Copyright Ⓒ 2013 JSPE

㪬㪈㪐A Bidirectional Rotary Actuator Using Shape Memory Alloy Wires
The University of Tokyo

Donghyun Hwang, Shinya Hattori, Toshiro Higuchi

A bidirectional rotary actuator using shape memory alloy (SMA) wires is proposed. To achieve more multifunctional andwider applicability in comparison with previously reported various types of SMA rotary actuators, a cycloidal driving
mechanism is devised and applied to the proposed rotary actuator. The cycloidal driving mechanism which consists of a rotor
gear, a wobbling annular gear with rollers, and three crankshafts can effectively convert the expansion and contraction of theSMA wires to the bidirectional rotary motion. And, with this mechanism, the proposed actuator quietly and smoothly generates
bidirectional high torque motion without interference. In this work, a conceptually designed actuator is fabricated as afunctional prototype, and driving characterization and working performance verification are experimentally conducted. Due toits structural simplicity, scalability, modularity, and bidirectional capability with high torque and low speed, the proposed
actuator is expected to apply to diverse scientific and industrial fields.

Bias spring, B(1)1. Introduction
Various types of SMA actuators have been practically used in

Crankshaft,C(1)many fields due to its attractive working characteristics as follows:

Annular gear
Rotary gear

high power density, silent operation with smooth motion, simple
driving mechanism, compatibility with corrosive environment, andY

so on [1]. Generally, the SMA actuators consist of SMA actuating
OA O

elements, parts for biasing force, driving mechanism, and electricalX

control units to electrically activate the SMA elements by Joule
heating. In the case of the rotary actuators using the SMA, theC(2)C (3)rotational driving mechanism, which can convert the motion ofB(3)SMA to rotary motion, is essentially required.
B(2)In this work, a cycloidal driving mechanism based SMA actuator

Rollerwhich can generate bidirectional rotary motion is proposed. With its
unique driving characteristic, the cycloidal driving mechanism isSMA wire (2)Pulleywidely used for high-torque rotary actuators or motors. To generate

Fig. 1. Configuration of the bidirectional rotary actuator which consists ofbidirectional motion with relatively high torque using SMA wires,a rotational driving mechanism and three SMA wires with bias springs

this driving mechanism is adopted to the proposed actuator. The
actuator is conceptually designed and fabricated as a functional

F1

F3

prototype. And, driving characterization and working performanceverification are experimentally carried out.

2. Configuration and operating principle of the actuator
F2

The proposed actuator, as shown in Figure 1, consists of a central
cycloidal driving mechanism and radially symmetrically placed
SMA wires which are linked in series with bias springs. As theFig. 2. Operating schematics of the rotational driving mechanism

cycloidal driving mechanism, a rotor gear is rotationally fixed on

kept during the wobbling motion, and the rotor gear is rotated. In

the center of actuator, and an annular gear supported by three

this principle, the function of the cycloidal driving mechanism is tocrankshafts is eccentrically engaged with the rotor gear. In thisconvert expansion and contraction of the SMA wires to the rotary

configuration, by adjustment of the preloads on the SMA wires andmotion. The proposed actuator generates bidirectional rotary motion.

the springs, the tensional forces can be balanced. And, the resultant

The rotational direction is given by activation sequence of the threeforce on the driving mechanism is minimized. It means that small

SMA wires, i.e., clockwise rotation and counterclockwise rotation

external force is needed to rotate the rotor gear.

for the S(1)–S(2)–S(3) activation sequence and S(1)–S(3)–S(2)Figure 2 schematically shows operating principle of the proposedrotary actuator. Due to the sequentially applied external forces F1,F2, and F3 which are generated by contraction of SMA wires S(1),S(2), and S(3), respectively, the annular gear is wobbled. This

activation sequence, respectively.
The transmission ratio, R, of this mechanism may be expressed asRPL L(1)

wobbling motion is guided as curvilinear motion, by triple

where P and L are the number of pins of the annular gear and theparallel-crank-mechanism formed by radially equally placed threenumber of lobes of the rotor gear, respectively. Therefore, thecrankshafts. Thus, the eccentrical engagement between two gears istransmission ratio of the actuator is 11 with 12 pins and 11 lobes.

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

31

Copyright Ⓒ 2013 JSPE

㪬㪈㪐Loadcell (3)SMA wire (3)10 mm

YLoadcell (1)SMA wire (1)X

Rotary encoder

Experimental base
SMA wire (2)Fabricated actuator

Loadcell (2)Fig. 3. A fabricated functional prototype of 160×150×25 mm with three
SMA wires of 0.1 mm diameter and 50 mm length (left) and planar
schematics of experimental setup (right)

0.6

Ks=0.15 N/mm

Ks=0.26 N/mm

Phase (1)0.4

Phase (2) Phase (3)Fig. 5.

Measured tensional forces and rotational displacement withdifferent amplitudes of applied voltage and pulse duty cycles

0.2
0.2

Force [N]Force of phase (2) [N]0.4

00-0.2-0.2-0.4
Ks=0.08 N/mm
-0.6
-0.6 -0.4 -0.2
00.2 0.4
Force of phase (1) [N]

Ks=0.15 N/mm

-0.40.6010.921.8
32.7
Rotational angle of a rotary gear [deg]Fig. 4. Balanced tensional forces in SMA wires and springs when therotor gear is rotated by an external force

Fig. 6. Pull out torque vs. speed for different stiffness of the bias springs

3. Fabrication and experimental setup

motion. Incremental step size is 10.9º approximately. This step size

For experimental verification of working performances, acan be theoretically expected with the transmission ratio of thefunctional prototype is fabricated. The used cycloidal gear set withactuator. As compared with this motion, when 1.30 V/0.4 Hz withrollers is disassembled from a polymeric prototype which is made to

50% PDC is applied to the actuator, the rotor gear is rotatedverify the cycloidal driving principle. And, three NiTiCu SMA

relatively smoothly by overlapping pulses. Also, peak-to-peakwires of 0.1 mm diameter and 50 mm length are utilized [2]. In theamplitudes of measured tensional forces are reduced.

proposed actuator, these SMA wires are electrically heated up, andExperimental result of output torque versus speed characteristic

cooled down by natural convection. The fabricated actuator iswith the bias springs with different stiffness is shown in Figure 6.

assembled on the experimental bench, as shown in Figure. 3. To

The graph shows the deterioration of torque capability withmeasure the tensional forces in SMA wires and springs, all SMA

rotational speed. And, with relatively high spring stiffness, morewires are straightly arranged without pulley. In this experimental

high torque can be achieved at the same rotational speed.

setup, the rotational displacement and the tensional forces are

5. Conclusion

measured using optical rotary encoder (K-1, Canon) and three

In this work, the bidirectional SMA rotary actuator based on theloadcells (LVS-1KA, Kyowa) set up on manual linear stages,

cycloidal driving mechanism, which converts the expansion andrespectively. The tensional forces in the SMA wires and the springs

contraction of the SMA wires to the bidirectional rotary motion, isare balanced by adjusting the positions of the loadcells. Figure 4

proposed. A conceptually designed actuator is fabricated, andshows balanced tensional forces, when the rotor is rotated by andriving characterization and working performance verification areexternal force. Under this condition, driving characterization test

experimentally conducted. Experimental results show that theand working performance verification are conducted.

proposed SMA rotary actuator generates relatively high torque with

4. Performance verification

low speed. The proposed actuator is expected to apply to diverse

To drive the actuator, three-phase pulsed voltage is applied to thescientific and industrial fields, as a high-torque low-speed actuator,

three SMA wires. With consideration for generating forces affectedwith its structural simplicity, scalability, modularity, and operational

by voltage amplitude, activation frequency, and pulse duty cycle

easiness for bidirectional rotary motion.

(PDC), various driving tests are carried out to characterize theReferencesperformance. Figure 5 shows results of the measured tensional[1] W. Huang, “On the selection of shape memory alloys forforces and the rotational displacement with different amplitudes ofactuators,” Materials and Design, Vol. 23, pp. 11–19, Feb. 2002.

applied voltage and pulse duty cycles at the same frequency. When

[2] NT-H7-TTR, Furukawa NT Alloys, Furukawa Techno Material,

1.38 V/0.4 Hz with 30% PDC is applied, the SMA wires arewww.furukawa-ftm.com

activated one by one. And, the actuator generates rotational stepping

32

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.