Sensors and Actuators A 219 (2014) 4757
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Sensors and Actuators A: Physical
j ourna l h o mepage: www.elsev ier .com/ locate /sna
esign of an impact drive actuator using a shape memory alloy wire
hinya Hattori, Masayuki Hara , Hiroyuki Nabae, Donghyun Hwang, Toshiro Higuchiepartment of Precision Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
r t i c l e i n f o
rticle history:eceived 2 May 2014eceived in revised form 11 July 2014ccepted 24 August 2014vailable online 3 September 2014
a b s t r a c t
This paper introduces an impact drive mechanism (IDM) that utilizes the rapid contraction property ofshape memory alloy (SMA) wire. In this study, possible structures for an SMA-wire-based IDM actuatorare investigated, and a prototype actuator comprising a main body, an inertia body, an SMA wire, anda bias spring is developed. To verify the applicability of the prototype actuator as a positioning device,driving experiments were conducted under various conditions. The experimental results demonstratedthat the prototype actuator enables bidirectional step-like movement with several step sizes by changing0-019-00
eywords:hape memory alloy
the profile of applied voltage, such as amplitude, duty cycle, and frequency. In addition, we developed PI-controller-based position control systems by using these three parameters as control input; the controlcharacteristics and potential applications of each method were discussed. These results implied that theSMA-wire-based IDM actuator has the potential to be used as a new linear actuator in sub-millimeterorder driving.mpact drive mechanisminear actuator
In recent years, technological advancements have acceler-ted the trend of downsizing of electrical products. In particular,obile phones have undergone rapid downsizing, which nowa-ays support several functions such as high-resolution imagingnd high-speed wireless communication. For developing compactroducts with multiple functions, a compact high-performancectuator that can be driven in the limited space should be onef the most essential components. Electromagnetic motors haveeen widely used in actuators because of their high performance,asy availability, and good controllability. However, the structureonsisting of magnets and coils prevents further size reduction.urrently, the worlds smallest commercialized motor has 2 mmn diameter and 5 mm in length . Thus, the application of elec-romagnetic motors is becoming increasingly difficult as electricalroducts are downsized. Hence, alternative actuators that aremaller but offer high actuation performance will be necessary inhe near future.Solid-state actuators have been also studied with the aim of real-zing smaller and high-power actuators. Solid-state actuators usective materials that generate micro displacement (or strain) under
Corresponding author. Tel.: +81 3 5841 6465.E-mail addresses: email@example.com (S. Hattori),
firstname.lastname@example.org (M. Hara), email@example.com (H. Nabae),firstname.lastname@example.org (D. Hwang), email@example.comT. Higuchi).
ttp://dx.doi.org/10.1016/j.sna.2014.08.013924-4247/ 2014 Elsevier B.V. All rights reserved. 2014 Elsevier B.V. All rights reserved.
the effect of external energy such as electrical energy elements.Materials commonly used in solid-state actuators are piezoelectricelements, magnetostrictive materials, and shape memory alloys(SMAs), etc. Among these materials, the piezoelectric actuator hasparticularly attracted our interest owing to its large generativeforce, precise sub-micron displacement, and fast responsiveness. Hence, various types of actuators based on these propertieshave been proposed for application to various fields . Forexample, impact drive mechanism (IDM) actuators, which are wellknown as precise positioning devices that utilize the impact (iner-tia) and friction force produced when the piezoelectric element israpidly deformed , have been studied for application to scanningtunneling microscopes  and cell micromanipulators . Further,Yoshida et al.  proposed smooth impact drive mechanism (SIDM)actuator by improving upon the IDM principle. Currently, SIDMactuators are implemented in digital cameras to facilitate autofocus, zoom, and image stabilization. The present study also focuseson the IDM because the application of the driving principle couldbe very useful in implementing actuation in limited space.
The IDM mainly utilizes the fundamental physical phenomenonthat movement of an object accelerated on a friction surface byexternal impact force stops due to the friction force between theobject and friction surface. In previous studies, various types ofactuators utilizing the impulsive force have been developed, inwhich the impulsive force was mainly generated by electromag-
netic force , inertia force caused by rapid deformations ofthe piezoelectric element or magnetostrictive material [3,11], airpressures , and thermal expansions [13,14]. However, IDMactuators that can offer better performance than piezo-based IDM
48 S. Hattori et al. / Sensors and Actu
Main body Piezo Inertia body
(2) Rapid contracti on
(3) Slow extension
(1) Start Base
ig. 1. Driving principle of impact drive mechanism . As an example, the illus-ration uses a piezo-based IDM actuator to explain the behavior in each step.
ctuator have not been developed so far. Thus, this study examineshe possibility of using an SMA as the driving source in an IDM.MAs can be restored to a predetermined shape by Joule heat-ng . The possible strain and generative stress of an SMA arebviously larger than those of a piezoelectric element and magne-ostrictive material , although the responsiveness is not so highbasically, the SMA is actuated less than 100 Hz ). Since SMAsossess excellent energy density and tolerability for galling and cor-osion, SMA-based actuators can potentially be applied in variousnvironments. In this study, an IDM actuator using an SMA wire isroposed, and a prototype device is developed to examine whetherhe proposed actuator performs the expected movement . Thisaper mainly discusses the possibility of the SMA-wire-based IDMctuator, investigating the basic performance and characteristicsf the prototype actuator. In addition, with a view to enabling theractical application of the proposed actuator, we attempt to pro-ose position control methods based on the characteristics of theroposed actuator.
. Impact drive mechanism
In this section, we introduce the basic principle of IDM tooughly grasp the behavior of an IDM actuator with an SMA wire;ere, we take a piezo-based IDM actuator as an example. Fig. 1hows the schematic diagram of a typical piezo-based IDM actuator.he IDM actuator is mainly composed of three components: a mainody in contact with a base, a piezoelectric element to produce thempulsive force, and an inertia body for causing the impulsive iner-ia force to the main body. Fig. 1 also illustrates the driving principlend the process sequence is as follows:
1) In the initial state, the main body remains stationary condition
on the base.
2) The inertia body experiences high acceleration toward themain body when the piezoelectric element is activated so asto rapidly contract, inducing an impulsive inertia force on theators A 219 (2014) 4757
main body. At this time, the main body slips slightly toward theinertia body if the inertia force exceeds the static friction forcebetween the main body and the base.
(3) The piezoelectric element is activated to gradually extend untilthe original length, keeping the inertia force below the staticfriction force; the main body never moves in this period.
(4) Finally, the IDM actuator returns to initial state (1) with a smalldrift toward the inertia body.
The process described in (1) to (4) produces a tiny step-likedisplacement toward the inertia body. The step size depends onthe friction and the impact force, which is equal to the inertiaforce generated by the deformation of the piezoelectric element. Byrepeating this process, the IDM actuator achieves linear movementconsisting of continuous step-like displacements. Additionally, theIDM actuator enables the movement in the opposite direction byinverting the contraction and extension speeds of the piezoelectricelement.
As shown in Fig. 1, it should be noted that the coupled actionof rapid contraction and gradual extension (or rapid extension andgradual contraction) is necessary for the IDM. With regard to theapplication of an SMA wire to the IDM, rapid contraction can beobtained by intensive heating of the SMA wire, and natural cool-ing due to the air after the heating could realize gradual extension.Thus, the piezoelectric element can be replaced with an SMA wire inthe structure shown in Fig. 1 and an SMA-wire-based IDM actuatorwould be feasible. However, the rapid extension cannot be realizedby the natural air cooling because the cooling speed is not enough;the use of other cooling sources such as a Peltier element mightallow faster cooling but the structure becomes more complicatedand further downsizing would be difficult. Thus, we expect that anIDM actuator using an SMA wire can achieve only one-way move-ment, i.e., movement toward the inertia body; the present paperdefines the direction of this movement as positive direction.
3. IDM actuator using an SMA wire
In general, the application of bias force is necessary to repeat thecontraction and extension of SMA wire; the SMA wire would slackafter heating without the bias force. The bias force can be achievedby applying an external force such as gravity, elastic restoring force,or tension of another SMA wire that is activated antagonistically. Using these bias methods, some possible designs (structures)of SMA-wire-based IDM actuator can be devised on the basis of thestructures of piezo-based IDM actuator. Fig. 2(a) shows the struc-ture commonly used for the piezo-based IDM actuator, in whicha piezoelectric element is replaced with an SMA wire and a biasspring. This structure would allow one-way movement toward theinertia body because the SMA wire cannot extend rapidly as men-tioned in the previous section. Hence, a symmetric structure ofFig. 2(b) would be available to achieve bidirectional movement byswitching the activation of the two SMA wires although two SMAwires, bias springs, and actuator drivers are necessary. The struc-ture shown in Fig. 2(c) is stable in comparison with the structureof Fig. 2(a). In this structure, it is not necessary to concern aboutthe instability due to the difference in weight between the mainand inertia bodies but the assembling and parameter tuning wouldbecome more complicated. Fig. 2(d) shows a unique structure thatutilizes the gravity acting on the inertia body, in which the inertia
body is suspended by two SMA wires diagonally extended owingto the weight of the inertia body. This structure could perform bidi-rectional movement by controlling the activation of the two SMAwires. Finally, Fig. 2(e) illustrates a structure based on the SIDM
S. Hattori et al. / Sensors and Actuators A 219 (2014) 4757 49
Spring Inertia bodyMain body
Fig. 2. Possible structures enabling SMA-wire-based IDM actuator. Blue arrows indi-ci
Flexible electric wire Linear bushing
Inertia bodyPlastic screw&
(a) Side and top views (unit: mm)
(b) Cross-section viewate expected direction of movement. (For interpretation of the references to colorn this figure legend, the reader is referred to the web version of this article.)
6,8] which consists of a movable object and a shaft that is sup-orted by a spring and horizontally driven by the SMA wire. Theovable object moves horizontally toward the tip of the shaft by
epeating rapid contraction and gradual extension of the SMA wire.s shown in Fig. 2, several SMA-wire-based actuators would be fea-ible. However, in the present study, we designed and developed anMA-wire-based IDM actuator based on the simplest structure (a)ince the main focus of present study is to examine the applicabilityf an SMA wire as a driving source in the IDM.Fig. 3 shows a prototype IDM actuator composed of a main body,
n inertia body, a coil spring, and a SMA wire. The main (19.2 g) andnertia (7.9 g) bodies were made of aluminum, and their surfacesere anodized for electrical insulation between the bodies and theMA wire. In general, SMAs exhibit thermal hysteresis between thehases of heating and cooling, which is induced by the differencen the phase transformations from austenite/martensite to marten-ite/austenite. In the case of large thermal hysteresis, faster coolings necessary to rapidly return the SMA wire to the initial length butt cannot be realized by the air cooling. Hence, the increase in thehermal hysteresis degrades the responsiveness of SMA wire in theroposed actuator because the air cooling cannot be accelerated.n the present study, a NiTiCu alloy wire (NT-H7-TTR, Furukawaechno Material), which had the smallest hysteresis among avail-ble SMA wires, was selected in order to allow the highest possibleesponsiveness. The NiTiCu alloy wire contains 42.6% of nickel and.0% of copper. The phase transformation temperature in each statere As = 74.3, Af = 86.7, Ms = 76.5, and Mf = 63.1 C under 200 MPa
oad. The diameter and length were 0.1 mm and 70 mm, respec-ively, and the resistance was approximately 12 at 20.0 C. In
Fig. 3. Photo and schematic of a prototype actuator.Fig. 4. Schematics of side, top, and cross-section views of the prototype actuator.
addition, a coil spring with 0.5 N/mm stiffness was applied, preload-ing the SMA wire with 1.5 N at room temperature.
Fig. 4(a) shows the side and top views of the prototype actua-tor. The size was approximately 20 mm 20 mm 40 mm, wherethe longitudinal length varied depending on the tension of the SMAwire. A few screw holes on the main body facilitated the adjustmentof the wire length. The SMA wire was attached in a U-s...