated bydisplacement of the steel ball/rod7). The 0.1 mm thick polyurethanerubber was formed by KrF excimer laser ablation. The performanceof the sensor is shown in Fig. 7. Resolution of touch sensing is lessthan 10mN. The results demonstrate that easy and safe tactilemeasurement is possible in narrow spaces. Three tactile sensorswere assembled in this microscopic observation system. The systemsconsist of the tactile sensor, an ordinary fiberscope and a contactscope. Fig.8 is a cross-sectional view of the microscopic observationand�measurement system. The contact scope does not have a focallens on the tip of the image-guide fiber. Thus, if there is any distancebetween the image-guide fiber and target, the contact scope cannotobtain the image. However, when the tip of the image-guide fibertouches the surface of the target using latch mechanism, themicroscopic image can be determined from the image-guide fiber.

Heat

Heat

Figure 5: Latch mechanism

Contact Scope

Imageguide

SMA Coil ActuatorOptical Fiber

Tactile Sensor

Figure 8: Construction of microscopic

Optical fiber (ø:155µm)

SUS pipe (ø:450µm)

SUS pipe (ø:600µm)

Rubber

Steel ball (ø:400µm)

Optical fiberSilicon rubber

SUS pipe(ø:360µm)

(a) Ball type

(b) Rod type

Figure 6: Structure of tactile

0

10

20

30

40

0 10 20 30

Road (mN)

Ball type

Rod type (L=6mm)

Rod type ( L=3mm)

Figure 7: Performance of tactile

An ordinary fiberscope obtains the far field image to direct the systems to the observation area. The contact scopeobtains the near field image and can observe small structure even less than outer diameter of the contact scope.

5.TUBE TYPE TIP ARTICULATOR

A novel tube type tip articulator consists of 4 sets of SMAmicrocoil (wire diameter: 0.125 mm, coil diameter: 0.5 mm) asthe driving source and super elastic alloy (SEA) microcoils (wirediameter: 0.1 mm, coil diameter: 0.5 mm) for the bias springs,support plates and flexible outer tube. A schematic diagram of thetube type tip articulator is shown in Fig.9. The SMA close woundcoils are driven by electric current, so the support plates must bemade of insulated material. Support plates made ofpolyamideimide were fabricated by excimer laser machining. Thewavelength of machining beam was 248nm of KrF. A photographof the support plate of 4mm outer diameter is shown in Fig.10.The flexible outer tube consists of stainless steel coil (wirediameter: 50 µm, coil diameter: 4.0 mm) and polyparaxylene thinfilm (thickness: 20 µm) was formed by thermal evaporation. Aphotograph of the flexible outer tube is shown in Fig.11.

Because the SMA coil springs are built into bending part of thetip articulator with SEA bias springs, there is no rigid partcompared with our previous work shown in Fig.1. The fullyflexible structure of the tip articulator allows it to be used for avery wide range of applications such as medical and industrialscopes. The tube type tip articulator was bent approximately 90degrees in any direction when a 200 mA current was applied.Bending motion of the tip articulator is shown in Fig.12. Althoughthe amplitude of the current was relatively large, it is thought thatthe current can be reduced. The SMA coil springs were controlledby the electric resistance feedback method. Thus, the tip articulatorcould be bent at any angle in any direction.

4mm2mm

0.6mm

SMAclose wound coil

SEA compressive coil

Support plate

Figure 9: Schematic diagram of tube type tiparticulator

Figure 11: Photograph of flexible outer tubeFigure 12: Bending motion of tube type tip articulator

Figure 10: Photograph of support plate

4mm

6. JOINT MECHANISM

We also applied the SMA microcoil actuator to a joint mechanism. Conventional joint mechanisms for robots used tobe driven by motors and wires, but such mechanisms occupy a rather large volume and generate much noise.Application of the SMA microcoil actuator to the mechanism may make it significantly quieter, smaller and lightercompared with the conventional one. The joint mechanism that we have developed consists of a base plate, universaljoint, reflection plate (diameter: 170 mm), SMA microcoil springs (wire diameter: 0.1 mm, coil diameter: 0.9 mm) andbias spring. Two laser position meters were attached to the base plate to measure the tilt angle of the reflection plate. Aschematic diagram and photograph of the joint mechanism are shown in Fig.13 and Fig.14, respectively.

Nine SMA microcoils were electrically connected in series and mechanically connected in parallel (ξ-array). Both endsof each SMA microcoil were bound to a sleeve that was fabricated by YAG laser processing (wavelength: 1.06 µm).SMA actuators having high electric resistance and high power can be achieved by using the ξ-array8). SMA microcoilunits were attached to the joint mechanism of 42mm in length. The recovery force of the SMA microcoil unit was 2.6N.A schematic diagram of the SMA microcoil units is shown in Fig.15. Stainless steel springs (wire diameter: 0.7 mm,coil diameter: 6.4 mm) for the bias springs were also attached to the joint mechanism with SMA actuator units. Foursets of SMA actuator unit and bias spring make it possible to drive the joint mechanism in any direction. The bending

Universal joint

Laser displacement meter

Reflection plate

90�

Base

100mm 100mm

70mm

100mm

170mm

Figure 13: Schematic diagram of joint mechanism Figure 14: Photograph of joint

0 20 40 60

-4

-2

0

2

4

Time (S)

Ben

ding

ang

le o

f jo

int

mec

hani

sm (

deg)

SMA microcoil Sleeve

Figure 15: SMA actuator unit Figure 16: Motion of reflection plate

angle of the joint mechanism was controlled by the distance between the reflection and base plates from two laserposition meters.

The SMA microcoil actuators were driven by a sine wave (T: 25-60s). The reflection plate circulated according to theperiod and magnitude of the applied sine wave. The laser position meter detected displacement between the reflectionplate and base plate, and the SMA microcoil actuators were controlled based on the displacement. The motion of thereflection plate is shown in Fig.16. The motion of the reflection plate followed the sine-wave driving current at lowfrequencies, but to drive at a high frequency, it is necessary to improve the method of cooling the SMA actuators. As aresult, the joint mechanism showed good response for control values with maximum tilt angle of 3.2 degrees.

7. THERMAL CYCLING BEHAVIOR OF SMA

To apply SMA microcoil actuators in practice, the cyclic behaviorof SMA microcoil actuators is very important. H. Tobushi et al. 9)

investigated the cyclic properties of SMA helical springs byapplying heat cycles using hot water and air. But our applicationsmentioned above are driven by electric current and controlledelectric resistance feedback. To estimate the thermal cyclicbehavior of SMA microcoil actuators, it is necessary to apply theheat cycle by electric current. For this purpose, we developed athermal cyclic test by electric resistance monitoring. A schematicdiagram of the test equipment is shown in Fig.17.

The specimen was a Ti-Ni SMA microcoil, 0.1mm in wirediameter and 0.5mm in coil outer diameter. The number ofwindings of the coil was 150. The microcoil was heat-treated at623 and 723 K for 1 hour in a furnace. After that, the specimenswere attached to the heat cycle test equipment with 3.0% shearstress. Details of the specimens are shown in Table 1.

Table 1. List of specimens

No.Composition

(at%)Heat Treatment

Temp.Transformation

Temperature (K)Ni Ti (K) Af As Ms Mf

(1) 50.03 49.97 623 338.3 316.7 335.8 307.0(2) 723 337.5 324.8 327.8 317.3(3) 50.95 49.05 623 342.1 292.9 340.5 287.1(4) 723 315.6 299.1 312.2 295.0

The SMA microcoils were heated by electric current while monitoring the electric resistance of the coils, then afterheating, the SMA microcoils were cooled spontaneously. The thermal cycle was repeated up to 10,000 cycles. Thechange of generated force of the SMA microcoil is shown in Fig.18. Samples except for No.3 showed good reliability.Also, samples of Ti-49.95at%Ni SMA show better characteristics than samples of Ti-49.03at%Ni SMA. It is obviousfrom Fig.18 that the long-term reliability is closely related with the composition of Ti-Ni alloy and temperature of heattreatment. Figure 19 shows the relationship between electric resistance and force of the SMA microcoil. There is nodifference between the initial state and after repeated thermal cycles. It is clear from Figs.18 and 19 that Ti-49.95at%NiSMA shows excellent characteristics both in terms of generated force and electric resistance, hence it can be used in awide range of applications for highly reliable actuators.

Figure 17: Experimental setup of heat cycle test

8. CONCLUSIONS

We have developed two types of shape memory alloy (SMA) actuator and estimated the long-term reliability of SMAmicrocoils. A tube type tip articulator was bent approximately 90 degrees in any direction when a 200 mA current wasapplied. The joint mechanism showed good response for control values with maximum tilt angle of 3.2 degrees. Wealso estimated the thermal cycling behavior of SMA microcoil actuators with the resistance monitoring method. For10,000 cycles, the force of the SMA microcoil actuators was approximately constant and showed good long-termreliability. Since the results show good characteristics and reliability, SMA microcoil actuators can be used in a widerange of industrial and medical applications such as microfactories, powerplant maintenance and minimally invasivesurgery.

ACKNOWLEDGEMENTS

The authors would like to express their appreciation to Prof. T. Saburi of the Department of Materials Science andEngineering of Kansai University for valuable discussions. A part of this work was performed by Mitsubishi CableIndustries, Ltd. under the management of the Micromachine Center as the ISTF of MITI supported by NEDO.

0 2000 4000 6000 8000 100000.8

0.9

1

Number of Cycles

No

rma

lize

d F

orc

e(a

t 3

53

K)

Sample No.(1) Ti-Ni:50.03at%,H.T:623KSample No.(2) Ti-Ni:50.03at%,H.T:723KSample No.(3) Ti-Ni:50.95at%,H.T:623KSample No.(4) Ti-Ni:50.95at%,H.T:723K

27 28 29 30 310

0.1

0.2

Elect rical Resistivity���

Fo

rce�

N�

27 28 29 30 310

0.1

0.2

Electrical Resistivity���

Fo

rce�

N

�

Figure 18: Change of force as a function of repetition thermal cycle

Figure 19: Relationship between force and electric resistance

(a) Initial (b) After 10,000cycles

REFERENCES

1. S. Maeda, O. Tohyama and H. Itoh :”Micro Actuators with Shape Memory Alloy (SMA) Coil Springs and theirApplication to Active Fiberscopes”, IEICE Trans. Electron., E80-C, pp.226-231, (1997).

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3. O. Tohyama, S. Maeda, K. Abe and M. Murayama :”Fiber-Optic Sensors and Actuators for EnvironmentalRecognition Devices”, IEICE Trans. Electron., E83-C, pp.475-480, (2000).

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6. O. Tohyama, M. Kohashi, M. Sugihara and H. Itoh :”A fiber-optic pressure microsensor for biomedicalapplications”, Sensors and Actuators,A66, pp.150-154, (1998)

7. H. Tobushi, K. Kimura, T. Sawada, T. Hattori and P.H. Lin :”Recovery Stress Associated with R-PhaseTransformation in TiNi Shape Memory Alloy”, JSME Int. J., Ser. A, 37, pp.138-142, (1994).

8. K. Ikuta, M. Tsukamoto, S. Hirose :”Shape Memory Alloy Servo Actuator System with Electric ResistanceFeedback and Application for Active Endoscope”, Proc. International Conference on Robotic and Automation,Philadelphia, 1988, pp.427-430.

9. H. Tobushi, Y Ohashi, T. Horii and H. Yamamoto :” Cyclic Deformation of TiNi Shape-memory Alloy HelicalSpring”, Experimental Mechanics, 32, pp.304-308, (1992)

* Osamu_Tooyama@mitsubishi-cable.co.jp; phone +81-727-81-8823; fax +81-727-81-8812; http://www.mitsubishi-cable.co.jp; Mitsubishi Cable Industries, Ltd., 4-3, Ikejiri, Itami, Hyogo 664-0027 Japan