XXVIII. ASR '2003 Seminar, Instruments and Control, Ostrava, May 6, 2003 363

Uncommon Actuators For Robotics – Shape Memory Alloy

VAŠINA, Michal

Ing., ÚAMT, FEKT - VUT Brno, Božetěchova 2, Brno – Kr. Pole, 612 66 vasinam@feec.vutbr.cz

1.IntroductionThe discovery of Shape Memory Alloy can be dated to the first half of twentieth

century. In 1932, a Swedish physicist Arne Olander was notice interesting phenomenon, whenhe worked with compound of gold(Au) and cadmium(Cd). This alloy could be plasticallydeformed when cool and it was returned to its original dimensions by the heating. Thisbehaviour is known as a Shape Memory Effect (SME) and alloys, which are able to exhibitthis behaviour, are called Shape Memory Alloys.

SME was for the first time presented in Brussel’s World’s Fair in 1958. There waspresented the SMA (Au-Cd) actuator, which cyclically lifted a weight. The U.S. NavalOrdnance Laboratory researches made in 1961 a significant discovery in the field of SMA andSME during the testing the Nickel and Titanium compound. The Ni-Ti alloy exhibited theSME and it has got a few advantages with respect to other previously discovered alloys.

In the 1970’s the first commercial products went to operation. SMA was implemented tostatic and dynamic systems. The examples of static applications are electrical connectors andmechanical couplings for piping systems. In 1971 there was made the first really sophisticatedmachine, it was artificial heart powered by electrical actuation of SMA elements.

2.Mechanism of shape memory effectShape Memory Alloys are compounds of two or more alloys with some special

properties. They belong to group of intermetallic compounds. The typical attribute of thesematerials is that their parameters can’t be determined by simple interpolation of properties ofalloys included in the compound. Superplasticity, superelasticty, acid resistance and SMEmake these compounds technically important. The mechanical motion in SMA is caused bythe changes of crystalline structure of SMA. Crystalline structure is affected by severalfactors. In robotics systems the most important are the temperature T and external stress σ.Changes of the T or σ cause phase transformation in SMA. During this transformation thematerial structure is changed according to (fig.1). There are differentiate two types ofcrystalline structure. There is highly symmetrical structure – austenite (analogy of steel) andstructure with less symmetrical crystalline organization (orthorhombic, tetragonal) –martensite. More types of martensite can arise from one austenite structure during martensitetransformation. But reverse martensite transformation is only to the one austenite type. Themartensite transformation is non-symmetrical due to three-dimensional crystalline structure.Non-symmetrical effect of the transformation is observed mainly in push – pull operation ofthe SMA.

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Figure 1: Changes in crystalline structure of

Crystalline structure is in deformable state (martensite) when its temperature is lowerthan Mt. In this state the SMA elements length can be changed about 10% as the consequenceof an external stress. If the material is heated to temperature At (line 1, fig.1) the crystallinestructure will change to hard non-deformable state (austenite). If the temperature is held at Atthe elements of the SMA stay in contracted state. When the temperature decreases under Mtthe material is deformable once again. SMA element is extended if it is actuated by arefreshing force (line 2, fig.1) else it stays in contracted state (line 3, fig.1). When thetemperature is increased over point At, SME of the material can be completely destroyed. Itmeans that the material loses SME (element will stay contracted forever). Conversion insidethe SMA materials isn’t thermodynamically reversible process. There is energy dissipationdue to internal friction and creation of structural defects. The result of these processes is atemperature hysteresis loop illustrated on figure 2. Relation between deformation andtemperature is similar. Parameters of hysteresis loop of SMA are given by SMA compositionand process of production.

Figure 2: Hysteresis loop in Shape Memory Alloys

There is another phenomenon with a similar behaviour such as SME. Two WayMemory Effect (TWME) was named by its attributes. The alloys exhibit this type of memory

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effect have a two states. Hi-temperature and low-temperature. The exact origin andmechanism of the TWME obtained after training cycling can be easily explained by use ofthermodynamic description. The processing to induce the TWME consists in general ofthermomechanical cycling through the transformation region to acquire the cold, martensiticshape and is therefore referred to as training.

3. Interesting Compound - NI-TI AlloyThe most flexible and beneficial Shape Memory Alloy, that has been discovered so far,

for engineering applications is a Nickel-Titanium (Ni-Ti) compound. NiTinol is name given toSMA developed by researchers from U.S. Naval Ordnance Laboratory (Nickel-TitaniumNational Ordnance Laboratory). The following characteristics of NiTinol compound make itstand out from the other SMA’s: ability to be electrically heated for shape recovery, stabletransformation temperature, excellent corrosion resistance, more recoverable motion, highbiocompatibility.

NiTinol is a binary, equiatomic intermetallic compound of Nickel and Titanium. Thisalloy consists of 50 atomic% on Nickel and 50 atomic% Titanium approximately.Metallurgical properties of NiTinol, allows to change its mechanical attributes andtemperatures of phases transformations. This change is executed by the addition anotherchemical element or by the change of ratio of alloys in compound. For example, when Nickelquantity in Ni-Ti increases by 1%, phase transformation temperature decreases, but yieldstrength in austenite increases. Transformation temperature can be also decreased by additioniron or chromium. Adding of these and other chemical elements transformation temperaturecan be varied between -200 and 110°C. Copper can be added to decrease the hysteresis andlower the deformation stress of the martensite.

Production and formation of NiTinol for specific functions is complicated problem.Titanium is a very reactive element. Because of this reason melting must be done in an inertatmosphere. Plasma-arc melting, electron-beam melting, and vacuum induction melting arecommon methods. For shaping Ni-Ti ingots there are used standard hot-forming and cold-working processes. During cold-working the alloy work hardens very quickly and must beannealed frequently. SMA´s characteristics are refined by hardening and correct hot forming.During work with NiTinol it is difficult to use welding and low or high temperature soldering.For creating specific shapes the grinding, shearing, and punching is often used.

4. SMA in engineering applicationsWhen using the SMA in the practical applications, it is necessary to reckon with certain

constraint conditions. The alloys, which are made for the usage of SME, usually have a high-melting point thanks to the metals used and therefore it is impossible to use classical low-temperature soldering. Thus a problem may arise in contacting the SMA drive but in mostcases this problem is solved by usage of mechanical joints. However, the contact resistance,parasitic heat dissipation and other effects have to be taken in consideration. The question ofheat shielding is another factor, which does not make the usage of SMA easier. The hysteresishas already been mentioned above. In most SMA cases the width of hysteresis ranges from10°C to 50°C. The SMA components can be made in almost any shape and they canessentially evolve forces of any direction. For practical reasons the majority of the elementsthat have been produced so far can develop only tensile force but the movement of mostmechanisms requires forces and moments that change the direction. Therefore, it is necessary

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to use a reverse force in SMA mechanisms, either by gravitation, spring or inversed-antagonistic SMA drive.

The choice of the SMA control is also dependent on a concrete usage. The possibilitiesof the SMA temperature changes have already been mentioned in the introduction. Theelectrical resistance of the alloy is in the overwhelming majority of cases used in the systemswhere the SME is classified as an actuator. By providing the current flow through the SMAdrive we achieve the change of temperature thanks to its electrical resistance and thereby itcauses the SME. Such a way of contraction control of the actuators is discussed in [5] [6].This method of control was also used in the construction of a six-legged stepping robot (fig.5)at the author’s department in Brno University of technology. Each leg of the robot has twodegrees of freedom and the movement of the leg is provided by three NiTinol drives.

Figure 5: A six-legged stepping robot with two degrees of freedom in each leg SMA actuated.

The horizontal movement is realized by the antagonistic connection of two wires. Thevertical movement is realized by connection of SMA wire and a spring. The method of PWM,which is generated by the microprocessor and which the peripheral electronic switches areconnected to, is used for the control of the actuator´s contraction. These electronic switcheshave to be of sufficient power dimension. NiTinol actuator in the form of a wire with thediameter of 0,35mm, resistance of 0,5W and maximal pressure force of 9,3 N, requires thecurrent of 1000 mA [8]. At the moment of testing the robot was supplied from a laboratorysource. The replacement of laboratory source by batteries is the question of sufficient reservefor maximal possible load of robot´s construction. During the testing this robot and anothercases of antagonistic force working was found the necessity to switch each single driveexactly. Concrete parameters of used SMA wires area given in technical specificationsprovided by the SMA producers [8] [10]. The Nitinol is distributed in many shapes. Theeasiest variant of the control of SMA actuators is based on the principle of ON / OFF control,where the effects of undesirable SMA properties are minimal and they can be eliminated bysophisticated electronics. However, this type of control is difficult to apply in morecomplicated systems where smooth motion is required. Continuous control of SMA actuatorsis relatively difficult. Its quality depends for example on the ability to create a suitable SMAmodel and its correct identification. The main problem of SMA components control is anextensive hysteresis of their static characteristics. The majority of current SMA models thatare used for their control are based on models and methods described in [2] [7]. Some other

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ways of SMA modelling are described in [3] [4]. The complexity of SMA behaviour and itsmodels is the main cause that no reliable and continuous quality control exists so far. Suchcontrol is in the process of rudimentary research.

5. ConclusionThe drives based on metals with the shape memory effect are the subject of research in

many institutions, which are interested in the research of robotics. Research in this area ispromising and attractive both from practical and theoretical point of view.

6. AcknowledgementsThis work was supported by the Grant Agency of Czech Republic under project

102/02/0782 “Research in Control of Smart Robotic Actuators”

7. References1. IKUTA, K., TSUKAMOTO, M., HIROSE, S.: Shape Memory Alloy Servo Actuator

System With Electric Resistance Feedback and Application for Active Endoscope, Proc.of the 1988 IEEE International Conference on Robotics and Automation, Vol.1.,Computer Society Press, Washington, DC.

2. BAR-COHEN,Y.: Topics on Nondestructive Evaluation. Vol. 4. American Society forNondestructive Testing. 2000. ISBN 1-57117-043-X

3. DRAHOŠ, P.: Model of Shape Memory Alloy Drive. In: Proceedings of 2nd ConferenceTASCOM 97 in SR (Žilina). Vol.3. University of Žilina 1997. pp.265-268

4. DRAHOŠ, P.: Thermodynamic Model of SMA Drive. In: Proceedings of 4ndConference Process Control 2000 in ČR (Kouty nad Desnou),University of Pardubice

5. CONRAD, J.M., MILLS, J.W.: Stiquito, Advanced experiments with a simple andinexpensive robot, IEEE Computer Society Press, Los Alamitos, Calofornia, 1998,ISBN 0-8186-7408-3

6. GILBERTSON, G.G.:Muscle wires project book, Mondo-tronics, inc., San Rafael,2000, ISBN 1-879896-16-8

7. OTSUKA, K., WAYMAN, C.M.: Shape Memory Materials, Cambridge UniversityPress, 1998, ISBN 0 521 44487

8. Dynalloy, Inc., Technical characteristics of Flexinol actuators wires,http://www.dynalloy.com//

9. Center for intelligent materials systems and structures, Department of MechanicalEngineering, Virginia Tech., Blacksburghttp://www.cimss.vt.edu//

10. Smart components and micro-technologies, Euroflex Schussler GmbHhttp://www.nitinol.de/contact/index.htm